Inflammasomes and their activation

Metabolism-inflammasome crosstalk shapes innate and adaptive immunity

Inflammasomes are a central component of innate immunity and play a vital role in regulating innate immune response. Activation of inflammasomes is also indispensable for adaptive immunity, modulating the development and response of adaptive immunity. Recently, increasing studies have shown that metabolic alterations and adaptations strongly influence and regulate the differentiation and function of the immune system. In this review, we will take a holistic view of how inflammasomes bridge innate and adaptive (especially T cell) immunity and how inflammasomes crosstalk with metabolic signals during the immune responses. And, special attention will be paid to the metabolic control of inflammasome-mediated interactions between innate and adaptive immunity in disease. Understanding the metabolic regulatory functions of inflammasomes would provide new insights into future research directions in this area and may help to identify potential targets for inflammasome-associated diseases and broaden therapeutic avenues.

The Neglected Sibling: NLRP2 Inflammasome in the Nervous System

While classical NOD-like receptor pyrin domain containing protein 1 (NLRP1) and NLRP3 inflammasomal proteins have been extensively investigated, the contribution of NLRP2 is still ill-defined in the nervous system. Given the putative significance of NLRP2 in orchestrating neuroinflammation, further inquiry is needed to gain a better understanding of its connectome, hence its specific targeting may hold a promising therapeutic implication. Therefore, bioinformatical approach for extracting information, specifically in the context of neuropathologies, is also undoubtedly preferred. To the best of our knowledge, there is no review study selectively targeting only NLRP2. Increasing, but still fragmentary evidence should encourage researchers to thoroughly investigate this inflammasome in various animal- and human models. Taken together, herein we aimed to review the current literature focusing on the role of NLRP2 inflammasome in the nervous system and more importantly, we provide an algorithm-based protein network of human NLRP2 for elucidating potentially valuable molecular partnerships that can be the beginning of a new discourse and future therapeutic considerations.

NLRP inflammasomes in health and disease

NLRP inflammasomes are a group of cytosolic multiprotein oligomer pattern recognition receptors (PRRs) involved in the recognition of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) produced by infected cells. They regulate innate immunity by triggering a protective inflammatory response. However, despite their protective role, aberrant NLPR inflammasome activation and gain-of-function mutations in NLRP sensor proteins are involved in occurrence and enhancement of non-communicating autoimmune, auto-inflammatory, and neurodegenerative diseases. In the last few years, significant advances have been achieved in the understanding of the NLRP inflammasome physiological functions and their molecular mechanisms of activation, as well as therapeutics that target NLRP inflammasome activity in inflammatory diseases. Here, we provide the latest research progress on NLRP inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRP7, NLRP2, NLRP9, NLRP10, and NLRP12 regarding their structural and assembling features, signaling transduction and molecular activation mechanisms. Importantly, we highlight the mechanisms associated with NLRP inflammasome dysregulation involved in numerous human auto-inflammatory, autoimmune, and neurodegenerative diseases. Overall, we summarize the latest discoveries in NLRP biology, their forming inflammasomes, and their role in health and diseases, and provide therapeutic strategies and perspectives for future studies about NLRP inflammasomes.

The Mechanism of Pyroptosis and Its Application Prospect in Diabetic Wound Healing

Pyroptosis defines a form of pro-inflammatory-dependent programmed cell death triggered by gasdermin proteins, which creates cytoplasmic pores and promotes the activation and accumulation of immune cells by releasing several pro-inflammatory mediators and immunogenic substances upon cell rupture. Pyroptosis comprises canonical (mediated by Caspase-1) and non-canonical (mediated by Caspase-4/5/11) molecular signaling pathways. Numerous studies have explored the contributory roles of inflammasome and pyroptosis in the progression of multiple pathological conditions such as tumors, nerve injury, inflammatory diseases and metabolic disorders. Accumulating evidence indicates that the activation of the NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome results in the activation of pyroptosis and inflammation. Current evidence suggests that pyroptosis-dependent cell death plays a progressive role in the development of diabetic complications including diabetic wound healing (DWH) and diabetic foot ulcers (DFUs). This review presents a brief overview of the molecular mechanisms underlying pyroptosis and addresses the current research on pyroptosis-dependent signaling pathways in the context of DWH. In this review, we also present some prospective therapeutic compounds/agents that can target pyroptotic signaling pathways, which may serve as new strategies for the effective treatment and management of diabetic wounds.

Inflammasome involvement in CS-induced damage in HaCaT keratinocytes

Cigarette smoke (CS) alters cutaneous biological processes such as redox homeostasis and inflammation response that might be involved in promoting skin inflammatory conditions. Exposure to CS has also been linked to a destabilization of the NLRP3 inflammasome in pollution target tissues such as the lung epithelium, resulting in a more vulnerable immunological response to several exogenous and endogenous stimuli related to oxidative stress. Thus, CS has an adverse effect on host defense, increasing the susceptibility to develop lung infections and pathologies. In the skin, another direct target of pollution, inflammasome disorders have been linked to an increasing number of diseases such as melanoma, psoriasis, vitiligo, atopic dermatitis, and acne, all conditions that have been connected directly or indirectly to pollution exposure. The inflammasome machinery is an important innate immune sensor in human keratinocytes. However, the role of CS in the NLRP1 and NLRP3 inflammasome in the cutaneous barrier has still not been investigated. In the present study, we were able to determine in keratinocytes exposed to CS an increased oxidative damage evaluated by 4-HNE protein adduct and carbonyl formation. Of note is that, while CS inhibited NLRP3 activation, it was able to activate NLRP1, leading to an increased secretion of the proinflammatory cytokines IL-1β and IL-18. This study highlights the importance of the inflammasome machinery in CS that more in general, in pollution, affects cutaneous tissues and the important cross-talk between different members of the NLRP inflammasome family.

Inflammasome Signaling in the Aging Brain and Age-Related Neurodegenerative Diseases

Inflammasomes are intracellular protein complexes, members of the innate immune system, and their activation and regulation play an essential role in maintaining homeostatic conditions against exogenous and endogenous stimuli. Inflammasomes occur as cytosolic proteins and assemble into a complex during the recognition of pathogen-associated or danger-associated molecular patterns by pattern-recognition receptors in host cells. The formation of the inflammasome complex elicits signaling molecules of proinflammatory cytokines such as interleukin-1β and interleukin 18 via activation of caspase-1 in the canonical inflammasome pathway whereas caspase-11 in the case of a mouse and caspase-4 and caspase-5 in the case of humans in the non-canonical inflammasome pathway, resulting in pyroptotic or inflammatory cell death which ultimately leads to neuroinflammation and neurodegenerative diseases. Inflammasome activation, particularly in microglial cells and macrophages, has been linked to aging as well as age-related neurodegenerative diseases. The accumulation of abnormal/ misfolded proteins acts as a ligand for inflammasome activation in neurodegenerative diseases. Although recent studies have revealed the inflammasomes' functionality in both in vitro and in vivo models, many inflammasome signaling cascade activations during biological aging, neuroinflammation, and neurodegeneration are still ambiguous. In this review, we comprehensively unveil the cellular and molecular mechanisms of inflammasome activation during neuronal aging and age-related neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion disease, and amyotrophic lateral sclerosis.

The Programmed Cell Death of Macrophages, Endothelial Cells, and Tubular Epithelial Cells in Sepsis-AKI

Sepsis is a systemic inflammatory response syndrome caused by infection, following with acute injury to multiple organs. Sepsis-induced acute kidney injury (AKI) is currently recognized as one of the most severe complications related to sepsis. The pathophysiology of sepsis-AKI involves multiple cell types, including macrophages, vascular endothelial cells (ECs) and renal tubular epithelial cells (TECs), etc. More significantly, programmed cell death including apoptosis, necroptosis and pyroptosis could be triggered by sepsis in these types of cells, which enhances AKI progress. Moreover, the cross-talk and connections between these cells and cell death are critical for better understanding the pathophysiological basis of sepsis-AKI. Mitochondria dysfunction and oxidative stress are traditionally considered as the leading triggers of programmed cell death. Recent findings also highlight that autophagy, mitochondria quality control and epigenetic modification, which interact with programmed cell death, participate in the damage process in sepsis-AKI. The insightful understanding of the programmed cell death in sepsis-AKI could facilitate the development of effective treatment, as well as preventive methods.

The cytokine storms of COVID-19, H1N1 influenza, CRS and MAS compared. Can one sized treatment fit all?

An analysis of published data appertaining to the cytokine storms of COVID-19, H1N1 influenza, cytokine release syndrome (CRS), and macrophage activation syndrome (MAS) reveals many common immunological and biochemical abnormalities. These include evidence of a hyperactive coagulation system with elevated D-dimer and ferritin levels, disseminated intravascular coagulopathy (DIC) and microthrombi coupled with an activated and highly permeable vascular endothelium. Common immune abnormalities include progressive hypercytokinemia with elevated levels of TNF-α, interleukin (IL)-6, and IL-1β, proinflammatory chemokines, activated macrophages and increased levels of nuclear factor kappa beta (NFκB). Inflammasome activation and release of damage associated molecular patterns (DAMPs) is common to COVID-19, H1N1, and MAS but does not appear to be a feature of CRS. Elevated levels of IL-18 are detected in patients with COVID-19 and MAS but have not been reported in patients with H1N1 influenza and CRS. Elevated interferon-γ is common to H1N1, MAS, and CRS but levels of this molecule appear to be depressed in patients with COVID-19. CD4+ T, CD8+ and NK lymphocytes are involved in the pathophysiology of CRS, MAS, and possibly H1N1 but are reduced in number and dysfunctional in COVID-19. Additional elements underpinning the pathophysiology of cytokine storms include Inflammasome activity and DAMPs. Treatment with anakinra may theoretically offer an avenue to positively manipulate the range of biochemical and immune abnormalities reported in COVID-19 and thought to underpin the pathophysiology of cytokine storms beyond those manipulated via the use of, canakinumab, Jak inhibitors or tocilizumab. Thus, despite the relative success of tocilizumab in reducing mortality in COVID-19 patients already on dexamethasone and promising results with Baricitinib, the combination of anakinra in combination with dexamethasone offers the theoretical prospect of further improvements in patient survival. However, there is currently an absence of trial of evidence in favour or contravening this proposition. Accordingly, a large well powered blinded prospective randomised controlled trial (RCT) to test this hypothesis is recommended.

NLRP7: From inflammasome regulation to human disease

Nucleotide-binding oligomerization domain (NOD) and leucine-rich repeat (LRR)-containing receptors or NOD-like receptors (NLRs) are cytosolic pattern recognition receptors, which sense conserved microbial patterns and host-derived danger signals to elicit innate immune responses. The activation of several prototypic NLRs, including NLR and pyrin domain (PYD) containing (NLRP) 1, NLRP3 and NLR and caspase recruitment domain (CARD) containing (NLRC) 4, results in the assembly of inflammasomes, which are large, cytoplasmic multiprotein signalling platforms responsible for the maturation and release of the pro-inflammatory cytokines IL-1β and IL-18, and for the induction of a specialized form of inflammatory cell death called pyroptosis. However, the function of other members of the NLR family, including NLRP7, are less well understood. NLRP7 has been linked to innate immune signalling, but its precise role is still controversial as it has been shown to positively and negatively affect inflammasome responses. Inflammasomes are essential for homeostasis and host defence, but inappropriate inflammasome responses due to hereditary mutations and somatic mosaicism in inflammasome components and defective regulation have been linked to a broad spectrum of human diseases. A compelling connection between NLRP7 mutations and reproductive diseases, and in particular molar pregnancy, has been established. However, the molecular mechanisms by which NLRP7 mutations contribute to reproductive diseases are largely unknown. In this review, we focus on NLRP7 and discuss the current evidence of its role in inflammasome regulation and its implication in human reproductive diseases.

Inflammasome Activation in Pollution-Induced Skin Conditions

SUMMARY

Exposure to air pollutants has been now associated with detrimental effects on a variety of organs, including the heart, lungs, GI tract, and brain. However, recently it has become clear that pollutant exposure can also promote the development/exacerbation of a variety of skin conditions, including premature aging, psoriasis, acne, and atopic dermatitis. Although the molecular mechanisms by which pollutant exposure results in these cutaneous pathological manifestations, it has been noticed that an inflammatory status is a common denominator of all those skin conditions. For this reason, recently, the activation of a cytosolic multiprotein complex involved in inflammatory responses (the inflammasome) that could promote the maturation of proinflammatory cytokines interleukin-1β and interleukin-18 has been hypothesized to play a key role in pollution-induced skin damage. In this review, we summarize and propose the cutaneous inflammasome as a novel target of pollutant exposure and the eventual usage of inflammasome inhibitor as new technologies to counteract pollution-induced skin damage. Possibly, the ability to inhibit the inflammasome activation could prevent cutaneous inflammaging and ameliorate the health and appearance of the skin.

The inhibitor effect of RKIP on inflammasome activation and inflammasome-dependent diseases

Aberrant inflammasome activation contributes to the pathogenesis of various human diseases, including atherosclerosis, gout, and metabolic disorders. Elucidation of the underlying mechanism involved in the negative regulation of the inflammasome is important for developing new therapeutic targets for these diseases. Here, we showed that Raf kinase inhibitor protein (RKIP) negatively regulates the activation of the NLRP1, NLRP3, and NLRC4 inflammasomes. RKIP deficiency enhanced caspase-1 activation and IL-1β secretion via NLRP1, NLRP3, and NLRC4 inflammasome activation in primary macrophages. The overexpression of RKIP in THP-1 cells inhibited NLRP1, NLRP3, and NLRC4 inflammasome activation. RKIP-deficient mice showed increased sensitivity to Alum-induced peritonitis and Salmonella typhimurium-induced inflammation, indicating that RKIP inhibits NLRP3 and NLRC4 inflammasome activation in vivo. Mechanistically, RKIP directly binds to apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC) and competes with NLRP1, NLRP3, or NLRC4 to interact with ASC, thus interrupting inflammasome assembly and activation. The depletion of RKIP aggravated inflammasome-related diseases such as monosodium urate (MSU)-induced gouty arthritis and high-fat diet (HFD)-induced metabolic disorders. Furthermore, the expression of RKIP was substantially downregulated in patients with gouty arthritis or type 2 diabetes (T2D) compared to healthy controls. Collectively, our findings suggest that RKIP negatively regulates NLRP1, NLRP3, and NLRC4 inflammasome activation and is a potential therapeutic target for the treatment of inflammasome-related diseases.

Inflammasome Regulation: Therapeutic Potential for Inflammatory Bowel Disease

Inflammasomes are multiprotein complexes formed to regulate the maturation of pro-inflammatory caspases, in response to intracellular or extracellular stimulants. Accumulating studies showed that the inflammasomes are implicated in the pathogenesis of inflammatory bowel disease (IBD), although their activation is not a decisive factor for the development of IBD. Inflammasomes and related cytokines play an important role in the maintenance of gut immune homeostasis, while its overactivation might induce excess immune responses and consequently cause tissue damage in the gut. Emerging studies provide evidence that some genetic abnormalities might induce enhanced NLRP3 inflammasome activation and cause colitis. In these cases, the colonic inflammation can be ameliorated by blocking NLRP3 activation or its downstream cytokine IL-1β. A number of natural products were shown to play a role in preventing colon inflammation in various experimental colitis models. On the other hand, lack of inflammasome function also causes intestinal abnormalities. Thus, an appropriate regulation of inflammasomes might be a promising therapeutic strategy for IBD intervention. This review aims at summarizing the main findings in these studies and provide an outline for further studies that might contribute to our understanding of the role of inflammasomes in the pathogenesis and therapeutic treatment of IBD.

Postoperative cognitive dysfunction is made persistent with morphine treatment in aged rats

Postoperative cognitive dysfunction (POCD) is the collection of cognitive impairments, lasting days to months, experienced by individuals following surgery. Persistent POCD is most commonly experienced by older individuals and is associated with a greater vulnerability to developing Alzheimer's disease, but the underlying mechanisms are not known. It is known that laparotomy (exploratory abdominal surgery) in aged rats produces memory impairments for 4 days. Here we report that postsurgical treatment with morphine extends this deficit to at least 2 months while having no effects in the absence of surgery. Indeed, hippocampal-dependent long-term memory was impaired 2, 4, and 8 weeks postsurgery only in aged, morphine-treated rats. Short-term memory remained intact. Morphine is known to have analgesic effects via μ-opioid receptor activation and neuroinflammatory effects through Toll-like receptor 4 activation. Here we demonstrate that persistent memory deficits were mediated independently of the μ-opioid receptor, suggesting that they were evoked through a neuroinflammatory mechanism and unrelated to pain modulation. In support of this, aged, laparotomized, and morphine-treated rats exhibited increased gene expression of various proinflammatory markers (IL-1β, IL-6, TNFα, NLRP3, HMGB1, TLR2, and TLR4) in the hippocampus at the 2-week time point. Furthermore, central blockade of IL-1β signaling with the specific IL-1 receptor antagonist (IL-1RA), at the time of surgery, completely prevented the memory impairment. Finally, synaptophysin and PSD95 gene expression were significantly dysregulated in the hippocampus of aged, laparotomized, morphine-treated rats, suggesting that impaired synaptic structure and/or function may play a key role in this persistent deficit. This instance of long-term memory impairment following surgery closely mirrors the timeline of persistent POCD in humans and may be useful for future treatment discoveries.

NOD2/RIG-I Activating Inarigivir Adjuvant Enhances the Efficacy of BCG Vaccine Against Tuberculosis in Mice

Tuberculosis (TB) caused by Mycobacterium tuberculosis (MTB) kills about 1.5 million people each year and the widely used Bacille Calmette-Guérin (BCG) vaccine provides a partial protection against TB in children and adults. Because BCG vaccine evades lysosomal fusion in antigen presenting cells (APCs), leading to an inefficient production of peptides and antigen presentation required to activate CD4 T cells, we sought to boost its efficacy using novel agonists of RIG-I and NOD2 as adjuvants. We recently reported that the dinucleotide SB 9200 (Inarigivir) derived from our small molecule nucleic acid hybrid (SMNH)^® platform, activated RIG-I and NOD2 receptors and exhibited a broad-spectrum antiviral activity against hepatitis B and C, Norovirus, RSV, influenza and parainfluenza. Inarigivir increased the ability of BCG-infected mouse APCs to secrete elevated levels of IL-12, TNF-α, and IFN-β, and Caspase-1 dependent IL-1β cytokine. Inarigivir also increased the ability of macrophages to kill MTB in a Caspase-1-, and autophagy-dependent manner. Furthermore, Inarigivir led to a Capsase-1 and NOD2- dependent increase in the ability of BCG-infected APCs to present an Ag85B-p25 epitope to CD4 T cells in vitro. Consistent with an increase in immunogenicity of adjuvant treated APCs, the Inarigivir-BCG vaccine combination induced robust protection against tuberculosis in a mouse model of MTB infection, decreasing the lung burden of MTB by 1-log10 more than that afforded by BCG vaccine alone. The Inarigivir-BCG combination was also more efficacious than a muramyl-dipeptide-BCG vaccine combination against tuberculosis in mice, generating better memory T cell responses supporting its novel adjuvant potential for the BCG vaccine.

An overview of the non-canonical inflammasome

Inflammasomes are large cytosolic multiprotein complexes assembled in response to infection and cellular stress, and are crucial for the activation of inflammatory caspases and the subsequent processing and release of pro-inflammatory mediators. While caspase-1 is activated within the canonical inflammasome, the related caspase-4 (also known as caspase-11 in mice) and caspase-5 are activated within the non-canonical inflammasome upon sensing of cytosolic lipopolysaccharide (LPS) from Gram-negative bacteria. However, the consequences of canonical and non-canonical inflammasome activation are similar. Caspase-1 promotes the processing and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the release of danger signals, as well as a lytic form of cell death called pyroptosis, whereas caspase-4, caspase-5 and caspase-11 directly promote pyroptosis through cleavage of the pore-forming protein gasdermin D (GSDMD), and trigger a secondary activation of the canonical NLRP3 inflammasome for cytokine release. Since the presence of the non-canonical inflammasome activator LPS leads to endotoxemia and sepsis, non-canonical inflammasome activation and regulation has important clinical ramifications. Here we discuss the mechanism of non-canonical inflammasome activation, mechanisms regulating its activity and its contribution to health and disease.

Fatty food, fatty acids, and microglial priming in the adult and aged hippocampus and amygdala

Short-term (3-day) consumption of a high fat diet (HFD) rich in saturated fats is associated with a neuroinflammatory response and subsequent cognitive impairment in aged, but not young adult, male rats. This exaggerated effect in aged rats could be due to a "primed" microglial phenotype observed in the normal aging process in rodents in which aged microglia display a potentiated response to immune challenge. Here, we investigated the impact of HFD on microglial priming and lipid composition in the hippocampus and amygdala of young and aged rats. Furthermore, we investigated the microglial response to palmitate, the main saturated fatty acid (SFA) found in HFD that is proinflammatory. Our results indicate that HFD increased gene expression of microglial markers of activation indicative of microglial priming, including CD11b, MHCII, CX3CR1, and NLRP3, as well as the pro-inflammatory marker IL-1β in both hippocampus and amygdala-derived microglia. Furthermore, HFD increased the concentration of SFAs and decreased the concentration of polyunsaturated fatty acids (PUFAs) in the hippocampus. We also observed a specific decrease in the anti-inflammatory PUFA docosahexaenoic acid (DHA) in the hippocampus and amygdala of aged rats. In a separate cohort of young and aged animals, isolated microglia from the hippocampus and amygdala exposed to palmitate in vitro induced an inflammatory gene expression profile mimicking the effects of HFD in vivo. These data suggest that palmitate may be a critical nutritional signal from the HFD that is directly involved in hippocampal and amygdalar inflammation. Interestingly, microglial activation markers were increased in response to HFD or palmitate in an age-independent manner, suggesting that HFD sensitivity of microglia, under these experimental conditions, is not the sole mediator of the exaggerated inflammatory response observed in whole tissue extracts from aged HFD-fed rats.

An Update on CARD Only Proteins (COPs) and PYD Only Proteins (POPs) as Inflammasome Regulators

Inflammasomes are protein scaffolds required for the activation of caspase-1 and the subsequent release of interleukin (IL)-1β, IL-18, and danger signals, as well as the induction of pyroptotic cell death to restore homeostasis following infection and sterile tissue damage. However, excessive inflammasome activation also causes detrimental inflammatory disease. Therefore, extensive control mechanisms are necessary to prevent improper inflammasome responses and inflammatory disease. Inflammasomes are assembled by sequential nucleated polymerization of Pyrin domain (PYD) and caspase recruitment domain (CARD)-containing inflammasome components. Once polymerization is nucleated, this process proceeds in a self-perpetuating manner and represents a point of no return. Therefore, regulation of this key step is crucial for a controlled inflammasome response. Here, we provide an update on two single domain protein families containing either a PYD or a CARD, the PYD-only proteins (POPs) and CARD-only proteins (COPs), respectively. Their structure allows them to occupy and block access to key protein-protein interaction domains necessary for inflammasome assembly, thereby regulating the threshold of these nucleated polymerization events, and consequently, the inflammatory host response.

NLRP3 inflammasome inhibitor glyburide expedites diabetic-induced impaired fracture healing

Localized inflammation is accompanied by the diabetic-induced fracture. The present study aims to investigate the therapeutic effects of glyburide, an NLRP3 inflammasome inhibitor, in a diabetic-induced fracture model. An animal model of diabetic-induced fracture was established and the mice were administrated with metformin or glyburide for 3 weeks. Quantitative polymerase chain reaction (qPCR) and Western blotting were used to evaluate the relative expressions of IFN-γ, TNF-α, and IL-6. Micro-computed tomography (μCT) scanning was applied to evaluate bone callus formation. Histopathology examinations of fractured femur sections were performed using Tartrate-resistant acid phosphatase (TRAP) staining and Alcian blue and orange G staining. Bone strength was evaluated using Torsional testing. Our results showed that treatment of glyburide significantly decreased the expressions of IFN-γ, TNF-α, and IL-6 in the fracture calluses in diabetic-induced fracture model, while bone callus volume and bone volume fraction were increased. Additionally, our results also demonstrated that treatment of glyburide rescued the increase of osteoclasts in the bone-cartilage interface. Apart from decreasing a percentage of cartilage area and increasing the percentage of bone and fibrotic tissue area, treatment of glyburide increased the maximum torque and yield torque of fractures. These results implied that glyburide might be used as a potential drug candidate for diabetic-induced fracture.

Short-term high-fat diet modulates several inflammatory, ER stress, and apoptosis markers in the hippocampus of young mice

The consumption of saturated fatty acids is one of the leading risk factors for Alzheimer's Disease (AD) development. Indeed, the short-term consumption of a high-fat diet (HFD) is related to increased inflammatory signals in the hippocampus; however, the potential molecular mechanisms linking it to AD pathogenesis are not fully elucidated. In our study, we investigated the effects of short-term HFD feeding (within 3, 7 and 10 days) in AD markers and neuroinflammation in the hippocampus of mice. The short period of HFD increased fasting glucose and HOMA-IR. Also, mice fed HFD increased the protein content of β-Amyloid, pTau, TNFα, IL1β, pJNK, PTP1B, peIF2α, CHOP, Caspase3, Cleaved-Caspase3 and Alzheimer-related genes (Bax, PS1, PEN2, Aph1b). At 10 days, both neuronal (N2a) and microglial (BV2) cells presented higher expression of inflammatory and apoptotic genes when stimulated with palmitate. These findings suggest that a short period of consumption of a diet rich in saturated fat is associated with activation of inflammatory, ER stress and apoptotic signals in the hippocampus of young mice.

NLRP3 inflammasome in ischemic stroke: As possible therapeutic target

Inflammation is a devastating pathophysiological process during stroke, a devastating disease that is the second most common cause of death worldwide. Activation of the NOD-like receptor protein (NLRP3)-infammasome has been proposed to mediate inflammatory responses during ischemic stroke. Briefly, NLRP3 inflammasome activates caspase-1, which cleaves both pro-IL-1 and pro-IL-18 into their active pro-inflammatory cytokines that are released into the extracellular environment. Several NLRP3 inflammasome inhibitors have been promoted, including small molecules, type I interferon, micro RNAs, nitric oxide, and nuclear factor erythroid-2 related factor 2 (Nrf2), some of which are potentially efficacious clinically. This review will describe the structure and cellular signaling pathways of the NLRP3 inflammasome during ischemic stroke, and current evidence for NLRP3 inflammasome inhibitors.

Beyond Autoantibodies: Biologic Roles of Human Autoreactive B Cells in Rheumatoid Arthritis Revealed by RNA-Sequencing

OBJECTIVE

To obtain the comprehensive transcriptome profile of human citrulline-specific B cells from patients with rheumatoid arthritis (RA).

METHODS

Citrulline- and hemagglutinin-specific B cells were sorted by flow cytometry using peptide-streptavidin conjugates from the peripheral blood of RA patients and healthy individuals. The transcriptome profile of the sorted cells was obtained by RNA-sequencing, and expression of key protein molecules was evaluated by aptamer-based SOMAscan assay and flow cytometry. The ability of these proteins to effect differentiation of osteoclasts and proliferation and migration of synoviocytes was examined by in vitro functional assays.

RESULTS

Citrulline-specific B cells, in comparison to citrulline-negative B cells, from patients with RA differentially expressed the interleukin-15 receptor α (IL-15Rα) gene as well as genes related to protein citrullination and cyclic AMP signaling. In analyses of an independent cohort of cyclic citrullinated peptide-seropositive RA patients, the expression of IL-15Rα protein was enriched in citrulline-specific B cells from the patients' peripheral blood, and surprisingly, all B cells from RA patients were capable of producing the epidermal growth factor ligand amphiregulin (AREG). Production of AREG directly led to increased migration and proliferation of fibroblast-like synoviocytes, and, in combination with anti-citrullinated protein antibodies, led to the increased differentiation of osteoclasts.

CONCLUSION

To the best of our knowledge, this is the first study to document the whole transcriptome profile of autoreactive B cells in any autoimmune disease. These data identify several genes and pathways that may be targeted by repurposing several US Food and Drug Administration-approved drugs, and could serve as the foundation for the comparative assessment of B cell profiles in other autoimmune diseases.

NLRP3 promotes tumor growth and metastasis in human oral squamous cell carcinoma

Background

Inflammasomes are reported to be abnormally expressed and activated in several malignancies and play important roles in tumor development. The present study was designed to investigate the expression and function of the NLR family pyrin domain containing protein 3 (NLRP3) inflammasome in oral squamous cell carcinoma (OSCC).

Methods

NLRP3 expression in OSCC cell lines and the normal human immortalized oral epithelial cells (HIOEC) was determined by real-time PCR and western blot. Immunohistochemistry was used to examine the expression of NLRP3 and IL-1β in the paraffin-embedded OSCC tissues. The proliferation of OSCC cells was detected by the 3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay and cell colony formation ability of the OSCC cells was also evaluated. Tumor cell migration or invasion was measured by the transwell assay and related protein markers were determined by western blot. A mouse xenograft model was established to investigate the OSCC tumor growth in vivo.

Results

Significant higher expression of NLRP3 was observed in the OSCC cells. Obvious expression of NLRP3 and IL-1β was found in the paraffin-embedded OSCC tissues, and the NLRP3 expression levels were correlated with the tumor size, lymphonode metastatic status and IL-1β expression. Downregulating NLRP3 expression markedly reduced the cleavage of caspase-1 and production of IL-1β in OSCC cells. NLRP3 knockdown also inhibited the proliferation, migration and invasion of OSCC cells. Further investigation indicated that expressions of E-cadherin and vimentin in OSCC cells were increased, while N-cadherin expression was decreased after NLRP3 knockdown. Downregulating NLRP3 expression in OSCC cells significantly reduced the tumor growth in vivo.

Conclusions

Our data suggested that the increased expression of NLRP3 in OSCC was associated with tumor growth and metastasis. NLRP3 may be considered as a potential target for OSCC therapy.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4403-9) contains supplementary material, which is available to authorized users.

The oxidized phospholipid oxPAPC protects from septic shock by targeting the non-canonical inflammasome in macrophages

Lipopolysaccharide (LPS) of Gram-negative bacteria can elicit a strong immune response. Although extracellular LPS is sensed by TLR4 at the cell surface and triggers a transcriptional response, cytosolic LPS binds and activates non-canonical inflammasome caspases, resulting in pyroptotic cell death, as well as canonical NLRP3 inflammasome-dependent cytokine release. Contrary to the highly regulated multiprotein platform required for caspase-1 activation in the canonical inflammasomes, the non-canonical mouse caspase-11 and the orthologous human caspase-4 function simultaneously as innate sensors and effectors, and their regulation is unclear. Here we show that the oxidized phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (oxPAPC) inhibits the non-canonical inflammasome in macrophages, but not in dendritic cells. Aside from a TLR4 antagonistic role, oxPAPC binds directly to caspase-4 and caspase-11, competes with LPS binding, and consequently inhibits LPS-induced pyroptosis, IL-1β release and septic shock. Therefore, oxPAPC and its derivatives might provide a basis for therapies that target non-canonical inflammasomes during Gram-negative bacterial sepsis.

The Emerging Role of Vitamin B6 in Inflammation and Carcinogenesis

Vitamin B6 serves as a coenzyme catalyzing more than 150 enzymes regulating metabolism and synthesis of proteins, carbohydrates, lipids, heme, and important bioactive metabolites. For several years vitamin B6 and its vitamers (B6) were recognized as antioxidant and antiinflammatory and in modulating immunity and gene expression. During the last 10 years, there were growing reports implicating B6 in inflammation and inflammation-related chronic illnesses including cancer. It is unclear if the deficiency of B6 or additional intake of B6, above the current requirement, should be the focus. Whether the current recommended daily intake for B6 is adequate should be revisited, since B6 is important to human health beyond its role as a coenzyme and its status is affected by many factors including but not limited to age, obesity, and inflammation associated with chronic illnesses. A link between inflammation B6 status and carcinogenesis is not yet completely understood. B6-mediated synthesis of H₂S, a gasotransmitter, and taurine in health and disease, especially in maintaining mitochondrial integrity and biogenesis and inflammation, remains an important area to be explored. Recent developments in the molecular role of B6 and its direct interaction with inflammasomes, and nuclear receptor corepressor and coactivator, receptor-interacting protein 140, provide a strong impetus to further explore the multifaceted role of B6 in carcinogenesis and human health.

Measuring NLR Oligomerization I: Size Exclusion Chromatography, Co-immunoprecipitation, and Cross-Linking

Oligomerization of nod-like receptors (NLRs) can be detected by several biochemical techniques dependent on the stringency of protein-protein interactions. Some of these biochemical methods can be combined with functional assays, such as caspase-1 activity assay. Size exclusion chromatography (SEC) allows separation of native protein lysates into different sized complexes by fast protein liquid chromatography (FPLC) for follow-up analysis. Using co-immunoprecipitation (co-IP), combined with SEC or on its own, enables subsequent antibody-based purification of NLR complexes and associated proteins, which can then be analyzed by immunoblot and/or subjected to functional caspase-1 activity assay. Chemical cross-linking covalently joins two or more molecules, thus capturing the oligomeric state with high sensitivity and stability. Apoptosis-associated speck-like protein containing a caspase activation domain (ASC) oligomerization has been successfully used as readout for NLR or AIM2-like receptor (ALR) inflammasome activation in response to various pathogen- or damage-associated molecular patterns (PAMPs or DAMPs) in human and mouse macrophages and THP-1 cells. Here, we provide a detailed description of the methods used for NLRP7 oligomerization in response to infection with Staphylococcus aureus (S. aureus) in primary human macrophages, co-immunoprecipitation and immunoblot analysis of NLRP7 and NLRP3 inflammasome complexes, as well as caspase-1 activity assays. Also, ASC oligomerization is shown in response to dsDNA, LPS/ATP, and LPS/nigericin in mouse bone marrow-derived macrophages (BMDMs) and/or THP-1 cells or human primary macrophages.

COPs and POPs Patrol Inflammasome Activation

Sensing and responding to pathogens and tissue damage is a core mechanism of innate immune host defense, and inflammasomes represent a central cytosolic pattern recognition receptor pathway leading to the generation of the pro-inflammatory cytokines interleukin-1β and interleukin-18 and pyroptotic cell death that causes the subsequent release of danger signals to propagate and perpetuate inflammatory responses. While inflammasome activation is essential for host defense, deregulated inflammasome responses and excessive release of inflammatory cytokines and danger signals are linked to an increasing spectrum of inflammatory diseases. In this review, we will discuss recent developments in elucidating the role of PYRIN domain-only proteins (POPs) and the related CARD-only proteins (COPs) in regulating inflammasome responses and their impact on inflammatory disease.

High-fat diet and aging interact to produce neuroinflammation and impair hippocampal- and amygdalar-dependent memory

More Americans are consuming diets higher in saturated fats and refined sugars than ever before, and based on increasing obesity rates, this is a growing trend among older adults as well. While high saturated fat diet (HFD) consumption has been shown to sensitize the inflammatory response to a subsequent immune challenge in young adult rats, the inflammatory effect of HFD in the already-vulnerable aging brain has not yet been assessed. Here, we explored whether short-term (3 days) consumption of HFD would serve as a neuroinflammatory trigger in aging animals, leading to cognitive deficits. HFD impaired long-term contextual (hippocampal dependent) and auditory-cued fear (amygdalar dependent) memory in aged, but not young adult rats. Short-term memory performance for both tasks was intact, suggesting that HFD impairs memory consolidation processes. Microglial markers of activation Iba1 and cd11b were only increased in the aged rats, while MHCII was further amplified by HFD. Furthermore, these HFD-induced long-term memory impairments were accompanied by IL-1β protein increases in both the hippocampus and amygdala in aged rats. Central administration of IL-1RA in aged rats following conditioning mitigated both contextual and auditory-cued fear memory impairments caused by HFD, strongly suggesting that IL-1β plays a critical role in these effects. Voluntary wheel running, known to have anti-inflammatory effects in the hippocampus, rescued hippocampal-dependent but not amygdalar-dependent memory impairments caused by HFD. Together, these data suggest that short-term consumption of HFD can lead to memory deficits and significant brain inflammation in the aged animal, and strongly suggest that appropriate diet is crucial for cognitive health.

The PYRIN domain-only protein POP2 inhibits inflammasome priming and activation

Inflammasomes are protein platforms linking recognition of microbe, pathogen-associated and damage-associated molecular patterns by cytosolic sensory proteins to caspase-1 activation. Caspase-1 promotes pyroptotic cell death and the maturation and secretion of interleukin (IL)-1β and IL-18, which trigger inflammatory responses to clear infections and initiate wound-healing; however, excessive responses cause inflammatory disease. Inflammasome assembly requires the PYRIN domain (PYD)-containing adaptor ASC, and depends on PYD–PYD interactions. Here we show that the PYD-only protein POP2 inhibits inflammasome assembly by binding to ASC and interfering with the recruitment of ASC to upstream sensors, which prevents caspase-1 activation and cytokine release. POP2 also impairs macrophage priming by inhibiting the activation of non-canonical IκB kinase ɛ and IκBα, and consequently protects from excessive inflammation and acute shock in vivo. Our findings advance our understanding of the complex regulatory mechanisms that maintain a balanced inflammatory response and highlight important differences between individual POP members.

Excessive inflammasome activation leads to inflammatory diseases, but how inflammasomes are regulated by PYD-only adaptors is unclear. Here the authors show that the PYD-only protein POP2 inhibits both inflammasome priming and assembly by interfering, respectively, with IκBα activation and NLRP3-ASC interaction.

A high-fat diet impairs reproduction by decreasing the IL1β level in mice treated at immature stage

Obesity causes low-grade inflammation that is involved in male infertility. Interleukin 1 beta (IL1β) plays an important role in this process. A high-fat diet (HFD) is the most common cause of obesity. However, the effect of a HFD on IL1β and its consequence in reproduction remain unclear. We established a HFD model in mice treated at immature stage (mice-TIS) and mice treated at mature stage (mice-TMS). Surprisingly, we found that a HFD decreased IL1β levels and was accompanied by an increase in testosterone in mice-TIS, while the reverse results were observed in mice-TMS. In addition, a HFD caused a reduction in testis macrophages and in the expression of inflammasome-related genes and proteins in mice-TIS. Furthermore, we found that IL1β inhibited testosterone secretion through down-regulating the gene expression of P450SCC and P450c17. However, the influence on mice-TIS that were induced by a HFD was recovered by stopping the HFD. In this study, we are the first to report that a HFD impairs the reproductive system by decreasing IL1β and enhancing testosterone levels in mice-TIS, which are different from the effects in mice-TMS. This provides new ideas for the treatment of obesity-induced infertility.

A Role for the Inflammasome in Spontaneous Labor at Term with Acute Histologic Chorioamnionitis

Inflammasomes are cytosolic signaling platforms that regulate the activation of caspase (CASP)-1, which induces the maturation of interleukin (IL)-1β and IL-18. Herein, we determined whether the chorioamniotic membranes from women in spontaneous labor at term with acute histologic chorioamnionitis express major inflammasome components and whether these changes are associated with the activation of CASP-1 and CASP-4 and the release of mature IL-1β and IL-18. When comparing the chorioamniotic membranes from women in spontaneous labor at term with acute histologic chorioamnionitis to those without this placental lesion, we found that (1) the messenger RNA (mRNA) abundance of NLR family pyrin domain containing 3 ( NLRP3), NLR family CARD domain containing 4 ( NLRC4), absent in melanoma 2 ( AIM2), and nucleotide binding oligomerization domain 2 ( NOD2) was higher; (2) the NLRP3 and NLRC4 protein quantities were increased; (3) the mRNA and protein expressions of CASP-1 and its active forms were greater; (4) CASP-4 was increased at the mRNA level only; (5) the mRNA and protein expressions of IL-1β and its mature form were higher; and (6) a modest increase in the total protein concentration and abundance of the mature form of IL-18 was observed. In vitro incubation of the chorioamniotic membranes with the CASP-1 inhibitor, VX765, decreased the release of endotoxin-induced IL-1β and IL-18 (2-fold) but not IL-6 or tumor necrosis factor α. In conclusion, spontaneous labor at term with acute histologic chorioamnionitis is characterized by an upregulation of inflammasome components which, in turn, may participate in the activation of CASP-1 and lead to the release of mature IL-1β by the chorioamniotic membranes. These results support a role for the inflammasome in the mechanisms responsible for spontaneous labor at term with acute histologic chorioamnionitis.

[Interleukin-1, inflammasome and autoinflammatory diseases]

Interleukin-1 is a major cytokine of innate immunity and inflammation. It exerts various systemic effects during the inflammatory response, such as fever induction, thrombopoiesis and granulopoiesis, or leukocyte recruitment. Its involvement has been demonstrated in many inflammatory-mediated diseases, such as diabetes or gout. Moreover, interleukin-1 plays a pivotal role in some autoinflammatory diseases, such as cryopyrinopathies or familial Mediterranean fever. In these diseases, a constitutional defect of the inflammasome, a protein complex responsible for the activation of interleukin-1, explains the hypersecretion of interleukin-1. Other autoinflammatory diseases have a more complex pathophysiology involving deregulation of the interleukin-1 pathway, upstream or downstream of the inflammasome, or through more complex mechanisms. In this review, we are detailing the synthesis, the activation, the signalling, and the regulation of interleukin-1. We then describe the autoinflammatory diseases or related-diseases where the pathological role of interleukin-1 has been demonstrated.

Glucocorticoids Mediate Short-Term High-Fat Diet Induction of Neuroinflammatory Priming, the NLRP3 Inflammasome, and the Danger Signal HMGB1

The impact of the foods we eat on metabolism and cardiac physiology has been studied for decades, yet less is known about the effects of foods on the CNS, or the behavioral manifestations that may result from these effects. Previous studies have shown that long-term consumption of high-fat foods leading to diet-induced obesity sensitizes the inflammatory response of the brain to subsequent challenging stimuli, causing deficits in the formation of long-term memories. The new findings reported here demonstrate that short-term consumption of a high-fat diet (HFD) produces the same outcomes, thus allowing the examination of mechanisms involved in this process long before obesity and associated comorbidities occur. Rats fed an HFD for 3 d exhibited increases in corticosterone, the inflammasome-associated protein NLRP3 (nod-like receptor protein 3), and the endogenous danger signal HMGB1 (high-mobility group box 1) in the hippocampus. A low-dose (10 μg/kg) lipopolysaccharide (LPS) immune challenge potentiated the neuroinflammatory response in the hippocampus of rats fed the HFD, and caused a deficit in the formation of long-term memory, effects not observed in rats fed regular chow. The blockade of corticosterone action with the glucocorticoid receptor antagonist mifepristone prevented the NLRP3 and HMGB1 increases in unchallenged animals, normalized the proinflammatory response to LPS, and prevented the memory impairment. These data suggest that short-term HFD consumption increases vulnerability to memory disruptions caused by an immune challenge by upregulating important neuroinflammatory priming and danger signals in the hippocampus, and that these effects are mediated by increases in hippocampal corticosterone.

Invited review: Role of bacterial endotoxins in the etiopathogenesis of periparturient diseases of transition dairy cows

The dairy industry continues to suffer severe economic losses due to the increased disease incidence cows experience during the transition period. It has long been the classical view that the major contributing factor to the development of these periparturient diseases is the considerable increase in nutritional demands for milk production. This classical view, however, fails to account for the substantial correlation between both metabolic and infectious diseases and the detrimental effects that can occur with the provision of high-energy diets to support these nutritional demands. Currently, increasing evidence implicates bacterial endotoxins in the etiopathology of most periparturient diseases. Bacterial endotoxins are components of the outer cell wall of gram-negative and gram-positive bacteria that are highly immunostimulatory and can trigger proinflammatory immune responses. The ability of endotoxins to translocate from the mucosal tissues, including the gastrointestinal tract, mammary gland, and uterus, into the systemic circulation has been observed. Once they have entered the circulation, endotoxins potentially contribute to disease either directly, through eliciting an inflammatory response, or indirectly through other factors such as the overreaction of the natural protective mechanisms of the host. Although the evidence implicating a role of endotoxins in the pathogenesis of transition diseases continues to grow, our current knowledge of the host response to mucosal endotoxin exposure and pathogenic mechanisms remain largely unknown. Developing our understanding of the connection between endotoxemia and dairy cattle disease holds significant potential for the future development of preventative measures that could benefit the productivity of the dairy industry as well as animal welfare.

A Role for the Inflammasome in Spontaneous Labor at Term

PROBLEM

Inflammasomes are signaling platforms that, upon sensing pathogens and sterile stressors, mediate the release of mature forms of interleukin (IL)-1β and IL-18. The aims of this study were to determine (i) the expression of major inflammasome components in the chorioamniotic membranes in spontaneous labor at term, (ii) whether there are changes in the inflammasome components associated with the activation of caspase-1 and caspase-4, and (iii) whether these events are associated with the release of the mature forms of IL-1β and IL-18.

METHOD OF STUDY

Chorioamniotic membranes were collected from women at term with and without spontaneous labor. mRNA abundance and protein concentrations of inflammasome components, nucleotide-binding oligomerization domain-containing (NOD)1 and NOD2 proteins, caspase-1, caspase-4, IL-1β, and IL-18 were quantified by qRT-PCR (n = 28-29 each), ELISA (n = 10 each) or immunoblotting (n = 8 each), and immunohistochemistry (n = 10 each). Active caspase-1 and caspase-4, as well as mature IL-18, were determined by immunoblotting (n = 4 each), and pro- and mature forms of IL-1β were determined by ELISA (n = 4-7 each).

RESULTS

Inflammasome components and NOD proteins were expressed in the chorioamniotic membranes obtained from women at term. The chorioamniotic membranes from women who underwent labor had (i) higher concentrations of NLRP3 (NOD-like receptor family, pyrin domain-containing protein 3) and NOD1 protein, (ii) greater immunoreactivity for caspase-1 and caspase-4, (iii) a greater quantity of the active form of caspase-1 (p20), and (iv) higher mRNA abundance and protein concentrations of pro- and mature IL-1β. However, mRNA abundance and protein concentrations of the mature form of IL-18 were not increased in tissues from women who underwent labor at term.

CONCLUSIONS

Spontaneous labor at term is characterized by the expression of inflammasome components, which may participate in the activation of caspase-1 and lead to the cleavage and release of mature IL-1β by the chorioamniotic membranes. These results support the participation of the inflammasome in the mechanisms responsible for spontaneous parturition at term.

The PYRIN Domain-only Protein POP1 Inhibits Inflammasome Assembly and Ameliorates Inflammatory Disease

In response to infections and tissue damage, ASC-containing inflammasome protein complexes are assembled that promote caspase-1 activation, IL-1β and IL-18 processing and release, pyroptosis, and the release of ASC particles. However, excessive or persistent activation of the inflammasome causes inflammatory diseases. Therefore, a well-balanced inflammasome response is crucial for the maintenance of homeostasis. We show that the PYD-only protein POP1 inhibited ASC-dependent inflammasome assembly by preventing inflammasome nucleation, and consequently interfered with caspase-1 activation, IL-1β and IL-18 release, pyroptosis, and the release of ASC particles. There is no mouse ortholog for POP1, but transgenic expression of human POP1 in monocytes, macrophages, and dendritic cells protected mice from systemic inflammation triggered by molecular PAMPs, inflammasome component NLRP3 mutation, and ASC danger particles. POP1 expression was regulated by TLR and IL-1R signaling, and we propose that POP1 provides a regulatory feedback loop that shuts down excessive inflammatory responses and thereby prevents systemic inflammation.

ATP binding by NLRP7 is required for inflammasome activation in response to bacterial lipopeptides

Nucleotide-binding oligimerization domain (NOD)-like receptors (NLRs) are pattern recognition receptors (PRRs) involved in innate immune responses. NLRs encode a central nucleotide-binding domain (NBD) consisting of the NAIP, CIITA, HET-E and TP1 (NACHT) domain and the NACHT associated domain (NAD), which facilitates receptor oligomerization and downstream inflammasome signaling. The NBD contains highly conserved regions, known as Walker motifs, that are required for nucleotide binding and hydrolysis. The NLR containing a PYRIN domain (PYD) 7 (NLRP7) has been recently shown to assemble an ASC and caspase-1-containing high molecular weight inflammasome complex in response to microbial acylated lipopeptides and Staphylococcus aureus infection. However, the molecular mechanism responsible for NLRP7 inflammasome activation is still elusive. Here we demonstrate that the NBD of NLRP7 is an ATP binding domain and has ATPase activity. We further show that an intact nucleotide-binding Walker A motif is required for NBD-mediated nucleotide binding and hydrolysis, oligomerization, and NLRP7 inflammasome formation and activity. Accordingly, THP-1 cells expressing a mutated Walker A motif display defective NLRP7 inflammasome activation, interleukin (IL)-1β release and pyroptosis in response to acylated lipopeptides and S. aureus infection. Taken together, our results provide novel insights into the mechanism of NLRP7 inflammasome assembly.

Shigella IpaH7.8 E3 ubiquitin ligase targets glomulin and activates inflammasomes to demolish macrophages

When nucleotide-binding oligomerization domain-like receptors (NLRs) sense cytosolic-invading bacteria, they induce the formation of inflammasomes and initiate an innate immune response. In quiescent cells, inflammasome activity is tightly regulated to prevent excess inflammation and cell death. Many bacterial pathogens provoke inflammasome activity and induce inflammatory responses, including cell death, by delivering type III secreted effectors, the rod component flagellin, and toxins. Recent studies indicated that Shigella deploy multiple mechanisms to stimulate NLR inflammasomes through type III secretion during infection. Here, we show that Shigella induces rapid macrophage cell death by delivering the invasion plasmid antigen H7.8 (IpaH7.8) enzyme 3 (E3) ubiquitin ligase effector via the type III secretion system, thereby activating the NLR family pyrin domain-containing 3 (NLRP3) and NLR family CARD domain-containing 4 (NLRC4) inflammasomes and caspase-1 and leading to macrophage cell death in an IpaH7.8 E3 ligase-dependent manner. Mice infected with Shigella possessing IpaH7.8, but not with Shigella possessing an IpaH7.8 E3 ligase-null mutant, exhibited enhanced bacterial multiplication. We defined glomulin/flagellar-associated protein 68 (GLMN) as an IpaH7.8 target involved in IpaH7.8 E3 ligase-dependent inflammasome activation. This protein originally was identified through its association with glomuvenous malformations and more recently was described as a member of a Cullin ring ligase inhibitor. Modifying GLMN levels through overexpression or knockdown led to reduced or augmented inflammasome activation, respectively. Macrophages stimulated with lipopolysaccharide/ATP induced GLMN puncta that localized with the active form of caspase-1. Macrophages from GLMN(+/-) mice were more responsive to inflammasome activation than those from GLMN(+/+) mice. Together, these results highlight a unique bacterial adaptation that hijacks inflammasome activation via interactions between IpaH7.8 and GLMN.

An Update on PYRIN Domain-Containing Pattern Recognition Receptors: From Immunity to Pathology

Cytosolic pattern recognition receptors (PRRs) sense a wide range of endogenous danger-associated molecular patterns as well as exogenous pathogen-associated molecular patterns. In particular, Nod-like receptors containing a pyrin domain (PYD), called NLRPs, and AIM2-like receptors (ALRs) have been shown to play a critical role in host defense by facilitating clearance of pathogens and maintaining a healthy gut microflora. NLRPs and ALRs both encode a PYD, which is crucial for relaying signals that result in an efficient innate immune response through activation of several key innate immune signaling pathways. However, mutations in these PRRs have been linked to the development of auto-inflammatory and autoimmune diseases. In addition, they have been implicated in metabolic diseases. In this review, we summarize the function of PYD-containing NLRPs and ALRs and address their contribution to innate immunity, host defense, and immune-linked diseases.

IL-1β processing in mechanical ventilation-induced inflammation is dependent on neutrophil factors rather than caspase-1

PURPOSE

Mechanical ventilation can cause ventilator-induced lung injury, characterized by a sterile inflammatory response in the lungs resulting in tissue damage and respiratory failure. The cytokine interleukin-1β (IL-1β) is thought to play an important role in the pathogenesis of ventilator-induced lung injury. Cleavage of the inactive precursor pro-IL-1β to form bioactive IL-1β is mediated by several types of proteases, of which caspase-1, activated within the inflammasome, is the most important. Herein, we studied the roles of IL-1β, caspase-1 and neutrophil factors in the mechanical ventilation-induced inflammatory response in mice.

METHODS

Untreated wild-type mice, IL-1αβ knockout and caspase-1 knockout mice, pralnacasan (a selective caspase-1 inhibitor)-treated mice, anti-keratinocyte-derived chemokine (KC)-treated mice and cyclophosphamide-treated neutrophil-depleted wild-type mice were ventilated using clinically relevant ventilator settings (tidal volume 8 ml/kg). The lungs and plasma were collected to determine blood gas values, cytokine profiles and neutrophil influx.

RESULTS

Mechanical ventilation resulted in increased pulmonary concentrations of IL-1β and KC and increased pulmonary neutrophil influx compared with non-ventilated mice. Ventilated IL-1αβ knockout mice did not demonstrate this increase in cytokines. No significant differences were observed between wild-type and caspase-1-deficient or pralnacasan-treated mice. In contrast, in anti-KC antibody-treated mice and neutropenic mice, inflammatory parameters decreased in comparison with ventilated non-treated mice.

CONCLUSIONS

Our results illustrate that IL-1 is indeed an important cytokine in the inflammatory cascade induced by mechanical ventilation. However, the inflammasome/caspase-1 appears not to be involved in IL-1β processing in this type of inflammatory response. The attenuated inflammatory response observed in ventilated anti-KC-treated and neutropenic mice suggests that IL-1β processing in mechanical ventilation-induced inflammation is mainly mediated by neutrophil factors.

Pathogenesis of acute stroke and the role of inflammasomes

Inflammation is an innate immune response to infection or tissue damage that is designed to limit harm to the host, but contributes significantly to ischemic brain injury following stroke. The inflammatory response is initiated by the detection of acute damage via extracellular and intracellular pattern recognition receptors, which respond to conserved microbial structures, termed pathogen-associated molecular patterns or host-derived danger signals termed damage-associated molecular patterns. Multi-protein complexes known as inflammasomes (e.g. containing NLRP1, NLRP2, NLRP3, NLRP6, NLRP7, NLRP12, NLRC4, AIM2 and/or Pyrin), then process these signals to trigger an effector response. Briefly, signaling through NLRP1 and NLRP3 inflammasomes produces cleaved caspase-1, which cleaves both pro-IL-1β and pro-IL-18 into their biologically active mature pro-inflammatory cytokines that are released into the extracellular environment. This review will describe the molecular structure, cellular signaling pathways and current evidence for inflammasome activation following cerebral ischemia, and the potential for future treatments for stroke that may involve targeting inflammasome formation or its products in the ischemic brain.

Post-translational control of IL-1β via the human papillomavirus type 16 E6 oncoprotein: a novel mechanism of innate immune escape mediated by the E3-ubiquitin ligase E6-AP and p53

Infections with high-risk human papillomaviruses (HPVs) are causally involved in the development of anogenital cancer. HPVs apparently evade the innate immune response of their host cells by dysregulating immunomodulatory factors such as cytokines and chemokines, thereby creating a microenvironment that favors malignancy. One central key player in the immune surveillance interactome is interleukin-1 beta (IL-1β) which not only mediates inflammation, but also links innate and adaptive immunity. Because of its pleiotropic physiological effects, IL-1β production is tightly controlled on transcriptional, post-translational and secretory levels. Here, we describe a novel mechanism how the high-risk HPV16 E6 oncoprotein abrogates IL-1β processing and secretion in a NALP3 inflammasome-independent manner. We analyzed IL-1β regulation in immortalized keratinocytes that harbor the HPV16 E6 and/or E7 oncogenes as well as HPV-positive cervical tumor cells. While in primary and in E7-immortalized human keratinocytes the secretion of IL-1β was highly inducible upon inflammasome activation, E6-positive cells did not respond. Western blot analyses revealed a strong reduction of basal intracellular levels of pro-IL-1β that was independent of dysregulation of the NALP3 inflammasome, autophagy or lysosomal activity. Instead, we demonstrate that pro-IL-1β is degraded in a proteasome-dependent manner in E6-positive cells which is mediated via the ubiquitin ligase E6-AP and p53. Conversely, in E6- and E6/E7-immortalized cells pro-IL-1β levels were restored by siRNA knock-down of E6-AP and simultaneous recovery of functional p53. In the context of HPV-induced carcinogenesis, these data suggest a novel post-translational mechanism of pro-IL-1β regulation which ultimately inhibits the secretion of IL-1β in virus-infected keratinocytes. The clinical relevance of our results was further confirmed in HPV-positive tissue samples, where a gradual decrease of IL-1β towards cervical cancer could be discerned. Hence, attenuation of IL-1β by the HPV16 E6 oncoprotein in immortalized cells is apparently a crucial step in viral immune evasion and initiation of malignancy.

Persistently high-risk HPV-infected individuals have an increased risk to develop anogenital cancer. HPV encodes the viral proteins E6 and E7 that interact with and induce the degradation of the cell cycle regulators p53 and pRb, respectively, priming immortalized keratinocytes towards malignant transformation. In early antiviral immune response, IL-1β is an important factor for the initiation of inflammation and activation of immune cells such as macrophages and T cells. Our study describes a post-translationally controlled pathway where E6 mediates proteasomal degradation of IL-1β in HPV16-immortalized human keratinocytes. This process depends on the cellular ubiquitin ligase E6-AP and p53 highlighting a novel molecular mechanism of a virus-host interaction that is critical for evading innate immune defense. IL-1β dysregulation is also found in tissue sections which represent different stages of virus-induced carcinogenesis, underlining the clinical relevance of our findings.

Chitinase 3-like-1 promotes Streptococcus pneumoniae killing and augments host tolerance to lung antibacterial responses

Host antibacterial responses include mechanisms that kill bacteria, but also those that protect or tolerize the host to potentially damaging antibacterial effects. We determined that Chitinase 3-like-1 (Chi3l1), a conserved prototypic chitinase-like protein, is induced by Streptococcus pneumoniae and plays central roles in promoting bacterial clearance and mediating host tolerance. S. pneumoniae-infected Chi3l1 null mice exhibit exaggerated lung injury, inflammation and hemorrhage, more frequent bacterial dissemination, decreased bacterial clearance, and enhanced mortality compared to controls. Chi3l1 augments macrophage bacterial killing by inhibiting caspase-1-dependent macrophage pyroptosis and augments host tolerance by controlling inflammasome activation, ATP accumulation, expression of ATP receptor P2X7R, and production of thymic stromal lymphopoietin and type 1, type 2, and type 17 cytokines. These data demonstrate that Chi3l1 is induced during infection, where it promotes bacterial clearance while simultaneously augmenting host tolerance, and that these roles likely contributed to the retention of Chi3l1 over species and evolutionary time.

Innate immunosenescence: effect of aging on cells and receptors of the innate immune system in humans

Components of the innate immune response, including neutrophils and macrophages, are the first line of defense against infections. Their role is to initiate an inflammatory response, phagocyte and kill pathogens, recruit natural killer cells (NK), and facilitate the maturation and migration of dendritic cells that will initiate the adaptive immune response. Extraordinary advances have been made in the last decade on the knowledge of the receptors and mechanisms used by cells of the innate immunity not only to sense and eliminate the pathogen but also to communicate each other and collaborate with cells of adaptive immunity to mount an effective immune response. The analysis of innate immunity in elderly humans has evidenced that aging has a profound impact on the phenotype and functions of these cells. Thus altered expression and/or function of innate immunity receptors and signal transduction leading to defective activation and decreased chemotaxis, phagocytosis and intracellular killing of pathogens have been described. The phenotype and function of NK cells from elderly individuals show significant changes that are compatible with remodeling of the different NK subsets, with a decrease in the CD56bright subpopulation and accumulation of the CD56dim cells, in particular those differentiated NK cells that co-express CD57, as well as a decreased expression of activating natural cytotoxicity receptors. These alterations can be responsible of the decreased production of cytokines and the lower per-cell cytotoxicity observed in the elderly. Considering the relevance of these cells in the initiation of the immune response, the possibility to reactivate the function of innate immune cells should be considered in order to improve the response to pathogens and to vaccination in the elderly.