Tumors undergoing rejection induced by monoclonal antibodies of the IgG2a isotype contain increased numbers of macrophages activated for a distinctive form of antibody-dependent cytolysis.

Monoclonal antibodies of the IgG2a isotype specifically inhibit the growth of human tumors in nude mice; tumors in mice given no antibody, an antibody of the IgG2a isotype that does bind to the tumor cells, or an antibody of the IgG2b isotype grow progressively. In the present work it is demonstrated that tumors of mice given the IgG2a antibody are essentially masses of necrosis, while tumors from the control mice are sheets of healthy tumor cells. Tumors of the treated mice, in comparison to controls, contain an increased number of macrophages, and these macrophages are activated for a distinct form of cytolysis dependent on antibodies of the IgG2a isotype. The data suggest that changes in the number and function of intratumoral macrophages are a major component of tumor destruction mediated by IgG2a antibodies.