A colony-stimulating factor (CSF) that stimulates the in vitro proliferation of human and murine bone marrow cells was purified from normal human urine to apparent electrophoretic homogeneity (CSG(HU)-5). The nature of its stimulating activity on colony formation was determined by using unfractionated and monocyte-depleted bone marrow cells from humans and mice. The factor was able to stimulate colony formation in the unfractionated, but not in the monocyte-depleted human bone marrow cells. However, it was able to stimulate colony formation in murine bone marrow cells irrespective of the presence or absence of bone marrow monocytic cells. Supernatant fluid from liquid cultures of human unfractionated bone marrow cells contained low colony-stimulating activity (CSA) on unfractionated as well as monocyte-depleted human bone marrow cells, and the addition of CSF(HU)-5 to these liquid cultures resulted in production of two or three times more CSA that was active on both unfractionated and monocyte-depleted human bone marrow cells. This indicates that CSF(HU)-5 stimulates CSF-producing cells to elaborate CSA that is active on human CFU-C. A binding assay system was established using 125I-CSF(HU). CSF(HU) was able to bind to human and murine unfractionated cells and murine monocyte-depleted bone marrow cells, but was not able to bind to human monocyte-depleted bone marrow cells. The number of CSF(HU) binding sites to human unfractionated cells was about eightfold fewer than that to murine unfractionated cells. The parallel between the binding and colony-stimulating activities of CSF(HU) demonstrates that this binding assay system provides a useful tool for understanding the colony-forming process, which consists of many partial reactions.
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