Cell cycle progression out of G1 sensitizes primary-cultured nontransformed T cells to TCR-mediated apoptosis.

Shortly after primary activation and IL-2-induced entry into cell cycle, splenic or lymph node T cells can be induced to undergo apoptosis by recrosslinking of the TCR complex using anti-TCR antibodies. We demonstrate here that primary-activated T cells induced to undergo apoptosis by TCR recrosslinking during the G1 phase of the cell cycle did not arrest in the G1 phase of the cell cycle. Instead, the cells continued to progress through the cell cycle and underwent at least one mitosis before dying. Rapamycin, an inhibitor of IL-2-induced S phase entry, prevented this apoptotic death. Prevention of cell death correlated with delayed entry into S phase from G1 following TCR religation in the rapamycin-treated cultures. Addition of rapamycin after cells had entered S phase or had already divided failed to prevent cell death. Treatment of activated T cells with dibutyryl cAMP or forskolin, which also block primary-activated T lymphocytes in G1, also inhibited TCR-induced cell death. In contrast, treatment of TCR-religated cells with reagents that blocked cell cycle progression in S phase (aphidicolin, deferoxamine) after TCR religation failed to prevent apoptotic cell death. Activated T cells sorted for S + G2/M DNA content following Hoechst 33342 staining were also found to be more sensitive to TCR-induced apoptosis than cells sorted for G1 DNA content. Rapamycin inhibited apoptosis in G1-sorted cells, but not in S + G2/M-sorted cells. Together, these results suggest that factors regulating cell cycle progression also control the induction of TCR-mediated apoptosis. Primary-activated T cells may become committed to programmed cell death only after progressing into S phase of the cell cycle.