CYCLIC GUANOSINE 3',5'-MONOPHOSPHATE MEDIATES 3-MORPHOLINOSYDNONIMINE-INDUCED INHIBITION OF HUMAN NATURAL-KILLER-CELLS

Nitric oxide (NO) donors were used to investigate the effect of NO on and the role of cyclic GMP in the regulation of human natural killer ( NK) cell function. NO-producing drugs, molsidomine and its metabolite 3-morpholinesydnonimine (SIN-1), inhibited NK cell-mediated cytotoxici ty significantly at 0.04-5 mM. At 1 mM, SIN-1 completely inhibited NK cell activity while molsidomine decreased NK cell-mediated cytolysis b y 35% of the control value. These data suggest that NO from exogenous NO-donors may down-regulate NK cell cytotoxic function. The stimulator y effect of interferon-gamma (IFN-gamma) on human NK cell-mediated kil ling could not overtake the NK cell inhibition induced by the NO relea sing drugs, indicating different modes of action for IFN-gamma and SIN -1. The results in the present study also showed that SIN-1 (1 mM) sti mulated cyclic GMP production 37-fold in NK cells. In the presence of 0.5 mM IBMX, a phosphodiesterase inhibitor, the increase in cyclic GMP was even more pronounced, demonstrating a relation between cyclic GMP stimulation and NK cell inhibition by SIN-1. Further evidence for med iation via cyclic GMP was provided by the finding that methylene blue (20 mu M), an inhibitor of soluble guanylate cyclase, decreased both t he inhibition of SIN-1-induced NK cell cytotoxicity as well as cyclic GMP formation. Moreover, membrane-penetrating cyclic GMP and its analo gues inhibited NK cell-mediated cytolysis significantly. Molsidomine w as without effect on cyclic GMP levels. Our data indicate that cyclic GMP may play a role in human NK cell regulation and suggest that the i nhibitory effect of cGMP may be elicited by NO.