Research

Introduction

Circulating immune cells and hematopoietic progenitors must exit blood vessels near specific target sites of injury, inflammation or tissue repair. The vessel wall at these sites displays specific combinations of traffic signals in the form of adhesion molecules (selectins, integrins) and chemotactic cytokines (chemokines) which operate in sequence to recruit only specific circulating subsets with proper receptors to these signals. As these processes take place under shear stress, these traffic molecules have evolved to operate under specialized kinetic and mechanical contexts.

The role of DC ICAM-1 in immune synapses in vivo

ICAM-1 is major cell adhesive ligand of the leukocyte integrin LFA-1. It is expressed on various vascular cells, pericytes, on different leukocytes including antigen presenting dendritic cells (DCs), macrophage subsets and B cells, as well as on various transformed epithelial cells. We have recently found that functional contacts between CD4 lymphocytes and adjuvant stimulated dendritic cells (DCs) inside lymph nodes (a model for skin vaccination) take place independently of ICAM expression on these DCs. Our work shows that priming and Th1 differentiation of these lymphocytes depend primarily on TCR signals rather than on T cell integrins.

Targeting killer T cells to metastatic lesions

Although it is well established that tumor-specific CTLs enter primary tumors (e.g., raised against patient specific neoantigens or tumor-enriched antigens), the ability to use similar tumor-specific CTLs to eradicate secondary tumors (metastasis) at remote organs has been very limited because these injected CTLs must be able to exit (extravasate) blood vessels nearby the metastatic lesions they are assigned to eliminate. The lung is a major organ of metastasis of multiple malignancies including melanoma, breast cancer and lung cancer. Currently, the feasibility of targeting tumor specific killing T cells through the dense vasculature of the lung to enter metastatic lesions and kill these lesions is unclear.

Breast cancer killing and metastasis

Malignant breast epithelial cells express elevated levels of the leukocyte integrin ligand ICAM-1. Tumor cells are targeted by NK cells and cytotoxic T cells (CTLs). These cells generate specialized killing synapses with both cancer cells and several stromal cells in their surroundings. Various studies implicated ICAM-1 on target tumor cells and LFA-1 or related integrins on killer leukocytes as essential for the generation of killing synapses but these killing synapses have been studied mainly in vitro focusing on malignant immune cells as targets rather than on solid tumors. Thus, evidence that this ligand is involved in the killing of specific solid tumors such as breast carcinoma by NK cells and CTL is still missing.

Chemokine receptors and adaptors on microvilli

Our earlier work has shed light on the earliest signals transmitted by chemokines through their G-protein coupled receptors (GPCRs) to distinct leukocyte integrins (Scheme 1). In lymphocytes, the integrin adaptor talin1 was found to control major conformational changes of both LFA-1 and VLA-4 integrins implicated in lymphocyte arrest on endothelial ligands.

TCR activation

Lymphocyte stoppage on dendritic cells (DC) requires activation of LFA-1 by T cell receptor (TCR) signals, but the molecular basis of this activation is elusive. Using antibodies for specific LFA-1 conformations, we found that TCR activation in resting lymphocytes was insufficient to trigger LFA-1 extension or headpiece opening. However, TCR signals facilitate rapid conformational activation of LFA-1 by immobile ICAM-1, and promote lymphocyte spreading without requirement for cytosolic Ca2+.