Abstract
Recognition of microorganisms by pattern-recognition receptors (PRRs) is the primary component of innate immunity that is responsible for the maintenance of host–microbial interactions in intestinal mucosa. Dysregulation in host–commensal interactions has been implicated as the central pathogenesis of inflammatory bowel disease (IBD), which predisposes to developing colorectal cancer. Recent animal studies have begun to outline some unique physiology and pathology involving each PRR signaling in the intestine. The major roles played by PRRs in the gut appear to be the regulation of the number and the composition of commensal bacteria, epithelial proliferation, and mucosal permeability in response to epithelial injury. In addition, PRR signaling in lamina propria immune cells may be involved in induction of inflammation in response to invasion of pathogens. Because some PRR-deficient mice have shown variable susceptibility to colitis, the outcome of intestinal inflammation may be modified depending on PRR signaling in epithelial cells, immune cells, and the composition of commensal flora. Through recent findings in animal models of IBD, this review will discuss how abnormal PRR signaling may contribute to the pathogenesis of inflammation and inflammation-associated tumorigenesis in the intestine.
Similar content being viewed by others
Introduction: The role of TLRs and NLRs in Healthy Gut—PRRs as Regulators of intestinal epithelial cell (IEC) Homeostatis
The innate immunity provides a primary host response to microbial invasion, which induces an inflammatory nidus to localize the infection and prevent systemic dissemination of pathogens. The key process in this is the recognition of microbial agents by pattern-recognition receptors (PRRs). The PRRs include Toll-like receptors (TLRs), nucleotide binding oligomerization domain (NOD)-like receptors (NLRs), RNA helicases (RIG-I (retinoid acid-inducible gene-I), MDA5 (melanoma differentiation-associated gene 5), and LGP2 (laboratory of genetics and physiology gene 2)), C-type lectin receptors, and cytosolic DNA sensors (DNA-dependent activator of IFN-regulatory factors, absent in melanoma-2, interferon inducible protein 16 (leucine-rich repeat flightless-interacting protein 1), RNA polymerase III, DExD/H box RNA helicases, and IFI16), which sense evolutionarily conserved pathogen-associated molecular patterns (PAMPs) of microorganisms.1 By detecting PAMPs, PRRs trigger sequential activation of intracellular signaling pathways leading to induction of a range of cytokines and chemokines that orchestrate the early host resistance to infection. Specifically, activation of NLRs results in the formation of a molecular scaffold complex (an inflammasome) that leads to the active release of interleukin (IL)-1β and IL-18 through caspase-1 activation (Figure 1). These PRRs signaling also facilitate the differentiation of T cells and B cells to establish antigen-specific adaptive immunity.2, 3, 4
The pattern-recognition receptor pathway inducing production of mature interleukin (IL)-18 and IL-1β. Upon recognition of pathogens, Toll-like receptor (TLR) signaling (mainly TLR4) induces transcription of IL-18 and IL-1β. Expression of mRNA for these genes produces pro-forms of IL-18 and IL-1β protein within the cytosol. At the same time, TLR signaling induces reactive oxygen species (ROS) generation and following release of mitochondrial DNA, which activates nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). NLRs are also activated by detecting various ligands introduced via phagocytosis followed by lysosomal stabilization, bacterial type III secretion system, or intracellular metabolic and/or enzymatic activities. The activated NLRs are homo- or hetero-oligomalized to form multi-protein complex termed “inflammasome” that activate caspase-1. Activated caspase-1 induces proteolytic conversion of pro-IL-18 and pro-IL-1β to mature IL-18 and IL-1β to release the active form of these cytokines. ASC, apoptosis-associated speck-like protein; CARD, caspase activation and recruitment domain; CIITA, MHC class II transcription activator; HET-E, incompatibility locus protein from podospora anserina; MAPK, mitogen-activated protein kinase; Mt DNA, mitochondrial DNA; NACHT, neuronal apoptosis inhibitory protein; NBD, nucleotide-binding domain; NF, nuclear factor; NLRP3, NLR protein 3; PYD, pyrin domain; TP1, telomerase-associated protein.
Since the discovery of TLRs as a major family of PRRs, it has been of great interest whether or not they are functionally expressed in intestinal epithelial interface and what roles they have in the gastrointestinal tract. Because of the unique nature of the gut where diverse microorganisms coexist, microbial-sensing TLRs may have special roles in mucosal homeostasis. Among the 13 TLRs discovered, TLR1 through TLR9 have been identified as being expressed in human IECs.5, 6 However, the functional consequences of these TLRs in healthy gut physiology have yet to be fully determined.
Although it has not been fully understood how the TLR responses in IECs are regulated in the face of commensal bacteria in the human intestine, in vitro data has demonstrated hyporesponsiveness of IECs to TLR2 and TLR4 ligands.5, 6 The underlying mechanism of this observation comprises a decrease in TLR surface expression and the induction of an inhibitory molecule of TLR signaling after ligand stimulation. Antigen-presenting cells in the lamina propria also appear to be unresponsive to TLR ligands.7 Other TLRs are normally expressed in endosomes (TLR3, TLR7 to TLR9) or basolateral membrane (TLR5), where these TLRs are not exposed to pathogens unless pathogens get into the cells or invade mucosa. NOD and NLRs are also expressed in the cytoplasm and thus do not recognize extracellular pathogens unless pathogens inject the cells’ effector proteins (Table 1). On the other hand, constitutive activation of selected TLR signaling in IECs may be required for epithelial homeostasis. IEC-specific deletion of the downstream molecules of TLR signaling such as myeloid differentiation primary response gene 88 (MyD88), transforming growth factor-β-activated kinase 1, and nuclear factor-κB essential modulator in mice results in spontaneous intestinal inflammation.8, 9, 10 TLR9 can be expressed on the apical or the basolateral sides of IECs, in which ligand recognition activates a tolerogenic or inflammatory responses, respectively.11 In addition to IECs, many TLRs are expressed and have unique roles in Paneth cells, goblet cells, and enteroendocrine cells in the intestine.12, 13, 14 These findings highlight a unique feature of PRRs in IECs that establishes selective responses and tolerance to the commensal flora at the mucosal interface.
Functionally, epithelial PRRs contribute to balancing the composition of luminal microorganisms by regulating the secretion of a range of antimicrobial peptides and mucosal immunoglobulin A (IgA). Mice deficient in MyD88 have demonstrated a significant defect in production of multiple antimicrobial peptides in Paneth cells, resulting in increased bacterial penetration to the mesenteric lymph nodes.12 TLR9−/− and NOD2−/− mice have impaired expression of Paneth cell cryptdin (mouse α-defensin) compared with wild-type (WT) mice.11, 15 Patients with Crohn’s disease (a chronic intestinal inflammatory condition) who carry NOD2 mutations have defective production of α-defensins (HD5, HD6) by Paneth cells along with an increased abundance of mucosa-associated bacteria.16, 17 In addition, signaling through TLR2, TLR3, TLR4, NOD1, NOD2, and NLR Protein 3 (NLRP3) have all been implicated with the expression of β-defensins in IECs.18, 19 Most TLR signaling in IECs induces B cell-activating factors that lead to immunoglobulin class switch recombination in lamina propria B cells without T-cell activation, resulting in IgA secretion.20, 21 In addition, activation of TLR3 and TLR4 has been shown to induce the expression of polymeric immunoglobulin receptor, an epithelial immunoglobulin transporter, by IECs that enhances luminal IgA secretion.22, 23 Therefore, TLR signaling in IECs is involved in multiple steps in intestinal IgA secretion. The antimicrobial peptides and the secretory IgA balance microbial composition, limiting penetration of commensal bacteria in the intestine.24 Therefore, regulation of commensal bacterial burden and composition is likely to be a major function of PRRs to maintain intestinal homeostasis.
TLRs and NLRs in Gut Inflammation—When Microbe Sensing Goes Awry
Targeted PRR gene knockout mice have provided important information regarding intestinal phenotypes of individual PRRs (Table 2). Despite their importance in regulation of commensal flora, only mice that are deficient either in TLR5, NLRP6, or RIG-I have shown to develop spontaneous intestinal inflammation.25, 26, 27 The tolerogenic nature of PRRs at the intestinal host–microbe interface may allow the absence of PRR signaling to keep normal intestinal integrity. Nevertheless, the lack of spontaneous phenotypes in most PRR-deficient mice suggests the possible involvement of compensatory mechanisms. As many pathogens carry multiple PAMPs, a pathogen can be redundantly recognized by multiple PRRs within a single cell of the host. Because PRRs share several key innate immune signaling pathways, intestinal immune homeostasis may be preserved by providing a sufficient host response to commensal microorganisms in most PRR-deficient mice (Figure 2). However, many PRR-deficient mice develop more severe intestinal inflammation than WT mice in the setting of epithelial injury or abnormal cytokine environments such as the absence of IL-10, highlighting the requirement of proper PRR responses for danger signaling during pathological conditions.
Pattern-recognition receptors share immune signaling pathways. Most Toll-like receptors (TLRs) except for TLR3 induce nuclear factor (NF)-κB activation through the myeloid differentiation primary response gene 88 (MyD88) pathway. TLR3 exclusively induces IRF3 activation through the TIR-domain-containing adapter-inducing interferon (IFN)-β (TRIF) pathway. The TRIF pathway is shared by TLR4 and TLR5 (in intestinal epithelial cells). Nucleotide-binding oligomerization domain 1 (NOD1) and NOD2 also activate NF-κB via the RICK (receptor-interacting serine/threonine kinase) pathway. TLRs and NODs also induce activate mitogen-activated protein (MAP)-kinases. AP-1, activator protein 1; CARD, caspase activation and recruitment domain; IRF, interferon regulatory factor; TAK1, transforming growth factor-β-activated kinase 1; TRAF, tumor necrosis factor receptor-associated factor; TRAM, TRIF-related adapter molecule.
The role of TLRs in intestinal inflammation
The role of each TLR signaling in the development of intestinal inflammation may vary depending on the cell types in mucosa and interactions between individual TLR signaling. The spontaneous intestinal inflammation in TLR5−/− mice implies a distinct role of TLR5 in maintenance of mucosal immune response to commensal bacteria. Induction of colitis in TLR5−/− mice is inhibited by cross breeding these mice with TLR4−/− mice, indicating that the mucosal inflammation is TLR4-dependent without TLR5 regulation.25 It is still unclear which cell types in intestinal mucosa are responsible for the TLR4-mediated induction of colitis. However, epithelial TLR4 signaling is unlikely to be involved, because constitutively active expression of epithelial TLR4 does not induce mucosal inflammation in the villin-TLR4 transgenic mice.21, 28 Though, selective deletion of MyD88 in IECs results in spontaneous inflammation of the small intestine,8 MyD88 signaling in myeloid cells is required for the development of intestinal inflammation during colonization of RAG2−/− mice with Helicobacter hepaticus.29 Consistently, TLR4 signaling in myeloid cells has been implicated with mucosal inflammation during enteric Citrobacter rodentium infection.30 Therefore, TLR signaling in the myeloid cell compartment has more significant roles than epithelial TLR signaling in induction of mucosal inflammation in the intestine.
Dysbiosis of intestinal microbiota has been implicated as a prerequisite for the development of inflammatory bowel disease (IBD).31 Given the importance of mucosal PRR signaling in the maintenance of commensal flora in the normal intestine, PRR deficiency may alter the composition of commensal flora that may be involved in the pathogenesis of colitis. The colitic TLR5−/− mice show increased intestinal bacterial burden and variability of commensal flora compared with non-colitic TLR5−/− mice.25 An increase of the Enterobacteriaceae composition has been shown during chemically induced colitis in mice, which correlates with the severity of intestinal inflammation.32 The extent of this shift in commensal composition during colitis is less in TLR2/TLR4 double-deficinet mice than WT mice.32 In addition, the TLR2/TLR4 double-deficient mice have significantly less burdens of Bacteroides, Prevotella, and Enterococcous species in the intestine compared with WT mice.32 However, the alteration of composition of commensal flora in TLR-deficient mice is limited at baseline and mostly dependent on maternal transmission rather than TLR regulation of host–commensal interactions.33 IL-10−/− mouse is a well-established model of colitis as they develop spontaneous colitis due to uncontrolled pro-inflammatory cytokine production in response to commensal flora.34 In fact, different types of commensal bacteria cause intestinal inflammation with different onset, location, and degree in IL-10−/− mice.35 Interestingly, nullifying MyD88 in IL-10−/− mice results in the prevention of colitis induction.36 There are conflicting reports regarding inhibition or acceleration of colitis in TLR4/IL-10 double-knockout mice, which may be explained by differences in flora across various facilities.37, 38, 39 Cross breeding IL-10−/− mice with TLR2−/− mice have shown exacerbation of colitis, but crossing IL-10−/− mice with TLR9−/− mice did not alter intestinal phenotype of IL-10−/− mice despite both TLR2 and TLR9 signal through MyD88.37, 40 Nullifying MyD88 in other commensal-dependent spontaneous colitis models, such as IL-2−/− mice or Tbx21−/−Rag−/− mice both show similar severity of colitis as their MyD88-sufficient counterparts.36, 41, 42 Interestingly, treatment of TLR4/TLR5 double-knockout mice with IL-10 receptor neutralizing antibody (IL-10R mAb) results in development of colitis but IL-1R-deficient TLR5−/− mice does not show signs of colitis in response to IL-10R mAb treatment.43 Because IL-1R shares MyD88 signaling with other TLRs, these results indicate that IL-1 and MyD88 signaling may have a role for induction of colitis seen in TLR5−/− mice, although in another study IL-1R−/− mice displayed worse colitis induced by dextran sulfate sodium (DSS).44 These conflicting results suggest that the role of MyD88 in comensal dependent colitis may differ depending on its upstream inputs and cytokine profile.
Demand for PRR signaling becomes higher in the setting of epithelial damage than homeostatic conditions in the gut. Regardless of the cause, disruption of the epithelial barrier integrity leads to exposure of mucosal immune system to the mass of commensals. During epithelial damage, PRRs also recognize damage-associated molecular patterns that are released from host tissues.45, 46 Mouse DSS-induced colitis is a well-established colitis model specifically relevant to investigate intestinal innate immune responses to epithelial injury as this model is induced by chemical damage of the epithelium and does not require adaptive immunity.47 In the DSS-induced colitis, it has been shown that germ-free mice demonstrate worse colitis compared with conventional mice, but similar severity of colitis has been noted between conventional mice and the commensal-depleted mice that are orally supplemented with TLR2 and TLR4 ligands.48, 49 Consistently, TLR2−/− and TLR4−/− mice show increased severity of DSS-induced colitis.49, 50 MyD88, but not TIR-domain-containing adapter-inducing interferon-β (TRIF)-deficient, mice also demonstrated more severe disease than WT mice in this model, suggesting the importance of MyD88-dependent TLR signaling in protection against mucosal damage.50, 51 However, the underlying mechanisms of protection differ between TLR2 and TLR4. TLR2 signaling in IECs induces tight junctional protein ZO-1 through activation of protein kinase C, which strengthen epithelial barrier integrity and resists cell apoptosis.52, 53 TLR2 signaling also induces cytoprotective trefoil factor production.54 By contrast, TLR4-mediated MyD88 signaling in IECs and subepithelial macrophages induces cyclooxygenase 2 (COX-2) that amplifies mucosal prostaglandin E2 (PGE2) synthesis, supporting epithelial survival.55, 56, 57, 58 TLR4 signaling in IECs also induces expression and release of amphyregulin and epiregulin that activate epidermal growth factor receptor.59 Therefore, multiple cytoprotective functions in IECs are covered by several TLRs signaling, but defects in different aspects of the cytoprotection may result in similar manifestations in the intestine.
The DSS colitis model may rely on different mechanisms to induce acute vs. chronic inflammation. For example, TLR9−/− mice have shown to be more susceptible to acute DSS colitis, but they demonstrate less severe manifestations of colitis during repeated cycles of DSS treatments compared with WT mice.11, 60 Suppression of TLR9 signaling by adenoviral oligodeoxynucleotides, which is known to block the effect of bacterial cytosine-phosphate-guanosine oligodeoxynucleotides, have demonstrated a suppressive effect of intestinal inflammation in various mouse models of chronic colitis.60 Therefore, bacterial DNA from commensal flora is one of the factors inducing intestinal inflammation through activation of TLR9 during chronic colitis. These results indicate that TLR signaling during DSS colitis contribute to cytoprotection and mucosal restitution, but at the same time, it may be involved in sustained mucosal inflammation in response to commensal bacteria.
The role of NLRs in intestinal inflammation
Intestinal inflammation in NLR-deficient mice has been extensively studied (Table 2). The most striking phenotype was observed in NLRP6−/− mice.27 Mice deficient in NLRP6 and its signaling adapter apoptosis-associated speck-like protein (ASC) show increased number of CD45+ cells in lamina propria, crypt hyperplasia in the colon and enlargement of Peyer’s patches with formation of germinal centers.27 These mice have increased mucosal permeability during DSS colitis and are unable to recover from colitis.61, 62 Bone marrow chimera experiments demonstrate that NLRP6 deficiency in non-hematopoietic cells is responsible for the increased susceptibility to DSS colitis.27 Among bone marrow chimera mice developed by WT mice and NLRP6−/− mice, mucosal IL-18 production in response to DSS treatment is significantly reduced only when NLRP6−/− mice are used as recipient.27 Similar increase in DSS susceptibility is observed when WT bone marow is transferred to IL-18−/− mice but not IL-1β−/− mice, indicating that NLRP6-mediated IL-18 production from intestinal non-hematopoietic cells contributes to the resistance against DSS-induced colitis.27 As the expression of NLRP6 in IECs is transcriptionally regulated by nuclear receptor peroxisome proliferator-activated receptor γ which is known to be induced by TLR4 signaling and has potent anti-inflammatory properties, NLRP6-mediated mucosal protection against DSS-induced injury involves epithelial response to commensal bacteria through TLR4 signaling.63, 64, 65 Furthermore, NLRP6−/− mice have significantly elevated expression of C-C motif chemokine ligand 5 (CCL5) chemokine in the colon compared with WT mice, suggesting that NLRP6 has an important role for the negative regulation of CCL5 to maintain intestinal host–commensal homeostasis.27 Interestingly, the susceptibility to intestinal inflammation and upregulation of CCL5 in NLRP6−/− mice is associated with alterations of commensal composition (especially high prevalence of genus Prevotellaceae and group TM7 bacteria).27 Co-housing with NLRP6−/− mice and hence adopting similar flora, WT mice experienced upregulation of CCL5 in the colon. Although this upregulation of intestinal CCL5 did not cause spontaneous intestinal inflammation in WT mice, these mice had worse colitis in response to DSS than WT mice that are housed by themselves. Increased susceptibility to DSS-induced colitis by co-housing with NLRP6−/− mice was not observed in CCL5−/− mice confirming CCL5 as an effector molecule in exacerbation of colitis by the commensal alteration. Although IL-18−/− mice have elevated CCL5 levels in the intestine, IL-18−/− mice carry distinct commensal bacteria from that in NLRP6−/− mice.27 Commensal bacteria-dependent increase of colitis susceptibility has also been observed in Tbx21−/−Rag−/−-deficient mice and angiotensin-converting enzyme 2 (ACE2)-deficient mice.66, 67 Therefore, IL-18-dependent and -independent mechanisms or multiple types of commensal bacteria may be involved in the intestinal phenotype of NLRP6−/− mice.
As NOD2 gene mutations are associated with Crohn’s disease, mice with genetically modified Nod2 gene have been engineered and studied. Although NOD2-knockout mice (NOD2−/− mice) do not develop spontaneous colitis, these mice have increased burdens of Bacteroides, Firmicutes, and Bacillus species in ileal mucosa compared with WT mice and are more susceptible to 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis.68, 69 Bone marrow chimera model demonstrated that NOD2 deficiency in hematopoietic cells increases severity of TNBS-induced colitis.68 Furthermore, transgenic mice that overexpress NOD2 under major histocompatibility complex class II promoter (antigen-presenting cells) demonstrate increased resistance to TNBS-induced colitis.70 Mice carrying the Nod2 mutation (Nod22939iC mice), noted in patients with Crohn’s disease (3020insC), have worse colitis secondary to DSS treatment compared with controls.71 Although this mouse strain is recently found to have a duplication of the 3′ end of the WT Nod2 locus, the altered function of NOD2 with this mutation that augments IL-1β production has been validated (http://www.sciencemag.org/content/333/6040/288.3.full.pdf). In addition, NOD1−/− mice and Nod1/Nod2 double-knockout mice have been reported to be more susceptible to DSS-induced colitis compared with control mice partly due to increased mucosal permiability during colitis.72, 73 NOD2 has shown to regulate intestinal commensal flora through the secretion of bacteria-killing factors.69 Therefore, impaired NOD2 signaling alters commensal composition and increases the susceptibility to intestinal inflammation that may be associated with defective secretion of antimicrobial peptides in the intestine and abnormal immune cell response to the altered commensal flora. The increased mucosal permeability may facilitate mucosal immune cell exposure to luminal contents.
The role of NLRP3 in colitis has been studied in several mouse colitis models with various results. Multiple reports have demonstrated that mice deficient in NLRP3 or its downstream signaling molecules ASC and caspase-1 have increased severity of DSS-induced colitis compared with WT mice.74, 75, 76, 77 NLRP3−/− mice are also more susceptible to TNBS-induced colitis than WT mice.74, 75 Bone marrow chimera experiments highlight the importance of NLRP3 in intestinal stromal cells rather than hematopoietic cells in resistance to DSS-induced colitis.75 Both NLRP3−/− mice and caspase-1−/− mice show increased mucosal permeability accompanied by impaired epithelial proliferation during DSS colitis.75 The number of proliferating epithelial cells and mucosal permeability in these mice are similar to WT mice at baseline, indicating that these mice have defective repair responses to mucosal injury. As intraperitoneal injection of recombinant IL-18 during DSS treatment reduced colitis severity in caspase-1−/− mice, NLRP3-mediated protection from DSS-induced mucosal injury might be mediated by IL-18 production through caspase-1 activation.
Earlier studies, however, have shown that blocking IL-18 hampers mucosal induction of pro-inflammatory cytokines and chemokines and attenuates severity of DSS-induced colitis in mice.78, 79 IL-18 transgenic mice exhibit an increased susceptibility to DSS-induced colitis.80 The other report has described that absence of caspase-1 protects mice from DSS-induced colitis especially in chronic phase.78 Finally, two reports have demonstrated that NLRP3−/− mice develop a less severe intestinal inflammation during DSS-induced colitis and that pharmacological inhibition of caspase-1 attenuated DSS-induced colitis.27, 81
There appears to be two phenotypes of NLRP3 in colitis. Worse DDS-induced colitis in NLRP3−/− and caspase-1−/− mice are likely due to impaired epithelial proliferation after epithelial injury.75, 77 The precise mechanism by which NLRP3 regulates epithelial proliferation is still obscure, though this possibly involves interferon (IFN)-γ and IL-18 in regulation of cell proliferation.82, 83, 84 By contrast, NLRP3−/− mice have reduced mucosal expression of IFN-γ and impaired neutrophil activity during DSS-induced colitis, which may attenuate intestinal inflammation in this model.74, 85 IL-18 is known to induce IFN-γ production directly by natural killer cells and synergistically with IL-12 by T cells.84, 86 Therefore, NLRP3 may have both promotive and suppressive role in DSS-induced colitis.
Which of these opposite NLRP3 phenotype dominates may be dictated by the composition of the intestinal flora. As demonstrated by experiments of WT mice co-housed with NLPR6−/− mice, severity of DSS-induced colitis can be substantially influenced by the composition of commensal flora.27 NLRP3−/− mice have shown a unique expression profile of intestinal antimicrobial peptides and distinct intestinal microbiota from WT mice.74 As development of intestinal commensal flora depends on maternal gut flora and immunity that differ between breeding facilities, it is possible that mice raised in different facilities have distinct flora profiles that lack some of the key populations responsible for each NLRP3-associated intestinal phenotype.32, 87, 88
NLR inflammasome is activated in cells of intestinal mucosa during colitis. DSS has been shown to activate NLRP3 inflammasome and releases IL-1β from lipopolysaccharide-stimulated macrophages through potassium efflux, lysosomal maturation, and production of reactive oxygen species.81 Effective cytokine release by inflammasome requires TLR signaling that induces precursors of IL-1β and IL-18 (Figure 1). Commensal bacteria through TLR signaling may individually activate NLR inflammasomes by inducing adenosine triphosphate (ATP), lysosomal leakage, and production of reactive oxygen species.89, 90, 91 Recently, we have described that oxidized mitochondrial DNA induced by the above stimuli binds to NLRP3 and induces inflammasome activation during cell apoptosis, but this is inhibited by autophagy activation.92 Therefore, host–microbial interactions in intestinal mucosal interface are orchestrated by TLRs and NLRs through regulation of the balance between autophagy and inflammasome activation.
TLRs and NLRs in Inflammation-associated Colorectal Cancer
Chronic intestinal inflammation has long been suggested to trigger tissue neoplastic transformation as higher incidence of intestinal cancer has been observed in patients with IBD. Extensive studies have identified the molecular pathogenesis of sporadic colon cancer based on genetic alterations. Although the process of developing colitis-associated cancer involves some of these genetic abnormalities, unique aspects of colon carcinogenesis in the setting of chronic colitis have been illuminated.93, 94 As inflamed intestinal cells in patients with IBD have these genetic alterations before developing histological features of dysplasia, genetic alterations in colitis-associated cancer may be a secondary step rather than primary cause of tumorigenesis.95 It is likely that abnormal signaling in some PRRs leads to uncontrolled expression of the genes and enzymes regulating cell proliferation, apoptosis, and DNA repair before the gene alterations. Although precise mechanisms underlying initiation and/or promotion of the inflammation-associated intestinal cancer have yet to be fully determined, frequent cycles of injury and repair of the epithelium in the presence of tumorigenic cytokines, chemokines, and prostaglandins may predispose to genetic mutations, which increases neoplastic risk.96, 97 As conventionalization of germ-free animals accelerates epithelial proliferation in the intestine, PRR signaling may have an important role in regulation of epithelial proliferation.98 It has been reported that TLR-mediated MyD88 signaling in subepithelial macrophages regulates crypt stem cell differentiation and epithelial proliferation through expression of COX-2 and PGE2.57, 58 TLR4 stimulation has also been shown to induce proliferation of human IECs via induction of epidermal growth factor receptor ligands.99, 100 The inflammatory milieu may enhance surface expression of TLR2 and TLR4, leading to IECs responsiveness to their ligands.101, 102 These results suggest that abnormal TLR signaling, especially TLR2 and TLR4 in both IECs and subepithelial macrophages, may induce aberrant epithelial proliferation and thus may contribute to cancer development in the setting of chronic inflammation.
Recent reports regarding PRR phenotypes in mouse models of colitis-associated (AOM-DSS) and spontaneous multiple intestinal (Apc/Min) neoplasia appear to show some important pathways in the regulation of intestinal tumorigenesis (Table 3). The AOM-DSS (a single injection of azoxymethane followed by repeated DSS treatment and recovery) model mimics human colitis-associated cancer as it represents repeated mucosal injury and repair, leading to epithelial proliferation and dysplastic transformation in the colon.103 In this model, TLR4−/− mice are protected from tumor development due to decreased expression of mucosal COX-2, PGE2, and amphiregulin.59 Supplementation of PGE2 during the recovery of colitis bypasses the protective phenotype of TLR4−/− mice against intestinal tumors along with sustained upregulation of COX-2 in subepithelial macrophages and epithelial amphiregulin production.56 Therefore, the upregulation of PGE2 during the recovery phase of colitis is a key for colitis-associated tumorigenesis involved in TLR4 signaling. TLR4 expression only in stroma but not in myeloid cells restored tumor incidence in TLR4−/− mice.104 In addition, transgenic expression of constitutively active TLR4 results in increased mucosal PGE2 production and tumor development in this model.28 These results indicate that epithelial TLR4 signaling contribute to tumor development through mucosal production of PGE2 in the setting of chronic colitis.
On the other hand, TLR2−/− mice have demonstrated increased tumor development in the AOM-DSS model.105 Although IECs in TLR2−/− mice are less proliferative and more apoptotic in normal mucosa, they become more proliferative and less apoptotic during chronic colitis compared with WT mice.105 Underlying mechanism of increased tumor burden in TLR2−/− mice is associated with strong activation of epithelial STAT3 (signal transducer and activator of transcription 3) and higher expression of tumorigenic cytokines (IL-6, tumor necrosis factor-α (TNF-α), and IL-17A) in intestinal mucosa. Mucosal TNF-α signaling and IL-6-mediated STAT3 activation are known to be indispensable during tumor development in this model.106 In addition, STAT3-mediated T helper type 17 cell generation has been shown to facilitate tumor development in Apc/Min mice.107 It has been reported that increased expression of these cytokines in TLR2−/− mice is due to compensatory activation of TLR4.30, 108 Therefore, enhanced tumorigenesis in TLR2−/− mice may involve TLR4 signaling.
MyD88−/− mice have demonstrated multiple phenotypes in the development of intestinal tumors. MyD88−/− mice develop more tumors in the AOM-DSS model than WT controls but fewer tumors in AOM-treated IL-10−/− model or AOM-oxazolone model compared with their WT counterparts.109, 110, 111 This discrepancy is likely due to differences in inflammation between the models, as MyD88 deficiency results in increased severity of colitis in the AOM-DSS model but reduced colitis in AOM-treated IL-10−/− model and no effect on colitis severity in the AOM-oxazolone model.109, 110, 111 It is important to note that there is no increase in IEC proliferation after AOM-DSS treatment in MyD88−/− mice.109 The underlying mechanism of the increased susceptibility of MyD88−/− mice to the AOM-DSS induced intestinal tumor is upregulation of the genes associated with Wnt signaling, DNA repair, and angiogenesis.109 In addition, MyD88−/− mice showed more frequent clonal mutations in the β-catenin gene in IECs during AOM-DSS treatment.109 Without chronic inflammation, MyD88 deficiency results in resistance to intestinal tumor development in the Apc/Min and AOM (without DSS) models.109, 112 Therefore, MyD88 signaling may act both in a tumorigenic and anti-tumorigenic capacity depending on the presence and the types of chronic inflammation in the intestine. This is possible because MyD88 transduces multiple receptor signaling pathways, including most TLRs, IL-1R, and IL-18R, which individually lead to the induction of a distinct set of genes.113 Therefore, tumorigenesis induced by AOM-DSS treatment in MyD88−/− mice may differ from that in TLR2- or TLR4-deficient mice.
Besides TLRs, IL-18R, another upstream of MyD88 signaling, appears to be an important regulator of inflammation-associated tumorigenesis in the AOM-DSS model.109 Mice deficient in IL-18 and IL-18R but not in IL-1R exhibit similar susceptibility to tumor development as MyD88−/− mice in the AOM-DSS model.109 Similar to the MyD88−/− mice, these mice that are deficient in IL-18 and IL-18R exhibit severe inflammation but do not have increased IEC proliferation in response to AOM-DSS, suggesting that IL-18-induced IL-18R activation is responsible for the protective role of MyD88 signaling during colitis-associated tumorigenesis in this model.
Higher incidence of intestinal tumors in the AOM-DSS model has been observed in NLRP3−/− mice and NLRP6−/− mice who are unable to produce mature forms of IL-18 and IL-1β.61, 76, 85, 114 Similar tumorigenic phenotype has been reported in NLRC4−/− (NLR family CARD domain-containing protein 4: an another cytosolic member of NLR family) mice and caspase-1−/− mice.76, 114 As caspase-1 activation is the final step of NLR signaling, NLR-dependent protection against colitis-associated tumorigenesis is mediated by their endproducts. It is likely that IL-18 signaling integrates the role of NLRs in the resistance to intestinal tumor development as IL-1R−/− mice have similar susceptibility to intestinal tumor in the AOM-DSS model compared with WT mice.109 NLRP12−/− mice also show increased susceptibility to AOM-DSS-induced colonic tumor that is associated with greater mucosal productions of IL-6, TNF-α, MIP2 (macrophage-inflammatory protein 2), IL-1β, and COX-2 due to increased activation of nuclear factor-κB, extracellular signal–regulated kinase, and STAT3.115, 116
The major defect in these NLR-deficient mice lies in the regulation of epithelial cell proliferation, as these mice demonstrate increased proliferation of IECs during AOM-DSS treatment.61, 76, 85, 114 Similar increase in IEC proliferation during intestinal tumorigenesis has been reported in caspase-1−/− and NOD1−/− mice.72, 114 Although precise mechanisms underlying increased IEC proliferation in NLR-deficient mice remain to be explored, NLRP6−/− mice have shown greater expression of the genes associated with Wnt-signaling pathway in tumor tissue than tumors in WT mice in the AOM-DSS model.62 Interestingly, a recent report demonstrated an increased susceptibility of NOD2−/− mice to intestinal tumorigenesis in response to AOM-DSS, which could be transmissible to WT mice by co-housing.117 As the increased severity of DSS colitis in NLRP6−/− mice is also transmissible via commensal bacteria, the unique compositions of commensal bacteria may be involved in tumor susceptibility in this model. Bone marrow chimera experiments have demonstrated important contributions of NLRP3 and NLRP6 in myeloid cells and NLRP12 in non-hematopoietic component to the protection against colitis-associated tumorigenesis.61, 76, 115, 116 Furthermore, an abnormal mucosal microenvironment has been proposed as a mechanism underlying increased tumorigenesis in these mice, because most NLR-deicient mice demonstrate more severe inflammation in response to DSS with greater expressions of tumorigenic cytokines (TNF-α, IL-6) and chemokines (keratinocyte chemoattractant, eotaxin, granulocyte colony-stimulating factor and monocyte chemotactic protein) in the intestinal mucosa compared with NLR-sufficient mice, while the lack of IL-18 production leads to a defective activation of anti-tumor signaling through IFN-γ and STAT1.61, 82 As IL-18 is known to facilitate anti-tumor immunity through induction of T helper type 1 and cytotoxic T-cell responses, IL-18-mediated suppression of intestinal tumorigenesis may also be involved in the regulation of anti-tumor immunity.118 These results indicate that NLR signaling in the gastrointestinal tract promote multiple immune and non-immune programs that regulate neoplastic development in the setting of chronic inflammation.
Manipulation of PRR as Novel Therapy for colitis and colitis-associated cancer
Several TLR agonists and antagonists have been applied for the prevention and/or treatment of murine models of colitis (Table 4). In general, PRR signaling contributes to induction of regional inflammation but may promote cytoprotective and repair responses during mucosal injury in the gut. In addition, a portion of TLR4 signaling and most nucleotide-sensing TLRs may contribute to immuno-modulation through induction of type I IFNs. Therefore, selection of signaling targets and the timing of intervention are important to establish successful therapeutics for colitis.
Oral administration of a TLR2 agonist Pam3CSK4 has shown its therapeutic potential in DSS-induced colitis.52 The protective effect of Pam3CSK4 is mediated through preservation of tight junctional epithelial barrier.52, 54, 119 TLR2 stimulation also increases the colonic production of trefoil factor that facilitates wound healing and blocks apoptotic signaling.54 Increased susceptibility of TLR2−/− mice to colitis-associated tumor further indicates a possible beneficial effect of Pam3CSK4 for cancer prevention during chronic colitis. Although TLR2 agonists have not yet entered clinical trials for human diseases, these results suggest that TLR2 signaling may be an ideal target for management strategy of IBD.
TLR4 antagonists (CRX-526; a synthetic lipid A mimetic molecule, 1A6; a specific monoclonal antibody) have been applied to prevent murine colitis.120, 121 Although TLR4 antagonist (1A6) suppressed induction of acute inflammatory infiltrate by blocking the expression of chemokines in the intestine when administered before colitis, it delayed mucosal healing when administered to established colitis.121 The TLR4 blocking strategy did not ameliorate the chronic model of T-cell transfer colitis, but administration of 1A6 during recovery phase of colitis significantly prevented tumor development in the AOM-DSS model.28, 121 Therefore, combination therapies with cytoprotective agents may be required for TLR4 blocking strategy to avoid the delay of mucosal healing. TLR4 antagonists are currently evaluated in clinical trials for sepsis cases.122
In vivo manipulation of NLRs have also been examined. NOD2 stimulation by systemic administration of muramyl dipeptide appears to be beneficial in mouse models of colitis.123 Glyburide (glibenclamide), a sulfonylurea drug for the treatment of type 2 diabetes, is known to inhibit ATP-sensitive K+ channels and thus can prevent NLRP3 inflammasome activation.124 Glyburide has been shown to prevent ventilator-induced lung injury in mice and delay endotoxin-induced lethality in mice, but it has not been applied for animal models of colitis.124, 125
Conclusion
PRR signaling has significant roles in intestinal homeostasis, which is mainly composed of the maintenance of commensals. Extensive studies in animal models of colitis have provided several key points of the contribution of individual PRRs and their effector pathways in the pathogenesis of colitis. Epithelial PRR signaling mainly promotes mucosal protection through induction of pathways that leads to cell proliferation and survival or cytoprotection in response to mucosal injury. By contrast, it is likely that PRR signaling in hematopoietic cells is responsible for induction of local inflammation in response to invading pathogens. These PRR signals may be enhanced during chronic inflammation in the intestine. Continuous activation of abnormal PRR signaling leads to the development of neoplasms. In this regard, PRR signaling is involved in multiple aspects of intestinal tumorigenesis as well as host anti-tumor immunity. Manipulation of PRR signaling as therapeutic strategies of human diseases has just begun. Not only targeting particular PRR signaling but targeting upstream signaling or downstream effector molecules such as caspase-1, IL-1β or IL-18 may be beneficial to develop novel strategies for managing IBD and prevention of colorectal cancer in those patients.
References
Thompson, M.R., Kaminski, J.J., Kurt-Jones, E.A. & Fitzgerald, K.A. Pattern recognition receptors and the innate immune response to viral infection. Viruses 3, 920–940 (2011).
Watts, C., West, M.A. & Zaru, R. TLR signalling regulated antigen presentation in dendritic cells. Curr. Opin. Immunol. 22, 124–130 (2010).
Rahman, A.H., Taylor, D.K. & Turka, L.A. The contribution of direct TLR signaling to T cell responses. Immunol. Res. 45, 25–36 (2009).
DeFranco, A.L., Rookhuizen, D.C. & Hou, B. Contribution of Toll-like receptor signaling to germinal center antibody responses. Immunol. Rev. 247, 64–72 (2012).
Otte, J.M., Cario, E. & Podolsky, D.K. Mechanisms of cross hyporesponsiveness to Toll-like receptor bacterial ligands in intestinal epithelial cells. Gastroenterology 126, 1054–1070 (2004).
Melmed, G. et al. Human intestinal epithelial cells are broadly unresponsive to Toll-like receptor 2-dependent bacterial ligands: implications for host-microbial interactions in the gut. J. Immunol. 170, 1406–1415 (2003).
Smythies, L.E. et al. Human intestinal macrophages display profound inflammatory anergy despite avid phagocytic and bacteriocidal activity. J. Clin. Invest. 115, 66–75 (2005).
Gong, J. et al. Epithelial-specific blockade of MyD88-dependent pathway causes spontaneous small intestinal inflammation. Clin. Immunol. 136, 245–256 (2010).
Nenci, A. et al. Epithelial NEMO links innate immunity to chronic intestinal inflammation. Nature 446, 557–561 (2007).
Kajino-Sakamoto, R. et al. Enterocyte-derived TAK1 signaling prevents epithelium apoptosis and the development of ileitis and colitis. J. Immunol. 181, 1143–1152 (2008).
Lee, J. et al. Maintenance of colonic homeostasis by distinctive apical TLR9 signalling in intestinal epithelial cells. Nat. Cell Biol. 8, 1327–1336 (2006).
Vaishnava, S., Behrendt, C.L., Ismail, A.S., Eckmann, L. & Hooper, L.V. Paneth cells directly sense gut commensals and maintain homeostasis at the intestinal host-microbial interface. Proc. Natl. Acad. Sci. USA 105, 20858–20863 (2008).
Sodhi, C.P. et al. Intestinal epithelial Toll-like receptor 4 regulates goblet cell development and is required for necrotizing enterocolitis in mice. Gastroenterology 143, 708–718 e701–705 (2012).
Palazzo, M. et al. Activation of enteroendocrine cells via TLRs induces hormone, chemokine, and defensin secretion. J. Immunol. 178, 4296–4303 (2007).
Kobayashi, K.S. et al. Nod2-dependent regulation of innate and adaptive immunity in the intestinal tract. Science 307, 731–734 (2005).
Wehkamp, J. et al. NOD2 (CARD15) mutations in Crohn's disease are associated with diminished mucosal alpha-defensin expression. Gut 53, 1658–1664 (2004).
Schultsz, C., Van Den Berg, F.M., Ten Kate, F.W., Tytgat, G.N. & Dankert, J. The intestinal mucus layer from patients with inflammatory bowel disease harbors high numbers of bacteria compared with controls. Gastroenterology 117, 1089–1097 (1999).
Vora, P. et al. Beta-defensin-2 expression is regulated by TLR signaling in intestinal epithelial cells. J. Immunol. 173, 5398–5405 (2004).
Uehara, A., Fujimoto, Y., Fukase, K. & Takada, H. Various human epithelial cells express functional Toll-like receptors, NOD1 and NOD2 to produce anti-microbial peptides, but not proinflammatory cytokines. Mol. Immunol. 44, 3100–3111 (2007).
He, B. et al. Intestinal bacteria trigger T cell-independent immunoglobulin A(2) class switching by inducing epithelial-cell secretion of the cytokine APRIL. Immunity 26, 812–826 (2007).
Shang, L. et al. Toll-like receptor signaling in small intestinal epithelium promotes B-cell recruitment and IgA production in lamina propria. Gastroenterology 135, 529–538 (2008).
Bruno, M.E. et al. Regulation of the polymeric immunoglobulin receptor in intestinal epithelial cells by Enterobacteriaceae: implications for mucosal homeostasis. Immunol. Invest. 39, 356–382 (2010).
Schneeman, T.A. et al. Regulation of the polymeric Ig receptor by signaling through TLRs 3 and 4: linking innate and adaptive immune responses. J. Immunol. 175, 376–384 (2005).
Macpherson, A.J. & Uhr, T. Induction of protective IgA by intestinal dendritic cells carrying commensal bacteria. Science 303, 1662–1665 (2004).
Vijay-Kumar, M. et al. Deletion of TLR5 results in spontaneous colitis in mice. J. Clin. Invest. 117, 3909–3921 (2007).
Wang, Y. et al. Rig-I−/− mice develop colitis associated with downregulation of G alpha i2. Cell Res. 17, 858–868 (2007).
Elinav, E. et al. NLRP6 inflammasome regulates colonic microbial ecology and risk for colitis. Cell 145, 745–757 (2011).
Fukata, M. et al. Constitutive activation of epithelial TLR4 augments inflammatory responses to mucosal injury and drives colitis-associated tumorigenesis. Inflamm. Bowel Dis. 17, 1464–1473 (2011).
Asquith, M.J., Boulard, O., Powrie, F. & Maloy, K.J. Pathogenic and protective roles of MyD88 in leukocytes and epithelial cells in mouse models of inflammatory bowel disease. Gastroenterology 139, 519–529 e511-512 (2010).
Gibson, D.L. et al. Interleukin-11 reduces TLR4-induced colitis in TLR2-deficient mice and restores intestinal STAT3 signaling. Gastroenterology 139, 1277–1288 (2010).
Tamboli, C.P., Neut, C., Desreumaux, P. & Colombel, J.F. Dysbiosis as a prerequisite for IBD. Gut 53, 1057 (2004).
Heimesaat, M.M. et al. Shift towards pro-inflammatory intestinal bacteria aggravates acute murine colitis via Toll-like receptors 2 and 4. PLoS One 2, e662 (2007).
Ubeda, C. et al. Familial transmission rather than defective innate immunity shapes the distinct intestinal microbiota of TLR-deficient mice. J. Exp. Med. 209, 1445–1456 (2012).
Kuhn, R., Lohler, J., Rennick, D., Rajewsky, K. & Muller, W. Interleukin-10-deficient mice develop chronic enterocolitis. Cell 75, 263–274 (1993).
Kim, S.C. et al. Variable phenotypes of enterocolitis in interleukin 10-deficient mice monoassociated with two different commensal bacteria. Gastroenterology 128, 891–906 (2005).
Rakoff-Nahoum, S., Hao, L. & Medzhitov, R. Role of toll-like receptors in spontaneous commensal-dependent colitis. Immunity 25, 319–329 (2006).
Gonzalez-Navajas, J.M. et al. TLR4 signaling in effector CD4+ T cells regulates TCR activation and experimental colitis in mice. J. Clin. Invest. 120, 570–581 (2010).
Biswas, A. et al. Negative regulation of Toll-like receptor signaling plays an essential role in homeostasis of the intestine. Eur. J. Immunol. 41, 182–194 (2011).
Matharu, K.S. et al. Toll-like receptor 4-mediated regulation of spontaneous Helicobacter-dependent colitis in IL-10-deficient mice. Gastroenterology 137, 1380–1390 e1381–1383 (2009).
Messlik, A. et al. Loss of Toll-like receptor 2 and 4 leads to differential induction of endoplasmic reticulum stress and proapoptotic responses in the intestinal epithelium under conditions of chronic inflammation. J. Proteome Res. 8, 4406–4417 (2009).
Garrett, W.S. et al. Colitis-associated colorectal cancer driven by T-bet deficiency in dendritic cells. Cancer Cell 16, 208–219 (2009).
Powell, N. et al. The transcription factor T-bet regulates intestinal inflammation mediated by interleukin-7 receptor+ innate lymphoid cells. Immunity 37, 674–684 (2012).
Carvalho, F.A. et al. Interleukin-1beta (IL-1beta) promotes susceptibility of Toll-like receptor 5 (TLR5) deficient mice to colitis. Gut 61, 373–384 (2012).
Gonzalez-Navajas, J.M. et al. Interleukin 1 receptor signaling regulates DUBA expression and facilitates Toll-like receptor 9-driven antiinflammatory cytokine production. J. Exp. Med. 207, 2799–2807 (2010).
Zheng, L., Riehl, T.E. & Stenson, W.F. Regulation of colonic epithelial repair in mice by Toll-like receptors and hyaluronic acid. Gastroenterology 137, 2041–2051 (2009).
Lim, D.M. & Wang, M.L. Toll-like receptor 3 signaling enables human esophageal epithelial cells to sense endogenous danger signals released by necrotic cells. Am. J. Physiol. Gastrointest. Liver Physiol. 301, G91–G99 (2011).
Dieleman, L.A. et al. Dextran sulfate sodium-induced colitis occurs in severe combined immunodeficient mice. Gastroenterology 107, 1643–1652 (1994).
Kitajima, S., Morimoto, M., Sagara, E., Shimizu, C. & Ikeda, Y. Dextran sodium sulfate-induced colitis in germ-free IQI/Jic mice. Exp. Anim. 50, 387–395 (2001).
Rakoff-Nahoum, S., Paglino, J., Eslami-Varzaneh, F., Edberg, S. & Medzhitov, R. Recognition of commensal microflora by toll-like receptors is required for intestinal homeostasis. Cell 118, 229–241 (2004).
Fukata, M. et al. Toll-like receptor-4 is required for intestinal response to epithelial injury and limiting bacterial translocation in a murine model of acute colitis. Am. J. Physiol. Gastrointest. Liver Physiol. 288, G1055–G1065 (2005).
Choi, Y.J., Im, E., Chung, H.K., Pothoulakis, C. & Rhee, S.H. TRIF mediates Toll-like receptor 5-induced signaling in intestinal epithelial cells. J. Biol. Chem. 285, 37570–37578 (2010).
Cario, E., Gerken, G. & Podolsky, D.K. Toll-like receptor 2 controls mucosal inflammation by regulating epithelial barrier function. Gastroenterology 132, 1359–1374 (2007).
Cario, E., Gerken, G. & Podolsky, D.K. Toll-like receptor 2 enhances ZO-1-associated intestinal epithelial barrier integrity via protein kinase C. Gastroenterology 127, 224–238 (2004).
Podolsky, D.K., Gerken, G., Eyking, A. & Cario, E. Colitis-associated variant of TLR2 causes impaired mucosal repair because of TFF3 deficiency. Gastroenterology 137, 209–220 (2009).
Fukata, M. et al. Cox-2 is regulated by Toll-like receptor-4 (TLR4) signaling: Role in proliferation and apoptosis in the intestine. Gastroenterology 131, 862–877 (2006).
Hernandez, Y. et al. The role of prostaglandin E2 (PGE 2) in toll-like receptor 4 (TLR4)-mediated colitis-associated neoplasia. BMC Gastroenterol. 10, 82 (2010).
Pull, S.L., Doherty, J.M., Mills, J.C., Gordon, J.I. & Stappenbeck, T.S. Activated macrophages are an adaptive element of the colonic epithelial progenitor niche necessary for regenerative responses to injury. Proc. Natl. Acad. Sci. USA 102, 99–104 (2005).
Brown, S.L. et al. Myd88-dependent positioning of Ptgs2-expressing stromal cells maintains colonic epithelial proliferation during injury. J. Clin. Invest. 117, 258–269 (2007).
Fukata, M. et al. Toll-like receptor-4 promotes the development of colitis-associated colorectal tumors. Gastroenterology 133, 1869–1881 (2007).
Obermeier, F. et al. CpG motifs of bacterial DNA essentially contribute to the perpetuation of chronic intestinal inflammation. Gastroenterology 129, 913–927 (2005).
Chen, G.Y., Liu, M., Wang, F., Bertin, J. & Nunez, G. A functional role for Nlrp6 in intestinal inflammation and tumorigenesis. J. Immunol. 186, 7187–7194 (2011).
Normand, S. et al. Nod-like receptor pyrin domain-containing protein 6 (NLRP6) controls epithelial self-renewal and colorectal carcinogenesis upon injury. Proc. Natl. Acad. Sci. USA 108, 9601–9606 (2011).
Kempster, S.L. et al. Developmental control of the Nlrp6 inflammasome and a substrate, IL-18, in mammalian intestine. Am. J. Physiol. Gastrointest. Liver Physiol. 300, G253–G263 (2011).
Eun, C.S. et al. Attenuation of colonic inflammation by PPARgamma in intestinal epithelial cells: effect on Toll-like receptor pathway. Dig. Dis. Sci. 51, 693–697 (2006).
Dubuquoy, L. et al. Impaired expression of peroxisome proliferator-activated receptor gamma in ulcerative colitis. Gastroenterology 124, 1265–1276 (2003).
Hashimoto, T. et al. ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation. Nature 487, 477–481 (2012).
Garrett, W.S. et al. Communicable ulcerative colitis induced by T-bet deficiency in the innate immune system. Cell 131, 33–45 (2007).
Penack, O. et al. NOD2 regulates hematopoietic cell function during graft-versus-host disease. J. Exp. Med. 206, 2101–2110 (2009).
Petnicki-Ocwieja, T. et al. Nod2 is required for the regulation of commensal microbiota in the intestine. Proc. Natl. Acad. Sci. USA 106, 15813–15818 (2009).
Yang, Z. et al. NOD2 transgenic mice exhibit enhanced MDP-mediated down-regulation of TLR2 responses and resistance to colitis induction. Gastroenterology 133, 1510–1521 (2007).
Maeda, S. et al. Nod2 mutation in Crohn's disease potentiates NF-kappaB activity and IL-1beta processing. Science 307, 734–738 (2005).
Chen, G.Y., Shaw, M.H., Redondo, G. & Nunez, G. The innate immune receptor Nod1 protects the intestine from inflammation-induced tumorigenesis. Cancer Res. 68, 10060–10067 (2008).
Natividad, J.M. et al. Commensal and probiotic bacteria influence intestinal barrier function and susceptibility to colitis in Nod1(−/−);Nod2(−/−) Mice. Inflamm. Bowel Dis. 18, 1434–1446 (2011).
Hirota, S.A. et al. NLRP3 inflammasome plays a key role in the regulation of intestinal homeostasis. Inflamm. Bowel Dis. 17, 1359–1372 (2011).
Zaki, M.H. et al. The NLRP3 inflammasome protects against loss of epithelial integrity and mortality during experimental colitis. Immunity 32, 379–391 (2010).
Allen, I.C. et al. The NLRP3 inflammasome functions as a negative regulator of tumorigenesis during colitis-associated cancer. J. Exp. Med. 207, 1045–1056 (2010).
Dupaul-Chicoine, J. et al. Control of intestinal homeostasis, colitis, and colitis-associated colorectal cancer by the inflammatory caspases. Immunity 32, 367–378 (2010).
Siegmund, B. et al. Neutralization of interleukin-18 reduces severity in murine colitis and intestinal IFN-gamma and TNF-alpha production. Am. J. Physiol. Regul. Integr. Comp. Physiol. 281, R1264–R1273 (2001).
Sivakumar, P.V. et al. Interleukin 18 is a primary mediator of the inflammation associated with dextran sulphate sodium induced colitis: blocking interleukin 18 attenuates intestinal damage. Gut 50, 812–820 (2002).
Ishikura, T. et al. Interleukin-18 overproduction exacerbates the development of colitis with markedly infiltrated macrophages in interleukin-18 transgenic mice. J. Gastroenterol. Hepatol. 18, 960–969 (2003).
Bauer, C. et al. Colitis induced in mice with dextran sulfate sodium (DSS) is mediated by the NLRP3 inflammasome. Gut 59, 1192–1199 (2010).
Zaki, M.H., Lamkanfi, M. & Kanneganti, T.D. The Nlrp3 inflammasome: contributions to intestinal homeostasis. Trends Immunol. 32, 171–179 (2011).
Nava, P. et al. Interferon-gamma regulates intestinal epithelial homeostasis through converging beta-catenin signaling pathways. Immunity 32, 392–402 (2010).
Fantuzzi, G., Puren, A.J., Harding, M.W., Livingston, D.J. & Dinarello, C.A. Interleukin-18 regulation of interferon gamma production and cell proliferation as shown in interleukin-1beta-converting enzyme (caspase-1)-deficient mice. Blood 91, 2118–2125 (1998).
Zaki, M.H., Vogel, P., Body-Malapel, M., Lamkanfi, M. & Kanneganti, T.D. IL-18 production downstream of the Nlrp3 inflammasome confers protection against colorectal tumor formation. J. Immunol. 185, 4912–4920 (2010).
Okamura, H., Kashiwamura, S., Tsutsui, H., Yoshimoto, T. & Nakanishi, K. Regulation of interferon-gamma production by IL-12 and IL-18. Curr. Opin. Immunol. 10, 259–264 (1998).
Donnet-Hughes, A. et al. Potential role of the intestinal microbiota of the mother in neonatal immune education. Proc. Nutr. Soc. 69, 407–415 (2010).
Diaz, R.L. et al. Maternal adaptive immunity influences the intestinal microflora of suckling mice. J. Nutr. 134, 2359–2364 (2004).
Atarashi, K. et al. ATP drives lamina propria T(H)17 cell differentiation. Nature 455, 808–812 (2008).
Ko, M.K., Saraswathy, S., Parikh, J.G. & Rao, N.A. The role of TLR4 activation in photoreceptor mitochondrial oxidative stress. Invest. Ophthalmol. Vis. Sci. 52, 5824–5835 (2011).
Suliman, H.B. et al. Toll-like receptor 4 mediates mitochondrial DNA damage and biogenic responses after heat-inactivated E. coli. FASEB J. 19, 1531–1533 (2005).
Shimada, K. et al. Oxidized mitochondrial DNA activates the NLRP3 inflammasome during apoptosis. Immunity 36, 401–414 (2012).
Itzkowitz, S.H. Microsatellite instability in colitis associated colorectal cancer. Gut 46, 304–305 (2000).
Itzkowitz, S.H. & Yio, X. Inflammation and cancer. IV. Colorectal cancer in inflammatory bowel disease: the role of inflammation. Am. J. Physiol. Gastrointest. Liver Physiol. 287, G7–17 (2004).
Ullman, T.A. & Itzkowitz, S.H. Intestinal inflammation and cancer. Gastroenterology 140, 1807–1816 (2011).
Kundu, J.K. & Surh, Y.J. Inflammation: gearing the journey to cancer. Mutat. Res. 659, 15–30 (2008).
Ono, M. Molecular links between tumor angiogenesis and inflammation: inflammatory stimuli of macrophages and cancer cells as targets for therapeutic strategy. Cancer Sci. 99, 1501–1506 (2008).
Cherbuy, C. et al. Microbiota matures colonic epithelium through a coordinated induction of cell cycle-related proteins in gnotobiotic rat. Am. J. Physiol. Gastrointest. Liver Physiol. 299, G348–G357 (2010).
Hsu, D. et al. Toll-like receptor 4 differentially regulates epidermal growth factor-related growth factors in response to intestinal mucosal injury. Lab. Invest. 90, 1295–1305 (2010).
Brandl, K. et al. MyD88 signaling in nonhematopoietic cells protects mice against induced colitis by regulating specific EGF receptor ligands. Proc. Natl. Acad. Sci. USA 107, 19967–19972 (2010).
Lin, Y. et al. The lipopolysaccharide-activated toll-like receptor (TLR)-4 induces synthesis of the closely related receptor TLR-2 in adipocytes. J. Biol. Chem. 275, 24255–24263 (2000).
Rehli, M. et al. PU.1 and interferon consensus sequence-binding protein regulate the myeloid expression of the human Toll-like receptor 4 gene. J. Biol. Chem. 275, 9773–9781 (2000).
Suzuki, R., Kohno, H., Sugie, S. & Tanaka, T. Dose-dependent promoting effect of dextran sodium sulfate on mouse colon carcinogenesis initiated with azoxymethane. Histol. Histopathol. 20, 483–492 (2005).
Fukata, M. et al. Innate immune signaling by Toll-like receptor-4 (TLR4) shapes the inflammatory microenvironment in colitis-associated tumors. Inflamm. Bowel Dis. 15, 997–1006 (2009).
Lowe, E.L. et al. Toll-like receptor 2 signaling protects mice from tumor development in a mouse model of colitis-induced cancer. PLoS One 5, e13027 (2010).
Grivennikov, S. et al. IL-6 and Stat3 are required for survival of intestinal epithelial cells and development of colitis-associated cancer. Cancer Cell 15, 103–113 (2009).
Wu, S. et al. A human colonic commensal promotes colon tumorigenesis via activation of T helper type 17 T cell responses. Nat. Med. 15, 1016–1022 (2009).
Fransen, F. et al. Differential effect of TLR2 and TLR4 on the immune response after immunization with a vaccine against Neisseria meningitidis or Bordetella pertussis. PLoS One 5, e15692 (2010).
Salcedo, R. et al. MyD88-mediated signaling prevents development of adenocarcinomas of the colon: role of interleukin 18. J. Exp. Med. 207, 1625–1636 (2010).
Uronis, J.M. et al. Modulation of the intestinal microbiota alters colitis-associated colorectal cancer susceptibility. PLoS One 4, e6026 (2009).
Schiechl, G. et al. Tumor development in murine ulcerative colitis depends on MyD88 signaling of colonic F4/80+CD11b(high)Gr1(low) macrophages. J. Clin. Invest. 121, 1692–1708 (2011).
Rakoff-Nahoum, S. & Medzhitov, R. Regulation of spontaneous intestinal tumorigenesis through the adaptor protein MyD88. Science 317, 124–127 (2007).
Watters, T.M., Kenny, E.F. & O’Neill, L.A. Structure, function and regulation of the Toll/IL-1 receptor adaptor proteins. Immunol. Cell Biol. 85, 411–419 (2007).
Hu, B. et al. Inflammation-induced tumorigenesis in the colon is regulated by caspase-1 and NLRC4. Proc. Natl. Acad. Sci. USA 107, 21635–21640 (2010).
Allen, I.C. et al. NLRP12 suppresses colon inflammation and tumorigenesis through the negative regulation of noncanonical NF-kappaB signaling. Immunity 36, 742–754 (2012).
Zaki, M.H. et al. The NOD-like receptor NLRP12 attenuates colon inflammation and tumorigenesis. Cancer Cell 20, 649–660 (2011).
Couturier-Maillard, A. et al. NOD2-mediated dysbiosis predisposes mice to transmissible colitis and colorectal cancer. J. Clin. Invest. in press (2013).
Balkow, S. et al. Dendritic cell activation by combined exposure to anti-CD40 plus interleukin (IL)-12 and IL-18 efficiently stimulates anti-tumor immunity. Exp. Dermatol. 18, 78–87 (2009).
Ey, B., Eyking, A., Gerken, G., Podolsky, D.K. & Cario, E. TLR2 mediates gap junctional intercellular communication through connexin-43 in intestinal epithelial barrier injury. J. Biol. Chem. 284, 22332–22343 (2009).
Fort, M.M. et al. A synthetic TLR4 antagonist has anti-inflammatory effects in two murine models of inflammatory bowel disease. J. Immunol. 174, 6416–6423 (2005).
Ungaro, R. et al. A novel Toll-like receptor 4 antagonist antibody ameliorates inflammation but impairs mucosal healing in murine colitis. Am. J. Physiol. Gastrointest. Liver Physiol. 296, G1167–G1179 (2009).
Raja, S.G. & Dreyfus, G.D. Eritoran: the evidence of its therapeutic potential in sepsis. Core Evid. 2, 199–207 (2008).
Watanabe, T. et al. Muramyl dipeptide activation of nucleotide-binding oligomerization domain 2 protects mice from experimental colitis. J. Clin. Invest. 118, 545–559 (2008).
Lamkanfi, M. et al. Glyburide inhibits the Cryopyrin/Nalp3 inflammasome. J. Cell Biol. 187, 61–70 (2009).
Kuipers, M.T. et al. Ventilator-induced lung injury is mediated by the NLRP3 inflammasome. Anesthesiology 116, 1104–1115 (2012).
Abreu, M.T., Fukata, M. & Arditi, M. TLR signaling in the gut in health and disease. J. Immunol. 174, 4453–4460 (2005).
Pott, J. et al. Age-dependent TLR3 expression of the intestinal epithelium contributes to rotavirus susceptibility. PLoS Pathog. 8, e1002670 (2012).
Wells, J.M., Loonen, L.M. & Karczewski, J.M. The role of innate signaling in the homeostasis of tolerance and immunity in the intestine. Int. J. Med. Microbiol. 300, 41–48 (2010).
Kim, J.G., Lee, S.J. & Kagnoff, M.F. Nod1 is an essential signal transducer in intestinal epithelial cells infected with bacteria that avoid recognition by toll-like receptors. Infect. Immun. 72, 1487–1495 (2004).
Inohara, Chamaillard, McDonald, C. & Nunez, G. NOD-LRR proteins: role in host-microbial interactions and inflammatory disease. Annu. Rev. Biochem. 74, 355–383 (2005).
Hirata, Y., Broquet, A.H., Menchen, L. & Kagnoff, M.F. Activation of innate immune defense mechanisms by signaling through RIG-I/IPS-1 in intestinal epithelial cells. J. Immunol. 179, 5425–5432 (2007).
Volman, J.J., Mensink, R.P., Buurman, W.A., Onning, G. & Plat, J. The absence of functional dectin-1 on enterocytes may serve to prevent intestinal damage. Eur. J. Gastroenterol. Hepatol. 22, 88–94 (2010).
Martinon, F. & Tschopp, J. NLRs join TLRs as innate sensors of pathogens. Trends Immunol. 26, 447–454 (2005).
Ohtani, K. et al. The membrane-type collectin CL-P1 is a scavenger receptor on vascular endothelial cells. J. Biol. Chem. 276, 44222–44228 (2001).
Nguyen, D.G. & Hildreth, J.E. Involvement of macrophage mannose receptor in the binding and transmission of HIV by macrophages. Eur. J. Immunol. 33, 483–493 (2003).
van Asbeck, E.C., Hoepelman, A.I., Scharringa, J., Herpers, B.L. & Verhoef, J. Mannose binding lectin plays a crucial role in innate immunity against yeast by enhanced complement activation and enhanced uptake of polymorphonuclear cells. BMC Microbiol. 8, 229 (2008).
Teh, C., Le, Y., Lee, S.H. & Lu, J. M-ficolin is expressed on monocytes and is a lectin binding to N-acetyl-D-glucosamine and mediates monocyte adhesion and phagocytosis of Escherichia coli. Immunology 101, 225–232 (2000).
den Dunnen, J., Gringhuis, S.I. & Geijtenbeek, T.B. Innate signaling by the C-type lectin DC-SIGN dictates immune responses. Cancer Immunol. Immunother. 58, 1149–1157 (2009).
Brown, G.D. et al. Dectin-1 mediates the biological effects of beta-glucans. J. Exp. Med. 197, 1119–1124 (2003).
Rachmilewitz, D. et al. Toll-like receptor 9 signaling mediates the anti-inflammatory effects of probiotics in murine experimental colitis. Gastroenterology 126, 520–528 (2004).
Vijay-Kumar, M. et al. Activation of toll-like receptor 3 protects against DSS-induced acute colitis. Inflamm. Bowel Dis. 13, 856–864 (2007).
Vijay-Kumar, M. et al. Flagellin treatment protects against chemicals, bacteria, viruses, and radiation. J. Immunol. 180, 8280–8285 (2008).
Rachmilewitz, D. et al. Immunostimulatory DNA ameliorates experimental and spontaneous murine colitis. Gastroenterology 122, 1428–1441 (2002).
Obermeier, F. et al. Contrasting activity of cytosin-guanosin dinucleotide oligonucleotides in mice with experimental colitis. Clin. Exp. Immunol. 134, 217–224 (2003).
Acknowledgements
This work was supported by NIH grants 1R01HL66436 and 1R01AI 067995 to M.A, and Senior Research Award from Crohn’s Colitis Foundation of America (3391) and a NIH grant 1R01AI095255 to M.F.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Competing interests
The authors declared no conflict of interest.
Rights and permissions
About this article
Cite this article
Fukata, M., Arditi, M. The role of pattern recognition receptors in intestinal inflammation. Mucosal Immunol 6, 451–463 (2013). https://doi.org/10.1038/mi.2013.13
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/mi.2013.13
This article is cited by
-
Analysis of colonic mucosa-associated microbiota using endoscopically collected lavage
Scientific Reports (2022)
-
Skin immunity: dissecting the complex biology of our body's outer barrier
Mucosal Immunology (2022)
-
Gut microbiota alternation under the intestinal epithelium-specific knockout of mouse Piga gene
Scientific Reports (2022)
-
Toll-like receptor 4 (TLR4) antagonists as potential therapeutics for intestinal inflammation
Indian Journal of Gastroenterology (2021)
-
Calprotectin, an available prognostic biomarker in systemic sclerosis: a systematic review
Clinical Rheumatology (2021)
