Ex vivo desequestration of Plasmodium falciparum-infected erythrocytes from human placenta by chondroitin sulfate A

Internalization and trafficking of CSPG-bound recombinant VAR2CSA lectins in cancer cells

Proteoglycans are proteins that are modified with glycosaminoglycan chains. Chondroitin sulfate proteoglycans (CSPGs) are currently being exploited as targets for drug-delivery in various cancer indications, however basic knowledge on how CSPGs are internalized in tumor cells is lacking. In this study we took advantage of a recombinant CSPG-binding lectin VAR2CSA (rVAR2) to track internalization and cell fate of CSPGs in tumor cells. We found that rVAR2 is internalized into cancer cells via multiple internalization mechanisms after initial docking on cell surface CSPGs. Regardless of the internalization pathway used, CSPG-bound rVAR2 was trafficked to the early endosomes in an energy-dependent manner but not further transported to the lysosomal compartment. Instead, internalized CSPG-bound rVAR2 proteins were secreted with exosomes to the extracellular environment in a strictly chondroitin sulfate-dependent manner. In summary, our work describes the cell fate of rVAR2 proteins in tumor cells after initial binding to CSPGs, which can be further used to inform development of rVAR2-drug conjugates and other therapeutics targeting CSPGs.

Progress and Insights Toward an Effective Placental Malaria Vaccine

In areas where Plasmodium falciparum transmission is endemic, clinical immunity against malaria is progressively acquired during childhood and adults are usually protected against the severe clinical consequences of the disease. Nevertheless, pregnant women, notably during their first pregnancies, are susceptible to placental malaria and the associated serious clinical outcomes. Placental malaria is characterized by the massive accumulation of P. falciparum infected erythrocytes and monocytes in the placental intervillous spaces leading to maternal anaemia, hypertension, stillbirth and low birth weight due to premature delivery, and foetal growth retardation. Remarkably, the prevalence of placental malaria sharply decreases with successive pregnancies. This protection is associated with the development of antibodies directed towards the surface of P. falciparum-infected erythrocytes from placental origin. Placental sequestration is mediated by the interaction between VAR2CSA, a member of the P. falciparum erythrocyte membrane protein 1 family expressed on the infected erythrocytes surface, and the placental receptor chondroitin sulfate A. VAR2CSA stands today as the leading candidate for a placental malaria vaccine. We recently reported the safety and immunogenicity of two VAR2CSA-derived placental malaria vaccines (PRIMVAC and PAMVAC), spanning the chondroitin sulfate A-binding region of VAR2CSA, in both malaria-naïve and P. falciparum-exposed non-pregnant women in two distinct Phase I clinical trials (ClinicalTrials.gov, NCT02658253 and NCT02647489). This review discusses recent advances in placental malaria vaccine development, with a focus on the recent clinical data, and discusses the next clinical steps to undertake in order to better comprehend vaccine-induced immunity and accelerate vaccine development.

VAR2CSA-Mediated Host Defense Evasion of Plasmodium falciparum Infected Erythrocytes in Placental Malaria

Over 30 million women living in P. falciparum endemic areas are at risk of developing malaria during pregnancy every year. Placental malaria is characterized by massive accumulation of infected erythrocytes in the intervillous space of the placenta, accompanied by infiltration of immune cells, particularly monocytes. The consequent local inflammation and the obstruction of the maternofetal exchanges can lead to severe clinical outcomes for both mother and child. Even if protection against the disease can gradually be acquired following successive pregnancies, the malaria parasite has developed a large panel of evasion mechanisms to escape from host defense mechanisms and manipulate the immune system to its advantage. Infected erythrocytes isolated from placentas of women suffering from placental malaria present a unique phenotype and express the pregnancy-specific variant VAR2CSA of the Plasmodium falciparum Erythrocyte Membrane Protein (PfEMP1) family at their surface. The polymorphic VAR2CSA protein is able to mediate the interaction of infected erythrocytes with a variety of host cells including placental syncytiotrophoblasts and leukocytes but also with components of the immune system such as non-specific IgM. This review summarizes the described VAR2CSA-mediated host defense evasion mechanisms employed by the parasite during placental malaria to ensure its survival and persistence.

Genesis of placental sequestration in malaria and possible targets for drugs for placental malaria

Malaria during pregnancy results in intrauterine growth restriction, fetal anemia, and infant mortality. Women are more susceptible to malaria during pregnancy due to malaria‐induced inflammation and the sequestration of infected red blood cells in the placenta, which bind to the chondroitin sulfate portion of syndecan‐1 on the syncytiotrophoblast and in the intervillous space. Syndecan‐1 is a dimeric proteoglycan with an extracellular ectodomain that is cleaved from the transmembrane domain (referred to as “shedding”) by matrix metalloproteinases (MMPs), likely the secreted MMP‐9. The ectodomain includes four binding sites for chondroitin sulfate, which are proximal to the transmembrane domain, and six distal binding sites primarily for heparan sulfate. This “shedding” of syndecan‐1 is inhibited by the presence of the heparan sulfate chains, which can be removed by heparanase. The intervillous space contains fibrin strands and syndecan‐1 ectodomains free of heparan sulfate. The following is proposed as the sequence of events that leads to and is primarily responsible for sequestration in the intervillous space of the placenta. Inflammation associated with malaria triggers increased heparanase activity that degrades the heparan sulfate on the membrane‐bound syndecan‐1. Inflammation also upregulates MMP‐9 and the removal of heparan sulfate gives MMP‐9 access to cleave syndecan‐1, thereby releasing dimeric syndecan‐1 ectodomains with at least four chondroitin sulfate chains attached. These multivalent ectodomains bind infected RBCs together leading to their aggregation and entrapment in intervillous fibrin. This mechanism suggests possible new targets for anti‐placental malaria drugs such as the inhibition of MMP‐9. Doxycycline is an antimalarial drug which inhibits MMP‐9.

Computational Fluid Dynamic Simulations of Maternal Circulation: Wall Shear Stress in the Human Placenta and Its Biological Implications

Introduction

In the human placenta the maternal blood circulates in the intervillous space (IVS). The syncytiotrophoblast (STB) is in direct contact with maternal blood. The wall shear stress (WSS) exerted by the maternal blood flow on the STB has not been evaluated. Our objective was to determine the physiological WSS exerted on the surface of the STB during the third trimester of pregnancy.

Material and Methods

To gain insight into the shear stress levels that the STB is expected to experience in vivo, we have formulated three different computational models of varying levels of complexity that reflect different physical representations of the IVS. Computations of the flow fields in all models were performed using the CFD module of the finite element code COMSOL Multiphysics 4.4. The mean velocity of maternal blood in the IVS during the third trimester was measured in vivo with dynamic MRI (0.94±0.14 mm.s^-1). To investigate if the in silico results are consistent with physiological observations, we studied the cytoadhesion of human parasitized (Plasmodium falciparum) erythrocytes to primary human STB cultures, in flow conditions with different WSS values.

Results

The WSS applied to the STB is highly heterogeneous in the IVS. The estimated average values are relatively low (0.5±0.2 to 2.3±1.1 dyn.cm^-2). The increase of WSS from 0.15 to 5 dyn.cm^-2 was associated with a significant decrease of infected erythrocyte cytoadhesion. No cytoadhesion of infected erythrocytes was observed above 5 dyn.cm^-2 applied for one hour.

Conclusion

Our study provides for the first time a WSS estimation in the maternal placental circulation. In spite of high maternal blood flow rates, the average WSS applied at the surface of the chorionic villi is low (<5 dyn.cm^-2). These results provide the basis for future physiologically-relevant in vitro studies of the biological effects of WSS on the STB.

Improved in vitro culture of Plasmodium falciparum permits establishment of clinical isolates with preserved multiplication, invasion and rosetting phenotypes

To be able to robustly propagate P. falciparum at optimal conditions in vitro is of fundamental importance for genotypic and phenotypic studies of both established and fresh clinical isolates. Cryo-preserved P. falciparum isolates from Ugandan children with severe or uncomplicated malaria were investigated for parasite phenotypes under different in vitro growth conditions or studied directly from the peripheral blood. The parasite cultures showed a minimal loss of parasite-mass and preserved percentage of multiple infected pRBCs to that in peripheral blood, maintained adhesive phenotypes and good outgrowth and multiplication rates when grown in suspension and supplemented with gas. In contrast, abnormal and greatly fluctuating levels of multiple infections were observed during static growth conditions and outgrowth and multiplication rates were inferior. Serum, as compared to Albumax, was found necessary for optimal presentation of PfEMP1 at the pRBC surface and/or for binding of serum proteins (immunoglobulins). Optimal in vitro growth conditions of P. falciparum therefore include orbital shaking (50 rev/min), human serum (10%) and a fixed gas composition (5% O2, 5% CO2, 90% N2). We subsequently established 100% of 76 frozen patient isolates and found rosetting with schizont pRBCs in every isolate (>26% schizont rosetting rate). Rosetting during schizogony was often followed by invasion of the bound RBC as seen by regular and time-lapse microscopy as well as transmission electron microscopy. The peripheral parasitemia, the level of rosetting and the rate of multiplication correlated positively to one another for individual isolates. Rosetting was also more frequent with trophozoite and schizont pRBCs of children with severe versus uncomplicated malaria (p<0.002; p<0.004). The associations suggest that rosetting enhances the ability of the parasite to multiply within the human host.

High avidity antibodies to full-length VAR2CSA correlate with absence of placental malaria

VAR2CSA mediates sequestration of Plasmodium falciparum-infected erythrocytes in the placenta, increasing the risk of poor pregnancy outcomes. Naturally acquired antibodies (Ab) to placental parasites at delivery have been associated with improved pregnancy outcomes, but Ab levels and how early in pregnancy Ab must be present in order to eliminate placental parasites before delivery remains unknown. Antibodies to individual Duffy-binding like domains of VAR2CSA have been studied, but the domains lack many of the conformational epitopes present in full-length VAR2CSA (FV2). Thus, the purpose of this study was to describe the acquisition of Ab to FV2 in women residing in high and low transmission areas and determine how Ab levels during pregnancy correlate with clearance of placental parasites. Plasma samples collected monthly throughout pregnancy from pregnant women living in high and low transmission areas in Cameroon were evaluated for Ab to FV2 and the proportion of high avidity Ab (i.e., Ab that remain bound in the presence of 3M NH₄SCN) was assessed. Ab levels and proportion of high avidity Ab were compared between women with placental malaria (PM⁺) and those without (PM⁻) at delivery. Results showed that PM⁻ women had significantly higher Ab levels (p = 0.0047) and proportion of high avidity Ab (p = 0.0009) than PM⁺ women throughout pregnancy. Specifically, women with moderate to high Ab levels (>5,000 MFI) and those with ≥35% high avidity Ab at 5–6 months were found to have 2.3 (95% CI, 1.0–4.9) and 7.6-fold (p = 0.0013, 95% CI: 1.2–50.0) reduced risk of placental malaria, respectively. These data show that high levels of Ab to FV2, particularly those with high avidity for FV2, produced by mid-pregnancy are important in clearing parasites from the placenta. Both high Ab levels and proportion of high avidity Ab to FV2 may serve as correlates of protection for assessing immunity against placental malaria.

High levels of antibodies to multiple domains and strains of VAR2CSA correlate with the absence of placental malaria in Cameroonian women living in an area of high Plasmodium falciparum transmission

Placental malaria, caused by sequestration of Plasmodium falciparum-infected erythrocytes in the placenta, is associated with increased risk of maternal morbidity and poor birth outcomes. The parasite antigen VAR2CSA (variant surface antigen 2-chondroitin sulfate A) is expressed on infected erythrocytes and mediates binding to chondroitin sulfate A, initiating inflammation and disrupting homeostasis at the maternal-fetal interface. Although antibodies can prevent sequestration, it is unclear whether parasite clearance is due to antibodies to a single Duffy binding-like (DBL) domain or to an extensive repertoire of antibodies to multiple DBL domains and allelic variants. Accordingly, plasma samples collected longitudinally from pregnant women were screened for naturally acquired antibodies against an extensive panel of VAR2CSA proteins, including 2 to 3 allelic variants for each of 5 different DBL domains. Analyses were performed on plasma samples collected from 3 to 9 months of pregnancy from women living in areas in Cameroon with high and low malaria transmission. The results demonstrate that high antibody levels to multiple VAR2CSA domains, rather than a single domain, were associated with the absence of placental malaria when antibodies were present from early in the second trimester until term. Absence of placental malaria was associated with increasing antibody breadth to different DBL domains and allelic variants in multigravid women. Furthermore, the antibody responses of women in the lower-transmission site had both lower magnitude and lesser breadth than those in the high-transmission site. These data suggest that immunity to placental malaria results from high antibody levels to multiple VAR2CSA domains and allelic variants and that antibody breadth is influenced by malaria transmission intensity.

Plasmodium falciparum infection-induced changes in erythrocyte membrane proteins

Over the past decade, advances in proteomic and mass spectrometry techniques and the sequencing of the Plasmodium falciparum genome have led to an increasing number of studies regarding the parasite proteome. However, these studies have focused principally on parasite protein expression, neglecting parasite-induced variations in the host proteome. Here, we investigated P. falciparum-induced modifications of the infected red blood cell (iRBC) membrane proteome, taking into account both host and parasite proteome alterations. Furthermore, we also determined if some protein changes were associated with genotypically distinct P. falciparum strains. Comparison of host membrane proteomes between iRBCs and uninfected red blood cells using fluorescence-based proteomic approaches, such as 2D difference gel electrophoresis revealed that more than 100 protein spots were highly up-represented (fold change increase greater than five) following P. falciparum infection for both strains (i.e. RP8 and Institut Pasteur Pregnancy Associated Malaria). The majority of spots identified by mass spectrometry corresponded to Homo sapiens proteins. However, infection-induced changes in host proteins did not appear to affect molecules located at the outer surface of the plasma membrane. The under-representation of parasite proteins could not be attributed to deficient parasite protein expression. Thus, this study describes for the first time that considerable host protein modifications were detected following P. falciparum infection at the erythrocyte membrane level. Further analysis of infection-induced host protein modifications will improve our knowledge of malaria pathogenesis.

Antibodies to the Plasmodium falciparum rhoptry protein RAP-2/RSP-2 in relation to anaemia in Cameroonian children

Previous studies have implicated reactive antibodies to the low molecular weight rhoptry-associated proteins (RAP-1, RAP-2/RSP-2 and RAP-3) in erythroid cell destruction during Plasmodium falciparum infection. In this pilot study, the frequency, specificity and functional capacity of naturally acquired anti-RAP-2/RSP-2 antibodies were investigated in the sera of anaemic and nonanaemic malaria-infected Cameroonian children. All sera recognized RAP-2/RSP-2 by FACS, irrespective of the clinical status of the subjects. However, the anaemic children showed higher levels of IgG antibodies than the nonanaemic group, while both groups showed similar levels of IgM antibodies. Only few individuals had detectable levels of RAP-2/RSP-2-specific IgG1 and IgG3 subclass antibodies, while no IgG2 and IgG4 subclass antibodies were detected in these subjects. By ELISA, the anaemic group tended to show higher levels of antibodies to RAP-2/RSP-2 regarding all antibody classes tested, except for IgG4 and IgE. Unexpectedly, sera from the nonanaemic group activated complement to a greater extent than those from the anaemic group. These results need to be confirmed in extended studies but indicate that the effector functions of the RAP-2/RSP-2-reactive antibodies may be more important than their amounts. Such antibodies could play a role in both immunity and pathogenesis during P. falciparum infection.

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.

Structural interactions in chondroitin 4-sulfate mediated adherence of Plasmodium falciparum infected erythrocytes in human placenta during pregnancy-associated malaria

Infection with Plasmodium falciparum during pregnancy results in the adherence of infected red blood cells (IRBCs) in placenta, causing pregnancy-associated malaria with severe health complications in mothers and fetuses. The chondroitin 4-sulfate (C4S) chains of very low sulfated chondroitin sulfate proteoglycans (CSPGs) in placenta mediate the IRBC adherence. While it is known that partially sulfated but not fully sulfated C4S effectively binds IRBCs, structural interactions involved remain unclear and are incompletely understood. In this study, structurally defined C4S oligosaccharides of varying sulfate contents and sizes were evaluated for their ability to inhibit the binding of IRBCs from different P. falciparum strains to CSPG purified from placenta. The results clearly show that, with all parasite strains studied, dodecasaccharide is the minimal chain length required for the efficient adherence of IRBCs to CSPG and two 4-sulfated disaccharides within this minimal structural motif are sufficient for maximal binding. Together, these data demonstrate for the first time that the C4S structural requirement for IRBC adherence is parasite strain-independent. We also show that the carboxyl group on nonreducing end glucuronic acid in dodecasaccharide motif is important for IRBC binding. Thus, in oligosaccharides containing terminal 4,5-unsaturated glucuronic acid, the nonreducing end disaccharide moiety does not interact with IRBCs due to the altered spatial orientation of carboxyl group. In such C4S oligosaccharides, 14-mer but not 12-mer constitutes the minimal motif for inhibition of IRBC binding to placental CSPG. These data have important implications for the development and evaluation of therapeutics and vaccine for placental malaria.

Polyanionic drugs and viral oncogenesis: a novel approach to control infection, tumor-associated inflammation and angiogenesis

Polyanionic macromolecules are extremely abundant both in the extracellular environment and inside the cell, where they are readily accessible to many proteins for interactions that play a variety of biological roles. Among polyanions, heparin, heparan sulfate proteoglycans (HSPGs) and glycosphingolipids (GSLs) are widely distributed in biological fluids, at the cell membrane and inside the cell, where they are implicated in several physiological and/or pathological processes such as infectious diseases, angiogenesis and tumor growth. At a molecular level, these processes are mainly mediated by microbial proteins, cytokines and receptors that exert their functions by binding to HSPGs and/or GSLs, suggesting the possibility to use polyanionic antagonists as efficient drugs for the treatment of infectious diseases and cancer. Polysulfated (PS) or polysulfonated (PSN) compounds are a heterogeneous group of natural, semi-synthetic or synthetic molecules whose prototypes are heparin and suramin. Different structural features confer to PS/PSN compounds the capacity to bind and inhibit the biological activities of those same heparin-binding proteins implicated in infectious diseases and cancer. In this review we will discuss the state of the art and the possible future development of polyanionic drugs in the treatment of infectious diseases and cancer.

Binding affinity of Plasmodium falciparum-infected erythrocytes from infected placentas and laboratory selected strains to chondroitin 4-sulfate

The adherence of Plasmodium falciparum-infected red blood cells (IRBCs) in human placenta is mediated by chondroitin 4-sulfate (C4S). The C4S-adherent parasites selected from laboratory strains have been widely used for determining the C4S structural elements involved in IRBC binding and for the identification of parasite adhesive protein(s). However, as far as we know, the relative binding strength of the placental versus laboratory-selected parasites has not been reported. In this study, we show that IRBCs from the infected placentas bind to C4S about 3-fold higher than those selected for C4S adherence from laboratory strains. Although adherent parasites selected from several laboratory strains have comparable binding strengths, the one obtained from 3D7 parasites designated as 3D7N61 used for malaria genome sequencing, exhibits markedly lower binding strength. Furthermore, 3D7N61-CSA parasites lose most of the binding capacity by tenth generation in continuous culture.

Cytoadhesion of Plasmodium falciparum-infected erythrocytes and the infected placenta: a two-way pathway

Malaria is undoubtedly the world's most devastating parasitic disease, affecting 300 to 500 million people every year. Some cases of Plasmodium falciparum infection progress to the deadly forms of the disease responsible for 1 to 3 million deaths annually. P. falciparum-infected erythrocytes adhere to host receptors in the deep microvasculature of several organs. The cytoadhesion of infected erythrocytes to placental syncytiotrophoblast receptors leads to pregnancy-associated malaria (PAM). This specific maternal-fetal syndrome causes maternal anemia, low birth weight and the death of 62,000 to 363,000 infants per year in sub-Saharan Africa, and thus has a poor outcome for both mother and fetus. However, PAM and non-PAM parasites have been shown to differ antigenically and genetically. After multiple pregnancies, women from different geographical areas develop adhesion-blocking antibodies that protect against placental parasitemia and clinical symptoms of PAM. The recent description of a new parasite ligand encoded by the var2CSA gene as the only gene up-regulated in PAM parasites renders the development of an anti-PAM vaccine more feasible. The search for a vaccine to prevent P. falciparum sequestration in the placenta by eliciting adhesion-blocking antibodies and a cellular immune response, and the development of new methods for evaluating such antibodies should be key priorities in mother-child health programs in areas of endemic malaria. This review summarizes the main molecular, immunological and physiopathological aspects of PAM, including findings related to new targets in the P. falciparum var gene family. Finally, we focus on a new methodology for mimicking cytoadhesion under blood flow conditions in human placental tissue.

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.

CD44, a signal receptor for the inhibition of the cytoadhesion of CD36-binding Plasmodium falciparum-infected erythrocytes by CSA-binding infected erythrocytes

The cytoadhesion of Plasmodium falciparum-infected erythrocytes (IEs) in organ microvessels is a key event in the pathogenesis of cerebral malaria and pulmonary edema. Identification of the molecules involved in the interaction between IEs and endothelial cells has been a major goal of research into severe forms of malaria. In contrast, the consequences of cytoadhesion for endothelial cells have been largely ignored. By combining phenotypic selection, cytoadhesion assays and flow cytometry, we demonstrated that the cytoadhesion of CSA-binding IEs inhibited the cytoadhesion of CD36-binding IEs. We identified CD44 as a signal receptor for CSA-binding IEs cytoadhesion, and demonstrated that the signal was transduced to CD36 through a pathway involving the Src-kinase family and MEK. CD36-mediated cytoadhesion was modulated independently of changes in CD36 expression. These results provide the first evidence that some IEs can downregulate the cytoadhesion of IEs of another phenotype, by modifying endothelial cells via a signaling pathway relating CD44 to CD36. Mimicking this phenomenon may constitute an interesting therapeutic strategy for inhibiting the adhesion of CD36-binding IEs -- the most abundant phenotype among field isolates -- and promoting their degradation in the spleen.

Structural Basis for Binding of Plasmodium falciparum Erythrocyte Membrane Protein 1 to Chondroitin Sulfate and Placental Tissue and the Influence of Protein Polymorphisms on Binding Specificity

Chondroitin sulfate (CS) A is a key receptor for adhesion of Plasmodium falciparum-infected erythrocytes (IEs) in the placenta and can also mediate adhesion to microvascular endothelial cells. IEs that adhere to CSA express var2csa-type genes, which encode specific variants of the IE surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1). We report direct binding of native PfEMP1, isolated from IEs and encoded by var2csa, to immobilized CSA. Binding of PfEMP1 was dependent on 4-O-sulfated disaccharides and glucuronic acid rather than iduronic acid, consistent with the specificity of intact IEs. Using immobilized CS oligosaccharides as neoglycolipid probes, the minimum chain length for direct binding of PfEMP1 was eight monosaccharide units. Similarly for IE adhesion to placental tissue there was a requirement for 4-O-sulfated GalNAc and glucuronic acid mixed with non-sulfated disaccharides; 6-O-sulfation interfered with the interaction between placental CSA and IEs. The minimum chain length for maximal inhibition of adhesion was 10 monosaccharide residues. Partially 4-O-sulfated CS oligosaccharides (45-55% sulfation) were highly effective inhibitors of placental adhesion (IC(50), 0.15 microg/ml) and may have potential for therapeutic development. We used defined P. falciparum isolates expressing different variants of var2csa in adhesion assays and found that there were isolate-specific differences in the preferred structural motifs for adhesion to CSA that correlated with polymorphisms in PfEMP1 encoded by var2csa-type genes. This may influence sites of IE sequestration or parasite virulence. These findings have significant implications for understanding the pathogenesis and biology of malaria, particularly during pregnancy, and the development of targeted interventions.

Developmental stage- and cell cycle number-dependent changes in characteristics of Plasmodium falciparum-infected erythrocyte adherence to placental chondroitin-4-sulfate proteoglycan

The adherence of Plasmodium falciparum-infected red blood cells (IRBCs) in the human placenta is mediated by chondroitin-4-sulfate (C4S). Although IRBC binding to C4S has been unequivocally established, the adherence characteristics of IRBCs at different stages of parasite development and through successive parasite generations after selection for C4S adherence are not known. Here we show that IRBCs acquire a significant capacity to bind to C4S at as early as 14 h and exhibit maximum binding at 22 to 26 h postinvasion. Surprisingly, the IRBC binding ability decreases by approximately 50% at the late trophozoite and schizont stages. The binding strength of the IRBCs also gradually decreases during successive generations after selection for C4S binding, and at the 32nd generation, the binding capacity was only approximately 31% of that of IRBCs at the 2nd generation, suggesting that IRBCs eventually lose their C4S-adherent capacity. We also tested the susceptibility of the adhesive protein(s) on the IRBC surface to trypsin treatment at different stages of parasite development. The data show that IRBCs with late trophozoites are more resistant to trypsin treatment than those containing early trophozoites, indicating that parasite proteins expressed on the IRBC surface during trophozoite maturation partially mask accessibility of adhesive protein for binding to C4S. These data provide important insights into the expression pattern of the C4S-adhesive protein(s) on the IRBC surface, emphasizing the need for understanding the regulation of genes involved in IRBC binding to C4S. Our data also define the parasite stage at which IRBCs are suitable for studying structural interactions with C4S.

IL-12 producing monocytes and IFN-γ and TNF-α producing T-lymphocytes are increased in placentas infected by Plasmodium falciparum

Placental Plasmodium falciparum sequestration is associated with dysregulated immune function. Placental inflammatory responses via IFN-gamma and TNF-alpha are implicated in functional damage. However, they are needed during placental infection to control asexual stage parasites. To test the hypothesis that placental immunomodulation associated with malaria disturbs cytokine secretion differently in monocytes and lymphocytes, we have determined the proportion of monocytes and/or lymphocytes secreting IFN-gamma, TNF-alpha, IL-10 and IL-12. Intervillous and peripheral blood monocyte (CD14+) and lymphocyte (CD3/CD4+; CD3/CD8+) cytokine production was compared between 17 P. falciparum-infected and 12 non-infected Senegalese women. After culture with phorbolmyristate acetate/ionomycin (PMA/iono), lipopolysaccharide (LPS) or P. falciparum-infected erythrocytes (IE), the intracellular expression of cytokines in lymphocytes (IFN-gamma, TNF-alpha) and monocytes (IL-10, IL-12, TNF-alpha), was detected. In response to IE, CD4+ and CD8+ T-cells produced IFN-gamma and TNF-alpha at similar rates in both compartments. In response to PMA/iono, the frequencies of CD4+ and CD8+ T-cells producing IFN-gamma and TNF-alpha were similar in both compartments, but increased in P. falciparum-infected placentas. In response to LPS or IE, IL-12 secreting monocytes were increased in infected women, while the frequency of TNF-alpha secreting monocytes was decreased compared to that in non-infected placenta. The monocyte IL-12 response is not impaired in infected women. IL-12 is an important factor for inducing IFN-gamma in T-cells. Thus, IL-12 and IFN-alpha responses may synergistically allow a protective immune response in placental malaria. TNF-alpha production by CD4+ and CD8+ T-cells is up-regulated in P. falciparum-infected placentas, suggesting that T-cells actively participate to inflammatory responses.

HIV impairs opsonic phagocytic clearance of pregnancy-associated malaria parasites

BACKGROUND

Primigravid (PG) women are at risk for pregnancy-associated malaria (PAM). Multigravid (MG) women acquire protection against PAM; however, HIV infection impairs this protective response. Protection against PAM is associated with the production of IgG specific for variant surface antigens (VSA-PAM) expressed by chondroitin sulfate A (CSA)-adhering parasitized erythrocytes (PEs). We hypothesized that VSA-PAM-specific IgG confers protection by promoting opsonic phagocytosis of PAM isolates and that HIV infection impairs this response.

METHODS AND FINDINGS

We assessed the ability of VSA-PAM-specific IgG to promote opsonic phagocytosis of CSA-adhering PEs and the impact of HIV infection on this process. Opsonic phagocytosis assays were performed using the CSA-adherent parasite line CS2 and human and murine macrophages. CS2 PEs were opsonized with plasma or purified IgG subclasses from HIV-negative or HIV-infected PG and MG Kenyan women or sympatric men. Levels of IgG subclasses specific for VSA-PAM were compared in HIV-negative and HIV-infected women by flow cytometry. Plasma from HIV-negative MG women, but not PG women or men, promoted the opsonic phagocytosis of CSA-binding PEs (p < 0.001). This function depended on VSA-PAM-specific plasma IgG1 and IgG3. HIV-infected MG women had significantly lower plasma opsonizing activity (median phagocytic index 46 [interquartile range (IQR) 18-195] versus 251 [IQR 93-397], p = 0.006) and levels of VSA-PAM-specific IgG1 (mean fluorescence intensity [MFI] 13 [IQR 11-20] versus 30 [IQR 23-41], p < 0.001) and IgG3 (MFI 17 [IQR 14-23] versus 28 [IQR 23-37], p < 0.001) than their HIV-negative MG counterparts.

CONCLUSIONS

Opsonic phagocytosis may represent a novel correlate of protection against PAM. HIV infection may increase the susceptibility of multigravid women to PAM by impairing this clearance mechanism.

Differential changes in Plasmodium falciparum var transcription during adaptation to culture

Plasmodium falciparum erythrocyte membrane protein 1, which is encoded by the var multigene family, is expressed on the surface of P. falciparum-infected erythrocytes and has been implicated in many of the complications associated with falciparum malaria. Transcriptional switching of var is commonly investigated using in vitro cultured parasites, because parasite material from patients is limited. We investigated the affect of short-term in vitro cultivation on var gene transcription in patient samples. A significant reduction in the overall abundance of var transcripts was observed during the first approximately 10 days of culture. The rate of down-regulation was not constant among all var genes; genes with an upsA, -D, and -E 5' flanking region had a significantly faster rate than genes with an upsB or -C flanking region. These results have significant implications for the investigation of associations between var transcription and clinical manifestations using parasites that have been enriched by in vitro culture.

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.

Immunogenicity of Duffy binding-like domains that bind chondroitin sulfate A and protection against pregnancy-associated malaria

Sequestration of Plasmodium falciparum-infected erythrocytes in the placenta is implicated in pathological outcomes of pregnancy-associated malaria (PAM). P. falciparum isolates that sequester in the placenta primarily bind chondroitin sulfate A (CSA). Following exposure to malaria during pregnancy, women in areas of endemicity develop immunity, and so multigravid women are less susceptible to PAM than primigravidae. Protective immunity to PAM is associated with the development of antibodies that recognize diverse CSA-binding, placental P. falciparum isolates. The epitopes recognized by such protective antibodies have not been identified but are likely to lie in conserved Duffy binding-like (DBL) domains, encoded by var genes, that bind CSA. Immunization of mice with the CSA-binding DBL3gamma domain encoded by var1CSA elicits cross-reactive antibodies that recognize diverse CSA-binding P. falciparum isolates and block their binding to placental cryosections under flow. However, CSA-binding isolates primarily express var2CSA, which does not encode any DBLgamma domains. Here, we demonstrate that antibodies raised against DBL3gamma encoded by var1CSA cross-react with one of the CSA-binding domains, DBL3X, encoded by var2CSA. This explains the paradoxical observation made here and earlier that anti-rDBL3gamma sera recognize CSA-binding isolates and provides evidence for the presence of conserved, cross-reactive epitopes in diverse CSA-binding DBL domains. Such cross-reactive epitopes within CSA-binding DBL domains can form the basis for a vaccine that provides protection against PAM.

Characterization of anti-var2CSA-PfEMP1 cytoadhesion inhibitory mouse monoclonal antibodies

Pregnancy-associated malaria (PAM) is associated with the massive sequestration of erythrocytes infected with CSA-binding parasites in the placenta. Natural protective immunity against PAM is acquired during the course of pregnancies, with the development of anti-PfEMP1 antibodies recognizing placental infected erythrocytes (IEs) from different geographical regions. Mouse monoclonal antibodies (mabs) were raised against Plasmodium falciparum variant surface proteins expressed by CSA-binding parasites. These mabs blocked 0-60% of CSA-binding parasite adhesion and immunoprecipitated a 350 kDa 125I-labeled PfEMP1(CSA). Two var2CSA domains expressed on the surface of CHO cells (DBL5epsilon and DBL6epsilon) were identified as the targets of three of four antibodies inhibiting CSA binding. Two of these antibodies also recognized either DBL2x or DBL3x, suggesting that some epitopes may be common to several var2CSA domains. These mabs also specifically selected CSA-binding IEs and facilitated the purification from IE extracts of the native var2CSA ligand. This purified ligand elicited antibodies in immunized mice inhibiting efficiently IE(CSA) cytoadhesion. Based on our findings, we provide the first demonstration that the parasite var2CSA surface protein can elicit inhibitory antibodies and define here the subunits of the var2CSA ligand suitable for use in vaccine development.

Nonimmune immunoglobulin binding and multiple adhesion characterize Plasmodium falciparum-infected erythrocytes of placental origin

The harmful effects of pregnancy-associated malaria (PAM) are engendered by the heavy sequestration of Plasmodium falciparum-parasitized RBCs in the placenta. It is well documented that this process is mediated by interactions of parasite-encoded variant surface antigens and placental receptors. A P. falciparum erythrocyte membrane protein 1 variant, VAR2CSA, and the placental receptor chondroitin sulfate A (CSA) are currently the focus of PAM research. A role for immunoglobulins (IgG and IgM) from normal human serum and hyaluronic acid as additional receptors in placental sequestration have also been suggested. We show here (i) that CSA and nonimmune IgG/IgM binding are linked phenotypes of in vitro-adapted parasites, (ii) that a VAR2CSA variant shown to bind CSA also harbors IgG- and IgM-binding domains (DBL2-X, DBL5-epsilon, and DBL6-epsilon), and (iii) that IgG and IgM binding and adhesion to multiple receptors (IgG/IgM/HA/CSA) rather than the exclusive binding to CSA is a characteristic of fresh Ugandan placental isolates. These findings are of importance for the understanding of the pathogenesis of placental malaria and have implications for the ongoing efforts to develop a global PAM vaccine.

Acquisition of antibodies to variant antigens on the surface of Plasmodium falciparum-infected erythrocytes during pregnancy

Pregnancy-associated malaria is characterized by Plasmodium falciparum adherence to chondroitin sulfate A (CSA) in placenta, through a particular variant surface antigen (VSA). VSA(CSA)-specific IgG are involved in protection against placental malaria. In order to assess the relationship between VSA(CSA)-specific antibody responses and parity as well as protection against placental malaria, the occurrence of P. falciparum infection was assessed in 306 pregnant women from a low malaria transmission area of Senegal. Anti-VSA(CSA) antibodies against three placental parasite isolates were measured by flow cytometry, at enrollment and delivery. Placental infection prevalence rates were highest in primigravidae, but no clear decreasing trend was observed from the second pregnancy onwards. Anti-VSA(CSA) antibody prevalence rates increased with parity. Both anti-VSA(CSA) antibody prevalence rates and levels increased during pregnancy only in women infected with P. falciparum. Although a single or a very limited number of P. falciparum infections were able to induce an anti-VSA(CSA) antibody response, the level or the quality of this response did not appear to confer protection against placental malaria infection.

Adherence of Plasmodium falciparum infected erythrocytes to CHO-745 cells and inhibition of binding by protein A in the presence of human serum

Adhesion of erythrocytes infected with the malaria parasite Plasmodium falciparum to human host receptors is a process associated with severe malarial pathology. A number of in vitro cell lines are available as models for these adhesive processes, including Chinese hamster ovary (CHO) cells which express the placental adhesion receptor chondroitin-4-sulphate (CSA) on their surface. CHO-745 cells, a glycosaminoglycan-negative mutant CHO cell line lacking CSA and other reported P. falciparum adhesion receptors, are often used for recombinant expression of host receptors and for receptor binding studies. In this study we show that P. falciparum-infected erythrocytes can be easily selected for adhesion to an endogenous receptor on the surface of CHO-745 cells, bringing into question the validity of using these cells as a tool for P. falciparum adhesin expression studies. The adhesive interaction between CHO-745 cells and parasitized erythrocytes described here is not mediated by the known P. falciparum adhesion receptors CSA, CD36, or ICAM-1. However, we found that CHO-745-selected parasitized erythrocytes bind normal human IgM and that adhesion to CHO-745 cells is inhibited by protein A in the presence of serum, but not in its absence, indicating a non-specific inhibitory effect. Thus, protein A, which has been used as an inhibitor for a recently described interaction between infected erythrocytes and the placenta, may not be an appropriate in vitro inhibitor for understanding in vivo adhesive interactions.

Plasmodium falciparum-infected Red Blood Cells Selected for Binding to Cultured Syncytiotrophoblast Bind to Chondroitin Sulfate A and Induce Tyrosine Phosphorylation in the Syncytiotrophoblast

An important pathogenic complication of malaria during human pregnancy is sequestration of Plasmodium-infected red blood cells (iRBCs) in the placental intervillous spaces. This sequestration is thought to be mediated in part by binding of the iRBCs to receptors expressed on the syncytiotrophoblast (ST) membrane. We report here the use of a dynamic system to study the consequences of this cytoadherence on ST function using human syncytiotrophoblast and the choriocarcinoma cell line, BeWo. Laboratory isolates of Plasmodium falciparum were selected for their ability to bind to ST and used to investigate binding-induced cellular changes in the ST. Treatment of the ST cells with chondroitinase ABC suggested that the selected parasites bind predominantly to chondroitin sulfate A, but other receptors for parasite binding may be involved. Intracellular signaling in the ST induced by iRBCs binding was investigated by assessing tyrosine phosphorylation of ST proteins following iRBC binding. We demonstrate for the first time that iRBC cytoadherence to syncytiotrophoblast enhances tyrosine phosphorylation of a series of proteins in these cells. This approach will be useful in further studies of ST function in the malaria-infected placenta, the dynamics of selection of syncytiotrophoblast-binding parasites, and the identification of new receptors for parasite cytoadherence in the placenta.

The immunology and pathogenesis of malaria during pregnancy

Women in endemic areas become highly susceptible to malaria during first and second pregnancies, despite immunity acquired after years of exposure. Recent insights have advanced our understanding of pregnancy malaria caused by Plasmodium falciparum, which is responsible for the bulk of severe disease and death. Accumulation of parasitized erythrocytes in the blood spaces of the placenta is a key feature of maternal infection with P. falciparum. Placental parasites express surface ligands and antigens that differ from those of other P. falciparum variants, facilitating evasion of existing immunity, and mediate adhesion to specific molecules, such as chondroitin sulfate A, in the placenta. The polymorphic and clonally variant P. falciparum erythrocyte membrane protein 1, encoded by var genes, binds to placental receptors in vitro and may be the target of protective antibodies. An intense infiltration of immune cells, including macrophages, into the placental intervillous spaces, and the production of pro-inflammatory cytokines often occur in response to infection, and are associated with low birth weight and maternal anemia. Expression of alpha and beta chemokines may initiate or facilitate this cellular infiltration during placental malaria. Specific immunity against placental-binding parasites may prevent infection or facilitate clearance of parasites prior to the influx of inflammatory cells, thereby avoiding a cascade of events leading to disease and death. Much less is known about pathogenic processes in P. vivax infections, and corresponding immune responses. Emerging knowledge of the pathogenesis and immunology of malaria in pregnancy will increasingly lead to new opportunities for the development of therapeutic and preventive interventions and new tools for diagnosis and monitoring.

Natural killer cell and macrophage cooperation in MyD88-dependent innate responses to Plasmodium falciparum

IFN-gamma secretion by natural killer (NK) cells is pivotal to several tumor and viral immune responses, during which NK and dendritic cells cooperation is required. We show here that macrophages are mandatory for NK cell IFN-gamma secretion in response to erythrocytes infected with Plasmodium falciparum (Pf), a causative agent of human malaria. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine CXCL8, without triggering their granule-mediated cytolytic programs. Despite their reported role in Pf recognition, Toll-like receptor (TLR) 2, TLR9, and TLR11 are individually dispensable for NK cell activation induced by Pf-infected erythrocytes. However, IL-18R expression on NK cells, IL-18 production by macrophages, and MyD88 on both cell types are essential components of this previously undescribed pathway of NK cell activation in response to a parasite infection.

Carrageenans inhibit the in vitro growth of Plasmodium falciparum and cytoadhesion to CD36

Carbohydrates are implicated in many of the invasive and adhesive interactions that occur between Plasmodium falciparum malaria parasites and human host cells, including invasion of sporozoites into hepatocytes, entry of merozoites into new host erythrocytes during asexual blood-stage replication, adhesion of infected erythrocytes to uninfected erythrocytes (rosetting) and to a number of host endothelial receptors including ICAM-1, CD36 and chondroitin-4-sulphate. In addition to increasing our understanding of host-parasite interactions, the investigation of carbohydrates with differing levels and patterns of sulphation as inhibitors may contribute to the development of novel therapeutics targeting malaria. Here we show that three polysaccharides derived from seaweed (carrageenans) with differing sulphation levels and patterns can inhibit the in vitro erythrocytic invasion and growth of both drug sensitive and drug resistant P. falciparum lines and the adhesion of parasitized erythrocytes to the human glycoprotein CD36.

Analysis of IgG with specificity for variant surface antigens expressed by placental Plasmodium falciparum isolates

Background

Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes that can sequester in placental intervillous space by expressing particular variant surface antigens (VSA) that can mediate adhesion to chondroitin sulfate A (CSA) in vitro. IgG antibodies with specificity for the VSA expressed by these parasites (VSA_PAM) are associated with protection from maternal anaemia, prematurity and low birth weight, which is the greatest risk factor for death in the first month of life.

Methods

In this study, the development of anti-VSA_PAM antibodies in a group of 151 women who presented to the maternity ward of Albert Schweitzer Hospital in Lambaréné, Gabon for delivery was analysed using flow cytometry assays. Plasma samples from placenta infected primiparous women were also investigated for their capacity to inhibit parasite binding to CSA in vitro.

Results

In the study cohort, primiparous as well as secundiparous women had the greatest risk of infection at delivery as well as during pregnancy. Primiparous women with infected placentas at delivery showed higher levels of VSA_PAM-specific IgG compared to women who had no malaria infections at delivery. Placental isolates of Gabonese and Senegalese origin tested on plasma samples from Gabon showed parity dependency and gender specificity patterns. There was a significant correlation of plasma reactivity as measured by flow cytometry between different placental isolates. In the plasma of infected primiparous women, VSA_PAM-specific IgG measured by flow cytometry could be correlated with anti-adhesion antibodies measured by the inhibition of CSA binding.

Conclusion

Recognition of placental parasites shows a parity- and sex- dependent pattern, like that previously observed in laboratory strains selected to bind to CSA. Placental infections at delivery in primiparous women appear to be sufficient to induce functional antibodies which can both recognize the surface of the infected erythrocytes as well as block their binding to CSA. The correlation between serum reactivities of placental field isolates from different geographic locations and collected at different times is indicative of the conserved nature of the antigen(s) mediating PAM.

Plasmodium falciparum-infected erythrocytes adhere both in the intervillous space and on the villous surface of human placenta by binding to the low-sulfated chondroitin sulfate proteoglycan receptor

In pregnant women infected with Plasmodium falciparum, the parasite-infected red blood cells (IRBCs) sequester in the placenta through chondroitin 4-sulfate (C4S)-mediated adherence. The pattern of IRBC adherence in P. falciparum-infected placenta has been controversial. Moreover, the identity of the chondroitin sulfate proteoglycan (CSPG) receptor, that mediates IRBC adherence, and its location in the placenta have not been established. This study, using immunohistochemical techniques, clearly shows, for the first time, that the low-sulfated CSPGs of the placenta are localized predominantly in the intervillous space. Ex vivo IRBC adherence analyses demonstrate that the IRBCs are adhered to the CSPG receptors in the placenta in a C4S-dependent manner. This IRBC binding pattern was similar to that observed in P. falciparum-infected placentas. These data and the results of dual-fluorescence staining of the endogenous RBCs and syncytiotrophoblasts, and co-localization of CSPG and IRBC adherence unequivocally establish that the low-sulfated CSPGs are the major natural receptors for IRBC adherence in the placenta. Further, it was found that IRBCs adhere mainly in the intervillous space and also at significant levels to the syncytiotrophoblasts. Finally, the ex vivo IRBC adherence method described herein provides a reliable procedure for future studies for the assessment of the efficacy of C4S inhibitors and adhesion inhibitory antibodies.

Divergent binding sites on intercellular adhesion molecule-1 (ICAM-1) for variant Plasmodium falciparum isolates

Adhesion of human erythrocytes infected with the malaria parasite Plasmodium falciparum to host endothelium has been associated with severe forms of this disease. A number of endothelial receptors have been identified, and there is evidence that one of these, intercellular adhesion molecule-1 (ICAM-1), may play an important role in the pathology of cerebral malaria. Mutagenesis of domain 1 of ICAM-1, which is involved in parasite adhesion, shows that the binding sites for different parasite variants overlap to a large extent, but that there are subtle differences between them that correlate with their adhesive phenotypes. This suggests that the ability to bind to ICAM-1 has arisen from a common variant, but that subsequent changes have led to differences in binding avidity, which may affect pathogenesis. The definition of common binding determinants and the elucidation of links between ICAM-1 binding phenotype and disease will provide new leads in the design of therapeutic interventions.

Placenta cryosections for study of the adhesion of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate A in flow conditions

The adhesion of Plasmodium falciparum-infected erythrocytes (IEs) to chondroitin-4-sulfate (CSA) via the PfEMP1-CSA parasite ligand domain is correlated with placental malaria in primigravidae. The recent identification of parasite genes encoding CSA adhesion molecules and the development of pan-reactive monoclonal antibodies against the Pf(CSA) ligand have opened up new avenues for the development of anti-IE sequestration therapies for the prevention of placental malaria. A model closely mimicking placental sequestration of IEs during pregnancy is needed for the preclinical and clinical evaluation of candidate molecules for the induction of antibodies that could protect pregnant women from placental malaria. We found that normal placenta cryosections were a specific and highly consistent support for the binding of IEs to CSA in flow conditions under physiological conditions. This model makes possible the quantitative and qualitative analysis of IE adhesion. We identified distinct CSA-binding phenotypes within the FCR3(CSA)-selected parasites in flow analyses, but not in static analyses. We also analyzed inhibitors of placental parasite binding such as soluble CSA and antibodies directed against the Pf(CSA) ligand. Our data demonstrate that placenta cryosections could be used to standardize assays between laboratories, potentially advancing the development of therapies against placental malaria.

Antibodies to variant surface antigens of Plasmodium falciparum-infected erythrocytes and adhesion inhibitory antibodies are associated with placental malaria and have overlapping and distinct targets

We measured antibodies to chondroitin sulfate A (CSA)-binding and placental Plasmodium falciparum-infected red blood cells (PRBCs) among pregnant women with or without placental malaria. Immunoglobulin G to PRBC surface antigens was rare in uninfected primigravidae (3.7%), more prevalent in infected primigravidae (70%; P<.001), and common in infected (77%) and uninfected (83%) multigravidae. Similar patterns were seen for agglutinating antibodies, and antibodies were similar among women with past or active placental infection. PRBC adhesion to CSA was inhibited 60% by serum from infected primigravidae but 24% by serum from uninfected primigravidae (P=.025), whereas infection did not alter adhesion inhibition by multigravidae (77% inhibition)[corrected]. There was substantial heterogeneity in antibody type and levels. Antibodies did not correlate with parasite density or pregnancy outcome. Comparisons between antibodies suggest that adhesion-inhibitory antibodies and those to PRBC variant antigens have distinct and overlapping epitopes, may be acquired independently, and have different roles in immunity.

Recovery of adhesion to chondroitin-4-sulphate in Plasmodium falciparum varCSA disruption mutants by antigenically similar PfEMP1 variants

Protection against maternal malaria has been associated with the acquisition of a specific antibody response that prevents adhesion of Plasmodium falciparum-infected erythrocytes to the glycosaminoglycan chondroitin-4-sulphate (CSA), which is present in the placental intervillous space. These antibodies are directed against variant forms of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) that mediate binding to CSA. We have generated insertional disruption mutants of the gene encoding the CSA-binding phenotype in the P. falciparum clone FCR3 (varCSA) to test the hypothesis that strategies targeting the parasite's determinant for this adhesive phenotype may prevent sequestration of infected erythrocytes in the placenta and hence the development of maternal malaria. The varCSA-disruption mutants were initially unable to adhere to CSA; however, they could recover the phenotype after repeated selection over CSA. We show that recovery of CSA binding is varCSA independent and mediated by the activation of a novel var variant. Importantly, the corresponding PfEMP1 protein reacts with a monoclonal antibody recognizing the DBL3 gamma domain of the varCSA gene product, indicating that the DBL3 gamma CSA-binding domains are conserved between these PfEMP1-binding variants. Our data support strategies exploring these conserved epitopes as vaccine candidates against maternal malaria.

Adhesion of normal and Plasmodium falciparum ring-infected erythrocytes to endothelial cells and the placenta involves the rhoptry-derived ring surface protein-2

Recent findings have challenged the current view of Plasmodium falciparum (P falciparum) blood-stage biology by demonstrating the cytoadhesion of early ring-stage-infected erythrocytes (rIEs) to host endothelial cells and placental syncytiotrophoblasts. The adhesion of rIEs was observed only in parasites that bind to the placenta via chondroitin sulfate A (CSA). In this work, a panel of mouse monoclonal antibodies (mAbs) that specifically inhibit cytoadhesion of rIEs but not of mature IEs was generated The previously described ring surface protein 2 (RSP-2), a 42-kDa protein, was identified as the target of the ring-stage-specific mAbs. Time course surface fluorescence experiments revealed a short overlap (approximately 4 hours) of expression between RSP-2 and P falciparum erythrocyte membrane protein 1 (PfEMP1). Their consecutive expression enables IEs to adhere to endothelial cells during the entire blood-stage cycle. During this study, a new phenotype was detected in parasite cultures, the adhesion of normal erythrocytes (nEs) to endothelial cells. All adherent nEs were coated with RSP-2. Immunolocalization studies show that RSP-2 is a rhoptry-derived protein that is discharged onto the erythrocyte membrane during contact with merozoites. Our results identify RSP-2 as a key molecule in sequestration of young blood-stage forms and nEs to endothelial cells.

Chondroitin sulfate proteoglycan expression and binding of Plasmodium falciparum-infected erythrocytes in the human placenta during pregnancy

A characteristic feature of malaria during pregnancy is the sequestration of Plasmodium falciparum-infected red blood cells (IRBCs) in the intervillous spaces of the placenta. We have recently shown that unusually low-sulfated chondroitin sulfate proteoglycans (CSPGs) present in the intervillous spaces mediate the adherence of IRBCs in the placenta. In areas of endemicity, the prevalence of P. falciparum infection in pregnant women peaks during weeks 13 to 20 and then gradually declines, implying that the placental CSPGs are available for IRBC adhesion early during the pregnancy. However, there is no information on the expression and composition of CSPGs during pregnancy. In this study, the expression pattern of CSPGs during the course of pregnancy was investigated. The CSPGs were purified from placentas of various gestational ages, characterized, and tested for the ability to bind IRBCs. The data demonstrate that the CSPGs are present in the intervillous spaces throughout the second and third trimesters. The levels of CSPGs expressed per unit tissue weight were similar in placentas of various gestational ages. However, the structures of the intervillous-space CSPGs changed considerably during the course of pregnancy. In particular, the molecular weight was decreased, with an accompanying gradual increase in the CSPG size polydispersity, from 16 weeks until 38 weeks. The sulfate content was increased considerably after 24 weeks. Despite these structural changes, the CSPGs of placentas of various gestational ages efficiently supported the binding of IRBCs. These results demonstrate that CSPGs can mediate the sequestration of IRBCs in the intervillous spaces of the placenta during the entire second and third trimesters and possibly during the later part of the first trimester as well.

The low sulfated chondroitin sulfate proteoglycans of human placenta have sulfate group-clustered domains that can efficiently bind Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum infection in pregnant women results in the chondroitin 4-sulfate-mediated adherence of the parasite-infected red blood cells (IRBCs) in the placenta, adversely affecting the health of the fetus and mother. We have previously shown that unusually low sulfated chondroitin sulfate proteoglycans (CSPGs) in the intervillous spaces of the placenta are the receptors for IRBC adhesion, which involves a chondroitin 4-sulfate motif consisting of six disaccharide moieties with approximately 30% 4-sulfated residues. However, it was puzzling how the placental CSPGs, which have only approximately 8% of the disaccharide 4-sulfated, could efficiently bind IRBCs. Thus, we undertook to determine the precise structural features of the CS chains of placental CSPGs that interact with IRBCs. We show that the placental CSPGs are a mixture of two major populations, which are similar by all criteria except differing in their sulfate contents; 2-3% and 9-14% of the disaccharide units of the CS chains are 4-sulfated, and the remainder are nonsulfated. The majority of the sulfate groups in the CSPGs are clustered in CS chain domains consisting of 6-14 repeating disaccharide units. While the sulfate-rich regions of the CS chains contain 20-28% 4-sulfated disaccharides, the other regions have little or no sulfate. Further, we find that the placental CSPGs are able to efficiently bind IRBCs due to the presence of 4-sulfated disaccharide clusters. The oligosaccharides corresponding to the sulfate-rich domains of the CS chains efficiently inhibited IRBC adhesion. Thus, our data demonstrate, for the first time, the unique distribution of sulfate groups in the CS chains of placental CSPGs and that these sulfate-clustered domains have the necessary structural elements for the efficient adhesion of IRBCs, although the CS chains have an overall low degree of sulfation.

Malaria pathogenesis: a jigsaw with an increasing number of pieces

Plasmodium falciparum malaria remains as one of the most devastating global health problems of today. It is estimated that around 150 million individuals get the disease every year and of these 2-3 million die from it. Our knowledge of the mechanisms underlying the pathology has expanded greatly over the last decades, but many aspects of the molecular biology, immunology and epidemiology that govern the pathogenesis and spread of this parasite are still unclear. As new insights are gained we are also revealing a challenging biological complexity. Piecing this information together is the key to vaccine development and production of new antimalarial drugs.

Sequestration of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate A, a receptor for maternal malaria: monoclonal antibodies against the native parasite ligand reveal pan-reactive epitopes in placental isolates

Plasmodium falciparum parasites express variant adhesion molecules on the surface of infected erythrocytes (IEs), which act as targets for natural protection. Recently it was shown that IE sequestration in the placenta is mediated by binding to chondroitin sulfate A via the duffy binding-like (DBL)-gamma 3 domain of P falciparum erythrocyte membrane protein 1 (PfEMP1(CSA)). Conventional immunization procedures rarely result in the successful production of monoclonal antibodies (mAbs) against such conformational vaccine candidates. Here, we show that this difficulty can be overcome by rendering Balb/c mice B cells tolerant to the surface of human erythrocytes or Chinese hamster ovary (CHO) cells before injecting P falciparum IEs or transfected CHO cells expressing the chondroitin sulfate A (CSA)-binding domain (DBL-gamma 3) of the FCR3 var(CSA) gene. We fused spleen cells with P3U1 cells and obtained between 20% and 60% mAbs that specifically label the surface of mature infected erythrocytes of the CSA phenotype (mIE(CSA)) but not of other adhesive phenotypes. Surprisingly, 70.8% of the 43 mAbs analyzed in this work were IgM. All mAbs immunoprecipitated PfEMP1(CSA) from extracts of (125)I surface-labeled IE(CSA). Several mAbs bound efficiently to the surface of CSA-binding parasites from different geographic areas and to placental isolates from West Africa. The cross-reactive mAbs are directed against the DBL-gamma 3(CSA), demonstrating that this domain, which mediates CSA binding, is able to induce a pan-reactive immune response. This work is an important step toward the development of a DBL-gamma 3-based vaccine that could protect pregnant women from pathogenesis. )

Short-chain aliphatic polysulfonates inhibit the entry of Plasmodium into red blood cells

Several steps in the pathogenesis of a Plasmodium falciparum infection depend on interactions of parasite surface proteins with negatively charged sugars on the surface of host cells such as sialate residues or glycosaminoglycans. For these reasons, our previous studies examining agents that interfere with heparan sulfate-protein binding during amyloidogenesis suggested that short-chain aliphatic polysulfonates may prove useful as antimalarial agents. A series of related polysulfonates were synthesized and assessed both in tissue culture with the asexual stages of P. falciparum in human red blood cells and in vivo by use of Plasmodium berghei infections in mice. Poly(vinylsulfonate sodium salt) (molecular weight range, 1,500 to 3,000) proved effective in interfering with P. falciparum merozoite entry into human red blood cells and significantly delaying the increase in the level of P. berghei parasitemia in mice. The concept that anionic molecules that mimic large polysaccharide structures may have antimalarial properties has been suggested and examined previously. Our results suggest that related anionic agents [poly(vinylsulfonate sodium salt)-like molecules] orders of magnitude smaller than those previously considered may prove useful in abrogating merozoite entry into erythrocytes and may potentially block sporozoite entry into liver cells. Structure-activity studies conducted to enhance these properties may provide compounds with scope for significant further analysis and development.

A distinct 5' flanking var gene region regulates Plasmodium falciparum variant erythrocyte surface antigen expression in placental malaria

The Plasmodium falciparum multigene var family codes for approximately 50 variant adhesive proteins expressed in a mutually exclusive manner at the surface of infected red blood cells (iRBCs). Switching expression of var genes can lead to fundamental changes in the adhesive and antigenic properties of iRBCs. For example, a specific phenotypic switch in adhesion from CD36 to chondroitin sulphate A (CSA) is associated with malaria pathogenesis in pregnant women. The factors and DNA elements that control the expression of a particular member of the var gene family during gestational malaria remains enigmatic. Here, we report that the subtelomeric FCR3 varCSA is expressed under the control of a unique DNA element of 1.8 kb, whereas the other members of the var multigene family are flanked by common regulatory elements. The 5' varCSA-type element is conserved as a single copy in laboratory strains and clinical isolates from Brazil and West Africa and contains two distinct repetitive elements of 150 bp and 60 bp respectively. The 5' varCSA-type sequence tags a var gene in the 3D7 genome that is homologous to the FCR3 varCSA gene. A recombinant DBL gamma domain of this var gene showed specific binding to CSA. This subtelomeric varCSA gene is transcribed in the opposite sense when compared with the usual orientation of telomere-adjacent var genes. This unique arrangement might explain why the varCSA gene is relatively conserved in genetically distinct parasites despite being located in a highly recombinogenic chromosome compartment. The 5' untranslated region (UTR) of the varCSA-type sequence is also transcribed in placental isolates that bind to CSA, illustrating an important role for the unique 5' varCSA-type sequence in the regulation of var genes involved in malaria pathogenesis in pregnant women. However, this promoter is not always found to be transcribing var genes selected for expression of products that bind to CSA in vitro. Our work identifies a sequence tag for the identification of varCSA genes in placental isolates for the first time.

Gravidity-dependent production of antibodies that inhibit binding of Plasmodium falciparum-infected erythrocytes to placental chondroitin sulfate proteoglycan during pregnancy

During pregnancy, Plasmodium falciparum-infected erythrocytes sequester in the placenta by adhering to chondroitin 4-sulfate, creating a risk factor for both the mother and the fetus. The primigravidae are at higher risk for placental malaria than the multigravidae. This difference in susceptibility has been attributed to the lack of antibodies that block the adhesion of infected erythrocytes to placental chondroitin 4-sulfate in primigravid women. However, recent results show that many primigravidae at term have antibody levels similar to those of multigravidae, and thus the significance of antiadhesion antibodies in providing protection against malaria during pregnancy remains unclear. In this study, we analyzed plasma samples from women of various gravidities at different gestational stages for antiadhesion antibodies. The majority of women, regardless of gravidity, had similar levels of antibodies at term. Most primigravidae had low levels of or no antiadhesion antibodies prior to ~20 weeks of pregnancy and then produced antibodies. Multigravidae also lacked antibodies until ~12 weeks of pregnancy, but thereafter they efficiently produced antibodies. In pregnant women who had placental infection at term, higher levels of antiadhesion antibodies correlated with lower levels of placental parasitemia. The difference in kinetics of antibody production between primigravidae and multigravidae correlated with the prevalence of malaria in these groups, suggesting that antibodies are produced during pregnancy in response to placental infection. The early onset of efficient antibody response in multigravidae and the delayed production to antibodies in primigravidae appear to account for the gravidity-dependent differential susceptibilities of pregnant women to placental malaria.

Plasmodium falciparum cytoadherence to human placenta: evaluation of hyaluronic acid and chondroitin 4-sulfate for binding of infected erythrocytes

Chondroitin 4-sulfate (C4S) is known to mediate the adherence of Plasmodium falciparum infected red blood cells (IRBCs) to human placenta. Recently, hyaluronic acid (HA) has also been reported to bind IRBCs, and HA has been suggested as an additional receptor for the sequestration of IRBCs in the placenta. In this study, we assessed the adherence of 3D7 parasite strain, which has been reported to bind both C4S and HA, using highly purified clinical grade rooster comb HA, Streptococcus HA, several preparations of human umbilical cord HA (hucHA), and bovine vitreous humor HA (bvhHA). While all hucHA preparations and bvhHA bound with moderate to high density to IRBCs, the rooster comb and bacterial HAs did not bind IRBCs. IRBCs binding to the hucHA and bvhHA could be abolished by pretreatment with testicular hyaluronidase but not with Streptomyces hyalurolyticus hyaluronidase, suggesting that IRBC binding to hucHA and bvhHA was due to chondroitin sulfate (CS) contaminants in HAs. Compositional analysis confirmed the presence of CS in both hucHA and bvhHA. The CSs present in these commercial hucHA and bvhHA samples were isolated, characterized, and studied for their ability to bind IRBCs. The data suggested that IRBC adherence to hucHA and bvhHA was mediated by the CS present in these samples. However, our data did not exclude the possibility of a minor population of distinct parasite subtype adhering to HA and further studies using pure HA conjugated to proteins or lipids and placental parasite isolates should clarify whether HA is an in vivo receptor for IRBC adherence.