Virulence differences in Toxoplasma mediated by amplification of a family of polymorphic pseudokinases

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.

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 gondii Infection in the Eurasian Beaver (Castor fiber) in Switzerland: Seroprevalence, Genetic Characterization, and Clinicopathologic Relevance

Toxoplasma gondii is a coccidian parasite able to infect all warm-blooded animals and humans. Rodents are one of the most important intermediate hosts for T. gondii, but little is known about infection in beavers and its clinical relevance. Toxoplasmosis was not considered an important waterborne disease until recently, but with increased outbreaks in humans and animals this perspective has changed. Serum samples from 247 Eurasian beavers (Castor fiber) collected from 2002 to 2022 were tested for antibodies to T. gondii by a commercial ELISA. Antibodies to T. gondii were found in 113 (45.8%) beavers. Higher weight and proximity to urban areas were found to be significant predictors for seropositivity. Additionally, T. gondii DNA was detected in 23/41 brain tissue samples by real-time PCR. Histopathologic examination of brain sections revealed inflammatory changes in 26/40 beavers, mainly characterized by encephalitis, meningitis, choroid plexitis, or a combination of them. In six of these cases the lesions were in direct association with parasitic stages. With an adapted nested PCR multilocus sequence typing and in silico restriction fragment length polymorphism analysis approach, three different T. gondii genotypes were detected in brain samples: the clonal Type II strain (ToxoDB 1), a Type II variant (ToxoDB 3), and a novel genotype exhibiting both Type II and I alleles in a further animal. Toxoplasma gondii infections in beavers have epidemiologic and clinical significance. The high seroprevalence indicates frequent contact with the parasite, and as competent intermediate hosts they may play an important role, contributing to maintaining the life cycle of T. gondii in semiaquatic habitats. In addition, although most beavers appear to develop subclinical to chronic disease courses, acute and fatal outcomes, mainly characterized by encephalitis and generalized infection, do also occur.

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.

A Combination of Four Nuclear Targeted Effectors Protects Toxoplasma Against Interferon Gamma Driven Human Host Cell Death During Acute Infection

In both mice and humans, Type II interferon-gamma (IFNγ) is crucial for regulation of Toxoplasma gondii (T. gondii) infection, during acute or chronic phases. To thwart this defense, T. gondii secretes protein effectors hindering the hosťs immune response. For example, T. gondii relies on the MYR translocon complex to deploy soluble dense granule effectors (GRAs) into the host cell cytosol or nucleus. Recent genome-wide loss-of-function screens in IFNγ-primed primary human fibroblasts identified MYR translocon components as crucial for parasite resistance against IFNγ driven vacuole clearance. However, these screens did not pinpoint specific MYR-dependent GRA proteins responsible for IFNγ signaling blockade, suggesting potential functional redundancy. Our study reveals that T. gondii depends on the MYR translocon complex to prevent host cell death and parasite premature egress in human cells stimulated with IFNγ postinfection, a unique phenotype observed in various human cell lines but not in murine cells. Intriguingly, inhibiting parasite egress did not prevent host cell death, indicating this mechanism is distinct from those described previously. Genome-wide loss-of-function screens uncovered TgIST, GRA16, GRA24, and GRA28 as effectors necessary for a complete block of IFNγ response. GRA24 and GRA28 directly influenced IFNγ driven transcription, GRA24's action depended on its interaction with p38 MAPK, while GRA28 disrupted histone acetyltransferase activity of CBP/p300. Given the intricate nature of the immune response to T. gondii, it appears that the parasite has evolved equally elaborate mechanisms to subvert IFNγ signaling, extending beyond direct interference with the JAK/STAT1 pathway, to encompass other signaling pathways as well.

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.

Genotyping of toxoplasma gondii isolates from México reveals non-archetypal and potentially virulent strains for mice

Genotyping and virulence studies of Toxoplasma gondii are essential to investigate the pathogenesis of strains circulating worldwide. In this study, eight T. gondii isolates obtained from a congenitally infected newborn, a calf, two cats, three dogs, and a wallaby from five states of México were genotyped by Mn-PCR-RFLP with 11 typing markers (SAG1, SAG2 5'3', alt. SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico), five virulence markers (CS3, ROP16, ROP17, ROP18 and ROP5), 15 microsatellite markers (TUB-2, W35, TgM-A, B18, B17, M33, IV.1, XI.1, M48, M102, N60, N82, AA, N61, N83), and sequencing. A phylogenetic network was built to determine the relationship between Mexican isolates and those reported worldwide. Six different genotypes were identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP), ToxoDB #8, #10, #28 (n = 3), #48, #116, and #282. Genotyping by microsatellite analysis differentiated the three PCR-RFLP genotype #28 isolates into two strains, revealing a total of seven microsatellite genotypes. Three different allele combinations of ROP18/ROP5 virulence markers were also found, 3/3, 1/1, and 4/1. The last two combinations are predicted to be highly virulent in the murine model. According to the phylogenetic network, the T. gondii strains studied here are related to archetypal strains I and III, but none are related to the strains previously reported in México. The genotypes identified in this study in different species of animals demonstrate the great genetic diversity of T. gondii in México. The ToxoDB-PCR-RFLP #28 genotype was found in three isolates from different hosts and states. Additionally, four of the isolates are predicted to be highly virulent in mice. The next step will be to perform in vitro and in vivo assays to determine the phenotype of these T. gondii isolates in murine models.

Host MOSPD2 enrichment at the parasitophorous vacuole membrane varies between Toxoplasma strains and involves complex interactions

Toxoplasma gondii is an obligate, intracellular parasite. Infection of a cell produces a unique niche for the parasite named the parasitophorous vacuole (PV) initially composed of host plasma membrane invaginated during invasion. The PV and its membrane (parasitophorous vacuole membrane [PVM]) are subsequently decorated with a variety of parasite proteins allowing the parasite to optimally grow in addition to manipulate host processes. Recently, we reported a proximity-labeling screen at the PVM-host interface and identified host endoplasmic reticulum (ER)-resident motile sperm domain-containing protein 2 (MOSPD2) as being enriched at this location. Here we extend these findings in several important respects. First, we show that the extent and pattern of host MOSPD2 association with the PVM differ dramatically in cells infected with different strains of Toxoplasma. Second, in cells infected with Type I RH strain, the MOSPD2 staining is mutually exclusive with regions of the PVM that associate with mitochondria. Third, immunoprecipitation and liquid chromatography tandem mass spectrometry (LC-MS/MS) with epitope-tagged MOSPD2-expressing host cells reveal strong enrichment of several PVM-localized parasite proteins, although none appear to play an essential role in MOSPD2 association. Fourth, most MOSPD2 associating with the PVM is newly translated after infection of the cell and requires the major functional domains of MOSPD2, identified as the CRAL/TRIO domain and tail anchor, although these domains were not sufficient for PVM association. Lastly, ablation of MOSPD2 results in, at most, a modest impact on Toxoplasma growth in vitro. Collectively, these studies provide new insight into the molecular interactions involving MOSPD2 at the dynamic interface between the PVM and the host cytosol. IMPORTANCE Toxoplasma gondii is an intracellular pathogen that lives within a membranous vacuole inside of its host cell. This vacuole is decorated by a variety of parasite proteins that allow it to defend against host attack, acquire nutrients, and interact with the host cell. Recent work identified and validated host proteins enriched at this host-pathogen interface. Here, we follow up on one candidate named MOSPD2 shown to be enriched at the vacuolar membrane and describe it as having a dynamic interaction at this location depending on a variety of factors. Some of these include the presence of host mitochondria, intrinsic domains of the host protein, and whether translation is active. Importantly, we show that MOSPD2 enrichment at the vacuole membrane differs between strains indicating active involvement of the parasite with this phenotype. Altogether, these results shed light on the mechanism and role of protein associations in the host-pathogen interaction.

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.

Unravelling Strain-Specific Modifications of Toxoplasma gondii tRNA and sncRNA Using LC-MS/MS

Many RNA modifications have been detected in rRNA, tRNA and small noncoding RNA (sncRNA) as well as in low-abundance RNA species such mRNA. Although RNA modifications play roles in many cellular and biological processes in various domains of life, knowledge about the diversity and role of RNA modifications in Toxoplasma gondii is limited. In this study, RNA modifications in three T. gondii strains (RH type I, PRU type II, and VEG type III) with distinct virulence abilities were determined by liquid chromatography-tandem mass spectrometry. We compared the levels of modifications of four nucleotides in tRNA and sncRNA, characterized RNA modification patterns of different T. gondii strains, and determined the diversity of RNA modifications. We detected and quantified 22 modified nucleosides in both tRNA and sncRNA. Significant differences in the diversity of the modified nucleosides were found between the three T. gondii strains. RNA modifications were correlated with the expression of many T. gondii virulence proteins. Some of the identified modifications (e.g., 2'-O-methylinosine, pseudouridine) play a role in mediating the host-parasite interaction. These results provide novel insight into the global modifications in tRNA and sncRNA, and the diversity of RNA modifications between T. gondii strains with different virulence backgrounds. IMPORTANCE Although RNA modifications play roles in many cellular and developmental processes in various domains of life, knowledge about the patterns and functions of RNA modifications in T. gondii is limited. Here, a quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach was used to study global RNA modifications in T. gondii strains of distinct virulence backgrounds. We quantified 22 modified nucleosides in both tRNA and sncRNA. Significant T. gondii strain-specific differences in RNA modifications were detected. More tRNA modifications correlated with T. gondii virulence proteins than sncRNA modifications. RNA modifications were significantly correlated with virulence proteins. Our data provide the first comprehensive profiling of the modifications tRNA and sncRNA in T. gondii, expanding the diversity of RNA modifications in this parasite and suggesting new regulators for modulating its virulence.

Forward genetic analysis of monensin and diclazuril resistance in Eimeria tenella

Worldwide distributed coccidiosis is caused by infection of both Eimeria species and Cystoisospora in the host intestine and causes huge economic losses to the livestock industry, especially the poultry industry. The control of such diseases relies mainly on chemoprophylaxis with anticoccidials, which has led to a very common drug resistance in this field. However, the genetic mechanisms underlying resistance to many anticoccidial drugs remain unknown. In this study, strains of E. tenella resistant to 250 mg/kg monensin were generated and characterized. Forward genetic approaches based on pooled genome sequencing, including experimental evolution and linkage group selection, were used to locate candidate targets responsible for resistance to monensin and diclazuril in E. tenella. A total of 16 nonsynonymous mutants in protein-coding genes were identified in monensin-resistant strains, and two genomic regions with strong selection signals were also detected in diclazuril-resistant strains. Our study reveals the genetic characterization of the experimental evolution and linkage group selection in Eimeria species, and also provides important information that contributes to the understanding of the molecular mechanism of drug resistance in coccidia.

Variation in CD8 T cell IFNγ differentiation to strains of Toxoplasma gondii is characterized by small effect QTLs with contribution from ROP16

Introduction

Toxoplasma gondii induces a strong CD8 T cell response characterized by the secretion of IFNγ that promotes host survival during infection. The initiation of CD8 T cell IFNγ responses in vitro differs widely between clonal lineage strains of T. gondii, in which type I strains are low inducers, while types II and III strains are high inducers. We hypothesized this phenotype is due to a polymorphic "Regulator Of CD8 T cell Response" (ROCTR).

Methods

Therefore, we screened F1 progeny from genetic crosses between the clonal lineage strains to identify ROCTR. Naïve antigen-specific CD8 T cells (T57) isolated from transnuclear mice, which are specific for the endogenous and vacuolar TGD057 antigen, were measured for their ability to become activated, transcribe Ifng and produce IFNγ in response to T. gondii infected macrophages.

Results

Genetic mapping returned four non-interacting quantitative trait loci (QTL) with small effect on T. gondii chromosomes (chr) VIIb-VIII, X and XII. These loci encompass multiple gene candidates highlighted by ROP16 (chrVIIb-VIII), GRA35 (chrX), TgNSM (chrX), and a pair of uncharacterized NTPases (chrXII), whose locus we report to be significantly truncated in the type I RH background. Although none of the chromosome X and XII candidates bore evidence for regulating CD8 T cell IFNγ responses, type I variants of ROP16 lowered Ifng transcription early after T cell activation. During our search for ROCTR, we also noted the parasitophorous vacuole membrane (PVM) targeting factor for dense granules (GRAs), GRA43, repressed the response suggesting PVM-associated GRAs are important for CD8 T cell activation. Furthermore, RIPK3 expression in macrophages was an absolute requirement for CD8 T cell IFNγ differentiation implicating the necroptosis pathway in T cell immunity to T. gondii.

Discussion

Collectively, our data suggest that while CD8 T cell IFNγ production to T. gondii strains vary dramatically, it is not controlled by a single polymorphism with strong effect. However, early in the differentiation process, polymorphisms in ROP16 can regulate commitment of responding CD8 T cells to IFNγ production which may have bearing on immunity to T. gondii.

A ring trial to harmonize Toxoplasma gondii microsatellite typing: comparative analysis of results and recommendations for optimization

A ring trial among five European laboratories was organized to reach consistency in microsatellite (MS) typing of the zoonotic parasite Toxoplasma gondii. Three sample sets were circulated and analyzed by each laboratory following a previously published method that is based on fragment length polymorphism of 15 MS markers. The first sample set compared typing results in general and focused on effects of DNA concentration; the second sample set focused on the polymorphic fingerprinting markers that can differentiate T. gondii strains within the same archetypal lineage; and the third set focused on non-archetypal genotypes. Methodological variations between laboratories, including the software programs used to determine MS fragment length, were collated using a questionnaire. Overall, lineage-level typing results reached a high level of agreement, especially in samples with the highest DNA concentrations. However, laboratory-specific differences were observed for particular markers. Major median differences in fragment length, of up to 6 base pairs, were related to the fluorophore used to label fragment-specific primers. In addition, primer pairs with identical sequences obtained from different suppliers resulted in fragments of differing length. Furthermore, differences in the way the sequencing profiles were assessed and interpreted may have led to deviating results in fragment length determination. Harmonization of MS typing, for example, by using the same fluorophores or by numerical adjustments applied to the fragment-lengths determined, could improve the uniformity of the results across laboratories. This is the first interlaboratory comparison, providing guidelines (added as a supplement) for the optimization of this technique.

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.

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.

Insight into the current Toxoplasma gondii DNA vaccine: a review article

INTRODUCTION

Toxoplasma gondii (T.gondii) is a widespread protozoan with significant economic losses and public health importance. But so far, the protective effect of reported DNA-based vaccines fluctuates widely, and no study has demonstrated complete protection.

AREAS COVERED

This review provides an inclusive summary of T. gondii DNA vaccine antigens, adjuvants, and some other parameters. A total of 140 articles from 2000 to 2021 were collected from five databases. By contrasting the outcomes of acute and chronic challenges, we aimed to investigate and identify viable immunological strategies for optimum protection. Furthermore, we evaluated and discussed the impact of several parameters on challenge outcomes in the hopes of developing some recommendations to assist better future horizontal comparisons among research.

EXPERT OPINION

In the coming five years of research, the exploration of vaccine cocktails combining invasion antigens and metabolic antigens with genetic adjuvants or novel DNA delivery methods may offer us desirable protection against this multiple stage of life parasite. In addition to finding a better immune strategy, developing better in silico prediction methods, solving problems posed by variables in practical applications, and gaining a more profound knowledge of T.gondii-host molecular interaction is also crucial towards a successful vaccine.

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.

Dense granule biogenesis, secretion, and function in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite and the causative agent of Toxoplasmosis. A key to understanding and treating the disease lies with determining how the parasite can survive and replicate within cells of its host. Proteins released from specialized secretory vesicles, named the dense granules (DGs), have diverse functions that are critical for adapting the intracellular environment, and are thus key to survival and pathogenicity. In this review, we describe the current understanding and outstanding questions regarding dense granule biogenesis, trafficking, and regulation of secretion. In addition, we provide an overview of dense granule protein ("GRA") function upon secretion, with a focus on proteins that have recently been identified.

Strain-specific pre-existing immunity: A key to understanding the role of chronic Toxoplasma infection in cognition and Alzheimer's diseases?

Toxoplasma exposure can elicit cellular and humoral immune responses. In the case of chronic Toxoplasma infection, these immune responses are long-lasting. Some studies suggest that pre-existing immunity from Toxoplasma infection can shape immune responses and resistance to other pathogens and brain insults later in life. Much evidence has been generated suggesting Toxoplasma infection may contribute to cognitive impairment in the elderly. However, there have also been studies that disagree with the conclusion. Toxoplasma has many strain types, with virulence being the most notable difference. There is also considerable variation in the outcomes following Toxoplasma exposure ranging from resolved to persistent infection. Therefore, the brain microenvironment, particularly cellular constituents, differs based on the infecting strain (virulent versus hypovirulent) and infection stage (resolved versus persistent). Such difference might play a critical role in determining the outcome of the host on subsequent challengings to the brain. The ability of Toxoplasma strains to set up distinct stages for neurodegenerative pathology through varying degrees of virulence provides unique experimental tools for characterizing these pathways.

Global profiling of protein lysine malonylation in Toxoplasma gondii strains of different virulence and genetic backgrounds

Lysine malonylation is a post-translational modification (PTM), which regulates many cellular processes. Limited information is available about the level of lysine malonylation variations between Toxoplasma gondii strains of distinct genetic lineages. Yet, insights into such variations are needed to understand the extent to which lysine malonylation contributes to the differences in the virulence and repertoire of virulence factors between T. gondii genotypes. In this study, we profiled lysine malonylation in T. gondii using quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) and immuno-affinity purification. This analysis was performed on three T. gondii strains with distinctive pathogenicity in mice, including RH strain (type I), PRU strain (type II), and VEG strain (type III). In total, 111 differentially malonylated proteins and 152 sites were upregulated, and 17 proteins and 17 sites were downregulated in RH strain versus PRU strain; 50 proteins and 59 sites were upregulated, 50 proteins and 53 sites were downregulated in RH strain versus VEG strain; and 72 proteins and 90 sites were upregulated, and 7 proteins and 8 sites were downregulated in VEG strain versus PRU strain. Differentially malonylated proteins were involved in key processes, such as those mediating the regulation of protein metabolism, stress response, glycolysis, and actin cytoskeleton. These results reveal an association between lysine malonylation and intra-species virulence differences in T. gondii and offer a new resource for elucidating the contribution of lysine malonylation to energy metabolism and virulence in T. gondii.

Lysine malonylation has been shown to play important roles in various biological processes in Toxoplasma gondii. Here, we used quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) and immuno-affinity purification to test the hypothesis that lysine malonylation underpins the inter-genotype differences in the virulence of T. gondii. Several up-regulated and down- regulated malonylated proteins were identified in the tachyzoites of RH (type I) strain, PRU (type II) strain, and VEG (type III) strain. Differentially regulated malonylated proteins were enriched in many biological and metabolic pathways, and were found to contribute T. gondii energy metabolism, stress response, and infectivity, suggesting the role of lysine malonylation in the regulation of T. gondii virulence. These findings expand our knowledge of lysine malonylation in T. gondii and provide more insight into the mechanisms mediating the virulence differences between T. gondii strains of different genotypes.

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.

Quality of clinical practice guidelines for gestational and congenital toxoplasmosis: A systematic review of the literature

OBJECTIVE

To evaluate the methodological quality and transparency of the clinical practice guidelines (CPGs) for the prevention, diagnosis, and treatment of gestational and congenital toxoplasmosis (CT).

METHODS

Systematic review of the literature on gestational and CT CPGs conducted in the MEDLINE, Embase, TripDatabase, Biblioteca Virtual en Salud databases and extensive manual searches in 19 CPG repositories. The characteristics of each of the guidelines were extracted using My AGREE PLUS on-line. Three reviewers assessed overall quality using the Appraisal of Guidelines for Research and Evaluation II (AGREE II) tool.

RESULTS

The combined systematic review found 8651 citations. Of them 46 full texts were reviewed, and eight documents were finally included: four toxoplasmosis CPGs, three prenatal care CPGs that included recommendations on toxoplasmosis, and one pregnancy infection guideline that also included recommendations on toxoplasmosis. The AGREE II domains found to have the highest scores were 'clarity of presentation' (85%; [37%-100%]), followed by 'scope and purpose' (73%; [33%-98%]), and 'editorial independence' (51%; [3%-94%]); the domains with the lowest scores were 'rigour of development' (36%; [11%-79%]), 'stakeholder involvement' (34%; [24%-85%]), and 'applicability' (17%; [6%-83%]). The Colombian and Spanish-Agencia de Evaluación de Tecnologías Sanitarias de Andalucía (AETSA) CPGs had the highest global AGREE II scores. Absolute interrater agreement was good to excellent.

CONCLUSION

Substantial quality variation was found among CPGs, which provided recommendations in accordance with the context of the disease in the corresponding country or region. Only two of the CPGs appraised obtained a good score and are classified as 'recommended'.

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.

Genetic architecture of transmission stage production and virulence in schistosome parasites

Both theory and experimental data from pathogens suggest that the production of transmission stages should be strongly associated with virulence, but the genetic bases of parasite transmission/virulence traits are poorly understood. The blood fluke Schistosoma mansoni shows extensive variation in numbers of cercariae larvae shed and in their virulence to infected snail hosts, consistent with expected trade-offs between parasite transmission and virulence. We crossed schistosomes from two populations that differ 8-fold in cercarial shedding and in their virulence to Biomphalaria glabrata snail hosts, and determined four-week cercarial shedding profiles in F0 parents, F1 parents and 376 F2 progeny from two independent crosses in inbred snails. Sequencing and linkage analysis revealed that cercarial production is polygenic and controlled by five QTLs (i.e. Quantitative Trait Loci). These QTLs act additively, explaining 28.56% of the phenotypic variation. These results demonstrate that the genetic architecture of key traits relevant to schistosome ecology can be dissected using classical linkage mapping approaches.

Global phosphoproteome analysis reveals significant differences between sporulated oocysts of virulent and avirulent strains of Toxoplasma gondii

In this study, the differences in the phosphoproteomic landscape of sporulated oocysts between virulent and avirulent strains of Toxoplasma gondii were examined using a global phosphoproteomics approach. Phosphopeptides from sporulated oocysts of the virulent PYS strain (Chinese ToxoDB#9) and the avirulent PRU strain (type II) were enriched by titanium dioxide (TiO₂) affinity chromatography and quantified using IBT approach. A total of 10,645 unique phosphopeptides, 8181 nonredundant phosphorylation sites and 2792 phosphoproteins were identified. We also detected 4129 differentially expressed phosphopeptides (DEPs) between sporulated oocysts of PYS strain and PRU strain (|log1.5 fold change| > 1 and p < 0.05), including 2485 upregulated and 1644 downregulated phosphopeptides. Motif analysis identified 24 motifs from the upregulated phosphorylated peptides including 22 serine motifs and two threonine motifs (TPE and TP), and 15 motifs from the downregulated phosphorylated peptides including 12 serine motifs and three threonine motifs (TP, RxxT and KxxT) in PYS strain when comparing PYS strain to PRU strain. Several kinases were consistent with motifs of overrepresented phosphopeptides, such as PKA, PKG, CKII, IKK, MAPK, EGFR, INSR, Jak, Syk, Src, Ab1. GO enrichment, KEGG pathway analysis and STRING analysis revealed DEPs significantly enriched in many biological processes and pathways. Kinase related network analysis showed that AGC kinase was the most connected kinase peptide. Our findings reveal significant difference in phosphopeptide profiles of sporulated oocysts between virulent and avirulent T. gondii strains, providing new resources for further elucidation of the mechanisms underpinning the virulence of T. gondii.

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).

High performance of a novel point-of-care blood test for Toxoplasma infection in women from diverse regions of Morocco

Point-of-care (POC) testing for Toxoplasma infection has the potential to revolutionize diagnosis and management of toxoplasmosis, especially in high-risk populations in areas with significant environmental contamination and poor health infrastructure precluding appropriate follow-up and preventing access to medical care. Toxoplasmosis is a significant public health challenge in Morocco, with a relatively heavy burden of infection and, to this point, minimal investment nationally to address this infection. Herein, we analyse the performance of a novel, low-cost rapid test using fingerstick-derived whole blood from 632 women (82 of whom were pregnant) from slums, educational centres, and from nomad groups across different geographical regions (i.e. oceanic, mountainous) of Morocco. The POC test was highly sensitive and specific from all settings. In the first group of 283 women, sera were tested by Platelia ELISA IgG and IgM along with fingerstick whole blood test. Then a matrix study with 349 women was performed in which fingerstick – POC test results and serum obtained by venipuncture contemporaneously were compared. These results show high POC test performance (Sensitivity: 96.4% [IC95 90.6–98.9%]; Specificity: 99.6% [IC95 97.3–99.9%]) and high prevalence of Toxoplasma infection among women living in rural and mountainous areas, and in urban areas with lower educational levels. The high performance of POC test confirms that it can reduce the need for venipuncture and clinical infrastructure in a low-resource setting. It can be used to efficiently perform seroprevalence determinations in large group settings across a range of demographics, and potentially expands healthcare access, thereby preventing human suffering.

Interaction Between the Complement System and Infectious Agents - A Potential Mechanistic Link to Neurodegeneration and Dementia

As part of the innate immune system, complement plays a critical role in the elimination of pathogens and mobilization of cellular immune responses. In the central nervous system (CNS), many complement proteins are locally produced and regulate nervous system development and physiological processes such as neural plasticity. However, aberrant complement activation has been implicated in neurodegeneration, including Alzheimer’s disease. There is a growing list of pathogens that have been shown to interact with the complement system in the brain but the short- and long-term consequences of infection-induced complement activation for neuronal functioning are largely elusive. Available evidence suggests that the infection-induced complement activation could be protective or harmful, depending on the context. Here we summarize how various infectious agents, including bacteria (e.g., Streptococcus spp.), viruses (e.g., HIV and measles virus), fungi (e.g., Candida spp.), parasites (e.g., Toxoplasma gondii and Plasmodium spp.), and prion proteins activate and manipulate the complement system in the CNS. We also discuss the potential mechanisms by which the interaction between the infectious agents and the complement system can play a role in neurodegeneration and dementia.

The Toxoplasma Polymorphic Effector GRA15 Mediates Seizure Induction by Modulating Interleukin-1 Signaling in the Brain

Toxoplasmic encephalitis can develop in individuals infected with the protozoan parasite Toxoplasma gondii and is typified by parasite replication and inflammation within the brain. Patients often present with seizures, but the parasite genes and host pathways involved in seizure development and/or propagation are unknown. We previously reported that seizure induction in Toxoplasma-infected mice is parasite strain dependent. Using quantitative trait locus mapping, we identify four loci in the Toxoplasma genome that potentially correlate with seizure development. In one locus, we identify the polymorphic virulence factor, GRA15, as a Toxoplasma gene associated with onset of seizures. GRA15 was previously shown to regulate host NF-κB-dependent gene expression during acute infections, and we demonstrate a similar role for GRA15 in brains of toxoplasmic encephalitic mice. GRA15 is important for increased expression of interleukin 1 beta (IL-1β) and other IL-1 pathway host genes, which is significant since IL-1 signaling is involved in onset of seizures. Inhibiting IL-1 receptor signaling reduced seizure severity in Toxoplasma-infected mice. These data reveal one mechanism by which seizures are induced during toxoplasmic encephalitis.

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.

Generation of Toxoplasma gondii and Hammondia hammondi Oocysts and Purification of Their Sporozoites for Downstream Manipulation

Toxoplasma gondii tachyzoites and bradyzoites are studied extensively in the laboratory due to the ease with which they can be cultured. In contrast, oocysts and the sporozoites within them are more difficult to work with, in that cat infections are required for their generation and isolating sporozoites requires a laborious excystation procedure. More over some parasite species such as Hammondia hammondi are obligately heteroxenous and require passage through a cat for completion of the life cycle. There is no debate that there is great value in studying this important life cycle stage, and we present here a detailed description of the current protocols used in our laboratories to generate and isolate T. gondii and H. hammondi oocysts, and to excyst and purify the sporozoites within them for use in downstream experimental applications.

The Host Autophagy During Toxoplasma Infection

Autophagy is an important homeostatic mechanism, in which lysosomes degrade and recycle cytosolic components. As a key defense mechanism against infections, autophagy is involved in the capture and elimination of intracellular parasites. However, intracellular parasites, such as Toxoplasma gondii, have developed several evasion mechanisms to manipulate the host cell autophagy for their growth and establish a chronic infection. This review provides an insight into the autophagy mechanism used by the host cells in the control of T. gondii and the host exploitation by the parasite. First, we summarize the mechanism of autophagy, xenophagy, and LC3-associated phagocytosis. Then, we illustrate the process of autophagy proteins-mediated T. gondii clearance. Furthermore, we discuss how the parasite blocks and exploits this process for its survival.

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.

An Important Role for CD4+ T Cells in Adaptive Immunity to Toxoplasma gondii in Mice Lacking the Transcription Factor Batf3

Immunity to Toxoplasma gondii at early stages of infection in C57BL/6 mice depends on gamma interferon (IFN-γ) production by NK cells, while at later stages it is primarily mediated by CD8 T cells. We decided to explore the requirement for CD4 T cells during T. gondii infection in Batf3-/- mice, which lack CD8α+ dendritic cells (DCs) that are necessary for cross-presentation of cell-associated antigens to CD8 T cells. We show that in this immunodeficient background on a BALB/c background, CD4 T cells become important effector cells and are able to protect Batf3-/- mice from infection with the avirulent strain RHΔku80Δrop5 Independently of the initial NK cell activation, CD4 T cells in wild-type and Batf3-/- mice were the major source of IFN-γ. Importantly, memory CD4 T cells were sufficient to provide protective immunity following transfer into Batf3-/- mice and secondary challenge with the virulent RHΔku80 strain. Collectively, these results show that under situations where CD8 cell responses are impaired, CD4 T cells provide an important alternative immune response to T. gondiiIMPORTANCEToxoplasma gondii is a widespread parasite of animals that causes zoonotic infections in humans. Although healthy individuals generally control the infection with only moderate symptoms, it causes serious illness in newborns and those with compromised immune systems such as HIV-infected AIDS patients. Because rodents are natural hosts for T. gondii, laboratory mice provide an excellent model for studying immune responses. Here, we used a combination of an attenuated mutant strain of the parasite that effectively vaccinates mice, with a defect in a transcriptional factor that impairs a critical subset of dendritic cells, to studying the immune response to infection. The findings reveal that in BALB/c mice, CD4 memory T cells play a dominant role in producing IFN-γ needed to control chronic infection. Hence, BALB/c mice may provide a more appropriate model for declining immunity seen in HIV-AIDS patients where loss of CD4 cells is associated with emergence of opportunistic infections.

Loss of rhoptry protein 9 impeded Toxoplasma gondii infectivity

Toxoplasma gondii is an obligatory intracellular parasite that critically depends on active invasion and egress from infected host cells to complete its propagation cycle. T. gondii rhoptry proteins (TgROPs) are virulent factors associated with host cell invasion, growth. In this study, we analyzed the functions of ROP9 in the process of T. gondii infection. The TgROP9 knockout RH strain (RH△ROP9) and its recovery strain (RH-ReROP9) were constructed using the CRISPR/Cas9 system. The invasion, proliferation, and egress efficiency of the RH△ROP9 strain were evaluated and their pathogenicity to mice was analyzed. Compared with RH wild-type (RH-WT) and RH-ReROP9 strains, the invasion percentage of RH△ROP9 to Vero cells was reduced by about 28.0% (p< 0.01) at 1.5 h, and the relative proliferation percentage was decreased by about 35.0% (p< 0.01) after infection with 10² or 10³ parasites. In addition, the RH△ROP9 strain also showed prolonged egress time from host cells. The survival time of the mice (12.6 ± 1.6 or 10.1 ± 1.1 days) were delayed (p < 0.001) after infection with either 200 or 1000 RH△ROP9 parasites. These evidences suggested that ROP9 facilitated T. gondii infection in vitro and in vivo.

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.

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.

In Vivo CRISPR Screen Identifies TgWIP as a Toxoplasma Modulator of Dendritic Cell Migration

Toxoplasma can reach distant organs, especially the brain, leading to a lifelong chronic phase. However, genes involved in related in vivo processes are currently unknown. Here, we use focused CRISPR libraries to identify Toxoplasma genes that affect in vivo fitness. We focus on TgWIP, whose deletion affects Toxoplasma dissemination to distant organs. We show that TgWIP is secreted into the host cell upon invasion and interacts with the host WAVE regulatory complex and SHP2 phosphatase, both of which regulate actin dynamics. TgWIP affects the morphology of dendritic cells and mediates the dissolution of podosomes, which dendritic cells use to adhere to extracellular matrix. TgWIP enhances the motility and transmigration of parasitized dendritic cells, likely explaining its effect on in vivo fitness. Our results provide a framework for systemic identification of Toxoplasma genes with in vivo effects at the site of infection or on dissemination to distant organs, including the brain.

Molecular and biological analysis revealed genetic diversity and high virulence strain of Toxoplasma gondii in Japan

Toxoplasma gondii is classified into 16 haplogroups based on a worldwide genotyping study of the parasite. However, only a few isolates from Japan were included in this analysis. To conduct more precise genotyping of T. gondii, we examined the genotypes of Japanese isolates in this study. DNA sequences of 6 loci were determined in 17 Japanese isolates and compared with those of strains of 16 haplogroups. As a result, Japanese isolates were classified into four groups. We investigated the virulence of some Japanese isolates and found a highly virulent strain in mice, comparable to that of RH strain, although this Japanese isolate was sister to strains of haplogroup 2, which show moderate virulence in mice. We further investigated whether this high virulence isolate had different virulence mechanism and strategy to adapt to Japanese host from other strains by comparing the virulence-related genes, ROP5, 18 and the immunomodulatory gene, ROP16 of the isolate with those of archetypical strains (GT1, ME49 and VEG). This analysis indicated the high virulence of the isolate in mice was partly explained by gene sequences of ROP5 and ROP16. These findings lead to the elucidation of biodiversity of T. gondii and have potential to optimize the diagnostic protocol.

Toxoplasma gondii Recruits Factor H and C4b-Binding Protein to Mediate Resistance to Serum Killing and Promote Parasite Persistence in vivo

Regulating complement is an important step in the establishment of infection by microbial pathogens. Toxoplasma gondii actively resists complement-mediated killing in non-immune human serum (NHS) by inactivating C3b, however the precise molecular basis is unknown. Here, a flow cytometry-based C3b binding assay demonstrated that Type II strains had significantly higher levels of surface-bound C3b than Type I strains. However, both strains efficiently inactivated C3b and were equally resistant to serum killing, suggesting that resistance is not strain-dependent. Toxoplasma activated both the lectin (LP) and alternative (AP) pathways, and the deposition of C3b was both strain and lectin-dependent. A flow cytometry-based lectin binding assay identified strain-specific differences in the level and heterogeneity of surface glycans detected. Specifically, increased lectin-binding by Type II strains correlated with higher levels of the LP recognition receptor mannose binding lectin (MBL). Western blot analyses demonstrated that Toxoplasma recruits both classical pathway (CP) and LP regulator C4b-binding proteins (C4BP) and AP regulator Factor H (FH) to the parasite surface to inactivate bound C3b-iC3b and C3dg and limit formation of the C5b-9 attack complex. Blocking FH and C4BP contributed to increased C5b-9 formation in vitro. However, parasite susceptibility in vitro was only impacted when FH was blocked, indicating that down regulation of the alternative pathway by FH may be more critical for parasite resistance. Infection of C3 deficient mice led to uncontrolled parasite growth, acute mortality, and reduced antibody production, indicating that both the presence of C3, and the ability of the parasite to inactivate C3, was protective. Taken together, our results establish that Toxoplasma regulation of the complement system renders mice resistant to acute infection by limiting parasite proliferation in vivo, but susceptible to chronic infection, with all mice developing transmissible cysts to maintain its life cycle.

Assays for Monitoring Toxoplasma gondii Infectivity in the Laboratory Mouse

Toxoplasma is a widespread parasite of animals including many rodents that are a natural part of the transmission cycle between cats, which serve as the definitive host. Although wild rodents, including house mice, are relatively resistant, laboratory mice are highly susceptible to infection. As such, laboratory mice have been used to compare pathogenesis of natural variants and to evaluate the contributions of both host and parasite genes to infection. Protocols are provided here for evaluating acute and chronic infection with different parasite strains in laboratory mice. These protocols should provide uniform standards for evaluating natural variants and attenuated mutants and for comparing outcomes across different studies and between different laboratories.

Virulence of atypical Toxoplasma gondii strains isolated in French Guiana in a murine model

Background. Toxoplasma gondii is an obligate intracellular protozoan parasite of warm-blooded vertebrates. Most infections in immunocompetent patients are asymptomatic. However, since 2000s, strains with particular genetic profiles that differ from the known clonal type (type I, II, III), have been described. In French Guiana, these strains are highly pathogenic in immunocompetent patients. They have defined a new clinical entity called Amazonian Toxoplasmosis. The present study aims to further improve our knowledge on the pathogenicity of these Amazonian T. gondii strains in comparison with three reference strains using Swiss strain mice. With these data, we tried to establish a predictive virulence score to classify these strains, but also to correlate this virulence with the severity of the disease in infected patients. Results. All the virulence indicators revealed that the Amazonian strains isolated in French Guiana presented a high virulence profile, but lower than the highly virulent type I reference RH strain. The findings reveal differences in virulence between human and animal strains, but also between anthropized and wild strains. Conclusion. In addition to being a clinically relevant animal model of Amazonian Toxoplasmosis, this model could also provide a solid experimental basis for future studies aiming to investigate the underlying mechanisms of Amazonian Toxoplasmosis disease.