Whereas interleukin-6 (IL-6) is a proinflammatory cytokine, IL-10 is an anti-inflammatory cytokine. Although signal transducer and activator of transcription 3 (STAT3) is essential for the function of both IL-6 and IL-10, it is unclear how these two cytokines have such opposing functions. Here we show that suppressor of cytokine signaling 3 (SOCS3) is a key regulator of the divergent action of these two cytokines. In macrophages lacking the Socs3 gene or carrying a mutation of the SOCS3-binding site in gp130, the lipopolysaccharide-induced production of tumor necrosis factor (TNF) and IL-12 is suppressed by both IL-10 and IL-6. SOCS3 specifically prevents activation of STAT3 by IL-6 but not IL-10. Taken together, these data indicate that SOCS3 selectively blocks signaling by IL-6, thereby preventing its ability to inhibit LPS signaling.

A balance between pro- and anti-inflammatory mechanisms at mucosal interfaces, sites of constitutive exposure to microbes and non-microbial foreign substances, allows for efficient protection against pathogens yet prevents adverse inflammatory responses associated with allergy, asthma, and intestinal inflammation1. Regulatory T (Treg) cells prevent systemic and tissue-specific autoimmunity and inflammatory lesions at mucosal interfaces. These cells are generated in the thymus (tTreg cells) and in the periphery (iTreg cells) and their dual origin implies a division of labor between tTreg and iTreg cells in immune homeostasis. Here we demonstrate that a highly selective blockage in differentiation of iTreg cells did not lead to unprovoked multi-organ autoimmunity, exacerbation of induced tissue-specific autoimmune pathology (EAE), or increased pro-inflammatory Th1 and Th17 cell responses. However, iTreg cell-deficient mice spontaneously developed pronounced Th2 type pathologies at mucosal sites — in the gastrointestinal tract and lungs — with hallmarks of allergic inflammation and asthma. Furthermore, iTreg cell deficiency altered gut microbial communities. These results suggest that whereas Treg cells generated in the thymus appear sufficient for control of systemic and tissue-specific autoimmunity, extrathymic differentiation of Treg cells impacts commensal microbiota composition and serves a distinct, essential function in restraint of allergic type inflammation at mucosal interfaces.

Regulatory T (Treg) cells, expressing abundant amounts of the IL-2 receptor (IL-2R), are reliant on IL-2 produced by activated T cells. This feature implied a key role for a simple network based on IL-2 consumption by Treg cells in their suppressor function. However, congenital deficiency in IL-2R results in reduced expression of the Treg cell lineage specification factor Foxp3, confounding experimental efforts to understand the role of IL-2R expression and signaling in Treg suppressor function. Using genetic gain and loss of function approaches, we demonstrate that IL-2 capture is dispensable for control of CD4+ T cells, but is important for limiting CD8+ T cell activation, and that IL-2R dependent STAT5 transcription factor activation plays an essential role in Treg cell suppressor function separable from T cell receptor signaling.

SUMMARY In multicellular organisms, specialized functions are delegated to distinct cell types whose identity and functional integrity is maintained upon challenge. However, little is known about the mechanisms enabling lineage inheritance and their biological implications. Regulatory T (Treg) cells, which express the transcription factor Foxp3, suppress fatal autoimmunity throughout the lifespan of animals. Here, we show that a dedicated Foxp3 intronic element CNS2 maintains Treg cell lineage identity by acting as a sensor of the essential Treg cell growth factor IL-2 and its downstream target STAT5. CNS2 sustains Foxp3 expression during division of mature Treg cells when IL-2 is limiting and counteracts pro-inflammatory cytokine signaling that leads to the loss of Foxp3. CNS2 mediated stable inheritance of Foxp3 expression is critical for adequate suppression of diverse types of chronic inflammation by Treg cells and prevents their differentiation into inflammatory effector cells. The described mechanism may represent a general principle of the inheritance of differentiated cell states.

The cytokine, transforming growth factor-␤1 (TGF-␤1), converts naive T cells into regulatory T cells that prevent autoimmunity. However, in the presence of interleukin (IL)-6, TGF-␤1 has also been found to promote differentiation into IL-17-producing helper T (Th17) cells that are deeply involved in autoimmunity and inflammation. However, it has not been clarified how TGF-␤1 and IL-6 determine such a distinct fate. Here we found that a master regulator for Th17, retinoic acid-related orphan receptor ␥t (ROR␥t), was rapidly induced by TGF-␤1 regardless of the presence of IL-6. IL-6 reduced Foxp3 expression, and overexpression of Foxp3 in a T cell line resulted in a strong reduction of IL-17A expression. We have characterized the IL-17A promoter and found that ROR␥t binding is sufficient for activation of the minimum promoter in the HEK 293T cells. ROR␥t-mediated IL-17A promoter activation was suppressed by forced expression of Foxp3. Foxp3 directly interacted with ROR␥t through exon 2 region of Foxp3. The exon 2 region and forkhead (FKH) domain of Foxp3 were necessary for the suppression of ROR␥t-mediated IL-17A promoter activation. We propose that induction of Foxp3 is the mechanism for the suppression of Th17 and polarization into inducible Treg.

Recently, DNA methylation and reduced expression of the suppressor of the cytokine signaling-3 (SOCS3) gene in human hepatocellular carcinoma (HCC) patients have been reported. However, the roles of SOCS3 in HCC development in vivo have not been clarified. Using RT-PCR analysis and Western blotting, we confirmed that SOCS3 expression was reduced in HCC patients. However, reduced expression of SOCS3 occurred not only in HCC but also in nontumor regions, and this reduction was stronger as the fibrosis grade increased. Furthermore, SOCS3 levels were inversely correlated with signal transducers and activators of transcription-3 (STAT3) activation as well as transforming growth factor (TGF)-b1 levels in the non-HCC region. To define the molecular consequences of SOCS3 silencing/STAT3 hyperactivation and liver fibrosis, we examined liverspecific SOCS3-deficient mice. We demonstrated that SOCS3 deletion in the liver resulted in hyperactivation of STAT3 and promoted ConA-and chemical-induced liver fibrosis. The expression of TGF-b1, a mediator of fibrosis, was enhanced by SOCS3 gene deletion, but suppressed by the overexpression of a dominant-negative STAT3 or SOCS3 both in vivo and in vitro. These data suggest that TGF-b1 is a target gene of STAT3 and could be one of the mechanisms for enhanced fibrosis in SOCS3-deficient mice. Thus, our present study provides a novel role of SOCS3 and STAT3 in HCC development: in addition to the previously characterized oncogenic potentials, STAT3 enhances hepatic fibrosis through the upregulation of TGF-b1 expression, and SOCS3 prevents this process.

SOCS1 is required to restrict IFN-γ and IL-17 expression and maintain Foxp3 expression in and function of regulatory T cells.

Approximately 20% of human cancers are estimated to develop from chronic inflammation. Recently, the NF-κB pathway was shown to play an essential role in promoting inflammation-associated cancer, but the role of the JAK/STAT pathway, another important signaling pathway of proinflammatory cytokines, remains to be investigated. Suppressor of cytokine signaling-1 (SOCS1) acts as an important physiological regulator of cytokine responses, and silencing of the SOCS1 gene by DNA methylation has been found in several human cancers. Here, we demonstrated that SOCS1-deficient mice (SOCS1−/−Tg mice), in which SOCS1 expression was restored in T and B cells on a SOCS1−/− background, spontaneously developed colorectal carcinomas carrying nuclear β-catenin accumulation and p53 mutations at 6 months of age. However, interferon (IFN)γ−/−SOCS1−/− mice and SOCS1−/−Tg mice treated with anti-IFNγ antibody did not develop such tumors. STAT3 and NF-κB activation was evident in SOCS1−/−Tg mice, but these were not sufficient for tumor development because these are also activated in IFNγ−/−SOCS1−/− mice. However, colons of SOCS1−/−Tg mice, but not IFNγ−/−SOCS1−/− mice, showed hyperactivation of STAT1, which resulted in the induction of carcinogenesis-related enzymes, cyclooxygenase-2 and inducible nitric oxide synthase. These data strongly suggest that SOCS1 is a unique antioncogene which prevents chronic inflammation-mediated carcinogenesis by regulation of the IFNγ/STAT1 pathways.