The contribution of host cell-directed vs. parasite-directed immunity to the disease and dynamics of malaria infections

Hosts defend themselves against pathogens by mounting an immune response. Fully understanding the immune response as a driver of host disease and pathogen evolution requires a quantitative account of its impact on parasite population dynamics. Here, we use a data-driven modeling approach to quantify the birth and death processes underlying the dynamics of infections of the rodent malaria parasite, Plasmodium chabaudi, and the red blood cells (RBCs) it targets. We decompose the immune response into 3 components, each with a distinct effect on parasite and RBC vital rates, and quantify the relative contribution of each component to host disease and parasite density. Our analysis suggests that these components are deployed in a coordinated fashion to realize distinct resource-directed defense strategies that complement the killing of parasitized cells. Early in the infection, the host deploys a strategy reminiscent of siege and scorched-earth tactics, in which it both destroys RBCs and restricts their supply. Late in the infection, a "juvenilization" strategy, in which turnover of RBCs is accelerated, allows the host to recover from anemia while holding parasite proliferation at bay. By quantifying the impact of immunity on both parasite fitness and host disease, we reveal that phenomena often interpreted as immunopathology may in fact be beneficial to the host. Finally, we show that, across mice, the components of the host response are consistently related to each other, even when infections take qualitatively different trajectories. This suggests the existence of simple rules that govern the immune system's deployment.

Time-of-day of blood-feeding: effects on mosquito life history and malaria transmission

BACKGROUND

Biological rhythms allow organisms to compartmentalise and coordinate behaviours, physiologies, and cellular processes with the predictable daily rhythms of their environment. There is increasing recognition that the biological rhythms of mosquitoes that vector parasites are important for global health. For example, whether perturbations in blood foraging rhythms as a consequence of vector control measures can undermine disease control. To address this, we explore the impacts of altered timing of blood-feeding on mosquito life history traits and malaria transmission.

METHODS

We present three experiments in which Anopheles stephensi mosquitoes were fed in the morning or evening on blood that had different qualities, including: (i) chemical-induced or (ii) Plasmodium chabaudi infection-induced anaemia; (iii) Plasmodium berghei infection but no anaemia; or (iv) stemming from hosts at different times of day. We then compared whether time-of-day variation in blood meal characteristics influences mosquito fitness proxies relating to survival and reproduction, and malaria transmission proxies.

RESULTS

Mosquito lifespan is not influenced by the time-of-day they received a blood meal, but several reproductive metrics are affected, depending on other blood characteristics. Overall, our data suggest that receiving a blood meal in the morning makes mosquitoes more likely to lay eggs, lay slightly sooner and have a larger clutch size. In keeping with previous work, P. berghei infection reduces mosquito lifespan and the likelihood of laying eggs, but time-of-day of blood-feeding does not impact upon these metrics nor on transmission of this parasite.

CONCLUSION

The time-of-day of blood-feeding does not appear to have major consequences for mosquito fitness or transmission of asynchronous malaria species. If our results from a laboratory colony of mosquitoes living in benign conditions hold for wild mosquitoes, it suggests that mosquitoes have sufficient flexibility in their physiology to cope with changes in biting time induced by evading insecticide-treated bed nets. Future work should consider the impact of multiple feeding cycles and the abiotic stresses imposed by the need to forage for blood during times of day when hosts are not protected by bed nets.

Insight into the Effects of Plasmodium chabaudi on Platelets Using Carbon-Fiber Microelectrode Amperometry

Platelets are anuclear circulating cell bodies within the bloodstream commonly known for their roles in clot formation during vascular injury to prevent blood loss. They also have significant impact in a range of diseases, including malaria. However, the role of platelets in malaria is controversial, with contradicting evidence suggesting either that they assist in destruction of malarial parasites or facilitate a severe form of malaria. Precedent work suggests that the timing of infection is critical in determining whether platelets switch roles from being protective to deleterious. As such, the work herein makes use of the unique mechanistic perspective offered by carbon-fiber microelectrode amperometry (CFMA) to understand how platelet secretion is impacted in malarial infection stages (ascending parasite count versus descending parasite count). Malarial platelet behavior was compared to platelets from noninfected control mice by probing their exocytotic function. Results suggest that mouse malaria caused by the parasite Plasmodium chabaudi, during both ascending and descending infection stages, reduces platelet exocytotic events and delays platelet granule fusion; in addition, platelets are more impacted by the disease early in the infection stages. In all, understanding platelet behavior in the malarial context may present new therapeutic routes to treat or cure malaria.

Genomic and transcriptomic comparisons of closely related malaria parasites differing in virulence and sequestration pattern

Background: Malaria parasite species differ greatly in the harm they do to humans. While P. falciparum kills hundreds of thousands per year, P. vivax kills much less often and P. malariae is relatively benign. Strains of the rodent malaria parasite Plasmodium chabaudi show phenotypic variation in virulence during infections of laboratory mice. This make it an excellent species to study genes which may be responsible for this trait. By understanding the mechanisms which underlie differences in virulence we can learn how parasites adapt to their hosts and how we might prevent disease. Methods: Here we present a complete reference genome sequence for a more virulent P. chabaudi strain, PcCB, and perform a detailed comparison with the genome of the less virulent PcAS strain. Results: We found the greatest variation in the subtelomeric regions, in particular amongst the sequences of the pir gene family, which has been associated with virulence and establishment of chronic infection. Despite substantial variation at the sequence level, the repertoire of these genes has been largely maintained, highlighting the requirement for functional conservation as well as diversification in host-parasite interactions. However, a subset of pir genes, previously associated with increased virulence, were more highly expressed in PcCB, suggesting a role for this gene family in virulence differences between strains. We found that core genes involved in red blood cell invasion have been under positive selection and that the more virulent strain has a greater preference for reticulocytes, which has elsewhere been associated with increased virulence. Conclusions: These results provide the basis for a mechanistic understanding of the phenotypic differences between Plasmodium chabaudi strains, which might ultimately be translated into a better understanding of malaria parasites affecting humans.

Genomic and transcriptomic comparisons of closely related malaria parasites differing in virulence and sequestration pattern

Background: Malaria parasite species differ greatly in the harm they do to humans. While P. falciparum kills hundreds of thousands per year, P. vivax kills much less often and P. malariae is relatively benign. Strains of the rodent malaria parasite Plasmodium chabaudi show phenotypic variation in virulence during infections of laboratory mice. This make it an excellent species to study genes which may be responsible for this trait. By understanding the mechanisms which underlie differences in virulence we can learn how parasites adapt to their hosts and how we might prevent disease.

Methods: Here we present a complete reference genome sequence for a more virulent P. chabaudi strain, PcCB, and perform a detailed comparison with the genome of the less virulent PcAS strain.

Results: We found the greatest variation in the subtelomeric regions, in particular amongst the sequences of the pir gene family, which has been associated with virulence and establishment of chronic infection. Despite substantial variation at the sequence level, the repertoire of these genes has been largely maintained, highlighting the requirement for functional conservation as well as diversification in host-parasite interactions. However, a subset of pir genes, previously associated with increased virulence, were more highly expressed in PcCB, suggesting a role for this gene family in virulence differences between strains. We found that core genes involved in red blood cell invasion have been under positive selection and that the more virulent strain has a greater preference for reticulocytes, which has elsewhere been associated with increased virulence.

Conclusions: These results provide the basis for a mechanistic understanding of the phenotypic differences between Plasmodium chabaudi strains, which might ultimately be translated into a better understanding of malaria parasites affecting humans.

The impact of within-host ecology on the fitness of a drug-resistant parasite

Background and objectives

The rate of evolution of drug resistance depends on the fitness of resistant pathogens. The fitness of resistant pathogens is reduced by competition with sensitive pathogens in untreated hosts and so enhanced by competitive release in drug-treated hosts. We set out to estimate the magnitude of those effects on a variety of fitness measures, hypothesizing that competitive suppression and competitive release would have larger impacts when resistance was rarer to begin with.

Methodology

We infected mice with varying densities of drug-resistant Plasmodium chabaudi malaria parasites in a fixed density of drug-sensitive parasites and followed infection dynamics using strain-specific quantitative PCR.

Results

Competition with susceptible parasites reduced the absolute fitness of resistant parasites by 50–100%. Drug treatment increased the absolute fitness from 2- to >10 000-fold. The ecological context and choice of fitness measure was responsible for the wide variation in those estimates. Initial population growth rates poorly predicted parasite abundance and transmission probabilities.

Conclusions and implications

(i) The sensitivity of estimates of pathogen fitness to ecological context and choice of fitness measure make it difficult to derive field-relevant estimates of the fitness costs and benefits of resistance from experimental settings. (ii) Competitive suppression can be a key force preventing resistance from emerging when it is rare, as it is when it first arises. (iii) Drug treatment profoundly affects the fitness of resistance. Resistance evolution could be slowed by developing drug use policies that consider in-host competition.

Gene expression of the liver of vaccination-protected mice in response to early patent infections of Plasmodium chabaudi blood-stage malaria

BACKGROUND

The role of the liver for survival of blood-stage malaria is only poorly understood. In experimental blood-stage malaria with Plasmodium chabaudi, protective vaccination induces healing and, thus, survival of otherwise lethal infections. This model is appropriate to study the role of the liver in vaccination-induced survival of blood-stage malaria.

METHODS

Female Balb/c mice were vaccinated with a non-infectious vaccine consisting of plasma membranes isolated in the form of erythrocyte ghosts from P. chabaudi-infected erythrocytes at week 3 and week 1 before infection with P. chabaudi blood-stage malaria. Gene expression microarrays and quantitative real-time PCR were used to investigate the response of the liver, in terms of expression of mRNA and long intergenic non-coding (linc)RNA, to vaccination-induced healing infections and lethal P. chabaudi malaria at early patency on day 4 post infection, when parasitized erythrocytes begin to appear in peripheral blood.

RESULTS

In vaccination-induced healing infections, 23 genes were identified to be induced in the liver by > tenfold at p < 0.01. More than one-third were genes known to be involved in erythropoiesis, such as Kel, Rhag, Ahsp, Ermap, Slc4a1, Cldn13 Gata1, and Gfi1b. Another group of > tenfold expressed genes include genes involved in natural cytotoxicity, such as those encoding killer cell lectin-like receptors Klrb1a, Klrc3, Klrd1, the natural cytotoxicity-triggering receptor 1 Ncr1, as well as the granzyme B encoding Gzmb. Additionally, a series of genes involved in the control of cell cycle and mitosis were identified: Ccnb1, Cdc25c, Ckap2l were expressed > tenfold only in vaccination-protected mice, and the expression of 22 genes was at least 100% higher in vaccination-protected mice than in non-vaccinated mice. Furthermore, distinct lincRNA species were changed by > threefold in livers of vaccination-protected mice, whereas lethal malaria induced different lincRNAs.

CONCLUSION

The present data suggest that protective vaccination accelerates the malaria-induced occurrence of extramedullary erythropoiesis, generation of liver-resident cytotoxic cells, and regeneration from malaria-induced injury in the liver at early patency, which may be critical for final survival of otherwise lethal blood-stage malaria of P. chabaudi.

Host Energy Source Is Important for Disease Tolerance to Malaria

Pathologic infections are accompanied by a collection of short-term behavioral perturbations collectively termed sickness behaviors [1, 2]. These include changes in body temperature, reduced eating and drinking, and lethargy and mimic behaviors of animals in torpor and hibernation [1, 3-6]. Sickness behaviors are important, pathogen-specific components of the host response to infection [1, 3, 7-9]. In particular, host anorexia has been shown to be beneficial or detrimental depending on the infection [7, 8]. While these studies have illuminated the effects of anorexia on infection, they consider this behavior in isolation from other behaviors and from its effects on host metabolism and energy. Here, we explored the temporal dynamics of multiple sickness behaviors and their effect on host energy and metabolism throughout infection. We used the Plasmodium chabaudi AJ murine model of malaria as it causes severe pathology from which most animals recover. We found that infected animals did become anorexic, skewing their metabolism toward fatty acid oxidation and ketosis. Metabolism of fats requires oxygen for the production of ATP. In this model, animals also suffer severe anemia, limiting their ability to carry oxygen concurrent with their switch toward fatty acid metabolism. We reasoned that the combination of anorexia and anemia would increase pressure on glycolysis as a critical energy pathway because it does not require oxygen. Treating infected mice when anorexic with the glycolytic inhibitor 2-deoxyglucose (2DG) reduced survival; treating animals with glucose improved survival. Peak parasite loads were unchanged, demonstrating changes in disease tolerance. Parasite clearance was reduced with 2DG treatment, suggesting altered resistance.

A Macrophage Colony-Stimulating-Factor-Producing γδ T Cell Subset Prevents Malarial Parasitemic Recurrence

Despite evidence that γδ T cells play an important role during malaria, their precise role remains unclear. During murine malaria induced by Plasmodium chabaudi infection and in human P. falciparum infection, we found that γδ T cells expanded rapidly after resolution of acute parasitemia, in contrast to αβ T cells that expanded at the acute stage and then declined. Single-cell sequencing showed that TRAV15N-1 (Vδ6.3) γδ T cells were clonally expanded in mice and had convergent complementarity-determining region 3 sequences. These γδ T cells expressed specific cytokines, M-CSF, CCL5, CCL3, which are known to act on myeloid cells, indicating that this γδ T cell subset might have distinct functions. Both γδ T cells and M-CSF were necessary for preventing parasitemic recurrence. These findings point to an M-CSF-producing γδ T cell subset that fulfills a specialized protective role in the later stage of malaria infection when αβ T cells have declined.

In vivo validation of anti-malarial activity of crude extracts of Terminalia macroptera, a Malian medicinal plant

BACKGROUND

Plasmodium falciparum malaria is still one of the most deadly pathology worldwide. Efficient treatment is jeopardized by parasite resistance to artemisinin and its derivatives, and by poor access to treatment in endemic regions. Anti-malarial traditional remedies still offer new tracks for identifying promising antiplasmodial molecules, and a way to ensure that all people have access to care. The present study aims to validate the traditional use of Terminalia macroptera, a Malian plant used in traditional medicine.

METHODS

Terminalia macroptera was collected in Mali. Leaves (TML) and roots ethanolic extracts (TMR) were prepared and tested at 2000 mg/kg for in vivo acute toxicity in Albino Swiss mice. Antiplasmodial activity of the extracts was assessed against a chloroquine resistant strain P. falciparum (FcB1) in vitro. In vivo, anti-malarial efficacy was assessed by a 4-day suppressive test at 100 mg/kg in two malaria murine models of uncomplicated malaria (Plasmodium chabaudi chabaudi infection) and cerebral malaria (Plasmodium berghei strain ANKA infection). Constituents of TMR were characterized by ultra-high-performance liquid chromatography coupled to high resolution mass spectrometry. Top ranked compounds were putatively identified using plant databases and in silico fragmentation pattern.

RESULTS

Lethal dose of TML and TMR were greater than 2000 mg/kg in Albino Swiss mice. According to the OECD's Globally Harmonized System of Classification, both extracts are non-toxic orally. Antiplasmodial activity of T. macroptera extracts was confirmed in vitro against P. falciparum FcB1 strain with IC50 values of 1.2 and 1.6 µg/mL for TML and TMR, respectively. In vivo, oral administration of TML and TMR induced significant reduction of parasitaemia (37.2 and 46.4% respectively) in P. chabaudi chabaudi infected mice at the 7th day of infection compared to untreated mice. In the cerebral malaria experimental model, mice treated with TMR and TML presented respectively 50 and 66.7% survival rates at day 9 post-infection when all untreated mice died. Eleven major compounds were found in TMR. Among them, several molecules already known could be responsible for the antiplasmodial activity of the roots extract of T. macroptera.

CONCLUSIONS

This study confirms both safety and anti-malarial activity of T. macroptera, thus validating its traditional use.

A nutrient mediates intraspecific competition between rodent malaria parasites in vivo

Hosts are often infected with multiple strains of a single parasite species. Within-host competition between parasite strains can be intense and has implications for the evolution of traits that impact patient health, such as drug resistance and virulence. Yet the mechanistic basis of within-host competition is poorly understood. Here, we demonstrate that a parasite nutrient, para-aminobenzoic acid (pABA), mediates competition between a drug resistant and drug susceptible strain of the malaria parasite, Plasmodium chabaudi. We further show that increasing pABA supply to hosts infected with the resistant strain worsens disease and changes the relationship between parasite burden and pathology. Our experiments demonstrate that, even when there is profound top-down regulation (immunity), bottom-up regulation of pathogen populations can occur and that its importance may vary during an infection. The identification of resources that can be experimentally controlled opens up the opportunity to manipulate competitive interactions between parasites and hence their evolution.

Resource limitation prevents the emergence of drug resistance by intensifying within-host competition

Slowing the evolution of antimicrobial resistance is essential if we are to continue to successfully treat infectious diseases. Whether a drug-resistant mutant grows to high densities, and so sickens the patient and spreads to new hosts, is determined by the competitive interactions it has with drug-susceptible pathogens within the host. Competitive interactions thus represent a good target for resistance management strategies. Using an in vivo model of malaria infection, we show that limiting a resource that is disproportionately required by resistant parasites retards the evolution of drug resistance by intensifying competitive interactions between susceptible and resistant parasites. Resource limitation prevented resistance emergence regardless of whether resistant mutants arose de novo or were experimentally added before drug treatment. Our work provides proof of principle that chemotherapy paired with an "ecological" intervention can slow the evolution of resistance to antimicrobial drugs, even when resistant pathogens are present at high frequencies. It also suggests that a broad range of previously untapped compounds could be used for treating infectious diseases.

VIRULENCE OF MIXED-CLONE AND SINGLE-CLONE INFECTIONS OF THE RODENT MALARIA PLASMODIUM CHABAUDI

Most evolutionary models treat virulence as an unavoidable consequence of microparasite replication and have predicted that in mixed-genotype infections, natural selection should favor higher levels of virulence than is optimal in genetically uniform infections. Increased virulence may evolve as a genetically fixed strategy, appropriate for the frequency of mixed infections in the population, or may occur as a conditional response to mixed infection, that is, a facultative strategy. Here we test whether facultative alterations in replication rates in the presence of competing genotypes occur and generate greater virulence. An important alternative, not currently incorporated in models of the evolution of virulence, is that host responses mounted against genetically diverse parasites may be more costly or less effective than those against genetically uniform parasites. If so, mixed clone infections will be more virulent for a given parasite replication rate. Two groups of mice were infected with one of two clones of Plasmodium chabaudi parasites, and three groups of mice were infected with 1:9, 5:5, or 9:1 mixtures of the same two clones. Virulence was assessed by monitoring mouse body weight and red blood cell density. Transmission stage densities were significantly higher in mixed- than in single-clone infections. Within treatment groups, transmission stage production increased with the virulence of the infection, a phenotypic correlation consistent with the genetic correlation assumed by much of the theoretical work on the evolution of virulence. Consistent with theoretical predictions of facultative alterations in virulence, we found that mice infected with both parasite clones lost more weight and had on average lower blood counts than those infected with single-clone infections. However, there was no consistent evidence of the mechanism invoked by evolutionary models that predict this effect. Replication rates and parasite densities were not always higher in ∗∗∗mixed-clone infections, and for a given replication rate or parasite density, mixed-clone infections were still more virulent. Instead, prolonged anemia and increased transmission may have occured because genetically diverse infections are less rapidly cleared by hosts. Differences in maximum weight loss occured even when there were comparable parasite densities in mixed- and single-clone infections. We suggest that mounting an immune response against more that one parasite genotype is more costly for hosts, which therefore suffer higher virulence.

GENETIC RELATIONSHIPS BETWEEN PARASITE VIRULENCE AND TRANSMISSION IN THE RODENT MALARIA PLASMODIUM CHABAUDI

Many parasites evolve to become virulent rather than benign mutualists. One of the major theoretical models of parasite virulence postulates that this is because rapid within-host replication rates are necessary for successful transmission (parasite fitness) and that virulence (damage to the host) is an unavoidable consequence of this rapid replication. Two fundamental assumptions underlying this so-called evolutionary trade-off model have rarely been tested empirically: (1) that higher replication rates lead to higher levels of virulence; and (2) that higher replication rates lead to higher transmission. Both of these relationships must have a genetic basis for this evolutionary hypothesis to be relevant. These assumptions were tested in the rodent malaria parasite, Plasmodium chabaudi, by examining genetic relationships between virulence and transmission traits across a population of eight parasite clones isolated from the wild. Each clone was injected into groups of inbred mice in a controlled laboratory environment, and replication rate (measured by maximum asexual parasitemia), virulence (measured by live-weight loss and degree of anemia in the mouse), and transmission (measured by density of sexual forms, gametocytes, in the blood and proportion of mosquitoes infected after taking a blood-meal from the mouse) were assessed. It was found that clones differed widely in these traits and these clone differences were repeatable over successive blood passages. Virulence traits were strongly phenotypically and genetically (i.e., across clones) correlated to maximum parasitemia thus supporting the first assumption that rapid replication causes higher virulence. Transmission traits were also positively phenotypically and genetically correlated to parasitemia, which supports the second assumption that rapid replication leads to higher transmission. Thus, two assumptions of the parasite-centered trade-off model of the evolution of virulence were shown to be justified in malaria parasites.

ICAM-1 is a key receptor mediating cytoadherence and pathology in the Plasmodium chabaudi malaria model

Background

Parasite cytoadherence within the microvasculature of tissues and organs of infected individuals is implicated in the pathogenesis of several malaria syndromes. Multiple host receptors may mediate sequestration. The identity of the host receptor(s), or the parasite ligand(s) responsible for sequestration of Plasmodium species other than Plasmodium falciparum is largely unknown. The rodent malaria parasites may be useful to model interactions of parasite species, which lack the var genes with their respective hosts, as other multigene families are shared between the species. The role of the endothelial receptors ICAM-1 and CD36 in cytoadherence and in the development of pathology was investigated in a Plasmodium chabaudi infection in C57BL/6 mice lacking these receptors. The schizont membrane-associated cytoadherence (SMAC) protein of Plasmodium berghei has been shown to exhibit reduced CD36-associated cytoadherence in P. berghei ANKA-infected mice.

Methods

Parasite tissue sequestration and the development of acute stage pathology in P. chabaudi infections of mice lacking CD36 or ICAM-1, their respective wild type controls, and in infections with mutant P. chabaudi parasites lacking the smac gene were compared. Peripheral blood parasitaemia, red blood cell numbers and weight change were monitored throughout the courses of infection. Imaging of bioluminescent parasites in isolated tissues (spleen, lungs, liver, kidney and gut) was used to measure tissue parasite load.

Results

This study shows that neither the lack of CD36 nor the deletion of the smac gene from P. chabaudi significantly impacted on acute-stage pathology or parasite sequestration. By contrast, in the absence of ICAM-1, infected animals experience less anaemia and weight loss, reduced parasite accumulation in both spleen and liver and higher peripheral blood parasitaemia during acute stage malaria. The reduction in parasite tissue sequestration in infections of ICAM-1 null mice is maintained after mosquito transmission.

Conclusions

These results indicate that ICAM-1-mediated cytoadherence is important in the P. chabaudi model of malaria and suggest that for rodent malarias, as for P. falciparum, there may be multiple host and parasite molecules involved in sequestration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1834-8) contains supplementary material, which is available to authorized users.

Protective vaccination and blood-stage malaria modify DNA methylation of gene promoters in the liver of Balb/c mice

Epigenetic mechanisms such as DNA methylation are increasingly recognized to be critical for vaccination efficacy and outcome of different infectious diseases, but corresponding information is scarcely available for host defense against malaria. In the experimental blood-stage malaria Plasmodium chabaudi, we investigate the possible effects of a blood-stage vaccine on DNA methylation of gene promoters in the liver, known as effector against blood-stage malaria, using DNA methylation microarrays. Naturally susceptible Balb/c mice acquire, by protective vaccination, the potency to survive P. chabaudi malaria and, concomitantly, modifications of constitutive DNA methylation of promoters of numerous genes in the liver; specifically, promoters of 256 genes are hyper(=up)- and 345 genes are hypo(=down)-methylated (p < 0.05). Protective vaccination also leads to changes in promoter DNA methylation upon challenge with P. chabaudi at peak parasitemia on day 8 post infection (p.i.), when 571 and 1013 gene promoters are up- and down-methylated, respectively, in relation to constitutive DNA methylation (p < 0.05). Gene set enrichment analyses reveal that both vaccination and P. chabaudi infections mainly modify promoters of those genes which are most statistically enriched with functions relating to regulation of transcription. Genes with down-methylated promoters encompass those encoding CX3CL1, GP130, and GATA2, known to be involved in monocyte recruitment, IL-6 trans-signaling, and onset of erythropoiesis, respectively. Our data suggest that vaccination may epigenetically improve parts of several effector functions of the liver against blood-stage malaria, as, e.g., recruitment of monocyte/macrophage to the liver accelerated liver regeneration and extramedullary hepatic erythropoiesis, thus leading to self-healing of otherwise lethal P. chabaudi blood-stage malaria.

Differential miRNA Expression in the Liver of Balb/c Mice Protected by Vaccination during Crisis of Plasmodium chabaudi Blood-Stage Malaria

MicroRNAs are increasingly recognized as epigenetic regulators for outcome of diverse infectious diseases and vaccination efficacy, but little information referring to this exists for malaria. This study investigates possible effects of both protective vaccination and P. chabaudi malaria on the miRNome of the liver as an effector against blood-stage malaria using miRNA microarrays and quantitative PCR. Plasmodium chabaudi blood-stage malaria takes a lethal outcome in female Balb/c mice, but a self-healing course after immunization with a non-infectious blood-stage vaccine. The liver robustly expresses 71 miRNA species at varying levels, among which 65 miRNA species respond to malaria evidenced as steadily increasing or decreasing expressions reaching highest or lowest levels toward the end of the crisis phase on day 11 p.i. in lethal malaria. Protective vaccination does not affect constitutive miRNA expression, but leads to significant (p < 0.05) changes in the expression of 41 miRNA species, however evidenced only during crisis. In vaccination-induced self-healing infections, 18 miRNA-species are up- and 14 miRNA-species are down-regulated by more than 50% during crisis in relation to non-vaccinated mice. Vaccination-induced self-healing and survival of otherwise lethal infections of P. chabaudi activate epigenetic miRNA-regulated remodeling processes in the liver manifesting themselves during crisis. Especially, liver regeneration is accelerated as suggested by upregulation of let-7a-5p, let-7b-5p, let-7c-5p, let-7d-5p, let-7f-5p, let-7g-5p, let-7i-5p, miR-26a, miR-122-5p, miR30a, miR27a, and mir-29a, whereas the up-regulated expression of miR-142-3p by more than 100% is compatible with the view of enhanced hepatic erythropoiesis, possibly at expense of megakaryopoiesis, during crisis of P. chabaudi blood-stage malaria.

Beneficial effect of Punica granatum peel extract on murine malaria-induced spleen injury

BACKGROUND

Multiple drug-resistant malaria parasites have been widely detected, which has encouraged research studies focused on discovering alternative therapies. Medicinal plants such as pomegranate, Punica granatum, have been proven to exhibit antiprotozoal effects and therefore, we examined its effects on murine malaria-induced splenic injury and oxidative stress in this study.

METHODS

Mice were divided into three groups, a vehicle control and two groups that were infected with 10(6) Plasmodium chabaudi-parasitized red blood cells (RBCs). The third group was gavaged with 100 μL of 300 mg/kg pomegranate peel extract for 6 days. All mice were euthanized on day 6 post-infection.

RESULTS

The results revealed the potential antimalarial, antioxidant, and anti-inflammatory effects of pomegranate. Furthermore, pomegranate peel extracts significantly reduced parasitemia and spleen index of the treated mice compared to the untreated group. Additionally, the spleen histology score supported the findings by showing better amelioration in the pomegranate-treated mice than in the untreated mice. Concomitantly, the spleen capsule thickness showed clear evidence of splenomegaly in the untreated mice, as evidenced by the reduced spleen capsule. However, pomegranate peel extract exhibited a remarkable restorative effect on the spleen capsules of the treated mice. Moreover, the extract significantly reduced the expression levels of the proinflammatory cytokines interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ as well as inducible nitric oxide synthase (iNOS). Moreover, our study showed that pomegranate extract profoundly affected oxidative stress levels by reducing the oxidant molecules, nitric oxide (NO) and malondialdehyde (MDA).

CONCLUSION

This study showed that pomegranate clearly induced antimalarial activity in the host by attenuating inflammatory and oxidative stress responses. Furthermore, pomegranate enhanced the innate immune responses and, therefore, could serve an alternative therapy to control clinical malaria episodes and may protect against malaria infection.

Protective Vaccination against Blood-Stage Malaria of Plasmodium chabaudi: Differential Gene Expression in the Liver of Balb/c Mice toward the End of Crisis Phase

Protective vaccination induces self-healing of otherwise fatal blood-stage malaria of Plasmodium chabaudi in female Balb/c mice. To trace processes critically involved in self-healing, the liver, an effector against blood-stage malaria, is analyzed for possible changes of its transcriptome in vaccination-protected in comparison to non-protected mice toward the end of the crisis phase. Gene expression microarray analyses reveal that vaccination does not affect constitutive expression of mRNA and lincRNA. However, malaria induces significant (p < 0.01) differences in hepatic gene and lincRNA expression in vaccination-protected vs. non-vaccinated mice toward the end of crisis phase. In vaccination-protected mice, infections induce up-regulations of 276 genes and 40 lincRNAs and down-regulations of 200 genes and 43 lincRNAs, respectively, by >3-fold as compared to the corresponding constitutive expressions. Massive up-regulations, partly by >100-fold, are found for genes as RhD, Add2, Ank1, Ermap, and Slc4a, which encode proteins of erythrocytic surface membranes, and as Gata1 and Gfi1b, which encode transcription factors involved in erythrocytic development. Also, Cldn13 previously predicted to be expressed on erythroblast surfaces is up-regulated by >200-fold, though claudins are known as main constituents of tight junctions acting as paracellular barriers between epithelial cells. Other genes are up-regulated by <100- and >10-fold, which can be subgrouped in genes encoding proteins known to be involved in mitosis, in cell cycle regulation, and in DNA repair. Our data suggest that protective vaccination enables the liver to respond to P. chabaudi infections with accelerated regeneration and extramedullary erythropoiesis during crisis, which contributes to survival of otherwise lethal blood-stage malaria.

Facilitation through altered resource availability in a mixed-species rodent malaria infection

A major challenge in disease ecology is to understand how co-infecting parasite species interact. We manipulate in vivo resources and immunity to explain interactions between two rodent malaria parasites, Plasmodium chabaudi and P. yoelii. These species have analogous resource-use strategies to the human parasites Plasmodium falciparum and P. vivax: P. chabaudi and P. falciparum infect red blood cells (RBC) of all ages (RBC generalist); P. yoelii and P. vivax preferentially infect young RBCs (RBC specialist). We find that: (1) recent infection with the RBC generalist facilitates the RBC specialist (P. yoelii density is enhanced ~10 fold). This occurs because the RBC generalist increases availability of the RBC specialist's preferred resource; (2) co-infections with the RBC generalist and RBC specialist are highly virulent; (3) and the presence of an RBC generalist in a host population can increase the prevalence of an RBC specialist. Thus, we show that resources shape how parasite species interact and have epidemiological consequences.

Altered renal immune complexes deposition in female BWF1 lupus mice following Plasmodium chabaudi infection

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease that has a mysterious relationship with malaria infection. The current study was designated to compare between the effect of the live and the gamma irradiated Plasmodium chabaudi infection on BWF1 lupus murine model. A total of 30 female BWF1 mice were randomly divided into three groups (10 mice/group) as follows: group (I) lupus group (lupus non infected); group (II) live malaria infected group (lupus + live malaria infection); and group (III) irradiated malaria-infected group (lupus + gamma irradiated malaria infection). Live P. chabaudi infection was accompanied with a decrease in survival rate and food consumption in comparison to the control group of mice while gamma irradiated P. chabaudi -infection was unable to do this effect. Additionally, live P. chabaudi infection was accompanied with an increased level of proteinuria and increased rate of immune complexes deposition in kidney. Moreover, infection with live, but not gamma-irradiated P. chabaudi was accompanied with an increase in nitric oxide (NO), hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) levels in plasma of lupus mice. The levels of both total cholesterol and triglycerides in plasma of lupus mice after live P. chabaudi infection were obviously decreased in comparison to the control group. On the other hand, gamma-irradiated P. chabaudi infection resembled the control group. Our data revealed that infection of lupus mice with live but not gamma-irradiated P. chabaudi has several histological and biochemical effects.

Protective effect of berberine chloride on Plasmodium chabaudi-induced hepatic tissue injury in mice

The present study aimed to investigate the protective role of berberine (BER) against Plasmodium chabaudi-induced infection in mice. Animals were divided into three groups. Group I served as a vehicle control. Group II and group III were infected with 1000 P. chabaudi infected erythrocytes. Group III was gavaged with 100 μl of 10 mg/kg berberine chloride for 10 days. All mice were sacrificed at day 10 post-infection. The percentage of parasitemia was significantly reduced more than 30%, after treatment of mice with BER. Infection caused marked hepatic injuries as indicated by histopathological alterations as evidenced by the presence of hepatic lobular inflammatory cellular infiltrations, dilated sinusoids, vacuolated hepatocytes, increased number of Kupffer cells and the malaria pigment, hemozoin. These changes in livers led to the increased histological score. Also, infection induced a significant increase in liver alanine aminotransferase and aspartate aminotransferase and a significant increase in the total leucocytic count. Moreover, mice became anemic as proved by the significant decrease in erythrocyte number and haemoglobin content. BER showed a significant protective potential by improving the above mentioned parameters. Based on these results, it is concluded that berberine could offer protection against hepatic tissue damage.

Understanding genetic variation in in vivo tolerance to artesunate: implications for treatment efficacy and resistance monitoring

Artemisinin-based drugs are the front-line weapon in the treatment of human malaria cases, but there is concern that recent reports of slow clearing infections may signal developing resistance to treatment. In the absence of molecular markers for resistance, current efforts to monitor drug efficacy are based on the rate at which parasites are cleared from infections. However, some knowledge of the standing variation in parasite susceptibility is needed to identify a meaningful increase in infection half-life. Here, we show that five previously unexposed genotypes of the rodent malaria parasite Plasmodium chabaudi differ substantially in their in vivo response to treatment. Slower clearance rates were not linked to parasite virulence or growth rate, going against the suggestion that drug treatment will drive the evolution of virulence in this system. The level of variation observed here in a relatively small number of genotypes suggests existing ‘resistant’ parasites could be present in the population and therefore, increased parasite clearance rates could represent selection on pre-existing variation rather than de novo resistance events. This has implications for resistance monitoring as susceptibility may depend on evolved traits unrelated to drug exposure.

Pharmacokinetics of artemisinin delivered by oral consumption of Artemisia annua dried leaves in healthy vs. Plasmodium chabaudi-infected mice

ETHNOPHARMACOLOGICAL RELEVANCE

The Chinese have used Artemisia annua as a tea infusion to treat fever for >2000 years. The active component is artemisinin. Previously we showed that when compared to mice fed an equal amount of pure artemisinin, a single oral dose of dried leaves of Artemisia annua (pACT) delivered to Plasmodium chabaudi-infected mice reduced parasitemia at least fivefold. Dried leaves also delivered >40 times more artemisinin in the blood with no toxicity. The pharmacokinetics (PK) of artemisinin delivered from dried plant material has not been adequately studied.

MATERIALS AND METHODS

Healthy and Plasmodium chabaudi-infected mice were oral gavaged with pACT to deliver a 100 mg kg(-1) body weight dose of artemisinin. Concentrations of serum artemisinin and one of its liver metabolites, deoxyartemisinin, were measured over two hours by GCMS.

RESULTS

The first order elimination rate constant for artemisinin in pACT-treated healthy mice was estimated to be 0.80 h(-1) with an elimination half-life (T½) of 51.6 min. The first order absorption rate constant was estimated at 1.39 h(-1). Cmax and Tmax were 4.33 mg L(-1) and 60 min, respectively. The area under the curve (AUC) was 299.5 mg min L(-1). In contrast, the AUC for pACT-treated infected mice was significantly greater at 435.6 mg min L(-1). Metabolism of artemisinin to deoxyartemisinin was suppressed in infected mice over the period of observation. Serum levels of artemisinin in the infected mice continued to rise over the 120 min of the study period, and as a result, the T½ was not determined; the Cmax and Tmax were estimated at ≥6.64 mgL(-1) and ≥120 min, respectively. Groups of healthy mice were also fed either artemisinin or artemisinin mixed in mouse chow. When compared at 60 min, artemisinin was undetectable in the serum of mice fed 100 mg AN kg(-1) body weight. When plant material was present either as mouse chow or Artemisia annua pACT, artemisinin levels in the serum rose to 2.44 and 4.32 mg L(-1), respectively, indicating that the presence of the plant matrix, even that of mouse chow, had a positive impact on the appearance of artemisinin in the blood.

CONCLUSIONS

These results showed that artemisinin and one of its drug metabolites were processed differently in healthy and infected mice. The results have implications for possible therapeutic use of pACT in treating malaria and other artemisinin-susceptible diseases.

Quantifying variation in the potential for antibody-mediated apparent competition among nine genotypes of the rodent malaria parasite Plasmodium chabaudi

Within-host competition among parasite genotypes affects epidemiology as well as the evolution of virulence. In the rodent malaria Plasmodium chabaudi, competition among genotypes, as well as clone-specific and clone-transcending immunity are well documented. However, variation among genotypes in the induction of antibodies is not well understood, despite the important role of antibodies in the clearance of malaria infection. Here, we quantify the potential for antibodies induced by one clone to bind another (i.e., to cause antibody-mediated apparent competition) for nine genetically distinct P. chabaudi clones. We hypothesised that clones would vary in the strength of antibody induction, and that the propensity for clone-transcending immunity between a pair of clones would increase with increasing genetic relatedness at key antigenic loci. Using serum collected from mice 35 days post-infection, we measured titres of antibody to an unrelated antigen, Keyhole Limpet Haemocyanin (KLH), and two malaria antigens: recombinant Apical Membrane Antigen-1 (AMA-1) and Merozoite Surface Protein-119 (MSP-119). Amino acid sequence homology within each antigenic locus was used as a measure of relatedness. We found significant parasite genetic variation for the strength of antibody induction. We also found that relatedness at MSP-119 but not AMA-1 predicted clone-transcending binding. Our results help explain the outcome of chronic-phase mixed infections and generate testable predictions about the pairwise competitive ability of P. chabaudi clones.