Evaluation of the immunocompetence of macrophages and the generation of T‐cell‐mediated cytotoxic response against allogeneic tumor cells in tumor bearers

The present study has examined the competence of peritoneal cells (PC) of tumor bearers to reverse the immunosuppressive effect of tumor cells, and the ability of the tumor bearers to generate a T‐cell‐mediated cytotoxic response against allogeneic spleen cells or allogeneic tumor cells. It was found that PC from tumor bearers possess a greater ability to reverse the tumor cell immunosuppression in normal mice. In contrast to normal hosts, the cytotoxic responses of the tumor bearers against allogeneic tumor cells cannot be restored by PC from normal mice or from tumor bearers. The ineffective cytotoxic response in tumor bearers was not due to deficient T responders or T helpers. Rather, it was due to the presence of suppressor cells which were adherent, radioresistant and resistant to anti‐Thy‐1.2 antibody lysis, and thus were likely to be macrophages. The suppressor macrophages were present only in the spleen and were absent from the peritoneal cavity of the tumor bearers. Their suppressive effect can only be exerted in the presence of tumor cells. These results demonstrate the dichotomy of macrophage function, peritoneal macrophages from the tumor bearers showing increased activity of augmentation of T‐cell‐response whereas the splenic macrophages cause suppression of the T‐cell‐response.