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

Exposing kinetic disparities between inflammasome readouts using time-resolved analysis

The NLRP3 inflammasome is an intracellular multiprotein complex described to be involved in both an effective host response to infectious agents and various diseases. Investigation into the NLRP3 inflammasome has been extensive in the past two decades, and often revolves around the analysis of a few specific readouts, including ASC-speck formation, caspase-1 cleavage or activation, and cleavage and release of IL-1β and/or IL-18. Quantification of these readouts is commonly undertaken as an endpoint analysis, where the presence of each positive outcome is assessed independently of the others. In this study, we apply time-resolved analysis of a human macrophage model (differentiated THP-1-ASC-GFP cells) to commonly accessible methods. This approach yields the additional quantifiable metrics time-resolved absolute change and acceleration, allowing comparisons between readouts. Using this methodological approach, we reveal (potential) discrepancies between inflammasome-related readouts that otherwise might go undiscovered. The study highlights the importance of time-resolved data in general and may be further extended as well as incorporated into other areas of research.

Imeglimin attenuates NLRP3 inflammasome activation by restoring mitochondrial functions in macrophages

Imeglimin is a novel oral antidiabetic drug for treating type 2 diabetes. However, the effect of imeglimin on NLRP3 inflammasome activation has not been investigated yet. Here, we aimed to investigate whether imeglimin reduces LPS-induced NLRP3 inflammasome activation in THP-1 macrophages and examine the associated underlying mechanisms. We analyzed the mRNA and protein expression levels of NLRP3 inflammasome components and IL-1β secretion. Additionally, reactive oxygen species (ROS) generation, mitochondrial membrane potential, and mitochondrial permeability transition pore (mPTP) opening were measured by flow cytometry. Imeglimin inhibited NLRP3 inflammasome-mediated IL-1β production in LPS-stimulated THP-1-derived macrophages. In addition, imeglimin reduced LPS-induced mitochondrial ROS production and mitogen-activated protein kinase phosphorylation. Furthermore, imeglimin restored the mitochondrial function by modulating mitochondrial membrane depolarization and mPTP opening. We demonstrated for the first time that imeglimin reduces LPS-induced NLRP3 inflammasome activation by inhibiting mPTP opening in THP-1 macrophages. These results suggest that imeglimin could be a promising new anti-inflammatory agent for treating diabetic complications.

Systematic Review of Brain-Eating Amoeba: A Decade Update

INTRODUCTION

Primary amoebic meningoencephalitis (PAM) is a rare but lethal infection of the brain caused by a eukaryote called Naegleria fowleri (N. fowleri). The aim of this review is to consolidate the recently published case reports of N. fowleri infection by describing its epidemiology and clinical features with the goal of ultimately disseminating this information to healthcare personnel.

METHODS

A comprehensive literature search was carried out using PubMed, Web of Science, Scopus, and OVID databases until 31 December 2022 by two independent reviewers. All studies from the year 2013 were extracted, and quality assessments were carried out meticulously prior to their inclusion in the final analysis.

RESULTS

A total of 21 studies were selected for qualitative analyses out of the 461 studies extracted. The cases were distributed globally, and 72.7% of the cases succumbed to mortality. The youngest case was an 11-day-old boy, while the eldest was a 75-year-old. Significant exposure to freshwater either from recreational activities or from a habit of irrigating the nostrils preceded onset. The symptoms at early presentation included fever, headache, and vomiting, while late sequalae showed neurological manifestation. An accurate diagnosis remains a challenge, as the symptoms mimic bacterial meningitis. Confirmatory tests include the direct visualisation of the amoeba or the use of the polymerase chain reaction method.

CONCLUSIONS

N. fowleri infection is rare but leads to PAM. Its occurrence is worldwide with a significant risk of fatality. The suggested probable case definition based on the findings is the acute onset of fever, headache, and vomiting with meningeal symptoms following exposure to freshwater within the previous 14 days. Continuous health promotion and health education activities for the public can help to improve knowledge and awareness prior to engagement in freshwater activities.

Pro-inflammatory cytokine responses to Naegleria fowleri infection

Naegleria fowleri, or the “brain-eating amoeba,” is responsible for a rare, but lethal, infection known as primary amoebic meningoencephalitis (PAM). Confirmed PAM cases have seen both a rise in numbers, as well as expansion of geographic range over the past several decades. There is no effective therapy for PAM and the clinical prognosis remains grim with a mortality rate over 95%. The role of the immune response in disease prevention and disease severity remains unclear. In this review, we explore potential roles of inflammatory immune responses to N. fowleri in disease pathogenesis with a primary focus on pro-inflammatory cytokines IL-1, IL-6, and TNFα. We also discuss modulating proinflammatory cytokines as an additional immune therapy in PAM treatment.

NLRP3 inflammasome signal pathway involves in Vibrio harveyi-induced inflammatory response in murine peritoneal macrophages in vitro

Vibrio harveyi, an important zoonotic pathogen, can infect wounds and cause inflammatory response. Understanding the inflammatory response pathways could facilitate the exploration of molecular mechanisms for treating V. harveyi infection. NLR family pyrin domain-containing 3 (NLRP3) inflammasome is involved in the interaction between hosts and pathogenic microorganisms and could be sensed by various pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs). Nonetheless, the function of NLRP3 inflammasome in V. harveyi infection remains unclear. In the present study, we established a V. harveyi infection model using murine peritoneal macrophages (PMs). Various techniques, including western blot analysis, enzyme-linked immunosorbent assay (ELISA), RT-qPCR, immunofluorescence, and inhibition assays, were used to explore the molecular mechanism of V. harveyi-induced inflammation. The results showed that many inflammatory cytokines participated in V. harveyi infection, with interleukin (IL)-1β being the most abundant. Pan-caspase inhibitor pretreatment significantly decreased the secretion of IL-1β in murine PMs. Moreover, the identification of V. harveyi involved a large number of NLR molecules, especially the NLRP3 receptor, and further studies revealed that NLPR3 inflammasome was activated by V. harveyi infection, as evidenced by puncta-like NLRP3 surrounding cell nuclear, ASC specks in the nucleus and cytoplasm, and ASC oligomerization. Inhibition of NLRP3 inflammasome impaired the release of mature IL-1β in V. harveyi-infected murine PMs. Furthermore, blocking the secretion of mature IL-1β could markedly decrease the release of other proinflammatory cytokines, including IL-6, IL-12, and tumor necrosis factor-α. Overall, these data indicated that NLRP3 inflammasome was activated in response to V. harveyi infection and enhanced inflammatory response by promoting IL-1β secretion in murine PMs.

Primary Amoebic Meningoencephalitis by Naegleria fowleri: Pathogenesis and Treatments

Naegleria fowleri is a free-living amoeba (FLA) that is commonly known as the "brain-eating amoeba." This parasite can invade the central nervous system (CNS), causing an acute and fulminating infection known as primary amoebic meningoencephalitis (PAM). Even though PAM is characterized by low morbidity, it has shown a mortality rate of 98%, usually causing death in less than two weeks after the initial exposure. This review summarizes the most recent information about N. fowleri, its pathogenic molecular mechanisms, and the neuropathological processes implicated. Additionally, this review includes the main therapeutic strategies described in case reports and preclinical studies, including the possible use of immunomodulatory agents to decrease neurological damage.

Oxysterols Modify NLRP2 in Epithelial Cells, Identifying a Mediator of Ozone-induced Inflammation

Ozone (O₃) is a prevalent air pollutant causing lung inflammation. Previous studies demonstrate that O₃ oxidizes lipids, such as cholesterol, in the airway to produce oxysterols, such as secosterol-A (SecoA), which are electrophiles capable of forming covalent linkages preferentially with lysine residues and consequently modify protein function. The breadth of proteins modified by this oxysterol as well as the biological consequences in the lung are unknown. Using an alkynyl-tagged form of SecoA and shotgun proteomics, we identified 135 proteins to be modified bronchial epithelial cells. Among them was NLR Family Pyrin Domain Containing 2 (NLRP2) forming a SecoA-protein adduct at lysine (K1019) in the terminal leucine-rich-repeat, a known regulatory region for NLR proteins. NLRP2 expression in airway epithelial cells was characterized and CRISPR-Cas9 knockout and shRNA knockdown of NLRP2 was used to determine its function in O₃-induced inflammation. No evidence for NLPR2 inflammasome formation or NLRP2-dependent increase in caspase-1 activity in response to O₃ was observed. O₃-induced pro-inflammatory gene expression for CXCL2 and CXCL8/IL8 was further enhanced in NLRP2 knockout cells, suggesting a negative regulatory role. Reconstitution of NLRP2 KO cells with K1019R mutant NLRP2 partially blocked SecoA adduction and enhanced O₃-induced IL-8 release as compared to wild type NLRP2. Together, our findings uncover NLRP2 as a highly abundant, key component of pro-inflammatory signaling pathways in airway epithelial cells and as a novel mediator of O₃-induced inflammation.

CD36 promotes NLRP3 inflammasome activation via the mtROS pathway in renal tubular epithelial cells of diabetic kidneys

Tubulointerstitial inflammation plays a key role in the pathogenesis of diabetic nephropathy (DN). Interleukin-1β (IL-1β) is the key proinflammatory cytokine associated with tubulointerstitial inflammation. The NLRP3 inflammasome regulates IL-1β activation and secretion. Reactive oxygen species (ROS) represents the main mediator of NLRP3 inflammasome activation. We previously reported that CD36, a class B scavenger receptor, mediates ROS production in DN. Here, we determined whether CD36 is involved in NLRP3 inflammasome activation and explored the underlying mechanisms. We observed that high glucose induced-NLRP3 inflammasome activation mediate IL-1β secretion, caspase-1 activation, and apoptosis in HK-2 cells. In addition, the levels of CD36, NLRP3, and IL-1β expression (protein and mRNA) were all significantly increased under high glucose conditions. CD36 knockdown resulted in decreased NLRP3 activation and IL-1β secretion. CD36 knockdown or the addition of MitoTempo significantly inhibited ROS production in HK-2 cells. CD36 overexpression enhanced NLRP3 activation, which was reduced by MitoTempo. High glucose levels induced a change in the metabolism of HK-2 cells from fatty acid oxidation (FAO) to glycolysis, which promoted mitochondrial ROS (mtROS) production after 72 h. CD36 knockdown increased the level of AMP-activated protein kinase (AMPK) activity and mitochondrial FAO, which was accompanied by the inhibition of NLRP3 and IL-1β. The in vivo experimental results indicate that an inhibition of CD36 could protect diabetic db/db mice from tubulointerstitial inflammation and tubular epithelial cell apoptosis. CD36 mediates mtROS production and NLRP3 inflammasome activation in db/db mice. CD36 inhibition upregulated the level of FAO-related enzymes and AMPK activity in db/db mice. These results suggest that NLRP3 inflammasome activation is mediated by CD36 in renal tubular epithelial cells in DN, which suppresses mitochondrial FAO and stimulates mtROS production.

Extracellular vesicles secreted by Giardia duodenalis regulate host cell innate immunity via TLR2 and NLRP3 inflammasome signaling pathways

Giardia duodenalis, also known as G. intestinalis or G. lamblia, is the major cause of giardiasis leading to diarrheal disease with 280 million people infections annually worldwide. Extracellular vesicles (EVs) have emerged as a ubiquitous mechanism participating in cells communications. The aim of this study is to explore the roles of G. duodenalis EVs (GEVs) in host-pathogen interactions using primary mouse peritoneal macrophages as a model. Multiple methods of electron microscopy, nanoparticle tracking analysis, proteomic assays, flow cytometry, immunofluorescence, qPCR, western blot, ELISA, inhibition assays, were used to characterize GEVs, and explore its effects on the host cell innate immunity as well as the underlying mechanism using primary mouse peritoneal macrophages. Results showed that GEVs displayed typical cup-shaped structure with 150 nm in diameter. GEVs could be captured by macrophages and triggered immune response by increasing the production of inflammatory cytokines Il1β, Il6, Il10, Il12, Il17, Ifng, Tnf, Il18, Ccl20 and Cxcl2. Furthermore, activation of TLR2 and NLRP3 inflammasome signaling pathways involved in this process. In addition, CA-074 methyl ester (an inhibitor of cathepsin B) or zVAD-fmk (an inhibitor of pan-caspase) pretreatment entirely diminished these effects triggered by GEVs exposure. Taken together, these findings demonstrated that GEVs could be internalized into mouse peritoneal macrophages and regulate host cell innate immunity via TLR2 and NLRP3 inflammasome signaling pathways.

G. duodenalis, one of the most common cause of diarrheal diseases, is widely existed in the contaminated water and threatening the public health especially in developing countries. Along with the increasing resistance to anti-G. duodenalis drugs occurs, new targets against giardiasis are of urgently needed. The innate immune system is the first defense line of organism to resist multiple pathogens invasion through recognizing pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs), termed Toll-like receptors (TLRs) on the surface of cell membrane and nucleotide oligomerization domain (Nod)-like receptors (NLRs) inside immune cells. Recently, extracellular vesicles have emerged as a ubiquitous mechanism participating in cells communications. In this study, EVs secreted by extracellular protozoan G. duodenalis were obtained and displayed typical cup-shaped structure with 150 nm in diameter. Moreover, GEVs could enter into primary mouse peritoneal macrophages and regulate host cell innate immunity by up-regulation of various inflammatory cytokines expression. Furthermore, TLR2 and NLRP3 inflammasome signaling pathways involved in this process. This study demonstrated that GEVs could be internalized into primary mouse peritoneal macrophages, regulate host cell innate immunity via TLR2 and NLRP3 inflammasome signaling pathways, and may provide new targets against giardiasis.

Giardia spp. and the Gut Microbiota: Dangerous Liaisons

Alteration of the intestinal microbiome by enteropathogens is commonly associated with gastrointestinal diseases and disorders and has far-reaching consequences for overall health. Significant advances have been made in understanding the role of microbial dysbiosis during intestinal infections, including infection with the protozoan parasite Giardia duodenalis, one of the most prevalent gut protozoa. Altered species composition and diversity, functional changes in the commensal microbiota, and changes to intestinal bacterial biofilm structure have all been demonstrated during the course of Giardia infection and have been implicated in Giardia pathogenesis. Conversely, the gut microbiota has been found to regulate parasite colonization and establishment and plays a critical role in immune modulation during mono and polymicrobial infections. These disruptions to the commensal microbiome may contribute to a number of acute, chronic, and post-infectious clinical manifestations of giardiasis and may account for variations in disease presentation within and between infected populations. This review discusses recent advances in characterizing Giardia-induced bacterial dysbiosis in the gut and the roles of dysbiosis in Giardia pathogenesis.

Macrophage Proinflammatory Responses to Microorganisms and Transplanted Organs

Tissue-resident macrophages and those conscripted from the blood/bone marrow are professional phagocytes. They play a role in tissue homeostasis, replacement, and healing, and are the first-line responders to microbial (viral, bacterial, and fungi) infections. Intrinsic ameboid-type motility allows non-resident macrophages to move to the site of inflammation or injury, where, in response to the inflammatory milieu they perform the anti-microbial and/or tissue repair functions. Depending on the need and the signaling from the surrounding tissue and other immune cells, macrophages acquire morphologically and functionally different phenotypes, which allow them to play either pro-inflammatory or anti-inflammatory functions. As such, the macrophages are also the major players in the rejection of the transplanted organs making an excellent target for the novel anti-rejection therapies in clinical transplantation. In this review, we describe some of the less covered aspects of macrophage response to microbial infection and organ transplantation.

Microglia in neuropathology caused by protozoan parasites

Involvement of the central nervous system (CNS) is the most severe consequence of some parasitic infections. Protozoal infections comprise a group of diseases that together affect billions of people worldwide and, according to the World Health Organization, are responsible for more than 500000 deaths annually. They include African and American trypanosomiasis, leishmaniasis, malaria, toxoplasmosis, and amoebiasis. Mechanisms underlying invasion of the brain parenchyma by protozoa are not well understood and may depend on parasite nature: a vascular invasion route is most common. Immunosuppression favors parasite invasion into the CNS and therefore the host immune response plays a pivotal role in the development of a neuropathology in these infectious diseases. In the brain, microglia are the resident immune cells active in defense against pathogens that target the CNS. Beside their direct role in innate immunity, they also play a principal role in coordinating the trafficking and recruitment of other immune cells from the periphery to the CNS. Despite their evident involvement in the neuropathology of protozoan infections, little attention has given to microglia-parasite interactions. This review describes the most prominent features of microglial cells and protozoan parasites and summarizes the most recent information regarding the reaction of microglial cells to parasitic infections. We highlight the involvement of the periphery-brain axis and emphasize possible scenarios for microglia-parasite interactions.

Treponema pallidum promotes macrophage polarization and activates the NLRP3 inflammasome pathway to induce interleukin-1β production

Background

The involvement of inflammasome activation and macrophage polarization during the process of syphilis infection remains unknown. In this study, A series of experiments were performed using human macrophages to research the role of NLRP3 inflammasome regulation in interleukin (IL)-1β production and its influence on macrophage polarization triggered by T. pallidum.

Results

The results showed that in M0 macrophages treated with T. pallidum, the M1-associated markers inducible nitric oxide synthase (iNOS), IL-1β and TNF-α were upregulated, and the M2-associated markers CD206 and IL-10 were downregulated. In addition, we observed NLRP3 inflammasome activation and IL-1β secretion in T. pallidum-treated macrophages, and the observed production of IL-1β occurred in a dose- and time-dependent manner. Moreover, the secretion of IL-1β by macrophages after T. pallidum treatment was notably reduced by anti-NLRP3 siRNA and caspase-1 inhibitor treatment. NAC, KCl, and CA074-ME treatment also suppressed IL-1β release from T. pallidum-treated macrophages.

Conclusions

These findings showed that T. pallidum induces M0 macrophages to undergo M1 macrophage polarization and elevate IL-1β secretion through NLRP3. Moreover, the process of NLRP3 inflammasome activation and IL-1β production in macrophages in response to T. pallidum infection involves K⁺ efflux, mitochondrial ROS production and cathepsin release. This study provides a new insight into the innate immune response to T. pallidum infection.

Metabolomics Reveals the Efficacy of Caspase Inhibition for Saikosaponin D-Induced Hepatotoxicity

Saikosaponin d (SSd) is a major hepatoprotective component of saikosaponins derived from Radix Bupleuri, which was also linked to hepatotoxicity. Previous studies have demonstrated that caspases play a key role in SSd-induced liver cell death. Our in vitro and in vivo studies also showed that treatment with caspase inhibitor z-VAD-fmk could significantly reduce the L02 hepatocyte cells death and lessen the degree of liver damage in mice caused by SSd. In order to further reveal the underlying mechanisms of caspase inhibition in SSd-induced hepatotoxicity, mass spectrometry based untargeted metabolomics was conducted. Significant alterations in metabolic profiling were observed in SSd-treated group, which could be restored by caspase inhibition. Bile acids and phospholipids were screened out to be most significant by spearman correlation analysis, heatmap analysis and S-Plot analysis. These findings were further confirmed by absolute quantitation of bile acids via targeted metabolomics approach. Furthermore, cytokine profiles were analyzed to identify potential associations between inflammation and metabolites. The study could provide deeper insight into the hepatotoxicity of SSd and the efficacy of caspase inhibition.

Leptospira interrogans infection leads to IL-1β and IL-18 secretion from a human macrophage cell line through reactive oxygen species and cathepsin B mediated-NLRP3 inflammasome activation

Leptospirosis is a worldwide zoonosis caused by spirochetes from the genus Leptospira. Although there is a large diversity of clinical signs and symptoms, a severe inflammatory response is common to all leptospirosis patients. The mechanism of IL-1β secretion during Leptospira infection has been previously studied in mouse macrophages. However, the outcome of Leptospira infection is very different in human and murine macrophages, and the mechanisms responsible for IL-1β secretion in human macrophages had not been investigated. This study therefore examines the effects of Leptospira interrogans infection on inflammasome activation and proinflammatory cytokine expression in human macrophages. Increased mRNA and protein expression of NLRP3 was observed by real time RT-PCR and flow cytometry at 1 h after co-cultivation. Enzyme-linked immunosorbent assay (ELISA) determination showed that IL-1β and IL-18 are released in the culture supernatants at 1 h after cultivation. The inhibition assay showed that glybenclamide (a K⁺ efflux inhibitor that blocks NLRP3 inflammasome activation) and N-benzyloxycarbony-Val-Ala-Asp (O-methyl)-fluoromethylketone (Z-VAD-FMK; a caspase-1 inhibitor) and NLRP3 depletion with siRNAs reduced the levels of IL-1β and IL-18 release. Moreover, the levels of IL-1β and IL-18 production decreased in CA-074 (a cathepsin B inhibitor) and NAC (an anti-oxidant) pretreated human macrophages, compared to untreated controls. This study suggests that L. interrogans infection leads to reactive oxygen species (ROS)- and cathepsin B-dependent NLRP3 inflammasome activation, which subsequently mediates caspase-1 activation and IL-1β and IL-18 release.

Toll-like receptors participate in Naegleria fowleri recognition

Naegleria fowleri is a protozoan that invades the central nervous system and causes primary amoebic meningoencephalitis. It has been reported that N. fowleri induces an important inflammatory response during the infection. In the present study, we evaluated the roles of Toll-like receptors in the recognition of N. fowleri trophozoites by human mucoepithelial cells, analyzing the expression and production of innate immune response mediators. After amoebic interactions with NCI-H292 cells, the expression and production levels of IL-8, TNF-α, IL-1β, and human beta defensin-2 were evaluated by RT-PCR, ELISA, immunofluorescence, and dot blot assays, respectively. To determine whether the canonical signaling pathways were engaged, we used different inhibitors, namely, IMG-2005 for MyD88 and BAY 11-7085 for the nuclear factor NFkB. Our results showed that the expression and production of the pro-inflammatory cytokines and beta defensin-2 were induced by N. fowleri mainly through the canonical TLR4 pathway in a time-dependent manner.