The mechanism of antigen presentation by endothelial cells.

Endothelial cells line the vessels and lymphatics forming a barrier between circulating T cells and the extravascular tissue site of antigen. We have suggested that circulating T cells recognize antigen on the surface of endothelial cells, resulting in the activation of the endothelium such that the endothelial cells then release the key mediators of a cell-mediated immune response. To test this hypothesis, we have evaluated the extent to which endothelial cells can signal antigen-specific T cell activation. We have shown that cultured endothelial cells are as effective as macrophages in lymphocyte activation and that this activation is HLA-DR restricted. In additional experiments we have established that endothelial cells synthesize both Ia and IL-1 early in the signaling process. To eliminate any possible contribution of other cell types participating in the T cell-endothelial cell interaction, we have shown that cloned endothelial cells present antigen to cloned T cells. Moreover, there appeared to be a preference of selected T-cell populations for different types of antigen presenting cells. These experiments document that endothelial cells are independently competent antigen presenting cells.