Twenty-five years of biologicals in IBD: What´s all the hype about?

Twenty-five years ago the field was revolutionized by the introduction of infliximab as the first hybrid anti-TNF-antibody. Subsequently, other humanized anti-TNFs were developed and marketed, followed by antibodies to new targets including integrins (vedolizumab) and interleukin 12/23 (ustekinumab). All these so-called biologicals were shown in registrational trials to induce remission superior to placebo but consistently were effective in only a minority of patients. Even though in most trials only the responders were selected to continue on the respective medication for maintenance, many experienced a secondary loss of response and only a minority of usually <25% of the initial cohort achieved long-term (1 year) remission. In 'real life studies', the outcome was somewhat better, probably due to proper selection of patients and open, mostly retrospective study designs. A clear benefit of biologicals is apparent in otherwise treatment refractory patients, in extraintestinal manifestations and in Crohn´s disease (CD) with fistulizing complications. Biologicals achieve mucosal healing (MH) more often than corticosteroids or thiopurines, and MH is associated with improved prognosis. However, this does not justify escalating treatment until MH is reached since controlled trials proving this point of 'treat to target' are lacking both in ulcerative colitis and CD. Surgical rates have decreased with increasing use of biologicals, but disease progression has not been proven to improve. With the exception of opportunistic infections, serious adverse events are rare. In conclusion, biologicals have changed the scene considerably and expanded our armamentarium, but there is also a marketing hype fostering expectations without evidence.

Steroid-refractory ulcerative colitis: a critical review of national and international guideline recommendations.

Es gibt zahlreiche nationale und internationale Leitlinien zu chronisch entzündlichen Darmerkrankungen, die auf vergleichbarer Evidenz sowie ähnlichen Prozeduren beruhen und daher homogen sein sollten. In dieser kritischen Übersicht wurden die Leitlinienempfehlungen aus Europa (ECCO), Deutschland, Großbritannien, Kanada, den USA und Japan zur Therapie der steroidrefraktären Colitis ulcerosa verglichen. Die meisten Leitlinien unterschieden zwischen moderater/schwerer (ambulanter) und schwerer/fulminanter Colitis in der Klinik. Die Empfehlungen zur ersten Kategorie weisen gravierende Unterschiede auf, während zur Behandlung des stationären Patienten weitgehende Übereinstimmung herrscht. Verschiedene Erklärungen für die Inkonsistenzen werden diskutiert.

Infodemiology of Crohn's disease and Ulcerative colitis using Google Trends - an approach to investigate patient needs.

INTRODUCTION: Inflammatory bowel diseases (IBD) highly affect quality of life. The course of disease and success of treatment are variable. These factors result in a high number of psychiatric comorbidities with patients requiring extensive medical consultation or additional psychotherapy. Unfortunately, time is often limited in daily clinical care, which leaves patients not feeling sufficiently informed about their disease. Patients often compensate by searching the internet, with possibly harmful information. We aim to identify information gaps to allow a more complete education of patients. METHODS: We analyzed the internet search behavior using the key words [Morbus Crohn] (CD) and [Colitis ulcerosa] (UC) using Google Trends. In addition, we investigated which websites are the first hits on Google, as those are most likely visited by patients. RESULTS: Symptoms, nutrition and therapy are central topics for persons interested in IBD. The searches concerning symptoms or therapies do not match the actual incidence or prevalence of comorbidities as well as the more commonly used therapies or established nutritional recommendations. We found a distinct impact of well-known personalities on disease related searches. The first suggested websites on google showed great heterogeneity of responsible publishers, often with possible conflict of interests. In line with those observations, quality of website content is highly heterogeneous. CONCLUSION: This study showed a need of information concerning symptoms, nutrition and therapy that should be considered during patient education. Since time in physician patient dialogue is short it may be helpful to further evaluate websites independently in order to give recommendations of websites offering reliable information.

Crohn's disease-derived monocytes fail to induce Paneth cell defensins.

Crohn's disease (CD) is associated with a multitude of genetic defects, many of which likely affect Paneth cell function. Paneth cells reside in the small intestine and produce antimicrobial peptides essential for the host barrier, principally human α-defensin 5 (HD5) and HD6. Patients with CD of the ileum are characterized by reduced constitutive expression of these peptides and, accordingly, compromised antimicrobial barrier function. Here, we present a previously unidentified regulatory mechanism of Paneth cell defensins. Using cultures of human ileal tissue, we showed that the secretome of peripheral blood mononuclear cells (PBMCs) from healthy controls restored the attenuated Paneth cell α-defensin expression characteristic of patients with ileal CD. Analysis of the Wnt pathway in both cultured biopsies and intestinal epithelial cells implicated Wnt ligands driving the PBMC effect, whereas various tested cytokines were ineffective. We further detected another defect in patients with ileal CD, because the PBMC secretomes derived from patients with CD were unable to restore the reduced HD5/HD6 expression. Accordingly, analysis of PBMC subtypes showed that monocytes of patients with CD express significantly lower levels of canonical Wnt ligands, including Wnt3, Wnt3a, Wnt1, and wntless Wnt ligand secretion mediator (Evi/Wls). These studies reveal an important cross-talk between bone marrow-derived cells and epithelial secretory Paneth cells. Defective Paneth cell-mediated innate immunity due to inadequate Wnt ligand stimulation by monocytes provides an additional mechanism in CD. Because defects of Paneth cell function stemming from various etiologies are overcome by Wnt ligands, this mechanism is a potential therapeutic target for this disease.

Improvement of a 'Leaky' Intestinal Barrier.

In Crohn's disease, the mucus layer appears to be defective in terms of low defensin levels and lack of antibacterial activity. These deficiencies actually explain the Montreal phenotypes and the stable localization of disease in the terminal ileum with low α-defensins from Paneth cells and/or low ß-defensins in colonic disease, respectively. Conversely, in ulcerative colitis (UC) the defensin production is normal or even induced, but the mucus layer is thinner and patchy, more in the liquid form and also chemically altered so that antibacterial peptides are not retained and lost into the luminal bacterial bulk. Therefore, both barrier problems allow slow bacterial attachment and invasion, ultimately triggering the massive response of adaptive immunity and tissue destruction. Therefore, leakiness should refer to the antibacterial barrier and not to the general barrier against small molecules, such as mannitol or lactulose, which are not antigenic. The most promising approach in UC seems to be the use of probiotics or the natural compound lecithin as a stabilizer of mucus structure to enhance the barrier. While a phase II study has yielded positive results, the results of the ongoing phase III study are eagerly awaited. It is quite possible that the protective effect of smoking in UC is related to mucus production in the colon also, but this is not an option. Another alternative would be to shift cell differentiation in the colon towards goblet cell; the relevant differentiation factors are known. In Crohn's disease, the direct oral application of defensins might be effective if release and binding to the mucus are achieved. In the experimental colitis model, this works quite well. In conclusion, in a situation where enthusiasm about so-called biologics is declining due to loss of response over time, searching for the primary defects in inflammatory bowel disease and treating them may well be worthwhile, although it is unlikely to provide rapid relief.

Reduction of disulphide bonds unmasks potent antimicrobial activity of human ß-defensin 1.

Human epithelia are permanently challenged by bacteria and fungi, including commensal and pathogenic microbiota. In the gut, the fraction of strict anaerobes increases from proximal to distal, reaching 99% of bacterial species in the colon. At colonic mucosa, oxygen partial pressure is below 25% of airborne oxygen content, moreover microbial metabolism causes reduction to a low redox potential of -200 mV to -300 mV in the colon. Defensins, characterized by three intramolecular disulphide-bridges, are key effector molecules of innate immunity that protect the host from infectious microbes and shape the composition of microbiota at mucosal surfaces. Human ß-defensin 1 (hBD-1) is one of the most prominent peptides of its class but despite ubiquitous expression by all human epithelia, comparison with other defensins suggested only minor antibiotic killing activity. Whereas much is known about the activity of antimicrobial peptides in aerobic environments, data about reducing environments are limited. Herein we show that after reduction of disulphide-bridges hBD-1 becomes a potent antimicrobial peptide against the opportunistic pathogenic fungus Candida albicans and against anaerobic, Gram-positive commensals of Bifidobacterium and Lactobacillus species. Reduced hBD-1 differs structurally from oxidized hBD-1 and free cysteines in the carboxy terminus seem important for the bactericidal effect. In vitro, the thioredoxin (TRX) system is able to reduce hBD-1 and TRX co-localizes with reduced hBD-1 in human epithelia. Hence our study indicates that reduced hBD-1 shields the healthy epithelium against colonisation by commensal bacteria and opportunistic fungi. Accordingly, an intimate interplay between redox-regulation and innate immune defence seems crucial for an effective barrier protecting human epithelia.

[German Society of Gastroenterology, Digestive and Metabolic Diseases (DGVS) position statement on endoscopic decompression in acute Ileus]. / Positionspapier der DGVS zur endoskopischen Dekompression bei akutem Ileus.

In patients with ileus with dilated intestine in imaging studies, endoscopic decompression appears a feasible option. However, its use is often uncritical and without scientific evidence. Before considering endoscopic intervention, CT-imaging should differentiate between mechanical obstruction and paralytic ileus/intestinal pseudo-obstruction. Tumor diagnosis and localisation are essential because the latter determines the choice of the decompression procedure. Coecal dilatation of more than 12 cm indicates an increased risk of perforation. In patients with toxic megacolon, dilation of the transverse colon to more than 6 cm is considered critical without much prospective evidence. Endoscopic decompression has a high complication rate and should be performed electively, and not as an emergency procedure, whenever possible. The use of CO2 insufflation rather than ambient air is strongly recommended, as is the availability of fluoroscopy. Prior trans-nasal or oral decompression-tube placement is routinely performed, and tracheobronchial intubation frequently required. In over 90 % of patients with pseudo-obstruction, conservative treatment is successful within 24 to 48 hours, and endoscopic decompression is, therefore, unnecessary. Placement of self-expanding metal stents to decompress a tumor stenosis is considered mostly for the left colon and rectum and burdened with significant risks of perforation and stent migration. Stent impact on oncological outcome is controversial because of possible tumor cell mobilization and increased postoperative cancer recurrence rates. Surgery, as primary intervention, achieves its objective in most cases. Decompression effect by endoscopic suctioning of gas and intestinal fluid is usually transient so that it is combined with transrectal decompression tubes insertion. This paper reviews the advantages and flaws of various decompression procedures in different clinical settings.

Association of a functional variant in the Wnt co-receptor LRP6 with early onset ileal Crohn's disease.

Ileal Crohn's Disease (CD), a chronic small intestinal inflammatory disorder, is characterized by reduced levels of the antimicrobial peptides DEFA5 (HD-5) and DEFA6 (HD-6). Both of these α-defensins are exclusively produced in Paneth cells (PCs) at small intestinal crypt bases. Different ileal CD-associated genes including NOD2, ATG16L1, and recently the ß-catenin-dependant Wnt transcription factor TCF7L2 have been linked to impaired PC antimicrobial function. The Wnt pathway influences gut mucosal homeostasis and PC maturation, besides directly controlling HD-5/6 gene expression. The herein reported candidate gene study focuses on another crucial Wnt factor, the co-receptor low density lipoprotein receptor-related protein 6 (LRP6). We analysed exonic single nucleotide polymorphisms (SNPs) in a large cohort (Oxford: n = 1,893) and prospectively tested 2 additional European sample sets (Leuven: n = 688, Vienna: n = 1,628). We revealed an association of a non-synonymous SNP (rs2302685; Ile1062Val) with early onset ileal CD (OR 1.8; p = 0.00034; for homozygous carriers: OR 4.1; p = 0.00004) and additionally with penetrating ileal CD behaviour (OR 1.3; p = 0.00917). In contrast, it was not linked to adult onset ileal CD, colonic CD, or ulcerative colitis. Since the rare variant is known to impair LRP6 activity, we investigated its role in patient mucosa. Overall, LRP6 mRNA was diminished in patients independently from the genotype. Analysing the mRNA levels of PC product in biopsies from genotyped individuals (15 controls, 32 ileal, and 12 exclusively colonic CD), we found particularly low defensin levels in ileal CD patients who were carrying the variant. In addition, we confirmed a direct relationship between LRP6 activity and the transcriptional expression of HD-5 using transient transfection. Taken together, we identified LRP6 as a new candidate gene in ileal CD. Impairments in Wnt signalling and Paneth cell biology seem to represent pathophysiological hallmarks in small intestinal inflammation and should therefore be considered as interesting targets for new therapeutic approaches.

Synergistic effects of antimicrobial peptides and antibiotics against Clostridium difficile.

Accelerating rates of health care-associated infections caused by Clostridium difficile, with increasing recurrence and rising antibiotic resistance rates, have become a serious problem in recent years. This study was conducted to explore whether a combination of antibiotics with human antimicrobial peptides may lead to an increase in antibacterial activity. The in vitro activities of the antimicrobial peptides HBD1 to HBD3, HNP1, HD5, and LL-37 and the antibiotics tigecycline, moxifloxacin, piperacillin-tazobactam, and meropenem alone or in combination against 10 toxinogenic and 10 nontoxinogenic C. difficile strains were investigated. Bacterial viability was determined by flow cytometry and toxin production by enzyme-linked immunosorbent assay (ELISA). When combined at subinhibitory concentrations, antimicrobial peptides and antibiotics generally led to an additive killing effect against toxinogenic and nontoxinogenic C. difficile strains. However, LL-37 and HBD3 acted in synergism with all the antibiotics that were tested. Electron microscopy revealed membrane perturbation in bacterial cell walls by HBD3. In 3 out of 10 toxinogenic strains, HBD3, LL-37, piperacillin-tazobactam, and meropenem administration led to an increased toxin release which was not neutralized by the addition of HNP1. Antimicrobial peptides increase the bacterial killing of antibiotics against C. difficile regardless of the antibiotics' mode of action. Membrane perturbation in or pore formation on the bacterial cell wall may enhance the uptake of antibiotics and increase their antibacterial effect. Therefore, a combination of antibiotics with antimicrobial peptides may represent a promising novel approach to the treatment of C. difficile infections.

TCF-1-mediated Wnt signaling regulates Paneth cell innate immune defense effectors HD-5 and -6: implications for Crohn's disease.

Wnt signaling regulates small intestinal stem cell maintenance and Paneth cell differentiation. In patients with ileal Crohn's disease (CD), a decrease of Paneth cell α-defensins has been observed that is partially caused by impaired TCF-4 and LRP6 function. Here we show reduced expression of the Wnt signaling effector TCF-1 (also known as TCF-7) in patients with ileal CD. Reporter gene assays and in vitro promoter binding analysis revealed that TCF-1 activates α-defensin HD-5 and HD-6 transcription in cooperation with ß-catenin and that activation is mediated by three distinct TCF binding sites. EMSA analysis showed binding of TCF-1 to the respective motifs. In ileal CD patients, TCF-1 mRNA expression levels were significantly reduced. Moreover, we found specifically reduced expression of active TCF-1 mRNA isoforms. Tcf-1 knockout mice exhibited reduced cryptdin expression in the jejunum, which was not consistently seen at other small intestinal locations. Our data provide evidence that TCF-1-mediated Wnt signaling is disturbed in small intestinal CD, which might contribute to the observed barrier dysfunction in the disease.

Dysbiosis in inflammatory bowel diseases: egg, not chicken.

There is agreement that inflammatory bowel diseases are, both in terms of species composition and function, associated with an altered intestinal microbiome. This is usually described by the term "dysbiosis," but this is a vague definition lacking quantitative precision. In this brief narrative review, the evidence concerning the primary or secondary role of this dysbiotic state is critically evaluated. Among others, the following facts argue against a primary etiological impact: 1) There is no specific dysbiotic microbiome in IBD, 2) the presence or absence of mucosal inflammation has a profound impact on the composition of the microbiome, 3) dysbiosis is not specific for IBD but linked to many unrelated diseases, 4) antibiotics, probiotics, and microbiome transfer have a very limited therapeutic effect, 5) the microbiome in concordant twins is similar to disease-discordant twins, and 6) the microbiome in relatives of IBD patients later developing IBD is altered, but these individuals already display subclinical inflammation.

To STRIDE or not to STRIDE: a critique of "treat to target" in ulcerative colitis.

INTRODUCTION: The STRIDE consensus intends to complement the clinical endpoint with an endoscopic endpoint of mucosal healing and others as treatment targets in ulcerative colitis. If these targets are not reached, STRIDE requires dose or timing adjustments or switching the medication. This narrative review provides a critique of this concept. AREAS COVERED: We analyze and discuss the limitations of current endpoints as targets, their currently limited achievability, and the lacking evidence from controlled trials relating to 'treat to target.' The relevant publications in PubMed were identified in a literature review with the key word 'ulcerative colitis.' EXPERT OPINION: In ulcerative colitis, the standard clinical target is measured traditionally by the MAYO-score, but in variable combinations of patient and physician reported outcomes as well as also different definitions of the endoscopic part. Only a score of 0 is more stringent than clinical remission but is only achieved by a minority of patients in first and even less in second line therapy. The concept is not based on clear evidence that patients indeed benefit from appropriate escalation of treatment. Until the STRIDE approach is proven to be superior to standard treatment focusing on clinical well-being, the field should remain reluctant.

Are Internet Information Sources Helpful for Adult Crohn's Disease Patients Regarding Nutritional Advice?

Background: Adult patients suffering from Crohn's disease (CD) are often dissatisfied with the information they receive from their physicians about nutrition and its impact on CD inflammation activity. Only a few publications are available about patients' internet research on nutrition in CD. The study aim is to elucidate the internet information sources of adult CD patients regarding nutritional advice via a questionnaire. Methods: A questionnaire with 28 (general and specific) questions for outpatients at our tertiary center with CD was created and used for an analysis of their information sources about nutrition in CD. Four CD and/or nutritional medicine experts examined the 21 most relevant websites referring to nutritional advice for CD patients. Results: One hundred and fifty CD patients reported their Internet research behavior for nutritional advice and their dietary habits. Many CD patients prefer to consult the Internet instead of asking their general practitioner (GP) for nutritional recommendations. Most of the websites providing nutritional advice for CD patients are of very poor quality and cannot be recommended. We found significant correlations between (a) nutritional habits of CD patients, (b) their information sources and several demographic or CD-related factors. There is a lack of websites which provide high-quality, good nutritional advice to CD patients. Conclusions: The majority of the examined websites did not provide sufficient information according to the CD guidelines and nutritional medicine guidelines. A higher quality level of website content (e.g., on social media or on university/center websites) provided by experienced physicians is required to secure trustworthy and reliable nutritional information in CD.

Antimicrobial activity of high-mobility-group box 2: a new function to a well-known protein.

The human intestinal tract is highly colonized by a vast number of microorganisms. Despite this permanent challenge, infections remain rare, due to a very effective barrier defense system. Essential effectors of this system are antimicrobial peptides and proteins (AMPs), which are secreted by intestinal epithelial and lymphoid cells, balance the gut microbial community, and prevent the translocation of microorganisms. Several antimicrobial proteins have already been identified in the gut. Nonetheless, we hypothesized that additional AMPs are yet to be discovered in this setting. Using biological screening based on antimicrobial function, here we identified competent antibacterial activity of high-mobility-group box 2 (HMGB2) against Escherichia coli. By recombinant expression, we confirmed this biologically new antimicrobial activity against different commensal and pathogenic bacteria. In addition, we demonstrated that the two DNA-binding domains (HMG boxes A and B) are crucial for the antibiotic function. We detected HMGB2 in several gastrointestinal tissues by mRNA analysis and immunohistochemical staining. In addition to the nuclei, we also observed HMGB2 in the cytoplasm of intestinal epithelial cells. Furthermore, HMGB2 was detectable in vitro in the supernatants of two different cell types, supporting an extracellular function. HMGB2 expression was not changed in inflammatory bowel disease but was detected in certain stool samples of patients, whereas it was absent from control individuals. Taken together, we characterized HMGB2 as an antimicrobial protein in intestinal tissue, complementing the diverse repertoire of gut mucosal defense molecules.

Intestinal bacterial translocation in rats with cirrhosis is related to compromised Paneth cell antimicrobial host defense.

UNLABELLED: Liver cirrhosis is associated with bacterial translocation (BT) and endotoxemia. Most translocating bacteria belong to the common intestinal microbiota, suggesting a breakdown of intestinal barrier function. We hypothesized that diminished mucosal antimicrobial host defense could predispose to BT. Two rodent models of portal hypertension with increased BT were used, CCl(4)-induced ascitic cirrhosis and 2-day portal vein-ligated (PVL) animals. BT was assessed by standard microbiological techniques on mesenteric lymph nodes. Total RNA was isolated systematically throughout the intestinal tract, and expression of Paneth cell α-cryptdins and ß-defensins was determined by real-time quantitative polymerase chain reaction (qPCR). To determine functional consequences, mucosal antimicrobial activity was assessed with a fluorescence-activated cell sorting assay. BT was detectable in 40% of rats with cirrhosis. Compared with the group without BT, these animals exhibited diminished intestinal Paneth cell α-cryptdin 5 and 7 expression. In contrast, PVL was associated with BT in all animals but did not affect antimicrobial peptides. The decrease in Paneth cell antimicrobials was most pronounced in the ileum and the coecum. Other antimicrobials showed no changes or even an induction in the case of BT at different sites. Antimicrobial activity toward different commensal strains was reduced, especially in the distal ileum and the cecum in experimental cirrhosis with BT (excluding PVL). CONCLUSION: Compromised Paneth cell antimicrobial host defense seems to predispose to BT in experimental cirrhosis. Understanding this liver-gut axis including the underlying mechanisms could help us to find new treatment avenues.

Role of the ABCG8 19H risk allele in cholesterol absorption and gallstone disease.

BACKGROUND: Gallstone disease is associated with p.D19H of ABCG8 as well as alterations of cholesterol and bile acid metabolism. However, molecular mechanisms have not been fully elucidated. It is important to understand the link between the sterol transporters ABCG5/8 and NPC1L1 and intestinal cholesterol absorption as well as de novo synthesis in gallstone patients stratified according to 19H risk allele. Moreover, the functional importance of the 19H variant on intestinal ABCG8 feature remains to be clarified. METHODS: Measurements of serum surrogate markers of cholesterol absorption (plant sterols: sitosterol, campesterol) and synthesis (cholesterol precursor: lathosterol) were carried out by gas chromatography/mass spectrometry (GC/MS). For expression studies, total RNA was isolated from 168 ileal biopsies of study participants with (34) and without gallstone disease (134). Messenger RNA was measured by LightCycler real-time PCR. Genomic DNA was obtained from blood leukocytes. Genotype frequencies of p.D19H were established using MALDI-TOF mass spectrometry. RESULTS: Compared to controls, cholesterol absorption but not synthesis in gallstone carriers was diminished by about 21% based on low serum sitosterol (P = 0.0269) and campesterol (P = 0.0231) to cholesterol ratios. D19H was found to be significantly associated with gallstones (odds ratio [OR] = 2.9, P = 0.0220, 95% confidence interval [CI]:1.22-6.89), particularly in the overweight cohort (OR = 3.2, P = 0.0430, 95% CI:1.07-9.26). Cholesterol absorption was about 24% lower in individuals carrying p.D19H compared to wild type (Psitosterol = 0.0080, Pcampesterol = 0.0206). Moreover, irrespective of phenotype, carriers of p.D19H displayed a significant lower absorption than carriers of the major allele. The most pronounced effect on cholesterol absorption ratio was observed for serum campesterol levels (wild type controls to mutated controls 28%, P = 0.0347 and wild type controls to gallstone carriers with 19H allele 37%, P = 0.0030). Notably, ABCG5/8 and NPC1L1 expression was similar in gallstone carriers and controls regardless of p.D19H presence. CONCLUSIONS: Both gallstone disease and p.D19H of ABCG8 are associated with diminished cholesterol absorption. However, p.D19H is not responsible for the differences in small intestinal sterol transporter expression.

Defective antibacterial barrier in inflammatory bowel disease.

The pathogenesis of inflammatory bowel disease (IBD) is very complex, including a variety of genetic and environmental contributing factors. In this context, over the past few years, a picture of IBD as a primary defect of the innate immune system rather than the adaptive immune system has evolved. The intestinal antimicrobial barrier morphologically consists of a single layer of epithelial cells and the mucus and constitutes the first defense mechanism against the microbial burden of the gut. From a more mechanistic point of view, this barrier additionally depends on a crucial interplay between the mucus and antimicrobial peptides like for instance defensins. Disturbances in this system are in the pathophysiological center stage of IBD genesis and progression. In this article we will give a short overview about some of the key mechanisms in this context with special attention on defensins and the mucus layer.

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.

Peroxisome proliferator-activated receptor gamma activation is required for maintenance of innate antimicrobial immunity in the colon.

Crohn's disease (CD), a major form of human inflammatory bowel disease, is characterized by primary immunodeficiencies. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is essential for intestinal homeostasis in response to both dietary- and microbiota-derived signals. Its role in host defense remains unknown, however. We show that PPARgamma functions as an antimicrobial factor by maintaining constitutive epithelial expression of a subset of beta-defensin in the colon, which includes mDefB10 in mice and DEFB1 in humans. Colonic mucosa of Ppargamma mutant animals shows defective killing of several major components of the intestinal microbiota, including Candida albicans, Bacteroides fragilis, Enterococcus faecalis, and Escherichia coli. Neutralization of the colicidal activity using an anti-mDefB10 blocking antibody was effective in a PPARgamma-dependent manner. A functional promoter variant that is required for DEFB1 expression confers strong protection against Crohn's colitis and ileocolitis (odds ratio, 0.559; P = 0.018). Consistently, colonic involvement in CD is specifically linked to reduced expression of DEFB1 independent of inflammation. These findings support the development of PPARgamma-targeting therapeutic and/or nutritional approaches to prevent colonic inflammation by restoring antimicrobial immunity in CD.