Etty N. Benveniste

Immune function of Astrocytes

Astrocytes are the major glial cell within the central nervous system (CNS) and have a number of important physiological properties related to CNS homeostasis. The aspect of astrocyte biology addressed in this review article is the astrocyte as an immunocompetent cell within the brain. The capacity of astrocytes to express class II major histocompatibility complex (MHC) antigens and costimulatory molecules (B7 and CD40) that are critical for antigen presentation and T‐cell activation are discussed. The functional role of astrocytes as immune effector cells and how this may influence aspects of inflammation and immune reactivity within the brain follows, emphasizing the involvement of astrocytes in promoting Th2 responses. The ability of astrocytes to produce a wide array of chemokines and cytokines is discussed, with an emphasis on the immunological properties of these mediators. The significance of astrocytic antigen presentation and chemokine/cytokine production to neurological diseases with an immunological component is described. GLIA 36:180–190, 2001.

Cytokines in inflammatory brain lesions: helpful and harmful

Multiple sclerosis (MS) is thought to be an autoimmune disease. In healthy individuals, the T cells of the immune system, when activated by an infectious agent, regularly traffic across an intact blood-brain barrier, survey the CNS and then leave. In MS, for reasons that are only gradually being understood, certain events in the peripheral immune response and in the brain cause some autoreactive T cells to stay in the CNS. Their presence initiates infiltration by other leukocytes and activation and recruitment of endogenous glia to the inflammatory process, ultimately leading to the destruction of myelin and the myelin-producing cell, the oligodendrocyte, and the dysfunction of axons. The key mediators in the subsequent cycles of histological damage and repair, and clinical relapse and remission are thought to be adhesion molecules, chemokines and cytokines.

Role of macrophages/microglia in multiple sclerosis and experimental allergic encephalomyelitis

Abstract One of the characteristic features of microglia is their rapid activation in response to injury, inflammation, neurodegeneration, infection, and brain tumors. This review focuses on the role of the microglia in multiple sclerosis (MS), a chronic inflammatory demyelinating disease of the central nervous system (CNS), and in the animal model of MS, experimental allergic encephalomyelitis (EAE). Microglial activation in MS and EAE is thought to contribute directly to CNS damage through several mechanisms, including production of proinflammatory cytokines, matrix metalloproteinases, and free radicals. In addition, activated microglia serve as the major antigen-presenting cell in the CNS, likely contributing to aberrant immune reactivity at this site. A mechanistic understanding of the way in which microglia are activated and ultimately inhibited is crucial for the formulation of therapeutic modalities to treat MS and other CNS autoimmune diseases.

Cytokine Actions in the Central Nervous System

Cytokines and chemokines have been implicated in contributing to the initiation, propagation and regulation of immune and inflammatory responses. Also, these soluble mediators have important roles in contributing to a wide array of neurological diseases such as multiple sclerosis, AIDS Dementia Complex, stroke and Alzheimers disease. Cytokines and chemokines are synthesized within the central nervous system by glial cells and neurons, and have modulatory functions on these same cells via interactions with specific cell-surface receptors. In this article, I will discuss the ability of glial cells and neurons to both respond to, and synthesize, a variety of cytokines. The emphasize will be on three select cytokines; interferon-gamma (IFN-gamma), a cytokine with predominantly proinflammatory effects; interleukin-6 (IL-6), a cytokine with both pro- and anti-inflammatory properties; and transforming growth factor-beta (TGF-beta), a cytokine with predominantly immunosuppressive actions. The significance of these cytokines to neurological diseases with an immunological component will be discussed.

Interleukin-6 expression and regulation in astrocytes.

The physiological function of interleukin-6 (IL-6) within the central nervous system (CNS) is complex; IL-6 exerts neurotrophic and neuroprotective effects, and yet can also function as a mediator of inflammation, demyelination, and astrogliosis, depending on the cellular context. In the normal brain, IL-6 levels remain low. However, elevated expression occurs in injury, infection, stroke, and inflammation. Given the diverse biological functions of IL-6 and its expression in numerous CNS conditions, it is critical to understand its regulation in the brain in order to control its expression and ultimately its effects. Accumulating data demonstrate that the predominant CNS source of IL-6 is the activated astrocyte. Furthermore, a wide range of factors have been demonstrated to be involved in IL-6 regulation by astrocytes. In this review, we summarize information concerning IL-6 regulation in astrocytes, focusing on the role of proinflammatory factors, neurotransmitters, and second messengers.

Induction and regulation of interleukin-6 gene expression in rat astrocytes

Cells that produce interleukin-6 (IL-6) require the presence of signaling molecules since this cytokine is not normally constitutively expressed. It is now established that astrocytes produce IL-6; however, the precise inducing molecules and the kinetics of their action have not yet been clearly identified. In the current study, we show that either interleukin-1 beta (IL-1 beta) or tumor necrosis factor-alpha (TNF-alpha) exert a strong inducing signal for IL-6 in primary rat astrocytes. When the two cytokines are added together the response is synergistic, suggesting that each cytokine may induce IL-6 gene expression by different pathways. Interferon-gamma (IFN-gamma) does not affect IL-6 expression although if it is added in conjunction with IL-1 beta, an augmented induction of IL-6 occurs. In addition to the cytokines, bacterial lipopolysaccharide (LPS) and the calcium ionophore, A23187, induce IL-6 expression. IL-6 expression can be blocked by the glucocorticoid analogue, dexamethasone. IL-6 induction by LPS/Ca2+ ionophore is more sensitive to the suppressive effects of dexamethasone than is IL-6 induction by TNF-alpha/IL-1 beta. Cycloheximide (CHX), an inhibitor of protein synthesis, markedly increased levels of IL-6 mRNA in both unstimulated and stimulated astrocytes, indicating that ongoing protein synthesis is not required for astrocyte IL-6 gene expression. We propose that astrocyte-produced IL-6 may have a role in augmenting intracerebral immune responses in neurological diseases such as multiple sclerosis (MS), AIDS dementia complex (ADC), and viral infections. These diseases are characterized by infiltration of lymphoid and mononuclear cells into the central nervous system (CNS), and intrathecal production of immunoglobulins. IL-6 may act to promote terminal differentiation of B cells in the CNS, leading to immunoglobulin synthesis.

Adhesion molecule expression and regulation on cells of the central nervous system

Cellular adhesion molecules were initially defined as cell surface structures mediating cell-cell and cell-extracellular matrix (ECM) interactions. Adhesion molecules involved in immune responses have been classified into three families according to their structure: selectins, immunoglobulin (Ig) superfamily, and integrins. It has been well documented that adhesion molecules of these family members (E-selectin, ICAM-1, and VCAM-1) are expressed on brain microvessel endothelial cells in active lesions of multiple sclerosis (MS) brain. In addition, accumulating data show that glial cells can express some of these adhesion molecules upon activation: astrocytes can express ICAM-1, VCAM-1, and E-selectin, and microglia express ICAM-1 and VCAM-1. In vitro studies show that these adhesion molecules are actively regulated by several cytokines which have relevance to MS or experimental autoimmune encephalomyelitis (EAE). In addition, soluble forms of adhesion molecules have been found in the serum and cerebrospinal fluid (CSF) of MS patients, and may be useful diagnostically. Experimental therapy of EAE using antibodies against several adhesion molecules clearly shows that adhesion molecules are critical for the pathogenesis of EAE. Thus far, the function of adhesion molecule expression on brain endothelial and glial cells has not been clearly elucidated. Studies on the possible role of adhesion molecules on brain endothelial and glial cells will be helpful in understanding their involvement in immune responses in the central nervous system (CNS).

The Transcription Factors Sp1, Sp3, and AP-2 Are Required for Constitutive Matrix Metalloproteinase-2 Gene Expression in Astroglioma Cells

Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases that contribute to pathological conditions associated with angiogenesis and tumor invasion. MMP-2 is highly expressed in human astroglioma cells, and contributes to the invasiveness of these cells. The human MMP-2 promoter contains potential cis-acting regulatory elements including cAMP response element-binding protein, AP-1, AP-2, PEA3, C/EBP, and Sp1. Deletion and site-directed mutagenesis analysis of the MMP-2 promoter demonstrates that the Sp1 site at −91 to −84 base pairs and the AP-2 site at −61 to −53 base pairs are critical for constitutive activity of this gene in invasive astroglioma cell lines. Electrophoretic gel shift analysis demonstrates binding of specific DNA-protein complexes to the Sp1 and AP-2 sites: Sp1 and Sp3 bind to the Sp1 site, while the AP-2 transcription factor binds the AP-2 element. Co-transfection expression experiments inDrosophilia SL2 cells lacking endogenous Sp factors demonstrate that Sp1 and Sp3 function as activators of the MMP-2 promoter and synergize for enhanced MMP-2 activation. Overexpression of AP-2 in AP-2-deficient HepG2 cells enhances MMP-2 promoter activation. These findings document the functional importance of Sp1, Sp3, and AP-2 in regulating constitutive expression of MMP-2. Delineation of MMP-2 regulation may have implications for development of new therapeutic strategies to arrest glioma invasion.

Interleukin-6 (IL-6) Production by Astrocytes: Autocrine Regulation by IL-6 and the Soluble IL-6 Receptor

In the CNS, astrocytes are a major inducible source of interleukin-6 (IL-6). Although IL-6 has beneficial effects in the CNS because of its neurotrophic properties, its overexpression is generally detrimental, adding to the pathophysiology associated with CNS disorders. Many factors have been shown to induce IL-6 expression by astrocytes, particularly the cytokines tumor necrosis factor-α (TNF-α) and interleukin-1 β (IL-1β). However, the role of IL-6 in its own regulation in astrocytes has not been determined. In this study, we examined the influence of IL-6 alone or in combination with TNF-α or IL-1β on IL-6 expression. IL-6 alone had no effect on IL-6 expression; however, the addition of the soluble IL-6 receptor (sIL-6R) induced IL-6 transcripts. Addition of TNF-α or IL-1β plus IL-6/sIL-6R led to synergistic increases in IL-6 expression. This synergy also occurred in the absence of exogenously added IL-6, attributable to TNF-α- or IL-1β-induced endogenous IL-6 protein production. IL-6 upregulation seen in the presence of TNF-α or IL-1β plus IL-6/sIL-6R was transcriptional, based on nuclear run-on analysis. Experiments were extended to other IL-6 family members to determine their role in IL-6 regulation in astrocytes. Oncostatin M (OSM) induced IL-6 alone and synergized with TNF-α for enhanced expression. These results demonstrate that IL-6/sIL-6R and OSM play an important role in the regulation of IL-6 expression within the CNS, particularly in conjunction with the proinflammatory cytokines TNF-α and IL-1β.

Signal transducer and activator of transcription-3: a molecular hub for signaling pathways in gliomas.

Glioblastoma is the most common and severe primary brain tumor in adults. Its aggressive and infiltrative nature renders the current therapeutics of surgical resection, radiation, and chemotherapy relatively ineffective. Accordingly, recent research has focused on the elucidation of various signal transduction pathways in glioblastoma, particularly aberrant activation. This review focuses on the signal transducer and activator of transcription-3 (STAT-3) signal transduction pathway in the context of this devastating tumor. STAT-3 is aberrantly activated in human glioblastoma tissues, and this activation is implicated in controlling critical cellular events thought to be involved in gliomagenesis, such as cell cycle progression, apoptosis, angiogenesis, and immune evasion. There are no reports of gain-of-function mutations in glioblastoma; rather, the activation of STAT-3 is thought to be a consequence of either dysregulation of upstream kinases or loss of endogenous inhibitors. This review provides detailed insight into the multiple mechanisms of STAT-3 activation in glioblastoma, as well as describing endogenous and chemical inhibitors of this pathway and their clinical significance. In glioblastoma, STAT-3 acts a molecular hub to link extracellular signals to transcriptional control of proliferation, cell cycle progression, and immune evasion. Because STAT-3 plays this central role in glioblastoma signal transduction, it has significant potential as a therapeutic target. (Mol Cancer Res 2008;6(5):675–84)