Increased expression of inducible nitric oxide synthase in circulating monocytes from patients with active inflammatory bowel disease

Background: Inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) synthesis are increased in epithelial cells and in tissue macrophages of the inflamed mucosa from patients with inflammatory bowel disease (IBD), Since tissue macrophages are derived from circulating monocytes. we studied iNOS expression in circulating monocytes and related this expression to disease activity. In view of the possible role of NO in monocyte function. we also studied iNOS expression in relation to markers of monocyte activation. Methods: The expression of iNOS in circulating monocytes from 15 patients with active IBD, 6 patients who went into remission and 18 healthy controls was quantified by flow cytometry and correlated with surface markers (CD63, CD11b, HLA-DR) for monocyte activation. In addition. NOS expression in circulating monocytes was assessed by Western blotting. immunocytochemistry and measurement of the NO metabolites nitrite and nitrate in plasma. Results: The expression of iNOS in circulating monocytes and the percentage of iNOS-positive monocytes were increased in patients with active IBD compared to healthy controls (fluorescence index: 1.3 (0.1-6.3) versus 0.8 (0.0-1.8) P < 0.05: percentage of iNOS positive monocyte : 37.3 (1.0-77.0)% versus 5.3 (0.0-43.3)% P < 0,01). The six patients who went into remission all had a marked reduction of iNOS expression. iNOS expression was confirmed by Western blotting and immunocytochemistry. Plasma nitrite and nitrate levels were elevated in three patients with active IBD, The surface markers for monocyte activation. CD63 and CD11b, were not elevated. HLA-DR expression was decreased on circulating monocytes from patient, with active ulcerative colitis. Conclusions: iNOS is increased in circulating monocytes from patients with active IBD and this increased expression correlates with disease activity. Considering the decreased HLA-DR expression and absence of monocyte activation markers, NO produced by iNOS may have a function in suppressing systemic monocyte activation.