Self-Renewal of Multipotent Long-Term Repopulating Hematopoietic Stem Cells Is Negatively Regulated by FAS and Tumor Necrosis Factor Receptor Activation

Multipotent self-renewing hematopoietic stem cells (HSCs) are responsible for reconstitution of all blood cell lineages. Whereas growth stimulatory cytokines have been demonstrated to promote HSC self-renewal, the potential role of negative regulators remains elusive. Receptors for tumor necrosis factor (TNF) and Fas ligand have been implicated as regulators of steady-state hematopoiesis, and if overexpressed mediate bone marrow failure. However, it has been proposed that hematopoietic progenitors rather than stem cells might be targeted by Fas activation. Here, murine Lin⁻Sca1⁺c-kit⁺ stem cells revealed little or no constitutive expression of Fas and failed to respond to an agonistic anti-Fas antibody. However, if induced to undergo self-renewal in the presence of TNF-α, the entire short and long-term repopulating HSC pool acquired Fas expression at high levels and concomitant activation of Fas suppressed in vitro growth of Lin⁻Sca1⁺c-kit⁺ cells cultured at the single cell level. Moreover, Lin⁻Sca1⁺c-kit⁺ stem cells undergoing self-renewal divisions in vitro were severely and irreversibly compromised in their short- and long-term multilineage reconstituting ability if activated by TNF-α or through Fas, providing the first evidence for negative regulators of HSC self-renewal.