Innate Antimicrobial Immunity in Inflammatory Bowel Diseases

[David in Seattle]

Julia Beisner; Eduard F Stange; Jan Wehkamp

Authors and Disclosures

Expert Rev Clin Immunol. 2010;6(5):809-818. © 2010 Expert Reviews Ltd.

Abstract and Introduction
Abstract

Inflammatory bowel diseases are characterized by chronic intestinal inflammation at different sites. Data from animal models as well as human patients including gene-association studies suggest that different components of the innate barrier function are primarily defective. These recent advances support the evolving hypothesis that intestinal bacteria induce inflammation predominantly as a result of a weakened innate mucosal barrier in genetically predisposed individuals. This article discusses our current understanding of the primary events of disease. Together, these findings should result in new therapeutic avenues aimed at restoring antimicrobial barrier function to prevent a bacterial-triggered inflammatory response.

Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is characterized by chronic inflammation of the GI tract. The two major types of IBD are Crohn's disease (CD) and ulcerative colitis (UC). Whereas in UC inflammation is typically restricted to the colon, CD can affect any part of the GI tract, most commonly the small intestine and the colon.[1] Currently, classical anti-inflammatory and immunosuppressive drugs are commonly used in first-line therapy. The conventional therapies primarily target the mucosal inflammatory response and associated symptoms rather than treating the primary cause of the disease.

Although the precise etiology is still unknown, it is clear that complex mechanisms involving epithelial barrier function and innate and adaptive immunity play an important role in the disease pathogenesis. Research during the last few decades has mainly concentrated on the role of the adaptive immune system. In recent years this focus has shifted towards innate immunity, including antimicrobial host defense. Given the central role of the epithelium as a first line of defense against the luminal microbiota, the importance of the intestinal barrier has strongly attracted attention. Several studies have revealed that IBD patients show a compromised mucosal host defense and that clinical phenotypes of disease location are characterized by different defects of antibacterial immune function.[2] Data from animal models as well as gene-association studies suggest that different components of barrier function are primarily defective. These recent advances support the evolving hypothesis that intestinal bacteria induce inflammation predominantly as a result of a weakened innate mucosal barrier in genetically predisposed individuals. It is now widely accepted that these defects in the intestinal barrier play a central role in disease pathogenesis. A key question still remains as to whether the dysfunction of the intestinal barrier is a primary factor causing inflammation or rather is a consequence of the action of inflammatory mediators, although both concepts are not mutually exclusive. Understanding further mechanistic details will resolve the interplay between the innate and adaptive immune responses. This article aims to explain and discuss the current view and concepts for the primary events in disease pathogenesis.

Role of Luminal & Mucosal Bacteria in Pathogenesis

The mammalian gut harbors approximately 500–1000 different species of bacteria, which make up several trillions of microorganisms.[3,4] The two major types of IBD, CD and UC, occur in the areas of the GI tract with the highest concentration of bacteria. Moving distally from the duodenum, the microbial density increases in the lumen. The distal ileum contains up to 108 primarily anaerobic bacteria per gram of luminal contents,[5] whereas up to 1011–1012 bacteria per gram of luminal contents colonize the colon. Thus, the mucosal immune system has to keep a homeostatic balance between maintaining a tolerance towards the commensal microflora and at the same time protecting the host against microbial invasion. This balance is achieved through a complex network of innate and adaptive mucosal immune responses. It is interesting that the onset of disease often starts with a bacterial infection; it has been discussed for many years whether specific pathogens including Mycobacterium tuberculosis, rubella virus or other microbes might play a role in IBD. However, the variety of identified mucosal pathogens makes it unlikely that a specific microbe is the cause of disease and suggests that patients with CD are generally more susceptible towards microbial infections. Current concepts assume rather that the inflammation in the gut is a reaction towards the microflora that leads to disproportionate activation of immune responses and thus chronic inflammation of, and damage to, the intestinal mucosa.[2,6] Analysis of the enteric luminal flora in IBD patients revealed differences in the composition compared with healthy controls. Swidsinski et al.,[7] as well as other groups from France and the UK, demonstrated that mucosa-associated bacteria, especially anaerobic Bacteroides species and aerobic Enterobacteriaceae (Escherichia coli), are dramatically increased in IBD mucosa. Similarly, concentrations of adherent E. coli and enterococci are increased in the neoterminal ileum of CD patients after surgical resection.[8] In addition, early disease recurrence was associated with increased numbers of E. coli, Bacteroides and Fusobacterium. Another study by Darfeuille-Michaud described an increased number of adherent invasive E. coli strains in the ileal mucosa of patients with CD that may disrupt the intestinal barrier by synthesizing an α-hemolysin.[9] Fitting well with these observations, the adaptive immune responses target microbial antigens from the intestinal microflora, arguing against the 'old' concept of an autoimmune disease.[10] In summary, these findings show that the commensal flora may cause inflammation in the absence of adequate epithelial barrier function. Thus, an imbalance between the commensal flora and the epithelium of the host seems to be crucial in disease pathogenesis (Figure 1).[2]

Figure 1.

Proposed model for the role of intestinal bacteria and host defensins in the pathogenesis and disease progression of Crohn's disease. In Crohn's disease, the intestinal tract is characterized by a disturbed balance of host antimicrobial peptides and intestinal microbes. Owing to insufficient expression and function of antimicrobial defensin molecules, intestinal microbes are able to invade the mucosa. With further progression of disease, the bacterial influx provokes an inflammatory response. Reproduced with permission from [2].

 

[David in Seattle]

Barrier Function of the Intestinal Epithelium

Despite the enormous number of bacteria in the luminal compartment, intestinal infections and bacterial invasions into mucosal surfaces are relatively rare. A first line of defense against luminal bacteria is the intestinal epithelium. This epithelial cell layer consists of absorptive cells and secretory cells – mainly goblet cells – and small intestinal Paneth cells. These rapidly renewing cells derive from stem cells, which are located at the base of the intestinal crypts and differentiate into other epithelial cell types. The enterocytes form a dense, well-ordered brush border at the apical area consisting of organized microvilli. The integrity of the layer of epithelial cells is maintained by intercellular junction proteins composed of tight junctions, adherens junctions and desmosomes. Active intestinal inflammation in IBD patients is characterized by transepithelial migration of neutrophils into the mucosal epithelium, which has been shown to be associated with differential expression of epithelial intercellular junction proteins. In the colonic mucosa of chronic active IBD patients, expression of intercellular junction proteins is reduced.[11] However, the epithelium does not only provide a mechanical barrier; it also actively participates in microbial sensing, which results in the production of several microbe-killing effector molecules. The enteroendocrine cells contain small secretory granules in which different peptide hormones can be stored. Mucins secreted by goblet cells coat the epithelial surface in association with the membrane-bound mucins and act as a physicochemical barrier for the protection of the epithelial cell surface against commensals and pathogens.

Importantly, epithelial cells of the mucosa produce a variety of antimicrobial peptides (AMPs), cytokines and chemokines that protect mucosal surfaces against invading microbes.[12] AMPs are a group of small and mainly cationic proteins[13] that show a broad spectrum of antimicrobial activity against Gram-positive and Gram-negative bacteria, as well as against fungi, viruses and protozoa. Most of them act by disrupting the structure or function of microbial cell membranes. The cationic charge allows these molecules to bind to negatively charged microbial surfaces and mediates the integration and destruction of negatively charged microbial membranes.[14] In mammals, defensins and cathelicidins are the two major classes of AMPs.[15,16] A total of six α-defensins and four epithelial β-defensins have been identified in the human intestinal mucosa so far. The α-defensins include human neutrophil peptides 1–4 produced by granulocytes and human defensin (HD) 5 and 6 produced by small intestinal Paneth cells. The β-defensins are either inducible or constitutively expressed by enterocytes throughout the intestinal tract. It has been demonstrated that Paneth cell defensins actively regulate the composition of the bacterial flora but similar data for other host antimicrobials are still lacking.

Role of Small Intestinal Paneth Cells

In the small intestine, Paneth cells residing at the bottom of the intestinal crypts are the key effectors of innate mucosal defense. Paneth cells produce large amounts of α-defensins and other antimicrobial peptides, such as lysozymes and secretory phospholipase A2 (sPLA2).[17,18] These substances are stored in secretory granules and released into the intestinal lumen upon stimulation with bacterial antigens such as lipopolysaccharide (LPS) and muramyl dipeptide (MDP).[19] The major antimicrobial peptides secreted by Paneth cells are the α-defensins HD-5 and HD-6[20,21] and expression levels of HD-5 exceed those of lysozyme and sPLA2 by a factor of up to 100.[22] HD-5 is stored in its precursor form and is activated by trypsin in the lumen of the intestinal crypts.[23] The functional role of Paneth cell α-defensins in the small intestine has been demonstrated in different mouse models. Intestinal extracts from mice deficient for the cryptdin-processing enzyme matrilysin and thus lacking functional mature mouse α-defensins (cryptdins) show decreased antimicrobial activity. Consequently, these mice are more susceptible to orally administered bacterial pathogens, as well as dextran sulfate sodium colitis.[24] Furthermore, human α-defensin HD-5 transgenic mice are resistant to infection from orally administered Salmonella typhimurium.[25] In addition to their direct host defense function, Paneth cell defensins regulate the composition and number of colonizing microbes present in the small intestine.[22,26] Together, the different mouse model data highlight the pivotal role of Paneth cell defensins in protecting the organism from pathogens and in controlling the host–commensal balance in the intestinal mucosal barrier.

In patients with ileal CD, a decrease of Paneth cell α-defensins (HD-5 and HD-6) has been observed, which consequently results in reduced antimicrobial activity.[27,22] By contrast, ileal levels of α-defensins from patients with colonic CD were unchanged. Furthermore, the specific decrease was independent of inflammation in the specimens and not observed in UC, or most importantly pouchitis, an inflammatory control of non-Crohn's ileitis.[28] Different mechanisms leading to diminished Paneth cell α-defensin levels in patients with ileal CD have been identified.

Role of Wnt Signaling

Alterations in the Wnt signaling pathway seem to play an important role in the pathogenesis of ileal CD. Wnt signaling leads to stabilization of β-catenin, which translocates into the nucleus and forms a complex with transcription factors of the lymphoid enhancer factor (Lef)/T-cell factor (TCF) family to activate transcription of Wnt target genes. In the small intestine, active Wnt signaling, which is mediated by β-catenin/TCF-4, maintains the undifferentiated state of intestinal crypt progenitor cells (reviewed in [29]) and, paradoxically, also induces maturation of Paneth cells in intestinal crypts.[30] By contrast, disruption of the Wnt signaling pathway leads to impaired Paneth cell differentiation in a mouse model, which is accompanied by a disordered localization of these cells within the crypts.[31] Consistent with the important role of TCF-4 in Paneth cell defensin expression, we found a reduced expression of the Wnt signaling transcription factor TCF-4 in patients with ileal CD.[32] Like the decrease of α-defensins, the reduction of TCF-4 expression was independent of current inflammation. Besides a specific reduced expression of the transcription factor TCF-4, a single-nucleotide polymorphism (SNP) in the TCF-4 promoter (rs3814570) was significantly associated with ileal CD but not with solely colonic CD or UC.[33] Recently, we extended the hypothesis of an important role of antimicrobial host defense to pediatric patients with CD. Children with ileal CD exhibit a lower expression of small intestinal HD-5 as well as TCF-4 at the age of onset, suggesting that these defects in mucosal barrier function might be a key factor in the early disease pathogenesis.[34] Thus, defects in Wnt signaling are important characteristics of ileal CD, which results in a compromised innate immune function of Paneth cells via defensin secretion.

 

[David in Seattle]

Role of Bacterial Recognition

The innate immune system in the intestine must be able to discriminate between commensal bacteria and pathogenic bacteria. The molecules that are mainly responsible for this pivotal distinction are pattern-recognition receptors (PRRs), which recognize microbial parts called pathogen-associated molecular patterns (PAMPs). Intestinal epithelial cells express various types of PRRs that allow them to respond to gut microorganisms by rapidly initiating innate immune responses of survival and activating defense strategies against luminal pathogens. The release of small intestinal Paneth cell antimicrobial peptides into the intestinal lumen follows stimulation of PRRs with PAMPs. The secretion of Paneth cell defensins is activated by bacterial products including LPS, lipoteichoic acid, lipid A and MDP.[19] One of the two classes of PRRs are the nucleotide-binding oligomerization domain/caspase recruitment domain isoforms (NOD/CARD) cytoplasmic receptors for bacterial molecules. NOD2/CARD15 is an intracellular sensor for MDP, the minimal bioactive peptidoglycan motif common to all bacteria.[35,36] Genetic studies have identified NOD2 as a susceptibility gene for CD.[37,38] Three common allelic variants of the NOD2 gene were correlated with an increased susceptibility for CD in Caucasians. Studies in different ethnic populations have shown clear differences in the genetic variability of the NOD2 gene. None of the three common variations were found in Asian populations[39,40] and in African–Americans mutation frequency attributable risk was much lower,[41] suggesting that ethnic variation could partially explain variations in the frequency of CD in different world populations. In terms of the disease mechanism, NOD2 has been linked to the innate immune response associated with altered epithelial bacterial defense. In the small intestinal epithelium, NOD2 is predominately expressed in Paneth cells.[42] Studies in humans and rodent models substantiate a link between NOD2 mutations, Paneth cell defensins and CD. Clinical analyses revealed that NOD2 mutations are associated with the ileal type of CD.[43] Patients with ileal CD show a decrease of Paneth cell α-defensins HD-5 and HD-6 and this decrease is most pronounced in patients with a NOD2 mutation. Kobayashi and colleagues reported a decrease of Paneth cell α-defensin mRNA expression in mice lacking the NOD2 gene.[44] The NOD2-knockout mice were unable to detect MDP and were more susceptible to an oral infection with the pathogenic bacterium Listeria monocytogenes, supporting the importance of NOD2 in epithelial antimicrobial function. A recent study showed that NOD2 plays an important role in the regulation of commensal microbiota in the intestine, implying a direct link between impairment of NOD2 function, defective defensin expression and bacterial homeostasis.[45] Taken together, these results suggest that there is a link between NOD2 function and α-defensin expression, but the mechanisms remain unclear.

In addition, NOD2 triggers the transcriptional activation of numerous genes involved in both innate and adaptive immune responses by the activation of the transcription factor NF-κB. Upon activation, NOD2 mediates the expression of the inducible antimicrobial peptide human β-defensin 2 (hBD2)[46] but no differences in colonic hBD2 expression could be observed with regard to NOD2 status [Wehkamp J, Unpublished Data]. A very recent study revealed a synergistic induction of expression of antimicrobial peptide hBD2 and cathelicidin by 1,25-dihydroxyvitamin D3 and MDP through stimulation of NOD2 expression.[47] In addition, MDP-NOD2 stimulation in epithelial cells has been shown to induce human neutrophil peptide 1 (HNP-1) expression.[48] In summary, these studies highlight the essential role of NOD2 in the maintenance of mucosal homeostasis.

The other class of PRRs are the Toll-like receptors (TLRs); transmembrane proteins present at the cell surface or on the membrane of endocytic vesicles or other intracellular organelles.[49] TLRs and members of the IL-1R family share a conserved stretch of approximately 200 amino acids in their cytoplasmic region known as the Toll/IL-1R (TIR) domain crucial for signaling. By contrast, the extracellular domains of TLRs are characterized by a leucine-rich repeat (LRR) motif, which is directly involved in ligand binding and therefore in the recognition of a variety of pathogens. After ligand binding, intracellular adapter molecules are recruited to the receptor complex, including MyD88, which then in turn activate certain protein kinases that lead to the induction or suppression of genes. In a mouse model, it has been shown that Paneth cells detect bacteria via cell-intrinsic MyD88-dependent TLR signaling, which limits bacterial penetration of host tissues, revealing a role for epithelial MyD88 in maintaining intestinal homeostasis.[50] TLR9, another important Paneth cell PRR, recognizes unmethylated cytidine–phosphate–guanosine (CpG). Oral administration of oligonucleotides containing a CpG sequence led to extensive Paneth cell degranulation and protected mice against subsequent treatment with S. typhimurium.[51] Furthermore, TLR9 might cooperate with NOD2 in Paneth cell function by promoting the degranulation of antimicrobial peptides. However, mouse Paneth cells respond to purified bacterial cell envelope glycolipids despite an impaired TLR4 pathway, suggesting that currently undiscovered mechanisms mediate secretory responses to glycolipids.[52]

Role of Autophagy & Endosomal Stress

Autophagy is principally a degradation mechanism of cellular structures but it also appears to be involved in the breakdown of phagocytosed or invasive bacteria. ATG16L1 is a key autophagy protein that has an important function in Paneth cell biology. Since Paneth cells secrete defensins and other antimicrobial peptides through exocytosis of granules, aberrations in this pathway might have the potential to impair antimicrobial activity in the small intestine. In a genome-wide scan, a rare coding ATG16L1 variant has been found to be associated with CD[53] and these findings could be reproduced in different cohorts. An increased frequency of the variant was found in the ileal phenotype,[54] further supporting the idea of a primary role for Paneth cells in the development of this disease. Cadwell et al. reported that disruption of the Paneth cell granule exocytosis pathway in CD patients homozygous for the disease risk allele T300A of ATG16L1 was similar as compared with the knockout mice.[55] Recently, a functional link between bacterial sensing by NOD proteins and induction of autophagy was provided. During bacterial invasion, NOD2 recruits the critical autophagy protein ATG16L1 to the plasma membrane at the site of bacterial entry.[56] The most common CD-associated NOD2 mutant (L1007fsinsC) results in an impaired recruitment of ATG16L1 and thus fails to induce bacterial autophagy.

Another factor required for the maintenance of secretory cells, is the transcription factor X-box-binding protein 1 (XBP1), a key component of the endoplasmic reticulum (ER) stress response.[57] Secretory functions of cells rely on the capacity of the ER to modify polypeptides and mediate trafficking of secretory proteins through the ER–Golgi network. XBP1 activates the expression of certain ER chaperone genes and initiates ER biogenesis. XBP1 deletion in mouse intestinal epithelial cells resulted in spontaneous enteritis and increased susceptibility to induced colitis.[58] The observed effects were secondary to Paneth cell dysfunction and increased epithelial reactivity to bacterial products as well as the cytokine TNF-α. An association of genetic XBP1 variants was identified in patients with CD and also in those with UC.

Role of epithelial β-defensins

The first defensin identified in the human large bowel was the β-defensin hBD1. Human β-defensins are mainly produced by epithelial cells.[59] This peptide is ubiquitously expressed at all surfaces of the human body, including the skin, and the respiratory, urogenital and GI tracts.[60–65] In the noninflamed colon, hBD1 is the major β-defensin. In the inflamed mucosa of IBD patients, a reduction of hBD1 expression has been observed.[66] Strongly supporting the important role of hBD1, Kocsis et al. have reported a genetic association of hBD1 SNPs with colonic CD in a Hungarian cohort.[67] In contrast to hBD1, hBD2 and hBD3 are normally absent in the healthy colon and are only induced during inflammation or infection by different stimuli such as cytokines and bacteria.[68,64] This induction is mediated by either proinflammatory cytokines such as IL-1β (mostly through NF-κB-dependent and activator protein-1-dependent pathways) or by a direct response to bacteria.[69–71] In patients with UC, hBD2 and hBD3 expression are strongly induced in the inflamed tissue. In comparison to UC, this induction is attenuated in CD[64,66,72] and in line with this, the colonic mucosa of CD patients is compromised in its killing capacity towards different commensal bacteria.[73] One possible mechanism for a reduced hBD2 expression in the colon could be a reduction of hBD2 copy numbers, which was found in patients with colonic CD.[74] Even though this link was seen in a European and US cohort, this association was absent in patients from New Zealand.[75] Other antimicrobial peptides including the antimicrobial antiproteases elafin, secretory leukocyte protease inhibitor and cathelicidin (LL37) show similar expression patterns to the inducible β-defensins, even though the mechanisms of this are still unclear.[76,77]

 

[David in Seattle]

Role of Chemokines in the Epithelial Barrier

Intestinal epithelial cells produce chemokines, which are small chemotactic peptides that regulate the recruitment and homeostatic migration of leukocytes into the intestinal mucosa. Besides their primary and well-characterized function in regulating the migration of immune cells, some chemokines have been shown to display direct antimicrobial activity.[78,79] Recent findings demonstrated that the chemokines CCL14 and CCL15, which are constitutively expressed in the intestinal epithelium, display strong antibacterial activity.[80] Expression of CCL14 and CCL15 was significantly enhanced in patients with CD and UC, suggesting that they contribute as AMPs to the innate mucosal immune defense in the steady state and additionally during chronic intestinal inflammation.

Role of Neutrophils

Neutrophils provide a first line of defense after microbes have penetrated the mucosa by phagocytosing, killing and digesting bacteria and fungi.[81] Both α-defensins and the cathelicidin LL37 contribute a significant proportion to the neutrophil granule content. A defect of both antimicrobial peptides has been described in Kostmann disease.[82] It has been suggested that patients with CD exhibit a reduced number of neutrophils migrating to the sites at which bacteria penetrate the mucosa.[83] Marks et al. implied that a defective response in neutrophil induction might lead to enhanced secretion of proinflammatory cytokines, which drives chronic inflammation.[83] However, these data are still discussed controversially. Other studies suggest that increased mucosal generation of IL-8 and other cytokines may attract neutrophils and thus lead to their increased accumulation and activation in the affected mucosa of IBD patients.[84] In our own investigations, we could not observe a difference in the number of neutrophils between CD and UC.[77] Consistent with this observation, Cunliffe and colleagues have demonstrated that neutrophil defensin expression is equally upregulated in active CD and UC.[85] Further studies will be necessary to precisely evaluate the role of neutrophil defense in patients with IBD.

Interaction between Innate & Adaptive Immunity

There is increasing evidence that defensins exhibit immunomodulatory functions and may be involved in adaptive immunity by attracting immunocompetent cells to sites of infection and inflammation.[86] As an example, human β-defensins hBD1, 2 and 3 are chemotactic for memory T lymphocytes and immature dendritic cells and attract cells through the chemokine receptor CCR6.[87] Other investigations have shown that hBD2 induces the migration of mast cells by activating G-protein-phospholipase C-coupled receptors and is a specific chemoattractant for human neutrophils.[88,89] These studies suggest that defensins might also function as a bridge between innate immunity and the adaptive immune system by recruiting inflammatory cells at the intestinal mucosa. Furthermore, crypt epithelial cells may further contribute to innate intestinal host defense by orchestrating the recruitment of immune cells via the secretion of chemokines.[19] It has been shown that Paneth cell α-defensin cryptdin 3 induces IL-8 secretion from intestinal cells via activation of the p38 mitogen-activated protein kinase (MAPK) and NF-κB signaling cascades but also other proinflammatory cytokines such as macrophage inhibitory protein (MIP)-1α, MIP-1β, MIP-1δ, IL-17, IL-12 and p70.[90] The proinflammatory activity of the cryptdins seems to depend on their pore-forming activities as cryptdin 4, which cannot form pores in eukaryotic cell membranes, does not induce an inflammatory response. In the same way, HD-5 may act as a regulator of the intestinal inflammatory response by binding to the cell membrane of intestinal epithelial cells and inducing the secretion of the chemokine IL-8 in a concentration- and structure-dependent fashion.[91] However, not only small intestinal α-defensins demonstrate these properties. Very early studies reported that human neutrophil defensins HNP1–3 are chemotactic for monocytes,[92] naive T cells and immature dendritic cells.[93] This immunostimulatory activity has been shown to depend on TLR4. In addition, LL37 has been shown to be chemotactic for monocytes, T cells, neutrophils and mast cells.[94–96] A main focus of future studies will be to better understand the complex interplay of innate and adaptive immune functions.

Therapeutic Consequences

Current standard treatments do not seem to have substantial effects on the expression of the main antimicrobial defensins.[28] However, the new insights into innate immune effector molecules such as defensins and other antimicrobial peptides could have an important influence on future therapeutic strategies. For the treatment of IBD, future strategies might aim to strengthen protective innate immunity. Antibiotic therapy is currently used for inducing remission and treating fistula, as well as maintenance therapy after surgery. Results from several clinical trials suggest that infection with helminths is protective in IBD. Oral therapeutic intervention with live ova from Trichuris suis has been shown to improve the clinical outcome of UC in a double-blind clinical study and CD in an open-label study.[97,98] In this context, the reported efficiency of T. suis therapy in CD provokes the testable question of whether the stimulation of Paneth cell defensins or other antimicrobials by parasitic worms might be an explanation for their therapeutic effect. However, recent animal studies have provided evidence that helminth infection H. diminuta caused a significant exacerbation.[99] Thus, as for all therapeutic approaches, the therapeutic use of helminths in IBD has to be carefully considered and more clinical studies will be necessary to evaluate the potential risks. Understanding the precise mechanisms by which helminths modulate the host's responses to enteric bacteria will further help to optimize its successful use in patients. Probiotic bacteria like E. coli Nissle 1917 (Mutaflor®, Ardeypharma GmbH, Herdecke, Germany), other therapeutic probiotic E. coli (Symbioflor®, SybioPharm GmbH, Herborn, Germany) as well as Lactobacilli have been shown to strongly induce antimicrobial peptides.[70,71,100] In the case of E. coli Nissle, the oldest known (in use since World War I, 1917) and probably the best studied probiotic strain, this induction is mediated by a specific flagellin.[101] Its efficacy in maintaining remission in UC has been shown to be as effective as standard treatment with mesalazine in a placebo-controlled double-blind study.[102] This strain is now recommended as a guideline therapy by the Deutsche Gesellschaft für Gastroenterologie. Even though E. coli Nissle is clearly effective in UC, probiotic treatment in CD seems to be limited and less useful. A possible but speculative explanation could be a general defect in the upregulation of defensins and other protective host molecules; CD patients show an attenuated induction of β-defensins in the colon and are characterized by a diminished functional antimicrobial activity towards various bacteria of the normal gut flora, suggesting defects in antibacterial barrier function. It is possible that a defective β-defensin induction due to different and partly unknown mechanisms contributes to the low efficacy of probiotic treatment in CD. By contrast, epithelial induction of β-defensins and other antimicrobials could enhance protective antimicrobial barrier function, especially in the case of clinical remission of UC. Probiotic bacteria are the first therapeutic agents for IBD that induce the production of antimicrobial peptides, and this might be an important mechanism to prevent bacterial invasion into the mucosa. It is likely that other therapeutic agents like worm eggs, vitamin D, specific bacteria, food, artificial components or possibly prebiotics could have similar effects.

Expert Commentary

Multiple lines of investigation support the central role of innate immunity and barrier function in IBD. Together, these studies represent a paradigm shift in the field. A decade ago, most scientists believed that chronic as well as active inflammation in IBD is due to a primary dysregulation of inflammatory cells. We believe that the inflammatory cascade is a more or less 'normal' reaction to the disturbed entry of commensal and pathogenic microbes. It will be a major aim of future studies to better understand the multiple and complex interactions between the adaptive and innate immune systems and between the different layers of defense. This should result in different treatment strategies aiming to treat the primary, and not secondary, problem of the disease.

 

[David in Seattle]

Five-year View

The main current interest of understanding IBD is shifting towards an appreciation of the epithelial barrier function. By contrast, based on the assumption of an autoimmune disease or overreaction of adaptive immunity, the current main treatment is aimed at suppressing the inflammatory cascade. These drugs include steroids, azathioprine and modern biologics like anti-TNF-α. We believe that a suppression of the inflammatory cascade will remain an important facet of treatment, especially in the active state. For maintaining remission, future treatment concepts should either directly substitute deficient antimicrobial host factors or indirectly aim to bolster this existent barrier. We anticipate that this approach will effectively prevent relapses in IBD and also display fewer side effects. Specific agents will hopefully be available within the next few years.

Sidebar
Key Issues

* The family of defensins are key effector molecules of host defense.
* Different defensins are expressed specifically at different sites of the GI tract (and other parts of the body).
* Crohn's disease (CD) is characterized by adherent mucosal bacteria and an adaptive inflammatory response (antibodies and T cells), which is directed against mostly commensal (not pathogenic) bacteria.
* The mucosa of CD patients from different anatomical sites is characterized by an impaired ability to kill a broad variety of different bacteria (including anaerobic strains).
* Different clinical phenotypes are characterized by distinct deficiencies in antimicrobial peptide classes, including defensins, cathelecidins and antiproteases.
* Paneth cells seem to be central for the development of small intestinal CD.
* Antimicrobial defensins, especially Paneth cell defensins, regulate the composition of the bacterial flora.
* Chemokines also function as antimicrobials and more of these peptides will be identified in the near future.
* Innate and adaptive immune components are closely linked and their interaction should be the focus of current and future investigations.

 

[David in Seattle]

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•• Paneth cell human defensin-5 regulates the intestinal microbiota. The composition of flora is host driven.
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•• Wnt signaling is compromised in ileal Crohn's disease, suggesting a dysfunctional epithelial stem cell regulation that affects Paneth cell function.
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• Nucleotide-binding oligomerization domain 2 compromises both Paneth cell antimicrobials and – as elegantly shown here – regulation of the commensal flora. Flora composition is host driven.
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• Another elegant observation demonstrating the important role of NOD2 in regulating the antimicrobial response.
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• An elegant study demonstrating that NOD2 is essential for the antimicrobial host response, and not only in Paneth cells.
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• Paneth cells regulate the intestinal microbiota. The composition of the flora is host driven.
51. Rumio C, Besusso D, Palazzo M et al. Degranulation of Paneth cells via Toll-like receptor 9. Am. J. Pathol. 165(2), 373–381 (2004).

 

[David in Seattle]

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[Dexky]

Well, as usual, most of that made my eyes glaze over but the Five Year View sounds very promising. Especially so, in light of EJ's age!!

Thanks David!!

 

[David in Seattle]

Well, as usual, most of that made my eyes glaze over but the Five Year View sounds very promising. Especially so, in light of EJ's age!!

Thanks David!!

I thought it sounded positive too. Glad you liked it