Topographical analysis of viral epitopes using monoclonal antibodies: Mechanism of virus neutralization

The mouse mammary tumor virus (MMTV) was used in neutralization studies with monoclonal antibodies (monoclones) to the major external glycoprotein gp52. Monoclones to MMTV gp52 showed that epitopes with different antigenic specificities were targets for virus neutralization. These epitopes were group specific (common to C3H, GR, RIII, and C3Hf MMTVs), class specific (common to C3H and GR MMTVs), and type specific (unique to C3H MMTV). Competition antibody binding assays using a radioiodinated C3H MMTV type-specific neutralizing monoclone revealed that all the monoclones that neutralized virus infectivity also blocked the binding of the type-specific monoclone to C3H MMTV. A topographically distinct site from this determinant was found using other monoclones which did not compete for the binding of the type-specific monoclone and their epitopes did not function as targets for neutralizing antibodies. However, these nonneutralizing monoclones could neutralize virus infectivity in the presence of antimouse IgG sera. Therefore, two topographically distinct sites could be identified on a single protein of the virus. One site functioned as a target for neutralization and the second site bound antibody but did not affect neutralization. The observation that virus bound to cells was no longer susceptible to neutralization by monoclones showed that neutralization affected virus prior to its binding to cell surface receptors. This result further indicated that antibody prevented virus adsorption to cells and that neutralization was mediated by steric hindrance from antibodies binding to epitopes adjacent to those determinants which functioned in virus binding to cell surface receptors.