Abstract

Our previous study showed that CD9 costimulation of TCR-triggered naive T cells elicits activation ([3H]TdR incorporation) that is similar to CD28 costimulation; however, unlike CD28 costimulation, CD9 costimulation results in apoptosis of these previously activated T cells. Here, we investigated whether the apoptosis occurring after TCR/CD9 stimulation is associated with a death pathway involving Fas stimulation and Fas-mediated caspase activation as observed in activation-induced cell death (AICD). In contrast to AICD, the apoptosis resulting from TCR/CD9 stimulation in C57BL/6 T cells was independent of Fas, because this form of apoptosis was not prevented by anti-Fas ligand mAb and was also induced in MRL/lpr T cells. AICD was observed at 12 h after the restimulation of activated T cells with anti-CD3 and reached a peak level at 24 h after this restimulation. CPP32-like protease activity was detected during AICD. Although TCR/CD9 stimulation-associated apoptosis was observed at 24 h after the stimulation of naive T cells and reached a peak level at 36 h after this stimulation, CPP32-like protease activity in these T cells was only marginal at all time points. Nevertheless, both forms of apoptosis were prevented similarly by two different peptide-based caspase inhibitors. These results indicate that the apoptosis that follows the T cell activation which is induced as a result of CD9 costimulation does not involve a Fas-CPP32-like protease pathway, but suggest that different caspase members are likely to be critical in this form of apoptosis.