Evidence of promiscuous endothelial binding by Plasmodium falciparum-infected erythrocytes

CD36-A Host Receptor Necessary for Malaria Parasites to Establish and Maintain Infection

Plasmodium falciparum-infected erythrocytes (PfIEs) present P. falciparum erythrocyte membrane protein 1 proteins (PfEMP1s) on the cell surface, via which they cytoadhere to various endothelial cell receptors (ECRs) on the walls of human blood vessels. This prevents the parasite from passing through the spleen, which would lead to its elimination. Each P. falciparum isolate has about 60 different PfEMP1s acting as ligands, and at least 24 ECRs have been identified as interaction partners. Interestingly, in every parasite genome sequenced to date, at least 75% of the encoded PfEMP1s have a binding domain for the scavenger receptor CD36 widely distributed on host endothelial cells and many other cell types. Here, we discuss why the interaction between PfIEs and CD36 is optimal to maintain a finely regulated equilibrium that allows the parasite to multiply and spread while causing minimal harm to the host in most infections.

A member of the tryptophan-rich protein family is required for efficient sequestration of Plasmodium berghei schizonts

Protein export and host membrane remodeling are crucial for multiple Plasmodium species to establish a niche in infected hosts. To better understand the contribution of these processes to successful parasite infection in vivo, we sought to find and characterize protein components of the intraerythrocytic Plasmodium berghei-induced membrane structures (IBIS) that form in the cytoplasm of infected erythrocytes. We identified proteins that immunoprecipitate with IBIS1, a signature member of the IBIS in P. berghei-infected erythrocytes. In parallel, we also report our data describing proteins that co-precipitate with the PTEX (Plasmodium translocon of exported proteins) component EXP2. To validate our findings, we examined the location of three candidate IBIS1-interactors that are conserved across multiple Plasmodium species, and we found they localized to IBIS in infected red blood cells and two further colocalized with IBIS1 in the liver-stage parasitophorous vacuole membrane. Successful gene deletion revealed that these two tryptophan-rich domain-containing proteins, termed here IPIS2 and IPIS3 (for intraerythrocytic Plasmodium-induced membrane structures), are required for efficient blood-stage growth. Erythrocytes infected with IPIS2-deficient schizonts in particular fail to bind CD36 as efficiently as wild-type P. berghei-infected cells and therefore fail to effectively sequester out of the circulating blood. Our findings support the idea that intra-erythrocytic membrane compartments are required across species for alterations of the host erythrocyte that facilitate interactions of infected cells with host tissues.

Ectopic Expression of Plasmodium vivax vir Genes in P. falciparum Affects Cytoadhesion via Increased Expression of Specific var Genes

Plasmodium falciparum-infected erythrocytes (PfIEs) adhere to endothelial cell receptors (ECRs) of blood vessels mainly via PfEMP1 proteins to escape elimination via the spleen. Evidence suggests that P. vivax-infected reticulocytes (PvIRs) also bind to ECRs, presumably enabled by VIR proteins, as shown by inhibition experiments and studies with transgenic P. falciparum expressing vir genes. To test this hypothesis, our study investigated the involvement of VIR proteins in cytoadhesion using vir gene-expressing P. falciparum transfectants. Those VIR proteins with a putative transmembrane domain were present in Maurer's clefts, and some were also present in the erythrocyte membrane. The VIR protein without a transmembrane domain (PVX_050690) was not exported. Five of the transgenic P. falciparum cell lines, including the one expressing PVX_050690, showed binding to CD36. We observed highly increased expression of specific var genes encoding PfEMP1s in all CD36-binding transfectants. These results suggest that ectopic vir expression regulates var expression through a yet unknown mechanism. In conclusion, the observed cytoadhesion of P. falciparum expressing vir genes depended on PfEMP1s, making this experimental unsuitable for characterizing VIR proteins.

The Cellular and Molecular Interaction Between Erythrocytes and Plasmodium falciparum Merozoites

Plasmodium falciparum is the most lethal human malaria parasite, partly due to its genetic variability and ability to use multiple invasion routes via its binding to host cell surface receptors. The parasite extensively modifies infected red blood cell architecture to promote its survival which leads to increased cell membrane rigidity, adhesiveness and permeability. Merozoites are initially released from infected hepatocytes and efficiently enter red blood cells in a well-orchestrated process that involves specific interactions between parasite ligands and erythrocyte receptors; symptoms of the disease occur during the life-cycle’s blood stage due to capillary blockage and massive erythrocyte lysis. Several studies have focused on elucidating molecular merozoite/erythrocyte interactions and host cell modifications; however, further in-depth analysis is required for understanding the parasite’s biology and thus provide the fundamental tools for developing prophylactic or therapeutic alternatives to mitigate or eliminate Plasmodium falciparum-related malaria. This review focuses on the cellular and molecular events during Plasmodium falciparum merozoite invasion of red blood cells and the alterations that occur in an erythrocyte once it has become infected.

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.

Adhesion between P. falciparum infected erythrocytes and human endothelial receptors follows alternative binding dynamics under flow and febrile conditions

Characterizing the adhesive dynamics of Plasmodium falciparum infected erythrocytes (IEs) to different endothelial cell receptors (ECRs) in flow is a big challenge considering available methods. This study investigated the adhesive dynamics of IEs to five ECRs (CD36, ICAM-1, P-selectin, CD9, CSA) using simulations of in vivo-like flow and febrile conditions. To characterize the interactions between ECRs and knobby and knobless IEs of two laboratory-adapted P. falciplarum isolates, cytoadhesion analysis over time was performed using a new tracking bioinformatics method. The results revealed that IEs performed rolling adhesion exclusively over CD36, but exhibited stationary binding to the other four ECRs. The absence of knobs affected rolling adhesion both with respect to the distance travelled by IEs and their velocity. Knobs played a critical role at febrile temperatures by stabilizing the binding interaction. Our results clearly underline the complexity of the IE-receptor interaction and the importance of knobs for the survival of the parasite at fever temperatures, and lead us to propose a new hypothesis that could open up new strategies for the treatment of malaria.

Stringent Selection of Knobby Plasmodium falciparum-Infected Erythrocytes during Cytoadhesion at Febrile Temperature

Changes in the erythrocyte membrane induced by Plasmodium falciparum invasion allow cytoadhesion of infected erythrocytes (IEs) to the host endothelium, which can lead to severe complications. Binding to endothelial cell receptors (ECRs) is mainly mediated by members of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family, encoded by var genes. Malaria infection causes several common symptoms, with fever being the most apparent. In this study, the effects of febrile conditions on cytoadhesion of predominately knobless erythrocytes infected with the laboratory isolate IT4 to chondroitin-4-sulfate A (CSA), intercellular adhesion molecule 1 (ICAM-1), and CD36 were investigated. IEs enriched for binding to CSA at 40 °C exhibited significantly increased binding capacity relative to parasites enriched at 37 °C. This interaction was due to increased var2csa expression and trafficking of the corresponding PfEMP1 to the IE surface as well as to a selection of knobby IEs. Furthermore, the enrichment of IEs to ICAM-1 at 40 °C also led to selection of knobby IEs over knobless IEs, whereas enrichment on CD36 did not lead to a selection. In summary, these findings demonstrate that knobs are crucial for parasitic survival in the host, especially during fever episodes, and thus, that selection pressure on the formation of knobs could be controlled by the host.

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

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

Towards an anti-disease malaria vaccine

Human infective parasites, such as those that cause malaria, are highly adapted to evade clearance by the immune system. In situations where they must maintain prolonged interactions with molecules of their host, they often use parasite surface protein families. These families are highly diverse to prevent immune recognition, and yet, to promote parasite survival, their members must retain the ability to interact with specific human receptors. One of the best understood of the parasite surface protein families is the PfEMP1 proteins of Plasmodium falciparum. These molecules cause infected erythrocytes to adhere to human receptors found on blood vessel and tissue surfaces. This protects the parasite within from clearance by the spleen and also causes symptoms of severe malaria. The PfEMP1 are exposed to the immune system during infection and are therefore excellent vaccine candidates for use in an approach to prevent severe disease. A key question, however, is whether their extensive diversity precludes them from forming components of the malaria vaccines of the future?

Host factors that modify Plasmodium falciparum adhesion to endothelial receptors

P. falciparum virulence is related to adhesion and sequestration of infected erythrocytes (IE) in deep vascular beds, but the endothelial receptors involved in severe malaria remain unclear. In the largest ever study of clinical isolates, we surveyed adhesion of freshly collected IE from children under 5 years of age in Mali to identify novel vascular receptors, and examined the effects of host age, hemoglobin type, blood group and severe malaria on levels of IE adhesion to a panel of endothelial receptors. Several novel molecules, including integrin α3β1, VE-cadherin, ICAM-2, junctional adhesion molecule-B (JAM-B), laminin, and cellular fibronectin, supported binding of IE from children. Severe malaria was not significantly associated with levels of IE adhesion to any of the 19 receptors. Hemoglobin AC, which reduces severe malaria risk, reduced IE binding to the receptors CD36 and integrin α5β1, while hemoglobin AS did not modify IE adhesion to any receptors. Blood groups A, AB and B significantly reduced IE binding to ICAM-1. Severe malaria risk varies with age, but age significantly impacted the level of IE binding to only a few receptors: IE binding to JAM-B decreased with age, while binding to CD36 and integrin α5β1 significantly increased with age.

Characterisation of Plasmodium falciparum populations selected on the human endothelial receptors P-selectin, E-selectin, CD9 and CD151

The ability of the parasite Plasmodium falciparum to evade the immune system and be sequestered within human small blood vessels is responsible for severe forms of malaria. The sequestration depends on the interaction between human endothelial receptors and P. falciparum erythrocyte membrane protein 1 (PfEMP1) exposed on the surface of the infected erythrocytes (IEs). In this study, the transcriptomes of parasite populations enriched for parasites that bind to human P-selectin, E-selectin, CD9 and CD151 receptors were analysed. IT4_var02 and IT4_var07 were specifically expressed in IT4 parasite populations enriched for P-selectin-binding parasites; eight var genes (IT4_var02/07/09/13/17/41/44/64) were specifically expressed in isolate populations enriched for CD9-binding parasites. Interestingly, IT4 parasite populations enriched for E-selectin- and CD151-binding parasites showed identical expression profiles to those of a parasite population exposed to wild-type CHO-745 cells. The same phenomenon was observed for the 3D7 isolate population enriched for binding to P-selectin, E-selectin, CD9 and CD151. This implies that the corresponding ligands for these receptors have either weak binding capacity or do not exist on the IE surface. Conclusively, this work expanded our understanding of P. falciparum adhesive interactions, through the identification of var transcripts that are enriched within the selected parasite populations.

Binding of Plasmodium falciparum to CD36 can be shielded by the glycocalyx

BACKGROUND

Plasmodium falciparum-infected erythrocytes sequester in the microcirculation due to interaction between surface-expressed parasite proteins and endothelial receptors. Endothelial cells are covered in a carbohydrate-rich glycocalyx that shields against undesired leukocyte adhesion. It was investigated if the cellular glycocalyx affects the binding of P. falciparum-infected erythrocytes to CD36 in vitro.

METHODS

Glycocalyx growth was followed in vitro by using azido sugars and cationized ferritin detecting O-glycoproteins and negatively charged proteoglycans, respectively. P. falciparum (clone FCR3/IT) was selected on Chinese hamster ovary (CHO) cells transfected with human CD36. Cytoadhesion to CHO CD36 at 1-4 days after seeding was quantified by using a static binding assay.

RESULTS

The glycocalyx thickness of CHO cells increased during 4 days in culture as assessed by metabolic labelling of glycans with azido sugars and with electron microscopy studying the binding of cationized ferritin to cell surfaces. The functional importance of this process was addressed in binding assays by using CHO cells transfected with CD36. In parallel with the maturation of the glycocalyx, antibody-binding to CD36 was inhibited, despite stable expression of CD36. P. falciparum selected for CD36-binding recognized CD36 on CHO cells on the first day in culture, but the binding was lost after 2-4 days.

CONCLUSION

The endothelial glycocalyx affects parasite cytoadhesion in vitro, an effect that has previously been ignored. The previously reported loss of glycocalyx during experimental malaria may play an important role in the pathogenesis of malaria complications by allowing the close interaction between infected erythrocytes and endothelial receptors.

The role of PfEMP1 as targets of naturally acquired immunity to childhood malaria: prospects for a vaccine

The Plasmodium falciparum erythrocyte membrane protein 1 antigens that are inserted onto the surface of P. falciparum infected erythrocytes play a key role both in the pathology of severe malaria and as targets of naturally acquired immunity. They might be considered unlikely vaccine targets because they are extremely diverse. However, several lines of evidence suggest that underneath this molecular diversity there are a restricted set of epitopes which may act as effective targets for a vaccine against severe malaria. Here we review some of the recent developments in this area of research, focusing on work that has assessed the potential of these molecules as possible vaccine targets.

Type of in vitro cultivation influences cytoadhesion, knob structure, protein localization and transcriptome profile of Plasmodium falciparum

In vitro cultivation of Plasmodium falciparum is critical for studying the biology of this parasite. However, it is likely that different in vitro cultivation conditions influence various aspects of the parasite’s life cycle. In the present study two P. falciparum isolates were cultivated using the two most common methods, in which AlbuMAX or human serum as additives are used, and the results were compared. The type of cultivation influenced the knob structure of P. falciparum-infected erythrocytes (IEs). IEs cultivated with AlbuMAX had fewer knobs than those cultivated with human serum. Furthermore, knob size varied between isolates and is also depended on the culture medium. In addition, there was a greater reduction in the cytoadhesion of IEs to various endothelial receptors in the presence of AlbuMAX than in the presence of human serum. Surprisingly, cytoadhesion did not correlate with the presence or absence of knobs. Greater numbers of the variant surface antigen families RIFIN, STEVOR, and PfMC-2TM were found at the IE membrane when cultivated in the presence of AlbuMAX. Moreover, the type of cultivation had a marked influence on the transcriptome profile. Compared with cultivation with human serum, cultivation with AlbuMAX increased the expression of approximately 500–870 genes.

PfEMP1 - A Parasite Protein Family of Key Importance in Plasmodium falciparum Malaria Immunity and Pathogenesis

Plasmodium falciparum causes the most severe form of malaria and is responsible for essentially all malaria-related deaths. The accumulation in various tissues of erythrocytes infected by mature P. falciparum parasites can lead to circulatory disturbances and inflammation, and is thought to be a central element in the pathogenesis of the disease. It is mediated by the interaction of parasite ligands on the erythrocyte surface and a range of host receptor molecules in many organs and tissues. Among several proteins and protein families implicated in this process, the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of high-molecular weight and highly variable antigens appears to be the most prominent. In this chapter, we aim to provide a systematic overview of the current knowledge about these proteins, their structure, their function, how they are presented on the erythrocyte surface, and how the var genes encoding them are regulated. The role of PfEMP1 in the pathogenesis of malaria, PfEMP1-specific immune responses, and the prospect of PfEMP1-specific vaccination against malaria are also covered briefly.

Endothelial protein C receptor gene variants not associated with severe malaria in ghanaian children

Background

Two recent reports have identified the Endothelial Protein C Receptor (EPCR) as a key molecule implicated in severe malaria pathology. First, it was shown that EPCR in the human microvasculature mediates sequestration of Plasmodium falciparum-infected erythrocytes. Second, microvascular thrombosis, one of the major processes causing cerebral malaria, was linked to a reduction in EPCR expression in cerebral endothelial layers. It was speculated that genetic variation affecting EPCR functionality could influence susceptibility to severe malaria phenotypes, rendering PROCR, the gene encoding EPCR, a promising candidate for an association study.

Methods

Here, we performed an association study including high-resolution variant discovery of rare and frequent genetic variants in the PROCR gene. The study group, which previously has proven to be a valuable tool for studying the genetics of malaria, comprised 1,905 severe malaria cases aged 1–156 months and 1,866 apparently healthy children aged 2–161 months from the Ashanti Region in Ghana, West Africa, where malaria is highly endemic. Association of genetic variation with severe malaria phenotypes was examined on the basis of single variants, reconstructed haplotypes, and rare variant analyses.

Results

A total of 41 genetic variants were detected in regulatory and coding regions of PROCR, 17 of which were previously unknown genetic variants. In association tests, none of the single variants, haplotypes or rare variants showed evidence for an association with severe malaria, cerebral malaria, or severe malaria anemia.

Conclusion

Here we present the first analysis of genetic variation in the PROCR gene in the context of severe malaria in African subjects and show that genetic variation in the PROCR gene in our study population does not influence susceptibility to major severe malaria phenotypes.

An analysis of the binding characteristics of a panel of recently selected ICAM-1 binding Plasmodium falciparum patient isolates

The basis of severe malaria pathogenesis in part includes sequestration of Plasmodium falciparum-infected erythrocytes (IE) from the peripheral circulation. This phenomenon is mediated by the interaction between several endothelial receptors and one of the main parasite-derived variant antigens (PfEMP1) expressed on the surface of the infected erythrocyte membrane. One of the commonly used host receptors is ICAM-1, and it has been suggested that ICAM-1 has a role in cerebral malaria pathology, although the evidence to support this is not conclusive. The current study examined the cytoadherence patterns of lab-adapted patient isolates after selecting on ICAM-1. We investigated the binding phenotypes using variant ICAM-1 proteins including ICAM-1^Ref, ICAM-1^Kilifi, ICAM-1^S22/A, ICAM-1^L42/A and ICAM-1^L44/A using static assays. The study also examined ICAM-1 blocking by four anti-ICAM-1 monoclonal antibodies (mAb) under static conditions. We also characterised the binding phenotypes using Human Dermal Microvascular Endothelial Cells (HDMEC) under flow conditions. The results show that different isolates have variant-specific binding phenotypes under both static and flow conditions, extending our previous observations that this variation might be due to variable contact residues on ICAM-1 being used by different parasite PfEMP1 variants.

Pathogenesis of cerebral malaria--inflammation and cytoadherence

Despite decades of research on cerebral malaria (CM) there is still a paucity of knowledge about what actual causes CM and why certain people develop it. Although sequestration of P. falciparum infected red blood cells has been linked to pathology, it is still not clear if this is directly or solely responsible for this clinical syndrome. Recent data have suggested that a combination of parasite variant types, mainly defined by the variant surface antigen, P. falciparum erythrocyte membrane protein 1 (PfEMP1), its receptors, coagulation and host endothelial cell activation (or inflammation) are equally important. This makes CM a multi-factorial disease and a challenge to unravel its causes to decrease its detrimental impact.

Brain endothelial cells increase the proliferation of Plasmodium falciparum through production of soluble factors

We here describe the novel finding that brain endothelial cells in vitro can stimulate the growth of Plasmodium falciparum through the production of low molecular weight growth factors. By using a conditioned medium approach, we show that the brain endothelial cells continued to release these factors over time. If this mirrors the in vivo situation, these growth factors potentially would provide an advantage, in terms of enhanced growth, for sequestered parasitised red blood cells in the brain microvasculature. We observed this phenomenon with brain endothelial cells from several sources as well as a second P. falciparum strain. The characteristics of the growth factors included: <3 kDa molecular weight, heat stable, and in part chloroform soluble. Future efforts should be directed at identifying these growth factors, since blocking their production or actions might be of benefit for reducing parasite load and, hence, malaria pathology.