Protective immunity to erythrocytic Plasmodium chabaudi AS infection involves IFNgamma-mediated responses and a cellular infiltrate to the liver

Emerging concepts in T follicular helper cell responses to malaria

Antibody responses to malaria and candidate malaria vaccines are short-lived in children, leaving them susceptible to repeated malaria episodes. Because T follicular helper (T_FH) cells provide critical help to B cells to generate long-lived antibody responses, they have become the focus of recent studies of Plasmodium-infected mice and humans. The emerging data converge on common themes, namely, that malaria-induced T_H1 cytokines are associated with the activation of (i) T-like memory T_FH cells with impaired B cell helper function, and (ii) pre-T_FH cells that acquire Th1-like features (T-bet expression, IFN-γ production), which impede their differentiation into fully functional T_FH cells, thus resulting in germinal center dysfunction and suboptimal antibody responses. Deeper knowledge of T_FH cells in malaria could illuminate strategies to improve vaccines through modulating T_FH cell responses. This review summarizes emerging concepts in T_FH cell responses to malaria.

Testosterone-induced persistent susceptibility to Plasmodium chabaudi malaria: long-term changes of lincRNA and mRNA expression in the spleen

Testosterone (T) is known to induce persistent susceptibility to blood-stage malaria of Plasmodium chabaudi in otherwise resistant female C57BL/6 mice, which is associated with permanent changes in mRNA expression of the liver. Here, we investigate the spleen as the major effector against blood-stage malaria for any possible T-induced long-term effects on lincRNA and mRNA expression. Female C57BL/6 mice were treated with T for 3 weeks, then T was withdrawn for 12 weeks before challenging with P. chabaudi. LincRNA and mRNA expression was examined after 12 weeks of T-withdrawal and after subsequent infections using Agilent whole mouse genome oligo microarrays. Our data show for the first time long-term effects of T on lincRNA expression evidenced directly as persistent changes after T-withdrawal for 12 weeks and indirectly as altered responsiveness of expression to P. chabaudi infections. There are 3 lincRNA-species upregulated and 10 lincRNAs downregulated by more than 2-fold (p<0.01). In addition, 11 and 10 mRNAs are persistently up- and downregulated by T, respectively. These changes remain not sustained during infections at peak parasitemia, when 15 other lincRNAs and 9 other mRNAs exhibit an altered expression. The only exception is the Tnk1-mRNA encoding the non-receptor tyrosine kinase 1 that is persistently downregulated by 0.34-fold after T-withdrawal and that becomes upregulated by 5.9-fold upon infection at peak parasitemia, suggesting an involvement of tyrosine phosphorylation by Tnk1 in mediating long-term effects of T in the spleen. The T-induced changes in splenic mRNA expression are totally different to those previously observed in the liver. Collectively, our data support the view that T induces long-term organ-specific changes in both lincRNA and mRNA expression, that presumably contribute to organ-specific dysfunctions upon infection with blood-stage malaria of P. chabaudi.

Age-dependent sex bias in clinical malarial disease in hypoendemic regions

BACKGROUND AND OBJECTIVES

Experimental models show a male bias in murine malaria; however, extant literature on biases in human clinical malaria is inconclusive. Studies in hyperendemic areas document an absence of sexual dimorphism in clinical malaria. Data on sex bias in clinical malaria in hypoendemic areas is ambiguous--some reports note a male bias but do not investigate the role of differential mosquito exposure in that bias. Moreover, these studies do not examine whether the bias is age related. This study investigates whether clinical malaria in hypoendemic regions exhibits a sex bias and whether this bias is age-dependent. We also consider the role of vector exposure in this bias.

METHODS

Retrospective passive clinical malaria datasets (2002-2007) and active surveillance datasets (2000-2009) were captured for the hypoendemic Mumbai region in Western India. To validate findings, passive retrospective data was captured from a primary malaria clinic (2006-2007) in hypoendemic Rourkela (Eastern India). Data was normalized by determining percent slide-positivity rates (SPRs) for males and females, and parasite-positivity distributions were established across age groups. The Mann-Whitney test, Wilcoxon Signed Rank test, and Chi-square analysis were used to determine statistical significances.

RESULTS

In both the Mumbai and Rourkela regions, clinical malaria exhibited an adult male bias (p<0.01). A sex bias was not observed in children aged ≤10. Post-puberty, male SPRs were significantly greater than females SPRs (p<0.01). This adult male bias was observed for both vivax and falciparum clinical disease. Analysis of active surveillance data did not reveal an age or sex bias in the frequency of parasite positivity.

CONCLUSION

This study demonstrates an age-dependent sex bias in clinical malaria in hypoendemic regions and enhanced incidence of clinical malaria in males following puberty. Possible roles of sex hormones, vector exposure, co-infections, and other factors in this enhanced susceptibility are discussed.

Loss of ability to self-heal malaria upon taurine transporter deletion

Deletion of the taurine transporter gene (taut) results in lowered levels of taurine, the most abundant amino acid in mammals. Here, we show that taut-/- mice have lost their ability to self-heal blood-stage infections with Plasmodium chabaudi malaria. All taut-/- mice succumb to infections during crisis, while about 90% of the control taut(+/+) mice survive. The latter retain unchanged taurine levels even at peak parasitemia. Deletion of taut, however, results in the lowering of circulating taurine levels from 540 to 264 micromol/liter, and infections cause additional lowering to 192 micromol/liter. Peak parasitemia levels in taut-/- mice are approximately 60% higher than those in taut(+/+) mice, an elevation that is associated with increased systemic tumor necrosis factor alpha (TNF-alpha) and interleukin-1beta (IL-1beta) levels, as well as with liver injuries. The latter manifest as increased systemic ammonia levels, a perturbed capacity to entrap injected particles, and increased expression of genes encoding TNF-alpha, IL-1beta, IL-6, inducible nitric oxide synthase (iNOS), NF-kappaB, and vitamin D receptor (VDR). Autopsy reveals multiorgan failure as the cause of death for malaria-infected taut-/- mice. Our data indicate that taut-controlled taurine homeostasis is essential for resistance to P. chabaudi malaria. Taurine deficiency due to taut deletion, however, impairs the eryptosis of P. chabaudi-parasitized erythrocytes and expedites increases in systemic TNF-alpha, IL-1beta, and ammonia levels, presumably contributing to multiorgan failure in P. chabaudi-infected taut-/- mice.

Immune responses of NIH mice infected with avirulent and virulent strains of Plasmodium chabaudi adami single and mixed infections

An understanding of the nature of the immune response to asexual erythrocytic stages of malaria parasites will facilitate vaccine development by identifying which responses the vaccine should preferentially induce. The present study examined and compared the immune responses of NIH mice in either single or mixed infections with avirulent (DK) or virulent (DS) strains of Plasmodium chabaudi adami using the ELISA test for detecting and measurement of cytokines and antibody production. In both single and mixed infections, the study showed that both cell- and antibody-mediated responses were activated. In all experiments, an early relatively high level of IFN-gamma and IgG2a during the acute phase of the infection, and later elevation of IL-4 and IgG1, suggested that there was a sequential Th1/Th2 response. However, in the avirulent DK strain infection a stronger Th1 response was observed compared to the virulent DS strain-infection or in mixed infections. In the virulent DS infection, there was a stronger Th2 response compared to that in the DK and mixed infections. The faster proliferation rate of the virulent DS strain compared to the DK strain was also evident.

Parasite-derived plasma microparticles contribute significantly to malaria infection-induced inflammation through potent macrophage stimulation

There is considerable debate as to the nature of the primary parasite-derived moieties that activate innate pro-inflammatory responses during malaria infection. Microparticles (MPs), which are produced by numerous cell types following vesiculation of the cellular membrane as a consequence of cell death or immune-activation, exert strong pro-inflammatory activity in other disease states. Here we demonstrate that MPs, derived from the plasma of malaria infected mice, but not naive mice, induce potent activation of macrophages in vitro as measured by CD40 up-regulation and TNF production. In vitro, these MPs induced significantly higher levels of macrophage activation than intact infected red blood cells. Immunofluorescence staining revealed that MPs contained significant amounts of parasite material indicating that they are derived primarily from infected red blood cells rather than platelets or endothelial cells. MP driven macrophage activation was completely abolished in the absence of MyD88 and TLR-4 signalling. Similar levels of immunogenic MPs were produced in WT and in TNF(-/-), IFN-gamma(-/-), IL-12(-/-) and RAG-1(-/-) malaria-infected mice, but were not produced in mice injected with LPS, showing that inflammation is not required for the production of MPs during malaria infection. This study therefore establishes parasitized red blood cell-derived MPs as a major inducer of systemic inflammation during malaria infection, raising important questions about their role in severe disease and in the generation of adaptive immune responses.

Gastrointestinal nematode infection exacerbates malaria-induced liver pathology

Mixed parasite infections are common in many parts of the world, but little is known of the effects of concomitant parasite infections on the immune response or severity of clinical disease. We have used the nonlethal malaria infection model of Plasmodium chabaudi AS in combination with the gastrointestinal nematode Heligmosomoides bakeri polygyrus to investigate the impact of nematode infections on malarial morbidity and antimalarial immunity. The data demonstrate that wild-type C57BL/6 mice coinfected with both parasites simultaneously exhibit a striking increase in mortality, while mice deficient in IFN-gamma or IL-23 survive coinfection. The increase in mortality in wild-type mice was associated with severe liver pathology characterized by extensive coagulative necrosis and an increase in hepatic IFN-gamma, IL-17, and IL-22 mRNA expression. This is the first demonstration of increased malaria-associated pathology associated with a switch toward a proinflammatory environment, involving not only IFN-gamma but also the IL-17/IL-23 axis, as a result of coinfection with a gastrointestinal helminth.

Augmented particle trapping and attenuated inflammation in the liver by protective vaccination against Plasmodium chabaudi malaria

BACKGROUND

To date all efforts to develop a malaria vaccine have failed, reflecting the still fragmentary knowledge about protective mechanisms against malaria. In order to evaluate if vaccination changes responses of the anti-malaria effectors spleen and liver to blood stage malaria, BALB/c mice succumbing to infection with Plasmodium chabaudi were compared to those surviving after vaccination.

METHODS

Mice were vaccinated with host cell plasma membranes isolated from P. chabaudi-infected erythrocytes. Hepatic and splenic capacity to trap particulate material was determined after injection of fluorescent polystyrol beads. Hepatic gene expression was measured using real-time RT-PCR and Northern blotting.

RESULTS

Survival of BALB/c mice was raised from 0% to 80% and peak parasitaemia was decreased by about 30% by vaccination. Vaccination boosted particle trapping capacity of the liver during crisis when splenic trapping is minimal due to spleen 'closing'. It also attenuated malaria-induced inflammation, thus diminishing severe damages and hence liver failure. Vaccination increased hepatic IFN-gamma production but mitigated acute phase response. Vaccination has a complex influence on infection-induced changes in expression of hepatic nuclear receptors (CAR, FXR, RXR, and PXR) and of the metabolic enzymes Sult2a and Cyp7a1. Although vaccination decreased CAR mRNA levels and prevented Cyp7a1 suppression by the CAR ligand 1,2-bis [2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) on day 8 p.i., Sult2a-induction by TCPOBOP was restored.

CONCLUSION

These data support the view that the liver is an essential effector site for a vaccine against blood stage malaria: vaccination attenuates malaria-induced inflammation thus improving hepatic metabolic activity and particle trapping activity of the liver.

Deletion of LTbetaR augments male susceptibility to Plasmodium chabaudi

SUMMARY Disruption of the lymphotoxin beta receptor (LTbetaR) gene has been shown to result in enhanced resistance of female mice to blood-stage Plasmodium chabaudi malaria. Here, we investigate the effect of LTbetaR deletion on host defence of males. In contrast to females, male LTbetaR(-/-) mice do not exhibit any increase in resistance. Conversely, they are even more susceptible than wild-type C57BL/6 mice, which becomes evident after lowering circulating levels of testosterone by castration, which makes C57BL/6 males resistant, whereas LTbetaR(-/-) remain susceptible. Gene-expression analysis using cDNA arrays revealed no differences in immunological responses in spleen of malaria-resistant female and malaria-susceptible castrated male LTbetaR(-/-) mice. In the liver, however, expression levels of plasminogen activator inhibitor PAI1, chemokine CXCL10, dual specificity phosphatase DUSP1, and hydroxysteroid-specific sulfotransferases Sult2a1/2 were decreased 6- to 85-fold in susceptible castrated male LTbetaR(-/-) mice in comparison to resistant female LTbetaR(-/-) mice at maximal parasitaemia, as evidenced by Northern blot analyses. The present data support our previous view that the liver is involved in the combat against malarial blood stages and that down-regulation of the genes DUSP1 and Sult2a1/2 signals dysregulation of protective liver responses, thus possibly contributing to male susceptibility of LTbetaR(-/-) mice.

Testosterone responsiveness of spleen and liver in female lymphotoxin β receptor-deficient mice resistant to blood-stage malaria

Disrupted signaling through lymphotoxin beta receptor (LTbetaR) results in severe defects of the spleen and even loss of all other secondary lymphoid tissues, making mice susceptible to diverse infectious agents. Surprisingly, however, we find that female LTbetaR-deficient mice are even more resistant to blood stages of Plasmodium chabaudi malaria than wild-type C57BL/6 mice. Higher resistance of LTbetaR-deficient mice correlates with an earlier onset of reticulocytosis, and the period of anemia is shorter. After surviving fulminant parasitemias of about 35%, mice develop long-lasting protective immunity against homologous rechallenge, with both spleen and liver acting as anti-malaria effectors. Testosterone suppresses resistance, i.e. all mice succumb to infections during or shortly after peak parasitemia. At peak parasitemia, testosterone does not essentially affect cellularity and apoptosis in the spleen, but aggravates liver pathology in terms of increased cell swelling, numbers of apoptotic and binucleated cells and reduced serum alkaline phosphatase levels, and conversely, reduces inflammatory lymphocytic infiltrates in the liver. In the spleen, hybridization of cDNA arrays identified only a few testosterone-induced changes in gene expression, in particular upregulation of INFgamma and IFN-regulated genes. By contrast, a much larger number of testosterone-affectable genes was observed in the liver, including genes involved in regulation of the extracellular matrix, in chemokine and cytokine signaling, and in cell cycle control. Collectively, our data suggest that testosterone dysregulates the inflammatory response in spleen and liver during their differentiation to anti-malaria effectors in malaria-resistant female LTbetaR-deficient mice, thus contributing to the testosterone-induced lethal outcome of malaria.

Testosterone suppresses protective responses of the liver to blood-stage malaria

Testosterone induces a lethal outcome in otherwise self-healing blood-stage malaria caused by Plasmodium chabaudi. Here, we examine possible testosterone effects on the antimalaria effectors spleen and liver in female C57BL/6 mice. Self-healing malaria activates gating mechanisms in the spleen and liver that lead to a dramatic reduction in trapping activity, as measured by quantifying the uptake of 3-mum-diameter fluorescent polystyrol particles. However, testosterone delays malaria-induced closing of the liver, but not the spleen. Coincidently, testosterone causes an approximately 3- to 28-fold depression of the mRNA levels of nine malaria-responsive genes, out of 299 genes tested, only in the liver and not in the spleen, as shown by cDNA arrays and Northern blotting. Among these are the genes encoding plasminogen activator inhibitor (PAI1) and hydroxysteroid sulfotransferase (STA2). STA2, which detoxifies bile acids, is suppressed 10-fold by malaria and an additional 28-fold by testosterone, suggesting a severe perturbation of bile acid metabolism. PAI1 is protective against malaria, since disruption of the PAI1 gene results in partial loss of the ability to control the course of P. chabaudi infections. Collectively, our data indicate that the liver rather than the spleen is a major target organ for testosterone-mediated suppression of resistance against blood-stage malaria.

Plasmodium chabaudi adami: interferon-gamma but not IL-2 is essential for the expression of cell-mediated immunity against blood-stage parasites in mice

Cell-mediated immunity (CMI) may be important in immunity against blood-stage malaria. Accordingly, we examined the role of type 1 cytokines in the resolution of Plasmodium chabaudi adami malaria in mice genetically modified to have type 1 cytokine gene defects. Parasitemia was prolonged in double knockout (IL-2(-/-), IFNgamma(-/-)) mice compared to control mice. Despite deficiencies in gammadelta T cell and B cell subsets, these mice produced anti-malarial antibodies and eventually cured their infections, possibly by antibody-mediated immunity. However, because acute P. c. adami parasitemia may also be suppressed by CMI, the requirements for IL-2 and IFNgamma were evaluated in mice lacking B cells and functional IL-2 or IFNgamma genes. Acute malaria in J(H)(-/-), IL-2(-/-) mice was prolonged, but eventually cured. In contrast, J(H)(-/-), IFNgamma(-/-) mice developed unremitting parasitemia. These data strongly suggest that IFNgamma, but not IL-2, plays an essential role in the expression of CMI against P. c. adami infections. This finding may prove useful in developing malarial vaccines aimed at inducing CMI.

Mice deficient in interleukin-4 (IL-4) or IL-4 receptor alpha have higher resistance to sporozoite infection with Plasmodium berghei (ANKA) than do naive wild-type mice

BALB/c interleukin-4 (IL-4(-/-)) or IL-4 receptor-alpha (IL-4ralpha(-/-)) knockout (KO) mice were used to assess the roles of the IL-4 and IL-13 pathways during infections with the blood or liver stages of plasmodium in murine malaria. Intraperitoneal infection with the blood-stage erythrocytes of Plasmodium berghei (ANKA) resulted in 100% mortality within 24 days in BALB/c mice, as well as in the mutant mouse strains. However, when infected intravenously with the sporozoite liver stage, 60 to 80% of IL-4(-/-) and IL-4ralpha(-/-) mice survived, whereas all BALB/c mice succumbed with high parasitemia. Compared to infected BALB/c controls, the surviving KO mice showed increased NK cell numbers and expression of inducible nitric oxide synthase (iNOS) in the liver and were able to eliminate parasites early during infection. In vivo blockade of NO resulted in 100% mortality of sporozoite-infected KO mice. In vivo depletion of NK cells also resulted in 80 to 100% mortality, with a significant reduction in gamma interferon (IFN-gamma) production in the liver. These results suggest that IFN-gamma-producing NK cells are critical in host resistance against the sporozoite liver stage by inducing NO production, an effective killing effector molecule against Plasmodium. The absence of IL-4-mediated functions increases the protective innate immune mechanism identified above, which results in immunity against P. berghei infection in these mice, with no major role for IL-13.

Homogeneous Assays Allow Direct "In Well" Cytokine Level Quantification

We have developed several homogeneous cytokine assays based on TR-FRET using europium cryptate- and cross-linked allophycocyanin-conjugated antibodies. Taking advantage of homogeneous assays, we have coincubated the cells with the antibody pair during the stimulation step. The results show that the presence of cells in wells at the time of reading does not alter the results. Furthermore, this experimental procedure avoids cumbersome transfer and storage steps streamlining sample analysis. Finally, it facilitates the follow-up of cytokine production.

Immunity to asexual blood stage malaria and vaccine approaches

The development of a malaria vaccine seems to be a definite possibility despite the fact that even individuals with a life time of endemic exposure do not develop sterile immunity. An effective malaria vaccine would be invaluable in preventing malaria-associated deaths in endemic areas, especially amongst children less than 5 years of age and pregnant women. This review discusses our current understanding of immunity against the asexual blood stage of malaria - the stage that is responsible for the symptoms of the disease - and approaches to the design of an asexual blood stage vaccine.