In the decade following their initial discovery, the suppressor of cytokine signaling (SOCS) proteins have been studied for their potential use as immunomodulators in disease. SOCS proteins, especially SOCS1 and SOCS3, are expressed by immune cells and cells of the central nervous system (CNS) and have the potential to impact immune processes within the CNS, including inflammatory cytokine and chemokine production, activation of microglia, macrophages and astrocytes, immune cell infiltration and autoimmunity. We describe CNS-relevant in vitro and in vivo studies that have examined the function of SOCS1 or SOCS3 under various neuroinflammatory or neuropathological conditions, including exposure of CNS cells to inflammatory cytokines or bacterial infection, demyelinating insults, stroke, spinal cord injury, multiple sclerosis and glioblastoma multiforme. The SOCS familySuppressor of cytokine signaling (SOCS) proteins are intracellular, cytokine-inducible proteins that inhibit cytokine signaling in numerous cell types, including cells of the immune and central nervous systems (CNS). The SOCS family is composed of eight members: cytokine inducible SRC homology 2 (SH2)-domain-containing protein (CIS) and SOCS1 to SOCS7 [1,2]. To exert their function, SOCS proteins associate with phosphorylated tyrosine residues on Janus kinases (JAKs) and/or cytokine receptor subunits through a central SH2 domain. A C-terminal SOCS box then interacts with components of the ubiquitin ligase machinery and mediates proteosomal degradation of associated proteins [3]. In addition, the N-terminus of SOCS1 and SOCS3, specifically, contains a kinase-inhibitory region (KIR) (Figure 1), which acts as a pseudosubstrate for JAKs, conferring inhibition of JAK kinase activity [1]. Through these interactions, SOCS proteins attenuate responses to cytokines and growth factors. Because studies of SOCS family members have established SOCS1 and SOCS3 as the most important in regulating innate and adaptive immune responses, they are the focus of this review. There is limited information regarding the role of other SOCS family members in CNS immunity. Therefore, they are not discussed here. SOCS regulation of JAK/STAT signalingSignal transduction through the JAK/signal transducer and activator of transcription (STAT) signaling pathway is a crucial mediator of inflammatory and immune responses in the CNS [4]. Activation of the JAK/STAT pathway is achieved by cytokines binding to their associated cell-surface receptors, leading to a series of phosphorylation events, culminating in phosphorylation of the STAT transcription factors (Figure 2). Activated STATs promote Corresponding author: Benveniste, E.N. (tika@uab.edu). Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Plea...

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.

Astrocytes play a number of important physiological roles in CNS homeostasis. Inflammation stimulates astrocytes to secrete cytokines and chemokines that guide macrophages/microglia and T cells to sites of injury/inflammation. Herein, we describe how these processes are controlled by the suppressor of cytokine signaling (SOCS) proteins, a family of proteins that negatively regulate adaptive and innate immune responses. In this study, we describe that the immunomodulatory cytokine IFN-β induces SOCS-1 and SOCS-3 expression in primary astrocytes at the transcriptional level. SOCS-1 and SOCS-3 transcriptional activity is induced by IFN-β through IFN-γ activation site (GAS) elements within their promoters. Studies in STAT-1α-deficient astrocytes indicate that STAT-1α is required for IFN-β-induced SOCS-1 expression, while STAT-3 small interfering RNA studies demonstrate that IFN-β-induced SOCS-3 expression relies on STAT-3 activation. Specific small interfering RNA inhibition of IFN-β-inducible SOCS-1 and SOCS-3 in astrocytes enhances their proinflammatory responses to IFN-β stimulation, such as heightened expression of the chemokines CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β), CCL5 (RANTES), and CXCL10 (IP-10), and promoting chemotaxis of macrophages and CD4+ T cells. These results indicate that IFN-β induces SOCS-1 and SOCS-3 in primary astrocytes to attenuate its own chemokine-related inflammation in the CNS.

T cell infiltration into the central nervous system (CNS) is a significant underlying pathogenesis in autoimmune inflammatory demyelinating diseases. Several lines of evidence suggest that glutamate dysregulation in the CNS is an important consequence of immune cell infiltration in neuroinflammatory demyelinating diseases; yet, the causal link between inflammation and glutamate dysregulation is not well understood. A major source of glutamate release during oxidative stress is the system xc− transporter, however, this mechanism has not been tested in animal models of autoimmune inflammatory demyelination. We find that pharmacological and genetic inhibition of system xc− attenuates chronic and relapsing-remitting experimental autoimmune encephalomyelitis (EAE). Remarkably, pharmacological blockade of system xc− seven days after induction of EAE attenuated T cell infiltration into the CNS, but not T cell activation in the periphery. Mice harboring a Slc7a11 (xCT) mutation that inactivated system xc− were resistant to EAE, corroborating a central role for system xc− in mediating immune cell infiltration. We next examined the role of the system xc− transporter in the CNS after immune cell infiltration. Pharmacological inhibitors of the system xc− transporter administered during the first relapse in a SJL animal model of relapsing-remitting EAE abrogated clinical disease, inflammation, and myelin loss. Primary co-culture studies demonstrate that myelin-specific CD4+ T helper type 1 (Th1) cells provoke microglia to release glutamate via the system xc− transporter causing excitotoxic death to mature myelin-producing OLs. Taken together these studies support a novel role for the system xc− transporter in mediating T cell infiltration into the CNS as well as promoting myelin destruction after immune cell infiltration in EAE.

Costimulation between T cells and APCs is required for adaptive immune responses. CD40, an important costimulatory molecule, is expressed on a variety of cell types, including macrophages and microglia. The aberrant expression of CD40 is implicated in diseases including multiple sclerosis, rheumatoid arthritis, and Alzheimer’s disease, and inhibition of CD40 signaling has beneficial effects in a number of animal models of autoimmune diseases. In this study, we discovered that IL-10, a cytokine with anti-inflammatory properties, inhibits LPS-induced CD40 gene expression. We previously demonstrated that LPS induction of CD40 in macrophages/microglia involves both NF-κB activation and LPS-induced production of IFN-β, which subsequently activates STAT-1α. IL-10 inhibits LPS-induced IFN-β gene expression and subsequent STAT-1α activation, but does not affect NF-κB activation. Our results also demonstrate that IL-10 inhibits LPS-induced recruitment of STAT-1α, RNA polymerase II, and the coactivators CREB binding protein and p300 to the CD40 promoter, as well as inhibiting permissive histone H3 acetylation (AcH3). IL-10 and LPS synergize to induce suppressor of cytokine signaling (SOCS)-3 gene expression in macrophages and microglia. Ectopic expression of SOCS-3 attenuates LPS-induced STAT activation, and inhibits LPS-induced CD40 gene expression, comparable to that seen by IL-10. These results indicate that SOCS-3 plays an important role in the negative regulation of LPS-induced CD40 gene expression by IL-10.