Conformational epitopes specific to carcinoembryonic antigen defined by monoclonal antibodies raised against colon cancer xenografts.

Four monoclonal antibodies (MoAbs) reactive with carcinoembryonic antigen (CEA) were obtained by hybridizing mouse myeloma cells (P3-X63-Ag8-U1) with spleen cells from nude mice (BALB/c, nu/nu) that had rejected transplanted human colonic adenocarcinomas Co-3 and Co-4 following intraperitoneal injection of spleen cells from immunocompetent mice (BALB/c). By solid-phase RIA with purified CEA and its related antigens, NCC-CO-413 (IgG2a, kappa) was shown to react with NCA and BGP-I as well as with CEA, whereas the reactivities of three other MoAbs, NCC-CO-308 (IgG1, kappa), -432 (IgG1 lambda), and -411 (IgG1, kappa) were limited to CEA. Immunohistochemical reactivities of these MoAbs to colonic carcinomas, granulocytes, and liver bile canaliculi on acetone-fixed paraffin-embedded sections ("AMeX" sections) confirmed the specificities of these MoAbs shown by the solid-phase RIA. By competition solid-phase RIA, the epitopes recognized by NCC-CO-308 and -432 were shown to be shared or located close to each other, whereas the other MoAbs were shown to recognize different epitopes. Thus, two epitopes specific to CEA and one shared by NCA and BGP-I as well as CEA were identified. Furthermore, reactivities of MoAbs with the two CEA-specific epitopes were easily abolished by heat denaturation or reduction of CEA, as revealed by solid-phase RIA and SDS-PAGE-immunoblotting, indicating that these two CEA-specific epitopes are based on the conformational structure of the CEA molecule.