Are human anti-idiotypic anti-OC125 antibodies formed after immunization with the anti-CA125 antibody B43.13?

The two monoclonal antibodies (MAb) OC125 and B43.13 both recognize antigenic determinants on the cancer antigen 125 (CA125) molecule. The aim of the present study was to clarify how far antibodies formed by patients treated with B43.13 may cross-react with idiotopes of OC125. Serum samples from 15 ovarian cancer patients treated with B43.13 were tested for the presence of anti-idiotypic anti-OC125 antibodies; 4 of these patients previously had received OC125 F(ab')2 fragments. Six patients treated only with B43.13 and all patients pretreated with OC125 fragments developed a considerable increase of human anti-mouse anti-bodies after B43.13 infusion, indicating an immune response to antibody infusion. However, none of the patients treated only with B43.13 developed detectable levels of anti-idiotypic anti-OC125 antibodies, whereas in 2 patients pretreated with OC125 fragments, the pre-existing concentration of anti-idiotypic anti-OC125 antibodies further increased after B43.13 infusion. The binding of these newly formed antibodies to OC125 was not inhibited by the B43.13, but about 70% of the binding was inhibited by the CA125 antigen. The present data suggest that the idiotopes expressed on the antibodies B43.13 and OC125, respectively, are completely different. Thus, anti-idiotypic anti-B43.13 antibodies may not cross-react with OC125. A greater number of cases is needed to clarify how far the increase of anti-idiotypic anti-OC125 antibodies observed in 2 patients pretreated with OC125 fragments really is due to B43.13 infusion.