A novel Toll-like receptor 4 antagonist antibody ameliorates inflammation but impairs mucosal healing in murine colitis

TLR-dependent T cell activation in autoimmunity

Autoimmune disease can develop as a result of a breakdown in immunological tolerance, leading to the activation of self-reactive T cells. There is an established link between infection and human autoimmune diseases. Furthermore, experimental autoimmune diseases can be induced by autoantigens that are administered together with complete Freund's adjuvant, which contains killed Mycobacterium tuberculosis; in some cases, these bacteria can be replaced by individual pathogen-associated molecular patterns (PAMPs). Exogenous PAMPs and endogenous danger signals from necrotic cells bind to pattern recognition receptors (including Toll-like receptors) and activate signalling pathways in innate immune cells and in T cells. This leads to pro-inflammatory cytokine production and T cell activation, which are now considered to be major factors in the development of autoimmunity.

Cathelicidin signaling via the Toll-like receptor protects against colitis in mice

BACKGROUND & AIMS

Cathelicidin (encoded by Camp) is an antimicrobial peptide in the innate immune system. We examined whether macrophages express cathelicidin in colons of mice with experimental colitis and patients with inflammatory bowel disease, and we investigated its signaling mechanisms.

METHODS

Quantitative, real-time, reverse-transcription polymerase chain reaction (PCR), bacterial 16S PCR, immunofluorescence, and small interfering RNA (siRNA) analyses were performed. Colitis was induced in mice using dextran sulfate sodium (DSS); levels of cathelicidin were measured in human primary monocytes.

RESULTS

Expression of cathelicidin increased in the inflamed colonic mucosa of mice with DSS-induced colitis compared with controls. Cathelicidin expression localized to mucosal macrophages in inflamed colon tissues of patients and mice. Exposure of human primary monocytes to Escherichia coli DNA induced expression of Camp messenger RNA, which required signaling by extracellular signal-regulated kinase (ERK); expression was reduced by siRNAs against Toll-like receptor (TLR)9 and MyD88. Intracolonic administration of bacterial DNA to wild-type mice induced expression of cathelicidin in colons of control mice and mice with DSS-induced colitis. Colon expression of cathelicidin was significantly reduced in TLR9(-/-) mice with DSS-induced colitis. Compared with wild-type mice, Camp(-/-) mice developed a more severe form of DSS-induced colitis, particularly after intracolonic administration of E coli DNA. Expression of cathelicidin from bone marrow-derived immune cells regulated DSS induction of colitis in transplantation studies in mice.

CONCLUSIONS

Cathelicidin protects against induction of colitis in mice. Increased expression of cathelicidin in monocytes and experimental models of colitis involves activation of TLR9-ERK signaling by bacterial DNA. This pathway might be involved in the pathogenesis of ulcerative colitis.

Colitis development during the suckling-weaning transition in mucin Muc2-deficient mice

The mucin Muc2 is the structural component of the colonic mucus layer. Adult Muc2 knockout (Muc2(-/-)) mice suffer from severe colitis. We hypothesized that Muc2 deficiency induces inflammation before weaning of mother's milk [postnatal day (P) 14] with aggravation of colitis after weaning (P28). Muc2(-/-) and wild-type mice were killed at embryonic day 18.5 and P1.5, P7.5, P14, P21, and P28. Colonic morphology, influx of T cells, and goblet cell-specific protein expression was investigated by (immuno)histochemistry. Cytokine and Toll-like receptor (TLR) profiles in the colon were analyzed by quantitative RT-PCR. Muc2(-/-) mice showed an increased and persistent influx of Cd3ε-positive T cells in the colonic mucosa as of P1.5. This was accompanied by mucosal damage at P28 in the distal colon but not in the proximal colon. At P14, the proinflammatory immune response [i.e., increased interleukin (IL)-12 p35, IL-12 p40, and tumor necrosis factor-α, expression] in the distal colon of Muc2(-/-) mice presented with an immune suppressive response [i.e., increased Foxp3, transforming growth factor (TGF)-β1, IL-10, and Ebi3 expression]. In contrast, at P28, a proinflammatory response remained in the distal colon, whereas the immune suppressive response (i.e., Foxp3 and TGF-β1 expression) declined. The proximal colon of Muc2(-/-) mice did not show morphological damage and was dominated by an immune suppressive response at P14 and P28. Interestingly, changes in expression of TLRs and TLR-related molecules were observed in the distal colon at P14 and P28 and in the proximal colon only at P28. Colitis in Muc2(-/-) mice is limited before weaning by immune suppressive responses and exacerbates in the distal colon after weaning because of the decline in the immune suppressive response.

Substance P induces CCN1 expression via histone deacetylase activity in human colonic epithelial cells

We have shown that substance P (SP) and its neurokinin-1 receptor (NK-1R) regulate intestinal angiogenesis by increasing expression of protein CYR61 (the cysteine-rich angiogenic inducer 61, or CCN1) in colonic epithelial cells. However, the mechanism involved in SP-induced CCN1 expression has not been studied, and the outcome of increased CCN1 expression in the development of colitis is not fully understood. Because histone deacetylase (HDAC) modulates transcription of several genes involved in inflammation, we investigated participation of HDAC in SP-induced CCN1 expression in human colonic epithelial NCM460 cells overexpressing NK-1R (NCM460-NK-1R) and in primary colonocytes. SP increased HDAC activity with deacetylation and dephosphorylation of nucleosome protein histone H3 in NCM460-NK-1R and/or primary colonocytes. Histone deacetylation and dephosphorylation was observed in colonic mucosa from irritable bowel disease patients. Similarly, colonic mucosal tissues from mice exposed to dextran sulfate sodium showed histone H3 deacetylation and dephosphorylation and increased HDAC activity that was reversed by the NK-1R antagonist CJ-12255. SP-induced increased CCN1 expression in NCM460-NK-1R cells was abolished by pharmacological HDAC inhibition. HDAC overexpression activated basal and SP-induced CCN1 promoter activity. Intracolonic CCN1 overexpression significantly ameliorated dextran sulfate sodium-induced colitis, with reduction of proinflammatory cytokine expression in mice. Thus, SP-mediated CCN1 expression in the inflamed human and mouse colon involves increased HDAC activity. Our results strongly suggest that increased CCN1 expression may be involved in mucosal healing during colitis.

Toll-like receptor 4 signaling integrates intestinal inflammation with tumorigenesis: lessons from the murine model of colitis-associated cancer

Chronic inflammation has long been implicated as a predisposition for cancer, but the underlying mechanism for how this occurs has remained obscure. Ulcerative colitis (UC) is a chronic inflammatory disorder of the large intestine which is known to be highly linked to colorectal cancer. During chronic inflammation the intestinal mucosa is in a constant cycle of injury and repair resulting in aberrant epithelial proliferation, a process that increases the risk of neoplastic transformation. In particular, the coexistence of commensal flora in the intestine plays an important role in the regulation of mucosal restitution after epithelial injury. It has become apparent that signaling through toll-like receptors (TLRs), the receptor family recognizing pathogen-associated molecular patterns, is crucial to intestinal epithelial proliferation and mucosal restitution. We have recently described two important downstream pathways underlying TLR4-mediated epithelial proliferation in a mouse model of colitis-associated cancer; i.e., cyclooxygenase 2 (COX-2)-mediated production of prostaglandin E₂ (PGE₂), and induction of specific ligands for epidermal growth factor receptor (EGFR). These two pathways are closely involved with mucosal levels of PGE₂ and other prostanoids such as 15-deoxy-delta 12,14-prostaglandin-J2 (15d-PGJ2). Understanding the fine interplay between the TLR signaling and intestinal tumorigenesis in the setting of chronic inflammation can contribute to establishing a novel treatment strategy for inflammation-associated cancers.

Rectal administration of lipopolysaccharide and ovalbumin ameliorates acute murine colitis

BACKGROUND

Repeated challenges of lipopolysaccharide (LPS) could reduce the expression of proinflammatory cytokines in vitro, and oral administration of ovalbumin (OVA) induces mucosal tolerance in vivo. However, the effect of local administration of LPS and OVA on experimental colitis in vivo remains unknown.

AIMS

This study was performed to elucidate the effect of rectal administration of LPS and OVA on an acute murine colitis induced by dextran sulfate sodium (DSS).

METHODS

BALB/c mice were rectally administered LPS with or without OVA followed by 3% DSS. Colitis was assessed by disease activity index (DAI) including weight loss, stool consistency and rectal bleeding, and histopathology. Primary colon epithelial cells were isolated and the expression of Toll-like receptor 4 (TLR4) was examined using the Western blot analysis. IL-6, IFN-γ and IL-10 mRNA levels in colonic tissue were assessed using real-time RT-PCR.

RESULTS

LPS administration significantly attenuated the severity of acute DSS-induced colitis as assessed by DAI and histopathologic scoring compared with the control group. Combined treatment of LPS and OVA restored body weight loss and further ameliorated the severity of acute DSS colitis. LPS pretreatment regardless of OVA administration decreased TLR4 expression. LPS and OVA pretreatment reduced IL-6 and IFN-γ mRNA expression and increased IL-10 mRNA expression compared with controls.

CONCLUSION

Rectal administration of LPS attenuated acute murine colitis, possibly through TLR4 down-regulation, and combined treatment of OVA additionally ameliorated colonic inflammation associated with up-regulation of IL-10.

Constitutive activation of epithelial TLR4 augments inflammatory responses to mucosal injury and drives colitis-associated tumorigenesis

BACKGROUND

Chronic intestinal inflammation culminates in cancer and a link to Toll-like receptor-4 (TLR4) has been suggested by our observation that TLR4 deficiency prevents colitis-associated neoplasia. In the current study we address the effect of the aberrant activation of epithelial TLR4 on induction of colitis and colitis-associated tumor development. We take a translational approach to address the consequences of increased TLR signaling in the intestinal mucosa.

METHODS

Mice transgenic for a constitutively active TLR4 under the intestine-specific villin promoter (villin-TLR4 mice) were treated with dextran sodium sulfate (DSS) for acute colitis and azoxymethane (AOM)-DSS TLR4 expression was analyzed by immunohistochemistry in colonic tissue from patients with ulcerative colitis (UC) and UC-associated cancer. The effect of an antagonist TLR4 antibody (Ab) was tested in prevention of colitis-associated neoplasia in the AOM-DSS model.

RESULTS

Villin-TLR4 mice were highly susceptible to both acute colitis and colitis-associated neoplasia. Villin-TLR4 mice had increased epithelial expression of COX-2 and mucosal PGE₂ production at baseline. Increased severity of colitis in villin-TLR4 mice was characterized by enhanced expression of inflammatory mediators and increased neutrophilic infiltration. In human UC samples, TLR4 expression was upregulated in almost all colitis-associated cancer and progressively increased with grade of dysplasia. As a proof of principle, a TLR4/MD-2 antagonist antibody inhibited colitis-associated neoplasia in the mouse model.

CONCLUSIONS

Our results show that regulation of TLRs can affect the outcome of both acute colitis and its consequences, cancer. Targeting TLR4 and other TLRs may ultimately play a role in prevention or treatment of colitis-associated cancer.

Worms, flies and four-legged friends: the applicability of biological models to the understanding of intestinal inflammatory diseases

Diseases of intestinal inflammation, including Crohn's disease, ulcerative colitis and necrotizing enterocolitis, cause substantial acute and chronic disability in a large proportion of the population. Crohn's disease and ulcerative colitis, which are collectively referred to as inflammatory bowel disease (IBD), lead to recurrent episodes of intestinal dysfunction and systemic illness, whereas necrotizing enterocolitis is characterized by the development of dramatic and all too often fatal intestinal necrosis in infants. To determine the molecular underpinnings of these disorders, investigators have explored a variety of animal models that vary widely in their complexity. These experimental systems include the invertebrate nematode Caenorhabditis elegans, the more complex invertebrate Drosophila melanogaster, and vertebrate systems including mice, rats and other mammals. This review explores the experimental models that are used to mimic and evaluate the pathogenic mechanisms leading to these diseases of intestinal inflammation. We then highlight, as an example, how the use of different experimental models that focus on the role of Toll-like receptor 4 (TLR4) signaling in the gut has revealed important distinctions between the pathogenesis of IBD and necrotizing enterocolitis. Specifically, TLR4-mediated signaling plays a protective role in the development of Crohn's disease and ulcerative colitis, whereas this signaling pathway plays a causative role in the development of necrotizing enterocolitis in the newborn small intestine by adversely affecting intestinal injury and repair mechanisms.

DNA attenuates enterocyte Toll-like receptor 4-mediated intestinal mucosal injury after remote trauma

Intestinal mucosal injury occurs after remote trauma although the mechanisms that sense remote injury and lead to intestinal epithelial disruption remain incompletely understood. We now hypothesize that Toll-like receptor 4 (TLR4) signaling on enterocytes after remote injury, potentially through the endogenous TLR4 ligand high-mobility group box-1 (HMGB1), could lead to intestinal dysfunction and bacterial translocation and that activation of TLR9 with DNA could reverse these effects. In support of this hypothesis, exposure of TLR4-expressing mice to bilateral femur fracture and systemic hypotension resulted in increased TLR4 expression and signaling and disruption of the ileal mucosa, leading to bacterial translocation, which was not observed in TLR4-mutant mice. TLR4 signaling in enterocytes, not immune cells, was required for this effect, as adenoviral-mediated inhibition of TLR4 in enterocytes prevented these findings. In seeking to identify the endogenous TLR4 ligands involved, the expression of HMGB1 was increased in the intestinal mucosa after injury in wild-type, but not TLR4-mutant, mice, and administration of anti-HMGB1 antibodies reduced both intestinal mucosal TLR4 signaling and bacterial translocation after remote trauma. Strikingly, mucosal injury was significantly increased in TLR9-mutant mice, whereas administration of exogenous DNA reduced the extent of TLR4-mediated enterocyte apoptosis, restored mucosal healing, and maintained the histological integrity of the intestinal barrier after remote injury. Taken together, these findings identify a novel link between remote injury and enterocyte TLR4 signaling leading to barrier injury, potentially through HMGB1 as a ligand, and demonstrate the reversal of these adverse effects through activation of TLR9.

TLR based therapeutics

Toll-like receptors (TLRs) play a crucial role in innate immune responses to infection. Binding of agonists to TLRs promotes maturation of antigen presenting cells, such as dendritic cells, which in turn directs the induction of adaptive immune responses. For this reason TLR agonists are being exploited as vaccine adjuvants for infectious disease or cancer and as therapeutics against tumors. However TLR agonists also promote inflammatory cytokine production and have a pathogenic role in many diseases with an inflammatory basis, including autoimmune diseases. Consequently, antibodies to TLRs and inhibitors of TLR signalling pathways have considerable potential as therapeutics for inflammatory disorders. Some have shown to be efficacious in pre-clinical models, and have now entered clinical trials.

Dietary fish oil and curcumin combine to modulate colonic cytokinetics and gene expression in dextran sodium sulphate-treated mice

Both fish oil (FO) and curcumin have potential as anti-tumour and anti-inflammatory agents. To further explore their combined effects on dextran sodium sulphate (DSS)-induced colitis, C57BL/6 mice were randomised to four diets (2 × 2 design) differing in fatty acid content with or without curcumin supplementation (FO, FO+2 % curcumin, maize oil (control, MO) or MO+2 % curcumin). Mice were exposed to one or two cycles of DSS in the drinking-water to induce either acute or chronic intestinal inflammation, respectively. FO-fed mice exposed to the single-cycle DSS treatment exhibited the highest mortality (40 %, seventeen of forty-three) compared with MO with the lowest mortality (3 %, one of twenty-nine) (P = 0·0008). Addition of curcumin to MO increased (P = 0·003) mortality to 37 % compared with the control. Consistent with animal survival data, following the one- or two-cycle DSS treatment, both dietary FO and curcumin promoted mucosal injury/ulceration compared with MO. In contrast, compared with other diets, combined FO and curcumin feeding enhanced the resolution of chronic inflammation and suppressed (P < 0·05) a key inflammatory mediator, NF-κB, in the colon mucosa. Mucosal microarray analysis revealed that dietary FO, curcumin and FO plus curcumin combination differentially modulated the expression of genes induced by DSS treatment. These results suggest that dietary lipids and curcumin interact to regulate mucosal homeostasis and the resolution of chronic inflammation in the colon.

Toll-like receptor modulators: a patent review (2006-2010)

INTRODUCTION

The immune response is mediated via two parallel immune components, innate and adaptive, whose effector functions are highly integrated and coordinated for the protection of the human body against invading pathogens and transformed cells. The discovery of pathogen recognition receptors (PRRs), most notably toll-like receptors (TLRs), in innate immunity has evoked increased interest in the therapeutic handling of the innate immune system. TLRs are germ line-encoded receptors that play a potent role in the recognition of a diverse variety of ligands ranging from hydrophilic nucleic acids to lipopolysaccharide (LPS) or peptidoglycan (PGN) structures in pathogens.

AREAS COVERED

This review discusses recent updates (2006-2010) in completed, ongoing and planned clinical trials of TLR immunomodulator-based therapies for the treatment of infectious diseases, inflammatory disorders and cancer.

EXPERT OPINION

Since the discovery of human TLRs, modulating immune responses using TLR agonists or antagonists for therapeutic purposes has provoked intense activity in the pharmaceutical industry. The ability of TLRs to initiate and propagate inflammation makes them attractive therapeutic targets. We are now at the stage of evaluating such molecules in human diseases. Additionally, there is also extensive literature available on TLRs in diseased states. These data provide a basis for the identification of novel immunomodulators (agonists and antagonists) for the therapeutic targeting of TLRs.

Systemic Toll-like receptor ligands modify B-cell responses in human inflammatory bowel disease

BACKGROUND

Bacteria have a central, although poorly understood, role in inflammatory bowel disease (IBD). Host-bacteria interactions primarily take place in the gastrointestinal tract, but cells may also encounter translocated bacteria in the bloodstream. IBD is associated with activated, circulating Toll-like receptor (TLR)2 and TLR4-expressing B cells suggesting that blood-borne microbial TLR ligands modulate B cell responses.

METHODS

Serum levels of lipopolysaccharide (LPS)/endotoxin and high mobility group box 1 (HMGB1), an endogenous TLR ligand, were quantified in Crohn's disease (CD) and ulcerative colitis (UC). Responses of purified B cells to LPS and HMGB1 were correlated with levels of systemic TLR ligands and clinical parameters of disease.

RESULTS

While IBD patients have increased levels of blood LPS, the net effect of endotoxemia has unexpected characteristics illustrating that LPS has both pro- and antiinflammatory roles through TLR4+ B cells. Experimental treatment of B cells demonstrates that the antiinflammatory effect of LPS is due to its hypo-acylation of lipid A suggesting an increased prevalence of systemic, hypo-acylated LPS in CD. In contrast, high levels of LPS are associated with disease activity in UC. HMGB1 activates B cells through TLR2 and CD36. Serum levels of HMGB1 correlate with spontaneous IL-8 production by B cells suggesting that blood-borne TLR2 ligands increase B-cell activation in vivo.

CONCLUSIONS

Systemic TLR ligands modulate B cells towards either proinflammatory or antiinflammatory activity depending on the predominant ligand(s). Further, the circulating B cell may represent an important proxy for quantifying the LPS lipid A acylation burden in patients with IBD.

Interleukin-11 reduces TLR4-induced colitis in TLR2-deficient mice and restores intestinal STAT3 signaling

BACKGROUND & AIMS

The roles of intestinal Toll-like receptors (TLR) in the pathogenesis of colitis are not known. TLR2 and TLR4 appear to protect against dextran sodium sulfate-induced colitis by promoting mucosal integrity, but it is not clear whether this method of protection occurs in other models of colitis. We investigated the roles of TLR2 and TLR4 and the cell types that express these receptors during infectious colitis.

METHODS

We generated chimeric mice with TLR2(-/-) or TLR4(-/-) bone marrow and infected them with the bacterial pathogen Citrobacter rodentium. We assessed their susceptibility to colitis and the mechanisms of TLR-mediated mucosal integrity.

RESULTS

TLR2-expressing tissue resident cells prevented lethal colitis, whereas TLR4-dependent inflammatory responses of hematopoietic cells mediated intestinal damage. TLR2 expression protected against intestinal damage by maintaining epithelial barrier function and inducing expression of interleukin (IL)-11 from tissue resident cells in the muscularis mucosae, concurrent with epithelial activation of the transcription factor STAT3. Addition of exogenous IL-11 protected against the lethal colitis in TLR2-deficient mice via STAT3 activation in intestinal epithelial cells.

CONCLUSIONS

TLR2-dependent cytoprotective responses from tissue resident cells maintain mucosal integrity against the ultimately lethal TLR4-dependent inflammatory responses of hematopoietic cells. Whereas TLR2 protects against various noxious agents, the role of TLR4 during colitis can be either protective or damaging, depending on the stimulus. Therefore, therapeutics that reduce innate immunity (TLR2 signaling in particular) may not be beneficial to patients with colitis; they could worsen symptoms. Therapies that stimulate cytoprotective responses, like IL-11, could have benefits for patients with colitis.

Toll-like receptors in inflammatory bowel diseases: a decade later

Differential alteration of Toll-like receptor (TLR) expression in inflammatory bowel disease (IBD) was first described 10 years ago. Since then, studies from many groups have led to the current concept that TLRs represent key mediators of innate host defense in the intestine, involved in maintaining mucosal as well as commensal homeostasis. Recent findings in diverse murine models of colitis have helped to reveal the mechanistic importance of TLR dysfunction in IBD pathogenesis. It has become evident that environment, genetics, and host immunity form a multidimensional and highly interactive regulatory triad that controls TLR function in the intestinal mucosa. Imbalanced relationships within this triad may promote aberrant TLR signaling, critically contributing to acute and chronic intestinal inflammatory processes in IBD colitis and associated cancer.

Targeting Toll-like receptors: emerging therapeutics?

There is a growing interest in the targeting of Toll-like receptors (TLRs) for the prevention and treatment of cancer, rheumatoid arthritis, inflammatory bowel disease and systemic lupus erythematosus (SLE). Several new compounds are now undergoing preclinical and clinical evaluation, with a particular focus on TLR7 and TLR9 activators as adjuvants in infection and cancer, and inhibitors of TLR2, TLR4, TLR7 and TLR9 for the treatment of sepsis and inflammatory diseases. Here, we focus on TLRs that hold the most promise for drug discovery research, highlighting agents that are in the discovery phase and in clinical trials,and on the emerging new aspects of TLR-mediated signalling - such as control by ubiquitination and regulation by microRNAs - that might offer further possibilities of therapeutic manipulation.

Toll-like receptors as therapeutic targets in gastrointestinal diseases

IMPORTANCE OF THE FIELD

Toll-like receptors (TLRs) are innate immunity receptors that recognize several different antigens, initiating immunological/inflammatory responses. Recent evidence associates numerous pathophysiological processes and diseases with dysregulated activation of these receptors, conferring a potential therapeutic value to their modulation.

AREAS COVERED IN THIS REVIEW

The aim of this systematic review that covers literature from the past 10 years is to address the role of TLRs in the pathophysiology of gastrointestinal (GI) diseases as well as the therapeutic potential of modulating TLRs' signaling pathways in GI pathology.

WHAT THE READER WILL GAIN

This review shows that TLRs play an important role in the pathophysiology of several GI diseases and that modulating TLRs signaling pathways may have an enormous therapeutic potential. Different methods for modulation of TLRs' activity in GI tract, with direct agonists/antagonists but also with non-specific substances, like antibiotics or probiotics, are presented.

TAKE HOME MESSAGE

Even though TLRs modulators have been used for therapy in some GI diseases, further research, particularly in humans, is needed in order to establish the precise role of the different TLRs in the diverse GI diseases and to motivate clinical trials that consider TLRs as therapeutic targets in GI pathology.

Intestinal stem cells and their roles during mucosal injury and repair

The ability of the host to respond to intestinal injury requires the regeneration of native tissue through a highly orchestrated response from the intestinal stem cells, a population of cells located within the intestinal crypts that have the capability to repopulate the entire villous. The field of intestinal stem cell biology is thus of great interest to surgeons and non-surgeons alike, given its relevance to diseases of intestinal injury and inflammation such as inflammatory bowel disease, trauma, and necrotizing enterocolitis. The field of intestinal stem cell research has been advanced recently by the identification of the putative marker, Lgr5, which has allowed for the isolation and further characterization of the intestinal stem cell. Under the control of the WNT signaling pathway, Lgr5 marks the rapidly dividing cells of the intestinal crypt, and identifies a population of cells that is capable of regenerating the entire villous. We now review the identification of Lgr5 as an intestinal stem cell marker, identify controversies in the intestinal stem cell field, and highlight the response of the intestinal stem cell to injury within the intestinal mucosa that may occur clinically.

Inflammatory bowel disease

Insights into inflammatory bowel disease (IBD) are advancing rapidly owing to immunologic investigations of a plethora of animal models of intestinal inflammation, ground-breaking advances in the interrogation of diseases that are inherited as complex genetic traits, and the development of culture-independent methods to define the composition of the intestinal microbiota. These advances are bringing a deeper understanding to the genetically determined interplay between the commensal microbiota, intestinal epithelial cells, and the immune system and the manner in which this interplay might be modified by relevant environmental factors in the pathogenesis of IBD. This review examines these interactions and, where possible, potential lessons from IBD-directed, biologic therapies that may allow for elucidation of pathways that are central to disease pathogenesis in humans.

Inflammatory bowel disease

Insights into inflammatory bowel disease (IBD) are advancing rapidly owing to immunologic investigations of a plethora of animal models of intestinal inflammation, ground-breaking advances in the interrogation of diseases that are inherited as complex genetic traits, and the development of culture-independent methods to define the composition of the intestinal microbiota. These advances are bringing a deeper understanding to the genetically determined interplay between the commensal microbiota, intestinal epithelial cells, and the immune system and the manner in which this interplay might be modified by relevant environmental factors in the pathogenesis of IBD. This review examines these interactions and, where possible, potential lessons from IBD-directed, biologic therapies that may allow for elucidation of pathways that are central to disease pathogenesis in humans.

Toll-like receptor signalling in the intestinal epithelium: how bacterial recognition shapes intestinal function

A single layer of epithelial cells lines the small and large intestines and functions as a barrier between commensal bacteria and the rest of the body. Ligation of Toll-like receptors (TLRs) on intestinal epithelial cells by bacterial products promotes epithelial cell proliferation, secretion of IgA into the gut lumen and expression of antimicrobial peptides. As described in this Review, this establishes a microorganism-induced programme of epithelial cell homeostasis and repair in the intestine. Dysregulation of this process can result in chronic inflammatory and over-exuberant repair responses, and it is associated with the development of colon cancer. Thus, dysregulated TLR signalling by intestinal epithelial cells may explain how colonic bacteria and inflammation promote colorectal cancer.

The role of TLRs, NLRs, and RLRs in mucosal innate immunity and homeostasis

The mucosal surfaces of the gastrointestinal tract are continually exposed to an enormous antigenic load of microbial and dietary origin, yet homeostasis is maintained. Pattern recognition molecules (PRMs) have a key role in maintaining the integrity of the epithelial barrier and in promoting maturation of the mucosal immune system. Commensal bacteria modulate the expression of a broad range of genes involved in maintaining epithelial integrity, inflammatory responses, and production of antimicrobial peptides. Mice deficient in PRMs can develop intestinal inflammation, which is dependent on the microbiota, and in humans, PRM polymorphisms are associated with exacerbated inflammatory bowel disease. Innate immune responses and epithelial barrier function are regulated by PRM-induced signaling at multiple levels, from the selective expression of receptors on mucosal cells or compartments to the expression of negative regulators. Here, we describe recent advances in our understanding of innate signaling pathways, particularly by Toll-like receptors and nucleotide-binding domain and leucine-rich repeat containing receptors at mucosal surfaces.

Pathogen recognition receptors, cancer and inflammation in the gut

The pathogen recognition receptors (PRRs) initiate immediate responses against infection and tissue damage to protect the host from microbial invasion. In response to mucosal damage, intestinal PRR signaling initiates damage repair processes. Recent advances appear to link PRR abnormalities and inflammatory as well as neoplastic intestinal disorders. Emerging evidence suggests a dual role of PRRs, in which they may simultaneously induce tumorigenesis and antitumor immunity. PRR may induce tumor cell proliferation by activating cell survival signaling mainly via NF-kappaB, but this signal can activate dendritic cells to promote antitumor immunity. TLR signaling within the tumor cells may result in evasion of immune surveillance, propagation of metastatic growth, or rather, induction of tumor cell apoptosis depending on ligands. Epithelial cells induce endogenous PRR ligands when damaged or during neoplastic transformation. Targeted manipulation of PRR signaling may provide emerging opportunities for the development of new therapeutic strategies for many gastrointestinal diseases.

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) in inflammatory disorders

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) are two major forms of innate immune sensors, which provide immediate responses against pathogenic invasion or tissue injury. Activation of these sensors induces the recruitment of innate immune cells such as macrophages and neutrophils, initiates tissue repair processes, and results in adaptive immune activation. Abnormalities in any of these innate sensor-mediated processes may cause excessive inflammation due to either hyper responsive innate immune signaling or sustained compensatory adaptive immune activation. Recent gene association studies appear to reveal strong associations of NLR gene mutations and development of several idiopathic inflammatory disorders. In contrast, TLR polymorphisms are less often associated with inflammatory disorders. Nevertheless, TLRs are up-regulated in the affected tissue of most inflammatory disorders, suggesting TLR signaling is involved in the pathogenesis of chronic and/or idiopathic inflammatory disorders. NLR signaling results in the formation of a molecular scaffold complex (termed an inflammasome) and orchestrates with TLRs to induce IL-1beta and IL-18, both of which are important mediators in the majority of inflammatory disorders. Therefore, understanding the roles of TLRs and NLRs in the pathogenesis of chronic and idiopathic inflammatory disorders may provide novel targets for the prevention and/or treatment of many common and uncommon diseases involving inflammation.