OP37 Fibrogenesis in chronic DSS colitis is driven by an innate lymphoid cell-independent innate immune response

Background Obtaining insights in the pathogenesis of intestinal fibrosis is a priority for improving outcomes in inflammatory bowel diseases (IBD). Studies in murine models and human organ fibrosis indicate a role for innate immunity pathways in fibrosis. The aim of this study was to dissect the role of innate lymphoid cells (ILC) in chronic intestinal inflammation and fibrosis. Methods A chronic 3-cycles dextran sulphate sodium (DSS) model was induced in wild-type (WT), recombinant activating gene (RAG)-deficient (lacking adaptive immunity), RAG−/− common γ-chain (ɣc)−/− (lacking ILC) and anti-CD90.2 treated (ILC depleted) C57Bl/6 RAG−/− mice. One cycle of DSS comprised 1 week of DSS administration followed by 2 weeks of recovery with normal drinking water. Colonic lamina propria cells were isolated and CD45+Lineage−CD127+CD90.2+ ILC, Ly6C+ monocytes and Ly6G+ neutrophils were identified after staining by flow cytometry. Inflammation and fibrosis were scored by macroscopic and HE and Martius Scarlet Blue staining and fibrosis was evaluated by hydroxyproline quantification. For analysis Kruskal–Wallis testing with multiple Dunn’s comparisons was performed. Results In RAG-1−/− mice chronic inflammation and fibrosis developed similarly as in WT mice, with elevated KLRG-1+ ILC2 (68.90 vs. 48.00% of ILC, p = 0.012) after repeated DSS exposure as compared with control mice (Figure 1). Chronic colitis could also be induced in RAG−/−ɣc−/− or ILC depleted RAG−/− mice (ILC: 0.99 vs. 25.70% of CD45+ cells, p = 0.029), with no attenuation of fibrosis (p > 0.99) as compared with chronic DSS exposed RAG-1−/− mice despite the absence of ILC (Figure 2). Colon length decrease was more pronounced in RAG−/−yc−/− as compared with RAG-1−/− mice after chronic DSS colitis (7.15 vs. 8.30 cm, p = 0.046), while hydroxyproline levels and thickness of mucosa and muscularis propria were not different in RAG−/−yc−/− as compared with RAG-1−/− mice after chronic DSS. Moreover, after the second cycle of DSS a slower recovery of weight was seen in RAG−/−yc−/− mice as compared with RAG-1−/− mice (d31: 88.64 vs. 111.0% of initial weight, p < 0.001; d35 p = 0.001; d39 p = 0.006 and d42 p = 0.002) (Figure 3). In absence of ILC, RAG−/−yc−/− mice increased lamina propria neutrophils (19.10 vs. 5.91% of CD45+ cells, p = 0.004) and monocytes (11.80 vs. 3.25% of CD45+ cells, p = 0.004) may represent an alternative source of inflammation. Conclusion These data argue against a pro-fibrotic role of ILC in the induction of fibrosis in chronic DSS colitis, and suggest a protective and recovery-enhancing role of ILC after repeated intestinal injury.