These results establish that constitutive expression of the TNF family cytokine TL1A leads to dose-dependent gastrointestinal inflammation marked by continuous epithelial and goblet cell hyperplasia, thickening of the submucosal layers of the intestine, and lamina propria inflammatory infiltrates. All of these changes are confined to the small bowel and most intense in the terminal ileum. The pathology induced by TL1A differed significantly from murine models of small bowel inflammation associated with overproduction of TNF (37
) or LIGHT (39
). In TL1A-driven inflammation, IL-13 was uniquely important for the pathogenesis of intestinal inflammation, as shown by significant amelioration of the pathology by treatment with anti-IL13 mAb. While IL-17 production was also modestly increased in mesenteric lymph node and lamina propria cells in CD2-TL1A transgenic mice, the pattern of increased IL-17 expression in various segments of the small intestine did not correlate with areas of maximum inflammation, and blocking IL-17 had little effect on the changes in the small intestine induced by TL1A, suggesting that IL-17 plays a secondary role in the pathogenesis of intestinal inflammation driven by TL1A.
Much of the intestinal pathology observed in TL1A transgenic mice may be attributed to IL-13, since this cytokine can act directly on epithelial cells to cause goblet cell hyperplasia and mucous production both in the lung and intestine (40
). Smooth muscle cells can also respond to IL-13 with hypertrophy and increased contractility (42
), suggesting that the hypertrophy of the muscularis layer of the small intestine seen in TL1A transgenic mice may also be due to IL-13. The colon may be spared from IL-13 driven pathology because of elevated levels of non-signaling IL-13R2 receptor subunit (43
). The lack of IFN-γ production we observed in TL1A transgenic mice may explain the relative longevity of these mice despite the inflammatory pathology observed, since IFN-γ is associated with enterocyte cell death that may lead to more severe clinical consequences (44
). Overall, the pathology induced by constitutive TL1A expression bears a striking resemblance to the intestinal response to nematode infections, where IL-13 mediates induction of goblet cell hyperplasia, mucous production and muscular hypertrophy that aids in worm expulsion, but spares the host from lethal inflammation (41
). The similarity between the pathology caused by the constitutive expression of TL1A and nematode infection raises the interesting possibility that TL1A might be an important inducer of IL-13 during such infections.
These data establish a novel link between TL1A, acting through its receptor DR3, to induce IL-13 and associated gastrointestinal pathology. Previous studies have shown that TL1A-DR3 interactions can enhance secretion of multiple cytokines, not just those associated with Th2 immune responses. IL-13 may be produced in CD2-TL1A transgenic mice by many cell types, including T cells, invariant NKT cells, intraepithelial intestinal NK cells, mast cells and basophils (41
). Since DR3 is mainly expressed on lymphocytes, T cells, NKT cells and NK cells, those cells are among likely cellular sources of IL-13. However, intracellular staining of T cells in the mLN and intestinal lamina propria only revealed small increases in Th2 cells, suggesting that other cell types may contribute to IL-13 production induced by TL1A. NKT cells have been found to produce IL-13 and implicated in IBD, particularly in ulcerative colitis (22
). However since these cells are relatively depleted in TL1A transgenic mice, they are less likely to be required. Intra-epithelial NK cells or recently identified novel ‘innate’ lymphocyte subsets in the mesenteric lymph nodes and fat-associated lymphoid clusters that produce IL-13 in the setting of helminth infections (48
) are other possible sources of IL-13 produced in response to TL1A.
CD4+ T cells are activated and expanded in TL1A transgenic mice and this is abrogated when the T cell repertoire is fixed through crossing these mice to the OT-II TCR transgenic mice on a Rag deficient background. These results strongly suggest that TL1A-driven T cell activation requires a concomitant TCR signal which is provided in vivo by antigens in the bowel lumen or other environmental antigens. Autoreactive T cells appear to be directly or indirectly required for some of the intestinal pathology in TL1A transgenic mice, because of the reduced histological scores in TL1A transgenic mice crossed to the OT-II TCR transgenic mice on a RAG1 deficient background. However, intestinal IL-13 and goblet cell hyperplasia are still present in TL1A transgenic mice on the OT-II/Rag1−/− background, suggesting that the TL1A present on resting T cells is sufficient to drive these pathological changes, likely acting through another cell type present in the small intestine.
Given the important role of Tregs in other models of intestinal inflammation we examined Treg function in CD2-TL1A transgenic mice. We as well as Taraban et al. (51
) noted that Tregs are numerically increased in mice constitutively expressing TL1A driven either by CD2 or CD11c promoters. Treg isolated from CD2-TL1A transgenic mice which constitutively produce TL1A are less effective at suppressing proliferation of naive T cells than wild-type Treg, and Taraban et al. (51
) show that soluble recombinant TL1A can also partially reverse proliferative suppression by Treg. This suggests that costimulation through DR3 can reverse suppression by Treg, either directly, or indirectly through promoting cytokine production. This is consistent with the known costimulatory effect of TL1A on T cell proliferation which is mediated through DR3. Costimulation by DR3 is associated with increased production of multiple cytokines, including IL-2 (3
), and the ability of IL-2 to reverse suppression by Treg (52
). Recent studies have shown that orally delivered antigen can induce de novo
generation of antigen-specific Treg through antigen presentation by a specialized population of dendritic cells capable of secreting retinoic acid and TGF-β (31
). We found that TL1A greatly inhibited TGF-β-induced differentiation of FoxP3+
T cells in vitro from naive T cell precursors. This suggests that peripheral induction of Treg in the gastrointestinal tract of transgenic mice might be defective in the setting of TL1A overexpresion. It is likely that in TL1A transgenic mice, a combination of Treg defects and TL1A-mediated costimulation of effector T cells results in failure of immunological tolerance to mucosal antigens, spontaneous T cell activation and intestinal inflammation.
The efficacy of blocking TL1A-DR3 interactions in the TNBS colitis model suggests that these interactions are necessary for inflammatory bowel disease involving a broader range of cytokines than is seen in TL1A overexpression. Antibody blockade of DR3-TL1A interactions by anti-TL1A, DR3-Fc and an antagonistic anti-DR3 Fab all led to a striking and virtually complete inhibition of TNBS-induced colitis. In the acute phase, TNBS-induced colitis is associated with T cells secreting IFN-γ (54
) and can be blocked by antibodies against the p40 common subunit of IL-12 and IL-23 (56
). More recently, IL-17 has also been implicated in the pathogenesis of acute TNBS colitis (57
). Crohn’s disease has been associated with IL-12 and IL-23, and Ulcerative Colitis with IL-13 (58
), and TL1A-DR3 interactions may contribute to inflammatory bowel disease through either of these two pathways. Because no major defects in systemic immunity have thus far been found in DR3 or TL1A deficient mice, treatment of inflammatory bowel disease by blocking TL1A-DR3 interactions may have a favorable therapeutic index.