Identification of a novel antigenic domain of Plasmodium falciparum merozoite surface protein-1 that specifically binds to human erythrocytes and inhibits parasite invasion, in vitro

Merozoite surface protein 1 (MSP-1) of Plasmodium falciparum is a promising candidate for vaccine development against malaria. Identification of protective epitopes within MSP-1 is an important step towards the elucidation of mechanisms of parasitic invasion and for the creation of a multi-subunit vaccine. In this study, we show that a 115 amino acid region (p115MSP-1) within the p38 domain of MSP-1 can: (i) specifically bind to human erythrocytes, independent of glycophorin A; (ii) inhibit parasite invasion at significant levels, in vitro; and (iii) be recognized by human sera of individuals from malaria-endemic regions of Africa. More importantly, we also show that polyclonal antibodies specific to this region prevent parasite invasion at levels approaching 90%, in vitro. Our data illustrate that not only is p115MSP-1 involved in parasite recognition/invasion of human erythrocytes, but that this region is highly antigenic, producing high titer antibodies. The delineation of the role of MSP-1 in parasite invasion is an important component of the development of a multi-subunit malaria vaccine, and this study identifies a candidate antigen in this context.     

Role of internal domains of glycophorin in Plasmodium falciparum invasion of human erythrocytes.

Human erythrocyte glycophorin, a putative receptor to Plasmodium falciparum malaria parasites, was studied in terms of its structural domains involved in mediating invasion. These domains were isolated from purified glycophorin A and from supernatants and membranes obtained from protease-treated erythrocytes. They were tested for invasion blocking capacity by using an in vitro assay system. The role of carbohydrate-rich domains was assessed with the following compounds: (i) sialoglycopeptides released by proteases either from whole cells or isolated glycophorin A; (ii) the sialoglycoproteins fetuin and alpha 1 acid glycoprotein and the N-acetylglucosamine-rich ovomucoid; and (iii) the saccharides N-acetylneuraminlactose, N-acetylglucosamine, and free sialic acid. With the exception of N-acetylglucosamine, all of the compounds failed to block invasion. The role of carbohydrate-poor domains of glycophorin was assessed with peptides isolated from membranes of proteolyzed cells and with the hydrophobic fragment of glycophorin A. Glycophorin and the derived hydrophobic peptides formed high-molecular-weight aggregates in physiological solutions. They all inhibited invasion to a comparable extent. The inhibitory potency of glycophorin A increased by sixfold after reconstitution into egg lecithin vesicles. The observations reported here underscore the role played by the hydrophobic domain in the glycophorin-mediated blockage of invasion. They also suggest that in the interactions between P. falciparum merozoites and the erythrocyte membrane, the exposed glycosylated domains of glycophorins provide the initial but rather weak binding sites, whereas the internal domains of the molecules provide the more stable attachment sites for merozoites.     

The RhopH complex is transferred to the host cell cytoplasm following red blood cell invasion by Plasmodium falciparum

The high-molecular mass rhoptry protein complex (PfRhopH), which comprises three distinct gene products, RhopH1, RhopH2, and RhopH3, is known to be secreted and transferred to the parasitophorous vacuole membrane upon invasion of a red blood cell by the malaria parasite Plasmodium falciparum. Here we show that the merozoite-acquired RhopH complex is also transferred to defined domains of the red blood cell cytoplasm, and possibly transiently associated with Maurer's clefts. This is the first report of trafficking in the host cell cytoplasm for P. falciparum rhoptry proteins secreted upon red blood cell invasion. Based on its newly identified sub-cellular location and the phenotype of RhopH1 mutants, we propose that the RhopH complex participate in the assembly of the cytoadherence complex.     

Characterization of permeation pathways appearing in the host membrane of Plasmodium falciparum infected red blood cells

The host cell membrane of Plasmodium falciparum infected cells becomes permeabilized at the trophozoite stage. A variety of otherwise impermeant substances such as carbohydrates, polyols, amino acids and anions easily gain access to the cytosol of infected cells. Using the isotonic-hemolysis method or uptake of labeled substances, we characterized the new permeation pathways as pores of approximately 0.7 nm equivalent radius. The pores bear a positively charged character which facilitates movement of small anions and excludes cations, so that the ionic composition and osmotic properties of infected cells are not drastically altered. Substances of a molecular size similar to that of disaccharides are fully excluded. Substances of limiting size might be accommodated in the pore, provided they bear a side group of hydrophobic character. The new permeation pathways may provide a vital route for acquisition or release of essential nutrients or catabolites.     

Fine mapping of Plasmodium falciparum ribosomal phosphoprotein PfP0 revealed sequences with highly specific binding activity to human red blood cells

The Plasmodium falciparum P0 ribosomal phosphoprotein (PfP0) was identified for the first time by screening a cDNA expression library of P. falciparum parasites with sera from malaria-immune individuals. Due to its localization on the surface of different parasite life-cycle stages (merozoites and gametocytes) and its recognition by invasion-blocking antibodies, PfP0 has been considered a potential malaria-vaccine component. In this study, 16 20-mer-long synthetic peptides spanning the entire PfP0 sequence were evaluated by means of receptor–ligand assays with human red blood cells (RBCs) in order to determine the role played by these peptides in the invasion process. Four RBC high-activity binding peptides (HABPs), located mostly toward the N-terminal region, were identified: HABP 33898 (¹MAKLSKQQKKQMYIEKLSSL²⁰), HABP 33900 (⁴¹ASVRKSLRGKATILMGKNTRY⁶⁰), HABP 33901 (⁶¹IRTALKKNLQAVPQIEKLLPY ⁸⁰), and HABP 33906 (¹⁶¹LIKQGEKVTASSATLLRKFNY¹⁸⁰). The binding pattern of HABPs 33898 and 33906 to enzyme-treated RBCs suggests receptors of protein nature for these two HABPs, one of which could correspond to a common 58-kDa RBC membrane protein, as indicated by results of cross-linking assays. Both HABPs exhibited high content of α-helical features and prevented P. falciparum merozoite invasion to RBCs in vitro by up to 91%. The invasion-blocking ability reported here for these PfP0 HABPs supports their inclusion in immunological studies with the aim of assessing their potential as candidates for a vaccine against P. falciparum malaria.     

Identification of blood group A and B antigens in human glycophorin

Glycophorin A (GPA), the major sialoglycoprotein of human erythrocyte membranes, was isolated separately from blood group A and B erythrocytes using phenol-water extraction. After purification, performed as gel filtration in the presence of SDS, two glycophorin samples GPA-A and GPA-B were run, in duplicate, in SDS-PAGE and electroblotted onto Immobilon P. After staining with 1) anti-glycophorin antibody and 2) with relevant anti-blood group (A or B) antibody it was shown that the band pattern of the samples in each duplicate was the same. GPA-A and GPA-B samples were also degraded using Carlson degradation (beta-elimination in mild alkaline/strong reducing conditions) and from reaction products the fractions of O-glycans and N-glycans were isolated; they were used in hemagglutination inhibition test. It was shown that both sugar fractions derived from GPA-A did inhibit agglutination of blood group A erythrocytes by anti-A antibody, whereas oligosaccharide fractions derived from GPA-B inhibited agglutination of blood group B erythrocytes by anti-B antibody. These results, obtained using immunochemical methods, confirm the presence of blood group A and B determinants in the carbohydrate moiety of human glycophorin, derived from the blood group A or B erythrocytes, respectively.     

Plasmodium falciparum is able to invade erythrocytes through a trypsin-resistant pathway independent of glycophorin B

Plasmodium falciparum invades erythrocytes through multiple ligand-receptor interactions, with redundancies in each pathway. One such alternate pathway is the trypsin-resistant pathway that enables P. falciparum to invade trypsin-treated erythrocytes. Previous studies have shown that this trypsin-resistant pathway is dependent on glycophorin B, as P. falciparum strains invade trypsin-digested glycophorin B-deficient erythrocytes at a highly reduced efficiency. Furthermore, in a recent study, the P. falciparum 7G8 strain did not invade glycophorin B-deficient erythrocytes, a finding that was not confirmed in the present study. To analyze the degree of dependence on glycophorin B for invasion by P. falciparum through the trypsin-resistant pathway, we have studied the invasion phenotypes of five parasite strains, 3D7, HB3, Dd2, 7G8, and Indochina I, on trypsin-treated normal and glycophorin B-deficient erythrocytes. Invasion was variably reduced in glycophorin B-deficient erythrocytes. Four strains, 3D7, HB3, Dd2, and Indochina I, invaded trypsin-treated erythrocytes, while invasion by the 7G8 strain was reduced by 90%. Among the four strains, invasion by 3D7, HB3, and Dd2 of trypsin-digested glycophorin B-deficient erythrocytes was further reduced. However, Indochina I invaded trypsin-digested glycophorin B-deficient erythrocytes at the same efficiency as its invasion of trypsin-digested normal erythrocytes. This strongly suggests that the Indochina I strain of P. falciparum is not dependent on glycophorin B to invade through a trypsin-resistant pathway as are the strains 3D7, HB3, and Dd2. Thus, P. falciparum is able to invade erythrocytes through a glycophorin B-independent, trypsin-resistant pathway.     

The human immune response to Plasmodium falciparum includes both antibodies that inhibit merozoite surface protein 1 secondary processing and blocking antibodies

Malaria merozoite surface protein 1 (MSP1) is cleaved in an essential step during erythrocyte invasion. The responses of children to natural malaria infection included antibodies that inhibit this cleavage and others that block the binding of these inhibitory antibodies. There was no correlation between the titer of the antibody to the 19-kDa fragment of MSP1 and its inhibitory activity. These findings have implications for the design of MSP1-based vaccines.     

Analysis of recombinant merozoite surface protein-1 of Plasmodium falciparum expressed in mammalian cells

Synthetic chimeric DNA constructs with a reduced A + T content coding for full-length merozoite surface protein-1 of Plasmodium falciparum (MSP1) and three fragments thereof were expressed in HeLa cells. To target the recombinant proteins to the surface of the host cell the DNA sequences coding for the N-terminal signal sequence and for the putative C-terminal recognition/attachment signal for the glycosyl-phosphatidyl-inositol (GPI)-anchor of MSP1 were replaced by the respective DNA sequences of the human decay-accelerating-factor (DAF). The full-length recombinant protein, hu-MSP1-DAF, was stably expressed and recognised by monoclonal antibodies that bind to the N-terminus or the C-terminus of the native protein, respectively. Its apparent molecular mass is higher as compared to the native protein and it is post-translationally modified by attachment of N-glycans whereas native MSP1 is not glycosylated. Immunofluorescence images of intact cells show a clear surface staining. After permeabilization hu-MSP1-DAF can be detected in the cytosol as well. As judged by protease treatment of intact cells 25% of recombinant MSP1 is located on the surface. This fraction of hu-MSP1-DAF can be cleaved off the cell membrane by phosphatidylinositol-specific phospholipase C indicating that the protein is indeed bound to the cell membrane via a GPI-anchor. Human erythrocytes do not adhere to the surface of mammalian cells expressing either of the constructs made in this study.     

Modification of a human monoclonal antibody Fab fragment specific for Plasmodium falciparum 19-kDa C-terminal merozoite surface protein 1 by site-directed mutagenesis

We recently produced human monoclonal antibody Fab fragments specific for the 19-kDa C-terminal merozoite surface protein 1 of Plasmodium falciparum in a bacterial expression system. The effect of single amino acid modifications in the third complementarity-determining regions of the heavy and light chains on affinity was examined in one of the Fab fragments, Pf25. Recombination polymerase chain reaction was used to modify Tyr(92) or Ile(97) in the light chain and Val(101) or Trp(107) in the heavy chain. No effective replacements for Tyr(92) and Val(101) were found, but possible substitutions of Ile(97) with Gly, Leu, Glu, Ala and Ser, and of Trp(107) with Arg and Ser were demonstrated. Of these modified Fab fragments, the affinities of Fabs with Ile(97)-Leu and Trp(107)-Ser mutations were slightly higher than that of the original Fab. The effects of these modifications on the antigen-antibody interaction are discussed.     

Mapping of the region predominantly recognized by antibodies to the Plasmodium falciparum merozoite surface antigen MSA 1.

The locations of the epitopes of a panel of mouse monoclonal antibodies directed against the Plasmodium falciparum merozoite surface antigen MSA 1 were mapped by using naturally occurring processed fragments, by chemical cleavage of the protein and by comparison of the isolate-specificity of binding with known sequence variation. By these criteria, the most antigenic region occurs in the cysteine-rich, invariant 19-kDa carboxyl terminal domain with 12/19 monoclonal antibodies (mAbs) binding to this region. One of these mAbs recognized an epitope near the C-terminal putative glycosylphosphatidylinositol anchor site. This was the only mAb which significantly inhibited parasite growth in vitro. The other mAbs recognized conformational epitopes involving the cysteine residues located throughout this fragment. This study has identified further naturally occurring processing sites and a consensus processing site sequence is now emerging.     

恶性疟原虫裂殖子表面蛋白1的17区片段基因复合核酸疫苗诱导小鼠抗体免疫性的研究

目的　检测以恶性疟原虫裂殖子表面蛋白 1的 17区片段基因为基础的复合核酸疫苗(分泌性的VR10 12 /TPA/HG MSP1 17和非分泌性的VR10 12 /HG MSP1 17)诱导小鼠的体液免疫反应和免疫血清对疟原虫生长的抑制能力。方法　以 2 0 0 μg/10 0 μl或 10 0 μg/10 0 μl每次每只VR10 12 /HG MSP1 17或VR10 12 /TPA/HG MSP1 17肌注免疫BALB/c或C5 7BL/6小鼠。用ELISA间接法测定小鼠血清的特异性抗体 ,用体外抑制试验检测免疫血清抑制疟原虫生长效果。结果　经 3次 10 0 μg/10 0 μl每次每只VR10 12 /HG MSP1 17免疫后 ,BALB/c小鼠和C5 7BL/6小鼠均产生了明显的HG和YMSP119抗体。但总体抗体水平不高。BALB/c小鼠经 3次 2 0 0 μg/10 0 μl每次每只的VR10 12 /HG MSP1 17免疫后 ,产生了较高的HG抗体 ,但MSP1 17的抗体无明显变化 ,经 3次 2 0 0 μg/10 0 μl每次每只VR10 12 /TPA/HG MSP1 17免疫后 ,仅产生较低的HG抗体 ,无MSP1 17抗体的产生。用 2 0 0 μg/10 0 μlVR10 12 /HG MSP1 17免疫的BALB/c小鼠血清做体外抑制试验 ,结果抑制效果明显。结论　VR10 12 /HG MSP1 17比VR10 12 /TPA/HG MSP 17具有更强的免疫原性 ,其免疫鼠血清能明显地抑制疟原虫生长     

Plasmodium falciparum 19-kilodalton merozoite surface protein 1 (MSP1)-specific antibodies that interfere with parasite growth in vitro can inhibit MSP1 processing, merozoite invasion, and intracellular parasite development

Merozoite surface protein 1 (MSP1) is a target for malaria vaccine development. Antibodies to the 19-kDa carboxy-terminal region referred to as MSP1(19) inhibit erythrocyte invasion and parasite growth, with some MSP1-specific antibodies shown to inhibit the proteolytic processing of MSP1 that occurs at invasion. We investigated a series of antibodies purified from rabbits immunized with MSP1(19) and AMA1 recombinant proteins for their ability to inhibit parasite growth, initially looking at MSP1 processing. Although significant inhibition of processing was mediated by several of the antibody samples, there was no clear relationship with overall growth inhibition by the same antibodies. However, no antibody samples inhibited processing but not invasion, suggesting that inhibition of MSP1 processing contributes to but is not the only mechanism of antibody-mediated inhibition of invasion and growth. Examining other mechanisms by which MSP1-specific antibodies inhibit parasite growth, we show that MSP1(19)-specific antibodies are taken up into invaded erythrocytes, where they persist for significant periods and result in delayed intracellular parasite development. This delay may result from antibody interference with coalescence of MSP1(19)-containing vesicles with the food vacuole. Antibodies raised against a modified recombinant MSP1(19) sequence were more efficient at delaying intracellular growth than those to the wild-type protein. We propose that antibodies specific for MSP1(19) can mediate inhibition of parasite growth by at least three mechanisms: inhibition of MSP1 processing, direct inhibition of invasion, and inhibition of parasite development following invasion. The balance between mechanisms may be modulated by modifying the immunogen used to induce the antibodies.     

Involvement of Pf155/RESA and cross-reactive antigens in Plasmodium falciparum merozoite invasion in vitro.

Lines of Plasmodium falciparum FCR3 either expressing or not expressing the blood-stage antigen Pf155/RESA were used to analyze the possible involvement of this antigen in the merozoite invasion process in vitro. Antibodies from human sera, affinity purified on synthetic peptides corresponding to C-terminal repeated sequences in Pf155/RESA, were shown to inhibit merozoite invasion of both types of parasites with similar efficiency. Reversal of the invasion inhibition by fusion proteins containing repeated sequences of Pf155/RESA but not of the cross-reactive antigens Ag332 and Pf11.1 indicated that the inhibitory antibodies had similar target antigens in both Pf155/RESA+ and Pf155/RESA- parasites that involved cross-reacting epitopes present in Pf155/RESA. Rabbit antibodies specific for Pf155/RESA repeats inhibited merozoite invasion of Pf155/RESA expressing parasites efficiently but had no or very small effect on the invasion of Pf155/RESA-deficient parasites. In contrast, rabbit antibodies specific for Ag332 repeats as well as human antibodies affinity purified on synthetic Ag332 peptides inhibited merozoite invasion of both types of parasites with high efficiency. A similar inhibition pattern was seen with the human monoclonal antibody 33G2, which has specificity for Ag332 but also cross-reacts with Pf155/RESA and Pf11.1. Taken together, our data suggest that Pf155/RESA and related cross-reactive antigens as well as Ag332 are involved in the merozoite invasion process and may constitute targets for invasion inhibitory antibodies.     

Plasmodium falciparum field isolates use complement receptor 1 (CR1) as a receptor for invasion of erythrocytes

A majority of Plasmodium falciparum strains invade erythrocytes through interactions with sialic acid (SA) on glycophorins. However, we recently reported that complement receptor 1 (CR1) is a SA-independent invasion receptor of many laboratory strains of P. falciparum. To determine the role of CR1 in erythrocyte invasion among P. falciparum field isolates, we tested eight isolates obtained from children in Kenya. All the parasites examined were capable of invading in a SA-independent manner, and invasion of neuraminidase-treated erythrocytes was nearly completely blocked by anti-CR1 and soluble CR1 (sCR1). In addition, anti-CR1 and sCR1 partially inhibited invasion of intact erythrocytes in a majority of isolates tested. Sequencing of the hypervariable region of P. falciparum AMA-1 showed considerable diversity among all the isolates. These data demonstrate that CR1 mediates SA-independent erythrocyte invasion in P. falciparum field isolates.     

Babesia divergens and Plasmodium falciparum use common receptors, glycophorins A and B, to invade the human red blood cell

Babesiosis has long been recognized as an economically important disease of cattle, but only in the last 30 years has Babesia been recognized as an important pathogen in humans. Invasion of erythrocytes is an integral part of the Babesia life cycle. However, very little information is available on the molecules involved in this process, in contrast to another hemoparasite, Plasmodium falciparum. Using invasion assays into normal red blood cells (RBCs), enzyme-treated cells, and clinically mutant cells, we showed that Babesia divergens uses neuraminidase- and trypsin-sensitive receptors to enter the RBCs, of which glycophorins A and B are the prominent ones. These results could have broad implications relating to evolutionarily conserved mechanisms of host cell entry in these related Apicomplexan parasites and pave the way toward a detailed molecular analysis of erythrocyte invasion in B. divergens.     

Human antibody response to the major merozoite surface antigen of Plasmodium falciparum is strain specific and short-lived.

The precursor of the major merozoite antigen of Plasmodium falciparum, gp190, is considered a candidate for inclusion in a malaria vaccine. This protein, which consists of conserved, dimorphic, and polymorphic sequences, is very immunogenic in humans. In a longitudinal study carried out with 94 inhabitants of a rural community in Mali, West Africa, we show that in this endemic area naturally acquired gp190-specific antibodies are predominantly directed against the dimorphic parts of one of the main alleles of gp190. The presence of antibodies against these dimorphic regions correlates with the prevalence of the corresponding antigen in the infecting parasite population. Moreover, qualitative as well as quantitative differences were found in the time course of the humoral immune response to the dimorphic regions in adults and children, who differ in their susceptibility to malaria infection.     

Molecular analysis of the cytoadherence phenotype of a Plasmodium falciparum field isolate that binds intercellular adhesion molecule-1

The ability of Plasmodium falciparum-infected erythrocytes to adhere to endothelial receptors and sequester in diverse host organs is an important pathogenic mechanism. Cytoadherence is mediated by variant surface antigens, which are referred to as PfEMP-1 and are encoded by var genes. The extracellular regions of PfEMP-1 contain multiple conserved cysteine-rich domains that are referred to as Duffy-binding-like (DBL) domains. Here, we analyze the adhesive phenotype of an Indian P. falciparum field isolate, JDP8, which binds ICAM-1 but does not bind CD36. This is a unique cytoadherence phenotype because P. falciparum strains that bind ICAM-1 described thus far usually also bind CD36. Moreover, binding to both receptors is thought to be important for static adhesion under flow. The ICAM-1 binding population of P. falciparum JDP8 adheres to endothelial cells under flow despite poor binding to CD36. We have also identified an expressed var gene, JDP8Icvar, which mediates the ICAM-1 binding phenotype of JDP8. Expression of different regions of JDP8Icvar on the surface of COS-7 cells followed by binding assays demonstrates that the ICAM-1 binding domain maps to the DBL2betaC2 domain of JDP8Icvar. Sequence comparison with two previously identified ICAM-1 binding domains of PfEMP-1, which also map to DBLbetaC2 domains, suggests that diverse P. falciparum isolates use a structurally conserved domain to bind ICAM-1. It thus appears that functional constraints may place limits on the extent of sequence diversity in receptor-binding domains of PfEMP-1.