A secreted serine-threonine kinase determines virulence in the eukaryotic pathogen Toxoplasma gondii

GRA12 is a common virulence factor across Toxoplasma gondii strains and mouse subspecies

Toxoplasma gondii parasites exhibit extraordinary host promiscuity owing to over 250 putative secreted proteins that disrupt host cell functions, enabling parasite persistence. However, most of the known effector proteins are specific to Toxoplasma genotypes or hosts. To identify virulence factors that function across different parasite isolates and mouse strains that differ in susceptibility to infection, we performed systematic pooled in vivo CRISPR-Cas9 screens targeting the Toxoplasma secretome. We identified several proteins required for infection across parasite strains and mouse species, of which the dense granule protein 12 (GRA12) emerged as the most important effector protein during acute infection. GRA12 deletion in IFNγ-activated macrophages results in collapsed parasitophorous vacuoles and increased host cell necrosis, which is partially rescued by inhibiting early parasite egress. GRA12 orthologues from related coccidian parasites, including Neospora caninum and Hammondia hammondi, complement TgΔGRA12 in vitro, suggesting a common mechanism of protection from immune clearance by their hosts.

Global research landscape and trends in Toxoplasma gondii from 2003 to 2022: A bibliometric analysis

Toxoplasmosis is a globally prevalent zoonotic disease with significant public health impact, yet effective prevention and control measures remain lacking. This study analyzed 10,737 publications from 2003 to 2022 in the Web of Science and Scopus databases using bibliometric methods. Key metrics, including annual publication trends, journals, core authors, contributing countries, citations, and keywords, were examined. The findings indicate that publication output in toxoplasmosis research has remained stable, with core journals predominantly in the parasitology field. The United States leads in research capacity and contributions, followed by Brazil, Germany, the United Kingdom, and France, while China shows promising potential. Epidemiological research has been a longstanding hotspot in this field, encompassing areas such as diagnosis, prevention, treatment, virulence analysis, and genotyping. The field is expanding towards mechanistic research and translational applications, shifting its emphasis from basic science to practical public health applications and disease prevention strategies. Current research frontiers and emerging trends focus on host-pathogen interactions, innovative prevention and control methods, and the global public health implications of toxoplasmosis. This study provides a comprehensive overview of the past two decades of progress in toxoplasmosis research, offering valuable insights for future studies and academic development.

Toxoplasma gondii ROP18 induces maternal-fetal dysfunction by downregulating CD73 expression on decidual macrophages

BACKGROUND

Decidual macrophages (dMφ) are pivotal in maintaining maternal-fetal immune tolerance during normal pregnancy by expressing a range of immune-suppressive molecules, including CD73. It has been demonstrated that Toxoplasma gondii (T. gondii) infection during pregnancy can impair dMφ function, potentially leading to adverse pregnancy outcomes, through downregulation of these inhibitory molecules. T. gondii rhoptry protein 18 (TgROP18), a key virulence factor of T. gondii, is associated with the incapacitation of the host's innate and adaptive immune responses to protect the parasite from elimination. However, the role of TgROP18 in modulating CD73 expression on dMφ and the underlying mechanisms remain to be elucidated.

METHODS

Wild-type (WT) and CD73-deficient (CD73-/-) pregnant mice were subjected to intraperitoneal injection of T. gondii RH or RH-Δrop18 on gestational day (Gd) 8, and subsequently euthanized on Gd 14. Pregnancy outcomes were then evaluated, and the expression levels of CD73, arginase 1 (Arg-1), and interleukin 10 (IL-10) were quantified by flow cytometry. Mononuclear cells isolated from the human aborted decidual tissues were also infected with T. gondii RH or RH-Δrop18 for the analysis of CD73 expression with flow cytometry. Additionally, infected human dMφ were used to assess the expression levels of CD73, Arg-1, IL-10, and their associated signaling molecules by western blot analysis. Furthermore, chromatin immunoprecipitation (ChIP) assays were performed to validate the involved signaling pathways.

RESULTS

Compared with the T. gondii RH-infected group, milder adverse pregnancy outcomes and attenuated expression levels of CD73 on dMφ were observed in T. gondii RH-Δrop18-infected pregnant mice and human decidual tissues. Lysine-specific histone demethylase1 (LSD1) and snail family transcriptional repressor 1 (SNAIL1) were found to be involved in the downregulation of CD73 expression on dMφ following T. gondii infection. Subsequently, reduced expression of CD73 contribute to the downregulation of Arg-1 and IL-10 expression through adenosine A2a receptor (A2AR) / protein kinase A (PKA) / phosphorylated cAMP-response element binding protein (p-CREB) / CCAAT enhancer binding protein B (C/EBPβ) pathway.

CONCLUSIONS

TgROP18 can significantly reduce CD73 expression on dMφ through LSD1/SNAIL1 pathway, subsequently leading to the decreased expression levels of Arg-1 and IL-10 via adenosine/A2AR/PKA/p-CREB/C/EBPβ pathway, which ultimately contributes to maternal-fetal tolerance dysfunction of dMφ.

In Silico Analysis and Characterization of the Immunogenicity of Toxoplasma gondii Rhoptry Protein 18

Rhoptry protein 18 (ROP18) is a key virulence factor secreted into host cells during the invasion of Toxoplasma gondii (T. gondii) and plays an important role in the pathogenesis of infection. Due to its potential as a vaccine candidate, this study aimed to characterize several properties of the T. gondii ROP18 (TgROP18) protein to support its inclusion in vaccine formulations. Using a range of bioinformatics tools, we investigated its T-cell and B-cell epitopes, physicochemical properties, subcellular localization, transmembrane domains, and tertiary and secondary structures. Our analysis revealed 48 post-translational modification sites in TgROP18. The secondary structure was composed of 4.87% beta-turns, 38.45% random coils, 42.42% alpha helices, and 14.26% extended strands. Several potential T- and B-cell epitopes were identified on ROP18. The Ramachandran plot of both crude and refined models showed that 85.8% and 95.3% of the amino acid residues, respectively, fell within favored regions, indicating energetically stable conformations. Allergenicity and antigenicity assessments indicated that TgROP18 is a nonallergenic, immunogenic protein. Predictions using the C-ImmSim server suggest that TgROP18 can stimulate humoral and cell-mediated immune responses, based on antibody titers and cytokine profiles following antigen administration. These findings provide baseline data for future investigations focused on the potential of TgROP18 in developing therapeutic strategies against toxoplasmosis.

Knockdown of DJ-1 Exacerbates Neuron Apoptosis Induced by TgCtwh3 through the NF-κB Pathway

Mutations or loss of function of DJ-1 and Toxoplasma gondii (T. gondii) infection has been linked to neurodegenerative diseases, which are often caused by oxidative stress. However, the relationship between DJ-1 and T. gondii infection is not yet fully understood. Therefore, this study aimed to investigate the expression of DJ-1 in the hippocampus tissue of mice or in HT22 infected with T. gondii Chinese 1 genotype Wh3 strain (TgCtwh3) and the effect of DJ-1 knockdown on neuronal apoptosis induced by TgCtwh3 tachyzoite, as well as the underlying mechanism at the cellular and molecular level. Firstly, we detected DJ-1 protein expression and cell apoptosis in the hippocampal tissue of mice infected by TgCtwh3. Then, we examined DJ-1 expression and apoptosis in HT22 challenged with TgCtwh3. Finally, we evaluated the apoptosis in HT22 with DJ-1 knockdown which was infected with TgCtwh3 and assayed the expression of NF-κBp65 and p-NF-κBp65. Our results showed that DJ-1 expression was reduced and neurons underwent apoptosis in the hippocampus of mice infected with TgCtwh3 tachyzoites. Additionally, the knockdown of DJ-1 followed by infection with TgCtwh3 tachyzoites led to increased apoptosis in HT22 cells through the NF-κB signaling pathway. Therefore, this study suggests that DJ-1 is an important target for preventing apoptosis caused by T. gondii TgCtwh3.

Kinase function of TgTKL1 is essential for its role in Toxoplasma propagation and pathogenesis

The Tyrosine Kinase-Like (TKL) family of proteins are a set of poorly studied kinases that have garnered attention in recent years for their role in Toxoplasma biology. The Toxoplasma genome contains eight TKL kinases, of which six have been predicted to be important for parasite propagation. We have previously shown that TgTKL1 is a nuclear kinase that is critical for the parasite lytic cycle and is essential for acute virulence in the animal model. However, the contribution of the kinase domain to the functioning of TgTKL1 was not known. Hence to determine the significance of its catalytic function, we first validated that TgTKL1 is a true kinase using purified recombinant protein. Furthermore, we successfully generated a TgTKL1 kinase mutant strain of Toxoplasma via CRISPR-Cas9 gene editing. Our studies revealed that the kinase mutant of TgTKL1 displays defects in parasite growth and host-cell invasion. Additionally, loss of kinase function alters the transcriptomic profile of the parasite, including downregulation of the invasion-related gene, TgSUB1. Importantly, this dysregulation of TgSUB1 expression leads to defects in post-exocytosis processing of micronemal proteins, an event critical for normal host-cell invasion. Furthermore, the TgTKL1 kinase mutant is completely avirulent in the mouse model of acute toxoplasmosis. Since the loss of kinase function leads to phenotypic manifestations seen previously with TgTKL1 knockout parasites, we conclude that kinase activity is important for TgTKL1 function in Toxoplasma propagation and virulence.

IMPORTANCE

Toxoplasma gondii is a protozoan parasite that can cause life-threatening disease in humans. Hence, identifying key factors required for parasite growth and pathogenesis is important to develop novel therapeutics. We have previously shown that a member of the TKL protein kinase family, TgTKL1, is a plant-like kinase that is required for effective Toxoplasma growth in vitro and essential for virulence in vivo. Herein, we show that the TgTKL1 is, indeed, a bona fide kinase, and loss of its kinase function in the Toxoplasma leads to similar defects seen in parasites with complete loss of TgTKL1. More specifically, the TgTKL1 kinase mutant exhibits defects in parasite growth, host-cell invasion, gene expression profile, and virulence in the animal model. Together, these findings suggest that TgTKL1 is a true kinase, and loss of its kinase activity leads to disruption of TgTKL1 function in Toxoplasma.

Perforation of the host cell plasma membrane during Toxoplasma gondii invasion requires rhoptry exocytosis

Toxoplasma gondii is an obligate intracellular parasite, and the delivery of effector proteins from the parasite into the host cell during invasion is critical for invasion itself and for parasite virulence. The effector proteins are released from specialized apical secretory organelles known as rhoptries. While much has been learned recently about the structure and composition of the rhoptry exocytic machinery and the function of individual rhoptry effector proteins that are exocytosed, virtually nothing is known about how the released proteins are translocated across the host cell plasma membrane. Previous electrophysiology experiments reported an unanticipated observation that invasion by T. gondii is preceded by a transient increase in host cell plasma membrane conductance. Here, we confirm this electrophysiological observation and propose that the conductance transient represents a parasite-induced perforation in the host cell plasma membrane through which rhoptry proteins are delivered. As a first step towards testing this hypothesis, and to provide higher throughput than patch clamp electrophysiology, we developed an alternative assay to detect the perforation. This assay utilizes high-speed, multi-wavelength fluorescence imaging to enable simultaneous visualization of host cell perforation and parasite invasion. Using this assay, we interrogated a panel of mutant parasites conditionally depleted of key invasion-related proteins. Parasites lacking signaling proteins involved in triggering rhoptry secretion (e.g., CLAMP) or components of the rhoptry exocytic machinery (e.g., Nd9, RASP2) are defective in their ability to induce the perforation. These data are consistent with a model in which the perforating agents that disrupt host cell membrane integrity during invasion - and may thereby provide the conduit for delivery of rhoptry effector proteins - are stored within the rhoptries themselves and released upon contact with the host cell.

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.

Toxoplasma WH3 Δrop18 acts as a live attenuated vaccine against acute and chronic toxoplasmosis

Toxoplasma gondii is a significant zoonotic pathogen of toxoplasmosis in humans and animals. Here a live attenuated Toxoplasma vaccine of WH3 Δrop18 was developed. The results showed that all mice vaccinated with WH3 Δrop18 were able to survive when challenge with various strains of Toxoplasma, including RH (type I), ME49 (type II), WH3 or WH6 (type Chinese 1). No cysts, if few, in the brain of the vaccinated animals were seen after challenge with cyst forming strains of ME49 or WH6. Vaccination with the WH3 Δrop18 triggered a strong immune response, including significantly increased level of the cytokines (IFN-γ, IL-12, TNF-α and IL-10) and the activation of CD4+ and CD8+ T-lymphocytes and long term of specific antibodies against Toxoplasma. Our results strongly indicate that vaccine of WH3 Δrop18 might provide effective immune protection against a wide range strains of Toxoplasma infections and be a promising live attenuated vaccine candidate.

Serological and molecular survey of Toxoplasma Gondii in aborted livestock fetuses from Northeast Iran

BACKGROUND

Toxoplasmosis not only leads to abortion in humans but also in herbivores, which causes significant financial and quality-adjusted life-year losses. The present study aimed to determine the prevalence of toxoplasmosis in aborted fetuses via serological and molecular assays. Moreover, the genotypes of the obtained isolates were detected.

METHODS

Serological and molecular methods were used to study aborted fetuses from Bojnourd City, North Khorasan Province, Iran, which included 52 ovines and 16 bovines. Nested PCR of the B1 gene was used to detect parasite DNA in brain tissues. The PCR-RFLP method for the GRA6 gene was used to determine the genotype of T. gondii.

RESULTS

Out of 68 aborted fetuses, 16.1% showed the presence of anti-T. gondii IgG. Among these, 11.7% were identified in bovine fetuses and 4.4% in ovine fetuses. Additionally, two (2.94%) samples of ovine tested positive for anti-T. gondii IgM. Our PCR analysis detected parasite DNA in two cases (2.94%) among 11 IgG-positive samples. All obtained isolates belong to type I of T. gondii.

CONCLUSION

Infection with Type I of T. gondii during the neonatal period may partly be responsible for abortion and economic losses in livestock farming in our studied region. To understand the molecular epidemiology and genotypes of T. gondii associated with abortion, further evaluation of aborted samples from different geographical locations is necessary.

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.

Effective factors in the pathogenesis of Toxoplasmagondii

Toxoplasma gondii (T. gondii) is a cosmopolitan protozoan parasite in humans and animals. It infects about 30 % of the human population worldwide and causes potentially fatal diseases in immunocompromised hosts and neonates. For this study, five English-language databases (ScienceDirect, ProQuest, Web of Science, PubMed, and Scopus) and the internet search engine Google Scholar were searched. This review was accomplished to draw a global perspective of what is known about the pathogenesis of T. gondii and various factors affecting it. Virulence and immune responses can influence the mechanisms of parasite pathogenesis and these factors are in turn influenced by other factors. In addition to the host's genetic background, the type of Toxoplasma strain, the routes of transmission of infection, the number of passages, and different phases of parasite life affect virulence. The identification of virulence factors of the parasite could provide promising insights into the pathogenesis of this parasite. The results of this study can be an incentive to conduct more intensive research to design and develop new anti-Toxoplasma agents (drugs and vaccines) to treat or prevent this infection. In addition, further studies are needed to better understand the key agents in the pathogenesis of T. gondii.

Whole-genome CRISPR screens to understand Apicomplexan-host interactions

Apicomplexan parasites are aetiological agents of numerous diseases in humans and livestock. Functional genomics studies in these parasites enable the identification of biological mechanisms and protein functions that can be targeted for therapeutic intervention. Recent improvements in forward genetics and whole-genome screens utilising CRISPR/Cas technology have revolutionised the functional analysis of genes during Apicomplexan infection of host cells. Here, we highlight key discoveries from CRISPR/Cas9 screens in Apicomplexa or their infected host cells and discuss remaining challenges to maximise this technology that may help answer fundamental questions about parasite-host interactions.

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.

Genetic crosses within and between species of Cryptosporidium

Parasites and their hosts are engaged in reciprocal coevolution that balances competing mechanisms of virulence, resistance, and evasion. This often leads to host specificity, but genomic reassortment between different strains can enable parasites to jump host barriers and conquer new niches. In the apicomplexan parasite Cryptosporidium, genetic exchange has been hypothesized to play a prominent role in adaptation to humans. The sexual lifecycle of the parasite provides a potential mechanism for such exchange; however, the boundaries of Cryptosporidium sex are currently undefined. To explore this experimentally, we established a model for genetic crosses. Drug resistance was engineered using a mutated phenylalanyl tRNA synthetase gene and marking strains with this and the previously used Neo transgene enabled selection of recombinant progeny. This is highly efficient, and genomic recombination is evident and can be continuously monitored in real time by drug resistance, flow cytometry, and PCR mapping. Using this approach, multiple loci can now be modified with ease. We demonstrate that essential genes can be ablated by crossing a Cre recombinase driver strain with floxed strains. We further find that genetic crosses are also feasible between species. Crossing Cryptosporidium parvum, a parasite of cattle and humans, and Cryptosporidium tyzzeri a mouse parasite resulted in progeny with a recombinant genome derived from both species that continues to vigorously replicate sexually. These experiments have important fundamental and translational implications for the evolution of Cryptosporidium and open the door to reverse- and forward-genetic analysis of parasite biology and host specificity.

Toxoplasma protein export and effector function

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

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

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

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

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

New Allele-Specific Oligonucleotide (ASO) amplifications for Toxoplasma gondii rop18 allele typing: Analysis of 86 human congenital infections in Brazil

This study aimed to detect and differentiate Toxoplasma gondii by the allele typing of its polymorphic rop18 gene. For this purpose, a novel genotyping system using allele-specific oligonucleotides (ASOs) was designed, consisting of three ASO pairs. The first and third pairs specifically amplify rop18 allele I and allele III, while the second pair amplify both allele I and II. Genomic DNA from 86 congenital infections was analyzed by ASO-PCRs, successfully typing 82 (95.35%) samples. The remaining 4 samples (4.65%) required sequencing and single nucleotide polymorphism (SNP) analysis of the amplification products. The distribution of samples according to rop18 alleles was: 39.5% of allele III, 38.4% of allele II, 19.8% of mixed rop18 alleles (I/III or II/III), and 2.3% of allele I. The six severely compromised infants exhibited the highest parasite load levels and were infected during the first and early second trimesters of pregnancy. Among these cases, two were associated with rop18 allele I parasites, two with mixed rop18 alleles (I/III), one with allele II, and one with allele III parasites. In conclusion, all severe cases of congenital toxoplasmosis were infected during early pregnancy, but they were not exclusively associated with rop18 allele I parasites, as observed in murine toxoplasmosis. Furthermore, nearly one-fifth of parasites were non-archetypal, exhibiting more than one rop18 allele, indicating a higher genetic diversity of Toxoplasma gondii in this South American sample. Overall, a robust T. gondii rop18 allele typing was developed and suggested that congenital toxoplasmosis in humans involves complex mechanisms beyond the parasite genotype.

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.

Genetic crosses within and between species of Cryptosporidium

Parasites and their hosts are engaged in rapid coevolution that balances competing mechanisms of virulence, resistance, and evasion. This often leads to host specificity, but genomic reassortment between different strains can enable parasites to jump host barriers and conquer new niches. In the apicomplexan parasite Cryptosporidium genetic exchange has been hypothesized to play a prominent role in adaptation to humans. The sexual lifecycle of the parasite provides a potential mechanism for such exchange; however, the boundaries of Cryptosporidium sex are currently undefined. To explore this experimentally, we established a model for genetic crosses. Drug resistance was engineered using a mutated phenylalanyl tRNA synthetase gene and marking strains with this and the previously used Neo transgene enabled selection of recombinant progeny. This is highly efficient, and genomic recombination is evident and can be continuously monitored in real time by drug resistance, flow cytometry, and PCR mapping. Using this approach multiple loci can now be modified with ease. We demonstrate that essential genes can be ablated by crossing a Cre recombinase driver strain with floxed strains. We further find that genetic crosses are also feasible between species. Crossing C. parvum, a parasite of cattle and humans, and C. tyzzeri a mouse parasite resulted in progeny with a recombinant genome derived from both species that continues to vigorously replicate sexually. These experiments have important fundamental and translational implications for the evolution of Cryptosporidium and open the door to reverse- and forward- genetic analysis of parasite biology and host specificity.

Mode of action studies confirm on-target engagement of lysyl-tRNA synthetase inhibitor and lead to new selection marker for Cryptosporidium

Introduction

Cryptosporidiosis is a leading cause of diarrheal-associated morbidity and mortality, predominantly affecting children under 5 years old in low-and-middle-income countries. There is no effective treatment and no vaccine. New therapeutics are emerging from drug discovery efforts. It is critical that mode of action studies are performed alongside drug discovery to ensure the best clinical outcomes. Unfortunately, technology to identify and validate drug targets for Cryptosporidium is severely lacking.

Methods

We used C. parvum lysyl-tRNA synthetase (CpKRS) and DDD01510706 as a target-compound pair to develop both chemical and genetic tools for mode of action studies for Cryptosporidium. We adapted thermal proteome profiling (TPP) for Cryptosporidium, an unbiased approach for target identification.

Results

Using TPP we identified the molecular target of DDD01510706 and confirm that it is CpKRS. Genetic tools confirm that CpKRS is expressed throughout the life cycle and that this target is essential for parasite survival. Parasites genetically modified to over-express CpKRS or parasites with a mutation at the compound-binding site are resistant to treatment with DDD01510706. We leveraged these mutations to generate a second drug selection marker for genetic modification of Cryptosporidium, KRSR. This second selection marker is interchangeable with the original selection marker, NeoR, and expands the range of reverse genetic approaches available to study parasite biology. Due to the sexual nature of the Cryptosporidium life cycle, parental strains containing different drug selection markers can be crossed in vivo.

Discussion

Selection with both drug markers produces highly efficient genetic crosses (>99% hybrid progeny), paving the way for forward genetics approaches in Cryptosporidium.

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.

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.

ROP16-mediated activation of STAT6 enhances cyst development of type III Toxoplasma gondii in neurons

Toxoplasma gondii establishes a long-lived latent infection in the central nervous system (CNS) of its hosts. Reactivation in immunocompromised individuals can lead to life threatening disease. Latent infection is driven by the ability of the parasite to convert from the acute-stage tachyzoite to the latent-stage bradyzoite which resides in long-lived intracellular cysts. While much work has focused on the parasitic factors that drive cyst development, the host factors that influence encystment are not well defined. Here we show that a polymorphic secreted parasite kinase (ROP16), that phosphorylates host cell proteins, mediates efficient encystment of T. gondii in a stress-induced model of encystment and primary neuronal cell cultures (PNCs) in a strain-specific manner. Using short-hairpin RNA (shRNA) knockdowns in human foreskin fibroblasts (HFFs) and PNCs from transgenic mice, we determined that ROP16's cyst enhancing abilities are mediated, in part, by phosphorylation-and therefore activation-of the host cell transcription factor STAT6. To test the role of STAT6 in vivo, we infected wild-type (WT) and STAT6KO mice, finding that, compared to WT mice, STAT6KO mice have a decrease in CNS cyst burden but not overall parasite burden or dissemination to the CNS. Finally, we found a similar ROP16-dependent encystment defect in human pluripotent stem cell-derived neurons. Together, these findings identify a host cell factor (STAT6) that T. gondii manipulates in a strain-specific manner to generate a favorable encystment environment.

TgTKL4 Is a Novel Kinase That Plays an Important Role in Toxoplasma Morphology and Fitness

Protein kinases of the protozoan parasite Toxoplasma gondii have been shown to play key roles in regulating parasite motility, invasion, replication, egress and survival within the host. The tyrosine kinase-like (TKL) kinase family of proteins are a set of poorly studied kinases that our recent studies have indicated play a critical role in Toxoplasma biology. In this study, we focused on TgTKL4, another member of the TKL family that is predicted to confer parasite fitness. Endogenous tagging of TgTKL4 identified it as a temporally oscillating kinase with dynamic localization in the parasite. Gene disruption experiments suggested that TgTKL4 is important for Toxoplasma propagation in vitro, and loss of this kinase resulted in replication and invasion defects. During parasite division, TgTKL4 expression was limited to the synthesis and mitosis-cytokinesis phases and, interestingly, loss of TgTKL4 led to defects in Toxoplasma morphology. Further analysis of the parasite cytoskeleton indicated that the subpellicular microtubules are shorter and more widely spaced in parasites lacking TgTKL4. Although loss of TgTKL4 caused only moderate changes in the gene expression profile, TgTKL4 null mutants exhibited significant changes in their global phospho-proteome, including in proteins that constitute the parasite cytoskeleton. Additionally, mice inoculated intraperitoneally with TgTKL4 knockout parasites showed increased survival rates, suggesting that TgTKL4 plays an important role in acute toxoplasmosis. Together, these findings suggest that TgTKL4 mediates a signaling pathway that regulates parasite morphology and is an important factor required for parasite fitness in vitro and in vivo. IMPORTANCE Toxoplasma gondii is a protozoan parasite that can cause life-threatening disease in mammals; hence, identifying key factors required for parasite growth and pathogenesis is important to develop novel therapeutics. In this study, we identified and characterized another member of the newly described TKL family, TgTKL4, a cell cycle-regulated kinase. By disrupting TgTKL4, we determined that this kinase is required for normal parasite growth in vitro and that loss of this kinase results in parasites with reduced competence in replication and invasion processes. Specifically, Toxoplasma parasites lacking TgTKL4 had defects in cytoskeletal arrangement, resulting in parasites with abnormal morphology. Phospho-proteome studies provided further clues that decreased phosphorylation of proteins that constitute the Toxoplasma cytoskeleton could be responsible for altered morphology in TgTKL4-deficient parasites. Additionally, loss of TgTKL4 resulted in attenuation of virulence in the animal model, suggesting that TgTKL4 is an important virulence factor. Hence, this study provides a novel insight into the importance of a TgTKL4 as a fitness-determining factor for Toxoplasma propagation in vitro and pathogenesis in vivo.

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

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

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.

Aptamer-Based Technologies for Parasite Detection

Centuries of scientific breakthroughs have brought us closer to understanding and managing the spread of parasitic diseases. Despite ongoing technological advancements in the detection, treatment, and control of parasitic illnesses, their effects on animal and human health remain a major concern worldwide. Aptamers are single-stranded oligonucleotides whose unique three-dimensional structures enable them to interact with high specificity and affinity to a wide range of targets. In recent decades, aptamers have emerged as attractive alternatives to antibodies as therapeutic and diagnostic agents. Due to their superior stability, reusability, and modifiability, aptamers have proven to be effective bioreceptors for the detection of toxins, contaminants, biomarkers, whole cells, pathogens, and others. As such, they have been integrated into a variety of electrochemical, fluorescence, and optical biosensors to effectively detect whole parasites and their proteins. This review offers a summary of the various types of parasite-specific aptamer-based biosensors, their general mechanisms and their performance.

An image-based high-content screening for compounds targeting Toxoplasma gondii repurposed inhibitors effective against the malaria parasite Plasmodium falciparum

Apicomplexa phylum includes numerous obligate intracellular protozoan parasites that are life threatening for humans and animals. In this context, Plasmodium falciparum and Toxoplasma gondii are of particular interest, as they are responsible for malaria and toxoplasmosis, respectively, for which efficient vaccines are presently lacking and therapies need to be improved. Apicomplexan parasites have a highly polarized morphology, with their apical end containing specific secretory organelles named rhoptries and micronemes, which depend on the unique receptor and transporter sortilin TgSORT for their biogenesis. In the present study, we took advantage of the subcellular polarity of the parasite to engineer a clonal transgenic Toxoplasma line that expresses simultaneously the green fluorescent protein TgSORT-GFP in the post-Golgi-endosome-like compartment and the red fluorescent protein rhoptry ROP1-mCherry near the apical end. We utilized this fluorescent transgenic T. gondii to develop a miniaturized image-based phenotype assay coupled to an automated image analysis. By applying this methodology to 1,120 compounds, we identified 12 that are capable of disrupting the T. gondii morphology and inhibiting intracellular replication. Analysis of the selected compounds confirmed that all 12 are kinase inhibitors and intramembrane pumps, with some exhibiting potent activity against Plasmodium falciparum. Our findings highlight the advantage of comparative and targeted phenotypic analysis involving two related parasite species as a means of identifying molecules with a conserved mode of action.

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.

Enhancement of immune responses by vaccine potential of three antigens, including ROP18, MIC4, and SAG1 against acute toxoplasmosis in mice

Toxoplasma gondii (T. gondii) causes considerable financial losses in the livestock industry and can present serious threats to pregnant women, as well as immunocompromised patients. Therefore, it is required to design and produce an efficient vaccine for controlling toxoplasmosis. The present study aimed to evaluate the protective immunity induced by RMS protein (ROP18, MIC4, and SAG1) with Freund adjuvant, calcium phosphate nanoparticles (CaPNs), and chitosan nanoparticles (CNs) in BALB/c mice. The RMS protein was expressed in Escherichia coli (E. coli) and purified using a HisTrap HP column. Thereafter, cellular and humoral immunity was assessed by injecting RMS protein on days 0, 21, and 35 into four groups [RMS, RMS-chitosan nanoparticles (RMS-CNs), RMS-calcium phosphate nanoparticles (RMS-CaPNs), and RMS-Freund]. Phosphate buffered saline (PBS), CNs, CaPNs, and Freund served as the four control groups. The results displayed that vaccination with RMS protein and adjuvants significantly elicited the levels of specific IgG antibodies and cytokines against toxoplasmosis. There were high levels of total IgG, IgG2a, and IFN-γ in vaccinated mice, compared to those in the control groups, especially in the RMS-Freund, indicating a Th-1 type response. The vaccinated and control mice were challenged intraperitoneally with 1 × 10³ tachyzoites of the T. gondii RH strain four weeks after the last injection, and in RMS-Freund and RMS-CaPNs groups, the highest increase in survival time was observed (15 days). The RMS can significantly increase Th1 and Th2 responses; moreover, multi-epitope vaccines with adjuvants can be a promising strategy for the production of a vaccine against toxoplasmosis.

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.

Interferon-Inducible E3 Ligase RNF213 Facilitates Host-Protective Linear and K63-Linked Ubiquitylation of Toxoplasma gondii Parasitophorous Vacuoles

The obligate intracellular protozoan pathogen Toxoplasma gondii infects a wide range of vertebrate hosts and frequently causes zoonotic infections in humans. Whereas infected immunocompetent individuals typically remain asymptomatic, toxoplasmosis in immunocompromised individuals can manifest as a severe, potentially lethal disease, and congenital Toxoplasma infections are associated with adverse pregnancy outcomes. The protective immune response of healthy individuals involves the production of lymphocyte-derived cytokines such as interferon gamma (IFN-γ), which elicits cell-autonomous immunity in host cells. IFN-γ-inducible antiparasitic defense programs comprise nutritional immunity, the production of noxious gases, and the ubiquitylation of the Toxoplasma-containing parasitophorous vacuole (PV). PV ubiquitylation prompts the recruitment of host defense proteins to the PV and the consequential execution of antimicrobial effector programs, which reduce parasitic burden. However, the ubiquitin E3 ligase orchestrating these events has remained unknown. Here, we demonstrate that the IFN-γ-inducible E3 ligase RNF213 translocates to Toxoplasma PVs and facilitates PV ubiquitylation in human cells. Toxoplasma PVs become decorated with linear and K63-linked ubiquitin and recruit ubiquitin adaptor proteins in a process that is RNF213 dependent but independent of the linear ubiquitin chain assembly complex (LUBAC). IFN-γ priming fails to restrict Toxoplasma growth in cells lacking RNF213 expression, thus identifying RNF213 as a potent executioner of ubiquitylation-driven antiparasitic host defense. IMPORTANCE Globally, approximately one out of three people become infected with the obligate intracellular parasite Toxoplasma. These infections are typically asymptomatic but can cause severe disease and mortality in immunocompromised individuals. Infections can also be passed on from mother to fetus during pregnancy, potentially causing miscarriage or stillbirth. Therefore, toxoplasmosis constitutes a substantial public health burden. A better understanding of mechanisms by which healthy individuals control Toxoplasma infections could provide roadmaps toward novel therapies for vulnerable groups. Our work reveals a fundamental mechanism controlling intracellular Toxoplasma infections. Cytokines produced during Toxoplasma infections instruct human cells to produce the enzyme RNF213. We find that RNF213 labels intracellular vacuoles containing Toxoplasma with the small protein ubiquitin, which functions as an "eat-me" signal, attracting antimicrobial defense programs to fight off infection. Our work therefore identified a novel antiparasitic protein orchestrating a central aspect of the human immune response to Toxoplasma.

Toxoplasma gondii rhoptry protein (TgROP18) enhances the expression of pro-inflammatory factor in LPS/IFN-γ-induced murine BV2 microglia cells via NF-κB signal pathway

Toxoplasma gondii, an opportunistic pathogenic protozoan, exhibits a strong predilection to infect the brain, causing severe neurological diseases, such as toxoplasmic encephalitis (TE), in immunocompromised patients. Microglia, the resident immune cells in the brain, is reported to play important roles in regulating the neuroinflammation mediated by T. gondii infection. Here we demonstrated that the tachyzoites of T. gondii RH strain could significantly upregulate the expression levels of microglial M1 phenotype markers including IL-1β, IL-6, TNF-α, iNOS and IL18 in activated murine BV2 microglia cells, which were regulated by T. gondii rhoptry protein 18 (TgROP18). Moreover, we found that TgROP18 could enhance the expression of M1 phenotype markers in activated murine BV2 microglia cells via activating NF-κB signal pathway. Additionally, TgROP18 was suggested to interact with the host p65 in activated murine BV2 microglia cells and induce the phosphorylation of p65 at S536. In summary, the present study demonstrated that TgROP18 could promote the activated microglia to polarize to M1 phenotype and enhanced the expression of pro-inflammatory factors via activating NF-κB signal pathway, which could contribute to elucidating the mechanism underlying the neuroinflammation mediated by activated microglia in the brain with T. gondii infection.

IFN-γ stimulated murine and human neurons mount anti-parasitic defenses against the intracellular parasite Toxoplasma gondii

Dogma holds that Toxoplasma gondii persists in neurons because neurons cannot clear intracellular parasites, even with IFN-γ stimulation. As several recent studies questioned this idea, here we use primary murine neuronal cultures from wild type and transgenic mice in combination with IFN-γ stimulation and parental and transgenic parasites to reassess IFN-γ dependent neuronal clearance of intracellular parasites. We find that neurons respond to IFN-γ and that a subset of neurons clear intracellular parasites via immunity regulated GTPases. Whole neuron reconstructions from mice infected with parasites that trigger neuron GFP expression only after full invasion reveal that ~50% of these T. gondii-invaded neurons no longer harbor parasites. Finally, IFN-γ stimulated human pluripotent stem cell derived neurons show an ~50% decrease in parasite infection rate when compared to unstimulated cultures. This work highlights the capability of human and murine neurons to mount cytokine-dependent anti-T. gondii defense mechanisms in vitro and in vivo.

Screening the Toxoplasma kinome with high-throughput tagging identifies a regulator of invasion and egress

Protein kinases regulate fundamental aspects of eukaryotic cell biology, making them attractive chemotherapeutic targets in parasites like Plasmodium spp. and Toxoplasma gondii. To systematically examine the parasite kinome, we developed a high-throughput tagging (HiT) strategy to endogenously label protein kinases with an auxin-inducible degron and fluorophore. Hundreds of tagging vectors were assembled from synthetic sequences in a single reaction and used to generate pools of mutants to determine localization and function. Examining 1,160 arrayed clones, we assigned 40 protein localizations and associated 15 kinases with distinct defects. The fitness of tagged alleles was also measured by pooled screening, distinguishing delayed from acute phenotypes. A previously unstudied kinase, associated with a delayed phenotype, was shown to be a regulator of invasion and egress. We named the kinase Store Potentiating/Activating Regulatory Kinase (SPARK), based on its impact on intracellular Ca2+ stores. Despite homology to mammalian 3-phosphoinositide-dependent protein kinase-1 (PDK1), SPARK lacks a lipid-binding domain, suggesting a rewiring of the pathway in parasites. HiT screening extends genome-wide approaches into complex cellular phenotypes, providing a scalable and versatile platform to dissect parasite biology.

Disruption of Toxoplasma gondii-Induced Host Cell DNA Replication Is Dependent on Contact Inhibition and Host Cell Type

The protozoan Toxoplasma gondii is a highly successful obligate intracellular parasite that, upon invasion of its host cell, releases an array of host-modulating protein effectors to counter host defenses and further its own replication and dissemination. Early studies investigating the impact of T. gondii infection on host cell function revealed that this parasite can force normally quiescent cells to activate their cell cycle program. Prior reports by two independent groups identified the dense granule protein effector HCE1/TEEGR as being solely responsible for driving host cell transcriptional changes through its direct interaction with the cyclin E regulatory complex DP1 and associated transcription factors. Our group independently identified HCE1/TEEGR through the presence of distinct repeated regions found in a number of host nuclear targeted parasite effectors and verified its central role in initiating host cell cycle changes. Additionally, we report here the time-resolved kinetics of host cell cycle transition in response to HCE1/TEEGR, using the fluorescence ubiquitination cell cycle indicator reporter line (FUCCI), and reveal the existence of a block in S-phase progression and host DNA synthesis in several cell lines commonly used in the study of T. gondii. Importantly, we have observed that this S-phase block is not due to additional dense granule effectors but rather is dependent on the host cell line background and contact inhibition status of the host monolayer in vitro. This work highlights intriguing differences in the host response to reprogramming by the parasite and raises interesting questions regarding how parasite effectors differentially manipulate the host cell depending on the in vitro or in vivo context.

IMPORTANCEToxoplasma gondii chronically infects approximately one-third of the global population and can produce severe pathology in immunologically immature or compromised individuals. During infection, this parasite releases numerous host-targeted effector proteins that can dramatically alter the expression of a variety of host genes. A better understanding of parasite effectors and their host targets has the potential to not only provide ways to control infection but also inform us about our own basic biology. One host pathway that has been known to be altered by T. gondii infection is the cell cycle, and prior reports have identified a parasite effector, known as HCE1/TEEGR, as being responsible. In this report, we further our understanding of the kinetics of cell cycle transition induced by this effector and show that the capacity of HCE1/TEEGR to induce host cell DNA synthesis is dependent on both the cell type and the status of contact inhibition.

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.

Comparative Phosphoproteomic Analysis of Sporulated Oocysts and Tachyzoites of Toxoplasma gondii Reveals Stage-Specific Patterns

Toxoplasma gondii is an obligate intracellular protozoan of severe threat to humans and livestock, whose life history harbors both gamic and apogamic stages. Chinese 1 (ToxoDB#9) was a preponderant genotype epidemic in food-derived animals and humans in China, with a different pathogenesis from the strains from the other nations of the world. Posttranslational modifications (PTMs) of proteins were critical mediators of the biology, developmental transforms, and pathogenesis of protozoan parasites. The phosphoprotein profiling and the difference between the developmental phases of T. gondii, contributing to development and infectivity, remain unknown. A quantitative phosphoproteomic approach using IBT integrated with TiO₂ affinity chromatography was applied to identify and analyze the difference in the phosphoproteomes between the sporulated oocysts and the tachyzoites of the virulent ToxoDB#9 (PYS) strain of T. gondii. A total of 4058 differential phosphopeptides, consisting of 2597 upregulated and 1461 downregulated phosphopeptides, were characterized between sporulated the oocysts and tachyzoites. Twenty-one motifs extracted from the upregulated phosphopeptides contained 19 serine motifs and 2 threonine motifs (GxxTP and TP), whereas 16 motifs identified from downregulated phosphopeptides included 13 serine motifs and 3 threonine motifs (KxxT, RxxT, and TP). Beyond the traditional kinases, some infrequent classes of kinases, including Ab1, EGFR, INSR, Jak, Src and Syk, were found to be corresponding to motifs from the upregulated and downregulated phosphopeptides. Remarkable functional properties of the differentially expressed phosphoproteins were discovered by GO analysis, KEGG pathway analysis, and STRING analysis. S8GFS8 (DNMT1-RFD domain-containing protein) and S8F5G5 (Histone kinase SNF1) were the two most connected peptides in the kinase-associated network. Out of these, phosphorylated modifications in histone kinase SNF1 have functioned in mitosis and interphase of T. gondii, as well as in the regulation of gene expression relevant to differentiation. Our study discovered a remarkable difference in the abundance of phosphopeptides between the sporulated oocysts and tachyzoites of the virulent ToxoDB#9 (PYS) strain of T. gondii, which may provide a new resource for understanding stage-specific differences in PTMs and may enhance the illustration of the regulatory mechanisms contributing to the development and infectivity of T. gondii.

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

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

The pathogenicity and virulence of Toxoplasma gondii

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

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.