Factors Promoting Development of Fibrosis in Crohn's Disease

Abstract

The concepts on the pathophysiology of intestinal fibrosis in Crohn's disease (CD) have changed in recent years. Some years ago fibrosis was regarded to be a consequence of long-standing inflammation with subsequent destruction of the gut wall matrix followed by scar formation and collagen deposition. Fibrosis in CD patients appeared to be an irreversible process that could hardly be influenced. Therefore, the main target in CD therapy was to control inflammation to avoid fibrosis development. Many of these assumptions seem to be only partially true. Inflammation may be a necessary prerequisite for the initiation of fibrosis. However, when the pathophysiologic processes that lead to fibrosis in CD patients have been initiated fibrosis development may be independent of inflammation and may continue even when inflammation is under good medical control. Fibrosis in CD also may be reversible. After strictureplasty local collagen deposits decrease or even disappear. With new animal models for intestinal fibrosis on the horizon, we need to spend more efforts on understanding the factors influencing fibrosis in CD patients to finally find specific therapies. In this context, it will be as important to find markers and quantitative imaging tools to have reliable endpoints for clinical trials in fibrosing CD.

Keywords: Crohn’s disease; animal model; collagen; fibrosis; stricture; therapy; transforming growth factor β.

Figure 1

Heterotopic transplantation, revascularization, and luminal occlusion of the graft. (A) Small bowel resections are extracted from C57BL/6 mice. (B) For isogeneic transplantation, the resection (arrow) is implanted into subcutaneous tissue in the neck of C57BL/6 mice. (C) The graft is freed from the pouch and harvested from the neck of the recipient 14 days posttransplantation. (D–F) Grafts in the neck of recipient animals observed in situ present a decreased length but are otherwise macroscopically intact. Blood vessels from the surrounding tissue stretch toward the graft where they form a dense network (twofold magnification). (G) Histologic cross sections of freshly isolated small intestine (day 0). Small bowel resections are extracted from C57BL/6 mice, implanted into C57BL/6 mice for isogeneic transplantation, and explanted at (H) day 2, (I) day 6, and (J) day 14 after transplantation. Transmitted light microscopy, H&E staining. Grafts revealed luminal occlusion.

Figure 2

Collagen layer thickness and Tgf-β, Col1a1, and Col3a1 mRNA are significantly increased in grafts from the heterotopic transplantation model in a time-dependent manner. Small bowel resections are extracted from RAG2 knockout mice and implanted into RAG2 knockout mice for isogeneic heterotopic transplantations. (A) Sirius Red staining. Transmission light microscopy and polarized light microscopy. (B) Collagen layer thickness measurement using transmission light microscopy confirmed significantly increased collagen layer thickness in a time-dependent manner (**p < 0.01, ***p < 0.001, ANOVA, Dunn’s multiple comparison test). Thickness was calculated from at least eight places in representative areas at 10-fold magnification for each single graft. (C) Tgf-β qPCR. (D) Col1a1. (E) Col1a3. *p < 0.05, ***p < 0.001, Kruskal–Wallis test, Dunn’s multiple comparison test.