Selective and strain-specific NFAT4 activation by the Toxoplasma gondii polymorphic dense granule protein GRA6

Recent advances in identifying and characterizing secretory proteins of Toxoplasma gondii by CRISPR-based screening

The apicomplexan parasite, Toxoplasma gondii, develops unique secretory organelles, such as micronemes, rhoptries, and dense granules, which do not exist in other well-studied eukaryotic organisms. These secretory organelles are key features of apicomplexan parasites and discharge various proteins that are essential for invasion, replication, egress, host-parasite interactions, and virulence. Many studies have therefore focused on identifying and characterizing the proteins secreted by T. gondii that play essential roles in pathology and that can be targeted for therapeutics and vaccine development. The recent development of functional genetic screens based on CRISPR/Cas9 technology has revolutionized this field and has enabled the identification of genes that contribute to parasite fitness in vitro and in vivo. Consequently, characterization of genes identified by unbiased CRISPR screens has revealed novel aspects of apicomplexan biology. In this review, we describe the development of CRIPSR-based screening technology for T. gondii, and recent advances in our understanding of secretory proteins identified and characterized by CRISPR-based screening.

Toxoplasma GRA15 expression on dendritic cells inhibits B cell differentiation and antibody production

One of the dense granule proteins named GRA15 in Toxoplasma gondii (T. gondii), is known to support an innate immune response in host through activation of NF-κB. However, little is known about advantages of GRA15 for parasites. By examining the role of GRA15 in the host-parasite interactions, it was clarified that GRA15 in T. gondii suppressed acquired immune responses in host. Wild-type parasite infection to C57BL/6 mice resulted in lower titers of T. gondii antibody and lower plasma cell counts compared to Δgra15 T. gondii. To identify host cells in which GRA15 acts to suppress antibody production, we generated conditional knock-in mice that express GRA15 in specific cell lineages. Anti-T. gondii antibodies were not reduced in macrophages of conditional knock-in mice after infection with Δgra15 T. gondii, while the production of T. gondii antibody was suppressed in dendritic cells of the conditional knock-in mice (CD11c-Cre/GRA15cKI). In the CD11c-Cre/GRA15cKI immunized with ovalbumin (OVA), the titers of anti-OVA antibody were reduced compared to control mice. Furthermore, the number of OVA antigen-specific T cells was also decreased in CD11c-Cre/GRA15cKI. These data showed that GRA15 in dendritic cells suppressed T cell-mediated humoral immunity. These findings might implicate the pathological significance of GRA15 and facilitate Toxoplasma vaccines production.

Human Retinal Organoid Model of Ocular Toxoplasmosis

The health burden of ocular toxoplasmosis is substantial, and there is an unmet need for safe and curative anti-microbial drugs. One major barrier to research on new therapeutics is the lack of in vitro human-based models beyond two-dimensional cultured cells and tissue explants. We aimed to address this research gap by establishing a human retinal organoid model of ocular toxoplasmosis. Retinal organoids, generated from human induced pluripotent stem cells and grown to two stages of organization, were incubated with a suspension of live or heat-killed GT-1 strain T. gondii tachyzoites, or medium without tachyzoites. Both developing (1 month post-isolation) and matured (6 months post-isolation) organoids were susceptible to infection. Spread of live parasites from the margin to the entire organoid over 1 week was indicated by immunolabelling for T. gondii surface antigen 1. This progression was accompanied by changes in the levels of selected tachyzoite transcripts-SAG1, GRA6, and ROP16-and human cytokine transcripts-CCL2, CXCL8, CXCL10, and IL6-in infected versus control conditions. Our human retinal organoid model of ocular toxoplasmosis offers the opportunity for many future lines of study, including tachyzoite interactions with retinal cell populations and leukocyte subsets, parasite stage progression, and disease processes of different T. gondii strains, as well as drug testing.

Toxoplasma gondii-Derived Exosomes: A Potential Immunostimulant and Delivery System for Tumor Immunotherapy Superior to Toxoplasma gondii

Immunotherapies such as immune checkpoint blockade (ICB) therapy and chimeric antigen receptor T-cell (CAR-T) therapy have ushered in a new era of tumor treatment. However, most patients do not benefit from immunotherapy due to limitations such as narrow indications, low response rates, and high rates of adverse effects. Toxoplasma gondii (T. gondii), a specialized intracellular protozoan, can modulate host immune responses by inhibiting or stimulating cytokines. The ability of T. gondii to enhance an organism's immune response was found to have a direct anti-tumor effect and enhance the sensitivity of patients with tumors to ICB therapy. However, the application of T. gondii for tumor therapy faces several challenges, such as biosafety concerns. Exosomes, a subtype of extracellular vesicle that contains active components such as proteins, nucleic acids, and lipids, have become effective therapeutic tools for various diseases, including tumors. Parasites, such as T. gondii, mediate the communication of pathogens with immune cells and modulate host cellular immune responses through exosomes. Growing evidence indicates that T. gondii-derived exosomes mediate communication between pathogens and immune cells, modulate host immune responses, and have great potential as new tools for tumor therapy. In this review, we highlight recent advances in isolation and identification techniques, profiling analysis, host immunomodulatory mechanisms, and the role of T. gondii-derived exosomes in tumor immunotherapy. Additionally, we emphasize the potential of T. gondii-derived exosomes as delivery platform to enhance anti-tumor efficacy in combination with other therapies. This review proposes that T. gondii-derived exosomes may serve as a novel tool for tumor immunotherapy owing to their ability to activate host immune function and properties such as high modifiability, stability, and low toxicity. This work will assist in promoting the application of parasite exosomes in tumor therapy.

Toxoplasma gondii infection induces the expression of the chemokine CXCL16 in macrophages to promote chemoattraction of CXCR6+ cells

CXCL16 is a multifaceted chemokine expressed by macrophages and other immune cells in response to viral and bacterial pathogens. However, few studies have investigated its role in parasitic infections. The obligate intracellular parasite Toxoplasma gondii (T. gondii) is the causative agent of toxoplasmosis, an infection with potentially deleterious consequences in immunocompromised individuals and the developing fetus of acutely infected pregnant women. Chemokines are critical mediators of host defense and, as such, dysregulation of their expression is a subversion strategy often employed by the parasite to ensure its survival. Herein, we report that types I and II T. gondii strains upregulated the expression of both transmembrane and soluble forms of CXCL16 in infected bone marrow-derived macrophages (BMDM). Exposure to soluble T. gondii antigens (STAg) and to excreted-secreted proteins (TgESP) led to the induction of CXCL16. Cxcl16 mRNA abundance and CXCL16 protein levels increased in a time-dependent manner upon T. gondii infection. Importantly, conditioned medium (CM) collected from T. gondii-infected wild-type (WT) macrophage cultures promoted the migration of RAW264.7 cells expressing CXCR6, the cognate receptor of CXCL16, an effect that was significantly reduced by a neutralizing anti-CXCL16 antibody or use of CM from CXCL16 knockout (KO) macrophages. Lastly, T. gondii-driven CXCL16 expression appeared to modulate cytokine-induced (IL-4 + IL-13) alternative macrophage activation and M2 phenotypic marker expression. Further investigation is required to determine whether this chemokine contributes to the pathogenesis of toxoplasmosis and to elucidate the underlying molecular mechanisms.

GRA47 is important for the morphology and permeability of the parasitophorous vacuole in Toxoplasma gondii

Establishing an intact intracellular parasitophorous vacuole (PV) that enables efficient nutrient uptake and protein trafficking is essential for the survival and proliferation of Toxoplasma gondii. Although the PV membrane (PVM)-localized dense granule protein 17 (GRA17) and GRA23 mediate the permeability of the PVM to small molecules, including nutrient uptake and excretion of metabolic by-products, the molecular mechanism by which T. gondii acquires nutrients remains unclear. In this study, we showed that the secreted protein GRA47 contributed to normal PV morphology, PVM permeability to small molecules, growth, and virulence in T. gondii. Co-immunoprecipitation analysis demonstrated potential interaction of GRA47 with GRA72, and the loss of GRA72 affected PV morphology, parasite growth and infectivity. To investigate the biological relationship among GRA47, GRA72, GRA17 and GRA23, attempts were made to construct strains with double gene deletion and overexpressing strains. Only Δgra23Δgra72 was successfully constructed. This strain exhibited a significant increase in the proportion of aberrant PVs compared with the Δgra23 strain. Overexpressing one of the three related GRAs partially rescued PVs with aberrant morphology in Δgra47, Δgra72 and Δgra17, while the expression of the Plasmodium falciparum PVM protein PfExp2, an ortholog of GRA17 and GRA23, fully rescued the PV morphological defect in all three Δgra strains. These results suggest that these GRA proteins may not be functionally redundant but rather work in different ways to regulate nutrient acquisition. These findings highlight the versatility of the nutrient uptake mechanisms in T. gondii, which may contribute to the parasite's remarkable ability to grow in different cellular niches in a very broad range of hosts.

Host E3 ubiquitin ligase ITCH mediates Toxoplasma gondii effector GRA35-triggered NLRP1 inflammasome activation and cell-autonomous immunity

Toxoplasma gondii is an intracellular parasite that can activate the NLRP1 inflammasome leading to macrophage pyroptosis in Lewis rats, but the underlying mechanism is not well understood. In this study, we performed a genome-wide CRISPR screen and identified the dense granule proteins GRA35, GRA42, and GRA43 as the Toxoplasma effectors mediating cell death in Lewis rat macrophages. GRA35 localizes on the parasitophorous vacuole membrane, where it interacts with the host E3 ubiquitin ligase ITCH. Inhibition of proteasome activity or ITCH knockout prevented pyroptosis in Toxoplasma-infected Lewis rat macrophages, consistent with the "NLRP1 functional degradation model." However, there was no evidence that ITCH directly ubiquitinates or interacts with rat NLRP1. We also found that GRA35-ITCH interaction affected Toxoplasma fitness in IFNγ-activated human fibroblasts, likely due to ITCH's role in recruiting ubiquitin and the parasite-restriction factor RNF213 to the parasitophorous vacuole membrane. These findings identify a new role of host E3 ubiquitin ligase ITCH in mediating effector-triggered immunity, a critical concept that involves recognizing intracellular pathogens and initiating host innate immune responses.IMPORTANCEEffector-triggered immunity represents an innate immune defense mechanism that plays a crucial role in sensing and controlling intracellular pathogen infection. The NLRP1 inflammasome in the Lewis rats can detect Toxoplasma infection, which triggers proptosis in infected macrophages and eliminates the parasite's replication niche. The work reported here revealed that host E3 ubiquitin ligase ITCH is able to recognize and interact with Toxoplasma effector protein GRA35 localized on the parasite-host interface, leading to NLRP1 inflammasome activation in Lewis rat macrophages. Furthermore, ITCH-GRA35 interaction contributes to the restriction of Toxoplasma in human fibroblasts stimulated by IFNγ. Thus, this research provides valuable insights into understanding pathogen recognition and restriction mediated by host E3 ubiquitin ligase.

Toxoplasma-host endoplasmic reticulum interaction: How T. gondii activates unfolded protein response and modulates immune response

Toxoplasma gondii is a neurotropic single-celled zoonotic parasite that can infect human beings and animals. Infection with T. gondii is usually asymptomatic in immune-competent individual, however, it can cause symptomatic and life-threatening conditions in immunocompromised individuals and in developing foetuses. Although the mechanisms that allow T. gondii to persist in host cells are poorly understood, studies in animal models have greatly improved our understanding of Toxoplasma-host cell interaction and how this interaction modulates parasite proliferation and development, host immune response and virulence of the parasite. T. gondii is capable of recruiting the host endoplasmic reticulum (ER), suggesting it may influence the host ER function. Herein, we provide an overview of T. gondii infection and the role of host ER during stressed conditions. Furthermore, we highlight studies that explore T. gondii's interaction with the host ER. We delve into how this interaction activates the unfolded protein response (UPR) and ER stress-mediated apoptosis. Additionally, we examine how T. gondii exploits these pathways to its advantage.

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.

Genetic diversity of Toxoplasma gondii in South America: occurrence, immunity, and fate of infection

Toxoplasma gondii is an intracellular parasite with a worldwide distribution. Toxoplasma gondii infections are of great concern for public health, and their impact is usually most severe in pregnant women and their foetuses, and in immunocompromised individuals. Displaying considerable genetic diversity, T. gondii strains differ widely according to geographical location, with archetypal strains predominantly found in the Northern Hemisphere and non-archetypal (atypical) strains, with highly diverse genotypes, found mainly in South America. In this review, we present an overview of the identification and distribution of non-archetypal strains of T. gondii. Special attention is paid to the strains that have been isolated in Brazil, their interaction with the host immunological response, and their impact on disease outcomes. The genetic differences among the strains are pivotal to the distinct immunological responses that they elicit. These differences arise from polymorphisms of key proteins released by the parasite, which represent important virulence factors. Infection with divergent non-archetypal strains can lead to unusual manifestations of the disease, even in immunocompetent individuals.

Host E3 ubiquitin ligase ITCH mediates Toxoplasma gondii effector GRA35-triggered NLRP1 inflammasome activation and cell-autonomous immunity

Toxoplasma gondii is an intracellular parasite that can activate the NLRP1 inflammasome leading to macrophage pyroptosis in Lewis rats, but the underlying mechanism is not well understood. In this study, we performed a genome-wide CRISPR screen and identified the dense granule proteins GRA35, GRA42, and GRA43 as the Toxoplasma effectors mediating cell death in Lewis rat macrophages. GRA35 localizes on the parasitophorous vacuole membrane, where it interacts with the host E3 ubiquitin ligase ITCH. Inhibition of proteasome activity or ITCH knockout prevented pyroptosis in Toxoplasma-infected Lewis rat macrophages, consistent with the "NLRP1 functional degradation model". However, there was no evidence that ITCH directly ubiquitinates or interacts with rat NLRP1. We also found that GRA35-ITCH interaction affected Toxoplasma fitness in IFNγ-activated human fibroblasts, likely due to ITCH's role in recruiting ubiquitin and the parasite-restriction factor RNF213 to the parasitophorous vacuole membrane. These findings identify a new role of host E3 ubiquitin ligase ITCH in mediating effector-triggered immunity, a critical concept that involves recognizing intracellular pathogens and initiating host innate immune responses.

High-throughput identification of Toxoplasma gondii effector proteins that target host cell transcription

Intracellular pathogens and other endosymbionts reprogram host cell transcription to suppress immune responses and recalibrate biosynthetic pathways. This reprogramming is critical in determining the outcome of infection or colonization. We combine pooled CRISPR knockout screening with dual host-microbe single-cell RNA sequencing, a method we term dual perturb-seq, to identify the molecular mediators of these transcriptional interactions. Applying dual perturb-seq to the intracellular pathogen Toxoplasma gondii, we are able to identify previously uncharacterized effector proteins and directly infer their function from the transcriptomic data. We show that TgGRA59 contributes to the export of other effector proteins from the parasite into the host cell and identify an effector, TgSOS1, that is necessary for sustained host STAT6 signaling and thereby contributes to parasite immune evasion and persistence. Together, this work demonstrates a tool that can be broadly adapted to interrogate host-microbe transcriptional interactions and reveal mechanisms of infection and immune evasion.

A genetic method specifically delineates Th1-type Treg cells and their roles in tumor immunity

Regulatory T (Treg) cells expressing the transcription factor (TF) Foxp3 also express other TFs shared by T helper (Th) subsets under certain conditions. Here, to determine the roles of T-bet-expressing Treg cells, we generate a mouse strain, called VeDTR, in which T-bet/Foxp3 double-positive cells are engineered to be specifically labeled and depleted by a combination of Cre- and Flp-recombinase-dependent gene expression control. Characterization of T-bet⁺Foxp3⁺ cells using VeDTR mice reveals high resistance under oxidative stress, which is involved in accumulation of T-bet⁺Foxp3⁺ cells in tumor tissues. Moreover, short-term depletion of T-bet⁺Foxp3⁺ cells leads to anti-tumor immunity but not autoimmunity, whereas that of whole Treg cells does both. Although ablation of T-bet⁺Foxp3⁺ cells during Toxoplasma infection slightly enhances Th1 immune responses, it does not affect the course of the infection. Collectively, the intersectional genetic method reveals the specific roles of T-bet⁺Foxp3⁺ cells in suppressing tumor immunity.

Host genetics highlights IFN-γ-dependent Toxoplasma genes encoding secreted and non-secreted virulence factors in in vivo CRISPR screens

Secreted virulence factors of Toxoplasma to survive in immune-competent hosts have been extensively explored by classical genetics and in vivo CRISPR screen methods, whereas their requirements in immune-deficient hosts are incompletely understood. Those of non-secreted virulence factors are further enigmatic. Here we develop an in vivo CRISPR screen system to enrich not only secreted but also non-secreted virulence factors in virulent Toxoplasma-infected C57BL/6 mice. Notably, combined usage of immune-deficient Ifngr1-/- mice highlights genes encoding various non-secreted proteins as well as well-known effectors such as ROP5, ROP18, GRA12, and GRA45 as interferon-γ (IFN-γ)-dependent virulence genes. The screen results suggest a role of GRA72 for normal GRA17/GRA23 localization and the IFN-γ-dependent role of UFMylation-related genes. Collectively, our study demonstrates that host genetics can complement in vivo CRISPR screens to highlight genes encoding IFN-γ-dependent secreted and non-secreted virulence factors in Toxoplasma.

Functional Characterization of 15 Novel Dense Granule Proteins in Toxoplasma gondii Using the CRISPR-Cas9 System

The analysis of the subcellular localization and function of dense granule proteins (GRAs) is of central importance for the understanding of host-parasite interaction and pathogenesis of Toxoplasma gondii infection. Here, we identified 15 novel GRAs and used C-terminal endogenous gene tagging to determine their localization at the intravacuolar network (IVN), parasitophorous vacuole (PV), or PV membrane (PVM) in the tachyzoites and at the periphery of the bradyzoites-containing cysts. The functions of the 15 gra genes were examined in type I RH strain and 5 of these gra genes were also evaluated in the cyst-forming type II Pru strain. The 15 novel gra genes were successfully disrupted by using CRISPR-Cas9 mediated homologous recombination and the results showed that 13 gra genes were not individually essential for T. gondii replication in vitro or virulence in mice during acute and chronic infection. Intriguingly, deletion of TGME49_266410 and TGME49_315910 in both RH and Pru strains decreased the parasite replication in vitro and attenuated its virulence, and also reduced the cyst-forming ability of the Pru strain in mice during chronic infection. Comparison of the transcriptomic profiles of the 15 gra genes suggests that they may play roles in other life cycle stages and genotypes of T. gondii. Taken together, our findings improve the understanding of T. gondii pathogenesis and demonstrate the involvement of two novel GRAs, TGME49_266410 and TGME49_315910, in the parasite replication and virulence. IMPORTANCE Dense granule proteins (GRAs) play important roles in Toxoplasma gondii pathogenicity. However, the functions of many putative GRAs have not been elucidated. Here, we found that 15 novel GRAs are secreted into intravacuolar network (IVN), parasitophorous vacuole (PV), or PV membrane (PVM) in tachyzoites and are located at the periphery of the bradyzoite-containing cysts. TGME49_266410 and TGME49_315910 were crucial to the growth of RH and Pru strains in vitro. Deletion of TGME49_266410 and TGME49_315910 attenuated the parasite virulence in mice. However, disruption of other 13 gra genes did not have a significant impact on the proliferation and pathogenicity of T. gondii in vitro or in vivo. The marked effects of the two novel GRAs (TGME49_266410 and TGME49_315910) on the in vitro growth and virulence of T. gondii are notable and warrant further elucidation of the temporal and spatial dynamics of translocation of these two novel GRAs and how do they interfere with host cell functions.

Overview of Apoptosis, Autophagy, and Inflammatory Processes in Toxoplasma gondii Infected Cells

Toxoplasma gondii (T. gondii) is an obligate intracellular parasite. During the parasitic invasion, T. gondii creates a parasitophorous vacuole, which enables the modulation of cell functions, allowing its replication and host infection. It has effective strategies to escape the immune response and reach privileged immune sites and remain inactive in a controlled environment in tissue cysts. This current review presents the factors that affect host cells and the parasite, as well as changes in the immune system during host cell infection. The secretory organelles of T. gondii (dense granules, micronemes, and rhoptries) are responsible for these processes. They are involved with proteins secreted by micronemes and rhoptries (MIC, AMA, and RONs) that mediate the recognition and entry into host cells. Effector proteins (ROP and GRA) that modify the STAT signal or GTPases in immune cells determine their toxicity. Interference byhost autonomous cells during parasitic infection, gene expression, and production of microbicidal molecules such as reactive oxygen species (ROS) and nitric oxide (NO), result in the regulation of cell death. The high level of complexity in host cell mechanisms prevents cell death in its various pathways. Many of these abilities play an important role in escaping host immune responses, particularly by manipulating the expression of genes involved in apoptosis, necrosis, autophagy, and inflammation. Here we present recent works that define the mechanisms by which T. gondii interacts with these processes in infected host cells.

The imbalance in the relationship between inflammatory and regulatory cytokines during gestational toxoplasmosis can be harmful to fetuses: A systematic review

Objective

To evaluate the available information on inflammatory and regulatory plasma mediators in pregnant women (PW) diagnosed with toxoplasmosis. Source: The PubMed, Embase, Scopus, and Lilacs databases were evaluated until October 2022. Study eligibility criteria: This review was carried out following the PRISMA and registered on the PROSPERO platform (CRD42020203951). Studies that reported inflammatory mediators in PW with toxoplasmosis were considered.

Evaluation methods

After excluding duplicate articles, two authors independently carried out the process of title and abstract exclusion, and a third resolved disagreements when necessary. The full text was evaluated to detect related articles. The extraction table was built from the following data: Author, year of publication, journal name and impact factors, country, study design, number of gestations and maternal age (years), gestational period, diagnosis of toxoplasmosis, levels of inflammatory markers, laboratory tests, and clinical significance. Methodological quality was assessed using Joanna Briggs Institute tools.

Results

Of the 1,024 studies reported, only eight were included. Of the 868 PW included in this review, 20.2% were IgM+/IgG- and 50.8% were IgM-/IgG+ to T. gondii, and 29.0% uninfected. Infected PW presented higher plasma levels ofIL-5, IL-6, IL-8, IL-17, CCL5, and IL-10. Regarding the methodological quality, four studies obtained high quality. Data from this review pointed out the maintenance of the inflammatory pattern during pregnancy with a closely related to the parasite.

Conclusion

Immune status in PW defined the course of the T. gondii infection, where the equilibrium between inflammatory and regulatory cytokines mitigated the harmful placenta and fetus effects.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD420203951.

Importance of aspartyl protease 5 in the establishment of the intracellular niche during acute and chronic infection of Toxoplasma gondii

Virulence and persistence of the obligate intracellular parasite Toxoplasma gondii involve the secretion of effector proteins belonging to the family of dense granule proteins (GRAs) that act notably as modulators of the host defense mechanisms and participate in cyst wall formation. The subset of GRAs residing in the parasitophorous vacuole (PV) or exported into the host cell, undergo proteolytic cleavage in the Golgi upon the action of the aspartyl protease 5 (ASP5). In tachyzoites, ASP5 substrates play central roles in the morphology of the PV and the export of effectors across the translocon complex MYR1/2/3. Here, we used N-terminal amine isotopic labeling of substrates to identify novel ASP5 cleavage products by comparing the N-terminome of wild-type and Δasp5 lines in tachyzoites and bradyzoites. Validated substrates reside within the PV or PVM in an ASP5-dependent manner. Remarkably, Δasp5 bradyzoites are impaired in the formation of the cyst wall in vitro and exhibit a considerably reduced cyst burden in chronically infected animals. More specifically two-photon serial tomography of infected mouse brains revealed a comparatively reduced number and size of the cysts throughout the establishment of persistence in the absence of ASP5.

The Toxoplasma effector GRA28 promotes parasite dissemination by inducing dendritic cell-like migratory properties in infected macrophages

Upon pathogen detection, macrophages normally stay sessile in tissues while dendritic cells (DCs) migrate to secondary lymphoid tissues. The obligate intracellular protozoan Toxoplasma gondii exploits the trafficking of mononuclear phagocytes for dissemination via unclear mechanisms. We report that, upon T. gondii infection, macrophages initiate the expression of transcription factors normally attributed to DCs, upregulate CCR7 expression with a chemotactic response, and perform systemic migration when adoptively transferred into mice. We show that parasite effector GRA28, released by the MYR1 secretory pathway, cooperates with host chromatin remodelers in the host cell nucleus to drive the chemotactic migration of parasitized macrophages. During in vivo challenge studies, bone marrow-derived macrophages infected with wild-type T. gondii outcompeted those challenged with MYR1- or GRA28-deficient strains in migrating and reaching secondary organs. This work reveals how an intracellular parasite hijacks chemotaxis in phagocytes and highlights a remarkable migratory plasticity in differentiated cells of the mononuclear phagocyte system.

Dense Granule Protein GRA64 Interacts with Host Cell ESCRT Proteins during Toxoplasma gondii Infection

The intracellular parasite Toxoplasma gondii adapts to diverse host cell environments within a replicative compartment that is heavily decorated by secreted proteins. In an attempt to identify novel parasite secreted proteins that influence host cell activity, we identified and characterized a transmembrane dense granule protein dubbed GRA64 (TGME49_202620). We found that GRA64 is on the parasitophorous vacuolar membrane (PVM) and is partially exposed to the host cell cytoplasm in both tachyzoite and bradyzoite parasitophorous vacuoles. Using co-immunoprecipitation and proximity-based biotinylation approaches, we demonstrated that GRA64 appears to interact with components of the host endosomal sorting complexes required for transport (ESCRT). Genetic disruption of GRA64 does not affect acute Toxoplasma virulence or encystation in mice, as observed via tissue cyst burdens in mice during chronic infection. However, ultrastructural analysis of Δgra64 tissue cysts using electron tomography revealed enlarged vesicular structures underneath the cyst membrane, suggesting a role for GRA64 in organizing the recruitment of ESCRT proteins and subsequent intracystic vesicle formation. This study uncovers a novel host-parasite interaction that contributes to an emerging paradigm in which specific host ESCRT proteins are recruited to the limiting membranes (PVMs) of tachyzoite and bradyzoite vacuoles formed during acute and chronic Toxoplasma infection. IMPORTANCE Toxoplasma gondii is a widespread foodborne parasite that causes congenital disease and life-threatening complications in immunocompromised individuals. Part of this parasite's success lies in its ability to infect diverse organisms and host cells and to persist as a latent infection within parasite-constructed structures called tissue cysts. In this study, we characterized a protein that is secreted by T. gondii into its parasitophorous vacuole during intracellular infection, which we dub GRA64. On the vacuolar membrane, this protein is exposed to the host cell cytosol and interacts with specific host ESCRT proteins. Parasites lacking the GRA64 protein exhibit ultrastructural changes in tissue cysts during chronic infection. This study lays the foundation for future studies on the mechanics and consequences of host ESCRT-parasite protein interactions.

Epigenetic Reprogramming in Host-Parasite Coevolution: The Toxoplasma Paradigm

Like many intracellular pathogens, the protozoan parasite Toxoplasma gondii has evolved sophisticated mechanisms to promote its transmission and persistence in a variety of hosts by injecting effector proteins that manipulate many processes in the cells it invades. Specifically, the parasite diverts host epigenetic modulators and modifiers from their native functions to rewire host gene expression to counteract the innate immune response and to limit its strength. The arms race between the parasite and its hosts has led to accelerated adaptive evolution of effector proteins and the unconventional secretion routes they use. This review provides an up-to-date overview of how T. gondii effectors, through the evolution of intrinsically disordered domains, the formation of supramolecular complexes, and the use of molecular mimicry, target host transcription factors that act as coordinating nodes, as well as chromatin-modifying enzymes, to control the fate of infected cells and ultimately the outcome of infection. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Single Cell Transcriptomes of In Vitro Bradyzoite Infected Cells Reveals Toxoplasma gondii Stage Dependent Host Cell Alterations

Toxoplasma gondii bradyzoites establish chronic infections within their host cells. Recent studies have demonstrated that several parasite effector proteins are translocated to host cells during the bradyzoite stage of chronic infection. To understand the interaction between host cells and bradyzoites at the transcriptomic landscape level, we utilized single-cell RNA-sequencing (scRNA-Seq) to characterize the bradyzoite-induced host cell response. Distinct gene expression profiles were observed in infected host, cells with low parasite mapped reads, and mock (non-exposed) control cells. Gene set enrichment analysis showed that c-Myc and NF-κB signaling and energy metabolic pathways were upregulated by infection. Type I and II interferon response pathways were upregulated in cells with low parasite mapped reads compared to the non-exposed host control cells, and this upregulation effect was reversed in infected cells. Differences were observed in the host cells depending on the differentiation status of the parasites, as determined by BAG1 and SAG1 expression. NF-κB, inflammatory response pathways, and IFN-γ response pathways were downregulated in host cells containing T. gondiiBAG1+/SAG1-, whereas this downregulation effect was reversed in case of T. gondiiBAG1-/SAG1+. We also identified two distinct host cell subsets that contained T. gondiiBAG1+/SAG1-, one of which displayed distinct transcriptomes with upregulated c-Myc expression. Overall, these data clearly demonstrate that host cell transcriptional alteration by bradyzoite infection is different from that of tachyzoite infection, indicating fine-tuning of the host immune response.

Depletion of Intracellular Glutamine Pools Triggers Toxoplasma gondii Stage Conversion in Human Glutamatergic Neurons

Toxoplasma gondii chronically infects the brain as latent cysts containing bradyzoites and causes various effects in the host. Recently, the molecular mechanisms of cyst formation in the mouse brain have been elucidated, but those in the human brain remain largely unknown. Here, we show that abnormal glutamine metabolism caused by both interferon-γ (IFN-γ) stimulation and T. gondii infection induce cyst formation in human neuroblastoma cells regardless of the anti-T. gondii host factor nitric oxide (NO) level or Indoleamine 2,3-dioxygenase-1 (IDO1) expression. IFN-γ stimulation promoted intracellular glutamine degradation in human neuronal cells. Additionally, T. gondii infection inhibited the mRNA expression of the host glutamine transporters SLC38A1 and SLC38A2. These dual effects led to glutamine starvation and triggered T. gondii stage conversion in human neuronal cells. Furthermore, these mechanisms are conserved in human iPSC-derived glutamatergic neurons. Taken together, our data suggest that glutamine starvation in host cells is an important trigger of T. gondii stage conversion in human neurons.

Transcriptional modification of host cells harboring Toxoplasma gondii bradyzoites prevents IFN gamma-mediated cell death

Toxoplasma gondii develops a latent infection in the muscle and central nervous system that acts as a reservoir for acute-stage reactivation in vulnerable patients. Little is understood about how parasites manipulate host cells during latent infection and the impact this has on survival. We show that bradyzoites impart a unique transcriptional signature on infected host cells. Many of these transcriptional changes rely on protein export and result in the suppression of type I interferon (IFN) and IFNγ signaling more so than in acute stages. Loss of the protein export component, MYR1, abrogates transcriptional remodeling and prevents suppression of IFN signaling. Among the exported proteins, the inhibitor of STAT1 transcription (IST) plays a key role in limiting IFNγ signaling in bradyzoites. Furthermore, bradyzoite protein export protects host cells from IFNγ-mediated cell death, even when export is restricted to latent stages. These findings highlight the functional importance of host manipulation in Toxoplasma's bradyzoite stages.

The pathogenicity and virulence of Toxoplasma gondii

Toxoplasma gondii is a parasitic protist infecting a wide group of warm-blooded animals, ranging from birds to humans. While this infection is usually asymptomatic in healthy individuals, it can also lead to severe ocular or neurological outcomes in immunocompromised individuals or in developing fetuses. This obligate intracellular parasite has the ability to infect a considerable range of nucleated cells and can propagate in the intermediate host. Yet, under the pressure of the immune system it transforms into an encysted persistent form residing primarily in the brain and muscle tissues. Encysted parasites, which are resistant to current medication, may reactivate and give rise to an acute infection. The clinical outcome of toxoplasmosis depends on a complex balance between the host immune response and parasite virulence factors. Susceptibility to the disease is thus determined by both parasite strains and host species. Recent advances on our understanding of host cell-parasite interactions and parasite virulence have brought new insights into the pathophysiology of T. gondii infection and are summarized here.

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

Secreted Effectors Modulating Immune Responses to Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that chronically infects a third of humans. It can cause life-threatening encephalitis in immune-compromised individuals. Congenital infection also results in blindness and intellectual disabilities. In the intracellular milieu, parasites encounter various immunological effectors that have been shaped to limit parasite infection. Parasites not only have to suppress these anti-parasitic inflammatory responses but also ensure the host organism's survival until their subsequent transmission. Recent advancements in T. gondii research have revealed a plethora of parasite-secreted proteins that suppress as well as activate immune responses. This mini-review will comprehensively examine each secreted immunomodulatory effector based on the location of their actions. The first section is focused on secreted effectors that localize to the parasitophorous vacuole membrane, the interface between the parasites and the host cytoplasm. Murine hosts are equipped with potent IFNγ-induced immune-related GTPases, and various parasite effectors subvert these to prevent parasite elimination. The second section examines several cytoplasmic and ER effectors, including a recently described function for matrix antigen 1 (MAG1) as a secreted effector. The third section covers the repertoire of nuclear effectors that hijack transcription factors and epigenetic repressors that alter gene expression. The last section focuses on the translocation of dense-granule effectors and effectors in the setting of T. gondii tissue cysts (the bradyzoite parasitophorous vacuole).

Toxoplasma gondii infection and its implications within the central nervous system

Toxoplasma gondii is a parasite that infects a wide range of animals and causes zoonotic infections in humans. Although it normally only results in mild illness in healthy individuals, toxoplasmosis is a common opportunistic infection with high mortality in individuals who are immunocompromised, most commonly due to reactivation of infection in the central nervous system. In the acute phase of infection, interferon-dependent immune responses control rapid parasite expansion and mitigate acute disease symptoms. However, after dissemination the parasite differentiates into semi-dormant cysts that form within muscle cells and neurons, where they persist for life in the infected host. Control of infection in the central nervous system, a compartment of immune privilege, relies on modified immune responses that aim to balance infection control while limiting potential damage due to inflammation. In response to the activation of interferon-mediated pathways, the parasite deploys an array of effector proteins to escape immune clearance and ensure latent survival. Although these pathways are best studied in the laboratory mouse, emerging evidence points to unique mechanisms of control in human toxoplasmosis. In this Review, we explore some of these recent findings that extend our understanding for proliferation, establishment and control of toxoplasmosis in humans.

Toxoplasma gondii Matrix Antigen 1 Is a Secreted Immunomodulatory Effector

Our studies on novel cyst wall proteins serendipitously led us to the discovery that cyst wall and vacuolar matrix protein MAG1, first identified a quarter of a century ago, functions as a secreted immunomodulatory effector. MAG1 is a dense granular protein that is found in the parasitophorous vacuolar matrix in tachyzoite vacuoles and the cyst wall and matrix in bradyzoite vacuoles. In the current study, we demonstrated that MAG1 is secreted beyond the parasitophorous vacuole into the host cytosol in both tachyzoites and bradyzoites. Secretion of MAG1 gradually decreases as the parasitophorous vacuole matures, but prominent MAG1 puncta are present inside host cells even at 4 and 6 days following infection. During acute murine infection, Δmag1 parasites displayed significantly reduced virulence and dissemination. In the chronic stage of infection, Δmag1 parasites generated almost no brain cysts. To identify the mechanism behind the attenuated pathology seen with Δmag1 parasites, various immune responses were screened in vitro using bone marrow-derived macrophages (BMDM). Infection of BMDM with Δmag1 parasites induced a significant increase in interleukin 1β (IL-1β) secretion, which is a hallmark of inflammasome activation. Heterologous complementation of MAG1 in BMDM cells prevented this Δmag1 parasite-induced IL-1β release, indicating that secreted MAG1 in host cytosol dampens inflammasome activation. Furthermore, knocking out GRA15 (an inducer of IL-1β release) in Δmag1 parasites completely inhibited all IL-1β release by host cells following infection. These data suggest that MAG1 has a role as an immunomodulatory molecule and that by suppressing inflammasome activation, it would favor survival of the parasite and the establishment of latent infection.IMPORTANCEToxoplasma gondii is an Apicomplexan that infects a third of humans, causing encephalitis in AIDS patients and intellectual disabilities in congenitally infected patients. We determined that one of the cyst matrix proteins, MAG1, which had been thought to be an innate structural protein, can be secreted into the host cell and suppress the host immune reaction. This particular immune reaction is initiated by another parasite-secreted protein, GRA15. The intricate balance of inflammasome activation by GRA15 and suppression by MAG1 protects mice from acute death while enabling parasites to disseminate and establish chronic cysts. Our finding contributes to our understanding of how parasites persist in the host and how T. gondii modulates the host immune system.

Comparative Proteomics Analysis for Elucidating the Interaction Between Host Cells and Toxoplasma gondii

Toxoplasma gondii, a representative model organism belonging to the phylum Apicomplexa, can infect almost all warm-blooded organisms, including humans. The invasion of host cells via host–parasite interaction is the key step for T. gondii to complete its life cycle. Herein we performed tandem mass tag analysis to investigate global proteomic changes in host cells (human foreskin fibroblasts, HFFs) [HFFs infected with T. gondii (HT) vs. HFFs (H)] and T. gondii [HT vs. T. gondii (T)] during intracellular infection. Overall, 3477 and 1434 proteins were quantified, of which 375 and 1099 proteins were differentially expressed (adjusted p-value < 0.05 and >1.5 or <0.67-fold change) in host cells and T. gondii, respectively. T. gondii invasion relies on the secretion of numerous secretory proteins, which originate from three secretory organelles: micronemes, rhoptries, and dense granules. In the HT vs. T group, few secretory proteins were upregulated, such as microneme proteins (MICs: MIC6, MIC10), rhoptry bulb proteins (ROPs: ROP5, ROP17), and dense granule proteins (GRAs: GRA4, GRA5, GRA12). In contrast, dozens of known secretory proteins were significantly downregulated in T. gondii-infected HFFs. In HFFs, gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed a large number of differentially expressed proteins (DEPs) enriched in metabolic processes and immune-associated signaling pathways, such as NF-κB, cAMP, and Rap1 signaling pathways. Further, in case of T. gondii, DEPs were involved in ribosome biogenesis, citrate cycle, and galactose metabolism, indicating that cell biosynthesis and metabolism of T. gondii were altered after host cell invasion. These findings reveal novel modifications in the proteome of host cells as well as T. gondii, helping us better understand the mechanisms underlying host–parasite interaction.

Uncovering a novel role of PLCβ4 in selectively mediating TCR signaling in CD8+ but not CD4+ T cells

Because of their common signaling molecules, the main T cell receptor (TCR) signaling cascades in CD4⁺ and CD8⁺ T cells are considered qualitatively identical. Herein, we show that TCR signaling in CD8⁺ T cells is qualitatively different from that in CD4⁺ T cells, since CD8α ignites another cardinal signaling cascade involving phospholipase C β4 (PLCβ4). TCR-mediated responses were severely impaired in PLCβ4-deficient CD8⁺ T cells, whereas those in CD4⁺ T cells were intact. PLCβ4-deficient CD8⁺ T cells showed perturbed activation of peripheral TCR signaling pathways downstream of IP₃ generation. Binding of PLCβ4 to the cytoplasmic tail of CD8α was important for CD8⁺ T cell activation. Furthermore, GNAQ interacted with PLCβ4, mediated double phosphorylation on threonine 886 and serine 890 positions of PLCβ4, and activated CD8⁺ T cells in a PLCβ4-dependent fashion. PLCβ4-deficient mice exhibited defective antiparasitic host defense and antitumor immune responses. Altogether, PLCβ4 differentiates TCR signaling in CD4⁺ and CD8⁺ T cells and selectively promotes CD8⁺ T cell–dependent adaptive immunity.

Interorganellar communication and membrane contact sites in protozoan parasites

A key characteristic of eukaryotic cells is the presence of organelles with discrete boundaries and functions. Such subcellular compartmentalization into organelles necessitates platforms for communication and material exchange between each other which often involves vesicular trafficking and associated processes. Another way is via the close apposition between organellar membranes, called membrane contact sites (MCSs). Apart from lipid transfer, MCSs have been implicated to mediate in various cellular processes including ion transport, apoptosis, and organelle dynamics. In mammalian and yeast cells, contact sites have been reported between the membranes of the following: the endoplasmic reticulum (ER) and the plasma membrane (PM), ER and the Golgi apparatus, ER and endosomes (i.e., vacuoles, lysosomes), ER and lipid droplets (LD), the mitochondria and vacuoles, the nucleus and vacuoles, and the mitochondria and lipid droplets, whereas knowledge of MCSs in non-model organisms such as protozoan parasites is extremely limited. Growing evidence suggests that MCSs play more general and conserved roles in cell physiology. In this mini review, we summarize and discuss representative MCSs in divergent parasitic protozoa, and highlight the universality, diversity, and the contribution of MCSs to parasitism.

Toxoplasma gondii manipulates host cell signaling pathways via its secreted effector molecules

The obligate intracellular parasite Toxoplasma gondii secretes a vast variety of effector molecules from organelles known as rhoptries (ROPs) and dense granules (GRAs). ROP proteins are released into the cytosol of the host cell where they are directed to the cell nucleus or to the parasitophorous vacuole (PV) membrane. ROPs secrete proteins that enable host cell penetration and vacuole formation by the parasites, as well as hijacking host-immune responses. After invading host cells, T. gondii multiplies within a PV that is maintained by the parasite proteins secreted from GRAs. Most GRA proteins remain within the PV, but some are known to access the host cytosol across the PV membrane, and a few are able to traffic into the host-cell nucleus. These effectors bind to host cell proteins and affect host cell signaling pathways to favor the parasite. Studies on host-pathogen interactions have identified many infection-altered host signal transductions. Notably, the relationship between individual parasite effector molecules and the specific targeting of host-signaling pathways is being elucidated through the advent of forward and reverse genetic strategies. Understanding the complex nature of the host-pathogen interactions underlying how the host-signaling pathway is manipulated by parasite effectors may lead to new molecular biological knowledge and novel therapeutic methods for toxoplasmosis. In this review, we discuss how T. gondii modulates cell signaling pathways in the host to favor its survival.

Toxoplasma Effectors that Affect Pregnancy Outcome

As an immune-privileged organ, the placenta can tolerate the introduction of antigens without inducing a strong inflammatory response that would lead to abortion. However, for the control of intracellular pathogens, a strong Th1 response characterized by the production of interferon-γ is needed. Thus, invasion of the placenta by intracellular parasites puts the maternal immune system in a quandary: The proinflammatory response needed to eliminate the pathogen can also lead to abortion. Toxoplasma is a highly successful parasite that causes lifelong chronic infections and is a major cause of abortions in humans and livestock. Here, we discuss how Toxoplasma strain type and parasite effectors influence host cell signaling pathways, and we speculate about how this might affect the outcome of gestation.

Influence of the Host and Parasite Strain on the Immune Response During Toxoplasma Infection

Toxoplasma gondii is an exceptionally successful parasite that infects a very broad host range, including humans, across the globe. The outcome of infection differs remarkably between hosts, ranging from acute death to sterile infection. These differential disease patterns are strongly influenced by both host- and parasite-specific genetic factors. In this review, we discuss how the clinical outcome of toxoplasmosis varies between hosts and the role of different immune genes and parasite virulence factors, with a special emphasis on Toxoplasma-induced ileitis and encephalitis.

Role of Gate-16 and Gabarap in Prevention of Caspase-11-Dependent Excess Inflammation and Lethal Endotoxic Shock

Sepsis is a life-threating multi-organ disease induced by host innate immunity to pathogen-derived endotoxins including lipopolysaccharide (LPS). Direct sensing of LPS by caspase-11 activates inflammasomes and causes lethal sepsis in mice. Inhibition of caspase-11 inflammasomes is important for the prevention of LPS-induced septic shock; however, whether a caspase-11 inflammasome-specific suppressive mechanism exists is unclear. Here we show that deficiency of GABARAP autophagy-related proteins results in over-activation of caspase-11 inflammasomes but not of canonical inflammasomes. Gate-16^−/−Gabarap^−/− macrophages exhibited elevated guanylate binding protein 2 (GBP2)-dependent caspase-11 activation and inflammatory responses. Deficiency of GABARAPs resulted in formation of GBP2-containing aggregates that promote IL-1β production. High mortality after low dose LPS challenge in Gate-16^−/−Gabarap^−/− mice primed with poly(I:C) or polymicrobial sepsis was ameliorated by compound GBP2 deficiency. These results reveal a critical function of Gate-16 and Gabarap to suppress GBP2-dependent caspase-11-induced inflammation and septic shock.

Toxoplasma Mechanisms for Delivery of Proteins and Uptake of Nutrients Across the Host-Pathogen Interface

Many intracellular pathogens, including the protozoan parasite Toxoplasma gondii, live inside a vacuole that resides in the host cytosol. Vacuolar residence provides these pathogens with a defined niche for replication and protection from detection by host cytosolic pattern recognition receptors. However, the limiting membrane of the vacuole, which constitutes the host-pathogen interface, is also a barrier for pathogen effectors to reach the host cytosol and for the acquisition of host-derived nutrients. This review provides an update on the specialized secretion and trafficking systems used by Toxoplasma to overcome the barrier of the parasitophorous vacuole membrane and thereby allow the delivery of proteins into the host cell and the acquisition of host-derived nutrients.

During host cell traversal and cell-to-cell passage, Toxoplasma gondii sporozoites inhabit the parasitophorous vacuole and posteriorly release dense granule protein-associated membranous trails

Toxoplasma gondii has a worldwide distribution and infects virtually all warm-blooded animals, including humans. Ingestion of the environmentally resistant oocyst stage, excreted only in the feces of cats, is central to transmission of this apicomplexan parasite. There is vast literature on the host and T. gondii tachyzoite (proliferative stage of the parasite) but little is known of the host-parasite interaction and conversion of the free-living stage (sporozoite inside the oocyst) to the parasitic stage. Here, we present events that follow invasion of host cells with T. gondii sporozoites by using immunofluorescence (IF) and transmission electron microscopy (TEM). Several human type cell cultures were infected with T. gondii sporozoites of the two genotypes (Type II, ME49 and Type III, VEG) most prevalent worldwide. For the first known time, using anti-rhoptry neck protein 4 (RON4) antibodies, the moving junction was visualized in sporozoites during the invasion process and shortly after its completion. Surprisingly, IF and TEM evaluation revealed that intracellular sporozoites release, at their posterior end, long membranous tails, herein named sporozoite-specific trails (SSTs). Differential permeabilization and IF experiments showed that the SSTs are associated with several dense granule proteins (GRAs) and that their membranous component is of parasite origin. Furthermore, TEM observations demonstrated that SST-associated sporozoites are delimited by a typical parasitophorous vacuole, which is retained during parasite exit from the host cell and during cell-to-cell passage. Our data strongly suggest that host cell traversal by T. gondii sporozoites relies on a novel force-producing mechanism, based on the massive extrusion at the parasite posterior pole of GRA-associated membranous material derived from the same pool of membranes forming the intravacuolar network.

Toxoplasmosis and Psychiatric and Neurological Disorders: A Step toward Understanding Parasite Pathogenesis

Toxoplasmosis, a disease that disrupts fetal brain development and severely affects the host's brain, has been linked to many behavioral and neurological disorders. There is growing interest in how a single-celled neurotropic parasite, Toxoplasma gondii, can control or change the behavior of the host as well as how it dominates the host's neurons. Secrets beyond these could be answered by decoding the Toxoplasma gondii genome, unravelling the function of genomic sequences, and exploring epigenetics and mRNAs alterations, as well as the postulated mechanisms contributing to various neurological and psychiatric symptoms caused by this parasite. Substantial efforts have been made to elucidate the action of T. gondii on host immunity and the biology of its infection. However, the available studies on the molecular aspects of toxoplasmosis that affect central nervous system (CNS) circuits remain limited, and much research is still needed on this interesting topic. In my opinion, this parasite is a gift for studying the biology of the nervous system and related diseases. We should utilize the unique features of Toxoplasma, such as its abilities to modulate brain physiology, for neurological studies or as a possible tool or approach to cure neurological disease.

Toxoplasma gondii Dense Granule Proteins 7, 14, and 15 Are Involved in Modification and Control of the Immune Response Mediated via NF-κB Pathway

Toxoplasma gondii infects almost all warm-blooded animals, including humans, leading to both cellular and humoral immune responses in the host. The virulence of T. gondii is strain specific and is defined by secreted effector proteins that disturb host immunity. Here, we focus on nuclear factor-kappa B (NFκB) signaling, which regulates the induction of T-helper type 1 immunity. A luciferase assay for screening effector proteins, including ROPs and GRAs that have biological activity against an NFκB-dependent reporter plasmid, found that overexpression of GRA7, 14, and 15 of a type II strain resulted in a strong activity. Thus, our study was aimed at understanding the involvement of NFκB in the pathogenesis of toxoplasmosis through a comparative analysis of these three molecules. We found that GRA7 and GRA14 were partially involved in the activation of NFκB, whereas GRA15 was essential for NFκB activation. The deletion of GRA7, GRA14, and GRA15 in the type II Prugniaud (Pru) strain resulted in a defect in the nuclear translocation of RelA. Cells infected with the PruΔgra15 parasite showed reduced phosphorylation of inhibitor-κBα. GRA7, GRA14, and GRA15 deficiency decreased the levels of interleukin-6 in RAW246.7 cells, and RNA-seq analysis revealed that GRA7, GRA14, and GRA15 deficiency predominantly resulted in downregulation of gene expression mediated by NFκB. The virulence of all mutant strains increased, but PruΔgra14 only showed a slight increase in virulence. However, the intra-footpad injection of the highly-virulent type I RHΔgra14 parasites in mice resulted in increased virulence. This study shows that GRA7, 14, and 15-induced host immunity via NFκB limits parasite expansion.

Effect of deletion of gra17 and gra23 genes on the growth, virulence, and immunogenicity of type II Toxoplasma gondii

The protozoan parasite Toxoplasma gondii secretes a number of dense granule proteins (GRAs) from the dense granule organelle to manipulate the host cell. Two of these effector proteins (GRA17 and GRA23) are involved in the trafficking of molecules between the parasitophorous vacuole (PV) and the host cell cytoplasm. However, their roles in establishing chronic infection remain obscured. In this study, CRISPR-Cas9 was used to delete gra17 or gra23 gene in T. gondii Pru strain (type II). The growth, the virulence, the ability to establish chronic infection, and the immunogenicity of the constructed mutant strains were investigated in Kunming mice. Pru:Δgra17 and Pru:Δgra23 mutants developed PVs with abnormal morphology and exhibited reduced growth rate, compared with the wild-type Pru strain. Deletion of gra17 abrogated acute infection and blocked cyst formation. Although the deletion of gra23 caused slight attenuation of the parasite virulence in mice, it caused a significant reduction in cyst formation. Immunization with Pru:Δgra17 induced high levels of IgG (IgG1 and IgG2a) antibodies and cytokines (interleukin-2 [IL-2], IL-10, IL-12, and interferon gamma [IFN-γ]), which conferred significant protection in mice challenged with virulent type I (RH), ToxoDB#9 (PYS) strains, or less virulent type II (Pru) strain of T. gondii. These findings show that GRA17 and GRA23 play important roles in T. gondii chronic infection and that irreversible deletion of gra17 in T. gondii type II Pru strain can be a viable option for stimulating protective immunity to T. gondii infection.

Toxoplasma GRA15 and GRA24 are important activators of the host innate immune response in the absence of TLR11

The murine innate immune response against Toxoplasma gondii is predominated by the interaction of TLR11/12 with Toxoplasma profilin. However, mice lacking Tlr11 or humans, who do not have functional TLR11 or TLR12, still elicit a strong innate immune response upon Toxoplasma infection. The parasite factors that determine this immune response are largely unknown. Herein, we investigated two dense granule proteins (GRAs) secreted by Toxoplasma, GRA15 and GRA24, for their role in stimulating the innate immune response in Tlr11-/- mice and in human cells, which naturally lack TLR11/TLR12. Our results show that GRA15 and GRA24 synergistically shape the early immune response and parasite virulence in Tlr11-/- mice, with GRA15 as the predominant effector. Nevertheless, acute virulence in Tlr11-/- mice is still dominated by allelic combinations of ROP18 and ROP5, which are effectors that determine evasion of the immunity-related GTPases. In human macrophages, GRA15 and GRA24 play a major role in the induction of IL12, IL18 and IL1β secretion. We further show that GRA15/GRA24-mediated IL12, IL18 and IL1β secretion activates IFNγ secretion by peripheral blood mononuclear cells (PBMCs), which controls Toxoplasma proliferation. Taken together, our study demonstrates the important role of GRA15 and GRA24 in activating the innate immune response in hosts lacking TLR11.

Cell type- and species-specific host responses to Toxoplasma gondii and its near relatives

Toxoplasma gondii is remarkably unique in its ability to successfully infect vertebrate hosts from multiple phyla and can successfully infect most cells within these organisms. The infection outcome in each of these species is determined by the complex interaction between parasite and host genotype. As techniques to quantify global changes in cell function become more readily available and precise, new data are coming to light about how (i) different host cell types respond to parasitic infection and (ii) different parasite species impact the host. Here we focus on recent studies comparing the response to intracellular parasitism by different cell types and insights into understanding host-parasite interactions from comparative studies on T. gondii and its close extant relatives.

Toxoplasma gondii dense granule protein GRA24 drives MyD88-independent p38 MAPK activation, IL-12 production and induction of protective immunity

The apicomplexan Toxoplasma gondii induces strong protective immunity dependent upon recognition by Toll-like receptors (TLR)11 and 12 operating in conjunction with MyD88 in the murine host. However, TLR11 and 12 proteins are not present in humans, inspiring us to investigate MyD88-independent pathways of resistance. Using bicistronic IL-12-YFP reporter mice on MyD88+/+ and MyD88-/- genetic backgrounds, we show that CD11c+MHCII+F4/80- dendritic cells, F4/80+ macrophages, and Ly6G+ neutrophils were the dominant cellular sources of IL-12 in both wild type and MyD88 deficient mice after parasite challenge. Parasite dense granule protein GRA24 induces p38 MAPK activation and subsequent IL-12 production in host macrophages. We show that Toxoplasma triggers an early and late p38 MAPK phosphorylation response in MyD88+/+ and MyD88-/- bone marrow-derived macrophages. Using the uracil auxotrophic Type I T. gondii strain cps1-1, we demonstrate that the late response does not require active parasite proliferation, but strictly depends upon GRA24. By i. p. inoculation with cps1-1 and cps1-1:Δgra24, we identified unique subsets of chemokines and cytokines that were up and downregulated by GRA24. Finally, we demonstrate that cps1-1 triggers a strong host-protective GRA24-dependent Th1 response in the absence of MyD88. Our data identify GRA24 as a major mediator of p38 MAPK activation, IL-12 induction and protective immunity that operates independently of the TLR/MyD88 cascade.

Comparative tachyzoite proteome analyses among six Neospora caninum isolates with different virulence

The biological variability among Neospora caninum isolates has been widely shown, however, the molecular basis that determines this diversity has not been thoroughly elucidated to date. The latest studies have focused on a limited number of isolates. Therefore, the goal of the present study was to compare the proteome of a larger number of N. caninum isolates with different origins and virulence. Label-free LC-MS/MS was used to investigate the tachyzoite proteomic differences among Nc-Bahia, Nc-Spain4H and Nc-Spain7, representing high virulence isolates and Nc-Ger6, Nc-Spain2H and Nc-Spain1H, representing low virulence isolates. Pairwise comparisons between all isolates and between high virulence and low virulence groups identified a subset of proteins with higher abundance in high virulence isolates. These proteins were involved in energy and redox metabolism, and DNA/RNA processing, which might determine the faster growth rates and parasite survival of the high virulence isolates. Highlighted proteins included a predicted member of the rhoptry kinase family ROP20 specific for N. caninum, Bradyzoite pseudokinase 1 and several dense granule proteins. DNA polymerase, which was more abundant in all high virulence isolates in all comparisons, might also be implicated in virulence. These results reveal insights into possible mechanisms involved in specific phenotypic traits and virulence in N. caninum, and the relevance of these candidate proteins for N. caninum virulence deserves further investigation.

Application of Toxoplasma gondii GRA15 peptides in diagnosis and serotyping

Previous studies showed that three clonal strain types (I, II, and III) of Toxoplasma gondii can be distinguished using serotyping based on a series of polymorphic proteins. However, to establish a systematic serotyping method with higher resolution even being equal to that of genotyping, more specific peptide markers are needed. The objective of the present study was to determine the possibility of the polymorphic dense granule protein 15 (GRA15) for diagnosis and serotyping of T. gondii infection. Three different T. gondii GT1 strain GRA15 gene fragments encoding a 584-residue peptide, a 199-residue peptide and a 84-residue peptide were amplified, expressed and purified, respectively. Anti-T. gondii GT1 strain antibodies, anti-T. gondii RH strain antibodies and anti-T. gondii PRU strain antibodies were used for immunoblotting analysis of the three peptides. Western blotting analysis showed that the 584-residue peptide of GT1 strain GRA15 was a potential candidate for serological diagnosis of T. gondii infection. RH strain from GT1 strain could be distinguished by serotyping based on the GRA15₁₉₉ or GRA15₈₄, and T. gondii GT1 strain could be distinguished from PRU strain by using serotyping based on the GRA15₈₄. These findings reveal, for the first time, a novel potential role of GRA15-derived peptides in diagnosis and serological differentiation of T. gondii infection.

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

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

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