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

Malaria blood stage infection suppresses liver stage infection via host-induced interferons but not hepcidin

Malaria-causing Plasmodium parasites first replicate as liver stages (LS), which then seed symptomatic blood stage (BS) infection. Emerging evidence suggests that these stages impact each other via perturbation of host responses, and this influences the outcome of natural infection. We sought to understand whether the parasite stage interplay would affect live-attenuated whole parasite vaccination, since the efficacy of whole parasite vaccines strongly correlates with their extend of development in the liver. We thus investigated the impact of BS infection on LS development of genetically attenuated and wildtype parasites in female rodent malaria models and observed that for both, LS infection suffered severe suppression during concurrent BS infection. Strikingly and in contrast to previously published studies, we find that the BS-induced iron-regulating hormone hepcidin is not mediating suppression of LS development. Instead, we demonstrate that BS-induced host interferons are the main mediators of LS developmental suppression. The type of interferon involved depended on the BS-causing parasite species. Our study provides important mechanistic insights into the BS-mediated suppression of LS development. This has direct implications for understanding the outcomes of live-attenuated Plasmodium parasite vaccination in malaria-endemic areas and might impact the epidemiology of natural malaria infection.

Immunogenicity, Efficacy, and Safety of a Novel Synthetic Microparticle Pre-Erythrocytic Malaria Vaccine in Multiple Host Species

We previously reported a protective antibody response in mice immunized with synthetic microparticle vaccines made using layer-by-layer fabrication (LbL-MP) and containing the conserved T1BT* epitopes from the P. falciparum circumsporozoite protein. To further optimize the vaccine candidate, a benchtop tangential flow filtration method (LbL-by-TFF) was developed and utilized to produce vaccine candidates that differed in the status of base layer crosslinking, inclusion of a TLR2 ligand in the antigenic peptide, and substitution of serine or alanine for an unpaired cysteine residue in the T* epitope. Studies in mice revealed consistent superiority of the Pam3Cys-modified candidates and a modest benefit of base layer crosslinking, as evidenced by higher and more persistent antibody titers (up to 18 months post-immunization), a qualitative improvement of T-cell responses toward a Th1 phenotype, and greater protection from live parasite challenges compared to the unmodified prototype candidate. Immunogenicity was also tested in a non-human primate model, the rhesus macaque. Base layer-crosslinked LbL-MP loaded with T1BT* peptide with or without covalently linked Pam3Cys elicited T1B-specific antibody responses and T1BT*-specific T-cell responses dominated by IFNγ secretion with lower levels of IL-5 secretion. The Pam3Cys-modified construct was more potent, generating antibody responses that neutralized wild-type P. falciparum in an in vitro hepatocyte invasion assay. IgG purified from individual macaques immunized with Pam3Cys.T1BT* LbL-MP protected naïve mice from challenges with transgenic P. berghei sporozoites that expressed the full-length PfCS protein, with 50-88% of passively immunized mice parasite-free for ≥15 days. Substitution of serine for an unpaired cysteine in the T* region of the T1BT* subunit did not adversely impact immune potency in the mouse while simplifying the manufacture of the antigenic peptide. In a Good Laboratory Practices compliant rabbit toxicology study, the base layer-crosslinked, Pam3Cys-modified, serine-substituted candidate was shown to be safe and immunogenic, eliciting parasite-neutralizing antibody responses and establishing the dose/route/regimen for a clinical evaluation of this novel synthetic microparticle pre-erythrocytic malaria vaccine candidate.

Does attenuated plasmodial sporozoite-mediated protection require peroxynitrite?

Attenuated plasmodial sporozoite-induced immune response includes intrahepatic nitric oxide (NO) production, which promotes apoptosis of infected hepatocytes and consequent parasite clearance. NO in excess reacts with superoxide, forming peroxynitrite, a powerful cytotoxic agent. Here, I suggest that peroxynitrite proapoptotic action may contribute to the attenuated malarial sporozoite-mediated protection.

The Complexity of Interferon Signaling in Host Defense against Protozoan Parasite Infection

Protozoan parasites, such as Plasmodium, Leishmania, Toxoplasma, Cryptosporidium, and Trypanosoma, are causative agents of health-threatening diseases in both humans and animals, leading to significant health risks and socioeconomic losses globally. The development of effective therapeutic and prevention strategies for protozoan-caused diseases requires a full understanding of the pathogenesis and protective events occurring in infected hosts. Interferons (IFNs) are a family of cytokines with diverse biological effects in host antimicrobial defense and disease pathogenesis, including protozoan parasite infection. Type II IFN (IFN-γ) has been widely recognized as the essential defense cytokine in intracellular protozoan parasite infection, whereas recent studies also revealed the production and distinct function of type I and III IFNs in host defense against these parasites. Decoding the complex network of the IFN family in host-parasite interaction is critical for exploring potential new therapeutic strategies against intracellular protozoan parasite infection. Here, we review the complex effects of IFNs on the host defense against intracellular protozoan parasites and the crosstalk between distinct types of IFN signaling during infections.

Nitric oxide in parasitic infections: a friend or foe?

The complex interaction between the host and the parasite remains a puzzling question. Control of parasitic infections requires an efficient immune response that must be balanced against destructive pathological consequences. Nitric oxide is a nitrogenous free radical which has many molecular targets and serves diverse functions. Apart from being a signaling messenger, nitric oxide is critical for controlling numerous infections. There is still controversy surrounding the exact role of nitric oxide in the immune response against different parasitic species. It proved protective against intracellular protozoa, as well as extracellular helminths. At the same time, it plays a pivotal role in stimulating detrimental pathological changes in the infected hosts. Several reports have discussed the anti-parasitic and immunoregulatory functions of nitric oxide, which could directly influence the control of the infection. Nevertheless, there is scarce literature addressing the harmful cytotoxic impacts of this mediator. Thus, this review provides insights into the most updated concepts and controversies regarding the dual nature and opposing sides of nitric oxide during the course of different parasitic infections.

Vaccination With Sporozoites: Models and Correlates of Protection

Despite continuous efforts, the century-old goal of eradicating malaria still remains. Multiple control interventions need to be in place simultaneously to achieve this goal. In addition to effective control measures, drug therapies and insecticides, vaccines are critical to reduce mortality and morbidity. Hence, there are numerous studies investigating various malaria vaccine candidates. Most of the malaria vaccine candidates are subunit vaccines. However, they have shown limited efficacy in Phase II and III studies. To date, only whole parasite formulations have been shown to induce sterile immunity in human. In this article, we review and discuss the recent developments in vaccination with sporozoites and the mechanisms of protection involved.

Cellular immune response to DNA and vaccinia prime-boost immunization kills Plasmodium yoelii-infected hepatocytes in vitro

Background

Plasmid DNA encoding Plasmodium yoelii circumsporozoite protein (PyCSP) followed by boosting with recombinant vaccinia virus containing the PyCSP elicited significant protective immunity in mice that was primarily mediated by CD8+ T-cell responses directed to P. yoelii -infected hepatocytes. This study was to further explore protection using in vitro cultures of P. yoelii parasites in mouse hepatocytes. Spleen cells from DNA/vaccinia virus-immunized mice were co-cultured in vitro with mouse hepatocytes containing developing P. yoelii liver stage parasites. A semipermeable membrane separating spleen cells and hepatocytes was used to demonstrate if cell-to-cell contact was required. Inhibitors of mediators likely involved in spleen cell killing were added to these co-cultures. Spleen cells from immunized mice inhibited in vitro P. yoelii parasite development, and inhibition was eliminated by separating effectors and targets with the semipermeable membrane. Additionally, inhibitors of inducible nitric oxide synthase, caspase activation, NF-κB activation as well as antibodies against interferon-gamma (IFN-γ) and ICAM-1 reduced parasite inhibition. These findings suggest that direct contact between spleen cells from immunized mice and P. yoelii-infected hepatocytes is required for eliminating liver stage parasites and provide more insight into CD8+ T-cell-mediated inhibition of malaria liver stage development.

Malaria Parasites: The Great Escape

Parasites of the genus Plasmodium have a complex life cycle. They alternate between their final mosquito host and their intermediate hosts. The parasite can be either extra- or intracellular, depending on the stage of development. By modifying their shape, motility, and metabolic requirements, the parasite adapts to the different environments in their different hosts. The parasite has evolved to escape the multiple immune mechanisms in the host that try to block parasite development at the different stages of their development. In this article, we describe the mechanisms reported thus far that allow the Plasmodium parasite to evade innate and adaptive immune responses.

Plasmodium cellular effector mechanisms and the hepatic microenvironment

Plasmodium falciparum malaria remains one of the most serious health problems globally. Immunization with attenuated parasites elicits multiple cellular effector mechanisms capable of eliminating Plasmodium liver stages. However, malaria liver stage (LS) immunity is complex and the mechanisms effector T cells use to locate the few infected hepatocytes in the large liver in order to kill the intracellular LS parasites remain a mystery to date. Here, we review our current knowledge on the behavior of CD8 effector T cells in the hepatic microvasculature, in malaria and other hepatic infections. Taking into account the unique immunological and lymphogenic properties of the liver, we discuss whether classical granule-mediated cytotoxicity might eliminate infected hepatocytes via direct cell contact or whether cytokines might operate without cell–cell contact and kill Plasmodium LSs at a distance. A thorough understanding of the cellular effector mechanisms that lead to parasite death hence sterile protection is a prerequisite for the development of a successful malaria vaccine to protect the 40% of the world’s population currently at risk of Plasmodium infection.

Antigen-driven focal inflammatory death of malaria liver stages

Multiple immunizations using live irradiated sporozoites, the infectious plasmodial stage delivered into the host skin during a mosquito bite, can elicit sterile immunity to malaria. CD8⁺ T cells seem to play an essential role in this protective immunity, since their depletion consistently abolishes sterilizing protection in several experimental models. So far, only a few parasite antigens are known to induce CD8⁺ T cell-dependent protection, but none of them can reach the levels of protection afforded by live attenuated parasites. Systematic attempts to identify novel antigens associated with this efficient cellular protection were so far unsuccessful. In addition, the precise mechanisms involved in the recognition and elimination of parasitized hepatocytes in vivo by CD8⁺ T cells still remain obscure. Recently, it has been shown that specific effector CD8⁺ T cells, after recognition of parasitized hepatocytes, recruit specific and non-specific activated CD8⁺ T cells to the site of infection, resulting in the formation of cellular clusters around and in the further elimination of intracellular parasites. The significance of this finding is discussed in the perspective of a general mechanism of antigen-dependent focalized inflammation and its consequences for the elimination of malaria liver stages.

In vivo, in vitro, and in silico studies suggest a conserved immune module that regulates malaria parasite transmission from mammals to mosquitoes

Human malaria can be caused by the parasite Plasmodium falciparum that is transmitted by female Anopheles mosquitoes. "Immunological crosstalk" between the mammalian and anopheline hosts for Plasmodium functions to control parasite numbers. Key to this process is the mammalian cytokine transforming growth factor-β1 (TGF-β1). In mammals, TGF-β1 regulates inducible nitric oxide (NO) synthase (iNOS) both positively and negatively. In some settings, high levels of NO activate latent TGF-β1, which in turn suppresses iNOS expression. In the mosquito, ingested TGF-β1 induces A. stephensi NOS (AsNOS), which limits parasite development and which in turn is suppressed by activation of the mosquito homolog of the mitogen-activated protein kinases MEK and ERK. Computational models linking TGF-β1, AsNOS, and MEK/ERK were developed to provide insights into this complex biology. An initial Boolean model suggested that, as occurs in mammalian cells, MEK/ERK and AsNOS would oscillate upon ingestion of TGF-β1. An ordinary differential equation (ODE) model further supported the hypothesis of TGF-β1-induced multiphasic behavior of MEK/ERK and AsNOS. To achieve this multiphasic behavior, the ODE model was predicated on the presence of constant levels of TGF-β1 in the mosquito midgut. Ingested TGF-β1, however, did not exhibit this behavior. Accordingly, we hypothesized and experimentally verified that ingested TGF-β1 induces the expression of the endogenous mosquito TGF-β superfamily ligand As60A. Computational simulation of these complex, cross-species interactions suggested that TGF-β1 and NO-mediated induction of As60A expression together may act to maintain multiphasic AsNOS expression via MEK/ERK-dependent signaling. We hypothesize that multiphasic behavior as represented in this model allows the mosquito to balance the conflicting demands of parasite killing and metabolic homeostasis in the face of damaging inflammation.

Natural acquisition of immunity to Plasmodium vivax: epidemiological observations and potential targets

Population studies show that individuals acquire immunity to Plasmodium vivax more quickly than Plasmodium falciparum irrespective of overall transmission intensity, resulting in the peak burden of P. vivax malaria in younger age groups. Similarly, actively induced P. vivax infections in malaria therapy patients resulted in faster and generally more strain-transcending acquisition of immunity than P. falciparum infections. The mechanisms behind the more rapid acquisition of immunity to P. vivax are poorly understood. Natural acquired immune responses to P. vivax target both pre-erythrocytic and blood-stage antigens and include humoral and cellular components. To date, only a few studies have investigated the association of these immune responses with protection, with most studies focussing on a few merozoite antigens (such as the Pv Duffy binding protein (PvDBP), the Pv reticulocyte binding proteins (PvRBPs), or the Pv merozoite surface proteins (PvMSP1, 3 & 9)) or the circumsporozoite protein (PvCSP). Naturally acquired transmission-blocking (TB) immunity (TBI) was also found in several populations. Although limited, these data support the premise that developing a multi-stage P. vivax vaccine may be feasible and is worth pursuing.

Chickens treated with a nitric oxide inhibitor became more resistant to Plasmodium gallinaceum infection due to reduced anemia, thrombocytopenia and inflammation

Malaria is a serious infectious disease caused by parasites of the Plasmodium genus that affect different vertebrate hosts. Severe malaria leads to host death and involves different pathophysiological phenomena such as anemia, thrombocytopenia and inflammation. Nitric oxide (NO) is an important effector molecule in this disease, but little is known about its role in avian malaria models. Plasmodium gallinaceum-infected chickens were treated with aminoguanidine (AG), an inhibitor of inducible nitric oxide synthase, to observe the role of NO in the pathogenesis of this avian model. AG increased the survival of chickens, but also induced higher parasitemia. Treated chickens demonstrated reduced anemia and thrombocytopenia. Moreover, erythrocytes at different stages of maturation, heterophils, monocytes and thrombocytes were infected by Plasmodium gallinaceum and animals presented a generalized leucopenia. Activated leukocytes and thrombocytes with elongated double nuclei were observed in chickens with higher parasitemia; however, eosinophils were not involved in the infection. AG reduced levels of hemozoin in the spleen and liver, indicating lower inflammation. Taken together, the results suggest that AG reduced anemia, thrombocytopenia and inflammation, explaining the greater survival rate of the treated chickens.

Interferon-inducible effector mechanisms in cell-autonomous immunity

Interferons (IFNs) induce the expression of hundreds of genes as part of an elaborate antimicrobial programme designed to combat infection in all nucleated cells - a process termed cell-autonomous immunity. As described in this Review, recent genomic and subgenomic analyses have begun to assign functional properties to novel IFN-inducible effector proteins that restrict bacteria, protozoa and viruses in different subcellular compartments and at different stages of the pathogen life cycle. Several newly described host defence factors also participate in canonical oxidative and autophagic pathways by spatially coordinating their activities to enhance microbial killing. Together, these IFN-induced effector networks help to confer vertebrate host resistance to a vast and complex microbial world.

Interferon-γ, a valuable surrogate marker of Plasmodium falciparum pre-erythrocytic stages protective immunity

Immunity against the pre-erythrocytic stages of malaria is the most promising, as it is strong and fully sterilizing. Yet, the underlying immune effectors against the human Plasmodium falciparum pre-erythrocytic stages remain surprisingly poorly known and have been little explored, which in turn prevents any rational vaccine progress. Evidence that has been gathered in vitro and in vivo, in higher primates and in humans, is reviewed here, emphasizing the significant role of IFN-γ, either as a critical immune mediator or at least as a valuable surrogate marker of protection. One may hope that these results will trigger investigations in volunteers immunized either by optimally irradiated or over-irradiated sporozoites, to quickly delineate better surrogates of protection, which are essential for the development of a successful malaria vaccine.

Naturally occurring triggers that induce apoptosis-like programmed cell death in Plasmodium berghei ookinetes

Several protozoan parasites have been shown to undergo a form of programmed cell death that exhibits morphological features associated with metazoan apoptosis. These include the rodent malaria parasite, Plasmodium berghei. Malaria zygotes develop in the mosquito midgut lumen, forming motile ookinetes. Up to 50% of these exhibit phenotypic markers of apoptosis; as do those grown in culture. We hypothesised that naturally occurring signals induce many ookinetes to undergo apoptosis before midgut traversal. To determine whether nitric oxide and reactive oxygen species act as such triggers, ookinetes were cultured with donors of these molecules. Exposure to the nitric oxide donor SNP induced a significant increase in ookinetes with condensed nuclear chromatin, activated caspase-like molecules and translocation of phosphatidylserine that was dose and time related. Results from an assay that detects the potential-dependent accumulation of aggregates of JC-1 in mitochondria suggested that nitric oxide does not operate via loss of mitochondrial membrane potential. L-DOPA (reactive oxygen species donor) also caused apoptosis in a dose and time dependent manner. Removal of white blood cells significantly decreased ookinetes exhibiting a marker of apoptosis in vitro. Inhibition of the activity of nitric oxide synthase in the mosquito midgut epithelium using L-NAME significantly decreased the proportion of apoptotic ookinetes and increased the number of oocysts that developed. Introduction of a nitric oxide donor into the blood meal had no effect on mosquito longevity but did reduce prevalence and intensity of infection. Thus, nitric oxide and reactive oxygen species are triggers of apoptosis in Plasmodium ookinetes. They occur naturally in the mosquito midgut lumen, sourced from infected blood and mosquito tissue. Up regulation of mosquito nitric oxide synthase activity has potential as a transmission blocking strategy.

Tomatine adjuvantation of protective immunity to a major pre-erythrocytic vaccine candidate of malaria is mediated via CD8+ T cell release of IFN-gamma

The glycoalkaloid tomatine, derived from the wild tomato, can act as a powerful adjuvant to elicit an antigen-specific cell-mediated immune response to the circumsporozoite (CS) protein, a major pre-erythrocytic stage malaria vaccine candidate antigen. Using a defined MHC-class-I-restricted CS epitope in a Plasmodium berghei rodent model, antigen-specific cytotoxic T lymphocyte activity and IFN-γ secretion ex vivo were both significantly enhanced compared to responses detected from similarly stimulated splenocytes from naive and tomatine-saline-immunized mice. Further, through lymphocyte depletion it is demonstrated that antigen-specific IFN-γ is produced exclusively by the CD8⁺ T cell subset. We conclude that the processing of the P. berghei CS peptide as an exogenous antigen and its presentation via MHC class I molecules to CD8⁺ T cells leads to an immune response that is an in vitro correlate of protection against pre-erythrocytic malaria. Further characterization of tomatine as an adjuvant in malaria vaccine development is indicated.

Natural cocoa as diet-mediated antimalarial prophylaxis

BACKGROUND

The Maya of Central America are credited with the first consumption of cocoa and maintaining its ancient Olmec name kakawa translated in English as "God Food", in recognition of its multiple health benefits. The legend of cocoa is receiving renewed attention in recent years, on account of epidemiological and scientific studies that support its cardiovascular health benefits. Increasing numbers of scientific reports corroborating cocoa's antiquated reputation as health food persuaded this author to promote regular consumption of cocoa in Ghana since 2004. Cocoa is readily available in Ghana; the country is the second largest producer accounting for 14% of the world's output. Numerous anecdotal reports of reduced episodic malaria in people who daily drink natural unsweetened cocoa beverage prompted a search for scientific mechanisms that possibly account for cocoa's antimalarial effects. This paper presents the outcome as a hypothesis.

METHODS

Internet search for literature on effects of cocoa's ingredients on malaria parasites and illness using a variety of search tools.

RESULTS

Evidential literature suggests five mechanisms that possibly underpin cocoa's anecdotal antimalarial effects. (i) Increased availability of antioxidants in plasma, (ii) membrane effects in general and erythrocyte membrane in particular, (iii) increased plasma levels of nitric oxide, (iv) antimalarial activity of cocoa flavanoids and their derivatives, and (v) boosted immune system mediated by components of cocoa including cocoa butter, polyphenols, magnesium, and zinc.

CONCLUSION

A hypothesis is formulated that cocoa offers a diet-mediated antimalarial prophylaxis; and an additional novel tool in the fight against the legendary scourge.

A retrospective evaluation of the role of T cells in the development of malaria vaccine

Due to the fact that the life cycle of malaria parasites is complex, undergoing both an extracellular and intracellular phases in its host, the human immune system has to mobilize both the humoral and cellular arms of immune responses to fight against this parasitic infection. Whereas humoral immunity is directed toward the extracellular stages which include sporozoites and merozoites, cell-mediated immunity (CMI), in which T cells play a major role, targets hepatic stages - liver stages - of the parasites. In this review, the role of T cells in protective immunity against liver stages of the malaria infection is being re-evaluated. Furthermore, this review intends to address how to translate the findings regarding the role of T cells obtained in experimental systems to actual development of malaria vaccine for humans.

The CCTTT pentanucleotide microsatellite in iNOS promoter influences the clinical outcome in P. falciparum infection

To assess the hypothesis that nitric oxide (NO) is critical in the pathogenesis of cerebral malaria, we analyzed those single nucleotide polymorphisms (SNPs) and microsatellite (MS) of the promoter region of inducible nitric oxide synthase (iNOS) gene which are known to enhance the NO production in vivo. A total of 428 (204 severe, 224 mild) adult patients living in the eastern part of India were analyzed. The single nucleotide substitutions -954G-->C was found to be very rare, and -1173C-->T was absent in this population. But interestingly, longer forms of MS were found to be significantly associated with severe malaria (OR = 2.89, 95% CI = 1.955-4.295, P < 0.0001), and the linear regression analysis revealed that the risk of severe malaria significantly increases as the summed repeat number in an individual increase (OR = 1.16, P = 0.0013). Further, the median plasma level of nitrate/nitrite (NOx) was observed to be high in mild patients compared to severe patients, and the level of parasitemia was significantly low among mild patients than severe ones. These findings suggest that the CCTTT repeats in iNOS may play a key role in the pathogenesis of severe malaria.

Sporozoite-mediated hepatocyte wounding limits Plasmodium parasite development via MyD88-mediated NF-kappa B activation and inducible NO synthase expression

Plasmodium sporozoites traverse several host cells before infecting hepatocytes. In the process, the plasma membranes of the cells are ruptured, resulting in the release of cytosolic factors into the microenvironment. This released endogenous material is highly stimulatory/immunogenic and can serve as a danger signal initiating distinct responses in various cells. Thus, our study aimed at characterizing the effect of cell material leakage during Plasmodium infection on cultured mouse primary hepatocytes and HepG2 cells. We observed that wounded cell-derived cytosolic factors activate NF-kappaB, a main regulator of host inflammatory responses, in cells bordering wounded cells, which are potential host cells for final parasite infection. This activation of NF-kappaB occurred shortly after infection and led to a reduction of infection load in a time-dependent manner in vitro and in vivo, an effect that could be reverted by addition of the specific NF-kappaB inhibitor BAY11-7082. Furthermore, no NF-kappaB activation was observed when Spect(-/-) parasites, which are devoid of hepatocyte traversing properties, were used. We provide further evidence that NF-kappaB activation causes the induction of inducible NO synthase expression in hepatocytes, and this is, in turn, responsible for a decrease in Plasmodium-infected hepatocytes. Furthermore, primary hepatocytes from MyD88(-/-) mice showed no NF-kappaB activation and inducible NO synthase expression upon infection, suggesting a role of the Toll/IL-1 receptor family members in sensing cytosolic factors. Indeed, lack of MyD88 significantly increased infection in vitro and in vivo. Thus, host cell wounding due to parasite migration induces inflammation which limits the extent of parasite infection.

Effect of active hexose correlated compound on the production of nitric oxide in hepatocytes

BACKGROUND

Active hexose correlated compound (AHCC) is a "complex compound" containing polysaccharides. AHCC has been reported to improve the prognosis of postoperative hepatocellular carcinoma patients. However, the molecular mechanism of this improvement is not fully understood. In the diseased liver, nitric oxide (NO) generated by inducible nitric oxide synthase (iNOS) is considered to be a causal factor for various hepatopathies. In this study, the possibility of AHCC regulation of NO production by iNOS was pursued as a potential liver-protecting mechanism.

METHODS

Primary cultured rat hepatocytes were treated with interleukin-1beta (IL-1beta) in the presence or absence of AHCC. NO production, iNOS induction, and iNOS signal were analyzed.

RESULTS

IL-1beta stimulated iNOS induction through the activation of nuclear factor kappaB (NFkappaB), leading to NO production. The addition of AHCC inhibited NO production, showing >80% inhibition at 8 mg/mL. AHCC also decreased the levels of iNOS protein and mRNA. However, AHCC influenced neither the degradation of inhibitory protein kappaB (IkappaB) nor the activation of NFkappaB stimulated by IL-1beta. Transfection experiments with an iNOS promoter-luciferase construct (iNOS-Luc) revealed that AHCC had no effect on the transactivation activity of the iNOS promoter. By contrast, AHCC inhibited the activity of iNOS-Luc containing a 3'untranslated region (UTR) with adenosine and uridine (AU)-rich elements, which shows the stabilizing activity of iNOS mRNA.

CONCLUSIONS

Results indicated that AHCC inhibits the induction of iNOS at the level of transcription, causing a decrease in NO production in hepatocytes. AHCC seems to decrease the levels of iNOS mRNA by reducing mRNA stabilization rather than inhibiting its synthesis.

Low levels of mammalian TGF-beta1 are protective against malaria parasite infection, a paradox clarified in the mosquito host

Nitric oxide (NO), derived from catalysis of inducible NO synthase (iNOS), limits malaria parasite growth in mammals. Transforming growth factor (TGF)-beta1 suppresses iNOS in cells in vitro as well as in vivo in mice, but paradoxically severe malaria in humans is associated with low levels of TGF-beta1. We hypothesized that this paradox is a universal feature of infection and occurs in the mosquito Anopheles stephensi, an invertebrate host for Plasmodium that also regulates parasite development with inducible NO synthase (AsNOS). We show that exogenous human TGF-beta1 dose-dependently regulates mosquito AsNOS expression and that parasite killing by low dose TGF-beta1 depends on AsNOS catalysis. Furthermore, induction of AsNOS expression by TGF-beta1 is regulated by NO synthesis. These results suggest that TGF-beta1 plays similar roles during parasite infection in mammals and mosquitoes and that this role is linked to the effects of TGF-beta1 on inducible NO synthesis.

Age-dependent effect of plasma nitric oxide on parasite density in Ghanaian children with severe malaria

Nitric oxide (NO) has toxic properties against Plasmodium falciparum. While high blood levels have been associated with protection against severe malarial disease, they may also contribute to the pathophysiology of cerebral malaria and severe anaemia. Promoter variants in the inducible nitric oxide synthase (iNOS) gene have been shown to influence NO concentrations and disease manifestation. However, findings are conflicting. We examined associations of plasma NO metabolites (NOx) with symptoms of severe malaria, particularly malarial anaemia and cerebral malaria, and with iNOS promoter variants. In 210 Ghanaian children with severe malaria, we measured plasma nitrite, nitrate, and S-nitrosothiol, and genotyped the iNOS promoter variants -954G-->C, -1173C-->T, and the -2.5 kb (CCTTT)(n) microsatellite. NOx levels decreased with age. In young children (<24 months), high NOx was associated with reduced parasite density. This was not seen in patients of 24-48 months of age and reversed in older children. Subgroup analysis revealed that in children with severe anaemia but without cerebral involvement (prostration, impaired consciousness, convulsions), high NOx levels correlated with low parasitaemia (P = 0.02). In these children, elevated NOx levels were also associated with the iNOS-954C-->T/(CCTTT)(8) haplotype (P = 0.03). No association between NOx or iNOS genotypes and cerebral malaria was observed. Our findings suggest that in young children with severe malaria NOx reduces parasitaemia. This effect wanes at higher ages and may reflect a predominance of unspecific immune responses to infection in early childhood. This finding may have importance for the understanding of associations between iNOS variants and severe malaria in regions of differing disease manifestation.

Induction of nitric oxide synthase in Anopheles stephensi by Plasmodium falciparum: mechanism of signaling and the role of parasite glycosylphosphatidylinositols

Malaria parasite (Plasmodium spp.) infection in the mosquito Anopheles stephensi induces significant expression of A. stephensi nitric oxide synthase (AsNOS) in the midgut epithelium as early as 6 h postinfection and intermittently thereafter. This induction results in the synthesis of inflammatory levels of nitric oxide (NO) in the blood-filled midgut that adversely impact parasite development. In mammals, P. falciparum glycosylphosphatidylinositols (PfGPIs) can induce NOS expression in immune and endothelial cells and are sufficient to reproduce the major effects of parasite infection. These effects are mediated in part by mimicry of insulin signaling by PfGPIs. In this study, we demonstrate that PfGPIs can induce AsNOS expression in A. stephensi cells in vitro and in the midgut epithelium in vivo. Signaling by P. falciparum merozoites and PfGPIs is mediated through A. stephensi Akt/protein kinase B and a pathway involving DSOR1, a mitogen-activated protein kinase kinase, and an extracellular signal-regulated kinase. However, despite the involvement of kinases that are also associated with insulin signaling in A. stephensi cells, signaling by P. falciparum and by PfGPIs is distinctively different from signaling by insulin. Therefore, although mimicry of insulin by PfGPIs appears to be restricted to mammalian hosts of P. falciparum, the conservation of PfGPIs as a prominent parasite-derived signal of innate immunity can now be extended to include Anopheles mosquitoes, indicating that parasite signaling of innate immunity is conserved in mosquito and mammalian cells.

Pirfenidone inhibits the induction of iNOS stimulated by interleukin-1beta at a step of NF-kappaB DNA binding in hepatocytes

BACKGROUND/AIMS

Pirfenidone has antiinflammatory effects in animals with endotoxemia. We reported that pirfenidone inhibits the enhancement of inflammatory cytokines and inducible nitric oxide synthase (iNOS) in liver of endotoxin-treated rats, leading to the prevention of hepatic injury. However, the mechanisms involved in suppression of these gene inductions are obscure. Studies were performed to investigate whether pirfenidone directly influences iNOS induction in hepatocytes.

METHODS

Cultured hepatocytes were treated with interleukin-1beta (IL-1beta) in the presence and absence of pirfenidone, and iNOS induction and its signal including NF-kappaB were analyzed.

RESULTS

Pirfenidone inhibited the induction of iNOS mRNA and protein, resulting in the decrease of nitric oxide production. Gel shift assay demonstrated that pirfenidone inhibited the activation of NF-kappaB. Consistent with this observation, transfection experiments revealed that pirfenidone decreased transcriptional activation of iNOS gene promoter. In contrast, pirfenidone had no effect on the degradation of IkappaB, and could not prevent nuclear translocation of p50/p65. Finally, pirfenidone inhibited the activation of Akt and up-regulation of IL-1 receptor stimulated by IL-1beta.

CONCLUSIONS

Results indicate that pirfenidone inhibits the induction of iNOS gene expression at a step of NF-kappaB DNA binding, but not its nuclear translocation, partly through the inhibition of IL-1 receptor induction in hepatocytes.

A small peptide (CEL-1000) derived from the beta-chain of the human major histocompatibility complex class II molecule induces complete protection against malaria in an antigen-independent manner

CEL-1000 (DGQEEKAGVVSTGLIGGG) is a novel potential preventative and therapeutic agent. We report that CEL-1000 confers a high degree of protection against Plasmodium sporozoite challenge in a murine model of malaria, as shown by the total absence of blood stage infection following challenge with 100 sporozoites (100% protection) and by a substantial reduction (400-fold) of liver stage parasite RNA following challenge with 50,000 sporozoites. CEL-1000 protection was demonstrated in A/J (H-2(a)) and C3H/HeJ (H-2(k)) mice but not in BALB/c (H-2(d)) or CAF1 (A/J x BALB/c F(1) hybrid) mice. In CEL-1000-treated and protected mice, high levels of gamma interferon (IFN-gamma) in serum and elevated frequencies of hepatic and splenic CD4+ IFN-gamma-positive T cells were detected 24 h after administration of an additional dose of CEL-1000. Treatment of A/J mice that received CEL-1000 with antibodies against IFN-gamma just prior to challenge abolished the protection, and a similar treatment with antibodies against CD4+ T cells partially reduced the level of protection, while treatment with control antibodies or antibodies specific for interleukin-12 (IL-12), CD8+ T cells, or NK cells had no effect. Our data establish that the protection induced by CEL-1000 is dependent on IFN-gamma and is partially dependent on CD4+ T cells but is independent of CD8+ T cells, NK cells, and IL-12 at the effector phase and does not induce a detectable antibody response.

Mammalian transforming growth factor beta1 activated after ingestion by Anopheles stephensi modulates mosquito immunity

During the process of bloodfeeding by Anopheles stephensi, mammalian latent transforming growth factor beta1 (TGF-beta1) is ingested and activated rapidly in the mosquito midgut. Activation may involve heme and nitric oxide (NO), agents released in the midgut during blood digestion and catalysis of L-arginine oxidation by A. stephensi NO synthase (AsNOS). Active TGF-beta1 persists in the mosquito midgut to extended times postingestion and is recognized by mosquito cells as a cytokine. In a manner analogous to the regulation of vertebrate inducible NO synthase and malaria parasite (Plasmodium) infection in mammals by TGF-beta1, TGF-beta1 regulates AsNOS expression and Plasmodium development in A. stephensi. Together, these observations indicate that, through conserved immunological cross talk, mammalian and mosquito immune systems interface with each other to influence the cycle of Plasmodium development.

Association of interferon-gamma responses to pre-erythrocytic stage vaccine candidate antigens of Plasmodium falciparum in young Kenyan children with improved hemoglobin levels: XV. Asembo Bay Cohort Project

Previous studies in animal models have revealed an association between interferon-gamma (IFN-gamma), produced by CD8+ T cells and irradiated sporozoite-induced sterile immunity. To determine whether IFN-gamma can serve as a marker of pre-erythrocytic protective immunity in individuals naturally exposed to malaria, we characterized IFN-gamma and lymphocyte proliferative responses to previously defined CD8+ cytotoxic T lymphocyte (CTL) epitopes from six pre-erythrocytic stage antigens in 107 children six months to two years old from a community-based birth cohort in western Kenya. We found that IFN-gamma positive responders had higher hemoglobin (Hb) levels and significantly reduced prevalence of severe malarial anemia one month after the test compared with IFN-gamma non-responders, suggesting that IFN-gamma immune responses to these pre-erythrocytic antigens were associated with protection against malarial anemia. Children who responded by lymphocyte proliferation had a significantly longer time to first documented malaria parasitemia after birth; however, there was no correlation between the presence of lymphocyte proliferative response and higher Hb levels. We propose that IFN-gamma production could be used as a potential marker of protective immunity against malaria associated anemia in young children living in malaria holoendemic areas.

Potentiation by a novel alkaloid glycoside adjuvant of a protective cytotoxic T cell immune response specific for a preerythrocytic malaria vaccine candidate antigen

We have recently demonstrated that the novel glycoalkaloid tomatine, derived from leaves of the wild tomato Lycopersicon pimpinellifolium, can act as a powerful adjuvant for the elicitation of antigen-specific CD8+ T cell responses. Here, we have extended our previous investigation with the model antigen ovalbumin to an established malaria infection system in mice and evaluated the cellular immune response to a major preerythrocytic stage malaria vaccine candidate antigen when administered with tomatine. The defined MHC H-2kd class I-binding 9-mer peptide (amino acids 252-260) from Plasmodium berghei circumsporozoite (CS) protein was prepared with tomatine to form a molecular aggregate formulation and this used to immunise BALB/c (H-2kd) mice. Antigen-specific IFN-gamma secretion and cytotoxic T lymphocyte activity in vitro were both significantly enhanced compared to responses detected from similarly stimulated splenocytes from naive and tomatine-saline-immunised control mice. Moreover, when challenged with P. berghei sporozoites, mice immunised with the CS 9-mer-tomatine preparation had a significantly delayed onset of erythrocytic infection compared to controls. The data presented validate the use of tomatine to potentiate a cellular immune response to antigenic stimulus by testing in an important biologically relevant system. Specifically, the processing of the P. berghei CS 9-mer as an exogenous antigen and its presentation via MHC class I molecules to CD8+ T cells led to an immune response that is an in vitro correlate of protection against preerythrocytic malaria. This was confirmed by the protective capacity of the 9-mer-tomatine combination upon in vivo immunisation. These findings merit further work to optimise the use of tomatine as an adjuvant in malaria vaccine development.

Protein kinase A- and C-dependent modulation of murine inducible nitric oxide synthase

Effects of pharmacological modulation of protein kinase A, C and G (PKA, PKC and PKG) were examined on inducible form of nitric oxide synthase (iNOS) expressed in COS cells to elucidate regulatory mechanism of iNOS by protein kinases. Formation of nitric oxide (NO), as an index of NOS activity, was assessed by measurement of nitrite in incubation medium in long term observation and by hemoglobin assay method in kinetic study. In long term observation (18 hours), activation of PKA by 8-Br-cAMP increased NO formation that was inhibited by N-(2-[p-bromocinnamylamino] ethyl)-5-isoquinolinesulfonamide (H89). Though activation of PKC by 12-O-tetradecanoyl phorbol-13-acetate (TPA) decreased NO formation, PKC inhibitor, chelerythrine, failed to inhibit the decrease. Activation of PKG with 8-Br-cGMP and inhibition with KT5823 resulted in no change in NO formation. Western blot analysis revealed that neither 8-Br-cAMP nor TPA affect iNOS expression. In kinetic study (short term perfusion study), no change in NO formation was observed by 8-Br-cAMP and TPA. These results indicate that, in living cells, PKG does not play a regulatory role in iNOS activity and that PKA and PKC do not directly modulate iNOS activity. However, PKA and PKC would possibly modify NOS activity indirectly via cofactors necessary for NO formation.

Interleukin-12- and gamma interferon-dependent protection against malaria conferred by CpG oligodeoxynucleotide in mice

Unmethylated CpG dinucleotides in bacterial DNA or synthetic oligodeoxynucleotides (ODNs) cause B-cell proliferation and immunoglobulin secretion, monocyte cytokine secretion, and activation of natural killer (NK) cell lytic activity and gamma interferon (IFN-gamma) secretion in vivo and in vitro. The potent Th1-like immune activation by CpG ODNs suggests a possible utility for enhancing innate immunity against infectious pathogens. We therefore investigated whether the innate immune response could protect against malaria. Treatment of mice with CpG ODN 1826 (TCCATGACGTTCCTGACGTT, with the CpG dinucleotides underlined) or 1585 (ggGGTCAACGTTGAgggggG, with g representing diester linkages and phosphorothioate linkages being to the right of lowercase letters) in the absence of antigen 1 to 2 days prior to challenge with Plasmodium yoelii sporozoites conferred sterile protection against infection. A higher level of protection was consistently induced by CpG ODN 1826 compared with CpG ODN 1585. The protective effects of both CpG ODNs were dependent on interleukin-12, as well as IFN-gamma. Moreover, CD8+ T cells (but not CD4+ T cells), NK cells, and nitric oxide were implicated in the CpG ODN 1585-induced protection. These data establish that the protective mechanism induced by administration of CpG ODN 1585 in the absence of parasite antigen is similar in nature to the mechanism induced by immunization with radiation-attenuated P. yoelii sporozoites or with plasmid DNA encoding preerythrocytic-stage P. yoelii antigens. We were unable to confirm whether CD8+ T cells, NK cells, or nitric oxide were required for the CpG ODN 1826-induced protection, but this may reflect differences in the potency of the ODNs rather than a real difference in the mechanism of action of the two ODNs. This is the first report that stimulation of the innate immune system by CpG immunostimulatory motifs can confer sterile protection against malaria.

Expression, extracellular secretion, and immunogenicity of the Plasmodium falciparum sporozoite surface protein 2 in Salmonella vaccine strains

Deleting transmembrane alpha-helix motifs from Plasmodium falciparum sporozoite surface protein (SSP-2) allowed its secretion from Salmonella enterica serovar Typhimurium SL3261 and S. enterica serovar Typhi CVD 908-htrA by the Hly type I secretion system. In mice immunized intranasally, serovar Typhimurium constructs secreting SSP-2 stimulated greater gamma interferon splenocyte responses than did nonsecreting constructs (P = 0.04).

Major hematologic diseases in the developing world- new aspects of diagnosis and management of thalassemia, malarial anemia, and acute leukemia

The three presentations in this session encompass clinical, pathophysiological and therapeutic aspects of hematologic diseases which impact most heavily on developing world countries. Dr. Victor Gordeuk discusses new insights regarding the multi-faceted pathogenesis of anemia in the complicated malaria occurring in Africa. He describes recent investigations indicating the possible contribution of immune dysregulation to this serious complication and the implications of these findings for disease management. Dr. Surapol Issaragrisil and colleagues describe epidemiologic and clinical characteristics of the thalassemic syndromes. In addition to being considered a major health problem in Southeast Asia, the migration throughout the world of people from this region has caused the disease to have global impact. A unique thalassemia variant, Hb Ebeta-thalassemia, with distinctive clinical features, has particular relevance for this demographic issue. Special focus will be reported regarding recent prenatal molecular screening methods in Thailand which have proven useful for early disease detection and disease control strategies. Dr. Raul Ribeiro describes a clinical model for providing effective treatment for a complex malignancy (childhood acute lymphoblastic leukemia) in countries with limited resources. With the multidisciplinary approach in Central American of the joint venture between St. Jude Children's Research Hospital International Outreach Program and indigenous health care personnel, major therapeutic advances for this disease have been achieved. Given the major demographic population shifts occurring worldwide, these illnesses also have important clinical implications globally. These contributions demonstrate that lessons learned within countries of disease prevalence aid our understanding and management of a number of disorders prominently seen in developed countries. They will show how effective partnerships between hematologists in more and less developed nations may work together to produce important advances for treating major hematologic diseases in less developed regions. A major focus relates to the socio-economic and medical burden of these diseases in developing countries with limited resources. As such, these problems provide a challenge and an opportunity for collaborative interaction between hematologists and policy makers worldwide.

Hypoxia and heat inhibit inducible nitric oxide synthase gene expression by different mechanisms in rat hepatocytes

Ischemia/reperfusion contributes to the hepatic injury in resection and transplantation of the liver. However, the precise mechanisms involved in hypoxia stress remain to be clarified. Pro-inflammatory cytokines including interleukin 1beta (IL-1beta) induce a gene expression of inducible nitric oxide synthase (iNOS) and produce nitric oxide, which exerts either a cytoprotective or toxic effect. In this report, we found that hypoxia and heat markedly inhibited the induction of nitric oxide production stimulated by IL-1beta in rat cultured hepatocytes. Both treatments also abolished the induction of iNOS protein and mRNA. However, hypoxia could not prevent either degradation of an inhibitory protein (IkappaBalpha) of nuclear factor-kappaB (NF-kappaB) or translocation of NF-kappaB to the nucleus, whereas heat inhibited both of the IkappaBalpha degradation and NF-kappaB translocation. Transfection experiments with iNOS promoter construct revealed that hypoxia as well as heat significantly inhibited the transactivation of iNOS gene. Further, a hypoxia-response element located in the promoter was not involved in the inhibition of iNOS induction by hypoxia. These results indicate that hypoxia and heat suppress iNOS gene induction at the transcriptional level through different mechanisms. Reduction of nitric oxide production under hypoxic conditions may be implicated in the cellular damage or protection during hepatic ischemia/reperfusion.

HLA-A*0201 restricted CD8+ T-lymphocyte responses to malaria: identification of new Plasmodium falciparum epitopes by IFN-gamma ELISPOT

The role of antigen specific CD8+ T-lymphocytes in mediating protection against sporozoite-induced malaria has been well established in murine models. In humans, indirect evidence has accumulated suggesting a similar protective role for antigen-specific CD8+ T-lymphocytes. Nevertheless, the low frequency of circulating specific cells together with the lack of sensitive methods to quantify them has hampered the direct assessment of their function. Using a combination of short-term cell culture and IFN-gamma ELISPOT, we studied CD8+ T-lymphocyte responses to a panel of HLA-A*0201 binding peptides. In addition to confirming the response to already described epitopes, we also identified five new CD8+ T-lymphocyte epitopes. These epitopes are presented in pre-erythrocytic stages gene products of Plasmodium falciparum 7G8 strain and correspond to the following protein segments: circumsporozoite (CS) 64-72, 104-113, 299-308 and 403-411; liver stage antigen (LSA-1) repeat region; sporozoite surface protein 2 or thrombospondin related anonymous protein (SSP2/TRAP) 78-88 and 504-513. Four of these peptides are conserved amongst all published sequences of P. falciparum strains. We conclude that the modified IFN-gamma ELISPOT assay is a sensitive technique to monitor antigen-specific CD8+ T-lymphocyte responses in human malaria which may help in the improvement and assessment of the efficacy of malaria subunit vaccines.

Improving protective immunity induced by DNA-based immunization: priming with antigen and GM-CSF-encoding plasmid DNA and boosting with antigen-expressing recombinant poxvirus

Intramuscular immunization with a naked DNA plasmid expressing the Plasmodium yoelii circumsporozoite protein (pPyCSP) protects mice against challenge with P. yoelii sporozoites. This protection can be improved either by coadministration of a plasmid expressing murine GM-CSF (pGMCSF) or by boosting with recombinant poxvirus expressing the PyCSP. We now report that combining these two strategies, by first mixing the priming dose of pPyCSP with pGMCSF and then boosting with recombinant virus, can substantially increase vaccine effectiveness. Not only were immune responses and protection improved but the pPyCSP dose could be lowered from 100 microg to 1 microg with little loss of immunogenicity after boost with recombinant poxvirus. Comparing mice primed by the 1-microg doses of pPyCSP plus 1 microg pGMCSF with mice primed by 1-microg doses of pPyCSP alone, the former were better protected (60% vs 0) and had higher concentrations of Abs (titers of 163, 840 vs 5, 120 by indirect fluorescent Ab test against sporozoites), more ex vivo CTL activity (25% vs 7% specific lysis), and more IFN-gamma-secreting cells by enzyme-linked immunospot assay (1460 vs 280 IFN-gamma spot-forming cells/106 cells). Priming with plasmid vaccine plus pGMCSF and boosting with recombinant poxviruses strongly improves the immunogenicity and protective efficacy of DNA vaccination and allows for significant reduction of dose.

Direct immunization of malaria DNA vaccine into the liver by gene gun protects against lethal challenge of Plasmodium berghei sporozoite

The liver is the first target organ for malaria parasites immediately after the bite of an infected mosquito. We studied local immunization of malaria DNA vaccines at the site of the liver using a gene gun as a useful tool for in vivo transfection of foreign genes. A malaria DNA vaccine consisting of the Plasmodium berghei circumsporozoite protein (PbCSP) gene plus the mouse IL-12 gene was bombarded directly by a gene gun into mouse liver once or into the skin twice. A marked protective effect was induced by gene bombardment into the liver (more than 71%) compared with that into the skin (less than 33%). A Th1-type immune response and high production of iNOS were observed in the hepatic lymphocytes from mice bombarded into the liver, resulting in more effective protection compared with those bombarded into the skin. These results provide an important implication on the development of efficient malaria vaccine strategies.

Implication of reactive oxygen species in the antibacterial activity against Salmonella typhimurium of hepatocyte cell lines

We recently described the antibacterial activity of a murine hepatocyte cell line stimulated with interferon-gamma (IFN-gamma), interleukin-1 (IL-1), and lipopolysaccharide (LPS) against intracellular Salmonella organisms. Here we show for the first time the existence of basal antibacterial activity in cultured hepatocyte cell lines. Thus treatment of resting and stimulated hepatocytes with catalase or superoxide dismutase increased bacterial number recovered per monolayer, which suggests that the mechanism involved with antibacterial activity of hepatocytes is mediated by reactive oxygen species (ROS). Also, the capacity of these cell lines to generate intracellular peroxides under resting and stimulated conditions was investigated. This revealed that IL-1 and LPS did not induce any increase in the amount of intracellular peroxides by themselves, but they primed IFN-gamma for maximal induction of peroxides. The intracellular amount of peroxides was highly increased on stimulation with IFN-gamma, IL-1, and LPS, and it was strongly inhibited by catalase. This explains that the mechanism whereby this enzyme inhibits antibacterial activity takes place by decreasing the intracellular pool of peroxides. In turn, experiments performed in the presence of several inhibitors of metabolic pathways involved in ROS generation suggested that cyclo-oxygenase are a source of these species in hepatocyte cell lines. These results attribute a prominent role to the generation of peroxides as effector molecules of antibacterial activity in hepatocyte cell lines. Thus these cells displayed a moderate basal level, which increased on stimulation with proinflammatory cytokines such as IFN-gamma, IL-1, and bacterial products such as LPS. Finally, it has been also shown for the first time that IFN-gamma stimulation induces production of peroxides in human and murine hepatocyte cell lines.

Blood mononuclear cell nitric oxide production and plasma cytokine levels in healthy gabonese children with prior mild or severe malaria

Plasmodium falciparum malaria is an important cause of morbidity and mortality in children. Factors that determine the development of mild versus severe malaria are not fully understood. Since host-derived nitric oxide (NO) has antiplasmodial properties, we measured NO production and NO synthase (NOS) activity in peripheral blood mononuclear cells (PBMC) from healthy Gabonese children with a history of prior mild malaria (PMM) or prior severe malaria (PSM) caused by P. falciparum. The PMM group had significantly higher levels of NOS activity in freshly isolated PBMC and higher NO production and NOS activity in cultured PBMC. The investigation of NO-modulating cytokines (e.g., interleukin 12, gamma interferon, tumor necrosis factor alpha [TNF-alpha], and transforming growth factor beta1) as an explanation for differing levels of NOS activity revealed that plasma levels of TNF-alpha were significantly higher in the PSM group. Our results suggest that NOS/ NO and TNF-alpha are markers for prior disease severity and important determinants of resistance to malaria.

IL-12 and NK Cells Are Required for Antigen-Specific Adaptive Immunity Against Malaria Initiated by CD8+ T Cells in the Plasmodium yoelii Model

CD8+ T cells have been implicated as critical effector cells in protection against preerythrocytic stage malaria, including the potent protective immunity of mice and humans induced by immunization with radiation-attenuated Plasmodium spp. sporozoites. This immunity is directed against the Plasmodium spp. parasite developing within the host hepatocyte and for a number of years has been presumed to be mediated directly by CD8+ CTL or indirectly by IFN-γ released from CD8+ T cells. In this paper, in BALB/c mice, we establish that after immunization with irradiated sporozoites or DNA vaccines parasite-specific CD8+ T cells trigger a novel mechanism of adaptive immunity that is dependent on T cell- and non-T cell-derived cytokines, in particular IFN-γ and IL-12, and requires NK cells but not CD4+ T cells. The absolute requirement for CD8+ T cells to initiate such an effector mechanism, and the requirement for IL-12 and NK cells in such vaccine-induced protective immunity, are unique and underscore the complexity of the immune responses that protect against malaria and other intracellular pathogens.

Iron chelation therapy for malaria: a review

Malaria is one of the major global health problems, and an urgent need for the development of new antimalarial agents faces the scientific community. A considerable number of iron(III) chelators, designed for purposes other than treating malaria, have antimalarial activity in vitro, apparently through the mechanism of withholding iron from vital metabolic pathways of the intra-erythrocytic parasite. Certain iron(II) chelators also have antimalarial activity, but the mechanism of action appears to be the formation of toxic complexes with iron rather than the withholding of iron. Several of the iron(III)-chelating compounds also have antimalarial activity in animal models of plasmodial infection. Iron chelation therapy with desferrioxamine, the only compound of this nature that is widely available for use in humans, has clinical activity in both uncomplicated and severe malaria in humans.

Chloroquine stimulates nitric oxide synthesis in murine, porcine, and human endothelial cells

Nitric oxide (NO) is a free radical involved in the regulation of many cell functions and in the expression of several diseases. We have found that the antimalarial and antiinflammatory drug, chloroquine, is able to stimulate NO synthase (NOS) activity in murine, porcine, and human endothelial cells in vitro: the increase of enzyme activity is dependent on a de novo synthesis of some regulatory protein, as it is inhibited by cycloheximide but is not accompanied by an increased expression of inducible or constitutive NOS isoforms. Increased NO synthesis is, at least partly, responsible for chloroquine-induced inhibition of cell proliferation: indeed, NOS inhibitors revert the drug-evoked blockage of mitogenesis and ornithine decarboxylase activity in murine and porcine endothelial cells. The NOS-activating effect of chloroquine is dependent on its weak base properties, as it is exerted also by ammonium chloride, another lysosomotropic agent. Both compounds activate NOS by limiting the availability of iron: their stimulating effects on NO synthesis and inhibiting action on cell proliferation are reverted by iron supplementation with ferric nitrilotriacetate, and are mimicked by incubation with desferrioxamine. Our results suggest that NO synthesis can be stimulated in endothelial cells by chloroquine via an impairment of iron metabolism.

The mosquito Anopheles stephensi limits malaria parasite development with inducible synthesis of nitric oxide

We have discovered that the mosquito Anopheles stephensi, a natural vector of human malaria, limits parasite development with inducible synthesis of nitric oxide (NO). Elevated expression of A. stephensi NO synthase (NOS), which is highly homologous to characterized NOS genes, was detected in the midgut and carcass soon after invasion of the midgut by Plasmodium. Early induction is likely primed by bacterial growth in the blood meal. Later increases in A. stephensi NOS expression and enzyme activity occurred at the beginning of sporozoite release. Circulating levels of nitrite/nitrate, end-products of NO synthesis, were significantly higher in Plasmodium-infected mosquitoes. Dietary provision of the NOS substrate L-arginine reduced Plasmodium infections in A. stephensi. In contrast, dietary provision of a NOS inhibitor significantly increased parasite numbers in infected mosquitoes, confirming that A. stephensi limits Plasmodium development with NO.

Pre-erythrocytic-stage immune effector mechanisms in Plasmodium spp. infections

The potent protective immunity against malaria induced by immunization of mice and humans with radiation-attenuated Plasmodium spp. sporozoites is thought to be mediated primarily by T-cell responses directed against infected hepatocytes. This has led to considerable efforts to develop subunit vaccines that duplicate this protective immunity, but a universally effective vaccine is still not available and in vitro correlates of protective immunity have not been established. Contributing to this delay has been a lack of understanding of the mechanisms responsible for the protection. There are now data indicating that CD8+ T cells, CD4+ T cells, cytokines, and nitric oxide can all mediate the elimination of infected hepatocytes in vitro and in vivo. By dissecting the protection induced by immunization with irradiated sporozoite, DNA and synthetic peptide-adjuvant vaccines, we have demonstrated that different T-cell-dependent immune responses mediate protective immunity in the same inbred strain of mouse, depending on the method of immunization. Furthermore, the mechanism of protection induced by a single method of immunization may vary among different strains of mice. These data have important implications for the development of pre-erythrocytic-stage vaccines designed to protect a heterogeneous human population, and of assays that predict protective immunity.