Granzymes trigger apoptosis in the target cell both by directly activating caspases and by damaging mitochondria, which also activates caspases. The two most abundant granzymes are granzymes A and B. Granzyme A triggers cell death by caspase-independent mitochondrial damage, through mechanisms that are not completely understood. Granzyme B, like the caspases, cleaves proteins after aspartic acid residues and activates caspase 3, thereby activating a caspase proteolytic cascade, which eventually activates the caspase-activated deoxyribonuclease (CAD) by cleaving an inhibitory protein (ICAD) that binds to and inactivates CAD. This nuclease is believed to be the enzyme that degrades DNA in target cells (Fig. 9.45). Granzyme B also targets mitochondria to activate the intrinsic apoptotic pathway; it cleaves the protein BID (for BH3-interacting domain death agonist protein), either directly recognition of its antigen on a virus-infected cell by a cytotoxic Cd8 T cell induces the T cell to release the contents of its cytotoxic granules in a directed fashion. perforin and granzymes, in a complex with the proteoglycan serglycin, are delivered to the membrane of the target cell (top panel). By an unknown mechanism, perforin directs the entry of the granule contents into the cytosol of the target cell without apparent pore formation, and the introduced granzymes then act on specific intracellular targets such as the proteins BId and pro-caspase 3 (second panel). either directly or indirectly, the granzymes cause the cleavage of BId into truncated BId (tBId) and the cleavage of pro-caspase 3 into an active caspase (third panel). tBId acts on mitochondria to release cytochrome c into the cytosol. This promotes apoptosis by inducing the formation of the apoptosome that activates procaspase 9, which in turn further amplifies caspase 3 activation. activated caspase 3 targets ICad to release caspase-activated dnase (Cad), which fragments the dna (bottom panel).
or indirectly by activated caspase 3, causing disruption of the mitochondrial outer membrane and the release from the mitochondrial intermembrane space of pro-apoptotic molecules such as cytochrome c. As discussed above (Section 9-30), cytochrome c is central to amplification of the intrinsic apoptotic cascade, as it initiates assembly of the apoptosome with Apaf-1, which in turn activates the initiator caspase 9. Thus, granzyme B acts directly to activate the effector caspase 3, and indirectly to activate the initiator caspase 9.
Cells undergoing programmed cell death are rapidly ingested by phagocytic cells, which recognize a change in the cell membrane: phosphatidylserine, which is normally found only in the inner leaflet of the membrane, replaces phosphatidylcholine as the predominant phospholipid in the outer leaflet. The ingested cell is broken down and completely digested by the phagocyte without the induction of co-stimulatory proteins. Thus, apoptosis is normally an immunologically ‘quiet’ process; that is, apoptotic cells do not normally contribute to or stimulate immune responses.
9-32 Cytotoxic T cells are selective serial killers of targets expressing a specific antigen.