AIM2 inflammasome in infection, cancer, and autoimmunity: Role in DNA sensing, inflammation, and innate immunity

AIM2 inflammasome-derived IL-1β induces postoperative ileus in mice

Postoperative ileus (POI) is an intestinal dysmotility frequently occurring after abdominal surgery. An orchestrated neuroimmune response within the muscularis externa (ME) involves activation of resident macrophages, enteric glia and infiltration of blood-derived leukocytes. Interleukin-1 receptor type-I (IL1R1) signalling on enteric glia has been shown to be involved in POI development. Herein we investigated the distinct role of the IL1R1 ligands interleukin (IL) -1α and IL-1β and focused on the mechanism of IL-1β production. IL-1α and IL-1β deficient mice were protected from POI. Bone-marrow transplantation studies indicated that IL-1α originated from radio-resistant cells while IL-1β was released from the radio-sensitive infiltrating leukocytes. Mouse strains deficient in inflammasome formation identified the absent in melanoma 2 (AIM2) inflammasome to be crucial for IL-1β production in POI. Mechanistically, antibiotic-treated mice revealed a prominent role of the microbiome in IL-1β production. Our study provides new insights into distinct roles of IL-1α and IL-1β signalling during POI. While IL-1α release is most likely an immediate passive response to the surgical trauma, IL-1β production depends on AIM2 inflammasome formation and the microbiome. Selective interaction in this pathway might be a promising target to prevent POI in surgical patients.

NLRP3, NLRP12, and IFI16 Inflammasomes Induction and Caspase-1 Activation Triggered by Virulent HSV-1 Strains Are Associated With Severe Corneal Inflammatory Herpetic Disease

The crosstalk between the host's inflammasome system and the invading virulent/less-virulent viruses determines the outcome of the ensuing inflammatory response. An appropriate activation of inflammasomes triggers antiviral inflammatory responses that clear the virus and heal the inflamed tissue. However, an aberrant activation of inflammasomes can result in a harmful and overwhelming inflammation that could damage the infected tissue. The underlying host's immune mechanisms and the viral virulent factors that impact severe clinical inflammatory disease remain to be fully elucidated. In this study, we used herpes simplex virus type 1 (HSV-1), the causative agent of corneal inflammatory herpetic disease, as a model pathogen to determine: (i) Whether and how the virulence of a virus affects the type and the activation level of the inflammasomes; and (ii) How triggering specific inflammasomes translates into protective or damaging inflammatory response. We showed that, in contrast to the less-virulent HSV-1 strains (RE, F, KOS, and KOS63), corneal infection of B6 mice with the virulent HSV-1 strains (McKrae, 17 or KOS79): (i) Induced simultaneous expression of the NLRP3, NLRP12, and IFI16 inflammasomes; (ii) Increased production of the biologically active Caspase-1 and pro-inflammatory cytokines IL-1β and IL-18; (iii) Heightened recruitment into the inflamed cornea of CD45highLy6C+Ly6G-F4/80+CD11b+CD11c- inflammatory monocytes and CD45highCD11b+F4/80-Ly6GhiLy6Cmed neutrophils; and (iv) This intensified inflammatory response was associated with a severe corneal herpetic disease, irrespective of the level of virus replication in the cornea. Similarly, in vitro infection of human corneal epithelial cells and human monocytic THP-1 cells with the virulent HSV-1 strains triggered a synchronized early expression of NLRP3, NLRP12 and IFI16, 2 h post-infection, associated with formation of single and dense specks of the adapter molecule ASC in HSV(+) cells, but not in the neighboring bystander HSV(-) cells. This was associated with increased cleavages of Caspase-1, IL-1β, and IL-18. These findings suggest a previously unappreciated role of viral virulence in a synchronized early induction of the NLRP3, NLRP12, and IFI16 inflammasomes that lead to a damaging inflammatory response. A potential role of common virus virulent factors that stimulate this harmful inflammatory corneal disease is currently under investigation.

Chrysanthemum indicum extract inhibits NLRP3 and AIM2 inflammasome activation via regulating ASC phosphorylation

ETHNOPHARMACOLOGICAL RELEVANCE

Chrysanthemum indicum (C. indicum), a perennial plant, has long been used to treat inflammation-related disorders, such as pneumonia, hypertension, gastritis, and gastroenteritis.

AIM OF THE STUDY

The inhibitory effect of C. indicum extract (C.I) on inflammasome activation was investigated to validate its potential in treating inflammation related disorders.

MATERIALS AND METHODS

LPS-primed bone marrow-derived macrophages (BMDMs) were used to confirm the inhibitory effect of C.I on selective inflammasome activation in vitro. A monosodium urate (MSU)-induced murine peritonitis model was employed to study the effect of C.I in vivo.

RESULTS

C.I inhibited activation of NLRP3 and AIM2 inflammasomes, leading to suppression of interleukin-1β secretion in vitro. Further, C.I regulates the phosphorylation of apoptosis-associated speck-like protein containing a CARD (ASC), which could be the main contribution to attenuate these inflammasomes activation. C.I also suppressed secretion of pro-inflammatory cytokines and neutrophils recruitment in MSU-induced murine peritonitis model.

CONCLUSIONS

This study provides scientific evidence substantiating the traditional use of C. indicum in the treatment of inflammatory diseases, including gout, which is induced by physiologically analogous cause to MSU-induced peritonitis.

Whole-Blood Gene Expression in Pulmonary Nontuberculous Mycobacterial Infection

The factors predisposing toward the development of pulmonary nontuberculous mycobacterial (pNTM) disease and influencing disease progression remain unclear. Impaired immune responses have been reported in individuals with pNTM disease, but data are limited and inconsistent. In this study, we sought to use gene expression profiling to examine the host response to pNTM disease. Microarray analysis of whole-blood gene expression was performed on 25 subjects with pNTM disease and 27 uninfected control subjects with respiratory disease. Gene expression results were compared with phenotypic variables and survival data. Compared with uninfected control subjects, pNTM disease was associated with downregulation of 213 transcripts enriched for terms related to T cell signaling, including IFNG. Reduced IFNG expression was associated with more severe computed tomography changes and impaired lung function. Mortality was associated with the expression of transcripts related to the innate immune response and inflammation, whereas transcripts related to T and B cell function were associated with improved survival. These findings suggest that pNTM disease is associated with an aberrant immune response, which may reflect an underlying propensity to infection or result from NTM infection itself. There were important differences in the immune response associated with survival and mortality in pNTM disease.

Absent in melanoma 2 inflammasome is required for host defence against Streptococcus pneumoniae infection

Streptococcus pneumoniae, a leading cause of invasive pneumococcal disease, is responsible for high mortality and morbidity worldwide. A previous study showed that the NLR family pyrin domain containing 3 (NLRP3) and absent in melanoma 2 (AIM2) inflammasomes are essential for caspase-1 activation and IL-1β production in the host response to S. pneumoniae infection. The function of NLRP3 in host innate immunity to S. pneumoniae was studied in vivo and in vitro. However, the role of AIM2 in host defence against S. pneumoniae remains unclear. Here, we show that AIM2-deficient (AIM2^–/–) mice display increased susceptibility to intra-nasal infection with S. pneumoniae in comparison to wild type mice and that this susceptibility was associated with defective IL-1β production. Macrophages from AIM2^–/– mice infected with S. pneumoniae showed impaired secretion of IL-1β as well as activation of the inflammasome, as determined by the oligomerisation of apoptosis-associated speck-like protein containing a CARD (ASC) and caspase-1 activation. Taken together, these results indicate that the AIM2 inflammasome is essential for caspase-1-dependent cytokine IL-1β production and eventual protection from pneumococcal infection in mice.

Knockdown of the AIM2 molecule attenuates ischemia-reperfusion-induced spinal neuronal pyroptosis by inhibiting AIM2 inflammasome activation and subsequent release of cleaved caspase-1 and IL-1β

Ischemia-reperfusion (IR) injury induces activation of several inflammasomes that widely affect neuroinflammation and, subsequently, neuronal viability. The absent in melanoma 2 (AIM2) inflammasome is highly expressed in neurons after traumatic injury. This study was performed to investigate whether the AIM2 molecule acts as an initiator to trigger AIM2 inflammasome activation and regulate neuronal pyroptosis in a mouse IR model. The early motor dysfunction that occurred within the first 8 h post-IR injury was closely associated with a massive increase in dsDNA in serum and cerebrospinal fluid (CSF) at the same observed timepoints. However, the subsequent persistent dysfunction was consistent with the continuously increasing protein levels of apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC), cleaved caspase-1 and IL-1β with time. Upregulated AIM2 immunoreactivity was primarily visualized in neurons. The si-AIM2 treatment in vivo preserved part of motor function, accompanied by decreased protein levels of AIM2, ASC, cleaved caspase-1 and IL-1β. In vitro, the direct interactions between the AIM2 molecule and caspase-1 were demonstrated by immunofluorescence staining and coimmunoprecipitation. In this context, both si-AIM2 and Ac-YVAD-CMK treatments effectively maintained neuronal viability, as demonstrated by the decreased percentage of cells with pyroptosis and release of lactate dehydrogenase (LDH), accompanied by weak immunoreactivity and a decreased number of AIM2-caspase-1 positive neurons. By contrast, poly(dA-dT) treatment exacerbated neuronal pyroptosis by reversing the above-mentioned effects. However, no significant differences were observed after si-Con treatment. These results suggest AIM2 molecule played an important role in initiating AIM2 inflammasome activation through IR-induced release of ectopic dsDNA.

PYHIN genes as potential biomarkers for prognosis of human papillomavirus-positive or -negative head and neck squamous cell carcinomas

The aim of the present study is to determine the expression levels of PYHIN (IFI16 and AIM2) and APOBEC3 (A3A, A3B, A3C, A3D, A3F, A3G, and A3H) gene family members in a cohort of patients with head and neck squamous cell carcinoma (HNSCC) and assess their potential correlation with human papillomavirus (HPV) infection status, clinical characteristics, and survival. For this purpose, 34 HNSCC tissue specimens along with healthy surrounding mucosa were collected from patients surgically treated for HNSCC. Nucleic acids were isolated to assess the presence of HPV and the expression levels of selected molecular markers. Survival analysis was carried out using the Kaplan-Meier method. In HPV-negative (HPV-) HNSCCs, we detected low mRNA expression levels of IFI16, A3A, and A3B, whereas these genes were upregulated of 2-100 folds in HPV-positive (HPV+) tumors (p < 0.05). Interestingly, AIM2 gene expression levels were predominantly unchanged in HPV+ HNSCCs compared to their HPV- counterparts, in which AIM2 was predominantly upregulated (10% vs. 50% of patients). In HPV- tumors, upregulation of TP53, NOTCH1, PD-L1, and IFI16 correlated with lower occurrence of nodal metastases. On the other hand, the expression of APOBEC family members did not correlate with clinical characteristics. Regarding survival, patients with upregulated A3F gene expression had a worse prognosis, while patients without changes in A3H expression had a lower survival rate. In conclusion, our findings indicate that the innate immune sensors IFI16 and AIM2 and some APOBEC family members could be potentially used as biomarkers for disease outcome in HNSCC patients regardless of HPV presence.

The Fire within: Cell-Autonomous Mechanisms in Inflammation-Driven Cancer

Inflammatory cells are important for tumor initiation and promotion, providing cancer cells with cytokines that enhance cell proliferation and survival. Although malignant epithelial cells were traditionally considered to be on the receiving end of these microenvironmental interactions, recent studies show that epithelial cells can undergo inflammatory reprogramming on their own. Such epigenetic switches are often triggered by chronic tissue injury and play important roles in tissue repair. By converting terminally differentiated cells that harbor even a single oncogenic mutation to a less differentiated state with a higher proliferative potential, cell-autonomous inflammation is an important contributor to tumor initiation.

The molecular machinery of regulated cell death

Cells may die from accidental cell death (ACD) or regulated cell death (RCD). ACD is a biologically uncontrolled process, whereas RCD involves tightly structured signaling cascades and molecularly defined effector mechanisms. A growing number of novel non-apoptotic forms of RCD have been identified and are increasingly being implicated in various human pathologies. Here, we critically review the current state of the art regarding non-apoptotic types of RCD, including necroptosis, pyroptosis, ferroptosis, entotic cell death, netotic cell death, parthanatos, lysosome-dependent cell death, autophagy-dependent cell death, alkaliptosis and oxeiptosis. The in-depth comprehension of each of these lethal subroutines and their intercellular consequences may uncover novel therapeutic targets for the avoidance of pathogenic cell loss.

Investigations of cellular immunity in juvenile idiopathic arthritis

The following was emphasised in an informative, educational issued on the American College of Rheumatology website in April 2017: “About one child in every 1000 develops some type of chronic arthritis. These disorders can affect children at any age, although rarely in the first six months of life. It is estimated that around 300,000 children in the United States have been diagnosed with the condition”. Therefore, knowledge of immunological investigations in patients with juvenile idiopathic arthritis is important for finding new treatment pathways. Our aim was to assess the immunological investigations and immune system implications in juvenile idiopathic arthritis. We will discuss: a) the specifically targeted proteins – the citrullinated peptide antibodies; b) non-specifically targeted proteins – heat-shock proteins (anti-HSP60, -65, and -70 antibodies), CLEC16A, inflammasomes, and phagocyte-derived S100; c) interleukins – IL-1, IL-6, IL-10, IL-17, and IL-18; d) innate immunity – macrophage activation syndrome, natural killer cells, complement activity, and immune complexes; and e) therapeutic targets – monoclonal antibodies, JAK inhibitors, and intravenous immune globulin.

Circulating Cell-Free mtDNA Contributes to AIM2 Inflammasome-Mediated Chronic Inflammation in Patients with Type 2 Diabetes

Mitochondrial dysfunction has been implicated in the pathogenesis of insulin resistance and type 2 diabetes. Damaged mitochondria DNA (mtDNA) may have a role in regulating hyperglycemia during type 2 diabetes. Circulating cell-free mitochondria DNA (ccf-mtDNA) was found in serum and plasma from patients and has been linked to the prognosis factors in various human diseases. However, the role of ccf-mtDNA in chronic inflammation in type 2 diabetes is unclear. In this study, we hypothesized that the ccf-mtDNA levels are associated with chronic inflammation in patients with type 2 diabetes. The mtDNA levels were elevated in the plasma from patients with type 2 diabetes compared to healthy subjects. The elevated mtDNA levels were associated with interleukin-1β (IL-1β) levels in patients with type 2 diabetes. The mtDNA, from patients with type 2 diabetes, induced absent in melanoma 2 (AIM2) inflammasome-dependent caspase-1 activation and IL-1β and IL-18 secretion in macrophages. Our results suggest that the ccf-mtDNA might contribute to AIM2 inflammasome-mediated chronic inflammation in type 2 diabetes.

The absent in melanoma 2 (AIM2) inflammasome in microbial infection

Inflammasomes play a very important role in the host defense against multiple pathogenic microbes, including bacteria and viruses. Inflammasomes are multiprotein complex platforms that mediate the processing of the two most important inflammatory cytokines, pro-IL-1β and pro-IL-18, to their active forms. The inflammasome is formed by the apoptosis-associated speck-like protein containing a CARD (ASC), procaspase-1 and a sensor protein, either a NOD-like receptor (NLR) or an absent in melanoma 2 (AIM2)-like receptor. The sensor molecule determines inflammasome specificity by detecting specific and conserved microbial products or cell stress signals. Compared with the other inflammasomes, there is much more unknown about the activation or regulation mechanisms of the AIM2 inflammasome. In this review, we will discuss these mechanisms and the specific roles of the AIM2 inflammasome in response to diverse pathogens.

Diverging inflammasome signals in tumorigenesis and potential targeting

Inflammasomes are molecular platforms that assemble upon sensing various intracellular stimuli. Inflammasome assembly leads to activation of caspase 1, thereby promoting the secretion of bioactive interleukin-1β (IL-1β) and IL-18 and inducing an inflammatory cell death called pyroptosis. Effectors of the inflammasome efficiently drive an immune response, primarily providing protection against microbial infections and mediating control over sterile insults. However, aberrant inflammasome signalling is associated with pathogenesis of inflammatory and metabolic diseases, neurodegeneration and malignancies. Chronic inflammation perpetuated by inflammasome activation plays a central role in all stages of tumorigenesis, including immunosuppression, proliferation, angiogenesis and metastasis. Conversely, inflammasome signalling also contributes to tumour suppression by maintaining intestinal barrier integrity, which portrays the diverse roles of inflammasomes in tumorigenesis. Studies have underscored the importance of environmental factors, such as diet and gut microbiota, in inflammasome signalling, which in turn influences tumorigenesis. In this Review, we deliver an overview of the interplay between inflammasomes and tumorigenesis and discuss their potential as therapeutic targets.

Inflammasomes: Emerging Central Players in Cancer Immunology and Immunotherapy

Inflammation has an established role in cancer development and progression and is a key player in regulating the entry and exit of immune cells in the tumor microenvironment, mounting a significant impact on anti-tumor immunity. Recent studies have shed light on the role of inflammasomes in the regulation of inflammation with a focus on the subsequent effects on the immunobiology of tumors. To generate strong anti-tumor immunity, cross-talk between innate, and adaptive immune cells is necessary. Interestingly, inflammasome bridges both arms of the immune system representing a unique opportunity to manipulate the role of inflammation in favor of tumor suppression. In this review, we discuss the impact of inflammasomes on the regulation of the levels of inflammatory cytokines-chemokines and the efficacy of immunotherapy response in cancer treatment.

Cytosolic Nucleic Acid Sensors in Inflammatory and Autoimmune Disorders

Innate immunity employs germline-encoded pattern recognition receptors (PRRs) to sense microbial pattern molecules. Recognition of pathogen-associated molecular patterns (PAMPs) by various PPRs located on the cell membrane or in the cytosol leads to the activation of cell signaling pathways and production of inflammatory mediators. Nucleic acids including DNA, RNA, and their derivatives are potent PAMPs which can be recognized by multiple PRRs to induce inflammatory responses. While nucleic acid sensors can also sense endogenous nucleic acids, they are capable of discriminating self from non-self. However, defects in nucleic acid sensing PRRs or dysregulation of nucleic acid sensing signaling pathways may cause excessive activation of the immune system resulting in the development of inflammatory and autoimmune diseases. This review will discuss the major pathways for sensing intracellular nucleic acids and how defects in these nucleic acid sensing are associated with different kinds of autoimmune and inflammatory disorders.

Nucleic Acid Sensing in Mammals and Plants: Facts and Caveats

The accumulation of nucleic acids in aberrant compartments is a signal of danger: fragments of cytosolic or extracellular self-DNA indicate cellular dysfunctions or disruption, whereas cytosolic fragments of nonself-DNA or RNA indicate infections. Therefore, nucleic acids trigger immunity in mammals and plants. In mammals, endosomal Toll-like receptors (TLRs) sense single-stranded (ss) or double-stranded (ds) RNA or CpG-rich DNA, whereas various cytosolic receptors sense dsDNA. Although a self/nonself discrimination could favor targeted immune responses, no sequence-specific sensing of nucleic acids has been reported for mammals. Specific immune responses to extracellular self-DNA versus DNA from related species were recently reported for plants, but the underlying mechanism remains unknown. The subcellular localization of mammalian receptors can favor self/nonself discrimination based on the localization of DNA fragments. However, autoantibodies and diverse damage-associated molecular patterns (DAMPs) shuttle DNA through membranes, and most of the mammalian receptors share downstream signaling elements such as stimulator of interferon genes (STING) and the master transcription regulators, nuclear factor (NF)-κB, and interferon regulatory factor 3 (IRF3). The resulting type I interferon (IFN) response stimulates innate immunity against multiple threats-from infection to physical injury or endogenous DNA damage-all of which lead to the accumulation of eDNA or cytoplasmatic dsDNA. Therefore, no or only low selective pressures might have favored a strict self/nonself discrimination in nucleic acid sensing. We conclude that the discrimination between self- and nonself-DNA is likely to be less strict-and less important-than assumed originally.

Cryo-EM structure of the NLRC4CARD filament provides insights into how symmetric and asymmetric supramolecular structures drive inflammasome assembly

Inflammasomes are supramolecular signaling platforms integral to innate immune defense against invading pathogens. The NOD-like receptor (NLR) family apoptosis inhibitory protein (NAIP)·NLR family caspase-recruiting domain (CARD) domain-containing 4 (NLRC4) inflammasome recognizes intracellular bacteria and induces the polymerization of the caspase-1 protease, which in turn executes maturation of interleukin-1β (IL-1β) and pyroptosis. Several high-resolution structures of the fully assembled NAIP·NLRC4 complex are available, but these structures do not resolve the architecture of the CARD filament in atomic detail. Here, we present the cryo-EM structure of the filament assembled by the CARD of human NLRC4 (NLRC4^CARD) at 3.4 Å resolution. The structure revealed that the helical architecture of the NLRC4^CARD filament is essentially identical to that of the downstream filament assembled by the CARD of caspase-1 (casp1^CARD), but deviates from the split washer-like assembly of the NAIP·NLRC4 oligomer. Our results suggest that architectural complementarity is a major driver for the recognition between upstream and downstream CARD assemblies in inflammasomes. Furthermore, a Monte Carlo simulation of the NLRC4^CARD filament assembly rationalized why an (un)decameric NLRC4 oligomer is optimal for assembling the helical base of the NLRC4^CARD filament. Together, our results explain how symmetric and asymmetric supramolecular assemblies enable high-fidelity signaling in inflammasomes.

Origin and Consequences of Necroinflammation

When cells undergo necrotic cell death in either physiological or pathophysiological settings in vivo, they release highly immunogenic intracellular molecules and organelles into the interstitium and thereby represent the strongest known trigger of the immune system. With our increasing understanding of necrosis as a regulated and genetically determined process (RN, regulated necrosis), necrosis and necroinflammation can be pharmacologically prevented. This review discusses our current knowledge about signaling pathways of necrotic cell death as the origin of necroinflammation. Multiple pathways of RN such as necroptosis, ferroptosis, and pyroptosis have been evolutionary conserved most likely because of their differences in immunogenicity. As the consequence of necrosis, however, all necrotic cells release damage associated molecular patterns (DAMPs) that have been extensively investigated over the last two decades. Analysis of necroinflammation allows characterizing specific signatures for each particular pathway of cell death. While all RN-pathways share the release of DAMPs in general, most of them actively regulate the immune system by the additional expression and/or maturation of either pro- or anti-inflammatory cytokines/chemokines. In addition, DAMPs have been demonstrated to modulate the process of regeneration. For the purpose of better understanding of necroinflammation, we introduce a novel classification of DAMPs in this review to help detect the relative contribution of each RN-pathway to certain physiological and pathophysiological conditions.

Activation and counteraction of antiviral innate immunity by KSHV: an Update

The innate immune responses triggering production of type I interferons and inflammatory cytokines constitute a nonspecific innate resistance that eliminates invading pathogens including viruses. The activation of innate immune signaling through pattern recognition receptors (PRRs) is by sensing pathogen-associated molecular patterns derived from viruses. According to their distribution within cells, PRRs are classified into three types of receptors: membrane, cytoplasmic, and nuclear. Kaposi's sarcoma-associated herpesvirus (KSHV), a large DNA virus, replicates in the nucleus. Its genome is protected by capsid proteins during transport in the cytosol. Multiple PRRs are involved in KSHV recognition. To successfully establish latent infection, KSHV has evolved to manipulate different aspects of the host antiviral innate immune responses. This review presents recent advances in our understanding about the activation of the innate immune signaling in response to infection of KSHV. It also reviews the evasion strategies used by KSHV to subvert host innate immune detection for establishing a persistent infection.

IFI16 filament formation in salivary epithelial cells shapes the anti-IFI16 immune response in Sjögren's syndrome

IFN-inducible protein 16 (IFI16) is an innate immune sensor that forms filamentous oligomers when activated by double-stranded DNA (dsDNA). Anti-IFI16 autoantibodies occur in patients with Sjögren's syndrome (SS) and associate with severe phenotypic features. We undertook this study to determine whether the structural and functional properties of IFI16 play a role in its status as an SS autoantigen. IFI16 immunostaining in labial salivary glands (LSGs) yielded striking evidence of filamentous IFI16 structures in the cytoplasm of ductal epithelial cells, representing the first microscopic description of IFI16 oligomerization in human tissues, to our knowledge. Transfection of cultured epithelial cells with dsDNA triggered the formation of cytoplasmic IFI16 filaments with similar morphology to those observed in LSGs. We found that a majority of SS anti-IFI16 autoantibodies immunoprecipitate IFI16 more effectively in the oligomeric dsDNA-bound state. Epitopes in the C-terminus of IFI16 are accessible to antibodies in the DNA-bound oligomer and are preferentially targeted by SS sera. Furthermore, cytotoxic lymphocyte granule pathways (highly enriched in the SS gland) induce striking release of IFI16•dsDNA complexes from cultured cells. Our studies reveal that IFI16 is present in a filamentous state in the target tissue of SS and suggest that this property of DNA-induced filament formation contributes to its status as an autoantigen in SS. These studies highlight the role that tissue-specific modifications and immune effector pathways might play in the selection of autoantigens in rheumatic diseases.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Regulatory Roles of Flavonoids on Inflammasome Activation during Inflammatory Responses

Inflammation is an innate immune response to noxious stimuli to protect the body from pathogens. Inflammatory responses consist of two main steps: priming and triggering. In priming, inflammatory cells increase expressions of inflammatory molecules, while in triggering, inflammasomes are activated, resulting in cell death and pro-inflammatory cytokine secretion. Inflammasomes are protein complexes comprising intracellular pattern recognition receptors (PRRs) (e.g., nucleotide-binding oligomerization domain-like receptors (NLRs), absent in melanoma 2 (AIM2), and caspases-4/5/11) and pro-caspase-1 with or without a bipartite adaptor molecule ASC. Inflammasome activation induces pyroptosis, inflammatory cell death, and stimulates caspase-1-mediated secretion of interleukin (IL)-1b and IL-18. Flavonoids are secondary metabolites found in various plants and are considered as critical ingredients promoting health and ameliorating various disease symptoms. Anti-inflammatory activity of flavonoids and underlying mechanisms have been widely studied. This review introduces current knowledge on different types of inflammasomes and their activation during inflammatory responses and discusses recent studies regarding anti-inflammatory roles of flavonoids as suppressors of inflammasomes in inflammatory conditions. Understanding the regulatory effects of flavonoids on inflammasome activation will increase our knowledge of flavonoid-mediated anti-inflammatory activity and provide new insights into the development of flavonoid preparations to prevent and treat human inflammatory diseases.

Cytokine Secretion and Pyroptosis of Thyroid Follicular Cells Mediated by Enhanced NLRP3, NLRP1, NLRC4, and AIM2 Inflammasomes Are Associated With Autoimmune Thyroiditis

Background

Inflammasomes, which mediate maturation of interleukin-1β (IL-β) and interleukin-18 (IL-18) and lead to pyroptosis, have been linked to various autoimmune disorders. This study investigated whether they are involved in the pathogenesis of autoimmune thyroiditis (AIT).

Methods

We collected thyroid tissues from 50 patients with AIT and 50 sex- and age-matched controls. Serum levels of free T3, free T4, thyrotropin, thyroid peroxidase antibody (TPOAb), and thyroglobulin antibody (TgAb) were measured by electrochemiluminescent immunoassays. Expression of several inflammasome components, the NOD-like receptor (NLR) family pyrin domain containing 1 (NLRP1), NLRP3, CARD-domain containing 4 (NLRC4), absent in melanoma 2 (AIM2), the apoptosis-associated speck-like protein that contains a caspase recruitment domain (ASC), caspase-1, IL-1β, and IL-18 was determined by real-time PCR and western blot. Immunohistochemistry was used to localize the expression of NLRP1, NLRP3, NLRC4, and AIM2. The Nthy-ori 3-1 thyroid cell line was stimulated with tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-17A, interleukin-6, and poly(dA:dT). The levels of IL-18 and IL-1β in the cell supernatant were measured by enzyme-linked immunosorbent assay, and lactate dehydrogenase was quantified by absorptiometry. ASC specks were examined by confocal immunofluorescence microscopic analysis. Cell death was examined by flow cytometry, and the N-terminal domain of gasdermin D was detected by western blot analysis.

Results

Expression of NLRP1, NLRP3, NLRC4, AIM2, ASC, caspase-1, pro IL-1β, pro IL-18, mRNA, and protein was significantly increased in thyroid tissues from patients with AIT, and enhanced posttranslational maturation of caspase-1, IL-18 and IL-1β was also observed. Expression of NLRP1, NLRP3, NLRC4, and AIM2 was localized mainly in thyroid follicular cells adjacent to areas of lymphatic infiltration. The thyroid mRNA level of NLRP1 and ASC was correlated to the serum TPOAb and TgAb levels in the AIT group. TNF-α and IFN-γ had a priming effect on the expression of multiple inflammasome components in thyroid cells. IFN-γ was found to strengthen poly(dA:dT)-induced cell pyroptosis and bioactive IL-18 release.

Conclusion

Our work has demonstrated for the first time that multiple inflammasomes are associated with AIT pathogenesis. The identified NLRP3, NLRP1, NLRC4, AIM2 inflammasomes and their downstream cytokines may represent potential therapeutic targets and biomarkers of AIT.

The Absent in Melanoma 2-Like Receptor IFN-Inducible Protein 16 as an Inflammasome Regulator in Systemic Lupus Erythematosus: The Dark Side of Sensing Microbes

Absent in melanoma 2 (AIM2)-like receptors (ALRs) are a newly characterized class of pathogen recognition receptors (PRRs) involved in cytosolic and nuclear pathogen DNA recognition. In recent years, two ALR family members, the interferon (IFN)-inducible protein 16 (IFI16) and AIM2, have been linked to the pathogenesis of various autoimmune diseases, among which systemic lupus erythematosus (SLE) has recently gained increasing attention. SLE patients are indeed often characterized by constitutively high serum IFN levels and increased expression of IFN-stimulated genes due to an abnormal response to pathogens and/or incorrect self-DNA recognition process. Consistently, we and others have shown that IFI16 is overexpressed in a wide range of autoimmune diseases where it triggers production of specific autoantibodies. In addition, evidence from mouse models supports a model whereby ALRs are required for IFN-mediated host response to both exogenous and endogenous DNA. Following interaction with cytoplasmic or nuclear nucleic acids, ALRs can form a functional inflammasome through association with the adaptor ASC [apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD)] and with procaspase-1. Importantly, inflammasome-mediated upregulation of IL-1β and IL-18 production positively correlates with SLE disease severity. Therefore, targeting ALR sensors and their downstream pathways represents a promising alternative therapeutic approach for SLE and other systemic autoimmune diseases.

Endosomal TLR3, TLR7, and TLR8 control neuronal morphology through different transcriptional programs

Neuroinflammation is associated with diverse neurological disorders. Endosomal Toll-like receptors (TLRs) including TLR3, TLR7, and TLR8 cell-autonomously regulate neuronal differentiation. However, the mechanisms by which these three TLRs affect neuronal morphology are unclear. In this study, we compare these TLRs in mouse neurons. By combining in vitro neuronal cultures, in utero electroporation, and transcriptomic profiling, we show that TLR8, TLR7, and TLR3 promote dendritic pruning via MYD88 signaling. However, they induce different transcriptomic profiles related to innate immunity, signaling, and neuronal development. The temporal expression patterns and the effects on neuronal morphology are not identical upon activation of these endosomal TLRs. Pathway analyses and in vitro studies specifically implicate mitogen-activated protein kinase signaling in TLR8-mediated dendritic pruning. We further show that TLR8 is more critical for dendritic arborization at a late development stage in vivo. The activation of TLR8, TLR7, or TLR3 results in dendritic shortening, and TLR7 and TLR3 but not TLR8 also control axonal growth. In-depth transcriptomic analyses show that TLRs use different downstream pathways to control neuronal morphology, which may contribute to neuronal development and pathological responses.

A study of single nucleotide polymorphisms in CD157, AIM2 and JARID2 genes in Han Chinese children with autism spectrum disorder

PURPOSE

Autism spectrum disorder (ASD) is a group of developmental brain disorders caused by genetic and environmental factors. The objective of this study was to investigate whether single nucleotide polymorphisms (SNPs) in genes related to immune function were associated with ASD in Chinese Han children.

MATERIALS AND METHODS

A total of 201 children with ASD and 200 age- and gender-matched healthy controls were recruited from September 2012 to June 2106. A TaqMan probe-based approach was used to genotype SNPs corresponding to rs28532698 and rs4301112 in CD157, rs855867 in AIM2, and rs2237126 in JARID2. Case-control and case-only studies were performed to determine the contribution of SNPs to the predisposition of disease and its severity, respectively.

RESULTS

Our results revealed that the genotypes and allele frequencies of these SNPs were not significantly associated with childhood ASD and its severity in this population.

CONCLUSIONS

Results of our study suggest that these SNPs are not predictors of childhood ASD in the Chinese Han population. The discrepant results suggest the predictor roles of SNPs have to be determined in different ethnic populations due to genetic heterogeneity of ASD.

Prognostic value of neutrophil-to-lymphocyte ratio and platelet-to-lymphocyte ratio in gastric cancer

The neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR) have been presented to be a prognostic indicator in several types of cancer. However, these issues have not been concluded yet. The present study was therefore performed to determine the prognostic value of NLR and PLR in gastric cancer (GC).A total of 182 GC patients, diagnosed between January 2011 and January 2014, were enrolled in the study. The clinicopathological parameters, laboratory analyses, and outcomes were collected. The association between NLR, PLR, and clinicopathological characters was analyzed with univariate and multivariate analyses.NLR was significantly related to age (P = .026), surgery (P = .006), node status (P = .004), and clinical stage (P = .009). The median overall survival (OS) and progression-free survival (PFS) were poor in the High-NLR group (OS: 36.0 vs 20.5 months, P < .001, PFS: 33.0 vs 12.0 months, P < .001) and High-PLR group (OS: 31.5 vs 18.5 months, P = .003, PFS: 26.0 vs 11.0 months, P = .01). Multivariate analyses indicated both surgery [for OS hazard ratio (HR) = 2.092, 95% confidence interval (95% CI): 1.345-3.253, P = .001; for PFS HR = 1.939, 95% CI: 1.259-2.988, P = .003] and NLR (for OS HR = 1.585, 95% CI: 1.011-2.485, P = .045) were independent prognostic factors.Elevated NLR and PLR were related with poor prognosis in GC patients before treatment. The NLR was an independent prognostic factor for OS. More studies should be conducted to address the potential prognostic value of NLR and PLR in GC.

The Inflammasome: Regulation of Nitric Oxide and Antimicrobial Host Defence

Nitric oxide (NO) is a gaseous signalling molecule that plays diverse physiological functions including antimicrobial host defence. During microbial infection, NO is synthesized by inducible NO synthase (iNOS), which is expressed by host immune cells through the recognition of microbial pattern molecules. Therefore, sensing pathogens or their pattern molecules by pattern recognition receptors (PRRs), which are located at the cell surface, endosomal and phagosomal compartment, or in the cytosol, is key in inducing iNOS and eliciting antimicrobial host defence. A group of cytosolic PRRs is involved in inducing NO and other antimicrobial molecules by forming a molecular complex called the inflammasome. Assembled inflammasomes activate inflammatory caspases, such as caspase-1 and caspase-11, which in turn process proinflammatory cytokines IL-1β and IL-18 into their mature forms and induce pyroptotic cell death. IL-1β and IL-18 play a central role in immunity against microbial infection through activation and recruitment of immune cells, induction of inflammatory molecules, and regulation of antimicrobial mediators including NO. Interestingly, NO can also regulate inflammasome activity in an autocrine and paracrine manner. Here, we discuss molecular mechanisms of inflammasome formation and the inflammasome-mediated regulation of host defence responses during microbial infections.

Digital signaling network drives the assembly of the AIM2-ASC inflammasome

The AIM2-ASC inflammasome is a filamentous signaling platform essential for mounting host defense against cytoplasmic dsDNA arising not only from invading pathogens but also from damaged organelles. Currently, the design principles of its underlying signaling network remain poorly understood at the molecular level. We show here that longer dsDNA is more effective in inducing AIM2 assembly, its self-propagation, and downstream ASC polymerization. This observation is related to the increased probability of forming the base of AIM2 filaments, and indicates that the assembly discerns small dsDNA as noise at each signaling step. Filaments assembled by receptor AIM2, downstream ASC, and their joint complex all persist regardless of dsDNA, consequently generating sustained signal amplification and hysteresis. Furthermore, multiple positive feedback loops reinforce the assembly, as AIM2 and ASC filaments accelerate the assembly of nascent AIM2 with or without dsDNA. Together with a quantitative model of the assembly, our results indicate that an ultrasensitive digital circuit drives the assembly of the AIM2-ASC inflammasome.

Inflammasomes in the gastrointestinal tract: infection, cancer and gut microbiota homeostasis

Inflammasome signalling is an emerging pillar of innate immunity and has a central role in the regulation of gastrointestinal health and disease. Activation of the inflammasome complex mediates both the release of the pro-inflammatory cytokines IL-1β and IL-18 and the execution of a form of inflammatory cell death known as pyroptosis. In most cases, these mediators of inflammation provide protection against bacterial, viral and protozoal infections. However, unchecked inflammasome activities perpetuate chronic inflammation, which underpins the molecular and pathophysiological basis of gastritis, IBD, upper and lower gastrointestinal cancer, nonalcoholic fatty liver disease and obesity. Studies have also highlighted an inflammasome signature in the maintenance of gut microbiota and gut-brain homeostasis. Harnessing the immunomodulatory properties of the inflammasome could transform clinical practice in the treatment of acute and chronic gastrointestinal and extragastrointestinal diseases. This Review presents an overview of inflammasome biology in gastrointestinal health and disease and describes the value of experimental and pharmacological intervention in the treatment of inflammasome-associated clinical manifestations.

Lung Diseases

Inflammasomes are large innate cytoplasmic complexes that play a major role in promoting inflammation in the lung in response to a range of environmental and infectious stimuli. Inflammasomes are critical for driving acute innate immune responses that resolve infection and maintain tissue homeostasis. However, dysregulated or excessive inflammasome activation can be detrimental. Here, we discuss the plethora of recent data from clinical studies and small animal disease models that implicate excessive inflammasome responses in the pathogenesis of a number of acute and chronic respiratory inflammatory diseases. Understanding of the role of inflammasomes in lung disease is of great therapeutic interest.

An inflammatory memory and angiogenic self-assembling nanofiber hydrogel scaffold seeded with Akkermansia muciniphila to accelerate the healing of diabetic ischemic ulcers

In this research, a polypeptide skeleton was synthesized based on 3,4-dihydroxyphenylalanine (DOPA) and conjugated with inflammatory and angiogenesis peptides, combined with A. Muciniphila as a novel strategy for diabetic ischemic ulcer treatment.

The risk of nontuberculous mycobacterial infection in patients with Sjögren's syndrome: a nationwide, population-based cohort study

BACKGROUND

Nontuberculous mycobacterial (NTM) infection in immunocompromized patients is currently a growing health concern, and we aimed to examine the relative risk of NTM infection in patients with Sjögren's syndrome (SS) compared with that in non-SS individuals.

METHODS

We used the 2003-2012 Taiwanese National Health Insurance Research Database to identify 6554 incident SS cases during 2007-2012 and selected 98,310 non-SS controls matched (1:15) for age, gender, and the year of first SS diagnosis date after excluding those who had rheumatoid arthritis or systemic lupus erythematosus.

RESULTS

We identified four NTM-infected patients in the SS group (three in the first year) and nine in the non-SS group (three in the first year). SS patients had a higher incidence rate of NTM infection than that in non-SS individuals (IRR, 7.56; 95% CI, 2.33-24.55), especially during the first year (IRR, 16.05; 95% CI, 3.24-79.51). After adjusting for potential confounders, the risk of NTM infection was not increased in SS patients during the entire follow-up period or during the first year, but the risk increased in SS patients treated with immunosuppressants during the entire follow-up period (HR, 17.77; 95% CI, 4.53-69.61), especially during the first year (HR, 33.33; 95% CI, 4.37-254.23).

CONCLUSION

An increased risk of NTM infection was found in SS patients treated with immunosuppressants during the first year after SS diagnosis.

Role of inflammasomes in inflammatory autoimmune rheumatic diseases

Inflammasomes are intracellular multiprotein complexes that coordinate anti-pathogenic host defense during inflammatory responses in myeloid cells, especially macrophages. Inflammasome activation leads to activation of caspase-1, resulting in the induction of pyroptosis and the secretion of pro-inflammatory cytokines including interleukin (IL)-1β and IL-18. Although the inflammatory response is an innate host defense mechanism, chronic inflammation is the main cause of rheumatic diseases, such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), ankylosing spondylitis (AS), and Sjögren's syndrome (SS). Since rheumatic diseases are inflammatory/autoimmune disorders, it is reasonable to hypothesize that inflammasomes activated during the inflammatory response play a pivotal role in development and progression of these diseases. Indeed, previous studies have provided important observations that inflammasomes are actively involved in the pathogenesis of inflammatory/autoimmune rheumatic diseases. In this review, we summarize the current knowledge on several types of inflammasomes during macrophage-mediated inflammatory responses and discuss recent research regarding the role of inflammasomes in the pathogenesis of inflammatory/autoimmune rheumatic diseases. This avenue of research could provide new insights for the development of promising therapeutics to treat inflammatory/autoimmune rheumatic diseases.

ATM-activated autotaxin (ATX) propagates inflammation and DNA damage in lung epithelial cells: a new mode of action for silica-induced DNA damage?

Silica exposure is a common risk factor for lung cancer. It has been claimed that key elements in cancer development are activation of inflammatory cells that indirectly induce DNA damage and proliferative stimuli in respiratory epithelial cells. We studied DNA damage induced by silica particles in respiratory epithelial cells and focused the role of the signaling enzyme autotaxin (ATX). A549 and 16 bronchial epithelial cells (16HBE) lung epithelial cells were exposed to silica particles. Reactive oxygen species (ROS), NOD-like receptor family pyrin domain containing-3 (NLRP3) inflammasome activation, ATX, ataxia telangiectasia mutated (ATM), and DNA damage (γH2AX, pCHK1, pCHK2, comet assay) were end points. Low doses of silica induced NLRP3 activation, DNA damage accumulation, and ATM phosphorylation. A novel finding was that ATM induced ATX generation and secretion. Not only silica but also rotenone, camptothecin and H2O2 activated ATX via ATM, suggesting that ATX is part of a generalized ATM response to double-strand breaks (DSBs). Surprisingly, ATX inhibition mitigated DNA damage accumulation at later time points (6-16 h), and ATX transfection caused NLRP3 activation and DNA damage. Furthermore, the product of ATX enzymatic activity, lysophosphatidic acid, recapitulated the effects of ATX transfection. These data indicate an ATM-ATX-dependent loop that propagates inflammation and DSB accumulation, making low doses of silica effective inducers of DSBs in epithelial cells. We conclude that an ATM-ATX axis interconnects DSBs with silica-induced inflammation and propagates these effects in epithelial cells. Further studies of this adverse outcome pathway may give an accurate assessment of the lowest doses of silica that causes cancer.

IL-1β Production by Intermediate Monocytes Is Associated with Immunopathology in Cutaneous Leishmaniasis

Cutaneous leishmaniasis due to Leishmania braziliensis infection is an inflammatory disease in which skin ulcer development is associated with mononuclear cell infiltrate and high levels of inflammatory cytokine production. Recently, NLRP3 inflammasome activation and IL-1β production have been associated with increased pathology in murine cutaneous leishmaniasis. We hypothesized that cutaneous leishmaniasis patients have increased expression of NLRP3, leading to high levels of IL-1β production. In this article we show high production of IL-1β in biopsy samples and Leishmania antigen-stimulated peripheral blood mononuclear cells from patients infected with L. braziliensis and reduced IL-1β levels after cure. IL-1β production positively correlated with the area of necrosis in lesions and duration of the lesions. The main source of IL-1β was intermediate monocytes (CD14⁺⁺CD16⁺). Furthermore, our murine experiments show that IL-1β production in response to L. braziliensis was dependent on NLRP3, caspase-1, and caspase-recruiting domain (ASC). Additionally, we observed an increased expression of the NLRP3 gene in macrophages and the NLRP3 protein in intermediate monocytes from cutaneous leishmaniasis patients. These results identify an important role for human intermediate monocytes in the production of IL-1β, which contributes to the immunopathology observed in cutaneous leishmaniasis patients.

Roles of ginsenosides in inflammasome activation

Inflammation is an innate immune response that protects the body from pathogens, toxins, and other dangers and is initiated by recognizing pathogen-associated molecular patterns or danger-associated molecular patterns by pattern-recognition receptors expressing on or in immune cells. Intracellular pattern-recognition receptors, including nucleotide-binding oligomerization domain-like receptors (NLRs), absent in melanoma 2, and cysteine aspartate–specific protease (caspase)-4/5/11 recognize various pathogen-associated molecular patterns and danger-associated molecular patterns and assemble protein complexes called “inflammasomes.” These complexes induce inflammatory responses by activating a downstream effector, caspase-1, leading to gasdermin D –mediated pyroptosis and the secretion of proinflammatory cytokines, such as interleukin (IL)-1β and IL-18. Ginsenosides are natural steroid glycosides and triterpene saponins found exclusively in the plant genus Panax. Various ginsenosides have been identified, and their abilities to regulate inflammatory responses have been evaluated. These studies have suggested a link between ginsenosides and inflammasome activation in inflammatory responses. Some types of ginsenosides, including Rh1, Rg3, Rb1, compound K, chikusetsu saponin IVa, Rg5, and Rg1, have been clearly demonstrated to inhibit inflammatory responses by suppressing the activation of various inflammasomes, including the NLRP3, NLRP1, and absent in melanoma 2 inflammasomes. Ginsenosides have also been shown to inhibit caspase-1 and to decrease the expression of IL-1β and IL-18. Given this body of evidence, the functional relationship between ginsenosides and inflammasome activation provides new insight into the understanding of the molecular mechanisms of ginsenoside-mediated antiinflammatory actions. This relationship also has applications regarding the development of antiinflammatory remedies by ginsenoside-mediated targeting of inflammasomes, which could be used to prevent and treat inflammatory diseases.

The emerging role of ASC in dendritic cell metabolism during Chlamydia infection

Chlamydia trachomatis is a bacterial agent that causes sexually transmitted infections worldwide. The regulatory functions of dendritic cells (DCs) play a major role in protective immunity against Chlamydia infections. Here, we investigated the role of ASC in DCs metabolism and the regulation of DCs activation and function during Chlamydia infection. Following Chlamydia stimulation, maturation and antigen presenting functions were impaired in ASC^-/- DCs compared to wild type (WT) DCs, in addition, ASC deficiency induced a tolerant phenotype in Chlamydia stimulated DCs. Using real-time extracellular flux analysis, we showed that activation in Chlamydia stimulated WT DCs is associated with a metabolic change in which mitochondrial oxidative phosphorylation (OXPHOS) is inhibited and the cells become committed to utilizing glucose through aerobic glycolysis for differentiation and antigen presenting functions. However, in ASC^-/- DCs Chlamydia-induced metabolic change was prevented and there was a significant effect on mitochondrial morphology. The mitochondria of Chlamydia stimulated ASC^-/- DCs had disrupted cristae compared to the normal narrow pleomorphic cristae found in stimulated WT DCs. In conclusion, our results suggest that Chlamydia-mediated activation of DCs is associated with a metabolic transition in which OXPHOS is inhibited, thereby dedicating the DCs to aerobic glycolysis, while ASC deficiency disrupts DCs function by inhibiting the reprogramming of DCs metabolism within the mitochondria, from glycolysis to electron transport chain.

cGAMP: A tale of two signals

In this issue of JEM, Swanson et al. (https://doi.org/10.1084/jem.20171749) report an unanticipated role for cGAMP in priming and activation of inflammasomes in addition to its well-characterized function as an endogenous second messenger inducing type I interferons in the cytosolic DNA-sensing pathway.

Inflammasomes in Mycobacterium tuberculosis-Driven Immunity

The development of effective innate and subsequent adaptive host immune responses is highly dependent on the production of proinflammatory cytokines that increase the activity of immune cells. The key role in this process is played by inflammasomes, multimeric protein complexes serving as a platform for caspase-1, an enzyme responsible for proteolytic cleavage of IL-1β and IL-18 precursors. Inflammasome activation, which triggers the multifaceted activity of these two proinflammatory cytokines, is a prerequisite for developing an efficient inflammatory response against pathogenic Mycobacterium tuberculosis (M.tb). This review focuses on the role of NLRP3 and AIM2 inflammasomes in M.tb-driven immunity.

"Toll-free" pathways for production of type I interferons

Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are recognized by different cellular pathogen recognition receptors (PRRs), which are expressed on cell membrane or in the cytoplasm of cells of the innate immune system. Nucleic acids derived from pathogens or from certain cellular conditions represent a large category of PAMPs/DAMPs that trigger production of type I interferons (IFN-I) in addition to pro-inflammatory cytokines, by specifically binding to intracellular Toll-like receptors or cytosolic receptors. These cytosolic receptors, which are not related to TLRs and we call them “Toll-free” receptors, include the RNA-sensing RIG-I like receptors (RLRs), the DNA-sensing HIN200 family, and cGAS, amongst others. Viruses have evolved myriad strategies to evoke both host cellular and viral factors to evade IFN-I-mediated innate immune responses, to facilitate their infection, replication, and establishment of latency. This review outlines these “Toll-free” innate immune pathways and recent updates on their regulation, with focus on cellular and viral factors with enzyme activities.