Macrophages as Regulators of Granulopoiesis

Abstract

Much evidence is available which indicates the ability of the macrophage to elaborate factors which can either increase (5, 9, 11, 13, 16, 22) or decrease (17, 20, 21, 25) the proliferation of hemopoietic cells. The question whether committed hemopoietic stem cells proliferation can be regulated by macrophages can now be answered by using the soft agar technique. The introduction of this technique for cloning granulocyte and macrophage precursor cells by Pluznik and Sachs (27) and Bradley and Metcalf (14) provided a new approach to the evaluation of the regulation of granulopoiesis and macrophage formation. The clonal growth in soft agar cultures of such precursor cells, termed colony forming units-culture (CFUC), is wholly dependent on the presence of a stimulatory substance designated colony stimulating factor (CSF) (28). In the mouse assay this substance is produced and/or released by cell suspensions from many organs including hemopoietic tissues, by cell lines and it is also present in the serum and urine (18, 29, 32). White blood cells are a major secretory source of CSF elaborated in vitro. However, the exact relationship between the various white blood cell populations in the production of CSF is unclear. Recently, different independent groups have shown that blood monocytes and tissue macrophages are the major hemopoietic source of CSF in humans and mice (12, 13, 19).