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

Dense granule protein 3 of Toxoplasma gondii plays a crucial role in the capability of the tissue cysts of the parasite to persist in the presence of anti-cyst CD8+ T cells during the chronic stage of infection

Toxoplasma gondii establishes chronic infection by forming tissue cysts, and this chronic infection is one of the most common parasitic infections in humans. Our recent studies revealed that whereas CD8+ T cells of genetically resistant BALB/c mice have the capability to remove the tissue cysts of the parasite through their perforin-mediated activities, small portions of the cysts are capable of persisting in the presence of the anti-cyst CD8+ T cells. It is currently unknown how those small portions of the cysts resist or escape the T-cell immunity and persist in the hosts. In the present study, we discovered that the cysts, which persisted in the presence of the perforin-mediated CD8+ T-cell immunity, have significantly greater mRNA levels for four dense granule proteins, GRA1, GRA2, GRA3, and GRA7, and one rhoptry protein, ROP35, than the total population of the cysts present in the absence of the T cells. In addition, increased levels of mRNA for GRA1, GRA3, and ROP35 in the cysts significantly correlated with their successful persistence through the condition in which greater degrees of reduction of the cyst burden occurred through anti-cyst CD8+ T cells. In addition, GRA3-deficient T. gondii displayed significantly enhanced elimination of the cysts by anti-cyst CD8+ T cells when compared to the wild-type parasite. These results indicate that GRA3 is a key molecule that mediates in the capability of T. gondii cysts to persist by resisting or evading the anti-cyst activity of CD8+ T cells during the later stage of infection.

Characterization of Neospora caninum virulence factors NcGRA7 and NcROP40 in bovine target cells

Bovine neosporosis is one of the major causes of reproductive failure in cattle worldwide, and differences in virulence between isolates have been widely shown. However, the molecular basis and mechanisms underlying virulence in Neospora caninum are mostly unknown. Recently, we demonstrated the involvement of NcGRA7 and NcROP40 in the virulence of N. caninum in a pregnant murine model using single knockout mutants in these genes generated by CRISR/Cas9 technology. In this study, the role of these proteins was investigated in two in vitro models using bovine target cells: trophoblast (F3 cell line) and monocyte-derived macrophages (BoMØ). The proliferation capacity of the single knockout mutant parasites was compared to the wild-type strain, the Nc-Spain7 isolate, using both cell populations. For the bovine trophoblast, no differences were observed in the growth of the defective parasites compared to the wild-type strain, neither in the proliferation kinetics nor in the competition assay. However, in naïve BoMØ, a significant decrease in the proliferation capacity of the mutant parasites was observed from 48 h pi onwards. Stimulation of BoMØ with IFN-γ showed a similar inhibition of tachyzoite growth in defective and wild-type strains in a dose-dependent manner. Finally, BoMØ infected with knockout parasites showed higher expression levels of TLR3, which is involved in pathogen recognition. These results suggest that NcGRA7 and NcROP40 may be involved in the manipulation of innate immune defense mechanisms against neosporosis and confirm the usefulness of the BoMØ model for the evaluation of N. caninum virulence mechanisms. However, the specific functions of these proteins remain unknown, opening the way for future research.

The determinants regulating Toxoplasma gondii bradyzoite development

Toxoplasma gondii is an obligate intracellular zoonotic pathogen capable of infecting almost all cells of warm-blooded vertebrates. In intermediate hosts, this parasite reproduces asexually in two forms, the tachyzoite form during acute infection that proliferates rapidly and the bradyzoite form during chronic infection that grows slowly. Depending on the growth condition, the two forms can interconvert. The conversion of tachyzoites to bradyzoites is critical for T. gondii transmission, and the reactivation of persistent bradyzoites in intermediate hosts may lead to symptomatic toxoplasmosis. However, the mechanisms that control bradyzoite differentiation have not been well studied. Here, we review recent advances in the study of bradyzoite biology and stage conversion, aiming to highlight the determinants associated with bradyzoite development and provide insights to design better strategies for controlling toxoplasmosis.

Transcriptional signatures of clonally derived Toxoplasma tachyzoites reveal novel insights into the expression of a family of surface proteins

Toxoplasma gondii has numerous, large, paralogous gene families that are likely critical for supporting its unparalleled host range: nearly any nucleated cell in almost any warm-blooded animal. The SRS (SAG1-related sequence) gene family encodes over 100 proteins, the most abundant of which are thought to be involved in parasite attachment and, based on their stage-specific expression, evading the host immune response. For most SRS proteins, however, little is understood about their function and expression profile. Single-parasite RNA-sequencing previously demonstrated that across an entire population of lab-grown tachyzoites, transcripts for over 70 SRS genes were detected in at least one parasite. In any one parasite, however, transcripts for an average of only 7 SRS genes were detected, two of which, SAG1 and SAG2A, were extremely abundant and detected in virtually all. These data do not address whether this pattern of sporadic SRS gene expression is consistently inherited among the progeny of a given parasite or arises independently of lineage. We hypothesized that if SRS expression signatures are stably inherited by progeny, subclones isolated from a cloned parent would be more alike in their expression signatures than they are to the offspring of another clone. In this report, we compare transcriptomes of clonally derived parasites to determine the degree to which expression of the SRS family is stably inherited in individual parasites. Our data indicate that in RH tachyzoites, SRS genes are variably expressed even between parasite samples subcloned from the same parent within approximately 10 parasite divisions (72 hours). This suggests that the pattern of sporadically expressed SRS genes is highly variable and not driven by inheritance mechanisms, at least under our conditions.

The Secreted Acid Phosphatase Domain-Containing GRA44 from Toxoplasma gondii Is Required for c-Myc Induction in Infected Cells

During host cell invasion, the eukaryotic pathogen Toxoplasma gondii forms a parasitophorous vacuole to safely reside within the cell, while it is partitioned from host cell defense mechanisms. From within this safe niche, parasites sabotage multiple host cell systems, including gene expression, apoptosis, and intracellular immune recognition, by secreting a large arsenal of effector proteins. Many parasite proteins studied for active host cell manipulative interactions have been kinases. The translocation of effectors from the parasitophorous vacuole into the host cell is mediated by a putative translocon complex, which includes the proteins MYR1, MYR2, and MYR3. Whether other proteins are involved in the structure or regulation of this putative translocon is not known. We have discovered that the secreted protein GRA44, which contains a putative acid phosphatase domain, interacts with members of this complex and is required for host cell effects downstream of effector secretion. We have determined that GRA44 is processed in a region with homology to sequences targeted by protozoan proteases of the secretory pathway and that both major cleavage fragments are secreted into the parasitophorous vacuole. Immunoprecipitation experiments showed that GRA44 interacts with a large number of secreted proteins, including MYR1. Importantly, conditional knockdown of GRA44 resulted in a lack of host cell c-Myc upregulation, which mimics the phenotype seen when members of the translocon complex are genetically disrupted. Thus, the putative acid phosphatase GRA44 is crucial for host cell alterations during Toxoplasma infection and is associated with the translocon complex which Toxoplasma relies upon for success as an intracellular pathogen.IMPORTANCE Approximately one-third of humans are infected with the parasite Toxoplasma gondiiToxoplasma infections can lead to severe disease in those with a compromised or suppressed immune system. Additionally, infections during pregnancy present a significant health risk to the developing fetus. Drugs that target this parasite are limited, have significant side effects, and do not target all disease stages. Thus, a thorough understanding of how the parasite propagates within a host is critical in the discovery of novel therapeutic targets. Toxoplasma replication requires that it enter the cells of the infected organism. In order to survive the environment inside a cell, Toxoplasma secretes a large repertoire of proteins, which hijack a number of important cellular functions. How these Toxoplasma proteins move from the parasite into the host cell is not well understood. Our work shows that the putative phosphatase GRA44 is part of a protein complex responsible for this process.

Identification of Fis1 Interactors in Toxoplasma gondii Reveals a Novel Protein Required for Peripheral Distribution of the Mitochondrion

Toxoplasma gondii's single mitochondrion is very dynamic and undergoes morphological changes throughout the parasite's life cycle. During parasite division, the mitochondrion elongates, enters the daughter cells just prior to cytokinesis, and undergoes fission. Extensive morphological changes also occur as the parasite transitions from the intracellular environment to the extracellular environment. We show that treatment with the ionophore monensin causes reversible constriction of the mitochondrial outer membrane and that this effect depends on the function of the fission-related protein Fis1. We also observed that mislocalization of the endogenous Fis1 causes a dominant-negative effect that affects the morphology of the mitochondrion. As this suggests that Fis1 interacts with proteins critical for maintenance of mitochondrial structure, we performed various protein interaction trap screens. In this manner, we identified a novel outer mitochondrial membrane protein, LMF1, which is essential for positioning of the mitochondrion in intracellular parasites. Normally, while inside a host cell, the parasite mitochondrion is maintained in a lasso shape that stretches around the parasite periphery where it has regions of coupling with the parasite pellicle, suggesting the presence of membrane contact sites. In intracellular parasites lacking LMF1, the mitochondrion is retracted away from the pellicle and instead is collapsed, as normally seen only in extracellular parasites. We show that this phenotype is associated with defects in parasite fitness and mitochondrial segregation. Thus, LMF1 is necessary for mitochondrial association with the parasite pellicle during intracellular growth, and proper mitochondrial morphology is a prerequisite for mitochondrial division.IMPORTANCEToxoplasma gondii is an opportunistic pathogen that can cause devastating tissue damage in the immunocompromised and congenitally infected. Current therapies are not effective against all life stages of the parasite, and many cause toxic effects. The single mitochondrion of this parasite is a validated drug target, and it changes its shape throughout its life cycle. When the parasite is inside a cell, the mitochondrion adopts a lasso shape that lies in close proximity to the pellicle. The functional significance of this morphology is not understood and the proteins involved are currently not known. We have identified a protein that is required for proper mitochondrial positioning at the periphery and that likely plays a role in tethering this organelle. Loss of this protein results in dramatic changes to the mitochondrial morphology and significant parasite division and propagation defects. Our results give important insight into the molecular mechanisms regulating mitochondrial morphology.

Identification of a Master Regulator of Differentiation in Toxoplasma

Toxoplasma gondii chronically infects a quarter of the world's population, and its recrudescence can cause life-threatening disease in immunocompromised individuals and recurrent ocular lesions in the immunocompetent. Acute-stage tachyzoites differentiate into chronic-stage bradyzoites, which form intracellular cysts resistant to immune clearance and existing therapies. The molecular basis of this differentiation is unknown, despite being efficiently triggered by stresses in culture. Through Cas9-mediated screening and single-cell profiling, we identify a Myb-like transcription factor (BFD1) necessary for differentiation in cell culture and in mice. BFD1 accumulates during stress and its synthetic expression is sufficient to drive differentiation. Consistent with its function as a transcription factor, BFD1 binds the promoters of many stage-specific genes and represents a counterpoint to the ApiAP2 factors that dominate our current view of parasite gene regulation. BFD1 provides a genetic switch to study and control Toxoplasma differentiation and will inform prevention and treatment of chronic infections.

Detection of Antibodies Against Toxoplasma gondii in Indian Cattle by Recombinant SAG2 Enzyme-Linked Immunosorbent Assay

BACKGROUND

A large number of recombinant proteins are tested over a period of time for diagnosis of toxoplasmosis throughout the world. However, such literature is very much limited from an Indian perspective.

PURPOSE

The aim of the study was to assess the sero prevalence of Toxoplasma gondii in adult cattle from Kerala, India, using recombinant surface antigen 2 (recSAG2) protein.

METHODS

An antibody detection recombinant ELISA specific for T gondii was laboratory-standardized using recSAG2 protein. The optimum antigen concentration, serum concentration, and conjugate dilutions were determined by the initial checkerboard titrations. Subsequently, the diagnostic potential of the recombinant protein was assessed with 258 field sera samples cattle and compared with indirect fluorescent antibody test (IFAT).

RESULTS

Among the cattle sera tested, 61.5% showed sero positivity of T. gondii-speciic IgG. When compared to IFAT, the sensitivity of the recSAG2 ELISA was found to be 80.00% with 88.57% specificity at 95% confidence interval with substantial agreement between the tests.

CONCLUSION

The results of present study have revealed the presence of high seroprevalence of the parasite and, hence, immense public health significance.

A systematic review of Toxoplasma gondii antigens to find the best vaccine candidates for immunization

At present, there is not any available accepted vaccine for prevention of Toxoplasma gondii (T. gondii) in human and animals. We conducted literature search through English (Google Scholar, PubMed, Science Direct, Scopus, EBSCO, ISI Web of Science) scientific paper databases to find the best vaccine candidates against toxoplasmosis among T. gondii antigens. Articles with information on infective stage, pathogenicity, immunogenicity and characterization of antigens were selected. We considered that the ideal and significant vaccines should include different antigens and been expressed in all infective stages of the parasite with a high pathogenicity and immunogenicity. Evaluation within this systematic review indicates that MIC 3, 4, 13, ROP 2, RON 5, GRA 1, 6, 8, 14 are expressed in all three infective stages and have pathogenicity and immunogenicity. MIC 5, ROM 4, GRA 2, 4, 15, ROP 5, 16, 17, 38, RON 4, MIC 1, GRA 10, 12, 16, SAG 3 are expressed in only tachyzoites and bradyzoites stages of T. gondii with pathogenicity/immunogenicity. Some antigens appeared to be expressed in a single stage (tachyzoites) but have high pathogenicity and induce immune response. They include enolase2 (ENO2), SAG 1, SAG5D, HSP 70, ROM 1, ROM 5, AMA 1, ROP 18, RON2 and GRA 24. In conclusion, current vaccination against T. gondii infection is not satisfactory, and with the increasing number of high-risk individuals, the development of an effective and safe specific vaccine is greatly valuable for toxoplasmosis prevention. This systematic review reveals prepare candidates for immunization studies.

Seroprevalence of sarcocystosis in the local communities of Pangkor and Tioman Islands using recombinant surface antigens 3 (rSAG3) of Sarcocystis falcatula

OBJECTIVE

To investigate the seroprevalence of Sarcocystosis in the local communities of Pangkor and Tioman islands, Malaysia, by using antigenic recombinant surface antigens 2 and 3 from Sarcocystis falcatula (rSfSAG2 and rSfSAG3) as the target proteins via Western blot and ELISA assays.

METHODS

SfSAG2 and SfSAG3 genes were isolated from S. falcatula and expressed in Escherichia coli expression system. A total of 348 serum samples [volunteers from both islands (n = 100), non-Sarcocystis parasitic infections patients (n = 50) and healthy donors (n = 100)] were collected and tested with purified SfSAGs in Western blot and ELISA assays to measure the seroprevalence of human sarcocystosis.

RESULTS

None of the sera in this study reacted with rSfSAG2 by Western blot and ELISA. For rSfSAG3, relatively high prevalence of sarcocystosis was observed in Tioman Island (75.5%) than in Pangkor Island (34%) by Western blot. In ELISA, the different prevalence rate was observed between Tioman Island (43.8%) and Pangkor Island (37%). The prevalence rate in other parasitic infections (amoebiasis, cysticercosis, filariasis, malaria, toxocariasis and toxoplasmosis) was 30% by Western blot and 26% by ELISA. Only 8% (by Western blot) and 10% (by ELISA) of healthy donors showed reactivity towards rSfSAG3.

CONCLUSION

This is the first study reporting a seroprevalence of sarcocystosis in Pangkor and Tioman Islands, Malaysia. The combination of Western blot and ELISA is suitable to be used for serodiagnosis of sarcocystosis. With further evaluations, SfSAG3 can potentially be used to confirm infection, asymptomatic screening, surveillance and epidemiological studies.

Observations on bradyzoite biology

Tachyzoites of the Apicomplexan Toxoplasma gondii cause acute infection, disseminate widely in their host, and eventually differentiate into a latent encysted form called bradyzoites that are found within tissue cysts. During latent infection, whenever transformation to tachyzoites occurs, any tachyzoites that develop are removed by the immune system. In contrast, cysts containing bradyzoites are sequestered from the immune system. In the absence of an effective immune response released organisms that differentiate into tachyzoites cause acute infection. Tissue cysts, therefore, serve as a reservoir for the reactivation of toxoplasmosis when the host becomes immunocompromised by conditions such as HIV infection, organ transplantation, or due to the impaired immune response that occurs when pathogens are acquired in utero. While tachyzoites and bradyzoites are well defined morphologically, there is no clear consensus on how interconversion occurs or what exact signal(s) mediate this transformation. Advances in research methods have facilitated studies on T. gondii bradyzoites providing important new insights into the biology of latent infection.

A latent ability to persist: differentiation in Toxoplasma gondii

A critical factor in the transmission and pathogenesis of Toxoplasma gondii is the ability to convert from an acute disease-causing, proliferative stage (tachyzoite), to a chronic, dormant stage (bradyzoite). The conversion of the tachyzoite-containing parasitophorous vacuole membrane into the less permeable bradyzoite cyst wall allows the parasite to persist for years within the host to maximize transmissibility to both primary (felids) and secondary (virtually all other warm-blooded vertebrates) hosts. This review presents our current understanding of the latent stage, including the factors that are important in bradyzoite induction and maintenance. Also discussed are the recent studies that have begun to unravel the mechanisms behind stage switching.

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.

Differential Roles for Inner Membrane Complex Proteins across Toxoplasma gondii and Sarcocystis neurona Development

The inner membrane complex (IMC) is a defining feature of apicomplexan parasites key to both their motility and unique cell division. To provide further insights into the IMC, we analyzed the dynamics and functions of representative alveolin domain-containing IMC proteins across developmental stages. Our work shows universal but distinct roles for IMC1, -3, and -7 during Toxoplasma asexual division but more specialized functions for these proteins during gametogenesis. In addition, we find that IMC15 is involved in daughter formation in both Toxoplasma and Sarcocystis tachyzoites, bradyzoites, and sporozoites. IMC14 and IMC15 function in limiting the number of Toxoplasma offspring per division. Furthermore, IMC7, -12, and -14, which are recruited in the G₁ cell cycle stage, are required for stress resistance of extracellular tachyzoites. Thus, although the roles of the different IMC proteins appear to overlap, stage- and development-specific behaviors indicate that their functions are uniquely tailored to each life stage requirement.

The inner membrane complex (IMC) of apicomplexan parasites contains a network of intermediate filament-like proteins. The 14 alveolin domain-containing IMC proteins in Toxoplasma gondii fall into different groups defined by their distinct spatiotemporal dynamics during the internal budding process of tachyzoites. Here, we analyzed representatives of different IMC protein groups across all stages of the Toxoplasma life cycle and during Sarcocystis neurona asexual development. We found that across asexually dividing Toxoplasma stages, IMC7 is present exclusively in the mother’s cytoskeleton, whereas IMC1 and IMC3 are both present in mother and daughter cytoskeletons (IMC3 is strongly enriched in daughter buds). In developing macro- and microgametocytes, IMC1 and -3 are absent, whereas IMC7 is lost in early microgametocytes but retained in macrogametocytes until late in their development. We found no roles for IMC proteins during meiosis and sporoblast formation. However, we observed that IMC1 and IMC3, but not IMC7, are present in sporozoites. Although the spatiotemporal pattern of IMC15 and IMC3 suggests orthologous functions in Sarcocystis, IMC7 may have functionally diverged in Sarcocystis merozoites. To functionally characterize IMC proteins, we knocked out IMC7, -12, -14, and -15 in Toxoplasma. IMC14 and -15 appear to be involved in switching between endodyogeny and endopolygeny. In addition, IMC7, -12, and -14, which are all recruited to the cytoskeleton outside cytokinesis, are critical for the structural integrity of extracellular tachyzoites. Altogether, stage- and development-specific roles for IMC proteins can be discerned, suggesting different niches for each IMC protein across the entire life cycle.

IMPORTANCE The inner membrane complex (IMC) is a defining feature of apicomplexan parasites key to both their motility and unique cell division. To provide further insights into the IMC, we analyzed the dynamics and functions of representative alveolin domain-containing IMC proteins across developmental stages. Our work shows universal but distinct roles for IMC1, -3, and -7 during Toxoplasma asexual division but more specialized functions for these proteins during gametogenesis. In addition, we find that IMC15 is involved in daughter formation in both Toxoplasma and Sarcocystis. IMC14 and IMC15 function in limiting the number of Toxoplasma offspring per division. Furthermore, IMC7, -12, and -14, which are recruited in the G₁ cell cycle stage, are required for stress resistance of extracellular tachyzoites. Thus, although the roles of the different IMC proteins appear to overlap, stage- and development-specific behaviors indicate that their functions are uniquely tailored to each life stage requirement.

Identification of microsporidia host-exposed proteins reveals a repertoire of rapidly evolving proteins

Pathogens use a variety of secreted and surface proteins to interact with and manipulate their hosts, but a systematic approach for identifying such proteins has been lacking. To identify these ‘host-exposed' proteins, we used spatially restricted enzymatic tagging followed by mass spectrometry analysis of Caenorhabditis elegans infected with two species of Nematocida microsporidia. We identified 82 microsporidia proteins inside of intestinal cells, including several pathogen proteins in the nucleus. These microsporidia proteins are enriched in targeting signals, are rapidly evolving and belong to large Nematocida-specific gene families. We also find that large, species-specific families are common throughout microsporidia species. Our data suggest that the use of a large number of rapidly evolving species-specific proteins represents a common strategy for microsporidia to interact with their hosts. The unbiased method described here for identifying potential pathogen effectors represents a powerful approach to study a broad range of pathogens.

Unbiased identification of proteins from pathogens that are exposed to a host can provide insight into host–pathogen interaction. Here, the authors use an enzymatic tagging method and mass spectrometry to identify rapidly evolving Nematocida microsporidia proteins when infecting C. elegans.

Detection of antibodies to Toxoplasma gondii in domesticated ruminants by recombinant truncated SAG2 enzyme-linked immunosorbent assay

An antibody detection recombinant enzyme-linked immunosorbent assay (ELISA) specific for Toxoplasma gondii was laboratory standardized using recombinant truncated surface antigen 2 (SAG2) protein of T. gondii. A 483-bp sequence coding for truncated tachyzoite stage-specific SAG2 protein was amplified and ligated in pPROExHT-b expression vector to transform Escherichia coli DH5α cells. A high-level expression of the histidine-tagged fusion protein was obtained after 8 h of incubation. The recombinant protein was affinity purified using Ni-NTA agarose column and characterized by SDS-PAGE and Western blot analysis. Subsequently, the diagnostic potential of the recombinant protein was assessed with 168 field sera samples from sheep, goats and cattle. Among the small ruminants, 50% (n = 60) sheep sera samples and 41.26% (n = 63) goat samples were detected positive for T. gondii-specific antibodies. As far as seroprevalence of toxoplasmosis in cattle is concerned, 64.44% (n = 45) of sera samples assayed were found to be positive. When compared to indirect fluorescent antibody test (IFAT), the sensitivity of the recombinant truncated SAG2 antigen-based ELISA (rec-SAG2-ELISA) ranged from 81.25 to 87.10% while the specificity was 85.71 to 91.43% with substantial agreement between the tests.

Dual transcriptional profiling of mice and Toxoplasma gondii during acute and chronic infection

BACKGROUND

The obligate intracellular parasite Toxoplasma gondii establishes a life-long chronic infection within any warm-blooded host. After ingestion of an encysted parasite, T. gondii disseminates throughout the body as a rapidly replicating form during acute infection. Over time and after stimulation of the host immune response, T. gondii differentiates into a slow growing, cyst form that is the hallmark of chronic infection. Global transcriptome analysis of both host and parasite during the establishment of chronic T. gondii infection has not yet been performed. Here, we conducted a dual RNA-seq analysis of T. gondii and its rodent host to better understand host and parasite responses during acute and chronic infection.

RESULTS

We obtained nearly one billion paired-end RNA sequences from the forebrains of uninfected, acutely and chronically infected mice, then aligned them to the genomic reference files of both T. gondii and Mus musculus. Gene ontology (GO) analysis of the 100 most highly expressed T. gondii genes showed less than half were shared between acute and chronic infection. The majority of the highly expressed genes common in both acute and chronic infection were involved in transcription and translation, underscoring that parasites in both stages are actively synthesizing proteins. Similarly, most of the T. gondii genes highly expressed during chronic infection were involved in metabolic processes, again highlighting the activity of the cyst stage at 28 days post-infection. Comparative analyses of host genes using uninfected forebrain revealed over twice as many immune regulatory genes were more abundant during chronic infection compared to acute. This demonstrates the influence of parasite development on host gene transcription as well as the influence of the host environment on parasite gene transcription.

CONCLUSIONS

RNA-seq is a valuable tool to simultaneously analyze host and microbe transcriptomes. Our data shows that T. gondii is metabolically active and synthesizing proteins at 28 days post-infection and that a distinct subset of host genes associated with the immune response are more abundant specifically during chronic infection. These data suggest host and pathogen interplay is still present during chronic infection and provides novel T. gondii targets for future drug and vaccine development.

DNA vaccine encoding the Toxoplasma gondii bradyzoite-specific surface antigens SAG2CDX protect BALB/c mice against type II parasite infection

The surface antigens SAG2C, SAG2D, and SAG2X, which expressed specifically on bradyzoite stage of Toxoplasma gondii, have been demonstrated to be important for persistence of cyst in the brain. In this study, DNA vaccines expressing SAG2C, SAG2D, and SAG2X of T. gondii were constructed and their protective efficacy were evaluated in BALB/c mice. Mice vaccinated with pVAX1-SAG2C (pSAG2C), pVAX1-2D (pSAG2D) or pVAX1-2X (pSAG2C) showed higher levels of serum IgG antibodies and lymphocyte proliferation response compared to PBS and pVAX1 treated mice (p<0.05). The immune response was characterized by a strong Th1 response and increased cytokine production of IL-2 and IFN-γ. Vaccinated mice displayed significant protection against the challenge with the cyst of T. gondii genotype II strain of PRU (cyst-forming in mouse). A significant reduction in the brain cyst burden was detected in the mice immunized with pSAG2C (72%), pSAG2D (23%), pSAG2X (69%) alone and even more reduction rate, 77%, was achieved in the combination group compared to PBS treated mice. The results implied that immunization with DNA vaccines expressing SAG2C, SAG2D, and SAG2X, and, in particular, a combination of all three DNA plasmids, could effectively protect the mice against T. gondii chronic infection.

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.

Analysis of structures and epitopes of surface antigen glycoproteins expressed in bradyzoites of Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite capable of infecting humans and animals. Surface antigen glycoproteins, SAG2C, -2D, -2X, and -2Y, are expressed on the surface of bradyzoites. These antigens have been shown to protect bradyzoites against immune responses during chronic infections. We studied structures of SAG2C, -2D, -2X, and -2Y proteins using bioinformatics methods. The protein sequence alignment was performed by T-Coffee method. Secondary structural and functional domains were predicted using software PSIPRED v3.0 and SMART software, and 3D models of proteins were constructed and compared using the I-TASSER server, VMD, and SWISS-spdbv. Our results showed that SAG2C, -2D, -2X, and -2Y are highly homologous proteins. They share the same conserved peptides and HLA-I restricted epitopes. The similarity in structure and domains indicated putative common functions that might stimulate similar immune response in hosts. The conserved peptides and HLA-restricted epitopes could provide important insights on vaccine study and the diagnosis of this disease.

Bradyzoite pseudokinase 1 is crucial for efficient oral infectivity of the Toxoplasma gondii tissue cyst

The tissue cyst formed by the bradyzoite stage of Toxoplasma gondii is essential for persistent infection of the host and oral transmission. Bradyzoite pseudokinase 1 (BPK1) is a component of the cyst wall, but nothing has previously been known about its function. Here, we show that immunoprecipitation of BPK1 from in vitro bradyzoite cultures, 4 days postinfection, identifies at least four associating proteins: MAG1, MCP4, GRA8, and GRA9. To determine the role of BPK1, a strain of Toxoplasma was generated with the bpk1 locus deleted. This BPK1 knockout strain (Δbpk1) was investigated in vitro and in vivo. No defect was found in terms of in vitro cyst formation and no difference in pathogenesis or cyst burden 4 weeks postinfection (wpi) was detected after intraperitoneal (i.p.) infection with Δbpk1 tachyzoites, although the Δbpk1 cysts were significantly smaller than parental or BPK1-complemented strains at 8 wpi. Pepsin-acid treatment of 4 wpi in vivo cysts revealed that Δbpk1 parasites are significantly more sensitive to this treatment than the parental and complemented strains. Consistent with this, 4 wpi Δbpk1 cysts showed reduced ability to cause oral infection compared to the parental and complemented strains. Together, these data reveal that BPK1 plays a crucial role in the in vivo development and infectivity of Toxoplasma cysts.

A forward genetic screen reveals that calcium-dependent protein kinase 3 regulates egress in Toxoplasma

Egress from the host cell is a crucial and highly regulated step in the biology of the obligate intracellular parasite, Toxoplasma gondii. Active egress depends on calcium fluxes and appears to be a crucial step in escaping the attack from the immune system and, potentially, in enabling the parasites to shuttle into appropriate cells for entry into the brain of the host. Previous genetic screens have yielded mutants defective in both ionophore-induced egress and ionophore-induced death. Using whole genome sequencing of one mutant and subsequent analysis of all mutants from these screens, we find that, remarkably, four independent mutants harbor a mis-sense mutation in the same gene, TgCDPK3, encoding a calcium-dependent protein kinase. All four mutations are predicted to alter key regions of TgCDPK3 and this is confirmed by biochemical studies of recombinant forms of each. By complementation we confirm a crucial role for TgCDPK3 in the rapid induction of parasite egress and we establish that TgCDPK3 is critical for formation of latent stages in the brains of mice. Genetic knockout of TgCDPK3 confirms a crucial role for this kinase in parasite egress and a non-essential role for it in the lytic cycle.

Toxoplasma gondii, an important human pathogen, is an obligate intracellular parasite, thus getting in and out of cells is key for its survival. The process by which Toxoplasma exits cells, known as egress, is controlled by calcium fluxes and can be triggered by ionophores. In vivo, rapid egress from the host cell has been identified as a means to escape attack by the innate immune system. At the molecular level, calcium dependent events in this parasite are regulated in part by plant like calcium dependent kinases, which share no homology to human kinases and are thus ideal drug targets. In this study we revisited 4 mutant parasite lines that were independently selected for an inability to undergo egress in response to ionophores. In all four mutants we have identified the Calcium Dependent Kinase 3 as the gene responsible for the defects. We have shown that two of these mutants, which are in a genetic background that allows virulence studies, also have a strong phenotype in vivo. That is, the parasites fail to form latent stages in mice. This work provides important information that a single kinase is responsible for the formation of latent stages that are important for transmission of the parasite.

A method to rapidly and accurately compare the relative efficacies of non-invasive imaging reporter genes in a mouse model and its application to luciferase reporters

PURPOSE

Our goal is to develop a simple, quantitative, robust method to compare the efficacy of imaging reporter genes in culture and in vivo. We describe an adenoviral vector-liver transduction procedure and compare the luciferase reporter efficacies.

PROCEDURES

Alternative reporter genes are expressed in a common adenoviral vector. Vector amounts used in vivo are based on cell culture titrations, ensuring that the same transduction efficacy is used for each vector. After imaging, in vivo and in vitro values are normalized to hepatic vector transduction using quantitative real-time PCR.

RESULTS

We assayed standard firefly luciferase (FLuc), enhanced firefly luciferase (EFLuc), luciferase 2 (Luc2), humanized Renilla luciferase (hRLuc), Renilla luciferase 8.6-535 (RLuc8.6), and a membrane-bound Gaussia luciferase variant (extGLuc) in cell culture and in vivo. We observed greater than 100-fold increase in bioluminescent signal for both EFLuc and Luc2 when compared to FLuc and greater than 10⁶-fold increase for RLuc8.6 when compared to hRLuc. ExtGLuc was not detectable in liver.

CONCLUSIONS

Our findings contrast, in some cases, with conclusions drawn in prior comparisons of these reporter genes and demonstrate the need for a standardized method to evaluate alternative reporter genes in vivo. Our procedure can be adapted for reporter genes that utilize alternative imaging modalities (fluorescence, bioluminescence, MRI, SPECT, PET).

β-1,3-Glucan, Which Can Be Targeted by Drugs, Forms a Trabecular Scaffold in the Oocyst Walls of Toxoplasma and Eimeria

The walls of infectious pathogens, which are essential for transmission, pathogenesis, and diagnosis, contain sugar polymers that are defining structural features, e.g., β-1,3-glucan and chitin in fungi, chitin in Entamoeba cysts, β-1,3-GalNAc in Giardia cysts, and peptidoglycans in bacteria. The goal here was to determine in which of three walled forms of Toxoplasma gondii (oocyst, sporocyst, or tissue cyst) is β-1,3-glucan, the product of glucan synthases and glucan hydrolases predicted by whole-genome sequences of the parasite. The three most important discoveries were as follows. (i) β-1,3-glucan is present in oocyst walls of Toxoplasma and Eimeria (a chicken parasite that is a model for intestinal stages of Toxoplasma) but is absent from sporocyst and tissue cyst walls. (ii) Fibrils of β-1,3-glucan are part of a trabecular scaffold in the inner layer of the oocyst wall, which also includes a glucan hydrolase that has a novel glucan-binding domain. (iii) Echinocandins, which target the glucan synthase and kill fungi, arrest development of the Eimeria oocyst wall and prevent release of the parasites into the intestinal lumen. In summary, β-1,3-glucan, which can be targeted by drugs, is an important component of oocyst walls of Toxoplasma but is not a component of sporocyst and tissue cyst walls.

We show here that walls of Toxoplasma oocysts, the infectious stage shed by cats, contain β-1,3-glucan, a sugar polymer that is a major component of fungal walls. In contrast to fungi, β-1,3-glucan is part of a trabecular scaffold in the inner layer of the oocyst wall that is independent of the permeability barrier formed by the outer layer of the wall. While glucan synthase inhibitors kill fungi, these inhibitors arrest the development of the oocyst walls of Eimeria (an important chicken pathogen that is a surrogate for Toxoplasma) and block release of oocysts into the intestinal lumen. The absence of β-1,3-glucan in tissue cysts of Toxoplasma suggests that drugs targeted at the glucan synthase might be used to treat Eimeria in chickens but not to treat Toxoplasma in people.

Virulence factors of Toxoplasma gondii

Toxoplasma gondii virulence is dependent on factors involved in either parasite-host cell interaction, or in host immune response. It is essentially defined in the mouse and little is known concerning human infection. The genetic dependence of virulence is a growing field, benefiting from the recent development of research of the population structure of T. gondii.

Sirtuins of parasitic protozoa: in search of function(s)

The SIR2 family of NAD⁺-dependent protein deacetylases, collectively called sirtuins, has been of central interest due to their proposed roles in life-span regulation and ageing. Sirtuins are one group of environment sensors of a cell interpreting external information and orchestrating internal responses at the sub-cellular level, through participation in gene regulation mechanisms. Remarkably conserved across all kingdoms of life SIR2 proteins in several protozoan parasites appear to have both conserved and intriguing unique functions. This review summarises our current knowledge of the members of the sirtuin families in Apicomplexa, including Plasmodium, and other protozoan parasites such as Trypanosoma and Leishmania. The wide diversity of processes regulated by SIR2 proteins makes them targets worthy of exploitation in anti-parasitic therapies.

Peptide microarray analysis of in silico-predicted epitopes for serological diagnosis of Toxoplasma gondii infection in humans

Toxoplasma gondii infections occur worldwide in humans and animals. In immunocompromised or prenatally infected humans, T. gondii can cause severe clinical symptoms. The identification of specific epitopes on T. gondii antigens is essential for the improvement and standardization of the serological diagnosis of toxoplasmosis. We selected 20 peptides mimicking linear epitopes on GRA1, GRA2, GRA4, and MIC3 antigenic T. gondii proteins in silico using the software ABCpred. A further 18 peptides representing previously published epitopes derived from GRA1, SAG1, NTPase1, and NTPase2 antigens were added to the panel. A peptide microarray assay was established to prove the diagnostic performance of the selected peptides with human serum samples. Seropositive human serum samples (n = 184) were collected from patients presenting with acute toxoplasmosis (n = 21), latent T. gondii infection (n = 53), and inactive ocular toxoplasmosis (n = 10) and from seropositive forest workers (n = 100). To adjust the cutoff values for each peptide, sera from seronegative forest workers (n = 75) and patients (n = 65) were used. Univariate logistic regression suggested the significant diagnostic potential of eight novel and two previously published peptides. A test based on these peptides had an overall diagnostic sensitivity of 69% (100% in ocular toxoplasmosis patients, 86% in acutely infected patients, 81% in latently infected patients, and 57% in seropositive forest workers). The analysis of seronegative sera performed with these peptides revealed a diagnostic specificity of 84%. The results of our study suggest that the use of a bioinformatic approach for epitope prediction in combination with peptide microarray testing is a powerful method for the selection of T. gondii epitopes as candidate antigens for serological diagnosis.

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.

Transcriptomic analysis of toxoplasma development reveals many novel functions and structures specific to sporozoites and oocysts

Sexual reproduction of Toxoplasma gondii occurs exclusively within enterocytes of the definitive felid host. The resulting immature oocysts are excreted into the environment during defecation, where in the days following, they undergo a complex developmental process. Within each oocyst, this culminates in the generation of two sporocysts, each containing 4 sporozoites. A single felid host is capable of shedding millions of oocysts, which can survive for years in the environment, are resistant to most methods of microbial inactivation during water-treatment and are capable of producing infection in warm-blooded hosts at doses as low as 1–10 ingested oocysts. Despite its extremely interesting developmental biology and crucial role in initiating an infection, almost nothing is known about the oocyst stage beyond morphological descriptions. Here, we present a complete transcriptomic analysis of the oocyst from beginning to end of its development. In addition, and to identify genes whose expression is unique to this developmental form, we compared the transcriptomes of developing oocysts with those of in vitro-derived tachyzoites and in vivo-derived bradyzoites. Our results reveal many genes whose expression is specifically up- or down-regulated in different developmental stages, including many genes that are likely critical to oocyst development, wall formation, resistance to environmental destruction and sporozoite infectivity. Of special note is the up-regulation of genes that appear “off” in tachyzoites and bradyzoites but that encode homologues of proteins known to serve key functions in those asexual stages, including a novel pairing of sporozoite-specific paralogues of AMA1 and RON2, two proteins that have recently been shown to form a crucial bridge during tachyzoite invasion of host cells. This work provides the first in-depth insight into the development and functioning of one of the most important but least studied stages in the Toxoplasma life cycle.

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 SnSAG merozoite surface antigens of Sarcocystis neurona are expressed differentially during the bradyzoite and sporozoite life cycle stages

Sarcocystis neurona is a two-host coccidian parasite whose complex life cycle progresses through multiple developmental stages differing at morphological and molecular levels. The S. neurona merozoite surface is covered by multiple, related glycosylphosphatidylinositol-linked proteins, which are orthologous to the surface antigen (SAG)/SAG1-related sequence (SRS) gene family of Toxoplasma gondii. Expression of the SAG/SRS proteins in T. gondii and another related parasite Neospora caninum is life-cycle stage specific and seems necessary for parasite transmission and persistence of infection. In the present study, the expression of S. neurona merozoite surface antigens (SnSAGs) was evaluated in the sporozoite and bradyzoite stages. Western blot analysis was used to compare SnSAG expression in merozoites versus sporozoites, while immunocytochemistry was performed to examine expression of the SnSAGs in merozoites versus bradyzoites. These analyses revealed that SnSAG2, SnSAG3 and SnSAG4 are expressed in sporozoites, while SnSAG5 was appeared to be downregulated in this life cycle stage. In S. neurona bradyzoites, it was found that SnSAG2, SnSAG3, SnSAG4 and SnSAG5 were either absent or expression was greatly reduced. As shown for T. gondii, stage-specific expression of the SnSAGs may be important for the parasite to progress through its developmental stages and complete its life cycle successfully. Thus, it is possible that the SAG switching mechanism by these parasites could be exploited as a point of intervention. As well, the alterations in surface antigen expression during different life cycle stages may need to be considered when designing prospective approaches for protective vaccination.

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.

Genetic manipulation of Neospora caninum to express the bradyzoite-specific protein NcSAG4 in tachyzoites

Neospora caninum is an apicomplexan parasite and the aetiological agent of bovine neosporosis, one of the main causes of reproductive failure worldwide. We have generated 2 independent transgenic knock-in clones, Nc-1SAG4c1.1 and Nc-1SAG4c2.1, that express the bradyzoite stage-specific protein NcSAG4 in the tachyzoite stage. These clones have similar growth rates in vitro as the wild-type (WT) strain Nc-1. Studies in a cerebral mouse model of infection revealed a slightly lower rate of detection of the transgenic strains in brains during the chronic phase of infection. However, a pregnant mouse model of infection revealed a reduction in the virulence of the Nc-1SAG4c1.1 strain despite the same tachyzoite expression of NcSAG4 and a similar anti-NcSAG4 response displayed by mice inoculated with Nc-1 SAG4c1.1 or Nc-1 SAG4c2.1 parasites. This behaviour may be related to the reduced ability of the Nc-1SAG4c1.1 parasites to invade host cells, which was observed in in vitro assays. The apparent reduction in persistence and the high growth rate of the transgenic strains, together with their constitutive expression of the protein NcSAG4, may be useful features for future immunoprophylaxis trials based on a safe live attenuated vaccine.

Strain-specific activation of the NF-kappaB pathway by GRA15, a novel Toxoplasma gondii dense granule protein

NF-κB is an integral component of the immune response to Toxoplasma gondii. Although evidence exists that T. gondii can directly modulate the NF-κB pathway, the parasite-derived effectors involved are unknown. We determined that type II strains of T. gondii activate more NF-κB than type I or type III strains, and using forward genetics we found that this difference is a result of the polymorphic protein GRA15, a novel dense granule protein which T. gondii secretes into the host cell upon invasion. A GRA15-deficient type II strain has a severe defect in both NF-κB nuclear translocation and NF-κB-mediated transcription. Furthermore, human cells expressing type II GRA15 also activate NF-κB, demonstrating that GRA15 alone is sufficient for NF-κB activation. Along with the rhoptry protein ROP16, GRA15 is responsible for a large part of the strain differences in the induction of IL-12 secretion by infected mouse macrophages. In vivo bioluminescent imaging showed that a GRA15-deficient type II strain grows faster compared with wild-type, most likely through its reduced induction of IFN-γ. These results show for the first time that a dense granule protein can modulate host signaling pathways, and dense granule proteins can therefore join rhoptry proteins in T. gondii's host cell-modifying arsenal.

Genomic data reveal Toxoplasma gondii differentiation mutants are also impaired with respect to switching into a novel extracellular tachyzoite state

Toxoplasma gondii pathogenesis includes the invasion of host cells by extracellular parasites, replication of intracellular tachyzoites, and differentiation to a latent bradyzoite stage. We present the analysis of seven novel T. gondii insertional mutants that do not undergo normal differentiation to bradyzoites. Microarray quantification of the variation in genome-wide RNA levels for each parasite line and times after induction allowed us to describe states in the normal differentiation process, to analyze mutant lines in the context of these states, and to identify genes that may have roles in initiating the transition from tachyzoite to bradyzoite. Gene expression patterns in wild-type parasites undergoing differentiation suggest a novel extracellular state within the tachyzoite stage. All mutant lines exhibit aberrant regulation of bradyzoite gene expression and notably some of the mutant lines appear to exhibit high proportions of the intracellular tachyzoite state regardless of whether they are intracellular or extracellular. In addition to the genes identified by the insertional mutagenesis screen, mixture model analysis allowed us to identify a small number of genes, in mutants, for which expression patterns could not be accounted for using the three parasite states – genes that may play a mechanistic role in switching from the tachyzoite to bradyzoite stage.

Human immunome, bioinformatic analyses using HLA supermotifs and the parasite genome, binding assays, studies of human T cell responses, and immunization of HLA-A*1101 transgenic mice including novel adjuvants provide a foundation for HLA-A03 restricted CD8+T cell epitope based, adjuvanted vaccine protective against Toxoplasma gondii

Background

Toxoplasmosis causes loss of life, cognitive and motor function, and sight. A vaccine is greatly needed to prevent this disease. The purpose of this study was to use an immmunosense approach to develop a foundation for development of vaccines to protect humans with the HLA-A03 supertype. Three peptides had been identified with high binding scores for HLA-A03 supertypes using bioinformatic algorhythms, high measured binding affinity for HLA-A03 supertype molecules, and ability to elicit IFN-γ production by human HLA-A03 supertype peripheral blood CD8⁺ T cells from seropositive but not seronegative persons.

Results

Herein, when these peptides were administered with the universal CD4⁺T cell epitope PADRE (AKFVAAWTLKAAA) and formulated as lipopeptides, or administered with GLA-SE either alone, or with Pam₂Cys added, we found we successfully created preparations that induced IFN-γ and reduced parasite burden in HLA-A*1101(an HLA-A03 supertype allele) transgenic mice. GLA-SE is a novel emulsified synthetic TLR4 ligand that is known to facilitate development of T Helper 1 cell (TH1) responses. Then, so our peptides would include those expressed in tachyzoites, bradyzoites and sporozoites from both Type I and II parasites, we used our approaches which had identified the initial peptides. We identified additional peptides using bioinformatics, binding affinity assays, and study of responses of HLA-A03 human cells. Lastly, we found that immunization of HLA-A*1101 transgenic mice with all the pooled peptides administered with PADRE, GLA-SE, and Pam₂Cys is an effective way to elicit IFN-γ producing CD8⁺ splenic T cells and protection. Immunizations included the following peptides together: KSFKDILPK (SAG1_224-232); AMLTAFFLR (GRA6_164-172); RSFKDLLKK (GRA7_134-142); STFWPCLLR (SAG2C_13-21); SSAYVFSVK(_SPA250-258); and AVVSLLRLLK(SPA_89-98). This immunization elicited robust protection, measured as reduced parasite burden using a luciferase transfected parasite, luciferin, this novel, HLA transgenic mouse model, and imaging with a Xenogen camera.

Conclusions

Toxoplasma gondii peptides elicit HLA-A03 restricted, IFN-γ producing, CD8⁺ T cells in humans and mice. These peptides administered with adjuvants reduce parasite burden in HLA-A*1101 transgenic mice. This work provides a foundation for immunosense based vaccines. It also defines novel adjuvants for newly identified peptides for vaccines to prevent toxoplasmosis in those with HLA-A03 supertype alleles.

Towards an immunosense vaccine to prevent toxoplasmosis: protective Toxoplasma gondii epitopes restricted by HLA-A*0201

The ideal vaccine to protect against toxoplasmosis in humans would include antigens that elicit a protective T helper cell type 1 immune response, and generate long-lived IFN-γ-producing CD8(+) T cells. Herein, we utilized a predictive algorithm to identify candidate HLA-A02 supertype epitopes from Toxoplasma gondii proteins. Thirteen peptides elicited production of IFN-γ from PBMC of HLA-A02 supertype persons seropositive for T. gondii infection but not from seronegative controls. These peptides displayed high-affinity binding to HLA-A02 proteins. Immunization of HLA-A*0201 transgenic mice with these pooled peptides, with a universal CD4(+) epitope peptide called PADRE, formulated with adjuvant GLA-SE, induced CD8(+) T cell IFN-γ production and protected against parasite challenge. Peptides identified in this study provide candidates for inclusion in immunosense epitope-based vaccines.

Toxoplasma gondii: 25 years and 25 major advances for the field

This article is an attempt to identify the most significant highlights of Toxoplasma research over the last 25 years. It has been a period of enormous progress and the top 25 most significant advances, in the view of this author, are described. These range from the bench to the bedside and represent a tremendous body of work from countless investigators. And, having laid out so much that has been discovered, it is impossible not to also reflect on the challenges that lie ahead. These, too, are briefly discussed. Finally, while every effort has been made to view the field as a whole, the molecular biology background of the author almost certainly will have skewed the relative importance attached to past and future advances. Despite this, it is hoped that the reader will agree with, or at least not disagree too strongly with, most of the choices presented here.

Identification of T. gondii epitopes, adjuvants, and host genetic factors that influence protection of mice and humans

Toxoplasma gondii is an intracellular parasite that causes severe neurologic and ocular disease in immune-compromised and congenitally infected individuals. There is no vaccine protective against human toxoplasmosis. Herein, immunization of L(d) mice with HF10 (HPGSVNEFDF) with palmitic acid moieties or a monophosphoryl lipid A derivative elicited potent IFN-gamma production from L(d)-restricted CD8(+) T cells in vitro and protected mice. CD8(+) T cell peptide epitopes from T. gondii dense granule proteins GRA 3, 6, 7, and Sag 1, immunogenic in humans for HLA-A02(+), HLA-A03(+), and HLA-B07(+) cells were identified. Since peptide repertoire presented by MHC class I molecules to CD8(+) T cells is shaped by endoplasmic reticulum-associated aminopeptidase (ERAAP), polymorphisms in the human ERAAP gene ERAP1 were studied and associate with susceptibility to human congenital toxoplasmosis (p<0.05). These results have important implications for vaccine development.

Advances in understanding immunity to Toxoplasma gondii

Toxoplasma gondii is an important cause of clinical disease in fetuses, infants and immunocompromised patients. Since the discovery of T. gondii 100 years ago, this pathogen and the host's immune response to toxoplasmosis have been studied intensely. This has led to the development of a working model of immunity to T. gondii, and has also resulted in fundamental new insights into the role of various cytokines in resistance to infection. By examining this organism, researchers have identified many of the requirements for resistance to intracellular pathogens and characterized numerous regulatory factors, including interleukin-10 (IL-10) and IL-27, which control inflammatory processes. In the next 100 years of T. gondii immunobiology, researchers will have the opportunity to answer some of the long-standing questions in the field using new techniques and reagents. These future studies will be vital in building a more comprehensive model of immunity to this pathogen and in advancing our understanding of immunoregulation, particularly in humans. Ultimately, the challenge will be to use this information to develop new vaccines and therapies to manage disease in affected patients.

Coordinated loading of IRG resistance GTPases on to the Toxoplasma gondii parasitophorous vacuole

The immunity-related GTPases (IRGs) constitute an interferon-induced intracellular resistance mechanism in mice against Toxoplasma gondii. IRG proteins accumulate on the parasitophorous vacuole membrane (PVM), leading to its disruption and to death of the parasite. How IRGs target the PVM is unknown. We show that accumulation of IRGs on the PVM begins minutes after parasite invasion and increases for about 1 h. Targeting occurs independently of several signalling pathways and the microtubule network, suggesting that IRG transport is diffusion-driven. The intensity of IRG accumulation on the PVM, however, is reduced in absence of the autophagy regulator, Atg5. In wild-type cells IRG proteins accumulate cooperatively on PVMs in a definite order reflecting a temporal hierarchy, with Irgb6 and Irgb10 apparently acting as pioneers. Loading of IRG proteins onto the vacuoles of virulent Toxoplasma strains is attenuated and the two pioneer IRGs are the most affected. The polymorphic rhoptry kinases, ROP16, ROP18 and the catalytically inactive proteins, ROP5A–D, are not individually responsible for this effect. Thus IRG proteins protect mice against avirulent strains of Toxoplasma but fail against virulent strains. The complex cooperative behaviour of IRG proteins in resisting Toxoplasma may hint at undiscovered complexity also in virulence mechanisms.

Advances in bioluminescence imaging of live animal models

Many of the obligate steps of physiology and disease are dynamic in time and space, and thus, end-point assays do not always provide a full understanding of these processes. Comprehensive understanding of the functional complexity of protein interactions and cell trafficking requires mapping of cellular and molecular function within complex systems over biologically relevant time scales. New approaches to bioluminescence imaging of cell migration, signaling pathways, drug action, and interacting protein partners in vivo allow the study of biology and disease within the context of living animals.

A pseudouridine synthase homologue is critical to cellular differentiation in Toxoplasma gondii

Toxoplasma gondii is a haploid protozoan parasite infecting about one in seven people in the United States. Key to the worldwide prevalence of T. gondii is its ability to establish a lifelong, chronic infection by evading the immune system, and central to this is the developmental switch between the two asexual forms, tachyzoites and bradyzoites. A library of mutants defective in tachyzoite-to-bradyzoite differentiation (Tbd(-)) was created through insertional mutagenesis. This library contains mutants that, compared to the wild type, are between 20% and 74% as efficient at stage conversion. Two mutants, TBD5 and TBD8, with disruptions in a gene encoding a putative pseudouridine synthase, PUS1, were identified. The disruption in TBD8 is in the 5' end of the PUS1 gene and appears to produce a null allele with a 50% defect in differentiation. This is about the same switch efficiency as obtained with an engineered pus1 deletion mutant (Deltapus1). The insertion in TBD5 is within the PUS1 coding region, and this appears to result in a more extreme phenotype of only approximately 10% switch efficiency. Complementation of TBD8 with the genomic PUS1 allele restored wild-type differentiation efficiency. Infection of mice with pus1 mutant strains results in increased mortality during the acute phase and higher cyst burdens during the chronic infection, demonstrating an aberrant differentiation phenotype in vivo due to PUS1 disruption. Our results suggest a surprising and important role for RNA modification in this biological process.

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.