Granzymes

Granzymes are serine proteases that are released by cytoplasmic granules within cytotoxic T cells and natural killer (NK) cells. They induce programmed cell death in the target cell, thus eliminating cells that have become cancerous or are infected with viruses or bacteria. The granzymes also kill bacteria and inhibit viral replication. In NK cells and T cells, the granzymes are packaged in cytotoxic granules with perforin. Other locations that granzymes can be detected are in the rough endoplasmic reticulum, golgi complex, and the trans-golgi reticulum. The contents of the cytotoxic granules function to permit entry of the granzymes into the target cell cytosol. The granules are released into an immune synapse formed with a target cell, where perforin mediates the delivery of the granzymes into endosomes in the target cell, and finally into the target cell cytosol. Granzymes are identified as being part of the serine esterase family. They are closely related to other immune serine proteases expressed by innate immune cells, such as neutrophil elastase and cathepsin G.

Granzyme B activates apoptosis by activating caspases (especially caspase-3), which cleaves many substrates, including caspase-activated DNase to execute cell death. Granzyme B also cleaves the protein Bid, which recruits the proteins Bax and Bak to change the membrane permeability of the mitochondria, causing the release of (which is one of the parts needed to activate caspase-9 via the apoptosome), Smac/Diablo and Omi/HtrA2 (which suppress the inhibitor of apoptosis proteins (IAPs)), among other proteins. Granzyme B also cleaves many of the proteins responsible for apoptosis in the absence of caspase activity. The other granzymes activate cell death by caspase-dependent and caspase-independent mechanisms.

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