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Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a specific humoral growth factor that stimulates both neutrophilic granulocyte and macrophage production by bone marrow hematopoietic progenitor cells. GM-CSF also stimulates the proliferation and clonal growth of both tissue macrophages and blood monocytes. Although at low concentrations GM-CSF was unable to support the long-term growth of tissue macrophages, it greatly enhanced their responsiveness to macrophage CSF (M-CSF, or CSF-1). This effect was also observed by treating macrophages with GM-CSF for a short time. GM-CSF did not compete with M-CSF for binding to M-CSF receptors nor was it inactivated by treatment with anti-M-CSF antiserum. Treatment of tissue macrophages with GM-CSF led to a rapid but transient downregulation of M-CSF receptors; prolonged incubation at 37 degrees C, however, resulted in a restoration and upregulation of M-CSF receptors. Identical effects were observed with both native or recombinant GM-CSF. This study suggests that GM-CSF regulates tissue macrophage production by two modes of action: (a) direct stimulation of macrophage proliferation, and (b) enhancement of their responsiveness to M-CSF.

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