Identification of Toxoplasma gondii bradyzoite-specific monoclonal antibodies

Bicyclic azetidines target acute and chronic stages of Toxoplasma gondii by inhibiting parasite phenylalanyl t-RNA synthetase

Toxoplasma gondii commonly infects humans and while most infections are controlled by the immune response, currently approved drugs are not capable of clearing chronic infection in humans. Hence, approximately one third of the world’s human population is at risk of reactivation, potentially leading to severe sequelae. To identify new candidates for treating chronic infection, we investigated a series of compounds derived from diversity-oriented synthesis. Bicyclic azetidines are potent low nanomolar inhibitors of phenylalanine tRNA synthetase (PheRS) in T. gondii, with excellent selectivity. Biochemical and genetic studies validate PheRS as the primary target of bicyclic azetidines in T. gondii, providing a structural basis for rational design of improved analogs. Favorable pharmacokinetic properties of a lead compound provide excellent protection from acute infection and partial protection from chronic infection in an immunocompromised mouse model of toxoplasmosis. Collectively, PheRS inhibitors of the bicyclic azetidine series offer promise for treatment of chronic toxoplasmosis.

Current treatments for toxoplasmosis are limited by adverse reactions and inability to cure chronic infections dominated by semi-dormant cyst forms. Here the authors demonstrate the potential of small molecule inhibitors of PheRS for controlling acute and chronic toxoplasmosis.

Substrate-mediated regulation of the arginine transporter of Toxoplasma gondii

Intracellular parasites, such as the apicomplexan Toxoplasma gondii, are adept at scavenging nutrients from their host. However, there is little understanding of how parasites sense and respond to the changing nutrient environments they encounter during an infection. TgApiAT1, a member of the apicomplexan ApiAT family of amino acid transporters, is the major uptake route for the essential amino acid L-arginine (Arg) in T. gondii. Here, we show that the abundance of TgApiAT1, and hence the rate of uptake of Arg, is regulated by the availability of Arg in the parasite's external environment, increasing in response to decreased [Arg]. Using a luciferase-based 'biosensor' strain of T. gondii, we demonstrate that the expression of TgApiAT1 varies between different organs within the host, indicating that parasites are able to modulate TgApiAT1-dependent uptake of Arg as they encounter different nutrient environments in vivo. Finally, we show that Arg-dependent regulation of TgApiAT1 expression is post-transcriptional, mediated by an upstream open reading frame (uORF) in the TgApiAT1 transcript, and we provide evidence that the peptide encoded by this uORF is critical for mediating regulation. Together, our data reveal the mechanism by which an apicomplexan parasite responds to changes in the availability of a key nutrient.

Inhibition of nitric oxide production of activated mice peritoneal macrophages is independent of the Toxoplasma gondii strain

BACKGROUND

Toxoplasma gondii causes toxoplasmosis and is controlled by activated macrophages. However, infection of macrophages by tachyzoites induces TGF-β signaling (TGF-s) inhibiting nitric oxide (NO) production. NO inhibition may be a general escape mechanism of distinct T. gondii strains.

OBJECTIVES

To evaluate in activated macrophages the capacity of T. gondii strains of different virulence and genetics (RH, type I; ME-49, type II; VEG, type III; P-Br, recombinant) to evade the NO microbicidal defense system and determine LC3 loading to the parasitophorous vacuole.

METHODS

Activated peritoneal macrophages were infected with the different T. gondii strains, NO-production was evaluated by the Griess reagent, and inducible nitric oxide synthase expression, TGF-s, and LC3 localisation assayed by immunofluorescence.

FINDINGS

Only RH persisted in macrophages, while VEG was more resistant than P-Br and ME-49. All strains induced TGF-s, degradation of inducible nitric oxide synthase, and NO-production inhibition from 2 to 24 h of infection, but only RH sustained these alterations for 48 h. By 24 h of infection, TGF-s lowered in macrophages infected by ME-49, and P-Br, and NO-production recovered, while VEG sustained TGF-s and NO-production inhibition longer. LC3 loading to parasitophorous vacuole was strain-dependent: higher for ME-49, P-Br and VEG, lower for RH. All strains inhibited NO-production, but only RH sustained this effect probably because it persisted in macrophages due to additional evasive mechanisms as lower LC3 loading to parasitophorous vacuole.

MAIN CONCLUSIONS

These results support that T. gondii can escape the NO microbicidal defense system at the initial phase of the infection, but only the virulent strain sustain this evasion mechanism.

MAG2, a Toxoplasma gondii Bradyzoite Stage-Specific Cyst Matrix Protein

Toxoplasma gondii causes a chronic infection that affects a significant portion of the world's population, and this latent infection is the source of reactivation of toxoplasmosis. An attribute of the slowly growing bradyzoite stage of the parasite is the formation of a cyst within infected cells, allowing the parasite to escape the host's immune response. In this study, a new bradyzoite cyst matrix antigen (MAG) was identified through a hybridoma library screen. This cyst matrix antigen, matrix antigen 2 (MAG2), contains 14 tandem repeats consisting of acidic, basic, and proline residues. Immunoblotting revealed that MAG2 migrates at a level higher than its predicted molecular weight, and computational analysis showed that the structure of MAG2 is highly disordered. Cell fractionation studies indicated that MAG2 was associated with both insoluble and soluble cyst matrix material, suggesting that it interacts with the intracyst network (ICN). Examination of the kinetics of MAG2 within the cyst matrix using fluorescence recovery after photobleaching (FRAP) demonstrated that MAG2 does not readily diffuse within the cyst matrix. Kinetic studies of MAG1 demonstrated that this protein has different diffusion kinetics in tachyzoite and bradyzoite vacuoles and that its mobility is not altered in the absence of MAG2. In addition, deletion of MAG2 does not influence growth, cystogenesis, or cyst morphology.IMPORTANCE This report expands on the list of characterized Toxoplasma gondii cyst matrix proteins. Using fluorescence recovery after photobleaching (FRAP), we have shown that matrix proteins within the cyst matrix are not mainly in a mobile state, providing further evidence of how proteins behave within the cyst matrix. Understanding the proteins expressed during the bradyzoite stage of the parasite reveals how the parasite functions during chronic infection.

Toxoplasma gondii-positive human sera recognise intracellular tachyzoites and bradyzoites with diverse patterns of immunoreactivity

Antibody detection assays have long been the first line test to confirm infection with the zoonotic parasite Toxoplasma gondii. However, challenges exist with serological diagnosis, especially distinguishing between acute, latent and reactivation disease states. The sensitivity and specificity of serological tests might be improved by testing for antibodies against parasite antigens other than those typically found on the parasite surface during the acute stage. To this end, we analysed the reactivity profile of human sera, identified as positive for anti-Toxoplasma gondii IgG in traditional assays, by indirect immunofluorescence reactivity to acute stage intracellular tachyzoites and in vitro-induced latent stage bradyzoites. The majority of anti-Toxoplasma gondii IgG positive sera recognised both intracellularly replicating tachyzoites and in vitro-induced bradyzoites with varying patterns of immune-reactivity. Furthermore, anti-bradyzoite antibodies were not detected in sera that were IgM-positive/IgG-negative. These results demonstrate that anti-Toxoplasma gondii-positive sera may contain antibodies to a variety of antigens in addition to those traditionally used in serological tests, and suggest the need for further investigations into the utility of anti-bradyzoite-specific antibodies to aid in diagnosis of Toxoplasma gondii infection.

Importance of serological cross-reactivity among Toxoplasma gondii, Hammondia spp., Neospora spp., Sarcocystis spp. and Besnoitia besnoiti

Toxoplasma gondii, Neospora spp., Sarcocystis spp., Hammondia spp. and Besnoitia besnoiti are genetically related cyst-forming coccidia. Serology is frequently used for the identification of T. gondii, Neospora spp. and B. besnoiti-exposed individuals. Serologic cross-reactions occur in different tests among animals infected with T. gondii and H. hammondi, as well as among animals infected by T. gondii and N. caninum. Infections caused by N. caninum and N. hughesi are almost indistinguishable by serology. Neospora caninum, B. besnoiti and Sarcocystis spp. infections in cattle show some degree of serologic cross-reactivity. Antibody cross-reactivity between Neospora spp. and H. heydorni-infected animals is suspected, but not proven to occur. We review serologic cross-reactivity among animals and/or humans infected with T. gondii, Neospora spp., Sarcocystis spp., Hammondia spp. and B. besnoiti. Emphasis is laid upon antigens and serological methods for N. caninum diagnosis which were tested for cross-reactivity with related protozoa. Species-specific antigens, as well as stage-specific proteins have been identified in some of these parasites and have promising use for diagnosis and epidemiological surveys.

Characterization of an IgG monoclonal antibody targeted to both tissue cyst and sporocyst walls of Toxoplasma gondii

Toxoplasma gondii infects animals habiting terrestrial and aquatic environments. Its oocysts and tissue cysts are important for the horizontal transmission of this parasite. The oocyst and tissue cyst walls are crucial for the ability of the parasite to persist in the environment or in animal tissues, respectively. However, the composition of these walls is not well understood. We report the generation of monoclonal antibodies directed against wall components using mice immunized with oocyst antigens of T. gondii. One monoclonal antibody (mAb) G1/19 reacted solely with T. gondii sporozoites. The respective antigen had a relative molecular weight (Mr) of 30 kDa. MAb G1/19 failed to react with sporozoites of any other coccidian parasite species tested (Hammondia hammondi, Hammondia heydorni, Cystoisospora felis, Eimeria bovis, Sarcocystis sp.). Another mAb, designated K8/15-15, recognized antigens in sporocyst walls of the parasite and in the walls of in vivo or in vitro produced tissue cysts, as demonstrated by immunofluorescence and immunoblot assays. Antigens of 80 to a high molecular weight protein of about 350 kDa Mr were recognized by this antibody using antigen extracts from sporocysts, and from in vitro or in vivo generated tissue cysts of the parasite. Tissue cyst and sporocyst walls of H. hammondi and H. heydorni, and tissue cysts of Neospora caninum were also recognized by mAb K8/15-15. Sporocyst walls of C. felis also reacted to this mAb. The cyst walls of Sarcocystis sp. and Besnoitia besnoiti were not recognized by mAb K8/15-15. Reactivity by a single mAb against T. gondii antigens in tissue cysts and sporocysts had not been reported previously. MAb K8/15-15 may be a practical tool for the identification of both cysts and sporocysts of the parasite, and may also be potentially employed in proteomic studies on the identification of new components of the cyst and sporocyst walls of T. gondii.

The Toxoplasma gondii cyst wall protein CST1 is critical for cyst wall integrity and promotes bradyzoite persistence

Toxoplasma gondii infects up to one third of the world's population. A key to the success of T. gondii as a parasite is its ability to persist for the life of its host as bradyzoites within tissue cysts. The glycosylated cyst wall is the key structural feature that facilitates persistence and oral transmission of this parasite. Because most of the antibodies and reagents that recognize the cyst wall recognize carbohydrates, identification of the components of the cyst wall has been technically challenging. We have identified CST1 (TGME49_064660) as a 250 kDa SRS (SAG1 related sequence) domain protein with a large mucin-like domain. CST1 is responsible for the Dolichos biflorus Agglutinin (DBA) lectin binding characteristic of T. gondii cysts. Deletion of CST1 results in reduced cyst number and a fragile brain cyst phenotype characterized by a thinning and disruption of the underlying region of the cyst wall. These defects are reversed by complementation of CST1. Additional complementation experiments demonstrate that the CST1-mucin domain is necessary for the formation of a normal cyst wall structure, the ability of the cyst to resist mechanical stress, and binding of DBA to the cyst wall. RNA-seq transcriptome analysis demonstrated dysregulation of bradyzoite genes within the various cst1 mutants. These results indicate that CST1 functions as a key structural component that confers essential sturdiness to the T. gondii tissue cyst critical for persistence of bradyzoite forms.

Toxoplasma gondii causes severe encephalitis in immune compromised hosts after reactivation of brain cysts that persist for the life span of the host. The biological mechanisms of bradyzoite persistence within cysts are not fully understood. The glycosylated cyst wall is thought to play a crucial role in survival of bradyzoites during chronic infection as well as successful oral transmission of infection. Here we have identified the gene encoding cyst wall glycoprotein CST1. When we delete the CST1 gene, parasites form dramatically fragile brain cysts. Parasites lacking CST1 develop fewer brain cysts, show dysregulation of bradyzoite-specific gene expression and are less able to grow under stressed conditions. The rescue of these phenotypes requires the heavily glycosylated mucin domain of CST1. These studies demonstrate that the glycosylation of CST1 plays a significant role in the structural integrity and persistence of brain cysts. Agents that perturb CST1 glycosylation have the potential to disrupt formation of latent brain cysts, preventing chronic Toxoplasma infection.

Targeted disruption of Toxoplasma gondii serine protease inhibitor 1 increases bradyzoite cyst formation in vitro and parasite tissue burden in mice

As an intracellular protozoan parasite, Toxoplasma gondii is likely to exploit proteases for host cell invasion, acquisition of nutrients, avoidance of host protective responses, escape from the parasitophorous vacuole, differentiation, and other activities. T. gondii serine protease inhibitor 1 (TgPI1) is the most abundantly expressed protease inhibitor in parasite tachyzoites. We show here that alternative splicing produces two TgPI1 isoforms, both of which are secreted via dense granules into the parasitophorous vacuole shortly after invasion, become progressively more abundant over the course of the infectious cycle, and can be detected in the infected host cell cytoplasm. To investigate TgPI1 function, the endogenous genomic locus was disrupted in the RH strain background. ΔTgPI1 parasites replicate normally as tachyzoites but exhibit increased bradyzoite gene transcription and labeling of vacuoles with Dolichos biflorus lectin under conditions promoting in vitro differentiation. The differentiation phenotype can be partially complemented by either TgPI1 isoform. Mice infected with the ΔTgPI1 mutant display ∼3-fold-increased parasite burden in the spleen and liver, and this in vivo phenotype is also complemented by either TgPI1 isoform. These results demonstrate that TgPI1 influences both parasite virulence and bradyzoite differentiation, presumably by inhibiting parasite and/or host serine proteases.

A Purification Method for Enrichment of the Toxoplasma gondii Cyst Wall

The tissue cyst wall of Toxoplasma gondii is a stage-specific structure that is produced by modification of the bradyzoite-containing parasitophorous vacuole. It is a limiting membrane structure and is critically important for cyst survival and transmission of infection. Studies on the structure and function of the cyst wall should provide new therapeutic strategies for the elimination or prevention of latency during T. gondii infection. The membrane proteins of the T. gondii cyst are an important target for studies of the biochemical and immunological function(s) of the cyst. However, the components of the cyst membrane have been poorly characterized due to the difficulty of purification of these membrane proteins. We developed a lectin DBA (Dolichos biflorus) coated magnetic bead isolation method to isolate T. gondii cyst wall proteins. Our data suggests that this method can isolate cyst wall proteins from both in vitro cell culture or in vivo mouse brain derived tissue cysts. Antibodies to these isolated protein preparations were shown to localize to the cyst wall.

Temporal and spatial distribution of Toxoplasma gondii differentiation into Bradyzoites and tissue cyst formation in vivo

During Toxoplasma gondii infection, a fraction of the multiplying parasites, the tachyzoites, converts into bradyzoites, a dormant stage, which form tissue cysts localized mainly in brain, heart, and skeletal muscles that persist for several years after infection. At this stage the parasite is protected from the immune system, and it is believed to be inaccessible to drugs. While the long persistence of tissue cysts does not represent a medical problem for healthy individuals, this condition represents a major risk for patients with a compromised immune system, who can develop recrudescent life-threatening T. gondii infections. We have investigated for the first time the dynamics and the kinetics of tachyzoite-to-bradyzoite interconversion and cyst formation in vivo by using stage-specific bioluminescent parasites in a mouse model. Our findings provide a new framework for understanding the process of bradyzoite differentiation in vivo. We have also demonstrated that complex molecules such as d-luciferin have access to tissue cysts and are metabolically processed, thus providing a rationale for developing drugs that attack the parasite at this developmental stage.

A novel actin-related protein is associated with daughter cell formation in Toxoplasma gondii

Cell division in Toxoplasma gondii occurs by an unusual budding mechanism termed endodyogeny, during which twin daughters are formed within the body of the mother cell. Cytokinesis begins with the coordinated assembly of the inner membrane complex (IMC), which surrounds the growing daughter cells. The IMC is compiled of both flattened membrane cisternae and subpellicular filaments composed of articulin-like proteins attached to underlying singlet microtubules. While proteins that comprise the elongating IMC have been described, little is known about its initial formation. Using Toxoplasma as a model system, we demonstrate that actin-like protein 1 (ALP1) is partially redistributed to the IMC at early stages in its formation. Immunoelectron microscopy localized ALP1 to a discrete region of the nuclear envelope, on transport vesicles, and on the nascent IMC of the daughter cells prior to the arrival of proteins such as IMC-1. The overexpression of ALP1 under the control of a strong constitutive promoter disrupted the formation of the daughter cell IMC, leading to delayed growth and defects in nuclear and apicoplast segregation. Collectively, these data suggest that ALP1 participates in the formation of daughter cell membranes during cell division in apicomplexan parasites.

Toxoplasma gondii strains defective in oral transmission are also defective in developmental stage differentiation

Toxoplasma gondii undergoes differentiation from rapidly growing tachyzoites to slowly growing bradyzoites during its life cycle in the intermediate host, and conversion can be induced in vitro by stress. Representative strains of the three clonal lineages showed equal capacity to differentiate into bradyzoites in vitro, as evidenced by induction of bradyzoite antigen 1, staining with Dolichos biflorus lectin (DBL), pepsin resistance, and oral infectivity in mice. We also examined several recently described exotic strains of T. gondii, which are genetically diverse and have a different ancestry from the clonal lineages. The exotic strain COUG was essentially like the clonal lineages and showed a high capacity to induce bradyzoites in vitro and in vivo, consistent with its ability to be efficiently transmitted by the oral route. In contrast, exotic strains MAS and FOU, which are defective in oral transmission, showed a decreased potential to develop into bradyzoites in vitro. This defect was evident from reduced staining with DBL and the cyst antigen CST1, failure to down-regulate tachyzoite antigens, such as tachyzoite surface antigens 1 and 2A, and decreased resistance to pepsin treatment. Despite normal in vitro differentiation, the exotic strains CAST and GPHT also showed decreased oral transmission, due to formation of smaller cysts and a lower tissue burden during chronic infection, traits also shared by MAS and FOU. Collectively, these findings reveal that the limited oral transmission in some strains of T. gondii is due to inefficient differentiation to the bradyzoite form, leading to defects in the formation of tissue cysts.

Developmental differentiation between tachyzoites and bradyzoites of Toxoplasma gondii

An important event in the pathogenesis of toxoplasmosis is the interconversion between the bradyzoite and the tachyzoite stage of Toxoplasma gondii within the intermediate host. The factors that influence either cyst formation (bradyzoites) or reactivation (tachyzoites) are unknown. Uwe Gross, Wolfgang Bohne, Martine Soête and Jean François Dubremetz here describe current knowledge about the mechanisms that might lead to the induction of stage differentiation of this protozoan parasite.

In vitro induction of Neospora caninum bradyzoites in vero cells reveals differential antigen expression, localization, and host-cell recognition of tachyzoites and bradyzoites

We report on an optimized method for the in vitro culture of tissue cyst-forming Neospora caninum bradyzoites in Vero cells and the separation of viable parasites from host cells. Treatment of tachyzoite-infected Vero cell cultures with 17 microM sodium nitroprusside for 8 days severely scaled down parasite proliferation, led to reduced expression of tachyzoite surface antigens, and induced the expression of the bradyzoite marker NcBAG1 and the cyst wall antigen recognized by the monoclonal antibody MAbCC2. Transmission electron microscopy demonstrated that intracellular parasites were located within parasitophorous vacuoles that were surrounded by a cyst wall-like structure, and the dense granule antigens NcGRA1, NcGRA2, and NcGRA7 were incorporated into the cyst wall. Adhesion-invasion assays employing purified tachyzoites and bradyzoites showed that tachyzoites adhered to, and invaded, Vero cells with higher efficiency than bradyzoites. However, removal of terminal sialic acid residues from either the host cell or the parasite surface increased the invasion of Vero cells by bradyzoites, but not tachyzoites.

The role of stage-specific oligonucleotide primers in providing effective laboratory support for the molecular diagnosis of reactivated Toxoplasma gondii encephalitis in patients with AIDS

The switch from bradyzoites to tachyzoites is the fundamental pathogenic event that leads to Toxoplasma gondii encephalitis (TE) in patients with AIDS. Distinction between these stages is difficult, particularly when specific treatment has been started. A new approach consisting of a nested PCR (n-PCR) assay was performed on cerebrospinal fluid (CSF) specimens collected from AIDS patients with TE before or after antiparasitic therapy was initiated, to assess the efficacy of primer sets which amplify target sequences expressed on bradyzoites (SAG4 and MAG1), tachyzoites (SAG1) or both stages (B1) of T. gondii. CSF specimens were obtained from 46 patients with AIDS, of whom 27 had TE (16 first episode, 11 relapse) and 19 had other AIDS-related brain lesions (AIDS-OBL) in the absence of TE. CSF specimens from 26 HIV-negative and immunocompetent patients were also checked. All samples were tested with different primer pairs targeting the B1, SAG-1, SAG-4 and MAG-1 genes. With B1, 75% of patients with first episodes of TE were positive, compared with 36.3% of those with relapse of TE and 5.2% of those with AIDS-OLB. The SAG1 gene yielded positive values in 28.7% and 45.4% of patients with first episodes of TE or relapse of TE, respectively, and in none of the controls. With the SAG4 and MAG1 genes, 72.7% of patients with relapse of TE were detected, compared with 25% of patients with first episodes of TE and 5.2% with AIDS-OLB. None of the HIV-negative subjects showed positive PCR reactions. These results demonstrate that specific primers for the genes SAG4, MAG1 and SAG1 may be useful in AIDS patients with relapse of TE, in whom the use of PCR targeting the B1 gene may fail to detect DNA, especially when prophylaxis or treatment has been started.

Differential expression of two plant-like enolases with distinct enzymatic and antigenic properties during stage conversion of the protozoan parasite Toxoplasma gondii

The precise molecular mechanisms underlying the switch between the two developmental stages of Toxoplasma gondii, and the metabolic adaptations occurring during this stage conversion are poorly understood. Because inhibitors of mitochondrial respiration are known to trigger differentiation from tachyzoite into bradyzoite stages, we believe that some of the switch components may be sought in the regulation of central carbohydrate metabolism. We have previously described a cDNA encoding a bradyzoite-specific enolase, ENO1. We now report the isolation and characterization of another enolase-encoding cDNA (ENO2) that is expressed preferentially in the tachyzoite stage. The deduced amino acid sequences of ENO1 and ENO2 share 73.65 % identity. They both display significant homologies to plant enolases with the presence of two plant-like peptide insertions, a pentapeptide EWGW(Y)C(S) and a dipeptide EK (or DK). We demonstrate that deletions of the ENO1 pentapeptide motif on its own or together with the dipeptide reduce drastically the affinity for the 2PGA substrate, suggesting that the evolutionary acquisition of these peptides in enolases of land plants and apicomplexan parasites contribute a specific function to their enzymatic activities. T. gondii ENO1 and ENO2 were also expressed as active recombinant enzymes in Escherichia coli. While ENO1 and ENO2 display similar K(m) values, the pure tachyzoite-specific enzyme (ENO2) has a threefold specific activity at V(max) compared with that of the bradyzoite-specific enolase (ENO1). Moreover, immunoblot analyses performed using polyclonal antibodies raised against the recombinant enzymes revealed that the native enolase in tachyzoite and bradyzoite are also antigenically distinct. Taken together, our results indicate that the differences witnessed between the two activities may be instrumental in maintaining glycolysis in pace with the distinct stage-specific requirements of carbohydrate metabolism.

Cyclic nucleotide signaling in Toxoplasma gondii bradyzoite differentiation

The ability of Toxoplasma gondii tachyzoites to differentiate into latent bradyzoite forms is essential for pathogenesis of clinical disease. We examined the effects of cyclic nucleotides on T. gondii bradyzoite differentiation in vitro. Differentiation of tachyzoites to bradyzoites was measured in an immunofluorescence assay using ME49 or its clonal derivative PLK, two well-characterized T. gondii strains. Treatment of human fibroblast cultures infected with T. gondii with 8-(4-chlorophenylthio)-cyclic GMP (CPT-cGMP), a membrane-permeable, nonhydrolyzable analogue of cGMP, resulted in an increased percentage of bradyzoite-positive vacuoles. Cyclic AMP (cAMP) also induced in vitro conversion of PLK, but the method of cAMP elevation was critical. Forskolin raises cAMP levels transiently and induced bradyzoites, whereas agents predicted to cause sustained elevation of cAMP were inhibitory to parasite conversion. Levels of cAMP were measured in host cells and extracellular tachyzoites. Forskolin, CPT-cGMP, and agents known to induce bradyzoite formation elevated cAMP in host cells and PLK parasites. These data suggest cyclic nucleotide signaling pathways are important in the stress-induced conversion of T. gondii tachyzoites to bradyzoites. Furthermore, because cAMP elevation was seen in PLK but not RH, a T. gondii strain that did not differentiate well in our assay, cAMP signaling within the parasite is likely to be critical.

The development and biology of bradyzoites of Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite of mammals and birds that is an important human pathogen. Infection with this Apicomplexan parasite results in its dissemination throughout its host via the tachyzoite life-stage. After dissemination these tachyzoites differentiate into bradyzoites within cysts that remain latent. These bradyzoites can transform back into tachyzoites and in immunosupressed individuals this often results in symptomatic disease. Both tachyzoites and bradyzoites develop in tissue culture and thus this crucial differentiation event can be studied. Recent advances in the genetic manipulation of T. gondii have expanded the molecular tools that can be applied to studies on bradyzoite differentiation. Evidence is accumulating that this differentiation event is stress mediated and may share common pathways with other stress-induced differentiation events in other eukaryotic organisms. Study of the stress response and signaling pathways are areas of active research in this organism. In addition, characterization of unique bradyzoite-specific structures, such as the cyst wall, should lead to a further understanding of T. gondii biology. This review focuses on the biology and development of bradyzoites and current approaches to the study of the tachyzoite to bradyzoite differentiation process.

The in vitro development of Neospora caninum bradyzoites

Neospora caninum is a recently identified apicomplexan protozoan parasite that is closely related to Toxoplasma gondii. Neospora caninum is of significant economic importance as it causes neurological disease and abortion in numerous animals. Antibodies to BAG1/hsp30 (also known as BAG5), a T. gondii bradyzoite-specific protein, have been demonstrated to react with N. caninum tissue cysts in vivo. Bradyzoite differentiation of N. caninum in vitro was investigated using culture conditions previously utilised for T. gondii in vitro bradyzoite development. Utilising the NC-Liverpool isolate of N. caninum, cyst-like structures developed within 3-4 days of culture of this parasite in human fibroblasts. In addition, an antigen reacting with mAb 74.1.8 (anti-BAG1) and rabbit anti-recombinant BAGI was demonstrable by immunofluorescence, fluorescence-activated cell sorter, and immunoblot analyses. Expression of this antigen was increased by stress conditions, similar to that which has been described for T. gondii bradyzoite induction. Cyst-wall formation in vitro, as assayed by lectin binding, did not occur as readily for N. caninum as it does for T. gondii.

Disruption of the Toxoplasma gondii bradyzoite-specific gene BAG1 decreases in vivo cyst formation

The bradyzoite stage of the Apicomplexan protozoan parasite Toxoplasma gondii plays a critical role in maintenance of latent infection. We reported previously the cloning of a bradyzoite-specific gene BAG1/hsp30 (previously referred to as BAG5) encoding a cytoplasmic antigen related to small heat shock proteins. We have now disrupted BAG1 in the T. gondii PLK strain by homologous recombination. H7, a cloned null mutant, and Y8, a control positive for both cat and BAG1, were chosen for further characterization. Immunofluorescence and Western blot analysis of bradyzoites with BAG1 antisera demonstrated expression of BAG1 in the Y8 and the PLK strain but no expression in H7. All three strains expressed a 116 kDa bradyzoite cyst wall antigen, a 29 kDa matrix antigen and the 65 kDa matrix reactive antigen MAG1. Mice inoculated with H7 parasites formed significantly fewer cysts than those inoculated with the Y8 and the PLK strains. H7 parasites were complemented with BAG1 using phleomycin selection. Cyst formation in vivo for the BAG1-complemented H7 parasites was similar to wild-type parasites. We therefore conclude that BAG1 is not essential for cyst formation, but facilitates formation of cysts in vivo.

Bradyzoite development in Toxoplasma gondii and the hsp70 stress response

Toxoplasma gondii is a well-described ubiquitous Apicomplexan protozoan parasite that is an important opportunistic pathogen. The factors affecting the transition of tachyzoites to the latent bradyzoite stage remain to be defined. The induction of bradyzoite development in vitro has been linked to temperature, pH, mitochondrial inhibitors, sodium arsenite, and many of the other stressors associated with heat shock protein (hsp) induction. There is evidence for other organisms that hsps are developmentally regulated. Therefore, we examined whether hsp induction is an early event in bradyzoite differentiation. Extracellular and intracellular T. gondii cells, after exposure to pH 8.1 or 7.1, were analyzed for the expression of inducible hsp70 by using monoclonal antibody C92F3A-5 (specific to hsp70). Western blotting demonstrated that a 72-kDa protein reactive with C92F3A-5 (hsp70), which we believe is part of the hsp70 family, is induced during bradyzoite development. By immunofluorescence and immunoelectron microscopy, we were able to demonstrate that hsp70 staining colocalized to T. gondii expressing bradyzoite-specific antigens and the presence of hsp70 in bradyzoites isolated from mouse brain. Quercetin, a bioflavonoid which inhibits the synthesis of hsp90, hsp70, and hsp27, suppresses the induction of bradyzoite development in vitro. Reverse transcription-PCR with conserved hsp70 primers demonstrated an increase in hsp70 in T. gondii on exposure to conditions which induce bradyzoite formation. A T. gondii hsp70 was subsequently cloned and sequenced by using this amplified fragment. We believe our evidence suggests that hsps are important in the process of bradyzoite differentiation.

Structures of Toxoplasma gondii tachyzoites, bradyzoites, and sporozoites and biology and development of tissue cysts

Infections by the protozoan parasite Toxoplasma gondii are widely prevalent world-wide in animals and humans. This paper reviews the life cycle; the structure of tachyzoites, bradyzoites, oocysts, sporocysts, sporozoites and enteroepithelial stages of T. gondii; and the mode of penetration of T. gondii. The review provides a detailed account of the biology of tissue cysts and bradyzoites including in vivo and in vitro development, methods of separation from host tissue, tissue cyst rupture, and relapse. The mechanism of in vivo and in vitro stage conversion from sporozoites to tachyzoites to bradyzoites and from bradyzoites to tachyzoites to bradyzoites is also discussed.

Toxoplasma gondii expresses two distinct lactate dehydrogenase homologous genes during its life cycle in intermediate hosts

Two Toxoplasma gondii genes were characterized that are differentially expressed during the parasite's life cycle. The genes named LDH1 and LDH2, respectively, encode polypeptides similar to the enzyme lactate dehydrogenase (LDH; L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) from a variety of organisms. They show 64.0% nucleotide identity in the coding region and both have an intron at the same relative position. The deduced amino acid sequences of LDH1 and LDH2 share 71.1% identity. LDH1 and LDH2 are most similar to an LDH of Plasmodium falciparum (46.5% and 48.5% amino acid identities, respectively). The mRNA of LDH2 was only detected in the bradyzoite stage, while the mRNA of LDH1 was detected in both the bradyzoite and tachyzoite stages. However, by isoelectric focusing and immunoblot analysis, only one LDH isoform was found to be expressed in each stage. Furthermore, the expression of a reporter gene carrying chloramphenicol acetyltransferase (CAT) coding sequence and the putative LDH2 promoter sequence was significantly up-regulated by growing parasites in tissue culture in media with alkaline pH (pH 8.2, a condition known to induce the expression of bradyzoite-specific antigens), while the expression of a CAT reporter construct carrying the putative LDH1 promoter sequence was down-regulated by similar treatment. These results indicate that LDH expression is developmentally regulated in T. gondii and suggest a possible correlation between stage conversion and alteration in carbohydrate or energy metabolism in this parasite.

Molecular cloning of the Toxoplasma gondii sag4 gene encoding an 18 kDa bradyzoite specific surface protein

An 18 kDa bradyzoite specific surface protein of Toxoplasma gondii (T. gondii) has been purified by affinity chromatography with a specific monoclonal antibody using parasites grown in vitro under conditions inducing the biosynthesis of bradyzoite specific proteins. N-terminal and internal amino acid sequences obtained by microsequencing enabled us to design degenerate oligonucleotides. A fragment of 187 bp was amplified by polymerase chain reaction (PCR). It was used as a probe to clone a 4 kb-Bam HI fragment encompassing the gene encoding the 18 kDa protein. Nucleotide sequence analysis revealed a single open reading frame of 516 nucleotides encoding a 172 amino acid protein. The deduced amino acid sequence matched perfectly the peptides microsequenced from the native protein. The N-terminal hydrophobic region was found to possess the characteristics of a signal peptide of 27 amino acids. The hydrophobic C-terminal part could represent a signal for a glycan-phosphoinositide anchor. The full-length cDNA was also isolated and both the 5' and 3' untranslated regions were determined. Reverse transcriptase-PCR (RT-PCR) using p18-specific primers showed a stage-specific expression of this gene. Comparison of the nucleic acid sequence and the predicted amino acid sequence with databases did not reveal significant homology with known genes or proteins. This gene is proposed to be named sag4, according to the existing T. gondii nomenclature.

A ubiquitous intracellular parasite: the cellular biology of Toxoplasma gondii

Toxoplasma gondii shares many features with other apicomplexan parasites but is unusual in its extremely broad host and tissue specificity. The parasite exhibits typical 'zoite' morphology, its highly polar structure being dictated by the complex cytoskeleton. Molecules on the surface of the zoite are prime candidates for interaction with the host cell and in vitro assays have implicated 2 of the 5 tachyzoite surface molecules in invasion: SAG1 as a ligand mediating host cell invasion, and SAG2 in enabling reorientation prior to invasion. The functional roles of other molecules, secreted from internal organelles during invasion and intracellular development, are also becoming clear through immuno-EM and biochemical studies, and from sequence data. Molecules from the rhoptries including the penetration enhancing factor ROP1 are secreted at the point of invasion and are integral to the newly formed parasitophorous vacuole membrane. Release of the dense granule molecules GRA 1-6, appears to be calcium regulated and occurs within 10 min of invasion leading to formation of the tubular membranous network and stabilization of the vacuole. The interaction between Toxoplasma and the host cell is stage specific. The tachyzoite divides rapidly and synchronously forming rosettes and causing host cell lysis, while the bradyzoite exhibits slow asynchronous division secreting a granular matrix and becoming enclosed within a cyst wall. This altered phenotype is a reflection of changes in gene expression. Bradyzoite specific molecules are found internally, on the parasite surface, and in the cyst matrix while important tachyzoite proteins such as SAG1 and SAG2 are downregulated. Differentiation between the 2 stages is reversible and is influenced by immunomodulatory agents. However a strong genetic element is involved and it is notable that virulent strains show a very low frequency of cyst production.

Monoclonal rat antibodies directed against Toxoplasma gondii suitable for studying tachyzoite-bradyzoite interconversion in vivo

We previously reported the in vitro analysis of stage differentiation of Toxoplasma gondii in murine bone marrow-derived macrophages. The purpose of this study was to generate monoclonal rat antibodies that might be suitable for investigating tachyzoite-bradyzoite interconversion in vivo with the murine model. Immunization of Fischer rats with cysts of T. gondii NTE resulted in the generation of seven monoclonal antibodies of the immunoglobulin G2a, G2b, or M isotype, which were further characterized by the immunoblot technique, immunofluorescence assay, immunohistology, and immunoelectron microscopy. Immunoblots demonstrated specific reactivity of five monoclonal antibodies with proteins with molecular masses of 40, 52, 55, 60, 64, 65, and 115 kDa. One antibody (CC2) appeared to recognize a differently expressed antigen depending on the parasite stage, reacting with a 40-kDa molecule in tachyzoites and a 115-kDa antigen in bradyzoites and oocysts. Several other monoclonal antibodies were shown to be stage specific and to react in immunofluorescence assays or in immunoblots with either tachyzoites or bradyzoites. Kinetics of stage conversion in vitro could be monitored by immunofluorescence with two of these monoclonal antibodies. Preliminary immunohistological investigations of tissue sections from infected mice demonstrated the possible usefulness of these monoclonal antibodies for future in vivo studies on stage differentiation of T. gondii in the murine system.

Cloning and characterization of a bradyzoite-specifically expressed gene (hsp30/bag1) of Toxoplasma gondii, related to genes encoding small heat-shock proteins of plants

Stage conversion between the tachyzoite and bradyzoite forms of the protozoan parasite Toxoplasma gondii is an important aspect in the pathogenesis of toxoplasmosis. In an initial investigation of molecular regulation of stage conversion in T. gondii, we describe the cloning and characterization of a bradyzoite-specifically expressed gene (hsp30/bag1). Bradyzoite formation was induced in cell culture by alkaline pH, and this was followed by purification of this parasitic stage using magnetic cell sorting. A bradyzoite cDNA library was constructed by random amplification using the polymerase chain reaction. Screening with a bradyzoite-specific monoclonal antibody identified a reactive clone. The amino acid sequence derived from the 687 bp open reading frame showed similarity to the conserved C-terminal region of small heat-shock proteins from plants. Stage-specific expression of the naturally occurring 30 kDa antigen in bradyzoites was confirmed by polyclonal antisera generated against the recombinant antigen. Immunoelectron microscopy indicated a cytosolic location of this antigen in bradyzoites. The expression of HSP30/BAG1 seems to be regulated at the mRNA level, since reverse polymerase chain reaction using bradyzoite-specific primers amplified transcripts in bradyzoites only, not in tachyzoites.

Identification of Eimeria bovis merozoite cDNAs using differential mRNA display

Differences in gene expression between Eimeria bovis sporozoites and first-generation merozoites were analyzed using the technique of differential mRNA display. Approx. 5% of the sequences detected in first-generation merozoites appear to be unique relative to sporozoites. Several of the bands corresponding to merozoite-specific gene expression were isolated and cloned. Northern blot analysis revealed that the cDNA fragments DMZ-7, DMZ-8 and NMZ-6 hybridized to mRNAs expressed at > 50-fold higher levels in merozoites relative to sporozoites. A fourth cDNA fragment, NMZ-4, hybridized to a mRNA expressed at 3-fold higher levels in merozoites. Further characterization demonstrated that expression of DMZ-8 in E. bovis-infected bovine cells begins as early as 12 h after sporozoite invasion and continues throughout the entire 14 days of first-generation schizogony. Sequence analysis of each of the four merozoite cDNAs failed to identify any significant similarity to any entries in the GenBank database, suggesting that these developmentally regulated genes may be unique to coccidian parasites.

Toxoplasma gondii: reactivity of murine sera against tachyzoite and cyst antigens via FAST-ELISA

The murine serological response to Toxoplasma gondii tachyzoite and cyst antigens was determined using FAST-ELISA. The serum IgG response to tachyzoite antigen was much stronger than that to cyst antigen. Adsorption of immune sera with tachyzoite antigen sharply reduced the reactivity in ELISA with tachyzoite antigen, but had no effect on the titre against cyst antigen, implying that there is virtually no antigenic overlap between the 2 stages. In sequential sera from infected mice the IgG antibody response against tachyzoites was always higher than the response to cyst antigen, whereas the IgM response to cysts was always higher than that to tachyzoites and remained detectable for at least 11 months.

Molecular characterization of a 65-kilodalton Toxoplasma gondii antigen expressed abundantly in the matrix of tissue cysts

We describe the cloning and characterization of a novel antigen expressed in the bradyzoite stage of Toxoplasma gondii. A cDNA library was constructed in bacteriophage lambda gt11 Sfi-Not using messenger RNA molecules isolated from cysts of the ME49 strain of T. gondii. The recombinant phage library was subjected to screening with polyclonal antibodies against bradyzoite antigens. This screening identified a recombinant antigen that was recognized strongly by polyclonal antibodies against bradyzoite antigens as well as by sera from mice chronically infected with T. gondii. The native antigen is a protein of 65 kDa that localized to the matrix of the cyst and the cyst wall surrounding the bradyzoites. The antigen was found to be expressed abundantly in cysts but could not be detected in tachyzoites or within the parasitophorous vacuole of tachyzoite infected host cells. Genomic and cDNA sequence of the gene revealed an open reading frame encoding 452 amino acids interrupted by 2 introns: a 503-bp intron located in the 5' untranslated region preceding the protein coding sequence and a 110-bp intron located 95 bp downstream of the first ATG.

Reduced replication of Toxoplasma gondii is necessary for induction of bradyzoite-specific antigens: a possible role for nitric oxide in triggering stage conversion

Stage conversion between tachyzoites and bradyzoites of Toxoplasma gondii was investigated in vitro by using murine bone marrow-derived macrophages (BMMs) as host cells. Following infection of untreated BMMs with tachyzoites, spontaneous expression of bradyzoite-specific antigens (Bsa) occurred at low frequency with Toxoplasma strain-dependent ratios from 0.03 to 2%. As previously described for peritoneal macrophages, activation of tachyzoite-infected BMMs with gamma interferon (IFN-gamma) or lipopolysaccharide resulted in the induction of Bsa. When bradyzoites were used for infection, prolonged expression of Bsa could be observed in IFN-gamma-activated BMMs. The induction of Bsa expression seemed to be closely linked to parasite multiplication and increased to maximal values of 50 to 70% in intermediately activated macrophages with nitric oxide (NO) levels that allowed reduced parasite replication. Identical results in stage conversion were obtained when sodium nitroprusside was used as a source of exogenous NO, indicating that NO might be a molecular trigger of stage conversion. NO is reactive with iron-sulfur centers in proteins, thereby inhibiting proteins involved in the mitochondrial respiratory chain. Using oligomycin and antimycin A as inhibitors of mitochondrial function, growth inhibition of parasites and induction of Bsa were obtained. Since microglia are the functional correlates of macrophages in the central nervous system and inhibit T. gondii upon activation with IFN-gamma, a similar mechanism might be involved during cyst development in the brain.

Induction of bradyzoite-specific Toxoplasma gondii antigens in gamma interferon-treated mouse macrophages

By using stage-specific monoclonal antibodies, an in vitro model has been developed to analyze the kinetics of expression of stage-specific antigens during the conversion process between tachyzoites and bradyzoites of Toxoplasma gondii. Following infection of murine macrophages with bradyzoites, the expression of bradyzoite-specific antigens declined, whereas the expression of tachyzoite-specific antigens increased during the first 72 h postinfection. Conversely, in gamma interferon-treated murine macrophages infected with tachyzoites, the inhibitory effect of gamma interferon on replication of parasites was accompanied by the induction of bradyzoite-specific antigens.