Invited review: NOD2 and defensins: translating innate to adaptive immunity in Crohn's disease

Alpha-defensins (α-Defs) in Crohn's disease: decrease of ileal α-Def 5 via permanent methylation and increase in plasma α-Def 1-3 concentrations offering biomarker utility

An impaired expression of α-defensins (α-Defs) in the ileal mucosa and, conversely, increased levels in plasma, have been reported in Crohn's disease (CD). However, the specificity and correlation of these findings with the degree of inflammation are unclear. We aimed to characterize the concentration and utility of ileal and plasma α-Defs in CD and to analyse a potential epigenetic mechanism of α-Def expression. Peripheral blood samples and ileal biopsies were obtained from patients at disease onset (aCD), from those who achieved remission (iCD) and from two control groups (healthy controls and non-CD-aetiology ileitis patients). Plasma α-Defs 1-3 and 4 were detected by enzyme-linked immunosorbent assay (ELISA); α-Def 5 by immunolocalization. Methylation analysis of the α-Def 5 gene was performed using the MassARRAY EpiTYPER system. Plasma α-Defs 1-3 concentrations were significantly higher in aCD with ileal involvement (L1, L3) versus iCD or the control groups. The α-Defs 1-3 concentrations were also similar to healthy controls in patients with non-CD ileitis. There was a significant positive correlation between plasma α-Defs 1-3 levels in aCD and the endoscopic index, as well as with C-reactive protein (CRP) levels. The immunopositivity scoring showed significantly reduced α-Def 5 expression in ileal inflamed (aCD) versus non-inflamed mucosa (iCD and healthy controls). The α-Def 5 gene showed a higher methylation status in CD patients than controls, regardless of the inflammation. Plasma α-Defs 1-3 concentrations correlate with the degree of inflammation and appear to be specific biomarkers of ileal-CD at diagnosis. Ileal α-Def 5 expression is down-regulated permanently by methylation.

How the Intricate Interaction among Toll-Like Receptors, Microbiota, and Intestinal Immunity Can Influence Gastrointestinal Pathology

The gut is able to maintain tolerance to microbial and food antigens. The intestine minimizes the number of harmful bacteria by shaping the microbiota through a symbiotic relationship. In healthy human intestine, a constant homeostasis is maintained by the perfect regulation of microbial load and the immune response generated against it. Failure of this balance may result in various pathological conditions. Innate immune sensors, such as Toll-like receptors (TLRs), may be considered an interface among intestinal epithelial barrier, microbiota, and immune system. TLRs pathway, activated by pathogens, is involved in the pathogenesis of several infectious and inflammatory diseases. The alteration of the homeostasis between physiologic and pathogenic bacteria of intestinal flora causes a condition called dysbiosis. The breakdown of homeostasis by dysbiosis may increase susceptibility to inflammatory bowel diseases. It is evident that environment, genetics, and host immunity form a highly interactive regulatory triad that controls TLR function. Imbalanced relationships within this triad may promote aberrant TLR signaling, critically contributing to acute and chronic intestinal inflammatory processes, such as in IBD, colitis, and colorectal cancer. The study of interactions between different components of the immune systems and intestinal microbiota will open new horizons in the knowledge of gut inflammation.

Inflammatory bowel disease: an impaired barrier disease

BACKGROUND

The intestinal barrier is a delicate structure composed of a single layer of epithelial cells, the mucus, commensal bacteria, immune cells, and antibodies. Furthermore, a wealth of antimicrobial peptides (AMPs) can be found in the mucus and defend the mucosa. Different lines of investigations now point to a prominent pathophysiological role of defensins, an important family of AMPs, in the pathogenesis of inflammatory bowel disease and, particularly, in small intestinal Crohn's disease.

PURPOSE

In this review, we introduce the different antimicrobial peptides of the intestinal mucosa and describe their function, their expression pattern along the gastrointestinal tract, and their spatial relationship to the mucus layer. We then focus on the alterations found in inflammatory bowel disease. Small intestinal Crohn's disease (CD) is closely linked to defects in Paneth cells (specialized secretory epithelial cells at the bottom crypts) which secrete α-defensin human defensin (HD)-5 in huge quantities in healthy individuals. Decreased expression of these antimicrobial peptides is found in ileal CD, and single nucleotide polymorphisms with the highest linkage to CD affect genes involved in Paneth cell biology and defensin secretion. Additionally, antimicrobial peptides have a role in ulcerative colitis, where the depleted mucus layer cannot fulfill its crucial function of binding defensins and other AMPs to their proper site of action.

CONCLUSION

Inflammatory bowel disease arises when the mucosal barrier is compromised in its defense against challenges from the intestinal microbiota. In ileal CD, a strong association can be found between diminished expression or defective function of defensins and the advent of intestinal inflammation.

Innate immune dysfunction in inflammatory bowel disease

The pathogenetic mechanisms that cause the two types of inflammatory bowel disease (IBD), Crohn's disease (CD) and ulcerative colitis (UC), are still under investigation. Nevertheless, there is broad agreement that luminal microbes are of particular relevance in the development of these conditions. In recent years, increasing evidence has shown that defects in the innate immunity are at the centre of both types of IBD. The innate intestinal barrier is provided by the epithelium which secretes antimicrobial peptides (so-called defensins) that are retained in the mucus layer. In ileal CD, the alpha-defensins are lacking owing to several Paneth cell defects. In colonic CD, the expression of beta-defensins is inadequate. This may be related to downregulation of the transcription factor peroxisome proliferator-activated receptor-gamma and in some cohorts is associated with a reduced HBD2 gene copy number. In UC, the mucus layer, which protects the host from the enormous amounts of luminal microbes, is defective. This is accompanied by an insufficient differentiation from intestinal stem cells towards goblet cells. All these disturbances in the gut barrier shift the balance from epithelial defence towards bacterial offence. The current treatment for CD and UC is based on suppression of this secondary inflammatory process. In future, patients may benefit from new therapeutic approaches stimulating the protective innate immune system.

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.

Altered expression of innate immunity genes in different intestinal sites of children with ulcerative colitis

BACKGROUND

Innate immunity has been very rarely investigated in ulcerative colitis and never in paediatrics. The present study was aimed at describing expression of innate immunity genes (NOD2, RIP2, α-defensins HD5 and HD6) in inflamed colon and in ileum of children with ulcerative colitis. Expression of TNFα and IL-1β was also analyzed.

METHODS

15 children with ulcerative colitis (9 pancolitis, 6 left-sided colitis) and 10 control children were enrolled. mRNA and protein expressions were detected by real time PCR and western blot assays.

RESULTS

NOD2, RIP2, IL-1β, TNFα expression levels were significantly increased in colonic mucosa of patients compared to controls (p<0.01). These genes were also upregulated (p<0.01) in the ileum of both pancolitis and left-sided colitis children. HD5 and HD6 were significantly upregulated (p<0.01) in the inflamed colon of patients as well as in the ileum of those with pancolitis.

CONCLUSIONS

An increased mucosal expression of innate immunity genes was found in the inflamed colon of children with ulcerative colitis, outlining the role of the innate immune response in disease pathogenesis. Involvement of the ileum in ulcerative colitis suggests that an immune activation can also be established in intestinal sites classically uninvolved by the inflammation, carrying implications for the treatment and course of the disease.

Antimicrobial peptides in gastrointestinal inflammation

Acute and chronic inflammations of mucosal surfaces are complex events in which the effector mechanisms of innate and adaptive immune systems interact with pathogenic and commensal bacteria. The role of constitutive and inducible antimicrobial peptides in intestinal inflammation has been investigated thoroughly over the recent years, and their involvement in various disease states is expanded ever more. Especially in the intestines, a critical balance between luminal bacteria and the antimicrobial peptides is essential, and a breakdown in barrier function by impaired production of defensins is already implicated in Crohn's disease. In this paper, we focus on the role of antimicrobial peptides in inflammatory processes along the gastrointestinal tract, while considering the resident and pathogenic flora encountered at the specific sites. The role of antimicrobial peptides in the primary events of inflammatory bowel diseases receives special attention.

Expression of nucleotide-binding oligomerization domain 2 in normal human dental pulp cells and dental pulp tissues

INTRODUCTION

The nucleotide-binding oligomerization domain (NOD) proteins belong to a distinct family of proteins that are implicated in the intracellular recognition of bacterial components. NOD2 appears to be a sensor of bacterial peptidoglycans because it recognizes a minimal motif present in all peptidoglycans. The interaction of NOD2 with downstream signaling molecules ultimately results in the activation of NF-kappaB and production of inflammatory mediators in innate immunity. As such, NOD2 may play an important role in the detection of bacterial pathogens and the initiation of inflammation within the dental pulp. This study was designed to evaluate the expression of NOD2 in normal human dental pulp cells (HDPCs) and human pulp tissues.

METHODS

Human pulp tissue samples were collected from freshly extracted human wisdom teeth, and HDPCs were prepared from the explants of normal human dental pulp tissues. Nested reverse-transcription polymerase chain reaction (Nested RT-PCR) and Western blotting were performed to detect the expression of NOD2 messenger RNA and protein, respectively. Immunohistochemical staining was used to determine the distribution of NOD2 in the pulp tissues.

RESULTS

The NOD2 messenger RNA and protein were present in normal human dental pulp tissues, with most NOD2 protein expression being localized to odontoblasts and some pulp vascular endothelial cells. In contrast, HDPCs only showed a low level of NOD2 protein expression.

CONCLUSIONS

Our results suggest that NOD2 protein expressed in HDPCs and pulp tissues may play an important role in dental immune defense.

Activation of NOD2-mediated intestinal pathway in a pediatric population with Crohn's disease

BACKGROUND

NOD2 is an intracellular protein involved in host recognition of specific bacterial molecules and is genetically associated with several inflammatory diseases, including Crohn's disease (CD). NOD2 stimulation activates the transcription factor, NF-kappaB, through RIP2, a caspase-recruitment domain-containing kinase. NOD2/RIP2 signaling also mediates the activation of antimicrobial peptides such as human alpha-defensin 5 (HD-5) and human alpha-defensin 6 (HD-6), both produced by Paneth cells. The present study is aimed at describing the downstream events triggered specifically by NOD2 induction in order to demonstrate that the protein, other than overexpressed, is also physiologically associated with RIP2 and Erbin in the bioptic intestinal inflamed specimens of children affected by CD.

METHODS

Fifteen children with CD and 10 children used as controls were entered in the study. Mucosal biopsy specimens were taken during endoscopy and mRNA and protein expressions were detected by using real-time polymerase chain reaction and Western blot.

RESULTS

NOD2 is able to form an immunocomplex with the kinase RIP2. As compared to controls, in the inflamed mucosa of patients both mRNA and protein expression levels of RIP2 are increased, and its active phosphorylated form is overexpressed.

CONCLUSIONS

In this study we provide for the first time ex vivo evidence of physiologically relevant protein interactions with NOD2, which are able to trigger the innate immune response in intestinal mucosal specimens of children with CD.

Bovine glycomacropeptide induces cytokine production in human monocytes through the stimulation of the MAPK and the NF-kappaB signal transduction pathways

BACKGROUND AND PURPOSE

Bovine glycomacropeptide (BGMP) is a natural milk peptide that is produced naturally in the gastrointestinal tract during digestion. Glycomacropepide has intestinal anti-inflammatory activity, but the mechanism of action is unknown. Here we have characterized the effects of BGMP on monocytes.

EXPERIMENTAL APPROACH

We have used human THP-1 cells as an in vitro monocyte model. The effect of BGMP on the secretion of tumour necrosis factor (TNF), interleukin (IL)-1beta and IL-8 was assessed, as well as the involvement of the NF-kappaB and MAP kinase signalling pathways. The stimulatory effect of BGMP was also tested in human peripheral blood monocytes.

KEY RESULTS

BGMP up-regulated the secretion of TNF, IL-1beta and IL-8 in a concentration-dependent fashion. The biological activity was exerted by the intact peptide, because cytokine secretion was not affected by protease inhibitors. The secretion of IL-8 and specially TNF and IL-1beta was blocked by PD98059, SP600125, SB203580 and Bay11-7082, suggesting the involvement of the MAP kinases p38, c-Jun N-terminal kinase and ERK and particularly the NF-kappaB pathway, although IL-8 secretion was independent of p38. BGMP was shown to elicit the phosphorylation of IkappaB-alpha and the nuclear translocation of the NF-kappaB subunits p50 and p65. The effect of BGMP on cytokine secretion was validated in human primary blood monocytes.

CONCLUSIONS AND IMPLICATIONS

BGMP stimulates human monocytes, operating via MAP kinase and NF-kappaB pathways. BGMP may exert an indirect intestinal anti-inflammatory effect by potentiating host defences against invading microorganisms.

Intestinal dendritic cells and epithelial barrier dysfunction in Crohn's disease

Crohn's disease (CD) is a chronic gastrointestinal inflammatory disorder considered to be the result of an inappropriate and exaggerated mucosal immune reaction to yet undefined triggers from the gut flora in genetically predisposed individuals. This inflammatory phenomenon has been characterized by an adaptive T-cell response in addition to an abnormal function of the innate immune system. Dendritic cells (DCs) are constituents of this innate system, inducing T-cell activation via antigen presentation. In the gut, mucosal DCs are separated from the luminal milieu by a monolayer of cylindrical epithelial cells that forms an anatomical and physiological barrier that controls the normal traffic of antigens between both compartments. An imbalance of colonic and ileal DC distribution in tissues from CD patients as well as functional differences between DCs isolated from normal and diseased intestinal samples have been demonstrated. Moreover, a gut barrier defect in the para- and transepithelial routes in addition to a significant reduction in the intestinal secretion of epithelial products involved in barrier function has been well documented in CD. Therefore, this may expose the diseased mucosa to overwhelming amounts of antigens, resulting in abnormal DC activation and a subsequent imbalance in their distribution. In conclusion, this review provides a summary of relevant progress in CD, intestinal epithelial permeability, and DCs highlighting a potential relationship between increased epithelial permeability and abnormal DC distribution during the pathogenesis of intestinal inflammation.

Population-based case-control study of alpha 1-antitrypsin and SLC11A1 in Crohn's disease and ulcerative colitis

BACKGROUND

Crohn's disease (CD) and ulcerative colitis (UC) are chronic inflammatory diseases of the digestive tract. Genetic factors and an abnormal immune response to infections are suspected to be involved in inflammatory bowel diseases.

METHODS

In the present study 300 blood samples from CD patients (n = 100), UC patients (n = 100), and healthy controls (n = 100) were taken from a population-based case-control study. PCR assays and capillary electrophoresis were used to detect alpha 1 antitrypsin M, S, and Z alleles and the C-to-T transition at the -237 nucleotide position of the SLC11A1 promoter. Additionally, length polymorphism of (gt)n alleles in the promoter region and TGTG and CAAA insertion/deletion in the untranslated region (3' UTR) of the SLC11A1 gene were evaluated.

RESULTS

The Z allele only for AAT was associated (P < 0.05) with CD. No other significant results were detected for AAT alleles. For SLC11A1, alleles 1 and 2 were significant (P < 0.05) for UC, but only allele 3 was significant (P < 0.05) for CD. There was a significant (P < 0.05) association of a CAAA insertion with CD but not for deletion in the 3' UTR. No differences (P < 0.05) were detected for TAAA.

CONCLUSIONS

Because AAT and SLC11A1 proteins directly or indirectly function as inhibitors of human leukocyte elastase, mutations in the AAT and SLC11A1 genes may change the balance between elastase produced by leukocytes during phagocytosis.

Infections and IBD

An interaction between infection and IBD was identified soon after Crohn's disease and ulcerative colitis were first described. Since then it has become apparent that infectious agents are involved with both the etiopathogenesis and clinical course of IBD on several levels. Whilst our understanding of this interplay is incomplete, it is clear that infections can initiate both the onset and relapse of IBD. Furthermore, the disease process itself predisposes patients to certain infections, and many drugs used to treat IBD also increase the risk of infectious complications. Attempts to establish the relative infectious risks associated with the drugs used to treat IBD remain in an early stage; but it seems that the greatest risks relate to the combined use of immunomodulating agents rather than to individual drugs. The risk of infections in patients with IBD might also be exacerbated by underuse of, and perhaps substandard response to, vaccinations. It is axiomatic that physicians treating patients with IBD must be aware of these infectious risks and of strategies to minimize them. Meanwhile, intriguing advances in the use of parasitic agents as a treatment for ulcerative colitis and Crohn's disease have introduced a new angle to the interplay between infections and IBD.

NLR, the nucleotide-binding domain leucine-rich repeat containing gene family

The NLR (nucleotide-binding domain leucine-rich repeat containing) family is found in plants and animals, and serves as crucial regulators of inflammatory and innate immune response, though its functions are likely to extend greatly beyond innate immunity, and even beyond the immune system. This review discusses recent findings regarding the function of NLR proteins in the control of IL-1, NF-kappaB, and host response to pathogens including distinct forms of cell death. The review also covers recent advances regarding the biochemical nature of NLRs, its regulation by intracellular nucleotides and extracellular ATP, by the chaperone protein HSP90, and the ubiquitin ligase-associated protein SGT1. Its role in inflammation is linked to the formation of biochemical complexes such as the inflammasome, and its roles in cell death might be linked to the proposed formation of pyroptosome and necrosome.

Host-microbe interaction: mechanisms of defensin deficiency in Crohn's disease

Defensins are endogenous antibiotics with microbicidal activity against Gram-negative and -positive bacteria, fungi, enveloped viruses and protozoa. A disturbed antimicrobial defense, as provided by Paneth and other epithelial cell defensins, seems to be a critical factor in the pathogenesis of Crohn's disease, an inflammatory disease of the intestinal tract. Different direct and indirect mechanisms leading to a breakdown of antimicrobial barrier function include direct changes in defensin gene numbers (e.g., copy number polymorphism), genetic mutations in pattern-recognition receptors (e.g., nucleotide-binding oligomerization domain 2) and, as described recently, a differentiation problem of epithelial stem cells mediated by the wingless type (Wnt) pathway. Knowledge regarding the regulation and biology of defensins provides an attractive target to open up new therapeutic avenues.