Serum IgE concentrations and the expression of the low-affinity receptor for IgE (FceRHl/CD23) Nitric oxide (NO) generated by activated murine macrophages is cytostatic or cytotoxic for a variety of pathogens, including Leishmania major, Plasmodium falciparum, Schistosoma mansoni, Trypanosoma cruzi, and Toxoplasma gondii (1-6). NO is generated from the oxidation of the terminal guanido nitrogen atom(s) of L-arginine by an NADPH-dependent enzyme, the NO synthase (NOS) (7-9). In murine macrophages, NOS activity is induced by lipopolysaccharide (LPS) (9) IgE-IC) or by a specific monoclonal antibody (mAb) to CD23 (CD23 mAb) promotes the generation of cGMP. Simultaneous measurement of the generation of nitrite (NO-j) indicated that the enhanced production of cGMP is consequent to activation of the L-arginine:NO pathway.The low-affinity receptor for IgE (FceRII/CD23) is also expressed on normal human monocytes/macrophages after their activation in vivo (22,23). Studies in patients infected with Leishmania braziliensis or in disease-free, immunoreactive donors have shown both in situ CD23 expression and serum IgE concentrations to be increased in these conditions (23). We have, therefore, studied whether cell activation through ligation of the membrane receptor CD23 induces the L-arginine:NO pathway and results in leishmanicidal activity in human macrophages. MATERIALS AND METHODSReagents. The following reagents were used: recombinant human interleukin 4 (IL-4; a gift from J. Banchereau, Schering-Plough); IFN--y (a gift from J. M. Mencia-Huerta, Institut Beaufour, Paris); TNF-a and anti-TNF-a mAb (Genzyme); human IgE (Stallergene, Paris), and goat anti-human IgE (Nordic, Tilburg, The Netherlands). Polymyxin B, LPS (Escherichia coli 055, L-2880), NG-monomethyl-L-arginine (L-NMMA), D-NMMA, L-arginine, D-arginine, superoxide dismutase (SOD), and catalase were all obtained from Sigma.Cells. Human mononuclear cells were obtained by Ficoll gradient separation of peripheral blood leukocytes from healthy volunteers. Monocytes were separated from lymphocytes by adherence to plastic dishes coated with fetal calf serum as described (24). After this procedure, >90% of cells expressed CD14 antigen and displayed cytochemical characteristics of monocytes (24). The cells were then incubated in Dulbecco's modified Eagle's medium (DMEM) supplemented with nonessential L-amino acids, sodium pyruvate, glutamine, penicillin, streptomycin, and 10% (vol/vol) fetal calf serum (all from GIBCO). The culture medium was routinely controlled for the absence of a direct activation effect on human monocytes (CD23 expression and TNF-a production as activation Abbreviations: mAb, monoclonal antibody; CD23 mAb, anti-CD23 mAb; IgE-IC, IgE-anti-IgE immune complexes; L-NMMA, NA3-monomethyl-L-arginine; LPS, lipopolysaccharide; NOS, NO synthase; iNOS, inducible NOS; IFN-y, interferon y; IL, interleukin; TNF-a, tumor necrosis factor ai; SOD, superoxide dismutase; HPRT, hypoxanthine phosphoribosyltransferase. tTo whom reprint requests should be ad...

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The host response to Leishmania infection is regulated by a specific pattern of local cytokine production. We investigated the effect of interleukin (IL)-10 and IL-4 on the leishmanicidal activity of human macrophages (M phi). As with L. major, intracellular killing of L. infantum by human M phi was obtained following ligation of surface CD23 or cell treatment with interferon-gamma (IFN-gamma). This leishmanicidal activity required nitric oxide (NO) generation by activated M phi, and it was partially mimicked by cell treatment with chemical NO donors. Addition of recombinant human IL-10 or IL-4 to CD23 mAb or IFN-gamma decreased L. infantum and L. major killing by infected M phi. IL-10 was more potent than IL-4 in inhibiting the leishmanicidal activity of human M phi. Inhibition of Leishmania killing by IL-4 and IL-10 correlated with decreased NO generation from M phi, and was reversed when exogenous NO was added to cell cultures. Therefore, IL-10 and IL-4 down-regulate leishmanicidal activity of human M phi, in part by inhibiting NO generation by these cells.