Heterogeneity of circulating carcinoembryonic antigen analyzed by sandwich-enzyme immunoassays with different specificities.

Nine monoclonal antibodies against carcinoembryonic antigen (CEA) were prepared and used to investigate the immunological and physicochemical heterogeneity of circulating CEA. Two of these, M221-73 and M272-11, recognized "CEA-distinctive" epitopes and they gave sandwich-enzyme immunoassays far less reactive with nonspecific cross-reacting antigen (NCA) and nonspecific cross-reacting antigen-2 (NCA-2). Sandwich-enzyme immunoassays selective for NCA-2 and NCA were also established using suitable monoclonal antibodies as competitive inhibitors against enzyme-labeled antibodies. The studies on the serum CEA levels determined by the "CEA-specific" assay indicated that CEA molecules recognized by M221-73 and M272-11 were generally found in the sera of both cancer patients and normal adults. CEA and related substances in sera were further analyzed by the sandwich-enzyme immunoassays after adsorption on M272-11-coupled immunosorbents and by gel filtration on an Ultrogel AcA-34 column. These studies revealed the presence of a CEA variant detected by the "NCA-2-selective" assay. The variant seemed to be closely related to NCA-2 because it lacked CEA-distinctive epitopes and had an apparent molecular weight similar to that of NCA-2. This variant appeared in the sera of some cancer patients and normal adults.