Regulation of erythroid colony formation by bone marrow macrophages

We present evidence that macrophages stimulate red cell production by human bone marrow cells in vitro and that this effect is mediated by soluble substances. Bone marrow cells from donors were cultured in liquid media; after 7-14 days, cells highly enriched for macrophages and supernatants from these cultures were collected and cocultured in plasma clots with fresh human bone marrow cells (CFU-e) for 7 days or peripheral blood mononuclear cells (BFU-e) for 1 5 days. When added in numbers generally noted in marrow aspirates. bone marrow cells enriched for macrophages enhanced erythroid colony (EC) formation. Thus. at macrophage concentrations of 0% versus 1 %, 2%, 4% and 10%, the numbers of EC/6 x i0� BM cells were: 677 ± 46 (mean ± SEM) versus 846 ± 98, 938 ± 85. 1039 ± 93. and 956 ± 68. respectively. Stimulation was noted until unphysiologicallyhigh ratios of macrophages (>1 5%) were added, when EC formation was progressively inhibited. The same erythroid growth curve was obtained whether macrophages and red cell precursors were from the same or different donors. Nonadherent cells from liquid culture did not modulate in vitro erythroid growth, whereas adherent cells at low concentrations enhanced and at higher concentrations inhibited EC formation. Supernatants from these macrophage cultures contained a substance, not erythropoietin, with similar erythropoietic effects. M ORPHOLOGICAL OBSERVATIONS by light and electron microscopy suggest that erythropoietic cells are not scattered randomly through the bone marrow matrix, but in fact are organized anatomically into erythroblastic islands.’ These islands consist of one or two central reticular cells or macrophages surrounded by a ring of developing erythroblasts.2 High resolution electron microscopy has shown that ferritin molecules essential to normal erythroblast development adhere to the surface ofthe erythroblast membrane. From this observation has come the hypothesis that the central macrophage, as the cell of origin of these molecules, assumes a “nursing” functionin erythropoiesis.3 The development of simple cloning techniques in semisolid mediumhas allowed further characterization of the physiology of hemopoiesis and has further clarified the function of peripheral blood and bone