Since tumor necrosis factor (TNF)-alpha, interferon (IFN)-gamma, and transforming growth factor (TGF)-beta have all been shown to be specific inhibitors of early human hematopoiesis, we wanted to investigate the interactions of these three cytokines on very primitive human adult bone marrow CD34++CD38- hematopoietic progenitor cells, using a pre-colony-forming cell (pre-CFC) assay, which detects the effects of these cytokines on the initial phases of the differentiation of these primitive progenitors, which are unresponsive to interleukin (IL) 3 alone. Surprisingly, TNF-alpha was a very potent stimulator of the proliferation of CD34++CD38- cells and was the most potent synergistic factor for the IL-3-induced proliferation of these cells of all cytokines tested (IL-1, IL-6, granulocyte colony-stimulating factor, kit ligand). TNF-alpha was the only cytokine that, as a single added factor, induced substantial proliferation in CD34++CD38- cells in the presence of IL-3, except for kit ligand, which induced very limited proliferation. TNF-alpha, moreover, induced a high degree of resistance to the inhibitory effects of TGF-beta in a dose-dependent way. The inhibitory effects of IFN-gamma, however, were not affected by the presence of TNF-alpha. We hypothesize that in situations of the hematopoietic stress, TNF-alpha may abrogate the inhibitory effect of ambient TGF-beta in the bone marrow microenvironment to allow primitive stem cells to proliferate and differentiate in response to an increased demand for mature blood cells.

Since tumor necrosis factor (TNF)-a, interferon (IFN)-% and transforming growth factor (TGF)-[3 have all been shown to be specific inhibitors of early human hematopoiesis, we wanted to investigate the interactions of these three cytokines on very primitive human adult bone marrow CD34 § +CD38-hematopoietic progenitor cells, using a pre-colony-forming cell (pre-CFC) assay, which detects the effects of these cytokines on the initial phases of the differentiation of these primitive progenitors, which are unresponsive to interleukin (IL) 3 alone. Surprisingly, TNF-cx was a very potent stimulator of the proliferation of CD34 + +CD38-cells and was the most potent synergistic factor for the IL-3-induced proliferation of these cells of all cytokines tested (IL-1, IL-6, granulocyte colony-stimulating factor, kit ligand). TNF-o~ was the only cytokine that, as a single added factor, induced substantial proliferation in CD34 + +CD38-cells in the presence of IL-3, except for kit ligand, which induced very limited proliferation. TNF-c~, moreover, induced a high degree of resistance to the inhibitory effects of TGF-13 in a dose-dependent way. The inhibitory effects oflFN-% however, were not affected by the presence of TNF-ot. We hypothesize that in situations of hematopoietic stress, TNF-oL may abrogate the inhibitory effect of ambient TGF-[3 in the bone marrow microenvironment to allow primitive stem cells to proliferate and differentiate in response to an increased demand for mature blood cells. T he proliferation and differentiation of primitive he-matopoietic progenitor cells are regulated by direct interaction of these cells with bone marrow stroma and by the stimulatory and inhibitory effects of cytokines present in the microenvironment of the bone'fnarrow (1). Among cytokines inhibiting hematopoiesis, the action of TGF-[3 on progenitor cells has been characterized the best (2-5). The sensitivity of progenitor cells to the inhibitory effects of TGF-[3 is inversely correlated with their matura-tion stage: the proliferation and differentiation of a very primitive progenitor are profoundly inhibited by TGF-[3 (2-4), whereas more mature progenitors are even stimulated in some circumstances (5). We recently identified IFN-~/as a selective inhibitor of primitive human CD34 ++ CD38-progenitor cells (6). TNF-o~ is a pleiotropic cyto-kine that is predominantly produced by macrophages (7) and has been shown by many investigators to be a bifunc-tional regulator of the proliferation and differentiation of more mature hematopoietic progenitor cells, depending on the cytokines present in the culture medium and on the degree of progenitor cell enrichment (8-10). A number of reports from the same group have recently demonstrated …