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Microscopic colitis is an inflammatory bowel disease divided into two subtypes: collagenous colitis and lymphocytic colitis. With an increasing incidence of microscopic colitis exceeding those of ulcerative and Crohn’s disease among elderly people in some countries, microscopic colitis is a debilitating life experience. Therefore, physicians should be familiar with its clinical features and management strategies because the disease deserves the same attention as the classical inflammatory bowel diseases. Here, state-of-the-art knowledge of microscopic colitis is provided from a global perspective with reference to etiopathology and how to establish the diagnosis with the overall aim to create awareness and improve rational management in clinical practice. The immune system and a dysregulated immune response seem to play a key role combined with risk factors (e.g. cigarette smoking) in genetically predisposed individuals. The symptoms are characterized by recurrent or chronic nonbloody, watery diarrhea, urgency, weight loss, and a female preponderance. As biomarkers are absent, the diagnosis relies on colonoscopy with a histological assessment of biopsy specimens from all parts of the colon. Although the disease is not associated with a risk of colorectal cancer, a recent nationwide, population-based cohort study found an increased risk of lymphoma and lung cancer. Budesonide is the first-line therapy for management, whereas immunomodulatory drugs (including biologics) and drugs with antidiarrheal properties may be indicated in those failing, dependent, or intolerant to budesonide. In microscopic colitis induced by checkpoint inhibitors, a drug class used increasingly for a wide range of malignancies, a more aggressive therapeutic approach with biologics introduced early seems reasonable. However, particular attention needs to be drawn to the existence of incomplete forms of microscopic colitis with the risk of being overlooked in routine clinical settings.

Microscopic colitis (MC) is a relatively common cause of chronic watery diarrhea, especially in older persons. Associated symptoms, including abdominal pain and arthralgias, are common. The diagnosis is based upon characteristic histological findings in the presence of diarrhea. The two types of MC, collagenous and lymphocytic colitis, share similar clinical features, with the main difference being the presence or absence of a thickened subepithelial collagen band. There are several treatment options for patients with MC, although only budesonide has been well studied in multiple controlled clinical trials. This review will describe the clinical features, epidemiology, pathophysiology, diagnostic criteria, and treatment of patients with MC.

Abstract Background For unidentified reasons, possibly due to increased immune surveillance, patients with collagenous colitis (CC) and lymphocytic colitis (LC), both forms of microscopic colitis (MC), have lower risk of colorectal cancer than controls and ulcerative colitis (UC) patients. Levels of secreted and cell-bound mediators in MC patients with active disease and in histological remission (HR) compared to UC patients and controls were investigated. Methods Median fluorescence intensity of 54 analytes in colonic biopsies from patients with active CC (n = 21), LC (n = 11), and UC (n = 19); CC-HR (n = 6), LC-HR (n = 9), UC in remission (n = 19), non-diarrhea controls (n = 48), and diarrhea controls (n = 25) was measured using Luminex. Results Granzyme B and CCL5 levels were higher in active CC than in UC, whereas CCL4 and CD163 levels were similar in CC and UC, and both groups had higher levels of matrix metalloproteinase (MMP)-1, MMP-3, and tumor necrosis factor receptor II than both control groups. APRIL, BAFF, BCMA, CCL20, CXCL8, chitinase 3-like 1, pentraxin-3, Fas, and IL-33 were higher in UC than MC. Increases in 4-1BB and perforin in MC compared to controls were lower than in UC. Levels of gp130 and IL-6Rα were decreased in MC but increased in UC compared to controls. Conclusions Microscopic colitis patients exhibit increased levels of several analytes, including some associated with CD8+ T lymphocytes, suggesting a different pathogenesis of MC compared to UC. Higher levels of MMP-1 and MMP-3 in CC than LC indicate separate disease entities. Lay Summary Microscopic colitis patients have elevated colonic levels of numerous analytes compared to controls, with some being higher than in ulcerative colitis patients and associated with CD8+ cytotoxic T lymphocytes. These differences imply distinct pathogeneses of these diseases. Graphical Abstract Graphical Abstract

ObjectiveMicroscopic colitis (MC) encompasses the two histopathological distinct entities of collagenous colitis (CC) and lymphocytic colitis (LC). In this Danish population-based cohort study, we examined the risk of MC following stool culture with Campylobacter concisus, C. jejuni, non-typhoidal Salmonella or a culture-negative stool test.DesignWe identified patients with a first-time positive stool culture with C. concisus, C. jejuni, non-typhoidal Salmonella or negative stool test, from 2009 through 2013 in North Denmark Region, Denmark, and matched each with 10 population comparisons. All subjects were followed up until 1 March 2018 using Systematised Nomenclature of Medicine codes from The Danish Pathology Register for incident diagnoses of CC and LC. We computed risk and adjusted HRs with 95% CIs for MC among patients and comparisons.ResultsWe identified 962 patients with C. concisus, 1725 with C. jejuni, 446 with Salmonella and 11 825 patients with culture-negative stools. The MC risk and HR versus comparisons were high for patients with C. concisus (risk 6.2%, HR 32.4 (95% CI 18.9 to 55.6)), less for C. jejuni (risk 0.6%, HR 3.7 (95% CI 1.8 to 7.7)), low for Salmonella (risk 0.4%, HR 2.2 (95% CI 0.5 to 10.8)) and for patients with negative stool testing (risk 3.3%, HR 19.6 (95% CI 16.4 to 23.4)). After exclusion of the first year of follow-up, the HRs were 9.3 (95% CI 4.1 to 20.1), 2.2 (95% CI 0.9 to 5.4), 1.3 (95% CI 0.2 to 11.1) and 5.6 (95% CI 4.6 to 7.2), respectively.ConclusionA high risk of MC was observed following C. concisus in stools. Further studies are needed to elucidate any underlying biological mechanisms.

Abstract Background The diagnosis of microscopic colitis (MC) is based on endoscopic biopsy with histological assessment. Histological outcomes (remission, response or improvement) are important treatment targets in clinical trials. Although a substantial body of research on MC has been published in recent years, no standardized criteria currently exist for its histological outcomes. We sought to review and summarize the histological evaluation of MC in published systematic reviews (SRs) assessing the efficacy of interventions and to examine the heterogeneity in histological evaluation among the randomized controlled trials (RCTs) included in those SRs. Methods We conducted an umbrella review (ie, an overview of systematic reviews) of published SRs. A literature search of the Cochrane Database of Systematic Reviews, MEDLINE, and Embase was performed up to May 2025. Definitions of histological evaluation and monitoring following interventions were extracted and summarized from the published SRs and the RCTs included within them. Results Fourteen SRs with meta-analyses that focused on interventions were included. Nineteen RCTs were included in these SRs. Of them, 12 fully published RCTs reported histological outcome data and met our inclusion criteria. The definitions for histological outcomes varied between RCTs but were generally based on reduction in lamina propria cellularity, intraepithelial lymphocytes, or collagen band thickness. Conclusions This umbrella review highlights the heterogeneity in the definitions of histological outcomes in MC RCTs. The summarized evidence will support ongoing efforts to develop consensus definitions for histological outcomes in order to facilitate clinical trials of medical therapies for MC. Lay Summary We conducted the first systematic review to evaluate the heterogeneity in definitions of histological outcomes in microscopic colitis across randomized controlled trials. The evidence synthesized in this review will help inform ongoing efforts to establish consensus definitions for histological outcomes in medical therapies for microscopic colitis.

Believed to be a rare cause of chronic diarrhoea, microscopic colitis (MC) is a condition with rising incidence. Many prevalent risk factors and the unknown pathogenesis of MC rationalise the need for studies on microbiota composition. PubMed, Scopus, Web of Science and Embase were searched. Eight case-control studies were included. The risk of bias was assessed with the Newcastle–Ottawa Scale. Clinical details on the study population and MC were poor. The most consistent result among the studies was a decreased Akkermansia genus in faecal samples. Other results were inconsistent due to the different taxonomic levels of the outcomes. Possible changes in different taxa were observed in patients who suffered from MC compared to healthy controls. The alpha diversity compared between MC and the diarrhoea control may suggest potential similarities. The beta diversity in MC compared to healthy and diarrhoeal populations showed no significant outcomes. The microbiome composition in MC possibly differed from the healthy control, but no agreement regarding taxa was made. It might be relevant to focus on possible factors influencing the microbiome composition and its relationship with other diarrhoeal diseases.

Microscopic colitis (MC) is a chronic inflammatory disease of the large intestine and a common cause of chronic diarrhea in older adults. Here, we use single-cell RNA sequencing analysis of colonic mucosal tissue to build a cellular and molecular model for MC. Our results show that in MC, there is a substantial expansion of tissue CD8 +  T cells, likely arising from local expansion following T cell receptor engagement. Within the T cell compartment, MC is characterized by a shift in CD8 tissue-resident memory T cells towards a highly cytotoxic and inflammatory phenotype and expansion of CD4 +  T regulatory cells. These results provide insight into inflammatory cytokines shaping MC pathogenesis and highlight notable similarities and differences with other immune-mediated intestinal diseases, including a common upregulation of IL26 and an MC-specific upregulation of IL10 . These data help identify targets against enteric T cell subsets as an effective strategy for treatment of MC. Microscopic colitis is a chronic inflammatory disease of the large intestine. Here the authors use single-cell RNA transcriptomic profiling and tissue localization studies to characterise the colon immune cell populations in MC, showing expansion of CD8 T cells with diverse TCR clonotypes and expression of CD4 T reg cell signatures.

Background Microscopic colitis (MC), comprising lymphocytic colitis (LC) and collagenous colitis (CC), is an inflammatory bowel disease with increasing incidence. MC etiopathogenesis remains unknown; however, altered colonic epithelial integrity may underlie uncontrolled luminal antigen passage, triggering immuno‐inflammatory responses. Objective The aim of this study was to further define the involvement of the colonic epithelium in MC. Methods A paired transcriptomic and proteomic analysis followed by epithelial ultrastructural examination was performed on colonic biopsies from LC and CC patients, and from irritable bowel syndrome with a predominance of diarrhoea (IBS‐D) and healthy subjects (H) as control groups. The impact of budesonide therapy on the epithelial structure was also evaluated in CC. Results MC patients exhibited decreased expression of inter‐microvilli adhesion and actin‐bundling proteins, accompanied by increased expression of actin‐membrane connection proteins compared to both control groups. Distinct molecular differentiated CC and LC, which translated into differential ultrastructure abnormalities. The colonic microvilli in CC patients were shorter in length and fewer in number, with partial restoration following budesonide treatment, whereas LC showed a reduction solely in microvilli number. A negative correlation was found between daily stool frequency and SPATN1 and ATP8B1 protein levels in CC patients. Conclusions Molecular dysregulation and aberrant ultrastructure of the colonic brush border feature the colonic epithelium in LC and CC. These previously undescribed findings provide new perspectives for further defining MC pathogenesis and identifying biomarkers for diagnosis, prognosis and treatment of this debilitating and prevalent disease. Key Summary Summarise the established knowledge on this subject ◦ Microscopic colitis (MC) is a chronic immune‐mediated disease characterized by colonic mucosal inflammation and barrier dysfunction, but a deeper characterization of colonocytes is lacking. What are the significant and/or new findings of this study? ◦ This study is the first to MC identify molecular and ultrastructural alterations of the brush border in colonocytes in lymphocytic colitis (LC) and collagenous colitis (CC). ◦ The uniqueness of this study relies on the identification of a molecular phenotype of the brush border that differentiates from healthy controls and, notably, from the closest diarrhoeal disorder, irritable bowel syndrome with a predominance of diarrhoea. ◦ Given the fundamental role of the brush border in maintaining intestinal homoeostasis and regulating epithelial barrier function, our study sets the basis for the identification of new predictive diagnostic and/or prognostic biomarkers for both LC and CC, as well as new potential therapeutic targets.

Background An association has been reported between celiac disease (CD) and microscopic colitis (MC). However, large, population‐based cohort studies are rare. Objective To systematically examine the association between CD and MC in a large, nationwide cohort. Methods We conducted a nationwide population‐based matched cohort study in Sweden of 45,138 patients with biopsy‐verified CD (diagnosed in 1990–2016), 223,149 reference individuals, and 51,449 siblings of CD patients. Data on CD and MC were obtained from all (n = 28) pathology departments in Sweden. Adjusted hazard ratios (aHRs) were calculated using Cox regression. Results During follow‐up, 452 CD patients and 197 reference individuals received an MC diagnosis (86.1 vs. 7.5 per 100,000 person‐years). This difference corresponded to an aHR of 11.6 (95% confidence interval [CI] = 9.8–13.8) or eight extra MC cases in 1000 CD patients followed up for 10 years. Although the risk of MC was highest during the first year of follow‐up (aHR 35.2; 95% CI = 20.1–61.6), it remained elevated even after 10 years (aHR 8.1; 95% CI = 6.0–10.9). Examining MC subtypes lymphocytic colitis (LC) and collagenous colitis (CC) separately, the aHR was 12.4 (95% CI = 10.0–15.3) for LC and 10.2 (95% CI = 7.7–13.6) for CC. MC was also more common before CD (adjusted odds ratio [aOR] = 52.7; 95% CI = 31.4–88.4). Compared to siblings, risk estimates decreased but remained elevated (CD and later MC: HR = 6.2; CD and earlier MC: aOR = 7.9). Conclusion Our study demonstrated a very strong association of MC with CD with an increased risk of future and previous MC in CD patients. The magnitude of the associations underscores the need to consider the concomitance of these diagnoses in cases in which gastrointestinal symptoms persist or recur despite a gluten‐free diet or conventional MC treatment. The comparatively lower risk estimates in sibling comparisons suggest that shared genetic and early environmental factors may contribute to the association between CD and MC.