Interleukin-4 down-regulates MHC class II antigens on cultured rat astrocytes

The role of astrocytes in multiple sclerosis pathogenesis

INTRODUCTION

Multiple sclerosis (MS) is a demyelinating autoimmune disease of the central nervous system (CNS), in which astrocytes play an important role as CNS immune cells. However, the activity of astrocytes as antigen-presenting cells (APC) continues to be subject to debate.

DEVELOPMENT

This review analyses the existing evidence on the participation of astrocytes in CNS inflammation in MS and on several mechanisms that modify astrocyte activity in the disease.

CONCLUSIONS

Astrocytes play a crucial role in the pathogenesis of MS because they express toll-like receptors (TLR) and major histocompatibility complex (MHC) classI andII. In addition, astrocytes participate in regulating the blood-brain barrier (BBB) and in modulating T cell activity through the production of cytokines. Future studies should focus on the role of astrocytes in order to find new therapeutic targets for the treatment of MS.

Interleukin-4 blocks thapsigargin-induced cell death in rat rod photoreceptors: involvement of cAMP/PKA pathway

Although the photoreceptors cell death is the main cause of some retinopathies diseases, the mechanisms involved in this process are poorly understood. The neuroprotective effects of interleukin-4 (IL-4) have been shown in several tissues, including retina. We demonstrate that treatment of rat retinal explants with IL-4 completely inhibited the thapsigargin-induced rod photoreceptor cell death after 24 hr in culture. We also showed that IL-4 receptor alpha subunit (IL-4Ralpha) is abundantly present in retina. Colocalization of IL-4Ralpha and rhodopsin indicate a direct effect of this cytokine in rod photoreceptor cells. Moreover, IL-4 increased the intracellular levels of cAMP in 7.4-fold, indicating that the neuroprotective effect of this cytokine was completely blocked by RpcAMP, an inhibitor of protein kinase (PKA). Our data demonstrate, for the first time, the neuroprotective effect of IL-4 through cAMP/PKA pathway in thapsigargin-induced photoreceptor cell death.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Differential selectivity of CIITA promoter activation by IFN-gamma and IRF-1 in astrocytes and macrophages: CIITA promoter activation is not affected by TNF-alpha

During demyelinating disease of the central nervous system (CNS), locally elevated cytokine levels may induce upregulation of MHC class II molecules on otherwise low expressing or negative cell types such as microglia and astrocytes, since IFN-gamma has been shown to induce MHC class II expression on these cell types in vitro. While many transcription factors are involved with MHC class II expression, only the class II transactivator (CIITA) is tightly coordinated with IFN-gamma-inducibility. Control of CIITA gene expression is complex, involving four distinct promoters, two of which (promoters III and IV) are IFN-gamma-inducible in certain cell types. Here we demonstrate that IFN-gamma treatment of rat astrocytes induces only CIITA promoter IV activity in contrast to the murine macrophage cell line RAW 264.7 that uses both IFN-gamma-inducible promoters. In contrast to previously published reports, promoter IV activation is completely dependent upon an intact interferon regulatory factor-1 (IRF-1) but not STAT binding site using promoter constructs specifically mutated at these positions. Importantly, while TNF-alpha is able to synergize with IFN-gamma to increase astrocyte MHC class II expression in vitro, we show that treatment of rat astrocytes with TNF-alpha has no effect on CIITA promoter activity. These data demonstrate that TNF-alpha augments MHC class II expression through a mechanism downstream or independent of CIITA induction.

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.

Erratic behavior of nitric oxide within the immune system: illustrative review of conflicting data and their immunopharmacological aspects

The literature data assembled in this article document the variation of immunobiological effects of nitric oxide (NO). A number of factors are obviously responsible for the diversity, ranging from inactivity, alleviation, but not rarely to exacerbation of certain pathogenetic processes. A better understanding of NO interactions with the immune system can only be reached if more complex experimental designs to study the effects of reactive nitrogen species are adopted in the future. They should integrate major participating variables and take into account pharmacodynamic/kinetic aspects of NO production in triggering the ultimate effects. If manipulation of NO in the organism by means of recently developed NO inhibitors and NO donors is to become a rational tool of immunopharmacological strategies, detailed knowledge of their pharmacologies and toxicologies is urgently needed in order to differentiate between the effects of NO and other side effects. Hopefully, this approach could improve the predictability of the clinical outcomes of NO manipulation.

Functional IL-4 receptors on mouse astrocytes: IL-4 inhibits astrocyte activation and induces NGF secretion

IL-4 is a Th2-derived cytokine which plays an important role in the function of various immunocompetent cells as well as in the pathophysiology of various CNS disorders. In this study we characterized the expression of IL-4R in cultured astrocytes and explored the effects of IL-4 on the function of these cells. We found that astrocytes express the mRNA of both the membrane-bound and the soluble forms of the IL-4R, whereas they do not secrete IL-4. IL-4 inhibited both NO production and iNOS expression induced by LPS stimulation and decreased the secretion of TNF-alpha and the expression of ICAM-1. In contrast, IL-4 induced the secretion of NGF by astrocytes and synergized with LPS and TNF-alpha in this effect. These results suggest an important role for IL-4 as an immunosuppressive and a neurotrophic factor in the CNS.

Therapeutic effects of interleukin-4 gene transfer in experimental inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by altered immunoregulation and augmented intestinal synthesis of nitric oxide. The purpose of this study was to determine the effects of exogenous IL-4, introduced by a recombinant human type 5 adenovirus (Ad5) vector, on the tissue injury associated with an experimental model of colonic immune activation and inflammation. Colitis was induced in rats by the intrarectal administration of trinitrobenzene sulfonic acid (TNB) dissolved in 50% ethanol, and control rats received saline via the same route. 1 h later, all rats were randomized into two groups. The first group was injected intraperitoneally (ip) with 3.0 x 10(6) plaque forming units (PFUs) of Ad5 transfected with murine interleukin-4 (Ad5IL-4) and the second group was injected ip with the same amount of Ad5 expressing the Escherichia coli Lac Z gene (Ad5LacZ). One-half of the colitic and control rats were injected again with 3.0 x 10(6) PFUs of Ad5IL-4 or Ad5LacZ on day 3 of the 6-d study. When introduced once or twice via the peritoneal route into control rats, Ad5LacZ was localized to the serosal lining of the peritoneal cavity, the diaphragm and the liver on day 6. One or two injections of Ad5IL-4 into rats also produced measurable levels of circulating IL-4. TNB-colitis in both Ad5LacZ-treated groups was associated with pronounced elevations in serum IFN-gamma, and mucosal ulceration of the distal colon. Myeloperoxidase and inducible nitric oxide synthase II (NOS II) synthetic activity were also increased by 30- and fivefold, respectively, above control levels in the distal colon. However, two injections of Ad5IL-4 into colitic rats caused the overexpression of IL-4, and significantly inhibited tissue damage, serum and colon IFN-gamma levels and myeloperoxidase activity in the distal colon. In addition, NOS II gene expression and NOS II nitric oxide synthesis was significantly inhibited. No therapeutic effect was observed in rats injected once with Ad5IL-4. Thus, IL-4, introduced by Ad5, is therapeutic during acute inflammation in the rat colon. The therapeutic effect of IL-4 was associated with an inhibition of inducible nitric oxide expression and a reduction in nitric oxide synthesis.