The surface antigen SAG3 mediates the attachment of Toxoplasma gondii to cell-surface proteoglycans

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

A cluster of four surface antigen genes specifically expressed in bradyzoites, SAG2CDXY, plays an important role in Toxoplasma gondii persistence

Toxoplasma gondii is one of the most successful protozoan parasites of warm-blooded animals. Stage-specific expression of its surface molecules is thought to be key to its ability to establish chronic infection in immunocompetent animals. The rapidly dividing tachyzoite stage displays a different subset of family of surface antigen 1 (SAG1)-related sequences (SRSs) from that displayed by the encysted bradyzoite stage. It is possible that this switch is necessary to protect the bradyzoites against an immune response raised against the tachyzoite stage. Alternatively, it might be that bradyzoite SRSs evolved to facilitate invasion of different cell types, such as those found in the brain, where cysts develop, or the small intestine, where bradyzoites must enter after oral infection. Here we studied the function of a cluster of four tandem genes, encoding bradyzoite SRSs called SAG2C, -D, -X, and -Y. Using bioluminescence imaging of mice infected with parasites expressing firefly luciferase (FLUC) driven by the SAG2D promoter, we show stage conversion for the first time in living animals. A truncated version of the SAG2D promoter (SAG2Dmin) gave efficient expression of FLUC in both tachyzoites and bradyzoites, indicating that the bradyzoite specificity of the complete SAG2D promoter is likely due to an element(s) that normally suppresses expression in tachyzoites. Comparing mice infected with the wild type or a mutant where the SAG2CDXY cluster of genes has been deleted (DeltaSAG2CDXY), we demonstrate that whereas DeltaSAG2CDXY parasites are less capable of maintaining a chronic infection in the brain, they do not show a defect in oral infectivity.

Investigation of SnSPR1, a novel and abundant surface protein of Sarcocystis neurona merozoites

An expressed sequence tag (EST) sequencing project has produced over 15,000 partial cDNA sequences from the equine pathogen Sarcocystis neurona. While many of the sequences are clear homologues of previously characterized genes, a significant number of the S. neurona ESTs do not exhibit similarity to anything in the extensive sequence databases that have been generated. In an effort to characterize parasite proteins that are novel to S. neurona, a seemingly unique gene was selected for further investigation based on its abundant representation in the collection of ESTs and the predicted presence of a signal peptide and glycolipid anchor addition on the encoded protein. The gene was expressed in E. coli, and monospecific polyclonal antiserum against the recombinant protein was produced by immunization of a rabbit. Characterization of the native protein in S. neurona merozoites and schizonts revealed that it is a low molecular weight surface protein that is expressed throughout intracellular development of the parasite. The protein was designated Surface Protein 1 (SPR1) to reflect its display on the outer surface of merozoites and to distinguish it from the ubiquitous SAG/SRS surface antigens of the heteroxenous Coccidia. Interestingly, infection assays in the presence of the polyclonal antiserum suggested that SnSPR1 plays some role in attachment and/or invasion of host cells by S. neurona merozoites. The work described herein represents a general template for selecting and characterizing the various unidentified gene sequences that are plentiful in the EST databases for S. neurona and other apicomplexans. Furthermore, this study illustrates the value of investigating these novel sequences since it can offer new candidates for diagnostic or vaccine development while also providing greater insight into the biology of these parasites.

Azurin-like protein blocks invasion of Toxoplasma gondii through potential interactions with parasite surface antigen SAG1

Some pathogenic bacteria produce factors that have evolved a capacity to neutralize competing microbes. The cupredoxin family protein azurin, produced by Pseudomonas aeruginosa, exhibits a remarkable ability to impede invasion of a number of diverse intracellular pathogens, including the human AIDS virus human immunodeficiency virus type 1 and the protozoan parasite Plasmodium falciparum (which causes malaria). Here we report that azurin and an azurin-like protein (Laz) from gonococci/meningococci have activity against Toxoplasma, an apicomplexan parasite that causes opportunistic infection in immunocompromised individuals. We demonstrate that the mechanism of action for Laz involves interfering with the ability of Toxoplasma to adhere to host cells. Computer structural analysis reveals that azurin shares structural features with the predominant surface antigen SAG1, which is known to play an important role in parasite attachment. Interestingly, azurin also has structural similarities to a monoclonal antibody to SAG1. Surface plasmon resonance binding studies validate that SAG1 interacts strongly with Laz and, to lesser extent, azurin. Moreover, Toxoplasma mutants lacking SAG1 are not as susceptible to the growth-inhibitory effects of Laz. Collectively, our data show that Toxoplasma adhesion can be significantly impaired by Laz, and to some extent by azurin, via interactions with SAG1. These observations indicate that Laz can serve as an important tool in the study of host-pathogen interactions and is worthy of further study for development into potential therapeutic agents.

Trypanosoma cruzi heparin-binding proteins and the nature of the host cell heparan sulfate-binding domain

Trypanosoma cruzi invasion is mediated by receptor-ligand recognition between the surfaces of both parasite and target cell. We have previously demonstrated the role of heparan sulfate proteoglycan in the attachment and invasion of T. cruzi in cardiomyocytes. Herein, we have isolated the T. cruzi heparin-binding proteins (HBP-Tc) and investigated the nature of cardiomyocyte heparan sulfate (HS)-binding site to the parasite surface ligand. Two major heparin-binding proteins with molecular masses of 65.8 and 59 kDa were observed in total extract of amastigote and trypomastigote forms of T. cruzi. Hydrophobic [S(35)]methionine labeled proteins eluted from heparin-sepharose affinity chromatography also revealed both proteins in trypomastigotes but only the 59 kDa is strongly recognized by biotin-conjugated glycosaminoglycans. Competition assays were performed to analyze the role of sulfated proteoglycans, including heparin, keratan sulfate and both acetylated and highly sulfated domains of heparan sulfate, in the recognition and invasion process of T. cruzi. Significant inhibitions of 84% and 35% in the percentage of infection were revealed after treatment of the parasites with heparin and the N-acetylated/ N-sulfated heparan sulfate domain, respectively, suggesting the important role of the glycuronic acid and NS glucosamine domain of the HS chain in the recognition of the HBP-Tc during the T. cruzi-cardiomyocyte interaction.

Kinetic modeling of Toxoplasma gondii invasion

The phylum Apicomplexa includes parasites responsible for global scourges such as malaria, cryptosporidiosis, and toxoplasmosis. Parasites in this phylum reproduce inside the cells of their hosts, making invasion of host cells an essential step of their life cycle. Characterizing the stages of host-cell invasion, has traditionally involved tedious microscopic observations of individual parasites over time. As an alternative, we introduce the use of compartment models for interpreting data collected from snapshots of synchronized populations of invading parasites. Parameters of the model are estimated via a maximum negative log-likelihood principle. Estimated parameter values and their 95% confidence intervals (95% CI), are consistent with reported observations of individual parasites. For RH strain parasites, our model yields that: (1) penetration of the host-cell plasma membrane takes 26s (95% CI: 22-30s); (2) parasites that ultimately invade, remain attached three times longer than parasites that eventually detach from the host cells, and (3) 25% (95% CI: 19-33%) of parasites invade while 75% (95% CI: 67-81%) eventually detach from their host cells without progressing to invasion. A key feature of the model is the incorporation of invasion stages that cannot be directly observed. This allows us to characterize the phenomenon of parasite detachment from host cells. The properties of this phenomenon would be difficult to quantify without a mathematical model. We conclude that mathematical modeling provides a powerful new tool for characterizing the stages of host-cell invasion by intracellular parasites.

Vaccination against murine toxoplasmosis using recombinant Toxoplasma gondii SAG3 antigen alone or in combination with Quil A

PURPOSE

Surface antigen 3 (SAG3) of Toxoplasma gondii is very similar in structure to the major surface antigen 1 (SAG1). Although numerous studies have supported the importance of SAG1 in protection against T. gondii infection, few reports exist on SAG3.

MATERIALS AND METHODS

Glutathione-S-transferase (GST)-fused SAG3 of T. gondii (rSAG3) were immunized into BALB/c mice alone or in combination with Quil A (rSAG3/Quil A), and then evaluated the protective immunity in vivo and in vitro against murine toxoplasmosis.

RESULTS

Immunization with rSAG3 or rSAG3/Quil A resulted in significantly more survival days and fewer brain cysts after challenge with T. gondii compared to an infected control group. Mice immunized with rSAG3 alone or in combination with Quil A produced significantly more specific IgG2a antibody, whereas specific IgG1 antibody titers did not increase. The percentage of CD8+ T cells, IFN-gamma mRNA expression, and nitric oxide production significantly increased in rSAG3- and rSAG3/Quil A-immunized mice.

CONCLUSION

These results indicate that vaccination with Toxoplasma rSAG3 results in partial protective immunity against T. gondii infection through induction of a Th1-type immune response, and that protective immunity is accelerated by the modulating effects of Quil A.

Whole-genome natural histories of apicomplexan surface proteins

The natural histories of free-living and pathogenic protozoans have been described in over a century of studies, spanning a range of disciplines such as microscopic, cellular, taxonomic, pathological, clinical and molecular. Only in the last decade has this landscape of work benefited from the availability of whole-genome nucleotide sequence data. For many pathogens, it is now possible to overlay analyses of protein repertoires onto the current spectrum of knowledge. This article illuminates protozoan natural histories, particularly the rapidly evolving and highly adaptive direct physical interface of apicomplexan parasites and their hosts, by providing a brief introduction to the origin and phylogenetic distribution of parasite-encoded surface proteins and their component domains.

Bradyzoite-specific surface antigen SRS9 plays a role in maintaining Toxoplasma gondii persistence in the brain and in host control of parasite replication in the intestine

Toxoplasma gondii is a ubiquitous parasite that persists for the life of a healthy mammalian host. A latent, chronic infection can reactivate upon immunosuppression and cause life-threatening diseases, such as encephalitis. A key to the pathogenesis is the parasite's interconversion between the tachyzoite (in acute infection) and bradyzoite (in chronic infection) stages. This developmental switch is marked by differential expression of numerous, closely related surface proteins belonging to the SRS (SAG1-related sequence) superfamily. To probe the functions of bradyzoite-specific SRSs, we created a bioluminescent strain lacking the expression of SRS9, one of the most abundant SRSs of the bradyzoite stage. Imaging of mice intraperitoneally infected with tachyzoites revealed that during an acute infection, wild-type and Deltasrs9 strains replicated at similar rates, disseminated systemically following similar kinetics, and initially yielded similar brain cyst numbers. However, during a chronic infection, Deltasrs9 cyst loads substantially decreased compared to those of the wild type, suggesting that SRS9 plays a role in maintaining parasite persistence in the brain. In oral infection with bradyzoite cysts, the Deltasrs9 strain showed oral infectivity and dissemination patterns indistinguishable from those of the wild type. When chronically infected mice were treated with the immunosuppressant dexamethasone, however, the Deltasrs9 strain reactivated in the intestinal tissue after only 8 to 9 days, versus 2 weeks for the wild-type strain. Thus, SRS9 appears to play an important role in both persistence in the brain and reactivation in the intestine. Possible mechanisms for this are discussed.

Erythrocyte invasion by Babesia parasites: Current advances in the elucidation of the molecular interactions between the protozoan ligands and host receptors in the invasion stage

During an asexual growth cycle of Babesia parasites in a natural host, the extracellular merozoites invade (i.e., attach to, penetrate, and internalize) the host erythrocytes (RBC) via multiple adhesive interactions of several protozoan ligands with the target receptors on the host cell surface. After internalizing the host RBC, they asexually multiply, egress from the RBC by rupturing the host cells, and then invade the new RBC again. In the invasion stage, several surface-coating molecules of merozoites might be involved in the initial attachment to the RBC, while proteins secreted from apical organelles (rhoptry, microneme, and spherical body) are proposed to play roles mainly in erythrocyte penetration or internalization. On the other hand, several components located on the surface of the RBC, such as sialic acid residues, protease-sensitive proteins, or sulphated glycosaminoglycans, are identified or suspected as the host receptors of erythrocyte invasion by Babesia parasites. The detailed molecular interactions between Babesia merozoites and the host RBC are incompletely understood. In this review, these identified or suspected molecules (protozoan ligands/erythrocyte receptors) are described by especially focusing on Babesia bovis.

Laser scanning cytometer-based assays for measuring host cell attachment and invasion by the human pathogen Toxoplasma gondii

BACKGROUND

Toxoplasma gondii is among the most common protozoan parasites of humans. Both attachment to and invasion of host cells by T. gondii are necessary for infection, yet little is known about the molecular mechanisms underlying these processes. T. gondii's etiological importance and its role as a model organism for studying invasion in related parasites necessitate a means to quantitatively assay host cell attachment and invasion.

METHODS

We present here Laser Scanning Cytometer (LSC)-based assays of T. gondii invasion and attachment. The invasion assay involves automated counting of invaded and non-invaded parasites, differentially labeled with distinct fluorochromes. The attachment assay compares the relative binding of differentially labeled parasites. The assays were evaluated using treatments known to decrease invasion or attachment.

RESULTS

The LSC-based assays are robust and reproducible, remove operator bias, and significantly increase the sample size that can be feasibly counted compared to other currently available microscope-based methods. In the first application of the new assays, we have shown that parasites attach to fixed and unfixed host cells using different mechanisms.

CONCLUSIONS

The LSC-based assays represent useful new methods for quantitatively measuring attachment and invasion by T. gondii, and can be readily adapted to study similar processes in other host-pathogen systems.

Neospora caninum: antibodies directed against tachyzoite surface protein NcSRS2 inhibit parasite attachment and invasion of placental trophoblasts in vitro

Polyclonal and monoclonal antibodies to native Neospora caninum tachyzoite surface protein NcSRS2 were generated and tested in vitro for their ability to neutralize tachyzoite attachment to and invasion of host cells. Host cells included Vero cells and a newly cloned, immortalized ovine trophoblast cell line obtained from primary cultures of ovine placenta. The ovine trophoblasts had morphology consistent with fetal trophoblasts and expressed mRNA for interferon-tau, a marker for trophoblasts. Native NcSRS2 was used to immunize mice to obtain monospecific anti-NcSRS2 polyclonal serum and anti-NcSRS2 monoclonal antibodies. Compared to irrelevant antibodies, monospecific anti-NcSRS2 serum and two anti-NcSRS2 monoclonal antibodies, 100.2.4.4 and 119.4.9.10, significantly blocked invasion of tachyzoites into both trophoblasts and Vero cells. Parasite attachment, assessed by IFA, was significantly reduced by anti-NcSRS2 mAb 100.2.4.4 and monospecific serum. The findings provide rationale to investigate a role for antibodies to NcSRS2 in prevention of N. caninum transplacental transmission in vivo.

Crystal Structure of the Complex between the Monomeric Form of Toxoplasma gondii Surface Antigen 1 (SAG1) and a Monoclonal Antibody that Mimics the Human Immune Response

Toxoplasma gondii, the intracellular parasite responsible for toxoplasmosis infects more than one-third of the world population and can be life-threatening for fetuses and immunocompromised patients. The surface protein SAG1 is an important immune target, which provides a strong immune response against the invasive tachyzoite while the other forms of the parasite, devoid of SAG1 at their surface, are multiplying. In addition to this role as a "hot spot" decoy, SAG1 is predicted to act as an adhesin during host-cell attachment through its binding to proteoglycans. To begin to understand the relationships between SAG1 epitopes and the ligand-binding site, we have solved the crystal structure of the monomeric form of T.gondii SAG1 complexed to a Fab derived from a monoclonal antibody raised against tachyzoite particles. This antibody competes strongly with human Toxoplasma-specific sera, suggesting that its epitope is part of an immunodominant region present on the surface of SAG1. The structure reveals that this conformational epitope, located within the SAG1 N-terminal domain, does not overlap with the proposed ligand-binding pocket. This study provides the first structural description of the monomeric form of SAG1, and significant insights into its dual role of adhesin and immune target during parasite infection.

Cell surface heparan sulfate promotes replication of Toxoplasma gondii

Previous work suggests that cell surface heparan sulfate acts as a receptor for the Apicomplexan parasite Toxoplasma gondii. Using Chinese hamster ovary cell mutants defective in heparan sulfate biosynthesis, we show that heparan sulfate is necessary and sufficient for infectivity. Further, we demonstrate that the parasite requires N sulfation of heparan sulfate initiated by N-deacetylase/N-sulfotransferase-1, but 2-O sulfation and 6-O sulfation appear to be dispensable. In order to study the role of heparan sulfate in other cell types, we created a conditional allele for N-deacetylase/N-sulfotransferase-1 by using Cre-loxP technology. Mammary tumor cells lacking N-deacetylase/N-sulfotransferase-1 exhibited reduced toxoplasma infectivity like Chinese hamster ovary cell mutants. Surprisingly, heparin, chemically modified heparinoids, and monoclonal antibodies to heparan sulfate had no effect on toxoplasma infection. T. gondii attachment and invasion were unchanged in N-deacetylase/N-sulfotransferase-1-inactivated cells as well, but replication was reduced. Thus, heparan sulfate does not appear to function as a receptor for T. gondii but instead facilitates parasite replication postinvasion.

Conditional expression of Toxoplasma gondii apical membrane antigen-1 (TgAMA1) demonstrates that TgAMA1 plays a critical role in host cell invasion

Toxoplasma gondii is an obligate intracellular parasite and an important human pathogen. Relatively little is known about the proteins that orchestrate host cell invasion by T. gondii or related apicomplexan parasites (including Plasmodium spp., which cause malaria), due to the difficulty of studying essential genes in these organisms. We have used a recently developed regulatable promoter to create a conditional knockout of T. gondii apical membrane antigen-1 (TgAMA1). TgAMA1 is a transmembrane protein that localizes to the parasite's micronemes, secretory organelles that discharge during invasion. AMA1 proteins are conserved among apicomplexan parasites and are of intense interest as malaria vaccine candidates. We show here that T. gondii tachyzoites depleted of TgAMA1 are severely compromised in their ability to invade host cells, providing direct genetic evidence that AMA1 functions during invasion. The TgAMA1 deficiency has no effect on microneme secretion or initial attachment of the parasite to the host cell, but it does inhibit secretion of the rhoptries, organelles whose discharge is coupled to active host cell penetration. The data suggest a model in which attachment of the parasite to the host cell occurs in two distinct stages, the second of which requires TgAMA1 and is involved in regulating rhoptry secretion.

Sarcocystis neurona merozoites express a family of immunogenic surface antigens that are orthologues of the Toxoplasma gondii surface antigens (SAGs) and SAG-related sequences

Sarcocystis neurona is a member of the Apicomplexa that causes myelitis and encephalitis in horses but normally cycles between the opossum and small mammals. Analysis of an S. neurona expressed sequence tag (EST) database revealed four paralogous proteins that exhibit clear homology to the family of surface antigens (SAGs) and SAG-related sequences of Toxoplasma gondii. The primary peptide sequences of the S. neurona proteins are consistent with the two-domain structure that has been described for the T. gondii SAGs, and each was predicted to have an amino-terminal signal peptide and a carboxyl-terminal glycolipid anchor addition site, suggesting surface localization. All four proteins were confirmed to be membrane associated and displayed on the surface of S. neurona merozoites. Due to their surface localization and homology to T. gondii surface antigens, these S. neurona proteins were designated SnSAG1, SnSAG2, SnSAG3, and SnSAG4. Consistent with their homology, the SnSAGs elicited a robust immune response in infected and immunized animals, and their conserved structure further suggests that the SnSAGs similarly serve as adhesins for attachment to host cells. Whether the S. neurona SAG family is as extensive as the T. gondii SAG family remains unresolved, but it is probable that additional SnSAGs will be revealed as more S. neurona ESTs are generated. The existence of an SnSAG family in S. neurona indicates that expression of multiple related surface antigens is not unique to the ubiquitous organism T. gondii. Instead, the SAG gene family is a common trait that presumably has an essential, conserved function(s).

The Toxoplasma gondii rhoptry protein ROP4 is secreted into the parasitophorous vacuole and becomes phosphorylated in infected cells

Many intracellular pathogens are separated from the cytosol of their host cells by a vacuole membrane. This membrane serves as a critical interface between the pathogen and the host cell, across which nutrients are imported, wastes are excreted, and communication between the two cells takes place. Very little is known about the vacuole membrane proteins mediating these processes in any host-pathogen interaction. During a screen for monoclonal antibodies against novel surface or secreted proteins of Toxoplasma gondii, we identified ROP4, a previously uncharacterized member of the ROP2 family of proteins. We report here on the sequence, posttranslational processing, and subcellular localization of ROP4, a type I transmembrane protein. Mature, processed ROP4 is localized to the rhoptries, secretory organelles at the apical end of the parasite, and is secreted from the parasite during host cell invasion. Released ROP4 associates with the vacuole membrane and becomes phosphorylated in the infected cell. Similar results are seen with ROP2. Further analysis of ROP4 showed it to be phosphorylated on multiple sites, a subset of which result from the action of either host cell protein kinase(s) or parasite kinase(s) activated by host cell factors. The localization and posttranslational modification of ROP4 and other members of the ROP2 family of proteins within the infected cell make them well situated to play important roles in vacuole membrane function.

Identification of a sporozoite-specific member of the Toxoplasma SAG superfamily via genetic complementation

Toxoplasma gondii sporozoites possess an array of stage-specific antigens that are localized to the membrane and internal cellular space, as well as secreted into the primary parasitophorous vacuole. Specific labelling of viable sporozoites excysted from oocysts reveals a complex admixture of surface proteins partially shared with tachyzoites. SAG1, SRS3 and SAG3 were detected on sporozoites as well as numerous minor antigens. In contrast, tachyzoite SAG2A and B were completely absent whereas a dominant 25 kDa protein was unique to the sporozoite surface. The sporozoite gene encoding this protein was identified in tachyzoites genetically complemented with a sporozoite cDNA library and cloned via site-specific recombination into a bacterial shuttle vector. The sporozoite cDNA identified in these experiments encoded a protein with conserved structural features of the prototypical T. gondii SAG1 (P30) and shared sequence identity with surface proteins from Sarcocystis spp. This new member of the SAG superfamily was designated SporoSAG. Expression of SporoSAG in tachyzoites conferred enhanced invasion on transgenic parasites suggesting a role for this protein in oocyst/sporozoite transmission to susceptible hosts.

Biosynthesis of glycosylphosphatidylinositol is essential to the survival of the protozoan parasite Toxoplasma gondii

The PIGA gene from Toxoplasma gondii has been cloned and characterized. Like mammalian PIGA, the transmembrane and C-terminal domains are sufficient to direct localization to the parasite endoplasmic reticulum. A functional copy of PIGA is required for tachyzoite viability, demonstrating that glycosylphosphatidylinositol biosynthesis is an essential process in T. gondii.

The SRS superfamily of Toxoplasma surface proteins

The surface of the protozoan parasite Toxoplasma gondii is coated with developmentally expressed, glycosylphosphatidylinositol-linked proteins structurally related to the highly immunogenic surface antigen SAG1. Collectively, these surface antigens are known as the SRS (SAG1-related sequences) superfamily of proteins. SRS proteins are thought to mediate attachment to host cells and activate host immunity to regulate the parasite's virulence. To better understand the number, evolution and developmental expression of SRS genes, this study has bioinformatically identified 161 unique SRS DNA sequences present in the T. gondii type II Me49 genome. The SRS superfamily of sequences phylogenetically bifurcates into two subfamilies, the prototypic members being SAG1 and SAG2A, respectively. Paralogous SRS sequences are 24-99% identical, are tandemly arrayed throughout the genome, and are present on most, if not all, chromosomes. All 11 SRS sequences on chromosomes Ia and Ib are clustered at sub-telomeric expression sites. Messenger RNA expression in the majority of SRS sequences for which multiple Expressed Sequence Tags exist is developmentally regulated. A consensus nucleotide sequence surrounding both the splice acceptor and donor sites was identified in those SRS sequences possessing an intron. Genotypic differences among SRS sequences are present at several loci (e.g. the absence of SAG5B, the truncation of SAG2D in Me49 compared with RH) indicating that different genotypes possess distinct sets of SRS sequences. Orthologous genes are restricted to tissue-dwelling coccidia (Neospora, Sarcocystis) with no related sequences present in other more distant apicomplexa such as Eimeria, Cryptosporidia, and Plasmodium spp.

Growth-inhibitory effect of heparin on Babesia parasites

We examined the inhibitory effects of three heparins on the growth of Babesia parasites. The multiplication of Babesia bovis, B. bigemina, B. equi, and B. caballi in in vitro cultures and that of B. microti in vivo were significantly inhibited in the presence of heparins, as determined by light microscopy. Treatment with various concentrations of heparin showed complete clearance of the intracellular parasites. Interestingly, a higher percentage of abnormally multidividing B. bovis parasites was observed in the presence of low concentrations of heparin. Furthermore, fluorescein isothiocyanate-labeled heparin was preferably found on the surfaces of extracellular merozoites, as detected by confocal laser scanning microscopy. These findings indicate that the heparin covers the surfaces of babesial merozoites and inhibits their subsequent invasion of erythrocytes.

Analysis of the SAG5 locus reveals a distinct genomic organisation in virulent and avirulent strains of Toxoplasma gondii

We have recently characterised, in the virulent strain RH of Toxoplasma gondii, three glycosylphosphatidylinositol-anchored surface antigens related to SAG1 (p30) and encoded by highly homologous, tandemly arrayed genes named SAG5A, SAG5B and SAG5C. In the present study, we compared the genomic organisation of the SAG5 locus in strains belonging to the three major genotypes of T. gondii. Southern blot analysis using a SAG5-specific probe produced two related but distinct hybridisation patterns, one exclusive of genotype I virulent strains, the other shared by avirulent strains of either genotype II or genotype III. To understand the molecular bases of this intergenotypic heterogeneity, we cloned and sequenced the SAG5 locus in the genotype II strain Me49. We found that in this isolate the SAG5B gene is missing, with SAG5A and SAG5C laying contiguously. This genomic arrangement explains the hybridisation profiles observed for all the avirulent strains examined and indicates that the presence of SAG5B is a distinctive trait of genotype I. Furthermore, we identified two novel SAG1-related genes, SAG5D and SAG5E, mapping respectively 1.8 and 4.0 kb upstream of SAG5A. SAG5D is transcribed in tachyzoites and encodes a polypeptide of 362 amino acids sharing 50% identity with SAG5A-C, whereas SAG5E is a transcribed pseudogene. We also evaluated polymorphisms at the SAG5 locus by comparing the coding regions of SAG5A-E from strains representative of the three archetypal genotypes. In agreement with the strict allelic dimorphism of T. gondii, we identified two alleles for SAG5D, whereas SAG5A, SAG5C and SAG5E were found to be three distinct nucleotide variants. The higher intergenotypic polymorphism of SAG5A, SAG5C and SAG5E suggests that these genes underwent a more rapid genetic drift than the other members of the SAG1 family. Finally, we developed a new PCR-restriction fragment length polymorphism method based on the SAG5C gene that is able to discriminate between strains of genotype I, II and III by a single endonuclease digestion.

Host but not parasite cholesterol controls Toxoplasma cell entry by modulating organelle discharge

Host cell cholesterol is implicated in the entry and replication of an increasing number of intracellular microbial pathogens. Although uptake of viral particles via cholesterol-enriched caveolae is increasingly well described, the requirement of cholesterol for internalization of eukaryotic pathogens is poorly understood and is likely to be partly organism specific. We examined the role of cholesterol in active host cell invasion by the protozoan parasite Toxoplasma gondii. The parasitophorous vacuole membrane (PVM) surrounding T. gondii contains cholesterol at the time of invasion. Although cholesterol-enriched parasite apical organelles termed rhoptries discharge at the time of cell entry and contribute to PVM formation, surprisingly, rhoptry cholesterol is not necessary for this process. In contrast, host plasma membrane cholesterol is incorporated into the forming PVM during invasion, through a caveolae-independent mechanism. Unexpectedly, depleting host cell plasma membrane cholesterol blocks parasite internalization by reducing the release of rhoptry proteins that are necessary for invasion. Cholesterol back-addition into host plasma membrane reverses this inhibitory effect of depletion on parasite secretion. These data define a new mechanism by which host cholesterol specifically controls entry of an intracellular pathogen.

Protein disulfide isomerase of Toxoplasma gondii is targeted by mucosal IgA antibodies in humans

Mass spectrometric analysis identified a 49 kDa antigen from Toxoplasma gondii as protein disulfide isomerase (PDI). This antigen is generally recognized by IgA in tears of healthy humans. We determined the complete open reading frame and expressed PDI recombinantly. Recombinant PDI was recognized by IgA in human tears known to contain antibodies specific for the 49 kDa antigen. High expression level and similarity to other protozoan PDIs suggest that T. gondii PDI might be a suitable target for recently described anti-protozoan drugs. PDI-specific antibodies clearly constitute part of the mucosal antibody repertoire possibly involved in defence against parasites.

Vero cell surface proteoglycan interaction with the microneme protein NcMIC(3) mediates adhesion of Neospora caninum tachyzoites to host cells unlike that in Toxoplasma gondii

Neospora caninum and Toxoplasma gondii are characterised by a very low host cell specificity, thus they are able to infect a wide range of different cells in vivo and in vitro. Infection of the host cell by tachyzoites is a process which is preceded by adhesion onto the host cell surface. The receptors on the host cell surface which would allow N. caninum to establish a physical interaction have not been investigated so far. Here we report the role of host cell surface proteoglycans as receptors for the adhesion of N. caninum tachyzoites to Vero cell monolayers. We found that N. caninum tachyzoites, similar to T. gondii tachyzoites, can bind to sulphated proteoglycans which naturally occur on the surface of mammalian cells, including heparin/heparan sulphate, chondroitin sulphates, as well as to the artificially sulphated glycosaminoglycan dextran sulphate. Although removal of heparan sulphate from the host cell surface results in decreased adhesion of T. gondii tachyzoites, binding of N. caninum tachyzoites is not affected by this treatment. Conversely, enzymatic removal of chondroitin sulphate A, B and C decreases N. caninum adhesion but does not affect T. gondii binding to Vero cells. Thus, T. gondii and N. caninum tachyzoites exhibit differential adhesive properties with regard to host cell surface glycosaminoglycans. Additional experiments employing Triton X-100 solubilised NcSRS2 and NcMIC3 showed that NcSRS2 binds to the host cell surface, but not through those sulphated glycosaminoglycans investigated in this study. In contrast, NcMIC3 binding to the host cell surface is dramatically influenced by these modifications. Further experiments showed that the NcMIC3 adhesive motif comprised of four consecutive epidermal growth factor-like domains expressed as a recombinant protein exhibits a high binding activity for sulphated glycosaminoglycans. These results suggest that host cell surface proteoglycan interaction of N. caninum differs from that observed for T. gondii, and that the epidermal growth factor-like adhesive motif in NcMIC3 could be involved in this process.

The SAG5 locus of Toxoplasma gondii encodes three novel proteins belonging to the SAG1 family of surface antigens

We have identified three novel Toxoplasma gondii proteins showing close structural similarity to molecules of the SAG1 family, a group of glycosylphosphatidylinositol-anchored surface antigens expressed by the invasive stages of T. gondii. The novel proteins, denominated SAG5A, SAG5B and SAG5C, are encoded by tandemly arrayed and tightly clustered genes containing no introns. The 367 amino acid-long SAG5B and SAG5C are 97.5% identical to each other, whereas SAG5A (362 amino acids) consists of a C-terminal domain sharing 98% identity with SAG5B and SAG5C, and an N-terminal domain whose identity to the other SAG5 polypeptides is only 42%. Expression analysis of the T. gondii strains RH (virulent) and 76 K (avirulent) showed that all members of the SAG5 cluster are transcribed in T. gondii tachyzoites and bradyzoites. However, immunoblot studies on the RH strain revealed that the synthesis of SAG5A does not occur in tachyzoites and is possibly controlled at the post-transcriptional level. On the contrary, SAG5B and SAG5C were detected by immunoblot in tachyzoite lysates and found to migrate in the 40-45 kDa range under reducing conditions or at approximately 34 kDa under unreduced conditions. Triton X-114 partitioning of tachyzoite protein lysates treated with phosphatidylinositol-specific phospholipase C indicated that SAG5B and SAG5C are glycosylphosphatidylinositol-anchored membrane-associated molecules. Consistently, immunofluorescence analysis of transformed tachyzoites over-expressing SAG5B or SAG5C showed that these molecules are targeted to the parasite surface. The characterisation of the SAG5 locus sheds further light on the complex repertoire of SAG1-related genes in T. gondii, that now comprises 14 highly homologous members and five distantly related genes belonging to the SAG2 family.