Nitric Oxide Synthase Is Not a Constituent of the Antimicrobial Armature of Human Mononuclear Phagocytes

Abstract

Nitric oxide synthase (NOS) has received immense interest as an antimicrobial and antitumoral effector system of mononuclear phagocytes from rodents. Because there is increasing doubt that an analogous system exists in human macrophages, NOS was reexamined in these cells. Under tightly controlled conditions, with murine macrophages as positive controls, human macrophages failed to secrete nitric oxide «0.1 #Lmol/106 cells/24 h), even after activation with endotoxin, intcrferon-v, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor-a, bacteria, or proliferating lymphocytes. The discrepancy between murine and human macrophages depended on neither the anatomic source (blood, peritoneum), the agent used for activation, nor the duration of activation. NOS activity was paralleled by metabolization of L-arginine to L-citrulline. Exogenous tetrahydrobiopterin, an essential cofactor of NOS not synthesized by human macrophages, did not support NOS activity in human macrophages. Also, no NOS activity was found in cellular subfractions of human macrophages. It appears that in humans, the inducible high-output NOS is not conserved as an antimicrobial system of macrophages.Nitric oxide (NO) has recently been brought into focus as an antimicrobial and antitumoral effector system ofmononuclear phagocytes with activity against fungi [1,2], bacteria [3,4], parasites [5][6][7][8], and tumor cells [9][10][11][12][13][14][15]. While there is general agreement that phagocytes from mice and rats synthesize abundant NO from L-arginine [I, 7, 16-19], demonstration of high-output NO synthase (NOS) activity in human mononuclear phagocytes remains controversial. On one hand, it has been proposed that the antimicrobial activity of human blood-derived macrophages, seen after prolonged activation against Mycobacterium aviuni-Mycobacteriurn intracellulare [3] or Trypanosoma cruzii [20], depends on NO. On the other hand, human peritoneal, alveolar, and bloodderived macrophages have not been found to secrete substantial amounts of NO [21][22][23][24], even after treatment with endotoxin (lipopolysaccharide, LPS) and interferon-v (IFN-'Y). Also nitrite, a descendant of NO that is unstable under physiologic conditions, was not detectable in supernatants from human macrophages stimulated with LPS and IFN-')" [23][24][25]. Furthermore, alveolar and peritoneal macrophages have not been found to metabolize appreciable amounts of L-arginine, the substrate of NOS [21].Because of the significance attributed to the effector function of NO produced by mouse and rat macrophages [26][27][28]

Arginine metabolism: nitric oxide and beyond

“…These observations reflect the fact that arginase activity is greater in murine macrophages than in rat macrophages [201,292]. In marked contrast with rodent macrophages, human macrophages normally express little arginase or iNOS [293], although instances of NO production in human macrophages have been reported (reviewed in [201,294]). These findings indicate either that there are species differences in the intrinsic capacity for expression of these enzymes in macrophages or that conditions for reproducibly eliciting arginase or iNOS expression in human macrophages have not been identified.…”

Indoleamine 2,3 dioxygenase–mediated tryptophan catabolism regulates accumulation of Th1/Th17 cells in the joint in collagen‐induced arthritis

“…There might, however, be differences in the mechanism underlying the inefficient activation of IDO in human and mouse systems. In particular, the regulation of IDO activation by NO can be affected by the inefficient activation of iNOS in human myeloid cells compared with that in their murine counterparts (48). An additional level of regulation in human RA can be provided by expression of IDO in the arthritic joint.…”

Human monocytic U937 cells transfected with human hepatic inducible nitric oxide synthase exhibit leishmanicidal activity

“…Non phagocytized parasites were removed by washing the cells, and remaining extracellular parasites were killed by incubation for 1 h at 37°C with 20% normal non-decomplemented human serum. Cells were washed once and cultured for different time periods in DMEM-c containing or not BH 4 and/or L-NIO as previously indicated. The rate of cellular infection was determined by microscopic examination of stained preparations.…”

“…1]. Indeed, according to several investigators, human M failed to synthesize NO in vitro in response to classical inducers of iNOS commonly used for activation of murine M [2][3][4]. A number of recent studies, however, have documented the presence of NO products and/or iNOS expression in vivo in different infectious conditions [1] and a role for NO in the killing of intracellular Leishmania by human M has been proposed [5].…”