In a previous communication, carcinoembryonic antigen (CEA) of rat, which is an analogue to human CEA, was demonstrated in gastrointestinal adenocarcinomas induced in inbred Fischer rats by injection of 1,2-dimethylhydrazine. Antibodies detectable by enzyme immunoassay were elicited in Fischer rats by immunization with a perchloric acid extract of the CEA-containing rat tumor, RCA-1, incorporated into Freund’s complete adjuvant. Specificity studies showed that activity of the rat antisera could be virtually abolished by inhibition with the tumor extract used at a concentration of 25 μg/ml. Inhibition by newborn rat tissues required extract at a concentration of 250 μg/ml. Extracts of normal adult tissues did not inhibit at these concentrations, but did inhibit at a concentration of 2,500 μg/ml. The results showed that rat CEA, though present in low concentration in normal adult rat tissue, is capable of eliciting an immune response in rats.
Active immunization of rats with an emulsion consisting of Freund’s complete adjuvant (FCA) and an extract of rat tumor containing carcinoembryonic antigen (CEA) induced clear-cut protection from growth of the syngeneic CEA-positive tumor, RCA-1. No protection was observed in rats treated with FCA alone nor was there protection against a tumor that had no serologically detectable CEA. The results suggested that the tumor immunity exhibited by the immunized rats was mediated by an immune response specific for rat CEA. It was shown further that multiparous rats were more resistant to growth of RCA-1 tumor than nulliparous rats. This suggested that immunization against rat CEA, which is an oncofetal antigen, may occur during pregnancy.
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