Carcinoembryonic antigen continuous epitopes determined by the SPOT method

Carcinoembryonic antigen (CEA) is a heavily glycosylated tumor-associated p rotein with an N-A1-B1-A2-B2-A3-B3 domain structure. Circulating CEA immuno assays are used for monitoring digestive cancer patients, and radiolabeled anti-CEA monoclonal antibodies (MAb) are used for the diagnosis and therapy of CEA-positive tumors. The five major nonoverlapping epitopes (Gold 1-5) have been broadly correlated with the domain organization, but there is no precise localization of the epitopes at the sequence level. In an attempt t o identify the peptide sequences corresponding to the five Gold epitopes on the CEA molecule, we prepared a set of 227 overlapping fifteen-mer peptide s corresponding to the complete CEA sequence with the SPOT method. Using fi ve high affinity MAbs directed against the five CEA Gold epitopes, we demon strated that none of these epitopes could be mimicked by a fifteen-mer pept ide sequence. However, using rabbit and goat anti-CEA sera, we identified s ix major continuous antigenic regions. All are included in the Ig-like doma ins of the CEA: two in the Al domain (residues 120-134 and 153-164), one ea ch in the A2 (329-337) and A3 domains (508-513), one at the junction betwee n the A3 and B3 domains (553-561) and one in the B3 domain (565-573). A ver y homologous sequence (common residues VSPRL) was mapped in each of the thr ee A domains. Thus, in terms of occurrence of continuous epitopes, the Ig-l ike domains Al, A2, A3 and B3 seem to be the most antigenic parts of CEA. T hese peptide sequences should be good candidates for the future development of site-specific anti-CEA MAbs. Copyright (C) 2001 S. Karger AG. Basel.