Innate resistance against Toxoplasma gondii: an evolutionary tale of mice, cats, and men

Toxoplasma gondii chitinase-like protein TgCLP1 regulates the parasite cyst burden

Toxoplasma, an important intracellular parasite of humans and animals, causes life-threatening toxoplasmosis in immunocompromised individuals. Although Toxoplasma secretory proteins during acute infection (tachyzoite, which divides rapidly and causes inflammation) have been extensively characterized, those involved in chronic infection (bradyzoite, which divides slowly and is surrounded by a cyst wall) remain uncertain. Regulation of the cyst wall is essential to the parasite life cycle, and polysaccharides, such as chitin, in the cyst wall are necessary to sustain latent infection. Toxoplasma secretory proteins during the bradyzoite stage may have important roles in regulating the cyst wall via polysaccharides. Here, we focused on characterizing the hypothetical T. gondii chitinase, chitinase-like protein 1 (TgCLP1). We found that the chitinase-like domain containing TgCLP1 is partially present in the bradyzoite microneme and confirmed, albeit partially, its previous identification in the tachyzoite microneme. Furthermore, although parasites lacking TgCLP1 could convert from tachyzoites to bradyzoites and make an intact cyst wall, they failed to convert from bradyzoites to tachyzoites, indicating that TgCLP1 is necessary for bradyzoite reactivation. Taken together, our findings deepen our understanding of the molecular basis of recrudescence and could contribute to the development of novel strategies for the control of toxoplasmosis.

Twelve toll-like receptor (TLR) genes in the family Equidae - comparative genomics, selection and evolution

Toll-like receptors (TLRs) represent an important part of the innate immune system. While human and murine TLRs have been intensively studied, little is known about TLRs in non-model species. The order Perissodactyla comprises a variety of free-living and domesticated species exposed to different pathogens in different habitats and is therefore suitable for analyzing the diversity and evolution of immunity-related genes. We analyzed TLR genes in the order Perissodactyla with a focus on the family Equidae. Twelve TLRs were identified by bioinformatic analyses of online genomic resources; their sequences were confirmed in equids by genomic DNA re-sequencing of a panel of nine species. The expression of TLR11 and TLR12 was confirmed in the domestic horse by cDNA sequencing. Phylogenetic reconstruction of the TLR gene family in Perissodactyla identified six sub-families. TLR4 clustered together with TLR5; the TLR1-6-10 subfamily showed a high degree of sequence identity. The average estimated evolutionary divergence of all twelve TLRs studied was 0.3% among the Equidae; the most divergent CDS were those of Equus caballus and Equus hemionus kulan (1.34%) in the TLR3, and Equus africanus somaliensis and Equus quagga antiquorum (2.1%) in the TLR1 protein. In each TLR gene, there were haplotypes shared between equid species, most extensively in TLR3 and TLR9 CDS, and TLR6 amino acid sequence. All twelve TLR genes were under strong negative overall selection. Signatures of diversifying selection in specific codon sites were detected in all TLRs except TLR8. Differences in the selection patterns between virus-sensing and non-viral TLRs were observed.

The IRAK1/IRF5 axis initiates IL-12 response by dendritic cells and control of Toxoplasma gondii infection

Activation of endosomal Toll-like receptor (TLR) 7, TLR9, and TLR11/12 is a key event in the resistance against the parasite Toxoplasma gondii. Endosomal TLR engagement leads to expression of interleukin (IL)-12 via the myddosome, a protein complex containing MyD88 and IL-1 receptor-associated kinase (IRAK) 4 in addition to IRAK1 or IRAK2. In murine macrophages, IRAK2 is essential for IL-12 production via endosomal TLRs but, surprisingly, Irak2-/- mice are only slightly susceptible to T. gondii infection, similar to Irak1-/- mice. Here, we report that upon T. gondii infection IL-12 production by different cell populations requires either IRAK1 or IRAK2, with conventional dendritic cells (DCs) requiring IRAK1 and monocyte-derived DCs (MO-DCs) requiring IRAK2. In both populations, we identify interferon regulatory factor 5 as the main transcription factor driving the myddosome-dependent IL-12 production during T. gondii infection. Consistent with a redundant role of DCs and MO-DCs, mutations that affect IL-12 production in both cell populations show high susceptibility to infection in vivo.

Genome-wide and targeted CRISPR screens identify RNF213 as a mediator of interferon gamma-dependent pathogen restriction in human cells

To define cellular immunity to the intracellular pathogen Toxoplasma gondii, we performed a genome-wide CRISPR loss-of-function screen to identify genes important for (interferon gamma) IFN-γ-dependent growth restriction. We revealed a role for the tumor suppressor NF2/Merlin for maximum induction of Interferon Stimulated Genes (ISG), which are positively regulated by the transcription factor IRF-1. We then performed an ISG-targeted CRISPR screen that identified the host E3 ubiquitin ligase RNF213 as necessary for IFN-γ-mediated control of T. gondii in multiple human cell types. RNF213 was also important for control of bacterial (Mycobacterium tuberculosis) and viral (Vesicular Stomatitis Virus) pathogens in human cells. RNF213-mediated ubiquitination of the parasitophorous vacuole membrane (PVM) led to growth restriction of T. gondii in response to IFN-γ. Moreover, overexpression of RNF213 in naive cells also impaired growth of T. gondii. Surprisingly, growth inhibition did not require the autophagy protein ATG5, indicating that RNF213 initiates restriction independent of a previously described noncanonical autophagy pathway. Mutational analysis revealed that the ATPase domain of RNF213 was required for its recruitment to the PVM, while loss of a critical histidine in the RZ finger domain resulted in partial reduction of recruitment to the PVM and complete loss of ubiquitination. Both RNF213 mutants lost the ability to restrict growth of T. gondii, indicating that both recruitment and ubiquitination are required. Collectively, our findings establish RNF213 as a critical component of cell-autonomous immunity that is both necessary and sufficient for control of intracellular pathogens in human cells.

Toxoplasma protein export and effector function

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

A 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 goats and sheep from the Northeast Region of Brazil destined for human consumption

This study aimed to genotype isolates of Toxoplasma gondii obtained from samples of brain, diaphragm and heart of goats and sheep intended for human consumption in the State of Paraíba, Brazil. Tissue samples from 14 animals, goats (n = 5) and lambs (n = 9), were sourced from public slaughterhouses in seven cities and bio-assayed in mice. The brains of the mice were utilized for DNA extraction. Genotyping was carried out by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) using 10 markers (SAG1, SAG2, SAG3, BTUB, c22-8, PK1, GRA6, L358, c-29-2 and Apico). A total of 10 isolates were fully genotyped (i.e. at all loci), three from goats and seven from sheep, revealing five distinct genotypes: #13 (n = 4); #48 (n = 3); #57 (n = 1); #273 (n = 1); and one new genotype that had not been previously described. Genotype #13 is frequently found in the Northeast of Brazil and represents a clonal lineage circulating in this region and was the most prevalent genotype identified (n = 4). Moreover, in the present study genotypes #13, #48, #57, and #273 were documented for the first time in sheep from Brazil, and the novel genotype was isolated from a goat. Our findings align with previous studies on T. gondii from Brazil, where new genotypes are continuously being identified, highlighting a high level of genetic diversity of T. gondii isolates in the country.

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.

Characterization of Neospora caninum virulence factors NcGRA7 and NcROP40 in bovine target cells

Bovine neosporosis is one of the major causes of reproductive failure in cattle worldwide, and differences in virulence between isolates have been widely shown. However, the molecular basis and mechanisms underlying virulence in Neospora caninum are mostly unknown. Recently, we demonstrated the involvement of NcGRA7 and NcROP40 in the virulence of N. caninum in a pregnant murine model using single knockout mutants in these genes generated by CRISR/Cas9 technology. In this study, the role of these proteins was investigated in two in vitro models using bovine target cells: trophoblast (F3 cell line) and monocyte-derived macrophages (BoMØ). The proliferation capacity of the single knockout mutant parasites was compared to the wild-type strain, the Nc-Spain7 isolate, using both cell populations. For the bovine trophoblast, no differences were observed in the growth of the defective parasites compared to the wild-type strain, neither in the proliferation kinetics nor in the competition assay. However, in naïve BoMØ, a significant decrease in the proliferation capacity of the mutant parasites was observed from 48 h pi onwards. Stimulation of BoMØ with IFN-γ showed a similar inhibition of tachyzoite growth in defective and wild-type strains in a dose-dependent manner. Finally, BoMØ infected with knockout parasites showed higher expression levels of TLR3, which is involved in pathogen recognition. These results suggest that NcGRA7 and NcROP40 may be involved in the manipulation of innate immune defense mechanisms against neosporosis and confirm the usefulness of the BoMØ model for the evaluation of N. caninum virulence mechanisms. However, the specific functions of these proteins remain unknown, opening the way for future research.

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

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

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

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

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

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

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.

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

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

Vaccination of squirrel monkeys (Saimiri spp.) with nanoparticle based-Toxoplasma gondii antigens: new hope for captive susceptible species

Squirrel monkeys (Saimiri spp.), new world primates from South America, are very susceptible to toxoplasmosis. Numerous outbreaks of fatal toxoplasmosis in zoos have been identified around the world, resulting in acute respiratory distress and sudden death. To date, preventive hygiene measures or available treatments are not able to significantly reduce this mortality in zoos. Therefore, vaccination seems to be the best long-term solution to control acute toxoplasmosis. Recently, we developed a nasal vaccine composed of total extract of soluble proteins of Toxoplasma gondii associated with muco-adhesive maltodextrin-nanoparticles. The vaccine, which generated specific cellular immune responses, demonstrated efficacy against toxoplasmosis in murine and ovine experimental models. In collaboration with six French zoos, our vaccine was used as a last resort in 48 squirrel monkeys to prevent toxoplasmosis. The full protocol of vaccination includes two intranasal sprays followed by combined intranasal and s.c. administration. No local or systemic side-effects were observed irrespective of the route of administration. Blood samples were collected to study systemic humoral and cellular immune responses up to 1 year after the last vaccination. Vaccination induced a strong and lasting systemic cellular immune response mediated by specific IFN-γ secretion by peripheral blood mononuclear cells. Since the introduction of vaccination, no deaths of squirrel monkeys due to T. gondii has been observed for more than 4 years suggesting the promising usage of our vaccine. Moreover, to explain the high susceptibility of naive squirrel monkeys to toxoplasmosis, their innate immune sensors were investigated. It was observed that Toll-like and Nod-like receptors appear to be functional following T. gondii recognition suggesting that the extreme susceptibility to toxoplasmosis may not be linked to innate detection of the parasite.

Inflammasome Activation Dampens Type I IFN Signaling to Strengthen Anti-Toxoplasma Immunity

Innate immunity acts as the first line of defense against pathogen invasion. During Toxoplasma gondii infection, multiple innate immune sensors are activated by invading microbes or pathogen-associated molecular patterns (PAMPs). However, how inflammasome is activated and its regulatory mechanisms during T. gondii infection remain elusive. Here, we showed that the infection of PRU, a lethal type II T. gondii strain, activates inflammasome at the early stage of infection. PRU tachyzoites, RNA and soluble tachyzoite antigen (STAg) mainly triggered the NLRP3 inflammasome, while PRU genomic DNA (gDNA) specially activated the AIM2 inflammasome. Furthermore, mice deficient in AIM2, NLRP3, or caspase-1/11 were more susceptible to T. gondii PRU infection, and the ablation of inflammasome signaling impaired antitoxoplasmosis immune responses by enhancing type I interferon (IFN-I) production. Blockage of IFN-I receptor fulfilled inflammasome-deficient mice competent immune responses as WT mice. Moreover, we have identified that the suppressor of cytokine signaling 1 (SOCS1) is a key negative regulator induced by inflammasome-activated IL-1β signaling and inhibits IFN-I production by targeting interferon regulatory factor 3 (IRF3). In general, our study defines a novel protective role of inflammasome activation during toxoplasmosis and identifies a critical regulatory mechanism of the cross talk between inflammasome and IFN-I signaling for understanding infectious diseases. IMPORTANCE As a key component of innate immunity, inflammasome is critical for host antitoxoplasmosis immunity, but the underlying mechanisms are still elusive. In this study, we found that inflammasome signaling was activated by PAMPs of T. gondii, which generated a protective immunity against T. gondii invasion by suppressing type I interferon (IFN-I) production. Mechanically, inflammasome-coupled IL-1β signaling triggered the expression of negative regulator SOCS1, which bound to IRF3 to inhibit IFN-I production. The role of IFN-I in anti-T. gondii immunity is little studied and controversial, and here we also found IFN-I is harmful to host antitoxoplasmosis immunity by using knockout mice and recombinant proteins. In general, our study identifies a protective role of inflammasomes to the host during T. gondii infection and a novel mechanism by which inflammasome suppresses IFN-I signaling in antitoxoplasmosis immunity, which will likely provide new insights into therapeutic targets for toxoplasmosis and highlight the cross talk between innate immune signaling in infectious diseases prevention.

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.

Immune responses to Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that can cause severe complications in the newborn and immunocompromised individuals. The parasite evokes a strong innate immune response in the infected hosts which is followed by a robust adaptive immunity. In the last few years, importance of innate immune mechanisms dependent on the role of MyD-88 independent pathways, inflammatory monocytes and innate lymphocyte have been identified. However, notwithstanding the strong immune response to the parasite, the chronic infection persists in the host. The inability to prevent chronic infection can be attributed to aberration in the memory CD8 T cell response caused by an increased expression of inhibitory receptors that leads to their dysfunctionality.

Epigenetic Reprogramming in Host-Parasite Coevolution: The Toxoplasma Paradigm

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

Neuroimmunology of Common Parasitic Infections in Africa

Parasitic infections of the central nervous system are an important cause of morbidity and mortality in Africa. The neurological, cognitive, and psychiatric sequelae of these infections result from a complex interplay between the parasites and the host inflammatory response. Here we review some of the diseases caused by selected parasitic organisms known to infect the nervous system including Plasmodium falciparum, Toxoplasma gondii, Trypanosoma brucei spp., and Taenia solium species. For each parasite, we describe the geographical distribution, prevalence, life cycle, and typical clinical symptoms of infection and pathogenesis. We pay particular attention to how the parasites infect the brain and the interaction between each organism and the host immune system. We describe how an understanding of these processes may guide optimal diagnostic and therapeutic strategies to treat these disorders. Finally, we highlight current gaps in our understanding of disease pathophysiology and call for increased interrogation of these often-neglected disorders of the nervous system.

TNFα rs1799964 TT genotype may be a susceptibility factor for vertical transmission of Toxoplasma gondii and clinical signs in newborns from pregnant women with acute toxoplasmosis

BACKGROUND

One of the main impacts of Toxoplasma gondii infection occurs during pregnancy and is related to the vertical transmission of the parasite (congenital toxoplasmosis), which can cause severe clinical outcomes and fetal death. During acute infection, in order to control the rapid replication of tachyzoites, different host immune response genes are activated, and these include cytokine-encoding genes. Considering that polymorphisms in cytokine genes may increase susceptibility to vertical transmission of T. gondii by determining the immune status of the pregnant woman, this study evaluated the influence of polymorphisms of tumor necrosis factor alpha (TNFα) rs1799964 (- 1031) and interleukin 1 beta (IL1β) rs16944 (- 511) genes on gestational toxoplasmosis and on the vertical transmission of the parasite and verified the allele and genotype frequency of these polymorphisms in pregnant patients whose respective newborn did or did not present clinical abnormalities suggestive of congenital toxoplasmosis.

METHODS AND RESULTS

A total of 204 pregnant patients with (n = 114) or without (n = 90) infection by T. gondii were enrolled. No associations were found involving the polymorphisms rs1799964 (- 1031) of the TNFα gene and rs16944 (- 511) of the IL1β gene with the increased chance of T. gondii infection during pregnancy. However, it was observed that the maternal TT genotype referring to the polymorphism of the TNFα gene seems to influence the vertical transmission of the parasite (P = 0.01; χ² = 6.05) and the presence of clinical manifestation in newborns from pregnancies with acute toxoplasmosis (P = 0.007; χ² = 9.68).

CONCLUSION

The TNFα rs1799964 TT genotype may act as a susceptibility factor for the vertical transmission of parasite and for the presence of clinical signs in newborns from pregnant women with acute toxoplasmosis.

Over-expression screen of interferon-stimulated genes identifies RARRES3 as a restrictor of Toxoplasma gondii infection

Toxoplasma gondii is an important human pathogen infecting an estimated one in three people worldwide. The cytokine interferon gamma (IFNγ) is induced during infection and is critical for restricting T. gondii growth in human cells. Growth restriction is presumed to be due to the induction of interferon-stimulated genes (ISGs) that are upregulated to protect the host from infection. Although there are hundreds of ISGs induced by IFNγ, their individual roles in restricting parasite growth in human cells remain somewhat elusive. To address this deficiency, we screened a library of 414 IFNγ induced ISGs to identify factors that impact T. gondii infection in human cells. In addition to IRF1, which likely acts through the induction of numerous downstream genes, we identified RARRES3 as a single factor that restricts T. gondii infection by inducing premature egress of the parasite in multiple human cell lines. Overall, while we successfully identified a novel IFNγ induced factor restricting T. gondii infection, the limited number of ISGs capable of restricting T. gondii infection when individually expressed suggests that IFNγ-mediated immunity to T. gondii infection is a complex, multifactorial process.

High resistance to Toxoplasma gondii infection in inducible nitric oxide synthase knockout rats

Nitric oxide (NO) is an important immune molecule that acts against extracellular and intracellular pathogens in most hosts. However, after the knockout of inducible nitric oxide synthase (iNOS^−/−) in Sprague Dawley (SD) rats, these iNOS^−/− rats were found to be completely resistant to Toxoplasma gondii infection. Once the iNOS^−/− rat peritoneal macrophages (PMs) were infected with T. gondii, they produced high levels of reactive oxygen species (ROS) triggered by GRA43 secreted by T. gondii, which damaged the parasitophorous vacuole membrane and PM mitochondrial membranes within a few hours post-infection. Further evidence indicated that the high levels of ROS caused mitochondrial superoxide dismutase 2 depletion and induced PM pyroptosis and cell death. This discovery of complete resistance to T. gondii infection, in the iNOS^−/−-SD rat, demonstrates a strong link between NO and ROS in immunity to T. gondii infection and showcases a potentially novel and effective backup innate immunity system.

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iNOS^−/−-SD rats show strong resistance to Toxoplasma gondii infection

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iNOS^−/−-SD rat PMs resist T. gondii infection through ROS upregulation

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The T. gondii infection results in PM pyroptosis in iNOS^−/−-SD rats

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GRAs play a key role in the activation of resistance in iNOS^−/−-SD rat PMs

Toxoplasma gondii-Induced Neutrophil Extracellular Traps Amplify the Innate and Adaptive Response

Toxoplasmosis affects one-third of the human population worldwide. Humans are accidental hosts and are infected after consumption of undercooked meat and water contaminated with Toxoplasma gondii cysts and oocysts, respectively. Neutrophils have been shown to participate in the control of T. gondii infection in mice through a variety of effector mechanisms, such as reactive oxygen species (ROS) and neutrophil extracellular trap (NET) formation. However, few studies have demonstrated the role of neutrophils in individuals naturally infected with T. gondii. In the current study, we evaluated the activation status of neutrophils in individuals with acute or chronic toxoplasmosis and determined the role of T. gondii-induced NET formation in the amplification of the innate and adaptive immune responses. We observed that neutrophils are highly activated during acute infection through increased expression of CD66b. Moreover, neutrophils from healthy donors (HDs) cocultured with tachyzoites produced ROS and formed NETs, with the latter being dependent on glycolysis, succinate dehydrogenase, gasdermin D, and neutrophil elastase. Furthermore, we observed elevated levels of the chemokines (CXC motif) CXCL8 and (CC motif) CCL4 ligands in plasma from patients with acute toxoplasmosis and production by neutrophils from HDs exposed to T. gondii. Finally, we showed that T. gondii-induced NETs activate neutrophils and promote the recruitment of autologous CD4⁺ T cells and the production of interferon gamma (IFN-γ), tumor necrosis factor (TNF), interleukin 6 (IL-6), IL-17, and IL-10 by peripheral blood mononuclear cells. In conclusion, we demonstrated that T. gondii activates neutrophils and promotes the release of NETs, which amplify human innate and adaptive immune responses.

Murine Irgm Paralogs Regulate Nonredundant Functions To Execute Host Defense to Toxoplasma gondii

Gamma interferon (IFN-γ)-induced immunity-related GTPases (IRGs) confer cell-autonomous immunity to the intracellular protozoan pathogen Toxoplasma gondii. Effector IRGs are loaded onto the Toxoplasma-containing parasitophorous vacuole (PV), where they recruit ubiquitin ligases, ubiquitin-binding proteins, and IFN-γ-inducible guanylate-binding proteins (Gbps), prompting PV lysis and parasite destruction. Host cells lacking the regulatory IRGs Irgm1 and Irgm3 fail to load effector IRGs, ubiquitin, and Gbps onto the PV and are consequently defective for cell-autonomous immunity to Toxoplasma. However, the role of the third regulatory IRG, Irgm2, in cell-autonomous immunity to Toxoplasma has remained unexplored. Here, we report that Irgm2 unexpectedly plays a limited role in the targeting of effector IRGs, ubiquitin, and Gbps to the Toxoplasma PV. Instead, Irgm2 is instrumental in the decoration of PVs with γ-aminobutyric acid receptor-associated protein-like 2 (GabarapL2). Cells lacking Irgm2 are as defective for cell-autonomous host defense to Toxoplasma as pan-Irgm-/- cells lacking all three Irgm proteins, and Irgm2-/- mice succumb to Toxoplasma infections as readily as pan-Irgm-/- mice. These findings demonstrate that, relative to Irgm1 and Irgm3, Irgm2 plays a distinct but critically important role in host resistance to Toxoplasma.

Induction of IL-12p40 and type 1 immunity by Toxoplasma gondii in the absence of the TLR-MyD88 signaling cascade

Toxoplasma gondii is an orally acquired pathogen that induces strong IFN-γ based immunity conferring protection but that can also be the cause of immunopathology. The response in mice is driven in part by well-characterized MyD88-dependent signaling pathways. Here we focus on induction of less well understood immune responses that do not involve this Toll-like receptor (TLR)/IL-1 family receptor adaptor molecule, in particular as they occur in the intestinal mucosa. Using eYFP-IL-12p40 reporter mice on an MyD88^-/- background, we identified dendritic cells, macrophages, and neutrophils as cellular sources of MyD88-independent IL-12 after peroral T. gondii infection. Infection-induced IL-12 was lower in the absence of MyD88, but was still clearly above noninfected levels. Overall, this carried through to the IFN-γ response, which while generally decreased was still remarkably robust in the absence of MyD88. In the latter mice, IL-12 was strictly required to induce type I immunity. Type 1 and type 3 innate lymphoid cells (ILC), CD4⁺ T cells, and CD8⁺ T cells each contributed to the IFN-γ pool. We report that ILC3 were expanded in infected MyD88^-/- mice relative to their MyD88^+/+ counterparts, suggesting a compensatory response triggered by loss of MyD88. Furthermore, bacterial flagellin and Toxoplasma specific CD4⁺ T cell populations in the lamina propria expanded in response to infection in both WT and KO mice. Finally, we show that My88-independent IL-12 and T cell mediated IFN-γ production require the presence of the intestinal microbiota. Our results identify MyD88-independent intestinal immune pathways induced by T. gondii including myeloid cell derived IL-12 production, downstream type I immunity and IFN-γ production by ILC1, ILC3, and T lymphocytes. Collectively, our data reveal an underlying network of immune responses that do not involve signaling through MyD88.

Toxoplasma gondii is an apicomplexan parasite estimated to infect 30–50% of humans worldwide. The parasite normally establishes latency in brain and muscle tissue marked by persistent asymptomatic infection. T. gondii masterfully strikes a balance between eliciting strong, anti-parasite immunity while also persisting in the host. Although the murine host recognizes Toxoplasma profilin via MyD88 and Toll-like receptors 11/12, humans lack these receptors and MyD88 deficient patients retain resistance to T. gondii infection. Given these observations, it is important to identify MyD88 independent pathways of immunity. Using an oral infection mouse model, we identified cellular sources of IL-12 and IFN-γ, two cytokines that are essential for host resistance to this microbial pathogen. We determined how these responses are impacted by the presence and absence of MyD88 and the intestinal microbiota. Our data demonstrate that T. gondii triggers MyD88-independent innate and adaptive immunity in the intestinal mucosa that requires the presence of intestinal microbes. These pathways may be conserved among species and understanding how they work in rodents will likely help determine how humans recognize and respond to T. gondii infection.

Secreted Effectors Modulating Immune Responses to Toxoplasma gondii

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

Toxoplasma gondii Infection in Swine: Implications for Public Health

Toxoplasmosis, due to Toxoplasma gondii, is a parasitic disease with global importance. Among livestock, chronic T. gondii infection has been reported in higher rates in pigs and small ruminants, but with subclinical infections in case commonly encountered in pigs. Seroprevalence in the global pig population ranges according to the age or species of pigs, geographical distribution, production programs, and systems. Generally, T. gondii infections are noticed in low prevalence rates in conventional pig farms with high hygiene standards. In contrast, higher prevalence is common on free-ranging farms, outdoor or backyard small pig fams, as well as in farmed or hunted wild boars. The T. gondii average worldwide seroprevalence in pigs is reported to be 13% in Europe, 21% in Africa, 25% in North America, 21% in Asia, and 23% in South America. Human toxoplasmosis outbreaks have been correlated with the consumption of raw or undercooked meat, especially from infected pigs or wild boars, as well as of contaminated drinking water. The risk of infection in processed pork products is lower compared with fresh pork, as meat processing can reduce or inactivate T. gondii tissue cysts. Hence, the prevalence of T. gondii in the pig population may be a useful indicator of the risk of human toxoplasmosis associated with the consumption of pork products. The lack of obligatory screening methods at farm level for the detection of antibodies in farmed animals or the viable T. gondii in carcasses at slaughterhouse level increases the risk of contaminated pork or meat products. For this reason, the application of biosecurity and surveillance programs at farm level is very important to prevent a T. gondii infection.

The developmental trajectories of Toxoplasma stem from an elaborate epigenetic rewiring

Toxoplasma gondii is considered to be one of the most successful parasitic pathogens. It owes this success to its flexibility in responding to signals emanating from the different environments it encounters during its multihost life cycle. The adaptability of this unicellular organism relies on highly coordinated and evolutionarily optimized developmental abilities that allow it to adopt the forms best suited to the requirements of each environment. Here we discuss recent outstanding studies that have uncovered how master regulators epigenetically regulate the cryptic process of sexual development and the transition to chronicity. We also highlight the molecular and technical advances that allow the field to embark on a new journey of epigenetic reprogramming of T. gondii development.

The role of IL-12 in stimulating NK cells against Toxoplasma gondii infection: a mini-review

Toxoplasma gondii is an intracellular protozoan parasite that can remarkably infect, survive, and replicate in almost all mammalian cells and can cause severe neurological and ocular damage in immunocompromised individuals. It is known that Natural Killer cells (NK cells), as a type of cytotoxic lymphocyte, have critical protective roles in innate immunity during the T. gondii infection through releasing interferon gamma (IFN-γ). Interleukin 12 (IL-12) is a pivotal critical cytokine for the generation of IFN-γ-producing NK cells. Several studies have shown cytokines' impact on NK cell activation; and IL-2 has an important role with a potent stimulatory factor for NK cells. In this review, we summarized the mechanism of interleukin-12 production stimulation by T. gondii tachyzoites and discussed several factors affecting this mechanism.

Dinoflagellate symbionts escape vomocytosis by host cell immune suppression

Alveolata comprises diverse taxa of single-celled eukaryotes, many of which are renowned for their ability to live inside animal cells. Notable examples are apicomplexan parasites and dinoflagellate symbionts, the latter of which power coral reef ecosystems. Although functionally distinct, they evolved from a common, free-living ancestor and must evade their host's immune response for persistence. Both the initial cellular events that gave rise to this intracellular lifestyle and the role of host immune modulation in coral-dinoflagellate endosymbiosis are poorly understood. Here, we use a comparative approach in the cnidarian endosymbiosis model Aiptasia, which re-establishes endosymbiosis with free-living dinoflagellates every generation. We find that uptake of microalgae is largely indiscriminate, but non-symbiotic microalgae are expelled by vomocytosis, while symbionts induce host cell innate immune suppression and form a lysosomal-associated membrane protein 1-positive niche. We demonstrate that exogenous immune stimulation results in symbiont expulsion and, conversely, inhibition of canonical Toll-like receptor signalling enhances infection of host animals. Our findings indicate that symbiosis establishment is dictated by local innate immune suppression, to circumvent expulsion and promote niche formation. This work provides insight into the evolution of the cellular immune response and key steps involved in mediating endosymbiotic interactions.

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.

Early immune responses and parasite tissue distribution in mice experimentally infected with oocysts of either archetypal or non-archetypal genotypes of Toxoplasma gondii

In most of the world Toxoplasma gondii is comprised of archetypal types (types I, II and III); however, South America displays several non-archetypal strains. This study used an experimental mouse model to characterize the immune response and parasite kinetics following infection with different parasite genotypes. An oral inoculation of 50 oocysts per mouse from T. gondii M4 type II (archetypal, avirulent), BrI or BrIII (non-archetypal, virulent and intermediate virulent, respectively) for groups (G)2, G3 and G4, respectively was used. The levels of mRNA expression of cytokines, immune compounds, cell surface markers and receptor adapters [interferon gamma (IFNγ), interleukin (IL)-12, CD8, CD4, CD25, CXCR3 and MyD88] were quantified by SYBR green reverse transcription-quantitative polymerase chain reaction. Lesions were characterized by histology and detection by immunohistochemistry established distribution of parasites. Infection in G2 mice was mild and characterized by an early MyD88-dependent pathway. In G3, there were high levels of expression of pro-inflammatory cytokines IFNγ and IL-12 in the mice showing severe clinical symptoms at 8–11 days post infection (dpi), combined with the upregulation of CD25, abundant tachyzoites and tissue lesions in livers, lungs and intestines. Significant longer expression of IFNγ and IL-12 genes, with other Th1-balanced immune responses, such as increased levels of CXCR3 and MyD88 in G4, resulted in survival of mice and chronic toxoplasmosis, with the occurrence of tissue cysts in brain and lungs, at 14 and 21 dpi. Different immune responses and kinetics of gene expression appear to be elicited by the different strains and non-archetypal parasites demonstrated higher virulence.

Transcriptomics analysis of Toxoplasma gondii-infected mouse macrophages reveals coding and noncoding signatures in the presence and absence of MyD88

BACKGROUND

Toxoplasma gondii is a globally distributed protozoan parasite that establishes life-long asymptomatic infection in humans, often emerging as a life-threatening opportunistic pathogen during immunodeficiency. As an intracellular microbe, Toxoplasma establishes an intimate relationship with its host cell from the outset of infection. Macrophages are targets of infection and they are important in early innate immunity and possibly parasite dissemination throughout the host. Here, we employ an RNA-sequencing approach to identify host and parasite transcriptional responses during infection of mouse bone marrow-derived macrophages (BMDM). We incorporated into our analysis infection with the high virulence Type I RH strain and the low virulence Type II strain PTG. Because the well-known TLR-MyD88 signaling axis is likely of less importance in humans, we examined transcriptional responses in both MyD88+/+ and MyD88-/- BMDM. Long noncoding (lnc) RNA molecules are emerging as key regulators in infection and immunity, and were, therefore, included in our analysis.

RESULTS

We found significantly more host genes were differentially expressed in response to the highly virulent RH strain rather than with the less virulent PTG strain (335 versus 74 protein coding genes for RH and PTG, respectively). Enriched in these protein coding genes were subsets associated with the immune response as well as cell adhesion and migration. We identified 249 and 83 non-coding RNAs as differentially expressed during infection with RH and PTG strains, respectively. Although the majority of these are of unknown function, one conserved lncRNA termed mir17hg encodes the mir17 microRNA gene cluster that has been implicated in down-regulating host cell apoptosis during T. gondii infection. Only a minimal number of transcripts were differentially expressed between MyD88 knockout and wild type cells. However, several immune genes were among the differences. While transcripts for parasite secretory proteins were amongst the most highly expressed T. gondii genes during infection, no differentially expressed parasite genes were identified when comparing infection in MyD88 knockout and wild type host BMDM.

CONCLUSIONS

The large dataset presented here lays the groundwork for continued studies on both the MyD88-independent immune response and the function of lncRNAs during Toxoplasma gondii infection.

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

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

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.

ISG15 Connects Autophagy and IFN-γ-Dependent Control of Toxoplasma gondii Infection in Human Cells

The intracellular protozoan parasite Toxoplasma gondii is capable of infecting most nucleated cells, where it survives in a specially modified compartment called the parasitophorous vacuole (PV). Interferon gamma (IFN-γ) is the major cytokine involved in activating cell-autonomous immune responses to inhibit parasite growth within this intracellular niche. In HeLa cells, IFN-γ treatment leads to ubiquitination of susceptible parasite strains, recruitment of the adaptors p62 and NDP52, and engulfment in microtubule-associated protein 1 light chain 3 (LC3)-positive membranes that restrict parasite growth. IFN-γ-mediated growth restriction depends on core members of the autophagy (ATG) pathway but not the initiation or degradative steps in the process. To explore the connection between these different pathways, we used permissive biotin ligation to identify proteins that interact with ATG5 in an IFN-γ-dependent fashion. Network analysis of the ATG5 interactome identified interferon-stimulated gene 15 (ISG15), which is highly upregulated by IFN treatment, as a hub connecting the ATG complex with other IFN-γ-induced genes, suggesting that it forms a functional link between the pathways. Deletion of ISG15 resulted in impaired recruitment of p62, NDP52, and LC3 to the PV and loss of IFN-γ-restricted parasite growth. The function of ISG15 required conjugation, and a number of ISGylated targets overlapped with the IFN-γ-dependent ATG5 interactome, including the adapter p62. Collectively, our findings establish a role for ISG15 in connecting the ATG pathway with IFN-γ-dependent restriction of T. gondii in human cells.IMPORTANCE Interferon(s) provide the primary defense against intracellular pathogens, a property ascribed to their ability to upregulate interferon-stimulated genes. Due to the sequestered niche occupied by Toxoplasma gondii, the host has elaborated intricate ways to target the parasite within its vacuole. One such mechanism is the recognition by a noncanonical autophagy pathway that envelops the parasite-containing vacuole and stunts growth in human cells. Remarkably, autophagy-dependent growth restriction requires interferon-γ, yet none of the classical components of autophagy are induced by interferon. Our studies draw a connection between these pathways by demonstrating that the antiviral protein ISG15, which is normally upregulated by interferons, links the autophagy-mediated control to ubiquitination of the vacuole. These findings suggest a similar link between interferon-γ signaling and autophagy that may underlie defense against other intracellular pathogens.

The zebrafish as a novel model for the in vivo study of Toxoplasma gondii replication and interaction with macrophages

Toxoplasma gondii is an obligate intracellular parasite capable of invading any nucleated cell. Three main clonal lineages (type I, II, III) exist and murine models have driven the understanding of general and strain-specific immune mechanisms underlying Toxoplasma infection. However, murine models are limited for studying parasite-leukocyte interactions in vivo, and discrepancies exist between cellular immune responses observed in mouse versus human cells. Here, we developed a zebrafish infection model to study the innate immune response to Toxoplasma in vivo By infecting the zebrafish hindbrain ventricle, and using high-resolution microscopy techniques coupled with computer vision-driven automated image analysis, we reveal that Toxoplasma invades brain cells and replicates inside a parasitophorous vacuole to which type I and III parasites recruit host cell mitochondria. We also show that type II and III strains maintain a higher infectious burden than type I strains. To understand how parasites are cleared in vivo, we further analyzed Toxoplasma-macrophage interactions using time-lapse microscopy and three-dimensional correlative light and electron microscopy (3D CLEM). Time-lapse microscopy revealed that macrophages are recruited to the infection site and play a key role in Toxoplasma control. High-resolution 3D CLEM revealed parasitophorous vacuole breakage in brain cells and macrophages in vivo, suggesting that cell-intrinsic mechanisms may be used to destroy the intracellular niche of tachyzoites. Together, our results demonstrate in vivo control of Toxoplasma by macrophages, and highlight the possibility that zebrafish may be further exploited as a novel model system for discoveries within the field of parasite immunity.This article has an associated First Person interview with the first author of the paper.

C5aR1 Activation Drives Early IFN-γ Production to Control Experimental Toxoplasma gondii Infection

Toxoplasma gondii (T. gondii) is a parasite infecting animals and humans. In intermediate hosts, such as humans or rodents, rapidly replicating tachyzoites drive vigorous innate and adaptive immune responses resulting in bradyzoites that survive within tissue cysts. Activation of the innate immune system is critical during the early phase of infection to limit pathogen growth and to instruct parasite-specific adaptive immunity. In rodents, dendritic cells (DCs) sense T. gondii through TLR11/12, leading to IL-12 production, which activates NK cells to produce IFN-γ as an essential mechanism for early parasite control. Further, C3 can bind to T. gondii resulting in limited complement activation. Here, we determined the role of C5a/C5aR1 axis activation for the early innate immune response in a mouse model of peritoneal T. gondii infection. We found that C5ar1^−/− animals suffered from significantly higher weight loss, disease severity, mortality, and parasite burden in the brain than wild type control animals. Severe infection in C5ar1^−/− mice was associated with diminished serum concentrations of IL-12, IL-27, and IFN-γ. Importantly, the serum levels of pro-inflammatory cytokines, including IL-1α, IL-6, and TNF-α, as well as several CXC and CC chemokines, were decreased in comparison to wt animals, whereas anti-inflammatory IL-10 was elevated. The defect in IFN-γ production was associated with diminished Ifng mRNA expression in the spleen and the brain, reduced frequency of IFN-γ⁺ NK cells in the spleen, and decreased Nos2 expression in the brain of C5ar1^−/− mice. Mechanistically, DCs from the spleen of C5ar1^−/− mice produced significantly less IL-12 in response to soluble tachyzoite antigen (STAg) stimulation in vivo and in vitro. Our findings suggest a model in which the C5a/C5aR1 axis promotes IL-12 induction in splenic DCs that is critical for IFN-γ production from NK cells and subsequent iNOS expression in the brain as a critical mechanism to control acute T. gondii infection.

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

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

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

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

Toxoplasma gondii is a protozoan parasite that infects over 1 billion people worldwide. Infection with the parasite is normally asymptomatic and Toxoplasma co-exists with its host in the form of latent cysts in brain and muscle tissue. The balance between immune recognition and immune evasion is likely a key factor in the outcome of this host-parasite interaction. It is therefore important to understand how Toxoplasma triggers immunity, and in particular how the protective cytokine IL-12 is induced during infection. While Toll-like receptor (TLR)/MyD88 signaling is important in mouse resistance to Toxoplasma, this pathway is likely less important in human infection. Here, we report that the parasite dense granule protein GRA24 triggers p38 MAPK activation and IL-12 production independently of TLR/MyD88 signaling. We identify additional cytokines and chemokines that are regulated by GRA24 during in vivo infection. Our data demonstrate that GRA24 initiates a protective MyD88-independent immune response during in vivo infection. The GRA24 molecule provides an example of a parasite molecule whose function is induction of a host protective immune response. From the standpoint of Toxoplasma, this likely reflects an evolutionary adaptation to ensure host survival and simultaneously enable latency to maximize the chance of transmission.

Immediate Interferon Gamma Induction Determines Murine Host Compatibility Differences between Toxoplasma gondii and Neospora caninum

Rodents are critical for the transmission of Toxoplasma gondii to the definitive feline host via predation, and this relationship has been extensively studied as a model for immune responses to parasites. Neospora caninum is a closely related coccidian parasite of ruminants and canines but is not naturally transmitted by rodents. We compared mouse innate immune responses to N. caninum and T. gondii and found marked differences in cytokine levels and parasite growth kinetics during the first 24 h postinfection (hpi). N. caninum-infected mice produced significantly higher levels of interleukin-12 (IL-12) and interferon gamma (IFN-γ) by as early as 4 hpi, but the level of IFN-γ was significantly lower or undetectable in T. gondii-infected mice during the first 24 hpi. "Immediate" IFN-γ and IL-12p40 production was not detected in MyD88^-/- mice. However, unlike IL-12p40^-/- and IFN-γ^-/- mice, MyD88^-/- mice survived N. caninum infections at the dose used in this study. Serial measures of parasite burden showed that MyD88^-/- mice were more susceptible to N. caninum infections than wild-type (WT) mice, and control of parasite burdens correlated with a pulse of serum IFN-γ at 3 to 4 days postinfection in the absence of detectable IL-12. Immediate IFN-γ was partially dependent on the T. gondii mouse profilin receptor Toll-like receptor 11 (TLR11), but the ectopic expression of N. caninum profilin in T. gondii had no impact on early IFN-γ production or parasite proliferation. Our data indicate that T. gondii is capable of evading host detection during the first hours after infection, while N. caninum is not, and this is likely due to the early MyD88-dependent recognition of ligands other than profilin.

What a Difference 30 Years Makes! A Perspective on Changes in Research Methodologies Used to Study Toxoplasma gondii

Toxoplasma gondii is a remarkable species with a rich cell, developmental, and population biology. It is also sometimes responsible for serious disease in animals and humans and the stages responsible for such disease are relatively easy to study in vitro or in laboratory animal models. As a result of all this, Toxoplasma has become the subject of intense investigation over the last several decades, becoming a model organism for the study of the phylum of which it is a member, Apicomplexa. This has led to an ever-growing number of investigators applying an ever-expanding set of techniques to dissecting how Toxoplasma "ticks" and how it interacts with its many hosts. In this perspective piece I first wind back the clock 30 years and then trace the extraordinary pace of methodologies that have propelled the field forward to where we are today. In keeping with the theme of this collection, I focus almost exclusively on the parasite, rather than host side of the equation. I finish with a few thoughts about where the field might be headed-though if we have learned anything, the only sure prediction is that the pace of technological advance will surely continue to accelerate and the future will give us still undreamed of methods for taking apart (and then putting back together) this amazing organism with all its intricate biology. We have so far surely just scratched the surface.

Toxoplasma gondii induces extracellular traps release in cat neutrophils

Neutrophils respond differently to violations of the body's physiological barriers during infections. Extracellular traps comprise one of the mechanisms used by these cells to reduce the spread of pathogens to neighboring tissues, as well as ensure a high concentration of antimicrobial agents at the site of infection. To date, this innate defense mechanism has not been previously demonstrated in neutrophils of cats exposed to Toxoplasma gondii. The aim of this study was to characterize the in vitro release of neutrophil extracellular traps (NETs) when neutrophils isolated from cats were exposed to T. gondii. First, cellular viability was tested at different time points after parasite exposure. The production of reactive oxygen species (ROS) and lactate dehydrogenase and the amount of extracellular DNA were quantified. In addition, the number of parasites associated with neutrophils was determined, and the observed NETs formed were microscopically characterized. Results showed that (i) in culture, neutrophils isolated from cats presented diminished cellular viability after 4 h of incubation, and when neutrophils were incubated with T. gondii, they displayed cytotoxic effects after 3 h of interaction; (ii) neutrophils were able to release structures composed of DNA and histones, characterized as NETs under optical, immunofluorescence, and electron scanning microscopy, when stimulated with T. gondii; (iii) only 11.4% of neutrophils were able to discharge NETs during 3 h of incubation; however, it was observed through extracellular quantification of DNA that this small number of cells were able to display different behavior compared to a negative control (no parasite) group; (iv) significant differences in ROS production were observed in neutrophils exposed to T. gondii. In conclusion, our results showed that neutrophils isolated from cats exposed to T. gondii release structures composed of DNA and histones, similar to what has already been described in other neutrophil species infected with the parasite.

C57BL/6 mice immunized with synthetic peptides from Toxoplasma gondii surface and microneme immunodominant antigens are able to decrease parasite burden in the brain tissues

Toxoplasmosis is a disease caused by Toxoplasma gondii, an intracellular protozoan able to infect a wide range of hosts. The infection is particularly severe in immunocompromised patients or during pregnancy, circumstances in which the parasite could find a more favorable microenvironment to replicate and invade host tissues. The current treatment consists in toxic drugs for the patients, being not appropriate for the fetuses and immunodeficient patients. So far, there is a lack of available vaccine to prevent the disease. The present study aimed to evaluate the immune response induced by peptides derived from parasite immunodominant proteins from key components, as surface, rhoptry, microneme and dense granule antigens. A panel of eleven peptides was selected considering the highest scores for B cell epitope prediction by in silico analyses. The peptides were divided in groups, according to the parasite organelle locations, and used to immunize C57BL/6 mice. The animals were submitted to three doses of immunization and infected by 10 cysts of T. gondii ME49 strain. Blood samples were collected and used to measure the production of antibodies and cytokines, while the brains were collected to determine the parasite burden by quantitative polymerase chain reaction (qPCR). It was found that synthetic peptides from all targets were able to induce IgG synthesis in immunized mice, as well as to modulate the Th1/Th2 cytokine production, particularly the MIC and SRS groups, which presented the IFN-γ/IL-10 and TNF-α/IL-10 ratios 30 and 10 times higher, respectively, when compared with non-immunized group. Interestingly, the animals from MIC and SRS groups had significantly lower levels of T. gondii DNA in their brains. In summary, it can be concluded that peptides mainly from SRS and MIC parasite components constitute relevant targets to design vaccine candidates against parasite burden observed during chronic toxoplasmosis.

Treatment with Bumped Kinase Inhibitor 1294 Is Safe and Leads to Significant Protection against Abortion and Vertical Transmission in Sheep Experimentally Infected with Toxoplasma gondii during Pregnancy

Previous studies on drug efficacy showed low protection against abortion and vertical transmission of Toxoplasma gondii in pregnant sheep. Bumped kinase inhibitors (BKIs), which are ATP-competitive inhibitors of calcium-dependent protein kinase 1 (CDPK1), were shown to be highly efficacious against several apicomplexan parasites in vitro and in laboratory animal models. Here, we present the safety and efficacy of BKI-1294 treatment (dosed orally at 100 mg/kg of body weight 5 times every 48 h) initiated 48 h after oral infection of sheep at midpregnancy with 1,000 TgShSp1 oocysts. BKI-1294 demonstrated systemic exposure in pregnant ewes, with maximum plasma concentrations of 2 to 3 μM and trough concentrations of 0.4 μM at 48 h after each dose. Oral administration of BKI-1294 in uninfected sheep at midpregnancy was deemed safe, since there were no changes in behavior, fecal consistency, rectal temperatures, hematological and biochemical parameters, or fetal mortality/morbidity. In ewes infected with a T. gondii oocyst dose lethal for fetuses, BKI-1294 treatment led to a minor rectal temperature increase after infection and a decrease in fetal/lamb mortality of 71%. None of the lambs born alive in the treated group exhibited congenital encephalitis lesions, and vertical transmission was prevented in 53% of them. BKI-1294 treatment during infection led to strong interferon gamma production after cell stimulation in vitro and a low humoral immune response to soluble tachyzoite antigens but high levels of anti-SAG1 antibodies. The results demonstrate a proof of concept for the therapeutic use of BKI-1294 to protect ovine fetuses from T. gondii infection during pregnancy.

Indoleamine 2,3-Dioxygenase Activity During Acute Toxoplasmosis and the Suppressed T Cell Proliferation in Mice

Toxoplasma gondii (T. gondii) is an obligate intracellular parasite and belongs to the phylum Apicomplexa. T. gondii is of medical and veterinary importance, because T. gondii causes the parasitic disease toxoplasmosis. In human cells, the interferon-gamma inducible indoleamine 2,3-dioxygenase 1 (IDO1) is an antimicrobial effector mechanism that degrades tryptophan to kynurenine and thus limits pathogen proliferation in vitro. Furthermore, IDO is described to have immunosuppressive properties, e.g., regulatory T cell differentiation and T cell suppression in humans and mice. However, there is only little known about the role of IDO1 in mice during acute toxoplasmosis. To shed further light on the role of mIDO1 in vivo, we have used a specifically adjusted experimental model. Therein, we infected mIDO1-deficient (IDO^−/−) C57BL/6 mice and appropriate wild-type (WT) control mice with a high dose of T. gondii ME49 tachyozoites (type II strain) via the intraperitoneal route and compared the phenotype of IDO^−/− and WT mice during acute toxoplasmosis. During murine T. gondii infection, we found mIDO1 mRNA and mIDO1 protein, as well as mIDO1-mediated tryptophan degradation in lungs of WT mice. IDO^−/− mice show no tryptophan degradation in the lung during infection. Even though T. gondii is tryptophan auxotroph and rapidly replicates during acute infection, the parasite load was similar in IDO^−/− mice compared to WT mice 7 days post-infection. IDO1 is described to have immunosuppressive properties, and since T cell suppression is observed during acute toxoplasmosis, we analyzed the possible involvement of mIDO1. Here, we did not find differences in the intensity of ex vivo mitogen stimulated T cell proliferation between WT and IDO^−/− mice. Concomitant nitric oxide synthase inhibition and interleukin-2 supplementation increased the T cell proliferation from both genotypes drastically, but not completely. In sum, we analyzed the involvement of mIDO1 during acute murine toxoplasmosis in our specifically adjusted experimental model and found a definite mIDO1 induction. Nevertheless, mIDO1 seems to be functional redundant as an antiparasitic defense mechanism during acute toxoplasmosis in mice. Furthermore, we suggest that the systemic T cell suppression observed during acute toxoplasmosis is influenced by nitric oxide activity and IL-2 deprivation.