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Crohn’s disease can affect any part of the gastrointestinal tract; however, current European and national guidelines worldwide do not differentiate between small-intestinal and colonic Crohn’s disease for medical treatment. Data from the past decade provide evidence that ileal Crohn’s disease is distinct from colonic Crohn’s disease in several intestinal layers. Remarkably, colonic Crohn’s disease shows an overlap with regard to disease behaviour with ulcerative colitis, underlining the fact that there is more to inflammatory bowel disease than just Crohn’s disease and ulcerative colitis, and that subtypes, possibly defined by location and shared pathophysiology, are also important. This Review provides a structured overview of the differentiation between ileal and colonic Crohn’s disease using data in the context of epidemiology, genetics, macroscopic differences such as creeping fat and histological findings, as well as differences in regard to the intestinal barrier including gut microbiota, mucus layer, epithelial cells and infiltrating immune cell populations. We also discuss the translation of these basic findings to the clinic, emphasizing the important role of treatment decisions. Thus, this Review provides a conceptual outlook on a new mechanism-driven classification of Crohn’s disease. Evidence suggests that ileal Crohn’s disease is distinct from colonic Crohn’s disease on a multitude of layers. This Review provides a structured overview of this evidence and its implications for clinical decision-making. Key points Emerging data from fields including clinical behaviour, epidemiology, genetics and the gut microbiota strongly suggest that ileal and colonic Crohn’s disease should be regarded as at least two subtypes of this disease entity. Creeping fat, which represents hyperplasia of the mesenteric fat and wrapping of inflamed intestinal segments, is unique to small-intestinal Crohn’s disease and absent in colonic Crohn’s disease. Predictive models applying a genetic risk score are able to differentiate between ileal and colonic Crohn’s disease. The ileum in Crohn’s disease is characterized by a type 1 or type 17 helper T cell profile, whereas the colon is characterized by a type 1 profile. There is an observable trend that patients with Crohn’s disease with isolated ileal manifestation are less likely to respond to biological therapies than patients with colonic disease.

Crohn’s disease (CD) is a persistent inflammatory disorder primarily affecting the terminal ileum. The Tnf Δ ΑRE mice, which spontaneously develop CD-like ileitis due to TNF overexpression, represent a faithful model of the human disease. Here, via single-cell RNA sequencing in Tnf Δ ΑRE mice, we show that murine TNF-dependent ileitis is characterized by cell expansion in tertiary lymphoid organs (TLO), T cell effector reprogramming, and accumulation of activated macrophages in the submucosal granulomas. Within the stromal cell compartment, fibroblast subsets (telocytes, trophocytes, Pdgfra lo Cd81 − cells) are less abundant while lymphatic endothelial cells (LEC) and fibroblastic reticular cells (FRC) show relative expansion compared to the wild type. All three fibroblast subsets show strong pro-inflammatory signature. TNFR1 loss or gain of function experiments in specific fibroblast subsets suggest that the Tnf Δ ΑRE -induced ileitis is initiated in the lamina propria via TNF pathway activation in villus-associated fibroblasts (telocytes and Pdgfra lo Cd81 − cells), which are responsible for the organization of TLOs. Trophocytes drive disease progression in the submucosal layer, accompanied by the excessive formation of granulomas. These findings provide evidence for spatial regulation of inflammation by fibroblast subsets and underscore the pivotal role of fibroblasts in the inception and advancement of ileitis. The Tnf ΔΑRE mice overexpress TNF, resulting in ileitis, showing strong similarity to Crohn’s disease in human patients. Here authors describe the immune pathology of the murine disease model by single-cell RNA sequencing, spatial and functional experiments to find that spatially distinct subsets of fibroblasts initiate the disease and drive disease progression.

The mortality rate for alimentary tract hemorrhage remains high due to a variety of contributing factors. In this report, we present a case of post-severe trauma patient with life-threatening gastrointestinal bleeding caused by cytomegalovirus (CMV)-induced damage to the terminal ileum. A 76-year-old female with a history of hypertension and gastrointestinal bleeding developed CMV ileitis post-severe trauma. Despite negative CMV IgM antibodies, PCR testing confirmed CMV infection in the biopsy tissue. Histopathological examination revealed viral inclusion bodies, with immunohistochemistry confirming CMV presence. Intravenous ganciclovir effectively managed symptoms and halted bleeding. CMV ileitis, typically seen in immunocompromised states, may occur sporadically in immunocompetent individuals, including post-orthopedic surgery patients. The exact mechanism remains unclear, possibly related to surgical stress. Diagnosis relies on histopathology and immunohistochemistry. Early recognition and treatment are vital for optimal outcomes, emphasizing the need for awareness among orthopedic surgeons regarding CMV as a potential cause of postoperative complications.

ObjectiveEvaluate the cost-effectiveness of laparoscopic ileocaecal resection compared with infliximab in patients with ileocaecal Crohn’s disease failing conventional therapy.DesignA multicentre randomised controlled trial was performed in 29 centres in The Netherlands and the UK. Adult patients with Crohn’s disease of the terminal ileum who failed >3 months of conventional immunomodulators or steroids without signs of critical strictures were randomised to laparoscopic ileocaecal resection or infliximab. Outcome measures included quality-adjusted life-years (QALYs) based on the EuroQol (EQ) 5D-3L Questionnaire and the Inflammatory Bowel Disease Questionnaire (IBDQ). Costs were measured from a societal perspective. Analyses were performed according to the intention-to-treat principle. Missing cost and effect data were imputed using multiple imputation. Cost-effectiveness planes and cost-effectiveness acceptability curves were estimated to show uncertainty.ResultsIn total, 143 patients were randomised. Mean Crohn’s disease total direct healthcare costs per patient at 1 year were lower in the resection group compared with the infliximab group (mean difference €−8931; 95% CI €−12 087 to €−5097). Total societal costs in the resection group were lower than in the infliximab group, however not statistically significant (mean difference €−5729, 95% CI €−10 606 to €172). The probability of resection being cost-effective compared with infliximab was 0.96 at a willingness to pay (WTP) of €0 per QALY gained and per point improvement in IBDQ Score. This probability increased to 0.98 at a WTP of €20 000/QALY gained and 0.99 at a WTP of €500/point of improvement in IBDQ Score.ConclusionLaparoscopic ileocaecal resection is a cost-effective treatment option compared with infliximab.Clinical trial registration numberDutch Trial Registry NTR1150; EudraCT number 2007-005042-20 (closed on 14 October 2015).

Understanding the interplay between genetic susceptibility, the microbiome, the environment and the immune system in Crohn's Disease (CD) is essential for developing optimal therapeutic strategies. We sought to examine the dynamics of the relationship between inflammation, the ileal microbiome, and host genetics in murine models of ileitis. We induced ileal inflammation of graded severity in C57BL6 mice by gavage with Toxoplasma gondii, Giardia muris, low dose indomethacin (LDI; 0.1 mg/mouse), or high dose indomethacin (HDI; 1 mg/mouse). The composition and spatial distribution of the mucosal microbiome was evaluated by 16S rDNA pyrosequencing and fluorescence in situ hybridization. Mucosal E. coli were enumerated by quantitative PCR, and characterized by phylogroup, genotype and pathotype. Moderate to severe ileitis induced by T. gondii (day 8) and HDI caused a consistent shift from >95% gram + Firmicutes to >95% gram - Proteobacteria. This was accompanied by reduced microbial diversity and mucosal invasion by adherent and invasive E. coli, mirroring the dysbiosis of ileal CD. In contrast, dysbiosis and bacterial invasion did not develop in mice with mild ileitis induced by Giardia muris. Superimposition of genetic susceptibility and T. Gondii infection revealed greatest dysbiosis and bacterial invasion in the CD-susceptible genotype, NOD2(-/-), and reduced dysbiosis in ileitis-resistant CCR2(-/-) mice. Abrogating inflammation with the CD therapeutic anti-TNF-α-mAb tempered dysbiosis and bacterial invasion. Acute ileitis induces dysbiosis and proliferation of mucosally invasive E. coli, irrespective of trigger and genotype. The identification of CCR2 as a target for therapeutic intervention, and discovery that host genotype and therapeutic blockade of inflammation impact the threshold and extent of ileal dysbiosis are of high relevance to developing effective therapies for CD.

ObjectiveAlthough immunoglobulin A (IgA) is abundantly expressed in the gut and known to be an important component of mucosal barriers against luminal pathogens, its precise function remains unclear. Therefore, we tried to elucidate the effect of IgA on gut homeostasis maintenance and its mechanism.DesignWe generated various IgA mutant mouse lines using the CRISPR/Cas9 genome editing system. Then, we evaluated the effect on the small intestinal homeostasis, pathology, intestinal microbiota, cytokine production, and immune cell activation using intravital imaging.ResultsWe obtained two lines, with one that contained a <50 base pair deletion in the cytoplasmic region of the IgA allele (IgA tail-mutant; IgAtm/tm) and the other that lacked the most constant region of the IgH α chain, which resulted in the deficiency of IgA production (IgA−/−). IgA−/− exhibited spontaneous inflammation in the ileum but not the other parts of the gastrointestinal tract. Associated with this, there were significantly increased lamina propria CD4+ T cells, elevated productions of IFN-γ and IL-17, increased ileal segmented filamentous bacteria and skewed intestinal microflora composition. Intravital imaging using Ca2+ biosensor showed that IgA−/− had elevated Ca2+ signalling in Peyer’s patch B cells. On the other hand, IgAtm/tm seemed to be normal, suggesting that the IgA cytoplasmic tail is dispensable for the prevention of the intestinal disorder.ConclusionIgA plays an important role in the mucosal homeostasis associated with the regulation of intestinal microbiota and protection against mucosal inflammation especially in the ileum.

ObjectiveAlthough perturbations in mitochondrial function and structure have been described in the intestinal epithelium of Crohn’s disease and ulcerative colitis patients, the role of epithelial mitochondrial stress in the pathophysiology of inflammatory bowel diseases (IBD) is not well elucidated. Prohibitin 1 (PHB1), a major component protein of the inner mitochondrial membrane crucial for optimal respiratory chain assembly and function, is decreased during IBD.DesignMale and female mice with inducible intestinal epithelial cell deletion of Phb1 (Phb1iΔIEC ) or Paneth cell-specific deletion of Phb1 (Phb1ΔPC ) and Phb1fl/fl control mice were housed up to 20 weeks to characterise the impact of PHB1 deletion on intestinal homeostasis. To suppress mitochondrial reactive oxygen species, a mitochondrial-targeted antioxidant, Mito-Tempo, was administered. To examine epithelial cell-intrinsic responses, intestinal enteroids were generated from crypts of Phb1iΔIEC or Phb1ΔPC mice.Results Phb1iΔIEC mice exhibited spontaneous ileal inflammation that was preceded by mitochondrial dysfunction in all IECs and early abnormalities in Paneth cells. Mito-Tempo ameliorated mitochondrial dysfunction, Paneth cell abnormalities and ileitis in Phb1iΔIEC ileum. Deletion of Phb1 specifically in Paneth cells (Phb1ΔPC ) was sufficient to cause ileitis. Intestinal enteroids generated from crypts of Phb1iΔIEC or Phb1ΔPC mice exhibited decreased viability and Paneth cell defects that were improved by Mito-Tempo.ConclusionOur results identify Paneth cells as highly susceptible to mitochondrial dysfunction and central to the pathogenesis of ileitis, with translational implications for the subset of Crohn’s disease patients exhibiting Paneth cell defects.

We previously showed that the probiotic mixture, VSL#3, prevents the onset of ileitis in SAMP/YitFc (SAMP) mice, and this effect was associated with stimulation of epithelial-derived TNF. The aim of this study was to determine the mechanism(s) of VSL#3-mediated protection on epithelial barrier function and to further investigate the \"paradoxical\" effects of TNF in preventing SAMP ileitis. Permeability was evaluated in SAMP mice prior to the onset of inflammation and during established disease by measuring transepithelial electrical resistance (TEER) on ex vivo-cultured ilea following exposure to VSL#3 conditioned media (CM), TNF or VSL#3-CM + anti-TNF. Tight junction (TJ) proteins were assessed by qRT-PCR, Western blot, and confocal microscopy, and TNFRI/TNFRII expression measured in freshly isolated intestinal epithelial cells (IEC) from SAMP and control AKR mice. Culture with either VSL#3-CM or TNF resulted in decreased ileal paracellular permeability in pre-inflamed SAMP, but not SAMP with established disease, while addition of anti-TNF abrogated these effects. Modulation of the TJ proteins, claudin-2 and occludin, occurred with a significant decrease in claudin-2 and increase in occludin following stimulation with VSL#3-CM or TNF. TNF protein levels increased in supernatants of SAMP ilea incubated with VSL#3-CM compared to vehicle, while IEC-derived TNFR mRNA expression decreased in young, and was elevated in inflamed, SAMP versus AKR mice. Our data demonstrate that the previously established efficacy of VSL#3 in preventing SAMP ileitis is due to direct innate and homeostatic effects of TNF on the gut epithelium, modulation of the TJ proteins, claudin-2 and occludin, and overall improvement of intestinal permeability.

Obesity has been associated with a more severe disease course in inflammatory bowel disease (IBD) and epidemiological data identified dietary fats but not obesity as risk factors for the development of IBD. Crohn's disease is one of the two major IBD phenotypes and mostly affects the terminal ileum. Despite recent observations that high fat diets (HFD) impair intestinal barrier functions and drive pathobiont selection relevant for chronic inflammation in the colon, mechanisms of high fat diets in the pathogenesis of Crohn's disease are not known. The aim of this study was to characterize the effect of HFD on the development of chronic ileal inflammation in a murine model of Crohn's disease-like ileitis. TNF(ΔARE/WT) mice and wildtype C57BL/6 littermates were fed a HFD compared to control diet for different durations. Intestinal pathology and metabolic parameters (glucose tolerance, mesenteric tissue characteristics) were assessed. Intestinal barrier integrity was characterized at different levels including polyethylene glycol (PEG) translocation, endotoxin in portal vein plasma and cellular markers of barrier function. Inflammatory activation of epithelial cells as well as immune cell infiltration into ileal tissue were determined and related to luminal factors. HFD aggravated ileal inflammation but did not induce significant overweight or typical metabolic disorders in TNF(ΔARE/WT). Expression of the tight junction protein Occludin was markedly reduced in the ileal epithelium of HFD mice independently of inflammation, and translocation of endotoxin was increased. Epithelial cells showed enhanced expression of inflammation-related activation markers, along with enhanced luminal factors-driven recruitment of dendritic cells and Th17-biased lymphocyte infiltration into the lamina propria. HFD feeding, independently of obesity, accelerated disease onset of small intestinal inflammation in Crohn's disease-relevant mouse model through mechanisms that involve increased intestinal permeability and altered luminal factors, leading to enhanced dendritic cell recruitment and promoted Th17 immune responses.

The pathogenesis of Crohn's disease (CD), an idiopathic inflammatory bowel disease, is attributed, in part, to intestinal bacteria that may initiate and perpetuate mucosal inflammation in genetically susceptible individuals. Paneth cells (PC) are the major source of antimicrobial peptides in the small intestine, including human α-defensins HD5 and HD6. We tested the hypothesis that reduced expression of PC α-defensins compromises mucosal host defenses and predisposes patients to CD of the ileum. We report that patients with CD of the ileum have reduced antibacterial activity in their intestinal mucosal extracts. These specimens also showed decreased expression of PC α-defensins, whereas the expression of eight other PC products either remained unchanged or increased when compared with controls. The specific decrease of a-defensins was independent of the degree of inflammation in the specimens and was not observed in either CD of the colon, ulcerative colitis, or pouchitis. The functional consequence of a-defensin expression levels was examined by using a transgenic mouse model, where we found changes in HD5 expression levels, comparable to those observed in CD, had a pronounced impact on the luminal micro-biota. Thus, the specific deficiency of PC defensins that characterizes ileal CD may compromise innate immune defenses of the ileal mucosa and initiate and/or perpetuate this disease.