Tumor necrosis factor-like cytokine 1A (TL1A, TNFSF15) is implicated in inflammatory bowel disease (IBD), modulating the location and severity of intestinal inflammation and fibrosis. TL1A expression is increased in inflamed gut mucosa and associated with fibrostenosing Crohn’s disease. TL1A-overexpression in mice leads to spontaneous ileitis, and exacerbated induced proximal colitis and fibrosis. Studies of the role of TNFSF15/TL1A in fibrogenesis will help define pathogenesis and identify potential therapeutic targets for those most severely affected by IBD. The precise mechanisms and specific contributors to TL1A-driven fibrosis require further investigation. IBD is associated with shifts in the gut microbiome, but the effect of differing microbial populations and their interaction with TL1A on fibrosis has not been determined. Intestinal fibroblasts express the TL1A receptor, DR3, and stimulation with exogenous TL1A induces activation and collagen expression in vitro, but the contribution of direct TL1A-DR3 signaling on fibroblasts to the development of fibrosis in vivo is unknown.
We show that the gut microbiome is required for TL1A-mediated intestinal fibrosis and optimal fibroblast activation into myofibroblasts. Moreover, we provide evidence that the TL1A-mediated intestinal fibrotic phenotype requires cues provided by unique bacterial populations, as opposed to any microbiome per se. Our analysis further identifies several candidate organisms that correlate with degree of fibrosis and directly impact fibroblast function. Thus, TL1A-mediated intestinal fibrosis and fibroblast phenotype are dependent on specific microbial populations.
To evaluate for a selective role of direct TL1A-DR3 signaling on fibroblasts in fibrostenosing IBD, TL1A over-expressing naï¿½ve T cells (Tl1a-Tg) were transferred into Rag-/- mice; Rag-/- mice lacking DR3 in all cell types (Rag-/-Dr3-/-); or Rag-/- mice lacking DR3 only on fibroblasts (Rag-/-Dr3delCol1a2). We show that Rag-/-Dr3-/- recipients demonstrate reduced disease activity, inflammation, and an accompanying reduction in fibrosis and fibroblast activation compared with DR3-sufficient Rag-/- recipients. In contrast to pan-DR3-deficient recipients, Rag-/-Dr3delCol1a2 recipients exhibit a similar degree of severe inflammatory disease as Rag-/- recipients. Despite the presence of abundant inflammation, however, fibroblast-selective DR3-deficiency significantly reduces intestinal fibrosis and decreases activation of fibroblasts. Ex vivo, DR3-deficient fibroblasts isolated from these colitic mice exhibit a significant reduction in gap-closure compared to those from DR3-intact Rag-/- mice. These data demonstrate that direct TL1A-DR3 signaling on fibroblasts in vivo significantly contributes to TL1A-mediated intestinal fibrosis, independent of inflammation.