Granzyme B-mediated apoptosis proceeds predominantly through a Bcl-2-inhibitable mitochondrial pathway

Cytotoxic T Lymphocytes kill virus-infected and tumor cell targets through the concerted action of proteins contained in cytolytic granules, primarily granzyme B and perforin, Granzyme B, a serine proteinase with substrate sp ecificity similar to the caspase family of apoptotic cysteine proteinases, is capable of cleaving and activating a number of death proteins in target cells. Despite the ability to engage the death pathway at multiple entry po ints, the preferred mechanism for rapid induction of apoptosis by granzyme B has yet to be clearly established. Here we use time lapse confocal micros copy to demonstrate that mitochondrial cytochrome c release is the primary mode of granzyme B-induced apoptosis and that Bcl-2 is a potent inhibitor o f this pivotal event. Caspase activation is not required for cytochrome c r elease, an activity that correlates with cleavage and activation of Bid, wh ich we have found to be cleaved more readily by granzyme B than either casp ase-3 or caspase-8. Bcl-2 blocks the rapid destruction of targets by granzy me B by blocking mitochondrial involvement in the process.