Cytotoxic T lymphocyte granules are secretory lysosomes, containing both perforin and granzymes.

Abstract

SummaryCytotoxic T lymphocytes (CTL) contain granules that are exocytosed during specific interaction with target cells (TC). In this process, the granule contents, including the lethal protein perforin, as well as granzymes, a family of serine esterases, are delivered to the TC . Information regarding the routing of these proteins towards the granule and their exact localization within the granule is of primary importance to resolve the mechanism of granule-mediated TC killing . In this study, the subcellular localization of perforin, granzymes, and known endosomal and lysosomal marker proteins was determined in human and murine CTL, by immunogold labeling of ultrathin cryosections followed by electron microscopy. Perforin and granzymes can be detected in rough endoplasmic reticulum, Golgi complex, trans-Golgi reticulum, and in all cytotoxic granules . Within the granules, they have a similar distribution and are localized not only in the so-called dense core but also over the region containing small internal vesicles. This finding implies that perforin and granzymes can be released in membrane-enveloped and/or -associated form into the intercellular cleft formed upon CTUTC interaction . On the basis of the present evidence, additional release of these molecules in soluble form cannot be excluded . The lysosomal membrane glycoproteins lamp-1, lamp-2, and CD63, are abundantly present on the granule-delimiting outer membrane, which becomes incorporated into the CTL plasma membrane during lethal hit delivery. In contrast, the cation-independent mannose 6-phosphate receptor, known to be present in endosomes and absent from lysosomes, is found only in a minority of the granules. Together with our previous findings that the granules are acidic and connected to the endocytic pathway, these observations define CTL granules as secretory lysosomes.

Migration-associated secretion of melanoma inhibitory activity at the cell rear is supported by KCa3.1 potassium channels

“…We determined that MIA protein secretion is a Ca 2+ -regulated process, similar to the previously described secretion of many other specialized exocytic vesicles such as azurophile granules in neutrophils [44], specific granules in mast cells [45] and lytic granules in cytotoxic Tlymphocytes [46,47]. The most thoroughly characterized Ca 2+ -triggered exocytosis is the release of neurotransmitters into the synaptic space [48].…”

Cell Death Mechanisms Induced by Cytotoxic Lymphocytes

“…Degranulation can be detected by exposure of the lysosomal-membrane-associated glycoproteins, CD107a, CD107b, and CD63, on the lymphocyte surface (53). These molecules are found in the granule-membrane inner surface and are exposed onto the lymphocyte surface through degranulation (53)(54)(55). It has been suggested that lysosomalmembrane-associated glycoproteins and the soluble protein cathepsin-B play an important role in avoiding lymphocyte self-destruction.…”

“…Perforin is found in a soluble monomer shape within granules and, after the cytotoxic-cell/target-cell junction, perforin is released by exocytosis (54). Once it is anchored, perforin begins the polymerization in the presence of Ca 2+ to form cylindrical pores with an internal diameter of 5 to 20 nm (63,65,66).…”

Heterogeneity of Effector Phenotype for Acute Phase and Memory Influenza A Virus-Specific CTL

“…Prominent in the CTL cytoplasm are secretory granules that contain a spectrum of cytotoxic molecules including perforin (Pfp) 4 and a group of serine proteases (11) called granzymes (granule enzymes; Gzm). After CTL engagement via TCR-mediated recognition of viral peptides (p) complexed with self-class I MHC glycoproteins (pMHCI), the granules move toward the CTL target interface where secretion of the cytotoxic contents results in apoptotic cell death.…”