Malaria's deadly grip: cytoadhesion of Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum Protein Microarray Antibody Profiles Correlate With Protection From Symptomatic Malaria in Kenya

BACKGROUND

Immunoglobulin G antibodies (Abs) to Plasmodium falciparum antigens have been associated with naturally acquired immunity to symptomatic malaria.

METHODS

We probed protein microarrays covering 824 unique P. falciparum protein features with plasma from residents of a community in Kenya monitored for 12 weeks for (re)infection and symptomatic malaria after administration of antimalarial drugs. P. falciparum proteins recognized by Abs from 88 children (aged 1-14 years) and 86 adults (aged ≥ 18 years), measured at the beginning of the observation period, were ranked by Ab signal intensity.

RESULTS

Abs from immune adults reacted with a total 163 of 824 P. falciparum proteins. Children gradually acquired Abs to the full repertoire of antigens recognized by adults. Abs to some antigens showed high seroconversion rates, reaching maximal levels early in childhood, whereas others did not reach adult levels until adolescence. No correlation between Ab signal intensity and time to (re)infection was observed. In contrast, Ab levels to 106 antigens were significantly higher in children who were protected from symptomatic malaria compared with those who were not. Abs to antigens predictive of protection included P. falciparum erythrocyte membrane protein 1, merozoite surface protein (MSP) 10, MSP2, liver-stage antigen 3, PF70, MSP7, and Plasmodium helical interspersed subtelomeric domain protein.

CONCLUSIONS

Protein microarrays may be useful in the search for malaria antigens associated with protective immunity.

WITHDRAWN: From within host dynamics to the epidemiology of infectious disease: Scientific overview and challenges

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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.

Discovery of a novel and conserved Plasmodium falciparum exported protein that is important for adhesion of PfEMP1 at the surface of infected erythrocytes

Plasmodium falciparum virulence is linked to its ability to sequester in post-capillary venules in the human host. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is the main variant surface antigen implicated in this process. Complete loss of parasite adhesion is linked to a large subtelomeric deletion on chromosome 9 in a number of laboratory strains such as D10 and T9-96. Similar to the cytoadherent reference line FCR3, D10 strain expresses PfEMP1 on the surface of parasitized erythrocytes, however without any detectable cytoadhesion. To investigate which of the deleted subtelomeric genes may be implicated in parasite adhesion, we selected 12 genes for D10 complementation studies that are predicted to code for proteins exported to the red blood cell. We identified a novel single copy gene (PF3D7_0936500) restricted to P. falciparum that restores adhesion to CD36, termed here virulence-associated protein 1 (Pfvap1). Protein knockdown and gene knockout experiments confirmed a role of PfVAP1 in the adhesion process in FCR3 parasites. PfVAP1 is co-exported with PfEMP1 into the host cell via vesicle-like structures called Maurer's clefts. This study identifies a novel highly conserved parasite molecule that contributes to parasite virulence possibly by assisting PfEMP1 to establish functional adhesion at the host cell surface.

Protein export into malaria parasite-infected erythrocytes: mechanisms and functional consequences

Phylum Apicomplexa comprises a large group of obligate intracellular parasites of high medical and veterinary importance. These organisms succeed intracellularly by effecting remarkable changes in a broad range of diverse host cells. The transformation of the host erythrocyte is particularly striking in the case of the malaria parasite Plasmodium falciparum. P. falciparum exports hundreds of proteins that mediate a complex cellular renovation marked by changes in the permeability, rigidity, and cytoadherence properties of the host erythrocyte. The past decade has seen enormous progress in understanding the identity and function of these exported effectors, as well as the mechanisms by which they are trafficked into the host cell. Here we review these advances, place them in the context of host manipulation by related apicomplexans, and propose key directions for future research.

Paths to a malaria vaccine illuminated by parasite genomics

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Discovery of vaccine candidate antigens by parasite genome sequence analyses.

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Genetic crosses, linkage group selection, and functional studies on parasites.

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Characterizing developmental and epigenetic variation alongside allelic polymorphism.

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Selection by naturally acquired immune responses helps to focus vaccine design.

More human death and disease is caused by malaria parasites than by all other eukaryotic pathogens combined. As early as the sequencing of the first human genome, malaria parasite genomics was prioritized to fuel the discovery of vaccine candidate antigens. This stimulated increased research on malaria, generating new understanding of the cellular and molecular mechanisms of infection and immunity. This review of recent developments illustrates how new approaches in parasite genomics, and increasingly large amounts of data from population studies, are helping to identify antigens that are promising lead targets. Although these results have been encouraging, effective discovery and characterization need to be coupled with more innovation and funding to translate findings into newly designed vaccine products for clinical trials.

Red cell adhesion in human diseases

PURPOSE OF REVIEW

This review discusses the unexpected role of red blood cell (RBC) adhesiveness in the pathophysiology of two red cell diseases, hereditary spherocytosis and polycythemia vera, and two 'nonerythroid' disorders, central retinal vein occlusion and Gaucher disease. These pathologies share common clinical manifestations, that is vaso-occlusion and/or thrombotic events.

RECENT FINDINGS

Recently, the direct involvement of RBC adhesion to the vascular endothelium has been demonstrated in the occurrence of vaso-occlusive events, in particular in sickle cell disease (SCD). Several erythroid adhesion molecules and their ligands have been identified that belong to different molecular classes (integrins, Ig-like molecules, lipids...) and are activated by a variety of signaling pathways. Among these, the laminin receptor, Lutheran/basal cell adhesion molecule, which is activated by phosphorylation, appears to play a central role in several pathologies.

SUMMARY

RBC adhesiveness might be involved in complications such as the vaso-occlusive crisis in SCD, thrombosis in polycythemia vera, splenic sequestration in hereditary spherocytosis, occlusions in central retinal vein occlusion and bone infarcts in Gaucher disease. Characterization of this pathological process at the cellular and molecular levels should prove useful to develop new therapeutic approaches based on the blockade of RBC abnormal interactions with vascular endothelium and/or circulating blood cells.

High-Throughput Testing of Antibody-Dependent Binding Inhibition of Placental Malaria Parasites

The particular virulence of Plasmodium falciparum manifests in diverse severe malaria syndromes as cerebral malaria, severe anemia and placental malaria. The cause of both the severity and the diversity of infection outcome, is the ability of the infected erythrocyte (IE) to bind a range of different human receptors through Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of the infected cell. As the var genes encoding the large PfEMP1 antigens are extensively polymorphic, vaccine development strategies are focused on targeting the functional binding epitopes. This involves identification of recombinant fragments of PfEMP1s that induce antibodies, which hinder the adhesion of the IE to a given receptor or tissue. Different assays to measure the blocking of adhesion have been described in the literature, each with different advantages. This chapter describes a high-throughput assay used in the preclinical and clinical development of a VAR2CSA based vaccine against placental malaria.

Generation of antigenic diversity in Plasmodium falciparum by structured rearrangement of Var genes during mitosis

The most polymorphic gene family in P. falciparum is the ∼60 var genes distributed across parasite chromosomes, both in the subtelomeres and in internal regions. They encode hypervariable surface proteins known as P. falciparum erythrocyte membrane protein 1 (PfEMP1) that are critical for pathogenesis and immune evasion in Plasmodium falciparum. How var gene sequence diversity is generated is not currently completely understood. To address this, we constructed large clone trees and performed whole genome sequence analysis to study the generation of novel var gene sequences in asexually replicating parasites. While single nucleotide polymorphisms (SNPs) were scattered across the genome, structural variants (deletions, duplications, translocations) were focused in and around var genes, with considerable variation in frequency between strains. Analysis of more than 100 recombination events involving var exon 1 revealed that the average nucleotide sequence identity of two recombining exons was only 63% (range: 52.7–72.4%) yet the crossovers were error-free and occurred in such a way that the resulting sequence was in frame and domain architecture was preserved. Var exon 1, which encodes the immunologically exposed part of the protein, recombined in up to 0.2% of infected erythrocytes in vitro per life cycle. The high rate of var exon 1 recombination indicates that millions of new antigenic structures could potentially be generated each day in a single infected individual. We propose a model whereby var gene sequence polymorphism is mainly generated during the asexual part of the life cycle.

Malaria kills >600,000 people each year, with most deaths caused by Plasmodium falciparum. A family of proteins known as P. falciparum erythrocyte membrane protein 1, PfEMP1, is expressed on the surface of infected erythrocytes and plays an important role in pathogenesis. Each P. falciparum genome contains approximately 60 highly polymorphic var genes encoding the PfEMP1 proteins, and monoallelic expression with periodic switching results in immune evasion. Var gene polymorphism is thus critical to this survival strategy. We investigated how var gene diversity is generated by performing an in vitro evolution experiment, tracking var gene mutation in ‘real-time’ with whole genome sequencing. We found that genome structural variation is focused in and around var genes. These genetic rearrangements created new ‘chimeric’ var gene sequences during the mitotic part of the life cycle, and were consistent with processes of mitotic non-allelic homologous recombination. The recombinant var genes were always in frame and with conserved overall var gene architecture, and the recombination rate implies that many millions of rearranged var gene sequences are produced every 48-hour life cycle within infected individuals. In conclusion, we provide a detailed description of how new var gene sequences are continuously generated in the parasite genome, helping to explain long-term parasite survival within infected human hosts.

Llama immunization with full-length VAR2CSA generates cross-reactive and inhibitory single-domain antibodies against the DBL1X domain

VAR2CSA stands today as the leading vaccine candidate aiming to protect future pregnant women living in malaria endemic areas against the severe clinical outcomes of pregnancy associated malaria (PAM). The rational design of an efficient VAR2CSA-based vaccine relies on a profound understanding of the molecular interactions associated with P. falciparum infected erythrocyte sequestration in the placenta. Following immunization of a llama with the full-length VAR2CSA recombinant protein, we have expressed and characterized a panel of 19 nanobodies able to recognize the recombinant VAR2CSA as well as the surface of erythrocytes infected with parasites originating from different parts of the world. Domain mapping revealed that a large majority of nanobodies targeted DBL1X whereas a few of them were directed towards DBL4ε, DBL5ε and DBL6ε. One nanobody targeting the DBL1X was able to recognize the native VAR2CSA protein of the three parasite lines tested. Furthermore, four nanobodies targeting DBL1X reproducibly inhibited CSA adhesion of erythrocytes infected with the homologous NF54-CSA parasite strain, providing evidences that DBL1X domain is part or close to the CSA binding site. These nanobodies could serve as useful tools to identify conserved epitopes shared between different variants and to characterize the interactions between VAR2CSA and CSA.

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

Fatal cerebral malaria: a venous efflux problem

Most Plasmodium falciparum-infected children with cerebral malaria (CM) die from respiratory arrest, but the underlying pathology is unclear. Here we present a model in which the ultimate cause of death from CM is severe intracranial hypertension. Dynamic imaging of mice infected with P. berghei ANKA, an accepted model for experimental CM, revealed that leukocyte adhesion impairs the venous blood flow by reducing the functional lumen of postcapillary venules (PCV). The resulting increase in intracranial pressure (ICP) exacerbates cerebral edema formation, a hallmark of both murine and pediatric CM. We propose that two entirely different pathogenetic mechanisms-cytoadherence of P. falciparum-infected erythrocytes in pediatric CM and leukocyte arrest in murine CM-result in the same pathological outcome: a severe increase in ICP leading to brainstem herniation and death from respiratory arrest. The intracranial hypertension (IH) model unifies previous hypotheses, applies to human and experimental CM alike, eliminates the need to explain any selective recognition mechanism Plasmodium might use to target multiple sensitive sites in the brain, and explains how an intravascular parasite can cause so much neuronal dysfunction.

High-throughput screening platform identifies small molecules that prevent sequestration of Plasmodium falciparum-infected erythrocytes

BACKGROUND

We developed a 2-step approach to screen molecules that prevent and/or reverse Plasmodium falciparum-infected erythrocyte (IE) binding to host receptors. IE adhesion and sequestration in vasculature causes severe malaria, and therefore antiadhesion therapy might be useful as adjunctive treatment. IE adhesion is mediated by the polymorphic family (approximately 60 members) of P. falciparum EMP1 (PfEMP1) multidomain proteins.

METHODS

We constructed sets of PfEMP1 domains that bind ICAM-1, CSA, or CD36, receptors that commonly support IE binding. Combinations of domain-coated beads were assayed by Bio-Plex technology as a high-throughput molecular platform to screen antiadhesion molecules (antibodies and small molecules). Molecules identified as so-called hits in the screen (first step) then could be assayed individually for inhibition of binding of live IE to receptors (second step).

RESULTS

In proof-of-principle studies, the antiadhesion activity of several antibodies was concordant in Bio-Plex and live IE assays. Using this 2-step approach, we identified several molecules in a small molecule library of 10 000 compounds that could inhibit and reverse binding of IEs to ICAM-1 and CSA receptors.

CONCLUSION

This 2-step screening approach should be efficient for identification of antiadhesion drug candidates for falciparum malaria.

Sequence variation and structural conservation allows development of novel function and immune evasion in parasite surface protein families

Trypanosoma and Plasmodium species are unicellular, eukaryotic pathogens that have evolved the capacity to survive and proliferate within a human host, causing sleeping sickness and malaria, respectively. They have very different survival strategies. African trypanosomes divide in blood and extracellular spaces, whereas Plasmodium species invade and proliferate within host cells. Interaction with host macromolecules is central to establishment and maintenance of an infection by both parasites. Proteins that mediate these interactions are under selection pressure to bind host ligands without compromising immune avoidance strategies. In both parasites, the expansion of genes encoding a small number of protein folds has established large protein families. This has permitted both diversification to form novel ligand binding sites and variation in sequence that contributes to avoidance of immune recognition. In this review we consider two such parasite surface protein families, one from each species. In each case, known structures demonstrate how extensive sequence variation around a conserved molecular architecture provides an adaptable protein scaffold that the parasites can mobilise to mediate interactions with their hosts.

Stuck in a rut? Reconsidering the role of parasite sequestration in severe malaria syndromes

Severe malaria defines individuals at increased risk of death from their infection. Proposed pathogenic mechanisms include parasite sequestration, inflammation, and endothelial dysfunction. Severe malaria is not a single entity, manifesting with distinct syndromes such as severe anemia, severe respiratory distress or coma, each characterized by differences in epidemiology, underlying biology, and risk of death. The relative contribution of the various pathogenic mechanisms may differ between syndromes, and this is supported by accumulating evidence, which challenges sequestration as the initiating event. Here we propose that high parasite biomass is the common initiating feature, but subtle variations in the interaction between the host and parasite exist, and understanding these differences may be crucial to improve outcomes in patients with severe malaria.