ACTIVATION OF CASPASE-3-LIKE ENZYMES IN NONAPOPTOTIC T-CELLS

The activation of the caspasae family of cysteine proteases is a key s tep in the implementation of apoptotic cell death leading to further d ownstream effects such as DNA fragmentation. In cultured tumor cells, caspase activity appears only when cells are undergoing apoptosis. Her e we show that human and murine T lymphocytes acquire high intracellul ar activities of cell death-specific caspases upon activation by mitog ens and IL-2 without evidence that apoptosis is proceeding. The highes t activity is seen when cells are mitogen activated for 3 days. On a p er cell basis, caspase activity in activated T cells is much higher th an in tumor cells induced to undergo apoptosis. In the presence of exo genously added IL-2 cells stay alive and maintain a high level of casp ase activity while IL-2 withdrawal results in cell death and decline o f caspase activity. Caspase activity can also be measured in extracts from spleen and lymph nodes from mice injected with superantigen. Whil e in tumor cell lines caspase activity correlates with cleavage of pol y(ADP)-ribose polymerase (PARP) and DNA fragmentation, in activated T cells cleavage products of cellular PARR can be detected whereas DNA f ragmenting activity appears only upon IL-2 withdrawal which coincides with cell death. These data show that caspase activation in intact cel ls does not necessarily lead to cell death and argue for a checkpoint in the apoptotic pathway downstream of caspases. Furthermore, they pro vide a molecular correlate for the high susceptibility of activated T cells for apoptosis.