Inhibition of in vitro lymphoproliferative responses to tumor‐associated antigens by suppressor cells from rats bearing progressively growing gross leukemia virus‐induced tumors

W/Fu rats were injected subcutaneously with low numbers of cells from the Gross leukemia virus‐induced lymphoma, (C58NT) D, which induced transient tumor growth and regression (regressors), or with high numbers of tumor cells resulting in progressive tumor growth (progressors). Spleen cells from regressors had a significant reactivity in the mixed leukocyte tumor cell interaction (MLTI), while spleen cells from progressors were unresponsive. Similarly, the responses to the non‐specific mitogens, phytohemagglutinin and concanavalin A, were suppressed in spleen‐cell cultures of progressors. Passage of spleen cells from progressors over rayon adherence columns or pretreatment with an iron/magnet technique resulted in almost complete restoration of MLTI and mitogen responses. Addition of spleen cells from progressors depressed the MLTI of spleen cells from regressors and the mitogen reactivity of normal spleen cells. Serum from progressors also suppressed MLTI and mitogen reactivity. These data indicate that, in spleens of rats bearing progressively growing tumors, suppressor cells can be demonstrated which inhibit specific reactivity to tumor‐associated antigens and non‐specific reactivity to mitogens. The presence of suppressor cells or of inhibitory factors in the serum may contribute to the immunosuppression frequently observed in tumor‐bearing hosts.