Resident and infiltrating central nervous system APCs regulate the emergence and resolution of experimental autoimmune encephalomyelitis

Uncovering mechanisms of brain inflammation in Alzheimer's disease with APOE4: Application of single cell-type lipidomics

A chronic state of unresolved inflammation in Alzheimer's disease (AD) is intrinsically involved with the remodeling of brain lipids. This review highlights the effect of carrying the apolipoprotein E ε4 allele (APOE4) on various brain cell types in promoting an unresolved inflammatory state. Among its pleotropic effects on brain lipids, we focus on APOE4's activation of Ca²⁺ -dependent phospholipase A2 (cPLA2) and its effects on arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid signaling cascades in the brain. During the process of neurodegeneration, various brain cell types, such as astrocytes, microglia, and neurons, together with the neurovascular unit, develop distinct inflammatory phenotypes that impact their functions and have characteristic lipidomic fingerprints. We propose that lipidomic phenotyping of single cell-types harvested from brains differing by age, sex, disease severity stage, and dietary and genetic backgrounds can be employed to probe mechanisms of neurodegeneration. A better understanding of the brain cellular inflammatory/lipidomic response promises to guide the development of nutritional and drug interventions for AD dementia.

Microglia and metastases to the central nervous system: victim, ravager, or something else?

Central nervous system (CNS) metastases are a major cause of death in patients with cancer. Tumor cells must survive during their migration and dissemination in various sites and niches. The brain is considered an immunological sanctuary site, and thus the safest place for metastasis establishment. The risk of brain metastases is highest in patients with melanoma, lung, or breast cancers. In the CNS, metastatic cancer cells exploit the activity of different non-tumoral cell types in the brain microenvironment to create a new niche and to support their proliferation and survival. Among these cells, microglia (the brain resident macrophages) display an exceptional role in immune surveillance and tumor clearance. However, upon recruitment to the metastatic site, depending on the microenvironment context and disease conditions, microglia might be turned into tumor-supportive or -unsupportive cells. Recent single-cell 'omic' analyses have contributed to clarify microglia functional and spatial heterogeneity during tumor development and metastasis formation in the CNS. This review summarizes findings on microglia heterogeneity from classical studies to the new single-cell omics. We discuss i) how microglia interact with metastatic cancer cells in the unique brain tumor microenvironment; ii) the microglia classical M1-M2 binary concept and its limitations; and iii) single-cell omic findings that help to understand human and mouse microglia heterogeneity (core sensomes) and to describe the multi-context-dependent microglia functions in metastases to the CNS. We then propose ways to exploit microglia plasticity for brain metastasis treatment depending on the microenvironment profile.

Immune Cell Contributors to the Female Sex Bias in Multiple Sclerosis and Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is a chronic, autoimmune, demyelinating disease of the central nervous system (CNS) that leads to axonal damage and accumulation of disability. Relapsing-remitting MS (RR-MS) is the most frequent presentation of MS and this form of MS is three times more prevalent in females than in males. This female bias in MS is apparent only after puberty, suggesting a role for sex hormones in this regulation; however, very little is known of the biological mechanisms that underpin the sex difference in MS onset. Experimental autoimmune encephalomyelitis (EAE) is an animal model of RR-MS that presents more severely in females in certain mouse strains and thus has been useful to study sex differences in CNS autoimmunity. Here, we overview the immunopathogenesis of MS and EAE and how immune mechanisms in these diseases differ between a male and female. We further describe how females exhibit more robust myelin-specific T helper (Th) 1 immunity in MS and EAE and how this sex bias in Th cells is conveyed by sex hormone effects on the T cells, antigen presenting cells, regulatory T cells, and innate lymphoid cell populations.

TH Cells and Cytokines in Encephalitogenic Disorders

The invasion of immune cells into the central nervous system (CNS) is a hallmark of the process we call neuroinflammation. Diseases such as encephalitides or multiple sclerosis (MS) are characterised by the dramatic influx of T lymphocytes and monocytes. The communication between inflammatory infiltrates and CNS resident cells is primarily mediated through cytokines. Over the years, numerous cytokine networks have been assessed to better understand the development of immunopathology in neuroinflammation. In MS for instance, many studies have shown that CD4+ T cells infiltrate the CNS and subsequently lead to immunopathology. Inflammatory CD4+ T cells, such as TH1, TH17, GM-CSF-producing helper T cells are big players in chronic neuroinflammation. Conversely, encephalitogenic or meningeal regulatory T cells (TREGs) and TH2 cells have been shown to drive a decrease in inflammatory functions in microglial cells and thus promote a neuroprotective microenvironment. Recent studies report overlapping as well as differential roles of these cells in tissue inflammation. Taken together, this suggests a more complex relationship between effector T cell subsets in neuroinflammation than has hitherto been established. In this overview, we review the interplay between helper T cell subsets infiltrating the CNS and how they actively contribute to neuroinflammation and degeneration. Importantly, in this context, we will especially focus on the current knowledge regarding the contribution of various helper cell subsets to neuroinflammation by referring to their helper T cell profile in the context of their target cell.

T cell transgressions: Tales of T cell form and function in diverse disease states

Insights into T cell form, function, and dysfunction are rapidly evolving. T cells have remarkably varied effector functions including protecting the host from infection, activating cells of the innate immune system, releasing cytokines and chemokines, and heavily contributing to immunological memory. Under healthy conditions, T cells orchestrate a finely tuned attack on invading pathogens while minimizing damage to the host. The dark side of T cells is that they also exhibit autoreactivity and inflict harm to host cells, creating autoimmunity. The mechanisms of T cell autoreactivity are complex and dynamic. Emerging research is elucidating the mechanisms leading T cells to become autoreactive and how such responses cause or contribute to diverse disease states, both peripherally and within the central nervous system. This review provides foundational information on T cell development, differentiation, and functions. Key T cell subtypes, cytokines that create their effector roles, and sex differences are highlighted. Pathological T cell contributions to diverse peripheral and central disease states, arising from errors in reactivity, are highlighted, with a focus on multiple sclerosis, rheumatoid arthritis, osteoarthritis, neuropathic pain, and type 1 diabetes.

Preclinical Therapy with Vitamin D3 in Experimental Encephalomyelitis: Efficacy and Comparison with Paricalcitol

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS). MS and its animal model called experimental autoimmune encephalomyelitis (EAE) immunopathogenesis involve a plethora of immune cells whose activation releases a variety of proinflammatory mediators and free radicals. Vitamin D3 (VitD) is endowed with immunomodulatory and antioxidant properties that we demonstrated to control EAE development. However, this protective effect triggered hypercalcemia. As such, we compared the therapeutic potential of VitD and paricalcitol (Pari), which is a non-hypercalcemic vitamin D analog, to control EAE. From the seventh day on after EAE induction, mice were injected with VitD or Pari every other day. VitD, but not Pari, displayed downmodulatory ability being able to reduce the recruitment of inflammatory cells, the mRNA expression of inflammatory parameters, and demyelination at the CNS. Lower production of proinflammatory cytokines by lymph node-derived cells and IL-17 by gut explants, and reduced intestinal inflammation were detected in the EAE/VitD group compared to the EAE untreated or Pari groups. Dendritic cells (DCs) differentiated in the presence of VitD developed a more tolerogenic phenotype than in the presence of Pari. These findings suggest that VitD, but not Pari, has the potential to be used as a preventive therapy to control MS severity.

Seasonal Variations in Macrophages/Microglia Underlie Changes in the Mouse Model of Multiple Sclerosis Severity

Both immune and neurodegenerative mechanisms underlie multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). MS/EAE are triggered by encephalitogenic immune cells, including Th1 and Th17 cells, whereas T regulatory (Treg) cells are involved in inflammation resolution. Pro-inflammatory macrophages/microglia also play a deleterious role in the disease. Seasonal variations in MS relapses, active lesions, and pro- and anti-inflammatory cytokine levels have been described in MS patients and have been related with both perinatal and adult exposure to sunlight and other environmental factors. However, some data in EAE mice suggest that these variations might be, at least partially, endogenously determined. Thus, our objective was to study the effect of the season of birth and disease induction on the course of EAE, and immune cell infiltration in the central nervous system (CNS) in myelin oligodendrocyte glycoprotein (MOG_35-55)-induced EAE in 8 weeks old, female C57BL/6N mice maintained under constant, controlled conditions. EAE severity as well as pathogenic (Th1, Th17, macrophages/microglia) and protective (Treg) subsets was found to vary according to the season of birth or of EAE induction. Summer-born or summer-immunized animals developed a milder disease, which coincided with variations in numbers of T effector/regulatory subsets, and significantly low numbers of macrophages/microglia. These results suggest that endogenous rhythms in immune responses might cause seasonal variations in EAE severity, and, maybe, in the course of MS, and that they might be related to macrophages/microglia.

Perspectives on the role of brain cellular players in cancer-associated brain metastasis: translational approach to understand molecular mechanism of tumor progression

Brain metastasis is one of the leading causes of death among cancer patients. Cancer cells migrate to various sites and harbor different niche in the body which help cancer cells in their survival. The brain is one of the safest place where cancer cells are protected from immune cells. Breast, lung, and melanoma cancer cells have high propensity to migrate towards the brain. To enter the brain, cancer cells have to cross the blood brain barrier. Survival and finding new niche in the brain are directed by several mechanisms in which different cellular players take part such as astrocytes, microglia, Schwann cells, satellite cells, oligodendrocytes, and ependymal cells. Usually, cancer cells highjack the machinery of brain cellular players to survive in the brain environment. It has been shown that co-culture of M2 macrophage with cancer cells leads to increased proliferation and survival of cancer cells. One of the challenges of understanding brain metastasis is appropriate model system to understand dynamic interaction of cancer cells and brain cellular players. To meet this challenge, microfluidic-based devices are employed which can mimic the dynamic conditions as well as can be used for culturing human cells for personalized therapy. In this review, we have systematically reviewed the current status of the role of cellular players in brain metastasis along with explaining how translational approach of microfluidics can be employed for finding new drug target as well as biomarker for brain metastasis. Finally, we have also commented on the mechanism of action of drugs against brain metastasis.

Fibronectin-adherent peripheral blood derived mononuclear cells as Paclitaxel carriers for glioblastoma treatment: An in vitro study

BACKGROUND

Glioblastoma (GBM) represents the most aggressive malignant brain tumor in adults, with a risible median life expectancy despite gold standard treatment. Novel drug-delivery methods have been explored. Here we evaluated the possibility to use mononuclear cells (MCs) belonging to the monocytic-dendritic lineage as drug-carrier.

METHODS

MCs were obtained from 10 patients harboring a GBM, and from healthy volunteers, considered as controls. GBM tissue was also obtained from patients. MCs were cultured and the adherent population on fibronectin (FN-MCs), after immunocytochemistry and flow cytometry characterization, was loaded with Paclitaxel (FN-MCs-PTX). Antiproliferative and migration activity of FN-MCs-PTX was evaluated in two-dimensional (2D) and three-dimensional (3D) co-culture assays with red fluorescent U87 Malignant Glioma cells and primary GBM cells. Antiangiogenic properties of FN-MCs-PTX were tested on cultures with endothelial cells.

RESULTS

Phenotypical characterization showed a high expression of monocytic-dendritic markers in GBM cells and FN-MCs. FN-MCs demonstrated to effectively uptake PTX and to strongly inhibit GBM growth in vitro (P <0.01). Moreover, tumor-induced migration of MCs, although partially affected by the PTX cargo, remained statistically significant when compared with unprimed cells and this was confirmed in a 3D Matrigel model (P <0.01) and in a Trans-well assay (P <0.01). FN-MCs-PTX also disclosed considerable antiangiogenic properties.

DISCUSSION

Our results suggest that the fibronectin-adherent population of MCs isolated from peripheral blood can be an effective tool to inhibit GBM growth. Given the relative facility to obtain such cells and the short time needed for their culture and drug loading this approach may have potential as an adjuvant therapy for GBM.

Clearance of cerebral Aβ in Alzheimer's disease: reassessing the role of microglia and monocytes

Deficiency in cerebral amyloid β-protein (Aβ) clearance is implicated in the pathogenesis of the common late-onset forms of Alzheimer’s disease (AD). Accumulation of misfolded Aβ in the brain is believed to be a net result of imbalance between its production and removal. This in turn may trigger neuroinflammation, progressive synaptic loss, and ultimately cognitive decline. Clearance of cerebral Aβ is a complex process mediated by various systems and cell types, including vascular transport across the blood–brain barrier, glymphatic drainage, and engulfment and degradation by resident microglia and infiltrating innate immune cells. Recent studies have highlighted a new, unexpected role for peripheral monocytes and macrophages in restricting cerebral Aβ fibrils, and possibly soluble oligomers. In AD transgenic (ADtg) mice, monocyte ablation or inhibition of their migration into the brain exacerbated Aβ pathology, while blood enrichment with monocytes and their increased recruitment to plaque lesion sites greatly diminished Aβ burden. Profound neuroprotective effects in ADtg mice were further achieved through increased cerebral recruitment of myelomonocytes overexpressing Aβ-degrading enzymes. This review summarizes the literature on cellular and molecular mechanisms of cerebral Aβ clearance with an emphasis on the role of peripheral monocytes and macrophages in Aβ removal.

Antigen presentation for priming T cells in central system

Generation of myelin antigen-specific T cells is a major event in neuroimmune responses that causes demyelination. The antigen-priming of T cells and its location is important in chronic and acute inflammation. In autoimmune multiple sclerosis, the effector T cells are considered to generate in periphery. However, the reasons for chronic relapsing-remitting events are obscure. Considering mechanisms, a feasible aim of research is to investigate the role of antigen-primed T cells in lupus cerebritis. Last thirty years of investigations emphasize the relevance of microglia and infiltrated dendritic cells/macrophages as antigen presenting cells in the central nervous system. The recent approach towards circulating B-lymphocytes is an important area in the context. Here, we analyze the existing findings on antigen presentation in the central nervous system. The aim is to visualize signaling events of myelin antigen presentation to T cells and lead to the strategy of future goals on immunotherapy research.

OGR1/GPR68 Modulates the Severity of Experimental Autoimmune Encephalomyelitis and Regulates Nitric Oxide Production by Macrophages

Ovarian cancer G protein-coupled receptor 1 (OGR1) is a proton-sensing molecule that can detect decreases in extracellular pH that occur during inflammation. Although OGR1 has been shown to have pro-inflammatory functions in various diseases, its role in autoimmunity has not been examined. We therefore sought to determine whether OGR1 has a role in the development of T cell autoimmunity by contrasting the development of experimental autoimmune encephalomyelitis between wild type and OGR1-knockout mice. OGR1-knockout mice showed a drastically attenuated clinical course of disease that was associated with a profound reduction in the expansion of myelin oligodendrocyte glycoprotein 35-55-reactive T helper 1 (Th1) and Th17 cells in the periphery and a reduced accumulation of Th1 and Th17 effectors in the central nervous system. We determined that these impaired T cell responses in OGR1-knockout mice associated with a reduced frequency and number of dendritic cells in draining lymph nodes during EAE and a higher production of nitric oxide by macrophages. Our studies suggest that OGR1 plays a key role in regulating T cell responses during autoimmunity.

The Pro-inflammatory Effects of Glucocorticoids in the Brain

Glucocorticoids are a class of steroid hormones derived from cholesterol. Their actions are mediated by the glucocorticoid and mineralocorticoid receptors, members of the superfamily of nuclear receptors, which, once bound to their ligands, act as transcription factors that can directly modulate gene expression. Through protein-protein interactions with other transcription factors, they can also regulate the activity of many genes in a composite or tethering way. Rapid non-genomic signaling was also demonstrated since glucocorticoids can act through membrane receptors and activate signal transduction pathways, such as protein kinases cascades, to modulate other transcriptions factors and activate or repress various target genes. By all these different mechanisms, glucocorticoids regulate numerous important functions in a large variety of cells, not only in the peripheral organs but also in the central nervous system during development and adulthood. In general, glucocorticoids are considered anti-inflammatory and protective agents due to their ability to inhibit gene expression of pro-inflammatory mediators and other possible damaging molecules. Nonetheless, recent studies have uncovered situations in which these hormones can act as pro-inflammatory agents depending on the dose, chronicity of exposure, and the structure/organ analyzed. In this review, we will provide an overview of the conditions under which these phenomena occur, a discussion that will serve as a basis for exploring the mechanistic foundation of glucocorticoids pro-inflammatory gene regulation in the brain.

Are Microglial Cells the Regulators of Lymphocyte Responses in the CNS?

The infiltration of immune cells in the central nervous system is a common hallmark in different neuroinflammatory conditions. Accumulating evidence indicates that resident glial cells can establish a cross-talk with infiltrated immune cells, including T-cells, regulating their recruitment, activation and function within the CNS. Although the healthy CNS has been thought to be devoid of professional dendritic cells (DCs), numerous studies have reported the presence of a population of DCs in specific locations such as the meninges, choroid plexuses and the perivascular space. Moreover, the infiltration of DC precursors during neuroinflammatory situations has been proposed, suggesting a putative role of these cells in the regulation of lymphocyte activity within the CNS. On the other hand, under specific circumstances, microglial cells are able to acquire a phenotype of DC expressing a wide range of molecules that equip these cells with all the necessary machinery for communication with T-cells. In this review, we summarize the current knowledge on the expression of molecules involved in the cross-talk with T-cells in both microglial cells and DCs and discuss the potential contribution of each of these cell populations on the control of lymphocyte function within the CNS.

Differential roles of resident microglia and infiltrating monocytes in murine CNS autoimmunity

Macrophages can be of dual origin. Most tissue-resident macrophage compartments are generated before birth and subsequently maintain themselves independently from each other locally in healthy tissue. Under inflammatory conditions, these cells can however be complemented by macrophages derived from acute monocyte infiltrates. Due to the lack of suitable experimental systems, differential functional contributions of central nervous system (CNS)-resident microglia and monocyte-derived macrophages (MoMF) to CNS inflammation, such as experimental autoimmune encephalomyelitis (EAE), the mouse model of multiple sclerosis (MS), remain poorly understood. Here, we will review recent progress in this field that suggest distinct roles of microglia and MoMF in disease induction and progression, capitalizing on novel transgenic mouse models. The latter finding could have major implications for the rationale development of therapeutic approaches to the management of brain inflammation and MS therapy.

Therapeutic effects of glatiramer acetate and grafted CD115⁺ monocytes in a mouse model of Alzheimer's disease

Weekly glatiramer acetate immunization of transgenic mice modelling Alzheimer's disease resulted in retained cognition (Morris water maze test), decreased amyloid-β plaque burden, and regulation of local inflammation through a mechanism involving enhanced recruitment of monocytes. Ablation of bone marrow-derived myeloid cells exacerbated plaque pathology, whereas weekly administration of glatiramer acetate enhanced cerebral recruitment of innate immune cells, which dampened the pathology. Here, we assessed the therapeutic potential of grafted CD115(+) monocytes, injected once monthly into the peripheral blood of transgenic APPSWE/PS1ΔE9 Alzheimer's disease mouse models, with and without weekly immunization of glatiramer acetate, as compared to glatiramer acetate alone. All immune-modulation treatment groups were compared with age-matched phosphate-buffered saline-injected control transgenic and untreated non-transgenic mouse groups. Two independent cohorts of mice were assessed for behavioural performance (6-8 mice/group); treatments started in 10-month-old symptomatic mice and spanned a total of 2 months. For all three treatments, our data suggest a substantial decrease in cognitive deficit as assessed by the Barnes maze test (P < 0.0001-0.001). Improved cognitive function was associated with synaptic preservation and reduction in cerebral amyloid-β protein levels and astrogliosis (P < 0.001 and P < 0.0001), with no apparent additive effects for the combined treatment. The peripherally grafted, green fluorescent protein-labelled and endogenous monocytes, homed to cerebral amyloid plaques and directly engulfed amyloid-β; their recruitment was further enhanced by glatiramer acetate. In glatiramer acetate-immunized mice and, moreover, in the combined treatment group, monocyte recruitment to the brain was coupled with greater elevation of the regulatory cytokine IL10 surrounding amyloid-β plaques. All treated transgenic mice had increased cerebral levels of MMP9 protein (P < 0.05), an enzyme capable of degrading amyloid-β, which was highly expressed by the infiltrating monocytes. In vitro studies using primary cultures of bone marrow monocyte-derived macrophages, demonstrated that glatiramer acetate enhanced the ability of macrophages to phagocytose preformed fibrillar amyloid-β1-42 (P < 0.0001). These glatiramer acetate-treated macrophages exhibited increased expression of the scavenger receptors CD36 and SCARA1 (encoded by MSR1), which can facilitate amyloid-β phagocytosis, and the amyloid-β-degrading enzyme MMP9 (P < 0.0001-0.001). Overall, our studies indicate that increased cerebral infiltration of monocytes, either by enrichment of their levels in the circulation or by weekly immunization with glatiramer acetate, resulted in substantial attenuation of disease progression in murine Alzheimer's models by mechanisms that involved enhanced cellular uptake and enzymatic degradation of toxic amyloid-β as well as regulation of brain inflammation.

Microglia Versus Myeloid Cell Nomenclature during Brain Inflammation

As immune sentinels of the central nervous system (CNS), microglia not only respond rapidly to pathological conditions but also contribute to homeostasis in the healthy brain. In contrast to other populations of the myeloid lineage, adult microglia derive from primitive myeloid precursors that arise in the yolk sac early during embryonic development, after which they self-maintain locally and independently of blood-borne myeloid precursors. Under neuro-inflammatory conditions such as experimental autoimmune encephalomyelitis, circulating monocytes invade the CNS parenchyma where they further differentiate into macrophages or inflammatory dendritic cells. Often it is difficult to delineate resident microglia from infiltrating myeloid cells using currently known markers. Here, we will discuss the current means to reliably distinguish between these populations, and which recent advances have helped to make clear definitions between phenotypically similar, yet functionally diverse myeloid cell types.

Focal transient CNS vessel leak provides a tissue niche for sequential immune cell accumulation during the asymptomatic phase of EAE induction

Peripheral immune cells are critical to the pathogenesis of neurodegenerative diseases including multiple sclerosis (MS) (Hendriks et al., 2005; Kasper and Shoemaker, 2010). However, the precise sequence of tissue events during the early asymptomatic induction phase of experimental autoimmune encephalomyelitis (EAE) pathogenesis remains poorly defined. Due to the spatial-temporal constrains of traditional methods used to study this disease, most studies had been performed in the spine during peak clinical disease; thus the debate continues as to whether tissue changes such as vessel disruption represent a cause or a byproduct of EAE pathophysiology in the cortex. Here, we provide dynamic, high-resolution information on the evolving structural and cellular processes within the gray matter of the mouse cortex during the first 12 asymptomatic days of EAE induction. We observed that transient focal vessel disruptions precede microglia activation, followed by infiltration of and directed interaction between circulating dendritic cells and T cells. Histamine antagonist minimizes but not completely ameliorates blood vessel leaks. Histamine H1 receptor blockade prevents early microglia function, resulting in subsequent reduction in immune cell accumulation, disease incidence and clinical severity.

CCR2-dependent dendritic cell accumulation in the central nervous system during early effector experimental autoimmune encephalomyelitis is essential for effector T cell restimulation in situ and disease progression

Dendritic cells (DCs)--although absent from the healthy CNS parenchyma--rapidly accumulate within brain and spinal cord tissue during neuroinflammation associated with experimental autoimmune encephalomyelitis (EAE; a mouse model of multiple sclerosis). Yet, although DCs have been appreciated for their role in initiating adaptive immune responses in peripheral lymphoid organ tissues, how DCs infiltrate the CNS and contribute to ongoing neuroinflammation in situ is poorly understood. In this study, we report the following: 1) CD11c(+) bone marrow-derived DCs and CNS-infiltrating DCs express chemokine receptor CCR2; 2) compared with CCR2(+/+) cells, adoptively transferred CCR2(-/-) bone marrow-derived DCs or DC precursors do not accumulate in the CNS during EAE, despite abundance in blood; 3) CCR2(-/-) DCs show less accumulation in the inflamed CNS in mixed bone marrow chimeras, when compared with CCR2(+/+) DCs; and 4) ablation of CCR2(+/+) DCs during EAE clinical onset delays progression and attenuates cytokine production by infiltrating T cells. Whereas the role of CCR2 in monocyte migration into the CNS has been implicated previously, the role of CCR2 in DC infiltration into the CNS has never been directly addressed. Our data suggest that CCR2-dependent DC recruitment to the CNS during ongoing neuroinflammation plays a crucial role in effector T cell cytokine production and disease progression, and signify that CNS-DCs and circulating DC precursors might be key therapeutic targets for suppressing ongoing neuroinflammation in CNS autoimmune diseases.

In acute experimental autoimmune encephalomyelitis, infiltrating macrophages are immune activated, whereas microglia remain immune suppressed

Multiple sclerosis (MS) is an autoimmune demyelinating disorder of the central nervous system (CNS) characterized by loss of myelin accompanied by infiltration of T-lymphocytes and monocytes. Although it has been shown that these infiltrates are important for the progression of MS, the role of microglia, the resident macrophages of the CNS, remains ambiguous. Therefore, we have compared the phenotypes of microglia and macrophages in a mouse model for MS, experimental autoimmune encephalomyelitis (EAE). In order to properly discriminate between these two cell types, microglia were defined as CD11b(pos) CD45(int) Ly-6C(neg) , and infiltrated macrophages as CD11b(pos) CD45(high) Ly-6C(pos) . During clinical EAE, microglia displayed a weakly immune-activated phenotype, based on the expression of MHCII, co-stimulatory molecules (CD80, CD86, and CD40) and proinflammatory genes [interleukin-1β (IL-1β) and tumour necrosis factor- α (TNF-α)]. In contrast, CD11b(pos) CD45(high) Ly-6C(pos) infiltrated macrophages were strongly activated and could be divided into two populations Ly-6C(int) and Ly-6C(high) , respectively. Ly-6C(high) macrophages contained less myelin than Ly-6C(int) macrophages and expression levels of the proinflammatory cytokines IL-1β and TNF-α were higher in Ly-6C(int) macrophages. Together, our data show that during clinical EAE, microglia are only weakly activated whereas infiltrated macrophages are highly immune reactive.

Angiotensin-converting enzyme overexpression in myelomonocytes prevents Alzheimer's-like cognitive decline

Cognitive decline in patients with Alzheimer's disease (AD) is associated with elevated brain levels of amyloid β protein (Aβ), particularly neurotoxic Aβ(1-42). Angiotensin-converting enzyme (ACE) can degrade Aβ(1-42), and ACE overexpression in myelomonocytic cells enhances their immune function. To examine the effect of targeted ACE overexpression on AD, we crossed ACE(10/10) mice, which overexpress ACE in myelomonocytes using the c-fms promoter, with the transgenic APP(SWE)/PS1(ΔE9) mouse model of AD (AD⁺). Evaluation of brain tissue from these AD⁺ACE(10/10) mice at 7 and 13 months revealed that levels of both soluble and insoluble brain Aβ(1-42) were reduced compared with those in AD⁺ mice. Furthermore, both plaque burden and astrogliosis were drastically reduced. Administration of the ACE inhibitor ramipril increased Aβ levels in AD⁺ACE(10/10) mice compared with the levels induced by the ACE-independent vasodilator hydralazine. Overall, AD⁺ACE(10/10) mice had less brain-infiltrating cells, consistent with reduced AD-associated pathology, though ACE-overexpressing macrophages were abundant around and engulfing Aβ plaques. At 11 and 12 months of age, the AD⁺ACE(10/WT) and AD⁺ACE(10/10) mice were virtually equivalent to non-AD mice in cognitive ability, as assessed by maze-based behavioral tests. Our data demonstrate that an enhanced immune response, coupled with increased myelomonocytic expression of catalytically active ACE, prevents cognitive decline in a murine model of AD.

Dichotomy of CCL21 and CXCR3 in nerve injury-evoked and autoimmunity-evoked hyperalgesia

The chemokine CCL21 is released from injured neurons and acts as a ligand of the chemokine receptor, CXCR3, which likely contributes to pro-inflammatory adaptations and secondary neuronal damage. CCL21-CXCR3 signalling may therefore impact on the development of neuropathic pain. By using the respective knockout mice we show that deficiency of CCL19/21 in plt/plt mice attenuates nerve injury evoked pain but not the hyperalgesia evoked by autoimmune encephalomyelitis (EAE). Oppositely, CXCR3-deficiency had no protective effect after traumatic nerve injury but reduced EAE-evoked hyperalgesia and was associated with reduced clinical EAE scores, a reduction of the pro-inflammatory cell infiltration and reduced upregulation of interferon gamma and interleukin-17 in the spinal cord. In contrast, microglia activation in the spinal cord after traumatic sciatic nerve injury was neither attenuated in CXCR3(-/-) nor plt/plt mice, nor in double knockouts. However, the severity of EAE, but not the hyperalgesia, was also reduced in plt/plt mice, which was associated with reduced infiltration of the spinal cord with CCR7+ T-cells, an increase of CD25+ T-cells and reduced upregulation of CXCL9 and 10, CCL11 and 12. The data show that CCL21 and CXCR3 have dichotomous functions in traumatic and EAE-evoked neuropathic pain suggesting diverse mechanisms likely requiring diverse treatments although both types of neuropathic pain are mediated in part through the immune activation.

Role of inactivated influenza vaccine in regulation of autoimmune processes in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is characterized by appearance of anti-myelin autoantibodies in the blood and with the increased expression of MHC (major histocompatibility complex) class I and II antigens in the brain tissue. Although there is an evidence of possible linkage between influenza vaccination and development of autoimmune processes, the precise mechanisms of action of this vaccine on EAE-induction is still unclear. In this study, effects of influenza vaccine on clinical sign, antimyelin antibody titer in the blood by ELISA test and expression of MHC class I and II molecules immunohistochemistry were examined in the brain of C57BL mice with EAE. EAE was induced by MOG 35-55 protein in 16 of 32 mice. Influenza split vaccine was administered to eight MOG-induced EAE mice and to eight previously nontreated mice. A significant increase of anti-influenza antibody was detected in vaccinated mice compared to nontreated mice. Also, significant increase of antimyelin antibodies was detected in mice with EAE compared to vaccinated group without EAE and control group, respectively. In EAE-influenza vaccinated mice, a mild but not significant increase of antimyelin antibodies was detected, compared to EAE mice. High expression of MHC-II and mild expression of MHC-I were detected in the brain of mice with EAE. No expressions were detected in vaccinated and normal intact brains. Similar staining was found between EAE-vaccinated and EAE group in both MHC-I and MHC-II expression. The results obtained show that influenza vaccine has no significant influence on EAE induction and severity of autoimmune processes.

Development of experimental autoimmune encephalomyelitis critically depends on CD137 ligand signaling

Multiple sclerosis (MS) is a degenerative autoimmune disease of the CNS. Experimental autoimmune encephalomyelitis (EAE) is a commonly used murine model for MS. Here we report that CD137 ligand (CD137L, 4-1BB ligand, TNFS9), a member of the TNF superfamily, is critical for the development of EAE. EAE symptoms were significantly ameliorated in CD137L(-/-) mice. In the absence of CD137L, myelin oligodendrocyte glycoprotein (MOG)-specific T-cells secreted lower levels of T(h)1/T(h)17 cell-associated cytokines. MOG-specific T-cells also trafficked less efficiently to the CNS in CD137L(-/-) mice, possibly as a consequence of reduced expression of vascular cell adhesion molecule-1 (VCAM-1), which regulates leukocyte extravasation. Thus, CD137L regulates many functions of MOG-specific T-cells that contribute to EAE and may represent a novel therapeutic target for the treatment of MS.

The role of glial cells in Alzheimer disease: potential therapeutic implications

INTRODUCTION

Alzheimer (AD) disease is a complex neurodegenerative disease characterised by inflammation, neurotoxicity, oxidative stress, and reactive gliosis. Microglia and astrocytes not only act as antigen-presenting cells, but also function as effector cells releasing pro-inflammatory molecules that promote excitotoxicity and neurodegeneration.

OBJECTIVE

In the present review we discuss the role of glia, specifically microglia and astrocytes, in the pathophysiology of AD and possible therapeutic implications.

DEVELOPMENT

The growing body of evidence suggesting that microglia and astrocytes play a pathogenic role and activate inflammation pathways, the neurotoxic factors released by these cells when activated, and the way these factors may disrupt the homeostasis of the central nervous system all support the hypothesis that glia-induced inflammation exacerbates AD.

CONCLUSIONS

Inhibiting inflammation by deactivating glial cells may reduce the production of factors which contribute to neurotoxicity, and therefore result in clinical improvement. Microglia and astrocytes are therapeutic targets for the development of new drugs to combat this disease. Therapeutic strategies designed to counter the detrimental effects of overactivation of these cell populations should be investigated.

Targeting metabotropic glutamate receptors in neuroimmune communication

L-Glutamate (L-Glu) is the principal excitatory neurotransmitter in the Central Nervous System (CNS), where it regulates cellular and synaptic activity, neuronal plasticity, cell survival and other relevant functions. Glutamatergic neurotransmission is complex and involves both ionotropic (ligand-gated ion channels; iGluRs) and metabotropic receptors (G-protein coupled receptors). Recent evidence suggests that glutamatergic receptors are also expressed by immune cells, regulating the degree of cell activation. In this review we primarily focus on mGluRs and their role in the crosstalk between the central nervous and immune systems during neuroinflammation.

Pathogenic CD8 T cells in multiple sclerosis and its experimental models

A growing body of evidence suggests that autoreactive CD8 T cells contribute to the disease process in multiple sclerosis (MS). Lymphocytes in MS plaques are biased toward the CD8 lineage, and MS patients harbor CD8 T cells specific for multiple central nervous system (CNS) antigens. Currently, there are relatively few experimental model systems available to study these pathogenic CD8 T cells in vivo. However, the few studies that have been done characterizing the mechanisms used by CD8 T cells to induce CNS autoimmunity indicate that several of the paradigms of how CD4 T cells mediate CNS autoimmunity do not hold true for CD8 T cells or for patients with MS. Thus, myelin-specific CD4 T cells are likely to be one of several important mechanisms that drive CNS disease in MS patients. The focus of this review is to highlight the current models of pathogenic CNS-reactive CD8 T cells and the molecular mechanisms these lymphocytes use when causing CNS inflammation and damage. Understanding how CNS-reactive CD8 T cells escape tolerance induction and induce CNS autoimmunity is critical to our ability to propose and test new therapies for MS.

Oral administration of the KATP channel opener diazoxide ameliorates disease progression in a murine model of multiple sclerosis

Background

Multiple Sclerosis (MS) is an acquired inflammatory demyelinating disorder of the central nervous system (CNS) and is the leading cause of nontraumatic disability among young adults. Activated microglial cells are important effectors of demyelination and neurodegeneration, by secreting cytokines and others neurotoxic agents. Previous studies have demonstrated that microglia expresses ATP-sensitive potassium (K_ATP) channels and its pharmacological activation can provide neuroprotective and anti-inflammatory effects. In this study, we have examined the effect of oral administration of K_ATP channel opener diazoxide on induced experimental autoimmune encephalomyelitis (EAE), a mouse model of MS.

Methods

Anti-inflammatory effects of diazoxide were studied on lipopolysaccharide (LPS) and interferon gamma (IFNγ)-activated microglial cells. EAE was induced in C57BL/6J mice by immunization with myelin oligodendrocyte glycoprotein peptide (MOG_35-55). Mice were orally treated daily with diazoxide or vehicle for 15 days from the day of EAE symptom onset. Treatment starting at the same time as immunization was also assayed. Clinical signs of EAE were monitored and histological studies were performed to analyze tissue damage, demyelination, glial reactivity, axonal loss, neuronal preservation and lymphocyte infiltration.

Results

Diazoxide inhibited in vitro nitric oxide (NO), tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) production and inducible nitric oxide synthase (iNOS) expression by activated microglia without affecting cyclooxygenase-2 (COX-2) expression and phagocytosis. Oral treatment of mice with diazoxide ameliorated EAE clinical signs but did not prevent disease. Histological analysis demonstrated that diazoxide elicited a significant reduction in myelin and axonal loss accompanied by a decrease in glial activation and neuronal damage. Diazoxide did not affect the number of infiltrating lymphocytes positive for CD3 and CD20 in the spinal cord.

Conclusion

Taken together, these results demonstrate novel actions of diazoxide as an anti-inflammatory agent, which might contribute to its beneficial effects on EAE through neuroprotection. Treatment with this widely used and well-tolerated drug may be a useful therapeutic intervention in ameliorating MS disease.

The critical role of antigen-presentation-induced cytokine crosstalk in the central nervous system in multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a debilitating disease of the central nervous system (CNS) that has been extensively studied using the animal model experimental autoimmune encephalomyelitis (EAE). It is believed that CD4(+) T lymphocytes play an important role in the pathogenesis of this disease by mediating the demyelination of neuronal axons via secretion of proinflammatory cytokines resulting in the clinical manifestations. Although a great deal of information has been gained in the last several decades about the cells involved in the inflammatory and disease mediating process, important questions have remained unanswered. It has long been held that initial neuroantigen presentation and T cell activation events occur in the immune periphery and then translocate to the CNS. However, an increasing body of evidence suggests that antigen (Ag) presentation might initiate within the CNS itself. Importantly, it has remained unresolved which antigen presenting cells (APCs) in the CNS are the first to acquire and present neuroantigens during EAE/MS to T cells, and what the conditions are under which this takes place, ie, whether this occurs in the healthy CNS or only during inflammatory conditions and what the related cytokine microenvironment is comprised of. In particular, the central role of interferon-γ as a primary mediator of CNS pathology during EAE has been challenged by the emergence of Th17 cells producing interleukin-17. This review describes our current understanding of potential APCs in the CNS and the contribution of these and other CNS-resident cells to disease pathology. Additionally, we discuss the question of where Ag presentation is initiated and under what conditions neuroantigens are made available to APCs with special emphasis on which cytokines may be important in this process.

Resident B cells regulate thymic expression of myelin oligodendrocyte glycoprotein

Thymic B cells represent a numerically minor cell population located primarily at the cortico-medullary junction. Their biological role is unclear. B cell-deficient μMT mice exhibited reduced medullary thymic epithelial cell (mTEC) numbers and reduced MOG and insulin mRNA expression. Lymphotoxin produced by B cells was critical for normal tissue restricted antigen (TRA) expression, suggesting that B cells regulate self-antigens through their production of LT. These results reveal an unexpected role of B cells in mTEC maintenance and expression of TRAs through their production of LT.

Isolation of brain and spinal cord mononuclear cells using percoll gradients

Isolation of immune cells that infiltrate the central nervous system (CNS) during infection, trauma, autoimmunity or neurodegeneration, is often required to define their phenotype and effector functions. Histochemical approaches are instrumental to determine the location of the infiltrating cells and to analyze the associated CNS pathology. However, in-situ histochemistry and immunofluorescent staining techniques are limited by the number of antibodies that can be used at a single time to characterize immune cell subtypes in a particular tissue. Therefore, histological approaches in conjunction with immune-phenotyping by flow cytometry are critical to fully characterize the composition of local CNS infiltration. This protocol is based on the separation of CNS cellular suspensions over discontinous percoll gradients. The current article describes a rapid protocol to efficiently isolate mononuclear cells from brain and spinal cord tissues that can be effectively utilized for identification of various immune cell populations in a single sample by flow cytometry.

Macrophages in Alzheimer's disease: the blood-borne identity

Alzheimer’s disease (AD) is a progressive and incurable neurodegenerative disorder clinically characterized by cognitive decline involving loss of memory, reasoning and linguistic ability. The amyloid cascade hypothesis holds that mismetabolism and aggregation of neurotoxic amyloid-β (Aβ) peptides, which are deposited as amyloid plaques, are the central etiological events in AD. Recent evidence from AD mouse models suggests that blood-borne mononuclear phagocytes are capable of infiltrating the brain and restricting β-amyloid plaques, thereby limiting disease progression. These observations raise at least three key questions: (1) what is the cell of origin for macrophages in the AD brain, (2) do blood-borne macrophages impact the pathophysiology of AD and (3) could these enigmatic cells be therapeutically targeted to curb cerebral amyloidosis and thereby slow disease progression? This review begins with a historical perspective of peripheral mononuclear phagocytes in AD, and moves on to critically consider the controversy surrounding their identity as distinct from brain-resident microglia and their potential impact on AD pathology.

FLT-3 expression and function on microglia in multiple sclerosis

Inflammatory cell infiltration and resident microglial activation within the central nervous system (CNS) are pathological events in multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). While MS therapies target the peripheral immune system, no treatment is currently known to also modulate microglia. FMS-like tyrosine-3 (FLT-3) is expressed on hematopoietic and dendritic cells. We reported that FLT-3 inhibition ameliorates early actively induced EAE by predominantly modulating dendritic cell function as compared to microglia. We demonstrate in this report that FLT-3 is expressed in perivascular cuffs, brain parenchyma and in non-lesioned gray and white matter within MS brain but not in these regions within control brain. Furthermore, we demonstrate that FLT-3 is expressed on two populations of cells within MS brain; one which expresses the dendritic cell marker CD209, and the other which does not, suggesting that FLT-3 within MS brain is expressed on infiltrating dendritic cells and a non-dendritic cell such as microglia. Additionally, we report that FLT-3 inhibition in murine microglia blocks, in a dose-dependent manner, IFN-γ-induced expression of MHC class II and CD86, and LPS-induced secretion of IL-6. These data suggest that FLT-3 is involved in microglial cells' capacity to respond to environmental cues to function as antigen presenting cells and mediate CNS inflammation. Furthermore these data suggest that FLT-3 may be a therapeutic target on microglia to mitigate CNS inflammation.

Activated microglial cells acquire an immature dendritic cell phenotype and may terminate the immune response in an acute model of EAE

Antigen presentation, a key mechanism in immune responses, involves two main signals: the first is provided by the engagement of a major histocompatibility complex (MHC), class I or class II, with their TCR receptor in lymphocytes, whereas the second demands the participation of different co-stimulatory molecules, such as CD28, CTLA-4 and their receptors B7.1 and B7.2. Specific T-cell activation and deactivation are achieved through this signalling. The aim of our study is to characterise, in the acute experimental autoimmune encephalomyelitis (EAE) model in Lewis rat, the temporal expression pattern of these molecules as well as the cells responsible for their expression. To accomplish that, MBP-immunised female Lewis rats were daily examined for the presence of clinical symptoms and sacrificed, according to their clinical score, at different phases during EAE. Spinal cords were cut with a cryostat and processed for immunohistochemistry: MHC-class I and MHC-class II, co-stimulatory molecules (B7.1, B7.2, CD28, CTLA-4) and markers of dendritic cells (CD1 for immature cells and fascin for mature cells). Our results show that microglial cells are activated in the inductive phase and, during this phase and peak, they are able to express MHC-class I, MHC-class II and CD1, but not B7.1 and B7.2. This microglial phenotype may induce the apoptosis or anergy of infiltrated CD28+ lymphocytes observed around blood vessels and in the parenchyma. During the recovery phase, microglial cells express high MHC-class I and class II and, those located in the surroundings of blood vessels, displayed the B7.2 co-stimulatory molecule. These cells are competent to interact with CTLA-4+ cells, which indicate an active role of microglial cells in modulating the ending of the immune response by inducing lymphocyte activity inhibition and Treg activation. Once clinical symptomatology disappeared, some foci of activated microglial cells (MHC-class II+/B7.2+) were still present in concomitance with CTLA-4+ cells, suggesting a prolonged involvement of microglia in lymphocyte inhibition and tolerance promotion. In addition to microglia, during the inductive and recovery phases, we also found perivascular ED2+ cells and fascin+ cells which are able to migrate to the parenchyma and may play a role in lymphocytic regulation. Further studies to understand the specific function played by these cells are warranted.

T cells facilitate recovery from Venezuelan equine encephalitis virus-induced encephalomyelitis in the absence of antibody

Venezuelan equine encephalitis virus (VEEV) is a mosquito-borne RNA virus of the genus Alphavirus that is responsible for a significant disease burden in Central and South America through sporadic outbreaks into human and equid populations. For humans, 2 to 4% of cases are associated with encephalitis, and there is an overall case mortality rate of approximately 1%. In mice, replication of the virus within neurons of the central nervous system (CNS) leads to paralyzing, invariably lethal encephalomyelitis. However, mice infected with certain attenuated mutants of the virus are able to control the infection within the CNS and recover. To better define what role T cell responses might be playing in this process, we infected B cell-deficient microMT mice with a VEEV mutant that induces mild, sublethal illness in immune competent mice. Infected microMT mice rapidly developed the clinical signs of severe paralyzing encephalomyelitis but were eventually able to control the infection and recover fully from clinical illness. Recovery in this system was T cell dependent and associated with a dramatic reduction in viral titers within the CNS, followed by viral persistence in the brain. Further comparison of the relative roles of T cell subpopulations within this system revealed that CD4(+) T cells were better producers of gamma interferon (IFN-gamma) than CD8(+) T cells and were more effective at controlling VEEV within the CNS. Overall, these results suggest that T cells, especially CD4(+) T cells, can successfully control VEEV infection within the CNS and facilitate recovery from a severe viral encephalomyelitis.

Role of IL-23 in mobilization of immunoregulatory nitric oxide- or superoxide-producing Gr-1+ cells from bone marrow

Spleens of mice injected with heat-killed Mycobacterium tuberculosis increase their Gr-1+ cell content and develop a system of interactive Ly-6G+ and Ly-6G-Gr-1+ populations or "Greg" subsets, which, upon stimulation by activated T cells, produce immunoregulatory superoxide (O2(-)) and nitric oxide (NO), respectively. The balance between immunosuppressive NO and its antagonist O2(-) regulates T cell expansion, similar to regulation of vasodilation. Reduction of NO levels by O2(-) is required for efficient T cell expansion and development of autoimmunity. We studied the source of Gr-1+ cells in bone marrow (BM), where their levels were higher than in spleen, with both Greg subsets expressing strong activity. In the spleens of primed IL-23-/- mice, Ly-6G+ cells remained at naïve levels and produced no O2(-). The complementary Ly-6G(-)Gr-1+ splenocytes and their suppressive activity were partially reduced. Surprisingly, Gr-1+ cell levels in BM of IL-23-/- mice were increased, as were their O2(-) and NO production. Transfer of primed BM cells partially restored regulatory function in the spleen of IL-23-/- recipients. The results suggest that IL-23 is involved in mobilization of O2(-)- and NO-producing Gr-1+ cells from BM, which may contribute to its widely studied role in (auto)immunity.

PARP-1 deficiency increases the severity of disease in a mouse model of multiple sclerosis

Poly(ADP-ribose) polymerase-1 (PARP-1) has been implicated in the pathogenesis of several central nervous system (CNS) disorders. However, the role of PARP-1 in autoimmune CNS injury remains poorly understood. Therefore, we studied experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis in mice with a targeted deletion of PARP-1. We identified inherent physiological abnormalities in the circulating and splenic immune composition between PARP-1(-/-) and wild type (WT) mice. Upon EAE induction, PARP-1(-/-) mice had an earlier onset and developed a more severe EAE compared with WT cohorts. Splenic response was significantly higher in PARP-1(-/-) mice largely because of B cell expansion. Although formation of Th1 and Th17 effector T lymphocytes was unaffected, PARP-1(-/-) mice had significantly earlier CD4+ T lymphocyte and macrophage infiltration into the CNS during EAE. However, we did not detect significant differences in cytokine profiles between PARP-1(-/-) and WT spinal cords at the peak of EAE. Expression analysis of different PARP isozymes in EAE spinal cords showed that PARP-1 was down-regulated in WT mice and that PARP-3 but not PARP-2 was dramatically up-regulated in both PARP-1(-/-) and WT mice, suggesting that these PARP isozymes could have distinct roles in different CNS pathologies. Together, our results indicate that PARP-1 plays an important role in regulating the physiological immune composition and in immune modulation during EAE; our finding identifies a new aspect of immune regulation by PARPs in autoimmune CNS pathology.

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.

Signal transduction inhibition of APCs diminishes th17 and Th1 responses in experimental autoimmune encephalomyelitis

IL-17- and IFN-gamma-secreting T cells play an important role in autoimmune responses in multiple sclerosis and the model system experimental autoimmune encephalomyelitis (EAE). Dendritic cells (DCs) in the periphery and microglia in the CNS are responsible for cytokine polarization and expansion of this T cell subset. Our results indicate that in vivo administration of a signal transduction inhibitor that targets DCs to mice with EAE led to a decrease in CNS infiltration of pathogenic Ag-specific T cells. Since this approach does not target T cells directly, we assessed the effects on the APCs that are involved in generating the T cell responses. Since in EAE and multiple sclerosis, both microglia and peripheral DCs are likely to contribute to disease, we utilized a bone marrow chimera system to distinguish between these two populations. These studies show that peripheral DCs are the primary target but that microglia are also modestly affected by CEP-701, as numbers and activation states of the cells in the CNS are decreased after therapy. Our results also showed a decrease in secretion of TNF-alpha, IL-6, and IL-23 by DCs as well as a decrease in expression of costimulatory molecules. We further determined that levels of phospho-Stat1, Stat3, Stat5, and NF-kappaB, which are signaling molecules that have been implicated in these pathways, were decreased. Thus, use of this class of signal transduction inhibitors may represent a novel method to treat autoimmunity by dampening the autoreactive polarizing condition driven by DCs.

Intracerebral dendritic cells critically modulate encephalitogenic versus regulatory immune responses in the CNS

Dendritic cells (DCs) appear in higher numbers within the CNS as a consequence of inflammation associated with autoimmune disorders, such as multiple sclerosis, but the contribution of these cells to the outcome of disease is not yet clear. Here, we show that stimulatory or tolerogenic functional states of intracerebral DCs regulate the systemic activation of neuroantigen-specific T cells, the recruitment of these cells into the CNS and the onset and progression of experimental autoimmune encephalomyelitis (EAE). Intracerebral microinjection of stimulatory DCs exacerbated the onset and clinical course of EAE, accompanied with an early T-cell infiltration and a decreased proportion of regulatory FoxP3-expressing cells in the brain. In contrast, the intracerebral microinjection of DCs modified by tumor necrosis factor alpha induced their tolerogenic functional state and delayed or prevented EAE onset. This triggered the generation of interleukin 10 (IL-10)-producing neuroantigen-specific lymphocytes in the periphery and restricted IL-17 production in the CNS. Our findings suggest that DCs are a rate-limiting factor for neuroinflammation.

Peripheral dendritic cells are essential for both the innate and adaptive antiviral immune responses in the central nervous system

Intranasal application of vesicular stomatitis virus (VSV) causes acute infection of the central nervous system (CNS). However, VSV encephalitis is not invariably fatal, suggesting that the CNS may contain a professional antigen-presenting cell (APC) capable of inducing or propagating a protective antiviral immune response. To examine this possibility, we first characterized the cellular elements that infiltrate the brain as well as the activation status of resident microglia in the brains of normal and transgenic mice acutely ablated of peripheral dendritic cells (DCs) in vivo. VSV encephalitis was characterized by a pronounced infiltrate of myeloid cells (CD45(high)CD11b(+)) and CD8(+) T cells containing a subset that was specific for the immunodominant VSV nuclear protein epitope. This T cell response correlated temporally with a rapid and sustained upregulation of MHC class I expression on microglia, whereas class II expression was markedly delayed. Ablation of peripheral DCs profoundly inhibited the inflammatory response as well as infiltration of virus-specific CD8(+) T cells. Unexpectedly, the VSV-induced interferon-gamma (IFN-gamma) response in the CNS remained intact in DC-deficient mice. Thus, both the inflammatory and certain components of the adaptive primary antiviral immune response in the CNS are dependent on peripheral DCs in vivo.

Toll-like receptor 7 mitigates lethal West Nile encephalitis via interleukin 23-dependent immune cell infiltration and homing

West Nile virus (WNV), a mosquito-transmitted single-stranded RNA (ssRNA) flavivirus, causes human disease of variable severity. We investigated Toll-like receptor 7-deficient (Tlr7(-/-)) and myeloid differentiation factor 88-deficient (Myd88(-/-)) mice, which both have defective recognition of ssRNA, and found increased viremia and susceptibility to lethal WNV infection. Despite increased tissue concentrations of most innate cytokines, CD45(+) leukocytes and CD11b(+) macrophages failed to home to WNV-infected cells and infiltrate into target organs of Tlr7(-/-) mice. Tlr7(-/-) mice and macrophages had reduced interleukin-12 (IL-12) and IL-23 responses after WNV infection, and mice deficient in IL-12 p40 and IL-23 p40 (Il12b(-/-)) or IL-23 p19 (Il23a(-/-)), but not IL-12 p35 (Il12a(-/-)), responded similarly to Tlr7(-/-) mice, with increased susceptibility to lethal WNV encephalitis. Collectively, these results demonstrate that TLR7 and IL-23-dependent WNV responses represent a vital host defense mechanism that operates by affecting immune cell homing to infected target cells.