The surface variant antigens of Plasmodium falciparum contain cross-reactive epitopes

Hiding in plain sight: an epitope-based strategy for a subunit malaria vaccine

Recent data suggest that approaches to developing a subunit blood-stage malaria vaccine may be misdirected. While antigenic polymorphism is recognized as a challenge, efforts to counter this have primarily involved enhancing the quantity and quality of antibody with potent adjuvants, identifying conserved target proteins, or combining multiple antigens to broaden the immune response. However, paradoxically, evidence has emerged that narrowing, rather than broadening, the immune response may be required to obtain an immune response protective against multiple Plasmodium strains. Non-immunodominant, conserved epitopes are crucial. The evidence comes from studying the immune response to red cell surface-expressed antigens but should also be applicable to merozoite surface antigens. Strategies to define the targets of these highly focused immune responses are provided.

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.

Identification and Characterization of the Antigens Expressed On the Surface of Human Erythrocytes Infected With Plasmodium falciparum

BACKGROUND

Molecules expressed on the surface of infected erythrocytes (IE) with Plasmodium falciparum play important roles in malaria pathogenesis and immune evasion. Some of these molecules are specific adhesive ligands mediating adhesion of IE to the vascular endothelium. In the current study, the antigens exposed on the surface of IE with different isolates and various binding subpopulations of P. falciparum were studied.

METHODS

A pooled hyper immune serum (HIS) from Malawian adults and eluted antibodies from the surface of the homologous and heterologous parasites were used. The parasite surface molecules were analyzed by Immuno-Gold-Silver enhancement (IGSE) and Western blotting. Mini-column cytoadherence method was used to select various parasite-binding subpopulations.

RESULTS

Surface antigens of all the isolates were recognized by HIS and high recognition of antigens was observed in all isolates with homologous eluted antibodies. Western blot analysis showed that the eluted antibodies reacted with a small subset of antigens compared with HIS. Three bands, PfEMP-1, were detected in the Triton X- insoluble fraction of the ICAM-1 binding subpopulation. Another interesting band was ∼ 52-55 kDa in various isolates of P. falciparum. This molecule as defined by its low molecular weight, Triton X-100 solubility, surface location and sensitivity to 1 mg/ml trypsin.

CONCLUSION

The IE's surface antigens differed in parental population compared with the selected subpopulations. These molecules could induce isolate-specific immunity. Antibodies purified from the surface of IE can be used as specific reagents to investigate parasite-derived proteins expressed on the surface of IE.

Subdomain 3 of Plasmodium falciparum VAR2CSA DBL3x is identified as a minimal chondroitin sulfate A-binding region

Molecular interactions between the VAR2CSA protein, expressed on the surface of Plasmodium falciparum-infected erythrocytes, and placental chondroitin sulfate A (CSA) are primarily responsible for pregnancy-associated malaria (PAM). Interrupting these interactions may prevent or ameliorate the severity of PAM. Several of the Duffy binding-like (DBL) domains of VAR2CSA, including the DBL3x domain, have been shown to bind CSA in vitro, but a more detailed understanding of how DBL domains bind CSA is needed. In this study, we demonstrate that subdomain 3 (S3), one of the three subdomains of VAR2CSA DBL3x by itself, is the major contributor toward CSA binding. NMR spectroscopy and flow cytometry analyses show that S3 and the intact DBL3x domain bind CSA similarly. Mutations within the S3 portion of DBL3x markedly affect CSA binding. Both recombinant molecules, S3 and DBL3x, are recognized by antibodies in the plasma of previously pregnant women living in malaria-endemic regions of Mali, but much less so by plasma from men of the same regions. As the S3 sequence is highly conserved in all known VAR2CSA proteins expressed by different parasite isolates obtained from various malaria endemic areas of the world, the identification of S3 as an independent CSA-binding region provides a compelling molecular basis for designing interventions against PAM.

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.

Agglutination of Trypanosoma cruzi in infected cells treated with serum from chronically infected mice

The protozoan parasite Trypanosoma cruzi is the causative agent of Chagas disease. The chronic stage of infection is characterized by a production of neutralizing antibodies in the vertebrate host. A polyclonal antibody, anti-egressin, has been found to inhibit egress of parasites from the host cell late in the intracellular cycle, after the parasites have transformed from the replicative amastigote into the trypomastigote. It has also been found that BALB/c mouse fibroblasts in the late stages of parasite infection become permeable to molecules as large as antibodies, leading to the possibility that anti-egressin affects the intracellular parasites. This project addresses the fate of the intracellular trypomastigotes that have been inhibited from egressing the host cell. Extended cultures of infected fibroblasts treated with chronic mouse serum reduced parasite egress at all time points measured. Parasites released from infected fibroblasts treated with chronic serum had a reduced ability to infect fibroblasts in culture, yet did not lose infectivity entirely. Absorption of chronic serum with living trypomastigotes removed the anti-egressin effect. The possibility that the target of anti-egressin is a parasite surface component is further indicated by the agglutination of extracellular trypomastigotes by chronic serum. The possibility that cross-linking by antibody occurs intracellularly, thus inhibiting egress, was reinforced by cleaving purified IgG into Fab fragments, which did not inhibit egress when added to infected cultures. From this work, it is proposed that the current, best explanation of the mechanism of egress inhibition by anti-egressin is intracellular agglutination, preventing normal parasite-driven egress.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

The cysteine-rich interdomain region from the highly variable plasmodium falciparum erythrocyte membrane protein-1 exhibits a conserved structure

Plasmodium falciparum malaria parasites, living in red blood cells, express proteins of the erythrocyte membrane protein-1 (PfEMP1) family on the red blood cell surface. The binding of PfEMP1 molecules to human cell surface receptors mediates the adherence of infected red blood cells to human tissues. The sequences of the 60 PfEMP1 genes in each parasite genome vary greatly from parasite to parasite, yet the variant PfEMP1 proteins maintain receptor binding. Almost all parasites isolated directly from patients bind the human CD36 receptor. Of the several kinds of highly polymorphic cysteine-rich interdomain region (CIDR) domains classified by sequence, only the CIDR1α domains bind CD36. Here we describe the CD36-binding portion of a CIDR1α domain, MC179, as a bundle of three α-helices that are connected by a loop and three additional helices. The MC179 structure, containing seven conserved cysteines and 10 conserved hydrophobic residues, predicts similar structures for the hundreds of CIDR sequences from the many genome sequences now known. Comparison of MC179 with the CIDR domains in the genome of the P. falciparum 3D7 strain provides insights into CIDR domain structure. The CIDR1α three-helix bundle exhibits less than 20% sequence identity with the three-helix bundles of Duffy-binding like (DBL) domains, but the two kinds of bundles are almost identical. Despite the enormous diversity of PfEMP1 sequences, the CIDR1α and DBL protein structures, taken together, predict that a PfEMP1 molecule is a polymer of three-helix bundles elaborated by a variety of connecting helices and loops. From the structures also comes the insight that DBL1α domains are approximately 100 residues larger and that CIDR1α domains are approximately 100 residues smaller than sequence alignments predict. This new understanding of PfEMP1 structure will allow the use of better-defined PfEMP1 domains for functional studies, for the design of candidate vaccines, and for understanding the molecular basis of cytoadherence.

Malaria parasites express proteins of the erythrocyte membrane protein-1 family (PfEMP1) on the surfaces of the human red blood cells that they infect. These large proteins vary in sequence extensively, yet bind to host receptors to allow infected cells to adhere to host tissues. PfEMP1 proteins help parasites evade the immune system, as the 60 PfEMP1 genes are expressed one at a time. Sequence comparisons predict that PfEMP1 molecules are modular, made up of Duffy binding-like (DBL) and cysteine-rich interdomain region (CIDR) domains. Many CIDR domains bind to the human receptor CD36. We have analyzed the structure of the CD36-binding portion, known as MC179, of a CIDR domain. The MC179 protein is composed of a bundle of three helices connected by a loop and three additional helices. Based on the structure and sequence similarities, MC179 is a good model for the hundreds of known CIDR sequences. In addition, the MC179 three-helix bundle is remarkably similar to subdomain 3 of the known DBL structures. MC179 provides insight into the relatedness of both kinds of PfEMP1 domains and predicts that the large PfEMP1 molecules are polymers of three-helix bundles and their connecting polypeptides.

Structure of the DBL3x domain of pregnancy-associated malaria protein VAR2CSA complexed with chondroitin sulfate A

Plasmodium falciparum–infected erythrocytes bind to chondroitin sulfate A (CSA) in the placenta via the VAR2CSA protein, a member of the P. falciparum erythrocyte membrane protein-1 family, leading to life-threatening malaria in pregnant women with severe effects on their fetuses and newborns. Here we describe the structure of the CSA binding DBL3x domain, a Duffy binding-like (DBL) domain of VAR2CSA. By forming a complex of DBL3x with CSA oligosaccharides and determining its structure, we have identified the CSA binding site to be a cluster of conserved positively charged residues on subdomain 2 and subdomain 3. Mutation or chemical modification of lysine residues at the site markedly diminished CSA binding to DBL3x. The location of the CSA binding site is an important step forward in the molecular understanding of pregnancy-associated malaria and offers a new target for vaccine development.

Malaria vaccines: immunity, models and monoclonal antibodies

Although experts in the field have agreed on the malaria vaccine technology roadmap that should be followed (http://www.malariavaccineroadmap.net/), the path towards an effective malaria vaccine remains littered with intellectual and practical pot-holes. The animal models that are currently available are problematic, and current understanding of the exact mechanisms and targets of protective immune responses is incomplete. However, recent technological advances might help overcome some of these hurdles.

The C-terminal segment of the cysteine-rich interdomain of Plasmodium falciparum erythrocyte membrane protein 1 determines CD36 binding and elicits antibodies that inhibit adhesion of parasite-infected erythrocytes

Attachment of erythrocytes infected by Plasmodium falciparum to receptors of the microvasculature is a major contributor to the pathology and morbidity associated with malaria. Adhesion is mediated by the P. falciparum erythrocyte membrane protein 1 (PfEMP-1), which is expressed at the surface of infected erythrocytes and is linked to both antigenic variation and cytoadherence. PfEMP-1 contains multiple adhesive modules, including the Duffy binding-like domain and the cysteine-rich interdomain region (CIDR). The interaction between CIDRalpha and CD36 promotes stable adherence of parasitized erythrocytes to endothelial cells. Here we show that a segment within the C-terminal region of CIDRalpha determines CD36 binding specificity. Antibodies raised against this segment can specifically block the adhesion to CD36 of erythrocytes infected with various parasite strains. Thus, small regions of PfEMP-1 that determine binding specificity could form suitable components of an antisequestration malaria vaccine effective against different parasite strains.

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.

Levels of plasma immunoglobulin G with specificity against the cysteine-rich interdomain regions of a semiconserved Plasmodium falciparum erythrocyte membrane protein 1, VAR4, predict protection against malarial anemia and febrile episodes

Antibodies to variant surface antigen have been implicated as mediators of malaria immunity in studies measuring immunoglobulin G (IgG) binding to infected erythrocytes. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important target for these antibodies, but no study has directly linked the presence of PfEMP1 antibodies in children to protection. We measured plasma IgG levels to the cysteine-rich interdomain region 1alpha (CIDR1alpha) of VAR4 (VAR4-CIDR1alpha), a member of a semiconserved PfEMP1 subfamily, by enzyme-linked immunosorbent assay in 561 Tanzanian individuals, who were monitored clinically for 7 months. The participants resided in Mkokola (a high-transmission village where malaria is holoendemic) or Kwamasimba (a moderate-transmission village). For comparison, plasma IgG levels to two merozoite surface protein 1 (MSP1) constructs, MSP1-19 and MSP1 block 2, and a control CIDR1 domain were measured. VAR4-CIDR1alpha antibodies were acquired at an earlier age in Mkokola than in Kwamasimba, but after the age of 10 years the levels were comparable in the two villages. After controlling for age and other covariates, the risk of having anemia at enrollment was reduced in VAR4-CIDR1alpha responders for Mkokola (adjusted odds ratio [AOR], 0.49; 95% confidence interval [CI], 0.29 to 0.88; P = 0.016) and Kwamasimba (AOR, 0.33; 95% CI, 0.16 to 0.68; P = 0.003) villages. The risk of developing malaria fever was reduced among individuals with a measurable VAR4-CIDR1alpha response from Mkokola village (AOR, 0.51; 95% CI, 0.29 to 0.89; P = 0.018) but not in Kwamasimba. Antibody levels to the MSP1 constructs and the control CIDR1alpha domain were not associated with morbidity protection. These data strengthen the concept of developing vaccines based on PfEMP1.

Adherence of Plasmodium falciparum infected erythrocytes to CHO-745 cells and inhibition of binding by protein A in the presence of human serum

Adhesion of erythrocytes infected with the malaria parasite Plasmodium falciparum to human host receptors is a process associated with severe malarial pathology. A number of in vitro cell lines are available as models for these adhesive processes, including Chinese hamster ovary (CHO) cells which express the placental adhesion receptor chondroitin-4-sulphate (CSA) on their surface. CHO-745 cells, a glycosaminoglycan-negative mutant CHO cell line lacking CSA and other reported P. falciparum adhesion receptors, are often used for recombinant expression of host receptors and for receptor binding studies. In this study we show that P. falciparum-infected erythrocytes can be easily selected for adhesion to an endogenous receptor on the surface of CHO-745 cells, bringing into question the validity of using these cells as a tool for P. falciparum adhesin expression studies. The adhesive interaction between CHO-745 cells and parasitized erythrocytes described here is not mediated by the known P. falciparum adhesion receptors CSA, CD36, or ICAM-1. However, we found that CHO-745-selected parasitized erythrocytes bind normal human IgM and that adhesion to CHO-745 cells is inhibited by protein A in the presence of serum, but not in its absence, indicating a non-specific inhibitory effect. Thus, protein A, which has been used as an inhibitor for a recently described interaction between infected erythrocytes and the placenta, may not be an appropriate in vitro inhibitor for understanding in vivo adhesive interactions.

Parasite immune escape: new views into host-parasite interactions

For parasites of humans and animals that rely on vectors or on sexual contact for transmission, it is particularly important that infection does not to terminate before the occurrence of the crucial event that completes its lifecycle (e.g. another mosquito bite). For chronic infection to occur, it is essential that the parasite avoids clearance by the host immune system. Much progress has been made in elucidating the immunological interactions and the molecular mechanisms involved in the process of immune evasion. Mathematical models have also been invaluable in understanding these processes, particularly in the generation of new ideas about a complex form of immune evasion known as antigenic variation whereby a major target of the host immune response is varied during the course of a single infection to avoid recognition.

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.

Replication slippage and the dynamics of the immune response in malaria: a formal model for immunity

A simple mathematical model of the dynamics of malaria invasion is proposed. The model is based on the hypothesis that replication slippage (slipped strand mispairing occurring in replicating DNAs), likely to take place within regular, low-complexity, regions of the plasmodium genome, is a significant mechanism of the ability for the parasite to escape the host immune defence. The model reconciles the conclusions of other formal approaches like periodic bursts of parasitaemia and, in particular, it predicts that antibody cross-reactivity affects the level and the time profile of malaria recrudescence. It also suggests that an efficient strategy for the parasite to escape host humoral immune defences is to express a single antigen. Linking local complexity to antigenic variability tentatively allows prediction of new potentially immunogenic sequences in the plasmodium genome.

Abnormal display of PfEMP-1 on erythrocytes carrying haemoglobin C may protect against malaria

Haemoglobin C, which carries a glutamate-to-lysine mutation in the beta-globin chain, protects West African children against Plasmodium falciparum malaria. Mechanisms of protection are not established for the heterozygous (haemoglobin AC) or homozygous (haemoglobin CC) states. Here we report a marked effect of haemoglobin C on the cell-surface properties of P. falciparum-infected erythrocytes involved in pathogenesis. Relative to parasite-infected normal erythrocytes (haemoglobin AA), parasitized AC and CC erythrocytes show reduced adhesion to endothelial monolayers expressing CD36 and intercellular adhesion molecule-1 (ICAM-1). They also show impaired rosetting interactions with non-parasitized erythrocytes, and reduced agglutination in the presence of pooled sera from malaria-immune adults. Abnormal cell-surface display of the main variable cytoadherence ligand, PfEMP-1 (P. falciparum erythrocyte membrane protein-1), correlates with these findings. The abnormalities in PfEMP-1 display are associated with markers of erythrocyte senescence, and are greater in CC than in AC erythrocytes. Haemoglobin C might protect against malaria by reducing PfEMP-1-mediated adherence of parasitized erythrocytes, thereby mitigating the effects of their sequestration in the microvasculature.

Immunoglobulin G isotype responses to erythrocyte surface-expressed variant antigens of Plasmodium falciparum predict protection from malaria in African children

We assessed immunoglobulin G (IgG) isotype responses to variant surface antigens (VSA) expressed on parasite-infected erythrocytes of a panel of heterologous isolates during and after acute episodes in groups of Gabonese children presenting with either mild or severe Plasmodium falciparum malaria. In the acute and convalescent phases IgG3 and IgG1 anti-VSA antibodies, respectively, predominated. In the absence of infection, the levels of both cytophilic isotypes waned, while those of IgG4 increased, particularly in those admitted with severe malaria. Prospective analyses showed significantly longer delays between malaria attacks associated both (i) with increasing IgG1 responses with specificity for VSA of isolates from children with mild malaria and (ii) with increasing IgG4 responses with specificity for VSA of isolates from children with severe malaria. These findings imply that the predictive value of prospectively measured cross-reactive VSA-specific IgG antibodies with respect to protection against malaria in African children depends both on their isotype and on their fine specificity.

Selection of peptides recognized by human antibodies against the surface of Plasmodium falciparum-infected erythrocytes

In an attempt to identify mimotopes of the surface antigens of P. falciparum-infected erythrocytes (iRBC), antibodies were eluted from iRBC that had been treated with a pool of sera from malaria-infected individuals (IHS), and were used to screen a phage display library (PDL). After repeated panning of the PDL on immobilized antibodies, phage that selectively bound to IHS were accumulated. Of 23 randomly chosen clones that were sequenced, 13 individual sequences were detected at varying frequencies and 3 of the 13 sequences had homology with membrane proteins known to exist on iRBC. The majority of phage clones (7 out of 8 clones) selected after the 4th panning bound selectively to IgG in IHS. Specific binding of the selected phage to IgG in IHS was also confirmed using 24 IHS and 11 sera from uninfected individuals. One phage clone was the most frequently found in the sequenced clones after the 4th panning, and the binding of this clone to IgG in all IHS was greater than in any serum from uninfected individuals. A rabbit antiserum against the peptide expressed on the clone specifically recognized the surface of iRBC and resulted in iRBC haemolysis.

Transient cross-reactive immune responses can orchestrate antigenic variation in malaria

The malaria parasite Plasmodium falciparum has evolved to prolong its duration of infection by antigenic variation of a major immune target on the surface of the infected red blood cell. This immune evasion strategy depends on the sequential, rather than simultaneous, appearance of immunologically distinct variants. Although the molecular mechanisms by which a single organism switches between variants are known in part, it remains unclear how an entire population of parasites within the host can synchronize expression to avoid rapidly exhausting the variant repertoire. Here we show that short-lived, partially cross-reactive immune responses to parasite-infected erythrocyte surface antigens can produce a cascade of sequentially dominant antigenic variants, each of which is the most immunologically distinct from its preceding types. This model reconciles several previously unexplained and apparently conflicting epidemiological observations by demonstrating that individuals with stronger cross-reactive immune responses can, paradoxically, be more likely to sustain chronic infections. Antigenic variation has always been seen as an adaptation of the parasite to evade host defence: we show that the coordination necessary for the success of this strategy might be provided by the host.

Antigenic variation and cytoadhesion in Babesia bovis and Plasmodium falciparum: different logics achieve the same goal

Babesia bovis is a protozoal hemoparasite of cattle which behaves in certain crucial respects like Plasmodium falciparum, despite being phylogenetically distant and having many differences in its life cycle. The shared behavioral attributes of rapid antigenic variation and cytoadhesion/sequestration are thought to contribute significantly to immune evasion, establishment of persistent infections, and disease pathology. Although differing in their genetic and biochemical strategies for achieving these behaviors, information from studies of each parasite may further our understanding of the overall host-parasite interaction. In this review we contrast the molecular basis and 'genetic logic' for these critical behaviors in the two parasites, with emphasis on the biology of B. bovis.

DNA vaccines and their application against parasites--promise, limitations and potential solutions

DNA or nucleic acid vaccines are being evaluated for efficacy against a range of parasitic diseases. Data from studies in rodent model systems have provided proof of principle that DNA vaccines are effective at inducing both humoral and T cell responses to a variety of candidate vaccine antigens. In particular, the induction of potent cellular responses often gives DNA vaccination an immunological advantage over subunit protein vaccination. Protection against parasite challenge has been demonstrated in a number of systems. However, application of parasite DNA vaccines in large animals including ruminants, primates and humans has been compromised by the relative lack of immune responsiveness to the vaccines, but the reasons for this hyporesponsiveness are not clear. Here, we review DNA vaccines against protozoan parasites, in particular vaccines for malaria, and the use of genomic approaches such as expression library immunization to generate novel vaccines. The application of DNA vaccines in ruminants is reviewed. We discuss some of the approaches being evaluated to improve responsiveness in large animals including the use of cytokines as adjuvants, targeting molecules as delivery ligands, electroporation and CpG oligonucleotides.

Induction of crossreactive antibodies against the Plasmodium falciparum variant protein

The variant antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), present on the surface of P. falciparum-parasitized erythrocytes (PE), plays a central role in naturally acquired immunity, although antibodies to PfEMP1 are predominantly variant specific. To overcome this major limitation for vaccine development, we immunized mice with three cysteine-rich interdomain 1 (CIDR1) domains of PfEMP1 that have the critical function of binding the PE to CD36 on endothelium and thus preventing spleen-dependent killing of the parasite. The immunizations consisted of different combinations of three CIDR1 encoded by DNA followed by recombinant protein boost. Immunizations with a single variant in a prime-boost regimen induced no or low cross-reactivity toward heterologous CIDR1; however, a broad range of crossreactivity was detected in mice that were immunized with all three variants simultaneously. The induced crossreactivity suggests that an anti-PfEMP1 vaccine may be possible.

DNA immunization with the cysteine-rich interdomain region 1 of the Plasmodium falciparum variant antigen elicits limited cross-reactive antibody responses

The variant surface antigens of Plasmodium falciparum are an important component of naturally acquired immunity and an important vaccine target. However, these proteins appear to elicit primarily variant-specific antibodies. We tested if naked DNA immunization can elicit more cross-reactive antibody responses and allow simultaneous immunization with several variant constructs. Mice immunized with plasmid DNA expressing variant cysteine-rich interdomain region 1 (CIDR1) domains of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) developed antibodies that were reactive to the corresponding PfEMP1s as measured by an enzyme-linked immunosorbent assay, flow cytometry, and agglutination of parasitized erythrocytes (PEs). We observed some cross-reactive immune responses; for example, sera from mice immunized with one domain agglutinated PEs of various lines and recognized heterologous domains expressed on the surface of Chinese hamster ovary (CHO) cells. We found no significant antigenic competition when animals were immunized with a mixture of plasmids or immunized sequentially with individual constructs. Moreover, mixed or sequential immunizations resulted in greater cross-reactive agglutination responses than immunization with a single domain. Recombinant protein (Sc y179) immunization after priming with DNA (prime-boost regimen) increased antibody titers to the homologous domain substantially but seemed to diminish the cross-reactive responses somewhat. The titer of agglutinating antibodies was previously shown to correlate with protection. Surprisingly, the agglutination titers of sera from DNA immunization were high, similar to those of pooled human hyperimmune sera. These sera also appeared to give limited low-titer variant transcending agglutination. Thus, DNA immunization appears to be a very useful tool for developing variant antigen vaccines.

A glycosylphosphatidylinositol anchor signal sequence enhances the immunogenicity of a DNA vaccine encoding Plasmodium falciparum sexual-stage antigen, Pfs230

Mammalian expression vectors encoding region C of malaria transmission-blocking vaccine candidate Pfs230 (aa 443-1132) with and without a 3' glycosylphosphatidylinositol (GPI) anchor signal sequence were tested for their immunogenicity in mice. The plasmid containing the GPI anchor signal sequence consistently induced higher titers of anti-Pfs230 antibodies using three delivery systems: intramuscular (i.m.), intradermal (i.d.), and gene gun (g.g.). In contrast, the isotype profile elicited varied depending on the delivery system and was not effected by the presence of the GPI anchor sequence. Both gene gun and intradermal administration induced primarily an IgG1 response, while intramuscular injection induced both IgG1 and IgG2a antibodies. Regardless of the mode of delivery, all the plasmids encoding Pfs230 region C primed for a mixed IgG1/IgG2a response to an intraperitoneal (i.p.) injection of E. coli-produced recombinant Pfs230 region C. None of these vaccination strategies were more effective than r230/MBP.C alone in generating malaria transmission-blocking immunity.

The low sulfated chondroitin sulfate proteoglycans of human placenta have sulfate group-clustered domains that can efficiently bind Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum infection in pregnant women results in the chondroitin 4-sulfate-mediated adherence of the parasite-infected red blood cells (IRBCs) in the placenta, adversely affecting the health of the fetus and mother. We have previously shown that unusually low sulfated chondroitin sulfate proteoglycans (CSPGs) in the intervillous spaces of the placenta are the receptors for IRBC adhesion, which involves a chondroitin 4-sulfate motif consisting of six disaccharide moieties with approximately 30% 4-sulfated residues. However, it was puzzling how the placental CSPGs, which have only approximately 8% of the disaccharide 4-sulfated, could efficiently bind IRBCs. Thus, we undertook to determine the precise structural features of the CS chains of placental CSPGs that interact with IRBCs. We show that the placental CSPGs are a mixture of two major populations, which are similar by all criteria except differing in their sulfate contents; 2-3% and 9-14% of the disaccharide units of the CS chains are 4-sulfated, and the remainder are nonsulfated. The majority of the sulfate groups in the CSPGs are clustered in CS chain domains consisting of 6-14 repeating disaccharide units. While the sulfate-rich regions of the CS chains contain 20-28% 4-sulfated disaccharides, the other regions have little or no sulfate. Further, we find that the placental CSPGs are able to efficiently bind IRBCs due to the presence of 4-sulfated disaccharide clusters. The oligosaccharides corresponding to the sulfate-rich domains of the CS chains efficiently inhibited IRBC adhesion. Thus, our data demonstrate, for the first time, the unique distribution of sulfate groups in the CS chains of placental CSPGs and that these sulfate-clustered domains have the necessary structural elements for the efficient adhesion of IRBCs, although the CS chains have an overall low degree of sulfation.

Widespread functional specialization of Plasmodium falciparum erythrocyte membrane protein 1 family members to bind CD36 analysed across a parasite genome

Plasmodium falciparum-infected erythrocytes sequester from blood circulation by binding host endothelium. A large family of variant proteins mediates cytoadherence and their binding specificity determines parasite sequestration patterns and potential for disease. The aim of the present study was to understand how binding properties are encoded into family members and to develop sequence algorithms for predicting binding. To accomplish these goals computational approaches and a binding assay were used to characterize adhesion across Plasmodium falciparum erythrocyte membrane 1 (PfEMP1) proteins in the 3D7 parasite genome. We report that most family members encode the capacity to bind CD36 in the protein's semi-conserved head structure and describe the sequence characteristics of a group of PfEMP1 proteins that do not. Structural and functional grouping of PfEMP1 proteins based upon head structure and additional domain architectural properties provide new insights into the protein family. These can be used to investigate the role of proteins in malaria pathogenesis and potentially tailor vaccines to recognize particular binding variants.

Recombinant PfEMP1 peptide inhibits and reverses cytoadherence of clinical Plasmodium falciparum isolates in vivo

The parasite ligand Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) and host endothelial receptors represent potential targets for antiadhesive therapy for cytoadherence. In the present study, the major host receptor CD36 was targeted in vitro and in vivo with a recombinant peptide, PpMC-179, corresponding to the minimal CD36-binding domain from the cysteine-rich interdomain region 1 (CIDR1) within the MCvar1 PfEMP1. The in vitro inhibitory effect of PpMC-179 on human dermal microvascular endothelial cells (HDMECs) expressing multiple relevant adhesion molecules was investigated using a parallel-plate flow chamber. Pretreatment of endothelial monolayers with PpMC-179 (2 microM) inhibited the adhesion of infected erythrocytes (IRBCs) from all clinical isolates tested by 84.4% on resting and 62.8% on tumor necrosis factor alpha (TNF-alpha)-stimulated monolayers. Adhesion to stimulated cells was further inhibited (90.4%) when PpMC-179 was administered with an inhibitory anti-intercellular adhesion molecule 1 (ICAM-1) monoclonal antibody 84H10 (5 microg/mL). To determine the in vivo effectiveness of PpMC-179, we used a human/severe combined immunodeficiency (SCID) mouse chimeric model that allowed direct visualization of cytoadherence on intact human microvasculature. In unstimulated skin grafts, PpMC-179 inhibited adhesion by 86.3% and by 84.6% in TNF-alpha-stimulated skin grafts. More importantly, PpMC-179 administration resulted in the detachment of already adherent IRBCs by 80.7% and 83.3% on resting and stimulated skin grafts, respectively. The antiadhesive effect of PpMC-179 was rapid and sustained in vivo for at least 30 minutes. Our data indicate that targeting cytoadhesion in vivo is feasible and may offer a rapid antimalarial therapy.

Immunity to asexual blood stage malaria and vaccine approaches

The development of a malaria vaccine seems to be a definite possibility despite the fact that even individuals with a life time of endemic exposure do not develop sterile immunity. An effective malaria vaccine would be invaluable in preventing malaria-associated deaths in endemic areas, especially amongst children less than 5 years of age and pregnant women. This review discusses our current understanding of immunity against the asexual blood stage of malaria - the stage that is responsible for the symptoms of the disease - and approaches to the design of an asexual blood stage vaccine.

Sequestration of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate A, a receptor for maternal malaria: monoclonal antibodies against the native parasite ligand reveal pan-reactive epitopes in placental isolates

Plasmodium falciparum parasites express variant adhesion molecules on the surface of infected erythrocytes (IEs), which act as targets for natural protection. Recently it was shown that IE sequestration in the placenta is mediated by binding to chondroitin sulfate A via the duffy binding-like (DBL)-gamma 3 domain of P falciparum erythrocyte membrane protein 1 (PfEMP1(CSA)). Conventional immunization procedures rarely result in the successful production of monoclonal antibodies (mAbs) against such conformational vaccine candidates. Here, we show that this difficulty can be overcome by rendering Balb/c mice B cells tolerant to the surface of human erythrocytes or Chinese hamster ovary (CHO) cells before injecting P falciparum IEs or transfected CHO cells expressing the chondroitin sulfate A (CSA)-binding domain (DBL-gamma 3) of the FCR3 var(CSA) gene. We fused spleen cells with P3U1 cells and obtained between 20% and 60% mAbs that specifically label the surface of mature infected erythrocytes of the CSA phenotype (mIE(CSA)) but not of other adhesive phenotypes. Surprisingly, 70.8% of the 43 mAbs analyzed in this work were IgM. All mAbs immunoprecipitated PfEMP1(CSA) from extracts of (125)I surface-labeled IE(CSA). Several mAbs bound efficiently to the surface of CSA-binding parasites from different geographic areas and to placental isolates from West Africa. The cross-reactive mAbs are directed against the DBL-gamma 3(CSA), demonstrating that this domain, which mediates CSA binding, is able to induce a pan-reactive immune response. This work is an important step toward the development of a DBL-gamma 3-based vaccine that could protect pregnant women from pathogenesis. )

Assignment of functional roles to parasite proteins in malaria-infected red blood cells by competitive flow-based adhesion assay

Adhesion of parasitized red blood cells (PRBCs) to endothelial cells and subsequent accumulation in the microvasculature are pivotal events in the pathogenesis of falciparum malaria. During intraerythrocytic development, numerous proteins exported from the parasite associate with the RBC membrane skeleton but the precise function of many of these proteins remain unknown. Their cellular location, however, suggests that some may play a role in adhesion. The adhesive properties of PRBCs are best studied under flow conditions in vitro; however, experimental variation in levels of cytoadherence in currently available assays make subtle alterations in adhesion difficult to quantify. Here, we describe a flow-based assay that can quantify small differences in adhesion and document the extent to which a number of parasite proteins influence adhesion using parasite lines that no longer express specific proteins. Loss of parasite proteins ring-infected erythrocyte surface antigen (RESA), knob-associated histidine-rich protein (KAHRP) or Plasmodium falciparum erythrocyte membrane protein 3 (PfEMP3) had a significant effect on the ability of PRBCs to adhere, whereas loss of mature parasite-infected erythrocyte surface antigen (MESA) had no effect. Our studies indicate that a number of membrane skeleton-associated parasite proteins, although not exposed on the RBC surface, can collectively affect the adhesive properties of PRBCs and further our understanding of pathophysiologically relevant structure/function relationships in malaria-infected RBCs.

Immunization of Aotus monkeys with a functional domain of the Plasmodium falciparum variant antigen induces protection against a lethal parasite line

Immunity to Plasmodium falciparum in African children has been correlated with antibodies to the P. falciparum erythrocyte membrane protein 1 (PfEMP1) variant gene family expressed on the surface of infected red cells. We immunized Aotus monkeys with a subregion of the Malayan Camp variant antigen (MCvar1) that mediates adhesion to the host receptor CD36 on the endothelial surface and present data that PfEMP1 is an important target for vaccine development. The immunization induced a high level of protection against the homologous strain. Protection correlated with the titer of agglutinating antibodies and occurred despite the expression of variant copies of the gene during recurrent waves of parasitemia. A second challenge with a different P. falciparum strain, to which there was no agglutinating activity, showed no protection but boosted the immune response to this region during the infection. The level of protection and the evidence of boosting during infection encourage further exploration of this concept for malaria vaccine development.

Progress and challenges for malaria vaccines

Malaria causes much physical and economic hardship in tropical regions, particularly in communities where medical care is rudimentary. Should a vaccine be developed, it is the residents of these areas that stand to benefit the most. But the vaccine, which has been promised to be 'just round the corner' for many years, remains elusive. It is important to ask why this is so, when effective vaccines exist for many other infectious diseases. What are the reasons for the slow rate of progress, and what has been learned from the first clinical trials of candidate malaria vaccines? What are the remaining challenges, and what strategies can be pursued to address them?

Cytoadherence of Plasmodium falciparum-infected erythrocytes is mediated by a redox-dependent conformational fraction of CD36

The adherence of Plasmodium falciparum-infected RBC (IRBC) to postcapillary venular endothelium is an important determinant of the pathogenesis of severe malaria complications. Cytoadherence of IRBC to endothelial cells involves specific receptor/ligand interactions. The glycoprotein CD36 expressed on endothelial cells is the major receptor involved in this interaction. Treatment of CD36-expressing cells with reducing agents, such as DTT and N-acetylcysteine, was followed by CD36 conformational change monitorable by the appearance of the Mo91 mAb epitope. Only a fraction of the surface expressed CD36 molecules became Mo91 positive, suggesting the presence of two subpopulations of molecules with different sensitivities to reduction. The Mo91 epitope has been localized on a peptide (residues 260-279) of the C-terminal, cysteine-rich region of CD36. Treatment with reducing agents inhibited the CD36-dependent cytoadherence of IRBC to CD36-expressing cells and dissolved pre-existent CD36-mediated IRBC/CD36-expressing cell aggregates. CD36 reduction did not impair the functionality of CD36, since the reactivity of other anti-CD36 mAbs as well as the binding of oxidized low density lipoprotein, a CD36 ligand, were maintained. The modifications induced by reduction were reversible. After 14 h CD36 was reoxidized, the cells did not express the Mo91 epitope, and cytoadherence to IRBC was restored. The results indicate that IRBCs bind only to a redox-modulated fraction of CD36 molecules expressed on the cell surface. The present data indicate the therapeutic potential of reducing agents, such as the nontoxic drug N-acetylcysteine, to prevent or treat malaria complications due to IRBC cytoadhesion.

Towards a blood-stage vaccine for malaria: are we following all the leads?

Although the malaria parasite was discovered more than 120 years ago, it is only during the past 20 years, following the cloning of malaria genes, that we have been able to think rationally about vaccine design and development. Effective vaccines for malaria could interrupt the life cycle of the parasite at different stages in the human host or in the mosquito. The purpose of this review is to outline the challenges we face in developing a vaccine that will limit growth of the parasite during the stage within red blood cells--the stage responsible for all the symptoms and pathology of malaria. More than 15 vaccine trials have either been completed or are in progress, and many more are planned. Success in current trials could lead to a vaccine capable of saving more than 2 million lives per year.

Decoding the language of var genes and Plasmodium falciparum sequestration

Sequestration and rosetting are key determinants of Plasmodium falciparum pathogenesis. They are mediated by a large family of variant proteins called P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 proteins are multispecific binding receptors that are transported to parasite-induced, 'knob-like' binding structures at the erythrocyte surface. To evade immunity and extend infections, parasites clonally vary their expressed PfEMP1. Thus, PfEMP1 are functionally selected for binding while immune selection acts to diversify the family. Here, we describe a new way to analyse PfEMP1 sequence that provides insight into domain function and protein architecture with potential implications for malaria disease.