In vitro induction of tumoricidal and suppressor macrophages by lymphokines: possible feedback regulation

Proteose-peptone-induced macrophages (pM phi) from C57BL/6 (B6) mice were treated in vitro with lymphokine, and 2 functions were evaluated: the ability to suppress lymphokine production and the cytotoxic capacity against tumor target cells. When 10% to 20% pM phi treated with lymphokine were added to a lymphokine-producing system, strong inhibition of MIF and MAF production occurred. The suppression was not specific, since pM phi inhibited the production of lymphokine obtained by either mitogenic stimulation of normal spleen cells (NSC) or alloantigen stimulation of immune spleen cells (ISC). The suppressor cells were adherent, phagocytic, Thy 1.2 negative, with monocyte-macrophage-like morphology. Lymphokine-treated pM phi were also tumoricidal when tested in 18 hr microcytotoxicity assay against RL male 1 lymphoma target cells. However, using endotoxin-free reagents (less than or equal to 0.01 ng/ml of endotoxin by the LAL assay), we found that small amounts of lipopolysaccharide (LPS) were required, in addition to lymphokine, to induce tumoricidal activity in pM phi. In contrast, noncytotoxic pM phi stimulated with lymphokine alone were also able to inhibit lymphokine production, although not as effectively as pM phi stimulated by lymphokine plus LPS. Therefore, we concluded that lymphokine treatment itself is sufficient to induce M phi to become suppressor cells, whereas an additional signal is necessary to induce cytolytic activity. We suggest that there is a negative feedback mechanism of control on the lymphokine-producing cells through the activation of suppressor M phi by lymphokine.