A novel erythrocyte binding antigen-175 paralogue from Plasmodium falciparum defines a new trypsin-resistant receptor on human erythrocytes

Malian field isolates provide insight into Plasmodium malariae intra-erythrocytic development and invasion

Plasmodium malariae is the third most prevalent human malaria parasite species and contributes significantly to morbidity. Nevertheless, our comprehension of this parasite's biology remains limited, primarily due to its frequent co-infections with other species and the lack of a continuous in vitro culture system. To effectively combat and eliminate this overlooked parasite, it is imperative to acquire a better understanding of this species. In this study, we embarked on an investigation of P. malariae, including exploring its clinical disease characteristics, molecular aspects of red blood cell (RBC) invasion, and host-cell preferences. We conducted our research using parasites collected from infected individuals in Mali. Our findings revealed anaemia in most of P. malariae infected participants presented, in both symptomatic and asymptomatic cases. Regarding RBC invasion, quantified by an adapted flow cytometry based method, our study indicated that none of the seven antibodies tested, against receptors known for their role in P. falciparum invasion, had any impact on the ability of P. malariae to penetrate the host cells. However, when RBCs were pre-treated with various enzymes (neuraminidase, trypsin, and chymotrypsin), we observed a significant reduction in P. malariae invasion, albeit not a complete blockade. Furthermore, in a subset of P. malariae samples, we observed the parasite's capability to invade reticulocytes. These results suggest that P. malariae employs alternative pathways to enter RBCs of different maturities, which may differ from those used by P. falciparum.

Mass Spectrometric and Artificial Intelligence-Based Identification of the Secretome of Plasmodium falciparum Merozoites to Provide Novel Candidates for Vaccine Development Pipeline

PURPOSE

Merozoites are the only extracellular form of blood stage parasites, making it a worthwhile target. Multiple invasins that are stored in the merozoite apical organelles, are secreted just prior to invasion, and mediates its interaction with RBC. A comprehensive identification of all these secreted invasins is lacking and this study addresses that gap.

EXPERIMENTAL DESIGN

Pf3D7 merozoites were enriched and triggered to discharge apical organelle contents by exposure to ionic conditions mimicking that of blood plasma. The secreted proteins were separated from cellular contents and both the fractions were subjected to proteomic analysis. Also, the identified secreted proteins were subjected to GO, PPI network analysis, and AI-based in silico approach to understand their vaccine candidacy.

RESULTS

A total of 63 proteins were identified in the secretory fraction with membrane and apical organellar localization. This includes various MSPs, micronemal EBAs and rhoptry bulb proteins, which play a crucial role in initial and late merozoite attachment, and majority of them qualified as vaccine candidates.

CONCLUSION AND CLINICAL RELEVANCE

We, for the first time, report the secretory repertoire of merozoite and its status for vaccine candidacy. This information can be utilized to develop better invasion blocking multisubunit vaccines, comprising of immunological epitopes from several secreted invasins.

Transcriptome analysis reveals molecular targets of erythrocyte invasion phenotype diversity in natural Plasmodium falciparum isolates from Cameroon

Further understanding of the molecular mediators of alternative RBC invasion phenotypes in endemic malaria parasites will support malaria blood-stage vaccine or drug development. This study investigated the prevalence of sialic acid (SA)-dependent and SA-independent RBC invasion pathways in endemic Plasmodium falciparum parasites from Cameroon and compared the schizont stage transcriptomes in these two groups to uncover the wider repertoire of transcriptional variation associated with the use of alternative RBC invasion pathway phenotypes. A two-color flow cytometry-based invasion-inhibition assay against RBCs treated with neuraminidase, trypsin, and chymotrypsin and deep RNA sequencing of schizont stages harvested in the first ex vivo replication cycle in culture were employed in this investigation. RBC invasion phenotypes were determined for 63 isolates from asymptomatic children with uncomplicated malaria. Approximately 80% of the isolates invaded neuraminidase-treated but not chymotrypsin-treated RBCs, representing SA-independent pathways of RBC invasion. The schizont transcriptome profiles of 16 isolates with invasion phenotypes revealed a total of 5,136 gene transcripts, with 85% of isolates predicted at schizont stages. Two distinct transcriptome profile clusters belonging to SA-dependent and SA-independent parasites were obtained by data reduction with principal component analysis. Differential analysis of gene expression between the two clusters implicated, in addition to the well-characterized adhesins, the upregulation of genes encoding proteins mediating merozoite organelle discharges as well as several conserved, virulent, merozoite-associated, and exported proteins. The latter majority have been shown to have structural and physiological relevance to RBC surface remodeling and immune evasion in malaria and thus have potential as anti-invasion targets.

Recent increase in low complexity polygenomic infections and sialic acid-independent invasion pathways in Plasmodium falciparum from Western Gambia

BACKGROUND

The malaria parasite Plasmodium falciparum utilizes multiple alternative receptor-ligand interactions for the invasion of human erythrocytes. While some P. falciparum clones make use of sialic acid (SA) residues on the surface of the human glycophorin receptors to invade the erythrocyte, others use alternative receptors independent of sialic acid residues. We hypothesized that over the years, intensified malaria control interventions and declining prevalence in The Gambia have resulted in a selection of parasites with a dominant invasion pathways and ligand expression profiles.

METHODS

Blood samples were collected from 65 malaria-infected participants with uncomplicated malaria across 3 years (2015, 2016, and 2021). Genetic diversity was determined by genotyping the merozoite surface protein 2 (msp2) polymorphic gene of P. falciparum. Erythrocyte invasion phenotypes were determined using neuraminidase, trypsin, and chymotrypsin enzymes, known to cleave different receptors from the surface of the erythrocyte. Schizont-stage transcript levels were obtained for a panel of 6 P. falciparum invasion ligand genes (eba175, eba181, Rh2b, Rh4, Rh5, and clag2) using 48 successfully cultured isolates.

RESULTS

Though the allelic heterozygosity of msp2 repeat region decreased as expected with reduced transmission, there was an increase in infections with more than a single msp2 allelotype from 2015 to 2021. The invasion phenotypes of these isolates were mostly SA independent with a continuous increase from 2015 to 2021. Isolates from 2021 were highly inhibited by chymotrypsin treatment compared to isolates from 2015 and 2016. Higher invasion inhibition for 2021 isolates was further obtained following erythrocyte treatment with a combination of chymotrypsin and trypsin. The transcript levels of invasion ligand genes varied across years. However, levels of clag2, a rhoptry-associated protein, were higher in 2015 and 2016 isolates than in 2021 isolates, while Rh5 levels were higher in 2021 compared to other years.

CONCLUSIONS

Overall, these findings suggest increasing mixed infections with an increase in the use of sialic-acid independent invasion pathways by P. falciparum clinical isolates in the Western part of Gambia.

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.

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.

Sialic Acids as Receptors for Pathogens

Carbohydrates have long been known to mediate intracellular interactions, whether within one organism or between different organisms. Sialic acids (Sias) are carbohydrates that usually occupy the terminal positions in longer carbohydrate chains, which makes them common recognition targets mediating these interactions. In this review, we summarize the knowledge about animal disease-causing agents such as viruses, bacteria and protozoa (including the malaria parasite Plasmodium falciparum) in which Sias play a role in infection biology. While Sias may promote binding of, e.g., influenza viruses and SV40, they act as decoys for betacoronaviruses. The presence of two common forms of Sias, Neu5Ac and Neu5Gc, is species-specific, and in humans, the enzyme converting Neu5Ac to Neu5Gc (CMAH, CMP-Neu5Ac hydroxylase) is lost, most likely due to adaptation to pathogen regimes; we discuss the research about the influence of malaria on this trait. In addition, we present data suggesting the CMAH gene was probably present in the ancestor of animals, shedding light on its glycobiology. We predict that a better understanding of the role of Sias in disease vectors would lead to more effective clinical interventions.

Biogenesis and discharge of the rhoptries: Key organelles for entry and hijack of host cells by the Apicomplexa

Rhoptries are specialized secretory organelles found in the Apicomplexa phylum, playing a central role in the establishment of parasitism. The rhoptry content includes membranous as well as proteinaceous materials that are discharged into the host cell in a regulated fashion during parasite entry. A set of rhoptry neck proteins form a RON complex that critically participates in the moving junction formation during invasion. Some of the rhoptry bulb proteins are associated with the membranous materials and contribute to the formation of the parasitophorous vacuole membrane while others are targeted into the host cell including the nucleus to subvert cellular functions. Here, we review the recent studies on Toxoplasma and Plasmodium parasites that shed light on the key steps leading to rhoptry biogenesis, trafficking, and discharge.

Rab5b-Associated Arf1 GTPase Regulates Export of N-Myristoylated Adenylate Kinase 2 From the Endoplasmic Reticulum in Plasmodium falciparum

Plasmodium falciparum extensively remodels human erythrocytes by exporting hundreds of parasite proteins. This remodeling is closely linked to the Plasmodium virulence-related functions and immune evasion. The N-terminal export signal named PEXEL (Plasmodium export element) was identified to be important for the export of proteins beyond the PVM, however, the issue of how these PEXEL-positive proteins are transported and regulated by Rab GTPases from the endoplasmic reticulum (ER) to the cell surface has remained poorly understood. Previously, we identified new aspects of the trafficking of N-myristoylated adenylate kinase 2 (PfAK2), which lacks the PEXEL motif and is regulated by the PfRab5b GTPase. Overexpression of PfRab5b suppressed the transport of PfAK2 to the parasitophorous vacuole membrane and PfAK2 was accumulated in the punctate compartment within the parasite. Here, we report the identification of PfRab5b associated proteins and dissect the pathway regulated by PfRab5b. We isolated two membrane trafficking GTPases PfArf1 and PfRab1b by coimmunoprecipitation with PfRab5b and via mass analysis. PfArf1 and PfRab1b are both colocalized with PfRab5b adjacent to the ER in the early erythrocytic stage. A super-resolution microgram of the indirect immunofluorescence assay using PfArf1 or PfRab1b- expressing parasites revealed that PfArf1 and PfRab1b are localized to different ER subdomains. We used a genetic approach to expresses an active or inactive mutant of PfArf1 that specifically inhibited the trafficking of PfAK2 to the parasitophorous vacuole membrane. While expression of PfRab1b mutants did not affect in the PfAK2 transport. In contrast, the export of the PEXEL-positive protein Rifin was decreased by the expression of the inactive mutant of PfRab1b or PfArf1. These data indicate that the transport of PfAK2 and Rifin were recognized at the different ER subdomain by the two independent GTPases: PfAK2 is sorted by PfArf1 into the pathway for the PV, and the export of Rifin might be sequentially regulated by PfArf1 and PfRab1b.

Progress towards the development of a P. vivax vaccine

INTRODUCTION

Plasmodium vivax causes significant public health problems in endemic regions. A vaccine to prevent disease is critical, considering the rapid spread of drug-resistant parasite strains, and the development of hypnozoites in the liver with potential for relapse. A minimally effective vaccine should prevent disease and transmission while an ideal vaccine provides sterile immunity.

AREAS COVERED

Despite decades of research, the complex life cycle, technical challenges and a lack of funding have hampered progress of P. vivax vaccine development. Here, we review the progress of potential P. vivax vaccine candidates from different stages of the parasite life cycle. We also highlight the challenges and important strategies for rational vaccine design. These factors can significantly increase immune effector mechanisms and improve the protective efficacy of these candidates in clinical trials to generate sustained protection over longer periods of time.

EXPERT OPINION

A vaccine that presents functionally-conserved epitopes from multiple antigens from various stages of the parasite life cycle is key to induce broadly neutralizing strain-transcending protective immunity to effectively disrupt parasite development and transmission.

Targeting and Inhibiting Plasmodium falciparum Using Ultra-small Gold Nanoparticles

Malaria, a mosquito-borne disease caused by Plasmodium species, claims more than 400,000 lives globally each year. The increasing drug resistance of the parasite renders the development of new anti-malaria drugs necessary. Alternatively, better delivery systems for already marketed drugs could help to solve the resistance problem. Herein, we report glucose-based ultra-small gold nanoparticles (Glc-NCs) that bind to cysteine-rich domains of Plasmodium falciparum surface proteins. Microscopy shows that Glc-NCs bind specifically to extracellular and all intra-erythrocytic stages of P. falciparum. Glc-NCs may be used as drug delivery agents as illustrated for ciprofloxacin, a poorly soluble antibiotic with low antimalarial activity. Ciprofloxacin conjugated to Glc-NCs is more water-soluble than the free drug and is more potent. Glyco-gold nanoparticles that target cysteine-rich domains on parasites may be helpful for the prevention and treatment of malaria.

Antibodies against a short region of PfRipr inhibit Plasmodium falciparum merozoite invasion and PfRipr interaction with Rh5 and SEMA7A

Plasmodium falciparum merozoite invasion into erythrocytes is an essential step of the blood-stage cycle, survival of parasites, and malaria pathogenesis. P. falciparum merozoite Rh5 interacting protein (PfRipr) forms a complex with Rh5 and CyRPA in sequential molecular events leading to erythrocyte invasion. Recently we described PfRipr as a conserved protein that induces strain-transcending growth inhibitory antibodies in in vitro assays. However, being a large and complex protein of 1086 amino acids (aa) with 87 cysteine residues, PfRipr is difficult to express in conventional expression systems towards vaccine development. In this study we sought to identify the most potent region of PfRipr that could be developed to overcome difficulties related to protein expression, as well as to elucidate the invasion inhibitory mechanism of anti-PfRipr antibodies. Using the wheat germ cell-free system, Ecto- PfRipr and truncates of approximately 200 aa were expressed as soluble proteins. We demonstrate that antibodies against PfRipr truncate 5 (PfRipr_5: C₇₂₀-D₉₃₄), a region within the PfRipr C-terminal EGF-like domains, potently inhibit merozoite invasion. Furthermore, the antibodies strongly block PfRipr/Rh5 interaction, as well as that between PfRipr and its erythrocyte-surface receptor, SEMA7A. Taken together, PfRipr_5 is a potential candidate for further development as a blood-stage malaria vaccine.

Dual Plasmepsin-Targeting Antimalarial Agents Disrupt Multiple Stages of the Malaria Parasite Life Cycle

Artemisin combination therapy (ACT) is the main treatment option for malaria, which is caused by the intracellular parasite Plasmodium. However, increased resistance to ACT highlights the importance of finding new drugs. Recently, the aspartic proteases Plasmepsin IX and X (PMIX and PMX) were identified as promising drug targets. In this study, we describe dual inhibitors of PMIX and PMX, including WM382, that block multiple stages of the Plasmodium life cycle. We demonstrate that PMX is a master modulator of merozoite invasion and direct maturation of proteins required for invasion, parasite development, and egress. Oral administration of WM382 cured mice of P. berghei and prevented blood infection from the liver. In addition, WM382 was efficacious against P. falciparum asexual infection in humanized mice and prevented transmission to mosquitoes. Selection of resistant P. falciparum in vitro was not achievable. Together, these show that dual PMIX and PMX inhibitors are promising candidates for malaria treatment and prevention.

Structural basis for neutralization of Plasmodium vivax by naturally acquired human antibodies that target DBP

The Plasmodium vivax Duffy-binding protein (DBP) is a prime target of the protective immune response and a promising vaccine candidate for P. vivax malaria. Naturally acquired immunity (NAI) protects against malaria in adults residing in infection-endemic regions, and the passive transfer of malarial immunity confers protection. A vaccine that replicates NAI will effectively prevent disease. Here, we report the structures of DBP region II in complex with human-derived, neutralizing monoclonal antibodies obtained from an individual in a malaria-endemic area with NAI. We identified protective epitopes using X-ray crystallography, hydrogen-deuterium exchange mass spectrometry, mutational mapping and P. vivax invasion studies. These approaches reveal that naturally acquired human antibodies neutralize P. vivax by targeting the binding site for Duffy antigen receptor for chemokines (DARC) and the dimer interface of P. vivax DBP. Antibody binding is unaffected by polymorphisms in the vicinity of epitopes, suggesting that the antibodies have evolved to engage multiple polymorphic variants of DBP. The human antibody epitopes are broadly conserved and are distinct from previously defined epitopes for broadly conserved murine monoclonal antibodies. A library of globally conserved epitopes of neutralizing human antibodies offers possibilities for rational design of strain-transcending DBP-based vaccines and therapeutics against P. vivax.

Moderately Neutralizing Epitopes in Nonfunctional Regions Dominate the Antibody Response to Plasmodium falciparum EBA-140

Plasmodium falciparum erythrocyte-binding antigen 140 (EBA-140) plays a role in tight junction formation during parasite invasion of red blood cells and is a potential vaccine candidate for malaria. Individuals in areas where malaria is endemic possess EBA-140-specific antibodies, and individuals with high antibody titers to this protein have a lower rate of reinfection by parasites. The red blood cell binding segment of EBA-140 is comprised of two Duffy-binding-like domains, called F1 and F2, that together create region II. The sialic acid-binding pocket of F1 is essential for binding, whereas the sialic acid-binding pocket in F2 appears dispensable. Here, we show that immunization of mice with the complete region II results in poorly neutralizing antibodies. In contrast, immunization of mice with the functionally relevant F1 domain of region II results in antibodies that confer a 2-fold increase in parasite neutralization compared to that of the F2 domain. Epitope mapping of diverse F1 and F2 monoclonal antibodies revealed that the functionally relevant F1 sialic acid-binding pocket is a privileged site inaccessible to antibodies, that the F2 sialic acid-binding pocket contains a nonneutralizing epitope, and that two additional epitopes reside in F1 on the opposite face from the sialic acid-binding pocket. These studies indicate that focusing the immune response to the functionally important F1 sialic acid binding pocket improves the protective immune response of EBA-140. These results have implications for improving future vaccine designs and emphasize the importance of structural vaccinology for malaria.

The Molecular Basis of Erythrocyte Invasion by Malaria Parasites

Plasmodium species cause malaria by proliferating in human erythrocytes. Invasion of immunologically privileged erythrocytes provides a relatively protective niche as well as access to a rich source of nutrients. Plasmodium spp. target erythrocytes of different ages, but share a common mechanism of invasion. Specific engagement of erythrocyte receptors defines target cell tropism, activating downstream events and resulting in the physical penetration of the erythrocyte, powered by the parasite's actinomyosin-based motor. Here we review the latest in our understanding of the molecular composition of this highly complex and fascinating biological process.

Receptor-ligand and parasite protein-protein interactions in Plasmodium vivax: Analysing rhoptry neck proteins 2 and 4

Elucidating receptor-ligand and protein-protein interactions represents an attractive alternative for designing effective Plasmodium vivax control methods. This article describes the ability of P. vivax rhoptry neck proteins 2 and 4 (RON2 and RON4) to bind to human reticulocytes. Biochemical and cellular studies have shown that two PvRON2- and PvRON4-derived conserved regions specifically interact with protein receptors on reticulocytes marked by the CD71 surface transferrin receptor. Mapping each protein fragment's binding region led to defining the specific participation of two 20 amino acid-long regions selectively competing for PvRON2 and PvRON4 binding to reticulocytes. Binary interactions between PvRON2 (ligand) and other parasite proteins, such as PvRON4, PvRON5, and apical membrane antigen 1 (AMA1), were evaluated and characterised by surface plasmon resonance. The results revealed that both PvRON2 cysteine-rich regions strongly interact with PvAMA1 Domains II and III (equilibrium constants in the nanomolar range) and at a lower extent with the complete PvAMA1 ectodomain and Domains I and II. These results strongly support that these proteins participate in P. vivax's complex invasion process, thus providing new pertinent targets for blocking P. vivax merozoites' specific entry to their target cells.

Comparative Heterochromatin Profiling Reveals Conserved and Unique Epigenome Signatures Linked to Adaptation and Development of Malaria Parasites

Heterochromatin-dependent gene silencing is central to the adaptation and survival of Plasmodium falciparum malaria parasites, allowing clonally variant gene expression during blood infection in humans. By assessing genome-wide heterochromatin protein 1 (HP1) occupancy, we present a comprehensive analysis of heterochromatin landscapes across different Plasmodium species, strains, and life cycle stages. Common targets of epigenetic silencing include fast-evolving multi-gene families encoding surface antigens and a small set of conserved HP1-associated genes with regulatory potential. Many P. falciparum heterochromatic genes are marked in a strain-specific manner, increasing the parasite's adaptive capacity. Whereas heterochromatin is strictly maintained during mitotic proliferation of asexual blood stage parasites, substantial heterochromatin reorganization occurs in differentiating gametocytes and appears crucial for the activation of key gametocyte-specific genes and adaptation of erythrocyte remodeling machinery. Collectively, these findings provide a catalog of heterochromatic genes and reveal conserved and specialized features of epigenetic control across the genus Plasmodium.

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).

Plasmodium reichenowi EBA-140 merozoite ligand binds to glycophorin D on chimpanzee red blood cells, shedding new light on origins of Plasmodium falciparum

Background

All symptoms of malaria are caused by the intraerythrocytic proliferation of Plasmodium merozoites. Merozoites invade erythrocytes using multiple binding ligands that recognise specific surface receptors. It has been suggested that adaptation of Plasmodium parasites to infect specific hosts is driven by changes in genes encoding Plasmodium erythrocyte-binding ligands (EBL) and reticulocyte-binding ligands (RBL). Homologs of both EBL and RBL, including the EBA-140 merozoite ligand, have been identified in P. falciparum and P. reichenowi, which infect humans and chimpanzees, respectively. The P. falciparum EBA-140 was shown to bind human glycophorin C, a minor erythrocyte sialoglycoprotein. Until now, the erythrocyte receptor for the P. reichenowi EBA-140 remained unknown.

Methods

The baculovirus expression vector system was used to obtain the recombinant EBA-140 Region II, and flow cytometry and immunoblotting methods were applied to characterise its specificity.

Results

We showed that the chimpanzee glycophorin D is the receptor for the P. reichenowi EBA-140 ligand on chimpanzee red blood cells.

Conclusions

We propose that the development of glycophorin C specificity is spurred by the P. falciparum lineage. We speculate that the P. falciparum EBA-140 evolved to hijack GPC on human erythrocytes during divergence from its ape ancestor.

Host Cell Tropism and Adaptation of Blood-Stage Malaria Parasites: Challenges for Malaria Elimination

Plasmodium falciparum and Plasmodium vivax account for most of the mortality and morbidity associated with malaria in humans. Research and control efforts have focused on infections caused by P. falciparum and P. vivax, but have neglected other malaria parasite species that infect humans. Additionally, many related malaria parasite species infect nonhuman primates (NHPs), and have the potential for transmission to humans. For malaria elimination, the varied and specific challenges of all of these Plasmodium species will need to be considered. Recent advances in molecular genetics and genomics have increased our knowledge of the prevalence and existing diversity of the human and NHP Plasmodium species. We are beginning to identify the extent of the reservoirs of each parasite species in humans and NHPs, revealing their origins as well as potential for adaptation in humans. Here, we focus on the red blood cell stage of human infection and the host cell tropism of each human Plasmodium species. Determinants of tropism are unique among malaria parasite species, presenting a complex challenge for malaria elimination.

Genetic Evidence for Erythrocyte Receptor Glycophorin B Expression Levels Defining a Dominant Plasmodium falciparum Invasion Pathway into Human Erythrocytes

Plasmodium falciparum, the parasite that causes the deadliest form of malaria, has evolved multiple proteins known as invasion ligands that bind to specific erythrocyte receptors to facilitate invasion of human erythrocytes. The EBA-175/glycophorin A (GPA) and Rh5/basigin ligand-receptor interactions, referred to as invasion pathways, have been the subject of intense study. In this study, we focused on the less-characterized sialic acid-containing receptors glycophorin B (GPB) and glycophorin C (GPC). Through bioinformatic analysis, we identified extensive variation in glycophorin B (GYPB) transcript levels in individuals from Benin, suggesting selection from malaria pressure. To elucidate the importance of the GPB and GPC receptors relative to the well-described EBA-175/GPA invasion pathway, we used an ex vivo erythrocyte culture system to decrease expression of GPA, GPB, or GPC via lentiviral short hairpin RNA transduction of erythroid progenitor cells, with global surface proteomic profiling. We assessed the efficiency of parasite invasion into knockdown cells using a panel of wild-type P. falciparum laboratory strains and invasion ligand knockout lines, as well as P. falciparum Senegalese clinical isolates and a short-term-culture-adapted strain. For this, we optimized an invasion assay suitable for use with small numbers of erythrocytes. We found that all laboratory strains and the majority of field strains tested were dependent on GPB expression level for invasion. The collective data suggest that the GPA and GPB receptors are of greater importance than the GPC receptor, supporting a hierarchy of erythrocyte receptor usage in P. falciparum.

Acquisition, maintenance and adaptation of invasion inhibitory antibodies against Plasmodium falciparum invasion ligands involved in immune evasion

Erythrocyte-binding antigens (EBAs) and P. falciparum reticulocyte-binding homologue proteins (PfRhs) are two important protein families that can vary in expression and utilization by P. falciparum to evade inhibitory antibodies. We evaluated antibodies at repeated time-points among individuals living in an endemic region in Nigeria over almost one year against these vaccine candidates. Antibody levels against EBA140, EBA175, EBA181, PfRh2, PfRh4, and MSP2, were measured by ELISA. We also used parasites with disrupted EBA140, EBA175 and EBA181 genes to show that all these were targets of invasion inhibitory antibodies. However, antigenic targets of inhibitory antibodies were not stable and changed substantially over time in most individuals, independent of age. Antibodies levels measured by ELISA also varied within and between individuals over time and the antibodies against EBA181, PfRh2 and MSP2 declined more rapidly in younger individuals (≤15 years) compared with older (>15). The breadth of high antibody responses over time was more influenced by age than by the frequency of infection. High antibody levels were associated with a more stable invasion inhibitory response, which could indicate that during the long process of formation of immunity, many changes not only in levels but also in functional responses are needed. This is an important finding in understanding natural immunity against malaria, which is essential for making an efficacious vaccine.

Red cell receptors as access points for malaria infection

PURPOSE OF REVIEW

Red cell receptors provide unique entry points for Plasmodium parasites to initiate blood-stage malaria infection. Parasites encode distinct ligands that bind specifically to both highly abundant and low-copy receptors. Recent advances in the understanding of molecular and structural mechanisms of these interactions provide fundamental insights into receptor-ligand biology and molecular targets for intervention.

RECENT FINDINGS

The review focuses on the requirements for known interactions, insight derived from complex structures, and mechanisms of receptor/ligand engagement. Further, novel roles for established red cell membrane proteins, parasite ligands and associated interacting partners have recently been established in red cell invasion.

SUMMARY

The new knowledge underlines the intricacies involved in invasion by a eukaryotic parasite into a eukaryotic host cell demonstrated by expanded parasite ligand families, redundancy in red cell receptor engagement, multitiered temporal binding, and the breadth of receptors engaged.

Baculovirus-expressed Plasmodium reichenowi EBA-140 merozoite ligand is host specific

Plasmodium reichenowi, an ape malaria parasite is morphologically identical and genetically similar to Plasmodium falciparum, infects chimpanzees but not humans. Genomic studies revealed that all primate malaria parasites belong to Laverania subgenus. Laverania parasites exhibit strict host specificity, but the molecular mechanisms underlying these host restrictions remain unexplained. Plasmodium merozoites express multiple binding ligands that recognize specific receptors on erythrocytes, including micronemal proteins belonging to P. falciparum EBL family. It was shown that erythrocyte binding antigen-175 (EBA-175), erythrocyte binding ligand-1 (EBL-1), erythrocyte binding antigen-140 (EBA-140) recognize erythrocyte surface sialoglycoproteins - glycophorins A, B, C, respectively. EBA-140 merozoite ligand hijacks glycophorin C (GPC), a minor erythrocyte sialoglycoprotein, to invade the erythrocyte through an alternative invasion pathway. A homolog of P. falciparum EBA-140 protein was identified in P. reichenowi. The amino acid sequences of both EBA-140 ligands are very similar, especially in the conservative erythrocyte binding region (Region II). It has been suggested that evolutionary changes in the sequence of EBL proteins may be associated with Plasmodium host restriction. In this study we obtained, for the first time, the recombinant P. reichenowi EBA-140 ligand Region II using baculovirus expression vector system. We show that the ape EBA-140 Region II is host specific and binds to chimpanzee erythrocytes in the dose and sialic acid dependent manner. Further identification of the erythrocyte receptor for this ape ligand is of great interests, since it may reveal the molecular basis of host restriction of both P. reichenowi and its deadliest human counterpart, P. falciparum.

A novel Plasmodium falciparum rhoptry associated adhesin mediates erythrocyte invasion through the sialic-acid dependent pathway

Erythrocyte invasion by Plasmodium falciparum merozoites is central to blood-stage infection and malaria pathogenesis. This intricate process is coordinated by multiple parasite adhesins that bind erythrocyte receptors and mediate invasion through several alternate pathways. P. falciparum expresses 2700 genes during the blood-stages, of which the identity and function of many remains unknown. Here, we have identified and characterized a novel P. falciparum rhoptry associated adhesin (PfRA) that mediates erythrocyte invasion through the sialic-acid dependent pathway. PfRA appears to play a significant functional role as it is conserved across different Plasmodium species. It is localized in the rhoptries and further translocated to the merozoite surface. Both native and recombinant PfRA specifically bound erythrocytes in a sialic-acid dependent, chymotrypsin and trypsin resistant manner, which was abrogated by PfRA antibodies confirming a role in erythrocyte invasion. PfRA antibodies inhibited erythrocyte invasion and in combination with antibodies against other parasite ligands produced an additive inhibitory effect, thus validating its important role in erythrocyte invasion. We have thus identified a novel P. falciparum adhesin that binds with a sialic acid containing erythrocyte receptor. Our observations substantiate the strategy to block P. falciparum erythrocyte invasion by simultaneously targeting multiple conserved merozoite antigens involved in alternate invasion pathways.

Malaria invasion ligand RH5 and its prime candidacy in blood-stage malaria vaccine design

With drug resistance to available therapeutics continuing to develop against Plasmodium falciparum malaria, the development of an effective vaccine candidate remains a major research goal. Successful interruption of invasion of parasites into erythrocytes during the blood stage of infection will prevent the severe clinical symptoms and complications associated with malaria. Previously studied blood stage antigens have highlighted the hurdles that are inherent to this life-cycle stage, namely that highly immunogenic antigens are also globally diverse, resulting in protection only against the vaccine strain, or that naturally acquired immunity to blood stage antigens do not always correlate with actual protection. The blood stage antigen reticulocyte binding homolog RH5 is essential for parasite viability, has globally limited diversity, and is associated with protection from disease. Here we summarize available information on this invasion ligand and recent findings that highlight its candidacy for inclusion in a blood-stage malaria vaccine.

Erythrocyte invasion receptors for Plasmodium falciparum: new and old

Understanding the complex process by which the invasive form of the Plasmodium falciparum parasite, the merozoite, attaches to and invades erythrocytes as part of its blood stage life cycle represents a key area of research in the battle to combat malaria. Central to this are efforts to determine the identity of receptors on the host cell surface, their corresponding merozoite-binding proteins and the functional relevance of these binding events as part of the invasion process. This review will provide an updated summary of studies identifying receptor interactions essential for or implicated in P. falciparum merozoite invasion of human erythrocytes, highlighting the recent identification of new receptors using groundbreaking high throughput approaches and with particular focus on the properties and putative involvement of the erythrocyte proteins targeted by these invasion pathways.

Merozoite surface proteins in red blood cell invasion, immunity and vaccines against malaria

Malaria accounts for an enormous burden of disease globally, with Plasmodium falciparum accounting for the majority of malaria, and P. vivax being a second important cause, especially in Asia, the Americas and the Pacific. During infection with Plasmodium spp., the merozoite form of the parasite invades red blood cells and replicates inside them. It is during the blood-stage of infection that malaria disease occurs and, therefore, understanding merozoite invasion, host immune responses to merozoite surface antigens, and targeting merozoite surface proteins and invasion ligands by novel vaccines and therapeutics have been important areas of research. Merozoite invasion involves multiple interactions and events, and substantial processing of merozoite surface proteins occurs before, during and after invasion. The merozoite surface is highly complex, presenting a multitude of antigens to the immune system. This complexity has proved challenging to our efforts to understand merozoite invasion and malaria immunity, and to developing merozoite antigens as malaria vaccines. In recent years, there has been major progress in this field, and several merozoite surface proteins show strong potential as malaria vaccines. Our current knowledge on this topic is reviewed, highlighting recent advances and research priorities.

The authors summarize current knowledge of merozoite surface proteins of malaria parasites; their function in invasion, processing of surface proteins before, during and after invasion, their importance as targets of immunity, and the current status of malaria vaccines that target merozoite surface proteins.

Plasmodium falciparum Adhesins Play an Essential Role in Signalling and Activation of Invasion into Human Erythrocytes

The most severe form of malaria in humans is caused by the protozoan parasite Plasmodium falciparum. The invasive form of malaria parasites is termed a merozoite and it employs an array of parasite proteins that bind to the host cell to mediate invasion. In Plasmodium falciparum, the erythrocyte binding-like (EBL) and reticulocyte binding-like (Rh) protein families are responsible for binding to specific erythrocyte receptors for invasion and mediating signalling events that initiate active entry of the malaria parasite. Here we have addressed the role of the cytoplasmic tails of these proteins in activating merozoite invasion after receptor engagement. We show that the cytoplasmic domains of these type 1 membrane proteins are phosphorylated in vitro. Depletion of PfCK2, a kinase implicated to phosphorylate these cytoplasmic tails, blocks P. falciparum invasion of red blood cells. We identify the crucial residues within the PfRh4 cytoplasmic domain that are required for successful parasite invasion. Live cell imaging of merozoites from these transgenic mutants show they attach but do not penetrate erythrocytes implying the PfRh4 cytoplasmic tail conveys signals important for the successful completion of the invasion process.

Homology-Based Prediction of Potential Protein-Protein Interactions between Human Erythrocytes and Plasmodium falciparum

Plasmodium falciparum, a causative agent of malaria, is a well-characterized obligate intracellular parasite known for its ability to remodel host cells, particularly erythrocytes, to successfully persist in the host environment. However, the current levels of understanding from the laboratory experiments on the host–parasite interactions and the strategies pursued by the parasite to remodel host erythrocytes are modest. Several computational means developed in the recent past to predict host–parasite/pathogen interactions have generated testable hypotheses on feasible protein–protein interactions. We demonstrate the utility of protein structure-based protocol in the recognition of potential interacting proteins across P. falciparum and host erythrocytes. In concert with the information on the expression and subcellular localization of host and parasite proteins, we have identified 208 biologically feasible interactions potentially brought about by 59 P. falciparum and 30 host erythrocyte proteins. For selected cases, we have evaluated the physicochemical viability of the predicted interactions in terms of surface complementarity, electrostatic complementarity, and interaction energies at protein interface regions. Such careful inspection of molecular and mechanistic details generates high confidence on the predicted host–parasite protein–protein interactions. The predicted host–parasite interactions generate many experimentally testable hypotheses that can contribute to the understanding of possible mechanisms undertaken by the parasite in host erythrocyte remodeling. Thus, the key protein players recognized in P. falciparum can be explored for their usefulness as targets for chemotherapeutic intervention.

Contrasting Patterns of Serologic and Functional Antibody Dynamics to Plasmodium falciparum Antigens in a Kenyan Birth Cohort

IgG antibodies to Plasmodium falciparum are transferred from the maternal to fetal circulation during pregnancy, wane after birth, and are subsequently acquired in response to natural infection. We examined the dynamics of malaria antibody responses of 84 Kenyan infants from birth to 36 months of age by (i) serology, (ii) variant surface antigen (VSA) assay, (iii) growth inhibitory activity (GIA), and (iv) invasion inhibition assays (IIA) specific for merozoite surface protein 1 (MSP1) and sialic acid-dependent invasion pathway. Maternal antibodies in each of these four categories were detected in cord blood and decreased to their lowest level by approximately 6 months of age. Serologic antibodies to 3 preerythrocytic and 10 blood-stage antigens subsequently increased, reaching peak prevalence by 36 months. In contrast, antibodies measured by VSA, GIA, and IIA remained low even up to 36 months. Infants sensitized to P. falciparum in utero, defined by cord blood lymphocyte recall responses to malaria antigens, acquired antimalarial antibodies at the same rate as those who were not sensitized in utero, indicating that fetal exposure to malaria antigens did not affect subsequent infant antimalarial responses. Infants with detectable serologic antibodies at 12 months of age had an increased risk of P. falciparum infection during the subsequent 24 months. We conclude that serologic measures of antimalarial antibodies in children 36 months of age or younger represent biomarkers of malaria exposure rather than protection and that functional antibodies develop after 36 months of age in this population.

Studies on Immunogenicity and Antigenicity of Baculovirus-Expressed Binding Region of Plasmodium falciparum EBA-140 Merozoite Ligand

The erythrocyte binding ligand 140 (EBA-140) is a member of the Plasmodium falciparum erythrocyte binding antigens (EBA) family, which are considered as prospective candidates for malaria vaccine development. EBA proteins were identified as important targets for naturally acquired inhibitory antibodies. Natural antibody response against EBA-140 ligand was found in individuals living in malaria-endemic areas. The EBA-140 ligand is a paralogue of the well-characterized P. falciparum EBA-175 protein. They both share homology of domain structure, including the binding region (Region II), which consists of two homologous F1 and F2 domains and is responsible for ligand-erythrocyte receptor interaction during merozoite invasion. It was shown that the erythrocyte receptor for EBA-140 ligand is glycophorin C-a minor human erythrocyte sialoglycoprotein. In studies on the immunogenicity of P. falciparum EBA ligands, the recombinant proteins are of great importance. In this report, we have demonstrated that the recombinant baculovirus-obtained EBA-140 Region II is immunogenic and antigenic. It can raise specific antibodies in rabbits, and it is recognized by natural antibodies present in sera of patients with malaria, and thus, it may be considered for inclusion in multicomponent blood-stage vaccines.

The apicomplexan glideosome and adhesins - Structures and function

The apicomplexan family of pathogens, which includes Plasmodium spp. and Toxoplasma gondii, are primarily obligate intracellular parasites and invade multiple cell types. These parasites express extracellular membrane protein receptors, adhesins, to form specific pathogen-host cell interaction complexes. Various adhesins are used to invade a variety of cell types. The receptors are linked to an actomyosin motor, which is part of a complex comprised of many proteins known as the invasion machinery or glideosome. To date, reviews on invasion have focused primarily on the molecular pathways and signals of invasion, with little or no structural information presented. Over 75 structures of parasite receptors and glideosome proteins have been deposited with the Protein Data Bank. These structures include adhesins, motor proteins, bridging proteins, inner membrane complex and cytoskeletal proteins, as well as co-crystal structures with peptides and antibodies. These structures provide information regarding key interactions necessary for target receptor engagement, machinery complex formation, how force is transmitted, and the basis of inhibitory antibodies. Additionally, these structures can provide starting points for the development of antibodies and inhibitory molecules targeting protein-protein interactions, with the aim to inhibit invasion. This review provides an overview of the parasite adhesin protein families, the glideosome components, glideosome architecture, and discuss recent work regarding alternative models.

Variation in Plasmodium falciparum erythrocyte invasion phenotypes and merozoite ligand gene expression across different populations in areas of malaria endemicity

Plasmodium falciparum merozoites use diverse alternative erythrocyte receptors for invasion and variably express cognate ligands encoded by the erythrocyte binding antigen (eba) and reticulocyte binding-like homologue (Rh) gene families. Previous analyses conducted on parasites from single populations in areas of endemicity revealed a wide spectrum of invasion phenotypes and expression profiles, although comparisons across studies have been limited by the use of different protocols. For direct comparisons within and among populations, clinical isolates from three different West African sites of endemicity (in Ghana, Guinea, and Senegal) were cryopreserved and cultured ex vivo after thawing in a single laboratory to assay invasion of target erythrocytes pretreated with enzymes affecting receptor subsets. Complete invasion assay data from 67 isolates showed no differences among the populations in the broad range of phenotypes measured by neuraminidase treatment (overall mean, 40.6% inhibition) or trypsin treatment (overall mean, 83.3% inhibition). The effects of chymotrypsin treatment (overall mean, 79.2% inhibition) showed heterogeneity across populations (Kruskall-Wallis P = 0.023), although the full phenotypic range was seen in each. Schizont-stage transcript data for a panel of 8 invasion ligand genes (eba175, eba140, eba181, Rh1, Rh2a, Rh2b, Rh4, and Rh5) were obtained for 37 isolates, showing similar ranges of variation in each population except that eba175 levels tended to be higher in parasites from Ghana than in those from Senegal (whereas levels of eba181 and Rh2b were lower in parasites from Ghana). The broad diversity in invasion phenotypes and gene expression seen within each local population, with minimal differences among them, is consistent with a hypothesis of immune selection maintaining parasite variation.

More than just immune evasion: Hijacking complement by Plasmodium falciparum

Malaria remains one of the world's deadliest diseases. Plasmodium falciparum is responsible for the most severe and lethal form of human malaria. P. falciparum's life cycle involves two obligate hosts: human and mosquito. From initial entry into these hosts, malaria parasites face the onslaught of the first line of host defence, the complement system. In this review, we discuss the complex interaction between complement and malaria infection in terms of hosts immune responses, parasite survival and pathogenesis of severe forms of malaria. We will focus on the role of complement receptor 1 and its associated polymorphisms in malaria immune complex clearance, as a mediator of parasite rosetting and as an entry receptor for P. falciparum invasion. Complement evasion strategies of P. falciparum parasites will also be highlighted. The sexual forms of the malaria parasites recruit the soluble human complement regulator Factor H to evade complement-mediated killing within the mosquito host. A novel evasion strategy is the deployment of parasite organelles to divert complement attack from infective blood stage parasites. Finally we outline the future challenge to understand the implications of these exploitation mechanisms in the interplay between successful infection of the host and pathogenesis observed in severe malaria.

The baculovirus-expressed binding region of Plasmodium falciparum EBA-140 ligand and its glycophorin C binding specificity

The erythrocyte binding ligand 140 (EBA-140) is a member of the Plasmodium falciparum DBL family of erythrocyte binding proteins, which are considered as prospective candidates for malaria vaccine development. The EBA-140 ligand is a paralogue of the well-characterized P. falciparum EBA-175 protein. They share homology of domain structure, including Region II, which consists of two homologous F1 and F2 domains and is responsible for ligand-erythrocyte receptor interaction during invasion. In this report we describe, for the first time, the glycophorin C specificity of the recombinant, baculovirus-expressed binding region (Region II) of P. falciparum EBA-140 ligand. It was found that the recombinant EBA-140 Region II binds to the endogenous and recombinant glycophorin C, but does not bind to Gerbich-type glycophorin C, neither normal nor recombinant, which lacks amino acid residues 36-63 of its polypeptide chain. Our results emphasize the crucial role of this glycophorin C region in EBA-140 ligand binding. Moreover, the EBA-140 Region II did not bind either to glycophorin D, the truncated form of glycophorin C lacking the N-glycan or to desialylated GPC. These results draw attention to the role of glycophorin C glycans in EBA-140 binding. The full identification of the EBA-140 binding site on glycophorin C molecule, consisting most likely of its glycans and peptide backbone, may help to design therapeutics or vaccines that target the erythrocyte binding merozoite ligands.

Plasmodium falciparum aldolase and the C-terminal cytoplasmic domain of certain apical organellar proteins promote actin polymerization

The current model of Apicomplexan motility and host cell invasion is that both processes are driven by an actomyosin motor located beneath the plasma membrane, with the force transduced to the outside of the cell via coupling through aldolase and the cytoplasmic tail domains (CTDs) of certain type 1 membrane proteins. In Plasmodium falciparum (Pf), aldolase is thought to bind to the CTD of members of the thrombospondin-related anonymous protein (TRAP) family, which are micronemal proteins and represented by MTRAP in merozoites. Other type 1 membrane proteins including members of the erythrocyte binding antigen (EBA) and reticulocyte binding protein homologue (RH) protein families, which are also apical organellar proteins, have also been implicated in host cell binding in erythrocyte invasion. However, recent studies with Toxoplasma gondii have questioned the importance of aldolase in these processes. Using biolayer interferometry we show that Pf aldolase binds with high affinity to both rabbit and Pf actin, with a similar affinity for filamentous (F-) actin and globular (G-) actin. The interaction between Pf aldolase and merozoite actin was confirmed by co-sedimentation assays. Aldolase binding was shown to promote rabbit actin polymerization indicating that the interaction is more complicated than binding alone. The CTDs of some but not all type 1 membrane proteins also promoted actin polymerization in the absence of aldolase; MTRAP and RH1 CTDs promoted actin polymerization but EBA175 CTD did not. Direct actin polymerization mediated by membrane protein CTDs may contribute to actin recruitment, filament formation and stability during motor assembly, and actin-mediated movement, independent of aldolase.

Critical glycosylated residues in exon three of erythrocyte glycophorin A engage Plasmodium falciparum EBA-175 and define receptor specificity

Erythrocyte invasion is an essential step in the pathogenesis of malaria. The erythrocyte binding-like (EBL) family of Plasmodium falciparum proteins recognizes glycophorins (Gp) on erythrocytes and plays a critical role in attachment during invasion. However, the molecular basis for specific receptor recognition by each parasite ligand has remained elusive, as is the case with the ligand/receptor pair P. falciparum EBA-175 (PfEBA-175)/GpA. This is due largely to difficulties in producing properly glycosylated and functional receptors. Here, we developed an expression system to produce recombinant glycosylated and functional GpA, as well as mutations and truncations. We identified the essential binding region and determinants for PfEBA-175 engagement, demonstrated that these determinants are required for the inhibition of parasite growth, and identified the glycans important in mediating the PfEBA-175–GpA interaction. The results suggest that PfEBA-175 engages multiple glycans of GpA encoded by exon 3 and that the presentation of glycans is likely required for high-avidity binding. The absence of exon 3 in GpB and GpE due to a splice site mutation confers specific recognition of GpA by PfEBA-175. We speculate that GpB and GpE may have arisen due to selective pressure to lose the PfEBA-175 binding site in GpA. The expression system described here has wider application for examining other EBL members important in parasite invasion, as well as additional pathogens that recognize glycophorins. The ability to define critical binding determinants in receptor-ligand interactions, as well as a system to genetically manipulate glycosylated receptors, opens new avenues for the design of interventions that disrupt parasite invasion.

Plasmodium falciparum uses distinct ligands that bind host cell receptors for invasion of red blood cells (RBCs) during malaria infection. A key entry pathway involves P. falciparum EBA-175 (PfEBA-175) recognizing glycophorin A (GpA) on RBCs. Despite knowledge of this protein-protein interaction, the complete mechanism for specific receptor engagement is not known. PfEBA-175 recognizes GpA but is unable to engage the related RBC receptor GpB or GpE. Understanding the necessary elements that enable PfEBA-175 to specifically recognize GpA is critical in developing specific and potent inhibitors of PfEBA-175 that disrupt host cell invasion and aid in malaria control. Here, we describe a novel system to produce and manipulate the host receptor GpA. Using this system, we probed the elements in GpA necessary for engagement and thus for host cell invasion. These studies have important implications for understanding how ligands and receptors interact and for the future development of malaria interventions.

Human erythrocyte band 3 functions as a receptor for the sialic acid-independent invasion of Plasmodium falciparum. Role of the RhopH3-MSP1 complex

Plasmodium falciparum takes advantage of two broadly defined alternate invasion pathways when infecting human erythrocytes: one that depends on and the other that is independent of host sialic acid residues on the erythrocyte surface. Within the sialic acid-dependent (SAD) and sialic acid-independent (SAID) invasion pathways, several alternate host receptors are used by P. falciparum based on its particular invasion phenotype. Earlier, we reported that two putative extracellular regions of human erythrocyte band 3 termed 5C and 6A function as host invasion receptor segments binding parasite proteins MSP1 and MSP9 via a SAID mechanism. In this study, we developed two mono-specific anti-peptide chicken IgY antibodies to demonstrate that the 5C and 6A regions of band 3 are exposed on the surface of human erythrocytes. These antibodies inhibited erythrocyte invasion by the P. falciparum 3D7 and 7G8 strains (SAID invasion phenotype), and the blocking effect was enhanced in sialic acid-depleted erythrocytes. In contrast, the IgY antibodies had only a marginal inhibitory effect on FCR3 and Dd2 strains (SAD invasion phenotype). A direct biochemical interaction between erythrocyte band 3 epitopes and parasite RhopH3, identified by the yeast two-hybrid screen, was established. RhopH3 formed a complex with MSP119 and the 5ABC region of band 3, and a recombinant segment of RhopH3 inhibited parasite invasion in human erythrocytes. Together, these findings provide evidence that erythrocyte band 3 functions as a major host invasion receptor in the SAID invasion pathway by assembling a multi-protein complex composed of parasite ligands RhopH3 and MSP1.

The Duffy binding protein as a key target for a Plasmodium vivax vaccine: lessons from the Brazilian Amazon

Plasmodium vivax infects human erythrocytes through a major pathway that requires interaction between an apical parasite protein, the Duffy binding protein (PvDBP) and its receptor on reticulocytes, the Duffy antigen/receptor for chemokines (DARC). The importance of the interaction between PvDBP (region II, DBPII) and DARC to P. vivax infection has motivated our malaria research group at Oswaldo Cruz Foundation (state of Minas Gerais, Brazil) to conduct a number of immunoepidemiological studies to characterise the naturally acquired immunity to PvDBP in populations living in the Amazon rainforest. In this review, we provide an update on the immunology and molecular epidemiology of PvDBP in the Brazilian Amazon - an area of markedly unstable malaria transmission - and compare it with data from other parts of Latin America, as well as Asia and Oceania.

Inside scoop on outside proteins

Invasion into red blood cells is an essential step in the life cycle of parasites that cause human malaria. Antibodies targeting the key parasite proteins in this process are important for developing a protective immune response. In the current issue, Boyle and colleagues provide a detailed examination of Plasmodium falciparum invasion and specifically illuminate the fate of surface-exposed parasite proteins during and immediately after invasion.

Specific phosphorylation of the PfRh2b invasion ligand of Plasmodium falciparum

Red blood cell invasion by the malaria parasite Plasmodium falciparum relies on a complex protein network that uses low and high affinity receptor–ligand interactions. Signal transduction through the action of specific kinases is a control mechanism for the orchestration of this process. In the present study we report on the phosphorylation of the CPD (cytoplasmic domain) of P. falciparum Rh2b (reticulocyte homologue protein 2b). First, we identified Ser³²³³ as the sole phospho-acceptor site in the CPD for in vitro phosphorylation by parasite extract. We provide several lines of evidence that this phosphorylation is mediated by PfCK2 (P. falciparum casein kinase 2): phosphorylation is cAMP independent, utilizes ATP as well as GTP as phosphate donors, is inhibited by heparin and tetrabromocinnamic acid, and is mediated by purified PfCK2. We raised a phospho-specific antibody and showed that Ser³²³³ phosphorylation occurs in the parasite prior to host cell egress. We analysed the spatiotemporal aspects of this phosphorylation using immunoprecipitated endogenous Rh2b and minigenes expressing the CPD either at the plasma or rhoptry membrane. Phosphorylation of Rh2b is not spatially restricted to either the plasma or rhoptry membrane and most probably occurs before Rh2b is translocated from the rhoptry neck to the plasma membrane.

Erythrocyte-binding antigens of Plasmodium falciparum are targets of human inhibitory antibodies and function to evade naturally acquired immunity

Abs that inhibit Plasmodium falciparum invasion of erythrocytes form an important component of human immunity against malaria, but key target Ags are largely unknown. Phenotypic variation by P. falciparum mediates the evasion of inhibitory Abs, contributing to the capacity of P. falciparum to cause repeat and chronic infections. However, Ags involved in mediating immune evasion have not been defined, and studies of the function of human Abs are limited. In this study, we used novel approaches to determine the importance of P. falciparum erythrocyte-binding Ags (EBAs), which are important invasion ligands, as targets of human invasion-inhibitory Abs and define their role in contributing to immune evasion through variation in function. We evaluated the invasion-inhibitory activity of acquired Abs from malaria-exposed children and adults from Kenya, using P. falciparum with disruption of genes encoding EBA140, EBA175, and EBA181, either individually or combined as EBA140/EBA175 or EBA175/EBA181 double knockouts. Our findings provide important new evidence that variation in the expression and function of the EBAs plays an important role in evasion of acquired Abs and that a substantial amount of phenotypic diversity results from variation in expression of different EBAs that contributes to immune evasion by P. falciparum. All three EBAs were identified as important targets of naturally acquired inhibitory Abs demonstrated by differential inhibition of parental parasites greater than EBA knockout lines. This knowledge will help to advance malaria vaccine development and suggests that multiple invasion ligands need to be targeted to overcome the capacity of P. falciparum for immune evasion.

Identification and prioritization of merozoite antigens as targets of protective human immunity to Plasmodium falciparum malaria for vaccine and biomarker development

The development of effective malaria vaccines and immune biomarkers of malaria is a high priority for malaria control and elimination. Ags expressed by merozoites of Plasmodium falciparum are likely to be important targets of human immunity and are promising vaccine candidates, but very few Ags have been studied. We developed an approach to assess Ab responses to a comprehensive repertoire of merozoite proteins and investigate whether they are targets of protective Abs. We expressed 91 recombinant proteins, located on the merozoite surface or within invasion organelles, and screened them for quality and reactivity to human Abs. Subsequently, Abs to 46 proteins were studied in a longitudinal cohort of 206 Papua New Guinean children to define Ab acquisition and associations with protective immunity. Ab responses were higher among older children and those with active parasitemia. High-level Ab responses to rhoptry and microneme proteins that function in erythrocyte invasion were identified as being most strongly associated with protective immunity compared with other Ags. Additionally, Abs to new or understudied Ags were more strongly associated with protection than were Abs to current vaccine candidates that have progressed to phase 1 or 2 vaccine trials. Combinations of Ab responses were identified that were more strongly associated with protective immunity than responses to their single-Ag components. This study identifies Ags that are likely to be key targets of protective human immunity and facilitates the prioritization of Ags for further evaluation as vaccine candidates and/or for use as biomarkers of immunity in malaria surveillance and control.

Defining the erythrocyte binding domains of Plasmodium vivax tryptophan rich antigen 33.5

Tryptophan-rich antigens play important role in host-parasite interaction. One of the Plasmodium vivax tryptophan-rich antigens called PvTRAg33.5 had earlier been shown to be predominantly of alpha helical in nature with multidomain structure, induced immune responses in humans, binds to host erythrocytes, and its sequence is highly conserved in the parasite population. In the present study, we divided this protein into three different parts i.e. N-terminal (amino acid position 24–106), middle (amino acid position 107–192), and C-terminal region (amino acid position 185–275) and determined the erythrocyte binding activity of these fragments. This binding activity was retained by the middle and C-terminal fragments covering 107 to 275 amino acid region of the PvTRAg33.5 protein. Eight non-overlapping peptides covering this 107 to 275 amino acid region were then synthesized and tested for their erythrocyte binding activity to further define the binding domains. Only two peptides, peptide P4 (at 171–191 amino acid position) and peptide P8 (at 255–275 amino acid position), were found to contain the erythrocyte binding activity. Competition assay revealed that each peptide recognizes its own erythrocyte receptor. These two peptides were found to be located on two parallel helices at one end of the protein in the modelled structure and could be exposed on its surface to form a suitable site for protein-protein interaction. Natural antibodies present in the sera of the P. vivax exposed individuals or the polyclonal rabbit antibodies against this protein were able to inhibit the erythrocyte binding activity of PvTRAg33.5, its fragments, and these two synthetic peptides P4 and P8. Further studies on receptor-ligand interaction might lead to the development of the therapeutic reagent.

Homozygosity for a null allele of SMIM1 defines the Vel-negative blood group phenotype

The Vel antigen is present on red blood cells (RBCs) from all humans except rare Vel-negative individuals who can form antibodies to Vel in response to transfusion or pregnancy. These antibodies may cause severe hemolytic reactions in blood recipients. We combined SNP profiling and transcriptional network modeling to link the Vel-negative phenotype to SMIM1, located in a 97-kb haplotype block on chromosome 1p36. This gene encodes a previously undiscovered, evolutionarily conserved transmembrane protein expressed on RBCs. Notably, 35 of 35 Vel-negative individuals were homozygous for a frameshift deletion of 17 bp in exon 3. Functional studies using antibodies raised against SMIM1 peptides confirmed a null phenotype in RBC membranes, and SMIM1 overexpression induced Vel expression. Genotype screening estimated that ~1 of 17 Swedish blood donors is a heterozygous deletion carrier and ~1 of 1,200 is a homozygous deletion knockout and enabled identification of Vel-negative donors. Our results establish SMIM1 as a new erythroid gene and Vel as a new blood group system.

Various PfRH5 polymorphisms can support Plasmodium falciparum invasion into the erythrocytes of owl monkeys and rats

Aotus nancymaae, the owl monkey, provides a useful laboratory model for research to develop drugs and vaccines against human falciparum malaria; however, many Plasmodium falciparum parasites are unable to invade A. nancymaae erythrocytes, rendering the parasites noninfective to the monkeys. In previous work, we identified a key polymorphism that determined the inheritance of erythrocyte invasion in a genetic cross of two P. falciparum clones that were virulent (GB4) or noninfective (7G8) to A. nancymaae. This polymorphism, an isoleucine-to-lysine polymorphism at position 204 (I204K) of the GB4 erythrocyte binding protein PfRH5, was nevertheless not found in several other P. falciparum lines that could also invade A. nancymaae erythrocytes. Alternative PfRH5 polymorphisms occur at different positions in these virulent parasites, and additional polymorphisms are found in P. falciparum parasites that cannot infect A. nancymaae. By allelic replacement methods, we have introduced the polymorphisms of these A. nancymaae-virulent or noninfective parasites at codons 204, 347, 358, 362, 410, and 429 of the endogenous PfRH5 gene in the noninfective 7G8 line. 7G8 transformants expressing the polymorphisms of the A. nancymaae-virulent parasites show neuraminidase-sensitive (sialic acid-dependent) invasion into the monkey erythrocytes, whereas 7G8 transformants expressing the PfRH5 alleles of noninfective parasites show little or no invasion of these erythrocytes. Parasites harboring PfRH5 polymorphisms 204K or 204R are also able to invade rat erythrocytes and are differentially sensitive to the removal of surface sialic acids by neuraminidase. These studies offer insights into the PfRH5 receptor-binding domain and interactions that support the invasion of various primate and rodent erythrocytes by P. falciparum.