Every T cell that infiltrates a rejecting allograft develops a transient, but intimate association with the allogeneic endothelial cells that line the graft microvasculature. This may provide significant T cell activation at the graft site. Indeed, we have provided evidence that there are several pathways by which allogeneic endothelia can provoke T cell activation. The proposed studies are designed to further characterize these pathways, to investigate the immunologic consequences of each pathway, and to determine the influence of inflammatory cytokines or immunosuppressants on pathway utilization. To facilitate these studies, we have developed endothelial cell lines (GVEC) for which autologous T cells, B cells and Mo are available. This provides an unique opportunity to ask many important experimental questions regarding lymphocyte-endothelial cross-talk during inflammatory responses, and allows endothelial cells to be evaluated in relation to other APC types. The proposed studies are outlined by the following Specific Aims: (I) To define the relative efficiency and immunologic consequences of HTL stimulation by allogeneic GVEC. These studies will employ cytokine-stimulated or unstimulated GVEC or matched allogeneic Mo or B cells as allogeneic stimuli, and purified CD4+ or CD8+, naive or memory T cells as responders. HTL responses will be defined with limiting dilution analysis of HTL frequency, and by PCR analysis of lymphokine mRNAs. (II) To define the relative efficiency and immunologic consequences of CTL stimulation by allogeneic GVEC. These studies will use limiting dilution analysis to quantitate CD4 + and CD8 +, naive and memory CTL responses to allogeneic GVEC vs. matched allogeneic Mo or B cells. They will also evaluate GVEC and cytokine-treated GVEC as target cells for alloactivated CTL. (II) To define the patterns of endothelial behavior caused by local HTL alloactivation. These studies will utilize transwells to model contact-independent lymphocyte- endothelial interactions driven by alloactivated T cells. Endothelial responses will be monitored by flow cytometric analyses of endothelial phenotype and PCR analyses of cytokine mRNA production. The transwell system will also be used to analyze the influence of selected regulatory cytokines (IL2, lL4, IL1O and IFNg) or selected clinically-relevant immunosuppressants (CsA, FK5O6, RS61447, and OKT3) on the efficiency and consequences of HTL and GVEC activation. (IV) To determine the relative ability of GVEC to serve as autologous APC for allogeneic peptides. These studies utilize the unique advantage of GVEC, i.e., the availability of autologous leukocytes, to develop a model of antigen processing and/or presentation by endothelial cells. A peptide fragment of HLA-DR1 that binds to HLA-DR11, tetanus toxoid (TT), and a TT peptide fragment that binds to HLA-DR2 and DR5, will be used as antigens. In both cases, HTL responses will be monitored as described in Specific Aim I. Together, these studies will provide a more complete understanding of alloantigen processing/presentation by endothelial cells, of T cell responses to endothelial alloantigens, and of endothelial responses to alloantigen- activated T cells.