Dysregulated cytokine expression and signaling are major contributors to a number of autoimmune diseases. Interleukin-17A (IL-17A) and IL-6 are important in many disorders characterized by immune self-recognition, and IL-6 is known to induce the differentiation of T helper 17 (Th17) cells. Here we described an IL-17A-triggered positive-feedback loop of IL-6 signaling, which involved the activation of the transcription factors nuclear factor (NF)-kappaB and signal transducer and activator of transcription 3 (STAT3) in fibroblasts. Importantly, enhancement of this loop caused by disruption of suppressor of cytokine signaling 3 (SOCS3)-dependent negative regulation of the IL-6 signal transducer gp130 contributed to the development of arthritis. Because this mechanism also enhanced experimental autoimmune encephalomyelitis (EAE) in wild-type mice, it may be a general etiologic process underlying other Th17 cell-mediated autoimmune diseases.

IL-17-producing Th (Th17) comprise a distinct lineage of pro-inflammatory Th that are major contributors to autoimmune diseases. Treatment with IL-6 and transforming growth factor beta (TGFbeta) induces naive CD4+ T cells to generate Th17, which also requires expression of the IL-6/TGFbeta target RORgammat. We reported that IL-6 transduces two signaling pathways via tyrosine redidues of the signal transducer gp130: one depends on signal transducers and activators of transcription (STAT)-3 activation and the other on Src homology region 2 domain-containing phosphatase 2 (SHP2)/Grb2 associated binder (Gab)/mitogen-activated protein kinase (MAPK) activation. Here, we showed that CD4+ T cells carrying a mutant gp130 that transduces the SHP2/Gab/MAPK pathway but not the STAT3-mediated one failed to develop into Th17, while CD4+ T cells whose mutant gp130 transduces the STAT3 signal only generated Th17, indicating that IL-6 acts directly on T cells through the tyrosine residues of gp130 required for STAT3 activation to promote the development of Th17. Moreover, we found that gp130-STAT3 pathway is essential for Th17 development and for the expression of RORgammat by using T cells specifically lacking gp130 and STAT3. Noteworthy is that the regulatory T cell (Treg) percentages and numbers were comparable between all mutant mice we tested in vivo, although we showed that IL-6-gp130-STAT3 pathway suppressed Treg development in vitro. Thus, we conclude that IL-6 acts directly to promote the development of Th17 by activating the T cell gp130-STAT3 pathway but has a minimum effect on Treg development at least in the steady state in vivo. Therefore, blockade of IL-6-gp130-STAT3 pathway in CD4+ T cells could be a good target for controlling unwanted Th17-mediated immune responses including autoimmune diseases.

Systemic cytokine activity in response to Toll-like receptor (TLR) signaling induces the expression of various proteins in the liver after infections. Here we show that Interleukin-7 (IL-7), the production of which was thought to occur at a constant rate in vivo, was a hepatically expressed protein that directly controled T cell responses. Depletion of IL-7 expression in the liver abrogated several TLR-mediated T cell events, including enhanced CD4+ T cell and CD8+ T cell survival, augmented CD8+ T cell cytotoxic activity, and the development of experimental autoimmune encephalitis, a Th17 cell-mediated autoimmune disease. Thus, T cell responses are regulated by hepatocyte-derived IL-7, which is expressed in response to TLR signaling in vivo. We suggested that TLR-induced IL-7 expression in the liver, which is an acute-phase response, may be a good diagnostic and therapeutic target for efficient vaccine developments and for conditions characterized by TLR-mediated T cell dysregulation, including autoimmune diseases.

Zinc (Zn) is an essential trace metal required by many enzymes and transcription factors for their activity or the maintenance of their structure. Zn has a variety of effects in the immune responses and inflammation, although it has not been well known how Zn affects these reactions on the molecular basis. We here showed that Zn suppresses T(h)17-mediated autoimmune diseases at lest in part by inhibiting the development of T(h)17 cells via attenuating STAT3 activation. In mice injected with type II collagen to induce arthritis, Zn treatment inhibited T(h)17 cell development. IL-6-mediated activation of STAT3 and in vitro T(h)17 cell development were all suppressed by Zn. Importantly, Zn binding changed the alpha-helical secondary structure of STAT3, disrupting the association of STAT3 with JAK2 kinase and with a phospho-peptide that included a STAT3-binding motif from the IL-6 signal transducer gp130. Thus, we conclude that Zn suppresses STAT3 activation, which is a critical step for T(h)17 development.