Transforming growth factor beta-1 (TGF-beta-1) is known to inhibit the growth of immature hematopoietic progenitor cells, whereas more mature, lineage-restricted progenitors are not inhibited. In contrast, in the presence of saturating concentrations of granulocyte/macrophage-colony-stimulating factor (GM-CSF), TGF-beta promoted a 3- to 5-fold increase in the number and size (> 0.5 mm) of bone marrow colonies in a dose-dependent manner with an ED50 Of 10-20 pM; TGF-beta-1 alone had no effect. Morphological examination showed an increase in granulocyte colonies. In suspension cultures, TGF-beta-1 and GM-CSF stimulated an increase in total viable cells with markedly enhanced neutrophilic differentiation and a concomitant decrease in the number of monocytes/macrophages by day 6 in culture. Limiting dilution analysis demonstrated a 2- to 5-fold increase in the frequency of progenitor cells that responded to GM-CSF plus TGF-beta-1 vs. GM-CSF alone. Bone marrow progenitors obtained from mice 3 days after treatment with 5-fluorouracil responded to a combination of GM-CSF and TGF-beta-1, whereas either factor alone had no effect. A single-cell assay identified a progenitor cell that directly responded to TGF-beta and GM-CSF. TGF-beta increased the number of GM-CSF receptors on bone marrow cells. Thus, TGF-beta-1 can act as a bifunctional mediator of hematopoietic cell growth, and TGF-beta-1 and GM-CSF act together to stimulate granulopoiesis as measured by large granulocyte colony formation; the progenitor cell is tentatively designated granulocyte burst-forming unit.
