Key Points
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Cytotoxic T lymphocytes (CTLs) kill their targets by both caspase-dependent and caspase-independent pathways.
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Redundant cell-death pathways provide protection from viruses or tumours that can evade one pathway or another, such as caspase-resistant tumours that overexpress BCL-2.
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CTLs and natural killer (NK) cells use the same mechanisms to induce cell death, releasing cytotoxic granules into the synapse formed between the CTL and its target; however, they have different profiles of expression of cytolytic molecules.
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Granzyme B, which activates the caspases, can also act as an initiator or effector caspase by directly cleaving BID (BH3-interacting domain death agonist) to activate a mitochondrial apoptotic pathway or by cleaving the inhibitor of caspase-activated DNase (ICAD) to activate DNA damage.
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Granzyme A, the most abundant granzyme, which is expressed by both innate and adaptive immune cytotoxic cells, activates a new caspase-independent cell-death pathway that targets a newly described cytoplasmic complex (the SET complex) to damage DNA by single-stranded nicks.
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Granzyme C activates yet another cell-death pathway, which is just beginning to be elucidated, that induces prominent mitochondrial swelling and dysfunction.
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The mechanism used by the membrane-perturbing protein perforin to deliver the granzymes to the cytosol of target cells is still not well understood.
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Granulysin, a membrane-perturbing protein in human (but not mouse) CTL and NK-cell granules, causes microbial and mammalian cell death by disrupting bacterial and mitochondrial membranes.
Abstract
Granule exocytosis is the main pathway for the immune elimination of virus-infected cells and tumour cells by cytotoxic T lymphocytes and natural killer cells. After target-cell recognition, release of the cytotoxic granule contents into the immunological synapse formed between the killer cell and its target induces apoptosis. The granules contain two membrane-perturbing proteins, perforin and granulysin, and a family of serine proteases known as granzymes, complexed with the proteoglycan serglycin. In this review, I discuss recent insights into the mechanisms of granule-mediated cytotoxicity, focusing on how granzymes A, B and C and granulysin activate cell death through caspase-independent pathways.
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Acknowledgements
I thank D. Martinvalet, Z. Fan, L. Shi, T. Ley, P. Henkart and C. Froelich for sharing unpublished data, and the members of my laboratory for helpful discussions. This work was supported by a grant from the National Institutes of Health.
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Glossary
- GRAFT-VERSUS-HOST DISEASE
-
(GVHD). An immune response mounted against the recipient of an allograft by immunocompetent donor T cells derived from the graft. Typically, it is seen in the context of allogeneic bone-marrow transplantation.
- CASPASES
-
A family of cytosolic proteases that contain a cysteine residue in the active site and that cleave their substrates after an aspartic-acid residue. They can be divided into pro-inflammatory caspases (caspases 1, 4, 5 and 11), which cleave and activate pro-inflammatory cytokines, and pro-apoptotic caspases, which cleave and activate pro-apoptotic substrates. Pro-apoptotic caspases comprise initiator caspases (caspases 2, 8 and 9), which, in turn, cleave and activate effector caspases (caspases 3, 6 and 7).
- TRYPTASE
-
An enzyme that (similar to trypsin) cuts after basic amino acids such as lysine and arginine.
- CHYMASE
-
A protease that (similar to chymotrypsin) cuts after hydrophobic amino acids.
- METASE
-
A protease that cuts after methionine residues.
- DEFENSINS
-
Small basic peptides produced by immune cells that are microbicidal and work by damaging bacterial membranes.
- KLENOW POLYMERASE
-
The large fragment of Escherichia coli DNA polymerase I produced after cleavage with subtilisin. The Klenow fragment has 5′→3′ polymerase activity and 3′→5′ exonuclease activity, but no 5′→3′ exonuclease activity. It is used to end-label free 3′ recessed ends of DNA.
- TERMINAL DEOXYNUCLEOTIDYL TRANSFERASE
-
(TdT). An enzyme expressed during lymphocyte development that adds nucleotides to the free 3′ end of DNA breaks. It is used to assay apoptosis by catalysing the addition of radiolabelled or biotinylated nucleotides to sites of DNA damage.
- ANNEXIN-V STAINING
-
Annexin V binds to phosphatidyl serine, which is normally located on the inner leaflet of the plasma membrane, but which flips to the outer layer during apoptosis. Annexin-V staining is often used as an indicator of apoptosis.
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Lieberman, J. The ABCs of granule-mediated cytotoxicity: new weapons in the arsenal. Nat Rev Immunol 3, 361–370 (2003). https://doi.org/10.1038/nri1083
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DOI: https://doi.org/10.1038/nri1083
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