The stimulatory effects of TNFα on very primitive CD34++CD38- human progenitor cells derived from adult bone marrow are mediated through an interruption of the autocrine inhibitory TGF-β loop

We previously showed TNFα to be a potent synergistic stimulator of very primitive CD34++CD38- human adult bone marrow (ASM) progenitor cells at low, but not at higher concentrations, where this cytokine has an inhibitory effect (Snoeck et al., J.Exp.Med. 183: 705, 1996). Moreover, TNFα induced resistance to the inhibitory effects of exogenously added TGF-β. We now wanted to investigate whether the stimulatory effects of TNFα at low concentrations are due to interference with an autocrine inhibitory TGF-β loop. It has indeed been shown that stem cells are kept in a quiescent state by TGF- β, and that TGF-β may be produced in an autocrine fashion by very primitive stem and progenitor cells (Hatzfeld et al., J.Exp.Med. 174:925, 1991). Culture of primitive CD34-H-CD38- ABM cells in a liquid pre-CFC assay (with IL-1, IL-6, IL-3 and SCF) in the presence of an optimal concentration of anti-TGF-β antibodies induced an increase of the proliferation and production of secondary CFC from these cells. Using the combination of both anti-TGF-β and TNFα however did not result in any additional stimulatory effect, but resulted in a TNFα induced dose-dependent inhibition of the proliferation and differentiation of CD34++CD38- ABM cells. From these and from our previously published data, we conclude that 1 ) TNFα and anti-TGF-β stimulate the proliferation of CD34++CD38- ABM cells by an overlapping mechanism, 2) that this mechanism is most probably an interruption of the autocrine inhibitory TGF-β loop, 3) that TNF-α is an inhibitory cytokine for CD34++CD38- ABM cells at higher concentrations, and 4) that the biphasic dose response of TNFα in the absence of anti-TGFβ is explained by an interruption of the TGF-β loop at lower concentrations and a direct inhibitory effect which becomes dominant at higher concentrations.