SUMMARY Innate lymphoid cells (ILCs) expressing the nuclear receptor RORγt are essential for gut immunity presumably through production of interleukin (IL)-22. The molecular mechanism underlying the development of RORγt+ ILCs is poorly understood. Here, we have shown that the aryl hydrocarbon receptor (Ahr) plays an essential role in RORγt+ ILC maintenance and function. Expression of Ahr in the hematopoietic compartment was important for accumulation of adult but not fetal intestinal RORγt+ ILCs. Without Ahr, RORγt+ ILCs had increased apoptosis and less production of IL-22. RORγt interacted with Ahr and promoted Ahr binding at the Il22 locus. Upon IL-23 stimulation, Ahr-deficient RORγt+ ILCs had reduced IL-22 expression, consistent with downregulation of IL-23R in those cells. Ahr-deficient mice succumbed to Citrobacter rodentium infection, while ectopic expression of IL-22 protected animals from early mortality. Our data uncover a previously unrecognized physiological role for Ahr in promoting innate gut immunity by regulating RORγt+ ILCs.
Group 3 innate lymphoid cells (ILC3s) expressing the transcription factor (TF) RORγt are important for the defense and homeostasis of host intestinal tissues. The zinc finger TF Ikaros encoded by Ikzf1 is essential for RORγt+ fetal lymphoid tissue inducer (LTi) cell development and lymphoid organogenesis, but its role in postnatal ILC3s is unknown. Here, we showed that small intestinal ILC3s had the lowest expression of Ikaros compared to ILC precursors and other ILC subsets. Ikaros inhibited ILC3s in a cell-intrinsic manner through zinc finger-dependent inhibition of transcriptional activity of the aryl hydrocarbon receptor, a key regulator of ILC3 maintenance and function. Ablation of Ikzf1 in RORγt+ ILC3s resulted in increased expansion and cytokine production of intestinal ILC3s and protection against infection and colitis. Therefore, in contrast to its requirement for LTi development, Ikaros inhibits postnatal ILC3 development and function to regulate gut immune responses at steady state and in disease.
Proper immune responses are needed for controlling pathogen infection at mucosal surfaces. IL-22-producing CD4+ T cells play an important role in controlling bacterial infection in the gut; however, transcriptional regulation of these cells remains elusive. Here, we show that mice with targeted deletion of the fourth DNA-binding zinc finger of the transcription factor Ikaros had increased IL-22- but not IL-17-producing CD4+ T cells in the gut. Adoptive transfer of CD4+ T cells from these Ikaros mutant mice conferred enhanced mucosal immunity against Citrobacter rodentium infection. Despite an intact in vivo thymic-derived Treg compartment in these Ikaros mutant mice, TGF-β, a cytokine well known for induction of Tregs, failed to induce Foxp3 expression in Ikaros mutant CD4+ T cells in vitro but instead promoted IL-22. Aberrant upregulation of IL-21 in CD4+ T cells expressing mutant Ikaros was at least in part responsible for the enhanced IL-22 expression in a Stat3-dependent manner. Genetic analysis using compound mutations further demonstrated that the aryl hydrocarbon receptor (Ahr), but not RORγt, was required for aberrant IL-22 expression by Ikaros mutant CD4+ T cells, whereas forced expression of Foxp3 was sufficient to inhibit this aberrant cytokine production. Together, our data has uncovered new functions for Ikaros in maintaining mucosal immune homeostasis by restricting IL-22 production by CD4+ T cells.
TGF-β is a common factor important for the differentiation of pro-inflammatory Th17 and anti-inflammatory inducible Treg cells. However, the precise molecular mechanisms underlying the fate decision of differentiating CD4+ T cells in the presence of TGF-β is poorly understood. Here, we show that distinctive N-terminal DNA-binding zinc fingers of Ikaros play essential roles in Treg and Th17 fate decision. Ikaros has a highly conserved DNA-binding domain near the N-terminus with four tandem zinc fingers. Zinc fingers 2 and 3 are required for stable binding to DNA, whereas fingers 1 and 4 appear to be important for differentially modulating binding properties to specific sites at target genes. Our data show that T cells lacking Ikaros zinc finger 4 but not 1 failed to differentiate into Foxp3+ Tregs upon TGF-β stimulation. Instead, TGF-β-skewed Ikaros zinc finger 4 mutant cells displayed aberrant upregulation of Th17-associated cytokines IL-17 and IL-22. IL-17 but not IL-22 upregulation is dependent on transcription factor RORγt. Aryl hydrocarbon receptor, an essential transcription factor required for IL-22 expression, was unexpectedly decreased. Together, our data uncover a novel selective requirement for Ikaros zinc fingers in the differentiation of Treg and Th17 cells and an intricate interplay among various transcription factors in programming Th17/Treg lineages. We are currently examining the role of Ikaros zinc finger 4 in infection and autoimmunity.
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