The primary mechanism by which cytotoxic T cells eliminate virally infected cells is by granule exocytosis. The release of cytotoxic granule contents by cytotoxic T lymphocytes (CTL) triggers apoptotic target cell death. CTL granules contain a pore-forming protein, perforin, and a group of serine proteases called granzymes. In the classic model, perforins create holes in the target cell membrane, allowing entrance of the granzymes. Granzyme A and B are the predominant granzymes activated after CTL activation, but each act via an independent apoptotic pathway, granzyme B is activated immediately, while granzyme A acts hours later. Granzyme B has been shown to induce apoptosis and to cleave a number of substrates which are similar in specificity to those of the caspase family of proteinases. Granzyme B can cleave substrates, such as DNA-PKcs, and nuclear mitotic apparatus protein (NuMA). Furthermore, Granzyme B can also cleave substrates such as Bid and DFF45 in a caspase-independent fashion. Studies involving mice which are deficient in both granzyme A and B suggest a model whereby the granzyme B pathway may have evolved as the major apoptotic pathway with the granzyme A pathway acting as a backup. However, further research is needed to delineate the components of these distinct pathways.
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