Identification of genetic diversity by cultivating influenza A(H3N2) virus in vitro in the presence of post-infection sera from small children.

Antigenic variants probably arise in the field by escaping herd immunity. We have earlier found that sera from small children are more strain-specific than sera from adults and could therefore, provide favourable conditions for selecting antigenic escape mutants. We had access to small volumes of anonymous sera collected in Norway after the epidemic season 1999/00, which was dominated by the A/Panama/2007/99 (H3N2) variant. The HA gene of the representative strain of that season was genetically identical to A/South Australia/147/99 (H3N2) and was selected for this study. Two sera from children aged 4 and 3 years, respectively, and one adult (64 years old) were used to attempt selecting antigenic escape mutants. Virus was grown in MDCK cells in the presence of human serum and escaped variants were tested by haemagglutination-inhibition tests. Although variant strains were occasionally identified, their HA1 genetic sequence did not identify obvious changes at known antigenic sites. However, by cloning and subsequent sequencing, the genetic diversity of the parent virus was found to be significantly reduced when grown in the presence of human sera. Data also showed that the two children's sera selected additional mutants from those already present in the parent pool and that the two sera selected different mutants. On a community level, it is possible that antigenic changes could be accumulated in a step-wise manner when epidemic virus is transmitted from one small child to the next, each with a restricted and possibly variant antibody repertoire.