Use of molecular and ultrastructural markers to evaluate stage conversion of Toxoplasma gondii in both the intermediate and definitive host

Determination of a Key Antigen for Immunological Intervention To Target the Latent Stage of Toxoplasma gondii

Toxoplasma gondii, an obligate intracellular protozoan parasite, establishes a chronic infection by forming cysts preferentially in the brain. Up to one third of the human population worldwide is estimated to be chronically infected with this parasite. However, there is currently no drug effective against the cyst form of the parasite. In addition, the protective immunity against the cysts remains largely unknown. We analyzed the molecular mechanisms by which the immune system detects host cells harboring the cysts to eliminate the latent stage of the parasite using mice with the H-2^d haplotype, which are genetically resistant to the infection. Our study revealed that CD8⁺ immune T cells bearing TCR Vβ8.1, 8.2 chain have a potent activity to remove T. gondii cysts from the brain. Our studies also uncovered that H-2L^d is the major Ag-presenting molecule to CD8⁺ T cells for initiating cyst elimination, and that CD8⁺Vβ8.1, 8.2⁺ immune T cells recognize the N-terminal region (aa 41-152) of dense granule protein 6 (GRA6Nt) of the parasite presented by the H-2L^d molecule. Furthermore, CD8⁺ immune T cells induced by immunization with recombinant GRA6Nt were eventually capable of removing the cysts from the brain when transferred to infected immunodeficient mice lacking T cells. Thus, GRA6Nt is a novel and potent Ag to activate CD8⁺ T cells capable of removing T. gondii cysts. These observations offer a basis for immunological intervention to combat chronic infection with T. gondii by targeting the persistent cysts of the parasite.

Reexamining Chronic Toxoplasma gondii Infection: Surprising Activity for a "Dormant" Parasite

PURPOSE OF REVIEW

Despite over a third of the world's population being chronically infected with Toxoplasma gondii, little is known about this largely asymptomatic phase of infection. This stage is mediated in vivo by bradyzoites within tissue cysts. The absence of overt symptoms has been attributed to the dormancy of bradyzoites. In this review, we reexamine the conventional view of chronic toxoplasmosis in light of emerging evidence challenging both the nature of dormancy and the consequences of infection in the CNS.

RECENT FINDINGS

New and emerging data reveal a previously unrecognized level of physiological and replicative capacity of bradyzoites within tissue cysts. These findings have emerged in the context of a reexamination of the chronic infection in the brain that correlates with changes in neuronal architecture, neurochemistry, and behavior that suggest that the chronic infection is not without consequence.

SUMMARY

The emerging data driven by the development of new approaches to study the progression of chronic toxoplasma infection reveals significant physiological and replicative capacity for what has been viewed as a dormant state. The emergence of bradyzoite and tissue cyst biology from what was viewed as a physiological "black box" offers exciting new areas for investigation with direct implications on the approaches to drug development targeting this drug-refractory state. In addition, new insights from studies on the neurobiology on chronic infection reveal a complex and dynamic interplay between the parasite, brain microenvironment, and the immune response that results in the detente that promotes the life-long persistence of the parasite in the host.

Macrophages facilitate the excystation and differentiation of Toxoplasma gondii sporozoites into tachyzoites following oocyst internalisation

Toxoplasma gondii is a common parasite of humans and animals, which is transmitted via oocysts in cat faeces or tissue cysts in contaminated meat. The robust oocyst and sporocyst walls protect the infective sporozoites from deleterious external attacks including disinfectants. Upon oocyst acquisition, these walls lose their integrity to let the sporozoites excyst and invade host cells following a process that remains poorly understood. Given the resistance of the oocyst wall to digestive enzymes and the ability of oocysts to cause parenteral infections, the present study investigated the possible contribution of macrophages in supporting sporozoite excystation following oocyst internalisation. By using single cell micromanipulations, real-time and time-point imaging techniques, we demonstrated that RAW macrophages could interact rapidly with oocysts and engulfed them by remodelling of their actin cytoskeleton. Internalised oocysts were associated to macrophage acidic compartments and showed evidences of wall disruption. Sporozoites were observed in macrophages containing oocyst remnants or in new macrophages, giving rise to dividing tachyzoites. All together, these results highlight an unexpected role of phagocytic cells in processing T. gondii oocysts, in line with non-classical routes of infection, and open new perspectives to identify chemical factors that lead to oocyst wall disruption under physiological conditions.

Phenotypes Associated with Knockouts of Eight Dense Granule Gene Loci (GRA2-9) in Virulent Toxoplasma gondii

Toxoplasma gondii actively invades host cells and establishes a parasitophorous vacuole (PV) that accumulates many proteins secreted by the dense granules (GRA proteins). To date, at least 23 GRA proteins have been reported, though the function(s) of most of these proteins still remains unknown. We targeted gene knockouts at ten GRA gene loci (GRA1-10) to investigate the cellular roles and essentiality of these classical GRA proteins during acute infection in the virulent type I RH strain. While eight of these genes (GRA2-9) were successfully knocked out, targeted knockouts at the GRA1 and GRA10 loci were not obtained, suggesting these GRA proteins may be essential. As expected, the Δgra2 and Δgra6 knockouts failed to form an intravacuolar network (IVN). Surprisingly, Δgra7 exhibited hyper-formation of the IVN in both normal and lipid-free growth conditions. No morphological alterations were identified in parasite or PV structures in the Δgra3, Δgra4, Δgra5, Δgra8, or Δgra9 knockouts. With the exception of the Δgra3 and Δgra8 knockouts, all of the GRA knockouts exhibited defects in their infection rate in vitro. While the single GRA knockouts did not exhibit reduced replication rates in vitro, replication rate defects were observed in three double GRA knockout strains (Δgra4Δgra6, Δgra3Δgra5 and Δgra3Δgra7). However, the virulence of single or double GRA knockout strains in CD1 mice was not affected. Collectively, our results suggest that while the eight individual GRA proteins investigated in this study (GRA2-9) are not essential, several GRA proteins may provide redundant and potentially important functions during acute infection.

Development of dual fluorescent stage specific reporter strain of Toxoplasma gondii to follow tachyzoite and bradyzoite development in vitro and in vivo

Toxoplasma gondii is a protozoan that infects 30% of humans as intermediate hosts. T Sexual reproduction can occur only within the intestinal tract of felines, however, infection in other mammals and birds is associated with asexual replication and interconversion between the tachyzoite and bradyzoite stages. Bradyzoites are slow growing forms found in tissue cysts in latent infection. Recently, our group described the biological behavior of the EGS strain that forms thick walled cysts spontaneously in tissue culture, constituting a useful tool for examining the developmental biology of T. gondii. To further improve the usefulness of this model, we constructed genetically modified EGS parasites that express fluorescent tags under the control of stage specific promoters. The promoter regions for SAG-1 (tachyzoite specific), BAG-1 and LDH-2 (bradyzoite specific) were amplified by PCR and plasmids were constructed with mCherry (redT) and sfGFP (greenB) sequences, respectively. Strains of parasites were selected using FACS to arrive at single fluorescent and dual fluorescent strains of EGS expressing tags in a stage specific manner. In cell cultures, vacuoles labeled by immunofluorescence assay using anti-CST-1 a marker for T. gondii cyst wall contained parasites that were positive for BAG1-GFP and negative for SAG1-mCherry. Tachyzoites and bradyzoites harvested from the mice expressed stage specific mCherry and GFP proteins, respectively. These new dual fluorescent transgenic EGS strains are a promising tool to elucidate the mechanisms of T. gondii differentiation both in vitro and in vivo.

Novel Approaches Reveal that Toxoplasma gondii Bradyzoites within Tissue Cysts Are Dynamic and Replicating Entities In Vivo

Despite their critical role in chronic toxoplasmosis, the biology of Toxoplasma gondii bradyzoites is poorly understood. In an attempt to address this gap, we optimized approaches to purify tissue cysts and analyzed the replicative potential of bradyzoites within these cysts. In order to quantify individual bradyzoites within tissue cysts, we have developed imaging software, BradyCount 1.0, that allows the rapid establishment of bradyzoite burdens within imaged optical sections of purified tissue cysts. While in general larger tissue cysts contain more bradyzoites, their relative “occupancy” was typically lower than that of smaller cysts, resulting in a lower packing density. The packing density permits a direct measure of how bradyzoites develop within cysts, allowing for comparisons across progression of the chronic phase. In order to capture bradyzoite endodyogeny, we exploited the differential intensity of TgIMC3, an inner membrane complex protein that intensely labels newly formed/forming daughters within bradyzoites and decays over time in the absence of further division. To our surprise, we were able to capture not only sporadic and asynchronous division but also synchronous replication of all bradyzoites within mature tissue cysts. Furthermore, the time-dependent decay of TgIMC3 intensity was exploited to gain insights into the temporal patterns of bradyzoite replication in vivo. Despite the fact that bradyzoites are considered replicatively dormant, we find evidence for cyclical, episodic bradyzoite growth within tissue cysts in vivo. These findings directly challenge the prevailing notion of bradyzoites as dormant nonreplicative entities in chronic toxoplasmosis and have implications on our understanding of this enigmatic and clinically important life cycle stage.

The protozoan Toxoplasma gondii establishes a lifelong chronic infection mediated by the bradyzoite form of the parasite within tissue cysts. Technical challenges have limited even the most basic studies on bradyzoites and the tissue cysts in vivo. Bradyzoites, which are viewed as dormant, poorly replicating or nonreplicating entities, were found to be surprisingly active, exhibiting not only the capacity for growth but also previously unrecognized patterns of replication that point to their being considerably more dynamic than previously imagined. These newly revealed properties force us to reexamine the most basic questions regarding bradyzoite biology and the progression of the chronic phase of toxoplasmosis. By developing new tools and approaches to study the chronic phase at the level of bradyzoites, we expose new avenues to tackle both drug development and a better understanding of events that may lead to reactivated symptomatic disease.

Asexual expansion of Toxoplasma gondii merozoites is distinct from tachyzoites and entails expression of non-overlapping gene families to attach, invade, and replicate within feline enterocytes

Background

The apicomplexan parasite Toxoplasma gondii is cosmopolitan in nature, largely as a result of its highly flexible life cycle. Felids are its only definitive hosts and a wide range of mammals and birds serve as intermediate hosts. The latent bradyzoite stage is orally infectious in all warm-blooded vertebrates and establishes chronic, transmissible infections. When bradyzoites are ingested by felids, they transform into merozoites in enterocytes and expand asexually as part of their coccidian life cycle. In all other intermediate hosts, however, bradyzoites differentiate exclusively to tachyzoites, and disseminate extraintestinally to many cell types. Both merozoites and tachyzoites undergo rapid asexual population expansion, yet possess different effector fates with respect to the cells and tissues they develop in and the subsequent stages they differentiate into.

Results

To determine whether merozoites utilize distinct suites of genes to attach, invade, and replicate within feline enterocytes, we performed comparative transcriptional profiling on purified tachyzoites and merozoites. We used high-throughput RNA-Seq to compare the merozoite and tachyzoite transcriptomes. 8323 genes were annotated with sequence reads across the two asexually replicating stages of the parasite life cycle. Metabolism was similar between the two replicating stages. However, significant stage-specific expression differences were measured, with 312 transcripts exclusive to merozoites versus 453 exclusive to tachyzoites. Genes coding for 177 predicted secreted proteins and 64 membrane- associated proteins were annotated as merozoite-specific. The vast majority of known dense-granule (GRA), microneme (MIC), and rhoptry (ROP) genes were not expressed in merozoites. In contrast, a large set of surface proteins (SRS) was expressed exclusively in merozoites.

Conclusions

The distinct expression profiles of merozoites and tachyzoites reveal significant additional complexity within the T. gondii life cycle, demonstrating that merozoites are distinct asexual dividing stages which are uniquely adapted to their niche and biological purpose.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1225-x) contains supplementary material, which is available to authorized users.

Toxoplasma secretory granules: one population or more?

In Toxoplasma gondii, dense granules are known as the storage secretory organelles of the so-called GRA proteins (for dense granule proteins), which are destined to the parasitophorous vacuole (PV) and the PV-derived cyst wall. Recently, newly annotated GRA proteins targeted to the host cell nucleus have enlarged this view. Here we provide an update on the latest developments on the Toxoplasma secreted proteins, which to date have been mainly studied at both the tachyzoite and bradyzoite stages, and we point out that recent discoveries could open the issue of a possible, yet uncharacterized, distinct secretory pathway in Toxoplasma.

Toxoplasma gondii merozoite gene expression analysis with comparison to the life cycle discloses a unique expression state during enteric development

BACKGROUND

Considerable work has been carried out to understand the biology of tachyzoites and bradyzoites of Toxoplasma gondii in large part due to in vitro culture methods for these stages. However, culturing methods for stages that normally develop in the gut of the definitive felid host, including the merozoite and sexual stages, have not been developed hindering the ability to study a large portion of the parasite's life cycle. Here, we begin to unravel the molecular aspects of enteric stages by providing new data on merozoite stage gene expression.

RESULTS

To profile gene expression differences in enteric stages we harvested merozoites from the intestine of infected cats and hybridized mRNA to the Affymetrix Toxoplasma GeneChip. We analyzed the merozoite data in context of the life cycle by comparing it to previously published data for the oocyst, tachyzoite, and bradyzoite stages. Principal component analysis highlighted the unique profile of merozoites, placing them approximately half-way on a continuum between the tachyzoite/bradyzoite and oocyst samples. Prior studies have shown that antibodies to surface antigen one (SAG1) and many dense granule proteins do not label merozoites: our microarray data confirms that these genes were not expressed at this stage. Also, the expression for many rhoptry and microneme proteins was drastically reduced while the expression for many surface antigens was increased at the merozoite stage. Gene Ontology and KEGG analysis revealed that genes involved in transcription/translation and many metabolic pathways were upregulated at the merozoite stage, highlighting unique growth requirements of this stage. To functionally test these predictions, we demonstrated that an upstream promoter region of a merozoite specific gene was sufficient to control expression in merozoites in vivo.

CONCLUSIONS

Merozoites are the first developmental stage in the coccidian cycle that takes place within the gut of the definitive host. The data presented here describe the global gene expression profile of the merozoite stage and the creation of transgenic parasite strains that show stage-specific expression of reporter genes in the cat intestine. These data and reagents will be useful in unlocking how the parasite senses and responds to the felid gut environment to initiate enteric development.

Phylogeny and virulence divergency analyses of Toxoplasma gondii isolates from China

BACKGROUND

Toxoplasma gondii (T. gondii) is a very successful parasite that can infect virtually all warm blooded animals with a worldwide distribution. It causes a large range of clinical manifestations in both humans and domesticated animals. In addition, marked biological differences exist among T. gondii strains in the pathogenicity and geographical distribution. Molecular epidemiology studies primarily based on restriction fragment length polymorphism (RFLP) method revealed that three main types are predominant in North America and Europe, whereas other diverse genotypes are found in other parts of the world. Microsatellite (MS) as a type of genetic marker has been widely used in many organisms. Limited MS genotyping, however, to fingerprint T. gondii isolates has been reported and little is known about the MS data of the strains predominantly prevalent in China.

METHODS

Genotyping of twenty-eight Chinese T. gondii isolates were performed using 15 MS markers located on 12 different chromosomes. Results were analyzed in terms of population structure by a Bayesian statistical approach. Phylogenetic analysis was obtained from a Neighbor-Net phylogenetic network. The virulence analyses of some representative isolates were determined by inoculation of mice and cell invasion assays. The gene expressions of some virulence-associated factors (VFs) were performed by quantitative real-time PCR (qRT- PCR).

RESULTS

Three haplogroups were clustered among the 28 isolates although minor genetic differences were found within haplogroups. The majority of strains belong to one haplogroup corresponding to the previously described Chinese 1 type (ToxoDB#9). Phylogenetic networks uncovered a limited diversity of T. gondii strains and the virulence differs in the strains sharing the same genotype. No remarkable difference, however, was noted in the tested VFs except for dense granule protein3 (GRA3), which was found to have a higher expression in low virulent TgCtwh6 (Wh6) strain than that in high virulent TgCtwh3 (Wh3) strain.

CONCLUSION

The profile of microsatellite typing data from Chinese T. gondii strains revealed a limited genetic diversity and the selected VFs and phylogenetic network analyses displayed less divergence, although the strain virulence differs in the Chinese 1 type of T. gondii predominantly prevalent in China.

Toxoplasma gondii merozoite gene expression analysis with comparison to the life cycle discloses a unique expression state during enteric development

Background

Considerable work has been carried out to understand the biology of tachyzoites and bradyzoites of Toxoplasma gondii in large part due to in vitro culture methods for these stages. However, culturing methods for stages that normally develop in the gut of the definitive felid host, including the merozoite and sexual stages, have not been developed hindering the ability to study a large portion of the parasite’s life cycle. Here, we begin to unravel the molecular aspects of enteric stages by providing new data on merozoite stage gene expression.

Results

To profile gene expression differences in enteric stages we harvested merozoites from the intestine of infected cats and hybridized mRNA to the Affymetrix Toxoplasma GeneChip. We analyzed the merozoite data in context of the life cycle by comparing it to previously published data for the oocyst, tachyzoite, and bradyzoite stages. Principal component analysis highlighted the unique profile of merozoites, placing them approximately half-way on a continuum between the tachyzoite/bradyzoite and oocyst samples. Prior studies have shown that antibodies to surface antigen one (SAG1) and many dense granule proteins do not label merozoites: our microarray data confirms that these genes were not expressed at this stage. Also, the expression for many rhoptry and microneme proteins was drastically reduced while the expression for many surface antigens was increased at the merozoite stage. Gene Ontology and KEGG analysis revealed that genes involved in transcription/translation and many metabolic pathways were upregulated at the merozoite stage, highlighting unique growth requirements of this stage. To functionally test these predictions, we demonstrated that an upstream promoter region of a merozoite specific gene was sufficient to control expression in merozoites in vivo.

Conclusions

Merozoites are the first developmental stage in the coccidian cycle that takes place within the gut of the definitive host. The data presented here describe the global gene expression profile of the merozoite stage and the creation of transgenic parasite strains that show stage-specific expression of reporter genes in the cat intestine. These data and reagents will be useful in unlocking how the parasite senses and responds to the felid gut environment to initiate enteric development.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-350) contains supplementary material, which is available to authorized users.

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.

Determination of stage interconversion in vitro and in vivo by construction of transgenic Toxoplasma gondii that stably express stage-specific fluorescent proteins

Detection of Toxoplasma gondii conversion from the tachyzoite stage to the bradyzoite stage in living brain tissue is difficult because the parasites are small and conversion and reactivation of the parasites are transient events. To better understand the mechanisms of T. gondii stage conversion between tachyzoites and bradyzoites, and to recognize stage conversion in an intermediate host, we constructed a transgenic cyst-forming strain (PLK) of T. gondii. The parasites stably expressed enhanced green fluorescence protein (EGFP) in the tachyzoite stage and red fluorescence protein (RFP) in the bradyzoite stage, under the control of the SAG1 and BAG1 promoters, respectively. The resulting transgenic parasite was designated as PLK/Bi. The PLK/Bi zoites expressed only green fluorescence in the tachyzoite stage and only red fluorescence in the bradyzoite stage in vitro and in vivo. Fluorescence analyses showed that recombinant GFP and RFP were located to the intracellular vacuolar spaces. In addition, an analysis of growth and culture conditions of transgenic T. gondii was performed in vitro and the virulence was evaluated in vivo. Our data suggested that the stage-specific fluorescence expression by PLK/Bi may be rationally designed for in vitro and in vivo studies on stage conversion and reactivation of T. gondii.

A nucleotide sugar transporter involved in glycosylation of the Toxoplasma tissue cyst wall is required for efficient persistence of bradyzoites

Toxoplasma gondii is an intracellular parasite that transitions from acute infection to a chronic infective state in its intermediate host via encystation, which enables the parasite to evade immune detection and clearance. It is widely accepted that the tissue cyst perimeter is highly and specifically decorated with glycan modifications; however, the role of these modifications in the establishment and persistence of chronic infection has not been investigated. Here we identify and biochemically and biologically characterize a Toxoplasma nucleotide-sugar transporter (TgNST1) that is required for cyst wall glycosylation. Toxoplasma strains deleted for the TgNST1 gene (Δnst1) form cyst-like structures in vitro but no longer interact with lectins, suggesting that Δnst1 strains are deficient in the transport and use of sugars for the biosynthesis of cyst-wall structures. In vivo infection experiments demonstrate that the lack of TgNST1 activity does not detectably impact the acute (tachyzoite) stages of an infection or tropism of the parasite for the brain but that Δnst1 parasites are severely defective in persistence during the chronic stages of the infection. These results demonstrate for the first time the critical role of parasite glycoconjugates in the persistence of Toxoplasma tissue cysts.

The Toxoplasma tissue cyst is essential to the persistence of the parasite during the chronic infection of an immunocompetent host. While significant efforts have been made to identify molecular factors that trigger and sustain parasite encystation, the role of the glycoconjugates that decorate the cyst wall has received little attention. Here we identify and characterize a bona fide nucleotide-sugar transporter, TgNST1, whose activity is required for the proper assembly of cyst wall glycoconjugates. We found that deletion of TgNST1 interferes with glycosylation during both the tachyzoite and bradyzoite stages of infection, and we observed substantial defects in the ability of Δnst1 parasites to maintain chronic infection. Surprisingly, Δnst1 parasites were not significantly defective in acute infection of mice, and showed wild type levels and migration rates to the brain. These results highlight the important role of cyst-wall glycosylation in parasite persistence during chronic infection, and suggest that drugs targeting nucleotide-sugar transporters and other enzymes required for glycosylation, perhaps in combination with drugs targeting other pathways, might be useful to prevent the establishment of chronic parasite infection.

CD73-generated adenosine facilitates Toxoplasma gondii differentiation to long-lived tissue cysts in the central nervous system

Toxoplasma gondii is an obligate intracellular protozoan pathogen that traffics to the central nervous system (CNS) following invasion of its host. In the CNS, T. gondii undergoes transformation from a rapidly dividing tachyzoite to a long-lived, slow-dividing bradyzoite contained within cysts. The role of extracellular adenosine in T. gondii pathogenesis has not been previously investigated. T. gondii uses host purines such as adenosine for its energy needs, as it is unable to make its own. Here, we show that CD73(-/-) mice, which lack the ability to generate extracellular adenosine, are protected from T. gondii chronic infection, with significantly fewer cysts and reduced susceptibility to reactivation of infection in the CNS independent of host effector function. Parasite dissemination to the brain was unimpaired in CD73(-/-) hosts, suggesting that the reduced cyst number is due to impaired parasite differentiation in the CNS. Confirming this, T. gondii tachyzoites formed fewer cysts following alkaline pH stress in astrocytes isolated from CD73(-/-) mice compared with wild type, and in fibroblasts treated with a CD73 inhibitor. Cyst formation was rescued in CD73(-/-) astrocytes supplemented with adenosine, but not with adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine. Furthermore, mice lacking adenosine receptors had no defect in cyst formation. Based on these findings, we conclude that CD73 expression promotes Toxoplasma bradyzoite differentiation and cyst formation by a mechanism dependent on the generation of adenosine, but independent of adenosine receptor signaling. Overall, these findings suggest that modulators of extracellular adenosine may be used to develop therapies aimed at defending against human toxoplasmosis.

Mechanisms of Toxoplasma gondii persistence and latency

Toxoplasma gondii is an obligate intracellular protozoan parasite that causes opportunistic disease, particularly in immunocompromised individuals. Central to its transmission and pathogenesis is the ability of the proliferative stage (tachyzoite) to convert into latent tissue cysts (bradyzoites). Encystment allows Toxoplasma to persist in the host and affords the parasite a unique opportunity to spread to new hosts without proceeding through its sexual stage, which is restricted to felids. Bradyzoite tissue cysts can cause reactivated toxoplasmosis if host immunity becomes impaired. A greater understanding of the molecular mechanisms orchestrating bradyzoite development is needed to better manage the disease. Here, we will review key studies that have contributed to our knowledge about this persistent form of the parasite and how to study it, with a focus on how cellular stress can signal for the reprogramming of gene expression needed during bradyzoite development.

Epidemiology of and diagnostic strategies for toxoplasmosis

The apicomplexan parasite Toxoplasma gondii was discovered a little over 100 years ago, but knowledge of its biological life cycle and its medical importance has grown in the last 40 years. This obligate intracellular parasite was identified early as a pathogen responsible for congenital infection, but its clinical expression and the importance of reactivations of infections in immunocompromised patients were recognized later, in the era of organ transplantation and HIV infection. Recent knowledge of host cell-parasite interactions and of parasite virulence has brought new insights into the comprehension of the pathophysiology of infection. In this review, we focus on epidemiological and diagnostic aspects, putting them in perspective with current knowledge of parasite genotypes. In particular, we provide critical information on diagnostic methods according to the patient's background and discuss the implementation of screening tools for congenital toxoplasmosis according to health policies.

Proteomic analysis of fractionated Toxoplasma oocysts reveals clues to their environmental resistance

Toxoplasma gondii is an obligate intracellular parasite that is unique in its ability to infect a broad range of birds and mammals, including humans, leading to an extremely high worldwide prevalence and distribution. This work focuses on the environmentally resistant oocyst, which is the product of sexual replication in felids and an important source of human infection. Due to the difficulty in producing and working with oocysts, relatively little is known about how this stage is able to resist extreme environmental stresses and how they initiate a new infection, once ingested. To fill this gap, the proteome of the wall and sporocyst/sporozoite fractions of mature, sporulated oocysts were characterized using one-dimensional gel electrophoresis followed by LC-MS/MS on trypsin-digested peptides. A combined total of 1021 non-redundant T. gondii proteins were identified in the sporocyst/sporozoite fraction and 226 were identified in the oocyst wall fraction. Significantly, 172 of the identified proteins have not previously been identified in Toxoplasma proteomic studies. Among these are several of interest for their likely role in conferring environmental resistance including a family of small, tyrosine-rich proteins present in the oocyst wall fractions and late embryogenesis abundant domain-containing (LEA) proteins in the cytosolic fractions. The latter are known from other systems to be key to enabling survival against desiccation.

Identification of tissue cyst wall components by transcriptome analysis of in vivo and in vitro Toxoplasma gondii bradyzoites

The Toxoplasma gondii bradyzoite is essential to establish persistent infection, yet little is known about what factors this developmental form secretes to establish the cyst or interact with its host cell. To identify candidate bradyzoite-secreted effectors, the transcriptomes of in vitro tachyzoites 2 days postinfection, in vitro bradyzoites 4 days postinfection, and in vivo bradyzoites 21 days postinfection were interrogated by microarray, and the program SignalP was used to identify signal peptides indicating secretion. One hundred two putative bradyzoite-secreted effectors were identified by this approach. Two candidates, bradyzoite pseudokinase 1 and microneme adhesive repeat domain-containing protein 4, were chosen for further investigation and confirmed to be induced and secreted by bradyzoites in vitro and in vivo. Thus, we report the first analysis of the transcriptomes of in vitro and in vivo bradyzoites and identify two new protein components of the Toxoplasma tissue cyst wall.

The organization of the wall filaments and characterization of the matrix structures of Toxoplasma gondii cyst form

The encystation process is a key step in Toxoplasma gondii life cycle, allowing the parasite to escape from the host immune system and the transmission among the hosts. A detailed characterization of the formation and structure of the cyst stage is essential for a better knowledge of toxoplasmosis. Here we isolated cysts from mice brains and analysed the cyst wall structure and cyst matrix organization using different electron microscopy techniques. Images obtained showed that the cyst wall presented a filamentous aspect, with circular openings on its surface. The filaments were organized in two layers: a compact one, facing the exterior of the whole cyst and a more loosen one, facing the matrix. Within the cyst wall, we observed tubules and a large number of vesicles. The cyst matrix presented vesicles of different sizes and tubules, which were organized in a network connecting the bradyzoites to each other and to the cyst wall. Large vesicles, with a granular material in their lumen of glycidic nature were observed. Similar vesicles were also found associated with the posterior pole of the bradyzoites and in proximity to the cyst wall.

Involvement of a Toxoplasma gondii chromatin remodeling complex ortholog in developmental regulation

The asexual cycle of the parasite Toxoplasma gondii has two developmental stages: a rapidly replicating form called a tachyzoite and a slow growing cyst form called a bradyzoite. While the importance of ATP-independent histone modifications for gene regulation in T. gondii have been demonstrated, ATP-dependent chromatin remodeling pathways have not been examined. In this study we characterized C9, an insertional mutant showing reduced expression of bradyzoite differentiation marker BAG1, in cultured human fibroblasts. This mutant contains an insertion in the gene encoding TgRSC8, which is homologous to the Saccharomyces cerevisiae proteins Rsc8p (remodel the structure of chromatin complex subunit 8) and Swi3p (switch/sucrose non-fermentable [SWI/SNF]) of ATP-dependent chromatin-remodeling complexes. In the C9 mutant, TgRSC8 is the downstream open reading frame on a dicistronic transcript. Though protein was expressed from the downstream gene of the dicistron, TgRSC8 levels were decreased in C9 from those of wild-type parasites, as determined by western immunoblot and flow cytometry. As TgRSC8 localized to the parasite nucleus, we postulated a role in gene regulation. Transcript levels of several markers were assessed by quantitative PCR to test this hypothesis. The C9 mutant displayed reduced steady state transcript levels of bradyzoite-induced genes BAG1, LDH2, SUSA1, and ENO1, all of which were significantly increased with addition of TgRSC8 to the mutant. Transcript levels of some bradyzoite markers were unaltered in C9, or unable to be increased by complementation with TgRSC8, indicating multiple pathways control bradyzoite-upregulated genes. Together, these data suggest a role for TgRSC8 in control of bradyzoite-upregulated gene expression. Thus chromatin remodeling, by both ATP-independent and dependent mechanisms, is an important mode of gene regulation during stage differentiation in parasites.

Type II Toxoplasma gondii KU80 knockout strains enable functional analysis of genes required for cyst development and latent infection

Type II Toxoplasma gondii KU80 knockouts (Δku80) deficient in nonhomologous end joining were developed to delete the dominant pathway mediating random integration of targeting episomes. Gene targeting frequency in the type II Δku80 Δhxgprt strain measured at the orotate (OPRT) and the uracil (UPRT) phosphoribosyltransferase loci was highly efficient. To assess the potential of the type II Δku80 Δhxgprt strain to examine gene function affecting cyst biology and latent stages of infection, we targeted the deletion of four parasite antigen genes (GRA4, GRA6, ROP7, and tgd057) that encode characterized CD8(+) T cell epitopes that elicit corresponding antigen-specific CD8(+) T cell populations associated with control of infection. Cyst development in these type II mutant strains was not found to be strictly dependent on antigen-specific CD8(+) T cell host responses. In contrast, a significant biological role was revealed for the dense granule proteins GRA4 and GRA6 in cyst development since brain tissue cyst burdens were drastically reduced specifically in mutant strains with GRA4 and/or GRA6 deleted. Complementation of the Δgra4 and Δgra6 mutant strains using a functional allele of the deleted GRA coding region placed under the control of the endogenous UPRT locus was found to significantly restore brain cyst burdens. These results reveal that GRA proteins play a functional role in establishing cyst burdens and latent infection. Collectively, our results suggest that a type II Δku80 Δhxgprt genetic background enables a higher-throughput functional analysis of the parasite genome to reveal fundamental aspects of parasite biology controlling virulence, pathogenesis, and transmission.

Discovery of three novel coccidian parasites infecting California sea lions (Zalophus californianus), with evidence of sexual replication and interspecies pathogenicity

Enteric protozoal infection was identified in 5 stranded California sea lions (Zalophus californianus). Microscopically, the apical cytoplasm of distal jejunal enterocytes contained multiple stages of coccidian parasites, including schizonts with merozoites and spherical gametocytes, which were morphologically similar to coccidians. By histopathology, organisms appeared to be confined to the intestine and accompanied by only mild enteritis. Using electron microscopy, both sexual (microgametocytes, macrogamonts) and asexual (schizonts, merozoites) coccidian stages were identified in enterocytes within parasitophorous vacuoles, consistent with apicomplexan development in a definitive host. Serology was negative for tissue cyst-forming coccidians, and immunohistochemistry for Toxoplasma gondii was inconclusive and negative for Neospora caninum and Sarcocystis neurona. Analysis of ITS-1 gene sequences amplified from frozen or formalin-fixed paraffin-embedded intestinal sections identified DNA sequences with closest homology to Neospora sp. (80%); these novel sequences were referred to as belonging to coccidian parasites "A," "B," and "C." Subsequent molecular analyses completed on a neonatal harbor seal (Phoca vitulina) with protozoal lymphadenitis, hepatitis, myocarditis, and encephalitis showed that it was infected with a coccidian parasite bearing the "C" sequence type. Our results indicate that sea lions likely serve as definitive hosts for 3 newly described coccidian parasites, at least 1 of which is pathogenic in a marine mammal intermediate host species.

Molecular characterization of Neospora caninum MAG1, a dense granule protein secreted into the parasitophorous vacuole, and associated with the cyst wall and the cyst matrix

SUMMARY

In Neospora caninum and Toxoplasma gondii, the parasitophorous vacuole (PV) is synthesized at the time of infection. During tachyzoite-to-bradyzoite stage conversion, the PV is later transformed into a tissue cyst that allows parasites to survive in their host for extended periods of time. We report on the characterization of NcMAG1, the N. caninum orthologue of T. gondii MAG1 (matrix antigen 1; TgMAG1). The 456 amino acid predicted NcMAG1 protein is 54% identical to TgMAG1. By immunoblotting, a rabbit antiserum raised against recombinant NcMAG1 detected a major product of approximately 67 kDa in extracts of N. caninum tachyzoite-infected Vero cells, which was stained more prominently in extracts of infected Vero cells treated to induce in vitro bradyzoite conversion. Immunofluorescence and TEM localized the protein mainly within the cyst wall and the cyst matrix. In both tachyzoites and bradyzoites, NcMAG1 was associated with the parasite dense granules. Comparison between NcMAG1 and TgMAG1 amino acid sequences revealed that the C-terminal conserved regions exhibit 66% identity, while the N-terminal variable regions exhibit only 32% identity. Antibodies against NcMAG1-conserved region cross-reacted with the orthologuous protein in T. gondii but those against the variable region did not. This indicates that the variable region possesses unique antigenic characteristics.

Primary culture of intestinal epithelial cells as a potential model for Toxoplasma gondii enteric cycle studies

The primary culture of intestinal epithelial cells from domestic cats is an efficient cellular model to study the enteric cycle of Toxoplasma gondii in a definitive host. The parasite-host cell ratio can be pointed out as a decisive factor that determines the intracellular fate of bradyzoites forms. The development of the syncytial-like forms of T. gondii was observed using the 1:20 bradyzoite-host cell ratio, resulting in similar forms described in in vivo systems. This alternative study potentially opens up the field for investigation into the molecular aspects of this interaction. This can contribute to the development of new strategies for intervention of a main route by which toxoplasmosis spreads.

Interaction and cystogenesis of Toxoplasma gondii within skeletal muscle cells in vitro

Infection by the protozoan parasite Toxoplasma gondii is widely prevalent in humans and animals. To prevent human infection, all meat should be well cooked before consumption, since the parasite is present in skeletal muscle. In this context, the use of skeletal muscle cells (SkMCs) as a cellular model opens up new approaches to investigate T. gondii-host cell interactions. Immunofluorescent detection of proteins that are stage-specific for bradyzoites indicated that complete cystogenesis of T. gondii in in vitro cultures of SkMCs occurs after 96 h of infection. Ultrastructural analysis showed that, after 48 h of interaction, there were alterations on the parasitophorous vacuole membrane, including greater thickness and increased electron density at the inner face of the membrane. The present study demonstrates the potential use of primary cultures of SkMCs to evaluate different molecular aspects of T. gondii invasion and cystogenesis and presents a promising in vitro model for the screening of drug activities toward tissue cysts and bradyzoites.

Understanding mechanisms and the role of differentiation in pathogenesis of Toxoplasma gondii: a review

Parasite differentiation from proliferating tachyzoites into latent bradyzoites is central to pathogenesis and transmission of the intracellular protozoan pathogen Toxoplasma gondii. The presence of bradyzoite-containing cysts in human hosts and their subsequent rupture can cause life-threatening recrudescence of acute infection in the immunocompromised and cyst formation in other animals contributes to zoonotic transmission and widespread dissemination of the parasite. In this review, we discuss the evidence showing how the clinically relevant process of bradyzoite differentiation is regulated at both transcriptional and post-transcriptional levels. Specific regulatory factors implicated in modulating bradyzoite differentiation include promoter-based cis-elements, epigenetic modifications and protein translation control through eukaryotic initiation factor -2 (eIF2). In addition to a summary of the current state of knowledge in these areas we discuss the pharmacological ramifications and pose some questions for future research.

Entamoeba invadens, encystation process and enolase

The reptilian parasite Entamoeba invadens is accepted as a model for the study of the Entamoeba encystation process. Here we describe the production and characterization of a mAb (B4F2), generated against a component of the E. invadens cyst wall. This mAb specifically recognizes a 48-kDa protein present in cytoplasmic vesicles of cells encysting for 24 h. In mature cysts (96 h), the antigen was detected on the cyst surface. By two-dimensional electrophoresis and mass spectrometry analysis, the B4F2 specific antigen was identified as enolase. Levels of enolase mRNA were increased in encysting cells and the B4F2 mAb was found to inhibit cyst formation. Therefore, these results strongly suggest a new role for enolase in E. invadens encystation, and the B4F2 mAb will be useful tool to study its role in the differentiation process.

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.

Host Cell Preference of Toxoplasma gondii Cysts in Murine Brain: A Confocal Study

Toxoplasma gondii is a protozoan parasite that is widely prevalent in humans and typically results in a chronic infection characterized by cysts located predominantly in the central nervous system. In immunosuppressed hosts, such as patients with HIV infection, the infection can be reactivated from the cysts in the brain resulting in a severe and potentially fatal encephalitis. Studies suggest that the chronic infection may also have neuropathological and behavioral effects in immune competent hosts. An improved understanding of tissue cyst behavior is of importance for understanding both the reactivation as well as the neurophysiological consequences of chronic infection. In vivo studies have identified neurons as host cells for cysts but in vitro studies have found that astrocytes can also foster development of the cysts. In this study we have addressed the question of which neural cell tissue cysts of T. gondii reside during chronic infection using a mouse model. Mice were infected with Me49 Strain T. gondii and the intracellular localization of the cysts analyzed during the development and establishment of a chronic infection at 1, 2, and 6 months post infection. Brains were fixed, cryosectioned, and stained with FITC-Dolichos biflorans to identify the Toxoplasma cysts and they were labeled with cell specific antibodies to neurons or astrocytes and then analyzed using confocal fluorescence microscopy. Cysts were found to occur almost exclusively in neurons throughout chronic infection. No cysts were identified in astrocytes, using the astrocyte marker, GFAP. Astrocyte interactions with neuronal-cysts, however, were frequently observed.

Isolation of Toxoplasma gondii development mutants identifies a potential proteophosphogylcan that enhances cyst wall formation

Within warm-blooded animals, Toxoplasma gondii switches from an actively replicating form called a tachyzoite into a slow growing encysted form called a bradyzoite. To uncover the genes involved in bradyzoite development, we screened over 8000 T. gondii insertional mutants by immunofluorescence microscopy. We identified nine bradyzoite development mutants that were defective in both cyst wall formation and expression of a bradyzoite specific heat shock protein. One of these mutants, named 42F5, contained an insertion into the predicted gene TGME49_097520. The disrupted protein is serine/proline-rich with homology to proteophosphoglycans from Leishmania. T. gondii proteophosphoglycan (GU182879) expressed from the native promoter was undetectable in tachyzoites, but bradyzoites show punctate spots within the parasite and staining around the parasitophorous vacuole. Complementation of the 42F5 mutant with GU182879 expressed from either the alpha-tubulin or native promoter restores cyst wall formation. Overall, GU182879 is upregulated in bradyzoites and enhances cyst wall component expression and assembly.

Toxoplasma gondii: Simple duplex RT-PCR assay for detecting SAG1 and BAG1 genes during stage conversion in immunosuppressed mice

Toxoplasmic encephalitis (TE) is caused by reactivation of dormant bradyzoites into rapidly dividing tachyzoites of the apicomplexan parasite Toxoplasma gondii in immune-compromised hosts. Diagnosis of this life-threatening disease is complicated, since it is difficult to distinguish between these two stages. It is, therefore, mainly based on a test positive for T. gondii antibodies, and specific clinical symptoms. We developed a duplex RT-PCR to detect the expression of bradyzoite (BAG1) and tachyzoite (SAG1) specific genes simultaneously during tachyzoite/bradyzoite stage conversion. The conversion reaction was observed in many organs of experimental mice, indicated by tachyzoites in the cerebrum, cerebellum, heart and lung, beginning in week 1 after the suppression period, and continuing until the end. Bradyzoites were also detected in nearly all organs throughout the study, suggesting that during the reactivation period, bradyzoites not only escape from cysts and reinvade neighboring cells as tachyzoites, but are also driven into developing new bradyzoites. The results of our study show that duplex RT-PCR is an easy, rapid, sensitive, and reproducible method, which is particularly valuable when numerous samples must be analyzed. This technique may usefully serve as an alternate tool for diagnosing TE in severely immunocompromised patients.

Toxoplasma gondii cyclophilin 18-mediated production of nitric oxide induces Bradyzoite conversion in a CCR5-dependent manner

Toxoplasma gondii modulates pro- and anti-inflammatory responses to regulate parasite multiplication and host survival. Pressure from the immune response causes the conversion of tachyzoites into slowly dividing bradyzoites. The regulatory mechanisms involved in this switch are poorly understood. The aim of this study was to investigate the immunomodulatory role of T. gondii cyclophilin 18 (TgCyp18) in macrophages and the consequences of the cellular responses on the conversion machinery. Recombinant TgCyp18 induced the production of nitric oxide (NO), interleukin-12 (IL-12), and tumor necrosis factor alpha through its binding with cysteine-cysteine chemokine receptor 5 (CCR5) and the production of gamma interferon and IL-6 in a CCR5-independent manner. Interestingly, the treatment of macrophages with TgCyp18 resulted in the inhibition of parasite growth and an enhancement of the conversion into bradyzoites via NO in a CCR5-dependent manner. In conclusion, T. gondii possesses sophisticated mechanisms to manipulate host cell responses in a TgCyp18-mediated process.

Stage-specific expression of Nc SAG4 as a marker of chronic Neospora caninum infection in a mouse model

Neospora caninum infection persists throughout the life of its intermediate host due to the conversion of tachyzoites to slowly dividing bradyzoites that encyst in the brain. This event results in persistent N. caninum infection in bovine herds and partially explains the poor efficacy of many chemotherapeutic agents and vaccine formulations. Thus, there is a need for greater understanding of the tachyzoite-to-bradyzoite conversion mechanisms. Here we studied for the first time the transcription kinetics of the N. caninum bradyzoite-specific gene NcSAG4 in brain samples from chronically infected mice by means of real-time RT-PCR. NcSAG4-messenger RNA (mRNA) levels increased significantly during the chronic phase but followed 2 different expression patterns depending on the isolate used for murine inoculation. NcSAG4-mRNA levels in brains from Nc-1-inoculated mice peaked during late chronic infection (on day 64 post-infection, p.i.), whereas those from Nc-Liv-inoculated mice peaked earlier during the chronic infection (on day 32 p.i.). This difference could be a reflection of the different abilities of these isolates to replicate and form cysts in parasitized brains. These results are consistent with our observations of anti-rNcSAG4 antibody production; low levels were present at seroconversion and slowly increased during the chronic phase. In contrast, NcSAG1 transcription levels, which mark the tachyzoite stage, were maintained without variation in both groups of mice. This suggests the presence of a significant amount of tachyzoites or intermediate zoites expressing NcSAG1 in the brain, even during the late chronic infection.

Visualization of Toxoplasma gondii stage conversion by expression of stage-specific dual fluorescent proteins

To recognize the stage conversion of Toxoplasma gondii between tachyzoite and bradyzoite in live host cells, a transgenic T. gondii line, which expressed stage-specific red and green fluorescence, was constructed. T. gondii PLK strain tachyzoites were stably transformed with genes encoding red fluorescent protein (DsRed Express) and green fluorescent protein (GFP) under the control of tachyzoite-specific SAG1 and bradyzoite-specific BAG1 promoters, respectively. The resulting transgenic parasite was designated PLK/DUAL. When PLK/DUAL was cultured in pH 7.0 medium, the PLK/DUAL zoites expressed red fluorescence, but no detectable levels of green fluorescence were observed. The PLK/DUAL zoites reacted with anti-SAG1 antibody, but not anti-BAG1 antiserum. When PLK/DUAL was cultured under high pH conditions, or in the presence of the p38 MAPK inhibitor SB202190, a small number of zoites expressed green fluorescence and were BAG1 positive. C57BL/6J mice were infected with PLK/DUAL tachyzoites. During the acute and reactivating phase, zoites expressed red fluorescence. However, green fluorescence was not detectable. By contrast, latent cysts expressed green fluorescence. The stage-specific dual fluorescence of PLK/DUAL facilitates identification of the parasitic stage in live cells, with the advantage that fixation or immunostaining is not required.

The role played by electron microscopy in advancing our understanding of Toxoplasma gondii and other apicomplexans

In many ways the history of the discovery of the life cycle of Toxoplasma gondii and the development of biological electron microscopy progressed in parallel through the 1950s and 1960s. Although Toxoplasma was discovered in 1908, it was only in the 1950s that the extent of the infection in humans and domestic animals was realised and work was undertaken to elucidate its life cycle (reviewed elsewhere in this edition). The development of ultrastructural techniques and their application to biological systems including Toxoplasma developed over the same period. This resulted in a synergistic effect with the re-classification of previously unrelated parasites within a single phylum, the Apicomplexa, which was based on the ultrastructural appearances of the infectious stages. This review will describe the central role played by electron microscopy and Toxoplasma in the developments associated with this progress.

Primary skeletal muscle cells trigger spontaneous Toxoplasma gondii tachyzoite-to-bradyzoite conversion at higher rates than fibroblasts

Toxoplasma gondii is one of the most common eukaryotic parasites and an important opportunistic pathogen of humans. The interconversion from proliferative tachyzoites into quiescent encysted bradyzoites plays pivotal roles in the lifelong persistence of T. gondii in its host and the pathogenesis of toxoplasmosis. Stage conversion and persistence in skeletal muscle tissue may be particularly important for the food-borne transmission of T. gondii to humans via raw or undercooked meat products. Here, we have followed the transition of tachyzoites to bradyzoites after infection of skeletal muscle cells (SkMC). Primary murine myoblasts were differentiated to multinucleated syncytial myotubes that displayed regular contractions in vitro and expressed myogenic markers Myf5 and MyoD. Tachyzoites of T. gondii invaded SkMC within 4h of infection and started to replicate within 24h of infection. Remarkably, intracellular tachyzoites readily differentiated to bradyzoites in SkMC without the need of exogenous stress factors. Double immunofluorescence labelling revealed significantly higher percentages of bradyzoite-containing vacuoles in SkMC than in murine fibroblasts at 24h until 6 days after infection. Furthermore, transcript levels of bradyzoite-specific ENO1 but not tachyzoite-specific ENO2 strongly increased in T. gondii-infected SkMC until 6 days of infection. These findings indicate that the commitment of T. gondii to differentiate to bradyzoites in SkMC does not require exogenous stress factors but could be rather regulated by cell-type specific factors.

Parasite stage-specific recognition of endogenous Toxoplasma gondii-derived CD8+ T cell epitopes

BACKGROUND

BALB/c mice control infection with the obligate intracellular parasite Toxoplasma gondii and develop a latent chronic infection in the brain, as do immunocompetent humans. Interferon-gamma-producing CD8+ T cells provide essential protection against T. gondii infection, but the epitopes recognized have so far remained elusive.

METHODS

We employed caged major histocompatibility complex molecules to generate approximately 250 H-2L(d) tetramers and to distinguish T. gondii-specific CD8+ T cells in BALB/c mice.

RESULTS

We identified 2 T. gondii-specific H-2L(d)-restricted T cell epitopes, one from dense granule protein GRA4 and the other from rhoptry protein ROP7. H-2L(d)/GRA4 reactive T cells from multiple organ sources predominated 2 weeks after infection, while the reactivity of the H-2L(d)/ROP7 T cells peaked 6-8 weeks after infection. BALB/c animals infected with T. gondii mutants defective in establishing a chronic infection showed altered levels of antigen-specific T cells, depending on the T. gondii mutant used.

CONCLUSIONS

Our results shed light on the identity and the parasite stage-specificity of 2 CD8+ T cell epitopes recognized in the acute and chronic phase of infection with T. gondii.

The timing of sulfadiazine therapy impacts the reactivation of latent Toxoplasma infection in IRF-8−/− mice

The process of reactivation of latent infection with Toxoplasma gondii in immunosuppressed hosts is yet not fully understood. In the past, a number of murine models of reactivation in immunocompromised mice have been described using sulfadiazine to establish latent infection before withdrawal and subsequent reactivation. We studied the process of reactivation in brains of mice with a targeted mutation in the interferon-regulatory factor (IRF)-8 gene after withdrawal of sulfadiazine therapy. IRF-8(-/-) mice were orally infected with five cysts of the ME 49 strain of T. gondii. To allow establishment of latent infection with cyst formation, mice were treated with sulfadiazine starting either 3, 5, 6, or 7 days postinfection. Sulfadiazine was withdrawn after 14-21 days to allow reactivation. We observed that timing of sulfadiazine therapy had a marked impact on the course of infection and reactivation. Mice treated late after infection (days 5-7) showed increased mortality and decreased time to death compared to mice treated early after infection (group A). In the blood of mice with late (days 5-7) but not early (day 3) initiation of treatment, T. gondii-specific deoxyribonucleic acid was detected by polymerase chain reaction. Using double staining with stage-specific antibodies, tachyzoites were detectable in brains of mice with late initiation of sulfadiazine treatment but rarely within cysts thus indicating continued invasion of parasites across the blood-brain barrier. Intracerebral cyst rupture or bradyzoite-tachyzoite conversion was not detectable in IRF-8(-/-) mice when sulfadiazine therapy was initiated late after infection. These results indicate that continued invasion of tachyzoites rather than reactivation of latent cerebral infection impacts the course of infection in this model of reactivated toxoplasmosis. In conclusion, the timing of sulfadiazine therapy is of utmost importance for the course of infection in immunocompromised mice.

Use of MAG1 recombinant antigen for diagnosis of Toxoplasma gondii infection in humans

This paper describes the cloning, purification, and serological applications of matrix antigen MAG1 of Toxoplasma gondii. The expression system used allows the production of a large amount of T. gondii recombinant protein, which was assessed for its potential use in an enzyme-linked immunosorbent assay (ELISA) for detection of T. gondii infection in humans. Serum samples from 117 patients with different stages of infection, along with 10 serum samples from seronegative patients obtained for routine diagnostic tests, were used. The results were compared with those of an ELISA that uses a native T. gondii antigen extract. The MAG1 antigen detected antibodies more frequently from the acute stage (97.3%) than from the chronic stage (7.5%) of toxoplasmosis. Hence, this antigen may be used as a tool for detection of T. gondii immunoglobulin G antibodies in persons with acute toxoplasmosis.

Infection with Toxoplasma gondii bradyzoites has a diminished impact on host transcript levels relative to tachyzoite infection

Toxoplasma gondii, an intracellular pathogen, has the potential to infect nearly every warm-blooded animal but rarely causes morbidity. The ability for the parasite to convert to the bradyzoite stage and live inside slow-growing cysts that can go unnoticed by the host immune system allows for parasite persistence for the life of the infected host. This intracellular survival likely necessitates host cell modulation, and tachyzoites are known to modify a number of signaling cascades within the host to promote parasite survival. Little is known, however, about how bradyzoites manipulate their host cell. Microarrays were used to profile the host transcriptional changes caused by bradyzoite infection and compared to those of tachyzoite-infected and uninfected hosts cells 2 days postinfection in vitro. Infection resulted in chemokine, cytokine, extracellular matrix, and growth factor transcript level changes. A small group of genes were specifically induced by tachyzoite infection, including granulocyte-macrophage colony-stimulating factor, BCL2-related protein A1, and interleukin-24. Bradyzoite infection yielded only about half the changes seen with tachyzoite infection, and those changes that did occur were almost all of lower magnitude than those induced by tachyzoites. These results suggest that bradyzoites lead a more stealthy existence within the infected host cell.

Expression analysis of immune response genes of Müller cells infected with Toxoplasma gondii

PURPOSE

To investigate changes in immune response genes following Toxoplasma gondii infection of Müller cells.

METHODS

Human Müller cells were infected or mock infected with two strains of T. gondii (RH and Prugniaud). RNA and supernatants were collected from infected and uninfected cells at 2 and 24 h. RNA from the two time points were compared using a custom made DNA microarray. Real time PCR or human cytokine antibody array was used to confirm up-regulation of immune molecules.

RESULTS

Gene expression in infected cells showed up-regulation of CCL2, IL-6, CXCL8, and CXCL2. CCL2 and CXCL2 gene expression was confirmed by real time PCR. IL-6 and CXCL8 protein production was confirmed by a cytokine antibody array. IL-4 production was observed by cytokine antibody array but not by DNA microarray. In contrast, infection with T. gondii did not induce interferon-gamma (IFNgamma) and IL-12 expression, molecules conventionally associated with the inter-conversion of tachyzoite to bradyzoite.

CONCLUSION

These results indicate that while in vitro infected Müller cells may be capable of inducing an immune response by attracting blood-borne leucocytes, they do not appear able to directly control the proliferation of T. gondii.

Dense granules: are they key organelles to help understand the parasitophorous vacuole of all apicomplexa parasites?

Together with micronemes and rhoptries, dense granules are specialised secretory organelles of Apicomplexa parasites. Among Apicomplexa, Plasmodium represents a model of parasites propagated by way of an insect vector, whereas Toxoplasma is a model of food borne protozoa forming cysts. Through comparison of both models, this review summarises data accumulated over recent years on alternative strategies chosen by these parasites to develop within a parasitophorous vacuole and explores the role of dense granules in this process. One of the characteristics of the Plasmodium erythrocyte stages is to export numerous parasite proteins into both the host cell cytoplasm and/or plasma membrane via the vacuole used as a step trafficking compartment. Whether this feature can be correlated to few storage granules and a restricted number of dense granule proteins, is not yet clear. By contrast, the Toxoplasma developing vacuole is decorated by abundantly expressed dense granule proteins and is characterised by a network of membranous nanotubes. Although the exact function of most of these proteins remains currently unknown, recent data suggest that some of these dense granule proteins could be involved in building the intravacuolar membranous network. Conserved expression of the Toxoplasma dense granule proteins throughout most of the parasite stages suggests that they could also be key elements of the cyst formation.

The transcriptome of Toxoplasma gondii

BACKGROUND

Toxoplasma gondii gives rise to toxoplasmosis, among the most prevalent parasitic diseases of animals and man. Transformation of the tachzyoite stage into the latent bradyzoite-cyst form underlies chronic disease and leads to a lifetime risk of recrudescence in individuals whose immune system becomes compromised. Given the importance of tissue cyst formation, there has been intensive focus on the development of methods to study bradyzoite differentiation, although the molecular basis for the developmental switch is still largely unknown.

RESULTS

We have used serial analysis of gene expression (SAGE) to define the Toxoplasma gondii transcriptome of the intermediate-host life cycle that leads to the formation of the bradyzoite/tissue cyst. A broad view of gene expression is provided by >4-fold coverage from nine distinct libraries (approximately 300,000 SAGE tags) representing key developmental transitions in primary parasite populations and in laboratory strains representing the three canonical genotypes. SAGE tags, and their corresponding mRNAs, were analyzed with respect to abundance, uniqueness, and antisense/sense polarity and chromosome distribution and developmental specificity.

CONCLUSION

This study demonstrates that phenotypic transitions during parasite development were marked by unique stage-specific mRNAs that accounted for 18% of the total SAGE tags and varied from 1-5% of the tags in each developmental stage. We have also found that Toxoplasma mRNA pools have a unique parasite-specific composition with 1 in 5 transcripts encoding Apicomplexa-specific genes functioning in parasite invasion and transmission. Developmentally co-regulated genes were dispersed across all Toxoplasma chromosomes, as were tags representing each abundance class, and a variety of biochemical pathways indicating that trans-acting mechanisms likely control gene expression in this parasite. We observed distinct similarities in the specificity and expression levels of mRNAs in primary populations (Day-6 post-sporozoite infection) that occur prior to the onset of bradyzoite development that were uniquely shared with the virulent Type I-RH laboratory strain suggesting that development of RH may be arrested. By contrast, strains from Type II-Me49B7 and Type III-VEGmsj contain SAGE tags corresponding to bradyzoite genes, which suggests that priming of developmental expression likely plays a role in the greater capacity of these strains to complete bradyzoite development.