The colony-to-cluster ratio in agar cultures of bone marrow. II. Influence of a GM-CFC inhibiting factor

An inhibitory activity for the proliferation of granulocyte-macrophage colony-forming cells (GM-CFC) of mouse bone marrow in soft-agar cultures was partly purified from rat bone marrow conditioned medium by ultrafiltration and gel chromatography. It was selective in that it also inhibited the proliferation of myeloid but not that of lymphatic mouse leukaemia cells. Even at strong (greater than 60%) inhibition of the formation of cell clones (colonies + clusters) the colony-to-cluster ratio (CClR) remained as in the controls. The data point to a long-lasting and selective effect of the inhibitor on the GM-CFC without influence on the proliferation of its progeny. This conclusion is confirmed by the temporal development of the inhibitory effect in the course of culture. The determination of the CClR enables one to discriminate between proliferation modulating effects (CClR alteration) and those acting via ON/OFF-switching mechanisms (CClR constancy) influencing the clone forming efficiency at all.