Cutting edge: Nlrp10 is essential for protective antifungal adaptive immunity against Candida albicans

Candida albicans and Candida glabrata: global priority pathogens

SUMMARYA significant increase in the incidence of Candida-mediated infections has been observed in the last decade, mainly due to rising numbers of susceptible individuals. Recently, the World Health Organization published its first fungal pathogen priority list, with Candida species listed in medium, high, and critical priority categories. This review is a synthesis of information and recent advances in our understanding of two of these species-Candida albicans and Candida glabrata. Of these, C. albicans is the most common cause of candidemia around the world and is categorized as a critical priority pathogen. C. glabrata is considered a high-priority pathogen and has become an increasingly important cause of candidemia in recent years. It is now the second most common causative agent of candidemia in many geographical regions. Despite their differences and phylogenetic divergence, they are successful as pathogens and commensals of humans. Both species can cause a broad variety of infections, ranging from superficial to potentially lethal systemic infections. While they share similarities in certain infection strategies, including tissue adhesion and invasion, they differ significantly in key aspects of their biology, interaction with immune cells, host damage strategies, and metabolic adaptations. Here we provide insights on key aspects of their biology, epidemiology, commensal and pathogenic lifestyles, interactions with the immune system, and antifungal resistance.

Exploiting antifungal immunity in the clinical context

The continuous expansion of immunocompromised patient populations at-risk for developing life-threatening opportunistic fungal infections in recent decades has helped develop a deeper understanding of antifungal host defenses, which has provided the foundation for eventually devising immune-based targeted interventions in the clinic. This review outlines how genetic variation in certain immune pathway-related genes may contribute to the observed clinical variability in the risk of acquisition and/or severity of fungal infections and how immunogenetic-based patient stratification may enable the eventual development of personalized strategies for antifungal prophylaxis and/or vaccination. Moreover, this review synthesizes the emerging cytokine-based, cell-based, and other immunotherapeutic strategies that have shown promise as adjunctive therapies for boosting or modulating tissue-specific antifungal immune responses in the context of opportunistic fungal infections.

Epithelial Nlrp10 inflammasome mediates protection against intestinal autoinflammation

Unlike other nucleotide oligomerization domain-like receptors, Nlrp10 lacks a canonical leucine-rich repeat domain, suggesting that it is incapable of signal sensing and inflammasome formation. Here we show that mouse Nlrp10 is expressed in distal colonic intestinal epithelial cells (IECs) and modulated by the intestinal microbiome. In vitro, Nlrp10 forms an Apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC)-dependent, m-3M3FBS-activated, polyinosinic:polycytidylic acid-modulated inflammasome driving interleukin-1β and interleukin-18 secretion. In vivo, Nlrp10 signaling is dispensable during steady state but becomes functional during autoinflammation in antagonizing mucosal damage. Importantly, whole-body or conditional IEC Nlrp10 depletion leads to reduced IEC caspase-1 activation, coupled with enhanced susceptibility to dextran sodium sulfate-induced colitis, mediated by altered inflammatory and healing programs. Collectively, understanding Nlrp10 inflammasome-dependent and independent activity, regulation and possible human relevance might facilitate the development of new innate immune anti-inflammatory interventions.

Pathogenesis and virulence of Candida albicans

Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.

Assessing Genomic Diversity and Productivity Signatures in Dianzhong Cattle by Whole-Genome Scanning

Dianzhong cattle is a classic Chinese indigenous cattle breed with historical records dating back to 200 BC. But with its genomic differences having not been clearly elucidated, the quest for genomic characterization will be an essential step towards understanding the genomic basis of productivity and adaptation to survival under Chinese farming systems. Here we compared 10 Dianzhong cattle (four newly sequenced and six downloaded) with 29 published genomes of three underlying ancestral populations (Chinese zebu, Indian zebu, and Yanbian cattle) to characterize the genomic variations of Dianzhong cattle. Dianzhong cattle has a high nucleotide diversity (0.0034), second only to Chinese zebu. Together with analyses of linkage disequilibrium decay and runs of homozygosity, Dianzhong cattle displayed higher genomic diversity and weaker artificial selection compared with Yanbian cattle. From a selective sweep analysis by four methods (Fst, π-ratio, XP-CLR, and XP-EHH), the positive selective signals were mainly manifested in candidate genes and pathways related to heat resistance, growth and development, fat deposition, and male reproduction. Missense mutations were detected in candidate genes, SDS (c.944C > A and p.Ala315Glu), PDGFD (c.473A > G and p.Lys158Arg), and DDX4 (rs460251486, rs722912933, and rs517668236), which related to heat resistance, fat deposition, and spermatogenesis, respectively. Our findings unravel, at the genome-wide level, the unique diversity of Dianzhong cattle while emphasizing the opportunities for improvement of livestock productivity in further breeding programs.

Nod-like Receptors: Critical Intracellular Sensors for Host Protection and Cell Death in Microbial and Parasitic Infections

Cell death is an essential immunological apparatus of host defense, but dysregulation of mutually inclusive cell deaths poses severe threats during microbial and parasitic infections leading to deleterious consequences in the pathological progression of infectious diseases. Nucleotide-binding oligomerization domain (NOD)-Leucine-rich repeats (LRR)-containing receptors (NLRs), also called nucleotide-binding oligomerization (NOD)-like receptors (NLRs), are major cytosolic pattern recognition receptors (PRRs), their involvement in the orchestration of innate immunity and host defense against bacteria, viruses, fungi and parasites, often results in the cleavage of gasdermin and the release of IL-1β and IL-18, should be tightly regulated. NLRs are functionally diverse and tissue-specific PRRs expressed by both immune and non-immune cells. Beyond the inflammasome activation, NLRs are also involved in NF-κB and MAPK activation signaling, the regulation of type I IFN (IFN-I) production and the inflammatory cell death during microbial infections. Recent advancements of NLRs biology revealed its possible interplay with pyroptotic cell death and inflammatory mediators, such as caspase 1, caspase 11, IFN-I and GSDMD. This review provides the most updated information that caspase 8 skews the NLRP3 inflammasome activation in PANoptosis during pathogen infection. We also update multidimensional roles of NLRP12 in regulating innate immunity in a content-dependent manner: novel interference of NLRP12 on TLRs and NOD derived-signaling cascade, and the recently unveiled regulatory property of NLRP12 in production of type I IFN. Future prospects of exploring NLRs in controlling cell death during parasitic and microbial infection were highlighted.

Local antifungal immunity in the kidney in disseminated candidiasis

Disseminated candidiasis is a hospital-acquired infection that results in high degree of mortality despite antifungal treatment. Autopsy studies revealed that kidneys are the major target organs in disseminated candidiasis and death due to kidney damage is a frequent outcome in these patients. Thus, the need for effective therapeutic strategies to mitigate kidney damage in disseminated candidiasis is compelling. Recent studies have highlighted the essential contribution of kidney-specific immune response in host defense against systemic infection. Crosstalk between kidney-resident and infiltrating immune cells aid in the clearance of fungi and prevent tissue damage in disseminated candidiasis. In this review, we provide our recent understanding on antifungal immunity in the kidney with an emphasis on IL-17-mediated renal defense in disseminated candidiasis.

Nod-Like Receptors in Host Defence and Disease at the Epidermal Barrier

The nucleotide-binding domain and leucine-rich-repeat-containing family (NLRs) (sometimes called the NOD-like receptors, though the family contains few bona fide receptors) are a superfamily of multidomain-containing proteins that detect cellular stress and microbial infection. They constitute a critical arm of the innate immune response, though their functions are not restricted to pathogen recognition and members engage in controlling inflammasome activation, antigen-presentation, transcriptional regulation, cell death and also embryogenesis. NLRs are found from basal metazoans to plants, to zebrafish, mice and humans though functions of individual members can vary from species to species. NLRs also display highly wide-ranging tissue expression. Here, we discuss the importance of NLRs to the immune response at the epidermal barrier and summarise the known role of individual family members in the pathogenesis of skin disease.

Candida albicans Virulence Factors and Pathogenicity for Endodontic Infections

Candida albicans (C. albicans) is the fungus most frequently isolated from endodontic root canal infections. Although recognized by dental pulp and periradicular tissue cells that elicit immune responses, it eludes host defenses and elicits cell death. Then, C. albicans binds tooth dentin, forms biofilms, and invades dentinal tubules to resist intracanal disinfectants and endodontic treatments. Insensitive to most common medicaments, it survives sequestered within biofilms and intratubular dentin. Thus, C. albicans has been associated with cases of persistent or refractory root canal infections. Its treatment strategies may require alternative intracanal irrigants, intracanal medicaments such as chlorhexidine gel or human beta defensin-3 (HBD3), Ca-Si-based obturating materials, and microsurgical procedures.

Autoinflammation in addition to combined immunodeficiency: SLC29A3 gene defect

INTRODUCTION

H Syndrome is an autosomal recessive (AR) disease caused by defects in SLCA29A3 gene. This gene encodes the equilibrative nucleoside transporter, the protein which is highly expressed in spleen, lymph node and bone marrow. Autoinflammation and autoimmunity accompanies H Syndrome (HS).

AIM

The aim was to further elucidate the mechanisms of disease by molecular studies in a patient with SLC29A3 gene defect.

PATIENT AND METHODS

Mitochondrial dysfunction, lysosomal integrity, cytokine response in response to stimulation with different pattern recognition receptor ligands, and circulating cell-free mitochondrial-DNA(ccf-mtDNA) level in plasma were analyzed compared to controls to understand the cellular triggers of autoinflammation. RNA sequencing (RS) analyses were also performed in monocytes before/after culture with lipopolysaccharide.

RESULTS

Patient had progressive destructive arthropathy in addition to clinical findings due to combined immunodeficiency. Pure red cell aplasia (PRCA), vitiligo, diabetes, multiple autoantibody positivity, lymphopenia, increased acute phase reactants were present. Recent thymic emigrants (RTE), naïve T cells were decreased, effector memory CD4 + T cells, nonclassical inflammatory monocytes were increased. Patient's peripheral blood mononuclear cells secreted more IL-1β and IL-6, showed lysosomal disruption and significant mitochondrial dysfunction compared to healthy controls. Plasma ccf-mtDNA level was significantly elevated compared to age-matched controls (p < 0.05). RNA sequencing studies revealed decreased expression of NLR Family Caspase Recrument-Domain Containing 4(NLRC4), 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4(PFKFB4), serine dehydratase(SDS), heparan sulfate(Glucosamine) 3-O-sulfotransferase 1(HS3ST1), neutral cholesterol ester hydrolase 1 (NCEH1), and interleukin-8 (IL-8) in patient's monocytes compared to controls. Longstanding PRCA, which is possibly autoimmune, resolved after initiating monthly intravenous immunoglobulins (IVIG) and low dose steroids to the patient.

CONCLUSION

Although autoinflammation and autoimmunity are reported in HS, by functional analyses we here show in the present patient that over-active inflammasome pathway in HS might be related with mitochondrial and lysosomal dysfunction. Increased plasma ccf-mtDNA may be used as a biomarker of inflammasomopathy in HS. HS should be included in the classification of primary immunodeficiency diseases.

NLRP10 ablation protects against ischemia/reperfusion-associated brain injury by suppression of neuroinflammation

Ischemic stroke leads to neuronal cell death and induces a cascade of inflammatory signals that results in secondary brain damage. Although constant efforts to develop therapeutic strategies and to reveal the molecular mechanism resulting in the physiopathology of this disease, much still remains unclear. Membrane-bound Toll-like receptors (TLRs) and cytosolic nucleotide binding oligomerization domain (NOD)-like receptors (NLRs) are two major families of pattern recognition receptors that initiate pro-inflammatory signaling pathways. In the present study, we explored the role of NLRP10 in regulating inflammatory responses in acute ischemic stroke using the wild type (WT) and NLRP10 knockout (KO) mice by inducing middle cerebral artery occlusion/reperfusion (MCAO) injuries. The study first showed that NLRP10 was over-expressed in the ischemic penumbra of WT mice. Then, the brain infarct volume was significantly decreased, and the moving activity was improved post-MCAO in mice with NLRP10 knockout. Apoptosis was also alleviated by NLRP10-knockout, as evidenced by the decreased number of TUNEL-staining cells. Further, NLRP10 deficiency attenuated the activation of glia cells in hippocampus of mice with MCAO operation. NLRP10 inhibition ameliorated the levels of inflammatory factors in peripheral blood serum and hippocampus of mice after stroke. The activation of toll-like receptor (TLR)-4/nuclear factor-κB (NF-κB) signaling pathways was markedly suppressed by NLRP10 ablation in mice after MCAO treatment. Importantly, inflammasome, including NLRP12, ASC and Caspase-1, induced by MCAO in hippocampus of mice was clearly impeded by the loss of NLRP10. The results above were mainly verified in LPS-incubated astrocytes in the absence of NLRP10. Correspondingly, in LPS-treated astrocytes, NLRP10 knockout-reduced inflammation via impairing TLR-4/NF-κB and NLRP12/ASC/Caspase-1 pathways was evidently restored by over-expressing NLRP10. Therefore, the results above indicated an essential role of NLRP10 in regulating ischemic stroke, presenting NLRP10 as a promising target to protect human against stroke.

Heat Shock Proteins and Inflammasomes

Heat shock proteins (HSP) regulate inflammation in many physiological contexts. However, inflammation is a broad process, involving numerous cytokines produced by different molecular pathways with multiple functions. In this review, we focused on the particular role of HSP on the inflammasomes intracellular platforms activated by danger signals and that enable activation of inflammatory caspases, mainly caspase-1, leading to the production of the pro-inflammatory cytokine IL-1β. Interestingly, some members of the HSP family favor inflammasomes activation whereas others inhibit it, suggesting that HSP modulators for therapeutic purposes, must be carefully chosen.

Differences in interleukin-1β release-inducing activity of Candida albicans toward dendritic cells and macrophages

OBJECTIVE

The purpose of this study is to elucidate differences in the mechanism of the IL-1β release-inducing activity of Candida albicans toward dendritic cells and macrophages because IL-1β is one of the proinflammatory cytokines which is crucial in host defense against candidiasis.

DESIGN

Two C. albicans strains were used in this study. One strain is uridine-auxotrophic (CAI4) that needs uridine to grow and form hyphae, and another is a strain without any specific auxotrophy (pACT1-GFP), which forms hyphae naturally by culturing with serum components. Murine macrophage and dendritic cell lines were primed with LPS and then stimulated with C. albicans CAI4 or pACT1-GFP.

RESULTS

Both strains of C. albicans induced IL-1β release from dendritic cells, and C. albicans pACT1-GFP induced IL-1β release but CAI4 induced little amounts in macrophages. These differences were suggested to be due to the difference in the amount of extracellular ATP released in the cell culture supernatants induced by C. albicans CAI4 or pACT1-GFP. For induction of IL-1β release from both macrophages and dendritic cells by C. albicans, direct contacts of the microbes with cells were required. In addition, macrophages required morphological change of C. albicans from yeast to hyphae for induction of IL-1β release, whereas dendritic cells did not require it. Dead C. albicans could induce IL-1β release from dendritic cells, but could not from macrophages.

CONCLUSIONS

There are different mechanisms by which C. albicans induces IL-1β release from dendritic cells and macrophages.

Cigarette smoke-induced inflammation: NLRP10-mediated mechanisms

Chronic obstructive pulmonary disease (COPD) is a progressive, life-threatening disease that causes irreversible lung damage. Cigarette smoking is the chief etiologic factor for the commencement of this condition. Despite constant efforts to develop therapeutic interventions and to ascertain the molecular mechanism leading to the pathophysiology of this disease, much remains unknown. However, pattern recognition receptors (PRRs), i.e., Toll-like-receptors (TLRs) and NOD-like receptors (NLRs) are believed to play important roles in COPD and could serve as effective therapeutic targets. Although the role of TLRs in COPD has been well studied, the importance of NLRs has not yet been explored in detail. The NLR family member NLRP10 (aka NOD8, PAN5, PYNOD) is the only member of this family of proteins that lacks the leucine rich repeat (LRR) domain responsible for detection of pathogen and danger-associated molecular patterns (PAMPs/DAMPs). Therefore, instead of functioning as a PRR, NLRP10 may have a broader regulatory role. To elucidate the role of NLRP10 in secondhand smoke (SHS)-induced inflammation, we exposed C57Bl/6 (WT) and Nlrp10-deficient mice (Nlrp10^-/-) on the C57Bl/6 background to filtered air- or SHS- for 6 weeks (acute exposure) and assessed the resulting molecular events. Leukocyte recruitment in SHS-exposed Nlrp10^-/- mice was found to be significantly lower compared to SHS-exposed WT mice. In addition, we observed an important role for NLRP10 in SHS-mediated caspase-1 activation, cytokine/chemokine production (IL-1β, IL-18, MCP-1 and IL-17A), and induction of NF-κB and MAPKs in the lungs of C57Bl/6 mice. The reduced influx of CD4⁺IL-17A⁺ and CD8⁺IL-17A⁺ cells into the lungs of SHS-exposed Nlrp10^-/- mice and impaired differentiation of Nlrp10^-/- Th0 cells into Th17 cells (ex vivo) provide insight into the mechanistic details underlying NLRP10-dependent IL-17 production. We further substantiated our in vivo findings by challenging human alveolar type II epithelial cells (A549) transfected with scrambled- or Nlrp10-siRNA with cigarette smoke extract (CSE). We observed an important role of NLRP10 in cytokine and chemokine production as well as expression of NF-κB and MAPKs in CSE-exposed A549 cells. Furthermore, replenishment of A549 cell culture with recombinant IL-17A (rIL-17A) during NLRP10 knockdown rescued CSE-induced inflammatory responses. To identify upstream mediators of NLRP10 regulation we investigated epigenetic markers within the Nlrp10 promoter following cigarette smoke exposure and observed significant changes in active as well as repressive gene markers on histone 3 and histone 4 using both in vivo and in vitro study models. Further, alterations in the respective histone acetyl- and methyltransferases (PCAF, SET1, ESET, SUV20H1) correlated well with the observed histone modifications. Overall, our findings suggest a novel role of epigenetically regulated NLRP10 in Th17/IL-17 signaling during CS exposure.

A young girl with severe cerebral fungal infection due to card 9 deficiency

Pattern recognition receptors (PRRs), receptors of the innate immune system, are important in interaction with pathogens. Caspase Recruitment Domain-containing protein 9 (CARD9), a member of PRRs, is an intracellular adaptor protein important in fungal defense. CARD9 deficiency causes a rare primary immunodeficiency (PID) characterized by superficial and deep fungal infections. We report a 17year-old female with a homozygous nonsense mutation in CARD9, who presented with severe cerebral fungal infection of the central nervous system. She was also found to have an heterozygous NLRP12 mutation, which may have had add-on effect on the severity of the infection.

NLRP10 Enhances CD4+ T-Cell-Mediated IFNγ Response via Regulation of Dendritic Cell-Derived IL-12 Release

NLRP10 is a nucleotide-binding oligomerization domain-like receptor that functions as an intracellular pattern recognition receptor for microbial products. Here, we generated a Nlrp10^-/- mouse to delineate the role of NLRP10 in the host immune response and found that Nlrp10^-/- dendritic cells (DCs) elicited sub-optimal IFNγ production by antigen-specific CD4⁺ T cells compared to wild-type (WT) DCs. In response to T-cell encounter, CD40 ligation or Toll-like receptor 9 stimulation, Nlrp10^-/- DCs produced low levels of IL-12, due to a substantial decrease in NF-κB activation. Defective IL-12 production was also evident in vivo and affected IFNγ production by CD4⁺ T cells. Upon Mycobacterium tuberculosis (Mtb) infection, Nlrp10^-/- mice displayed diminished T helper 1-cell responses and increased bacterial growth compared to WT mice. These data indicate that NLRP10-mediated IL-12 production by DCs is critical for IFNγ induction in T cells and contributes to promote the host defense against Mtb.

Involvement of NLRP10 in IL-1α induction of oral epithelial cells by periodontal pathogens

This study investigated the pathogenesis of periodontitis and the role of nucleotide-binding oligomerization domain-like receptor protein 10 (NLRP10). The human oral epithelial cell line HOK-16B was infected with two periodontal pathogens, Tannerella forsythia and Fusobacterium nucleatum, at various MOIs. RT-PCR and immunoblotting demonstrated that infection increased mRNA and protein expression of NLRP10, respectively. The siRNA-mediated NLRP10 knockdown significantly reduced IL-1α expression and secretion. Both bacteria induced phosphorylation of ERK, JNK and p38 MAP kinases in HOK-16B cells. NLRP10 knockdown impaired ERK phosphorylation only. ERK inhibition significantly decreased the expression of T. forsythia- and F. nucleatum-induced IL-1α. Our data suggest that NLRP10 is involved in activating the ERK signalling pathway in HOK-16B cells infected with T. forsythia and F. nucleatum. This pathway likely augments the pro-inflammatory cytokine IL-1α levels, which may play a critical role in periodontitis.

Candida albicans infection and intestinal immunity

Fungal infections cause high rates of morbidity and mortality in intensive care and immunocompromised patients, and can represent a life-threatening disease. As a microorganism commonly found in the intestine, Candida albicans (C. albicans) can invade the gut epithelium barrier via microfold cells and enter the bloodstream. The defensive potential of the intestinal barrier against invasive C. albicans is dependent on innate and adaptive immune responses which enable the host to eliminate pathogenic fungi. The lamina propria layer of the intestine contains numerous immune cells capable of inducing an innate cellular immune response against invasive fungi. This review focuses on the immune response triggered by a C. albicans infection in the intestine.

Cell biology of Candida albicans-host interactions

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The cell biology of Candida albicans is adapted both for life as a commensal and as a pathogen.

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C. albicans can either downregulate or upregulate virulence properties in the human host.

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This fungus modulates the activity of phagocytes to enable its own survival.

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Candida is metabolically flexible enabling it to survive in multiple niches in the host.

Candida albicans is a commensal coloniser of most people and a pathogen of the immunocompromised or patients in which barriers that prevent dissemination have been disrupted. Both the commensal and pathogenic states involve regulation and adaptation to the host microenvironment. The pathogenic potential can be downregulated to sustain commensalism or upregulated to damage host tissue and avoid and subvert immune surveillance. In either case it seems as though the cell biology of this fungus has evolved to enable the establishment of different types of relationships with the human host. Here we summarise latest advances in the analysis of mechanisms that enable C. albicans to occupy different body sites whilst avoiding being eliminated by the sentinel activities of the human immune system.

NLRC4 suppresses melanoma tumor progression independently of inflammasome activation

Members of the NLR family can assemble inflammasome complexes with the adaptor protein ASC and caspase-1 that result in the activation of caspase-1 and the release of IL-1β and IL-18. Although the NLRC4 inflammasome is known to have a protective role in tumorigenesis, there is an increased appreciation for the inflammasome-independent actions of NLRC4. Here, we utilized a syngeneic subcutaneous murine model of B16F10 melanoma to explore the role of NLRC4 in tumor suppression. We found that NLRC4-deficient mice exhibited enhanced tumor growth that was independent of the inflammasome components ASC and caspase-1. Nlrc4 expression was critical for cytokine and chemokine production in tumor-associated macrophages and was necessary for the generation of protective IFN-γ-producing CD4+ and CD8+ T cells. Tumor progression was diminished when WT or caspase-1-deficient, but not NLRC4-deficient, macrophages were coinjected with B16F10 tumor cells in NLRC4-deficient mice. Finally, examination of human primary melanomas revealed the extensive presence of NLRC4+ tumor-associated macrophages. In contrast, there was a paucity of NLRC4+ tumor-associated macrophages observed in human metastatic melanoma, supporting the concept that NLRC4 expression controls tumor growth. These results reveal a critical role for NLRC4 in suppressing tumor growth in an inflammasome-independent manner.

Epidermal NLRP10 contributes to contact hypersensitivity responses in mice

The nucleotide binding and oligomerization domain-like receptor (NLR) protein NLRP10 is highly expressed in the epidermis and contributes to cell-autonomous responses against invasive bacteria. To investigate the role of NLRP10 in inflammatory responses of the skin we analyzed the effect of full-body and keratinocyte-specific depletion of NLRP10 in croton oil-induced irritant contact dermatitis (ICD) and 1-fluoro-2,4-dinitrobenzene (DNFB)-induced contact hypersensitivity (CHS) in mice. Nlrp10(-/-) mice were phenotypically normal and skin repair after wounding was not affected by lack of NLRP10. Similarly, we did not detect a contribution of NLRP10 to the ICD response induced by croton oil. In contrast, Nlrp10(-/-) mice showed significantly reduced inflammation in the DNFB-induced CHS response as compared to control animals. Microscopic analysis revealed significantly reduced numbers of CD4(+) and CD8(+) T cells in the infiltrates of animals lacking NLRP10 expression after CHS challenge. Epidermis-specific deletion of Nlrp10 by keratin-14 promotor driven Cre-recombinase was sufficient to account for this phenotype, although lymphocyte recruitment seemed to be unaltered in animals lacking NLRP10 expression in keratinocytes. Taken together, we provide evidence that NLRP10 contributes to T-cell-mediated inflammatory responses in the skin and highlight a physiological role of NLRP10 in epidermal keratinocytes.

Antibody blockade of IL-17 family cytokines in immunity to acute murine oral mucosal candidiasis

Antibodies targeting IL-17A or its receptor, IL-17RA, are approved to treat psoriasis and are being evaluated for other autoimmune conditions. Conversely, IL-17 signaling is critical for immunity to opportunistic mucosal infections caused by the commensal fungus Candida albicans, as mice and humans lacking the IL-17R experience chronic mucosal candidiasis. IL-17A, IL-17F, and IL-17AF bind the IL-17RA-IL-17RC heterodimeric complex and deliver qualitatively similar signals through the adaptor Act1. Here, we used a mouse model of acute oropharyngeal candidiasis to assess the impact of blocking IL-17 family cytokines compared with specific IL-17 cytokine gene knockout mice. Anti-IL-17A antibodies, which neutralize IL-17A and IL-17AF, caused elevated oral fungal loads, whereas anti-IL-17AF and anti-IL-17F antibodies did not. Notably, there was a cooperative effect of blocking IL-17A, IL-17AF, and IL-17F together. Termination of anti-IL-17A treatment was associated with rapid C. albicans clearance. IL-17F-deficient mice were fully resistant to oropharyngeal candidiasis, consistent with antibody blockade. However, IL-17A-deficient mice had lower fungal burdens than anti-IL-17A-treated mice. Act1-deficient mice were much more susceptible to oropharyngeal candidiasis than anti-IL-17A antibody-treated mice, yet anti-IL-17A and anti-IL-17RA treatment caused equivalent susceptibilities. Based on microarray analyses of the oral mucosa during infection, only a limited number of genes were associated with oropharyngeal candidiasis susceptibility. In sum, we conclude that IL-17A is the main cytokine mediator of immunity in murine oropharyngeal candidiasis, but a cooperative relationship among IL-17A, IL-17AF, and IL-17F exists in vivo. Susceptibility displays the following hierarchy: IL-17RA- or Act1-deficiency > anti-IL-17A + anti-IL-17F antibodies > anti-IL-17A or anti-IL-17RA antibodies > IL-17A deficiency.

Skin Immunity to Candida albicans

Candida albicans is a dimorphic commensal fungus that colonizes healthy human skin, mucosa, and the reproductive tract. C. albicans is also a predominantly opportunistic fungal pathogen, leading to disease manifestations such as disseminated candidiasis and chronic mucocutaneous candidiasis (CMC). The differing host susceptibilities for the sites of C. albicans infection have revealed tissue compartmentalization with tailoring of immune responses based on the site of infection. Furthermore, extensive studies of host genetics in rare cases of CMC have identified conserved genetic pathways involved in immune recognition and the response to the extracellular pathogen. We focus here on human and mouse skin as a site of C. albicans infection, and we review established and newly discovered insights into the cellular pathways that promote cutaneous antifungal immunity.

Innate immune cell response upon Candida albicans infection

Candida albicans is a polymorphic fungus which is the predominant cause of superficial and deep tissue fungal infections. This microorganism has developed efficient strategies to invade the host and evade host defense systems. However, the host immune system will be prepared for defense against the microbe by recognition of receptors, activation of signal transduction pathways and cooperation of immune cells. As a consequence, C. albicans could either be eliminated by immune cells rapidly or disseminate hematogenously, leading to life-threatening systemic infections. The interplay between Candida albicans and the host is complex, requiring recognition of the invaded pathogens, activation of intricate pathways and collaboration of various immune cells. In this review, we will focus on the effects of innate immunity that emphasize the first line protection of host defense against invaded C. albicans including the basis of receptor-mediated recognition and the mechanisms of cell-mediated immunity.

The role of pattern recognition receptors in the innate recognition of Candida albicans

Candida albicans is both a commensal microorganism in healthy individuals and a major fungal pathogen causing high mortality in immunocompromised patients. Yeast-hypha morphological transition is a well known virulence trait of C. albicans. Host innate immunity to C. albicans critically requires pattern recognition receptors (PRRs). In this review, we summarize the PRRs involved in the recognition of C. albicans in epithelial cells, endothelial cells, and phagocytic cells separately. We figure out the differential recognition of yeasts and hyphae, the findings on PRR-deficient mice, and the discoveries on human PRR-related single nucleotide polymorphisms (SNPs).

The interplay between inflammasome activation and antifungal host defense

Fungal infections cause significant morbidity and mortality in humans, and they are a growing problem due to the increased usage of broad-spectrum antibiotics and immunosuppressive therapies. The equilibrium between the commensal microbial flora and the immune system that protects the host against invasive fungal infection is disturbed during disease, and understanding this disturbed balance is important to develop new therapeutic interventions for the treatment of fungal infection. In the context of tolerating fungi during colonization and eliciting a vigorous immune response to eliminate invading fungal pathogens when needed, the inflammasome has been identified as an integral component of antifungal host defense. It contributes to mucosal host defense by regulating T-helper 17 (Th17) cell responses, and contributes to protective responses such as neutrophil influx during fungal sepsis. Several aspects are important for understanding the role of the inflammasome for antifungal host defense, such as the role of fungal cell wall morphology and its components in triggering the inflammasome, the pattern recognition pathways and downstream signaling cascades involved in the activation of the inflammasome, and the effects of inflammasome activation during fungal infection. The future perspectives of inflammasome research in fungal immunology, with emphasis on targeting the inflammasome for the design of future immunotherapies, is also discussed.

NLR proteins and parasitic disease

Parasitic diseases are a serious global health concern. Many of the most common and most severe parasitic diseases, including Chagas' disease, leishmaniasis, and schistosomiasis, are also classified as neglected tropical diseases and are comparatively less studied than infectious diseases prevalent in high income nations. The NLRs (nucleotide-binding domain leucine-rich-repeat-containing proteins) are cytosolic proteins known to be involved in pathogen detection and host response. The role of NLRs in the host response to parasitic infection is just beginning to be understood. The NLR proteins NOD1 and NOD2 have been shown to contribute to immune responses during Trypanosoma cruzi infection, Toxoplasma gondii infection, and murine cerebral malaria. The NLRP3 inflammasome is activated by T. cruzi and Leishmania amazonensis but also induces pathology during infection with schistosomes or malaria. Both the NLRP1 and NLRP3 inflammasomes respond to T. gondii infection. The NLRs may play crucial roles in human immune responses during parasitic infection, usually acting as innate immune sensors and driving the inflammatory response against invading parasites. However, this inflammatory response can either kill the invading parasite or be responsible for destructive pathology. Therefore, understanding the role of the NLR proteins will be critical to understanding the host defense against parasites as well as the fine balance between homeostasis and parasitic disease.

NOD-like receptors: versatile cytosolic sentinels

Nucleotide binding oligomerization domain (NOD)-like receptors are cytoplasmic pattern-recognition receptors that together with RIG-I-like receptor (retinoic acid-inducible gene 1), Toll-like receptor (TLR), and C-type lectin families make up the innate pathogen pattern recognition system. There are 22 members of NLRs in humans, 34 in mice, and even a larger number in some invertebrates like sea urchins, which contain more than 200 receptors. Although initially described to respond to intracellular pathogens, NLRs have been shown to play important roles in distinct biological processes ranging from regulation of antigen presentation, sensing metabolic changes in the cell, modulation of inflammation, embryo development, cell death, and differentiation of the adaptive immune response. The diversity among NLR receptors is derived from ligand specificity conferred by the leucine-rich repeats and an NH2-terminal effector domain that triggers the activation of different biological pathways. Here, we describe NLR genes associated with different biological processes and the molecular mechanisms underlying their function. Furthermore, we discuss mutations in NLR genes that have been associated with human diseases.

Coincidental loss of DOCK8 function in NLRP10-deficient and C3H/HeJ mice results in defective dendritic cell migration

Dendritic cells (DCs) are the primary leukocytes responsible for priming T cells. To find and activate naïve T cells, DCs must migrate to lymph nodes, yet the cellular programs responsible for this key step remain unclear. DC migration to lymph nodes and the subsequent T-cell response are disrupted in a mouse we recently described lacking the NOD-like receptor NLRP10 (NLR family, pyrin domain containing 10); however, the mechanism by which this pattern recognition receptor governs DC migration remained unknown. Using a proteomic approach, we discovered that DCs from Nlrp10 knockout mice lack the guanine nucleotide exchange factor DOCK8 (dedicator of cytokinesis 8), which regulates cytoskeleton dynamics in multiple leukocyte populations; in humans, loss-of-function mutations in Dock8 result in severe immunodeficiency. Surprisingly, Nlrp10 knockout mice crossed to other backgrounds had normal DOCK8 expression. This suggested that the original Nlrp10 knockout strain harbored an unexpected mutation in Dock8, which was confirmed using whole-exome sequencing. Consistent with our original report, NLRP3 inflammasome activation remained unaltered in NLRP10-deficient DCs even after restoring DOCK8 function; however, these DCs recovered the ability to migrate. Isolated loss of DOCK8 via targeted deletion confirmed its absolute requirement for DC migration. Because mutations in Dock genes have been discovered in other mouse lines, we analyzed the diversity of Dock8 across different murine strains and found that C3H/HeJ mice also harbor a Dock8 mutation that partially impairs DC migration. We conclude that DOCK8 is an important regulator of DC migration during an immune response and is prone to mutations that disrupt its crucial function.

Adaptive immune responses to Candida albicans infection

Fungal infections are becoming increasingly prevalent in the human population and contribute to morbidity and mortality in healthy and immunocompromised individuals respectively. Candida albicans is the most commonly encountered fungal pathogen of humans, and is frequently found on the mucosal surfaces of the body. Host defense against C. albicans is dependent upon a finely tuned implementation of innate and adaptive immune responses, enabling the host to neutralise the invading fungus. Central to this protection are the adaptive Th1 and Th17 cellular responses, which are considered paramount to successful immune defense against C. albicans infections, and enable tissue homeostasis to be maintained in the presence of colonising fungi. This review will highlight the recent advances in our understanding of adaptive immunity to Candida albicans infections.

Structural mechanisms of inflammasome assembly

Inflammasomes are supramolecular signaling complexes that activate a subset of caspases known as the inflammatory caspases, an example of which is caspase 1. Upon stimulation by microbial and damage-associated signals, inflammasomes assemble to elicit the first line of host defense via the proteolytic maturation of cytokines interleukin-1β and interleukin-18, and by induction of pyroptotic cell death. Inflammasome assembly requires activation of an upstream sensor, a downstream effector and, in most cases, an adaptor molecule such as apoptosis-associate speck-like protein containing a caspase recruitment domain (ASC). Depending on whether ASC is required, inflammasomes can be categorized into ASC-dependent and ASC-independent inflammasomes. Here, we review current understandings of the structures of inflammasomes, as probed using traditional structural methods, as well as biochemical, biophysical and single-molecule methods. The key structural scaffold for inflammasome assembly is composed of filaments of Pyrin domains and caspase recruitment domains (CARD) in the sensor, adaptor and effector components. Nucleated polymerization appears to govern the ordered assembly process from activation of a Pyrin domain-containing sensor such as AIM2 by dsDNA or NLRP3 by extracellular particulates, to recruitment of the Pyrin domain and CARD-containing adaptor ASC, and finally to activation of CARD-containing caspase 1. The underlying filamentous architecture of inflammasomes and the cooperativity in the assembly may explain the 'all-or-none' response in inflammasome activation. Inflammasomes are tightly regulated by a number of cytosolic inhibitors, which may change the morphology and assembly kinetics of inflammasomes. Biochemical and cellular studies suggest that Pyrin domain and CARD filaments possess prion-like properties in propagating inflammasome activation within and between cells.

Innate Defense against Fungal Pathogens

Human fungal infections have been on the rise in recent years and proved increasingly difficult to treat as a result of the lack of diagnostics, effective antifungal therapies, and vaccines. Most pathogenic fungi do not cause disease unless there is a disturbance in immune homeostasis, which can be caused by modern medical interventions, disease-induced immunosuppression, and naturally occurring human mutations. The innate immune system is well equipped to recognize and destroy pathogenic fungi through specialized cells expressing a broad range of pattern recognition receptors (PRRs). This review will outline the cells and PRRs required for effective antifungal immunity, with a special focus on the major antifungal cytokine IL-17 and recently characterized antifungal inflammasomes.

Caspase-8 modulates dectin-1 and complement receptor 3-driven IL-1β production in response to β-glucans and the fungal pathogen, Candida albicans

Inflammasomes are central mediators of host defense to a wide range of microbial pathogens. The nucleotide-binding domain and leucine-rich repeat containing family (NLR), pyrin domain-containing 3 (NLRP3) inflammasome plays a key role in triggering caspase-1-dependent IL-1β maturation and resistance to fungal dissemination in Candida albicans infection. β-Glucans are major components of fungal cell walls that trigger IL-1β secretion in both murine and human immune cells. In this study, we sought to determine the contribution of β-glucans to C. albicans-induced inflammasome responses in mouse dendritic cells. We show that the NLRP3-apoptosis-associated speck-like protein containing caspase recruitment domain protein-caspase-1 inflammasome is absolutely critical for IL-1β production in response to β-glucans. Interestingly, we also found that both complement receptor 3 (CR3) and dectin-1 play a crucial role in coordinating β-glucan-induced IL-1β processing as well as a cell death response. In addition to the essential role of caspase-1, we identify an important role for the proapoptotic protease caspase-8 in promoting β-glucan-induced cell death and NLRP3 inflammasome-dependent IL-1β maturation. A strong requirement for CR3 and caspase-8 also was found for NLRP3-dependent IL-1β production in response to heat-killed C. albicans. Taken together, these results define the importance of dectin-1, CR3, and caspase-8, in addition to the canonical NLRP3 inflammasome, in mediating β-glucan- and C. albicans-induced innate responses in dendritic cells. Collectively, these findings establish a novel link between β-glucan recognition receptors and the inflammatory proteases caspase-8 and caspase-1 in coordinating cytokine secretion and cell death in response to immunostimulatory fungal components.

The porcine innate immune system: an update

Over the last few years, we have seen an increasing interest and demand for pigs in biomedical research. Domestic pigs (Sus scrofa domesticus) are closely related to humans in terms of their anatomy, genetics, and physiology, and often are the model of choice for the assessment of novel vaccines and therapeutics in a preclinical stage. However, the pig as a model has much more to offer, and can serve as a model for many biomedical applications including aging research, medical imaging, and pharmaceutical studies to name a few. In this review, we will provide an overview of the innate immune system in pigs, describe its anatomical and physiological key features, and discuss the key players involved. In particular, we compare the porcine innate immune system to that of humans, and emphasize on the importance of the pig as model for human disease.

New insights into innate immune control of systemic candidiasis

Systemic infection caused by Candida species is the fourth leading cause of nosocomial bloodstream infection in modern hospitals and carries high morbidity and mortality despite antifungal therapy. A recent surge of immunological studies in the mouse models of systemic candidiasis and the parallel discovery and phenotypic characterization of inherited genetic disorders in antifungal immune factors that are associated with enhanced susceptibility or resistance to the infection have provided new insights into the cellular and molecular basis of protective innate immune responses against Candida. In this review, the new developments in our understanding of how the mammalian immune system responds to systemic Candida challenge are synthesized and important future research directions are highlighted.

Advances in Nod-like receptors (NLR) biology

The innate immune system is composed of a wide repertoire of conserved pattern recognition receptors (PRRs) able to trigger inflammation and host defense mechanisms in response to endogenous or exogenous pathogenic insults. Among these, nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are intracellular sentinels of cytosolic sanctity capable of orchestrating innate immunity and inflammatory responses following the perception of noxious signals within the cell. In this review, we elaborate on recent advances in the signaling mechanisms of NLRs, operating within inflammasomes or through alternative inflammatory pathways, and discuss the spectrum of their effector functions in innate immunity. We describe the progressive characterization of each NLR with associated controversies and cutting edge discoveries.

Candida albicans and cancer: Can this yeast induce cancer development or progression?

There is currently increasing concern about the relation between microbial infections and cancer. More and more studies support the view that there is an association, above all, when the causal agents are bacteria or viruses. This review adds to this, summarizing evidence that the opportunistic fungus Candida albicans increases the risk of carcinogenesis and metastasis. Until recent years, Candida spp. had fundamentally been linked to cancerous processes as it is an opportunist pathogen that takes advantage of the immunosuppressed state of patients particularly due to chemotherapy. In contrast, the most recent findings demonstrate that C. albicans is capable of promoting cancer by several mechanisms, as described in the review: production of carcinogenic byproducts, triggering of inflammation, induction of Th17 response and molecular mimicry. We underline the need not only to control this type of infection during cancer treatment, especially given the major role of this yeast species in nosocomial infections, but also to find new therapeutic approaches to avoid the pro-tumor effect of this fungal species.

Candida albicans triggers NLRP3-mediated pyroptosis in macrophages

Pyroptosis is an inflammasome-mediated programmed cell death pathway triggered in macrophages by a variety of stimuli, including intracellular bacterial pathogens. Activation of pyroptosis leads to the secretion of interleukin-1β (IL-1β) and pore-mediated cell lysis. Although not considered an intracellular pathogen, Candida albicans is able to kill and, thereby, escape from macrophages. Here, we show that C. albicans-infected bone marrow-derived macrophages (BMDM) and murine J774 macrophages undergo pyroptotic cell death that is suppressed by glycine and pharmacologic inhibition of caspase-1. Infection of BMDM harvested from mice lacking components of the inflammasome revealed that pyroptosis was dependent on caspase-1, ASC, and NLRP3 and independent of NLRC4. In contrast to its role during intracellular bacterial infection, pyroptosis does not restrict C. albicans replication. Nonfilamentous Candida spp. did not trigger pyroptosis, while Candida krusei, which forms pseudohyphae in macrophages, triggered much lower levels than did C. albicans. Interestingly, a Saccharomyces cerevisiae strain from the filamentous background Σ1278 also triggered low, but significant, levels of pyroptosis. We have found that deletion of the transcription factor UPC2 decreases pyroptosis but has little effect on filamentation in the macrophage. In addition, a gain-of-function mutant of UPC2 induces higher levels of pyroptosis than does a matched control strain. Taken together, these data are most consistent with a model in which filamentation is necessary but not sufficient to trigger NLRP3 inflammasome-mediated pyroptosis. This is the first example of a fungal pathogen triggering pyroptosis and indicates that C. albicans-mediated macrophage damage is not solely due to hypha-induced physical disruption of cellular integrity.

Licensing Adaptive Immunity by NOD-Like Receptors

The innate immune system is composed of a diverse set of host defense molecules, physical barriers, and specialized leukocytes and is the primary form of immune defense against environmental insults. Another crucial role of innate immunity is to shape the long-lived adaptive immune response mediated by T and B lymphocytes. The activation of pattern recognition receptors (PRRs) from the Toll-like receptor family is now a classic example of innate immune molecules influencing adaptive immunity, resulting in effective antigen presentation to naïve T cells. More recent work suggests that the activation of another family of PRRs, the NOD-like receptors (NLRs), induces a different set of innate immune responses and accordingly, drives different aspects of adaptive immunity. Yet how this unusually diverse family of molecules (some without canonical PRR function) regulates immunity remains incompletely understood. In this review, we discuss the evidence for and against NLR activity orchestrating adaptive immune responses during infectious as well as non-infectious challenges.

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.

Pathogenic fungus Microsporum canis activates the NLRP3 inflammasome

Microsporum canis is a pathogenic fungus with worldwide distribution that causes tinea capitis in animals and humans. M. canis also causes invasive infection in immunocompromised patients. To defy pathogenic fungal infection, the host innate immune system is the first line of defense. As an important arm of innate immunity, the inflammasomes are intracellular multiprotein complexes that control the activation of caspase-1, which cleaves proinflammatory cytokine pro-interleukin-1β (IL-1β) into its mature form. To determine whether the inflammasome is involved in the host defense against M. canis infection, we challenged human monocytic THP-1 cells and mouse dendritic cells with a clinical strain of M. canis isolated from patients with tinea capitis. We found that M. canis infection triggered rapid secretion of IL-1β from both THP-1 cells and mouse dendritic cells. Moreover, by using gene-specific shRNA and competitive inhibitors, we determined that M. canis-induced IL-1β secretion was dependent on NLRP3. The pathways proposed for NLRP3 inflammasome activation, namely, cathepsin B activity, K(+) efflux, and reactive oxygen species production, were all required for the inflammasome activation triggered by M. canis. Meanwhile, Syk, Dectin-1, and Card9 were found to be involved in M. canis-induced IL-1β secretion via regulation of pro-IL-1β transcription. More importantly, our data revealed that M. canis-induced production of IL-1β was dependent on the NLRP3 inflammasome in vivo. Together, this study unveils that the NLRP3 inflammasome exerts a critical role in host innate immune responses against M. canis infection, and our data suggest that diseases that result from M. canis infection might be controlled by regulating the activation of inflammasomes.

Functions of NOD-Like Receptors in Human Diseases

Nucleotide-binding and oligomerization domain NOD-like receptors (NLRs) are highly conserved cytosolic pattern recognition receptors that perform critical functions in surveying the intracellular environment for the presence of infection, noxious substances, and metabolic perturbations. Sensing of these danger signals by NLRs leads to their oligomerization into large macromolecular scaffolds and the rapid deployment of effector signaling cascades to restore homeostasis. While some NLRs operate by recruiting and activating inflammatory caspases into inflammasomes, others trigger inflammation via alternative routes including the nuclear factor-κB, mitogen-activated protein kinase, and regulatory factor pathways. The critical role of NLRs in development and physiology is demonstrated by their clear implications in human diseases. Mutations in the genes encoding NLRP3 or NLRP12 lead to hereditary periodic fever syndromes, while mutations in CARD15 that encodes NOD2 are linked to Crohn’s disease or Blau’s syndrome. Genome-wide association studies (GWASs) have identified a number of risk alleles encompassing NLR genes in a host of diseases including allergic rhinitis, multiple sclerosis, inflammatory bowel disease, asthma, multi-bacillary leprosy, vitiligo, early-onset menopause, and bone density loss in elderly women. Animal models have allowed the characterization of underlying effector mechanisms in a number of cases. In this review, we highlight the functions of NLRs in health and disease and discuss how the characterization of their molecular mechanisms provides new insights into therapeutic strategies for the management of inflammatory pathologies.

Unsolved Mysteries in NLR Biology

NOD-like receptors (NLRs) are a class of cytoplasmic pattern-recognition receptors. Although most NLRs play some role in immunity, their functions range from regulating antigen presentation (NLRC5, CIITA) to pathogen/damage sensing (NLRP1, NLRP3, NLRC1/2, NLRC4) to suppression or modulation of inflammation (NLRC3, NLRP6, NLRP12, NLRX1). However, NLRP2, NLRP5, and NLRP7 are also involved in non-immune pathways such as embryonic development. In this review, we highlight some of the least well-understood aspects of NLRs, including the mechanisms by which they sense pathogens or damage. NLRP3 recognizes a diverse range of stimuli and numerous publications have presented potential unifying models for NLRP3 activation, but no single mechanism proposed thus far appears to account for all possible NLRP3 activators. Additionally, NLRC3, NLRP6, and NLRP12 inhibit NF-κB activation, but whether direct ligand sensing is a requirement for this function is not known. Herein, we review the various mechanisms of sensing and activation proposed for NLRP3 and other inflammasome activators. We also discuss the role of NLRC3, NLRP6, NLRP12, and NLRX1 as inhibitors and how they are activated and function in their roles to limit inflammation. Finally, we present an overview of the emerging roles that NLRP2, NLRP5, and NLRP7 play during embryonic development and postulate on the potential pathways involved.

Activation and regulation of the inflammasomes

Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform - the apoptosome - that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

NLRP6 in infection and inflammation

NLRs play fundamental roles in host-defense and inflammatory disorders. NLRP6 is a newly characterized member of this family that inhibits NF-κB and MAP-kinase dependent immune signaling to hamper anti-microbial defense. Further, NLRP6 regulates intestinal inflammation by maintaining gut microbiota composition. In this review, we examine the recent studies and emphasize the key functions regulated by NLRP6.

Genome-wide association studies of allergic diseases

Allergic diseases are complex diseases caused by a combination of genetic and environmental factors. To determine the genetic components of these diseases and to discover the genes and cellular pathways underlying them, a large number of genetic studies have been conducted. Progress in genetics enables us to conduct genome-wide association studies (GWASs), which is a comprehensive and unbiased approach to identify susceptibility loci for multifactorial diseases. Recent GWASs have convincingly detected a large number of loci associated with allergic diseases. Candidate genes in the susceptibility loci suggest roles for epithelial barrier functions, innate-adaptive immunity, IL-1 family signaling, regulatory T cells and the vitamin D pathway in the pathogenesis of allergic diseases. Interestingly, the IL1RL1, HLA, IL13 and C11orf30 regions are overlapping susceptibility loci among atopic dermatitis and asthma or allergic rhinitis. Although a more complete collection of associated genes and pathways is needed, biologic insights revealed by GWASs improve our understanding of the pathophysiology of human allergic diseases and contribute to the development of better treatment and preventive strategies.