In vivo antitumor effects of monoclonal antibodies with different immunoglobulin classes.

Monoclonal antibodies were produced against MM46, an MM antigen-positive, ascitic mouse mammary tumor of C3H/He mice, and 14 clones were found to produce antibodies reactive with MM46, but not with an MM antigen-negative MM48 tumor. Among these 14 antibodies, 10 reacted also with lymph node cells of C3H.B6-Ly-6b, Ly-6.2 congenic mice. The antigen defined by the 10 antibodies was provisionally designated MM1, and the one defined by the other 4 was designated MM2. Further analysis of MM1 by antibody binding inhibition assay with 3H-labeled MM1-gamma 2b-1 antibody revealed that 8 of the 10 antibodies as well as monoclonal anti-Ly-6.2 showed significant inhibition. This result indicated that there were more than two antigenic determinants on MM1 antigen. The immunoglobulin class of eight antibodies detecting the same antigenic determinant on MM1 was examined and was found to cover major classes of mouse immunoglobulin (mu, gamma 1, gamma 2a, gamma 2b, gamma 3, and alpha). Therefore, the in vivo effect against MM46 tumor cells by these antibodies was studied by a serological tumor neutralization assay. Tumor cells (4 X 10(5)) were treated with antibodies and then injected s.c. into syngeneic C3H/He mice. Ten days later, the tumor was weighed. gamma 2a antibody showed significant suppression of tumor growth, and both gamma 2b and gamma 1 antibodies also revealed suppression. However, mu, gamma 3, and alpha antibodies did not show any significant effect on the tumor growth. To elucidate the mechanisms of tumor suppression by antibodies, the role of macrophages was studied by the antibody-dependent macrophage-mediated cytotoxicity test. In accordance with the in vivo tumor effects, gamma 2a antibody showed 40 to 60% cytotoxicity up to the concentration of 1 microgram/ml, and both gamma 2b and gamma 1 antibodies were also cytotoxic, although less so than gamma 2a. Neither mu nor alpha antibody showed any significant cytotoxicity. gamma 3 antibody showed very weak cytotoxicity against MM46 tumor cells. Thus, a good correlation was observed between the in vivo antitumor effects and in vitro antibody-dependent macrophage-mediated cytotoxicity activity with regard to each class of immunoglobulin, which suggested that macrophages may play an important role in the in vivo antitumor effect of the antibodies used.