Nitric oxide-mediated antiplasmodial activity in human and murine hepatocytes induced by gamma interferon and the parasite itself: enhancement by exogenous tetrahydrobiopterin

The role of nitric oxide as an effector of macrophage-mediated cytotoxicity against Trichomonas vaginalis

The purpose of this study is to determine whether nitric oxide is involved in the extracellular killing of Trichomonas vaginalis by mouse (BALB/c) peritoneal macrophages and RAW264.7 cells activated with LPS or rIFN-gamma and also to observe the effects of various chemicals which affect the production of reactive nitrogen intermediates (RNI) in the cytotoxicity against T. vaginalis. The cytotoxicity was measured by counting the release of [3H]-thymidine from labelled protozoa and NO2- was assayed by Griess reaction. NG-monomethyl-L-arginine (L-NMMA), NG-nitro-L-arginine methyl ester (NAME) and arginase inhibited cytotoxicity to T. vaginalis and nitrite production by activated mouse perioneal macrophages and RAW 264.7 cells. The addition of excess L-arginine competitively restored trichomonacidal activity of macrophages. Exogenous addition of FeSO4 inhibited cytotoxicity to T. vaginalis and nitric products of macrophages. From above results, it is assumed that nitric oxide plays an important role in the host defense mechanism of macrophages against T. vaginalis.

Toward clinical trials of DNA vaccines against malaria

In mid 1997 the first malaria DNA vaccine will enter clinical trials. This single gene DNA vaccine encoding the Plasmodium falciparum circumsporozoite protein (PfCSP) will be studied for safety and immunogenicity. If these criteria are met, a multi-gene DNA vaccine designed to induce protective CD8+ T cell responses against P. falciparum infected hepatocytes will be subsequently assessed for safety, immunogenicity and capacity to protect immunized volunteers against experimental challenge with P. falciparum sporozoites. Our perspectives on malaria vaccine development in general, and on a multi-gene DNA vaccine in particular, have been recently reviewed. Herein, we review the rationale and experimental foundation for the anticipated P. falciparum DNA vaccine trials.

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.

Sterile protection of monkeys against malaria after administration of interleukin-12

An estimated 300-500 million new infections and 1.5-2.7 million deaths attributed to malaria occur annually in the developing world, and every year tens of millions of travelers from countries where malaria is not transmitted visit countries with malaria. Because the parasites that cause malaria have developed resistance to many antimalarial drugs, new methods for prevention are required. Intraperitoneal injection into mice of one dose of 150 ng (approximately 7.5 micrograms per kg body weight) recombinant mouse interleukin-12 (rmIL-12) 2 days before challenge with Plasmodium yoelii sporozoites protects 100% of mice against malaria. We report that one subcutaneous injection of 10 micrograms/kg recombinant human IL-12 (rhIL-12) 2 days before challenge with P. cynomolgi sporozoites protected seven of seven rhesus monkeys. Protection was associated with marked increases in plasma levels of interferon-gamma (IFN-gamma), and relative increases of lymphoid cell messenger RNA coding for IFN-gamma and several other cytokines. We speculate that rIL-12 protects monkeys through IFN-gamma-dependent elimination of P. cynomolgi-infected hepatocytes. This first report of rIL-12-induced protection of primates against an infectious agent supports assessment of rhIL-12 for immunoprophylaxis of human malaria.

Sera from patients with falciparum malaria induce substance P gene expression in cultured human brain microvascular endothelial cells

Substance P is a pluripotent neuropeptide capable of inducing neurogenic inflammation, immunoregulation, and vasodilatation. In an effort to contribute to the understanding of the pathophysiology of cerebral malaria, we have evaluated the effects of sera obtained from patients suffering from severe or mild malaria and from a healthy donor with no previous history of exposure to malaria on the expression of the substance P gene by cultured human brain microvascular endothelial cells (HBMEC) and human umbilical-vein endothelial cells. PCR, Southern blotting, hybridization with an internal probe, and densitometry demonstrated that treatment of HBMEC with sera from patients with severe malaria caused remarkably increased expression of the substance P gene. In HBMEC, substance P was not significantly influenced by serum from a healthy donor. Substance P was expressed at almost undetectable levels in untreated HBMEC. Treatment of cultured human umbilical-vein endothelial cells with the same sera produced no signal. The influence of different sera on the expression of substance P by HBMEC suggests that substance P expression may be involved in events leading to the development of severe malaria.

DNA vaccines against malaria: immunogenicity and protection in a rodent model

Since the first demonstration of the technology a few years ago, DNA vaccines have emerged as a promising method of vaccination. In a variety of experimental systems, DNA vaccines have been shown not only to induce potent immune responses, but also to offer many advantages in terms of ease of construction, testing, and production. In this article we summarize the progress achieved in development of DNA vaccines that can protect mice from infection by the rodent malaria parasite Plasmodium yoelii, describe initial studies of immunogenicity of a malaria DNA vaccine in a primate model, and outline the strategies being employed to design the next generation of malaria DNA vaccines.

Competition for tetrahydrobiopterin between phenylalanine hydroxylase and nitric oxide synthase in rat liver

Tetrahydrobiopterin (BH4) is an important cofactor for two hepatic enzymes, inducible nitric oxide synthase (iNOS) and phenylalanine hydroxylase (PAH), and competition for BH4 between the two enzymes might limit hepatic iNOS or PAH activity. To test this hypothesis, we determined whether conversion of phenylalanine to tyrosine was modified by changes in NO synthase activity, and conversely whether NO synthesis was limited by the rate of phenylalanine conversion to tyrosine in rat hepatocytes and perfused livers. NO production was decreased only slightly, when flux through PAH was maximized in isolated perfused livers, and in isolated hepatocytes only when BH4 synthesis was inhibited. Increases in NO synthesis did not reduce tyrosine formation from phenylalanine. Phenylalanine markedly increased biopterin synthesis, whereas arginine had no effect. Thus, basal BH4 synthesis appears to be adequate to support iNOS activity, whereas BH4 synthesis is increased to support PAH activity.

High-performance liquid chromatographic methods for the quantification of tetrahydrobiopterin biosynthetic enzymes

Tetrahydrobiopterin is a cofactor in hydroxylation reactions, including phenylalanine 4-monooxygenase, tyrosine 3-monooxygenase, tryptophan 5-monooxygenase, alkyl glycol ether monooxygenase and nitric oxide synthase. Determination of its biosynthesis is carried out to diagnose inherited diseases leading to partial defects in tetrahydrobiopterin synthesis. In addition, tetrahydrobiopterin synthesis is induced by proinflammatory cytokines, and intracellular levels of tetrahydro-biopterin in many cases limit the activity of tetrahydrobiopterin-dependent reactions, such as nitric oxide synthase in intact cells. Biosynthesis of tetrahydrobiopterin from guanosine 5'-triphosphate (GTP) requires the action of three enzymes, GTP-cyclohydrolase I (E.C. 3.5.4.16), 6-pyruvoyl tetrahydropterin synthase (EC, 4.6.1.10) and sepiapterin reductase (E.C. 1.1.1.153). Methods for quantification of biopterin and related pteridines in biological matrices by HPLC and application of these for determining the activity of the three tetrahydrobiopterin biosynthetic enzymes are reviewed in this article.

Nitric oxide in Tanzanian children with malaria: inverse relationship between malaria severity and nitric oxide production/nitric oxide synthase type 2 expression

Nitric oxide (NO)-related activity has been shown to be protective against Plasmodium falciparum in vitro. It has been hypothesized, however, that excess NO production contributes to the pathogenesis of cerebral malaria. The purpose of this study was to compare markers of NO production [urinary and plasma nitrate + nitrite (NOx)], leukocyte-inducible nitric oxide synthase type 2 (NOS2), and plasma TNF-alpha and IL-10 levels with disease severity in 191 Tanzanian children with and without malaria. Urine NOx excretion and plasma NOx levels (corrected for renal impairment) were inversely related to disease severity, with levels highest in subclinical infection and lowest in fatal cerebral malaria. Results could not be explained by differences in dietary nitrate ingestion among the groups. Plasma levels of IL-10, a cytokine known to suppress NO synthesis, increased with disease severity. Leukocyte NOS2 antigen was detectable in all control children tested and in all those with subclinical infection, but was undetectable in all but one subject with cerebral malaria. This suppression of NO synthesis in cerebral malaria may contribute to pathogenesis. In contrast, high fasting NOx levels and leukocyte NOS2 in healthy controls and asymptomatic infection suggest that increased NO synthesis might protect against clinical disease. NO appears to have a protective rather than pathological role in African children with malaria.

Induction of nitric oxide synthesis in murine macrophages by Helicobacter pylori

Intact Helicobacter pylori cells, as well as cellular components, stimulated nitric oxide (NO) synthesis in an in vitro murine macrophage system by the L-arginine-nitric oxide pathway. Macrophage-mediated NO formation was dependent on the presence of H. pylori and exhibited a dose-dependent increase at H. pylori concentrations between 10(6) and 5 and 10(7) cells/ml. H. pylori mediated NO synthesis also required L-arginine and was inhibited by NG-monomethyl-L-arginine (NMMA), a selective inhibitor of nitric oxide synthase. NO synthesis was induced by whole H. pylori cells. H. pylori media filtrate, extracted membrane proteins, and H. pylori lipopolysaccharide (LPS). Maximal NO synthesis was induced by viable H. pylori cells with media filtrate and membrane protein extracts inducing significant NO responses. NO stimulation by media filtrate and membrane protein extracts support secreted H. pylori products as potential activators of inflammatory cell NO synthesis in vivo. NO synthesis in response to H. pylori suggests that chronic H. pylori infection may increase endogenous formation of NO. Elevated NO exposure may represent an etiologic factor explaining the epidemiologic association between long-term H. pylori infection and gastric cancer.

Circumventing genetic restriction of protection against malaria with multigene DNA immunization: CD8+ cell-, interferon gamma-, and nitric oxide-dependent immunity

Despite efforts to develop vaccines that protect against malaria by inducing CD8+ T cells that kill infected hepatocytes, no subunit vaccine has been shown to circumvent the genetic restriction inherent in this approach, and little is known about the interaction of subunit vaccine-induced immune effectors and infected hepatocytes. We now report that immunization with plasmid DNA encoding the plasmodium yoelii circumsporozoite protein protected one of five strains of mice against malaria (H-2d, 75%); a PyHEP17 DNA vaccine protected three of the five strains (H-2a, 71%; H-2k, 54%; H-2d, 26%); and the combination protected 82% of H-2a, 90% of H-2k, and 88% of H-2d mice. Protection was absolutely dependent on CD8+ T cells, INF-gamma, or nitric oxide. These data introduce a new target of protective preerythrocytic immune responses, PyHEP 17 and its P. falciparum homologue, and provide a realistic perspective on the opportunities and challenges inherent in developing malaria vaccines that target the infected hepatocyte.

Elevated levels of methaemoglobin in Tanzanian children with severe and uncomplicated malaria

Elevated levels of methaemoglobin, the ferric form of haemoglobin incapable of oxygen transport, have been previously found during Plasmodium vivax infections and in acidotic infants. We measured methaemoglobin in the following 5 groups of children with P. falciparum malaria admitted to Muhimbili Medical Centre, Dar es Salaam, Tanzania. (i) Cerebral malaria (CM) with unrousable coma (n = 50), including 32 with complete recovery (CMCR) and 18 with death or neurological sequelae (CMDS); (ii) malaria with severe anaemia but without severe respiratory distress (SA; n = 6); (iii) uncomplicated malaria (UM; n = 37); (iv) asymptomatic parasitaemia (AP; n = 5); and (v) healthy controls (HC; n = 34). Mean methaemoglobin levels were elevated in all groups with malaria, forming up to 16.4% of circulating haemoglobin. The degree of methaemoglobinaemia correlated with disease severity and severity of anaemia. Mean methaemoglobin levels in children with AP, UM, SA, CMCR and CMDS were 3.3%, 4.1%, 5.6%, 4.7% and 5.8% respectively; the mean levels in those with clinical disease were significantly higher than those in healthy controls (2.0%). Methaemoglobinaemia > 10% was found in 5.4%, 16.7%, 12.5%, and 22.2% of those with UM, SA, CMCR and CMDS, respectively. In the presence of parasite sequestration, impaired tissue perfusion, and a reduction in oxygen carrying capacity of blood due to anaemia, a further reduction in oxygen carrying capacity from even a modest concentration of methaemoglobin is likely to exacerbate tissue hypoxia, perhaps critically so in a minority of anaemic and acidotic patients with severe falciparum malaria.

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.

Erythrocyte stages of Plasmodium falciparum exhibit a high nitric oxide synthase (NOS) activity and release an NOS-inducing soluble factor

Nitric oxide (NO), a highly diffusible cellular mediator involved in a wide range of biological effects, has been indicated as one of the cytotoxic agents released by leukocytes to counteract malaria infection. On the other hand, NO has been implicated as a mediator of the neuropathological symptoms of cerebral malaria. In such circumstances NO production has been thought to be induced in host tissues by host-derived cytokines. Here we provide evidence for the first time that human red blood cells infected by Plasmodium falciparum (IRBC) synthesize NO. The synthesis of NO (measured as citrulline and nitrate production) appeared to be very high in comparison with human endothelial cells; no citrulline and nitrate production was detectable in noninfected red blood cells. The NO synthase (NOS) activity was very high in the lysate of IRBC (while not measurable in that of normal red blood cells) and was inhibited in a dose-dependent way by three different NOS inhibitors (L-canavanine, NG-amino-L-arginine, and NG-nitro-L-arginine). NOS activity in P. falciparum IRBC is Ca++ independent, and the enzyme shows an apparent molecular mass < 100 kD, suggesting that the parasite expresses an isoform different from those found in mammalian cells. IRBC release a soluble factor able to induce NOS in human endothelial cells. Such NOS-inducing activity is not tissue specific, is time and dose dependent, requires de novo protein synthesis, and is probably associated with a thermolabile protein having a molecular mass > 100 kD. Our data suggest that an increased NO synthesis in P. falciparum malaria can be directly elicited by soluble factor(s) by the blood stages of the parasite, without necessarily requiring the intervention of host cytokines.

Serum nitrite plus nitrate in infection with human immunodeficiency virus type-1

To get a measure of the extent of induction of nitric oxide synthase in infection with human immunodeficiency virus type-1 (HIV-1) in vivo, we estimated serum nitrite plus nitrate concentrations in 110 HIV-1 infected individuals compared to 76 blood donors. To monitor cytokine action and to measure induction of pteridine synthesis, we determined in parallel neopterin, biopterin, soluble tumor necrosis factor-alpha receptor 55 and 75, and beta 2-microglobulin. Serum nitrite plus nitrate concentrations were elevated in patients as compared to blood donor controls. In sera of patients, nitrite plus nitrate levels correlated significantly with neopterin, soluble tumor necrosis factor receptor 55 and 75, and beta 2-microglobulin. Nitrite plus nitrate levels were higher and correlations were stronger in groups of patients with lower CD4+ cell count. These results suggest that cytokine-mediated nitric oxide synthesis occurs in individuals with HIV-1 infection.

In search of a function for tetrahydrobiopterin in the biosynthesis of nitric oxide

(6R)-5,6,7,8-Tetrahydro-L-biopterin(H4biopterin) is well known as a cofactor of enzymes that hydroxylate aromatic amino acids. More recent work has revealed an essential role of H4biopterin in the biosynthesis of nitric oxide (NO), an intercellular messenger molecule synthesized from L-arginine by different NO synthase isozymes in many species and tissues. While the function of H4biopterin in aromatic amino acid hydroxylation is well established, the role of this pteridine in NO synthesis is, as yet, elusive. Current experimental evidence hints at a dual mode of action of H4biopterin, involving both an allosteric effect on the NO synthase protein and participation as a reactant in L-arginine oxidation. As discussed in detail in the present article, the latter effect of this pteridine may be related to the protection of NO synthase from feedback inhibition by NO.

Nitric oxide in the liver: physiopathological roles

Many of the known roles of arginine (e.g. in immune function, wound healing, and protection against ammonia intoxication) are mediated by a metabolic pathway synthesising nitric oxide (NO) in the liver. Contrary to some of the current views, liver-produced NO may be basically beneficial, as it exerts both protective actions against tissue injury and cytotoxic effects on invading microorganisms, parasites, or tumor cells. An ongoing equilibrium between NO and other NO-reactive compounds (e.g. O2 and non-heme iron-sulphur-containing moieties) appears to be important in this respect, even under critical conditions. Thus, NO may prevent liver tissue harm from oxidant stress. Only when this putative counterbalance is upset by an uncontrolled, prolonged and/or massive production of NO, liver tissue damage may occur leading to hepatic inflammation or even tumor development. Moreover, the currently available data support the working hypothesis that hepatocytes partake not only to immunoregulatory processes, but even to immune defence mechanisms. Thus, the liver constitutes an excellent model for investigations into the crosstalks regulating the production of NO which take place among not only the various networks operating inside a single hepatic cell, but even the individual types of liver cells.

Interleukin 12 induction of interferon gamma-dependent protection against malaria

Intraperitoneal injection of recombinant Interleukin 12 (rIL-12) at 30 ng/day for 5 days beginning 1 to 2 days before sporozoite challenge or administration of a single dose of 150 ng of rIL-122 days before challenge protected 100% of BALB/c mice against challenge with 10(2) Plasmodium yoelii sporozoites. rIL-12-induced protection was eliminated in all mice by administration of a monoclonal antibody against interferon gamma and in 50% of mice by administration of NG-monomethyl-L-arginine, a competitive inhibitor of nitric oxide synthase. rIL-12 protected BALB/c mice treated with cytotoxic anti-CD4 and anti-CD8 monoclonal antibodies, as well as T-cell- and B-cell-deficient severe combined immunodeficiency mice. These data suggest that rIL-12 stimulates non-B, non-T cells to produce interferon gamma that kills intrahepatic parasites by stimulating nitric oxide production. If rIL-12 proves to be well tolerated by humans, our findings support consideration of rIL-12 as an immunoprophylactic against malaria.