Glia-T cell dialogue

Vitamin D3 inhibits proinflammatory cytokines and nitric oxide production by the EOC13 microglial cell line

In recent years, a neuroimmunomodulatory role for 1,25-dihydroxyvitamine D(3) [1,25(OH)(2)D(3)] has emerged. Microglial cells present a potential target for the effects of this hormone in the brain. This study focuses on the effect of 1,25(OH)(2)D(3) on the expression and production of inflammatory cytokines and nitric oxide (NO) by the EOC13 microglial cell line. The presence of the vitamin D3 receptor in microglia was demonstrated by RT-PCR. 1,25(OH)(2)D(3) inhibited the production of tumor necrosis factor-alpha, interleukin-6, and NO by stimulated microglia in a concentration-related fashion. The production of transforming growth factor-beta1 (TGF-beta1), an anti-inflammatory cytokine, was not modified in the presence of 1,25(OH)(2)D(3), indicating that the effects of 1,25(OH)(2)D(3) may not involve TGF-beta1 regulation. These results show that 1,25(OH)(2)D(3) has direct anti-inflammatory properties on microglia. It further supports the hypothesis that 1,25(OH)(2)D(3) could be involved in the maintenance of the brain homeostasis and may have a therapeutic potential in inflammatory pathologies of the central nervous system.

IL-2 gene knockout affects T lymphocyte trafficking and the microglial response to regenerating facial motor neurons

Following facial nerve axotomy in mice, T cells cross the intact blood-brain barrier (BBB), home to nerve cell bodies in the facial motor nucleus (FMN), and augment neuroregenerative processes. The pivotal T cell immunoregulatory cytokine, IL-2, appears to have bidirectional effects on neuronal and microglial cell function, suggesting rival hypotheses that IL-2 could either enhance or disrupt processes associated with regeneration of axotomized facial motor neurons. We tested these competing hypotheses by comparing the effect of facial nerve axotomy on C57BL/6-IL-2(-/-) knockout and C57BL/6-IL-2(+/+) wild-type littermates. Since IL-2 may also be produced endogenously in the brain, we also sought to determine whether differences between the knockout and wild-type mice were attributable to loss of IL-2 gene expression in the CNS, loss of peripheral sources of IL-2 and the associated effects on T cell function, or a combination of these factors. To address this question, we bred novel congenic mice with the SCID mutation (mice lacking T cell derived IL-2) that were homozygous for either the IL-2 knockout or wild-type gene alleles (C57BL/6scid-IL-2(-/-) and C57BL/6scid-IL-2(+/+) littermates, respectively). Groups were assessed for differences in (1) T lymphocytes entering the axotomized FMN; (2) perineuronal CD11b(+) microglial phagocytic clusters, a measure of motor neuron death; and (3) activated microglial cells as measured by MHC-II positivity. C57BL/6-IL-2(-/-) knockout mice had significantly higher numbers of T cells and lower numbers of activated MHC-II-positive microglial cells in the regenerating FMN than wild-type littermates, although the number of CD11b(+) phagocytic microglia clusters did not differ. Thus, despite the significant impairment of T cell function known to be associated with loss of peripheral IL-2, the increased number of T cells entering the axotomized FMN appears to have sufficient activity to support neuroregenerative processes. Congenic C57BL/6scid-IL-2(-/-) knockout mice had lower numbers of CD11b(+) microglial phagocytic clusters than congenic C57BL/6scid-IL-2(+/+) wild-type littermates, suggesting that loss of the IL-2 gene in the CNS (and possibly the loss of other unknown sources of the gene) enhanced neuronal regeneration. Further study of IL-2's complex actions in neuronal injury may provide greater understanding of key variables that determine whether or not immunological processes in the brain are proregenerative.

Slit proteins, potential endogenous modulators of inflammation

Recent studies suggest that molecules important for guiding neuronal migration and axon path-finding also play a role in modulating leukocyte chemotaxis. Neuronal migration and leukocyte chemotaxis may share some common regulatory mechanisms. Intracellular signal transduction mechanisms guiding neuronal migration and leukocyte chemotaxis are beginning to be elucidated. Studying molecular mechanisms modulating cell migration may provide new insights into understanding of endogenous inhibitors of inflammation.

Autoimmunity as the body's defense mechanism against the enemy within: Development of therapeutic vaccines for neurodegenerative disorders

Insults to the central nervous system (CNS), whether of microbial or microbe-free origin, result in tissue damage. Until recently, it was generally believed that only microbe-related damage elicits an adaptive immune response, the purpose of which is to eliminate the offending microorganisms. Recent studies in the author's laboratory suggest, however, that the body exhibits an adaptive immune response to microbe-free injuries as well. The immune response in this case is directed against dominant self-antigens residing in the damaged site, where such an adaptive anti-self immune response reinforces the protective activity of local resident cells by providing them with factors that can augment and regulate their capacity for buffering troublemakers such as destructive self-compounds emerging from the injured neural tissue. Because the specificity of this autoimmune response apparently depends not on the type but on the site of lesion, the response can be boosted by therapeutic vaccination for acute and chronic neurodegenerative conditions irrespective of their primary etiology. The results have far-reaching implications, both for microbial infections and for neurodegenerative diseases of the CNS.

Mycophenolic acid downregulates inducible nitric oxide synthase induction in astrocytes

Free radical nitric oxide (NO), generated by inducible nitric oxide synthase (iNOS) in astrocytes and macrophages, has been implicated in CNS inflammatory disorders such as multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). Mycophenolic acid (MPA), a selective inhibitor of inosine monophosphate dehydrogenase (IMPDH), inhibited interferon-gamma (IFN-gamma) + lipopolysaccharide (LPS)-induced NO production dose-dependently in astrocytes, but not in macrophages. The effect of MPA was not mediated through interference with IMPDH-dependent synthesis of iNOS cofactor BH4 and subsequent suppression of iNOS enzymatic activity, as direct BH4 precursor sepiapterin failed to block the action of the drug. However, MPA markedly inhibited IFN-gamma + LPS-triggered astrocyte expression of mRNA for iNOS and its transcription factor IRF-1, while the expression of tumor necrosis factor-alpha (TNF-alpha) gene was not altered. The observed MPA suppression of NO release and iNOS and IRF-1 induction in astrocytes were efficiently prevented by exogenous guanosine, indicating that the drug acted through reduction of IMPDH-dependent synthesis of guanosine nucleotides. This IRF-1-dependent inhibition of iNOS activation might be partly responsible for the protective effect of MPA in EAE, prompting investigation of its potential use in multiple sclerosis.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit chemokine production in activated microglia

Microglia react to even minor disturbances in CNS homeostasis and function as critical regulators of CNS inflammation. Activated microglia secrete inflammatory mediators such as cytokines and chemokines, which contribute to the pathophysiological changes associated with several neuroimmunologic disorders. Microglia-derived inflammatory chemokines recruit various populations of immune cells, which initiate and maintain the inflammatory response against foreign antigens. Entry and retention of activated immune cells in the CNS is a common denominator in a variety of traumatic, ischemic, and degenerative diseases. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two structurally related neuropeptides that function as potent anti-inflammatory factors in the periphery. Here we investigated the effects of VIP and PACAP on chemokine production by activated microglia. VIP and PACAP inhibit the expression of the microglia-derived CXC chemokines MIP-2 and KC, and of the CC chemokines MIP-1alpha, -1beta, MCP-1, and RANTES. The inhibition of chemokine gene expression correlates with an inhibitory effect of VIP/PACAP on NFkB binding. The VIP/PACAP inhibition of both chemokine production and of NFkB binding is mediated through the specific receptor VPAC1 and involves a cAMP-dependent intracellular pathway. Of biological significance is the fact that the inhibition of chemokine production by VIP/PACAP leads to a significant reduction in the chemotactic activity generated by activated microglia for peripheral leukocytes, i.e., neutrophils, macrophages, and lymphocytes. Because reduction in the number and activation of infiltrating leukocytes represents an important factor in the control of inflammation in the CNS, VIP and/or PACAP released by neurons during an inflammatory response could serve as neuronal survival factors by limiting the inflammatory process.

Dual action of glatiramer acetate (Cop-1) in the treatment of CNS autoimmune and neurodegenerative disorders

Protective autoimmunity is the body's defense mechanism against destructive self-compounds such as those commonly associated with neurodegenerative disorders. Autoimmune disease and neurodegenerative disorders can thus be viewed as two extreme manifestations of the same process. Therefore, when designing therapy, it is important to avoid an approach that will cure the one by invoking the other. One way to stop, or at least slow down, the progression of neurodegeneration without risking development of an autoimmune disease is by boosting protective autoimmunity in a well-controlled way. Copolymer 1 (Cop-1), an approved drug for the treatment of multiple sclerosis, can be used as a treatment for autoimmune diseases and as a therapeutic vaccine for neurodegenerative diseases. We propose that the protective effect of Cop-1 vaccination is obtained through a well-controlled inflammatory reaction, and that the activity of Cop-1 in driving this reaction derives from its ability to serve as a 'universal antigen' by weakly activating a wide spectrum of self-reactive T cells.

Interleukin-13 and -4 induce death of activated microglia

When the brain suffers injury, microglia migrate to the damaged sites and become activated. These activated microglia are not detected several days later and the mechanisms underlying their disappearance are not well characterized. In this study, we demonstrate that interleukin (IL)-13, an anti-inflammatory cytokine, selectively induces cell death of activated microglia in vitro. Cell death was detected 4 days after the coaddition of IL-13 with any one of the microglial activators, lipopolysaccharide (LPS), ganglioside, or thrombin. This cell death occurred in a time-dependent manner. LPS, ganglioside, thrombin, or IL-13 alone did not induce cell death. Among anti-inflammatory cytokines, IL-4 mimicked the effect of IL-13, while TGF-beta did not. Cells treated with IL-13 plus LPS, or IL-13 plus ganglioside, showed the characteristics of apoptosis when analyzed by electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling staining. Electron micrographs also showed microglia engulfing neighboring dead cells. We propose that IL-13 and IL-4 induce death of activated microglia, and that this process is important for prevention of chronic inflammation that can cause tissue damage.

Systemic administration of kainic acid in adult rat stimulates expression of the chemokine receptor CCR5 in the forebrain

As chemokines and their receptors are primarily expressed by glial cells in brain parenchyma, a model of glial cell proliferation may be useful to study the regulation of their expression in the brain. The well-established kainic acid seizure model was used in this study, focusing on the expression of the CCR5 chemokine receptor. Adult Sprague-Dawley rats were injected intraperitoneally with kainic acid (12 mg/kg), and in situ hybridization of CCR5 mRNA was performed at 12 h, 1, 3, or 7 days, posttreatment. Autoradiographic films and wet photographic emulsions demonstrated the very low expression of CCR5 mRNA in normal brain parenchyma, as well as in the microvasculature and ventricular/choroid plexus systems. After kainic acid treatment, brain CCR5 mRNA expression increased progressively from 12 h to 7 days, especially in the olfactory system, amygdaloid complex, thalamus, hippocampal formation, septum, and neocortex. This increase paralleled that of activated microglial cells as shown, using the microglial marker, OX-42. Moreover, CCR5 mRNA ISH combined with neuron-specific enolase immunocytochemistry showed that, in addition to its glial expression, CCR5 mRNA is expressed in neurons in the normal brain and, to a lesser extent, after kainate treatment due to neuronal losses. Finally, CCR5 protein is detected by immunocytochemistry in neurodegenerative areas in numerous glial cells, as well as in neurons, as clearly shown in the hippocampal formation. In summary, the chemokine receptor CCR5 is expressed by neuronal and non-neuronal cell types in the normal brain and is upregulated in both cell types after an insult.

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.

Immune function of microglia

During the past decade, mechanisms involved in the immune surveillance of the central nervous system (CNS) have moved to the forefront of neuropathological research mainly because of the recognition that most neurological disorders involve activation and, possibly, dysregulation of microglia, the intrinsic macrophages of the CNS. Increasing evidence indicates that, in addition to their well-established phagocytic function, microglia may also participate in the regulation of non specific inflammation as well as adaptive immune responses. This article focuses on the signals regulating microglia innate immune functions, the role of microglia in antigen presentation, and their possible involvement in the development of CNS immunopathology.

Identification and IFNgamma-regulation of differentially expressed mRNAs in murine microglial and CNS-associated macrophage subpopulations

CNS-resident macrophages (microglia and CNS-associated macrophages) are the main immunocompetent cells of the central nervous system (CNS) and respond by rapid activation to brain injury. Molecular events occurring during IFNgamma-activation and identification of potential markers of the CNS-resident macrophage subsets were investigated using microglial-derived clones (EOC) differing in their morphology and their antigen presenting activities for CD4+ and CD8+ T-cells. By applying the subtractive process of cDNA representational difference analysis (cRDA), 16 differentially expressed mRNAs were isolated and sequenced, revealing 8 known and 8 novel molecules; 15 of these messages were unpreviously reported in microglia. Two markers of all activated microglial EOC cells were identified (iNOS; IRG-1) and specific subpopulation markers were highlighted, including molecules known to be closely expressed in perivascular spaces. Moreover, some messages could support the distinct morphology, adhesive characteristics, and potential functions of the different clones.

Regulation of T-cell responses by CNS antigen-presenting cells: different roles for microglia and astrocytes

Analysis of the mechanisms underlying CNS immune surveillance and immunopathology have provided new insights into the intracerebral regulation of immune responses. Here, Francesca Aloisi, Francesco Ria and Luciano Adorini review the role of CNS antigen presenting cells and focus on the control of Th1 and Th2 responses by microglia and astrocytes.