IMMUNOLOGICAL CROSS-REACTIVITY OF ANTIBODIES TO A SYNTHETIC UNDECAPEPTIDE ANALOGOUS TO AMINO TERMINAL SEGMENT OF CARCINOEMBRYONIC ANTIGEN, WITH INTACT PROTEIN AND WITH HUMAN SERA

A peptide corresponding to the 11 amino acid residues of the NH2-terminal portion in the sequence of carcinoembryonic antigen (CEA) was synthesized by the solid phase technique. The synthetic CEA(1-11) peptide was attached by means of a water-soluble carbodiimide reagent to multichain poly(DL-alanine) and to bovine serum albumin. Both macromolecular conjugates provoked rabbit anti-CEA(1-11) peptide antibodies. The specificity of this immunological system and the crossreactivity between the peptide and intact CEA were investigated by 2 methods, passive hemagglutination and modified bacteriophage [T4] inactivation. Hemagglutination experiments showed that not only anti-CEA(1-11) sera, but also anti-CEA sera, agglutinated CEA(1-11)-coated sheep erythrocytes, and both these reactions were inhibited with CEA(1-11) peptide. In experiments with the chemically modified bacteriophage technique, CEA(1-11)-coated phage was efficiently inactivated with antisera against the CEA(1-11) conjugates, and the inactivation reaction could be totally inhibited with the free peptide. The semipure CEA, but not the pure protein, could also inhibit the phage inactivation, even though less efficiently. Sera of some cancer patients were tested for their capacity to inhibit the inactivation of CEA(1-11)-coated phage by means of anti-CEA(1-11) antiserum. Sera from a large proportion of patients with adenocarcinomas of the digestive tract, pancreas and breast are apparently capable of inhibiting the above inactivation, whereas most normal sera do not inhibit.