Heterogeneous spontaneous and interleukin-4-induced nitric oxide production by human monocytes

Abstract: The generation of nitric oxide by human monocytes has long been a subject of controversy because of the difficulty of rationalizing this production. In this work we evaluated the capacity of human monocytes to produce nitric oxide (NO) as measured by nitrite (N02) release. Resting unstimulated monocytes (2 x 106 cells/ml) were found to produce significant amounts of N02 after 8 to 12 days in culture This production appeared to be highly heterogeneous. Indeed, approximatively, 75 % of monocytes from the different donors produced up to 10 jiM NO2 and were considered low producers; the last 25 % produced higher amounts of N02 (from 10 to 110 jsM) and were considered high producers. In any case the spontaneous production of N02 by monocytes was over-come in the presence of 1 mM N’-monomethyl-L-arginine (LNMMA). This inhibitory effect was reversed in the presence of an excess of L-arginine (5 mM), in-dicating that this process is effectively dependent on L-arginine metabolism. Because interleukin-4 (IL-4) is considered an important NO-regulatory cytokine, its regulatory effect on this spontaneous production of NO was also evaluated. In the presence of a defined dose of IL-4 (1 to 100 ng/ml) the spontaneous production of the high-producing population of monocytes was abrogated, whereas IL-4 stimulated the production by the low-producing population of monocytes, which was sup-pressed in the presence of LNMMA. The present data in-dicate that NO production by human monocytes is heter-ogeneous and that IL-4 can be a potent inducer or inhibitor of this production, suggesting a variability in the activation state of these cells. J Leukoc. Biol. 56