Rapid induction of apoptosis in CD8+ HIV-1 envelope-specific murine CTLs by short exposure to antigenic peptide

During primary viral infection, in vivo exposure to high doses of virus causes a loss of Ag-specific CD8+ T cells. This phenomenon, termed clonal exhaustion, and other mechanisms by which CTLs are deleted are poorly understood. Here we show evidence for a novel form of cell death in which recently stimulated CD8+ HIV-1 envelope gp160-specific murine CTLs become apoptotic in vitro after brief exposure to free antigenic peptide (P18-I10). Peak apoptosis occurred within 3 h of treatment with peptide, and the level of apoptosis was dependent on both the time after initial stimulation with target cells and the number of targets. Using T cell-specific H-2Dd/P18-I10 tetramers, we observed that the apoptosis was induced by such complexes. Induction of apoptosis was blocked by cyclosporin A, a caspase 3 inhibitor, and a mitogen-activated protein kinase inhibitor, but not by Abs to either Fas ligand or to TNF-α. Thus, these observations suggest the existence of a Fas- or TNF-α-independent pathway initiated by TCR signaling that is involved in the rapid induction of CTL apoptosis. Such a pathway may prove important in the mechanism by which virus-specific CTLs are deleted in the presence of high viral burdens.