Bone marrow natural suppressor cells inhibit the growth of myeloid progenitor cells and the synthesis of colony-stimulating factors.

Natural suppressor (NS) cells, which nonspecifically suppress immune responses, are generally found at sites of hemopoietic generation or regeneration. Murine bone marrow NS cells were activated by recombinant interleukin 3 (rIL-3) or recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) and produced a soluble suppressor factor. In the present study, the soluble suppressor factor from bone marrow NS cells was found to be a potent inhibitor of myeloid colony formation at concentrations below those required for immunosuppression. NS cell supernatants inhibited the growth of granulocyte-macrophage colony-forming units (CFU-GM), granulocyte erythrocyte macrophage megakaryocyte colony-forming units (CFU-GEMM), and erythroid colony-forming units (CFU-E) to a similar extent. Neutralizing anti-transforming growth factor beta (TGF-beta) reversed the suppressive effects of the supernatants, suggesting that TGF-beta was involved in the suppression. The NS cell supernatants also inhibited the production of colony-stimulating activity by bone marrow stromal cells and the transcription of GM-CSF mRNA by activated T cells. These data suggest that NS cells are important regulators of hemopoiesis. NS cells, which are non-adherent, radioresistant non-T cells resident in the bone marrow, were shown to be sensitive to treatment with the lysosomotropic agent, L-leucine methyl ester, suggesting that the NS cells may be of large granular lymphocytic or monocytic lineage. Cytotoxicity studies revealed that cells in the NS population had natural cytotoxic (NC), but not natural killer (NK) activity.