P061 Identification of the mesocolic mesothelium as a novel source of fibroblasts – implications for Crohn's disease

Abstract

Background: Regulatory Foxp3+ T cells (Treg) maintain immune tolerance by controlling systemic autoimmune and allergic diseases but are also crucial for control of intestinal homeostasis and oral tolerance to dietary antigens and to the microbiota. Breakdown of intestinal homeostasis in chronic inflammatory bowel disease is associated with inability of Treg to control inflammation, suggesting that the inflamed intestinal microenvironment is deleterious for Treg. Methods: In this study we explored, using two models of colitis (DSS colitis and TNBS colitis), the impact of intestinal inflammation on the number, survival, phenotype and regulatory function of Treg and conversion from CD4+ T cells. Results: We found that intestinal inflammation increased the number Foxp3+ Treg in Mesenteric Lymph Nodes (MLN), although their frequency decreased due to the inflammatory infiltrate. Colitis was associated with increased proliferation of Foxp3+Ki67+ Treg in MLN and LP. MLN Foxp3+ Treg at the steady state comprised both Helios+Neuropilin+ and Helios Neuropilin , which are believed to represent natural (nTreg) versus induced (iTreg) Treg, respectively and intestinal inflammation decreased the frequency of nTreg. Ex vivo Treg suppression assay showed that MLN Treg from colitic mice maintain their suppressive function, when removed from the inflamed intestine. Adoptive transfer of naive CD4+ T cells (containing nTreg) in congenic recipient mice showed that colon inflammation exacerbated recruitment of CD4+ T cells into the lamina propria. However, in vivo conversion of CD4+CD25+ Foxp3 pre-Treg (sorted from Foxp3-egfp transgenic mice) into Foxp3+ Treg was impaired in colitic recipient mice. Finally, transfer of Foxp3-egfp+ cells in DSS mice showed a significant decrease of colitis severity. Conclusions: Taken together our data illustrate that Treg can be activated and recruited in the inflamed intestine but that intestinal inflammation impairs Treg conversion from CD4+ T cells.