New assay to detect shedding antigen with use of anti-idiotypic monoclonal antibodies

Anti‐idiotypic (Anti‐Id) monoclonal antibodies (MoAbs) against anti‐carcinoembryonic antigen (CEA) monoclonal antibody 5B3 (IgG 1) were prepared and characterized. Five anti‐Id MoAbs recognized the private idiotype of MoAb 5B3. Idiotype mapping suggested that MoAb 5B3 had at least three distinct idiotopes at its antigen combining site. Utilizing the inhibition of idiotype‐anti‐idiotype interaction with use of anti‐Id MoAb T4‐305 or T4‐212, an immunoassay to detect shedding CEA was established. The results of this inhibition assay correlated with those of double determinant immunoassay and inhibition percentages linearly increased in a CEA concentration‐dependent manner. Since it requires only one epitope of a given antigen molecule, this assay could be widely used for detection of shedding antigens, including tumor‐associated antigens.