OC 38.4 - Platelet PAR4 (F2RL3) plays protective role in Coxsackievirus B3 myocarditis

Background: Lack of platelets results in increased Coxsackievirus B3 (CVB3) myocarditis. PAR4 is essential for normal platelet functions. We showed that global PAR4 deficiency resulted in increased CVB3 myocarditis. Toll-like receptor 4 (TLR4) and M1 macrophages were shown to contribute to myocarditis pathology.

Aims: To investigate the role of platelet PAR4 in Coxsackievirus B3 myocarditis

Methods: To investigate the role of platelet PAR4 in CVB3 myocarditis, we infected PF4Cre;PAR4fl/fl (PAR4ΔPlt) and their control (PAR4fl/fl) with CVB3 (Nancy strain) and analyzed myocarditis in the heart 8 days post infection (dpi). For in vitro studies, we tested the anti-inflammatory effect of thrombin-stimulated platelet releasate (Plt-R) on TLR3 and TLR4 stimulated macrophages with regard to NFκB pathway activation, cytokine production and their polarization.

Results: We found that PAR4ΔPlt mice exhibited increased cardiac infiltrations and increased cardiac virus load 8dpi compared to controls. In line with this, Plt-R reduced both TLR3- and TLR4-induced NFκB pathway activation in RawDual cells. This reduction in NFκB activation was associated with reduced TNFα levels of TLR3 and TLR4 stimulated macrophages. Plt-R increased the marker of M2 polarization of macrophages.

Conclusion(s): Our data demonstrates that PAR4 on platelets limits CVB3 myocarditis in mice. Platelets release anti-inflammatory factors reducing macrophage activation. Ongoing studies are performed to link extracellular vesicles (EVs) and their cargo to the anti-inflammatory effect of Plt-R. Indeed, treatment of mice with human platelet EVs reduced CVB3 myocarditis in mice