Increased uptake of non-pathogenic E. coli via the follicle-associated epithelium in longstanding ileal Crohn's disease

Bacterial induction of proinflammatory cytokines in inflammatory bowel disease

It has become increasingly clear that inflammatory bowel disease (IBD) develops on the background of genetic defects in the host, conveying an increased susceptibility to an environmental antigen or antigens. The environmental factor implicated in the pathophysiology of gut inflammation, which is undergoing increased scrutiny, is the intestinal flora. The intestinal flora as a whole and specific bacteria and their products have been found to trigger cytokine expression in various cell types. Consistently, multiple bacterial strains were found to induce tumor necrosis factor alpha (TNF-α) and interleukin-8 (IL-8) in macrophage and epithelial cell systems, respectively, in particular in Crohn's disease. Interestingly, various cell types from patients with IBD display an increased susceptibility to specific bacterial products, including flagellin, pili, and lipopolysaccharides. It remains to be determined whether additional effector proteins regulate cytokine expression and the aberrant mucosal immune response in IBD.

Campylobacter jejuni induces transcytosis of commensal bacteria across the intestinal epithelium through M-like cells

BACKGROUND

Recent epidemiological analyses have implicated acute Campylobacter enteritis as a factor that may incite or exacerbate inflammatory bowel disease (IBD) in susceptible individuals. We have demonstrated previously that C. jejuni disrupts the intestinal barrier function by rapidly inducing epithelial translocation of non-invasive commensal bacteria via a transcellular lipid raft-mediated mechanism ('transcytosis'). To further characterize this mechanism, the aim of this current study was to elucidate whether C. jejuni utilizes M cells to facilitate transcytosis of commensal intestinal bacteria.

RESULTS

C. jejuni induced translocation of non-invasive E. coli across confluent Caco-2 epithelial monolayers in the absence of disrupted transepithelial electrical resistance or increased permeability to a 3 kDa dextran probe. C. jejuni-infected monolayers displayed increased numbers of cells expressing the M cell-specific marker, galectin-9, reduced numbers of enterocytes that stained with the absorptive enterocyte marker, Ulex europaeus agglutinin-1, and reduced activities of enzymes typically associated with absorptive enterocytes (namely alkaline phosphatase, lactase, and sucrase). Furthermore, in Campylobacter-infected monolayers, E. coli were observed to be internalized specifically within epithelial cells displaying M-like cell characteristics.

CONCLUSION

These data indicate that C. jejuni may utilize M cells to promote transcytosis of non-invasive bacteria across the intact intestinal epithelial barrier. This mechanism may contribute to the inflammatory immune responses against commensal intestinal bacteria commonly observed in IBD patients.

Campylobacter jejuni induces transcytosis of commensal bacteria across the intestinal epithelium through M-like cells

BACKGROUND

Recent epidemiological analyses have implicated acute Campylobacter enteritis as a factor that may incite or exacerbate inflammatory bowel disease (IBD) in susceptible individuals. We have demonstrated previously that C. jejuni disrupts the intestinal barrier function by rapidly inducing epithelial translocation of non-invasive commensal bacteria via a transcellular lipid raft-mediated mechanism ('transcytosis'). To further characterize this mechanism, the aim of this current study was to elucidate whether C. jejuni utilizes M cells to facilitate transcytosis of commensal intestinal bacteria.

RESULTS

C. jejuni induced translocation of non-invasive E. coli across confluent Caco-2 epithelial monolayers in the absence of disrupted transepithelial electrical resistance or increased permeability to a 3 kDa dextran probe. C. jejuni-infected monolayers displayed increased numbers of cells expressing the M cell-specific marker, galectin-9, reduced numbers of enterocytes that stained with the absorptive enterocyte marker, Ulex europaeus agglutinin-1, and reduced activities of enzymes typically associated with absorptive enterocytes (namely alkaline phosphatase, lactase, and sucrase). Furthermore, in Campylobacter-infected monolayers, E. coli were observed to be internalized specifically within epithelial cells displaying M-like cell characteristics.

CONCLUSION

These data indicate that C. jejuni may utilize M cells to promote transcytosis of non-invasive bacteria across the intact intestinal epithelial barrier. This mechanism may contribute to the inflammatory immune responses against commensal intestinal bacteria commonly observed in IBD patients.

The universe of immune deficiencies in Crohn's disease: a new viewpoint for an old disease?

Crohn's disease (CD) is generally considered a multifactorial disorder, since different genetic and environmental factors are thought to play a role in its pathogenesis. Recently, genome wide linkage studies allowed to identify the association of several loci with the increased risk of CD, although it is still unclear how they interact with environmental factors in causing the disease. The fact that many CD-risk-related genes are involved in the function of phagocytes seems in agreement with the well known role of these cells in CD histopathology. Functional defects in cytokine production or in clearance of bacteria in CD patients have recently been reported. Growing evidence that CD could arise from primary phagocyte immunodeficiency is also coming from the study of cases with early onset in infancy. We review such evidences starting from selected cases and discuss the clinical implications of these findings.

The role of bacteria in the pathogenesis of inflammatory bowel disease

Crohn's disease (CD) and ulcerative colitis (UC) have features that suggest bacterial involvement, and all genetic models of inflammatory bowel disease (IBD) require the presence of commensal bacteria. CD is associated with innate immune response genes such as NOD2/CARD15 and the autophagy genes ATG16L1 and IRGM. However, IBD responds to immunosuppression, suggesting that any bacteria involved are not acting as conventional pathogens. Molecular techniques are rapidly advancing our knowledge of the gut microbiota. In CD there is reduced diversity, and notably a reduction in the probiotic Faecalibacterium prausnitzii, the presence of which in the terminal ileum is associated with a reduced risk of recurrence following surgery. There is also a consistent increase in mucosa-associated Escherichia coli with an "adherent and invasive" phenotype, which allows them to replicate inside macrophages and induce granulomas. Speculation that CD could be caused by the Mycobacterium avium subspecies paratuberculosis (MAP) continues. The response to antitumor necrosis factor treatments suggests that, if relevant at all, MAP is not acting as a conventional pathogen. However, there is increased colonization by MAP in CD, and there is evidence that it could have an indirect effect mediated by the suppression of macrophage function. UC relapse is frequently associated with infection by pathogens, but there is less evidence for involvement of a specific bacterial species. Poor barrier integrity followed by an inflammatory reaction to bacterial components, with chronicity maintained by an autoimmune process, seems a plausible pathogenic model. Bacterial theories of pathogenesis are now becoming testable by targeted therapeutic interventions.

Peyer's Patches: The Immune Sensors of the Intestine

The gut-associated lymphoid tissue (GALT) consists of isolated or aggregated lymphoid follicles forming Peyer's patches (PPs). By their ability to transport luminal antigens and bacteria, PPs can be considered as the immune sensors of the intestine. PPs functions like induction of immune tolerance or defense against pathogens result from the complex interplay between immune cells located in the lymphoid follicles and the follicle-associated epithelium. This crosstalk seems to be regulated by pathogen recognition receptors, especially Nod2. Although TLR exerts a limited role in PP homeotasis, Nod2 regulates the number, size, and T-cell composition of PPs, in response to the gut flora. In turn, CD4⁺ T-cells present in the PP are able to modulate the paracellular and transcellular permeabilities. Two human disorders, Crohn's disease and graft-versus-host disease are thought to be driven by an abnormal response toward the commensal flora. They have been associated with NOD2 mutations and PP dysfunction.

Mucosal dendritic cell diversity in the gastrointestinal tract

The discovery of dendritic cells (DCs) in skin by Paul Langerhans in 1868 identified a cell type which has since been recognized as a key link between innate and adaptive immunity. DCs originate from bone marrow and disseminate through blood to all tissues in the body, and distinct DC subpopulations have been identified in many different tissues. DC diversity is apparent throughout all mucosal surfaces of the body, but the focus of this review article is DC diversity throughout the gastro-intestinal tract (GIT). DC subpopulations have been well characterized in the oral cavity and small intestine, but DC characterization in other regions, such as the esophagus and stomach, is limited. Substantial research has focused on DC function during disease, but understanding the regulation of inflammation and the induction of acquired immune responses requires combined phenotypic and functional characterization of individual DC subpopulations. Furthermore, little is known regarding mucosal DC subpopulations in the GIT of the neonate and how these DC populations change following colonization by commensal microflora. The current review will highlight mucosal DC diversity and discuss factors that may influence mucosal DC differentiation.

Functional changes induced by psychological stress are not enough to cause intestinal inflammation in Sprague-Dawley rats

BACKGROUND

It is well known that stress contributes to the perpetuation of several gastrointestinal diseases. However, its role as a trigger of the inflammatory process in absence of other putative contributing factors remains controversial. Our aim was to elucidate whether stress per se can induce a primary gut inflammation in non-predisposed rats.

METHODS

Male Sprague-Dawley rats were divided in sham and stress groups. Chronic stress was induced by subjecting animals 1 h day(-1) to wrap restraint or water avoidance stress alternatively for five consecutive days, as a model of ongoing life stress.

KEY RESULTS

Chronic stress induced a significant decrease in body weight gain without changes in food intake and an increase in frequency of defecation. Electromiografic (EMG) study showed that the duration of the migrating motor cycles (MMCs), but not its frequency, was shortened in stressed animals compared with non-stress conditions. Moreover, stressful stimulus caused mucosal mast cell hyperplasia and a decrease of iNOS mRNA expression. Bacterial translocation observed in stressed animals was not related to changes in epithelial barrier function and was not enough to induce intestinal inflammation.

CONCLUSIONS & INFERENCES

Decreased MMC duration, mast cell hyperplasia and decreased mRNA iNOS expression, but not altered epithelial barrier function, could be factors implicated in bacterial translocation-induced by chronic stress. However, these changes are not sufficient to induce intestinal inflammation in stress non-susceptible strain of rats.

Stress-induced barrier disruption of rat follicle-associated epithelium involves corticotropin-releasing hormone, acetylcholine, substance P, and mast cells

BACKGROUND

The follicle-associated epithelium (FAE) is specialized in uptake and sampling of luminal antigens and bacteria. We previously showed that stress increased FAE permeability in rats. An increased uptake may alter antigen exposure in Peyer's patches leading to intestinal disease. The aim of this study was to elucidate mechanisms involved in the acute stress-induced increase in FAE permeability.

METHODS

Rats were pretreated i.p. with corticotropin-releasing hormone receptor (CRH-R) antagonist, neurokinin receptor 1 (NK-1R) antagonist, atropine, the mast cell stabilizer doxantrazole (DOX), or NaCl, and submitted to 1-h acute water avoidance stress. FAE tissues were mounted in Ussing chambers for measurements of permeability to (51)Cr-EDTA, horseradish peroxidase (HRP) and chemically killed Escherichia coli K-12. Further, FAE segments were exposed in vitro in chambers to CRH, substance P (SP), carbachol, and DOX. Neurotransmitter- and receptor distribution was studied by immunohistochemistry.

KEY RESULTS

Stress-induced increases in uptake across FAE of HRP and E. coli were reduced by DOX, CRH-R antagonist and atropine, whereas the NK-1R antagonist decreased (51)Cr-EDTA permeability. Exposure to CRH and carbachol increased HRP and E. coli passage, whereas SP increased bacterial and (51)Cr-EDTA permeability. DOX counteracted all of these effects. Immunohistochemistry revealed CRH, acetylcholine, SP, and their receptors on mast cells within the Peyer's patches, subepithelial dome, and adjacent villi.

CONCLUSIONS & INFERENCES

Corticotropin-releasing hormone and acetylcholine signaling affect mainly transcellular permeability while SP seems more selective toward the paracellular pathways. Our findings may be of importance for the understanding of the pathogenesis of stress-related intestinal disorders.

Enhanced translocation of bacteria across metabolically stressed epithelia is reduced by butyrate

BACKGROUND

The gut microflora in some patients with Crohn's disease can be reduced in numbers of butyrate-producing bacteria and this could result in metabolic stress in the colonocytes. Thus, we hypothesized that the short-chain fatty acid, butyrate, is important in the maintenance and regulation of the barrier function of the colonic epithelium.

METHODS

Confluent monolayers of the human colon-derived T84 or HT-29 epithelial cell lines were exposed to dinitrophenol (DNP (0.1 mM), uncouples oxidative phosphorylation) + Escherichia coli (strain HB101, 10(6) cfu) +/- butyrate (3-50 mM). Transepithelial resistance (TER), and bacterial internalization and translocation were assessed over a 24-hour period. Epithelial ultrastructure was assessed by transmission electron microscopy.

RESULTS

Epithelia under metabolic stress display decreased TER and increased numbers of pseudopodia that is consistent with increased internalization and translocation of the E. coli. Butyrate (but not acetate) significantly reduced the bacterial translocation across DNP-treated epithelia but did not ameliorate the drop in TER in the DNP+E. coli exposed monolayers. Inhibition of bacterial transcytosis across metabolically stressed epithelia was associated with reduced I-kappaB phosphorylation and hence NF-kappaB activation.

CONCLUSIONS

Reduced butyrate-producing bacteria could result in increased epithelial permeability particularly in the context of concomitant exposure to another stimulus that reduces mitochondria function. We speculate that prebiotics, the substrate for butyrate synthesis, is a valuable prophylaxis in the regulation of epithelial permeability and could be of benefit in preventing relapses in IBD.

The intestinal barrier and its regulation by neuroimmune factors

BACKGROUND

The ability to control uptake across the mucosa and protect from damage of harmful substances from the lumen is defined as intestinal barrier function. A disturbed barrier dysfunction has been described in many human diseases and animal models, for example, inflammatory bowel disease, irritable bowel syndrome, and intestinal hypersensitivity. In most diseases and models, alterations are seen both of the paracellular pathway, via the tight junctions, and of the transcellular routes, via different types of endocytosis. Recent studies of pathogenic mechanisms have demonstrated the important role of neuroimmune interaction with the epithelial cells in the regulation of barrier function. Neural impulses from extrinsic vagal and/or sympathetic efferent fibers or intrinsic enteric nerves influence mucosal barrier function via direct effects on epithelial cells or via interaction with immune cells. For example, by nerve-mediated activation by corticotropin-releasing hormone or cholinergic pathways, mucosal mast cells release a range of mediators with effects on transcellular, and/or paracellular permeability (for example, tryptase, TNF-alpha, nerve growth factor, and interleukins).

PURPOSE

In this review, we discuss current physiological and pathophysiological aspects of the intestinal barrier and, in particular, its regulation by neuroimmune factors.

Multiple facets of intestinal permeability and epithelial handling of dietary antigens

The intestinal epithelium, the largest interface between the host and environment, regulates fluxes of ions and nutrients and limits host contact with the massive load of luminal antigens. Local protective and tolerogenic immune responses toward luminal content depend on antigen sampling by the gut epithelial layer. Whether, and how exaggerated, the entrance of antigenic macromolecules across the gut epithelium might initiate and/or perpetuate chronic inflammation as well as the respective contribution of paracellular and transcellular permeability remains a matter of debate. To this extent, experimental studies involving the in vivo assessment of intestinal permeability using small inert molecules do not necessarily correlate with the uptake of larger dietary antigens. This review analyzes both the structural and functional aspects of intestinal permeability with special emphasis on antigen handling in healthy and diseased states and consequences on local immune responses to food antigens.

Postmortem morphology and viability of human Peyer's patches in distal ileum: a technical note

The intestinal mucosa contains a highly specialized immune system which plays a central role in the induction of immune reactions. In the small bowel, Gut-Associated Lymphoid Tissue (GALT) is organized in lymphoid aggregates which are known as Peyer's Patches (PP). Even though human PP involvement in systemic immunity has been described, little is known about their anatomy and morphology and viability. The aim of this study was to examine PP according to their macroscopic anatomy, distribution and cell viability after death. Specimens from the distal ileum were obtained from 72 serial autopsy cases: PP were identified and, parts of them were analyzed for histological examination. Moreover, viability of recovered PP cells was assessed by the trypan blue exclusion test. Most of the PP (90%) were situated on the antimesenteric border of ileum, and the greatest density of PP occurred in the most distal segment. The number of PP varied with age, with the maximum number observed in 21- to 30-years old cadavers. Histological examination showed their remarkable architectural preservation at different post-mortem intervals (PMI), while the mucosal surface underwent autolysis. In 56% of cases PP cells were still viable, especially at PMI < 24 hours after death. These data confirm that human PP are still well preserved in a remarkable percentage of cadavers also several hours after death, and their availability may be helpful in various fields of research.

Commensal bacteria, traditional and opportunistic pathogens, dysbiosis and bacterial killing in inflammatory bowel diseases

PURPOSE OF REVIEW

The authors present evidence published during the past 2 years of the roles of commensal and pathogenic bacteria in the pathogenesis of the inflammatory bowel diseases.

RECENT FINDINGS

Rodent models conclusively implicate commensal enteric bacteria in chronic, immune-mediated, experimental colitis, and genetically determined defects in bacterial killing by innate immune cells are found in a subset of patients with Crohn's disease. There is no evidence that a single pathogen, including Mycobacterium avium subspecies paratuberculosis, causes Crohn's disease or ulcerative colitis. However, adherent/invasive Escherichia coli are associated with ileal Crohn's disease, with the mechanisms and genetics of adherent/invasive E. coli virulence being elucidated. Molecular characterization of the microbiota in patients with inflammatory bowel diseases reveals decreased biodiversity of commensal bacteria, most notably the phyla Bacteroidetes and Firmicutes, including the clinically relevant Faecalibacterium prausnitzii, and increased E. coli concentrations. VSL#3 is one probiotic preparation shown to be efficacious in certain clinical situations in small clinical trials.

SUMMARY

Further characterization of altered microbiota in patients with inflammatory bowel diseases and linking dysbiosis with host genetic alterations in immunoregulation, innate microbial killing and barrier function are critical, so that individualized treatments to increase beneficial commensals and their metabolic products (probiotic and prebiotic administration) and diminish deleterious species such as adherent/invasive E. coli can be tailored for defined patient subsets.

CD83+CCR7- dendritic cells accumulate in the subepithelial dome and internalize translocated Escherichia coli HB101 in the Peyer's patches of ileal Crohn's disease

Recurrent Crohn's disease originates with small erosions in the follicle-associated epithelium overlying the Peyer's patches. Animal studies have illustrated mucosal immune regulation by dendritic cells located in the subepithelial dome. The aim of this study was to characterize the dendritic cells at this specific site in patients with Crohn's disease. Ileal tissues were obtained after surgery performed on Crohn's patients; ileal samples from noninflammatory bowel disease and ulcerative colitis served as standard and inflammatory controls, respectively. Flow cytometry of isolated intestinal mononuclear cells showed a larger subset of dendritic cells in Crohn's samples compared with controls. This finding was corroborated by confocal microscopy, showing enhanced infiltrates of cells positive for the dendritic cell markers, DC-SIGN(+) and CD83(+), in the subepithelial dome. Moreover, the CD83(+) cells in Crohn's tissues showed reduced expression of the lymph node migratory receptor, CCR7, possibly contributing to the high numbers of dendritic cells. After exposure to nonpathogenic Escherichia coli in Ussing chambers, dendritic cells in the subepithelial dome of Crohn's disease demonstrated increased co-localization with translocated bacteria. Immunohistochemical results revealed that DC-SIGN(+) cells in Crohn's tissues were found to express toll-like receptor 4 and produce tumor necrosis factor-alpha. In conclusion, nonmigrating dendritic cells that accumulate in the subepithelial dome and internalize nonpathogenic bacteria may be important for the onset and perpetuation of mucosal inflammation in Crohn's disease.

Intestinal barrier dysfunction in inflammatory bowel diseases

The etiology of human inflammatory bowel diseases (IBDs) is believed to involve inappropriate host responses to the complex commensal microbial flora in the gut, although an altered commensal flora is not completely excluded. A multifunctional cellular and secreted barrier separates the microbial flora from host tissues. Altered function of this barrier remains a major largely unexplored pathway to IBD. Although there is evidence of barrier dysfunction in IBD, it remains unclear whether this is a primary contributor to disease or a consequence of mucosal inflammation. Recent evidence from animal models demonstrating that genetic defects restricted to the epithelium can initiate intestinal inflammation in the presence of normal underlying immunity has refocused attention on epithelial dysfunction in IBD. We review the components of the secreted and cellular barrier, their regulation, including interactions with underlying innate and adaptive immunity, evidence from animal models of the barrier's role in preventing intestinal inflammation, and evidence of barrier dysfunction in both Crohn's disease and ulcerative colitis.

Yersinia pseudotuberculosis induces transcytosis of nanoparticles across human intestinal villus epithelium via invasin-dependent macropinocytosis

Crohn's disease is characterized by a defect in intestinal barrier function, where bacteria are considered the most important inflammation-driving factor. Enteric bacteria, including E. coli and Yersinia spp, affect tight junctions in enterocytes, but little is known about bacterial effects on the transcellular pathway. Our objective was to study the short-term effects of Y. pseudotuberculosis on uptake of nanoparticles across human villus epithelium. Monolayers of human colon epithelium-derived Caco-2 cells and biopsies of normal human ileum were studied after 2 h exposure to Y. pseudotuberculosis expressing (inv+) or lacking (inv-) the bacterial adhesion molecule, invasin. Transepithelial transport of fluorescent nanoparticles (markers of transcytosis) was quantified by flow cytometry, and mechanisms explored by using inhibitors of endocytosis. Epithelial expressions of beta1-integrin and particle uptake pathways were studied by confocal microscopy. The paracellular pathway was assessed by electrical resistance (TER), mannitol flux, and expression of tight junction proteins occludin and caludin-4 by confocal microscopy. Inv+ Y. pseudotuberculosis adhered to the apical surface of epithelial cells and induced transcytosis of exogenous nanoparticles across Caco-2 monolayers (30-fold increase, P<0.01) and ileal mucosa (268+/-47% of control; P<0.01), whereas inv bacteria had no effect on transcytosis. The transcytosis was concentration-, particle size- and temperature-dependent, and possibly mediated via macropinocytosis. Y. pseudotuberculosis also induced apical expression of beta1-integrin on epithelial cells. A slight drop in TER was seen after exposure to inv+ Y. pseudotuberculosis, whereas mannitol flux and tight junction protein expression was unchanged. In summary, Y. pseudotuberculosis induced apical expression of beta1-integrin and stimulated uptake of nanoparticles via invasin-dependent transcytosis in human intestinal epithelium. Our findings suggest that bacterial factors may initiate transcytosis of luminal exogenous particles across human ileal mucosa, thus presenting a novel mechanism of intestinal barrier dysfunction.