Morphological characterisation of Crohn's disease fistulae

Background: Fistulae are a common complication in up to 35% of all patients with Crohn's disease. Their therapy is difficult and frequently unsatisfactory. To date, no histological comparison of Crohn's disease fistulae with non-inflammatory bowel disease fistulae has been performed. In addition, Crohn's disease fistulae have not been well characterised morphologically. Methods: Eighty four fistulae from Crohn's disease patients were compared with 13 fistulae from controls. Haematoxylin-eosin staining, electron microscopy, and immunohistochemistry for panCytokeratin (epithelial cells), CD20 (B cells), CD45R0 (T cells), and CD68 (macrophages) were performed according to standard techniques. In addition, histopathological findings were compared with clinical and laboratory data. Results: In 31.0% of controls and 27.4% of Crohn's disease specimens, fistulae had a lining of flattened intestinal epithelium without goblet cells or, in the case of cutaneous/perianal disease, narrow squamous epithelium. Non-epithelialised fistulae were covered by a thin layer of (myo)fibroblasts, focally forming a new basement membrane, as demonstrated by electron microscopy. All fistulae were surrounded by granulation tissue. Crohn's disease fistulae presented with central infiltration by CD45R0(+) T cells, followed by a small band of CD68(+) macrophages and dense accumulation of CD20(+) B cells. In contrast, in controls, there was dense infiltration by CD68(+) macrophages with only few CD20(+) B cells and CD45R0(+) T lymphocytes. Conclusions: Fistulae in Crohn's disease differ markedly from non-Crohn's disease fistulae with regard to their cellular composition. The presence of an epithelial lining in a subgroup of fistulae may be important for the therapeutic approach and healing process.