Nitric oxide suppression of human hematopoiesis in vitro: Contribution to inhibitory action of interferon-γ and tumor necrosis factor-α

IFN-γ and TNF-α, potent inhibitors of hematopoiesis, induce nitric oxide synthase (NOS) in various cell types. When normal human bone marrow (BM) or CD34⁺ cells were exposed to NO, inhibition of colony formation was dose dependent and direct. NO induced apoptosis in BM progenitors, as shown by electrophoretic detection of DNA degradation and deoxynucleotidyl transferase assay. Using PCR and immunoprecipitation, we found inducible NOS (iNOS) mRNA and iNOS protein in BM after stimulation with IFN-γ or TNF-α. iNOS mRNA was also detected by PCR in highly purified CD34⁺ cells; TNF-α or IFN-γ increased iNOS expression. The presence of iNOS in CD34⁺ cells was confirmed in single cells by immunochemical staining. N^G-Monomethyl-L-arginine (MM-Arg), an NOS inhibitor, partially reversed the effects of TNF-α and, to a lesser extent, IFN-γ in methylcellulose culture of total BM and CD34⁺ cells, and inhibited apoptosis of BM cells induced by these cytokines. When the effects of competitive iNOS inhibition were tested on more immature progenitors, MM-Arg increased the number of long-term BM culture-initiating cells in control cultures but failed to protect these cells from the inhibitory action of IFN-γ and TNF-α. Our results suggest that NO may be one mediator of cytokine-induced hematopoietic suppression.