Class II transactivator and class II MHC gene expression in microglia: modulation by the cytokines TGF-beta, IL-4, IL-13 and IL-10

Microglia are the resident macrophages of the brain, and when activated, have functions including cytokine production, phagocytosis and antigen presentation. The class II MHC genes encode proteins that present antigenic peptides to helper T cells, leading to T cell activation and the development of an antigen-specific immune response. Class II MHC gene expression is strictly regulated by the class II transactivator (CIITA) transcription factor. In this study, we investigated the effects of various immunomodulatory cytokines on IFN-gamma induction of class II MHC and CIITA gene expression in microglia, both primary microglia and a murine microglial cell line, EOC 20. By flow cytometry analysis we show that IFN-gamma-induced surface expression of class II MHC molecules on EOC 20 cells can be inhibited by the cytokines TGF-beta1, IL-4 and IL-10, but not IL-13. Using a ribonuclease protection assay, we have found that TGF-beta1, IL-4 and IL-10 act by inhibiting the expression of IFN-gamma-induced CIITA mRNA and, in turn, class II MHC mRNA. TGF-beta1, IL-4, and IL-10 inhibition of IFN-gamma-induced CIITA mRNA accumulation was not due to destabilization of CIITA mRNA, suggesting an effect at the level of transcription. In primary murine microglia, IL-10 and TGF-beta1 inhibited IFN-gamma-induced CIITA and class II MHC expression. However, a discordant effect of IL-4 was noted in that IL-4 enhanced IFN-gamma-induced CIITA and class II MHC expression in primary microglia. Although some differences are observed between EOC 20 cells and primary microglia in terms of responsiveness to TGF-beta, IL-4 and IL-10, CIITA and class II MHC gene expression are coordinately modulated.

Cytokine regulation of CC and CXC chemokine expression by human astrocytes

Chemokines constitute a large family of secreted proteins that function as chemoattractants and activators of leukocytes. Astrocytes, the major glial cell type in the central nervous system (CNS), are a source of chemokine production within diseased brain. As such, we have examined the production of chemokines by human astroglioma cell lines and primary human astrocytes treated with a variety of stimuli, including LPS, TNF-alpha, IFN-gamma and IL-1beta. In addition, IL-6 in conjunction with the soluble IL-6 receptor (sIL-6R), and hybrid IL-6 (H-IL-6), a highly active fusion protein of sIL-6R and IL-6, were tested for their ability to induce chemokine expression. The findings presented herein demonstrate that both human astroglioma cell lines and primary human astrocytes express the CXC chemokines IP-10 and IL-8 and the CC chemokines MCP-1 and RANTES in response to TNF-alpha and IL-1beta. IFN-gamma induced the expression of IP-10, but not of IL-8, MCP-1 or RANTES. Surprisingly, IL-6/sIL-6R and H-IL-6 had little or no effect on chemokine expression in these cells. The effect of TGF-beta on chemokine expression in human astroglioma cell lines and astrocytes was also examined. TGF-beta alone had little or no effect on RANTES, MCP-1 and IL-8 expression; however, TGF-beta synergized with TNF-alpha to enhance MCP-1 expression in both astroglioma cells and primary astrocytes. An inhibitory effect of TGF-beta on TNF-alpha and IL-1beta induced RANTES and IL-8 expression was observed in human astroglioma cells. In contrast, TGF-beta enhanced TNF-alpha and IL-1beta induction ofIL-8 production by human astrocytes. These findings document a complex pattern of chemokine regulation by the pleiotropic cytokine TGF-beta with both enhancing and inhibitory effects.

IL-1 beta inhibits IFN-gamma-induced class II MHC expression by suppressing transcription of the class II transactivator gene

Class II MHC Ags are critical for the initiation of immune responses by presenting Ag to T lymphocytes, leading to their activation and differentiation. The transcriptional activation of class II MHC genes requires the induction of the class II transactivator (CIITA) protein, a master regulator that is essential for both constitutive and IFN-gamma-inducible class II MHC expression. The cytokine IL-1beta has been shown to inhibit IFN-gamma-induced class II MHC expression in various cell types. We investigated the underlying mechanism of this inhibitory effect of IL-1beta using human astroglioma cell lines. Our findings demonstrate that IL-1beta prevents the expression of class II MHC mRNA and protein upon treatment with IFN-gamma. Furthermore, we demonstrate that IFN-gamma induction of CIITA mRNA expression is inhibited by treatment of cells with IL-1beta. IL-1beta suppressed IFN-gamma activation of the type IV CIITA promoter in astroglioma cells, indicating that the inhibitory influence of IL-1beta is mediated by inhibition of CIITA transcription. IL-1beta did not interfere with IFN-gamma receptor signal transduction, since tyrosine phosphorylation, nuclear translocation, and DNA binding of STAT-1alpha to an IFN-gamma activation sequence of the type IV CIITA promoter were not affected by IL-1beta. As well, IL-1beta treatment did not affect the ability of IFN-gamma-induced interferon-regulatory factor-1 (IRF-1) to bind the IRF-1 element within the type IV CIITA promoter. This study suggests that IL-1beta may play a role in regulating immunoreactivity by inhibiting transcription of the CIITA gene, thereby reducing subsequent class II MHC expression.

IL-1β Inhibits IFN-γ-Induced Class II MHC Expression by Suppressing Transcription of the Class II Transactivator Gene

Class II MHC Ags are critical for the initiation of immune responses by presenting Ag to T lymphocytes, leading to their activation and differentiation. The transcriptional activation of class II MHC genes requires the induction of the class II transactivator (CIITA) protein, a master regulator that is essential for both constitutive and IFN-γ-inducible class II MHC expression. The cytokine IL-1β has been shown to inhibit IFN-γ-induced class II MHC expression in various cell types. We investigated the underlying mechanism of this inhibitory effect of IL-1β using human astroglioma cell lines. Our findings demonstrate that IL-1β prevents the expression of class II MHC mRNA and protein upon treatment with IFN-γ. Furthermore, we demonstrate that IFN-γ induction of CIITA mRNA expression is inhibited by treatment of cells with IL-1β. IL-1β suppressed IFN-γ activation of the type IV CIITA promoter in astroglioma cells, indicating that the inhibitory influence of IL-1β is mediated by inhibition of CIITA transcription. IL-1β did not interfere with IFN-γ receptor signal transduction, since tyrosine phosphorylation, nuclear translocation, and DNA binding of STAT-1α to an IFN-γ activation sequence of the type IV CIITA promoter were not affected by IL-1β. As well, IL-1β treatment did not affect the ability of IFN-γ-induced interferon-regulatory factor-1 (IRF-1) to bind the IRF-1 element within the type IV CIITA promoter. This study suggests that IL-1β may play a role in regulating immunoreactivity by inhibiting transcription of the CIITA gene, thereby reducing subsequent class II MHC expression.

Transcriptional regulation of the intercellular adhesion molecule-1 gene by proinflammatory cytokines in human astrocytes

Intercellular adhesion molecule-1 (ICAM-1) expression is upregulated by cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interferon-gamma (IFN-gamma) in numerous cell types including the astrocyte, which functions as an immunoregulatory cell within the central nervous system. We investigated the mechanism by which ICAM-1 is transcriptionally regulated by proinflammatory cytokines in human fetal astrocytes. TNF-alpha and IL-1beta enhanced ICAM-1 expression at both the mRNA and protein levels, while IFN-gamma had a modest enhancing effect. However, a synergistic response was noted when IFN-gamma was added with either TNF-alpha or IL-1beta. Using human ICAM-1 deletion constructs and linker scanning mutants, we determined that the NF-kappaB element (-186 bp region) is critical for both TNF-alpha- and IL-1beta-mediated ICAM-1 expression, while the IFN-gamma activation sequence (GAS) element at -75 bp region is important for IFN-gamma stimulation. The synergistic effect between TNF-alpha and IFN-gamma is dependent on both NF-kappaB and GAS elements. Upon TNF-alpha and IL-1beta stimulation, p65 homodimers and p65/p50 heterodimers bind to the NF-kappaB site, and STAT-1alpha homodimers bind to the GAS element upon IFN-gamma stimulation. Transient transfection assays demonstrated that overexpression of the p65 protein transactivated the promoter activity of an ICAM-1 reporter construct, while p50 overexpression inhibited, in a dose-dependent manner, p65-mediated ICAM-1 expression. These data collectively suggest that in human astrocytes, the p65 homodimer is responsible for ICAM-1 upregulation upon TNF-alpha or IL-1beta stimulation, and that IFN-gamma enhancement of ICAM-1 involves activation of STAT-1alpha homodimers.

Transcriptional Suppression of Matrix Metalloproteinase-2 Gene Expression in Human Astroglioma Cells by TNF-α and IFN-γ

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that function in the turnover of extracellular matrix components during development. In addition, MMPs also contribute to pathological conditions associated with inflammation, angiogenesis, and tumor invasion. A 72-kDa type IV collagenase, also referred to as gelatinase A or MMP-2, has been proposed to potentiate the invasion and metastasis of malignant tumors. In particular, MMP-2 activity has been shown to constitute an important component of human astroglioma invasion. We investigated the influence of various cytokines, both proinflammatory and immunosuppressive, on MMP-2 gene expression in two human astroglioma cell lines (U251-MG and CRT). Our results indicate that the cell lines constitutively express high levels of MMP-2 mRNA, protein, and bioactivity as assessed by ribonuclease protection assay, immunoblotting, and zymography assays, respectively. The proinflammatory cytokines TNF-α and IFN-γ individually can inhibit constitutive MMP-2 expression, and function in an additive manner for near-complete inhibition of MMP-2 expression. Inhibition of MMP-2 mRNA levels by TNF-α and IFN-γ is not due to destabilization of the MMP-2 message; rather, inhibition is mediated at the transcriptional level. Furthermore, TNF-α/IFN-γ inhibition of MMP-2 expression results in decreased invasiveness of the human astroglioma cells through an extracellular matrix. These results raise the possibility that TNF-α and IFN-γ may have beneficial effects in attenuating astroglioma invasive properties.

Transcriptional suppression of matrix metalloproteinase-2 gene expression in human astroglioma cells by TNF-alpha and IFN-gamma

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that function in the turnover of extracellular matrix components during development. In addition, MMPs also contribute to pathological conditions associated with inflammation, angiogenesis, and tumor invasion. A 72-kDa type IV collagenase, also referred to as gelatinase A or MMP-2, has been proposed to potentiate the invasion and metastasis of malignant tumors. In particular, MMP-2 activity has been shown to constitute an important component of human astroglioma invasion. We investigated the influence of various cytokines, both proinflammatory and immunosuppressive, on MMP-2 gene expression in two human astroglioma cell lines (U251-MG and CRT). Our results indicate that the cell lines constitutively express high levels of MMP-2 mRNA, protein, and bioactivity as assessed by ribonuclease protection assay, immunoblotting, and zymography assays, respectively. The proinflammatory cytokines TNF-alpha and IFN-gamma individually can inhibit constitutive MMP-2 expression, and function in an additive manner for near-complete inhibition of MMP-2 expression. Inhibition of MMP-2 mRNA levels by TNF-alpha and IFN-gamma is not due to destabilization of the MMP-2 message; rather, inhibition is mediated at the transcriptional level. Furthermore, TNF-alpha/IFN-gamma inhibition of MMP-2 expression results in decreased invasiveness of the human astroglioma cells through an extracellular matrix. These results raise the possibility that TNF-alpha and IFN-gamma may have beneficial effects in attenuating astroglioma invasive properties.

Transcriptional regulation of intercellular adhesion molecule-1 in astrocytes involves NF-kappaB and C/EBP isoforms

ICAM-1 is an inducible cell surface protein that is involved in cell extravasation into inflamed tissues as well as immune responses. ICAM-1 expression is upregulated by proinflammatory cytokines such as TNF-alpha and IL-1beta in numerous cell types including the astrocyte, which functions as an immune effector cell in the central nervous system (CNS). We investigated the mechanism by which the ICAM-1 gene is transcriptionally regulated in astrocytes in response to TNF-alpha and IL-1beta. Human ICAM-1 promoter constructs linked to the reporter gene luciferase were transiently transfected into astrocytes, stimulated with TNF-alpha and IL-1beta, and ICAM-1 promoter activity examined. We determined that binding sites for both NF-kappaB (-186 bp region) and C/EBP (-198 bp region) are involved in TNF-alpha and IL-1beta-mediated ICAM-1 upregulation. Electrophoretic mobility shift assays using antibodies against NF-kappaB and C/EBP isoforms showed that p65 homodimers and p65/p50 heterodimers bind to the NF-kappaB site, and C/EBPdelta homodimers and C/EBPbeta/delta heterodimers bind to the C/EBP site. Transient transfection assays demonstrated that overexpression of p65 could transactivate the promoter activity of ICAM-1 reporter constructs. p50 overexpression had no effect on the basal levels of ICAM-1 transcription, but inhibited, in a dose dependent manner, p65 mediated transcription. Overexpression of C/EBPbeta slightly inhibited basal levels of ICAM-1 promoter activity, however, when C/EBPbeta and p65 were cotransfected, C/EBPbeta completely abolished the transactivating effects of p65. These results demonstrate that cytokine-induced ICAM-1 expression in astrocytes is regulated by interactions between NF-kappaB and C/EBP transcription factors.

Role of IL-6 and the Soluble IL-6 Receptor in Inhibition of VCAM-1 Gene Expression

Adhesion molecules such as VCAM-1 and ICAM-1 are increased in the central nervous system (CNS) during inflammatory responses and contribute to extravasation of leukocytes across the blood-brain barrier (BBB) and into CNS parenchyma. Astrocytes contribute to the structural integrity of the BBB and can be induced to express VCAM-1 and ICAM-1 in response to cytokines such as TNF-α, IL-1β, and IFN-γ. In this study, we investigated the influence of IL-6 on astroglial adhesion molecule expression. IL-6, the soluble form of the IL-6R (sIL-6R), or both IL-6 plus sIL-6R, had no effect on VCAM-1 or ICAM-1 gene expression. Interestingly, the IL-6/sIL-6R complex inhibited TNF-α-induced VCAM-1 gene expression but did not affect TNF-α-induced ICAM-1 expression. The inhibitory effect of IL-6/sIL-6R complex was reversed by the inclusion of anti-IL-6R and gp130 Abs, demonstrating the specificity of the response. A highly active fusion protein of sIL-6R and IL-6, covalently linked by a flexible peptide, which is designated H-IL-6, also inhibited TNF-α-induced VCAM-1 expression. sIL-6R alone was an effective inhibitor of TNF-α-induced VCAM-1 due to endogenous IL-6 production. These results indicate that the IL-6 system has an unexpected negative effect on adhesion molecule expression in glial cells and may function as an immunosuppressive cytokine within the CNS.

Role of IL-6 and the soluble IL-6 receptor in inhibition of VCAM-1 gene expression

Adhesion molecules such as VCAM-1 and ICAM-1 are increased in the central nervous system (CNS) during inflammatory responses and contribute to extravasation of leukocytes across the blood-brain barrier (BBB) and into CNS parenchyma. Astrocytes contribute to the structural integrity of the BBB and can be induced to express VCAM-1 and ICAM-1 in response to cytokines such as TNF-alpha, IL-1beta, and IFN-gamma. In this study, we investigated the influence of IL-6 on astroglial adhesion molecule expression. IL-6, the soluble form of the IL-6R (sIL-6R), or both IL-6 plus sIL-6R, had no effect on VCAM-1 or ICAM-1 gene expression. Interestingly, the IL-6/sIL-6R complex inhibited TNF-alpha-induced VCAM-1 gene expression but did not affect TNF-alpha-induced ICAM-1 expression. The inhibitory effect of IL-6/sIL-6R complex was reversed by the inclusion of anti-IL-6R and gp130 Abs, demonstrating the specificity of the response. A highly active fusion protein of sIL-6R and IL-6, covalently linked by a flexible peptide, which is designated H-IL-6, also inhibited TNF-alpha-induced VCAM-1 expression. sIL-6R alone was an effective inhibitor of TNF-alpha-induced VCAM-1 due to endogenous IL-6 production. These results indicate that the IL-6 system has an unexpected negative effect on adhesion molecule expression in glial cells and may function as an immunosuppressive cytokine within the CNS.

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 Alzheimer's 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.

Post-transcriptional inhibition of CD40 gene expression in microglia by transforming growth factor-beta

Microglia are one of the major glial cell types within the central nervous system, and can function as immune effector cells upon activation. CD40 is a cell surface receptor belonging to the TNF receptor family that plays a critical role in the regulation of immune responses. In this study, we investigated the expression of CD40 on microglia, and the role of transforming growth factor-beta (TGF-beta), an immunosuppressive cytokine, in regulating CD40 expression. Microglia constitutively express very low levels of CD40, and IFN-gamma enhances CD40 mRNA and protein expression in these cells. IFN-gamma-induced CD40 mRNA expression is partially sensitive to the protein synthesis inhibitor puromycin, suggesting that ongoing protein synthesis is necessary for optimal induction of CD40 mRNA by IFN-gamma. TGF-beta inhibits IFN-gamma-induced CD40 protein and mRNA expression. Inhibition of IFN-gamma-induced CD40 mRNA levels by TGF-beta in microglia is not due to inhibition of CD40 transcription; rather, inhibition is due to enhanced degradation of CD40 mRNA. These results indicate that TGF-beta can inhibit expression of an immunologically important receptor, CD40, in microglia, and does so at the post-transcriptional level by destabilizing CD40 mRNA. TGF-beta inhibition of CD40 expression may be one of the mechanisms by which TGF-beta exerts its suppressive effects on immune responses.

Kinase inhibitors abrogate IFN-gamma-induced class II transactivator and class II MHC gene expression in astroglioma cell lines

Multiple kinase events, involving both tyrosine (tyr) kinase and serine/threonine (ser/thr) kinase activity, are required for IFN-gamma-induced class II MHC mRNA and protein expression in primary rat astrocytes. In this study, we examined the necessity of ser/thr and tyr kinase activity for IFN-gamma-induced stimulation of class II MHC gene expression in the human astroglioma cell lines CRT and CH235, as well as the involvement of these kinases in IFN-gamma-induced expression of the class II transactivator (CIITA), a protein critical for IFN-gamma-induced transcription of class II MHC genes. We show that general ser/thr kinase inhibitors, inhibitors of the ser/thr kinase mitogen-activated protein kinase (MAPK), and tyr kinase inhibitors reduce IFN-gamma-induced class II MHC mRNA and protein expression in a dose-dependent manner. As well, these inhibitors abrogate IFN-gamma-induced CIITA mRNA expression in the astroglioma cell lines. We have further demonstrated that cells constitutively expressing the CIITA protein (2fTGH.CIITA) show no decrease in CIITA or class II MHC mRNA expression in the presence of ser/thr and tyr kinase inhibitors. Collectively, these data indicate that ser/thr kinase activity, possibly MAPK, and tyr kinase activity are required for IFN-gamma-induced expression of CIITA mRNA, and the subsequent expression of class II MHC genes.

Cleavage of membrane-associated ICAM-1 from astrocytes: involvement of a metalloprotease

Disease of the central nervous system (CNS) with immune-mediated pathogenesis is frequently associated with enhanced expression of intercellular adhesion molecule-1 (ICAM-1) on resident glial cells, including astrocytes. Recently, a soluble form of ICAM-1 (sICAM-1) has been demonstrated within the CNS and cerebrospinal fluid (CSF), arising from an intrathecal source. In this study, we investigated the ability of TNF-alpha treated astrocytes to generate sICAM-1 from a population of membrane-associated ICAM-1. To determine the ability of ICAM-1 to be released from the cell surface, generating sICAM-1, cell cultures were treated with TNF-alpha for 21 h prior to cell surface protein iodination or biotinylation. We show that the membrane-associated form of ICAM-1 (approximately 90 KD) is converted to a soluble form (approximately 83 KD) in cell culture supernatants. The half-life of TNF-alpha induced membrane-associated ICAM-1 on rat astrocytes is approximately 5 h. The proteolytic cleavage process for the conversion of membrane-associated ICAM-1 to sICAM-1 was sensitive to Batimastat (BB94) and phosphoramidon, two inhibitors of metalloproteases, whereas inhibitors of serine-, cysteine-, aspartic-, and chymotrypsin-like proteases had no effect on this process. These results indicate that astrocytes can be induced to produce sICAM-1, and this process involves a metalloprotease that is induced/activated in a TNF-alpha-dependent fashion. It is proposed that astrocytes may be a source of intrathecal sICAM-1 under inflammatory conditions.

Cleavage of membrane-associated ICAM-1 from astrocytes: involvement of a metalloprotease

Disease of the central nervous system (CNS) with immune-mediated pathogenesis is frequently associated with enhanced expression of intercellular adhesion molecule-1 (ICAM-1) on resident glial cells, including astrocytes. Recently, a soluble form of ICAM-1 (sICAM-1) has been demonstrated within the CNS and cerebrospinal fluid (CSF), arising from an intrathecal source. In this study, we investigated the ability of TNF-alpha treated astrocytes to generate sICAM-1 from a population of membrane-associated ICAM-1. To determine the ability of ICAM-1 to be released from the cell surface, generating sICAM-1, cell cultures were treated with TNF-alpha for 21 h prior to cell surface protein iodination or biotinylation. We show that the membrane-associated form of ICAM-1 (approximately 90 KD) is converted to a soluble form (approximately 83 KD) in cell culture supernatants. The half-life of TNF-alpha induced membrane-associated ICAM-1 on rat astrocytes is approximately 5 h. The proteolytic cleavage process for the conversion of membrane-associated ICAM-1 to sICAM-1 was sensitive to Batimastat (BB94) and phosphoramidon, two inhibitors of metalloproteases, whereas inhibitors of serine-, cysteine-, aspartic-, and chymotrypsin-like proteases had no effect on this process. These results indicate that astrocytes can be induced to produce sICAM-1, and this process involves a metalloprotease that is induced/activated in a TNF-alpha-dependent fashion. It is proposed that astrocytes may be a source of intrathecal sICAM-1 under inflammatory conditions.

Elevation of cyclic AMP levels in astrocytes antagonizes cytokine-induced adhesion molecule expression

We have examined the effect of elevating cyclic AMP levels on cytokine-mediated enhancement of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) gene expression by astrocytes. Treatment of astrocytes with the cyclic AMP mimetic dibutyryl-cyclic AMP, or the agonists norepinephrine, forskolin, prostaglandin E2, and cholera toxin alone had no effect on ICAM-1 or VCAM-1 mRNA gene expression. However, elevating cyclic AMP levels within the cells by these agents suppressed interleukin-1beta- and tumor necrosis factor-alpha-induced adhesion molecule expression at both the mRNA and protein levels. The phosphodiesterase type IV inhibitor, rolipram, was able to potentiate the inhibitory effect of forskolin on ICAM-1 and VCAM-1 gene expression. Inhibition of tumor necrosis factor-alpha-induced VCAM-1 mRNA levels by forskolin was partially due to enhanced degradation of VCAM-1 message, whereas the decay rates of tumor necrosis factor-alpha-induced ICAM-1 message and interleukin-1beta-induced ICAM-1/VCAM-1 message were not affected by forskolin treatment. These results demonstrate that the pathways used by interleukin-1beta and tumor necrosis factor-alpha to induce adhesion molecule expression are antagonized by cyclic AMP-dependent protein kinase-mediated signaling pathways.

TGF-beta suppresses IFN-gamma induction of class II MHC gene expression by inhibiting class II transactivator messenger RNA expression

Recently, a non-DNA binding protein, class II transactivator (CIITA), has been shown to be required for constitutive and IFN-gamma-inducible class II MHC transcription. The cytokine TGF-beta inhibits IFN-gamma-induced class II MHC expression at the transcriptional level. In this study, we provide evidence that TGF-beta blocks IFN-gamma-induced CIITA mRNA accumulation. TGF-beta down-regulates class II MHC and CIITA mRNA accumulation in human astroglioma and fibrosarcoma cell lines, but TGF-beta does not destabilize the CIITA message, suggesting an effect at the transcriptional level. In cells that stably overexpressed CIITA, leading to a constitutive class II MHC-positive phenotype, the inhibitory effect of TGF-beta on class II MHC was abrogated, but the cells remained responsive for expression of TGF-beta-inducible genes. Cell lines that possessed defects in TGF-beta signaling also became refractory to inhibition of IFN-gamma-induced CIITA and class II MHC expression. Our data indicate that TGF-beta suppresses IFN-gamma-induced class II MHC expression by inhibiting accumulation of CIITA mRNA.

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

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 disease.

TGF-beta suppresses IFN-gamma induction of class II MHC gene expression by inhibiting class II transactivator messenger RNA expression

Recently, a non-DNA binding protein, class II transactivator (CIITA), has been shown to be required for constitutive and IFN-gamma-inducible class II MHC transcription. The cytokine TGF-beta inhibits IFN-gamma-induced class II MHC expression at the transcriptional level. In this study, we provide evidence that TGF-beta blocks IFN-gamma-induced CIITA mRNA accumulation. TGF-beta down-regulates class II MHC and CIITA mRNA accumulation in human astroglioma and fibrosarcoma cell lines, but TGF-beta does not destabilize the CIITA message, suggesting an effect at the transcriptional level. In cells that stably overexpressed CIITA, leading to a constitutive class II MHC-positive phenotype, the inhibitory effect of TGF-beta on class II MHC was abrogated, but the cells remained responsive for expression of TGF-beta-inducible genes. Cell lines that possessed defects in TGF-beta signaling also became refractory to inhibition of IFN-gamma-induced CIITA and class II MHC expression. Our data indicate that TGF-beta suppresses IFN-gamma-induced class II MHC expression by inhibiting accumulation of CIITA mRNA.

Induction of RANTES expression by astrocytes and astrocytoma cell lines

The cellular infiltrate found during the acute phase of multiple sclerosis (MS) consists of monocytes and activated T cells, suggesting the presence of cell-specific chemotactic signals during the inflammatory response. We examined the ability of human astrocytoma cell lines, as well as primary human and rat astrocytes, to generate a specific member of the intercrine/chemokine family of cytokines, RANTES, when exposed to TNF-alpha, IL-1 beta and IFN-gamma. Astrocytoma cells as well as primary astrocytes produced RANTES upon incubation with TNF-alpha or IL-1 beta. IFN-gamma alone did not induce RANTES production by astrocytes, but it potentiated the effects of either TNF-alpha or IL-1 beta. Induction of RANTES by TNF-alpha was mediated by the p55 receptor since a specific anti-p55 antiserum mimicked the effect of TNF-alpha. These results indicate that human astrocytes are capable of generating a cell-specific chemokine that can account for the inflammatory cellular infiltrate observed during the acute phase of MS, in a process that is regulated by cytokines.

A 3' cis-acting element is involved in tumor necrosis factor-alpha gene expression in astrocytes

Tumor necrosis factor-alpha (TNF-alpha) contributes to demyelinating diseases in the central nervous system. Astrocytes, the major glial cells in the CNS, do not constitutively express TNF-alpha, but the TNF-alpha gene is transcriptionally activated in response to a variety of stimuli, including TNF-alpha itself. Because of the importance of TNF-alpha in the CNS, we examined the mechanisms underlying transcriptional regulation of the TNF-alpha gene in astrocytes. In transient transfection assays, a plasmid construct containing 1.3 kilobase pairs (kb) of 5' flanking sequence of the rat TNF-alpha gene showed high basal activity that could not be further enhanced by TNF-alpha stimulation. A "marked" 10-kb TNF-alpha gene construct, which contains the whole TNF-beta gene with 1.2 kb of 5' flanking sequence, 1.1 kb of intergenic sequence, and the whole TNF-alpha gene with 3 kb of 3' flanking sequence, was able to respond to TNF-alpha stimulation. Analysis of a series of 5' and 3' deletion constructs of the marked TNF-alpha genes demonstrated that upstream sequence elements such as NF-kappaB are not required for TNF-alpha induction and that TNF-alpha responsive elements are located in the 3' flanking region of the TNF-alpha gene. We also found that a TNF-alpha-inducible DNase I-hypersensitive (DH) site is present in this 3' region whose deletion abolishes TNF-alpha inducibility of the marked TNF-alpha gene. Electrophoresis mobility shift assays showed that TNF-alpha-inducible nuclear proteins, consisting of p50 and p65 NF-kappaB proteins, specifically bind to two consecutive NF-kappaB binding sites within the 3' DH site. These results indicate that TNF-alpha-induced TNF-alpha gene expression in astrocytes involves p50 and p65 NF-kappaB proteins binding to downstream NF-kappaB sites and concomitant modulation of the chromatin structure.

The central nervous system as an immunocompetent organ: role of glial cells in antigen presentation

The central nervous system has the capacity to initiate and regulate immune responses by allowing restricted entry of peripheral immune cells and providing a microenvironment that is conducive to the activation of immune effector cells. In this commentary, we discuss the ability of two glial cell types, astrocytes and microglia, to function as APCs. In particular, we focus on the phenotypic and functional distinctions between astrocytes and microglia that may contribute to their differing abilities to effectively present Ag to CD4+ cells.

The central nervous system as an immunocompetent organ: role of glial cells in antigen presentation

The central nervous system has the capacity to initiate and regulate immune responses by allowing restricted entry of peripheral immune cells and providing a microenvironment that is conducive to the activation of immune effector cells. In this commentary, we discuss the ability of two glial cell types, astrocytes and microglia, to function as APCs. In particular, we focus on the phenotypic and functional distinctions between astrocytes and microglia that may contribute to their differing abilities to effectively present Ag to CD4+ cells.

Stat1 alpha expression is involved in IFN-gamma induction of the class II transactivator and class II MHC genes

Class II MHC Ags are critical in the regulation of immune responses by presenting Ag to T lymphocytes, resulting in their activation and differentiation. Class II expression is rare in the normal central nervous system, but elevated expression on glial cells has been observed in several neurologic diseases. We have previously demonstrated that IFN-gamma-induced class II expression in glial cells involves activation of both tyrosine kinase and protein kinase C. IFN-gamma induces tyrosine phosphorylation of the tyrosine kinases Jak1 and Jak2 and of Stat1 alpha. In addition, IFN-gamma enhances expression of Stat1 alpha mRNA and protein. We utilized antisense oligonucleotides against Stat1 alpha to determine directly whether IFN-gamma-induced activation and/or enhancement of Stat1 alpha is involved in class II expression. Antisense oligonucleotides complementary to Stat1 alpha mRNA were introduced in CH235-MG astroglioma cells by transient transfection; such treatment inhibited both constitutive and IFN-gamma-enhanced expression of Stat1 alpha. IFN-gamma-induced class II MHC expression was also inhibited in cells exposed to Stat1 alpha antisense oligonucleotides. The fact that the class II promoter does not contain IFN-gamma-activated sequences for binding Stat1 alpha suggests that Stat1 alpha must activate another protein that is directly involved in class II expression. A likely candidate is the class II MHC transactivator (CIITA). IFN-gamma induction of CIITA mRNA was also inhibited in cells treated with antisense oligonucleotides against Stat1 alpha. These findings demonstrate that Stat1 alpha is involved in IFN-gamma induction of CIITA expression, resulting in class II MHC expression.