Infectivity phenotypes of H3N2 influenza A viruses in primary swine respiratory epithelial cells are controlled by sialic acid binding

BACKGROUND In the late 1990s, triple reassortant H3N2 influenza A viruses emerged and spread widely in the US swine population. We have shown previously that an isolate representative of this virus-lineage, A/Swine/Minnesota/593/99 (Sw/MN), exhibits phenotypic differences compared to a wholly human-lineage H3N2 virus isolated during the same time period, A/Swine/Ontario/00130/97 (Sw/ONT). Specifically, Sw/MN was more infectious for pigs and infected a significantly higher proportion of cultured primary swine respiratory epithelial cells (SRECs). In addition, reverse genetics-generated Sw/MN × Sw/ONT reassortant and point mutant viruses demonstrated that the infectivity phenotypes in SRECs were strongly dependent on three amino acids within the hemagglutinin (HA) gene. OBJECTIVES To determine the mechanism by which Sw/MN attains higher infectivity than Sw/ONT in SRECs. METHODS A/Swine/Minnesota/593/99, Sw/ONT, and mutant (reverse genetics-generated HA reassortant and point mutant) viruses were compared at various HA-mediated stages of infection: initial sialic acid binding, virus entry, and the pH of virus-endosome fusion. RESULTS/CONCLUSIONS Sialic acid binding was the sole stage where virus differences directly paralleled infectivity phenotypes in SRECs, indicating that binding is the primary mechanism responsible for differences in the infectivity levels of Sw/MN and Sw/ONT.