The intestinal epithelium is strategically placed at the frontline of the mucosal immune system where it forms a bipolar interface between the diversity of luminal antigens and subjacent immune cells present in the lamina propria. This delicate barrier is comprised of only a single cell layer throughout the gastrointestinal tract which is continually replaced throughout lifetime. There is emerging evidence that in addition to forming a highly selective anatomic barrier which mechanically restricts penetration and invasion of any deleterious luminal constituents, the intestinal epithelium essentially serves as a highly dynamic immunologic frontier – exhibiting both innate and adaptive immune features. Thus, the former simple concept of just a passive frontier has been replaced with an active ‘gate alert model’ (Fig. 1). In order to accomplish this fundamental task, the intestinal epithelium is uniquely equipped with a variety of versatile and sophisticated talents to sense, recognize and, if necessary, combat potential threats to mucosal immune homeostasis. Studies over the past year have provided novel insights into the key intrinsic processes of the intestinal epithelium to closely monitor its environment, police the gates, communicate messages to neighbouring cells and rapidly initiate active defensive and repair measures. Exogenous or endogenous danger signals serve as the important driving force in directing intestinal epithelial cells (IEC) to mount host-protective immune responses. The current understanding is that several regulatory platforms of signalling mechanisms exist which are closely related and interact. With the inflammasome as ‘new kid on the block’, investigators have begun to carve out its potential important role in this context.
As frontline barrier of the intestinal mucosa, IEC must watch the extracellular and intracellular environment closely and constantly. IEC must exert a rigorous process of rapid and precise discrimination between ‘self’ and ‘nonself’ based on the recognition of broadly conserved molecular patterns by germ-line encoded pattern recognition receptors (PRRs) , which include the families of Toll-like receptors (TLRs) and NOD-like receptors (NLRs). PRRs are central regulators of intestinal epithelial innate immunity and thus pivotal to the control of host defence by maintaining mucosal and commensal homeostasis. They play key roles in recognition and sensing of non/bacterial danger signals, inhibition of invasion of facultative/obligate pathogens and other threats, induction of antimicrobial effector pathways, and control of adaptive immune responses, by acting through a series of inter-dependent signalling events. Environmental factors and genetic variations may alter PRR function. Intestinal PRR malfunction may lead to changes in gut microbiota composition, potentially promoting the development of inflammatory diseases in the intestine and systemic involvement, such as metabolic syndrome [2•], in the genetically susceptible host.
TLRs comprise a class of 13 mammalian type I transmembrane glycoproteins (10 in humans and 12 in mice) which all contain multiple leucine-rich repeat motifs (LRR) in the large, divergent ectodomain and a highly conserved region in the short intracellular tail, called the Toll-interleukin-1 receptor domain. Intestinal epithelial TLR alterations have been associated with several intestinal disorders, for example, IBD [1,3]. The NLR (nucleotide-binding domain, leucine-rich repeat-containing) family consists of more than 23 human and 34 murine cytosolic members, including NODs (nucleotide-binding oligomerization domain-1), NALPs, IPAF, NAIPs and CITTA, however the physiological function of most NLRs is so far poorly understood. NLRs also contain a LRR-domain at the C-terminal end, a central nucleotide-binding domain, and an effector domain (CARD, PYD or BIR) for protein–protein interaction at the N-terminal end (summarized in [4••]). NOD2 was the first characterized member of the NLR family present in IEC  and associated with IBD [6,7].
In recent months, much attention has focussed on a specific member of the NLR family, the NLRP3 (NLR family, pyrin domain-containing 3; also called cryopyrin) inflammasome, and its regulatory interaction with other innate immune processes in the intestinal mucosa. The NLRP3 inflammasome is a multiprotein, cytoplasmic complex, composed of a NLR protein, the adaptor ASC and procaspase-1, which regulates processing and secretion of cytokines belonging to the interleukin-1 (IL-1) family. NLRP3 protein is widely expressed in the gastrointestinal tract and can be found in epithelial cells at mucosal sites, including oral cavity and esophagus, but also granulocytes, T and B cells . The NLRP3 inflammasome may be stimulated by the presence of microbial-associated products and toxins in conjunction with a vast variety of endogenous and exogenous danger-associated signals (summarized in [9••]), thus serving as a general sensor of any form of cellular stress triggered by signalling intermediates, including potassium efflux, reactive oxygen species (ROS) and cathepsins. Three fundamentally different models (‘channel’, ‘ROS’, ‘lysosome rupture’) for activation of the NLRP3 inflammasome have been proposed (reviewed in ), but integration and relative contribution of the main signalling elements remain so far largely unresolved. Signalling crosstalk between TLRs and NLRP3 occurs, which leads to generation of pro-inflammatory IL-1β, IL-18 and IL-33 [11••]. Conversion of pro-IL-1β to its active form appears to require not only activation of caspase-1 but also a second stimulus which induces formation of the inflammasome module (Fig. 2). However, although TLR signalling may be required for transcriptional upregulation of the IL-1β precursor in some cell types (such as macrophages), NLRP3-mediated caspase-1 activation by TLR agonists (e.g. LPS or heat-killed bacteria) can proceed through pannexin-1-independent of the ‘classical’ TLR pathway . Signalling pathways via TLRs and distinct NLRs may converge in NF-κB activation, regulating the expression of numerous immune and inflammatory genes. Yet, the outcome of NF-κB activation is ambiguous, depending on cell type and context involved (Fig. 2).
The gate keeper of the paracellular pathway is the intercellular junctional complex which is comprised of apically located tight junctions and lateral adherens junctions and desmosomes. Control of IEC barrier permeability vs. physical integrity is essential to selectively permit the entry of required nutrients yet protect the host from pathogens and potentially harmful antigens present in the lumen. Recent studies have demonstrated that selective commensal microbiota may provide the host with critical assistance to maintain protective tight-junctions-associated barrier integrity of the intestinal epithelium. Bacterial signals like indole  or lipopeptides  may directly enhance transepithelial resistance of IEC barrier, ameliorating colonic inflammation . In contrast, excess IL-1β secretion due to inflammasome hyperactivity could secondarily alter tight junctions regulation and expression in the intestinal epithelium, augmenting colonic inflammation . Tight junctions defects may also be endogenously caused by depletion of cellular protective mediators, like loss of epithelial matriptase during conditions of hypoxic or inflammatory stress leading to aberrant turnover of claudin-2 . Tight junctions-associated barrier impairment is thought to contribute to the aberrant inflammatory response seen in IBD. Although tight junctions dysfunction may not result per se in spontaneous colitis in mice, it can critically activate pro-inflammatory and immunoregulatory responses at subclinical levels in the underlying lamina propria [18•]. Thus, compromised tight junctions-associated integrity of the intestinal epithelium may act as a determining ‘first-hit’ that increases susceptibility to immune-mediated colitis and accelerates mucosal disease progression.
Regulatory control of healthy IEC barrier function depends on consistent and immediate communication processes that ensure effective cell–cell crosstalk in feedback loops. Emerging evidence shows that IEC can communicate with each other through chemical signals directly transmitted via cell–cell contacts. Gap junction channels comprise a large family of transmembrane connexion (Cx) proteins which are responsible for direct cell–cell communication through coordination of intercellular transfer of ions, small metabolites and peptides (<1 kDa). Gap junction-mediated cell–cell ‘coupling’ via Cx43 represents a key mechanism in gap junctional intercellular communication (GJIC). Alteration of GJIC caused by changes in Cx43 has been proposed to be involved in the pathophysiology of diverse IEC barrier diseases. We have recently shown that activation of TLR2 preserves gap junction-associated architecture and GJIC via Cx43 between juxtaposed IEC during acute and chronic inflammatory injury in the intestine [19•]. IEC-specific knockdown of Cx43 by mucosal RNA interference in vivo leads to abrogation of TLR2-mediated IEC restitution and delays wound closure in acute inflammatory stress-induced damage in the intestine. These data highlight a novel mechanism of how commensal-mediated IEC–IEC communication may help to sustain mucosal barrier homeostasis via Cx43 [19•].
In addition, IEC coordinate alert messages to other neighbouring cell subsets through release of mediators into the extracellular environment. TLR-mediated IEC activation induces dendritic cell sampling of luminal bacteria . IEC-derived factors can drive dendritic cell-induced differentiation of adaptive Foxp3+ regulatory T cells in the local microenvironment. After IEC contact, dendritic cells acquire a tolerogenic phenotype and critically protect against TH1/TH17-mediated colitis [21•]. Future studies will need to determine whether IEC may be coupled by ‘heterotypic’ gap junctions, thus forming dynamic message conduits with dendritic cells, and other adjacent cell types, such as intraepithelial lymphocytes or myofibroblasts.
Mice deficient in TLR or NLR signalling exhibit delayed or diminished wound healing during various mucosal insults of the intestine, including acute DSS colitis. Recent studies have identified several new mechanisms by which TLR or NLR signalling exerts cyto-protective functions in the intestinal epithelium (and adjacent cell subsets) which are required for mucosal barrier preservation, repair and restitution during injury.
TLR4 activation results in increased cyclooxygenase (COX)-2 expression and prostaglandin E2 (PGE2) synthesis which promotes increased IEC survival and proliferation in the colon [22,23]. It has now been shown that endogenous hyaluronic acid participates in this protective response to DSS-mediated colitis via TLR4-MyD88 in a positive feedback loop [24•]. Of note, TLR4-induced IEC proliferation appears to be site-specific (small intestine vs. colon) and developmentally regulated (immature vs. mature intestine) [25•], probably due to mucosal differences in interactions with the commensal microbiota and changes in commensal composition during early lifetime.
Goblet cells facilitate mucosal protection and epithelial barrier repair predominantly through production of TFF3 which mediates intestinal epithelial antiapoptosis and migration . Loss of IEC barrier protection via TFF3 leads to enhanced leukocyte recruitment in the lamina propria through fucosyltransferase VII during intestinal inflammation . Recently, we identified an essential link between TLR2 and modulation of GC-derived TFF3 in the healthy and diseased intestine [28•]. Collective data from in-vitro and ex-vivo models of murine and human GC revealed that TLR2 stimulation rapidly induces synthesis of TFF3, but not MUC2. In-vivo studies using mice deficient in TFF3 demonstrated that the ability for oral treatment with a synthetic TLR2 agonist to confer antiapoptotic protection of the intestinal mucosa against inflammatory stress-induced damage is mediated through TFF3 induction. Conversely, mice lacking TLR2 exhibit a selective deficiency in TFF3 expression during intestinal GC differentiation. Supplementation of recombinant TFF3 rescues TLR2-deficient mice from increased morbidity and mortality during acute DSS-induced injury through inhibition of excessive mucosal apoptosis and associated reduction in leukocyte influx [28•].
Besides TLRs, new findings imply that central components of the NLRP3 inflammasome also modulate IEC barrier homeostasis during inflammatory-induced injury of the intestinal mucosa. NLRP3 inflammasome activators all require ROS production. High levels of ROS, which are abundantly released from macrophages during acute DSS colitis, aggravate IEC barrier disruption and perpetuate mucosal inflammation. Inhibition of excess oxidative stress, for example by prohibitin which functions as an antioxidant in IEC [29•], ameliorates colonic inflammation. Downstream, inappropriate secretion of IL-1β increases, while therapy with IL-1 receptor antagonist decreases, severity of colitis [11••]. Mice that are knocked-in with the human Crohn's disease-associated NOD2 variant 3020insC demonstrate more efficient processing and secretion of IL-1β which triggers hyperinflammation in acute DSS colitis . IEC-derived IL-33, another member of the IL-1 family and inflammasome product, is also significantly increased during human IBD [31•]. Based on these previous findings, one may have therefore suspected that blockage of the NLRP3 inflammasome activation should protect against oxidant stress-induced inflammatory damage via IL-1β (and related family members) in acute DSS colitis. However, unexpectedly, the opposite has now been shown.
Four independent studies [32•–35•] found in parallel that mice deficient in NLRP3, ASC or caspase-1 are highly susceptible to acute DSS-mediated inflammatory injury, as demonstrated by increased mortality and morbidity (body weight loss, diarrhoea and rectal bleeding) when compared to DSS-WT. Absence of any of the inflammasome components impairs IEC proliferation and restitution after acute DSS-induced inflammation, leading to increased barrier permeability with enhanced bacterial translocation [32•]. Studies using NLRP3-/- bone marrow chimeras implied that presence of NLRP3 in nonhematopoietic cells, but not in leukocytes, is critical for protection against DSS-induced injury. At mechanistic levels, loss of intestinal epithelial IL-18 may be the culprit that perpetuates inflammatory injury in the context of NLRP3 deficiency. NLRP3-deficient IEC failed to produce IL-18 and systemic administration of recombinant IL-18 markedly reduced severity of DSS-mediated damage [32•,33•]. This finding is in agreement with previous data implying that deficiency in IL-18 or caspase-1 exacerbates acute DSS-induced colonic inflammation [36,37]. IL-18 may signal through MyD88  which is critical for appropriate tissue repair responses after acute injury [39,40]. Therefore the authors proposed that (possibly IEC-derived) NLRP3-mediated IL-18 production may significantly contribute to preservation of IEC barrier integrity during the early phase of wound healing in acute DSS colitis. In this context, it remains to be investigated whether IL-18 may directly affect the assembly of tight junctions in IEC or rather indirectly, for example through inhibition of pro-inflammatory IL-1β activity via nitric oxide production , which blocks excessive caspase-1 activation in an autoregulatory negative feedback loop . Collectively, these data imply that absence of NLRP3 inflammasome-mediated anti-inflammatory and barrier-protective immune responses may play an important role in the pathogenesis of this chemically induced damage model of acute colitis in mice.
It must be noted that the outcome of inflammasome-mediated innate immune responses may be ambiguous which could primarily result from the variable involvement of affected cell populations within the intestinal mucosa at different stages of disease. It is important to stress that IL-18 is a pleiotropic mediator exhibiting both beneficial and harmful effects. IL-18 may induce pro-inflammatory as well as anti-inflammatory immune responses, depending on the predominant cell types involved, the pathogenesis (genetic variations) and the phase of colitis in murine models of IBD. For instance, macrophage-derived IL-18 drives TH1-mediated mucosal inflammation in the TNBS colitis model . Signalling via MyD88 contributes to the development and persistence of chronic colitis by promoting the expansion of colitogenic CD4+ TH17 cells in inflamed mucosa [44,45]. Moreover, mice deficient in caspase-12 (a negative regulator of the caspase-1 inflammasome and NLR-NFκB pathways) are more susceptible to repetititve DSS treatments than wild type, implying that deregulation of the caspase-1-IL-1R axis may turn deleterious during the development of chronic DSS colitis by allowing aberrant inflammasome signalling to trigger excessive pro-inflammatory immune responses in the lamina propria [33•]. Future studies will need to determine whether loss of NLRP3 may rather exert mucosa-protective effects in TH1/TH17-mediated spontaneous murine models of colitis (such IL-10-/- mice). Other models of intestinal injury, such as radiation-mucositis, ischemia/perfusion-mucositis or NEC-mucositis, must be addressed as well.
The pathophysiological relevance of these murine findings for human IBD remains to be clarified. Substantial effort is currently devoted to delineate the mechanistic functions and related signalling cascades of the inflammasome in the context of IBD pathogenesis. An open question is whether the inflammasome is activated primarily or secondarily, or both, in IBD. The NLRP3 region was recently reported to be associated with Crohn's disease susceptibility [46•]. ‘Loss-of-function’ variants could predispose IBD patients to become colonized with facultative-pathogenic commensals or pathogens that trigger initiation or perpetuation of disease, while ‘gain-of-function’ mutations may be associated with auto-inflammatory effects leading to prolonged episodes of chronic inflammation. Functional in-vitro studies demonstrated that Crohn's disease-associated SNPs in the NLRP3 area alter protein expression and IL-1β production in monocytes [46•], implying a defective innate immune response that may impair clearance of luminal antigens and/or pathogens. Based on the above findings obtained from studies using acute DSS colitis as murine model, one may hypothesize that NLRP3 reduction does not serve as trigger for initiation but rather as basis for aggravation of mucosal inflammation in human IBD.
NLRP3 mRNA expression was shown to be increased in active human Crohn's disease and murine TNBS-induced colitis samples [46•], yet the precise cause and consequence of this observation remains elusive. It remains to be determined whether IBD patients in remission or prior to onset of active disease already demonstrate alterations in protein expression and subcellular distribution as well as dysfunction of distinct inflammasome components. NLRP3 upregulation could result from abundant lipopolysaccharide or danger signals released from damaged cells during ongoing inflammation. In addition, pro-inflammatory TNFα, which plays a significant pathophysiological role in triggering IBD, has been found to sensitize macrophages to aberrant inflammasome hyperresponsiveness by promoting caspase-1 activation via NLRP3 [47•]. Enhanced NLRP3 expression may perpetuate chronic inflammation through TH17 polarization by inflammasome hyperactivation. Two recent studies generated mice carrying knock-in mutations of the NLRP3 gene found to increase susceptibility to auto-inflammatory diseases (such as Muckle–Wells syndrome) in humans [48•,49•]. These mice show severe spontaneous skin lesions characterized by a TH17 cell – skewed phenotype, but (like humans with compatible mutations) do not develop spontaneous intestinal inflammation. This result suggests that the impact of NLRP3 gain-of-function may be organ/barrier site-dependent and environment-dependent. Cell-type specific differences are likely and must be delineated. It is possible that inflammasome upregulation in IBD may ‘just’ reflect relative immuno-incompetence or rather dysbalance in a complex network of interactive innate immune signalling modules. Future studies will need to functionally analyse the intestinal immune responses of gene-targeted mice carrying a variant in the NLRP3 gene equivalent to one of the human mutations previously associated with IBD.
Both TLRs and NLRs influence autophagy which represents a fundamental component of the innate immune process to maintain cellular homeostasis in the intestinal epithelial and mucosal barrier during health and disease. However, our knowledge regarding potential signalling interconnections between TLRs and NLRs/inflammasome through the autophagic machinery remains so far very limited. Variants in the autophagy genes Atg16L1 and IRGM have previously been associated with increased susceptibility to Crohn's disease [50–52]. NOD2 recruits Atg16L1 to the plasma membrane to initiate bacterial autophagy, while the Crohn's disease-associated NOD2 mutant fails [53•]. Impaired autophagy confers functional hyperresponsiveness of the inflammasome in the intestine . It is likely, though not yet investigated, that a reverse correlation may occur as well. Exposure of Atg16L1-deficient macrophages to commensal bacteria or the TLR4 ligand LPS causes TRIF-dependent activation of caspase-1, leading to abnormally high concentrations of IL-1β and IL-18, which exacerbates DSS-induced acute colitis . However, NOD2-induced autophagy in dendritic cells appears to be NLRP3-independent [55•], implying differences in autophagy regulation between distinct PRR members. In addition, deregulated autophagy compromises Paneth cell-mediated antimicrobial signalling in colonic inflammation . MyD88-dependent signalling is crucial for limiting mucosal adherence and penetration of commensals through production of Paneth cell-derived α-defensins . Of note, Paneth cell α-defensins may block release of IL-1ß from LPS-activated monocytes , yet the responsible mechanism is so far not understood. In this context, it would be interesting to know whether Atg16L1-deficiency induces directly Paneth cell abnormalities or rather affects indirectly Paneth cell function by targeting the inflammasome. It also remains elusive how the interaction between NOD2 and autophagy vs. the inflammasome may alter Paneth cell signalling. Collectively, these studies provide first evidence that an important linkage between autophagy and inflammasome activation may exist in host defence which maintains mucosal barrier homeostasis in the intestine. A preliminary study recently implied that mice deficient in NLRP3 may exhibit alterations in commensal composition due to defensin deficiency [35•]. Future studies will need to investigate whether imbalances in the inflammasome platform may alter Paneth cell physiology and its autophagic processes (Fig. 3), thus causing reduced defensin production and leading to changes in commensal composition.
Furthermore, it will be important to examine how aberrant modulation of the inflammasome contributes to initiation and progression of colitis-associated neoplasia. A recent study showed that mice deficient in PYCARD, caspase-1 or NLRP3 are highly susceptible to chemically (AOM)-induced colon carcinogenesis in chronic DSS colitis [34•]. These data imply that the inflammasome may function as an important tumour suppressor. Of note, bone marrow chimeric studies suggested that NLRP3 regulates colitis-driven tumorigenesis through the hematopoietic/myeloid compartment, yet the mechanisms remain to be resolved in detail. However, TLR/MyD88/ERK-dependent Apc-mediated tumorigenesis appears to be not affected by inhibition of IL-1R1 or caspase-1 [59•], suggesting that the inflammasome may not be necessarily involved in the pathogenesis of other murine models of intestinal tumour growth. Manipulation of the intestinal microbiota has been shown to trigger the development of colorectal tumours [60•]. TLR4 activation promotes hyperproliferative tumour-responses in colonic EC, at least in the AOM/DSS model . It will be important to examine whether aberrant modulation of the inflammasome imbalances specific TLR signals due to changes in commensal composition, which may contribute to initiation and progression of DSS colitis-associated neoplasia. Damage to the IEC layer by toxic AOM/DSS may release various NLRP3 ligands, and future studies will need to dissect whether failure of the hematopoietic compartment to recognize these signals may block antitumour responses in the microenvironment of the intestinal mucosa.
Exciting progress has been made over the past year in better understanding the complex diversity of physiological functions of innate immunity in the intestinal epithelial barrier. It has become clear that many more signalling components and mediators of the innate immune system exist, than previously appreciated, which mediate host-protective responses of the intestinal epithelium. In addition, first evidence is now provided that the NLRP3 inflammasome can trigger anti-inflammatory responses in IEC and enhance tissue repair during acute injury in the intestinal mucosa. But numerous questions arise from these initial results: How does signalling via the NLRP3 inflammasome regulate intestinal lamina propria mononuclear cells during inflammation and cancer? Is IL-18 really the only protective mediator induced by NLRP3 in IEC during injury? How does loss of inflammasome function modulate TH1/TH17-mediated chronic colitis? How is a possible balance between autophagy and the inflammasome coordinated at molecular signalling levels? Does defensin reduction seen in NLRP3 deficiency represent a primary or secondary effect? Does NLRP3 dysfunction sensitize otherwise tolerogenic mucosal immune cells to TLR ligands? What is the clinical relevance of these findings for human IBD pathogenesis – and other intestinal diseases? In-depth clarification of the molecular components involved and their cell-type specific regulation will be essential to elucidate the mechanisms by which mucosal innate immunity is governed in human health and disease. Answers to all these questions may not only help to provide further insights into the biological dynamics of how the intestinal epithelium safeguards mucosal barrier immunity, but may also lead to novel approaches for treatment of inflammation and cancer in the intestine.
This study was supported by grants from the Crohn's and Colitis Foundation of America (SRA #1790) and the Deutsche Forschungsgemeinschaft (CA226/4-2; CA226/8-1; CA226/9-1) and IFORES.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
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Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 660).
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