Spleen cells from mice with syngeneic tumor transplants are hyporesponsive in mixed-lymphocyte culture. The hyporesponsiveness is due to the activation of suppressor cells. Spleen from tumor-bearing mice, treated with mitomycin and added to normal mixed lymphocyte culture (with responding cells syngeneic to the added cells), inhibits proliferation of the responding cells. Suppressor activity in the spleen cells can be detected as early as 5 days after s.c. transplantation of P-815 mastocytoma into DBA/2 mice. Tumor cells placed in cell-impermeable i.p. diffusion chambers can also activate splenic suppressor cells.

Suppressor cells can be activated in syngeneic mice by (DBA/2) P-815 cells, by (C3H) L25-cells, or by recent (C57BL/6) methylcholanthrene-induced tumors. The latter tumors retain their ability to activate suppressor cells after passaging in syngeneic mice. Only one tumor, induced with methylcholanthrene in DBA/2 mice, failed to activate suppressor cells.

Suppressor activity in the spleen cells from mice with 20-day s.c. tumor transplants is not reduced after removal of glass-adherent cells. Suppressor activity is significantly decreased after removal of thymus-derived cells with anti-θ treatment and complement.

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This investigation was supported by USPHS Research Grant CA 15462 from the National Cancer Institute and General Research Support Grant 5 S07RR 05402.

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