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[Antigenic structures of CEA and CEA-related normal antigens and problems related to the determination of blood CEA
Abstract:
The antigenic structures of carcinoembryonic antigen (CEA) from tumor tissues, and of CEA-related antigens in normal adult feces and meconium were analyzed. Four CEA-related antigens were isolated from normal adult feces, three of them were inclusively called as normal fecal antigen (NFA) consisted of NFA-1, NFA-2 and NFCA (normal fecal cross-reacting antigen), the other was fecal NCA. NCA-2 was also isolated from meconium. CEA, NFA-2 and NCA-2 were very similar to each other physiochemically and antigenically, but CEA-distinctive and NFA-2-distinctive determinants were detected by using specifically prepared antisera. No distinctive determinant on NCA-2 was found so far tested with our anti-NCA-2 antisera. Both NFA-1 and NCA were partially cross-reactive with CEA, NFA-2 or NCA-2, but they were antigenically independent of each other. NFCA were partially cross-reactive with both NFA-2 and NCA. From these results, it was concluded that CEA and NFA-2 could be divided into four antigenic moieties, that is, NCA-common, NFCA-common, NFA-1-common determinant and CEA-distinctive or NFA-2-distinctive determinant. Among these, NFA-1-common and NFCA-common determinants seem to be dominant as judged from the reactivities with conventional anti-CEA antisera. Recently, we established a new solid-phase radioimmunoassay for CEA. It was found that the reactivity of the system with several purified CEA preparations could vary due to differences in nature, especially in the affinity to antigen, of antibody preparations used either for solid-phase or for tracer antibody. Among purified CEA preparations, some differences in reactivity with our assay system were also found. Reactivities of four CEA-assay systems, three commercially available and ours, with purified CEA and related antigen preparations were tested. It was found that the order of reaction intensity among antigen preparations varied due to assay systems, e.g. a given CEA preparation which reacted most strongly with an assay system reacted very weakly with another assay system, and vice versa. From these results, the problems concerned with variations in estimated CEA values due to assay methods were discussed.