Platelet activation and aggregation promote lung inflammation and influenza virus pathogenesis.

RATIONALE: The hallmark of severe influenza virus infection is excessive inflammation of the lungs. Platelets are activated during influenza, but their role in influenza virus pathogenesis and inflammatory responses is unknown. OBJECTIVES: To determine the role of platelets during influenza A virus infections and propose new therapeutics against influenza. METHODS: We used targeted gene deletion approaches and pharmacologic interventions to investigate the role of platelets during influenza virus infection in mice. MEASUREMENTS AND MAIN RESULTS: Lungs of infected mice were massively infiltrated by aggregates of activated platelets. Platelet activation promoted influenza A virus pathogenesis. Activating protease-activated receptor 4, a platelet receptor for thrombin that is crucial for platelet activation, exacerbated influenza-induced acute lung injury and death. In contrast, deficiency in the major platelet receptor glycoprotein IIIa protected mice from death caused by influenza viruses, and treating the mice with a specific glycoprotein IIb/IIIa antagonist, eptifibatide, had the same effect. Interestingly, mice treated with other antiplatelet compounds (antagonists of protease-activated receptor 4, MRS 2179, and clopidogrel) were also protected from severe lung injury and lethal infections induced by several influenza strains. CONCLUSIONS: The intricate relationship between hemostasis and inflammation has major consequences in influenza virus pathogenesis, and antiplatelet drugs might be explored to develop new antiinflammatory treatment against influenza virus infections.

Annexin V incorporated into influenza virus particles inhibits gamma interferon signaling and promotes viral replication.

UNLABELLED: During the budding process, influenza A viruses (IAVs) incorporate multiple host cell membrane proteins. However, for most of them, their significance in viral morphogenesis and infectivity remains unknown. We demonstrate here that the expression of annexin V (A5) is upregulated at the cell surface upon IAV infection and that a substantial proportion of the protein is present in lipid rafts, the site of virus budding. Western blotting and immunogold analysis of highly purified IAV particles showed the presence of A5 in the virion. Significantly, gamma interferon (IFN-γ)-induced Stat phosphorylation and IFN-γ-induced 10-kDa protein (IP-10) production in macrophage-derived THP-1 cells was inhibited by purified IAV particles. Disruption of the IFN-γ signaling pathway was A5 dependent since downregulation of its expression or its blockage reversed the inhibition and resulted in decreased viral replication in vitro. The functional significance of these results was also observed in vivo. Thus, IAVs can subvert the IFN-γ antiviral immune response by incorporating A5 into their envelope during the budding process. IMPORTANCE: Many enveloped viruses, including influenza A viruses, bud from the plasma membrane of their host cells and incorporate cellular surface proteins into viral particles. However, for the vast majority of these proteins, only the observation of their incorporation has been reported. We demonstrate here that the host protein annexin V is specifically incorporated into influenza virus particles during the budding process. Importantly, we showed that packaged annexin V counteracted the antiviral activity of gamma interferon in vitro and in vivo. Thus, these results showed that annexin V incorporated in the viral envelope of influenza viruses allow viral escape from immune surveillance. Understanding the role of host incorporated protein into virions may reveal how enveloped RNA viruses hijack the host cell machinery for their own purposes.

Switch from protective to adverse inflammation during influenza: viral determinants and hemostasis are caught as culprits.

Influenza viruses cause acute respiratory infections, which are highly contagious and occur as seasonal epidemic and sporadic pandemic outbreaks. Innate immune response is activated shortly after infection with influenza A viruses (IAV), affording effective protection of the host. However, this response should be tightly regulated, as insufficient inflammation may result in virus escape from immunosurveillance. In contrast, excessive inflammation may result in bystander lung tissue damage, loss of respiratory capacity, and deterioration of the clinical outcome of IAV infections. In this review, we give a comprehensive overview of the innate immune response to IAV infection and summarize the most important findings on how the host can inappropriately respond to influenza.

Protease-activated receptor 1 inhibition protects mice against thrombin-dependent respiratory syncytial virus and human metapneumovirus infections.

BACKGROUND AND PURPOSE: Protease-activated receptor 1 (PAR1) has been demonstrated to be involved in the pathogenesis of viral diseases. However, its role remains controversial. The goal of our study was to investigate the contribution of PAR1 to respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) infections. EXPERIMENTAL APPROACH: Pharmacological approaches were used to investigate the role of PAR1 during RSV and hMPV infection, in vitro using epithelial A549 cells and in vivo using a mouse model of virus infection. KEY RESULTS: In vitro, the PAR1 antagonist RWJ-56110 reduced the replication of RSV and hMPV in A549 cells. In agreement with these results, RWJ-56110-treated mice were protected against RSV and hMPV infections, as indicated by less weight loss and mortality. This protective effect in mice correlated with decreased lung viral replication and inflammation. In contrast, hMPV-infected mice treated with the PAR1 agonist TFLLR-NH2 showed increased mortality, as compared to infected mice, which were left untreated. Thrombin generation was shown to occur downstream of PAR1 activation in infected mice via tissue factor exposure as part of the inflammatory response, and thrombin inhibition by argatroban reduced the pathogenicity of the infection with no additive effect to that induced by PAR1 inhibition. CONCLUSION AND IMPLICATIONS: These data show that PAR1 plays a detrimental role during RSV and hMPV infections in mice via, at least, a thrombin-dependent mechanism. Thus, the use of PAR1 antagonists and thrombin inhibitors may have potential as a novel approach for the treatment of RSV and hMPV infections.