Characterization of the gene encoding sporozoite surface protein 2, a protective Plasmodium yoelii sporozoite antigen

CD8+ Trms against malaria liver-stage: prospects and challenges

Attenuated sporozoites provide a valuable model for exploring protective immunity against the malarial liver stage, guiding the design of highly efficient vaccines to prevent malaria infection. Liver tissue-resident CD8+ T cells (CD8+ Trm cells) are considered the host front-line defense against malaria and are crucial to developing prime-trap/target strategies for pre-erythrocytic stage vaccine immunization. However, the spatiotemporal regulatory mechanism of the generation of liver CD8+ Trm cells and their responses to sporozoite challenge, as well as the protective antigens they recognize remain largely unknown. Here, we discuss the knowledge gap regarding liver CD8+ Trm cell formation and the potential strategies to identify predominant protective antigens expressed in the exoerythrocytic stage, which is essential for high-efficacy malaria subunit pre-erythrocytic vaccine designation.

Analysis of Theileria orientalis draft genome sequences reveals potential species-level divergence of the Ikeda, Chitose and Buffeli genotypes

BACKGROUND

Theileria orientalis (Apicomplexa: Piroplasmida) has caused clinical disease in cattle of Eastern Asia for many years and its recent rapid spread throughout Australian and New Zealand herds has caused substantial economic losses to production through cattle deaths, late term abortion and morbidity. Disease outbreaks have been linked to the detection of a pathogenic genotype of T. orientalis, genotype Ikeda, which is also responsible for disease outbreaks in Asia. Here, we sequenced and compared the draft genomes of one pathogenic (Ikeda) and two apathogenic (Chitose, Buffeli) isolates of T. orientalis sourced from Australian herds.

RESULTS

Using de novo assembled sequences and a single nucleotide variant (SNV) analysis pipeline, we found extensive genetic divergence between the T. orientalis genotypes. A genome-wide phylogeny reconstructed to address continued confusion over nomenclature of this species displayed concordance with prior phylogenetic studies based on the major piroplasm surface protein (MPSP) gene. However, average nucleotide identity (ANI) values revealed that the divergence between isolates is comparable to that observed between other theilerias which represent distinct species. Analysis of SNVs revealed putative recombination between the Chitose and Buffeli genotypes and also between Australian and Japanese Ikeda isolates. Finally, to inform future vaccine studies, dN/dS ratios and surface location predictions were analysed. Six predicted surface protein targets were confirmed to be expressed during the piroplasm phase of the parasite by mass spectrometry.

CONCLUSIONS

We used whole genome sequencing to demonstrate that the T. orientalis Ikeda, Chitose and Buffeli variants show substantial genetic divergence. Our data indicates that future researchers could potentially consider disease-associated Ikeda and closely related genotypes as a separate species from non-pathogenic Chitose and Buffeli.

Discriminating Protective from Nonprotective Plasmodium-Specific CD8+ T Cell Responses

Despite decades of research, malaria remains a global health crisis. Current subunit vaccine approaches do not provide efficient long-term, sterilizing immunity against Plasmodium infections in humans. Conversely, whole parasite vaccinations with their larger array of target Ags have conferred long-lasting sterilizing protection to humans. Similar studies in rodent models of malaria reveal that CD8(+) T cells play a critical role in liver-stage immunity after whole parasite vaccination. However, it is unknown whether all CD8(+) T cell specificities elicited by whole parasite vaccination contribute to protection, an issue of great relevance for enhanced subunit vaccination. In this article, we show that robust CD8(+) T cell responses of similar phenotype are mounted after prime-boost immunization against Plasmodium berghei glideosome-associated protein 5041-48-, sporozoite-specific protein 20318-325-, thrombospondin-related adhesion protein (TRAP) 130-138-, or circumsporozoite protein (CSP) 252-260-derived epitopes in mice, but only CSP252-260- and TRAP130-138-specific CD8(+) T cells provide sterilizing immunity and reduce liver parasite burden after sporozoite challenge. Further, CD8(+) T cells specific to sporozoite surface-expressed CSP and TRAP proteins, but not intracellular glideosome-associated protein 50 and sporozoite-specific protein 20, efficiently recognize sporozoite-infected hepatocytes in vitro. These results suggest that: 1) protection-relevant antigenic targets, regardless of their immunogenic potential, must be efficiently presented by infected hepatocytes for CD8(+) T cells to eliminate liver-stage Plasmodium infection; and 2) proteins expressed on the surface of sporozoites may be good target Ags for protective CD8(+) T cells.

Enhanced vaccine-induced CD8+ T cell responses to malaria antigen ME-TRAP by fusion to MHC class ii invariant chain

The orthodox role of the invariant chain (CD74; Ii) is in antigen presentation to CD4+ T cells, but enhanced CD8+ T cells responses have been reported after vaccination with vectored viral vaccines encoding a fusion of Ii to the antigen of interest. In this study we assessed whether fusion of the malarial antigen, ME-TRAP, to Ii could increase the vaccine-induced CD8+ T cell response. Following single or heterologous prime-boost vaccination of mice with a recombinant chimpanzee adenovirus vector, ChAd63, or recombinant modified vaccinia virus Ankara (MVA), higher frequencies of antigen-specific CD4+ and CD8+ T cells were observed, with the largest increases observed following a ChAd63-MVA heterologous prime-boost regimen. Studies in non-human primates confirmed the ability of Ii-fusion to augment the T cell response, where a 4-fold increase was maintained up to 11 weeks after the MVA boost. Of the numerous different approaches explored to increase vectored vaccine induced immunogenicity over the years, fusion to the invariant chain showed a consistent enhancement in CD8+ T cell responses across different animal species and may therefore find application in the development of vaccines against human malaria and other diseases where high levels of cell-mediated immunity are required.

Identification of targets of CD8⁺ T cell responses to malaria liver stages by genome-wide epitope profiling

CD8⁺ T cells mediate immunity against Plasmodium liver stages. However, the paucity of parasite-specific epitopes of CD8⁺ T cells has limited our current understanding of the mechanisms influencing the generation, maintenance and efficiency of these responses. To identify antigenic epitopes in a stringent murine malaria immunisation model, we performed a systematic profiling of H(2b)-restricted peptides predicted from genome-wide analysis. We describe the identification of Plasmodium berghei (Pb) sporozoite-specific gene 20 (S20)- and thrombospondin-related adhesive protein (TRAP)-derived peptides, termed PbS20₃₁₈ and PbTRAP₁₃₀ respectively, as targets of CD8⁺ T cells from C57BL/6 mice vaccinated by whole parasite strategies known to protect against sporozoite challenge. While both PbS20₃₁₈ and PbTRAP₁₃₀ elicit effector and effector memory phenotypes in both the spleens and livers of immunised mice, only PbTRAP₁₃₀-specific CD8⁺ T cells exhibit in vivo cytotoxicity. Moreover, PbTRAP₁₃₀-specific, but not PbS20₃₁₈-specific, CD8⁺ T cells significantly contribute to inhibition of parasite development. Prime/boost vaccination with PbTRAP demonstrates CD8⁺ T cell-dependent efficacy against sporozoite challenge. We conclude that PbTRAP is an immunodominant antigen during liver-stage infection. Together, our results underscore the presence of CD8⁺ T cells with divergent potencies against distinct Plasmodium liver-stage epitopes. Our identification of antigen-specific CD8⁺ T cells will allow interrogation of the development of immune responses against malaria liver stages.

A retrospective evaluation of the role of T cells in the development of malaria vaccine

Due to the fact that the life cycle of malaria parasites is complex, undergoing both an extracellular and intracellular phases in its host, the human immune system has to mobilize both the humoral and cellular arms of immune responses to fight against this parasitic infection. Whereas humoral immunity is directed toward the extracellular stages which include sporozoites and merozoites, cell-mediated immunity (CMI), in which T cells play a major role, targets hepatic stages - liver stages - of the parasites. In this review, the role of T cells in protective immunity against liver stages of the malaria infection is being re-evaluated. Furthermore, this review intends to address how to translate the findings regarding the role of T cells obtained in experimental systems to actual development of malaria vaccine for humans.

Functional immunoassays using an in-vitro malaria liver-stage infection model: where do we go from here?

For more than 25 years, the ISI assay and ILSDA have been used to study the development of the malaria parasite in the liver, to discover and characterize sporozoite and liver-stage antigens, to support the development of malaria vaccine candidates, and to search for immunological correlates of protection in animals and in humans. Although both assays have been limited by low sporozoite invasion rates, significant biological variability, and the subjective nature of manually counting hepatocytes containing parasites as the read-out, they have nevertheless been useful tools for exploring parasite biology. This review describes the origin, application and current status of these assays, critically discusses the need for improvements, and explores the roles of these assays in supporting the development of an effective vaccine against Plasmodium falciparum malaria.

Differential transcriptome profiling identifies Plasmodium genes encoding pre-erythrocytic stage-specific proteins

Invasive sporozoite and merozoite stages of malaria parasites that infect mammals enter and subsequently reside in hepatocytes and red blood cells respectively. Each invasive stage may exhibit unique adaptations that allow it to interact with and survive in its distinct host cell environment, and these adaptations are likely to be controlled by differential gene expression. We used suppression subtractive hybridization (SSH) of Plasmodium yoelii salivary gland sporozoites versus merozoites to identify stage-specific pre-erythrocytic transcripts. Sequencing of the SSH library and matching the cDNA sequences to the P. yoelii genome yielded 25 redundantly tagged genes including the only two previously characterized sporozoite-specific genes encoding the circumsporozoite protein (CSP) and thrombospondin-related anonymous protein (TRAP). Twelve novel genes encode predicted proteins with signal peptides, indicating that they enter the secretory pathway of the sporozoite. We show that one novel protein bearing a thrombospondin type 1 repeat (TSR) exhibits an expression pattern that suggests localization in the sporozoite secretory rhoptry organelles. In addition, we identified a group of four genes encoding putative low-molecular-mass proteins. Two proteins in this group exhibit an expression pattern similar to TRAP, and thus possibly localize in the sporozoite secretory micronemes. Proteins encoded by the differentially expressed genes identified here probably mediate specific interactions of the sporozoite with the mosquito vector salivary glands or the mammalian host hepatocyte and are not used during merozoite-red blood cell interactions.

The cytoplasmic domain of the Plasmodium falciparum ligand EBA-175 is essential for invasion but not protein trafficking

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.

Gene expression analysis during liver stage development of Plasmodium

The complex life cycle of malaria parasites requires significant changes in gene expression as the parasites move from vector to host and back to the vector. Although recognised as an important vaccine and drug target, the liver stage parasite has remained difficult to study. One of the major impediments in identifying parasite gene expression at the liver stage has remained the large number of uninfected hepatocytes relative to the number of infected hepatocytes in the liver after sporozoite inoculation. This article describes several of the approaches that have been utilised to overcome this difficulty in rodent models of malaria. While significant progress has been made to identify genes that are expressed during liver stage parasite development, a great deal more work remains to be done.

The binding of the circumsporozoite protein to cell surface heparan sulfate proteoglycans is required for plasmodium sporozoite attachment to target cells

The major surface protein of malaria sporozoites, the circumsporozoite protein, binds to heparan sulfate proteoglycans on the surface of hepatocytes. It has been proposed that this binding event is responsible for the rapid and specific localization of sporozoites to the liver after their injection into the skin by an infected anopheline mosquito. Previous in vitro studies performed under static conditions have failed to demonstrate a significant role for heparan sulfate proteoglycans during sporozoite invasion of cells. We performed sporozoite attachment and invasion assays under more dynamic conditions and found a dramatic decrease in sporozoite attachment to cells in the presence of heparin. In contrast to its effect on attachment, heparin does not appear to have an effect on sporozoite invasion of cells. When substituted heparins were used as competitive inhibitors of sporozoite attachment, we found that sulfation of the glycosaminoglycan chains at both the N- and O-positions was important for sporozoite adhesion to cells. We conclude that the binding of the circumsporozoite protein to hepatic heparan sulfate proteoglycans is likely to function during sporozoite attachment in the liver and that this adhesion event depends on the sulfated glycosaminoglycan chains of the proteoglycans.

Expression, extracellular secretion, and immunogenicity of the Plasmodium falciparum sporozoite surface protein 2 in Salmonella vaccine strains

Deleting transmembrane alpha-helix motifs from Plasmodium falciparum sporozoite surface protein (SSP-2) allowed its secretion from Salmonella enterica serovar Typhimurium SL3261 and S. enterica serovar Typhi CVD 908-htrA by the Hly type I secretion system. In mice immunized intranasally, serovar Typhimurium constructs secreting SSP-2 stimulated greater gamma interferon splenocyte responses than did nonsecreting constructs (P = 0.04).

Two conserved amino acid motifs mediate protein targeting to the micronemes of the apicomplexan parasite Toxoplasma gondii

The micronemal protein 2 (MIC2) of Toxoplasma gondii shares sequence and structural similarities with a series of adhesive molecules of different apicomplexan parasites. These molecules accumulate, through a yet unknown mechanism, in secretory vesicles (micronemes), which together with tubular and membrane structures form the locomotion and invasion machinery of apicomplexan parasites. Our findings indicated that two conserved motifs placed within the cytoplasmic domain of MIC2 are both necessary and sufficient for targeting proteins to T. gondii micronemes. The first motif is based around the amino acid sequence SYHYY. Database analysis revealed that a similar sequence is present in the cytoplasmic tail of all transmembrane micronemal proteins identified so far in different apicomplexan species. The second signal consists of a stretch of acidic residues, EIEYE. The creation of an artificial tail containing only the two motifs SYHYY and EIEYE in a preserved spacing configuration is sufficient to target the surface protein SAG1 to the micronemes of T. gondii. These findings shed new light on the molecular mechanisms that control the formation of the microneme content and the functional relationship that links these organelles with the endoplasmic reticulum of the parasite.

The complexity of protective immunity against liver-stage malaria

Sterile protective immunity against challenge with Plasmodium spp. sporozoites can be induced in multiple model systems and humans by immunization with radiation-attenuated Plasmodium spp. sporozoites. The infected hepatocyte has been established as the primary target of this protection, but the underlying mechanisms have not been completely defined. Abs, CD8+ T cells, CD4+ T cells, cytokines (including IFN-gamma and IL-12), and NO have all been implicated as critical effectors. Here, we have investigated the mechanisms of protective immunity induced by immunization with different vaccine delivery systems (irradiated sporozoites, plasmid DNA, synthetic peptide/adjuvant, and multiple Ag peptide) in genetically distinct inbred strains, genetically modified mice, and outbred mice. We establish that there is a marked diversity of T cell-dependent immune responses that mediate sterile protective immunity against liver-stage malaria. Furthermore, we demonstrate that distinct mechanisms of protection are induced in different strains of inbred mice by a single method of immunization, and in the same strain by different methods of immunization. These data underscore the complexity of the murine host response to a parasitic infection and suggest that an outbred human population may behave similarly. Data nevertheless suggest that a pre-erythrocytic-stage vaccine should be designed to induce CD8+ T cell- and IFN-gamma-mediated immune responses and that IFN-gamma responses may represent an in vitro correlate of pre-erythrocytic-stage protective immunity.

Developmental arrest of the human malaria parasite Plasmodium falciparum within the mosquito midgut via CTRP gene disruption

Apicomplexan protozoa possess a family of micronemal and cell surface-associated proteins, each comprised a combination of cell-adhesive vertebrate von Willebrand factor (vWF)-like A domains and thrombospondin (TSP) type 1-like domains. The human malaria parasite Plasmodium falciparum has in the extracellular portion of the CS protein TRAP-related protein (CTRP) six tandemly arrayed A domains followed by seven TSP type 1-like domains, whereas a second member of this family, thrombospondin-related anonymous protein (TRAP), contains a single vWF-like A domain and a single TSP type 1-like domain. Here we show that CTRP transcripts are present within the infected mosquito midgut and that CTRP protein is expressed with a punctate distribution and a predominance at the apical end of mosquito midgut-stage ookinetes. This expression pattern is analogous to micronemal expression of TRAP in Plasmodium sporozoites. Disruption of the CTRP gene by homologous recombination in cultures of the human malaria parasite P. falciparum demonstrates that CTRP is essential for mosquito midgut development. Oocyst formation was never observed following membrane feeds of CTRP disruptant lines to Anopheline mosquitoes, despite the development of mature ookinetes. We propose that CTRP is involved in essential recognition or motility processes at the ookinete cell surface within the mosquito midgut.

Conservation of a gliding motility and cell invasion machinery in Apicomplexan parasites

Most Apicomplexan parasites, including the human pathogens Plasmodium, Toxoplasma, and Cryptosporidium, actively invade host cells and display gliding motility, both actions powered by parasite microfilaments. In Plasmodium sporozoites, thrombospondin-related anonymous protein (TRAP), a member of a group of Apicomplexan transmembrane proteins that have common adhesion domains, is necessary for gliding motility and infection of the vertebrate host. Here, we provide genetic evidence that TRAP is directly involved in a capping process that drives both sporozoite gliding and cell invasion. We also demonstrate that TRAP-related proteins in other Apicomplexa fulfill the same function and that their cytoplasmic tails interact with homologous partners in the respective parasite. Therefore, a mechanism of surface redistribution of TRAP-related proteins driving gliding locomotion and cell invasion is conserved among Apicomplexan parasites.

CD4(+) T-cell- and gamma interferon-dependent protection against murine malaria by immunization with linear synthetic peptides from a Plasmodium yoelii 17-kilodalton hepatocyte erythrocyte protein

Most work on protective immunity against the pre-erythrocytic stages of malaria has focused on induction of antibodies that prevent sporozoite invasion of hepatocytes, and CD8(+) T-cell responses that eliminate infected hepatocytes. We recently reported that immunization of A/J mice with an 18-amino-acid synthetic linear peptide from Plasmodium yoelii sporozoite surface protein 2 (SSP2) in TiterMax adjuvant induces sterile protection that is dependent on CD4(+) T cells and gamma interferon (IFN-gamma). We now report that immunization of inbred A/J mice and outbred CD1 mice with each of two linear synthetic peptides from the 17-kDa P. yoelii hepatocyte erythrocyte protein (HEP17) in the same adjuvant also induces protection against sporozoite challenge that is dependent on CD4(+) T cells and IFN-gamma. The SSP2 peptide and the two HEP17 peptides are recognized by B cells as well as T cells, and the protection induced by these peptides appears to be directed against the infected hepatocytes. In contrast to the peptide-induced protection, immunization of eight different strains of mice with radiation-attenuated sporozoites induces protection that is absolutely dependent on CD8(+) T cells. Data represented here demonstrate that CD4(+) T-cell-dependent protection can be induced by immunization with linear synthetic peptides. These studies therefore provide the foundation for an approach to pre-erythrocytic-stage malaria vaccine development, based on the induction of protective CD4(+) T-cell responses, which will complement efforts to induce protective antibody and CD8(+) T-cell responses.

Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

The 104 kDa microneme-rhoptry protein (p104) is the only known apical complex organelle-specific protein of Theileria parva. p104 exhibits striking structural similarities to circumsporozoite protein and sporozoite surface protein 2 of Plasmodium yoelii. Their primary sequences contain two hydrophobic segments, located at the amino-and the carboxy-terminus. The p104 amino-terminal hydrophobic region was suggested to be a signal peptide for entry into the endoplasmic reticulum and the extreme carboxy-terminal region to function as a membrane anchor. We have studied the biogenesis of p104 in a cell-free expression system and found that p104 is co-translationally transported into membranes derived from endoplasmic reticulum. The amino-terminal signal peptide is not cleaved off and anchors the protein in the membrane with the carboxy-terminal portion translocated into the lumen. We suggest that in vivo p104 is co-translationally integrated into the membrane of the endoplasmic reticulum, from where it is further transported to the microneme-rhoptry complex. Thus, p104 appears to be a suitable marker to study the development of micronemes and rhoptries in T. parva.

The Thrombospondin-related Protein Family of Apicomplexan Parasites: The Gears of the Cell Invasion Machinery

A number of severe diseases of medical and veterinary importance are caused by parasites of the phylum Apicomplexa. These parasites invade host cells using similar subcellular structures, organelles and molecular species. Proteins containing one or more copies of the type I repeat of human platelet thrombospondin (TSP1), are crucial components of both locomotion and invasion machinery. Members of this family have been identified in Eimeria tenella, E. maxima, Toxoplasma gondii, Cryptosporidium parvum and in all Plasmodium species so far analysed. Here, Andrea Crisanti and colleagues discuss the structure, localization and current understanding of the function of TSP family members in the invasion of target cells by apicomplexan parasites.

Adhesive proteins of the malaria parasite

Malaria infection of the host cells requires host-parasite recognition events mediated by adhesion and signaling molecules. Recent development of systems for stable transformation and targeted integration of exogenous DNA in malaria parasites provides a powerful tool to study the structure and function of Plasmodium attachment motifs, and their role in infection and disease.

Induction of protective immune responses by immunization with linear multiepitope peptides based on conserved sequences from Plasmodium falciparum antigens

A cysteine-containing peptide motif, EWSPCSVTCG, is found highly conserved in the circumsporozoite protein (CSP) and the thrombospondin-related anonymous protein (TRAP) of all the Plasmodium species analyzed so far and has been shown to be crucially involved in the sporozoite invasion of hepatocytes. We have recently shown that peptide sequences containing this motif, and also the antibodies raised against the motif, inhibit the merozoite invasion of erythrocytes. However, during natural infection, and upon immunization with recombinant CSP, this motif represents a cryptic epitope. Here we present the results of immunization studies with two linear multiepitopic constructs, a 60-residue (P60) and a 32-residue (P32) peptide, containing the conserved motif sequence. Both the peptides per se generated high levels of specific antibodies in BALB/c mice. P32 was found to be genetically restricted to H-2(d) and H-2(b) haplotypes of mice, whereas P60 was found to be immunogenic in five different strains of mice. The antibody response was predominantly targeted to the otherwise cryptic, conserved motif sequence in P60. Anti-P60 antibodies specifically stained the asexual blood stages of Plasmodium falciparum and Plasmodium yoelii in an immunofluorescence assay, recognized a 60- to 65-kDa parasite protein in an immunoblot assay, and blocked P. falciparum merozoite invasion of erythrocytes in a dose-dependent manner. Immunization with P60 also induced significant levels of the cytokines interleukin-2 (IL-2), IL-4, and gamma interferon in BALB/c mice. Moreover, >60% of mice immunized with P60 survived a heterologous challenge infection with a lethal strain of P. yoelii. These results indicate that appropriate medium-sized synthetic peptides might prove useful in generating specific immune responses to an otherwise cryptic but critical and putatively protective epitope in an antigen and could form part of a multicomponent malaria vaccine.

Molecular cloning and expression analysis of a Cryptosporidium parvum gene encoding a new member of the thrombospondin family

The apicomplexan parasite Cryptosporidium parvum invades and multiplies primarily in the brush border cells of the intestinal mucosa causing in AIDS patients a severe diarrhoea that represents a significant contributing factor leading to death. Morphological analysis indicates that the invasion machinery of C. parvum is similar to the apical complex of other parasites of the phylum Apicomplexa. We provide here evidence indicating that C. parvum also shares with these parasites a molecule crucial for the invasion of host cells. We have cloned a 3894 bp-long C. parvum cDNA encoding a protein characterised by sequence and structural similarities with members of the thrombospondin (TSP) family previously described in apicomplexan parasites of the genera Toxoplasma, Eimeria and Plasmodium. This novel C. partum molecule, the TSP-related adhesive protein of Cryptosporidium-1 (TRAP-C1), is encoded by a single copy gene containing no introns. TRAP-C1 is localised in the apical end of C. parvum sporozoites and is structurally related to the micronemal proteins MIC2 of Toxoplasma and Etp100 of Eimeria, which are involved in host-cell attachment and/or invasion. The identification of TRAP-C1 sheds new light on the molecules possibly involved in the invasion process of intestinal cells by C. parvum. We have also analysed the sequence variation of TRAP-C1 among C. parvum isolates and in the closely related species C. wrairi.

Combined preparative enzymatic synthesis of dTDP-6-deoxy-4-keto-D-glucose from dTDP and sucrose

dTDP-6-deoxy-4-keto-D-glucose (1), the common intermediate in the biosyntheses of the manifold deoxysugars, was synthesized on a gram-scale by the combination of sucrose synthase and dTDP-D-glucose 4,6-dehydratase in a fed batch, starting the reaction with dTDP. This process allowed a dTDP conversion with a 100% rate. An easy and efficient three-step purification with anion-exchange chromatography and gel filtration gave 1.1 g of 1 in an overall yield of 73%. This work realizes a first step for an economic access to activated deoxysugars.

TRAP is necessary for gliding motility and infectivity of plasmodium sporozoites

Many protozoans of the phylum Apicomplexa are invasive parasites that exhibit a substrate-dependent gliding motility. Plasmodium (malaria) sporozoites, the stage of the parasite that invades the salivary glands of the mosquito vector and the liver of the vertebrate host, express a surface protein called thrombospondin-related anonymous protein (TRAP) that has homologs in other Apicomplexa. By gene targeting in a rodent Plasmodium, we demonstrate that TRAP is critical for sporozoite infection of the mosquito salivary glands and the rat liver, and is essential for sporozoite gliding motility in vitro. This suggests that in Plasmodium sporozoites, and likely in other Apicomplexa, gliding locomotion and cell invasion have a common molecular basis.

Development of two monoclonal antibodies against Plasmodium falciparum sporozoite surface protein 2 and mapping of B-cell epitopes

The Plasmodium yoelii sporozoite surface protein 2 (PySSP2) is the target of protective cellular immunity. Cytotoxic T cells specific for the Plasmodium falciparum analog PfSSP2, also known as thrombospondin-related anonymous protein (TRAP), are induced in human volunteers immunized with irradiated sporozoites. PfSSP2 is an important candidate antigen for a multicomponent malaria vaccine. We generated and characterized three monoclonal antibodies (MAbs) specific for PfSSP2/TRAP. The MAbs PfSSP2.1 (immunoglobulin G1 [IgG1]), PfSSP2.2 (IgG2a), and PfSSP2.3 (IgM) were species specific and identified three distinct B-cell epitopes containing sequences DRYI, CHPSDGKC, and TRPHGR, respectively. PfSSP2.1 partially inhibited P. falciparum liver-stage parasite development in human hepatocyte cultures (42 and 86% in two experiments at 100 microg/ml). Mice immunized with vaccinia virus expressing full-length PfSSP2 protein produced antibodies to (DRYIPYSP)3, and humans living in malaria-endemic areas (Indonesia and Kenya), who have lifelong exposure and partial clinical immunity to malaria, had antibodies to both (DRYIPYSP)3 and (CHPSDGKCN)2. Mice immunized with multiple antigen peptides MAP4 (DRYIPYSP)3P2P30 and MAP4 (CHPSDGKCN)3P2P30 in TiterMax developed antibodies to sporozoites that partially inhibited sporozoite invasion of human hepatoma cells (39 to 71% at a serum dilution of 1:50 in three different experiments). The modest inhibitory activities of the MAbs and the polyclonal antibodies to PfSSP2/TRAP epitopes do not suggest that a single-component vaccine designed to induce antibodies against PfSSP2/TRAP will be protective. Nonetheless, the MAbs directed against PfSSP2, and the peptides recognized by these MAbs, will be essential reagents in the development of PfSSP2/TRAP as a component of a multivalent P. falciparum human malaria vaccine.

Cell adhesion to a motif shared by the malaria circumsporozoite protein and thrombospondin is mediated by its glycosaminoglycan-binding region and not by CSVTCG

The malaria circumsporozoite protein (CS), thrombospondin (TSP), and several other proteins including the terminal complement proteins and the neural adhesion molecules F-spondin and Unc-5, share a cell adhesive sequence. In CS this sequence is designated as region II-plus (EWSPCSVTCGNGIQVRIK) and in TSP it is found in the type I repeats. Previous studies aimed at fine mapping the amino acid residues required for cell adhesion have yielded discrepant results. Here we show in three different cell lines that the downstream basic residues are required for cell adhesion whereas the CSVTCG sequence is not. Using mutant Chinese hamster ovary cells selected for deficiencies in proteoglycan synthesis, we show that in wild type cells, heparan sulfate proteoglycans are the binding sites for this motif. This finding is supported by additional experiments with two other cell lines demonstrating that treatment with heparitinase but not chondroitinase abolishes cell adhesion to peptides representing this motif. Using Chinese hamster ovary cell mutants deficient in heparan sulfate proteoglycans but possessing chondroitin sulfate proteoglycans, we show that cell surface chondroitin sulfate proteoglycans can also mediate binding to this motif although higher concentrations of peptides are required for adhesion. Chondroitinase, but not heparitinase, treatment of these cells destroys cell surface-binding sites. Taken together, these results indicate that cell adhesion to this motif involves an interaction between the downstream positively-charged residues and the negatively charged glycosaminoglycan chains of heparan sulfate, or in some cases chondroitin sulfate, proteoglycans on the cell surface.

Cloning and cross-species comparison of the thrombospondin-related anonymous protein (TRAP) gene from Plasmodium knowlesi, Plasmodium vivax and Plasmodium gallinaceum

To examine the structure of the Plasmodium sporozoite micronemal protein, thrombospondin-related anonymous protein (TRAP) we have isolated TRAP genes from three species of Plasmodium: P. gallinaceum (PgTRAP), P. knowlesi (PkTRAP) and P. vivax (PvTRAP). Thus it is now possible to compare the TRAP gene from a total of six species of Plasmodium. The overall structure of TRAP is conserved in all species; specifically, an amino-terminal A-domain similar to magnesium-binding domains of mammalian integrins; a thrombospondin-like sulfatide-binding domain similar to region II in Plasmodium circumsporozoite protein; an acidic asparagine/proline-rich repeat region; a trans-membrane domain and a short acidic cytoplasmic region with a highly conserved carboxy terminus. The overall structure of TRAP from P. gallinaceum and P. falciparum (PfTRAP) is conserved and phylogenetic analysis suggests a monophyletic relationship of avian P. gallinaceum and human P. falciparum. Comparison of the amino acid sequences of the A-domain of PgTRAP and PfTRAP indicates a more rapid divergence of this domain with respect to the rest of the protein in these two species. The structural differences of PgTRAP and PfTRAP may relate to the distinct invasion pathways, macrophage and endothelial cell invasion of P. gallinaceum sporozoites versus hepatocyte invasion of P. falciparum.

Cloning and expression of the thrombospondin related adhesive protein gene of Plasmodium berghei

Sporozoite recognition of host cells is a key step in the life-cycle of malaria parasites. Two sporozoite proteins have so far been characterized in some detail, the circumsporozoite protein (CS) and thrombospondin related adhesive protein (TRAP). We report here the cloning and expression of the TRAP gene homologue from Plasmodium berghei, PbTRAP. The PbTRAP gene encodes a protein of 606 amino acids having a deduced molecular mass of 66 kDa. The overall structure is clearly that of the TRAP family having a signal sequence followed by an integrin A domain, a sulphatide binding motif, followed by a proline based repeat before a transmembrane domain and helical cytoplasmic tail. The observed molecular mass is almost 50% larger than expected, this can be explained almost entirely by the abnormal behaviour in SDS-PAGE of the proline based repeat. As would be expected PbTRAP shows greatest similarity with the P. yoelli TRAP homologue sporozoite surface protein 2 (SSP2) than with PfTRAP, the TRAP gene from P. falciparum. The pattern of expression is similar to that of SSP2.

Remnant lipoproteins inhibit malaria sporozoite invasion of hepatocytes

Remnants of lipoproteins, intestinal chylomicrons, and very low density lipoprotein (VLDL), are rapidly cleared from plasma and enter hepatocytes. It has been suggested that remnant lipoproteins are initially captured in the space of Disse by heparan sulfate proteoglycans (HSPGs), and that their subsequent internalization into hepatocytes is mediated by members of the LDL-receptor gene family. Similarly to lipoprotein remnants, malaria sporozoites are removed from the blood circulation by the liver within minutes after injection by Anopheles mosquitoes. The sporozoite's surface is covered by the circumsporozoite protein (CS), and its region II-plus has been implicated in the binding of the parasites to glycosaminoglycan chains of hepatocyte HSPGs. Lactoferrin, a protein with antibacterial properties found in breast milk and neutrophil granules, is also rapidly cleared from the circulation by hepatocytes, and can inhibit the hepatic uptake of lipoprotein remnants. Here we provide evidence that sporozoites, lactoferrin, and remnant lipoproteins are cleared from the blood by similar mechanisms. CS, lactoferrin, and remnant lipoproteins compete in vitro and in vivo for binding sites on liver cells. The relevance of this binding event for sporozoite infectivity is highlighted by our demonstration that apoliprotein E-enriched beta-VLDI and lactoferrin inhibit sporozoite invasion of HepG2 cells. In addition, malaria sporozoites are less infective in LDL-receptor knockout (LDLR -/-) mice maintained on a high fat diet, as compared with littermates maintained on a normal diet. We conclude that the clearance of lipoprotein remnants and sporozoites from the blood is mediated by the same set of highly sulfated HSPGs on the hepatocyte plasma membrane.

The biology of malarial parasite in the mosquito--a review

The purpose of this review is to summarize the biology of Plasmodium in the mosquito including recent data to contribute to better understanding of the developmental interaction between mosquito and malarial parasite. The entire sporogonic cycle is discussed taking into consideration different parasite/vector interactions and factors affecting parasite development to the mosquito.

Identification and characterization of the protective hepatocyte erythrocyte protein 17 kDa gene of Plasmodium yoelii, homolog of Plasmodium falciparum exported protein 1

We recently reported the discovery of a 17-kDa Plasmodium yoelii protein expressed in infected hepatocytes and erythrocytes, P. yoelii hepatocyte erythrocyte protein 17 (PyHEP17), and have demonstrated that this protein is a target of protective antibodies and T cells. Here, we report the identification and characterization of the gene encoding this protein and reveal that it is composed of two exons. Immunization of mice with PyHEP17 plasmid DNA induces antibodies, cytotoxic T lymphocytes, and protective immunity directed against the infected hepatocyte. Based on extensive sequence homology, expression pattern, and antigenic cross-reactivity, the Plasmodium falciparum homolog of PyHEP17 is identified as the protein known as exported protein-1 (PfExp-1), also called antigen 5.1, circumsporozoite related antigen, or QF116. Identity between PyHEP17 and PfExp-1 is 37% at the amino acid level (60/161 residues), mapping primarily to two regions within the second exon of 73% (16/22 residues) and 71% (25/35 residues) identity. On this basis, PfExp-1 is proposed as an important component of pre-erythrocytic human malaria vaccines.

Antibodies to a conserved-motif peptide sequence of the Plasmodium falciparum thrombospondin-related anonymous protein and circumsporozoite protein recognize a 78-kilodalton protein in the asexual blood stages of the parasite and inhibit merozoite invasion in vitro

Athrombospondin-related anonymous protein (TRAP) of the human malaria parasite Plasmodium falciparum shares highly conserved amino acid sequence motifs with the circumsporozoite protein of all plasmodia sequenced so far, as well as with unrelated proteins like thrombospondin and properdin. Although it was first described as an asexual blood stages protein, there has been some controversy about its expression in these stages. Pursuant to our interest in the conserved sequences within the malaria antigens, we synthesized an 18-residue peptide (18-mer) representing a conserved motif of TRAP and raised polyclonal antibodies against it. In an immunoblot assay in which we probed proteins from the asexual blood stages of the parasite, we found that this antibody recognized predominantly a 78-kDa protein in the whole parasite lysate. Furthermore, in another immunoblot, the recombinant TRAP constructs containing the conserved-motif sequence were distinctly recognized by the antipeptide antibodies, whereas a construct lacking the motif sequence was not, suggesting that the antibodies specifically cross-reacted with a protein which might be a TRAP-like protein present in the asexual blood stages of the parasite. Also, in an immunofluorescence assay, this antibody brightly stained the acetone-fixed trophozoites of the parasite. Most significantly, anti-18-mer immunoglobulin G, as well as antipeptide antibody against a smaller (nonamer) construct representing the most conserved motif within the 18-mer, inhibited the merozoite invasion of erythrocytes in a dose-dependent manner. These results provide evidence of the expression of TRAP or a TRAP-like protein in the asexual blood stages of the parasite and of a possible role of the conserved motifs in the parasite-host cell interaction during the process of invasion.

In vitro biosynthesis and in vivo processing of the major microneme antigen of Sarcocystis muris cyst merozoites

The cDNA clone pSM/1.6 encoding the 26.5-kDa precursor molecule of the 16/17-kDa microneme antigen of Sarcocystis muris cyst merozoites was expressed in a cell-free translation/translocation system to study translocation of the protein across membranes. The antigen was found to be translocated across heterologous endoplasmic reticulum membranes. Translocation was accompanied by cleavage of a signal peptide to create a 23-kDa polypeptide that was completely protected from digestion with proteinase K. Pulse-chase analysis of [35S]-methionine-labeled S. muris cyst merozoites demonstrated that the 16/17-kDa antigen derived from a 23-kDa precursor molecule and that its processing occurred at between a few minutes and 2 h after biosynthesis. This leads to the conclusion that the native microneme antigen is secreted from the parasite cell via the endoplasmic reticulum. Sorting into micronemes might occur during transition through a Golgi-like structure, involving cleavage of the hydrophilic propeptide to create the mature 16/17-kDa protein.

The large difference in infectivity for mice of Plasmodium berghei and Plasmodium yoelii sporozoites cannot be correlated with their ability to enter into hepatocytes

Sporozoites of P. yoelii nigeriensis are 50-100-times more infective to mice than the strain NK65 of P. berghei. To study the mechanisms involved in this striking difference in the infectivity of these closely related species of malaria parasites, we have developed a quantitative PCR targeted to parasite-specific ribosomal RNA. Using this method, we detect RNA from a single sporozoite, and exo-erythorcytic forms of RNA in the livers of mice injected with 200 sporozoites. We find that 20 h after sporozoite injection, there is no significant difference between the amounts of P. berghei and P. yoelii rRNA in the livers of C57/BL6 mice, indicating that these two parasite species invade hepatocytes with similar efficiency. Between 20 and 40 h, however, P. yoelii RNA increases 11 times, while P. berghei RNA increases only 1.6 times. We conclude that the greater infectivity of P. yoelii sporozoites in these mice reflects, at least in part, their superior development in hepatocytes. These data provide for the first time in vivo evidence supporting the notion that species-specificity of malaria is not determined by mechanisms associated with sporozoite attachment and penetration into the hepatocytes.

Conserved location of genes on polymorphic chromosomes of four species of malaria parasites

The number of chromosomes and the chromosomal location and linkage of more than 50 probes, mainly of genes, have been established in four species of Plasmodium which infect African murine rodents. We expected that the location and linkage of genes would not be conserved between these species of malaria parasites since extensive inter- and intraspecific size differences of the chromosomes existed and large scale internal rearrangements and chromosome translocations in parasites from laboratory lines had been reported. Our study showed that all four species contained 14 chromosomes, ranging in size between 0.5 and 3.5 Mb, which showed extensive size polymorphisms. The location and linkage of the genes on the polymorphic chromosomes, however, was conserved and nearly identical between these species. These results indicate that size polymorphisms of the chromosomes are more likely due to variation in non-coding (subtelomeric, repeat) sequences and show that a high plasticity of internal regions of chromosomes that may exist does not frequently affect chromosomal location and linkage of genes.

Protection against malaria by immunization with plasmid DNA encoding circumsporozoite protein

Immunization with irradiated sporozoites protects animals and humans against malaria, and the circumsporozoite protein is a target of this protective immunity. We now report that adjuvant-free intramuscular injection of mice with plasmid DNA encoding the Plasmodium yoelii circumsporozoite protein induced higher levels of antibodies and cytotoxic T lymphocytes against the P. yoelii circumsporozoite protein than did immunization with irradiated sporozoites. Mice immunized with this vaccine had an 86% reduction in liver-stage parasite burden after challenge with 5 x 10(5) sporozoites (> 10(5) median infectious doses). Eighteen (68%) of 28 mice that received two or three doses of vaccine were protected against challenge with 10(2) sporozoites, and the protection was dependent on CD8+ T cells. These studies demonstrate the utility of plasmid DNA immunization against a nonviral infection. By obviating the requirement for peptide synthesis, expression and purification of recombinant proteins, and adjuvants, this method of immunization provides an important alternative for rapid identification of protective B- and T-cell epitopes and for construction of vaccines to prevent malaria and other infectious diseases.

Complete protection against Plasmodium yoelii by adoptive transfer of a CD8+ cytotoxic T-cell clone recognizing sporozoite surface protein 2

BALB/c mice immunized with irradiated Plasmodium yoelii sporozoites produce antibodies and cytotoxic T lymphocytes against the circumsporozoite protein and against a 140-kDa protein, sporozoite surface protein 2 (PySSP2). Approximately 50% of mice immunized with P815 cells transfected with the gene encoding PySSP2 are protected against malaria, and this protection is reversed by in vivo depletion of CD8+ T cells. To determine if CD8+ T cells against PySSP2 are adequate to protect against malaria in the absence of other malaria-specific immune responses, we produced three CD8+ T-cell clones by stimulating spleen cells from mice immunized with irradiated P. yoelii sporozoites with a mitomycin-treated P815 cell clone transfected with the PySSP2 gene. Adoptive transfer of clone TSLB7 protected 100% of mice against P. yoelii. The second clone protected 58% of mice, and the third clone provided no protection. Clone TSLB7 protected even when administered 3 h after sporozoite inoculation at a time when sporozoites had entered hepatocytes, suggesting that it is recognizing and eliminating infected hepatocytes. These studies demonstrate that cytotoxic T lymphocytes against PySSP2 can protect against P. yoelii sporozoite challenge in the absence of other parasite-specific immune responses.

Induction of murine cytotoxic T lymphocytes against Plasmodium falciparum sporozoite surface protein 2

Sporozoite surface protein 2 has been identified as a target of malaria vaccines designed to produce protective CD8+ cytotoxic T lymphocytes (CTL) because mice immunized with mastocytoma cells expressing a fragment of Plasmodium yoelii sporozoite surface protein 2 (PySSP2) are protected against malaria by an immune response that requires CD8+ CTL. To define CTL epitopes in the Plasmodium falciparum sporozoite surface protein 2 (PfSSP2), spleen cells (SC) from mice immunized with irradiated sporozoites (irr spz) were stimulated with synthetic peptides, and these effectors were tested for cytolytic activity against peptide-pulsed, major histocompatibility complex (MHC)-matched targets. Two peptides containing CTL epitopes, A6 (Pf SSP2 3D7 214-233) and BH1 (Pf SSP2 3D7 3-11) were identified in bulk cultures of SC from immune C57BL/6 mice, and by production of CTL lines. Immunization with recombinant vaccinia expressing the full length PfSSP2 induced antigen specific, MHC-restricted, CD8+ T cell-dependent cytolytic activity against these two peptides. Finally, CTL were induced by immunization with a bacteria-derived recombinant fragment of PfSSP2 (rPfSSP2) mixed with a liposomal formulation containing a cationic lipid (Lipofectin Reagent, LPF). Induced CTL lysed target cells pulsed with peptide A6 or with LPF/rPfSSP2, but not targets pulsed with only rPfSSP2. These studies demonstrate that CTL specific to PfSSP2 are present in C57BL/6 mice and that immunization with purified rPfSSP2 delivered with LPF induces a cytotoxic T cell response.

Inducing protective immune responses against the sporozoite and liver stages of Plasmodium

Work on vaccines against the pre-erythrocytic stages of the Plasmodium life cycle is based on the observation that immunization with irradiated sporozoites (IRR SPZ) is protective. Antibodies against several SPZ surface proteins can prevent SPZ from effectively invading hepatocytes; antibodies and cytolytic-T lymphocytes directed against at least 3 parasite proteins expressed in infected hepatocytes can kill infected hepatocytes; and cytokines can activate infected hepatocytes to kill the intracellular parasite. Work is in progress to identify additional pre-erythrocytic parasite targets and to develop methods for optimally inducing protective immunity against SPZ and infected hepatocytes. The goal is to construct a vaccine that protects by inducing antibody and cellular immune responses against multiple parasite proteins.

Structural and functional properties of region II-plus of the malaria circumsporozoite protein

During feeding, infected mosquitos inject malaria sporozoites into the host circulation. Within minutes, the parasites are found in the liver where they initiate the first stage of malaria infection. All species of malaria sporozoites are uniformly covered by the circumsporozoite protein (CS), which contains a conserved COOH-terminal sequence called region II-plus. We have previously shown that region II-plus is the parasite's hepatocyte-binding ligand and that this ligand binds to heparan sulfate proteoglycans (HSPGs) on the hepatocyte membrane. Using a series of substituted region II-plus peptides, we show here that the downstream basic amino acids as well as the interdispersed hydrophobic residues are required for binding of CS to hepatocyte HSPGs. We also show that this positively charged stretch of amino acids must be aggregated in order to bind to the receptor. On the basis of this information, we have synthesized a multiple antigen peptide that mimics the hepatocyte-binding ligand. This construct inhibits both CS binding to HepG2 cells in vitro as well as CS clearance in mice.

Malaria vaccine development

The malaria parasite life cycle presents several targets for attack, but these different parts of the life cycle are susceptible to different types of host immune response. For example, the sporozoite is most sensitive to immune antibody, while liver stage parasites can be eliminated by cytotoxic T lymphocytes. Attachment of merozoites to erythrocytes, on the other hand, can be blocked by antibody. Convincing experimental evidence shows that completely protective immunity to malaria can be induced. The challenge now is to design recombinant or synthetic vaccines that induce the right types of immune responses to specific life cycle stages. This requires the identification and characterization of B- and T-lymphocyte epitopes expressed by the parasite or by parasitized host cells. These epitopes must be incorporated into a delivery system that maximizes the interaction between the vaccine epitopes and the host immune system. Many epitopes from several parts of the life cycle are already characterized; development of multivalent vaccines, that is, vaccines which contain immunogens from more than one part of the life cycle, is a promising area for research efforts.

Proteins on the surface of the malaria parasite and cell invasion

The malaria parasite exists in an extracellular form at several stages in its life cycle. Within the vertebrate host, sporozoites and merozoites have to invade specific cell types. Proteins on the surface of the parasite or externalized from specialized organelles have been implicated as ligands for receptors on the host cell surface. Direct binding studies have identified parasite proteins that interact with the target cell surface. Examination of the deduced amino acid sequences has allowed the identification of primary structural motifs which may have roles in this process. On the sporozoite, the circumsporozoite protein and sporozoite surface protein-2, a protein initially located within micronemes, have been found to contain an amino acid sequence thought to be involved in mediating recognition of sulphated polysaccharides on the surface of a liver cell. On the merozoite, merozoite surface protein-1 may be involved in the initial recognition of red blood cells; this protein undergoes a complex series of modifications in the time between its synthesis as a precursor molecule and successful erythrocyte invasion. Other merozoite proteins located at the apical end of the parasite have been identified as erythrocyte or reticulocyte binding proteins.

Analysis of the human antibody response to thrombospondin-related anonymous protein of Plasmodium falciparum

Thrombospondin-related anonymous protein (TRAP) of the malaria parasite Plasmodium falciparum shares two sequence motifs with other proteins which possess adhesive properties. Recently, findings indicate that TRAP is an antigen which contributes to antisporozoite immunity. We have cloned and expressed the TRAP coding sequences in Escherichia coli to investigate the human humoral immune response against this protein in a region of malaria endemicity of West Africa characterized by a seasonal transmission. Our results show that antibodies against TRAP are present in infected individuals. The anti-TRAP antibodies were analyzed in both a longitudinal and a prospective study. The longitudinal analysis shows seasonal fluctuations of the levels of specific antibodies as well as age-dependent quantitative differences. The immune response is long-lived in most of the adults and some of the older children but short-lived in young children. More importantly, the prospective analysis suggests that the presence of anti-TRAP antibodies in older children before the beginning of malaria transmission correlates with the subsequent control of parasite densities.