Immunity, Granzymes and Cell Killing

Nigel J Waterhouse, Apoptosis and Natural Toxicity, Melbourne, Victoria, Australia
Olivia Susanto, Apoptosis and Natural Toxicity, Melbourne, Victoria, Australia
Karin A Sedelies, Apoptosis and Natural Toxicity, Melbourne, Victoria, Australia
Joseph A Trapani, Cancer Cell Death Laboratory, Melbourne, Victoria, Australia

Published online: December 2009

DOI: 10.1002/9780470015902.a0021982

Full Article on Wiley Online Library

Cytotoxic lymphocytes protect us from viral infection and cancer by directly killing tumour cells, or cells harbouring a virus. One crucial mechanism they use to kill their targets is known as the ‘granule exocytosis’ pathway. This involves secretion of a potent mix of toxins, resulting in transfer of granule proteases (granzymes) from the killer cell into the target cell, where they cleave various intracellular substrates to activate diverse signalling pathways to cell death. Access to the target cell cytosol depends on a pore-forming protein toxin, perforin. Because granule exocytosis is a crucial part of the body's natural defence against such dangerous cells, understanding how granzymes kill their targets may yield novel strategies and identify new molecular targets for anticancer or antiviral therapies.

Key Concepts:

The critical role of cytotoxic T lymphocytes and natural killer cells is to eliminate abnormal cells, thereby defending higher organisms against intracellular infection.
An important additional function of cytotoxic lymphocytes also encompasses their role in surveillance against transformed cells.
Perforin and granzymes synergize to kill target cells. Perforin is critical to this process and its role is to enable granzyme proteases to access key substrates in the target cell.
Granzyme B is the most potent pro-apoptotic granzyme, as it shares with caspases the capacity to cleave its substrates after key aspartate residues.
Cytotoxic lymphocytes have developed a plethora of different signalling pathways to bring about target cell death. This is critically important as viruses are adept at blocking cell death to maximize their opportunity for viral replication and spread to uninfected cells.
Apart from inflicting cell death, granzymes have additional functions such as directly interfering with viral replication and in cytokine processing, leading to amplification of the inflammatory response to pathogens.
Dysregulation of cytotoxic T cell function can sometimes lead to adverse effect such as autoimmune tissue damage.

Keywords: granzyme; cytotoxic lymphocyte; cell death; perforin

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Article Title:	Immunity, Granzymes and Cell Killing
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	Figure 1. Cytotoxic lymphocytes kill their targets through granule exocytosis, and granzyme B initiates apoptosis via two main pathways. Granzyme B cleaves Bid to induce MOMP and subsequent release of cytochrome c from the mitochondria results in caspase activation and ‘classic’ apoptosis. However, granzyme B may also activate caspases directly (particularly in the mouse) and can also cleave many other substrates directly, potentially leading to caspase-independent death. Granzyme B also shows species variation in substrate specificity, with human granzyme B preferentially cleaving Bid, and mouse granzyme B favouring direct caspases cleavage.
	Figure 2. Mouse NK cells kill their targets via diverse pathways. Mouse NK cells were incubated with plastic-adherent mouse embryonic fibroblasts (MEF) at a 2:1 ratio for 2 h. The NK cells were then washed off, and the target cells fixed, permeabilized and stained for cytochrome c. The cells were then visualized by confocal microscopy for morphological changes and cytochrome c localization. MEF cells showed rounding and blebbing consistent with apoptosis, and released cytochrome c from the mitochondria indicating that MOMP has occurred. Although the apoptotic phenotype was being blocked by the caspase inhibitor zVAD-fmk, clonogenic survival was not rescued (data not shown) indicating that caspase-independent death was taking place.
	Figure 3. Granzymes cleave many substrates to initiate diverse pathways to cell death. Of the granzyme family, granzyme B is well characterized and initiates apoptosis of target cells (Figure 1). The other granzymes have also been shown to induce target cell death when applied directly to target cells in vitro; however, the morphology of cell death is not classically apoptotic and does not involve caspase activation. Although these other granzymes cleave a diverse range of substrates, granzymes A, C, K and M also lead to reactive oxygen species generation as a key feature. Many of the noncaspase substrates include critical cytoskeletal proteins and DNA repair proteins, cleavage of which is implicated in cell death.

Immunity, Granzymes and Cell Killing

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