A Role for Tumor Necrosis Factor and Bacterial Antigens in the Pathogenesis of Crohnʼs Disease–Associated Fistulae

Intestinal fistulae represent a severe complication of Crohn's disease (CD). The authors have demonstrated that epithelial-to-mesenchymal transition plays a pivotal role in their pathogenesis. High levels of interleukin-13 and tumor necrosis factor (TNF) are detected in myofibroblast-like transitional cells covering the fistula tracts. Here, a functional role was investigated for the transcription factor Ets-1, TNF, and the bacterial wall component (muramyl dipeptide [MDP]) in the pathogenesis of CD-associated fistulae.Perianal fistulae from CD patients were analyzed by immunohistochemistry. Primary colonic lamina propria fibroblasts (CLPFs) were isolated from CD patients with or without fistulizing disease. Messenger RNA (mRNA) levels were assessed by real-time polymerase chain reaction in CLPF or HT29 intestinal epithelial cells (IECs) grown as monolayers or spheroids.Strong expression of Ets-1 transcription factor was demonstrated in transitional cell covering the fistula tracts by immunohistochemistry. TNF induced mRNA expression of ETS-1 and β6-integrin in HT29 IEC and in CLPF from fistulizing CD patients. These effects could be fully blocked by administration of anti-TNF antibodies. In HT29 cells, TNF further induced mRNA levels of TNF and transforming growth factor beta by treatment for 24 hours. In fistula CLPF derived from CD patients, TNF induced expression of β6-integrin, TNF, and transforming growth factor beta. Of note, the bacterial wall component, MDP, induced mRNA levels of ETS-1, transforming growth factor beta, interleukin-13, SNAIL1, and β6-integrin in HT29 IEC monolayers and fistula CLPF by treatment for 24 hours.TNF and MDP induce the expression of factors associated with epithelial-to-mesenchymal transition and invasion in IEC and fistula CLPF. Our findings indicate that TNF and MDP might synergize in the pathogenesis of CD-associated fistulae.