Effects of conditioned medium from non-differentiating or differentiated mouse myeloid leukemia cells on formation of macrophage-granulocyte colonies of normal mouse bone marrow cells

The effects of conditioned media (CM) of various clones of mouse myeloid leukemia cells (M1) on colony formation of normal mouse bone marrow cells were examined under various conditions. The CM of sensitive M1 cells, which could be induced to differentiate into macrophage-like and granulocyte-like cells by various factors stimulating the differentiation (D-factor), slightly inhibited colony formation of normal bone marrow cells by colony stimulating factor (GM-CSF). The CM of resistant M1 cells, which were resistant to the induction of differentiation even with high concentrations of the D-factor, significantly inhibited the colony formation of bone marrow cells by GM-CSF. The inhibitory activity of the CM from resistant cells decreased on treatment of the cells with a low concentration of actinomycin D, which could sensitize the resistant cells to induction of differentiation. The CM of differentiated sensitive M1 cells induced by dexamethasone stimulated the colony formation of bone marrow cells without exogenously added GM-CSF. Most of the GM-CSF activity in the CM was separated from the D-factor of M1 cells by Sephadex G-75 gel filtration.