Uncovering Pathogenic Mechanisms of Inflammatory Bowel Disease Using Mouse Models of Crohnâs DiseaseâLike Ileitis: What is the Right Model?Summary

Crohnâs disease and ulcerative colitis, together known as inflammatory bowel disease, are debilitating chronic disorders of unknown cause and cure. Our evolving understanding of these pathologies is enhanced greatly by the use of animal models of intestinal inflammation that allow in vivo mechanistic studies, preclinical evaluation of new therapies, and investigation into the causative factors that underlie disease pathogenesis. Several animal models, most commonly generated in mice, exist for the study of colitis. The appropriateness of their use often can be determined by their mode of generation (ie, chemical induction, T-cell transfer, targeted genetic manipulation, spontaneously occurring, and so forth), the type of investigation (mechanistic studies, pathogenic experiments, preclinical evaluations, and so forth), and the type of inflammation that occurs in the model (acute vs chronic colitis, tissue injury/repair, and so forth). Although most murine models of inflammatory bowel disease develop inflammation in the colon, Crohnâs disease most commonly occurs in the terminal ileum, where a very limited number of mouse models manifest disease. This review discusses appropriate experimental applications for different mouse models of colitis, and highlights the particular utility of 2 highly relevant models of Crohnâs-like ileitisâthe spontaneous SAMP1/YitFc inbred mouse strain and the genetically engineered TnfÎAU-rich element/+ mouse model of tumor necrosis factor overexpression, both of which bear strong resemblance to the human condition. Similar to patients with Crohnâs disease, SAMP1/YitFc ileitis develops spontaneously, without chemical, genetic, or immunologic manipulation, making this model particularly relevant for studies aimed at identifying the primary defect underlying the occurrence of Crohnâs ileitis, as well as preclinical testing of novel treatment modalities. Keywords: Crohnâs Disease, Ulcerative Colitis, SAMP1/YitFc, TnfÎARE/

Intestinal-specific TNFα overexpression induces Crohn's-like ileitis in mice.

Human and animal studies have clearly established tumor necrosis factor (TNF)α as an important mediator of Crohn's disease pathogenesis. However, whether systemic or only local TNFα overproduction is required for the development of chronic intestinal inflammation and Crohn's disease remains unclear. The aim of this study was to assess the contribution of intestinal epithelial-derived TNFα to the development of murine Crohn's-like ileitis.We adapted the well-established TNF(∆ARE/+) mouse model of Crohn's disease (which systemically overexpresses TNFα) to generate a homozygous mutant strain that overexpress TNFα only within the intestinal epithelium. Intestinal-specific TNF(i∆ARE/i∆ARE) mice were examined for histopathological signs of gut inflammation and extraintestinal manifestations of Crohn's disease. The mucosal immune phenotype was characterized, and the contribution of specific lymphocyte populations to the pathogenesis of TNF(i∆ARE/i∆ARE) ileitis was assessed.TNF(i∆ARE/i∆ARE) mice had increased mucosal and systemic TNFα levels compared to wild-type controls (P<0.001), as well as severe chronic ileitis with increased neutrophil infiltration and villous distortion, but no extraintestinal manifestations (P<0.001 vs. wild-type controls). The gut mucosal lymphocytic compartment was also expanded in TNF(i∆ARE/i∆ARE) mice (P<0.05), consisting of activated CD69(+) and CD4(+)CD62L(-) lymphocytes (P<0.05). FasL expression was significantly elevated in the mesenteric lymph nodes of TNF(i∆ARE/i∆ARE) mice (P<0.05). Adoptive transfer of mucosal TNF(i∆ARE/i∆ARE) lymphocytes resulted in ileitis in immunologically naïve severe combined immunodeficiency recipients (P<0.05 vs. wild-type controls), indicating an effector phenotype that was associated with increased production of both Th1 (IFNγ) and Th2 (IL-5, IL-13) cytokines.Intestinal epithelial-derived TNFα is sufficient for the induction of Crohn's-like ileitis, but not for the occurrence of extraintestinal manifestations, in TNF(i∆ARE/i∆ARE) mice. These effects were associated with generation of effector lymphocytes within the intestinal mucosa and dysregulated apoptosis. Thus, targeted intestinal blockade of TNFα may provide an effective means to neutralize gut-derived TNFα with reduced side effects

SAMP1/YitFc Mouse Strain: A Spontaneous Model of Crohn&apos;s Disease-like Ileitis

The SAMP1/YitFc mouse strain represents a model of Crohn’s disease (CD)-like ileitis that is ideal for investigating the pathogenesis of chronic intestinal inflammation. Different from the vast majority of animal models of colitis, the ileal-specific phenotype characteristic of SAMP1/YitFc mice occurs spontaneously, without genetic, chemical, or immunological manipulation. In addition, SAMP1/YitFc mice possess remarkable similarities to the human condition with regard to disease location, histologic features, incidence of extraintestinal manifestations, and response to conventional therapies. SAMP1/YitFc mice also display a well-defined time course of a predisease state and phases of acute and chronic ileitis. As such, the SAMP1/YitFc model is particularly suitable for elucidating pathways that precede the clinical phenotype that may lead to preventive, and therefore more efficacious, intervention with the natural course of disease, or alternatively, for the development of therapeutic strategies directed against chronic, established ileitis. In this review we summarize important contributions made by our group and others that uncover potential mechanisms in the pathogenesis of CD using this unique murine model of chronic intestinal inflammation

Histopathological features of TNF^{i∆ARE/i∆ARE} mice.

<p>Histological assessment of inflammation in the terminal ileum was done using a validated scoring system. Indices were calculated for (A) villous distortion, (B) active inflammation (neutrophil infiltration), and (C) chronic inflammation (mononuclear cell infiltration). (D) The total inflammatory score represents the sum of all 3 individual indices. Values for all indices were significantly elevated in homozygous TNF^{i∆ARE/i∆ARE} mice (n=11), whereas no inflammatory changes were detected in heterozygous TNF^i∆ARE/+ mice (n=10) or wt mice (n=10); all mice were evaluated at 16-20 weeks of age. Graphs represent mean values ± SEM for each group of mice. *P<0.05, ** < P<0.01, *P<0.001. (E) Representative photomicrographs of H&E stained sections from wt TNF^+/+, TNF^i∆ARE/+, and TNF^{i∆ARE/i∆ARE} mice. 1) wt TNF^+/+ ileum showing normal villous architecture with no blunting or evidence of acute or chronic inflammatory infiltration, 20x. (2) TNF^i∆ARE/+ ileum showing mild villous blunting with no significant inflammation, 20x. (3) TNF^{i∆ARE/i∆ARE} ileum showing evidence of chronic ileitis with villous blunting; acute and chronic inflammatory infiltration is seen within the LP and extending into the submucosa, 20x. (4) Higher magnification of ileum histology from a TNF^{i∆ARE/i∆ARE} mouse showing evidence of marked inflammatory infiltration and distortion of normal villous architecture and crypts, 40x. (5) Colon histology of wt TNF^+/+ mice showing no evidence of colonic inflammation, 20x. (6) Colon histology of a TNF^{i∆ARE/i∆ARE} mouse showing normal colonic histology with only a few lymphocytes infiltrating the LP.</p

Increased apoptosis of lymphocytes in the MLNs of TNF^{i∆ARE/i∆ARE} mice.

<p>Single cell suspensions were obtained from MLNs, and the expression of various surface markers was analyzed by flow cytometry. Fluorochrome-tagged monoclonal antibodies against CD4, Fas, and FasL were utilized for identification of the respective populations. Multiple color flow cytometry was performed on a FACS calibur system to determine the percentage of cells expressing surface markers and the intensity of expression. The vast majority of MLN cells stained positive for Fas and no significant differences detected between wt and TNF^{i∆ARE/i∆ARE} mice. Conversely, expression of FasL was significantly elevated in MLN cells from TNF^{i∆ARE/i∆ARE} mice as compared to wt controls. This difference remained significant when expression of FasL was examined separately in the CD4⁺ or CD4^- populations. Mice were processed individually. Three separate experiments with 3-4 mice per group gave similar results, and one representative experiment is shown. Graphs represent mean values ± SEM for each group of experimental mice. *P<0.05, ** < P<0.01, *P<0.001.</p