The role of adhesion molecules in endothelial cell accessory function

Influence of nutrients and cytokines on endothelial cell metabolism

The vascular endothelium plays an active role in physiological processes such as hemostasis, regulation of vessel tone and vascular permeability. Cell injury, or any event which disrupts endothelial integrity and thus endothelial permeability properties, may be involved in the early events leading to atherosclerotic lesion formation. Because of its constant exposure to blood components, including prooxidants, diet-derived fats and their derivatives, the endothelium is susceptible to oxidative stress and to injury mediated by blood lipid components. It is likely that these events potentiate the overall inflammatory response to injury by increasing cytokine release in proximity to the endothelium, which then could further disrupt endothelial barrier function. Even though mechanisms associated with lipid/cytokine-mediated endothelial cell dysfunction are unclear, our data suggest that they may be both oxidative and non-oxidative in nature. We suggest that dietary fats, rich in certain unsaturated fatty acids are atherogenic by enhancing the formation of reactive oxygen intermediates. These intermediates can activate oxidative stress-responsive transcription factors, such as NF-kappa B, which in turn may promote cytokine production, adhesion molecule expression and ultimately endothelial barrier dysfunction. The resulting disturbances in endothelial integrity possibly allow increased penetration of cholesterol-rich lipoprotein remnants into the arterial wall, a critical event in the etiology of atherosclerosis. Data suggest that certain nutrients, which have antioxidant and/or membrane stabilizing properties, protect endothelial cells by interfering with the above proposed mechanisms of endothelial cell dysfunction.

Endothelial cells promote anti-CD3-induced T-cell proliferation via cell-cell contact mediated by LFA-1 and CD2

T-cell activation requires not only T-cell receptor (TCR) engagement and subsequent TCR/CD3 cross-linking, but also one or more secondary activation signals generated by accessory cells (AC). We investigated the accessory function of endothelial cells (EC) in an in vitro model for T-cell activation where the first cross-linking signal was delivered to peripheral human T lymphocytes by either immobilized anti-CD3 monoclonal antibody (MoAb) or by PHA. In a previous report, we showed that EC provided a potent costimulatory signal in this model system. We have now analysed the nature of the EC-derived costimulatory signal by testing whether EC could be substituted by cytokines, by studying the effect of EC fixation and by testing the involvement of a number of adhesion molecules. Our findings indicate that EC accessory function is mediated mainly by membrane-bound factors. The nature of these membrane-bound factors was analysed by studying the inhibitory properties of a series of MoAbs directed against several adhesion molecules. Antibodies directed against CD44, E-selectin, CD31, CD26, B7/BB1, VLA-4 or VCAM-1 were not inhibitory. However, an inhibition, was clearly observed with antibodies against LFA-1 and CD2. Remarkably, this inhibition was not found with MoAbs to their respective counterstructures ICAM-1 and LFA-3. In summary, we postulate that both LFA-1/ICAM-1, and CD2/LFA-3 interactions are involved in EC accessory function, although the role of the EC-associated adhesion partners is not fully understood.

The proliferative response of human T cells to allogeneic IFN-gamma-treated endothelial cells is mediated via both CD2/LFA-3 and LFA-1/ICAM-1 and -2 adhesion pathways

We studied the proliferative response of purified human peripheral blood T lymphocytes (contaminated with less than 0.1% monocytes) to allogeneic MHC class II molecules expressed by endothelial cells (EC) or fibroblasts (FB). In vitro expression of MHC class II molecules was induced by gamma-interferon (IFN-gamma) treatment. The MHC class II expression levels after IFN-gamma treatment on both cell types were comparable. No T cell proliferation was found in the presence of either untreated or IFN-gamma-treated FB, and a marginal proliferation in the presence of untreated EC. IFN-gamma-treated EC, however, were able to induce significant T cell growth. The previously established role of MHC class II molecules in allogeneic T cell proliferation was confirmed in inhibition experiments with monoclonal antibody (mAb) against MHC class II or CD4. In this model, we tested the involvement of a number of adhesion molecules by adding mAbs to cocultures of T cells and IFN-gamma-treated EC. Monoclonal antibodies directed against CD31, CD26, B7/BB1, E-selectin, CD44, VLA-4 alpha-chain and VCAM-1 had no effect, whereas moderate inhibition was observed with anti-VLA-beta-chain and anti-LFA-3. A distinct inhibition of T cell proliferation was observed with mAbs directed against LFA-1, CD2, or a combination of anti-ICAM-1 and -2. Combinations of mAbs directed against T cell adhesion molecules (LFA-1, CD2, VLA-4) or EC adhesion molecules (ICAM-1, and -2, LFA-3, VCAM-1) were able to block T cell proliferation for 100 and 80% respectively. We conclude that CD2/LFA-3 and LFA-1/ICAM interactions are crucially involved in allogeneic T cell/EC interactions.