Differences in spleen suppressor cell content in rats with progressing and regressing tumors

Young New Zealand Black rats (less than 35 wk old) developed progressively growing tumors when given injections of cells from a syngeneic 3-methylcholanthrene (MCA)-induced fibrosarcoma. Spontaneous tumor regression occurred in animals more than 10 months of age. Compared to the responses of spleen cells from age-matched controls, spleen cells from rats in which tumor progression was present progressor rats) showed decreased proliferative responses to concanavalin (Con A), phytohemagglutinin (PHA), and syngeneic MCA tumor cells. Spleen cells from rats in which the tumors had regressed (regressor rats), however, produced significant increases in proliferative responses when compared with the responses of spleen cells from age-matched controls and from progressor rats. Cell-mixing experiments implicated the presence of two spleen suppressor cell populations in progressor rats, one of which was not present in regressor rats. The unique suppressor cell found in progressor rats appeared to be specific for tumor cell-induced proliferative responses. Spleen cells from both progressor and regressor rats produced similar suppressive effects in the PHA and Con A responses of normal cells.