Abstract : Background Innate immune responses to influenza A virus (IAV) infection are initiated in part by the Toll-like receptor 3 (TLR3). TLR3-dependent signaling induces an antiviral immune response and an NFκB-dependent inflammatory response. Protease-activated receptor 2 (PAR2) inhibits the antiviral response and enhances the inflammatory response. Mice protected from PAR2 deficiency during IAV infection. However, the PAR2-expressing cell types that contribute to IAV pathology in mice and the mechanism by which PAR2 contributes to IAV infection are unknown. Methods: IAV infection was analyzed in global ( Par2 -/- ), myeloid ( Par2 fl/fl ;LysM Cre+ ) and Par2-deficient lung epithelial cell (EpC) mice ( Par2 fl/fl ;SPC Cre+ ) and in their respective controls ( Par2 + /+ and Par2 fl/fl ). Furthermore, the effect of PAR2 activation on polyinosinic-polycytidylic acid (poly I:C) activation of TLR3 was analyzed in bone marrow-derived macrophages (BMDM). Finally, we determined the effect of PAR2 inhibition in wild-type (WT) mice. Results After IAV infection, Par2-/- and myeloid Par2-deficient mice showed increased survival compared to infected controls. Improved survival was associated with reduced proinflammatory mediators and reduced bronchoalveolar lavage fluid (BALF) cellular infiltration of Par2 -/- and Par2 fl/fl; LysM Cre+ 3 days post infection (dpi) compared to infected control mice. Interestingly, Par2 fl/fl ;SPC Cre+ mice did not show any survival advantage over Par2 fl/fl mice. In vitro studies showed that Par2 -/- BMDM produced less IL6 and IL12p40 than Par2 +/+ BMDM after poly I:C stimulation. Furthermore, activation of PAR2 on Par2+/+ BMDM increased poly I:C induction of IL6 and IL12p40 compared with poly I:C stimulation alone. Importantly, inhibition of PAR2 prior to IAV infection protects WT mice. Conclusion Global Par2 or myeloid cell but not lung EpC Par2 deficiency was associated with reduced BALF inflammatory markers and reduced IAV-induced mortality. Our study suggests that PAR2 may be a therapeutic target to reduce IAV pathology.
