Platelet EVs contain an active proteasome involved in protein processing for antigen presentation via MHC-I molecules

Abstract

In addition to their hemostatic role, platelets play a significant role in immunity. Once activated, platelets release extracellular vesicles (EVs) formed by budding of their cytoplasmic membranes. Because of their heterogeneity, platelet EVs (PEVs) are thought to perform diverse functions. It is unknown, however, whether the proteasome is transferred from platelets to PEVs or whether its function is retained. We hypothesized that functional protein processing and antigen presentation machinery is transferred to PEVs by activated platelets. Using molecular and functional assays, we show that the active 20S proteasome is enriched in PEVs along with MHC-I and lymphocyte costimulatory molecules (CD40L and OX40L). Proteasome-containing PEVs were identified in healthy donor blood, but did not increase in platelet concentrates that caused adverse transfusion reactions. They were, however, augmented after immune complex injections in mice. The complete biodistribution of murine PEVs following injection into mice revealed that they could principally reach lymphoid organs such as spleen and lymph nodes, in addition to the bone marrow, and to a lesser extent liver and lungs. The PEV proteasome processed exogenous ovalbumin (OVA) and loaded its antigenic peptide onto MHC-I molecules which promoted OVA-specific CD8+ T lymphocyte proliferation. These results suggest that PEVs contribute to adaptive immunity through cross-presentation of antigens and have privileged access to immune cells through the lymphatic system, a tissue location that is inaccessible to platelets.

Platelet-Released Factors: Their Role in Viral Disease and Applications for Extracellular Vesicle (EV) Therapy

“…Platelet derived pEVs have not yet been confirmed to cross the blood-brain barrier. However, pEVs have been found in the synovial fluid associated with inflammation and rheumatoid arthritis (RA) [136]. pEVs can also reach lymphoid organs, such as the spleen and lymph nodes, with some ability to enter the bone marrow, liver, and lungs, as found in an ovalbumin-based mouse model [136].…”

Platelets in ITP: Victims in Charge of Their Own Fate?

“…Hence, autoantibodies could be a likely trigger for platelets to shed PMPs in ITP. In this regard, injection with immune complexes has been shown to trigger PMP release in mice expressing the FcγRIIA transgene [65]. Other triggers have been described to activate platelets, such as shear forces from the blood flow and activation of glycoprotein VI [14,66].…”

“…Unlike platelets, PMPs are readily able to enter the lymphatic system and the bone marrow, where they can modulate target cells [65]. PMPs have been described to exert a proinflammatory function in autoimmune diseases [69,91].…”

“…However, substantially more evidence is needed to support this theory, which could be gathered from larger cohort studies investigating the effects of PMPs on different T cell subsets. A recent study from Marcoux et al showed that PMPs, similar to platelets themselves, present antigens via their MHC class I receptor to CD8+ T cells [65]. They suggest that platelet-antigens may also be presented by PMPs, which might contribute to the T cell mediated anti-platelet response in ITP as well [65].…”

“…A recent study from Marcoux et al showed that PMPs, similar to platelets themselves, present antigens via their MHC class I receptor to CD8+ T cells [65]. They suggest that platelet-antigens may also be presented by PMPs, which might contribute to the T cell mediated anti-platelet response in ITP as well [65]. In summary, PMPs may contribute to the altered immune balance and the loss of immune tolerance in ITP through interactions with myeloid and lymphoid cells.…”