Serologic mapping and biochemical characterization of the carcinoembryonic antigen epitopes using fourteen distinct monoclonal antibodies

Abstract

A series of 14 monoclonal anti‐carcinoembryonic antigen (CEA, M_r 180,000) antibodies (MAbs) that show a strong degree of selective reactivity for human colon carcinomas versus normal adult tissues were used to construct a serological map of the CEA molecule. The MAbs were generated using extracts of colon carcinomas as immunogen and are thus given a COL designation. None of the 14 COL‐MAbs tested were reactive with purified non‐specific cross‐reacting antigen (NCA, M_r 55,000) from normal lung, although some showed reactivity to human granulocytes. All the COL‐MAbs tested were reactive with normal fecal antigen‐2 (NFA‐2, M_r170,000); however, many of the COL‐MAbs demonstrated a higher affinity constant to CEA than to NFA‐2. Cross‐competition radioimmunoassays classified the 14 COL‐MAbs into 5 groups. The chemical nature of the COL‐binding domains was tested using chemically or enzymatically treated CEA; all reacted with periodate‐treated CEA and deglycosy‐lated CEA, indicating that the COL‐reactive epitopes appear to be of a proteinaceous nature. Heat treatment, reduction, alkylation, pepsin digestion or pronase treatment of CEA, however, gave differential results with respect to COL binding. Antibody titration experiments were carried out to define differential reactivities to colorectal carcinoma versus NCA‐containing granulocyte extracts; these results were compared with results obtained using several anti‐CEA MAbs that have been used in clinical trials. Granulocyte binding and biochemical studies showed that the COL MAbs may distinguish as many as 7 to 10 CEA epitopes.