Characterization of a novel bispecific antibody that mediates Fcgamma receptor type I-dependent killing of tumor-associated glycoprotein-72-expressing tumor cells.

A bispecific antibody was made by chemical conjugation of Fab' fragments from humanized antibodies specific for tumor-associated glycoprotein-72 (TAG-72) and high-affinity immunoglobulin receptor, FcgammaA receptor type I (FcgammaRI). The purified anti-TAG-72 x anti-FcgammaRI (HCC49xH22) bispecific antibody had an approximate Mr of 111,000, consistent with a F(ab')2, and bound specifically to KLEB and LS174T tumor cell lines, which express the TAG-72 tumor antigen. Furthermore, HCC49x H22 was shown to simultaneously bind to KLEB cells and a soluble FcgammaRI fusion protein, demonstrating the bifunctional nature of the molecule. Using IFN-gamma-treated monocytes as effector cells, concentrations of the bispecific antibody in the range of 1-10,000 ng/ml mediated specific lysis of TAG-72-positive tumor cells. In contrast, the bispecific antibody did not promote antibody-dependent cellular cytotoxicity of a cell line that was negative for TAG-72 antigen. Importantly, the antibody-dependent cellular cytotoxicity activity of the bispecific antibody was significantly greater than that of the monoclonal antibody HCC49. These in vitro data indicate that the humanized bispecific antibody HCC49xH22 has the appropriate specificity and functional activity for further evaluation as potential immunotherapy for TAG-72-positive malignancies.