CLEAVAGE OF FLICE (CASPASE-8) BY GRANZYME-B DURING CYTOTOXIC T-LYMPHOCYTE-INDUCED APOPTOSIS

Cytotoxic T lymphocytes induce apoptosis in target cells through the C D95(APO-1/Fas) and the perforin/granzyme B (GrB) pathway. The exact su bstrate of GrB in vivo is still unknown, but to induce apoptosis GrB r equires the activity of caspases in target cells. We show here that in HeLa target cells induction of apoptosis through the perforin/GrB pat hway resulted in minor direct cleavage of CPP32 (caspase-3) by GrB. Mo st caspase-3 cleavage resulted from activation of an upstream caspase. Moreover, target cells derived from caspase-3(-/-) mice displayed GrB -induced poly(ADP-ribose) polymerase (PARP) cleavage with only partial ly reduced efficiency compared to wild-type target cells. This indicat es that other PARP-cleaving caspases can be activated during perforin/ GrB-induced cell death. In contrast to caspase-3, FLICE (caspase-8) wa s directly cleaved by GrB in HeLa cells. We therefore conclude that FL ICE not only plays a central role in CD95(APO-1/Fas)-induced apoptosis but can also be directly activated during perforin/GrB-induced apopto sis.