Inhibitory activity of IL-6 on malaria hepatic stages

Does hydrogen peroxide contribute to the immunity against Malaria induced by whole attenuated plasmodial sporozoites?

Plasmodium sporozoites can block apoptotic pathways within host hepatocytes, ensuring the survival of the parasite. However, attenuated plasmodial sporozoites are unable to prevent apoptosis, which provides many parasite antigens to immune cells. This exposure leads to protection against Malaria in both human and animal models. If these hosts are later inoculated with infectious sporozoites, apoptosis of infected hepatocytes will occur, preventing parasite development. Considering that hydrogen peroxide can induce apoptosis, it is plausible that it plays a role in the mechanisms associated with the protection mediated by attenuated plasmodial sporozoites. Based on published results that describe the relationship between Plasmodium, hydrogen peroxide, and apoptosis, a rational explanation can be provided for this hypothesis.

The distinctive role of membrane fibrinogen-like protein 2 in the liver stage of rodent malaria infections

Viral infection often induce the expression of murine fibrinogen-like protein 2 (mFGL2) triggering immune coagulation, which causes severe liver pathogenesis via increased fibrin deposition and thrombosis in the microvasculature. We aimed to investigate the role of mFGL2 in the liver stage of malaria infections. We reveal that infection with malaria sporozoites also induces increased expression of mFGL2 and that this expression is primarily located within the liver Kupffer and endothelial cells. In addition, we report that inhibition of FGL2 has no significant effect on immune coagulation but increases the expression of inflammatory cytokines in the livers of infected mice. Interestingly, FGL2 deficiency had no significant impact on the development of liver stage malaria parasites or the pathogenesis of the infected liver. In contrast to viral infections, we conclude that mFGL2 does not contribute to either parasite development or liver pathology during these infections, revealing the unique features of this protein in liver-stage malaria 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.

Protection against malaria in mice is induced by blood stage-arresting histamine-releasing factor (HRF)-deficient parasites

Although most vaccines against blood stage malaria in development today use subunit preparations, live attenuated parasites confer significantly broader and more lasting protection. In recent years, Plasmodium genetically attenuated parasites (GAPs) have been generated in rodent models that cause self-resolving blood stage infections and induce strong protection. All such GAPs generated so far bear mutations in housekeeping genes important for parasite development in red blood cells. In this study, using a Plasmodium berghei model compatible with tracking anti-blood stage immune responses over time, we report a novel blood stage GAP that lacks a secreted factor related to histamine-releasing factor (HRF). Lack of HRF causes an IL-6 increase, which boosts T and B cell responses to resolve infection and leave a cross-stage, cross-species, and lasting immunity. Mutant-induced protection involves a combination of antiparasite IgG2c antibodies and FcγR(+) CD11b(+) cell phagocytes, especially neutrophils, which are sufficient to confer protection. This immune-boosting GAP highlights an important role of opsonized parasite-mediated phagocytosis, which may be central to protection induced by all self-resolving blood stage GAP infections.

The TLR2 is activated by sporozoites and suppresses intrahepatic rodent malaria parasite development

TLRs (Toll-like receptors) play an important role in the initiation of innate immune responses against invading microorganisms. Although several TLRs have been reported to be involved in the innate immune response against the blood-stage of malaria parasites, the role of TLRs in the development of the pre-erythrocytic stage is still largely unknown. Here, we found that sporozoite and its lysate could significantly activate the TLR2, and induce macrophages to release proinflammatory cytokines, including IL-6, MCP-1 and TNF-α, in a TLR2-dependent manner. Further studies showed that sporozoite and its lysate could be recognized by either TLR2 homodimers or TLR2/1 and TLR2/6 heterodimers, implicating the complexity of TLR2 agonist in sporozoite. Interestingly, the TLR2 signaling can significantly suppress the development of the pre-erythrocytic stage of Plasmodium yoelii, as both liver parasite load and subsequent parasitemia were significantly elevated in both TLR2- and MyD88-deficient mice. Additionally, the observed higher level of parasite burden in TLR2^−/− mice was found to be closely associated with a reduction in proinflammatory cytokines in the liver. Therefore, we provide the first evidence that sporozoites can activate the TLR2 signaling, which in turn significantly inhibits the intrahepatic parasites. This may provide us with novel clues to design preventive anti-malaria therapies.

The antimalarial drug primaquine targets Fe-S cluster proteins and yeast respiratory growth

Malaria is a major health burden in tropical and subtropical countries. The antimalarial drug primaquine is extremely useful for killing the transmissible gametocyte forms of Plasmodium falciparum and the hepatic quiescent forms of P. vivax. Yet its mechanism of action is still poorly understood. In this study, we used the yeast Saccharomyces cerevisiae model to help uncover the mode of action of primaquine. We found that the growth inhibitory effect of primaquine was restricted to cells that relied on respiratory function to proliferate and that deletion of SOD2 encoding the mitochondrial superoxide dismutase severely increased its effect, which can be countered by the overexpression of AIM32 and MCR1 encoding mitochondrial enzymes involved in the response to oxidative stress. This indicated that ROS produced by respiratory activity had a key role in primaquine-induced growth defect. We observed that Δsod2 cells treated with primaquine displayed a severely decreased activity of aconitase that contains a Fe–S cluster notoriously sensitive to oxidative damage. We also showed that in vitro exposure to primaquine impaired the activity of purified aconitase and accelerated the turnover of the Fe–S cluster of the essential protein Rli1. It is suggested that ROS-labile Fe–S groups are the primary targets of primaquine. Aconitase activity is known to be essential at certain life-cycle stages of the malaria parasite. Thus primaquine-induced damage of its labile Fe–S cluster – and of other ROS-sensitive enzymes – could inhibit parasite development.

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The mode of action of the antimalarial drug primaquine is poorly understood.

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The yeast model is used to decipher its mechanism of action.

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SOD and respiratory function are key for yeast sensitivity to primaquine.

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Primaquine treatment impairs Fe–S containing enzyme aconitase.

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Its attack on Fe–S clusters could explain the primaquine-induced growth inhibition.

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.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

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.

Plasmodium-host interactions directly influence the threshold of memory CD8 T cells required for protective immunity

Plasmodium infections are responsible for millions of cases of malaria and ∼1 million deaths annually. Recently, we showed that sterile protection (95%) in BALB/c mice required Plasmodium berghei circumsporozoite protein (CS(252-260))-specific memory CD8 T cells exceeding a threshold of 1% of all PBLs. Importantly, it is not known if Plasmodium species affect the threshold of CS-specific memory CD8 T cells required for protection. Furthermore, C57BL/6 mice immunized with radiation-attenuated parasites are more difficult to protect against Plasmodium sporozoite challenge than similarly immunized BALB/c mice; however, it is not known whether this is the result of different CD8 T cell specificity, functional attributes of CD8 T cells, or mouse strain-specific factors expressed in nonhematopoietic cells. In this article, we show that more CS-specific memory CD8 T cells are required for protection against P. yoelii sporozoite challenge than for protection against P. berghei sporozoite challenge. Furthermore, P. berghei CS(252)-specific CD8 T cells exhibit reduced protection against P. berghei sporozoite challenge in the context of C57BL/6 and C57BL/10 non-MHC-linked genes in CB6F1 and B10.D2 mice, respectively. Generation and immunization of reciprocal chimeric mice between BALB/c and B10.D2 strains revealed that B10 background factors expressed by nonhematopoietic cells increased the threshold required for protection through a CD8 T cell-extrinsic mechanism. Finally, reduced CS-specific memory CD8 T cell protection in P. yoelii-infected BALB/c or P. berghei-infected B10.D2 mice correlated with increased rates of Plasmodium amplification in the liver. Thus, both Plasmodium species and strain-specific background genes in nonhematopoietic cells determine the threshold of memory CD8 T cells required for protection.

Inhibitory effect of TNF-α on malaria pre-erythrocytic stage development: influence of host hepatocyte/parasite combinations

Background

The liver stages of malaria parasites are inhibited by cytokines such as interferon-γ or Interleukin (IL)-6. Binding of these cytokines to their receptors at the surface of the infected hepatocytes leads to the production of nitric oxide (NO) and radical oxygen intermediates (ROI), which kill hepatic parasites. However, conflicting results were obtained with TNF-α possibly because of differences in the models used. We have reassessed the role of TNF-α in the different cellular systems used to study the Plasmodium pre-erythrocytic stages.

Methods and Findings

Human or mouse TNF-α were tested against human and rodent malaria parasites grown in vitro in human or rodent primary hepatocytes, or in hepatoma cell lines. Our data demonstrated that TNF-α treatment prevents the development of malaria pre-erythrocytic stages. This inhibitory effect however varies with the infecting parasite species and with the nature and origin of the cytokine and hepatocytes. Inhibition was only observed for all parasite species tested when hepatocytes were pre-incubated 24 or 48 hrs before infection and activity was directed only against early hepatic parasite. We further showed that TNF-α inhibition was mediated by a soluble factor present in the supernatant of TNF-α stimulated hepatocytes but it was not related to NO or ROI. Treatment TNF-α prevents the development of human and rodent malaria pre-erythrocytic stages through the activity of a mediator that remains to be identified.

Conclusions

Treatment TNF-α prevents the development of human and rodent malaria pre-erythrocytic stages through the activity of a mediator that remains to be identified. However, the nature of the cytokine-host cell-parasite combination must be carefully considered for extrapolation to the human infection.

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.

Differential effector pathways regulate memory CD8 T cell immunity against Plasmodium berghei versus P. yoelii sporozoites

Malaria results in >1,000,000 deaths per year worldwide. Although no licensed vaccine exists, much effort is currently focused on subunit vaccines that elicit CD8 T cell responses directed against Plasmodium parasite liver stage Ags. Multiple immune-effector molecules play a role in antimicrobial immunity mediated by memory CD8 T cells, including IFN-gamma, perforin, TRAIL, Fas ligand, and TNF-alpha. However, it is not known which pathways are required for memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. In this study, we used a novel immunization strategy to generate memory CD8 T cells in the BALB/c mouse model of P. berghei or P. yoelii sporozoite infection to examine the role of immune-effector molecules in resistance to the liver stage infection. Our studies reveal that endogenous memory CD8 T cell-mediated protection against both parasite species is, in part, dependent on IFN-gamma, whereas perforin was only critical in protection against P. yoelii. We further show that neutralization of TNF-alpha in immunized mice markedly reduces memory CD8 T cell-mediated protection against both parasite species. Thus, our studies identify IFN-gamma and TNF-alpha as important components of the noncytolytic pathways that underlie memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. Our studies also show that the effector pathways that memory CD8 T cells use to eliminate liver stage infection are, in part, Plasmodium species specific.

Vaccination with live Plasmodium yoelii blood stage parasites under chloroquine cover induces cross-stage immunity against malaria liver stage

Immunity to malaria has long been thought to be stage-specific. In this study we show that immunization of BALB/c mice with live erythrocytes infected with nonlethal strains of Plasmodium yoelii under curative chloroquine cover conferred protection not only against challenge by blood stage parasites but also against sporozoite challenge. This cross-stage protection was dose-dependent and long lasting. CD4(+) and CD8(+) T cells inhibited malaria liver but not blood stage. Their effect was mediated partially by IFN-gamma, and was completely dependent of NO. Abs against both pre-erythrocytic and blood parasites were elicited and were essential for protection against blood stage and liver stage parasites. Our results suggest that Ags shared by liver and blood stage parasites can be the foundation for a malaria vaccine that would provide effective protection against both pre-erythrocytic and erythrocytic asexual parasites found in the mammalian host.

Elevated basal hepcidin levels in the liver may inhibit the development of malaria infection: another piece towards solving the malaria puzzle?

BACKGROUND

Inflammatory cytokines play a crucial role in the human immune response to infection by malaria. During the initial sporozoite infection of the liver the presence of Interleukin-6 (IL-6) can be determinant. IL-6 controls systemic iron homeostasis through hepcidin, which is produced mainly by hepatocytes. An elevated basal hepcidin level in the liver can be induced by chronic inflammatory disease. Hepcidin is also a peptide with antimicrobial properties.

PRESENTATION OF THE HYPOTHESIS

We hypothesize that elevated basal hepcidin levels in the liver inhibit the development of malaria infection. When hepcidin is abundant, hepatocytes sequester iron, and this inhibits sporozoite development in liver-stage malaria infection.

TESTING THE HYPOTHESIS

The validity of our hypothesis can be proven by observing sporozoite growth in hepcidin-treated hepatocytes, or in hepatocytes, stimulated with IL-6 to increase hepcidin levels before incubation with malaria sporozoites and observing the effect the hepcidin knockout function has on the infection.

IMPLICATIONS OF THE HYPOTHESIS

Confirmation of our hypothesis could help to understand the complexity of the malaria infection.

Molecular interactions between Plasmodium and its insect vectors

Our understanding of the intricate interactions between the malarial parasite and the mosquito vector is complicated both by the number and diversity of parasite and vector species, and by the experimental inaccessibility of phenomena under investigation. Steady developments in techniques to study the parasite in the mosquito have recently been augmented by methods to culture in their entirety the sporogonic stages of some parasite species. These, together with the new saturation technologies, and genetic transformation of both parasite and vector will permit penetrating studies into an exciting and largely unknown area of parasite-host interactions, an understanding of which must result in the development of new intervention strategies. This microreview highlights key areas of current basic molecular interest, and identifies numerous lacunae in our knowledge that must be filled if we are to make rational decisions for future control strategies. It will conclude by trying to explain why in the opinion of this reviewer understanding malaria-mosquito interactions may be critical to our future attempts to limit a disease of growing global importance.

Plasmodium vivax malaria vaccine development

Plasmodium vivax represents the most widespread malaria parasite worldwide. Although it does not result in as high a mortality rate as P. falciparum, it inflicts debilitating morbidity and consequent economic impact in endemic communities. In addition, the relapsing behavior of this malaria parasite and the recent resistance to anti-malarials contribute to making its control more difficult. Although the biology of P. vivax is different from that of P. falciparum and the human immune response to this parasite species has been rather poorly studied, significant progress is being made to develop a P. vivax-specific vaccine based on the information and experience gained in the search for a P. falciparum vaccine. We have devoted great effort to antigenically characterize the P. vivax CS protein and to test its immunogenicity using the Aotus monkey model. Together with other groups we are also assessing the immunogenicity and protective efficacy of the asexual blood stage vaccine candidates MSP-1 and DBP in the monkey model, as well as the immunogenicity of Pvs25 and Pvs28 ookinete surface proteins. The transmission-blocking efficacy of the responses induced by these latter antigens is being assessed using Anopheles albimanus mosquitoes. The current status of these vaccine candidates and other antigens currently being studied is described.

Host-virus interactions during malaria infection in hepatitis B virus transgenic mice

We have previously shown that hepatitis B virus (HBV) replication is abolished in the liver of HBV transgenic mice by inflammatory cytokines induced by HBV-specific cytotoxic T cells and during unrelated viral infections of the liver. We now report that intrahepatic HBV replication is also inhibited in mice infected by the malaria species Plasmodium yoelii 17X NL. P. yoelii infection triggers an intrahepatic inflammatory response characterized by the influx of natural killer cells, macrophages, and T cells. During this process, interferon (IFN)-gamma and IFN-alpha/beta suppress HBV gene expression and replication in the liver. Collectively, the data suggest that malaria infection might influence the course and pathogenesis of HBV infection in coinfected humans.

Liver CD4-CD8- NK1.1+ TCR alpha beta intermediate cells increase during experimental malaria infection and are able to exhibit inhibitory activity against the parasite liver stage in vitro

Experimental infection of C57BL/6 mice by Plasmodium yoelii sporozoites induced an increase of CD4-CD8- NK1.1+ TCR alpha beta int cells and a down-regulation of CD4+ NK1.1+ TCR alpha beta int cells in the liver during the acute phase of the infection. These cells showed an activated CD69+, CD122+, CD44high, and CD62Lhigh surface phenotype. Analysis of the expressed TCRV beta segment repertoire revealed that most of the expanded CD4-CD8- (double-negative) T cells presented a skewed TCRV beta repertoire and preferentially used V beta 2 and V beta 7 rather than V beta 8. To get an insight into the function of expanded NK1.1+ T cells, experiments were designed in vitro to study their activity against P. yoelii liver stage development. P. yoelii-primed CD3+ NK1.1+ intrahepatic lymphocytes inhibited parasite growth within the hepatocyte. The antiplasmodial effector function of the parasite-induced NK1.1+ liver T cells was almost totally reversed with an anti-CD3 Ab. Moreover, IFN-gamma was in part involved in this antiparasite activity. These results suggest that up-regulation of CD4-CD8- NK1.1+ alpha beta T cells and down-regulation of CD4+ NK1.1+ TCR alpha beta int cells may contribute to the early immune response induced by the Plasmodium during the prime infection.

Status of Plasmodium falciparum towards catalase

The role of endogenous and internalized catalase in the protection of Plasmodium against oxidant stress was studied. Catalase activities were measured in isolated Plasmodium falciparum at different stages of intererythrocytic development. Activities measured at late schizont stages were compared to parasite markers (glutamate dehydrogenase, SOD) and to red blood cell markers (haemoglobin, Cu/Zn-SOD). The fate of the host cell catalase in the parasite digestive system was studied by immunoelectron microscopy using monoclonal antibodies. The internalized catalase appeared to be dissociated in the digestive system of the parasite and inactivated. To examine the protective role of the endogenous and internalized catalase in the parasite protection against oxidant stress, parasites were cultivated at two oxygen concentrations (5% and 20%) in inhibited catalase red blood cells. These experiments suggested that the catalases present both in red blood cell and parasite are not essential when parasites are cultivated under 5% oxygen, but are necessary to protect the parasite under 20% oxygen. Catalase may not be the main protective enzyme involved in the protection of P. falciparum in standard in vitro culture conditions, but may become critical under the higher oxygen tensions conditions encountered in vivo.

Immunization with a recombinant C-terminal fragment of Plasmodium yoelii merozoite surface protein 1 protects mice against homologous but not heterologous P. yoelii sporozoite challenge

It has been reported previously that immunization with recombinant protein containing the two epidermal growth factor (EGF)-like modules from merozoite surface protein 1 (MSP-1) of Plasmodium yoelii (strain YM) protects mice against a lethal blood-stage challenge with the same parasite strain. Since MSP-1 is expressed in both liver- and blood-stage schizonts and on the surface of merozoites, we evaluated the effectiveness of immunization with recombinant proteins containing either the individual or the two combined EGF-like modules in producing a protective response against a sporozoite challenge. The recombinant protein expressing the combined EGF-like modules of the YM strain protected mice against a homologous sporozoite challenge, and sterile protection, as defined by the absence of detectable blood-stage parasites, was observed in the majority of the mice. In contrast, mice immunized with recombinant P. yoelii YM MSP-1 were not protected against a heterologous challenge with sporozoites from strain 265 BY of P. yoelii. The lack of protection may be explained by differences identified in the amino acid sequences of MSP-1 for the two strains. A recombinant protein containing the two EGF-like modules of MSP-1 from P. yoelii 265 BY was produced and used to immunize mice. These mice were protected against a homologous challenge with sporozoites of P. yoelii 265 BY. The results suggest that a recombinant MSP-1 has potential as a vaccine against malaria, but its efficacy may be limited by sequence polymorphism and selection of variants.

Tumour necrosis factor and associated cytokines in the host's response to malaria

Tumour Necrosis Factor (TNF) is produced at the initiation of malaria infections (pre-erythrocytic phase), as demonstrated by the release of bioactive TNF by peripheral blood mononuclear cells from individuals residing in endemic areas after stimulation with stage specific sporozoite antigens. During the erythrocytic phase, TNF production is greatly augmented by parasite antigens at the time of schizont rupture and merozoite release from infected erythrocytes. Some of the strongest inducers of TNF synthesis and release are malaria toxins, e.g. glycosylphosphatidylinositol moieties and malaria pigment. Because of TNF's well-known cytotoxic activity it was originally hypothesized that it alone was responsible for killing parasites directly or within host cells. Though earlier reports of the capability of serum containing TNF to kill plasmodia supported this idea, later experiments with recombinant TNF showed a lack of significant parasiticidal activity. Recent studies investigating related factors showed that they were involved with TNF in the control of infection. These factors included -ther cytokines, such as interleukin (IL)-1, IL-6, IL-12, interferon-gamma (IFN gamma) as well as nitric oxide intermediates (NOI) and reactive oxygen intermediates (ROI). This positioned TNF as a key regulator of the immune response against the malaria parasite. However, it must be noted that TNF and its associated factors are also responsible for the fever, aches and pains of acute illness, as well as the hypoglycemia, shock, bleeding and reversible coma of severe malaria seen in approximately 1 percent of individuals with malaria. Therein lies the rub; factors important in the control of malaria also appear to have detrimental properties. Research presented in this review characterizes TNF and associated cytokines' importance in the immune response to malaria.

Molecular characterization of the glutathione peroxidase gene of the human malaria parasite Plasmodium falciparum

In this paper we report the isolation and the characterization of a gene encoding the antioxidant enzyme glutathione peroxidase from the human malaria parasite Plasmodium falciparum. This gene contains two introns of 208 and 168 bp and is present in a single copy on chromosome 13. The open reading frame encodes a protein with a predicted length of 205 amino acids, which possesses a potential cleavage site between residues 21 and 22 after a hydrophobic region with the characteristics of a signal sequence. Therefore, the mature protein is predicted to be 184 residues long with a molecular mass of 21404 Da. In comparison with other known glutathione peroxidases many amino acid residues implicated in catalysis are conserved in the malarial enzyme. Phylogenetic analysis indicates that the deduced protein sequence is more closely related to plant glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase. A 1.5-kb transcript was identified in asynchronous erythrocytic stages.

Characterization of iron-dependent endogenous superoxide dismutase of Plasmodium falciparum

Two main superoxide dismutase activities at isoelectric points (pI) 6.2 and 6.8 and two minor at pI 5.6 and 6.4 were found in crude extracts of Plasmodium falciparum. These activities were cyanide-resistant and hydrogen peroxide-sensitive and represented 20-30% of the total SOD activity found in the crude extract. A fragment of 424 bp, amplified from genomic DNA from P. falciparum, was cloned and sequenced. The deduced amino acid sequence identified this fragment as a coding region of an SOD gene. A cDNA corresponding to SOD was then isolated from a P. falciparum cDNA library and sequenced. The deduced amino acid sequence of SOD (197 aa) was compared with 32 known Feor Mn-SODs by the 'DARWIN' system. This analysis showed that the parasitic enzyme was related to typical Fe-SODs. The SOD subunit was purified and the N-terminal sequence, determined up to 29 residues, corresponded to that of cDNA isolated. The iron-dependent SOD activity found in Plasmodium falciparum represents the first level of the antioxidant defence system of the parasite. It is also the first SOD characterized in the parasitic Apicomplexa phylum whose sequence can be compared to equivalent iron-dependent enzymes known in other protozoa and bacteria.

Increase in glutathione peroxidase activity in malaria parasite after selenium supplementation

Glutathione peroxidase (GPx), a key enzyme involved in the detoxification of many peroxides, has been investigated in two malaria parasite species: P. yoelii in vivo (murine malaria) and P. falciparum in vitro (human malaria). We demonstrate the presence of an endogenous GPx activity in these two Plasmodia species. Enzymatic assays and the use of specific substrates and inhibitors allowed us to determine that the activity is selenium dependent. As this activity was shown to be lower in P. falciparum than in P. yoelii, and selenium levels were found to be low in culture medium and culture red blood cells, we hypothesized that a severe selenium deficiency could be responsible for this difference. After selenium supplementation, with either sodium selenite or selenocystine, we observed an increase in growth of P. falciparum only in with sodium selenite, whereas higher GPx activities were noted in parasites grown in media supplemented with both. An increase in GPx activities was also observed in parasites that had undergone an experimental oxidative stress with TBOOH. As the erythrocyte is unable to synthesize new proteins, these results provide further evidence for the existence of an endogenous parasitic selenium-dependent glutathione peroxidase.

Susceptibility to Plasmodium berghei infection in rats is modulated by the acute phase response

Brown Norway (BN) and Sprague Dawley (SD) rats are known to differ in their susceptibility to infection with sporozoites of Plasmodium berghei, as measured by the density of liver schizonts. Because of the known inhibitory effect of non-specific immunomodulators on schizont development, we compared some aspects of the acute phase response in these two rat strains. LPS induced IL-6 production was measured in supernatants of spleen cells and peritoneal macrophages of both strains. SD rats, which are the least susceptible to P. berghei sporozoites, showed significantly higher IL-6 production by macrophages from both sources. When LPS was administered in vivo, SD rats also had a significantly higher IL-6 response. Hepatocytes from both strains were cultured in the presence of IL-6. After three days of culture, alpha 2-Macroglobulin concentrations in the supernatants of SD hepatocytes were much higher than those from BN rats. Kupffer cell depletion in both BN and SD rats was correlated with a significant increase in liver schizont density, but did not abrogate the difference in susceptibility. From these results we conclude that the higher cytokine production capacity of SD rats compared to BN rats, may contribute to the difference in susceptibility to P. berghei sporozoites between these strains, but that other yet unknown factors are also involved.

Protective immunity against malaria: cellular changes in the liver vary according to the method of immunization

Characterization of cells present in the extravascular compartment of murine liver was performed after different immunization procedures against the malaria parasite Plasmodium yoelii. Mice were immunized with live or irradiated sporozoites or with parasitized erythrocytes. Whatever the immunization protocol used, the mice were protected against a sporozoite challenge but each immunization procedure induced a specific profile of cell types. Immunization with irradiated sporozoite induce a significant increase in CD8+ lymphocytes, parasitized erythrocytes stimulates production of monocytes/macrophages and CD8+ lymphocytes while, after live sporozoites immunization, polymorphonuclear cells, macrophages/monocytes, B cells and a range of T cell subsets were increased in number.

Immunization with irradiated Plasmodium berghei sporozoites induces IL-2 and IFN gamma but not IL-4

Protective immunity against Plasmodium induced by immunization with irradiated sporozoites (SPZ) depends on both humoral and cellular responses. Although circumsporozoite protein (CSP)-specific cytolytic T lymphocyte responses have been established as an effector system, other cell types are required for protection. We have previously demonstrated that although protective immunity and T cell proliferative reactivity to SPZ are mouse strain- and SPZ dose-dependent, no correlation between the two responses could be found. Since protective immunity involves functionally diverse T cell subsets, we asked whether the discordance between proliferative responses to SPZ and protective immunity might have resulted from selective activation of either the Th1 or Th2 cell subset. Protective immunity, in vitro proliferative responses, and lymphokine production were tested in BALB/c, C57Bl/6, and C3H/HeN mice immunized according to different SPZ regimens. The levels of IL-2 paralleled the proliferative reactivities in each mouse strain examined. Although IFN gamma levels were present in the unprimed lymphocyte cultures, they increased following each SPZ immunization, in C57Bl/6, moderate in C3H/HeN, and lowest in BALB/c splenic cultures. Surprisingly, no IL-4 was detected in splenic cultures from any mouse strain during proliferative activity or protective immunity. In contrast, elevated IL-6 production was noted after each immunization, regardless of the protective status and it correlated with anti-CSP IgG serum levels. These data establish that lymphokine profiles corresponding primarily to the Th1 cells were induced by immunization with P. berghei SPZ and that IL-4 secreting T cells were not induced by the SPZ-stage berghei antigens.

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

Expression of inducible nitric oxide (NO) synthase has been shown to inhibit the development of several pathogens, including fungi, bacteria, parasites, and viruses. However, there is still controversy as to whether this effector mechanism can inhibit the development of human pathogens. We now report that gamma interferon (IFN-gamma) induces the elimination of Plasmodium falciparum-infected primary human hepatocytes from cultures and that the antimalarial activity is dependent on NO. Infection with the parasite alone in the absence of added IFN-gamma caused a 10-fold increase in NO formation. Both spontaneous inhibition and IFN-gamma-induced inhibition of Plasmodium yoelii-infected murine hepatocytes were increased with the addition of the NO synthase cofactor tetrahydrobiopterin, or sepiapterin, which is converted to tetrahydrobiopterin. These results indicate that under in vitro conditions the parasite itself provides a signal that triggers induction of the NO pathway in human and murine hepatocytes and that NO formation in infected hepatocytes is limited by tetrahydrobiopterin availability.

Inducing protective immune responses against the sporozoite and liver stages of Plasmodium

Work on vaccines against the pre-erythrocytic stages of the Plasmodium life cycle is based on the observation that immunization with irradiated sporozoites (IRR SPZ) is protective. Antibodies against several SPZ surface proteins can prevent SPZ from effectively invading hepatocytes; antibodies and cytolytic-T lymphocytes directed against at least 3 parasite proteins expressed in infected hepatocytes can kill infected hepatocytes; and cytokines can activate infected hepatocytes to kill the intracellular parasite. Work is in progress to identify additional pre-erythrocytic parasite targets and to develop methods for optimally inducing protective immunity against SPZ and infected hepatocytes. The goal is to construct a vaccine that protects by inducing antibody and cellular immune responses against multiple parasite proteins.

Natural amino acid polymorphisms of the circumsporozoite protein of Plasmodium falciparum abrogate specific human CD4+ T cell responsiveness

Sequence polymorphism has been reported for virtually all malaria antigens and, in the case of the circumsporozoite (CS) protein, this variation is in the form of point mutations concentrated primarily in several regions recognized by T cells. The factors responsible for the variation are unknown. We studied the T cell responses to all known variants in malaria-exposed Thais. Memory CD4+ T cells responded to variants of a polymorphic immunodominant region (denoted Th2R), and CD4+ T cell clones specific for one Thai Th2R variant were generated. There was minimal cross-reactivity to any of the naturally occurring variants, including the other Thai variant, and competition studies performed with the clones using analog peptides demonstrated that all the substitutions of the polymorphic residues modulate either the binding of the peptide to major histocompatibility complex (MHC) molecules or the recognition by the T cell receptor of the peptide-MHC complex. Our data suggest that CD4+ T cells may be able to select parasites expressing variant sequences and have implications for development of a CS-based vaccine.

Evidence for limited activation of distinct CD4+ T cell subsets in response to the Plasmodium falciparum circumsporozoite protein in Papua New Guinea

Both CD4+ and CD8+ T cells, as well as antibody, are known to be important in sporozoite immunity. Data from animal studies suggest that cytokines, in particular gamma-interferon and interleukin-6, are involved. The interplay of these various factors and their importance in vaccine development has, however, not yet been elucidated. In this study, we have studied cellular and humoral responses of individuals naturally exposed to malaria in a highly endemic region of Papua New Guinea to the circumsporozoite protein of Plasmodium falciparum, a prime vaccine candidate antigen. A paucity of any CD4+ lymphoproliferative response to this protein by Papua New Guineans was notable which parallels our recent observation of a paucity of CD8+ T cell response and contrasts markedly with the responses of other endemic populations. There was nevertheless a significant antibody response to the central conserved B cell epitope, (NANP)n, as well as to other critical epitopes. An inverse relationship between gamma-interferon production and interleukin-6 production and a positive correlation between gamma-interferon production and CS peptide-specific lymphoproliferation was observed. High levels of peptide-specific IL-6 production were associated with high levels of peptide-specific serum antibodies. Our data provide evidence for the limited activation of distinct CD4+ T cell subsets and for the existence of functionally distinct subpopulations of human CD4+ T cells with respect to cytokines known to be important in sporozoite immunity.

The role of Kupffer cells in the clearance of malaria sporozoites from the circulation

In the past decade, one of the most intriguing subjects in understanding the mechanism of malaria infection has been explanation of the role of Kupffer cells. These liver cells, which play an important part in the body's defense against infection, seemed to have on essential supportive role in the homing o f sporozoites. Do Kupffer cells favor the establishment of primary malaria infection? Extensive research has revealed much, but still not everything we need to know about the sporozoite-Kupffer cell affair.

Kupffer cell elimination enhances development of liver schizonts of Plasmodium berghei in rats

We investigated the development of exoerythrocytic forms (EEF) of Plasmodium berghei in livers of normal and macrophage-depleted Brown Norway rats. Macrophages were depleted by use of liposome-encapsulated dichloromethylene diphosphonate. Upon inoculation of sporozoites, macrophage-depleted rats had significantly larger numbers of EEF than untreated rats. We also investigated the effect of macrophage impairment by silica treatment on the development of EEF and confirmed that silica induces a significant reduction of EEF development. Intravenous administration of silica induced high levels of interleukin-6 in plasma within a few hours. The seemingly contradictory results for EEF development may be explained by our previous observation that interleukin-6 strongly inhibits sporozoite penetration and EEF development in vivo. We conclude that in experimental infections with sporozoites, Kupffer cells inhibit rather than enhance EEF development.

In vivo induction of the nitric oxide pathway in hepatocytes after injection with irradiated malaria sporozoites, malaria blood parasites or adjuvants

The mechanisms responsible for malarial immunity induced by repetitive injections of X-irradiated sporozoites have not been fully established. We demonstrate here that a single injection of irradiated sporozoites induced, as soon as 24 h after, a non-permissive state to hepatocyte reinfection with sporozoites in vitro. The same effect was observed when malarial blood forms, irradiated promastigotes of Leishmania infantum, adjuvants (muramyl dipeptide, poly acidylic uridylic) or interferon-gamma was injected. Activation of the nitric oxide (NO) pathway in the hepatocyte by these factors was found to be responsible for hepatocyte refractory status. Additionally, this metabolic pathway is involved in protection given by repeated injections of irradiated sporozoites since protection could be reversed by treating mice at the time of sporozoite challenge with a competitive inhibitor (NG-monomethyl-L-arginine) of the NO pathway. These results suggest that, in view of an antisporozoite vaccine, further studies are needed to find out how to activate specifically a long-lasting nonspecific immune response.

'Original antigenic sin', T cell memory, and malaria sporozoite immunity: an hypothesis for immune evasion

Prior to any exposure to malaria, most adults have T cells specific for malaria parasites and various malaria proteins. The protein for which this has been shown more than any other is the circumsporozoite protein (CSP) of Plasmodium falciparum. These T cells can be present in high frequency and appear to have arisen through exposure to other (non-malaria) organisms. Although T cells are thought to provide protection against sporozoites, these T cells specific for cross-reactive organisms are clearly unable to protect against malaria, and may be preferentially expanded following exposure to malaria sporozoites. Thus, cross-reactive organisms have the potential to skew the repertoire of sporozoite-induced T cells and affect the induction of protective immunity. This is analogous to the concept of 'original antigenic sin' whereby prior exposure to one strain of influenza virus was shown to be able to divert the antibody response to a second challenging strain to focus on the shared (cross-reactive) epitopes.

Cytokines inhibit the development of liver schizonts of the malaria parasite Plasmodium berghei in vivo

The effect of induction of an acute-phase response and its mediators on the development of liver schizonts of the rodent malaria parasite Plasmodium berghei was investigated in Brown Norway rats. Subcutaneous injection of turpentine oil 24 h or 5 min before inoculation of sporozoites resulted in 80% and 35% reduction of schizont development, respectively. Turpentine oil induced high plasma levels of interleukin-6 (IL-6). Intraperitoneal administration of IL-1, IL-6 or both, significantly reduced liver schizont development. This reduction was also present if IL-6 had been administered 24 h after sporozoite inoculation. Inhibition induced by IL-1 could be prevented by simultaneous administration of polyclonal anti-IL-6. Administration of polyclonal anti-IL-6 without IL-1 resulted in a 40% increase of liver schizonts compared to control animals. We conclude that induction of an acute-phase response during experimental Plasmodium berghei infections in Brown Norway rats, strongly inhibits liver schizont development and that IL-6 is a key mediator in this process.

Malaria: becoming more specific about non-specific immunity

For the hundreds of millions of people presently infected with malaria, survival may depend on relatively non-specific immune effector mechanisms. Progress has been made in understanding the anti-parasitic properties of tumor necrosis factor-alpha, interferon-gamma and nitric oxide, in defining the parasite toxins that induce tumor necrosis factor-alpha production, and in exploring the role of cytokines and adhesion molecules in the pathogenesis of cerebral malaria.

High frequency of malaria-specific T cells in non-exposed humans

A major goal of current candidate malaria vaccines is to stimulate the expansion of clones of malaria-specific lymphocytes. We have examined the in vitro T cell responses of a group of malaria exposed and non-exposed adult Caucasian donors to recombinant circumsporozoite (CS) proteins, one of which is undergoing clinical trials, to blood-stage parasites, and to synthetic peptides copying the CS protein and defined blood-stage proteins. In nearly all individuals tested, CD4 T cell proliferation or lymphokine production occurred in response to whole parasite or CS protein stimulation, and T cells from many individuals responded to synthetic peptides. T cell responses were major histocompatibility complex-restricted, and stimulation of T cells with malaria parasites or CS protein did not appear to expand a population of T cell receptor gamma/delta cells. Malaria-specific responses were independent of prior malaria exposure, and in some cases exceeded the magnitude of response to tetanus toxoid. Specific T cells are present in high frequency in the peripheral blood of many donors who have never been exposed to malaria. Although malaria-specific CD4 T cells play an important role in immunity, these data question whether vaccines need to stimulate such cells, and focus attention on other aspects of malaria immunity which may be more critical to a successful vaccine.

Immunomodulatory agents in the laboratory and clinic

This paper reviews naturally occurring and synthetic compounds that either enhance immune defences or lower both natural and acquired immunity. Immunomodulatory agents used both for laboratory study and clinically for the management of immunologically based diseases are considered.

In vitro activity of CD4+ and CD8+ T lymphocytes from mice immunized with a synthetic malaria peptide

In previous work, a T-helper epitope was mapped within the circumsporozoite protein of the murine malaria parasite Plasmodium yoelii. A 21-mer synthetic peptide corresponding to this epitope (amino acid positions 59-79; referred to as Py1) induced a specific T-cell proliferation in BALB/c and C57BL/6 mice and provided help for the production of antibodies to peptides from the repetitive region, (Gln-Gly-Pro-Gly-Ala-Pro)n, of the P. yoelii circumsporozoite protein when mice were immunized with the Py1 peptide conjugated to the repetitive peptide. Experiments were then designed to study the in vitro antiparasite efficacy of T cells elicited in vivo by peptide immunization. T-cell activity was evaluated on cultured hepatic stages of P. yoelii. Peptide immunizations led to the preferential activation of CD8+ T cells in BALB/c mice and of both CD4+ and CD8+ T cells in C57BL/6 mice. Parasite elimination was mediated directly by these cells and did not seem to be dependent on lymphokine secretion. These data suggest that peptide-primed CD4+ T cells as well as CD8+ T cells could be cytolytic for the hepatic phase of malaria parasites. The fact that the same peptide could activate different lymphocyte populations, depending on the strain of mouse, highlights the importance of a better understanding of the fine mechanisms behind the immune responses to synthetic peptides being tested for malaria vaccine development.

The implications for malaria vaccine programs if memory T cells from non-exposed humans can respond to malaria antigens

Although the goal of current candidate vaccines is to expand a population of malaria antigen-specific lymphocytes, accumulating evidence suggests that peripheral blood of adult humans contains significant numbers of malaria-specific T cells prior to any exposure to vaccine or actual infection. The reason why such naive humans are susceptible to malaria infection may thus relate not to inadequate T-cell surveillance but to some other factor--possibly lack of suitable splenic modification. It is possible that current vaccine programs are misdirected because these other factors are not being addressed. The possibility of an attenuated vaccine should be re-examined.