Macrophages participate in both the amplification of inflammation at the time of injury, and downregulation of the inflammatory response to avoid excess tissue damage. These divergent functions of macrophages are dictated by their microenvironment, especially cytokines, which promote a spectrum of macrophage phenotypes. The M1 proinflammatory phenotype is induced by LPS, IFN-γ and GM-CSF, and IL-4, IL-13 and M-CSF induce anti-inflammatory M2 macrophages. Suppressors Of Cytokine Signaling (SOCS) proteins function as feedback inhibitors of the JAK/STAT signaling pathway, and can terminate innate and adaptive immune responses. In this study, we have evaluated the influence of SOCS3 on macrophage polarization and function. Macrophages obtained from LysMCre-SOCS3fl/fl mice, which lack SOCS3 in myeloid lineage cells, exhibit enhanced and prolonged activation of JAK/STAT pathway compared to macrophages from SOCS3fl/fl mice. Furthermore, SOCS3-deficient macrophages have higher levels of the M1 genes IL-1β, IL-6, IL-12, IL-23 and iNOS, due to enhanced transcriptional activation and chromatin modifications. SOCS3-deficient M1 macrophages also have a stronger capacity to induce Th1 and Th17 cell differentiation than M1 macrophages from SOCS3fl/fl mice. Lastly, LPS-induced sepsis is exacerbated in LysMCre-SOCS3fl/fl mice, and is associated with enhanced STAT1/3 activation and increased plasma levels of M1 cytokines/chemokines such as IL-1β, TNF-α, IL-6, CCL3, CCL4 and CXCL11. These findings collectively indicate that SOCS3 is involved in repressing the M1 proinflammatory phenotype, thereby deactivating inflammatory responses in macrophages.

Suppressor of cytokine signaling (SOCS) proteins are feedback inhibitors of the JAK/STAT pathway. SOCS3 has a crucial role in inhibiting STAT3 activation, cytokine signaling, and inflammatory gene expression in macrophages/microglia. To determine the role of SOCS3 in myeloid cells in neuroinflammation, mice with conditional SOCS3 deletion in myeloid cells (LysMCre-SOCS3fl/fl ) were tested for experimental autoimmune encephalomyelitis (EAE). The myeloid-specific SOCS3-deficient mice are vulnerable to myelin oligodendrocyte glycoprotein (MOG)-induced EAE, with a severe, nonresolving atypical form of disease. In vivo, enhanced infiltration of inflammatory cells and demyelination is prominent in the cerebellum of myeloid-specific SOCS3-deficient mice, as is enhanced STAT3 signaling and expression of inflammatory cytokines/chemokines and an immune response dominated by Th1 and Th17 cells. In vitro, SOCS3-deficient macrophages exhibit heightened STAT3 activation and are polarized toward the classical M1 phenotype. SOCS3-deficient M1 macrophages provide the microenvironment to polarize Th1 and Th17 cells and induce neuronal death. Furthermore, adoptive transfer of M2 macrophages into myeloid SOCS3-deficient mice leads to delayed onset and reduced severity of atypical EAE by decreasing STAT3 activation, Th1/Th17 cells, and proinflammatory mediators in the cerebellum. These findings indicate that myeloid cell SOCS3 provides protection from EAE through deactivation of neuroinflammatory responses. Th1 cells | M1 polarization

TGF-β, together with IL-6 and IL-21, promotes Th17 cell development. IL-6 and IL-21 induce activation of STAT3, which is crucial for Th17 cell differentiation, as well as the expression of suppressor of cytokine signaling (SOCS)3, a major negative feedback regulator of STAT3-activating cytokines that negatively regulates Th17 cells. However, it is still largely unclear how TGF-β regulates Th17 cell development and which TGF-β signaling pathway is involved in Th17 cell development. In this report, we demonstrate that TGF-β inhibits IL-6- and IL-21-induced SOCS3 expression, thus enhancing as well as prolonging STAT3 activation in naive CD4+CD25− T cells. TGF-β inhibits IL-6-induced SOCS3 promoter activity in T cells. Also, SOCS3 small interfering RNA knockdown partially compensates for the action of TGF-β on Th17 cell development. In mice with a dominant-negative form of TGF-β receptor II and impaired TGF-β signaling, IL-6-induced CD4+ T cell expression of SOCS3 is higher whereas STAT3 activation is lower compared with wild-type B6 CD4+ T cells. The addition of a TGF-β receptor I kinase inhibitor that blocks Smad-dependent TGF-β signaling greatly, but not completely, abrogates the effect of TGF-β on Th17 cell differentiation. Our data indicate that inhibition of SOCS3 and, thus, enhancement of STAT3 activation is at least one of the mechanisms of TGF-β promotion of Th17 cell development.

Astrocytes have important physiological roles in CNS homeostasis, and serve as a bridge between the CNS and the immune system. IL-17 and IL-6 are important in many CNS disorders characterized by neuroinflammation. We examined the role of IL-17 on the IL-6 signaling cascade in primary astrocytes. IL-17 functioned in a synergistic manner with IL-6 to induce IL-6 expression in astrocytes. The synergistic effect involved numerous signaling pathways including NF-κB, JNK MAPK and p38 MAPK. The NF-κB pathway inhibitor BAY-11, JNK inhibitor JNKiII, and p38 inhibitor SB203580 suppressed the synergistic effect of IL-6 and IL-17 on IL-6 expression. IL-17 synergized with IL-6 to enhance the recruitment of activated NF-κB p65, c-Fos, c-Jun, and the histone acetyltransferases CBP and p300 to the IL-6 promoter in vivo to induce IL-6 transcription. This was accompanied by enhanced acetylation of Histones H3 and H4 on the IL-6 promoter. Moreover, we elucidated an important role for SOCS3 in IL-17 enhancement of IL-6 signaling in astrocytes. SOCS3 siRNA knockdown and SOCS3 deletion in astrocytes augmented the synergistic effect of IL-6 and IL-17, due to an enhancement of activation of the NF-κB and MAPK pathways. These results indicate that astrocytes can serve as a target of Th17 cells and IL-17 in the CNS, and SOCS3 participates in IL-17 functions in the CNS as a negative feedback regulator.

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