Fas Antigen Expression on CD34’ Human Marrow Cells Is Induced by Interferon

Activation of Fas antigen, a cell surface receptor molecule. by its ligand results in transduction of a signal for cell death. The Fas system has been implicated in target cell recognition, clonal development of immune effector cells, and termination of the cellular immune response. Fas antigen expression on lymphocytes is regulated by interferon y (IFNy) and tumor necrosis factor a (TNFa), cytokines that also have inhibitory effects on hematopoiesis. We investigated Fas antigen expression on human marrow cells and the effects of Fas activation on hematopoiesis in vitro. Freshly isolated immature hematopoietic cells, as defined by the CD34 marker, did not express Fas antigen at levels detectable by fluorescent staining. CD34+ cells, which include progenitors and stem cells, showed low levels of Fas expression in culture, even in the presence of growth factors. Stimulation by TNFa and IFNy markedly increased Fas antigen expression on CD34+ cells. Anti-Fas antibody, which mimics the action