Appearance of L90I and N205S Mutations in Effector Domain of NS1 Gene of pdm (09) H1N1 Virus from India during 2009-2013

Dual R108K and G189D Mutations in the NS1 Protein of A/H1N1 Influenza Virus Counteract Host Innate Immune Responses

Influenza A viruses (IAV) modulate host antiviral responses to promote growth and pathogenicity. Here, we examined the multifunctional IAV nonstructural protein 1 (NS1) of influenza A virus to better understand factors that contribute to viral replication efficiency or pathogenicity. In 2009, a pandemic H1N1 IAV (A/California/07/2009 pH1N1) emerged in the human population from swine. Seasonal variants of this virus are still circulating in humans. Here, we compared the sequence of a seasonal variant of this H1N1 influenza virus (A/Urumqi/XJ49/2018(H1N1), first isolated in 2018) with the pandemic strain A/California/07/2009. The 2018 virus harbored amino acid mutations (I123V and N205S) in important functional sites; however, 108R and 189G were highly conserved between A/California/07/2009 and the 2018 variant. To better understand interactions between influenza viruses and the human innate immune system, we generated and rescued seasonal 2009 H1N1 IAV mutants expressing an NS1 protein harboring a dual mutation (R108K/G189D) at these conserved residues and then analyzed its biological characteristics. We found that the mutated NS1 protein exhibited systematic and selective inhibition of cytokine responses via a mechanism that may not involve binding to cleavage and polyadenylation specificity factor 30 (CPSF30). These results highlight the complexity underlying host–influenza NS1 protein interactions.

First genome report and analysis of chicken H7N9 influenza viruses with poly-basic amino acids insertion in the hemagglutinin cleavage site

We report the full-length sequence of two chicken source influenza A (H7N9) viruses found in Guangdong live poultry market (LPM) during the most recent wave of human infections (from October 2016 to the present time). These viruses carry insertion of poly-basic amino acids (KGKRTAR/G) at the protease cleavage site of the HA protein, which were previously found in the highly pathogenic (HP) human influenza A (H7N9) [IAV(H7N9)] strains. Phylogenetic analysis of these two novel avian influenza viruses (AIVs) suggested that their genomes reassorted between the Yangtze River Delta (YRD) and Pearl River Delta (PRD) clades. Molecular clock analysis indicated that they emerged several months before the HP human strains. Collectively, our results suggest that IAV(H7N9) viruses evolve in chickens through antigenic drift to include a signature HP sequence in the HA gene, which highlights challenges in risk assessment and public health management of IAV(H7N9) infections at the human-animal interface.