The role of defensins in the pathogenesis of chronic-inflammatory bowel disease

Defensins are endogenous antimicrobial peptides with a broad activity spectrum. Even at micromolar concentrations gramnegative and grampositive bacteria, but also mycobacteria, as well as fungi (candida), viruses (herpes) and protozoa (giardia lamblia) are destroyed. As part of the innate immune system defensins are expressed by the intestinal epithelium and contribute to the maintenance of the mucosal barrier. This barrier appears to be defective in inflammatory bowel diseases since on one hand, the immune response is directed against the "normal" luminal bacterial flora and on the other hand, mucosal adherent and invasive bacteria have been observed in these diseases. A defective defensin expression may well explain these phenomena. Indeed, Crohn's disease of the terminal ileum, especially if associated with a NOD2 mutation, is characterised by a diminished alpha-defensin (human defensin 5 and 6) expression, and in inflamed Crohn's colitis, in contrast to ulcerative colitis, the beta-defensin (human beta-defensins 2 and 3) response is reduced. Through a deficient chemical mucosal barrier this defect could lead to increased bacterial invasion into the intestinal mucosa and might well explain an adequate inflammatory response. Although the final proof that this deficient defensin response leads to a reduced antibacterial activity of the intestinal mucosa is still lacking, the most plausible concept of pathogenesis of Crohn's disease is a defensin deficiency syndrome.     

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

The nucleotide-binding oligomerisation protein 2 (NOD2) is a sensor for bacterial muramyl dipeptide, which ensures ileal expression of antimicrobial peptides (so-called alpha-defensins) and promotes cytokine and chemokine production by immunocytes and enterocytes. Defective NOD2 signaling pathway and impaired expression of defensins were inextricably linked to the pathogenesis of Crohn's disease, a common form of inflammatory bowel disease. NOD2 and defensin deficiency at the level of the epithelial barrier and gut-associated lymphoid tissue may favour Crohn's disease by failing to protect from enteropathogens and to instruct adaptive immune response in the gut micro-environment. Herein, we provide an overview on the key role of NOD2 and defensins in antigen-presenting function of dendritic cells and antigen-specific immunity. We also outline the urgent need for a better understanding of the regulators of NOD2 function and defensin biogenesis to support the development of a rational immunostimulatory treatment for restoring long-lasting immunity in Crohn's disease.     

Defensins and inflammation: The role of defensins in inflammatory bowel disease

Defensins are antimicrobial peptides produced at a variety of epithelial surfaces. In the intestinal tract, they contribute to host immunity and assist in maintaining the balance between protection from pathogens and tolerance to normal flora. However, attenuated expression of defensins compromises host immunity and hence may alter the balance toward inflammation. Altered defensin production is suggested to be an integral element in the pathogenesis of inflammatory bowel disease (IBD). Evidence for this is shown in Crohn's disease where reduced α-defensin levels are seen in patients with ileal disease and reduced β-defensin levels in those with colonic involvement. Further evidence is provided by research linking nucleotide oligomerization domain 2 (NOD2) mutations and deficient defensin expression. However, alternate studies suggest that NOD2 status and defensin expression are independent, and that defensin deficiency is due to mucosal surface destruction as a result of inflammatory changes, indicating that reduced defensin expression is a symptom of the disease and not the cause. Although it is clear that defensin expression is altered in IBD, it is less clear whether defensin deficiency is implicated in the pathogenesis of IBD or is a consequence of the disease process. The aim of this article is to review the current knowledge of defensins in IBD and discuss their potential role in IBD pathogenesis.     

Reduced mucosal antimicrobial activity in Crohn's disease of the colon

OBJECTIVES: In order to maintain the mucosal barrier against luminal microorganisms the intestinal epithelial cells synthesise various broad-spectrum antimicrobial peptides including defensins and cathelicidins. Recent studies indicate that both may be deficient in Crohn's disease. To elucidate the possible functional consequences of this deficiency antimicrobial activity in colonic mucosa from patients with inflammatory bowel disease and healthy controls was investigated. METHODS: A flow cytometric assay was established to quantitate bacterial killing and test the antibacterial activity of cationic peptide extracts from colonic biopsies taken from patients with active or inactive ileocolonic or colonic Crohn's disease (n = 22), ulcerative colitis (n = 29) and controls (n = 13) against clinical isolates of Bacteroides vulgatus and Enterococcus faecalis or the reference strains Escherichia coli American Type Culture Collection (ATCC) 25922 and Staphylococcus aureus ATCC 25923. RESULTS: Compared with controls and ulcerative colitis there was a reduced antimicrobial effect in Crohn's disease extracts that was most evident against B. vulgatus. The antimicrobial effect against E. coli and E. faecalis was significantly lower in Crohn's disease compared with ulcerative colitis. Activity against S. aureus disclosed a similar pattern, but was less pronounced. The differences were independent of the inflammation status or concurrent steroid treatment. Bacteria incubated with biopsy extracts from ulcerative colitis patients frequently showed a characteristic change in cell size and granularity, compatible with more extensive membrane disintegration, compared with bacteria incubated with extracts from controls or Crohn's disease. CONCLUSION: Crohn's disease of the colon is characterized by a diminished functional antimicrobial activity that is consistent with the reported low antibacterial peptide expression.     

Functional consequences of NOD2 (CARD15) mutations

Polymorphisms in NOD2 (CARD15) are associated with ileal and ileocolonic Crohn's disease, increased mortality from graft-versus-host disease, and Blau syndrome. NOD2 activation by peptidoglycan components initiates various signaling pathways and CD-associated NOD2 mutations are associated with decreased activation of NF-kappaB. NOD2 may be important for both initial defenses against commensal and pathogenic bacteria and tolerance mechanisms for maintaining controlled activation of the intestinal immune system. Significant progress has been made in defining NOD2 signaling partners and pathways and functional consequences of NOD2 mutations with respect to its activation, expression, signaling, synergistic effects with Toll-like receptor signaling, and antimicrobial effects. However, NOD2 contributions to human intestinal inflammation are complex and incompletely understood. Improved understanding of NOD2-mediated pathways may lead to identification of other molecules that can also contribute to the development of Crohn's disease in humans.     

肠黏膜屏障损伤与保护分子机制研究进展

介绍肠黏膜屏障损伤因素的分子机制,如内毒素及氧自由基、炎症介质和细胞因子如白细胞介素（interleukin）、肿瘤坏死因子-α（TNF-α）、核转录因子kappa B、TOLL样受体（TLRs）和NOD受体通路、高迁移率族蛋白B1、烟酰胺腺嘌呤二核苷酸磷酸（NADPH）氧化酶（NOX）等。对肠黏膜屏障的保护分子机制如肠上皮紧密连接蛋白、白介素家族、其它保护性调控因子等作出介绍。     

Bacterial interactions with cells of the intestinal mucosa: Toll-like receptors and NOD2

Toll-like receptors (TLR) and NOD2 are emerging as key mediators of innate host defence in the intestinal mucosa, crucially involved in maintaining mucosal as well as commensal homeostasis. Recent observations suggest new (patho-) physiological mechanisms of how functional versus dysfunctional TLRx/NOD2 pathways may oppose or favour inflammatory bowel disease (IBD). In health, TLRx signalling protects the intestinal epithelial barrier and confers commensal tolerance whereas NOD2 signalling exerts antimicrobial activity and prevents pathogenic invasion. In disease, aberrant TLRx and/or NOD2 signalling may stimulate diverse inflammatory responses leading to acute and chronic intestinal inflammation with many different clinical phenotypes.     

Why does Crohn's disease usually occur in terminal ileum?

Crohn's disease can affect any part of the gastrointestinal tract, but terminal ileum is the most frequent localization. The reason why Crohn's disease is primarily located in the distal part of the ileum remains unexplained.In this article it has been attempted to provide a compelling explanation why Crohn's disease usually occurs in terminal ileum. Recent data indicate that some individuals are genetically predisposed to develop ileal Crohn's disease. Two genetic alterations, the polymorphism of Caspase Associated Recruitment Domain (CARD15) and Carcinoembryonic Antigen-related Cell Adhesion Molecule 6 (CEACM6), favour the colonization of terminal ileum by entero adherent-invasive Escherichia coli (AIEC). The adhesion of these bacteria to epithelial intestinal cells depends on Carcinoembryonic Antigen-related Cell Adhesion Molecule 6 expression in ileal epithelial cells and on the reduced ileal defensins expressed in a CARD15 dependent manner. Genetic defects in Authophagy-related 16-like gene (ATG16L1) and Immunity-related Guanosine Triphospatase (IRGM) recently found in ileal CD patients lead to a reduction of bacterial killing by macrophages and consequent continuous immunological upstimulation, cytokine secretion, chronic inflammation of the ileum and tissue injury. On the basis of all these data Crohn's disease of the ileum seems to be a subset of the disease mainly genetically determined.     

Epithelial restitution and wound healing in inflammatory bowel disease

Inflammatory bowel disease is characterized by a chronic inflammation of the intestinal mucosa. The mucosal epithelium of the alimentary tract constitutes a key element of the mucosal barrier to a broad spectrum of deleterious substances present within the intestinal lumen including bacterial microorganisms, various dietary factors, gastrointestinal secretory products and drugs. In addition, this mucosal barrier can be disturbed in the course of various intestinal disorders including inflammatory bowel diseases. Fortunately, the integrity of the gastrointestinal surface epithelium is rapidly reestablished even after extensive destruction. Rapid resealing of the epithelial barrier following injuries is accomplished by a process termed epithelial restitution, followed by more delayed mechanisms of epithelial wound healing including increased epithelial cell proliferation and epithelial cell differentiation. Restitution of the intestinal surface epithelium is modulated by a range of highly divergent factors among them a broad spectrum of structurally distinct regulatory peptides, variously described as growth factors or cytokines. Several regulatory peptide factors act from the basolateral site of the epithelial surface and enhance epithelial cell restitution through TGF-13-dependent pathways. In contrast, members of the trefoil factor family （TFF peptides） appear to stimulate epithelial restitution in conjunction with mucin glycoproteins through a TGF-13-independent mechanism from the apical site of the intestinal epithelium. In addition, a number of other peptide molecules like extracellular matrix factors and blood clotting factors and also non- peptide molecules including phospholipids~ short-chain fatty acids （SCFA）, adenine nucleotides, trace elements and pharmacological agents modulate intestinal epithelial repair mechanisms. Repeated damage and injury of the intestinal surface are key features of various intestinal disorders including inflammatory bowel diseases and require constant repair of the epi     

Role of bacteria in the etiopathogenesis of inflammatory bowel disease

Increased numbers of mucosa-associated Escherichia coli are observed in both of the major inflammatory bowel diseases, Crohn＇s disease （CD） and ulcerative colitis （UC）. A potential pathophysiological link between the presence of pathogenic invasive bacteria and genetic host susceptibility of patients with ileal CD is suspected. In CD patients, with increased ileal expression of the CEACAM6 molecule acting as a receptor recognized by type 1 pilus bacterial adhesin, and with the identification of mutations in the NOD2-encoding gene, the presence of pathogenic invasive bacteria could be the link between abnormal ileal bacterial colonization and innate immune responses to invasive bacteria. In a susceptible host, the sequential etiological steps of the disease induced by adherent-invasive E. coli （AIEC） are： （1） abnormal colonization via binding to the CEACAM6 receptor, which is overexpressed in the ileal mucosa of CD patients; （2） ability to adhere to and to invade intestinal epithelial cells, which allows bacteria to cross the mucosal barrier; （3） survival and replication within infected macrophages in the lamina propria; and （4） induction of tumor necrosis factor-α secretion and granuloma formation.