Toll-like receptor 9-induced type I IFN protects mice from experimental colitis

STAT2 contributes to promotion of colorectal and skin carcinogenesis

Signal transducer and activator of transcription 2 (STAT2) is an essential transcription factor in the type I IFN (IFN-alpha/beta) signal transduction pathway and known for its role in mediating antiviral immunity and cell growth inhibition. Unlike other members of the STAT family, IFNs are the only cytokines known to date that can activate STAT2. Given the inflammatory and antiproliferative dual nature of IFNs, we hypothesized that STAT2 prevents inflammation-induced colorectal and skin carcinogenesis by altering the inflammatory immune response. Contrary to our hypothesis, deletion of STAT2 inhibited azoxymethane/dextran sodium sulfate-induced colorectal carcinogenesis as measured by prolonged survival, lower adenoma incidence, smaller polyps, and less chronic inflammation. STAT2 deficiency also inhibited 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate-induced skin carcinogenesis as indicated by reduced papilloma multiplicity. A potential mechanism by which STAT2 promotes carcinogenesis is through activation of proinflammatory mediators. Deletion of STAT2 decreased azoxymethane/dextran sodium sulfate-induced expression and release of proinflammatory mediators, such as interleukin-6 and CCL2, and decreased interleukin-6 release from skin carcinoma cells, which then decreased STAT3 activation. Our findings identify STAT2 as a novel contributor to colorectal and skin carcinogenesis that may act to increase the gene expression and secretion of proinflammatory mediators, which in turn activate the oncogenic STAT3 signaling pathway.

Effect of natural commensal-origin DNA on toll-like receptor 9 (TLR9) signaling cascade, chemokine IL-8 expression, and barrier integritiy of polarized intestinal epithelial cells

BACKGROUND AND AIM

The intestinal epithelium is constantly exposed to high levels of genetic material like bacterial DNA. Under normal physiological conditions, the intestinal epithelial monolayer as a formidable dynamic barrier with a high-polarity structure facilitates only a controlled and selective flux on components between the lumen and the underlining mucosa and even is able to facilitate structure-based macromolecules movement. The aim of this study was to test the effect of natural commensal-origin DNA on the TLR9 signaling cascade and the barrier integrity of polarized intestinal epithelial cells (IECs).

METHODS

: Polarized HT-29 and T84 cells were treated with TNF-alpha in the presence or absence of DNA from Lactobacillus rhamnosus GG (LGG) and Bifidobacterium longum. TLR9 and interleukin-8 (IL-8) mRNA expression was assessed by semiquantitative and TaqMan real-time reverse-transcription polymerase chain reaction. Expression of TLR9 protein, degradation of inhibitor of kappa B alpha (IkappaBalpha), and p38 mitogen-activated protein kinase (p38 MAP) phosphorylation were assessed by Western blotting. To further reveal the role of TLR9 signaling, the TLR9 gene was silenced by siRNA. IL-8 secretion was measured by an enzyme-linked immunosorbent assay. Nuclear factor-kappa B (NF-kappaB) activity was assessed by the electrophoretic mobility shift assay (EMSA) and NF-kappaB-dependent luciferase reporter gene assays. As an indicator of tight junction formation and monolayer integrity of epithelial cell monolayers, transepithelial electrical resistance (TER) was repetitively monitored. Transmonolayer movement of natural commensal-origin DNA across monolayers was monitored using qRT-PCR and nested PCR based on bacterial 16S rRNA genes.

RESULTS

In response to apically applied natural commensal-origin DNA, polarized HT-29 and T84 cells enhanced expression of TLR9 in a specific manner, which was subsequently associated with attenuation of TNF-alpha-induced NF-kappaB activation and NF-kappaB-mediated IL-8 expression. TLR9 silencing abolished this inhibitory effect. Apically applied LGG DNA attenuated TNF-alpha-enhanced NF-kappaB activity by reducing IkappaBalpha degradation and p38 phosphorylation. LGG DNA did not decrease the TER but rather diminished the TNF-alpha-induced TER reduction. Translocation of natural commensal-origin DNA into basolateral compartments did not occur under tested conditions.

CONCLUSIONS

Our study indicates that TLR9 signaling mediates, at least in part, the anti-inflammatory effects of natural commensal-origin DNA on the gut because TLR9 silencing abolished the inhibitory effect of natural commensal-origin DNA on TNF-alpha-induced IL-8 secretion in polarized IECs. The nature of the TLR9 agonist, the polarity of cells, and the tight junction integrity of IECs has to be taken into account in order to predict the outcome of TLR9 signaling. (Inflamm Bowel Dis 2010).

Microbe-dendritic cell dialog controls regulatory T-cell fate

Each microenvironment is controlled by a specific set of regulatory elements that have to be finely and constantly tuned to maintain local homeostasis. These environments could be site specific, such as the gut environment, or induced by chronic exposure to microbes. Various populations of dendritic cells are central to the orchestration of this control. In this review, we discuss some new findings associating dendritic cells from defined compartments with the induction and control of regulatory T cells in the context of exposure to both commensal and pathogenic microbes.

Induction of IDO-1 by immunostimulatory DNA limits severity of experimental colitis

The chronic inflammatory bowel diseases are characterized by aberrant innate and adaptive immune responses to commensal luminal bacteria. In both human inflammatory bowel disease and in experimental models of colitis, there is an increased expression of the enzyme IDO. IDO expression has the capacity to exert antimicrobial effects and dampen adaptive immune responses. In the murine trinitrobenzene sulfonic acid model of colitis, inhibition of this enzyme leads to worsened disease severity, suggesting that IDO acts as a natural break in limiting colitis. In this investigation, we show that induction of IDO-1 by a TLR-9 agonist, immunostimulatory (ISS) DNA, critically contributes to its colitis limiting capacities. ISS DNA induces intestinal expression of IDO-1 but not the recently described paralog enzyme IDO-2. This induction occurred in both epithelial cells and in subsets of CD11c(+) and CD11b(+) cells of the lamina propria, which also increase after ISS-oligodeoxynucleotide. Signaling required for intestinal IDO-1 induction involves IFN-dependent pathways, as IDO-1 was not induced in STAT-1 knockout mice. Using both the trinitrobenzene sulfonic acid and dextran sodium sulfate models of colitis, we show the importance of IDO-1s induction in limiting colitis severity. The clinical parameters and histological correlates of colitis in these models were improved by administration of the TLR-9 agonist; however, when the function of IDO is inhibited, the colitis limiting effects of ISS-oligodeoxynucleotide were abrogated. These findings support the possibility that targeted induction of IDO-1 is an approach deserving further investigation as a therapeutic strategy for diseases of intestinal inflammation.

Synthetic oligodeoxynucleotides induce gastritis in mice

To investigate whether DNA directly induces gastritis and/or peptic ulcer, we injected synthetic DNA including CpG motif (CpG-DNA) to mouse stomach. BALB/c mice were injected with either saline, acetic acid (AA), CpG-DNA, or Control-DNA. Mice were sacrificed, and sections of the stomachs were stained with hematoxylin and eosin. The lesions were histopathologically scored from 0 to 4 based on the extent of the inflammation. Populations of neutrophils and mononuclear cells infiltrated to the lesion were calculated. IFN-y mRNA expression at the injection site was analyzed by RT-PCR. The number of CpG motifs included in the complete genomes of H. pylori HP26695 and J99, Escherichia coli O157, and Salmonella Typhi was determined by genomic analysis of these bacteria. Intragastric injection with CpG-DNA induced gastritis, and statistical analysis of histological scores revealed a significant difference between saline vs CpG-DNA (p = 0.037). The population of mononuclear cells infiltrated to the lesions was significantly higher in mice injected with CpG-DNA than that injected with AA (p = 0.0061). IFN-gamma mRNA expression was detected in the CpG-DNA group. While H. pylori includes multiple CpG motifs in its genome, it has fewer than the other pathogenic gram-negative bacilli. We conclude that synthetic DNA including CpG motif directly causes gastritis in mice and induces IFN-gamma production in the stomach. Bacterial DNA including CpG motif is known to stimulate innate immunity and to cause inflammation. Thus, H. pylori genomic DNA may be one of the virulent factors involved in H. pylori infection.

TLR4 signaling in effector CD4+ T cells regulates TCR activation and experimental colitis in mice

TLRs sense various microbial products. Their function has been best characterized in DCs and macrophages, where they act as important mediators of innate immunity. TLR4 is also expressed on CD4+ T cells, but its physiological function on these cells remains unknown. Here, we have shown that TLR4 triggering on CD4+ T cells affects their phenotype and their ability to provoke intestinal inflammation. In a model of spontaneous colitis, Il10-/-Tlr4-/- mice displayed accelerated development of disease, with signs of overt colitis as early as 8 weeks of age, when compared with Il10-/- and Il10-/-Tlr9-/- mice, which did not develop colitis by 8 months. Similar results were obtained in a second model of colitis in which transfer of naive Il10-/-Tlr4-/- CD4+ T cells into Rag1-/- recipients sufficient for both IL-10 and TLR4 induced more aggressive colitis than the transfer of naive Il10-/- CD4+ T cells. Mechanistically, LPS stimulation of TLR4-bearing CD4+ T cells inhibited ERK1/2 activation upon subsequent TCR stimulation via the induction of MAPK phosphatase 3 (MKP-3). Our data therefore reveal a tonic inhibitory role for TLR4 signaling on subsequent TCR-dependent CD4+ T cell responses.

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.

Latent cytomegalovirus infection exacerbates experimental colitis

Inflammatory bowel disease (IBD) severity is positively correlated with cytomegalovirus (CMV) infection. This may reflect CMV triggering and/or exacerbating flares of IBD and/or IBD or immunosuppressive drugs administered to patients with IBD increasing susceptibility to CMV infection. Herein, we performed studies in mice to investigate these possibilities. Mice administered murine CMV (MCMV) developed signs of acute viral infection (malaise and weight loss) and had MCMV loads that were readily detected in numerous organs including the intestine. By 4 weeks, these mice manifested a "latent" infection in which MCMV levels were low but detectable by PCR. Such MCMV infection did not induce acute colitis in either wild-type mice or IL-10(-/-) mice, which are highly prone to developing colitis. However, underlying MCMV infection in an acute or latent state exacerbated the severity of colitis induced by dextran sodium sulfate (DSS). Such potentiation of DSS colitis by latent MCMV infection seemed to occur without viral reactivation. Whereas initial MCMV infection induced acute alterations in serum and intestinal cytokines, such cytokine levels returned to baseline before administration of DSS. However, the initial infection resulted in lasting elevation of antibodies to commensal bacterial antigens, suggesting that MCMV infection may have potentiated colitis via priming of the intestinal immune response to gut microbiota. Thus, underlying CMV infection can alter mucosal immunity, potentially increasing the tendency of CMV-infected hosts to develop colitis.

TLR8-mediated activation of human monocytes inhibits TL1A expression

TLR play important roles in inflammation and innate immune response to pathogens. TLR8 recognizes ssRNA and induces NF-kappaB via MyD88 signaling. TL1A is a member of the TNF superfamily that markedly enhances IFN-gamma production by IL-12/IL-18-stimulated peripheral and mucosal CD4(+) T cells. TL1A expression is increased in the mucosa of patients with inflammatory bowel disease and is considered a key mediator of Crohn's disease (CD). We have previously shown that TL1A is strongly induced by immune complexes (IC) but not TLR ligands in antigen-presenting cells. However, a potential interaction between these pro-inflammatory signaling pathways has not been investigated. IC-induced TL1A expression of monocytes was potently inhibited by a TLR8 or TLR7/8 ligand (R848) in a dose-dependent manner. Furthermore, when co-cultured with CD4(+) T cells, TLR8 ligands inhibited TL1A production, resulting in almost complete inhibition of IFN-gamma production by the CD4(+) T cells. Furthermore, we demonstrate that IFN-alpha is not required for this suppressive effect by TLR8 signaling. Our data demonstrate for the first time a direct interaction between TLR and TL1A signaling pathways. TLR8 activation may be an important, novel pathway for targeted treatment of Th1-mediated diseases, such as CD.

A role for CD47 in the development of experimental colitis mediated by SIRPalpha+CD103- dendritic cells

Mesenteric lymph node (mLN) CD103 (alphaE integrin)(+) dendritic cells (DCs) induce regulatory T cells and gut tolerance. However, the function of intestinal CD103(-) DCs remains to be clarified. CD47 is the ligand of signal regulatory protein alpha (SIRPalpha) and promotes SIRPalpha(+) myeloid cell migration. We first show that mucosal CD103(-) DCs selectively express SIRPalpha and that their frequency was augmented in the lamina propria and mLNs of mice that developed Th17-biased colitis in response to trinitrobenzene sulfonic acid. In contrast, the percentage of SIRPalpha(+)CD103(-) DCs and Th17 responses were decreased in CD47-deficient (CD47 knockout [KO]) mice, which remained protected from colitis. We next demonstrate that transferring wild-type (WT), but not CD47 KO, SIRPalpha(+)CD103(-) DCs in CD47 KO mice elicited severe Th17-associated wasting disease. CD47 expression was required on the SIRPalpha(+)CD103(-) DCs for efficient trafficking to mLNs in vivo, whereas it was dispensable on both DCs and T cells for Th17 polarization in vitro. Finally, administration of a CD47-Fc molecule resulted in reduced SIRPalpha(+)CD103(-) DC-mediated Th17 responses and the protection of WT mice from colitis. We thus propose SIRPalpha(+)CD103(-) DCs as a pathogenic DC subset that drives Th17-biased responses and colitis, and the CD47-SIRPalpha axis as a potential therapeutic target for inflammatory bowel disease.

Metabolic assessment of gradual development of moderate experimental colitis in IL-10 deficient mice

Evidence has linked genetic predisposition and environmental exposures to the worldwide pandemic of inflammatory bowel diseases (IBD), but underlying biochemical events remain largely undefined. Here, we studied the gradual development of colitis in Interleukin 10 deficient mice using a combination of (i) histopathological analysis of intestinal sections, (ii) metabolic profiling of blood plasma, and (iii) measurement of plasma inflammatory biomarkers. Data integration using chemometric tools, including Independent Component Analysis, provided a new strategy for measuring and mapping the metabolic effects associated with the development of intestinal inflammation at the age of 1, 8, 16, and 24 weeks. Chronic inflammation appeared at 8 weeks and onward, and was associated with altered cecum and colon morphologies and increased inflammatory cell infiltration into the mucosa and the submucosa. Blood plasma profiles provided additional evidence of loss of energy homeostasis, impaired metabolism of lipoproteins and glycosylated proteins. In particular, IL-10-/-mice were characterized by decreased levels of VLDL and increased concentrations of LDL and polyunsaturated fatty acids, which are related to the etiology of IBD. Moreover, higher levels of lactate, pyruvate, citrate and lowered glucose suggested increased fatty acid oxidation and glycolysis, while higher levels of free amino acids reflected muscle atrophy, breakdown of proteins and interconversions of amino acids to produce energy. These integrated system investigations demonstrate the potential of metabonomics for investigating the mechanistic basis of IBD, and it will provide novel avenues for management of IBD.

Activation of interferon regulatory factor-3 via toll-like receptor 3 and immunomodulatory functions detected in A549 lung epithelial cells exposed to misplaced U1-snRNA

U1-snRNA is an integral part of the U1 ribonucleoprotein pivotal for pre-mRNA splicing. Toll-like receptor (TLR) signaling has recently been associated with immunoregulatory capacities of U1-snRNA. Using lung A549 epithelial/carcinoma cells, we report for the first time on interferon regulatory factor (IRF)-3 activation initiated by endosomally delivered U1-snRNA. This was associated with expression of the IRF3-inducible genes interferon-β (IFN-β), CXCL10/IP-10 and indoleamine 2,3-dioxygenase. Mutational analysis of the U1-snRNA-activated IFN-β promoter confirmed the crucial role of the PRDIII element, previously proven pivotal for promoter activation by IRF3. Notably, expression of these parameters was suppressed by bafilomycin A₁, an inhibitor of endosomal acidification, implicating endosomal TLR activation. Since resiquimod, an agonist of TLR7/8, failed to stimulate A549 cells, data suggest TLR3 to be of prime relevance for cellular activation. To assess the overall regulatory potential of U1-snRNA-activated epithelial cells on cytokine production, co-cultivation with peripheral blood mononuclear cells (PBMC) was performed. Interestingly, A549 cells activated by U1-snRNA reinforced phytohemagglutinin-induced interleukin-10 release by PBMC but suppressed that of tumor necrosis factor-α, indicating an anti-inflammatory potential of U1-snRNA. Since U1-snRNA is enriched in apoptotic bodies and epithelial cells are capable of performing efferocytosis, the present data in particular connect to immunobiological aspects of apoptosis at host/environment interfaces.

Regulatory T cells in the control of host-microorganism interactions (*)

Each microenvironment requires a specific set of regulatory elements that are finely and constantly tuned to maintain local homeostasis. Various populations of regulatory T cells contribute to the maintenance of this equilibrium and establishment of controlled immune responses. In particular, regulatory T cells limit the magnitude of effector responses, which may result in failure to adequately control infection. However, regulatory T cells also help limit collateral tissue damage caused by vigorous antimicrobial immune responses against pathogenic microbes as well as commensals. In this review, we describe various situations in which the balance between regulatory T cells and effector immune functions influence the outcome of host-microorganism coexistence and discuss current hypotheses and points of polemic associated with the origin, target, and antigen specificity of both endogenous and induced regulatory T cells during these interactions.

Interleukin-12 family members and type I interferons in Th17-mediated inflammatory disorders

Cytokines produced by antigen-presenting cells govern the fate of helper T-cell responses. Herein, we review the impact of interleukin (IL)-23 and IL-27 on the outcome of T-helper (Th) 17 cell responses and discuss their impact in the pathogenesis of T-cell-mediated inflammatory disorders of autoimmune or allergic origin. We then discuss how type I interferons might influence the course of autoimmune diseases by tipping the balance between IL-12 family members.

Current concepts regarding the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating disease that predominantly affects premature neonates. The mortality associated with NEC has not changed appreciably over the past several decades. The underlying etiology of NEC remains elusive, although bacterial colonization of the gut, formula feeding, and perinatal stress have been implicated as putative risk factors. The disease is characterized by massive epithelial destruction, which results in gut barrier failure. The exact molecular and cellular mechanisms involved in this complex disease are poorly understood. Recent studies have provided significant insight into our understanding of the pathogenesis of NEC. Endogenous mediators such as prostanoids, cyclooxygenases, and nitric oxide may play a role in the development of gut barrier failure. Understanding the structural architecture of the gut barrier and the cellular mechanisms that are responsible for gut epithelial damage could lead to the development of novel diagnostic, prophylactic and therapeutic strategies in NEC.

Probiotics and immunity

Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host, including the gastrointestinal tract. While this beneficial effect was originally thought to stem from improvements in the intestinal microbial balance, there is now substantial evidence that probiotics can also provide benefits by modulating immune functions. In animal models, probiotic supplementation is able to provide protection from spontaneous and chemically induced colitis by downregulating inflammatory cytokines or inducing regulatory mechanisms in a strain-specific manner. In animal models of allergen sensitization and murine models of asthma and allergic rhinitis, orally administered probiotics can strain-dependently decrease allergen-specific IgE production, in part by modulating systemic cytokine production. Certain probiotics have been shown to decrease airway hyperresponsiveness and inflammation by inducing regulatory mechanisms. Promising results have been obtained with probiotics in the treatment of human inflammatory diseases of the intestine and in the prevention and treatment of atopic eczema in neonates and infants. However, the findings are too variable to allow firm conclusions as to the effectiveness of specific probiotics in these conditions.

Reduced microbial diversity and high numbers of one single Escherichia coli strain in the intestine of colitic mice

Commensal bacteria play a role in the aetiology of inflammatory bowel diseases (IBD). High intestinal numbers of Escherichia coli in IBD patients suggest a role of this organism in the initiation or progression of chronic gut inflammation. In addition, some E. coli genotypes are more frequently detected in IBD patients than others. We aimed to find out whether gut inflammation in an IBD mouse model is associated with a particular E. coli strain. Intestinal contents and tissue material were taken from 1-, 8-, 16- and 24-week-old interleukin 10-deficient (IL-10(-/-)) mice and the respective wild-type animals. Caecal and colonic inflammation was observed in IL-10(-/-) animals from the 8 weeks of life on accompanied by a lower intestinal microbial diversity than in the respective wild-type animals. Culture- based and molecular approaches revealed that animals with gut inflammation harboured significantly higher numbers of E. coli than healthy controls. Phylogenetic grouping according to the E. coli Reference Collection (ECOR) system and strain typing by random-amplified polymorphic DNA and pulsed-field gel electrophoresis revealed that all mice were colonized by one single E. coli strain. The strain was shown to have the O7:H7:K1 serotype and to belong to the virulence-associated phylogenetic group B2. In a co-association experiment with gnotobiotic mice, the strain outnumbered E. coli ECOR strains belonging to the phylogenetic group A and B2 respectively. A high number of virulence- and fitness-associated genes were detected in the strain's genome possibly involved in the bacterial adaptation to the murine intestine.

New insights into the roles of dendritic cells in intestinal immunity and tolerance

Dendritic cells (DCs) play a critical key role in the initiation of immune responses to pathogens. Paradoxically, they also prevent potentially damaging immune responses being directed against the multitude of harmless antigens, to which the body is exposed daily. These roles are particularly important in the intestine, where only a single layer of epithelial cells provides a barrier against billions of commensal microorganisms, pathogens, and food antigens, over a huge surface area. In the intestine, therefore, DCs are required to perform their dual roles very efficiently to protect the body from the dual threats of invading pathogens and unwanted inflammatory reactions. In this review, we first describe the biology of DCs and their interactions with other cells types, paying particular attention to intestinal DCs. We, then, examine the ways in which this biology may become misdirected, resulting in inflammatory bowel disease. Finally, we discuss how DCs potentiate immune responses against viral, bacterial, parasitic infections, and their importance in the pathogenesis of prion diseases. We, therefore, provide an overview of the complex cellular interactions that affect intestinal DCs and control the balance between immunity and tolerance.

Reciprocal expression and signaling of TLR4 and TLR9 in the pathogenesis and treatment of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a common and often fatal inflammatory disorder affecting preterm infants that develops upon interaction of indigenous bacteria with the premature intestine. We now demonstrate that the developing mouse intestine shows reciprocal patterns of expression of TLR4 and TLR9, the receptor for bacterial DNA (CpG-DNA). Using a novel ultrasound-guided in utero injection system, we administered LPS directly into the stomachs of early and late gestation fetuses to induce TLR4 signaling and demonstrated that TLR4-mediated signaling within the developing intestine follows its expression pattern. Murine and human NEC were associated with increased intestinal TLR4 and decreased TLR9 expression, suggesting that reciprocal TLR4 and TLR9 signaling may occur in the pathogenesis of NEC. Enteral administration of adenovirus expressing mutant TLR4 to neonatal mice reduced the severity of NEC and increased TLR9 expression within the intestine. Activation of TLR9 with CpG-DNA inhibited LPS-mediated TLR4 signaling in enterocytes in a mechanism dependent upon the inhibitory molecule IRAK-M. Strikingly, TLR9 activation with CpG-DNA significantly reduced NEC severity, whereas TLR9-deficient mice exhibited increased NEC severity. Thus, the reciprocal nature of TLR4 and TLR9 signaling within the neonatal intestine plays a role in the development of NEC and provides novel therapeutic approaches to this disease.

Post-translational inhibition of IP-10 secretion in IEC by probiotic bacteria: impact on chronic inflammation

Background

Clinical and experimental studies suggest that the probiotic mixture VSL#3 has protective activities in the context of inflammatory bowel disease (IBD). The aim of the study was to reveal bacterial strain-specific molecular mechanisms underlying the anti-inflammatory potential of VSL#3 in intestinal epithelial cells (IEC).

Methodology/Principal Findings

VSL#3 inhibited TNF-induced secretion of the T-cell chemokine interferon-inducible protein (IP-10) in Mode-K cells. Lactobacillus casei (L. casei) cell surface proteins were identified as active anti-inflammatory components of VSL#3. Interestingly, L. casei failed to block TNF-induced IP-10 promoter activity or IP-10 gene transcription at the mRNA expression level but completely inhibited IP-10 protein secretion as well as IP-10-mediated T-cell transmigration. Kinetic studies, pulse-chase experiments and the use of a pharmacological inhibitor for the export machinery (brefeldin A) showed that L. casei did not impair initial IP-10 production but decreased intracellular IP-10 protein stability as a result of blocked IP-10 secretion. Although L. casei induced IP-10 ubiquitination, the inhibition of proteasomal or lysosomal degradation did not prevent the loss of intracellular IP-10. Most important for the mechanistic understanding, the inhibition of vesicular trafficking by 3-methyladenine (3-MA) inhibited IP-10 but not IL-6 expression, mimicking the inhibitory effects of L. casei. These findings suggest that L. casei impairs vesicular pathways important for the secretion of IP-10, followed by subsequent degradation of the proinflammatory chemokine. Feeding studies in TNF^ΔARE and IL-10^−/− mice revealed a compartimentalized protection of VSL#3 on the development of cecal but not on ileal or colonic inflammation. Consistent with reduced tissue pathology in IL-10^−/− mice, IP-10 protein expression was reduced in primary epithelial cells.

Conclusions/Significance

We demonstrate segment specific effects of probiotic intervention that correlate with reduced IP-10 protein expression in the native epithelium. Furthermore, we revealed post-translational degradation of IP-10 protein in IEC to be the molecular mechanism underlying the anti-inflammatory effect.

Microbes in gastrointestinal health and disease

Most, if not all, animals coexist with a complement of prokaryotic symbionts that confer a variety of physiologic benefits. In humans, the interaction between animal and bacterial cells is especially important in the gastrointestinal tract. Technical and conceptual advances have enabled rapid progress in characterizing the taxonomic composition, metabolic capacity, and immunomodulatory activity of the human gut microbiota, allowing us to establish its role in human health and disease. The human host coevolved with a normal microbiota over millennia and developed, deployed, and optimized complex immune mechanisms that monitor and control this microbial ecosystem. These cellular mechanisms have homeostatic roles beyond the traditional concept of defense against potential pathogens, suggesting these pathways contribute directly to the well-being of the gut. During their coevolution, the bacterial microbiota has established multiple mechanisms to influence the eukaryotic host, generally in a beneficial fashion, and maintain their stable niche. The prokaryotic genomes of the human microbiota encode a spectrum of metabolic capabilities beyond that of the host genome, making the microbiota an integral component of human physiology. Gaining a fuller understanding of both partners in the normal gut-microbiota interaction may shed light on how the relationship can go awry and contribute to a spectrum of immune, inflammatory, and metabolic disorders and may reveal mechanisms by which this relationship could be manipulated toward therapeutic ends.

Mechanism of cytokine modulation of epithelial tight junction barrier

Cytokines play a crucial role in the modulation of inflammatory response in the gastrointestinal tract. Pro-inflammatory cytokines including tumor necrosis factor-alpha, interferon-gamma, interleukin-1beta?IL-1beta?, and interleukin-12 are essential in mediating the inflammatory response, while anti-inflammatory cytokines including interleukin-10 and transforming growth factor-beta are important in the attenuation or containment of inflammatory process. It is increasingly recognized that cytokines have an important physiological and pathological effect on intestinal tight junction (TJ) barrier. Consistent with their known pro-inflammatory activities, pro-inflammatory cytokines cause a disturbance in intestinal TJ barrier, allowing increased tissue penetration of luminal antigens. Recent studies indicate that the inhibition of cytokine induced increase in intestinal TJ permeability has an important protective effect against intestinal mucosal damage and development of intestinal inflammation. In this review, the effects of various pro-inflammatory and anti-inflammatory cytokines on intestinal TJ barrier and the progress into the mechanisms that mediate the cytokine modulation of intestinal TJ barrier are reviewed.

CpG motifs of bacterial DNA exert protective effects in mouse models of IBD by antigen-independent tolerance induction

BACKGROUND & AIMS

Prophylactic treatment of mice with CpG motifs of bacterial DNA protects from experimental inflammatory bowel disease, at least partly via induction of inhibitory T-cells. The aim of this study was to elucidate whether these CpG-dependent protective effects require presence of bacterial flora suggesting antigen-specific regulatory activity.

METHODS

Germ-free BALB/c and IL-10(-/-) mice were treated with CpG-oligodeoxynucleotides (ODN), control-ODN, or PBS. CD4(+)CD62L(+) cells of these mice were transferred into SCID recipients. CpG-ODN-treated germ-free IL-10(-/-) mice were transferred into colitogenic environment. Monoclonal antibodies were used to neutralize TGF-beta and IFN-alpha/beta during CpG-ODN treatment. CD4(+)CD62L(+) cells of donors were evaluated for cytokine secretion and FOXP3, PD-1, and CD25 expression.

RESULTS

Compared to PBS or control-ODN treatment, CpG-ODN application to germ-free donors led to decreased intestinal inflammation as indicated by histology, decreased proinflammatory cytokines, and increased IL-10 secretion. Protection was also observed after cotransfer of cells from PBS and CpG-ODN treated donors. Anti-TGF-beta and anti-INF-alpha/beta partly reversed the protective CpG-ODN effect. CpG-ODN-treated germ-free IL-10(-/-) mice transferred into colitogenic environment developed significantly less colitis than controls but not recipients of IL-10(-/-)CD4(+)CD62L(+)cells. CD4(+)CD62L(+)cells of CpG-treated germ-free animals displayed increased expression of regulatory markers.

CONCLUSIONS

Even without pre-existence of bacterial flora CpG-ODN exposition induces tolerance, indicating that CpG-ODN-induced regulatory T-cells are not bacterial antigen specific. TGF-beta and IFN-alpha/beta play major roles in induction of regulatory cells, and although IL10-independent mechanisms play a role in CpG-ODN protection, this cytokine likely is important for the effector mechanism of CpG-ODN-induced regulatory T-cells.

Live and ultraviolet-inactivated Lactobacillus rhamnosus GG decrease flagellin-induced interleukin-8 production in Caco-2 cells

Probiotics are widely used in the treatment and prevention of gastrointestinal problems. However, in some immune-compromised populations, the administration of live microorganisms may not be appropriate. A potential alternative to live microorganisms is to inactivate them as long as the beneficial function is retained. We hypothesized that UV-inactivated Lactobacillus rhamnosus GG (LGG) could downregulate interleukin-8 (IL-8) production in intestinal epithelial cells stimulated by the pathogenic ligand, flagellin, using similar mechanisms as live LGG. Caco-2 cells were pretreated with live or UV-inactivated LGG at 10(11) colony-forming units/L and stimulated by flagellin at a dose of 500 mug/L. IL-8 production was measured by ELISA, inhibitor of kappaB (IkappaB) and ubiquitinated-IkappaB (Ub-IkappaB) expression by immunoblotting and nuclear factor (NF) kappaB localization by immunofluorescence staining. Flagellin induced a 17-fold increase in IL-8 production compared with control (P < 0.05), whereas pretreatment with either live LGG or UV-inactivated LGG resulted in 66 and 59% decreases, respectively, compared with the flagellin group (P < 0.05). Flagellin-induced NFkappaB nuclear translocation was prevented by both live and UV-inactivated LGG. Flagellin decreased IkappaB, which was reversed by either live or UV-inactivated LGG (P < 0.05). UV-inactivated LGG decreased Ub-IkappaB expression (P < 0.05), although live LGG had no effect. This study supports the concept that UV-inactivated and live LGG are equally effective in decreasing IL-8 production in the intestinal epithelium. Although the mechanism involves different pathways, both alter cytoplasmic IkappaB, thereby inhibiting NFkappaB nuclear translocation.

Toll-like receptor-4 coordinates the innate immune response of the kidney to renal ischemia/reperfusion injury

Toll-like receptors (TLRs) can detect endogenous danger molecules released upon tissue injury resulting in the induction of a proinflammatory response. One of the TLR family members, TLR4, is constitutively expressed at RNA level on renal epithelium and this expression is enhanced upon renal ischemia/reperfusion (I/R) injury. The functional relevance of this organ-specific upregulation remains however unknown. We therefore investigated the specific role of TLR4 and the relative contribution of its two downstream signaling cascades, the MyD88-dependent and TRIF-dependent cascades in renal damage by using TLR4−/−, MyD88−/− and TRIF-mutant mice that were subjected to renal ischemia/reperfusion injury. Our results show that TLR4 initiates an exaggerated proinflammatory response upon I/R injury, as reflected by lower levels of chemokines and infiltrating granulocytes, less renal damage and a more preserved renal function in TLR4−/− mice as compared to wild type mice. In vitro studies demonstrate that renal tubular epithelial cells can coordinate an immune response to ischemic injury in a TLR4-dependent manner. In vivo we found that epithelial- and leukocyte-associated functional TLR4 contribute in a similar proportion to renal dysfunction and injury as assessed by bone marrow chimeric mice. Surprisingly, no significant differences were found in renal function and inflammation in MyD88−/− and TRIF-mutant mice compared with their wild types, suggesting that selective targeting of TLR4 directly may be more effective for the development of therapeutic tools to prevent I/R injury than targeting the intracellular pathways used by TLR4. In conclusion, we identified TLR4 as a cellular sentinel for acute renal damage that subsequently controls the induction of an innate immune response.

Commensal DNA limits regulatory T cell conversion and is a natural adjuvant of intestinal immune responses

The intestinal tract is in intimate contact with the commensal microflora. Nevertheless, how commensals communicate with cells to ensure immune homeostasis is still unclear. In this study, we found that gut flora DNA (gfDNA) plays a major role in intestinal homeostasis through Toll-like receptor 9 (TLR9) engagement. Tlr9(-/-) mice displayed increased frequencies of CD4(+)Foxp3(+) regulatory T (Treg) cells within intestinal effector sites and reduced constitutive IL-17- and IFN-gamma-producing effector T (Teff) cells. Complementing this, gfDNA limited lamina propria dendritic cell-induced Treg cell conversion in vitro. Further, Treg/Teff cell disequilibrium in Tlr9(-/-) mice led to impaired immune responses to oral infection and to oral vaccination. Impaired intestinal immune responses were recapitulated in mice treated with antibiotics and were reversible after reconstitution with gfDNA. Together, these data point to gfDNA as a natural adjuvant for priming intestinal responses via modulation of Treg/Teff cell equilibrium.

Opposing functions of IKKbeta during acute and chronic intestinal inflammation

NF-kappaB is a key transcriptional regulator of inflammatory responses, but also controls expression of prosurvival genes, whose products protect tissues from damage and may thus act indirectly in an antiinflammatory fashion. The variable importance of these two distinct NF-kappaB-controlled responses impacts the potential utility of NF-kappaB inhibition as a treatment strategy for intractable inflammatory conditions, such as inflammatory bowel disease. Here, we show in murine models that inhibition of IKKbeta-dependent NF-kappaB activation exacerbates acute inflammation, but attenuates chronic inflammatory disease in the intestinal tract. Acute ulcerating inflammation is aggravated because of diminished NF-kappaB-mediated protection against epithelial cell apoptosis and delayed mucosal regeneration secondary to reduced NF-kappaB-dependent recruitment of inflammatory cells that secrete cytoprotective factors. In contrast, in IL-10-deficient mice, which serve as a model of chronic T cell-dependent colitis, ablation of IKKbeta in the intestinal epithelium has no impact, yet IKKbeta deficiency in myeloid cells attenuates inflammation and prolongs survival. These results highlight the striking context and tissue dependence of the proinflammatory and antiapoptotic functions of NF-kappaB. Our findings caution against the therapeutic use of IKKbeta/NF-kappaB inhibitors in acute inflammatory settings dominated by cell loss and ulceration.

Important role for Toll-like receptor 9 in host defense against meningococcal sepsis

Neisseria meningitidis is a leading cause of meningitis and sepsis. The pathogenesis of meningococcal disease is determined by both bacterial virulence factors and the host inflammatory response. Toll-like receptors (TLRs) are prominent activators of the inflammatory response, and TLR2, -4, and -9 have been reported to be involved in the host response to N. meningitidis. While TLR4 has been suggested to play an important role in early containment of infection, the roles of TLR2 and TLR9 in meningococcal disease are not well described. Using a model for meningococcal sepsis, we report that TLR9(-/-) mice displayed reduced survival and elevated levels of bacteremia compared to wild-type mice. In contrast, TLR2(-/-) mice controlled the infection in a manner comparable to that of wild-type mice. TLR9 deficiency was also associated with reduced bactericidal activity in vitro, which was accompanied by reduced production of nitric oxide by TLR9-deficient macrophages. Interestingly, TLR9(-/-) mice recruited more macrophages to the bloodstream than wild-type mice and produced elevated levels of cytokines at late time points during infection. At the cellular level, activation of signal transduction and induction of cytokine gene expression were independent of TLR2 or TLR9 in macrophages and conventional dendritic cells. In contrast, plasmacytoid dendritic cells relied entirely on TLR9 to induce these activities. Thus, our data demonstrate an important role for TLR9 in host defense against N. meningitidis.

Toll-like receptors in inflammatory bowel disease-stepping into uncharted territory

Ulcerative colitis and Crohn’s disease are chronic relapsing-remitting inflammatory processes of the intestinal tract. The etiology of these diseases is currently unknown. However, inflammation is hypothesized to result from inappropriate activation of mucosal immunity by luminal antigens in genetically susceptible individuals. Toll-like receptors (TLRs) are a family of transmembrane proteins that act as microbial pattern recognition receptors. They are crucial initiators of innate immune responses. The role of TLRs in the pathogenesis of inflammatory bowel disease (IBD) has not been fully elucidated. In this review, we aim to analyze the available data connecting individual TLRs to intestinal inflammation and IBD.

Lactobacillus reuteri 100-23 transiently activates intestinal epithelial cells of mice that have a complex microbiota during early stages of colonization

Monoassociations of germ-free animals with colitogenic and probiotic bacterial strains trigger intestinal epithelial cell (IEC) activation and host-derived feedback mechanisms. To characterize the impact of a single nonpathogenic bacterial strain on the intestinal epithelium in the presence of an established microbiota, we inoculated reconstituted Lacotobacillus-free (RLF) mice at 8 wk of age with Lactobacillus reuteri 100-23. Primary IEC from the small intestine of L. reuteri-inoculated and control RLF mice were isolated 2, 6, and 21 d after inoculation followed by gene expression analysis (real-time PCR; Affymetrix microarrays) as well as 2-dimensional-gel electrophoreses (2D SDS-PAGE) and peptide mass fingerprinting via matrix-assisted laser desorption/ionization time of flight MS. At d 6, gene expression of proinflammatory cytokines and chemokines including interleukin (IL)-1alpha, IL-6, interferon-gamma-inducible protein 10, and macrophage inflammatory protein 2 was transiently induced, whereas gene expression levels of regulatory proteins A20 and Toll-interacting protein decreased. In addition, 8 target proteins with changes in the steady-state protein expression levels were identified at d 2 and 6 of L. reuteri colonization. Consistent with the absence of histopathology, L. reuteri-induced activation of primary IEC returned to control levels by d 21 after inoculation of RLF mice. The capability of L. reuteri 100-23 to directly trigger epithelial cell activation was confirmed in small IEC cultures using the murine cell line Mode-K. These results clearly indicate that the intestinal epithelium is reactive toward environmental changes induced by the commensal bacterial strain L. reuteri even in the presence of an already-established microbiota. The induction of transient IEC activation may help to maintain mucosal homeostasis.

Toll-like receptors, transduction-effector pathways, and disease diversity: evidence of an immunobiological paradigm explaining all human illness?

Membrane-bound Toll-like receptors (TLRs) are frontline guardians in the mammalian innate immune system. They primarily function to recognize pathogen-associated molecular patterns (PAMPs) of invading microorganisms and on activation mount rapid, nonspecific innate responses and trigger sequential delayed specific adaptive cellular responses, which are mediated by complex signal transduction pathways involving adaptor molecules, costimulatory ligands and receptors, kinases, transcription factors, and modulated gene expression. Increasing evidence of multiple functionality and diversity suggests TLRs play critical roles in noninfective medical conditions such as cardiovascular, gastrointestinal, neurologic, musculoskeletal, obstetric, renal, liver, and dermatologic diseases, allergy, autoimmunity, and tissue regeneration. The significance of TLR heterogeneity underscores the possibility for establishing a universal immunobiological model to explain all human disease. Novel immunomodulatory therapies targeting specific or multiple TLRs may in the future offer new tools to combat or eradicate pathogenesis potentially transforming the landscape of current medical treatments.

Body traffic: ecology, genetics, and immunity in inflammatory bowel disease

The abundant bacteria and other microbial residents of the human intestine play important roles in nutrient absorption, energy metabolism, and defense against microbial pathogens. The mutually beneficial relationship of host and commensal microbiota represents an ancient and major coevolution in composition and mutual regulation of the human mucosa and the resident microbial community. Inflammatory bowel disease (IBD) is a set of chronic, relapsing inflammatory intestinal diseases in which rules of normal host-microbial interaction have been violated. This review considers the components of this host-microbial mutualism and the ways in which it is undermined by pathogenic microbial traits and by host immune and epithelial functions that confer to them susceptibility in patients with IBD. Recent advances in understanding the genetics of IBD and the immunology of host-microbial interaction are opening new strategies for treatments that target host susceptibility, candidate microbial pathogens, and intestinal ecology.

Synergistic benefit in inflammatory arthritis by targeting I kappaB kinase epsilon and interferon beta

OBJECTIVES

The I kappaB kinase (IKK)-related kinase IKKepsilon regulates type I interferon expression and responses as well as proinflammatory mediator production. We examined the role of IKKepsilon in arthritis and its ability to enhance the therapeutic response to systemic interferon (IFN) beta therapy in passive murine K/BxN arthritis.

METHODS

IKKepsilon(-/-), IFN alpha(approximately)beta R(-/-) and wild type mice were given K/BxN serum and treated with polyinosinic polycytidylic acid (poly(I:C)), IFN beta, or normal saline. Clinical response and histological scores were assessed. Gene expression in the paws was measured by quantitative PCR. Serum interleukin 1a receptor agonist (IL1Ra) and IL10 were measured by ELISA and multiplex bead array.

RESULTS

Arthritis was almost completely blocked in wild type mice if arthritogenic K/BxN serum and the Toll-like receptor (TLR)3 ligand, poly(I:C), were coadministered at the onset of the model, but not in established disease. Mice deficient in IFN alpha(approximately)beta R had an accelerated course of arthritis, and did not respond to poly(I:C). IKKepsilon null mice had a modest decrease in clinical arthritis compared with heterozygous mice. Low doses of IFN beta that were ineffective in wild type mice significantly decreased clinical arthritis in IKKepsilon null mice. Articular chemokine gene expression was reduced in the IKKepsilon(-/-) mice with arthritis and secreted IL1Ra (sIL1Ra) mRNA was significantly increased. Serum levels of IL1Ra were increased in low dose IFN beta-treated IKKepsilon(-/-) mice.

CONCLUSIONS

Subtherapeutic doses of IFN beta enhance the anti-inflammatory effects of IKKepsilon deficiency, possibly by increasing production of IL1Ra and unmasking the antichemokine effects. Combination therapy with low dose IFN beta and an IKKepsilon inhibitor might improve efficacy of either agent alone and offers a novel approach to RA.

Mucosal macrophages and the regulation of immune responses in the intestine

The healthy intestinal mucosa is home to one of the largest populations of macrophages (mvarphi) in the body [Lee SH, Starkey PM, Gordon S. Quantitative analysis of total macrophage content in adult mouse tissues. Immunochemical studies with monoclonal antibody F4/80. J Exp Med 1985;161:475-89], yet little is known about their function. Resident mvarphi in the large and small intestine are distinct from other mvarphi populations in the body, with regards to both their functional properties and surface phenotype. They respond in an unconventional manner to inflammatory stimuli, with little upregulation of proteins involved in antigen presentation and T cell co-stimulation, and no production of pro-inflammatory cytokines. This suggests that under resting conditions, intestinal mvarphi may be conditioned to be anti-inflammatory in response to local stimuli such as commensal bacteria. In contrast, during inflammation, intestinal mvarphi exhibit increased bactericidal and inflammatory abilities, promote protective immunity and/or mediate pathology. Thus the status of this cell may be the key to understanding how the intestine maintains a balance between being able to generate protective immunity against pathogens, but still prevent pathological inflammation under normal conditions. In this review, we discuss the current knowledge of intestinal mvarphi biology, and highlight the different levels of immunoregulation which influence these cells, with particular focus on innate pathogen recognition receptor (PRR) function and responsiveness to microbial stimuli.

Secretion of human interferon-beta 1b by recombinant Lactococcus lactis

Interferon-beta has anti-viral, anti-proliferation and multifunctional immunomodulatory activities and shows promising clinical effects for treatment of inflammatory disorders. The recombinant human interferon-beta (huIFN-beta) 1b was expressed in the food-grade lactic acid bacterium, Lactococcus lactis, using a nisin-controlled gene expression system. huIFN-beta production from recombinant strains (with and without LEISSTCDA propeptide) was approximately 21 and 7 microg l(-1), respectively. Moreover, 95% (former strain) and 88% (latter strain) of total recombinant proteins were secreted into the culture medium. The biological activities of huIFN-beta from recombinant strains revealed similar antiviral activities of 10(7) I.U. mg(-1). These results demonstrate the potential application of recombinant strains as a food grade vehicle to deliver bioactive huIFN-beta in vivo.

Systemic control of plasmacytoid dendritic cells by CD8+ T cells and commensal microbiota

The composition of the intestinal microbial community is a distinctive individual trait that may divergently influence host biology. Because dendritic cells (DC) regulate the quality of the host response to microbiota, we evaluated DC in mice bearing distinct enteric microbial communities divergent for colitis susceptibility. Surprisingly, a selective, systemic reduction of plasmacytoid dendritic cells (pDC) was observed in isogenic mice with different microbiota: restricted flora (RF) vs specific pathogen free (SPF). This reduction was not observed in germfree mice, suggesting that the pDC deficiency was not simply due to a lack of intestinal microbial products. The microbial action was linked to cytotoxic CD8(+) T cells, since pDC in RF mice were preserved in the CD8(-/-) and perforin(-/-) genotypes, partially restored by anti-CD8beta Ab, and augmented in SPF mice bearing the TAP(-/-) genotype. Direct evidence for pDC cytolysis was obtained by rapid and selective pDC depletion in SPF mice transferred with RF CD8(+) T cells. These data indicate that commensal microbiota, via CTL activation, functionally shape systemic immune regulation that may modify risk of inflammatory disease.

Inflammatory bowel disease, past, present and future: lessons from animal models

Accumulating data from animal models indicate that Inflammatory bowel disease (IBD) is mediated by a much more complicated mechanism than previously predicted. For example, the role of an individual molecule in the pathogenesis of IBD distinctly differs depending on several factors, including the fundamental mechanism of induction of the disease, the target cell type, the phase of disease, and the environment. Therefore, it has been difficult in the past to fully explain the complicated mechanism. Novel concepts have recently been proposed to further explain the complicated mechanism of IBD. In this review, we introduce past, current, and possible future concepts for IBD models regarding T helper (Th) 1, Th2, and Th17, antigen sampling and presentation, regulatory cell networks, NOD2, Toll-like receptors, bacteria/epithelia interaction, stem cells, autophagy, microRNAs, and glycoimmunology, and we also discuss the relevance of these new concepts, developed at the bench (in animal models), to the bedside.

Muramyl dipeptide activation of nucleotide-binding oligomerization domain 2 protects mice from experimental colitis

The mechanisms underlying the susceptibility of individuals with caspase recruitment domain 15 (CARD15) mutations and corresponding abnormalities of nucleotide-binding oligomerization domain 2 (NOD2) protein to Crohn disease are still poorly understood. One possibility is based on previous studies showing that muramyl dipeptide (MDP) activation of NOD2 negatively regulates TLR2 responses and that absence of such regulation leads to heightened Th1 responses. We now report that administration of MDP protects mice from the development of experimental colitis by downregulating multiple TLR responses, not just TLR2. The basis of these in vivo findings was suggested by in vitro studies of DCs, in which we showed that prestimulation of cells with MDP reduces cytokine responses to multiple TLR ligands and this reduction is dependent on enhanced IFN regulatory factor 4 (IRF4) activity. Further studies of mouse models of colitis showed that this inhibitory role of IRF4 does in fact apply to MDP-mediated protection from colitis, since neither IRF4-deficient mice nor mice treated with siRNA specific for IRF4 were protected. These findings indicate that MDP activation of NOD2 regulates innate responses to intestinal microflora by downregulating multiple TLR responses and suggest that the absence of such regulation leads to increased susceptibility to Crohn disease.

Therapeutic impact of toll-like receptors on inflammatory bowel diseases: a multiple-edged sword

Recent studies have begun to define the mechanisms through which Toll-like receptors (TLRs) regulate intestinal homeostasis in health and disease. Current therapies for inflammatory bowel diseases (IBDs) mostly aim at interrupting the inflammatory cascade through agents that regulate TH1 or TH2 cytokine responses. As recognition grows for TLR dysfunction to play a role in IBD pathogenesis, TLRs could provide another valid interventional target for novel therapy development. However, seemingly contradictory results from studying different murine models of colitis have so far confounded whether therapeutically useful modulation of TLRs is best accomplished by activating, inhibiting, or rather a combination of both at different stages of mucosal disease. This review evaluates potential strategies as well as their rationale and future prospects.

Therapeutic applications of synthetic CpG oligodeoxynucleotides as TLR9 agonists for immune modulation

Vertebrate toll-like receptors (TLRs) sense invading pathogens by recognizing bacterial and viral structures and, as a result, activate innate and adaptive immune responses. Ten human functional TLRs have been reported so far; three of these (TLR7, 8, and 9) are expressed in intracellular compartments and respond to single-stranded nucleic acids as natural ligands. The pathogen structure selectively recognized by TLR9 in bacterial or viral DNA was identified to be CpG dinucleotides in specific sequence contexts (CpG motifs). Short phosphorothioate-stabilized oligodeoxynucleotides (ODNs) containing such motifs are used as synthetic TLR9 agonists, and different classes of ODN TLR9 agonists have been identified with distinct immune modulatory profiles. The TLR9-mediated activation of the vertebrate immune system suggests using such TLR9 agonists as effective vaccine adjuvants for infectious disease, and for the treatment of cancer and asthma/allergy. Immune activation by CpG ODNs has been demonstrated to be beneficial in animal models as a vaccine adjuvant and for the treatment of a variety of viral, bacterial, and parasitic diseases. Antitumor activity of CpG ODNs has also been established in numerous mouse models. In clinical vaccine trials in healthy human volunteers or in immunocompromised HIV-infected patients, CpG ODNs strongly enhanced vaccination efficiency. Most encouraging results in the treatment of cancers have come from human phase I and II clinical trials using CpG ODNs as a tumor vaccine adjuvant, monotherapy, or in combination with chemotherapy. Therefore, CpG ODNs represent targeted immune modulatory drugs with a broad range of potential applications.

TLR ligand suppression or enhancement of Treg cells? A double-edged sword in immunity to tumours

Toll-like receptor (TLR) agonists are potent activators of innate immune responses, activating dendritic cell (DC) maturation and inflammatory cytokine secretion by innate immune cells and as a consequence they promote adaptive immune response when coadministered with foreign antigens. There is also some evidence from mouse models that TLR ligands can help to break tolerance to self-antigens and promote immune responses to tumour antigens. Therefore, they have been exploited as adjuvants for tumour vaccines or as immunotherapeutics against cancer. Clinical evaluation of TLR agonists has resulted in a licensed immunotherapeutic for basal cell carcinoma, but there have also been disappointing results from clinical trials, with one pharmaceutical company recently halting its clinical programme. A major obstacle to the development of any active immunotherapeutic approach to cancer is the immunosuppressive environment of the growing tumour, including the induction of tolerogenic DCs and regulatory T (Treg) cells, which suppress the development of protective effector T-cell responses. This can be compounded by the use of TLR ligands as immunotherapeutics. A problem with TLR agonists that has not been fully appreciated is that they can generate suppressive as well as inflammatory responses in innate immune cells and can promote the induction of regulatory as well as effector T cells. This is part of a normal mechanism for limiting collateral damage during infection or sterile inflammation, but can constrain their ability to induce protective antitumour immunity, especially in the immune suppressed environment of the tumour. Alternatively, manipulating the TLR-activated innate immune responses to selectively blocking immunosuppressive arm, as well as that induced by the tumour, may hold the key to enhancing their efficacy as tumour immunotherapeutics and as adjuvants for cancer vaccines.

Microbial influences in inflammatory bowel diseases

The predominantly anaerobic microbiota of the distal ileum and colon contain an extraordinarily complex variety of metabolically active bacteria and fungi that intimately interact with the host's epithelial cells and mucosal immune system. Crohn's disease, ulcerative colitis, and pouchitis are the result of continuous microbial antigenic stimulation of pathogenic immune responses as a consequence of host genetic defects in mucosal barrier function, innate bacterial killing, or immunoregulation. Altered microbial composition and function in inflammatory bowel diseases result in increased immune stimulation, epithelial dysfunction, or enhanced mucosal permeability. Although traditional pathogens probably are not responsible for these disorders, increased virulence of commensal bacterial species, particularly Escherichia coli, enhance their mucosal attachment, invasion, and intracellular persistence, thereby stimulating pathogenic immune responses. Host genetic polymorphisms most likely interact with functional bacterial changes to stimulate aggressive immune responses that lead to chronic tissue injury. Identification of these host and microbial alterations in individual patients should lead to selective targeted interventions that correct underlying abnormalities and induce sustained and predictable therapeutic responses.

Immunopathogenesis of inflammatory bowel disease

Crohn’s disease and ulcerative colitis are chronic relapsing immune mediated disorders that results from an aberrant response to gut luminal antigen in genetically susceptible host. The adaptive immune response that is then triggered was widely considered to be a T-helper-1 mediated condition in Crohn’s disease and T-helper-2 mediated condition in ulcerative colitis. Recent studies in animal models, genome wide association, and basic science has provided important insights in in the immunopathogenesis of inflammatory bowel disease, one of which was the characterization of the interleukin-23/Th-17 axis.

Granulocyte macrophage colony-stimulating factor ameliorates DSS-induced experimental colitis

BACKGROUND

Sargramostim, granulocyte macrophage colony-stimulating factor (GM-CSF), a hematopoietic growth factor, stimulates cells of the intestinal innate immune system. Clinical trials show that sargramostim induces clinical response and remission in patients with active Crohn's disease. To study the mechanism, we examined the effects of GM-CSF in the dextran sulfate sodium (DSS)-induced acute colitis model. We hypothesized that GM-CSF may work through effects on dendritic cells (DCs).

METHODS

Acute colitis was induced in Balb/c mice by administration of DSS in drinking water. Mice were treated with daily GM-CSF or phosphate-buffered saline (PBS). To probe the role of plasmacytoid DCs (pDCs) in the response to GM-CSF, we further examined the effects of monoclonal antibody 440c, which is specific for a sialic acid-binding immunoglobulin (Ig)-like lectin expressed on pDCs.

RESULTS

GM-CSF ameliorates acute DSS-induced colitis, resulting in significantly improved clinical parameters and histology. Microarray analysis showed reduced expression of proinflammatory genes including TNF-alpha and IL1-beta; the results were further confirmed by real-time reverse-transcriptase polymerase chain reaction and serum Bio-plex analysis. GM-CSF treatment significantly expands pDCs and type 1 IFN production. Administration of mAb 440c completely blocked the therapeutic effect of GM-CSF. GM-CSF is also effective in RAG1(-/-) mice, demonstrating activity-independent effects on T and B cells. IFN-beta administration mimics the therapeutic effect of GM-CSF in DSS-treated mice. GM-CSF increases systemic and mucosal type 1 IFN expression and exhibits synergy with pDC activators, such as microbial cytosine-phosphate-guanosine (CpG) DNA.

CONCLUSIONS

GM-CSF is effective in the treatment of DSS colitis in a mechanism involving the 440c(+) pDC population.

TLR9-mediated recognition of DNA

The mammalian immune system senses pathogens through pattern recognition receptors (PRRs) and responds with activation. The Toll-like receptor (TLR) family that consists of 13 receptors plays a critical role in this process. TLRmediated signaling activates immune cells and leads to an innate immune response with subsequent initiation of an adaptive immune response. Toll-like receptor 9 (TLR9) recognizes deoxyribonucleic acid (DNA) leading to cellular activation and cytokine production influencing the immune response against viruses and bacteria. The stimulation of TLR9 will be exploited for adjuvant therapy and treatment of cancer or allergy. In this review we will discuss TLR9 ligands, TLR9 expression, signaling, and the therapeutic potential of TLR9 ligands in treatment of infectious or allergic diseases and cancer.