Antibodies to the strain-specific and cross-reactive determinants of the haemagglutinin of influenza H3N2 viruses. 1. Preparation of antibodies and in vitro studies.

The serological analysis of antibodies to the haemagglutinin (HA) of influenza A viruses of the Hong Kong (H3N2) subtype is described, using haemagglutination-inhibition, immuno-double-diffusion and single-radial-diffusion techniques. By cross-absorption of antisera to purified HA antigens, different populations of antibody molecules were obtained, which are designated strain-specific and cross-reactive and characterized in terms of their antigenic specificities for HA antigens of the homologous and antigenically variant H3N2 viruses. A narrowly strain-specific population of antibodies (SS"HK) was obtained as the residual antibody in antiserum to A/Hong Kong/1/68 HA after absorption with the closely related A/England/42/72 virus, whilst a contrasting broadly cross-reactive population (CR'HK) was obtained by absorption of the anti-A/Hong Kong/1/68 HA serum with the more distantly related strain A/Victoria/3/75 and eluting the cross-reactive antibodies from the absorbing virus. Similarly, specific and cross-reactive antibodies were derived from antiserum to A/Victoria/3/75 HA antigen by absorption with A/Hong Kong/1/68 virus. Single-radial-diffusion tests were performed, involving sequential application of different antibody preparations in the same wells in immunoplates containing intact virus particles. The cross-reactive and strain-specific antibodies differed in their property of mutual interference of attachment ot antigen. The results suggested that the cross-reactive antigenic determinants on the HA subunit may be located closer to the distal end of the molecule than the strain-specific determinants. Further tests employing single-radial-diffusion showed that there are more cross-reactive than strain-specific sites available for antibody in the intact virus particle. The strain-specific antibodies also gave higher haemagglutination-inhibition titres per microgram IgG than the cross-reactive antibodies.