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Influenza A viruses (H3N2) isolated in MDCK cells after 1992/1993 influenza season have changed in that these viruses agglutinate human red blood cells (HRBC) but not chicken RBC (CRBC) (Ch− virus). The yield of Ch− virus in MDCK cells was comparable with that of the virus that could agglutinate CRBC (Ch+ virus); however, binding assays revealed reduced binding of the Ch− virus to MDCK cells. Replacement of the HA gene of A/Aichi/2/68 (Ch+) with that of A/Aichi/30/97 (Ch−) led to a reduction in binding of the resultant virus to MDCK cells. Ch− viruses, however, efficiently bound to derivative MDCK cells resialylated with SAα2,6Galβ1,4GlcNAc, indicating that HA of Ch− viruses distinguished the sialyloligosaccharides on native MDCK cells from those on resialylated MDCK cells. Ch− viruses exhibited limited sensitivity to NA-specific inhibitor in plaque reduction assays using MDCK cells, despite the fact that NA activity of the Ch− viruses was susceptible to the drug. These findings showed that change in the receptor-binding specificity of Ch− viruses affected the NA-dependent virus released from MDCK cells. [Copyright &y& Elsevier]

We introduced 146 single-point amino acid changes into the HA1 protein of the A/Aichi/2/68 (A/Aichi/68)(H3N2) strain. These changes were classified as positive (do not abrogate hemadsorption) or negative according to their effect on hemadsorption activity. The percentage of positive was 47%. Out of 26 amino acid positions that were changed in vitro and matched with those of mainstream amino acid changes of the HA1 polypeptide, 25 were positive. From the analysis of amino acids participating in the mainstream amino acid changes, each antigenic site was further divided into smaller parts. The amino acid substitutions in the gaps between the smaller parts of the antigenic sites mostly resulted in hemadsorption-negative changes. [Copyright &y& Elsevier]