Cytotoxic lymphocytes employ Granzyme B as a potent initiator of apoptosis to cleave and activate effector caspases. Unexpectedly, cells transfected with Bcl-2 were resistant to granzyme B-induced killing, suggesting that a mitochondrial pathway was critical. Utilizing cells expressing a dominant-negative caspase 9, the current study demonstrated that caspase activation via the apoptosome was not required. Indeed, cleavage of caspase 3 to p20 still occurred in Bcl-2-transfectants but processing to p17 was blocked. This blockade was recapitulated by the Inhibitor-of-Apoptosis-Protein XIAP and relieved by Smac/DIABLO. Thus granzyme B mediates direct cleavage of caspase 3 and also activates mitochondrial disruption, resulting in the release of proapoptotic proteins that suppress caspase inhibition. Engagement of both pathways is critical for granzyme-induced killing.

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