Toxoplasma gondii: 25 years and 25 major advances for the field

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.

Toxoplasma gondii Causes Adverse Pregnancy Outcomes by Damaging Uterine Tissue-Resident NK Cells That Secrete Growth-Promoting Factors

Vertical transmission of the intracellular parasite, Toxoplasma gondii can lead to adverse pregnancy outcomes especially when infection occurs in early pregnancy. Decidual natural killer (dNK) cells accumulate at the maternal-fetal interface in large numbers during early pregnancy. Their nutritional roles during infection with T. gondii remain poorly defined. In the present study, we demonstrated that a functional deficiency of the uterine tissue-resident NK (trNK) cells, a subset of dNK cells, contributes to the adverse pregnancy outcomes induced by T. gondii in early pregnancy. Adverse pregnancy outcomes could be ameliorated by adoptive transfer of trNK cells. Moreover, fetal growth restriction could be improved after supplementation of growth-promoting factors. In addition to the widely recognized disturbance of the immune balance at the interface between the mother and the fetus, our study reveals a novel mechanism in T. gondii that contributes to the adverse pregnancy outcomes.

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

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

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.

Toxoplasma IWS1 Determines Fitness in Interferon-γ-Activated Host Cells and Mice by Indirectly Regulating ROP18 mRNA Expression

Toxoplasma gondii secretes various virulence effector molecules into host cells to disrupt host interferon-γ (IFN-γ)-dependent immunity. Among these effectors, ROP18 directly phosphorylates and inactivates IFN-inducible GTPases, such as immunity-related GTPases (IRGs) and guanylate-binding proteins (GBPs), leading to the subversion of IFN-inducible GTPase-induced cell-autonomous immunity. The modes of action of ROP18 have been studied extensively; however, little is known about the molecular mechanisms by which ROP18 is produced in the parasite itself. Here, we report the role of T. gondii transcription factor IWS1 in ROP18 mRNA expression in the parasite. Compared with wild-type virulent type I T. gondii, IWS1-deficient parasites showed dramatically increased loading of IRGs and GBPs onto the parasitophorous vacuole membrane (PVM). Moreover, IWS1-deficient parasites displayed decreased virulence in wild-type mice but retained normal virulence in mice lacking the IFN-γ receptor. Furthermore, IWS1-deficient parasites showed severely decreased ROP18 mRNA expression; however, tagged IWS1 did not directly bind with genomic regions of the ROP18 locus. Ectopic expression of ROP18 in IWS1-deficient parasites restored the decreased loading of effectors onto the PVM and in vivo virulence in wild-type mice. Taken together, these data demonstrate that T. gondii IWS1 indirectly regulates ROP18 mRNA expression to determine fitness in IFN-γ-activated host cells and mice. IMPORTANCE The parasite Toxoplasma gondii has a counterdefense system against interferon-γ (IFN-γ)-dependent host immunity which relies on the secretion of parasite effector proteins. ROP18 is one of the effector, which is released into host cells to inactivate IFN-γ-dependent anti-Toxoplasma host proteins. The mechanism by which Toxoplasma ROP18 subverts host immunity has been extensively analyzed, but how Toxoplasma produces this virulence factor remains unclear. Here, we show that Toxoplasma transcription factor IWS1 is important for ROP18 mRNA expression in the parasite. Loss of IWS1 from virulent Toxoplasma leads to dramatically decreased ROP18 mRNA expression, resulting in profoundly decreased virulence due to greater activity of IFN-γ-dependent host immune responses. Thus, Toxoplasma prepares the critical virulence factor ROP18 via an IWS1-dependent system to negate IFN-γ-dependent antiparasitic immunity and thus survive in the host.

Toxoplasmosis in Human and Animals Around the World. Diagnosis and Perspectives in the One Health Approach

Toxoplasmosis is a unique health disease that significantly affects the health of humans, domestic animals, wildlife and is present in ecosystems, including water, soil and food. Toxoplasma gondii is one of the best-adapted parasites in the word. This parasite is able to persist for long periods in its hosts, in different geographic regions of the word. This review summarizes the current literature of these themes, focusing on: (1) toxoplasmosis, a zoonotic infection; (2) One health approach and toxoplasmosis; (3) human toxoplasmosis; (4) animal toxoplasmosis; (5) toxoplasmosis diagnosis, as immunological, parasitological and molecular diagnosis; (6) T. gondii outbreaks caused by infected meat, milk and dairy products, as well as, vegetables and water consume; (7) studies in experimental models; (8) genetic characterization of T. gondii strains; (9) extracellular vesicles and miRNA; and (10) future perspectives on T. gondii and toxoplasmosis. The vast prevalence of toxoplasmosis in both humans and animals and the dispersion and resistence of T. gondii parasites in environment highlight the importance of the one health approach in diagnostic and control of the disease. Here the different aspects of the one health approach are presented and discussed.

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

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

Structural basis of membrane recognition of Toxoplasma gondii vacuole by Irgb6

Upon Toxoplasma gondii infection, Irgb6 is recruited to the parasitophorous vacuole membrane (PVM) where Irgb6 disrupts it. We solved the atomic structures of Irgb6 in two distinct nucleotide states, revealing a unique PVM-binding interface sensitive to GTPase cycling.

The p47 immunity-related GTPase (IRG) Irgb6 plays a pioneering role in host defense against Toxoplasma gondii infection. Irgb6 is recruited to the parasitophorous vacuole membrane (PVM) formed by T. gondii and disrupts it. Despite the importance of this process, the molecular mechanisms accounting for PVM recognition by Irgb6 remain elusive because of lack of structural information on Irgb6. Here we report the crystal structures of mouse Irgb6 in the GTP-bound and nucleotide-free forms. Irgb6 exhibits a similar overall architecture to other IRGs in which GTP binding induces conformational changes in both the dimerization interface and the membrane-binding interface. The membrane-binding interface of Irgb6 assumes a unique conformation, composed of N- and C-terminal helical regions forming a phospholipid binding site. In silico docking of phospholipids further revealed membrane-binding residues that were validated through mutagenesis and cell-based assays. Collectively, these data demonstrate a novel structural basis for Irgb6 to recognize T. gondii PVM in a manner distinct from other IRGs.

Toxoplasma gondii serine hydrolases regulate parasite lipid mobilization during growth and replication within the host

The intracellular protozoan parasite Toxoplasma gondii must scavenge cholesterol and other lipids from the host to facilitate intracellular growth and replication. Enzymes responsible for neutral lipid synthesis have been identified but there is no evidence for enzymes that catalyze lipolysis of cholesterol esters and esterified lipids. Here, we characterize several T. gondii serine hydrolases with esterase and thioesterase activities that were previously thought to be depalmitoylating enzymes. We find they do not cleave palmitoyl thiol esters but rather hydrolyze short-chain lipid esters. Deletion of one of the hydrolases results in alterations in levels of multiple lipids species. We also identify small-molecule inhibitors of these hydrolases and show that treatment of parasites results in phenotypic defects reminiscent of parasites exposed to excess cholesterol or oleic acid. Together, these data characterize enzymes necessary for processing lipids critical for infection and highlight the potential for targeting parasite hydrolases for therapeutic applications.

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

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

Cytologic detection of Toxoplasma gondii in the cerebrospinal fluid of a dog and in vitro isolation of a unique mouse-virulent recombinant strain

Parasites resembling Neospora caninum or Toxoplasma gondii were detected by cytologic examination of cerebrospinal fluid (CSF) from a dog with neurologic disease. The dog became severely ill and was euthanized. Canine tissue homogenates were used for direct parasite isolation in cell culture, bioassay in 2 mouse lineages, and PCR. T. gondii was isolated in monkey kidney cells, and species identity was confirmed by PCR. Inoculated parasites were highly virulent for mice, which developed clinical signs and were euthanized immediately. PCR-RFLP for T. gondii using the cultured isolate (TgDgBA22) was conducted with 12 genetic markers, and a unique recombinant strain was identified. Detection of T. gondii by CSF cytology, although described in humans, had not been reported previously in dogs, to our knowledge, and was crucial for the diagnosis of toxoplasmosis in the examined dog.

Medial Amygdala Arginine Vasopressin Neurons Regulate Innate Aversion to Cat Odors in Male Mice

Aversion to environmental cues of predators is an integral part of defensive behaviors in many prey animals. It enhances their survival and probability of future reproduction. At the same time, animals cannot be maximally defended because imperatives of defense usually trade-off with behaviors required for sexual reproduction like display of dominance and production of sexual pheromones. Here, we approach this trade-off through the lens of arginine vasopressin (AVP) neurons within the posterodorsal medial amygdala (MePD) of mice. This neuronal population is known to be involved in sexual behaviors like approach to sexually salient cues. We show that chemogenetic partial ablation of this neuronal population increases aversion to predator odors. Moreover, overexpression of AVP within this population is sufficient to reduce aversion to predator odors. The loss of fear of the predator odor occurs in parallel with increased recruitment of AVP neurons within the MePD. These observations suggest that AVP neurons in the medial aspect of the extended amygdala are a proximate locus for the reduction in innate fear during life stages dominated by reproductive efforts.

Immunological, histopathological, and ultrastructural evidence of steroid-induced reactivation of chronic murine toxoplasmosis

We developed a model of steroid-induced reactivation of chronic murine toxoplasmosis to mirror similar effects of steroids or other immunosuppressants in infected humans. Immunological, histopathological, and ultrastructural parameters were reported. Prior to steroid administration, mice were infected with 10 cysts of the Me49 strain of Toxoplasma gondii. Mice were treated with dexamethasone (DXM, 2.5 mg/kg/day in drinking water), alone or combined with Solu-Cortef (SOLU, 50 mg/kg by subcutaneous injection 3 times a week) for 7 weeks or left untreated as control. Histopathological changes and ultrastructural effects of steroids on the course of chronic toxoplasmosis were recorded. By electron microscopy, the brains of infected combined treated mice showed an increase in number of tachyzoites and bradyzoites, degeneration, and necrosis of neural cells and hydropic degeneration besides the observed rupture of toxoplasma cysts releasing free tachyzoites in brain tissue. DXM+SOLU-combined treatment also significantly increased mortality, mean brain cyst count as compared to infected untreated mice (P = .01 and). Moreover, 3/12 (25%) treated animals developed clinical signs of toxoplasmic encephalitis. This simple model of drug-induced reactivation of chronic toxoplasmosis permits investigation of host-parasite interaction and may be used for the evaluation of chemotherapeutics in immunocompromised infected patients.

Decision by injection without infection

The protozoan pathogen Toxoplasma gondii co-opts host immunity by secreting various effector proteins into host cells. In this issue of JEM, Chen et al. (https://doi.org/10.1084/jem.20181757) report that T. gondii injects an effector called ROP16 without invasion to polarize macrophages toward M2 cells.

Production and characterization of monoclonal antibodies against Toxoplasma gondii ROP18 with strain-specific reactivity

The rhoptry kinase 18 of Toxoplasma gondii (TgROP18) has been identified as a key virulence factor that allows the parasite to escape from host immune defences and promotes its proliferation in host cells. Although much research is focused on the interaction between host cells and TgROP18, the development of monoclonal antibodies (mAbs) against TgROP18 has not been reported till date. To produce mAbs targeting TgROP18, two hybridomas secreting mAbs against TgROP18, designated as A1 and T2, were generated using cell fusion technology. The subtypes of the A1 and T2 mAbs were identified as IgG3 λ and IgM κ, and peptide scanning revealed that the core sequences of the antigenic epitopes were 180LRAQRRRSELVFE192 and 351NYFLLMMRAEADM363, respectively. The T2 mAb specifically reacted with both T. gondii type I and Chinese I, but not with T. gondii type II, Plasmodium falciparum or Schistosoma japonicum. Finally, the sequences of heavy chain and light chain complementarity-determining regions of T2 were amplified, cloned and characterized, making the modification of the mAb feasible in the future. The development of mAbs against TgROP18 would aid the investigation of the molecular mechanisms underlying the modulation of host cellular functions by TgROP18, and in the development of strategies to diagnose and treat Toxoplasmosis.

T cell-derived interferon-γ is required for host defense to Toxoplasma gondii

Interferon-γ (IFN-γ) is important for host defense against various intracellular organisms including a protozoan pathogen Toxoplasma gondii. Various immune cells are recently shown to produce IFN-γ in T. gondii infection, however, it remains elusive which cell types are important for anti-T. gondii host defense so far. Here we generate a new IFN-γ reporter "GREVEN" mouse line in which a fusion protein of Venus and NanoLuc to analyze IFN-γ producing cells during T. gondii infection and find that CD4⁺, CD8⁺, γδ T cells and natural killer cells express Venus in a time dependent manner. Furthermore, Lck-Cre/Ifng^fl/fl mice are highly susceptible to T. gondii infection. Taken together, our results demonstrate that T cell-derived IFN-γ plays an important role in anti-T. gondii host defense.

Host-parasite interaction associated with major mental illness

Clinical studies frequently report that patients with major mental illness such as schizophrenia and bipolar disorder have co-morbid physical conditions, suggesting that systemic alterations affecting both brain and peripheral tissues might underlie the disorders. Numerous studies have reported elevated levels of anti-Toxoplasma gondii (T. gondii) antibodies in patients with major mental illnesses, but the underlying mechanism was unclear. Using multidisciplinary epidemiological, cell biological, and gene expression profiling approaches, we report here multiple lines of evidence suggesting that a major mental illness-related susceptibility factor, Disrupted in schizophrenia (DISC1), is involved in host immune responses against T. gondii infection. Specifically, our cell biology and gene expression studies have revealed that DISC1 Leu607Phe variation, which changes DISC1 interaction with activating transcription factor 4 (ATF4), modifies gene expression patterns upon T. gondii infection. Our epidemiological data have also shown that DISC1 607 Phe/Phe genotype was associated with higher T. gondii antibody levels in sera. Although further studies are required, our study provides mechanistic insight into one of the few well-replicated serological observations in major mental illness.

A one health approach to vaccines against Toxoplasma gondii

Toxoplasmosis is a serious disease with global impact, now recognised as one of the most important food borne diseases worldwide and a major cause of production loss in livestock. A one health approach to develop a vaccination programme to tackle toxoplasmosis is an attractive and realistic prospect. Knowledge of disease epidemiology, parasite transmission routes and main risk groups has helped to target key host species and outcomes for a vaccine programme and these would be to prevent/reduce congenital disease in women and sheep; prevent/reduce T. gondii tissue cysts in food animal species and to prevent/reduce T. gondii oocyst shedding in cats. Most animals, including humans, develop good protective immunity following infection, involving cell mediated immune responses, which may explain why live vaccines are generally more effective to protect against T. gondii. Recent advances in our knowledge of parasite genetics and gene manipulation, strain variation, key antigenic epitopes, delivery systems and induction of immune responses are all contributing to the prospects of developing new vaccines which may be more widely applicable. A key area in progressing vaccine development is to devise standard vaccine efficacy models in relevant animal hosts and this is where a one health approach bringing together researchers across different disciplines can be of major benefit. The tools and technologies are in place to make a real impact in tackling toxoplasmosis using vaccination and it just requires a collective will to make it happen.

Toxoplasma Effector GRA15-Dependent Suppression of IFN-γ-Induced Antiparasitic Response in Human Neurons

Toxoplasma gondii is an important human and animal pathogen that causes life-threatening toxoplasmosis. The host immune system produces interferon-γ (IFN-γ) to inhibit T. gondii proliferation. IFN-γ-inducible indole-2,3-dioxygenase 1 (IDO1), which mediates tryptophan degradation, has a major role in anti-T. gondii immune responses in various human cells. In response to the host's immune system, T. gondii secretes many virulence molecules into the host cells to suppress IFN-γ-dependent antiparasitic immune responses. The GRA15-induced proparasitic mechanism for suppressing IDO1-dependent immune responses has previously been tested only in human hepatocyte and monocyte co-cultures. Thus, whether human cells other than hepatocytes contain this virulence mechanism remains unclear. Here, we show that the GRA15-dependent virulence mechanism for suppressing the IDO1-dependent anti-T. gondii response operates in human neuronal cell lines and primary human neurons. Analysis of various human cell lines revealed that IL-1β-induced iNOS-dependent reduction of IDO1 mRNA expression occurred in brain cell lines (A172; glioblastoma, IMR-32; neuroblastoma, and T98G; glioblastoma) and liver cell lines (Huh7 and HepG2), but not in other cell lines. Moreover, co-culturing type II T. gondii-infected THP-1 human monocytes with the brain cell lines inhibited the IDO1-mediated anti-T. gondii response in a GRA15-dependent manner. These data suggest that a GRA15-dependent virulence mechanism antagonizes the IDO1-dependent host immune response in human brain cells.

Structural basis of Toxoplasma gondii perforin-like protein 1 membrane interaction and activity during egress

Intracellular pathogens must egress from the host cell to continue their infectious cycle. Apicomplexans are a phylum of intracellular protozoans that have evolved members of the membrane attack complex and perforin (MACPF) family of pore forming proteins to disrupt cellular membranes for traversing cells during tissue migration or egress from a replicative vacuole following intracellular reproduction. Previous work showed that the apicomplexan Toxoplasma gondii secretes a perforin-like protein (TgPLP1) that contains a C-terminal Domain (CTD) which is necessary for efficient parasite egress. However, the structural basis for CTD membrane binding and egress competency remained unknown. Here, we present evidence that TgPLP1 CTD prefers binding lipids that are abundant in the inner leaflet of the lipid bilayer. Additionally, solving the high-resolution crystal structure of the TgPLP1 APCβ domain within the CTD reveals an unusual double-layered β-prism fold that resembles only one other protein of known structure. Three direct repeat sequences comprise subdomains, with each constituting a wall of the β-prism fold. One subdomain features a protruding hydrophobic loop with an exposed tryptophan at its tip. Spectrophotometric measurements of intrinsic tryptophan fluorescence are consistent with insertion of the hydrophobic loop into a target membrane. Using CRISPR/Cas9 gene editing we show that parasite strains bearing mutations in the hydrophobic loop, including alanine substitution of the tip tryptophan, are equally deficient in egress as a strain lacking TgPLP1 altogether. Taken together our findings suggest a crucial role for the hydrophobic loop in anchoring TgPLP1 to the membrane to support its cytolytic activity and egress function.

The intracellular parasite Toxoplasma gondii infects many hosts including humans. Infected people with a weak immune system can suffer severe disease when the parasite replicates uncontrolled via repeated cycles of cell invasion, intracellular growth, and exit, resulting in cell death. Previous studies showed that T. gondii encodes a pore-forming protein, TgPLP1, which contains an unusual domain that is crucial for efficient exit from both the parasite containing vacuole and the host cell. However, how TgPLP1 recognizes and binds to the appropriate membrane is unclear. Here we use a combination of biochemistry, structural biology, and parasitology to identify a preference of TgPLP1 for specific lipids and show that a loop within the structure of the membrane-binding domain inserts into the target membrane and is necessary for exit from the parasite containing vacuole. Our study sheds light into the determinants of membrane binding in TgPLP1 and may inform the overall mechanism of pore formation in similar systems.

Inducible Nitric Oxide Synthase Is a Key Host Factor for Toxoplasma GRA15-Dependent Disruption of the Gamma Interferon-Induced Antiparasitic Human Response

Although Toxoplasma virulence mechanisms targeting gamma interferon (IFN-γ)-induced cell-autonomous antiparasitic immunity have been extensively characterized in mice, the virulence mechanisms in humans remain uncertain, partly because cell-autonomous immune responses against Toxoplasma differ markedly between mice and humans. Despite the identification of inducible nitric oxide synthase (iNOS) as an anti-Toxoplasma host factor in mice, here we show that iNOS in humans is a pro-Toxoplasma host factor that promotes the growth of the parasite. The GRA15 Toxoplasma effector-dependent disarmament of IFN-γ-induced parasite growth inhibition was evident when parasite-infected monocytes were cocultured with hepatocytes. Interleukin-1β (IL-1β), produced from monocytes in a manner dependent on GRA15 and the host's NLRP3 inflammasome, combined with IFN-γ to strongly stimulate iNOS expression in hepatocytes; this dramatically reduced the levels of indole 2,3-dioxygenase 1 (IDO1), a critically important IFN-γ-inducible anti-Toxoplasma protein in humans, thus allowing parasite growth. Taking the data together, Toxoplasma utilizes human iNOS to antagonize IFN-γ-induced IDO1-mediated cell-autonomous immunity via its GRA15 virulence factor.IMPORTANCE Toxoplasma, an important intracellular parasite of humans and animals, causes life-threatening toxoplasmosis in immunocompromised individuals. Gamma interferon (IFN-γ) is produced in the host to inhibit the proliferation of this parasite and eventually cause its death. Unlike mouse disease models, which involve well-characterized virulence strategies that are used by Toxoplasma to suppress IFN-γ-dependent immunity, the strategies used by Toxoplasma in humans remain unclear. Here, we show that GRA15, a Toxoplasma effector protein, suppresses the IFN-γ-induced indole-2,3-dioxygenase 1-dependent antiparasite immune response in human cells. Because NLRP3-dependent production of IL-1β and nitric oxide (NO) in Toxoplasma-infected human cells is involved in the GRA15-dependent virulence mechanism, blocking NO or IL-1β production in the host could represent a novel therapeutic approach for treating human toxoplasmosis.

Toxoplasma Effector TgIST Targets Host IDO1 to Antagonize the IFN-γ-Induced Anti-parasitic Response in Human Cells

Toxoplasma gondii is an important human and animal pathogen that causes life-threatening toxoplasmosis. Interferon-γ (IFN-γ) is critical for anti-T. gondii cell-autonomous immunity in both humans and mice. To proliferate efficiently within the hosts, virulent strains of T. gondii can suppress IFN-γ-dependent immunity. During parasite infection, it is well-characterized that various virulence effectors are secreted to transcriptionally or post-translationally target IFN-γ-inducible GTPases, which are essential for anti-parasite responses in mice. However, the role of IFN-γ-inducible GTPases in anti-T. gondii responses in human cells is controversial since they are non-functional or absent in humans. Instead, IFN-γ-induced tryptophan degradation by indole-2,3-dioxygenase (IDO) is important for the anti-T. gondii human response. To date, the T. gondii virulent mechanism targeting IDO in human cells remains elusive. Here we show that although humans possess two IDO isozymes, IDO1 and IDO2, human cells of various origins require IDO1 but not IDO2 for IFN-γ-induced cell-autonomous immunity to T. gondii. T. gondii secretes an effector TgIST to inhibit IDO1 mRNA expression. Taken together, the data suggests that T. gondii possesses virulence programs operated by TgIST to antagonize IFN-γ-induced IDO1-mediated anti-parasite cell-autonomous immunity in human cells.

New approach to differentiate primary from latent Toxoplasma gondii abortion through immunoglobulin and DNA interpretation

BACKGROUND

Toxoplasma gondii is an acute or latent zoonotic abortifacient human protozoan. Women may be aborted due to recent or latent infection during pregnancy or order to flare up of the dormant bradyzoites to acute tachyzoites (latent opportunistic relapse).

AIMS

1) to validate the interpretation of IgM and IgG immunoglobulins seromonotoring with DNA comparative results in differentiating recent from latent T. gondii abortion.

METHOD

Blood with the corresponding placental or uterine wash samples were collected from 73 aborted Egyptian women from Cairo and Giza labour wards. Patients aborted in any of the phases (Ph-1, Ph-2, Ph-3 and Ph-4 were corresponding to abortion at the 1st, 2nd and 3rd trimesters plus females who gave birth with congenital anomalies), respectively. All aborted patients were assayed serologically by Enzyme Linked Immunosorbent Assay (ELISA) for IgM and IgG titers and the compatible DNA from placenta and uterine wash tissues by conventional Polymerase Chain Reaction (PCR) specific for T. gondii.

RESULTS

Sero-positive aborted women were 50.7% by ELISA versus 37% by PCR. Not all T. gondii sero-positive aborted women were having T. gondii DNA or harboring compatible placental T. gondii cysts. This denotes that immunoglobulins alone are insufficient criteria for confirming toxoplasma abortion.

CONCLUSION

Immunoglobulins with DNA comparative results can possibly differentiate recent from latent T. gondii abortion at higher precision. We recommend the need for routine monitoring of T. gondii i.e. (pre-, during and post-delivery).

Comparative studies of Toxoplasma gondii transcriptomes: insights into stage conversion based on gene expression profiling and alternative splicing

Background

Toxoplasma gondii is one of the most important apicomplexan parasites and infects one-third of the human population worldwide. Transformation between the tachyzoite and bradyzoite stages in the intermediate host is central to chronic infection and life-long risk. There have been some transcriptome studies on T. gondii; however, we are still early in our understanding of the kinds and levels of gene expression that occur during the conversion between stages.

Results

We used high-throughput RNA-sequencing data to assemble transcripts using genome-based and de novo strategies. The expression-level analysis of 6996 T. gondii genes showed that over half (3986) were significantly differentially expressed during stage conversion, whereas 2205 genes were upregulated, and 1778 genes were downregulated in tachyzoites compared with bradyzoites. Several important gene families were expressed at relatively high levels. Comprehensive functional annotation and gene ontology analysis revealed that stress response-related genes are important for survival of bradyzoites in immune-competent hosts. We compared Trinity-based de novo and genome-based strategies, and found that the de novo assembly strategy compensated for the defects of the genome-based strategy by filtering out several transcripts with low expression or those unannotated on the genome. We also found some inaccuracies in the ToxoDB gene models. In addition, our analysis revealed that alternative splicing can be differentially regulated in response to life-cycle change. In depth analysis revealed a 20-nt, AG-rich sequence, alternative splicing locus from alt_acceptor motif search in tachyzoite.

Conclusion

This study represents the first large-scale effort to sequence the transcriptome of bradyzoites from T. gondii tissue cysts. Our data provide a comparative view of the tachyzoite and bradyzoite transcriptomes to allow a more complete dissection of all the molecular regulation mechanisms during stage conversions. A better understanding of the processes regulating stage conversion may guide targeted interventions to disrupt the transmission of T. gondii.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2983-5) contains supplementary material, which is available to authorized users.

Estradiol Attenuates the Severity of Primary Toxoplasma gondii Infection-Induced Adverse Pregnancy Outcomes Through the Regulation of Tregs in a Dose-Dependent Manner

Estradiol (E2) plays a crucial and intricate role during pregnancy to mediate several aspects of the pregnancy process. A perplexing phenomenon in congenital toxoplasmosis is that the severity of Toxoplasma gondii (T. gondii)-mediated adverse pregnancy outcome is closely related with time of primary maternal infection during pregnancy. In this study, the results showed that T. gondii infection in early pregnancy was more likely to induce miscarriage in mice than in late pregnancy, which may be related to inflammation of the maternal–fetal interface. Meanwhile, the T. gondii infection-induced-apoptotic rate of Tregs was higher and the expression of programmed death-1 (PD-1) on Tregs was lower in early pregnancy than in late pregnancy. As the level of E2 in mouse serum gradually increased with the development of pregnancy, we proposed that E2 may contribute to the discrepancy of Tregs at different stages of pregnancy. Thus, we investigated in vitro and in vivo effects of E2 in regulating Tregs. We found that E2 in vitro could protect Tregs against apoptosis and upregulate the expression of PD-1 on Tregs in a dose-dependent manner through ERα. Likewise, the simulated mid-pregnancy level of E2 in nonpregnant mice also alleviated the T. gondii infection-induced apoptosis of Tregs and potentiated the PD-1 expression on Tregs. Therefore, in the pathogenesis of T. gondii-induced abnormal pregnancy, E2 helped maintain the immune balance and improve the pregnancy outcome through regulating Tregs. This finding illustrates the intricate working of hormone–immune system interaction in infection-induced abnormal pregnancy.

Identification of Antigenic and Immunogenic Proteins of Toxoplasma gondii in Human and Sheep by Immunoproteomics

BACKGROUND

Toxoplasmosis is a parasitic disease caused by the intracellular protozoan parasite, Toxoplasma gondii, which can infect humans and warm-blooded animals. This infection can lead to still birth and abortion among some susceptible hosts especially sheep and human in pregnancy. Development of a vaccine against T. gondii infection is very important-especially for use in immunocompromised patients, pregnant women, and sheep. Different antigens of T. gondii can be potential candidates for immunization. The aims of this study were to identify the immunodominant and antigenic proteins of T. gondii in sheep and man.

METHODS

Tachyzoites' proteins were separated by two-dimensional polyacrylamide gel electrophoresis (2-DE), and subjected to western blot analysis probed with T. gondii positive sera of sheep and human (Biotechnology Department of Pasteur Institute of Tehran, Iran, from April 2016 to March 2017). Finally, the immunoreactive proteins were identified by mass spectrometry (MALDI-TOF/MS and MS/MS) technique.

RESULTS

Five immunoreactive and antigenic proteins were recognized by Toxoplasma positive sera of human and sheep. These identified proteins were Enolase 2, rhoptry protein 4 (ROP4), dense granular protein 14 (GRA14), rhoptry protein 15 (ROP15) and rhoptry protein 9 (ROP9).

CONCLUSION

The identified immunodominant proteins have potential to be used as diagnostic antigens and as diagnostic markers of Toxoplasma infection in sheep and human.

Evaluation of Immune Responses Induced by GRA7 and ROP2 Genes by DNA Vaccine Cocktails Against Acute Toxoplasmosis in BALB/c Mice

BACKGROUND

The severe damages of toxoplasmosis clearly indicate the need for the development of a more effective vaccine. Immunization with plasmid DNA is a promising vaccination technique. Therefore, GRA7 plasmid was prepared to be used as a vaccine. The purpose of this study was evaluation of immunization with cocktail DNA vaccine including plasmids encoding Toxoplasma gondii ROP2 and GRA7 in BALB/c mice.

METHODS

In this study, 733 bp of GRA7 gene was cloned in pCDNA3.1 plasmid as an expression vector. The plasmids containing GRA7 and ROP2 genes were administered via IM according to immunized mice three times with a 3 week interval. For lymphocyte proliferation and cytokine assay, splenocytes of immunized mice were cultured for proliferation and cytokine assay. The other mice in each group were inoculated by the parasite and mortality of the mice was evaluated on a daily basis.

RESULTS

The cytokine assay results and lymphocyte proliferation response in cocktail DNA vaccines showed that IFN-γ levels were significantly higher than controls (p<0.05), whereas IL-4 expression level in BALB/c mice immunized with cocktail was lower than that in control groups and these results are confirmed by MTT test. Predominance of the levels of IgG2a over IgG1 was observed in sera of the immunized mice. Furthermore, IgG2a values in cocktail DNA vaccines pcGRA7 were significantly higher than control group (p<0.01). In contrast, IgG1 antibodies were similar between the two groups (p>0.5). So, survival time in the immune groups was significantly prolonged in comparison to control ones (p<0.01).

CONCLUSION

The immunized mice by DNA vaccine produce higher titration of IFNγ, indicated with Th1 response which is confirmed by high level of IgG2a. These data demonstrate that the cocktail GRA7/ROP2 is a potential vaccine candidate against toxoplasmosis.

Toxoplasma gondii-positive human sera recognise intracellular tachyzoites and bradyzoites with diverse patterns of immunoreactivity

Antibody detection assays have long been the first line test to confirm infection with the zoonotic parasite Toxoplasma gondii. However, challenges exist with serological diagnosis, especially distinguishing between acute, latent and reactivation disease states. The sensitivity and specificity of serological tests might be improved by testing for antibodies against parasite antigens other than those typically found on the parasite surface during the acute stage. To this end, we analysed the reactivity profile of human sera, identified as positive for anti-Toxoplasma gondii IgG in traditional assays, by indirect immunofluorescence reactivity to acute stage intracellular tachyzoites and in vitro-induced latent stage bradyzoites. The majority of anti-Toxoplasma gondii IgG positive sera recognised both intracellularly replicating tachyzoites and in vitro-induced bradyzoites with varying patterns of immune-reactivity. Furthermore, anti-bradyzoite antibodies were not detected in sera that were IgM-positive/IgG-negative. These results demonstrate that anti-Toxoplasma gondii-positive sera may contain antibodies to a variety of antigens in addition to those traditionally used in serological tests, and suggest the need for further investigations into the utility of anti-bradyzoite-specific antibodies to aid in diagnosis of Toxoplasma gondii infection.

A rare case of pulmonary toxoplasmosis in a patient with undifferentiated inflammatory arthritis on chronic methotrexate and corticosteroid therapy

Pulmonary toxoplasmosis is a serious pulmonary condition caused by the protozoan Toxoplasma gondii It typically affects immunocompromised patients presenting acutely with cough, fever, myalgias, arthralgias and lymphadenopathy, and chronically with persistent cough and dyspnoea. Because of its protean features, it can mimic many more common lung conditions in the immunocompromised patient, including atypical pneumonia, Pneumocystis pneumonia and interstitial lung disease. In this article, we present the case of a 55-year-old woman who presented to our hospital with persistent dyspnoea and cough, initially suspected to have an arthritis-related interstitial lung disease. She received a final diagnosis of pulmonary toxoplasmosis after lung biopsy demonstrated Toxoplasma cysts, later confirmed by serology. Treatment with trimethoprim-sulfamethoxazole resulted in significant improvement of her respiratory symptoms after 3 months.

Essential role for GABARAP autophagy proteins in interferon-inducible GTPase-mediated host defense

Mammalian autophagy-related 8 (Atg8) homologs consist of LC3 proteins and GABARAPs, all of which are known to be involved in canonical autophagy. In contrast, the roles of Atg8 homologs in noncanonical autophagic processes are not fully understood. Here we show a unique role of GABARAPs, in particular gamma-aminobutyric acid (GABA)-A-receptor-associated protein-like 2 (Gabarapl2; also known as Gate-16), in interferon-γ (IFN-γ)-mediated antimicrobial responses. Cells that lacked GABARAPs but not LC3 proteins and mice that lacked Gate-16 alone were defective in the IFN-γ-induced clearance of vacuolar pathogens such as Toxoplasma. Gate-16 but not LC3b specifically associated with the small GTPase ADP-ribosylation factor 1 (Arf1) to mediate uniform distribution of interferon-inducible GTPases. The lack of GABARAPs reduced Arf1 activation, which led to formation of interferon-inducible GTPase-containing aggregates and hampered recruitment of interferon-inducible GTPases to vacuolar pathogens. Thus, GABARAPs are uniquely required for antimicrobial host defense through cytosolic distribution of interferon-inducible GTPases.

Immuno-Efficacy of a T. gondii Secreted Protein with an Altered Thrombospondin Repeat (TgSPATR) As a Novel DNA Vaccine Candidate against Acute Toxoplasmosis in BALB/c Mice

Toxoplasma gondii (T.gondii) is distributed worldwide and infects most species of warm-blooded animals, including humans. Toxoplasmosis has serious consequences, especially in people with an impaired or immature immune system. Thus, an effective vaccine is urgently required. Secretory microneme proteins are essential for the adhesion and invasion of T. gondii. The gene encoding the microneme protein, T. gondii secreted protein with an altered thrombospondin repeat (TgSPATR), we constructed a recombinant eukaryotic plasmid, pVAX1-TgSPATR, as a DNA vaccine, injected it intramuscularly into BALB/c mice and evaluated the induced immune response. Lymphocyte proliferation assays, cytokine (IFN-γ, IL-2, IL-4, IL-10), and antibody determinations showed that mice immunized with pVAX1-TgSPATR produced humoral and mixed Th1/Th2 type cellular immune responses. The survival times of mice immunized with pVAX1-TgSPATR were also significantly prolonged (15.7 ± 1.42 days) compared with control groups, which died within 7 days of challenge (p < 0.05). The current study indicated that pVAX1-TgSPATR induce a T. gondii specific immune response and might be a promising vaccine candidate against toxoplasmosis. To the best of our knowledge, this is the first report to evaluate the immunoprotective value of TgSPATR against T. gondii.

A Human Proteome Array Approach to Identifying Key Host Proteins Targeted by Toxoplasma Kinase ROP18

Toxoplasma kinase ROP18 is a key molecule responsible for the virulence of Toxoplasma gondii; however, the mechanisms by which ROP18 exerts parasite virulence via interaction with host proteins remain limited to a small number of identified substrates. To identify a broader array of ROP18 substrates, we successfully purified bioactive mature ROP18 and used it to probe a human proteome array. Sixty eight new putative host targets were identified. Functional annotation analysis suggested that these proteins have a variety of functions, including metabolic process, kinase activity and phosphorylation, cell growth, apoptosis and cell death, and immunity, indicating a pleiotropic role of ROP18 kinase. Among these proteins, four candidates, p53, p38, UBE2N, and Smad1, were further validated. We demonstrated that ROP18 targets p53, p38, UBE2N, and Smad1 for degradation. Importantly, we demonstrated that ROP18 phosphorylates Smad1 Ser-187 to trigger its proteasome-dependent degradation. Further functional characterization of the substrates of ROP18 may enhance understanding of the pathogenesis of Toxoplasma infection and provide new therapeutic targets. Similar strategies could be used to identify novel host targets for other microbial kinases functioning at the pathogen-host interface.

Circadian transcription factor BMAL1 regulates innate immunity against select RNA viruses

BMAL1 (brain and muscle ARNT-like protein 1, also known as MOP3 or ARNT3) belongs to the family of the basic helix-loop-helix (bHLH)-PAS domain-containing transcription factors, and is a key component of the molecular oscillator that generates circadian rhythms. Here, we report that BMAL1-deficient cells are significantly more susceptible to infection by two major respiratory viruses of the Paramyxoviridae family, namely RSV and PIV3. Embryonic fibroblasts from Bmal1^-/- mice produced nearly 10-fold more progeny virus than their wild type controls. These results were supported by animal studies whereby pulmonary infection of RSV produced a more severe disease and morbidity in Bmal1^-/-mice. These results show that BMAL1 can regulate cellular innate immunity against specific RNA viruses.

Is Predominant Clonal Evolution a Common Evolutionary Adaptation to Parasitism in Pathogenic Parasitic Protozoa, Fungi, Bacteria, and Viruses?

We propose that predominant clonal evolution (PCE) in microbial pathogens be defined as restrained recombination on an evolutionary scale, with genetic exchange scarce enough to not break the prevalent pattern of clonal population structure. The main features of PCE are (1) strong linkage disequilibrium, (2) the widespread occurrence of stable genetic clusters blurred by occasional bouts of genetic exchange ('near-clades'), (3) the existence of a "clonality threshold", beyond which recombination is efficiently countered by PCE, and near-clades irreversibly diverge. We hypothesize that the PCE features are not mainly due to natural selection but also chiefly originate from in-built genetic properties of pathogens. We show that the PCE model obtains even in microbes that have been considered as 'highly recombining', such as Neisseria meningitidis, and that some clonality features are observed even in Plasmodium, which has been long described as panmictic. Lastly, we provide evidence that PCE features are also observed in viruses, taking into account their extremely fast genetic turnover. The PCE model provides a convenient population genetic framework for any kind of micropathogen. It makes it possible to describe convenient units of analysis (clones and near-clades) for all applied studies. Due to PCE features, these units of analysis are stable in space and time, and clearly delimited. The PCE model opens up the possibility of revisiting the problem of species definition in these organisms. We hypothesize that PCE constitutes a major evolutionary strategy for protozoa, fungi, bacteria, and viruses to adapt to parasitism.

Identification and characterization of an immunogenic antigen, enolase 2, among excretory/secretory antigens (ESA) of Toxoplasma gondii

An immunogenic protein, enolase 2, was identified among the secreted excretory/secretory antigens (ESAs) from Toxoplasma gondii strain RH using immunoproteomics based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Enolase 2 was cloned, sequenced, and heterologously expressed. BLAST analysis revealed 75-96% similarity with enolases from other parasites. Immunoblotting demonstrated good immunoreactivity of recombinant T. gondii enolase (Tg-enolase 2) to T. gondii-infected animal serum. Purified Tg-enolase 2 was found to catalyze dehydration of 2-phospho-d-glycerate to phosphoenolpyruvate. In vitro studies revealed maximal activity at pH 7.5 and 37 °C, and activity was inhibited by K(+), Ni(2+), Al(3+), Na(+), Cu(2+) and Cr(3+). A monoclonal antibody against Tg-enolase 2 was prepared, 1D6, with the isotype IgG2a/κ. Western blotting revealed that 1D6 reacts with Tg-enolase 2 and native enolase 2, present among T. gondii ESAs. The indirect immunofluorescence assays showed that enolase 2 could be specifically detected on the growing T. gondii tachyzoites. Immunoelectron microscopy revealed the surface and intracellular locations of enolase 2 on T. gondii cells. In conclusion, our results clearly show that the enzymatic activity of T. gondii enolase 2 is ion dependent and that it could be influenced by environmental factors. We also provide evidence that enolase 2 is an important immunogenic protein of ESAs from T. gondii and that it is a surface-exposed protein with strong antigenicity and immunogenicity. Our findings indicate that enolase 2 could play important roles in metabolism, immunogenicity and pathogenicity and that it may serve as a novel drug target and candidate vaccine against T. gondii infection.

Deciphering the Draft Genome of Toxoplasma gondii RH Strain

Toxoplasmosis is a widespread parasitic infection by Toxoplasma gondii, a parasite with at least three distinct clonal lineages. This article reports the whole genome sequencing and de novo assembly of T. gondii RH (type I representative strain), as well as genome-wide comparison across major T. gondii lineages. Genomic DNA was extracted from tachyzoites of T. gondii RH strain and its identity was verified by PCR and LAMP. Subsequently, whole genome sequencing was performed, followed by sequence filtering, genome assembly, gene annotation assignments, clustering of gene orthologs and phylogenetic tree construction. Genome comparison was done with the already archived genomes of T. gondii. From this study, the genome size of T. gondii RH strain was found to be 69.35Mb, with a mean GC content of 52%. The genome shares high similarity to the archived genomes of T. gondii GT1, ME49 and VEG strains. Nevertheless, 111 genes were found to be unique to T. gondii RH strain. Importantly, unique genes annotated to functions that are potentially critical for T. gondii virulence were found, which may explain the unique phenotypes of this particular strain. This report complements the genomic archive of T. gondii. Data obtained from this study contribute to better understanding of T. gondii and serve as a reference for future studies on this parasite.

Recombinant Toxoplasma gondii phosphoglycerate mutase 2 confers protective immunity against toxoplasmosis in BALB/c mice

Toxoplasmosis is one of the most widespread zoonoses worldwide. It has a high incidence and can result in severe disease in humans and livestock. Effective vaccines are needed to limit and prevent infection with Toxoplasma gondii. In this study, we evaluated the immuno-protective efficacy of a recombinant Toxoplasma gondii phosphoglycerate mutase 2 (rTgPGAM 2) against T. gondii infection in BALB/c mice. We report that the mice nasally immunised with rTgPGAM 2 displayed significantly higher levels of special IgG antibodies against rTgPGAM 2 (including IgG1, IgG2a and IgAs) and cytokines (including IFN-γ, IL-2 and IL-4) in their blood sera and supernatant of cultured spleen cells compared to those of control animals. In addition, an increased number of spleen lymphocytes and enhanced lymphocyte proliferative responses were observed in the rTgPGAM 2-immunised mice. After chronic infection and lethal challenge with the highly virulent T. gondii RH strain by oral gavage, the survival time of the rTgPGAM 2-immunised mice was longer (P < 0.01) and the survival rate (70%) was higher compared with the control mice (P < 0.01). The reduction rate of brain and liver tachyzoites in rTgPGAM 2-vaccinated mice reached approximately 57% and 69% compared with those of the control mice (P < 0.01). These results suggest that rTgPGAM 2 can generate protective immunity against T. gondii infection in BALB/c mice and may be a promising antigen in the further development of an effective vaccine against T. gondii infection.

Ubiquitination of pathogen-containing vacuoles promotes host defense to Chlamydia trachomatis and Toxoplasma gondii

Many intracellular bacterial and protozoan pathogens reside within host cell vacuoles customized by the microbial invaders to fit their needs. Within such pathogen-containing vacuoles (PVs) microbes procure nutrients and simultaneously hide from cytosolic host defense systems. Among the many PV-resident human pathogens are the bacterium Chlamydia trachomatis and the protozoan Toxoplasma gondii. Immune responses directed against their PVs are poorly characterized. We reported that activation of host cells with IFNγ triggers the attachment of polyubiquitin chains to Toxoplasma- and Chlamydia-containing vacuoles and thereby marks PVs for destruction. In murine cells PV ubiquitination is dependent on IFNγ-inducible Immunity Related GTPases (IRGs). Human cells also decorate PVs with ubiquitin upon IFNγ priming; however, the molecular machinery promoting PV ubiquitination in human cells remains unknown and is likely to be distinct from the IRG-dependent pathway we described in murine cells. Thus, IFNγ-inducible PV ubiquitination constitutes a critical event in cell-autonomous immunity to C. trachomatis and T. gondii in mice and humans, but the molecular machinery underlying PV ubiquitination is expected to be multifaceted and possibly host species-specific.

RabGDIα is a negative regulator of interferon-γ-inducible GTPase-dependent cell-autonomous immunity to Toxoplasma gondii

IFN-γ orchestrates cell-autonomous host defense against various intracellular vacuolar pathogens. IFN-γ-inducible GTPases, such as p47 immunity-related GTPases (IRGs) and p65 guanylate-binding proteins (GBPs), are recruited to pathogen-containing vacuoles, which is important for disruption of the vacuoles, culminating in the cell-autonomous clearance. Although the positive regulation for the proper recruitment of IRGs and GBPs to the vacuoles has been elucidated, the suppressive mechanism is unclear. Here, we show that Rab GDP dissociation inhibitor α (RabGDIα), originally identified as a Rab small GTPase inhibitor, is a negative regulator of IFN-γ-inducible GTPases in cell-autonomous immunity to the intracellular pathogen Toxoplasma gondii. Overexpression of RabGDIα, but not of RabGDIβ, impaired IFN-γ-dependent reduction of T. gondii numbers. Conversely, RabGDIα deletion in macrophages and fibroblasts enhanced the IFN-γ-induced clearance of T. gondii. Furthermore, upon a high dose of infection by T. gondii, RabGDIα-deficient mice exhibited a decreased parasite burden in the brain and increased resistance in the chronic phase than did control mice. Among members of IRGs and GBPs important for the parasite clearance, Irga6 and Gbp2 alone were more frequently recruited to T. gondii-forming parasitophorous vacuoles in RabGDIα-deficient cells. Notably, Gbp2 positively controlled Irga6 recruitment that was inhibited by direct and specific interactions of RabGDIα with Gbp2 through the lipid-binding pocket. Taken together, our results suggest that RabGDIα inhibits host defense against T. gondii by negatively regulating the Gbp2-Irga6 axis of IFN-γ-dependent cell-autonomous immunity.

Pathobiology of human RH strain induced experimental toxoplasmosis in murine model

Of late, toxoplasmosis has gained immense importance as an opportunist parasite in immunocompromised patients. In immunocompromised subjects, the disease is supposed to occur in acute form and causes acute toxoplasmic encephalitis. However, the exact pathogenesis of other vital organs, particularly in acute form of infection, is still a matter of debate. Therefore, an attempt was made to study the pathogenesis of acute form of toxoplasmosis using cryopreserved human RH strain of the parasite in murine models. For this, 100 tachyzoites were given to individual mice and upon the setup of acute form of infection, the mice were euthanized and the organs were processed for histopathology. Histopathology revealed tachyzoites in liver only while severe necrosis due to multiplication of tachyzoites were visible in liver, spleen, lungs and brain. Kidneys and heart appeared more or less normal. Finally, the pathology of disease in these organs is described in detail. The present research has generated some vital information regarding necrotic changes in tissues due to acute toxoplasmosis and will defiantly help the researchers in the better understanding of disease particularly in humans and putting up of suitable treatment regime for human subjects infected with acute toxoplasmosis.

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

Ma et al. show that the Toxoplasma gondii polymorphic dense granule protein GRA6 triggers the activation of the host transcription factor NFAT4, thus affecting the host immune response and maximizing parasite virulence.

Toxoplasma gondii infection results in co-option and subversion of host cellular signaling pathways. This process involves discharge of T. gondii effector molecules from parasite secretory organelles such as rhoptries and dense granules. We report that the T. gondii polymorphic dense granule protein GRA6 regulates activation of the host transcription factor nuclear factor of activated T cells 4 (NFAT4). GRA6 overexpression robustly and selectively activated NFAT4 via calcium modulating ligand (CAMLG). Infection with wild-type (WT) but not GRA6-deficient parasites induced NFAT4 activation. Moreover, GRA6-deficient parasites failed to exhibit full virulence in local infection, and the treatment of WT mice with an NFAT inhibitor mitigated virulence of WT parasites. Notably, NFAT4-deficient mice displayed prolonged survival, decreased recruitment of CD11b⁺ Ly6G⁺ cells to the site of infection, and impaired expression of chemokines such as Cxcl2 and Ccl2. In addition, infection with type I parasites culminated in significantly higher NFAT4 activation than type II parasites due to a polymorphism in the C terminus of GRA6. Collectively, our data suggest that GRA6-dependent NFAT4 activation is required for T. gondii manipulation of host immune responses to maximize the parasite virulence in a strain-dependent manner.

Immunization of Wistar female rats with 255-Gy-irradiated Toxoplasma gondii: preventing parasite load and maternofoetal transmission

Toxoplasmosis, caused by an obligate intracellular protozoan parasite, Toxoplasma gondii, is an worldwide parasitic disease, with significant importance for animal production and considerable impact to the public health. This study was aimed to evaluate the dynamic of the distribution of T.gondii in tissues of female Wistar rats and their puppies tissues, after the immunization by oral rote with irradiated tachyzoites. One week after pregnancy confirmation, rats was challenged by gavage with T. gondii bradyzoites, oocysts or tachyzoites of T. gondii. Forty-eight pregnant rats were grouped as follow: immunized and challenged with bradyzoites (BZ*); non-immunized and challenged with bradyzoites (BZ); immunized and challenged with oocysts (OC*); non-immunized and challenged with oocysts (OC); immunized and challenged with tachyzoites (TZ*); non-immunized and challenged with tachyzoites (TZ); only immunized (I); control group (C). After parturition the rats were sacrificed and the tissues were researched for the DNA of T. gondii by polymerase chain reaction (PCR) and the parasite load determined by the quantitative PCR (qPCR). It was verified that the immunization with irradiated tachyzoites of T. gondii induced the reduction of parasitic load in most organs analyzed, although not prevent the establishment of infection with the parasite. And also, the immunization showed a favorable effect on the birth rate and litter size.

New insights into clonality and panmixia in Plasmodium and toxoplasma

Until the 1990s, Plasmodium and Toxoplasma were widely considered to be potentially panmictic species, because they both undergo a meiotic sexual cycle in their definitive hosts. We have proposed that both parasites are able of clonal (nonrecombining) propagation, at least in some cycles. Toxoplasma was soon shown to be a paradigmatic case of clonal population structure in North American and in European cycles. But the proposal provoked an outcry in the case of Plasmodium and still appears as doubtful to many scientists. However, the existence of Plasmodium nonrecombining lines has been fully confirmed, although the origin of these lines is debatable. We discuss the current state of knowledge concerning the population structure of both parasites in the light of the recent developments of pathogen clonal evolution proposed by us and of new hypotheses presented here.

Role of mouse and human autophagy proteins in IFN-γ-induced cell-autonomous responses against Toxoplasma gondii

IFN-γ mediates cellular innate immunity against an intracellular parasite, Toxoplasma gondii, by inducing immunity-related GTPases such as p47 IFN-γ-regulated GTPases (IRGs) and p65 guanylate-binding proteins (GBPs), which also participate in antibacterial responses via autophagy. An essential autophagy protein, Atg5, was previously shown to play a critical role in anti-T. gondii cell-autonomous immunity. However, the involvement of other autophagy proteins remains unknown. In this study, we show that essential autophagy proteins differentially participate in anti-T. gondii cellular immunity by recruiting IFN-γ-inducible GTPases. IFN-γ-induced suppression of T. gondii proliferation and recruitment of an IRG Irgb6 and GBPs are profoundly impaired in Atg7- or Atg16L1-deficient cells. In contrast, cells lacking other essential autophagy proteins, Atg9a and Atg14, are capable of mediating the anti-T. gondii response and recruiting Irgb6 and GBPs to the parasites. Although IFN-γ also stimulates anti-T. gondii cellular immunity in humans, whether this response requires GBPs and human autophagy proteins remains to be seen. To analyze the role of human ATG16L1 and GBPs in IFN-γ-mediated anti-T. gondii responses, human cells lacking ATG16L1 or GBPs were generated by the Cas9/CRISPR genome-editing technique. Although both ATG16L1 and GBPs are dispensable for IFN-γ-induced inhibition of T. gondii proliferation in the human cells, human ATG16L1 is also required for the recruitment of GBPs. Taken together, human ATG16L1 and mouse autophagy components Atg7 and Atg16L1, but not Atg9a and Atg14, participate in the IFN-γ-induced recruitment of the immunity-related GTPases to the intracellular pathogen.

Strategies to discover the structural components of cyst and oocyst walls

Cysts of Giardia lamblia and Entamoeba histolytica and oocysts of Toxoplasma gondii and Cryptosporidium parvum are the infectious and sometimes diagnostic forms of these parasites. To discover the structural components of cyst and oocyst walls, we have developed strategies based upon a few simple assumptions. Briefly, the most abundant wall proteins are identified by monoclonal antibodies or mass spectrometry. Structural components include a sugar polysaccharide (chitin for Entamoeba, β-1,3-linked glucose for Toxoplasma, and β-1,3-linked GalNAc for Giardia) and/or acid-fast lipids (Toxoplasma and Cryptosporidium). Because Entamoeba cysts and Toxoplasma oocysts are difficult to obtain, studies of walls of nonhuman pathogens (E. invadens and Eimeria, respectively) accelerate discovery. Biochemical methods to dissect fungal walls work well for cyst and oocyst walls, although the results are often unexpected. For example, echinocandins, which inhibit glucan synthases and kill fungi, arrest the development of oocyst walls and block their release into the intestinal lumen. Candida walls are coated with mannans, while Entamoeba cysts are coated in a dextran-like glucose polymer. Models for cyst and oocyst walls derive from their structural components and organization within the wall. Cyst walls are composed of chitin fibrils and lectins that bind chitin (Entamoeba) or fibrils of the β-1,3-GalNAc polymer and lectins that bind the polymer (Giardia). Oocyst walls of Toxoplasma have two distinct layers that resemble those of fungi (β-1,3-glucan in the inner layer) or mycobacteria (acid-fast lipids in the outer layer). Oocyst walls of Cryptosporidium have a rigid bilayer of acid-fast lipids and inner layer of oocyst wall proteins.

Pediatric uveitis: An update

Because of their varied spectrum of clinical presentation and difficulty in management, pediatric uveitis remains a challenge to the ophthalmologist. Variations in clinical presentation, difficulties in eye examination, extended burden of the inflammation over quality of life, limited treatment modalities, risk of amblyopia are the main challenges in the management of pediatric uveitis. Pediatric uveitis is a cause of significant ocular morbidity and severe vision loss is found in 25-33% of such cases. This article summarizes the common causes of uveitis in children with special approach to the evaluation and diagnosis of each clinical entity.