Malaria circumsporozoite protein binds to heparan sulfate proteoglycans associated with the surface membrane of hepatocytes

Sulfated glycoconjugates enhance CD36-dependent adhesion ofPlasmodium falciparum–infected erythrocytes to human microvascular endothelial cells

A novel adhesive pathway that enhances the adhesion ofPlasmodium falciparum-infected erythrocytes (IEs) to endothelial cells has been identified. The sulfated glycoconjugates heparin, fucoidan, dextran sulfate 5000, and dextran sulfate 500 000 caused a dramatic increase in adhesion of IEs to human dermal microvascular endothelial cells. The same sulfated glycoconjugates had little effect on IE adhesion to human umbilical vein endothelial cells, a CD36-negative cell line. The effect was abolished by a monoclonal antibody directed against CD36, suggesting that enhanced adhesion to endothelium is dependent on CD36. No effect was observed on adhesion to purified platelet CD36 cells immobilized on plastic. The same sulfated glycoconjugates enhanced adhesion of infected erythrocytes to COS cells transfected with CD36, and this was inhibited by the CD36 monoclonal antibody. These findings demonstrate a role for sulfated glycoconjugates in endothelial adherence that may be important in determining the location and magnitude of sequestration through endogenous carbohydrates. In addition, they highlight possible difficulties that may be encountered from the proposed use of sulfated glycoconjugates as antiadhesive agents in patients with severe malaria.

Antibodies against thrombospondin-related anonymous protein do not inhibit Plasmodium sporozoite infectivity in vivo

Thrombospondin-related anonymous protein (TRAP), a candidate malaria vaccine antigen, is required for Plasmodium sporozoite gliding motility and cell invasion. For the first time, the ability of antibodies against TRAP to inhibit sporozoite infectivity in vivo is evaluated in detail. TRAP contains an A-domain, a well-characterized adhesive motif found in integrins. We modeled here a three-dimensional structure of the TRAP A-domain of Plasmodium yoelii and located regions surrounding the MIDAS (metal ion-dependent adhesion site), the presumed business end of the domain. Mice were immunized with constructs containing these A-domain regions but were not protected from sporozoite challenge. Furthermore, monoclonal and rabbit polyclonal antibodies against the A-domain, the conserved N terminus, and the repeat region of TRAP had no effect on the gliding motility or sporozoite infectivity to mice. TRAP is located in micronemes, secretory organelles of apicomplexan parasites. Accordingly, the antibodies tested here stained cytoplasmic TRAP brightly by immunofluorescence. However, very little TRAP could be detected on the surface of sporozoites. In contrast, a dramatic relocalization of TRAP onto the parasite surface occurred when sporozoites were treated with calcium ionophore. This likely mimics the release of TRAP from micronemes when a sporozoite contacts its target cell in vivo. Contact with hepatoma cells in culture also appeared to induce the release of TRAP onto the surface of sporozoites. If large amounts of TRAP are released in close proximity to its cellular receptor(s), effective competitive inhibition by antibodies may be difficult to achieve.

The journey of the malaria sporozoite through its hosts: two parasite proteins lead the way

Malaria is transmitted to a mammalian host when the sporozoite stage of the Plasmodium parasite is injected by a mosquito vector. Sporozoites are unique in being able to interact with both hosts. Formed and released in the mosquito midgut, sporozoites bind to the salivary glands and invade their secretory cells. Once injected into the mammalian host, they home to the liver and invade hepatocytes. Recent work has shown that two sporozoite surface proteins, CS and TRAP, act in both hosts, perform multiple functions, and are each essential for the parasite at more than one step of its life cycle.

Cellular invasion of Orientia tsutsugamushi requires initial interaction with cell surface heparan sulfate

Role of transmembrane heparan sulfate proteoglycans on invasion of Orientia tsutsugamushi into host cells was investigated. Pretreatment with heparan sulfate and heparin inhibited the infection of O. tsutsugamushi for L cell, mouse fibroblast, whereas other glycosaminoglycans had little effect. These same treatments were also shown to reduce the infection in a dose-dependent manner, and enzymatic treatment of cells with heparitinase, but not chondroitinase ABC, inhibited the infection. In addition, mutant cell lines of Chinese hamster ovarian cell defective in heparan sulfate synthesis but not chondrotin sulfate synthesis and defective in all glycosaminoglycan synthesis showed marked reduction in susceptibility to infection by O. tsutsugamushi. Also mutant cell lines, which express heparan sulfate proteoglycans at low level, showed intermediate level of infectivity. Finally O. tsutsugamushi bind to(35)S-labelled heparin. Collectively, these findings provide strong evidence that heparan sulfate proteoglycans contribute to the attachment of O. tsutsugamushi to the cells.

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.

Antiangiogenic domains shared by thrombospondins and metallospondins, a new family of angiogenic inhibitors

The growth of solid tumors has been shown to depend on neovascularization. By understanding the mechanisms that control the neovascular response, it may be possible to design therapeutic strategies to selectively prevent or halt pathologic vascular growth and restrain cancer progression. Thrombospondin-1 is an extracellular matrix protein that among several functions suppresses capillary growth in angiogenesis assays. We have demonstrated that within the context of the mammary gland TSP1 can modulate normal development of blood vessels. Expression of TSP1 in transgenic animals under the control of the MMTV promoter was associated with a 50-72% reduction in capillary growth. In addition, TSP1 reduced tumor size in transgenic overexpressors. The data suggest an important role for TSP1 in modulating vascular growth in both normal and pathologic tissues. The antiangiogenic region of TSP1 has been mapped to the type I (properdin) repeats. To identify novel proteins with such a domain, we have cloned two cDNAs (METH-1 and METH-2) which also have antiangiogenic properties. In addition to carboxyterminal thrombospondin-like domains they also contain metalloproteinase and disintegrin sequences. Expression of both proteins is broad but nonoverlapping. Recombinant fragments from these sequences have strong antiangiogenic potential in the CAM and cornea pocket assays. At the same molar ratio, METH-1 and METH-2 are about 20-fold more potent than TSP1. We predict that these proteins are likely endogenous modulators of vascular growth with relevant therapeutic potential in cancer and other disease states.

Eukaryotic cell uptake of heparin-coated microspheres: a model of host cell invasion by Chlamydia trachomatis

Using polystyrene microspheres coated with heparin or heparan sulfate, it was shown that coated microspheres specifically bound eukaryotic cells and were endocytosed by nonprofessional phagocytic cells. Coated microspheres displayed properties of binding to eukaryotic cells that were similar to those of chlamydiae, and the microspheres were competitively inhibited by chlamydial organisms. Endocytosis of heparin-coated beads resulted in the tyrosine phosphorylation of a similar set of host proteins as did endocytosis of chlamydiae; however, unlike viable chlamydial organisms, which prevent phagolysosomal fusion, endocytosed beads were trafficked to a lysosomal compartment. These findings suggest that heparin-coated beads and Chlamydia trachomatis enter eukaryotic cells by similar pathways.

Mechanisms of attachment and internalization of Cryptosporidium parvum to biliary and intestinal epithelial cells

BACKGROUND & AIMS

Although infection of the intestinal and biliary tracts by Cryptosporidium parvum is a major problem in patients with the acquired immunodeficiency syndrome, the specific microbial and host molecules involved in C. parvum infection are unknown. We tested the hypothesis that lectin-carbohydrate interactions and cytoskeleton reorganization are involved in the infection of biliary and intestinal epithelia by C. parvum.

METHODS

In vitro models of cryptosporidial infection using human biliary and intestinal epithelial cell lines were used to assay C. parvum attachment and invasion.

RESULTS

Exposure of C. parvum sporozoites to the sugar, galactose-N-acetylgalactosamine (Gal/GalNAc), and to bovine mucin reduced C. parvum attachment to biliary and intestinal epithelia up to 70%. Preincubation of cell monolayers with either lectins specific to Gal/GalNAc, or glycosidases that specifically release Gal/GalNAc oligosaccharides from glycoproteins, decreased attachment up to 80%. Cytochalasin B and cytochalasin D, but not nocodazole, decreased invasion of cells by C. parvum up to 70% without affecting attachment. During cell invasion (but not attachment), confocal microscopy showed recruitment of actin (but not tubulin) in biliary and intestinal epithelia directly adjacent to C. parvum.

CONCLUSIONS

Gal/GalNAc epitopes of glycoproteins on the epithelial apical membrane and Gal/GalNAc-specific sporozoite surface lectins are involved in the mechanism(s) of C. parvum attachment to intestinal and biliary epithelial cells, and actin remodeling in host cells is required for C. parvum invasion.

The cytotoxic T-lymphocyte epitope of the Plasmodium falciparum circumsporozoite protein also modulates the efficiency of receptor-ligand interaction with hepatocytes

Malaria sporozoites are transmitted from the mosquito salivary gland to host hepatocytes within minutes of an infectious bite. The circumsporozoite protein (CS), which covers the surface of Plasmodium sporozoites, functions during these minutes in the targeting of host liver cells. The protein's potentially important role in an antimalaria vaccine has spawned interest in both the host immune responses to the parasite's presence and the actual functional role of the protein in the targeting of host liver cells. Here we show that the region of CS known to elicit a cytotoxic T-lymphocyte (CTL) response to irradiated sporozoites also, somewhat ironically, mediates the receptor-ligand interaction essential to parasite invasion of the host. Hence, the structure of CS represents a balance of potentially counterdirectional forces. Polymorphism in the CTL epitope appears to be a product of this balanced state as opposed to an "arms race" as it is so often portrayed. The conceptual difference between the theories regarding the maintainance of polymorphism in CTL epitopes may have significant implication for vaccine design.

Cryptosporidium parvum apical complex glycoprotein CSL contains a sporozoite ligand for intestinal epithelial cells

Cryptosporidiosis, caused by the apicomplexan parasite Cryptosporidium parvum, has become a well-recognized diarrheal disease of humans and other mammals throughout the world. No approved parasite-specific drugs, vaccines, or immunotherapies for control of the disease are currently available, although passive immunization with C. parvum-specific antibodies has some efficacy in immunocompromised and neonatal hosts. We previously reported that CSL, an approximately 1,300-kDa conserved apical glycoprotein of C. parvum sporozoites and merozoites, is the antigenic species mechanistically bound by neutralizing monoclonal antibody 3E2 which elicits the circumsporozoite precipitate (CSP)-like reaction and passively protects against C. parvum infection in vivo. These findings indicated that CSL has a functional role in sporozoite infectivity. Here we report that CSL has properties consistent with being a sporozoite ligand for intestinal epithelial cells. For these studies, native CSL was isolated from whole sporozoites by isoelectric focusing (IEF) following observations that the approximately 1,300-kDa region containing CSL as seen by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was comprised of approximately 15 molecular species (pI 3 to 10) when examined by two-dimensional (2-D) electrophoresis and silver staining. A subset of six approximately 1,300-kDa species (pI 4.0 to 6.5) was specifically recognized by 3E2 in 2-D Western immunoblots of IEF-isolated CSL. Isolated native CSL bound specifically and with high affinity to permissive human intestinal epithelial Caco-2 cells in a dose-dependent, saturable, and self-displaceable manner. Further, CSL specifically bound to the surface of live Caco-2 cells inhibited sporozoite attachment and invasion. In addition, sporozoites having released CSL after incubation with 3E2 and occurrence of the CSP-like reaction did not attach to and invade Caco-2 cells. These findings indicate that CSL contains a sporozoite ligand which facilitates attachment to and invasion of Caco-2 cells and, further, that ligand function may be disrupted by CSL-reactive monoclonal antibody. We conclude that CSL is a rational target for passive or active immunization against cryptosporidiosis.

Chromatin clearance in C57Bl/10 mice: interaction with heparan sulphate proteoglycans and receptors on Kupffer cells

Chromatin is an important autoantigen in the pathogenesis of systemic lupus erythematosus (SLE) as an immunogen and as a part of nephritogenic immune complexes. Earlier studies focused on clearance of DNA. However, DNA released into the circulation from dying cells is found associated with histones in nucleosomes. The liver is the major organ involved in clearance of chromatin from the circulation of mice. Heparan sulphate proteoglycans (HSPG) have been implicated in the clearance of various charged molecules. Receptor-mediated clearance of ssDNA by the liver has also been reported. Because chromatin contains positively charged histones in addition to DNA, we wished to determine if HSPG and/or DNA receptors are involved in chromatin clearance. The rate of clearance of H1-stripped chromatin from the bloodstream of C57Bl/10 mice was markedly decreased by prior treatment of mice with Heparinase I. Clearance was also inhibited by heparin, heparan sulphate, and DNA, but not by colominic acid. DNA was the most effective inhibitor of clearance and released chromatin from sites of clearance. Depletion of Kupffer cells and splenic macrophages using liposome-encapsulated Clodronate (dichloromethylene bisphosphonate) markedly inhibited chromatin clearance. These data suggest that chromatin clearance is mediated by charge interactions with cell surface HSPG and by DNA receptors. Clearance and degradation of chromatin require functional macrophages in the liver and spleen.

Identification of heparin as a ligand for the A-domain of Plasmodium falciparum thrombospondin-related adhesion protein

Thrombospondin-related adhesion protein (TRAP) is a Plasmodium falciparum transmembrane protein that is expressed within the micronemes of sporozoites, and is implicated in host cell invasion and motility. Contained within the extracellular region of TRAP is an A-domain, a module found in a number of membrane, plasma and matrix proteins, that is often involved in ligand recognition. In order to determine the role of the TRAP A-domain, it has been expressed as a glutathione S-transferase fusion protein and its ligand binding compared with that of other characterised glutathione S-transferase A-domain fusion proteins. Using a solid phase assay to screen for binding to known A-domain ligands, the TRAP A-domain was found to bind heparin. Binding to heparin appeared to be specific as it was saturable, and was inhibited by soluble heparin, fucoidan and dextran sulfate, but not by other negatively charged sulfated glycosaminoglycans such as chondroitin sulfates. Furthermore, unlike some A-domain ligand interactions, the A-domain of both TRAP and the leukocyte integrin, Mac-1, bound to heparin in the absence of divalent cations. It has been shown previously that another domain within TRAP, which is homologous to region II-plus of circumsporozoite protein, binds to sulfatide and to heparan sulfate on the immortalised hepatocyte line HepG2. The TRAP A-domain also bound to sulfatide and to HepG2 cells. Thus the A-domain shares certain binding properties already attributed to the region II-plus-like domain of TRAP, and may contribute to the binding of TRAP to heparan sulfate on hepatocytes.

Plasmodium gallinaceum ookinetes adhere specifically to the midgut epithelium of Aedes aegypti by interaction with a carbohydrate ligand

During the course of its development in the mosquito and transmission to a new vertebrate host, the malaria parasite must interact with the mosquito midgut and invade the gut epithelium. To investigate how the parasite recognizes the midgut before invasion, we have developed an in vitro adhesion assay based on combining fluorescently labelled ookinetes with isolated midgut epithelia from blood-fed mosquitoes. Using this assay, we found that Plasmodium gallinaceum ookinetes readily adhered to midguts of Aedes aegypti, mimicking the natural recognition of the epithelium by the parasite. This interaction is specific: the ookinetes preferentially adhered to the lumen (microvillar) side of the gut epithelium and did not bind to other mosquito tissues. Conversely, the binding was not due to a non-specific adhesive property of the midguts, because a variety of other cell types, including untransformed P. gallinaceum zygotes or macrogametes, did not show similar binding to the midguts. High concentrations of glycosylated (fetuin, orosomucoid, ovalbumin) or non-glycosylated (bovine serum albumin) proteins, added as non-specific competitors, failed to compete with the ookinetes in binding assays. We also found that the adhesion of ookinetes to the midgut surface is necessary for sporogonic development of the parasite in the mosquito. Antibodies and other reagents that blocked adhesion in vitro also reduced oocyst formation when these reagents were combined with mature ookinetes and fed to mosquitoes. Chemical modification of the midguts with sodium periodate at pH 5.5 destroyed adhesion, indicating that the ookinete binds to a carbohydrate ligand on the surface of the midgut. The ligand is sensitive to periodate concentrations of less than 1 mmol l-1, suggesting that it may contain sialic-acid-like sugars. Furthermore, free N-acetylneuraminic acid competed with the ookinetes in binding aasays, while other monosaccharides had no effect. However, in agreement with the current belief that adult insects do not contain sialic acids, we were unable to detect any sialic acids in mosquito midguts using the most sensitive HPLC-based fluorometric assay currently available. We postulate that a specific carbohydrate group is used by the ookinete to recognize the midgut epithelium and to attach to its surface. This is the first receptor-ligand interaction demonstrated for the ookinete stage of a malaria parasite. Further characterization of the midgut ligand and its parasite counterpart may lead to novel strategies of blocking oocyst development in the mosquito.

Identification of carbohydrates on Eimeria stiedai sporozoites and their role in the invasion of cultured cells in vitro

The carbohydrates present on Eimeria stiedai sporozoites and their functional role in the process of invasion of host cells were examined. Lectin-binding sites on the surface of sporozoites were detected by means of peroxidase-conjugated lectins. Sporozoites showed specific binding with UEA-I and PNA lectins, which bind L-fucose and D-galactose, respectively. Exposure of sporozoites to 100 microg/ml UEA-I significantly reduced their ability to invade primary rabbit liver biliary epithelial cells, but similar treatment with PNA had no such effect. Pre-incubation of these cells in Dulbecco's minimum essential medium containing 10% fetal bovine serum and 1% L-fucose suppressed the invasion activity of the sporozoites, but pre-incubation of the sporozoites in the same medium without L-fucose had no effect on cell penetration. D-galactose added to the medium had no effect on the invasion activity of sporozoites. These results indicate that L-fucose residues on E. stiedai sporozoites and L-fucose-binding sites on host cells both are associated with the recognition and/or invasion process.

Plasmodium falciparum: molecular background to strain-specific rosette disruption by glycosaminoglycans and sulfated glycoconjugates

Rosetting, the adhesion of Plasmodium falciparum-infected erythrocytes to uninfected erythrocytes, is a virulent parasite phenotype associated with the occurrence of severe malaria, e.g., cerebral malaria. Compounds with specific anti-rosetting activity are potential therapeutic agents. Glycosaminoglycans and sulfated glycoconjugates were found to disrupt rosettes in a strain- and isolate-specific manner. Rosette disruption was strongly connected to the presence of N-sulfate groups in heparin/heparan sulfate as demonstrated by modified heparin preparations. This finding was corroborated by the disruption of rosettes with mono- and disaccharides derived from heparin/heparan sulfate that contained N-sulfated glucosamine. Furthermore, heparinase III treatment of erythrocyte cultures infected by FCR3S1 (and to some extent TM 284) P. falciparum strains abolished rosetting. Heparinase III treatment of the uninfected erythrocytes prior to mixing with the infected culture impeded formation of rosettes, indicating that the rosetting receptors at least partially are of glycosaminoglycan nature.

Plasmodium vivax: a monoclonal antibody recognizes a circumsporozoite protein precursor on the sporozoite surface

The major surface circumsporozoite (CS) proteins are known to play a role in malaria sporozoite development and invasion of invertebrate and vertebrate host cells. Plasmodium vivax CS protein processing during mosquito midgut oocyst and salivary gland sporozoite development was studied using monoclonal antibodies which recognize different CS protein epitopes. Monoclonal antibodies which react with the CS amino acid repeat sequences by ELISA recognized a 50-kDa precursor protein in immature oocyst and additional 47- and 42-kDa proteins in older oocysts. A 42-kDa CS protein was detected after initial sporozoite invasion of mosquito salivary glands and an additional 50-kDa precursor CS protein observed later in infected salivary glands. These data confirm previous results with other Plasmodium species, in which more CS protein precursors were detected in oocysts than in salivary gland sporozoites. A monoclonal antibody (PvPCS) was characterized which reacts with an epitope found only in the 50-kDa precursor CS protein. PvPCS reacted with all P. vivax sporozoite strains tested by indirect immunofluorescent assay, homogeneously staining the sporozoite periphery with much lower intensity than that produced by anti-CS repeat antibodies. Immunoelectron microscopy using PvPCS showed that the CS protein precursor was associated with peripheral cytoplasmic vacuoles and membranes of sporoblast and budding sporozoites in development oocysts. In salivary gland sporozoites, the CS protein precursor was primarily associated with micronemes and sporozoite membranes. Our results suggest that the 50-kDa CS protein precursor is synthesized intracellularly and secreted on the membrane surface, where it is proteolytically processed to form the 42-kDa mature CS protein. These data indicate that differences in CS protein processing in oocyst and salivary gland sporozoites development may occur.

Molecular characterization of the mycobacterial heparin-binding hemagglutinin, a mycobacterial adhesin

Although it generally is accepted that the interaction of Mycobacterium tuberculosis with alveolar macrophages is a key step in the pathogenesis of tuberculosis, interactions with other cell types, especially epithelial cells, also may be important. In this study we describe the molecular characterization of a mycobacterial heparin-binding hemagglutinin (HBHA), a protein that functions as an adhesin for epithelial cells. The structural gene was cloned from M. tuberculosis and bacillus Calmette-Guérin, and the sequence was found to be identical between the two species. The calculated Mr was smaller than the observed Mr when analyzed by SDS/PAGE. This difference can be attributed to the Lys/Pro-rich repeats that occur at the C-terminal end of the protein and to a putative carbohydrate moiety. Glycosylation of HBHA appears to protect the protein from proteolytic degradation, which results in the removal of the C-terminal Lys/Pro-rich region responsible for binding of HBHA to sulfated carbohydrates. Evidence suggests that glycosylation is also important for HBHA-mediated hemagglutination and for certain immunologic properties of the protein. Finally, the absence of a signal peptide in the coding region of HBHA raises the possibility that this protein is not secreted via the general secretion pathway.

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.

Malaria circumsporozoite protein inhibits protein synthesis in mammalian cells

Native Plasmodium circumsporozoite (CS) protein, translocated by sporozoites into the cytosol of host cells, as well as recombinant CS constructs introduced into the cytoplasm by liposome fusion or transient transfection, all lead to inhibition of protein synthesis in mammalian cells. The following findings suggest that this inhibition of translation is caused by a binding of the CS protein to ribosomes. (i) The distribution of native CS protein translocated by sporozoites into the cytoplasm as well as microinjected recombinant CS protein suggests association with ribosomes. (ii) Recombinant CS protein binds to RNase-sensitive sites on rough microsomes. (iii) Synthetic peptides representing the conserved regions I and II-plus of the P.falciparum CS protein displace recombinant CS protein from rough microsomes with dissociation constants in the nanomolar range. (iv) Synthetic peptides representing region I from the P.falciparum CS protein and region II-plus from the P.falciparum, P.berghei or P.vivax CS protein inhibit in vitro translation. We propose that Plasmodium manipulates hepatocyte protein synthesis to meet the requirements of a rapidly developing schizont. Since macrophages appear to be particularly sensitive to the presence of CS protein in the cytosol, inhibition of translation may represent a novel immune evasion mechanism of Plasmodium.

Apical organelles and host-cell invasion by Apicomplexa

Host-cell invasion by apicomplexan parasites involves the successive exocytosis of three different secretory organelles; namely micronemes, rhoptries and dense granules. The findings of recent studies have extended the structural homologies of each set of organelles between most members of the phylum and suggest shared functions of each set. Micronemes are apparently used for host-cell recognition, binding, and possibly motility; rhoptries for parasitophorous vacuole formation; and dense granules for remodeling the vacuole into a metabolically active compartment. In addition, gene cloning and sequencing have demonstrated conserved domains, which are likely to serve similar functions in the invasion process. This is especially true for microneme proteins containing thrombospondin-like domains, which are likely to be involved in binding to sulphated glycoconjugates. One such protein was recently shown to be required for the motility of Plasmodium sporozoites. These molecules have been shown to be shed on the parasite and/or cell surfaces during the invasion process in Plasmodium, Toxoplasma and Eimeria. For rhoptries and dense granules, the association between exocytosed proteins and the parasitophorous vacuole membrane had been analyzed extensively in Toxoplasma, as these proteins are likely to play a crucial role in metabolic interactions between the parasites and their host cells. The development of parasite transformation by gene transfection has provided powerful tools to analyze the fate and function(s) of the corresponding proteins.

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.

Heparin interferes with translocation of Yop proteins into HeLa cells and binds to LcrG, a regulatory component of the Yersinia Yop apparatus

Yersiniae are equipped with the Yop virulon, an apparatus that allows extracellular bacteria to deliver toxic Yop proteins inside the host cell cytosol in order to sabotage the communication networks of the host cell or even to cause cell death. LcrG is a component of the Yop virulon involved in the regulation of secretion of the Yops. In this paper, we show that LcrG can bind HeLa cells, and we analyse the role of proteoglycans in this phenomenon. Treatment of the HeLa cells with heparinase I, but not chondroitinase ABC, led to inhibition of binding. Competition assays indicated that heparin and dextran sulphate strongly inhibited binding, but that other glycosaminoglycans did not. This demonstrated that binding of HeLa cells to purified LcrG is caused by heparan sulphate proteoglycans. LcrG could bind directly to heparin-agarose beads and, in agreement with these results, analysis of the protein sequence of Yersinia enterocolitica LcrG revealed heparin-binding motifs. In vitro production and secretion by Y. enterocolitica of the Yops was unaffected by the addition of heparin. However, the addition of exogenous heparin decreased the level of YopE-Cya translocation into HeLa cells. A similar decrease was seen with dextran sulphate, whereas the other glycosaminoglycans tested had no significant effect. Translocation was also decreased by treatment of HeLa cells with heparinitase, but not with chondroitinase. Thus, heparan sulphate proteoglycans have an important role to play in translocation. The interaction between LcrG and heparan sulphate anchored at the surface of HeLa cells could be a signal triggering deployment of the Yop translocation machinery. This is the first report of a eukaryotic receptor interacting with the type III secretion and associated translocation machinery of Yersinia or of other bacteria.

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.

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.

Fine epitope specificity of antibodies to region II of the Plasmodium vivax circumsporozoite protein correlates with ability to bind recombinant protein and sporozoites

Recent work has suggested that important B- and T-cell epitopes on the circumsporozoite protein (CSP) of Plasmodium vivax lie external to the major repeat regions of the protein. We have studied two naturally exposed human populations (Caucasian and Papua New Guineans) and determined the antibody response to yeast-derived recombinant CSPs, overlapping synthetic peptides spanning amino acids 76 348 of the Belem P. vivax CSP and overlapping peptides representing the variant repeats of the VK247 strain of P. vivax. We have demonstrated that the P. vivax CSP-specific antibody response is directed towards areas within the repeat region as well as areas external to this; but the dominant epitopes recognized by the two populations studied, were distinct. One epitope, lying external to the repeats and recognized by both populations, partially overlaps an area of the protein referred to as region II-plus. Sera from malaria-exposed Papua New Guineans and Thais contained antibodies to this epitope (V22, single letter amino acid sequence TCGVGVRVRRRVNAANKKPE) which were capable of recognizing sporozoites, as determined by quantitative inhibition IFA. Seventeen percent of PNG sera had antibodies to this peptide compared with 33% who had antibodies to the central repeats of the protein. Immunization of mice with recombinant CSP did not induce antibodies to V22. However, immunization with overlapping peptide epitopes representing this region (V21 or V22) induced specific antibodies but only two sera recognized both V21 and V22 and, by inference, the overlapping peptide sequence (TCGVGVRVRR). Antibodies in these two sera could bind recombinant CSP in ELISA; however, in contrast, nine sera which recognized either V21 or V22 alone did not bind CSP. Only one of two sera containing antibodies recognizing CSP stained P. vivax sporozoites. This serum also recognized an epitope dependent upon two amino acids aminoterminal to V22. These data suggest that the fine specificity of antibodies is a critical determinant for binding to both recCSP and sporozoites.

Leishmania mexicana: binding of promastigotes to type I collagen

During leishmania infection, parasites are inoculated to the human host through the bite of a sandfly vector into the dermis, where they first interact with tissue components, cells and extracellular matrix molecules. Since collagen is the most abundant component of the skin matrix, we investigated whether there is a specific interaction of Leishmania mexicana promastigotes with this host component. Promastigotes were able to attach to collagen fibrils and move through the matrix of mouse skin sections and to penetrate easily into a type I collagen gel. Denatured type I collagen coated beads (Cytodex 3) readily bound to the parasite surface. The interaction of promastigotes with type I collagen was dose dependent and saturable and was competitively and specifically inhibited with increasing concentrations of gelatin. Biotin-labeled parasite surface molecules were able to associate with both denatured collagen from microcarriers and native type I collagen from bovine kidney. It is suggested that the presence of parasite cell membrane receptors to collagen may confer a specific tropism for the skin, where collagen is the most abundant component of the matrix.

Cell invasion by the vertebrate stages of Plasmodium

Protozoans of the genus Plasmodium are the causative agents of malaria; they have a complex life cycle involving vertebrate and arthropod hosts and have three distinct invasive stages. Although the invasive stages probably invade cells using similar mechanisms, each stage has a different host cell specificity and utilizes different receptors to enter cells.

The malaria circumsporozoite protein: interaction of the conserved regions I and II-plus with heparin-like oligosaccharides in heparan sulfate

The malaria circumsporozoite (CS) protein binds to glycosaminoglycans from heparan sulfate proteoglycans on the cell surface of hepatocytes and is specifically cleared from the bloodstream by the liver. We show here that the two conserved regions, I and II-plus, of the CS protein, in a concerted action, preferentially bind to highly sulfated heparin-like oligosaccharides in heparan sulfate. In a concentration-dependent manner, peptides representing region I and region II-plus inhibited the binding of recombinant CS protein to HepG2 cells by 62 and 84%, respectively. Furthermore, the action of endoproteinase Arg-C, which cleaves the recombinant CS constructs CS27IVC and CSFZ(Cys) predominantly at the conserved region I, was inhibited by heparin in a concentration-dependent fashion. CSFZ(Cys), which has a higher affinity to HSPGs than CS27IVC, was stabilized by heparin at a w/w ratio (CS protein:glycosaminoglycan) of 20/1, whereas full protection of CS27IVC required more heparin (5/1). Heparan sulfate provided full protection of CSFZ(Cys) only at a ratio of 1/10. Native fucoidan as well as normally sulfated fuco-oligosaccharides (0.76 mol sulfate/mol fucose) inhibited Plasmodium berghei development in HepG2 cells by 84 and 66%, respectively, in a concentration-dependent manner and sporozoite invasion into CHO cells by 80%. Desulfated fucoidan oligosaccharides were inactive. These results may explain the selective interaction between the CS protein and the unique heparan sulfate from liver, which is noted for its unusually high degree of sulfation, and may provide a plausible explanation for the selective targeting of the malaria CS protein to the liver.

Circumsporozoite protein is required for development of malaria sporozoites in mosquitoes

Malaria parasites undergo a sporogonic cycle in the mosquito vector. Sporozoites, the form of the parasite injected into the host during a bloodmeal, develop inside oocysts in the insect midgut, then migrate to and eventually invade the salivary glands. The circumsporozoite protein (CS), one of the major proteins synthesized by salivary gland sporozoites, is a surface-associated molecule which is important in sporozoite infectivity to the host. Here, by gene targeting, we created Plasmodium berghei lines in which the single-copy CS gene was disrupted. The CS(-) and wild-type parasites produced similar numbers of oocysts of comparable size in the mosquito midgut. In the CS(-) oocysts, however, sporozoite formation was profoundly inhibited. CS therefore appears to have a pleiotropic role and to be vital for malaria parasites in both the vector and the host: in mosquitoes, CS is essential for sporozoite development within oocysts, and in the vertebrate host it promotes sporozoite attachment to hepatocytes.

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.

Host cell heparan sulfate proteoglycans mediate attachment and entry of Listeria monocytogenes, and the listerial surface protein ActA is involved in heparan sulfate receptor recognition

The mechanisms by which the intracellular pathogen Listeria monocytogenes interacts with the host cell surface remain largely unknown. In this study, we investigated the role of heparan sulfate proteoglycans (HSPG) in listerial infection. Pretreatment of bacteria with heparin or heparan sulfate (HS), but not with other glycosaminoglycans, inhibited attachment and subsequent uptake by IC-21 murine macrophages and CHO epithelial-like cells. Specific removal of HS from target cells with heparinase III significantly impaired listerial adhesion and invasion. Mutant CHO cells deficient in HS synthesis bound and internalized significantly fewer bacteria than wild-type cells did. Pretreatment of target cells with the HS-binding proteins fibronectin and platelet factor 4, or with heparinase III, impaired listerial infectivity only in those cells expressing HS. Moreover, a synthetic peptide corresponding to the HS-binding ligand in Plasmodium falciparum circumsporozoite protein (pepPf1) inhibited listerial attachment to IC-21 and CHO cells. A motif very similar to the HS-binding site of pepPf1 was found in the N-terminal region of ActA, the L. monocytogenes surface protein responsible for actin-based bacterial motility and cell-to-cell spread. In the same region of ActA, several clusters of positively charged amino acids which could function as HS-binding domains were identified. An ActA-deficient mutant was significantly impaired in attachment and entry due to altered HS recognition functions. This work shows that specific interaction with an HSPG receptor present on the surface of both professional and nonprofessional phagocytes is involved in L. monocytogenes cytoadhesion and invasion and strongly suggests that the bacterial surface protein ActA may be a ligand mediating HSPG receptor recognition.

Dual interaction of the malaria circumsporozoite protein with the low density lipoprotein receptor-related protein (LRP) and heparan sulfate proteoglycans

Speed and selectivity of hepatocyte invasion by malaria sporozoites have suggested a receptor-mediated mechanism and the specific interaction of the circumsporozoite (CS) protein with liver-specific heparan sulfate proteoglycans (HSPGs) has been implicated in the targeting to the liver. Here we show that the CS protein interacts not only with cell surface heparan sulfate, but also with the low density lipoprotein receptor-related protein (LRP). Binding of 125I-CS protein to purified LRP occurs with a Kd of 4.9 nM and can be inhibited by the receptor-associated protein (RAP). Blockage of LRP by RAP or anti-LRP antibodies on heparan sulfate-deficient CHO cells results in more than 90% inhibition of binding and endocytosis of recombinant CS protein. Conversely, blockage or enzymatic removal of the cell surface heparan sulfate from LRP-deficient embryonic mouse fibroblasts yields the same degree of inhibition. Heparinase-pretreatment of LRP-deficient fibroblasts or blockage of LRP on heparan sulfate-deficient CHO cells by RAP, lactoferrin, or anti-LRP antibodies reduces Plasmodium berghei invasion by 60-70%. Parasite development in heparinase-pretreated HepG2 cells is inhibited by 65% when RAP is present during sporozoite invasion. These findings suggest that malaria sporozoites utilize the interaction of the CS protein with HSPGs and LRP as the major mechanism for host cell invasion.

Release of malaria circumsporozoite protein into the host cell cytoplasm and interaction with ribosomes

To date, the circumsporozoite (CS) protein has been implicated in guiding malaria sporozoites to the liver [Cerami et al., Cell 70, 1992, 1021-1033]. Here we show that shortly after invasion, P. berghei and P. yoelii sporozoites lie free in the invaded cell and release considerable amounts of CS protein into the cytoplasm. The intracytoplasmic deposition of CS protein begins during the attachment of the sporozoite to the host cell surface and reaches its peak during the first 4-6 h after invasion. Initially, the CS protein spreads over the entire cytoplasm of the infected cell where it interacts with cytosolic as well as endoplasmic reticulum-associated ribosomes. During the subsequent development of the parasites to exoerythrocytic forms, the CS protein binding becomes gradually restricted to ribosomes lining the outer membrane of the nuclear envelope of the host cell. The distribution pattern of the parasite-released CS protein in the host cell cytoplasm is independent of the permissiveness of the host cell for the development of the parasites to exoerythrocytic forms. It requires neither the host cell metabolism nor does it involve the endocytotic machinery. Recombinant P. falciparum CS protein interacts with RNAse-sensitive sites on endoplasmic reticulum-associated ribosomes as shown by microinjection and immunoelectron microscopy. The generalized interaction of the CS protein with host cell ribosomes suggests that the CS protein has an intracellular function during the hepatic phase in the life cycle of Plasmodium and may also explain the generation of a CD8+ T cell response in the course of rodent malaria infections.

A recombinant Chlamydia trachomatis major outer membrane protein binds to heparan sulfate receptors on epithelial cells

Chlamydial attachment to columnar conjunctival or urogenital epithelial cells is an initial and critical step in the pathogenesis of chlamydial mucosal infections. The chlamydial major outer membrane protein (MOMP) has been implicated as a putative chlamydial cytoadhesin; however, direct evidence supporting this hypothesis has not been reported. The function of MOMP as a cytoadhesin was directly investigated by expressing the protein as a fusion with the Escherichia coli maltose binding protein (MBP-MOMP) and studying its interaction with human epithelial cells. The recombinant MBP-MOMP bound specifically to HeLa cells at 4 degrees C but was not internalized after shifting the temperature to 37 degrees C. The MBP-MOMP competitively inhibited the infectivity of viable chlamydiae for epithelial cells, indicating that the MOMP and intact chlamydiae bind the same host receptor. Heparan sulfate markedly reduced binding of the MBP-MOMP to cells, whereas chondroitin sulfate had no effect on binding. Enzymatic treatment of cells with heparitinase but not chondroitinase inhibited the binding of MBP-MOMP. These same treatments were also shown to reduce the infectivity of chlamydiae for epithelial cells. Mutant cell lines defective in heparan sulfate synthesis but not chondroitin sulfate synthesis showed a marked reduction in the binding of MBP-MOMP and were also less susceptible to infection by chlamydiae. Collectively, these findings provide strong evidence that the MOMP functions as a chlamydial cytoadhesin and that heparan sulfate proteoglycans are the host-cell receptors to which the MOMP binds.

Characterization of the parasite-host cell interactions involved in Theileria parva sporozoite invasion of bovine lymphocytes

Sporozoite invasion of bovine lymphocytes by Theileria parva is a pH-dependent process that occurs without the need for de novo protein synthesis. The process was inhibited by RGD(S) peptides, fibronectin and, in the presence of serum, by antibodies reactive with fibronectin. Invasion was also blocked by a range of sulphated glycoconjugates, but treatment of lymphocytes with heparitinase did not inhibit entry. Enzymic modifications of the lymphocyte surface demonstrated that trypsin-insensitive glycoproteins containing O- and N-linked carbohydrates as well as phospholipase-sensitive molecules on the host cell surface were critical to sporozoite entry. Modification of the lymphocyte surface with NEM and DTT had only marginal effects on sporozoite binding but blocked parasite internalization. Invasion was also blocked by several antibodies which cross-reacted with sporozoite surface molecules. While only a few experimental conditions specifically blocked sporozoite binding, a wider range of reagents and treatments inhibited parasite entry. The reasons for this are discussed in terms of the nature of the zippering process that facilitates sporozoite internalization.

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.

Identification of a heparin-binding hemagglutinin present in mycobacteria

Adherence to mammalian host tissues is an important virulence trait in microbial pathogenesis, yet little is known about the adherence mechanisms of mycobacteria. Here, we show that binding of mycobacteria to epithelial cells but not to macrophages can be specifically inhibited by sulfated carbohydrates. Using heparin-Sepharose chromatography, a 28-kD heparin-binding protein was purified from culture supernatants and cell extracts of Mycobacterium bovis and Mycobacterium tuberculosis. This protein, designated heparin-binding hemagglutinin (HBHA), promotes the agglutination of rabbit erythrocytes, which is specifically inhibited by sulfated carbohydrates. HBHA also induce mycobacterial aggregation, suggesting that it can mediate bacteria-bacteria interactions as well. Hemagglutination, mycobacterial aggregation, as well as attachment to epithelial cells are specifically inhibited in the presence of anti-HBHA antibodies. Immunoelectron microscopy using anti-HBHA monoclonal antibodies revealed that the protein is surface exposed, consistent with a role in adherence. Immunoblot analyses using antigen-specific antibodies indicated that HBHA is different from the fibronectin-binding proteins of the antigen 85 complex and p55, and comparison of the NH2-terminal amino acid sequence of purified HBHA with the protein sequence data bases did not reveal any significant similarity with other known proteins. Sera from tuberculosis patients but not from healthy individuals were found to recognize HBHA, indicating its immunogenicity in humans during mycobacterial infections. Identification of putative mycobacterial adhesins, such as the one described in this report, may provide the basis for the development of new therapeutic and prophylactic strategies against mycobacterial diseases.

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.

The receptor-binding domain of pseudorabies virus glycoprotein gC is composed of multiple discrete units that are functionally redundant

Many herpesviruses attach to cells in a two-step process, using the glycoprotein gC family of homologs to bind the primary receptor, heparan sulfate (HS) proteoglycan, and glycoprotein gD homologs to bind an unknown secondary receptor. We have previously shown by deletion analysis that the amino-terminal one-third of gC from pseudorabies virus (PRV), a swine herpesvirus, includes at least the principal HS receptor-binding domain. This portion of PRV gC contains three discrete clusters of basic residues that exactly or nearly match proposed consensus sequences for heparin-binding domains (HBDs); four additional potential HBDs lie in the distal two-thirds of the glycoprotein. We now specifically implicate each of the three amino-terminal HBDs in virus attachment. Mutational analysis demonstrated that any one of the three HBDs could mediate efficient virus infectivity; HS-dependent PRV attachment to cells was eliminated only after all three amino-terminal HBDs were altered. Furthermore, the binding dysfunction was due to a disruption of the specific HBDs and not to total charge loss. Thus, unlike previously described viral receptor-binding domains, the PRV gC receptor-binding domain is composed of multiple, discrete units that can function independently of one another. These units may function redundantly either to increase binding affinity or perhaps to effectively increase the virus's host range.

Cell surface glycosaminoglycans are not obligatory for Plasmodium berghei sporozoite invasion in vitro

The malaria circumsporozoite (CS) protein binds to glycosaminoglycan chains from heparan sulfate proteoglycans present on the basolateral surface of hepatocytes and hepatoma cells in vitro. When injected into mice, CS protein is rapidly cleared from the blood circulation by hepatocytes. The binding region for the HSPGs is the evolutionarily conserved region II-plus of the CS protein. Here we have asked whether the presence of glycosaminoglycans on the plasma membrane of target cells is required for sporozoite invasion in vitro. Two types of target cells were used: HepG2 cells, which are permissive for Plasmodium berghei sporozoite development into mature exoerythrocytic forms, and CHO cells, in which the intracellular development of the parasites is arrested early after penetration. The invasion of mutant CHO cells expressing undersulfated glycosaminoglycans or no glycosaminoglycans was only inhibited 41-49% or 24-32%, respectively, in comparison to invasion of CHO-K1 cells. Previous cleavage of HepG2 surface membrane glycosaminoglycans with heparinase or heparitinase had no significant inhibitory effect on subsequent P. berghei sporozoite invasion and EEF development in these cells, although the glycosaminoglycan lyase treatments removed over 80% of CS binding sites from the cell surface. These results suggest that although the presence of glycosaminoglycans on the target cell surface enhances sporozoite invasion, glycosaminoglycans are not required for sporozoite penetration or the development of exoerythrocytic forms in vitro.

A conserved peptide sequence of the Plasmodium falciparum circumsporozoite protein and antipeptide antibodies inhibit Plasmodium berghei sporozoite invasion of Hep-G2 cells and protect immunized mice against P. berghei sporozoite challenge

Minutes after injection into the circulation, malaria sporozoites enter hepatocytes. The speed and specificity of the invasion process suggest that it is receptor mediated. The region II sequence of Plasmodium falciparum circumsporozoite (CS) protein includes a nonapeptide (WSPCSVTCG) which is highly conserved in all of the CS proteins sequenced to data, including the one from Plasmodium berghei. We have found that two peptides based on the P. falciparum region II sequence, P18 (EWSPCSVTCGNGIQVRIK) and P32 (IEQYLKKIKNS ISTEWSPCSVTCGNGIQVRIK), significantly inhibited P. berghei sporozoite invasion into Hep-G2 cells in vitro. This inhibition was enhanced if either peptide was preincubated with Hep-G2 cells prior to sporozoite invasion. We confirm that region II is a sporozoite ligand for the hepatocyte receptor; moreover, despite the few differences between P. falciparum and P. berghei region II sequences around the nonapeptide sequence (66% homology), the functional characteristics of the motif sequences are not affected. Since the conserved motifs represent a crucial sequence involved in Plasmodium sporozoite invasion of hepatocytes, antibodies to region II should inhibit sporozite invasion into hepatocytes. Indeed, we found that polyclonal antibodies generated to the P. falciparum-based peptide P32 inhibited P. berghei sporozoite invasion of Hep-G2 cells. Furthermore, inbred mice (C57BL/6) immunized with P32 were protected against a lethal challenge of P. berghei sporozoites. Our results suggest that the conserved region II of the CS protein contains crucial B- and T-cell epitopes, that such peptide sequences from the human malaria parasite P. falciparum can be screened in the P. berghei rodent model, and, finally, that region II can be considered useful as one of the components of a malaria vaccine.