Kinase fusion proteins: intracellular R-proteins in plant immunity

Resistance (R) genes in the Triticeae tribe include not only genes encoding the canonical intracellular nucleotide-binding leucine-rich-repeat proteins (NLRs) but also genes encoding kinase fusion proteins (KFPs). Exploring these unconventional KFPs may expand the scope of effector-triggered immunity (ETI) and will have significant implications for crop improvement.

Efficacy of Jiangzhi Xiaoban tablet on toll-like receptor 4/nuclear factor-kappa B/nod-like receptor protein 3 signaling pathway in mice with atherosclerosis induced by high-fat diet

OBJECTIVE

To study the effect of Jiangzhi Xiaoban tablet (, JZXB) on toll-like receptor 4 (TLR4)/nuclear factor-kappa B (NF-κB)/Nod-like receptor protein 3 (NLRP3) signaling pathway expression in atherosclerosis (AS) mice by establishing a mouse model of AS, and to explore its mechanism of prevention and treatment of AS.

METHODS

Sixty-four male C57BL/6J mice were randomly divided into two groups, 12 in the normal control group and 52 in the model group (MOD). Seven weeks later, two mice in each of the above two groups were randomly sacrificed, and the whole aortic tissue of the mice was taken out for hematoxylin-eosin staining. After successful modeling, 50 mice in the modeling group were randomly divided into 5 groups: MOD, atorvastatin group (ATO), low-dose group of JZXB (JZXB-L), middle-dose group of JZXB (JZXB-M), and high-dose group of JZXB (JZXB-H), 10 mice in each group. The mice in each group were killed after 6 weeks of preventive administration. HE staining was used to observe the pathological changes of aorta in AS mice. The levels of serum triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) were detected by automatic biochemical analyzer. The levels of inflammatory factor interleukin-1β (IL-1β) were detected by enzyme linked immunosorbent assay. The expression of TLR4, NF-κB and NLRP3 proteins in aortic tissue was detected by immunohistochemistry.

RESULTS

Compared with the MOD, the levels of serum TC, TG and LDL-C in the JZXB-H and ATO were significantly decreased, while the level of HDL-C was significantly increased. The levels of serum TG, LDL-C in the JZXB-M were significantly decreased, and the level of HDL-C was significantly increased. Compared with the MOD, the levels of IL-1β were significantly decreased, aortic lesions were significantly improved, and the expression of TLR4, NF-κB, and NLRP3 proteins in the aortic tissue was significantly decreased in the JZXB-H, JZXB-M, and ATO.

CONCLUSION

JZXB has inhibitory effect on atherosclerosis in mice, and its mechanism may be through regulating the TLR4/NF-κB/NLRP3 signaling pathway and reducing the inflammatory response, so as to play a role in inhibiting atherosclerosis.

Regulation of plant immunity via small RNA-mediated control of NLR expression

Plants use different receptors to detect potential pathogens: membrane-anchored pattern recognition receptors (PRRs) activated upon perception of pathogen-associated molecular patterns (PAMPs) that elicit pattern-triggered immunity (PTI); and intracellular nucleotide-binding leucine-rich repeat proteins (NLRs) activated by detection of pathogen-derived effectors, activating effector-triggered immunity (ETI). The interconnections between PTI and ETI responses have been increasingly reported. Elevated NLR levels may cause autoimmunity, with symptoms ranging from fitness cost to developmental arrest, sometimes combined with run-away cell death, making accurate control of NLR dosage key for plant survival. Small RNA-mediated gene regulation has emerged as a major mechanism of control of NLR dosage. Twenty-two nucleotide miRNAs with the unique ability to trigger secondary siRNA production from target transcripts are particularly prevalent in NLR regulation. They enhance repression of the primary NLR target, but also bring about repression of NLRs only complementary to secondary siRNAs. We summarize current knowledge on miRNAs and siRNAs in the regulation of NLR expression with an emphasis on 22 nt miRNAs and propose that miRNA and siRNA regulation of NLR levels provides additional links between PTI and NLR defense pathways to increase plant responsiveness against a broad spectrum of pathogens and control an efficient deployment of defenses.

Suppression of NLR-mediated plant immune detection by bacterial pathogens

The plant immune system is constituted of two functionally interdependent branches that provide the plant with an effective defense against microbial pathogens. They can be considered separate since one detects extracellular pathogen-associated molecular patterns by means of receptors on the plant surface, while the other detects pathogen-secreted virulence effectors via intracellular receptors. Plant defense depending on both branches can be effectively suppressed by host-adapted microbial pathogens. In this review we focus on bacterially driven suppression of the latter, known as effector-triggered immunity (ETI) and dependent on diverse NOD-like receptors (NLRs). We examine how some effectors secreted by pathogenic bacteria carrying type III secretion systems can be subject to specific NLR-mediated detection, which can be evaded by the action of additional co-secreted effectors (suppressors), implying that virulence depends on the coordinated action of the whole repertoire of effectors of any given bacterium and their complex epistatic interactions within the plant. We consider how ETI activation can be avoided by using suppressors to directly alter compromised co-secreted effectors, modify plant defense-associated proteins, or occasionally both. We also comment on the potential assembly within the plant cell of multi-protein complexes comprising both bacterial effectors and defense protein targets.

Construction and Identification of an NLR-Associated Prognostic Signature Revealing the Heterogeneous Immune Response in Skin Cutaneous Melanoma

Background

Skin cutaneous melanoma (SKCM) is the deadliest dermatology tumor. Ongoing researches have confirmed that the NOD-like receptors (NLRs) family are crucial in driving carcinogenesis. However, the function of NLRs signaling pathway-related genes in SKCM remains unclear.

Objective

To establish and identify an NLRs-related prognostic signature and to explore its predictive power for heterogeneous immune response in SKCM patients.

Methods

Establishment of the predictive signature using the NLRs-related genes by least absolute shrinkage and selection operator-Cox regression analysis (LASSO-COX algorithm). Through univariate and multivariate COX analyses, NLRs signature's independent predictive effectiveness was proven. CIBERSORT examined the comparative infiltration ratios of 22 distinct types of immune cells. RT-qPCR and immunohistochemistry implemented expression validation for critical NLRs-related prognostic genes in clinical samples.

Results

The prognostic signature, including 7 genes, was obtained by the LASSO-Cox algorithm. In TCGA and validation cohorts, SKCM patients with higher risk scores had remarkably poorer overall survival. The independent predictive role of this signature was confirmed by multivariate Cox analysis. Additionally, a graphic nomogram demonstrated that the risk score of the NLRs signature has high predictive accuracy. SKCM patients in the low-risk group revealed a distinct immune microenvironment characterized by the significantly activated inflammatory response, interferon-α/γ response, and complement pathways. Indeed, several anti-tumor immune cell types were significantly accumulated in the low-risk group, including M1 macrophage, CD8 T cell, and activated NK cell. It is worth noting that our NLRs prognostic signature could serve as one of the promising biomarkers for predicting response rates to immune checkpoint blockade (ICB) therapy. Furthermore, the results of expression validation (RT-qPCR and IHC) were consistent with the previous analysis.

Conclusion

A promising NLRs signature with excellent predictive efficacy for SKCM was developed.

Structural basis for flagellin induced NAIP5 activation

The NAIP/NLRC4 inflammasome is activated when NAIP binds to a gram-negative bacterial ligand. Initially, NAIP exists in an inactive state with a wide-open conformation. Upon ligand binding, the winged helix domain (WHD) of NAIP is activated and forms steric clash with NLRC4 to open it up. However, how ligand binding induces the conformational change of NAIP is less clear. To understand this process, we investigated the dynamics of the ligand binding region of inactive NAIP5 and solved the cryo-EM structure of NAIP5 in complex with its specific ligand, FliC from flagellin, at 2.93 Å resolution. The structure revealed a "trap and lock" mechanism in FliC recognition, whereby FliC-D0C is first trapped by the hydrophobic pocket of NAIP5, then locked in the binding site by the insertion domain (ID) and C-terminal tail (CTT) of NAIP5. The FliC-D0N domain further inserts into the loop of ID to stabilize the complex. According to this mechanism, FliC activates NAIP5 by bringing multiple flexible domains together, particularly the ID, HD2, and LRR domains, to form the active conformation and support the WHD loop in triggering NLRC4 activation.

Prevotella genus and its related NOD-like receptor signaling pathway in young males with stage III periodontitis

Background

As periodontitis progresses, the oral microbiota community changes dynamically. In this study, we evaluated the dominant bacteria and their roles in the potential pathway in young males with stage III periodontitis.

Methods

16S rRNA sequencing was performed to evaluate variations in the composition of oral bacteria between males with stage I and III periodontitis and identify the dominant bacteria of each group. Function prediction was obtained based on 16S rRNA sequencing data. The inhibitor of the predominant pathway for stage III periodontitis was used to investigate the role of the dominant bacteria in periodontitis in vivo and in vitro.

Results

Chao1 index, Observed Species and Phylogenetic Diversity (PD) whole tree values were significantly higher in the stage III periodontitis group. β-diversity suggested that samples could be divided according to the stages of periodontitis. The dominant bacteria in stage III periodontitis were Prevotella, Prevotella_7, and Dialister, whereas that in stage I periodontitis was Cardiobacterium. KEGG analysis predicted that variations in the oral microbiome may be related to the NOD-like receptor signaling pathway. The inhibitor of this pathway, NOD-IN-1, decreased P. intermedia -induced Tnf-α mRNA expression and increased P. intermedia -induced Il-6 mRNA expression, consistent with the ELISA results. Immunohistochemistry confirmed the down-regulation of TNF-α and IL-6 expressions by NOD-IN-1 in P. intermedia-induced periodontitis.

Conclusion

The composition of the oral bacteria in young males varied according to the stage of periodontitis. The species richness of oral microtia was greater in young males with stage III periodontitis than those with stage I periodontitis. Prevotella was the dominant bacteria in young males with stage III periodontitis, and inhibition of the NOD-like receptor signaling pathway can decrease the periodontal inflammation induced by P. intermedia.

NLRC4 Inflammasome-Mediated Regulation of Eosinophilic Functions

Eosinophils play critical roles in the maintenance of homeostasis in innate and adaptive immunity. Although primarily known for their roles in parasitic infections and the development of Th2 cell responses, eosinophils also play complex roles in other immune responses ranging from anti-inflammation to defense against viral and bacterial infections. However, the contributions of pattern recognition receptors in general, and NOD-like receptors (NLRs) in particular, to eosinophil involvement in these immune responses remain relatively underappreciated. Our in vivo studies demonstrated that NLRC4 deficient mice had a decreased number of eosinophils and impaired Th2 responses after induction of an allergic airway disease model. Our in vitro data, utilizing human eosinophilic EoL-1 cells, suggested that TLR2 induction markedly induced pro-inflammatory responses and inflammasome forming NLRC4 and NLRP3. Moreover, activation by their specific ligands resulted in caspase-1 cleavage and mature IL-1β secretion. Interestingly, Th2 responses such as secretion of IL-5 and IL-13 decreased after transfection of EoL-1 cells with short interfering RNAs targeting human NLRC4. Specific induction of NLRC4 with PAM3CSK4 and flagellin upregulated the expression of IL-5 receptor and expression of Fc epsilon receptors (FcεR1α, FcεR2). Strikingly, activation of the NLRC4 inflammasome also promoted expression of the costimulatory receptor CD80 as well as expression of immunoregulatory receptors PD-L1 and Siglec-8. Concomitant with NLRC4 upregulation, we found an increase in expression and activation of matrix metalloproteinase (MMP)-9, but not MMP-2. Collectively, our results present new potential roles of NLRC4 in mediating a variety of eosinopilic functions.

The role of chaperone complex HSP90-SGT1-RAR1 as the associated machinery for hybrid inviability between Nicotiana gossei Domin and N. tabacum L

Two cultured cell lines (GTH4 and GTH4S) of a Nicotiana interspecific F1 hybrid (N. gossei × N. tabacum) were comparatively analyzed to find genetic factors related to hybrid inviability. Both cell lines proliferated at 37 °C, but after shifting to 26 °C, GTH4 started to die similar to the F1 hybrid seedlings, whereas GTH4S survived. As cell death requires de novo expression of genes and proteins, we compared expressed protein profiles between the two cell lines, and found that NgSGT1, a cochaperone of the chaperone complex (HSP90-SGT1-RAR1), was expressed in GTH4 but not in GTH4S. Agrobacterium-mediated transient expression of NgSGT1, but not NtSGT1, induced cell death in leaves of N. tabacum, suggesting its possible role in hybrid inviability. Cell death in N. tabacum was also induced by transient expression of NgRAR1, but not NtRAR1. In contrast, transient expression of any parental combinations of three components revealed that NgRAR1 promoted cell death, whereas NtRAR1 suppressed it in N. tabacum. A specific inhibitor of HSP90, geldanamycin, inhibited the progression of hypersensitive response-like cell death in GTH4 and leaf tissue after agroinfiltration. The present study suggested that components of the chaperone complex are involved in the inviability of Nicotiana interspecific hybrid.

A novel homozygous variant in NLRP5 is associate with human early embryonic arrest in a consanguineous Chinese family

Early embryonic arrest is one of the major causes of recurrent assisted reproduction failure. It is characterized by delayed embryonic development and failure to form viable eight-cell stage embryos on day 3 of an assisted reproduction cycle. A recent study reported that biallelic mutations in NLRP5 can cause early embryonic arrest. NLRP5 is a member of subcortical maternal complex, which plays a significant role in embryogenesis. In this study, we described a female in a consanguineous Chinese family who displayed clinical features of early embryonic arrest and identified a novel homozygous variant c.1061C>T (p.Pro354Leu) in NLRP5. This is the second report of the biallelic NLRP5 variant that associates with early embryonic arrest in humans, further confirming the role of NLRP5 variants in early embryonic arrest and expanding the spectrum of known pathogenic variants in NLRP5.

Class I transactivator, NLRC5: a central player in the MHC class I pathway and cancer immune surveillance

Major histocompatibility complex (MHC) class I and class II molecules play critical roles in the activation of the adaptive immune system by presenting antigens to CD8+ and CD4+ T cells, respectively. Although it has been well known that CIITA (MHC class II transactivator), an NLR (nucleotide-binding domain, leucine-rich-repeat containing) protein, as a master regulator of MHC class II gene expression, the mechanism of MHC class I gene transactivation was unclear. Recently, another NLR protein, NLRC5 (NLR family, CARD domain-containing 5), was identified as an MHC class I transactivator (CITA). NLRC5 is a critical regulator for the transcriptional activation of MHC class I genes and other genes involved in the MHC class I antigen presentation pathway. CITA/NLRC5 plays a crucial role in human cancer immunity through the recruitment and activation of tumor killing CD8+ T cells. Here, we discuss the molecular function and mechanism of CITA/NLRC5 in the MHC class I pathway and its role in cancer.

Autoimmunity in plants

Attenuation in the activity of the negative regulators or the hyperactivity of plant innate immune receptors often causes ectopic defense activation manifested in severe growth retardation and spontaneous lesion formations, referred to as autoimmunity. In this review, we have described the cellular and molecular basis of the development of autoimmune responses for their useful applications in plant defense. Plants are exposed to diverse disease-causing pathogens, which bring infections by taking over the control on host immune machineries. To counter the challenges of evolving pathogenic races, plants recruit specific types of intracellular immune receptors that mostly belong to the family of polymorphic nucleotide-binding oligomerization domain-containing leucine-rich repeat (NLR) proteins. Upon recognition of effector molecules, NLR triggers hyperimmune signaling, which culminates in the form of a typical programmed cell death, designated hypersensitive response. Besides, few plant NLRs also guard certain host proteins known as 'guardee' that are modified by effector proteins. However, this fine-tuned innate immune system can be lopsided upon knock-out of the alleles that correspond to the host guardees, which mimick the presence of pathogen. The absence of pathogens causes inappropriate activation of the respective NLRs and results in the constitutive activation of plant defense and exhibiting autoimmunity. In plants, autoimmune mutants are readily scorable due to their dwarf phenotype and development of characteristic macroscopic disease lesions. Here, we summarize recent reports on autoimmune response in plants, how it is triggered, and phenotypic consequences associated with this phenomenon.

Structural, Functional, and Genomic Diversity of Plant NLR Proteins: An Evolved Resource for Rational Engineering of Plant Immunity

Plants employ a diverse intracellular system of NLR (nucleotide binding-leucine-rich repeat) innate immune receptors to detect pathogens of all types. These receptors represent valuable agronomic traits that plant breeders rely on to maximize yield in the face of devastating pathogens. Despite their importance, the mechanistic underpinnings of NLR-based disease resistance remain obscure. The rapidly increasing numbers of plant genomes are revealing a diverse array of NLR-type immune receptors. In parallel, mechanistic studies are describing diverse functions for NLR immune receptors. In this review, we intend to broadly describe how the structural, functional, and genomic diversity of plant immune receptors can provide a valuable resource for rational engineering of plant immunity.

Inflammasome as a Therapeutic Target for Cancer Prevention and Treatment

Chronic inflammation is a critical modulator of carcinogenesis through secretion of inflammatory cytokines, which leads to the formation of an inflammatory microenvironment. In this process, the inflammasome plays an important role in the expression and activation of interleukin (IL)-1β and IL-18 to promote cancer development. The inflammasome is a multiprotein complex consisting of several nucleotide-binding domain and leucine-rich repeat containing receptor, adaptor proteins, and caspase 1 (CASP1). It senses the various intracellular (damage-associated molecular patterns) and extracellular (pathogen-associated molecular patterns) stimuli. A primed inflammasome recruits adaptor proteins, activates CASP1 to enhance the proteolytic cleavage of pro-IL-1β and IL-18, and sends the signal to respond to each insult. Depending on stimuli and cell contexts, several inflammasomes are closely associated with the initiation and promotion of carcinogenesis. In contrast, inflammasomes also show an ambivalent effect on carcinogenesis by enhancing inflammatory cell death (pyroptosis) and repairing damaged tissues. Although the inflammasome plays a controversial role in carcinogenesis, it may be a promising target for human cancer prevention and treatment. A more in-depth study on the role of the inflammasome in carcinogenesis, based on stimuli, cell contexts, and cancer stages, can lead to the development of novel therapeutic strategies against malignant human cancers.

NLRP3 inflammasome blockade reduces liver inflammation and fibrosis in experimental NASH in mice

BACKGROUND & AIMS

NOD-like receptor protein 3 (NLRP3) inflammasome activation occurs in Non-alcoholic fatty liver disease (NAFLD). We used the first small molecule NLRP3 inhibitor, MCC950, to test whether inflammasome blockade alters inflammatory recruitment and liver fibrosis in two murine models of steatohepatitis.

METHODS

We fed foz/foz and wild-type mice an atherogenic diet for 16weeks, gavaged MCC950 or vehicle until 24weeks, then determined NAFLD phenotype. In mice fed an methionine/choline deficient (MCD) diet, we gavaged MCC950 or vehicle for 6weeks and determined the effects on liver fibrosis.

RESULTS

In vehicle-treated foz/foz mice, hepatic expression of NLRP3, pro-IL-1β, active caspase-1 and IL-1β increased at 24weeks, in association with cholesterol crystal formation and NASH pathology; plasma IL-1β, IL-6, MCP-1, ALT/AST all increased. MCC950 treatment normalized hepatic caspase 1 and IL-1β expression, plasma IL-1β, MCP-1 and IL-6, lowered ALT/AST, and reduced the severity of liver inflammation including designation as NASH pathology, and liver fibrosis. In vitro, cholesterol crystals activated Kupffer cells and macrophages to release IL-1β; MCC950 abolished this, and the associated neutrophil migration. MCD diet-fed mice developed fibrotic steatohepatitis; MCC950 suppressed the increase in hepatic caspase 1 and IL-1β, lowered numbers of macrophages and neutrophils in the liver, and improved liver fibrosis.

CONCLUSION

MCC950, an NLRP3 selective inhibitor, improved NAFLD pathology and fibrosis in obese diabetic mice. This is potentially attributable to the blockade of cholesterol crystal-mediated NLRP3 activation in myeloid cells. MCC950 reduced liver fibrosis in MCD-fed mice. Targeting NLRP3 is a logical direction in pharmacotherapy of NASH.

LAY SUMMARY

Fatty liver disease caused by being overweight with diabetes and a high risk of heart attack, termed non-alcoholic steatohepatitis (NASH), is the most common serious liver disease with no current treatment. There could be several causes of inflammation in NASH, but activation of a protein scaffold within cells termed the inflammasome (NLRP3) has been suggested to play a role. Here we show that cholesterol crystals could be one pathway to activate the inflammasome in NASH. We used a drug called MCC950, which has already been shown to block NLRP3 activation, in an attempt to reduce liver injury in NASH. This drug partly reversed liver inflammation, particularly in obese diabetic mice that most closely resembles the human context of NASH. In addition, such dampening of liver inflammation in NASH achieved with MCC950 partly reversed liver scarring, the process that links NASH to the development of cirrhosis.

Mitochondrial DNA oxidation induces imbalanced activity of NLRP3/NLRP6 inflammasomes by activation of caspase-8 and BRCC36 in dry eye

The concept of innate immunity has been expanded to recognize environmental pathogens other than microbial components. However, whether and how the innate immunity is initiated by epithelium in response to environmental physical challenges such as low humidity and high osmolarity in an autoimmune disease, dry eye, is still largely unknown. Using two experimental dry eye models, primary human corneal epithelial cultures exposed to hyperosmolarity and mouse ocular surface facing desiccating stress, we uncovered novel innate immunity pathway by ocular surface epithelium, where oxidized mitochondrial DNA induces imbalanced activation of NLRP3/NLRP6 inflammasomes via stimulation of caspase-8 and BRCC36 in response to environmental stress. Activated NLRP3 with suppressed NLRP6 stimulates caspase-1 activation that leads to IL-1β and IL-18 maturation and secretion. NLRP3-independent caspase-8 noncanonically activates caspase-1 via reciprocal regulation of NLRP3/NLRP6-mediated inflammasomes. Reactive oxygen species-induced mitochondrial DNA oxidative damage and BRCC36 deubiquitinating activity provide a missing link and mechanism by which innate immunity responds to environmental stress via caspase-8-involved NLRP3/NLRP6 inflammasomes.

The CC domain structure from the wheat stem rust resistance protein Sr33 challenges paradigms for dimerization in plant NLR proteins

Plants use intracellular immunity receptors, known as nucleotide-binding oligomerization domain-like receptors (NLRs), to recognize specific pathogen effector proteins and induce immune responses. These proteins provide resistance to many of the world's most destructive plant pathogens, yet we have a limited understanding of the molecular mechanisms that lead to defense signaling. We examined the wheat NLR protein, Sr33, which is responsible for strain-specific resistance to the wheat stem rust pathogen, Puccinia graminis f. sp. tritici We present the solution structure of a coiled-coil (CC) fragment from Sr33, which adopts a four-helix bundle conformation. Unexpectedly, this structure differs from the published dimeric crystal structure of the equivalent region from the orthologous barley powdery mildew resistance protein, MLA10, but is similar to the structure of the distantly related potato NLR protein, Rx. We demonstrate that these regions are, in fact, largely monomeric and adopt similar folds in solution in all three proteins, suggesting that the CC domains from plant NLRs adopt a conserved fold. However, larger C-terminal fragments of Sr33 and MLA10 can self-associate both in vitro and in planta, and this self-association correlates with their cell death signaling activity. The minimal region of the CC domain required for both cell death signaling and self-association extends to amino acid 142, thus including 22 residues absent from previous biochemical and structural protein studies. These data suggest that self-association of the minimal CC domain is necessary for signaling but is likely to involve a different structural basis than previously suggested by the MLA10 crystallographic dimer.

CITA/NLRC5: A critical transcriptional regulator of MHC class I gene expression

Major histocompatibility complex (MHC) class I and class II molecules play essential roles in the development and activation of the human adaptive immune system. An NLR protein, CIITA (MHC class II transactivator) has been recognized as a master regulator of MHC class II gene expression, albeit knowledge about the regulatory mechanism of MHC class I gene expression had been limited. Recently identified MHC class I transactivator (CITA), or NLRC5, also belongs to the NLR protein family and constitutes a critical regulator for the transcriptional activation of MHC class I genes. In addition to MHC class I genes, CITA/NLRC5 induces the expression of β2 -microglobulin, TAP1 and LMP2, essential components of the MHC class I antigen presentation pathway. Therefore, CITA/NLRC5 and CIITA are transcriptional regulators that orchestrate the concerted expression of critical components in the MHC class I and class II pathways, respectively. © 2016 BioFactors, 42(4):349-357, 2016.

Mitochondrial damage contributes to Pseudomonas aeruginosa activation of the inflammasome and is downregulated by autophagy

The nucleotide-binding domain, leucine-rich repeat containing family caspase recruitment domain containing 4 (NLRC4) inflammasome can be activated by pathogenic bacteria via products translocated through the microbial type III secretion apparatus (T3SS). Recent work has shown that activation of the NLRP3 inflammasome is downregulated by autophagy, but the influence of autophagy on NLRC4 activation is unclear. We set out to determine how autophagy might influence this process, using the bacterium Pseudomonas aeruginosa, which activates the NLRC4 inflammasome via its T3SS. Infection resulted in T3SS-dependent mitochondrial damage with increased production of reactive oxygen intermediates and release of mitochondrial DNA. Inhibiting mitochondrial reactive oxygen release or degrading intracellular mitochondrial DNA abrogated NLRC4 inflammasome activation. Moreover, macrophages lacking mitochondria failed to activate NLRC4 following infection. Removal of damaged mitochondria by autophagy significantly attenuated NLRC4 inflammasome activation. Mitochondrial DNA bound specifically to NLRC4 immunoprecipitates and transfection of mitochondrial DNA directly activated the NLRC4 inflammasome; oxidation of the DNA enhanced this effect. Manipulation of autophagy altered the degree of inflammasome activation and inflammation in an in vivo model of P. aeruginosa infection. Our results reveal a novel mechanism contributing to NLRC4 activation by P. aeruginosa via mitochondrial damage and release of mitochondrial DNA triggered by the bacterial T3SS that is downregulated by autophagy.

The Interplay between NLRs and Autophagy in Immunity and Inflammation

Since they were first described as cytosolic sensors of microbial molecules a decade ago, the Nod-like receptors (NLRs) have been shown to have many different and important roles in various aspects of immune and inflammatory responses, ranging from antimicrobial mechanisms to control of adaptive responses. In this review, we focus on the interplay between NLRs and autophagy, an evolutionarily conserved mechanism that is crucial for homeostasis and has recently been shown to be involved in the protective response against infections. Furthermore, the association between mutations of NLRs as well as proteins that form the autophagic machinery and inflammatory diseases such as Crohn’s disease highlight the importance of these proteins and their interactions in the regulation of inflammation.