Nod-like receptor pyrin domain-containing protein 6 (NLRP6) controls epithelial self-renewal and colorectal carcinogenesis upon injury. Proc Natl Acad Sci U S A. 2011;108:9601-9606. [PMID: 21593405 DOI: 10.1073/pnas.1100981108]

Immune reaction and colorectal cancer: Friends or foes?

The potential clinical impact of enhancing antitumor immunity is increasingly recognized in oncology therapeutics for solid tumors. Colorectal cancer is one of the most studied neoplasms for the tumor-host immunity relationship. Although immune cell populations involved in such a relationship and their prognostic role in colorectal cancer development have clearly been identified, still no approved therapies based on host immunity intensification have so far been introduced in clinical practice. Moreover, a recognized risk in enhancing immune reaction for colitis-associated colorectal cancer development has limited the emphasis of this approach. The aim of the present review is to discuss immune components involved in the host immune reaction against colorectal cancer and analyze the fine balance between pro-tumoral and anti-tumoral effect of immunity in this model of disease.

Human fecal microbiome-based biomarkers for colorectal cancer

Colorectal cancer may develop slowly over years from precursor lesions, and thus screening combined with early diagnosis is the key to disease prevention. Recent studies have elucidated specific traits in the gut microbiome associated with colorectal cancer and suggested that the microbiome may be useful in screening for colorectal cancer purposes but failed to provide protocols that can be applied in a practical situation. A recent study by Zackular and colleagues, presented on page 1112, provides an important way forward here in showing that specific analysis of multiple aspects of the microbiome composition in toto provides reliable detection of both precancerous and cancerous lesions. This important achievement when combined with other noninvasive techniques promises to provide highly effective tools for early colorectal cancer diagnosis and its prevention.

Diet, metabolites, and "western-lifestyle" inflammatory diseases

One explanation for the increased incidence of allergies, asthma, and even some autoimmune diseases has been the hygiene hypothesis. However, recent studies also highlight an important role for diet and bacterial metabolites in controlling various immune pathways, including gut and immune homeostasis, regulatory T cell biology, and inflammation. Dietary-related metabolites engage "metabolite-sensing" G-protein-coupled receptors, such as GPR43, GPR41, GPR109A, GPR120, and GPR35. These receptors are expressed on immune cells and some gut epithelial cells and generally mediate a direct anti-inflammatory effect. Insufficient intake of "healthy foodstuffs" adversely affects the production of bacterial metabolites. These metabolites and those derived directly from food drive beneficial downstream effects on immune pathways. We propose that insufficient exposure to dietary and bacterial metabolites might underlie the development of inflammatory disorders in Western countries. This review highlights what is currently known about diet, metabolites, and their associated immune pathways in relation to the development of inflammatory disease.

Beyond canonical inflammasomes: emerging pathways in IL-1-mediated autoinflammatory disease

In recent years, non-communicable chronic diseases that are potentiated by sterile inflammation have replaced infectious diseases as the major threat to human health. Sterile inflammation that results from aberrant tissue damage plays pivotal roles in the pathogenesis of numerous acute and chronic inflammatory diseases including atherosclerosis, type 2 diabetes, cancer, obesity, and multiple neurodegenerative diseases. The cellular events and molecular signaling pathways that govern sterile inflammation currently remain poorly defined; however, emerging data suggest central roles for IL-1 in driving autoimmune and inflammatory disease pathogenesis. Improved characterization of the immunological pathways that contribute to sterile inflammation are desperately needed to develop effective therapeutics to treat these devastating diseases. In this review, we discuss recent advances in our understanding of how IL-1 is regulated in response to tissue damage. In particular, we highlight recent studies that describe novel roles for conventional cell death molecules in the regulation of IL-1β production.

Inflammasome activation and metabolic disease progression

Innate pattern recognition receptors NLRs are cytosolic sensors that detect endogenous metabolic stress and form a multiprotein complex called the inflammasome, that recruits and activates caspase enzymes mediating the activation of the cytokines IL-1β and IL-18. The innate immune system and metabolic system are evolutionarily conserved, intimately integrated, and functionally dependent. In recent decades, obesity-associated metabolic diseases have been become a worldwide epidemic. Here we review recent evidence that demonstrates the important roles of NLRs and inflammasomes in response to metabolic stress in different tissues.

The fire within: microbes inflame tumors

The immune system and the microbiota mutually interact to maintain homeostasis in the intestine. However, components of the microbiota can alter this balance and promote chronic inflammation, promoting intestinal tumor development. We review recent advances in understanding the complex interactions between the microbiota and the innate and adaptive immune systems and discuss their potential to lead us in new directions for understanding cancer biology and treatment.

NOD-Like Receptors: Master Regulators of Inflammation and Cancer

Cytosolic NOD-like receptors (NLRs) have been associated with human diseases including infections, cancer, and autoimmune and inflammatory disorders. These innate immune pattern recognition molecules are essential for controlling inflammatory mechanisms through induction of cytokines, chemokines, and anti-microbial genes. Upon activation, some NLRs form multi-protein complexes called inflammasomes, while others orchestrate caspase-independent nuclear factor kappa B (NF-κB) and mitogen activated protein kinase (MAPK) signaling. Moreover, NLRs and their downstream signaling components engage in an intricate crosstalk with cell death and autophagy pathways, both critical processes for cancer development. Recently, increasing evidence has extended the concept that chronic inflammation caused by abberant NLR signaling is a powerful driver of carcinogenesis, where it abets genetic mutations, tumor growth, and progression. In this review, we explore the rapidly expanding area of research regarding the expression and functions of NLRs in different types of cancers. Furthermore, we particularly focus on how maintaining tissue homeostasis and regulating tissue repair may provide a logical platform for understanding the liaisons between the NLR-driven inflammatory responses and cancer. Finally, we outline novel therapeutic approaches that target NLR signaling and speculate how these could be developed as potential pharmaceutical alternatives for cancer treatment.

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.

Inflammasome-microbiota interplay in host physiologies

Host defense responses against microbes are most often thought of in terms of effectors of microbial destruction. However, recent evidence demonstrates that the more complex interactions between the microbiota and innate immune mechanisms, such as the inflammasome-mediated response, cannot be readily explained within just the traditional paradigms of microbial killing mechanisms. In this review, the concepts of both resistance and tolerance are applied to inflammasome-microbiota interactions, and the various physiological consequences of this interplay, including roles in inflammation, tissue repair, tumorigenesis, and metabolism, are discussed.

Progress in understanding the relationship between intestinal microbial flora and colorectal cancer

There is a great density of and many kinds of bacteria in the human gut. They participate in a number of human physiological activities and metabolism and are closely related to human health and diseases. Together with other changes, a part of gut bacteria and their products cause inflammation and finally lead to the formation of colorectal cancer through binding to the receptors and stimulating the secretion of inflammatory cytokines. Probiotics protect the intestinal mucosa through the barrier effect and by inhibiting DNA damage, are beneficial to human beings, prevent the occurrence of many diseases and play a role in adjuvant treatment of certain disease. In addition, the positive effect of prebiotics and synbiotics on human health can not be ignored. Overall, human and intestinal microbial flora interact with and influence each other, which is very important for human health and thus deserves more detailed research to guide future clinical work.

CCL17 production by dendritic cells is required for NOD1-mediated exacerbation of allergic asthma

RATIONALE

Pattern recognition receptors are attractive targets for vaccine adjuvants, and polymorphisms of the innate receptor NOD1 have been associated with allergic asthma.

OBJECTIVES

To elucidate whether NOD1 agonist may favor allergic asthma in humans through activation of dendritic cells, and to evaluate the mechanisms involved using an in vivo model.

METHODS

NOD1-primed dendritic cells from allergic and nonallergic donors were characterized in vitro on their phenotype, cytokine secretion, and Th2 polarizing ability. The in vivo relevance was examined in experimental allergic asthma, and the mechanisms were assessed using transfer of NOD1-conditioned dendritic cells from wild-type or CCL17-deficient mice.

MEASUREMENTS AND MAIN RESULTS

NOD1 priming of human dendritic cells promoted a Th2 polarization profile that involved the production of CCL17 and CCL22 in nonallergic subjects but only CCL17 in allergic patients, without requiring allergen costimulation. Moreover, NOD1-primed dendritic cells from allergic donors exhibited enhanced maturation that led to abnormal CCL22 and IL-10 secretion compared with nonallergic donors. In mice, systemic NOD1 ligation exacerbated allergen-induced experimental asthma by amplifying CCL17-mediated Th2 responses in the lung. NOD1-mediated sensitization of purified murine dendritic cells enhanced production of CCL17 and CCL22, but not of thymic stromal lymphopoietin and IL-33, in vitro. Consistently, adoptive transfer of NOD1-conditioned dendritic cells exacerbated the Th2 pulmonary response in a CCL17-dependent manner in vivo.

CONCLUSIONS

Data from this study unveil a deleterious role of NOD1 in allergic asthma through direct induction of CCL17 by dendritic cells, arguing for a need to address vaccine formulation safety issues related to allergy.

NOD-like receptors in intestinal homeostasis and epithelial tissue repair

The intestinal epithelium constitutes a dynamic physical barrier segregating the luminal content from the underlying mucosal tissue. Following injury, the epithelial integrity is restored by rapid migration of intestinal epithelial cells (IECs) across the denuded area in a process known as wound healing. Hence, through a sequence of events involving restitution, proliferation and differentiation of IECs the gap is resealed and homeostasis reestablished. Relapsing damage followed by healing of the inflamed mucosa is a hallmark of several intestinal disorders including inflammatory bowel diseases (IBD). While several regulatory peptides, growth factors and cytokines stimulate restitution of the epithelial layer after injury, recent evidence in the field underscores the contribution of innate immunity in controlling this process. In particular, nucleotide-binding and oligomerization domain-like receptors (NLRs) play critical roles in sensing the commensal microbiota, maintaining homeostasis, and regulating intestinal inflammation. Here, we review the process of intestinal epithelial tissue repair and we specifically focus on the impact of NLR-mediated signaling mechanisms involved in governing epithelial wound healing during disease.

NLRP6 inflammasome orchestrates the colonic host-microbial interface by regulating goblet cell mucus secretion

Mucus production by goblet cells of the large intestine serves as a crucial antimicrobial protective mechanism at the interface between the eukaryotic and prokaryotic cells of the mammalian intestinal ecosystem. However, the regulatory pathways involved in goblet cell-induced mucus secretion remain largely unknown. Here, we demonstrate that the NLRP6 inflammasome, a recently described regulator of colonic microbiota composition and biogeographical distribution, is a critical orchestrator of goblet cell mucin granule exocytosis. NLRP6 deficiency leads to defective autophagy in goblet cells and abrogated mucus secretion into the large intestinal lumen. Consequently, NLRP6 inflammasome-deficient mice are unable to clear enteric pathogens from the mucosal surface, rendering them highly susceptible to persistent infection. This study identifies an innate immune regulatory pathway governing goblet cell mucus secretion, linking nonhematopoietic inflammasome signaling to autophagy and highlighting the goblet cell as a critical innate immune player in the control of intestinal host-microbial mutualism. PAPERCLIP:

Gut microbiome and anticancer immune response: really hot Sh*t!

The impact of gut microbiota in eliciting innate and adaptive immune responses beneficial for the host in the context of effective therapies against cancer has been highlighted recently. Chemotherapeutic agents, by compromising, to some extent, the intestinal integrity, increase the gut permeability and selective translocation of Gram-positive bacteria in secondary lymphoid organs. There, anticommensal pathogenic Th17 T-cell responses are primed, facilitating the accumulation of Th1 helper T cells in tumor beds after chemotherapy as well as tumor regression. Importantly, the redox equilibrium of myeloid cells contained in the tumor microenvironment is also influenced by the intestinal microbiota. Hence, the anticancer efficacy of alkylating agents (such as cyclophosphamide) and platinum salts (oxaliplatin, cis-platin) is compromised in germ-free mice or animals treated with antibiotics. These findings represent a paradigm shift in our understanding of the mode of action of many compounds having an impact on the host-microbe mutualism.

NLRPs, microbiota, and gut homeostasis: unravelling the connection

Within the NOD-like receptor (NLR) family, there are several NLRP (NLR family, pyrin domain-containing) proteins that are involved in the formation of inflammasomes. These multi-protein complexes are a key part of the network of cellular events required for secretion of the pro-inflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome is the best-characterized member of the family and has recently been implicated in gut homeostasis and determining the severity of inflammation in inflammatory bowel disease (IBD) and inflammation-associated colorectal cancer. This led to the discovery that NLRP6 and NLRP12 also contribute to the maintenance of intestinal homeostasis and modulation of the gut microbiota, which in turn influences the intestine and distant organs. In this review, we bring together the latest data on the potential roles of NLRP family members in gut health and disease and identify the most pressing questions that remain to be answered to further our understanding of human diseases including IBD, inflammation-associated cancers, and metabolic syndromes linked with obesity.

Nod-like receptors: key molecular switches in the conundrum of cancer

It is believed the immune system can contribute to oncogenic transformation especially in settings of chronic inflammation, be activated during immunosurveillance to destroy early neoplastic cells before they undergo malignant outgrowth, and finally, can assist growth of established tumors by preventing clearance, remodeling surrounding tissue, and promoting metastatic events. These seemingly opposing roles of the immune system at the different stages of cancer development must all be mediated by innate signaling mechanisms that regulate the overall state of immune activation. Recently, the cytosolic nod-like receptor (NLR) pathway of innate immunity has gained a lot of attention in the tumor immunology field due to its known involvement in promoting inflammation and immunity, and conversely, in regulating tissue repair processes. In this review, we present all the current evidence for NLR involvement in the different stages of neoplasia to understand how a single molecular pathway can contribute to conflicting immunological interactions with cancer.

Non-Inflammasome Forming NLRs in Inflammation and Tumorigenesis

Aberrant inflammation is an enabling characteristic of tumorigenesis. Thus, signaling cascades that alter inflammatory activation and resolution are of specific relevance to disease pathogenesis. Pattern recognition receptors (PRRs) are essential mediators of the host immune response and have emerged as critical elements affecting multiple facets of tumor pathobiology. The nucleotide-binding domain and leucine-rich repeat containing (NLR) proteins are intracellular PRRs that sense microbial and non-microbial products. Members of the NLR family can be divided into functional sub-groups based on their ability to either positively or negatively regulate the host immune response. Recent studies have identified a novel sub-group of non-inflammasome forming NLRs that negatively regulate diverse biological pathways associated with both inflammation and tumorigenesis. Understanding the mechanisms underlying the function of these unique NLRs will assist in the rationale design of future therapeutic strategies targeting a wide spectrum of inflammatory diseases and cancer. Here, we will discuss recent findings associated with this novel NLR sub-group and mechanisms by which these PRRs may function to alter cancer pathogenesis.

The complex role of inflammasomes in the pathogenesis of Inflammatory Bowel Diseases - lessons learned from experimental models

Inflammasomes are a large family of multiprotein complexes recognizing pathogen-associated molecular pattern molecules (PAMPs) and damage-associated molecular patterns (DAMPs). This leads to caspase-1 activation, promoting the secretion of mature IL-1β, IL-18 and under certain conditions even induce pyroptosis. Inflammatory Bowel Diseases (IBD) is associated with alterations in microbiota composition, inappropriate immune responses and genetic predisposition associated to bacterial sensing and autophagy. Besides their acknowledged role in mounting microbial induced host responses, a crucial role in maintenance of intestinal homeostasis was revealed in inflammasome deficient mice. Further, abnormal activation of these functions appears to contribute to the pathology of intestinal inflammation including IBD and colitis-associated cancer. Herein, the current literature implicating the inflammasomes, microbiota and IBD is comprehensively reviewed.

Intestinal epithelial cells: regulators of barrier function and immune homeostasis

The abundance of innate and adaptive immune cells that reside together with trillions of beneficial commensal microorganisms in the mammalian gastrointestinal tract requires barrier and regulatory mechanisms that conserve host-microbial interactions and tissue homeostasis. This homeostasis depends on the diverse functions of intestinal epithelial cells (IECs), which include the physical segregation of commensal bacteria and the integration of microbial signals. Hence, IECs are crucial mediators of intestinal homeostasis that enable the establishment of an immunological environment permissive to colonization by commensal bacteria. In this Review, we provide a comprehensive overview of how IECs maintain host-commensal microbial relationships and immune cell homeostasis in the intestine.

NLRC3, a member of the NLR family of proteins, is a negative regulator of innate immune signaling induced by the DNA sensor STING

Stimulator of interferon genes (STING, also named MITA, MYPS, or ERIS) is an intracellular DNA sensor that induces type I interferon through its interaction with TANK-binding kinase 1 (TBK1). Here we found that the nucleotide-binding, leucine-rich-repeat-containing protein, NLRC3, reduced STING-dependent innate immune activation in response to cytosolic DNA, cyclic di-GMP (c-di-GMP), and DNA viruses. NLRC3 associated with both STING and TBK1 and impeded STING-TBK1 interaction and downstream type I interferon production. By using purified recombinant proteins, we found NLRC3 to interact directly with STING. Furthermore, NLRC3 prevented proper trafficking of STING to perinuclear and punctated region, known to be important for its activation. In animals, herpes simplex virus 1 (HSV-1)-infected Nlrc3(-/-) mice exhibited enhanced innate immunity and reduced morbidity and viral load. This demonstrates the intersection of two key pathways of innate immune regulation, NLR and STING, to fine tune host response to intracellular DNA, DNA virus, and c-di-GMP.

Inflammasomes in cancer: a double-edged sword

Chronic inflammatory responses have long been observed to be associated with various types of cancer and play decisive roles at different stages of cancer development. Inflammasomes, which are potent inducers of interleukin (IL)-1β and IL-18 during inflammation, are large protein complexes typically consisting of a Nod-like receptor (NLR), the adapter protein ASC, and Caspase-1. During malignant transformation or cancer therapy, the inflammasomes are postulated to become activated in response to danger signals arising from the tumors or from therapy-induced damage to the tumor or healthy tissue. The activation of inflammasomes plays diverse and sometimes contrasting roles in cancer promotion and therapy depending on the specific context. Here we summarize the role of different inflammasome complexes in cancer progression and therapy. Inflammasome components and pathways may provide novel targets to treat certain types of cancer; however, using such agents should be cautiously evaluated due to the complex roles that inflammasomes and pro-inflammatory cytokines play in immunity.

Identification of mutations in the PYRIN-containing NLR genes (NLRP) in Head and Neck Squamous Cell Carcinoma

Head and Neck Squamous Cell Carcinoma (HNSCC) encompasses malignancies that arise in the mucosa of the upper aerodigestive tract. Recent high throughput DNA sequencing revealed HNSCC genes mutations that contribute to several cancer cell characteristics, including dysregulation of cell proliferation and death, intracellular proinflammatory signaling, and autophagy. The PYRIN-domain containing NLR (Nucleotide-binding domain, Leucine rich Repeats – containing) proteins have recently emerged as pivotal modulators of cell death, autophagy, inflammation, and metabolism. Their close physiologic association with cancer development prompted us to determine whether mutations within the NLRP (PYRIN-containing NLR) gene family were associated with HNSCC genome instability and their clinicopathologic correlations. Catastrophic mutational events underlie cancer cell genome instability and mark a point-of-no-return in cancer cell development and generation of heterogeneity. The mutation profiles of 62 patients with primary conventional type HNSCC excluding other histologic variants were analyzed. Associations were tested using Fisher's Exact test or Mann-Whitney U test. Mutations in NLRP were associated with elevated genome instability as characterized by higher mutation rates. Clinically, NLRP mutations were more frequently found in HNSCC arising in the floor of mouth (50.0%) in comparison with HNSCC at other head and neck locations (14.8%). These mutations were clustered at the leucine rich repeats region of NLRP proteins, and affected NLRP genes were mostly localized at chromosomes 11p15.4 and 19q13.42-19q13.43. Twenty novel NLRP mutations were identified in HNSCC, and mutations in this group of genes were correlated with increased cancer cell genome mutation rates, and such features could be a potential molecular biomarker of HNSCC genome instability.

Role of Nlrp6 and Nlrp12 in the maintenance of intestinal homeostasis

There has been significant interest in understanding how interactions between the host immune system and the gut microbiota regulate intestinal homeostasis. Recent data suggest that the Nod-like receptor (NLR) family of PRRs regulate both the composition of the gut microbiota and innate immune signaling pathways that prevent pathologic intestinal inflammation and tumorigenesis. In this review, we will focus on NLRP6 and NLRP12, two members of the NLR family that have emerged as important players in the maintenance of intestinal homeostasis, and discuss the signaling pathways engaged by these receptors as well as the current models of how these receptors protect against the development of colitis and tumorigenesis.

Debug Your Bugs - How NLRs Shape Intestinal Host-Microbe Interactions

The host's ability to discriminate friend and foe and to establish a precise homeostasis with its associated microbiota is crucial for its survival and fitness. Among the mediators of intestinal host-microbe interactions, NOD-like receptor (NLR) proteins take center stage. They are present in the epithelial lining and innate immune cells that constantly monitor microbial activities at the intestinal barrier. Dysfunctional NLRs predispose to intestinal inflammation as well as sensitization to extra-intestinal immune-mediated diseases and are linked to the alteration of microbial communities. Here, we review advances in our understanding of their reciprocal relationship in the regulation of intestinal homeostasis and implications for intestinal health.

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.

Interactions between Nod-Like Receptors and Intestinal Bacteria

Nucleotide oligomerization domain (Nod)-like Receptors (NLRs) are cytosolic sensors that mediate the activation of Caspase-1 and the subsequent processing and secretion of the pro-inflammatory cytokines IL-1β and IL-18, as well as an inflammatory cell death termed pyroptosis. While a multitude of bacteria have been shown to activate one or more NLRs under in vitro conditions, the exact impact of NLR activation during the course of colonization, both of pathogenic and commensal nature, is less understood. In this review, we will focus on the role of intestinal NLRs during the various stages of infection with common gastrointestinal bacterial pathogens, as well as NLR function in controlling and shaping the microbiota.

Alarmins, inflammasomes and immunity

The elaboration of an effective immune response against pathogenic microbes such as viruses, intracellular bacteria or protozoan parasites relies on the recognition of microbial products called pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs). Ligation of the PRRs leads to synthesis and secretion of pro-inflammatory cytokines and chemokines. Infected cells and other stressed cells also release host-cell derived molecules, called damage-associated molecular patterns (DAMPs, danger signals, or alarmins), which are generic markers for damage. DAMPs are recognized by specific receptors on both immune and nonimmune cells, which, depending on the target cell and the cellular context, can lead to cell differentiation or cell death, and either inflammation or inhibition of inflammation. Recent research has revealed that DAMPs and PAMPs synergize to permit secretion of pro-inflammatory cytokines such as interleukin-1β (IL-1β): PAMPs stimulate synthesis of pro-IL-1β, but not its secretion; while DAMPs can stimulate assembly of an inflammasome containing, usually, a Nod-like receptor (NLR) member, and activation of the protease caspase-1, which cleaves pro-IL-1β into IL-1β, allowing its secretion. Other NLR members do not participate in formation of inflammasomes but play other essential roles in regulation of the innate immune response.

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.

Mechanisms of NOD-like receptor-associated inflammasome activation

A major function of a subfamily of NLR (nucleotide-binding domain, leucine-rich repeat containing, or NOD-like receptor) proteins is in inflammasome activation, which has been implicated in a multitude of disease models and human diseases. This work will highlight key progress in understanding the mechanisms that activate the best-studied NLRs (NLRP3, NLRC4, NAIP, and NLRP1) and in uncovering inflammasome NLRs.

NOD-Like Receptors in Lung Diseases

The lung is a particularly vulnerable organ at the interface of the body and the exterior environment. It is constantly exposed to microbes and particles by inhalation. The innate immune system needs to react promptly and adequately to potential dangers posed by these microbes and particles, while at the same time avoiding extensive tissue damage. Nucleotide-binding oligomerization domain-like receptors (NLRs) represent a group of key sensors for microbes and damage in the lung. As such they are important players in various infectious as well as acute and chronic sterile inflammatory diseases, such as pneumonia, chronic obstructive pulmonary disease (COPD), acute lung injury/acute respiratory distress syndrome, pneumoconiosis, and asthma. Activation of most known NLRs leads to the production and release of pro-inflammatory cytokines, and/or to the induction of cell death. We will review NLR functions in the lung during infection and sterile inflammation.

Beneficial and Detrimental Roles of NLRs in Carcinogenesis

Inflammation plays a critical role in tumorigenesis and can contribute to oncogenic mutations, tumor promotion, and angiogenesis. Tumor-promoting inflammation is driven by many factors including the presence of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18. One major source of IL-1β and IL-18 secretion is through the activation of inflammasomes. Inflammasomes are multi-protein complexes that upon activation lead to the processing and secretion of IL-1β and IL-18 mediated by the cysteine protease caspase-1. Several inflammasomes, including NLRP3, NLRC4, and NLRP6, have been implicated in tumorigenesis. However, inflammasomes play divergent roles in different types of cancer reflecting the complexity of inflammation during tumorigenesis. Understanding the role of inflammasome activation during specific stages of tumorigenesis and also during cancer immunotherapy will help identify novel therapeutic targets that could improve treatment strategies for cancer patients. Here we will discuss recent advances in understanding the mechanism by which NLRs regulate carcinogenesis.

Inflammation-induced cancer: crosstalk between tumours, immune cells and microorganisms

Inflammation is a fundamental innate immune response to perturbed tissue homeostasis. Chronic inflammatory processes affect all stages of tumour development as well as therapy. In this Review, we outline the principal cellular and molecular pathways that coordinate the tumour-promoting and tumour-antagonizing effects of inflammation and we discuss the crosstalk between cancer development and inflammatory processes. In addition, we discuss the recently suggested role of commensal microorganisms in inflammation-induced cancer and we propose that understanding this microbial influence will be crucial for targeted therapy in modern cancer treatment.

Gut microbiota protects against gastrointestinal tumorigenesis caused by epithelial injury

Inflammation is a critical player in the development of both colitis-associated and sporadic colon cancers. Several studies suggest that the microbiota contribute to inflammation and tumorigenesis; however, studies to understand the role of the microbiota in colon tumor development in germ-free (GF) mice are limited. We therefore studied the effects of the microbiota on the development of inflammation and tumors in GF and conventionally raised specific pathogen-free (SPF) mice treated with azoxymethane (AOM) and dextran sulfate sodium (DSS). We discovered that GF mice developed significantly more and larger tumors compared with that in SPF mice after AOM and DSS treatment despite the lack of early acute inflammation in response to chemically induced injury by DSS. Although the extent of intestinal epithelial damage and apoptosis was not significantly different in GF and SPF mice, there was a delay in intestinal epithelial repair to DSS-induced injury in GF mice resulting in a late onset of proinflammatory and protumorigenic responses and increased epithelial proliferation and microadenoma formation. Recolonization of GF mice with commensal bacteria or administration of lipopolysaccharide reduced tumorigenesis. Thus, although commensal bacteria are capable of driving chronic inflammation and tumorigenesis, the gut microbiota also have important roles in limiting chemically induced injury and proliferative responses that lead to tumor development.

An epithelial armamentarium to sense the microbiota

Intestinal epithelial cells were once thought to be inert, non-responsive cells that simply acted as a physical barrier that prevents the contents of the intestinal lumen from accessing the underlying tissue. However, it is now clear that these cells express a full repertoire of Toll- and Nod-like receptors, and that their activation by components of the microbiota is vital for the development of a functional epithelium, maintenance of barrier integrity, and defense against pathogenic organisms. Additionally, mounting evidence suggests that epithelial sensing of bacteria plays a significant role in the management of the numbers and types of microbes present in the gut microbiota via the production of antimicrobial peptides and other microbe-modulatory products. This is a critical process, as it is now becoming apparent that alterations in the composition of the microbiota can predispose an individual to a wide variety of chronic diseases. In this review, we will discuss the bacterial pattern recognition receptors that are known to be expressed by the intestinal epithelium, and how each of them individually contributes to these vital protective functions. Moreover, we will review what is known about the communication between epithelial cells and various classes of underlying leukocytes, and discuss how they interact with the microbiota to form a three-part relationship that maintains homeostasis in the gut.

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.

CD90(+) Stromal Cells are Non-Professional Innate Immune Effectors of the Human Colonic Mucosa

Immune responses at the intestinal mucosa must allow for host protection whilst simultaneously avoiding inappropriate inflammation. Although much work has focused on the innate immune functionality of hematopoietic immune cells, non-hematopoietic cell populations – including epithelial and stromal cells – are now recognized as playing a key role in innate defense at this site. In this study we examined the innate immune capacity of primary human intestinal stromal cells (iSCs). CD90⁺ iSCs isolated from human colonic mucosa expressed a wide array of innate immune receptors and functionally responded to stimulation with bacterial ligands. iSCs also sensed infection with live Salmonella typhimurium, rapidly expressing IL-1 family cytokines via a RIPK2/p38MAPK-dependent signaling process. In addition to responding to innate immune triggers, primary iSCs exhibited a capacity for bacterial uptake, phagocytosis, and antigen processing, although to a lesser extent than professional APCs. Thus CD90⁺ iSCs represent an abundant population of “non-professional” innate immune effector cells of the human colonic mucosa and likely play an important adjunctive role in host defense and immune regulation at this site.

Microbiota regulation of inflammatory bowel disease and colorectal cancer

The host and microbiota have evolved mechanisms for coexistence over millions of years. Accumulating evidence indicates that a dynamic mutualism between the host and the commensal microbiota has important implications for health, and microbial colonization contributes to the maintenance of intestinal immune homeostasis. However, alterations in communication between the mucosal immune system and gut microbial communities have been implicated as the core defect that leads to chronic intestinal inflammation and cancer development. We will discuss the recent progress on how gut microbiota regulates intestinal homeostasis and the pathogenesis of inflammatory bowel disease and colorectal cancer.

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.

Impact of the gut microbiome on mucosal inflammation

In the past 10 years it has become increasingly apparent that the gut microbiome has profound effects on the immune system to which it is juxtaposed, the mucosal immune system. Here, I explore recent studies in which the effects of the microbiota expand or facilitate anti-inflammatory or regulatory immunological machinery or which favor development of proinflammatory immunological machinery in this system. I then focus on how these opposing processes play out in inflammatory bowel disease (IBD); a disease in which normal immune homeostasis is disturbed and inflammation takes hold.

Inflammasomes in inflammatory bowel disease pathogenesis

PURPOSE OF REVIEW

Inflammasomes are molecular platforms assembled in response to infection or danger signals, and they regulate the activation of caspase-1 and the maturation of the inflammatory cytokines IL-1β and IL-18. In this review, we will summarize the centrality of Nod-like receptor proteins that assemble inflammasomes and regulate intestinal homeostasis by controlling host defense responses, microbiota composition, intestinal inflammation and tissue damage.

RECENT FINDINGS

In the intestine, the innate immune system evolved to tolerate commensal microorganisms while maintaining the capacity to trigger host defense responses to invading pathogens. Recent findings suggest that inflammasomes play a critical role in the intricate interplay between the local microbial community and the mucosal immune system by sensing commensal bacteria, regulating microbial ecology, establishing the host defense response that discriminates pathogenic from commensal microbes and preventing the emergence of pathobionts. A model to reconcile the conflicting results in the literature on the role of inflammasomes in experimental colitis will be discussed.

SUMMARY

A better understanding of the relationship between inflammasome signaling and the intestinal microbiota might provide insight into the complex interaction of the innate immune system with the intestinal environment, and identify new approaches for the treatment of inflammatory bowel disease and gastrointestinal cancer.

Stress-induced corticotropin-releasing hormone-mediated NLRP6 inflammasome inhibition and transmissible enteritis in mice

BACKGROUND & AIMS

Stress alters brain-gut interactions and could exacerbate intestinal disorders, including irritable bowel syndrome. Alterations in the intestinal microbiota have been associated with irritable bowel syndrome. Maintenance of healthy microbiota requires nucleotide-binding oligomerization domain protein-like receptors, pyrin-domain containing (NLRP)-6 inflammasomes. We investigated the involvement of NLRP6 in water-avoidance stress (WAS)-induced intestinal disorders in mice.

METHODS

B57BL6 mice were subjected to WAS for 1 hour each day for 10 days; body weights and intestinal inflammation and permeability were analyzed. We investigated signaling via the NLRP3 and NLRP6 inflammasomes, and the role of corticotropin-releasing hormone (CRH) in WAS-associated inflammation and NLRP6 inhibition. Mice that were not exposed to stress were co-housed with mice subjected to WAS to determine the effects of WAS-induced dysbiosis, measured by sequencing bacterial 16S ribosomal RNA. We also assessed the effects of a peroxisome proliferator-activated receptor-γ agonist and probiotics.

RESULTS

WAS-induced small-bowel inflammation (enteritis) was associated with inhibition of NLRP6, but not NLRP3, and was prevented by a peroxisome proliferator-activated receptor-γ agonist, which induced epithelial expression of NLRP6. CRH was released during WAS and inhibited NLRP6 expression. WAS induced alterations in the gut microbiota of mice; co-housed nonstressed mice developed enteritis associated with increased CRH and decreased levels of NLRP6. Probiotic therapy reduced intestinal inflammation in mice with WAS-induced enteritis.

CONCLUSIONS

Exposure of mice to stress inhibits NLRP6 and alters the composition of the gut microbiota, leading to intestinal inflammation. These findings might explain the benefits of probiotics for patients with stress-associated gastrointestinal disorders.

Of inflammasomes and pathogens--sensing of microbes by the inflammasome

Inflammasomes are signalling platforms that sense a diverse range of microbial products and also a number of stress and damage associated endogenous signals. Inflammasome complexes can be formed by members of the Nod-like receptor family or the PYHIN family member AIM2. Upon formation, inflammasomes trigger proteolysis of caspase-1, which subsequently leads to a potent inflammatory response through the maturation and secretion of IL-1 family cytokines, which can be accompanied by an inflammatory cell death termed pyroptosis. Here, we review the sensing mechanisms of the currently characterized inflammasome complexes and discuss how they are involved in the innate immune response against microbial pathogens. We especially highlight recent advances in the molecular understanding of how microbial patterns are detected and discriminated from endogenous compounds by inflammasome sensors. Further, we review how inflammasomes contribute to the anti microbial host defense by cytokine-dependent and cell autonomous mechanisms.

Myofibroblast activation in colorectal cancer lymph node metastases

BACKGROUND

Myofibroblasts have an important role in regulating the normal colorectal stem cell niche. While the activation of myofibroblasts in primary colorectal cancers has been previously described, myofibroblast activation in lymph node metastases has not been described before.

METHODS

Paraffin-embedded lymph node sections from patients with macrometastases, micrometastases and isolated tumour cells were stained to identify myofibroblasts and to characterise the distribution of different cell types in tumour-containing lymph nodes. The extent of myofibroblast presence was quantified and compared with the size of the metastasis and degree of proliferation and differentiation of the cancer cells.

RESULTS

We show substantial activation of myofibroblasts in the presence of colorectal metastases in lymph nodes, which is intimately associated with glandular structures, both in micro- and macrometastases. The degree of activation is positively associated with the size of the metastases and the proportion of Ki67+ve cancer cells, and negatively associated with the degree of enterocyte differentiation as measured by CK20 expression.

CONCLUSION

The substantial activation of myofibroblasts in tumour-containing lymph nodes strongly suggests that these metastatic cancer cells are still significantly dependent on their microenvironment. Further understanding of these epithelial-mesenchymal interactions could lead to the development of new therapies in metastatic disease.

Inflammasome in intestinal inflammation and cancer

The activation of specific cytosolic pathogen recognition receptors, the nucleotide-binding-oligomerization-domain- (NOD-) like receptors (NLRs), leads to the assembly of the inflammasome, a multimeric complex platform that activates caspase-1. The caspase-1 pathway leads to the upregulation of important cytokines from the interleukin (IL)-1 family, IL-1β, and IL-18, with subsequent activation of the innate immune response. In this review, we discuss the molecular structure, the mechanisms behind the inflammasome activation, and its possible role in the pathogenesis of inflammatory bowel diseases and intestinal cancer. Here, we show that the available data points towards the importance of the inflammasome in the innate intestinal immune response, being the complex involved in the maintenance of intestinal homeostasis, correct intestinal barrier function and efficient elimination of invading pathogens.

Enterocyte loss of polarity and gut wound healing rely upon the F-actin-severing function of villin

Efficient wound healing is required to maintain the integrity of the intestinal epithelial barrier because of its constant exposure to a large variety of environmental stresses. This process implies a partial cell depolarization and the acquisition of a motile phenotype that involves rearrangements of the actin cytoskeleton. Here we address how polarized enterocytes harboring actin-rich apical microvilli undergo extensive cell remodeling to drive injury repair. Using live imaging technologies, we demonstrate that enterocytes in vitro and in vivo rapidly depolarize their microvilli at the wound edge. Through its F-actin-severing activity, the microvillar actin-binding protein villin drives both apical microvilli disassembly in vitro and in vivo and promotes lamellipodial extension. Photoactivation experiments indicate that microvillar actin is mobilized at the lamellipodium, allowing optimal migration. Finally, efficient repair of colonic mechanical injuries requires villin severing of F-actin, emphasizing the importance of villin function in intestinal homeostasis. Thus, villin severs F-actin to ensure microvillus depolarization and enterocyte remodeling upon injury. This work highlights the importance of specialized apical pole disassembly for the repolarization of epithelial cells initiating migration.