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

Binding of Plasmodium falciparum-Infected Red Blood Cells to Engineered 3D Microvessels

P. falciparum-infected red blood cell (iRBC) sequestration in the microvasculature is a pivotal event in severe malaria pathogenesis. In vitro binding assays using endothelial cell monolayers under static and flow conditions have revealed key ligand-receptor interactions for iRBC sequestration. However, mechanisms remain elusive for iRBC sequestration in specific vascular locations, which prevents further development of effective therapies. New models are needed to better recapitulate the complex geometry of blood flow in human blood vessels and organ-specific vascular signatures. Recent advances in engineering 3D microvessels in vitro have emerged as promising technologies to not only model complex human vascular structures but also allow for precise and step-wise control of individual biological and biomechanical parameters. By designing networks with different branching structures and change of vessel diameter along the flow path, these models recapitulate pressure and flow changes occurring in vivo. Here, we describe the methodology employed to build 3D microvessels using soft lithography and injection molding techniques, as well as the protocol to fabricate capillary-size vessels through collagen photoablation. Furthermore, we describe the methodology of using these models to study malaria and narrate necessary steps for perfusion of P. falciparum through 3D microvessels and different options to quantify P. falciparum-iRBC binding.

A new heparan sulfate from the mollusk Nodipecten nodosus inhibits merozoite invasion and disrupts rosetting and cytoadherence of Plasmodium falciparum

BACKGROUND

Despite treatment with effective antimalarial drugs, the mortality rate is still high in severe cases of the disease, highlighting the need to find adjunct therapies that can inhibit the adhesion of Plasmodium falciparum-infected erythrocytes (Pf-iEs).

OBJECTIVES

In this context, we evaluated a new heparan sulfate (HS) from Nodipecten nodosus for antimalarial activity and inhibition of P. falciparum cytoadhesion and rosetting.

METHODS

Parasite inhibition was measured by SYBR green using a cytometer. HS was assessed in rosetting and cytoadhesion assays under static and flow conditions using Chinese hamster ovary (CHO) and human lymphatic endothelial cell (HLEC) cells expressing intercellular adhesion molecule-1 (ICAM1) and chondroitin sulfate A (CSA), respectively.

FINDINGS

This HS inhibited merozoite invasion similar to heparin. Moreover, mollusk HS decreased cytoadherence of P. falciparum to CSA and ICAM-1 on the surface of endothelial cells under static and flow conditions. In addition, this glycan efficiently disrupted rosettes.

CONCLUSIONS

These findings support a potential use for mollusk HS as adjunct therapy for severe malaria.

Pre-clinical and clinical development of the first placental malaria vaccine

INTRODUCTION

Malaria during pregnancy is a massive health problem in endemic areas. Placental malaria infections caused by Plasmodium falciparum are responsible for up to one million babies being born with a low birth weight every year. Significant efforts have been invested into preventing the condition. Areas covered: Pub Med was searched using the broad terms 'malaria parasite placenta' to identify studies of interactions between parasite and host, 'prevention of placental malaria' to identify current strategies to prevent placental malaria, and 'placental malaria vaccine' to identify pre-clinical vaccine development. However, all papers from these searches were not systematically included. Expert commentary: The first phase I clinical trials of vaccines are well underway. Trials testing efficacy are more complicated to carry out as only women that are exposed to parasites during pregnancy will contribute to endpoint measurements, further it may require extensive follow-up to establish protection. Future second generation vaccines may overcome the inherent challenges in making an effective placental malaria vaccine.

Comparison of functional assays used in the clinical development of a placental malaria vaccine

BACKGROUND

Malaria in pregnancy is associated with significant morbidity in pregnant women and their offspring. Plasmodium falciparum infected erythrocytes (IE) express VAR2CSA that mediates binding to chondroitin sulphate A (CSA) in the placenta. Two VAR2CSA-based vaccines for placental malaria are in clinical development. The purpose of this study was to evaluate the robustness and comparability of binding inhibition assays used in the clinical development of placental malaria vaccines.

METHODS

The ability of sera from animals immunised with different VAR2CSA constructs to inhibit IE binding to CSA was investigated in three in vitro assays using 96-well plates, petri dishes, capillary flow and an ex vivo placental perfusion assay.

RESULTS

The inter-assay variation was not uniform between assays and ranged from above ten-fold in the flow assay to two-fold in the perfusion assay. The intra-assay variation was highest in the petri dish assay. A positive correlation between IE binding avidity and the level of binding after antibody inhibition in the petri dish assay indicate that high avidity IE binding is more difficult to inhibit. The highest binding inhibition sensitivity was found in the 96-well and petri dish assays compared to the flow and perfusion assays where binding inhibition required higher antibody titers.

CONCLUSIONS

The inhibitory capacity of antibodies is not easily translated between assays and the high sensitivity of the 96-well and petri dish assays stresses the need for comparing serial dilutions of serum. Furthermore, IE binding avidity must be in the same range when comparing data from different days. There was an overall concordance in the capacity of antibody-mediated inhibition, when comparing the in vitro assays with the perfusion assay, which more closely represents in vivo conditions. Importantly the ID1-ID2a protein in a liposomal formulation, currently in a phase I trial, effectively induced antibodies that inhibited IE adhesion in placental tissue.

Adhesion of Plasmodium falciparum infected erythrocytes in ex vivo perfused placental tissue: a novel model of placental malaria

Background

Placental malaria occurs when Plasmodium falciparum infected erythrocytes sequester in the placenta. Placental parasite isolates bind to chondroitin sulphate A (CSA) by expression of VAR2CSA on the surface of infected erythrocytes, but may sequester by other VAR2CSA mediated mechanisms, such as binding to immunoglobulins. Furthermore, other parasite antigens have been associated with placental malaria. These findings have important implications for placental malaria vaccine design. The objective of this study was to adapt and describe a biologically relevant model of parasite adhesion in intact placental tissue.

Results

The ex vivo placental perfusion model was modified to study adhesion of infected erythrocytes binding to CSA, endothelial protein C receptor (EPCR) or a transgenic parasite where P. falciparum erythrocyte membrane protein 1 expression had been shut down. Infected erythrocytes expressing VAR2CSA accumulated in perfused placental tissue whereas the EPCR binding and the transgenic parasite did not. Soluble CSA and antibodies specific against VAR2CSA inhibited binding of infected erythrocytes.

Conclusion

The ex vivo model provides a novel way of studying receptor-ligand interactions and antibody mediated inhibition of binding in placental malaria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1342-2) contains supplementary material, which is available to authorized users.

Modeling cytoadhesion of Plasmodium falciparum-infected erythrocytes and leukocytes-common principles and distinctive features

Cytoadhesion of Plasmodium falciparum‐infected erythrocytes to the microvascular endothelial lining shares striking similarities to cytoadhesion of leukocytes. In both cases, adhesins are presented in structures that raise them above the cell surface. Another similarity is the enhancement of adhesion under physical force (catch bonding). Here, we review recent advances in our understanding of the molecular and biophysical mechanisms underlying cytoadherence in both cellular systems. We describe how imaging, flow chamber experiments, single‐molecule measurements, and computational modeling have been used to decipher the relevant processes. We conclude that although the parasite seems to induce processes that resemble the cytoadherence of leukocytes, the mechanics of erythrocytes is such that the resulting behavior in shear flow is fundamentally different.

Cytoadhesion of Plasmodium falciparum-infected erythrocytes to chondroitin-4-sulfate is cooperative and shear enhanced

Infections with the human malaria parasite Plasmodium falciparum during pregnancy can lead to severe complications for both mother and child, resulting from the cytoadhesion of parasitized erythrocytes in the intervillous space of the placenta. Cytoadherence is conferred by the specific interaction of the parasite-encoded adhesin VAR2CSA with chondroitin-4-sulfate (CSA) present on placental proteoglycans. CSA presented elsewhere in the microvasculature does not afford VAR2CSA-mediated cytoadhesion of parasitized erythrocytes. To address the placenta-specific binding tropism, we investigated the effect of the receptor/ligand arrangement on cytoadhesion, using artificial membranes with different CSA spacing intervals. We found that cytoadhesion is strongly dependent on the CSA distance, with half-maximal adhesion occurring at a CSA distance of 9 ± 1 nm at all hydrodynamic conditions. Moreover, binding to CSA was cooperative and shear stress induced. These findings suggest that the CSA density, together with allosteric effects in VAR2CSA, aid in discriminating between different CSA milieus.

Fucosylated chondroitin sulfate inhibits Plasmodium falciparum cytoadhesion and merozoite invasion

Sequestration of Plasmodium falciparum-infected erythrocytes (Pf-iEs) in the microvasculature of vital organs plays a key role in the pathogenesis of life-threatening malaria complications, such as cerebral malaria and malaria in pregnancy. This phenomenon is marked by the cytoadhesion of Pf-iEs to host receptors on the surfaces of endothelial cells, on noninfected erythrocytes, and in the placental trophoblast; therefore, these sites are potential targets for antiadhesion therapies. In this context, glycosaminoglycans (GAGs), including heparin, have shown the ability to inhibit Pf-iE cytoadherence and growth. Nevertheless, the use of heparin was discontinued due to serious side effects, such as bleeding. Other GAG-based therapies were hampered due to the potential risk of contamination with prions and viruses, as some GAGs are isolated from mammals. In this context, we investigated the effects and mechanism of action of fucosylated chondroitin sulfate (FucCS), a unique and highly sulfated GAG isolated from the sea cucumber, with respect to P. falciparum cytoadhesion and development. FucCS was effective in inhibiting the cytoadherence of Pf-iEs to human lung endothelial cells and placenta cryosections under static and flow conditions. Removal of the sulfated fucose branches of the FucCS structure virtually abolished the inhibitory effects of FucCS. Importantly, FucCS rapidly disrupted rosettes at high levels, and it was also able to block parasite development by interfering with merozoite invasion. Collectively, these findings highlight the potential of FucCS as a candidate for adjunct therapy against severe malaria.

Relevant assay to study the adhesion of Plasmodium falciparum-infected erythrocytes to the placental epithelium

In placental malaria, Plasmodium falciparum-infected erythrocytes adhere to the apical plasma membrane of the placental epithelium, triggering an impairment of placental function detrimental to the fetus. The design of anti-adhesion intervention strategies requires a detailed understanding of the mechanisms involved. However, most adhesion assays lack in vivo relevance and are hardly quantitative. Here, we describe a flow cytometry-based adhesion assay that is fully relevant by using apical epithelial plasma membrane vesicles as the adhesion matrix, and being applicable to infected erythrocytes directly isolated from patients. Adhesion is measured both as the percentage of pathogens bound to epithelial membrane vesicles as well as the mean number of vesicles bound per infected erythrocytes. We show that adhesins alternative to those currently identified could be involved. This demonstrates the power of this assay to advance our understanding of epithelial adhesion of infected erythrocytes and in the design of intervention strategies.

Strain-transcendent immune response to recombinant Var2CSA DBL5-ε domain block P. falciparum adhesion to placenta-derived BeWo cells under flow conditions

Background

Pregnancy-associated malaria (PAM) is a serious consequence of the adhesion to the placental receptor chondroitin sulfate A (CSA) of Plasmodium falciparum-infected erythrocytes (PE) expressing the large cysteine-rich multi-domain protein var2CSA. Women become resistant to PAM, and develop strain-transcending immunity against CSA-binding parasites. The identification of var2CSA regions that could elicit broadly neutralizing and adhesion-blocking antibodies is a key step for the design of prophylactic vaccine strategies.

Methodology

Escherichia coli expressed var2CSA DBL domains were refolded and purified prior to immunization of mice and a goat. Protein-G-purified antibodies were tested for their ability to block FCR3^CSA-infected erythrocytes binding to placental (BeWo) and monkey brain endothelial (ScC2) cell lines using a flow cytoadhesion inhibition assay mimicking closely the physiological conditions present in the placenta at shear stress of 0.05 Pa. DBL5-ε, DBL6-ε and DBL5-6-ε induced cross-reactive antibodies using Alum and Freund as adjuvants, which blocked cytoadhesion at values ranging between 40 to 96% at 0.5 mg IgG per ml. Importantly, antibodies raised against recombinant DBL5-ε from 3 distinct parasites genotypes (HB3, Dd2 and 7G8) showed strain-transcending inhibition ranging from 38 to 64% for the heterologuous FCR3^CSA.

Conclusions

Using single and double DBL domains from var2CSA and Alum as adjuvant, we identified recombinant subunits inducing an immune response in experimental animals which is able to block efficiently parasite adhesion in a flow cytoadhesion assay that mimics closely the erythrocyte flow in the placenta. These subunits show promising features for inclusion into a vaccine aiming to protect against PAM.

Microfluidic approaches to malaria pathogenesis

Malaria is a major poverty-related human infectious disease of the world. Over a billion individuals are under threat and several million die from malaria every year. The nature of disease, especially fatal disease, has been the subject of many studies. The consensus is that parasite-induced cytoadherance of red blood cells precipitates capillary blockage and inflammatory responses in affected organs. Reduced deformability of infected erythrocytes may also contribute to disease. What is not very clear is why people with significant parasite burdens display large variations in disease outcomes. Technologies which allow a detailed description of the cytoadherance properties of infected erythrocytes in individual patients, and which allow a complete description of the flow capabilities of red blood cell populations in that patient, would be very useful. Here we review the recent introduction of microfluidic technology to study malaria pathogenesis, including the fabrication processes. The devices are cheap, versatile, portable and require very small patient samples. With greater use in research laboratories and field sites, we eventually expect microfluidic methods to play important roles in malaria diagnosis, as well as prognosis.

Plasmodium falciparum during pregnancy: a puzzling parasite tissue adhesion tropism

P. falciparum malaria severely affects pregnant women and children. Despite immunity through lifelong exposure to malaria, pregnant women become susceptible to infections causing anaemia, abortions and low birth weight. They experience massive accumulation of infected erythrocytes (IEs) in the placenta. Adhesion of IEs to host endothelial receptors is mediated by members of a large diverse protein family called P. falciparum erythrocyte membrane protein 1 (PfEMP1). Pregnancy malaria is generally associated with the emergence of a distinct subset of parasites expressing a unique PfEMP1 that binds to the host-receptor chondroitin sulfate A (CSA). Resistance to pregnancy malaria is associated with the acquisition of antibodies that block IEs binding to placental CSA. The absence (or rare occurrence) of CSA-binding parasites in malaria patients (children, men and non-pregnant women) suggests that these parasites become virulent only during pregnancy. The molecular mechanisms used by P. falciparum to achieve the timely expression of the Pf-CSA ligand in pregnant women remain puzzling. In this review we will discuss two hypothetical mechanisms by which CSA-binding parasites may arise during pregnancy. The first, a selection process by the placenta of a distinct sub-population of P. falciparum expressing a particular PfEMP1. The second, an induction mechanism that facilitates the expression of a particular PfEMP1 protein by specific host factor(s) present only during pregnancy.

Microfluidic modeling of cell-cell interactions in malaria pathogenesis

The clinical outcomes of human infections by Plasmodium falciparum remain highly unpredictable. A complete understanding of the complex interactions between host cells and the parasite will require in vitro experimental models that simultaneously capture diverse host-parasite interactions relevant to pathogenesis. Here we show that advanced microfluidic devices concurrently model (a) adhesion of infected red blood cells to host cell ligands, (b) rheological responses to changing dimensions of capillaries with shapes and sizes similar to small blood vessels, and (c) phagocytosis of infected erythrocytes by macrophages. All of this is accomplished under physiologically relevant flow conditions for up to 20 h. Using select examples, we demonstrate how this enabling technology can be applied in novel, integrated ways to dissect interactions between host cell ligands and parasitized erythrocytes in synthetic capillaries. The devices are cheap and portable and require small sample volumes; thus, they have the potential to be widely used in research laboratories and at field sites with access to fresh patient samples.

Disruption of var2csa gene impairs placental malaria associated adhesion phenotype

Infection with Plasmodium falciparum during pregnancy is one of the major causes of malaria related morbidity and mortality in newborn and mothers. The complications of pregnancy-associated malaria result mainly from massive adhesion of Plasmodium falciparum-infected erythrocytes (IE) to chondroitin sulfate A (CSA) present in the placental intervillous blood spaces. Var2CSA, a member of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family is the predominant parasite ligand mediating CSA binding. However, experimental evidence suggests that other host receptors, such as hyaluronic acid (HA) and the neonatal Fc receptor, may also support placental binding. Here we used parasites in which var2csa was genetically disrupted to evaluate the contribution of these receptors to placental sequestration and to identify additional adhesion receptors that may be involved in pregnancy-associated malaria. By comparison to the wild-type parasites, the FCR3Δvar2csa mutants could not be selected for HA adhesion, indicating that var2csa is not only essential for IE cytoadhesion to the placental receptor CSA, but also to HA. However, further studies using different pure sources of HA revealed that the previously observed binding results from CSA contamination in the bovine vitreous humor HA preparation. To identify CSA-independent placental interactions, FCR3Δvar2csa mutant parasites were selected for adhesion to the human placental trophoblastic BeWo cell line. BeWo selected parasites revealed a multi-phenotypic adhesion population expressing multiple var genes. However, these parasites did not cytoadhere specifically to the syncytiotrophoblast lining of placental cryosections and were not recognized by sera from malaria-exposed women in a parity dependent manner, indicating that the surface molecules present on the surface of the BeWo selected population are not specifically expressed during the course of pregnancy-associated malaria. Taken together, these results demonstrate that the placental malaria associated phenotype can not be restored in FCR3Δvar2csa mutant parasites and highlight the key role of var2CSA in pregnancy malaria pathogenesis and for vaccine development.

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.

Neural cell adhesion molecule, a new cytoadhesion receptor for Plasmodium falciparum-infected erythrocytes capable of aggregation

The cytoadhesion of Plasmodium falciparum-infected erythrocytes (IEs) to the endothelial cells lining the microvasculature, clogging the microvessels of various organs, is a key event in the pathogenesis of certain severe forms of malaria, such as cerebral malaria and pulmonary edema. Studies aiming to identify possible correlations between the severity of clinical cases and the presence of particular cytoadhesion phenotypes have been largely unsuccessful. One of the possible reasons for this failure is that some of the key receptors and/or mechanisms involved have yet to be identified. By combining IE selection, cell transfection, and adhesion inhibition assays, we identified a new cytoadhesion receptor, neural cell adhesion molecule (NCAM). NCAM is a member of the immunoglobulin superfamily and has nonpolysialylated and polysialylated isoforms, the latter being rare in adults. The nonpolysialylated form is present on the surfaces of endothelial cells in the microvessels of various organs in which IE sequestration occurs. We found that multiphenotypic IEs interacted with nonpolysialylated NCAM and with another, as yet unidentified receptor. These IEs also displayed cytoadhesion in flow conditions, presenting the unique ability to form adherent macroaggregates composed of hundreds of IEs. These features may act as virulence factors, increasing the capacity of IEs to clog microvessels via receptor synergy and macroaggregate formation, thereby facilitating the pathogenesis of severe forms of malaria.

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.

Plasmodium falciparum: Chondroitin sulfate A is the major receptor for adhesion of parasitized erythrocytes in the placenta

Plasmodium falciparum parasites that sequester in the placenta bind to the molecule chondroitin sulfate A (CSA). Women become resistant to malaria during pregnancy as they acquire antibodies that inhibit parasite adhesion to CSA, suggesting that a vaccine against placental malaria is feasible. Hyaluronic acid (HA) and non-immune IgG have also been proposed as receptors for P. falciparum adhesion in the placenta, but evidence for their roles is inconclusive. In this study, CSA, HA, and IgG were simultaneously assessed for their relative contributions to placental adhesion. Placental parasites collected in Tanzania uniformly adhered to the molecule CSA, and soluble CSA completely inhibited adhesion of most samples to placental cryosections. Three of 46 placental parasite samples also adhered to immobilized HA, but HA failed to inhibit adhesion of any placental parasites to placental cryosections. Similarly, non-immune IgG and protein A failed to inhibit adhesion of parasite samples to placental cryosection. P. falciparum adhesion in the placenta appears to be a non-redundant process that requires CSA as a receptor. Vaccines that elicit functional antibodies against CSA-binding parasites may confer resistance to pregnancy malaria.

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.