Publications

The contemporary understanding that the immune response significantly supports higher brain functions has emphasized the notion that the brain's condition is linked in a complex manner to the state of the immune system. It is therefore not surprising that immunity is a key factor in shaping brain aging. In this perspective article, we propose amending the Latin phrase \u201cmens sana in corpore sano\u201d (\u201ca healthy mind in a healthy body\u201d) to \u201ca healthy mind in a healthy immune system.\u201d Briefly, we discuss the emerging understanding of the pivotal role of the immune system in supporting lifelong brain maintenance, how the aging of the immune system impacts the brain, and how the potential rejuvenation of the immune system could, in turn, help revitalize brain function, with the ultimate ambitious goal of developing an anti-aging immune therapy.

The central nervous system (CNS) is endowed with its own resident innate immune cells, the microglia. They constitute approximately 10% of the total cells within the CNS parenchyma and act as sentinels, sensing and mitigating any deviation from homeostasis. Nevertheless, under severe acute or chronic neurological injury or disease, microglia are unable to contain the damage, and the reparative activity of monocyte-derived macrophages (MDMs) is required. The failure of the microglia under such conditions could be an outcome of their prolonged exposure to hostile stimuli, leading to their exhaustion or senescence. Here, we describe the conditions under which the microglia fall short, focusing mainly on the context of Alzheimers disease, and shed light on the functions performed by MDMs. We discuss whether and how MDMs engage in cross-talk with the microglia, why their recruitment is often inadequate, and potential ways to augment their homing to the brain in a well-controlled manner.

Reciprocal communication between the brain and the immune system is essential for maintaining lifelong brain function. This interaction is mediated, at least in part, by immune cells recruited from both the circulation and niches at the borders of the brain. Here, we describe how immune exhaustion and senescence, even if not primary causative factors, can accelerate neurodegenerative diseases. We emphasize the role of a compromised peripheral immune system in driving neurodegeneration and discuss strategies for harnessing peripheral immunity to effectively treat neurodegenerative diseases, including the underlying mechanisms and opportunities for clinical translation. Specifically, we highlight the potential of boosting the immune system by blocking inhibitory checkpoint molecules to harness reparative immune cells in helping the brain to fight against neurodegeneration.

The brain's choroid plexus (CP), which operates as an anatomical and functional checkpoint, regulates the communication between brain and periphery and contributes to the maintenance of healthy brain homeostasis throughout life. Evidence from mouse models and humans reveals a link between loss of CP checkpoint properties and dysregulation of the CP immune milieu as a conserved feature across diverse neurological conditions. In particular, we suggest that an imbalance between different immune signals at the CP, including CD4⁺ T cell-derived cytokines, type-I interferon, and complement components, can perpetuate brain inflammation and cognitive deterioration in aging and neurodegeneration. Furthermore, we highlight the role of CP metabolism in controlling CP inflammation, and propose that targeting molecules that regulate CP metabolism could be effective in safeguarding brain function.

Alzheimers disease (AD) and dementia in general are age-related diseases with multiple contributing factors, including brain inflammation. Microglia, and specifically those expressing the AD risk gene TREM2, are considered important players in AD, but their exact contribution to pathology remains unclear. In this study, using high-throughput mass cytometry in the 5×FAD mouse model of amyloidosis, we identified senescent microglia that express high levels of TREM2 but also exhibit a distinct signature from TREM2-dependent disease-associated microglia (DAM). This senescent microglial protein signature was found in various mouse models that show cognitive decline, including aging, amyloidosis and tauopathy. TREM2-null mice had fewer microglia with a senescent signature. Treating 5×FAD mice with the senolytic BCL2 family inhibitor ABT-737 reduced senescent microglia, but not the DAM population, and this was accompanied by improved cognition and reduced brain inflammation. Our results suggest a dual and opposite involvement of TREM2 in microglial states, which must be considered when contemplating TREM2 as a therapeutic target in AD.

It is increasingly clear that the central nervous system (CNS) relies significantly on both adaptive and innate immune cells for its repair and lifelong maintenance. These interactions hold profound implications for brain aging and neurodegeneration. Recent work by Smyth et al. describes newfound anatomical connections between the brain and dura mater, which they named the arachnoid cuff exit points.

Systemic immunity supports lifelong brain function. Obesity posits a chronic burden on systemic immunity. Independently, obesity was shown as a risk factor for Alzheimers disease (AD). Here we show that high-fat obesogenic diet accelerated recognition-memory impairment in an AD mouse model (5xFAD). In obese 5xFAD mice, hippocampal cells displayed only minor diet-related transcriptional changes, whereas the splenic immune landscape exhibited aging-like CD4⁺ T-cell deregulation. Following plasma metabolite profiling, we identified free N-acetylneuraminic acid (NANA), the predominant sialic acid, as the metabolite linking recognition-memory impairment to increased splenic immune-suppressive cells in mice. Single-nucleus RNA-sequencing revealed mouse visceral adipose macrophages as a potential source of NANA. In vitro, NANA reduced CD4⁺ T-cell proliferation, tested in both mouse and human. In vivo, NANA administration to standard diet-fed mice recapitulated high-fat diet effects on CD4⁺ T cells and accelerated recognition-memory impairment in 5xFAD mice. We suggest that obesity accelerates disease manifestation in a mouse model of AD via systemic immune exhaustion.

The choroid plexus (CP) plays a key role in remotely controlling brain function in health, aging, and disease. Here, we report that CP epithelial cells express the brain-specific cholesterol 24-hydroxylase (CYP46A1) and that its levels are decreased under different mouse and human brain conditions, including amyloidosis, aging, and SARS-CoV-2 infection. Using primary mouse CP cell cultures, we demonstrate that the enzymatic product of CYP46A1, 24(S)-hydroxycholesterol, downregulates inflammatory transcriptomic signatures within the CP, found here to be elevated across multiple neurological conditions. In vitro, the pro-inflammatory cytokine tumor necrosis factor α (TNF-α) downregulates CYP46A1 expression, while overexpression of CYP46A1 or its pharmacological activation in mouse CP organ cultures increases resilience to TNF-α. In vivo, overexpression of CYP46A1 in the CP in transgenic mice with amyloidosis is associated with better cognitive performance and decreased brain inflammation. Our findings suggest that CYP46A1 expression in the CP impacts the role of this niche as a guardian of brain immune homeostasis.

Contemporary studies have completely changed the view of brain immunity from envisioning the brain as isolated and inaccessible to peripheral immune cells to an organ in close physical and functional communication with the immune system for its maintenance, function, and repair. Circulating immune cells reside in special niches in the brains borders, the choroid plexus, meninges, and perivascular spaces, from which they patrol and sense the brain in a remote manner. These niches, together with the meningeal lymphatic system and skull microchannels, provide multiple routes of interaction between the brain and the immune system, in addition to the blood vasculature. In this Review, we describe current ideas about brain immunity and their implications for brain aging, diseases, and immune-based therapeutic approaches.

In the version of this article originally published, there was an error in the topmost label (\u201cLPS injection\u201d) in Figure 4f. The citation to Lee et al. 2021 should have been to Hasel et al. 2021 (Nat. Neurosci. 24, 14751487 (2021)). The figure has been corrected in the HTML and PDF versions of the article.

The type I interferon (IFN) response is the body's typical immune defense against viruses. Previous studies linked high expression of genes encoding type I IFNs in the brain's choroid plexus to cognitive decline under virus-free conditions in aging and neurodegeneration. Multiple reports have documented persisting cognitive symptoms following recovery from COVID-19. Cumulative evidence shows that the choroid plexus is one of the brain regions most vulnerable to infection with the coronavirus SARS-CoV-2, and manifests increased expression of genes encoding type I IFNs even in the absence of viral traces within the brain. In this Perspective, we propose that the type I IFN defensive immune response to SARS-CoV-2 infection in the choroid plexus poses a risk to cognitive function if not resolved in a timely manner.

Recent functional and anatomical discoveries of brain-immune relationships have overturned previous beliefs regarding the brain's immune privilege. Here, we propose that the brain and immune cells at its borders operate as an \u201cecosystem\u201d to support the brain's robustness and resilience. Modulation of this ecosystem can be harnessed in the clinic.

Microglial research has advanced considerably in recent decades yet has been constrained by a rolling series of dichotomies such as \u201cresting versus activated\u201d and \u201cM1 versus M2.\u201d This dualistic classification of good or bad microglia is inconsistent with the wide repertoire of microglial states and functions in development, plasticity, aging, and diseases that were elucidated in recent years. New designations continuously arising in an attempt to describe the different microglial states, notably defined using transcriptomics and proteomics, may easily lead to a misleading, although unintentional, coupling of categories and functions. To address these issues, we assembled a group of multidisciplinary experts to discuss our current understanding of microglial states as a dynamic concept and the importance of addressing microglial function. Here, we provide a conceptual framework and recommendations on the use of microglial nomenclature for researchers, reviewers, and editors, which will serve as the foundations for a future white paper.

The incidence of age-related dementia is growing with increased longevity, yet there are currently no disease-modifying therapies for these devastating disorders. Studies over the last several years have led to an evolving awareness of the role of the immune system in supporting brain maintenance and repair, displaying a diverse repertoire of functions while orchestrating the crosstalk between the periphery and the brain. Here, we provide insights into the current understanding of therapeutic targets that could be adopted to modulate the immune system, either systemically or locally, to defeat brain aging and neurodegeneration.

Alzheimers disease (AD) is a complex neurodegenerative disease, perturbing neuronal and non-neuronal cell populations. In this study, using single-cell transcriptomics, we mapped all non-immune, non-neuronal cell populations in wild-type and AD model (5xFAD) mouse brains. We identified an oligodendrocyte state that increased in association with brain pathology, which we termed disease-associated oligodendrocytes (DOLs). In a murine model of amyloidosis, DOLs appear long after plaque accumulation, and amyloid-beta (Aβ) alone was not sufficient to induce the DOL signature in vitro. DOLs could be identified in a mouse model of tauopathy and in other murine neurodegenerative and autoimmune inflammatory conditions, suggesting a common response to severe pathological conditions. Using quantitative spatial analysis of mouse and postmortem human brain tissues, we found that oligodendrocytes expressing a key DOL marker (SERPINA3N/SERPINA3 accordingly) are present in the cortex in areas of brain damage and are enriched near Aβ plaques. In postmortem human brain tissue, the expression level of this marker correlated with cognitive decline. Altogether, this study uncovers a shared signature of oligodendrocytes in central nervous system pathologies.

For decades, neurodegenerative diseases were thought to be caused by the accumulation of toxic compounds, exacerbated by local inflammation, which together lead to neuronal loss and cognitive impairment . An additional factor that was long overlooked , is the role of the systemic immune system, which provides a defense mechanism against internal and external intruders in all bodily tissues. The evolving understanding of the life-long cross-talk between the CNS and the immune system led to an awareness of the function of systemic adaptive immunity in containing emerging destructive factors within the brain. . This includes harnessing of circulating myeloid cells to help the brain. However, as damage accumulates within the brain, the systemic immune system loses its protective capacity. Under such conditions, the dysregulated immune system becomes an escalating factor itself, thereby driving a vicious cycle that must be arrested. The brain's privilege does not imply immune isolation, but rather, a unique mode of brainimmune communication.Both adaptive and innate immune support are needed for brain maintenance and repair.Neurodegenerative diseases have an essential peripheral component, which is often manifested by immune dysfunction.While not a primary cause, the immune dysfunction develops at an early stage in neurodegenerative disease, presumably before symptoms appear, resulting in a vicious cycle that further escalates disease progression.Breaking the vicious cycle by activating the systemic immune system can harness bone marrow-derived macrophages and regulatory T cells to the brain to help defeat disease.

Microglia and monocyte-derived macrophages (MDM) are key players in dealing with Alzheimers disease. In amyloidosis mouse models, activation of microglia was found to be TREM2 dependent. Here, using Trem2^−/−5xFAD mice, we assessed whether MDM act via a TREM2-dependent pathway. We adopted a treatment protocol targeting the programmed cell death ligand-1 (PD-L1) immune checkpoint, previously shown to modify Alzheimers disease via MDM involvement. Blockade of PD-L1 in Trem2^−/−5xFAD mice resulted in cognitive improvement and reduced levels of water-soluble amyloid beta₁₄₂ with no effect on amyloid plaque burden. Single-cell RNA sequencing revealed that MDM, derived from both Trem2^−/− and Trem2^+/+5xFAD mouse brains, express a unique set of genes encoding scavenger receptors (for example, Mrc1, Msr1). Blockade of monocyte trafficking using anti-CCR2 antibody completely abrogated the cognitive improvement induced by anti-PD-L1 treatment in Trem2^−/−5xFAD mice and similarly, but to a lesser extent, in Trem2^+/+5xFAD mice. These results highlight a TREM2-independent, disease-modifying activity of MDM in an amyloidosis mouse model.

Removal of excess cholesterol from the brain in the form of 24-hydroxycholesterol (24-OH), produced by the CYP46A1 hydroxylase, is critical in diverse aspects of brain function, including clearance of toxic amyloid-β (Aβ). In the present study, we show that CYP46A1 is expressed by the choroid plexus (CP), where it decreases with Alzheimer's disease (AD) in both human patients and a transgenic mouse model of AD (5xFAD). We hypothesized, that CYP46A1 expression at the CP has a functional role, which may partake in disease modification. In addition, we have previously found that PD-1/PD-L1 immunotherapy improves cognition and reduces pathology in mouse models of neurodegeneration, and that the mechanism of action entails IFNγ-dependent mechanisms at the CP. Here, we investigated whether anti-PD-L1 treatment induces molecular pathways at the CP related to CYP46A1 activity as a regulator of cholesterol transport. We used an AAV viral vector for targeted CYP46A1 overexpression at the CP in order to examine the implications in AD pathology. We systemically injected 5XFAD mice with anti-PD-L1 and excised the CP for qPCR and ELISA in order to investigate potential molecular pathways at the CP related to cholesterol transport. We stimulated primary CP cultures with 24-OH and IFNγ and measured targets of interest via qPCR. Using a viral construct to target the CP, we demonstrate that CYP46A1 overexpression in 5xFAD mice ameliorates pathology. Furthermore, we show that systemic PD-L1 blockade in 5xFAD mice induces Cyp46a1 gene expression at the CP, followed by increased levels of apolipoprotein E (APOE), the main cholesterol carrier in the brain and a partaker in Aβ clearance. Using primary cultures of CP epithelium, we prove that 24-OH induces Apoe gene expression. Furthermore, we propose the transcription factor SP1 to mediate the IFNγ-dependent induction of Cyp46a1. Altogether, our findings suggest the CYP46A1-APOE axis at the CP as an amenable target to rescue AD.

Background: For decades, dementia has been characterized by accumulation of waste in the brain and low-grade inflammation. Over the years, emerging studies highlighted the involvement of the immune system in neurodegenerative disease emergence and severity. Numerous studies in animal models of amyloidosis demonstrated the beneficial role of monocyte-derived macrophages in mitigating the disease, though less is known regarding tauopathy. Boosting the immune system in animal models of both amyloidosis and tauopathy, resulted in improved cognitive performance and in a reduction of pathological manifestations. However, a full understanding of the chain of events that is involved, starting from the activation of the immune system, and leading to disease mitigation, remained elusive. Here, we hypothesized that the brain-immune communication pathway that is needed to be activated to combat tauopathy involves monocyte mobilization via the C-C chemokine receptor 2 (CCR2)/CCL2 axis, and additional immune cells, such as CD4⁺ T cells, including FOXP3⁺ regulatory CD4⁺ T cells. Methods: We used DM-hTAU transgenic mice, a mouse model of tauopathy, and applied an approach that boosts the immune system, via blocking the inhibitory Programmed cell death protein-1 (PD-1)/PD-L1 pathway, a manipulation previously shown to alleviate disease symptoms and pathology. An anti-CCR2 monoclonal antibody (αCCR2), was used to block the CCR2 axis in a protocol that partially eliminates monocytes from the circulation at the time of anti-PD-L1 antibody (αPD-L1) injection, and for the critical period of their recruitment into the brain following treatment. Results: Performance of DM-hTAU mice in short-term and working memory tasks, revealed that the beneficial effect of αPD-L1, assessed 1 month after a single injection, was abrogated following blockade of CCR2. This was accompanied by the loss of the beneficial effect on disease pathology, assessed by measurement of cortical aggregated human tau load using Homogeneous Time Resolved Fluorescence-based immunoassay, and by evaluation of hippocampal neuronal survival. Using both multiparametric flow cytometry, and Cytometry by Time Of Flight, we further demonstrated the accumulation of FOXP3⁺ regulatory CD4⁺ T cells in the brain, 12 days following the treatment, which was absent subsequent to CCR2 blockade. In addition, measurement of hippocampal levels of the T-cell chemoattractant, C-X-C motif chemokine ligand 12 (Cxcl12), and of inflammatory cytokines, revealed that αPD-L1 treatment reduced their expression, while blocking CCR2 reversed this effect. Conclusions: The CCR2/CCL2 axis is required to modify pathology using PD-L1 blockade in a mouse model of tauopathy. This modification involves, in addition to monocytes, the accumulation of FOXP3⁺ regulatory CD4⁺ T cells in the brain, and the T-cell chemoattractant, Cxcl12.

For decades, it was commonly accepted that the brain is secluded from peripheral immune activity and is self-sufficient for its maintenance and repair. This simplistic perception was based on the presence of resident immune cells, the microglia, and barrier systems within the brain, and the assumption that the central nervous system (CNS) lacks lymphatic drainage. This view was revised with the discoveries that higher functions of the CNS, homeostasis and repair are supported by peripheral innate and adaptive immune cells. The findings of bone marrow-derived immune cells in specialized niches, and the renewed observation that a lymphatic drainage system exists within the brain, further contributed to this revised model. In this Review, we describe the immune niches within the brain, the contribution of professional immune cells to brain functions, the bidirectional relationships between the CNS and the immune system and the relevance of immune components to brain aging and neurodegenerative diseases.

Tertiary lymphoid structures (TLS) are organized aggregates of B and T cells formed ectopically during different stages of life in response to inflammation, infection, or cancer. Here, we describe formation of structures reminiscent of TLS in the spinal cord meninges under several central nervous system (CNS) pathologies. After acute spinal cord injury, B and T lymphocytes locally aggregate within the meninges to form TLS-like structures, and continue to accumulate during the late phase of the response to the injury, with a negative impact on subsequent pathological conditions, such as experimental autoimmune encephalomyelitis. Using a chronic model of spinal cord pathology, the mSOD1 mouse model of amyotrophic lateral sclerosis, we further showed by single-cell RNA-sequencing that a meningeal lymphocyte niche forms, with a unique organization and activation state, including accumulation of pre-B cells in the spinal cord meninges. Such a response was not found in the CNS-draining cervical lymph nodes. The present findings suggest that a special immune response develops in the meninges during various neurological pathologies in the CNS, a possible reflection of its immune privileged nature.

An interaction network exists among cells within the brain, maintaining brain homeostasis and ensuring its functional plasticity. In addition to neurons, participating cells include astrocytes, oligodendrocytes, and microglia. Peripheral immune cells, such as monocytes and lymphocytes, have also been found to play an important role in supporting the brain in health and assisting in its repair. Here, we describe the multiple immune-specific modes of cellular dialogue among cells within the mammalian brain and their crosstalk with the periphery in both health and disease. We further suggest that interventions directed at boosting the peripheral immune response can restore the balance between the brain and the immune system and can rewire their communication to modify chronic neurodegenerative diseases.

The role of non-neuronal cells in Alzheimer's disease progression has not been fully elucidated. Using single-nucleus RNA sequencing, we identified a population of disease-associated astrocytes in an Alzheimer's disease mouse model. These disease-associated astrocytes appeared at early disease stages and increased in abundance with disease progression. We discovered that similar astrocytes appeared in aged wild-type mice and in aging human brains, suggesting their linkage to genetic and age-related factors.

The understanding of the dialogue between the brain and the immune system has undergone dramatic changes over the last two decades, with immense impact on the perception of neurodegenerative diseases, mental dysfunction, and many other brain pathologic conditions. Accumulated results have suggested that optimal function of the brain is dependent on support from the immune system, provided that this immune response is tightly controlled. Moreover, in contrast to the previous prevailing dogma, it is now widely accepted that circulating immune cells are needed for coping with brain pathologies and that their optimal effect is dependent on their type, location, and activity. In this perspective, we describe our own scientific journey, reviewing the milestones in attaining this understanding of the brain-immune axis integrated with numerous related studies by others. We then explain their significance in demonstrating the possibility of harnessing the immune system in a well-controlled manner for the treatment of neurodegenerative diseases.

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disorder, in which the clinical manifestations may be influenced by genetic and unknown environmental factors. Here we show that ALS-prone Sod1 transgenic (Sod1-Tg) mice have a pre-symptomatic, vivarium-dependent dysbiosis and altered metabolite configuration, coupled with an exacerbated disease under germ-free conditions or after treatment with broad-spectrum antibiotics. We correlate eleven distinct commensal bacteria at our vivarium with the severity of ALS in mice, and by their individual supplementation into antibiotic-treated Sod1-Tg mice we demonstrate that Akkermansia muciniphila (AM) ameliorates whereas Ruminococcus torques and Parabacteroides distasonis exacerbate the symptoms of ALS. Furthermore, Sod1-Tg mice that are administered AM are found to accumulate AM-associated nicotinamide in the central nervous system, and systemic supplementation of nicotinamide improves motor symptoms and gene expression patterns in the spinal cord of Sod1-Tg mice. In humans, we identify distinct microbiome and metabolite configurations-including reduced levels of nicotinamide systemically and in the cerebrospinal fluid-in a small preliminary study that compares patients with ALS with household controls. We suggest that environmentally driven microbiome-brain interactions may modulate ALS in mice, and we call for similar investigations in the human form of the disease.

The immune system supports brain plasticity and homeostasis, yet it is prone to changes following psychological stress. Thus, it remains unclear whether and how stress-induced immune alterations contribute to the development of mental pathologies. Here, we show that following severe stress in mice, leukocyte trafficking through the choroid plexus (CP), a compartment that mediates physiological immune-brain communication, is impaired. Blocking glucocorticoid receptor signaling, either systemically or locally through its genetic knockdown at the CP, facilitated the recruitment of Gata3- and Foxp3-expressing T cells to the brain and attenuated post-traumatic behavioral deficits. These findings functionally link post-traumatic stress behavior with elevated stress-related corticosteroid signaling at the brain-immune interface and suggest a novel therapeutic target to attenuate the consequences of severe psychological stress.

Emerging results support the concept that Alzheimer disease (AD) and age-related dementia are affected by the ability of the immune system to contain the brain's pathology. Accordingly, well-controlled boosting, rather than suppression of systemic immunity, has been suggested as a new approach to modify disease pathology without directly targeting any of the brain's disease hallmarks. Here, we provide a short review of the mechanisms orchestrating the cross-talk between the brain and the immune system. We then discuss how immune checkpoint blockade directed against the PD-1/PD-L1 pathways could be developed as an immunotherapeutic approach to combat this disease using a regimen that will address the needs to combat AD. (C) 2019, AICH-Servier Group

Alzheimer's disease (AD) is a heterogeneous disorder with multiple etiologies. Harnessing the immune system by blocking the programmed cell death receptor (PD)-1 pathway in an amyloid beta mouse model was shown to evoke a sequence of immune responses that lead to disease modification. Here, blocking PD-L1, a PD-1 ligand, was found to have similar efficacy to that of PD-1 blocking in disease modification, in both animal models of AD and of tauopathy. Targeting PD-L1 in a tau-driven disease model resulted in increased immunomodulatory monocyte-derived macrophages within the brain parenchyma. Single cell RNA-seq revealed that the homing macrophages expressed unique scavenger molecules including macrophage scavenger receptor 1 (MSR1), which was shown here to be required for the effect of PD-L1 blockade in disease modification. Overall, our results demonstrate that immune checkpoint blockade targeting the PD-1/PD-L1 pathway leads to modification of common factors that go awry in AD and dementia, and thus can potentially provide an immunotherapy to help combat these diseases.

Immune cells patrol the brain and can support its function, but can we modulate brainimmune communication to fight neurological diseases? Here, we briefly discuss the mechanisms orchestrating the cross-talk between the brain and the immune system and describe how targeting this interaction in a well-controlled manner could be developed as a universal therapeutic approach to treat neurodegeneration.

Microglia differentiate from progenitors that infiltrate the nascent CNS during early embryonic development. They then remain in this unique immune-privileged environment throughout life. Multiple immune mechanisms, which we collectively refer to as microglial checkpoints, ensure efficient and tightly regulated microglial responses to perturbations in the CNS milieu. Such mechanisms are essential for proper CNS development and optimal physiological function. However, in chronic disease or aging, when a robust immune response is required, such checkpoint mechanisms may limit the ability of microglia to protect the CNS. Here we survey microglial checkpoint mechanisms and their roles in controlling microglial function throughout life and in disease, and discuss how they may be targeted therapeutically.Author correction:  In the version of this article initially published, the annotation accompanying ref. 47 ended with \u201cthough the modulation of microglia.\u201d The first word of this phrase should have been \u201cthrough.\u201d The error has been corrected in the HTML and PDF versions of the article.

A major challenge in the field of neurodegenerative diseases and brain aging is to identify the body's intrinsic mechanism that could sense the central nervous system (CNS) damage early and protect the brain from neurodegeneration. Accumulating evidence suggests that disease-associated microglia (DAM), a recently identified subset of CNS resident macrophages found at sites of neurodegeneration, might play such a protective role. Here, we propose that microglia are endowed with a dedicated sensory mechanism, which includes the Trem2 signaling pathway, to detect damage within the CNS in the form of neurodegeneration-associated molecular patterns (NAMPs). Combining data from transcriptional analysis of DAM at single-cell level and from human genome-wide association studies (GWASs), we discuss potential function of different DAM pathways in the diseased brain and outline how manipulating DAM may create new therapeutic opportunities. Recent analyses of CNS immune cells in neurodegenerative conditions have identified a subset of microglia showing a unique transcriptional and functional signature, disease-associated microglia (DAM). This perspective proposes a role for DAM as a sensor of early CNS damage and discusses the therapeutic potential of modulating DAM function in CNS diseases.

During ageing, microglia acquire a phenotype that may negatively affect brain function. Here we show that ageing microglial phenotype is largely imposed by interferon type I (IFN-I) chronically present in aged brain milieu. Overexpression of IFN-β in the CNS of adult wild-type mice, but not of mice lacking IFN-I receptor on their microglia, induces an ageing-like transcriptional microglial signature, and impairs cognitive performance. Furthermore, we demonstrate that age-related IFN-I milieu downregulates microglial myocyte-specific enhancer factor 2C (Mef2C). Immune challenge in mice lacking Mef2C in microglia results in an exaggerated microglial response and has an adverse effect on mice behaviour. Overall, our data indicate that the chronic presence of IFN-I in the brain microenvironment, which negatively affects cognitive function, is mediated via modulation of microglial activity. These findings may shed new light on other neurological conditions characterized by elevated IFN-I signalling in the brain.

Alzheimer's disease (AD) is a detrimental neurodegenerative disease with no effective treatments. Due to cellular heterogeneity, defining the roles of immune cell subsets in AD onset and progression has been challenging. Using transcriptional single-cell sorting, we comprehensively map all immune populations in wild-type and AD-transgenic (Tg-AD) mouse brains. We describe a novel microglia type associated with neurodegenerative diseases (DAM) and identify markers, spatial localization, and pathways associated with these cells. Immunohistochemical staining of mice and human brain slices shows DAM with intracellular/phagocytic Aβ particles. Single-cell analysis of DAM in Tg-AD and triggering receptor expressed on myeloid cells 2 (Trem2)^−/− Tg-AD reveals that the DAM program is activated in a two-step process. Activation is initiated in a Trem2-independent manner that involves downregulation of microglia checkpoints, followed by activation of a Trem2-dependent program. This unique microglia-type has the potential to restrict neurodegeneration, which may have important implications for future treatment of AD and other neurodegenerative diseases.

Hypoxia occurs physiologically in the developing body, and changing oxygen tensions are known to direct tissue differentiation; however, in the context of pathology, the same hypoxiaactivated mechanisms may negatively affect tissue function. In this issue of The EMBO Journal, EstebanMartínez et al (2017) report that programmed mitophagy, dependent on hypoxiainduced NIP3like protein X (BNIP3L, best known as NIX), is an essential step in differentiation of both retinal neurons and inflammatory macrophages.

The central nervous system (CNS) is endowed with several immune-related mechanisms that contribute to its protection and maintenance in homeostasis and under pathology. Here, we discovered an additional mechanism that controls inflammatory responses within the CNS milieu under injurious conditions, involving CD200 ligand (CD200L) expressed by newly formed endothelial cells. We observed that CD200L is constitutively expressed in the mouse healthy CNS by endothelial cells of the bloodcerebrospinal fluid barrier and of the spinal cord meninges, but not by the endothelium of the bloodspinal cord barrier. Following spinal cord injury (SCI), newly formed endothelial cells, located only at the epicenter of the lesion site, expressed CD200L. Moreover, in the absence of CD200L expression by CNS-resident cells, functional recovery of mice following SCI was impaired. High throughput single-cell flow cytometry image analysis following SCI revealed CD200L-dependent direct interaction between endothelial and local CD200R⁺ myeloid cells, including activated microglia and infiltrating monocyte-derived macrophages (mo-MΦ). Absence of CD200L signaling, both in vitro and in vivo, resulted in a higher inflammatory response of the encountering macrophages, manifested by elevation in mRNA expression of Tnfα and Il1β, increased intracellular TNFα immunoreactivity, and reduced expression levels of macrophage factors that are associated with resolution of inflammation, Dectin-1, CD206 (mannose receptor), and IL-4R. Collectively, our results highlight the importance of CD200-mediated immune dialogue between endothelial cells and the local resident microglia and infiltrating mo-MΦ within the lesion area, as a mechanism that contributes to regulation of inflammation following acute CNS injury.

For decades, the irreversible functional loss following central nervous system injury has been perceived with a high degree of pessimism, and researchers challenging this assumption have been considered either heroic figures, or misguided fanatics. This high degree of pessimism regarding recovery evolved - in part - from many assumptions, among which that repair in the central nervous system (CNS) was likely to obey rules different from those governing wound healing or the repair of tissues outside the CNS. Therefore, it is not surprising that many phenomena, which were observed at the site of CNS injury and take part in the repair process outside the CNS, were believed to contribute to the failure of CNS repair and were therefore considered targets for elimination; these include the key processes of local inflammation and scar formation. In addition, the common manner in which the blood-brain barrier and the blood-cerebrospinal barriers were perceived led to the assumed linkage between brain pathologies and inflammation. Accordingly, it was believed that the CNS optimally functions and repairs itself in isolation from any circulating immune cells and has no need for assistance from peripheral immune cells to support repair; if immune cells infiltrate to the CNS territory, this was taken as a sign of pathology that should be mitigated. Nevertheless, through intensive studies over the last decade, many of these assumptions were challenged or modified. It became clear that the local immune response, encompassing activated microglia and infiltrating circulating immune cells, and scar formation are interim processes that are pivotal for overall repair, but require timely resolution. In addition, it became clear that resident microglia and infiltrating blood macrophages have distinct attributes and roles under pathological conditions; it is the timing and location of their activity that determine the outcome of their function. Furthermore, the requirements for neuronal rescue, protection, restoration, renewal, and regeneration differ in time, location, and the molecular and cellular players involved. According to this new development in the field of CNS injuries, the failure of recovery is not due to the occurrence of inflammation and scar formation, which are essential, but rather due to their insufficient timely resolution. This chapter will provide a historical perspective of the field, describe the emerging new understandings, and discuss their implications to therapies for spinal cord injury.

Neuroinflammation is common to various diseases of the central nervous system (CNS), but its imprecise definition has led to many misconceptions in research and clinical approaches. It is now recognized that neuroinflammation in chronic neurodegenerative conditions, including Alzheimer's disease (AD) and age-related dementia, is distinct from the inflammation that accompanies relapsingremitting multiple sclerosis (RRMS), and its experimental animal model, experimental autoimmune encephalomyelitis (EAE). Here, we discuss the discrete features of inflammation in different CNS pathologies, given the current understanding of the CNSimmune crosstalk; the roles of the immune cells that are involved, their phenotypes, and their location and route of entry to the CNS. Understanding the term neuroinflammation to encompass a broad range of disease-specific conditions is essential for finding effective therapeutic approaches for these pathologies.

For decades, the brain was considered to be a tissue behind barriers, and thus autonomous with respect to its maintenance needs. Several years ago, our group suggested that the ability of the brain to operate optimally, with relatively little deterioration throughout life, is dependent on continuous support from circulating immune cells. We envisioned that such immune support is provided from specific permissive sites that lie between the brain and the immune system, enabling immune-brain dialogue without the risk of an inflammatory autoimmune response. The brain's choroid plexus has been identified as an active neuroimmunological interface that is continuously exposed to signals from the brain and from the periphery. The interplay between these bidirectional signals shapes brain function in health and disease, including in multiple sclerosis. Accumulating evidence suggests that autoimmune T cells that recognize brain antigens are involved in this dialogue. Here, we discuss the fate of this dialogue under neurodegenerative conditions, and how boosting autoimmunity, rather than suppressing it, by either active vaccination with central nervous system-derived antigens, or by breaking peripheral immune suppression, could be harnessed for mitigating neurodegenerative diseases.

Microglia, the resident myeloid cells of the central nervous system, play important roles in life-long brain maintenance and in pathology. Despite their importance, their regulatory dynamics during brain development have not been fully elucidated. Using genome-wide chromatin and expression profiling coupled with single-cell transcriptomic analysis throughout development, we found that microglia undergo three temporal stages of development in synchrony with the brain-early, pre-, and adult microglia-which are under distinct regulatory circuits. Knockout of the gene encoding the adult microglia transcription factor MAFB and environmental perturbations, such as those affecting the microbiome or prenatal immune activation, led to disruption of developmental genes and immune response pathways. Together, our work identifies a stepwise microglia developmental program integrating immune response pathways that may be associated with several neurodevelopmental disorders.

Hippocampal neurogenesis is affected throughout life by factors external to the brain, including what we eat, our gut microbiota, and the immune system. However, the mechanisms that link microbiota to neurogenesis are still puzzling. Now in Cell Reports, Möhle et al. (2016) attribute a role to Ly6C^hi monocytes in this gut-immune-brain axis.

The choroid plexus epithelium within the brain ventricles orchestrates blood-derived monocyte entry to the central nervous system under injurious conditions, including when the primary injury site is remote from the brain. Here, we hypothesized that the retinal pigment epithelium (RPE) serves a parallel role, as a gateway for monocyte trafficking to the retina following direct or remote injury. We found elevated expression of genes encoding leukocyte trafficking determinants in mouse RPE as a consequence of retinal glutamate intoxication or optic nerve crush (ONC). Blocking VCAM-1 after ONC interfered with monocyte infiltration into the retina and resulted in a local pro-inflammatory cytokine bias. Live imaging of the injured eye showed monocyte accumulation first in the RPE, and subsequently in the retina, and peripheral leukocytes formed close contact with the RPE. Our findings further implied that the ocular milieu can confer monocytes a phenotype advantageous for neuroprotection. These results suggest that the eye utilizes a mechanism of crosstalk with the immune system similar to that of the brain, whereby epithelial barriers serve as gateways for leukocyte entry. Synopsis The retinal pigment epithelium (RPE) acts as the primary entry point for infiltrating monocytes during retinal injury to positively contribute to the restoration of immunological balance in the retinal milieu. A video of this synopsis is available online at http://embopress.org/video-EMBOJ-2016-94202 The epithelial barriers of the central nervous system display immunomodulatory capacity. The expression of leukocyte trafficking determinants is elevated in the retinal pigment epithelium (RPE) of the eye upon direct or remote injury to retinal ganglion cells. Live imaging of the injured eye demonstrates the sequential accumulation of monocytes, first in the RPE and then in the retina. The ocular milieu can confer monocytes a phenotype that is advantageous for restoration of immunological balance and for neuroprotection. The retinal pigment epithelium acts as the primary entry point for infiltrating monocytes during retinal injury to positively contribute to the restoration of immunological balance in the retinal milieu.

Hemorrhagic stroke, primarily caused by rupture of blood vessels in the brain, is a leading cause of death and disability in adults. In this issue of Immunity, Liu et al. (2016) demonstrate that repair of cerebrovascular ruptures can be directly mediated by myeloid cells. Hemorrhagic stroke, primarily caused by rupture of blood vessels in the brain, is a leading cause of death and disability in adults. In this issue of Immunity, Liu et al. (2016) demonstrate that repair of cerebrovascular ruptures can be directly mediated by myeloid cells.

Stroke is a leading cause of disability and currently lacks effective therapy enabling long-term functional recovery. Ischemic brain injury causes local inflammation, which involves both activated resident microglia and infiltrating immune cells, including monocytes. Monocyte-derived macrophages (MDMs) exhibit a high degree of functional plasticity. Here, we determined the role of MDMs in long-term spontaneous functional recovery after middle cerebral artery occlusion in mice. Analyses by flow cytometry and immunocytochemistry revealed that monocytes home to the stroke-injured hemisphere., and that infiltration peaks 3 d after stroke. At day 7, half of the infiltrating MDMs exhibited a bias toward a proinflammatory phenotype and the other half toward an anti-inflammatory phenotype, but during the subsequent 2 weeks, MDMs with an anti-inflammatory phenotype dominated. Blocking monocyte recruitment using the anti-CCR2 antibody MC-21 during the first week after stroke abolished long-term behavioral recovery, as determined in corridor and staircase tests, and drastically decreased tissue expression of anti-inflammatory genes, including TGF beta, CD163, and Ym1. Our results show that spontaneously recruited monocytes to the injured brain early after the insult contribute to long-term functional recovery after stroke.

Recent findings have revealed distinct roles for type I and II interferons (IFN-I and IFN-γ) in the recruitment of immune cells to the central nervous system (CNS) and highlighted the importance of this process for brain maintenance and protection/repair. Furthermore, manipulation of IFN-I and IFN-γ pathways in pathological contexts has yielded conflicting results. We discuss these findings, focusing on two distinct conditions; relapsing remitting multiple sclerosis (RRMS) and brain aging. Using these examples, we propose that regulation of immune cell entry to the CNS is a mechanism through which interaction between IFN-I and -II can affect brain function from its anatomical borders. Deviation from homeostatic IFN-I/-II balance may contribute to distinct brain pathologies, resulting from either insufficient immune surveillance of the CNS and loss of immune-dependent protection, or overwhelming leukocyte entry and immune-mediated destruction. Homeostasis: IFN-γ signaling at the choroid plexus within the brain controls CNS immune surveillance important for brain maintenance.Relapsing remitting multiple sclerosis: endogenously expressed as well as therapeutically delivered IFN-I ameliorates disease progression by indirectly attenuating IFN-γ signaling at the borders of the brain and restraining excess infiltration of pathological immune cells to the CNS.Aging: excess expression of IFN-I by the CNS choroid plexus is associated with insufficient IFN-γ dependent CNS immune surveillance as well as with loss of cognitive function and neurogenesis.

In the article \u201cPassive or Active Immunization with Myelin Basic Protein Promotes Recovery from Spinal Cord Injury Contusion\u201d by Ehud Hauben, Oleg Butovsky, Uri Nevo, Eti Yoles, Gila Moalem, Eugenia Agranov, Felix Mor, Raya Leibowitz-Amit, Evgenie Pevsner, Solange Akselrod, Michal Neeman, Irun R. Cohen, and Michal Schwartz, which appeared on pages 64216430 of the September 1, 2000 issue, the following error was discovered:In the legend of Figure 4A, a reference is missing. Hauben E, Nevo U, Yoles E, Moalem G, Agranov E, Mor F, Akselrod S, Neeman M, Cohen IR, Schwartz M (2000) Autoimmune T cells as potential neuroprotective therapy for spinal cord injury. Lancet 355:28287, which is cited in the text of the article, should be cited in the legend as well. The sentence in the legend that reads \u201cFor comparison, a similar experiment using five PBS-treated and six rats treated immediately with anti-MBP T cells is shown here\u201d should be replaced by \u201cFor comparison, a similar experiment, taken from Hauben et al. (2000), of six rats treated immediately with anti-MBP is shown.\u201d

Systemic immune suppression may curtail the ability to mount the protective, cell-mediated immune responses that are needed for brain repair. By using mouse models of Alzheimer's disease (AD), we show that immune checkpoint blockade directed against the programmed death-1 (PD-1) pathway evokes an interferon (IFN)-γ-dependent systemic immune response, which is followed by the recruitment of monocyte-derived macrophages to the brain. When induced in mice with established pathology, this immunological response leads to clearance of cerebral amyloid-β (Aβ) plaques and improved cognitive performance. Repeated treatment sessions were required to maintain a long-lasting beneficial effect on disease pathology. These findings suggest that immune checkpoints may be targeted therapeutically in AD.

Alzheimers disease (AD) is a neurodegenerative disorder in which chronic neuroinflammation contributes to disease escalation. Nevertheless, while immunosuppressive drugs have repeatedly failed in treating this disease, recruitment of myeloid cells to the CNS was shown to play a reparative role in animal models. Here we show, using the 5XFAD AD mouse model, that transient depletion of Foxp3+ regulatory T cells (Tregs), or pharmacological inhibition of their activity, is followed by amyloid-β plaque clearance, mitigation of the neuroinflammatory response and reversal of cognitive decline. We further show that transient Treg depletion affects the brains choroid plexus, a selective gateway for immune cell trafficking to the CNS, and is associated with subsequent recruitment of immunoregulatory cells, including monocyte-derived macrophages and Tregs, to cerebral sites of plaque pathology. Our findings suggest targeting Treg-mediated systemic immunosuppression for treating AD.

Chronic neuroinflammation is evident in brain aging and neurodegenerative disorders and is often associated with excessive nitric oxide (NO) production within the central nervous system (CNS). Under such conditions, increased NO levels are observed at the choroid plexus (CP), an epithelial layer that forms the blood-cerebrospinal fluid barrier (BCSFB) and serves as a selective gateway for leukocyte entry to the CNS in homeostasis and following injury. Here, we hypothesized that elevated cerebral NO levels interfere with CP gateway activity. We found that induction of leukocyte trafficking determinants by the CP and sequential leukocyte entry to the CSF are dependent on the CP epithelial NFκB/p65 signaling pathway, which was inhibited upon exposure to NO. Examining the CP in 5XFAD transgenic mouse model of Alzheimer's disease (AD-Tg) revealed impaired ability to mount an NFκB/p65-dependent response. Systemic administration of an NO scavenger in AD-Tg mice alleviated NFκB/p65 suppression at the CP and augmented its gateway activity. Together, our findings identify cerebral NO as a negative regulator of CP gateway activity for immune cell trafficking to the CNS.

Neuropsychiatric disease is one of the most common manifestations of human systemic lupus erythematosus, but the mechanisms remain poorly understood. In human brain microvascular endothelial cells invitro, TNF-like weak inducer of apoptosis (TWEAK) decreases tight junction ZO-1 expression and increases the permeability of monolayer cell cultures. Furthermore, knockout (KO) of the TWEAK receptor, Fn14, in the MRL/lpr lupus mouse strain markedly attenuates neuropsychiatric disease, as demonstrated by significant reductions in depressive-like behavior and improved cognitive function. The purpose of the present study was to determine the mechanisms by which TWEAK signaling is instrumental in the pathogenesis of neuropsychiatric lupus (NPSLE). Evaluating brain sections of MRL/lpr Fn14WT and Fn14KO mice, we found that Fn14KO mice displayed significantly decreased cellular infiltrates in the choroid plexus. To evaluate the integrity of the blood brain barrier (BBB) in MRL/lpr mice, Western blot for fibronectin, qPCR for iNOS, and immunohistochemical staining for VCAM-1/ICAM-1 were performed. We found preserved BBB permeability in MRL/lpr Fn14KO mice, attributable to reduced brain expression of VCAM-1/ICAM-1 and iNOS. Additionally, administration of Fc-TWEAK intravenously directly increased the leakage of a tracer (dextran-FITC) into brain tissue. Furthermore, MRL/lpr Fn14KO mice displayed reduced antibody (IgG) and complement (C3, C6, and C4a) deposition in the brain. Finally, we found that MRL/lpr Fn14KO mice manifested reduced neuron degeneration and hippocampal gliosis. Our studies indicate that TWEAK/Fn14 interactions play an important role in the pathogenesis of NPSLE by increasing the accumulation of inflammatory cells in the choroid plexus, disrupting BBB integrity, and increasing neuronal damage, suggesting a novel target for therapy in this disease.

Amyotrophic lateral sclerosis (ALS) is a devastating fatal motor neuron disease, for which there is currently no cure or effective treatment. In this disease, local neuroinflammation develops along the disease course and contributes to its rapid progression. In several models of CNS pathologies, circulating immune cells were shown to display an indispensable role in the resolution of the neuroinflammatory response. The recruitment of such cells to the CNS involves activation of the choroid plexus (CP) of the brain for leukocyte trafficking, through a mechanism that requires IFN-γ signaling. Here, we found that in the mutant SOD1^G93A (mSOD1) mouse model of ALS, the CP does not support leukocyte trafficking during disease progression, due to a local reduction in IFN-γ levels. Therapeutic immunization of mSOD1 mice with a myelin-derived peptide led to CP activation, and was followed by the accumulation of immunoregulatory cells, including IL-10-producing monocyte-derived macrophages and Foxp3⁺ regulatory T cells, and elevation of the neurotrophic factors IGF-1 and GDNF in the diseased spinal cord parenchyma. The immunization resulted in the attenuation of disease progression and an increased life expectancy of the mSOD1 mice. Collectively, our results demonstrate that recruitment of immunoregulatory cells to the diseased spinal cord in ALS, needed for fighting off the pathology, can be enhanced by transiently boosting peripheral immunity to myelin antigens.

Current therapy for the treatment of central nervous system (CNS) injuries and disorders is limited and commonly involves the use of immunosuppressive therapy. Nevertheless, controversy surrounds the use of indiscriminate blockers of the immune system, firstly, due to their limited efficacy, and secondly, due to their harmful side effects. It is well known that incomplete resolution of local inflammation following acute injury, which is initially evoked as a physiological inflammatory mechanism, often leads to chronic neuroinflammatory pathology in the CNS. In contrast, if such response is timely resolved, it can promote neuroprotection and tissue repair. In this chapter, we review the different components and pathways comprising the immune network needed for CNS repair following acute injury. The complexity of this network is evident by the presence of intertwined subsets of inflammatory and regulatory cells; their beneficial role in the repair is critically dependent not only on the type of participating cells, but also on the timing and location of their activities. Such a network, evoked in response to injury, if synchronized with the needs of the tissue, ensures repair and return to homeostasis.

In the past, the brain was considered an autonomous organ, selfcontained and completely separate from the body's immune system. But over the past twenty years, neuroimmunologist Michal Schwartz, together with her research team, not only has overturned this misconception but has brought to light revolutionary new understandings of brain health and repair. In this book Schwartz describes her research journey, her experiments, and the triumphs and setbacks that led to the discovery of connections between immune system and brain. Michal Schwartz, with Anat London, also explains the significance of the findings for future treatments of brain disorders and injuries, spinal cord injuries, glaucoma, depression, and other conditions such as brain aging and Alzheimer's and Parkinson's diseases. Scientists, physicians, medical students, and all readers with an interest in brain function and its relationship to the immune system in health and disease will find this book a valuable resource. With general readers in mind, the authors provide a useful primer to explain scientific terms and concepts discussed in the book.

Tissue microenvironment influences the function of resident and infiltrating myeloid-derived cells. In the central nervous system (CNS), resident microglia and freshly recruited infiltrating monocyte-derived macrophages (mo-MΦ) display distinct activities under pathological conditions, yet little is known about the microenvironment-derived molecular mechanism that regulates these differences. Here, we demonstrate that long exposure to transforming growth factor-β1 (TGFβ1) impaired the ability of myeloid cells to acquire a resolving anti-inflammatory phenotype. Using genome-wide expression analysis and chromatin immunoprecipitation followed by next-generation sequencing, we show that the capacity to undergo pro- to anti-inflammatory (M1-to-M2) phenotype switch is controlled by the transcription factor interferon regulatory factor 7 (IRF7) that is down-regulated by the TGFβ1 pathway. RNAi-mediated perturbation of Irf7 inhibited the M1-to-M2 switch, while IFNβ1 (an IRF7 pathway activator) restored it. In vivo induction of Irf7 expression in microglia, following spinal cord injury, reduced their pro-inflammatory activity. These results highlight the key role of tissue-specific environmental factors in determining the fate of resident myeloid-derived cells under both physiological and pathological conditions. Synopsis Chronic exposure to the abundant CNS cytokine, TGFβ1, impairs the ability of myeloid cells, specifically microglia, to acquire an inflammation-resolving, anti-inflammatory (M2), phenotype under pathological conditions. The transcription factor IRF7 is a key regulator of the M1-to-M2 phenotype switch and is down-regulated by the TGFβ1 signaling pathway. Induction of IRF7 expression by IFN-β1 under pathological conditions reduces microglial pro-inflammatory response following injury. Extended exposure to TGFβ1 prevents myeloid cells to switch phenotype under inflammatory conditions. IRF7 regulates the M1-to-M2 phenotype switch in myeloid cells through down-regulation of pro-inflammatory genes. The prevalent cytokine in the adult CNS milieu, TGFβ1, suppresses IRF7 expression. IRF7 induction restores the ability of microglia to acquire an M2-like phenotype under inflammatory conditions. TGFβ1 regulates the ability of microglia to acquire an inflammation resolving phenotype under pathological conditions by down-regulating the expression of the transcription factor IRF7.

Immune cell infiltration to the brain's territory was considered for decades to reflect a pathological process in which immune cells attack the central nervous system (CNS); such a process is observed in the inflammatory autoimmune disease, multiple sclerosis (MS). As neuroinflammatory processes within the CNS parenchyma are also common to other CNS pathologies, regardless of their etiology, including neurodegenerative disorders such as Alzheimer's disease (AD) and Amyotrophic lateral sclerosis (ALS), these pathologies have often been compared to MS, a disease that benefits from immunosuppressive therapy. Yet, over the last decade, it became clear that autoimmunity has a bright side, and that it plays a pivotal role in CNS repair following damage. Specifically, autoimmune T cells were found to facilitate CNS healing processes, such as in the case of sterile mechanical injuries to the brain or the spinal cord, mental stress, or biochemical insults. Even more intriguingly, autoimmune T cells were found to be involved in supporting fundamental processes of brain functional integrity, such as in the maintenance of life-long brain plasticity, including spatial learning and memory, and neurogenesis. Importantly, autoimmune T cells are part of a cellular network which, to operate efficiently and safely, requires tight regulation by other immune cell populations, such as regulatory T cells, which are indispensable for maintenance of immunological self-tolerance and homeostasis. Here, we suggest that dysregulation of the balance between peripheral immune suppression, on one hand, and protective autoimmunity, on the other, is an underlying mechanism in the emergence and progression of the neuroinflammatory response associated with chronic neurodegenerative diseases and brain aging. Mitigating chronic neuroinflammation under these conditions necessitates activation, rather than suppression, of the peripheral immune response directed against self. Accordingly, we propose that fighting off acute and chronic neurodegenerative conditions requires breaking peripheral immune tolerance to CNS self-antigens, in order to boost protective autoimmunity. Nevertheless, the optimal approach to fine tune such immune response must be individually explored for each condition.

Inadequate axonal regeneration is a common phenomenon occurring following acute injury to the central nervous system (CNS), and is often associated with permanent neurological deficits. The injured axons attempting to regenerate face the inhospitable environment of the CNS scar, which can hinder axonal growth and sprouting. In addition, in response to the insult, intense activation and infiltration of immune cells take place. Both the scar tissue and immune response, which have received a bad reputation in the context of CNS repair are essential for the overall recovery from CNS injuries, but are not optimally controlled. The glial scar contributes to protection of the spared neural tissues by establishing a boundary between damaged and salvageable tissue, and by educating the immune cells to promote the healing of the CNS tissue. In turn, the immune cells, and in particular the infiltrating macrophages, exert several functions at the lesion site, including resolution of the microglial response, control of scar tissue degradation, and production of growth factors; thereby, promoting neuronal survival, axonal regeneration, and tissue remodeling. As axonal regeneration and tissue remodeling are viewed as critical steps for the overall functional recovery following CNS injury, a detailed understanding of the mechanisms underlying the timely formation and degradation of the CNS scar, and its crosstalk with the inflammatory response, are of great importance, both biologically and clinically.

Aging-associated cognitive decline is affected by factors produced inside and outside the brain. By using multiorgan genome-wide analysis of aged mice, we found that the choroid plexus, an interface between the brain and the circulation, shows a type I interferon (IFN-I)-dependent gene expression profile that was also found in aged human brains. In aged mice, this response was induced by brain-derived signals, present in the cerebrospinal fluid. Blocking IFN-I signaling within the aged brain partially restored cognitive function and hippocampal neurogenesis and reestablished IFN-II-dependent choroid plexus activity, which is lost in aging. Our data identify a chronic aging-induced IFN-I signature, often associated with antiviral response, at the brain's choroid plexus and demonstrate its negative influence on brain function, thereby suggesting a target for ameliorating cognitive decline in aging.

Keywords: Immunology; Neurosciences

Keywords: Immunology; Neurosciences

Keywords: Immunology; Neurosciences

Keywords: Immunology; Neurosciences

Immune activity in the CNS parenchyma under various acute and chronic neurodegenerative conditions has been often interpreted as a sign of pathological inflammation. The apparent resemblance of the local neuroinflammatory processes to autoimmune diseases, such as multiple sclerosis (MS), generated the view that, despite differences in etiology and pathology, neurodegenerative disorders with a local inflammatory component can benefit from systemic anti-inflammatory therapy. In addition, as CNS self-reactive T cells are associated with the etiology of MS, autoimmunity was assumed to solely reflect pathology, and therefore, was universally linked to autoimmune disease. Yet, it is becoming increasingly clear that CNS-specific T cells, along with circulating and local innate immune cells, can enhance CNS healing processes following non-infectious injuries, or any deviation from homeostasis, including chronic pathological conditions. Here, we discuss the theory of "protective autoimmunity," which describes the activity of an immune cell network encompassing effector and regulatory T cells with specificity for CNS antigens, in CNS maintenance and repair. Such an immune network, evoked in response to external and internal threats, functions in a tightly regulated way, ensuring restoration of the brain's equilibrium and return to homeostasis.

Inflammation is an integral part of the body's physiological repair mechanism, unless it remains unresolved and becomes pathological, as evident in the progressive nature of neurodegeneration. Based on studies from outside the central nervous system (CNS), it is now understood that the resolution of inflammation is an active process, which is dependent on well-orchestrated innate and adaptive immune responses. Due to the immunologically privileged status of the CNS, such resolution mechanism has been mostly ignored. Here, we discuss resolution of neuroinflammation as a process that depends on a network of immune cells operating in a tightly regulated sequence, involving the brain's choroid plexus (CP), a unique neuro-immunological interface, positioned to integrate signals it receives from the CNS parenchyma with signals coming from circulating immune cells, and to function as an onalert gate for selective recruitment of inflammation-resolving leukocytes to the inflamed CNS parenchyma. Finally, we propose that functional dysregulation of the CP reflects a common underlying mechanism in the pathophysiology of neurodegenerative diseases, and can thus serve as a potential novel target for therapy.

Monocyte-derived macrophages (mo-MΦs) and T cells have been shown to contribute to spinal cord repair. Recently, the remote brain choroid plexus epithelium (CP) was identified as a portal for monocyte recruitment, and its activation for leukocyte trafficking was found to be IFN-γ-dependent. Here, we addressed how the need for effector T cells can be reconciled with the role of inflammation-resolving immune cells in the repair process. Using an acute spinal cord injury model, we show that in mice deficient in IFN-γ-producing T cells, the CP was not activated, and recruitment of inflammation-resolving mo-MΦ to the spinal cord parenchyma was limited. We further demonstrate that mo-MΦ locally regulated recruitment of thymic-derived Foxp3⁺ regulatory T (Treg) cells to the injured spinal cord parenchyma at the subacute/chronic phase. Importantly, an ablation protocol that resulted in reduced Tregs at this stage interfered with tissue remodeling, in contrast to Treg transient ablation, restricted to the 4 d period before the injury, which favored repair. The enhanced functional recovery observed following such a controlled decrease of Tregs suggests that reduced systemic immunosuppression at the time of the insult can enhance CNS repair. Overall, our data highlight a dynamic immune cell network needed for repair, acting in discrete compartments and stages, and involving effector and regulatory T cells, interconnected by mo-MΦ. Any of these populations may be detrimental to the repair process if their level or activity become dysregulated. Accordingly, therapeutic interventions must be both temporally and spatially controlled.

Adaptive immunity was repeatedly shown to play a role in maintaining lifelong brain function. Under physiological conditions, this activity was associated with CD4⁺ T cells specific for brain self-antigens. Nevertheless, direct interactions of T cells with the healthy neuronal parenchyma are hardly detectable. Recent studies have identified the brain's choroid plexus (CP) as an active neuro-immunological interface, enriched with CNS-specific CD4⁺ T cells. Strategically positioned for receiving signals from both the central nervous system (CNS) through the cerebrospinal fluid (CSF), and from the circulation through epithelium-immune cell interactions, the CP has recently been recognized as an important immunological compartment in maintaining and restoring brain homeostasis/allostasis. Here, we propose that CNS-specific T cells shape brain function via the CP, and suggest this immunological control to be lost as part of aging, in general, and immune senescence, in particular. Accordingly, the CP may serve as a novel target for immunomodulation to restore brain equilibrium.

Infiltrating T cells and monocyte-derived macrophages support central nervous system repair. Although infiltration of leucocytes to the injured central nervous system has recently been shown to be orchestrated by the brain's choroid plexus, the immunological mechanism that maintains this barrier and regulates its activity as a selective gate is poorly understood. Here, we hypothesized that CD4⁺ effector memory T cells, recently shown to reside at the choroid plexus stroma, regulate leucocyte trafficking through this portal through their interactions with the choroid plexus epithelium. We found that the naïve choroid plexus is populated by T helper 1, T helper 2 and regulatory T cells, but not by encephalitogenic T cells. In vitro findings revealed that the expression of immune cell trafficking determinants by the choroid plexus epithelium is specifically induced by interferon-γ. Tumour necrosis factor-a and interferon-γ reciprocally controlled the expression of their receptors by the choroid plexus epithelium, and had a synergistic effect in inducing the epithelial expression of trafficking molecules. In vivo, interferonc-dependent signalling controlled trafficking through the choroid plexus; interferon-γ receptor knockout mice exhibited reduced levels of T cells and monocyte entry to the cerebrospinal fluid and impaired recovery following spinal cord injury. Moreover, reduced expression of trafficking molecules by the choroid plexus was correlated with reduced CD4⁺ T cells in the choroid plexus and cerebrospinal fluid of interferon-γ receptor knockout mice. Similar effect on the expression of trafficking molecules by the choroid plexus was found in bone-marrow chimeric mice lacking interferon-γ receptor in the central nervous system, or reciprocally, lacking interferon-γ in the circulation. Collectively, our findings attribute a novel immunological plasticity to the choroid plexus epithelium, allowing it to serve, through interferon-γ signalling, as a tightly regulated entry gate into the central nervous system for circulating leucocytes immune surveillance under physiological conditions, and for repair following acute injury.

Functional macrophage heterogeneity is well appreciated outside the CNS in wound healing and cancer, and was recently also demonstrated in several CNS compartments after "sterile" insults. Yet, such heterogeneity was largely overlooked in the context of inflammatory autoimmune pathology, in which macrophages were mainly associated with disease induction and propagation. In this article, we show the diversity of monocyte-derived macrophages along the course of experimental autoimmune uveitis, an inflammatory condition affecting the ocular system, serving as a model for CNS autoimmune pathology. Disease induction resulted in the appearance of a distinct myeloid population in the retina, and in the infiltration of monocyte-derived macrophages that were absent from control eyes. During the disease course, the frequency of CX ³CR1^high infiltrating macrophages that express markers associated with inflammation-resolving activity was increased, along with a decrease in the frequency of inflammation-associated Ly6C⁺ macrophages. Inhibition of monocyte infiltration at the induction phase of experimental autoimmune uveitis prevented disease onset, whereas monocyte depletion at the resolution phase resulted in a decrease in Foxp3⁺ regulatory T cells and in exacerbated disease. Thus, monocyte-derived macrophages display distinct phenotypes throughout the disease course, even in an immune-induced pathology, reflecting their differential roles in disease induction and resolution.

Monocyte-derived macrophages are essential for recovery after spinal cord injury, but their homing mechanism is poorly understood. Here, we show that although of common origin, the homing of proinflammatory (M1) and the " alternatively activated" anti-inflammatory (M2) macrophages to traumatized spinal cord (SC) was distinctly regulated, neither being through breached blood-brain barrier. The M1 macrophages (Ly6c^hiCX3CR1^lo) derived from monocytes homed in a CCL2 chemokine-dependent manner through the adjacent SC leptomeninges. The resolving M2 macrophages (Ly6c^loCX3CR1^hi) derived from monocytes trafficked through a remote blood-cerebrospinal-fluid (CSF) barrier, the brain-ventricular choroid plexus (CP), via VCAM-1-VLA-4 adhesion molecules and epithelial CD73 enzyme for extravasation and epithelial transmigration. Blockage of these determinants, or mechanical CSF flow obstruction, inhibited M2 macrophage recruitment and impaired motor-function recovery. The CP, along with the CSF and the central canal, provided an anti-inflammatory supporting milieu, potentially priming the trafficking monocytes. Overall, our finding demonstrates that the route of monocyte entry to central nervous system provides an instructional environment to shape their function.

Functional macrophage heterogeneity is recognized outside the central nervous system (CNS), where alternatively-activated macrophages can perform immuneresolving functions. Such functional heterogeneity was largely ignored in the CNS, with respect to the resident microglia and the myeloid-derived cells recruited from the blood following injury or disease, previously defined as blood-derived microglia; both were indistinguishably perceived detrimental. Our studies have led us to view the myeloid-derived infiltrating cells as functionally distinct from the resident microglia, and accordingly, to name them monocyte-derived macrophages (mo-MΦ). Although microglia perform various maintenance and protective roles, under certain conditions when they can no longer provide protection, mo-MΦ are recruited to the damaged CNS; there, they act not as microglial replacements but rather assistant cells, providing activities that cannot be timely performed by the resident cells. Here, we focus on the functional heterogeneity of microglia/mo-MΦ, emphasizing that, as opposed to the mo-MΦ, microglia often fail to timely acquire the phenotype essential for CNS repair.

Complex barriers separate immune-privileged tissues from the circulation. Here, we propose that cell entry to immune-privileged sites through barriers composed of tight junction-interconnected endothelium is associated with destructive inflammation, whereas border structures comprised of fenestrated vasculature enveloped by tightly regulated epithelium serve as active and selective immune-skewing gates in the steady state. Based on emerging knowledge of the central nervous system and information from other immune-privileged sites, we propose that these sites are endowed either with absolute endothelial-based barriers and epithelial gates that enable selective and educative transfer of trafficking leukocytes or with selective epithelial gates only.

The poor recovery of the central nervous system (CNS) after injury, coupled with its complex and immunologically-privileged nature, led to the belief that CNS repair is different from the repair of other tissues. Here, we consider CNS repair from a novel perspective, suggesting that CNS responses to injury resemble wound healing. Extrapolating the classical wound healing model suggests that poor CNS recovery is an outcome of insufficient resolution of interim reparative events that precede tissue regeneration and renewal, a state reminiscent of chronic/unresolved wounds. This comparison requires reevaluation of the inflammatory response, glial scarring, and barrier permeability, traditionally considered obstacles to CNS repair. Understanding the similarity to wound healing suggests new research directions and therapeutic avenues for CNS injuries.

The adaptive arm of the immune system has been suggested as an important factor in brain function. However, given the fact that interactions of neurons or glial cells with T lymphocytes rarely occur within the healthy CNS parenchyma, the underlying mechanism is still a mystery. Here we found that at the interface between the brain and blood circulation, the epithelial layers of the choroid plexus (CP) are constitutively populated with CD4⁺ effector memory cells with a T-cell receptor repertoire specific to CNS antigens. With age, whereas CNS specificity in this compartment was largely maintained, the cytokine balance shifted in favor of the T helper type 2 (Th2) response; the Th2-derived cytokine IL-4 was elevated in the CP of old mice, relative to IFN-_γ, which decreased. We found this local cytokine shift to critically affect the CP epithelium, triggering it to produce the chemokine CCL11 shown to be associated with cognitive dysfunction. Partial restoration of cognitive ability in aged mice, by lymphopenia-induced homeostasis-driven proliferation of memory T cells, was correlated with restoration of the IL-4:IFN-_γ ratio at the CP and modulated the expression of plasticity-related genes at the hippocampus. Our data indicate that the cytokine milieu at the CP epithelium is affected by peripheral immunosenescence, with detrimental consequences to the aged brain. Amenable to immunomodulation, this interface is a unique target for arresting age-related cognitive decline.

The central nervous system (CNS) tissues, including the brain, the eye, and the spinal cord, are immune-privileged, secluded from the circulation by a complex of barriers, and equipped with their own myeloid cell population, the resident microglia. Based on the classical perspective of immune-brain interactions and on the contribution of such interactions to the progression of multiple sclerosis, an autoimmune inflammatory disease of the CNS, infiltrating macrophages were traditionally viewed as an enemy of the nervous system. However, over the past two decades, research has revealed the pivotal role of monocyte-derived macrophages in CNS repair, and opened up a new era in understanding and treating CNS pathologies. Here, we gather current knowledge regarding macrophage broad spectrum of activities in the CNS, whose two poles correspond to the classical pro-inflammatory M1 and the lternatively-activated' M2 cells previously described in various non-CNS pathologies, and their diverse, multi-functional contribution in various neurological conditions, ranging from acute traumas to neurodegenerative disorders, and autoimmune diseases. The diverse functions are manifested by induction and resolution of inflammation as well as their involvement in neural tissue regeneration and renewal, matrix remodelling, debris clearance, and angiogenesis. A special focus is devoted to current evidence suggesting that resident microglia and infiltrating monocyte-derived macrophages are functionally non-redundant cell types. Taken together, these recent advances reveal a dramatic therapeutic opportunity for controlled harnessing of macrophages for repair of the damaged CNS following acute insults, in neurodegenerative conditions, and in psychiatric disorders.

Philosophers defined the eye as a window to the soul long before scientists addressed this cliché to determine its scientific basis and clinical relevance. Anatomically and developmentally, the retina is known as an extension of the CNS; it consists of retinal ganglion cells, the axons of which form the optic nerve, whose fibres are, in effect, CNS axons. The eye has unique physical structures and a local array of surface molecules and cytokines, and is host to specialized immune responses similar to those in the brain and spinal cord. Several well-defined neurodegenerative conditions that affect the brain and spinal cord have manifestations in the eye, and ocular symptoms often precede conventional diagnosis of such CNS disorders. Furthermore, various eye-specific pathologies share characteristics of other CNS pathologies. In this Review, we summarize data that support examination of the eye as a noninvasive approach to the diagnosis of select CNS diseases, and the use of the eye as a valuable model to study the CNS. Translation of eye research to CNS disease, and deciphering the role of immune cells in these two systems, could improve our understanding and, potentially, the treatment of neurodegenerative disorders.

For decades, the irreversible functional loss following central nervous system injury has been perceived with a high degree of pessimism, and researchers challenging this assumption have been considered either heroic figures, or misguided fanatics. This high degree of pessimism regarding recovery evolved - in part - from many assumptions, among which that repair in the central nervous system (CNS) was likely to obey rules different from those governing wound healing or the repair of tissues outside the CNS. Therefore, it is not surprising that many phenomena, which were observed at the site of CNS injury and take part in the repair process outside the CNS, were believed to contribute to the failure of CNS repair and were therefore considered targets for elimination; these include the key processes of local inflammation and scar formation. In addition, the common manner in which the blood-brain barrier and the blood-cerebrospinal barriers were perceived led to the assumed linkage between brain pathologies and inflammation. Accordingly, it was believed that the CNS optimally functions and repairs itself in isolation from any circulating immune cells and has no need for assistance from peripheral immune cells to support repair; if immune cells infiltrate to the CNS territory, this was taken as a sign of pathology that should be mitigated. Nevertheless, through intensive studies over the last decade, many of these assumptions were challenged or modified. It became clear that the local immune response, encompassing activated microglia and infiltrating circulating immune cells, and scar formation are interim processes that are pivotal for overall repair, but require timely resolution. In addition, it became clear that resident microglia and infiltrating blood macrophages have distinct attributes and roles under pathological conditions; it is the timing and location of their activity that determine the outcome of their function. Furthermore, the requirements for neuronal rescue, protection, restoration, renewal, and regeneration differ in time, location, and the molecular and cellular players involved. According to this new development in the field of CNS injuries, the failure of recovery is not due to the occurrence of inflammation and scar formation, which are essential, but rather due to their insufficient timely resolution. This chapter will provide a historical perspective of the field, describe the emerging new understandings, and discuss their implications to therapies for spinal cord injury.

Reduction in T cell receptor (TCR) diversity in old age is considered as a major cause for immune complications in the elderly population. Here, we explored the consequences of aging on the TCR repertoire in mice using high-throughput sequencing (TCR-seq). We mapped the TCR beta repertoire of CD4+ T cells isolated from bone marrow (BM) and spleen of young and old mice. We found that TCR beta diversity is reduced in spleens of aged mice but not in their BM. Splenic CD4+ T cells were also skewed toward an effector memory phenotype in old mice, while BM cells preserved their memory phenotype with age. Analysis of V beta and J beta gene usage across samples, as well as comparison of CDR3 length distributions, showed no significant age dependent changes. However, comparison of the frequencies of amino-acid (AA) TCR beta sequences between samples revealed repertoire changes that occurred at a more refined scale. The BM-derived TCR beta repertoire was found to be similar among individual mice regardless of their age. In contrast, the splenic repertoire of old mice was not similar to those of young mice, but showed an increased similarity with the BM repertoire. Each old-mouse had a private set of expanded TCR beta sequences. Interestingly, a fraction of these sequences was found also in the BM of the same individual, sharing the same nucleotide sequence. Together, these findings show that the composition and phenotype of the CD4+ T cell BM repertoire are relatively stable with age, while diversity of the splenic repertoire is severely reduced. This reduction is caused by idiosyncratic expansions of tens to hundreds of T cell clonotypes, which dominate the repertoire of each individual. We suggest that these private and abundant clonotypes are generated by sporadic clonal expansions, some of which correspond to pre-existing BM clonotypes. These organ- and age-specific changes of the TCR beta repertoire have implications for understanding and manipulating age-associated immune decline.

For decades, several axioms have prevailed with respect to the relationships between the CNS and circulating immune cells. Specifically, immune cell entry was largely considered to be pathological or to mark the beginning of pathology within the brain. Moreover, local inflammation associated with neurodegenerative diseases such Alzheimer's disease or amyotrophic lateral sclerosis, were considered similar in their etiology to inflammatory diseases, such as remitting relapsing-multiple sclerosis. The ensuing confusion reflected a lack of awareness that the etiology of the disease as well as the origin of the immune cells determines the nature of the inflammatory response, and that inflammation resolution is an active cellular process. The last two decades have seen a revolution in these prevailing dogmas, with a significant contribution made by the authors. Microglia and infiltrating monocyte-derived macrophages are now known to be functionally distinct and of separate origin. Innate and adaptive immune cells are now known to have protective/healing properties in the CNS, as long as their activity is regulated, and their recruitment is well controlled; their role is appreciated in maintenance of brain plasticity in health, aging, and chronic neurodevelopmental and neurodegenerative diseases. Moreover, it is now understood that the barriers of the brain are not uniform in their interactions with the circulating immune cells. The implications of these new findings to the basic understanding of CNSrepair processes, brain aging, and a wide spectrum of CNSdisorders, including acute injuries, Rett syndrome, Alzheimer's disease, and multiple sclerosis, will be discussed.

Toll-like receptors (TLRs) are traditionally associated with immune-mediated host defense. Here, we ascribe a novel extra-immune, hypothalamic-associated function to TLR2, a TLR-family member known to recognize lipid components, in the protection against obesity. We found that TLR2-deficient mice exhibited mature-onset obesity and susceptibility to high-fat diet (HFD)-induced weight gain, via modulation of food intake. Age-related obesity was still evident in chimeric mice, carrying comparable TLR2 + immune cells, suggesting a non-hematopoietic-related involvement of this receptor. TLR2 was up-regulated with age or HFD in pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus, a brain area participating in central-metabolic regulation, possibly modulating the hypothalamic- anorexigenic peptide, α-melanocyte-stimulating hormone (α-MSH). Direct activation of TLR2 in a hypothalamic-neuronal cell-line via its known ligands, further supports its capacity to mediate non-immune related metabolic regulation. Thus, our findings identify TLR2 expressed by hypothalamic neurons as a potential novel regulator of age-related weight gain and energy expenditure.

Stressful episodes or chronic stress can shape our brain, leaving behind their unfavorable biochemical signature on the neural tissue parenchyma. Mitigating such a signature on the central nervous system (CNS) would be advantageous for coping with stress. While the underlying mechanisms that facilitate this response are still a mystery, recent studies demonstrate that modulation of circulating immunity can potentially enhance the ability to deal with stressors. Accordingly, boosting immunity by active immunization with CNS-derived peptides was shown to reduce anxiety levels and to modulate hippocampal plasticity. Yet, the fact that the adaptive arm of the immune system is largely excluded from directly interacting with the healthy CNS raises a key question as to how these beneficial effects take place. Certain insight towards a mechanism underlying these effects emerged from the observation of increased immune surveillance at the borders of the brain under conditions of mental stress; specifically, at the choroid plexus (CP), an epithelial layer that resides at the junction between the blood circulation and the brain, and plays a key role in maintaining and restoring brain homeostasis, regulating cerebrospinal-fluid (CSF) production and neurotropic factors composition. Here, we suggest that immunomodulation of this site by active immunization could protect against stressful episodes, thereby providing a therapeutic, as well as preventive, vaccine for the mind against stress and depression.

Understanding of ocular diseases and the search for their cure have been based on the common assumption that the eye is an immune privileged site, and the consequent conclusion that entry of immune cells to this organ is forbidden. Accordingly, it was assumed that when immune cell entry does occur, this reflects an undesired outcome of breached barriers. However, studies spanning more than a decade have demonstrated that acute insults to the retina, or chronic conditions resulting in retinal ganglion cell loss, such as in glaucoma, result in an inferior outcome in immunocompromised mice; likewise, steroidal treatment was found to be detrimental under these conditions. Moreover, even conditions that are associated with inflammation, such as age-related macular degeneration, are not currently believed to require immune suppression for treatment, but rather, are thought to benefit from immune modulation. Here, we propose that the immune privilege of the eye is its ability to enable, upon need, the entry of selected immune cells for its repair and healing, rather than to altogether prevent immune cell entry. The implications for acute and chronic degenerative diseases, as well as for infection and inflammatory diseases, are discussed.

Systemic or intracerebral delivery of neural stem and progenitor cells (NSPCs) and activation of endogenous NSPCs hold much promise as potential treatments for diseases in the human CNS. Recent studies have shed new light on the interaction between the NSPCs and cells belonging to the innate and adaptive arms of the immune system. According to these studies, the immune cells can be both beneficial and detrimental for cell genesis from grafted and endogenous NSPCs in the CNS, and the NSPCs exert their beneficial effects not only by cell replacement but also by immunomodulation and trophic support. The cross-talk between immune cells and NSPCs and their progeny seems to determine both the efficacy of endogenous regenerative responses and the mechanism of action as well as the fate and functional integration of grafted NSPCs. Better understanding of the dialog between NSPCs and innate and adaptive immune cells is crucial for further development of effective strategies for CNS repair.

Accidental organophosphate poisoning resulting from environmental or occupational exposure, as well as the deliberate use of nerve agents on the battlefield or by terrorists, remain major threats for multi-casualty events, with no effective therapies yet available. Even transient exposure to organophosphorous compounds may lead to brain damage associated with microglial activation and to long-lasting neurological and psychological deficits. Regulation of the microglial response by adaptive immunity was previously shown to reduce the consequences of acute insult to the central nervous system (CNS). Here, we tested whether an immunization-based treatment that affects the properties of T regulatory cells (Tregs) can reduce brain damage following organophosphate intoxication, as a supplement to the standard antidotal protocol. Rats were intoxicated by acute exposure to the nerve agent soman, or the organophosphate pesticide, paraoxon, and after 24h were treated with the immunomodulator, poly-YE. A single injection of poly-YE resulted in a significant increase in neuronal survival and tissue preservation. The beneficial effect of poly-YE treatment was associated with specific recruitment of CD4 ⁺ T cells into the brain, reduced microglial activation, and an increase in the levels of brain derived neurotrophic factor (BDNF) in the piriform cortex. These results suggest therapeutic intervention with poly-YE as an immunomodulatory supplementary approach against consequences of organophosphate-induced brain damage.

The brain has been commonly regarded as a "tissue behind walls." Appearance of immune cells in the brain has been taken as a sign of pathology. Moreover, since infiltrating monocyte-derived macrophages and activated resident microglia were indistinguishable by conventional means, both populations were considered together as inflammatory cells that should be mitigated.Yet, because the microglia permanently reside in the brain, attributing to them negative properties evoked an ongoing debate; why cells that are supposed to be the brain guardians acquire only destructive potential? Studies over the last two decades in the immune arena in general, and in the context of central nervous system pathology in particular, have resulted in a paradigm shift toward a more balanced appreciation of the contributions of immune cells in the context of brain maintenance and repair, and toward the recognition of distinct roles of resident microglia and infiltrating monocyte-derived macrophages.

The inflammatory response in the injured spinal cord, an immune privileged site, has been mainly associated with the poor prognosis. However, recent data demonstrated that, in fact, some leukocytes, namely monocytes, are pivotal for repair due to their alternative anti-inflammatory phenotype. Given the pro-inflammatory milieu within the traumatized spinal cord, known to skew monocytes towards a classical phenotype, a pertinent question is how parenchymal-invading monocytes acquire resolving properties essential for healing, under such unfavorable conditions. In light of the spatial association between resolving (interleukin (IL)-10 producing) monocytes and the glial scar matrix chondroitin sulfate proteoglycan (CSPG), in this study we examined the mutual relationship between these two components. By inhibiting the de novo production of CSPG following spinal cord injury, we demonstrated that this extracellular matrix, mainly known for its ability to inhibit axonal growth, serves as a critical template skewing the entering monocytes towards the resolving phenotype. In vitro cell culture studies demonstrated that this matrix alone is sufficient to induce such monocyte polarization. Reciprocal conditional ablation of the monocyte-derived macrophages concentrated at the lesion margins, using diphtheria toxin, revealed that these cells have scar matrix-resolving properties. Replenishment of monocytic cell populations to the ablated mice demonstrated that this extracellular remodeling ability of the infiltrating monocytes requires their expression of the matrix-degrading enzyme, matrix metalloproteinase 13 (MMP-13), a property that was found here to be crucial for functional recovery. Altogether, this study demonstrates that the glial scar-matrix, a known obstacle to regeneration, is a critical component skewing the encountering monocytes towards a resolving phenotype. In an apparent feedback loop, monocytes were found to regulate scar resolution. This cross-regulation between the glial scar and monocytes primes the resolution of this interim phase of spinal cord repair, thereby providing a fundamental platform for the dynamic healing response.

Background: Circulating immune cells including autoreactive T cells and monocytes have been documented as key players in maintaining, protecting and repairing the central nervous system (CNS) in health and disease. Here, we hypothesized that neurodegenerative diseases might be associated, similarly to tumors, with increased levels of circulating peripheral myeloid derived suppressor cells (MDSCs), representing a subset of suppressor cells that often expand under pathological conditions and inhibit possible recruitment of helper T cells needed for fighting off the disease. Methods and Findings: We tested this working hypothesis in amyotrophic lateral sclerosis (ALS) and its mouse model, which are characterized by a rapid progression once clinical symptoms are evident. Adaptive transfer of alternatively activated myeloid (M2) cells, which homed to the spleen and exhibited immune suppressive activity in G93A mutant superoxide dismutase-1 (mSOD1) mice at a stage before emergence of disease symptoms, resulted in earlier appearance of disease symptoms and shorter life expectancy. The same protocol mitigated the inflammation-induced disease model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), which requires circulating T cells for disease induction. Analysis of whole peripheral blood samples obtained from 28 patients suffering from sporadic ALS (sALS), revealed a two-fold increase in the percentage of circulating MDSCs (LIN ^-/LowHLA-DR ^-CD33 ⁺) compared to controls. Conclusions: Taken together, these results emphasize the distinct requirements for fighting the inflammatory neurodegenerative disease, multiple sclerosis, and the neurodegenerative disease, ALS, though both share a local inflammatory component. Moreover, the increased levels of circulating MDSCs in ALS patients indicates the operation of systemic mechanisms that might lead to an impairment of T cell reactivity needed to overcome the disease conditions within the CNS. This high level of suppressive immune cells might represent a risk factor and a novel target for therapeutic intervention in ALS at least at the early stage.

Regenerative processes occurring under physiological (maintenance) and pathological (reparative) conditions are a fundamental part of life and vary greatly among different species, individuals, and tissues. Physiological regeneration occurs naturally as a consequence of normal cell erosion, or as an inevitable outcome of any biological process aiming at the restoration of homeostasis. Reparative regeneration occurs as a consequence of tissue damage. Although the central nervous system (CNS) has been considered for years as a "perennial" tissue, it has recently become clear that both physiological and reparative regeneration occur also within the CNS to sustain tissue homeostasis and repair. Proliferation and differentiation of neural stem/progenitor cells (NPCs) residing within the healthy CNS, or surviving injury, are considered crucial in sustaining these processes. Thus a large number of experimental stem cell-based transplantation systems for CNS repair have recently been established. The results suggest that transplanted NPCs promote tissue repair not only via cell replacement but also through their local contribution to changes in the diseased tissue milieu. This review focuses on the remarkable plasticity of endogenous and exogenous (transplanted) NPCs in promoting repair. Special attention will be given to the cross-talk existing between NPCs and CNS-resident microglia as well as CNS-infiltrating immune cells from the circulation, as a crucial event sustaining NPC-mediated neuroprotection. Finally, we will propose the concept of the context-dependent potency of transplanted NPCs (therapeutic plasticity) to exert multiple therapeutic actions, such as cell replacement, neurotrophic support, and immunomodulation, in CNS repair.

Purpose: Evaluation of the neuroprotective effect of weekly glatiramer acetate (GA) on retinal structure and function in diabetic patients who underwent panretinal photocoagulation (PRP). Patients and methods: Patients with severe nonproliferative or early proliferative diabetic retinopathy and no previous laser treatment were randomly divided into two groups: (1) those who received four GA treatments and (2) those who received placebo treatment. The subcutaneous injections were administered 1 week prior to laser and weekly in the subsequent three sessions of PRP in both groups. All patients underwent a full ophthalmic examination (best-corrected logMAR visual acuity, slit lamp examination, applanation tonometry, fundus biomicroscopy and indirect fundus examination); functional examination (standard automated perimetry, electroretinography and frequency-doubling technology C-20 visual field) and anatomic examination (color photography, optical coherence tomography (OCT) and Heidelberg retinal tomography). The examinations were performed before the photocoagulation and repeated 1,3,6, and 12 months after treatment (in a double-masked manner). To compare the two groups, generalized estimating equation models were performed to account for the dependence between eyes of the same patient. Results: Thirteen patients (23 eyes) were included in the study group and 13 patients (24 eyes) were included in the control group. OCT showed a statistically significant difference in retinal nerve fiber layer (RNFL) thickness in the inferior peripapillary region and average thickness with thinner measurements in the control group at 1-year post-PRP. Functional analysis demonstrated a difference between groups, but it did not reach statistical significance. Conclusion: The results of this study suggest that weekly GA treatment has a potential neuroprotective effect on the RNFL following photocoagulation for diabetic retinopathy.

An organism's behavior is determined by the way it senses and perceives the surrounding environment, and by its responses to these stimuli. The major factors known to affect the behavioral response to an event are genetic background, environmental factors, and past experiences, and their imprinting on the relevant brain circuits. Recently, circulating immune cells were introduced as novel players into this system. It was proposed that the brain and circulating immune cells engage in a continuous dialogue that takes place within the brain's territory, though outside the parenchyma (occurring within the brain's borders - the choroid plexi, the brain meninges and the cerebrospinal fluid (CSF)). The cytokines secreted by activated leukocytes residing at the borders were shown to affect neurotrophic factors production within the parenchyma. Here, we suggest that such a dialogue is stimulated at the brain's borders, upon need, by a " danger" signal that originates in the parenchyma in response to any destabilizing event, and discuss the potential role of reactive oxygen species (ROS) in transmitting this signal. Accordingly, a failure to restore balance is likely to lead to aberrant responses to subsequent events. This view thus supports the contention that circulating immune cells are required to maintain the brain's balanced activity and suggests a novel mechanism whereby the surveying immune cells are sensing the brain's status and needs.

The ability to respond to a wide range of novel touch sensations and to habituate upon repeated exposures is fundamental for effective sensation. In this study we identified adult spinal cord neurogenesis as a potential novel player in the mechanism of tactile sensation. We demonstrate that a single exposure to a novel mechanosensory stimulus induced immediate proliferation of progenitor cells in the spinal dorsal horn, whereas repeated exposures to the same stimulus induced neuronal differentiation and survival. Most of the newly formed neurons differentiated toward a GABAergic fate. This touch-induced neurogenesis reflected the novelty of the stimuli, its diversity, as well as stimulus duration. Introducing adult neurogenesis as a potential mechanism of response to a novel stimulus and for habituation to repeated sensory exposures opens up potential new directions in treating hypersensitivity, pain and other mechanosensory disorders.

The death of retinal ganglion cells (RGCs) is a hallmark of many retinal neuropathies. Neuroprotection, axonal regeneration, and cell renewal are vital for the integrity of the visual system after insult but are scarce in the adult mammalian retina. We hypothesized that monocytederived macrophages, known to promote healing in peripheral tissues, are required after an insult to the visual system, where their role has been largely overlooked. We found that after glutamate eye intoxication, monocyte-derived macrophages infiltrated the damaged retina of mice. Inhibition of this infiltration resulted in reduced survival of RGCs and diminished numbers of proliferating retinal progenitor cells (RPCs) in the ciliary body. Enhancement of the circulating monocyte pool led to increased RGC survival and RPC renewal. The infiltrating monocyte-derived macrophages skewed the milieu of the injured retina toward an antiinflammatory and neuroprotective one and down-regulated accumulation of other immune cells, thereby resolving local inflammation. The beneficial effect on RGC survival depended on expression of interleukin 10 and major histocompatibility complex class II molecules by monocyte-derived macrophages. Thus, we attribute to infiltrating monocyte-derived macrophages a novel role in neuroprotection and progenitor cell renewal in the injured retina, with far-reaching potential implications to retinal neuropathies and other neurodegenerative disorders.

Noninvasive monitoring of β-amyloid (Aβ) plaques, the neuropathological hallmarks of Alzheimer's disease (AD), is critical for AD diagnosis and prognosis. Current visualization of Aβ plaques in brains of live patients and animal models is limited in specificity and resolution. The retina as an extension of the brain presents an appealing target for a live, noninvasive optical imaging of AD if disease pathology is manifested there. We identified retinal Aβ plaques in postmortem eyes from AD patients (n=8) and in suspected early stage cases (n=5), consistent with brain pathology and clinical reports; plaques were undetectable in age-matched non-AD individuals (n=5). In APP_SWE/PS1_βE9 transgenic mice (AD-Tg; n=18) but not in non-Tg wt mice (n=10), retinal Aβ plaques were detected following systemic administration of curcumin, a safe plaque-labeling fluorochrome. Moreover, retinal plaques were detectable earlier than in the brain and accumulated with disease progression. An immune-based therapy effective in reducing brain plaques, significantly reduced retinal Aβ plaque burden in immunized versus non-immunized AD mice (n=4 mice per group). In live AD-Tg mice (n=24), systemic administration of curcumin allowed noninvasive optical imaging of retinal Aβ plaques in vivo with high resolution and specificity; plaques were undetectable in non-Tg wt mice (n=11). Our discovery of Aβ specific plaques in retinas from AD patients, and the ability to noninvasively detect individual retinal plaques in live AD mice establish the basis for developing high-resolution optical imaging for early AD diagnosis, prognosis assessment and response to therapies.

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing fatal neurodegenerative disorder characterized by the selective death of motor neurons (MN) in the spinal cord, and is associated with local neuroinflammation. Circulating CD4 ⁺ T cells are required for controlling the local detrimental inflammation in neurodegenerative diseases, and for supporting neuronal survival, including that of MN. T-cell deficiency increases neuronal loss, while boosting T cell levels reduces it. Here, we show that in the mutant superoxide dismutase 1 G93A (mSOD1) mouse model of ALS, the levels of natural killer T (NKT) cells increased dramatically, and T-cell distribution was altered both in lymphoid organs and in the spinal cord relative to wild-type mice. The most significant elevation of NKT cells was observed in the liver, concomitant with organ atrophy. Hepatic expression levels of insulin-like growth factor (IGF)-1 decreased, while the expression of IGF binding protein (IGFBP)-1 was augmented by more than 20-fold in mSOD1 mice relative to wild-type animals. Moreover, hepatic lymphocytes of pre-symptomatic mSOD1 mice were found to secrete significantly higher levels of cytokines when stimulated with an NKT ligand, ex-vivo. Immunomodulation of NKT cells using an analogue of α-galactosyl ceramide (α-GalCer), in a specific regimen, diminished the number of these cells in the periphery, and induced recruitment of T cells into the affected spinal cord, leading to a modest but significant prolongation of life span of mSOD1 mice. These results identify NKT cells as potential players in ALS, and the liver as an additional site of major pathology in this disease, thereby emphasizing that ALS is not only a non-cell autonomous, but a non-tissue autonomous disease, as well. Moreover, the results suggest potential new therapeutic targets such as the liver for immunomodulatory intervention for modifying the disease, in addition to MN-based neuroprotection and systemic treatments aimed at reducing oxidative stress.

Amyotrophic lateral sclerosis (ALS) is a devastating disease, characterized by extremely rapid loss of motor neurons. Our studies over the last decade have established CD4⁺ T cells as important players in central nervous system maintenance and repair. Those results, together with recent findings that CD4⁺ T cells play a protective role in mouse models of ALS, led us to the current hypothesis that in ALS, a rapid T-cell malfunction may develop in parallel to the motor neuron dysfunction. Here, we tested this hypothesis by assessing thymic function, which serves as a measure of peripheral T-cell availability, in an animal model of ALS (mSOD1 [superoxide dismutase] mice; G93A) and in human patients. We found a significant reduction in thymic progenitor-cell content, and abnormal thymic histology in 3-4-month-old mSOD1 mice. In ALS patients, we found a decline in thymic output, manifested in the reduction in blood levels of T-cell receptor rearrangement excision circles, a non-invasive measure of thymic function, and demonstrated a restricted T-cell repertoire. The morbidity of the peripheral immune cells was also manifested in the increase of pro-apoptotic BAX/BCXL2 expression ratio in peripheral blood mononuclear cells (PBMCs) of these patients. In addition, gene expression screening in the same PBMCs, revealed in the ALS patients a reduction in key genes known to be associated with T-cell activity, including: CD80, CD86, IFNG and IL18. In light of the reported beneficial role of T cells in animal models of ALS, the present observation of thymic dysfunction, both in human patients and in an animal model, might be a co-pathological factor in ALS, regardless of the disease aetiology. These findings may lead to the development of novel therapeutic approaches directed at overcoming the thymic defect and T-cell deficiency.

Until recently, the local inflammation that occurs in response to spinal cord injury has received a negative reputation; overall, it was assumed to be one of the major causes of a vicious neurotoxic cycle that leads to impaired recovery following injury. This local inflammation involves both the activated tissue-resident microglia and monocyte-derived macrophages infiltrating from the blood. Ten years ago, we proposed that the blood-derived macrophages, reminiscent of "alternatively activated" macrophages (also known as tissue repairing, M2), are not spontaneously recruited in sufficient numbers to sites of injured central nervous system (CNS). We further demonstrated that their exogenous administration to the margins of injured spinal cord improved functional outcome. However, our suggestions evoked criticism, claiming that we were adding macrophages to a site that is already overwhelmed with inflammatory cells. Using experimental paradigms that enabled functional distinction between the resident and infiltrating cells, our most recent studies further corroborated our repair perception, showing that (a) infiltrating monocyte-derived macrophages are recruited following injury and localize to the margins of the lesion, unlike the activated resident microglia that are not compartmentalized, and (b) activated resident microglia and infiltrating monocyte-derived macrophages perform distinct roles; recruited blood-derived macrophages display an (IL-10-dependent) anti-inflammatory phenotype when they become co-localized with the glial scar. We further found that post-injury recruitment of blood monocytes is indeed suboptimal. Augmentation of the levels of naïve blood monocytes leads to their increased recruitment to the same zones that are the targets of the infiltrated endogenous monocytes, and they acquire the same anti-inflammatory activity, leading to improved recovery. Thus, boosting the levels of the relevant blood monocytes reinforces the body's own repair mechanisms that, for reasons that are currently under investigation, are not optimally triggered within the critical post-injury period.

Treatment of Alzheimer disease or amyotrophic lateral sclerosis with anti-inflammatory drugs (to prevent disease or slow its progression) has yielded mixed results, despite evidence indicating that local cytotoxic inflammation occurs in these conditions. Here, through consideration of the importance of immune cell origin (resident versus blood-derived immune cells) and activity (pro-inflammatory versus anti-inflammatory activity) under neurodegenerative conditions, we propose a model that reconciles these seemingly inconsistent data. We suggest that systemic immune cells (CD4+ T cells and peripheral blood-derived monocytes) must be recruited to the CNS to modify potentially destructive local inflammation, and that the failure of systemic anti-inflammatory drug therapies to arrest neurodegenerative disease progression might result from drug-induced suppression of such recruitment. Thus, we propose that an appreciation of the distinctive temporal and spatial contributions of resident and systemic leukocytes to disease progression is essential for the development of effective therapeutic regimens.

Neuropsychological syndromes including schizophrenia often do not manifest until late adolescence or early adulthood. Studies attributing a role in brain maintenance to the immune system led us to propose that malfunction of immune-dependent regulation of brain functions at adolescence underlies the late onset of such diseases/syndromes. One such function is sensorimotor gating, the ability to segregate a continuous stream of sensory and cognitive information, and to selectively allocate attention to a significant event by silencing the background (measured by prepulse inhibition; PPI). This activity is impaired in schizophrenia, as well as in several other neuropsychological diseases. Using a model of prenatal immune activation (maternal polyriboinosinic-polyribocytidylic acid (poly I:C) injection), often used as a model for schizophrenia, and in which abnormal PPI has a delayed appearance, we demonstrated a form of immune deficit in the adult offspring. Similar abnormal PPI with a delayed appearance was found in congenitally immune-deficient mice (severe combined immune deficient, SCID), and could be reversed by immune reconstitution. This functional deficit correlated with impairment of both hippocampal neurogenesis and expression of the gene encoding kisspeptin (Kiss1) that manifested at adulthood. Moreover, exogenous administration of a kisspeptin-derived peptide partially reversed the gating deficits in the SCID mice. Our results suggest that a form of congenital immune deficiency may be a key factor that determines manifestation of developmental neuropsychological disorders with onset only at early adulthood.

Circulating immune cells support hippocampal neurogenesis, spatial memory, expression of brain-derived neurotrophic factor, and resilience to stress. Nevertheless, considering the immune privileged status of the central nervous system (CNS), such cells were assumed to be excluded from the healthy brain. It is evident, however, that the CNS is continuously surveyed by leukocytes, though their function is still a mystery. Coupling this routine leukocyte trafficking with the function attributed to circulating T cells in brain plasticity led us to propose here that CNS immunosurveillance is an integral part of the functioning brain. Anatomical restriction of selected self-recognizing leukocytes to the brain's borders and fluids (cerebrospinal fluid) not only supports the brain's activity, but also controls the potential aggressiveness of such cells. Accordingly, the brain's privilege is its acquisition of a private peripheral immunological niche under its own control, which supports brain function. Immune malfunction may comprise a missing link between a healthy and diseased mind.

Keywords: Immunology

Glaucoma, although once thought of as a single disease, is actually a group of diseases of the optic nerve involving loss of retinal ganglion cells. The process of cell death occurs in a characteristic pattern of optic neuropathy - a broad term for a certain pattern of damage to the optic nerve (the bundle of nerve fibers that carries information from the eye to the brain). Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss that can progress to permanent blindness.

Immunization with an altered myelin-derived peptide (MOG45D) improves recovery from acute CNS insults, partially via recruitment of monocyte-derived macrophages that locally display a regulatory activity. Here, we investigated the local alterations in the cellular and molecular immunological milieu associated with attenuation of Alzheimer's disease-like pathology following immunotherapy. We found that immunization of amyloid precursor protein/presenilin 1 double-transgenic mice with MOG45D peptide, loaded on dendritic cells, led to a substantial reduction of parenchymal and perivascular amyloid beta (Aβ)-plaque burden and soluble Aβ_(1-42) peptide levels as well as reduced astrogliosis and levels of a key glial scar protein (chondroitin sulphate proteoglycan). These changes were associated with a shift in the local innate immune response, manifested by increased Iba1 ⁺/CD45^high macrophages that engulfed Aβ, reduced pro-inflammatory (tumor necrosis factor-α) and increased anti-inflammatory (interleukin-10) cytokines, as well as a significant increase in growth factors (IGF-1 and TGFβ) in the brain. Furthermore, the levels of matrix metalloproteinase-9, an enzyme shown to degrade Aβ and is associated with glial scar formation, were significantly elevated in the brain following immunization. Altogether, these results indicate that boosting systemic immune cells leads to a local immunomodulation manifested by elevated levels of anti-inflammatory cytokines and metalloproteinases that contribute to ameliorating Alzheimer's disease pathology.

The factors that determine brain aging remain a mystery. Do brain aging and memory loss reflect processes occurring only within the brain? Here, we present a novel view, linking aging of adaptive immunity to brain senescence and specifically to spatial memory deterioration. Inborn immune deficiency, in addition to sudden imposition of immune malfunction in young animals, results in cognitive impairment. As a corollary, immune restoration at adulthood or in the elderly results in a reversal of memory loss. These results, together with the known deterioration of adaptive immunity in the elderly, suggest that memory loss does not solely reflect chronological age; rather, it is an outcome of the gap between an increasing demand for maintenance (age-related risk-factor accumulation) and the reduced ability of the immune system to meet these needs.

Background: Although macrophages (MΦ) are known as essential players in wound healing, their contribution to recovery from spinal cord injury (SCI) is a subject of debate. The difficulties in distinguishing between different MΦ subpopulations at the lesion site have further contributed to the controversy and led to the common view of MΦ as functionally homogenous. Given the massive accumulation in the injured spinal cord of activated resident microglia, which are the native immune occupants of the central nervous system (CNS), the recruitment of additional infiltrating monocytes from the peripheral blood seems puzzling. A key question that remains is whether the infiltrating monocyte-derived MΦ contribute to repair, or represent an unavoidable detrimental response. The hypothesis of the current study is that a specific population of infiltrating monocyte-derived MΦ is functionally distinct from the inflammatory resident microglia and is essential for recovery from SCI. Methods and Findings: We inflicted SCI in adult mice, and tested the effect of infiltrating monocyte-derived MΦ on the recovery process. Adoptive transfer experiments and bone marrow chimeras were used to functionally distinguish between the resident microglia and the infiltrating monocyte-derived MΦ. We followed the infiltration of the monocyte-derived MΦ to the injured site and characterized their spatial distribution and phenotype. Increasing the naïve monocyte pool by either adoptive transfer or CNS-specific vaccination resulted in a higher number of spontaneously recruited cells and improved recovery. Selective ablation of infiltrating monocyte-derived MΦ following SCI while sparing the resident microglia, using either antibody-mediated depletion or conditional ablation by diphtheria toxin, impaired recovery. Reconstitution of the peripheral blood with monocytes resistant to ablation restored the lost motor functions. Importantly, the infiltrating monocyte-derived MΦ displayed a local anti-inflammatory beneficial role, which was critically dependent upon their expression of interleukin 10. Conclusions: The results of this study attribute a novel anti-inflammatory role to a unique subset of infiltrating monocyte-derived MΦ in SCI recovery, which cannot be provided by the activated resident microglia. According to our results, limited recovery following SCI can be attributed in part to the inadequate, untimely, spontaneous recruitment of monocytes. This process is amenable to boosting either by active vaccination with a myelin-derived altered peptide ligand, which indicates involvement of adaptive immunity in monocyte recruitment, or by augmenting the naïve monocyte pool in the peripheral blood. Thus, our study sheds new light on the long-held debate regarding the contribution of MΦ to recovery from CNS injuries, and has potentially far-reaching therapeutic implications.

Glaucoma, a slow progressive neurodegenerative disorder associated with death of retinal ganglion cells and degeneration of their connected optic nerve fibers, has been classically linked to high intraocular pressure. Regardless of the primary risk factor, degeneration may continue, resulting in further loss of neurons and subsequent glaucomatous damage. During the past decade, scientists and clinicians began to accept that, in addition or as an alternative to fighting off the primary risk factor(s), there is a need to protect the tissue from the ongoing spread of damage-an approach collectively termed "neuroprotection." We found that the immune system, the body's own defense mechanism, plays a key role in the ability of the optic nerve and the retina to withstand glaucomatous conditions. This defense involves recruitment of both innate and adaptive immune cells that together create a protective niche and thereby halt disease progression. The spontaneous immune response might not be sufficient, and therefore, we suggest boosting it by immunization (with the appropriate antigen, at specific timing and predetermined optimal dosing) which may be developed into a suitable therapeutic vaccination to treat glaucoma. This view of immune system involvement in glaucoma will raise new challenges in glaucoma research, changing the way in which clinicians perceive the disease and the approach to therapy.

The psychobiological mechanisms that contribute to the development of stress resilience are not fully elucidated. One potential approach for enhancing resilience is the exposure to mild challenges. According to this approach, a mildly stressful episode may immunize the individual, thereby strengthening resistance to subsequent stressors. This phenomenon is often viewed as a form of behavioral immunization. Although, the term 'behavioral immunization' was borrowed from the field of immunology, the involvement of the adaptive immune system in stress resilience was never investigated. However, based on accumulated new data, we suggest that the immunological memory does have a significant role in developing coping responses to stress. Although, immune deficiency results in an impaired ability to cope with stress, boosting immunological memory can increase stress resilience. Therefore, we propose that defense against mental challenge, similarly to defense against intruders, involves an immunological mechanism, which establishes stress resilience to a later challenge. Here, we review the involvement of the adaptive immune system in coping mechanisms in response to psychological stress, and discuss the connection between cognitive memory and immunological memory in establishing ability to efficiently cope with stressful episodes.

Following CNS injury, in an apparently counterintuitive response, scar tissue formation inhibits axonal growth, imposing a major barrier to regeneration. Accordingly, scar-modulating treatments have become a leading therapeutic goal in the field of spinal cord injury. However, increasing evidence suggests a beneficial role for this scar tissue as part of the endogenous local immune regulation and repair process. How can these opposing effects be reconciled? Perhaps it is all a matter of timing.

Our research group has been working for more than a decade on the cross-talk between the immune and the nervous systems. Due to the unique nature of the central nervous system (CNS) as an immune privileged site, it was commonly believed that the nervous system functions optimally without any immune intervention, and that any immune cell infiltration to the CNS is a sign of pathology. However, since the immune system constitutes the body's major defense and repair mechanism, it seemed unreasonable that the CNS would have completely lost the need for assistance from this system. This insight prompted us to revisit the entire question of whether immune cells are required for recovery from CNS injuries. We subsequently made numerous fundamental observations that led us to formulate a unified theory linking all neurodegenerative conditions; thus, we suggested that "T-cell immunity to self maintains the self," at least in the CNS. According to this view, immunity to self ("protective autoimmunity") provides a pivotal role in maintenance, protection, and repair of the healthy and diseased CNS. We further showed that the T cells mediate their effect, at least under pathological conditions, by controlling the recruitment of blood-borne monocytes, which play a crucial local role that cannot be replaced by the resident microglia. Boosting of such a T cell response specific for brain proteins, while carefully choosing the antigen, the carrier, timing, dosing, and regimen should be considered as a way of augmenting a physiological repair mechanism needed to ameliorate disease conditions while restoring equilibrium needed for protection, repair and renewal; such therapy is not intended to modify a single mediator of a single disease, but rather, would serve as an approach for adjusting the levels of the immune response needed to restore a lost balance.

Background: Depressive behavior in animals is often associated with reduced levels of brain-derived neurotrophic factor (BDNF) and impaired neurogenesis in the hippocampus. Recent studies showed that T cells recognizing central nervous system (CNS)-specific antigens can regulate adult hippocampal neurogenesis and expression of BDNF. On the basis of these findings, we hypothesized that controlling CNS specific immune activity by immunization with a myelin-related peptide may have an antidepressant effect. Methods: We investigated the impact of immunization with a CNS related peptide, on the behavioral and cellular outcomes of chronic mild stress (CMS; an animal model for depression) in rats. Results: Immunization with a weak agonist of a myelin-derived peptide ameliorated depressive behavior such as anhedonia (measured by sucrose preference), induced by CMS in rats. The behavioral outcome was accompanied by restoration of hippocampal BDNF levels and neurogenesis. Conclusions: The results of this study introduce a novel approach of immunization with CNS-related antigens as a therapeutic means for fighting depression. Vaccination, as an antidepressant therapy, may invoke several molecular and cellular pathways that are known to be regulated by antidepressant drugs. Therefore, we suggest that immune-based therapies should be considered for treatment of depression.

Objective. The phenotypic complexity, together with the multifarious nature of the so-called "schizophrenic psychoses", limits our ability to form a simple and logical biologically based hypothesis for the disease group. Biological markers are defined as biochemical, physiological or anatomical traits that are specific to particular conditions. An important aim of biomarker discovery is the detection of disease correlates that can be used as diagnostic tools. Method. A selective review of the WFSBP Task Force on Biological Markers in schizophrenia is provided from the central nervous system to phenotypes, functional brain systems, chromosomal loci with potential genetic markers to the peripheral systems. Results. A number of biological measures have been proposed to be correlated with schizophrenia. At present, not a single biological trait in schizophrenia is available which achieves sufficient specificity, selectivity and is based on causal pathology and predictive validity to be recommended as diagnostic marker. Conclusions. With the emergence of new technologies and rigorous phenotypic subclassification the identification of genetic bases and assessment of dynamic disease related alterations will hopefully come to a new stage in the complex field of psychiatric research.

Retinal neurogenesis ceases by the early postnatal period, although retinal progenitor cells (RPCs) persist throughout life. In this study, we show that in the mammalian eye, the function of Toll-like receptor 4 (TLR4) extends beyond regulation of the innate immune response; it restricts RPC proliferation. In TLR4-defi cient mice, enhanced proliferation of cells reminiscent of RPCs is evident during the early postnatal period. In vitro experiments demonstrate that TLR4 acts as an intrinsic regulator of RPC fate decision. Increased TLR4 expression in the eye correlates with the postnatal cessation of cell proliferation. However, defi cient TLR4 expression is not suffi cient to extend the proliferative period but rather contributes to resumption of proliferation in combination with growth factors. Proliferation in vivo is inhibited by both MyD88- dependent and -independent pathways, similar to the mechanisms activated by TLR4 in immune cells. Thus, our study attributes a novel role to TLR4 as a negative regulator of RPC proliferation.

Purpose: Drusen formation in age-related macular degeneration (AMD) shares some similarities with Alzheimer's disease (AD), which is associated with amyloid deposits. Aggregated beta-amyloid induces microglia to become cytotoxic and block neurogenesis. Recent evidence showed that T cell-based vaccination with Copaxone in AD mice model resulted in modulation of microglia into neuroprotective phenotype and as a result in reduction of cognitive decline, elimination of plaque formation, and induction of neuronal survival and neurogenesis. The aim was to investigate whether the effect of Copaxone on drusen in dry AMD is similar to that on deposits of other age-related chronic neurodegenerative diseases such as Alzheimer disease (AD). Materials and Methods: Patients over 50 years of age with intermediate dry AMD in both eyes were randomized to receive Copaxone or sham injections and were weekly treated by subcutaneous injections of Copaxone (dose of 20 mg) or sham injections for 12 weeks. At baseline, 6-week, and 12-week visits, visual acuity, contrast sensitivity, fundus examination and photography, fluorescein angiography, and ocular coherent tomography were performed. Main outcome measure was a change in total drusen area (TDA) measured by Image-Pro software and presented in arbitrary units (AU). Results: Eight studied eyes of four treated patients showed a decrease in TDA from 48130 to 16205 AU at 12 weeks as compared to baseline. In contrast, four control eyes (two patients) demonstrated almost no change in TDA (from 32294 to 32781 AU). Conclusion: These preliminary results show that Copaxone reduces drusen area.

Immune cells and immune molecules have recently been shown to support neurogenesis from neural stem and progenitor cells in the adult brain. This non-classical immune activity takes place constantly under normal physiological conditions and is extended under acute pathological conditions to include the attraction of progenitor cells and induction of neurogenesis in regions of the adult central nervous system (CNS) in which formation of new neurons does not normally occur. We suggest that the immune system should be viewed as a novel player in the adult neural stem cell niche and a coordinator of cell renewal processes after injury. We discuss these notions in light of the well-known facts that both immune-cell activity and cell renewal are inherently limited in the adult CNS and that immune and stem cells provide the body's mechanisms of repair.

Aging is often associated with a decline in hippocampus-dependent spatial memory. Here, we show that functional cell-mediated immunity is required for the maintenance of hippocampus-dependent spatial memory. Sudden imposition of immune compromise in young mice caused spatial memory impairment, whereas immune reconstitution reversed memory deficit in immune-deficient mice. Analysis of hippocampal gene expression suggested that immune-dependent spatial memory performance was associated with the expression of insulin-like growth factor (Igf1) and of genes encoding proteins related to presynaptic activity (Syt10, Cplx2). We further showed that memory loss in aged mice could be attributed to age-related attenuation of the immune response and could be reversed by immune system activation. Homeostatic-driven proliferation of lymphocytes, which expands the existing T cell repertoire, restored spatial memory deficits in aged mice. Thus, our results identify a novel function of the immune system in the maintenance of spatial memory and suggest an original approach for arresting or reversing age-associated memory loss.

Trafficking of T lymphocytes to specific organs, such as the skin and lungs, is part of the body's defense mechanism following acute psychological stress. Here we demonstrate that T lymphocytes are also trafficking to the brain in response to psychological stress and are needed to alleviate its negative behavioral consequences. We show that short exposure of mice to a stressor (predator odor) enhanced T-cell infiltration to the brain, especially to the choroid plexus, and that this infiltration was associated with increased ICAM-1 expression by choroid plexus cells. Systemic administration of corticosterone could mimic the effects of psychological stress on ICAM-1 expression. Furthermore, we found that the ability to cope with this stress is interrelated with T-cell trafficking and with the brain and hippocampal BDNF levels. Immunization with a CNS-related peptide reduced the stress-induced anxiety and the acoustic startle response, and restored levels of BDNF, shown to be important for stress resilience. These results identified T cells as novel players in coping with psychological stress, and offers immunization with a myelin-related peptide as a new therapeutic approach to alleviate chronic consequences of acute psychological trauma, such as those found in posttraumatic stress disorder.

The burden of neurological diseases in western societies has accentuated the need to develop effective therapies to stop the progression of chronic neurological diseases. Recent discoveries regarding the role of the immune system in brain damage coupled with the development of new technologies to manipulate the immune response make immunotherapies an attractive possibility to treat neurological diseases. The wide repertoire of immune responses and the possibility to engineer such responses, as well as their capacity to promote tissue repair, indicates that immunotherapy might offer benefits in the treatment of neurological diseases, similar to the benefits that are being associated with the treatment of cancer and autoimmune diseases. However, before applying such strategies to patients it is necessary to better understand the pathologies to be targeted, as well as how individual subjects may respond to immunotherapies, either in isolation or in combination. Due to the powerful effects of the immune system, one priority is to avoid tissue damage due to the activity of the immune system, particularly considering that the nervous system does not tolerate even the smallest amount of tissue damage.

Background: Chondroitin sulfate proteoglycan (CSPG) is a major component of the glial scar. It is considered to be a major obstacle for central nervous system (CNS) recovery after injury, especially in light of its well-known activity in limiting axonal growth. Therefore, its degradation has become a key therapeutic goal in the field of CNS regeneration. Yet, the abundant de novo synthesis of CSPG in response to CNS injury is puzzling. This apparent dichotomy led us to hypothesize that CSPG plays a beneficial role in the repair process, which might have been previously overlooked because of nonoptimal regulation of its levels. This hypothesis is tested in the present study. Methods and Findings: We inflicted spinal cord injury in adult mice and examined the effects of CSPG on the recovery process. We used xyloside to inhibit CSPG formation at different time points after the injury and analyzed the phenotype acquired by the microglia/macrophages in the lesion site. To distinguish between the resident microglia and infiltrating monocytes, we used chimeric mice whose bone marrow-derived myeloid cells expressed GFP. We found that CSPG plays a key role during the acute recovery stage after spinal cord injury in mice. Inhibition of CSPG synthesis immediately after injury impaired functional motor recovery and increased tissue loss. Using the chimeric mice we found that the immediate inhibition of CSPG production caused a dramatic effect on the spatial organization of the infiltrating myeloid cells around the lesion site, decreased insulin-like growth factor 1 (IGF-1) production by microglia/macrophages, and increased tumor necrosis factor alpha (TNF-α) levels. In contrast, delayed inhibition, allowing CSPG synthesis during the first 2 d following injury, with subsequent inhibition, improved recovery. Using in vitro studies, we showed that CSPG directly activated microglia/ macrophages via the CD44 receptor and modulated neurotrophic factor secretion by these cells. Conclusions: Our results show that CSPG plays a pivotal role in the repair of injured spinal cord and in the recovery of motor function during the acute phase after the injury; CSPG spatially and temporally controls activity of infiltrating blood-borne monocytes and resident microglia. The distinction made in this study between the beneficial role of CSPG during the acute stage and its deleterious effect at later stages emphasizes the need to retain the endogenous potential of this molecule in repair by controlling its levels at different stages of post-injury repair.

Mounting evidence from the last decade has shown that the immune system not only fights pathogens but also protects the body against cancer. More recently, immune surveillance has been shown to be important for maintaining the functional integrity of the central nervous system. The immune system, however, does not always prevail; tumors do grow and eventually kill their host, and devastating neurodegenerative conditions do develop. Neurodegenerative diseases, like tumors, lie dormant long before clinical symptoms appear. We propose that at this dormant stage, an ongoing competition between the local disease-causing factors and the immune system's attempts to contain them takes place. Onset of clinical symptoms occurs after disease-causing factors escape immune surveillance. Identifying the immune escape mechanisms and circumventing them soon after the emergence of clinical symptoms could lead to the development of novel therapeutic intervention for some of the most devastating neurodegenerative disorders.

Although there is a growing acceptance that immune cells could play a protective role under various injurious or pathological conditions of the central nervous system (CNS), the possibility that the immune system is constantly involved in day-to-day maintenance of CNS functional integrity has not been acknowledged. Here, we propose a unifying hypothesis, based on a recent collection of experimental results, suggesting that the loss of immunity to certain self-antigens or its insufficiency when encountering increased levels of risk factors is an important underlying factor in the onset or escalation of neurodegenerative processes, age-related dementia or mental dysfunction. We further suggest a model that explains how immunity to self exerts its roles in the special immune-privileged context of the CNS.

Neurogenesis, the formation of new neurons from stem/progenitor cells, occurs in the hippocampal dentate gyrus throughout life. Although the exact function of adult hippocampal neurogenesis is currently unknown, recent studies suggest that the newly formed neuronal population plays an important role in hippocampal-dependent cognitive abilities, including declarative memory. The process of adult neurogenesis is greatly influenced by the interaction between cells of the adaptive immune system and CNS-resident immune cells. Our laboratory has recently demonstrated that immune cells contribute to maintaining life-long hippocampal neurogenesis. The regulation of such immune-cell activity is crucial: too little immune activity (as in immune deficiency syndromes) or too much immune activity (as in severe inflammatory diseases) can lead to impaired hippocampal neurogenesis, which could then result in impaired hippocampal-dependent cognitive abilities. From these converging discoveries arise a mechanism that can explain one route by which our body affects our mind.

Neurodegenerative diseases are generally considered to be non-inflammatory, unlike autoimmune diseases such as multiple sclerosis, which are neurodegenerative diseases that are inflammatory in nature (Trapp et al., 1999a, b; Hohlfeld and Wiendl, 2001; Groom et al., 2003). Nevertheless, most neurodegenerative diseases are accompanied by a local inflammatory response, widely assumed to be unfavorable for CNS recovery (Kurosinski and Gotz, 2002; Jellinger, 2003; Popovich and Jones, 2003). Moreover, the progressive degeneration seen in such diseases is often mediated by compounds and processes that are secondary to the primary risk, e.g., misfolding and aggregation of self-proteins (Shastry, 2003). These primary and secondary risk factors represent a continuous threat to any viable neurons embedded in a chronically diseased tissue; they induce abnormalities in cells in their vicinity, thereby contributing to the chaos rather than helping to resolve it.

Adaptive and innate immunity, if well controlled, contribute to the maintenance of the CNS, as well as to downregulation of adverse acute and chronic neurological conditions. T cells that recognize CNS antigens are needed to activate resident immune cells and to recruit blood-borne monocytes, which act to restore homeostasis and facilitate repair. However, boosting such a T-cell response in a risk-free way requires a careful choice of the antigen, carrier, and regimen. A single vaccination with CNS-derived peptides or their weak agonists reduces neuronal loss in animal models of acute neurodegeneration. Repeated injections are needed to maintain a long-lasting effect in chronic neurodegenerative conditions, yet the frequency of the injections seems to have a critical effect on the outcome. An example is glatiramer acetate, a compound that is administered in a daily regimen to patients with multiple sclerosis. A single injection of glatiramer acetate, with or without an adjuvant, is neuroprotective in some animal models of acute CNS injuries. However, in an animal model of amyotrophic lateral sclerosis, a single injection of adjuvant-free glatiramer acetate is insufficient, while daily injections are not only ineffective but can carry an increased risk of mortality in female mice. Thus, considering immune-based therapies as a single therapy, rather than as a family of therapies that are regimen dependent, may be misleading. Moreover, the vaccination regimen and administration of a compound, even one shown to be safe in humans for the treatment of a particular neurodegenerative disease, must be studied in preclinical experiments before it is tested in a clinical trial for a novel indication; otherwise, an effective drug in a certain regimen for one disease may be ineffective or even carry risks when used for another disorder.

Glaucoma, once thought as a single disease, is actually a group of diseases of the optic nerve involving loss of retinal ganglion cells. The process of cell death occurs in a characteristic pattern of optic neuropathy, a broad term for a certain pattern of damage to the optic nerve (the bundle of nerve fibers that carries information from the eye to the brain). Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness. Worldwide, it is estimated that about 66.8 million people have visual impairment as a result of glaucoma, with 6.7 million suffering from blindness.

This symposium aims at summarizing some of the scientific bases for current or planned clinical trials in patients with spinal cord injury (SCI). It stems from the interactions of four researchers involved in basic and clinical research who presented their work at a dedicated Symposium of the Society for Neuroscience in San Diego. After SCI, primary and secondary damage occurs and several endogenous processes are triggered that may foster or hinder axonal reconnection from supralesional structures. Studies in animals show that some of these processes can be enhanced or decreased by exogenous interventions using drugs to diminish repulsive barriers (anti-Nogo, anti-Rho) that prevent regeneration and/or sprouting of axons. Cell grafts are also envisaged to enhance beneficial immunological mechanisms (autologous macrophages, vaccines) or remyelinate axons (oligodendrocytes derived from stem cells). Some of these treatments could be planned concurrently with neurosurgical approaches that are themselves beneficial to decrease secondary damage (e.g., decompression/ reconstructive spinal surgery). Finally, rehabilitative approaches based on the presence of functional networks (i.e., central pattern generator) below the lesion combined with the above neurobiological approaches may produce significant functional recovery of some sensorimotor functions, such as locomotion, by ensuring an optimal function of endogenous spinal networks and establishing new dynamic interactions with supralesional structures. More work is needed on all fronts, but already the results offer great hope for functional recovery after SCI based on sound basic and clinical neuroscience research.

Neurogenesis - the formation of new neurons in the adult brain - is considered to be one of the mechanisms by which the brain maintains its lifelong plasticity in response to extrinsic and intrinsic changes. The mechanisms underlying the regulation of neurogenesis are largely unknown. Here, we show that Toll-like receptors (TLRs), a family of highly conserved pattern-recognizing receptors involved in neural system development in Drosophila and innate immune activity in mammals, regulate adult hippocampal neurogenesis. We show that TLR2 and TLR4 are found on adult neural stem/progenitor cells (NPCs) and have distinct and opposing functions in NPC proliferation and differentiation both in vitro and in vivo. TLR2 deficiency in mice impaired hippocampal neurogenesis, whereas the absence of TLR4 resulted in enhanced proliferation and neuronal differentiation. In vitro studies further indicated that TLR2 and TLR4 directly modulated self-renewal and the cell-fate decision of NPCs. The activation of TLRs on the NPCs was mediated via MyD88 and induced PKCα/β-dependent activation of the NF-κB signalling pathway. Thus, our study identified TLRs as players in adult neurogenesis and emphasizes their specified and diverse role in cell renewal.

Neural stem/progenitor cells are known to exist in the intact spinal cord, but the presence of newly formed neurons during adulthood has not been documented there to date. Here, we report the appearance of newly formed neurons under normal physiological conditions. These neurons are immature, express a GABAergic phenotype, and are primarily located in the dorsal part of the spinal cord. This localization appeared to be mediated by stromal-derived factor-1/CXC-chemokine receptor-4 signaling in the dorsal region. The extent of spinal cord neurogenesis was found to be greatly influenced by immune system integrity and in particular by myelin-specific T cells. These observations provide evidence for in vivo spinal cord neurogenesis under nonpathological conditions and introduce novel mechanisms regulating adult spinal cord plasticity.

We have recently shown that the ability of microglia to effectively fight off aggregated β-amyloid plaque formation and cognitive loss in transgenic mouse models of Alzheimer's disease (Tg-AD), is augmented in response to T-cell-based immunization, using glatiramer acetate (GA). The immunization increases incidence of local CD11c⁺ dendritic-like cells. It is unclear, however, whether these dendritic cells are derived from resident microglia or from the bone marrow. To determine the origin of this dendritic-cell population, we used chimeric mice whose bone marrow-derived cells express a transgene that allows the cells to be specifically ablated by diphtheria toxin. We show here that T-cell-based immunization of these mice, using GA, induced the recruitment of bone marrow-derived dendritic cells. Depletion of the dendritic cells by systemic injection of diphtheria toxin resulted in significantly increased formation of amyloid plaques. Thus, recruitment of bone marrow-derived dendritic cells evidently plays a role in reducing plaque formation in a mouse model of Alzheimer's disease.

Microglia are resident cells in the central nervous system (CNS), of hematopoietic origin with a high plasticity. In this study, we examined whether adaptive immune system, involving in CNS maintenance and repair, can induce microglia to express markers of neural cells. We show that long exposure (above 10 days) of microglia to low doses (10 ng/ml) of the 'proinflammatory' T-cell derived cytokine, IFN-γ, induced them to express neuronal markers including γ-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD-67). In contrast, exposure of microglia to low doses (10 ng/ml) of the 'anti-inflammatory' T-cell derived cytokine, IL-4, induced the expression of oligodendrocyte markers and dendritic cell (DC) marker, CD11c. The microglial origin of the neural-like cells was confirmed using microglia from transgenic mice expressing GFP under promoter of the chemokine fractalkine receptor CX₃CR1, and diphtheria toxin receptor, under CD11c promoter. This study emphasizes that microglial plasticity includes their ability to give rise to neural-like cells and shows that cytokines produced by the adaptive immune system are involved in these processes.

Glaucomatous neuropathy, like other neurodegenerative diseases, is a multi-dimensional disease in which various molecular and cellular factors contribute to the pathological process. These factors, while not initially causative, are key elements in disease progression and may continue to contribute even after the primary pathology is alleviated. An entire field of research of neuroprotection and restoration has opened up as part of the search for additional ways of slowing disease progression. We have proposed, on the basis of experimental evidence, that a vaccine could be a means of recruiting the immune system to help eliminate many of the factors associated with glaucomatous neurodegeneration and thus prevent disease progression, though not its onset. This immune defence involves lymphocytes, resident and infiltrating innate immune cells, the microglia, and macrophages. The antigens of choice are synthetic antigens, such as glatiramer acetate, that weakly cross-react with self-antigens in the retina and optic nerves. The vaccine induces a beneficial immune response that recruits immune effector cells to counteract or neutralize many of the compounds and factors that contribute to ongoing destruction, and in addition supports cell renewal and repair.

PURPOSE. A disaccharide (DS) derived from the naturally occurring compound chondroitin sulfate proteoglycan (CSPG) was recently shown to have neuroprotective activity. The authors examined the ability of this compound (CSPG-DS) to protect retinal ganglion cells (RGCs) from death caused by elevated intraocular pressure (IOP). METHODS. With the use of chronic and acute models of elevated IOP, the authors examined the effects of CSPG-DS on RGC survival in adult (∼2 months old), aged (10-12 months old), and immunocompromised Lewis rats. Systemic, topical, and oral routes of administration were examined. RESULTS. CSPG-DS protected RGCs from IOP-induced death. Treatment was effective in all three examined rat populations (normal adult, aged, and immunocompromised rats) and with all routes of administration, possibly in part through its control of microglial activity. CONCLUSIONS. Results point to the therapeutic potential of CSPG-DS for glaucoma, particularly in elderly populations for whom disease prevalence is high.

The ability of the central nervous system to cope with stressful conditions was shown to be dependent on proper T-cell-mediated immune response. Because the therapeutic window for neuroprotection after acute insults such as stroke is relatively narrow, we searched for a procedure that would allow the relevant T cells to be recruited rapidly. Permanent middle cerebral artery occlusion was induced in adult rats. To facilitate a rapid poststroke T cell activity, rats were treated with poly-YE using different regimens. Control and poly-YE-treated rats were assessed for functional recovery using neurological severity score and Morris water maze. Neuroprotection, neurogenesis, growth factor expression, and microglial phenotype were assessed using histological and immunofluorescence methods. Administration of poly-YE as late as 24 hours after middle cerebral artery occlusion yielded a beneficial effect manifested by better neurological performance, reduced neuronal loss, attenuation of behavioral deficits, and increased hippocampal and cortical neurogenesis. This compound affected the subacute phase by modulating microglial response and by increasing local production of insulin-like growth factor-I, known to be a key player in neuronal survival and neurogenesis. The relative wide therapeutic window, coupled with its efficacy in attenuating further degeneration and enhancing restoration, makes poly-YE a promising immune-based candidate for stroke therapy. (Stroke. 2007;38[part 2]:774-782.)

The ability to recover from CNS injuries is strain dependent. Transgenic mice that weakly express the p41 CD74 isoform (an integral membrane protein functioning as a MHC class II chaperone) on an I-A^b genetic background have normal CD4⁺ T cell populations and normal surface expression of MHC class II, but their B cell development is arrested while the cells are still immature. After a CNS injury, these mice recover better than their matched wild-type controls. We generated p41-transgenic mice on an I-A^d background (p41-I-A^d mice), and found that their recovery from CNS injuries was worse than that of controls. A correlative inverse effect was seen with respect to the kinetics of T cell and B cell recruitment to the injured CNS and the expression of insulin-like growth factor at the lesion site. These results, besides verifying previous findings that B cells function in the damaged CNS, demonstrate that the outcome of a particular genetic manipulation may be strain dependent.

A major causative factor in the paralysis that often follows an acute injury to the central nervous system (CNS) is the paradoxical inability of the CNS to tolerate its own mechanism of self-repair. The dismal result is often a wider spread of damage (part of the inevitable "secondary" or "delayed" degeneration) rather than contribution toward a cure. Ever since the phenomenon of posttraumatic damage spread in the CNS was first recognized, neuroscientists have attempted to identify the players in this destructive process and have sought ways to neutralize or bypass them with the object of rescuing any neurons that are still viable. This approach is collectively termed neuroprotection. In this chapter, we present a view of experimental paradigms used to study neuroprotection.

Recent studies of correlations of intensity in databases of natural images revealed a remarkable property. The two point correlations are described in terms of power law behavior, with an exponent which seems to be robust. In the present Letter we consider the statistical meaning of that result. We study many individual images of one of the databases considered. We find that the same law characterizing the correlations in the whole database governs also images randomly chosen from that database, with one essential difference. The exponent characterizing each image is specific and differs from the exponent characterizing the whole database. The distribution of single image exponents has been measured and found to exhibit a rather heavy tail. The database exponent cannot, thus, be considered as a statistical representative of a single image exponent. Possible reasons for the diversity in image exponents are discussed.

Neurogenesis is known to continuously take place in certain neurogenic areas of the adult central nervous system (CNS) and can be induced in non-neurogenic areas under traumatic or degenerative conditions. Here we introduce T cells and CNS-resident microglia as important players in the regulation of adult neurogenesis. Under normal conditions, immune deficient mice (SCID and nude) and transgenic mice that most of their T-cell pool is specific for an irrelevant antigen (ovalbumin) exhibited impaired hippocampal neurogenesis. In contrast, mice in which the majority of T cells specifically recognize the CNSabundant antigen myelin basic protein showed normal neurogenesis. CNSspecific T cells were also found to be important for spatial learning abilities and for brain-derived neurotrophic factor expression in the dentate gyrus. Environmental enrichment did not evoke enhanced neurogenesis in immune-deficient animals, whereas in wild-type animals it led to enhanced hippocampal neurogenesis coupled with recruitment of T cells and activation of microglia. The clinical implications of these findings were tested using a rat model of cerebral ischemic insult. We demonstrate that Tcell based immune activation following stroke induces a robust elevation of neurogenesis in the hippocampus as well as in the cerebral cortex. Our results suggest that T cells, acting via resident antigen presenting cells, are important regulators of adult neurogenesis under both physiological and pathological conditions.

The well regulated activities of microglia and T cells specific to central nervous system (CNS) antigens can contribute to the protection of CNS neural cells and their renewal from adult neural stem/progenitor cells (aNPCs). Here we report that T cell-based vaccination of mice with a myelin-derived peptide, when combined with transplantation of aNPCs into the cerebrospinal fluid (CSF), synergistically promoted functional recovery after spinal cord injury. The synergistic effect was correlated with modulation of the nature and intensity of the local T cell and microglial response, expression of brain-derived neurotrophic factor and noggin protein, and appearance of newly formed neurons from endogenous precursor-cell pools. These results substantiate the contention that the local immune response plays a crucial role in recruitment of aNPCs to the lesion site, and suggest that similar immunological manipulations might also serve as a therapeutic means for controlled migration of stem/progenitor cells to other acutely injured CNS sites.

Alzheimer's disease (AD) is characterized by plaque formation, neuronal loss, and cognitive decline. The functions of the local and systemic immune response in this disease are still controversial. Using AD double-transgenic (APP/PS1) mice, we show that a T cell-based vaccination with glatiramer acetate, given according to a specific regimen, resulted in decreased plaque formation and induction of neurogenesis. It also reduced cognitive decline, assessed by performance in a Morris water maze. The vaccination apparently exerted its effect by causing a phenotype switch in brain microglia to dendritic-like (CD11c) cells producing insulin-like growth factor 1. In vitro findings showed that microglia activated by aggregated β-amyloid, and characterized as CD11b⁺/CD11c^-/ MHC class II^-/TMF- α⁺ cells, impeded neurogenesis from adult neural stem/progenitor cells, whereas CD11b⁺/CD11c⁺/MHC class II⁺/TNF-α^- microglia, a phenotype induced by IL-4, counteracted the adverse β-amyloid-induced effect. These results suggest that dendritic-like microglia, by facilitating the necessary adjustment, might contribute significantly to the brain's resistance to AD and argue against the use of antiinflammatory drugs.

In recent years there has been a growing interest in the statistical properties of surfaces growing under deposition of material. Yet it is clear that a theory describing the evolution of a surface should at the same time describe the properties of the bulk buried underneath. Clearly, the structure of the bulk is relevant for many practical purposes, such as the transport of electric current in devices, transport of fluids in geological formations and stress transmission in granular systems. The present paper demonstrates explicitly how models describing deposition can provide us with information on the structure of the bulk. Comparison of an analytic model with a simulation of a discrete growth model reveals an interesting long-range tail in the density-density correlation in the direction of deposition.

The ability to cope with ongoing neurodegeneration after injury to the central nervous system of mammals differs among strains and depends in part on the animal's ability to manifest a T-cell-mediated protective response. After CNS injury, strain-related differences were observed. Moreover, the post-injury effect of naturally occurring regulatory CD4+CD25+ T cells was found to differ in different strains. In this study, using partially injured optic nerves of Balb/c/OLA and C57BL/6J mice as models, we observed strain-related differences in the T-cell-mediated protection obtained by antigens administered via the nasal route. Active immunization with myelin-related antigens emulsified in complete Freund's adjuvant had a beneficial effect on both strains, whereas mucosal administration of the same antigens was destructive in mice of the Balb/c/OLA strain but protective in C57BL/6J mice.

Peripheral cellular immunity was recently shown to play a critical role in brain plasticity and performance. The antigenic specificity of the participating T cells, however, was not investigated, and nor was their relevance to psychological stress. Here we show, using a mouse model, that adaptive immunity mitigates maladaptation to the acute psychological stress known to trigger abnormal behaviors reminiscent of human post-traumatic stress disorder. Assessment of behavioral adaptation (measured by the acoustic startle response and avoidance behavior) in mice after their exposure to predator odor revealed that maladaptation was several times more prevalent in T cell-deficient mice than in their wild-type counterparts. A single population of T cells reactive to central nervous system (CNS)-associated self-protein was sufficient to endow immune-deficient mice with the ability to withstand the psychological stress. Naturally occurring CD4+CD25+ regulatory T cells were found to suppress this endogenous anti-stress attribute. These findings suggest that T cells specific to abundantly expressed CNS antigens are responsible for brain tissue homeostasis and help the individual to cope with stressful life episodes. They might also point the way to development of immune-based therapies for mental disorders, based either on up-regulation of T cells that partially cross-react with selfantigens or on weakening of the activity of regulatory T cells.

The role of activated microglia (MG) in demyelinating neurodegenerative diseases such as multiple sclerosis is controversial. Here we show that high, but not low, levels of IFN-γ (a cytokine associated with inflammatory autoimmune diseases) conferred on rodent MG a phenotype that impeded oligodendrogenesis from adult neural stem/progenitor cells. IL-4 reversed the impediment, attenuated TNF-α production, and overcame blockage of IGF-I production caused by IFN-γ. In rodents with acute or chronic EAE, injection of IL-4-activated MG into the cerebrospinal fluid resulted in increased oligodendrogenesis in the spinal cord and improved clinical symptoms. The newly formed oligodendrocytes were spatially associated with MG expressing MHC class II proteins and IGF-I. These results point to what we believe to be a novel role for MG in oligodendrogenesis from the endogenous stem cell pool.

Chondroitin sulfate proteoglycan (CSPG), a matrix protein that occurs naturally in the central nervous system (CNS), is considered to be a major inhibitor of axonal regeneration and is known to participate in activation of the inflammatory response. The degradation of CSPG by a specific enzyme, chondroitinase ABC, promotes repair. We postulated that a disaccharidic degradation product of this glycoprotein (CSPG-DS), generated following such degradation, participates in the modulation of the inflammatory responses and can, therefore, promote recovery in immune-induced neuropathologies of the CNS, such as experimental autoimmune encephalomyelitis (EAE) and experimental autoimmune uveitis (EAU). In these pathologies, the dramatic increase in T cells infiltrating the CNS is far in excess of the numbers needed for regular maintenance. Here, we show that CSPG-DS markedly alleviated the clinical symptoms of EAE and protected against the neuronal loss in EAU. The last effect was associated with a reduction in the numbers of infiltrating T cells and marked microglia activation. This is further supported by our in vitro results indicating that CSPG-DS attenuated T cell motility and decreased secretion of the cytokines interferon-gamma and tumor necrosis factor-alpha. Mechanistically, these effects are associated with an increase in SOCS-3 levels and a decrease in NF-kappa B. Our results point to a potential therapeutic modality, in which a compound derived from an endogenous CNS-resident molecule, known for its destructive role in CNS recovery, might be helpful in overcoming inflammation-induced neurodegenerative conditions.

Accumulating evidence suggests that neurodegenerative diseases of the central nervous system (CNS) are associated with a local inflammatory response. CNS autoimmune diseases are also associated with inflammation. Does this mean that all neurodegenerative diseases are autoimmune in nature? Does it imply that autoimmune and neurodegenerative diseases are both eligible for the same therapy? What distinguishes between the two types of disease? Do they differ both in etiology and in pathology, or do they have different etiologies but similar pathology and progression? In this minireview we offer a new view of the inflammatory differences between neurodegenerative and autoimmune diseases in the CNS and discuss the implications for therapy.

Spinal cord injury is a devastating condition of the central nervous system (CNS), often resulting in severe loss of tissue, functional impairment, and only limited repair. Studies over the last few years have shown that response to the insult and spontaneous attempts at repair are multiphasic processes, with varying and sometimes conflicting requirements. This knowledge has led to novel strategies of therapeutic intervention. Our view is that a pivotal role in repair, maintenance, healing, and cell renewal in the CNS, as in other tissues, is played by the immune system. The mode and timing of intervention must be carefully selected, however, as the capacity of the CNS to tolerate local repair mechanisms is limited. Studies have shown that the spontaneously evoked early innate response to CNS injury is characterized by invasion of neutrophils and is unfavorable for cell survival. This is followed by a response of the resident innate immune cells (microglia), which however cannot supply all the needs of the damaged tissue; moreover, once evoked, and for as long as the damage persists, the microglial response remains beyond the capacity of the CNS to tolerate it. Immune-based clinical intervention is most effective in improving functional and morphological recovery when delayed for a certain period. Effective intervention might be in the form of (1) local injection of "alternatively activated" macrophages, (2) systemic injection of dendritic cells specific to CNS antigens, or (3) T-cell-based vaccination. The treatment of choice depends on the severity of the insult, the site of injury, the therapeutic window, and safety considerations.

T-cell-mediated autoimmunity participates in physiological defense, maintenance and repair of the adult brain. However, unless such autoimmune responses to insults are rigorously controlled, they might lead to an autoimmune disease or other immune-related defects, where destructive activity outweighs the beneficial effect. Here, we discuss these apparently contradictory effects of autoimmunity in schizophrenic patients, whose typical immune aberrations have prompted recent speculation about an autoimmune-related etiology. We found that, although schizophrenic patients have active immune systems, they often lack autoimmune clones specifically reactive to a major myelin protein, myelin basic protein (MBP). This, in conjunction with our discovery in rodents that T cells that recognize brain-resident proteins are needed for normal cognitive functioning, led us to propose an immune-based neurodevelopmental hypothesis, in which autoimmune-T-cell deficiency is suggested to cause onset or progression of schizophrenia.

Neurogenesis is known to take place in the adult brain. This work identifies T lymphocytes and microglia as being important to the maintenance of hippocampal neurogenesis and spatial learning abilities in adulthood. Hippocampal neurogenesis induced by an enriched environment was associated with the recruitment of T cells and the activation of microglia. In immune-deficient mice, hippocampal neurogenesis was markedly impaired and could not be enhanced by environmental enrichment, but was restored and boosted by T cells recognizing a specific CNS antigen. CNS-specific T cells were also found to be required for spatial learning and memory and for the expression of brain-derived neurotrophic factor in the dentate gyrus, implying that a common immune-associated mechanism underlies different aspects of hippocampal plasticity and cell renewal in the adult brain.

Microglia, the standby cells for immune defense in the CNS, have a reputation for exacerbating the neural damage that occurs in neurodegenerative diseases. However, research over the past few years has established that microglia do not constitute a single, uniform cell population, but rather comprise a family of cells with diverse phenotypes - some that are beneficial and others that the CNS can barely tolerate and that are therefore destructive. This finding raised several questions. What instructs microglia to acquire a particular phenotype, and how do these phenotypes differ? How committed are microglia to a specific phenotype? Can destructive microglia become protective, and can protective microglia retain their beneficial phenotype even when they encounter a destructive environment? Here, we address these questions, and the background of research that elicited them.

Cell renewal in the adult central nervous system (CNS) is limited, and is blocked in inflammatory brain conditions. We show that both neurogenesis and oligodendrogenesis of adult neural progenitor cells in mice are blocked by inflammation-associated (endotoxin-activated) microglia, but induced by microglia activated by cytokines (IL-4 or low level of IFN-γ) associated with T-helper cells. Blockage was correlated with up-regulation of microglial production of tumor necrosis factor-α. The effect induced by IL-4-activated microglia was mediated, at least in part, by insulin-like growth factor-I. The IL-4-activated microglia showed a bias towards oligodendrogenesis whereas the IFN-γ-activated microglia showed a bias towards neurogenesis. It thus appears that microglial phenotype critically affects their ability to support or impair cell renewal from adult stem cell.

Acute or chronic glaucoma is often associated with an increase in intraocular pressure (IOP). In many patients, however, therapeutic pressure reduction does not halt disease progression. Neuroprotection has been proposed as a complementary therapeutic approach. We previously demonstrated effective T-cell-based neuroprotection in experimental animals vaccinated with the synthetic copolymer glatiramer acetate (copolymer-1, Cop-1), a weak agonist of self-antigens. This study was undertaken to test different routes and modes of vaccination with Cop-1 as treatment modalities for protection against retinal ganglion cell (RGC) death caused by chronic elevation of IOP in rats, and to determine whether anatomical neuroprotection is accompanied by functional neuroprotection. In a chronic model of unilaterally high IOP, Cop-1 vaccination, with or without an adjuvant, protected rats against IOP-induced loss of RGCs by eliciting a systemic T-cell-mediated response capable of cross-reacting with self-antigens residing in the eye. In rats deprived of T cells, Cop-1 (unlike treatment with α ₂-adrenoreceptor agonists) was not protective of RGCs, substantiating the contention that its beneficial effect is not conferred directly but is T-cell-mediated. Pattern electroretinography provided evidence of functional protection. Thus, vaccination with adjuvant-free Cop-1 can protect RGCs from the consequences of elevated IOP in rats. This protection is manifested both morphologically and functionally. These findings can be readily implemented for the development of a therapeutic vaccination to arrest the progression of glaucoma.

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (±s.d.) IOP to increase significantly from 18.3±4.5 (baseline) to 29.8±8.8 mmHg within 3 weeks (p

Nerve agents are highly toxic organophosphates (OPs) that can cause severe damage to the central and peripheral nervous systems. The central nervous system insult results in seizures and neuronal death. The glutamatergic system apparently contributes to the neuropathology. Using a model of OP intoxication causing death of retinal ganglion cells in the mouse eye, we show here that intoxication is exacerbated if the mice are devoid of mature T cells. The retinal neurons could be protected from these effects by vaccination, 7 days before or immediately after intoxication, with the copolymer glatiramer acetate (Cop-1), recently found to limit the usual consequences of an acute glutamate insult to the eye. These findings underlie a new therapeutic approach to protection against OP intoxication, based on the rationale that boosting of the adaptive immunity recruited at the site of intoxication helps the local cellular machinery such as resident microglia to withstand the neurotoxic effects.

The past decade has seen growing acceptance that glaucoma should be viewed as a slowly progressive neurodegenerative disease. According to this view, in glaucoma (as in other such diseases), whatever the primary risk factors, at any given time some neurons are still healthy but are threatened with destruction owing to the toxicity emanating from the degenerating neurons. It follows that any intervention that protects surviving neurons and rescues the marginally damaged ones should slow down progression of the disease. This novel view of glaucoma prompted scientists to compare glaucoma with other neurodegenerative diseases with respect to mediators of disease progression and ways in which the spread of damage, or 'secondary degeneration', can be attenuated. Studies of partial crush injury of the rat optic nerve, a model of secondary degeneration established in our laboratory, led us to conceptualize the 'enemy within' as a flood of neurotoxic self-compounds issuing from the degenerating nerve. With this model, pharmacological and molecular approaches were employed to identify and test potentially therapeutic neuroprotective compounds and methodologies, leading us ultimately to the serendipitous discovery of protective autoimmunity as the body's defense against destructive self-compounds. Mediators of self-perpetuating acute and chronic degeneration identified in the injured optic nerve were also detected in other sites of central nervous system (CNS) damage. This finding led scientists to screen drugs that had proven to be beneficial in other disease models for their use in glaucoma therapy. It also opened the way to studies of the direct effects of these toxic mediators on retinal ganglion cell survival and ways to prevent the degenerative outcome. Although no single model can fully simulate human glaucoma or any other neurodegenerative disease, the availability of different models of optic nerve damage and the similarity of findings in the optic nerve and in other parts of the CNS have led to significant progress toward development of a cure for glaucoma.

'Protective autoimmunity' refers to a well-controlled anti-self response that helps the body resist neurodegeneration. The response is mediated by autoimmune T cells, which produce cytokines and growth factors. Using an in vitro assay of hippocampal slices, we show that the cytokines interferon-γ and (especially) interleukin-4, characteristic of pro-inflammatory and anti-inflammatory T cells, respectively, can make microglia neuroprotective. Aggregated β-amyloid, like bacterial cell wall-derived lipopolysaccharide, rendered the microglia cytotoxic. Cytotoxicity was correlated with a signal transduction pathway that down-regulates expression of class-II major histocompatibility proteins (MHC-II) through the MHC-II-transactivator and the invariant chain. Protection by interleukin-4 was attributed to down-regulation of tumor necrosis factor-α and up-regulation of insulin-like growth factor I. These findings suggest that beneficial or harmful expression of the local immune response in the damaged CNS depends on how microglia interpret the threat, and that a well-regulated T-cell-mediated response enables microglia to alleviate rather than exacerbate stressful situations in the CNS.

Autoimmune diseases are traditionally viewed as an outcome of a malfunctioning of the immune system, in which an individual's immune system reacts against the body's own proteins. In multiple sclerosis (MS), a disease of the white matter of the central nervous system (CNS), the attack is directed against myelin proteins. In this article we summarize a paradigm shift proposed by us in the perception of autoimmune disease. Observations by our group indicating that an autoimmune response is the body's mechanism for coping with CNS damage led us to suggest that all individuals are apparently endowed with a purposeful autoimmune response to CNS injuries, but have only limited inherent ability to control this response so that its effect will be beneficial. In animals susceptible to autoimmune diseases, the same autoimmune T cells are responsible both for neuroprotection and for disease development; the timing and strength of their activity will determine which of these effects is expressed. Individuals with non-inflammatory neurodegenerative diseases need a heightened autoimmunity. We discovered that autoimmunity could be boosted without risk of disease induction, even in susceptible strains, by the use of Copolymer-1 (Copaxone (R)), a weak agonist of a wide range of self-reactive T cells. Here we summarize the basic findings that led us to formulate the concept of protective autoimmunity, the mechanisms underlying its constitutive presence and its on/off regulation, and its therapeutic implications. We also offer an explanation for the commonly observed presence of cells and antibodies directed against self-components in healthy individuals. (c) 2005 Elsevier B.V. All rights reserved.

The trigger that leads to the pathogenesis of type 1 diabetes is currently unknown. It is well established that the pathophysiology of the disease is biphasic. In the first stage, leukocytes infiltrate the pancreatic islets in a response that does not cause damage. In the second phase, which occurs only in diabetes-prone individuals and strains, autoreactive T cells acquire aggressive potential and destroy the majority of the pancreatic islets. Rodents and humans exhibit a physiological ripple of apoptotic β-cell death shortly after birth, which induces an adaptive autoimmune response towards islet-antigens, both in diabetes-prone non-obese diabetic (NOD) mice and in mice that do not develop diabetes. Here, we propose that the early T cell-mediated autoimmune response towards islet-antigens is physiological, purposeful and beneficial.

Glutamate in excessive amounts is a major contributor to neuronal degeneration, and its removal is attributed mainly to astrocytes. Traumatic injury to the central nervous system (CNS) is often accompanied by disappearance of astrocytes from the lesion site and failure of the remaining cells to withstand the ensuing toxicity. Microglia that repopulate the lesion site are the usual suspects for causing redox imbalance and inflammation and thus further exacerbating the neurotoxicity. However, our group recently demonstrated that early post-injury activation of microglia as antigen-presenting cells correlates with an ability to withstand injurious conditions. Moreover, we found that T cells reactive to CNS-specific self-antigens protected neurons against glutamate toxicity. Here, we show that antigen-specific autoimmune T cells, by tailoring the microglial phenotype, can increase the ability of microglia-enriched cultures to remove glutamate. This T-cell-mediated effect could not be achieved by the potent microglia-activating agent lipopolysaccharide (LPS), but was dose-dependently reproduced by the Th1 cytokine interferon (IFN)-γ and significantly reduced by neutralizing anti-IFN-γ antibodies. Under the same conditions, IFN-γ had no effect on cultured astrocytes. Up-regulation of glutamate uptake induced by IFN-γ activation was not accompanied by the acute inflammatory response seen in LPS-activated cultures. These findings suggest that T cells or their cytokines can cause microglia to adopt a phenotype that facilitates rather than impairs glutamate clearance, possibly contributing to restoration of homeostasis.

A well-controlled local immune response is needed for recovery from CNS injury, and autoimmune T cells play a key role in mediating this response. Autoimmunity, at least in the context of the CNS, is the body's righting force against self-derived destructive compounds, and "tolerance to self" (traditionally though mistakenly equated with nonresponsiveness) should be viewed as the ability to tolerate autoimmune responses without developing an autoinumme disease. This physiological repair mechanism is controlled by naturally occurring CD4(+)CD25(+) regulatory T cells (Treg cells), with an on/off switch regulated by brain-derived compounds. We suggest that ongoing neurodegeneration after acute injuries results from insufficiency of the endogenous fighting force, and chronic neurodegenerative diseases reflect age-related deterioration of the body's two principal regulators: the central nervous system (CNS) and the immune system. If so, then the restoration or boosting of immune function might bridge the gap between such manifestations of CNS-related risk factors and the defensive capacity of the immune system. Boosting of peripheral immunity by the use of weak agonists of CNS self-antigens, might offer a promising potential therapy for different neurodegenerative conditions. This could be accomplished either by vaccination with a universal weak T cell-reactive antigen or with altered peptide ligands of self-antigens, or by weakening the suppressive effect of autoimmunity (e.g., by eliminating Treg cells). T cells that home to the damaged CNS might then help restore local homeostasis by controlling the local microglial response. Because neurodegenerative diseases possess common features (deriving from the local chaos), the same vaccine might protect against several different disorders associated with impaired motor, sensory, cognitive, or mental functions of the brain.

The failure of the spinal cord to recover after injury has been associated with the immune privilege mechanism that suppresses immune activity throughout the central nervous system. Primed macrophages and dendritic cells were shown to promote neurological recovery in preclinical models of spinal cord injury. A cell therapy consisting of autologous incubated macrophages is now being tested on spinal cord injury patients in clinical trials.

T-cells directed to self-antigens ("autoimmune" T-cells) have traditionally been perceived as tending to attack the body's own tissues, and likely to exert their destructive effects unless they undergo deletion in the thymus during ontogeny. Naturally occurring CD4+CD25+ regulatory T-cells were viewed as thymus-derived cells that constitutively suppress any autoimmune T-cells that escaped thymic deletion. Studies in recent years suggest, however, that some autoimmune T-cells are necessary, at least in the central nervous system for neural maintenance and repair, possibly in part by rendering the resident microglia capable of fighting off adverse conditions, as well as for neural maintenance and repair. In line with this notion, the regulatory T-cells are thought to allow autoimmunity to exist in healthy individuals without causing an autoimmune disease. This proposed immune scenario and its implications for therapy are discussed.

OBJECT: A Phase I, open-label nonrandomized study was conducted to assess the safety and tolerability of incubated autologous macrophages administered to patients with acute complete spinal cord injury (SCI). METHODS: This therapy was first tested in rat models of spinal cord transection and contusion, in which it was shown to promote motor recovery. The procedure developed for clinical use consists of isolating monocytes from patient blood and incubating them ex vivo with autologous dermis. The resulting incubated autologous macrophages were injected into the patient's spinal cord immediately caudal to the lesion within 14 days of injury. Patients underwent preoperative and follow-up neurological assessment (American Spinal Injury Association [ASIA] standards), electrophysiological monitoring (motor evoked and/or somatosensory evoked potentials), magnetic resonance imaging, and safety monitoring. Before macrophage administration, complete neurological functional loss (ASIA Grade A) was confirmed in all patients. Of the eight patients in the study, three recovered clinically significant neurological motor and sensory function (ASIA Grade C status). During the study period, some adverse events were encountered, the most serious of which involved two cases of pulmonary embolism and one case of osteomyelitis that were treated and resolved without further complication. These and other adverse events appear to be similar to those encountered in other spinal cord-injured patients and are not considered a consequence of the experimental therapy. CONCLUSIONS: It is concluded that incubated autologous macrophage cell therapy is well tolerated in patients with acute SCI. Further clinical evaluation is warranted.

Under acute or chronic neurodegenerative conditions in the central nervous system (CNS), where surviving neurons are mortally threatened by progressive degeneration mediated by a toxic excess of normally beneficial self-compounds, neuroprotection is an important therapeutic target. Pharmacologically, this is attainable by increasing the neuronal resistance or disabling the toxicity mediators. Immunological neuroprotection, based on our discovery that the systemic immune system helps the CNS withstand degenerative conditions, requires well-controlled boosting of the peripheral T-cell response to injury, directed against self-antigens residing in the damaged site. The latter provides global protection against multiple threatening compounds.

Neurodegenerative diseases differ in etiology but are propagated similarly. We show that neuronal loss caused by intraocular injection of aggregated β-amyloid was significantly greater in immunodeficient mice than in normal mice. The neurodegeneration was attenuated or augmented by elimination or addition, respectively, of naturally occurring CD4⁺CD25⁺ regulatory T cells (Treg). Vaccination with retina-derived antigens or with the synthetic copolymer glatiramer acetate (Copolymer-1, Cop-1), but not with β-amyloid, reduced the ocular neuronal loss. In mouse hippocampal slices, microglia encountering activated T cells overcame the cytotoxicity of aggregated β-amyloid. These findings support the concept of "protective autoimmunity", show that a given T cell-based vaccination is protective at a particular site irrespective of toxicity type, and suggest that locally activated T cells induce a microglial phenotype that helps neurons withstand the insult. Alzheimer's and other neurodegenerative diseases might be arrested or retarded by vaccination with Cop-1 or related compounds or by treatment with compounds that weaken Treg suppression.

Autoimmune CD4⁺ T cells can mediate the ability to withstand neurodegenerative conditions. Here we show that the ability to spontaneously manifest a T cell-dependent protective response is restricted by naturally occurring CD4⁺CD25⁺ regulatory T cells (Treg); depletion of Treg was beneficial in two mouse strains (C57BL/6J and BALB/c/OLA) differing in their spontaneous T cell-dependent ability to withstand the consequences of optic nerve injury. Passive transfer of exogenous Treg was destructive in BALB/c/OLA mice (which can spontaneously manifest a T cell-dependent protective anti-self response to injury) but beneficial in C57BL/6J mice (which have only limited ability to manifest such a response). This dichotomy was resolved by the finding that, in severe combined immunodeficient mice, a beneficial effect is obtained by passive transfer of either Treg-free CD4⁺ T cells (Teff) or Treg alone, indicating that neuroprotection can be achieved by either Treg or Teff in the absence of the other. We attribute these disparate effects of Treg to their differential interaction (in part via IL-10 and transforming growth factor β) with local innate immune cells (microglia) in the presence and in the absence of effector T cells. Activation of microglia by pro- and antiinflammatory cytokines in suitably controlled amounts might trigger different signal transduction pathways, each of which induces a neuroprotective microglial phenotype. These results suggest that, under neurodegenerative conditions, the effects of Treg, and possibly also of other regulatory T cells, might not be uniform, and that their expression in different individuals might be genetically determined. Therefore, therapeutic intervention based on induction of regulatory T cells might have limitations.

Chondroitin sulphate proteoglycan (CSPG) inhibits axonal regeneration in the central nervous system (CNS) and its local degradation promotes repair. We postulated that the enzymatic degradation of CSPG generates reparative products. Here we show that an enzymatic degradation product of CSPG, a specific disaccharide (CSPG-DS), promoted CNS recovery by modulating both neuronal and microglial behaviour. In neurons, acting via a mechanism that involves the PKCα and PYK2 intracellular signalling pathways, CSPG-DS induced neurite outgrowth and protected against neuronal toxicity and axonal collapse in vitro. In microglia, via a mechanism that involves ERK1/2 and PYK2, CSPG-DS evoked a response that allowed these cells to manifest a neuroprotective phenotype ex vivo. In vivo, systemically or locally injected CSPG-DS protected neurons in mice subjected to glutamate or aggregated β-amyloid intoxication: Our results suggest that treatment with CSPG-DS might provide a way to promote post-traumatic recovery, via multiple cellular targets.

Autoimmune T cells have been viewed for decades as an outcome of immune system malfunction, and specifically as a failure to distinguish between components of self and non-self. The need for discrimination between self and non-self as a way to avoid autoimmunity has been repeatedly debated over the years. Recent studies suggest that autoimmunity, at least in the nervous system, is the body's defense mechanism against deviations from the normal. The ability to harness neuroprotective autoimmunity upon need is evidently allowed by naturally occurring CD4+CD25+ regulatory T cells, which are themselves controlled by brain-derived compounds. These findings challenge widely accepted concepts of the need for discrimination between self and non-self, as they suggest that while such discrimination is indeed required, it is needed not as a way to avoid an anti-self response but to ensure its proper regulation. Whereas a response to non-self can be self-limited by a decreased presence of the relevant antigen, the response to self needs a mechanism for strict control, such as that provided by the naturally occurring regulatory T cells.

Neurodegenerative conditions share common primary risk factors and mediators of disease progression. Because many degenerative disorders are age related, deteriorating immunity in aging patients might impose additional risk. Adaptive (T-cell-mediated) immunity is a defense mechanism that instructs microglia to fight off and clear away self-derived enemies. Such adaptive immunity can be boosted, without risking the development of autoimmune disease, by injecting weak agonists of self-antigens or by weakening the suppressive CD4⁺CD25⁺ regulatory T cells. If widely cross-reactive, the agonist might effectively counteract a variety of neurodegenerative disorders. Boosting of relevant T cells by vaccination could thus 'recharge' a deteriorating immune system that has to contend with an increasing number of risk factors.

Fighting off neuronal degeneration requires a well controlled T-cell response against self-antigens residing in sites of the CNS damage. The ability to evoke this response is normally suppressed by naturally occurring CD4 ⁺CD25⁺ regulatory T-cells (Treg). No physiological compound that controls Treg activity has yet been identified. Here, we show that dopamine, acting via type 1 dopamine receptors (found here to be preferentially expressed by Treg), reduces the suppressive activity and the adhesive and migratory abilities of Treg. Treg activity was correlated with activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) signaling pathway. Systemic injection of dopamine or an agonist of its type 1 receptors significantly enhanced, via a T-cell-dependent mechanism, protection against neuronal death after CNS mechanical and biochemical injury. These findings shed light on the physiological mechanisms controlling Treg and might open the way to novel therapeutic strategies for downregulating Treg activity (e.g., in neuronal degeneration) or for strengthening it (in autoimmune diseases).

Immune system activity has traditionally been considered harmful for recovery after spinal cord injury (SCI). Recent evidence suggests, however, that immune activity - and specifically autoimmune activity - is evoked by the insult, is beneficial if properly regulated and is amenable to boosting. Thus, for example, vaccination with an altered peptide ligand derived from myelin basic protein reduces the progressive degeneration of neurons that escaped the initial insult, thereby promoting recovery after SCI. As the steroid drug methylprednisolone (MP) is currently the only treatment available for patients with SCI, our purpose in the present study was to examine the mutual compatibility of the two treatments within the post-traumatic therapeutic window. We show, using rats of two different strains, that if MP is injected concomitantly with the therapeutic vaccination, the beneficial effect of the vaccination is diminished. However, if MP is given immediately after the insult and the vaccination 48 h later, MP does not detract from the beneficial effect of the vaccination. These results demonstrate that the therapeutic window after SCI can accommodate immediate administration of MP plus a delayed therapeutic vaccination.

The effects of the adaptive immune system on the cognitive performance and abnormal behaviors seen in mental disorders such as schizophrenia have never been documented. Here, we show that mice deprived of mature T cells manifested cognitive deficits and behavioral abnormalities, which were remediable by T cell restoration. T cell-based vaccination, using glatiramer acetate (copolymer-1, a weak agonist of numerous self-reactive T cells), can overcome the behavioral and cognitive abnormalities that accompany neurotransmitter imbalance induced by (+)dizocilpine maleate (MK-801) or amphetamine. The results, by suggesting that peripheral T cell deficit can lead to cognitive and behavioral impairment, highlight the importance of properly functioning adaptive immunity in the maintenance of mental activity and in coping with conditions leading to cognitive deficits. These findings point to critical factors likely to contribute to age- and AIDS-related dementias and might herald the development of a therapeutic vaccination for fighting off cognitive dysfunction and psychiatric conditions.

Self-tolerance, or the ability of the immune system to refrain from destroying the organism's own tissues, is a prerequisite for proper immune system operation. How such self-tolerance is achieved is still a subject of debate. The belief that autoimmunity poses a continuous threat to the organism was challenged by data demonstrating that autoimmunity has a protective function after traumatic injury to the central nervous system. This finding led us to suggest the 'comprehensive immunity' approach by which autoimmunity is viewed as a special case of immunity, namely as a defense mechanism that operates by fighting against the threat of potential destructive activity originated or mediated within the organism, similarly to the immune defense that operates against the threat from exogenous pathogens. We present a primary mathematical spatio-temporal model that supports this concept. The numerical solutions of this model illustrate the beneficial operation of a well-controlled immune response specific to self-antigens residing in the site of lesion. The model also explains how the response to self might be tolerated on a day-to-day basis. In addition, we demonstrate that the same autoimmune response, operating at different levels of regulation, can lead to either an autoimmune disease or a degenerative disorder. This preliminary qualitative model supports our contention that the way autoimmunity is perceived should be revised.

Protective autoimmunity was only recently recognized as a mechanism for attenuating the progression of neurodegeneration. Using a rat model of optic nerve crush or contusive spinal cord injury, and a mouse model of neurodegenerative conditions caused by injection of a toxic dose of intraocular glutamate, we show that a single low dose of whole-body or lymphoid-organ γ-irradiation significantly improved the spontaneous recovery. Animals with severe immune deficiency or deprived of mature T cells were unable to benefit from this treatment, suggesting that the irradiation-induced neuroprotection is immune mediated. This suggestion received further support from the findings that irradiation was accompanied by an increased incidence of activated T cells in the lymphoid organs and peripheral blood and an increase in mRNA encoding for the pro-inflammatory cytokines interleukin-12 and interferon-γ, and that after irradiation, passive transfer of a subpopulation of suppressive T cells (naturally occurring regulatory CD4 ⁺CD25⁺ T cells) wiped out the irradiation-induced protection. These results suggest that homeostasis-driven proliferation of T cells, induced by a single low-dose irradiation, leads to boosting of T cell-mediated neuroprotection and can be utilized clinically to fight off neurodegeneration and the threat of other diseases in which defense against toxic self-compounds is needed.

In neurodegenerative disorders, as well as in acute central nervous system (CNS) injuries, the initial impairment triggers a cascade of destructive events, collectively termed secondary degeneration, which eventually cause much more extensive damage. To investigate the process of secondary degeneration and ways to prevent it, we designed a well-calibrated model of optic nerve crush injury. Until recently, the main purpose of the immune system was thought to be protection of the body against alien pathogens. Since mechanical or biochemical insults do not involve exogenous pathogens, recruitment of the adaptive immune system was not considered relevant in such cases. We recently demonstrated, however, that a T-cell-mediated immune response directed against self-antigens residing in the site of damage can be beneficial for the injured optic nerve or spinal cord. This protective autoimmune response was found to be spontaneously evoked in some individuals, but not strongly enough to significantly affect recovery. Our aim was to boost this protective response in those individuals capable of spontaneously manifesting it, and to induce it in those incapable of manifesting it spontaneously. Optimal functional recovery requires the application of a proper combination of neuroprotection and neuroregeneration.

Glaucoma,is a neurodegenerative disease of the optic nerve, which continues to progress even if the primary cause of degeneration is identified and alleviated. At any given time the affected optic nerve contains fibers that are amenable to neuroprotection, and will escape degeneration provided that the proper pharmacological or other intervention is applied. Autoimmunity has long been viewed as. a deleterious phenomenon that should be terminated or at least minimized in order to preserve health. We recently demonstrated, however, that T cells specific to self-proteins residing in the site of CNS insult can be protective. With the aim of boosting autoimmunity for neuroprotection without risking the induction of an autoimmune disease, we developed the use of Cop-1 (an FDA-approved drug for the treatment of multiple sclerosis) as an active vaccination for neuroprotection. Cop-1 is a synthetic polymer that weakly cross-reacts with a wide range of self-reacting T cells. Vaccination with Cop-1 resulted in significant neuroprotection in rat models of optic nerve crush and chronic glaucoma. Thus,boosting of a T cell-based mechanism, which we have termed 'protective autoimmunity', promotes recovery of the damaged optic nerve. Current studies in our laboratory are aimed at translating this treatment into a clinical therapy. (C) 2003 Elsevier Inc. All rights reserved.

After an injury to the central nervous system (CNS), activated microglia have been shown to contribute to the ongoing destructive processes leading to secondary neuronal degeneration. They can, however, also express neuroprotective activity. Studies from our laboratory point to the existence of a physiological T cell-mediated neuroprotective mechanism (adaptive immunity) that is amenable to boosting. We postulate that the beneficial or destructive outcome of the local microglial (innate) response is determined by a well-controlled dialog between the innate and the adaptive immune players. Here, we show that spontaneous or exogenously boosted T cell-mediated neuroprotection is correlated with early activation of microglia as antigen-presenting cells. We suggest that such microglial activity, if well controlled, is a crucial step in determining the fate of the neurons in a hostile environment.

Keywords: SPINAL-CORD-INJURY; CENTRAL-NERVOUS-SYSTEM; AMYOTROPHIC-LATERAL-SCLEROSIS; MYELIN BASIC-PROTEIN; MULTIPLE-SCLEROSIS; T-CELLS; IN-VITRO; GLATIRAMER ACETATE; OPTIC NEUROPATHIES; PARKINSONS-DISEASE

Injury-induced self-destructive processes cause significant functional loss after incomplete spinal cord injury (SCI). Cellular elements of both the innate (macrophage) and the adaptive (T-cell) immune response can, if properly activated and controlled, promote post-traumatic regrowth and protection after SCI. Dendritic cells (DCs) trigger activation of effector and regulatory T-cells, providing a link between the functions of the innate and the adaptive immune systems. They also initiate and control the body's response to pathogenic agents and regulate immune responses to both foreign and self-antigens. Here we show that post-injury injection of bone marrow-derived DCs pulsed with encephalitogenic or nonencephalitogenic peptides derived from myelin basic protein, when administered (either systemically or locally by injection into the lesion site) up to 12 d after the injury, led to significant and pronounced recovery from severe incomplete SCI. No significant protection was seen in DC recipients deprived of mature T-cells. Flow cytometry, RT-PCR, and proliferation assays indicated that the DCs prepared and used here were mature and immunogenic. Taken together, the results suggest that the DC-mediated neuroprotection was achieved via the induction of a systemic T-cell-dependent immune response. Better preservation of neural tissue and diminished formation of cysts and scar tissue accompanied the improved functional recovery in DC-treated rats. The use of antigen-specific DCs may represent an effective way to obtain, via transient induction of an autoimmune response, the maximal benefit of immune-mediated repair and maintenance as well as protection against self-destructive compounds.

Axonal injury in the central nervous system (CNS) results in the degeneration of directly damaged fibers and also in the secondary degeneration of fibers that escaped the primary insult. Studies have shown that a protective T cell-mediated autoimmunity directed against myelin-related self-antigens is a physiological response to CNS insult, spontaneously elicited in strains that are constitutionally resistant to experimental autoimmune encephalomyelitis (EAE) but not in EAE-susceptible strains. The protective response following axonal injury can be induced in susceptible rats and boosted in resistant rats by passive or active immunization with myelin-related antigens. Here we show that oral administration of low-dose myelin basic protein (MBP) over a 5-day period is beneficial for post-traumatic survival of neurons in Lewis (EAE-susceptible) rats. Protection was accompanied by increased expression of the costimulatory molecule B7.2 in the traumatized nerves, similar to that seen after passive transfer of MBP-specific T cells. These results support the contention that properly controlled autoimmunity is the body's defense mechanism against non-infective insults. Oral immunization with MBP can be viewed as a way to control the autoimmunity capable of fighting off the consequences of CNS injury in EAE-susceptible strains.

PURPOSE. TO investigate the antigenic specificity of the immune neuroprotective mechanism that can protect retinal ganglion cells (RGCs) against death caused by high intraocular pressure (IOP). METHODS. A unilateral increase in IOP was induced in rats by argon laser photocoagulation of the episcleral veins and limbal plexus. Rats with high IOP were immunized with glatiramer acetate (Cop-1, a synthetic copolymer) or with myelin-derived or uveitogenic peptides. When the steroid drug methylprednisolone was used, it was administered intraperitoneally every other day for 12 days. RESULTS. Vaccination with myelin-derived peptides that reside in the axons failed to protect RGCs from death caused by high IOP. In contrast, IOP-induced RGC loss was reduced by vaccination with R16, a peptide derived from interphotoreceptor retinoid-binding protein, an immunodominant antigen residing in the eye. The benefit of protection against IOP-induced RGC loss outweighed the cost of the monophasic experimental autoimmune uveitis (EAU) that transiently developed in a susceptible rat strain. Treatment with methylprednisolone alleviated the disease symptoms, but caused further loss of RGCs. Cop-1 vaccination was effective in both EAU-resistant and EAU-susceptible strains. CONCLUSIONS. To benefit damaged neurons, immune neuroprotection should be directed against immunodominant antigens that reside in the site of damage. In a rat model of high IOP, RGCs can benefit from vaccination with peptides derived from proteins that are immunodominant in the eye but not from myelin-associated proteins. This suggests that the site of primary degeneration in IOP-induced RGC loss is in the eye. Cop-1 vaccination apparently circumvents the site-specificity barrier and provides protection without risk of inducing autoimmune disease.

Glutamate, a key neurotransmitter, is pivotal to CNS function. Alterations in its concentration can be dangerous, as seen for example in acute injuries of the CNS, chronic neurodegenerative disorders and mental disorders. Its homeostasis is attributed to the efficient removal of glutamate from the extracellular milieu by reuptake via local transport mechanisms. Our recent studies suggest that glutamate, either directly or indirectly, elicits a purposeful systemic T-cell-mediated immune response directed against immunodominant self-antigens that reside at the site of glutamate-induced damage. We suggest that the harnessed autoimmunity (which we have termed 'protective autoimmunity') helps the resident microglia in their dual function as antigen-presenting cells (serving the immune system) and as cells that clear the damaged site of potentially harmful material (serving the nervous system). The interplay between glutamate and an adaptive immune response illustrates the bidirectional dialog between the immune and nervous systems, under both physiological and pathological conditions. These results point to the possible development of a therapeutic vaccination with self-antigens, or with antigens cross-reactive with self-antigens, as a way to augment autoimmunity without inducing an autoimmune disease, thus providing a safe method of limiting degeneration. This approach, which boosts a physiological mechanism for the regulation of glutamate, and possibly also that of other self-compounds, might prove to be a feasible strategy for therapeutic protection against glutamate-associated neurodegenerative or mental disorders.

Closed head injury often has a devastating outcome, partly because the insult, like other injuries to the central nervous system (CNS), triggers self-destructive processes. During studies of the response to other CNS insults, it was unexpectedly discovered that the immune system, if well controlled, provides protection against self-destructive activities. Here we show that in mice with closed head injury, the immune system plays a key role in the spontaneous recovery. Strain-related differences were observed in the ability to harness a T cell-dependent protective mechanism against the effects of the injury. We further show that the trauma-induced deficit could be reduced, both functionally and anatomically, by post-traumatic vaccination with Cop-1, a synthetic copolymer used to treat patients with multiple sclerosis and found (using a different treatment protocol) to effectively counteract the loss of neurons caused by axonal injury or glutamate-induced toxicity. We suggest that a compound such as Cop-1 can be safely developed as a therapeutic vaccine to boost the body's immune repair mechanisms, thereby providing multifactorial protection against the consequences of brain trauma.

Therapeutic vaccination with Copaxone (glatiramer acetate, Cop-1) protects motor neurons against acute and chronic degenerative conditions. In acute degeneration after facial nerve axotomy, the number of surviving motor neurons was almost two times higher in Cop-1-vaccinated mice than in nonvaccinated mice, or in mice injected with PBS emulsified in complete Freund's adjuvant (P < 0.05). In mice that express the mutant human gene Cu/Zn superoxide dismutase G93A (SOD1), and therefore simulate the chronic human motor neuron disease amyotrophic lateral sclerosis, Cop-1 vaccination prolonged life span compared to untreated matched controls, from 211 ± 7 days (n = 15) to 263 ± 8 days (n = 14; P < 0.0001). Our studies show that vaccination significantly improved motor activity. In line with the experimentally based concept of protective autoimmunity, these findings suggest that Cop-1 vaccination boosts the local immune response needed to combat destructive self-compounds associated with motor neuron death. Its differential action in CNS autoimmune diseases and neurodegenerative disorders, depending on the regimen used, allows its use as a therapy for either condition. Daily administration of Cop-1 is an approved treatment for multiple sclerosis. The protocol for non-autoimmune neurodegenerative diseases such as amyotrophic lateral sclerosis, remains to be established by future studies.

Inflammation has been widely perceived as participating in the etiology of acute and chronic neurodegenerative conditions. Accordingly, in the context of traumatic injuries or chronic neurodegenerative diseases in the central nervous system (CNS), activated microglia have been viewed as detrimental and attempts have been made to treat both conditions by antiinflammatory therapy. Recent studies have suggested that microglia act as stand-by cells in the service of both the immune and the nervous systems. In the healthy CNS these cells are quiescent, but in the event of injury to axons or cell bodies they exercise their neural function by buffering harmful self-compounds and clearing debris from the damaged site, and their immune function by providing immune-related requirements for recovery. Proper regulation of the inflammatory (autoimmune) response to injury will arrest degeneration and promote regrowth, whereas inappropriate regulation will lead to ongoing degeneration. Regulation is achieved by the operation of a T cell-mediated response directed to abundant selfantigens residing in the damaged site. Since this immune-dependent mechanism was found to protect against glutamate toxicity (a major factor in neurodegenerative disorders), boosting of this response might constitute the basis for development of a therapeutic vaccination against neurodegenerative diseases, all of which exhibit similar pathways and patterns of progression.

The function of the adaptive immune response against exogenous (non-self) agents is to help the innate arm of the immune system (represented by phagocytic cells) to fight and eliminate these agents. We suggest that the body also protects itself against potentially harmful self components using mechanisms similar to those used for fighting and eliminating non-self agents, and that the protective immune activity against self-components competes with the activity of self-destructive compounds. Tolerance to self is thus not a lack of response to self, but the ability to tolerate an active defense response to self without developing an autoimmune disease.

Inflammation is thought to exacerbate the outcome of spinal cord injury. However, our findings have led us to view inflammation as a healing response that needs the help of a systemic immune response mediated by T helper 1 (Th1) cells that are specific to the abundant antigens residing in the lesion site. Strains differ in their ability to manifest, at the right time and intensity, a spontaneous T-cell response to antigens at the lesion site and therefore in their ability to generate a local inflammatory response whose outcome is beneficial (maintenance and repair). All strains, however, can benefit from immune intervention that boosts and regulates the inflammatory response. Because recovery comprises multi-step processes, pharmacological intervention will be less effective than well-synchronized, self-healing immune activity. Risk-free neuroprotective intervention might be achieved by post-traumatic vaccination with a weak, non-pathogenic, auto-antigen, causing autoimmune T cells to home to the lesion site where they become activated and therefore activate local phagocytic cells to remove hostile elements and provide growth factors.

Spinal cord injury often has devastating consequences, due to the poor regenerative capacity of the central nervous system (CNS), and because the injury triggers self-destructive processes that lead to degeneration of directly injured fibers, and fibers that survived the lesion but subsequently undergo "secondary degeneration." The highly complex, multifaceted dialogue between the immune system and the damaged spinal cord is complicated further by factors related to species, strain, and gender. It is suggested that following an injury to the CNS, the resident and systemic immune cells are recruited and activated, to reduce the self-destruction and facilitate the repair of the damaged nerve. Both of these functions can benefit from a well-controlled immune response that critically involves the innate and adaptive arms of the immune system, represented by macrophages/microglia and autoimmune T cells, respectively. A poorly controlled immune response can be non-efficient or even destructive, leading in extreme cases even to autoimmune disease. The beneficial autoimmune response can be boosted by posttraumatic T cell-based vaccination with peptides that activate weak self-reactive T cells, thereby ensuring timely and adequate immune activation without the risk of autoimmune disease. It is proposed that the notion of "bad" or "good" inflammation be replaced by a concept that views inflammation in the CNS as a repair mechanism and thus as beneficial if well regulated, and supports therapy for spinal cord and other CNS injuries by immunomodulation.

Primary open-angle glaucoma is a chronic, progressive optic neuropathy associated with a gradual decline in visual function, which may lead to blindness. In most cases, the optic neuropathy is associated with increased intraocular pressure. It is now generally accepted, however, that normalization of pressure, although necessary, is often not-sufficient as a remedial measure. This is because of the existence of additional factors, some of which emerge as a consequence of the initial damage. This situation is reminiscent of the response to a traumatic axonal insult, in which some of the damage is immediate and is caused by the insult itself, and some is secondary and is caused by a deficiency of growth-supportive factors as well as by toxic factors derived from the damaged tissue. Accordingly, the author has suggested that glaucoma may be viewed as a neurodegenerative disease and consequently amenable to any therapeutic intervention applicable to neurodegenerative diseases. There is evidence that neuroprotection can be achieved both pharmacologically and immunologically. Pharmacologic intervention neutralizes some of the effects of the nerve-derived toxic factors and possibly increases the ability of the remaining healthy neurons, at any given time, to cope with the stressful conditions. Immunologic intervention boosts the body's repair mechanisms for counteracting the toxicity of physiologic compounds acting as stress signals.

Purpose: To evaluate the neuroprotective effect of memantine, an NMDA receptor channel blocker, in two retinal ganglion cell (RGC) injury models in rats. Methods: Neuroprotective effect of memantine was tested in partial optic nerve injury and chronic ocular hypertensive models. In the optic nerve injury model, memantine (0.1 - 30 mg/kg) was injected intraperitoneally immediately after injury. Two weeks later, optic nerve function was determined by measuring compound action potential and surviving RGC was determined by retrograde labeling with dextran tetramethyl rhodamine. Chronic ocular hypertension was attained by laser photocoagulation of episcleral and limbal veins. Memantine (5 or 10 mg/kg) was administered continuously each day with an osmotic pump, either immediately after or 10 days after first laser photocoagulation, for 3 weeks, after which RGC survival was determined. Results: Two weeks after partial optic nerve injury, there was = 80% reduction in RGC number. Memantine (5 mg/kg) caused a twofold increase in compound action potential amplitude and a 1.7-fold increase in survival of RGCs, respectively. In the chronic ocular hypertension model there was 37% decrease in RGCs after 3 weeks of elevated intraocular pressure. Memantine (10 mg/kg daily) reduced ganglion cell loss to 12% when applied immediately after first laser photocoagulation, and prevented any further loss when applied 10 days after first laser photocoagulation. Conclusion: The protective effect of memantine suggests that excessive stimulation of NMDA receptors by glutamate is involved in causing cell damage in these RGC injury models.

Insults to the central nervous system (CNS), whether of microbial or microbefree origin, result in tissue damage. Until recently, it was generally believed that only microbe-related damage elicits an adaptive immune response, the purpose of which is to eliminate the offending microorganisms. Recent studies in the author's laboratory suggest, however, that the body exhibits an adaptive immune response to microbe-free injuries as well. The immune response in this case is directed against dominant self-antigens residing in the damaged site, where such an adaptive anti-self immune response reinforces the protective activity of local resident cells by providing them with factors that can augment and regulate their capacity for buffering troublemakers such as destructive self-compounds emerging from the injured neural tissue. Because the specificity of this autoimmune response apparently depends not on the type but on the site of lesion, the response can be boosted by therapeutic vaccination for acute and chronic neurodegenerative conditions irrespective of their primary etiology. The results have far-reaching implications, both for microbial infections and for neurodegenerative diseases of the CNS.

The ability of rats or mice to withstand the consequences of injury to myelinated axons in the CNS was previously shown to depend on the ability to manifest a T cell-mediated protective immune response, which is amenable to boosting by myelin-specific T cells. Here we show that this ability, assessed by retinal ganglion cell survival after optic nerve injury or locomotor activity after spinal cord contusion, is decreased if the animals were immunized as neonates with myelin proteins (resulting in their nonresponsiveness as adults to myelin proteins) or injected with naturally occurring regulatory CD4⁺CD25⁺ T cells immediately after the injury, and is improved by elimination of these regulatory T cells. In nude BALB/c mice replenished with a splenocyte population lacking CD4⁺CD25⁺ regulatory T cells, significantly more neurons survived after optic nerve injury than in nude mice replenished with a complete splenocyte population or in matched wild-type controls. In contrast, neuronal survival in wild-type BALB/c mice injected with CD4⁺CD25⁺ regulatory T cells immediately after injury was significantly worse than in noninjected controls. These findings suggest that the ability to cope with the sequelae of a CNS insult is affected unfavorably by nonresponsiveness to myelin self-antigens and favorably by conditions allowing rapid expression of an autoimmune response. The regulatory T cells might represent an evolutionary compromise between the need to avoid autoimmune diseases and the need for autoimmunity on alert for the purpose of tissue maintenance.

Vaccination with peptides derived from interphotoreceptor retinoid-binding protein (a self-Ag that can cause experimental autoimmune uveoretinitis) resulted in protection of retinal ganglion cells from glutamate-induced death or death as a consequence of optic nerve injury. In the case of glutamate insult, no such protection was obtained by vaccination with myelin Ags (self-Ags associated with an autoimmune disease in the brain and spinal cord that evokes a protective immune response against consequences of injury to myelinated axons). We suggest that protective autoimmunity is the body's defense mechanism against destructive self-compounds, and an autoimmune disease is the outcome of a failure to properly control such a response. Accordingly, the specific self-Ag (although not necessarily its particular epitopes) used by the body for protection against potentially harmful self-compounds (e.g., glutamate) can be inferred from the specificity of the autoimmune disease associated with the site at which the stress occurs (irrespectively of the type of stress) and is in need of help.

Autoimmunity, at least in the central nervous system (CNS), is not only an outcome of immune system malfunction, but is the body's own protective mechanism against destructive self-compounds. Likewise, the naturally occurring regulatory CD4⁺CD25⁺ T cells have a physiological function, and are not merely an evolutionary adaptation to suppress self-reactive T-cell clones that escaped deletion in the thymus. We postulate that the regulatory T (Tr) cells are the product of an evolutionary compromise between the need for autoimmunity on alert for tissue maintenance and the need to control autoimmunity to avoid autoimmune disease. In the event of an insult to the CNS, the balance between self-reactive (effector) T cells and Tr cells determines the time of onset, the intensity and the duration of the autoimmune response. This response might thus represent an adaptive mechanism, which is optimal for day-to-day maintenance, but insufficient in extreme cases of CNS damage or failure of regulation. Downregulation or upregulation of CD4⁺CD25⁺ Tr cells might be a way to achieve better protection from neurodegenerative conditions induced by self-destruction or avoid autoimmune inflammatory disease development, respectively.

Autoimmune diseases are traditionally viewed as an outcome of a chaotic situation in which an individual's immune system reacts against the body's own proteins. In multiple sclerosis, a disease of the white matter of the central nervous system (CNS), the immune attack is directed against myelin proteins. In this article, the authors propose a paradigm shift in the perception of autoimmune disease. They suggest that an autoimmune disease may be viewed as a by-product of the malfunctioning of a physiological autoimmune response whose purpose is protective. The proposed view is based on observations by their group suggesting that an autoimmune response is the body's own mechanism for coping with CNS damage. According to this view, all individuals are endowed with the potential ability to evoke an autoimmune response to CNS injuries. However, the inherent ability to control this response so that its beneficial effect will be expressed is limited and is correlated with the individual's inherent ability to resist autoimmune disease induction. The same autoimmune T cells are responsible for neuroprotection and for disease development. In patients with CNS trauma or neurodegenerative disorders, it might be possible to gain maximal autoimmune protection and avoid autoimmune disease induction by boosting the immune response, using myelin-associated peptides that are nonpathogenic or antigens that simulate the activities of such peptides. In patients with multiple sclerosis and other neurodegenerative diseases, where the aim is to block the autoimmune disorder while deriving the potential benefit of the autoimmune response, the effect of treatment should be immunomodulatory rather than immunosuppressive. In this article, the authors present a novel concept of protective autoimmunity and propose that autoimmune disease is a by-product of failure to sustain it. They summarize the basic findings that led them to formulate the new concept and offer an explanation for the commonly observed presence of cells and antibodies directed against self-components in healthy individuals. The therapeutic implications of the new concept and their experimental findings are discussed.

The resistance of rats or mice to glutamate-induced toxicity depends on their ability to spontaneously manifest a T cell-dependent response to the insult. Survival of retinal ganglion cells (RGCs) exposed to glutamate in BALB/c SCID mice (a strain relatively resistant to glutamate toxicity) was significantly worse than in the wild type. In the susceptible C57BL/6J mouse strain, however, significantly more RGCs survived among SCID mutants than in the matched wild type. RGC survival in the SCID mutants of the two strains was similar. These results suggest 1) that immunodeficiency might be an advantage in strains incapable of spontaneously manifesting protective T cell-dependent immunity and 2) that B cells might be destructive in such cases. After exposure of RGCs to toxic glutamate concentrations in three variants of B cell-deficient C57BL/6J mice, namely muMT^-/- (B cell knockout mice) and Ii^-/- mice reconstituted with transgenically expressed low levels of Ii p31 isoforms (p31 mice) or Ii p41 isoforms (p41 mice), significantly more RGCs survived in these mice than in the wild type. The improved survival was diminished by replenishment of the B cell-deficient mice with B cells derived from the wild type. It thus seems that B cells have an adverse effect on neuronal recovery after injury, at least in a strain that is unable to spontaneously manifest a T cell-dependent protective mechanism. These findings have clear implications for the design of immune-based therapies for CNS injury.

Myelin-specific encephalitogenic T cells, when passively transferred into rats or mice, cause an experimental autoimmune disease. Previous studies by our group have shown that (a) the same cells also significantly reduce post-traumatic degeneration in these animals after injury to the central nervous system, (b) this beneficial autoimmunity is a physiological response, and (c) animals differ in their ability to resist injurious conditions, and the ability to resist post-traumatic degeneration correlates with resistance to the development of an autoimmune disease. Here we show that optic nerve neurons in both resistant and susceptible rat strains can be protected from secondary degeneration after crush injury by immunization with myelin basic protein emulsified in complete or incomplete Freund's adjuvant. We provide evidence that potentially destructive autoimmunity (causing autoimmune disease) and beneficial autoimmunity (causing improved neuronal survival) both result from activity of the same myelin-specific, proinflammatory Th1 cells. We further show that following passive transfer of such Th1 cells, the expression of their beneficial potential depends on the activity of an additional T cell (CD4⁺) population. By identifying the additional cellular component of autoimmune neuroprotection, we may be able to take meaningful steps toward achieving neuroprotection without risk of accompanying autoimmune disease.

Accumulating evidence suggests that activation of the immune system in the central nervous system (CNS) after trauma protects the CNS from damage propagation and facilitates regeneration. Studies by our group have shown that passive transfer of autoimmune T cells specific to myelin basic protein (T(MBP)) can protect injured neurons in the rat CNS from secondary degeneration. In this study, we investigated the effects of T(MBP) treatment on the local immune response (by B cells and macrophages) and on the expression of neurotrophic factors after crush injury of the rat optic nerve. Systemic injection of activated T(MBP) caused an increase in the accumulation of macrophages/microglia and B cells in the injured nerve, which was greater than that seen in the injured optic nerves of untreated animals. This accumulation was accompanied by a transient, but massive, increase in the expression of neurotrophic factors. Immunocytochemical analysis demonstrated differential expression of neurotrophins by resident astrocytes and by infiltrating B cells, T cells, and macrophages. Because postinjury neuronal survival and maintenance are known to be affected by neurotrophins, our findings point to a possible contribution of a neurotrophin-related mechanism to the protective effect conferred by T cell-mediated autoimmunity on injured neurons.

Neurodegenerative diseases, whatever their primary causes, are characterized by certain common features, one of which is their self-perpetuating nature. The ongoing progression of the disorder is due to the effects of destructive self-compounds, whose presence in the tissues is an outcome of the early phase of the disease and which gradually destroy remaining functional neurons. Studies in our laboratory have led to the recent formulation of a novel concept of protective autoimmunity as the body's mechanism of defense against these destructive self-compounds. This autoimmune response to central nervous system (CNS) insults is mediated by T-cells and presumably operates by activating and regulating local microglia and infiltrating macrophages (inflammatory response) to carry out their function of clearing destructive material from the tissue at risk. We suggest that a well-controlled autoimmunity counteracts and overcomes the destructive effects of the potentially harmful self-compounds, at the cost of some loss of tissue. An additional risk to the individual is the induction of an autoimmune disease, which is likely to occur if the autoimmune response is malfunctioning. An optimal balance of the various factors will lead to an outcome of maximal benefit at minimal cost to the tissue. A procedure for safely boosting the autoimmune response, by vaccination with a weak self-crossreactive antigen such as glatiramer acetate (also known as Cop-1) was found to protect rats from glutamate toxicity, a major mediator of the spread of damage and a well-known causative factor in neurodegenerative disorders. Cop-1, when administered according to a different regimen, is an FDA-approved drug for the treatment of multiple sclerosis. Different formulations of the same drug can therefore be used to treat two extreme manifestations of chronic degenerative diseases of the CNS.

Immune cells have been shown to contribute to spontaneous recovery from central nervous system (CNS) injury. Here we show that adult female rats and mice recover significantly better than their male littermates from incomplete spinal cord injury (ISCI). This sexual dimorphism is wiped out and recovery is worse in adult mice deprived of mature T cells. After spinal cord contusion in adult rats, functional recovery (measured by locomotor scores in an open field) was significantly worse in females treated with dihydrotestosterone prior to the injury than in placebo-treated controls, and significantly better in castrated males than in their noncastrated male littermates. Post-traumatic administration of the testosterone receptor antagonist flutamide promoted the functional recovery in adult male rats. These results, in line with the known inhibitory effect of testosterone on cell-mediated immunity, suggest that androgen-mediated immunosuppression plays a role in ISCI-related immune dysfunction and can therefore partly explain the worse outcome of ISCI in males than in female. We suggest that females, which are more prone to develop autoimmune response than males, benefit from this response in cases of CNS insults.

Spinal cord injury results in a massive loss of neurons, due not only to the direct effects of the primary injury but also to self-destructive processes triggered by the insult. Our group has recently reported that traumatic injury of the central nervous system (CNS) spontaneously evokes a purposeful T cell-mediated autoimmune response that reduces the injury-induced degeneration in the CNS; in its absence, the outcome of the injury is worse. Using a rat model of spinal cord contusion, we show here that this autoimmune protection can be induced and/or boosted by post-traumatic immunization with CNS myelin-associated self antigens such as myelin basic protein (MBP). In an attempt to reduce the risk of pathogenic autoimmunity while retaining the benefit of the immunization, we immunized spinally injured rats with MBP-derived peptides with attenuated pathogenic properties created by replacement of one amino acid in the T cell receptor-binding site. Immunization with these altered peptide ligands immediately after spinal cord contusion resulted in a significant improvement in recovery, assessed by locomotor activity in an open field. The feasibility of T cell-based vaccination, as opposed to vaccination mediated by antibodies for the treatment of nerve trauma, is further suggested by the relatively rapid onset of the T cell response following immunization. Such cell-mediated therapy is not only a way to evoke and boost a physiological remedy; it also has the advantage of being mediated by mobile cells, which can produce a variety of neurotrophic factors and cytokines in accordance with the tissue needs. T cells can also regulate other immune cells in a way that favors tissue maintenance and repair.

PURPOSE. Glaucoma is widely accepted as a neurodegenerative disease in which retinal ganglion cell (RGC) loss is initiated by a primary insult to the optic nerve head, caused, for example, by increased intraocular pressure (IOP). In some cases, the surviving RGCS, despite adequate IOP control, may continue to degenerate as a result of their heightened susceptibility to self-destructive processes evoked by the initial damage. In animal models of mechanical or biochemical injury to the optic nerve or retina, a T-cell-mediated immune response evoked by the insult helps to reduce this ongoing loss. The current study was conducted to find out whether the ability to resist the IOP-induced loss of RGCs in a rat model is affected by the immune system. METHODS. The ocular veins and limbal plexus of rats of two strains differing in their resistance to experimental autoimune encephalomyelitis (EAE) and in their ability to manifest a beneficial autoimmune response were laser irradiated twice to induce an increase in IOP. The pressure was measured weekly, and RGC losses were assessed 3 and 6 weeks after the first irradiation. To verify the existence of a relationship between the immune system and RGC survival, we assessed neuronal survival in Sprague-Dawley (SPD) rats devoid of mature T cells as well as after transferring splenocytes from Fisher rats, an EAE-resistant rat strain capable of manifesting T-cell-mediated neuroprotection, to rats of a major histocompatibility complex (MHC)-matched EAE-susceptible strain (Lewis), in which the ability to manifest such protective immunity is limited RESULTS. Both 3 and 6 weeks after the increase in IOP was initiated, the number of surviving RGCs in SPD rats, a strain in which a beneficial autoimmune response can be evoked spontaneously, was significantly higher than in Lewis rats. Moreover, in SPD rats that were thymectomized at birth, the number of surviving RGCs after an increase in IOP as adults was significantly diminished. Passive transfer of splenocytes from Fisher rats to Lewis rats significantly reduced the IOP-induced loss of RGCs in the latter. CONCLUSIONS. In rats of different strains, a similar increase in IOP results in differing amounts of RGC loss. This disparity was found to correlate with immune potency. These findings may explain why patients with glaucoma experience different degrees of visual loss after pressure reduction, even when the severity of the disease at the time of diagnosis is similar. The results have far-reaching prognostic and therapeutic implications.

Glutamate is an essential neurotransmitter in the CNS. However, at abnormally high concentrations it becomes cytotoxic. Recent studies in our laboratory showed that glutamate evokes T cell-mediated protective mechanisms. The aim of the present study was to examine the nature of the glutamate receptors and signalling pathways that participate in immune protection against glutamate toxicity. We show, using the mouse visual system, that glutamate-induced toxicity is strain dependent, not only with respect to the amount of neuronal loss it causes, but also in the pathways it activates. In strains that are genetically endowed with the ability to manifest a T cell-dependent neuroprotective response to glutamate insult, neuronal losses due to glutamate toxicity were relatively small, and treatment with NMDA-receptor antagonist worsened the outcome of exposure to glutamate. In contrast, in mice devoid of T cell-dependent endogenous protection, NMDA receptor antagonist reduced the glutamate-induced neuronal loss. In all strains, blockage of the AMPA/KA receptor was beneficial. Pharmacological (with α2-adrenoceptor agonist) or molecular intervention (using either mice overexpressing Bcl-2, or DAP-kinase knockout mice) protected retinal ganglion cells from glutamate toxicity but not from the toxicity of NMDA. The results suggest that glutamate-induced neuronal toxicity involves multiple glutamate receptors, the types and relative contributions of which, vary among strains. We suggest that a multifactorial protection, based on an immune mechanism independent of the specific pathway through which glutamate exerts its toxicity, is likely to be a safer, more comprehensive, and hence more effective strategy for neuroprotection. It might suggest that, because of individual differences, the pharmacological use of NMDA-antagonist for neuroprotective purposes might have an adverse effect, even if the affinity is low.

Protective autoimmunity is the body's defense mechanism against destructive self-compounds such as those commonly associated with neurodegenerative disorders. Autoimmune disease and neurodegenerative disorders can thus be viewed as two extreme manifestations of the same process. Therefore, when designing therapy, it is important to avoid an approach that will cure the one by invoking the other. One way to stop, or at least slow down, the progression of neurodegeneration without risking development of an autoimmune disease is by boosting protective autoimmunity in a well-controlled way. Copolymer 1 (Cop-1), an approved drug for the treatment of multiple sclerosis, can be used as a treatment for autoimmune diseases and as a therapeutic vaccine for neurodegenerative diseases. We propose that the protective effect of Cop-1 vaccination is obtained through a well-controlled inflammatory reaction, and that the activity of Cop-1 in driving this reaction derives from its ability to serve as a'universal antigen'by weakly activating a wide spectrum of self-reactive T cells.

Functional loss after injury to the mammalian central nervous system (CNS) has been attributed not only to the immediate loss of neurons but also to secondary neuronal degeneration caused by the toxicity of physiological compounds present in abnormally high amounts as a result of the injury. One such compound appears to be the protease thrombin. Here we show that the beneficial effect of T cells directed against myelin self-antigens can be attributed, at least in part, to the ability of these 'autoimmune' T cells to produce antithrombin III. Using transgenic mice lacking the thrombin receptor PAR-1, we also present molecular evidence indicating that down-regulation of PAR-1 by genetic manipulation leads to increased post-traumatic survival of CNS neurons. We further show that the ability of autoimmune T cells to produce thrombin inhibitors and to exert post-traumatic neuroprotection are both independent of their PAR-1 expression. These findings suggest that thrombin plays a key role in post-injury neuronal survival, and that its post-traumatic toxicity can be down-regulated by appropriate alteration of the amounts of PAR-1 receptors or of antithrombin III, the latter achieved by T cell-mediated autoimmunity.

The myelin-associated protein Nogo-A has received more research attention than any other inhibitor of axonal regeneration in the injured central nervous system (CNS). Circumvention of its inhibitory effect, by using antibodies specific to Nogo-A, has been shown to promote axonal regrowth. Studies in our laboratory have demonstrated that active or passive immunization of CNS-injured rats or mice with myelin-associated peptides induces a T-cell-mediated protective autoimmune response, which promotes recovery by reducing posttraumatic degeneration. Here, we show that neuronal degeneration after incomplete spinal-cord contusion in rats was substantially reduced, and hence recovery was significantly promoted, by posttraumatic immunization with p472, a peptide derived from Nogo-A. The observed effect seemed to be mediated by T cells and could be reproduced by passive transfer of a T cell line directed against the Nogo-A peptide. Thus, it seems that after incomplete spinal-cord injury, immunization with a variety of myelin-associated peptides, including those derived from Nogo-A, can be used to evoke a T cell-mediated response that promotes recovery. The choice of peptide(s) for clinical treatment of spinal-cord injuries should be based on safety considerations; in particular, the likelihood that the chosen peptide will not cause an autoimmune disease or interfere with essential functions of this peptide or other proteins. From a therapeutic point of view, the fact that the active cellular agents are T cells rather than antibodies is an advantage, as T cell production commences within the time window required for a protective effect after spinal-cord injury, whereas antibody production takes longer.

Injuries to the central nervous system (CNS) evoke self-destructive processes, which eventually lead to a much greater loss of tissue than that caused by the trauma itself. The agents of self-destruction include physiological compounds, such as glutamate, which are essential for the proper functioning of the CNS, but become cytotoxic when their normal concentrations are exceeded. The CNS is equipped with buffering mechanisms that are specific for each compound. Here we show, using Balb/c mice (a strain resistant to induction of experimental autoimmune encephalomyelitis), that after intravitreal injection of any concentration of glutamate (a neurotransmitter that becomes toxic when in excess) or ammonium-ferrous sulfate hexahydrate (which increases the formation of toxic oxygen species), the loss of retinal ganglion cells in mice devoid of mature T cells (nude mice) is significantly greater than in matched wild-type controls. We further show that this outcome can be partially reversed by supplying the T cell-defective mice with splenocytes derived from the wild-type mice. The results suggest that potentially toxic physiological compounds, when present in excessive amounts, can recruit and activate a T-cell-dependent self-protective immune mechanism. This may represent a prototype mechanism for the physiological regulation of potentially destructive CNS events by T-cell-mediated immune activity, when the local buffering mechanisms cannot adequately cope with them.

Axonal injury initiates a process of neuronal degeneration, with resulting death of neuronal cell bodies. We show here that in C57BL/6J mice, previously shown to have a limited ability to manifest a post-traumatic protective immunity, the rate of neuronal survival is increased if IL-6 is deficient during the first 24 hours after optic nerve injury. Immunocytochemical staining preformed 7 days after the injury revealed an increased number of activated microglia in the IL-6-deficient mice compared to the wild-type mice. In addition, IL-6-deficient mice showed an increased resistance to glutamate toxicity. These findings suggest that the presence of IL-6 during the early post-traumatic phase, at least in mice that are susceptible to autoimmune disease development, has a negative effect on neuronal survival. This further substantiates the contention that whether immune-derived factors are beneficial or harmful for nerve recovery after injury depends on the phenotype of the immune cells and the timing and nature of their dialog with the damaged neural tissue.

Injury to the CNS is often followed by a spread of damage (secondary degeneration), resulting in neuronal losses that are substantially greater than might have been predicted from the severity of the primary insult. Studies in our laboratory have shown that injured CNS neurons can benefit from active or passive immunization with CNS myelin-associated antigens. The fact that autoimmune T-cells can be both beneficial and destructive, taken together with the established phenomenon of genetic predisposition to autoimmune diseases, raises the question: will genetic predisposition to autoimmune diseases affect the outcome of traumatic insult to the CNS? Here we show that the survival rate of retinal ganglion cells in adult mice or rats after crush injury of the optic nerve or intravitreal injection of a toxic dosage of glutamate is up to twofold higher in strains that are resistant to the CNS autoimmune disease experimental autoimmune encephalomyelitis (EAE) than in susceptible strains. The difference was found to be attributed, at least in part, to a beneficial T-cell response that was spontaneously evoked after CNS insult in the resistant but not in the susceptible strains. In animals of EAE-resistant but not of EAE-susceptible strains devoid of mature T-cells (as a result of having undergone thymectomy at birth), the numbers of surviving neurons after optic nerve injury were significantly lower (by 60%) than in the corresponding normal animals. Moreover, the rate of retinal ganglion cell survival was higher when the optic nerve injury was preceded by an unrelated CNS (spinal cord) injury in the resistant strains but not in the susceptible strains. It thus seems that, in normal animals of EAE-resistant strains (but not of susceptible strains), the injury evokes an endogenous protective response that is T-cell dependent. These findings imply that a protective T-cell-dependent response and resistance to autoimmune disease are regulated by a common mechanism. The results of this study compel us to modify our understanding of autoimmunity and autoimmune diseases, as well as the role of autoimmunity in non-autoimmune CNS disorders. They also obviously have far-reaching clinical implications in terms of prognosis and individual therapy.

Primary damage caused by injury to the CNS is often followed by delayed degeneration of initially spared neurons. Studies in our laboratory have shown that active or passive immunization with CNS myelin-associated self-antigens can reduce this secondary loss. Here we show, using four experimental paradigms in rodents, that CNS trauma spontaneously evokes a beneficial T cell-dependent immune response, which reduces neuronal loss. (1) Survival of retinal ganglion cells in rats was significantly higher when optic nerve injury was preceded by an unrelated CNS (spinal cord) injury. (2) Locomotor activity of rat hindlimbs (measured in an open field using a locomotor rating scale) after contusive injury of the spinal cord (T8) was significantly better (by three to four score grades) after passive transfer of myelin basic protein (MBP)-activated splenocytes derived from spinally injured rats than in untreated injured control rats or rats similarly treated with splenocytes from naive animals or with splenocytes from spinally injured rats activated ex vivo with ovalbumin or without any ex vivo activation. (3) Neuronal survival after optic nerve injury was 40% lower in adult rats devoid of mature T cells (caused by thymectomy at birth) than in normal rats. (4) Retinal ganglion cell survival after optic nerve injury was higher (119 ± 3.7%) in transgenic mice overexpressing a T cell receptor (TcR) for MBP and lower (85 ± 1.3%) in mice overexpressing a T cell receptor for the non-self antigen ovalbumin than in matched wild types. Taken together, the results imply that CNS injury evokes a T cell-dependent neuroprotective response.

Our group recently demonstrated that autoimmune T cells directed against central nervous system-associated myelin antigens protect neurons from secondary degeneration. We further showed that the synthetic peptide copolymer 1 (Cop-1), known to suppress experimental autoimmune encephalomyelitis, can be safely substituted for the natural myelin antigen in both passive and active immunization for neuroprotection of the injured optic nerve. Here we attempted to determine whether similar immunizations are protective from retinal ganglion cell loss resulting from a direct biochemical insult caused, for example, by glutamate (a major mediator of degeneration in acute and chronic optic nerve insults) and in a rat model of ocular hypertension. Passive immunization with T cells reactive to myelin basic protein or active immunization with myelin oligodendrocyte glycoprotein-derived peptide, although neuroprotective after optic nerve injury, was ineffective against glutamate toxicity in mice and rats. In contrast, the number of surviving retinal ganglion cells per square millimeter in glutamate-injected retinas was significantly larger in mice immunized 10 days previously with Cop-1 emulsified in complete Freund's adjuvant than in mice injected with PBS in the same adjuvant (2,133 ± 270 and 1,329 ± 121, respectively, mean ± SEM; P < 0.02). A similar pattern was observed when mice were immunized on the day of glutamate injection (1,777 ± 101 compared with 1,414 ± 36; P < 0.05), but not when they were immunized 48 h later. These findings suggest that protection from glutamate toxicity requires reinforcement of the immune system by antigens that are different from those associated with myelin. The use of Cop-1 apparently circumvents this antigen specificity barrier. In the rat ocular hypertension model, which simulates glaucoma, immunization with Cop-1 significantly reduced the retinal ganglion cell loss from 27.8% ± 6.8% to 4.3% ± 1.6%, without affecting the intraocular pressure. This study may point the way to a therapy for glaucoma, a neurodegenerative disease of the optic nerve often associated with increased intraocular pressure, as well as for acute and chronic degenerative disorders in which glutamate is a prominent participant.

The progression of degeneration in chronic optic neuropathies or in animal models of optic nerve injury is thought to be caused at least in part by an increase in glutamate to abnormally high concentrations. We show here that glutamate when injected in subtoxic amounts into the vitreal body of the rat eye transduces a self-protecting signal that renders the retinal ganglion cells resistant to further toxicity whether glutamate-derived or not. This neuroprotective effect is attained within 24 h and lasts at least 4 days. Western blot analysis of rat retinas revealed increased amounts of bcl-2 four days after injection of glutamate in either subtoxic or toxic (120 nmol) amounts but not after saline injection. The effects of intravitreal glutamate or saline injection on the secretion of neurotrophins by retinal ganglion cells was evaluated in rat aqueous humor 6 h 1 day and 4 days after injection. Nerve growth factor brain-derived neurotrophic factor and neurotrophin-3 showed similar kinetic patterns in all of the eyes; that is they increased to a peak 1 day after the injection and returned to normal by day 4. However increased amounts the neurotrophin receptor TrkA within the retinal ganglion cell layer and nerve fiber layer were detected 1 day after injection of glutamate in either toxic or subtoxic amounts but not after saline injection. This finding points to the possible involvement of neurotrophin receptors in regulation of the cellular responses to glutamate challenge. Identification of the intracellular signals that trigger the glutamate-induced self-protective mechanism would shed light on the genetic balance needed for survival and guide the development of drugs for the up-regulation of desired genes and their products.

A recent study in our laboratory showed, against all expectations, that macrophages and a particular type of T cell, by promoting regrowth and reducing the post-traumatic spread of damage in the injured rat optic nerve or spinal cord, have a beneficial effect on the injured CNS. Macrophages in the CNS have long been thought to have predominantly destructive effects. Autoimmunity in general, and in the CNS in particular, has never been documented as a purposeful physiological response of benign character. Our results suggest that after traumatic injury to the central nervous system (CNS), both of these immune cell types potentially have beneficial effects: macrophages can promote repair and T cells of a particular specificity can reduce the spread of damage. However, possibly because of the immune-privileged character of the CNS, the spontaneously evoked physiological activities of both macrophages and T cells in the CNS are restricted, and appear to need well-controlled boosting in order to be effective. It thus appears that (i) a stress signal transmitted from the traumatized tissue (in this case the CNS) for recruitment of the adaptive immune system does not have to be pathogen-related in order to evoke a response, (ii) a response to self is not necessarily a quirk of nature, and (iii) an autoimmune response, provided that it is well-regulated, helps the individual to cope with stress signals from the traumatized CNS, and thus plays a role in maintenance of the injured tissue without posing a threat to the organism.

T-cell autoimmunity to myelin basic protein was recently shown to be neuroprotective in injured rat optic nerves. In the present study, using the mouse optic nerve, we examined whether active immunization rather than passive transfer of T-cells can be beneficial in protecting retinal ganglion cells (RGCs) from post-traumatic death. Before severe crush injury of the optic nerve, SJL/J and C3H.SW mice were actively immunized with encephalitogenic or nonencephalitogenic peptides of proteolipid protein (PLP) or myelin oligodendrocyte glycoprotein (MOG), respectively. At different times after the injury, the numbers of surviving RGCs in both strains immunized with the nonencephalitogenic peptides pPLP 190-209 or pMOG 1-22 were significantly higher than in injured controls treated with the non-self-antigen ovalbumin or with a peptide derived from β-amyloid, a non-myelin-associated protein. Immunization with the encephalitogenic myelin peptide pPLP 139-151 was beneficial only when the disease it induced, experimental autoimmune encephalomyelitis, was mild. The results of this study show that survival of RGCs after axonal injury can be enhanced by vaccination with an appropriate self-antigen. Furthermore, the use of nonencephalitogenic myelin peptides for immunization apparently allows neuroprotection without incurring the risk of an autoimmune disease. Application of these findings might lead to a promising new approach for treating optic neuropathies such as glaucoma.

Spinal cord injury and its devastating consequences are the subject of intensive research aimed at reversing or at least minimizing functional loss. Research efforts focus on either attenuating the post-injury spread of damage (secondary degeneration) or inducing some regeneration. In most of these studies, as well as in clinical situations, evaluation of the state of the injured spinal cord poses a serious difficulty. To address this problem, we carried out a diffusion-weighted MRI experiment and developed an objective routine for quantifying anisotropy in injured rat spinal cords. Rats were subjected to a contusive injury of the spinal cord caused by a controlled weight drop. Untreated control rats were compared with rats treated with T cells specific to the central nervous system self-antigen myelin basic protein, a form of therapy recently shown to be neuroprotective. After the rats were killed their excised spinal cords were fixed in formalin and imaged by multislice spin echo MRI, using two orthogonal diffusion gradients. Apparent diffusion coefficient (ADC) values and anisotropy ratio (AI) maps were extracted on a pixel-by-pixel basis. The calculated sum of AI values (SAI) for each slice was defined as a parameter representing the total amount of anisotropy. The mean-AI and SAI values increased gradually with the distance from the site of the lesion. At the site itself, the mean-AI and SAI values were significantly higher in the spinal cords of the treated animals than in the controls (P = 0.047, P = 0.028, respectively). These values were consistent with the score of functional locomotion. The difference was also manifested in the AI maps, which revealed well-organized neural structure in the treated rats but not in the controls. The SAI values, AI histograms, and AI maps proved to be useful parameters for quantifying injury and recovery in an injured spinal cord. These results encourage the development of diffusion anisotropy MRI as a helpful approach for quantifying the extent of secondary degeneration and measuring recovery after spinal cord injury.

The physiological conditions under which beneficial autoimmunity is evoked have never been documented. We recently demonstrated that autoimmune T cells directed against myelin-associated self-proteins, when passively transferred into rats or mice, reduce the spread of damage after traumatic injury to central nervous system axons. This finding raised a fundamental question: Does this beneficial effect represent a physiological neuroprotective response that normally is too weak to be effective and requires boosting, or is it simply the welcome result of an ex vivo manipulation? It appears from our studies that trauma, at least in the central nervous system, evokes a stress signal that activates a T cell dependent response directed against self antigens, and that this response is physiological in nature, beneficial in intent, and amenable to boosting by active or passive immunization.

The functional loss that often follows injury of the mammalian CNS has been attributed not only to the immediate neural loss, but also to secondary neuronal degeneration caused by toxic biochemical mediators in the environment of the injured nerve. We report here that a high thrombin content, produced as a result of injury-induced activation of prothrombin, appears to be an important mediator of secondary damage. Measurement of post-traumatic neuronal survival in vivo revealed that post-traumatic local application of the thrombin inhibitor N-α-(2-naphthylsulphonylglycyl)-4-(D,L)-amidinophenylalanine piperidide acetate in the rat optic nerve subjected to mild partial crush injury left twice as many retinal ganglion cells with functioning axons as in controls. Thus, by readjusting thrombin activity, thereby possibly obtaining a moderate post-traumatic increase and thus gaining the benefit of thrombin without its toxic effects, it may be possible to create an environment that is more favourable towards post-traumatic survival.

Spinal cord injury results in a massive loss of neurons, and thus of function. We recently reported that passive transfer of autoimmune T cells directed against myelin-associated antigens provides acutely damaged spinal cords with effective neuroprotection. The therapeutic time window for the passive transfer of T cells was found to be at least 1 week. Here we show that posttraumatic T cell-based active vaccination is also neuroprotective. Immunization with myelin-associated antigens such as myelin basic protein (MBP) significantly promoted recovery after spinal cord contusion injury in the rat model. To reduce the risk of autoimmune disease while retaining the benefit of the immunization, we vaccinated the rats immediately after severe incomplete spinal cord injury with MBP-derived altered pepdde ligands. Immunization with these peptides resulted in significant protection from neuronal loss and thus in a reduced extent of paralysis, assessed by an open-field behavioral test. Retrograde labeling of the rubrospinal tracts and magnetic resonance imaging supported the behavioral results. Further optimization of nonpathogenic myelin-derived peptides can be expected to lead the way to the development of an effective therapeutic vaccination protocol as a strategy for the prevention of total paralysis after incomplete spinal cord injury.

PURPOSE. Primary open-angle glaucoma is a chronic, progressive optic neuropathy associated with a gradual decline in visual functions, which may lead to blindness. Methods. In most cases, the optic neuropathy is associated with increased intraocular pressure. However, it is now generally accepted, that normalization of pressure, although necessary, is often not sufficient as a remedial measure. This is because of the existence of additional risk factors, some of which emerge as a consequence of the initial damage. This situation is reminiscent of the response to a traumatic axonal insult: Some of the damage is immediate and is caused by the insult itself, while some is secondary and is caused by a deficiency of growth-supportive factors as well as by toxic factors derived from the damaged tissue. Accordingly, we have suggested that glaucoma may be viewed as a neurodegenerative disease and consequently is amenable to any therapeutic intervention applicable to these diseases. CONCLUSIONS. There is evidence that neuroprotection can be achieved both pharmacologically and immunologically. Pharmacological intervention (e.g. by using selective alpha-2 adrenergic receptor agonists) neutralizes some of the effects of the nerve-derived toxic factors and possibly increases the ability of the remaining healthy neurons, at any given time, to cope with the stressful conditions. Immunological intervention boosts the body's own repair mechanisms for counteracting the toxicity of physiological compounds acting as stress signals.

In glaucoma, as in other degenerative diseases of the central nervous system (CNS), there are some neurons which, although susceptible to degeneration, are amenable to neuroprotection. Until very recently, attempts to attenuate the spread of damage after CNS trauma or in neurodegenerative diseases did not include recruitment of the immune system, because it was assumed that in the CNS any immune activity, particularly autoimmune activity, would be harmful. Using the injured optic nerve of the rat as a model, we recently showed, however, that this 'secondary degeneration' can be slowed down by a well-controlled adaptive immune response, which is mediated by T cells against CNS self-antigens, such as myelin basic protein (MBP), and can be achieved either by passive transfer of MBP-activated T cells or by active immunization with MBP. Accordingly, we suggested that autoimmune T cells can be neuroprotective. We further showed that the neuroprotective autoimmunity is a physiological response to the injury, perhaps insufficient in its natural state, but amenable to boosting. The neuroprotective activity of these T cells probably depends on their reactivation by their specific antigen after they are targeted to the injured nerve. As nerve degeneration is initiated and sustained by many factors, it would probably best be counteracted by a comprehensive type of therapy rather than treatment that addresses only some aspects of nerve damage. T cell therapy, being physiological rather than pharmaceutical in nature, may provide a global answer. However, since the neuroprotective immune response is directed against the self, it must be rigorously regulated to avoid inducing an autoimmune disease. We showed that the synthetic copolymer Cop-1 can passively or actively evoke T cell-mediated neuroprotection, probably by cross-reacting with MBP. Safe synthetic peptides that resemble self-antigens and are cross-activated by CNS-associated self antigens may be a useful starting point for the development of anti-self immunity for neuroprotective purposes.

Immune activity in general, and autoimmunity in particular, have long been considered as harmful in the context of central nervous system (CNS) trauma. Increasing evidence suggests, however, that the injured CNS can benefit from autoimmune manipulations. Active or passive immunization with CNS-associated self antigens was shown to promote recovery from a CNS insult. It is now also evident that this beneficial 'autoimmunity' is not solely an outcome of immune manipulation but is also a physiological response, evoked by a non-pathogenic insult and apparently designed to counteract the insult-related toxicity which is induced in part by essential physiological compounds present in excess of their normal levels. It appears that when the buffering capacity of constitutive local mechanisms (transporters, enzymes, etc.) that normally regulate these compounds is exceeded, assistance is recruited from the immune system. Like the overactive physiological compounds themselves, the immune system needs to be rigorously regulated in order to produce adequate phagocytic activity and the required quantity of cytokines and growth factors at the right time and place. Boosting of this autoimmune response is potentially a powerful strategy for neuroprotective therapy.

Nerve injury causes degeneration of directly injured neurons and the damage spreads to neighboring neurons. Research on containing the damage has been mainly pharmacological, and has not recruited the immune system. We recently discovered that after traumatic injury to the central nervous system (spinal cord or optic nerve), the immune system apparently recognizes certain injury-associated self-compounds as potentially destructive and comes to the rescue with a protective antiself response mediated by a T-cell subpopulation that can recognize self-antigens. We further showed that individuals differ in their ability to manifest this protective autoimmunity, which is correlated with their ability to resist the development of autoimmune diseases. This finding led us to suggest that the antiself response must be tightly regulated to be expressed in a beneficial rather than a destructive way. In seeking to develop a neuroprotective therapy by boosting the beneficial autoimmune response to injury-associated self-antigens, we looked for an antigen that would not induce an autoimmune disease. Candidate vaccines were the safe synthetic copolymer Cop-1, known to cross-react with self-antigens, or altered myelin-derived peptides. Using these compounds as vaccines, we could safely boost the protective autoimmune response in animal models of acute and chronic insults of mechanical or biochemical origin. Since this vaccination is effective even when given after the insult, and because it protects against the toxicity of glutamate (the most common mediator of secondary degeneration), it can be used to treat chronic neurodegenerative disorders such as glaucoma, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

The innate and adaptive arms of the immune system, represented principally by macrophages and by T and B cells, respectively, provide body tissues with mechanisms of defence, protection and repair. In the central nervous system (CNS), probably because of its status of 'immune privilege', any immune activity has long been viewed as detrimental. Recent studies have provided evidence, however, that immune activity after traumatic CNS injury may have a beneficial effect, manifested by promotion of regeneration and reduction in the secondary degeneration of neurons that escaped direct injury. Rigorous regulation of immune system activity allows the individual to derive the benefit of such neuroprotection without the risk of detrimental side effects. Recently, our research group found a way to boost the T-cell-mediated autoimmune protection while avoiding the risk of autoimmune disease.

Neuronal degeneration after traumatic injury to the central nervous system (CNS) can be reduced by active immunization or passive transfer of T cells against CNS-associated myelin antigens. We propose that a protective autoimmunity is evoked by CNS insult when non-immunological local protective mechanisms cannot adequately buffer the injury-induced toxicity. The ability of a particular strain to develop a protective autoimmune response appears to be inversely related to its susceptibility to autoimmune disease. We also propose that vaccination with specific CNS-derived 'safe' (non-pathogenic) peptides after traumatic CNS insult, and possibly at any stage of chronic neurodegenerative disease, can be used to boost the protective autoimmunity and thereby to reduce further injury-induced damage. Such therapeutic vaccination ensures that the augmented beneficial autoimmunity will be free of accompanying disease.

Partial injury to the spinal cord can propagate itself, sometimes leading to paralysis attributable to degeneration of initially undamaged neurons. We demonstrated recently that autoimmune T cells directed against the CNS antigen myelin basic protein (MBP) reduce degeneration after optic nerve crush injury in rats. Here we show that not only transfer of T cells but also active immunization with MBP promotes recovery from spinal cord injury. Anesthetized adult Lewis rats subjected to spinal cord contusion at T7 or T9, using the New York University impactor, were injected systemically with anti-MBP T cells at the time of contusion or 1 week later. Another group of rats was immunized, 1 week before contusion, with MBP emulsified in incomplete Freund's adjuvant (IFA). Functional recovery was assessed in a randomized, double-blinded manner, using the open-field behavioral test of Basso, Beattie, and Bresnahan. The functional outcome of contusion at T7 differed from that at T9 (2.9 ± 0.4, n = 25, compared with 8.3 ± 0.4, n = 12; p < 0.003). In both cases, a single T cell treatment resulted in significantly better recovery than that observed in control rats treated with T cells directed against the nonself antigen ovalbumin. Delayed treatment with T cells (1 week after contusion) resulted in significantly better recovery (7.0 ± 1; n = 6) than that observed in control rats treated with PBS (2.0 ± 0.8; n = 6; p < 0.01; nonparametric ANOVA). Rats immunized with MBP obtained a recovery score of 6.1 ± 0.8 (n = 6) compared with a score of 3.0 ± 0.8 (n = 5; p < 0.05) in control rats injected with PBS in IFA. Morphometric analysis, immunohistochemical staining, and diffusion anisotropy magnetic resonance imaging showed that the behavioral outcome was correlated with tissue preservation. The results suggest that T cell-mediated immune activity, achieved by either adoptive transfer or active immunization, enhances recovery from spinal cord injury by conferring effective neuroprotection. The autoimmune T cells, once reactivated at the lesion site through recognition of their specific antigen, are a potential source of various protective factors whose production is locally regulated.

We recently demonstrated that autoimmune T cells protect neurons from secondary degeneration after central nervous system (CNS) axotomy in rats. Here we show, using both morphological and electrophysiological analyses, that the neuroprotection is long-lasting and is manifested functionally. After partial crush injury of the rat optic nerve, systemic injection of autoimmune T cells specific to myelin basic protein significantly diminished the loss of retinal ganglion cells and conducting axons, and significantly retarded the loss of the visual response evoked by light stimulation. These results support our challenge to the traditional concept of autoimmunity as always harmful, and suggest that in certain situations T cell autoimmunity may actually be beneficial. It might be possible to employ T cell intervention to slow down functional loss in the injured CNS. (C) 2000 Published by Elsevier Science B.V.

We recently reported that the posttraumatic spread of degeneration in the damaged optic nerve can be attenuated by the adoptive transfer of autoimmune T cells specific to myelin basic protein. However, it would be desirable to obtain immune neuroprotection free of any possible autoimmune disease. In an attempt to obtain disease-free immune neuroprotection, we used the synthetic four-amino acid polymer copolymer 1 (Cop-1), which is known not to be encephalitogenic despite its cross-reactivity with myelin basic protein. We show here that active immunization with Cop-1 administered in adjuvant, as well as adoptive transfer of T cells reactive to Cop-1, can inhibit the progression of secondary degeneration after crush injury of the rat optic nerve. These results have implications for the treatment of optic neuropathies.

Autoimmunity is usually considered only as a cause of disease; nevertheless, human T-cell repertoires are filled naturally with autoimmune lymphocytes. Here, we review evidence that autoimmune T cells can help heal damaged tissues, indicating that natural autoimmunity could also be a cause of health.

It is now commonly accepted that glaucoma is a neurodegenerative disease of the optic nerve. Thus, at any given time, there are neurons that, though still viable, are vulnerable to the hostile extracellular milieu and are therefore amenable to neuroprotective therapy. Neuroprotection refers to any intervention, either external to the optic nerve or internally, that will lead to an intracellular change in the balance between survival and death signals in favor of survival. Several potential sites and modalities for such intervention may exist. When designing neuroprotective therapy, ways must be sought to recruit the physiologic self-repair mechanisms awakened by the primary or secondary risk factors. These mechanisms appear to be insufficiently effective when in their natural state, but they may be simulated or boosted by appropriate therapeutic compounds or cells. (C) 2000 Lippincott Williams and Wilkins, Inc.

Autoimmune T cells against central nervous system myelin associated peptide reduce the spread of damage and promote recovery in injured rat spinal cord, findings that might lead to neuroprotective cell therapy without risk of autoimmune disease.

PURPOSE. To establish a method for morphometric analysis of retrogradely labeled retinal ganglion cells (RGCs) of the mouse retina, to be used for the study of molecular aspects of RGC survival and neuroprotection in this model; to evaluate the effect of overexpression of Cu-Zn-superoxide dismutase (CuZnSOD) on RGC survival after severe crush injury to the optic nerve, and to assess the effect of the α2-adrenoreceptor agonist brimonidine, recently shown to be neuroprotective, on RGC survival. METHODS. A severe crush injury was inflicted unilaterally in the orbital portion of the optic nerves of wild-type and transgenic (Tg-SOD) mice expressing three to four times more human CuZnSOD than the wild type. In each mouse all RGCs were labeled 72 hours before crush injury by stereotactic injection of the neurotracer dye FluoroGold (Fluorochrome, Denver, CO) into the superior colliculus. Survival of RGCs was then assessed morphometrically, with and without systemic injection of brimonidine. RESULTS. Two weeks after crush injury, the number of surviving RGCs was significantly lower in the Tg-SOD mice (596.6 ± 71.9 cells/mm²) than in the wild-type control mice (863.5 ± 68 cells/mm²). There was no difference between the numbers of surviving RGCs in the uninjured retinas of the two strains (3708 ± 231.3 cells/mm² and 3904 ± 120 cells/mm², respectively). Systemic injections of brimonidine significantly reduced cell death in the Tg-SOD mice, but not in the wild type. CONCLUSIONS. Overexpression of CuZnSOD accelerates RGC death after optic nerve injury in mice. Activation of the α2-adrenoreceptor pathway by brimonidine enhances survival of RGCs in an in vivo transgenic model of excessive oxidative stress.

Keywords: CENTRAL-NERVOUS-SYSTEM; SPINAL-CORD INJURY; PERIPHERAL-NERVE; T-CELLS; WALLERIAN DEGENERATION; MACROPHAGE RESPONSES; AXONAL REGENERATION; WOUND REPAIR; RECOVERY; INFLAMMATION

Injuries of the central nervous system (CNS) lead to an inevitable and irreversible loss of function because of the lack of neurogenesis, poor regeneration, and the spread of degeneration. In most tissues, protection and repair are the function of the immune system. It has long been thought that this does not apply to the CNS, where - because of its immune-privileged character - any immune activity was assumed to be detrimental. We have recently proposed, however, that provided care is taken to avoid the attendant risks, both repair and protection of injured CNS neurons can benefit from immune intervention. In the following I will summarize the data that led to this concept and describe the evidence supporting it.

Traumatic injury to the optic nerve is accompanied by a process of degeneration that leads to apoptotic death of the corresponding cell bodies. In the traumatized nerves, a process of self-propagating degeneration takes place and is mediated by toxic substances, some of them well-documented and others not yet identified. Using the partially injured rat optic nerve as a model, we show that glutamate is in above-normal concentrations in the aqueous humor of eyes with impaired optic nerves. Neurons which had escaped the primary insult and were still viable were more susceptible to glutamate than normal uninjured nerves, and were likely to die as a result of exposure to glutamate even at low concentrations. A transient accumulation of autoimmune T cells was observed at the site of the optic nerve injury. We show that autoimmune T cells that react specifically with myelin basic protein and are known to be detrimental to the central nervous system (CNS) are beneficial in reducing the deficit caused by secondary degeneration. The protective effect of the T cells is demonstrated both morphologically and functionally. We suggest that these autoimmune T cells may induce a benign immune response to pathogen-free CNS lesions, and that it may be possible to boost this response therapeutically in a way that will allow the neuroprotective effect to be manifested without incurring the risk of autoimmune disease.

Degenerative diseases of the central nervous system (CNS) are characterized by progressive degeneration, which continues even after the primary causative factor has been identified and neutralized or removed. The progressive degeneration is thought to be a self-perpetuating process, attributable in part to factors derived from the degenerating nerves themselves. If this is so, the mediators of toxicity causing secondary degeneration in the various degenerative diseases are likely to be similar. Common mediators include excitatory amino acids, compounds that cause oxidative or metabolic stress, and factors that disturb the ionic balance of the nerve's extracellular milieu. It is proposed here, based on recent work by the author and by others on CNS trauma and autoimmunity, as well as on accumulated information about the professional role of the adaptive and the immune response in general, that active or passive T cell-mediated autoimmunity directed against self-antigens associated with the disorder will be beneficial in halting the spread of damage. (C) 2000 Wiley-Liss, Inc.

Neurotrophins (NTs) promote neuronal survival and maintenance during development and after injury. However, their role in the communication between the nervous system and the immune system is not yet clear. We observed recently that passively transferred activated T cells of various antigen specificities home to the injured central nervous system (CNS), yet only autoimmune T cells specific to a CNS antigen, myelin basic protein (MBP), protect neurons from secondary degeneration after crush injury of the rat optic nerve. Here we examined the involvement of NTs in T-cell-mediated neuroprotection, and the possible significance of the antigen specificity of the T cells in this activity. Analysis of cytokine and NT expression in various rat T cell lines showed that the T cells express mRNA for cytokines of Th1, Th2, and Th3 phenotypes. In addition, the T cells express mRNA and protein specific to nerve growth factor, brain-derived neurotrophic factor, NT-3, and NT-4/5. Antigen activation significantly increased NT secretion. Thus, reactivation of CNS autoimmune T cells by locally presented antigens to which they are specific can lead to enhanced secretion of NTs and possibly also of other factors in injured optic nerves. mRNA for Trka, TrkB and p75 receptors was expressed in the injured nerve, suggesting that these specific receptors can mediate the effects of the T-cell-derived NTs. The neuroprotective effect of the passively transferred autoimmune anti-MBP T cells in injured optic nerves was significantly decreased after local application of a tyrosine kinase inhibitor known to be associated with NT-receptor activity. These results suggest that the neuroprotective effect of autoimmune T cells involves the secretion of factors such as NTs by the T cells reactivated by their specific antigen in the injured CNS. T cell intervention in the injured CNS might prove to be a useful means of promoting post-injury CNS maintenance and recovery, possibly via supply of NTs and other factors. (C) 2000 Academic Press.

The genesis of immune privilege high in the evolutionary tree suggests that immune privilege is necessary, if not advantageous for the progressive development of the CNS. Upon reaching a certain degree of complexity, it seems as if the CNS was obliged to restrain the immune system from penetrating the blood-brain barrier. CNS autoimmunity against myelin proteins is known to be a contributory factor in the pathophysiology of multiple sclerosis and in the animal model of experimental autoimmune encephalomyelitis (EAE) (). Such autoimmunity has therefore been regarded as detrimental and hence obviously undesirable. However, recent findings in our laboratory suggest that T-cell autoimmunity to CNS self-antigens (Moalem et al., 1999), if expressed at the right time and the right place, can do much good in the CNS. We shall review the experiments briefly, and then discuss their implications for our understanding of immune privilege and CNS maintenance after injury. Copyright (C) 1999 Elsevier Science B.V.

Neurodegenerative diseases are characterized by a relentless loss of specific groups of neuronal subtypes. Many of these diseases share similar molecular mechanisms and extracellular mediators of neuronal loss. We now suggest that neurodegeneration originating in the neuronal cell bodies (e.g. in Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis) should be distinguished from that originating in the axons (e.g. in glaucoma, certain peripheral neuropathies and spinal stenosis). We propose that the former group of diseases be defined as 'somagenic' and the latter as 'axogenic'. Although axogenic disorders may share common symptoms and mediators of toxicity with somagenic disorders, they have distinct temporal, subcellular and signal-transduction features. We further suggest that, by adopting this classification of disorders based on pathophysiological processes, we will come to recognize additional diseases (in particular, those defined as axogenic) as being neurodegenerative and therefore possibly amenable to neuroprotective therapy.

The limitation of immune responsiveness in the mammalian CNS has been attributed to the intricate nature of neuronal networks, which would appear to be more susceptible than other tissues to the threat of permanent disorganization when exposed to massive inflammation. This line of logic led to the conclusion that all forms of CNS inflammation would do more harm than good and, hence, the less immune intervention the better. However, mounting evidence indicates that some forms of immune-system intervention can help to protect or restore CNS integrity. We have shown that the innate immune system, represented by activated macrophages, can facilitate the processes of regeneration in the severed spinal cord. More recently, we found that autoimmune T cells that are specific for a component of myelin can protect CNS neurons from the catastrophic secondary degeneration, which extends traumatic lesions to adjacent CNS areas that did not suffer direct damage. The challenge, therefore, is to learn how to modify immune interactions in the traumatized CNS in order to promote its post-injury maintenance and repair.

THE FAILURE OF the adult mammalian central nervous system (CNS) to regenerate after injury has long been viewed as a unique phenomenon resulting from the specific nature of this system. The finding that some CNS axons could be induced to regrow if provided with a permissive environment suggested that this failure is a result, at least in part, of the nature of the postinjury neuronal environment. It was further shown that the involvement of inflammatory cells, particularly macrophages, in postinjury processes in the CNS is limited. We have suggested that, to achieve recovery after injury, the adult mammalian CNS may require the assistance of the same postinjury factors as those involved in the recovery of spontaneously regenerating systems but that its accessibility to such assistance is restricted. Accordingly, we proposed that it might be possible to circumvent the restriction, allowing regeneration to occur. We showed that the implantation of autologous macrophages, which had been prestimulated by exposure to a regenerative (sciatic) nerve, into completely transected spinal cords of adult rats led to partial motor recovery. This treatment intervenes in the postinjury process by simulating in the axotomized CNS the events that occur naturally in spontaneously regenerating systems.

It is now widely accepted that injured nerves, like any other injured tissue, need assistance from their extracellular milieu in order to heal. We compared the postinjury activities of thrombin and gelatinases, two types of proteolytic activities known to be critically involved in tissue healing, in nonregenerative (rat optic nerve) and regenerative (fish optic nerve and rat sciatic nerve) neural tissue. Unlike gelatinases, whose induction pattern was comparable in all three nerves, thrombin-like activity differed clearly between regenerating and nonregenerating nervous systems. Postinjury levels of this latter activity seem to dictate whether it will display beneficial or detrimental effects on the capacity of the tissue for repair. The results of this study further highlight the fact that tissue repair and nerve regeneration are closely linked and that substances that are not unique to the nervous system, but participate in wound healing in general, are also crucial for regeneration or its failure in the nervous system.

The adult mammalian central nervous system (CNS) fails to regenerate its axons following injury. A comparison between its postinjury response and that of axons of nervous systems capable of regeneration reveals major differences with respect to inflammation. In regenerative systems, a large number of macrophages rapidly invade the injured site during the first few hours and days after the injury. Following their activation/differentiation through interaction with the host tissue, they play a central role in tissue healing through phagocytosis of cell debris and communication with cellular and molecular elements of the damaged tissue. Relative to the peripheral nervous system (PNS), macrophage recruitment in the adult mammalian CNS is delayed and is restricted in amount and activity. It was recently proposed that in injured mammalian CNS tissue, implantation of macrophages stimulated by prior co-culture with segments of peripheral (sciatic) nerves can compensate, at least in part, for the restricted postinjury inflammatory reaction. In the present study, this experimental paradigm is further explored and shows that there is no conflict between the systemic use of anti-inflammatory compounds and treatment with stimulated macrophages to promote regrowth of neuronal tissue.

In order to study the course of optic nerve degeneration and devise possible ways to achieve neuroprotection, a well-controlled, animal model of partial crush injury of the optic nerve was used. Following the controlled partial crush injury of the rat optic nerve, quantitative morphological and electrophysiological measurements were made of primary and secondary neuronal losses. The neuroprotective effects of NMDA-receptor antagonists and α₂- adrenoreceptor agonists were also studied. The results suggested that the ongoing progression of the optic nerve degeneration in glaucoma might be a consequence of the toxic extracellular environment produced by neurons that degenerate as a result of the primary cause of the disease (such as increased IOP).

The irreversible loss of function after axonal injury in the central nervous system (CNS) is a result of the lack of neurogenesis, poor regeneration, and the spread of damage caused by toxicity emanating from the degenerating axons to uninjured neurons in the vicinity. Now, 100 years after Ramon y Cajal's discovery that CNS neurons - unlike neurons of the peripheral nervous system fail to regenerate, it has become evident that (a) CNS tissue is indeed capable of regenerating, at least in part, provided that it acquires the appropriate conditions for growth support, and (b) that the spread of damage can be stopped and the postinjury rescue of neurons thus achieved, if ways are found to neutralize the mediators of toxicity, either by inhibiting their action or by increasing tissue resistance to them. In most physiological systems the processes of tissue maintenance and repair depend on the active assistance of immune cells. In the CNS, however, communication with the immune system is restricted. The accumulated evidence from our previous studies suggests that the poor posttraumatic repair and maintenance in the CNS is due at least in part to this restriction. Key factors in the recovery of injured tissues, but missing or deficient in the CNS, are the processes of recruitment and activation of immune cells. We therefore propose the development of immune cell therapies in which the injured CNS is exogenously provided with an adequate number of appropriately activated immune cells (macrophages for regrowth and autoimmune T cells for maintenance), controlled in such a way as to derive maximal benefit with minimal risk of disease. It is expected that these self-adjusting cells will communicate with the damaged tissue, monitor tissue needs, and control the dynamic course of CNS healing.

Autoimmunity to antigens of the central nervous system is usually considered detrimental. T cells specific to a central nervous system self antigen, such as myelin basic protein, can indeed induce experimental autoimmune encephalomyelitis, but such T cells may nevertheless appear in the blood of healthy individuals. We show here that autoimmune T cells specific to myelin basic protein can protect injured central nervous system neurons from secondary degeneration. After a partial crush injury of the optic nerve, rats injected with activated anti-myelin basic protein T cells retained approximately 300% more retinal ganglion cells with functionally intact axons than did rats injected with activated T cells specific for other antigens. Electrophysiological analysis confirmed this finding and suggested that the neuroprotection could result from a transient reduction in energy requirements owing to a transient reduction in nerve activity. These findings indicate that T-cell autoimmunity in the central nervous system, under certain circumstances, can exert a beneficial effect by protecting injured neurons from the spread of damage.

The central nervous system (CNS), unlike the peripheral nervous system (PNS), is an immune-privileged site in which local immune responses are restricted. Whereas immune privilege in the intact CNS has been studied intensively, little is known about its effects after trauma. In this study, we examined the influence of CNS immune privilege on T cell response to central nerve injury. Immunocytochemistry revealed a significantly greater accumulation of endogenous T cells in the injured rat sciatic nerve than in the injured rat optic nerve (representing PNS and CNS white matter trauma, respectively). Use of the in situ terminal deoxytransferase-catalyzed DNA nick end labeling (TUNEL) procedure revealed extensive death of accumulating T cells in injured CNS nerves as well as in CNS nerves of rats with acute experimental autoimmune encephalomyelitis, but not in injured PNS nerves. Although Fas ligand (FasL) protein was expressed in white matter tissue of both systems, it was more pronounced in the CNS. Expression of major histocompatibility complex (MHC) class II antigens was found to be constitutive in the PNS, but in the CNS was induced only after injury. Our findings suggest that the T cell response to central nerve injury is restricted by the reduced expression of MHC class II antigens, the pronounced FasL expression, and the elimination of infiltrating lymphocytes through cell death.

We have previously demonstrated that the failure of the mammalian central nervous system (CNS) to regenerate following axonal injury is related to its immunosuppressive nature, which restricts the ability of both recruited blood-borne monocytes and CNS-resident microglia to support a process of repair. In this study we show that transected optic nerve transplanted with macrophages stimulated by spontaneously regenerating nerve tissue, e.g., segments of peripheral nerve (sciatic nerve), exhibit axonal regrowth at least as far as the optic chiasma. Axonal regrowth was confirmed by double retrograde labeling of the injured optic axons, visualized in their cell bodies. Transplanted macrophages exposed to segments of CNS (optic) nerve were significantly less effective in inducing regrowth. Immunocytochemical analysis showed that the induced regrowth was correlated with a wide distribution of macrophages within the transplanted-transected nerves. It was also correlated with an enhanced clearance of myelin, known to be inhibitory for regrowth and poorly eliminated after injury in the CNS. These results suggest that healing of the injured mammalian CNS, like healing of any other injured tissue, requires the partnership of the immune system, which is normally restricted, but that the restriction can be circumvented by transplantation of peripheral nerve-stimulated macrophages.

GTP and Ca²⁺, two well-known modulators of intracellular signaling pathways, control a structural/functional switch between two vital and mutually exclusive activities, cross-linking and G(α) activity, in the same enzyme. The enzyme, a brain-derived tissue-type transglutaminase (TGase), was recently cloned by us in two forms, one of which (s-TGN) lacks a C-terminal region that is present in the other (1-TGN). Immunoreaction with antibodies directed against a peptide present in the C-terminus of 1-TGN but missing in s-TGN suggested that this site, which is located in the C-terminal fourth domain, undergoes conformational changes as a result of interaction between 1-TGN and GTP. Site-directed mutagenesis suggested that the third domain is involved in mediating the inhibition of the cross-linking activity. These results were supported by molecular modeling, which further suggested that domains III and IV both participate in conformational changes leading to the functional switch between the Ca²⁺-dependent cross-linking activity and the G(α) activity.

Neuroprotective therapy is a relatively new development in the approach to the treatment of acute and chronic brain damage. Though initially viewed in the framework of acute CNS injuries, the concept was recently extended to include chronic injuries, in which at any given time there are some neurons in an acute phase of degeneration coexisting with others that are healthy, marginally damaged, or dead. The healthy neurons and those that are only marginally damaged are the potential targets for neuroprotection. For the development of neuroprotective therapies, it is essential to employ an animal model in which the damage resulting from secondary degeneration can be quantitatively distinguished from primary degeneration. This is of particular relevance when the site of the damage is in the white matter (nerve fibers) rather than in the gray matter (cell bodies). In the present work we reexamine the concepts of secondary degeneration and neuroprotection in white matter lesions. Using a partial crush injury of the adult rat optic nerve as a model, we were able to assess both primary and secondary nerve damage. We show that neurons whose axons were not damaged or only marginally damaged after an acute insult will eventually degenerate as a consequence of their existence in the degenerative environment produced by the injury. This secondary degeneration does not occur in all of the neurons at once, but affects them in a stepwise fashion related to the severity of the damage inflicted. These findings, which may be applicable to the progression of acute or chronic neuropathy, imply that neuroprotective therapy may have a beneficial effect even if there is a time lag between injury and treatment.

The central nervous system (CNS) enjoys a unique relationship with the immune system. Under non-pathological conditions, T cells move through the CNS but do not accumulate there. CNS trauma has been shown to trigger a response to CNS self-antigens such as myelin basic protein (MBP). Here, we examined whether the injured CNS tissue undergoes changes that permit T cell accumulation. We found that injury to CNS white matter, such as the optic nerve, led to a transiently increased accumulation of T cells (between days 3 and 21). In Lewis rats with unilaterally injured optic nerves, systemic administration of passively transferred T cells recognizing either self- antigen (MBP) or non-self-antigen (ovalbumin) resulted in accumulation of the T cells in injured optic nerve, irrespective of their antigenic specificity. The effect of the T cells on the damaged nerve, the lack of selectivity in T cell accumulation and the mechanism underlying non-selective accumulation are discussed.

The poor ability of injured central nervous system (CNS) axons to regenerate has been correlated, at least partially, with a limited and suppressed postinjury inflammatory response. A key cell type in the inflammatory process is the macrophage, which can respond in various ways, depending on the conditions of stimulation. The aim of this study is to compare the activities of macrophages or microgila when encountering CNS and peripheral nervous systems (PNS), on the assumption that nerve-related differences in the inflammatory response may have implications for tissue repair and thus for nerve regeneration. Phagocytic activity of macrophages or of isolated brain-derived microglia was enhanced upon their exposure to sciatic (PNS) nerve segments, but inhibited by exposure to optic (CNS) nerve segments. Similarly, nitric oxide production by macrophages or microglia was induced by sciatic nerve segments but not by optic nerve segments. The previously demonstrated presence of a resident inhibitory activity in CNS nerve, could account, at least in part, for the inhibited phagocytic activity of blood-borne macrophages in CNS nerve as well as of microglia resident in the brain. It seems that the CNS microglia are reversibly immunosuppressed by the CNS environment, at least with respect to the activities examined here. It also appears from this study that the weak induction of early healing-related activities of macrophages/microglia in the environment of CNS might explain the subsequent failure of this environment to acquire growth-supportive properties in temporal and spatial synchrony with the needs of regrowing axons.

Postinjury recovery in most tissues requires an effective dialog with macrophages; however, in the mammalian central nervous system, this dialog may be restricted (possibly due to its immune-privileged status), which probably contributes to its regeneration failure. We circumvented this by implanting macrophages, pre-exposed ex vivo to peripheral nerve segments, into transected rat spinal cord. This stimulated tissue repair and partial recovery of motor function, manifested behaviorally by movement of hind limbs, plantar placement of the paws and weight support, and electrophysiologically by cortically evoked hind-limb muscle response. We substantiated these findings immunohistochemically by demonstrating continuity of labeled nerve fibers across the transected site, and by tracing descending fibers distally to it by anterograde labeling. In recovered rats, retransection of the cord above the primary transection site led to loss of recovery, indicating the involvement of long descending spinal tracts. Injection of macrophages into the site of injury is relatively non-invasive and, as the cells are autologous, it may be developed into a clinical therapy.

Axons in the central nervous system (CNS) of adult mammals do not regenerate after injury. Mammalian CNS differs in this respect from other mammalian tissues, including the peripheral nervous system (PNS), and from the CNS of lower vertebrates. In most parts of the body, including the nervous system, injury triggers an inflammatory reaction involving macrophages. This reaction is needed for tissue healing; when it is delayed or insufficient, healing is incomplete. The CNS, although needing an efficient inflammatory reaction resembling that in the periphery for tissue healing, appears to have lost the ability to supply it. We suggest that restricted CNS recruitment and activation of macrophages are linked to regeneration failure and might reflect the immune privilege that characterizes the mammalian CNS. As macrophages play a critical role in tissue restoration, and because their recruitment and activation are among the most upstream of the events leading to tissue healing, overcoming the deficiencies in these steps might trigger a self-repair process leading to recovery after CNS injury.

Background: Acute partial lesion of the rat optic nerve, although not a model for glaucoma, mimics some of the features of the disease. Objective: To learn whether degeneration of rat optic nerve fibers and death of their retinal ganglion cells induced by an acute partial lesion are associated with elevated levels of glutamate, known to occur in the eyes of humans and monkeys with glaucoma. Materials and Methods: Rat optic nerve was subjected to a partial crush injury. Aqueous humor samples were aspirated from the anterior and posterior aqueous chambers at specified times and their amino acid contents were determined by means of high-performance liquid chromatography. Results: Three and 7 days after injury, intraocular glutamate and aspartate levels were found to be significantly higher than in normal or sham-operated-on eyes, and returned to normal by day 14. Conclusions: Degeneration of the optic nerve, induced by a mechanical injury of the axons, leads to intraocular elevation of glutamate and aspartate levels. These results illustrate that the model of the partial optic nerve lesion exhibits another feature typical of a long-term optic neuropathy, such as glaucoma.

Accumulating evidence points to the existence of a mechanism that may explain wily glaucomatous neuropathy continues to progress even after its primary cause, e.g., high intraocular pressure, has been alleviated or attenuated. We suggest that such a mechanism involves processes collectively termed secondary degeneration, an inevitable outcome of acute injury of the central nervous system. Secondary degeneration refers to the spread of degeneration to apparently healthy neurons that escaped the primary insult, but are adjacent to the injured neurons and are thus exposed to the degenerative milieu that the latter create. Neuroprotection, i.e., protection of undamaged neurons from secondary degeneration, would therefore require that the extracellular elements associated with the degeneration be neutralized, balanced off, or inhibited. In seeking an experimental framework for testing treatment modalities for neuroprotection, we have developed all animal model in which a well-calibrated, reproducible, partial lesion is inflicted on the optic nerve of the adult rat. Using this model, the extent of the primary damage can be quantified and the secondary degeneration demonstrated and assessed. Damage inflicted directly on the optic nerve fibers inevitably leads to their degeneration and the eventual death of their cell bottles. Over time, neurons that initially escaped the injury undergo self-perpetuating secondary degeneration, the extent of which is a function of the severity of the primary insult. We suggest that a similar mechanism may underlie the propagation of damage seen in glaucoma at any given time after alleviation of the primary cause of the disease, and might explain why patients with severe pre-existing damage are much more likely to deteriorate even if their intraocular pressure is the same or lower than that of patients without visual loss at the time of diagnosis. The model can be used to screen compounds for their efficacy in protecting initially spared neurons from undergoing secondary degeneration, thereby achieving a better functional outcome. The findings obtained using this model support the attempt to develop neuroprotective therapy for glaucoma. Such therapy would need to be applied in combination with treatment (e.g., antihypertensive therapy) directed against the primary cause of the neuropathy.

Recent findings have led to changes in the traditional concept of nerve recovery, including the realization that injured nerves, like any other injured tissue, need the assistance of blood-derived cells and factors in order to heal. We show that factor XIIIa (FXIIIa, the potentially active a₂- subunit of factor XIII), an enzyme that participates in blood coagulation by stabilizing the fibrin clot, is also active in the nervous system where it may play a key role in the healing of injured tissue. We demonstrate that the plasma, macrophages and nerves of fish contain a 55 kDa form of transglutaminase that cross-reacts immunologically with the a-subunit of FXIII in mammals (80 kDa). The fish enzyme in the plasma, unlike its mammalian counterpart, is active, pointing to a difference in control of the coagulation pathway in the two species. Analysis of FXIIIa expression in mammalian neural tissues and their response to injury revealed high levels of the enzyme in media conditioned by peripheral nerves as compared with medium conditioned by nerves of the central nervous system. Furthermore, similarity was observed in the postinjury behavior of FXIIIa in regenerating nerve tissues (peripheral nervous system of mammals and the central nervous system of fish). We suggest that the postinjury level of factor XIIIa in the nervous system may be related to the tissue's regenerative capacity, and that FXIIIa may therefore be a link underlying a possible association between the processes of blood coagulation and nerve healing.

Damage resulting from a partial acute lesion of white matter in the central nervous system (CNS) gradually spreads also to neurons that escaped the primary injury, resulting in their degeneration. Such spreading has been referred to as secondary degeneration. In order to demonstrate that this degeneration is indeed secondary to that caused by the acute insult, as well as to investigate the mechanism underlying the spread of damage and ways in which to protect neurons from such damage, we have proposed the use of partial lesion of the rodent optic nerve as a model. In this model we examined whether an antagonist of a receptor-mediated channel, shown to be beneficial in gray matter lesions, can protect neurons from undergoing secondary degeneration following white matter legion. A well-calibrated partial crush lesion inflicted on the optic nerve of adult rats was immediately followed by a single intraperitoneal injection of the N-methyl- D-aspartate receptor antagonist, MK-801 (1 mg/kg). Protection of neurons from secondary degeneration was assessed by retrograde labeling and by measurement of the visual evoked potential (VEP) response to light. Two weeks after the injury, the mean number of neurons that were still intact was about threefold higher in the MK-801-treated group than in the saline-treated control group, indicating a treatment-induced protection of neurons that had escaped primary injury. A positive VEP response to light was obtained in 90% of the MK-801 treated animals and in only 50% of injured controls. The questions regarding whether the secondary degeneration of initially spared neurons starts in their cell bodies or in their axons, and consequently the identity of the primary site of their protection by MK-801, are discussed in relation to the absence of N-methyl-D-aspartate receptors on nerve fibers. The present findings may have implications for both acute and chronic injuries of the CNS.Errata: Journal of Neurotrauma Volume 16, Issue 4, Pages 345.1999

Keywords: Cell Biology; Neurosciences

Tissue-type transglutaminases (TGases) were recently shown to exert dual enzymatic activities; they catalyze the posttranslational modification of proteins by transamidation, and they also act as guanosine triphosphatase (GTPase). Here we show that a tissue-type TGase is expressed in rat brain astrocytes in vitro, and is induced by the inflammation-associated cytokines interleukin-1β and to a lesser extent by tumor necrosis factor-α. Induction is accompanied by overexpression and appearance of an additional shorter clone, which does not contain the long 3'-untranslated region and encodes for a novel TGase enzyme whose C terminus lacks a site that affects the enzyme's interaction with guanosine triphosphate (GTP). Expression of two clones revealed that the long form is inhibited noncompetitively by GTP, but the short form significantly less so. The different affinities for GTP may account for the difference in physiological function between these two enzymes.Errata:Page 3725, right column, and page 3726, left column: The putrescine concentrations should be expressed in μM instead of mM. The corrected sentences are as follows:The reaction mixture contained 50 mM Tris-HCl, pH 8.0, 5 mM dithiothreitol, 5 mM CaCl₂, 0.37 μM putrescine (1:5, [³H]putrescine:putrescine), increasing amounts of N,N-dimethylated casein (0.057 ± 5.7 mg/ml) and samples of HEK-293 cytosolic fraction.In the GTP experiments, 0.075 μM putrescine was used.

In this study we present a method to achieve a complete transection of optic nerve axons in adult rat, while preserving the vasculature and retaining the continuity of the meninges. Under deep anesthesia, the optic nerve of adult rat is exposed. Using specially designed instruments built from disposable glass microsampling pipets, a small opening is created in the meninges of the optic nerve, 2-3 mm behind the eye globe. A glass dissector is introduced through the opening and is used to cut all the axons through the whole width of the nerve. Complete transfection of the optic nerve axons was achieved, while retaining the continuity of the meninges and avoiding damage to the nerve's vascular supply. Transection was confirmed by transillumination showing a complete gap in the continuity of the nerve axons, and by both morphological and electrophysiological criteria. Nerve transection performed by the conventional technique leads to neuroma formation and hampers regeneration. Crush injury may cause nerve ischemia, which is detrimental to axonal recovery. Both of these disadvantages are avoided by the method of transection presented here. The opening created in the 'meningeal tube' can be used to inject substances that may be of benefit in recovery, rescue and/or regeneration of the injured axons. The model is particularly suitable for in vivo studies on nerve regeneration, and especially for screening of putative therapeutic agents.

Preclinical studies of brimonidine show that it is a potent alpha(2)-adrenoceptor agonist that is 1000-fold more selective for the alpha(2)- vs. the alpha(1)-adrenoceptor, and is 7-12-fold more alpha(2)-selective than clonidine and 23- to 32-fold more alpha(2)-selective than apraclonidine (p-aminoclonidine). Brimonidine decreased intraocular pressure (IOP) in various animal models but, unlike apraclonidine, brimonidine was not mydriatic. The site and pharmacology of the IOP response depends on the animal species. In rabbits, the IOP response to brimonidine is mediated by an ocular alpha(2)-adrenoceptor while in monkeys, a central nervous system (CNS) 'imidazoline' receptor appears to be involved. Brimonidine decreased IOP by suppressing the rate of aqueous humor flow and enhancing uveoscleral outflow. Topical brimonidine resulted in posterior segment drug levels adequate to activate alpha(2)-adrenoceptors, but was not vasoconstrictive in a model designed to assess the vasoactivity of the human retinal microvasculature. Brimonidine protected the rat optic nerve from secondary damage following mechanical injury to the optic nerve and was nontoxic in an array of experiments designed to evaluate ocular and organ toxicity. Taken together, the high alpha(2)-adrenoceptor selectivity, ocular hypotensive efficacy, retinal bioavailability and neuroprotective properties make brimonidine an important addition to the field of antiglaucoma agents.

Purpose: To determine if α₂-adrenoceptor agonists have potential therapeutic activity in reducing injury-induced secondary degeneration of mammalian optic nerve. Methods: By using a well-calibrated partial injury of the rat optic nerve as a model it is possible to determine in terms of morphological and electrophysiological criteria, the extent of rescue of fibers that have escaped the primary lesion and hence to examine the potential neurorescue of different compounds. The survival parameters examined were compound action potential (CAP), visual evoked potential response, and retrograde labeling of the retina by injection of a fluorescent dye distally to the injury site 2 weeks after injury, and prompt treatment with α₂-adrenoceptor agonists. One week after dye application, retinae were excised and their labeled ganglion cells counted. The agonists were also examined in vitro for their ability to protect neurons from glutamate-induced death. Results: Both compounds caused a dose-dependent reduction of injury-induced deficit when injected intraperitoneally at the time of the injury. For example, AGN 191103 (10μg/kg) and brimonidine (100μg/kg) increased the CAP from 215 to 1450 uVs, and from 453 to 766 uVs, respectively. Both compounds also protected hippocampal nerves from glutamate cytotoxicity in vitro. Conclusion: The mechanism underlying the observed beneficial effects of the tested agonists on functional outcome after optic nerve injury has yet to be elucidated. Nevertheless, AGN 191103 and brimonidine have actions that spare injured optic nerves in addition to decreasing intraocular pressure.

The poor ability of mammalian central nervous system (CNS) axons to regenerate has been attributed, in part, to astrocyte behavior after axonal injury. This behavior is manifested by the limited ability of astrocytes to migrate and thus repopulate the injury site. Here, the migratory behavior of astrocytes in response to injury of CNS axons in vivo was simulated in vitro using a scratch-wounded astrocytic monolayer and soluble substances derived from injured rat optic nerves. The soluble substances, applied to the scratch-wounded astrocytes, blocked their migration whereas some known wound- associated factors such as transforming growth factor-β₁ (TGF-β₁), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), transforming growth factor-α (TGF-α), and heparin-binding epidermal growth factor in combination with insulin-like growth factor-1 (HB-EGF + IGF-1) stimulated intensive migration with consequent closure of the wound. Migration was not dominated by proliferating cells. Both bFGF and HB-EGF + IGF-1, but not TGF- β1, could overcome the blocking effect of the optic nerve-derived substances on astrocyte migration. The induced migration appeared to involve proteoglycans. It is suggestive that appropriate choice of growth factors at the appropriate postinjury period may compensate for the endogenous deficiency in glial supportive factors and/or presence of glial inhibitory factors in the CNS.

HU-211 is a novel synthetic analog of tetrahydrocannabinol that was recently shown in animal models to be nonpsychotropic. In this study we show that HU-211 can potentially be used as a neuroprotective compound in the CNS. Using a calibrated crush injury of adult rat optic nerve, we show that HU- 211 can reduce injury-induced metabolic and electrophysiological deficits. Energy metabolism was monitored by measuring the intramitochondrial nicotine- amine adenine dinucleotide redox state hourly for 6 h after injury and treatment. Electrophysiological activity was assessed by compound action potential and visual evoked potential response. Beneficial effects were dose- dependent, being optimal at 7 mg/kg, administered intraperitoneally. The time window during which treatment was effective was found to be from the time of injury for at least 5 h, with treatment most effective at the time of injury. These results strongly suggest that HU-211, given immediately after CNS injury at the optimal dosage, may possess neuroprotective activities.

Macrophages have long been known to play a key role in the healing processes of tissues that regenerate after injury; however, the nature of their involvement in healing of the injured central nervous system (CNS) is still a subject of controversy. Here we show that the absence of regrowth in transected rat optic nerve (which, like all other CNS nerves in mammals, cannot regenerate after injury) can be overcome by local transplantation of macrophages preincubated ex vivo with segments of a nerve (e.g., sciatic nerve) that can regenerate after injury. The observed effect of the transplanted macrophages was found to be an outcome of their stimulated activity, as indicated by phagocytosis. Thus, macrophage phagocytic activity was stimulated by their preincubation with sciatic nerve segments but inhibited by their preincubation with optic nerve segments. We conclude that the inability of nerves of the mammalian CNS to regenerate is related to the failure of their macrophages recruited after injury to acquire growth- supportive activity. We attribute this failure to the presence of a CNS resident macrophage inhibitory activity, which may be the biochemical basis underlying the immune privilege of the CNS. The transplantation of suitably activated macrophages into injured nerves may overcome multiple malfunctioning aspects of the CNS response to trauma, and thus may be eveloped into a novel, practical, and multipotent therapy for CNS injuries.

This study demonstrated the involvement of the tumor suppressor protein p53 in differentiation and programmed cell death of neurons and oligodendrocytes, two cell types that leave the mitotic cycle early in development and undergo massive-scale cell death as the nervous system matures. We found that primacy cultures of rat oligodendrocytes and neurons, as well as of the neuronal PC12 pheochromocytoma cell line, constitutively express the p53 protein. At critical points in the maturation of these cells in vitro, the subcellular localization of p53 changes: during differentiation it appears mainly in the nucleus, whereas in mature differentiated cells it is present mainly in the cytoplasm. These subcellular changes were correlated with changes in levels of immunoprecipitated p53. Infection of cells with a recombinant retrovirus encoding a C-terminal p53 miniprotein (p53 DD), previously shown to act as a dominant negative inhibitor of endogenous wild-type p53 activity, inhibited the differentiation of oligodendrocytes and of PC12 cells and protected neurons from spontaneous apoptotic death. These findings suggest that p53, upon receiving appropriate signals, is recruited into the nucleus, where it plays a regulatory role in directing primacy neurons, oligodendrocytes, and PC12 cells toward either differentiation or apoptosis in vitro.

Purpose: This article presents the rationale and an experimental strategy for the development of new treatment modalities for glaucomatous neuropathy. Accumulating evidence suggests that regardless of the primary trigger of the retinal and optic nerve damage in glaucoma, the disease will continue to progress even when the cause is removed. The resulting damage can be mimicked by the progression of damage secondary to an acute partial crush injury at the optic nerve head. Such secondary damage includes degeneration of the directly injured optic nerve fibers culminating in death of their cell bodies, as well as degeneration of nerve fibers that escaped acute injury but nevertheless deteriorate as a result of their exposure to injury-induced mediators of secondary degeneration released by the directly affected neurons. Conclusion: We therefore propose that substances found to be effective in rescuing fibers from secondary degeneration and in increasing the survival rate or prolonging survival of retinal ganglion cells in the partially lesioned optic nerve may be useful for the treatment of glaucoma. The new approach does not replace hypotensive therapy, but addresses the glaucoma-induced damage by promoting nerve protection and neuroregeneration.

Morphogenesis and tissue repair require appropriate cross-talk between the cells and their surrounding milieu, which includes extracellular components and soluble factors, e.g., cytokines and growth factors. The present work deals with this communication needed for recovery after axotomy in the central nervous system (CNS). The failure of CNS axons to regenerate after axonal injury has been attributed, in part, to astrocyte failure to repopulate the injury site. The goal of this work was to provide an in vitro model to mimic the in vivo response of astrocytes to nerve injury and to find ways to modulate this response and create a milieu that favors astrocyte migration and repopulation of the injury site. In an astrocyte scratch wound model, we blocked astrocyte migration by tumor necrosis factor α (TNF-α). This effect could not be reversed by astrocyte migration-inducing factors such as transforming growth factor β₁ (TGF-β₁) or by any of the tested extracellular matrix (ECM) components (laminin and fibronectin) except for vitronectin (Vn). Vn, added together with TNF-α, counteracted the TNF-α blockage and allowed a massive migration of astrocytes (not due to cell proliferation) beyond that allowed by Vn only. Heparan sulfate proteoglycans (HSPG) were shown to be involved in the migration. The results may be relevant to regeneration of CNS axons, and may also provide an example that an extracellular component (Vn) can overcome and neutralize a negative effect of a growth factor/cytokine (TNF-α) and can act in synergy with other features of this cytokine to promote a necessary function (e.g., cell migration) that is other-wise inhibited.

Migration of astrocytes is thought to play a role in nerve regeneration and to be mediated, at least in part, by inflammation-associated cytokines, Plasminogen activators are secreted proteases that function in fibrinolysis and participate in cellular migration and invasion and, in some cases, are modulated by cytokines, Here, we show that two cytokines, tumor necrosis factor-alpha and interleukin-1 beta, can modulate plasminogen activation in astrocytes, each causing 90% reduction of total plasminogen activator activity. Direct and reverse zymography indicated that this reduction resulted from two simultaneous events, a pronounced decrease in tissue-type plasminogen activator activity and an induction of plasminogen activator inhibitor-1. Northern hybridization analysis indicated a 30-fold increase of the steady-state level of plasminogen activator inhibitor-1 mRNA following treatment with each of the two cytokines. Both of the cytokine-induced effects could be blocked by cycloheximide or actinomycin D. When signal transduction pathways were blocked, the results indicated the involvement of reduction in cyclic AMP levels, protein kinase activity, and arachidonic metabolites of the lipoxygenase pathway. The results thus show that the two cytokines reduce the ability of astrocytes to conduct fibrinolysis and extracellular proteolysis, and suggest that the effect of these cytokines on members of the plasminogen activation system is through a common signal transduction pathway.

Compared to the peripheral nervous system (PNS), the central nervous system (CNS) of mammals has a poor prospect for regeneration. Accumulating evidence suggests that this is due, in part, to differences in how the immune and nervous systems communicate in response to injury. The macrophage is one of the central cells in this communication with the capacity to respond in a variety of ways depending on the conditions of stimulation. After injury, macrophages enter the CNS much later and in fewer numbers than they do the PNS. It is possible that this late and reduced response is not sufficient to modify the CNS environment to one that is conducive to successful regeneration. In the present study we investigated whether the limited macrophage invasion of injured CNS is due to the presence of an endogenous inhibitory factor that is persistent after injury. Using an in vitro migration assay, we show that rat optic nerve (CNS) is deficient in its ability to attract monocytes as compared to rat sciatic nerve (PNS). We further demonstrate that this deficiency is due to the presence of a soluble inhibitory factor in the CNS. This factor may also cause a subsequent effective difference in those macrophages that are recruited, as is shown by morphological data. The brain-resident factor that inhibits macrophage migration may be the physiological basis of an immune-brain barrier underlying the known phenomenon of immune privilege.

We have tested the effects of early posttraumatic low-energy laser irradiation on injured neural tissues. Rat optic nerves were crushed by a calibrated forceps and the ensuing degenerative processes were followed up electrophysiologically and by on-line metabolism measurements, using nicotinamide adenine dinucleotide autofluorescence in response to anoxia. The irradiation not only decelerated the posttraumatic degenerative processes but also increased axonal survival as shown by visual evoked potential response recording. The irradiation also reduced significantly the very early posttraumatic reduction in the metabolic activity of the optic nerve. It seems that the low-energy irradiation, possibly by a photochemical mechanism and by modifying the cellular metabolism, prevents the spread of the injury effects from the injury site and along the axons. This action of low-energy laser irradiation is akin to the neuroprotective effects ascribed to various drugs, such as corticosteroids.

Cytokines have been suggested to be involved in the cross talk between the immune and the nervous systems, under normal and pathological conditions, For example, the cytokine interleukin-2 was suggested to be involved in response to CNS trauma and spontaneous regeneration. Here, we examined whether mammalian CNS has an intrinsic potential to produce interleukin-2 and, if so, what its cellular origin is. mRNA sequences encoding for interleukin-2 were detected in brains of humans and rodents. Northern blot analysis revealed the presence of several interleukin-2 transcripts of different sizes in the brain, all recognized by lymphocyte-derived interleukin-2 cDNA probes. One of the transcripts, a high molecular weight form of similar to 5 kb, appeared to be unique to the brain. Reverse transcription and amplification by PCR of human fetal brain mRNA revealed one cDNA product that, upon sequence analysis, showed a high degree of homology with the human lymphocyte-derived interleukin-2 coding sequence, To identify the possible cellular source of the interleukin-2 transcripts within the mammalian brain, we similarly analyzed mRNA of rat brain cells in culture. Northern blot analysis revealed that astrocytes contain transcripts that hybridize with interleukin-2 cDNA probe. These findings point to the astrocytes as a possible source of brain interleukin-2.

A covalent dimer of interleukin (IL)-2, produced in vitro by the action of a nerve-derived transglutaminase, has been shown previously to be cytotoxic to mature rat brain oligodendrocytes. Here we report that this cytotoxic effect operates via programmed cell death (apoptosis) and that the p53 tumor suppressor gene is involved directly in the process. The apoptotic death of mature rat brain oligodendrocytes in culture following treatment with dimeric IL-2 was demonstrated by chromatin condensation and internucleosomal DNA fragmentation. The peak of apoptosis was observed 16-24 h after treatment, while the commitment to death was already observed after 3-4 h. An involvement of p53 in this process was indicated by the shift in location of constitutively expressed endogenous p53 from the cytoplasm to the nucleus, as early as 15 min after exposure to dimeric IL-2. Moreover, infection with a recombinant retrovirus encoding a C-terminal p53 miniprotein, shown previously to act as a dominant negative inhibitor of endogenous wild-type p53 activity, protected these cells from apoptosis.

Axons of the mammalian central nervous system do not regenerate spontaneously after axonal injury, unlike the central nervous system axons of fish and amphibians and the peripheral nervous system of mammals, which possess a good regenerative ability (Grafstein: The Retina: A Model for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155197, 1979; Murray: J Comp Neurol 168:175196, 1976; Ramony Cajal: Degeneration and Regeneration of the Nervous System, 1928; Reier and Webster: J Neurocytol 3:591618, 1974; Sperry: Physiol Zool 23:351361, 1948). It was previously believed that intrinsic differences between the central nervous system neurons of mammals and fish account for their differences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David and Aguayo: Science 214:931933, 1981; Kierstead et al: Science 246:255257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peripheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293314, 1990; Eitan et al: Science 264:17641768, 1994). It is generally believed today that the glial cells surrounding the axons play a crucial role in determining the regenerative capacity of any central nervous system. In this review, we discuss various aspects of glial cells in the context of regeneration, with special emphasis on similarities and differences between the glial cells of rat and fish optic nerves, representing a nonregenerative and a regenerative system, respectively. © 1995 WileyLiss, Inc.

Fish optic nerve sections were recently shown to be nonpermissive to growth of adult retinal axons. In addition, fish optic nerve myelin was found to inhibit growth of adult retinal axons and this inhibition was neutralized by IN-1 antibodies (known to block rat myelin-associated inhibitors). In this study we examined whether the growth nonpermissiveness of fish optic nerves which had not been injured prior to their excision results, at least in part, from the presence of myelin-associated growth inhibitors. It was found that preincubation of the sections with IN-1 antibodies, known to recognize and neutralize the myelin-associated growth inhibitors of the rat central nervous system, increases sixfold the number of axons that grow on these sections. This demonstrates that fish myelin-associated growth inhibitors, which are similar to those of rat, are at least partly responsible for the growth nonpermissiveness of normal fish optic nerves.

An interleukin-2 dimer, produced enzymatically by a nerve-derived transglutaminase in vitro, is cytotoxic to oligodendrocytes, unlike the immune-derived monomeric interleukin-2. The object of this study was to establish a way to produce a dimer of interleukin-2 in quantities, by means of genetic engineering, and to confirm that the structure of the resulting molecule is critical for its function. A defective herpes simplex virus vector was utilized for overproduction of a dimeric interleukin-2. The resulting linear dimer, which is a translational product, differs from the enzymatically produced dimer, which is a posttranslational modification of interleukin-2. The linear dimer, while retaining the known interleukin-2 activity of monomeric interleukin-2 with respect to mitogenicity on T cells, was not cytotoxic to oligodendrocytes. This finding suggests that the lack of cytotoxicity of the linear dimeric interleukin-2 is not caused by a loss of activity during its preparation but is related to its conformational structure, which evidently does not meet the requirements for cytotoxicity. This study opens the way to the design at the transcriptional level of modified proteins and their efficient production, provided that the new transcript encodes for the desired modification in the protein at the appropriate sites.

Neu differentiation factor (NDF, also called heregulin) was isolated from mesenchymal cells on the basis of its ability to elevate phosphorylation of ErbB proteins. Earlier in situ hybridization analysis showed that NDF was transcribed predominantly in the central nervous system during embryonic development. To gain insights into the role of NDF in brain we analyzed its distribution by immunohistochemistry and in situ hybridization. Late- gestation (day 17) rat embryos displayed high NDF immunoreactivity in both motor (e.g., putamen) and limbic (e.g., septum) regions. Lower levels of the factor were exhibited by adult brain, except for the cerebellum, where NDF expression was increased postnatally. Both neurons and glial cells were identified by immunohistochemistry as NDF-producing cells (e.g., pyramidal neurons in the cerebral cortex and glial cells in the corpus callosum). By establishment of primary cultures of rat brain cells we confirmed that NDF was expressed in neurons as well as in astrocytes. In addition, by using such primary cultures we observed that NDF treatment exerted only a limited mitogenic effect, which was accompanied by significant acceleration of astrocyte maturation. Furthermore, long-term incubation with the factor specifically protected astrocytes from apoptosis, implying that NDF functions in brain as a survival and maturation factor for astrocytes.

This study shows that the fish optic nerve, which is able to regenerate after injury, contains myelinassociated growth inhibitors similar to the growth inhibitors present in mammalian central nervous system (CNS) myelin. The ability of nerves to regenerate was previously correlated with the ability of sections from these nerves to support neuronal attachment to and axonal growth in vitro. Thus neuroblastoma cells or embryonic neurons became attached to and grew axons on sections of rat sciatic nerve or fish optic nerve, which are spontaneously regenerating systems, but not on sections of rat optic nerve, a nongenerating system. Failure of the latter to support axonal growth has been attributed, at least in part, to growth inhibitors. Recently it was shown that adult neurons, which differ in their growth requirement from embryonic neurons, are unable to extend neurites on sections of normal sciatic nerve but are able to extend neurites on sections of sciatic nerve that was injured prior to its excision. We found a similar in the fish optic nerve, i.e. that the nerve is normally not permissive to growth of adult retinal axons but becomes growth permissive after injury. The nonpermissiveness of the normal fish optic nerve was found to correlate with the presence of myelinassociated growthinhibitory molecules. This inhibitory activity of fish myelin was neutralized by IN1 antibodies, known to neutralize rat myelin growth inhibitors apparently similar or even identical to those of rat, but possibly present in lower amounts than in the rat. Results are discussed with respect to the possibility that fish optic nerve, like the rat sciatic regeneration of adult neurons. Such changes might include elimination or neutralization of growth inhibitors. © 1994 WileyLiss, Inc.

The reported effects of low-energy laser irradiation on the nervous system are manifested in alterations in cellular and extracellular biochemical constituents and reactions, as well as in changes in cell division rates. These bioeffects were observed in both in vivo and in vitro experiments. Other observed phenomena relate to the function of the nervous system and consist mainly of induced alteration in electrical conduction, stimulation thresholds, and behavioral effects. Clinical aspects of low-energy laser bioeffects relate mainly to pain mitigation and postponement of the posttraumatic neural degeneration processes. Many of the reported observations were obtained by experiments apparently conducted according to less than rigorous scientific criteria, and some could not be duplicated. On the whole, however, there is little doubt that low-energy laser irradiation exerts some effects on the nervous system under specific conditions of irradiation and tissue exposure via a mechanism which is probably photochemical in nature.

The expression of intermediate filaments is developmentally regulated. In the mammalian embryo keratins are the first to appear, followed by vimentin, while the principal intermediate filament of the adult brain is glial fibrillary acidic protein. The intermediate filaments expressed by a cell thus reflect its state of differentiation. The differentiation state of cells, and especially of glial cells, in turn determines their ability to support axonal growth. In this study we used three new antibodies directed against three fish intermediate filaments (glial fibrillary acidic protein, keratin 8 and vimentin), in order to determine the identity and level of expression of intermediate filaments present in fish glial cells in culture. We found that fish astrocytes and oligodendrocytes are both able to express keratin 8 and vimentin. We further demonstrate that under proliferative conditions astrocytes express high keratin 8 levels and most oligodendrocytes also express keratin 8, whereas under nonproliferative conditions the astrocytes express only low keratin 8 levels and most oligodendrocytes do not express keratin 8 at all. These results suggest that the fish glial cells retain characteristics of immature cells. The findings are also discussed in relation to the fish glial lineage.

The ability or inability of nerves to regenerate their injured axons depends on the cellular milieu surrounding the axons and its response to axonal injury. Recent research has shed more light on the nature of these cells and their associated soluble and insoluble substances in the mature nerve in the resting state and after injury. Several studies have helped to elucidate key cellular processes and substances in regeneration, and have demonstrated the involvement of regeneration-related cross-talk between the immune and the nervous systems. These findings are based on two independent lines of research, one involving studies of the fish optic nerve, and the other involving studies of mammalian sciatic nerves. Both systems regenerate readily after injury. These results suggest that macrophages and/or their products (cytokines) at the site of the injury affect local glial cells in a way that benefits regeneration. Such effects presumably include elimination of oligodendrocytes and proper activation of astrocytes. Taken together, the observations that inflammation is beneficial for regeneration and that anti-inflammatory agents promote posttraumatic rescue of fibers from secondary degeneration lead to propose that inflammation has dissimilar effects on axonal rescue and regeneration.

The poor regenerative ability of neurons of the central nervous system in mammals, as compared with their counterpart in fish or amphibians, is thought to stem from differences in their immediate nonneuronal environment and its response to axonal injury. We describe one aspect of the environmental response to axonal injury in a spontaneously regonerating systemthe fish optic nerve. The aspect under investigation was the reaction of glial cells at the injury site. This was examined by the use of antibodies that specifically recognize vimentin in fish glial cells. In the present study, affinitypurified vimentin antibodies were raised against a nonconserved Nterminal 14amino acid peptide, which was predicted from the nucleotide sequence of vimentin. These antibodies were found to react specifically with glial cells in vitro. Moreover, the antivimentin antibodies stained both the optic nerve and the optic tract, but with different patterns. Specificity of the antibodies was verified by protein immunoblotting, tissue distribution, and labeling patterns. After injury, vimentin immunoreactivity initially disappeared from the site of the lesion due to cell death. Early signs of glial cell migration toward the injury site were evident a few days later. It is suggested that the reappearance of vimentinpositive glial cells at the site of injury is associated with axonal elongation across it, and that they contribute to the regenerative ability of the fish optic nerve. © 1994 WileyLiss, Inc.

The central nervous system has long been regarded as an immunologically privileged site. Accumulating evidence suggests, however, that the privilege is not total, and that certain immune functions involving immune components and resident glial cells can operate in the central nervous system. The nervous and immune systems interact during normal development, but in the mature brain their interaction is restricted mainly to cases of pathogenic infections and traumatic lesions. The focus of this review is on bidirectional interactions between immune and neuroglial components in response to nerve injury. The macrophage is the most ubiquitous of the immune-derived cell types associated with injury. Its role, as in any other organ, is tissue remodeling and promotion of healing. Macrophage activities include removal of dead tissue and debris by phagocytosis, lipid recycling, and secretion of a wide spectrum of cytokines possessing trophic, mitogenic, and chemotactic properties. These activities affect the behavior of resident cells in the vicinity of the wound. We discuss the possible association of these cytokines with the ability of injured nerves to regenerate. Finally, we consider the apparently conflicting effects of posttraumatic inflammation on the recovery of function.

Axonal injury of peripheral nerves has been shown to be followed by rapid and massive invasion of the nerves by macrophages, which appear to play an important role in the subsequent ability of these nerves to regenerate. In contrast, macrophage invasion of injured nerves of the central nervous system is limited, and the relationship between the post-traumatic inflammatory response of central nervous system nerves and their poor ability to regenerate is not fully understood. We used the proinflammatory cytokine tumor necrosis factor-α and the macrophage growth factor, colony stimulating factor-1, to examine whether the inflammatory response can be augmented in the optic nerve following injury, and whether such augmentation is accompanied by regeneration-associated changes. It appeared that the two cytokines caused a significant increase in the number of macrophages invading the optic nerve immediately after injury. Interestingly, however, in the nerve treated with tumor necrosis factor-α (but not in the nerve treated with colony stimulating factor-1) this increase was accompanied by an increased permissiveness of the nerve to neuronal adhesion, which we examined in vitro using longitudinal sections of the nerve on which PC12 cells were seeded. The results are discussed with respect to the ability of tumor necrosis factor-α to modify the nonpermissive nature of central nervous system white matter.

Failure of axons of the central nervous system in adult mammals to regenerate spontaneously after injury is attributed in part to inhibitory molecules associated with oligodendrocytes. Regeneration of central nervous system axons in fish is correlated with the presence of a transglutaminase. This enzyme dimerizes interleukin-2, and the product is cytotoxic to oligodendrocytes in vitro. Application of this nerve-derived transglutaminase to rat optic nerves, in which the injury had caused the loss of visual evoked potential response to light, promoted the recovery of that response within 6 weeks after injury. Transmission electron microscopy analysis revealed the concomitant appearance of axons in the distal stump of the optic nerve.

Laser irradiation at subthreshold energies exerts various effects on the eye and other parts of the body, mainly the skin and nervous system, through a mechanism that has yet to be adequately explained. The ocular bioeffects are manifested mostly in the retina, but also in other ocular tissues. This review outlines the reported effects of low-energy laser irradiation on nonophthalmological tissues and organs, including those of the nervous system, with special emphasis on the optic nerve. It also details the ophthalmic phenomena induced by low-energy laser irradiation and examines claims of its therapeutic efficacy in several eye diseases, such as keratitis, glaucoma and macular degeneration.

Recent results shed new light on how some nervous systems can regenerate after injury while others cannot. Until recently, it was widely believed that the main difference between systems that regenerate and those that do not lies in the normal state of their permissiveness to the regenerating axons. Thus, while nonregenerative systems, such as the rat optic nerve, were shown to contain myelin-associated growth inhibitors, regenerative systems, such as the fish optic nerve, were thought to have no such inhibitors. However, it has now been demonstrated that spontaneously regenerating systems do contain growth inhibitors, though their levels seem to be lower than in nonregenerative systems. The main difference, however, appears to reside in the system's response to injury. This article discusses the involvement of myelin-associated growth inhibitors in the spontaneously regenerating nervous system of fish, traces the apparent discrepancy, and shows how it has been resolved recently.

The elements that control neuronal proliferation are largely unknown. Proliferating neurons in cultures of goldfish brain were studied in an attempt to identify the cell types involved. Neuronal proliferation was found to occur only when the neuronal stem cells were in direct contact with astrocytes, and never directly on the substrate. The regulation of neuronal proliferation thus appears to be mediated, at least in part, by contact with astrocytes. In addition, neurite extension was inhibited by medium conditioned by fish astrocytes. Since neurite extension and neuronal proliferation are mutually exclusive processes, inhibition of neurite extension by soluble substances derived from the astrocytes is probably one of the mechanisms controlling neuronal proliferation. The complex reciprocal relationship between neurons and astrocytes is also demonstrated by an observed inhibition of astrocytic proliferation by medium conditioned by differentiating fish neurons. This inhibition of astrocytic proliferation might be part of a mechanism through which interference with neuronal differentiation by astrocytes is avoided. The results of this study thus suggest that astrocytes, in addition to their known roles in controlling neuronal migration, neuronal differentiation and neurite elongation, may also play a role in the control of neuronal proliferation.

Analysis of the shape of the cross sections of adult rat optic nerve axons reveals that the majority of axons do not have a true circular shape. Therefore, determination of axonal size has to utilize methods of approximation. The method presented here utilizes three calculated parameters for expression of axonal size: (i) axonal diameter, as calculated from its area, or (ii) axonal diameter, as calculated from its perimeter, both assuming axonal shape to be a perfect circle and (iii) axonal shape factor, which represents the divergence of the axon from a perfect circular shape. The use of the calculated axonal diameter, with a correction for its shape factor, provides a normalized way of expressing axonal size.

Mammalian central nervous system neurons do not regenerate after axonal injury, unlike their counterparts in fish and amphibians. After axonal injury, glial cells in mammals do not support regrowth of axons, while in fish they support the regeneration process. Controversy exists as to whether or not the intact fish optic nerve expresses glial fibrillary acidic protein, a wellknown marker for mature astrocytes, and thus whether its astrocytes differ in this respect from those of the brain and spinal cord, as well as from optic nerve astrocytes of other species. In an attempt to resolve this question we cloned fish glial fibrillary acidic protein. Two different complementary DNA clones were isolated from a carp brain complementary DNA library, each encoding a different form of glial fibrillary acidic protein apparently originating from different genes. Monospecific polyclonal antibodies were raised against a peptide synthesized according to the predicted amino acid sequence, and used to identify and localize the fish glial fibrillary acidic protein. Two glial fibrillary acidic proteins (of 49 kDa and 51 kDa) were identified by the antibodies in all tested fish central nervous system tissues. The antibodies were then used to examine glial fibrillary acidic protein immunoreactivity in sections taken from uninjured and injured optic nerves of goldfish. Injury was followed by an elevation in glial fibrillary acidic protein immunoreactivity along the whole length of the nerve, except at the site of the injury, whereas in the case of vimentinno immunoreactivity was detectable. However, in contrast to vimentinpositive glial cells, which repopulate the site of the injury soon after the optic nerve is injured, glial fibrillary acidic proteinpositive glial cells remained outside the injury site for as long as 6 weeks after the injury. Despite the injuryinduced changes in glial fibrillary acidic protein immunoreactivity, no change was observed in the level of transcript encoding glial fibrillary acidic protein after injury, while there was an increase in the amount of glial fibrillary acidic protein associated with the cytoskeleton and a reduction in the soluble form. These results suggest that the injuryinduced changes in immunoreactivity on sections involve changes not in transcription or translation of glial fibrillary acidic protein, but in glial fibrillary acidic protein compartmentalization. © 1993 WileyLiss, Inc.

The use of low-energy helium-neon laser irradiation has recently attracted attention concerning the treatment of nerve injury. Studies in the rat optic nerve have yielded several key observations which support the notion that treatment with low-energy laser is beneficial to injured rat optic nerves, provided that all parameters are well-calibrated and optimal. Irradiation employing a nonoptimal parameter could have either a devastating effect or no effect at all. Most laser irradiation studies using calibrated parameters have employed an electrophysiological setup in which measurements of compound action potential are carried out 2 weeks after injury. After this time period the electrophysiological activity of the injured treated nerve itself is very low, so that any beneficial effect resulting from any treatment is detectable.

The central nervous systems of mammals and fish differ significantly in their ability to regenerate. Central nervous system axons in the fish readily regenerate after injury, while in mammals they begin to elongate but their growth is aborted at the site of injury, an area previously shown to contain no glial cells. In the present study we compared the ability of glial cells to migrate and thus to repopulate the injured area in fish and rats, and used light and electron microscopy in an attempt to correlate such migration with the ability of axons to traverse this area. One week after the optic nerve was crushed, both axonal and glial responses to injury were similar in fish and rat. In both species glial cells were absent in the injured area (indicated by the disappearance of glial fibrillary acidic protein and vimentin immunoreactive cells from the site of injury in rat and fish, respectively), while at the same time axonal growth, indicated by expression of the growth-associated protein GAP-43, was restricted to the proximal part of the nerve. In fish, 2 weeks after the crush, GAP-43 staining (i.e., growing axons) was seen at the site of injury, in association with migrating vimentin-positive glial cells. One week later the site of injury in the fish optic nerve was repopulated by vimentin-positive glial cells, and GAP-43-positive axons had already traversed the site of injury and reached the distal part of the nerve. In contrast, the site of injury in the rat remained devoid of glial fibrillary acidic protein immunoreactive cells, and the expression of GAP-43 by growing axons was still restricted to the proximal part of the nerve. Double-labeling experiments and transmission electron microscopy performed 2 weeks after crush injury of the fish optic nerve revealed that the frontier of axonal growth (i.e., the leading growth cones) appeared to be 200-300 mum ahead of the nearest vimentin-positive glial cells. The leading growth cones were associated with other

It has long been known that while damaged nerves of the peripheral nervous system in mammals are able to regenerate, this does not occur in the central nervous system. Recent studies have shown that a chemical factor present in regenerating fish optic nerves facilitates regeneration of nerve axons in rodents. Further research encourages the hope that this may ultimately lead to the successful treatment of nerve damage in humans.

1. 1. The present review describes the results of a cloning that was successfully employed in the study of optic nerve regeneration in fish. 2. 2. Three intermediate filaments (IFs) expressed by the glial fish optic nerve were cloned. 3. 3. By the use of this approach it was possible to resolve the controversial question of whether glial fibrillary acidic protein (GFAP) is expressed in the fish optic nerve. 4. 4. Morever, as a result of the information that emerged from the cloning, it was possible to raise well-characterized monospecific antibodies and successfully exploit them in order to determine glial cell maturation, lineage and plasticity, monitor the glial cell response to injury of the fish optic nerve, and compare it to that of a non-regenerative system.

Gangliosides have been shown to be capable of protecting nerve tissue from mechanical and biochemical insults and promoting their repair. The present study provides morphologic evidence that monosialogangliosides attenuate the degenerative process at the distal stump of the rat optic nerve after crush injury. Injured rat optic nerves were treated for 7 days after injury with daily intraperitoneal injections of monosialogangliosides (30 mg/kg/day), and compared with untreated injured controls with respect to the number of viable axons 2 and 4 weeks after injury, as indicated by transmission electron microscopy. After 2 weeks, the mean number of viable axons in the treated optic nerves (n = 5) was slightly higher than in the controls (n = 5). Four weeks after injury, although the absolute number in both the experimental and the control groups had dropped, it was about seven-fold higher in the treated animals (1696 ± 1149, n = 7) than in the untreated animals (216 ± 65, n = 6); this difference was statistically significant. These findings, which offer some insight as to how monosialogangliosides affect injured nerves, may have important implications for treatment in cases of optic nerve injury.

Lowenergy laser irradiation has been reported to postpone the degenerative processes in crushed optic nerves of rats, which are part of the nonregenerable mammalian central nervous system. In the present study, we evaluated the optimal irradiation parameters for this purpose. Optic nerves of 141 rats were subjected to crush injury and then irradiated through the eye, starting at different points of time before or after the injury, for different durations and periods, using various intensities of either heliumneon laser or noncoherent infrared light (904 nm). The effect was evaluated by measurements of the compound action potentials of the nerve segments between the site of injury and the optic chiasm. The compound action potential amplitude of the crushed nonirradiated nerves, as measured 2 weeks after the injury, was found to be 0.51 ± 0.30 mV, in contrast to 3.10 ± 1.03 mV measured in 232 normal nerves. Irradiation with a 10.5 mW heliumneon laser for 2 and 3 min once a day for 14 consecutive days resulted in maximal preservation of action potentials (1.78 ± 0.72 and 1.95 ± 0.71 mV, respectively). Irradiations beginning immediately prior to the injury were as effective as irradiations beginning soon after it. Irradiations for longer than 3 min or twice a day aggravated the damage. Noncoherent infrared light was ineffective or adversely affected the injured nerves. Our experiments suggest that optimal delay of posttraumatic optic nerve degeneration in rats is attainable with 10.5 mW heliumneon laser irradiations for 2 or 3 min once a day for 14 consecutive days. © 1993 WileyLiss, Inc.

Regenerating optic nerves from fish produce a factor that is cytotoxic to oligodendrocytes. The cytotoxic factor is recognized by antibodies to interteukin-2 (IL-2) and has the apparent molecular size of a dimer of IL-2. An enzyme, identified as a nerve transglutaminase, was purified from regenerating optic nerves of fish and was found to catalyze dimerization of human IL-2. The dimerized IL-2, unlike monomeric IL-2, is cytotoxic to oligodendrocytes from rat brain in culture. The results suggest that posttranslational modification of a cytokine can alter its activity. Under conditions in which oligodendrocytes inhibit neuronal regeneration, dimerization of IL-2 might provide a mechanism to permit nerve growth.

Injury to the mammalian central nervous system results in loss of function because of its inability to regenerate. It has been postulated that some axons in the mammalian central nervous system have the ability to regenerate but fail to do so because of the inhospitable nature of surrounding glial cells. For example, mature oligodendrocytes were shown to inhibit axonal growth, and astrocytes were shown to form scar tissue that is nonsupportive for growth. In the present study we report an additional phenomenon which might explain the failure of axons to elongate across the site of the injury, namely, the absence of astrocytes from the crush site between the glial scar and the distal stump. Astrocytes began to disappear from the injury site as early as 2 days after the injury. After 1 week the site was necrotic and contained very few glial cells and numerous macrophages. Disappearance of glial cells was demonstrated in both rabbit and rat optic nerves by light microscopy, using antibodies directed against glial fibrillary acidic protein, and by transmission electron microscopy. Results are discussed with reference to possible implications of the long-lasting absence of astrocytes from the injury site, especially in view of the differences between the present findings in rodents and our recent observations in fish.

Current views suggest that the extracellular environment is critically important for successful axonal regeneration in the CNS. The goldfish optic nerve readily regenerates, indicating the presence of an environment that supports regeneration. An analysis of changes that occur during regeneration, in this model may help identify those molecules that contribute to a favourable environment for axonal regrowth. We examined the distribution and expression of two extracellular matrix molecules, laminin and chondroitin sulphate proteoglycan, and a carbohydrate epitope shared by a family of adhesion molecules (HNK-1), using immunocytochemical detection in sections from the normal adult goldfish optic nerve and in nerves from one hour to five months following optic nerve crush. We also used in vitro preparations to determine if neurites in retinal explants could express these same molecules. The linear distributions of laminin and chondroitin sulphate proteoglycan immunoreactivity in control optic nerves are co-extensive with the glia limitans, suggesting both are expressed by non-neuronal components surrounding the axon fascicles. Between one and three weeks postoperatively when axons elongate and reach their target, laminin and chondroitin sulphate proteoglycan immunoreactivity increases around the crush site and distally. At six weeks postoperatively the pattern of immunoreactivity has returned to normal. While the temporal pattern of changes in immunoreactivity is similar, the spatial pattern of these two extracellular proteins in the regenerating nerve differs. Chondroitin sulphate proteoglycan immunoreactivity is organized in discrete columns associated with regenerating axons while laminin immunoreactivity is more diffusely distributed. Examination of retinal explants reveals growing neurites express chondroitin sulphate proteoglycan but not laminin. Our results suggest that laminin is only associated with non-neuronal cells, while chondroitin sulphate proteoglycan is associated with axons as well as non-neuronal cells. HNK-1 immunoreactivity is co-extensive with both the glia limitans and axon fascicles and is more extensively distributed in the intact nerve than either laminin or chondroitin sulphate proteoglycan immunoreactivity. In contrast to laminin and chondroitin sulphate proteoglycan, HNK-1 immunoreactivity is substantially decreased at the crush site within one week following optic nerve crush. HNK-1 immunoreactivity reappears through the crush site during the next several weeks, although non-immunoreactive regions, co-extensive with areas predominantly containing non-neuronal cells, persist both proximal and distal to the crush, up to six weeks postoperatively. The pattern suggests that HNK-1 epitope expression by these non-neuronal cells is decreased during axonal regeneration. Our results show that each of these molecules is constitutively expressed with a unique distribution in the normal goldfish optic nerve and each exhibits different patterns of change during regeneration. It thus appears that each may contribute to modifications of the environment that supports axonal regeneration. Both neurons and non-neuronal cells contribute to these changes.

The poor regenerative ability of the CNS of mammals has been attributed, at least in part, to the presence of mature oligodendrocytes, which have been shown to inhibit axonal growth. Proliferation of oligodendrocyte progenitor cells in the rat optic nerve during development, and thereby the timing of oligodendrocyte differentiation, has been shown to depend on a factor derived from type 1 astrocytes, later characterized as platelet-derived growth factor (PDGF). In the present study we examine whether injury to the optic nerve induces changes in the levels of PDGF in spontaneously regenerating systems, compared with nonregenerating systems. Soluble substances, derived from nonneuronal cells surrounding injured fish and rat optic nerves, were prepared and examined for the presence of PDGF immunoreactivity and biological mitogenic activity on PDGF-responsive cells. The results suggest that PDGF-like mitogenic activity and immunoreactivity are present in both fish and rat optic nerves. However, in the rat optic nerve PDGF levels increased after axonal injury, whereas in the fish optic nerve injury was accompanied by an apparent decrease in PDGF-like levels. The results are discussed with respect to the possible role of PDGF in regeneration.

Previous studies have suggested that L1, the cell adhesion molecule, is present in the regenerating fish optic nerve. The present study was undertaken in order to determine whether L1 is expressed by fish neurons and specifically by non-myelinated axons, using fish retinal explants in vitro and the developing fish visual system in vivo. In vitro, the nonmyelinated axons emerging from retinal explants showed L1 immunoreactivity and in vivo, L1 immunoreactive sites were found to be associated with areas rich in non-myelinated axons. At embryonic stage 23, as the eye developed and optic nerve axons began to elongate towards the tectum, L1-like immunoreactivity was seen both in optic nerve and in plexiform layers of the retina. At this stage and until hatching, the cellular layers within the retina showed little or no staining relative to the layers containing axons or dendrites. After hatching, L1 immunoreactivity was also observed in the ganglion cell layer, but upon maturation both the retina and the optic nerve lost most of their L1 immunoreactivity. We therefore suggest that non-myelinated axons of the fish visual system express L1 during development, which is lost after myelination and presumably reappears during regeneration.

Spontaneous growth of injured axons in the mammalian central nervous system is limited. We have previously shown an apparently regenerative growth of injured optic axons in the adult rabbit, achieved by supplying them with soluble substances originating from growing axons, followed by low energy helium-neon laser irradiation. The growing unmyelinated and thinly myelinated axons were embedded in astrocytes, and some were in the process of remyelination by oligodendrocytes. They were shown to have originated from the retinal ganglion cells. The present study further supports evidence relating to the origin and nature of these axons. Light microscopic analysis of these axons labeled with anterogradely transported horseradish peroxidase revealed that many of these axons have varicosities and bear growth cone-like swellings in their tips. These axons traverse the lesion site and extend into the distal stump in a disorganized pattern.

This study demonstrates the earliest reported effects of GM1 treatment on crush-injured axons of the mammalian optic nerve. GM1, administered intraperitoneally immediately after injury, was found to reduce the injury- induced metabolic deficit in nerve activity within 2 hr of injury, as measured by changes in the nicotine-amine adenine dinucleotide redox state. After 4 wk, transmission electron microscopy 1 mm distal to the site of injury revealed a sevenfold increase in axonal survival in GM1-treated compared to untreated injured nerves. These results emphasize the beneficial effect of GM1 on injured optic nerves as well as the correlation between immediate and long-term consequences of the injury. Thus, these results have implications for treating damaged optic nerves.

Regeneration of injured central nervous system axons is largely dependent on the response of the associated nonneuronal glial cells to injury. Glial cells of the mammalian central nervous system, unlike those of fish, are apparently not conducive to axonal regeneration. While the lineage of rat glial cells is well characterized and its role in the support or inhibition of regenerative growth is beginning to be understood, little is known about fish glial cells. Accordingly, glial cells in cultures of adult goldfish brain and of newly hatched goldfish larvae were studied in an attempt to establish their lineage. The cells were identified by means of indirect immunofluorescence, using antibodies against fish astrocytes and oligodendrocytes. The cell count in the cultures increased from a small number of cells at 24 h after plating to a large number of both astrocytes and oligodendrocytes after 1 week in culture. Both of these cell types had originated from proliferating cells, as shown by their uptake of tritiated thymidine and by the inhibition of cell proliferation by 5fluoro2deoxyuridine. Both astrocytes, i.e., glial fibrillary acidic proteinpositive cells, and oligodendrocytes, i.e., 6D2positive cells, were positively labeled also by A₂B₅ antibodies, which are known to label progenitors of type2 astrocytes and oligodendrocytes in the rat optic nerve. The results suggest that A₂B₅ positive progenitor cells in the goldfish central nervous system, as in the rat optic nerve, might be a common progenitor of astrocytes and oligodendrocytes. © 1992 WileyLiss, Inc.

Keywords: Biochemistry & Molecular Biology; Physiology

Axons of the central nervous system in adult mammals do not regenerate spontaneously after injury, partly because of the presence of oligodendrocytes that inhibit axonal growth. This is not the case in lower vertebrates (e.g., in fish), where regeneration of the optic nerve does occur spontaneously and has been correlated with the presence of factors cytotoxic to oligodendrocytes. The present study provides evidence that the substance originating from the fish optic nerves, which is cytotoxic to oligodendrocytes, is an interleukin 2-like substance.

Despite numerous reports of beneficial effects of GM1 ganglioside treatment following brain lesions in animals, the underlying neurobiological mechanism of ganglioside-induced functional restoration is still unclear. In order to obtain a better insight into this question, we have made use of a newly developed animal model of brain injury that would potentially permit us to determine the causal relationships(s) among behavioral and neuroanatomical/neurochemical parameters of restoration of function. Following graded crush of the adult rat optic nerve, we have treated the rats with intraperitoncally injected gangliosides and studied the functional outcome with electrophysiological and behavioral parameters. The electrophysiological recprding of the compound action potential (CAP) from excised rat optic nerve revealed a significant loss of CAP throughout the first 2 weeks after the injury. However, when rats were treated daily for 7 days with GM1-gangliosides, the CAP measured 10 days after the crush was significantly larger compared to operated controls without treatment. Thus, GM1 appeared to be capable of delaying or partially preventing retinal ganglion cells or their axons from secondary degeneration. Loss of visual function was also evident on the behavioral level of analysis: when rats with unilateral optic nerve crush were evaluated in a visual orienting paradigm, the rats revealed deficits in their ability to orient towards small, moving visual stimuli. However, within about 2 weeks, the animals recovered spontaneously to near normal performance. Daily treatment with GM1-gangliosides was found to significantly improve outcome, largely due to a reduction of the immediate post-lesion deficit. In a second behavioral experiment we also created graded crush in rats bilaterally and evaluated the animals visual capacities in a two-choice brightness discrimination task. In this park, an initial loss of function was followed by recovery within about 2 weeks, but GM1 treatment was without beneficial effects in this paradigm. It is concluded that GM1 improves outcome after graded crush of the adult rat optic nerve, although it appears that improved function needs to be documented with sufficiently sensitive behavioral assays.

The high post-traumatic regenerative ability of fish central nervous system has been partially attributed to the hospitable nature of the surrounding non-neuronal cells and their appropriate response to injury. Uncovering the correlation between fish non-neuronal cell structure and behavior might yield a better understanding of what makes them supportive to axonal growth. Towards this goal, structural proteins expressed by fish non-neuronal cells need to be characterized. In the present study we isolated cDNA clones encoding fish intermediate filaments which are prominent structural proteins in astrocytes. Among the isolated clones, one was identified as fish vimentin and another was found identical to the cloned fish keratin 8. Results are discussed with respect to the use of these cDNAs for further understanding of fish non-neuronal cell plasticity.

The results of this study attribute to tumor necrosis factor (TNF) a role in regeneration of injured mammalian central nervous system (CNS) axons which grow into their own degenerating environment. This is the first time that a specific factor involved in axonal regeneration has been identified. The axonal environment is occupied mostly by glia cells, i.e., astrocytes and oligodendrocytes. Previous studies have shown that mature oligodendrocytes are inhibitory to axonal growth. Therefore, it seemed likely that application of a factor such as TNF, which has been shown to be cytotoxic to oligodendrocytes, would contribute to the creation of permissive conditions for axonal regeneration. In the present work, injured adult rabbit optic nerves were treated with human recombinant TNF (rhTNF). As a result, abundant newly growing axons (circa 9000, about 4% of the total estimated number of axons in an intact adult rabbit) were observed traversing the site of injury.

We present a new method for creating conditions conducive to axonal growth in injured optic nerves of adult rabbits. The surgical approach consists of making a cavity in the adult rabbit optic nerve, into which a piece of nitrocellulose soaked with conditioned medium originating from regenerating fish optic nerves is implanted. In addition, daily irradiation (10 days, 5 min, 35 mW) with low energy He-Ne laser is carried out. Such a combined treatment may open a door to neurobiologists and clinicians, hoping to unravel the enigma of mammalian CNS regeneration.

Changes in arachidonic acid metabolism were studied in the optic nerve, the chorioretina, and in the vitreous following crush injury to the optic nerve of rats. Crush injury led to: 1. (i) a 3.9-fold increase in optic nerve prostaglandin type E₂ in vitro production which peaked on day 5 and was followed by a gradual decline, but was still significantly higher than baseline levels by day 12; 2. (ii) a two-fold increase in the chorioretina prostaglandin type E₂ in vitro production which peaked on day 1, and resumed baseline levels by day 3; 3. (iii) a 3.5-fold increase in vitreous prostaglandin type E₂ levels on day 1 which remained at 1.5-2 times higher than baseline levels for the rest of the study period (12 days). The findings indicate that the pattern of changes in prostaglandin type E₂ production by the optic nerve (consisting mostly of white matter) is different from that described for injured brain tissues. The prolonged accumulation of vitreal prostaglandin type E₂ in eyes with damaged optic nerve may lead to undesirable effects on the retina beyond those directly manifested in the retina by altered axonal flow in the injured optic nerve.

The intermediate filament glial fibrillary acidic protein (GFAP) is the predominant cytoskeletal protein of mature glial cells in the mammalian nervous system. The nervous systems of lower vertebrates, such as fish, have been examined for the presence of GFAP and several investigators have shown that goldfish (Carassius auratus) brain contains GFAPpositive astrocytes. The same studies have demonstrated that, in contrast to the brain, the optic nerve of goldfish did not show any GFAP immunoreactivity, suggesting that this intermediate filament protein is not expressed in fish optic nerve astrocytes. The present study shows, however, that the monoclonal antibodies to porcine GFAP react with the optic nerve of carp (Cyprinus carpio), another member of the goldfish family. These antibodies to porcine GFAP cross react with rat brain and carp optic nerve, yielding a band of approximately 52 kDa in both species. Northern blot analysis using mouse GFAP DNA probe revealed that carp optic nerve RNA contains two transcripts, of 2.3 and 2.1 kb, which hybridize with the mouse GFAP probe. Injury to the carp optic nerve was followed by a decrease of GFAP immunoreactivity from neural tissue and a strong expression around blood vessels and connective tissues. On the basis of these observations and within the limitation of the techniques it is reasonable to conclude that the carp optic nerve expresses GFAP immunoreactivity and that the pattern of expression of this intermediate filament protein is altered after injury. Such an alteration might be relevant to the process of regeneration.

Mammalian central nervous system (CNS) axons are virtually incapable of regenerating after injury. However, CNS neurons of lower vertebrates, such as fish and amphibians, are endowed with a high regenerative capacity. Lately, the glial cells have been credited with the regenerative ability of any specific CNS. We have previously demonstrated that many oligodendrocytes are recovered in cultures of injured rat optic nerve, while only a few oligodendrocytes are recovered from injured fish optic nerve in culture. We further demonstrated that medium conditioned by regenerating fish optic nerves (CM), which has been shown to cause axonal elongation in injured rabbit optic nerves, causes a decrease in the number of oligodendrocytes in rat glial cultures. In the present study, we demonstrate that soluble factors in the CM are capable of reducing the number of fish oligodendrocytes in fish optic nerve cultures. In addition, an inverse relationship was found between the number of macrophages and the number of oligodendrocytes. These results thus suggest that macrophages and/or activated resident microglial cells are directly or indirectly responsible for the presence of these soluble factor(s) that regulate the postinjury number of oligodendrocytes in the fish optic nerves.

The limited capacity for regenerative axonal growth by adult mammalian central neurons has been attributed, at least in part, to the presence of mature oligodendrocytes, which are non-permissive for axonal growth. These cells do not interfere with growth during development, as developmental growth is largely completed before the maturation of the oligodendrocytes. Unlike mammals, fish central nervous system is endowed with a high regenerative capability. When soluble substances derived from regenerating fish optic nerves are applied to injured adult rabbit optic nerves, regenerative axonal growth is permitted. Therefore, in the present study, we tested whether the fish optic nerve, after injury, is endowed with a mechanism by which it avoids the possible inhibitory effect of the process-bearing mature oligodendrocytes. Specifically, we looked for the possible presence of soluble substances that can regulate the number of process-bearing mature oligodendrocytes. We found that soluble substances derived from regenerating fish optic nerve, when added to cultures of oligodendrocytes derived from newborn or injured adult rat optic nerves, caused a decrease in the number of process-bearing mature oligodendrocytes. Soluble substances derived from normal noninjured fish optic nerves, had a significantly lower effect. The observed decrease in the number of mature oligodendrocytes could not be mimicked by the addition of platelet-derived growth factor (PDGF), a known mitogen of oligodendrocyte progenitors which transiently inhibits their maturation. This study suggests a role to oligodendrocyte inhibitory/cytotoxic factor(s) in regeneration.

In the mammalian peripheral nervous system (PNS), expression of the neural adhesion molecule L1 on Schwann cells and neurons has been correlated with axonal growth during development and regeneration. The present study was undertaken to examine whether a similar correlation exists between a lesion-induced increase of L1 expression and regenerative capacity in the central nervous system (CNS). The fish optic nerve was used as a model for a succesfully regenerating region of the CNS. Immunochemical and immunohistological experiments carried out with immunoaffinity purified polyclonal antibodies, generated against L1 from mouse brain, showed that carp optic nerve and brain, but not liver, contained L1 immunoreactivity. Western blot analysis of brain tissue yielded one distinct band at 200 kDa, while a double band at 200 kDa and two low-molecular weight bands at 120 and 100 kDa, possibly degradation products, were seen in the optic nerve. Immunohistological examination of normal optic nerves revealed L1 immunoreactivity, predominantly associated with connective tissue boundaries of nerve fascicles and with blood vessels, as well as inside axonal fascicles. L1 immunoreactivity was increased by 25%, 8 days after crushing of the optic nerve, as determined by radioimmunoassay on a nerve segment distal to the site of injury and compared with untreated control nerves. Increased levels of L1 were also seen by immunohistology and found to be predominantly associated, as in the normal nerve, with connective tissue boundaries and blood vessels. These observations suggest that a lesion-induced increase in L1 expression in the fish optic nerve is associated with axonal regrowth in the CNS.

Abstract: This study provides evidence that apolipoproteinAI (apoAI), derived from fish plasma and nerve, has heparin binding activity. We have shown previously that injury in a regenerative CNS, such as that of fish optic nerves, leads to increased levels of apoAI in media conditioned by these nerves, as compared with media conditioned by noninjured nerves. In the present study, we have purified and characterized apoAI from both fish plasma and optic nerves. Sequence analysis of the 15 Nterminal amino acids revealed that at least 14 amino acids are identical in these two purified apoAI samples. The purified apoAI derived from both fish plasma and optic nerves binds to heparin. Binding measurements using [³H]heparin followed by Scatchard analysis revealed that apoAI binds to heparin with relatively low affinity (K_D= 2.8 × 10⁻⁶M). Results are discussed with respect to the possibility that accumulation of apoAI in the extracellular matrix of fish optic nerves is made possible via heparin binding, like that to apolipoproteinE in mammals.

Spontaneous growth of axons after injury is extremely limited in the mammalian central nervous system (CNS). It is now clear, however, that injured CNS axons can be induced to elongate when provided with a suitable environment. Thus injured CNS axons can elongate, but they do not do so unless their environment is altered. We now show apparent regenerative growth of injured optic axons. This growth is achieved in the adult rabbit optic nerve by the use of a combined treatment consisting of: (1) supplying soluble substances originating from growing axons to the injured rabbit optic nerves (Schwartz et al., Science, 228:600603, 1985), and (2) application of low energy HeNe laser irradiation, which appears to delay degenerative changes in the injured axons (Schwartz et al., Lasers Surg. Med., 7:5155. 1985; Assia et al., Brain Res., 476:205212, 1988). Two to 8 weeks after this treatment, unmyelinated and thinly myelinated axons are found at the lesion site and distal to it. Morphological and immunocytochemical evidence indicate that these thinly myelinated and unmyelinated axons are growing in close association with glial cells. Only these axons are identified as being growing axons. These newly growing axons traverse the site of injury and extend into the distal stump of the nerve, which contains degenerating axons. Axons of this type could be detected distal to the lesion only in nerves subjected to the combined treatment. No unmyelinated or thinly myelinated axons in association with glial cells were seen at 6 or 8 weeks postoperatively in nerves that were not treated, or in nerves in which the two stumps were completely disconnected. Two millimeters distal to the site of injury, the growing axons are confined to a compartment comprising 5%30% of the cross section of the nerve. A temporal analysis indicates that axons have grown as far as 6 mm distal to the site of injury, by 8 weeks postoperatively. Anterograde labeling with horseradish peroxidase, injected intraocularly, indicates that some of these newly growing axons arise from retinal ganglion cells.

Heliumneon lowenergy laser (LEL) irradiation, known for its therapeutic effects in arthritis, wound healing, and muscular strain, was applied to eyes of rats that had sustained and optic nerve crush injury. Crush injury to the optic nerve resulted in a longlasting in vitro elevation of prostaglandin E₂(PGE₂) production (1.83.9fold above control values during the 12 day study period). LEL irradiation per se had no effect in vitro on PGE₂ production by the optic nerve; however, LEL irradiation of eyes with crushed optic nerve inhibited the enhanced production of PGE₂ and leukotriene B₄ in vitro by 55% and 75% during the late and immediate phase after trauma, respectively. Our findings of the suppressive effect of LEL irradiation on arachidonic acid metabolism are reminiscent of the actions of steroidal and nonsteroidal antiinflammatory drugs, and might indicate, for the first time, the possible biochemical mechanisms associated with the clinical antiinflammatory effects of LEL irradiation.

Crushed fish optic axons readily regenerate, while similarly injured rat optic axons do not; the reasons for the differences in regeneration ability may lie in differences in the environment of the axons. We have cultured glial cells from previously crushed optic nerves of fish and rat to determine whether a relationship exists between the ability to regenerate and the nature of the responses of the associated nonneuronal cells to injury. The glial cells were examined using indirect immunofluorescence with antibodies to known glial markers. In the rat cultures, mature GalC oligodendrocytes, which are known to be nonpermissive for axonal growth, were abundant. In contrast, in the fish cultures mature oligodendrocytes were rare, but A₂B₅ positive cells were abundant. The high number of A₂B₅ positive cells in the fish may suggest a high number of immature cells. This interpretation, however, should wait until evidence for glial cell lineage of the fish is available. Additional indication is provided also in the present study that the number of mature oligodendrocytes in the fish is regulated by elements external to the nerve. This study thus demonstrates an important difference between rat and fish optic nerves in the response of glial cells to the optic nerve injury.

Keywords: Developmental Biology; Neurosciences; Pharmacology & Pharmacy

In the present study, we have developed an animal model for central nervous system (CNS) damage using the graded crush injury of the adult rat optic nerve. This injury model has the following characteristics: (a) the injuries are reproducible; (b) the injuries can be created with controlled severity; and (c) variations in the severity of the injuries correlate with definable electrophysiological and behavioral outcomes. Self-closing forceps, modified by attaching a screw to the handle, were used as a lesion-causing device. Rat optic nerves were crushed in vitro. The severity of the injury, calibrated by a measured applied force, was determined, electrophysiologically, in vitro. Compound action potential was assessed and recorded by suction electrodes connected to both sides of the nerve. In vivo studies correlated the calibrated crush injuries with behavioral outcome. After severe crush (applied force: 205 g or higher), the rats were unable to orient towards visual stimuli presented in the visual field of the damaged nerve and only minor transient recovery was observed. After moderate crush (applied force: 120 g), however, there was a significant recovery of orienting performance up to the 6th postoperative day, followed by a gradual, secondary loss of function. In the mildly injured group (applied force lower than 30 g), functional improvement continued for up to 10 days and remained stable thereafter, with only minor secondary loss. This CNS graded injury model may be valuable to study the molecular and anatomical mechanisms underlying secondary degeneration and the potency of various posttraumatic treatments in leading to recovery of function.

Abstract Fish optic nerves, unlike mammalian optic nerves, are endowed with a high capacity to regenerate. Injury to fish optic nerves causes pronounced changes in the composition of pulselabeled substances derived from the surrounding nonneuronal cells. The most prominent of these injuryinduced changes is in a 28kilodalton (kDa) polypeptide whose level increases after injury, as revealed by onedimensional gel electrophoresis and autoradiography. The present study identified as apolipoprotein AI (apoAI) a polypeptide of 28 kDa in media conditioned by regenerating fish optic nerves. The level of this polypeptide increased after injury by approximately 35%. ApoAI was isolated by gelpermeation chromatography from delipidated highdensity lipoproteins (HDL) that had been obtained from carp plasma by sequential ultracentrifugation. Further identification of the purified protein as apoAI was based on its molecular mass (28 kDa) as determined by gel electrophoresis, amino acid composition, and microheterogeneity studies. The isolated protein was further analyzed by immunoblots of twodimensional gels and was found to contain six isoforms. Western blot analysis using antibodies directed against the isolated plasma protein showed that the 28kDa polypeptide in the preparation of soluble substances derived from the fish optic nerves (conditioned media, CM) crossreacted immunologically with the isolated fish plasma apoAI. Immunoblots of twodimensional gels revealed the presence of three apoAI isoforms in the CM of regenerating fish optic nerves (pIs: 6.49, 6.64, and 6.73). At least some of the apoAI found in the CM is derived from the nerve, as was shown by pulse labeling with [³⁵S]methionine, followed by immunoprecipitation. The apoAI immunoactive polypeptides in the CM of the fish optic nerve were found in high molecularweight, putative HDLlike particles. Immunocytochemical staining revealed that apoAI immunoreactive sites were present in the fish optic nerves. Higher labeling was found in injured nerves (between the site of injury and the brain) than in noninjured nerves. The accumulation of apoAI in nerves that are capable of regenerating may be similar to that of apoE in sciatic nerves of mammals (a regenerative system); in contrast, although its synthesis is increased, apoAI does not accumulate in avian optic nerves nor does apoE in rat optic nerves (two nonregenerative systems).

Adult injured optic nerves of mammals show an increased distribution of laminin in the extracellular matrix after application of soluble substances in the form of conditioned media originating from growing nerves (Zak, et al., 1987). This increase could result from direct or an indirect activation of glial cells or from activation of other cellular elements. In the present study, we show that the conditioned media derived from regenerating fish optic nerves contain factors that directly modulate the level of laminin in C6 glioma cells. The identity of the laminin was confirmed by metabolic labeling of the treated cells with [³⁵S]methionine and by subsequent immunoprecipitation and gel electrophoresis. The level of other extracellular matrix proteins, such as fibronectin, is also increased. The significance of the presence of glialactivating substances in the conditioned media of regenerating nerves for the process of regeneration is discussed.

Compression injury of a central nerve results in its degeneration with irreversible loss of function due to the inability of the mammalian central nervous system (CNS) to regenerate. In contrast, the CNS of lower vertebrates has a high capacity to regenerate. Recently, low energy laser irradiation was shown to attenuate degeneration in injured CNS nerves. The optic nerves of rats were subjected to moderate crush, calibrated so that some electrophysiological activity was preserved. The nerves were then subjected to low energy laser irradiation (10.5 mW, 2 min daily) for various periods. The electrical activity of the nerves, distal to the site of injury, was determined by measuring the compound action potential at the termination of the experiment. Two weeks of irradiation begun immediately after injury and continued daily thereafter, resulted in a compound action potential which was significantly higher (mean ± S.E.M. 1856 ± 535 μV) than that of non-irradiated injured nerves (351 ± 120 μV). The effect was temporary and subsided within a week. This two-week irradiation was slightly more effective than a treatment lasting one week (1406 ± 225 μV) and was significantly more effective than 4 days of irradiation (960 ± 133 μV). The number of treatments is therefore important. The time at which the treatment commences relative to the injury is also critical. Irradiation initiated two hours after the crush was about half as effective as immediate irradiation (810 ± 42 μV). No apparent effect was evident when the laser was applied for the first time 5 h, or longer, after the crush. Pretreatment with a laser irradiation immediately before the crush was an effective (1430 ± 281 μV) as irradiation soon after the injury. It is important to note that the laser effect is demonstrable only in moderately injured nerves. Severely injured nerves are not affected by treatment even after prolonged (3 and 4 weeks) irradiation. Our results further suggest that low energy laser irradiation only delays degenerative processes but does not prevent them. Optimal results are obtained when nerve crush is moderate, and the irradiation is initiated immediately before or soon after the injury and continued for at least one week.

Neurons in the mammalian central nervous system (CNS) have a poor capacity for regenerating their axons after injury. In contrast, neurons in the CNS of lower vertebrates and in the peripheral nervous system (PNS) of mammals are endowed with a high posttraumatic capacity to regenerate. The differences in regenerative capacity have been attributed to the different compositions of the respective cellular environments and to different responses to injury the nonneuronal cells display, which range from supportive and permissive to nonsupportive and hostile for regeneration. The same cell type may support or inhibit regeneration, depending on its state of maturity or differentiation. Astrocytes and oligodendrocytes are examples of cells in which such a dichotomy is manifested. In developing and in spontaneously regenerating nerves, these cells support (astrocytes) and permit (oligodendrocytes) growth. However, in nonregenerating adult mammalian nerves, astrocytes form the nonsupportive scar tissue; and the mature oligodendrocytes inhibit axonal growth. Maturation of these cells may be regulated differently during development than after injury. Among the putative regulators are factors derived from astrocytes, resident microglia; or cytokines produced by macrophages. During development, regulation leads to a temporal separation between axonal growth and maturation of the cellular environment, which might not occur spontaneously after injury in a nonregenerating CNS without intervention at the appropriate time. Data suggest that temporal intervention aimed at the glial cells might enhance the poor regenerative capacity of the mammalian CNS. Possible regulation of the nonneuronal cell response to injury via involvement of protooncogenes is proposed.

1. 1. We have compared the concentration and chemical composition of carp and human plasma lipoproteins and studied their interaction with human fibroblast LDL receptors. 2. 2. The main lipoproteins in carp are of high density (HDL) in contrast to low density lipoproteins (LDL) in human. 3. 3. Carp lipoproteins are devoid of apolipoprotein (apo) E, a major ligand for interaction with LDL receptors in mammals. 4. 4. Carp very low density lipoproteins (VLDL) and LDL but not HDL nor apoA-I cross react with human LDL in their interaction with LDL receptors on human cultured fibroblasts. 5. 5. Carp liver membranes possess high affinity receptors that are saturable and have calcium dependent ligand specificity (apoB and apoE) similar to human LDL receptor. Carp VLDL and LDL but not HDL nor its major apolipoprotein complexed to L-α-phosphatidylcholine dimyristoyl (apoA-I-DMPC) competed with the specific binding of human LDL to this receptor.

Adult rabbit retina can express regeneration-associated characteristics after optic nerve injury, provided it is supplied with appropriate diffusible substances originating from media conditioned by regenerating fish optic nerves or by optic nerves of a newborn rabbit [Hadani et al., Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 7965; Schwartz et al., Science, 228 (1985) 600]. This was shown by applying the active substances to the injured axons in the form of 'wrap-around' implants, consisting of collagen-coated silicone tubes which had been soaked in the conditioned media (CM). The regeneration-associated response was manifested biochemically and by sprouting of nerve fibers in culture. The present work provides morphological evidence that the implantation prolongs survival of ganglion cells and optic nerve fibers and induces new growth. Light microscopic analysis (using horseradish peroxidase (HRP) for labeling the fibers) revealed, 1 week following optic nerve injury, labeled fibers and ganglion cells in both the implanted and control (injured only or injured and implanted with collagen-coated silicone tubes free of CM) nerves. However, from the second week after the injury, distinct differences in the appearance of viable ganglion cells and labeled fibers, were seen between experimental and control preparations. In sections taken through the optic nerve, at the region distal to the site of injury, HRP-labeled fibers were seen in the experimental nerves 1 week, 2 weeks and to a significantly lesser extent 1 month after injury. In the control nerves, HRP-labeled fibers were scarcely seen from 2 weeks on after the injury. The fibers seemed to have a structure of growing tips at their terminals. Electron microscopic analysis revealed the presence of fibers with growth cones in the experimental nerves. These growth cones were found to be embedded in a scaffold-like structure formed by astrocytic processes. These results emphasize that active conditioned media implanted around the injured optic nerve of an adult rabbit causes a morphological response in addition to the already observed biochemical response.

Central nervous system (CNS) neurons of mammals regenerate poorly after axonal injury. However, if an injured CNS neuron (rabbit optic nerve) is supplied with appropriate soluble substances ('growth-associated triggering factors') derived from medium conditioned by regenerating fish optic nerve or newborn rabbit optic nerve, it can express regeneration-associated characteristics. Such characteristics include a general increase in protein synthesis, changes in synthesis of specific polypeptides, and sprouting of nerve fibers in culture. The present study of rabbit optic nerves demonstrates that such active substances affect the neuronal environment (i.e., the non-neuronal cells), thereby perhaps causing a shift in the environment from an inhibitory to a regenerative supportive one. Apparently, such an environment is spontaneously achieved in injured CNS nerves of lower vertebrates (e.g., fish optic nerves), which are regenerable. Treatment of injured rabbit optic nerve with soluble factors from medium conditioned by regenerating carp optic nerve resulted in a selective increase in proliferation ([³H]thymidine incorporation) of perineural cells and the appearance of a 12-kDa polypeptide in a homogenate derived from the nerve and its associated cells. This polypeptide may be related to growth, since it comigrates in NaDodSO₄/polyacrylamide gel electrophoresis with a 12-kDa polypeptide that is continuously present in a regenerative system. In addition, there were injury-induced changes in the polypeptides of the nerve that were independent of treatment with conditioned medium and were correlated with nerve maturation. The most prominent changes of this type were in 18-kDa and 25-kDa polypeptides whose levels were reduced after injury and were found to be correlated with the nerve maturation (myelination) state.

Lowenergy HeNe laser irradiation (LELI) was found to affect the electric activity and morphology in both intact and severely injured peripheral nerves in rats. Action potential (AP) in the healthy nerve increased by 33% following a single transcutaneous irradiation. Similar irradiation in crushed nerves caused AP to increase significantly over the AP of nonirradiated crushed nerve. Morphological observations revealed that a laserirradiated injured nerve had diminished scar tissue as compared to an injured but not an irradiated nerve.

Axons of the mammalian peripheral and central nervous systems degenerate after nerve injury. We have recently found that HeNe laser irradiation may prevent some of the consequences of the injury in peripheral nerves of mammals. In the present study, the efficacy of the laser in treating injured neurons of the mammalian CNS was tested. Optic nerves of adult rabbits were exposed daily for 814 days to HeNe laser irradiation (14 min, 15W) through the overlying muscles and skin. As a result of this treatment, the injured nerves maintained their histological integrity, which is invariably lost in injured mammalian CNS neurons.

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state (2); second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state (3) only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers (4) depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and evetually cease their elongation (5).

The relationships of neurons and non-neuronal cells are vital for the maintenance and function of neurons. Trauma alters these relationships causing proliferation of non-neuronal cells and, in adult mammalian CNS, presumably disturbs the environmental support needed for regeneration. A supportive environment can be restored by introducing a regenerating nerve to injured mammalian CNS. This response is probably due, at least in part, to diffusible substances secreted by the non-neuronal cells. We have obtained diffusible substances from either regenerating fish optic nerves or neonatal rabbit optic nerves and applied them around crushed adult rabbit optic nerves. This manipulation caused the adult nerve to show regenerative changes: a general increase of protein synthesis in the retinas; selective increase in synthesis of a few polypeptides in the retinas; sprouting from the retinas in vitro; increased viability of nerve fibers as shown by HRP staining; and the appearance of growth cones adjacent to glial limitans in the injured nerves. We termed these diffusible, active substances 'Growth Associated Triggering Factors' (GATFs). In addition to the phenomena described above, the active substances (obtained in the form of media conditioned by regenerating fish optic nerve or neonatal rabbit optic nerve) caused various other changes in the injured nerve itself: acceleration of non-neuronal cell proliferation; changes in the protein pattern, e.g. an increase in a 12 kDa polypeptide which might be a second mediator in the cascade of events leading to regeneration; increased laminin immunoreactive sites in the nerve; and the acquisition of growth supportive activity in media conditioned by the implanted injured nerves. Nerve growth factor (NGF) failed to induce such an effect whereas fibroblast growth factor (FGF) implanted around injured nerve had a significant effect on survival of the injured fibers. We would like to suggest that the non-neuronal cells have the capacity to control neuronal regeneration potential, provided they undergo the changes required for regeneration at the appropriate time. Any non-neuronal cell malfunction may lead to failure of regeneration. This brings an optimistic view to the study of regeneration as it suggests that mammalian CNS glial cells have the capacity to support regeneration provided they are stimulated at the right time with appropriate stimuli.

The effect of low energy CW HeNe laser irradiation on normal and dissected nerves in the rat was examined. The methods are described. Results are compared to the laser effect on other living tissues. HeNe irradiation was found to increase significantly the action potentials of the nerves. It was found to be a long-lasting effect, keeping an increase in the nerves action potential for more than eight months after irradition has been stopped. A possible explanation for the way the irradiation acts on the nerve is suggested.

Previous studies carried out in our laboratory have demonstrated that goldfish brain contains substances that promote neurite extension from regenerating retinae in culture. Fractionation of the brain extract by molecular sieving chromatography revealed the presence of several molecular species, including two peaks that have neurotrophic activity, representing low-molecular-weight substances. One peak was eluted (P-a) with an apparent molecular weight of about 13 kDa and was designated substratum neurite extension factor (SNEF) because it retained its neurotrophic activity when adsorbed onto the substratum. This recovered Sephadex fraction (P-a) when applied in vivo intraocularly caused an earlier capacity of the corresponding retinae to sprout in vitro. Thus, at 3 and 5 days after injury the neuritic growth indices from the factor-treated retinae were of 0.9 +/- 0.2 and 2.8 +/- 0.5, respectively, as compared with indices of 0.3 +/- 0.1 and 0.9 +/- 0.2, respectively, in retinae of injured but nontreated nerves. The factor was further purified by two steps of HPLC (ion exchange followed by reversed phase). The results showed that it is an acidic glycoprotein with an apparent molecular weight of 10 kDa.

Abstract: Translation products of mRNA from retinas of goldfish optic nerve (representing a regenerative CNS) and adult rabbit optic nerve (representing a nonregenerative CNS which can be induced to express regenerative characteristics) were examined by one and twodimensional gel electrophoresis. Translation products from retinas of the regenerating goldfish optic nerve included polypeptides barely detectable in the translation products of mRNA derived from retinas of uninjured controls. Some of these polypeptides, of apparent molecular weights 2428, 4349, 60, and 65 kilodaltons can be considered as growthassociated polypeptides described in other regenerative and developing systems. The induction of regenerationassociated characteristics in the injured adult rabbit optic nerve, \u201cimplanted\u201d with diffusible substances from nonneuronal cells of regenerative or growing nerve, is reflected by changes in the mRNA translation products of the retina. Among such translation products are those of the following molecular weights: 1618, 28, 3235, 4347, and 5660 kilodaltons, and some highermolecularweight species.

This work describes a surgical approach which establishes the rabbit's visual system as an experimental model for studying CNS regeneration. Using this model, the optic nerve, its cell bodies, and the axons, are easily accessible through an orbital approach, without the need for craniotomy and brain retraction. This surgical approach allows transplantation and 'wrap around' implantations of nerve segments from xenogeneic and syngeneic systems and diffusible substances derived from them, respectively. Furthermore, it enables studies aimed at determining deficiencies in mammalian CNS and investigating methods of augmentating mammalian CNS regeneration.

Regeneration of fish optic nerve (representing regenerative central nervous system) was accompanied by increased activity of regeneration-triggering factors produced by nonneuronal cells. A graft of regenerating fish optic nerve, or a "wrap-around" implant containing medium conditioned by it, induced a response associated with regeneration in injured optic nerves of adult rabbits (representing a nonregenerative central nervous system). This response was manifested by an increase of general protein synthesis and of selective polypeptides in the retinas and by the ability of the retina to sprout in culture.

Goldfish retinae deprived of their target by tectal ablation showed characteristics of regeneration, such as sprouting activity in culture and changes in protein synthesis. These features resemble changes occurring in regenerating retinae of optic nerves which were subjected only to crush injury. Retinae of tectal ablated nerves maintained their ability to sprout in culture longer than retinae of crushed-injured optic nerves. This may indicate a possible absence of the machinery which regulates growth termination as reflected in this case, by the enduring growth activity in culture. Among the changes in protein synthesis which occurred in retinae following either tectal ablation or crush injury, the most pronounced were in tubulin and in some other polypeptides of the following molecular weights: 46-49, 65 and 74 kdalton. Results are discussed with respect to the possible role of the target in initiating regeneration upon disconnection of the nerve, and in terminating growth upon reconnection.

In our previous work we established conditions to study the contribution of non-neuronal cells to the process of goldfish optic nerve regeneration. This issue has been studied successfully by adapting the use of X-irradiation to manipulate division of non-neuronal cells associated with the injured nerve. The regenerative capacity of the goldfish retinal ganglion cells was determined subsequent to the X-ray treatment. In this work we present an analysis of the molecular events associated with regeneration and enhanced regenerative capacity which follows X-irradiation. Under normal conditions the non-neuronal cells surrounding an intact nerve released axonal growth inhibitors, the level of which was only slightly sensitive to X-irradiation. In contrast, regenerating nerves showed a marked decrease in substances having an inhibitory effect on sprouting in vitro. Moreover, their level was significantly reduced following X-irradiation which was accompanied by increased accumulation of fibrous collagen adjacent to astrocytes. The alteration in the reciprocal relationship between the axon and the surrounding non-neuronal cells was also manifested by changes in the profile of labeled proteins released by the non-neuronal cells. The results of this work, therefore, indicate the sensitivity of the retinal ganglion cells to environment provided by the cells surrounding the regenerating fibers and thus may open a new perspective relating to the capacity of neurons to regenerate.

Monoclonal antibodies directed primarily against antigenic determinants associated with the goldfish optic nerve were prepared and characterized. One selected clone 234C(IgG2a), detected antigenic determinants of glycoprotein nature with an apparent mol. wt. of 140 000. Following injury the level of these molecules increased with a peak at 57 days after the lesion (2 to 3fold higher than the basal level). The results strongly suggest that the increase derives, at least partially, from a real increment in the level of these molecules in the retinal ganglion cells rather than from changes in accessibility. Immunofluorescence studies indicate that the retinal ganglion cells carry the antigenicity. Intraocular injection of the monoclonal antibodies, concomitantly with crush injury, resulted in an earlier and higher regenerative response, reflected by sprouting capacity, protein synthesis and accumulation of radiolabeled material in the tectum contralateral to the side of injury. This may indicate that the antibodies directly activate retinal cells via interaction with surface molecules. Alternatively, the antibodies may be directed against surface molecules which are associated with axonal growth inhibitors, and may therefore mask these surface antigens from further interaction with their native substrate.

The neurites growing out of the goldfish regenerating retinas, in vitro, provide a system in which the mechanism of growth induction, fiber maintenance and elongation can be investigated. In the present study we have demonstrated that pulsed electromagnetic fields, varying in strength, affect the extent of growth from the regenerating retinas, when the retina) explants are kept under sub-optimal conditions (0.5 % fetal calf serum, FCS). The effect ranges from facilitation and promotion of growth to the inhibition of it, depending upon the parameters of the applied electromagnetic field. The results are discussed with respect to the inter-relation between the different field parameters and the extent of growth.

An association between gangliosides and neuronal regeneration in goldfish is demonstrated in the present study. A single intraocular injection of affinity purified anti-GM₁ antibodies administered simultaneously with crush injury of the optic nerve, inhibits the regenerating process as expressed by two parameters: protein synthesis in the retina and in vitro sprouting ability from the retina. The retinal level of several gangliosides (such as GD₃, GD_1a, GD_1b and GT_1b) is enhanced during regeneration. Although GM₁ appears to be a minor retinal ganglioside, antibodies to GM₁ exert a marked effect on retinal regenerative process. It is assumed that such antibodies could interact with more abundant retinal gangliosides such as GD_1b which shows enhanced biosynthesis during regeneration and which shares a similar disaccharide terminal residue with GM₁.

We have recently shown that cell bodies of an injured optic nerve of adult rabbit can be induced to express regeneration-associated response by external signals derived from nonneuronal cells of regenerating nerves of lower vertebrates. In this study it is shown that even substances derived from a nonregenerating mammalian system also can trigger such a regenerative response. Thus, substances derived from intact nerves of neonatal rabbits and of adult rabbits, to a lesser extent, were active in triggering a regeneration-associated response, whereas substances derived from injured nerves of adult were not. However, if subsequent to the injury the nerve was implanted with a silicone tube containing medium conditioned by neonatal optic nerves, the substances derived from the implanted injured nerve were active. Thus, it appears that the ability of a periaxonal environment to provide triggering substances correlates with axonal growth. Therefore, we named these substances 'growth-associated triggering factors' (GATFs). It is suggested that mammalian cells are unable to express a regenerative response after an injury due to the failure of their nonneuronal cells to produce regeneration-triggering substances. This disability may be circumvented by an appropriate implantation procedure.

Intraocular injection of antiserum to mixed ganglioside or to GM1 inhibited the regeneration of goldfish optic axons following an optic nerve crush. For example, injections of antiserum on 5 consecutive days beginning the day before the crush resulted in a decrease of about 40% in the axonal outgrowth distance measured at 10 days after the crush. The inhibition was observed even when the treatment was begun a few days after the lesion, and greater degrees of inhibition were observed when the treatment was given later in regeneration. This indicates that the antiganglioside serum interfered with axonal elongation more than with the initial sprout formation. The antiganglioside treatment did not impair the enlargement of the cell bodies and nucleoli that accompanies regeneration, nor did it affect fast axonal transport of protein, glycoprotein, or glycolipid in regenerating nerves. Thus inhibition of outgrowth by antiganglioside treatment was not mediated by a gross change in the metabolism of the regenerating neurons. Treatment of normal neurons with the antiserum produced a 2030% increase in the amount of ³Hglucosaminelabeled glycoproteins and glycolipids conveyed by fast axonal transport. These results suggest that membrane gangliosides may normally influence the supply of axonally transported glycosylated macromolecules. However, the effect of antiganglioside on axonal transport of glycosylated molecules and on axonal outgrowth are not necessarily related to each other.

The molecular regulation of tubulin synthesis was investigated in the regenerating goldfish retina. Previous in vivo studies pointed to an increase in tubulin synthesis in the retina during regeneration of the injured goldfish optic nerve. Using labeled cDNA probes, we showed that this increase occurs as a result of enhanced tubulin mRNA levels. Analysis of labeled in vivo products revealed enhanced β₂-tubulin synthesis accompanied by an increase in the level of the low-M_r microtubule-associated proteins identified as TAU factors. The results are discussed with respect to the possible involvement of these proteins in the process of nerve regeneration.

Systemic humoral factors have been studied in traumatic, chronic and acute spinal cord injured patients. Antibodies specific to nervous system autoantigens were detected in a majority of the sera obtained from these patients, at different periods after injury. Limited in vitro sprouting of dorsal root ganglia in chicken embryos was observed in the presence of serum from these patients. The possible association between growth inhibiting factors and the presence of antibodies against nervous tissue autoantigens is discussed.

Proliferating cells associated with the visual pathway were found in the present study to affect the regenerative capacity of the goldfish retina following optic nerve injury. The contribution of these cells to the process of regeneration was investigated in the goldfish visual system by reducing their proliferation in the optic tract and tecta, using X-irradiation. The regenerative ability of the retina was then evaluated by the following parameters: sprouting from retinal explants, protein synthesis in the retina and accumulation of radiolabeled transported components in the tectum. X-irridiation of the visual system at an early stage of the regeneration process had a promoting effect whereas irradiation at a later stage resulted in a reduced capacity to regenerate. The results are discussed with respect to the possibility that proliferating cells, possibly glia, exert two contradictory contributions: an inhibitory effect at the site of injury, whereas distal to it, a supportive, perhaps trophic effect.

An image analysis technique was developed which provides an automatic way of measuring the fiber outgrowth from regenerating retinal explants.