An abrupt and concordant initiation of apoptosis: antigen‐dependent death of CD8+ CTL

The ability of CD8+ cytotoxic T lymphocytes (CTL) to clear viral infections may be limited when high avidity CTL encounter supra‐optimal antigen density on antigen‐presenting cells (APC) and undergo antigen‐dependent apoptosis of CTL (ADAC). Previously, we have shown ADAC in CD8+ populations to be Fas independent, TNF‐α receptor 2 (TNFR2) mediated, caspase dependent, and accompanied by a decrease in Bcl‐2. We now employ flow cytometry to follow ADAC within individual CD8+ cells to demonstrate that the intense TCR signal induced in high avidity CTL by supra‐optimal antigen density results 8 – 16 h later in a caspase‐independent TNFR2 down‐modulation that is directly related to the stimulating APC antigen density and concludes in a rapid onset of apoptosis by 18 – 24 h. Individual CTL undergoing apoptosis exhibit a dramatic and concurrent: (1) positive staining with Annexin V and propidium iodide; (2) transformation to a smaller cell size characteristic of apoptosis; and (3) a nearly complete loss of Bcl‐2, c‐IAP1, and TRAF2. We conclude that the antigen‐dependent apoptosis of CD8+ CTL occurs when a tandem TCR / TNFR2 signalinitiates an abrupt and concordant onset of multiple apoptotic events.