IL-13 Promotes Collagen Accumulation in Crohn's Disease Fibrosis by Down-Regulation of Fibroblast MMP Synthesis: A Role for Innate Lymphoid Cells?

Background: Fibrosis is a serious consequence of Crohn's disease (CD), often necessitating surgical resection. We examined the hypothesis that IL-13 may promote collagen accumulation within the CD muscle microenvironment. Methods: Factors potentially modulating collagen deposition were examined in intestinal tissue samples from fibrotic (f) CD and compared with cancer control (C), ulcerative colitis (UC) and uninvolved (u) CD. Mechanisms attributable to IL-13 were analysed using cell lines derived from uninvolved muscle tissue and tissue explants. Results: In fCD muscle extracts, collagen synthesis was significantly increased compared to other groups, but MMP-2 was not co-ordinately increased. IL-13 transcripts were highest in fCD muscle compared to muscle from other groups. IL-13 receptor (R) alpha 1 was expressed by intestinal muscle smooth muscle, nerve and KIR+ cells. Fibroblasts from intestinal muscle expressed R alpha 1, phosphorylated STAT6 in response to IL-13, and subsequently down-regulated MMP-2 and TNF-alpha-induced MMP-1 and MMP-9 synthesis. Cells with the phenotype KIR(+)CD45(+)CD56(+/-)CD3(-) were significantly increased in fCD muscle compared to all other groups, expressed Ra1 and membrane IL-13, and transcribed high levels of IL-13. In explanted CD muscle, these cells did not phosphorylate STAT6 in response to exogenous IL-13. Conclusions: The data indicate that in fibrotic intestinal muscle of Crohn's patients, the IL-13 pathway is stimulated, involving a novel population of infiltrating IL-13R alpha 1(+), KIR+ innate lymphoid cells, producing IL-13 which inhibits fibroblast MMP synthesis. Consequently, matrix degradation is down-regulated and this leads to excessive collagen deposition.