Changes in the antigenicity of the hemagglutinin molecule of H3 influenza virus at acidic pH.

In order to determine the location and biological significance of the acid-induced conformational change in influenza virus hemagglutinin (HA) reported by Skehel et al., monoclonal antibodies were prepared to the molecule before and after treatment at pH 5.0. These antibodies together with monoclonal antibodies to the different antigenic regions of the H3 HA were used in immunoprecipitation and ELISA binding studies to show that antigenic changes accompanied the conformational change in the HA. Treatment at pH 5.2 or less exposed new determinants on the HA while two antigenic regions, located at the tip and interface of the molecule at neutral pH, were lost or modified. Antigenic sites in the loop and hinge regions defined by the available monoclonal antibodies were not affected by the conformational change. Monoclonal antibodies specific for the acid-induced conformation efficiently inhibited hemagglutination of the virus at low pH but were extremely poor inhibitors of virus-induced red blood cell hemolysis at its pH optimum of 5.1. These antibodies were unable to neutralize viral infectivity under neutral or acidic conditions. Antibodies specific for the non-acid-treated HA conformation failed to inhibit hemagglutination at low pH values but were able to both inhibit hemolysis of red blood cells and neutralize virus infectivity. Residual unmodified HA after pH 5.0 treatment could explain the inhibition of hemolysis and infectivity by monoclonal antibodies in each of the different antigenic areas.