Characterization of Sindbis virus epitopes important for penetration in cell culture and pathogenesis in animals

Two anti-glycoprotein E2 monoclonal (MC) antibodies, designated R6 and R13, prepared against an attenuated mutant (SB-RL) of Sindbis virus, preferentially neutralize attenuated, rapidly penetrating strains of Sindbis as opposed to virulent, more slowly penetrating strains. An antigenic variant of SB-RL which displayed reduced reactivity with R6 and R13, demonstrated virulence in suckling mice and slow penetration in baby hamster kidney cells. Anti-E2 MC antibodies to SB-RL and to an independently propagated wild-type Sindbis strain, SIN, were used to compare the topographical relationship of the R6 and R13 epitopes to other E2 antigenic sites. The R6 and R13 epitopes constituted a previously undescribed E2 site, E2-c. Antibody competition experiments using virions in solid phase and in suspension, as well as binding of MC antibody to antigenic variants, demonstrated that E2-c was a spatially distinct and independently mutable site at the surface of Sindbis virions. Of three E2 antigenic sites, E2-a and E2-c were conserved among the Sindbis strains examined, while E2-b was strain specific. Although attenuated strains were preferentially neutralized by E2-c specific MC antibodies, the critical mutation in these strains did not alter their ability to bind E2-c MC antibody. Rather, the mutation was responsible for the altered biological effect of antibody binding at E2-c; the in vitro rapid penetration phenotype; and the in vivo phenotype of attenuation in suckling mice.