Changes in the influenza virus haemagglutinin at acid pH detected by monoclonal antibodies to glycopolypeptides HA1 and HA2.

Monoclonal antibodies (Mabs) specific to the HA1 and HA2 subunits of the influenza virus haemagglutinin (HA) were used to show that changes in the antigenicity of the HA molecule at acid pH involve both HA subunits. In solid phase RIA (intact virus adsorbed) the acid-induced change was detected in the form of greatly increased binding of anti-HA 1 Mabs (IVA 1 and IVG 6) and anti-HA2 Mab (IIF 4). This increased binding could be most probably explained by alterations in accessibility of epitopes to the corresponding Mabs. Other Mabs examined (including 7 anti-HA2 Mabs specific to 3 independent antigenic sites) had either similar reactivities with both untreated and pH 5-treated virus or slightly but significantly increased binding to pH 5-treated virus. No effect of pH 5 treatment on antibody binding was observed with purified BHA in solid phase RIA. Nevertheless a similar pH 5-induced conformational change in the isolated BHA (like in intact viral HA in solid phase RIA) was detected in competitive binding assay carried out in liquid phase.