Intra- and Extracellular Effector Vesicles From Human T And NK Cells: Same-Same, but Different?

Abstract

Cytotoxic T lymphocytes (CTL) and Natural Killer (NK) cells utilize an overlapping effector arsenal for the elimination of target cells. It was initially proposed that all cytotoxic effector proteins are stored in lysosome-related effector vesicles (LREV) termed “secretory lysosomes” as a common storage compartment and are only released into the immunological synapse formed between the effector and target cell. The analysis of enriched LREV, however, revealed an uneven distribution of individual effectors in morphologically distinct vesicular entities. Two major populations of LREV were distinguished based on their protein content and signal requirements for degranulation. Light vesicles carrying FasL and 15 kDa granulysin are released in a PKC-dependent and Ca2+-independent manner, whereas dense granules containing perforin, granzymes and 9 kDa granulysin require Ca2+-signaling as a hallmark of classical degranulation. Notably, both types of LREV do not only contain the mentioned cytolytic effectors, but also store and transport diverse other immunomodulatory proteins including MHC class I and II, costimulatory and adhesion molecules, enzymes (i.e. CD26/DPP4) or cytokines. Interestingly, the recent analyses of CTL- or NK cell-derived extracellular vesicles (EV) revealed the presence of a related mixture of proteins in microvesicles or exosomes that in fact resemble fingerprints of the cells of origin. This overlapping protein profile indicates a direct relation of intra- and extracellular vesicles. Since EV potentially also interact with cells at distant sites (apart from the IS), they might act as additional effector vesicles or intercellular communicators in a more systemic fashion.

T Lymphocyte and CAR-T Cell-Derived Extracellular Vesicles and Their Applications in Cancer Therapy

“…Although the timing and modes of activation and maturation are different, both CTL and Natural Killer cells (NK) utilize an overlapping arsenal consisting of cytotoxic effector proteins including FasL [ 102 ], perforins, granzymes and granulysin contained in their secretory lysosomes [ 78 , 144 ] (recently reviewed in [ 142 ]). The molecular mechanisms controlling the maturation and traffic of the secretory lysosomes and the polarized secretion of exosomes towards the synapse are, in great part, common to both cell types [ 44 , 102 , 145 ]. Thus, it is conceivable that approaches directed to increase exosome secretion in CTL ex vivo can also be useful to boost exosome secretion by CAR NK cells.…”

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

“…Therefore, expression of these death receptor ligands at the surface of NK cells is achieved by degranulation of LGs ( 66 , 70 ). In the case of FasL, several studies suggested that FasL is contained within distinct LG vesicles that do not contain cytotoxic proteins such as perforin and granzymes ( 18 , 71 – 73 ). In addition, it was also suggested that these LG subsets present different signaling requirements for degranulation and rely on distinct molecular processes for their secretion ( 18 , 74 ).…”

“…In the case of FasL, several studies suggested that FasL is contained within distinct LG vesicles that do not contain cytotoxic proteins such as perforin and granzymes ( 18 , 71 – 73 ). In addition, it was also suggested that these LG subsets present different signaling requirements for degranulation and rely on distinct molecular processes for their secretion ( 18 , 74 ). Future studies are required to better elucidate the identity and molecular regulation of FasL-containing vesicles, and it will be interesting to see whether TRAIL is also stored within the same (or a similar) subset of LG vesicles along with FasL.…”

“…Like perforin and granzymes, FasL is initially synthesized in the ER, trafficked to the Golgi, and finally sorted to the LGs. A proline-rich domain at the C-terminal end of FasL was found to be essential in this process by mediating interaction of FasL with various SH3 domain-containing proteins ( 18 , 75 ). FasL becomes phosphorylated by Src kinases recruited to this proline-rich domain and FasL is also ubiquitinated at lysine residues close to the proline-rich domain ( 76 ).…”

“…These secreted vesicles seem to have immune regulatory functions and present anti-tumor effects. For a discussion of the similarities and distinctions between the LGs and NK-EVs regarding composition and molecular processes, the reader is referred to a recent excellent review ( 18 ). In line with this, both CD8 + cytotoxic T lymphocytes (CTLs) and NK cells were also found to secrete cytotoxic supramolecular attack particles (SMAP) composed of thrombospondin-1 (TSP-1), perforin, and granzyme B ( 19 , 20 ).…”