| Abstract: | Three different monoclonal antibody isotypes, IgG1, IgG2b, and IgG2a, were derived by selection of isotype-switch variants from the CO-19-9 hybridoma. All three antibodies retained their binding specificities and affinities and bound to the same epitope--as defined by the anti-idiotype analysis. Availability of gamma 1, gamma 2b, and gamma 2a Ig heavy chain variants directed against the same epitope on the monosialoganglioside antigen permitted detailed analysis of their Fc fragment receptor (FcR) binding affinities, their cytolytic activities (antibody-dependent, macrophage-mediated cytotoxicity) in vitro, and their tumoricidal activities in vivo. Analysis of the binding of three isotypes to the human FcR expressed by U937 cells induced by gamma interferon has shown that only IgG2a proteins bound to high-affinity FcR, but not IgG1 or IgG2b variants. Although all three isotypes were active in the antibody-dependent, macrophage-mediated cytotoxicity assay with murine thioglycolate-elicited macrophages, the IgG2a gave the highest percentage of lysis; similar results were obtained in the same assay with human monocytes as effector cells. In the nude mice experiment, only the IgG2a variant inhibited growth of human colorectal carcinoma, while IgG1 and IgG2b were ineffective. Thus, selection of isotype-switch variants resulted in the conversion of monoclonal antibody from noncytolytic to cytolytic with possible immunotherapeutic application. |
