Antigenic epitopes in the hemagglutinin of Qinghai-type influenza H5N1 virus.

The highly pathogenic avian influenza H5N1 viruses have become widespread and evolved into several clades. In our previous studies, the antigenic sites of the H5 hemagglutinin (HA) were characterized by selection and sequencing of escape mutants. In the present studies we analyzed the antigenic epitopes recognized by monoclonal antibodies against avian influenza A/Duck/Novosibirsk/56/05 (H5N1) virus isolated in western Siberia and belonging to subclade 2.2 of the H5N1 viruses. The analysis revealed several antigenically relevant positions of amino acid residues in the globular head of the HA not encountered earlier in the escape mutants of the H5 subtype. The newly recognized positions (113, 117, 118, 120, and 123, mature H5 numbering) are concentrated in an area adjacent to the region described in earlier studies as corresponding to site B in H3 HA, but extending far beyond this area. The amino acid positions recognized by the monoclonal antibodies against A/Duck/Novosibirsk/56/05 (H5N1) virus differ from the positions recognized by the monoclonal antibodies against H5N2 influenza viruses. The data suggest that the evolution of the HA of H5 avian influenza viruses is associated not only with the changes of antigenic epitopes recognized by antibodies, but also with a change in the dominance of the immunogenicity of different sites in the HA.