Antigenic alterations of carcinoembryonic antigen involved in the secretion from various human tumor cell lines

Antigenic variations of carcinoembryonic antigens (CEAs) produced by 8 human tumor cell lines of various organ origins--4 derived from colonic, one from gastric, one from pancreatic, and 2 from lung cancers--were investigated on the basis of reactivity with several monoclonal antibodies recognizing either the peptide or the carbohydrate moiety of the CEA molecule by using a sandwich-type solid-phase radioimmunoassay (RIA). CEAs from all the cell lines revealed nearly uniform reactivities with the monoclonal antibodies recognizing the peptide epitopes, and no difference was observed between the reactivities of CEAs released into culture medium (m-CEAs) and of those extracted from cells (c-CEAs). On the other hand, with each of 4 monoclonal antibodies recognizing the carbohydrate epitopes, the reactivities of CEAs were found to be highly variable from one cell line to another, and a marked difference was detected between the reactivities of m-CEA and of c-CEA in every cell line. In general, c-CEA reacted much more strongly than m-CEA with these antibodies. The reactivities of some of the CEAs with 2 out of 4 antibodies to sugar epitopes increased significantly after treatment of CEAs with neuraminidase. These results suggest that distinct structural changes in the carbohydrate moiety may occur during the releasing process of CEA from cells, and that, in some cases, some epitopes may be masked by attachment of sialic acid residues.