Role of the amino acid mutations in the HA gene of H9N2 avian influenza virus under selective pressure in escape vaccine antibodies

Abstract

It has been well-documented that some amino acid mutations in hemagglutinin (HA) of H9N2 avian influenza virus (H9N2 virus) alter the viral antigenicity, but little is reported about the role of mutations in escape vaccine antibodies. In this study, we found that the evolution of F/98 strain in chicken embryos or chickens resulted in significant differences in immune escape, and identify the contribution of HA mutations to the antigenic variation and immune escape of H9N2 virus. Among HA mutations the antigen variants occurring in embryonated chicken eggs and/or chickens with or without the selection pressure of vaccine antibodies, the mutations, S145N, Q164L, A168T, A198V, M224K and Q234L, affect the antigen drift of H9N2 virus. Specially, the A198V mutation, located at the receptor-binding site on the head domain of HA, significantly contributed the antigenic variation. The mutation A198V or Q234L significantly improved the receptor binding activity, while S145N mutation decreased the receptor binding activity. Single S145N mutation could promote viral escape from polyclonal antibodies (pAbs) by preventing Ab binding physically, and single A198V mutation could promote viral escape from pAbs by enhancing the receptor binding activity. Additionally, either the mutation S145N or A198V did interfere with the immunogenicity of the inactivated vaccine, resulting in reduction of the protective efficiency of H9N2 inactivated vaccine, which contributed escape from the antibody-based immunity. Our findings provided an important reference for the accurate evaluation of the role of the amino acids mutation in HA affecting the antigenicity of H9N2 virus on immune escape