Radioimmunoassay of influenza A virus haemagglutinin. I. Preparation and properties of radioactive 125I-labelled bromelain-released haemagglutinin.

Haemagglutinin released from influenza A virus recombinant MRC11 [antigenically identical to the strain A/Port Chalmers/1/73 (H3N2)] by bromelain treatment and purified by rate zonal centrifugation (further on B-HA) was examined for eventual contamination by neuraminidase. According to specific enzymatic activities corresponding to MRC11 virus and B-HA alone respectively, B-HA contained less than 0.1% of enzymatically active neuraminidase orginally present in the virus. Gel double diffusion tests, specifities of rabbit antisera induced by B-HA, as well as radioimmunoprecipitation experiments demonstrated that B-HA was devoid of any antigenically active neuraminidase. Precipitation of 125I-labelled B-HA with antisera to influenza virus recombinants with N2 neuraminidase has been evidently caused by antibodies to host antigenic determinaant(s) present in these sera. With respect to purity as well as radioimmunoprecipitation properties, B-HA is quite suitable for radioimmunoassay experiments.