Apoptosis of inflammatory cells in immune control of the nervous system: role of glia

Neuropathologic study of experimental classical swine fever

The aim of this study was to report on the lesions occurring in the central nervous system (CNS) during experimental classical swine fever (CSF) to clarify the spatial and chronologic distribution of the lesions and virus antigen in the CNS. To learn more about the pathogenetic mechanisms of the lesions during CSF in the CNS and to investigate the role of the virus in these mechanisms, cellular infiltrates and infected cells have been characterized. Twenty-eight pigs were inoculated with the virulent CSF virus isolate Alfort 187 and slaughtered from 2 to 15 postinoculation days; 4 animals of similar background served as a control group. Immunohistochemistry, electron microscopy, and the transferase-mediated deoxyuridine triphosphate nick-end labeling method were used to detect viral antigens and apoptosis. The results showed the presence of nonpurulent meningoencephalitis, occasional microhemorrhages, and apoptosis of the lymphocytes forming the perivascular and interstitital infiltrate in swine with CSF. Macrophages appeared to display little involvement in CNS lesions. The infected cells observed at the early stage of disease were lymphocytes and microglial cells in the rostral portion of the telencephalon, with infection of these cells in other areas in the next stages. The relationship between these lesions and the presence of viral antigen varied according to the type of lesion: hemorrhages were not associated with the presence of antigen in endothelial cells, but infiltrate-cell apoptosis was temporally and spacially associated to viral infection. However, the link between viral infection and the presence of cell infiltrate was far from clear.

Antigen presentation in autoimmunity and CNS inflammation: how T lymphocytes recognize the brain

The central nervous system (CNS) is traditionally viewed as an immune privileged site in which overzealous immune cells are prevented from doing irreparable damage. It was believed that immune responses occurring within the CNS could potentially do more damage than the initial pathogenic insult itself. However, virtually every aspect of CNS tissue damage, including degeneration, tumors, infection, and of course autoimmunity, involves a significant cellular inflammatory component. While the blood-brain barrier (BBB) inhibits diffusion of hydrophilic (immune) molecules across brain capillaries, activated lymphocytes readily pass the endothelial layer of postcapillary venules without difficulty. In classic neuro-immune diseases such as multiple sclerosis or acute disseminated encephalomyelitis, it is thought that neuroantigen-reactive lymphocytes, which have escaped immune tolerance, now invade the CNS and are responsible for tissue damage, demyelination, and axonal degeneration. The developed animal model for these disorders, experimental autoimmune encephalomyelitis (EAE), reflects many aspects of the human conditions. Studies in EAE proved that auto-reactive encephalitogenic T helper (Th) cells are responsible for the onset of the disease. Th cells recognize their cognate antigen (Ag) only when presented by professional Ag-presenting cells in the context of major histocompatibility complex class II molecules. The apparent target structures of EAE immunity are myelinating oligodendrocytes, which are not capable of presenting Ag to invading encephalitogenic T cells. A compulsory third party is thus required to mediate between the attacking T cells and the myelin-expressing target. This review will discuss the recent advances in this field of research and we will discuss the journey of an auto-reactive T cell from its site of activation into perivascular spaces and further into the target tissue.

T-cell apoptosis in human glioblastoma multiforme: Implications for immunotherapy

We used immunohistochemistry and flow cytometry to assess apoptosis in human glioblastoma multiforme (GBM). Our immunohistochemical study revealed apoptosis of glioma cells expressing glial fibrillary acidic protein and of CD3(+) T cells infiltrating GBM. To quantify and phenotype the apoptotic T cells, we performed flow cytometry on lymphocytes separated from GBM. The cells were stained with annexin-V-FLUOS/propidium iodide to identify apoptosis. We found that high proportions of both the CD4(+) and CD8(+) T cells were apoptotic. In particular, we found that T cells expressing Fas ligand (Fas-L, CD95L) were eight times more vulnerable to apoptosis than those not expressing Fas-L, which suggests that the T-cell apoptosis is induced by overactivation of the T-cell receptor, possibly in the absence of appropriate costimulation. Our results have implications for the design of immunotherapies for GBM.

Failed central nervous system regeneration: a downside of immune privilege?

Immunity is required to eliminate dangerous or degenerated material and to support regeneration, but also causes significant parenchymal damage. In the eye and the brain, in which cornea and lens poorly regenerate and neurons are hardly replaceable, early transplantation experiments demonstrated remarkable tolerance to various grafts. This "immunologically privileged status" (Billingham and Boswell, 1953) may reflect evolutionary pressure to downmodulate certain actions of immune cells within particularly vulnerable tissues. As an example, tolerating certain "neurotrophic" viruses may often be a more successful strategy for survival than the elimination of all infected neurons. While several constitutive and inducible signals maintaining or re-establishing immune tolerance within the brain have been identified, it has also become evident that the resulting anti-inflammatory environment limits certain beneficial effects of neuroinflammation such as neurotrophin secretion or glutamate buffering by T-cells and the clearance of growth-inhibiting myelin or amyloid. Following spinal cord injury, the costs and benefits of neuroinflammation seem to come close because enhancing as well as suppressing innate or adaptive immunity caused amelioration and aggravation of functional regeneration in similar experiments. Evaluating such balances has also begun in (animal models of) Alzheimer's disease, central nervous system trauma, and stroke, and the appreciation of the beneficial side of neuroinflammation has caused a rethinking of the ill-defined use of immune suppressants. As dual roles for individual molecules have been recognized (Merrill and Benveniste, 1996), we are uncovering an already fine-tuned system, but the challenge remains to further support beneficial immune cascades without causing additional damage, and vice versa.

Apoptosis inhibition in T cells triggers the expression of proinflammatory cytokines--implications for the CNS

Stimulation of death receptors such as CD95 or TNF-R1 results in rapid onset of apoptosis. Here we show that inhibition of death receptor-induced apoptosis by the broad range caspase inhibitor ZVAD causes a switch from apoptotic to proinflammatory signaling. In previous studies we have reported that caspase inhibitors induce expression of various proinflammatory cytokines in CD95-stimulated primary T cells, such as TNF-alpha, IFN-gamma and GM-CSF. In this study we provide further evidence for the proinflammatory activity of CD95. Stimulation of CD95 by agonistic antibodies (7C11) resulted in expression of IL-2 in primary T cells, which was further enhanced when caspase activity was blocked by ZVAD. Moreover, CD95 triggered expression of IL-4 and IL-8 when caspase activity was inhibited, but not in the absence of ZVAD. Our findings are of significant importance for the CNS as changes in the cytokine pattern in the periphery affects the entry of various immune cells into the brain. Moreover, invading activated T cells can also directly influence the cytokine profile within the brain, triggering signaling cascades that eventually lead to neuronal cell death. The use of caspase inhibitors to prevent apoptotic cell death should be carefully evaluated in the management of systemic and CNS diseases.

Ezrin Immunoreactivity Reveals Specific Astrocyte Activation in Cerebral HIV

The actin-binding protein ezrin is associated with cellular shape changes, motility, tumor invasion, and lymphocyte activation. We have earlier shown that ezrin immunoreactivity (IR) is faintly present in normal astrocytes but increased in malignant human astrogliomas. We studied the role of ezrin in astrocyte activation, applying immunostaining on serial paraffin sections from human autopsied brain tissues (51 cases). Cerebral HIV infection was chosen as a model displaying consistent exemplary astrocyte activation. Semiquantitative ezrin-IR was compared with the common glial markers GFAP, ferritin, and HLA-DR in relation to clinical and morphologic criteria of HIV encephalopathy. In all cases with HIV infection, GFAP-, HLA-DR-, and ferritin-IR were elevated in comparison to normal brain tissues. In contrast, high ezrin-IR in HIV infection strictly correlated with additional HIV encephalopathy. HIV encephalopathy with particularly high ezrin-IR was correlated with neuronal apoptosis (TUNEL). Combined ezrin-IR and GFAP-IR thus reveals 2 distinct states of astrocytic activation. Normal ezrin-IR, when paralleled by upregulated GFAP, reflects astroglial activation not associated with neuronal apoptosis. High ezrin-IR indicates specific astrocyte stressors related to cellular damage within the central nervous system. Ezrin-IR might also provide a diagnostic tool for the classification of HIV encephalopathy.

Neurosphere-derived multipotent precursors promote neuroprotection by an immunomodulatory mechanism

In degenerative disorders of the central nervous system (CNS), transplantation of neural multipotent (stem) precursor cells (NPCs) is aimed at replacing damaged neural cells. Here we show that in CNS inflammation, NPCs are able to promote neuroprotection by maintaining undifferentiated features and exerting unexpected immune-like functions. In a mouse model of chronic CNS inflammation, systemically injected adult syngeneic NPCs use constitutively activated integrins and functional chemokine receptors to selectively enter the inflamed CNS. These undifferentiated cells survive repeated episodes of CNS inflammation by accumulating within perivascular areas where reactive astrocytes, inflamed endothelial cells and encephalitogenic T cells produce neurogenic and gliogenic regulators. In perivascular CNS areas, surviving adult NPCs induce apoptosis of blood-borne CNS-infiltrating encephalitogenic T cells, thus protecting against chronic neural tissue loss as well as disease-related disability. These results indicate that undifferentiated adult NPCs have relevant therapeutic potential in chronic inflammatory CNS disorders because they display immune-like functions that promote long-lasting neuroprotection.

An essential role for tumor necrosis factor in the formation of experimental murine Staphylococcus aureus-induced brain abscess and clearance

Tumor necrosis factor-alpha (TNF-alpha) is a central mediator of the immune response to pathogens, but may also exert neurotoxic effects, thereby contributing to immunopathology. To define the role of TNF during the course of brain abscess, TNF-deficient (TNF(0/0) mice were stereotaxically infected with Staphylococcus (S.) aureus-laden agarose beads. In comparison to 100% survival of wild type (WT) mice, TNF(0/0) mice displayed high mortality rates (54%) in the initial phase of abscess development as well as significantly increased morbidity in the course of the disease. The worse clinical outcome was due to an increased intracerebral (i.c.) bacterial load in TNF(0/0) mice as compared to WT mice. The impaired control of S. aureus was associated with reduced inductible nitric oxide synthase (iNOS) mRNA and protein expression in TNF(0/0)mice. Similarly, numbers of inflammatory leukocytes, cytokine expression of IL-6, IL-12p40, IFNgamma IL-beta mRNA, and brain edema were significantly increased in TNF(0/0)mice as compared to WT animals. In addition, resolution of i.c. infiltrates was delayed in TNF(0/0)mice correlating with reduced apoptosis of inflammatory leukocytes and formation of a fibrous abscess capsule. Collectively, these data demonstrate that TNF is of key importance for the control of S. aureus-induced brain abscess and regulates the ensuing host immune response.

Hormone regulation of microglial cell activation: relevance to multiple sclerosis

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of proteins. The role of PPARs in regulating the transcription of genes involved in glucose and lipid metabolism has been extensively characterized. Interestingly, PPARs have also been demonstrated to mediate inflammatory responses. Microglia participate in pathology associated with multiple sclerosis (MS). Upon activation, microglia produce molecules including NO and TNF-alpha that can be toxic to CNS cells including myelin-producing oligodendrocytes and neurons, which are compromised in the course of MS. Previously, we and others demonstrated that PPAR-gamma agonists including 15d-PGJ(2) are effective in the treatment of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. PPAR-gamma modulation of EAE may occur, at least in part, by inhibition of microglial cell activation. Here, we indicate that 15d-PGJ(2) is a more potent inhibitor of microglial activation than thiazolidinediones, which are currently used to treat diabetes. Furthermore, 15d-PGJ(2) acts cooperatively with 9-cis retinoic acid, the ligand for the retinoid X receptor (RXR), in inhibiting microglial cell activation. This suggests that 15d-PGJ(2) and 9-cis RA inhibit cell activation through the formation of PPAR-gamma/RXR heterodimers. Interestingly, PGA(2), which like 15d-PGJ(2) is a cyclopentenone prostaglandin, but which unlike 15d-PGJ(2) does not bind PPAR-gamma, is a potent inhibitor of microglial cell activation. Collectively, these studies suggest that 15d-PGJ(2) inhibits microglial cell activation by PPAR-gamma-dependent as well as PPAR-gamma-independent mechanisms. The studies further suggest that the PPAR-gamma agonist 15d-PGJ(2) in combination with retinoids may be effective in the treatment of MS.

At the interface of the immune system and the nervous system: how neuroinflammation modulates the fate of neural progenitors in vivo

Neural stem and progenitor cells express a variety of receptors that enable them to sense and react to signals emanating from physiological and pathophysiological conditions in the brain as well as elsewhere in the body. Many of these receptors and were first described in investigations of the immune system, particularly with respect to hematopoietic stem cells. This emerging view of neurobiology has two major implications. First, many phenomena known from the hematopoietic system may actually be generalizable to stem cells from many organ systems, reflecting the cells' progenitor-mediated regenerative potential. Second, regenerative interfaces may exist between diverse organ systems; populations of cells of neuroectodermal and hematopoietic origin may interact to play a crucial role in normal brain physiology, pathology, and repair. An understanding of the origins of signals and the neural progenitors' responses might lead to the development of effective therapeutic strategies to counterbalance acute and chronic neurodegenerative processes. Such strategies may include modifying and modulating cells with regenerative potential in subtle ways. For example, stem cells might be able to detect pathology-associated signals and be used as "interpreters" to mediate drug and other therapeutic interventions. This review has focused on the role of inflammation in brain repair. We propose that resident astroglia and blood-born cells both contribute to an inflammatory signature that is unique to each kind of neuronal degeneration or injury. These cells play a key role in coordinating the neural progenitor cell response to brain injury by exerting direct and indirect environmentally mediated influence on neural progenitor cells. We suggest that investigations of the neural progenitor-immunologic interface will provide valuable data related to the mechanisms by which endogenous and exogenous neural progenitor cells react to brain pathology, ultimately aiding in the design of more effective therapeutic applications of stem cell biology. Such improvements will include: (1) ascertaining the proper timing for implanting exogenous neural progenitor cells in relation to the administration of anti-inflammatory agents; (2) identifying what types of molecules might be administered during injury to enhance the mobilization and differentiation of endogenous and exogenous neural progenitor cells while also inhibiting the detrimental aspects of the inflammatory reaction; (3) divining clues as to which molecules may be required to change the lesioned environment in order to invite the homing of reparative neural progenitor cells.

Apoptotic Cell Death in Experimental Autoimmune Encephalomyelitis

Particularly in the vulnerable CNS with a low capacity for regeneration specialized mechanisms must be active for the fast and gentle elimination of dysregulated autoaggressive immune cells. In EAE, local apoptosis of autoimmune T-cells has been identified as a safe means for the removal of these unwanted cells. T-cell apoptosis in situ followed by phagocytic clearance of apoptotic remnants by glia assures a minimum of detrimental bystander damage to the local parenchyma and down-regulates the local inflammatory reaction. The pharmacological augmentation of local apoptosis of inflammatory effector cells might gain therapeutic importance also in human neuroimmunological diseases such as multiple sclerosis.

Nitric-oxide-dependent and independent mechanisms of protection from CNS inflammation during Th1-mediated autoimmunity: evidence from EAE in iNOS KO mice

Experimental autoimmune encephalomyelitis (EAE) disease was accelerated iNOS-deficient (KO) mice: coinciding with greatly increased numbers of Ag-specific Th1 cells in the periphery that appeared to rapidly shift from the spleen to the CNS during onset of disease symptoms. iNOS KO mice had significantly increased Th1 cells in the CNS versus wild-type mice. Apoptosis of CNS-infiltrating CD4(+) T cells was impaired in iNOS KO mice at peak of disease; consequently, these mice had more CNS-infiltrating CD4(+) T cells. Subsequently, iNOS KO mice up-regulated apoptosis of CNS-CD4(+) T cells. During chronic EAE, CNS macrophages were greatly decreased, suggesting elimination of CNS-infiltrating CD4(+) T cells and activated macrophages by iNOS-independent mechanisms. INOS is not only required for apoptosis of CNS-CD4(+) T cells but also prevents overexpansion of autoreactive Th1 cells in the periphery and the CNS.

T-cell properties determine disease site, clinical presentation, and cellular pathology of experimental autoimmune encephalomyelitis

Two distinct clinical phenotypes of experimental autoimmune encephalomyelitis are observed in BALB interferon-gamma knockout mice immunized with encephalitogenic peptides of myelin basic protein. Conventional disease, characterized by ascending weakness and paralysis, occurs with greater frequency after immunizing with a peptide comprising residues 59 to 76. Axial-rotatory disease, characterized by uncontrolled axial rotation, occurs with greater frequency in mice immunized with a peptide corresponding to exon 2 of the full length 21.5-kd protein. The two clinical phenotypes are histologically distinguishable. Conventional disease is characterized by inflammation and demyelination primarily in spinal cord, whereas axial-rotatory disease involves inflammation and demyelination of lateral medullary areas of brain. Both types have infiltrates in which neutrophils are a predominating component. By isolating T cells and transferring disease to naive recipients, we show here that the type of disease is determined entirely by the inducing T cell. Furthermore, studies using CXCR2 knockout recipients, unable to recruit neutrophils to inflammatory sites, show that although neutrophils are critical for some of these T cells to effect disease, there are also interferon-gamma-deficient T cells that induce disease in the absence of both interferon-gamma and neutrophils. These results highlight the multiplicity of T-cell-initiated effector pathways available for inflammation and demyelination.

Ibuprofen and apigenin induce apoptosis and cell cycle arrest in activated microglia

In case of injury or disease, microglia are recruited to the site of the pathology and become activated as evidenced by morphological changes and expression of pro-inflammatory cytokines. Evidence suggests that microglia proliferate by cell division to create gliosis at the site of pathological conditions such as the amyloid plaques in Alzheimer's disease and the substantia nigra of Parkinson's disease patients. The hyperactivation of microglia contributes to neurotoxicity. In the present study we tested the hypothesis that anti-inflammatory compounds modulate the progression of cell cycle and induce apoptosis of the activated cells. We investigated the effects of ibuprofen (non-steroidal anti-inflammatory drug) and apigenin (a flavonoid with anti-inflammatory and anti-proliferative properties) on the cell cycle of the murine microglial cell line BV-2. The findings indicate that apigenin-induced cell cycle arrest preferentially in the G2/M phase and ibuprofen caused S phase arrest. The binding of annexin V-FITC to the membranes of cells which indicates the apoptotic process were examined, whereas the DNA was stained with propidium iodide. Both apigenin and ibuprofen induced apoptosis significantly in early and late stages. The induction of apoptosis by ibuprofen and apigenin was confirmed using TUNEL assay, revealing that 25 microM apigenin and 250 microM ibuprofen significantly increased apoptosis in BV-2 cells. The results from the present study suggest that anti-inflammatory compounds might inhibit microglial proliferation by modulating the cell cycle progression and apoptosis.

Apoptosis and rabies virus neuroinvasion

Rabies virus (RV) causes a non-lytic infection of neurons leading to a fatal myeloencephalitis in mammals including humans. By comparing the infection of the nervous system of mice by a highly pathogenic neuroinvasive strain of RV (CVS) and by a strain of attenuated pathogenicity (PV) with restricted brain invasion, we showed that RV neuroinvasiveness results of three factors: not only neurotropic RV avoids induced neuron cell death but also "protective" T cells that migrate into the infected nervous system are killed by apoptosis and finally inflammation of the infected nervous system is limited. Our data suggest that the preservation of the neuronal network, the limitation of the inflammation and the destruction of T cells that invade the CNS in response to the infection are crucial events for RV neuroinvasion and for transmission of RV to another animal.

Gene expression analysis suggests that 1,25-dihydroxyvitamin D3reverses experimental autoimmune encephalomyelitis by stimulating inflammatory cell apoptosis

Multiple sclerosis (MS) is a debilitating autoimmune disease of the central nervous system (CNS) that develops in genetically susceptible individuals who are exposed to undefined environmental risk factors. Epidemiological, genetic, and biological evidence suggests that insufficient vitamin D may be an MS risk factor. However, little is known about how vitamin D might be protective in MS. We hypothesized that 1,25-dihydroxyvitamin D3[1,25-(OH)2D3] might regulate gene expression patterns in a manner that would resolve inflammation. To test this hypothesis, experimental autoimmune encephalomyelitis (EAE) was induced in mice, 1,25-(OH)2D3or a placebo was administered, and 6 h later, DNA microarray hybridization was performed with spinal cord RNA to analyze the gene expression patterns. At this time, clinical, histopathological, and biological studies showed that the two groups did not differ in EAE disease, but changes in several 1,25-(OH)2D3-responsive genes indicated that the 1,25-(OH)2D3had reached the CNS. Compared with normal mice, placebo-treated mice with EAE showed increased expression of many immune system genes, confirming the acute inflammation. When 1,25-(OH)2D3was administered, several genes like glial fibrillary acidic protein and eukaryotic initiation factor 2α kinase 4, whose expression increased or decreased with EAE, returned to homeostatic levels. Also, two genes with pro-apoptotic functions, calpain-2 and caspase-8-associated protein, increased significantly. A terminal deoxynucleotidyl transferase-mediated dUTP nicked end labeling study detected increased nuclear fragmentation in the 1,25-(OH)2D3-treated samples, confirming increased apoptosis. Together, these results suggest that sensitization of inflammatory cells to apoptotic signals may be one mechanism by which the 1,25-(OH)2D3resolved EAE.

T Cell Ig- and Mucin-Domain-Containing Molecule-3 (TIM-3) and TIM-1 Molecules Are Differentially Expressed on Human Th1 and Th2 Cells and in Cerebrospinal Fluid-Derived Mononuclear Cells in Multiple Sclerosis

T cell Ig- and mucin-domain-containing molecules (TIMs) comprise a recently described family of molecules expressed on T cells. TIM-3 has been shown to be expressed on murine Th1 cell clones and has been implicated in the pathogenesis of Th1-driven experimental autoimmune encephalomyelitis. In contrast, association of TIM-1 polymorphisms to Th2-related airway hyperreactivity has been suggested in mice. The TIM molecules have not been investigated in human Th1- or Th2-mediated diseases. Using real-time (TaqMan) RT-PCR, we show that human Th1 lines expressed higher TIM-3 mRNA levels, while Th2 lines demonstrated a higher expression of TIM-1. Analysis of cerebrospinal fluid mononuclear cells obtained from patients with multiple sclerosis revealed significantly higher mRNA expression of TIM-1 compared with controls. Moreover, higher TIM-1 expression was associated with clinical remissions and low expression of IFN-gamma mRNA in cerebrospinal fluid mononuclear cells. In contrast, expression of TIM-3 correlated well with high expression of IFN-gamma and TNF-alpha. These data imply the differential expression of human TIM molecules by Th1 and Th2 cells and may suggest their differential involvement in different phases of a human autoimmune disease.

Effects of Complement C5 on Apoptosis in Experimental Autoimmune Encephalomyelitis

Complement activation is involved in the initiation of Ab-mediated inflammatory demyelination in experimental autoimmune encephalomyelitis (EAE). At a sublytic dose, the C5b-9 membrane attack complex protects oligodendrocytes (OLG) from apoptosis. Using C5-deficient (C5-d) mice, we previously showed a dual role for C5: enhancement of inflammatory demyelination in acute EAE, and promotion of remyelination during recovery. In this study, we investigated the role of C5 in apoptosis in myelin-induced EAE. In acute EAE, C5-d and C5-sufficient (C5-s) mice had similar numbers of total apoptotic cells, whereas C5-s had significantly fewer than C5-d during recovery. In addition, although both groups of mice displayed TUNEL(+) OLG, there were significantly fewer in C5-s than in C5-d during both acute EAE and recovery. Gene array and immunostaining of apoptosis-related genes showed that Fas ligand expression was higher in C5-s. In C5-s mice, Fas(+) cells were also higher than in C5-d mice in acute EAE; however, these cells were significantly reduced during recovery. Together, these findings are consistent with the role of C5, possibly by forming the membrane attack complex, in limiting OLG apoptosis in EAE, thus promoting remyelination during recovery.

Astrocyte-induced regulatory T cells mitigate CNS autoimmunity

Although astrocytes presumably participate in maintaining the immune privilege of the central nervous system (CNS), the mechanisms behind their immunoregulatory properties are still largely undefined. In this study, we describe the development of regulatory T cells upon contact with astrocytes. Rat T cells pre-incubated with astrocytes completely lost the ability to proliferate in response to mitogenic stimuli. The cells were blocked in G0/G1 phase of the cell cycle, expressed less IL-2R, and produced significantly lower amounts of interferon-gamma (IFN-gamma), but not interleukin-2 (IL-2), IL-10, or tumor necrosis factor (TNF). These anergic cells completely prevented mitogen-induced growth of normal T lymphocytes, as well as CNS antigen-driven proliferation of autoreactive T cells. The suppressive activity resided in both CD4+ and CD8+ T-cell compartments. Heat-sensitive soluble T-cell factors, not including transforming growth factor-beta (TGF-beta) or IL-10, were solely responsible for the observed suppression, as well as for the transfer of suppressive activity to normal T cells. The administration of astrocyte-induced regulatory T cells markedly alleviated CNS inflammation and clinical symptoms of CNS autoimmunity in rats with experimental allergic encephalomyelitis. Finally, the cells with suppressive properties were readily generated from human lymphocytes after contact with astrocytes. Taken together, these data indicate that astrocyte-induced regulatory T cells might represent an important mechanism for self-limitation of excessive inflammation in the brain.

CD95 polymorphisms are associated with susceptibility to MS in women

CD95/CD95L interaction results in activation-induced apoptosis thereby regulating clonal expansion of T cells outside the thymus. Genetic defects in this system result in autoimmune lymphoproliferation in mice and men. CD95-induced cell death may be defective in MS. We studied the association of CD95 and CD95L polymorphisms with MS in 221 unique patients representing 79% ascertainment in Olmsted County, MN, and 442 gender-, age- and ethnicity-matched controls. Being a homozygote for the G allele of CD95 5'(-670)*A-->G SNP (p=0.034; OR: 1.59, 95% CI: 1.06-2.38) and for the C allele of CD95 E7(74)*C-->T SNP (p=0.007; OR: 1.73, 95% CI: 1.17-2.56) increased susceptibility to MS exclusively in women. There was strong but incomplete linkage disequilibrium between the two markers (p<0.001; D'=0.546). Homozygosity for 5'(-670)*A or E7(74)*C explained 28% of risk of MS in women but 0% of the risk in men in Olmsted County, MN. Our results agree with the previously published studies and highlight that the association of the polymorphisms is restricted to women with MS. We did not find an association between CD95L and susceptibility to MS nor CD95 or CD95L and age of onset, disease course and disease severity.

Phagocytosis of apoptotic inflammatory cells by microglia and its therapeutic implications: termination of CNS autoimmune inflammation and modulation by interferon-beta

Apoptosis of autoaggressive T-cells in the CNS is an effective, noninflammatory mechanism for the resolution of T-cell infiltrates, contributing to clinical recovery in T-cell-mediated neuroinflammatory diseases. The clearance of apoptotic leukocytes by tissue-specific phagocytes is critical in the resolution of the inflammatory infiltrate and leads to a profound downregulation of phagocyte immune functions. Adult human microglia from surgically removed normal brain tissue was used in a standardized, light-microscopic in vitro phagocytosis assay of apoptotic autologous peripheral blood-derived mononuclear cells (MNCs). Microglia from five different patients had a high capacity for the uptake of apoptotic MNCs in contrast to nonapoptotic target cells with the phagocytosis rate for nonapoptotic MNCs amounting to only 61.6% of the apoptotic MNCs. A newly described phosphatidylserine receptor, critical in the phagocytosis of apoptotic cells by macrophages, is also expressed at similar levels on human microglia. The effects of the therapeutically used immunomodulatory agent interferon-beta (IFNbeta) were investigated using Lewis rat microglia and apoptotic, encephalitogenic, myelin basic protein-specific autologous T-cells. Also, rat microglia had a high capacity to phagocytose apoptotic T-cells specifically. IFNbeta increased the phagocytosis of apoptotic T-cells to 36.8% above the untreated controls. The enhanced phagocytic activity was selective for apoptotic T-cells and was not mediated by increased IL-10 secretion. Apoptotic inflammatory cells may be efficiently and rapidly removed by microglial cells in the autoimmune-inflamed human CNS. The in vitro increase of phagocytosis by IFNbeta merits further investigations whether this mechanism could also be therapeutically exploited.

Anti-inflammatory properties of pro-inflammatory interferon-gamma

Production of interferon-gamma (IFNgamma) in response to infection is a hallmark of innate and adaptive immunity. In addition to the pivotal role of IFNgamma in host defense, its excessive release has been associated with the pathogenesis of chronic inflammatory and autoimmune diseases. In fact, knockout models reveal that IFNgamma plays a key role in mediating a number of pathological processes related to chronic immune activation. On the other hand, evidence has been accumulated in recent years that supports the concept of a dual role of IFNgamma in inflammation. Here, we review anti-inflammatory aspects of IFNgamma in the regulatory network of cytokine biology. These include induction of anti-inflammatory molecules such as interleukin (IL)-1 receptor antagonist (IL-1Ra) and IL-18 binding protein (IL-18BP), modulation of pro-inflammatory cytokine production, activation of apoptosis, and interference with the signal transduction machinery by induction of suppressors of cytokine signaling (SOCS).

Microglia-mediated nitric oxide cytotoxicity of T cells following amyloid beta-peptide presentation to Th1 cells

Alzheimer's disease is marked by progressive accumulation of amyloid beta-peptide (Abeta) which appears to trigger neurotoxic and inflammatory cascades. Substantial activation of microglia as part of a local innate immune response is prominent at sites of Abeta plaques in the CNS. However, the role of activated microglia as Abeta APCs and the induction of adaptive immune responses has not been investigated. We have used primary microglial cultures to characterize Abeta-Ag presentation and interaction with Abeta-specific T cells. We found that IFN-gamma-treated microglia serve as efficient Abeta APCs of both Abeta1-40 and Abeta1-42, mediating CD86-dependent proliferation of Abeta-reactive T cells. When cultured with Th1 and Th2 subsets of Abeta-reactive T cells, Th1, but not Th2, cells, underwent apoptosis after stimulation, which was accompanied by increased levels of IFN-gamma, NO, and caspase-3. T cell apoptosis was prevented in the presence of an inducible NO synthase type 2 inhibitor. Microglia-mediated proliferation of Abeta-reactive Th2 cells was associated with expression of the Th2 cytokines IL-4 and IL-10, which counterbalanced the toxic levels of NO induced by Abeta. Our results demonstrate NO-dependent apoptosis of T cells by Abeta-stimulated microglia which may enhance CNS innate immune responses and neurotoxicity in Alzheimer's disease. Secretion of NO by stimulated microglia may underlie a more general pathway of T cell death in the CNS seen in neurodegenerative diseases. Furthermore, Th2 type T cell responses may have a beneficial effect on this process by down-regulation of NO and the proinflammatory environment.

Vulnerability of human neurons to T cell-mediated cytotoxicity

Axonal and neuronal loss occurs in inflammatory diseases of the CNS such as multiple sclerosis. The cause of the loss remains unclear. We report that polyclonally activated T cells align along axons and soma of cultured human neurons leading to substantial neuronal death. This occurs in an allogeneic and syngeneic manner in the absence of added Ag, requires T cells to be activated, and is mediated through cell contact-dependent mechanisms involving FasL, LFA-1, and CD40 but not MHC class I. Activated CD4(+) and CD8(+) T cell subsets are equally neuronal cytotoxic. In contrast to neurons, other CNS cell types (oligodendrocytes and astrocytes) are not killed by T cells. These results demonstrate for the first time the high and selective vulnerability of human neurons to T cells, and suggest that when enough activated T cells accumulate in the CNS, neuronal cytotoxicity can result through Ag-independent non-MHC class I mechanisms.

MCP-1 (CCL2) protects human neurons and astrocytes from NMDA or HIV-tat-induced apoptosis

Acquired immunodeficiency syndrome (AIDS)-associated dementia is often characterized by chronic inflammation, with infected macrophage infiltration of the CNS resulting in the production of human immunodeficiency virus type 1 (HIV-1) products, including tat, and neurotoxins that contribute to neuronal loss. In addition to their established role in leukocyte recruitment and activation, we identified an additional role for chemokines in the CNS. Monocyte chemoattractant protein-1 (MCP-1 or CCL2) and regulated upon activation normal T cell expressed and secreted (RANTES) were found to protect mixed cultures of human neurons and astrocytes from tat or NMDA-induced apoptosis. Neuronal and astrocytic apoptosis in these cultures was significantly inhibited by co-treatment with MCP-1 or RANTES but not IP-10. The protective effect of RANTES was blocked by antibodies to MCP-1, indicating that RANTES protection is mediated by the induction of MCP-1. The NMDA blocker, MK801, also abolished the toxic effects of both tat and NMDA. Tat or NMDA treatment of mixed cultures for 24 h resulted in increased extracellular glutamate ([Glu]e) and NMDA receptor 1 (NMDAR1) expression, potential contributors to apoptosis. Co-treatment with MCP-1 inhibited tat and NMDA-induced increases in [Glu]e and NMDAR1, and also reduced the levels and number of neurons containing intracellular tat. These data indicate that MCP-1 may play a novel role as a protective agent against the toxic effects of glutamate and tat.

Regulatory role of p53 in experimental autoimmune encephalomyelitis

The role of p53, a pro-apoptotic protein, in experimental autoimmune encephalomyelitis (EAE) was investigated using p53-deficient C57BL/6J mice. p53-deficient mice immunised with myelin oligodendrocyte glycoprotein (MOG) exhibited a more severe clinical course of EAE with more severe inflammation in the central nervous system (CNS) compared to wild-type littermates. While T and B cell responses of p53-deficient mice to MOG were comparable to those of wild-type littermates, significantly higher production of IL-6, granulocyte macrophage colony-stimulating factor and IL-10 was observed in lymphocytes exposed to MOG from p53-deficient mice than those from wild-type littermates. Furthermore, a flow cytometric analysis of Annexin V staining showed that apoptosis of CNS-infiltrating cells was less in p53-deficient mice with EAE compared to wild-type littermates. These results suggest that p53 may be involved in the regulatory process of EAE through the control of cytokine production and/or the apoptotic elimination of inflammatory cells.

Effects of polyclonal immunoglobulins and other immunomodulatory agents on microglial phagocytosis of apoptotic inflammatory T-cells

T-cell apoptosis in the CNS is an effective mechanism for the noninflammatory resolution of autoimmune T-cell infiltrates. Ingestion of apoptotic leukocytes by microglia results in an efficient clearance of the inflammatory infiltrate, followed by a profound downregulation of proinflammatory phagocyte immune functions. The effects of different immunomodulatory agents on Lewis rat microglial phagocytosis of apoptotic autologous thymocytes or myelin-basic protein (MBP)-specific, encephalitogenic T-cells were investigated using a standardized, light microscopical in vitro phagocytosis assay. Pretreatment of microglia with polyclonal 7S immunoglobulins (IVIg) decreased the phagocytosis of apoptotic thymocytes by 38.2% (p<0.0001). Also, immunoglobulin F(ab')(2) fragments decreased microglial phagocytosis, suggesting an Fc receptor-independent mechanism. Similar results were obtained using MBP-specific T-cells. Pretreatment of microglia with IFN-gamma increased the phagocytosis of apoptotic cells by 65.4%, which was to a large extent counteracted by IVIg. Glatiramer acetate (GLAT) did not exert an effect on microglial phagocytosis, while methylprednisolone (MP) induced microglial apoptosis in vitro. These results indicate that IVIg has a high potential to inhibit microglial phagocytosis of apoptotic inflammatory T-cells even under proinflammatory conditions and extend our view of the complex immunomodulatory effects of IVIg.

Effects of peroxisome proliferator-activated receptor-gamma agonists on central nervous system inflammation

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a critical role in glucose and lipid metabolism. More recently, PPAR-gamma ligands have been reported to inhibit the expression of proinflammatory molecules by monocytes/macrophages. Of relevance to CNS disease is that PPAR-gamma agonists have been demonstrated to have similar effects on microglia. PPAR-gamma agonists also ameliorate experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. This Mini-Review summarizes the effects of PPAR-gamma agonists in mediating immune responses and the potential of these agonists in the treatment of inflammatory disorders of the CNS.

Microglia as liaisons between the immune and central nervous systems: functional implications for multiple sclerosis

Multiple sclerosis is a chronic demyelinating inflammatory disease of the central nervous system (CNS). As the tissue macrophage of the CNS, microglia have the potential to regulate and be regulated by cells of the CNS and by CNS-infiltrating immune cells. The exquisite sensitivity of microglia to these signals, coupled with their ability to develop a broad range of effector functions, allows the CNS to tailor microglial function for specific physiological needs. However, the great plasticity of microglial responses can also predispose these cells to amplify disproportionately the irrelevant or dysfunctional signals provided by either the CNS or immune systems. The consequences of such an event could be the conversion of self-limiting inflammatory responses into chronic neurodegeneration and may explain in part the heterogeneous nature of multiple sclerosis.

Prevention of autoimmune attack and disease progression in multiple sclerosis: current therapies and future prospects

Multiple sclerosis (MS) is an important cause of progressive neurological disability, typically commencing in early adulthood. There is a need for safe and effective therapy to prevent the progressive central nervous system (CNS) damage and resultant disability that characterize the disease course. Increasing evidence supports a chronic autoimmune basis for CNS damage in MS. In the present study, we review current concepts of autoimmune pathogenesis in MS, assess current therapies aimed at countering autoimmune attack and discuss potential therapeutic strategies. Among currently available therapies, beta-interferon and glatiramer acetate have a modest effect on reducing relapses and slowing the accumulation of disability in relapsing-remitting MS. Beta-interferon is of doubtful efficacy in secondary progressive MS and appears to aggravate primary progressive MS, possibly by increasing antibody-mediated CNS damage through inhibition of B-cell apoptosis. Mitoxantrone may reduce relapses and slow disability progression in relapsing-remitting and secondary progressive MS, but its use is limited by the risk of cardiomyopathy. There are currently no effective treatments for primary progressive MS. Many therapies that are effective in the animal model, experimental autoimmune encephalomyelitis (EAE), are either ineffective in MS or--in the case of gamma-interferon, lenercept and altered peptide ligands--actually make MS worse. This discrepancy may be explained by the occurrence in MS of defects in immunoregulatory mechanisms, the integrity of which is essential for the efficacy of these treatments in EAE. It is likely that the development of safe, effective therapy for MS will depend on a better understanding of immunoregulatory defects in MS.

The role of macrophage/microglia and astrocytes in the pathogenesis of three neurologic disorders: HIV-associated dementia, Alzheimer disease, and multiple sclerosis

Macrophage/microglia (M phi) are the principal immune cells in the central nervous system (CNS) concomitant with inflammatory brain disease and play a significant role in the host defense against invading microorganisms. Astrocytes, as a significant component of the blood-brain barrier, behave as one of the immune effector cells in the CNS as well. However, both cell types may play a dual role, amplifying the effects of inflammation and mediating cellular damage as well as protecting the CNS. Interactions of the immune system, M phi, and astrocytes result in altered production of neurotoxins and neurotrophins by these cells. These effects alter the neuronal structure and function during pathogenesis of HIV-1-associated dementia (HAD), Alzheimer disease (AD), and multiple sclerosis (MS). HAD primarily involves subcortical gray matter, and both HAD and MS affect sub-cortical white matter. AD is a cortical disease. The process of M phi and astrocytes activation leading to neurotoxicity share similarities among the three diseases. Human Immunodeficiency Virus (HIV)-1-infected M phi are involved in the pathogenesis of HAD and produce toxic molecules including cytokines, chemokines, and nitric oxide (NO). In AD, M phis produce these molecules and are activated by beta-amyloid proteins and related oligopeptides. Demyelination in MS involves M phi that become lipid laden, spurred by several possible antigens. In these three diseases, cytokine/chemokine communications between M phi and astrocytes occur and are involved in the balance of protective and destructive actions by these cells. This review describes the role of M phi and astrocytes in the pathogenesis of these three progressive neurological diseases, examining both beneficent and deleterious effects in each disease.

Phagocytotic removal of apoptotic, inflammatory lymphocytes in the central nervous system by microglia and its functional implications

Apoptotic cell death of inflammatory T cells is an established mechanism to terminate an autoimmune inflammatory response in the rodent and human central nervous system (CNS). The efficient clearance of apoptotic cells protects the tissue from leakage of potentially harmful substances from secondary necrotic cells. As the resident phagocyte, the microglial cell is the primary candidate for the clearance of apoptotic lymphocytes. Furthermore, the phagocytosis of apoptotic cells is accompanied by a spectrum of anti-inflammatory effects. In this review, we focus on the mechanisms for removal of apoptotic inflammatory cells by microglia in the central nervous system and their functional consequences.

Induction of macrophage-derived chemokine/CCL22 expression in experimental autoimmune encephalomyelitis and cultured microglia: implications for disease regulation

Macrophage-derived chemokine (MDC/CCL22) and its receptor CCR4 have been implicated in chronic inflammatory processes and in the homing of monocytes, Th2 cells and regulatory T-cell subsets. Here, we demonstrate that MDC and CCR4 mRNAs are expressed in the central nervous system (CNS) of mice developing relapsing-remitting and chronic-relapsing forms of experimental autoimmune encephalomyelitis (EAE). By immunohistochemistry, we show that MDC is produced by CNS-infiltrating leukocytes and intraparenchymal microglia, whereas CCR4 is expressed on some invading leukocytes. Upon in vitro activation, mouse microglia express MDC transcripts and secrete bioactive MDC that induces chemotaxis of Th2, but not Th1 cells. We suggest that MDC produced by microglia could regulate Th1-mediated CNS inflammation by facilitating the homing of Th2 and, possibly, regulatory T cells into the lesion site.

An examination of the Apo-1/Fas promoter Mva I polymorphism in Japanese patients with multiple sclerosis

BACKGROUND

The Apo-1/Fas (CD95) molecule is an apoptosis-signaling cell surface receptor belonging to the tumor necrosis factor (TNF) receptor family. Both Fas and Fas ligand (FasL) are expressed in activated mature T cells, and prolonged cell activation induces susceptibility to Fas-mediated apoptosis. The Apo-1/Fas gene is located in a chromosomal region that shows linkage in multiple sclerosis (MS) genome screens, and studies indicate that there is aberrant expression of the Apo-1/Fas molecule in MS.

METHODS

Mva I polymorphism on the Apo-1/Fas promoter gene was detected by PCR-RFLP from the DNA of 114 Japanese patients with conventional MS and 121 healthy controls. We investigated the association of the Mva I polymorphism in Japanese MS patients using a case-control association study design.

RESULTS

We found no evidence that the polymorphism contributes to susceptibility to MS. Furthermore, there was no association between Apo-1/Fas gene polymorphisms and clinical course (relapsing-remitting course or secondary-progressive course). No significant association was observed between Apo-1/Fas gene polymorphisms and the age at disease onset.

CONCLUSIONS

Overall, our findings suggest that Apo-1/Fas promoter gene polymorphisms are not conclusively related to susceptibility to MS or the clinical characteristics of Japanese patients with MS.

Inhibitory effect of apoptosis in human astrocytes CCF-STTG1 cells by lemon oil

The effects of lemon pure essential oils on the heat shock-induced apoptosis in human astrocytes cell line CCF-STTG1 were examined. In previous studies, heat shock has been reported to induce the apoptosis or programmed cell death through the activation of caspase-3. Treatment of heat shock on CCF-STTG1 cells markedly induced apoptotic cell death as determined by flow cytometry. Interestingly, pre-treatment with lemon pure essential oils on CCF-STTG1 cells inhibited the heat shock-induced apoptosis. Lemon oil also inhibited the heat shock-induced apoptosis in primary cultured rat astrocytes. To determine whether lemon oil inhibits the heat shock-induced activation of the apoptotic proteases, activation of caspase-3 was assessed by Western blotting. DNA fragmentation, giemsa staining, and heat shock-induced activation of caspase-3 were blocked by lemon pure essential oil, which is consistent with flow cytometry. Poly-ADP-ribose polymerase (PARP), the cysteine protease substrate, was fragmented as a consequence of apoptosis by heat shock. Lemon oil inhibited the PARP fragmentation. These results suggest that lemon pure essential oils may modulate the apoptosis through the activation of the interleukin-1 beta -converting enzyme-like caspases.

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.