Ability of flt3 Ligand to Stimulate the In Vitro Growth of Primitive Murine Hematopoietic Progenitors Is Potently and Directly Inhibited by Transforming Growth Factor-β and Tumor Necrosis Factor-α

Abstract

The recently cloned flt3 ligand (FL) stimulates the growth of primitive hematopoietic progenitor cells through synergistic interactions with multiple other cytokines. The present study is the first demonstrating cytokines capable of inhibiting FL-stimulated hematopoietic cell growth. Tumor necrosis factor-α (TNF-α) and transforming growth factor-β1 (TGF-β1) potently inhibited the clonal growth of murine Lin–Sca-1+ bone marrow progenitors stimulated by FL alone or in combination with granulocyte colony-stimulating factor (G-CSF), stem cell factor (SCF), interleukin (IL)-3, IL-6, IL-11, or IL-12. TGF-β1 inhibited more than 96% of the myeloid colony formation in response to these cytokine combinations, whereas TNF-α reduced the number of colonies by 58% to 96% depending on the cytokine by which FL was combined. In addition, both TNF-α and TGF-β1 inhibited more than 90% of B220+ cell production from B220– bone marrow cells stimulated by FL + IL-7. The effects of TNF-α and TGF-β1 appeared to be due to a direct effect and on the early progenitors because the inhibition was observed at the single cell level, and because delayed addition of the two inhibitors for only 48 hours dramatically reduced their inhibitory effects. A neutralizing anti-TGF-β antibody showed the presence of endogenous TGF-β in the cultures and potently enhanced the ability of FL to stimulate progenitor cell growth in the absence of other cytokines. Agonistic antibodies specifically activating the p75 TNF receptors were more efficient than wild type murine TNF-α in signaling growth inhibition of Lin–Sca-1+ progenitor cells, whereas the p55 agonist had less effect than murine TNF-α. Finally, TGF-β increased the number of FL + IL-11-stimulated Lin–Sca-1+ cells in the G, phase of the cell cycle with 76%, whereas TNF-α only had a marginal effect on cell cycle distribution. Thus, TGF-J3, TNF-α, and p75 TNF receptor agonists are potent direct inhibitors of FL-stimulated progenitor cell growth in vitro.