key: cord-0001635-fk9ww8tm authors: Huber, Amanda K.; Duncker, Patrick C.; Irani, David N. title: Immune Responses to Non-Tumor Antigens in the Central Nervous System date: 2014-11-13 journal: Front Oncol DOI: 10.3389/fonc.2014.00328 sha: dc4a3df470bfd502db850deec4219e39f7eadb63 doc_id: 1635 cord_uid: fk9ww8tm The central nervous system (CNS), once viewed as an immune-privileged site protected by the blood–brain barrier (BBB), is now known to be a dynamic immunological environment through which immune cells migrate to prevent and respond to events such as localized infection. During these responses, endogenous glial cells, including astrocytes and microglia, become highly reactive and may secrete inflammatory mediators that regulate BBB permeability and recruit additional circulating immune cells. Here, we discuss the various roles played by astrocytes, microglia, and infiltrating immune cells during host immunity to non-tumor antigens in the CNS, focusing first on bacterial and viral infections, and then turning to responses directed against self-antigens in the setting of CNS autoimmunity. The central nervous system (CNS) was previously viewed as an immune-privileged area, fully isolated from the immune system by the blood-brain barrier (BBB). In early studies, Ehrlich reported that while various organs were strongly stained following intravenous, intra-arterial, or subcutaneous injection of intravital dyes, the brain was only weakly stained or not at all (1) . Other studies found that tissue grafts were not rejected when implanted into the brains of test animals (2) , leading to the idea that the CNS was fully "immune-privileged." This viewpoint had to be altered, however, after it was discovered that a graft within the CNS could be rejected if a second graft was placed subcutaneously into the same animal (3) . This finding clearly demonstrated that foreign antigens are recognized in the CNS if peripheral priming occurs (3) . It is now accepted that the BBB is a dynamic, interactive, and regulatory tissue interface that allows bi-directional communication between the CNS and the immune system (4, 5) . The BBB, formed by complex interactions between capillary endothelial cells (ECs), astrocyte end-feet, pericytes, and microglia (6, 7) , is the largest and most stringent barrier that impedes the paracellular movement of ions, solutes, proteins, water, and leukocytes into the CNS (8) . However, the BBB can also be influenced by peripheral immune events, creating what has now come to be known as the neuro-immune axis (4, 9, 10) . The neuro-immune axis is not only responsible for establishing the blood-CNS barrier at baseline, but it also regulates communication between the CNS and the immune system during pathological conditions such as viral or bacterial infections, ischemia, or inflammatory autoimmune disorders such as multiple sclerosis (MS) (11) . It achieves this state by responding to secreted factors from both immune and CNS cells, as well as by regulating the exchange of chemokines, cytokines, and immune cells between the blood and the CNS (4, 9, 10) . Therefore, the original concept of the BBB being a purely anatomical barrier has now shifted to one where the BBB is considered a highly reactive interface controlled by signals from ECs, glial cells, pericytes, and neurons in the CNS, as well as from immune responses in the periphery (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) . The BBB is composed of capillary ECs ensheathed by astrocyte end-feet, pericytes, and microglia (6, 7) . Astrocyte end-feet completely surround the abluminal surface of brain capillaries forming a layer known as the glial limitans, but direct contact with EC is inhibited by a dense basement membrane (22) . While astrocytes are necessary to maintain BBB integrity by secreting factors that alter barrier permeability (6, 23) , they are not actually required to form the BBB, which develops even before these astrocytic processes are present (24) (25) (26) . Astrocytes control blood flow to the CNS by regulating vascular tone through fluctuating calcium currents (27) . Pericytes are essential to barrier formation, as the BBB is compromised in pericyte-deficient mice (28, 29) . These cells regulate gene expression in EC and induce the polarization of astrocyte end-feet (28) . Microglia play a role at the BBB by regulating substrate transport across EC and by linking the brain to systemic immune activity (30). Blood-brain barrier EC forms a highly sophisticated barrier via a network of tight junctions (TJ) and adherens junctions (AJ) (8, 31, 32) . The EC of the CNS are unique in that the TJ restrict the paracellular passage of solutes, have no pinocytic activity, and have few if any fenestrations (33-39). This causes the BBB to have high endothelial electrical resistance (40, 41), some 50-100 times higher than peripheral microvessels (42-44). The TJ are composed of a parallel network of intramembranous protein strands, composed of claudins, occludin, and zonula occludin (ZO) proteins (37). Claudins, specifically claudin-3, -4, and -12, compose the TJ backbone (45-47). Occludin is not required for TJ formation (48); instead, it plays a role in "sealing" the junction thereby increasing electrical resistance (49, 50). CNS microvessel TJ are also abundant in ZO-2, and to a lesser extent, ZO-1, that are cytoplasmic accessory proteins that serve to anchor the transmembrane proteins of the TJ to the actin cytoskeleton of the ECs (51, 52). The choroid plexus (CP) is a villous structure located on the roof of the four cerebral ventricles where cerebrospinal fluid (CSF) is actively secreted. The CP is highly vascular and contains the blood-CSF barrier (BCSFB) (51). Unlike the BBB, however, the BCSFB arises from cuboidal choroid plexus epithelial cells (CPE) with a very different TJ structure. The CPE express ZO-1 and ZO-2 in different amounts (51), and have a different claudin signature, expressing claudin-1, -2, -3, and -11 (51, 53, 54). Furthermore, capillaries within the CP villi are fenestrated (51, 55, 56), reflected by a much lower endothelial electrical resistance than the BBB (57). For these reasons, the BBB is considered more of an absolute barrier, while the BCSFB may be where most normal immune surveillance of the CNS occurs (58). It is now accepted there is a constant need for immune surveillance of the normal CNS as part of host defense (11, 59, 60) , with mechanisms present that simultaneously keep excessive inflammation in check (61). To assist in maintaining this control, the healthy CNS is relatively devoid of antigen-presenting cells (APC), lacks constitutive human leukocyte antigen (HLA) class I and II protein expression on parenchymal cells, and does not maintain typical lymphatic vessels (11) . CD4+ T cells, having first encountered antigens in deep cervical lymph nodes (62), carry out routine surveillance of the CNS by searching for their cognate antigens presented by macrophages in the CSF (11, 63) . Resting lymphocytes fail to enter the CNS (64), while activated T cells of all specificities can traverse the BBB and/or BCSFB (65). Those cells that do not encounter their cognate antigen within a few hours then circulate out of the CNS (66, 67). The first steps of pathogenic neuroinflammation involve changes at the BBB, including increased production of chemokines and up-regulation of adhesion molecules by the EC resulting in leukocytes traversing the BBB and accumulating in the perivascular space of post-capillary venules (11, 68) . Even during these early events, however, cellular recruitment remains tightly controlled as parenchymal lymphocytes express a unique adhesion molecule profile, different from peripheral T cells (69-71). Once in the perivascular space, T cells encounter the glial limitans as well as astrocytes that express and release factors that induce apoptosis (72), inhibit proliferation (72), induce differentiation into a regulatory (Treg) phenotype (73). Microglia and neurons also assist in controlling neuroinflammation. Microglia do so by expressing a homolog of the co-stimulatory molecule B7, programed death protein (PD)-1, which negatively regulates T cell activation and cytokine production (74). Neurons secrete transforming growth factor (TGF)-β, exert cell contact-dependent effects that support the conversion of CD4 T cells to Tregs, and can be induced to express the PD-1 ligand, PD-L1 (75) . Thus, while the BBB is not the impenetrable barrier it was once thought to be, CD4+ T cell surveillance of the CNS is still a tightly controlled process. Bacterial infections of the CNS are rare, but often life threatening, events (76) . Excluding direct inoculation following CNS trauma, bacteria typically gain CNS entry following hematogenous dissemination from distant sites (lungs and heart valves) or by direct extension from parameningeal foci of infection (inner ear and sinuses). Penetration of the BBB may occur via three potential mechanisms: (1) direct destruction of capillary ECs (77, 78), (2) disruption of intercellular TJ and migration in between ECs (79), and (3) transcytosis via intracellular vesicles directly through ECs (80) . Once inside, numerous innate immune receptors and pathways are activated (Figure 1 ). Analogous to peripheral tissues, resident CNS immune cells known as microglia bear a wide range of innate immune receptors. Common bacterial motifs, referred to as pathogen associated molecular patterns (PAMP), are recognized by cognate pattern recognition receptors (PRR), including Toll-like receptors (TLR), on the surface and in the cytoplasm of microglia, and to a lesser extent, on astrocytes (81) (82) (83) . Microglial activation, triggered either by intact bacteria or bacterial cell wall antigens (84, 85) , results in rapid changes in cellular morphology in vivo (86) . Similar to tissue resident macrophages found in the periphery, microglia can phagocytize bacteria and present bacterial antigens via HLA to infiltrating CD4 T cells in vivo (84, 87, 88) . These cells also rapidly produce pro-inflammatory cytokines and chemokines that recruit peripheral leukocytes to the area of infection and activate astrocytes. For example, during both experimental Streptococcus pneumoniae and Staphylococcus aureus infections of the CNS, microglia produce tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-12, C-X-C motif ligand (CXCL)1, CXCL2, C-C motif ligand (CCL)2, CCL3, and CCL5 ex vivo, mediators that recruit neutrophils (CXCL1 and CXCL2), monocytes (CCL2 and CCL3), and T cells (CCL5) (84, 85, (89) (90) (91) . These activated microglia also secrete matrix metalloproteinases (MMP) that enhance BBB breakdown and facilitate additional leukocyte extravasation into the CNS (92) . Finally, microglia can have direct bactericidal activity, being capable of producing reactive oxygen species (ROS), reactive nitrogen intermediates, and other proteases that kill bacteria in vivo (93) (94) (95) (96) . Microglia partner with astrocytes to eliminate infection as quickly as possible in order to minimize neuronal damage (86, 97, 98) . In the normal CNS, astrocytes contribute to gap junction stability of the BBB (99) . Their release of pro-inflammatory mediators such as IL-1β (100, 101), nitric oxide (102), TGF-β (103), and MMPs (92) in vitro suggest these cells may compromise BBB integrity in the setting of bacterial infection. Astrocytes are activated by bacterial PAMP or mediators produced by microglia; this changes their morphology and further triggers their release of innate inflammatory mediators both in vitro and in vivo. These mediators can include complement proteins, IL-1β, IL-6, and the chemokines, CCL2, CCL3, CXCL1, and CXCL10 (104) (105) (106) (107) (108) (109) (110) (111) , which further help recruit neutrophils, monocytes, and T cells. In response to interferon (IFN)-γ, TNF-α, and/or IL-1β, astrocytes Frontiers in Oncology | Neuro-Oncology (2). Common bacterial motifs (PAMPs) are recognized by pattern recognition receptors (PRRs) on microglia and astrocytes resulting in their activation. This causes changes in glial cell morphology, enhanced production of inflammatory mediators that recruit neutrophils, monocytes, and T cells, and increased endothelial cell expression of adhesion molecules, including ICAM-1 and VCAM-1 (3). Circulating neutrophils, monocytes, and T cells then bind and extravasate into the infected CNS (4). Neutrophils directly phagocytize and kill bacteria through the release of defensins, lytic enzymes, and anti-microbial peptides (5) . Neutrophils also produce MMPs, IL-6, IL-8, CXCL9, and CXCL10 that further open the BBB and shift the chemotactic profile toward the recruitment of T cells. Bacterial antigens are presented to T cells by microglia and/or infiltrating monocytes, transitioning from innate immunity toward an adaptive immune response (6) . Resolution of bacterial infection returns tight junctions to normal and microglia and astrocytes to a resting state (7). also up-regulate the cell surface adhesion molecules, intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion molecule (VCAM)-1 in vitro (112) (113) (114) (115) (116) , which would enhance the infiltration of monocytes and T cells into the CNS in vivo. As in the periphery (117, 118) , neutrophils are one of the primary lines of host defense during CNS bacterial infections (112, 119, 120) . Studies in knockout mice show that the main chemokines driving neutrophil recruitment to the CNS are the C-X-C motif receptor (CXCR)-2 ligands, CXCL1 and CXCL2 (121) . Furthermore, CSF samples from patients with bacterial meningitis show elevated levels of neutrophil attracting chemokines compared to controls (122, 123) . Neutrophils, like microglia, respond to PAMP through various TLR, and are activated by cytokines such as TNF-α and IFN-γ in vitro (124) . Neutrophils activated in the periphery up-regulate adhesion molecules that enhance their migration into tissues (125) , while BBB EC express E-selectin and P-selectin during CNS bacterial infection (126) , suggesting a mechanism that allows for the migration of neutrophils during these infections. Once neutrophils recognize a bacterial pathogen, they can directly phagocytize these organisms (127) , as well as release MMP, defensins, lytic enzymes, and anti-microbial peptides that aid in clearing the infection (128) . The inflammatory cytokine, TNF-α, induces neutrophils to produce IL-6, IL-8, CXCL9, and CXCL10 in vivo (129, 130) , thereby shifting the chemotactic profile toward the recruitment of T cells and driving the adaptive immune response. Adaptive immune responses are important in fighting CNS bacterial infections (131) . During bacterial meningitis, T cell production of IFN-γ leads to the generation of chemokines that preferentially recruit monocytes and more T cells (132) , supporting the transition from an innate to an adaptive immune response. Furthermore, IFN-γ, potentially made locally by T cells, increases the antigen-presenting capacity of microglial cells in vitro via upregulation of HLA class I and II molecules, the co-stimulatory molecules, B7-1 and B7-2, and CD40 (133, 134) . Bacterial antigen presentation by microglia activates T cells (135) , driving further T cell proliferation and greater production of IFN-γ. Viruses use a variety of mechanisms to gain entry into the CNS. In the case of alphaherpesviruses (i.e., herpes simplex virus and varicella-zoster virus) and rabies virus, infection of peripheral nerves allows viral particles to travel by anterograde axonal transport into the CNS. Human immunodeficiency virus and human T cell leukemia virus-I enter the CNS parenchyma by infecting host immune cells in the periphery, and using them as "Trojan horses" to carry viral particles across the BBB. Finally, Epstein-Barr virus and West Nile virus directly infect the ECs of the BBB, resulting in barrier disruption and enhanced migration of immune cells into the parenchyma (136) . Because viruses can infect microglia, astrocytes, oligodendrocytes, as well as terminally differentiated and non-renewable cells such as neurons, the ensuing immune response within the CNS must avoid extensive cytolytic damage of virus-infected target cells (137) . In general, innate anti-viral immunity such as the generation of type-I IFN occurs very rapidly, while the adaptive immune response is slower because it must first develop in the periphery (138) . Important components of adaptive anti-viral immunity involve IFN-γ production by T cells as well as the www.frontiersin.org expansion and migration of virus-specific antibody secreting cells (ASC) (138, 139) (Figure 2) . During CNS viral infections, virus-specific PAMP activate individual TLR present on microglia, astrocytes, and oligodendrocytes. The former two cell populations, in particular, respond by producing anti-viral and pro-inflammatory mediators. During experimental mouse hepatitis virus (MHV) infection, astrocytes and microglia produce both type-I IFN (IFN-α and IFN-β), as well as IL-6, TNF-α, IL-12, IL-1α, and IL-1β in vivo (140) (141) (142) . Furthermore, MHV infection triggers MMP-3 and MMP-12 release from astrocytes and oligodendrocytes (142) , which along with (1), viruses can gain entry into the CNS by infecting peripheral nerves and traveling by anterograde axonal transport into the CNS, by infecting host immune cells in the periphery and using these cells as "Trojan horses" to carry them across the BBB, or by directly infecting BBB endothelial cells (2) . Viral PAMPs then activate microglia, astrocytes, and oligodendrocytes (3). Microglia and astrocytes produce a range of anti-viral/pro-inflammatory cytokines, including type-I IFNs, IL-6, TNF-α, IL-12, IL-1α, and IL-1β (3). Astrocytes also produce MMP-3 and MMP-12 resulting in the up-regulation of adhesion molecules on endothelial cells (3) . Interactions between adhesion molecules and neutrophils contribute to BBB breakdown via the production of MMP-9 and the disassembly of the tight junctions (4). DCs are seen in the CNS within several days and migrate to draining lymph nodes where they activate and expand virus-specific T cells (5) . Chemokines produced by astrocytes are responsible for recruiting virus-specific CD4+ and CD8+ T cells as well as ASCs to the CNS (6). CD8+ T cells produce IFN-γ and lytic molecules, including granzyme B and perforin, to eliminate virus from astrocytes, while IFN-γ controls viral replication in oligodendrocytes (7). Virus-specific antibodies control virus replication in cells such as neurons via complement-independent, non-cytolytic mechanisms. These antibodies inhibit virus budding and replication, viral RNA transcription, and cell-to-cell virus spread. IL-6 and the up-regulation of adhesion molecules on cerebrovascular endothelium, enhance cellular migration across the BBB (143) . Astrocytes produce CXCL10, CXCL11, and CCL5 in vivo that recruit virus-specific CD4+ and CD8+ T cells (144) (145) (146) , as well as ASC (147, 148) , to the CNS to promote viral clearance. CXCL9 production from microglia is dependent on IFN-γ, while CXCL10 and CXCL11 are up-regulated by type-I IFN and TNF-α (149) (150) (151) (152) . Neutrophils and macrophages are recruited to the CNS following viral infection (153, 154) . Thus far, macrophages appear to have more limited anti-viral activity in the CNS (155), but neutrophils contribute to the breakdown of the BBB by interacting with EC via adhesion molecules to promote the disassembly of tight junction complexes (156) . Neutrophils also secrete MMP-9 that degrades the extracellular matrix and basal lamina of the BBB and further opens the BBB (157) . This has been most clearly demonstrated in the MHV model, where depletion of MMP-9 inhibited lymphocyte infiltration into the CNS (157, 158) . Dendritic cells (DC) are seen in the CNS within a few days after CNS viral infection. These cells rely on the chemokine CCL3 to migrate to cervical lymph nodes draining the CNS, where they prime virus-specific T cells (159) . In the MHV model, virus-specific CD8+ T cells are detected in local lymph nodes prior to CNS infiltration and then accumulate in the CNS (160) . Both CD4+ and CD8+ T cells are in part recruited to the CNS by the chemokines, CXCL9 and CXCL10, acting through their cognate receptor, CXCR3 (161) (162) (163) . T cell expression of CCR2 and CCR5 likely contribute to CNS recruitment as well (164, 165) . The role of CD4+ T cells in this setting is to support CD8+ T cell function via the production of IFN-γ (166) . CD8+ T cells are the main anti-viral effector cells in the CNS during infection and are essential for clearing virus from glial cells (142, 167, 168) . CD8+ T cells produce IFN-γ and lytic molecules, including granzyme B and perforin (169) . These lytic molecules eliminate virally infected astrocytes (170) , while IFN-γ serves to control viral replication in oligodendrocytes (171, 172) . In both the MHV and Sindbis virus (SINV) encephalitis models, T cells promote B cell proliferation and differentiation (173, 174) , in part by secreting the cytokines, IL-10 and IL-21 (175) (176) (177) . Virus-specific ASC help control viruses in the CNS through potent complement-independent, non-cytolytic mechanisms (141, (178) (179) (180) (181) (182) (183) . These ASC arise either from ectopic lymphoid follicle-like structures within the CNS (152) or migrate from cervical lymph nodes where they have expanded and up-regulated CXCR3 and CXCR4 on their surface prior to entering the CNS (184) . ASC recruitment to the CNS has been most extensively studied in the SINV encephalitis model. The initial ASC entering the CNS have an HLA class II positive, plasmablast-like phenotype, but these cells gradually lose HLA class II expression and acquire a more plasma cell-like phenotype (139, 141) . Virus-specific antibodies function to neutralize both extracellular virus as well as virus particles budding from infected cell membranes. During SINV infection, Frontiers in Oncology | Neuro-Oncology antibodies that bind the E2 viral envelope glycoprotein inhibit virus replication (185) and prevent viral budding from infected neurons without actually killing target cells (182, 186) . Similarly, during rabies virus infection, antibodies against the RV glycoprotein inhibit viral RNA transcription and prevent cell-to-cell viral spread (187) . Antibodies can also trigger natural killer (NK) cells and macrophages to induce antibody dependent cell-mediated cytolysis of virally infected cells (152) . Finally, in exchange for non-cytolytic viral clearance in the acute setting, virus-specific ASC must persist in the CNS long term to prevent viral reactivation at a later date since viral RNA is never fully eradicated from target tissues (139) . Multiple sclerosis, an autoimmune disease characterized by infiltrating immune cells targeting myelin antigens in the CNS, is the most common cause of neurologic disability in persons younger than 40 years of age (188) . Pathologically, MS lesions are characterized by focal inflammation, demyelination, and axonal damage (189) . MS is a complex disease whose occurrence and progression are influenced by both genetic (190) (191) (192) and environmental (193, 194) risk factors. Evidence derived from both human genetic studies and a related mouse model, experimental autoimmune encephalomyelitis (EAE), suggest that encephalitogenic CD4+ T cells are primary initiators of disease. Genome-wide association studies show that MS risk alleles are confined to immune related genes governing antigen presentation as well as the proliferation and survival of T cells, including HLA class II (HLA-DRB1*1501), IL-2R, and IL-7R (190) (191) (192) . Moreover, EAE in mice is induced by immunizing animals with various myelin peptides (195) , or via the adoptive transfer of myelin-specific CD4+ T cells, resulting in a disease having some clinical and pathological similarities to human MS (196, 197) . In MS patients, CD4+ T cells localize within CNS lesions present in the brain (198) and spinal cord (199) , and elevated frequencies of myelin-reactive CD4+ T cells can be found in circulating the blood (200, 201) . Although not described in detail here due to space constraints, many MS lesions also contain abundant CD8+ T cells whose specificity and role in disease pathogenesis remain poorly understood. Likewise, therapies targeted specifically at B cells have proven highly effective in MS patients, highlighting an emerging role for this cell type in both relapsing and progressive forms of disease. During both MS and EAE, self-reactive T cells are likely activated in the periphery (189) , where they undergo initial differentiation and expansion (124) . Upon entry into the CNS, these cells are reactivated by myelin epitopes presented by an as of yet unidentified local DC (202, 203) . Production of cytokines such as IFN-γ and TNF-α from activated CD4+ T cells results in local activation of resting microglia, leading to the up-regulation of HLA class I and II as well as co-stimulatory molecules (B7-1, B7-2, and CD40) on the surface of these cells (133, 134, 204, 205) . These activated microglia are capable of serving as APC for infiltrating myelin-specific CD4+ T cells in vivo thus sustaining this pathogenic local T cell response (97) . Production of cytokines, chemokines, and MMPs by microglia (206) facilitate local inflammation by causing BBB breakdown and recruiting more immune cells into the CNS. These include circulating monocytes capable of differentiating into inflammatory DC and macrophages upon tissue entry (207) , culminating in demyelination (124) . Furthermore, microglial production of IL-23 and IL-1β promotes granulocyte macrophage colony-stimulating factor (GM-CSF) secretion by CD4+ T cells (208) . GM-CSF has been shown in EAE to promote CNS inflammation by mobilizing Ly6C hi monocytes from the bone marrow into the periphery, thereby increasing the number of circulating monocytes available for recruitment to the CNS (207) . GM-CSF can also increase HLA class II expression and proinflammatory cytokine production by microglia, macrophages, and DC in vitro (209, 210) . IL-17 producing T cells have been detected within CNS lesions during both EAE and MS (211, 212) . IL-17 promotes brain inflammation, inducing the production of pro-inflammatory cytokines, TNF-α, IL-6, and IL-1β most probably from astrocytes, microglia, or macrophages. It also stimulates the release of chemokines responsible for recruiting neutrophils to the CNS, particularly CXCL1 and CXCL2 (213, 214) . Finally, IL-17 can disrupt TJ in the BBB, allowing further migration of CD4+ T cells to the CNS (212, 215) . Microglia play important roles in augmenting CNS inflammation, demyelination, and neuronal damage in both EAE and MS (67, [216] [217] [218] . Activation of microglia occurs rapidly following the induction of EAE and results in the release of cytokines, chemokines, ROS, and tissue-degrading MMP (206) . One mediator, TNF-like weak inducer of apoptosis (TWEAK), triggers proliferation, angiogenesis, inflammation, is associated with extensive myelin loss, and induces astrocyte cell death during MS (219). IL-17 produced by microglia (220) worsens brain inflammation by stimulating GM-CSF production, as well as increasing IL-6, inflammatory proteins, nitric oxide, and adhesion molecule expression by macrophages. Moreover, expression of myeloperoxidase (MPO) and ROS by microglia results in direct myelin degradation and neuronal damage (216, 218) . Paradoxically, microglia also can play a neuroprotective role during CNS autoimmunity. These cells can promote remyelination, protect neurons, and suppress the adaptive immune response within the CNS (221, 222) . Within MS lesions, microglia and macrophages express the neurotrophic factors, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF), supporting neuronal survival (220, 223, 224) . Furthermore, microglia secrete the anti-inflammatory cytokines, IL-10 and TGF-β, and express the inhibitory receptor, PD-L1, responsible for inhibiting T cell proliferation and cytokine production (74, 225). Astrocytes are a major source of CCL2 and CXCL10 in the CNS, regulating the migration of monocytes into the brain (CCL2) and microglia into the lesion site (CXCL10) (111, (226) (227) (228) . One study suggested these cells play a more prominent role in regulating the recruitment of peripheral monocytes into the CNS (229) . CXCL12, www.frontiersin.org a chemokine that induces the expression of CXCL8 and CCL2, is also expressed by astrocytes in MS lesions (230) . CXCL12 can be cleaved by MMP-2, also expressed by astrocytes in MS and EAE, into a neurotoxic peptide that causes further neuronal damage (231) . Similar to microglia, astrocytes also play a protective role during MS and EAE. Homeostatic astrocyte functions include buffering potassium, removing extracellular glutamate that can accumulate to toxic levels, adjusting water balance, and controlling synaptic activity and blood flow in the CNS (8). These cells are also able to produce neurotrophins and the anti-inflammatory cytokine, IL-10 (232). The vast complexity of cellular interconnections within the CNS, and the non-renewable nature of many neural cells, mandate that some local immune responses be tightly controlled while others (i.e., cytolytic ones) be excluded to the fullest extent possible. The BBB is a dynamic and highly regulated tissue interface that helps make the CNS a unique immunological environment. It responds to signals from both neurons and glial cells on one side while simultaneously being able to sample immunological events passing through intravascular compartments. Immune cells perform normal surveillance of the CNS by searching for antigens previously encountered in extraneural sites such as the deep cervical lymph nodes. Pathological conditions such as infections caused by viruses or bacteria elicit changes at the BBB, including the up-regulation of a unique subset of adhesion molecules as well as heightened release of chemokines by ECs. Mediators produced by astrocytes and microglia further increase BBB permeability and recruit additional circulating leukocytes into the CNS. The ensuing immune response must then be tightly controlled in order to avoid collateral tissue damage. As such, astrocytes and microglia maintain mechanisms to dampen inflammatory responses. In some settings, immune cells such as ASC persist long term within the CNS to prevent viral reactivation. When normal control mechanisms fail, neuroinflammatory diseases such as MS can result. For this reason alone, it is imperative that the complexity of immune reactions within the CNS be better understood. 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Molecular analysis of a novel bidirectional pathway Toll-like receptors in bacterial meningitis Broad expression of Toll-like receptors in the human central nervous system Intracisternally localized bacterial DNA containing CpG motifs induces meningitis The protein tyrosine kinase inhibitor AG126 prevents the massive microglial cytokine induction by pneumococcal cell walls Role of microglia in central nervous system infections the first line of defence in brain pathologies CD14 receptor-mediated uptake of nonopsonized Mycobacterium tuberculosis by human microglia Mycobacterium tuberculosis-induced cytokine and chemokine expression by human microglia and astrocytes: effects of dexamethasone Characterization of microglial responses to Staphylococcus aureus: effects on cytokine, costimulatory molecule, and tolllike receptor expression Toll-like receptor 2 (TLR2) is pivotal for recognition of S. aureus peptidoglycan but not intact bacteria by microglia MyD88-dependent signals are essential for the host immune response in experimental brain abscess Matrix metalloproteinases: multifunctional effectors of inflammation in multiple sclerosis and bacterial meningitis Cytotoxicity of microglia Detection of lysosomal cysteine proteinases in microglia: flow cytometric measurement and histochemical localization of cathepsin B and L Microglia, an in vivo source of reactive oxygen species in the brain Protease production by cultured microglia: substrate gel analysis and immobilized matrix degradation Immune function of microglia Astrocytes: biology and pathology Glial influence on the blood brain barrier IL-1beta differentially regulates calcium wave propagation between primary human fetal astrocytes via pathways involving P2 receptors and gap junction channels Reciprocal regulation of the junctional proteins claudin-1 and connexin43 by interleukin-1beta in primary human fetal astrocytes Induction of nitric oxide synthase inhibits gap junction permeability in cultured rat astrocytes Regulation of gap junction communication by growth factors from non-neural cells to astroglia: a brief review Astrocyte-neuron metabolic relationships: for better and for worse Astrocytes modulate the chemokine network in a pathogen-specific manner Glial cells as intrinsic components of non-cellautonomous neurodegenerative disease Glutamate release from astrocytes in physiological conditions and in neurodegenerative disorders characterized by neuroinflammation Role of complement in neurodegeneration and neuroinflammation P2X7-like receptor activation in astrocytes increases chemokine monocyte chemoattractant protein-1 expression via mitogen-activated protein kinase The T cell chemoattractant IFN-inducible protein 10 is essential in host defense against viral-induced neurologic disease Astrocyte expression of mRNA encoding cytokines IP-10 and JE/MCP-1 in experimental autoimmune encephalomyelitis Differential activation of astrocytes by innate and adaptive immune stimuli Differential abilities of central nervous system resident endothelial cells and astrocytes to serve as inducible antigen-presenting cells Transcriptional regulation of the intercellular adhesion molecule-1 gene by proinflammatory cytokines in human astrocytes Cytokineinduced expression of vascular cell adhesion molecule-1 (VCAM-1) by astrocytes and astrocytoma cell lines Regulation of intercellular adhesion molecule-1 gene expression by tumor necrosis factor-alpha, interleukin-1 beta, and interferon-gamma in astrocytes Neutrophils, from marrow to microbes Neutrophils and immunity: challenges and opportunities The bacterial endotoxin lipopolysaccharide has the ability to target the brain in upregulating its membrane CD14 receptor within specific cellular populations Cytokines and chemokines at the crossroads of neuroinflammation, neurodegeneration, and neuropathic pain CXC chemokine receptor-2 ligands are required for neutrophil-mediated host defense in experimental brain abscesses Patterns of protein expression in infectious meningitis: a cerebrospinal fluid protein array analysis chemokines are expressed in the cerebrospinal fluid in bacterial meningitis and mediate chemotactic activity on peripheral blood-derived polymorphonuclear and mononuclear cells in vitro Innate immunity in the central nervous system Neutrophils in the activation and regulation of innate and adaptive immunity Recruitment of neutrophils across the blood-brain barrier: the role of E-and P-selectins How does the brain limit the severity of inflammation and tissue injury during bacterial meningitis? Antimicrobial peptides important in innate immunity Chemokines in and out of the central nervous system: much more than chemotaxis and inflammation Review: the chemokine receptor CXCR3 and its ligands CXCL9, CXCL10 and CXCL11 in neuroimmunity -a tale of conflict and conundrum Protective immunosurveillance of the central nervous system by Listeria-specific CD4 and CD8 T cells in systemic listeriosis in the absence of intracerebral Listeria Interferon-gamma differentially modulates the release of cytokines and chemokines in lipopolysaccharide-and pneumococcal cell wall-stimulated mouse microglia and macrophages Microglia are more efficient than astrocytes in antigen processing and in Th1 but not Th2 cell activation Antigen presentation and tumor cytotoxicity by interferon-gamma-treated microglial cells Microglia induce CD4 T lymphocyte final effector function and death Virus infections in the nervous system The adaptive immune system in diseases of the central nervous system Clearance of virus infection from the CNS Alphavirus-induced encephalomyelitis: antibodysecreting cells and viral clearance from the nervous system Neuroimmunology of central nervous system viral infections: the cells, molecules and mechanisms involved CNS viral infection diverts homing of antibody-secreting cells from lymphoid organs to the CNS Coronavirus infection of the central nervous system: host-virus stand-off IL-6 in autoimmune disease and chronic inflammatory proliferative disease Functional diversity of chemokines and chemokine receptors in response to viral infection of the central nervous system Neuronal CXCL10 directs CD8+ T-cell recruitment and control of West Nile virus encephalitis Chemokine receptor CCR5 promotes leukocyte trafficking to the brain and survival in West Nile virus infection Dynamic regulation of alpha-and beta-chemokine expression in the central nervous system during mouse hepatitis virus-induced demyelinating disease Astrocyte-derived CXCL10 drives accumulation of antibody-secreting cells in the central nervous system during viral encephalomyelitis Cytokine regulation of CC and CXC chemokine expression by human astrocytes Interferon-inducible T cell alpha chemoattractant (I-TAC): a novel non-ELR CXC chemokine with potent activity on activated T cells through selective high affinity binding to CXCR3 Biochemical characterization of a gamma interferoninducible cytokine (IP-10) Intrathecal humoral immunity to encephalitic RNA viruses Maturation and localization of macrophages and microglia during infection with a neurotropic murine coronavirus Mouse hepatitis virus pathogenesis in the central nervous system is independent of IL-15 and natural killer cells Depletion of blood-borne macrophages does not reduce demyelination in mice infected with a neurotropic coronavirus Isolation and characterization of gelatinase granules from human neutrophils Neutrophils promote mononuclear cell infiltration during viral-induced encephalitis Matrix metalloproteinase expression correlates with virulence following neurotropic mouse hepatitis virus infection The CC chemokine ligand 3 regulates CD11c+ CD11b+CD8alpha-dendritic cell maturation and activation following viral infection of the central nervous system: implications for a role in T cell activation Kinetics of virus-specific CD8+ -T-cell expansion and trafficking following central nervous system infection Differential roles for CXCR3 in CD4+ and CD8+ T cell trafficking following viral infection of the CNS CXCL10 and trafficking of virusspecific T cells during coronavirus-induced demyelination Expression of CXC chemokine ligand 10 from the mouse hepatitis virus genome results in protection from viral-induced neurological and liver disease Lack of CCR2 results in increased mortality and impaired leukocyte activation and trafficking following infection of the central nervous system with a neurotropic coronavirus Functional expression of chemokine receptor CCR5 on CD4+ T cells during virus-induced central nervous system disease The chemokine receptor CXCR2 and coronavirus-induced neurologic disease Inverted immunodominance and impaired cytolytic function of CD8+ T cells during viral persistence in the central nervous system Perforin-mediated effector function within the central nervous system requires IFN-gamma-mediated MHC up-regulation Differential regulation of primary and secondary CD8+ T cells in the central nervous system Mouse hepatitis virus is cleared from the central nervous systems of mice lacking perforin-mediated cytolysis Inhibition of interferon-gamma signaling in oligodendroglia delays coronavirus clearance without altering demyelination IFNgamma is required for viral clearance from central nervous system oligodendroglia IL-21 acts directly on B cells to regulate Bcl-6 expression and germinal center responses Biology of interleukin-10 Factors supporting intrathecal humoral responses following viral encephalomyelitis Recruitment and retention of B cells in the central nervous system in response to alphavirus encephalomyelitis Shifting hierarchies of interleukin-10-producing T cell populations in the central nervous system during acute and persistent viral encephalomyelitis The role of antibody in recovery from alphavirus encephalitis Control of central nervous system viral persistence by neutralizing antibody The production of antibody by invading B cells is required for the clearance of rabies virus from the central nervous system In Semliki Forest virus encephalitis, antibody rapidly clears infectious virus and is required to eliminate viral material from the brain, but is not required to generate lesions of demyelination Antibody-mediated clearance of alphavirus infection from neurons Experimental intracerebral vaccination protects mouse from a neurotropic virus by attracting antibody secreting cells to the CNS CXCR3-dependent plasma blast migration to the central nervous system during viral encephalomyelitis Recovery from viral encephalomyelitis: immune-mediated noncytolytic virus clearance from neurons Roles of immunoglobulin valency and the heavy-chain constant domain in antibody-mediated downregulation of Sindbis virus replication in persistently infected neurons Delineation of putative mechanisms involved in antibody-mediated clearance of rabies virus from the central nervous system Epidemiology and clinical record of multiple sclerosis in selected countries: a systematic review Mechanisms regulating regional localization of inflammation during CNS autoimmunity Differential twin concordance for multiple sclerosis by latitude of birthplace Fine-mapping the genetic association of the major histocompatibility complex in multiple sclerosis: HLA and non-HLA effects Analysis of immune-related loci identifies 48 new susceptibility variants for multiple sclerosis Past exposure to sun, skin phenotype, and risk of multiple sclerosis: casecontrol study Epstein-Barr virus in the multiple sclerosis brain: a controversial issue -report on a focused workshop held in the Centre for Brain Research of the Medical University of Active induction of experimental allergic encephalomyelitis Passive induction of experimental allergic encephalomyelitis IL-12-and IL-23-modulated T cells induce distinct types of EAE based on histology, CNS chemokine profile, and response to cytokine inhibition Selection for T-cell receptor V beta-D beta-J beta gene rearrangements Frontiers in Oncology | Neuro-Oncology with specificity for a myelin basic protein peptide in brain lesions of multiple sclerosis Lower motor neuron loss in multiple sclerosis and experimental autoimmune encephalomyelitis T-cell recognition of an immunodominant myelin basic protein epitope in multiple sclerosis T cells responsive to myelin basic protein in patients with multiple sclerosis Cutting edge: IL-6-dependent autoimmune disease: dendritic cells as a sufficient, but transient, source Dendritic cells permit immune invasion of the CNS in an animal model of multiple sclerosis Microglia initiate central nervous system innate and adaptive immune responses through multiple TLRs Direct activation of innate and antigenpresenting functions of microglia following infection with Theiler's virus The innate immune system in demyelinating disease Circulating Ly-6C+ myeloid precursors migrate to the CNS and play a pathogenic role during autoimmune demyelinating disease Differential expression and regulation of IL-23 and IL-12 subunits and receptors in adult mouse microglia The encephalitogenicity of T(H)17 cells is dependent on IL-1-and IL-23-induced production of the cytokine GM-CSF RORgammat drives production of the cytokine GM-CSF in helper T cells, which is essential for the effector phase of autoimmune neuroinflammation IL-17 plays an important role in the development of experimental autoimmune encephalomyelitis Human TH17 lymphocytes promote blood-brain barrier disruption and central nervous system inflammation Requirement of interleukin 17 receptor signaling for lung CXC chemokine and granulocyte colony-stimulating factor expression, neutrophil recruitment, and host defense Roles of interleukin-17 in an experimental Legionella pneumophila pneumonia model Cellular mechanisms of IL-17-induced blood-brain barrier disruption Phagocytic activity of macrophages and microglial cells during the course of acute and chronic relapsing experimental autoimmune encephalomyelitis Cytokines: influence on glial cell gene expression and function Phagocytosis of neuronal debris by microglia is associated with neuronal damage in multiple sclerosis Expression of TWEAK and its receptor Fn14 in the multiple sclerosis brain: implications for inflammatory tissue injury Production and functions of IL-17 in microglia The good, the bad and the ugly. Macrophages/microglia with a focus on myelin repair Physiology of microglia Microglial physiology: unique stimuli, specialized responses BDNF and gp145trkB in multiple sclerosis brain lesions: neuroprotective interactions between immune and neuronal cells? CNS expression of B7-H1 regulates pro-inflammatory cytokine production and alters severity of Theiler's virus-induced demyelinating disease Expression of MCP-1 by reactive astrocytes in demyelinating multiple sclerosis lesions Transcriptional regulation of chemokine gene expression in astrocytes Chemokine monocyte chemoattractant protein-1 is expressed by astrocytes after mechanical injury to the brain Inhibition of reactive astrocytosis in established experimental autoimmune encephalomyelitis favors infiltration by myeloid cells over T cells and enhances severity of disease A role for CXCL12 (SDF-1alpha) in the pathogenesis of multiple sclerosis: regulation of CXCL12 expression in astrocytes by soluble myelin basic protein HIVinduced metalloproteinase processing of the chemokine stromal cell derived factor-1 causes neurodegeneration Astrocyte phenotypes and their relationship to myelination The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.