Inhibitory activity in mouse lung-conditioned medium studied in the agar assay for bone marrow colony-forming cells: removal by vitamin C

The growth of granulocyte/macrophage colony-forming cells (CFC) from bone marrow, with mouse lung-conditioned medium (LCM) as source of colony-stimulating factor, was tested in five strains of mice. Maximum stimulation of growth was usually obtained at a LCM dilution of 1:5 (final concentration, 1.8% vol/vol). Higher concentrations nearly always caused inhibition. The inhibitory activity was resistant to heating (56 degrees C, 30 min) and was not removed by dialysis, but could be extracted with chloroform, suggesting a lipophilic inhibitor of high molecular weight. On gel filtration (Sephadex G50) the stimulatory activity eluted in the first fraction, whereas the inhibitory activity could not be recovered. Ascorbic acid reduced or prevented the inhibition (optimum concentration, 0.45 mmol/l), and provides a convenient means of stabilizing the CFC agar assay system. In physiological conditions the vitamin might influence the responsiveness of CFC to growth-regulating factors.