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Human leukocyte antigen (HLA)-DR ligation mediates cell death of antigen-presenting cells (APC), including mature B cells, macrophages, and dendritic cells. This study investigates the apoptotic effects of HLA class II ligation mediated by anti-HLA antibodies on activated human vascular graft endothelial cells (ECs).HLA class II expression was examined by flow cytometry using a panel of HLA-typed vascular ECs isolated from transplant donors and compared with that of B lymphocytes. The apoptotic effects of anti-HLA-DR monoclonal antibodies (mAbs) were investigated using viability assays, DNA content analysis, and annexin-V labeling. Intracellular signaling pathways mediated by HLA-DR ligation on ECs were examined by Western blotting.Even with optimal stimulation, the expression of HLA-DR on interferon (IFN)–γ-treated ECs was quantitatively lower (3–5-fold) than that on B cells. Whereas anti-HLA-DR monomorphic mAbs induced apoptosis of B cells (approximately 22%), no significant apoptosis of IFN–γ-activated (DR-positive) ECs (<5%), collected from the same donor, was observed under the same conditions. Similarly, specific polymorphic anti-HLA-DR11 or -DR16 antibodies were unable to induce EC apoptosis. Nevertheless, antibody-binding to HLA-DR on ECs is sufficient to induce intracellular signaling, as evident in the modulation of tyrosine phosphorylation and protein kinase (PK)C-α/β and PKB/Akt activation. Our results suggest that HLA-DR ligation induces both common and divergent signaling events in ECs and B cells.Collectively, our data suggest that, in contrast with professional APC, graft ECs evade apoptosis mediated by HLA-DR ligation, not as a result of moderate HLA-DR expression but rather as a result of a specific signaling pathway.