Decoding the language of var genes and Plasmodium falciparum sequestration

Genetic diversity and natural selection analysis of VAR2CSA and vir genes: implication for vaccine development

Variable surface antigens (VSAs) encoded by var and vir genes in Plasmodium falciparum and Plasmodium vivax, respectively, are known to be involved in malaria pathogenesis and host immune escape through antigenic variations. Knowledge of the genetic diversity of these antigens is essential for malaria control and effective vaccine development. In this study, we analysed the genetic diversity and evolutionary patterns of two fragments (DBL2X and DBL3X) of VAR2CSA gene and four vir genes (vir 4, vir 12, vir 21 and vir 27) from different endemic regions, including Southeast Asia and sub-Saharan Africa. High levels of segregating sites (S) and haplotype diversity (Hd) were observed in both var and vir genes. Among vir genes, vir 12 (S = 131, Hd = 0.996) and vir 21 (S = 171, Hd = 892) were found to be more diverse as compared to vir 4 (S = 11, Hd = 0.748) and vir 27 (S = 23, Hd = 0.814). DBL2X (S = 99, Hd = 0.996) and DBL3X (S = 307, Hd = 0.999) fragments showed higher genetic diversity. Our analysis indicates that var and vir genes are highly diverse and follow the similar evolutionary pattern globally. Some codons showed signatures of positive or negative selection pressure, but vir and var genes are likely to be under balancing selection. This study highlights the high variability of var and vir genes and underlines the need of functional experimental studies to determine the most relevant allelic forms for effective progress towards vaccine formulation and testing.

A model of pregnancy-associated malaria for inducing adverse pregnancy outcomes in ICR mouse

BACKGROUND

Based on understanding of placental pathological features and safe medication in pregnancy-associated malaria (PAM), establishment of a stable pregnant mouse infection model with Plasmodium was urgently needed.

METHODS

ICR mice with vaginal plugs detected were randomly divided into post-pregnancy infection (Malaria+) and uninfected pregnancy (Malaria-) cohorts. Age-matched mice that had not been mated were infected as pre-pregnancy infection group (Virgin control), which were subsequently mated with ICR males. All mice were inoculated with 1 × 106Plasmodium berghei ANKA-infected RBCs by intraperitoneal injection, and the same amount of saline was given to Malaria- group. We recorded the incidence of adverse pregnancy outcomes and the amounts of offspring in each group.

RESULTS

The Virgin group mice were unable to conceive normally, and vaginal bleeding, abortion, or stillbirth appeared in the Malaria+ group. The incidence of adverse pregnancy outcomes was extremely high and statistically significant compared with the control (Malaria-) group (P < 0.05), of which placenta exhibited pathological features associated with human gestational malaria.

CONCLUSIONS

The intraperitoneal injection of 1 × 106Plasmodium berghei ANKA-infected RBCs could establish a model of pregnancy-associated malaria in ICR mouse.

Investigation of Trypanosoma-induced vascular damage sheds insights into Trypanosoma vivax sequestration

Multiple blood-borne pathogens infecting mammals establish close interactions with the host vascular endothelium as part of their life cycles. In this work, we investigate differences in the interactions of three Trypanosoma species: T. brucei, T. congolense and T. vivax with the blood vasculature. Infection with these species results in vastly different pathologies, including different effects on vascular homeostasis, such as changes in vascular permeability and microhemorrhages. While all three species are extracellular parasites, T. congolense is strictly intravascular, while T. brucei is capable of surviving both extra- and intravascularly. Our knowledge regarding T. vivax tropism and its capacity of migration across the vascular endothelium is unknown. In this work, we show for the first time that T. vivax parasites sequester to the vascular endothelium of most organs, and that, like T. congolense, T. vivax Y486 is largely incapable of extravasation. Infection with this parasite species results in a unique effect on vascular endothelium receptors including general downregulation of ICAM1 and ESAM, and upregulation of VCAM1, CD36 and E-selectin. Our findings on the differences between the two sequestering species (T. congolense and T. vivax) and the non-sequestering, but extravasating, T. brucei raise important questions on the relevance of sequestration to the parasite's survival in the mammalian host, and the evolutionary relevance of both sequestration and extravasation.

Different PfEMP1-expressing Plasmodium falciparum variants induce divergent endothelial transcriptional responses during co-culture

The human malaria parasite Plasmodium falciparum is responsible for the majority of mortality and morbidity caused by malaria infection and differs from other human malaria species in the degree of accumulation of parasite-infected red blood cells in the microvasculature, known as cytoadherence or sequestration. In P. falciparum, cytoadherence is mediated by a protein called PfEMP1 which, due to its exposure to the host immune system, undergoes antigenic variation resulting in the expression of different PfEMP1 variants on the infected erythrocyte membrane. These PfEMP1s contain various combinations of adhesive domains, which allow for the differential engagement of a repertoire of endothelial receptors on the host microvasculature, with specific receptor usage associated with severe disease. We used a co-culture model of cytoadherence incubating human brain microvascular endothelial cells with erythrocytes infected with two parasite lines expressing different PfEMP1s that demonstrate different binding profiles to vascular endothelium. We determined the transcriptional profile of human brain microvascular endothelial cells (HBMEC) following different incubation periods with infected erythrocytes, identifying different transcriptional profiles of pathways previously found to be involved in the pathology of severe malaria, such as inflammation, apoptosis and barrier integrity, induced by the two PfEMP1 variants.

Genomic analysis of Indian isolates of Plasmodium falciparum: Implications for drug resistance and virulence factors

The emergence of drug resistance to frontline treatments such as Artemisinin-based combination therapy (ACT) is a major obstacle to the control and eradication of malaria. This problem is compounded by the inherent genetic variability of the parasites, as many established markers of resistance do not accurately predict the drug-resistant status. There have been reports of declining effectiveness of ACT in the West Bengal and Northeast regions of India, which have traditionally been areas of drug resistance emergence in the country. Monitoring the genetic makeup of a population can help to identify the potential for drug resistance markers associated with it and evaluate the effectiveness of interventions aimed at reducing the spread of malaria. In this study, we performed whole genome sequencing of 53 isolates of Plasmodium falciparum from West Bengal and compared their genetic makeup to isolates from Southeast Asia (SEA) and Africa. We found that the Indian isolates had a distinct genetic makeup compared to those from SEA and Africa, and were more similar to African isolates, with a high prevalence of mutations associated with antigenic variation genes. The Indian isolates also showed a high prevalence of markers of chloroquine resistance (mutations in Pfcrt) and multidrug resistance (mutations in Pfmdr1), but no known mutations associated with artemisinin resistance in the PfKelch13 gene. Interestingly, we observed a novel L152V mutation in PfKelch13 gene and other novel mutations in genes involved in ubiquitination and vesicular transport that have been reported to support artemisinin resistance in the early stages of ACT resistance in the absence of PfKelch13 polymorphisms. Thus, our study highlights the importance of region-specific genomic surveillance for artemisinin resistance and the need for continued monitoring of resistance to artemisinin and its partner drugs.

Special Issue: "Parasitic Infection and Host Immunity": Editorial

Parasite-host interactions depend on a complex interplay between the metabolism of the parasite, their antigens, and the host immune response system [...].

Neutrophils impose strong immune pressure against PfEMP1 variants implicated in cerebral malaria

Plasmodium falciparum, the deadliest form of human malaria, remains one of the major threats to human health in endemic regions. Its virulence is attributed to its ability to modify infected red blood cells (iRBC) to adhere to endothelial receptors by placing variable antigens known as PfEMP1 on the iRBC surface. PfEMP1 expression determines the cytoadhesive properties of the iRBCs and is implicated in severe malaria. To evade antibody‐mediated responses, the parasite undergoes continuous switches of expression between different PfEMP1 variants. Recently, it became clear that in addition to antibody‐mediated responses, PfEMP1 triggers innate immune responses; however, the role of neutrophils, the most abundant white blood cells in the human circulation, in malaria remains elusive. Here, we show that neutrophils recognize and kill blood‐stage P. falciparum isolates. We identify neutrophil ICAM‐1 and specific PfEMP1 implicated in cerebral malaria as the key molecules involved in this killing. Our data provide mechanistic insight into the interactions between neutrophils and iRBCs and demonstrate the important influence of PfEMP1 on the selective innate response to cerebral malaria.

Vivax Malaria and the Potential Role of the Subtelomeric Multigene vir Superfamily

Vivax malaria, caused by Plasmodium vivax, remains a public health concern in Central and Southeast Asia and South America, with more than two billion people at risk of infection. Compared to Plasmodium falciparum, P. vivax is considered a benign infection. However, in recent decades, incidences of severe vivax malaria have been confirmed. The P. falciparum erythrocyte membrane protein 1 family encoded by var genes is known as a mediator of severe falciparum malaria by cytoadherence property. Correspondingly, the vir multigene superfamily has been identified as the largest multigene family in P. vivax and is implicated in cytoadherence to endothelial cells and immune response activation. In this review, the functions of vir genes are reviewed in the context of their potential roles in severe vivax malaria.

Malaria-induced bacteremia as a consequence of multiple parasite survival strategies

Globally, malaria continues to be an enormous public health burden, with concomitant parasite-induced damage to the gastrointestinal (GI) barrier resulting in bacteremia-associated morbidity and mortality in both adults and children. Infected red blood cells sequester in and can occlude the GI microvasculature, ultimately leading to disruption of the tight and adherens junctions that would normally serve as a physical barrier to translocating enteric bacteria. Mast cell (MC) activation and translocation to the GI during malaria intensifies damage to the physical barrier and weakens the immunological barrier through the release of enzymes and factors that alter the host response to escaped enteric bacteria. In this context, activated MCs release Th2 cytokines, promoting a balanced Th1/Th2 response that increases local and systemic allergic inflammation while protecting the host from overwhelming Th1-mediated immunopathology. Beyond the mammalian host, recent studies in both the lab and field have revealed an association between a Th2-skewed host response and success of parasite transmission to mosquitoes, biology that is evocative of parasite manipulation of the mammalian host. Collectively, these observations suggest that malaria-induced bacteremia may be, in part, an unintended consequence of a Th2-shifted host response that promotes parasite survival and transmission. Future directions of this work include defining the factors and mechanisms that precede the development of bacteremia, which will enable the development of biomarkers to simplify diagnostics, the identification of therapeutic targets to improve patient outcomes and better understanding of the consequences of clinical interventions to transmission blocking strategies.

Malaria in Pregnancy: From Placental Infection to Its Abnormal Development and Damage

Malaria remains a global health burden with Plasmodium falciparum accounting for the highest mortality and morbidity. Malaria in pregnancy can lead to the development of placental malaria, where P. falciparum-infected erythrocytes adhere to placental receptors, triggering placental inflammation and subsequent damage, causing harm to both mother and her infant. Histopathological studies of P. falciparum-infected placentas revealed various placental abnormalities such as excessive perivillous fibrinoid deposits, breakdown of syncytiotrophoblast integrity, trophoblast basal lamina thickening, increased syncytial knotting, and accumulation of mononuclear immune cells within intervillous spaces. These events in turn, are likely to impair placental development and function, ultimately causing placental insufficiency, intrauterine growth restriction, preterm delivery and low birth weight. Hence, a better understanding of the mechanisms behind placental alterations and damage during placental malaria is needed for the design of effective interventions. In this review, using evidence from human studies and murine models, an integrated view on the potential mechanisms underlying placental pathologies in malaria in pregnancy is provided. The molecular, immunological and metabolic changes in infected placentas that reflect their responses to the parasitic infection and injury are discussed. Finally, potential models that can be used by researchers to improve our understanding on the pathogenesis of malaria in pregnancy and placental pathologies are presented.

Organotypic endothelial adhesion molecules are key for Trypanosoma brucei tropism and virulence

Trypanosoma brucei is responsible for lethal diseases in humans and cattle in Sub-Saharan Africa. These extracellular parasites extravasate from the blood circulation into several tissues. The importance of the vasculature in tissue tropism is poorly understood. Using intravital imaging and bioluminescence, we observe that gonadal white adipose tissue and pancreas are the two main parasite reservoirs. We show that reservoir establishment happens before vascular permeability is compromised, suggesting that extravasation is an active mechanism. Blocking endothelial surface adhesion molecules (E-selectin, P-selectins, or ICAM2) significantly reduces extravascular parasite density in all organs and delays host lethality. Remarkably, blocking CD36 has a specific effect on adipose tissue tropism that is sufficient to delay lethality, suggesting that establishment of the adipose tissue reservoir is necessary for parasite virulence. This work demonstrates the importance of the vasculature in a T. brucei infection and identifies organ-specific adhesion molecules as key players for tissue tropism.

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Our study investigates the blood vasculature for T. brucei reservoir establishment

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We show the pancreas is a large extravascular reservoir

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We establish that T. brucei tropism is linked to organotypic adhesion molecules

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Interfering with adhesion molecules impacts parasite virulence and host survival

Tissue tropism in parasitic diseases

Parasitic diseases, such as sleeping sickness, Chagas disease and malaria, remain a major cause of morbidity and mortality worldwide, but particularly in tropical, developing countries. Controlling these diseases requires a better understanding of host-parasite interactions, including a deep appreciation of parasite distribution in the host. The preferred accumulation of parasites in some tissues of the host has been known for many years, but recent technical advances have allowed a more systematic analysis and quantifications of such tissue tropisms. The functional consequences of tissue tropism remain poorly studied, although it has been associated with important aspects of disease, including transmission enhancement, treatment failure, relapse and clinical outcome. Here, we discuss current knowledge of tissue tropism in Trypanosoma infections in mammals, describe potential mechanisms of tissue entry, comparatively discuss relevant findings from other parasitology fields where tissue tropism has been extensively investigated, and reflect on new questions raised by recent discoveries and their potential impact on clinical treatment and disease control strategies.

Sticking for a Cause: The Falciparum Malaria Parasites Cytoadherence Paradigm

After a successful invasion, malaria parasite Plasmodium falciparum extensively remodels the infected erythrocyte cellular architecture, conferring cytoadhesive properties to the infected erythrocytes. Cytoadherence plays a central role in the parasite's immune-escape mechanism, at the same time contributing to the pathogenesis of severe falciparum malaria. In this review, we discuss the cytoadhesive interactions between P. falciparum infected erythrocytes and various host cell types, and how these events are linked to malaria pathogenesis. We also highlight the limitations faced by studies attempting to correlate diversity in parasite ligands and host receptors with the development of severe malaria.

Phosphorylation of the VAR2CSA extracellular region is associated with enhanced adhesive properties to the placental receptor CSA

Plasmodium falciparum is the main cause of disease and death from malaria. P. falciparum virulence resides in the ability of infected erythrocytes (IEs) to sequester in various tissues through the interaction between members of the polymorphic P. falciparum erythrocyte membrane protein 1 (PfEMP1) adhesin family to various host receptors. Here, we investigated the effect of phosphorylation of variant surface antigen 2-CSA (VAR2CSA), a member of the PfEMP1 family associated to placental sequestration, on its capacity to adhere to chondroitin sulfate A (CSA) present on the placental syncytium. We showed that phosphatase treatment of IEs impairs cytoadhesion to CSA. MS analysis of recombinant VAR2CSA phosphosites prior to and after phosphatase treatment, as well as of native VAR2CSA expressed on IEs, identified critical phosphoresidues associated with CSA binding. Site-directed mutagenesis on recombinant VAR2CSA of 3 phosphoresidues localised within the CSA-binding region confirmed in vitro their functional importance. Furthermore, using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein-9 nuclease (CRISPR/Cas9), we generated a parasite line in which the phosphoresidue T934 is changed to alanine and showed that this mutation strongly impairs IEs cytoadhesion to CSA. Taken together, these results demonstrate that phosphorylation of the extracellular region of VAR2CSA plays a major role in IEs cytoadhesion to CSA and provide new molecular insights for strategies aiming to reduce the morbidity and mortality of PM.

Transmission-Blocking Vaccines: Old Friends and New Prospects

In the progression of the life cycle of Plasmodium falciparum, a small proportion of asexual parasites differentiate into male or female sexual forms called gametocytes. Just like their asexual counterparts, gametocytes are contained within the infected host's erythrocytes (RBCs). However, unlike their asexual partners, they do not exit the RBC until they are taken up in a blood meal by a mosquito. In the mosquito midgut, they are stimulated to emerge from the RBC, undergo fertilization, and ultimately produce tens of thousands of sporozoites that are infectious to humans. This transmission cycle can be blocked by antibodies targeting proteins exposed on the parasite surface in the mosquito midgut, a process that has led to the development of candidate transmission-blocking vaccines (TBV), including some that are in clinical trials. Here we review the leading TBV antigens and highlight the ongoing search for additional gametocyte/gamete surface antigens, as well as antigens on the surfaces of gametocyte-infected erythrocytes, which can potentially become a new group of TBV candidates.

New var reconstruction algorithm exposes high var sequence diversity in a single geographic location in Mali

BACKGROUND

Encoded by the var gene family, highly variable Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP1) proteins mediate tissue-specific cytoadherence of infected erythrocytes, resulting in immune evasion and severe malaria disease. Sequencing and assembling the 40-60 var gene complement for individual infections has been notoriously difficult, impeding molecular epidemiological studies and the assessment of particular var elements as subunit vaccine candidates.

METHODS

We developed and validated a novel algorithm, Exon-Targeted Hybrid Assembly (ETHA), to perform targeted assembly of var gene sequences, based on a combination of Pacific Biosciences and Illumina data.

RESULTS

Using ETHA, we characterized the repertoire of var genes in 12 samples from uncomplicated malaria infections in children from a single Malian village and showed them to be as genetically diverse as vars from isolates from around the globe. The gene var2csa, a member of the var family associated with placental malaria pathogenesis, was present in each genome, as were vars previously associated with severe malaria.

CONCLUSION

ETHA, a tool to discover novel var sequences from clinical samples, will aid the understanding of malaria pathogenesis and inform the design of malaria vaccines based on PfEMP1. ETHA is available at: https://sourceforge.net/projects/etha/ .

Cytoadhesion to gC1qR through Plasmodium falciparum Erythrocyte Membrane Protein 1 in Severe Malaria

Cytoadhesion of Plasmodium falciparum infected erythrocytes to gC1qR has been associated with severe malaria, but the parasite ligand involved is currently unknown. To assess if binding to gC1qR is mediated through the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family, we analyzed by static binding assays and qPCR the cytoadhesion and var gene transcriptional profile of 86 P. falciparum isolates from Mozambican children with severe and uncomplicated malaria, as well as of a P. falciparum 3D7 line selected for binding to gC1qR (Pf3D7^gC1qR). Transcript levels of DC8 correlated positively with cytoadhesion to gC1qR (rho = 0.287, P = 0.007), were higher in isolates from children with severe anemia than with uncomplicated malaria, as well as in isolates from Europeans presenting a first episode of malaria (n = 21) than Mozambican adults (n = 25), and were associated with an increased IgG recognition of infected erythrocytes by flow cytometry. Pf3D7^gC1qR overexpressed the DC8 type PFD0020c (5.3-fold transcript levels relative to Seryl-tRNA-synthetase gene) compared to the unselected line (0.001-fold). DBLβ12 from PFD0020c bound to gC1qR in ELISA-based binding assays and polyclonal antibodies against this domain were able to inhibit binding to gC1qR of Pf3D7^gC1qR and four Mozambican P. falciparum isolates by 50%. Our results show that DC8-type PfEMP1s mediate binding to gC1qR through conserved surface epitopes in DBLβ12 domain which can be inhibited by strain-transcending functional antibodies. This study supports a key role for gC1qR in malaria-associated endovascular pathogenesis and suggests the feasibility of designing interventions against severe malaria targeting this specific interaction.

Plasmodium falciparum sequesters in vital organs. This phenomenon mediated by cytoadhesion of infected-erythrocytes to host receptors in the microvasculature, contributes to the development of severe malaria. Although cytoadhesion to Endothelial Protein-C Receptor has a central role in severe malaria, other host receptors are also likely to be involved. Our results generated by the analysis of P. falciparum isolates from Mozambican patients and laboratory parasite lines indicate that a specific domain (DBLβ12) from DC8-type PfEMP1s can bind to the human receptor gC1qR, previously associated with severe malaria. Our findings revealed that antibodies against PfEMP1 could provide strain-transcending inhibition of gC1qR-binding. Overall, these results support a key role for the adhesion to gC1qR in malaria-associated endovascular pathogenesis and the feasibility of new interventions targeting this specific interaction.

Antigenic Variation and the Genetics and Epigenetics of the PfEMP1 Erythrocyte Surface Antigens in Plasmodium falciparum Malaria

How immunity to malaria develops remains one of the great unresolved issues in bio-medicine and resolution of its various paradoxes is likely to be the key to developing effective malaria vaccines. The basic epidemiological observations are; under conditions of intense natural transmission, humans do become immune to P. falciparum malaria, but this is a slow process requiring multiple disease episodes which many, particularly young children, do not survive. Adult survivors are immune to the symptoms of malaria, and unless pregnant, can control the growth of most or all new inoculations. Sterile immunity is not achieved and chronic parasitization of apparently healthy adults is the norm. In this article, we analyse the best understood malaria "antigenic variation" system, that based on Plasmodium falciparum's PfEMP1-type cytoadhesion antigens, and critically review recent literature on the function and control of this multi-gene family of parasite variable surface antigens.

Malaria Parasites in the Asymptomatic: Looking for the Hay in the Haystack

With malaria elimination back on the international agenda, programs face the challenge of targeting all Plasmodium infections, not only symptomatic cases. As asymptomatic individuals are unlikely to seek treatment, they are missed by passive surveillance while remaining infectious to mosquitoes, thus acting as silent reservoirs of transmission. To estimate the risk of asymptomatic infections in various phases of malaria elimination, we need a deeper understanding of the underlying mechanisms favoring carriage over disease, which may involve both pathogen and host factors. Here we review our current knowledge on the determinants leading to Plasmodium falciparum symptomless infections. Understanding the host-pathogen interactions that are most likely to affect transitions between malaria disease states could guide the development of tools to tackle asymptomatic carriers in elimination settings.

Comparative study of Plasmodium falciparum erythrocyte membrane protein 1-DBLα domain variants with respect to antigenic variations and docking interaction analysis with glycosaminoglycans

The variant surface antigen PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) encoded by the polymorphic multi-copy var gene family plays an important role in parasite biology and the host-parasite interactions. Sequestration and antigenic variation is an essential component in the survival and pathogenesis of Plasmodium falciparum and contributes to chronic infection. The DBLα domain of PfEMP1 is a potential target for immuno-epidemiological studies and has been visualized as a vaccine candidate against severe malaria. Specific host receptors like heparin, heparan sulphate, blood group A and complement receptor 1 have been reported to bind the DBLα domain. Although heparin has been experimentally shown to disrupt the parasite-host interaction and effectively disrupt rosetting, the binding sites for the DBLα domain and the mechanism behind heparin-mediated rosette inhibition have not been elucidated. In this study, 3D structures and epitopes of the DBLα domain in 3D7 and in two Indian isolates have been predicted and compared. We have carried out docking studies on DBLα domains with human GAG receptors (heparin and heparan sulphate) to predict the strength of association between the protein-ligand interactions. The DBLα domain structures showed extensive diversity and polymorphism in their binding sites. The docking results indicate that heparin binds more effectively with high affinity as compared to heparan sulphate with some common interacting residues. These common residues can play an important role in rosetting and will aid in the designing of inhibitors specific to the interactions between DBLα and heparin or heparan sulphate would be important in malaria treatment. Thus it may lead to the development of novel interference strategies to block red blood cell invasion and provide protection against malaria.

Large screen approaches to identify novel malaria vaccine candidates

Until recently, malaria vaccine development efforts have focused almost exclusively on a handful of well characterized Plasmodium falciparum antigens. Despite dedicated work by many researchers on different continents spanning more than half a century, a successful malaria vaccine remains elusive. Sequencing of the P. falciparum genome has revealed more than five thousand genes, providing the foundation for systematic approaches to discover candidate vaccine antigens. We are taking advantage of this wealth of information to discover new antigens that may be more effective vaccine targets. Herein, we describe different approaches to large-scale screening of the P. falciparum genome to identify targets of either antibody responses or T cell responses using human specimens collected in Controlled Human Malaria Infections (CHMI) or under conditions of natural exposure in the field. These genome, proteome and transcriptome based approaches offer enormous potential for the development of an efficacious malaria vaccine.

Plasmodium falciparum complicated malaria: Modulation and connectivity between exportome and variant surface antigen gene families

In temperate and sub-tropical regions of Asia and Latin America, complicated malaria manifested as hepatic dysfunction or renal dysfunction is seen in all age groups. There has been a concerted focus on understanding the patho-physiological and molecular basis of complicated malaria in children, much less is known about it in adults. We report here, the analysis of data from a custom, cross strain microarray (Agilent Platform) using material from adult patient samples, showing hepatic dysfunction or renal failure. These are the most common manifestations seen in adults along with cerebral malaria. The data has been analyzed with reference to variant surface antigens, encoded by the var, rifin and stevor gene families. The differential regulation profiles of key genes (comparison between Plasmodium falciparum complicated and uncomplicated isolates) have been observed. The exportome has been analyzed using similar parameters. Gene ontology term based functional enrichment of differentially regulated genes identified, up-regulated genes statistically enriched (P<0.05) to critical biological processes like generation of precursor metabolite and energy, chromosome organization and electron transport chain. Systems network based functional enrichment of overall differentially regulated genes yielded a similar result. We are reporting here, up-regulation of var group B and C genes whose proteins are predicted to interact with CD36 receptor in the host, the up-regulation of domain cassette 13 (DC13) containing var group A, as also the up-regulation of group A rifins and many of the stevors. This is contrary to most other reports from pediatric patients, with cerebral malaria where the up-regulation of mostly var A group genes have been seen. A protein-protein interaction based network has been created and analysis performed. This co-expression and text mining based network has shown overall connectivity between the variant surface antigens (VSA) and the exportome. The up-regulation of var group B and C genes encoding PfEMP1 with different domain architecture would be important for deciding strategies for disease prevention.

Evaluating IgG Antibody to Variant Surface Antigens Expressed on Plasmodium falciparum Infected Erythrocytes Using Flow Cytometry

Constant exposure to Plasmodium falciparum leads to acquisition of malarial antibodies that can protect against the clinical consequences of infection. One important target of such antibodies is against the parasite-infected erythrocyte (IEs). Current established assays to test the efficacy of antibodies in preventing parasite growth include direct parasite growth inhibition, the agglutination of IEs, and the inhibition of adhesion of IEs (to quantify antibody that inhibits adhesion to purified receptors or cells).However, many of these assays are labor-intensive and low-output which limits study sizes to small cohorts. Here we present an alternative assay that measures the levels of protective antibodies to variant surface antigens (VSA) of IEs. This assay can be performed using a microtitre plate and requires only a small volume of test serum sample. Serum samples are incubated with IEs and are then analyzed using a semi-automated autosampler attached to a flow cytometer. The assay is accurate, quick, and reproducible. Ultimately this assay could be used on large population-based studies, which could increase the statistical power of clinical studies.

Differential PfEMP1 expression is associated with cerebral malaria pathology

Plasmodium falciparum is unique among human malarias in its ability to sequester in post-capillary venules of host organs. The main variant antigens implicated are the P. falciparum erythrocyte membrane protein 1 (PfEMP1), which can be divided into three major groups (A–C). Our study was a unique examination of sequestered populations of parasites for genetic background and expression of PfEMP1 groups. We collected post-mortem tissue from twenty paediatric hosts with pathologically different forms of cerebral malaria (CM1 and CM2) and parasitaemic controls (PC) to directly examine sequestered populations of parasites in the brain, heart and gut. Use of two different techniques to investigate this question produced divergent results. By quantitative PCR, group A var genes were upregulated in all three organs of CM2 and PC cases. In contrast, in CM1 infections displaying high levels of sequestration but negligible vascular pathology, there was high expression of group B var. Cloning and sequencing of var transcript tags from the same samples indicated a uniformly low expression of group A-like var. Generally, within an organ sample, 1–2 sequences were expressed at dominant levels. 23% of var tags were detected in multiple patients despite the P. falciparum infections being genetically distinct, and two tags were observed in up to seven hosts each with high expression in the brains of 3–4 patients. This study is a novel examination of the sequestered parasites responsible for fatal cerebral malaria and describes expression patterns of the major cytoadherence ligand in three organ-derived populations and three pathological states.

One of the most severe forms of malarial disease is cerebral malaria, which disproportionally affects young children. In this disease, the parasite places proteins on the red blood cell surface, providing a “smokescreen” by which they evade host immunity and hide in organ blood vessels, blocking them and causing tissue damage. It is impossible to study parasites in the organs during life and autopsy studies on children with malaria are exceedingly rare. In Malawi, we examined parasites from the brain, heart and intestine of twenty cases of fatal malaria including controls with low numbers of malaria parasites but another identified cause of death. We found little difference in the category of proteins the parasites used in controls and cerebral malaria, although a small number of specific proteins were detected in multiple infections. In an alternative form of malaria in which the brain is heavily infected but shows no evidence of damage, we found a different set of proteins at high proportion. However, as these children were typically older and most were infected with HIV, we could not determine which of these factors was most important. Interactions between host and parasite have the potential to influence disease outcomes.

Cross-reactive immune responses as primary drivers of malaria chronicity

The within-host dynamics of an infection with the malaria parasite Plasmodium falciparum are the result of a complex interplay between the host immune system and parasite. Continual variation of the P. falciparum erythrocyte membrane protein (PfEMP1) antigens displayed on the surface of infected red blood cells enables the parasite to evade the immune system and prolong infection. Despite the importance of antigenic variation in generating the dynamics of infection, our understanding of the mechanisms by which antigenic variation generates long-term chronic infections is still limited. We developed a model to examine the role of cross-reactivity in generating infection dynamics that are comparable to those of experimental infections. The hybrid computational model we developed is attuned to the biology of malaria by mixing discrete replication events, which mimics the synchrony of parasite replication and invasion, with continuous interaction with the immune system. Using simulations, we evaluated the dynamics of a single malaria infection over time. We then examined three major mechanisms by which the dynamics of a malaria infection can be structured: cross-reactivity of the immune response to PfEMP1, differences in parasite clearance rates, and heterogeneity in the rate at which antigens switch. The results of our simulations demonstrate that cross-reactive immune responses play a primary role in generating the dynamics observed in experimentally untreated infections and in lengthening the period of infection. Importantly, we also find that it is the primary response to the initially expressed PfEMP1, or small subset thereof, that structures the cascading cross-immune dynamics and allows for elongation of the infection.

Plasmodium falciparum origin recognition complex subunit 1 (PfOrc1) functionally complements Δsir3 mutant of Saccharomyces cerevisiae

Telomere position effect efficiently controls silencing of subtelomeric var genes, which are involved in antigenic variation in human malaria parasite Plasmodium falciparum. Although, PfOrc1 has been found to be associated with PfSir2 in the silencing complex, its function in telomere silencing remained uncertain especially due to an apparent lack of BAH domain at its amino-terminal region. Here we report that PfOrc1 possesses a Sir3/Orc1 like silencing activity. Using yeast as a surrogate organism we have shown that PfOrc1 could complement yeast Sir3 activity during telomere silencing in a Sir2 dependent manner. By constructing a series of chimera between PfOrc1 and ScSir3 we have observed that the amino-terminal domain of PfOrc1 harbors silencing activity similar to that present in the amino-terminal domain of ScSir3. We further generated several amino-terminal deletion mutants to dissect out such silencing activity and found that the first seventy amino acids at the amino-terminal domain are dispensable for its activity. Thus our results strongly supports that PfOrc1 may have a role in telomere silencing in this parasite. This finding will help to decipher the mechanism of telomere position effect in P. falciparum.

CD36 recruits α₅β₁ integrin to promote cytoadherence of P. falciparum-infected erythrocytes

The adhesion of Plasmodium falciparum-infected erythrocytes (IRBC) to receptors on different host cells plays a divergent yet critical role in determining the progression and outcome of the infection. Based on our ex vivo studies with clinical parasite isolates from adult Thai patients, we have previously proposed a paradigm for IRBC cytoadherence under physiological shear stress that consists of a recruitment cascade mediated largely by P-selectin, ICAM-1 and CD36 on primary human dermal microvascular endothelium (HDMEC). In addition, we detected post-adhesion signaling events involving Src family kinases and the adaptor protein p130CAS in endothelial cells that lead to CD36 clustering and cytoskeletal rearrangement which enhance the magnitude of the adhesive strength, allowing adherent IRBC to withstand shear stress of up to 20 dynes/cm². In this study, we addressed whether CD36 supports IRBC adhesion as part of an assembly of membrane receptors. Using a combination of flow chamber assay, atomic force and confocal microscopy, we showed for the first time by loss- and gain-of function assays that in the resting state, the integrin α₅β₁ does not support adhesive interactions between IRBC and HDMEC. Upon IRBC adhesion to CD36, the integrin is recruited either passively as part of a molecular complex with CD36, or actively to the site of IRBC attachment through phosphorylation of Src family kinases, a process that is Ca²⁺-dependent. Clustering of β₁ integrin is associated with an increase in IRBC recruitment as well as in adhesive strength after attachment (∼40% in both cases). The adhesion of IRBC to a multimolecular complex on the surface of endothelial cells could be of critical importance in enabling adherent IRBC to withstand the high shear stress in the microcirculations. Targeting integrins may provide a novel approach to decrease IRBC cytoadherence to microvascular endothelium.

Of the several species of malaria parasites that infect humans, disease caused by Plasmodium falciparum is responsible for most of the deaths. The unique pathological finding of this infection is the intense adhesion of infected red blood cells (IRBC) in the microcirculation, resulting in obstruction to blood flow and organ dysfunction. The focus of our research is to identify the molecules on host endothelial cells that support the adhesion as potential therapeutic targets. In this report, we showed for the first time a functional association between CD36, a well studied adhesion molecule for parasite adhesion, and α₅β₁, a member of the integrin family of adhesion molecules that are important for adhesion of leukocytes to blood vessels and cell adhesion to the extracellular matrix. We found that by itself, α₅β₁ integrin does not support IRBC adhesion. When IRBC adhere to CD36, the integrin is recruited to the site of adhesion through activation of the Src family kinase signaling pathway. As a result, both the number of adherent IRBC and the strength with which they adhere is greatly increased. These results demonstrate that IRBC adhesion is a dynamic and complex process. We need to identify each of the functional components to design anti-adhesive therapy.

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.

Assessing the genetic diversity of the vir genes in Indian Plasmodium vivax population

Variant surface antigens (VSAs) present on the surface of parasitized erythrocytes facilitate many Plasmodium spp. to escape the host immune system during infection. Multigene families coding for VSAs exist in several Plasmodium spp. and are located on telomeric and subtelomeric regions of the chromosomes. P. vivax genome also contains a multigene superfamily vir (variant interspersed repeats), present in the subtelomeric region with a possible role in immune evasion like the var gene in P. falciparum. Blood samples from 148 symptomatic malaria cases were collected from five different regions of India, viz. Mangalore, Rourkela, Goa, Delhi and Jabalpur. P. vivax isolates (74 single infections) were sequenced for four vir genes (viz. vir 27, vir 4, vir 12 and vir 21) and analyzed for the genetic variability existing in different populations of India. The results indicate that vir genes in different P. vivax populations in India are highly divergent both within and between the isolates. High levels of single nucleotide polymorphisms (SNPs) were observed attributing to the existing polymorphism for all the four vir genes studied across the population. Detailed knowledge of the genetic variation among the vir genes will help in understanding the evolutionary aspects of vir genes and may also provide basis for understanding the disease chronicity.

Analysis of var genes cloned from a Plasmodium falciparum isolate in China

OBJECTIVE

To analyse the var gene repertoire and characterise the chondroitin sulphate A (CSA)-binding activity of the Duffy-binding like (DBL) domains encoded by the var2csa gene of a Plasmodium falciparum (P. falciparum) isolate in Hainan Province, China.

METHODS

The sequences of var DBL1 regions were PCR-amplified, sequenced and the sequence characteristics was bioinformatically analysed. Recombinant proteins encoded by the var2csa genes were expressed and purified. The binding activities of the recombinant proteins to CSA receptor was detected by ELISA assays.

RESULTS

Fifty six unique DBL α sequences were obtained, and the sequences represented similar diversity to the var genes of the genome parasite 3D7. There are two var2csa genes in the P. falciparum isolated from Hainan Province. Unlike in other falciparum parasites such as HB3, the two var2csa genes are more diverged. The receptor-binding capacity of DBL-5ε and DBL-6ε domains of HN var2CSA was studied.

CONCLUSIONS

This work represented the diversity of var genes of a P. falciparum isolate in China.

Investigating the host binding signature on the Plasmodium falciparum PfEMP1 protein family

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family plays a central role in antigenic variation and cytoadhesion of P. falciparum infected erythrocytes. PfEMP1 proteins/var genes are classified into three main subfamilies (UpsA, UpsB, and UpsC) that are hypothesized to have different roles in binding and disease. To investigate whether these subfamilies have diverged in binding specificity and test if binding could be predicted by adhesion domain classification, we generated a panel of 19 parasite lines that primarily expressed a single dominant var transcript and assayed binding against 12 known host receptors. By limited dilution cloning, only UpsB and UpsC var genes were isolated, indicating that UpsA var gene expression is rare under in vitro culture conditions. Consequently, three UpsA variants were obtained by rosette purification and selection with specific monoclonal antibodies to create a more representative panel. Binding assays showed that CD36 was the most common adhesion partner of the parasite panel, followed by ICAM-1 and TSP-1, and that CD36 and ICAM-1 binding variants were highly predicted by adhesion domain sequence classification. Binding to other host receptors, including CSA, VCAM-1, HABP1, CD31/PECAM, E-selectin, Endoglin, CHO receptor “X”, and Fractalkine, was rare or absent. Our findings identify a category of larger PfEMP1 proteins that are under dual selection for ICAM-1 and CD36 binding. They also support that the UpsA group, in contrast to UpsB and UpsC var genes, has diverged from binding to the major microvasculature receptor CD36 and likely uses other mechanisms to sequester in the microvasculature. These results demonstrate that CD36 and ICAM-1 have left strong signatures of selection on the PfEMP1 family that can be detected by adhesion domain sequence classification and have implications for how this family of proteins is specializing to exploit hosts with varying levels of anti-malaria immunity.

The malaria parasite Plasmodium falciparum persists in the human host partly by avoiding elimination in the spleen during blood stage infection. This strategy depends principally upon members of the large and diverse PfEMP1 family of proteins that are exported to the surface of infected erythrocytes. PfEMP1 proteins are important targets for host protective antibody responses and encode binding to several different host receptor proteins. Switches in PfEMP1 expression allow parasites to evade host antibodies and may precipitate severe disease when infected erythrocytes accumulate in brain or placenta. Consequently, the severity of malaria infection may depend on the type of PfEMP1 protein expressed. In this study, we employ a representative panel of distinct PfEMP1 types and host receptor proteins to demonstrate that CD36 and ICAM-1 binding properties of full-length PfEMP1 are highly predicted by their domain composition. We also find that CD36 binding is under strong selection in many PfEMP1 proteins, but that a group of PfEMP1s associated with more severe infections does not bind CD36 and may utilize alternative means to sequester infected erythrocytes. These findings have implications for understanding the molecular basis for severe malaria.

Uncertainty in mapping malaria epidemiology: implications for control

Malaria is a location-specific, dynamic infectious disease transmitted by mosquitoes to humans and is influenced by environmental, vector, parasite, and host factors. The principal purposes of malarial epidemiology are 1) to describe the malarial distribution in space and time along with the physical, biologic, and social etiologic factors and 2) to guide control objectives for either modeling impact or measuring progress of control tactics. Mapping malaria and many of its causative factors has been achieved on many different levels from global distribution to biologic quantitative trait localization in humans, parasites, and mosquitoes. Despite these important achievements, a large degree of uncertainty still exists on the annual burden of malarial cases. Accurate, sensitive detection and treatment of asymptomatic reservoirs important to infectious transmission are additional components necessary for future control measures. Presently spurred by the leadership and funding of Bill and Melinda Gates, the malarial community is developing and implementing plans for elimination of malaria. The challenge for malariologists is to digitally integrate and map epidemiologic factors and intervention measures in space and time to target effective, sustainable control alongside research efforts.

A novel Plasmodium falciparum erythrocyte binding protein associated with the merozoite surface, PfDBLMSP

Proteins on the surface of the merozoite, the invasive form of the malaria parasite Plasmodium falciparum,and those secreted from its apical secretory organelles are promising vaccine candidates against blood stage malaria. In the present study, we have identified a novel parasite protein (PfDBLMSP; Gene IDPF10_0348), that harbors a predicted signal sequence, a central Duffy binding-like (DBL) domain and a secreted polymorphic antigen associated with merozoites (SPAM) domain in its C-terminal half. Transcription and translation of pfdblmsp is up-regulated specifically in schizont stage parasites, similar to other well-chararacterized merozoite proteins involved in invasion of red blood cells (RBCs). PfDBLMSPwas localized on the merozoite surface with a GFP targeting approach using schizont-stage specific expression systems, and by immunofluorescence assays of the endogenous protein. PfDBLMSP expressed on the surface of mammalian cells (COS-7) showed binding with human RBCs and this binding was sensitive to trypsin and neuraminidase treatments. The recombinant proteins corresponding to the DBL and SPAM domains showed reactivity with immune sera from individuals residing in P. falciparum endemic areas. Polymorphism in PfDBLMSP sequences from different P. falciparum strains and field isolates suggested that its DBL domain is under natural immune pressure. Our data on localization and functional assays suggest a possible role of PfDBLMSP in binding of merozoites with erythrocytes during invasion.

Biochemical and biophysical characterisation of DBL1alpha1-varO, the rosetting domain of PfEMP1 from the VarO line of Plasmodium falciparum

Rosetting of erythrocytes infected with Plasmodium falciparum is frequently observed in children with severe malaria. This adhesion phenomenon has been linked to the DBL1alpha domain of P. falciparum erythrocyte membrane protein 1 (PfEMP1) in three laboratory clones: FCR3S1.2, IT4R29 and Palo Alto varO. Here, we compare the soluble recombinant NTS-DBL1alpha(1)-varO domain (NTS: N-terminal segment) obtained from E. coli, Pichia pastoris and baculovirus/insect cell expression systems. In each case, the presence of NTS was necessary for obtaining a soluble product. Successful expression in E. coli required maltose-binding protein as an N-terminal fusion partner. Each expression system produced an identical, correctly folded protein, as judged by biochemical and biophysical characterisations, and by the capacity to elicit antibodies that react with the surface of VarO-infected erythrocytes and disrupt VarO rosettes. Binding studies using surface plasmon resonance (SPR) techniques showed that NTS-DBL1alpha(1) produced in E. coli binds to heparin with micromolar affinity. IC(50) constants for other sulphated oligosaccharides were determined using SPR by measuring their competitive binding to the soluble protein in the presence of immobilized heparin. The affinity to NTS-DBL1alpha(1) was related to the degree of sulphation of the oligosaccharide, although the position of the sulphate groups on the sugar rings was also important. VarO rosettes could be disrupted by sulphated oligosaccharides with an efficacy that correlated with their binding affinity to recombinant NTS-DBL1alpha(1). Thus high yields of soluble NTS-DBL1alpha(1) with native conformation have been produced, opening novel perspectives for both structure-function studies and vaccine development.

Genetic and epigenetic mechanisms underlying cell-surface variability in protozoa and fungi

Eukaryotic microorganisms have evolved ingenious mechanisms to generate variability at their cell surface, permitting differential adherence, rapid adaptation to changing environments, and evasion of immune surveillance. Fungi such as Saccharomyces cerevisiae and the pathogen Candida albicans carry a family of mucin and adhesin genes that allow adhesion to various surfaces and tissues. Trypanosoma cruzi, T. brucei, and Plasmodium falciparum likewise contain large arsenals of different cell surface adhesion genes. In both yeasts and protozoa, silencing and differential expression of the gene family results in surface variability. Here, we discuss unexpected similarities in the structure and genomic location of the cell surface genes, the role of repeated DNA sequences, and the genetic and epigenetic mechanisms-all of which contribute to the remarkable cell surface variability in these highly divergent microbes.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

The varieties of gene amplification, diversification and hypervariability in the human malaria parasite, Plasmodium falciparum

The human malaria parasite, Plasmodium falciparum, is able to evade host cell-mediated and humoral immunity to maintain both persistent and repeated infections. Immune evasion is in part due to a robust repertoire of proteins which participate in host-parasite interactions but also exhibit profound antigenic diversity, and in some instances switches in gene expression. The antigenic diversity occurs both at the parasite level within families of amplified proteins, and within populations of parasites in which mechanisms of recombination and gene conversion conspire to create a broad plasticity in the antigenic exposure to the host. This review will introduce the spectrum of amplified protein families in P. falciparum and focus on three sub-telomeric encoded families, RIFIN, STEVOR and Pfmc-2TM which exhibit hypervariability with respect to their antigenic diversity.

Six genes are preferentially transcribed by the circulating and sequestered forms of Plasmodium falciparum parasites that infect pregnant women

In areas of stable malaria transmission, susceptibility to Plasmodium falciparum malaria increases during first pregnancy. Women become resistant to pregnancy malaria over successive pregnancies as they acquire antibodies against the parasite forms that sequester in the placenta, suggesting that a vaccine is feasible. Placental parasites are antigenically distinct and bind receptors, like chondroitin sulfate A (CSA), that are not commonly bound by other parasites. We used whole-genome-expression analysis to find transcripts that distinguish parasites of pregnant women from other parasites and employed a novel approach to define and adjust for cell cycle timing of parasites. Transcription of six genes was substantially higher in both placental parasites and peripheral parasites from pregnant women, and each gene encodes a protein with a putative export sequence and/or transmembrane domain. This cohort of genes includes var2csa, a member of the variant PfEMP1 gene family previously implicated in pregnancy malaria, as well as five conserved genes of unknown functions. Women in East Africa acquire antibodies over successive pregnancies against a protein encoded by one of these genes, PFD1140w, and this protein shows seroreactivity similar to that of VAR2CSA domains. These findings suggest that a suite of genes may be important for the genesis of the placental binding phenotype of P. falciparum and may provide novel targets for therapeutic intervention.

Serum lipoproteins promote efficient presentation of the malaria virulence protein PfEMP1 at the erythrocyte surface

The virulence of the malaria parasite Plasmodium falciparum is related to its ability to express a family of adhesive proteins known as P. falciparum erythrocyte membrane protein 1 (PfEMP1) at the infected red blood cell surface. The mechanism for the transport and delivery of these adhesins to the erythrocyte membrane is only poorly understood. In this work, we have used specific immune reagents in a flow cytometric assay to monitor the effects of serum components on the surface presentation of PfEMP1. We show that efficient presentation of the A4 and VAR2CSA variants of PfEMP1 is dependent on the presence of serum in the bathing medium during parasite maturation. Lipid-loaded albumin supports parasite growth but allows much less efficient presentation of PfEMP1 at the red blood cell surface. Analysis of the serum components reveals that lipoproteins, especially those of the low-density lipoprotein fraction, promote PfEMP1 presentation. Cytoadhesion of infected erythrocytes to the host cell receptors CD36 and ICAM-1 is also decreased in infected erythrocytes cultured in the absence of serum. The defect appears to be in the transfer of PfEMP1 from parasite-derived structures known as the Maurer's clefts to the erythrocyte membrane or in surface conformation rather than a down-regulation or switching of particular PfEMP1 variants.

The ABO blood group system and Plasmodium falciparum malaria

In the century since the discovery of the ABO blood groups, numerous associations between ABO groups and disease have been noted. However, the selection pressures defining the ABO distributions remain uncertain. We review published information on Plasmodium falciparum infection and ABO blood groups. DNA sequence information dates the emergence and development of the group O allele to a period of evolution before human migration out of Africa, concomitant with P falciparum's activity. The current geographic distribution of group O is also consistent with a selection pressure by P falciparum in favor of group O individuals in malaria-endemic regions. We critically review clinical reports of ABO and P falciparum infection, documenting a correlation between disease severity and ABO group. Finally, we review published data on the pathogenesis of P falciparum infection, and propose a biologic model to summarize the role of ABO blood groups in cytoadherence biology. Such ABO-related mechanisms also point to a new hypothesis to account for selection of the Le(a-b-) phenotype. Taken together, a broad range of available evidence suggests that the origin, distribution, and relative proportion of ABO blood groups in humans may have been directly influenced by selective genetic pressure from P falciparum infection.

Pregnancy-associated malaria: Parasite binding, natural immunity and vaccine development

Humans living in areas of high malaria transmission gradually acquire, during the early years of life, protective clinical immunity to Plasmodium falciparum, limiting serious complications of malaria to young children. However, pregnant women become more susceptible to severe P. falciparum infections during their first pregnancy. Pregnancy associated malaria is coupled with massive accumulation of parasitised erythrocytes and monocytes in the placental intervillous blood spaces, contributing to disease and death in pregnant women and developing infants. Indirect evidence suggests that prevention may be possible by vaccinating women of childbearing age before their first pregnancy. This review aims to introduce the reader to the implications of malaria infection during pregnancy and to analyse recent findings towards the identification and characterisation of parasite encoded erythrocyte surface proteins expressed in malaria-infected pregnant women that are likely targets of protective immunity and have potential for vaccine development.

Natural killer cells and malaria

Malaria, caused by the infection with parasites of the germs Plasmodium, is one of the three most important infectious diseases worldwide, along with tuberculosis and infection with human immunodeficiency virus. Natural killer (NK) cells are lymphocytes classically involved in the early defense against viral infections and intracytoplasmic bacterial infections and are also implicated during the course of tumor development and allogeneic transplantation. These cells display important cytotoxic activity and produce high levels of proinflammatory cytokines. In both mouse and human models of malaria, NK cells appear to be a major source of interferon-gamma during the early phase of infection. In humans, indirect signaling through monocytes/macrophages required to optimally stimulate NK cell activity. However, the in vivo functions of NK cells during malaria are still enigmatic, and many issues remain to be dissected, such as the molecular basis of the direct recognition of iRBCs by NK cells.

Antimalarial drugs - host targets (re)visited

Every year, forty percent of the world population is at risk of contracting malaria. Hopes for the erradication of this disease during the 20th century were dashed by the ability of Plasmodium falciparum, its most deadly causative agent, to develop resistance to available drugs. Efforts to produce an effective vaccine have so far been unsuccessful, enhancing the need to develop novel antimalarial drugs. In this review, we summarize our knowledge concerning existing antimalarials, mechanisms of drug-resistance development, the use of drug combination strategies and the quest for novel anti-plasmodial compounds. We emphasize the potential role of host genes and molecules as novel targets for newly developed drugs. Recent results from our laboratory have shown Hepatocyte Growth Factor/MET signaling to be essential for the establishment of infection in hepatocytes. We discuss the potential use of this pathway in the prophylaxis of malaria infection.

A family affair: var genes, PfEMP1 binding, and malaria disease

An immunovariant adhesion protein family in Plasmodium falciparum named erythrocyte membrane protein 1 (PfEMP1), encoded by var genes, is responsible for both antigenic variation and cytoadhesion of infected erythrocytes at blood microvasculature sites throughout the body. Elucidation of the genome sequence of P. falciparum has revealed that var genes can be classified into different groups, each with distinct 5' flanking sequences, chromosomal locations and gene orientations. Recent binding and serological comparisons suggest that this genomic organization might cause var genes to diversify into separately recombining adhesion groups that have different roles in infection and disease. Detailed understanding of PfEMP1 expression and receptor binding mechanisms during infection and of the antigenic relatedness of disease variants might lead to new approaches in prevention of malaria disease.

Variant antigen gene expression in malaria

Pathogens of the genus Plasmodium are unicellular parasites that infect a variety of animals, including reptiles, birds and mammals. All Plasmodium species target host erythrocytes and replicate asexually within this niche. In humans, proliferation within erythrocytes causes disease symptoms ranging from asymtomatic infection to severe disease, including mild to severe febrile and respiratory symptoms, profound anaemia and obstruction of blood flow. The most serious form of human malaria is caused by Plasmodium falciparum, a pathogen that is responsible for several million deaths annually throughout the developing world. Malaria parasites succeed in evading the host immune response to establish long-term, persistent infections, thus increasing the efficiency by which they are transmitted to the mosquito vector. The ability to evade the host immune system, in particular the avoidance of antibody-mediated immunity against parasite-encoded surface proteins, is the result of amplification of extensive repertoires of multicopy, hypervariable gene families that encode infected erythrocyte or merozoite surface proteins. Via switching between antigenically diverse genes within these large families, populations of parasites have the capacity for rapid variation in antigenicity and virulence over the course of an infection. Here we review the amplification and generation of antigenic diversity within the Plasmodium variant gene families, as well as discuss the mechanisms underlying their tightly controlled gene expression and antigenic switching.

Delivery of the malaria virulence protein PfEMP1 to the erythrocyte surface requires cholesterol-rich domains

The particular virulence of the human malaria parasite Plasmodium falciparum derives from export of parasite-encoded proteins to the surface of the mature erythrocytes in which it resides. The mechanisms and machinery for the export of proteins to the erythrocyte membrane are largely unknown. In other eukaryotic cells, cholesterol-rich membrane microdomains or "rafts" have been shown to play an important role in the export of proteins to the cell surface. Our data suggest that depletion of cholesterol from the erythrocyte membrane with methyl-beta-cyclodextrin significantly inhibits the delivery of the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). The trafficking defect appears to lie at the level of transfer of PfEMP1 from parasite-derived membranous structures within the infected erythrocyte cytoplasm, known as the Maurer's clefts, to the erythrocyte membrane. Thus our data suggest that delivery of this key cytoadherence-mediating protein to the host erythrocyte membrane involves insertion of PfEMP1 at cholesterol-rich microdomains. GTP-dependent vesicle budding and fusion events are also involved in many trafficking processes. To determine whether GTP-dependent events are involved in PfEMP1 trafficking, we have incorporated non-membrane-permeating GTP analogs inside resealed erythrocytes. Although these nonhydrolyzable GTP analogs reduced erythrocyte invasion efficiency and partially retarded growth of the intracellular parasite, they appeared to have little direct effect on PfEMP1 trafficking.

Plasmodium falciparum genotype population dynamics in asymptomatic children from Senegal

In areas where malaria is endemic, infected individuals generally harbor a mixture of genetically distinct Plasmodium falciparum parasite populations. For the first time, we studied temporal variations of blood parasite densities and circulating genotypes in asymptomatic Senegalese children, at time intervals as short as 4-12 h. Twenty-one Senegalese children, presenting with an asymptomatic P. falciparum infection, were sampled eight times within three days. Parasite density was assessed by thick blood smears, and all infecting genotypes were quantified by the fragment-analysis method. Parasite densities showed dramatic fluctuations up to a 1 to 1,000 ratio, with at least one peak of parasite density. Polyclonal infections were detected in all children, with a multiplicity of infection of 5.2-6.8 genotypes per child. A single sample never reflected the full complexity of the parasite populations hosted by a given individual. Genotypes with different behaviors were detected in all children, some genotypes undergoing major fluctuations, while others were highly stable during the follow-up. A single peripheral blood sampling does not reflect the total parasite load. Repeated sampling is needed to have a more detailed scheme of parasite population dynamics and a better knowledge of the true complexity of an infection.

A single member of the Plasmodium falciparum var multigene family determines cytoadhesion to the placental receptor chondroitin sulphate A

In high-transmission regions, protective clinical immunity to Plasmodium falciparum develops during the early years of life, limiting serious complications of malaria in young children. Pregnant women are an exception and are especially susceptible to severe P. falciparum infections resulting from the massive adhesion of parasitized erythrocytes to chondroitin sulphate A (CSA) present on placental syncytiotrophoblasts. Epidemiological studies strongly support the feasibility of an intervention strategy to protect pregnant women from disease. However, different parasite molecules have been associated with adhesion to CSA. In this work, we show that disruption of the var2csa gene of P. falciparum results in the inability of parasites to recover the CSA-binding phenotype. This gene is a member of the var multigene family and was previously shown to be composed of domains that mediate binding to CSA. Our results show the central role of var2CSA in CSA adhesion and support var2CSA as a leading vaccine candidate aimed at protecting pregnant women and their fetuses.