Human monocyte interleukin-1beta posttranslational processing. Evidence of a volume-regulated response

Unraveling the blood microbiome: novel insights into inflammasome responses in Crohn's disease

OBJECTIVE

Crohn's disease (CD), an inflammatory bowel disease with unknown etiology, is influenced by genetic, environmental, and immunological factors. This study aimed to analyze the blood microbiome and inflammasome responses, emphasizing NLRP3 protein expression and IL-1β and IL-18 plasma levels, between Crohn's patients and healthy subjects.

METHODS

A total of 40 volunteers were included in this study. The 16S rRNA technique was used to sequence the V3-V4 regions of the blood sample. NLRP3 protein levels in plasma were ascertained through Western Blot, and IL-1β and IL-18 plasma profiles were examined using ELISA.

RESULTS

Analysis highlighted five unique phyla in patients' plasma, emphasizing the role of the blood microbiome in CD. Compared to controls, Crohn's patients exhibited elevated NLRP3 protein expression. Plasma IL-1β levels were diminished in patients ( P  = 0.0041), whereas IL-18 levels were comparably higher ( P  = 0.8209). In patients with CD, the presence of Staphylococcus sciuri in blood samples highlights its potential role in the disease's onset. The study also underscored the interplay between dietary habits, specifically increased meat consumption, and the progression of CD.

CONCLUSION

Our pioneering research discerns the variations in the blood microbiome and inflammasome responses between Crohn's patients and healthy individuals. Significant microbiome alterations and the detection of the Staphylococcus sciuri pathogen in Crohn's patients were notable. The pronounced NLRP3 protein in patients suggests its potential as a diagnostic biomarker. Future explorations into IL-1β and IL-18 pathways promise to unveil innovative insights into CD.

The Role of the NLRP3 Inflammasome in the Molecular and Biochemical Mechanisms of Cervical Ripening: A Comprehensive Review

The uterine cervix is one of the key factors involved in ensuring a proper track of gestation and labor. At the end of the gestational period, the cervix undergoes extensive changes, which can be summarized as a transformation from a non-favorable cervix to one that is soft and prone to dilation. During a process called cervical ripening, fundamental remodeling of the cervical extracellular matrix (ECM) occurs. The cervical ripening process is a derivative of many interlocking and mutually driving biochemical and molecular pathways under the strict control of mediators such as inflammatory cytokines, nitric oxide, prostaglandins, and reactive oxygen species. A thorough understanding of all these pathways and learning about possible triggering factors will allow us to develop new, better treatment algorithms and therapeutic goals that could protect women from both dysfunctional childbirth and premature birth. This review aims to present the possible role of the NLRP3 inflammasome in the cervical ripening process, emphasizing possible mechanisms of action and regulatory factors.

Inflammasomes as regulators of mechano-immunity

Mechano-immunity, the intersection between cellular or tissue mechanics and immune cell function, is emerging as an important factor in many inflammatory diseases. Mechano-sensing defines how cells detect mechanical changes in their environment. Mechano-response defines how cells adapt to such changes, e.g. form synapses, signal or migrate. Inflammasomes are intracellular immune sensors that detect changes in tissue and cell homoeostasis during infection or injury. We and others recently found that mechano-sensing of tissue topology (swollen tissue), topography (presence and distribution of foreign solid implant) or biomechanics (stiffness), alters inflammasome activity. Once activated, inflammasomes induce the secretion of inflammatory cytokines, but also change cellular mechanical properties, which influence how cells move, change their shape, and interact with other cells. When overactive, inflammasomes lead to chronic inflammation. This clearly places inflammasomes as important players in mechano-immunity. Here, we discuss a model whereby inflammasomes integrate pathogen- and tissue-injury signals, with changes in tissue mechanics, to shape the downstream inflammatory responses and allow cell and tissue mechano-adaptation. We will review the emerging evidence that supports this model.

NOD-like Receptors-Emerging Links to Obesity and Associated Morbidities

Obesity and its associated metabolic morbidities have been and still are on the rise, posing a major challenge to health care systems worldwide. It has become evident over the last decades that a low-grade inflammatory response, primarily proceeding from the adipose tissue (AT), essentially contributes to adiposity-associated comorbidities, most prominently insulin resistance (IR), atherosclerosis and liver diseases. In mouse models, the release of pro-inflammatory cytokines such as TNF-alpha (TNF-α) and interleukin (IL)-1β and the imprinting of immune cells to a pro-inflammatory phenotype in AT play an important role. However, the underlying genetic and molecular determinants are not yet understood in detail. Recent evidence demonstrates that nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family proteins, a group of cytosolic pattern recognition receptors (PRR), contribute to the development and control of obesity and obesity-associated inflammatory responses. In this article, we review the current state of research on the role of NLR proteins in obesity and discuss the possible mechanisms leading to and the outcomes of NLR activation in the obesity-associated morbidities IR, type 2 diabetes mellitus (T2DM), atherosclerosis and non-alcoholic fatty liver disease (NAFLD) and discuss emerging ideas about possibilities for NLR-based therapeutic interventions of metabolic diseases.

Blocking the Hormone Receptors Modulates NLRP3 in LPS-Primed Breast Cancer Cells

NOD-like receptor protein 3 (NLRP3) may contribute to the growth and propagation of breast cancer (BC). The effect of estrogen receptor-α (ER-α), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) on NLRP3 activation in BC remains unknown. Additionally, our knowledge of the effect of blocking these receptors on NLRP3 expression is limited. We used GEPIA, UALCAN, and the Human Protein Atlas for transcriptomic profiling of NLRP3 in BC. Lipopolysaccharide (LPS) and adenosine 5'-triphosphate (ATP) were used to activate NLRP3 in luminal A MCF-7 and in TNBC MDA-MB-231 and HCC1806 cells. Tamoxifen (Tx), mifepristone (mife), and trastuzumab (Tmab) were used to block ER-α, PR, and HER2, respectively, on inflammasome activation in LPS-primed MCF7 cells. The transcript level of NLRP3 was correlated with ER-ɑ encoding gene ESR1 in luminal A (ER-α⁺, PR⁺) and TNBC tumors. NLRP3 protein expression was higher in untreated and LPS/ATP-treated MDA-MB-231 cells than in MCF7 cells. LPS/ATP-mediated NLRP3 activation reduced cell proliferation and recovery of wound healing in both BC cell lines. LPS/ATP treatment prevented spheroid formation in MDA-MB-231 cells but did not affect MCF7. HGF, IL-3, IL-8, M-CSF, MCP-1, and SCGF-b cytokines were secreted in both MDA-MB-231 and MCF7 cells in response to LPS/ATP treatment. Tx (ER-α inhibition) promoted NLRP3 activation and increased migration and sphere formation after LPS treatment of MCF7 cells. Tx-mediated activation of NLRP3 was associated with increased secretion of IL-8 and SCGF-b compared to LPS-only-treated MCF7 cells. In contrast, Tmab (Her2 inhibition) had a limited effect on NLRP3 activation in LPS-treated MCF7 cells. Mife (PR inhibition) opposed NLRP3 activation in LPS-primed MCF7 cells. We have found that Tx increased the expression of NLRP3 in LPS-primed MCF7. These data suggest a link between blocking ER-α and activation of NLRP3, which was associated with increased aggressiveness of the ER-α⁺ BC cells.

The mechanism of NLRP3 inflammasome activation and its pharmacological inhibitors

NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3) is a cytosolic pattern recognition receptor (PRR) that recognizes multiple pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Once activated, NLRP3 initiates the inflammasome assembly together with the adaptor ASC and the effector caspase-1, leading to caspase-1 activation and subsequent cleavage of IL-1β and IL-18. Aberrant NLRP3 inflammasome activation is linked with the pathogenesis of multiple inflammatory diseases, such as cryopyrin-associated periodic syndromes, type 2 diabetes, non-alcoholic steatohepatitis, gout, and neurodegenerative diseases. Thus, NLRP3 is an important therapeutic target, and researchers are putting a lot of effort into developing its inhibitors. The review summarizes the latest advances in the mechanism of NLRP3 inflammasome activation and its pharmacological inhibitors.

Purinergic P2X7 receptor antagonist ameliorates intestinal inflammation in postoperative ileus

Postoperative ileus (POI) is a postsurgical gastrointestinal motility dysfunction caused by mechanical stress to the intestine during abdominal surgery. POI leads to nausea and vomiting reduced patient quality of life, as well as high medical costs and extended hospitalization. Intestinal inflammation caused by macrophages and neutrophils is thought to be important in the mechanism of POI. Surgery-associated tissue injury and inflammation induce the release of adenosine triphosphate (ATP) from injured cells. Released ATP binds the purinergic P2X7 receptor (P2X7R) expressed on inflammatory cells, inducing the secretion of inflammatory mediators. P2X7R antagonists are thought to be important mediators of the first step in the inflammation process, and studies in chemically induced colitis models confirmed that P2X7R antagonists exhibit anti-inflammatory effects. Therefore, we hypothesized that P2X7R plays an important role in POI. POI models were generated from C57BL/6J mice. Mice were treated with P2X7R antagonist A438079 (34 mg/kg) 30 min before and 2 hr after intestinal manipulation (IM). Inflammatory cell infiltration and gastrointestinal transit were measured. A438079 ameliorated macrophage and neutrophil infiltration in the POI model. Impaired intestinal transit improved following A438079 treatment. P2X7R was expressed on both infiltrating and resident macrophages in the inflamed ileal muscle layer. The P2X7R antagonist A438079 exhibits anti-inflammatory effects via P2X7R expressed on macrophages and therefore could be a target in the treatment of POI.

The two pore potassium channel THIK-1 regulates NLRP3 inflammasome activation

The NLRP3 (NLR family, pyrin domain containing 3) inflammasome is a multi-protein complex responsible for the activation of caspase-1 and the subsequent cleavage and activation of the potent proinflammatory cytokines IL-1β and IL-18, and pyroptotic cell death. NLRP3 is implicated as a driver of inflammation in a range of disorders including neurodegenerative diseases, type 2 diabetes, and atherosclerosis. A commonly reported mechanism contributing to NLRP3 inflammasome activation is potassium ion (K⁺ ) efflux across the plasma membrane. Identification of K⁺ channels involved in NLRP3 activation remains incomplete. Here, we investigated the role of the K⁺ channel THIK-1 in NLRP3 activation. Both pharmacological inhibitors and cells from THIK-1 knockout (KO) mice were used to assess THIK-1 contribution to macrophage NLRP3 activation in vitro. Pharmacological inhibition of THIK-1 inhibited caspase-1 activation and IL-1β release from mouse bone-marrow-derived macrophages (BMDMs), mixed glia, and microglia in response to NLRP3 agonists. Similarly, BMDMs and microglia from THIK-1 KO mice had reduced NLRP3-dependent IL-1β release in response to P2X7 receptor activation with ATP. Overall, these data suggest that THIK-1 is a regulator of NLRP3 inflammasome activation in response to ATP and identify THIK-1 as a potential therapeutic target for inflammatory disease.

Volume-activated chloride channels contribute to lipopolysaccharide plus nigericin-induced pyroptosis in bone marrow-derived macrophages

The representative morphological features of pyroptosis are excessive cell swelling and subsequent membrane rupture. However, the mechanism underlying the cell's inherent inability to regulate volume during the progression of pyroptosis is poorly understood. In the current study, we found that both volume-activated chloride currents (I_{cl, vol}) and the regulatory volume decrease (RVD) were markedly decreased in bone marrow-derived macrophages (BMDMs) undergoing pyroptosis induced by lipopolysaccharides (LPS) and nigericin. The inhibition of I_{Cl, vol} and RVD by the chloride channel blockers, tamoxifen or DCPIB, led to the emergence of pyroptosis-like phenotypes such as activated-caspase-1, pyroptotic-body-like bubbles, and a fried-egg-like appearance. Moreover, the expression of the volume-activated chloride channel (VRAC) constituent protein Leucine-Rich Repeat-Containing 8A (LRRC8A) was significantly down-regulated in pyroptotic BMDMs treated with LPS and nigericin. The silencing of LRRC8A expression by small interfering RNA (si)-LRRC8A transfection not only reduced I_{Cl, vol} and RVD, but also caused BMDMs to show pyroptosis-like manifestations such as activated-caspase-1, membrane bubbles, and have a fried-egg-like appearance. These results reveal a new mechanism for the loss of volume regulation in the process of pyroptotic cell swelling and strongly suggest that a functional deficiency of VRAC/LRRC8A plays a key role in this disorder.

Discovery of dronedarone and its analogues as NLRP3 inflammasome inhibitors with potent anti-inflammation activity

Inhibiting NLRP3 inflammasome activation is a prospective therapeutic strategy for uncontrolled inflammatory diseases. It is the first time that dronedarone, a multiply ion channel blocker, was identified as a NLRP3-inflammasome inhibitor with an IC₅₀ value of 6.84 μM against IL-1β release. A series of novel 5-amide benzofuran derivatives were designed and synthesized as NLRP3-inflammasome inhibitors. Compound 8c showed slightly increased activity (IC₅₀ = 3.85 μM) against IL-1β release. Notably, treatment with 8c could significantly inhibit NLRP3-mediated IL-1β release and ameliorate peritoneal inflammation in a mouse model of sepsis. Collectively, 8c is a promising lead compound for further chemical development as a NLRP3 inhibitor with anti-inflammation effects.

Therapeutic Potential of Pharmacological Targeting NLRP3 Inflammasome Complex in Cancer

Introduction

Dysregulation of NLRP3 inflammasome complex formation can promote chronic inflammation by increased release of IL-1β. However, the effect of NLRP3 complex formation on tumor progression remains controversial. Therefore, we sought to determine the effect of NLRP3 modulation on the growth of the different types of cancer cells, derived from lung, breast, and prostate cancers as well as neuroblastoma and glioblastoma in-vitro.

Method

The effect of Caspase 1 inhibitor (VX765) and combination of LPS/Nigericin on NLRP3 inflammasome activity was analyzed in A549 (lung cancer), MCF-7 (breast cancer), PC3 (prostate cancer), SH-SY5Y (neuroblastoma), and U138MG (glioblastoma) cells. Human fibroblasts were used as control cells. The effect of VX765 and LPS/Nigericin on NLRP3 expression was analyzed using western blot, while IL-1β and IL-18 secretion was detected by ELISA. Tumor cell viability and progression were determined using Annexin V, cell proliferation assay, LDH assay, sphere formation assay, transmission electron microscopy, and a multiplex cytokine assay. Also, angiogenesis was investigated by a tube formation assay. VEGF and MMPs secretion were detected by ELISA and a multiplex assay, respectively. Statistical analysis was done using one-way ANOVA with Tukey’s analyses and Kruskal–Wallis one-way analysis of variance.

Results

LPS/Nigericin increased NRLP3 protein expression as well as IL-1β and IL-18 secretion in PC3 and U138MG cells compared to A549, MCF7, SH-SY5Y cells, and fibroblasts. In contrast, MIF expression was commonly found upregulated in A549, PC3, SH-SY5Y, and U138MG cells and fibroblasts after Nigericin treatment. Nigericin and a combination of LPS/Nigericin decreased the cell viability and proliferation. Also, LPS/Nigericin significantly increased tumorsphere size in PC3 and U138MG cells. In contrast, the sphere size was reduced in MCF7 and SH-SY5Y cells treated with LPS/Nigericin, while no effect was detected in A549 cells. VX765 increased secretion of CCL24 in A549, MCF7, PC3, and fibroblasts as well as CCL11 and CCL26 in SH-SY5Y cells. Also, VX765 significantly increased the production of VEGF and MMPs and stimulated angiogenesis in all tumor cell lines.

Discussion

Our data suggest that NLRP3 activation using Nigericin could be a novel therapeutic approach to control the growth of tumors producing a low level of IL-1β and IL-18.

MiR-146a-5p Mimic Inhibits NLRP3 Inflammasome Downstream Inflammatory Factors and CLIC4 in Neonatal Necrotizing Enterocolitis

Objective: Necrotizing enterocolitis (NEC) is a gastrointestinal emergency with a severe inflammation storm, intestinal necrosis, and perforation. MicroRNA-146a-5p (miR-146a-5p) has been reported to be a valuable anti-inflammatory factor in various intestinal inflammatory disorders. However, the role of miR-146a-5p in NEC, its effects on nucleotide-binding domain and leucine-rich repeat-containing protein 3 (NLRP3) inflammasome, and its downstream inflammatory factors remain unknown. This study aimed to investigate the role of miR-146a-5p and NLRP3 inflammasome and its downstream inflammatory factors in NEC development.

Methods: The expression levels of miR-146a and NLRP3 inflammasome were investigated in intestinal tissues. Next, the mechanism by which miR-146a-5p regulates NLRP3 inflammasome activation was explored in vitro in THP-1 cells. Finally, to identify the effects of miR-146a-5p on NEC in vivo, NEC mice were transinfected with miR-146a-5p overexpression adenovirus before the occurrence of NEC.

Results: NLRP3 inflammasome enzymatic protein caspase-1 and its downstream inflammatory factors increased in NEC intestinal samples in both humans and mice, and miR-146a-5p expression level was increased and mainly expressed in the macrophages of the affected intestine. In vitro, only miR-146a-5p mimic inhibited NLRP3 inflammasome downstream inflammatory factors and its upstream protein chloride intracellular channel protein 4 (CLIC4) expression in cellular membrane in the THP-1 cell line, and this only occurred under mild/moderate LPS concentration. MiR-146a-5p overexpression adenovirus transfection reduced CLIC4 cellular membrane expression and inhibited NLRP3 downstream factors increasing in vivo. After the transfection of miR-146a-5p adenovirus, the survival rate of NEC mice was increased, and intestinal injury was ameliorated.

Conclusion: MiR-146a-5p inhibited NLRP3 inflammasome downstream inflammatory factors and CLIC4 membrane expression in NEC. Additionally, miR-146a-5p could attenuate inflammation and intestinal injury in the NEC-affected intestine.

Single-cell RNA-seq reveals CD16- monocytes as key regulators of human monocyte transcriptional response to Toxoplasma

Monocytes are among the major myeloid cells that respond to Toxoplasma, a ubiquitous foodborne that infects ≥ 1 billion people worldwide, in human peripheral blood. As such, a molecular understanding of human monocyte-Toxoplasma interactions can expedite the development of novel human toxoplasmosis control strategies. Current molecular studies on monocyte-Toxoplasma interactions are based on average cell or parasite responses across bulk cell populations. Although informative, population-level averages of monocyte responses to Toxoplasma have sometimes produced contradictory results, such as whether CCL2 or IL12 define effective monocyte responses to the parasite. Here, we used single-cell dual RNA sequencing (scDual-Seq) to comprehensively define, for the first time, the monocyte and parasite transcriptional responses that underpin human monocyte-Toxoplasma encounters at the single cell level. We report extreme transcriptional variability between individual monocytes. Furthermore, we report that Toxoplasma-exposed and unexposed monocytes are transcriptionally distinguished by a reactive subset of CD14⁺CD16^- monocytes. Functional cytokine assays on sorted monocyte populations show that the infection-distinguishing monocytes secrete high levels of chemokines, such as CCL2 and CXCL5. These findings uncover the Toxoplasma-induced monocyte transcriptional heterogeneity and shed new light on the cell populations that largely define cytokine and chemokine secretion in human monocytes exposed to Toxoplasma.

LRRC8A is essential for hypotonicity-, but not for DAMP-induced NLRP3 inflammasome activation

The NLRP3 inflammasome is a multi-molecular protein complex that converts inactive cytokine precursors into active forms of IL-1β and IL-18. The NLRP3 inflammasome is frequently associated with the damaging inflammation of non-communicable disease states and is considered an attractive therapeutic target. However, there is much regarding the mechanism of NLRP3 activation that remains unknown. Chloride efflux is suggested as an important step in NLRP3 activation, but which chloride channels are involved is still unknown. We used chemical, biochemical, and genetic approaches to establish the importance of chloride channels in the regulation of NLRP3 in murine macrophages. Specifically, we identify LRRC8A, an essential component of volume-regulated anion channels (VRAC), as a vital regulator of hypotonicity-induced, but not DAMP-induced, NLRP3 inflammasome activation. Although LRRC8A was dispensable for canonical DAMP-dependent NLRP3 activation, this was still sensitive to chloride channel inhibitors, suggesting there are additional and specific chloride sensing and regulating mechanisms controlling NLRP3.

Bafilomycin A1 enhances NLRP3 inflammasome activation in human monocytes independent of lysosomal acidification

The release of interleukin (IL)‐1β from primary human monocytes in response to extracellular LPS occurs through the NACHT, LRR and PYD domains‐containing protein 3 (NLRP3) inflammasome. In primary monocytes, in response to LPS, NLRP3 inflammasome activation is characterized by an independence of K⁺ efflux and ASC speck formation and has been termed the ‘alternative’ pathway. Here, we report that pharmacological inhibition of V‐ATPase with bafilomycin A1 exacerbated LPS‐induced NLRP3 inflammasome activation in primary human monocytes. Inhibition of V‐ATPase in the presence of extracellular LPS led to NLRP3‐dependent, K⁺ efflux‐independent, ASC oligomerization and caspase‐1 activation. Although V‐ATPases are required for lysosomal acidification, we found that acidic lysosomal pH and protease activity were dispensable for this altered response, suggesting that V‐ATPase inhibition triggered alternative signalling events. Therefore, V‐ATPases may serve additional roles during NLRP3 inflammasome activation in primary human monocytes.

Selective inhibition of the K+ efflux sensitive NLRP3 pathway by Cl- channel modulation

The NLRP3 inflammasome regulates production of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and contributes to inflammation exacerbating disease. Fenamate non-steroidal anti-inflammatory drugs (NSAIDs) were recently described as NLRP3 inflammasome inhibitors via chloride channel inhibition. Fenamate NSAIDs inhibit cyclooxygenase (COX) enzymes, limiting their potential as therapeutics for NLRP3-associated diseases due to established side effects. The aim here was to develop properties of the fenamates that inhibit NLRP3, and at the same time to reduce COX inhibition. We synthesised a library of analogues, with feedback from in silico COX docking potential, and IL-1β release inhibitory activity. Through iterative screening and rational chemical design, we established a collection of chloride channel inhibiting active lead molecules with potent activity at the canonical NLRP3 inflammasome and no activity at COX enzymes, but only in response to stimuli that activated NLRP3 by a K⁺ efflux-dependent mechanism. This study identifies a model for the isolation and removal of unwanted off-target effects, with the enhancement of desired activity, and establishes a new chemical motif for the further development of NLRP3 inflammasome inhibitors.

The NLRP3 inflammasome regulates production of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and contributes to inflammation exacerbating disease.

The NLRP3-inflammasome as a sensor of organelle dysfunction

Diverse pathogen- and damage-associated stresses drive inflammation via activation of the multimolecular NLRP3-inflammasome complex. How the effects of diverse stimuli are integrated by the cell to regulate NLRP3 has been the subject of intense research, and yet an accepted unifying hypothesis for the control of NLRP3 remains elusive. Here, we review the literature on the effects of NLRP3-activating stimuli on subcellular organelles and conclude that a shared feature of NLRP3-activating stresses is an organelle dysfunction. In particular, we propose that the endosome may be more important than previously recognized as a signal-integrating hub for NLRP3 activation in response to many stimuli and may also link to the dysfunction of other organelles. In addition, NLRP3-inflammasome-activating stimuli trigger diverse posttranslational modifications of NLRP3 that are important in controlling its activation. Future research should focus on how organelles respond to specific NLRP3-activating stimuli, and how this relates to posttranslational modifications, to delineate the organellar control of NLRP3.

The NLRP3 Inflammasome as a Pharmacological Target

NLRP3 is a cytosolic receptor member of the nucleotide-binding oligomerization domain NOD-like receptor family that surveys the intracellular environment for the presence of infection, pathogens, and metabolic alarms. Although the surveillance activity of NLRP3 is required to protect the host from several pathogens, uncontrolled activity can be detrimental to the host. Pharmacological and genetic strategies limiting NLRP3 inflammasome activation have been shown to be beneficial in a wide range of experimental models, from common pathologies such as arthritis, cardiovascular disease, and metabolic syndromes to rare genetic disorders such as cryopyrin-associated periodic syndrome. Thus, compounds that prevent NLRP3 inflammasome activation are of common interest with relevant therapeutic potential. The focus of this review is recent developments in NLRP3 inflammasome inhibitors.

Altered Monocyte and Langerhans Cell Innate Immunity in Patients With Recurrent Respiratory Papillomatosis (RRP)

The micromilieu within respiratory papillomas supports persistent human papillomavirus (HPV) infection and disease recurrence in patients with recurrent respiratory papillomatosis (RRP). These patients show polarized (T_H2-/Treg) adaptive immunity in papillomas and blood, enriched immature Langerhans cell (iLC) numbers, and overexpression of cyclooxygenase-2/prostaglandin E₂ (PGE₂) in the upper airway. Blood monocyte-derived, and tissue-derived iLCs from RRP patients and controls were now studied to more fully understand innate immune dysregulation in RRP. Patients' monocytes generated fewer iLCs than controls, due to a reduced fraction of classical monocytes that generated most but not all the iLCs. Prostaglandin E₂, which was elevated in RRP plasma, reduced monocyte-iLC differentiation from controls to the levels of RRP patients, but had no effect on subsequent iLC maturation. Cytokine/chemokine responses by iLCs from papillomas, foreskin, and abdominal skin differed significantly. Freshly derived tissue iLCs expressed low CCL-1 and high CCL-20 mRNAs and were unresponsive to IL-36γ stimulation. Papilloma iLCs uniquely expressed IL-36γ at baseline and expressed CCL1 when cultured overnight outside their immunosuppressive microenvironment without additional stimulation. We conclude that monocyte/iLC innate immunity is impaired in RRP, in part due to increased PGE₂ exposure in vivo. The immunosuppressive papilloma microenvironment likely alters iLC responses, and vice versa, supporting T_H2-like/Treg HPV-specific adaptive immunity in RRP.

The NLRP3 Inflammasome: An Overview of Mechanisms of Activation and Regulation

The NLRP3 inflammasome is a critical component of the innate immune system that mediates caspase-1 activation and the secretion of proinflammatory cytokines IL-1β/IL-18 in response to microbial infection and cellular damage. However, the aberrant activation of the NLRP3 inflammasome has been linked with several inflammatory disorders, which include cryopyrin-associated periodic syndromes, Alzheimer's disease, diabetes, and atherosclerosis. The NLRP3 inflammasome is activated by diverse stimuli, and multiple molecular and cellular events, including ionic flux, mitochondrial dysfunction, and the production of reactive oxygen species, and lysosomal damage have been shown to trigger its activation. How NLRP3 responds to those signaling events and initiates the assembly of the NLRP3 inflammasome is not fully understood. In this review, we summarize our current understanding of the mechanisms of NLRP3 inflammasome activation by multiple signaling events, and its regulation by post-translational modifications and interacting partners of NLRP3.

From Inflammasome to Exosome-Does Extracellular Vesicle Secretion Constitute an Inflammasome-Dependent Immune Response?

Inflammasomes are intracellular protein complexes of pattern recognition receptors and caspase-1, with essential functions in regulating inflammatory responses of macrophages and dendritic cells. The primary role of inflammasomes is to catalyze processing and secretion of pro-inflammatory cytokines IL-1β and IL-18. Recently, intracellular non-canonical inflammasome activation by caspases-4/5, which are also regulators of pyroptosis via processing gasdermin D, has been elucidated. Caspase-1, the effector protease of inflammasome complex, is also known to modulate secretion of large number of other proteins. Thereby, besides its known role in processing pro-inflammatory cytokines, the inflammasome turns into a universal regulator of protein secretion, which allows the danger-exposed cells to release various proteins in order to alert and guide neighboring cells. Majority of these proteins are not secreted through the conventional ER-Golgi secretory pathway. Instead, they are segregated in membrane-enclosed compartment and secreted in nanosized extracellular vesicles, which protect their cargo and guide it for delivery. Growing evidence indicates that inflammasome activity correlates with enhanced secretion of extracellular vesicles and modulation of their protein cargo. This inflammasome-driven unconventional, vesicle-mediated secretion of multitude of immunoregulatory proteins may constitute a novel paradigm in inflammatory responses. In this mini review we discuss the current knowledge and highlight unsolved questions about metabolic processes, signals, and mechanisms linking inflammasome activity with regulated extracellular vesicle secretion of proteins. Further investigations on this relationship may in the future help understanding the significance of extracellular vesicle secretion in inflammatory diseases such as atherosclerosis, gouty arthritis, asthma, Alzheimer's and many others.

Chloride regulates dynamic NLRP3-dependent ASC oligomerization and inflammasome priming

The NLRP3 inflammasome is an important regulator of inflammation and immunity. It is a multimolecular platform formed within cells that facilitates the activation of proinflammatory caspases to drive secretion of cytokines such as interleukin-1β (IL-1β). Knowledge of the mechanisms regulating formation of the NLRP3 inflammasome is incomplete. Here we report Cl^- channel-dependent formation of dynamic ASC oligomers and inflammasome specks that remain inactive in the absence of K⁺ efflux. Formed after Cl^- efflux exclusively, ASC specks are NLRP3 dependent, reversible, and inactive, although they further prime inflammatory responses, accelerating and enhancing release of IL-1β in response to a K⁺ efflux-inducing stimulus. NEK7 is a specific K⁺ sensor and does not associate with NLRP3 under conditions stimulating exclusively Cl^- efflux, but does after K⁺ efflux, activating the complex driving inflammation. Our investigation delivers mechanistic understanding into inflammasome activation and the regulation of inflammatory responses.

Hypo-osmotic Stress Drives IL-33 Production in Human Keratinocytes-An Epidermal Homeostatic Response

Although inflammation has traditionally been considered a response to either exogenous pathogen-associated signals or endogenous signals of cell damage, other perturbations of homeostasis, generally referred to as stress, may also induce inflammation. The relationship between stress and inflammation is, however, not well defined. Here, we describe a mechanism of IL-33 induction driven by hypo-osmotic stress in human keratinocytes and also report interesting differences when comparing the responsiveness of other inflammatory mediators. The induction of IL-33 was completely dependent on EGFR and calcium signaling, and inhibition of calcium signaling not only abolished IL-33 induction but also dramatically changed the transcriptional pattern of other cytokines upon hypo-osmotic stress. IL-33 was not secreted but instead showed nuclear sequestration, conceivably acting as a failsafe mechanism whereby it is induced by potential danger but released only upon more extreme homeostatic perturbations that result in cell death. Finally, stress-induced IL-33 was also confirmed in an ex vivo human skin model, translating this mechanism to a potential tissue-relevant signal in the human epidermis. In conclusion, we describe hypo-osmotic stress as an inducer of IL-33 expression, linking cellular stress to nuclear accumulation of a strong proinflammatory cytokine.

The intracellular chloride channel proteins CLIC1 and CLIC4 induce IL-1β transcription and activate the NLRP3 inflammasome

The NLRP3 inflammasome is a multiprotein complex that regulates the activation of caspase-1 leading to the maturation of the proinflammatory cytokines IL-1β and IL-18 and promoting pyroptosis. Classically, the NLRP3 inflammasome in murine macrophages is activated by the recognition of pathogen-associated molecular patterns and by many structurally unrelated factors. Understanding the precise mechanism of NLRP3 activation by such a wide array of stimuli remains elusive, but several signaling events, including cytosolic efflux and influx of select ions, have been suggested. Accordingly, several studies have indicated a role of anion channels in NLRP3 inflammasome assembly, but their direct involvement has not been shown. Here, we report that the chloride intracellular channel proteins CLIC1 and CLIC4 participate in the regulation of the NLRP3 inflammasome. Confocal microscopy and cell fractionation experiments revealed that upon LPS stimulation of macrophages, CLIC1 and CLIC4 translocated into the nucleus and cellular membrane. In LPS/ATP-stimulated bone marrow-derived macrophages (BMDMs), CLIC1 or CLIC4 siRNA transfection impaired transcription of IL-1β, ASC speck formation, and secretion of mature IL-1β. Collectively, our results demonstrate that CLIC1 and CLIC4 participate both in the priming signal for IL-1β and in NLRP3 activation.

The complex cascade of cellular events governing inflammasome activation and IL-1β processing in response to inhaled particles

The innate immune system is the first line of defense against inhaled particles. Macrophages serve important roles in particle clearance and inflammatory reactions. Following recognition and internalization by phagocytes, particles are taken up in vesicular phagolysosomes. Intracellular phagosomal leakage, redox unbalance and ionic movements induced by toxic particles result in pro-IL-1β expression, inflammasome complex engagement, caspase-1 activation, pro-IL-1β cleavage, biologically-active IL-1β release and finally inflammatory cell death termed pyroptosis. In this review, we summarize the emerging signals and pathways involved in the expression, maturation and secretion of IL-1β during these responses to particles. We also highlight physicochemical characteristics of particles (size, surface and shape) which determine their capacity to induce inflammasome activation and IL-1β processing.

The effects of arginine glutamate, a promising excipient for protein formulation, on cell viability: Comparisons with NaCl

The effects of an equimolar mixture of l-arginine and l-glutamate (Arg·Glu) on cell viability and cellular stress using in vitro cell culture systems are examined with reference to NaCl, in the context of monoclonal antibody formulation. Cells relevant to subcutaneous administration were selected: the human monocyte cell line THP-1, grown as a single cell suspension, and adherent human primary fibroblasts. For THP-1 cells, the mechanism of cell death caused by relatively high salt concentrations was investigated and effects on cell activation/stress assessed as a function of changes in membrane marker and cytokine (interleukin-8) expression. These studies demonstrated that Arg·Glu does not have any further detrimental effects on THP-1 viability in comparison to NaCl at equivalent osmolalities, and that both salts at higher concentrations cause cell death by apoptosis; there was no significant effect on measures of THP-1 cellular stress/activation. For adherent fibroblasts, both salts caused significant toxicity at ~400 mOsm/kg, although Arg·Glu caused a more precipitous subsequent decline in viability than did NaCl. These data indicate that Arg·Glu is of equivalent toxicity to NaCl and that the mechanism of toxicity is such that cell death is unlikely to trigger inflammation upon subcutaneous injection in vivo.

Initiation and perpetuation of NLRP3 inflammasome activation and assembly

The NLRP3 (NOD-like receptor family, pyrin domain containing 3) inflammasome is a multiprotein complex that orchestrates innate immune responses to infection and cell stress through activation of caspase-1 and maturation of inflammatory cytokines pro-interleukin-1β (pro-IL-1β) and pro-IL-18. Activation of the inflammasome during infection can be protective, but unregulated NLRP3 inflammasome activation in response to non-pathogenic endogenous or exogenous stimuli can lead to unintended pathology. NLRP3 associates with mitochondria and mitochondrial molecules, and activation of the NLRP3 inflammasome in response to diverse stimuli requires cation flux, mitochondrial Ca(2+) uptake, and mitochondrial reactive oxygen species accumulation. It remains uncertain whether NLRP3 surveys mitochondrial integrity and senses mitochondrial damage, or whether mitochondria simply serve as a physical platform for inflammasome assembly. The structure of the active, caspase-1-processing NLRP3 inflammasome also requires further clarification, but recent studies describing the prion-like properties of ASC have advanced the understanding of how inflammasome assembly and caspase-1 activation occur while raising new questions regarding the propagation and resolution of NLRP3 inflammasome activation. Here, we review the mechanisms and pathways regulating NLRP3 inflammasome activation, discuss emerging concepts in NLRP3 complex organization, and expose the knowledge gaps hindering a comprehensive understanding of NLRP3 activation.

Spleen Tyrosine Kinase (Syk) Mediates IL-1β Induction by Primary Human Monocytes during Antibody-enhanced Dengue Virus Infection

Approximately 500,000 people are hospitalized with severe dengue illness annually. Antibody-dependent enhancement (ADE) of dengue virus (DENV) infection is believed to contribute to the pathogenic cytokine storm described in severe dengue patients, but the precise signaling pathways contributing to elevated cytokine production are not elucidated. IL-1β is a potent inflammatory cytokine that is frequently elevated during severe dengue, and the unique dual regulation of IL-1β provides an informative model to study ADE-induced cytokines. This work utilizes patient-derived anti-DENV mAbs and primary human monocytes to study ADE-induced IL-1β and other cytokines. ADE of DENV serotype 2 (DENV-2) elevates mature IL-1β secretion by monocytes independent of DENV replication by 4 h postinoculation (hpi). Prior to this, DENV immune complexes activate spleen tyrosine kinase (Syk) within 1 hpi. Syk induces elevated IL1B, TNF, and IL6 mRNA by 2 hpi. Syk mediates elevated IL-1β secretion by activating ERK1/2, and both Syk and ERK1/2 inhibitors ablated ADE-induced IL-1β secretion. Maturation of pro-IL-1β during ADE requires caspase-1 and NLRP3, but caspase-1 is suboptimally increased by ADE and can be significantly enhanced by a typical inflammasome agonist, ATP. Importantly, this inflammatory Syk-ERK signaling axis requires DENV immune complexes, because DENV-2 in the presence of serotype-matched anti-DENV-2 mAb, but not anti-DENV-1 mAb, activates Syk, ERK, and IL-1β secretion. This study provides evidence that DENV-2 immune complexes activate Syk to mediate elevated expression of inflammatory cytokines. Syk and ERK may serve as new therapeutic targets for interfering with ADE-induced cytokine expression during severe dengue.

Diverse Activators of the NLRP3 Inflammasome Promote IL-1β Secretion by Triggering Necrosis

The NLRP3 inflammasome is involved in caspase-1-dependent maturation of IL-1β in many contexts. A two-signal model has emerged for IL-1β maturation, with LPS providing "signal I" and diverse agents such as ATP, Nigericin, streptolysin O, uric acid crystals, and alum salts capable of acting as "signal II." In the absence of signal II, pro-IL-1β is upregulated but typically fails to be processed or released. What unites signal II stimuli has been debated, with the ability to promote K+ efflux suggested as a common factor, but the mechanism of IL-1β release remains unclear. Here, we show that all examined inflammasome signal II agents triggered necrosis, which was highly correlated with their ability to promote IL-1β release. IL-1β secretion occurred in tandem with the release of many additional proteins and was confined to necrotic cells. Thus, signal II agents initiate inflammation by promoting necrosis-driven IL-1β release, suggesting that IL-1β represents an inducible danger signal.

Danger- and pathogen-associated molecular patterns recognition by pattern-recognition receptors and ion channels of the transient receptor potential family triggers the inflammasome activation in immune cells and sensory neurons

An increasing number of studies show that the activation of the innate immune system and inflammatory mechanisms play an important role in the pathogenesis of numerous diseases. The innate immune system is present in almost all multicellular organisms and its activation occurs in response to pathogens or tissue injury via pattern-recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). Intracellular pathways, linking immune and inflammatory response to ion channel expression and function, have been recently identified. Among ion channels, the transient receptor potential (TRP) channels are a major family of non-selective cation-permeable channels that function as polymodal cellular sensors involved in many physiological and pathological processes.

In this review, we summarize current knowledge of interactions between immune cells and PRRs and ion channels of TRP families with PAMPs and DAMPs to provide new insights into the pathogenesis of inflammatory diseases. TRP channels have been found to interfere with innate immunity via both nuclear factor-kB and procaspase-1 activation to generate the mature caspase-1 that cleaves pro-interleukin-1β cytokine into the mature interleukin-1β.

Sensory neurons are also adapted to recognize dangers by virtue of their sensitivity to intense mechanical, thermal and irritant chemical stimuli. As immune cells, they possess many of the same molecular recognition pathways for danger. Thus, they express PRRs including Toll-like receptors 3, 4, 7, and 9, and stimulation by Toll-like receptor ligands leads to induction of inward currents and sensitization in TRPs. In addition, the expression of inflammasomes in neurons and the involvement of TRPs in central nervous system diseases strongly support a role of TRPs in inflammasome-mediated neurodegenerative pathologies. This field is still at its beginning and further studies may be required.

Overall, these studies highlight the therapeutic potential of targeting the inflammasomes in proinflammatory, autoinflammatory and metabolic disorders associated with undesirable activation of the inflammasome by using specific TRP antagonists, anti-human TRP monoclonal antibody or different molecules able to abrogate the TRP channel-mediated inflammatory signals.

Critical role of aquaporins in interleukin 1β (IL-1β)-induced inflammation

Rapid changes in cell volume characterize macrophage activation, but the role of water channels in inflammation remains unclear. We show here that, in vitro, aquaporin (AQP) blockade or deficiency results in reduced IL-1β release by macrophages activated with a variety of NLRP3 activators. Inhibition of AQP specifically during the regulatory volume decrease process is sufficient to limit IL-1β release by macrophages through the NLRP3 inflammasome axis. The immune-related activity of AQP was confirmed in vivo in a model of acute lung inflammation induced by crystals. AQP1 deficiency is associated with a marked reduction of both lung IL-1β release and neutrophilic inflammation. We conclude that AQP-mediated water transport in macrophages constitutes a general danger signal required for NLRP3-related inflammation. Our findings reveal a new function of AQP in the inflammatory process and suggest a novel therapeutic target for anti-inflammatory therapy.

Angiopoietin-1 upregulates de novo expression of IL-1β and Il1-Ra, and the exclusive release of Il1-Ra from human neutrophils

The expression of the angiopoietin (Ang) receptor, Tie2, on both endothelial and inflammatory cells supports the idea that Ang signaling may play a fundamental role in initiating and maintaining the inflammatory response. We have previously shown that Ang1 and/or Ang2 alter the innate immune response by enhancing human neutrophil survival, chemotaxis and production of inflammatory cytokine interleukin-8 (IL-8) in vitro. Thus, we hypothesized that Ang1 and Ang2 could modulate other inflammatory signals in neutrophils, a possibility we explored through a gene-based assay looking at changes in the mRNA expression of 84 inflammatory cytokines and their receptors. We observed that Ang1 (10(-8) M), but not Ang2, increased mRNA expression of prominent pro-inflammatory cytokine IL-1β and its natural antagonist IL-1RA, by up to 32.6- and 10.0-fold respectively, compared to PBS-control. The effects of Ang1 extended to the proteins, as Ang1 increased intracellular levels of precursor and mature IL-1β, and extracellular levels of IL-1RA proteins, by up to 4.2-, 5.0- and 4.4-fold respectively, compared to PBS-control. Interestingly, Ang1 failed at inducing IL-1β protein release or at increasing intracellular IL-1RA, but the ratio of IL-1RA to mature IL-1β remained above 100-fold molar excess inside and outside the cells. The above-noted effects of Ang1 were mediated by MAP kinases, whereby inhibiting MEK1/2 lead to up to 70% effect reduction, whereas the blockade of p38MAPK activity doubled Ang1's effect. These findings suggest that Ang1 selectively alters the balance of neutrophil-derived inflammatory cytokines, favoring the blockade of IL-1 activity, a consideration for future therapies of inflammatory diseases.

Purinergic signaling in inflammatory cells: P2 receptor expression, functional effects, and modulation of inflammatory responses

Extracellular ATP and related nucleotides promote a wide range of pathophysiological responses via activation of cell surface purinergic P2 receptors. Almost every cell type expresses P2 receptors and/or exhibit regulated release of ATP. In this review, we focus on the purinergic receptor distribution in inflammatory cells and their implication in diverse immune responses by providing an overview of the current knowledge in the literature related to purinergic signaling in neutrophils, macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells. The pathophysiological role of purinergic signaling in these cells include among others calcium mobilization, actin polymerization, chemotaxis, release of mediators, cell maturation, cytotoxicity, and cell death. We finally discuss the therapeutic potential of P2 receptor subtype selective drugs in inflammatory conditions.

Extracellular acidosis is a novel danger signal alerting innate immunity via the NLRP3 inflammasome

BACKGROUND

Local acidosis has been demonstrated in ischemic tissues and at inflammatory sites.

RESULTS

Acidic extracellular pH triggers NLRP3 inflammasome activation and interleukin-1β secretion in human macrophages.

CONCLUSION

Acidic pH represents a novel danger signal alerting the innate immunity.

SIGNIFICANCE

Local acidosis may promote inflammation at ischemic and inflammatory sites. Local extracellular acidification has been demonstrated at sites of ischemia and inflammation. IL-1β is one of the key proinflammatory cytokines, and thus, its synthesis and secretion are tightly regulated. The NLRP3 (nucleotide-binding domain leucine-rich repeat containing family, pyrin domain containing 3) inflammasome complex, assembled in response to microbial components or endogenous danger signals, triggers caspase-1-mediated maturation and secretion of IL-1β. In this study, we explored whether acidic environment is sensed by immune cells as an inflammasome-activating danger signal. Human macrophages were exposed to custom cell culture media at pH 7.5-6.0. Acidic medium triggered pH-dependent secretion of IL-1β and activation of caspase-1 via a mechanism involving potassium efflux from the cells. Acidic extracellular pH caused rapid intracellular acidification, and the IL-1β-inducing effect of acidic medium could be mimicked by acidifying the cytosol with bafilomycin A1, a proton pump inhibitor. Knocking down the mRNA expression of NLRP3 receptor abolished IL-1β secretion at acidic pH. Remarkably, alkaline extracellular pH strongly inhibited the IL-1β response to several known NLRP3 activators, demonstrating bipartite regulatory potential of pH on the activity of this inflammasome. The data suggest that acidic environment represents a novel endogenous danger signal alerting the innate immunity. Low pH may thus contribute to inflammation in acidosis-associated pathologies such as atherosclerosis and post-ischemic inflammatory responses.

Extracellular ATP and Toll-like receptor 2 agonists trigger in human monocytes an activation program that favors T helper 17

Strategically-paired Toll-like receptor (TLR) ligands induce a unique dendritic cell (DC) phenotype that polarizes Th1 responses. We therefore investigated pairing single TLR ligands with a non TLR-mediated danger signal to cooperatively induce distinct DC properties from cultured human monocytes. Adenosine triphosphate (ATP) and the TLR2 ligand lipoteichoic acid (LTA) selectively and synergistically induced expression of IL-23 and IL-1β from cultured monocytes as determined by ELISA assays. Flow cytometric analysis revealed that a sizable sub-population of treated cells acquired DC-like properties including activated surface phenotype with trans-well assays showing enhanced migration towards CCR7 ligands. Such activated cells also preferentially deviated, in an IL-23 and IL-1-dependent manner, CD4^pos T lymphocyte responses toward the IL-22^hi, IL-17^hi/IFN-γ^lo Th17 phenotype in standard in vitro allogeneic sensitization assays. Although pharmacological activation of either ionotropic or cAMP-dependent pathways acted in synergy with LTA to enhance IL-23, only inhibition of the cAMP-dependent pathway antagonized ATP-enhanced cytokine production. ATP plus atypical lipopolysaccharide from P. gingivalis (signaling through TLR2) was slightly superior to E. coli-derived LPS (TLR4 ligand) for inducing the high IL-23-secreting DC-like phenotype, but greatly inferior for inducing IL-12 p70 production when paired with IFN-γ, a distinction reflected in activated DCs’ ability to deviate lymphocytes toward Th1. Collectively, our data suggest TLR2 ligands encountered by innate immune cells in an environment with physiologically-relevant levels of extracellular ATP can induce a distinct activation state favoring IL-23- and IL-1β-dependent Th17 type response.

Cell volume regulation modulates NLRP3 inflammasome activation

Cell volume regulation is a primitive response to alterations in environmental osmolarity. The NLRP3 inflammasome is a multiprotein complex that senses pathogen- and danger-associated signals. Here, we report that, from fish to mammals, the basic mechanisms of cell swelling and regulatory volume decrease (RVD) are sensed via the NLRP3 inflammasome. We found that a decrease in extracellular osmolarity induced a K(+)-dependent conformational change of the preassembled NLRP3-inactive inflammasome during cell swelling, followed by activation of the NLRP3 inflammasome and caspase-1, which was controlled by transient receptor potential channels during RVD. Both mechanisms were necessary for interleukin-1β processing. Increased extracellular osmolarity prevented caspase-1 activation by different known NLRP3 activators. Collectively, our data identify cell volume regulation as a basic conserved homeostatic mechanism associated with the formation of the NLRP3 inflammasome and reveal a mechanism for NLRP3 inflammasome activation.

Critical role for calcium mobilization in activation of the NLRP3 inflammasome

The NLRP3 (nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3) inflammasome mediates production of inflammatory mediators, such as IL-1β and IL-18, and as such is implicated in a variety of inflammatory processes, including infection, sepsis, autoinflammatory diseases, and metabolic diseases. The proximal steps in NLRP3 inflammasome activation are not well understood. Here we elucidate a critical role for Ca(2+) mobilization in activation of the NLRP3 inflammasome by multiple stimuli. We demonstrate that blocking Ca(2+) mobilization inhibits assembly and activation of the NLRP3 inflammasome complex, and that during ATP stimulation Ca(2+) signaling is pivotal in promoting mitochondrial damage. C/EPB homologous protein, a transcription factor that can modulate Ca(2+) release from the endoplasmic reticulum, amplifies NLRP3 inflammasome activation, thus linking endoplasmic reticulum stress to activation of the NLRP3 inflammasome. Our findings support a model for NLRP3 inflammasome activation by Ca(2+)-mediated mitochondrial damage.

Biochemical regulation of the inflammasome

The extensively studied cytokine IL-1β is an important mediator of the inflammatory response. However, dysregulated release of IL-1β can be detrimental and is attributed to the progression and pathogenesis of multiple inflammatory diseases including, rhuematoid arthritis (RA), atherosclerosis, type 2 diabetes (T2D), Alzheimers disease and gout. IL-1β is encoded as a pro-protein. A multi-protein molecular scaffold termed the "Inflammasome" is responsible for the tightly controlled and coordinated processing of pro-IL-1β. The activation of several NLR (nucleotide-binding oligomerization domain (NOD)-like receptor) family members and PYHIN (pyrin and HIN domain) proteins can drive the formation of inflammasomes. However, the exact biochemical mechanisms governing their activation have been the subject of much research. Different inflammasomes have been demonstrated to respond to the same pathogen inducing a cooperative immune response accountable for the clearance of infection. Here, we review current knowledge surrounding the biochemical regulation of the NLRP1, NLRP3, NLRC4, AIM2 and IFI16 inflammasomes.

Understanding the mechanism of IL-1β secretion

The cytokine interleukin-1β (IL-1β) is a key mediator of the inflammatory response. Essential for the host-response and resistance to pathogens, it also exacerbates damage during chronic disease and acute tissue injury. It is not surprising therefore that there is a huge level of interest in how this protein is produced and exported from cells. However, the mechanism of IL-1β release has proven to be elusive. It does not follow the conventional ER-Golgi route of secretion. A literature full of disparate observations arising from numerous experimental systems, has contributed to a complicated mix of diverse proposals. Here we summarise these observations and propose that secretion of IL-1β occurs on a continuum, dependent upon stimulus strength and the extracellular IL-1β requirement.

Ginger phenylpropanoids inhibit IL-1beta and prostanoid secretion and disrupt arachidonate-phospholipid remodeling by targeting phospholipases A2

The rhizome of ginger (Zingiber officinale) is employed in Asian traditional medicine to treat mild forms of rheumatoid arthritis and fever. We have profiled ginger constituents for robust effects on proinflammatory signaling and cytokine expression in a validated assay using human whole blood. Independent of the stimulus used (LPS, PMA, anti-CD28 Ab, anti-CD3 Ab, and thapsigargin), ginger constituents potently and specifically inhibited IL-1β expression in monocytes/macrophages. Both the calcium-independent phospholipase A(2) (iPLA(2))-triggered maturation and the cytosolic phospholipase A(2) (cPLA(2))-dependent secretion of IL-1β from isolated human monocytes were inhibited. In a fluorescence-coupled PLA(2) assay, most major ginger phenylpropanoids directly inhibited i/cPLA(2) from U937 macrophages, but not hog pancreas secretory phospholipase A(2). The effects of the ginger constituents were additive and the potency comparable to the mechanism-based inhibitor bromoenol lactone for iPLA(2) and methyl arachidonyl fluorophosphonate for cPLA(2), with 10-gingerol/-shogaol being most effective. Furthermore, a ginger extract (2 μg/ml) and 10-shogaol (2 μM) potently inhibited the release of PGE(2) and thromboxane B2 (>50%) and partially also leukotriene B(4) in LPS-stimulated macrophages. Intriguingly, the total cellular arachidonic acid was increased 2- to 3-fold in U937 cells under all experimental conditions. Our data show that the concurrent inhibition of iPLA(2) and prostanoid production causes an accumulation of free intracellular arachidonic acid by disrupting the phospholipid deacylation-reacylation cycle. The inhibition of i/cPLA(2), the resulting attenuation of IL-1β secretion, and the simultaneous inhibition of prostanoid production by common ginger phenylpropanoids uncover a new anti-inflammatory molecular mechanism of dietary ginger that may be exploited therapeutically.

Sodium dependence of lysophosphatidylcholine-induced caspase-1 activity and reactive oxygen species generation

The proinflammatory cytokines interleukin (IL)-1β and IL-18 play pivotal roles in neuroinflammatory diseases. Caspase-1-mediated proteolytic cleavage is required to convert the premature, biologically inactive cytokines to their biologically active forms capable of promoting tissue inflammation. Although caspases have been recognized as potential therapeutic targets in inflammatory diseases, mechanisms regulating caspase-1 activation are not fully understood. Here we demonstrate that the proinflammatory lipid lysophosphatidylcholine (LPC) initiates microglial caspase-1 activation in a Na(+)-dependent manner. LPC-induced caspase-1 activity was almost completely inhibited upon omission of extracellular Na(+), but was unaffected by inhibition of Na(+)/K(+)-ATPase with ouabain or by inhibition of Na(+)/H(+) antiport with amiloride. Inhibition of caspase-1-mediated IL-1β processing by Na(+)-free medium led to reduced amounts of mature IL-1β released from LPC-stimulated microglia. Furthermore, LPC-induced production of reactive oxygen species (ROS) was abolished by Na(+)-free medium, indicating Na(+) dependence of NADPH oxidase activity in LPC-stimulated microglia. Since ROS production was found to be crucial to caspase-1 activation in LPC-stimulated microglia, the Na(+) dependence of caspase-1 can be related to the Na(+) dependence of NADPH oxidase. In summary, it is suggested that in LPC-activated microglia, Na(+) influx is required for the production of NADPH oxidase-mediated ROS, which subsequently stimulate caspase-1 activity.

Molecular mechanism of NLRP3 inflammasome activation

The inflammasome is an intracellular multimolecular complex that controls caspase-1 activity in the innate immune system. NLRP3, a member of the NLR family of cytosolic pattern recognition receptors, along with the adaptor protein ASC, mediates caspase-1 activation via assembly of the inflammasome in response to various pathogen-derived factors as well as danger-associated molecules. The active NLRP3 inflammasome drives innate immune response towards invading pathogens and cellular damage, and regulates adaptive immune response. Here, we review identified agonists of the NLRP3 inflammasome and the molecular mechanism by which they induce NLRP3 inflammasome activation. Three signaling pathways involving potassium efflux, generation of reactive oxygen species, and cathepsin B release are discussed.

Temporal interleukin-1beta secretion from primary human peripheral blood monocytes by P2X7-independent and P2X7-dependent mechanisms

The processing and regulated secretion of IL-1β are critical points of control of the biological activity of this important pro-inflammatory cytokine. IL-1β is produced by both monocytes and macrophages, but the rate and mechanism of release differ according to the differentiation status and the origin of these cells. We aimed to study the control of processing and release in human blood monocytes and human monocyte-derived macrophages. Toll-like receptor (TLR)-induced IL-1β production and release were investigated for dependence upon caspase-1, P2X7 receptor activation, and loss of membrane asymmetry associated with microvesicle shedding. TLR agonists induced P2X7 receptor-dependent IL-1β release in both monocytes and macrophages; however, only monocytes also showed P2X7 receptor-independent release of mature IL-1β. Furthermore, in monocytes ATP-mediated PS exposure could be activated independently of IL-1β production. Release of IL-1β from monocytes showed selectivity for specific TLR agonists and was accelerated by P2X7 receptor activation. Human monocytes released more IL-1β/cell than macrophages. These data have important implications for inflammatory diseases that involve monocyte activation and IL-1 release.

Externalization of the leaderless cytokine IL-1F6 occurs in response to lipopolysaccharide/ATP activation of transduced bone marrow macrophages

An interesting trait shared by many members of the IL-1 cytokine family is the absence of a signal sequence that can direct the newly synthesized polypeptides to the endoplasmic reticulum. As a result, these cytokines accumulate intracellularly. Recent studies investigating IL-1beta export established that its release is facilitated via activation of an intracellular multiprotein complex termed the inflammasome. The purpose of the current study was to explore the mechanism by which murine IL-1F6 is released from bone marrow-derived macrophages (BMDMs) and to compare this mechanism to that used by IL-1beta. BMDMs were engineered to overexpress IL-1F6 by retroviral transduction; cells overexpressing GFP also were generated to provide a noncytokine comparator. The transduced cells constitutively expressed IL-1F6 and GFP, but they did not constitutively release these polypeptides to the medium. Enhanced release of IL-1F6 was achieved by treating with LPS followed by ATP-induced activation of the P2X(7) receptor; GFP also was released under these conditions. No obvious proteolytic cleavage of IL-1F6 was noted following P2X(7) receptor-induced release. Stimulus-induced release of IL-1F6 and GFP demonstrated comparable susceptibility to pharmacological modulation. Therefore, transduced IL-1F6 is released in parallel with endogenous mature IL-1beta from LPS/ATP-treated BMDMs, but this externalization process is not selective for cytokines as a noncytokine (GFP) shows similar behavior. These findings suggest that IL-1F6 can be externalized via a stimulus-coupled mechanism comparable to that used by IL-1beta, and they provide additional insight into the complex cellular processes controlling posttranslational processing of the IL-1 cytokine family.

Inflammasomes: guardians of cytosolic sanctity

The innate immune system is critical in recognizing bacterial and viral infections to evoke a proper immune response. Certain members of the intracellular nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family detect microbial components in the cytosol and trigger the assembly of large caspase-1-activating complexes termed inflammasomes. Autoproteolytic maturation of caspase-1 zymogens within these inflammasomes leads to maturation and secretion of the pro-inflammatory cytokines interleukin-1 beta (IL-1 beta) and IL-18. The NLR proteins ICE protease-activating factor (IPAF), NALP1b (NACHT domain-, leucine-rich repeat-, and PYD-containing protein 1b), and cryopyrin/NALP3 assemble caspase-1-activating inflammasomes in a stimulus-dependent manner. Bacterial flagellin is sensed by IPAF, whereas mouse NALP1b detects anthrax lethal toxin. Cryopyrin/NALP3 mediates caspase-1 activation in response to a wide variety of microbial components and in response to crystalline substances such as the endogenous danger signal uric acid. Genetic variations in Nalp1 and cryopyrin/Nalp3 are associated with autoinflammatory disorders and increased susceptibility to microbial infection. Further understanding of inflammasomes and their role in innate immunity should provide new insights into the mechanisms of host defense and the pathogenesis of autoimmune diseases.

A novel role for defensins in intestinal homeostasis: regulation of IL-1beta secretion

Impaired expression of alpha-defensin antimicrobial peptides and overproduction of the proinflammatory cytokine IL-1beta have been associated with inflammatory bowel disease. In this study, we examine the interactions between alpha-defensins and IL-1beta and the role of defensin deficiency in the pathogenesis of inflammatory bowel disease. It was found that matrix metalloproteinase-7-deficient (MMP-7(-/-)) mice, which produce procryptdins but not mature cryptdins (alpha-defensins) in the intestine, were more susceptible to dextran sulfate sodium-induced colitis. Furthermore, both baseline and dextran sulfate sodium-induced IL-1beta production in the intestine were significantly up-regulated in MMP-7(-/-) mice compared with that in control C57BL/6 mice. To elucidate the molecular mechanism for the increased IL-1beta production in defensin deficiency in vivo, we evaluated the effect of defensins on IL-1beta posttranslational processing and release. It was found that alpha-defensins, including mouse Paneth cell defensins cryptdin-3 and cryptdin-4, human neutrophil defensin HNP-1, and human Paneth cell defensin HD-5, blocked the release of IL-1beta from LPS-activated monocytes, whereas TNF-alpha expression and release were not affected. Unlike alpha-defensins, human beta-defensins and mouse procryptdins do not have any effect on IL-1beta processing and release. Thus, alpha-defensins may play an important role in intestinal homeostasis by controlling the production of IL-1beta.

Differential requirement of P2X7 receptor and intracellular K+ for caspase-1 activation induced by intracellular and extracellular bacteria

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases. The maturation and secretion of IL-1beta are mediated by caspase-1, a protease that processes pro-IL-1beta into biologically active IL-1beta. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration of K(+) potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and intracellular K(+) in IL-1beta secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists such as lipopolysaccharide or infection with extracellular bacteria Staphylococcus aureus and Escherichia coli, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through depletion of intracellular K(+) caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K(+). Consistently, caspase-1 activation induced by intracellular Salmonella or Listeria was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally in the absence of P2X7 receptor-mediated cytoplasmic K(+) perturbations.

Mechanisms of regulation for interleukin-1beta in neurodegenerative disease

The interleukin-1 family of cytokines are central to the pathology of acute and chronic diseases of the central nervous system. We describe current evidence on the transcriptional and post-transcriptional regulation of interleukin-1beta production, secretion and activity in the brain. Regarding the induction of protein synthesis, the possible involvement of Toll like receptor-4 is discussed including evidence that ischemic brain damage is reduced in Toll like receptor-4 knockout mice. The post-translational involvement of the P2X7-receptor and caspase-1 in the processing and release of active IL-1beta is also considered, as is evidence suggesting a possible extracellular cleavage of pro-IL-1beta by neutrophil derived proteases. We provide some fresh perspectives on how interleukin-1beta may be regulated and how these mechanisms could be targeted in disease.