NOD2 is a negative regulator of Toll-like receptor 2–mediated T helper type 1 responses

Toll-like receptor 2 (TLR2)-TLR9 crosstalk dictates IL-12 family cytokine production in microglia

Microglia are the resident mononuclear phagocytes of the CNS parenchyma and represent an initial line of defense against invading microorganisms. Microglia utilize Toll-like receptors (TLRs) for pathogen recognition and TLR2 specifically senses conserved motifs of gram-positive bacteria including lipoproteins, lipoteichoic acids, and peptidoglycan (PGN) leading to cytokine/chemokine production. Interestingly, primary microglia derived from TLR2 knockout (KO) mice over-expressed numerous IL-12 family members, including IL-12p40, IL-12p70, and IL-27 in response to intact S. aureus, but not the less structurally complex TLR2 ligands Pam3CSK4 or PGN. The ability of intact bacteria to augment IL-12 family member expression was specific for gram-positive organisms, since numerous gram-negative strains were unable to elicit exaggerated responses in TLR2 KO microglia. Inhibition of SYK or IRAK4 signaling did not impact heightened IL-12 family member production in S. aureus-treated TLR2 KO microglia, whereas PI3K, MAPK, and JNK inhibitors were all capable of restoring exaggerated cytokine expression to wild type levels. Additionally, elevated IL-12 production in TLR2 KO microglia was ablated by a TLR9 antagonist, suggesting that TLR9 drives IL-12 family member production following exposure to intact bacteria that remains unchecked in the absence of TLR2 signaling. Collectively, these findings indicate crosstalk between TLR2 and TLR9 pathways to regulate IL-12 family member production by microglia. The summation of TLR signals must be tightly controlled to ensure the timely cessation and/or fine tuning of cytokine signaling to avoid nonspecific bystander damage due to sustained IL-12 release.

Muramyl dipeptide and its derivatives: peptide adjuvant in immunological disorders and cancer therapy

Muramyl dipeptide (MDP) is a synthetic immunoreactive peptide consisting of N-acetyl muramic acid attached to a short amino acid chain of L-Ala-D-isoGln. It was first identified in bacterial cell wall peptidoglycan as an active component in Freund's complete adjuvant. In the cell, MDP is detected by NOD2, a cytoplasmic receptor belonging to the human innate immune system. NOD2 mutations are frequently observed in patients with Crohn's disease, an autoimmune disorder, suggesting the significance of the MDP-NOD2 pathway in activating immunity. For this reason, structural modifications of MDP and its derivatives have been extensively studied in an attempt to increase adjuvant activity and boost the immune response effectively for clinical use in the treatment of cancer and other diseases. This review summarizes the synthetic chemistry of MDP and its derivatives and discusses their pharmacological action and stereoselective synthesis.

Beneficial effect of probiotics in IBD: are peptidogycan and NOD2 the molecular key effectors?

Although the beneficial capacities of probiotics are more and more substantiated, their effects clearly depend on the strains used and their mechanisms of action remain poorly understood. Recent evidences have highlighted the potential role of cell-wall components in the anti-inflammatory capacity of selected lactobacilli. In this addendum, we summarize our recent results concerning the role of peptidoglycan (PGN) and NOD2 signaling in the regulation of intestinal inflammation. We showed that the protective effect of Lactobacillus PGN is strain-specific and linked to the induction of diverse immune regulatory pathways. Moreover the beneficial effect of Lactobacillus PGN correlated with the release of a specific muropeptide sensed by NOD2. These findings allow for a better understanding of how probiotic lactobacilli exert their beneficial effect and will help guide for more successful strain selection.

NOD2, an intracellular innate immune sensor involved in host defense and Crohn's disease

Nucleotide-binding oligomerization domain 2 (NOD2) is an intracellular sensor for small peptides derived from the bacterial cell wall component, peptidoglycan. Recent studies have uncovered unexpected functions of NOD2 in innate immune responses such as induction of type I interferon and facilitation of autophagy; moreover, they have disclosed extensive cross-talk between NOD2 and Toll-like receptors, which has an indispensable role both in host defense against microbial infection and in the development of autoimmunity. Of particular interest, polymorphisms of CARD15 encoding NOD2 are associated with Crohn's disease and other autoimmune states such as graft vs. host disease. In this review, we summarize recent findings regarding normal functions of NOD2 and discuss the mechanisms by which NOD2 polymorphisms associated with Crohn's disease lead to intestinal inflammation.

Nod2: a key regulator linking microbiota to intestinal mucosal immunity

The human intestine harbors a large number of bacteria that are constantly interacting with the intestinal immune system, eliciting non-pathological basal level immune responses. Increasing evidence points to dysbiosis of microbiota in the intestine as an underlying factor in inflammatory bowel disease susceptibility. Loss-of-function mutations in NOD2 are among the stronger genetic factors linked to ileal Crohn's disease. Indeed, Nod2 is a key regulator of microbiota in the intestine, as microflora in the terminal ileum is dysregulated in Nod2-deficient mice. Nod2 is highly expressed in Paneth cells, which are responsible for the regulation of ileal microflora by anti-microbial compounds, and Nod2-deficient ileal intestinal epithelia are unable to kill bacteria efficiently. It is therefore likely that NOD2 mutations in Crohn's disease may increase disease susceptibility by altering interactions between ileal microbiota and mucosal immunity.

NLRs in immune privileged sites

Innate immune receptors such as the nucleotide-binding domain, leucine-rich repeat-containing (NBD-LRR) receptors, referred to as NLRs, are known to serve as a critical component of host defense. However, their participation in inflammatory responses within immune privileged sites such as the brain and eye is less understood. The potential importance of NLRs in regulation of inflammation within these particular sites is further underscored by their association with autoinflammatory disorders, wherein localized inflammation can occur within the brain or eye as neuroinflammation or uveitis, respectively. Many NLRs are expressed within the brain and eye and in this review, we discuss their roles in the inflammation of the central nervous system (CNS) and uveitis.

Regulation of the antimicrobial response by NLR proteins

Nucleotide-binding, oligomerization domain (NOD)-like receptor (NLR) proteins are a family of innate immune receptors that play a pivotal role in microbial sensing, leading to the initiation of antimicrobial immune responses. Dysregulation of the function of multiple NLR family members has been linked, both in mice and humans, to a propensity for infection and autoinflammatory disease. Despite our increased understanding of NLR function and interactions, many aspects related to mechanisms of sensing, downstream signaling, and in vivo functions remain elusive. In this review, we focus on key members of the NLR family, describing their activation by diverse microbes, downstream effector functions, and interactions with each other and with other innate sensor protein families. Also discussed is the role of microbial sensing by NLR receptors leading to activation of the adaptive immune arm that collaborates in the antimicrobial defense.

Innate and adaptive immunity in inflammatory bowel disease

Inflammatory bowel diseases are the consequence of a dysregulated mucosal immune system. The mucosal immune system consists of two arms, innate and adaptive immunity, that have been studied separately for a long time. Functional studies from in vivo models of intestinal inflammation as well as results from genome-wide association studies strongly suggest a cross-regulation of both arms. The present review will illustrate this interaction by selecting examples from innate immunity and adaptive immunity, and their direct impact on each other. Broadening our view by focusing on the cross-regulated areas of the mucosal immune system will not only facilitate our understanding of disease, but furthermore will allow identification of future therapeutic targets.

β2-Adrenergic agonists bias TLR-2 and NOD2 activated dendritic cells towards inducing an IL-17 immune response

This study tested the hypothesis that activation of β2-adrenoceptors on DCs influences NOD2 signaling along with its cross-talk with Toll-like receptor-2 resulting in altered Th cell priming ability. Th17 cells are a newly discovered lineage of CD4(+) T cells involved in defense against extracellular bacteria and also implicated in autoimmune disorders. Initiation and polarization of the adaptive immune response is controlled by innate immune recognition mediated by DCs. Previous studies demonstrated that adrenergic receptors modulate cytokine production by DCs and affect their Th cell priming ability. We show that the β2-adrenoceptor agonist salbutamol enhanced IL-6 production in murine bone marrow-derived DCs stimulated with the nucleotide-binding oligomerization domain 2 ligand muramyl dipeptide. However, when the Toll-like receptor-2 ligand Pam3CysSK4 was added, salbutamol inhibited IL-12 but did not alter IL-6 and IL-23 expression. Gene expression analysis showed that salbutamol inhibited the p40 subunit as well as IL-12p35, while IL-23p19 and IL-6 were stimulated. Therefore, β2-adrenoceptors modulated cytokine production resulting in a Th17 cell priming cytokine pattern. Indeed, when antigen-pulsed DCs stimulated by muramyl dipeptide or Pam3CysSK4+muramyl dipeptide in the presence of salbutamol were used for in vivo immunization, the resulting Th17/Th1 cell ratio was increased as evaluated by IL-17 and IFN-γ production. In addition, intradermal injection of norepinephrine along with Pam3CysSK4+muramyl dipeptide increased the Th17 response to an immunogenic protein and this effect was reversed by a β2-adrenoceptor antagonist. Thus, β2-adrenoceptors may be involved in the regulation of defense against extracellular bacteria and the pathogenesis of inflammatory diseases.

NLR functions beyond pathogen recognition

The last 10 years have witnessed the identification of a new class of intracellular pattern-recognition molecules--the nucleotide-binding domain and leucine-rich repeat-containing family (NLR). Members of this family garnered interest as pattern-recognition receptors able to trigger inflammatory responses against pathogens. Many studies support a pathogen-recognition function for human NLR proteins and shed light on their role in the broader control of adaptive immunity and various disease states. Other evidence suggests that NLRs function in processes unrelated to pathogen detection. Here we discuss recent advances in our understanding of the biology of the human NLR proteins and their non-pathogen-recognition function in tissue homeostasis, apoptosis, graft-versus-host disease and early development.

Organic dust augments nucleotide-binding oligomerization domain expression via an NF-{kappa}B pathway to negatively regulate inflammatory responses

Nucleotide-binding oligomerization domain 2 (NOD2) is involved in innate immune responses to peptidoglycan degradation products. Peptidoglycans are important mediators of organic dust-induced airway diseases in exposed agriculture workers; however, the role of NOD2 in response to complex organic dust is unknown. Monocytes/macrophages were exposed to swine facility organic dust extract (ODE), whereupon NOD2 expression was evaluated by real-time PCR and Western blot. ODE induced significant NOD2 mRNA and protein expression at 24 and 48 h, respectively, which was mediated via a NF-κB signaling pathway as opposed to a TNF-α autocrine/paracrine mechanism. Specifically, NF-κB translocation increased rapidly following ODE stimulation as demonstrated by EMSA, and inhibition of the NF-κB pathway significantly reduced ODE-induced NOD2 expression. However, there was no significant reduction in ODE-induced NOD2 gene expression when TNF-α was inhibited or absent. Next, it was determined whether NOD2 regulated ODE-induced inflammatory cytokine production. Knockdown of NOD2 expression by small interfering RNA resulted in increased CXCL8 and IL-6, but not TNF-α production in response to ODE. Similarly, primary lung macrophages from NOD2 knockout mice demonstrated increased IL-6, CXCL1, and CXCL1, but not TNF-α, expression. Lastly, a higher degree of airway inflammation occurred in the absence of NOD2 following acute (single) and repetitive (3 wk) ODE exposure in an established in vivo murine model. In summary, ODE-induced NOD2 expression is directly dependent on NF-κB signaling, and NOD2 is a negative regulator of complex, organic dust-induced inflammatory cytokine/chemokine production in mononuclear phagocytes.

Distinct roles for Nod2 protein and autocrine interleukin-1beta in muramyl dipeptide-induced mitogen-activated protein kinase activation and cytokine secretion in human macrophages

Elucidating factors regulating Crohn's disease-associated nucleotide-binding oligomerization domain 2 (Nod2) responses is critical to understanding the mechanisms of intestinal immune homeostasis. Stimulation of primary monocyte-derived macrophages by muramyl dipeptide (MDP), a component of bacterial peptidoglycan and specific Nod2 ligand, produces cytokines, including IL-1β. We found that IL-1β blockade profoundly inhibits MDP-induced cytokine production in human monocyte-derived macrophages, demonstrating a key role for IL-1β autocrine secretion in Nod2-mediated responses. Importantly, although MAPK activation has previously been attributed directly to Nod2 signaling, we determined that the IL-1β autocrine loop is responsible for the majority of MDP-induced MAPK activation. Because the critical effects of IL-1β autocrine secretion on MAPK activation are observed as early as 10 min after Nod2 stimulation, we hypothesized that secretion of IL-1β from preexisting intracellular pro-IL-1β stores is necessary for optimal MDP-mediated cytokine induction. Consistently, we detected IL-1β secretion within 10 min of MDP treatment. Moreover, caspase-1 inhibition significantly attenuates MDP-mediated early MAPK activation. Importantly, selective JNK/p38 activation is sufficient to rescue the decreased cytokine secretion during Nod2 stimulation in the absence of autocrine IL-1β. Finally, we found that the IL-1β autocrine loop significantly enhances responses by a broad range of pattern recognition receptors. Taken together, MDP stimulation activates Nod2 to process and release preexisting pro-IL-1β stores in a caspase-1-dependent fashion; this secreted IL-1β, in turn, contributes to the majority of MDP-initiated MAPK activation and leads to subsequent cytokine secretion. Our findings clarify mechanisms of IL-1β contributions to Nod2 responses and elucidate the dominant role of IL-1β in MDP-initiated MAPK and cytokine secretion.

NOD2 deficiency results in increased susceptibility to peptidoglycan-induced uveitis in mice

PURPOSE

The innate immune receptor NOD2 is a genetic cause of uveitis (Blau syndrome). Intriguingly, in the intestine where polymorphisms of NOD2 predispose to Crohn's disease, NOD2 reportedly suppresses inflammation triggered by the bacterial cell wall component, peptidoglycan (PGN). Whether NOD2 exerts a similar capacity in the regulation of ocular inflammation to PGN has not been explored.

METHODS

NOD2, NOD1, or MyD88 knockout (KO) mice and their wild-type (WT) controls were administered an intravitreal injection of PGN (a metabolite of which is the NOD2 agonist, muramyl dipeptide), or synthetic TLR2/1 and TLR2/6 agonists, Pam₃CSK4 and FSL-1. Ocular inflammation was assessed by intravital microscopy and histopathology. Cytokine production in eye tissue homogenates was measured by ELISA.

RESULTS

PGN triggered uveitis in mice. This inflammation was abolished in the absence of the TLR signaling mediator MyD88. NOD2 exerted a negative regulatory role because PGN-triggered eye inflammation was exacerbated in NOD2 KO mice. Increased intravascular response coincided with enhanced leukocytes within the aqueous and vitreous humors. The enhanced susceptibility of NOD2 KO mice to PGN uveitis coincided with increased cytokine production of IL-12p40, IL-17, and IL-23 but not IL-12p70, TNFα, or IFNγ. NOD1 deficiency did not result in the same sensitivity to PGN. Ocular inflammation induced by synthetic TLR2 agonists required MyD88 but not NOD2 or NOD1.

CONCLUSIONS

NOD2 may serve differential roles in the eye to promote inflammation while also tempering cell responses to PGN akin to what has been reported in colitis.

Experimental inflammatory bowel disease: insights into the host-microbiota dialog

Inflammatory bowel disease appears to result from an abnormal host immune response to the intestinal microbiota. Experimental models have allowed the dissection of the complex dialog between the host and its microbiota. Through genetic manipulation of the host genome the immune compartments, cells, molecules, and genes that are critical for maintenance of intestinal homeostasis are being identified. Genetic association studies in humans have identified over 100 susceptibility loci. Although there is remarkable coherence between the experimental model and the human genetic data, a full understanding of the mechanisms involved in genetic susceptibility to IBD and of gene-gene and gene-environmental interactions will require a "next generation" of experimental models.

Inflammatory bowel disease

This review focuses on the prominent etiological and pathogenetic aspects of inflammatory bowel disease (IBD), with particular attention being paid to the mucosal immune response to commensal micro-organisms in health and disease. Pathogenetic implications for target therapy will also be discussed. The clinical presentation, diagnostic aspects, and currently recommended therapeutic options for the two main types of IBD are also taken into consideration, including manifestations of these conditions in the oral cavity.

Autophagy, microbial sensing, endoplasmic reticulum stress, and epithelial function in inflammatory bowel disease

Increasing evidence has emerged that supports an important intersection between 3 fundamental cell biologic pathways in the pathogenesis of inflammatory bowel disease. These include the intersection between autophagy, as revealed by the original identification of ATG16L1 and IRGM as major genetic risk factors for Crohn's disease, and intracellular bacterial sensing, as shown by the importance of NOD2 in autophagy induction upon bacterial entry into the cell. A pathway closely linked to autophagy and innate immunity is the unfolded protein response, initiated by endoplasmic reticulum stress due to the accumulation of misfolded proteins, which is genetically related to ulcerative colitis and Crohn's disease (XBP1 and ORMDL3). Hypomorphic ATG16L1, NOD2, and X box binding protein-1 possess the common attribute of profoundly affecting Paneth cells, specialized epithelial cells at the bottom of intestinal crypts involved in antimicrobial function. Together with their functional juxtaposition in the environmentally exposed intestinal epithelial cell, their remarkable functional convergence on Paneth cells and their behavior in response to environmental factors, including microbes, these 3 pathways are of increasing importance to understanding the pathogenesis of inflammatory bowel disease. Moreover, in conjunction with studies that model deficient nuclear factor-κB function, these studies suggest a central role for altered intestinal epithelial cell function as one of the earliest events in the development of inflammatory bowel disease.

Streptococcus cristatus modulates the Fusobacterium nucleatum-induced epithelial interleukin-8 response through the nuclear factor-kappa B pathway

BACKGROUND AND OBJECTIVE

We previously reported that the interleukin-8 (IL-8) response to Fusobacterum nucleatum was attenuated in the presence of Streptococcus cristatus. Here, we further examined the underlying mechanism(s) involved in the modulating effect of S. cristatus by looking specifically at its impact on the nuclear factor-kappa B (NF-κB) pathway under the toll-like receptor (TLR) signaling background.

MATERIAL AND METHODS

OKF6/TERT-2 and KB cells were co-cultured with F. nucleatum and S. cristatus, either alone or in combination. Secretion of IL-8 protein was measured by ELISA. The nuclear translocation of NF-κB was evaluated by confocal microscopy, while DNA-binding activity was quantified using TransAM™ ELISA kits. Western blot analysis was performed to determine whether the anti-inflammatory effect of S. cristatus is related to the modulation of the NF-κB inhibitory protein IκB-α.

RESULTS

Incubation with F. nucleatum significantly enhanced the nuclear translocation of NF-κB. Exposure to S. cristatus alone did not cause detectable NF-κB translocation and was able to inhibit the F. nucleatum-induced NF-κB nuclear translocation. The TransAM assay further confirmed that S. cristatus blocked the nuclear translocation of NF-κB in response to F. nucleatum stimulation. In contrast to the nearly complete degradation of IκB-α induced by F. nucleatum alone, the presence of S. cristatus stabilized IκB-α. Pre-incubation with TLR2 and TLR4 antibodies, however, did not affect the epithelial response to either species alone or in combination.

CONCLUSION

The mechanism by which S. cristatus attenuates F. nucleatum-induced proinflammatory responses in oral epithelial cells appears to involve blockade of NF-κB nuclear translocation at the level of IκB-α degradation.

Defects of pattern recognition: primary immunodeficiencies of the innate immune system

Genetic defects leading to impaired recognition of invading pathogens by the innate immune system, and hence to increased susceptibility to specific classes of microorganisms have been recently recognized. To date, defects have been described in three of the major families of pattern recognition receptors (PRRs): the Toll-like receptors (TLRs), the C-type lectin receptors (CLRs), and the nucleotide binding domain leucine-rich repeat containing receptors (NLRs). By contrast, defects in the viral receptors RigI helicases have not been found. PRR defects vary greatly in severity, display a narrow susceptibility profile towards specific pathogens, and when severe in infancy and childhood, often decrease in severity thereafter. Their discovery leads to crucial insight in the pathophysiology of infections, and offer therapeutic targets for future immunotherapy.

Nod2 suppresses Borrelia burgdorferi mediated murine Lyme arthritis and carditis through the induction of tolerance

The internalization of Borrelia burgdorferi, the causative agent of Lyme disease, by phagocytes is essential for an effective activation of the immune response to this pathogen. The intracellular, cytosolic receptor Nod2 has been shown to play varying roles in either enhancing or attenuating inflammation in response to different infectious agents. We examined the role of Nod2 in responses to B. burgdorferi. In vitro stimulation of Nod2 deficient bone marrow derived macrophages (BMDM) resulted in decreased induction of multiple cytokines, interferons and interferon regulated genes compared with wild-type cells. However, B. burgdorferi infection of Nod2 deficient mice resulted in increased rather than decreased arthritis and carditis compared to control mice. We explored multiple potential mechanisms for the paradoxical response in in vivo versus in vitro systems and found that prolonged stimulation with a Nod2 ligand, muramyl dipeptide (MDP), resulted in tolerance to stimulation by B. burgdorferi. This tolerance was lost with stimulation of Nod2 deficient cells that cannot respond to MDP. Cytokine patterns in the tolerance model closely paralleled cytokine profiles in infected Nod2 deficient mice. We propose a model where Nod2 has an enhancing role in activating inflammation in early infection, but moderates inflammation after prolonged exposure to the organism through induction of tolerance.

NOD2 polymorphisms in clinical phenotypes of common variable immunodeficiency disorders

Common variable immunodeficiency disorders (CVIDs) are a heterogeneous group of diseases characterized by hypogammaglobulinaemia and consequent susceptibility to infection. CVID patients commonly develop a variety of additional manifestations for which the causative factors are not fully understood. Two such manifestations are granulomatous disease and enteropathy. Because the ability to predict complications would aid clinical management, we continue to search for possible disease modifier genes. NOD2 acts a microbial sensor and is involved in proinflammatory signalling. Particular mutations of the NOD2 gene are associated with Crohn's disease including gly908arg, leu1007finsc and arg702trp polymorphisms. We hypothesized that NOD2 polymorphisms may be a disease modifier gene towards an enteropathic or granulomatous phenotype within CVIDs. Sequence-specific primers returned genotypes for 285 CVID patients from centres across the United Kingdom and Europe. We present the frequencies of the different phenotypes of patients within our international cohort. Arg702trp polymorphisms were significantly less frequent than wild-type (WT) (P = 0·038) among international CVID patients with splenomegaly. Gly908arg polymorphisms were more prevalent than WT in UK patients with autoimmune disorders (P = 0·049) or enteropathy (P = 0·049). NOD2 polymorphisms were not more prevalent than WT in CVID patients with clinical phenotypes of granulomata. UK allele frequencies of 0·014, 0·056 and 0·026 were found for gly908arg, arg702trp and leu1007finsc NOD2 polymorphisms, respectively. These do not differ significantly from UK immunocompetent controls confirming, as expected, that in addition these NOD2 polymorphisms do not confer susceptibility to CVIDs per se.

The ever-expanding function of NOD2: autophagy, viral recognition, and T cell activation

The identification of several families of innate pattern recognition receptors has greatly enhanced our understanding of the host innate immune response against a variety of pathogens. One such family of innate receptors is the nucleotide-binding domain and leucine rich repeat containing receptors (NLRs). NOD2 has been characterized as a cytosolic sensor of bacteria peptidoglycan (PGN). For almost 10 years, NOD2 was assigned with the function of mediating the RICK- and nuclear factor-κB induced proinflammatory response triggered by PGN. Recent studies have extended the biological activity of NOD2 to include the induction of autophagy and antiviral responses, as well as mediating direct T cell activation. Here, we highlight and discuss these new findings in the context of immune activation and pathogen detection.

The control of adaptive immune responses by the innate immune system

The mammalian immune system comprises an adaptive and an innate component. The innate immune system employs a limited number of germ-line-encoded pattern-recognition receptors (PRRs) that recognize invariant pathogen-associated molecular patterns (PAMPs). In contrast, the adaptive immune system depends on the generation of a diverse repertoire of antigen receptors on T and B lymphocytes and subsequent activation and clonal expansion of cells carrying the appropriate antigen-specific receptors. Induction of adaptive immunity not only depends on direct antigen recognition by the antigen receptors but also relies on essential signals that are delivered by the innate immune system. In recent years, we have witnessed the discovery of a still expanding array of different PRR systems that govern the generation of adaptive immunity. Here, we review our current understanding of innate control of adaptive immunity. In particular, we discuss how PRRs initiate adaptive immune responses in general, discuss specific mechanisms that shape the ensuing T and B cell responses, and highlight open questions that are still awaiting answers.

NOD2 gene polymorphism rs2066844 associates with need for combined liver-intestine transplantation in children with short-gut syndrome

OBJECTIVES

The nucleotide-binding oligomerization protein 2 (NOD2) gene single nucleotide polymorphisms (SNPs) associated with Crohn's disease were recently associated with severe rejection after small-bowel transplantation (SBTx). The purpose of this study was to re-test this association and explore whether deficient innate immunity suggested by the NOD2 SNPs predisposes to intestine failure requiring isolated SBTx or combined liver-intestine failure requiring combined liver-SBTx (LSBTx).

METHODS

Archived DNA from 85 children with primary isolated SBTx or LSBTx was genotyped with Taqman biallelic discrimination assays. To minimize confounding effects of racial differences in minor allele frequencies (MAFs), allelic associations were tested in 60 Caucasian recipients (discovery cohort). Replication was sought in an independent cohort of 39 Caucasian pediatric and adult SBTx patients.

RESULTS

MAF for rs2066845 and rs2066847 was similar to that seen in 538 healthy North American Caucasians. In the discovery cohort, MAF for rs2066844 was significantly higher in LSBTx (13.5 vs. 3.6%, P=0.0007, Fisher's exact test), but not in isolated SBTx recipients (2.2 vs. 3.6%, P=NS), when compared with 538 healthy Caucasians. In addition, among LSBTx recipients who received identical immunosuppression, the minor allele of rs2066844 associated with early rejection in linear regression analysis (P=0.028) (all but one of the risk alleles were found in rejectors), decreased survival (P=0.015, log-rank, Kaplan-Meier analysis), and a 20-fold greater hazard of septic death in proportional hazard analysis (P=0.030). Steroid-resistant (severe) rejection and graft loss were associated with isolated SBTx (P=0.036 and 0.082, respectively), but not with NOD2 SNPs. The association between rs2066844 and combined liver-intestine failure requiring LSBTx was significant in the replication cohort (P=0.014), and achieved greater significance in the combined cohort (P=0.00006).

CONCLUSIONS

The NOD2 SNP rs2066844 associates with combined liver and intestinal failure in subjects with short-gut syndrome, who require combined liver-intestine transplantation, and secondarily with early rejection and septic deaths.

The NOD2 single nucleotide polymorphisms rs2066843 and rs2076756 are novel and common Crohn's disease susceptibility gene variants

Background

The aims were to analyze two novel NOD2 variants (rs2066843 and rs2076756) in a large cohort of patients with inflammatory bowel disease and to elucidate phenotypic consequences.

Methodology/Principal Findings

Genomic DNA from 2700 Caucasians including 812 patients with Crohn's disease (CD), 442 patients with ulcerative colitis (UC), and 1446 healthy controls was analyzed for the NOD2 SNPs rs2066843 and rs2076756 and the three main CD-associated NOD2 variants p.Arg702Trp (rs2066844), p.Gly908Arg (rs2066847), and p.Leu1007fsX1008 (rs2066847). Haplotype and genotype-phenotype analyses were performed. The SNPs rs2066843 (p = 3.01×10⁻⁵, OR 1.48, [95% CI 1.23-1.78]) and rs2076756 (p = 4.01×10⁻⁶; OR 1.54, [95% CI 1.28-1.86]) were significantly associated with CD but not with UC susceptibility. Haplotype analysis revealed a number of significant associations with CD susceptibility with omnibus p values <10⁻¹⁰. The SNPs rs2066843 and rs2076756 were in linkage disequilibrium with each other and with the three main CD-associated NOD2 mutations (D'>0.9). However, in CD, SNPs rs2066843 and rs2076756 were more frequently observed than the other three common NOD2 mutations (minor allele frequencies for rs2066843 and rs2076756: 0.390 and 0.380, respectively). In CD patients homozygous for these novel NOD2 variants, genotype-phenotype analysis revealed higher rates of a penetrating phenotype (rs2076756: p = 0.015) and fistulas (rs2076756: p = 0.015) and significant associations with CD-related surgery (rs2076756: p = 0.003; rs2066843: p = 0.015). However, in multivariate analysis only disease localization (p<2×10⁻¹⁶) and behaviour (p = 0.02) were significantly associated with the need for surgery.

Conclusion/Significance

The NOD2 variants rs2066843 and rs2076756 are novel and common CD susceptibility gene variants.

Peptidoglycan recognition proteins protect mice from experimental colitis by promoting normal gut flora and preventing induction of interferon-gamma

There are multiple mechanisms that protect the intestine from an excessive inflammatory response to intestinal microorganisms. We report here that all four mammalian peptidoglycan recognition proteins (PGRPs or Pglyrps) protect the host from colitis induced by dextran sulfate sodium (DSS). Pglyrp1(-/-), Pglyrp2(-/-), Pglyrp3(-/-), and Pglyrp4(-/-) mice are all more sensitive than wild-type mice to DSS-induced colitis due to a more inflammatory gut microflora, higher production of interferon-gamma, higher expression of interferon-inducible genes, and an increased number of NK cells in the colon upon initial exposure to DSS, which leads to severe hyperplasia of the lamina propria, loss of epithelial cells, and ulceration in the colon. Thus, during experimental colitis, PGRPs protect the colon of wild-type mice from an early inflammatory response and the loss of the barrier function of intestinal epithelium by promoting normal bacterial flora and by preventing damaging production of interferon-gamma by NK cells in response to injury.

Innate immune mechanisms of colitis and colitis-associated colorectal cancer

The innate immune system provides first-line defences in response to invading microorganisms and endogenous danger signals by triggering robust inflammatory and antimicrobial responses. However, innate immune sensing of commensal microorganisms in the intestinal tract does not lead to chronic intestinal inflammation in healthy individuals, reflecting the intricacy of the regulatory mechanisms that tame the inflammatory response in the gut. Recent findings suggest that innate immune responses to commensal microorganisms, although once considered to be harmful, are necessary for intestinal homeostasis and immune tolerance. This Review discusses recent findings that identify a crucial role for innate immune effector molecules in protection against colitis and colitis-associated colorectal cancer and the therapeutic implications that ensue.

Lactobacillus plantarum lipoteichoic acid down-regulated Shigella flexneri peptidoglycan-induced inflammation

Bacterial peptidoglycans (PGNs) are recognized by the host's innate immune system. This process is mediated by the NOD/CARD family of proteins, which induces inflammation by activating nuclear factor (NF)-κB. Excessive activation of monocytes by Shigella flexneri PGN (flexPGN) leads to serious inflammatory diseases such as intestinal bowel diseases (IBD) and Crohn's disease. In this study, we examined whether Lactobacillus plantarum lipoteichoic acid (pLTA) could attenuate the pro-inflammatory signaling induced by flexPGN in human monocytic THP-1 cells. Compared to control THP-1 cells, pLTA-tolerant cells showed a significant reduction in TNF-α and IL-1β production in response to flexPGN. We also examined the inhibition of NF-κB and the activation of mitogen-activated protein kinase (MAPK) in pLTA-tolerant cells. We found that the expression of NOD2 in pLTA-tolerant cells was down-regulated at the mRNA and protein levels, suggesting that pLTA is a potent modulator of the pro-inflammatory NOD2-related signaling pathways induced by flexPGN. Together, these data indicate that pLTA induces cross-tolerance against flexPGN. Notably, these effects are related not only to IL-1 signaling, which is known to play a role in LPS tolerance, but also to NOD-Rick signaling. This study provides insight into how commensal microflora may contribute to homeostasis of the host intestinal tract.

Immunosuppressive effects of tacrolimus on macrophages ameliorate experimental colitis

BACKGROUND

Tacrolimus is a novel immunomodulator for inflammatory bowel diseases. Immunosuppressive effects of tacrolimus on T cells are well known; however, the effects of tacrolimus on macrophages remain unclear. The aim of this study was to investigate the effects of tacrolimus on activated macrophages and to examine its efficacy in murine colitis models.

METHODS

Proinflammatory cytokine production from lipopolysaccharide (LPS)-stimulated peritoneal macrophages of IL-10-knockout (KO) mice with and without tacrolimus was measured. We investigated the effects of tacrolimus on nuclear factor-κB (NF-κB), mitogen-activated protein kinase (MAPK), and caspase activation in macrophages and the induction of apoptosis in macrophages in vitro and examined the in vivo apoptotic effect of tacrolimus on colonic macrophages in IL-10-KO mice. We evaluated the effect of the rectal administration of tacrolimus on colonic inflammation in IL-10-KO mice and dextran sulfate sodium (DSS)-induced colitis in CB.17/SCID mice.

RESULTS

Proinflammatory cytokine production from tacrolimus-treated macrophages was significantly lower than that from untreated cells. Tacrolimus suppressed LPS-induced activation of both NF-κB and MAPK in macrophages and induced apoptosis of macrophages via activation of caspases 3 and 9. Rectal administration of tacrolimus evoked apoptosis of colonic macrophages in IL-10-KO mice. Moreover, the rectal administration of tacrolimus ameliorated colitis in IL-10-KO mice and DSS-induced colitis in CB.17/SCID mice. Gene expression of inflammatory cytokines in colonic mucosa was significantly lower in tacrolimus-treated mice than in untreated mice.

CONCLUSIONS

Tacrolimus suppresses the function of activated macrophages and promotes their apoptosis, which may lead to the amelioration of colonic inflammation.

Overlap, common features, and essential differences in pediatric granulomatous inflammatory bowel disease

Overlap in the clinical presentation of pediatric granulomatous inflammatory bowel disease may be substantial, depending on the mode of presentation. Chronic granulomatous disease (CGD) may present with granulomatous colitis, perianal abscesses, hepatic abscesses or granulomas, failure to thrive, and obstruction of the gastrointestinal tract (including esophageal strictures and dysmotility, delayed gastric emptying, and small bowel obstruction). Anemia, thrombocytosis, elevated C-reactive protein and erythrocyte sedimentation rate, and hypoalbuminemia are nonspecific and may occur in any of the granulomatous inflammatory bowel diseases. In histology, macrophages with cytoplasmic inclusions will be rather specific for CGD. Sarcoidosis may present with abdominal pain or discomfort, diarrhea, weight loss, growth failure, delayed puberty, erythema nodosum, arthritis, uveitis, and hepatic granulomata. Only in 55% of the patients will angiotensin-converting enzyme be elevated. The noncaseating epithelioid granulomata will be unspecific. Bronchoalveolar lymphocytosis and abnormalities in pulmonary function are reported in sarcoidosis and in Crohn disease (CD) and CGD. Importantly, patients with CD may present with granulomatous lung disease, fibrosing alveolitis, and drug-induced pneumonitis. Sarcoidosis and concomitant gastrointestinal CD have been reported in patients, as well as coexistence of CD and sarcoidosis in siblings. Common susceptibility loci have been identified in CD and sarcoidosis. CD and CGD share defects in the defense mechanisms against different microbes. In the present review, common features and essential differences are discussed in clinical presentation and diagnostics--including histology--in CGD, sarcoidosis, and CD, together with 2 other granulomatous inflammatory bowel diseases, namely abdominal tuberculosis and Hermansky-Pudlak syndrome. Instructions for specific diagnosis and respective treatments are provided.

The innate immune system in the intestine

The innate immune system provides the first line of host defense against invading pathogens. Innate immune responses are initiated by germline-encoded PRR, which recognize specific structures expressed by microorganisms. TLR are a family of PRR which sense a wide range of microorganisms, including bacteria, fungi, protozoa and viruses. TLR are also expressed in the intestine and are critical for intestinal homeostasis. Recently, cytoplasmic PRR, such as NLR and RLR, have been shown to detect pathogens that have invaded the cytosol. One of the NLR, NOD2, is thought to be involved in the pathogenesis of Crohn's disease. This review focuses on the innate immune responses triggered by PRR in the intestine.

Controversy over NOD2, inflammation, and defensins

Simms LA, Doecke JD, Walsh MD, et al. Reduced alpha-defensin expression is associated with inflammation and not NOD2 mutation status in ileal Crohn’s disease. Gut. 2008;57:903–910.

Intertwined pathways of programmed cell death in immunity

Programmed cell death (PCD) occurs widely in species from every kingdom of life. It has been shown to be an integral aspect of development in multicellular organisms, and it is an essential component of the immune response to infectious agents. An analysis of the phylogenetic origin of PCD now shows that it evolved independently several times, and it is fundamental to basic cellular physiology. Undoubtedly, PCD pervades all life at every scale of analysis. These considerations provide a backdrop for understanding the complexity of intertwined, but independent, cell death programs that operate within the immune system. In particular, the contributions of apoptosis, autophagy, and necrosis in the resolution of an immune response are considered.

Role of negative regulators of TLR/IL-1 signaling in the pathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is an autoimmune-like disorder characterized by chronic, idiopathic inflammation of the intestinal mucosa. Ulcerative colitis and Crohn's disease are known as the two forms of IBD. Although the etiology of IBD is still unclear, many experimental and clinical observations suggest the involvement of environmental, hereditary and immunological factors in the pathogenesis of IBD. Recent studies have showed that IBD is associated with immunologic dysfunction. Toll-like receptors (TLR) are pathogen pattern recognition receptors expressed by immune and nonimmune cells in the intestinal mucosa that play a central role in the initiation of innate immune responses and subsequent adaptive immune responses to microbial pathogens. In recent years there has been rapid progress in our understanding of the role of positive regulation of TLR signaling in the pathogenesis of IBD. However, the role of negative regulators of TLR signaling in this process remains unclear and will therefore be summarized in this paper.

Peroxisome proliferator-activated receptor γ 2 mutation may cause a subset of ulcerative colitis

AIM

Previous studies suggest the homeostasis between acquisition of tolerance to the indigenous microflora and protective immune responses appears to be disrupted in inflammatory bowel disease (IBD). Some experimental studies indicate peroxisome proliferator-activated receptor γ (PPARγ) has been implicated as a regulator of intestinal inflammatory responses. In addition, the toll-like receptor (TLR)-4 can regulate expression of PPARγ in colonic epithelial cells. We attempted to demonstrate whether the functional imbalance between TLRs and PPARγ could lead to the onset and some polymorphisms of those genes could contribute to susceptibility to IBD.

METHODS

RT-PCR analysis were performed to detect TLR4 and PPARγ mRNA associated with those of P65 of NFκB, TNFα, MyD88, NOD2/CARD15, TLR-2,5,9, in the diseased colonic mucosa in ulcerative colitis (UC; n = 13) and Crohn's disease (CD; n = 7) compared with normal controls (n = 18). Consequently, we genotyped UC (n = 29) and CD (n = 10) compared with normal controls (n = 134) for the prevalence of suspicious mutations.

RESULTS

In a subset of UC patients who were revealed to carry PPARγ Pro12Ala mutation later, impaired expression of normal PPARγ mRNA was noted in the diseased mucosa accompanied with upregulations of MyD88 TLR-4, 5, 9, P65 and TNFα in mRNA levels. The prevalence of PPARγ Pro12Ala mutation was more frequently found in UC patients compared with CD patients and normal controls (P < 0.05).

CONCLUSIONS

These findings suggested that imbalances between TLRs and PPARγ in response to luminal bacteria could lead to colonic inflammation in some UC patients. Alternative explanations will be needed for the onset of the rest of UC and CD.

Abnormalities in the handling of intracellular bacteria in Crohn's disease

Ileal lesions in Crohn's disease (CD) patients are colonized by pathogenic adherent-invasive Escherichia coli (AIEC) able to invade and to replicate within intestinal epithelial cells. Recent advances have highlighted the importance of the innate immune system and the critical relationship between the gut flora and the intestinal mucosa. Several combinations of genetic predisposing factors to CD have been described, with the most significant replicable associations including genes for intracellular receptor of bacterial cell walls (NOD2/CARD15), and for bacterial clearance and antigen processing through autophagy (ATG16L1 and IRGM). We recently reported that in IRGM and ATG16L1 deficient cells, intracellular AIEC LF82 bacteria have enhanced replication and that autophagy deficiency surprisingly did not interfere with the ability of intracellular bacteria to survive and/or replicate for any other E. coli strains tested, including nonpathogenic, environmental, commensal, or pathogenic strains involved in gastroenteritis. As autophagy is an innate defense mechanism acting as a cell-autonomous system for elimination of intracellular pathogens, these findings lead weight to the notion that intracellular bacteria including AIEC might play a role in CD pathogenesis.

Toll-like receptor 2 deficiency results in impaired antibody responses and septic shock during Borrelia hermsii infection

Overwhelming bacteremia is a leading cause of death. To understand the mechanisms involved in protective antibody and pathological inflammatory responses during bacteremia, we have been studying the murine model of Borrelia hermsii infection. Toll-like receptor (TLR) signaling plays an important role in generating the rapid anti-B. hermsii antibody responses required for the resolution of bacteremia. Using NF-κB reporter assays, we found that B. hermsii activates TLR2 and TLR9. However, TLR2(-/-) TLR9(-/-) mice exhibited an impairment in anti-B. hermsii antibody responses similar to that of TLR2(-/-) mice. Moreover, the impairment in the antibody responses of TLR2(-/-) mice or TLR2(-/-) TLR9(-/-) mice coincides with an order-of-magnitude-higher bacteremia, and death results from septic shock, as evidenced by a dysregulated systemic cytokine response and characteristic organ pathology. Since TLR2 appears to be the major extracellular sensor stimulated by B. hermsii, we hypothesized that during elevated bacteremia the activation of intracellular sensors of bacteria triggers dysregulated inflammation in TLR2(-/-) mice. Indeed, blocking the internalization of B. hermsii prevented the induction of inflammatory cytokine responses in TLR2-deficient cells. Furthermore, we found that B. hermsii activates the cytoplasmic sensor nucleotide-binding oligomerization domain 2 (NOD2). Macrophages deficient in both TLR2 and NOD2 have impaired cytokine responses to B. hermsii compared to cells lacking TLR2 alone, and B. hermsii-infected TLR2(-/-) NOD2(-/-) mice exhibited improved survival compared to TLR2(-/-) mice. These data demonstrate that TLR2 is critical for protective immunity and suggest that, during heightened bacteremia, recognition of bacterial components by intracellular sensors can lead to pathological inflammatory responses.

The protein Nod2: an innate receptor more complex than previously assumed

For almost 10 years, Nod2 has been known as a cytosolic innate receptor able to sense peptidoglycan from Gram-positive and -negative bacteria and to trigger RIP2- and NF-κB-mediated pro-inflammatory and antibacterial response. Mutations in the gene encoding Nod2 in humans have been associated with Crohn's disease (CD). Mechanisms by which Nod2 variants can lead to CD development are still under investigation. The most admitted hypothesis suggests that the impaired function of Nod2 variants in intestinal epithelial and phagocytic cells results in deficiencies in epithelial-barrier function which subsequently lead to increased bacterial invasion and inflammation at intestinal sites. Very recent results have just reinforced this hypothesis by demonstrating that Nod2 wild-type (unlike Nod2 variants) could mediate autophagy, allowing an efficient bacterial clearance and adaptative immune response. Other recent data have attributed new roles to Nod2. Indeed, Nod2 has been shown to activate antiviral innate immune responses involving IRF3-dependent IFN-β production after viral ssRNA recognition through a RIP2-independent mechanism requiring the mitochondrial adaptor protein MAVS. Recently, Nod2 has been also shown to be exquisitely tuned to detect mycobacterial infections and mount a protective immunity against these pathogens.

[Immunopathogenesis of inflammatory bowel disease]

Inflammatory bowel diseases (IBD) are idiopathic, chronic and relapsing inflammatory conditions of the gastrointestinal tract. New insights into the pathogenesis of IBD have been provided by three lines of research: (1) studying susceptibility genes involved in the detection of bacterial components and in the regulation of the host immune response, (2) highlighting the disruption of tolerance towards the commensal microbiota and (3) unravelling the critical role of environmental factors such as sanitation and hygiene. This review presents current etiological hypothesis of IBD which argue that pathogenic intestinal bacteria and/or infectious agents initiate and perpetuate the inflammation of the gut in an individual with genetic vulnerability leading to impaired epithelial barrier function and abnormal mucosal immune responses.

Dissecting genetic predisposition to inflammatory bowel disease: current progress and prospective application

Over the last 10 years, sensitive advancement has been made in the study of genetic susceptibility to inflammatory bowel disease (IBD). Complementary methodologies of linkage, fine-mapping and candidate-gene studies have led to the identification of a number of susceptibility genes and loci, including caspase activation and recruitment domain 15 (CARD15), major histocompatibility complex (MHC) and IBD5, whereas many other genes (nucleotide oligomerization domain 1 [NOD1], tumor-upregulated CARD-containing antagonist of caspase-9 [TUCAN], Toll-like receptors [TLR], interleukin 23 receptor [IL23R], multidrug resistance 1 [MDR1], myosin IXb [MYO9B], chemokine [C-Cmotif] ligand 20 [CCL20], human beta-defensin 2 [HBD-2], autophagy-related 16-like 1 [ATG16L1]) are still awaiting confirmation. The CARD15 gene is currently the most widely replicated and investigated gene. The exact sequence of events that link CARD15 variants to early pathogenetic changes is unknown. However, the role of the encoded protein confirms the relevance of appropriate responses by the innate immune system to intestinal bacteria, including the production of antimicrobial peptides (defensins). With the implementation of new genomics and proteomics methodologies, genetic research will advance our further understanding of the clinical heterogeneity of IBD and tackle the complex interactions between genetic and environmental risk factors.

Toll-like receptors: a new target in rheumatoid arthritis?

Rheumatoid arthritis (RA) is one of the most prevalent autoimmune diseases. It is characterized by chronic inflammation of the joint leading to its destruction. Although the initiating cause remains elusive, environmental factors and genetic background are known to contribute to the etiology of RA. The role of Toll-like receptors (TLRs) in innate immunity and their ability to recognize microbial products has been well characterized. TLRs are able to recognize endogenous molecules released upon cell damage and necrosis, and are present in RA synovial fluid. Although it appears unlikely that a pathogen underlies the pathogenesis or progression of RA, the release of endogenous TLR ligands during inflammation may activate TLRs and perpetuate the disease. An increasing body of circumstantial evidence implicates TLR signaling in RA, although, at present, their involvement is not defined comprehensively. Targeting individual TLRs or their signaling transducers may provide a more specific therapy without global suppression of the immune system.