Astrocytes are active players incerebral innate immunityCinthia Farina1, Francesca Aloisi2 and Edgar Meinl3,4

1 Neuroimmunology and Neuromuscular Disorders Unit, National Neurological Institute Carlo Besta, 20133 Milan, Italy2 Department of Cell Biology and Neurosciences, Istituto Superiore di Sanita, 00161 Rome, Italy3 Institute of Clinical Neuroimmunology, Ludwig-Maximilians University, 81377 Munich, Germany4 Department of Neuroimmunology, Max-Planck-Institute of Neurobiology, 82152 Martinsried-Planegg, Germany

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Innate immunity is a constitutive component of thecentral nervous system (CNS) and relies strongly on resi-dent myeloid cells, the microglia. However, evidence isemerging that the most abundant glial cell population ofthe CNS, the astrocyte, participates in the local innateimmune response triggered by a variety of insults. Astro-cytes display an array of receptors involved in innateimmunity, including Toll-like receptors, nucleotide-bind-ing oligomerization domains, double-stranded RNA-dependent protein kinase, scavenger receptors, mannosereceptor and components of the complement system.Following activation, astrocytes are endowed with theability to secrete soluble mediators, such as CXCL10,CCL2, interleukin-6 and BAFF, which have an impact onboth innate and adaptive immune responses. The role ofastrocytes in inflammation and tissue repair is elaboratedby recent in vivo studies employing cell-type specific genetargeting.

IntroductionThe central nervous system (CNS) has been commonlyregarded as an immune-privileged site [1]. However,important studies published during the past ten yearsindicate that the CNS can offer the setting for innateimmune responses [2]. This might reflect the ability ofthe CNS to fight infections despite its immune-privilegedstate. The recognition of infectious non-self is mediated bya limited number of germline-encoded pattern-recognitionreceptors (PRRs) and triggers rapid responses. Some PRRscan also recognize endogenous ‘danger signals’ that alertthe immune system to cell damage, independently from thecontext of infection. Consistently, the activation of innateimmune pathways occurs not only in infectious CNS dis-eases but also after brain injury and ischemia, and inautoimmune and neurodegenerative disorders of theCNS (Table 1), indicating that the relevance of these path-ways extends beyond antimicrobial defense. To date, theneurotoxic and neuroprotective roles of innate immunereactions in non-infectious CNS diseases are an intensivelyinvestigated and debated issue.

The expression of PRRs is found in various immune andnon-immune cell types of the CNS, similarly to peripheraltissues.Microglia andastrocytesare themainCNS-residentcell types for which consistent data have been published

Corresponding author: Farina, C. (c.farina@istituto-besta.it).Available online 2 February 2007.

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(Table 2). Microglia are myeloid lineage cells and areconsidered ‘the CNS professional macrophages’, owing totheir phenotype and reactivity following injury and inflam-mation [3]. For this reason, most of the studies on innateimmune responses in the CNS have focused on microglia,which express a wide range of PRRs (Table 2). The contri-bution of other cell types to these processes has often beenneglected. However, recent evidence suggests that astro-cytes have a complex, dual role in the local regulation ofimmune reactivity. Astrocytes, the most abundant glial cellpopulation, are of neuroectodermal origin and are essentialfor brain homeostasis and neuronal function [4] (Box 1).They form the glia limitans around blood vessels restrictingthe access of immune cells to the CNS parenchyma [5] (seeFigure I inBox 2). In contrast to other brain cells, astrocytesare resistant to death receptor-induced apoptosis [6], indi-cating that they are well equipped to survive inflammatoryinsults.

In this article, we aim to answer the following questions:(i) do astrocytes participate in innate immune reactions?(ii) Which are the activating signals and functionalresponses involved?

The Toll-like receptor system in astrocytes: expression,signaling and biological relevanceToll-like receptors (TLRs) are evolutionarily conservedtype I membrane glycoproteins characterized by leucine-rich-repeat motifs in the extracellular domain and by acytoplasmic signaling domain similar to that of the inter-leukin (IL)-1 receptor [7]. TLR1, TLR2 and TLR6 recognizebacterial lipoproteins; TLR3, TLR7, TLR8 and TLR9 arespecific for nucleic acids; TLR5 binds to flagellin, the mainconstituent of bacterial flagella; TLR4 has a wide spectrumof ligands, including bacterial lipopolysaccharide (LPS)and fungal zymosan. TLR expression has been detectedin cells of the innate and adaptive immune system and innon-immune cells.

But are TLRs important for pathogen recognition in theCNS, and where are they expressed? Increasing evidenceindicates that TLRs have a major role in several inflam-matory CNS pathologies (Table 1). For example, geneticpolymorphisms in TLR4 have been associated withincreased susceptibility of humans tomeningococcal infec-tion [8]. In animal models of infectious CNS diseases,TLR2 deficiency can lead to opposite outcomes, such asincreased susceptibility to Streptococcus pneumoniae

d. doi:10.1016/j.it.2007.01.005

Table 1. Innate immune pathways involved in CNS pathologies

PRR system CNS disease Refs

TLR Neisseria meningitides meningitis [8]

Streptococcus pneumoniae meningitis [9]

Listeria monocytogenes meningitis [9]

Herpes simplex virus encephalitis [11]

West Nile virus encephalitis [77]

Multiple sclerosis or

experimental autoimmune encephalitis

[22,78]

Nerve injury [79]

Ischemia [80]

Scavenger

receptors

Alzheimer’s disease [48,81]

N. meningitides meningitis [50]

Nerve injury [82]

Ischemia [83]

Complement Alzheimer’s disease [40,84]

Parkinson’s disease [85]

Huntington’s disease [40]

Pick’s disease [40]

Multiple sclerosis or

experimental autoimmune encephalitis

[86,87]

Rasmussen’s encephalitis [88]

Prion disease [85]

N. meningitides meningitis [8,44,89]

HIV encephalitis [89]

Epstein-Barr virus encephalitis [89]

Measles virus encephalitis [89]

West Nile virus encephalitis [46,47]

Nerve injury [90]

Ischemia [91]

Box 1. Features and functions of astrocytes

Features

� Most abundant glial cell in the CNS

� Neuroectodermal origin

� Largely resistant to death receptor (Fas, TRAIL)-mediated apop-

tosis

Functions

� Metabolic support of neurons; glycogen storage and export of

lactate

� Uptake of neurotransmitters, including glutamate

� Production of neurotrophic factors

� Ion homeostasis (i.e. potassium uptake)

� Blood–brain barrier induction and maintenance

� Scar formation and tissue repair

� Regulation of immune responses in the CNS

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meningitis [9,10] and protection from herpes simplexvirus encephalitis [11]. However, in these models, owingto PRR expression on peripheral immune cells and onbrain-resident cells, it is difficult to distinguish the con-tributions to infection control of each cell type.

Under physiological conditions, basal expression ofTLR2 and TLR4 was demonstrated in the meninges, chor-oid plexus and circumventricular organs of the brain,namely in CNS areas that lack a blood–brain barrier(BBB) and are more exposed to invading pathogens [12–15]. More recently, low basal levels of TLR4 expressionwere shown in microglia in vivo [15]. Accordingly, thesystemic administration of LPS results in rapid upregula-tion of TLR2 in microglia and generates an innate inflam-matory response that is readily detected and moreprominent in BBB-free areas of the CNS but also extendsinto the brain parenchyma [12–14].

Table 2. PRRs in astrocytes and microgliaa

PRR Astrocytes

TLR TLR2 [22,23], TLR3 [16], TLR4 [20,23],

TLR5 [20,23], TLR9 [20,23]

CD14 Not expressed

NOD NOD1, NOD2 [36]

PKR Expressed [35]

Scavenger receptors SR-BI [94], SR-MARCO [51], RAGE [95], S

Mannose receptor Expressed [38]

Complement factors C1q, C1r, C1s, C4, C2, C3, factor B, facto

C5, C6, C7, C8, C9 [40]

Complement receptors CR1, CR2, C3aR, C5aR [40]

Complement inhibitors C1-INH, DAF/CD55, MCP/CD46, CD59, fac

factor I, S protein, clusterin [40]aBlue text indicates in vitro evidence (not yet supported by in vivo studies); red text ind

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The brain displays basal expression of TLR3, which isdetected on glial fibrillary acidic protein (GFAP)-positiveastrocytes in the hippocampus and striatum [16]. Severalrecent in vitro studies have shown that TLR3 is thepredominant TLR expressed in astrocytes. In two studies,the complete human TLR repertoire in human fetal astro-cytes was analyzed by quantitative polymerase chainreaction and indicated TLR3 as the only TLR with con-sistent expression in the resting state [17,18] and withupregulated levels following treatment with inflamma-tory cytokines such as IL-1b, interferon (IFN)b and IFNg

[17]. The analysis of cultured human adult astrocytesshowed basal levels of TLR2, TLR3 and TLR4; however,exposure to inflammatory cytokines strongly inducedTLR3, while having no effect on TLR4 and downregulat-ing TLR2 [19]. Similarly, cultured mouse and rat astro-cytes displayed high constitutive levels of TLR3 [20,21].TLR3 is commonly found in intracellular compartmentssuch as endosomes, and signaling requires ligand intern-alization. Interestingly, in human astrocytes, TLR3was detected not only intracellularly [18] but also onthe cell membrane [18,22], and its protein expressionwas induced following ligand binding [18]. TLR2, TLR4,TLR5 and TLR9 have been described in humanastrocyte-enriched cultures [19,20, 22,23] (Table 2); how-ever, expression of these TLRs on astrocytes in vivo has

Microglia

TLR1 [24], TLR2 [13], TLR3 [24], TLR4 [15],

TLR5 [24], TLR6 [24], TLR7 [24], TLR8 [24],

TLR9 [24]

Expressed [92]

Unknown

Expressed [93]

RCL [53] SR-A [48], SR-BI [48], SR-MARCO [51], RAGE [51],

CD36 [48], SRCL [53]

Expressed [3]

r D, C1q, C3, C4 [40]

CR1, CR3/CD11b, CR4/CD11c, C3aR, C5aR,

C1qRp [40]

tor H, C1-INH, CD59, clusterin [40]

icates in vivo evidence (mostly supported by in vitro studies not cited in the table).

Box 2. Mediators of astrocyte function

1. Cytokines: IL-6, IFNb, TGFb, GM-CSF, BAFF, IL-1b and TNF

� Increased BBB permeability

� Endothelial cell activation

� Microglial and monocytic activation, differentiation and proliferation

� Astrocyte activation

� B cell survival and differentiation

� Immunosuppression

� Release of neuroprotective mediators

2. Chemokines: CCL2, CCL5, CCL20, CXCL10, CXCL12, CXCL1,

CXCL2 and CX3CL1

� Recruitment of monocytes and macrophages, dendritic cells, T and

B lymphocytes, and neutrophils

� Regulation of myelination and microglial activity

� Astrocyte proliferation and survival

� Migration of microglia and neural progenitors

3. Neurotrophic factors: NGF, CNTF, BDNF, VEGF, IGF1 and LIF

� Neuronal survival, differentiation, function and regeneration

� Oligodendrocyte survival

� Remyelination

� Neurogenesis

Mediators released by astrocytes (bold text) following pathogen

recognition trigger two types of events (Figure I): (i) they activate

neighboring cells and amplify the local, initial innate immune

response further; and (ii) they modify BBB permeability and attract

immune cells from the blood circulation into the neural tissue, thus

supporting an adaptive immune response. The balance between

inflammatory (tissue damaging; red text) and immunosuppressive

(tissue regenerating; green text) pathways is fundamental for

controlled reactions to CNS trauma. Dysregulation of these pathways

might lead to pathogenic chronic neuroinflammation and neurode-

generation.

Astrocytes themselves can be the direct target of such factors.

Astrocytic responses to chemokines involve chemotaxis, cell prolif-

eration and survival. Additionally, chemokines, such as CXCL12 and

CCL5, induce glutamate release and cytokine and chemokine synth-

esis, indicating a role for this class of molecules in regulating glia–glia

and glia–neuron communication [72,96]. Among cytokines, TNF, IFNg

and IL-1 are the main astrocytic activators. Gene profiling studies in

astrocytes exposed to inflammatory cytokines in vitro showed

profound regulations in genes involved in several cellular pathways,

such as innate and adaptive immunity, antigen presentation,

apoptosis, leukocyte migration and BBB permeability [97–99]. Astro-

cyte-derived factors, such as GM-CSF, IL-6, CCL2 and CCL5 regulate

microglial migration, activation and proliferation.

Some glia-derived soluble mediators, including CCL2, CCL5,

CXCL10, CXCL12 and BAFF, are also responsible for triggering

adaptive immunity in the inflamed CNS.

Figure I. Astrocyte activation generates waves of innate and adaptive immunity. The astrocyte has a strategic location, being in close contact with CNS-resident cells

(neurons, microglia, oligodendrocytes and other astrocytes) and with blood vessels. The astrocyte-derived glia limitans (dark green) forms a barrier between the brain

parenchyma and the vascular system. The perivascular space, a site of immune cell accumulation in CNS inflammation, is delimited by the endothelial basement

membrane (dark pink) and the glia limitans [5]. Following PRR engagement, astrocytes secrete factors that target bystander brain cells and promote changes in the

permeability of the BBB, resulting in the recruitment of professional immune cells into the CNS parenchyma. This leads to an amplification of the initial innate immune

reaction, which can result in the elimination of the insult, with restoration of tissue integrity or scar formation. Relative sizes of the distinct cell types reflect in situ

observations. T and B lymphocytes, macrophages and microglia have the same color to indicate their common hematopoietic origin.

140 Review TRENDS in Immunology Vol.28 No.3

not yet been demonstrated. By contrast, similarly tomonocytes and macrophages, microglia (the professionalimmune cells of the brain) display a broader repertoire ofTLRs in vitro [18,24] (Table 2).

Human astrocytes express mRNAs for the adaptormolecules TIRAP (also known as Mal), TICAM (alsoknown as TRIF), MyD88 and the shorter splice variantMyD88S [17]. The MyD88-dependent pathway involves

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the sequential activation of IL-1 receptor-associatedkinase (IRAK) proteins, tumor necrosis factor (TNF)receptor-associated factor (TRAF)6 and mitogen-acti-vated protein (MAP) kinases down to activator protein(AP)-1 and nuclear factor (NF)-kB transcription factors.The TRIF-dependent pathway activates NF-kB on one sidethrough receptor-interacting protein (Rip)1 and interferonregulatory factor (IRF)3 and IRF7 on the other side through

Review TRENDS in Immunology Vol.28 No.3 141

Tank-binding kinase (TBK)1 and phosphatidylinositol3-kinase (PI3K), resulting in the expression ofIFN-inducible genes [7].

Double-stranded (ds)RNA-triggered biological effectsare mediated largely by TLR3. Injecting poly(I:C), a syn-thetic analog of dsRNA, into the brain triggered microglialand astrocytic activation in wild-type mice but not in TLR3knockout (KO) mice [25], indicating the important role ofTLR3 in the CNS response to viral infection.

In cultured astrocytes, poly(I:C) induces theexpression of several cytokines [TNF, IL-6, IFNb,granulocyte–macrophage colony-stimulating factor(GM-CSF) and transforming growth factor (TGF)b] andchemokines (CCL2, CCL5, CCL20, CXCL8 and CXCL10)[16–19].

The astrocytic response to dsRNA involves induciblenitric oxide synthase (iNOS) upregulation with nitric oxideproduction [20,21,26], a decrease in glutamate uptake [21],and downregulation of connexin 43 with a disturbance ofgap junction-mediated intercellular communication [27].This suggests that astrocytes contribute to tissue damageand neurotoxicity. Similarly, in vitro infection of mouseastrocytes with Theiler’s murine encephalomyelitis virus(TMEV) triggered an initial inflammatory response (CCL2and CXCL10 secretion) through TLR3 [28]. However, viralreplication was equal in wild-type and TLR3 KO astro-cytes, indicating that TLR3-mediated activation is insuffi-cient to block TMEV replication [28]. Gene profiling ofpoly(I:C)-treated adult human astrocytes indicated theinduction of an antiviral program [29] and of growth,differentiation and neuroprotective mediators, includingGM-CSF, vascular endothelial growth factor (VEGF)-C,neurotrophin (NT)-4 and ciliary neurotrophic factor(CNTF), suggesting that TLR stimulation in astrocytesalso activates tissue repair pathways [19].

In summary, astrocytes express TLR3 in vivo and invitro, and react to TLR3 ligands by producing mediatorsthat promote, on the one hand, the local inflammatoryresponse and, on the other hand, tissue repair. Becauserecent studies indicate that mRNA from necrotic cellsfunctions as an endogenous ligand for TLR3 [30], thepossibility that neural tissue damage activates astrocytesis intriguing and deserves further investigation.

Astrocytes express PKR, NOD proteins, scavengerreceptors, mannose receptor and components of thecomplement systemPRRs other than TLRs are involved in innate immuneresponses. TLR3 is not the only weapon in antiviral immu-nity, because TLR3 KO mice were able to mount a normalperipheral immune response to lymphocytic choriomenin-gitis virus, vesicular stomatitis virus (VSV) and murinecytomegalovirus [31]. Following intracerebral inoculationof reovirus, a dsRNA virus, wild-type and TLR3 KO miceshowed similar patterns of survival, viral titer and neuro-pathology [31]. Sendai virus was able to activate TLR3-negative cells and to induce interferon-stimulated andinterferon-related genes [32]. Indeed, many other PRRsthat mediate RNA recognition exist [33] and each mightrecognize distinct dsRNA structures [34]. One of thesePRRs is dsRNA-dependent protein kinase (PKR). Its acti-

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vation leads to the phosphorylation of the a subunit of theeukaryotic initiation factor 2 resulting in the blockade ofviral translation and subsequent shutdown of protein syn-thesis in virus-infected cells. Although no data are yetavailable on the distribution of PKR in the CNS, recentstudies showed that PKR is involved in the dsRNA-inducedresponse in cultured astrocytes [16,21,35].

Nucleotide-binding oligomerization domain (NOD)1and NOD2 proteins, intracellular proteins that recognizedistinct motifs of bacterial peptidoglycan, have beendescribed in cultured mouse astrocytes, and NOD2 wasfound to mediate IL-6 secretion in response to its specificligand [36].

Expression of the mannose receptor (MR) has also beendemonstrated in the CNS [37,38]. MR enables recognitionof mannosylated ligands of endogenous or microbial ori-gin, resulting in receptor-mediated endocytosis andenhanced microbicidal activity. Both cultured astrocytesand microglia express MR protein on the cell surface andin intracellular pools [37]. MR was identified as the recep-tor responsible for CD4-independent HIV-1 entry intoastrocytes [38]. This interaction triggered intracellularsignaling leading to matrix metalloproteinase 2 pro-duction [39].

Complement is another important component of innateimmunity. It comprises soluble and surface proteinsexpressed in almost every cell type. Recent studies havehighlighted the role of complement in the normal andpathological CNS.Notably, brain cells express a full comp-lement system to kill pathogens but are somewhat pro-tected from complement-induced lysis because of theexpression of complement inhibitors [40,41]. Reactiveastrocytes can produce most of the complement factorsafter activation and express in vivo complement receptorsand complement-regulatory proteins [40,42] (Table 2).Interestingly, pathogens can use some of these factorsto promote infection. For example, Epstein–Barr virusbinds to CR2, whereas CD46 serves as a receptor forhuman herpesvirus 6, measles virus andNeisseria menin-gitidis [43,44]. Although these pathogens were shown toinfect astrocytes in vitro, formal in vivo evidence is stillrequired. Studies of human pathological samples andanimal models of CNS infection have, in some instances,demonstrated the contribution of complement to anti-viralimmunity [45–47] (Table 1). Furthermore, geneticpolymorphisms in genes encoding complement com-ponents have been associated with N. meningitidis infec-tion [8] (Table 1).

Scavenger receptors (SRs), initially described ashigh-affinity receptors on macrophages for acetylatedlow-density lipoproteins, comprise several receptor clas-ses, each including numerous members [48,49]. SRs have arole in the binding and internalization of many unrelatedligands, such as fibrillar b-amyloid, lipids, glycatedcollagen and apoptotic cells, and, therefore, are importantfor tissue homeostasis. Astrocytes were shown to expressvarious SRs in situ using immunohistochemistry (Table 2).SR-MARCO is important for host defense, because it canbind to Gram-positive and Gram-negative bacteria. Inter-estingly, SR-MARCO is involved in the recognition anduptake of N. meningitidis [50], an important cause of

142 Review TRENDS in Immunology Vol.28 No.3

bacterial meningitis; therefore, its expression in astrocytes[51] might be an additional mechanism through whichCNS-resident cells can fight bacterial infections. The otherSRs present on astrocytes recognize endogenous signals.For example, SR-BI exerts its scavenger activity throughbinding to low-density lipoproteins, apoptotic cells andanionic phospholipids. SR-BI, RAGE and SRCL bind toand mediate the internalization of b-amyloid peptides andfibrils [48,52,53]. These observations, together with thefinding that astrocytes associate with and degrade b-amy-loid deposits in situ [54], highlight an important role forastrocytes in counteracting neurodegeneration. A deficit inthis function could have direct implications in Alzheimer’sdisease.

Astrocytes are important regulators ofneuroinflammationIn the previous sections, we have seen that astrocytes carrya series of PRRs, which are important for the primaryrecognition of infectious agents and of endogenous dangersignals. Factors produced by activated astrocytes (andmicroglia) target neighboring cells and promote leukocyterecruitment, resulting in the local amplification of inflam-matory responses (Box 2). For example, following stimu-lation with inflammatory cytokines, astrocytes in vitroproduce high amounts of the B cell survival factor BAFF(B cell-activating factor of the TNF family) [55]. Notably,BAFF is expressed in astrocytes in the normal human CNSand is strongly upregulated in activated astrocytes in thedemyelinated lesions of multiple sclerosis (MS), a chronicinflammatory disease of the CNS [55]. BAFF secretioncould be relevant in sustaining intrathecal B cell responsesin autoimmune and infectious diseases of the CNS. Accord-ingly, BAFF expression is readily induced in the CNS byneurotropic viruses and correlates with the recruitment ofantibody-secreting cells [56].

The central role of astrocytes in regulatingneuroinflammation was demonstrated recently in vivo[57,58]. Transgenic mice were generated in which NF-kB, a fundamental transcription factor family in innateresponses, was selectively inactivated in astrocytes [57].These mice displayed normal spinal cord architecture andcellular turnover; however, functional recovery after injurywas increased dramatically, and lesion volume and whitematter injury was reduced. These observations correlatedwith the drop in leukocyte recruitment into the lesionedarea owing to the reduced NF-kB-dependent expression ofCXCL10 and CCL2 [57]. Similarly, blockading the NF-kBpathway in neuroectodermal cells of the CNS (includingneurons, astrocytes and oligodendrocytes) led to a consist-ent decrease in proinflammatory gene expression duringexperimental autoimmune encephalomyelitis and clinical–pathological amelioration [58]. In summary, the NF-kBpathway in astrocytes is a key regulator of inflammation inthe CNS, and its inhibition has beneficial effects on tissueregeneration.

Astrocytes contribute to neuroprotective innateinflammationInnate immune responses are not necessarily detrimentalfor the cerebral tissue. The innate immune system also

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performs the task of restricting the lesioned area, removingthe insult and restoring tissue homeostasis.

A major, therapeutically relevant question is: what roledo astrocytes have in the innate neuroprotective immuneresponse? A hallmark of CNS injury, whatever its origin(i.e. infectious, autoimmune, mechanical or toxic) is theformation of scar tissue composed of activated (‘reactive’)astrocytes andmicroglia. An inflammatory response occurswithin this clearly demarcated area. This phenomenon iscommonly regarded as harmful and responsible for therestriction of axonal regrowth within the lesion. However,several studies indicate that activated astrocytes (inaddition to activated microglia) and their secretedproducts might exert neuroprotective actions.

Subsequent to trauma, astrocytes proliferate,accumulate glycogen and undergo fibrosis by accumulatingintermediate filaments, expressed as an increase in GFAP.Transgenic mice expressing herpes simplex virus-thymi-dine kinase (HSV-TK) under the GFAP promoter weregenerated. HSV-TK-expressing cells metabolize the anti-viral drug ganciclovir (GCV) to toxic nucleotide analogs,leading preferentially to the death of proliferating cells. Inuninjured GFAP-TK mice, astrocyte proliferation is rare;therefore, GCV administration does not kill resting astro-cytes. After CNS insults, local scar formation occurs andastrocytes proliferate, resulting in the loss of these cells inGCV-treated GFAP-TK mice [59,60]. This experimentalmodel unraveled a fundamental neuroprotective role forreactive astrocytes in: (i) demarcating the damaged areaand limiting leukocyte extravasation, because the ablationof reactive astrocytes resulted in improper scar formationand more extensive and long-lasting inflammation; (ii)promoting BBB repair, because in GCV-treated transgenicanimals, immunoglobulin entry into the brain parenchymapersisted after injury; and (iii) sustaining neuronal survi-val, because enhanced neuronal degeneration occurredafter astrocyte loss.

Several cytokines, including IL-1 and IL-6, have beenimplicated in the induction and modulation of reactiveastrogliosis and pathological inflammatory responses.However, the same factors in different settings mightalso behave as mediators of neuroprotection and remye-lination. For example, in various injury models, IL-1b

and IL-6 KO mice displayed delayed astrocyte activationand increased BBB permeability, indicating that cyto-kine-induced astrogliosis following trauma is importantto restore the integrity of the BBB and to repair thelesion [61–65]. Interestingly, IL-1b KO mice failed toupregulate the neurotrophic factors CNTF [61] and insu-lin-like growth factor (IGF)1 [63] following CNS trauma,indicating that the initial IL-1b-dependent inflammatoryresponse mediates the release of these neurotrophicfactors in the injured CNS. In wild-type mice, bothmicroglia [61,63] and astrocytes [63] express IL-1b,whereas mainly astrocytes were found to be positivefor IGF1 in a demyelination model [63]. Several in vitrodata demonstrate that cytokines such as IL-1, IL-6 andTNF support the production of neuroprotectivemediators [66]. In this regard, the observation thatTLR engagement on cultured astrocytes triggers theproduction of neurotrophic factors supports a role for

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these cells in the neuroprotective innate immuneresponse [19].

A Janus-like action is displayed also by astrocyte-derivedchemokines. CCL2 and CXCL12 not only promote therecruitment of inflammatory immune cells into the CNSparenchyma but also have a role in the migration of neuralprogenitors in the developing brain [67] and towards areasof brain injury [68–70]. CCL2 is readily upregulated invarious pathological conditions in microglia and astrocytes[71,72], and increased CXCL12 immunoreactivity was dete-cted in astrocytes inMS lesions [73,74]. CXCL1was seen onreactive astrocytes in MS lesions in the vicinity of CXCR2-positive oligodendrocytes, suggesting a role for thischemokine in remyelination [75]. Another chemokineconstitutively present in the CNS is CX3CL1, which isproduced by astrocytes and neurons. CX3CL1 is an import-ant inhibitor of microglial toxicity, as demonstrated inthreedifferent in vivomodels [76]. Therefore, theproductionof particular chemokines might reflect the attempt ofreactive astrocytes to promote regeneration in the lesionedCNS.

Concluding remarksResearch in the field of innate immunity in the CNS hasdeveloped during the past decade and has concentratedmainly on the reactivity of theprofessional resident immunecells, themicroglia. However, based on current evidence, wecan definitely consider the astrocyte as the innate-immune-competent cell, because it bears several PRRs involved intheprimaryrecognitionofmicrobial agents orof endogenousdanger signals. This cell type reactswithauniqueactivationprogram thatmodulates or amplifies the local inflammatoryreaction. Invivo studieshaverevealedJanus-like featuresofastrocytes.On theonehand, theycanpromote inflammationthrough NF-kB-dependent pathways; on the other hand,proliferating reactive astrocytes confine lesions and restorebrain homeostasis. Increasing knowledge on the involve-ment of astrocytes in shaping the innate immune responsecan provide new insights into the intrinsic capacity of theCNS tissue to face pathogenic insults and can aid in theidentification of molecular targets for therapeutic interven-tions in a variety of neuroinflammatory and neurodegen-erative diseases.

AcknowledgementsOur original work was supported by the Deutsche Forschungsgemeinschaft(SFB 571), the Gemeinnutzige Hertie-Foundation, the Deutsche MultipleSklerose Gesellschaft, the Verein zur Therapieforschung fur MS-Kranke,and the Italian Ministry of Health. The Institute for ClinicalNeuroimmunology is supported by the Hermann and Lilly SchillingFoundation.

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92 Nadeau, S. and Rivest, S. (2000) Role of microglial-derived tumornecrosis factor in mediating CD14 transcription and nuclear factor kBactivity in the brain during endotoxemia. J. Neurosci. 20, 3456–3468

93 Lee, J.H. et al. (2005) Double-stranded RNA-activated protein kinase isrequired for the LPS-induced activation of STAT1 inflammatorysignaling in rat brain glial cells. Glia 50, 66–79

94 Husemann, J. and Silverstein, S.C. (2001) Expression of scavengerreceptor class B, type I, by astrocytes and vascular smooth muscle cellsin normal adult mouse and human brain and in Alzheimer’s diseasebrain. Am. J. Pathol. 158, 825–832

95 Sasaki, N. et al. (2001) Immunohistochemical distribution of thereceptor for advanced glycation end products in neurons andastrocytes in Alzheimer’s disease. Brain Res. 888, 256–262

96 Dorf, M.E. et al. (2000) Astrocytes express functional chemokinereceptors. J. Neuroimmunol. 111, 109–121

97 John, G.R. et al. (2002) Multiple sclerosis: re-expression of adevelopmental pathway that restricts oligodendrocyte maturation.Nat. Med. 8, 1115–1121

98 John, G.R. et al. (2005) IL-1-regulated responses in astrocytes:relevance to injury and recovery. Glia 49, 161–176

99 Rivieccio, M.A. et al. (2005) The cytokine IL-1b activates IFN responsefactor3 inhumanfetalastrocytes inculture.J. Immunol.174,3719–3726

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