Postreassortment changes in a model system: HA-NA adjustment in an H3N2 avian-human reassortant influenza virus

In our previous studies we described the postreassortment changes in the hemagglutinin (HA) of H2N1, H3N1, H4N1 and H13N1 influenza A virus reassortants with HAs derived from avian viruses and low-functional neuraminidase (NA) of a human parent virus A/USSR/90/77 (H1N1). The changes involved amino acid substitutions that increased the negative local charge in the vicinity of the receptor-binding pocket and decreased the affinity of HA to sialic acid receptors. In the present report we describe the studies performed with H3N2 reassortant viruses having HA of A/Duck/Ukraine/1/63 (H3N8) virus and NA of A/Aichi/2/68 (H3N2) virus. Amino acid changes in the HA gene registered in virus variants selected in the course of serial passages resulted in a decrease in the affinity to sialic acid-containing substrates and cell receptors. However, the decrease was less expressed than in the reassortants containing the low-functional NA of N1 subtype described in our earlier studies, and the amino acid changes were not necessarily associated with an increase of negative charge. In one passage variant an amino acid substitution in NA was detected. The relevance of these results for the evolution of the H3N2 virus of the 1968 pandemic is discussed.