Interaction of murine colony-stimulating factor (CSF-1) with alveolar mononuclear phagocytes

Colony-stimulating factor (CSF-1) was purified from serum-free L-cell-conditioned medium (LCM) and iodinated so that we could study its interaction with murine alveolar macrophages. At 0 °C, the binding of ¹²⁵ICSF-1 to alveolar macrophages reached a stable maximum within 16 h. Under this condition, the binding of ¹²⁵ICSF-1 at various concentrations was saturated at about 3 ng/ml. The binding sites of ¹²⁵ICSF-1 were sensitive to trypsin but not to DNase or RNase treatment. At 37 °C, the trypsin-treated cells regenerated more than 90% of their original binding sites within 12 h. Whereas more than 97% of these alveolar macrophages were phagocytic and esterase-positive, autoradiographic studies showed that only 10–31 % of them were capable of binding to ¹²⁵ICSF-1. These results indicate that the frequencies of CSF-1-binding cells and alveolar macrophage colony-forming cells (AL-CFC) are closely correlated, but no causal relationship has been established.