Plasmodium falciparum: further characterization of a functionally active region of the merozoite invasion ligand EBA-175

Non-synonymous amino acid alterations in PfEBA-175 modulate the merozoite ligand's ability to interact with host's Glycophorin A receptor

The pathological outcome of malaria due to Plasmodium falciparum infection depends largely on erythrocyte invasion by blood-stage merozoites which employ a cascade of interactions occurring between parasite ligands and RBC receptors. In a previous study exploring the genetic diversity of region-II of PfEBA-175, a ligand that plays a crucial part in parasite's RBC entry through Glycophorin A (GPA) receptor, we demonstrated that F2 domain of region-II underwent positive selection in Indian P. falciparum population through the accumulation of non-synonymous polymorphisms. Here, we examine the functional impact of two highly prevalent non-synonymous alterations in F2, namely Q584E & E592A, using a battery of molecular, biophysical and in-silico techniques. Application of circular dichroism, FTIR, fluorescence spectroscopy reveals that secondary and three-dimensional folding of recombinant-F2 protein carrying 584E and 592A residues (F2-Mut) differs significantly from that carrying 584Q and 592E (F2-3D7). A comparison of spectroscopic and thermodynamic parameters shows that F2-Mut is capable of forming a complex with GPA with higher efficiency compared to F2-3D7. In silico docking predicts both artemisinin and artesunate possess the capacity of slipping into the GPA binding crevices of PfEBA-175 and disrupt PfEBA-GPA association. However, the estimated affinity of artesunate towards PfEBA-175 with 584E and 592A residues is higher than that of artemisinin. Thermodynamic parameters computed using isotherms are concordant with this in-silico prediction. Together, our data suggest that the presence of amino acid alterations in F2 provide structural and functional stability favoring PfEBA-GPA interaction and artesunate can efficiently disrupt the interaction between GPA and PfEBA-175 even carrying altered amino acid residues. The present study alerts the malaria research community by presenting evidence that the parasite is gaining evolutionary fitness by cultivating genetic alterations in many of its proteins.

Genetic structure of two erythrocyte binding antigens of Plasmodium falciparum reveals a contrasting pattern of selection

Erythrocyte binding antigens 175 (EBA-175) and 140 (EBA-140) play key roles in erythrocyte invasion by binding to glycophorin A (GPA) and C (GPC) respectively in human malaria. Since antigenic variation in malaria endemic region is a major barrier to development of effective vaccine, we explore the nature and pattern of sequence diversity of these two vaccine candidates in Kolkata, India. Population genetic parameters based on parasite sequences representing region II of Pfeba-175 and Pfeba-140 genes were estimated using DnaSP V.5.10 and MEGA version 6.0. A novel molecular docking approach was implemented to assess the binding affinities of Kolkata Pfeba-175 variants with GPA. P. falciparum Kolkata isolates experienced a recent population expansion as documented by negative Tajima's D, Fu & Li's statistics, unimodal mismatch distribution and star-like median-joining network for both loci. Positive selection seemed to play a major role in shaping the diversity of Pfeba-175 (d_N/d_S=2.45, and McDonald-Kreitman P-value=0.04) with successive accumulation of Q584K/E, E592A and R664S deriving high frequency haplotypes designated here as F2KH3 and F2KH1. In silico molecular docking demonstrated that polypeptides encoded by F2KH1 and F2KH3 were capable of engaging the parasite ligand into energetically favorable interaction with GPA. Our data demonstrated emergence of Pfeba-175 sequences harboring selectively advantageous nonsynonymous substitutions on Pf3D7 sequence background in the Kolkata parasite isolates. A contrasting pattern of Pf3D7-centric expansion of parasite sequences was noted for Pfeba-140. Together, this study provides a firm genetic and biological support favoring a dominant role of EBA-175 in erythrocyte invasion.

Multiple Plasmodium falciparum Merozoite Surface Protein 1 Complexes Mediate Merozoite Binding to Human Erythrocytes

Successful invasion of human erythrocytes byPlasmodium falciparummerozoites is required for infection of the host and parasite survival. The early stages of invasion are mediated via merozoite surface proteins that interact with human erythrocytes. The nature of these interactions are currently not well understood, but it is known that merozoite surface protein 1 (MSP1) is critical for successful erythrocyte invasion. Here we show that the peripheral merozoite surface proteins MSP3, MSP6, MSPDBL1, MSPDBL2, and MSP7 bind directly to MSP1, but independently of each other, to form multiple forms of the MSP1 complex on the parasite surface. These complexes have overlapping functions that interact directly with human erythrocytes. We also show that targeting the p83 fragment of MSP1 using inhibitory antibodies inhibits all forms of MSP1 complexes and disrupts parasite growthin vitro.

Merozoite surface protein 1 recognition of host glycophorin A mediates malaria parasite invasion of red blood cells

Plasmodium falciparum invasion of human red blood cells (RBCs) is an intricate process requiring a number of distinct ligand-receptor interactions at the merozoite-erythrocyte interface. Merozoite surface protein 1 (MSP1), a highly abundant ligand coating the merozoite surface in all species of malaria parasites, is essential for RBC invasion and considered a leading candidate for inclusion in a multiple-subunit vaccine against malaria. Our previous studies identified an interaction between the carboxyl-terminus of MSP1 and RBC band 3. Here, by employing phage display technology, we report a novel interaction between the amino-terminus of MSP1 and RBC glycophorin A (GPA). Mapping of the binding domains established a direct interaction between malaria MSP1 and human GPA within a region of MSP1 known to potently inhibit P falciparum invasion of human RBCs. Furthermore, a genetically modified mouse model lacking the GPA- band 3 complex in RBCs is completely resistant to malaria infection in vivo. These findings suggest an essential role of the MSP1-GPA-band 3 complex during the initial adhesion phase of malaria parasite invasion of RBCs.

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

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

Inhibitory humoral responses to the Plasmodium falciparum vaccine candidate EBA-175 are independent of the erythrocyte invasion pathway

Plasmodium falciparum utilizes multiple ligand-receptor interactions for invasion. The invasion ligand EBA-175 is being developed as a major blood-stage vaccine candidate. EBA-175 mediates parasite invasion of host erythrocytes in a sialic acid-dependent manner through its binding to the erythrocyte receptor glycophorin A. In this study, we addressed the ability of naturally acquired human antibodies against the EBA-175 RII erythrocyte-binding domain to inhibit parasite invasion of ex vivo isolates, in relationship to the sialic acid dependence of these parasites. We have determined the presence of antibodies to the EBA-175 RII domain by enzyme-linked immunosorbent assay (ELISA) in individuals from areas of Senegal where malaria is endemic with high and low transmission. Using affinity-purified human antibodies to the EBA-175 RII domain from pooled patient plasma, we have measured the invasion pathway as well as the invasion inhibition of clinical isolates from Senegalese patients in ex vivo assays. Our results suggest that naturally acquired anti-EBA-175 RII antibodies significantly inhibit invasion of Senegalese parasites and that these responses can be significantly enhanced through limiting other ligand-receptor interactions. However, the extent of this functional inhibition by EBA-175 antibodies is not associated with the sialic acid dependence of the parasite strain, suggesting that erythrocyte invasion pathway usage by parasite strains is not driven by antibodies targeting the EBA-175/glycophorin A interaction. This work has implications for vaccine design based on the RII domain of EBA-175 in the context of alternative invasion pathways.

Targeting sialic acid dependent and independent pathways of invasion in Plasmodium falciparum

The pathology of malaria is a consequence of the parasitaemia which develops through the cyclical asexual replication of parasites in a patient's red blood cells. Multiple parasite ligand-erythrocyte receptor interactions must occur for successful Plasmodium invasion of the human red cell. Two major malaria ligand families have been implicated in these variable ligand-receptor interactions used by Plasmodium falciparum to invade human red cells: the micronemal proteins from the Erythrocyte Binding Ligands (EBL) family and the rhoptry proteins from the Reticulocyte binding Homolog (PfRH) family. Ligands from the EBL family largely govern the sialic acid (SA) dependent pathways of invasion and the RH family ligands (except for RH1) mediate SA independent invasion. In an attempt to dissect out the invasion inhibitory effects of antibodies against ligands from both pathways, we have used EBA-175 and RH5 as model members of each pathway. Mice were immunized with either region II of EBA-175 produced in Pichia pastoris or full-length RH5 produced by the wheat germ cell-free system, or a combination of the two antigens to look for synergistic inhibitory effects of the induced antibodies. Sera obtained from these immunizations were tested for native antigen recognition and for efficacy in invasion inhibition assays. Results obtained show promise for the potential use of such hybrid vaccines to induce antibodies that can block multiple parasite ligand-red cell receptor interactions and thus inhibit parasite invasion.

Erythrocyte invasion and functionally inhibitory antibodies in Plasmodium falciparum malaria

Malaria is a disease that kills several million people every year. P. falciparum merozoites invade new erythrocytes every 48 h, causing fever, anemia and cerebral malaria. Effective immunity against malaria develops slowly and only after repeated exposure. Antibodies are an important part of this immunity. However, the antigens that mediate immunity by inducing functionally imperative antibodies have not yet been identified. This review gives an overview of the erythrocyte invasion process, which has been described to include several different antigens. Invasion inhibitory antibodies can inhibit merozoite penetration of new erythrocytes, and different methods for measurement of the presence of functionally important antibodies have been employed. ELISA, Invasion inhibition assays and ADCI are some of the methods discussed.

Polymorphisms in erythrocyte binding antigens 140 and 181 affect function and binding but not receptor specificity in Plasmodium falciparum

Invasion of human erythrocytes by the malaria parasite Plasmodium falciparum utilizes multiple ligand-receptor interactions involving erythrocyte receptors and parasite erythrocyte binding proteins of the Duffy binding-like family. Erythrocyte binding antigen 175 (EBA-175) binds to glycophorin A, the most abundant protein on the human erythrocyte surface and EBA-140 (also known as BAEBL) binds to glycophorin C, while the receptor for EBA-181 (also known as JESEBL) remains unknown. EBA binding is mediated via region II, a highly structured extracellular domain that shows a degree of sequence variability between different laboratory strains/isolates. Here, we determined the influence of region II polymorphisms on host cell receptor binding and overall function during invasion of EBA-140, EBA-175, and EBA-181. Polymorphisms in the binding domains of EBA-140 and EBA-181 have been suggested previously to alter their respective receptor specificities. In our hands, these polymorphisms affected the levels of EBA-140 and EBA-181 binding to receptors but, critically, not the receptor specificities of these proteins. The degree of EBA-140 binding to glycophorin C correlates with the level of function for this ligand-receptor interaction in merozoite invasion. In contrast, EBA-175, which is highly polymorphic in region II, shows no variability in its ability to bind to its receptor, glycophorin A. Combined, these data highlight the importance of sequence variability in EBAs as driven by immune selection but not by receptor specificity.

Variation in use of erythrocyte invasion pathways by Plasmodium falciparum mediates evasion of human inhibitory antibodies

Antibodies that inhibit Plasmodium falciparum invasion of erythrocytes are believed to be an important component of immunity against malaria. During blood-stage infection, P. falciparum can use different pathways for erythrocyte invasion by varying the expression and/or utilization of members of 2 invasion ligand families: the erythrocyte-binding antigens (EBAs) and reticulocyte-binding homologs (PfRhs). Invasion pathways can be broadly classified into 2 groups based on the use of sialic acid (SA) on the erythrocyte surface by parasite ligands. We found that inhibitory antibodies are acquired by malaria-exposed Kenyan children and adults against ligands of SA-dependent and SA-independent invasion pathways, and the ability of antibodies to inhibit erythrocyte invasion depended on the pathway used by P. falciparum isolates. Differential inhibition of P. falciparum lines that varied in their use of specific EBA and PfRh proteins pointed to these ligand families as major targets of inhibitory antibodies. Antibodies against recombinant EBA and PfRh proteins were acquired in an age-associated manner, and inhibitory antibodies against EBA175 appeared prominent among some individuals. These findings suggest that variation in invasion phenotype might have evolved as a mechanism that facilitates immune evasion by P. falciparum and that a broad inhibitory response against multiple ligands may be required for effective immunity.

Naturally acquired immunity and reduced susceptibility to falciparum malaria in two subpopulations of endemic eastern India

This study was aimed to assess the prevalence of naturally acquired humoral immune responses and their association with reduced susceptibility to malaria in children and adults with differential clinical conditions from an Indian zone where malaria is endemic. The study was undertaken in an eastern province of India (Keonjhar, Orissa) in a group of 341 children (both younger and older) and 98 adults living in two different areas, Town area and Forest area. They were studied for their parasitological and immunological profiles. Sera from different age-matched groups were screened by ELISA to measure IgG reactivities for characterizing humoral immune responses to the B-cell epitopes of Plasmodium falciparum MSP1, AMA1, RAP1 and EBA175 peptides and P. falciparum-infected erythrocyte lysate. In Town area, overall P. falciparum cases were 5.5%, whereas those in Forest area were 26.7%. We observed an age-wise increasing trend of immunity in these two populations. It was also noticed that the frequency of responders to stage-specific antigens was higher in individuals from the Town area where the frequency of malaria was lower. The naturally acquired humoral immune responses to different stage-specific antigens of P. falciparum reflect the reduced risk of malaria in the study groups. The higher frequency of seroresponders showed correlation with lower risk of developing malaria.

Immunogenicity and in vitro protective efficacy of a polyepitope Plasmodium falciparum candidate vaccine constructed by epitope shuffling

A polyepitope chimeric antigen incorporating multiple protective and conservative epitopes from multiple antigens of Plasmodium falciparum has been considered to be more effective in inducing multiple layers of immunity against malaria than a single stage- or single antigen-based vaccine. By modifying the molecular breeding approach to epitope shuffling, we have constructed a polyepitope chimeric gene that encodes 11 B-cell and T-cell proliferative epitope peptides derived from eight key antigens mostly in the blood stage of Plasmodium falciparum. A 35-kDa antigen encoded by this gene, named Malaria RCAg-1, was purified from an E. coli expression system. Immunization of rabbits and mice with the purified protein in the presence of Freund's adjuvant strongly generated long-lasting antibody responses that recognized the corresponding individual epitope peptide in this vaccine as well as blood stage parasites. CD4(+) T-cell responses were also elicited as shown by the enhancement of T-cell proliferation, IFN-gamma and IL-4 level. In vitro assay of protection revealed that vaccine-elicited antibodies could efficiently inhibit the growth of blood-stage parasites. Additionally, the chimeric antigen was recognized by human serum specimens from malaria patients and individuals living in the endemic area. Our studies indicate the potential of M.RCAg-1 recombinant protein as malaria candidate vaccines as well as the rationale of the epitope shuffling technology applied in designing malaria vaccines.

Antibody responses to Plasmodium falciparum vaccine candidate antigens in three areas distinct with respect to altitude

Antibody levels against malaria antigens were measured among patients presenting with uncomplicated malaria at health centers from three locations in Zimbabwe (Bindura, Chiredzi and Kariba) that are distinct with regard to altitude and climatic conditions. Antibody levels were determined by ELISA using the antigens, apical membrane antigen 1 (AMA-1), erythrocyte binding antigen 175 (EBA-175), circumsporozoite surface protein (CSP), merozoite surface protein 1 (MSP-1) and Pfg27. For all the antigens tested, IgG and IgM levels were higher for Bindura (altitude 1100 m) compared to Kariba (<600 m, altitude) and Chiredzi (approximately 600 m, altitude) with the exception of IgG and IgM to AMA-1 and EBA-175 which were similar between Chiredzi and Bindura. Plasma samples were further analyzed for their functional activity by testing their ability to inhibit the growth of Plasmodium falciparum in culture. Our results, determined by microscopy and verified by the LDH assay revealed that plasma from the three locations had similar inhibitory activity against the growth of P. falciparum in vitro. Our data revealed that highest growth inhibition correlated with the highest levels of MSP-1 antibody values.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

Malaria and the red blood cell membrane

Malaria is the most serious and widespread parasitic disease of humans and is arguably the commonest disease of red blood cells (RBCs). Malaria has exerted a powerful effect on human evolution and selection for resistance has led to the appearance and persistence of a number of inherited diseases. After parasite invasion, RBCs are progressively and dramatically modified. New structures appear inside the RBC and novel parasite proteins are exported to the erythrocyte cytoplasm and membrane skeleton. Radical biochemical, morphological, and rheological alterations manifest as increased membrane rigidity, reduced cell deformability, and greater adhesiveness for the vascular endothelium and other blood cells. Numerous protein-protein interactions between the malaria-parasite and the host RBC are important for many aspects of parasite biology and the pathogenesis of malaria. In addition, there are many other parasite proteins located within the infected red cell and at the membrane skeleton, for which no precise functional roles have yet been elucidated. Sequencing and annotation of the complete genome of Plasmodium falciparum, the production of proteomic and transcriptomic profiles of parasites, and the development of a transfection system for the asexual stage of the parasite are all recent achievements that should advance understanding of the molecular mechanisms that underlie the parasite-induced functional alterations in red cells.

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

The recognition and invasion of human erythrocytes by the most lethal malaria parasite Plasmodium falciparum is dependent on multiple ligand-receptor interactions. Members of the erythrocyte binding-like (ebl) family, including the erythrocyte binding antigen-175 (EBA-175), are responsible for high affinity binding to glycoproteins on the surface of the erythrocyte. Here we describe a paralogue of EBA-175 and show that this protein (EBA-181/JESEBL) binds in a sialic acid-dependent manner to erythrocytes. EBA-181 is expressed at the same time as EBA-175 and co-localizes with this protein in the microneme organelles of asexual stage parasites. The receptor binding specificity of EBA-181 to erythrocytes differs from other members of the ebl family and is trypsin-resistant and chymotrypsin-sensitive. Furthermore, using glycophorin B-deficient erythrocytes we show that binding of EBA-181 is not dependent on this sialoglycoprotein. The level of expression of EBA-181 differs among parasite lines, and the importance of this ligand for invasion appears to be strain-dependent as the EBA-181 gene can be disrupted in W2mef parasites, without affecting the invasion phenotype, but cannot be targeted in 3D7 parasites.

Plasmodium falciparum erythrocyte invasion through glycophorin C and selection for Gerbich negativity in human populations

Geographic overlap between malaria and the occurrence of mutant hemoglobin and erythrocyte surface proteins has indicated that polymorphisms in human genes have been selected by severe malaria. Deletion of exon 3 in the glycophorin C gene (called GYPCDeltaex3 here) has been found in Melanesians; this alteration changes the serologic phenotype of the Gerbich (Ge) blood group system, resulting in Ge negativity. The GYPCDeltaex3 allele reaches a high frequency (46.5%) in coastal areas of Papua New Guinea where malaria is hyperendemic. The Plasmodium falciparum erythrocyte-binding antigen 140 (EBA140, also known as BAEBL) binds with high affinity to the surface of human erythrocytes. Here we show that the receptor for EBA140 is glycophorin C (GYPC) and that this interaction mediates a principal P. falciparum invasion pathway into human erythrocytes. EBA140 does not bind to GYPC in Ge-negative erythrocytes, nor can P. falciparum invade such cells using this invasion pathway. This provides compelling evidence that Ge negativity has arisen in Melanesian populations through natural selection by severe malaria.

Three multigene families in Plasmodium parasites: facts and questions

Multigene families optimise fitness by providing a set of related genes with possibly different temporal and/or topological expression patterns. We analyse here the structural organisation and sequence diversity of the rDNA, sera and var C Plasmodium falciparum families, and discuss their consequences for parasite biology. The low rDNA copy number, which reduces reshuffling, is probably the corollary of the need for functionally distinct rRNAs in the insect and in the vertebrate host. The unusual intra-genome and population rDNA sequence diversity results in cells equipped with mosaic ribosome sets. The functional constraints are such that ribosome compatibility could influence parasite fitness and contribute to population structuring. Unlike the dispersed rDNA units, the sera family is arranged as a tandem gene cluster, with seven contiguous similar genes, and one more distantly related paralog. We address the question of the inclusion criteria in family definition. We discuss the results concerning the SERA proteins expression and function in the context of the long overlooked multigene family. The var C module is shared by var genes, 'orphan' var C and var C pseudogenes. Analysis of 125 var C deduced protein sequences highlights a well-conserved framework, including putative phosphorylation sites, consistent with the proposed function of mediating interaction with cytoskeletal proteins. The 5' and 3' flanking sequences of the var C pseudogenes are heterogeneous. In contrast, the flanking sequences of the uninterrupted var C modules show remarkable conservation. This is interesting in view of the silencing activity of the var intronic sequence on var expression. The 5' flanking sequence dichotomy reported for internal and sub-telomeric var genes extends to the 3' flanking sequences. This has profound implications for transcription regulation and generation of diversity. The var C family suggests a role for pseudogenes as a diversity reservoir and in genome dynamics by promoting ectopic recombination.

Polyethylene glycol-coated red blood cells fail to bind glycophorin A-specific antibodies and are impervious to invasion by the Plasmodium falciparum malaria parasite

This study was designed to assess the binding of glycophorin A-specific antibodies to polyethylene glycol (PEG)-modified red blood cells (RBCs) and evaluate their resistance to invasion by Plasmodium falciparum malaria parasites. RBCs were conjugated with a range of concentrations (0.05 to 7.5 mM) of activated PEG derivatives of either 3.35 or 18.5 kd molecular mass. The binding of glycophorin A-specific antibodies was assessed by hemagglutination and flow cytometry. PEG-modified RBCs were assessed for their ability to form rosettes around Chinese hamster ovary (CHO) cells transiently expressing the glycophorin A binding domain of EBA-175, a P falciparum ligand crucial to RBC invasion. PEG-RBCs were also tested for their ability to be invaded by the malaria parasite. RBCs coated with 3.35 and 18.5 kd PEG demonstrated a dose-dependent inhibition of glycophorin A-specific antibody binding, CHO cell rosetting, and P falciparum invasion. These results indicate that glycophorin A epitopes responsible for antibody and parasite binding are concealed by PEG coating, rendering these cells resistant to P falciparum invasion. These studies confirm the effectiveness of PEG modification for masking RBC-surface glycoproteins. This may provide a means to prevent alloimmunization in the setting of RBC transfusion and suggests a novel method to enhance the effectiveness of exchange transfusion for the treatment of cerebral malaria.

The apical organelles of malaria merozoites: host cell selection, invasion, host immunity and immune evasion

Malaria is caused by protozoan parasites belonging to the phylum Apicomplexa. These obligate intracellular parasites depend on the successful invasion of an appropriate host cell for their survival. This article is a broad overview of the molecular strategies employed by the merozoite, an invasive form of the malaria parasite, to successfully invade a suitable red blood cell.

Functional analysis of proteins involved in Plasmodium falciparum merozoite invasion of red blood cells

Plasmodium falciparum causes the most lethal form of malaria in humans and is responsible for over two million deaths per year. The development of a vaccine against this parasite is an urgent priority and potential protein targets include those on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to transfect P. falciparum has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. In this review, we describe the use of this technology to examine the role of merozoite antigens in erythrocyte invasion and to address their potential as vaccine candidates.

Development, expression, and murine testing of a multistage Plasmodium falciparum malaria vaccine candidate

A synthetic gene encoding twelve B cell epitopes, six T-cell proliferative epitopes, and three cytotoxic T lymphocyte (CTL) epitopes from nine stage-specific antigens, representing the sporozoite, liver stage, asexual blood-stage, and sexual-stage antigens of Plasmodium falciparum, was constructed by assembling overlapping oligonucleotides followed by PCR extension and annealing. A three-step PCR protocol using twelve long oligonucleotides was employed to generate a 1053 base-pair synthetic gene, the identity of which was confirmed by sequencing. This synthetic gene, named CDC/NII MAL VAC-1, was cloned, and the recombinant protein was expressed in the Baculovirus Expression Vector System (BEVS). The selection of malarial epitopes for inclusion in this vaccine construct was based on immunoepidemiological studies in malaria endemic area, in vitro, and in vivo protection studies in model systems. The 41 kDa BEVS-expressed recombinant protein reacted with mouse antibodies specific for individual B cell epitopes in the vaccine construct and with sera from clinically immune Kenyan adults. An immunization study in three strains of mice that differ at the H-2 locus demonstrated that the BEVS-expressed recombinant protein is immunogenic; the candidate vaccine antigen induced high titer antibodies, and lymphocyte proliferative and IFN-gamma responses. These results demonstrate that individual B and T cell epitopes can be assembled to create synthetic genes that encode proteins capable of eliciting specific antibody and T cell responses.

Antigenic variation in Plasmodium falciparum: mechanisms and consequences

In the past year, the major advances in malaria antigenic variation have been concerned with the transcription and switching of variant antigen genes, and the functional expression of regions of the major variant antigen. Also, new variant gene families have been discovered as a result of the Malaria Genome Project.

Immunogenicity and in vitro protective efficacy of a recombinant multistage Plasmodium falciparum candidate vaccine

Compared with a single-stage antigen-based vaccine, a multistage and multivalent Plasmodium falciparum vaccine would be more efficacious by inducing "multiple layers" of immunity. We have constructed a synthetic gene that encodes for 12 B cell, 6 T cell proliferative, and 3 cytotoxic T lymphocyte epitopes derived from 9 stage-specific P. falciparum antigens corresponding to the sporozoite, liver, erythrocytic asexual, and sexual stages. The gene was expressed in the baculovirus system, and a 41-kDa antigen, termed CDC/NIIMALVAC-1, was purified. Immunization in rabbits with the purified protein in the presence of different adjuvants generated antibody responses that recognized vaccine antigen, linear peptides contained in the vaccine, and all stages of P. falciparum. In vitro assays of protection revealed that the vaccine-elicited antibodies strongly inhibited sporozoite invasion of hepatoma cells and growth of blood-stage parasites in the presence of monocytes. These observations demonstrate that a multicomponent, multistage malaria vaccine can induce immune responses that inhibit parasite development at multiple stages. The rationale and approach used in the development of a multicomponent P. falciparum vaccine will be useful in the development of a multispecies human malaria vaccine and vaccines against other infectious diseases.

Identification of an erythrocyte binding peptide from the erythrocyte binding antigen, EBA-175, which blocks parasite multiplication and induces peptide-blocking antibodies

A biotinylated peptide covering a sequence of 21 amino acids (aa) from the erythrocyte binding antigen (EBA-175) of Plasmodium falciparum bound to human glycophorin A, an erythrocyte receptor for merozoites, as demonstrated by enzyme-linked immunosorbent assay (ELISA) and to erythrocytes as demonstrated by flow cytometry analysis. The peptide, EBA(aa1076-96), also bound to desialylated glycophorin A and glycophorin B when tested by ELISA. The peptide blocked parasite multiplication in vitro. The glycophorin A binding sequence was further delineated to a 12-aa sequence, EBA(aa1085-96), by testing the binding of a range of truncated peptides to immobilized glycophorin A. Our data indicate that EBA(aa1085-96) is part of a ligand on the merozoite for binding to erythrocyte receptors. This binding suggests that the EBA(aa1085-96) peptide is involved in a second binding step, independent of sialic acid. Antibody recognition of this peptide sequence may protect against merozoite invasion, but only a small proportion of sera from adults from different areas of malaria transmission showed antibody reactivities to the EBA(aa1076-96) peptide, indicating that this sequence is only weakly immunogenic during P. falciparum infections in humans. However, Tanzanian children with acute clinical malaria showed high immunoglobulin G reactivity to the EBA(aa1076-96) peptide compared to children with asymptomatic P. falciparum infections. The EBA(aa1076-96) peptide sequence from EBA-175 induced antibody formation in mice after conjugation of the peptide with purified protein derivative. These murine sera inhibited EBA(aa1076-96) peptide binding to glycophorin A.

Interaction of Salmonella typhi strains with cultured human monocyte-derived macrophages

Human monocyte-derived macrophages (MDM) provided this laboratory with a tool to develop a primary-cell assay for evaluating the relative virulence of newly constructed Salmonella typhi carrier strains. In this study, the interaction with and survival within MDM were compared for delta aroA143-attenuated strains, wild-type virulent strains, and the current oral-vaccine strain, Ty21a.