Pre-exposure of murine macrophages to lipopolysaccharide inhibits the induction of nitric oxide synthase and reduces leishmanicidal activity

A transcriptomic analysis of the effects of macrophage polarization and endotoxin tolerance on the response to Salmonella

Salmonella is an intracellular pathogen causing significant morbidity and mortality. Its ability to grow inside macrophages is important to virulence, and is dependent on the activation state of the macrophages. Classically activated M1 macrophages are non-permissive for Salmonella growth, while alternatively activated M2 macrophages are permissive for Salmonella growth. Here we showed that endotoxin-primed macrophages (MEP), such as those associated with sepsis, showed similar levels of Salmonella resistance to M1 macrophages after 2 hr of intracellular infection, but at the 4 hr and 24 hr time points were susceptible like M2 macrophages. To understand this mechanistically, transcriptomic sequencing, RNA-Seq, was performed. This showed that M1 and MEP macrophages that had not been exposed to Salmonella, demonstrated a process termed here as primed activation, in expressing relatively higher levels of particular anti-infective genes and pathways, including the JAK-STAT (Janus kinase-signal transducer and activator of transcription) pathway. In contrast, in M2 macrophages these genes and pathways were largely expressed only in response to infection. Conversely, in response to infection, M1 macrophages, but not MEP macrophages, modulated additional genes known to be associated with susceptibility to Salmonella infection, possibly contributing to the differences in resistance at later time points. Application of the JAK inhibitor Ruxolitinib before infection reduced resistance in M1 macrophages, supporting the importance of early JAK-STAT signalling in M1 resistance to Salmonella.

Endotoxin tolerance is not a LPS-specific phenomenon: partial mimicry with IL-1, IL-10 and TGFβ

Administration of non-lethal doses of lipopolysaccharide (LPS) to experimental animals and humans results for a short period of time in a state of hyporesponsiveness to a second LPS challenge. This phenomenon, known as endotoxin tolerance, has been reproduced in vitro using human monocytes, rendered endotoxin-tolerant following a first incubation with LPS. A further activation by LPS was manifested by a dramatically reduced production of tumor necrosis factor α (TNFα). We report this low responsiveness of LPS pretreated monocytes as an endotoxin non-specific phenomenon. Indeed, TNFα release upon further activation with either killed Gram-positive bacteria (Staphylococci, Streptococci) or zymosan was also significantly diminished. This was not the case when phorbol myristate acetate (PMA) was used as a second triggering agent, suggesting that the monocyte hyporesponsiveness due to LPS does not affect all activation pathways, particularly that of protein kinase C. On the other hand, both PMA and zymosan pretreatment could reduce a further activation of monocytes by LPS. We investigated whether the first signal(s) delivered by LPS, could be mimicked by some of the LPS-induced cytokines. Preincubation of monocytes with either interleukin-1 (IL-1), IL-10 or transforming growth factor β (TGFβ) lower the TNFα production upon further activation with LPS. None of the cytokines alone was as efficient as the LPS molecule, but high levels of tolerization were obtained with combination of IL-1, IL-10 and TGFβ. Neither IL-6, IL-8 nor TNFα led to hyporeactive cells. Our data indicate that endotoxin tolerance is not an LPS-specific phenomenon and that more than one cytokine can contribute to render human monocytes hyporeactive to further activation by LPS.

Effects of radiodetoxified endotoxin on nitric oxide production in J774 macrophages and in endotoxin shock

Radiodetoxified lipopolysaccharide (RD-LPS) is a 60Co-gamma-irradiated LPS with a modified structure, which decreases its toxic effects. To obtain a better understanding of the mechanism of the reduced toxicity of RD-LPS, here we studied the effect of RD-LPS on the regulation of nitric oxide (NO) production in vitro and in vivo. In control cells, stimulation by native LPS (10 μg/ml) induced the expression of the inducible NO synthase (iNOS) and production of NO, as measured by increase in the concentration of nitrite, breakdown product of NO. Pre-exposure of the cells for 24 h to a subthreshold concentration of RD-LPS (10 ng/ml) induced a complete desensitization to the LPS-induced NO production in comparison to control cells (P < 0.01). On the contrary, pre-exposure of the cells with native LPS (10 ng/ml) did not reduce LPS-induced NO synthesis. RD-LPS induced a smaller production of tumor necrosis factor (TNF) than native LPS, but did not induce a desensitization against subsequent LPS-induced TNF synthesis. ln in vivo studies, pretreatment of rats with repeated doses of sublethal RD-LPS (1 mg/kg/day i.p. for 4 days) inhibited increase of plasma nitrate/nitrite levels, NO production in peritoneal macrophages ex vivo and induction of lung iNOS activity, in response to a high-dose LPS challenge (15 mg/kg i.p.) given at the fifth day. Pretreatment with repeated sublethal doses of the native LPS (1 mg/kg/day i.p.) did not affect NO production in rats subjected to endotoxic shock. The results demonstrate that RD-LPS induces tolerance to the stimulatory effect of LPS on NO synthesis. Suppression of iNOS induction was only observed with RD-LPS, but not with native LPS in the models used herein. It remains to be further investigated whether suppression of iNOS induction by RD-LPS contributes to the protective effects of this compound in shock and inflammation.

Nitric oxide synthesis by hepatic cells is down regulated in endotoxin tolerant rats

The administration of a non-lethal dose of lipopolysaccharide (LPS) to experimental animals and human subjects results in a state of hyporesponsiveness to a second lethal challenge. Relatively little is known about the mechanisms of this endotoxin tolerance, especially about the induction of nitric oxide formation after LPS under these condition. Male Sprague-Dawley rats were divided into four groups: 1) rats that received a non-lethal i.v. injection of Escherichia coli LPS (0.5 mg/kg i.v., 'low dose'); 2) rats given a single injection of 'high dose' of LPS (3 mg/kg i.v.); 3) rats administered a low dose of LPS 12-168 h before they were challenged by a second injection of a high dose LPS (0.5 mg/kg followed by 3 mg/kg i.v., 'double injection'); and 4) rats treated with saline instead of LPS (1 ml/kg i.v., 'control'). 6 h after the high dose LPS, the livers were perfused with Krebs Henseleit buffer in a recirculating system at 37°C for 1 h, or hepatic cells were isolated. The isolated hepatocytes, Kupffer and hepatic endothelial cells were incubated in Hank's balanced salt solution (HBSS), containing 1 mM L-arginine, at 37°C for 3 h. The liver perfusate and supernatant from cell incubation were collected for determination of nitrite plus nitrate. Transcripts for inducible nitric oxide synthase (iNOS) were measured by cDNA equalized reverse transcription polymerase chain reaction in freshly isolated hepatic cells. Plasma glucose, lactate, alanine aminotransferase (ALT) and reactive nitrogen intermediates (RNIs) were also determined. High dose LPS alone caused a significant hypoglycemia (from 121.6 ± 3.0 to 84.5 ± 9.2 mg/dl), lactacidemia (from 8.3 ± 0.7 to 40.2 ± 5.7 mg/dl) and increase in plasma ALT (from 20.5 ± 2.8 to 477.8 ± 105.4 u/l). RNI levels in plasma also increased after 3 h and reached the maximum at 24 h after LPS (from 32.0 ± 1.3 to 795.3 ± 121.5 μM). RNI release from the perfused liver 6 h after high dose LPS was increased from 9.0 ± 2.0 to 156.6 ± 24.6 nmoles/g.h. Freshly isolated hepatic cells from control or low dose LPS treated rats released only small amounts of RNI. After high dose LPS administration, however, RNI release by hepatocytes, Kupffer and hepatic endothelial cells was increased 2.5, 14 and 4.5 fold, respectively. The 'high dose' LPS-induced increase of RNI production was associated with upregulation of iNOS mRNA in Kupffer and endothelial cells. After double injection of LPS (group 3), a protective effect was demonstrated by attenuated mortality, glucose changes, lactacidemia, and amino transferase activity, as compared to the high dose group. LPS tolerance with regard to RNI production by the liver was observed by 12 h, reached its maximum at about 72 h and was still evident even 120 h after the first injection of LPS. An attenuated RNI production in the supernatant from isolated hepatic cell cultures was also observed in the double injection group as compared to RNI release following the 'high dose' alone. This was associated with suppression of upregulation of iNOS mRNA induced by high dose LPS in Kupffer and hepatic endothelial cells. In contrast to the attenuated hepatic release of RNI during acute tolerance, RNI levels in plasma did not show hyporesponsiveness.

Effect of acute endotoxin tolerance on NO production by isolated hepatic parenchymal and nonparenchymal cells and alveolar macrophages in rats

The changes in NO production induced by endotoxin (LPS) tolerance are controversial. The aim of this study was to explore modulation of NO production by LPS tolerance in different liver cell types and alveolar macrophages. Such cells were studied in three groups of male Sprague Dawley rats: non-tolerant rats (sal-LPS) received saline or no treatment 48 h before a 3 mg/kg LPS injection, tolerant rats received low dose LPS (0.5 mg/kg) 48 h before a second injection of saline (LPS-sal) or LPS 3 mg/kg (LPS-LPS). All injections were delivered i.v. Animals were studied 1 and 6 h after the second injection. NO production (assessed by nitrite release) by hepatocytes, Kupffer cells, endothelial cells and alveolar macrophages was simultaneously determined after 20 h of culture in the presence or in the absence of LPS, interferon-y (IFN) or both. Basal NO production by hepatocytes of tolerant and nontolerant LPS injected rats was high 1 h after the second injection, and was dramatically reduced 6 h after the second injection. Hepatocytes of tolerized LPSinjected (LPS-LPS) rats were significantly less sensitive to in vitro stimulation by LPS and IFN at 1 h than hepatocytes of tolerized saline-injected rats and this difference disappeared by 6 h. In Kupffer cells of tolerant rats 6 h after the second LPS injection, basal NO generation was significantly less than in nontolerant rats. In both cell types of tolerant LPS-LPS rats, in vitro stimulated NO production was moderately upregulated at 1 h and then highly upregulated at 6 h, whereas in nontolerant (sal-LPS) animals, stimulated NO production was only slightly upregulated or not at all. Sensitivity to LPS and IFN stimulation of Kupper cells of LPS-LPS rats was not different from Kupffer cells of LPS-sal rats at 1 h, but became significantly higher at 6 h relative to both LPS-sal and sal-LPS animals. In endothelial cells of tolerant saline-injected (LPS-sal) rats, basal NO production was significantly less than in the sal-LPS group both at 1 and 6 h after the second injection. In endothelial cells of tolerant LPS-LPS animals, a significant upregulation of stimulated NO production higher than in the other groups was observed only at 1 h. No difference was evident in basal or stimulated NO production by alveolar macrophages of the different treatment groups, except for a significant increase in basal NO production in tolerant rats (LPS-LPS) at 6 h relative to 1 h after the second LPS injection.

Endotoxin tolerance: is there a clinical relevance?

Beeson (1946) first defined endotoxin tolerance as a reduced endotoxin-induced fever following repeated injections of typhoid vaccine. Freudenberg and Galanos (1988) demonstrated that endotoxin tolerance that can protect against a lethal challenge of lipopolysaccharide (LPS) involves the participation of macrophages. Evans and Zuckerman (1991) reported a role for glucocorticoids in endotoxin tolerance. Prostaglandins, interleukin-(IL-)10, and transforming growth factor-β are other players of in vivo endotoxin tolerance. Dramatic reduction of plasma tumor necrosis factor (TNF) (Mathison et al. 1990) and other cytokines in response to LPS parallels endotoxin tolerance. The reduced capacity to produce TNF and other cytokines can be mimicked in vitro by pretreatment of monocytes or macrophages with LPS. It is not a specific phenomenon and can be induced by other agents or events. Cross-tolerance between LPS, TLR2 specific ligands, IL-1 and TNF has been regularly reported. A similar loss of LPS-reactivity has been repeatedly reported in leukocytes of septic patients and in patients with non-infectious systemic inflammation response syndrome (SIRS; e.g . surgery, trauma, cardiac arrest and resuscitation, etc.). Studies on cellular signaling within leukocytes from septic and SIRS patients reveal numerous alterations of the activation pathways reminiscent of those observed in endotoxin-tolerant cells. While endotoxin tolerance prevents severity of infections and ischemia-reperfusion damage, it has been suggested that the immune dysregulation observed in SIRS patients was associated with an enhanced sensitivity to nosocomial infections. In conclusion, in vitro and in vivo endotoxin tolerance, either experimental or due to clinical status, are similar but not identical.

Review: Molecular mechanisms of endotoxin tolerance

The phenomenon of endotoxin tolerance has been widely investigated, but to date, the molecular mechanisms of endotoxin tolerance remain to be resolved clearly. The discovery of the Toll-like receptor (TLR) family as the major receptors for lipopolysaccharide (LPS) and other bacterial products has prompted a resurgence of interest in endotoxin tolerance mechanisms. Changes of cell surface molecules, signaling proteins, pro-inflammatory and anti -inflammatory cytokines and other mediators have been examined. During tolerance expression of LPS-binding protein (LBP), CD14, myeloid differentiation protein-2 (MD-2) and TLR2 are unchanged or up-regulated, whereas TLR4 is transiently suppressed or unchanged. Proximal post-receptor signaling proteins that are altered in tolerance include augmented degradation of interleukin-1 receptor-associated kinase (IRAK), and decreased TLR4-myeloid differentiation factor 88 (MyD88) and IRAK-MyD88 association. Tolerance has also been shown to be associated with decreased Gi protein content and activity, decreased protein kinase C (PKC) activity, reduction in mitogen-activated protein kinase (MAP kinase) activity, and reduced activator protein-1 (AP-1) and nuclear factor kappa B (NF-κB) induced gene transactivation. However, not all signaling proteins and pathways are suppressed in tolerance and induction of specific anti-inflammatory proteins and signaling pathways may serve important counter inflammatory functions. The latter include induction of IRAK-M and suppressor of cytokine-signaling-1 (SOCS-1), phosphoinositide-3-kinase (PI3K) signaling, and increased or maintained expression of inhibitor-κB (IκB) isoforms. Also at the nuclear level, increase in the NFκB subunit p50 homodimer expression and increased activation of peroxisome-proliferatoractivated receptors-γ (PPARγ) have been linked to tolerance phenotype. Although there are species and cellular variations in manifestation of the LPS tolerant phenotype, it is clear that the tolerance phenomena have evolved as a complex orchestrated counter regulatory response to inflammation.

Modulation of nitric oxide synthase activity in macrophages

L-Arginine is converted to the highly reactive and unstable nitric oxide (NO) and L-citrulline by an enzyme named nitric oxide synthase (NOS). NO decomposes into other nitrogen oxides such as nitrite (NO₂^-) and nitrate (NO₂^-), and in the presence of superoxide anion to the potent oxidizing agent peroxynitrite (ONOO⁻). Activated rodent macrophages are capable of expressing an inducible form of this enzyme (iNOS) in response to appropriate stimuli, i.e., lipopolysaccharide (LPS) and interferon-γ (IFNγ). Other cytokines can modulate the induction of NO biosynthesis in macrophages. NO is a major effector molecule of the anti-microbial and cytotoxic activity of rodent macrophages against certain micro-organisms and tumour cells, respectively. The NO synthesizing pathway has been demonstrated in human monocytes and other cells, but its role in host defence seems to be accessory. A delicate functional balance between microbial stimuli, host-derived cytokines and hormones in the microenvironment regulates iNOS expression. This review will focus mainly on the known and proposed mechanisms of the regulation of iNOS induction, and on agents that can modulate NO release once the active enzyme has been expressed in the macrophage.

Human monocytes tolerant to LPS retain the ability to phagocytose bacteria and generate reactive oxygen species

Tolerance to lipopolysaccharide (LPS) occurs when animals or cells exposed to LPS become hyporesponsive to a subsequent challenge with LPS. This mechanism is believed to be involved in the down-regulation of cellular responses observed in septic patients. The aim of this investigation was to evaluate LPS-induced monocyte tolerance of healthy volunteers using whole blood. The detection of intracellular IL-6, bacterial phagocytosis and reactive oxygen species (ROS) was determined by flow cytometry, using anti-IL-6-PE, heat-killed Staphylococcus aureus stained with propidium iodide and 2',7'-dichlorofluorescein diacetate, respectively. Monocytes were gated in whole blood by combining FSC and SSC parameters and CD14-positive staining. The exposure to increasing LPS concentrations resulted in lower intracellular concentration of IL-6 in monocytes after challenge. A similar effect was observed with challenge with MALP-2 (a Toll-like receptor (TLR)2/6 agonist) and killed Pseudomonas aeruginosa and S. aureus, but not with flagellin (a TLR5 agonist). LPS conditioning with 15 ng/mL resulted in a 40% reduction of IL-6 in monocytes. In contrast, phagocytosis of P. aeruginosa and S. aureus and induced ROS generation were preserved or increased in tolerant cells. The phenomenon of tolerance involves a complex regulation in which the production of IL-6 was diminished, whereas the bacterial phagocytosis and production of ROS was preserved. Decreased production of proinflammatory cytokines and preserved or increased production of ROS may be an adaptation to control the deleterious effects of inflammation while preserving antimicrobial activity.

Proinflammatory clearance of apoptotic neutrophils induces an IL-12(low)IL-10(high) regulatory phenotype in macrophages

Clearance of apoptotic exudate neutrophils (efferocytosis) induces either pro- or anti-inflammatory responses in mouse macrophages depending on host genetic background. In this study, we investigated whether neutrophil efferocytosis induces a stable macrophage phenotype that could be recalled by late restimulation with LPS. Bone marrow-derived macrophages previously stimulated by pro- but not anti-inflammatory neutrophil efferocytosis expressed a regulatory/M2b phenotype characterized by low IL-12 and high IL-10 production following restimulation, increased expression of LIGHT/TNF superfamily 14, Th2-biased T cell responses, and permissive replication of Leishmania major. Induction of regulatory/M2b macrophages required neutrophil elastase activity and was partially dependent on TLR4 signaling. These results suggested that macrophage differentiation to a regulatory phenotype plays a role in resolution of inflammation but could contribute to increased humoral Ab responses and parasite persistence in the infected host.

Calcineurin inactivation leads to decreased responsiveness to LPS in macrophages and dendritic cells and protects against LPS-induced toxicity in vivo

Microbial components such as lipopolysaccharide (LPS) bind to Toll-like receptors (TLRs) and activate innate and inflammatory responses. Responses to LPS and other microbial components are limited by the activation of negative feedback mechanisms that reduce responsiveness to subsequent LPS exposure, often termed LPS tolerance. Our laboratory has previously shown that calcineurin, a phosphatase known for its activation of T cells via NFAT, negatively regulates the TLR pathway in macrophages; consequently, calcineurin inhibitors (FK506 and cyclosporin A) mimic TLR ligands in activating the TLR pathway, NF-KB, and associated innate and inflammatory responses. This study investigated the physiological consequences of calcineurin inactivation for LPS-induced inflammatory responses in vitro and in vivo using two models: calcineurin inhibition by FK506 (tacrolimus) and myeloid cell-specific calcineurin deletion. Activation of dendritic cells and macrophages with FK506 in vitro was shown to induce a state of reduced responsiveness to LPS (i.e. a form of LPS tolerance). Similarly, macrophages from FK506-treated mice or from mice in which the calcineurin B1 (CnB1) subunit was conditionally knocked out in myeloid cells were found to have diminished LPS-induced inflammatory responses. In addition, mice with CnB1-deficient myeloid cells and mice undergoing FK506 treatment showed improved survival and recovery when challenged with high doses of systemic LPS compared to controls. These results demonstrate that inactivation of calcineurin in macrophages and other myeloid cells by inhibition or deletion can induce a form of LPS tolerance and protect the host from LPS toxicity in vivo.

Lipid A-mediated tolerance and cancer therapy

The term "tolerance" from an immunological perspective, broadly encompasses a number of phenomena, but generally refers to a diminished responsiveness to LPS and/or other microbial products. With the discovery that many of the immunological, physiological and/or pathophysiological effects of LPS can be attributed to the lipid A moiety of the LPS molecule, a number of different lipid A analogs were synthesized with the goal of developing a drug that could be used clinically to treat cancer. In many instances, the development of tolerance to the lipid A congeners confounded the utility of these analogs as cancer therapeutics. In certain circumstances, however, the development of tolerance in patients has been utilized therapeutically to protect immunosuppressed patients from sepsis. Although numerous studies have been designed to investigate the development of tolerance, the underlying molecular mechanism remains unclear. This may be due, in part, to differences in the experimental models used, the sources and types of microbes and microbial products studied, kinetics of responses, and/or other experimental conditions. Nonetheless, a number of different signaling pathways have been identified as potentially modulating and/or triggering the development of tolerance. Though complex and incompletely understood, the capacity of tolerance to impact lipid A-based therapeutics, either positively or negatively, is inarguable, thus underscoring the necessity for further investigation toward elucidating the mechanisms contributing to the development of tolerance to lipid A and its analogs.

Effective clearance of intracellular Leishmania major in vivo requires Pten in macrophages

Leishmaniases are a major international public health problem, and macrophages are crucial for host resistance to this parasite. To determine if phosphatase and tensin homologue deleted on chromosome ten (Pten), a negative regulator of the PI3K pathway, plays a role in macrophage-mediated resistance to Leishmania, we generated C57BL/6 mice lacking Pten specifically in macrophages (LysMCrePten(flox/flox) mice). Examination of lesions resulting from Leishmania major infection showed that LysMCrePten(flox/flox) mice were more susceptible to the parasite than wild-type (WT) mice in the early phase of the infection, but were eventually able to eliminate the pathogen. In vitro Pten-deficient macrophages showed a reduced ability to kill parasites in response to IFN-gamma treatment, possibly because the mutant cells exhibited decreased TNF secretion that correlated with reductions in inducible nitric oxide synthase expression and nitric oxide production. In response to various TLR ligands, Pten-deficient macrophages produced less TNF and IL-12 but more IL-10 than WT cells. However, analysis of cells in the lymph nodes draining L. major inoculation sites indicated that both LysMCrePten(flox/flox) and WT mice developed normal Th1 responses following L. major infection, in line with the ability of LysMCrePten(flox/flox) mice to eventually eliminate the parasite. Our results indicate that the efficient clearance of intracellular parasites requires Pten in macrophages.

Bench-to-bedside review: endotoxin tolerance as a model of leukocyte reprogramming in sepsis

Endotoxin tolerance is defined as a reduced responsiveness to a lipopolysaccharide (LPS) challenge following a first encounter with endotoxin. Endotoxin tolerance protects against a lethal challenge of LPS and prevents infection and ischemia-reperfusion damage. Endotoxin tolerance is paralleled by a dramatic reduction of tumor necrosis factor (TNF) production and some other cytokines in response to LPS. Endotoxin tolerance involves the participation of macrophages and mediators, such as glucocorticoids, prostaglandins, IL-10, and transforming growth factor-β. Endotoxin tolerance is accompanied by the up-regulation of inhibitory molecules that down-regulate the Toll-like receptor (TLR)4-dependent signaling pathway. Cross-tolerance between LPS and other TLR specific ligands, as well as IL-1 and TNF, has been regularly reported. A similar loss of LPS reactivity has been repeatedly reported in circulating leukocytes of septic patients and in patients with non-infectious systemic inflammation response syndrome (SIRS). Studies on cellular signaling within leukocytes from septic and SIRS patients reveal numerous alterations reminiscent of those observed in endotoxin tolerant cells. However, altered responsiveness to LPS of leukocytes from sepsis and SIRS patients is not synonymous with a global down-regulation of cellular reactivity. The term 'cellular reprogramming', which has been proposed to qualify the process of endotoxin tolerance, defines well the immune status of circulating leukocytes in septic and SIRS patients.

Deficient control of Trypanosoma cruzi infection in C57BL/6 mice is related to a delayed specific IgG response and increased macrophage production of pro-inflammatory cytokines

Earlier work in Trypanosoma cruzi-infected C57BL/6 and BALB/c mice revealed an acute disease, of lethal outcome in the former group and lesser severity in BALB/c mice. Fatal course was not accompanied by an increased parasite load, but by a substantial imbalance between pro- and anti-inflammatory cytokine serum levels. To better characterise the mechanisms allowing the host to restrain the infection, we have now studied the specific IgG production and in vitro behaviour of peritoneal macrophages (PMs) when exposed to T. cruzi. BALC/c mice displayed higher serum levels of specific immunoglobulins in the first weeks of acute infection. In vitro infected PMs showed no between-group differences in the number of intracellular parasites, although TNFalpha levels were significantly higher in culture supernatants from C57BL/6 mice. Because an LPS-based pretreatment (desensitisation protocol followed by a sublethal LPS dose) reduced disease severity of C57BL/6 mice, we next explored the features of the in vitro infection in PMs from mice subjected to such protocol. PMs from LPS-pretreated mice had a decreased production of TNFalpha and IL-1beta, becoming more permissive to parasite replication. It is concluded that deficient control of T. cruzi infection in C57BL/6 mice may also involve a less satisfactory specific IgG response and increased TNFalpha production by PMs. Improved disease outcome in LPS-pretreated mice may be associated with the reduced inflammatory cytokine production by PMs, but the impaired ability of these cells to control parasite growth suggests that compensatory mechanisms are operating in the in vivo situation.

Noradrenaline up-regulates the neuronal and the inducible nitric oxide synthase isoforms in magnocellular neurons of rat brain slices

Nitric oxide (NO) and noradrenaline (NA) are suggested to be implicated in the regulation of neuropeptide secretion in the supraoptic nuclei (SON) and the paraventricular nuclei (PVN) of the hypothalamus. Our study demonstrates short-term interactions between NA and the activity and expression of NO synthase (NOS) in magnocellular neurons, by using an ex vivo model of hypothalamic slices. In the SON as well as in the PVN, total NOS activity exhibited a time-dependant increase after an incubation with NA. In the SON, this increase of total NOS activity was in part the consequence of stimulation of the iNOS activity. Coimmunodetections showed that cells expressing the inducible form of NOS were not astrocytes but magnocellular neurons. Steady-state levels of iNOS and nNOS mRNA were dramatically enhanced by NA, particularly in the SON. Consequently, we provide new evidence that iNOS could play an important role in multiple physiological functions, including extracellular fluid balance, lactation, and parturition.

The effect of Brucella abortus on hydrogen peroxide and nitric oxide production by bovine polymorphonuclear cells

The effect of Brucella on the generation of microbicidal reactive oxygen and nitrogen metabolites by bovine peripheral polymorphonuclear cells (PMNs) was investigated. The PMNs were recovered from the peripheral blood of control calves and experimental calves previously vaccinated against brucellosis. Significantly larger quantities of NO and H2O2 were generated by PMNs from control and experimental calves following activation by heat-killed whole cells or outer membrane protein of Brucella abortus than by non-activated cells (p<0.05-0.01). In contrast, generation of H2O2 and NO decreased when PMNs were exposed to the lipopolysaccharide of Brucella. However, the generation of H2O2 and NO by activated PMNs from the control and experimental calves did not differ significantly.

In vitro effects of cAMP-elevating agents and glucocorticoid either alone or in combination on the production of nitric oxide, interleukin-12 and interleukin-10 in IFN-gamma- and LPS-activated mouse peritoneal macrophages

The effects of cAMP-elevating agents, N6-2'-O-dibutyryl cAMP (Bu2cAMP), and glucocorticoid (dexamethasone) on the production of inflammatory mediators--nitric oxide and interleukin-12 (IL-12) and anti-inflammatory mediator interleukin-10 (IL-10) were demonstrated in murine peritoneal macrophages. Inducible nitric oxide synthase (iNOS) and iNOS mRNA were detected by northern blot and western blot, respectively. The cAMP elevating agents Bu2cAMP and prostaglandin E2 each alone did not show any effect on NO production but along with IFN-gamma and lipolysaccharide (LPS) they slightly enhanced NO production. Dexamethasone inhibited NO production in IFN-gamma- and LPS-treated cells; cAMP elevating agents interfered with the NO production inhibited by dexamethasone. Inhibition was revealed at the mRNA level as well as at protein level. Bu2cAMP or dexamethasone either alone or synergistically inhibited IL-12 production; Bu2cAMP interfered with dexamethasone-mediated inhibition of IL-10 production in IFN-gamma- and LPS-treated macrophages. The use of glucocorticoids along with cAMP elevating agents was beneficial in lowering the level of inflammatory mediator IL-12 and producing high levels of the anti-inflammatory mediator IL-10 active in cell protection. On the other hand, interference of Bu2cAMP with dexamethasone-mediated NO inhibition may have adverse effect. Therefore, adverse effects due to cAMP-mediated interference (inhibition) with NO synthesis may occur in many inflammatory diseases during combined drug therapy by glucocorticoids and cAMP elevating agents.

Antimicrobial mechanisms of fish phagocytes and their role in host defense

Phagocytosis is a primitive defense mechanism in all multicellular animals. Phagocytes such as macrophages and neutrophils play an important role in limiting the dissemination of infectious agents, and are responsible for the eventual destruction of phagocytosed pathogens. These cells have evolved elaborate killing mechanisms for destroying pathogens. In addition to their repertoire of degradative enzymes and antimicrobial peptides, macrophages and neutrophils can be activated to produce a number of highly toxic molecules. Production of reactive oxygen and nitrogen intermediates by these cells are potent cytotoxic mechanisms against bacteria and protozoan pathogens. Studies in fish suggest that the biological basis of these inducible killing mechanisms is similar to those described in mammals. More recent work suggest novel roles for regulating these killing responses in fish. In this review, we describe the biological basis of these killing mechanisms and how they are regulated in fish.

Systemic macrophage activation in locally-induced experimental arthritis

Local and systemic macrophage activation was examined during the course of monoarticular murine antigen-induced arthritis (AIA), induced by systemic immunization and subsequent local induction. The levels of tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), IL-6, IL-12p70, and nitric oxide (NO) were determined in joints, sera, and supernatants of peritoneal macrophages (the latter unstimulated or stimulated ex vivo with LPS/IFN-gamma). In comparison with normal mice, systemic immunization (day 0) was associated to significant rise of TNF-alpha in serum, IL-1beta in the joints, IL-6 in unstimulated macrophages and IL-12p70 in stimulated macrophages. Local induction led to a further significant increase of: (i) TNF-alpha, IL-1beta, and IL-6 in the joints; and (ii) IL-1beta, and IL-6 in sera and stimulated macrophages during acute and/or early chronic AIA (days 1 to 7). Unstimulated macrophages showed increased NO release (day 3), while stimulated macrophages significantly increased secretion of IL-12p70 (day 1). In late chronic AIA (day 21), cytokine/NO expression returned to immunization levels or below at all sites; solely IL-1beta in the joints remained significantly above normal levels. Therefore, the prevalently local AIA model is characterized by a mixture of local and systemic activation of the mononuclear phagocyte system (MPS). While systemic MPS activation preceding arthritis induction can be attributed to systemic immunization, further systemic activation during arthritis appears an integral pathogenetic component of AIA.

IL-12 suppression during experimental endotoxin tolerance: dendritic cell loss and macrophage hyporesponsiveness

Endotoxin tolerance, the transient, secondary down-regulation of a subset of endotoxin-driven responses after exposure to bacterial products, is thought to be an adaptive response providing protection from pathological hyperactivation of the innate immune system during bacterial infection. However, although protecting from the development of sepsis, endotoxin tolerance also can lead to fatal blunting of immunological responses to subsequent infections in survivors of septic shock. Despite considerable experimental effort aimed at characterizing the molecular mechanisms responsible for a variety of endotoxin tolerance-related phenomena, no consensus has been achieved yet. IL-12 is a macrophage- and dendritic cell (DC)-derived cytokine that plays a key role in pathological responses to endotoxin as well as in the induction of protective responses to pathogens. It recently has been shown that IL-12 production is suppressed in endotoxin tolerance, providing a likely partial mechanism for the increased risk of secondary infections in sepsis survivors. We examined the development of IL-12 suppression during endotoxin tolerance in mice. Decreased IL-12 production in vivo is clearly multifactorial, involving both loss of CD11c(high) DCs as well as alterations in the responsiveness of macrophages and remaining splenic DCs. We find no demonstrable mechanistic role for B or T lymphocytes, the soluble mediators IL-10, TNF-alpha, IFN-alphabeta, or nitric oxide, or the NF-kappaB family members p50, p52, or RelB.

Mice lacking tartrate-resistant acid phosphatase (Acp 5) have disordered macrophage inflammatory responses and reduced clearance of the pathogen, Staphylococcus aureus

Tartrate-resistant acid phosphatase (TRAP) is a lysosomal di-iron protein of mononuclear phagocytes and osteoclasts. Hitherto, no role for the enzyme in immunity has been identified; however, knockout mice lacking TRAP have a skeletal phenotype caused by an intrinsic osteoclast defect. To investigate a putative function for TRAP in macrophages (Mphi), we investigated proinflammatory responses and systemic microbial clearance in knockout mice compared with age- and gender-matched congenic wild-type mice. Phorbol 12-myristate 13-acetate (PMA)-stimulated and interferon-gamma (IFN-gamma)-induced superoxide formation was enhanced in peritoneal Mphi lacking TRAP; nitrite production in response to stimulation with lipopolysaccharide (LPS) and IFN-gamma was also increased. In addition, secretion of the proinflammatory cytokines, tumour necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta and IL-12, was significantly greater in TRAP-deficient Mphi when stimulated with LPS, with or without addition of either TNF-alpha or IFN-gamma. The activity of tartrate-sensitive (lysosomal) acid phosphatase was increased in Mphi from the knockout mice but activities of the lysosomal hydrolases N-acetyl beta-glucosaminidase and acid beta-glucuronidase were unchanged, indicating selective activation of compensatory acid phosphatase activity. Evidence of impaired Mphi function in vivo was obtained in TRAP knockout mice, which showed delayed clearance of the microbial pathogen, Staphylococcus aureus, after sublethal intraperitoneal inoculation. After microbial challenge, peritoneal exudates obtained from TRAP knockout mice had a reduced population of Mphi. As peritoneal Mphi and neutrophils lacking TRAP were able to phagocytose and kill S. aureus normally in vitro, TRAP may directly or indirectly influence recruitment of Mphi to sites of microbial invasion. Our study shows that TRAP participates in the inflammatory response of the Mphi and influences effector signalling pathways in innate immunity.

Requirements for nitric oxide production by turbot (Scophthalmus maximus) head kidney macrophages

The effect of different cytokines and bacterial lipopolysaccharide (LPS) on turbot (Scophthalmus maximus) macrophage nitric oxide (NO) production has been studied. We have found two different responses concerning NO production in response to LPS. We have studied 43 turbot and only macrophage cultures derived from 30.2% of these turbot were significantly stimulated by LPS. The macrophage populations that did not respond to LPS, showed a constitutive production that was significantly reversed by NO inhibitors like N(G)-methyl-L-arginine (L-NMMA) and N-omega-nitro-L-arginine (L-NAME), and was dependent on intracellular calcium concentration. We studied the effect of other stimuli combined with LPS on the NO production of these otherwise non-responsive macrophages. LPS combined with turbot macrophage activating factor (MAF) containing supernatants, was capable of significantly stimulating some of these macrophage populations. The same response was observed when LPS was combined with turbot IFN-alphabeta-like substances. When LPS was combined with human recombinant tumor necrosis factor alpha (hrTNF-alpha), the NO production was significantly induced in all macrophage populations studied.

Indirect induction of suppressor of cytokine signalling-1 in macrophages stimulated with bacterial lipopolysaccharide: partial role of autocrine/paracrine interferon-alpha/beta

It has previously been reported by us that a brief prior exposure of mouse bone marrow culture-derived macrophages to bacterial lipopolysaccharide (LPS) resulted in a dramatic reduction in their ability to produce NO in response to a subsequent stimulus with either interferon-gamma (IFN-gamma) or IFN-gamma plus LPS. We show here that this brief exposure to LPS results in an impaired response to subsequently added IFN-gamma. A 2--4 h pretreatment with LPS leads to a dramatic reduction in the IFN-gamma-induced DNA-binding of the transcription factor, signal transducer and activator of transcription 1 alpha (STAT1 alpha). This loss in ability to activate STAT1 alpha temporally correlates with the LPS-induced accumulation of mRNA encoding the suppressor of cytokine signalling-1 (SOCS-1). However, LPS does not directly induce the synthesis of SOCS-1. Rather, LPS induces the synthesis of autocrine/paracrine factors that are the true mediators of SOCS-1 induction. IFN-alpha/beta is one of these mediators, but plays only a partial role in the induction of SOCS-1 because neutralization of LPS-induced IFN-alpha/beta production incompletely inhibits the induction of SOCS-1. We show that mouse IFN-beta directly induces the synthesis of SOCS-1, without the need for prior protein synthesis, and does so with faster kinetics than does LPS. Our results are consistent with the non-specific nature of LPS-induced tolerance and provide a mechanistic insight into nonspecificity; LPS indirectly induces the synthesis of a protein mediator, SOCS-1, which inhibits the signalling that is induced by IFN-gamma.

Inhibition of glutathione synthesis as a chemotherapeutic strategy for leishmaniasis

This study focuses on the use of buthionine sulphoximine (BSO), a gamma-glutamylcysteine synthetase inhibitor, on Leishmania donovani growth. The effect of BSO on amastigote multiplication within macrophages showed that 5 mM BSO decreased infectivity by about 50% and the mean number of amastigotes per 100 infected macrophages by 21%. The mechanism may be that BSO resulted in enhanced nitric oxide (NO) levels within macrophages, probably due to inhibition of GSH content since GSH (10 mm) given after BSO treatment led to a decrease in NO compared to macrophages treated with BSO alone which were preexposed to the Leishmania surface molecule lipophosphoglycan.

Administration of endotoxin associated with lipopolysaccharide tolerance protects mice against fungal infection

Lipopolysaccharide (LPS) pretreatment of mice resulted in a significantly enhanced survival after disseminated Cryptococcus neoformans infection. The survival was associated with reduced fungal burden in tissues. LPS-pretreated mice had lower levels of cytokines in blood, spleen, and lungs and higher levels in brain. Pentoxifylline abolished the beneficial effect of LPS pretreatment.

Interferon-gamma- and lipopolysaccharide-induced tumor necrosis factor-alpha is required for nitric oxide production: tumor necrosis factor-alpha and nitric oxide are independently involved in the killing of Mycobacterium microti in interferon-gamma- and lipopolysaccharide-treated J774A.1 cells

A comparative study was done using J774A.1 and J774A.1-derived transfected cells (J774A.1 C.1) containing antisense tumor necrosis factor alpha (TNF-alpha) plasmid to determine the role of endogenous TNF-alpha on nitric oxide production as well as on the growth of Mycobacterium microti in interferon gamma (IFN-gamma)- and lipopolysaccharide (LPS)-treated cells. On stimulation with IFN-gamma and LPS a higher level of NO was observed in J774A.1 cells compared to J774A.1 C.1 which indicated that endogenous TNF-alpha is required for the production of NO. Comparing the effect of IFN-gamma and LPS on the intracellular growth of M. microti, the growth-reducing activity was higher in J774A.1 cells than in J774A.1 C.1 cells and was not completely abrogated in the presence of the nitric oxide inhibitor NG-methyl-L-arginine (L-NMA). J774A.1 C.1 cells infected with M. microti produced a significant amount of NO when exogenous TNF-alpha was added along with IFN-gamma and LPS and the concentration of intracellular bacteria decreased almost to that in IFN-gamma and LPS treated parental J774A.1 cells. Addition of exogenous TNF-alpha even in the presence of L-NMA in J774A.1 C.1 cells could also partially restore intracellular growth inhibition of M. microti caused by IFN-gamma and LPS. TNF-alpha is probably required for the production of NO in J774A.1 cells by IFN-gamma and LPS but TNF-alpha and NO are independently involved in the killing of intracellular M. microti with IFN-gamma and LPS.

Avian macrophage metabolism

This review considers the role of avian macrophages as a source of immune effector and immunoregulatory metabolites. Although considerable attention has been given to the importance of leukocytic cytokines, particularly the monokines such as interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta), metabolites produced by macrophages appear to be of equal importance in determining the progression of immune responses. The three metabolite categories that have received the greatest attention are the reactive oxygen species (ROS), the reactive nitrogen intermediates (RNI), and the eicosanoids. Additionally, the xenobiotic metabolites produced via cytochrome P450 activity mediate some immune-environmental interactions. Each of these four metabolite categories is subject to different requirements for metabolite production, and each has distinct effector functions. An understanding of macrophage metabolite regulation could allow improvements in avian health management and production via the effective control of metabolite production. The present review considers prior and recent information on the production of the metabolites by avian macrophages. Additionally, the potential ramifications of metabolite production and regulation are discussed.

Involvement of the membrane form of tumour necrosis factor-alpha in lipopolysaccharide-induced priming of mouse peritoneal macrophages for enhanced nitric oxide response to lipopolysaccharide

We studied the pathways of macrophage response to lipopolysaccharide (LPS). When mouse macrophages pre-exposed to LPS were restimulated with this agent, reduced tumour necrosis factor-alpha (TNF-alpha) responses (desensitization/endotoxin tolerance) were accompanied by increased (priming) nitric oxide (NO) responses. Priming was also inducible with recombinant interferon-beta (IFN-beta). The requirement of TNF-alpha biosynthesis in the LPS-induced priming was also suggested by the observation that both anti-TNF-alpha serum and pentoxifylline inhibited this effect. However, addition of mouse recombinant TNF-alpha (mrTNF-alpha) did not enhance the priming induced by LPS or IFN-beta, and preincubation with mrTNF-alpha alone, or in association with other cytokines produced by macrophages (interleukin-1 beta, interleukin-6, or leukaemia inhibitory factor), did not induce a priming effect. We found however, that pentoxifylline, which blocked the priming, also decreased the level of membrane-bound TNF-alpha. Furthermore, exposure to compound BB-3103 (a metalloproteinase inhibitor that blocks the processing of membrane-bound TNF-alpha yielding to the secreted cytokine) enhanced the priming effect, the expression of membrane TNF-alpha and the specific binding of LPS. These observations suggest that the membrane form of TNF-alpha is involved in the interaction of LPS with a receptor required for LPS-induced priming.

Glomerular and vascular tissues do not down-regulate nitric oxide synthesis during protracted endotoxemia

Down-regulation of cytokines is implicated as an important component of the phenomenon of tolerance to bacterial products in humans and animals. Since many effects of endotoxin and cytokines are mediated by nitric oxide, this study was designed to evaluate in vivo the L-arginine:NO pathway in endotoxin tolerance. Sublethal injections of E. coli lipopolysaccharide (LPS, 1 mg/kg body wt, i.p.) were given to rats daily for five days. Blood levels of NO2-/NO3-, stable metabolites of nitric oxide (NO), significantly increased on day 1 (baseline, 89.64 +/- 40, day 1, 260.32 +/- 36 nmol/ml; P < 0.05) but returned to baseline levels on day 5 (77.60 +/- 5 nmol/ml). However, urinary NO2-/NO3- remained significantly elevated several-fold throughout the study period (baseline, 121.25 +/- 11.4, day 1, 899.35 +/- 15.8, day 5, 250.23 +/- 21.4 nmol/hr/100 g body wt). Glomeruli and aortae obtained from these rats showed increased NO production that was maintained at similar levels even at day 5 (glomeruli: control, 0.01 +/- 0.0, day 1, 22.4 +/- 0.3, and day 5, 22.0 +/- 2.5, P < 0.05 vs. control; aortae; control, 0.01 +/- 0.0, day 1, 4.3 +/- 2.2, and day 5, 5.4 +/- 1.0 nmol/hr/mg protein, P < 0.05 vs. control, respectively); this further increased significantly in response to in vitro LPS challenge. However, peritoneal macrophages, liver and spleen showed an initial increase in NO production that decreased significantly by the fifth day of LPS and could not be further stimulated by in vitro LPS challenge. Thus, in vivo NO synthesis is down-regulated during protracted LPS. Our results show that the process is relatively specific to the liver, spleen and macrophages, and is qualitatively and quantitatively different in vascular tissues such as aortae and glomeruli.

Excretory/secretory products of plerocercoids of Spirometra erinaceieuropaei induce the expression of inducible nitric oxide synthase mRNA in murine hepatocytes

In this study, we observed the level of normal murine hepatocyte inducible NOS (iNOS) mRNA by semi-quantitative polymerase chain reaction (SQ-PCR) analysis after stimulation with ES products (ESP) and/or ESP fractions from the plerocercoids. We found that ESP are able to induce the expression of iNOS gene in a dose-dependent fashion. Treatment of ESP with polymyxin B did not affect their ability to induce the expression of iNOS gene, suggesting that bacterial lipopolysaccharide (LPS) is not involved. The iNOS-inducing factor (a) is soluble, and may be a component whose molecular mass exceeds 94 kDa as analyzed by a combination of SDS-PAGE and SQ-PCR. The peak of iNOS mRNA level was detected 3 h after stimulation with ESP; the mRNA level decreased sharply from 9 h. Dexamethasone inhibited the induction of mRNA for hepatocyte iNOS. In contrast, cycloheximide stimulated the induction; this suggests that de nova protein synthesis is important in the regulation of the ESP-induced expression of iNOS mRNA. Actinomycin D blocked the induction. In addition, the results of Northern blot analysis showed that ESP suppressed the LPS (10 micrograms/ml) and interferon-gamma (IFN-gamma, 100 U/ml)-induced hepatocyte iNOS mRNA expression in a dose-dependent fashion and the suppressing effect was more marked when hepatocytes were exposed to ESP 3 h prior to LPS and IFN-gamma. These results demonstrate that the soluble factor(s) of ESP is capable of inducing murine iNOS gene expression in hepatocytes in the absence of added cytokines.

Regulation of the expression of nitric oxide synthase and leishmanicidal activity by glycoconjugates of Leishmania lipophosphoglycan in murine macrophages

Lipophosphoglycan (LPG) glycoconjugates from promastigotes of Leishmania were not able to induce the expression of the cytokine-inducible nitric oxide synthase (iNOS) by the murine macrophage cell line, J774. However, they synergize with interferon gamma to stimulate the macrophages to express high levels of iNOS. This synergistic effect was critically time-dependent. Preincubation of J774 cells with the LPG glycans 4-18 h before stimulation with interferon gamma resulted in a significant reduction in the expression of iNOS mRNA and of NO synthesis, compared with cells preincubated with culture medium alone. The regulatory effect on the induction of iNOS by LPG is located in the LPG phosphoglycan disaccharide backbone. Synthetic fragments of this backbone had a similar regulatory effect on NO synthesis. Further, the production of NO by activated macrophages in the present system was correlated directly with the leishmanicidal capacity of the cells. These data therefore demonstrate that LPG glycoconjugates have a profound effect on the survival of Leishmania parasites through their ability to regulate the expression of iNOS by macrophages.

Preexposure of mouse peritoneal macrophages to lipopolysaccharide and other stimuli enhances the nitric oxide response to secondary stimuli

The aim of this study was to compare the regulation of the production of tumor necrosis factor-alpha (TNF-alpha) and secondary nitric oxide (NO) in macrophages submitted to a sequence of two stimulations. Pre-exposure for 18 h of mouse thioglycollate-elicited peritoneal macrophages to low doses (1-10 ng/ml) of lipopolysaccharide (LPS), in the presence or absence of serum, induces on one hand a desensitization (endotoxin tolerance) for secondary TNF-alpha responses to LPS and, on the other hand, a 4 fold increase (priming) of secondary NO responses. Preexposure to components from Gram-positive bacteria (lipoteichoic acid, peptidoglycan) and to a synthetic lipid structurally related to lipid A (compound M4), induced similar effects. In contrast to the desensitization for TNF-alpha secretion, the priming for NO production was not mimicked by sodium nitroprusside, a generator of NO. The results suggest that concomitant but distinct activation pathways induced by LPS and other agents can be dissociated by serum-independent modulation processes elicited by pre-exposure of the cells to LPS itself, or to other stimuli.

Interleukin-10 inhibits IgE-mediated nitric oxide synthase induction and cytokine synthesis in normal human keratinocytes

Human keratinocytes (HK) generate nitric oxide (NO) and proinflammatory mediators following activation with either IgE/anti-IgE immune complexes or a combination of lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). Recently, interleukin-10 (IL-10) has been shown to down-regulate various inflammatory responses and to be secreted by lymphocytes and dendritic cells during skin inflammatory reactions. We show here that IL-10 down-regulates the production of tumor necrosis factor (TNF)-alpha and IL-6 by activated HK. Also, induction of inducible nitric oxide synthase (iNOS) expression in HK by IgE/anti-IgE or LPS/IFN-gamma is significantly reduced by the addition of IL-10. This effect is dose dependent and correlates with reduction of iNOS mRNA production and enzyme level. Therefore, IL-10 down-regulates NO-mediated HK inflammatory responses and may thus participate in the regulation of the skin immune network.

The killing of Leishmania major by human macrophages is mediated by nitric oxide induced after ligation of the Fc epsilon RII/CD23 surface antigen

Serum IgE concentrations and the expression of the low-affinity receptor for IgE (Fc epsilon RII/CD23) are increased in cutaneous leishmaniasis or after immune challenge with Leishmania antigens. In vitro, the ligation of CD23 by IgE-anti-IgE immune complexes (IgE-IC) or by anti-CD23 monoclonal antibody (mAb) induces nitric oxide (NO) synthase and the generation of various cytokines by human monocytes/macrophages. The present study shows that IgE-IC, via CD23 binding, induce intracellular killing of Leishmania major in human monocyte-derived macrophages through the induction of the L-arginine:NO pathway. This was demonstrated by increased generation of nitrite (NO2-), the stable oxidation product of NO, and by the ability of NG-monomethyl-L-arginine to block both NO generation and parasite killing. A similar NO-dependent effect was observed with interferon gamma-treated cells. Tumor necrosis factor alpha is involved in this process, since both the induction of NO synthase and the killing of parasites caused by anti-CD23 mAb were inhibited by an anti-tumor necrosis factor alpha mAb. Treatment of noninfected CD23+ macrophages with IgE-IC provided protection against subsequent in vitro infection of these cells by Leishmania major promastigotes. Thus, IgE-IC promote killing of L. major by inducing NO synthase in human macrophages.

The nonspecific nature of endotoxin tolerance

Re-exposure of organisms or cells to endotoxin after a previous challenge is not accompanied by the profound metabolic changes that are induced by the first encounter with endotoxin. Endotoxin tolerance is not specific to the action of lipopolysaccharide, and crossreactivity with other exogenous stimuli occurs. Various cytokines can mimic the effects of endotoxin in vivo and/or in vitro.

Upregulation of respiratory burst of polymorphonuclear leukocytes by a bleomycin derivative, peplomycin

The influence of peplomycin (PLM) on the respiratory burst of peripheral blood polymorphonuclear leukocytes (PMN) was investigated. Short-term (5 min) treatment of human PMN with 0.1mu g/ml to 100mu g/ml of PLM increased phorbol myristate acetate (PMA)- and formyl-methionyl-leucyl-phenylalanine (FMLP)-induced luminol-dependent chemiluminescence. PMN, as well as alveolar macrophages from rabbits treated with 0.5 to 1.0 mg/kg of peplomycin per day for 5 days, generated more superoxide (O2-) than the cells from untreated rabbits. In both PLM-treated and untreated PMN, chemiluminescence induced by FMLP and PMA was decreased to less than 50% of the control by staurosporine, superoxide dismutase (SOD) and catalase. However, the peak intensity in PLM-untreated PMN was decreased to about 30% of the control by genistein, while this agent induced a slight decrease in peak intensity in the PLM-treated PMN. Inositol triphosphate and diacyl glycerol levels were not clearly increased by PLM, but an increase of intracellular Ca++ and a shift of protein kinase C (PKC) to the membrane occurred in PMN within 1 min after PLM treatment. Western blotting revealed that the tyrosine phosphorylation of a 115 kDa protein was upregulated by 5 to 50mu g/ml of PLM. While, PLM suppressed SOD activity in alveolar macrophages and PMN. These results seem to indicate that PLM increases the respiratory burst of PMN and macrophages both by way of direct PKC activation and by the upregulation of protein tyrosine phosphorylation. This increased reactive oxygen generation, together with the suppression of SOD activity seems to be tissue-impairing.

Suppressor macrophages in Trypanosoma brucei infection: nitric oxide is related to both suppressive activity and lifespan in vivo

African trypanosome infections cause immunosuppression in both experimental rodent and natural hosts. One characteristic of this is an eliciting of suppressor macrophages which results in an unresponsiveness in lymphocytes. Macrophages from Trypanosoma brucei-infected mice have previously been shown to produce high levels of nitric oxide (NO). Using model systems based on in vivo macrophage transfer and drug cure, we have sought to determine the relationship between NO and suppressed lymphocyte responses. Peritoneal macrophages from T. brucei-infected mice inhibited the Concanavalin A (Con-A) response of spleen cells from syngeneic recipients 3-4 days after transfer in vivo due to the activity of suppressor macrophages. When macrophage NO synthesis was inhibited either in vitro or in vivo the suppressive effects were partially abrogated. These data provide evidence of a role for NO in mediating immunosuppression during murine T. brucei infection. Suppression in spleens of mice receiving suppressor macrophages was transient, with total recovery of spleen cell mitogen responses six days after transfer. Suppression and recovery was found to coincide with the presence or absence (respectively) of donor macrophages in recipient spleens. When T. brucei-infected mice were treated with a curative dose of a trypanocide there followed a recovery of lymphocyte responsiveness after a period of 4-5 days, and this directly correlated with a reduction of macrophage NO synthesis to control levels both in vivo and in vitro. The apparent loss of suppressor macrophage activity after 4-6 days in both drug cured animals and recipients of macrophage transfer was shown to be due to NO-mediated apoptosis of these cells.

Glycoinositolphospholipids of Leishmania major inhibit nitric oxide synthesis and reduce leishmanicidal activity in murine macrophages

Murine macrophages express high levels of inducible nitric oxide (NO) synthase and produce large amounts of nitric oxide when activated with interferon-gamma and lipopolysaccharide in vitro. Nitric oxide is a mediator of a variety of biological functions including microbicidal activity against the protozoan parasite Leishmania species. Glycoinositolphospholipids (GIPL) are the predominant surface glycolipids in both developmental stages of Leishmania major. We report here that GIPL can inhibit the synthesis of NO in a time- and dose-dependent manner. In contrast, lipophosphoglycan, which is present in the promastigote stage did not inhibit NO synthesis. GIPL-treated macrophages also showed markedly reduced leishmanicidal activity. The majority of the inhibitory activity of GIPL was found within the alkylacylglycerol moiety of the GIPL molecule. These data, therefore, suggest that GIPL may contribute towards the survival of the parasite in the immune hosts.

Interactions between cytokines and nitric oxide

There is now an impressive range of evidence supporting the important role of cytokines in sleep regulation (see Krueger et al., 1995; De Simoni et al., 1995). It has also been reported that inhibition of nitric oxide (NO) synthesis suppresses sleep in rabbits (Kapás et al., 1994). This is not surprising, since NO is closely involved in neurotransmission (Garthwaite, 1991; Schuman and Madison, 1994) and cytokines are the major inducers of NO synthesis (Hibbs et al., 1990). Further, it is now clear that NO plays an important role in modulating immune responses, possibly through the differential regulation of cytokine synthesis (Taylor-Robinson et al., 1994). In this article, I will provide evidence for the interactions between cytokines and nitric oxide, and discuss their implications in the regulation of immune responses. I shall illustrate these mainly with results from my coworkers and I, from our laboratory rather than attempting an exhaustive review of the subject.

Regulation of nitric oxide synthesis in infectious and autoimmune diseases

Nitric oxide (NO) is an essential mediator for a variety of biological functions, including defence against a range of pathogens. However, excessive production of NO as a result of immunological stimulation, will lead to important immunopathologies. Therefore the production of NO is necessarily under tight regulation. The regulatory mechanisms so far known to control NO synthesis include cytokines (induction of NO synthase by IFN-gamma, TNF alpha, MIF and LPS, and down regulation by IL-4, IL-10 and TGF beta), feedback inhibition by NO itself, inhibition by pretreatment with LPS and glycoinositol-phospholipids and up regulation by lipophosphoglycan from the protozoa parasite, Leishmania major.

Attenuation of the induction of nitric oxide synthase by endogenous glucocorticoids accounts for endotoxin tolerance in vivo

An enhanced formation of nitric oxide (NO) due to induction of a calcium-independent (inducible) NO synthase (iNOS) contributes importantly to the cardiovascular failure caused by bacterial endotoxin. Repeated challenges with small doses of endotoxin result in tolerance to both peripheral vascular failure and death caused by subsequent injection of a higher dose of endotoxin. Here we investigate whether tolerance to endotoxin is associated with a lack of induction of iNOS in vivo and whether endogenous glucocorticoids play a role in the development of endotoxin tolerance. In anesthetized rats, i.v. administration of Escherichia coli endotoxin [lipopolysaccharide (LPS); 2 mg.kg-1] resulted in a prolonged decrease in mean arterial blood pressure (MAP) and hyporeactivity to the contractile responses elicited by norepinephrine (NE; 10 nM) in aortic rings ex vivo. Hyporeactivity to NE was partially reversed by NG-nitro-L-arginine methyl ester (0.3 mM) in vitro, suggesting that an enhanced formation of NO contributes to this hyporeactivity. There was a substantial increase in the activity of iNOS in the lung 3 h after i.v. injection of LPS (0.2 +/- 0.1 to 6.6 +/- 0.6 pmol.mg-1.min-1; n = 5; P < 0.01). Rats injected i.p. with LPS (0.5 mg.kg-1) for 4 consecutive days became tolerant to an i.v. injection of LPS (2 mg.kg-1) in that both hypotension and vascular hyporeactivity to NE were significantly attenuated. Moreover, in these endotoxin-tolerant rats, the induction of iNOS by LPS in the lung was attenuated by 63% +/- 6%. Injection of LPS caused a 9-fold increase in plasma corticosterone (CCS) levels within 2 h and CCS levels remained significantly elevated 6 and 24 h after LPS. Animals rendered tolerant to endotoxin by administration of a low dose of LPS (0.5 mg.kg-1, i.p.) for 4 days still had a 6-fold increase in plasma CCS levels 24 h after the last injection of LPS. When endotoxin-tolerant rats were treated with the glucocorticoid receptor antagonist RU 486 (50 mg.kg-1, p.o. 3 h prior to LPS), there was a restoration of the effects of LPS (2 mg.kg-1, i.v.) in causing hypotension, vascular hyporeactivity to NE, and iNOS induction in the lung. However, in control rats RU 486 enhanced neither the decrease in MAP nor the induction of iNOS in response to LPS (2 mg.kg-1, i.v.). Thus, cardiovascular tolerance to endotoxin is accompanied and explained by reduced induction of iNOS in vivo due to the elevation of endogenous glucocorticoid levels.

Trypanosoma brucei is protected from the cytostatic effects of nitric oxide under in vivo conditions

In mice infected with Trypanosoma brucei, splenic and peritoneal macrophages release substantial amounts of nitric oxide (NO). The production of NO by activated macrophages has been reported to be a nonspecific immune-effector mechanism against several parasites, and in this work we investigate the role of NO in killing T. brucei. Addition of bloodstream trypanosomes to peritoneal macrophages activated in vitro resulted in an NO-dependent inhibition of parasite growth. This effect was totally abrogated when dilutions of whole blood were included in the cultures, suggesting that bloodstream parasites such as T. brucei are not susceptible to NO-mediated killing in vivo.