A specific sequence of stimulation is required to induce synthesis of the antimicrobial molecule nitric oxide by mouse macrophages

TLR agonists that induce IFN-beta abrogate resident macrophage suppression of T cells

Resident tissue macrophages (Mφs) continually survey the microenvironment, ingesting Ags and presenting them on their surface for recognition by T cells. Because these Ags can be either host cell- or pathogen-derived, Mφs must be able to distinguish whether a particular Ag should provoke an immune response or be tolerated. However, the mechanisms that determine whether Mφs promote or inhibit T cell activation are not well understood. To investigate this, we first determined the mechanism by which murine resident peritoneal Mφs suppress in vitro T cell proliferation in the absence of pathogens and then explored the effects of different pathogen-derived molecules on Mφ immunosuppression. Our results suggest that, in response to IFN-γ, which is secreted by TCR-activated T cells, resident peritoneal Mφs acquire immunosuppressive properties that are mediated by NO. However, pretreatment of Mφs with LPS or dsRNA, but not CpG or peptidoglycan, eliminates their suppressive properties, in part via the induction of autocrine-acting IFN-β. These results suggest TLR agonists that activate TRIF, and consequently induce IFN-β, but not those that exclusively signal through MyD88, abrogate the immunosuppressive properties of Mφs, and thus promote T cell expansion and elimination of invading microorganisms.

Resistance of Haemophilus influenzae to reactive nitrogen donors and gamma interferon-stimulated macrophages requires the formate-dependent nitrite reductase regulator-activated ytfE gene

Haemophilus influenzae efficiently colonizes and persists at the human nasopharyngeal mucosa, causing disease when it spreads to other sites. Nitric oxide (NO) represents a major antimicrobial defense deployed by host cells in locations colonized by H. influenzae during pathogenesis that are likely to vary in oxygen levels. Formate-dependent nitrite reductase regulator (FNR) is an oxygen-sensitive regulator in several bacterial pathogens. We report that fnr of H. influenzae is required for anaerobic defense against exposure to NO donors and to resist NO-dependent effects of gamma interferon (IFN-gamma)-activated murine bone marrow-derived macrophages. To understand the mechanism of resistance, we investigated the role of FNR-regulated genes in defense against NO sources. Expression analysis revealed FNR-dependent activation of nrfA, dmsA, napA, and ytfE. Nonpolar deletion mutants of nrfA and ytfE exhibited sensitivity to NO donors, and the ytfE gene was more critical for survival. Compared to the wild-type strain, the ytfE mutant exhibited decreased survival when exposed to macrophages, a defect that was more pronounced after prior stimulation of macrophages with IFN-gamma or lipopolysaccharide. Complementation restored survival of the mutant to the level in the parental strain. Increased sensitivity of the ytfE mutant relative to that of the parent was abrogated by treatment of macrophages with a NO synthase inhibitor, implicating YtfE in resistance to a NO-dependent pathway. These results identify a requirement for FNR in positive control of ytfE and indicate a critical role for ytfE in resistance of H. influenzae to reactive nitrogen species and the antibacterial effects of macrophages.

Effects of polyporus polysaccharide on activity and mRNA expression of inducible nitric oxide synthase in peritoneal macrophages of mice

OBJECTIVE

To study the mechanisms of the antitumor and immunoregulation functions of polyporus polysaccharide (PPS).

METHODS

The production of nitric oxide (NO), the activity and mRNA expression of inducible nitric oxide synthase (iNOS) in peritoneal macrophages of mice administered with different dose of PPS were observed by Griess reaction, fluorimetry assay and RT-PCR, respectively.

RESULTS

PPS could elevate the iNOS activity with dose-dependence and stimulate the iNOS mRNA expression of peritoneal macrophages in mice.

CONCLUSION

The regulation of PPS on the production of NO in peritoneal macrophages of mice may occur at transcriptional level of iNOS. This indicates that the mechanism of PPS's antitumor and immunoregulation functions may be related to increasing NO output of macrophages through stimulating iNOS's denovo synthesis.

FK 506 and aminoguanidine suppress iNOS induction in orthotopic corneal allografts and prolong graft survival in mice

The aim of this study was to compare the effectiveness of immunosuppressant FK 506 and the specific inhibitor of inducible nitric oxide synthase (iNOS) aminoguanidine (AG) in prevention of corneal graft rejection and to investigate the iNOS expression in the rejection process. Orthotopic corneal allografting in mice was performed (C57BL/10; H-2(b) to BALB/c; H-2(d)). FK 506 (0.3 mg/kg per day) or AG (100 mg/kg per day) was injected intraperitoneally for 4 weeks. Grafted mice without therapy served as controls. Immunohistological evaluation of iNOS-positive cells and macrophage infiltration in grafts 27th day after grafting was performed. Within 4 weeks FK 506 prevented graft rejection in 71% and AG in 57% of animals compared to 29% of clear grafts in controls. A significant proportion of iNOS-positive cells was detected in the rejected grafts of the control and AG-treated groups. The treatment with FK 506 resulted in the inhibition of iNOS expression to a high degree in the rejected corneas. Non-rejected corneas of all groups and non-transplanted corneas exhibited no iNOS-positive cells. A massive infiltration of macrophages was detected in the rejected grafts, whereas non-rejected grafts exhibited only slight infiltration of macrophages. The presented data suggest that overexpression of iNOS and/or activation of iNOS is one of the several influential factors that contribute to the rejection process and that iNOS suppression delays corneal allograft rejection. FK 506 and AG are effective drugs in preventing corneal allograft rejection. Higher beneficial effect of FK 506 on graft survival could be explained by its well-known selective T-cell immunosuppression.

Induction of iNOS expression and antimicrobial activity by interferon (IFN)-beta is distinct from IFN-gamma in Burkholderia pseudomallei-infected mouse macrophages

Burkholderia pseudomallei is a causative agent of melioidosis. This Gram-negative bacterium is able to survive and multiple inside both phagocytic and nonphagocytic cells. We previously reported that exogenous interferons (both type I and type II) enhanced antimicrobial activity of the macrophages infected with B. pseudomallei by up-regulating inducible nitric oxide synthase (iNOS). This enzyme thus plays an essential role in controlling intracellular growth of bacteria. In the present study we extended our investigation, analysing the mechanism(s) by which the two types of interferons (IFNs) regulate antimicrobial activity in the B. pseudomallei-infected macrophages. Mouse macrophage cell line (RAW 264.7) that was exposed simultaneously to B. pseudomallei and type I IFN (IFN-beta) expressed high levels of iNOS, leading to enhanced intracellular killing of the bacteria. However, neither enhanced iNOS expression nor intracellular bacterial killing was observed when the macrophages were preactivated with IFN-beta prior to being infected with B. pseudomallei. On the contrary, the timing of exposure was not critical for the type II IFN (IFN-gamma) because when the cells were either prestimulated or co-stimulated with IFN-gamma, both iNOS expression and intracellular killing capacity were enhanced. The differences by which these two IFNs regulate antimicrobial activity may be related to the fact that IFN-gamma was able to induce more sustained interferon regulatory factor-1 (IRF-1) expression compared with the cells activated with IFN-beta.

STAT1 plays a critical role in the regulation of antimicrobial effector mechanisms, but not in the development of Th1-type responses during toxoplasmosis

The production of IFN-gamma by T cells and the ability of this cytokine to activate the transcription factor STAT1 are implicated in the activation of antimicrobial mechanisms required for resistance to intracellular pathogens. In addition, recent studies have suggested that the ability of STAT1 to inhibit the activation of STAT4 prevents the development of Th1 responses. However, other studies suggest that STAT1 is required to enhance the expression of T-bet, a transcription factor that promotes Th1 responses. To address the role of STAT1 in resistance to T. gondii, Stat1-/- mice were infected with this pathogen, and their response to infection was assessed. Although Stat1-/- mice produced normal serum levels of IL-12 and IFN-gamma, these mice were unable to control parasite replication and rapidly succumbed to this infection. Susceptibility to toxoplasmosis was associated with an inability to up-regulate MHC expression on macrophages, defects in NO production, and the inability to up-regulate some of the IFN-inducible GTPase family of proteins, molecules associated with antitoxoplasma activity. Analysis of T cell responses revealed that STAT1 was not required for the development of a Th1 response, but was required for the infection-induced up-regulation of T-bet. Together these studies suggest that during toxoplasmosis the major role of STAT1 is not in the development of protective T cell responses, but, rather, STAT1 is important in the development of antimicrobial effector mechanisms.

A pathway through interferon-gamma is the main pathway for induction of nitric oxide upon stimulation with bacterial lipopolysaccharide in mouse peritoneal cells

Production of nitric oxide (NO) in response to bacterial lipopolysaccharide (LPS) was investigated using cultures of mouse peritoneal exudate cells (PEC) and the macrophage cell line RAW264.7. In the presence of anti-(interferon-gamma) (IFN-gamma), NO production was markedly suppressed in the PEC culture but not in the RAW264.7 culture. In the PEC culture, LPS induced both IFN-gamma production and activation of IFN response factor-1, which leads to the gene expression of inducible NO synthase, but neither was induced in the culture of RAW264.7 cells. In addition to anti-(IFN-gamma), antibodies against interleukin (IL)-12 and IL-18 showed a suppressive effect on LPS-induced NO production in the PEC culture, and these antibodies in synergy showed strong suppression. Stimulation of the PEC culture with IL-12 or IL-18 induced production of IFN-gamma and NO, and these cytokines, in combination, exhibited marked synergism. Stimulation of the culture with IFN-gamma induced production of NO, but not IL-12. The macrophage population in the PEC, prepared as adherent cells, responded well to LPS for IL-12 production, but weakly for production of IFN-gamma and NO. The macrophages also responded well to IFN-gamma for NO production. For production of IFN-gamma by stimulation with LPS or IL-12 + IL-18, nonadherent cells were required in the PEC culture. Considering these results overall, the indirect pathway, through the production of intermediates (such as IFN-gamma-inducing cytokines and IFN-gamma) by the cooperation of macrophages with nonadherent cells, was revealed to play the main role in the LPS-induced NO production pathway, as opposed to the direct pathway requiring only a macrophage population.

Local inflammatory reaction induced by Bothrops jararaca venom differs in mice selected for acute inflammatory response

Bothrops jararaca venom (BjV) causes severe systemic and local reactions, characterized by an acute inflammatory reaction with accumulation of leukocytes and release of endogenous mediators. The systemic and local effects of BjV were compared in lines of mice genetically selected for maximal (AIR(max)) or minimal (AIR(min)) acute inflammatory reactivity (AIR). The systemic reaction was evaluated by LD(50) and the local reaction by edema formation, cellular influx, release of PGE(2), NO and H(2)O(2) and the production of the pro-inflammatory cytokines IL-6, Tumor necrosis factor-alpha (TNF-alpha) and IFN-gamma. Both mouse lines were equally susceptible to the lethal effects of the venom showing similar LD(50) but differed significantly in terms of the local inflammatory reaction. Footpad edema and leukocyte influx in the peritoneum after BjV inoculation was higher in AIR(max) compared to AIR(min), BALB/c or outbred Swiss mice. Coincidently, higher levels of the soluble mediators PGE(2), IFN-gamma and TNF-alpha were detected in the inflammatory exudate induced by BjV in AIR(max) mice. Cytokines levels were correlated to in vitro NO and H(2)O(2) production. The results demonstrate that the genetic factors selected in AIR(max) and AIR(min) lines of mice interfere in the control of the acute local reaction triggered by BjV venom.

Different mechanisms in inhibition of rat macrophage nitric oxide synthase expression by FK 506 and cyclosporin A

The modulatory effect of FK 506 and cyclosporin A (CsA) on the expression of inducible nitric oxide synthase (iNOS) in macrophages and mechanisms of their action were analysed. Isolated rat peritoneal macrophages were cultured for 12 or 24 h with or without lipopolysaccharide (LPS) (5 microg/ml) and in the absence or presence of FK 506 or CsA (0.1 and 1 microg/ml). Total RNA from macrophages was isolated and the expression of the gene for iNOS was assessed by using RT-PCR. The concentration of NO2- in culture supernatants was taken as a measure of nitric oxide (NO) production. FK 506 (0.1 and 1 microg/ml) reduced the LPS-induced increase of NO2- levels by 68% and 81%, respectively. CsA (0.1 and 1 microg/ml) decreased levels of nitrites by 39% and 69%, respectively. The results obtained suggest that both immunosuppressive drugs exhibit dose-dependent inhibitory effect on NO production and that FK 506 is more potent agent than CsA, in this respect. FK 506 exhibits its inhibitory effect on a phosphatase at the transcriptional level in macrophages. iNOS expression down-regulation by CsA is occurred post-transcriptionally.

NO contributes to proliferative suppression in a murine model of filariasis

Infection of BALB/c mice with microfilariae (mf) of Brugia pahangi leads to the suppression of antigen (Ag)-specific proliferative responses in the spleen. The proliferative defect is dependent on inducible nitric oxide synthase (iNOS) activity, since inhibition of iNOS with either L-N-monomethyl arginine (L-NMMA) or aminoguanidine reversed defective proliferation. Splenocytes from mf-infected animals produce high levels of gamma interferon (IFN-gamma) upon in vitro restimulation with Ag, and experiments in IFN-gamma receptor-deficient (IFN-gamma R(-/-)) mice demonstrated that signaling via the IFN-gamma R is essential in the induction of NO production and subsequent proliferative suppression. Restimulation of splenocytes from mf-infected animals with an extract of Acanthocheilonema viteae, a related filarial worm which lacks endosymbiotic bacteria, also resulted in NO production and proliferative suppression, demonstrating that lipopolysaccharide of bacterial origin is not essential to the induction of iNOS activity. These results extend previous observations that infection with different life cycle stages of Brugia leads to the development of differentially polarized immune responses and demonstrate one method by which these differences may exert their effects on the proliferative potential of cells from infected animals.

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.

Role of Pasteurella multocida, Pasteurella haemolytica and Salmonella typhimurium porins on inducible nitric oxide release by murine macrophages

The aim of this study was to verify whether Pasteurella haemolytica, P. multocida and Salmonella typhimurium porins could affect the inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) release by murine resident peritoneal macrophages in vitro. We also compared their effect with that elicited by P. haemolytica, P. multocida and S. typhimurium lipopolysaccharide (LPS) whose biological activity is well known. Variations in NO release and iNOS mRNA expression due to variable concentrations of porins were recorded and compared. We also investigated the synergism between bacterial products and interferon gamma (IFN-gamma). With this aim cells were incubated with porins together with murine rIFN-gamma prior to assessing the presence of NO in the supernatant and mRNA analysis. Porins in themselves were not able to induce NO release by resident peritoneal macrophages. Incubation of macrophages with IFN-gamma in the presence of porins increased NO release, whereas incubation in the presence of the arginine analog N(G)-monomethyl-L-arginine (NMA) inhibited NO release. The greatest NO release was obtained using porins at a concentration of 5 microg/mL. Porins, together with IFN-gamma, were also able to upregulate the mRNA expression of iNOS. Our findings suggest that gram-negative porins are able to modulate inflammatory and immunological responses by affecting the release of NO and the expression of iNOS gene in activated macrophages.

The gills are an important site of iNOS expression in rainbow trout Oncorhynchus mykiss after challenge with the gram-positive pathogen Renibacterium salmoninarum

Following injection challenge of rainbow trout with the Gram-positive pathogen Renibacterium salmoninarum, serum nitrate levels increased indicative of NO production. The timing and amount of nitrate produced varied with the virulence of the bacterial strain used, with the highest levels seen in fish challenged with the most virulent (autoaggregating) strain. Immunization with a killed R. salmoninarum preparation in Freund's incomplete adjuvant significantly increased nitrate levels after challenge. Inducible nitric oxide synthase (iNOS) transcript expression was detectable in rainbow trout tissues after injection challenge with R. salmoninarum, and its induction in the gills was both quick (between 3 and 6 hr) and relatively prolonged (lasting several days). iNOS expression in the kidney was also seen at a later stage (24 hr) but appeared to switch off relatively rapidly. Bath challenge with R. salmoninarum also induced iNOS expression in gill, and a variable expression in the gut and kidney also occurred. These results highlight the importance of the gills, not only as a point of entry of pathogens but also as a tissue capable of mounting an immune response.

Effect of injection of L-NAME on drinking response

The drinking behavior responses to centrally administered N G-nitro-L-arginine methyl ester (L-NAME; 10, 20 or 40 microg/microl), an inhibitor of nitric oxide synthase, were studied in satiated rats, with cannulae stereotaxically implanted into the lateral ventricle (LV) and subfornical organ (SFO). Water intake increased in all animals after angiotensin II (ANG II) injection into the LV, with values of 14.2 +/- 1.4 ml/h. After injection of L-NAME at doses of 10, 20 or 40 microg/microl into the SFO before injection of ANG II (12 ng/microl) into the LV, water intake decreased progressively and reached basal levels after treatment with 0.15 M NaCl and with the highest dose of L-NAME (i.e., 40 microg). The water intake obtained after 40 microg/microl L-NAME was 0.8 +/- 0.01 ml/h. Also, the injection of L-NAME, 10, 20 or 40 microg/microl, into the LV progressively reduced the water intake induced by hypertonic saline, with values of 5.3 +/- 0.8, 3.2 +/- 0.8 and 0.7 +/- 0.01 ml/h, respectively. These results indicate that nitric oxide is involved in the regulation of drinking behavior induced by centrally administered ANG II and cellular dehydration and that the nitric oxide of the SFO plays an important role in this regulation.

Thioglycollate-elicited murine macrophages are cytotoxic to Mycoplasma arginini-infected YAC-1 tumor cells

Macrophages are important components of natural immunity involved in inhibition of tumor growth and destruction of tumor cells. It is known that these cells can be activated for tumoricidal activity by lymphokines and bacterial products. We investigated whether YAC-1 tumor cells infected with Mycoplasma arginini stimulate nitric oxide (NO) release and macrophage cytotoxic activity. Thioglycollate-elicited macrophages from male BALB/c mice were co-cultured for 20 h with YAC-1 tumor cells infected or not with Mycoplasma arginini. The cytotoxic activity was evaluated by MTT assay and nitrite levels were determined with the Griess reagent. Thioglycollate-elicited macrophages co-cultured with noninfected YAC-1 cells showed low cytotoxic activity (34.7 +/- 8.6%) and low production of NO (4.7 +/- 3.1 microM NO2-). These macrophages co-cultured with mycoplasma-infected YAC-1 cells showed significantly higher cytotoxic activity (61.4 +/- 9.1%; P < 0.05) and higher NO production (48.5 +/- 13 microM NO2-; P < 0.05). Addition of L-NAME (10 mM), an inhibitor of NO synthesis, to these co-cultures reduced the cytotoxic activity to 37.4 +/- 2% (P < 0.05) and NO production to 3 +/- 4 microM NO2- (P < 0.05). The present data show that Mycoplasma arginini is able to induce macrophage cytotoxic activity and that this activity is partially mediated by NO.

Trypanosoma cruzi: Tc52 Released Protein-Induced Increased Expression of Nitric Oxide Synthase and Nitric Oxide Production by Macrophages

Trypanosoma cruzi target molecules that might regulate the host immune responses have not yet been fully identified. In the present study, we demonstrate that the parasite-released molecule (Tc52) was able to synergize with IFN-γ to stimulate nitric oxide production by macrophages. This synergistic effect was also observed at the level of inducible nitric oxide synthase gene expression. Furthermore, Tc52 was also shown to induce gene expression for IL-1α, IL-12, and IL-10. Moreover, the combination of Tc52 and IFN-γ down-regulates IL-1α and IL-10 gene expression, but not IL-12. Isotype profiles and Tc52 or anti-CD3-induced T cell proliferation were also analyzed, indicating that active immunization with Tc52 partially relieves the immunosuppression observed during the acute phase of the disease. Moreover, under conditions of experimental infection, the Tc52 appears immunologically silent, failing to elicit Ab response and lymphocyte proliferation during the initial acute phase infection. Following active immunization, Tc52 was capable of stimulating T cell proliferation and Ab production with a predominance of IgG1, IgG2a, IgG2b, IgG3, and to a lesser extent IgA. Taken together, these results demonstrate that T. cruzi Tc52-released Ag could be involved in the immunoregulatory processes. The immune response against Tc52 that appears late in the T. cruzi infection may play a role in the modulation of its biological function(s).

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.

Structure and biological activities of acapsular Cryptococcus neoformans 602 complemented with the CAP64 gene

The extracellular polysaccharide capsule of Cryptococcus neoformans is a well-recognized virulence factor. Strain 602 is an acapsular clinical isolate of unknown serotype which has been widely used in studies of virulence and host-parasite interactions. In previous studies, strain 602 was compared with genetically unrelated strains of various serotypes because the wild-type equivalent of strain 602 was not available. We created an encapsulated strain, TYCC38-602, by transforming strain 602 with the CAP64 gene which was isolated from a serotype D strain. Serological tests and chemical analysis of the major polysaccharide capsule of TYCC38-602 indicated that strain 602 was originally derived from a serotype A strain. Restoration of the ability to produce a capsule enabled strain 602 to cause fatal infection in mice, whereas the acapsular strain 602 remained avirulent. Capsule-restored yeast cells of strain 602 activated the human complement system and bound C3 fragments in a manner that is characteristic of encapsulated cryptococci. In addition, the capsule in TYCC38-602 masked the ability of the organism to induce tumor necrosis factor alpha and subsequent nitric oxide synthase production in primed macrophage-like cells. These results indicate that the lack of capsule in strain 602 is the reason for its inability to cause fatal infection. Moreover, the acapsular phenotype accounts for differences in various biological activities of strain 602 compared to encapsulated strains. The results also indicate that the gene product of CAP64 does not contribute to serotype specificity of capsules in C. neoformans.

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.

Recombinant human prolactin induces protection against Salmonella typhimurium infection in the mouse: role of nitric oxide

In the present study, we demonstrated that repeated treatment with recombinant human prolactin (rhPRL) protected mice against Salmonella typhimurium infection. The protective activity was statistically significant, dose-dependent and present only when rhPRL treatments were performed before the infection. This activity was probably related to the observed increases in phagocytosis and intracellular killing of peritoneal macrophages induced by the hormonal treatment. The number of peripheral leukocytes was not modified, excluding a mobilization of cells from other compartments. A decrease in the mortality rate after challenge was also observed in mice treated with the monoclonal antibody anti-PRL receptor U5, confirming that the protective activity was associated with receptor activation. Our studies also suggest that nitric oxide (NO) production was involved in the protective effect of rhPRL since pre-treatment of the animals with L-NAME, an inhibitor of NO-synthase, was able to completely revert the protective activity, whereas D-NAME, the inactive D-isomer, was without effect.

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.

Drinking and blood pressure responses to central injection of L-NAME in conscious rats

The drinking behavior and blood pressure responses to i.c.v. administration of artificial cerebrospinal fluid (aCSF) or NG-nitro-L-arginine methyl ester (L-NAME, 10, 250, or 500 micrograms), an inhibitor of nitric oxide synthase, were examined in conscious rats following either osmotic stimulation (1.0 M NaCl, 15 ml/kg, s.c.) or induction of hemorrhage (0.7 ml/min to a 20% blood volume loss). Water intake increased in all animals. L-NAME at doses of 250 and 500 micrograms, but not 10 micrograms, significantly attenuated water consumption induced by both stimuli. The mean arterial blood pressure (MABP), which increased after osmotic stimulation, was maintained at pressor levels by 250 and 500 micrograms of L-NAME, but decreased progressively and reached basal levels after treatment with aCSF and the lowest dose of L-NAME (i.e., 10 micrograms). Hemorrhage significantly decreased MABP in all rats. The fall in blood pressure associated with hemorrhage returned to control levels in animals treated with 250 and 500 micrograms of L-NAME but not in those treated with aCSF or 10 micrograms of L-NAME. These results indicate that nitric oxide is involved in the regulation of drinking behavior and may play an important role in the central control of blood pressure during osmotic stimulation and hypotensive hemorrhage.

Role of nitric oxide in parasitic infections

Nitric oxide is produced by a number of different cell types in response to cytokine stimulation and thus has been found to play a role in immunologically mediated protection against a growing list of protozoan and helminth parasites in vitro and in animal models. The biochemical basis of its effects on the parasite targets appears to involve primarily inactivation of enzymes crucial to energy metabolism and growth, although it has other biologic activities as well. NO is produced not only by macrophages and macrophage-like cells commonly associated with the effector arm of cell-mediated immune reactivity but also by cells commonly considered to lie outside the immunologic network, such as hepatocytes and endothelial cells, which are intimately involved in the life cycle of a number of parasites. NO production is stimulated by gamma interferon in combination with tumor necrosis factor alpha or other secondary activation signals and is regulated by a number of cytokines (especially interleukin-4, interleukin-10, and transforming growth factor beta) and other mediators, as well as through its own inherent inhibitory activity. The potential for design of prevention and/or intervention approaches against parasitic infection (e.g., vaccination or combination chemo- and immunotherapy strategies) on the basis of induction of cell-mediated immunity and NO production appears to be great, but the possible pathogenic consequences of overproduction of NO must be taken into account. Moreover, more research on the role and regulation of NO in human parasitic infection is needed before its possible clinical relevance can be determined.

Gamma delta T cell-induced nitric oxide production enhances resistance to mucosal candidiasis

Despite the prevalence of gamma delta T cells in mucosae that are typically colonized by Candida albicans, little is known of the possible role of these cells in resistance to candidiasis. A sharp increase in the number of gamma delta T cells and macrophages following intraperitoneal inoculation of mice with C. albicans led us to examine the role of these cells in the immune response to C. albicans. We show that the gamma delta T cells enhance macrophage nitric oxide (NO) production and anti-candida activity, in vitro. We also propose that the gamma delta T cells regulate macrophage function during candidiasis in vivo as well, because depletion of these cells abrogated inducible NO synthase expression in mucosae and enhanced murine susceptibility to candidiasis.

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.

Lack of involvement of nitric oxide in killing of Aspergillus fumigatus conidia by pulmonary alveolar macrophages

Nitric oxide is an important antimicrobial mechanism of phagocytes from mice and rats, but in the case of human phagocytes, secretion is still controversial. We investigated whether nitric oxide is involved in the killing of Aspergillus fumigatus conidia by human or murine pulmonary alveolar macrophages. Stimulation of the macrophages with gamma interferon and Escherichia coli lipopolysaccharide had no effect on fungicidal activity against conidia in vitro, with or without the addition of tetrahydrobiopterin. Killing of conidia (means +/- standard deviations) by murine or human alveolar macrophages, before and after stimulation, was 44% +/- 13% and 49% +/- 12% (P = 0.34) and 24% +/- 5% and 29% +/- 10% (P = 0.20), respectively. Fungicidal activity was unaltered in the presence of the competitive inhibitor NG-monomethyl L-arginine, and nitrite was undetectable in cell supernatants. Peritoneal macrophages from B6C3F1 mice produced 18 mumol of nitrite per 10(6) cells in 18 h. In conclusion, nitric oxide does not appear to be involved in the fungicidal activity of murine or human alveolar macrophages against A. fumigatus conidia.

Cryptococcus neoformans fails to induce nitric oxide synthase in primed murine macrophage-like cells

Nitric oxide (NO) is a microbiostatic gas generated by activated murine macrophages. Cytokine signals, gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) act synergistically to induce production of a macrophage nitric oxide synthase (NOS). A variety of intracellular pathogens, when recognized by macrophages primed with IFN-gamma, induce NOS by eliciting TNF-alpha secretion, which then functions as a positive autocrine signal. In cell culture assays, a murine macrophage cell line (J774), primed with IFN-gamma, was tested for NOS induction upon challenge with virulent Cryptococcus neoformans. C. neoformans failed to induce macrophage NOS as measured by nitrite production. This was true irrespective of the C. neoformans-to-J774 ratio. Other nonpathogenic Cryptococcus species likewise failed to induce NOS, yet Saccharomyces cerevisiae, Histoplasma capsulatum, and Candida albicans were efficient inducers of NOS. Conditions which promoted attachment and/or phagocytosis of C. neoformans did not lead to NOS induction (including opsonization with specific antibodies against C. neoformans). Assays for transcriptional repressors of NOS were negative. Tests for consumption of nitrite by measurement of additional products of NOS induction were negative. No TNF-alpha was detected by enzyme-linked immunosorbent assay in supernatants from C. neoformans-J774 cocultures. A mutant C. neoformans strain with a minimal, but visible, polysaccharide capsule also failed to induce NOS; however, several nonencapsulated mutants of C. neoformans did induce NOS. Failure of C. neoformans to act as an inducer of NOS may be related to the virulence of this pathogen in mice; C. neoformans is a unique example of a facultative intracellular pathogen which fails to induce NOS in primed macrophages. The mechanism appears to involve the failure of TNF-alpha secretion once the macrophage comes in contact with the fungus. The presence of the polysaccharide capsule appears to mask the signal necessary for TNF-alpha secretion and, ultimately, NOS induction.

Productions of interferon-gamma and nitrite are induced in mouse splenic cells by a heteroglycan-protein fraction from culture medium of Lentinus edodes mycelia

A xylose-rich heteroglycan-protein fraction (LAP1) was prepared from a solid culture medium in which Lentinus edodes mycelia were growing actively. Mouse splenic cells (SPs) were incubated with [3H]TdR in the presence of LAP1. The incubated SPs were fractionated into plastic adherent splenic cells (ADs), nylon-column effluent splenic cells (NEs) and sIg-expressed splenic cells (SIs), which are rich in Mac-1+, Thy-1.2+ and Ly-5+ cells, respectively. The incorporation of [3H]TdR in response to LAP1 was enhanced in each of the fractionated cell populations. Northern or dot blot hybridization showed that productions of IFN-gamma and its receptor mRNAs are induced predominantly in NEs. In another experiment, SPs were fractionated into ADs, NEs and SIs. Then, NE-AD, SI-AD and NE-SI mixtures were prepared and incubated in the same manner. A significant incorporation of [3H]TdR was shown only in the NE-AD mixture. The enzyme-linked immunosorbent assay showed that IFN-gamma production in response to LAP1 is induced in SPs or in the NE-AD mixture, but not in NEs alone. The level of the production was about 5 times higher in the mixture than in SPs after a 72 h incubation. Moreover, LAP1 was capable of inducing NO2- production in SPs. Thus, the present studies imply that this heteroglycan-protein fraction stimulates productions of IFN-gamma and nitrite in mouse splenic cells, augmenting antitumor immune response(s).

Involvement of nitric oxide synthase in antiproliferative activity of macrophages: induction of the enzyme requires two different kinds of signal acting synergistically

Activated rodent macrophages inhibit micro-organism and tumour cell growth through a high output of nitric oxide; generated by an isoform of nitric oxide synthase which is induced, for example, in murine macrophages, by concomitant stimulation with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS). We show here that LPS could be replaced as a co-stimulant by the mycobacterial derivative muramyl dipeptide (MDP) in macrophages, and by interleukin-1 (IL-1) in EMT-6 adenocarcinoma cells. Moreover, our results indicate that nitric oxide synthase RNA synthesis required either simultaneous or sequential exposure to IFN-gamma and MDP/IL-1; whereas exposure to MDP/IL-1 followed by exposure to IFN-gamma was ineffective. Thus, two kinds of signal could be distinguished: IFN-gamma on the one hand, acting first in an irreversible way, and LPS, MDP, IL-1 on the other hand, which seemed to be permanently required for continuous transcription of the nitric oxide synthase gene.

Interferon-gamma enhances monocyte cytotoxicity via enhanced reactive oxygen intermediate production. Absence of an effect on macrophage cytotoxicity is due to failure to enhance reactive nitrogen intermediate production

Interferon-gamma (IFN-gamma) enhanced the cytotoxic capability of freshly isolated human blood monocytes but failed to enhance the tumoricidal competence of monocyte-derived macrophages. Treatment of monocytes with IFN-gamma (100 U/ml) caused a significant increase (P < 0.001) in lucigenin-dependent chemiluminescence and O2- production stimulated by N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) during the first few days in culture but IFN-gamma was unable to prevent the decline to negligible levels of chemiluminescence and O2- production which occurred during the later days in vitro. Culture of monocytes in the presence of IFN-gamma had no effect on phorbol 12-myristate 13-acetate (PMA)-stimulated O2- production. However, IFN-gamma decreased PMA-stimulated lucigenin-dependent chemiluminescence during the first 24 hr in vitro but then significantly enhanced (P < 0.001) chemiluminescence after 2-4 days in culture. IFN-gamma was unable to prevent the eventual decline to undetectable levels in PMA-stimulated chemiluminescence during the later days in vitro. Nitrite production by macrophages was unaffected by IFN-gamma treatment. It is concluded therefore, that IFN-gamma enhanced the cytotoxicity of freshly isolated human blood monocytes by increasing reactive oxygen intermediate generation but was unable to enhance the tumoricidal competence of macrophages as reactive nitrogen intermediate production was unaffected.

Protection against lethal bacterial infection in mice by monocyte-chemotactic and -activating factor

Chemotactic factors regulate the recruitment of neutrophils, lymphocytes, or monocytes-macrophages to infectious and inflammatory sites. The purpose of this study was to determine whether monocyte-chemotactic and -activating factor (MCAF [MCP-1], a JE gene product) also influences the host defense mechanism against microbial infection. We evaluated the effect of recombinant human MCAF on the survival rate of mice systemically infected with Pseudomonas aeruginosa or Salmonella typhimurium. The administration of 2.5 micrograms of MCAF 6 h before infection completely protected the mice from lethal infection. Mice with cyclophosphamide-induced leukopenia exhibiting increased susceptibility to P. aeruginosa were also endowed with resistance by the same dose of MCAF. Administration of MCAF at -6 h was critical, since MCAF given either earlier or later than -6 h failed to rescue mice from lethal infection. The in vivo effect on the survival of mice paralleled the reduced recovery of viable P. aeruginosa or S. typhimurium from the peritoneal cavity, i.e., the number of recovered bacteria from the MCAF (2.5 micrograms per mouse)-treated mice was reduced to less than 2% of control mice for P. aeruginosa and 4% of control mice for S. typhimurium at 24 h. Since MCAF exhibited chemotaxis on murine macrophages as well as enhanced phagocytosis and killing of bacteria in vitro, the activation of macrophages, followed by the recruitment into the peritoneal cavity, is responsible for eliminating bacteria and thus enhancing the survival rate.

Evidence for the involvement of macrophage-derived nitric oxide in the modulation of immune status by a conditioned aversive stimulus

Prior work in our laboratory has demonstrated that exposure to a conditioned aversive stimulus developed through pairings with electric shock results in pronounced alterations of immune status. These conditioned alterations of immune status include a decreased in natural killer cell activity, decreased production of interleukin-2 and gamma-interferon by concanavalin A (ConA)-stimulated splenocytes and a profound suppression of the mitogenic responsiveness of T and B lymphocytes to mitogens. The present study examines the role of macrophage-derived nitric oxide in the conditioned stimulus-induced suppression of lymphocyte proliferation by measuring the level of nitrite accumulation in culture, determining the effect of macrophage depletion, and assessing the effect of NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of the L-arginine-dependent nitric-oxide synthesizing pathway, alone and in combination with L- or D-arginine. The results show that the conditioned suppression of the mitogenic responsiveness of splenocytes to ConA is accompanied by a marked increase in nitrite accumulation. Both the depletion of macrophages and the addition of L-NMMA attenuates the conditioned suppression of ConA-stimulated lymphocyte proliferation. Furthermore, the addition of excess L-arginine, but not D-arginine, counteracts the effect of L-NMMA. The present findings show that the neuroendocrine alterations induced by a conditioned aversive stimulus suppress lymphocyte proliferation through alteration of the production of nitric oxide by macrophages.

Differential regulation of inducible nitric oxide synthase by fibroblast growth factors and transforming growth factor beta in bovine retinal pigmented epithelial cells: inverse correlation with cellular proliferation

Bovine retinal pigmented epithelial (RPE) cells express, after activation with interferon gamma (IFN-gamma) and lipopolysaccharide (LPS), an inducible nitric oxide synthase (NOS). Experiments were done to investigate the effects of the transforming growth factor beta 1, epidermal growth factor, and fibroblast growth factors (FGFs), which are abundant in the retina, on NOS activity. Transforming growth factor beta 1 slightly increases the production of nitrite, an oxidation product of NO, induced by LPS plus IFN-gamma, whereas acidic and basic FGFs markedly inhibit the nitrite release due to LPS/IFN-gamma in a concentration-dependent manner, and epidermal growth factor did not modify LPS/IFN-gamma-induced NOS activity. The growth factors alone did not stimulate nitrite release. We have attempted to elucidate the mechanism of FGF inhibition. Results with heparin, suramin, and tyrphostin suggest involvement of the high-affinity receptor for FGF in its inhibition of LPS/IFN-gamma-stimulated NOS activity. Continued stimulation of RPE cells with LPS/IFN-gamma was essential for the induction of NO synthesis, and maximal inhibition was obtained when FGF was present during stimulation with LPS/IFN-gamma, suggesting that FGF inhibits NOS induction. Furthermore, an antiproliferative action of NO was demonstrated by an inverse correlation between the amounts of nitrite or citrulline produced in response to different stimuli (LPS/IFN-gamma or LPS/IFN-gamma with growth factors) and the level of cellular proliferation. Similar inhibition of growth was obtained when RPE cells were incubated with an NO donor, sydnonimide. Because NO acts as a cytotoxic compound in the retina, FGF, by inhibiting the induction of NOS in RPE cells, may have beneficial effects in protecting the retina from cytokine and endotoxin-mediated tissue damage.