H5N1 avian influenza virus (AIV) undergoes rapid evolution, and its antigenicity needs to be constantly evaluated in order to update the vaccine strain. In this report, a clade 7.2 AIV isolate named A/Chicken/Huadong/4/2008 (DT) is identified. Antigenic analysis revealed that DT had a significant low cross-reactive HI titre with antiserum against a clade 7 representative AIV, A/Chicken/Shanxi/2/2006 (Re-4). Animal experiments demonstrated that the Re-4+Re-5 vaccine provided 80% protection against DT challenge in chickens. Antisera cross-reactivity showed that a mutant with a change of residues 129, 139, 140 in site A in the HA protein had reduced reactivity with DT antiserum and increased reactivity with Re-4 antiserum. Furthermore, residue Leu129 in site A of the HA protein was confirmed to be critical for maintenance of the reactivity with the DT antiserum, and Asn140, possessing a new glycosylation site, was confirmed to be critical for reducing reactivity with the Re-4 antiserum. These results imply that there is an antigenic drift within clade 7 viruses, and insertion and glycosylation of amino acid residues in site A of the HA protein may contribute to the antigenic variation.
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