Inhibition of rat mixed lymphocyte cultures by suppressor macrophages.

Normal rat spleens contain suppressor cells which can inhibit proliferative and cytotoxic responses of lymphocytes to alloantigens ijz vitro. The suppressor cells are adherent, phagocytic, resistant to treatment with ATS and C, radioresistant, resistant to treatment with mitomycin C, apparently absent from the thymus, and found in very high concentrations in peritoneal exudates. These characteristics indicate that the suppressor cell is a macrophages and not a T cell. When suppressor cells were removed from spleen cell suspensions, strong ilz vitro proliferative and cytotoxic responses to alloantigens could consistently be observed.