Suppression of mixed lymphocyte response in mice bearing primary tumors induced by murine sarcoma virus.

Mixed lymphocyte culture (MLC) responses of spleen cells from mice bearing tumors induced by murine sarcoma virus of the Moloney type (MSV) against allogeneic spleen cells were found to be markedly depressed, as estimated by three parameters: lymphoblast counts, 3H-thymidine incorporation (TI), and cell-mediated lysis (CML). While all three parameters were definitely depressed in comparison to the normal controls, TI was depressed to a greater degree than lymphoblast counts and CML. MSV spleen cells also had a suppressive effect on the MLC responses of normal spleen cells in third party type of experiments. The cells mediating this effect were radioresistant non-T cells, which were removed by adherence columns and were inactivated by carrageenan. These data suggested that the supressor cells were macrophages. Serum from MSV tumor-bearing mice had a greater suppressive effect than did normal mouse serum. Both the suppressor cells and the suppressive serum factors may play an important role in the immunosuppression observed in tumor-bearing hosts.