Understanding the Role of MxA in Controlling Influenza A Virus Replication in Primates

The relationship between influenza A virus replication and innate antiviral immune responses in primates was investigated both in vivo and in vitro. First, the effect of Influenza virus suppression on the induction of the innate immune response was determined. Administration of the Influenza A virus therapeutic drug oseltamivir prior to influenza A/Memphis/7/01 (H1N1) challenge significantly reduced virus replication in the trachea of rhesus macaques, and correlated with the induction of high levels of Type I IFN (IFNalpha and beta) and interferon stimulated genes. Surprisingly, one IFN-stimulated gene, MxA, was significantly lower in when the virus was allowed to replicate uncontrolled, suggesting a beneficial viral mechanism that suppresses IFNalpha induced MxA control of influenza virus replication in primates. In addition, induction of MxA by administration of exogenous IFNalpha resulted to suppression of influenza A virus replication in rhesus macaques. IFNalpha administration 24 hours prior to influenza virus challenge significantly lowered peak tracheal vRNA levels, reduced weight loss, and blunted the spike in body temperature after influenza infection, as compared to untreated control animals.;The role of MxA in blocking human Influenza A virus replication was determined by manipulation of MxA expression in primate cells. IFNalpha treatment of the rhesus kidney epithelial cell line (LLC-MK2) prior to influenza virus infection suppressed virus replication and induced the expression of many ISGs, including MxA. Knockdown of MxA expression in these IFN-treated cells with a MxA siRNA abolished IFNalpha induced suppression of virus replication. In addition, influenza virus replication was drastically inhibited in Vero cells stably transfected with MxA. A strand-specific RT-PCR assay specific for the three species of Influenza A virus RNA generated during replication within the host cell, showed that in Vero cells stably transfected with MxA, positive-stranded viral messenger RNA (mRNA), complimentary positive-strand influenza genome RNA (cRNA), and negative-stranded influenza genomic RNA (gRNA) were significantly reduced as compared to control Vero cells transfected with the empty vector. These results indicate that MxA inhibits influenza virus replication at a step prior to primary transcription of the viral genome within the primate cell.;Taken together, these findings demonstrate for the first time that IFNalpha induced MxA gene expression is the key effector molecule in controlling Influenza A virus replication in primate cells, and emphasize the importance of in vivo investigation MxA as a possible therapeutic drug targeting Influenza A virus infection.