Immune function of microglia

Selective neuronal vulnerability to oxidative stress in the brain

Oxidative stress (OS), caused by the imbalance between the generation and detoxification of reactive oxygen and nitrogen species (ROS/RNS), plays an important role in brain aging, neurodegenerative diseases, and other related adverse conditions, such as ischemia. While ROS/RNS serve as signaling molecules at physiological levels, an excessive amount of these molecules leads to oxidative modification and, therefore, dysfunction of proteins, nucleic acids, and lipids. The response of neurons to this pervasive stress, however, is not uniform in the brain. While many brain neurons can cope with a rise in OS, there are select populations of neurons in the brain that are vulnerable. Because of their selective vulnerability, these neurons are usually the first to exhibit functional decline and cell death during normal aging, or in age-associated neurodegenerative diseases, such as Alzheimer's disease. Understanding the molecular and cellular mechanisms of selective neuronal vulnerability (SNV) to OS is important in the development of future intervention approaches to protect such vulnerable neurons from the stresses of the aging process and the pathological states that lead to neurodegeneration. In this review, the currently known molecular and cellular factors that contribute to SNV to OS are summarized. Included among the major underlying factors are high intrinsic OS, high demand for ROS/RNS-based signaling, low ATP production, mitochondrial dysfunction, and high inflammatory response in vulnerable neurons. The contribution to the selective vulnerability of neurons to OS by other intrinsic or extrinsic factors, such as deficient DNA damage repair, low calcium-buffering capacity, and glutamate excitotoxicity, are also discussed.

Role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination of the CNS

As the resident innate immune cells of the central nervous system (CNS), microglia fulfil a critical role in maintaining tissue homeostasis and in directing and eliciting molecular responses to CNS damage. The human disease Multiple Sclerosis and animal models of inflammatory demyelination are characterized by a complex interplay between degenerative and regenerative processes, many of which are regulated and mediated by microglia. Cellular communication between microglia and other neural and immune cells is controlled to a large extent by the activity of cytokines. Here we review the role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination, highlighting their importance in potentiating cell damage, promoting neuroprotection and enhancing cellular repair in a context-dependent manner.

A confocal and electron microscopic comparison of interferon beta-induced changes in vesicular stomatitis virus infection of neuroblastoma and nonneuronal cells

Vesicular stomatitis virus (VSV) replication is highly sensitive to interferon (IFN)-induced antiviral responses. Pretreatment of sensitive cultured cells with IFNbeta results in a 10(4)-fold reduction in the release of infectious VSV particles. However, differences exist between the mechanisms of reduced infectious particle titers in cell lines of neuroblastoma and nonneuronal lineage. In L929-fibroblast-derived cells, using immunofluorescence confocal microscopy, infection under control conditions reveals the accumulation of VSV matrix, phosphoprotein (P), and nucleocapsid (N) proteins over time, with induced cellular morphological changes indicative of cytopathic effects (CPEs). Upon observing L929 cells that had been pretreated with IFNbeta, neither detectable VSV proteins nor CPEs were seen, consistent with type I IFN antiviral protection. When using the same techniques to observe VSV infections of NB41A3 cells, a neuroblastoma cell line, aside from similar viral progression in the untreated control cells, IFNbeta-treated cells illustrated a severely attenuated VSV infection. Attenuated VSV progression was observed through detection of VSV matrix, P, and N proteins in isolated cells during the first 8 h of infection. However, by 18-24 h postinfection all neuroblastomas had succumbed to the viral infection. Finally, upon closer inspection of IFNbeta-treated NB41A3 cells, no detectable changes in VSV protein localization were identified compared with untreated, virally infected neuroblastomas. Next, to extend our study to test our hypothesis that virion assembly is compromised within type I IFN-treated neuroblastoma cells, we employed electron microscopy to examine our experimental conditions at the ultrastructural level. Using VSV-specific antibodies in conjunction with immuno-gold reagents, we observed several similarities between the two cell lines, such as identification of viroplasmic regions containing VSV N and P proteins and signs of stress-induced CPEs of VSV-infected cells, which had either been mock-treated or pretreated with interferon-beta (IFNbeta). One difference we observed between nonneuronal and neuroblastoma cells was more numerous actively budding VSV virions across untreated L929 plasma membranes compared with untreated NB41A3 cells. Additionally, IFNbeta-treated, VSV-infected L929 cells exhibited neither cytoplasmic viroplasm nor viral protein expression. In contrast, IFNbeta-treated, VSV-infected NB41A3 cells showed evidence of VSV infection at a very low frequency as well as small-scale viroplasmic regions that colocalized with viral N and P proteins. Finally, we observed that VSV viral particles harvested from untreated VSV-infected L929 and NB41A3 cells were statistically similar in size and shape. A portion of VSV virions from IFNbeta-treated, virally infected NB41A3 cells were similar in size and shape to virus from both untreated cell types. However, among the sampling of virions, pleomorphic viral particles that were identified from IFNbeta-treated, VSV-infected NB41A3 cells were different enough to suggest a misassembly mechanism as part of the IFNbeta antiviral state in neuroblastoma cells.

Microglial action in glioma: a boon turns bane

Microglia has the potential to shape the neuroimmune defense with vast array of functional attributes. The cells prime infiltrated lymphocytes to retain their effector functions, play crucial role in controlling microenvironmental milieu and significantly participate in glioma. Reports demonstrate microglial accumulation in glioma and predict their assistance in glioma growth and spreading. Clarification of the 'double-edged' appearance of microglia is necessary to unfold its role in glioma biology. In this article the interpretation of microglial activities has been attempted to reveal their actual function in glioma. Contrary to the trendy acceptance of its glioma promoting infamy, accumulated evidences make an effort to view the state of affairs in favor of the cell. Critical scrutiny indicates that microglial immune assaults are intended to demolish the neoplastic cells in brain. But the weaponry of microglia has been tactically utilized by glioma in their favor as the survival strategy. Hence the defender appears as enemy in advanced glioma.

Astrocyte-restricted ablation of interleukin-17-induced Act1-mediated signaling ameliorates autoimmune encephalomyelitis

Interleukin-17 (IL-17) secreted by T helper 17 (Th17) cells is essential in the development of experimental autoimmune encephalomyelitis (EAE). However, it remains unclear how IL-17-mediated signaling in different cellular compartments participates in the central nervous system (CNS) inflammatory process. We examined CNS inflammation in mice with specific deletion of Act1, a critical component required for IL-17 signaling, in endothelial cells, macrophages and microglia, and neuroectoderm (neurons, astrocytes, and oligodendrocytes). In Act1-deficient mice, Th17 cells showed normal infiltration into the CNS but failed to recruit lymphocytes, neutrophils, and macrophages. Act1 deficiency in endothelial cells or in macrophages and microglia did not substantially impact the development of EAE. However, targeted Act1 deficiency in neuroectoderm-derived CNS-resident cells resulted in markedly reduced severity in EAE. Specifically, Act1-deficient astrocytes showed impaired IL-17-mediated inflammatory gene induction. Thus, astroctyes are critical in IL-17-Act1-mediated leukocyte recruitment during autoimmune-induced inflammation of the CNS.

Dexamethasone prevents LPS-induced microglial activation and astroglial impairment in an experimental bacterial meningitis co-culture model

We analyzed the effect of dexamethasone on gram-negative bacteria derived lipopolysaccharide (LPS) induced inflammation in astroglial/microglial co-cultures. At the cellular level the microglial phenotype converted to an activated type after LPS incubation. Furthermore, LPS compromised functional astroglial properties like membrane resting potential, intracellular coupling and connexin 43 (Cx43) expression. This change in Cx43 expression was not due to a downregulation of Cx43 mRNA expression. Morphological and functional changes were accompanied by a time-dependent release of inflammation related cytokines. Co-incubation of dexamethasone with LPS prevented these LPS-induced changes within our glial co-culture model. The ability of dexamethasone to reconstitute astrocytic properties and to decrease microglial activation in vitro could be one possible explanation for the beneficial effects of dexamethasone in the treatment of acute bacterial meningitis in vivo.

Intravitreous delivery of the corticosteroid fluocinolone acetonide attenuates retinal degeneration in S334ter-4 rats

PURPOSE

To study the neuroprotective properties of low-dose, sustained-release intravitreous fluocinolone acetonide (FA) in transgenic S334ter-4 rats.

METHODS

S334ter-4 rats aged 4 weeks were divided into four groups: 0.5 microg/d FA-loaded intravitreous drug delivery implant (IDDI); 0.2 microg/d FA-loaded IDDI; inactive IDDI; and unoperated controls. Electroretinography (ERG) was performed before surgery and every 2 weeks after surgery for 8 weeks. When the rats were 12 weeks of age, outer nuclear layer (ONL) and inner nuclear layer (INL) thicknesses were measured. Microglial cell counts were obtained from retinal wholemounts labeled for Iba-1.

RESULTS

At the end of the study, unoperated and inactive IDDI-implanted rats demonstrated 50% to 60% reductions in ERG amplitudes compared with those recorded at 4 weeks (P < 0.001 for both groups). FA 0.2-microg/d animals demonstrated 15% amplitude attenuation, while FA 0.5-microg/d animals showed 30% reduction. ONL thickness in FA 0.2-microg/d-treated eyes was 25.8% +/- 2.3% higher than in control group eyes (P < 0.001) and 30.0% +/- 2.1% higher than in inactive IDDI-implanted eyes (P < 0.001). In FA 0.5-microg/d-treated eyes, ONL thickness was 22.4% +/- 2.8% higher than in control group eyes (P < 0.001) and 22.3% +/- 3.7% higher than in inactive IDDI-implanted eyes (P < 0.01). No statistically significant difference was observed between the two control groups. No statistically significant difference between the two FA-treated groups was found. FA-treated groups demonstrated significantly fewer activated microglial cells than control groups.

CONCLUSIONS

Chronic intravitreous infusion of FA preserves ONL cell morphology and ERG a- and b-wave amplitudes and reduces retinal neuroinflammation in S334ter rats. Based on these findings, the synthetic corticosteroid FA may promise a therapeutic role in patients with retinal degeneration.

Mechanism of the anti-inflammatory effect Of 17beta-estradiol on brain following trauma-hemorrhage

Although 17beta-estradiol (E2) is reported to improve the inflammatory response after trauma-hemorrhage (T-H), it remains unknown whether E2 plays any role in the central nervous system after T-H. Microglial cells, resident central macrophages, are thought to play a central role in exacerbating cell-mediated inflammation. We hypothesized that T-H up-regulates microglial cell-mediated inflammatory response in the brain, and E2 produces central anti-inflammatory effects via negative regulation of microglial cells. Male Sprague-Dawley rats were subjected to sham operation (cannulation plus laparotomy) or T-H (midline laparotomy; mean blood pressure, 35 +/- 5 mmHg for 90 min followed by resuscitation) and immediately killed after resuscitation. Rats received vehicle or E2 (1 mg/kg body weight i.v.) at the onset of resuscitation. In other experiments, minocycline (40 mg/kg body weight i.p.), microglia inhibitor, was administered 1 h before T-H to prevent inflammatory response in the microglia after T-H. The plasma and hypothalamic tumor necrosis factor (TNF-alpha) levels were increased, along with the activation of microglial cells in T-H rats compared with shams. Furthermore, T-H increased microglial TNF-alpha productive capacity in vitro. 17beta administration after T-H prevented these inflammatory responses. In rats pretreated with minocycline, decreased microglial TNF-alpha production and hypothalamic TNF-alpha levels were observed, but plasma TNF-alpha levels were not altered after T-H. Thus, T-H induces inflammatory responses even in the hypothalamus, and E2 seems to be a useful adjunct for down-regulating microglial cell-mediated inflammatory response after T-H.

Neuroinflammation in Parkinson's disease: its role in neuronal death and implications for therapeutic intervention

Parkinson's disease (PD) is the second most common neurodegenerative disease, after Alzheimer's disease. The potential causes of PD remain uncertain, but recent studies suggest neuroinflammation and microglia activation play important roles in PD pathogenesis. Major unanswered questions include whether protein aggregates cause the selective loss of dopaminergic neurons in the substantia nigra that underlies the clinical symptoms and whether neuroinflammation is a consequence or a cause of nigral cell loss. Within the microenvironment of the brain, glial cells play a critical role in homeostatic mechanisms that promote neuronal survival. Microglia have a specialized immune surveillance role and mediate innate immune responses to invading pathogens by secreting a myriad of factors that include, cytokines, chemokines, prostaglandins, reactive oxygen and nitrogen species, and growth factors. Some of these factors have neuroprotective and trophic activities and aid in brain repair processes; while others enhance oxidative stress and trigger apoptotic cascades in neurons. Therefore, pro- and anti-inflammatory responses must be in balance to prevent the potential detrimental effects of prolonged or unregulated inflammation-induced oxidative stress on vulnerable neuronal populations. In this review, we discuss potential triggers of neuroinflammation and review the strongest direct evidence that chronic neuroinflammation may have a more important role to play in PD versus other neurodegenerative diseases. Alternatively, we propose that genetic deficiency is not the only way to reduce protective factors in the brain which may function to keep microglial responses in check or regulate the sensitivity of DA neurons. If chronic inflammation can be shown to decrease the levels of neuroprotective factors in the midbrain, in essence genetic haploinsufficiency of protective factors such as Parkin or RGS10 may result from purely environmental triggers (aging, chronic systemic disease, etc.), increasing the vulnerability to inflammation-induced nigral DA neuron death and predisposing an individual to development of PD. Lastly, we review the latest epidemiological and experimental evidence supporting the potential use of anti-inflammatory and immunomodulatory drugs as neuroprotective agents to delay the progressive nigrostriatal degeneration that leads to motor dysfunction in PD.

Interleukin-10/Ceftriaxone prevents E. coli-induced delays in sensorimotor task learning and spatial memory in neonatal and adult Sprague-Dawley rats

Intrauterine infection during pregnancy is associated with early activation of the fetal immune system and poor neurodevelopmental outcomes. Immune activation can lead to alterations in sensorimotor skills, changes in learning and memory and neural plasticity. Both interleukin-10 (IL-10) and Ceftriaxone have been shown to decrease immune system activation and increase memory capacity, respectively. Using a rodent model of intrauterine infection, we examined sensorimotor development in pups, learning and memory, via the Morris water maze, and long-term potentiation in adult rats. Pregnant rats at gestational day 17 were inoculated with 1 x 10(5) colony forming units of Escherichia coli (E. coli) or saline. Animals in the treatment group received IL-10/Ceftriaxone for 3 days following E. coli administration. Intrauterine infection delayed surface righting, negative geotaxis, startle response and eye opening. Treatment with IL-10/Ceftriaxone reduced the delay in these tests. Intrauterine infection impaired performance in the probe trial in the Morris water maze (saline 25.13+/-1.01; E. coli 20.75+/-1.01; E. coli+IL-10/Ceftriaxone 20.2+/-1.62) and reduced the induction of long-term potentiation (saline 141.5+/-4.3; E. coli 128.7+/-3.9; E. coli+IL-10/Ceftriaxone 140.0+/-10). In summary, the results of this study indicate that E. coli induced intrauterine infection delays sensorimotor and learning and memory, while IL-10/Ceftriaxone rescues some of these behaviors. These delays were also accompanied by an increase in interleukin-1beta levels, which indicates immune activation. IL-10/Ceftriaxone prevents these delays as well as decreases E. coli-induced interleukin-1beta activation and may offer a window of time in which suitable treatment could be administered.

Brucella abortus induces the secretion of proinflammatory mediators from glial cells leading to astrocyte apoptosis

Central nervous system (CNS) invasion by bacteria of the genus Brucella results in an inflammatory disorder called neurobrucellosis. In this study we present in vivo and in vitro evidence that B. abortus and its lipoproteins activate the innate immunity of the CNS, eliciting an inflammatory response that leads to astrogliosis, a characteristic feature of neurobrucellosis. Intracranial injection of heat-killed B. abortus (HKBA) or outer membrane protein 19 (Omp19), a B. abortus lipoprotein model, induced astrogliosis in mouse striatum. Moreover, infection of astrocytes and microglia with B. abortus induced the secretion of interleukin (IL)-6, IL-1beta, tumor necrosis factor (TNF)-alpha, macrophage chemoattractant protein-1, and KC (CXCL1). HKBA also induced these inflammatory mediators, suggesting the involvement of a structural component of the bacterium. Accordingly, Omp19 induced the same cytokine and chemokine secretion pattern. B. abortus infection induced astrocyte, but not microglia, apoptosis. Indeed, HKBA and Omp19 elicited not only astrocyte apoptosis but also proliferation, two features observed during astrogliosis. Apoptosis induced by HKBA and L-Omp19 was completely suppressed in cells of TNF receptor p55-/- mice or when the general caspase inhibitor Z-VAD-FMK was added to cultures. Hence, TNF-alpha signaling via TNF receptor (TNFR) 1 through the coupling of caspases determines apoptosis. Our results provide proof of the principle that Brucella lipoproteins could be key virulence factors in neurobrucellosis and that astrogliosis might contribute to neurobrucellosis pathogenesis.

Challenges of stem cell therapy for spinal cord injury: human embryonic stem cells, endogenous neural stem cells, or induced pluripotent stem cells?

Spinal cord injury (SCI) causes myelopathy, damage to white matter, and myelinated fiber tracts that carry sensation and motor signals to and from the brain. The gray matter damage causes segmental losses of interneurons and motoneurons and restricts therapeutic options. Recent advances in stem cell biology, neural injury, and repair, and the progress toward development of neuroprotective and regenerative interventions are the basis for increased optimism. This review summarizes the pathophysiological mechanisms following SCI and compares human embryonic, adult neural, and the induced pluripotent stem cell-based therapeutic strategies for SCI.

MAPK signal transduction underlying brain inflammation and gliosis as therapeutic target

A majority, if not all, acute and progressive neurodegenerative diseases are accompanied by local microglia-mediated inflammation, astrogliosis, infiltration of immune cells, and activation of the adaptive immunity. These processes progress by the expression of cytokines, adhesion molecules, proteases, and other inflammation mediators. In response to brain injury or infection, intracellular signaling pathways are activated in microglia, which turn on inflammatory and antigen-presenting cell functions. Different extrinsic signals shape microglial activation toward neuroprotective or neurotoxic phenotype under pathological conditions. This review discusses recent advances regarding molecular mechanisms of inflammatory signal transduction in neurological disorders and in in vitro models of inflammation/gliosis. Mitogen-activated protein kinases (MAPKs) are a family of serine/threonine protein kinases responsible for most cellular responses to cytokines and external stress signals and crucial for regulation of the production of inflammation mediators. Increased activity of MAPKs in activated microglia and astrocytes, and their regulatory role in the synthesis of inflammatory cytokines mediators, make them potential targets for novel therapeutics. MAPK inhibitors emerge as attractive anti-inflammatory drugs, because they are capable of reducing both the synthesis of inflammation mediators at multiple levels and are effective in blocking inflammatory cytokine signaling. Small molecule inhibitors targeting of p38 MAPK and JNK pathways have been developed and offer a great potential as potent modulators of brain inflammation and gliosis in neurological disorders, where cytokine overproduction contributes to disease progression. Many of the pharmacological MAPK inhibitors can be administered orally and initial results show therapeutic benefits in preclinical animal models.

Anti-inflammatory effect by lentiviral-mediated overexpression of IL-10 or IL-1 receptor antagonist in rat glial cells and macrophages

Neuroinflammation, as defined by activation of local glial cells and production of various inflammatory mediators, is an important feature of many neurological disorders. Expression of pro-inflammatory mediators produced by glial cells in the central nervous system (CNS) is considered to contribute to the neuropathology observed in those diseases. To diminish the production or action of pro-inflammatory mediators, we have used lentiviral (LV) vector-mediated encoding rat interleukin-10 (rIL-10) or rat interleukin-1 receptor antagonist (rIL-1ra) to direct the local, long-term expression of these anti-inflammatory cytokines in the CNS. We have shown that cultured macrophages or astroglia transduced with LV-rIL-10 or LV-rIL-1ra produced far less tumor necrosis factor (TNF)alpha or IL-6, respectively in response to pro-inflammatory stimuli. Moreover, intracerebroventricular (i.c.v.) administration of LV-rIL-10 or LV-rIL-1ra resulted in transduction of glial cells and macrophages and, subsequently reduced TNFalpha, IL-6 and inducible nitric oxide synthase (iNOS) expression in various brain regions induced by inflammatory stimuli, whereas peripheral expression of these mediators remained unaffected. In addition, expression levels of the anti-inflammatory cytokines IL-4 and transforming growth factor-beta were not altered in either brain or pituitary gland. Furthermore, i.c.v. administration of LV-rIL-10 or LV-rIL-1ra given during the remission phase of chronic-relapsing experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, improved the clinical outcome of the relapse phase. Thus, local application of LV vectors expressing anti-inflammatory cytokines could be of therapeutic interest to counteract pro-inflammatory processes in the brain without interfering with the peripheral production of inflammatory mediators.

Inflammatory mechanisms in ischemic stroke: role of inflammatory cells

Inflammation plays an important role in the pathogenesis of ischemic stroke and other forms of ischemic brain injury. Experimentally and clinically, the brain responds to ischemic injury with an acute and prolonged inflammatory process, characterized by rapid activation of resident cells (mainly microglia), production of proinflammatory mediators, and infiltration of various types of inflammatory cells (including neutrophils, different subtypes of T cells, monocyte/macrophages, and other cells) into the ischemic brain tissue. These cellular events collaboratively contribute to ischemic brain injury. Despite intense investigation, there are still numerous controversies concerning the time course of the recruitment of inflammatory cells in the brain and their pathogenic roles in ischemic brain injury. In this review, we provide an overview of the time-dependent recruitment of different inflammatory cells following focal cerebral I/R. We discuss how these cells contribute to ischemic brain injury and highlight certain recent findings and currently unanswered questions about inflammatory cells in the pathophysiology of ischemic stroke.

Aging exacerbates intracerebral hemorrhage-induced brain injury

Aging may be an important factor affecting brain injury by intracerebral hemorrhage (ICH). In the present study, we investigated the responses of glial cells and monocytes to intracerebral hemorrhage in normal and aged rats. ICH was induced by microinjecting autologous whole blood (15 microL) into the striatum of young (4 month old) and aged (24 month old) Sprague-Dawley rats. Age-dependent relations of brain tissue damage with glial and macrophageal responses were evaluated. Three days after ICH, activated microglia/macrophages with OX42-positive processes and swollen cytoplasm were more abundantly distributed around and inside the hemorrhagic lesions. These were more dramatic in aged versus the young rats. Western blot and immunohistochemistry analyses showed that the expression of interleukin-1beta protein after ICH was greater in aged rats, whereas the expression of GFAP and ciliary neurotrophic factor protein after ICH was significantly lower in aged rats. These results suggest that ICH causes more severe brain injury in aged rats most likely due to overactivation of microglia/macrophages and concomitant repression of reactive astrocytes.

Upregulation in rat spinal cord microglia of the nonintegrin laminin receptor 37 kDa-LRP following activation by a traumatic lesion or peripheral injury

The molecular mechanisms triggering microglial activation after injury to the central nervous system, involving cell-extracellular matrix interactions and cytokine signaling, are not yet fully understood. Here, we report that resident microglia in spinal cord express low levels of the non-integrin laminin receptor precursor (LRP), also found on certain neurons and glial cells in the peripheral nervous system. 37LRP/p40 and its 67-kDa isoform laminin receptor (LR) were the first high-affinity laminin binding proteins identified. While the role of laminin receptor was later attributed to integrins, LRP/LR gained new interest as receptors for prions, and their interaction with laminin seems important for migration of metastatic cancer cells. Using immunohistochemistry and Western blotting, we demonstrate that traumatic spinal cord injury leads to a strong and rapid increase in LRP levels in relation to activated microglia/macrophages. Associated with laminin re-expression in the lesion epicenter, LRP-positive microglia/macrophages exhibit a rounded, ameboid-like shape characteristic of phagocytic cells, whereas in more distant loci they reveal a hypertrophied cell body and short ramifications. The same morphological difference is observed in vitro for purified microglia cultured with or without laminin. Strong, transient upregulation of LRP by activated spinal cord microglia is also induced by transection of the sciatic nerve that affects the spinal cord circuitry without blood-brain barrier dysruption. LRP expression is maximal by 1 week post-lesion, before becoming restricted to dorsal and ventral horns, sites of major structural reorganization. Our findings strongly suggest the involvement of LRP in lesion-induced activation and migration of microglia.

Fractalkine and CX 3 CR1 regulate hippocampal neurogenesis in adult and aged rats

Microglia have neuroprotective capacities, yet chronic activation can promote neurotoxic inflammation. Neuronal fractalkine (FKN), acting on CX(3)CR1, has been shown to suppress excessive microglia activation. We found that disruption in FKN/CX(3)CR1 signaling in young adult rodents decreased survival and proliferation of neural progenitor cells through IL-1β. Aged rats were found to have decreased levels of hippocampal FKN protein; moreover, interruption of CX(3)CR1 function in these animals did not affect neurogenesis. The age-related loss of FKN could be restored by exogenous FKN reversing the age-related decrease in hippocampal neurogenesis. There were no measureable changes in young animals by the addition of exogenous FKN. The results suggest that FKN/CX(3)CR1 signaling has a regulatory role in modulating hippocampal neurogenesis via mechanisms that involve indirect modification of the niche environment. As elevated neuroinflammation is associated with many age-related neurodegenerative diseases, enhancing FKN/CX(3)CR1 interactions could provide an alternative therapeutic approach to slow age-related neurodegeneration.

Japanese encephalitis virus induce immuno-competency in neural stem/progenitor cells

Background

The low immunogenicity of neural stem/progenitor cells (NSPCs) coupled with negligible expression of MHC antigens has popularized their use in transplantation medicine. However, in an inflammatory environment, the NSPCs express costimulatory molecules and MHC antigens, and also exhibit certain immunomodulatory functions. Since NSPCs are the cellular targets in a number of virus infections both during postnatal and adult stages, we wanted to investigate the immunological properties of these stem cells in response to viral pathogen.

Methodology/Principal Findings

We utilized both in vivo mouse model and in vitro neurosphere model of Japanese encephalitis virus (JEV) infection for the study. The NSPCs residing in the subventricular zone of the infected brains showed prominent expression of MHC-I and costimulatory molecules CD40, CD80, and CD86. Using Flow cytometry and fluorescence microscopy, we observed increased surface expression of co-stimulatory molecule and MHC class I antigen in NSPCs upon progressive JEV infection in vitro. Moreover, significant production of pro-inflammatory cyto/chemokines was detected in JEV infected NSPCs by Cytokine Bead Array analysis. Interestingly, NSPCs were capable of providing functional costimulation to allogenic T cells and JEV infection resulted in increased proliferation of allogenic T cells, as detected by Mixed Lymphocyte reaction and CFSE experiments. We also report IL-2 production by NSPCs upon JEV infection, which possibly provides mitogenic signals to T cells and trigger their proliferation.

Conclusion/Significance

The in vivo and in vitro findings clearly indicate the development of immunogenicity in NSPCs following progressive JEV infection, in our case, JEV infection. Following a neurotropic virus infection, NSPCs possibly behave as immunogenic cells and contribute to both the innate and adaptive immune axes. The newly discovered immunological properties of NSPCs may have implications in assigning a new role of these cells as non-professional antigen presenting cells in the central nervous system.

Microglia in infectious diseases of the central nervous system

Microglia are the resident macrophage population in the central nervous system (CNS) parenchyma and, as such, are poised to provide a first line of defense against invading pathogens. Microglia are endowed with a vast repertoire of pattern recognition receptors that include such family members as Toll-like receptors and phagocytic receptors, which collectively function to sense and eliminate microbes invading the CNS parenchyma. In addition, microglial activation elicits a broad range of pro-inflammatory cytokines and chemokines that are involved in the recruitment and subsequent activation of peripheral immune cells infiltrating the infected CNS. Studies from several laboratories have demonstrated the ability of microglia to sense and respond to a wide variety of pathogens capable of colonizing the CNS including bacterial, viral, and fungal species. This review will highlight the role of microglia in microbial recognition and the resultant antipathogen response that ensues in an attempt to clear these infections. Implications as to whether microglial activation is uniformly beneficial to the CNS or in some circumstances may exacerbate pathology will also be discussed.

Intracerebral granulocyte-macrophage colony-stimulating factor induces functionally competent dendritic cells in the mouse brain

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor and a proinflammatory cytokine. While GM-CSF is lacking in normal brain tissue, it is expressed under pathological conditions and correlates with the presence of dendritic cells (DC). However, the role of GM-CSF for the onset of immune responses in the brain is still unclear. To analyze the role of GM-CSF for the induction and functional activity of immune cells in the brain, we performed chronic intracerebroventricular administration of GM-CSF to the brains of adult mice. After GM-CSF administration, intracerebral leukocytes (ICL) were characterized by means of flow cytometry, immunohistochemistry, and an ex vivo functional assay. GM-CSF treatment significantly increased the number of leukocytes expressing high levels of CD45, indicative of peripheral, blood-derived cells. The infiltrating cells were preferentially DC of the myeloid lineage (CD45(high) CD11c+ CD11b+) with an activated phenotype characterized by upregulated expression of MHCII and the costimulatory ligand CD80. Furthermore, DC from GM-CSF treated mice were fully competent to activate naive allogeneic T cells in a mixed leukocyte reaction. In contrast, intracerebroventricular IFN-gamma administration stimulated MHCII expression on cells resembling resident microglia, but did not induce comparable presence of DC. Taken together, intracerebroventricular GM-CSF treatment results in high numbers of DC in the brain. Moreover, these GM-CSF-induced DC display an activated phenotype and exhibit the capacity to act as fully competent DC even without a further inflammatory stimulus.

Mesenchymal stem cell transplantation attenuates blood brain barrier damage and neuroinflammation and protects dopaminergic neurons against MPTP toxicity in the substantia nigra in a model of Parkinson's disease

Immunomodulatory effects of transplanted mesenchymal stem cells (MSCs) in the treatment of Parkinson's disease were studied in the MPTP-induced mouse model. MPTP treatment induced a significant loss of dopaminergic neurons, decreased expressions of claudin 1, claudin 5 and occludin in the substantia nigra compacta (SNc), and functional damage of the blood brain barrier (BBB). Our study further discovered that infiltration of MBLs into the brain to bind with microglia was detected in the SNc of MPTP-treated mice, suggesting that the BBB compromise and MBL infiltration might be involved in the pathogenesis of MPTP-induced PD. In addition, MPTP treatment also increased the expression of mannose-binding lectins (MBLs) in the liver tissue. Intravenous transplantation of MSCs into MPTP-treated mice led to recovery of BBB integrity, suppression of MBL infiltration at SNc and MBL expression in the liver, suppression of microglial activation and prevention of dopaminergic neuron death. No transplanted MSCs were observed to differentiate into dopaminergic neurons, while the MSCs migrated into the SNc and released TGF-beta1 there. Therefore, intravenous transplantation of MSCs which protect dopaminergic neurons from MPTP toxicity may be engaged in anyone or a combination of these mechanisms: repair of the BBB, reduction of MBL in the brain, inhibition of microglial cytotoxicity, and direct protection of dopaminergic neurons.

Microglial activation by Citrobacter koseri is mediated by TLR4- and MyD88-dependent pathways

Citrobacter koseri is a Gram-negative bacterium that can cause a highly aggressive form of neonatal meningitis, which often progresses to establish multifocal brain abscesses. Despite its tropism for the brain parenchyma, microglial responses to C. koseri have not yet been examined. Microglia use TLRs to recognize invading pathogens and elicit proinflammatory mediator expression important for infection containment. In this study, we investigated the importance of the LPS receptor TLR4 and MyD88, an adaptor molecule involved in the activation of the majority of TLRs in addition to the IL-1 and IL-18 receptors, for their roles in regulating microglial activation in response to C. koseri. Proinflammatory mediator release was significantly reduced in TLR4 mutant and MyD88 knockout microglia compared with wild-type cells following exposure to either live or heat-killed C. koseri, indicating a critical role for both TLR4- and MyD88-dependent pathways in microglial responses to this pathogen. However, residual proinflammatory mediator expression was still observed in TLR4 mutant and MyD88 KO microglia following C. koseri exposure, indicating a contribution of TLR4- and MyD88-independent pathway(s) for maximal pathogen recognition. Interestingly, C. koseri was capable of surviving intracellularly in both primary microglia and macrophages, suggesting that these cells may serve as a reservoir for the pathogen during CNS infections. These results demonstrate that microglia respond to C. koseri with the robust expression of proinflammatory molecules, which is dictated, in part, by TLR4- and MyD88-dependent signals.

IL-13-induced oxidative stress via microglial NADPH oxidase contributes to death of hippocampal neurons in vivo

In the present study, we investigated the effects of IL-13, a well-known anti-inflammatory cytokine, on the thrombin-treated hippocampus in vivo. NeuN immunohistochemistry and Nissl staining revealed significant loss of hippocampal CA1 neurons upon intrahippocampal injection of thrombin. This neurotoxicity was accompanied by substantial microglial activation, as evident from OX-42 immunohistochemistry results. In parallel, Western blot analysis and hydroethidine histochemistry disclosed activation of NADPH oxidase, generation of reactive oxygen species, and oxidative damage in the hippocampal CA1 area showing hippocampal neuron degeneration. Interestingly, immunohistochemical and biochemical experiments showed that intrahippocampal injection of thrombin increased IL-13 immunoreactivity and IL-13 levels as early as 8 h after thrombin, reaching a peak at 7 days, which was maintained up to 14 days. Moreover, double-label immunohistochemistry revealed IL-13 immunoreactivity exclusively in activated microglia. IL-13-neutralizing Abs significantly rescued CA1 hippocampal neurons from thrombin neurotoxicity. In parallel, neutralization of IL-13 inhibited activation of NADPH oxidase, reactive oxygen species production, and oxidative damage. Additionally, IL-13 neutralization suppressed the expression of inducible NO synthase and several proinflammatory cytokines. To our knowledge, the present study is the first to show that IL-13 triggers microglial NADPH oxidase-derived oxidative stress, leading to the degeneration of hippocampal neurons in vivo, as occurs in cases of Alzheimer's disease.

Enhanced hippocampal neurogenesis in the absence of microglia T cell interaction and microglia activation in the murine running wheel model

Recently, activated microglia have been shown to be involved in the regulation of several aspects of neurogenesis under certain experimental conditions both in vitro and in vivo. A neurogenesis supportive microglia phenotype has been suggested to arise from the interaction of microglia with homing encephalitogenic T cells. However, a unified hypothesis regarding the exact nature of microglia activity that is supportive of neurogenesis is yet missing from the field. Our aim was to investigate the connection between microglia activity and adult hippocampal neurogenesis under physiological conditions. To address this question we compared the level of microglia activation in the hippocampus of mice, which had access to a running wheel for 10 days and that of sedentary controls. Surprisingly, despite elevated levels of proliferation of neural precursors and survival of newborn neurons in the dentate gyrus microglia remained in a "resting" state morphologically, antigenically, and at the transcriptional level. Moreover, neither T cells nor MHCII expressing microglia were present in the hippocampal brain parenchyma. Though microglia in the dentate gyrus of the runners proliferated at a higher level than in the sedentary controls, this difference was also present in non-neurogenic sites. Therefore, our findings suggest that classical signs of microglia activation and microglia activation arising from interaction with T cells in particular are not a prerequisite for the activity-induced increase in adult hippocampal neurogenesis in C57Bl/6 mice. Thus, our results draw attention on the species and model differences that might exist regarding the regulation of adult hippocampal neurogenesis.

The cortical innate immune response increases local neuronal excitability leading to seizures

Brain glial cells, five times more prevalent than neurons, have recently received attention for their potential involvement in epileptic seizures. Microglia and astrocytes, associated with inflammatory innate immune responses, are responsible for surveillance of brain damage that frequently results in seizures. Thus, an intriguing suggestion has been put forward that seizures may be facilitated and perhaps triggered by brain immune responses. Indeed, recent evidence strongly implicates innate immune responses in lowering seizure threshold in experimental models of epilepsy, yet, there is no proof that they can play an independent role in initiating seizures in vivo. Here, we show that cortical innate immune responses alone produce profound increases of brain excitability resulting in focal seizures. We found that cortical application of lipopolysaccharide, binding to toll-like receptor 4 (TLR4), triples evoked field potential amplitudes and produces focal epileptiform discharges. These effects are prevented by pre-application of interleukin-1 receptor antagonist. Our results demonstrate how the innate immune response may participate in acute seizures, increasing neuronal excitability through interleukin-1 release in response to TLR4 detection of the danger signals associated with infections of the central nervous system and with brain injury. These results suggest an important role of innate immunity in epileptogenesis and focus on glial inhibition, through pharmacological blockade of TLR4 and the pro-inflammatory mediators released by activated glia, in the study and treatment of seizure disorders in humans.

Innate and adaptive immunity for the pathobiology of Parkinson's disease

Innate and adaptive immunity affect the pathogenesis of Parkinson's disease (PD). In particular, activation of microglia influences degeneration of dopaminergic neurons. Cell-to-cell interactions and immune regulation critical for neuronal homeostasis also influence immune responses. The links between T cell immunity and nigrostriatal degeneration are supported by laboratory, animal model, and human pathologic investigations. Immune-associated biomarkers in spinal fluids and brain tissue of patients with idiopathic or familial forms of PD provide means to improve diagnosis and therapeutic monitoring. Relationships between oxidative stress, inflammation, and immune-mediated cell death pathways are examined in this review as they are linked to PD pathogenesis. Harnessing the immune system by drugs or by vaccination remain promising future therapeutic options.

The multifaceted profile of activated microglia

Although relatively neglected previously, research efforts in the past decade or so have identified a pivotal role for glial cells in regulating neuronal function. Particular emphasis has been placed on increasing our understanding of the function of microglia because a change from the ramified "resting" state of these cells has been associated with the pathogenesis of several neurodegenerative diseases, notably Alzheimer's disease. However, it is not clear whether activation of microglia and the associated inflammatory changes play a part in triggering disease processes or whether cell activation is a response to the early changes associated with the disease. In either case, the possibility exists that modulation of microglial activation may be beneficial in some circumstances, underlying the need to pursue research in this area. The original morphological categorization of microglia by Del Rio Hortega into ameboid, ramified, and intermediate forms, must now be elaborated to encompass a functional description. The evidence which has been generated recently suggests that microglia are probably never in a "resting" state and that several intermediate transitional states, based on function and morphology, probably exist. A more complete understanding of these states and the triggers which lead to a change from one to another state, and the factors which modulate the molecular switch that determines the persistence of the "activated" state remain to be identified.

NSA9, a human prothrombin kringle-2-derived peptide, acts as an inhibitor of kringle-2-induced activation in EOC2 microglia

In neurodegenerative diseases, such as Alzheimer's and Parkinson's, microglial cell activation is thought to contribute to CNS injury by producing neurotoxic compounds. Prothrombin and kringle-2 increase levels of NO and the mRNA expression of iNOS, IL-1beta, and TNF-alpha in microglial cells. In contrast, the human prothrombin kringle-2 derived peptide NSA9 inhibits NO release and the production of pro-inflammatory cytokines such as IL-1beta, TNF-alpha, and IL-6 in LPS-activated EOC2 microglia. In this study, we investigated the anti-inflammatory effects of NSA9 in human prothrombin- and kringle-2-stimulated EOC2 microglia. Treatment with 20-100 muM of NSA9 attenuated both prothrombin- and kringle-2-induced microglial activation. NO production induced by MAPKs and NF-kappaB was similarly reduced by inhibitors of ERK (PD98059), p38 (SB203580), NF-kappaB (N-acetylcysteine), and NSA9. These results suggest that NSA9 acts independently as an inhibitor of microglial activation and that its effects in EOC2 microglia are not influenced by the presence of kringle-2.

Effects of macrophage colony-stimulating factor on microglial responses to lipopolysaccharide and beta amyloid

A challenge for studies involving microglia cultures is obtaining sufficient cells for downstream experiments. Macrophage colony-stimulating factor (M-CSF) has been used to improve yield of microglia in culture. However, the effects of M-CSF on activation profiles of microglia cultures are still unclear. Microglia activation is characterised by upregulation of co-stimulatory molecules and an inflammatory phenotype. The aim of this study is to demonstrate whether M-CSF supplementation alters microglial responses in resting and activated conditions. Microglia derived from mixed glia cultures and the BV-2 microglia cell line were cultivated with/without M-CSF and activated with lipopolysaccharide (LPS) and beta amyloid (Abeta). We show M-CSF expands primary microglia without affecting microglial responses to LPS and Abeta, as shown by the comparable expression of MHC class II and CD40 to microglia grown without this growth factor. M-CSF supplementation in BV-2 cells had no effect on nitric oxide (NO) production. Therefore, M-CSF can be considered for improving microglia yield in culture without introducing activation artefacts.

Autoimmune T cell responses in the central nervous system

Autoreactive T cell responses have a crucial role in central nervous system (CNS) diseases such as multiple sclerosis. Recent data indicate that CNS autoimmunity can be mediated by two distinct lineages of CD4+ T cells that are defined by the production of either interferon-gamma or interleukin-17. The activity of these CD4+ T cell subsets within the CNS influences the pathology and clinical course of disease. New animal models show that myelin-specific CD8+ T cells can also mediate CNS autoimmunity. This Review focuses on recent progress in delineating the pathogenic mechanisms, regulation and interplay between these different T cell subsets in CNS autoimmunity.

Microglial physiology: unique stimuli, specialized responses

Microglia, the macrophages of the central nervous system parenchyma, have in the normal healthy brain a distinct phenotype induced by molecules expressed on or secreted by adjacent neurons and astrocytes, and this phenotype is maintained in part by virtue of the blood-brain barrier's exclusion of serum components. Microglia are continually active, their processes palpating and surveying their local microenvironment. The microglia rapidly change their phenotype in response to any disturbance of nervous system homeostasis and are commonly referred to as activated on the basis of the changes in their morphology or expression of cell surface antigens. A wealth of data now demonstrate that the microglia have very diverse effector functions, in line with macrophage populations in other organs. The term activated microglia needs to be qualified to reflect the distinct and very different states of activation-associated effector functions in different disease states. Manipulating the effector functions of microglia has the potential to modify the outcome of diverse neurological diseases.

Lipopolysaccharide extends the lifespan of mouse primary-cultured microglia

Microglial activation has been implicated in the recognition and phagocytic removal of degenerating neurons; however, this process must be tightly regulated in the central nervous system, because prolonged activation could damage normal neurons. We report that mouse primary-cultured microglia, which are destined to die within a few days under ordinary culture conditions, can live for more than 1 month when kept activated by lipopolysaccharide (LPS) treatment. Primary-cultured microglia treated with sublethal doses of LPS remained viable, without any measurable increase in apoptotic or necrotic cell death. LPS-treated microglia had an arborescent shape, with enlarged somata and thickened cell bodies. Although the amount of intracellular ATP in these microglia was reduced by 2 h after the start of LPS treatment, this had no effect on the viability of the cells. LPS treatment of microglia increased the antiapoptotic factor Bcl-xL protein level at day 1, although the level of the proapoptotic Bcl-associated X-protein was unaffected. Furthermore, the level of microtubule-associated light chain 3, a marker protein for autophagy, decreased at 3 h after exposure to LPS. These data show that the optimal dose of LPS suppresses the induction of both apoptosis and autophagy in primary-cultured microglia, allowing the cells to stay alive for more than 1 month. Because long-lived microglia may play critical roles in the exacerbation of neurodegeneration, our findings suggest that inducing a resting stage in active microglia could be a new and promising strategy to inhibit the deterioration of neurodegenerative disease.

Induction of macrophage migration by neurotoxic prion protein fragment

Prion diseases are characterized by accumulation of protease resistant isoforms of prion protein (PrP), and infiltration and activation of mononuclear phagocytes at the brain lesions. Interactions between prion proteins and immune cells during disease progression are still not very well understood. In the present study, multiwell chamber chemotaxis assay was carried out to assess the migratory response of macrophage cell line Ana-1 to a synthetic peptide homologous to residues 106-126 of the human prion protein. Specific protein kinase inhibitors were used to elucidate the signaling events underlying PrP106-126-induced macrophages migration, and a comparison with the signaling pattern of macrophage migration induced by substance P (SP) and N-formyl-methionyl-leucyl-phenylalanine (fMLP), respectively, was carried out. The results showed that PrP106-126 had a potent chemotactic effect on murine macrophage cell line Ana-1; that multiple signaling pathways might be involved in the PrP106-126-induced macrophage migrations; and that PrP106-126-induced chemotactic activity was similar to that induced by SP. These findings provide new insights into the mechanisms underlying the interaction between PrP and macrophages.

Interleukin-4 as a neuromodulatory cytokine: roles and signaling in the nervous system

Although interleukin (IL)-4 is described as a prototypical anti-inflammatory cytokine, in recent years its role as a neuromodulatory cytokine has been extensively discussed. This review highlights the pivotal contributions of IL-4 during the development and normal physiology of neural cells as well as IL-4 connections with the pathophysiology of degenerative or inflammatory processes observed in the central and peripheral nervous system.

A role for TREM2 ligands in the phagocytosis of apoptotic neuronal cells by microglia

Following neuronal injury, microglia initiate repair by phagocytosing dead neurons without eliciting inflammation. Prior evidence indicates triggering receptor expressed by myeloid cells-2 (TREM2) promotes phagocytosis and retards inflammation. However, evidence that microglia and neurons directly interact through TREM2 to orchestrate microglial function is lacking. We here demonstrate that TREM2 interacts with endogenous ligands on neurons. Staining with TREM2-Fc identified TREM2 ligands (TREM2-L) on Neuro2A cells and on cultured cortical and dopamine neurons. Apoptosis greatly increased the expression of TREM2-L. Furthermore, apoptotic neurons stimulated TREM2 signaling, and an anti-TREM2 mAb blocked stimulation. To examine the interaction between TREM2 and TREM2-L in phagocytosis, we studied BV2 microglial cells and their engulfment of apoptotic Neuro2A. One of our anti-TREM2 mAb, but not others, reduced engulfment, suggesting the presence of a functional site on TREM2 interacting with neurons. Further, Chinese hamster ovary cells transfected with TREM2 conferred phagocytic activity of neuronal cells demonstrating that TREM2 is both required and sufficient for competent uptake of apoptotic neuronal cells. Finally, while TREM2-L are expressed on neurons, TREM2 is not; in the brain, it is found on microglia. TREM2 and TREM2-L form a receptor-ligand pair connecting microglia with apoptotic neurons, directing removal of damaged cells to allow repair.

Replicon particles of Venezuelan equine encephalitis virus as a reductionist murine model for encephalitis

Venezuelan equine encephalitis virus (VEE) replicon particles (VRP) were used to model the initial phase of VEE-induced encephalitis in the mouse brain. VRP can target and infect cells as VEE, but VRP do not propagate beyond the first infected cell due to the absence of the structural genes. Direct intracranial inoculation of VRP into mice induced acute encephalitis with signs similar to the neuronal phase of wild-type VEE infection and other models of virus-induced encephalitis. Using the previously established VRP-mRNP tagging system, a new method to distinguish the host responses in infected cells from those in uninfected bystander cell populations, we detected a robust and rapid innate immune response in the central nervous system (CNS) by infected neurons and uninfected bystander cells. Moreover, this innate immune response in the CNS compromised blood-brain barrier integrity, created an inflammatory response, and directed an adaptive immune response characterized by proliferation and activation of microglia cells and infiltration of inflammatory monocytes, in addition to CD4(+) and CD8(+) T lymphocytes. Taken together, these data suggest that a naïve CNS has an intrinsic potential to induce an innate immune response that could be crucial to the outcome of the infection by determining the composition and dynamics of the adaptive immune response. Furthermore, these results establish a model for neurotropic virus infection to identify host and viral factors that contribute to invasion of the brain, the mechanism(s) whereby the adaptive immune response can clear the infection, and the role of the host innate response in these processes.