Enhanced binding activity observed between anti-carcinoembryonic monoclonal antibodies

Six anti-carcinoembryonic monoclonal antibodies (CEA MAbs) were isolated and characterized. Through blocking studies, only two were found to share the same epitope, indicating that CEA has at least five different epitopic sites. One of the group, 31C5A4 was found to enhance greatly the binding of CEA by 20C2H1. When (31C5A4) was fragmented, both the F(ab')2 and Fab bound CEA and blocked the binding of CEA by intact 31C5A4; however, only F(ab')2 was able to enhance. Fab could be induced to enhance after being restructured into a divalent form with rabbit anti-mouse (RAM) (heavy and light chains). The addition of the enhancing MAb (31C5A4) to the CEA-20C2H1 RAM-staph A (SA) complex caused a striking decrease in the rate at which the CEA dissociated from the complex. The data suggest that the enhancement observed with these anti-CEA MAbs was the result of the formation of a stable cyclic complex.