Based on differences in the amino acid sequence of the protein haemagglutinin (HA), the H9N2 avian influenza virus (H9N2 virus) has been clustered into multiple lineages, and its rapidly ongoing evolution increases the difficulties faced by prevention and control programs. The HA protein, a major antigenic protein, and the amino acid mutations that alter viral antigenicity in particular have always been of interest. Likewise, it has been well documented that some amino acid mutations in HA alter viral antigenicity in the H9N2 virus, but little has been reported regarding how these antibody escape mutations affect antigenic variation. In this study, we were able to identify 15 HA mutations that were potentially relevant to viral antigenic drift, and we also found that a key amino acid mutation, A180V, at position 180 in HA (the numbering for mature H9 HA), the only site of the receptor binding sites that is not conserved, was directly responsible for viral antigenic variation. Moreover, the recombinant virus with alanine to valine substitution at position 180 in HA in the SH/F/98 backbone (rF/HAA180V virus) showed poor cross-reactivity to immune sera from animals immunized with the SH/F/98 (F/98, A180), SD/SS/94 (A180), JS/Y618/12 (T180), and rF/HAA180V (V180) viruses by microneutralization (MN) assay. The A180V substitution in the parent virus caused a significant decrease in cross-MN titres by enhancing the receptor binding activity, but it did not physically prevent antibody (Ab) binding. The strong receptor binding avidity prevented viral release from cells. Moreover, the A180V substitution promoted H9N2 virus escape from an in vitro pAb-neutralizing reaction, which also slightly affected the cross-protection in vivo. Our results suggest that the A180V mutation with a strong receptor binding avidity contributed to the low reactors in MN/HI assays and slightly affected vaccine efficacy but was not directly responsible for immune escape, which suggested that the A180V mutation might play a key role in the process of the adaptive evolution of H9N2 virus.

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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