Neutralizing epitopes specific for influenza B virus Yamagata group strains are in the 'loop'

To study the neutralizing epitopes of influenza B virus Yamagata group strains, two monoclonal antibodies (mAbs) were used to select escape mutants of the virus. mAbs 5H4 and 3A12 were found to react with B/Yamagata group strains in haemagglutination inhibition and neutralization tests; no reactivity with B/Victoria group strains was observed. Most of the mutants reacted poorly to polyclonal ferret antibody against the 1998 isolate. Analysis of the deduced amino acid sequences identified a single amino acid substitution at residue 141 (Gly-->Arg) or 149 (Arg-->Gly) in 5H4-escape mutants and 141 (Gly-->Arg), 147 (Thr-->Ile) or 148 (Ser-->Gly) in 3A12-escape mutants. These residues are situated in close proximity in the 'loop' of the haemagglutinin molecule. These epitopes have been conserved in B/Yamagata group strains for almost 10 years in Japan but amino acid substitutions in the loop have been observed in clinical isolates only since 1999.