Cell‐mediated immune reaction against tumors induced by oncornaviruses. II. Nature of the effector cells in tumor‐cell cytolysis

The nature of the effector cells detected by the chromium release test (CRT) has been studied in BALB/c and C57Bl/6 mice bearing murine sarcoma virus (MSV)‐induced tumors. Anti‐θ‐C3H immune sera completely inhibited the cytotoxic activity of lymphoid cells in the presence of complement; anti‐immunoglobulin sera failed to decrease this activity. No activation of normal non‐sensitized lymphoid cells in the presence of heat‐decomplemented sera from mice bearing MSV‐induced sarcomas could be obtained. Identical results have been found in allogeneic systems with major H‐2 histocompatibility antigens. It can be concluded that a thymus‐processed lymphoid cell sub‐population sharing the θ antigen is exclusively or very predominantly responsible for the immune cytolysis both in syngeneic tumor systems and in allogeneic transplantation systems.