Platelet-derived extracellular vesicles convey mitochondrial DAMPs in platelet concentrates and their levels are associated with adverse reactions

BACKGROUND

Whereas platelet transfusion is a common medical procedure, inflammation still occurs in a fraction of transfused individuals despite the absence of any apparent infectious agents. Platelets can shed membrane vesicles, called extracellular vesicles (EVs), some of which contain mitochondria (mito+EV). With its content of damage-associated molecular pattern (DAMP), the mitochondrion can stimulate the innate immune system. Mitochondrial DNA (mtDNA) is a recognized DAMP detected in the extracellular milieu in numerous inflammatory conditions and in platelet concentrates. We hypothesized that platelet-derived mitochondria encapsulated in EVs may represent a reservoir of mtDNA.

STUDY DESIGN AND METHODS

Herein, we explored the implication of mito+EVs in the occurrence of mtDNA quantified in platelet concentrate supernatants that induced or did not induce transfusion adverse reactions.

RESULTS

We observed that EVs were abundant in platelet concentrates, and platelet-derived mito+EVs were more abundant in platelet concentrates that induced adverse reactions. A significant correlation (r_s  = 0.73; p < 0.0001) between platelet-derived mito+EV levels and mtDNA concentrations was found. However, there was a nonsignificant correlation between the levels of EVs without mitochondria and mtDNA concentrations (r_s  = -0.11; p = 0.5112). The majority of the mtDNA was encapsulated into EVs.

CONCLUSION

This study suggests that platelet-derived EVs, such as those that convey mitochondrial DAMPs, may be a useful biomarker for the prediction of potential risk of adverse transfusion reactions. Moreover, our work implies that investigations are necessary to determine whether there is a causal pathogenic role of mitochondrial DAMP encapsulated in EVs as opposed to mtDNA in solution.

Role of platelets and megakaryocytes in adaptive immunity

The immune system is comprised of two principal interconnected components called innate and adaptive immunity. While the innate immune system mounts a nonspecific response that provides protection against the spread of foreign pathogens, the adaptive immune system has developed to specifically recognize a given pathogen and lead to immunological memory. Platelets are small fragments produced from megakaryocytes in bone marrow and lungs. They circulate throughout the blood to monitor the integrity of the vasculature and to prevent bleeding. Given their large repertoire of immune receptors and inflammatory molecules, platelets and megakaryocytes can contribute to both innate and adaptive immunity. In adaptive immunity, platelets and megakaryocytes can process and present antigens to lymphocytes. Moreover, platelets, via FcγRIIA, rapidly respond to pathogens in an immune host when antibodies are present. This manuscript reviews the reported contributions of platelets and megakaryocytes with emphasis on antigen presentation and antibody response in adaptive immunity.

Phenotypic Characterization of Acoustically Enriched Extracellular Vesicles from Pathogen-Activated Platelets

Extracellular vesicles (EVs) are derived from the membrane of platelets and released into the circulation upon activation or injury. Analogous to the parent cell, platelet-derived EVs play an important role in hemostasis and immune responses by transfer of bioactive cargo from the parent cells. Platelet activation and release of EVs increase in several pathological inflammatory diseases, such as sepsis. We have previously reported that the M1 protein released from the bacterial pathogen Streptococcus pyogenes directly mediates platelet activation. In this study, EVs were isolated from these pathogen-activated platelets using acoustic trapping, and their inflammation phenotype was characterized using quantitative mass spectrometry-based proteomics and cell-based models of inflammation. We determined that M1 protein mediated release of platelet-derived EVs that contained the M1 protein. The isolated EVs derived from pathogen-activated platelets contained a similar protein cargo to those from physiologically activated platelets (thrombin) and included platelet membrane proteins, granule proteins, cytoskeletal proteins, coagulation factors, and immune mediators. Immunomodulatory cargo, complement proteins, and IgG3 were significantly enriched in EVs isolated from M1 protein-stimulated platelets. Acoustically enriched EVs were functionally intact and exhibited pro-inflammatory effects on addition to blood, including platelet-neutrophil complex formation, neutrophil activation, and cytokine release. Collectively, our findings reveal novel aspects of pathogen-mediated platelet activation during invasive streptococcal infection.

Tartrate-Resistant Acid Phosphatase 5 (TRAP5) Promotes Eosinophil Migration During Allergic Asthma

Allergic asthma is characterized by type 2 inflammation and eosinophilia. Tartrate-resistant acid phosphatase 5 (TRAP5/ACP5) is a metallophosphatase expressed by alveolar macrophages that dephosphorylates osteopontin (OPN), a phosphoglycoprotein with increased expression in asthma. To investigate the role of TRAP5 during asthma, we used a murine model of ovalbumin (OVA) induced allergic airway inflammation as well as IL-33 induced airway inflammation including Trap5-/- and wild-type (WT) mice. Histological analyses of murine lung revealed that OVA-induced inflammation induced the formation of inflammatory lesions and increased mucus production in both WT and Trap5-/- mice. However, lower cytokine levels (including IL-5 and IL-13) were detected by multiplex immunoassay in Trap5-/- mice after OVA-induced inflammation. Furthermore, qPCR analysis detected different gene expression profile of Trap5-/-/OVA mice, including upregulation of Il-17a and downregulation of Il-33. Lower eosinophil numbers were measured in bronchoalveolar lavage fluid of Trap5-/-/OVA mice using flow cytometry analysis, whereas immunofluorescence staining revealed high eosinophil number in lung tissue of both groups with OVA challenge. In the IL-33 model of type 2 inflammation, both WT and Trap5-/- mice showed similar inflammatory responses with regard to cytokine levels and cell recruitment patterns. In vitro, eosinophil chemotaxis was facilitated by non-phosphorylated but not phosphorylated OPN, an effect inhibited by an α4β1 integrin inhibitor. The results suggest that TRAP5 is important in the recruitment of immune cells, including eosinophils, as well as in shaping the profile and amplification of the inflammatory response during allergic airway inflammation. Thus, TRAP5 may serve as a therapeutic target in allergic asthma. This article is open access and distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).