TH17 cells are recognized as a unique subset of T helper cells that have critical roles in the pathogenesis of autoimmunity and tissue inflammation. Although RORγt is necessary for the generation of TH17 cells, the molecular mechanisms underlying the functional diversity of TH17 cells are not fully understood. Here we show that a member of interferon regulatory factor (IRF) family of transcription factors, IRF8, has a critical role in silencing TH17-cell differentiation. Mice with a conventional knockout, as well as a T cell-specific deletion, of the Irf8 gene exhibited more efficient TH17 cells. Indeed, studies of an experimental model of colitis showed that IRF8 deficiency resulted in more severe inflammation with an enhanced TH17 phenotype. IRF8 was induced steadily and inhibited TH17-cell differentiation during TH17 lineage commitment at least in part through its physical interaction with RORγt. These findings define IRF8 as a novel intrinsic transcriptional inhibitor of TH17-cell differentiation.

The ability of cancer cells to evade apoptosis is dictated by a shift in the balance between pro- and anti-apoptotic gene expression programs. Monocyte chemotactic protein induced protein 1 (MCPIP1) is a zinc finger RNA binding protein with important roles in mediating inflammatory responses. Overexpression of MCPIP1 in different cancer cell types has been implicated in eliciting an antitumor response, but a direct role of MCPIP1 in apoptosis has not been established. In this study, we demonstrate that MCPIP1 functions as a potent tumor suppressor that induces apoptosis of breast tumor cells by selectively enhancing mRNA decay of anti-apoptotic gene transcripts including Bcl2L1, Bcl2A1, RelB, Birc3, and Bcl3. Mechanistically, MCPIP1 physically interacted with a stem-loop structure in the 3'UTR of these transcripts through its PIN domain, causing mRNA destabilization. Furthermore, we found that MCPIP1 expression was repressed in breast tumor cells, and overexpression of MCPIP1 induced apoptosis, whereas its depletion enhanced cancer cell proliferation. Moreover, MCPIP1 induction in vivo resulted in complete regression of established tumors and a significant reduction in metastatic disease. Notably, low MCPIP1 expression in tumor samples from breast cancer patients was strongly associated with poor survival over 13 years of follow up. Collectively, our results highlight MCPIP1 is a new tumor suppressor in breast cancer that induces cell death by tipping the balance in favor of pro-apoptotic gene expression.

Interferon regulatory factor (IRF) family members, especially interferon regulatory factor-1 (IRF-1) and interferon regulatory factor-8 (IRF-8 or ICSBP), play important roles in interferon signaling in a wide range of host responses to infection and tumor growth. Interleukin-27 (IL-27), as a member of the IL-12 cytokine family, not only acts as a proinflammatory cytokine that regulates the differentiation of naive T helper cells but also possesses anti-inflammatory properties. IL-27 consists of EBI3 (Epstein-Barr virus-induced gene 3) and p28 subunits. Our previous work has shown that IRF-1 regulates IL-27 p28 gene transcription by specifically binding to the IRF-1 response element in the p28 promoter. In this study, we found that IRF-8-deficient macrophages were highly defective in the production of IL-27 p28 at both mRNA and protein levels. Circulating IL-27 p28 in serum was also decreased in IRF-8 ؊/؊ mice in a septic shock model. Lipopolysaccharide, as a potent inducer of IL-27 p28 expression, could activate IRF-8 expression in a MyD88-dependent pathway, which in turn induced p28 gene transcription through NF-B and/or IRF-8. Transcriptional analyses revealed that IRF-8 activated p28 gene transcription through binding to a site located at ؊57 to ؊48 in the p28 promoter overlapping the IRF-1 binding site. Consistent with this observation, overexpression of both IRF-8 and IRF-1 additively activated IL-27 p28 promoter. This study provides further mechanistic information regarding how signals initiated during innate and adaptive immune responses synergize to yield greater IL-27 production and sustained cellular immunity.

Tumor cell-derived molecules such as cytokines and lipid mediators play a critical role in inducing chronic inflammation in the tumor microenvironment. We found that Th17 cells were increased in the peripheral blood, spleen and tumor tissues of mammary gland tumor-bearing mice. The Th17 cell survival factor, IL-23, was also overexpressed in tumor tissues isolated from mice and human breast cancer patients. Soluble molecules secreted from breast tumor cells but not normal breast epithelial cells induced IL-23 protein secretion in DCs via induction of p19 mRNA expression. Our data further indicate that tumor-secreted PGE2 through EP2 and EP4 receptors enhanced IL-23 p19 gene transcription through binding to the cAMP-response element in the p19 promoter. Blocking PGE2 synthesis by NS398, a COX2 inhibitor, abrogated the enhancement of p19 expression both in vitro and in vivo. Furthermore, blocking PKA by H89 completely abrogated the inductive effects of tumor conditioned medium and PGE2 on p19 transcription, whereas the cAMP active analog, Forskolin mimics the PGE2 effect. Taken together, our results indicate that tumor-secreted PGE2 induces IL-23 but not IL-12 production in the tumor microenvironment leading to Th17 cell expansion. This inductive effect of PGE2 on IL-23 p19 transcription is mediated through cAMP/PKA signaling transduction pathway.

Interleukin-23 (IL-23) plays an essential role in maintenance of IL-17-producing T helper (Th17) cells that are involved in the pathogenesis of several autoimmune diseases. Regulation of Th17 cells is tightly controlled by multiple factors such as IL-27 and IFN-γ. However, the detailed mechanisms responsible for IFN-γ-mediated Th17 cell inhibition are still largely unknown. In this study, we demonstrate that IFN-γ differentially regulates IL-12 and IL-23 production in both dendritic cells and macrophages. IFN-γ suppresses IL-23 expression by selectively targeting p19 mRNA stability through its 3′untranslated region (3′UTR). Furthermore, IFN-γ enhances LPS-induced tristetraprolin (TTP) mRNA expression and protein production. Overexpression of TTP suppresses IL-23 p19 mRNA expression and p19 3′UTR-dependent luciferase activity. In addition, deletion of TTP completely abolishes IFN-γ-mediated p19 mRNA degradation. We further demonstrate that IFN-γ suppresses LPS-induced p38 phosphorylation and blockade of p38 MAPK signaling pathway with SB203580 inhibits IFN-γ and LPS induced p19 mRNA expression whereas overexpression of p38 increases p19 mRNA expression via reducing TTP binding to the p19 3′UTR. Finally, inhibition of p38 phosphorylation by IFN-γ leads to TTP dephosphorylation that could result in stronger binding of the TTP to the adenosine/uridine-rich elements in the p19 3′UTR and p19 mRNA degradation. In summary, our results reveal a direct link among TTP, IFN-γ and IL-23, indicating that IFN-γ-mediated Th17 cell suppression might act through TTP by increasing p19 mRNA degradation and therefore IL-23 inhibition.