Biochemical and biological characteristics of epitopes on the E1 glycoprotein of western equine encephalitis virus.

Antigenic determinants identified by monoclonal antibodies (Mabs) on the E1 glycoprotein of western equine encephalitis (WEE) virus have been characterized by their serological activity, requirements for secondary structure, expression on the mature virion, and their role in protecting animals from WEE virus challenge. On the basis of a cross-reactivity enzyme-linked immunosorbent assay (ELISA) and hemagglutination inhibition assay, eight antigenic determinants (epitopes) on the E1 glycoprotein have been identified, ranging in reactivity from WEE-specific to alphavirus group reactive. No neutralization of virus infectivity was demonstrable with any of the Mabs. An alphavirus group-reactive hemagglutination (HA) site, a WEE complex-reactive HA site, and a WEE virus-specific HA site were identified. Spatial arrangement of these epitopes was determined by a competitive binding ELISA. Four competition groups defining three distinct antigenic domains were identified. Antibodies directed against four E1 epitopes were capable of precipitating the E1/E2 heterodimer from infected cells or purified virus disrupted with nonionic detergents. These same antibodies precipitated only E1 in the presence of 0.1% SDS. That E1 conformation was important was shown by the inability of antibodies specific for seven of the epitopes to bind to virus denatured in 0.5% SDS. As determined by equilibrium gradient analysis of virus-antibody mixtures, four epitopes were found to be fully accessible on the mature virion, three epitopes were inaccessible, and one epitope was partially accessible to antibody binding. Antibodies specific for three epitopes were able to passively protect mice from WEE virus challenge.