Various cytokines are involved in the regulation of the immune system and inflammation. Dysregulation of cytokine signaling can cause a variety of diseases, including allergy, autoimmune diseases, inflammation, and cancer. Most cytokines use the so-called janus kinase/signal transducer and activator of transcription pathway, and this pathway is negatively regulated by suppressors of cytokine signaling (SOCS) proteins. SOCS proteins bind to janus kinase and to certain cytokine receptors and signaling molecules, thereby suppressing further signaling events. Studies have shown that SOCS proteins are key physiological regulators of inflammation. Recent studies have also demonstrated that SOCS1 and SOCS3 are important regulators of adaptive immunity.

Regulatory T cells (Treg cells) maintain immune homeostasis by limiting inflammatory responses. SOCS1 (suppressor of cytokine signaling 1), a negative regulator of cytokine signaling, is necessary for the suppressor functions of Treg cells in vivo, yet detailed mechanisms remain to be clarified. We found that Socs1−/− Treg cells produced high levels of IFN-γ and rapidly lost Foxp3 when transferred into Rag2−/− mice or cultured in vitro, even though the CNS2 (conserved noncoding DNA sequence 2) in the Foxp3 enhancer region was fully demethylated. Socs1−/− Treg cells showed hyperactivation of STAT1 and STAT3. Because Foxp3 expression was stable and STAT1 activation was at normal levels in Ifnγ−/−Socs1−/− Treg cells, the restriction of IFN-γ–STAT1 signaling by SOCS1 is suggested to be necessary for stable Foxp3 expression. However, Ifnγ−/−Socs1−/− Treg cells had hyperactivated STAT3 and higher IL-17A (IL-17) production compared with Ifnγ−/−Socs1+/+ Treg cells and could not suppress colitis induced by naive T cells in Rag2−/− mice. In vitro experiments suggested that cytokines produced by Socs1−/− Treg cells and Ifnγ−/−Socs1−/− Treg cells modulated antigen-presenting cells for preferential Th1 and Th17 induction, respectively. We propose that SOCS1 plays important roles in Treg cell integrity and function by maintaining Foxp3 expression and by suppressing IFN-γ and IL-17 production driven by STAT1 and STAT3, respectively.

IL-9 is a pleiotropic cytokine that can regulate autoimmune and allergic responses. Th9 cells can develop from naive T cells or Th2 cells through stimulation by TGF-β in vitro. In this study, we demonstrated that Smad2 and Smad3 are necessary for IL-9 production from T cells in an OVA-induced asthma model using T cell–specific Smad2- and Smad3-deficient mice. Smad2 and Smad3 were also redundantly essential for TGF-β signaling to induce histone modifications for Il9 transcription. Although Smad2/3 was recruited to the Il9 promoter by TGF-β stimulation, they are not sufficient to activate the Il9 promoter. By the screening the transcription factors, we found that IFN regulatory factor 4 (IRF4) was essential for the Smad2/3-mediated Il9 promoter activation. In addition, Smad2/3 physically interacted with IRF4, and Smad2/3 did not bind to the Il9 promoter and could not induce Th9 in IRF4-deficient T cells. Similarly, IRF4 could not stimulate Il9 transcription in the absence of Smad2/3, and TGF-β enhanced IRF4 recruitment to the Il9 promoter in a Smad2/3-dependent manner. We propose that Smad2/3 and IRF4 cooperatively transactivate the Il9 promoter and play an important role in regulating allergic immune responses by inducing Th9 cells.

Suppression of IL-2 βproduction from T cells is an important process for the immune regulation by TGF-β. However, the mechanism by which this suppression occurs remains to be established. Here, we demonstrate that Smad2 and Smad3, two major TGF-β-downstream transcription factors, are redundantly essential for TGF-β-mediated suppression of IL-2 production in CD4(+) T cells using Smad2- and Smad3-deficient T cells. Both Smad2 and Smad3 were recruited into the proximal region of the IL-2 promoter in response to TGF-β. We then investigated the histone methylation status of the IL-2 promoter. Although both histone H3 lysine 9 (H3K9) and H3K27 trimethylation have been implicated in gene silencing, only H3K9 trimethylation was increased in the proximal region of the IL-2 promoter in a Smad2/3-dependent manner, whereas H3K27 trimethylation was not. The H3K9 methyltransferases Setdb1 and Suv39h1 bound to Smad3 and suppressed IL-2 promoter activity in collaboration with Smad3. Overexpression of Suv39h1 in 68-41 T cells strongly inhibited IL-2 production in response to T cell receptor stimulation irrespective of the presence or absence of TGF-β, whereas Setdb1 overexpression only slightly suppressed IL-2 production. Silencing of Suv39h1 by shRNA reverted the suppressive effect of TGF-β on IL-2 production. Furthermore, TGF-β induced Suv39h1 recruitment to the proximal region of the IL-2 promoter in wild type primary T cells; however, this was not observed in Smad2(-/-)Smad3(+/-) T cells. Thus, we propose that Smads recruit H3K9 methyltransferases Suv39h1 to the IL-2 promoter, thereby inducing suppressive histone methylation and inhibiting T cell receptor-mediated IL-2 transcription.