Macrophages specifically regulate the concentration of their own growth factor in the circulation.

The physiological mechanism of clearance of the mononuclear phagocyte growth factor, colony-stimulating factor 1 (CSF-1), from the circulation of normal mice was investigated by following the fate of a trace amount of i.v. injected 125I-labeled CSF-1. Macrophages selectively cleared CSF-1 by CSF-1 receptor-mediated endocytosis and degraded the growth factor intracellularly. This manner of clearance provides a feedback control mechanism whereby the rate of macrophage production is determined by the number of mature macrophages.