Stage differentiation of the protozoan parasite Toxoplasma gondii

Effective factors in the pathogenesis of Toxoplasmagondii

Toxoplasma gondii (T. gondii) is a cosmopolitan protozoan parasite in humans and animals. It infects about 30 % of the human population worldwide and causes potentially fatal diseases in immunocompromised hosts and neonates. For this study, five English-language databases (ScienceDirect, ProQuest, Web of Science, PubMed, and Scopus) and the internet search engine Google Scholar were searched. This review was accomplished to draw a global perspective of what is known about the pathogenesis of T. gondii and various factors affecting it. Virulence and immune responses can influence the mechanisms of parasite pathogenesis and these factors are in turn influenced by other factors. In addition to the host's genetic background, the type of Toxoplasma strain, the routes of transmission of infection, the number of passages, and different phases of parasite life affect virulence. The identification of virulence factors of the parasite could provide promising insights into the pathogenesis of this parasite. The results of this study can be an incentive to conduct more intensive research to design and develop new anti-Toxoplasma agents (drugs and vaccines) to treat or prevent this infection. In addition, further studies are needed to better understand the key agents in the pathogenesis of T. gondii.

Innate, adaptive, and cell-autonomous immunity against Toxoplasma gondii infection

Hosts have been fighting pathogens throughout the evolution of all infectious diseases. Toxoplasma gondii is one of the most common infectious agents in humans but causes only opportunistic infection in healthy individuals. Similar to antimicrobial immunity against other organisms, the immune response against T. gondii activates innate immunity and in turn induces acquired immune responses. After activation of acquired immunity, host immune cells robustly produce the proinflammatory cytokine interferon-γ (IFN-γ), which activates a set of IFN-γ-inducible proteins, including GTPases. IFN-inducible GTPases are essential for cell-autonomous immunity and are specialized for effective clearance and growth inhibition of T. gondii by accumulating in parasitophorous vacuole membranes. Recent studies suggest that the cell-autonomous immune response plays a protective role in host defense against not only T. gondii but also various intracellular bacteria. Moreover, the negative regulatory mechanisms of such strong immune responses are also important for host survival after infection. In this review, we will discuss in detail recent advances in the understanding of host defenses against T. gondii and the roles played by cell-autonomous immune responses.

Researchers are extensively studying immune responses to the single-celled parasite Toxoplasma gondii, which infects around one-third of humans, often harmlessly, but can cause life-threatening toxoplasmosis infections in patients with weakened immune systems. Masahiro Yamamoto and Miwa Sasai at Osaka University in Japan review recent advances in understanding the interactions between the immune system and the parasite. They consider non-specific ‘innate’ immune responses and also the ‘acquired’ responses that target specific parts of the parasite, referred to as antigens. Methods that selectively switch off genes in mice are revealing details presumed to also be relevant for humans. Significant molecules, molecular signaling pathways and immune-regulating processes are being identified. Recent studies suggest cell-autonomous immunity, the ability of host cells to defend themselves against attack, plays a significant role in fighting Toxoplasma gondii infection.

Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis

Tachyzoite to bradyzoite development in Toxoplasma is marked by major changes in gene expression resulting in a parasite that expresses a new repertoire of surface antigens hidden inside a modified parasitophorous vacuole called the tissue cyst. The factors that control this important life cycle transition are not well understood. Here we describe an important transcriptional repressor mechanism controlling bradyzoite differentiation that operates in the tachyzoite stage. The ApiAP2 factor, AP2IV-4, is a nuclear factor dynamically expressed in late S phase through mitosis/cytokinesis of the tachyzoite cell cycle. Remarkably, deletion of the AP2IV-4 locus resulted in the expression of a subset of bradyzoite-specific proteins in replicating tachyzoites that included tissue cyst wall components BPK1, MCP4, CST1 and the surface antigen SRS9. In the murine animal model, the mis-timing of bradyzoite antigens in tachyzoites lacking AP2IV-4 caused a potent inflammatory monocyte immune response that effectively eliminated this parasite and prevented tissue cyst formation in mouse brain tissue. Altogether, these results indicate that suppression of bradyzoite antigens by AP2IV-4 during acute infection is required for Toxoplasma to successfully establish a chronic infection in the immune-competent host.

The Toxoplasma biology that underlies the establishment of a chronic infection is developmental conversion of the acute tachyzoite stage into the latent bradyzoite-tissue cyst stage. Despite the important clinical consequences of this developmental pathway, the molecular basis of the switch mechanisms that control formation of the tissue cyst is still poorly understood. A fundamental feature of tissue cyst formation is the expression of bradyzoite-specific genes. Here we show the transcription factor AP2IV-4 directly silences bradyzoite mRNA and protein expression in the acute tachyzoite stage demonstrating that developmental control of tissue cyst formation is as much about when not to express bradyzoite genes as it is about when to activate them. Losing the suppression of bradyzoite gene expression in the acute tachyzoite stage caused by deleting AP2IV-4 blocked the establishment of chronic disease in healthy animals via increased protective immunity suggesting a possible strategy for preventing chronic Toxoplasma infections.

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.

Proteomic Differences between Developmental Stages of Toxoplasma gondii Revealed by iTRAQ-Based Quantitative Proteomics

Toxoplasma gondii has a complex two-host life-cycle between intermediate host and definitive host. Understanding proteomic variations across the life-cycle stages of T. gondii may improve the understanding of molecular adaption mechanism of T. gondii across life-cycle stages, and should have implications for the development of new treatment and prevention interventions against T. gondii infection. Here, we utilized LC–MS/MS coupled with iTRAQ labeling technology to identify differentially expressed proteins (DEPs) specific to tachyzoite (T), bradyzoites-containing cyst (C) and sporulated oocyst (O) stages of the cyst-forming T. gondii Prugniuad (Pru) strain. A total of 6285 proteins were identified in the three developmental stages of T. gondii. Our analysis also revealed 875, 656, and 538 DEPs in O vs. T, T vs. C, and C vs. O, respectively. The up- and down-regulated proteins were analyzed by Gene Ontology enrichment, KEGG pathway and STRING analyses. Some virulence-related factors and ribosomal proteins exhibited distinct expression patterns across the life-cycle stages. The virulence factors expressed in sporulated oocysts and the number of up-regulated virulence factors in the cyst stage were about twice as many as in tachyzoites. Of the 79 ribosomal proteins identified in T. gondii, the number of up-regulated ribosomal proteins was 33 and 46 in sporulated oocysts and cysts, respectively, compared with tachyzoites. These results support the hypothesis that oocyst and cystic stages are able to adapt to adverse environmental conditions and selection pressures induced by the host's immune response, respectively. These findings have important implications for understanding of the developmental biology of T. gondii, which may facilitate the discovery of novel therapeutic targets to better control toxoplasmosis.

Time patterns of recurrences and factors predisposing for a higher risk of recurrence of ocular toxoplasmosis

PURPOSE

To ascertain time patterns of recurrences and factors predisposing for a higher risk of recurrence of ocular toxoplasmosis.

METHODS

Retrospective observational case series with follow-up examination. Database of 4,381 patients with uveitis was used. Data of 84 patients with ocular toxoplasmosis (sample group) could be included.

RESULTS

Two hundred and eighty active lesions in the first affected eye were detected. The mean number of recurrences per year was 0.29 (standard deviation, 0.24). Median recurrence-free survival time was 2.52 years (95% confidence interval, 2.03-3.02 years). Risk of recurrence was highest in the first year after the most recent episode (26%) implying a decrease with increasing recurrence-free interval. The risk of recurrence decreased with the duration of disease (P < 0.001). Treatment of the first active lesion influenced the risk of recurrence (P = 0.048). Furthermore, the risk of recurrence was influenced by patient age at the time of the first active lesion (P = 0.021) and the most recent episode (P = 0.002).

CONCLUSION

A secondary antibiotic prophylaxis could be discussed 1) during the first year after an active lesion has occurred, especially in case of the first active lesion of ocular toxoplasmosis, and 2) in older patients, especially if primarily infected with Toxoplasma gondii at an older age.

Oxidative stress control by apicomplexan parasites

Apicomplexan parasites cause infectious diseases that are either a severe public health problem or an economic burden. In this paper we will shed light on how oxidative stress can influence the host-pathogen relationship by focusing on three major diseases: babesiosis, coccidiosis, and toxoplasmosis.

Risk of Recurrence of Preexisting Ocular Toxoplasmosis during Pregnancy

PURPOSE

Comparison between the risk of recurrence during pregnancy with nonpregnant periods in patients with ocular toxoplasmosis (OT).

METHODS

Records of 40 women were retrospectively reviewed. The women were subdivided into either a pregnant group or a control group formed by nonpregnant women during follow-up. Comparable cases from the literature were added. Mean number of recurrences per year was compared.

RESULTS

Eleven women with pregnancies could be included. Six cases from the literature were added (total n = 17). Mean number of recurrences per year during versus not during pregnancy was 0.16 (SD 0.39) versus 0.32 (SD 0.30; p = 0.088), respectively. In the control group (n = 29) the mean number of recurrences per year (0.31; SD 0.27) was significantly higher when compared to cases during pregnant periods (p = 0.002).

CONCLUSION

Adding to published series, the presented data do not provide evidence for a relevant impact of pregnancy on recurrence behavior in OT.

Characterization of mouse brain microRNAs after infection with cyst-forming Toxoplasma gondii

Background

The obligate intracellular parasite Toxoplasma gondii can interfere with host cell signaling pathways, alter host defense systems and cell cycle control, and establish a chronic infection in the central nervous system. T. gondii infection may alter the expression profile of host microRNAs (miRNAs) which have key regulatory functions at the post-transcriptional level.

Methods

Using high-throughput sequencing and real-time quantitative PCR technology, we compared the miRNA expression profiles of uninfected mouse brains with brains from mice at 14 days and 21 days after infection with cyst-forming T. gondii (Type II).

Results

A total of 51.30 million raw reads were obtained from all samples and 495 (14d infected mouse sample), 511 (14d sham-infected control), 504 (21d infected mouse sample) and 514 (21d sham-infected control) miRNA candidates identified. Among these, 414 miRNAs were consistent across all the studied groups, 17 were specific to the 14d infected group and 32 were specific to the 21d infected group. In addition, 9 miRNAs were common to both the 14d- and 21d-infected groups. Enrichment analysis for the targets of these miRNAs showed a high percentage of “protein tag” functions. Immune related targets including chemokines, cytokines, growth factors and interleukins were also found.

Conclusions

These results not only showed that the miRNA expression of the host can be changed by the invasion of cyst-forming T. gondii, but also indicated that the host attempts to respond using two tactics: marking proteins with “protein tags” and adaptation of immune related systems.

Possible role of Toxoplasma gondii in brain cancer through modulation of host microRNAs

Background

The obligate intracellular protozoan parasite Toxoplasma gondii infects humans and other warm-blooded animals and establishes a chronic infection in the central nervous system after invasion. Studies showing a positive correlation between anti-Toxoplasma antibodies and incidences of brain cancer have led to the notion that Toxoplasma infections increase the risk of brain cancer. However, molecular events involved in Toxoplasma induced brain cancers are not well understood.

Presentation of the hypothesis

Toxoplasma gains control of host cell functions including proliferation and apoptosis by channelizing parasite proteins into the cell cytoplasm and some of the proteins are targeted to the host nucleus. Recent studies have shown that Toxoplasma is capable of manipulating host micro RNAs (miRNAs), which play a central role in post-transcriptional regulation of gene expression. Therefore, we hypothesize that Toxoplasma promotes brain carcinogenesis by altering the host miRNAome using parasitic proteins and/or miRNAs.

Testing the hypothesis

The miRNA expression profiles of brain cancer specimens obtained from patients infected with Toxoplasma could be analyzed and compared with that of normal tissues as well as brain cancer tissues from Toxoplasma uninfected individuals to identify dysregulated miRNAs in Toxoplasma-driven brain cancer cells. Identified miRNAs will be further confirmed by studying cancer related miRNA profiles of the different types of brain cells before and after Toxoplasma infection using cell lines and experimental animals.

Expected outcome

The miRNAs specifically associated with brain cancers that are caused by Toxoplasma infection will be identified.

Implications of the hypothesis

Toxoplasma infection may promote initiation and progression of cancer by modifying the miRNAome in brain cells. If this hypothesis is true, the outcome of this research would lead to the development of novel biomarkers and therapeutic tools against Toxoplasma driven brain cancers.

Detection of murine toxoplasmosis using magnetic bead-based serum peptide profiling by MALDI-TOF MS

Establishment of a rapid, highly specific, and accurate method for diagnosis of Toxoplasma gondii infection is essential to control and prevent zoonotic toxoplasmosis. In this study, a novel diagnostic strategy using magnetic bead-based serum peptide profiling by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was developed. The serum peptides (samples I, II, and III) from T. gondii RH strain-infected mice at days 3, 6, and 9 post-infection (p.i.), and healthy mice were enriched by the optimized magnetic bead-based hydrophobic interaction (MB-HIC8). The mass spectrograms were acquired by MALDI-TOF MS, and analyzed by ClinProTools bioinformatics software from Bruker Daltonics. The diagnostic models from T. gondii RH-infected serum peptide profiling of samples I, II, and III were produced by genetic algorithms, and verified by cross-validation. The sample II model could correctly recognize T. gondii RH strain infection in mice at days 3, 6, and 9 p.i. with a sensitivity of 91.1% and a specificity of 96.7%., and also detect T. gondii ME49 strain-infected serum samples at days 3, 6, 9, and 12 p.i. with a sensitivity of 91.7%. The results of the present study suggest that serum peptide profiling by MALDI-TOF MS is a novel potential tool for the clinical diagnosis of acute T. gondii infection.

Toxoplasma on the brain: understanding host-pathogen interactions in chronic CNS infection

Toxoplasma gondii is a prevalent obligate intracellular parasite which chronically infects more than a third of the world's population. Key to parasite prevalence is its ability to form chronic and nonimmunogenic bradyzoite cysts, which typically form in the brain and muscle cells of infected mammals, including humans. While acute clinical infection typically involves neurological and/or ocular damage, chronic infection has been more recently linked to behavioral changes. Establishment and maintenance of chronic infection involves a balance between the host immunity and parasite evasion of the immune response. Here, we outline the known cellular interplay between Toxoplasma gondii and cells of the central nervous system and review the reported effects of Toxoplasma gondii on behavior and neurological disease. Finally, we review new technologies which will allow us to more fully understand host-pathogen interactions.

Integrative genomic approaches highlight a family of parasite-specific kinases that regulate host responses

Apicomplexan parasites release factors via specialized secretory organelles (rhoptries, micronemes) that are thought to control host cell responses. In order to explore parasite-mediated modulation of host cell signaling pathways, we exploited a phylogenomic approach to characterize the Toxoplasma gondii kinome, defining a 44 member family of coccidian-specific secreted kinases, some of which have been previously implicated in virulence. Comparative genomic analysis suggests that "ROPK" genes are under positive selection, and expression profiling demonstrates that most are differentially expressed between strains and/or during differentiation. Integrating diverse genomic-scale analyses points to ROP38 as likely to be particularly important in parasite biology. Upregulating expression of this previously uncharacterized gene in transgenic parasites dramatically suppresses transcriptional responses in the infected cell. Specifically, parasite ROP38 downregulates host genes associated with MAPK signaling and the control of apoptosis and proliferation. These results highlight the value of integrative genomic approaches in prioritizing candidates for functional validation.

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.

The differential protein expression profiles and immunogenicity of tachyzoites and bradyzoites of in vitro cultured Neospora caninum

We report a study on the variations in the protein expression profiles of tachyzoites and bradyzoites of Neospora caninum. The in vitro stage conversion of N. caninum-infected Vero cells was induced by continuous treatment of infected cultures with 70 muM sodium nitroprusside (SNP) for up to 9 days. The stage conversion indicated by the expression of the bradyzoite-specific antigen BAG1 was analyzed by immunofluoresence assay. Morphological changes between tachyzoites and bradyzoites and localization of nuclei were demonstrated by transmission electron microscopy. Notably, we showed the differential protein expression profiles of tachyzoites and bradyzoites of N. caninum upon treatment with SNP. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated different protein patterns between tachyzoites and bradyzoites. Furthermore, Western blotting using rabbit polyclonal antibodies directed against tachyzoites revealed several reactive bands, one of which represented a tachyzoite-specific antigen of approximately 40 kDa remarkably expressed in the tachyzoite stage, but was absent from bradyzoites. Moreover, rabbit polyclonal serum raised against bradyzoites recognized a significant increased expression of an antigen with a MW of approximately 25 kDa in bradyzoites by Western blotting, suggesting that this protein is specifically expressed at the bradyzoite stage. Taken together, our data showed that differential protein expression profiling is a useful tool for discriminating between the two stages during tachyzoite-bradyzoite interconversion in N. caninum infections.

Stress-related and spontaneous stage differentiation of Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular parasitic protozoan that infects a variety of avian and mammalian hosts including up to one third of the human population worldwide. Developmental differentiation between distinct stages, i.e. sporozoites, tachyzoites and bradyzoites is fundamental for the parasite life cycle and for transmission between hosts. It is also interconnected with the pathogenesis of overt toxoplasmosis and makes T. gondii an important opportunistic pathogen of humans. In order to delineate the underlying mechanisms, several cell culture differentiation systems have been developed which mimic the transition from fast-replicating tachyzoites to slowly proliferating bradyzoites in vitro. Since exogenous stress factors, i.e. alkaline pH, IFN-gamma and other proinflammatory cytokines, chemicals or drugs, heat shock, and deprivation of nutrients have been shown to increase the efficacy of bradyzoite development in vitro, Toxoplasma stage differentiation is largely viewed as a stress-related response to hostile environmental conditions. However, tachyzoite to bradyzoite differentiation also occurs spontaneously in vitro and this raises questions about the importance of stress conditions for triggering stage conversion. High frequencies of spontaneous bradyzoite development in primary and permanent skeletal muscle cells, i.e. cells that preferentially harbour bradyzoite-containing tissue cysts in vivo suggest that the host cell type may be critical. Furthermore, the host cell transcriptome, including the expression of distinct host cell genes, has recently been shown to trigger bradyzoite development and cyst formation. Together, these results strongly indicate that the complex cellular environment, besides exogenous stress factors, may govern the developmental differentiation of T. gondii.

Anionic sites on Toxoplasma gondii tissue cyst wall: Expression, uptake and characterization

Toxoplasmosis, caused by Toxoplasma gondii, is an important parasitic disease worldwide, which causes widespread human and animal diseases. The need for new therapeutic agents along with the biology of these parasites has fueled a keen interest in the understanding of the nutrients acquisition by these parasites. Studies on the characterization of the T. gondii cyst wall as well as the contribution of the host cell to this formation have been little explored. The aim of this paper was to investigate the electric surface charge of the T. gondii tissue cysts by ultrastructural cytochemistry, through polycationic markers, employing ruthenium red (RR) and cationized ferritin (CF). Glycosaminoglycans revealed by RR were localized on the cyst wall as a homogeneous granular layer electrondense, all over its surface. The incubation of living tissue cysts with CF for 20 min at 4 degrees C followed by the increase of temperature to 37 degrees C indicated that T. gondii cyst wall is negatively charged and that occurs an incorporation of anionic sites by the cyst wall, through vesicles and tubules, and their posterior location in the cyst matrix. So, as to identify which group of molecules produces negative charge in the cyst wall, we used enzymes for cleavage on different types of molecules, demonstrating that the negative charge in the cyst wall is mainly produced by phospholipids. Our results, described in this work show, for the first time, the negativities of the cyst wall, the incorporation and the traffic of intracellular surface molecules by T. gondii cyst wall. Our model of study can give an important contribution to the knowledge of the biology and the processes involved in nutrients acquisition by bradyzoites living inside the cysts and, and also be applied as a target for the direct action of drugs against the cyst.

Subversion of innate and adaptive immune responses by Toxoplasma Gondii

The intracellular apicomplexan parasite Toxoplasma gondii is able to survive and persist in immunocompetent intermediate hosts for the host's life span. This is despite the induction of a vigorous humoral and -- more importantly -- cell-mediated immune response during infection. In order to establish and maintain such chronic infections, however, T. gondii has evolved multiple strategies to avoid or to interfere with potentially efficient anti-parasitic immune responses of the host. Such immune evasion includes (1) indirect mechanisms by altering the expression and secretion of immunomodulatory cytokines or by altering the viability of immune cells and (2) direct mechanisms by establishing a lifestyle within a suitable intracellular niche and by interference with intracellular signaling cascades, thereby abolishing a number of antimicrobial effector mechanisms of the host. Despite the parasite's ability to interfere successfully with the host's efforts to eradicate the infection, the immune response is, however, not completely abrogated but is rather partially diminished after infection. T. gondii thus keeps a delicate balance between induction and suppression of the host's immune response in order to guarantee the survival of the host as a safe harbor for parasite development and to allow its transmission to the definitive host.

Detection of a novel parasite kinase activity at the Toxoplasma gondii parasitophorous vacuole membrane capable of phosphorylating host IkappaBalpha

Toxoplasma gondii activates the NF-kappaB pathway in the infected host cell resulting in upregulation of pro-survival genes and prevention of apoptosis. Manipulation of the NF-kappaB cascade by T. gondii correlates with the localization of phosphorylated IkappaB at the parasitophorous vacuole membrane (PVM). This suggests a parasite-mediated event, involving the recruitment and activation of the host IkappaB kinase (IKK) complex, as has been observed with the related protozoan Theileria parva. In contrast to Theileria, confocal microscopy studies showed no apparent hijacking of IKKalpha, IKKbeta, or their activated phosphorylated forms at the T. gondii PVM. Remarkably, phosphorylation of IkappaBalpha at Ser 32/36 was observed at the PVM of T. gondii-infected IKKalpha-/-, IKKbeta-/- and IKKalpha/beta double-knockout (IKKalpha/beta-/-) fibroblasts, suggesting the involvement of a parasite kinase activity independent of host IKK. The presence of a putative T. gondii IkappaB kinase was examined by in vitro kinase assays using GST-IkappaBalpha constructs and protein extracts from both extracellular parasites and PVM fractions. Interestingly, an activity capable of phosphorylating IkappaBalpha at the critical Ser 32/36 sites was identified in parasite extracts, a property restricted to the IKK signalosome. Taken together, our data support the role for a T. gondii kinase involved in phosphorylation of host cell IkappaBalpha and suggest an unusual mechanism utilized by an intracellular pathogen capable of manipulating the NF-kappaB pathway.

Performance of a Western immunoblot assay to detect specific anti-Toxoplasma gondii IgG antibodies in human saliva

Toxoplasma gondii represents the most prominent infectious parasitic organism found in humans. While normally asymptomatic in healthy individuals, toxoplasmosis can cause abortion in patients during pregnancy, or can be fatal in immunosupressed individuals such as persons suffering from acquired immunodeficiency syndrom (AIDS). Toxoplasma gondii infection in humans is routinely assesssed by serological means. Here, we show that detection of anti-T. gondii IgG is also possible using a non-invasive methodology employing saliva. Sera and saliva of 201 healthy volunteers were investigated for the presence of anti-T. gondii-IgG antibodies by immunoblotting. The sera of 59 (29.4%) individuals showed IgG antibodies against T. gondii by ELISA, Vidas, and immunoblotting; 58 (98.3%) of these were also positive for anti-T. gondii IgG in the saliva immunoblot, with diagnostic relevant bands of Mr of 32-35 kDa and 40-45 kDa. The saliva immunoblot test exhibits a specificity of 100% and a sensitivity of 98.5%. Thus, saliva could be used as an alternative, non-invasive means for the detection of specific anti-T. gondii IgG in humans.

DIFFERENTIAL EFFECTS OF INTERFERON-γ AND TUMOR NECROSIS FACTOR-α ON TOXOPLASMA GONDII PROLIFERATION IN ORGANOTYPIC RAT BRAIN SLICE CULTURES

Organotypic slice culture explants of rat cortical tissue infected with Toxoplasma gondii tachyzoites were applied as an in vitro model to investigate host-pathogen interactions in cerebral toxoplasmosis. The kinetics of parasite proliferation and the effects of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) in infected organotypic cultures were monitored by light microscopy, transmission electron microscopy (TEM), and quantitative polymerase chain reaction (PCR) assay. As assessed by the loss of the structural integrity of the glial fibrillary acidic protein-intermediate filament network, tachyzoites infected and proliferated mainly within astrocytes, whereas neurons and microglia remained largely unaffected. Toxoplasma gondii proliferation was severely inhibited by IFN-y. However, this inhibition was not linked to tachyzoite-to-bradyzoite stage conversion. In contrast, TNF-alpha treatment resulted in a dramatically enhanced proliferation rate of the parasite. The cellular integrity in IFN-gamma-treated organotypic slice cultures was severely impaired compared with untreated and TNF-alpha-treated cultures. Thus, on infection of organotypic neuronal cultures, IFN-gamma and TNF-alpha exhibit largely detrimental effects, which could contribute to either inhibition or acceleration of parasite proliferation during cerebral toxoplasmosis.

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

Toxoplasma gondii has a complex life cycle involving definite (cat) and intermediate (all warm blooded animals) hosts. This gives rise to four infectious forms each of which has a distinctive biological role. Two (tachyzoite and merozoite) are involved in propagation within a host and two (bradyzoite and sporozoite) are involved in transmission to new hosts. The various forms can be identified by their structure, host parasite relationship and distinctive developmental processes. In the present in vivo study, the various stages have been evaluated by electron microscopy and immunocytochemistry using a panel of molecular markers relating to surface and cytoplasmic molecules, metabolic iso-enzymes and secreted proteins that can differentiate between tachyzoite, bradyzoite and coccidian development. Tachyzoites were characterised as being positive for surface antigen 1, enolase isoenzyme 2, lactic dehydrogenase isoenzyme 1 and negative for bradyzoite antigen 1. In contrast, bradyzoites were negative for SAG1 but positive for BAG1, ENO1 and LDH2. When stage conversion was followed in brain lesion at 10 and 15 days post-infection, tachyzoites were predominant but a number of single intermediate organisms displaying tachyzoite and certain bradyzoite markers were observed. At later time points, small groups of organisms displaying only bradyzoite markers were also present. A number (9) of dense granule proteins (GRA1-8, NTPase) have also been identified in both tachyzoites and bradyzoites but there were differences in their location during parasite development. All the dense granule proteins extensively label the parasitophorous vacuole during tachyzoite development. In contrast the tissue cyst wall displays variable staining for the dense granule proteins, which also expresses an additional unique cyst wall protein. The molecular differences could be identified at the single cell stage consistent with conversion occurring at the time of entry into a new cell. These molecular differences were reflected in the structural differences in the parasitophorous vacuoles observed by electron microscopy. Stage conversion to enteric (coccidian) development was limited to the enterocytes of the cat small intestine. Although no specific markers were available, this form of development can be identified by the absence of specific tachyzoite (SAG1) and bradyzoite (BAG1) markers although the isoenzymes ENO2 and LHD1 were expressed. There was also a significant difference in the expression of the dense granule proteins. The coccidian stages and merozoites only expressed two (GRA7 and NTPase) of the nine dense granule proteins and this was reflected in significant differences in the structure of the parasitophorous vacuole. The coccidian stages also undergo conversion from asexual to sexual development. The mechanism controlling this process is unknown but does not involve any change in the host cell type or parasitophorous vacuole and may be pre-programmed, since the number of asexual cycles was self-limiting. In conclusion, it was possible using a combination of molecular markers to identify tachyzoite, bradyzoite and coccidian development in tissue sections.

Identification and characterisation of a regulatory region in the Toxoplasma gondii hsp70 genomic locus

Toxoplasma gondii is an important human and veterinary pathogen. 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 induction. Heat shock or stress induced activation of a set of heat shock protein genes, is characteristic of almost all eukaryotic and prokaryotic cells. Studies in other organisms indicate that heat shock proteins are developmentally regulated. We have established that increases in the expression of bag1/hsp30 and hsp70 are associated with bradyzoite development. The T. gondii hsp70 gene locus was cloned and sequenced. The regulatory regions of this gene were analysed by deletion analysis using beta-galactosidase expression vectors transiently transfected into RH strain T. gondii. Expression was measured at pH 7.1 and 8.1 (i.e. pH shock) and compared to the expression obtained with similar constructs using BAG1 and SAG1 promoters. A pH-regulated region of the Tg-hsp70 gene locus was identified which has some similarities to heat shock elements described in other eukaryotic systems. Green fluorescent protein expression vectors driven by the Tg-hsp70 regulatory region were constructed and stably transfected into T. gondii. Expression of green fluorescent protein in these parasites was induced by pH shock in those lines carrying the Tg-hsp70 regulatory constructs. Gel shift analysis was carried out using oligomers corresponding to the pH-regulated region and a putative DNA binding protein was identified. These data support the identification of a pH responsive cis-regulatory element in the T. gondii hsp70 gene locus. A model of the interaction of hsp70 and small heat shock proteins (e.g. BAG1) in development is presented.

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.

Toxoplasma gondii eukaryotic translation initiation factor 4A associated with tachyzoite virulence is down-regulated in the bradyzoite stage

Comparative proteome analysis of mouse-virulent and attenuated Toxoplasma gondii strain revealed that steady-state synthesis of an unknown 53 kDa protein is markedly reduced in attenuated parasites. The results from protein microsequencing allowed isolation of a single-copy gene encoding a T. gondii homologue of eukaryotic translation initiation factor (eIF)4A. The deduced primary structure exhibits all sequence motifs typical of eIF4A. Differential expression of eIF4A between virulent and attenuated parasites was reconfirmed by immunoblot. Consistent with an involvement in the ribosomal preinitiation complex, the protein was localised in the tachyzoite extranuclear cytosol, being loosely associated with microsomal particles. Immunofluorescence detection of eIF4A in T. gondii stages of the intermediate host indicated that the protein is tachyzoite-specific. Stage-dependent expression is regulated at the transcriptional level as determined by reverse transcription-polymerase chain reaction and immunoblot. The down-regulation of eIF4A in attenuated T. gondii parasites and in the bradyzoite stage implies a role in tuning of the homeostasis of protein biosynthesis.

Characterisation of hexokinase in Toxoplasma gondii tachyzoites

We have cloned the hexokinase [E.C. 2.7.1.1] gene of Toxoplasma gondii tachyzoite and obtained an active recombinant enzyme with a calculated molecular mass of 51,465Da and an isoelectric point of 5.82. Southern blot analysis indicated that the hexokinase gene existed as a single copy in the tachyzoites of T. gondii. The sequence of T. gondii hexokinase exhibited the highest identity (44%) to that of Plasmodium falciparum hexokinase and lower identity of less than 35% to those of hexokinases from other organisms. The specific activity of the homogeneously purified recombinant enzyme was 4.04 micromol/mg protein/min at 37 degrees C under optimal conditions. The enzyme could use glucose, fructose, and mannose as substrates, though it preferred glucose. Adenosine triphosphate was exclusively the most effective phosphorus donor, and pyrophosphate did not serve as a substrate. K(m) values for glucose and adenosine triphosphate were 8.0+/-0.8 microM and 1.05+/-0.25mM, respectively. No allosteric effect of substrates was observed, and the products, glucose 6-phosphate and adenosine diphosphate, had no inhibitory effect on T. gondii hexokinase activity. Other phosphorylated hexoses, fructose 6-phosphate, trehalose 6-phosphate which is an inhibitor of yeast hexokinase, and pyrophosphate, also did not affect T. gondii hexokinase activity. Native hexokinase activity was recovered in both the cytosol and membrane fractions of the whole lysate of T. gondii tachyzoites. This result suggests that T. gondii hexokinase weakly associates with the membrane or particulate fraction of the tachyzoite cell.

Characterization of TgROP9 (p36), a novel rhoptry protein of Toxoplasma gondii tachyzoites identified by T cell clone

T cell clone 3Tx19 detects a Toxoplasma gondii tachyzoite protein which, in high resolution 2D gel electrophoresis, runs at 36 kDa apparent MW with two spots of pI 5.9 and 6.5, thus exhibiting a migration pattern distinct from those of other known Toxoplasma antigens. The sequences of peptide fragments from tryptic digestion of the more prominent protein spot allowed the design of oligonucleotide primers to obtain the coding cDNA sequence. Sequence analysis of cDNA from strain BK revealed a 363 amino acid open reading frame, defined by all nine peptide sequences determined. The deduced protein sequence contains two hydrophobic segments, one near the N-terminus including a predicted signal peptide and a shorter second at the carboxy terminus, but homology to any other known protein is lacking. With synthetic peptides covering the complete primary structure, the epitope for clone 3Tx19 was mapped within the deduced partial sequence, which had remained unconfirmed by tryptic peptides. Antibodies raised against another, putative B cell epitope peptide detected the same two protein spots in 2D gel, indicating that they are antigenically related isoforms. The protein p36 is expressed by T. gondii isolates of all three intraspecies subgroups, but not in the bradyzoite stage. In intracellular tachyzoites, p36 colocalizes with rhoptry proteins and has a distribution pattern disparate from that of dense granule and microneme proteins. Subcellular fractionation indicated that p36 is a soluble constituent of tachyzoites. We suggest that this T cell-stimulatory novel rhoptry protein of T. gondii be named ROP9. It represents a marker of the tachyzoite stage.

Toxoplasma gondii and MHC-restricted antigen presentation: on degradation, transport and modulation

Resistance against Toxoplasma gondii, an obligate intracellular protozoan parasite surrounded by a parasitophorous vacuolar membrane, is mediated by the cellular arm of the immune system, namely CD8+ and CD4+ T cells. Thus, priming and activation of these cells by presentation of antigenic peptides in the context of major histocompatibility complex class I and class II molecules have to take place. This is despite the fact that the vacuolar membrane avoids fusion with the endocytic compartment and acts like a molecular sieve, restricting passive diffusion of larger molecules. This raises several cell biological and immunological questions which will be discussed in this review in the context of our current knowledge about major histocompatibility complex-restricted antigen presentation in other systems: (1) By which pathways are parasite-derived antigens presented to T cells? (2) Has the parasite evolved mechanisms to interfere with major histocompatibility complex-restricted antigen presentation in order to avoid immune recognition? (3) To what extent and by which mechanism is antigenic material, originating from the parasite, able to pass through the vacuolar membrane into the cytosol of the infected cell and is it then accessible to the antigen presentation machinery of the infected cell? (4) What are the actual antigen-presenting cells which prime specific T cells in lymphoid organs? An understanding of these mechanisms will not only provide new insights into the pathogenesis of Toxoplasma gondii and possibly other intravacuolar parasites, but will also improve vaccination strategies.

Tachyzoite-specific isoform of Toxoplasma gondii lactate dehydrogenase is the target antigen of a murine CD4(+) T-cell clone

In two-dimensionally separated Toxoplasma gondii lysate, mouse Th1 clone 3Tx15 detects two proteins of apparent molecular weight 40000 and pI of 5.8 and 5.9. Microsequencing of peptide fragments from tryptic digestion of one of these proteins yielded partial sequences of T. gondii lactate dehydrogenase (LDH)1. As shown by Western blot, toxoplasmic LDH co-migrates in two-dimensional gel electrophoresis with both T-cell antigenic proteins. With synthetic peptides spanning the complete primary structure of T. gondii LDH1, the T-cell epitope was mapped to a nine amino acid partial sequence which exhibits a motif for binding to I-E(k), the class II restriction element of antigen recognition by clone 3Tx15. From the two known isoforms of T. gondii LDH, clone 3Tx15 specifically recognises tachyzoite LDH1, but not bradyzoite LDH2, as shown with the corresponding epitope peptides and recombinant proteins. Antigen-presenting cells infected with live bradyzoites stimulate 3Tx15 T cells, while killed bradyzoites provide no antigenic stimulus. This finding implies that a transformation into the tachyzoite stage occurs in cells challenged with bradyzoites. Although LDH1 represents one major constituent of the tachyzoite proteome, the protein does not seem to be immunogenic in T. gondii infection of mice. This is evident from the lack of serum anti-LDH immunoreactivity and the failure of adoptively transferred 3Tx15 T cells to protect against lethal challenge. In conclusion, a T-cell-stimulatory Toxoplasma antigen is identified by means of a novel, high-resolution T-cell blot technique, the clones antigenic fine specificity allowing detection of parasite-stage conversion.

Initial characterization of CST1, a Toxoplasma gondii cyst wall glycoprotein

Toxoplasma gondii is an important protozoan pathogen of humans that can cause encephalitis in immunocompromised individuals such as those with AIDS. This encephalitis is due to reactivation of latent infection in T. gondii-seropositive patients. Latent organisms survive within tissue cysts, which are specialized parasitophorous vacuoles containing bradyzoites. The cyst wall of this structure is produced by modification of the parasitophorous vacuole by the parasite and is important in cyst survival. The components of the cyst wall have been poorly characterized. By using immunofluorescence and immunoelectron microscopy, we have identified a monoclonal antibody (MAb 93.18) that reacts with the cyst wall. This antibody recognizes a 116-kDa glycoprotein, which we have termed CST1, containing sugar residues that bind Dolichos biflorans lectin (DBA). CST1 is distinct from T. gondii antigen labeled with succinyl Triticum vulgare lectin (S-WGA) and represents the major DBA-binding component in T. gondii. The carbohydrate components of the tissue cyst, such as CST1, are probably important in both providing stability and facilitating persistence in its host. As is seen in the carbohydrate capsules of fungi, glycoproteins in the T. gondii cyst wall may protect cysts from the immune response of the host. Further characterization of the formation of the cyst wall and its components should lead to insights into the mechanism of tissue cyst persistence and may suggest novel therapeutic approaches to eliminate tissue cysts of this organism.