Monoclonal antibodies directed against human Ia antigens detect an evolutionary conserved epitope on guinea pig Ia antigens with unique functional properties.

Three murine monoclonal antihuman Ia antibodies were found to cross react with guinea pig Ia antigens. Sequential immunoprecipitation studies demonstrated that these antibodies recognized a subset of guinea pig Ia antigens. In competition binding studies, the antihuman Ia antibodies competed with each other, but not with a number of murine monoclonals raised directly to guinea pig Ia antigens. When the antihuman Ia monoclonals were tested for their ability to inhibit the proliferative response of primed guinea pig T cells to antigen-pulsed macrophages, they completely inhibited the response of strain 2 T cells to the complex antigen ovalbumin while having no effect on the response to the copolymer L-glutamic acid, L-lysine. In contrast, the monoclonals raised against guinea pig Ia had much less dramatic effects on the response of uncloned T cells to these antigens. These results suggest that the cross-reactive monoclonals may recognize an evolutionary conserved epitope on Ia that has a regulatory function in T-cell activation.