Hemagglutinin with a polybasic cleavage site confers high virulence on H7N9 avian influenza viruses

H7N9 avian influenza virus (AIV) first emerged in February 2013 in China, and early isolates were all low pathogenic (LP). After circulation for a few years in live poultry markets of China, LP H7N9 AIVs evolved into a highly pathogenic (HP) form in late 2016. Deduced amino acid sequence analysis of hemagglutinin (HA) gene revealed that all HP H7N9 AIVs have obtained four-amino-acid insertion at position 339-342 (H7 numbering), making the cleavage site from a monobasic motif (LP AIVs) to a polybasic form (HP AIVs). Notably, the polybasic cleavage site motifs are diversified, of which PEVPKRKRTAR↓GLF motif is prevalent. To elucidate the reasons accounting for its dominance, recombinant H7N9 virus carrying PEVPKRKRTAR↓GLF (rJT157-2) motif was generated based on LP H7N9 virus A/chicken/Eastern China/JT157/2016 (JT157). Besides, another two viruses containing PEVPKGKRTAR↓GLF (rJT157-1) and PEIPKRKRTAR↓GLF (rJT157-3) cleavage site motifs were also constructed as comparisons. We found that rJT157-2 showed better biological characterizations in vitro including replication kinetics, plaque size, thermal and acid stability. In addition, animal experiments demonstrated that rJT157-2 was more pathogenic to both chickens and mice with higher virus titers and induced more severe changes in the lungs. These results suggested that HP H7N9 viruses carrying PEVPKRKRTAR↓GLF motif in the HA cleavage site were most likely adaptive mutants during the evolution of H7N9 AIVs.