Neutralizing and protective monoclonal antibodies identify antigenic sites in influenza H7 hemagglutinin and select for amino acid substitutions rarely observed at the global level

Abstract

ABSTRACT Avian influenza viruses of the H7 subtype infect a variety of avian as well as mammalian species, and a better understanding of H7 virus antigenicity and immunogenicity is vital to the development of effective countermeasures for a potential H7 pandemic threat. We have developed a diverse panel of murine monoclonal antibodies (mAbs) to the H7 hemagglutinin (HA) and characterized these mAbs for the HA epitope(s) that they bind, their neutralizing activity in vitro against several strains of H7, and their ability to provide protection against virus challenge in vivo. The majority of the isolated mAbs recognized antigenic sites A and B on the globular head of the influenza HA, but mAbs were also isolated to non-canonical sites on HA. Escape viruses generated against the mAbs were used to further define the antigenic sites on the H7 HA, and notably, we find that most amino acid substitutions identified in these escape viruses are rare at the global level. Finally, we used a polyclonal antibody generated by virus infection to assess whether antibody binding and neutralization were impacted by individual amino acid substitutions in the HA of escape viruses generated by the mAbs. The results showed that all of the escape viruses, each of which carried a single amino acid substitution, were still effectively neutralized by polyclonal antibodies, suggesting that the antibody response to H7 infection was broad and redundant. IMPORTANCE Influenza H7 viruses infect a variety of avian and mammalian species, and present a potential threat to human health. We used a panel of murine monoclonal antibodies (mAbs) to facilitate the identification of important protective epitopes on the H7 hemagglutinin (HA). While most of the antibodies bind to antigenic sites A and B on the H7 HA that correspond to well-characterized cognate sites on influenza H3, some antibodies appear to recognize new epitopes in HA distinct from those known antigenic sites. We also identified amino acid substitutions in escape viruses selected by these mAbs and evaluated their impact in the context of polyclonal responses and natural H7 virus evolution over a >60-year period. Taken together, the results add to our understanding of influenza H7 virus antigenicity and immunogenicity, and may have positive implications for successful H7 vaccine development. Influenza H7 viruses infect a variety of avian and mammalian species, and present a potential threat to human health. We used a panel of murine monoclonal antibodies (mAbs) to facilitate the identification of important protective epitopes on the H7 hemagglutinin (HA). While most of the antibodies bind to antigenic sites A and B on the H7 HA that correspond to well-characterized cognate sites on influenza H3, some antibodies appear to recognize new epitopes in HA distinct from those known antigenic sites. We also identified amino acid substitutions in escape viruses selected by these mAbs and evaluated their impact in the context of polyclonal responses and natural H7 virus evolution over a >60-year period. Taken together, the results add to our understanding of influenza H7 virus antigenicity and immunogenicity, and may have positive implications for successful H7 vaccine development.