Determination of the number of nonoverlapping antigenic areas on Hong Kong (H3N2) influenza virus hemagglutinin with monoclonal antibodies and the selection of variants with potential epidemiological significance

Monoclonal antibodies provided evidence for at least three nonoverlapping antigenic areas on the hemagglutinin molecule of A/Mem/V71 (H3N2) influenza virus. This was established by determining the reactivity patterns of 30 different monoclonal antibodies in hemagglutination-inhibition tests and by the failure to select antigenic variants of influenza virus when monoclonal antibodies from two nonoverlapping areas were used in combination. Antigenic analysis showed that most of the variants selected with monoclonal antibodies could not be distinguished from the parental virus with heterogeneous sera, suggesting that they are probably epidemiologically irrelevant. One variant, however, could be distinguished from the parental virus with heterogeneous sera and this variant had a change in sequence at residue 144 of the HAl polypeptide, from glycine in the parent to aspartic acid in the variant. A similar amino acid change has been found in naturally occurring variants at this residue. These studies suggest that some amino acid substitutions are more important than others for producing viruses with epidemiological potential. Antigenic analysis of naturally occurring H3N2 strains with monoclonal antibodies showed that antigenic variation occurs in each nonoverlapping antigenic area of the HA molecule and established that two distinct variants cocirculated in 1968, Hong Kong/1/68 being distinguishable from Aichi/2/68 in at least two antigenic areas. It appears that there may have been two separate lineages of H3N2 viruses, Hong Kong/1/68 giving rise to variants in England and Aichi/2/68 to variants in the USA and Australia.