The polymorphic pseudokinase ROP5 controls virulence in Toxoplasma gondii by regulating the active kinase ROP18

TgMIC6 inhibition of autophagy is partially responsible for the phenotypic differences between Chinese 1 Toxoplasma gondii strains

Chinese1 is the predominant Toxoplasma gondii lineage in China, and significant phenotypic differences are observed within the lineage. WH3 and WH6 are two representative strains of Chinese 1, which exhibit divergent virulence and pathogenicity in mice. However, virulence determinants and their modulating mechanisms remain elusive. A global genome expression analysis of the WH3 and WH6 transcriptional profiles identified microneme secretory protein 6 (MIC6), which may be associated with the phenotypic difference observed in WH3. In the present study, the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 genome-editing technique was used to generate a T. gondii microneme secretory protein (TgMIC6) knockout in WH3. Wild-type mice and different mouse and human cell lines were infected with the WH3, WH3-Δmic6, and WH6 strains. The survival rate of mice, related cytokine levels in serum, and the proliferation of parasites were observed. These results suggested that TgMIC6 is an important effector molecule that determines the differential virulence of WH3 in vivo and in vitro. Furthermore, MIC6 may enhance WH3 virulence via inhibition of host cell autophagy and activation of key molecules in the epidermal growth factor receptor (EGFR)-Akt-mammalian target of rapamycin (mTOR) classical autophagy pathway. CD40L was cleared in vivo by i.p injection of CD40L monoclonal antibody, and it was found that the virulence of WH3-Δmic6 to mice was restored to a certain extent in the absence of CD40L. This study elucidates the virulence determinants and immune escape strategies of Toxoplasma gondii in China. Moreover, these data will aid the development of effective strategies for the prevention and control of toxoplasmosis.

CRISPR screens identify genes essential for in vivo virulence among proteins of hyperLOPIT-unassigned subcellular localization in Toxoplasma

The research field to identify and characterize genes essential for in vivo virulence in Toxoplasma gondii has been dramatically advanced by a series of in vivo clustered regularly interspaced short palindromic repeats (CRISPR) screens. Although subcellular localizations of thousands of proteins were predicted by the spatial proteomic method called hyperLOPIT, those of more than 1,000 proteins remained unassigned, and their essentiality in virulence was also unknown. In this study, we generated two small-scale gRNA libraries targeting approximately 600 hyperLOPIT-unassigned proteins and performed in vivo CRISPR screens. As a result, we identified several genes essential for in vivo virulence that were previously unreported. We further characterized two candidates, TgGTPase and TgRimM, which are localized in the cytoplasm and the apicoplast, respectively. Both genes are essential for parasite virulence and widely conserved in the phylum Apicomplexa. Collectively, our current study provides a resource for estimating the in vivo essentiality of Toxoplasma proteins with previously unknown localizations.IMPORTANCEToxoplasma gondii is a protozoan parasite that causes severe infection in immunocompromised patients or newborns. Toxoplasma possesses more than 8,000 genes; however, the genes essential for in vivo virulence were not fully identified. The apicomplexan parasites, including Toxoplasma, developed unique organelles that do not exist in other model organisms; thus, determining the subcellular location of parasite proteins is important for understanding their functions. Here, we used in vivo genetic screens that enabled us to investigate hundreds of genes in Toxoplasma during mouse infection. We screened approximately 600 parasite proteins with previously unknown subcellular localizations. We identified many novel genes that confer parasite virulence in mice. Among the top hits, we characterized two genes essential for in vivo virulence, TgGTPase and TgRimM, which are widely conserved in the phylum Apicomplexa. Our findings will contribute to understanding how apicomplexans adapt to the host environment and cause disease.

CRISPR-based functional profiling of the Toxoplasma gondii genome during acute murine infection

Examining host-pathogen interactions in animals can capture aspects of infection that are obscured in cell culture. Using CRISPR-based screens, we functionally profile the entire genome of the apicomplexan parasite Toxoplasma gondii during murine infection. Barcoded gRNAs enabled bottleneck detection and mapping of population structures within parasite lineages. Over 300 genes with previously unknown roles in infection were found to modulate parasite fitness in mice. Candidates span multiple axes of host-parasite interaction. Rhoptry Apical Surface Protein 1 was characterized as a mediator of host-cell tropism that facilitates repeated invasion attempts. GTP cyclohydrolase I was also required for fitness in mice and druggable through a repurposed compound, 2,4-diamino-6-hydroxypyrimidine. This compound synergized with pyrimethamine against T. gondii and malaria-causing Plasmodium falciparum parasites. This work represents a complete survey of an apicomplexan genome during infection of an animal host and points to novel interfaces of host-parasite interaction.

Decoding Toxoplasma gondii virulence: the mechanisms of IRG protein inactivation

Toxoplasmosis is a common parasitic zoonosis that can be life-threatening in immunocompromised patients. About one-third of the human population is infected with Toxoplasma gondii. Primary infection triggers an innate immune response wherein IFN-γ-induced host cell GTPases, namely IRG and GBP proteins, serve as a vital component for host cell resistance. In the past decades, interest in elucidating the function of these GTPase families in controlling various intracellular pathogens has emerged. Numerous T. gondii effectors were identified to inactivate particular IRG proteins. T. gondii is re-optimizing its effectors to combat IRG function and in this way secures transmission. We discuss the IRG-specific effectors employed by the parasite in murine infections, contributing to a better understanding of T. gondii virulence.

Proteomic approaches for protein kinase substrate identification in Apicomplexa

Apicomplexa is a phylum of protist parasites, notable for causing life-threatening diseases including malaria, toxoplasmosis, cryptosporidiosis, and babesiosis. Apicomplexan pathogenesis is generally a function of lytic replication, dissemination, persistence, host cell modification, and immune subversion. Decades of research have revealed essential roles for apicomplexan protein kinases in establishing infections and promoting pathogenesis. Protein kinases modify their substrates by phosphorylating serine, threonine, tyrosine, or other residues, resulting in rapid functional changes in the target protein. Post-translational modification by phosphorylation can activate or inhibit a substrate, alter its localization, or promote interactions with other proteins or ligands. Deciphering direct kinase substrates is crucial to understand mechanisms of kinase signaling, yet can be challenging due to the transient nature of kinase phosphorylation and potential for downstream indirect phosphorylation events. However, with recent advances in proteomic approaches, our understanding of kinase function in Apicomplexa has improved dramatically. Here, we discuss methods that have been used to identify kinase substrates in apicomplexan parasites, classifying them into three main categories: i) kinase interactome, ii) indirect phosphoproteomics and iii) direct labeling. We briefly discuss each approach, including their advantages and limitations, and highlight representative examples from the Apicomplexa literature. Finally, we conclude each main category by introducing prospective approaches from other fields that would benefit kinase substrate identification in Apicomplexa.

Rhoptry proteins affect the placental barrier in the context of Toxoplasma gondii infection: Signaling pathways and functions

Toxoplasma gondii is an opportunistic and pathogenic obligate intracellular parasitic protozoan that is widespread worldwide and can infect most warm-blooded animals, seriously endangering human health and affecting livestock production. Toxoplasmosis caused by T. gondii infection has different clinical manifestations, which are mainly determined by the virulence of T. gondii and host differences. Among the manifestations of this condition, abortion, stillbirth, and fetal malformation can occur if a woman is infected with T. gondii in early pregnancy. Here, we discuss how the T. gondii rhoptry protein affects host pregnancy outcomes and speculate on the related signaling pathways involved. The effects of rhoptry proteins of T. gondii on the placental barrier are complex. Rhoptry proteins not only regulate interferon-regulated genes (IRGs) to ensure the survival of parasites in activated cells but also promote the spread of worms in tissues and the invasive ability of the parasites. The functions of these rhoptry proteins and the associated signaling pathways highlight relevant mechanisms by which Toxoplasma crosses the placental barrier and influences fetal development and will guide future studies to uncover the complexity of the host-pathogen interactions.

Early immune response to Toxoplasma gondii lineage III isolates of different virulence phenotype

Introduction

Toxoplasma gondii is an intracellular parasite of importance to human and veterinary health. The structure and diversity of the genotype population of T. gondii varies considerably with respect to geography, but three lineages, type I, II and III, are distributed globally. Lineage III genotypes are the least well characterized in terms of biology, host immunity and virulence. Once a host is infected with T.gondii, innate immune mechanisms are engaged to reduce the parasite burden in tissues and create a pro-inflammatory environment in which the TH1 response develops to ensure survival. This study investigated the early cellular immune response of Swiss-Webster mice post intraperitoneal infection with 10 tachyzoites of four distinct non-clonal genotypes of lineage III and a local isolate of ToxoDB#1. The virulence phenotype, cumulative mortality (CM) and allele profiles of ROP5, ROP16, ROP18 and GRA15 were published previously.

Methods

Parasite dissemination in different tissues was analyzed by real-time PCR and relative expression levels of IFNγ, IL12-p40, IL-10 and TBX21 in the cervical lymph nodes (CLN), brain and spleen were calculated using the ΔΔCt method. Stage conversion was determined by detection of the BAG1 transcript in the brain.

Results

Tissue dissemination depends on the virulence phenotype but not CM, while the TBX21 and cytokine levels and kinetics correlate better with CM than virulence phenotype. The earliest detection of BAG1 was seven days post infection. Only infection with the genotype of high CM (69.4%) was associated with high T-bet levels in the CLN 24 h and high systemic IFNγ expression which was sustained over the first week, while infection with genotypes of lower CM (38.8%, 10.7% and 6.8%) is characterized by down-regulation and/or low systemic levels of IFNγ. The response intensity, as assessed by cytokine levels, to the genotype of high CM wanes over time, while it increases gradually to genotypes of lower CM.

Discussion

The results point to the conclusion that the immune response is not correlated with the virulence phenotype and/or allele profile, but an early onset, intense pro-inflammatory response is characteristic of genotypes with high CM. Additionally, high IFNγ level in the brain may hamper stage conversion.

TBC9, an essential TBC-domain protein, regulates early vesicular transport and IMC formation in Toxoplasma gondii

Apicomplexan parasites harbor a complex endomembrane system as well as unique secretory organelles. These complex cellular structures require an elaborate vesicle trafficking system, which includes Rab GTPases and their regulators, to assure the biogenesis and secretory of the organelles. Here we exploit the model apicomplexan organism Toxoplasma gondii that encodes a family of Rab GTPase Activating Proteins, TBC (Tre-2/Bub2/Cdc16) domain-containing proteins. Functional profiling of these proteins in tachyzoites reveals that TBC9 is the only essential regulator, which is localized to the endoplasmic reticulum (ER) in T. gondii strains. Detailed analyses demonstrate that TBC9 is required for normal distribution of proteins targeting to the ER, and the Golgi apparatus in the parasite, as well as for the normal formation of daughter inner membrane complexes (IMCs). Pull-down assays show a strong protein interaction between TBC9 and specific Rab GTPases (Rab11A, Rab11B, and Rab2), supporting the role of TBC9 in daughter IMC formation and early vesicular transport. Thus, this study identifies the only essential TBC domain-containing protein TBC9 that regulates early vesicular transport and IMC formation in T. gondii and potentially in closely related protists.

iNOS is necessary for GBP-mediated T. gondii clearance in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular parasite of rodents and humans. Interferon-inducible guanylate binding proteins (GBPs) are mediators of T. gondii clearance, however, this mechanism is incomplete. Here, using automated spatially targeted optical micro proteomics we demonstrate that inducible nitric oxide synthetase (iNOS) is highly enriched at GBP2+ parasitophorous vacuoles (PV) in murine macrophages. iNOS expression in macrophages is necessary to limit T. gondii load in vivo and in vitro. Although iNOS activity is dispensable for GBP2 recruitment and PV membrane ruffling; parasites can replicate, egress and shed GBP2 when iNOS is inhibited. T. gondii clearance by iNOS requires nitric oxide, leading to nitration of the PV and collapse of the intravacuolar network of membranes in a chromosome 3 GBP-dependent manner. We conclude that reactive nitrogen species generated by iNOS cooperate with GBPs to target distinct structures in the PV that are necessary for optimal parasite clearance in macrophages.

IFNs in host defence and parasite immune evasion during Toxoplasma gondii infections

Interferons (IFNs) are a family of cytokines with diverse functions in host resistance to pathogens and in immune regulation. Type II IFN, i.e. IFN-γ, is widely recognized as a major mediator of resistance to intracellular pathogens, including the protozoan Toxoplasma gondii. More recently, IFN-α/β, i.e. type I IFNs, and IFN-λ (type III IFN) have been identified to also play important roles during T. gondii infections. This parasite is a widespread pathogen of humans and animals, and it is a model organism to study cell-mediated immune responses to intracellular infection. Its success depends, among other factors, on the ability to counteract the IFN system, both at the level of IFN-mediated gene expression and at the level of IFN-regulated effector molecules. Here, I review recent advances in our understanding of the molecular mechanisms underlying IFN-mediated host resistance and immune regulation during T. gondii infections. I also discuss those mechanisms that T. gondii has evolved to efficiently evade IFN-mediated immunity. Knowledge of these fascinating host-parasite interactions and their underlying signalling machineries is crucial for a deeper understanding of the pathogenesis of toxoplasmosis, and it might also identify potential targets of parasite-directed or host-directed supportive therapies to combat the parasite more effectively.

Toxoplasma membrane inositol phospholipid binding protein TgREMIND is essential for secretory organelle function and host infection

Apicomplexan parasites possess specialized secretory organelles called rhoptries, micronemes, and dense granules that play a vital role in host infection. In this study, we demonstrate that TgREMIND, a protein found in Toxoplasma gondii, is necessary for the biogenesis of rhoptries and dense granules. TgREMIND contains a Fes-CIP4 homology-Bin/Amphiphysin/Rvs (F-BAR) domain, which binds to membrane phospholipids, as well as a novel uncharacterized domain that we have named REMIND (regulator of membrane-interacting domain). Both the F-BAR domain and the REMIND are crucial for TgREMIND functions. When TgREMIND is depleted, there is a significant decrease in the abundance of dense granules and abnormal transparency of rhoptries, leading to a reduction in protein secretion from these organelles. The absence of TgREMIND inhibits host invasion and parasite dissemination, demonstrating that TgREMIND is essential for the proper function of critical secretory organelles required for successful infection by Toxoplasma.

Susceptibility of Toxoplasma gondii to autophagy in human cells relies on multiple interacting parasite loci

IMPORTANCE

Autophagy is a process used by cells to recycle organelles and macromolecules and to eliminate intracellular pathogens. Previous studies have shown that some stains of Toxoplasma gondii are resistant to autophagy-dependent growth restriction, while others are highly susceptible. Although it is known that autophagy-mediated control requires activation by interferon gamma, the basis for why parasite strains differ in their susceptibility is unknown. Our findings indicate that susceptibility involves at least five unlinked parasite genes on different chromosomes, including several secretory proteins targeted to the parasite-containing vacuole and exposed to the host cell cytosol. Our findings reveal that susceptibility to autophagy-mediated growth restriction relies on differential recognition of parasite proteins exposed at the host-pathogen interface, thus identifying a new mechanism for cell-autonomous control of intracellular pathogens.

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.

Structural basis of Irgb6 inactivation by Toxoplasma gondii through the phosphorylation of switch I

Irgb6 is a priming immune-related GTPase (IRG) that counteracts Toxoplasma gondii. It is known to be recruited to the low virulent type II T. gondii parasitophorous vacuole (PV), initiating cell-autonomous immunity. However, the molecular mechanism by which immunity-related GTPases become inactivated after the parasite infection remains obscure. Here, we found that Thr95 of Irgb6 is prominently phosphorylated in response to low virulent type II T. gondii infection. We observed that a phosphomimetic T95D mutation in Irgb6 impaired its localization to the PV and exhibited reduced GTPase activity in vitro. Structural analysis unveiled an atypical conformation of nucleotide-free Irgb6-T95D, resulting from a conformational change in the G-domain that allosterically modified the PV membrane-binding interface. In silico docking corroborated the disruption of the physiological membrane binding site. These findings provide novel insights into a T. gondii-induced allosteric inactivation mechanism of Irgb6.

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.

Histone variant H2B.Z acetylation is necessary for maintenance of Toxoplasma gondii biological fitness

Through regulation of DNA packaging, histone proteins are fundamental to a wide array of biological processes. A variety of post-translational modifications (PTMs), including acetylation, constitute a proposed histone code that is interpreted by "reader" proteins to modulate chromatin structure. Canonical histones can be replaced with variant versions that add an additional layer of regulatory complexity. The protozoan parasite Toxoplasma gondii is unique among eukaryotes in possessing a novel variant of H2B designated H2B.Z. The combination of PTMs and the use of histone variants are important for gene regulation in T. gondii, offering new targets for drug development. In this work, T. gondii parasites were generated in which the 5 N-terminal acetylatable lysines in H2B.Z were mutated to either alanine (c-Myc-A) or arginine (c-Myc-R). The c-Myc-A mutant displayed no phenotype over than a mild defect in its ability to kill mice. The c-Myc-R mutant presented an impaired ability to grow and an increase in differentiation to latent bradyzoites. The c-Myc-R mutant was also more sensitive to DNA damage, displayed no virulence in mice, and provided protective immunity against future infection. While nucleosome composition was unaltered, key genes were abnormally expressed during in vitro bradyzoite differentiation. Our results show that regulation of the N-terminal positive charge patch of H2B.Z is important for these processes. We also show that acetylated N-terminal H2B.Z interacts with some unique proteins compared to its unacetylated counterpart; the acetylated peptide pulled down proteins associated with chromosome maintenance/segregation and cell cycle, suggesting a link between H2B.Z acetylation status and mitosis.

Functional screening reveals Toxoplasma prenylated proteins required for endocytic trafficking and rhoptry protein sorting

In the apicomplexans, endocytosed cargos (e.g., hemoglobin) are trafficked to a specialized organelle for digestion. This follows a unique endocytotic process at the micropore/cytostome in these parasites. However, the mechanism underlying endocytic trafficking remains elusive, due to the repurposing of classical endocytic proteins for the biogenesis of apical organelles. To resolve this issue, we have exploited the genetic tractability of the model apicomplexan Toxoplasma gondii, which ingests host cytosolic materials (e.g., green fluorescent protein[GFP]). We determined an association between protein prenylation and endocytic trafficking, and using an alkyne-labeled click chemistry approach, the prenylated proteome was characterized. Genome editing, using clustered regularly interspaced short palindromic repaet/CRISPR-associated nuclease 9 (CRISPR/Cas9), was efficiently utilized to generate genetically modified lines for the functional screening of 23 prenylated candidates. This identified four of these proteins that regulate the trafficking of endocytosed GFP vesicles. Among these proteins, Rab1B and YKT6.1 are highly conserved but are non-classical endocytic proteins in eukaryotes. Confocal imaging analysis showed that Rab1B and Ras are substantially localized to both the trans-Golgi network and the endosome-like compartments in the parasite. Conditional knockdown of Rab1B caused a rapid defect in secretory trafficking to the rhoptry bulb, suggesting a trafficking intersection role for the key regulator Rab1B. Further experiments confirmed a critical role for protein prenylation in regulating the stability/activity of these proteins (i.e., Rab1B and YKT6.1) in the parasite. Our findings define the molecular basis of endocytic trafficking and reveal a potential intersection function of Rab1B on membrane trafficking in T. gondii. This might extend to other related protists, including the malarial parasites. IMPORTANCE The protozoan Toxoplasma gondii establishes a permissive niche, in host cells, that allows parasites to acquire large molecules such as proteins. Numerous studies have demonstrated that the parasite repurposes the classical endocytic components for secretory sorting to the apical organelles, leaving the question of endocytic transport to the lysosome-like compartment unclear. Recent studies indicated that endocytic trafficking is likely to associate with protein prenylation in malarial parasites. This information promoted us to examine this association in the model apicomplexan T. gondii and to identify the key components of the prenylated proteome that are involved. By exploiting the genetic tractability of T. gondii and a host GFP acquisition assay, we reveal four non-classical endocytic proteins that regulate the transport of endocytosed cargos (e.g., GFP) in T. gondii. Thus, we extend the principle that protein prenylation regulates endocytic trafficking and elucidate the process of non-classical endocytosis in T. gondii and potentially in other related protists.

Anti-Toxoplasma gondii Effects of Lipopeptide Derivatives of Lycosin-I

Toxoplasmosis, caused by Toxoplasma gondii (T. gondii), is a serious zoonotic parasitic disease. We previously found that Lycosin-I exhibited anti-T. gondii activity, but its serum stability was not good enough. In this study, we aimed to improve the stability and activity of Lycosin-I through fatty acid chain modification, so as to find a better anti-T. gondii drug candidate. The α/ε-amino residues of different lysine residues of Lycosin-I were covalently coupled with lauric acid to obtain eight lipopeptides, namely L-C12, L-C12-1, L-C12-2, L-C12-3, L-C12-4, L-C12-5, L-C12-6, and L-C12-7. Among these eight lipopeptides, L-C12 showed the best activity against T. gondii in vitro in a trypan blue assay. We then conjugated a shorter length fatty chain, aminocaproic acid, at the same modification site of L-C12, namely L-an. The anti-T. gondii effects of Lycosin-I, L-C12 and L-an were evaluated via an invasion assay, proliferation assay and plaque assay in vitro. A mouse model acutely infected with T. gondii tachyzoites was established to evaluate their efficacy in vivo. The serum stability of L-C12 and L-an was improved, and they showed comparable or even better activity than Lycosin-I did in inhibiting the invasion and proliferation of tachyzoites. L-an effectively prolonged the survival time of mice acutely infected with T. gondii. These results suggest that appropriate fatty acid chain modification can improve serum stability and enhance anti-T. gondii effect of Lycosin-I. The lipopeptide derivatives of Lycosin-I have potential as a novel anti-T. gondii drug candidate.

Inducible nitric oxide synthase (iNOS) is necessary for GBP-mediated T. gondii restriction in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular, protozoan pathogen of rodents and humans. T. gondii's ability to grow within cells and evade cell-autonomous immunity depends on the integrity of the parasitophorous vacuole (PV). Interferon-inducible guanylate binding proteins (GBPs) are central mediators of T. gondii clearance, however, the precise mechanism linking GBP recruitment to the PV and T. gondii restriction is not clear. This knowledge gap is linked to heterogenous GBP-targeting across a population of vacuoles and the lack of tools to selectively purify the intact PV. To identify mediators of parasite clearance associated with GBP2-positive vacuoles, we employed a novel protein discovery tool automated spatially targeted optical micro proteomics (autoSTOMP). This approach identified inducible nitric oxide synthetase (iNOS) enriched at levels similar to the GBPs in infected bone marrow-derived myeloid cells. iNOS expression on myeloid cells was necessary for mice to control T. gondii growth in vivo and survive acute infection. T. gondii infection of IFNγ-primed macrophage was sufficient to robustly induce iNOS expression. iNOS restricted T. gondii infection through nitric oxide synthesis rather than arginine depletion, leading to robust and selective nitration of the PV. Optimal parasite restriction by iNOS and vacuole nitration depended on the chromosome 3 GBPs. Notably, GBP2 recruitment and ruffling of the PV membrane occurred in iNOS knockouts, however, these vacuoles contained dividing parasites. iNOS activity was necessary for the collapse of the intravacuolar network of nanotubular membranes which connects parasites to each other and the host cytosol. Based on these data we conclude reactive nitrogen species generated by iNOS cooperate with the chromosome 3 GBPs to target distinct biology of the PV that are necessary for optimal parasite clearance in murine myeloid cells.

A heterotrimeric complex of Toxoplasma proteins promotes parasite survival in interferon gamma-stimulated human cells

Toxoplasma gondii secretes protein effectors to subvert the human immune system sufficiently to establish a chronic infection. Relative to murine infections, little is known about which parasite effectors disarm human immune responses. Here, we used targeted CRISPR screening to identify secreted protein effectors required for parasite survival in IFNγ-activated human cells. Independent screens were carried out using 2 Toxoplasma strains that differ in virulence in mice, leading to the identification of effectors required for survival in IFNγ-activated human cells. We identify the secreted protein GRA57 and 2 other proteins, GRA70 and GRA71, that together form a complex which enhances the ability of parasites to persist in IFNγ-activated human foreskin fibroblasts (HFFs). Components of the protein machinery required for export of Toxoplasma proteins into the host cell were also found to be important for parasite resistance to IFNγ in human cells, but these export components function independently of the identified protein complex. Host-mediated ubiquitination of the parasite vacuole has previously been associated with increased parasite clearance from human cells, but we find that vacuoles from GRA57, GRA70, and GRA71 knockout strains are surprisingly less ubiquitinated by the host cell. We hypothesise that this is likely a secondary consequence of deletion of the complex, unlinked to the IFNγ resistance mediated by these effectors.

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.

CRISPR Screens Identify Toxoplasma Genes That Determine Parasite Fitness in Interferon Gamma-Stimulated Human Cells

Toxoplasma virulence depends on its ability to evade or survive the toxoplasmacidal mechanisms induced by interferon gamma (IFNγ). While many Toxoplasma genes involved in the evasion of the murine IFNγ response have been identified, genes required to survive the human IFNγ response are largely unknown. In this study, we used a genome-wide loss-of-function screen to identify Toxoplasma genes important for parasite fitness in IFNγ-stimulated primary human fibroblasts. We generated gene knockouts for the top six hits from the screen and confirmed their importance for parasite growth in IFNγ-stimulated human fibroblasts. Of these six genes, three have homology to GRA32, localize to dense granules, and coimmunoprecipitate with each other and GRA32, suggesting they might form a complex. Deletion of individual members of this complex leads to early parasite egress in IFNγ-stimulated cells. Thus, prevention of early egress is an important Toxoplasma fitness determinant in IFNγ-stimulated human cells. IMPORTANCE Toxoplasma infection causes serious complications in immunocompromised individuals and in the developing fetus. During infection, certain immune cells release a protein called interferon gamma that activates cells to destroy the parasite or inhibit its growth. While most Toxoplasma parasites are cleared by this immune response, some can survive by blocking or evading the IFNγ-induced restrictive environment. Many Toxoplasma genes that determine parasite survival in IFNγ-activated murine cells are known but parasite genes conferring fitness in IFNγ-activated human cells are largely unknown. Using a Toxoplasma adapted genome-wide loss-of-function screen, we identified many Toxoplasma genes that determine parasite fitness in IFNγ-activated human cells. The gene products of four top hits play a role in preventing early parasite egress in IFNγ-stimulated human cells. Understanding how IFNγ-stimulated human cells inhibit Toxoplasma growth and how Toxoplasma counteracts this, could lead to the development of novel therapeutics.

Functional profiling of the Toxoplasma genome during acute mouse infection

Within a host, pathogens encounter a diverse and changing landscape of cell types, nutrients, and immune responses. Examining host-pathogen interactions in animal models can therefore reveal aspects of infection absent from cell culture. We use CRISPR-based screens to functionally profile the entire genome of the model apicomplexan parasite Toxoplasma gondii during mouse infection. Barcoded gRNAs were used to track mutant parasite lineages, enabling detection of bottlenecks and mapping of population structures. We uncovered over 300 genes that modulate parasite fitness in mice with previously unknown roles in infection. These candidates span multiple axes of host-parasite interaction, including determinants of tropism, host organelle remodeling, and metabolic rewiring. We mechanistically characterized three novel candidates, including GTP cyclohydrolase I, against which a small-molecule inhibitor could be repurposed as an antiparasitic compound. This compound exhibited antiparasitic activity against T. gondii and Plasmodium falciparum, the most lethal agent of malaria. Taken together, we present the first complete survey of an apicomplexan genome during infection of an animal host, and point to novel interfaces of host-parasite interaction that may offer new avenues for treatment.

Histone variant H2B.Z acetylation is necessary for maintenance of Toxoplasma gondii biological fitness

Through regulation of DNA packaging, histone proteins are fundamental to a wide array of biological processes. A variety of post-translational modifications (PTMs), including acetylation, constitute a proposed histone code that is interpreted by "reader" proteins to modulate chromatin structure. Canonical histones can be replaced with variant versions that add an additional layer of regulatory complexity. The protozoan parasite Toxoplasma gondii is unique among eukaryotes in possessing a novel variant of H2B designated H2B.Z. The combination of PTMs and the use of histone variants is important for gene regulation in T. gondii, offering new targets for drug development. In this work, T. gondii parasites were generated in which the 5 N-terminal acetylatable lysines in H2B.Z were mutated to either alanine (c-Myc-A) or arginine (c-Myc-R). c-Myc-A mutant only displayed a mild effect in its ability to kill mice. c-Myc-R mutant presented an impaired ability to grow and an increase in differentiation to latent bradyzoites. This mutant line was also more sensitive to DNA damage, displayed no virulence in mice, and provided protective immunity against future infection. While nucleosome composition was unaltered, key genes were abnormally expressed during in vitro bradyzoite differentiation. Our results show that the N-terminal positive charge patch of H2B.Z is important for these procceses. Pull down assays with acetylated N-terminal H2B.Z peptide and unacetylated one retrieved common and differential interactors. Acetylated peptide pulled down proteins associated with chromosome maintenance/segregation and cell cycle, opening the question of a possible link between H2B.Z acetylation status and mitosis.

ROP39 is an Irgb10-specific parasite effector that modulates acute Toxoplasma gondii virulence

Toxoplasma gondii (T. gondii) is a zoonotic apicomplexan parasite that is an important cause of clinical disability in humans. On a global scale, one third of the human population is infected with T. gondii. Mice and other small rodents are believed to be responsible for transmission of T. gondii to the domestic cat, its definitive host. Interferon-inducible Immunity-Related GTPases (IRG proteins) are important for control of murine T. gondii infections. Virulence differences between T. gondii strains are linked to polymorphic rhoptry proteins (ROPs) that cooperate to inactivate individual IRG family members. In particular, the pseudokinase ROP5 isoform B is critically important in laboratory strains of mice. We identified T. gondii ROP39 in complex with ROP5B and demonstrate its contribution to acute T. gondii virulence. ROP39 directly targets Irgb10 and inhibits homodimer formation of the GTPase leading to an overall reduction of IRG protein loading onto the parasitophorous vacuolar membrane (PVM). Maintenance of PVM integrity rescues the parasite from IRG protein-mediated clearance in vitro and in vivo. This study identifies a novel T. gondii effector that is important for specific inactivation of the IRG resistance system. Our data reveal that yet unknown T. gondii effectors can emerge from identification of direct interaction partners of ROP5B.

Toxoplasma gondii virulence factor ROP1 reduces parasite susceptibility to murine and human innate immune restriction

Toxoplasma gondii is an intracellular parasite that can infect many host species and is a cause of significant human morbidity worldwide. T. gondii secretes a diverse array of effector proteins into the host cell which are critical for infection. The vast majority of these secreted proteins have no predicted functional domains and remain uncharacterised. Here, we carried out a pooled CRISPR knockout screen in the T. gondii Prugniaud strain in vivo to identify secreted proteins that contribute to parasite immune evasion in the host. We demonstrate that ROP1, the first-identified rhoptry protein of T. gondii, is essential for virulence and has a previously unrecognised role in parasite resistance to interferon gamma-mediated innate immune restriction. This function is conserved in the highly virulent RH strain of T. gondii and contributes to parasite growth in both murine and human macrophages. While ROP1 affects the morphology of rhoptries, from where the protein is secreted, it does not affect rhoptry secretion. Finally, we show that ROP1 co-immunoprecipitates with the host cell protein C1QBP, an emerging regulator of innate immune signaling. In summary, we identify putative in vivo virulence factors in the T. gondii Prugniaud strain and show that ROP1 is an important and previously overlooked effector protein that counteracts both murine and human innate immunity.

Interactions of Autophagy and the Immune System in Health and Diseases

Autophagy is a highly conserved process that utilizes lysosomes to selectively degrade a variety of intracellular cargo, thus providing quality control over cellular components and maintaining cellular regulatory functions. Autophagy is triggered by multiple stimuli ranging from nutrient starvation to microbial infection. Autophagy extensively shapes and modulates the inflammatory response, the concerted action of immune cells, and secreted mediators aimed to eradicate a microbial infection or to heal sterile tissue damage. Here, we first review how autophagy affects innate immune signaling, cell-autonomous immune defense, and adaptive immunity. Then, we discuss the role of non-canonical autophagy in microbial infections and inflammation. Finally, we review how crosstalk between autophagy and inflammation influences infectious, metabolic, and autoimmune disorders.

Unifying Virulence Evaluation in Toxoplasma gondii: A Timely Task

Toxoplasma gondii, a major zoonotic pathogen, possess a significant genetic and phenotypic diversity that have been proposed to be responsible for the variation in clinical outcomes, mainly related to reproductive failure and ocular and neurological signs. Different T. gondii haplogroups showed strong phenotypic differences in laboratory mouse infections, which provide a suitable model for mimicking acute and chronic infections. In addition, it has been observed that degrees of virulence might be related to the physiological status of the host and its genetic background. Currently, mortality rate (lethality) in outbred laboratory mice is the most significant phenotypic marker, which has been well defined for the three archetypal clonal types (I, II and III) of T. gondii; nevertheless, such a trait seems to be insufficient to discriminate between different degrees of virulence of field isolates. Many other non-lethal parameters, observed both in in vivo and in vitro experimental models, have been suggested as highly informative, yielding promising discriminatory power. Although intra-genotype variations have been observed in phenotypic characteristics, there is no clear picture of the phenotypes circulating worldwide; therefore, a global overview of T. gondii strain mortality in mice is presented here. Molecular characterization has been normalized to some extent, but this is not the case for the phenotypic characterization and definition of virulence. The present paper proposes a baseline (minimum required information) for the phenotypic characterization of T. gondii virulence and intends to highlight the needs for consistent methods when a panel of T. gondii isolates is evaluated for virulence.

Involvement of Urm1, a Ubiquitin-Like Protein, in the Regulation of Oxidative Stress Response of Toxoplasma gondii

Ubiquitin-related modifier 1 (Urm1) is a ubiquitin-like molecule (UBL) with the ability to act as a posttranslational protein modifier. Here, we characterized the Toxoplasma gondii homolog of URM1 located in the tachyzoite cytoplasm. The total loss of the TgURM1 resulted in a significant reduction in parasite invasion, replication, and virulence in mice, revealing that TgURM1 plays a pivotal role in T. gondii survival. For TgURM1, urmylation was significantly induced by oxidative stress, and mutations of the C-terminal glycine-glycine motif of TgURM1 blocked the urmylation process. Furthermore, the TgURM1 knockout strain was intolerant to oxidative stress, suggesting that TgURM1 is involved in the oxidative stress process. TgAHP1, an alkyl hydroperoxide reductase, was screened via proximity-based protein labeling techniques and proteomics and was shown to interact with TgURM1 under oxidative stress conditions. In conclusion, TgURM1 is a UBL protein involved in the response of Toxoplasma to oxidative stress. IMPORTANCE T. gondii has an intricate life cycle which involves multiple morphologically and physiologically distinct stages, and posttranslational modifications (PTMs) may be key regulators of protein expression at relevant life cycle stages. In recent years, ubiquitin-like proteins with modification functions have been discovered and studied, including Sumo, Rub1, ATG8, and ATG12. Ubiquitin-related modifier 1 (Urm1) is a ubiquitin-like molecule (UBL), which is considered to be the oldest ubiquitin-like system. In this study, we identified the Urm1 gene in Toxoplasma and explored that the urmylation of Urm1 was significantly induced by oxidative stress. Fewer studies have been conducted on ubiquitin-like proteins of parasites, and our results provide theoretical support for the research of metabolic regulation and antioxidative stress processes in T. gondii.

Parasitomimetics: Can We Utilize Parasite-Derived Immunomodulatory Molecules for Interventions to Immunological Disorders?

Because our immune system has ability to expel microorganisms invading our body, parasites need evolution to maintain their symbiosis with the hosts. One such strategy of the parasites is to manipulate host immunity by producing immunomodulatory molecules and the ability of parasites to regulate host immunity has long been a target of research. Parasites can not only manipulate host immune response specific to them, but also influence the host's entire immune system. Such ability of the parasites may sometimes bring benefit to the hosts as many studies have indicated the "hygiene hypothesis" that a decreased opportunity of parasitic infections is associated with an increased incidence of allergy and autoimmune diseases. In other words, elucidating the mechanisms of parasites to regulate host immunity could be applied not only to resolution of parasitic infections but also to treatment of non-parasitic immunological disorders. In this review, we show how much progress has been made in the research on immunomodulation of host immunity by parasites. Here, we define the word 'parasitomimetics' as emulation of parasites' immunomodulatory systems to solve immunological problems in humans and discuss potential applications of parasite-derived molecules to other diseases.

Released Parasite-Derived Kinases as Novel Targets for Antiparasitic Therapies

The efficient manipulation of their host cell is an essential feature of intracellular parasites. Most molecular mechanisms governing the subversion of host cell by protozoan parasites involve the release of parasite-derived molecules into the host cell cytoplasm and direct interaction with host proteins. Among these released proteins, kinases are particularly important as they govern the subversion of important host pathways, such as signalling or metabolic pathways. These enzymes, which catalyse the transfer of a phosphate group from ATP onto serine, threonine, tyrosine or histidine residues to covalently modify proteins, are involved in numerous essential biological processes such as cell cycle or transport. Although little is known about the role of most of the released parasite-derived kinases in the host cell, they are examples of kinases hijacking host cellular pathways such as signal transduction or apoptosis, which are essential for immune response evasion as well as parasite survival and development. Here we present the current knowledge on released protozoan kinases and their involvement in host-pathogen interactions. We also highlight the knowledge gaps remaining before considering those kinases - involved in host signalling subversion - as antiparasitic drug targets.

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.

Toxoplasmosis: Current and Emerging Parasite Druggable Targets

Toxoplasmosis is a prevalent disease affecting a wide range of hosts including approximately one-third of the human population. It is caused by the sporozoan parasite Toxoplasma gondii (T. gondii), which instigates a range of symptoms, manifesting as acute and chronic forms and varying from ocular to deleterious congenital or neuro-toxoplasmosis. Toxoplasmosis may cause serious health problems in fetuses, newborns, and immunocompromised patients. Recently, associations between toxoplasmosis and various neuropathies and different types of cancer were documented. In the veterinary sector, toxoplasmosis results in recurring abortions, leading to significant economic losses. Treatment of toxoplasmosis remains intricate and encompasses general antiparasitic and antibacterial drugs. The efficacy of these drugs is hindered by intolerance, side effects, and emergence of parasite resistance. Furthermore, all currently used drugs in the clinic target acute toxoplasmosis, with no or little effect on the chronic form. In this review, we will provide a comprehensive overview on the currently used and emergent drugs and their respective parasitic targets to combat toxoplasmosis. We will also abridge the repurposing of certain drugs, their targets, and highlight future druggable targets to enhance the therapeutic efficacy against toxoplasmosis, hence lessening its burden and potentially alleviating the complications of its associated diseases.

Nanos gigantium humeris insidentes: old papers informing new research into Toxoplasma gondii

Since Nicolle, Manceaux and Splendore first described Toxoplasma gondii as a parasite of rodents and rabbits in the early 20th century, a diverse and vigorous research community has been built around studying this fascinating intracellular parasite. In addition to its importance as a pathogen of humans, livestock and wildlife, modern researchers are attracted to T. gondii as a facile experimental system to study many aspects of evolutionary biology, cellular biology, host-microbe interactions, and host immunity. For new researchers entering the field, the extensive literature describing the biology of the parasite, and the interactions with its host, can be daunting. In this review, we examine four foundational studies that describe various aspects of T. gondii biology, presenting a 'journal club'-style analysis of each. We have chosen a paper that established the beguiling life cycle of the parasite (Hutchison et al., 1971), a paper that described key features of its cellular biology that the parasite shares with related organisms (Gustafson et al., 1954), a paper that characterised the origin of the unique compartment in which the parasite resides within host cells (Jones and Hirsch, 1972), and a paper that established a key mechanism in the host immune response to parasite infection (Pfefferkorn, 1984). These interesting and far-reaching studies set the stage for subsequent research into numerous facets of parasite biology. As well as providing new researchers with an entry point into the literature surrounding the parasite, revisiting these studies can remind us of the roots of our discipline, how far we have come, and the new directions in which we might head.

Lessons from Toxoplasma: Host responses that mediate parasite control and the microbial effectors that subvert them

The intracellular parasite Toxoplasma gondii has long provided a tractable experimental system to investigate how the immune system deals with intracellular infections. This review highlights the advances in defining how this organism was first detected and the studies with T. gondii that contribute to our understanding of how the cytokine IFN-γ promotes control of vacuolar pathogens. In addition, the genetic tractability of this eukaryote organism has provided the foundation for studies into the diverse strategies that pathogens use to evade antimicrobial responses and now provides the opportunity to study the basis for latency. Thus, T. gondii remains a clinically relevant organism whose evolving interactions with the host immune system continue to teach lessons broadly relevant to host–pathogen interactions.

Unraveling Toxoplasma gondii GT1 Strain Virulence and New Protein-Coding Genes with Proteogenomic Analyses

Toxoplasma gondii is one of the most widespread parasites of great relevance to planetary health. It infects approximately one-third of the world population. T. gondii establishes itself in warm-blooded animals and causes adverse health outcomes, particularly in immunocompromised patients. T. gondii is also widely used as a model organism to study other related apicomplexan parasites, which requires a deeper understanding of its molecular biology. Type I strains (GT1 and RH) of T. gondii are considered the most virulent forms. The whole-genome sequencing of T. gondii annotated 8460 predicted gene models in the parasite. To this end, the proteogenomics technology allows harnessing of mass spectrometry (MS)-derived proteomic data to unravel new protein-coding genes, not to mention validation and correction of the existing gene models. In this study using the proteogenomic approach, we report the identification of 31 novel protein-coding genes while reannotating 88 existing gene models. Notably, the genome annotations were corrected for genes, such as SAG5C, GRA6, ROP4, ROP5, and ROP26. The associated proteins are known to play important roles in host-parasite interactions, particularly in relation to parasite virulence, suppression of host immune response, and distinctively pertinent for the survival of the parasite inside the host system. These new findings offer new insights, informing planetary health broadly and the knowledge base on T. gondii virulence specifically. The proteogenomics approach also provides a concrete example to study related apicomplexan organisms of relevance to planetary health, and so as to develop new diagnostics and therapeutics against toxoplasmosis and related diseases.

Genome-Wide Expression Patterns of Rhoptry Kinases during the Eimeria tenella Life-Cycle

Kinome from apicomplexan parasites is composed of eukaryotic protein kinases and Apicomplexa specific kinases, such as rhoptry kinases (ROPK). Ropk is a gene family that is known to play important roles in host–pathogen interaction in Toxoplasma gondii but is still poorly described in Eimeria tenella, the parasite responsible for avian coccidiosis worldwide. In the E. tenella genome, 28 ropk genes are predicted and could be classified as active (n = 7), inactive (incomplete catalytic triad, n = 12), and non-canonical kinases (active kinase with a modified catalytic triad, n = 9). We characterized the ropk gene expression patterns by real-time quantitative RT-PCR, normalized by parasite housekeeping genes, during the E. tenella life-cycle. Analyzed stages were: non-sporulated oocysts, sporulated oocysts, extracellular and intracellular sporozoites, immature and mature schizonts I, first- and second-generation merozoites, and gametes. Transcription of all those predicted ropk was confirmed. The mean intensity of transcription was higher in extracellular stages and 7–9 ropk were specifically transcribed in merozoites in comparison with sporozoites. Transcriptional profiles of intracellular stages were closely related to each other, suggesting a probable common role of ROPKs in hijacking signaling pathways and immune responses in infected cells. These results provide a solid basis for future functional analysis of ROPK from E. tenella.

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.

In vivo and in vitro models show unexpected degrees of virulence among Toxoplasma gondii type II and III isolates from sheep

Toxoplasma gondii is an important zoonotic agent with high genetic diversity, complex epidemiology, and variable clinical outcomes in animals and humans. In veterinary medicine, this apicomplexan parasite is considered one of the main infectious agents responsible for reproductive failure in small ruminants worldwide. The aim of this study was to phenotypically characterize 10 Spanish T. gondii isolates recently obtained from sheep in a normalized mouse model and in an ovine trophoblast cell line (AH-1) as infection target cells. The panel of isolates met selection criteria regarding such parameters as genetic diversity [types II (ToxoDB #1 and #3) and III (#2)], geographical location, and sample of origin (aborted foetal brain tissues or adult sheep myocardium). Evaluations of in vivo mortality, morbidity, parasite burden and histopathology were performed. Important variations between isolates were observed, although all isolates were classified as “nonvirulent” (< 30% cumulative mortality). The isolates TgShSp16 (#3) and TgShSp24 (#2) presented higher degrees of virulence. Significant differences were found in terms of in vitro invasion rates and tachyzoite yield at 72 h post-inoculation (hpi) between TgShSp1 and TgShSp24 isolates, which exhibited the lowest and highest rates, respectively. The study of the CS3, ROP18 and ROP5 loci allelic profiles revealed only type III alleles in ToxoDB #2 isolates and type II alleles in the #1 and #3 isolates included. We concluded that there are relevant intra- and inter-genotype virulence differences in Spanish T. gondii isolates, which could not be inferred by genetic characterization using currently described molecular markers.

Genome-Wide CRISPR/Cas9 Screen Identifies New Genes Critical for Defense Against Oxidant Stress in Toxoplasma gondii

Toxoplasma gondii is one of the most widespread apicomplexans and can cause serious infections in humans and animals. Its antioxidant system plays an important role in defending against oxidant stress imposed by the host. Some genes encoding the antioxidant enzymes of T. gondii have been identified; however, critical genes that function in response to reactive oxygen species (ROS) stress are still poorly understood. Here, we performed genome-wide CRISPR/Cas9 loss-of-function screening in the T. gondii RH strain to identify potential genes contributing to the ROS stress response. Under hydrogen peroxide treatment, 30 single guide RNAs targeting high-confidence genes were identified, including some known important antioxidant genes such as catalase and peroxiredoxin PRX3. In addition, several previously uncharacterized genes were identified, among which five hypothetical protein-coding genes, namely, HP1–HP5, were selected for further functional characterization. Targeted deletion of HP1 in T. gondii RH led to significant sensitivity to H₂O₂, suggesting that HP1 is critical for oxidative stress management. Furthermore, loss of HP1 led to decreased antioxidant capacity, invasion efficiency, and proliferation in vitro. In vivo results also revealed that the survival time of mice infected with the HP1-KO strain was significantly prolonged relative to that of mice infected with the wild-type strain. Altogether, these findings demonstrate that the CRISPR/Cas9 system can be used to identify potential genes critical for oxidative stress management. Furthermore, HP1 may confer protection against oxidative damage and contributes to T. gondii virulence in mice.

Differential expression of TgMIC1 in isolates of Chinese 1 Toxoplasma with different virulence

BACKGROUND

The predominant genotype of Toxoplasma in China is the Chinese 1 (ToxoDB#9) lineage. TgCtwh3 and TgCtwh6 are two representative strains of Chinese 1, exhibiting high and low virulence to mice, respectively. Little is known regarding the virulence mechanism of this non-classical genotype. Our previous RNA sequencing data revealed differential mRNA levels of TgMIC1 in TgCtwh3 and TgCtwh6. We aim to further confirm the differential expression of TgMIC1 and its significance in this atypical genotype.

METHODS

Quantitative real-time PCR was used to verify the RNA sequencing data; then, polyclonal antibodies against TgMIC1 were prepared and identified. Moreover, the invasion and proliferation of the parasite in HFF cells were observed after treatment with TgMIC1 polyclonal antibody or not.

RESULTS

The data showed that the protein level of TgMIC1 was significantly higher in high-virulence strain TgCtwh3 than in low-virulence strain TgCtwh6 and that the invasion and proliferation of TgCtwh3 were inhibited by TgMIC1 polyclonal antibody.

CONCLUSION

Differential expression of TgMIC1 in TgCtwh3 and TgCtwh6 may explain, at least partly, the virulence mechanism of this atypical genotype.

Protein kinases in Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular pathogen that causes life threatening illness in immunodeficient individuals, miscarriage in pregnant woman, and blindness in newborn children. Similar to any other eukaryotic cell, protein kinases play critical and essential roles in the Toxoplasma life cycle. Accordingly, many studies have focused on identifying and defining the mechanism of function of these signalling proteins with a long-term goal to develop anti-Toxoplasma therapeutics. In this review, we briefly discuss classification and key components of the catalytic domain which are critical for functioning of kinases, with a focus on domains, families, and groups of kinases within Toxoplasma. More importantly, this article provides a comprehensive, current overview of research on kinase groups in Toxoplasma including the established eukaryotic AGC, CAMK, CK1, CMGC, STE, TKL families and the apicomplexan-specific FIKK, ROPK and WNG family of kinases. This work provides an overview and discusses current knowledge on Toxoplasma kinases including their localization, function, signalling network and role in acute and chronic pathogenesis, with a view towards the future in probing kinases as viable drug targets.

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.

Inhibition of nitric oxide production of activated mice peritoneal macrophages is independent of the Toxoplasma gondii strain

BACKGROUND

Toxoplasma gondii causes toxoplasmosis and is controlled by activated macrophages. However, infection of macrophages by tachyzoites induces TGF-β signaling (TGF-s) inhibiting nitric oxide (NO) production. NO inhibition may be a general escape mechanism of distinct T. gondii strains.

OBJECTIVES

To evaluate in activated macrophages the capacity of T. gondii strains of different virulence and genetics (RH, type I; ME-49, type II; VEG, type III; P-Br, recombinant) to evade the NO microbicidal defense system and determine LC3 loading to the parasitophorous vacuole.

METHODS

Activated peritoneal macrophages were infected with the different T. gondii strains, NO-production was evaluated by the Griess reagent, and inducible nitric oxide synthase expression, TGF-s, and LC3 localisation assayed by immunofluorescence.

FINDINGS

Only RH persisted in macrophages, while VEG was more resistant than P-Br and ME-49. All strains induced TGF-s, degradation of inducible nitric oxide synthase, and NO-production inhibition from 2 to 24 h of infection, but only RH sustained these alterations for 48 h. By 24 h of infection, TGF-s lowered in macrophages infected by ME-49, and P-Br, and NO-production recovered, while VEG sustained TGF-s and NO-production inhibition longer. LC3 loading to parasitophorous vacuole was strain-dependent: higher for ME-49, P-Br and VEG, lower for RH. All strains inhibited NO-production, but only RH sustained this effect probably because it persisted in macrophages due to additional evasive mechanisms as lower LC3 loading to parasitophorous vacuole.

MAIN CONCLUSIONS

These results support that T. gondii can escape the NO microbicidal defense system at the initial phase of the infection, but only the virulent strain sustain this evasion mechanism.

Function of Neospora caninum dense granule protein 7 in innate immunity in mice

The intracellular parasite Neospora caninum can parasitize all nucleated cells of the host. Dense granule proteins (GRAs) secreted by dense granules are an important material involved in the formation of parasitophorous vacuoles (PVs), which facilitate parasite survival and replication in host cells. Due to the secretory and immune properties of NcGRA7, it is considered to be a promising serodiagnosis marker and an effective neosporosis vaccine candidate. However, the intracellular regulatory mechanisms involved in NcGRA7-induced host responses have rarely been examined. Here, we used the CRISPR/Cas9 genome editing system to obtain a NcGRA7 knockout strain (ΔNcGRA7) and a NcGRA7 complementary strain (iΔNcGRA7) to study their function. We found that ΔNcGRA7 exhibited slower growth in vitro and weakened virulence in mice compared with Nc1 and iΔNcGRA7. All parasite strains can stimulate host immune cells to produce IFN-γ, and the amount of IFN-γ production stimulated by Nc1 was significantly higher than that stimulated by ΔNcGRA7. The transcription levels of the cellular immune factors GBP1, GBP2, IRGa6, and IRGb6 were significantly higher after stimulation with ΔNcGRA7 parasites than after stimulation with Nc1. Furthermore, ΔNcGRA7 infection resulted in greater IRGa6 recruitment to the PVM than Nc1 infection. ΔNcGRA7 parasites were more easily cleared by macrophages than Nc1 parasites. Collectively, these results showed that NcGRA7 plays an important role in regulating the immune factors of mice and the aggregation of IRGa6 at the PVM, which affects the pathogenicity of N. caninum.

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.

Genome-wide screens identify Toxoplasma gondii determinants of parasite fitness in IFNγ-activated murine macrophages

Macrophages play an essential role in the early immune response against Toxoplasma and are the cell type preferentially infected by the parasite in vivo. Interferon gamma (IFNγ) elicits a variety of anti-Toxoplasma activities in macrophages. Using a genome-wide CRISPR screen we identify 353 Toxoplasma genes that determine parasite fitness in naїve or IFNγ-activated murine macrophages, seven of which are further confirmed. We show that one of these genes encodes dense granule protein GRA45, which has a chaperone-like domain, is critical for correct localization of GRAs into the PVM and secretion of GRA effectors into the host cytoplasm. Parasites lacking GRA45 are more susceptible to IFNγ-mediated growth inhibition and have reduced virulence in mice. Together, we identify and characterize an important chaperone-like GRA in Toxoplasma and provide a resource for the community to further explore the function of Toxoplasma genes that determine fitness in IFNγ-activated macrophages.

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.

Naïve CD8 T cell IFNγ responses to a vacuolar antigen are regulated by an inflammasome-independent NLRP3 pathway and Toxoplasma gondii ROP5

Host resistance to Toxoplasma gondii relies on CD8 T cell IFNγ responses, which if modulated by the host or parasite could influence chronic infection and parasite transmission between hosts. Since host-parasite interactions that govern this response are not fully elucidated, we investigated requirements for eliciting naïve CD8 T cell IFNγ responses to a vacuolar resident antigen of T. gondii, TGD057. Naïve TGD057 antigen-specific CD8 T cells (T57) were isolated from transnuclear mice and responded to parasite-infected bone marrow-derived macrophages (BMDMs) in an antigen-dependent manner, first by producing IL-2 and then IFNγ. T57 IFNγ responses to TGD057 were independent of the parasite’s protein export machinery ASP5 and MYR1. Instead, host immunity pathways downstream of the regulatory Immunity-Related GTPases (IRG), including partial dependence on Guanylate-Binding Proteins, are required. Multiple T. gondii ROP5 isoforms and allele types, including ‘avirulent’ ROP5A from clade A and D parasite strains, were able to suppress CD8 T cell IFNγ responses to parasite-infected BMDMs. Phenotypic variance between clades B, C, D, F, and A strains suggest T57 IFNγ differentiation occurs independently of parasite virulence or any known IRG-ROP5 interaction. Consistent with this, removal of ROP5 is not enough to elicit maximal CD8 T cell IFNγ production to parasite-infected cells. Instead, macrophage expression of the pathogen sensors, NLRP3 and to a large extent NLRP1, were absolute requirements. Other members of the conventional inflammasome cascade are only partially required, as revealed by decreased but not abrogated T57 IFNγ responses to parasite-infected ASC, caspase-1/11, and gasdermin D deficient cells. Moreover, IFNγ production was only partially reduced in the absence of IL-12, IL-18 or IL-1R signaling. In summary, T. gondii effectors and host machinery that modulate parasitophorous vacuolar membranes, as well as NLR-dependent but inflammasome-independent pathways, determine the full commitment of CD8 T cells IFNγ responses to a vacuolar antigen.

Parasites are excellent “students” of our immune system as they can deflect, antagonize and confuse the immune response making it difficult to vaccinate against these pathogens. In this report, we analyzed how a widespread parasite of mammals, Toxoplasma gondii, manipulates an immune cell needed for immunity to many intracellular pathogens, the CD8 T cell. Host pathways that govern CD8 T cell production of the immune protective cytokine, IFNγ, were also explored. We hypothesized the secreted T. gondii virulence factor, ROP5, work to inhibit the MHC 1 antigen presentation pathway therefore making it difficult for CD8 T cells to see T. gondii antigens sequestered inside a parasitophorous vacuole. However, manipulation through T. gondii ROP5 does not fully explain how CD8 T cells commit to making IFNγ in response to infection. Importantly, CD8 T cell IFNγ responses to T. gondii require the pathogen sensor NLRP3 to be expressed in the infected cell. Other proteins associated with NLRP3 activation, including members of the conventional inflammasome activation cascade pathway, are only partially involved. Our results identify a novel pathway by which NLRP3 regulates T cell function and underscore the need for NLRP3-activating adjuvants in vaccines aimed at inducing CD8 T cell IFNγ responses to parasites.