The rat Toxo1 locus directs toxoplasmosis outcome and controls parasite proliferation and spreading by macrophage-dependent mechanisms

Capers with caspases: Toxoplasma gondii tales of inflammation and survival

Intracellular pathogens strike a delicate balance between maintaining their survival within infected cells, while also activating host defense mechanisms. Toxoplasma gondii is a protozoan parasite that initiates a variety of host signaling pathways as it invades host cells and establishes residence in a parasitophorous vacuole. Recent work has highlighted the interplay between T. gondii infection and innate immune pathways that lead to inflammation, several of which converge on caspases. This family of cysteine proteases function at the crossroads of inflammation and cell death and serve as a key target for parasite manipulation. This review focuses on the interaction of T. gondii with caspase-dependent inflammatory and cell death pathways and the role of parasite effector proteins in modulating these processes.

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.

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

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

Forward Genetics in Apicomplexa Biology: The Host Side of the Story

Forward genetic approaches have been widely used in parasitology and have proven their power to reveal the complexities of host-parasite interactions in an unbiased fashion. Many aspects of the parasite’s biology, including the identification of virulence factors, replication determinants, antibiotic resistance genes, and other factors required for parasitic life, have been discovered using such strategies. Forward genetic approaches have also been employed to understand host resistance mechanisms to parasitic infection. Here, we will introduce and review all forward genetic approaches that have been used to identify host factors involved with Apicomplexa infections, which include classical genetic screens and QTL mapping, GWAS, ENU mutagenesis, overexpression, RNAi and CRISPR-Cas9 library screens. Collectively, these screens have improved our understanding of host resistance mechanisms, immune regulation, vaccine and drug designs for Apicomplexa parasites. We will also discuss how recent advances in molecular genetics give present opportunities to further explore host-parasite relationships.

Unifying Virulence Evaluation in Toxoplasma gondii: A Timely Task

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

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

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

Progesterone Suppresses Neisseria gonorrhoeae-Induced Inflammation Through Inhibition of NLRP3 Inflammasome Pathway in THP-1 Cells and Murine Models

Asymptomatic/subclinical gonococcal infections in females continue to be prevalent within the general population, thus emerging as a global health problem. However, the reasons for these clinical manifestations are unknown. Our group had previously found out that in females, asymptomatic gonococcal infections correlate with higher serum progesterone (P4) levels and lower IL-1β levels in cervical secretions. We used murine infection model and THP-1 cells to determine whether P4 exerts anti-inflammatory effects on gonococcal infections. In the murine infection model, P4 (1 mg/day) inhibited the inflammatory effects induced by gonococcal infections which led to decreased neutrophil infiltration, reduced polymorphonuclear neutrophils (PMNs) numbers, IL-1β, TNF-α, and IL-6 levels in vaginal secretions. In addition, P4 down-regulated the mRNA and protein levels of NLRP3, associated with lower mRNA levels of pro-IL-1β, repressed caspase-1 activity in genital tissues and THP-1 cells. Moreover, P4 suppressed the phosphorylation levels of NF-κB and attenuated Neisseria gonorrhoeae (N. gonorrhoeae, gonococci or GC)-induced ROS generation. This is consistent with the two signals required for activation of the NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3) inflammasome. In conclusion, our result shows that P4 suppresses the gonococci induced-inflammation, especially through the NLRP3 inflammasome pathway, and partially explains the pathogenesis of asymptomatic GC infection in women.

Insight Into Inflammasome Signaling: Implications for Toxoplasma gondii Infection

Inflammasomes are multimeric protein complexes regulating the innate immune response to invading pathogens or stress stimuli. Recent studies have reported that nucleotide-binding leucine-rich repeat-containing (NLRs) proteins and DNA sensor absent in melanoma 2 (AIM2) serve as inflammasome sentinels, whose stimulation leads to the proteolytic activation of caspase-1, proinflammatory cytokine secretion, and pyroptotic cell death. Toxoplasma gondii, an obligate intracellular parasite of phylum Apicomplexans, is reportedly involved in NLRP1, NLRP3 and AIM2 inflammasomes activation; however, mechanistic evidence regarding the activation of these complexes is preliminary. This review describes the current understanding of inflammasome signaling in rodent and human models of T. gondii infection.

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.

Transcriptional Profiling Suggests T Cells Cluster around Neurons Injected with Toxoplasma gondii Proteins

Like other persistent intracellular pathogens, Toxoplasma gondii, a protozoan parasite, has evolved to evade the immune system and establish a chronic infection in specific cells and organs, including neurons in the CNS. Understanding T. gondii’s persistence in neurons holds the potential to identify novel, curative drug targets. The work presented here offers new insights into the neuron-T. gondii interaction in vivo. By transcriptionally profiling neurons manipulated by T. gondii, we unexpectedly revealed that immune cells, and specifically CD8⁺ T cells, appear to cluster around these neurons, suggesting that CD8⁺ T cells specifically recognize parasite-manipulated neurons. Such a possibility supports evidence from other labs that questions the long-standing dogma that neurons are often persistently infected because they are not directly recognized by immune cells such as CD8⁺ T cells. Collectively, these data suggest we reconsider the broader role of neurons in the context of infection and neuroinflammation.

Toxoplasma gondii’s tropism for and persistence in the central nervous system (CNS) underlies the symptomatic disease that T. gondii causes in humans. Our recent work has shown that neurons are the primary CNS cell with which Toxoplasma interacts and which it infects in vivo. This predilection for neurons suggests that T. gondii’s persistence in the CNS depends specifically upon parasite manipulation of the host neurons. Yet, most work on T. gondii-host cell interactions has been done in vitro and in nonneuronal cells. We address this gap by utilizing our T. gondii-Cre system that allows permanent marking and tracking of neurons injected with parasite effector proteins in vivo. Using laser capture microdissection (LCM) and RNA sequencing using RNA-seq, we isolated and transcriptionally profiled T. gondii-injected neurons (TINs), Bystander neurons (nearby non-T. gondii-injected neurons), and neurons from uninfected mice (controls). These profiles show that TIN transcriptomes significantly differ from the transcriptomes of Bystander and control neurons and that much of this difference is driven by increased levels of transcripts from immune cells, especially CD8⁺ T cells and monocytes. These data suggest that when we used LCM to isolate neurons from infected mice, we also picked up fragments of CD8⁺ T cells and monocytes clustering in extreme proximity around TINs and, to a lesser extent, Bystander neurons. In addition, we found that T. gondii transcripts were primarily found in the TIN transcriptome, not in the Bystander transcriptome. Collectively, these data suggest that, contrary to common perception, neurons that directly interact with or harbor parasites can be recognized by CD8⁺ T cells.

IMPORTANCE Like other persistent intracellular pathogens, Toxoplasma gondii, a protozoan parasite, has evolved to evade the immune system and establish a chronic infection in specific cells and organs, including neurons in the CNS. Understanding T. gondii’s persistence in neurons holds the potential to identify novel, curative drug targets. The work presented here offers new insights into the neuron-T. gondii interaction in vivo. By transcriptionally profiling neurons manipulated by T. gondii, we unexpectedly revealed that immune cells, and specifically CD8⁺ T cells, appear to cluster around these neurons, suggesting that CD8⁺ T cells specifically recognize parasite-manipulated neurons. Such a possibility supports evidence from other labs that questions the long-standing dogma that neurons are often persistently infected because they are not directly recognized by immune cells such as CD8⁺ T cells. Collectively, these data suggest we reconsider the broader role of neurons in the context of infection and neuroinflammation.

DPP8/9 inhibitors activate the CARD8 inflammasome in resting lymphocytes

Canonical inflammasomes are innate immune signaling platforms that are formed in response to intracellular pathogen-associated signals and trigger caspase-1-dependent pyroptosis. Inflammasome formation and signaling is thought to mainly occur in myeloid cells, and in particular monocytes and macrophages. Here we show that small molecule inhibitors of dipeptidyl peptidases 8 and 9 (DPP8/9), which activate the related CARD8 and NLRP1 inflammasomes, also activate pyroptosis in human and rodent resting lymphocytes. We found that both CD4⁺ and CD8⁺ T cells were particularly sensitive to these inhibitors, although the sensitivity of T cells, like macrophages, varied considerably between species. In human T cells, we show that CARD8 mediates DPP8/9 inhibitor-induced pyroptosis. Intriguingly, although activated human T cells express the key proteins known to be required for CARD8-mediated pyroptosis, these cells were completely resistant to DPP8/9 inhibitors. Overall, these data show that resting lymphoid cells can activate at least one inflammasome, revealing additional cell types and states poised to undergo rapid pyroptotic cell death in response to danger-associated signals.

The NLRP1 and CARD8 inflammasomes

Inflammasomes are multiprotein complexes that activate inflammatory cytokines and induce pyroptosis in response to intracellular danger-associated signals. NLRP1 and CARD8 are related germline-encoded pattern recognition receptors that form inflammasomes, but their activation mechanisms and biological purposes have not yet been fully established. Both NLRP1 and CARD8 undergo post-translational autoproteolysis to generate two non-covalently associated polypeptide chains. NLRP1 and CARD8 activators induce the proteasome-mediated destruction of the N-terminal fragment, liberating the C-terminal fragment to form an inflammasome. Here, we review the danger-associated stimuli that have been reported to activate NLRP1 and/or CARD8, including anthrax lethal toxin, Toxoplasma gondii, Shigella flexneri and the small molecule DPP8/9 inhibitor Val-boroPro, focusing on recent mechanistic insights and highlighting unresolved questions. In addition, we discuss the recently identified disease-associated mutations in NLRP1 and CARD8, the potential role that DPP9's protein structure plays in inflammasome regulation, and the emerging link between NLRP1 and metabolism. Finally, we summarize all of this latest research and consider the possible biological purposes of these enigmatic inflammasomes.

The role of macrophages in protective and pathological responses to Toxoplasma gondii

The ability of Toxoplasma gondii to cause clinical disease in immune-competent and immune-deficient hosts coupled with its ease of use in vitro and availability of murine models has led to its use as a model organism to study how the immune system controls an intracellular infection. This article reviews the studies that established the role of the cytokine IFN-γ in the activation of macrophages to control T gondii and the events that lead to the mobilization and expansion of macrophage populations and their ability to limit parasite replication. Macrophages also have pro-inflammatory functions that promote protective NK and T-cell activities as well as regulatory properties that facilitate the resolution of inflammation. Nevertheless, while macrophages are important in determining the outcome of infection, T gondii has evolved mechanisms to subvert macrophage activation and can utilize their migratory activities to promote dissemination and these two properties underlie the ability of this parasite to persist and cause disease.

DPP8/9 inhibitors are universal activators of functional NLRP1 alleles

Intracellular pathogenic structures or activities stimulate the formation of inflammasomes, which recruit and activate caspase-1 and trigger an inflammatory form of cell death called pyroptosis. The well-characterized mammalian inflammasome sensor proteins all detect one specific type of signal, for example double-stranded DNA or bacterial flagellin. Remarkably, NLRP1 was the first protein discovered to form an inflammasome, but the pathogenic signal that NLRP1 detects has not yet been identified. NLRP1 is highly polymorphic, even among inbred rodent strains, and it has been suggested that these diverse NLRP1 alleles may have evolved to detect entirely different stimuli. Intriguingly, inhibitors of the serine proteases DPP8 and DPP9 (DPP8/9) were recently shown to activate human NLRP1, its homolog CARD8, and several mouse NLRP1 alleles. Here, we show now that DPP8/9 inhibitors activate all functional rodent NLRP1 alleles, indicating that DPP8/9 inhibition induces a signal detected by all NLRP1 proteins. Moreover, we discovered that the NLRP1 allele sensitivities to DPP8/9 inhibitor-induced and Toxoplasma gondii-induced pyroptosis are strikingly similar, suggesting that DPP8/9 inhibition phenocopies a key activity of T. gondii. Overall, this work indicates that the highly polymorphic NLRP1 inflammasome indeed senses a specific signal like the other mammalian inflammasomes.

TLR11-independent inflammasome activation is critical for CD4+ T cell-derived IFN-γ production and host resistance to Toxoplasma gondii

Innate recognition of invading intracellular pathogens is essential for regulating robust and rapid CD4+ T cell effector function, which is critical for host-mediated immunity. The intracellular apicomplexan parasite, Toxoplasma gondii, is capable of infecting almost any nucleated cell of warm-blooded animals, including humans, and establishing tissue cysts that persist throughout the lifetime of the host. Recognition of T. gondii by TLRs is essential for robust IL-12 and IFN-γ production, two major cytokines involved in host resistance to the parasite. In the murine model of infection, robust IL-12 and IFN-γ production have been largely attributed to T. gondii profilin recognition by the TLR11 and TLR12 heterodimer complex, resulting in Myd88-dependent IL-12 production. However, TLR11 or TLR12 deficiency failed to recapitulate the acute susceptibility to T. gondii infection seen in Myd88^-/- mice. T. gondii triggers inflammasome activation in a caspase-1-dependent manner resulting in cytokine release; however, it remains undetermined if parasite-mediated inflammasome activation impacts IFN-γ production and host resistance to the parasite. Using mice which lack different inflammasome components, we observed that the inflammasome played a limited role in host resistance when TLR11 remained functional. Strikingly, in the absence of TLR11, caspase-1 and -11 played a significant role for robust CD4+ T_H1-derived IFN-γ responses and host survival. Moreover, we demonstrated that in the absence of TLR11, production of the caspase-1-dependent cytokine IL-18 was sufficient and necessary for CD4+ T cell-derived IFN-γ responses. Mechanistically, we established that T. gondii-mediated activation of the inflammasome and IL-18 were critical to maintain robust CD4+ T_H1 IFN-γ responses during parasite infection in the absence of TLR11.

It is currently estimated that one third of the world’s population is seropositive for the parasite Toxoplasma gondii and this parasite can lead to serious illness and death in immunocompromised patients, and is one of the leading causes of foodborne-related deaths in the United States. Host immunity against the parasite has largely been attributed to recognition of the parasite-derived protein, profilin, by the innate Toll-like receptors (TLRs), TLR11 and TLR12. T. gondii also triggers inflammasome activation in a caspase-1-dependent manner resulting in cytokine release. However, how these innate recognition systems regulate T_H1 immunity and host resistance remains largely unknown. Therefore, using genetically modified mice, we investigated TLR11-dependent and -independent host immunity against the parasite. Our research establishes that in the absence of TLR11, inflammasome activation and subsequent production of the inflammasome-dependent molecule, IL-18 are critical for host immunity to the parasite. These data provide novel mechanistic insight into how TLR and inflammasomes cooperate in regulation of T_H1 immunity and host protection.

Toxoplasma Hypervirulence in the Rat Model Parallels Human Infection and Is Modulated by the Toxo1 Locus

Toxoplasmosis is considered as an opportunistic parasitic disease. If post-natally acquired in children or adults, it may pass unnoticed, at least with strains of European origin. However, in the wild biotopes especially in South America, Toxoplasma gondii strains display a greater genetic diversity, which correlates to higher virulence for humans, particularly along the Amazon River and its tributaries. In French Guiana, several atypical strains have been associated with severe clinical forms: ocular toxoplasmosis and acute respiratory distress syndrome both of which can result in death. Among these, the GUY008-ABE strain was responsible for an epidemic of severe disseminated toxoplasmosis in Suriname, which led to the death of one immunocompetent individual. To better understand the mechanism underlying the hypervirulence of the GUY008-ABE strain, we have tested the rat model which compared to the mouse, better reflects the immune resistance of humans to Toxoplasma infection. Here we compare the outcome of toxoplasmosis in F344 rats infected either by the GUY008-ABE strain or the type II Prugniaud strain. We show that the GUY008-ABE strain displays a higher virulence phenotype leading to the death of all infected rats observed in this study. GUY008-ABE infection was characterized by an increase of the parasite load in several organs, especially the heart and lung, and was mainly associated with severe histological changes in lungs. Moreover, correlating with its hypervirulence trait, the GUY008-ABE strain was able to form cysts in the LEW rat model otherwise known to be refractory to infection by other Toxoplasma strains. Together, these results show that the rat is a discriminating experimental model to study Toxoplasma virulence factors relevant to the pathogenesis of human infection and that the degree of virulence is linked to the Toxo1 locus.

Strategies Developed by Toxoplasma gondii to Survive in the Host

One of the most successful intracellular parasites, Toxoplasma gondii has developed several strategies to avoid destruction by the host. These include approaches such as rapid and efficient cell invasion to avoid phagocytic engulfment, negative regulation of the canonical CD40-CD40L-mediated autophagy pathway, impairment of the noncanonical IFN-γ-dependent autophagy pathway, and modulation of host cell survival and death to obtain lifelong parasite survival. Different virulent strains have even evolved different ways to cope with and evade destruction by the host. This review aims to illustrate every aspect of the game between the host and Toxoplasma during the process of infection. A better understanding of all aspects of the battle between Toxoplasma and its hosts will be useful for the development of better strategies and drugs to control the parasite.

Three Toxoplasma gondii Dense Granule Proteins Are Required for Induction of Lewis Rat Macrophage Pyroptosis

Upon invasion of Lewis rat macrophages, Toxoplasma rapidly induces programmed cell death (pyroptosis), which prevents Toxoplasma replication, possibly explaining the resistance of the Lewis rat to Toxoplasma Using a chemical mutagenesis screen, we identified Toxoplasma mutants that no longer induced pyroptosis. Whole-genome sequencing led to the identification of three Toxoplasma parasitophorous vacuole-localized dense granule proteins, GRA35, GRA42, and GRA43, that are individually required for induction of Lewis rat macrophage pyroptosis. Macrophage infection with Δgra35, Δgra42, and Δgra43 parasites led to greatly reduced cell death rates and enhanced parasite replication. Lewis rat macrophages infected with parasites containing a single, double, or triple deletion of these GRAs showed similar levels of cell viability, suggesting that the three GRAs function in the same pathway. Deletion of GRA42 or GRA43 resulted in GRA35 (and other GRAs) being retained inside the parasitophorous vacuole instead of being localized to the parasitophorous vacuole membrane. Despite having greatly enhanced replication in Lewis rat macrophages in vitro, Δgra35, Δgra42, and Δgra43 parasites did not establish a chronic infection in Lewis rats. Toxoplasma did not induce F344 rat macrophage pyroptosis, but F344 rats infected with Δgra35, Δgra42, and Δgra43 parasites had reduced cyst numbers. Thus, these GRAs determined parasite in vivo fitness in F344 rats. Overall, our data suggest that these three Toxoplasma dense granule proteins play a critical role in establishing a chronic infection in vivo, independently of their role in mediating macrophage pyroptosis, likely due to their importance in regulating protein localization to the parasitophorous vacuole membrane.IMPORTANCE Inflammasomes are major components of the innate immune system and are responsible for detecting various microbial and environmental danger signals. Upon invasion of Lewis rat macrophages, the parasite rapidly activates the NLRP1 inflammasome, resulting in pyroptosis and elimination of the parasite's replication niche. The work reported here revealed that Toxoplasma GRA35, GRA42, and GRA43 are required for induction of Lewis rat macrophage pyroptosis. GRA42 and GRA43 mediate the correct localization of other GRAs, including GRA35, to the parasitophorous vacuole membrane. These three GRAs were also found to be important for parasite in vivo fitness in a Toxoplasma-susceptible rat strain, independently of their role in NLRP1 inflammasome activation, suggesting that they perform other important functions. Thus, this study identified three GRAs that mediate the induction of Lewis rat macrophage pyroptosis and are required for pathogenesis of the parasite.

Inherent Oxidative Stress in the Lewis Rat Is Associated with Resistance to Toxoplasmosis

The course of Toxoplasma gondii infection in rats closely resembles that in humans. However, compared to the Brown Norway (BN) rat, the Lewis (LEW) rat is extremely resistant to T. gondii infection. Thus, we performed RNA sequencing analysis of the LEW rat versus the BN rat, with or without T. gondii infection, in order to unravel molecular factors directing robust and rapid early T. gondii-killing mechanisms in the LEW rat. We found that compared to the uninfected BN rat, the uninfected LEW rat has inherently higher transcript levels of cytochrome enzymes (Cyp2d3, Cyp2d5, and Cybrd1, which catalyze generation of reactive oxygen species [ROS]), with concomitant higher levels of ROS. Interestingly, despite having higher levels of ROS, the LEW rat had lower transcript levels for antioxidant enzymes (lactoperoxidase, microsomal glutathione S-transferase 2 and 3, glutathione S-transferase peroxidase kappa 1, and glutathione peroxidase) than the BN rat, suggesting that the LEW rat maintains cellular oxidative stress that it tolerates. Corroboratively, we found that scavenging of superoxide anion by Mn(III) tetrakis (4-benzoic acid) porphyrin (MnTBAP) decreased the refractoriness of LEW rat peritoneal cells to T. gondii infection, resulting in proliferation of parasites in LEW rat peritoneal cells which, in turn, led to augmented cell death in the infected cells. Together, our results indicate that the LEW rat maintains inherent cellular oxidative stress that contributes to resistance to invading T. gondii, and they thus unveil new avenues for developing therapeutic agents targeting induction of host cell oxidative stress as a mechanism for killing T. gondii.

NLRP3 and Potassium Efflux Drive Rapid IL-1β Release from Primary Human Monocytes during Toxoplasma gondii Infection

IL-1β is produced by myeloid cells and acts as a critical mediator of host defense during infection and injury. We found that the intracellular protozoan parasite Toxoplasma gondii induced an early IL-1β response (within 4 h) in primary human peripheral blood monocytes isolated from healthy donors. This process involved upregulation of IL-1β, IL-1RN (IL-1R antagonist), and NLRP3 transcripts, de novo protein synthesis, and the release of pro- and mature IL-1β from infected primary monocytes. The released pro-IL-1β was cleavable to mature bioactive IL-1β in the extracellular space by the protease caspase-1. Treatment of primary monocytes with the NLRP3 inhibitor MCC950 or with extracellular potassium significantly reduced IL-1β cleavage and release in response to T. gondii infection, without affecting the release of TNF-α, and indicated a role for the inflammasome sensor NLRP3 and for potassium efflux in T. gondii-induced IL-1β production. Interestingly, T. gondii infection did not induce an IL-1β response in primary human macrophages derived from the same blood donors as the monocytes. Consistent with this finding, NLRP3 was downregulated during the differentiation of monocytes to macrophages and was not induced in macrophages during T. gondii infection. To our knowledge, these findings are the first to identify NLRP3 as an inflammasome sensor for T. gondii in primary human peripheral blood cells and to define an upstream regulator of its activation through the release of intracellular potassium.

Toxoplasma Modulates Signature Pathways of Human Epilepsy, Neurodegeneration & Cancer

One third of humans are infected lifelong with the brain-dwelling, protozoan parasite, Toxoplasma gondii. Approximately fifteen million of these have congenital toxoplasmosis. Although neurobehavioral disease is associated with seropositivity, causality is unproven. To better understand what this parasite does to human brains, we performed a comprehensive systems analysis of the infected brain: We identified susceptibility genes for congenital toxoplasmosis in our cohort of infected humans and found these genes are expressed in human brain. Transcriptomic and quantitative proteomic analyses of infected human, primary, neuronal stem and monocytic cells revealed effects on neurodevelopment and plasticity in neural, immune, and endocrine networks. These findings were supported by identification of protein and miRNA biomarkers in sera of ill children reflecting brain damage and T. gondii infection. These data were deconvoluted using three systems biology approaches: "Orbital-deconvolution" elucidated upstream, regulatory pathways interconnecting human susceptibility genes, biomarkers, proteomes, and transcriptomes. "Cluster-deconvolution" revealed visual protein-protein interaction clusters involved in processes affecting brain functions and circuitry, including lipid metabolism, leukocyte migration and olfaction. Finally, "disease-deconvolution" identified associations between the parasite-brain interactions and epilepsy, movement disorders, Alzheimer's disease, and cancer. This "reconstruction-deconvolution" logic provides templates of progenitor cells' potentiating effects, and components affecting human brain parasitism and diseases.

Toxoplasma gondii seroprevalence varies by cat breed

Toxoplasma gondii is a widespread zoonotic parasite that is relevant for veterinary and public health. The domestic cat, the definitive host species with the largest worldwide population, has become evolutionarily and epidemiologically the most important host of T. gondii. The outcome of T. gondii infection is influenced by congenital and acquired host characteristics. We detected differences in T. gondii seroprevalence by cat breed in our previous studies. The aims of this study were to estimate T. gondii seroprevalence in selected domestic cat breeds, and to evaluate whether being of a certain breed is associated with T. gondii seropositivity, when the age and lifestyle of the cat are taken into account. The studied breeds were the Birman, British Shorthair, Burmese, Korat, Norwegian Forest Cat, Ocicat, Persian, and Siamese. Plasma samples were analyzed for the presence of immunoglobulin G antibodies against T. gondii with a commercial direct agglutination test at dilution 1:40. The samples were accompanied by owner-completed questionnaires that provided background data on the cats. Overall, 41.12% of the 1121 cats tested seropositive, and the seroprevalence increased with age. The Burmese had the lowest seroprevalence (18.82%) and the Persian had the highest (60.00%). According to the final multivariable logistic regression model, the odds to test seropositive were four to seven times higher in Birmans, Ocicats, Norwegian Forest Cats, and Persians when compared with the Burmese, while older age and receiving raw meat were also risk factors for T. gondii seropositivity. This study showed that T. gondii seroprevalence varies by cat breed and identified being of certain breeds, older age, and receiving raw meat as risk factors for seropositivity.

Toxoplasma Effectors Targeting Host Signaling and Transcription

Early electron microscopy studies revealed the elaborate cellular features that define the unique adaptations of apicomplexan parasites. Among these were bulbous rhoptry (ROP) organelles and small, dense granules (GRAs), both of which are secreted during invasion of host cells. These early morphological studies were followed by the exploration of the cellular contents of these secretory organelles, revealing them to be comprised of highly divergent protein families with few conserved domains or predicted functions. In parallel, studies on host-pathogen interactions identified many host signaling pathways that were mysteriously altered by infection. It was only with the advent of forward and reverse genetic strategies that the connections between individual parasite effectors and the specific host pathways that they targeted finally became clear. The current repertoire of parasite effectors includes ROP kinases and pseudokinases that are secreted during invasion and that block host immune pathways. Similarly, many secretory GRA proteins alter host gene expression by activating host transcription factors, through modification of chromatin, or by inducing small noncoding RNAs. These effectors highlight novel mechanisms by which T. gondii has learned to harness host signaling to favor intracellular survival and will guide future studies designed to uncover the additional complexity of this intricate host-pathogen interaction.

Toxoplasma gondii Infection Induces High Mobility Group Box 1 Released from Mouse Macrophages

High mobility group box 1 (HMGB1) is abundantly expressed in intracellular engaged DNA binding ability. However, more importantly, it is a weapon against infection through proinflammatory response and immune regulation while released to extracellular. Toxoplasma gondii causes inflammatory pathological changes including ileitis and encephalitis in chronic infection. To investigate whether HMGB1 contributes to the toxoplasmosis lesions, we examined HMGB1 changes during T. gondii infection. The results showed that HMGB1 transcription was down-regulated in the murine macrophage ANA1 cell line and mouse peritoneal macrophages (PMΦs) after T. gondii inoculation, but up-regulated in the IFN-γ treated macrophages and the intraperitoneal exudate cells from the T. gondii infected mice. The content of intracellular HMGB1 are basically consistent with the transcription levels in ANA1 assay, while there were no obvious changes in the mouse PMΦs. Both ANA1 and mouse PMΦs released HMGB1 after parasites infection, and no obvious HMGB1 aggregation in cytoplasm compare to the IFN-γ treatment group. Furthermore, we demonstrated that T. gondii invasion led to HMGB1 release, which was dependent on the Caspase 1 activity. These finding should promote to further investigate the functions of extracellular HMGB1 in the toxoplasmosis.

Host and Toxoplasma gondii genetic and non-genetic factors influencing the development of ocular toxoplasmosis: A systematic review

Toxoplasmosis is a cosmopolitan infection caused by the apicomplexan parasite Toxoplasma gondii. This infectious disease is widely distributed across the world where cats play an important role in its spread. The symptomatology caused by this parasite is diverse but the ocular affectation emerges as the most important clinical phenotype. Therefore, we conducted a systematic review of the current knowledge of ocular toxoplasmosis from the genetic diversity of the pathogen towards the treatment available for this infection. This review represents an update to the scientific community regarding the genetic diversity of the parasite, the genetic factors of the host, the molecular pathogenesis and its association with disease, the available diagnostic tools and the available treatment of patients undergoing ocular toxoplamosis. This review will be an update for the scientific community in order to encourage researchers to deploy cutting-edge investigation across this field.

The NLRP1 inflammasomes

Inflammasomes are cytosolic protein complexes that serve as platforms for the recruitment and activation of the pro-inflammatory CASPASE-1 protease. CASPASE-1 activation leads to processing and maturation of the cytokines interleukin-1β and interleukin-18 and a lytic form of cell death termed pyroptosis. Inflammasome assembly is initiated by cytosolic sensors in response to microbial infections. Many of these sensors, including NLRP1 (NLR family, pyrin domain containing 1), are described to form an inflammasome, but until recently, the mechanism of inflammasome activation and its physiological functions in host defense have remained unclear. In the last few years, important advances in our understanding of NLRP1 biology have been achieved. In this review, we discuss the activation of NLRP1 by various stimuli, including Bacillus anthracis lethal toxin, Toxoplasma gondii, muramyl dipeptide, and host intracellular ATP depletion. The role NLRP1 plays in pathogen recognition and resistance during infection is also discussed, as is the regulation of NLRP1 by host and viral proteins. We conclude by discussing the unexpected differences in the mechanism of NLRP1 inflammasome activation, as compared to the activation of other inflammasomes, such as the NAIP (NLR family, apoptosis inhibitory protein)/NLRC4 inflammasomes.

Acid-sensing ion channel 1a contributes to the effect of extracellular acidosis on NLRP1 inflammasome activation in cortical neurons

Background

Acid-sensing ion channels (ASICs) are cation channels which were activated by extracellular acidosis and involved in various physiological and pathological processes in the nervous system. Inflammasome is a key component of the innate immune response in host against harmful and irritable stimuli. As the first discovered molecular platform, NLRP1 (nucleotide-binding oligomerization domain (NOD)-like receptor protein 1) inflammasome is expressed in neurons and implicated in many nervous system diseases such as brain injury, nociception and epilepsy. However, little is known about the effect of ASICs on NLRP1 inflammasome activation under acidosis.

Methods

The expression of inflammasome complex protein (NLRP1, ASC (apoptosis-associated speck-like protein containing a caspase-activating recruitment domain) and caspase-1), inflammatory cytokines (IL-1β and IL-18), and apoptosis-related protein (Bax, Bcl-2, and activated caspase-3) was detected by Western blot. Large-conductance Ca²⁺ and voltage-activated K⁺ (BK) channel currents were recorded by whole-cell patch-clamp technology. Measurement of [K⁺]_i was performed by fluorescent ion imaging system. Co-expression of ASICs and BK channels was determined by dual immunofluorescence. Cell viability was assessed by MTT and LDH kit.

Results

ASICs and BK channels were co-expressed in primary cultured cortical neurons. Extracellular acidosis increased the expression of NLRP1, ASC, caspase-1, IL-1β, and IL-18. Further mechanistic studies revealed that acidosis-induced ASIC1a activation results in the increase of BK channel currents, with the subsequent K⁺ efflux and a low concentration of intracellular K⁺, which activated NLRP1 inflammasome. Furthermore, these effects of acidosis could be blocked by specific ASIC1a inhibitor PcTX1 and BK channel inhibitor IbTX. The data also demonstrated neutralization of NLRP1-protected cortical neurons against injury induced by extracellular acidosis.

Conclusions

Our data showed that NLRP1 inflammasome could be activated by extracellular acidosis though ASIC-BK channel K⁺ signal pathway and was involved in extracellular acidosis-induced cortical neuronal injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0465-7) contains supplementary material, which is available to authorized users.

Lytic Cycle of Toxoplasma gondii: 15 Years Later

Toxoplasmosis is the clinical and pathological consequence of acute infection with the obligate intracellular apicomplexan parasite Toxoplasma gondii. Symptoms result from tissue destruction that accompanies lytic parasite growth. This review updates current understanding of the host cell invasion, parasite replication, and eventual egress that constitute the lytic cycle, as well as the ways T. gondii manipulates host cells to ensure its survival. Since the publication of a previous iteration of this review 15 years ago, important advances have been made in our molecular understanding of parasite growth and mechanisms of host cell egress, and knowledge of the parasite's manipulation of the host has rapidly progressed. Here we cover molecular advances and current conceptual frameworks that include each of these topics, with an eye to what may be known 15 years from now.

Cancer in the parasitic protozoans Trypanosoma brucei and Toxoplasma gondii

Cancer is a general name for more than 100 malignant diseases. It is postulated that all cancers start from a single abnormal cell that grows out of control. Untreated cancers can cause serious consequences and deaths. Great progress has been made in cancer research that has significantly improved our knowledge and understanding of the nature and mechanisms of the disease, but the origins of cancer are far from being well understood due to the limitations of suitable model systems and to the complexities of the disease. In view of the fact that cancers are found in various species of vertebrates and other metazoa, here, we suggest that cancer also occurs in parasitic protozoans such as Trypanosoma brucei, a blood parasite, and Toxoplasma gondii, an obligate intracellular pathogen. Without treatment, these protozoan cancers may cause severe disease and death in mammals, including humans. The simpler genomes of these single-cell organisms, in combination with their complex life cycles and fascinating life cycle differentiation processes, may help us to better understand the origins of cancers and, in particular, leukemias.

Toxoplasma gondii development of its replicative niche: in its host cell and beyond

Intracellular pathogens can replicate efficiently only after they manipulate and modify their host cells to create an environment conducive to replication. While diverse cellular pathways are targeted by different pathogens, metabolism, membrane and cytoskeletal architecture formation, and cell death are the three primary cellular processes that are modified by infections. Toxoplasma gondii is an obligate intracellular protozoan that infects ∼30% of the world's population and causes severe and life-threatening disease in developing fetuses, in immune-comprised patients, and in certain otherwise healthy individuals who are primarily found in South America. The high prevalence of Toxoplasma in humans is in large part a result of its ability to modulate these three host cell processes. Here, we highlight recent work defining the mechanisms by which Toxoplasma interacts with these processes. In addition, we hypothesize why some processes are modified not only in the infected host cell but also in neighboring uninfected cells.

A highly conserved Toxo1 haplotype directs resistance to toxoplasmosis and its associated caspase-1 dependent killing of parasite and host macrophage

Natural immunity or resistance to pathogens most often relies on the genetic make-up of the host. In a LEW rat model of refractoriness to toxoplasmosis, we previously identified on chromosome 10 the Toxo1 locus that directs toxoplasmosis outcome and controls parasite spreading by a macrophage-dependent mechanism. Now, we narrowed down Toxo1 to a 891 kb interval containing 29 genes syntenic to human 17p13 region. Strikingly, Toxo1 is included in a haplotype block strictly conserved among all refractory rat strains. The sequencing of Toxo1 in nine rat strains (5 refractory and 4 susceptible) revealed resistant-restricted conserved polymorphisms displaying a distribution gradient that peaks at the bottom border of Toxo1, and highlighting the NOD-like receptor, Nlrp1a, as a major candidate. The Nlrp1 inflammasome is known to trigger, upon pathogen intracellular sensing, pyroptosis programmed-cell death involving caspase-1 activation and cleavage of IL-1β. Functional studies demonstrated that the Toxo1-dependent refractoriness in vivo correlated with both the ability of macrophages to restrict T. gondii growth and a T. gondii-induced death of intracellular parasites and its host macrophages. The parasite-induced cell death of infected macrophages bearing the LEW-Toxo1 alleles was found to exhibit pyroptosis-like features with ROS production, the activation of caspase-1 and IL1-β secretion. The pharmacological inactivation of caspase-1 using YVAD and Z-VAD inhibitors prevented the death of both intravacuolar parasites and host non-permissive macrophages but failed to restore parasite proliferation. These findings demonstrated that the Toxo1-dependent response of rat macrophages to T. gondii infection may trigger two pathways leading to the control of parasite proliferation and the death of parasites and host macrophages. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. These data represent new insights towards the identification of a major pathway of innate resistance to toxoplasmosis and the prediction of individual resistance.

Toxoplasmosis is a ubiquitous parasitic infection causing a wide spectrum of diseases. It is usually asymptomatic but can lead to severe ocular and neurological disorders. The host factors that determine natural resistance to toxoplasmosis are yet poorly characterized. Among the animal models to study susceptibility to toxoplasmosis, rats develop like humans a subclinical chronic infection. The finding of a total resistance in the LEW rat strain has allowed genetic studies leading to the identification of Toxo1, a unique locus that controls the outcome of toxoplasmosis. In this report, a panel of recombinant inbred rat strains was used to genetically reduce the Toxo1 locus, on chromosome 10, to a limited region containing 29 genes. This locus is highly conserved among five resistant, by comparison to four susceptible, rat strains, indicating that refractoriness to toxoplasmosis could be predicted. The Toxo1-controlled refractoriness depends on the ability of macrophages to restrict parasite proliferation and the rapid death of both T. gondii and host macrophages in vitro. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. Our data represent new insights towards the identification of a major pathway of innate immunity that protects from toxoplasmosis.

ALOX12 in human toxoplasmosis

ALOX12 is a gene encoding arachidonate 12-lipoxygenase (12-LOX), a member of a nonheme lipoxygenase family of dioxygenases. ALOX12 catalyzes the addition of oxygen to arachidonic acid, producing 12-hydroperoxyeicosatetraenoic acid (12-HPETE), which can be reduced to the eicosanoid 12-HETE (12-hydroxyeicosatetraenoic acid). 12-HETE acts in diverse cellular processes, including catecholamine synthesis, vasoconstriction, neuronal function, and inflammation. Consistent with effects on these fundamental mechanisms, allelic variants of ALOX12 are associated with diseases including schizophrenia, atherosclerosis, and cancers, but the mechanisms have not been defined. Toxoplasma gondii is an apicomplexan parasite that causes morbidity and mortality and stimulates an innate and adaptive immune inflammatory reaction. Recently, it has been shown that a gene region known as Toxo1 is critical for susceptibility or resistance to T. gondii infection in rats. An orthologous gene region with ALOX12 centromeric is also present in humans. Here we report that the human ALOX12 gene has susceptibility alleles for human congenital toxoplasmosis (rs6502997 [P, <0.000309], rs312462 [P, <0.028499], rs6502998 [P, <0.029794], and rs434473 [P, <0.038516]). A human monocytic cell line was genetically engineered using lentivirus RNA interference to knock down ALOX12. In ALOX12 knockdown cells, ALOX12 RNA expression decreased and levels of the ALOX12 substrate, arachidonic acid, increased. ALOX12 knockdown attenuated the progression of T. gondii infection and resulted in greater parasite burdens but decreased consequent late cell death of the human monocytic cell line. These findings suggest that ALOX12 influences host responses to T. gondii infection in human cells. ALOX12 has been shown in other studies to be important in numerous diseases. Here we demonstrate the critical role ALOX12 plays in T. gondii infection in humans.

Inflammasome sensor NLRP1 controls rat macrophage susceptibility to Toxoplasma gondii

Toxoplasma gondii is an intracellular parasite that infects a wide range of warm-blooded species. Rats vary in their susceptibility to this parasite. The Toxo1 locus conferring Toxoplasma resistance in rats was previously mapped to a region of chromosome 10 containing Nlrp1. This gene encodes an inflammasome sensor controlling macrophage sensitivity to anthrax lethal toxin (LT) induced rapid cell death (pyroptosis). We show here that rat strain differences in Toxoplasma infected macrophage sensitivity to pyroptosis, IL-1β/IL-18 processing, and inhibition of parasite proliferation are perfectly correlated with NLRP1 sequence, while inversely correlated with sensitivity to anthrax LT-induced cell death. Using recombinant inbred rats, SNP analyses and whole transcriptome gene expression studies, we narrowed the candidate genes for control of Toxoplasma-mediated rat macrophage pyroptosis to four genes, one of which was Nlrp1. Knockdown of Nlrp1 in pyroptosis-sensitive macrophages resulted in higher parasite replication and protection from cell death. Reciprocally, overexpression of the NLRP1 variant from Toxoplasma-sensitive macrophages in pyroptosis-resistant cells led to sensitization of these resistant macrophages. Our findings reveal Toxoplasma as a novel activator of the NLRP1 inflammasome in rat macrophages.

Inflammasomes are multiprotein complexes that are a major component of the innate immune system. They contain “sensor” proteins that are responsible for detecting various microbial and environmental danger signals and function by activating caspase-1, an enzyme that mediates cleavage and release of the pro-inflammatory cytokines, IL-1β and IL-18. Toxoplasma gondii is a highly successful protozoan parasite capable of infecting a wide range of host species that have variable levels of resistance. Rat strains have been previously shown to vary in their susceptibility to this parasite. We report here that rat macrophages from different inbred strains also vary in sensitivity to Toxoplasma induced lysis. We find that NLRP1, an inflammasome sensor whose only known agonist is anthrax LT, is also activated by Toxoplasma infection. In rats there is a perfect correlation between NLRP1 sequence and macrophage sensitivity to Toxoplasma-induced rapid cell death, inhibition of parasite proliferation, and IL-1β/IL-18 processing. Nlrp1 genes from sensitive rat macrophages can confer sensitivity to this rapid cell death when expressed in Toxoplasma resistant rat macrophages. Our findings suggest Toxoplasma is a new activator of the NLRP1 inflammasome.

Dual role for inflammasome sensors NLRP1 and NLRP3 in murine resistance to Toxoplasma gondii

Induction of immunity that limits Toxoplasma gondii infection in mice is critically dependent on the activation of the innate immune response. In this study, we investigated the role of cytoplasmic nucleotide-binding domain and leucine-rich repeat containing a pyrin domain (NLRP) inflammasome sensors during acute toxoplasmosis in mice. We show that in vitro Toxoplasma infection of murine bone marrow-derived macrophages activates the NLRP3 inflammasome, resulting in the rapid production and cleavage of interleukin-1β (IL-1β), with no measurable cleavage of IL-18 and no pyroptosis. Paradoxically, Toxoplasma-infected mice produced large quantities of IL-18 but had no measurable IL-1β in their serum. Infection of mice deficient in NLRP3, caspase-1/11, IL-1R, or the inflammasome adaptor protein ASC led to decreased levels of circulating IL-18, increased parasite replication, and death. Interestingly, mice deficient in NLRP1 also displayed increased parasite loads and acute mortality. Using mice deficient in IL-18 and IL-18R, we show that this cytokine plays an important role in limiting parasite replication to promote murine survival. Our findings reveal T. gondii as a novel activator of the NLRP1 and NLRP3 inflammasomes in vivo and establish a role for these sensors in host resistance to toxoplasmosis.

Inflammasomes are multiprotein complexes that are a major component of the innate immune system. They contain “sensor” proteins that are responsible for detecting various microbial and environmental danger signals and function by activating caspase-1, an enzyme that mediates cleavage and release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. Toxoplasma gondii is a highly successful protozoan parasite capable of infecting a wide range of host species that have variable levels of resistance. We report here that T. gondii is a novel activator of the NLRP1 and NLRP3 inflammasomes in vivo and establish a role for these sensors in host resistance to toxoplasmosis. Using mice deficient in IL-18 and IL-18R, we show that the IL-18 cytokine plays a pivotal role by limiting parasite replication to promote murine survival.

NLRP1 is an inflammasome sensor for Toxoplasma gondii

The obligate intracellular parasite Toxoplasma gondii is able to infect nearly all nucleated cell types of warm-blooded animals. This is achieved through the injection of hundreds of parasite effectors into the host cell cytosol, allowing the parasite to establish a vacuolar niche for growth, replication, and persistence. Here we show that Toxoplasma infection actives an inflammasome response in mice and rats, an innate immune sensing system designed to survey the host cytosol for foreign components leading to inflammation and cell death. Oral infection with Toxoplasma triggers an inflammasome response that is protective to the host, limiting parasite load and dissemination. Toxoplasma infection is sufficient to generate an inflammasome response in germfree animals. Interleukin 1β (IL-1β) secretion by macrophage requires the effector caspases 1 and 11, the adapter ASC, and NLRP1, the sensor previously described to initiate the inflammasome response to Bacillus anthracis lethal factor. The allele of NLRP1b derived from 129 mice is sufficient to enhance the B6 bone marrow-derived macrophage (BMDM) inflammasome response to Toxoplasma independent of the lethal factor proteolysis site. Moreover, N-terminal processing of NLRP1b, the only mechanism of activation known to date, is not observed in response to Toxoplasma infection. Cumulatively, these data indicate that NLRP1 is an innate immune sensor for Toxoplasma infection, activated via a novel mechanism that corresponds to a host-protective innate immune response to the parasite.

Reciprocal virulence and resistance polymorphism in the relationship between Toxoplasma gondii and the house mouse

Virulence in the ubiquitous intracellular protozoon Toxoplasma gondii for its natural intermediate host, the mouse, appears paradoxical from an evolutionary standpoint because death of the mouse before encystment interrupts the parasite life cycle. Virulent T. gondii strains secrete kinases and pseudokinases that inactivate the immunity-related GTPases (IRG proteins) responsible for mouse resistance to avirulent strains. Such considerations stimulated a search for IRG alleles unknown in laboratory mice that might confer resistance to virulent strains of T. gondii. We report that the mouse IRG system shows extraordinary polymorphic complexity in the wild. We describe an IRG haplotype from a wild-derived mouse strain that confers resistance against virulent parasites by interference with the virulent kinase complex. In such hosts virulent strains can encyst, hinting at an explanation for the evolution of virulence polymorphism in T. gondii.

DOI:http://dx.doi.org/10.7554/eLife.01298.001

The parasite Toxoplasma gondii is one of the most common parasites worldwide and is known for its unusual life cycle. It reproduces sexually inside its primary host—the cat—and produces eggs that are released in faeces. Other animals, most often rodents, can then become infected when they unknowingly eat the eggs while foraging. Once inside its new host, the parasite reproduces asexually until the rodent’s immune system begins to fight back. It then becomes semi-dormant and forms cysts within the brain and muscle cells of its host. In an added twist, the parasite also causes rodents to lose their fear of cats. This increases their chances of being caught and eaten, thereby helping the parasite to return to its primary host and complete its life cycle.

Previous work has shown that virulent strains of T. gondii can evade the host immune system in mice by secreting enzymes that inactivate immune-related proteins called IRG proteins. This prevents the infection being cleared and leads to death of the host within a few days. The existence of these virulent strains is intriguing because parasites that kill their host, and thus prevent their own reproduction, should be eliminated from the population. The fact that they are fairly common suggests that there must be a hitherto unknown mechanism that allows rodents to survive these virulent strains.

Lilue et al. now report the existence of such a mechanism in strains of mice found in the wild. In contrast to laboratory mice, wild mice produce IRG proteins that inhibit the enzymes secreted by the virulent strains of T. gondii. Moreover, the IRG genes in wild mice are highly variable, whereas laboratory mice all have virtually identical IRG genes.

By uncovering the complexity and variability of IRG genes in wild mice—complexity that has been lost from laboratory strains—Lilue et al. solve the conundrum of how highly virulent T. gondii strains can persist in the mouse population, and offer an explanation for the evolution of parasitic strains with differing levels of virulence.

DOI:http://dx.doi.org/10.7554/eLife.01298.002

Differences in iNOS and arginase expression and activity in the macrophages of rats are responsible for the resistance against T. gondii infection

Toxoplasma gondii infects humans and warm blooded animals causing devastating disease worldwide. It has long been a mystery as to why the peritoneal macrophages of rats are naturally resistant to T. gondii infection while those of mice are not. Here, we report that high expression levels and activity of inducible nitric oxide synthase (iNOS) and low levels of arginase-1 (Arg 1) activity in the peritoneal macrophages of rats are responsible for their resistance against T. gondii infection, due to high nitric oxide and low polyamines within these cells. The opposite situation was observed in the peritoneal macrophages of mice. This discovery of the opposing functions of iNOS and Arg 1 in rodent peritoneal macrophages may lead to a better understanding of the resistance mechanisms of mammals, particularly humans and livestock, against T. gondii and other intracellular pathogens.

Epidemiology of and diagnostic strategies for toxoplasmosis

The apicomplexan parasite Toxoplasma gondii was discovered a little over 100 years ago, but knowledge of its biological life cycle and its medical importance has grown in the last 40 years. This obligate intracellular parasite was identified early as a pathogen responsible for congenital infection, but its clinical expression and the importance of reactivations of infections in immunocompromised patients were recognized later, in the era of organ transplantation and HIV infection. Recent knowledge of host cell-parasite interactions and of parasite virulence has brought new insights into the comprehension of the pathophysiology of infection. In this review, we focus on epidemiological and diagnostic aspects, putting them in perspective with current knowledge of parasite genotypes. In particular, we provide critical information on diagnostic methods according to the patient's background and discuss the implementation of screening tools for congenital toxoplasmosis according to health policies.

A spontaneous mutation of the rat Themis gene leads to impaired function of regulatory T cells linked to inflammatory bowel disease

Spontaneous or chemically induced germline mutations, which lead to Mendelian phenotypes, are powerful tools to discover new genes and their functions. Here, we report an autosomal recessive mutation that occurred spontaneously in a Brown-Norway (BN) rat colony and was identified as causing marked T cell lymphopenia. This mutation was stabilized in a new rat strain, named BN^m for “BN mutated.” In BN^m rats, we found that the T cell lymphopenia originated in the thymus, was intrinsic to CD4 T lymphocytes, and was associated with the development of an inflammatory bowel disease. Furthermore, we demonstrate that the suppressive activity of both peripheral and thymic CD4⁺ CD25^bright regulatory T cells (Treg) is defective in BN^m rats. Complementation of mutant animals with BN Treg decreases disease incidence and severity, thus suggesting that the impaired Treg function is involved in the development of inflammatory bowel disease in BN^m rats. Moreover, the cytokine profile of effector CD4 T cells is skewed toward Th2 and Th17 phenotypes in BN^m rats. Linkage analysis and genetic dissection of the CD4 T cell lymphopenia in rats issued from BN^m×DA crosses allowed the localization of the mutation on chromosome 1, within a 1.5 megabase interval. Gene expression and sequencing studies identified a frameshift mutation caused by a four-nucleotide insertion in the Themis gene, leading to its disruption. This result is the first to link Themis to the suppressive function of Treg and to suggest that, in Themis-deficient animals, defect of this function is involved in intestinal inflammation. Thus, this study highlights the importance of Themis as a new target gene that could participate in the pathogenesis of immune diseases characterized by chronic inflammation resulting from a defect in the Treg compartment.

Deciphering the genetic basis of human diseases and understanding the function of mammalian genes are among the main challenges for today's geneticists. In this regard, rodent models represent invaluable tools to identify new genes and to study the mechanisms of action of genes implicated in human diseases. Here, we identified a spontaneous mutation responsible for a reduction of blood CD4 T lymphocyte counts in a rat strain. The mutant rats showed a high incidence of inflammatory bowel disease, which was associated with skewed cytokine secretion by effector CD4 T cells towards Th2 and Th17 and with impairment of the suppressive activity of the regulatory CD4 T cells (Treg). The contribution of Treg was further evidenced by experiments showing that transfer of Treg from normal BN rats to mutant animals prevented the occurrence of bowel lesions. By genetic mapping the lymphopenia, we identified a disruption of the Themis gene. This result is the first to link Themis to the suppressive function of Treg and to suggest that, in Themis-deficient animals, a defect of this function predisposes to intestinal inflammation. Thus, this new rat model highlights key roles of Themis both in regulating the immune system and in maintaining intestinal homeostasis.

Innate responses to Toxoplasma gondii in mice and humans

Primary infection with Toxoplasma gondii stimulates production of high levels of interleukin 12 (IL-12) and interferon γ (IFN-γ) by cells of the innate immune system. These two cytokines are central to resistance to T. gondii. Signaling through the Toll-like receptor (TLR) adaptor protein MyD88 is indispensible for activating early innate immune responses. Recent studies have established that TLR11 plays a dominant role in sensing T. gondii. At the same time, TLR11 is represented in humans only by a pseudogene, and the major question of how innate and adaptive immune responses occur in the absence of TLR11 remains unanswered. In this article, similarities and differences in sensors and effector molecules that determine host resistance to the parasite in humans and mice are discussed.

Toxoplasma gondii effectors are master regulators of the inflammatory response

Toxoplasma is a highly successful parasite that establishes a life-long chronic infection. To do this, it must carefully regulate immune activation and host cell effector mechanisms. Here we review the latest developments in our understanding of how Toxoplasma counteracts the immune response of the host, and in some cases provokes it, through the use of specific parasite effector proteins. An emerging theme from these discoveries is that Toxoplasma effectors are master regulators of the pro-inflammatory response, which elicits many of the toxoplasmacidal mechanisms of the host. We speculate that combinations of these effectors present in certain Toxoplasma strains work to maintain an optimal parasite burden in different hosts to ensure parasite transmission.

Development of high-throughput methods to quantify cysts of Toxoplasma gondii

Toxplasma is a protozoan parasite, which forms persistent cysts in tissues of chronically infected animals and humans. Cysts can reactivate leading to severe pathologies. They also contribute to the transmission of Toxoplasma infection in humans by ingestion of undercooked meat. Classically, the quantification of cyst burden in tissues uses microscopy methods, which are laborious and time consuming. Here, we have developed automated protocols to quantify cysts, based on flow cytometry or high-throughput microscopy. Brains of rodents infected with cysts of Prugniaud strain were incubated with the FITC-Dolichos biflorus lectin and analyzed by flow cytometry and high-throughput epifluorescence microscopy. The comparison of cyst counts by manual epifluorescence microscopy to flow cytometry or to high-throughput epifluorescence microscopy revealed a good correlation (r = 0.934, r = 0.993, P < 0.001 respectively). High-throughput epifluorescence microscopy was found to be more specific and sensitive than flow cytometry and easier to use for large series of samples. This reliable and easy protocol allow the specific detection of Toxoplasma cysts in brain, even at low concentrations; it could be a new way to detect them in water and in contaminate food.

Systems genetics approach reveals candidate genes for parasite resistance from quantitative trait loci studies in agricultural species

A systems genetics approach combining pathway analysis of quantitative trait loci (QTL) and gene expression information has provided strong evidence for common pathways associated with genetic resistance to internal parasites. Gene data, collected from published QTL regions in sheep, cattle, mice, rats and humans, and microarray data from sheep, were converted to human Entrez Gene IDs and compared to the KEGG pathway database. Selection of pathways from QTL data was based on a selection index that ensured that the selected pathways were in all species and the majority of the projects overall and within species. Pathways with either up- and down-regulated genes, primarily up-regulated genes or primarily down-regulated genes, were selected from gene expression data. After comparing the data sets independently, the pathways from each data set were compared and the common set of pathways and genes was identified. Comparisons within data sets identified 21 pathways from QTL data and 66 pathways from gene expression data. Both selected sets were enriched with pathways involved in immune functions, disease and cell responses to signals. The analysis identified 14 pathways that were common between QTL and gene expression data, and four directly associated with IFNγ or MHCII, with 31 common genes, including three MHCII genes. In conclusion, a systems genetics approach combining data from multiple QTL and gene expression projects led to the discovery of common pathways associated with genetic resistance to internal parasites. This systems genetics approach may prove significant for the discovery of candidate genes for many other multifactorial, economically important traits.

NALP1 influences susceptibility to human congenital toxoplasmosis, proinflammatory cytokine response, and fate of Toxoplasma gondii-infected monocytic cells

NALP1 is a member of the NOD-like receptor (NLR) family of proteins that form inflammasomes. Upon cellular infection or stress, inflammasomes are activated, triggering maturation of proinflammatory cytokines and downstream cellular signaling mediated through the MyD88 adaptor. Toxoplasma gondii is an obligate intracellular parasite that stimulates production of high levels of proinflammatory cytokines that are important in innate immunity. In this study, susceptibility alleles for human congenital toxoplasmosis were identified in the NALP1 gene. To investigate the role of the NALP1 inflammasome during infection with T. gondii, we genetically engineered a human monocytic cell line for NALP1 gene knockdown by RNA interference. NALP1 silencing attenuated progression of T. gondii infection, with accelerated host cell death and eventual cell disintegration. In line with this observation, upregulation of the proinflammatory cytokines interleukin-1β (IL-1β), IL-18, and IL-12 upon T. gondii infection was not observed in monocytic cells with NALP1 knockdown. These findings suggest that the NALP1 inflammasome is critical for mediating innate immune responses to T. gondii infection and pathogenesis. Although there have been recent advances in understanding the potent activity of inflammasomes in directing innate immune responses to disease, this is the first report, to our knowledge, on the crucial role of the NALP1 inflammasome in the pathogenesis of T. gondii infections in humans.

A history of studies that examine the interactions of Toxoplasma with its host cell: Emphasis on in vitro models

This review is a historical look at work carried out over the past 50 years examining interactions of Toxoplasma with the host cell and attempts to focus on some of the seminal experiments in the field. This early work formed the foundation for more recent studies aimed at identifying the host and parasite factors mediating key Toxoplasma-host cell interactions. We focus especially on those studies that were performed in vitro and provide discussions of the following general areas: (i) establishment of the parasitophorous vacuole, (ii) the requirement of specific host cell molecules for parasite replication, (iii) the scenarios under which the host cell can resist parasite replication and/or persistence, (iv) host species-specific and host strain-specific responses to Toxoplasma infection, and (v) Toxoplasma-induced immune modulation.

Genomics studies of immune-mediated diseases using the BN-LEW rat model

LEW and BN rats, that behave in opposite ways for their susceptibility to various immune-mediated diseases, provide a powerful model to investigate the molecular and genetic bases of immune system physiology and dysregulation. Using this model, we addressed the question of the genetic control of central nervous system autoimmunity, of xenobiotic-induced allergic diseases, and of T cell subsets that differ by their cytokine profiles. By linkage analysis and genetic dissection, using a panel of congenic rats, we identified a 120 Kb region on chromosome 9 that controls all these phenotypes, indicating that this region contains a gene or set of genes that plays an important role in the immune system homeostasis and susceptibility to immune mediated diseases. In this review, we will describe these rat genomics studies and will discuss the cellular and genetic factors that may be involved in the differences between these rat strains.

Toxoplasma gondii and the Immunity-Related GTPase (IRG) resistance system in mice: a review

The Immunity Related GTPases (IRG proteins) constitute a large family of interferon-inducible proteins that mediate early resistance to Toxoplasma gondii infection in mice. At least six members of this family are required for resistance of mice to virulent T. gondii strains. Recent results have shown that the complexity of the resistance arises from complex regulatory interactions between different family members. The mode of action against T. gondii depends on the ability of IRG proteins to accumulate on the parasitophorous vacuole of invading tachyzoites and to induce local damage to the vacuole resulting in disruption of the vacuolar membrane. Virulent strains of T. gondii overcome the IRG resistance system, probably by interfering with the loading of IRG proteins onto the parasitophorous vacuole membrane. It may be assumed that T. gondii strains highly virulent for mice will be disadvantaged in the wild due to the rapid extinction of the infected host, while it is self-evident that susceptibility to virulent strains is disadvantageous to the mouse host. We consider the possibility that this double disadvantage is compensated in wild populations by segregating alleles with different resistance and susceptibility properties in the IRG system.

Clinical relevance of placenta examination for the diagnosis of congenital toxoplasmosis

BACKGROUND

Neonates with congenital toxoplasmosis, even asymptomatic at birth, should be treated early to reduce long-term sequelae. Postnatal diagnosis of congenital toxoplasmosis is essential because prenatal diagnosis fails to detect approximately 15% of cases or cannot be performed when maternal infection is acquired in late pregnancy. Detection of parasites in the placenta is one diagnostic approach to the early neonatal diagnosis of congenital toxoplasmosis.

METHODS

The parasitic analyses of 102 placentas from cases of toxoplasmosis acquired during gestation were reviewed, with complete biologic follow-up of neonates. The value of quantitative PCR and mouse inoculation was assessed, and results are discussed in light of prenatal treatment and postnatal outcome.

RESULTS

Congenital toxoplasmosis was diagnosed in 28 of the 102 cases. A prenatal diagnosis was obtained in only 16 cases. Specific IgM was detected in 57% of the babies at birth. A positive placental examination by PCR and mouse inoculation was the only evidence of infection in 3 neonates (11%) who were asymptomatic at birth. The sensitivities of PCR and mouse inoculation were 71% and 67%, respectively, and the specificities were 97% and 100%. Parasites were detected more often when maternal infection was acquired during the third trimester of pregnancy (P < 0.01), regardless the type of treatment. The sensitivity of IgM detection appeared to be related to maternal treatment since IgM was positive in 43% and 75% when mothers were treated or not, respectively (P < 0.01). Though 5/7 symptomatic infants had a positive placenta examination, there was no correlation between a positive placenta and the presence of clinical signs during the first year of life. The positive and negative predictive values of placental examination were 91% and 90%, respectively.

CONCLUSION

Placental examination is an efficient tool for the early diagnosis of congenital toxoplasmosis.