MONOPHOSPHORYL LIPID-A STIMULATED UP-REGULATION OF NITRIC-OXIDE SYNTHASE AND NITRIC-OXIDE RELEASE BY HUMAN MONOCYTES IN-VITRO

Monophosphoryl lipid A (MPL) is a derivative of lipopolysaccharide (LP S) with reduced toxicity which has been shown to modulate various immu ne functions in monocytes. We examined whether human monocytes can be stimulated to produce nitric oxide (NO) and its catalytic enzyme nitri c oxide synthase (NOS). Monocytes were stimulated with LPS or MPL and both NOS and NO (as nitrite) production were measured. MPL at high dos es (>100 mu g/ml) stimulated monocytes to release NO that was signific antly greater than both the control and LPS-treated monocytes (p < 0.0 5). NO release by control cells and the LPS treated cells was not sign ificantly different. Both arginase and N-monomethyl arginine (NMLA) in hibited the MPL stimulated release of NO (p < 0.01). MPL significantly increased inducible NOS (iNOS) expression as measured by both fluores cent microscopy and flow cytometry (p < 0.05). Similarly, both soluble NOS (sNOS) and particulate NOS (pNOS) activity were significantly up- regulated by MPL (p < 0.05). Significant correlations were found betwe en pNOS expression and sNOS release (r = 0.72, p < 0.0001) and between 12 h NO release and sNOS production (r = 0.44, p < 0.005). These expe riments confirm that human monocytes can be stimulated with MPL to pro duce NO in vitro and suggest that up-regulation of pNOS does not precl ude NO release.