Enhanced cytotoxicity against colon carcinoma by combinations of noncompeting monoclonal antibodies to the 17-1A antigen

The purpose of this report was to investigate the binding specificities and ability to generate antibody dependent cell mediated cytotoxicity (ADCC) or antibody dependent complement mediated cytotoxicity of three novel monoclonal antibodies (mAbs), designated M74, M77, and M79, raised against the colorectal carcinoma antigen 17-1A. As determined by indirect radioimmunoassay, all three mAbs, as well as mAb 17-1A, bound to a similar extent to adherent cultures of the human colon carcinoma cell line, HT-29. Scatchard analysis of direct binding data for mAbs 17-1A, M74, M77, and M79 to HT-29 cells demonstrated equivalent association constants (7.54-9.77 X 10(7) liters/mol) and molecules bound per cell (2.15-2.69 X 10(5)). In contrast to mAbs M77 and M79, mAb M74 inhibited the binding of 125I-labeled mAb 17-1A to HT-29 cells. Similar to mAb 17-1A, incubation of human lymphocytes and blood monocytes with mAbs M74, M77, or M79 generated ADCC activity against HT-29 colon carcinoma cells. Various combinations of noncompeting mAbs to the 17-1A antigen (17-1A and M74; M77 and M79; M74 and either M77 or M79) but not competing mAbs (17-1A and M74; M77 and M79) resulted in a heightened level of ADCC activity. Under optimum conditions (saturation of antigenic sites with mAb), ADCC generated by the combination of noncompeting mAbs to the 17-1A antigen was additive to the activity seen with the respective mAbs alone. Under suboptimum conditions, the combination of noncompeting mAbs to the 17-1A antigen resulted in tumor cell cytotoxicity which was synergistic to the lysis obtained with the respective mAbs alone. No mAb used alone was able to generate antibody dependent complement mediated cytotoxicity against a panel of 17-1A positive colon carcinoma cells. Similarly, no antibody dependent complement mediated cytotoxicity activity was obtained with the combination of competing mAbs to the 17-1A antigen. However, HT-29 cells treated with noncompeting mAbs to the 17-1A antigen antigen were rendered susceptible to lysis by human complement. We conclude that the combination of mAbs recognizing different epitopes on the same tumor antigen could have important implications for the passive immunotherapy of cancer.