The Expanding Arsenal of Cytotoxic T Cells

Abstract

Cytotoxic T cells (CTLs) are the main cellular mediators of the adaptive immune defenses against intracellular pathogens and malignant cells. Upon recognition of specific antigen on their cellular target, CTLs assemble an immunological synapse where they mobilise their killing machinery that is released into the synaptic cleft to orchestrate the demise of their cell target. The arsenal of CTLs is stored in lysosome-like organelles that undergo exocytosis in response to signals triggered by the T cell antigen receptor following antigen recognition. These organelles include lytic granules carrying a cargo of cytotoxic proteins packed on a proteoglycan scaffold, multivesicular bodies carrying the death receptor ligand FasL, and the recently discovered supramolecular attack particles that carry a core of cytotoxic proteins encased in a non-membranous glycoprotein shell. Here we will briefly review the main features of these killing entities and discuss their interrelationship and interplay in CTL-mediated killing.

Keywords: FasL; SMAP; cytotoxic T cell; granzyme; lytic granule; perforin.

Figure 1

The three pathways to target cell killing by CTLs. (A) Following routing to the secretory pathway at the endoplamic reticulum (ER), the components of lytic granules (LG) -granzymes (Gzm), perforin (Prf) and serglycin (Srgn)- are sorted at the Golgi apparatus for transport to early endosomes (EE), wherefrom they transit through multivesicular bodies (MVB) and late endosomes (LE) to mature LGs. Gzms are tagged for transport by the cation-independent mannose-6P receptor (CI-MPR) through N-glysosylation and the addition of a M-6P moiety, while Prf is sorted through clathrin-dependent and -independent pathways. At LEs Gzms and Prf become activated but remain in an inactive state until their release and eventually localize in two types of mature LGs, single-core granules (SCG) or multiple-core granules (MCG), accumulating as multimolecular complexes held together by Srgn. Upon formation of the immunological synapse (IS) with the cognate cell target SCGs are mobilized to the cell-cell contact and fuse with the IS membrane, releasing soluble Gzm-Prf complexes that are taken up by the target cell through the pore-forming activity of Prf. In MSGs Gzm-Prf-Srgn complexes are encased in a glycoprotein shell enriched in thrombospondin-1 to form the SMAPs. Following CTL activation, MCGs undergo fusion with the IS membrane with a delayed kinetics compared to SCGs and release their cargo of SMAPs, which are taken up by the cell target through an as yet unidentified mechanism. (B) FasL transits through the ER, Golgi apparatus and EEs to MVBs, where it becomes associated both to the limiting membrane and to intraluminal vesicles that mature into EVs. FasL may also be partly segregated to Gzm- and Prf-containing LGs. On encounter of their cognate cell target CTLs mobilize MVBs to the IS, releasing FasL both at the synaptic membrane and into the synaptic cleft as FasL-containing EVs. Both plasma membrane-associated and EV-associated FasL can interact with Fas on the target cell membrane, triggering the Fas- and caspase-dependent death pathway. Fas-dependent killing is delayed compared to Gzm/Pfr-dependent killing and is essential for the serial killing activity of CTLs.