Characterization of influenza A-1983 epidemic strains by polyclonal and monoclonal antibodies and detection of two co-circulating antigenic variants.

Influenza virus strains isolated during 1985 epidemic in Czechoslovakia proved to be antigenically closely related to A/Bangkok/79, A/Philippines/2/83 and A/Texas/77 (all H3N2) viruses, if examined in haemagglutination inhibition (HI) tests with standard polyclonal antisera. If examined in HI tests with monoclonal antibody (MAb) IIB4, the virus isolates could be separated into two groups: those reacting to high titres (about two thirds of the isolates) and those negative with IIB4 (titre of less than 20; rest of the strains). A relationship to MAb IIB4 similar to that of freshly isolated A-H3 influenza virus strains was found with prototype strains A/Belgium/2/81 (highly positive with IIB4, HI titre up to 20 000 per 0.025 ml) and A/Philippines/2/82 (titre less than 20). Examination of the isolate labelled A/Prague/2/83, obtained from a single individual, suggested the existence of two stable and passage-independent lines of a single virus strain, namely one HI+ and the second HI- (highly positive and negative in HI tests with MAb IIB4, respectively). Solid-phase radio-immunoassay with 125I-labelled MAb IIB4 of the viruses under consideration showed that binding of virus with antibody had occurred in all cases and that, therefore, the negative results of HI tests with HI- strains were not due to the absence of binding of MAb IIB4 to the respective viral antigen.