Antigenic variants with amino acid deletions clarify a neutralizing epitope specific for influenza B virus Victoria group strains

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Abstract
To study the neutralizing epitopes of influenza B virus Victoria group strains, two monoclonal antibodies (MAbs) were used to select antigenic variants of the virus. MAbs 10B8 and 8E6 were found to react with B/Victoria group strains in three tests, peroxidase–antiperoxidase staining, haemagglutination inhibition and neutralization tests; no reactivity with B/Yamagata group strains was observed. Analysis of the deduced amino acid sequences of 10B8-induced variants identified a single amino acid deletion at residue 165 or 170, as well as a single amino acid substitution at residues 164 (Asp→Tyr), 165 (Asn→Ser or Thr) or 203 (Lys→Thr or Asn). A single amino acid substitution at residue 241 (Pro→Ser) was observed in 8E6-induced variants. Three-dimensional analysis showed that the epitopes for both MAbs were situated in close proximity to each other. Since B/Yamagata group strains are characterized by amino acid deletions at residues 164–166, the epitope for MAb 10B8 is strictly specific for B/Victoria group strains.