NITRIC-OXIDE SUPPRESSION OF HUMAN HEMATOPOIESIS IN-VITRO - CONTRIBUTION TO INHIBITORY-ACTION OF INTERFERON-GAMMA AND TUMOR-NECROSIS-FACTOR-ALPHA

IFN-gamma and TNF-cu, potent inhibitors of hematopoiesis, induce nitri c oxide synthase (NOS) in various cell types. When normal human bone m arrow (BM) or CD34(+) cells were exposed to NO, inhibition of colony f ormation was dose dependent and direct, NO induced apoptosis in BM pro genitors, 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 stim ulation with IFN-gamma or TNF-alpha. iNOS mRNA was also detected by PC R in highly purified CD34(+) cells; TNF-alpha or lFN-gamma increased N OS 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-cu and, to a lesser extent, IFN-gamma in methylcellulose culture of total BM and CD34(+) cells, and inhibited apoptosis of BM cells induced by thes e cytokines, When the effects of competitive iNOS inhibition were test ed on more immature progenitors, MM-Arg increased the number of long-t erm BM culture-initiating cells in control cultures but failed to prot ect these cells from the inhibitory action of IFN-gamma and TNF-alpha. Our results suggest that NIO may be one mediator of cytokine-induced hematopoietic suppression.