An inside job: hacking into Janus kinase/signal transducer and activator of transcription signaling cascades by the intracellular protozoan Toxoplasma gondii

Toxoplasma protein export and effector function

Toxoplasma gondii is a single-celled eukaryotic parasite with a considerable host range that must invade the cells of warm-blooded hosts to survive and replicate. The challenges and opportunities that such a strategy represent have been met by the evolution of effectors that are delivered into host cells, counter host defences and co-opt host cell functions for their own purposes. These effectors are delivered in two waves using distinct machinery for each. In this Review, we focus on understanding the architecture of these protein-export systems and how their protein cargo is recognized and selected. We discuss the recent findings on the role that host manipulation has in latent Toxoplasma infections. We also discuss how these recent findings compare to protein export in the related Plasmodium spp. (the causative agent of malaria) and how this can inform our understanding of host manipulation in the larger Apicomplexa phylum and its evolution.

A candidate virulence factor of Eimeria tenella (EtROP30) predicted by virulence enhancement of transgenic Toxoplasma gondii

Difficulties of in vitro culture and genetic manipulation of Eimeria tenella have hindered the screening of virulence factors in this parasite. In this study, the E. tenella rhoptry protein 30 (EtROP30) was expressed in Toxoplasma gondii (RH∆Ku80-EtROP30), and its effect on the proliferation and virulence of parasites was investigated. The results revealed that the expression of EtROP30 had no impact on the invasion and egress processes. However, the RH∆Ku80-EtROP30 strain formed larger plaques compared to the RH∆Ku80, indicating that the EtROP30 expression promotes T. gondii proliferation. Furthermore, the RH∆Ku80-EtROP30 strain exhibited greater pathogenicity, resulting in earlier mortality and shorter overall survival time compared to RH∆Ku80. These results imply that EtROP30 expression facilitates parasite intracellular proliferation and virulence in mice, suggesting that EtROP30 might be a candidate virulence factor of E. tenella.

Trained immunity: a cutting edge approach for designing novel vaccines against parasitic diseases?

The preventive situation of parasitosis, a global public health burden especially for developing countries, is not looking that good. Similar to other infections, vaccines would be the best choice for preventing and controlling parasitic infection. However, ideal antigenic molecules for vaccine development have not been identified so far, resulting from the complicated life history and enormous genomes of the parasites. Furthermore, the suppression or down-regulation of anti-infectious immunity mediated by the parasites or their derived molecules can compromise the effect of parasitic vaccines. Comparing the early immune profiles of several parasites in the permissive and non-permissive hosts, a robust innate immune response is proposed to be a critical event to eliminate the parasites. Therefore, enhancing innate immunity may be essential for designing novel and effective parasitic vaccines. The newly emerging trained immunity (also termed innate immune memory) has been increasingly recognized to provide a novel perspective for vaccine development targeting innate immunity. This article reviews the current status of parasitic vaccines and anti-infectious immunity, as well as the conception, characteristics, and mechanisms of trained immunity and its research progress in Parasitology, highlighting the possible consideration of trained immunity in designing novel vaccines against parasitic diseases.

In vitro and in vivo susceptibility to sulfadiazine and pyrimethamine of Toxoplasma gondii strains isolated from Brazilian free wild birds

Little is known about the existence of drug-resistant Toxoplasma gondii strains and their possible impact on clinic outcomes. To expand our knowledge about the existence of natural variations on drug susceptibility of T. gondii strains in Brazil, we evaluated the in vitro and in vivo susceptibility to sulfadiazine (SDZ) and pyrimethamine (PYR) of three atypical strains (Wild2, Wild3, and Wild4) isolated from free-living wild birds. In vitro susceptibility assay showed that the three strains were equally susceptible to SDZ and PYR but variations in the susceptibility were observed to SDZ plus PYR treatment. Variations in the proliferation rates in vitro and spontaneous conversion to bradyzoites were also accessed for all strains. Wild2 showed a lower cystogenesis capacity compared to Wild3 and Wild4. The in vivo analysis showed that while Wild3 was highly susceptible to all SDZ and PYR doses, and their combination, Wild2 and Wild4 showed low susceptibility to the lower doses of SDZ or PYR. Interestingly, Wild2 presented low susceptibility to the higher doses of SDZ, PYR and their combination. Our results suggest that the variability in treatment response by T. gondii isolates could possibly be related not only to drug resistance but also to the strain cystogenesis capacity.

Glycosylation Analysis of Feline Small Intestine Following Toxoplasma gondii Infection

Simple Summary

Toxoplasma gondii has a serious impact on public health and the economic development of animal husbandry. Glycosylation, especially N-glycosylation, the pattern modification of proteins, is closely related to the biological functions of proteins, and our study used it to analyze glycosylation alterations in the small intestine of cats infected with T. gondii. The results of the present study showed that 56 glycosylated peptides were upregulated and 37 glycosylated peptides were downregulated. Additionally, we also identified eight N-glycosylated proteins of T. gondii including eight N-glycopeptides and eight N-glycosylation sites. Moreover, the protein eEF2 and its corresponding peptide sequence were identified, with GO terms (i.e., cellular process and metabolic process, cell and cell part, and catalytic activity) that were significantly enriched in the T. gondii MAPK pathway. In addition, the Clusters of Orthologous Groups of proteins (COG) function prediction results showed that posttranslational modification, protein turnover, and chaperones (11%) had the highest enrichment for T. gondii. The host proteins ICAM-1 and PPT1 and the endoplasmic reticulum stress pathway may play an important role in the glycosylation of T. gondii-infected hosts. Our study may provide a new target for T. gondii detection to prevent the spread of T. gondii oocysts in the future.

Abstract

Toxoplasma gondii (T. gondii) is responsible for severe human and livestock diseases, huge economic losses, and adversely affects the health of the public and the development of animal husbandry. Glycosylation is a common posttranslational modification of proteins in eukaryotes, and N-glycosylation is closely related to the biological functions of proteins. However, glycosylation alterations in the feline small intestine following T. gondii infection have not been reported. In this study, the experimental group was intragastrically challenged with 600 brain cysts of the Prugniuad (Pru) strain that were collected from infected mice. The cats’ intestinal epithelial tissues were harvested at 10 days post-infection and then sent for protein glycosylation analysis. High-performance liquid chromatography coupled to tandem mass spectrometry was used to analyze the glycosylation alterations in the small intestine of cats infected with T. gondii. The results of the present study showed that 56 glycosylated peptides were upregulated and 37 glycosylated peptides were downregulated in the feline small intestine infected by T. gondii. Additionally, we also identified eight N-glycosylated proteins of T. gondii including eight N-glycopeptides and eight N-glycosylation sites. The protein A0A086JND6_TOXGO (eEF2) and its corresponding peptide sequence were identified in T. gondii infection. Some special GO terms (i.e., cellular process and metabolic process, cell and cell part, and catalytic activity) were significantly enriched, and the Clusters of Orthologous Groups of proteins (COG) function prediction results showed that posttranslational modification, protein turnover, and chaperones (11%) had the highest enrichment for T. gondii. Interestingly, eEF2, a protein of T. gondii, is also involved in the significantly enriched T. gondii MAPK pathway. The host proteins ICAM-1 and PPT1 and the endoplasmic reticulum stress pathway may play an important role in the glycosylation of Toxoplasma-infected hosts. This is the first report showing that T. gondii oocysts can undergo N-glycosylation in the definitive host and that eEF2 is involved, which may provide a new target for T. gondii detection to prevent the spread of T. gondii oocysts in the future.

Immune response against toxoplasmosis-some recent updates RH: Toxoplasma gondii immune response

AIMS

Cytokines, soluble mediators of immunity, are key factors of the innate and adaptive immune system. They are secreted from and interact with various types of immune cells to manipulate host body's immune cell physiology for a counter-attack on the foreign body. A study was designed to explore the mechanism of Toxoplasma gondii (T. gondii) resistance from host immune response.

METHODS AND RESULTS

The published data on aspect of host (murine and human) immune response against T. gondii was taken from Google scholar and PubMed. Most relevant literature was included in this study. The basic mechanism of immune response starts from the interactions of antigens with host immune cells to trigger the production of cytokines (pro-inflammatory and anti-inflammatory) which then act by forming a cytokinome (network of cytokine). Their secretory equilibrium is essential for endowing resistance to the host against infectious diseases, particularly toxoplasmosis. A narrow balance lying between Th1, Th2, and Th17 cytokines (as demonstrated until now) is essential for the development of resistance against T. gondii as well as for the survival of host. Excessive production of pro-inflammatory cytokines leads to tissue damage resulting in the production of anti-inflammatory cytokines which enhances the proliferation of Toxoplasma. Stress and other infectious diseases (human immunodeficiency virus (HIV)) that weaken the host immunity particularly the cellular component, make the host susceptible to toxoplasmosis especially in pregnant women.

CONCLUSION

The current review findings state that in vitro harvesting of IL12 from DCs, Np and MΦ upon exposure with T. gondii might be a source for therapeutic use in toxoplasmosis. Current review also suggests that therapeutic interventions leading to up-regulation/supplementation of SOCS-3, IL12, and IFNγ to the infected host could be a solution to sterile immunity against T. gondii infection. This would be of interest particularly in patients passing through immunosuppression owing to any reason like the ones receiving anti-cancer therapy, the ones undergoing immunosuppressive therapy for graft/transplantation, the ones suffering from immunodeficiency virus (HIV) or having AIDS. Another imortant suggestion is to launch the efforts for a vaccine based on GRA6Nt or other similar antigens of T. gondii as a probable tool to destroy tissue cysts.

Unraveling the Elusive Rhoptry Exocytic Mechanism of Apicomplexa

Apicomplexan parasites are unicellular eukaryotes that invade the cells in which they proliferate. The development of genetic tools in Toxoplasma, and then in Plasmodium, in the 1990s allowed the first description of the molecular machinery used for motility and invasion, revealing a crucial role for two different secretory organelles, micronemes and rhoptries. Rhoptry proteins are injected directly into the host cytoplasm not only to promote invasion but also to manipulate host functions. Nonetheless, the injection machinery has remained mysterious, a major conundrum in the field. Here we review recent progress in uncovering structural components and proteins implicated in rhoptry exocytosis and explain how revisiting early findings and considering the evolutionary origins of Apicomplexa contributed to some of these discoveries.

The Immunoproteasome Subunits LMP2, LMP7 and MECL-1 Are Crucial Along the Induction of Cerebral Toxoplasmosis

Cell survival and function critically relies on the fine-tuned balance of protein synthesis and degradation. In the steady state, the standard proteasome is sufficient to maintain this proteostasis. However, upon inflammation, the sharp increase in protein production requires additional mechanisms to limit protein-associated cellular stress. Under inflammatory conditions and the release of interferons, the immunoproteasome (IP) is induced to support protein processing and recycling. In antigen-presenting cells constitutively expressing IPs, inflammation-related mechanisms contribute to the formation of MHC class I/II-peptide complexes, which are required for the induction of T cell responses. The control of Toxoplasma gondii infection relies on Interferon-γ (IFNγ)-related T cell responses. Whether and how the IP affects the course of anti-parasitic T cell responses along the infection as well as inflammation of the central nervous system is still unknown. To answer this question we used triple knockout (TKO) mice lacking the 3 catalytic subunits of the immunoproteasome (β1i/LMP2, β2i/MECL-1 and β5i/LMP7). Here we show that the numbers of dendritic cells, monocytes and CD8⁺ T cells were reduced in Toxoplasma gondii-infected TKO mice. Furthermore, impaired IFNγ, TNF and iNOS production was accompanied by dysregulated chemokine expression and altered immune cell recruitment to the brain. T cell differentiation was altered, apoptosis rates of microglia and monocytes were elevated and STAT3 downstream signaling was diminished. Consequently, anti-parasitic immune responses were impaired in TKO mice leading to elevated T. gondii burden and prolonged neuroinflammation. In summary we provide evidence for a critical role of the IP subunits β1i/LMP2, β2i/MECL-1 and β5i/LMP7 for the control of cerebral Toxoplasma gondii infection and subsequent neuroinflammation.

Modelling Toxoplasma gondii infection in human cerebral organoids

Pluripotent stem cell-derived cerebral organoids have the potential to recapitulate the pathophysiology of in vivo human brain tissue, constituting a valuable resource for modelling brain disorders, including infectious diseases. Toxoplasma gondii, an intracellular protozoan parasite, infects most warm-blooded animals, including humans, causing toxoplasmosis. In immunodeficient patients and pregnant women, infection often results in severe central nervous system disease and fetal miscarriage. However, understanding the molecular pathophysiology of the disease has been challenging due to limited in vitro model systems. Here, we developed a new in vitro model system of T. gondii infection using human brain organoids. We observed that tachyzoites can infect human cerebral organoids and are transformed to bradyzoites and replicate in parasitophorous vacuoles to form cysts, indicating that the T. gondii asexual life cycle is efficiently simulated in the brain organoids. Transcriptomic analysis of T. gondii-infected organoids revealed the activation of the type I interferon immune response against infection. In addition, in brain organoids, T. gondii exhibited a changed transcriptome related to protozoan invasion and replication. This study shows cerebral organoids as physiologically relevant in vitro model systems useful for advancing the understanding of T. gondii infections and host interactions.

From Initiators to Effectors: Roadmap Through the Intestine During Encounter of Toxoplasma gondii With the Mucosal Immune System

The gastrointestinal tract is a major portal of entry for many pathogens, including the protozoan parasite Toxoplasma gondii. Billions of people worldwide have acquired T. gondii at some point in their life, and for the vast majority this has led to latent infection in the central nervous system. The first line of host defense against Toxoplasma is located within the intestinal mucosa. Appropriate coordination of responses by the intestinal epithelium, intraepithelial lymphocytes, and lamina propria cells results in an inflammatory response that controls acute infection. Under some conditions, infection elicits bacterial dysbiosis and immune-mediated tissue damage in the intestine. Here, we discuss the complex interactions between the microbiota, the epithelium, as well as innate and adaptive immune cells in the intestinal mucosa that induce protective immunity, and that sometimes switch to inflammatory pathology as T. gondii encounters tissues of the gut.

The Toxoplasma gondii virulence factor ROP16 acts in cis and trans, and suppresses T cell responses

The ability of Toxoplasma gondii to inject the rhoptry kinase ROP16 into host cells results in the activation of the transcription factors STAT3 and STAT6, but it is unclear how these events impact infection. Here, parasites that inject Cre-recombinase with rhoptry proteins were used to distinguish infected macrophages from those only injected with parasite proteins. Transcriptional profiling revealed that injection of rhoptry proteins alone was sufficient to induce an M2 phenotype that is dependent on STAT3 and STAT6, but only infected cells displayed reduced expression of genes associated with antimicrobial activity and protective immunity. In vivo, the absence of STAT3 or STAT6 improved parasite control, while the loss of ROP16 resulted in a marked reduction in parasite numbers and heightened parasite-specific T cell responses. Thus, ROP16 is a virulence factor that can act in cis and trans to promote M2 programs and which limits the magnitude of parasite-specific T cell responses.

Vaccination with gamma radiation-attenuated Toxoplasma gondii protects against ovarian infiltration in mice-bearing Ehrlich ascites carcinoma

Purpose: Cancer is one of the most common causes of mortality and morbidity worldwide. Vaccines have been emerged as an attractive approach for their capacity of eliciting long-term immune response targeting cancer cells. Attenuated avirulent Toxoplasma gondii stimulate immunity and activate antitumor cells thereby eliciting rejection of some established cancer. The purpose of this study was to evaluate the antitumor-protective capacity of vaccination with gamma radiation-attenuated T. gondii against ovarian penetration in Ehrlich ascites carcinoma (EAC)-bearing mice.Materials and methods: Forty-five mice were randomly divided into three groups as follows: nontumor-bearing (normal control); EAC-bearing group (EAC); and mice vaccinated orally with gamma radiation-attenuated T. gondii then inoculated 2 weeks later with EAC (TG + EAC). Survival rate, serum interleukin-12 (IL-12), and levels of IFN-γ mRNA, CD4, and CD8 in ovarian tissues homogenate were assessed. Also, ovarian histopathology and immunohistochemical expressions of metalloproteinase-2, CD34, and vimentin were determined.Results: The group vaccinated with attenuated T. gondii showed significantly increased survival rates, serum IL-12, and levels of IFN-γ, CD4, and CD8 in ovarian tissue homogenates as well as an enhancement of histopathological and immunohistochemical changes compared to EAC-bearing group.Conclusion: Vaccination with gamma radiation-attenuated T. gondii has the capacity to supply immunoprotective impact against ovarian invasion in EAC-bearing mice.

Structural predication and antigenic analysis of ROP16 protein utilizing immunoinformatics methods in order to identification of a vaccine against Toxoplasma gondii: An in silico approach

Toxoplasmosis, caused by Toxoplasma gondii, is a common parasitic disease, affecting almost one-third of the world's population. Currently, there are no effective treatments for inhibiting the formation of chronic tissue cysts in infected hosts. Thus, the production of appropriate vaccines against this pathogen is an important goal to avoid toxoplasmosis. considering the role of rhoptry antigens like ROP16 in virulence and satisfactory immunogenicity, they can be used as promising vaccine candidates against T. gondii. In the present study, an in silico approach was used to analyze various aspects of the ROP16 protein, including physicochemical characteristics, the potential epitopes of B and T-cells, the secondary and tertiary structure, the subcellular localization, the transmembrane domain, and other important features of this protein using several bioinformatics tools to design a proper vaccine against T. gondii. The results showed that ROP16 protein includes 93 potential post-translational modification sites. The secondary structure of the ROP16 protein comprises 34.23% alpha-helix, 54.46% random coil, and 11.32% extended strand. Moreover, several potential B- and T-cell epitopes were identified for ROP16. Based on the results of Ramachandran plot, 84.64% of the amino acid residues were located in the favored, 10.34% in allowed, and 5.02% in outlier regions. Furthermore, the results of the antigenicity and allergenicity assessment noted that this protein was immunogenic and non-allergenic. Our findings suggested that structural and functional predictions applied to ROP16 protein using in silico tools can reduce the failure risk of the laboratory studies. This research provided an important basis for further studies and also developed an effective vaccine against acute and chronic toxoplasmosis by various strategies. Further studies are needed on the development of vaccines in vivo using ROP16 alone or in combination with other antigens in the future.

Influence of Two Major Toxoplasma Gondii Virulence Factors (ROP16 and ROP18) on the Immune Response of Peripheral Blood Mononuclear Cells to Human Toxoplasmosis Infection

Toxoplasma gondii ROP16 and ROP18 proteins have been identified as important virulence factors for this parasite. Here, we describe the effect of ROP16 and ROP18 proteins on peripheral blood mononuclear cells (PBMCs) from individuals with different clinical status of infection. We evaluated IFN-γ, IL-10, and IL-1β levels in supernatants from PBMCs cultures infected with tachyzoites of the T. gondii wild-type RH strain or with knock-out mutants of the rop16 and rop18 encoding genes (RHΔrop16 and RHΔrop18). Cytokine secretion was compared between PBMCs obtained from seronegative individuals (n = 10), with those with chronic asymptomatic (n = 8), or ocular infection (n = 12). We also evaluated if polymorphisms in the genes encoding for IFN-γ, IL-10, IL-1β, Toll-like receptor 9 (TLR9), and purinoreceptor P2RX7 influenced the production of the encoded proteins after ex vivo stimulation. In individuals with chronic asymptomatic infection, only a moderate effect on IL-10 levels was observed when PBMCs were infected with RHΔrop16, whereas a significant difference in the levels of inflammatory cytokines IFN-γ and IL-1β was observed in seronegative individuals, but this was also dependent on the host's cytokine gene polymorphisms. Infection with ROP16-deficient parasites had a significant effect on IFN-γ production in previously non-infected individuals, suggesting that ROP16 which is considered as a virulence factor plays a role during the primary infection in humans, but not in the secondary immune response.

Rhoptry and Dense Granule Secreted Effectors Regulate CD8+ T Cell Recognition of Toxoplasma gondii Infected Host Cells

Toxoplasma gondii secretes rhoptry (ROP) and dense granule (GRA) effector proteins to evade host immune clearance mediated by interferon gamma (IFN-γ), immunity-related GTPase (IRG) effectors, and CD8⁺ T cells. Here, we investigated the role of parasite-secreted effectors in regulating host access to parasitophorous vacuole (PV) localized parasite antigens and their presentation to CD8⁺ T cells by the major histocompatibility class I (MHC-I) pathway. Antigen presentation of PV localized parasite antigens by MHC-I was significantly increased in macrophages and/or dendritic cells infected with mutant parasites that lacked expression of secreted GRA (GRA2, GRA3, GRA4, GRA5, GRA7, GRA12) or ROP (ROP5, ROP18) effectors. The ability of various secreted GRA or ROP effectors to suppress antigen presentation by MHC-I was dependent on cell type, expression of IFN-γ, or host IRG effectors. The suppression of antigen presentation by ROP5, ROP18, and GRA7 correlated with a role for these molecules in preventing PV disruption by IFN-γ-activated host IRG effectors. However, GRA2 mediated suppression of antigen presentation was not correlated with PV disruption. In addition, the GRA2 antigen presentation phenotypes were strictly co-dependent on the expression of the GRA6 protein. These results show that MHC-I antigen presentation of PV localized parasite antigens was controlled by mechanisms that were dependent or independent of IRG effector mediated PV disruption. Our findings suggest that the GRA6 protein underpins an important mechanism that enhances CD8⁺ T cell recognition of parasite-infected cells with damaged or ruptured PV membranes. However, in intact PVs, parasite secreted effector proteins that associate with the PV membrane or the intravacuolar network membranes play important roles to actively suppress antigen presentation by MHC-I to reduce CD8⁺ T cell recognition and clearance of Toxoplasma gondii infected host cells.

Transcriptome Sequencing Investigated the Tumor-Related Factors Changes After T. gondii Infection

Toxoplasma gondii is an intracellular parasite and causes a global epidemic parasitic disease. T. gondii-infection could inhibit the growth of tumor. In this study, the transcriptomes of samples were detected by deep sequencing analysis. The transcriptome data was compared with reference genome to perform sequence alignment and the further analysis. The analyses of differential expression and the differentially expressed genes were performed in the present study. Genes involved in P53 signaling pathway, COLORECTAL cancer pathway, NON-SMALL CELL LUNG cancer signaling pathway, and BREAST cancer signaling pathway were up-regulated or down-regulated among the samples. The KEGG analysis indicated that the cancer pathways changed after infection of T. gondii. Furthermore, tumor-related mRNAs from different samples had a large difference, which suggested that the difference might provide important information in resisting cancer. The protein results indicated that tumor-related protein changes occurred after infection of T. gondii. In conclusion, the infection changed the cancer pathways, which could possibly inhibit the growth of tumor.

Toxoplasma gondii Manipulates Expression of Host Long Noncoding RNA during Intracellular Infection

Long noncoding RNA (lncRNA) are non-protein-coding transcripts greater than 200 nucleotides that regulate gene expression. The field of transcriptomics is only beginning to understand the role of lncRNA in host defense. Little is known about the role of lncRNA in the response to infection by intracellular pathogens such as Toxoplasma gondii. Using a microarray, we examined the differential expression of 35,923 lncRNAs and 24,881 mRNAs in mouse bone-marrow-derived macrophages during infection with high- and low-virulence T. gondii strains. We found that 1,522 lncRNA molecules were differentially regulated during infection with the high-virulence Type I strain, versus 528 with the less-virulent Type II strain. Of these lncRNAs, 282 were co-regulated with a nearby or overlapping mRNA–including approximately 60 mRNAs with immune-related functions. We validated the microarray for 4 lncRNAs and 4 mRNAs using qRT-PCR. Using deletion strains of T. gondii, we found that the secretory kinase ROP16 controls upregulation of lncRNAs Csf1-lnc and Socs2-lnc, demonstrating that the parasite directly manipulates host lncRNA expression. Given the number of regulated lncRNAs and the magnitude of the expression changes, we hypothesize that these molecules constitute both an additional regulatory layer in the host response to infection and a target for manipulation by T. gondii.

From inflammatory reactions to neurotransmitter changes: Implications for understanding the neurobehavioral changes in mice chronically infected with Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite that can cause a latent infection in the central nervous system, leading to neurobehavioral abnormalities in the host. However, the mechanism underlying these changes remains relatively unexplored. In this study, we detected behavioral changes, pathological injury, secretion of neurotransmitters and related signal pathway in mice infected by T. gondii using behavioral test, histopathology, immunofluorescence staining, western blotting, HPLC and real time PCR. Mice showed neurobehavioral disturbances two months after infection with T. gondii. Histopathology revealed the activation of astrocytes and microglia, apoptosis of neurons and decreases in synapses in the brain of infected mice. Excessive secretion of cytokines and chemokines was detected in the brains of mice infected by T. gondii compared to uninfected mice. Furthermore, T. gondii infection led to abnormalities in neurotransmitters and the activation of NF-κB and dopamine (DA) signaling pathways in the infected mice. In conclusion, excessive activation of the inflammation in the brain could induce neuronal apoptosis in mice chronically infected with T. gondii. Dysregulation of the dopaminergic neurotransmitter could provide an explanation of neurobehavioral disorders in infected hosts.

Targeted disruption of CK1α in Toxoplasma gondii increases acute virulence in mice

Toxoplasma gondii, the causative agent of toxoplasmosis, encodes two casein kinase 1 (CK1) isoforms, CK1α and CK1β, with only CK1α having enzyme activity. Here we investigated the biological role of CK1α by construction of a CK1α deletion mutant (Δck1α) based on the type I parasite, and complement the mutant with restored expression of CK1α. Deletion of CK1α resulted in markedly defective parasite replication in vitro. Infected mice with Δck1α parasite caused suppression of IL-12 production, severe liver damage, higher tissue burdens, and short survival time relative to the CK1α-positive parental strain. Western blot analysis revealed that deletion of CK1α led to increased activation of the signal transducer and activator of transcription (STAT)-3 in infected mice and bone marrow-derived microphages. The transcriptome analysis showed that deletion of CK1α may increase expression of rhoptry proteins (ROPs). Western blot showed enhanced expression of ROP16 in the Δck1α parasite as compared with the wild-type and complemented parasites. These findings demonstrated that deletion of CK1α may increase acute virulence of T. gondii in mice by increased expression of ROPs, activation of STAT3, and suppression of IL-12 production, which have important implications for elucidating regulation mechanism of virulence factors for T. gondii.

Secretion of Rhoptry and Dense Granule Effector Proteins by Nonreplicating Toxoplasma gondii Uracil Auxotrophs Controls the Development of Antitumor Immunity

Nonreplicating type I uracil auxotrophic mutants of Toxoplasma gondii possess a potent ability to activate therapeutic immunity to established solid tumors by reversing immune suppression in the tumor microenvironment. Here we engineered targeted deletions of parasite secreted effector proteins using a genetically tractable Δku80 vaccine strain to show that the secretion of specific rhoptry (ROP) and dense granule (GRA) proteins by uracil auxotrophic mutants of T. gondii in conjunction with host cell invasion activates antitumor immunity through host responses involving CD8α+ dendritic cells, the IL-12/interferon-gamma (IFN-γ) TH1 axis, as well as CD4+ and CD8+ T cells. Deletion of parasitophorous vacuole membrane (PVM) associated proteins ROP5, ROP17, ROP18, ROP35 or ROP38, intravacuolar network associated dense granule proteins GRA2 or GRA12, and GRA24 which traffics past the PVM to the host cell nucleus severely abrogated the antitumor response. In contrast, deletion of other secreted effector molecules such as GRA15, GRA16, or ROP16 that manipulate host cell signaling and transcriptional pathways, or deletion of PVM associated ROP21 or GRA3 molecules did not affect the antitumor activity. Association of ROP18 with the PVM was found to be essential for the development of the antitumor responses. Surprisingly, the ROP18 kinase activity required for resistance to IFN-γ activated host innate immunity related GTPases and virulence was not essential for the antitumor response. These data show that PVM functions of parasite secreted effector molecules, including ROP18, manipulate host cell responses through ROP18 kinase virulence independent mechanisms to activate potent antitumor responses. Our results demonstrate that PVM associated rhoptry effector proteins secreted prior to host cell invasion and dense granule effector proteins localized to the intravacuolar network and host nucleus that are secreted after host cell invasion coordinately control the development of host immune responses that provide effective antitumor immunity against established ovarian cancer.

The Toxoplasma gondii Rhoptry Kinome Is Essential for Chronic Infection

Ingestion of the obligate intracellular protozoan parasite Toxoplasma gondii causes an acute infection that leads to chronic infection of the host. To facilitate the acute phase of the infection, T. gondii manipulates the host response by secreting rhoptry organelle proteins (ROPs) into host cells during its invasion. A few key ROP proteins with signatures of kinases or pseudokinases (ROPKs) act as virulence factors that enhance parasite survival against host gamma interferon-stimulated innate immunity. However, the roles of these and other ROPK proteins in establishing chronic infection have not been tested. Here, we deleted 26 ROPK gene loci encoding 31 unique ROPK proteins of type II T. gondii and show that numerous ROPK proteins influence the development of chronic infection. Cyst burdens were increased in the Δrop16 knockout strain or moderately reduced in 11 ROPK knockout strains. In contrast, deletion of ROP5, ROP17, ROP18, ROP35, or ROP38/29/19 (ROP38, ROP29, and ROP19) severely reduced cyst burdens. Δrop5 and Δrop18 knockout strains were less resistant to host immunity-related GTPases (IRGs) and exhibited >100-fold-reduced virulence. ROP18 kinase activity and association with the parasitophorous vacuole membrane were necessary for resistance to host IRGs. The Δrop17 strain exhibited a >12-fold defect in virulence; however, virulence was not affected in the Δrop35 or Δrop38/29/19 strain. Resistance to host IRGs was not affected in the Δrop17, Δrop35, or Δrop38/29/19 strain. Collectively, these findings provide the first definitive evidence that the type II T. gondii ROPK proteome functions as virulence factors and facilitates additional mechanisms of host manipulation that are essential for chronic infection and transmission of T. gondii.

Reactivation of chronic Toxoplasma gondii infection in individuals with weakened immune systems causes severe toxoplasmosis. Existing treatments for toxoplasmosis are complicated by adverse reactions to chemotherapy. Understanding key parasite molecules required for chronic infection provides new insights into potential mechanisms that can interrupt parasite survival or persistence in the host. This study reveals that key secreted rhoptry molecules are used by the parasite to establish chronic infection of the host. Certain rhoptry proteins were found to be critical virulence factors that resist innate immunity, while other rhoptry proteins were found to influence chronic infection without affecting virulence. This study reveals that rhoptry proteins utilize multiple mechanisms of host manipulation to establish chronic infection of the host. Targeted disruption of parasite rhoptry proteins involved in these biological processes opens new avenues to interfere with chronic infection with the goal to either eliminate chronic infection or to prevent recrudescent infections.

Attenuated Toxoplasma gondii Stimulates Immunity to Pancreatic Cancer by Manipulation of Myeloid Cell Populations

Suppressive myeloid cells represent a significant barrier to the generation of productive antitumor immune responses to many solid tumors. Eliminating or reprogramming suppressive myeloid cells to abrogate tumor-associated immune suppression is a promising therapeutic approach. We asked whether treatment of established aggressive disseminated pancreatic cancer with the immunotherapeutic attenuated Toxoplasma gondii vaccine strain CPS would trigger tumor-associated myeloid cells to generate therapeutic antitumor immune responses. CPS treatment significantly decreased tumor-associated macrophages and markedly increased dendritic cell infiltration of the pancreatic tumor microenvironment. Tumor-resident macrophages and dendritic cells, particularly cells actively invaded by CPS, increased expression of costimulatory molecules CD80 and CD86 and concomitantly boosted their production of IL12. CPS treatment increased CD4(+) and CD8(+) T-cell infiltration into the tumor microenvironment, activated tumor-resident T cells, and increased IFNγ production by T-cell populations. CPS treatment provided a significant therapeutic benefit in pancreatic tumor-bearing mice. This therapeutic benefit depended on IL12 and IFNγ production, MyD88 signaling, and CD8(+) T-cell populations. Although CD4(+) T cells exhibited activated effector phenotypes and produced IFNγ, CD4(+) T cells as well as natural killer cells were not required for the therapeutic benefit. In addition, CD8(+) T cells isolated from CPS-treated tumor-bearing mice produced IFNγ after re-exposure to pancreatic tumor antigen, suggesting this immunotherapeutic treatment stimulated tumor cell antigen-specific CD8(+) T-cell responses. This work highlights the potency and immunotherapeutic efficacy of CPS treatment and demonstrates the significance of targeting tumor-associated myeloid cells as a mechanism to stimulate more effective immunity to pancreatic cancer.

Nonreplicating, cyst-defective type II Toxoplasma gondii vaccine strains stimulate protective immunity against acute and chronic infection

Live attenuated vaccine strains, such as type I nonreplicating uracil auxotroph mutants, are highly effective in eliciting lifelong immunity to virulent acute infection by Toxoplasma gondii. However, it is currently unknown whether vaccine-elicited immunity can provide protection against acute infection and also prevent chronic infection. To address this problem, we developed nonreverting, nonreplicating, live attenuated uracil auxotroph vaccine strains in the type II Δku80 genetic background by targeting the deletion of the orotidine 5'-monophosphate decarboxylase (OMPDC) and uridine phosphorylase (UP) genes. Deletion of OMPDC induced a severe uracil auxotrophy with loss of replication, loss of virulence in mice, and loss of the ability to develop cysts and chronic infection. Vaccination of mice using type II Δku80 Δompdc mutants stimulated a fully protective CD8(+) T cell-dependent immunity that prevented acute infection by type I and type II strains of T. gondii, and this vaccination also severely reduced or prevented cyst formation after type II challenge infection. Nonreverting, nonreplicating, and non-cyst-forming Δompdc mutants provide new tools to examine protective immune responses elicited by vaccination with a live attenuated type II vaccine.

Secreted Toxoplasma gondii molecules interfere with expression of MHC-II in interferon gamma-activated macrophages

The obligate intracellular protozoan parasite Toxoplasma gondii interferes with major histocompatibility complex class II antigen presentation to dampen host CD4(+) T cell responses. While it is known that T. gondii inhibits major histocompatibility complex class II gene transcription and expression in infected host cells, the mechanism of this host manipulation is unknown. Here, we show that soluble parasite proteins inhibit IFNγ-induced expression of major histocompatibility complex class II on the surface of the infected cell in a dose-dependent response that was abolished by protease treatment. Subcellular fractionation of T. gondii tachyzoites revealed that the major histocompatibility complex class II inhibitory activity co-partitioned with rhoptries and/or dense granules. However, parasite mutants deleted for single rhoptries or dense granules genes (ROP1, 4/7, 14, 16 and 18 or GRA 2-9 and 12 knock-out strains) retained the ability to inhibit expression of major histocompatibility complex class II. In addition, excreted/secreted antigens released by extracellular tachyzoites displayed immunomodulatory activity characterized by an inhibition of major histocompatibility complex class II expression, and reduced expression and release of TNFα by macrophages. Tandem MS analysis of parasite excreted/secreted antigens generated a list of T. gondii secreted proteins that may participate in major histocompatibility complex class II inhibition and the modulation of host immune functions.

Host response profile of human brain proteome in toxoplasma encephalitis co-infected with HIV

Background

Toxoplasma encephalitis is caused by the opportunistic protozoan parasite Toxoplasma gondii. Primary infection with T. gondii in immunocompetent individuals remains largely asymptomatic. In contrast, in immunocompromised individuals, reactivation of the parasite results in severe complications and mortality. Molecular changes at the protein level in the host central nervous system and proteins associated with pathogenesis of toxoplasma encephalitis are largely unexplored. We used a global quantitative proteomic strategy to identify differentially regulated proteins and affected molecular networks in the human host during T. gondii infection with HIV co-infection.

Results

We identified 3,496 proteins out of which 607 proteins were differentially expressed (≥1.5-fold) when frontal lobe of the brain from patients diagnosed with toxoplasma encephalitis was compared to control brain tissues. We validated differential expression of 3 proteins through immunohistochemistry, which was confirmed to be consistent with mass spectrometry analysis. Pathway analysis of differentially expressed proteins indicated deregulation of several pathways involved in antigen processing, immune response, neuronal growth, neurotransmitter transport and energy metabolism.

Conclusions

Global quantitative proteomic approach adopted in this study generated a comparative proteome profile of brain tissues from toxoplasma encephalitis patients co-infected with HIV. Differentially expressed proteins include previously reported and several new proteins in the context of T. gondii and HIV infection, which can be further investigated. Molecular pathways identified to be associated with the disease should enhance our understanding of pathogenesis in toxoplasma encephalitis.

Electronic supplementary material

The online version of this article (doi:10.1186/1559-0275-11-39) contains supplementary material, which is available to authorized users.

Evaluation of immuno-efficacy of a novel DNA vaccine encoding Toxoplasma gondii rhoptry protein 38 (TgROP38) against chronic toxoplasmosis in a murine model

Background

Toxoplasma gondii is an obligate intracellular parasite which can infect almost all mammalian animals, leading to toxoplasmosis. T. gondii rhoptry protein 38 (TgROP38) is an active rhoptry protein kinase which is involved in the inhibitory effect on host cell transcription by down-regulating the MAPK signaling track.

Methods

TgROP38 gene was amplified and inserted into eukaryotic vector pVAX I and formed the DNA vaccine pVAX-ROP38. Mice in the experimental group were intramuscularly immunized with pVAX-ROP38 and those injected with pVAX I, PBS or nothing were treated as controls. After three injections at two week intervals, all mouse groups were challenged intraperitoneally with 1000 tachyzoites of the virulent T. gondii RH strain (Type I, ToxoDB #10) and 10 cysts of the PRU strain (Type II, ToxoDB #1), respectively.

Results

Mice inoculated with pVAX-ROP38 vaccine had a higher level of IgG antibodies (P < 0.01) and T lymphoproliferative response. The high ratio of IgG2a/IgG1 and the increasing levels of IFN-γ and IL-2 (P < 0.05) indicated an activated Th1 cell-mediated immune responses. Furthermore, the CD4⁺ and CD8⁺ proportions in vaccinated mice were also increased significantly compared with that in mice of the three control groups (P < 0.01). In the model of acute infection, the average survival time of mice in the pVAX-ROP38 group (8.1 days ± 0.75) was no statistically different compared to that in the PBS, pVAX I and blank control groups which died within 7 days. However, in the model of chronic infection, the brain cyst reduction in the pVAX-ROP38 group reached 76.6%, compared to controls (P < 0.01).

Conclusions

The present study revealed that the pVAX-ROP38 vaccine could elicit strong humoral and cell immunity response against chronic T. gondii infection in mice, resulting in the reduction of the brain cyst formation effectively, which suggests that TgROP38 is a desirable vaccine candidate against chronic T. gondii infection.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-525) contains supplementary material, which is available to authorized users.

Protective efficacy of Toxoplasma gondii calcium-dependent protein kinase 1 (TgCDPK1) adjuvated with recombinant IL-15 and IL-21 against experimental toxoplasmosis in mice

Background

Toxoplasma gondii can infect all warm-blooded animals including humans. Infection with T. gondii is probably the leading cause of posterior uveitis in humans and the most comment route of transmission is raw and undercooked meat from infected animals. T. gondii calcium-dependent protein kinase 1 (TgCDPK1) plays a critical role in direct parasite motility, host-cell invasion, and egress.

Methods

We constructed a DNA vaccine expressing TgCDPK1 inserted into eukaryotic expression vector pVAX I and evaluated the immune protection induced by pVAX-CDPK1 in Kunming mice. Mice immunized with pVAX-CDPK1 intramuscularly and/or with a plasmid encoding IL-15 and IL-21 (pVAX-IL-21-IL-15). The immune responses were analyzed including lymphoproliferative assay, cytokine, antibody measurements, lymphocyte surface markers by flow cytometry and protective efficacy were measured as survival and cysts numbers after challenge 1 to 2 months post vaccination.

Results

Immunization with pVAX-CDPK1 or pVAX-IL-21-IL-15 alone developed strong humoral responses and Th1 type cellular immune responses, and the significantly (P < 0.05) increase of both the percentages of CD4+ and CD8+ T cells compared with all the controls (blank control, PBS, and pVAX). Co-injection of pVAX-IL-21-IL-15 significantly increased humoral and cellular immune responses compared to the group of pVAX-CDPK1 or pVAX-IL-21-IL-15. Challenge experiments showed that co-administration of pVAX-IL-21-IL-15 and pVAX-CDPK1 significantly (P < 0.05) increased survival time (19.2 ± 5.1 days) compared with pVAX-CDPK1 (17.3 ± 4.3 days) or pVAX-IL-21-IL-15 (12.0 ± 2.0 days) alone, and pVAX-IL-21-IL-15 + pVAX-CDPK1 significantly reduced the number of brain cysts (72.7%) in contrast to pVAX-ROP13 (45.7%) or pVAX-IL-21-IL-15 alone (43.6%).

Conclusions

TgCDPK1 is identified to be a promising vaccine candidate for inducing a strong humoral and cellular response against T. gondii infection, and thus synergistic of mIL-21 and mIL-15 can induce non-specific immune responses, but also facilitate specific humoral as well as cellular immune responses elicited by DNA vaccine against acute and chronic T. gondii infection in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-487) contains supplementary material, which is available to authorized users.

Regulation of STATs by polycystin-1 and their role in polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a common genetic disease caused by mutations in the gene coding for polycystin-1 (PC1). PC1 can regulate STAT transcription factors by a novel, dual mechanism. STAT3 and STAT6 are aberrantly activated in renal cysts. Genetic and pharmacological approaches to inhibit STAT3 or STAT6 have led to promising results in ADPKD mouse models. Here, we review current findings that lead to a model of PC1 as a key regulator of STAT signaling in renal tubule cells. We discuss how PC1 may orchestrate appropriate epithelial responses to renal injury, and how this system may lead to aberrant STAT activation in ADPKD thereby causing inappropriate activation of tissue repair programs that culminate in renal cyst growth and fibrosis.

Patterns of Toxoplasma gondii cyst distribution in the forebrain associate with individual variation in predator odor avoidance and anxiety-related behavior in male Long-Evans rats

Toxoplasma gondii (T. gondii) is one of the world's most successful brain parasites. T. gondii engages in parasite manipulation of host behavior and infection has been epidemiologically linked to numerous psychiatric disorders. Mechanisms by which T. gondii alters host behavior are not well understood, but neuroanatomical cyst presence and the localized host immune response to cysts are potential candidates. The aim of these studies was to test the hypothesis that T. gondii manipulation of specific host behaviors is dependent on neuroanatomical location of cysts in a time-dependent function post-infection. We examined neuroanatomical cyst distribution (53 forebrain regions) in infected rats after predator odor aversion behavior and anxiety-related behavior in the elevated plus maze and open field arena, across a 6-week time course. In addition, we examined evidence for microglial response to the parasite across the time course. Our findings demonstrate that while cysts are randomly distributed throughout the forebrain, individual variation in cyst localization, beginning 3 weeks post-infection, can explain individual variation in the effects of T. gondii on behavior. Additionally, not all infected rats develop cysts in the forebrain, and attenuation of predator odor aversion and changes in anxiety-related behavior are linked with cyst presence in specific forebrain areas. Finally, the immune response to cysts is striking. These data provide the foundation for testing hypotheses about proximate mechanisms by which T. gondii alters behavior in specific brain regions, including consequences of establishment of a homeostasis between T. gondii and the host immune response.

Severe South American ocular toxoplasmosis is associated with decreased Ifn-γ/Il-17a and increased Il-6/Il-13 intraocular levels

In a cross sectional study, 19 French and 23 Colombian cases of confirmed active ocular toxoplasmosis (OT) were evaluated. The objective was to compare clinical, parasitological and immunological responses and relate them to the infecting strains. A complete ocular examination was performed in each patient. The infecting strain was characterized by genotyping when intraocular Toxoplasma DNA was detectable, as well as by peptide-specific serotyping for each patient. To characterize the immune response, we assessed Toxoplasma protein recognition patterns by intraocular antibodies and the intraocular profile of cytokines, chemokines and growth factors. Significant differences were found for size of active lesions, unilateral macular involvement, unilateral visual impairment, vitreous inflammation, synechiae, and vasculitis, with higher values observed throughout for Colombian patients. Multilocus PCR-DNA sequence genotyping was only successful in three Colombian patients revealing one type I and two atypical strains. The Colombian OT patients possessed heterogeneous atypical serotypes whereas the French were uniformly reactive to type II strain peptides. The protein patterns recognized by intraocular antibodies and the cytokine patterns were strikingly different between the two populations. Intraocular IFN-γ and IL-17 expression was lower, while higher levels of IL-13 and IL-6 were detected in aqueous humor of Colombian patients. Our results are consistent with the hypothesis that South American strains may cause more severe OT due to an inhibition of the protective effect of IFN-γ.

Non-replicating Toxoplasma gondii reverses tumor-associated immunosuppression

We examined the efficacy of using attenuated non-replicating Toxoplasma gondii uracil auxotrophs that can be safely delivered as anticancer immunotherapeutics. This strategy exerted remarkable therapeutic activity in murine models of melanoma and ovarian carcinoma, and holds broad potential for the development of novel, highly effective anticancer vaccines.

Toxoplasma gondii: immune response and protective efficacy induced by ROP16/GRA7 multicomponent DNA vaccine with a genetic adjuvant B7-2

Toxoplasma gondii infection occurs commonly in humans and other warm-blooded animals. Its serious impact on public health and livestock sectors makes the development of an effective vaccine particularly important. In the current study, we constructed a multiantigenic DNA vaccine expressing ROP16 and GRA7 of T. gondii and evaluated the protective efficacy of these two fragments with or without a plasmid encoding murine costimulatory molecule B7-2. These recombinant eukaryotic expression plasmids were termed pROP16, pGRA7, pROP16-GRA7 and pB7-2, respectively. After intramuscular immunization in Kunming mice, we assessed the immune response using cytokine and antibody determinations, T lymphocyte subsets analysis, and the survival times of mice post acute T. gondii challenge. The results showed that mice immunized with the multiantigenic DNA vaccine pROP16-GRA7 gained higher levels of IgG titers and IgG2a subclass titers, production of IFN-γ, percentage of CD8+ T cells and median survival times against the acute infection of T. gondii compared with those of mice administered with pROP16 or pGRA7 and those in control groups. Moreover, the adjuvant pB7-2 formulated with DNA vaccine boosted these humoral and cellular (Th1, CD8+ T cell) immune responses. Therefore, it might be a promising genetic adjuvant to DNA vaccine against T. gondii for further investigation.

Targeting tumors with nonreplicating Toxoplasma gondii uracil auxotroph vaccines

Toxoplasma gondii is an intracellular parasite that has evolved to actively control its invaded host cells. Toxoplasma triggers then actively regulates host innate interleukin-12 (IL-12) and interferon-γ (IFN-γ) responses that elicit T cell control of infection. A live, nonreplicating avirulent uracil auxotroph vaccine strain (cps) of Toxoplasma triggers novel innate immune responses that stimulate amplified CD8(+) T cell responses and life-long immunity in vaccinated mice. Here, we review recent reports showing that intratumoral treatment with cps activated immune-mediated regression of established solid tumors in mice. We speculate that a better understanding of host-parasite interaction at the molecular level and applying improved genetic models based on Δku80 Toxoplasma strains will stimulate development of highly effective immunotherapeutic cancer vaccine strategies using engineered uracil auxotrophs.

Avirulent Toxoplasma gondii generates therapeutic antitumor immunity by reversing immunosuppression in the ovarian cancer microenvironment

Reversing tumor-associated immunosuppression seems necessary to stimulate effective therapeutic immunity against lethal epithelial tumors. Here, we show this goal can be addressed using cps, an avirulent, nonreplicating uracil auxotroph strain of the parasite Toxoplasma gondii (T. gondii), which preferentially invades immunosuppressive CD11c(+) antigen-presenting cells in the ovarian carcinoma microenvironment. Tumor-associated CD11c(+) cells invaded by cps were converted to immunostimulatory phenotypes, which expressed increased levels of the T-cell receptor costimulatory molecules CD80 and CD86. In response to cps treatment of the immunosuppressive ovarian tumor environment, CD11c(+) cells regained the ability to efficiently cross-present antigen and prime CD8(+) T-cell responses. Correspondingly, cps treatment markedly increased tumor antigen-specific responses by CD8(+) T cells. Adoptive transfer experiments showed that these antitumor T-cell responses were effective in suppressing solid tumor development. Indeed, intraperitoneal cps treatment triggered rejection of established ID8-VegfA tumors, an aggressive xenograft model of ovarian carcinoma, also conferring a survival benefit in a related aggressive model (ID8-Defb29/Vegf-A). The therapeutic benefit of cps treatment relied on expression of IL-12, but it was unexpectedly independent of MyD88 signaling as well as immune experience with T. gondii. Taken together, our results establish that cps preferentially invades tumor-associated antigen-presenting cells and restores their ability to trigger potent antitumor CD8(+) T-cell responses. Immunochemotherapeutic applications of cps might be broadly useful to reawaken natural immunity in the highly immunosuppressive microenvironment of most solid tumors.

Modulation of innate immunity by Toxoplasma gondii virulence effectors

Toxoplasma gondii is a common parasite of animals and humans and can cause serious opportunistic infections. However, the majority of infections are asymptomatic, possibly because the organism has co-evolved with its many vertebrate hosts and has developed multiple strategies to persist asymptomatically for the lifetime of the host. Over the past two decades, infection studies in the mouse, combined with forward-genetics approaches aimed at unravelling the molecular basis of infection, have revealed that T. gondii virulence is mediated, in part, by secretion of effector proteins into the host cell during invasion. Here, we review recent advances that illustrate how these virulence factors disarm innate immunity and promote survival of the parasite.