Specific suppressor cells in mice bearing a syngeneic mastocytoma

P815 X2 mastocytoma cells, when injected subcutaneously into syngeneic DBA/2 mice, induced T lymphocyte mediated cytotoxicity in the mice. During the course of tumor growth this cytotoxic activity decreased and ultimately the tumor killed the mice. General immunological reactivity of lymphocytes from tumor bearing mice measured by their proliferative response to mitogens remained unaffected. In order to elucidate the mechanism of the decrease in cytotoxicity in the later stages of tumor growth, an in vitro method to generate cytotoxic cells against syngeneic P815 cells was developed. When spleen cells from mice with tumors in early stages of growth were incubated in vitro with mitomycin C-treated tumor cells, specific cytotoxicity mediated by T lymphocytes was greatly enhanced. In contrast, spleen cells, taken from mice with tumors at a later stage in their growth or from normal untreated mice, did not develop cytotoxic activity. Serum from P815 tumor bearing mice did not have a direct inhibitory effect on the cytotoxicity. The unresponsiveness of spleen cells from mice with tumors in the later stages of growth seemed to be due to the presence of suppressor cells since in vitro generation of cytotoxicity by spleen cells from early tumor-bearing mice was inhibited by the addition of spleen cells or thymocytes from mice with progressively growing tumors. Normal spleen cells or thymocytes did not affect the response. Suppressive lymphoid cells from tumor bearing mice did not inhibit the mitogen responses of normal spleen cells. The suppressor cells in P815 tumor bearing DBA/2 mice were further characterized. Suppressive activity was almost completely eliminated by treating these cells with anti Θ serum and complement. Treatment with anti mouse Ig serum and complement or with carbonyl iron did not affect their suppressive activity indicating that the suppressor cells are T lymphocytes. It was found that cytotoxicity against L1210 leukemia line in DBA/2 mice could also be generated in vitro by the same method as described for the P815 tumor by incubating spleen cells from mice with L1210 tumors in an early stage of their growth with mitomycin C-treated L1210 cells. When suppressive thymocytes from P815 tumor bearing mice were tested for their capacity to inhibit the generation of anti-L1210 cytotoxicity, they did not affect the activity, indicating that the suppressor cells in P815 tumor bearing mice are specific to the tumor. When ficoll-hypaque density cell separation was carried out using cytotoxic spleen cells and suppressive spleen cells from P815 tumor bearing mice, the dense fraction was enriched for killer cells while the suppressive activity was mainly recovered in the light fraction. Therefore, killer cells and suppressor cells in P815 tumor bearing mice are thought to be distinct populations although they both belong to the T lymphocyte group. Lymphoid cells from P815 tumor bearing mice were tested for suppressive activity at various stages of tumor growth. Suppressive activity was first detected in thymuses in the early stages of tumor growth when spleens and lymph nodes had some cytotoxic activity. The suppressive activity of thymocytes persisted during the stage of slowed tumor growth when highly cytotoxic activity could be detected in the spleens and lymph nodes. After the tumors resumed accelerated growth, lymph node cells became suppressive. In the late stage of tumor growth spleen cells as well as lymph node cells and thymocytes were suppressive. A cell-free extract, prepared by freeze-thawing suppressive thymocytes from P815 tumor bearing mice, was also suppressive and its activity was specific, i.e. it inhibited the generation of anti-P815 cytotoxicity but not anti-L1210 cytotoxicity. A possible role of the suppressor cells in the present study in the escape of tumors from the host's immune system and in the regulation of the cellular immune response is discussed.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat