Abstract
Currently, the continuous evolution of influenza A virus (IAV) still presents a major threat to human and animal health. Our sequence analysis reveals high variation at site 94 of the PA/PA-X gene in recent H5N1 and H7N9 IAV strains. To be noted, I94V mutation has become prevalent in recent human H7N9 viruses. However, the specific role of PA I94V mutation in regulating viral pathogenicity of H7N9 virus both in mammals and avian species is currently unknown. We first demonstrated that PA/PA-X I94V mutation increases the pathogenicity and viral fitness in mice while attenuating viral virulence in chickens. To determine the contribution of PA-I94V and PA-X-I94V in regulating the host adaptation of the H7N9 virus, we then systematically compared the phenotypes of the PA-X-deficient virus and the PA-X-deficient virus with PA I94V mutation. Notably, when deleted PA-X expression, the PA I94V mutation has no obvious effect on viral fitness in mice and chickens, suggesting the direct role of PA-X I94V in modulating viral virulence of H7N9 virus. Mechanistically, PA-X I94V mutation significantly increases viral polymerase activity and viral replication in mammalian cells. In addition, PA-X I94V mutation also modulates the host shutoff activity of PA-X and exerts an obvious role in inhibiting antiviral and cell death-related response. Collectively, our study advances our understanding of how the adaptive site in PA-X regulates host response and viral fitness in mice and chickens, offering deeper insights into the complex pathogenesis of influenza A virus.
