Detection of endogenous macrophage colony-stimulating factor (M-CSF) in human blood

We have detected endogenous human macrophage colony-stimulating factor (M-CSF) in blood of normal individuals, using a novel RIA that accurately measures M-CSF concentrations as low as 60 U/ml (1.2 ng/ml) in the presence of serum proteins. The RIA uses an antibody to highly purified recombinant human M-CSF and is calibrated to a mouse bone marrow colony-forming assay. Ten samples of normal human blood plasma contained an average 118 +/- 9 U/ml of M-CSF, and similar concentrations were detected in serum prepared from the same individuals. RIA-positive samples contained biologically active M-CSF, as determined in a colony assay performed on mouse bone marrow cells. The M-CSF biological activity was removed by specific immune precipitation and inhibited by addition of M-CSF antibody. Physical characterization of plasma M-CSF was done by immunoblotting after partial purification on controlled pore glass and immunoaffinity chromatography. The major reduced protein species of plasma M-CSF had an apparent molecular weight of about 24 kd, and minor species of 30, 45, and 60-70 kd were also present. The RIA results on ten normal individuals suggest that endogenous circulating M-CSF is present at a low but detectable concentration. This RIA can be used to measure M-CSF in clinical samples that contain serum proteins and other growth factors that may interfere with accurate bioassay determinations.