VAR2CSA and protective immunity against pregnancy-associated Plasmodium falciparum malaria

Host-parasite interactions during Plasmodium infection: Implications for immunotherapies

Malaria is a global infectious disease that remains a leading cause of morbidity and mortality in the developing world. Multiple environmental and host and parasite factors govern the clinical outcomes of malaria. The host immune response against the Plasmodium parasite is heterogenous and stage-specific both in the human host and mosquito vector. The Plasmodium parasite virulence is predominantly associated with its ability to evade the host's immune response. Despite the availability of drug-based therapies, Plasmodium parasites can acquire drug resistance due to high antigenic variations and allelic polymorphisms. The lack of licensed vaccines against Plasmodium infection necessitates the development of effective, safe and successful therapeutics. To design an effective vaccine, it is important to study the immune evasion strategies and stage-specific Plasmodium proteins, which are targets of the host immune response. This review provides an overview of the host immune defense mechanisms and parasite immune evasion strategies during Plasmodium infection. Furthermore, we also summarize and discuss the current progress in various anti-malarial vaccine approaches, along with antibody-based therapy involving monoclonal antibodies, and research advancements in host-directed therapy, which can together open new avenues for developing novel immunotherapies against malaria infection and transmission.

Towards identification and development of alternative vaccines against pregnancy-associated malaria based on naturally acquired immunity

Pregnant women are particularly susceptible to Plasmodium falciparum malaria, leading to substantial maternal and infant morbidity and mortality. While highly effective malaria vaccines are considered an essential component towards malaria elimination, strides towards development of vaccines for pregnant women have been minimal. The leading malaria vaccine, RTS,S/AS01, has modest efficacy in children suggesting that it needs to be strengthened and optimized if it is to be beneficial for pregnant women. Clinical trials against pregnancy-associated malaria (PAM) focused on the classical VAR2CSA antigen are ongoing. However, additional antigens have not been identified to supplement these initiatives despite the new evidence that VAR2CSA is not the only molecule involved in pregnancy-associated naturally acquired immunity. This is mainly due to a lack of understanding of the immune complexities in pregnancy coupled with difficulties associated with expression of malaria recombinant proteins, low antigen immunogenicity in humans, and the anticipated complications in conducting and implementing a vaccine to protect pregnant women. With the accelerated evolution of molecular technologies catapulted by the global pandemic, identification of novel alternative vaccine antigens is timely and feasible. In this review, we discuss approaches towards novel antigen discovery to support PAM vaccine studies.

Plasmodium falciparum VAR2CSA-Specific IgG Subclass Responses Reflect Protection Against Low Birth Weight and Pregnancy-Associated Malaria

Sequestration of Plasmodium falciparum-infected erythrocytes expressing the VAR2CSA antigen in the placenta results in poor pregnancy outcomes, including low birth weight and maternal anemia. Antigen-specific antibody-mediated immunity is acquired during successive pregnancies. Thus, evaluating VAR2CSA-specific IgG profiles among pregnant women will increase knowledge on the immunological mechanisms associated with protection, and help in the development of VAR2CSA-based placental malaria vaccines. Using the PAMVAC candidate vaccine antigen, we assessed anti-VAR2CSA IgG subclass responses of a cohort of pregnant Beninese, and analyzed their relationships with pregnancy outcomes. Cytophilic IgG1 and IgG3 responses were the most frequent, with prevalences ranging from 28% (IgG3) up to 50% (IgG1). Elevated levels of VAR2CSA-specific total IgG and cytophilic IgG3 during pregnancy were consistently associated with higher birth weights, whilst high levels of IgG4 were associated with a reduced risk of placental infections. This suggests that protective anti-VAR2CSA IgG responses are coordinated between both cytophilic and non-cytophilic antibodies.

First-in-human, Randomized, Double-blind Clinical Trial of Differentially Adjuvanted PAMVAC, A Vaccine Candidate to Prevent Pregnancy-associated Malaria

BACKGROUND

Malaria in pregnancy has major impacts on mother and child health. To complement existing interventions, such as intermittent preventive treatment and use of impregnated bed nets, we developed a malaria vaccine candidate with the aim of reducing sequestration of asexual "blood-stage" parasites in the placenta, the major virulence mechanism.

METHODS

The vaccine candidate PAMVAC is based on a recombinant fragment of VAR2CSA, the Plasmodium falciparum protein responsible for binding to the placenta via chondroitin sulfate A (CSA). Healthy, adult malaria-naive volunteers were immunized with 3 intramuscular injections of 20 μg (n = 9) or 50 μg (n = 27) PAMVAC, adjuvanted with Alhydrogel or glucopyranosyl lipid adjuvant in stable emulsion (GLA-SE) or in a liposomal formulation with QS21 (GLA-LSQ). Allocation was random and double blind. The vaccine was given every 4 weeks. Volunteers were observed for 6 months following last immunization.

RESULTS

All PAMVAC formulations were safe and well tolerated. A total of 262 adverse events (AEs) occurred, 94 (10 grade 2 and 2 grade 3) at least possibly related to the vaccine. No serious AEs occurred. Distribution and severity of AEs were similar in all arms. PAMVAC was immunogenic in all participants. PAMVAC-specific antibody levels were highest with PAMVAC-GLA-SE. The antibodies inhibited binding of VAR2CSA expressing P. falciparum-infected erythrocytes to CSA in a standardized functional assay.

CONCLUSIONS

PAMVAC formulated with Alhydrogel or GLA-based adjuvants was safe, well tolerated, and induced functionally active antibodies. Next, PAMVAC will be assessed in women before first pregnancies in an endemic area.

CLINICAL TRIALS REGISTRATION

EudraCT 2015-001827-21; ClinicalTrials.gov NCT02647489.

Imported Malaria in Countries where Malaria Is Not Endemic: a Comparison of Semi-immune and Nonimmune Travelers

The continuous increase in long-distance travel and recent large migratory movements have changed the epidemiological characteristics of imported malaria in countries where malaria is not endemic (here termed non-malaria-endemic countries). While malaria was primarily imported to nonendemic countries by returning travelers, the proportion of immigrants from malaria-endemic regions and travelers visiting friends and relatives (VFRs) in malaria-endemic countries has continued to increase. VFRs and immigrants from malaria-endemic countries now make up the majority of malaria patients in many nonendemic countries. Importantly, this group is characterized by various degrees of semi-immunity to malaria, resulting from repeated exposure to infection and a gradual decline of protection as a result of prolonged residence in non-malaria-endemic regions. Most studies indicate an effect of naturally acquired immunity in VFRs, leading to differences in the parasitological features, clinical manifestation, and odds for severe malaria and clinical complications between immune VFRs and nonimmune returning travelers. There are no valid data indicating evidence for differing algorithms for chemoprophylaxis or antimalarial treatment in semi-immune versus nonimmune malaria patients. So far, no robust biomarkers exist that properly reflect anti-parasite or clinical immunity. Until they are found, researchers should rigorously stratify their study results using surrogate markers, such as duration of time spent outside a malaria-endemic country.

Cerebral Plasmodium falciparum malaria: The role of PfEMP1 in its pathogenesis and immunity, and PfEMP1-based vaccines to prevent it

Malaria, a mosquito-borne infectious disease caused by parasites of the genus Plasmodium continues to be a major health problem worldwide. The unicellular Plasmodium-parasites have the unique capacity to infect and replicate within host erythrocytes. By expressing variant surface antigens Plasmodium falciparum has evolved to avoid protective immune responses; as a result in endemic areas anti-malaria immunity develops gradually over many years of multiple and repeated infections. We are studying the role of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed by asexual stages of P. falciparum responsible for the pathogenicity of severe malaria. The immunopathology of falciparum malaria has been linked to cyto-adhesion of infected erythrocytes to specific host receptors. A greater appreciation of the PfEMP1 molecules important for the development of protective immunity and immunopathology is a prerequisite for the rational discovery and development of a safe and protective anti-disease malaria vaccine. Here we review the role of ICAM-1 and EPCR receptor adhering falciparum-parasites in the development of severe malaria; we discuss our current research to understand the factors involved in the pathogenesis of cerebral malaria and the feasibility of developing a vaccine targeted specifically to prevent this disease.

The Impact of Plasmodium Infection on Placental Histomorphology: A Stereological Preliminary Study

Background

Malaria during pregnancy may threaten the mother's health and cause serious structural damage to the internal architecture of the placenta, which subsequently affects the pregnancy outcome. A better understanding of the impact of malaria parasites on the placenta morphology is crucial for better management of pregnant women and their babies.

Aim

To assess by stereology the histomorphology of selected placental structures in placenta malaria compared with normal placentae at term.

Method

A total of 10 placentae comprising 5 controls and 5 cases were selected from 50 placentae that were collected at term (38 weeks ± 2 weeks) from the maternal delivery suit of Korle-Bu Teaching Hospital in Accra, Ghana. Blood from the placentae was collected for both rapid diagnostic test and microscopic examinations. Samples collected were examined for Plasmodium parasites, after which they were classified as study group (Plasmodium positive) or control (Plasmodium negative). Stereological quantification using systematic uniform random sampling technique with test point and intersection counting of photomicrographs were employed to estimate the mean volume densities of syncytial knots, syncytial necrosis, foetal capillaries, and intervillous spaces of the placentae on a total of 1,600 photomicrographs.

Results

Out of the fifty placental samples from the maternal side tested for Plasmodium, six representing 12% were found to be infected with the parasite by both rapid diagnostic test and microscopy. On stereological assessment, the mean volume density of syncytial knots was significantly higher in the placental malaria group compared with the control placentae at term (P = 0.0080), but foetal capillaries (P = 0.7813), intervillous spaces (P = 0.8078), and syncytial necrosis (P = 0.8249) were not significantly different.

Conclusion

This preliminary result indicates that placental malaria may cause significant increase in the syncytial knots but not foetal capillaries, intervillous spaces, or syncytial necrosis. This finding signifies early maturation of the placenta and may be crucial in understanding perinatal outcomes.

Exploring parasite heterogeneity using single-cell RNA-seq reveals a gene signature among sexual stage Plasmodium falciparum parasites

The malaria parasite has a complex lifecycle, including several events of differentiation and stage progression, while actively evading immunity in both its mosquito and human hosts. Important parasite gene expression and regulation during these events remain hidden in rare populations of cells. Here, we combine a capillary-based platform for cell isolation with single-cell RNA-sequencing to transcriptionally profile 165 single infected red blood cells (iRBCs) during the intra-erythrocytic developmental cycle (IDC). Unbiased analyses of single-cell data grouped the cells into eight transcriptional states during IDC. Interestingly, we uncovered a gene signature from the single iRBC analyses that can successfully discriminate between developing asexual and sexual stage parasites at cellular resolution, and we verify five, previously undefined, gametocyte stage specific genes. Moreover, we show the capacity of detecting expressed genes from the variable gene families in single parasites, despite the sparse nature of data. In total, the single parasite transcriptomics holds promise for molecular dissection of rare parasite phenotypes throughout the malaria lifecycle.

IgG Responses to the Plasmodium falciparum Antigen VAR2CSA in Colombia Are Restricted to Pregnancy and Are Not Induced by Exposure to Plasmodium vivax

Clinical immunity to malaria is associated with the acquisition of IgG specific for members of the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family of clonally variant antigens on the surface of infected erythrocytes (IEs). The VAR2CSA subtype of PfEMP1 mediates IE binding in the placenta.

Clinical immunity to malaria is associated with the acquisition of IgG specific for members of the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family of clonally variant antigens on the surface of infected erythrocytes (IEs). The VAR2CSA subtype of PfEMP1 mediates IE binding in the placenta. VAR2CSA-specific IgG is normally acquired only after exposure to placental parasites. However, it was recently reported that men and children from Colombia often have high levels of functional VAR2CSA-specific IgG. This potentially undermines the current understanding of malaria immunity in pregnant women, and we thus conducted a study to assess further the levels of VAR2CSA-specific IgG in pregnant and nonpregnant Colombians. Plasma IgG against two full-length recombinant PfEMP1 proteins (one of the VAR2CSA type and one not) produced in baculovirus-transfected insect cells was detected frequently among Colombian men, children, and pregnant women with acute or previous malaria exposure. In contrast, IgG reactivity to a homologous full-length VAR2CSA-type protein expressed in Chinese hamster ovary (CHO) cells was low and infrequent among the Colombian plasma samples, as was reactivity to both corresponding native PfEMP1 proteins. Moreover, human and rabbit antibodies specific for Plasmodium vivax Duffy-binding protein (PvDBP), a protein with some homology to PfEMP1, did not react with VAR2CSA-type recombinant or native proteins, although the mouse monoclonal and PvDBP-specific antibody 3D10 was weakly reactive with recombinant proteins expressed in baculovirus-transfected insect cells. Our data indicate that the previously reported Colombian IgG reactivity to recombinant VAR2CSA is not malaria specific and that the acquisition of VAR2CSA-specific IgG is restricted to pregnancy, in Colombia and elsewhere.

An upstream open reading frame (uORF) signals for cellular localization of the virulence factor implicated in pregnancy associated malaria

Plasmodium falciparum, the causative agent of the deadliest form of human malaria, alternates expression of variable antigens, encoded by members of a multi-copy gene family named var. In var2csa, the var gene implicated in pregnancy-associated malaria, translational repression is regulated by a unique upstream open reading frame (uORF) found only in its 5' UTR. Here, we report that this translated uORF significantly alters both transcription and posttranslational protein trafficking. The parasite can alter a protein's destination without any modifications to the protein itself, but instead by an element within the 5' UTR of the transcript. This uORF-dependent localization was confirmed by single molecule STORM imaging, followed by fusion of the uORF to a reporter gene which changes its cellular localization from cytoplasmic to ER-associated. These data point towards a novel regulatory role of uORF in protein trafficking, with important implications for the pathology of pregnancy-associated malaria.

Virus-Like-Vaccines against HIV

Protection against chronic infections has necessitated the development of ever-more potent vaccination tools. HIV seems to be the most challenging foe, with a remarkable, poorly immunogenic and fragile surface glycoprotein and the ability to overpower the cell immune system. Virus-like-particle (VLP) vaccines have emerged as potent inducers of antibody and helper T cell responses, while replication-deficient viral vectors have yielded potent cytotoxic T cell responses. Here, we review the emerging concept of merging these two technologies into virus-like-vaccines (VLVs) for the targeting of HIV. Such vaccines are immunologically perceived as viruses, as they infect cells and produce VLPs in situ, but they only resemble viruses, as the replication defective vectors and VLPs cannot propagate an infection. The inherent safety of such a platform, despite robust particle production, is a distinct advantage over live-attenuated vaccines that must balance safety and immunogenicity. Previous studies have delivered VLVs encoded in modified Vaccinia Ankara vectors and we have developed the concept into a single-reading adenovirus-based technology capable of eliciting robust CD8⁺ and CD4⁺ T cells responses and trimer binding antibody responses. Such vaccines offer the potential to display the naturally produced immunogen directly and induce an integrated humoral and cellular immune response.

Increased risk of low birth weight in women with placental malaria associated with P. falciparum VAR2CSA clade

Pregnancy associated malaria (PAM) causes adverse pregnancy and birth outcomes owing to Plasmodium falciparum accumulation in the placenta. Placental accumulation is mediated by P. falciparum protein VAR2CSA, a leading PAM-specific vaccine target. The extent of its antigen diversity and impact on clinical outcomes remain poorly understood. Through amplicon deep-sequencing placental malaria samples from women in Malawi and Benin, we assessed sequence diversity of VAR2CSA’s ID1-DBL2x region, containing putative vaccine targets and estimated associations of specific clades with adverse birth outcomes. Overall, var2csa diversity was high and haplotypes subdivided into five clades, the largest two defined by homology to parasites strains, 3D7 or FCR3. Across both cohorts, compared to women infected with only FCR3-like variants, women infected with only 3D7-like variants delivered infants with lower birthweight (difference: −267.99 g; 95% Confidence Interval [CI]: −466.43 g,−69.55 g) and higher odds of low birthweight (<2500 g) (Odds Ratio [OR] 5.41; 95% CI:0.99,29.52) and small-for-gestational-age (OR: 3.65; 95% CI: 1.01,13.38). In two distinct malaria-endemic African settings, parasites harboring 3D7-like variants of VAR2CSA were associated with worse birth outcomes, supporting differential effects of infection with specific parasite strains. The immense diversity coupled with differential clinical effects of this diversity suggest that an effective VAR2CSA-based vaccine may require multivalent activity.

Knudsen L, Damm P, G. Theander T, R. Hansson S, A. Nielsen M, Salanti A. Placental Sequestration of Plasmodium falciparum Malaria Parasites Is Mediated by the Interaction Between VAR2CSA and Chondroitin Sulfate A on Syndecan-1

During placental malaria, Plasmodium falciparum infected erythrocytes sequester in the placenta, causing health problems for both the mother and fetus. The specific adherence is mediated by the VAR2CSA protein, which binds to placental chondroitin sulfate (CS) on chondroitin sulfate proteoglycans (CSPGs) in the placental syncytium. However, the identity of the CSPG core protein and the cellular impact of the interaction have remain elusive. In this study we identified the specific CSPG core protein to which the CS is attached, and characterized its exact placental location. VAR2CSA pull-down experiments using placental extracts from whole placenta or syncytiotrophoblast microvillous cell membranes showed three distinct CSPGs available for VAR2CSA adherence. Further examination of these three CSPGs by immunofluorescence and proximity ligation assays showed that syndecan-1 is the main receptor for VAR2CSA mediated placental adherence. We further show that the commonly used placental choriocarcinoma cell line, BeWo, express a different set of proteoglycans than those present on placental syncytiotrophoblast and may not be the most biologically relevant model to study placental malaria. Syncytial fusion of the BeWo cells, triggered by forskolin treatment, caused an increased expression of placental CS-modified syndecan-1. In line with this, we show that rVAR2 binding to placental CS impairs syndecan-1-related Src signaling in forskolin treated BeWo cells, but not in untreated cells.

Plasmodium falciparum is the most deadly malaria parasite, causing more than 500,000 deaths each year. The parasite infects the host’s red blood cells. In placental malaria infected red blood cells accumulate in placenta. The parasite protein VAR2CSA mediates this adherence, which causes complications for both mother and child. VAR2CSA binds a carbohydrate chain termed chondroitin sulfate (CS). CS is not a well-defined biochemical entity but constitute a family of oligosaccharides which each have unique sulfation patterns. The CS binding VAR2CSA is attached to proteoglycans expressed on the surface of placental cells. While much work has gone into understanding the nature of VAR2CSA and its interaction with placental CS, the protein to which the placental CS is attached is not known. To further the understanding of the molecular pathology of PM we characterized the CSPG receptor that the parasites adhere to by defining the exact proteoglycan that carries the placental CS. We further investigated the molecular and cellular consequences of VAR2CSA binding to the receptor. This work provides novel insights into the pathology of placental malaria and the nature of the parasite receptor. This may aid development of strategies to treat or prevent placental malaria.

The influence of pregnancy on the pharmacokinetic properties of artemisinin combination therapy (ACT): a systematic review

Background

Pregnancy has been reported to alter the pharmacokinetic properties of anti-malarial drugs, including the different components of artemisinin-based combination therapy (ACT). However, small sample sizes make it difficult to draw strong conclusions based on individual pharmacokinetic studies. The aim of this review is to summarize the evidence of the influence of pregnancy on the pharmacokinetic properties of different artemisinin-based combinations.

Methods

A PROSPERO-registered systematic review to identify clinical trials that investigated the influence of pregnancy on the pharmacokinetic properties of different forms of ACT was conducted, following PRISMA guidelines. Without language restrictions, Medline/PubMed, Embase, Cochrane Central Register of Controlled Trials, Web of Science, LILACS, Biosis Previews and the African Index Medicus were searched for studies published up to November 2015. The following components of ACT that are currently recommend by the World Health Organization as first-line treatment of malaria in pregnancy were reviewed: artemisinin, artesunate, dihydroartemisinin, lumefantrine, amodiaquine, mefloquine, sulfadoxine, pyrimethamine, piperaquine, atovaquone and proguanil.

Results

The literature search identified 121 reports, 27 original studies were included. 829 pregnant women were included in the analysis. Comparison of the available studies showed lower maximum concentrations (C_max) and exposure (AUC) of dihydroartemisinin, the active metabolite of all artemisinin derivatives, after oral administration of artemether, artesunate and dihydroartemisinin in pregnant women. Low day 7 concentrations were commonly seen in lumefantrine studies, indicating a low exposure and possibly reduced efficacy. The influence of pregnancy on amodiaquine and piperaquine seemed not to be clinically relevant. Sulfadoxine plasma concentration was significantly reduced and clearance rates were higher in pregnancy, while pyrimethamine and mefloquine need more research as no general conclusion can be drawn based on the available evidence. For atovaquone, the available data showed a lower maximum concentration and exposure. Finally, the maximum concentration of cycloguanil, the active metabolite of proguanil, was significantly lower, possibly compromising the efficacy.

Conclusion

These findings suggest that reassessment of the dose of the artemisinin derivate and some components of ACT are necessary to ensure the highest possible efficacy of malaria treatment in pregnant women. However, for most components of ACT, data were insufficient and extensive research with larger sample sizes will be necessary to identify the exact influences of pregnancy on the pharmacokinetic properties of different artemisinin-based combinations. In addition, different clinical studies used diverse study designs with various reported relevant outcomes. Future pharmacokinetic studies could benefit from more uniform designs, in order to increase quality, robustness and effectiveness.

Study registration: CRD42015023756 (PROSPERO)

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1160-6) contains supplementary material, which is available to authorized users.

Prevalence of Plasmodium falciparum Malaria among Pregnant Students in Dodoma Region, Tanzania: No Cases Have Been Detected

Malaria in pregnancy, being often asymptomatic, is a major problem in endemic African countries. It is characterized by anemia and placental malaria leading to poor pregnancy outcomes. In 2001 Tanzania adopted an intermittent-preventive treatment of malaria in pregnancy (IPTp) policy, which recommends receiving doses of antimalarial drugs every planned visit to the antenatal care centre (ANC), starting from the second trimester. Currently the policy is valid across the whole country, regardless that there are regions with very low malaria endemicity in Tanzania, such as Dodoma region. The current study aimed to show the real prevalence of malaria among young pregnant women in Dodoma region, by measuring the prevalence of malaria among University of Dodoma (UDOM) students, and to describe the social health care features of student female community. Two methods of malaria diagnostic, microscopy, and rapid test, as well as retrospective inspection of ANC registry book, showed the very low prevalence of malaria disease among pregnant students, approximately 0.3%. Additionally, the sociodemographic data from the questionnaires showed that all students use different malaria preventive measures, and most of them have the regular sexual partner. This fact approves the correlation between illiteracy of woman and the risk of malaria infection transmission.

Malaria and immunity during pregnancy and postpartum: a tale of two species

It is well established that pregnant women are at an increased risk of Plasmodium falciparum infection when compared to non-pregnant individuals and limited epidemiological data suggest Plasmodium vivax risk also increases with pregnancy. The risk of P. falciparum declines with successive pregnancies due to the acquisition of immunity to pregnancy-specific P. falciparum variants. However, despite similar declines in P. vivax risk with successive pregnancies, there is a paucity of evidence P. vivax-specific immunity. Cross-species immunity, as well as immunological and physiological changes that occur during pregnancy may influence the susceptibility to both P. vivax and P. falciparum. The period following delivery, the postpartum period, is relatively understudied and available epidemiological data suggests that it may also be a period of increased risk of infection to Plasmodium spp. Here we review the literature and directly compare and contrast the epidemiology, clinical pathogenesis and immunological features of P. vivax and P. falciparum in pregnancy, with a particular focus on studies performed in areas co-endemic for both species. Furthermore, we review the intriguing epidemiology literature of both P. falciparum and P. vivax postpartum and relate observations to the growing literature pertaining to malaria immunology in the postpartum period.

Parity-dependent recognition of DBL1X-3X suggests an important role of the VAR2CSA high-affinity CSA-binding region in the development of the humoral response against placental malaria

Plasmodium falciparum multidomain protein VAR2CSA stands today as the leading vaccine candidate against pregnancy-associated malaria (PAM). Most of the studies aiming to decrypt how naturally acquired immunity develops have assessed the immune recognition of individual VAR2CSA Duffy-binding-like (DBL) domains, thus overlooking the presence of conformational epitopes resulting from the overall folding of the full-length protein. In order to characterize the development of humoral immunity toward VAR2CSA, we made use of a large cohort of 293 Senegalese pregnant women to assess the level of recognition by plasma IgG of the full-length VAR2CSA protein of the 3D7 parasite strain (3D7-VAR2CSA), the CSA-binding multidomains 3D7-DBL1X to -DBL3X (3D7-DBL1X-3X), and the CSA nonbinding multidomains 3D7-DBL4ε to -DBL6ε (3D7-DBL4ε-6ε), as well as individual 3D7-DBL domains. Our results revealed a parity-dependent recognition of the full-length 3D7-VAR2CSA and of the CSA-binding region, 3D7-DBL1X-3X. Indeed, multigravid women possess significantly higher levels of antibodies directed against these constructs than primigravidae. Our results suggest an important role of antibodies targeting the CSA-binding region in the development of immunity against PAM, therefore providing new insights on how natural protection might be acquired and further information for the design of VAR2CSA-based vaccines.

A molecular switch in the efficiency of translation reinitiation controls expression of var2csa, a gene implicated in pregnancy-associated malaria

Plasmodium falciparum malaria parasites export the protein PfEMP1 to the surface of infected erythrocytes, enabling them to adhere to receptors in the microvasculature and thereby avoid clearance by the spleen. The gene var2csa encodes the form of PfEMP1 that binds specifically within the placenta, causing pregnancy-associated malaria, and appears to not be expressed in the absence of a placenta. We previously described an upstream open reading frame (uORF) that is responsible for repression of translation of the downstream ORF (dORF) that encodes VAR2CSA, thus keeping the gene silent when parasites infect non-pregnant individuals. To elucidate the molecular mechanism by which this repression is overcome during pregnancy, we stably transformed parasites with reporter gene constructs designed to detect switches in the efficiency of dORF translation. We found that proper regulation of switching relies on two separate components, (i) active translation of the uORF and (ii) sequence-specific characteristics of the surrounding transcript, which together control the ability of the ribosome complex to reinitiate a second round of translation and thus express VAR2CSA. These results provide the first details of a molecular switch that allows parasites take advantage of the unique niche provided by the placenta.

Histopathologies, immunolocalization, and a glycan binding screen provide insights into Plasmodium falciparum interactions with the human placenta

During pregnancy, Plasmodium falciparum-infected erythrocytes cytoadhere to the placenta. Infection is likely initiated at two sites where placental trophoblasts contact maternal blood: 1) via syncytiotrophoblast (STB), a multicellular transporting and biosynthetic layer that forms the surface of chorionic villi and lines the intervillous space, and 2) through invasive cytotrophoblasts, which line uterine vessels that divert blood to the placenta. Here, we investigated mechanisms of infected erythrocyte sequestration in relationship to the microanatomy of the maternal-fetal interface. Histological analyses revealed STB denudation in placental malaria, which brought the stromal cores of villi in direct contact with maternal blood. STB denudation was associated with hemozoin deposition (P = 0.01) and leukocyte infiltration (P = 0.001) and appeared to be a feature of chronic placental malaria. Immunolocalization of infected red blood cell receptors (CD36, ICAM1/CD54, and chondroitin sulfate A) in placentas from uncomplicated pregnancies showed that STB did not stain, while the underlying villous stroma was immunopositive. Invasive cytotrophoblasts expressed ICAM1. In malaria, STB denudation exposed CD36 and chondroitin sulfate A in the villous cores to maternal blood, and STB expressed ICAM1. Finally, we investigated infected erythrocyte adherence to novel receptors by screening an array of 377 glycans. Infected erythrocytes bound Lewis antigens that immunolocalized to STB. Our results suggest that P. falciparum interactions with STB-associated Lewis antigens could initiate placental malaria. Subsequent pathologies, which expose CD36, ICAM1, and chondroitin sulfate A, might propagate the infection.

Peripheral blood cell signatures of Plasmodium falciparum infection during pregnancy

Sequestration of Plasmodium falciparum-infected erythrocytes in placental intervillous spaces causes inflammation and pathology. Knowledge of the profiles of immune cells associated with the physiopathology of pregnancy-associated malaria (PAM) is scarce. We conducted a longitudinal, prospective study, both in Benin and Tanzania, including ∼1000 pregnant women in each site with systematic follow-up at scheduled antenatal visits until delivery. We used ex vivo flow cytometry to identify peripheral blood mononuclear cell (PBMC) profiles that are associated with PAM and anaemia, determining the phenotypic composition and activation status of PBMC in selected sub-groups with and without PAM both at inclusion and at delivery in a total of 302 women. Both at inclusion and at delivery PAM was associated with significantly increased frequencies both of B cells overall and of activated B cells. Infection-related profiles were otherwise quite distinct at the two different time-points. At inclusion, PAM was associated with anaemia, with an increased frequency of immature monocytes and with a decreased frequency of regulatory T cells (Treg). At delivery, infected women presented with significantly fewer plasmacytoid dendritic cells (DC), more myeloid DC expressing low levels of HLA-DR, and more effector T cells (Teff) compared to uninfected women. Independent associations with an increased risk of anaemia were found for altered antigen-presenting cell frequencies at inclusion, but for an increased frequency of Teff at delivery. Our findings emphasize the prominent role played by B cells during PAM whenever it arises during pregnancy, whilst also revealing signature changes in other circulating cell types that, we conclude, primarily reflect the relative duration of the infections. Thus, the acute, recently-acquired infections present at delivery were marked by changes in DC and Teff frequencies, contrasting with infections at inclusion, considered chronic in nature, that were characterized by an abundance of immature monocytes and a paucity of Treg in PBMC.

First-trimester Plasmodium falciparum infections display a typical "placental" phenotype

BACKGROUND

Plasmodium falciparum-infected erythrocytes (IEs) adhere to host cell receptors, allowing parasites to sequester into deep vascular beds of various organs. This defining phenomenon of malaria pathogenesis is key to the severe clinical complications associated with cerebral and placental malaria. The principal ligand associated with the binding to chondroitin sulfate A (CSA) that allows placental sequestration of IEs is a P. falciparum erythrocyte membrane protein 1 (PfEMP1) family member encoded by the var2csa gene.

METHODS

Here, we investigated the transcription pattern of var genes by real-time polymerase chain reaction, the expression of VAR2CSA, protein by flow cytometry, and the CSA-binding ability of IEs collected at different stages of pregnancy using a static-based Petri dish assay.

RESULTS

Through comparison with the profiles of isolates from nonpregnant hosts, we report several lines of evidence showing that parasites infecting women during pregnancy preferentially express VAR2CSA protein, and that selection for the capacity to adhere to CSA via VAR2CSA expression occurs early in pregnancy.

CONCLUSIONS

Our data suggest that the placental tropism of P. falciparum is already established in the first trimester of pregnancy, with consequent implications for the development of the pathology associated with placental malaria.

Identification and characterization of B-cell epitopes in the DBL4ε domain of VAR2CSA

Malaria during pregnancy in Plasmodium falciparum endemic regions is a major cause of mortality and severe morbidity. VAR2CSA is the parasite ligand responsible for sequestration of Plasmodium falciparum infected erythrocytes to the receptor chondroitin sulfate A (CSA) in the placenta and is the leading candidate for a placental malaria vaccine. Antibodies induced in rats against the recombinant DBL4ε domain of VAR2CSA inhibit the binding of a number of laboratory and field parasite isolates to CSA. In this study, we used a DBL4ε peptide-array to identify epitopes targeted by DBL4ε-specific antibodies that inhibit CSA-binding of infected erythrocytes. We identified three regions of overlapping peptides which were highly antigenic. One peptide region distinguished itself particularly by showing a clear difference in the binding profile of highly parasite blocking IgG compared to the IgG with low capacity to inhibit parasite adhesion to CSA. This region was further characterized and together these results suggest that even though antibodies against the synthetic peptides which cover this region did not recognize native protein, the results using the mutant domain suggest that this linear epitope might be involved in the induction of inhibitory antibodies induced by the recombinant DBL4ε domain.

Prospects and Pitfalls of Pregnancy-Associated Malaria Vaccination Based on the Natural Immune Response to Plasmodium falciparum VAR2CSA-Expressing Parasites

Pregnancy-associated malaria, a manifestation of severe malaria, is the cause of up to 200,000 infant deaths a year, through the effects of placental insufficiency leading to growth restriction and preterm delivery. Development of a vaccine is one strategy for control. Plasmodium falciparum-infected red blood cells accumulate in the placenta through specific binding of pregnancy-associated parasite variants that express the VAR2CSA antigen to chondroitin sulphate A on the surface of syncytiotrophoblast cells. Parasite accumulation, accompanied by an inflammatory infiltrate, disrupts the cytokine balance of pregnancy with the potential to cause placental damage and compromise foetal growth. Multigravid women develop immunity towards VAR2CSA-expressing parasites in a gravidity-dependent manner which prevents unfavourable pregnancy outcomes. Although current vaccine design, targeting VAR2CSA antigens, has succeeded in inducing antibodies artificially, this candidate may not provide protection during the first trimester and may only protect those women living in areas endemic for malaria. It is concluded that while insufficient information about placental-parasite interactions is presently available to produce an effective vaccine, incremental progress is being made towards achieving this goal.

Parasitologic Assessment of Two-Dose and Monthly Intermittent Preventive Treatment of Malaria during Pregnancy with Sulphadoxine-Pyrimethamine (IPTP-SP) in Lagos, Nigeria

Intermittent preventive treatment of malaria with sulphadoxine-pyrimethamine (IPTP-SP) is a key strategy in the control of malaria in pregnancy. However, reports of increasing level of resistance to SP using nonpregnant populations have made it imperative for the continuous monitoring of the efficacy of SP in pregnant women. This study assessed using microscopy, monthly dosing and the standard two-dose regimen among 259 pregnant women attending antenatal clinics in Lagos, Nigeria that consented 122 in the two-dose arm (Arm A) and 137 in the monthly dose arm (Arm B). Baseline parasitaemia in the two groups was 5 (4.1%) and 3 (2.2%) in Arms A and B, respectively. Few of the women developed parasitaemia after the initial SP dose in Arms A 4 (3.3%) and B 2 (1.5%). However, none of the women had malaria infection after the second dose in both Arms. Although IPTP-SP is suggestive of protecting the women from malaria infection, there was no significant difference observed between the two dosing schemes.

How specific is Plasmodium falciparum adherence to chondroitin 4-sulfate?

Plasmodium falciparum infection during pregnancy results in the sequestration of infected red blood cells (IRBCs) in the placenta, contributing to pregnancy associated malaria (PAM). IRBC adherence is mediated by the binding of a variant Plasmodium falciparum erythrocyte binding protein 1 named VAR2CSA to the low sulfated chondroitin 4-sulfate (C4S) proteoglycan (CSPG) present predominantly in the intervillous space of the placenta. IRBC binding is highly specific to the level and distribution of 4-sulfate groups in C4S. Given the strict specificity of IRBC-C4S interactions, it is better to use either placental CSPG or CSPGs bearing structurally similar C4S chains in defining VAR2CSA structural architecture that interact with C4S, evaluating VAR2CSA constructs for vaccine development or studying structure-based inhibitors as therapeutics for PAM.

Chondroitin sulfate A-adhering Plasmodium falciparum-infected erythrocytes express functionally important antibody epitopes shared by multiple variants

Acquired protection from Plasmodium falciparum placental malaria, a major cause of maternal, fetal, and infant morbidity, is mediated by IgG specific for the P. falciparum erythrocyte membrane protein 1 variant VAR2CSA. This protein enables adhesion of P. falciparum-infected erythrocytes to chondroitin sulfate A in the intervillous space. Although interclonal variation of the var2csa gene is lower than that among var genes in general, VAR2CSA-specific Abs appear to target mainly polymorphic epitopes. This has raised doubts about the feasibility of VAR2CSA-based vaccines. We used eight human monoclonal IgG Abs from affinity-matured memory B cells of P. falciparum-exposed women to study interclonal variation and functional importance of Ab epitopes among placental and peripheral parasites from East and West Africa. Most placental P. falciparum isolates were labeled by several mAbs, whereas peripheral isolates from children were essentially nonreactive. The mAb reactivity of peripheral isolates from pregnant women indicated that some were placental, whereas others had alternative sequestration foci. Most of the mAbs were comparable in their reactivity with bound infected erythrocytes (IEs) and recombinant VAR2CSA and interfered with IE and/or VAR2CSA binding to chondroitin sulfate A. Pair-wise mAb combinations were more inhibitory than single mAbs, and all of the mAbs together was the most efficient combination. Each mAb could opsonize IEs for phagocytosis, and a combination of the eight mAbs caused phagocytosis similar to that of plasma IgG-opsonized IEs. We conclude that functionally important Ab epitopes are shared by the majority of polymorphic VAR2CSA variants, which supports the feasibility of VAR2CSA-based vaccines against placental malaria.

Structural insights into chondroitin sulfate binding in pregnancy-associated malaria

Malaria during pregnancy is caused when parasite-infected erythrocytes accumulate within the placenta through interactions between the VAR2CSA protein on the infected erythrocyte surface and placental CSPGs (chondroitin sulfate proteoglycans). This interaction is the major target for therapeutics to treat or prevent pregnancy-associated malaria. Here we review the structural characterization of CSPG-binding DBL (Duffy-binding like) domains from VAR2CSA and summarize the growing evidence that the exquisite ligand specificity of VAR2CSA results from the adoption of higher-order architecture in which these domains fold together to form a ligand-binding pocket.

Carbohydrate binding molecules in malaria pathology

Interactions between parasite-encoded proteins and host carbohydrate molecules are essential at multiple stages in the life cycle of the malaria-causing parasite, Plasmodium falciparum, and these interactions are targets for the development of therapeutics to treat the disease. Here we review recent structural studies of carbohydrate binding modules that mediate recognition events important for cell invasion and cytoadhesion. In particular we focus on the structures of two molecules; the erythrocyte binding antigen, EBA-175 involved in erythrocyte invasion and the VAR2CSA protein that mediates binding of infected erythrocytes to the placenta. These proteins both use Duffy-binding like (DBL) domains, a Plasmodium specific fold, to bind host carbohydrates, but recent results show that they differ significantly in their architectures and modes of ligand recognition.

Molecular interactions in the placenta during malaria infection

Placental malaria is the placental sequestration of Plasmodium falciparum infected erythrocytes that accumulate in the intervillous space, resulting in pathological alterations. The intervillous space, the main compartment for exchange of nutrients and delivery of oxygen to the fetus, is of utmost importance for fetal development. Events leading to adverse outcomes of placental malaria can be summarized in four steps: (1) accumulation of P. falciparum infected erythrocytes; (2) infiltration of monocytes and macrophages; (3) alteration of the placental cytokine balance and (4) pathogenesis of adverse pregnancy outcomes. These events are triggered by chemokines and cytokines leading to impaired materno-fetal exchange and damage to the placenta. This review describes the events during placental malaria infection at molecular level and presents a simplified model describing all crucial steps leading to adverse pregnancy outcomes based on a review of recent literature (August 2009).

Assessing the burden of pregnancy-associated malaria under changing transmission settings

Background

The clinical presentation of pregnancy-associated malaria, or PAM, depends crucially on the particular epidemiological settings. This can potentially lead to an underestimation of its overall burden on the female population, especially in regions prone to epidemic outbreaks and where malaria transmission is generally low.

Methods

Here, by re-examining historical data, it is demonstrated how excess female mortality can be used to evaluate the burden of PAM. A simple mathematical model is then developed to highlight the contrasting signatures of PAM within the endemicity spectrum and to show how PAM is influenced by the intensity and stability of transmission.

Results

Both the data and the model show that maternal malaria has a huge impact on the female population. This is particularly pronounced in low-transmission settings during epidemic outbreaks where excess female mortality/morbidity can by far exceed that of a similar endemic setting.

Conclusion

The results presented here call for active intervention measures not only in highly endemic regions but also, or in particular, in areas where malaria transmission is low and seasonal.

Multiple var2csa-type PfEMP1 genes located at different chromosomal loci occur in many Plasmodium falciparum isolates

Background

The var2csa gene encodes a Plasmodium falciparum adhesion receptor which binds chondroitin sulfate A (CSA). This var gene is more conserved than other PfEMP1/var genes and is found in all P. falciparum isolates. In isolates 3D7, FCR3/It4 and HB3, var2csa is transcribed from a sub-telomeric position on the left arm of chromosome 12, but it is not known if this location is conserved in all parasites. Genome sequencing indicates that the var2csa gene is duplicated in HB3, but whether this is true in natural populations is uncertain.

Methodology/Principal Findings

To assess global variation in the VAR2CSA protein, sequence variation in the DBL2X region of var2csa genes in 54 P.falciparum samples was analyzed. Chromosome mapping of var2csa loci was carried out and a quantitative PCR assay was developed to estimate the number of var2csa genes in P.falciparum isolates from the placenta of pregnant women and from the peripheral circulation of other malaria patients. Sequence analysis, gene mapping and copy number quantitation in P.falciparum isolates indicate that there are at least two loci and that both var2csa-like genes can be transcribed. All VAR2CSA DBL2X domains fall into one of two distinct phylogenetic groups possessing one or the other variant of a large (∼26 amino acid) dimorphic motif, but whether either motif variant is linked to a specific locus is not known.

Conclusions/Significance

Two or more related but distinct var2csa-type PfEMP1/var genes exist in many P. falciparum isolates. One gene is on chromosome 12 but additional var2csa-type genes are on different chromosomes in different isolates. Multiplicity of var2csa genes appears more common in infected placentae than in samples from non-pregnant donors indicating a possible advantage of this genotype in pregnancy associated malaria.

Structural comparison of two CSPG-binding DBL domains from the VAR2CSA protein important in malaria during pregnancy

Severe malaria during pregnancy is associated with accumulation of parasite-infected erythrocytes in the placenta due to interactions between VAR2CSA protein, expressed on the surface of infected-erythrocytes, and placental chondroitin sulfate proteoglycans (CSPG). VAR2CSA contains multiple CSPG-binding domains, including DBL3X and DBL6ɛ. Previous structural studies of DBL3X suggested CSPG to bind to a positively charged patch and sulfate-binding site on the concave surface of the domain. Here we present the structure of the DBL6ɛ domain from VAR2CSA. This domain displays the same overall architecture and secondary structure as that of DBL3X but differs in loop structures, disulfide bond positions and surface charge distribution. In particular, despite binding to CSPG, DBL6ɛ lacks the key features of the CSPG-binding site of DBL3X. Instead DBL6ɛ binds to CSPG through a positively charged surface on the distal side of subdomain 2 that is exposed in intact VAR2CSA on the erythrocyte surface. Finally, unlike intact VAR2CSA, both DBL3X and DBL6ɛ bind to various carbohydrates, with greatest affinity for ligands with high sulfation and negative charge. These studies provide further insight into the structure of DBL domains and suggest a model for the role of individual domains in CSPG binding by VAR2CSA in placental malaria.

Human monoclonal IgG selection of Plasmodium falciparum for the expression of placental malaria-specific variant surface antigens

Pregnancy-associatedPlasmodium falciparum malaria (PAM) is a major cause of morbidity and mortality in African women and their offspring. PAM is characterized by accumulation of infected erythrocytes (IEs) that adhere to chondroitin sulphate A (CSA) in the placental intervillous space. We show here that human monoclonal IgG antibodies with specificity for variant surface antigens (VSA) specifically expressed by CSA-adhering IEs (VSA_PAM) can be used in vitro to select parasites from nonpregnant donors to express VSA_PAM and that this selection for VSA_PAM expression results in preferential transcription of var2csa. The results corroborate current efforts to develop PAM-specific vaccines based on VAR2CSA.

Plasmodium falciparum: Assessment of parasite-infected red blood cell binding to placental chondroitin proteoglycan and bovine tracheal chondroitin sulfate A

In pregnant women infected with Plasmodium falciparum, the infected red blood cells (IRBCs) sequester in placenta by binding to the chondroitin 4-sulfate (C4S) chains of low sulfated chondroitin sulfate proteoglycan (CSPG). Placental CSPG, the natural receptor for IRBC adherence in the placenta, is the ideal molecule for studying structural interactions in IRBC adhesion to C4S, adhesion inhibitory antibody responses, and identification of parasite adhesive protein(s). However, because of difficulty involved in purifying placental CSPG, the commercially available bovine tracheal chondroitin sulfate A (bCSA), a copolymer having structural features of both C4S and C6S, has been widely used. To determine the validity of bCSA for C4S-IRBC interaction studies, we comparatively evaluated the characteristics of IRBC binding to placental CSPG and bCSA using three commonly used parasite strains. The results indicate that, in all three parasites studied, the characteristics of IRBC binding to placental CSPG and bCSA are qualitatively similar, but the binding capacity with respect to both the number of IRBCs bound per unit area of coated surface and binding strength is significantly higher for CSPG than bCSA regardless of whether parasites were selected on CSPG or bCSA. These results demonstrate that placental CSPG is best suited for studying interactions between parasite adhesive protein(s) and C4S, and have implications in understanding C4S-IRBC structural interactions.

Recrudescent Plasmodium berghei from pregnant mice displays enhanced binding to the placenta and induces protection in multigravida

Pregnancy-associated malaria (PAM) is associated with placenta pathology and poor pregnancy outcome but the mechanisms that control the malaria parasite expansion in pregnancy are still poorly understood and not amenable for study in human subjects. Here, we used a set of new tools to re-visit an experimental mouse model of pregnancy-induced malaria recrudescence, BALB/c with chronic Plasmodium berghei infection. During pregnancy 60% of the pre-exposed primiparous females showed pregnancy-induced malaria recrudescence and we demonstrated that the recrudescent P. berghei show an unexpected enhancement of the adherence to placenta tissue sections with a marked specificity for CSA. Furthermore, we showed that the intensity of parasitemia in primigravida was quantitatively correlated with the degree of thickening of the placental tissue and up-regulation of inflammation-related genes such as IL10. We also confirmed that the incidence of pregnancy-induced recrudescence, the intensity of the parasitemia peak and the impact on the pregnancy outcome decreased gradually from the first to the third pregnancy. Interestingly, placenta pathology and fetal impairment were also observed at low frequency among non-recrudescent females. Together, the data raise the hypothesis that recrudescent P. berghei displays selected specificity for the placenta tissue enabling on one hand, the triggering of the pathological process underlying PAM and on the other hand, the induction of PAM protection mechanisms that are revealed in subsequent pregnancies. Thus, by exploiting P. berghei pregnancy-induced recrudescence, this experimental system offers a mouse model to study the susceptibility to PAM and the mechanisms of disease protection in multigravida.

Acquired immunity to malaria

Naturally acquired immunity to falciparum malaria protects millions of people routinely exposed to Plasmodium falciparum infection from severe disease and death. There is no clear concept about how this protection works. There is no general agreement about the rate of onset of acquired immunity or what constitutes the key determinants of protection; much less is there a consensus regarding the mechanism(s) of protection. This review summarizes what is understood about naturally acquired and experimentally induced immunity against malaria with the help of evolving insights provided by biotechnology and places these insights in the context of historical, clinical, and epidemiological observations. We advocate that naturally acquired immunity should be appreciated as being virtually 100% effective against severe disease and death among heavily exposed adults. Even the immunity that occurs in exposed infants may exceed 90% effectiveness. The induction of an adult-like immune status among high-risk infants in sub-Saharan Africa would greatly diminish disease and death caused by P. falciparum. The mechanism of naturally acquired immunity that occurs among adults living in areas of hyper- to holoendemicity should be understood with a view toward duplicating such protection in infants and young children in areas of endemicity.

Structure of the DBL3x domain of pregnancy-associated malaria protein VAR2CSA complexed with chondroitin sulfate A

Plasmodium falciparum–infected erythrocytes bind to chondroitin sulfate A (CSA) in the placenta via the VAR2CSA protein, a member of the P. falciparum erythrocyte membrane protein-1 family, leading to life-threatening malaria in pregnant women with severe effects on their fetuses and newborns. Here we describe the structure of the CSA binding DBL3x domain, a Duffy binding-like (DBL) domain of VAR2CSA. By forming a complex of DBL3x with CSA oligosaccharides and determining its structure, we have identified the CSA binding site to be a cluster of conserved positively charged residues on subdomain 2 and subdomain 3. Mutation or chemical modification of lysine residues at the site markedly diminished CSA binding to DBL3x. The location of the CSA binding site is an important step forward in the molecular understanding of pregnancy-associated malaria and offers a new target for vaccine development.

The structure of a chondroitin sulfate-binding domain important in placental malaria

Adhesive PfEMP1 proteins are displayed on the surface of malaria-infected red blood cells. They play a critical role in the disease, tethering infected cells away from destruction by the spleen and causing many severe symptoms. A molecular understanding of how these domains maintain their binding properties while evading immune detection will be important in developing therapeutics. In malaria of pregnancy, domains from the var2csa-encoded PfEMP1 protein interact with chondroitin sulfate on the placenta surface. This causes accumulation of infected red blood cells, leading to placental inflammation and block of blood flow to the developing fetus. This is associated with maternal anemia, low birth weight, and premature delivery and can lead to the death of mother and child. Here I present the structure of the chondroitin sulfate-binding DBL3X domain from a var2csa-encoded PfEMP1 protein. The domain adopts a fold similar to malarial invasion proteins, with extensive loop insertions. One loop is flexible in the unliganded structure but observed in the presence of sulfate or disaccharide, where it completes a sulfate-binding site. This loop, and others surrounding this putative carbohydrate-binding site, are flexible and polymorphic, perhaps protecting the binding site from immune detection. This suggests a model for how the domain maintains ligand binding while evading the immune response and will guide future drug and vaccine development.

Malaria vaccines: immunity, models and monoclonal antibodies

Although experts in the field have agreed on the malaria vaccine technology roadmap that should be followed (http://www.malariavaccineroadmap.net/), the path towards an effective malaria vaccine remains littered with intellectual and practical pot-holes. The animal models that are currently available are problematic, and current understanding of the exact mechanisms and targets of protective immune responses is incomplete. However, recent technological advances might help overcome some of these hurdles.

A method for visualizing surface-exposed and internal PfEMP1 adhesion antigens in Plasmodium falciparum infected erythrocytes

Background

The insertion of parasite antigens into the host erythrocyte membrane and the structure and distribution of Plasmodium falciparum adhesion receptors on that membrane are poorly understood. Laser scanning confocal microscopy (LSCM) and a novel labelling and fixation method have been used to obtain high resolution immuno-fluorescent images of erythrocyte surface PfEMP1 and internal antigens which allow analysis of the accumulation of PfEMP1 on the erythrocyte membrane during asexual development.

Methods

A novel staining technique has been developed which permits distinction between erythrocyte surface PfEMP1 and intracellular PfEMP1, in parasites whose nuclear material is exceptionally well resolved. Primary antibody detection by fluorescence is carried out on the live parasitized erythrocyte. The surface labelled cells are then fixed using paraformaldehyde and permeabilized with a non-ionic detergent to permit access of antibodies to internal parasite antigens. Differentiation between surface and internal antigens is achieved using antibodies labelled with different fluorochromes and confocal microscopy

Results

Surface exposed PfEMP1 is first detectable by antibodies at the trophozoite stage of intracellular parasite development although the improved detection method indicates that there are differences between different laboratory isolates in the kinetics of accumulation of surface-exposed PfEMP1.

Conclusion

A sensitive method for labelling surface and internal PfEMP1 with up to three different fluorochromes has been developed for laser scanning confocal optical microscopy and the analysis of the developmental expression of malaria adhesion antigens.