Endothelial Cell Interactions with the Immune System
Wnt regulation of T cell migration
Wnts are a family of secreted glycoproteins with diverse roles in development, including regulation of cell migration, however little is known about wnt signaling in mature T cells. We have found that endothelial cell-derived wnts, acting through Frizzled (Fz) receptors, induce MMP2 and MMP9 expression in effector T cells. Blocking wnt signaling, or MMP activity, reduces T cell migration through basement membrane in vitro, and into inflamed skin in vivo. Wnt signaling stabilizes beta-catenin protein in T cells and directly targets the MMP promoters through tandem LEF/TCF sites. Thus our data support a necessary, novel, and previously unexpected role for wnt signaling in T cell extravasation. This work is now in press at Immunity (Wu, Crampton and Hughes, 2007). We are currently extending this work into various mouse models of immunopathogenesis.
Differences between mouse and human immunology
We have recently reviewed the numerous differences between mouse and human immunology (Mestas and Hughes). Mice are the experimental tool of choice for the majority of immunologists and the study of their immune responses has yielded tremendous insight into the workings of the human immune system. However, as 65 million years of evolution might suggest, there are significant differences. Here we outline known discrepancies in both innate and adaptive immunity, including: balance of leukocyte subsets, defensins, toll receptors, iNOS, the NK inhibitory receptor families Ly49 and KIR, Fc receptors, Ig subsets, the B cell (BLNK, Btk and lambda5) and T cell (ZAP70 and gamma-c) signaling pathway components, Thy-1, gamma/delta T cells, cytokines and cytokine receptors, Th1/Th2 differentiation, costimulatory molecule expression and function, antigen-presenting function of endothelial cells, and chemokine and chemokine receptor expression. We also provide examples, such as multiple sclerosis and DTH, where complex, multicomponent processes differ. Such differences should be taken into account when using mice as preclinical models of human disease.
Endothelial cell expression of ITAC (CXCL11)
Endothelial cells actively recruit circulating immune cells into sites of active inflammation. We have identified ITAC as being a major CXC chemokine expressed by EC, along with Mig (CXCL9) and IP-10 (CXCL10) (Mazanet et al). ITAC binds with extremely high affinity to the CXCR3 receptor, which is also the receptor for Mig and IP-10. CXCR3 is expressed exclusively by memory T cells, suggesting that the ITAC-CXCR3 interaction may be particularly important in the recruitment of memory T cells across the endothelial barrier.
Early signalling events leading to T cell IL-2 synthesis in response to endothelial cells
Within a couple of hours after activation T cells begin to synthesize large amounts of the growth factor/cytokine IL-2. We are interested in the factors that mediate IL-2 transcription and in the signals that lead to their activation. For example, AP-1 is a major transcription factor involved in IL-2 activation and we have previously shown that endothelial cell signaling upregulates the AP-1 family member c-fos in T cells. Our recent data show that the earliest events in T cell activation by EC involve the induction of lipid raft aggregation (Mestas et al.). Lipid rafts are enriched for signaling proteins and their aggregation is essential for proper signaling through the T cell receptor. We have shown that CD2-CD58 intereaction is essential for EC-mediated aggregation.
Late signalling events leading to increased cytokine mRNA stability in T cells costimulated by endothelial cells
Recent studies have shown an important role for OX40L expression on EC in costimulation of T cell cytokine synthesis. Our studies suggest that signaling through T cell OX40 leads to stabilization of the mRNA for several cytokines including IL-2 and IFN-gamma. These signals do not affect transcription of IL-2. The enhanced synthesis of cytokines later in the T cell response prolongs T cell proliferation and may therefore extend immune responses. (Mestas et al. in press)
Endothelial cells, but not smooth muscle cells or fibroblasts, stimulate resting, memory T cells
Endothelial cells are often called semi-professional antigen-presenting cells. We have shown that they can present antigen to (activate) resting memory, but not naive, T cells (Salazar Murphy et al). Professional antigen-presenting cells, such as dendritic cells, can also activate naive T cells. Smooth muscle cells and fibroblasts, also components of the vessel wall, do not appear to be capable of activating resting or naive T cells. Using antibody screening and molecular biological techniques we are attempting to identify the surface molecules on endothelial cells that confer this ability. Recently we have identified B7-H1 and GL-50 on EC (Mazanet et al). B7-H1 negatively regulates cytokine expression and may be involved in setting thresholds for T cell activation. We are currently investigating the importance of multiple pro- and anti-inflammatory molecules on EC and testing the hypothesis that changes in the balance of these molecules may affect the costimulatory ability of EC.
Endothelial cells confer Cyclosporin A resistance on T cells
T cell activation, as measured by IL-2 synthesis, is normally blocked by the immunosuppresive drug CsA. However, we have found that when stimulated by endothelial cells T cells become resistant to the effects of the drug and secrete IL-2. Resistance develops between 8 and 12 hours and requires cell-cell contact. We are interested to know what surface receptor-ligand pairs mediate the CsA resistant signaling and how these signals relate to transcription factor activation. The transcription factor NFAT is a major target of CsA and is the molecule we are currently focussing our attention on. Our recent data suggest that wnt signaling through a non beta catenin-dependent pathway may be involved in maintaining NFAT in the nucleus of T cells, even in the presence of CsA (Salazar Murphy et al.).
Transmission electron micrograph of an endothelial cell monolayer