Identification and characterization of a novel antigenic epitope in the hemagglutinin of the escape mutants of H9N2 avian influenza viruses.

H9N2 avian influenza virus (AIV) evolves rapidly in both genovariation and antigenicity. It is essential to monitor the change of antigenicity, in particular in the hemagglutinin (HA) protein. Here we reported the selection of antigenic variants from A/Chicken/Shanghai/F/98 (H9N2) and A/chicken/Taixing/10/2010 (H9N2) viruses using HA-specific monoclonal antibodies (MAbs). Based on the reactivity of these variant and wild-type strains with the MAbs, we identified 6 critical amino acid positions (92, 145, 166, 167, 168, and 197) in the H9 antigenic sites, including the position 92 that has never been reported. Among AIVs originated from chicken in mainland China, the rates of Gly and Arg at position 92 within BJ/94-like (A/chicken/Beijing/1/1994) lineage viruses were 62.2% (28/45) and 37.8% (17/45), respectively; whereas the rates of Gly and Arg at position 92 within Y280-like (A/duck/Hong Kong/Y280/97) lineage viruses were 0.3% (2/670) and 99.1% (673/679), respectively. Our study suggests that G92R mutation together with other identified antigenic sites may serve as molecular markers for H9N2 virus evolution, and may aid improving AIV vaccine effectiveness.