Influenza A viruses (IAVs) cause seasonal epidemics and occasional pandemics, representing a serious public health concern. It has been described that one mechanism used by some IAV strains to escape the host innate immune responses and modulate virus pathogenicity involves the ability of the PA-X and NS1
proteins to inhibit the host
protein synthesis in infected cells. It was reported that for the 2009 pandemic H1N1 IAV (pH1N1) only the PA-X
protein had this inhibiting capability, while the NS1
protein did not. In this work, we have evaluated, for the first time, the combined effect of PA-X- and NS1-mediated inhibition of general gene expression on virus pathogenesis, using a temperature-sensitive, live-attenuated 2009 pandemic H1N1 IAV (pH1N1 LAIV). We found that viruses containing PA-X and NS1
proteins that simultaneously have (PAWT+/NS1MUT+) or do not have (PAMUT-/NS1WT-) the ability to block host gene expression showed reduced pathogenicity in vivo However, a virus where the ability to inhibit host
protein expression was switched between PA-X and NS1 (PAMUT-/NS1MUT+) presented pathogenicity similar to that of a virus containing both wild-type
proteins (PAWT+/NS1WT-). Our findings suggest that inhibition of host
protein expression is subject to a strict balance, which can determine the successful progression of IAV
infection. Importantly, knowledge obtained from our studies could be used for the development of new and more effective
vaccine approaches against IAV.IMPORTANCE Influenza A viruses (IAVs) are one of the most common causes of
respiratory infections in humans, resulting in thousands of deaths annually. Furthermore, IAVs can cause unpredictable pandemics of great consequence when viruses not previously circulating in humans are introduced into humans. The defense machinery provided by the host innate immune system limits IAV replication; however, to counteract host
antiviral activities, IAVs have developed different inhibition mechanisms, including prevention of host gene expression mediated by the viral PA-X and NS1
proteins. Here, we provide evidence demonstrating that optimal control of host
protein synthesis by IAV PA-X and/or NS1
proteins is required for efficient IAV replication in the host. Moreover, we demonstrate the feasibility of genetically controlling the ability of IAV PA-X and NS1
proteins to inhibit host immune responses, providing an approach to develop more effective
vaccines to combat disease caused by this important respiratory pathogen.