IL-33 ... promotes TReg cell function in the intestine

Regulatory T (TReg) cells expressing the transcription factor forkhead box P3 (FOXP3) are important for maintaining immune homeostasis in the intestine, but the key factors that control TReg cell activity at this site are poorly defined. Now, Fiona Powrie and colleagues show that interleukin-33 (IL-33) — which is an alarmin produced in response to tissue damage — promotes TReg cell function in the intestine.

Credit: NPG/S.Bradbrook

The authors set out to identify potential tissue-specific regulators of colonic TReg cells; mRNA expression profiles of TReg cells from the mesenteric lymph nodes and the colon of mice showed that one of the genes that was most highly upregulated in colonic TReg cells was Il1rl1, which encodes the IL-33 receptor ST2 (also known as IL-1 receptor-like 1). ST2+ TReg cells are abundant in the intestine, so the authors hypothesized that IL-33 might modulate TReg cell differentiation. Naive CD4+ T cells were isolated from Foxp3gfp reporter mice and activated in the presence of transforming growth factor β1 (TGFβ1). The addition of IL-33 to these cultures increased the percentage and number of FOXP3-expressing cells, but only in the presence of TGFβ1. Thus, IL-33 is a novel cofactor for TGFβ1-mediated in vitro generation of TReg cells.

Next, the authors investigated whether IL-33 has a role in the in vivo proliferation of thymus-derived TReg cells, which constitute a substantial proportion of the ST2+ colonic TReg cells. Indeed, IL-33 induced proliferation in splenic TReg cells but not in effector T cells. Furthermore, Il1rl1−/− TReg cells showed impaired proliferation after injection of IL-33 into chimeric mice that contained a mixture of wild-type and Il1rl1−/− haematopoietic cells. Hence, IL-33 promotes the proliferation and accumulation of thymus-derived TReg cells in vivo.

The T-cell-specific transcription factor GATA-binding protein 3 (GATA3) is known to be important for TReg cell stability and function, and to regulate ST2 expression in T helper 2 (TH2) cells. Hence, the authors investigated the role of GATA3 in IL-33-mediated regulation of intestinal TReg cells and found that GATA3 was phosphorylated after IL-33 stimulation of TGFβ1-activated naive CD4+ T cells. This stimulation induced the recruitment of GATA3 and RNA polymerase II to the Foxp3 promoter, which indicates that IL-33 regulates Foxp3 expression. In addition, the authors found that GATA3 was recruited to the Il1rl1 enhancer element. Thus, IL-33 seems to have a role in Foxp3 induction, as well as in promoting the expression of its own receptor.

Using the T cell transfer model of colitis, the authors examined the ability of Il1rl1−/− TReg cells to protect mice from disease; naive T cells were injected into Rag1−/− mice (which lack the gene encoding recombination-activating gene 1) either alone, or together with wild-type or Il1rl1−/− TReg cells. The ability of Il1rl1−/− TReg cells to prevent colitis was substantially impaired, which indicates that IL-33 signalling is important for the suppressive function of TReg cells in vivo.

Finally, the authors examined whether IL-23 — which can promote intestinal inflammation by inhibiting TReg cell differentiation — limits T cell responsiveness to IL-33. Indeed, the ability of IL-33 to promote FOXP3 expression in vitro in the presence of TGFβ1 was abolished after the addition of IL-23. The authors showed that IL-23 prevents T cells from responding to IL-33 by inhibiting ST2 expression and ST2 signal transduction.

Taken together, this study shows that IL-33 produced in response to tissue damage can enhance local intestinal TReg cell responses. Notably, IL-23 can limit this regulatory mechanism by inhibiting the responsiveness of TReg cells to IL-33.