Increased Expression of Inducible Nitric Oxide Synthase in Circulating Monocytes from Patients with Active Inammatory Bowel Disease G. Dijkstra, A. J. H. Zandvoort, A. C. Muller Kobold, A. de Jager-Krikken, P. Heeringa, H. van Goor, H. M. van Dullemen, J. W. Cohen Tervaert, A. van de Loosdrecht, H. Moshage & P. L. M. Jansen Dept. of Internal Medicine, Division of Gastroenterolog y and Hepatology; Depts. of Pathology and Internal Medicine, Division of Clinical Immunology; Dept. of Internal Medicine, Division of Hematology, University Hospital, Groningen, The Netherlands Dijkstra G, Zandvoort AJH, Muller Kobold AC, de Jager-Krikken A, Heeringa P, van Goor H, van Dullemen HM, Cohen Tervaert JW, van de Loosdrech t A, Moshage H, Jansen PLM. Increase d expressio n of inducible nitric oxide synthase in circulating monocytes from patients with active inflammatory bowel disease. Scand J Gastroenterol 2002;37:546 –554. Background: Inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) synthesis are increased in epithelial cells and in tissue macrophage s of the inamed mucosa from patients with inammatory 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 quantied by ow cytometry and correlated with surface markers (CD63, CD11b, HLA-DR) for monocyte activation . In addition, iNOS expressio n in circulating monocytes was assessed by Western blotting, immunocyto- chemistry and measurement of the NO metabolites nitrite and nitrate in plasma. Results: The expression of iNOS in circulatin g monocytes and the percentag e of iNOS-positive monocytes were increased in patients with active IBD compared to healthy controls (uorescenc e index: 1.3 (0.1–6.3) versus 0.8 (0.0– 1.8); P < 0.05; percentage of iNOS positive monocytes: 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 conrmed 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 expressio n was decreased on circulatin g monocytes from patients with active ulcerative colitis. Conclusions: iNOS is increased in circulating monocytes from patients with active IBD and this increased expressio n correlates with disease activity. Considering the decreased HLA-DR expressio n and absence of monocyte activation markers, NO produced by iNOS may have a function in suppressing systemic monocyte activation . Key words: Crohn disease; inducible nitric oxide synthase; inammatory bowel disease; monocytes; neopterin; nitric oxide; NO; ulcerative colitis Gerard Dijkstra, University Hospital Groningen, Dept. of Internal Medicine, Division of Gastroenter - ology and Hepatology, P.O. Box 30.001, 9700 RB Groningen, The Netherlands (fax. ‡31 50 361 9306, e-mail. G.Dijkstra@int.azg.nl ) C onicting data have been reported concerning activa- tion of circulating monocytes from patients with inammatory bowel disease (IBD). Increased num- bers (1) and increased turnover (2) of circulating monocytes were found in patients with active IBD. Evidence of monocyte activation was found in their increased motility, phagocytosis, spontaneous giant cell formation, superoxide production and release of lysosomal enzymes (3, 4). In contrast, a decient spontaneous cell-mediated cytotoxicity, antibody-dependen t cellular cytotoxicity (ADCC) and hypor- esponsiveness to activation with interferon-gamma (IFN-g) and mitogenic lectins have also been reported (5). Nitric oxide (NO) may be involved in the regulation of monocyte function. Both NO-mediated induction (6) and NO-mediated suppres- sion (7) of lipopolysaccharide (LPS) induced tumour necrosis factor alpha (TNF-a) production by human macrophages have been described. Nitric oxide synthase (NOS) is an enzyme that catalyses the synthesis of NO and L-citrulline using oxygen and L-arginine as substrates. The enzyme exists in three isoforms encoded by distinct genes. Two constitutively expressed isoforms, neu- ronal NOS (nNOS, type I) and endothelial NOS (eNOS, type III), produce small amounts of NO (picomolar range) for neuronal and cardiovascular regulatory processes. The third, ORIGINAL ARTICLE Ó 2002 Taylor & Francis