Regulation of constitutive bone marrow cell proliferation by bone marrow suppressor cells

Bone marrow (BM) cells have previously been shown to suppress specific immune responses of cells from peripheral lymphoid organs. The present report describes a suppressor cell present in normal rabbit BM, which regulated the constitutive proliferation of other BM cells. The suppressor cells were Fc gamma-receptor-positive (Fc gamma R+) complement-receptor-negative, and nonadherent or weakly adherent. Similar suppressive activity was not detected among rabbit spleen cells. Removal of Fc gamma R+ suppressor cells allowed greater than 10-fold increases in the proliferation of Fc gamma R- BM cells. Addition of Fc gamma R+ BM cells to Fc gamma R- cells efficiently blocked proliferation. The suppressor cells acted by inhibiting the elaboration of a soluble growth-promoting factor by cells in the Fc gamma R- population. The growth-promoting factor enhanced proliferation of unseparated rabbit BM cells and peripheral blood mononuclear cells.