SPECIFICITY OF ANTICARCINOEMBRYONIC ANTIGEN MONOCLONAL-ANTIBODIES ANDTHEIR EFFECTS ON CEA-MEDIATED ADHESION

The carcinoembryonic antigen (CEA) domain specificities of a library o f monoclonal anti-CEA antibodies were determined to investigate the me chanisms of homophilic binding involved in CEA-mediated intercellular adhesion. Using an indirect immunofluorescence cell surface staining t echnique, the reactivities of these antibodies were tested systematica lly on Chinese hamster ovary cells stably transfected with constructs of CEA gene family members CEA, nonspecific cross-reacting antigen, CG M6, and biliary glycoprotein, as well as on stable Chinese hamster ova ry transfectants expressing truncated CEA and CEA/nerve cell adhesion molecule chimeric proteins. Epitopes for these antibodies were thus lo calized on the CEA molecule as follows: monoclonal antibody groups 1, 2, and 6 react with epitopes in the N-terminal domain of CEA, whereas groups 3, 5a, 5b, and 9 react with determinants found in the internal repeating domains located in the C-terminal part of the CEA molecule. Groups 4 and 8 appear to react with a repeating epitope in the interna l domains of CEA and in all CEA subfamily members. Fab fragments of mo noclonal antibody group 1 block aggregation of CEA transfectant cells, indicating that a region in the N domain is involved in CEA homophili c interaction. Fab fragments of monoclonal antibody group 9 recognize an epitope dependent on the B2 and A3 domains and stimulate aggregatio n. These results support the recently reported CEA-CEA homophilic reci procal binding model in which anti-parallel molecules are bound by 2 b onds between the N domain of one molecule and the A3B3 domain of the i nteracting partner.