Collaborative control of induced regulators

Two related papers published in Nature Immunology show that the ligand-activated transcription factor aryl hydrocarbon receptor (AHR) functions with MAF (also known as c-Maf) or other transcription factors to induce the development of mouse and human type 1 regulatory T cells (T_R1 cells).

T_R1 cells are an inducible subset of regulatory T cells that produce interleukin-10 (IL-10) and are involved in the prevention of inflammatory and autoimmune disorders. Although it is known that IL-27 drives the expansion of mouse T_R1 cell subsets, the molecular mechanisms of their induction are not fully defined.

Apetoh et al. first observed that AHR expression was upregulated in mouse T_R1 cells that had been differentiated in vitro with IL-27 and transforming growth factor-β (TGFβ). MAF expression was also upregulated in these cells, consistent with previous studies that showed a role for this transcription factor in T_R1 cell differentiation and the activation of Il10 and Il21 transcription. Moreover, IL-10 production was increased when T_R1 cells were generated in the presence of AHR ligands, whereas it was reduced when Ahr expression by T_R1 cells was downregulated through the use of small interfering RNA (siRNA). When T cells that overexpressed a Maf transgene were cultured in the presence of IL-27, TGFβ and AHR ligands, even more IL-10 was produced, suggesting that MAF and AHR act together to enhance IL-10 production. AHR and MAF also increased the expression of IL-21 by T_R1 cells. Indeed, the Il10 and Il21 promoters were both found to contain AHR- and MAF-binding sites, and chromatin immunoprecipitation studies and reporter assays confirmed direct interaction of AHR and MAF with the Il10 and Il21 promoters in T_R1 cells, indicating that they control the transcriptional activity of both promoters. IL-21 is an autocrine T_R1 cell growth factor that was shown to promote expression of IL-10, AHR and MAF, thereby acting as a positive feedback mechanism in T_R1 cell development. Last, Apetoh et al. showed that AHR is also required in vivo for the induction of IL-27-driven T_R1 cells, which could inhibit the incidence of experimental autoimmune encephalomyelitis.

Similar to the observations in mouse T_R1 cells, Gandhi et al. found that AHR activation also promotes the in vitro differentiation of human IL-10-producing T_R1 cells by acting with MAF to transactivate the IL10 promoter. Human T_R1 cells that were generated in the presence of AHR ligands suppressed responder T cells in a cell contact-dependent manner involving the cytotoxic effector granzyme B (rather than through IL-10 production). When naive human T cells were cultured with both AHR ligand and TGFβ, they differentiated into forkhead box P3 (FOXP3)⁺ regulatory T cells that expressed high levels of AHR and the ectonucleotidase CD39. CD39, which hydrolyses ATP, was shown to be responsible for the suppression mediated by these induced FOXP3⁺ regulatory T cells. Further analysis revealed that in the FOXP3⁺ cells AHR activation enhanced expression of the transcription factors SMAD1 and Aiolos. SMAD1, which is involved in TGFβ signalling, was found to interact with the FOXP3 enhancer in T cells that were treated with TGFβ and AHR ligand and to promote FOXP3 transcription. Aiolos, which is a member of the Ikaros family, was shown to interact with the FOXP3 protein and repress IL2 transcription.

Together these papers provide important insights into the factors that synergize to control the generation of inducible regulatory T cells, which could prove useful for the therapeutic application of these cells in autoimmune disorders.