Research Projects

• In vivo Origins of Mononuclear Phagocytes

  
  
  We investigate M and DC development in physiological context by using adoptive precursor cell transfers into Mononuclear Phagocyte-depleted mice. Grafts are isolated from donor mice that harbor a specific GFP label in MP precursors (CX₃CR1^GFP mice). When applied to blood monocytes, this enabled us to define two functionally distinct circulating blood monocyte subsets (Geissmann et al. 2003, Landsman et al. 2007). Extension of this approach to BM monocytes and BM-resident progenitor cells that give exclusive rise to M and DC (termed MDP; Fogg et al. 2006) allowed us to establish a complete myeloid in vivo differentiation sequence from MDP via BM and blood monocyte intermediates into peripheral CD11c^high DC in lymphoid and non-lymphoid tissues (Varol et al. 2006).

  Interestingly in steady state, grafted monocytes give efficiently rise to lung and intestinal lamina propria MP, but not splenic CD11c^high DC. Instead, the latter appear to develop in homeostasis directly from local MDP without monocytic intermediate. To investigate the molecular cues that guide MP differentiation, we will use the transfer of genetically manipulated MP precursors. Besides providing insights in the organization of the MP system, these studies aim to explore the potential of a MP precursor transfer for a future therapeutic manipulation of the peripheral MP composition.

  

• In vivo Functions of Mononuclear Phagocytes

  

  To probe for differential functions of murine macrophages, DC and DC subsets, we developed an in vivo system that allows the conditional ablation of defined MP subpopulations in the intact mouse. Our model is based on restricted expression of a human diphtheria toxin (DTx) receptor transgene in mice that are naturally resistant to the bacterial exotoxin. DTx injection of CD11c-DTR mice, in which DTR expression is targeted to CD11c^high cells, results in rapid depletion of conventional CD11c^high DC (cDC) and allowed to establish the requirement of DC for cytotoxic CD8⁺ T cell responses to intracellular pathogens, incl. Listeria, LCMV and Malaria parasites. Interestingly, CD4⁺ T cell responses in the lymph nodes, but not the spleen, persist in the absence of cDC and are primed by Plasmacytoid DC suggesting further specialization within the APC compartment.

  To extend our ablation strategy to DC subsets, we are developing a Cre/loxP technology-based binary transgenic system that will restrict DTR expression to CD8α⁺ DC, which are suspected to play a unique role in cross-presentation and tolerance establishment.

  

• The Physiological Role of the CX₃C Chemokine Family

  
  

  The CX₃C chemokine family is comprised of the 7TM receptor CX₃CR1 and its sole ligand Fractalkine/ CX₃CL1. Both CX₃CR1 and CX₃CL1 are widely expressed throughout the organism, but in given tissues expression is highly cell type-restricted. Taking advantage of mice that harbor a targeted replacement of the CX₃CR1 gene by a GFP reporter (Jung et al. 2000), we could f.i. show that CX₃CR1 expression in the blood is largely restricted to circulating blood monocytes and might play a role in atherosclerosis.

  In the gut CX₃CR1 is specific for villus lamina propria DC that interact with CX₃CL1-expressing columnar epithelial cells to form trans-epithelial dendrites (TED) (Niess et al. 2005, see Figure). In the brain CX₃CL1/R1 interactions are confined to the neuronal-microglial interface, though their function remains unknown. More recently we have focused on the lung, where engagement CX₃CR1-expressing leukocytes might play a critical role in the development of Airway Hyper-reactivity and asthma.

  The key to the enigmatic physiological role of CX₃CR1/L1 might lie in the unique structure of CX₃CL1, which - unlike classical small peptide chemokines - is synthesized as a large, membrane-tethered protein and presented on an extended mucin-like stalk. Constitutive or inducible proteolytic cleavage by the metalloproteases ADAM 10 and 17 (TACE) can however result in release of a soluble CX₃CL1 entity. To determine the physiological role of membrane-anchored vs. shed CX₃CL1 we plan to reconstitute CX₃CL1-deficient mice with BAC-transgenes encoding cleavable, non-cleavable and secreted CX₃CL1 variants.