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P052. Inhibition of Th17 cells but not of Tregs affects extracellular matrix protein deposition in chronic DSS colitis

C. Breynaert1, C. Perrier1, J. Cremer1, L. Coorevits2, M. Ferrante1, S. Vermeire1, P. Rutgeerts1, C. Uyttenhove3, J. Van Snick3, J. Ceuppens2, G. Van Assche1, 1KU Leuven, TARGID, Leuven, Belgium, 2KU Leuven, Clinical immunology, Leuven, Belgium, 3Brussels Branch, Ludwig Institute for Cancer Research, Brussels, Belgium

Background

Studies in experimental colitis have greatly advanced our knowledge of the immune processes driving mucosal inflammation in IBD, but have generally failed to unravel the pathophysiology of connective tissue changes related to chronic colitis. This study aimed to investigate the effect of downregulation of Tregs and Th17 cells on fibrosis in a murine colitis model, characterized by segmental, transmural colitis with persistent lymphoid aggregates and collagen deposition.

Methods

A chronic colitis was induced with 3 cycles of dextran sodium sulphate (DSS) in 6 week-old C57BL/6 mice. One cycle of DSS was defined as 7 days of DSS followed by a recovery period of 14 days with normal drinking water. Weekly intraperitoneal injection of 500 µg rat IgG1anti-CD25 (n = 9), mouse IgG1 anti-IL17A (n = 8), mouse IgG1 anti-IL17F (n = 8) or corresponding isotype IgG (n = 9 and n = 8) was started after cycle 1. After euthanasia, the distal colon was harvested for histology. Blood and mesenteric lymph nodes (MLN) were studied using flow cytometry. Collagen deposition was quantified with Martius-Scarlett-Blue staining and hydroxyprolin assay.

Results

Although no significant differences in relative weight or disease activity index were observed, spleen weight (p = 0.002), colon weight (p = 0.024) and colon weight/length ratio (p = 0.030) were significantly higher in the anti-CD25 group, but colon length and macroscopic damage score were not different. Despite enhanced inflammation, collagen deposition in mucosa and submucosa was not affected. Anti-IL17A/F administration did not influence weight curves, colon or spleen weight or macroscopic damage scores, but colon length tended to be shorter in the anti-IL17A group vs isotype. No significant differences were observed in expression of CD4+Foxp3+ Tregs in blood and MLN after administration of anti-CD25 nor in expression of CD4+IL17A+/F+ cells in MLN after administration of aIL17A/F. Collagen deposition in mucosa and submucosa tended to be higher in the anti-IL17A group. This was confirmed using a hydroxyprolin assay (p = 0.027 vs isotype). No differences in thickness of the muscularis propria were observed in all conditions.

Conclusion

Although administration of anti-CD25 results in more colonic and systemic inflammation, collagen deposition is not altered, suggesting that Tregs are not crucial in the induction of fibrosis. In contrast, inhibition of IL17A unexpectedly increases collagen deposition in chronic DSS colitis, without reducing colonic or systemic inflammation.