854 VOLUME 11 NUMBER 9 SEPTEMBER 2010 NATURE IMMUNOLOGY ARTICLES Type 1 regulatory T cells (Tr1 cells) have emerged as an important subset of CD4 + T cells that help to control excessive inflammatory responses 1 . The anti-inflammatory effects of Tr1 cells rely on their secretion of interleukin 10 (IL-10), which dampens the function of antigen-presenting cells and antigen-specific effector T cells to sup- press tissue inflammation and autoimmunity. However, progress in the molecular analysis and understanding of the biological functions of Tr1 cells has been hampered by the lack of appropriate methods for generating large numbers of IL-10-producing T cells in vitro. IL-27, a heterodimeric cytokine of the IL-12 family, was initially sug- gested to induce the expansion of proinflammatory T helper type 1 (T H 1) cells by activating the transcription factors STAT1 and T-bet, similarly to IL-12 (ref. 2). However, mice that lack the IL-27 receptor (Il27ra -/- mice) develop exaggerated proinflammatory T cell responses 3 and autoimmunity 4 , which suggests that IL-27 might be directly involved in the inhibition of tissue inflammation. Indeed, IL-27 has been shown to be a growth and differentiation factor for Tr1 cells 5–7 . The activation of naive CD4 + cells in the presence of IL-27 or transforming growth factor-β (TGF-β) plus IL-27 results in the differentiation of IL-10-producing Tr1 cells that can potently suppress inflammation and autoimmunity. In addition to its effects on the differentiation of Tr1 cells, IL-27 directly inhibits the differentiation of IL-17-producing helper T cells (T H 17 cells) 4,8 and TGF-β-induced regulatory T cells (T reg cells) positive for the transcription factor Foxp3 (ref. 5). IL-27 drives the population expansion of Tr1 cells by inducing the expression of IL-21, a member of the IL-2 family of cytokines, which acts as an autocrine growth factor for Tr1 cells 9–11 . As with IL-10, IL-21 expression was initially reported to be specific for T H 2 cells 12 , but later studies showed that IL-21 is also produced by Tr1 cells 9 , T H 17 cells 13 and follicular helper T cells 14 . All these cell types produce IL-10, albeit in different amounts, which suggests that the production of IL-21 and of IL-10 might be linked. Even though IL-21 promotes the population expansion of T H 17 cells 15 , Il21r -/- mice, like Il27ra -/- mice, show greater susceptibility to experimental autoimmune encephalomyelitis (EAE), which indicates that IL-21 might have an important regulatory role in vivo 16 . In an effort to delineate the molecular mechanisms by which IL-27 induces Tr1 cells, IL-27 was found to directly induce the transcrip- tion factor c-Maf, which is crucial for Tr1 cell differentiation 9 . In the absence of c-Maf, the generation and population expansion of Tr1 cells are compromised. Indeed, c-Maf directly transactivates the Il10 and Il21 promoters 9,17 . Although these findings highlight the impor- tance of c-Maf and IL-21 for the biology of Tr1 cells, the addition of recombinant IL-21 to naive CD4 + T cells alone fails to generate Tr1 cells, which suggests that additional IL-27-driven molecular signals contribute to Tr1 cell differentiation. To explore the molecular mechanisms that account for the effects of IL-27 in Tr1 cell differentiation, we have analyzed the gene expres- sion of IL-27-induced Tr1 cells and found that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR) was induced by IL-27 in Tr1 cells. Furthermore, during Tr1 cell differentiation, AhR physically associated with c-Maf and transactivated the Il10 and Il21 1 Center for Neurologic Diseases, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA. 2 Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, USA. 3 Department of Environmental Health, Boston University School of Public Health, Boston, Massachusetts, USA. 4 These authors equally contributed to this work. Correspondence should be addressed to V.K.K. (vkuchroo@rics.bwh.harvard.edu) or H.L.W. (hweiner@rics.bwh.harvard.edu). Received 5 April; accepted 1 July; published online 1 August 2010; doi:10.1038/ni.1912 The aryl hydrocarbon receptor interacts with c-Maf to promote the differentiation of type 1 regulatory T cells induced by IL-27 Lionel Apetoh 1,4 , Francisco J Quintana 1,4 , Caroline Pot 1,4 , Nicole Joller 1 , Sheng Xiao 1 , Deepak Kumar 2 , Evan J Burns 1 , David H Sherr 3 , Howard L Weiner 1 & Vijay K Kuchroo 1 Type 1 regulatory T cells (Tr1 cells ) that produce interleukin 10 (IL-10) are instrumental in the prevention of tissue inflammation, autoimmunity and graft-versus-host disease. The transcription factor c-Maf is essential for the induction of IL-10 by Tr1 cells, but the molecular mechanisms that lead to the development of these cells remain unclear. Here we show that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR), which was induced by IL-27, acted in synergy with c-Maf to promote the development of Tr1 cells. After T cell activation under Tr1-skewing conditions, the AhR bound to c-Maf and promoted transactivation of the Il10 and Il21 promoters, which resulted in the generation of Tr1 cells and the amelioration of experimental autoimmune encephalomyelitis. Manipulating AhR signaling could therefore be beneficial in the resolution of excessive inflammatory responses. © 2010 Nature America, Inc. All rights reserved.