BIOCHEMICAL-CHARACTERIZATION OF 25 DISTINCT CARCINOEMBRYONIC ANTIGEN (CEA) EPITOPES RECOGNIZED BY 57 MONOCLONAL-ANTIBODIES AND CATEGORIZED INTO 7 GROUPS IN TERMS OF DOMAIN-STRUCTURE OF THE CEA MOLECULE

The chemical nature of 25 distinct carcinoembryonic antigen (CEA) epitopes, which were recognized by 57 different monoclonal antibodies and categorized into 7 groups (Groups A to G) in terms of domain structure of the CEA molecule, was analyzed and the findings obtained were compared with the results of our previous studies using recombinant CEA proteins. All 21 epitopes of Groups A to F defined by 48 MAbs were resistant to periodate oxidation and were to a greater or lesser extent retained after deglycosylation of CEA, indicating that they are all protein in nature. The 21 epitopes were detected in recombinant CEA proteins expressed in Chinese hamster ovary (CHO) cells. Seven of the 21 epitopes of protein nature were partially or completely sensitive to reduction and alkylation of CEA and not detected or only slightly revealed in the recombinant CEA proteins expressed in E. coli, indicating that those epitopes are dependent on the tertiary structure of the peptide chain, which is formed by disulfide bonds. All 4 epitopes of Group G defined by 9 MAbs were sensitive to mild periodate oxidation and deglycosylation, but resistant to reduction and alkylation and to digestion with pepsin or pronase, indicating that those 4 epitopes are carbohydrate in nature. Although none of the 4 epitopes of Group G were detected in the recombinant CEA proteins expressed in E. coli, two were detected in those expressed in CHO cells. The biochemical studies reported here thus provide information as to the nature of the epitopes on the CEA molecule and help form the basis for selecting the anti-CEA MAbs for use in biological study and potential clinical applications of CEA.