Immunization with PfEMP1-DBL1alpha generates antibodies that disrupt rosettes and protect against the sequestration of Plasmodium falciparum-infected erythrocytes

Multi-functional antibody profiling for malaria vaccine development and evaluation

INTRODUCTION

A vaccine would greatly accelerate current global efforts toward malaria elimination. While a partially efficacious vaccine has been achieved for Plasmodium falciparum, a major bottleneck in developing highly efficacious vaccines is a lack of reliable correlates of protection, and the limited application of assays that quantify functional immune responses to evaluate and down-select vaccine candidates in pre-clinical studies and clinical trials.

AREAS COVERED

In this review, we describe the important role of antibodies in immunity against malaria and detail the nature and functional activities of antibodies against the malaria-causing parasite. We highlight the growing understanding of antibody effector functions against malaria and in vitro assays to measure these functional antibody responses. We discuss the application of these assays to quantify antibody functions in vaccine development and evaluation.

EXPERT OPINION

It is becoming increasingly clear that multiple antibody effector functions are involved in immunity to malaria. Therefore, we propose that evaluating vaccine candidates needs to move beyond individual assays or measuring IgG magnitude alone. Instead, vaccine evaluation should incorporate the full breadth of antibody response types and harness a wider range of assays measuring functional antibody responses. We propose a 3-tier approach to implementing assays to inform vaccine evaluation.

Longitudinal analysis of naturally acquired PfEMP1 CIDR domain variant antibodies identifies associations with malaria protection

BACKGROUNDMalaria pathogenicity is determined, in part, by the adherence of Plasmodium falciparum-infected erythrocytes to the microvasculature mediated via specific interactions between P. falciparum erythrocyte membrane protein (PfEMP1) variant domains and host endothelial receptors. Naturally acquired antibodies against specific PfEMP1 variants can play an important role in clinical protection against malaria.METHODSWe evaluated IgG responses against a repertoire of PfEMP1 CIDR domain variants to determine the rate and order of variant-specific antibody acquisition and their association with protection against febrile malaria in a prospective cohort study conducted in an area of intense, seasonal malaria transmission.RESULTSUsing longitudinal data, we found that IgG antibodies against the pathogenic domain variants CIDRα1.7 and CIDRα1.8 were acquired the earliest. Furthermore, IgG antibodies against CIDRγ3 were associated with reduced prospective risk of febrile malaria and recurrent malaria episodes.CONCLUSIONThis study provides evidence that acquisition of IgG antibodies against PfEMP1 variants is ordered and demonstrates that antibodies against CIDRα1 domains are acquired the earliest in children residing in an area of intense, seasonal malaria transmission. Future studies will need to validate these findings in other transmission settings and determine the functional activity of these naturally acquired CIDR variant-specific antibodies.TRIAL REGISTRATIONClinicalTrials.gov NCT01322581.FUNDINGDivision of Intramural Research, National Institute of Allergy and Infectious Diseases, NIH.

Protective Immunity against Severe Malaria in Children Is Associated with a Limited Repertoire of Antibodies to Conserved PfEMP1 Variants

Extreme diversity of the major Plasmodium falciparum antigen, PfEMP1, poses a barrier to identifying targets of immunity to malaria. Here, we used protein microarrays containing hundreds of variants of the DBLα domain of PfEMP1 to cover the diversity of Papua New Guinean (PNG) parasites. Probing the plasma of a longitudinal cohort of malaria-exposed PNG children showed that group 2 DBLα antibodies were moderately associated with a lower risk of uncomplicated malaria, whereas individual variants were only weakly associated with clinical immunity. In contrast, antibodies to 85 individual group 1 and 2 DBLα variants were associated with a 70%-100% reduction in severe malaria. Of these, 17 variants were strong predictors of severe malaria. Analysis of full-length PfEMP1 sequences from PNG samples shows that these 17 variants are linked to pathogenic CIDR domains. This suggests that whereas immunity to uncomplicated malaria requires a broad repertoire of antibodies, immunity to severe malaria targets a subset of conserved variants. These findings provide insights into antimalarial immunity and potential antibody biomarkers for disease risk.

Exploring Plasmodium falciparum Var Gene Expression to Assess Host Selection Pressure on Parasites During Infancy

In sub-Saharan Africa, children below 5 years bear the greatest burden of severe malaria because they lack naturally acquired immunity that develops following repeated exposure to infections by Plasmodium falciparum. Antibodies to the surface of P. falciparum infected erythrocytes (IE) play an important role in this immunity. In children under the age of 6 months, relative protection from severe malaria is observed and this is thought to be partly due to trans-placental acquired protective maternal antibodies. However, the protective effect of maternal antibodies has not been fully established, especially the role of antibodies to variant surface antigens (VSA) expressed on IE. Here, we assessed the immune pressure on parasites infecting infants using markers associated with the acquisition of naturally acquired immunity to surface antigens. We hypothesized that, if maternal antibodies to VSA imposed a selection pressure on parasites, then the expression of a relatively conserved subset of var genes called group A var genes in infants should change with waning maternal antibodies. To test this, we compared their expression in parasites from children between 0 and 12 months and above 12 months of age. The transcript quantity and the proportional expression of group A var subgroup, including those containing domain cassette 13, were positively associated with age during the first year of life, which contrasts with above 12 months. This was accompanied by a decline in infected erythrocyte surface antibodies and an increase in parasitemia during this period. The observed increase in group A var gene expression with age in the first year of life, when the maternal antibodies are waning and before acquisition of naturally acquired antibodies with repeated exposure, is consistent with the idea that maternally acquired antibodies impose a selection pressure on parasites that infect infants and may play a role in protecting these infants against severe malaria.

Plasmodium falciparum malaria parasite var gene expression is modified by host antibodies: longitudinal evidence from controlled infections of Kenyan adults with varying natural exposure

Background

The PfEMP1 family of Plasmodium falciparum antigens play a key role in pathogenesis of severe malaria through their insertion into the surface of parasite infected erythrocytes, and adhesion to host cells. Previous studies have suggested that parasites expressing PfEMP1 subclasses group A and DC8, associated with severe malaria, may have a growth advantage in immunologically naïve individuals. However, this idea has not been tested in longitudinal studies.

Methods

Here we assessed expression of the var genes encoding PfEMP1, in parasites sampled from volunteers with varying prior exposure to malaria, following experimental infection by sporozoites (PfSPZ). Using qPCR, we tested for associations between the expression of various var subgroups in surviving parasite populations from each volunteer and 1) the levels of participants’ antibodies to infected erythrocytes before challenge infection and 2) the apparent in vivo parasite multiplication rate.

Results

We show that 1) expression of var genes encoding for group A and DC8-like PfEMP1 were associated with low levels of antibodies to infected erythrocytes (αIE) before challenge, and 2) expression of a DC8-like CIDRα1.1 domain was associated with higher apparent parasite multiplication rate in a manner that was independent of levels of prior antibodies to infected erythrocytes.

Conclusions

This study provides insight into the role of antibodies to infected erythrocytes surface antigens in the development of naturally acquired immunity and may help explain why specific PfEMP1 variants may be associated with severe malaria.

Trial registration

Pan African Clinical Trial Registry: PACTR201211000433272. Date of registration: 10th October 2012.

Diversity of vir Genes in Plasmodium vivax from Endemic Regions in the Republic of Korea: an Initial Evaluation

Variant surface antigens (VSAs) encoded by pir families are considered to be the key proteins used by many Plasmodium spp. to escape the host immune system by antigenic variation. This attribute of VSAs is a critical issue in the development of a novel vaccine. In this regard, a population genetic study of vir genes from Plasmodium vivax was performed in the Republic of Korea (ROK). Eighty-five venous blood samples and 4 of the vir genes, namely vir 27, vir 21, vir 12, and vir 4, were selected for study. The number of segregating sites (S), number of haplotypes (H), haplotype diversity (Hd), DNA diversity (π and Θ_w), and Tajima’s D test value were conducted. Phylogenetic trees of each gene were constructed. The vir 21 (S=143, H=22, Hd=0.827) was the most genetically diverse gene, and the vir 4 (S=6, H=4, Hd=0.556) was the opposite one. Tajima’s D values for vir 27 (1.08530, P>0.1), vir 12 (2.89007, P<0.01), and vir 21 (0.40782, P>0.1) were positive, and that of vir 4 (−1.32162, P>0.1) was negative. All phylogenetic trees showed 2 clades with no particular branching according to the geographical differences and cluster. This study is the first survey on the vir genes in ROK, providing information on the genetic level. The sample sequences from vir 4 showed a clear difference to the Sal-1 reference gene sequence, whereas they were very similar to those from Indian isolates.

Effects of sevuparin on rosette formation and cytoadherence of Plasmodium falciparum infected erythrocytes

In severe falciparum malaria cytoadherence of parasitised red blood cells (PRBCs) to vascular endothelium (causing sequestration) and to uninfected red cells (causing rosette formation) contribute to microcirculatory flow obstruction in vital organs. Heparin can reverse the underlying ligand-receptor interactions, but may increase the bleeding risks. As a heparin-derived polysaccharide, sevuparin has been designed to retain anti-adhesive properties, while the antithrombin-binding domains have been eliminated, substantially diminishing its anticoagulant activity. Sevuparin has been evaluated recently in patients with uncomplicated falciparum malaria, and is currently investigated in a clinical trial for sickle cell disease. The effects of sevuparin on rosette formation and cytoadherence of Plasmodium falciparum isolates from Thailand were investigated. Trophozoite stages of P. falciparum-infected RBCs (Pf-iRBCs) were cultured from 49 patients with malaria. Pf-iRBCs were treated with sevuparin at 37°C and assessed in rosetting and in cytoadhesion assays with human dermal microvascular endothelial cells (HDMECs) under static and flow conditions. The proportion of Pf-iRBCs forming rosettes ranged from 6.5% to 26.0% (median = 12.2%). Rosetting was dose dependently disrupted by sevuparin (50% disruption by 250 μg/mL). Overall 57% of P. falciparum isolates bound to HDMECs under static conditions; median (interquartile range) Pf-iRBC binding was 8.5 (3.0–38.0) Pf-iRBCs/1000 HDMECs. Sevuparin in concentrations ≥ 100 μg/mL inhibited cytoadherence. Sevuparin disrupts P. falciparum rosette formation in a dose dependent manner and inhibits cytoadherence to endothelial cells. The data support assessment of sevuparin as an adjunctive treatment to the standard therapy in severe falciparum malaria.

Development of drug-loaded immunoliposomes for the selective targeting and elimination of rosetting Plasmodium falciparum-infected red blood cells

Parasite proteins exported to the surface of Plasmodium falciparum-parasitized red blood cells (pRBCs) have a major role in severe malaria clinical manifestation, where pRBC cytoadhesion and rosetting processes have been strongly linked with microvascular sequestration while avoiding both spleen filtration and immune surveillance. The parasite-derived and pRBC surface-exposed PfEMP1 protein has been identified as one of the responsible elements for rosetting and, therefore, considered as a promising vaccine candidate for the generation of rosette-disrupting antibodies against severe malaria. However, the potential role of anti-rosetting antibodies as targeting molecules for the functionalization of antimalarial drug-loaded nanovectors has never been studied. Our manuscript presents a proof-of-concept study where the activity of an immunoliposomal vehicle with a dual performance capable of specifically recognizing and disrupting rosettes while simultaneously eliminating those pRBCs forming them has been assayed in vitro. A polyclonal antibody against the NTS-DBL1α N-terminal domain of a rosetting PfEMP1 variant has been selected as targeting molecule and lumefantrine as the antimalarial payload. After 30min incubation with 2μM encapsulated drug, a 70% growth inhibition for all parasitic forms in culture (IC50: 414nM) and a reduction in ca. 60% of those pRBCs with a rosetting phenotype (IC50: 747nM) were achieved. This immunoliposomal approach represents an innovative combination therapy for the improvement of severe malaria therapeutics having a broader spectrum of activity than either anti-rosetting antibodies or free drugs on their own.

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

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

Comparative study of Plasmodium falciparum erythrocyte membrane protein 1-DBLα domain variants with respect to antigenic variations and docking interaction analysis with glycosaminoglycans

The variant surface antigen PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) encoded by the polymorphic multi-copy var gene family plays an important role in parasite biology and the host-parasite interactions. Sequestration and antigenic variation is an essential component in the survival and pathogenesis of Plasmodium falciparum and contributes to chronic infection. The DBLα domain of PfEMP1 is a potential target for immuno-epidemiological studies and has been visualized as a vaccine candidate against severe malaria. Specific host receptors like heparin, heparan sulphate, blood group A and complement receptor 1 have been reported to bind the DBLα domain. Although heparin has been experimentally shown to disrupt the parasite-host interaction and effectively disrupt rosetting, the binding sites for the DBLα domain and the mechanism behind heparin-mediated rosette inhibition have not been elucidated. In this study, 3D structures and epitopes of the DBLα domain in 3D7 and in two Indian isolates have been predicted and compared. We have carried out docking studies on DBLα domains with human GAG receptors (heparin and heparan sulphate) to predict the strength of association between the protein-ligand interactions. The DBLα domain structures showed extensive diversity and polymorphism in their binding sites. The docking results indicate that heparin binds more effectively with high affinity as compared to heparan sulphate with some common interacting residues. These common residues can play an important role in rosetting and will aid in the designing of inhibitors specific to the interactions between DBLα and heparin or heparan sulphate would be important in malaria treatment. Thus it may lead to the development of novel interference strategies to block red blood cell invasion and provide protection against malaria.

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

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

Self-replicating alphavirus RNA vaccines

Recombinant nucleic acids are considered as promising next-generation vaccines. These vaccines express the native antigen upon delivery into tissue, thus mimicking live attenuated vaccines without having the risk of reversion to pathogenicity. They also stimulate the innate immune system, thus potentiating responses. Nucleic acid vaccines are easy to produce at reasonable cost and are stable. During the past years, focus has been on the use of plasmid DNA for vaccination. Now mRNA and replicon vaccines have come into focus as promising technology platforms for vaccine development. This review discusses self-replicating RNA vaccines developed from alphavirus expression vectors. These replicon vaccines can be delivered as RNA, DNA or as recombinant virus particles. All three platforms have been pre-clinically evaluated as vaccines against a number of infectious diseases and cancer. Results have been very encouraging and propelled the first human clinical trials, the results of which have been promising.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Analysis of antibody induction upon immunization with distinct NTS-DBL1α-domains of PfEMP1 from rosetting Plasmodium falciparum parasites

Background

Rosette-formation of Plasmodium falciparum parasitized erythrocytes is of importance in the development of severe malaria. The parasite-derived molecule PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1), central to rosetting, is suggested to be included in a multimeric vaccine targeting severe disease.

Methods

Three recombinant NTS-DBL1α-domains of PfEMP1 were generated in Escherichia coli, purified and used for immunization of rats and goats. Antibody titres were determined in ELISA assays and responses were compared in-between different individual animals and species. Reactivity with the parasites was tested in live pRBC using FACS. B-cell epitopes prediction was carried out in silico and compared to the results obtained by peptide microarray. Screening for serological cross-reactivity with heterologous NTS-DBL1α variants was carried out by ELISA, peptide array and FACS on pRBC of different laboratory strains and patient isolates.

Results

All three NTS-DBL1α-domains induced high titres of antibodies that were biologically active with no apparent difference between constructs covering slightly different parts of the DBL1α-sequence. The different animal species showed comparable titres of antibodies, while variations within individuals of the species could be observed.

Mapping of the recognized epitopes revealed that most parts of the molecule were able to induce an antibody response with a tendency for the N and C terminal parts of the molecule for slightly higher recognition. Important differences to the epitopes predicted were found as some of the most conserved parts of the DBL1α-domain contained the main epitopes for antibody reactivity. ELISA assays and peptide microarray demonstrated substantial cross-reactivity to heterologous variants, while binding to native PfEMP1 was observed only in few combinations on the pRBC surface, underlining that mainly internal, conserved and not surface exposed parts of the DBL1α-domain are responsible for this observation.

Conclusion

Biologically active antibodies can be induced consistently, with high titres, in different animal species and the antibodies elicited by different constructs react with similar epitopes. Induced antibodies recognize epitopes localized in all subdomains of the DBL1α-sequence. Cross-reactivity between NTS-DBL1α-variants is common in ELISA, but rare with live pRBC emphasizing that also internal, conserved areas of PfEMP1 carry important highly immunogenic epitopes of the molecule.

Evolution of the multi-domain structures of virulence genes in the human malaria parasite, Plasmodium falciparum

The var gene family of Plasmodium falciparum encodes the immunodominant variant surface antigens PfEMP1. These highly polymorphic proteins are important virulence factors that mediate cytoadhesion to a variety of host tissues, causing sequestration of parasitized red blood cells in vital organs, including the brain or placenta. Acquisition of variant-specific antibodies correlates with protection against severe malarial infections; however, understanding the relationship between gene expression and infection outcome is complicated by the modular genetic architectures of var genes that encode varying numbers of antigenic domains with differential binding specificities. By analyzing the domain architectures of fully sequenced var gene repertoires we reveal a significant, non-random association between the number of domains comprising a var gene and their sequence conservation. As such, var genes can be grouped into those that are short and diverse and genes that are long and conserved, suggesting gene length as an important characteristic in the classification of var genes. We then use an evolutionary framework to demonstrate how the same evolutionary forces acting on the level of an individual gene may have also shaped the parasite's gene repertoire. The observed associations between sequence conservation, gene architecture and repertoire structure can thus be explained by a trade-off between optimizing within-host fitness and minimizing between-host immune selection pressure. Our results demonstrate how simple evolutionary mechanisms can explain var gene structuring on multiple levels and have important implications for understanding the multifaceted epidemiology of P. falciparum malaria.

Plasmodium falciparum, the most severe of the human malarias, contains within its genome a family of ∼60 var genes which play an important role in disease pathology and maintenance of chronic infections. Var genes have a modular genetic architecture and encode varying numbers of binding domains with specific preferences to a range of host tissues. Given the availability of host receptors for binding and the immunogenic properties of each domain it is not clear why genes encode multiple domains at once and how these are structured within each parasite's antigenic repertoire. Here we investigate the domain architecture of these important virulence genes and highlight an evolutionary trade-off between maintaining within-host fitness and optimizing between-host transmission success as an important driver in structuring var genes and var gene repertoires alike.

Adjunctive therapy for cerebral malaria and other severe forms of Plasmodium falciparum malaria

Severe malaria due to Plasmodium falciparum causes more than 800,000 deaths every year. Primary therapy with quinine or artesunate is generally effective in controlling P. falciparum parasitemia, but mortality from cerebral malaria and other forms of severe malaria remains unacceptably high. Long-term cognitive impairment is also common in children with cerebral malaria. Of the numerous adjunctive therapies for cerebral malaria and severe malaria studied over the past five decades, only one (albumin) was associated with a reduction in mortality. In this article, we review past and ongoing studies of adjunctive therapy, and examine the evidence of efficacy for newer therapies, including inhibitors of cytoadherence (e.g., levamisole), immune modulators (e.g., rosiglitazone), agents that increase nitric oxide levels (e.g., arginine) and neuroprotective agents (e.g., erythropoietin).

Immunisation with recombinant PfEMP1 domains elicits functional rosette-inhibiting and phagocytosis-inducing antibodies to Plasmodium falciparum

Background

Rosetting is a Plasmodium falciparum virulence factor implicated in the pathogenesis of life-threatening malaria. Rosetting occurs when parasite–derived P. falciparum Erythrocyte Membrane Protein One (PfEMP1) on the surface of infected erythrocytes binds to human receptors on uninfected erythrocytes. PfEMP1 is a possible target for a vaccine to induce antibodies to inhibit rosetting and prevent severe malaria.

Methodology/Findings

We examined the vaccine potential of the six extracellular domains of a rosette-mediating PfEMP1 variant (ITvar9/R29var1 from the R29 parasite strain) by immunizing rabbits with recombinant proteins expressed in E. coli. Antibodies raised to each domain were tested for surface fluorescence with live infected erythrocytes, rosette inhibition and phagocytosis-induction. Antibodies to all PfEMP1 domains recognized the surface of live infected erythrocytes down to low concentrations (0.02–1.56 µg/ml of total IgG). Antibodies to all PfEMP1 domains except for the second Duffy-Binding-Like region inhibited rosetting (50% inhibitory concentration 0.04–4 µg/ml) and were able to opsonize and induce phagocytosis of infected erythrocytes at low concentrations (1.56–6.25 µg/ml). Antibodies to the N-terminal region (NTS-DBL1α) were the most effective in all assays. All antibodies were specific for the R29 parasite strain, and showed no functional activity against five other rosetting strains.

Conclusions/Significance

These results are encouraging for vaccine development as they show that potent antibodies can be generated to recombinant PfEMP1 domains that will inhibit rosetting and induce phagocytosis of infected erythrocytes. However, further work is needed on rosetting mechanisms and cross-reactivity in field isolates to define a set of PfEMP1 variants that could induce functional antibodies against a broad range of P. falciparum rosetting parasites.

var gene transcription and PfEMP1 expression in the rosetting and cytoadhesive Plasmodium falciparum clone FCR3S1.2

Background

The pathogenicity of Plasmodium falciparum is in part due to the ability of the parasitized red blood cell (pRBC) to adhere to intra-vascular host cell receptors and serum-proteins. Binding of the pRBC is mediated by Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a large multi-variant molecule encoded by a family of ≈60 var genes.

Methods

The study of var gene transcription in the parasite clone FCR3S1.2 was performed by semi-quantitative PCR and quantitative PCR (qPCR). The expression of the major PfEMP1 in FCR3S1.2 pRBC was analysed with polyclonal sera in rosette disruption assays and immunofluorecence.

Results

Transcripts from var1 (FCR3S1.2_var1; IT4var21) and other var genes were detected by semi-quantitative PCR but results from qPCR showed that one var gene transcript dominated over the others (FCR3S1.2_var2; IT4var60). Antibodies raised in rats to the recombinant NTS-DBL1α of var2 produced in E. coli completely and dose-dependently disrupted rosettes (≈95% at a dilution of 1/5). The sera reacted with the Maurer's clefts in trophozoite stages (IFA) and to the infected erythrocyte surface (FACS) indicating that FCR3S1.2_var2 encodes the dominant PfEMP1 expressed in this parasite.

Conclusion

The major transcript in the rosetting model parasite FCR3S1.2 is FCR3S1.2_var2 (IT4var60). The results suggest that this gene encodes the PfEMP1-species responsible for the rosetting phenotype of this parasite. The activity of previously raised antibodies to the NTS-DBL1α of FCR3S1.2_var1 is likely due to cross-reactivity with NTS-DBL1α of the var2 encoded PfEMP1.

Blood groups and malaria: fresh insights into pathogenesis and identification of targets for intervention

PURPOSE OF REVIEW

This review summarizes recent advances in our understanding of the interaction between malaria parasites and blood group antigens and discusses how the knowledge gleaned can be used to target the development of new antimalarial treatments and vaccines.

RECENT FINDINGS

Studies of the interaction between Plasmodium vivax and the Duffy antigen provide the clearest example of the potential for basic research on blood groups and malaria to be translated into a vaccine that could have a major impact on global health. Progress is also being made in understanding the effects of other blood group antigens on malaria. After years of controversy, the effect of ABO blood groups on falciparum malaria has been clarified, with the non-O blood groups emerging as significant risk factors for life-threatening malaria, through the mechanism of enhanced rosette formation. The Knops blood group system may also influence malaria susceptibility, although conflicting results from different countries mean that further research is required. Unanswered questions remain about the interactions between malaria parasites and other blood group antigens, including the Gerbich, MNS and Rhesus systems.

SUMMARY

The interplay between malaria parasites and blood group antigens remains a fascinating subject with potential to contribute to the development of new interventions to reduce the global burden of malaria.

Viral vectors in malaria vaccine development

Traditional vaccine technologies have resulted in an impressive array of efficacious vaccines against a variety of infectious agents. However, several potentially deadly pathogens, including retroviruses and parasites, have proven less amenable to the application of traditional vaccine platforms, indicating the need for new approaches. Viral vectors represent an attractive way to deliver and present vaccine antigens that may offer advantages over traditional platforms. Due to their ability to induce strong cell-mediated immunity (CMI) in addition to antibodies, viral vectors may be suitable for infectious agents, such as malaria parasites, where potent CMI is required for protection. Poxvirus-vectored malaria vaccines have been the most extensively studied in the clinic, achieving significant reductions in liver-stage parasite burden. More recently, adenovirus-vectored malaria vaccines have entered clinical testing. The most promising approach - heterologous prime-boost regimens, in which different viral vectors are sequentially paired with each other or with DNA or recombinant protein vaccines - is now being explored, and could provide high-grade protection, if findings in animal models are translatable to humans. Significant barriers remain, however, such as pre-existing immunity to the vector particle and an unexplained safety signal observed in one trial suggesting an increased risk of HIV acquisition in volunteers with pre-existing immunity to the vector.

In vitro selection of RNA aptamers against a conserved region of the Plasmodium falciparum erythrocyte membrane protein 1

The var-gene encoding Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is known to play a major role in the pathogenicity of the P. falciparum parasite. The protein enables the parasite to adhere to the endothelial linings of small blood vessels (cytoadherence) as well as to non-infected erythrocytes (rosetting), thus preventing clearance from the bloodstream. The development and spread of resistance towards most anti-malarial drugs used for treatment and prevention of the most severe form of malaria truly emphasise the importance of a continuous research and development of new drugs. In this study we use Systematic Evolution of Ligands by EXponential enrichment (SELEX) methodology to isolate high-affinity ligands (aptamers). To validate the results from the SELEX in vitro selection, different aptamers have been selected against PfEMP1 in a live cell assay of P. falciparum strain FCR3S1.2, a highly rosetting strain. We have been able to show the rosette disrupting capacity of these SELEX-aptamers at concentrations of 33 nM and with 100% disruption at 387 nM. The described results show that RNA aptamers are promising candidates for adjunct therapy in severe malaria.

Functional and immunological characterization of a Duffy binding-like alpha domain from Plasmodium falciparum erythrocyte membrane protein 1 that mediates rosetting

The Duffy binding-like (DBL) domains are common adhesion modules present in Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) variants, which are responsible for immune evasion and cytoadherence. Knowledge about how immune responses are acquired against polymorphic DBL domains of PfEMP1 can aid in the development of vaccines for malaria. A recombinant DBLalpha domain, encoded by R29 var1, which binds complement receptor 1 to mediate rosetting by the P. falciparum laboratory strain R29, was expressed in Escherichia coli, renatured by oxidative refolding to its native form, and purified to homogeneity. Antibody levels in 704 plasmas obtained from residents of areas of different levels of malaria endemicity in Orissa (India) and Manhiça (Mozambique) were assessed by enzyme-linked immunosorbent assay. The refolded DBLalpha domain was pure, homogeneous, and functional in that it bound human erythrocytes with specificity and was capable of inhibiting rosetting. The proportion of individuals who had measurable anti-DBLalpha immunoglobulin G responses was low in areas of low malaria endemicity in Orissa (6.7%) but high in areas of high endemicity in Orissa (87.5%) and Manhiça (74.5%). Seroprevalence and antibody levels against the recombinant protein increased with the age of inhabitants from areas with high transmission rates (P < 0.001). Half of the children in these areas had seroconverted by the age of 5 years. These findings suggest that in spite of the extreme polymorphism of PfEMP1 DBLalpha domains, the acquisition of specific antibodies is rapid and age related and reflects the reduced risk of malaria in areas with high transmission rates. Further studies are required to elucidate the role of these antibodies in protection from malaria.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

Plasmodium vivax and the importance of the subtelomeric multigene vir superfamily

Plasmodium vivax is responsible for more than 100 million clinical cases yearly. Unlike P. falciparum, in which infected red blood cells cytoadhere via variant proteins, avoiding passage through the spleen, P.-vivax-infected reticulocytes seem not to cytoadhere. However, a variant subtelomeric multigene vir family has been identified in P. vivax. Thus, questions remain about how P. vivax circulates through the spleen and the role of Vir proteins. In this review, the importance of the vir multigene superfamily is reviewed in the light of the completion of the entire genome sequence of P. vivax and from data gathered from experimental infections in reticulocyte-prone non-lethal malaria parasites and natural P. vivax infections.

An in vivo and in vitro model of Plasmodium falciparum rosetting and autoagglutination mediated by varO, a group A var gene encoding a frequent serotype

In the Saimiri sciureus monkey, erythrocytes infected with the varO antigenic variant of the Plasmodium falciparum Palo Alto 89F5 clone bind uninfected red blood cells (rosetting), form autoagglutinates, and have a high multiplication rate, three phenotypic characteristics that are associated with severe malaria in human patients. We report here that varO parasites express a var gene having the characteristics of group A var genes, and we show that the varO Duffy binding-like 1alpha(1) (DBL1alpha(1)) domain is implicated in the rosetting of both S. sciureus and human erythrocytes. The soluble varO N-terminal sequence (NTS)-DBL1alpha(1) recombinant domain, produced in a baculovirus-insect cell system, induced high titers of antibodies that reacted with varO-infected red blood cells and disrupted varO rosettes. varO parasites were culture adapted in vitro using human erythrocytes. They formed rosettes and autoagglutinates, and they had the same surface serotype and expressed the same varO gene as the monkey-propagated parasites. To develop an in vitro model with highly homogeneous varO parasites, rosette purification was combined with positive selection by panning with a varO NTS-DBL1alpha(1)-specific mouse monoclonal antibody. The single-variant, clonal parasites were used to analyze seroprevalence for varO at the village level in a setting where malaria is holoendemic (Dielmo, Senegal). We found 93.6% (95% confidence interval, 89.7 to 96.4%) seroprevalence for varO surface-reacting antibodies and 86.7% (95% confidence interval, 82.8 to 91.6%) seroprevalence for the recombinant NTS-DBL1alpha(1) domain, and virtually all permanent residents had seroconverted by the age of 5 years. These data imply that the varO model is a relevant in vivo and in vitro model for rosetting and autoagglutination that can be used for rational development of vaccine candidates and therapeutic strategies aimed at preventing malaria pathology.

Plasmodium falciparum malaria vaccines in development

The development and implementation of a malaria vaccine would constitute a major breakthrough for global health. Recently, numerous new candidates have entered clinical testing, following strategies that are as diverse as the malaria cycle is complex. While promising results have been obtained, some candidate vaccines have not fulfilled expectations. The challenges are not merely scientific; further progresses will require the development of competent investigator networks, partnerships between academics, industry and funding agencies, and continuous political commitment. In this review, we present the developmental status of all malaria vaccine candidates that are currently in human clinical testing against Plasmodium falciparum, as well as selected malaria vaccine candidates at preclinical development stage, and discuss the main challenges facing the field of malaria vaccine development.

PfEMP1-DBL1alpha amino acid motifs in severe disease states of Plasmodium falciparum malaria

An infection with Plasmodium falciparum may lead to severe malaria as a result of excessive binding of infected erythrocytes in the microvasculature. Vascular adhesion is mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP1), which is encoded for by highly polymorphic members of the var-gene family. Here, we profile var gene transcription in fresh P. falciparum trophozoites from Ugandan children with malaria through var-specific DBL1alpha-PCR amplification and sequencing. A method for subsectioning region alignments into homology areas (MOTIFF) was developed to examine collected sequences. Specific PfEMP1-DBL1alpha amino acid motifs correlated with rosetting and severe malaria, with motif location corresponding to distinct regions of receptor interaction. The method is potentially applicable to other families of variant proteins and may be useful in identifying sequence-phenotype relationships. The results suggest that certain PfEMP1 sequences are predisposed to inducing severe malaria.

A novel DBL-domain of the P. falciparum 332 molecule possibly involved in erythrocyte adhesion

Plasmodium falciparum malaria is brought about by the asexual stages of the parasite residing in human red blood cells (RBC). Contact between the erythrocyte surface and the merozoite is the first step for successful invasion and proliferation of the parasite. A number of different pathways utilised by the parasite to adhere and invade the host RBC have been characterized, but the complete biology of this process remains elusive. We here report the identification of an open reading frame (ORF) representing a hitherto unknown second exon of the Pf332 gene that encodes a cysteine-rich polypeptide with a high degree of similarity to the Duffy-binding-like (DBL) domain of the erythrocyte-binding-ligand (EBL) family. The sequence of this DBL-domain is conserved and expressed in all parasite clones/strains investigated. In addition, the expression level of Pf332 correlates with proliferation efficiency of the parasites in vitro. Antibodies raised against the DBL-domain are able to reduce the invasion efficiency of different parasite clones/strains. Analysis of the DBL-domain revealed its ability to bind to uninfected human RBC, and moreover demonstrated association with the iRBC surface. Thus, Pf332 is a molecule with a potential role to support merozoite invasion. Due to the high level of conservation in sequence, the novel DBL-domain of Pf332 is of possible importance for development of novel anti-malaria drugs and vaccines.

Comparison of cellular and humoral responses to recombinant protein and synthetic peptides of exon2 region of Plasmodium falciparum erythrocyte membrane protein1 (PfEMP1) among malaria patients from an endemic region

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed on the surface of parasitized red blood cells (PRBCs) mediate adhesion of PRBCs to host vascular endothelial receptors and is considered responsible for pathogenesis of severe P. falciparum malaria. The present study was undertaken to measure cellular immune responses and serum antibody responses against recombinant exon2 protein, the most conserved region of PfEMP1, and its synthetic peptides. T cell recognizing this domain could provide universal help to B cells in recognizing variant epitopes located in the extracellular region of PfEMP1. Human peripheral blood mononuclear cells from malaria-exposed immune adults (IA), malaria patients with varying severity, and malaria unexposed healthy donors were stimulated with recombinant exon2 protein and six synthetic peptides from its sequence to estimate the proliferative, IFN-gamma, and IL-4 responses. Antibody responses against these synthetic peptides and exon2 protein were also studied. Positive proliferative, IFN-gamma, and IL-4 responses in IA group each were 60% with recombinant exon2 protein and 27-47% with different synthetic peptides. Antibody recognition was observed in 67% with exon2 and between 40 and 53% with different peptides. In malaria patients, frequency and magnitude of proliferative response, IL-4 concentration, and antibody recognition were far less than immune adults but IFN-gamma response was almost similar. Proportion of positive responders and the magnitude of response to synthetic peptides were low. Also, there was no consistency in response of different peptides towards proliferative, cytokine, and antibody responses in IA and malaria patient groups except for peptide 1. We presume peptide 1 is a potential vaccine candidate and different cocktails containing peptide 1 are being evaluated for their T cell immunogenicity.

Structural modifications to a high-activity binding peptide located within the PfEMP1 NTS domain induce protection against P. falciparum malaria in Aotus monkeys

Binding of P. falciparum-infected erythrocytes to vascular endothelium and to uninfected erythrocytes is mediated by the parasite-derived variant erythrocyte membrane protein PfEMP-1 and various receptors, both on the vascular endothelium and on the erythrocyte surface. Consecutive, non-overlapping peptides spanning the N-terminal segment (NTS) and Duffy-binding-like PfEMP1 sequence alpha-domain (DBLalpha) of this protein were tested in erythrocyte and C32 cell binding assays. Eight peptides specifically bound to C32 cells, and were named high-activity binding peptides (HABPs). No erythrocyte binding HABPs were found in this region. Strikingly, three HABPs [6504 ((1)MVELA KMGPK EAAGG DDIED(20)), 6505 ((21)ESAKH MFDRI GKDVY DKVKE(40)) and 6506 ((41)YRAKE RGKGL QGRLS EAKFEK(60))] are located within the NTS, for which no specific function has yet been described. HABP 6505 is neither immunogenic nor protection-inducing; therefore, based on our previous reports, critical amino acids (shown in bold) in HABP-C32 cell binding were identified and replaced to modify HABP immunogenicity and protectivity. Analogue peptide 12722 (ESAKH KFDRI GKDVY DMVKE) produced high antibody titres and completely protected three out of 12 vaccinated Aotus monkeys and 23410 (KHKFD FIGKI VYDMV KER) also produced high protection-inducing titres and completely protected one out of eight monkeys. (1)H NMR studies showed that all peptides were helical. Binding of these peptides to isolated HLADRbeta1 molecules did not reveal any preference, suggesting that they could bind to molecules not studied here.

Complement factor D, albumin, and immunoglobulin G anti-band 3 protein antibodies mimic serum in promoting rosetting of malaria-infected red blood cells

Rosetting of Plasmodium falciparum-infected red blood cells (parasitized RBC [pRBC]) with uninfected RBC has been associated in many studies with malaria morbidity and is one form of cytoadherence observed with malarial parasites. Rosetting is serum dependent for many isolates of P. falciparum, including the strains FCR3S1.2 and Malayan Camp studied here. We identified the three naturally occurring components of sera which confer rosetting. Complement factor D alone induced 30 to 40% of de novo rosetting. Its effect was additive to that of 0.5 mg/ml albumin and to that of 15 ng/ml of naturally occurring antibodies to the anion transport protein, band 3. The three components together mediated rosetting as effectively as 10% serum. De novo rosetting experiments showed that naturally occurring anti-band 3 antibodies as well as factor D were effective only when added to pRBC. Factor D appeared to cleave a small fraction of a protein expressed on the surface of pRBC.

Release of sequestered malaria parasites upon injection of a glycosaminoglycan

Severe human malaria is attributable to an excessive sequestration of Plasmodium falciparum–infected and uninfected erythrocytes in vital organs. Strains of P. falciparum that form rosettes and employ heparan sulfate as a host receptor are associated with development of severe forms of malaria. Heparin, which is similar to heparan sulfate in that it is composed of the same building blocks, was previously used in the treatment of severe malaria, but it was discontinued due to the occurrence of serious side effects such as intracranial bleedings. Here we report to have depolymerized heparin by periodate treatment to generate novel glycans (dGAG) that lack anticoagulant-activity. The dGAGs disrupt rosettes, inhibit merozoite invasion of erythrocytes and endothelial binding of P. falciparum–infected erythrocytes in vitro, and reduce sequestration in in vivo models of severe malaria. An intravenous injection of dGAGs blocks up to 80% of infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestered parasites into circulation. P. falciparum–infected human erythrocytes that sequester in the non-human primate Macaca fascicularis were similarly found to be released in to the circulation upon a single injection of 500 μg of dGAG. We suggest dGAGs to be promising candidates for adjunct therapy in severe malaria.

Severe Plasmodium falciparum malaria is common and in part the result of an excessive binding of infected erythrocytes in the microvasculature. The parasite employs heparan sulfate during the adherence to the vascular endothelium and to erythrocytes. Heparin, which is related to heparan sulfate in that it is composed of the same building blocks, was here periodate-treated to generate depolymerized glycosaminoglycans (dGAGs) that possess no anticoagulant activity. The dGAGs disrupt erythrocyte and endothelial binding of P. falciparum–infected erythrocytes in vitro. An intravenous injection of dGAGs blocks infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestrated parasites into circulation both in the rat and in a non-human primate. If this approach is successfully translated to the clinical setting, it may offer help to patients whereby the injection of a dGAG releases already sequestered parasite-infected erythrocytes and re-establishes the micro-vascular blood flow. The authors suggest dGAGs to be promising candidates of adjunct therapy that may have an important impact on malaria mortality.

Modulation of malaria virulence by determinants of Plasmodium falciparum erythrocyte membrane protein-1 display

PURPOSE OF REVIEW

Plasmodium falciparum malaria parasites carry approximately 60 var genes that encode variable adhesins termed P. falciparum erythrocyte membrane protein-1. Clonal expression of a single P. falciparum erythrocyte membrane protein-1 variant on the surface of the parasitized host erythrocyte promotes binding of the cell to blood elements (including noninfected erythrocytes, leukocytes) and walls of microvessels. These binding events enable parasitized erythrocytes to sequester and avoid clearance by the spleen, and they also contribute to disease by causing microvascular inflammation and obstruction.

RECENT FINDINGS

Steps by which P. falciparum erythrocyte membrane protein-1 is exported to the parasitized erythrocyte surface have recently been elucidated. The ability of parasites to cytoadhere and cause disease depends on the variant of P. falciparum erythrocyte membrane protein-1 as well as its amount and distribution at the erythrocyte surface. An example of a host polymorphism that affects P. falciparum erythrocyte membrane protein-1 display is hemoglobin C, which may protect against malaria by impairing the parasite's ability to adhere to microvessels and induce inflammation. Interference with P. falciparum erythrocyte membrane protein-1-mediated phenomena appears to diminish cytoadherence in vivo and to protect against disease in animal models.

SUMMARY

Plasmodium falciparum erythrocyte membrane protein-1-mediated sequestration of parasitized erythrocytes plays a central role in malaria pathogenesis. Clinical interventions aimed at reducing cytoadherence and microvascular inflammation may improve disease outcome.

Generation of cross-protective antibodies against Plasmodium falciparum sequestration by immunization with an erythrocyte membrane protein 1-duffy binding-like 1 alpha domain

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important virulence factor on the surface of infected erythrocytes. Naturally acquired antibodies to PfEMP1 expressed by parasites causing severe malaria are suggested to be protective and of major interest for the development of a vaccine against severe disease. In this study, the PfEMP1 expressed by a parasite clone displaying a multiadhesive phenotype associated with severe malaria was well recognized by sera of malaria semi-immune children. The efficiency of the Duffy binding-like 1 alpha (DBL1 alpha) domain of this PfEMP1 was therefore, alone or in combination with two additional DBL1 alpha domains, evaluated as a potential vaccine candidate using both a rodent model and a primate model. Antibodies against the DBL1 alpha domain were generated by immunization with recombinant DBL1 alpha-Semliki Forest virus particles and recombinant protein and analyzed in vitro. The immunized animals were challenged in vivo with various parasite strains or clones. Immunization with the PfEMP1-DBL1 alpha domain abolished the PfEMP1-dependent sequestration of the homologous strain in immunized rats and substantially inhibited parasite adhesion in immunized monkeys. Protection against sequestration of heterologous parasite strains was also confirmed by direct or indirect challenge in the rat model. These results strongly support the use of the DBL1 alpha domain in the development of a vaccine targeting severe malaria.

A review of human vaccine research and development: malaria

The last several years have seen significant progress in the development of vaccines against malaria. Most recently, proof-of-concept of vaccine-induced protection from malaria infection and disease was demonstrated in African children. Pursued by various groups and on many fronts, several other candidate vaccines are in early clinical trials. Yet, despite the optimism and promise, an effective malaria vaccine is not yet available, in part because of the lack of understanding of the types of immune responses needed for protection, added to the difficulty of identifying, selecting and producing the appropriate protective antigens from a parasite with a genome of well over five thousand genes and to the frequent need to enhance the immunogenicity of purified antigens through the use of novel adjuvants or delivery systems. Insufficient clinical trial capacity and normative research functions such as local ethical committee reviews also contribute to slow down the development process. This article attempts to summarize the state of the art of malaria vaccine development.

Viral vectors for malaria vaccine development

A workshop on viral vectors for malaria vaccine development, organized by the PATH Malaria Vaccine Initiative, was held in Bethesda, MD on October 20, 2005. Recent advancements in viral-vectored malaria vaccine development and emerging vector technologies were presented and discussed. Classic viral vectors such as poxvirus, adenovirus and alphavirus vectors have been successfully used to deliver malaria antigens. Some of the vaccine candidates have demonstrated their potential in inducing malaria-specific immunity in animal models and human trials. In addition, emerging viral-vector technologies, such as measles virus (MV), vesicular stomatitis virus (VSV) and yellow fever (YF) virus, may also be useful for malaria vaccine development. Studies in animal models suggest that each viral vector is unique in its ability to induce humoral and/or cellular immune responses. Those studies have also revealed that optimization of Plasmodium genes for mammalian expression is an important aspect of vaccine design. Codon-optimization, surface-trafficking, de-glycosylation and removal of toxic domains can lead to improved immunogenicity. Understanding the vector's ability to induce an immune response and the expression of malaria antigens in mammalian cells will be critical in designing the next generation of viral-vectored malaria vaccines.

Induction of cross-reactive immune responses to NTS-DBL-1alpha/x of PfEMP1 and in vivo protection on challenge with Plasmodium falciparum

The interactions of Plasmodium falciparum infected erythrocytes parasitized red blood cells (pRBC) with endothelial receptors and erythrocytes are mediated by multiple Duffy-binding like (DBL) and cysteine-rich interdomain region (CIDR) domains harboured in the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). The success of a subunit vaccine based on PfEMP1 depends on its ability to elicit cross-reactive responses to a substantial number of PfEMP1 variants. We have here evaluated serological PfEMP1 cross-reactivity by immunizing rats with phylogenetically diverse recombinant NTS-DBL-1alpha/x fusion domains from the 3D7 genome parasite emulsified in Montanide ISA 720. Cross-reactivity was elicited to these diverse DBL-1alpha/x domains as measured by ELISA and by immunoblotting. Employing a novel in vivo model of human infected erythrocyte sequestration, immunized animals were challenged with the FCR3S1.2 clone and cross-protection in terms of reduction in lung sequestration amounting to approximately 50% was demonstrated. Our results suggest that immunization with phylogenetically distant DBL-1alpha/x variants, can elicit partial cross-protection to challenge with the parasites harbouring a distant variant. These observations have implications for the design of multi-component vaccines against P. falciparum malaria.

Recombinant alphaviruses as vectors for anti-tumour and anti-microbial immunotherapy

BACKGROUND

Vectors derived from alphaviruses are gaining interest for their high transfection potency and strong immunogenicity.

OBJECTIVES

After a brief introduction on alphaviruses and their vectors, an overview is given on current preclinical immunotherapy studies using vector systems based on alphaviruses. The efficacy of alphavirus vectors in inducing immune responses will be illustrated by a more detailed description of immunization studies using recombinant Semliki Forest virus for the treatment of human papilloma virus-induced cervical cancer.

RESULTS

Immunization with recombinant alphavirus results in the induction of humoral and cellular immune responses against microbes, infected cells and cancer cells. Preclinical studies demonstrate that infectious diseases and cancer can be treated prophylactically as well as therapeutically.

CONCLUSIONS

Alphavirus-based genetic immunization strategies are highly effective in animal model systems, comparing quite favourably with any other approach. Therefore, we hope and expect to see an efficient induction of tumour-or microbial immunity and a positive outcome in future clinical efficacy studies.

Whole-body imaging of sequestration of Plasmodium falciparum in the rat

The occlusion of vessels by packed Plasmodium falciparum-infected (iRBC) and uninfected erythrocytes is a characteristic postmortem finding in the microvasculature of patients with severe malaria. Here we have employed immunocompetent Sprague-Dawley rats to establish sequestration in vivo. Human iRBC cultivated in vitro and purified in a single step over a magnet were labeled with 99mtechnetium, injected into the tail vein of the rat, and monitored dynamically for adhesion in the microvasculature using whole-body imaging or imaging of the lungs subsequent to surgical removal. iRBC of different lines and clones sequester avidly in vivo while uninfected erythrocytes did not. Histological examination revealed that a multiadhesive parasite adhered in the larger microvasculature, inducing extensive intravascular changes while CD36- and chondroitin sulfate A-specific parasites predominantly sequester in capillaries, inducing no or minor pathology. Removal of the adhesive ligand Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), preincubation of the iRBC with sera to PfEMP1 or preincubation with soluble PfEMP1-receptors prior to injection significantly reduced the sequestration. The specificity of iRBC binding to the heterologous murine receptors was confirmed in vitro, using primary rat lung endothelial cells and rat lung cryosections. In offering flow dynamics, nonmanipulated endothelial cells, and an intact immune system, we believe this syngeneic animal model to be an important complement to existing in vitro systems for the screening of vaccines and adjunct therapies aiming at the prevention and treatment of severe malaria.

Clinical and molecular aspects of severe malaria

The erythrocytic cycle of Plasmodium falciparum presents a particularity in relation to other Plasmodium species that infect man. Mature trophozoites and schizonts are sequestered from the peripheral circulation due to adhesion of infected erythrocytes to host endothelial cells. Modifications in the surface of infected erythrocytes, termed knobs, seem to facilitate adhesion to endothelium and other erythrocytes. Adhesion provides better maturation in the microaerophilic venous atmosphere and allows the parasite to escape clearance by the spleen which recognizes the erythrocytes loss of deformability. Adhesion to the endothelium, or cytoadherence, has an important role in the pathogenicity of the disease, causing occlusion of small vessels and contributing to failure of many organs. Cytoadherence can also describe adhesion of infected erythrocytes to uninfected erythrocytes, a phenomenon widely known as rosetting. Clinical aspects of severe malaria, as well as the host receptors and parasite ligands involved in cytoadherence and rosetting, are reviewed here. The erythrocyte membrane protein 1 of P. falciparum (PfEMP1) appears to be the principal adhesive ligand of infected erythrocytes and will be discussed in more detail. Understanding the role of host receptors and parasite ligands in the development of different clinical syndromes is urgently needed to identify vaccination targets in order to decrease the mortality rates of this disease.

SURFIN is a polymorphic antigen expressed on Plasmodium falciparum merozoites and infected erythrocytes

The surfaces of the infected erythrocyte (IE) and the merozoite, two developmental stages of malaria parasites, expose antigenic determinants to the host immune system. We report on surface-associated interspersed genes (surf genes), which encode a novel polymorphic protein family, SURFINs, present on both IEs and merozoites. A SURFIN expressed in 3D7 parasites, SURFIN_4.2, was identified by mass spectrometric analysis of peptides cleaved off the surface of live IEs with trypsin. SURFINs are encoded by a family of 10 surf genes, including three predicted pseudogenes, located within or close to the subtelomeres of five of the chromosomes. SURFINs show structural and sequence similarities with exported surface-exposed proteins (PvSTP1, PkSICAvar, PvVIR, Pf332, and PfEMP1) of several Plasmodium species. SURFIN_4.2 of a parasite other than 3D7 (FCR3S1.2) showed polymorphisms in the extracellular domain, suggesting sequence variability between genotypes. SURFIN_4.2 not only was found cotransported with PfEMP1 and RIFIN to the IE surface, but also accumulated in the parasitophorous vacuole. In released merozoites, SURFIN_4.2 was present in an amorphous cap at the parasite apex, where it may be involved in the invasion of erythrocytes. By exposing shared polymorphic antigens on IEs and merozoites, the parasite may coordinate the antigenic composition of these attachment surfaces during growth in the bloodstream.

Identification, expression, localization and serological characterization of a tryptophan-rich antigen from the human malaria parasite Plasmodium vivax

Plasmodium vivax is most common but non-cultivable human malaria parasite which is poorly characterized at the molecular level. Here, we describe the identification and characterization of a P. vivax Tryptophan-Rich Antigen (PvTRAg) which contains unusually high (8.28%) tryptophan residues and is expressed by all blood stages of the parasite. The pvtrag gene comprises a 978bp open reading frame interrupted by two introns. The first intron is located in the 5'-untranslated region while the second one is positioned 174bp downstream to the ATG codon. The encoded approximately 40kDa protein contains a transmembrane domain near the N-terminus followed by a tryptophan-rich domain with significantly high surface probability and antigenic index. It is localized in the parasite cytoplasm as well as in the cytoplasm of the parasitized erythrocyte. The purified E. coli expressed recombinant PvTRAg protein showed a very high seropositivity rate for the presence of antibodies amongst the P. vivax patients, indicating that the antigen generates significant humoral immune response during the natural course of P. vivax infection. Analysis of various field isolates revealed that the tryptophan-rich domain is highly conserved except for three-point mutations. The PvTRAg could be a potential vaccine candidate since similar tryptophan-rich antigens of P. yoelii have shown protection against malaria in murine model.

Optimized expression of Plasmodium falciparum erythrocyte membrane protein 1 domains in Escherichia coli

Background

The expression of recombinant proteins in Escherichia coli is an important and frequently used tool within malaria research, however, this method remains problematic. High A/T versus C/G content and frequent lysine and arginine repeats in the Plasmodium falciparum genome are thought to be the main reason for early termination in the mRNA translation process. Therefore, the majority of P. falciparum derived recombinant proteins is expressed only as truncated forms or appears as insoluble inclusion bodies within the bacterial cells.

Methods

Several domains of PfEMP1 genes obtained from different P. falciparum strains were expressed in E. coli as GST-fusion proteins. Expression was carried out under various culture conditions with a main focus on the time point of induction in relation to the bacterial growth stage.

Results and conclusions

When expressed in E. coli recombinant proteins derived from P. falciparum sequences are often truncated and tend to aggregate what in turn leads to the formation of insoluble inclusion bodies. The analysis of various factors influencing the expression revealed that the time point of induction plays a key role in successful expression of A/T rich sequences into their native conformation. Contrary to recommended procedures, initiation of expression at post-log instead of mid-log growth phase generated significantly increased amounts of soluble protein of a high quality. Furthermore, these proteins were shown to be functionally active. Other factors such as temperature, pH, bacterial proteases or the codon optimization for E. coli had little or no effect on the quality of the recombinant protein, nevertheless, optimizing these factors might be beneficial for each individual construct. In conclusion, changing the timepoint of induction and conducting expression at the post-log stage where the bacteria have entered a decelerated growth phase, greatly facilitates and improves the expression of sequences containing rare codons.