Structural basis for the ABO blood-group dependence of Plasmodium falciparum rosetting

Do Blood group and Sickle cell trait protect against placental malaria?

Blood group O is reported to confer some degree of protection from severe malaria in endemic setting. This protection is believed to be due to reduced and smaller rosette formation in people of blood group O which can easily be cleared by the host immune system. Also, sickle cell trait (HbAS) is reported to disrupt the adhesion of infected erythrocytes to microvascular endothelial walls, which could protect pregnant women from placental malaria. We determined the association between HbAS and ABO blood group, and placental malaria amongst pregnant women of all parities. The study enrolled 221 pregnant women. Peripheral blood samples were taken for malaria smears, ABO blood grouping and haemoglobin (Hb) electrophoresis. A structured questionnaire was used to age, bed net usage, and the number of Sulphadoxine-pyrimethamine (SP) doses taken by a pregnant woman. Two hundred and twenty-one (221) pregnant women were enrolled and out of this number, 110 (49.8%) were primiparae and 111 (50.2%) multiparae, with a mean age of 23.7±5.2. Placental malaria (PM) prevalence by PCR detection was 19.4% (43/221). Of those who were malaria positive 58.1% (25/43) were primiparae. Primiparae who are of blood group O were more susceptible to PM [P=0.04, (OR); 2.85, 95% (Cl), 1.12-9.01]. But sickle cell trait did not reduce the prevalence of PM [P=0.84 (OR); 0.92, 95% (Cl), 0.43-1.99]. Non-blood group O primiparae women were protected against placental malaria. This could be why some primiparae women are protected from PM, just like multiparae women.

Identification of potential molecular mimicry in pathogen-host interactions

Pathogens have evolved sophisticated strategies to manipulate host signaling pathways, including the phenomenon of molecular mimicry, where pathogen-derived biomolecules imitate host biomolecules. In this study, we resurrected, updated, and optimized a sequence-based bioinformatics pipeline to identify potential molecular mimicry candidates between humans and 32 pathogenic species whose proteomes' 3D structure predictions were available at the start of this study. We observed considerable variation in the number of mimicry candidates across pathogenic species, with pathogenic bacteria exhibiting fewer candidates compared to fungi and protozoans. Further analysis revealed that the candidate mimicry regions were enriched in solvent-accessible regions, highlighting their potential functional relevance. We identified a total of 1,878 mimicked regions in 1,439 human proteins, and clustering analysis indicated diverse target proteins across pathogen species. The human proteins containing mimicked regions revealed significant associations between these proteins and various biological processes, with an emphasis on host extracellular matrix organization and cytoskeletal processes. However, immune-related proteins were underrepresented as targets of mimicry. Our findings provide insights into the broad range of host-pathogen interactions mediated by molecular mimicry and highlight potential targets for further investigation. This comprehensive analysis contributes to our understanding of the complex mechanisms employed by pathogens to subvert host defenses and we provide a resource to assist researchers in the development of novel therapeutic strategies.

Endothelial protein C receptor binding induces conformational changes to severe malaria-associated group A PfEMP1

Severe Plasmodium falciparum malaria infections are caused by microvascular sequestration of parasites binding to the human endothelial protein C receptor (EPCR) via the multi-domain P. falciparum erythrocyte membrane protein 1 (PfEMP1) adhesion ligands. Using cryogenic electron microscopy (Cryo-EM) and PfEMP1 sequence diversity analysis, we found that group A PfEMP1 CIDRα1 domains interact with the adjacent DBLα1 domain through central, conserved residues of the EPCR-binding site to adopt a compact conformation. Upon EPCR binding, the DBLα1 domain is displaced, and the EPCR-binding helix of CIDRα1 is turned, kinked, and twisted to reach a rearranged, stable EPCR-bound conformation. The unbound conformation and the required transition to the EPCR-bound conformation may represent a conformational masking mechanism of immune evasion for the PfEMP1 family.

Non-O ABO blood group genotypes differ in their associations with Plasmodium falciparum rosetting and severe malaria

Blood group O is associated with protection against severe malaria and reduced size and stability of P. falciparum-host red blood cell (RBC) rosettes compared to non-O blood groups. Whether the non-O blood groups encoded by the specific ABO genotypes AO, BO, AA, BB and AB differ in their associations with severe malaria and rosetting is unknown. The A and B antigens are host RBC receptors for rosetting, hence we hypothesized that the higher levels of A and/or B antigen on RBCs from AA, BB and AB genotypes compared to AO/BO genotypes could lead to larger rosettes, increased microvascular obstruction and higher risk of malaria pathology. We used a case-control study of Kenyan children and in vitro adhesion assays to test the hypothesis that "double dose" non-O genotypes (AA, BB, AB) are associated with increased risk of severe malaria and larger rosettes than "single dose" heterozygotes (AO, BO). In the case-control study, compared to OO, the double dose genotypes consistently had higher odds ratios (OR) for severe malaria than single dose genotypes, with AB (OR 1.93) and AO (OR 1.27) showing most marked difference (p = 0.02, Wald test). In vitro experiments with blood group A-preferring P. falciparum parasites showed that significantly larger rosettes were formed with AA and AB host RBCs compared to OO, whereas AO and BO genotypes rosettes were indistinguishable from OO. Overall, the data show that ABO genotype influences P. falciparum rosetting and support the hypothesis that double dose non-O genotypes confer a greater risk of severe malaria than AO/BO heterozygosity.

Malaria vaccines

INTRODUCTION

: Eradication of malaria remains one of the main aims of medicine. Despite progress in malaria treatment, mortality rate remains high, especially in the poorest parts of the world. Therefore, prevention through vaccines is fundamental and recent approval of the first effective vaccine reinforced this assumption. However, since the parasite cycle is complex, being composed of three stages, different types of vaccine targeting stage-specific antigens shall be developed. Moreover, the beneficial effect on vaccinated subjects can be tuned using compositions targeting different disease stages.

AREA COVERED

: We analysed the malaria vaccine patent landscape describing the most significant patents published after 2016, classified according to the different parasite stages targeted focusing on selected protein antigens or epitopes. We searched "malaria vaccine" on Patentscope and Espacenet.

EXPERT OPINION

: Pre-erythrocytic vaccines were boosted by RTS,S approval, but its partial efficacy, limited to sporozoites, calls for compositions active against other disease stages. In particular, multi-antigens vaccines could be more effective than single-stage ones, as they would activate an immune response more similar to that acquired in endemic regions. Furthermore, vaccine storage is another factor to be taken into account given the climate of the areas where malaria is widespread. More advanced technologies can lead to more effective and safer vaccines.

Rosette formation by Plasmodium vivax gametocytes favors the infection in Anopheles aquasalis

Plasmodium vivax is a public health problem and the most common type of malaria outside sub-Saharan Africa. The capacity of cytoadhesion, rosetting, and liver latent phase development could impact treatment and disease control. Although the ability to P. vivax gametocyte develop rosetting is known, it is not yet clear which role it plays during the infection and transmission process to the mosquito. Here, we used ex vivo approaches for evaluate the rosetting P. vivax gametocytes capacity and we have investigated the effect of this adhesive phenotype on the infection process in the vector Anopheles aquasalis mosquito. Rosette assays were performed in 107 isolates, and we have observed an elevated frequency of cytoadhesive phenomena (77,6%). The isolates with more than 10% of rosettes have presented a higher infection rate in Anopheles aquasalis (p=0.0252). Moreover, we found a positive correlation between the frequency of parasites in rosetting with the infection rate (p=0.0017) and intensity (p=0.0387) in the mosquito. The disruption of P. vivax rosette formation through mechanical rupture assay confirmed the previously findings, since the paired comparison showed that isolates with disrupted rosettes have a lower infection rate (p<0.0001) and intensity (p=0.0003) compared to the control group (no disruption). Herein we have demonstrated for the first time a potential effect of the rosette phenomenon on the infection process in the mosquito vector An. aquasalis, favoring its capacity and intensity of infection, thus allowing the perpetuation of the parasite cycle life.

Red Blood Cells Oligosaccharides as Targets for Plasmodium Invasion

The key element in developing a successful malaria treatment is a good understanding of molecular mechanisms engaged in human host infection. It is assumed that oligosaccharides play a significant role in Plasmodium parasites binding to RBCs at different steps of host infection. The formation of a tight junction between EBL merozoite ligands and glycophorin receptors is the crucial interaction in ensuring merozoite entry into RBCs. It was proposed that sialic acid residues of O/N-linked glycans form clusters on a human glycophorins polypeptide chain, which facilitates the binding. Therefore, specific carbohydrate drugs have been suggested as possible malaria treatments. It was shown that the sugar moieties of N-acetylneuraminyl-N-acetate-lactosamine and 2,3-didehydro-2-deoxy-N-acetylneuraminic acid (DANA), which is its structural analog, can inhibit P. falciparum EBA-175-GPA interaction. Moreover, heparin-like molecules might be used as antimalarial drugs with some modifications to overcome their anticoagulant properties. Assuming that the principal interactions of Plasmodium merozoites and host cells are mediated by carbohydrates or glycan moieties, glycobiology-based approaches may lead to new malaria therapeutic targets.

Evolving perspectives on rosetting in malaria

The ability of the intraerythrocytic Plasmodium spp. to form spontaneous rosettes with uninfected red blood cells (URBCs) has been observed in the medically important malaria parasites. Since the discovery of rosettes in the late 1980s, different formation mechanisms and pathobiological roles have been postulated for rosetting; most of which have focused on Plasmodium falciparum. Recent breakthroughs, including new data from Plasmodium vivax, have highlighted the multifaceted roles of rosetting in the immunopathobiology and the development of drug resistance in human malaria. Here, we provide new perspectives on the formation and the role of rosetting in malaria rheopathobiology.

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.

Influence of α2-Macroglobulin, Anti-Parasite IgM and ABO Blood Group on Rosetting in Plasmodium falciparum Clinical Isolates and Their Associations with Disease Severity in a Ghanaian Population

Purpose

The severity of Plasmodium falciparum infections is associated with the ability of the infected red blood cells to cytoadhere to host vascular endothelial surfaces and to uninfected RBCs. Host blood group antigens and two serum proteins α₂-macroglobulin (α₂M) and IgM have been implicated in rosette formation in laboratory-adapted P. falciparum. However, there is only limited information about these phenotypes in clinical isolates.

Methods

This was a hospital-based study involving children under 12 years-of-age reporting to the Hohoe Municipal Hospital with different clinical presentations of malaria. Parasite isolates were grown and rosette capabilities and characteristics were investigated by fluorescence microscopy. α₂M and IgM were detected by ELISA.

Results

Rosette formation was observed in 46.8% (75/160) of the parasite isolates from all the blood groups tested. Rosettes were more prevalent (55%) among isolates from patients with severe malaria compared to isolates from patients with uncomplicated malaria (45%). Rosette prevalence was highest (30%) among patients with blood group O (30%) and B (29%), while the mean rosette frequency was higher in isolates from patients with blood group A (28.7). Rosette formation correlated negatively with age (r = −0.09, P= 0.008). Participants with severe malaria had a lower IgM concentration (3.683±3.553) than those with uncomplicated malaria (5.256±4.294) and the difference was significant (P= 0.0228). The mean concentrations of anti-parasite IgM measured among the clinical isolates which formed rosettes was lower (4.2 ±3.930 mg/mL), than that in the non rosetting clinical isolates (4.604 ±4.159 mg/mL) but the difference was not significant (P=0.2733). There was no significant difference in plasma α₂M concentration between rosetting and non rosetting isolates (P=0.442).

Conclusion

P. falciparum parasite rosette formation was affected by blood group type and plasma concentration of IgM. A lower IgM concentration was associated with severe malaria whilst a higher α₂M concentration was associated with uncomplicated malaria.

ABO blood group and risk of malaria during pregnancy: a systematic review and meta-analysis

The association between the ABO blood group and the risk of malaria during pregnancy has not been clearly established. The present study summarised relevant knowledge and reassessed the association through meta-analysis. Articles in MEDICINE and PubMed published before 30 November 2021 were searched. Five studies satisfied the inclusion criteria and were enrolled in the meta-analysis. It was shown that primiparae with different ABO blood group, multiparae with blood group A and non-A, AB and non-AB had a comparable risk of malaria. However, multiparae with blood group B had a significantly higher risk than non-B group [odds ratio (OR) = 1.23, 95% confidence interval (CI) was 1.01 to 1.50, P = 0.04], while multiparae with blood group O had a significantly lower risk than non-O group (OR = 0.78, 95% CI was 0.63 to 0.97, P = 0.03). Therefore, the ABO blood group may not result in a different risk of malaria in primiparae. Blood group B is potentially a risk factor while blood group O is a protective factor for multiparae.

Measuring Rosetting Inhibition in Plasmodium falciparum Parasites Using a Flow Cytometry-Based Assay

Rosetting is the ability of Plasmodium falciparum-infected erythrocytes (IEs) to bind to host receptors on the surface of uninfected erythrocytes (uE) leading to the formation of a cluster of cells with a central IE surrounded by uE. It is a hallmark event during the pathogenesis of P. falciparum malaria, the most severe species causing malaria, which affects mostly young children in Africa. There are no current treatments effectively targeting and disrupting parasite rosette formation. Here, we detail a high-throughput, flow cytometry based assay that allows testing and identification of potential rosetting-inhibitory compounds that could be used in combination with anti-plasmodial drugs to reduce malaria morbidity and mortality.

Red blood cell blood group A antigen level affects the ability of heparin and PfEMP1 antibodies to disrupt Plasmodium falciparum rosettes

Background

The histo-blood group ABO system has been associated with adverse outcomes in COVID-19, thromboembolic diseases and Plasmodium falciparum malaria. An integral part of the severe malaria pathogenesis is rosetting, the adherence of parasite infected red blood cells (RBCs) to uninfected RBCs. Rosetting is influenced by the host’s ABO blood group (Bg) and rosettes formed in BgA have previously been shown to be more resilient to disruption by heparin and shield the parasite derived surface antigens from antibodies. However, data on rosetting in weak BgA subgroups is scarce and based on investigations of relatively few donors.

Methods

An improved high-throughput flow cytometric assay was employed to investigate rosetting characteristics in an extensive panel of RBC donor samples of all four major ABO Bgs, as well as low BgA expressing samples.

Results

All non-O Bgs shield the parasite surface antigens from strain-specific antibodies towards P. falciparum erythrocyte membrane protein 1 (PfEMP1). A positive correlation between A-antigen levels on RBCs and rosette tightness was observed, protecting the rosettes from heparin- and antibody-mediated disruption.

Conclusions

These results provide new insights into how the ABO Bg system affects the disease outcome and cautions against interpreting the results from the heterogeneous BgA phenotype as a single group in epidemiological and experimental studies.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03975-w.

Structural basis for placental malaria mediated by Plasmodium falciparum VAR2CSA

Plasmodium falciparum VAR2CSA binds to chondroitin sulfate A (CSA) on the surface of the syncytiotrophoblast during placental malaria. This interaction facilitates placental sequestration of malaria parasites resulting in severe health outcomes for both the mother and her offspring. Furthermore, CSA is presented by diverse cancer cells and specific targeting of cells by VAR2CSA may become a viable approach for cancer treatment. In the present study, we determined the cryo-electron microscopy structures of the full-length ectodomain of VAR2CSA from P. falciparum strain NF54 in complex with CSA, and VAR2CSA from a second P. falciparum strain FCR3. The architecture of VAR2CSA is composed of a stable core flanked by a flexible arm. CSA traverses the core domain by binding within two channels and CSA binding does not induce major conformational changes in VAR2CSA. The CSA-binding elements are conserved across VAR2CSA variants and are flanked by polymorphic segments, suggesting immune selection outside the CSA-binding sites. This work provides paths for developing interventions against placental malaria and cancer.

Placental malaria vaccine candidate antigen VAR2CSA displays atypical domain architecture in some Plasmodium falciparum strains

Two vaccines based on Plasmodium falciparum protein VAR2CSA are currently in clinical evaluation to prevent placental malaria (PM), but a deeper understanding of var2csa variability could impact vaccine design. Here we identified atypical extended or truncated VAR2CSA extracellular structures and confirmed one extended structure in a Malian maternal isolate, using a novel protein fragment assembly method for RNA-seq and DNA-seq data. Extended structures included one or two additional DBL domains downstream of the conventional NTS-DBL1X-6ɛ domain structure, with closest similarity to DBLɛ in var2csa and non-var2csa genes. Overall, 4/82 isolates displayed atypical VAR2CSA structures. The maternal isolate expressing an extended VAR2CSA bound to CSA, but its recombinant VAR2CSA bound less well to CSA than VAR2CSANF54 and showed lower reactivity to naturally acquired parity-dependent antibody. Our protein fragment sequence assembly approach has revealed atypical VAR2CSA domain architectures that impact antigen reactivity and function, and should inform the design of VAR2CSA-based vaccines.

Rosetting revisited: a critical look at the evidence for host erythrocyte receptors in Plasmodium falciparum rosetting

Malaria remains a major cause of mortality in African children, with no adjunctive treatments currently available to ameliorate the severe clinical forms of the disease. Rosetting, the adhesion of infected erythrocytes (IEs) to uninfected erythrocytes, is a parasite phenotype strongly associated with severe malaria, and hence is a potential therapeutic target. However, the molecular mechanisms of rosetting are complex and involve multiple distinct receptor–ligand interactions, with some similarities to the diverse pathways involved in P. falciparum erythrocyte invasion. This review summarizes the current understanding of the molecular interactions that lead to rosette formation, with a particular focus on host uninfected erythrocyte receptors including the A and B blood group trisaccharides, complement receptor one, heparan sulphate, glycophorin A and glycophorin C. There is strong evidence supporting blood group A trisaccharides as rosetting receptors, but evidence for other molecules is incomplete and requires further study. It is likely that additional host erythrocyte rosetting receptors remain to be discovered. A rosette-disrupting low anti-coagulant heparin derivative is being investigated as an adjunctive therapy for severe malaria, and further research into the receptor–ligand interactions underlying rosetting may reveal additional therapeutic approaches to reduce the unacceptably high mortality rate of severe malaria.

Sticking for a Cause: The Falciparum Malaria Parasites Cytoadherence Paradigm

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

SMIM1 at a glance; discovery, genetic basis, recent progress and perspectives

Recent elucidation of the genetic basis of the Vel blood group system has offered the field of blood transfusion medicine an additional consideration in determining the causes of hemolytic reactions after a patient is transfused. The identification of the SMIM1 gene to be responsible for the Vel blood group allows molecular based tools to be developed to further dissect the function of this antigen. Genetic signatures such as the homozygous 17 bp deletion and the heterozygous 17 bp deletion in combination with other single nucleotide polymorphisms (SNPs) and insertion sequences regulate the expression level of the gene. With this knowledge, it is now possible to study this antigen in-depth.

Duffy binding-like 1α adhesin from Plasmodium falciparum recognizes ABH histo-blood group saccharide in a type specific manner

The ability of erythrocytes, infected by Plasmodium falciparum, to adhere to endothelial cells (cytoadherence) and to capture uninfected erythrocyte (rosetting) is the leading cause of death by severe malaria. Evidences link the binding of the adhesin Duffy Binding Like1-α (DBL1α) domain to the ABH histo-blood antigens with formation of rosettes. Inspired by this very close relationship between the disease susceptibility and individual blood type, here we investigate the structural requirements involved in the interaction of DBL1α with A, B and H histo-blood determinants and their subtypes. Our results evidence the high preference of DBL1α to A epitopes, in comparison to B and H epitopes. DBL1α interacts with ABH epitopes in subtype specific manner, presenting a remarkable affinity for type 2 structures, Fucα1-2Galβ1-4GlcNAcβ1, particularly the A2 epitope. The contacts made by DBL1α binding pocket and the ABH histo-blood groups were mapped by theoretical methods and supported by NMR experiments.

Shed EBA-175 mediates red blood cell clustering that enhances malaria parasite growth and enables immune evasion

Erythrocyte Binding Antigen of 175 kDa (EBA-175) has a well-defined role in binding to glycophorin A (GpA) during Plasmodium falciparum invasion of erythrocytes. However, EBA-175 is shed post invasion and a role for this shed protein has not been defined. We show that EBA-175 shed from parasites promotes clustering of RBCs, and EBA-175-dependent clusters occur in parasite culture. Region II of EBA-175 is sufficient for clustering RBCs in a GpA-dependent manner. These clusters are capable of forming under physiological flow conditions and across a range of concentrations. EBA-175-dependent RBC clustering provides daughter merozoites ready access to uninfected RBCs enhancing parasite growth. Clustering provides a general method to protect the invasion machinery from immune recognition and disruption as exemplified by protection from neutralizing antibodies that target AMA-1 and RH5. These findings provide a mechanistic framework for the role of shed proteins in RBC clustering, immune evasion, and malaria.

Effect of the ABO blood group on susceptibility to severe malaria: A systematic review and meta-analysis

Understanding how ABO blood group interacts with Plasmodium falciparum (P. falciparum) infection may facilitate development of antimalarial treatments and vaccines. This study systematically summarizes information on the relationship of ABO blood group with severe P. falciparum infection, level of parasitemia and haemoglobin. A total of 1923 articles were retrieved from five databases. After removal of duplicates, and two levels of screening, 21 articles were selected for inclusion in the meta-analysis. A meta-analysis of the studies showed an increased odds of severe P. falciparum infection among individuals with blood group A, B, AB or non-O compared with blood group O. However, the difference in the level of P. falciparum parasitemia was not significant among individuals with blood group A or non-O compared with blood group O. The difference in haemoglobin level among P. falciparum infected individuals was also not significant between those with blood group A, B or AB versus those with blood group O.

Host immune evasion strategies of malaria blood stage parasite

Host immune evasion is a key strategy for the continual survival of many microbial pathogens including Apicomplexan protozoan: Plasmodium spp., the causative agent of Malaria. The malaria parasite has evolved a variety of mechanisms to evade the host immune responses within its two hosts: the female Anopheles mosquito vector and vertebrate host. In this review, we will focus on the molecular mechanisms of the immune evasion strategies used by the Plasmodium parasite at the blood stage which is responsible for the clinical manifestations of human malaria. We also aim to provide some insights on the potential targets for malaria interventions through the recent advancement in understanding the molecular biology of the parasite.

Position of human blood group O(H) and phenotype-determining enzymes in growth and infectious disease

The human ABO(H) blood group phenotypes arise from the evolutionarily oldest genetic system found in primate populations. While the blood group antigen A is considered the ancestral primordial structure, under the selective pressure of life‐threatening diseases blood group O(H) came to dominate as the most frequently occurring blood group worldwide. Non‐O(H) phenotypes demonstrate impaired formation of adaptive and innate immunoglobulin specificities due to clonal selection and phenotype formation in plasma proteins. Compared with individuals with blood group O(H), blood group A individuals not only have a significantly higher risk of developing certain types of cancer but also exhibit high susceptibility to malaria tropica or infection by Plasmodium falciparum. The phenotype‐determining blood group A glycotransferase(s), which affect the levels of anti‐A/Tn cross‐reactive immunoglobulins in phenotypic glycosidic accommodation, might also mediate adhesion and entry of the parasite to host cells via trans‐species O‐GalNAc glycosylation of abundantly expressed serine residues that arise throughout the parasite's life cycle, while excluding the possibility of antibody formation against the resulting hybrid Tn antigen. In contrast, human blood group O(H), lacking this enzyme, is indicated to confer a survival advantage regarding the overall risk of developing cancer, and individuals with this blood group rarely develop life‐threatening infections involving evolutionarily selective malaria strains.

The Gerbich blood group system: old knowledge, new importance

Antigens of the Gerbich blood group system are expressed on glycophorin C (GPC) and glycophorin D (GPD), minor sialoglycoproteins of human erythrocytes. GPC and GPD help maintain erythrocyte shape of and contributes to the stability of its membrane. There are six high-prevalence Gerbich antigens: Ge2, Ge3, Ge4, GEPL (GE10), GEAT (GE11), GETI (GE12) and five low-prevalence Gerbich antigens: Wb (GE5), Ls^a (GE6), An^a (GE7), Dh^a (GE8), GEIS (GE9). Some Gerbich antigens (Ge4, Wb, Dh^a, GEAT) are expressed only on GPC, two (Ge2, An^a) are expressed only on GPD, while others (Ge3, Ls^a, GEIS, GEPL, GETI) are expressed on both GPC and GPD. Antibodies recognizing GPC/GPD may arise naturally (so-called "naturally-occurring RBC antibodies") or as the result of alloimmunization, and some of them may be clinically relevant. Gerbich antibodies usually do not cause serious hemolytic transfusion reactions (HTR); autoantibodies of anti-Ge2- or anti-Ge3 specificity can cause autoimmune hemolytic anemia (AIHA).

Antibodies in children with malaria to PfEMP1, RIFIN and SURFIN expressed at the Plasmodium falciparum parasitized red blood cell surface

Naturally acquired antibodies to proteins expressed on the Plasmodium falciparum parasitized red blood cell (pRBC) surface steer the course of a malaria infection by reducing sequestration and stimulating phagocytosis of pRBC. Here we have studied a selection of proteins representing three different parasite gene families employing a well-characterized parasite with a severe malaria phenotype (FCR3S1.2). The presence of naturally acquired antibodies, impact on rosetting rate, surface reactivity and opsonization for phagocytosis in relation to different blood groups of the ABO system were assessed in a set of sera from children with mild or complicated malaria from an endemic area. We show that the naturally acquired immune responses, developed during malaria natural infection, have limited access to the pRBCs inside a blood group A rosette. The data also indicate that SURFIN_4.2 may have a function at the pRBC surface, particularly during rosette formation, this role however needs to be further validated. Our results also indicate epitopes differentially recognized by rosette-disrupting antibodies on a peptide array. Antibodies towards parasite-derived proteins such as PfEMP1, RIFIN and SURFIN in combination with host factors, essentially the ABO blood group of a malaria patient, are suggested to determine the outcome of a malaria infection.

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.

Histo-blood group antigens as mediators of infections

The critical first step of a microbial infection is usually the attachment of pathogens to host cell glycans. Targets on host tissues are in particular the histo-blood group antigens (HBGAs), which are present in rich diversity in the mucus layer and on the underlying mucosa. Recent structural and functional studies have revealed significant new insight into the molecular mechanisms, explaining why individuals with certain blood groups are at increased risk of some infections. The most prominent example of blood-group-associated diseases is cholera, caused by infection with Vibrio cholerae. Many other microbial pathogens, for example Pseudomonas aeruginosa infecting the airways, and enterotoxigenic Escherichia coli (ETEC) causing traveler's diarrhea, also bind to histo-blood group antigens, but show a less clear correlation with blood group phenotype. Yet other pathogens, for example norovirus and Helicobacter pylori, recognize HBGAs differently depending on the strain. In all cases, milk oligosaccharides can aid the hosts' defenses, acting as natural receptor decoys, and anti-infectious therapy can be designed along similar strategies. In this review, we focus on important infections of humans, but the molecular mechanisms are of general relevance to a broad range of microbial infections of humans and animals.

The structural basis for CD36 binding by the malaria parasite

CD36 is a scavenger receptor involved in fatty acid metabolism, innate immunity and angiogenesis. It interacts with lipoprotein particles and facilitates uptake of long chain fatty acids. It is also the most common target of the PfEMP1 proteins of the malaria parasite, Plasmodium falciparum, tethering parasite-infected erythrocytes to endothelial receptors. This prevents their destruction by splenic clearance and allows increased parasitaemia. Here we describe the structure of CD36 in complex with long chain fatty acids and a CD36-binding PfEMP1 protein domain. A conserved hydrophobic pocket allows the hugely diverse PfEMP1 protein family to bind to a conserved phenylalanine residue at the membrane distal tip of CD36. This phenylalanine is also required for CD36 to interact with lipoprotein particles. By targeting a site on CD36 that is required for its physiological function, PfEMP1 proteins maintain the ability to tether to the endothelium and avoid splenic clearance.

Targeting of the CD36 scavenger receptor by the malaria parasite effector PfEMP1 prevents splenic clearance of infected erythrocytes. Here, the authors propose that diverse PfEMP1 achieve this by binding to a conserved phenylalanine residue in CD36 that is also required for lipoprotein binding.

Functional analysis of monoclonal antibodies against the Plasmodium falciparum PfEMP1-VarO adhesin

Background

Rosetting, namely the capacity of the Plasmodium falciparum-infected red blood cells to bind uninfected RBCs, is commonly observed in African children with severe malaria. Rosetting results from specific interactions between a subset of variant P. falciparum erythrocyte membrane protein 1 (PfEMP1) adhesins encoded by var genes, serum components and RBC receptors. Rosette formation is a redundant phenotype, as there exists more than one var gene encoding a rosette-mediating PfEMP1 in each genome and hence a diverse array of underlying interactions. Moreover, field diversity creates a large panel of rosetting-associated serotypes and studies with human immune sera indicate that surface-reacting antibodies are essentially variant-specific. To gain better insight into the interactions involved in rosetting and map surface epitopes, a panel of monoclonal antibodies (mAbs) was investigated.

Methods

Monoclonal antibodies were isolated from mice immunized with PfEMP1-VarO recombinant domains. They were characterized using ELISA and reactivity with the native PfEMP1-VarO adhesin on immunoblots of reduced and unreduced extracts, as well as SDS-extracts of Palo Alto 89F5 VarO schizonts. Functionality was assessed using inhibition of Palo Alto 89F5 VarO rosette formation and disruption of Palo Alto 89F5 VarO rosettes. Competition ELISAs were performed with biotinylated antibodies against DBL1 to identify reactivity groups. Specificity of mAbs reacting with the DBL1 adhesion domain was explored using recombinant proteins carrying mutations abolishing RBC binding or binding to heparin, a potent inhibitor of rosette formation.

Results

Domain-specific, surface-reacting mAbs were obtained for four individual domains (DBL1, CIDR1, DBL2, DBL4). Monoclonal antibodies reacting with DBL1 potently inhibited the formation of rosettes and disrupted Palo Alto 89F5 VarO rosettes. Most surface-reactive mAbs and all mAbs interfering with rosetting reacted on parasite immunoblots with disulfide bond-dependent PfEMP1 epitopes. Based on competition ELISA and binding to mutant DBL1 domains, two distinct binding sites for rosette-disrupting mAbs were identified in close proximity to the RBC-binding site.

Conclusions

Rosette-inhibitory antibodies bind to conformation-dependent epitopes located close to the RBC-binding site and distant from the heparin-binding site. These results provide novel clues for a rational intervention strategy that targets rosetting.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-1016-5) contains supplementary material, which is available to authorized users.

Architecture of Human IgM in Complex with P. falciparum Erythrocyte Membrane Protein 1

Plasmodium falciparum virulence is associated with sequestration of infected erythrocytes. Microvascular binding mediated by PfEMP1 in complex with non-immune immunoglobulin M (IgM) is common among parasites that cause both severe childhood malaria and pregnancy-associated malaria. Here, we present cryo-molecular electron tomography structures of human IgM, PfEMP1 and their complex. Three-dimensional reconstructions of IgM reveal that it has a dome-like core, randomly oriented Fab2s units, and the overall shape of a turtle. PfEMP1 is a C- shaped molecule with a flexible N terminus followed by an arc-shaped backbone and a bulky C terminus that interacts with IgM. Our data demonstrate that the PfEMP1 binding pockets on IgM overlap with those of C1q, and the bulkiness of PfEMP1 limits the capacity of IgM to interact with PfEMP1. We suggest that P. falciparum exploits IgM to cluster PfEMP1 into an organized matrix to augment its affinity to host cell 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.

Evasion of Immunity to Plasmodium falciparum: Rosettes of Blood Group A Impair Recognition of PfEMP1

The ABO blood group antigens are expressed on erythrocytes but also on endothelial cells, platelets and serum proteins. Notably, the ABO blood group of a malaria patient determines the development of the disease given that blood group O reduces the probability to succumb in severe malaria, compared to individuals of groups A, B or AB. P. falciparum rosetting and sequestration are mediated by PfEMP1, RIFIN and STEVOR, expressed at the surface of the parasitized red blood cell (pRBC). Antibodies to these antigens consequently modify the course of a malaria infection by preventing sequestration and promoting phagocytosis of pRBC. Here we have studied rosetting P. falciparum and present evidence of an immune evasion mechanism not previously recognized. We find the accessibility of antibodies to PfEMP1 at the surface of the pRBC to be reduced when P. falciparum forms rosettes in blood group A RBC, as compared to group O RBC. The pRBC surrounds itself with tightly bound normal RBC that makes PfEMP1 inaccessible to antibodies and clearance by the immune system. Accordingly, pRBC of in vitro cloned P. falciparum devoid of ABO blood group dependent rosetting were equally well detected by anti-PfEMP1 antibodies, independent of the blood group utilized for their propagation. The pathogenic mechanisms underlying the severe forms of malaria may in patients of blood group A depend on the ability of the parasite to mask PfEMP1 from antibody recognition, in so doing evading immune clearance.

Immunoglobulin M: Restrainer of Inflammation and Mediator of Immune Evasion by Plasmodium falciparum Malaria

Immunoglobulin M (IgM) is an ancient antibody class that is found in all vertebrates, with the exception of coelacanths, and is indispensable in both innate and adaptive immunity. The equally ancient human malaria parasite, Plasmodium falciparum, formed an intimate relationship with IgM with which it co-evolved. In this article, we discuss the association between IgM and human malaria parasites, building on several recent publications that implicate IgM as a crucial molecule that determines both host and parasite survival. Consequently, a better understanding of this association may lead to the development of improved intervention strategies.

Immunogenicity of the Plasmodium falciparum PfEMP1-VarO Adhesin: Induction of Surface-Reactive and Rosette-Disrupting Antibodies to VarO Infected Erythrocytes

Adhesion of Plasmodium falciparum-infected red blood cells (iRBC) to human erythrocytes (i.e. rosetting) is associated with severe malaria. Rosetting results from interactions between a subset of variant PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) adhesins and specific erythrocyte receptors. Interfering with such interactions is considered a promising intervention against severe malaria. To evaluate the feasibility of a vaccine strategy targetting rosetting, we have used here the Palo Alto 89F5 VarO rosetting model. PfEMP1-VarO consists of five Duffy-Binding Like domains (DBL_1-5) and one Cysteine-rich Interdomain Region (CIDR₁). The binding domain has been mapped to DBL1 and the ABO blood group was identified as the erythrocyte receptor. Here, we study the immunogenicity of all six recombinant PfEMP1-VarO domains and the DBL1- CIDR1 Head domain in BALB/c and outbred OF1 mice. Five readouts of antibody responses are explored: ELISA titres on the recombinant antigen, VarO-iRBC immunoblot reactivity, VarO-iRBC surface-reactivity, capacity to disrupt VarO rosettes and the capacity to prevent VarO rosette formation. For three domains, we explore influence of the expression system on antigenicity and immunogenicity. We show that correctly folded PfEMP1 domains elicit high antibody titres and induce a homogeneous response in outbred and BALB/c mice after three injections. High levels of rosette-disrupting and rosette-preventing antibodies are induced by DBL1 and the Head domain. Reduced-alkylated or denatured proteins fail to induce surface-reacting and rosette-disrupting antibodies, indicating that surface epitopes are conformational. We also report limited cross-reactivity between some PfEMP1 VarO domains. These results highlight the high immunogenicity of the individual domains in outbred animals and provide a strong basis for a rational vaccination strategy targeting rosetting.

Multiple Plasmodium falciparum Erythrocyte Membrane Protein 1 Variants per Genome Can Bind IgM via Its Fc Fragment Fcμ

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) adhesive proteins expressed on the surfaces of infected erythrocytes (IEs) are of key importance in the pathogenesis of P. falciparum malaria. Several structurally and functionally defined PfEMP1 types have been associated with severe clinical manifestations, such as cerebral malaria in children and placental malaria in pregnant women. PfEMP1 that can bind the Fc part of IgM (Fcμ) characterizes one such type, although the functional significance of this IgM binding to PfEMP1 remains unclear. In this study, we report the identification and functional analysis of five IgM-binding PfEMP1 proteins encoded by P. falciparum NF54. In addition to the VAR2CSA-type PFL0030c protein, already known to bind Fcμ and to mediate chondroitin sulfate A (CSA)-specific adhesion of IEs in the placenta, we found four PfEMP1 proteins not previously known to bind IgM this way. Although they all contained Duffy binding-like ε (DBLε) domains similar to those in VAR2CSA-type PfEMP1, they did not mediate IE adhesion to CSA, and IgM binding did not shield IEs from phagocytosis of IgG-opsonized IEs. In this way, these new IgM-binding PfEMP1 proteins resemble the rosette-mediating and IgM-binding PfEMP1 HB3VAR06, but none of them mediated formation of rosettes. We could map the capacity for Fc-specific IgM binding to DBLε domains near the C terminus for three of the four PfEMP1 proteins tested. Our study provides new evidence regarding Fc-dependent binding of IgM to PfEMP1, which appears to be a common and multifunctional phenotype.

α2-Macroglobulin Can Crosslink Multiple Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) Molecules and May Facilitate Adhesion of Parasitized Erythrocytes

Rosetting, the adhesion of Plasmodium falciparum-infected erythrocytes to uninfected erythrocytes, involves clonal variants of the parasite protein P. falciparum erythrocyte membrane protein 1 (PfEMP1) and soluble serum factors. While rosetting is a well-known phenotypic marker of parasites associated with severe malaria, the reason for this association remains unclear, as do the molecular details of the interaction between the infected erythrocyte (IE) and the adhering erythrocytes. Here, we identify for the first time a single serum factor, the abundant serum protease inhibitor α2-macroglobulin (α2M), which is both required and sufficient for rosetting mediated by the PfEMP1 protein HB3VAR06 and some other rosette-mediating PfEMP1 proteins. We map the α2M binding site to the C terminal end of HB3VAR06, and demonstrate that α2M can bind at least four HB3VAR06 proteins, plausibly augmenting their combined avidity for host receptors. IgM has previously been identified as a rosette-facilitating soluble factor that acts in a similar way, but it cannot induce rosetting on its own. This is in contrast to α2M and probably due to the more limited cross-linking potential of IgM. Nevertheless, we show that IgM works synergistically with α2M and markedly lowers the concentration of α2M required for rosetting. Finally, HB3VAR06+ IEs share the capacity to bind α2M with subsets of genotypically distinct P. falciparum isolates forming rosettes in vitro and of patient parasite isolates ex vivo. Together, our results are evidence that P. falciparum parasites exploit α2M (and IgM) to expand the repertoire of host receptors available for PfEMP1-mediated IE adhesion, such as the erythrocyte carbohydrate moieties that lead to formation of rosettes. It is likely that this mechanism also affects IE adhesion to receptors on vascular endothelium. The study opens opportunities for broad-ranging immunological interventions targeting the α2M--(and IgM-) binding domains of PfEMP1, which would be independent of the host receptor specificity of clinically important PfEMP1 antigens.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

Rifins, rosetting, and red blood cells

The binding of multiple uninfected erythrocytes to a central malaria parasite-infected erythrocyte (IE) is called rosetting. Rosetting has been associated with severe disease, but its functional significance,and the host receptors and parasite ligands involved are only partially known. A recent study, which describes yet another piece in this already complex puzzle, provides a welcome boost and a broadening of an important malaria research field.

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.

RIFINs are adhesins implicated in severe Plasmodium falciparum malaria

Rosetting is a virulent Plasmodium falciparum phenomenon associated with severe malaria. Here we demonstrate that P. falciparum-encoded repetitive interspersed families of polypeptides (RIFINs) are expressed on the surface of infected red blood cells (iRBCs), where they bind to RBCs--preferentially of blood group A--to form large rosettes and mediate microvascular binding of iRBCs. We suggest that RIFINs have a fundamental role in the development of severe malaria and thereby contribute to the varying global distribution of ABO blood groups in the human population.

Binding of subdomains 1/2 of PfEMP1-DBL1α to heparan sulfate or heparin mediates Plasmodium falciparum rosetting

The capacity of Plasmodium falciparum parasitized erythrocytes (pRBC) to adhere to the endothelial lining in the microvasculature and to red blood cells (RBC) is associated with the virulence of the parasite, the pathogenesis and development of severe malaria. Rosetting, the binding of uninfected RBC to pRBC, is frequently observed in individuals with severe malaria and is mediated by the N-terminal NTS-DBL1α domain of the adhesin Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed at the surface of the pRBC. Heparan sulfate has been suggested to be an important receptor for the NTS-DBL1α variant IT4_var60 expressed by the parasite FCR3S1.2. Here, we have determined the binding site of NTS-DBL1α (IT4_var60) to the RBC and heparin using a set of recombinant, mutated proteins expressed in and purified from E. coli. All the variants were studied for their ability to bind to RBC, their capacities to disrupt FCR3S1.2 rosettes, their affinities for heparin and their binding to rosette-disruptive mAbs. Our results suggest that NTS-DBL1α mediates binding to RBC through a limited number of basic amino acid residues localized on the surface of subdomains 1 (SD1) and 2 (SD2). The SD2-binding site is localized in close proximity to one of two previously identified binding sites in the rosetting PfEMP1 of the parasite PaloAlto-varO. The binding site in SD2 of NTS-DBL1α could represent a template for the development of anti-rosetting drugs.

Investigating the function of Fc-specific binding of IgM to Plasmodium falciparum erythrocyte membrane protein 1 mediating erythrocyte rosetting

Acquired protection from Plasmodium falciparum malaria takes years to develop, probably reflecting the ability of the parasites to evade immunity. A recent example of this is the binding of the F_c region of IgM to VAR2CSA‐type PfEMP1. This interferes with specific IgG recognition and phagocytosis of opsonized infected erythrocytes (IEs) without compromising the placental IE adhesion mediated by this PfEMP1 type. IgM also binds via F_c to several other PfEMP1 proteins, where it has been proposed to facilitate rosetting (binding of uninfected erythrocytes to a central IE). To further dissect the functional role of F_c‐mediated IgM binding to PfEMP1, we studied the PfEMP1 protein HB3VAR06, which mediates rosetting and binds IgM. Binding of IgM to this PfEMP1 involved the F_c domains Cμ3‐Cμ4 in IgM and the penultimate DBL domain (DBLζ2) at the C‐terminus of HB3VAR06. However, IgM binding did not inhibit specific IgG labelling of HB3VAR06 or shield IgG‐opsonized IEs from phagocytosis. Instead, IgM was required for rosetting, and each pentameric IgM molecule could bind two HB3VAR06 molecules. Together, our data indicate that the primary function of F_c‐mediated IgM binding in rosetting is not to shield IE from specific IgG recognition and phagocytosis as in VAR2CSA‐type PfEMP1. Rather, the function appears to be strengthening of IE–erythrocyte interactions. In conclusion, our study provides new evidence on the molecular details and functional significance of rosetting, a long‐recognized marker of parasites that cause severe P. falciparum malaria.

Structural conservation despite huge sequence diversity allows EPCR binding by the PfEMP1 family implicated in severe childhood malaria

The PfEMP1 family of surface proteins is central for Plasmodium falciparum virulence and must retain the ability to bind to host receptors while also diversifying to aid immune evasion. The interaction between CIDRα1 domains of PfEMP1 and endothelial protein C receptor (EPCR) is associated with severe childhood malaria. We combine crystal structures of CIDRα1:EPCR complexes with analysis of 885 CIDRα1 sequences, showing that the EPCR-binding surfaces of CIDRα1 domains are conserved in shape and bonding potential, despite dramatic sequence diversity. Additionally, these domains mimic features of the natural EPCR ligand and can block this ligand interaction. Using peptides corresponding to the EPCR-binding region, antibodies can be purified from individuals in malaria-endemic regions that block EPCR binding of diverse CIDRα1 variants. This highlights the extent to which such a surface protein family can diversify while maintaining ligand-binding capacity and identifies features that should be mimicked in immunogens to prevent EPCR binding.

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EPCR binding is retained by PfEMP1 CIDRα1 domains despite huge sequence variation

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Diverse CIDRα1 domains retain structural and chemical features to bind to EPCR

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CIDRα1 domains mimic features of a natural ligand of EPCR and block its binding

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Patient sera contain neutralizing antibodies that prevent parasite binding to EPCR

B-cell epitopes in NTS-DBL1α of PfEMP1 recognized by human antibodies in Rosetting Plasmodium falciparum

Plasmodium falciparum is the most lethal of the human malaria parasites. The virulence is associated with the capacity of the infected red blood cell (iRBC) to sequester inside the deep microvasculature where it may cause obstruction of the blood-flow when binding is excessive. Rosetting, the adherence of the iRBC to uninfected erythrocytes, has been found associated with severe malaria and found to be mediated by the NTS-DBL1α-domain of Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1). Here we show that the reactivity of plasma of Cameroonian children with the surface of the FCR3S1.2-iRBC correlated with the capacity to disrupt rosettes and with the antibody reactivity with a recombinant PfEMP1 (NTS-DBL1α of IT4_var60) expressed by parasite FCR3S1.2. The plasma-reactivity in a microarray, consisting of 96 overlapping 15-mer long peptides covering the NTS-DBL1α domain from IT4var60 sequence, was compared with their capacity to disrupt rosettes and we identified five peptides where the reactivity were correlated. Three of the peptides were localized in subdomain-1 and 2. The other two peptide-sequences were localized in the NTS-domain and in subdomain-3. Further, principal component analysis and orthogonal partial least square analysis generated a model that supported these findings. In conclusion, human antibody reactivity with short linear-peptides of NTS-DBL1α of PfEMP1 suggests subdomains 1 and 2 to hold anti-rosetting epitopes recognized by anti-rosetting antibodies. The data suggest rosetting to be mediated by the variable areas of PfEMP1 but also to involve structurally relatively conserved areas of the molecule that may induce biologically active antibodies.

Cytoadherence phenotype of Plasmodium falciparum-infected erythrocytes is associated with specific pfemp-1 expression in parasites from children with cerebral malaria

Background

Cytoadherence of Plasmodium falciparum-infected erythrocytes (IEs) in deep microvasculature endothelia plays a major role in the pathogenesis of cerebral malaria (CM). This biological process is thought to be mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP-1) and human receptors such as CD36 and ICAM-1. The relationship between the expression of PfEMP-1 variants and cytoadherence phenotype in the pathology of malaria is not well established.

Methods

Cytoadherence phenotypes of IEs to CD36, ICAM-1, CSPG and the transcription patterns of A, B, var2csa, var3, var gene groups and domain cassettes DC8 and DC13 were assessed in parasites from children with CM and uncomplicated malaria (UM) to determine if cytoadherence is related to a specific transcription profile of pfemp-1 variants.

Results

Parasites from CM patients bind significantly more to CD36 than those from UM patients, but no difference was observed in their binding ability to ICAM-1 and CSPG. CM isolates highly transcribed groups A, B, var2csa, var3, DC8 and DC13 compared to UM parasites. The high transcription levels of var genes belonging to group B positively correlated with increased binding level to CD36.

Conclusion

CM isolates bind significantly more to CD36 than to ICAM-1, which was correlated with high transcription level of group B var genes, supporting their implication in malaria pathogenesis.

Glycophorin C (CD236R) mediates vivax malaria parasite rosetting to normocytes

Rosetting phenomenon has been linked to malaria pathogenesis. Although rosetting occurs in all causes of human malaria, most data on this subject has been derived from Plasmodium falciparum. Here, we investigate the function and factors affecting rosette formation in Plasmodium vivax. To achieve this, we used a range of novel ex vivo protocols to study fresh and cryopreserved P vivax (n = 135) and P falciparum (n = 77) isolates from Thailand. Rosetting is more common in vivax than falciparum malaria, both in terms of incidence in patient samples and percentage of infected erythrocytes forming rosettes. Rosetting to P vivax asexual and sexual stages was evident 20 hours postreticulocyte invasion, reaching a plateau after 30 hours. Host ABO blood group, reticulocyte count, and parasitemia were not correlated with P vivax rosetting. Importantly, mature erythrocytes (normocytes), rather than reticulocytes, preferentially form rosetting complexes, indicating that this process is unlikely to directly facilitate merozoite invasion. Although antibodies against host erythrocyte receptors CD235a and CD35 had no effect, Ag-binding fragment against the BRIC 4 region of CD236R significantly inhibited rosette formation. Rosetting assays using CD236R knockdown normocytes derived from hematopoietic stem cells further supports the role of glycophorin C as a receptor in P vivax rosette formation.