NLRP3 controls Trypanosoma cruzi infection through a caspase-1-dependent IL-1R-independent NO production

Mammalian Target of Rapamycin Inhibition in Trypanosoma cruzi-Infected Macrophages Leads to an Intracellular Profile That Is Detrimental for Infection

The causative agent of Chagas’ disease, Trypanosoma cruzi, affects approximately 10 million people living mainly in Latin America, with macrophages being one of the first cellular actors confronting the invasion during T. cruzi infection and their function depending on their proper activation and polarization into distinct M1 and M2 subtypes. Macrophage polarization is thought to be regulated not only by cytokines and growth factors but also by environmental signals. The metabolic checkpoint kinase mammalian target of rapamycin (mTOR)-mediated sensing of environmental and metabolic cues influences macrophage polarization in a complex and as of yet incompletely understood manner. Here, we studied the role of the mTOR pathway in macrophages during T. cruzi infection. We demonstrated that the parasite activated mTOR, which was beneficial for its replication since inhibition of mTOR in macrophages by different inhibitors decreased parasite replication. Moreover, in rapamycin pretreated and infected macrophages, we observed a decreased arginase activity and expression, reduced IL-10 and increased interleukin-12 production, compared to control infected macrophages treated with DMSO. Surprisingly, we also found a reduced iNOS activity and expression in these macrophages. Therefore, we investigated possible alternative mechanisms involved in controlling parasite replication in rapamycin pretreated and infected macrophages. Although, cytoplasmic ROS and the enzyme indoleamine 2, 3-dioxygenase (IDO) were not involved, we observed a significant increase in IL-6, TNF-α, and IL-1β production. Taking into account that IL-1β is produced by activation of the cytoplasmic receptor NLRP3, which is one of the main components of the inflammasome, we evaluated NLRP3 expression during mTOR inhibition and T. cruzi infection. We observed that rapamycin-pretreated and infected macrophages showed a significant increase in NLRP3 expression and produced higher levels of mitochondrial ROS (mtROS) compared with control cells. Moreover, inhibition of mtROS production partially reversed the effect of rapamycin on parasite replication, with there being a significant increase in parasite load in rapamycin pretreated and infected macrophages from NLRP3 KO mice compared to wild-type control cells. Our findings strongly suggest that mTOR inhibition during T. cruzi infection induces NLRP3 inflammasome activation and mtROS production, resulting in an inflammatory-like macrophage profile that controls T. cruzi replication.

Activation of NLRP3 inflammasomes in mouse hepatic stellate cells during Schistosoma J. infection

The major pathological changes during Schistosoma J. infection are characterized by granulomatous inflammation in the liver, a cellular immune response to schistosomal egg antigens. The molecular mechanisms initiating or promoting this schistosomal granulomatous inflammation remain poorly understood. In the present study, we first demonstrated that in mice infected with Schistosoma J. for 6 weeks exhibited increased levels of IL-1β in liver, a major product of NLRP3 inflammasomes and collagen deposition around the eosinophilic granuloma with Schistosoma J. eggs, which was substantially attenuated by caspase-1 inhibitor, YVAD. This activation of the NLRP3 inflammasome occurred in hepatic stellate cells (HSCs), as shown by a marked increase in co-localization of IL-1β with HSCs marker, desmin. Using isolated, cultured mouse HSCs, we further explored the mechanisms by which soluble egg antigen (SEA) from Schistosoma J. activates NLRP3 inflammasomes. SEA induced the formation and activation of NLRP3 inflammasomes, which was associated with both redox regulation and lysosomal dysfunction, but not with potassium channel activation. These results suggest that NLRP3 inflammasome activation in HSCs may serve as an early mechanism to turn on the inflammatory response and thereby instigate liver fibrosis during Schistosoma J. infection.

New Insights into the Immunobiology of Mononuclear Phagocytic Cells and Their Relevance to the Pathogenesis of Cardiovascular Diseases

Macrophages are the primary immune cells that reside within the myocardium, suggesting that these mononuclear phagocytes are essential in the orchestration of cardiac immunity and homeostasis. Independent of the nature of the injury, the heart triggers leukocyte activation and recruitment. However, inflammation is harmful to this vital terminally differentiated organ with extremely poor regenerative capacity. As such, cardiac tissue has evolved particular strategies to increase the stress tolerance and minimize the impact of inflammation. In this sense, growing evidences show that mononuclear phagocytic cells are particularly dynamic during cardiac inflammation or infection and would actively participate in tissue repair and functional recovery. They respond to soluble mediators such as metabolites or cytokines, which play central roles in the timing of the intrinsic cardiac stress response. During myocardial infarction two distinct phases of monocyte influx have been identified. Upon infarction, the heart modulates its chemokine expression profile that sequentially and actively recruits inflammatory monocytes, first, and healing monocytes, later. In the same way, a sudden switch from inflammatory macrophages (with microbicidal effectors) toward anti-inflammatory macrophages occurs within the myocardium very shortly after infection with Trypanosoma cruzi, the causal agent of Chagas cardiomyopathy. While in sterile injury, healing response is necessary to stop tissue damage; during an intracellular infection, the anti-inflammatory milieu in infected hearts would promote microbial persistence. The balance of mononuclear phagocytic cells seems to be also dynamic in atherosclerosis influencing plaque initiation and fate. This review summarizes the participation of mononuclear phagocyte system in cardiovascular diseases, keeping in mind that the immune system evolved to promote the reestablishment of tissue homeostasis following infection/injury, and that the effects of different mediators could modulate the magnitude and quality of the immune response. The knowledge of the effects triggered by diverse mediators would serve to identify new therapeutic targets in different cardiovascular pathologies.

Association of caspase-1 polymorphisms with Chagas cardiomyopathy among individuals in Santa Cruz, Bolivia

INTRODUCTION:

Trypanosoma cruzi (Tc) infection is usually acquired in childhood in endemic areas, leading to Chagas disease, which progresses to Chagas cardiomyopathy in 20-30% of infected individuals over decades. The pathogenesis of Chagas cardiomyopathy involves the host inflammatory response to T. cruzi, in which upstream caspase-1 activation prompts the cascade of inflammatory chemokines/cytokines, cardiac remodeling, and myocardial dysfunction. The aim of the present study was to examine the association of two caspase-1 single nucleotide polymorphisms (SNPs) with cardiomyopathy.

METHODS:

We recruited infected (Tc+, n = 149) and uninfected (Tc-, n = 87) participants in a hospital in Santa Cruz, Bolivia. Cardiac status was classified (I, II, III, IV) based on Chagas cardiomyopathy-associated electrocardiogram findings and ejection fractions on echocardiogram. Genotypes were determined using Taqman probes via reverse transcription-polymerase chain reaction of peripheral blood DNA. Genotype frequencies were analyzed according to three inheritance patterns (dominant, recessive, additive) using logistic regression adjusted for age and sex.

RESULTS:

The AA allele for the caspase-1 SNP rs501192 was more frequent in Tc+ cardiomyopathy (classes II, III, IV) patients compared to those with a normal cardiac status (class I) [odds ratio (OR) = -2.18, p = 0.117]. This trend approached statistical significant considering only Tc+ patients in class I and II (OR = -2.64, p = 0.064).

CONCLUSIONS:

Caspase-1 polymorphisms may play a role in Chagas cardiomyopathy development and could serve as markers to identify individuals at higher risk for priority treatment.

How Inflammasomes Inform Adaptive Immunity

An immune response consists of a finely orchestrated interplay between initial recognition of potential microbial threats by the innate immune system and subsequent licensed adaptive immune neutralization. The initial recognition integrates environmental cues derived from pathogen-associated molecular patterns and cell-intrinsic damage-associated molecular patterns to contextualize the insult and inform a tailored adaptive response via T and B lymphocytes. While there are much data to support the role of transcriptional responses downstream of pattern recognition receptors in informing the adaptive immune response, markedly less attention has been paid to the role of post-translational responses to pathogen-associated molecular pattern and damage-associated molecular pattern recognition by the innate immune system, and how this may influence adaptive immunity. A well-characterized post-translational consequence of pattern recognition receptor signaling is the assembly of a multimeric signaling platform, termed the inflammasome, by members of the nucleotide-binding oligomerization domain (Nod), leucine-rich repeat-containing receptors (NLRs), and pyrin and HIN domain (PYHIN) families. Inflammasomes assemble in response to cytosolic perturbations, such as mitochondrial dysfunction and aberrant ion fluxes in the case of the canonical NLRP3 inflammasome or the presence of bacterial lipopolysaccharides in the case of the non-canonical inflammasome. Assembly of the inflammasome allows for the cleavage and activation of inflammatory caspases. These activated inflammatory caspases in turn cleave pro-form inflammatory cytokines into their mature bioactive species and lead to unconventional protein secretion and lytic cell death. In this review, we discuss evidence for inflammasome-mediated instruction and contextualization of infectious and sterile agents to the adaptive immune system.

Ebi3 Prevents Trypanosoma cruzi-Induced Myocarditis by Dampening IFN-γ-Driven Inflammation

The identification of anti-inflammatory mediators can reveal important targetable molecules capable of counterbalancing Trypanosoma cruzi-induced myocarditis. Composed of Ebi3 and IL-27p28 subunits, IL-27 is produced by myeloid cells and is able to suppress inflammation by inducing IL-10-producing Tr1 cells, thus emerging as a potential candidate to ameliorate cardiac inflammation induced by T. cruzi. Although IL-27 has been extensively characterized as a suppressive cytokine that prevents liver immunopathogenesis after T. cruzi infection, the mechanisms underlying its effects on T. cruzi-induced myocarditis remain largely unknown. Here, wild-type (WT) and Ebi3-deficient animals were intraperitoneally infected with trypomastigotes of T. cruzi Y strain and used to evaluate the potential anti-inflammatory properties of Ebi3 during T. cruzi infection. The survival rates of mice were daily recorded, the frequency of inflammatory cells was analyzed by flow cytometry and inflammatory mediators were measured by ELISA, real-time PCR and PCR array. We reported that T. cruzi-induced myocarditis was prevented by Ebi3. Stressors mainly recognized by TLR2 and TLR4 receptors on myeloid cells were essential to trigger IL-27p28 production. In addition, Ebi3 regulated IFN-γ-mediated myocarditis by promoting an anti-inflammatory environment through IL-10, which was most likely produced by Tr1 cells rather than classical regulatory T cells (Tregs), in the heart tissue of T. cruzi-infected animals. Furthermore, in vivo IFN-γ blockade ameliorated the host survival without compromising the parasite control in the bloodstream. In humans, IL-27p28 was correlated with cardiac protection during Chagas disease. Patients with mild clinical forms of the disease produced high levels of IL-27p28, whereas lower levels were found in those with severe forms. In addition, polymorphic sites at Ebi3 gene were associated with severe cardiomyopathy in patients with Chagas disease. Collectively, we describe a novel regulatory mechanism where Ebi3 dampens cardiac inflammation by modulating the overproduction of IFN-γ, the bona fide culprit of Chagas disease cardiomyopathy.

Therapeutic effects of sphingosine kinase inhibitor N,N-dimethylsphingosine (DMS) in experimental chronic Chagas disease cardiomyopathy

Chagas disease cardiomyopathy is a parasite-driven inflammatory disease to which there are no effective treatments. Here we evaluated the therapeutic potential of N,N-dimethylsphingosine(DMS), which blocks the production of sphingosine-1-phosphate(S1P), a mediator of cellular events during inflammatory responses, in a model of chronic Chagas disease cardiomyopathy. DMS-treated, Trypanosoma cruzi-infected mice had a marked reduction of cardiac inflammation, fibrosis and galectin-3 expression when compared to controls. Serum concentrations of galectin-3, IFNγ and TNFα, as well as cardiac gene expression of inflammatory mediators were reduced after DMS treatment. The gene expression of M1 marker, iNOS, was decreased, while the M2 marker, arginase1, was increased. DMS-treated mice showed an improvement in exercise capacity. Moreover, DMS caused a reduction in parasite load in vivo. DMS inhibited the activation of lymphocytes, and reduced cytokines and NO production in activated macrophage cultures in vitro, while increasing IL-1β production. Analysis by qRT-PCR array showed that DMS treatment modulated inflammasome activation induced by T. cruzi on macrophages. Altogether, our results demonstrate that DMS, through anti-parasitic and immunomodulatory actions, can be beneficial in the treatment of chronic phase of T. cruzi infection and suggest that S1P-activated processes as possible therapeutic targets for the treatment of Chagas disease cardiomyopathy.

Molecular mechanisms and functions of pyroptosis, inflammatory caspases and inflammasomes in infectious diseases

Cell death is a fundamental biological phenomenon that is essential for the survival and development of an organism. Emerging evidence also indicates that cell death contributes to immune defense against infectious diseases. Pyroptosis is a form of inflammatory programmed cell death pathway activated by human and mouse caspase-1, human caspase-4 and caspase-5, or mouse caspase-11. These inflammatory caspases are used by the host to control bacterial, viral, fungal, or protozoan pathogens. Pyroptosis requires cleavage and activation of the pore-forming effector protein gasdermin D by inflammatory caspases. Physical rupture of the cell causes release of the pro-inflammatory cytokines IL-1β and IL-18, alarmins and endogenous danger-associated molecular patterns, signifying the inflammatory potential of pyroptosis. Here, we describe the central role of inflammatory caspases and pyroptosis in mediating immunity to infection and clearance of pathogens.

Epigenetic regulation of nitric oxide synthase 2, inducible (Nos2) by NLRC4 inflammasomes involves PARP1 cleavage

Nitric oxide synthase 2, inducible (Nos2) expression is necessary for the microbicidal activity of macrophages. However, NOS2 over-activation causes multiple inflammatory disorders, suggesting a tight gene regulation is necessary. Using cytosolic flagellin as a model for inflammasome-dependent NOS2 activation, we discovered a surprising new role for NLRC4/caspase-1 axis in regulating chromatin accessibility of the Nos2 promoter. We found that activation of two independent mechanisms is necessary for NOS2 expression by cytosolic flagellin: caspase-1 and NF-κB activation. NF-κB activation was necessary, but not sufficient, for NOS2 expression. Conversely, caspase-1 was necessary for NOS2 expression, but dispensable for NF-κB activation, indicating that this protease acts downstream NF-κB activation. We demonstrated that epigenetic regulation of Nos2 by caspase-1 involves cleavage of the chromatin regulator PARP1 (also known as ARTD1) and chromatin accessibility of the NF-κB binding sites located at the Nos2 promoter. Remarkably, caspase-1-mediated Nos2 transcription and NO production contribute to the resistance of macrophages to Salmonella typhimurium infection. Our results uncover the molecular mechanism behind the constricted regulation of Nos2 expression and open new therapeutic opportunities based on epigenetic activities of caspase-1 against infectious and inflammatory diseases.

Deviant Behavior: Tick-Borne Pathogens and Inflammasome Signaling

In the face of an assault, host cells mount an immediate response orchestrated by innate immunity. Two of the best described innate immune signaling networks are the Toll- and the Nod-like receptor pathways. Extensive work has been done characterizing both signaling cascades with several recent advances on the forefront of inflammasome biology. In this review, we will discuss how more commonly-studied pathogens differ from tick-transmitted microbes in the context of Nod-like receptor signaling and inflammasome formation. Because pathogens transmitted by ticks have unique characteristics, we offer the opinion that these microbes can be used to uncover novel principles of Nod-like receptor biology.

NLRP3 Inflammasome Activation in THP-1 Target Cells Triggered by Pathogenic Naegleria fowleri

Naegleria fowleri, known as the brain-eating amoeba, causes acute primary amoebic meningoencephalitis. During swimming and other recreational water activities, N. fowleri trophozoites penetrate the nasal mucosa and invade the olfactory bulbs, resulting in intense inflammatory reactions in the forebrain tissue. To investigate what kinds of inflammasome molecules are expressed in target cells due to N. fowleri infection, human macrophage cells (THP-1 cells) were cocultured with N. fowleri trophozoites in a noncontact system, and consequently, interleukin-1β (IL-1β) production was estimated. Caspase-1 activation and IL-1β production from THP-1 cells by Western blotting and the culture supernatant by enzyme-linked immunosorbent assay analysis were observed at 3 h after cocultivation. In addition, the increased expression of ASC and NLRP3, which make up an inflammasome complex, was also observed at 3 h after cocultivation. To confirm the caspase-1 activation and IL-1β production via the NLRP3 inflammasome in THP-1 cells triggered by N. fowleri trophozoites, THP-1 cells were pretreated with several inhibitors. The inhibition assay showed that CA-074 (a cathepsin B inhibitor), glybenclamide (an NLRP3 molecule inhibitor), and N-benzyloxycarbony-Val-Ala-Asp(O-methyl)-fluoromethylketone (Z-VAD-FMK; a caspase-1 inhibitor) reduced the levels of caspase-1 activation and IL-1β production from THP-1 cells. This study suggests that N. fowleri infection induces the NLRP3 inflammasome, which activates caspase-1 and subsequently produces IL-1β, thus resulting in inflammation.

Immune responses against protozoan parasites: a focus on the emerging role of Nod-like receptors

Nod-like receptors (NLRs) have gained attention in recent years because of the ability of some family members to assemble into a multimeric protein complex known as the inflammasome. The role of NLRs and the inflammasome in regulating innate immunity against bacterial pathogens has been well studied. However, recent studies show that NLRs and inflammasomes also play a role during infections caused by protozoan parasites, which pose a significant global health burden. Herein, we review the diseases caused by the most common protozoan parasites in the world and discuss the roles of NLRs and inflammasomes in host immunity against these parasites.

Trichomonas vaginalis induces IL-1β production in a human prostate epithelial cell line by activating the NLRP3 inflammasome via reactive oxygen species and potassium ion efflux

BACKGROUND

Trichomonas vaginalis is a sexually transmitted protozoan parasite that causes vaginitis in women, and urethritis and prostatitis in men. IL-1β is synthesized as immature pro-IL-1β, which is cleaved by activated caspase-1. Caspase-1 is, in turn, activated by a multi-protein complex known as an inflammasome. In this study, we investigated the inflammatory response of a prostate epithelial cell line (RWPE-1) to T. vaginalis and, specifically, the capacity of T. vaginalis to activate the NLRP3 inflammasome.

METHODS

RWPE-1 cells were stimulated by live T. vaginalis, and subsequent expression of pro-IL-1β, IL-1β, NLRP3, ASC and caspase-1 was determined by real-time PCR and Western blotting. IL-1β and caspase-1 production was also measured by ELISA. To evaluate the effects of NLRP3 and caspase-1 on IL-1β production, the activated RWPE-1 cells were transfected with small interfering RNAs to silence the NLRP3 and caspase-1 genes. Activation of the NLRP3 inflammasome was observed by fluorescence microscopy. Intracellular reactive oxygen species (ROS) were evaluated by spectrofluorometry.

RESULTS

When RWPE-1 cells were stimulated with live T. vaginalis, the mRNA and protein expression of IL-1β, NLRP3, ASC, and caspase-1 increased. Moreover, silencing of NLRP3 and caspase-1 attenuated T. vaginalis-induced IL-1β secretion. The NADPH oxidase inhibitor DPI and high extracellular potassium ion suppressed the production of IL-1β, caspase-1, and the expression of NLRP3 and ASC proteins. The specific NF-κB inhibitor, Bay 11-7082, inhibited IL-1β production, and also inhibited the production of caspase-1, ASC and NLRP3 proteins.

CONCLUSIONS

T. vaginalis induces the formation of the NLRP3 inflammasome in human prostate epithelial cells via ROS and potassium ion efflux, and this results in IL-1β production. This is the first evidence for activation of the NLRP3 inflammasome in the inflammatory response by prostate epithelial cells infected with T. vaginalis. Prostate 76:885-896, 2016. © 2016 Wiley Periodicals, Inc.

Reassessing the Evolutionary Importance of Inflammasomes

Inflammasomes monitor the cytosol for microbial contamination or perturbation and, thus, are predicted to provide potent defense against infection. However, the compendium of data from murine infection models suggests that inflammasomes merely delay the course of disease, allowing the host time to mount an adaptive response. Interpretations of such results are confounded by inflammasome-evasion strategies of vertebrate-adapted pathogens. Conversely, environmental opportunistic pathogens have not evolved in the context of inflammasomes and, therefore, are less likely to evade them. Indeed, opportunistic pathogens do not normally cause disease in wild-type animals. Accordantly, the extreme virulence of two opportunistic bacterial pathogens, Burkholderia thailandensis and Chromobacterium violaceum, is fully counteracted by inflammasomes in murine models. This leads us to propose a new hypothesis: perhaps animals maintain inflammasomes over evolutionary time not to defend against vertebrate-adapted pathogens but instead to counteract infection by a plethora of undiscovered opportunistic pathogens residing in the environment.

Transcriptome Remodeling in Trypanosoma cruzi and Human Cells during Intracellular Infection

Intracellular colonization and persistent infection by the kinetoplastid protozoan parasite, Trypanosoma cruzi, underlie the pathogenesis of human Chagas disease. To obtain global insights into the T. cruzi infective process, transcriptome dynamics were simultaneously captured in the parasite and host cells in an infection time course of human fibroblasts. Extensive remodeling of the T. cruzi transcriptome was observed during the early establishment of intracellular infection, coincident with a major developmental transition in the parasite. Contrasting this early response, few additional changes in steady state mRNA levels were detected once mature T. cruzi amastigotes were formed. Our findings suggest that transcriptome remodeling is required to establish a modified template to guide developmental transitions in the parasite, whereas homeostatic functions are regulated independently of transcriptomic changes, similar to that reported in related trypanosomatids. Despite complex mechanisms for regulation of phenotypic expression in T. cruzi, transcriptomic signatures derived from distinct developmental stages mirror known or projected characteristics of T. cruzi biology. Focusing on energy metabolism, we were able to validate predictions forecast in the mRNA expression profiles. We demonstrate measurable differences in the bioenergetic properties of the different mammalian-infective stages of T. cruzi and present additional findings that underscore the importance of mitochondrial electron transport in T. cruzi amastigote growth and survival. Consequences of T. cruzi colonization for the host include dynamic expression of immune response genes and cell cycle regulators with upregulation of host cholesterol and lipid synthesis pathways, which may serve to fuel intracellular T. cruzi growth. Thus, in addition to the biological inferences gained from gene ontology and functional enrichment analysis of differentially expressed genes in parasite and host, our comprehensive, high resolution transcriptomic dataset provides a substantially more detailed interpretation of T. cruzi infection biology and offers a basis for future drug and vaccine discovery efforts.

In-depth knowledge of the functional processes governing host colonization and transmission of pathogenic microorganisms is essential for the advancement of effective intervention strategies. This study focuses on Trypanosoma cruzi, the vector-borne protozoan parasite responsible for human Chagas disease and the leading cause of infectious myocarditis worldwide. To gain global insights into the biology of this parasite and its interaction with mammalian host cells, we have exploited a deep-sequencing approach to generate comprehensive, high-resolution transcriptomic maps for mammalian-infective stages of T. cruzi with the simultaneous interrogation of the human host cell transcriptome across an infection time course. We demonstrate that the establishment of intracellular T. cruzi infection in mammalian host cells is accompanied by extensive remodeling of the parasite and host cell transcriptomes. Despite the lack of transcriptional control mechanisms in trypanosomatids, our analyses identified functionally-enriched processes within sets of developmentally-regulated transcripts in T. cruzi that align with known or predicted biological features of the parasite. The novel insights into the biology of intracellular T. cruzi infection and the regulation of amastigote development gained in this study establish a unique foundation for functional network analyses that will be instrumental in providing functional links between parasite dependencies and host functional pathways that have the potential to be exploited for intervention.

Avian leukosis virus subgroup J triggers caspase-1-mediated inflammatory response in chick livers

Many pathogens trigger caspase-1-mediated innate immune responses. Avian leukosis virus subgroup J (ALV-J) causes serious immunosuppression and diverse tumors in chicks. The caspase-1 inflammasome mechanism of response to ALV-J invading remains unclear. Here we investigated the expression of caspase-1, the inflammasome adaptor NLRP3, IL-1β and IL-18 in response to ALV-J infection in the liver of chick. We found caspase-1 mRNA expression was elevated at 5 dpi and peaked at 7 dpi in ALV-J infected animals. Corresponding to this, the expressions of NLRP3 and proinflammatory cytokines IL-1β and IL-18 were significantly increased at 5 or 7 dpi. In addition, caspase-1 protein expression and inflammatory cell infiltration were induced after virus infection. These results indicated that ALV-J infection could trigger the caspase-1- mediated inflammatory response in chicks. Thus, an understanding of the inflammatory responses can provide a better insight into the pathogenicity of ALV-J and a possible anti-virus target for ALV-J infection.

PKC/ROS-Mediated NLRP3 Inflammasome Activation Is Attenuated by Leishmania Zinc-Metalloprotease during Infection

Parasites of the Leishmania genus infect and survive within macrophages by inhibiting several microbicidal molecules, such as nitric oxide and pro-inflammatory cytokines. In this context, various species of Leishmania have been reported to inhibit or reduce the production of IL-1β both in vitro and in vivo. However, the mechanism whereby Leishmania parasites are able to affect IL-1β production and secretion by macrophages is still not fully understood. Dependent on the stimulus at hand, the maturation of IL-1β is facilitated by different inflammasome complexes. The NLRP3 inflammasome has been shown to be of pivotal importance in the detection of danger molecules such as inorganic crystals like asbestos, silica and malarial hemozoin, (HZ) as well as infectious agents. In the present work, we investigated whether Leishmania parasites modulate NLRP3 inflammasome activation. Using PMA-differentiated THP-1 cells, we demonstrate that Leishmania infection effectively inhibits macrophage IL-1β production upon stimulation. In this context, the expression and activity of the metalloprotease GP63 - a critical virulence factor expressed by all infectious Leishmania species - is a prerequisite for a Leishmania-mediated reduction of IL-1β secretion. Accordingly, L. mexicana, purified GP63 and GP63-containing exosomes, caused the inhibition of macrophage IL-1β production. Leishmania-dependent suppression of IL-1β secretion is accompanied by an inhibition of reactive oxygen species (ROS) production that has previously been shown to be associated with NLRP3 inflammasome activation. The observed loss of ROS production was due to an impaired PKC-mediated protein phosphorylation. Furthermore, ROS-independent inflammasome activation was inhibited, possibly due to an observed GP63-dependent cleavage of inflammasome and inflammasome-related proteins. Collectively for the first time, we herein provide evidence that the protozoan parasite Leishmania, through its surface metalloprotease GP63, can significantly inhibit NLRP3 inflammasome function and IL-1β production.

Leishmania parasites are the causative agent of leishmaniasis, a wide spread disease in tropical and subtropical areas. The microorganisms have been shown to be well-adapted to their hosts and are able to enter their target cells where they replicate themselves. To ensure these processes, Leishmania disrupts a multitude of cellular signals and protective mechanisms, which overall attenuates immune responses against the parasites. A key factor for inflammatory processes, also during infections, is IL-1β. As previous studies suggested a dysregulation of IL-1β levels after infection with Leishmania parasites, we herein investigated the underlying mechanisms. Our work reveals that Leishmania suppressing IL-1β production through its virulence factor GP63. Furthermore, our data suggests that the parasites can dampen the maturation of IL-1β after different stimuli. In this regard we established a role for the suppression of the kinase PKC and the generation of reactive oxygen species, as well as the cleavage of cellular proteins that are important for IL-1β-generation. Thus, we here present a novel aspect for how Leishmania parasites can counteract host protective mechanisms.

A Two-Component DNA-Prime/Protein-Boost Vaccination Strategy for Eliciting Long-Term, Protective T Cell Immunity against Trypanosoma cruzi

In this study, we evaluated the long-term efficacy of a two-component subunit vaccine against Trypanosoma cruzi infection. C57BL/6 mice were immunized with TcG2/TcG4 vaccine delivered by a DNA-prime/Protein-boost (D/P) approach and challenged with T. cruzi at 120 or 180 days post-vaccination (dpv). We examined whether vaccine-primed T cell immunity was capable of rapid expansion and intercepting the infecting T. cruzi. Our data showed that D/P vaccine elicited CD4⁺ (30-38%) and CD8⁺ (22-42%) T cells maintained an effector phenotype up to 180 dpv, and were capable of responding to antigenic stimulus or challenge infection by a rapid expansion (CD8>CD4) with type 1 cytokine (IFNγ⁺ and TFNα⁺) production and cytolytic T lymphocyte (CTL) activity. Subsequently, challenge infection at 120 or 180 dpv, resulted in 2-3-fold lower parasite burden in vaccinated mice than was noted in unvaccinated/infected mice. Co-delivery of IL-12- and GMCSF-encoding expression plasmids provided no significant benefits in enhancing the anti-parasite efficacy of the vaccine-induced T cell immunity. Booster immunization (bi) with recombinant TcG2/TcG4 proteins 3-months after primary vaccine enhanced the protective efficacy, evidenced by an enhanced expansion (1.2-2.8-fold increase) of parasite-specific, type 1 CD4⁺ and CD8⁺ T cells and a potent CTL response capable of providing significantly improved (3-4.5-fold) control of infecting T. cruzi. Further, CD8⁺T cells in vaccinated/bi mice were predominantly of central memory phenotype, and capable of responding to challenge infection 4-6-months post bi by a rapid expansion to a poly-functional effector phenotype, and providing a 1.5-2.3-fold reduction in tissue parasite replication. We conclude that the TcG2/TcG4 D/P vaccine provided long-term anti-T. cruzi T cell immunity, and bi would be an effective strategy to maintain or enhance the vaccine-induced protective immunity against T. cruzi infection and Chagas disease.

Chagas disease, caused by Trypanosoma cruzi infection, represents the third greatest tropical disease burden in the world. No vaccine or suitable treatment is available for control of this infection. Based upon several studies we have conducted, we believe that TcG2 and TcG4 candidate antigens that are highly conserved in T. cruzi, expressed in clinically relevant forms of the parasite, and recognized by both B and T cell responses in multiple hosts, are an excellent choice for subunit vaccine development. In this study, we demonstrate that the delivery of TcG2 and TcG4 as a DNA-prime/protein-boost vaccine provided long-term protection from challenge infection, and this protection was associated with elicitation of long-lived CD8⁺ effector T cells. The longevity and efficacy of vaccine could be enhanced by booster immunization. We believe that this is the first report demonstrating a) a subunit vaccine can be useful in achieving long-term protection against T. cruzi infection and Chagas disease, and b) the effector T cells can be long-lived and play a role in vaccine elicited protection from parasitic infection.

NLR proteins and parasitic disease

Parasitic diseases are a serious global health concern. Many of the most common and most severe parasitic diseases, including Chagas' disease, leishmaniasis, and schistosomiasis, are also classified as neglected tropical diseases and are comparatively less studied than infectious diseases prevalent in high income nations. The NLRs (nucleotide-binding domain leucine-rich-repeat-containing proteins) are cytosolic proteins known to be involved in pathogen detection and host response. The role of NLRs in the host response to parasitic infection is just beginning to be understood. The NLR proteins NOD1 and NOD2 have been shown to contribute to immune responses during Trypanosoma cruzi infection, Toxoplasma gondii infection, and murine cerebral malaria. The NLRP3 inflammasome is activated by T. cruzi and Leishmania amazonensis but also induces pathology during infection with schistosomes or malaria. Both the NLRP1 and NLRP3 inflammasomes respond to T. gondii infection. The NLRs may play crucial roles in human immune responses during parasitic infection, usually acting as innate immune sensors and driving the inflammatory response against invading parasites. However, this inflammatory response can either kill the invading parasite or be responsible for destructive pathology. Therefore, understanding the role of the NLR proteins will be critical to understanding the host defense against parasites as well as the fine balance between homeostasis and parasitic disease.

Global metabolomic profiling of acute myocarditis caused by Trypanosoma cruzi infection

Chagas disease is caused by Trypanosoma cruzi infection, being cardiomyopathy the more frequent manifestation. New chemotherapeutic drugs are needed but there are no good biomarkers for monitoring treatment efficacy. There is growing evidence linking immune response and metabolism in inflammatory processes and specifically in Chagas disease. Thus, some metabolites are able to enhance and/or inhibit the immune response. Metabolite levels found in the host during an ongoing infection could provide valuable information on the pathogenesis and/or identify deregulated metabolic pathway that can be potential candidates for treatment and being potential specific biomarkers of the disease. To gain more insight into those aspects in Chagas disease, we performed an unprecedented metabolomic analysis in heart and plasma of mice infected with T. cruzi. Many metabolic pathways were profoundly affected by T. cruzi infection, such as glucose uptake, sorbitol pathway, fatty acid and phospholipid synthesis that were increased in heart tissue but decreased in plasma. Tricarboxylic acid cycle was decreased in heart tissue and plasma whereas reactive oxygen species production and uric acid formation were also deeply increased in infected hearts suggesting a stressful condition in the heart. While specific metabolites allantoin, kynurenine and p-cresol sulfate, resulting from nucleotide, tryptophan and phenylalanine/tyrosine metabolism, respectively, were increased in heart tissue and also in plasma. These results provide new valuable information on the pathogenesis of acute Chagas disease, unravel several new metabolic pathways susceptible of clinical management and identify metabolites useful as potential specific biomarkers for monitoring treatment and clinical severity in patients.

Endothelial Nlrp3 inflammasome activation associated with lysosomal destabilization during coronary arteritis

Inflammasomes play a critical role in the development of vascular diseases. However, the molecular mechanisms activating the inflammasome in endothelial cells and the relevance of this inflammasome activation is far from clear. Here, we investigated the mechanisms by which an Nlrp3 inflammasome is activated to result in endothelial dysfunction during coronary arteritis by Lactobacillus casei (L. casei) cell wall fragments (LCWE) in a mouse model for Kawasaki disease. Endothelial dysfunction associated with increased vascular cell adhesion protein 1 (VCAM-1) expression and endothelial-leukocyte adhesion was observed during coronary arteritis in mice treated with LCWE. Accompanied with these changes, the inflammasome activation was also shown in coronary arterial endothelium, which was characterized by a marked increase in caspase-1 activity and IL-1β production. In cultured endothelial cells, LCWE induced Nlrp3 inflammasome formation, caspase-1 activation and IL-1β production, which were blocked by Nlrp3 gene silencing or lysosome membrane stabilizing agents such as colchicine, dexamethasone, and ceramide. However, a potassium channel blocker glibenclamide or an oxygen free radical scavenger N-acetyl-l-cysteine had no effects on LCWE-induced inflammasome activation. LCWE also increased endothelial cell lysosomal membrane permeability and triggered lysosomal cathepsin B release into cytosol. Silencing cathepsin B blocked LCWE-induced Nlrp3 inflammasome formation and activation in endothelial cells. In vivo, treatment of mice with cathepsin B inhibitor also abolished LCWE-induced inflammasome activation in coronary arterial endothelium. It is concluded that LCWE enhanced lysosomal membrane permeabilization and consequent release of lysosomal cathepsin B, resulting in activation of the endothelial Nlrp3 inflammasome, which may contribute to the development of coronary arteritis.

Caspase-1/ASC inflammasome-mediated activation of IL-1β-ROS-NF-κB pathway for control of Trypanosoma cruzi replication and survival is dispensable in NLRP3-/- macrophages

In this study, we have utilized wild-type (WT), ASC-/-, and NLRP3-/- macrophages and inhibition approaches to investigate the mechanisms of inflammasome activation and their role in Trypanosoma cruzi infection. We also probed human macrophages and analyzed published microarray datasets from human fibroblasts, and endothelial and smooth muscle cells for T. cruzi-induced changes in the expression genes included in the RT Profiler Human Inflammasome arrays. T. cruzi infection elicited a subdued and delayed activation of inflammasome-related gene expression and IL-1β production in mφs in comparison to LPS-treated controls. When WT and ASC-/- macrophages were treated with inhibitors of caspase-1, IL-1β, or NADPH oxidase, we found that IL-1β production by caspase-1/ASC inflammasome required reactive oxygen species (ROS) as a secondary signal. Moreover, IL-1β regulated NF-κB signaling of inflammatory cytokine gene expression and, subsequently, intracellular parasite replication in macrophages. NLRP3-/- macrophages, despite an inability to elicit IL-1β activation and inflammatory cytokine gene expression, exhibited a 4-fold decline in intracellular parasites in comparison to that noted in matched WT controls. NLRP3-/- macrophages were not refractory to T. cruzi, and instead exhibited a very high basal level of ROS (>100-fold higher than WT controls) that was maintained after infection in an IL-1β-independent manner and contributed to efficient parasite killing. We conclude that caspase-1/ASC inflammasomes play a significant role in the activation of IL-1β/ROS and NF-κB signaling of cytokine gene expression for T. cruzi control in human and mouse macrophages. However, NLRP3-mediated IL-1β/NFκB activation is dispensable and compensated for by ROS-mediated control of T. cruzi replication and survival in macrophages.

Trypanosoma cruzi evades the protective role of interferon-gamma-signaling in parasite-infected cells

The protozoan parasite Trypanosoma cruzi is responsible for the zoonotic Chagas disease, a chronic and systemic infection in humans and warm-blooded animals typically leading to progressive dilated cardiomyopathy and gastrointestinal manifestations. In the present study, we report that the transcription factor STAT1 (signal transducer and activator of transcription 1) reduces the susceptibility of human cells to infection with T. cruzi. Our in vitro data demonstrate that interferon -γ (IFNγ) pre-treatment causes T. cruzi-infected cells to enter an anti-parasitic state through the activation of the transcription factor STAT1. Whereas stimulation of STAT1-expressing cells with IFNγ significantly impaired intracellular replication of parasites, no protective effect of IFNγ was observed in STAT1-deficient U3A cells. The gene encoding indoleamine 2, 3-dioxygenase (ido) was identified as a STAT1-regulated target gene engaged in parasite clearance. Exposure of cells to T. cruzi trypomastigotes in the absence of IFNγ resulted in both sustained tyrosine and serine phosphorylation of STAT1 and its increased DNA binding. Furthermore, we found that in response to T. cruzi the total amount of intracellular STAT1 increased in an infectious dose-dependent manner, both at the mRNA and protein level. While STAT1 activation is a potent strategy of the host in the fight against the invading pathogen, amastigotes replicating intracellularly antagonize this pathway by specifically promoting the dephosphorylation of STAT1 serine 727, thereby partially circumventing its protective effects. These findings point to the crucial role of the IFNγ/STAT1 signal pathway in the evolutionary combat between T. cruzi parasites and their host.

Trypanosoma cruzi infection in genetically selected mouse lines: genetic linkage with quantitative trait locus controlling antibody response

Trypanosoma cruzi infection was studied in mouse lines selected for maximal (AIRmax) or minimal (AIRmin) acute inflammatory reaction and for high (H_III) or low (L_III) antibody (Ab) responses to complex antigens. Resistance was associated with gender (females) and strain—the high responder lines AIRmax and H_III were resistant. The higher resistance of H_III as compared to L_III mice extended to higher infective doses and was correlated with enhanced production of IFN-γ and nitric oxide production by peritoneal and lymph node cells, in H_III males and females. We also analyzed the involvement of previously mapped Ab and T. cruzi response QTL with the survival of Selection III mice to T. cruzi infections in a segregating backcross [F1(H_III×L_III) ×L_III] population. An Ab production QTL marker mapping to mouse chromosome 1 (34.8 cM) significantly cosegregated with survival after acute T. cruzi infections, indicating that this region also harbors genes whose alleles modulate resistance to acute T. cruzi infection.

Chagas disease: still many unsolved issues

Over the past 20 years, the immune effector mechanisms involved in the control of Trypanosoma cruzi, as well as the receptors participating in parasite recognition by cells of the innate immune system, have been largely described. However, the main questions on the physiopathology of Chagas disease remain unanswered: “Why does the host immune system fail to provide sterile immunity?” and “Why do only a proportion of infected individuals develop chronic pathology?” In this review, we describe the mechanisms proposed to explain the inability of the immune system to eradicate the parasite and the elements that allow the development of chronic heart disease. Moreover, we discuss the possibility that the inability of infected cardiomyocytes to sense intracellular T. cruzi contributes to parasite persistence in the heart and the development of chronic pathology.

Parasite-derived neurotrophic factor/trans-sialidase of Trypanosoma cruzi links neurotrophic signaling to cardiac innate immune response

The Chagas' disease parasite Trypanosoma cruzi elicits a potent inflammatory response in acutely infected hearts that keeps parasitism in check and triggers cardiac abnormalities. A most-studied mechanism underlying innate immunity in T. cruzi infection is Toll-like receptor (TLR) activation by lipids and other parasite molecules. However, yet-to-be-identified pathways should exist. Here, we show that T. cruzi strongly upregulates monocyte chemoattractant protein 1 (MCP-1)/CCL2 and fractalkine (FKN)/CX3CL1 in cellular and mouse models of heart infection. Mechanistically, upregulation of MCP-1 and FKN stems from the interaction of parasite-derived neurotrophic factor (PDNF)/trans-sialidase with neurotrophic receptors TrkA and TrkC, as assessed by pharmacological inhibition, neutralizing antibodies, and gene silencing studies. Administration of a single dose of intravenous PDNF to naive mice results in a dose-dependent increase in MCP-1 and FKN in the heart and liver with pulse-like kinetics that peak at 3 h postinjection. Intravenous PDNF also augments MCP-1 and FKN in TLR signaling-deficient MyD88-knockout mice, underscoring the MyD88-independent action of PDNF. Although single PDNF injections do not increase MCP-1 and FKN receptors, multiple PDNF injections at short intervals up the levels of receptor transcripts in the heart and liver, suggesting that sustained PDNF triggers cell recruitment at infection sites. Thus, given that MCP-1 and FKN are chemokines essential to the recruitment of immune cells to combat inflammation triggers and to enhance tissue repair, our findings uncover a new mechanism in innate immunity against T. cruzi infection mediated by Trk signaling akin to an endogenous inflammatory and fibrotic pathway resulting from cardiomyocyte-TrkA recognition by matricellular connective tissue growth factor (CTGF/CCN2).

How is inflammation initiated? Individual influences of IL-1, IL-18 and HMGB1

Pro-inflammatory cytokines are crucial for fighting infection and establishing immunity. Recently, other proteins, such as danger-associated molecular patterns (DAMPs), have also been appreciated for their role in inflammation and immunity. Following the formation and activation of multiprotein complexes, termed inflammasomes, two cytokines, IL-1β and IL-18, along with the DAMP High Mobility Group Box 1 (HMGB1), are released from cells. Although these proteins all lack classical secretion signals and are released by inflammasome activation, they each lead to different downstream consequences. This review examines how various inflammasomes promote the release of IL-1β, IL-18 and HMGB1 to combat pathogenic situations. Each of these effector molecules plays distinct roles during sterile inflammation, responding to viral, bacterial and parasite infection, and tailoring the innate immune response to specific threats.