Host innate immune response to influenza A virus infection : role of LGP2 and importance of NS1:CPSF30 interaction for virulence
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Influenza A viruses can cause a highly contagious respiratory illness in humans. Immediately after virus infection the innate immune response is initiated by binding of viral RNA species to RIG-I that leads to activation of IRF3 and NF-κB transcription factors and activation of interferon (IFN) transcription. LGP2 is a member of the RIG-I like receptor (RLR) family and is induced after virus infection. The role of LGP2 in virus infection is controversial: it has been reported to either positively or negatively affect RIG-I mediated signaling. The goal of this study was to determine whether LGP2 has a role during infection with influenza A viruses that have circulated in humans. We focused on two viruses expressing NS1 proteins that differ in their ability to inhibit IRF3 activation and IFN transcription; a H1N1 virus (Tx91) that inhibits IRF3 activation and a H3N2 virus (Ud) that does not. This study revealed that LGP2 has strikingly different roles during infection of mouse embryonic fibroblasts and human cells with these viruses. Specifically, LGP2 has no detectable role in H1N1 virus-infected cells, whereas it downregulates IFN synthesis in H3N2 virus-infected cells. Our results indicate that LGP2 acts as a negative regulator of the IFN response in influenza A viruses that activate IRF3. The NS1 protein also binds the 30kDa-subunit of the cleavage and polyadenylation specificity factor-CPSF30, a protein required for 3′-end processing of cellular pre-mRNAs, thereby inhibiting production of mature IFN-β mRNA. The NS1 proteins of pathogenic 1997 H5N1 viruses lack two highly conserved residues (F103 and M106) that are needed to stabilize the NS1-CPSF30 complex. Instead their NS1 proteins have L at 103 and I at 106, resulting in non-optimal CPSF30 binding in infected cells. We demonstrated that strengthening CPSF30 binding by changing L and I to the consensus residues (F and M respectively) leads to a dramatic (300-fold) increase in lethality of the virus in mice. This increased virulence is associated with faster systemic spread of the virus. Microarray analyses revealed increased cytokine levels in extrapulmonary tissues, particularly the brain. These results highlight the importance of NS1:CPSF30 binding in modulating virulence in H5N1 viruses.