In vitro induction of cytotoxicity against syngeneic mastocytoma and its suppression by spleen and thymus cells from tumor-bearing mice.

P815 times 2 mastocytoma cells, when injected subcutaneously into syngeneic DBA/2 mice, induced T lymphocytemediated cytotoxicity in the mice. During the course of tumor growth this cytotoxic activity decreased and ultimately the tumor killed the mice. When spleen cells from mice with tumors in the early stages of growth were incubated in vitro with mitomycin C-treated tumor cells, specific cytotoxicity mediated by T lymphocytes was developed. In contrast, spleen cells, taken from mice with tumors at a later stage in their growth, did not develop cytotoxic activity. The unresponsiveness of spleen cells from mice with tumors in the latter stages of growth seemed to be due to the presence of suppressor cells since in vitro development of cytotoxicity by spleen cells from early tumor-bearing mice were 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.