Immunity

Review

Cross-regulation of Signaling Pathwaysby Interferon-g: Implicationsfor Immune Responses and Autoimmune Diseases

Xiaoyu Hu1,2 and Lionel B. Ivashkiv1,2,3,*1Arthritis and Tissue Degeneration Program, Hospital for Special Surgery, New York, NY 10021, USA2Department of Medicine, Weill Cornell Medical College, New York, NY 10021, USA3Graduate Program in Immunology and Microbial Pathogenesis, Weill Cornell Graduate School of Medical Sciences, New York,NY 10021, USA*Correspondence: ivashkivl@hss.eduDOI 10.1016/j.immuni.2009.09.002

Interferon-g (IFN-g) is an important mediator of immunity and inflammation that utilizes the JAK-STATsignaling pathway to activate the STAT1 transcription factor. Many functions of IFN-g have been ascribedto direct STAT1-mediated induction of immune effector genes, but recently it has become clear that keyIFN-g functions are mediated by cross-regulation of cellular responses to other cytokines and inflammatoryfactors. Here, we review mechanisms by which IFN-g and STAT1 regulate signaling by Toll-like receptors,inflammatory factors, tissue-destructive cytokines, anti-inflammatory cytokines, and cytokines that activateopposing STATs. These signaling mechanisms reveal insights about how IFN-g regulates macrophage acti-vation, inflammation, tissue remodeling, and helper and regulatory T cell differentiation and how Th1 andTh17 cell responses are integrated in autoimmune diseases.

IntroductionSince the first description of a type II interferon (IFN) activity more

than three decades ago, much has been learned about the

biological effects and signal transduction mechanisms of the

sole type II IFN, IFN-g. IFN-g is one of the most important endog-

enous mediators of immunity and inflammation. IFN-g plays

a key role in macrophage activation, inflammation, host defense

against intracellular pathogens, T helper 1 (Th1) cell responses,

and tumor surveillance and immunoediting. In parallel, IFN-g

exerts regulatory functions to limit tissue damage associated

with inflammation and to modulate Th and regulatory T (Treg)

cell differentiation. IFN-g can either augment or suppress

autoimmunity and the associated pathology in a context- and

disease-specific manner.

IFN-g signals mainly through the Janus kinase (JAK)-signal

transducer and activator of transcription (STAT) intracellular

signal transduction pathway to achieve transcriptional activation

of IFN-g-inducible genes. The STAT family of transcription

factors consists of seven members, all of which are involved in

receptor signaling by various cytokines and growth factors. The

major STAT protein activated by IFN-g is STAT1. Many IFN-g

functions are mediated by direct activation of immune effector

genes by STAT1, including genes encoding antiviral proteins,

microbicidal molecules, phagocytic receptors, chemokines,

cytokines, and antigen-presenting molecules. Canonical Jak-

STAT signaling mechanisms leading to activation of well-charac-

terized STAT1 target genes have been previously reviewed

(Stark, 2007) and will not be discussed here. A wide spectrum

of IFN-g activities cannot be explained on the basis of activation

and direct effector functions of STAT1 target genes. Activation of

other STATs and alternative signaling pathways can contribute to

IFN-g function in certain cell contexts (reviewed in van Boxel-

Dezaire and Stark, 2007 and Gough et al., 2008). Importantly,

many key IFN-g functions are mediated by cross-regulation of

cellular responses to other cytokines and inflammatory factors.

The capacity of IFN-g to cross-regulate signaling pathways

induced by other endogenous and exogenous factors is less

appreciated, and the underlying mechanisms are more recently

described. The mechanisms and (patho)physiological impact of

IFN-g-mediated cross-regulation of signal transduction will be

the main focus of the current review.

IFN-g-Induced Jak-STAT1 SignalingIn canonical IFN-g-Jak-STAT1 signaling (recently reviewed in

Stark, 2007), ligand engagement of the IFN-g receptor leads to

activation of receptor-associated Jak1 and Jak2 and phosphor-

ylation of a receptor tyrosine residue (Y440) that serves as a dock-

ing site for STAT1, which exists in a latent state in the cytoplasm.

STAT1 is then activated by phosphorylation of tyrosine 701,

translocates to the nucleus, binds to a regulatory DNA element

termed gamma-activated sequence (GAS), and stimulates

transcription of STAT1 target genes. STAT1 binds to DNA as a

dimer composed of two STAT1 subunits in a parallel configura-

tion, such that amino- and carboxy-termini are aligned (Figure 1).

Transcriptional activity of STAT1 is augmented by MAP kinase-

mediated phosphorylation of a serine residue in the carboxy-

terminal transcription activation domain, and the amplitude of

activation is fine tuned by feedback inhibition mediated by

various negative regulators of Jak-STAT signaling such as

SOCS1 (O’Shea and Murray, 2008). SOCS1 is a key suppressor

of IFN-g activities in vivo given that SOCS1 deficiency in mice

leads to neonatal lethality characterized by uncontrolled IFN-g

responses (Alexander et al., 1999; Marine et al., 1999). Recent

evidence has highlighted that STAT1 undergoes cycles of activa-

tion-inactivation that are coupled with nuclear-cytoplasmic shut-

tling and regulated by posttranslational modifications, including

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dephosphorylation of tyrosine 701 and acetylation of lysine resi-

dues (Figure 1).

Inactivation of nuclear STAT1 occurs rapidly after binding to

chromatin and activation of target gene transcription. STAT1

dissociates from DNA and the STAT1 dimer undergoes a confor-

mational change, such that the parallel orientation of STAT1

monomers changes to an antiparallel configuration that exposes

phosphotyrosine residues and thus facilitates dephosphoryla-

tion of STAT1 by phosphatases (Mao et al., 2005; Mertens

et al., 2006; Zhong et al., 2005). Subsequently, STAT1 is dephos-

phorylated by phosphatases such as TCP45, and dephosphory-

lated STAT1 returns to the cytoplasm, in which it can potentially

serve as the substrate for subsequent rounds of activation and

inactivation (Figure 1). There is accumulating evidence that

cytoplasmic STATs do not exist predominantly as monomers

(Braunstein et al., 2003; Ota et al., 2004) but instead as

homodimers with the two STAT1 subunits in an antiparallel

configuration (Mao et al., 2005). In this model, STAT1 tyrosine

phosphorylation triggers or stabilizes a conformational change

of pre-existing STAT1 dimers from antiparallel to parallel config-

uration and results in increased abundance of parallel dimers

with an exposed nuclear localization sequence and high DNA-

binding activity (Wenta et al., 2008).

Figure 1. IFN-g-Induced STAT1 Activation Cycle Mediated byTyrosine Phosphorylation and Lysine AcetylationUnphosphorylated STAT1 dimers are present in the cytoplasm in an equilib-rium state between a parallel and antiparallel configuration of STAT1 subunits.Upon activation of IFNGR signaling, STAT1 is phosphorylated by JAK kinaseson tyrosine 701 and phosphorylation stabilizes a parallel dimer configurationthat exhibits DNA binding activity. Phosphorylated STAT1 translocates tonucleus, binds to GAS DNA sequences, and activates transcription ofSTAT1 target genes. Active STAT1 in the nucleus undergoes acetylation onlysines 410 and 413, a process catalyzed by histone acetyltransferases(HATs) such as CBP. Acetylation flags STAT1 for dephosphorylation by theSTAT1 phosphatase TCP45; an antiparallel dimer configuration facilitates effi-cient dephosphorylation and thus deactivation. Dephosphorylated STAT1shuttles back to cytoplasm, where histone deacetylases (HDACs) such asHDAC3 and possibly Sirtuins deacetylate STAT1 and complete the phosphor-ylation-acetylation cycle. Deacetylation of STAT1 results in less efficientTCP45-mediated dephosphorylation and thus licenses STAT1 for IFNGR-JAK-mediated tyrosine phosphorylation.

540 Immunity 31, October 16, 2009 ª2009 Elsevier Inc.

The function of STATs and the transit of STAT1 through the

activation-inactivation cycle are regulated by lysine acetylation

(Kramer et al., 2006; Kramer et al., 2009; Nie et al., 2009; Tang

et al., 2007; Yuan et al., 2005). The acetylation status of several

STATs including STAT1, STAT2, and STAT3 is dynamically deter-

mined by opposing activities of histone acetyltransferases (HATs)

versus histone deacetylases (HDACs). However, the impact of

STAT acetylation on signaling is not well understood, given that

both positive and negative roles of STAT acetylation on cytokine

receptor signaling have been reported. The preponderance of

evidence suggests that acetylation of STAT3 is often, although

not exclusively, associated with positive regulation of signal

transduction (Nie et al., 2009; Wang et al., 2005; Yuan et al.,

2005), whereas acetylation of STAT1 is associated with inhibitory

effects (Kramer et al., 2006; Kramer et al., 2009). STAT3 acetyla-

tion by the HAT CBP has been correlated with increased DNA-

binding and transactivation activity (Wang et al., 2005; Yuan

et al., 2005) and potentially with its anti-inflammatory properties

(Sun et al., 2009). Conversely, deacetylation of STAT3 by the

HDAC Sirtuin 1 correlates with decreased STAT3 tyrosine phos-

phorylation and activity (Nie et al., 2009). Similar to STAT3, STAT1

is also acetylated by CBP (Kramer et al., 2006; Kramer et al.,

2009). However, in contrast to STAT3, STAT1 acetylation seems

to play a negative role in signaling. It was recently reported that

acetylation of STAT1 on lysine residues 410 and 413 in the

nucleus results in enhanced interaction with TCP45 and

increased dephosphorylation (Kramer et al., 2009). Thus, acety-

lation ‘‘flags’’ STAT1 for inactivation. The mechanism by which

acetylation promotes interaction of STAT1 with TCP45 is not

clear. One possibility is that acetylation promotes a change to

the antiparallel configuration of STAT1 subunits that facilitates

dephosphorylation by TCP45. In this speculative model, acety-

lated cytoplasmic STAT1 is refractory to activation because of

association with TCP45. Deacetylation of STAT1 that is mediated

by HDACs such as HDAC3 (Kramer et al., 2009) thus promotes

increased tyrosine phosphorylation and stabilization of the

active parallel configuration STAT1 dimer (Figure 1). This require-

ment for HDAC activity for STAT1 activation could potentially

explain the paradoxical observation that HDAC inhibitors

suppress STAT1-dependent transcription (Chang et al., 2004;

Nusinzon and Horvath, 2003). This acetylation-mediated nega-

tive regulatory mechanism can potentially be bypassed by de

novo synthesis of STAT1, which is an important mechanism for

augmenting long-term STAT1 activity (Hu et al., 2002).

The role of acetylation in regulating the STAT1 activation cycle

opens new avenues for regulation and modulation of STAT1

function and crosstalk with heterologous signaling pathways.

For example, the activity of certain STAT HDACs, such as

Sirtuin1, is regulated by the overall cellular metabolic state as

reflected in the NAD/NADH ratio and can be selectively and ther-

apeutically modulated by small molecule compounds (Finkel

et al., 2009). Other, as yet unknown, mechanisms control the

translocation of HDACs and HATs to the cytoplasm where they

can modify STATs. Despite recent progress, many unanswered

questions remain regarding STAT acetylation. One outstanding

question is what underlies the differential functional outcomes

of acetylation of different STAT molecules. Plausible explana-

tions include different acetylation sites (lysines 410 and 413 of

STAT1 versus lysines 679, 685, 707, and 709 of STAT3) and

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Figure 2. IFN-g Signaling Disrupts TLR-Induced Feedback Inhibitory LoopsAs shown on the left, IFN-g enhances TLR-induced TNF production by disrupting an IL-10-mediated inhibitory loop. IFN-g signaling leads toincreased activity of GSK3, which negatively regu-lates Il10 expression by suppressing activation oftranscription factors CREB and AP-1. As shownon the right, IFN-g enhances TLR-induced IL-6and IL-12 production by disrupting an inhibitoryloop mediated by canonical Notch target genesHes1 and Hey1. IFN-g signaling downregulatesintracellular NICD2 amounts and thus inhibitsexpression of Hes1 and Hey1. Hes1 and Hey1are transcription repressors that negatively regu-late Il6 and Il12 gene expression.

different structural changes induced by acetylation. Because

STAT1 and STAT3 often antagonize each other’s functions in

many processes including inflammation and tumorigenesis,

differential regulation of these STATs by acetylation may repre-

sent a mechanism to regulate the balance of STAT function

downstream of cytokine receptors.

Enhancement of Innate Immune ActivationIt has been long appreciated that IFN-g promotes innate immune

responses by activating macrophages. One mechanism of IFN-

g-mediated macrophage activation is direct effector gene acti-

vation via STAT1 as discussed above. Another way for IFN-g

to achieve strong activation effects is by enhancing macrophage

responsiveness to other inflammatory stimuli, such as Toll-like

receptor (TLR) ligands, tumor necrosis factor (TNF), and type I

IFNs; this phenomenon is termed ‘‘priming.’’ IFN-g primes

macrophages for enhanced type I IFN responses characterized

by increased STAT1 activation (Hu et al., 2002). Conversely,

type I IFNs augment IFN-g signaling via association of type I

and type II IFN receptor subunits (Takaoka et al., 2000). Priming

of TLR responses by IFN-g greatly augments TLR-induced

expression of inflammatory mediators and immune effectors,

including multiple cytokines and chemokines, and profoundly

affects biological outcomes of innate immunity and inflamma-

tion. The mechanisms underlying IFN-g-mediated priming have

been the subject of extensive investigation, and it has been sug-

gested that IFN-g priming enhances TLR-activated signal trans-

duction. For example, IFN-g priming increases TLR expression,

promotes NF-kB activation, and induces transcription factors

that are essential for expression of certain TLR-responsive

genes. However, enhancement of TLR signaling can not explain

the full spectrum of activation achieved by IFN-g priming, given

that accumulating evidence suggests that inactivation of feed-

back inhibition pathways by IFN-g is important for the broad

and sustained activation of macrophage effector genes and

mechanisms that is characteristic of primed cells. Of note,

enhancement of positive signaling and inactivation of feedback

inhibition are two complementary mechanisms that reinforce

each other to achieve the robust priming effects seen with

IFN-g. The enhancement of positive TLR signaling by IFN-g has

been reviewed elsewhere (Schroder et al., 2006); herein we

review recent progress regarding IFN-g-mediated abrogation

of TLR-induced feedback inhibitory loops.

IL-10 is a major anti-inflammatory cytokine induced by TLR

signaling and functions to inhibit production of TLR-induced

proinflammatory mediators, such as TNF, in a STAT3-dependent

manner (Mosser and Zhang, 2008). IFN-g priming disrupts this

IL-10-STAT3 feedback inhibitory loopand thus leads to increased

production of inflammatory cytokines. One mechanism by

which IFN-g suppresses the IL-10-STAT3 axis involves inhibition

of TLR-induced Il10 gene expression. IFN-g suppresses IL-10

production by increasing the activity of glycogen synthase kinase

3b (GSK3b), a serine-threonine kinase that inhibits the function of

AP-1 and CREB, two transcription factors critical for Il10 expres-

sion. Upon activation of TLRs, GSK3b is phosphorylated and

inactivated by the PI3K-Akt signaling pathway, and inactivation

of GSK3b allows Il10 to be expressed. IFN-g priming overcomes

this TLR-induced inhibition of GSK3b and thus restores the

capacity of GSK3b to inhibit Il10 expression (Hu et al., 2006)

(Figure 2, left panel). IFN-g-GSK3b-mediated regulation of TLR

responses is best characterized with IL-10 as a target. However,

given that GSK3b controls the function of CREB and AP-1, key

transcription factors involved in expression of many TLR-induced

genes, it is likely that IFN-g regulates expression of a subset of

TLR-inducible genes via GSK3 (Ho et al., 2008; Hu et al., 2007).

One unanswered question is the mechanism by which IFN-g

activates GSK3b. A potential mechanism is IFN-g-mediated

suppression of TLR-induced PI3K-Akt signaling, with resultant

decreased inhibitory phosphorylation of GSK3b (Hu et al.,

2006). However, the mechanism whereby IFN-g inhibits the

PI3K pathway is not clear. Alternatively, IFN-g can inactivate

GSK3 phosphatases or promote alternative GSK3 activation via

the tyrosine kinase Pyk2 (Tsai et al., 2009). As GSK3 is involved

in various signaling pathways including Wnt-b-catenin signaling,

IFN-g regulation of GSK3b has broader implications for signal

transduction crosstalk, such as potential cross-regulation

between IFN-g and Wnt pathways (Kockeritz et al., 2006).

In addition to inactivation of the IL-10-STAT3 axis, IFN-g

disrupts another feedback inhibitory loop involving Notch target

genes Hes1 and Hey1, which are transcriptional repressors (Hu

et al., 2008a). The Notch pathway, whose functions have been

predominantly characterized in developmental biology systems,

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was recently described to modulate macrophage activation

and to be regulated by IFN-g. In macrophages, expression of

canonical Notch target genes Hes1 and Hey1 is induced by

TLR stimulation. Expression of Notch target genes is synergisti-

cally activated by TLR and Notch pathways by cooperation

between RBP-J, a master transcription factor downstream of

Notch signaling, and the TLR signaling components IKKb and

p38. After induction by TLRs, transcription repressors Hes1

and Hey1 suppress TLR-induced IL-6 and IL-12 expression,

constituting another feedback inhibitory loop that dampens

cytokine production (Figure 2, right). IFN-g signaling inhibits

expression of Hes1 and Hey1 at least in part by downregulating

amounts of NICD2, the intracellular cleaved fragment of Notch2

receptor that binds RBP-J and activates Notch target gene

expression. Potential mechanisms by which IFN-g downregu-

lates NICD2 include modulation of proteases that generate and

degrade NICD2 and activation of GSK3 that destabilizes of

NICD proteins (Espinosa et al., 2003). Thus, IFN-g primes for

augmented TLR-induced IL-6 and IL-12 production by disrupt-

ing an inhibitory loop mediated by Hes1 and Hey1 (Hu et al.,

2008a). The above examples suggest that inactivation of feed-

back inhibitory pathways by IFN-g is a common mechanism of

priming, and additional examples are likely to be uncovered.

Another notion emerging from these studies is that IFN-g selec-

tively and differentially regulates expression of subsets of TLR

target genes by targeting distinct TLR-induced signaling mole-

cules such as AP-1 (Hu et al., 2007). This provides an additional

mechanism for selective regulation of TLR responses; the impor-

tance of such selective regulation has recently been highlighted

by Medzhitov and colleagues (Foster et al., 2007; Foster and

Medzhitov, 2009).

IFN-g also directly inhibits signaling pathways downstream of

anti-inflammatory cytokines to antagonize their suppressive

functions. IFN-g antagonizes anti-inflammatory effects of IL-10

both by attenuating IL-10 production, as discussed above, and

by suppressing IL-10 signaling. Anti-inflammatory action of

IL-10 is predominantly mediated by STAT3 and IFN-g cross-

regulates IL-10 signaling by abrogating expression of STAT3

target genes. IFN-g does not suppress upstream IL-10 signaling,

including activation of Jaks. Instead, IFN-g shifts the balance of

IL-10 STAT activation from STAT3 to STAT1, resulting in induc-

tion of STAT1 target genes by IL-10 (Herrero et al., 2003). Inhibi-

tion of IL-10-STAT3 signaling has important biological impact

because the anti-inflammatory activity of IL-10 is diminished

after IFN-g priming (Herrero et al., 2003). The mechanisms of

STAT1-STAT3 cross-regulation are discussed below. TGF-b is

another cytokine with important anti-inflammatory function that

is subject to the antagonistic action of IFN-g. IFN-g induces

expression of Smad7, an inhibitory Smad, and thus inhibits

TGF-b-induced activation of the activating Smad3 and of

TGFb-responsive genes (Ulloa et al., 1999). STAT1 also directly

binds Smad3 and inhibits its function (Ghosh et al., 2001). In

summary, inhibition of expression and function of anti-inflamma-

tory molecules represents an important mechanism of IFN-g-

mediated priming of enhanced innate immune responses.

Attenuation of Tissue DestructionThe activating effects of IFN-g on immunity and inflammation

have been extensively studied and are well established. At the

542 Immunity 31, October 16, 2009 ª2009 Elsevier Inc.

same time, IFN-g possesses crucial homeostatic functions that

limit inflammation-associated tissue damage. This enables the

host to utilize one mediator, IFN-g, to regulate the balance

between clearance of invading pathogens and limiting collateral

damage to the host. IFN-g plays an important role in limiting

tissue damage associated with acute infections and with chronic

inflammation in autoimmune diseases such as inflammatory

arthritis and experimental allergic encephalomyelitis (EAE).

Mechanisms underlying the homeostatic functions of IFN-g,

which include inhibition of gene expression, inhibition of migra-

tion and differentiation of tissue-destructive cells, and inhibition

of signaling by tissue-destructive cytokines, are reviewed in

this section.

One mechanism by which IFN-g attenuates tissue destruction

is inhibition of expression of genes that encode tissue-destruc-

tive factors, such as matrix metalloproteinases (MMPs), serine

proteases, coagulation factors, complement components, and

enzymes involved in prostaglandin metabolism (Barrios-Rodiles

and Chadee, 1998; Ho et al., 2008; Ma et al., 2001; Sanceau

et al., 2002; Zhou et al., 2003; L.B.I., unpublished data). IFN-g

broadly suppresses expression of multiple MMPs including

MMP1, MMP2, MMP3, MMP7, MMP9, and MMP10 induced

by various receptors such as TLRs and IL-1R. IFN-g-mediated

suppression of MMPs requires STAT1. However, to date there

is no compelling evidence that STAT1 directly suppresses gene

expression, including expression of genes that encode MMPs.

Instead, IFN-g inhibits receptors and signals that induce

MMP expression (Figure 3). IFN-g suppresses IL-1-induced

MMP expression in macrophages by STAT1-dependent down-

regulation of IL-1RI (Hu et al., 2005a). Inhibition at this proximal

step inactivates all signaling cascades downstream of the IL-1

receptor and results in a global block in macrophage responses

to IL-1 (Browning and Ribolini, 1987; Dickensheets and Donnelly,

1997; Hu et al., 2005a). IFN-g-mediated inhibition of TLR-

induced genes targets downstream signaling components and

is more selective in inhibiting a subset of�15% of TLR-inducible

genes, including those that encode MMPs (Hu et al., 2007). For

TLRs, the inhibitory effects of IFN-g are achieved by superinduc-

tion of transcriptional repressors, such as ATF-3. IFN-g and TLR

signaling synergistically induces expression of ATF-3 that binds

to and inhibits the MMP1 promoter (Ho et al., 2008). In addition,

IFN-g also inhibits AP-1 transcription factors that are required for

MMP expression (Ho et al., 2008). This inhibition of AP-1 and

downstream target genes is reminiscent of the above-discussed

findings that IFN-g inhibits IL-10 expression in part by inhibiting

AP-1 (Hu et al., 2006). IFN-g can transiently activate AP-1 in

certain cell types (Gough et al., 2007), but priming with IFN-g

suppresses AP-1 activity by several mechanisms, including

attenuation of upstream MAPK pathways that induce expression

and increase transcriptional activity of AP-1 proteins (Hu et al.,

2006), suppression of transcription of genes encoding AP-1

components (Hu et al., 2006), downregulation of AP-1 mRNA

at the posttranscriptional level (Radzioch and Varesio, 1991),

and regulation of AP-1 protein stability (Hu et al., 2007). Destabi-

lization of the AP-1 protein c-Jun by IFN-g appears to be medi-

ated by GSK3, which phosphorylates c-Jun and creates a

binding site for the E3 ubiquitin ligase Fbw7 (Wei et al., 2005).

Overall, differential regulation of transcription factors down-

stream of TLR signaling by IFN-g (inhibiting AP-1 versus

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Figure 3. Signaling MechanismsAssociated with IFN-g-MediatedAttenuation of Tissue DestructionIFN-g suppresses inflammatory tissue destructionvia regulation of IL-1R and TLR signaling. IFN-ginhibits IL-1 signaling and subsequent inductionof destructive factors in macrophages by downre-gulating IL-1RI expression. In addition, IFN-gblocks induction of MMP downstream of TLRsignaling by superinducing transcription repressorATF3 and inhibiting transcription activators CREBand AP-1. IFN-g inhibits CREB activity by sup-pressing its serine phosphorylation and inhibitsAP-1 by downregulating nuclear protein levels ofits subunits. IFN-g inhibits osteoclastogenesisand bone resorption via regulation of RANK,CSF-1R, and TREM2 signaling. In osteoclastprogenitor cells, IFN-g suppresses expression aswell as signal transduction of RANK, CSF-1R,and TREM2, receptors critical for the process ofosteoclastogenesis. IFN-g also attenuates fibrosisvia inhibition of TGF-bR and IL-4R signaling. IFN-gsuppresses TGF-bR signaling by induction ofinhibitory SMAD (SMAD7) and by direct inhibitionof SMAD3 by STAT1. IFN-g inhibits IL-4R signalingby induction of SOCS1.

augmenting NF-kB) provides a means to augment inflammatory

cytokine production yet to limit expression of tissue destructive

factors such as MMPs. A more universal mechanism of suppres-

sion that is independent of upstream signaling involves STAT1-

mediated sequestration of the coactivator CBP, which is then

not available to activate MMP gene promoters (Ma et al., 2005).

Another way by which IFN-g exerts homeostatic functions is

attenuation of tissue infiltration by neutrophils and monocytes.

In several models of human autoimmune disorders such as

experimental arthritis and EAE, deficiency of IFN-g signaling

results in increased accumulation of neutrophils and other

myeloid cells at sites of inflammation (Ferber et al., 1996; Irmler

et al., 2007; Manoury-Schwartz et al., 1997; Vermeire et al.,

1997). Several mechanisms may account for the suppressive

effects of IFN-g on inflammatory cell infiltration: (1) IFN-g atten-

uates myelopoiesis and granulopoiesis and thus limits avail-

ability of infiltrating cells at their source. Several reports have

shown that IFN-g-deficient mice undergo deregulated expan-

sion of macrophages and granulocytes during infections (Mat-

thys et al., 1999; Murray et al., 1998). (2) IFN-g inhibits expression

of chemokines that attract cells to inflammatory sites. One

example of such regulation is IFN-g-mediated inhibition of

expression of MCP-2, a major neutrophil chemoattractant in

mice (Kelchtermans et al., 2007). (3) IFN-g alters cellular respon-

siveness to chemokines. This phenomenon is exemplified by the

observation that IFN-g arrests monocyte migration and alters

cellular responses to CCL2 by modulating the activities of

signaling molecules Pyk2, Jnk, Rac, and Cdc42 and inhibiting

CCL2-induced activation of PAK kinase that regulates cytoskel-

eton rearrangement and cell polarization (Hu et al., 2008b).

Inflammation often leads to tissue remodeling and bone

resorption, processes that are subject to inhibition by IFN-g.

Bone resorption is mediated by myeloid lineage cells called

osteoclasts, and IFN-g is a potent inhibitor of osteoclastogene-

sis (Takahashi et al., 1986; Takayanagi et al., 2005). IFN-g sup-

presses osteoclastogenesis in vitro and in vivo by regulating

the expression and signaling by two key receptors required for

osteoclast generation and differentiation, c-Fms (the receptor

for macrophage colony-stimulating factor [M-CSF], also termed

CSF-1) and receptor activator of nuclear factor-kB (RANK),

a member of the TNF receptor family that binds its cognate

ligand RANKL. IFN-g interferes with RANK signaling by sup-

pressing expression of RANK and by targeting the key adaptor

molecule TRAF6 for proteasome-mediated degradation, result-

ing in diminished activation of downstream signaling events

(Takayanagi et al., 2000) (Figure 3). Similar to IFN-g, a type I

IFN, IFN-b, also inhibits RANK signaling in a STAT1-dependent

manner. However, instead of targeting TRAF6, IFN-b inhibits

translation of c-Fos, an AP-1 family transcription factor essential

for the induction of NFATc1, the master regulator of osteoclasto-

genesis (Takayanagi et al., 2002). Given that IFN-g suppresses

c-Fos expression in closely related cell types such as macro-

phages, it is possible that IFN-g targets c-Fos in osteoclasts in

addition to targeting RANK and TRAF6. One interesting possi-

bility awaiting assessment is the effect of IFN-g on CREB activa-

tion and function in the context of osteoclast differentiation.

Given the precedent of inhibition of TLR-induced CREB activity

by IFN-g in macrophages (Hu et al., 2006) and the critical role

of CREB in osteoclastogenesis (Sato et al., 2006), inhibition of

CREB may contribute to IFN-g-mediated inhibition of osteoclas-

togenesis. IFN-g also inhibits expression of c-Fms, thus confer-

ring resistance to M-CSF stimulation (Baccarini et al., 1992;

Inaba et al., 1995) (Figure 3). Diminished M-CSF responses result

in decreased production of osteoclast precursors and may also

explain the suppressive effects of IFN-g on myelopoiesis.

Fibrosis results from aberrant tissue remodeling and excessive

connective tissue formation after injury or during chronic inflam-

mation. IFN-g suppresses fibrosis in several models including

viral hepatitis, bleomycin-induced pulmonary fibrosis, and

schistosomiasis-induced fibrosis (Wynn, 2004) at least in part

by inhibiting signaling by the major profibrotic factors IL-4,

IL-13, and TGF-b (Figure 3). These suppressive effects can be

mediated at least in part by the IFN-g-induced T-bet transcription

factor (Aliprantis et al., 2007). Alternatively activated or M2

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Figure 4. Signaling MechanismsAssociated with IFN-g-Mediated Regulationof T Cell Differentiation and FunctionIn Th1 cell differentiation, IFN-g-STAT1 signaling iscritical for induction of T-bet and thus forsustaining the positive feedback loop that leadsto heightened production of IFN-g. IFN-g blocksTh2 cell differentiation by inhibiting IL-4-STAT6 signaling. IFN-g and STAT1 also blockTh17 cell differentiation. IFN-g-STAT1 signalingcan potently inhibit Th17 cell differentiation, butthe mechanism of action is not clear. BecauseSTAT3 signaling from multiple cytokines includingIL-6, IL-23, and IL-21 plays a pivotal role in medi-ating Th17 cell differentiation, it is possible thatIFN-g-STAT1 suppresses Th17 cell by targetingSTAT3 (as shown by dotted lines). IFN-g andSTAT1 also inhibit the aryl hydrocarbon nuclearreceptor (AHR) important for Th17 cell differentia-tion, and it is possible that suppression of TGF-band IL-1 signaling by IFN-g contributes to inhibi-tion of Th17 cell differentiation (not depicted).IFN-g further regulates Treg cell differentiationand function. IFN-g can block TGFb-mediated

Treg cell differentiation. In Foxp3+ Treg cells, IFN-g upregulates expression of T-bet, which in turns promotes expression of CXCR3 that regulates homing ofT-bet+ FoxP3+ Treg cells to sites of Th1 cell inflammation. T-bet also increases suppressive function of Treg cells, and T-bet+ FoxP3+ Treg cells effectivelysuppress Th1 cell inflammation in vivo.

macrophages have been proposed to play a key role in promoting

fibrosis (Mosser and Edwards, 2008; Wynn, 2004), and IFN-g-

mediated diversion of macrophage differentiation away from a

wound-healing profibrotic M2 phenotype also probably contrib-

utes to suppression of fibrosis. In addition to its effects on

fibrosis, IFN-g-mediated modulation of the balance between

M1 and M2 macrophage differentiation also contributes to other

important processes such as host defense, immune regulation,

and tissue remodeling (Mosser and Edwards, 2008). Finally,

IFN-g suppresses fibrosis by inhibiting collagen synthesis.

In summary, IFN-g attenuates tissue destruction by modu-

lating the expression, signaling, and function of tissue-destruc-

tive cytokines and their receptors, with resulting suppression

of gene expression and of cell recruitment and differentiation.

Where studied, these suppressive effects are dependent on

STAT1, suggesting indirect regulation mediated by STAT1 target

genes such as ATF3. Identification and characterization of

STAT1 target genes that regulate tissue-destructive pathways

represents a fruitful area for future research.

Regulation of Adaptive Immunity: Th and Treg CellDifferentiationAs a major effector cytokine of Th1 cell immunity, it is no surprise

that IFN-g autoamplifies Th1 cell responses (Figure 4) and cross-

inhibits differentiation and function of other Th cell subsets

including Th2 and Th17 cells. This regulation by IFN-g represents

a mechanism for maintaining Th1 cell lineage commitment and

stabilizing Th cell phenotypes (Szabo et al., 2003). One general

theme underlying IFN-g-mediated cross-inhibition is interfer-

ence with signal transduction pathways and transcription factors

downstream of cytokines that drive differentiation of other Th cell

subtypes. For example, IFN-g suppresses the IL-4-STAT6

pathway that is required for Th2 cell differentiation, mediated in

part by induction of SOCS1 that inhibits IL-4 receptor signaling

(Naka et al., 2001; Yu et al., 2004) (Figure 4). In addition, IFN-g-

induced T-bet suppresses Th2 cell differentiation by inhibiting

544 Immunity 31, October 16, 2009 ª2009 Elsevier Inc.

the expression and/or function of the Th2 cell transcription

factor GATA3 (Hwang et al., 2005; Yu et al., 2004). Another

SOCS-independent inhibitory mechanism is posttranscriptional

downregulation of IL-4-induced IL-4R gene expression (So

et al., 2000).

Differentiation of Th17cells, which is driven by IL-6, IL-1, TGF-b,

IL-21, and IL-23 (Zhou et al., 2009), is strongly suppressed by

IFN-g in vitro and in vivo. In vitro, treatment with IFN-g neutralizing

antibody during the course of Th17 cell differentiation leads to

increased frequency of Th17 cells, whereas exogenous IFN-g

reduces the Th17 cell population (Harrington et al., 2005). In vivo,

IFN-g deficient mice exhibit enhanced Th17 cell responses in

several disease models including mycobacterial infection and

collagen-induced arthritis (Chu et al., 2007; Cruz et al., 2006; Irm-

ler et al., 2007). Aside from its effects on Th17 cell development, it

was recently reported that IFN-g inhibits effector functions of

Th17 cells (Kelchtermans et al., 2009). Although cross-inhibition

of Th17 cell development by IFN-g is relatively well established,

the mechanisms are not clear. Inhibition by IFN-g is likely to be

dependent on STAT1, given that STAT1-deficient mice mount

enhanced Th17 cell responses, and another STAT1-activating

cytokine, IL-27, potently suppresses Th17 cell development in

a STAT1-dependent manner (Stumhofer et al., 2006). The mole-

cules that are important for Th17 cell differentiation and that

are inhibited by IFN-g have not been unequivocally identified.

Possibilities include inhibition of Smad signaling downstream of

TGF-b (Tanaka et al., 2008), downregulation of T cell IL-1R

expression (X.H., unpublished data), and inhibition of the aryl

hydrocarbon receptor (Kimura et al., 2008). In addition, STAT1

inhibits STAT3 (see below), which is activated by IL-6, IL-23,

and IL-21 and is important for Th17 cell differentiation; it is

possible that STAT1 suppresses Th17 cell differentiation by

targeting STAT3 (Figure 4).

Th17 cell responses are important for host defense against

extracellular bacteria and yeast and are characterized by neutro-

phil infiltration and the potential for severe tissue destruction

Immunity

Review

(McGeachy and Cua, 2008). Thus, counterregulation of Th17 cell

differentiation by IFN-g may represent an important pathway to

limit tissue inflammation and damage. Emerging evidence

suggests a greater complexity in IFN-g-mediated regulation of

Th17 cells than previously appreciated. Many Th cells at sites

of inflammation, such as the central nervous system in EAE,

coexpress IFN-g and IL-17, and recent evidence supports

plasticity in the Th17 cell lineage, with the potential to evolve

into IFN-g-expressing cells (Lee et al., 2009). Thus, different

from Th1 and Th2 cells, the relationship of Th1 and Th17 cells

is not limited to cross-inhibition. Instead, there is a potential for

ongoing generation and differentiation of Th cells with a changing

or mixed effector phenotype. This allows fine tuning of Th1-Th17

cell effector functions to achieve the most effective host

response during the course of infections and to balance immu-

nity with preservation of tissue integrity.

Regulatory T (Treg) cells serve to restrain overactivation of

effector T cells and maintain homeostasis. Interest in the role

of IFN-g in Treg cell development was prompted by the initially

paradoxical findings that IFN-g is protective in models of autoim-

mune diseases such as EAE (Ferber et al., 1996; Willenborg

et al., 1996). Exacerbation of EAE in mice deficient in IFN-g

signaling correlates with reduced numbers and function of

Treg cells (Nishibori et al., 2004; Wang et al., 2006). Moreover,

adoptive transfer of IFN-g-treated Treg cells is sufficient to

ameliorate EAE symptoms (Nishibori et al., 2004), supporting

an essential role of IFN-g in Treg cell development, at least in

EAE model. However, it is difficult to reconcile the above findings

with the observations that Treg cell development proceeds nor-

mally in the absence of IFN-g signaling under many conditions

(Kelchtermans et al., 2005). Recently, the emerging concept of

Treg cell diversity and polarization has shed light on the contro-

versial issue of the involvement of IFN-g in Treg cell development

(Barnes and Powrie, 2009). Two elegant studies suggest that,

similar to effector T cells, Treg cells undergo polarization into

specialized phenotypes and that factors important for effector

T cell development may also play a critical role in Treg cell polar-

ization (Koch et al., 2009; Zheng et al., 2009). For example, IRF4,

a transcription factor key for differentiation of Th2 cells, is

required for differentiation and function of a Treg cell subset

that specifically suppresses Th2 cell responses (Zheng et al.,

2009). In parallel, T-bet, a master regulator of Th1 cell differenti-

ation, is upregulated by IFN-g-STAT1 signaling in Foxp3+ Treg

cells, and Foxp3+T-bet+ cells represent a novel subset of Treg

cells that selectively dampens Th1 cell responses (Koch et al.,

2009) (Figure 4). The existence of specialized Treg cell subsets

may help to explain the apparent discrepancy that IFN-g is

necessary for Treg cell development under certain circum-

stances but not under others. Interestingly, as a major effector

of Th1 cell responses, IFN-g promotes differentiation of

Foxp3+T-bet+ Treg cells that suppress Th1 cell responses, and

such a differentiation constitutes a negative feedback loop that

contributes to homeostatic action of IFN-g. Overall, recent

developments implicate a regulatory role of IFN-g in modulating

many aspects of T cell biology aside from its classic activating

role in Th1 cell responses. In addition to its action on T cells,

IFN-g suppresses early B cell development in the bone marrow

and also promotes isotype switching to IgG2a, underscoring

its diverse effects on adaptive immunity (Schroder et al., 2004).

Cross-inhibition of the Opposing STATsMechanisms by which IFN-g and STAT1 regulate the function

of receptors that activate distinct signaling pathways were

described above. In this section, we will review mechanisms

by which IFN-g and STAT1 regulate signaling by cytokines that

utilize the Jak-STAT pathway but have different and opposite

functions from IFN-g. Cytokines that oppose each other often

activate different STATs that antagonize each other. A good

example of antagonistic STATs is STAT1 and STAT3, which

are activated by the opposing cytokines IFN-g and IL-10,

respectively. STAT1 and STAT3 oppose each other in many

biological processes including macrophage activation that is

enhanced by STAT1 and inhibited by STAT3, cell proliferation

that is suppressed by STAT1 and promoted by STAT3, and Th

cell differentiation in which STAT1 promotes Th1 cell responses

and STAT3 drives Th17 cell responses (O’Shea and Murray,

2008).

The best-established mechanism by which STATs oppose one

another is indirect regulation mediated by SOCS proteins that

suppress signaling by cytokine receptors by inhibiting

receptor-associated Jaks, binding to and blocking STAT dock-

ing sites, and targeting receptors for proteosomal degradation

(Yoshimura et al., 2007). IFN-g and STAT1 activate expression

of SOCS1, a potent feedback inhibitor of IFN-g signaling that

also cross-inhibits signaling by the type I IFN receptor and the

IL-4 receptor (Dickensheets et al., 1999; Fenner et al., 2006;

Losman et al., 1999; Naka et al., 2001; Yu et al., 2004; Zimmerer

et al., 2007). Thus, SOCS1-mediated inhibition can explain

the suppressive properties of IFN-g on Th2 cell differentiation

(Figure 4). However, SOCS1 does not effectively inhibit signaling

by the IL-10 receptor or IL-6-related receptors that utilize gp130

and is not known to inhibit signaling by IL-21 or IL-23. Thus, IFN-

g-mediated antagonism of IL-10 function (Herrero et al., 2003;

Lauw et al., 2000; Pajkrt et al., 1997) can not be explained by

a SOCS1-dependent mechanism; it also appears likely that

regulation of Th17 cell differentiation by IFN-g can not be

explained solely by induction of SOCS1 or other SOCS proteins.

STAT1 also suppresses STAT3 by alternative and more direct

mechanisms, as was first suggested by genetic evidence

showing increased STAT3 activation in STAT1-deficient cells

(Gil et al., 2001; Qing and Stark, 2004; Ramana et al., 2001).

Mechanisms by which STAT1 can potentially directly inhibit

STAT3 include competition for binding to docking sites on recep-

tors or to target DNA sequences in promoters, competition for

binding to other proteins or cofactors, sequestration of STAT3

from active complexes, and direct transcriptional repression of

STAT3 target genes. These mechanisms are relevant not only

for cross-inhibition of signaling by other cytokines but also for

establishing the balance of STAT activation downstream of the

IFNGR. Ligation of IFNGR leads to activation of both STAT1

and STAT3. However, in most cells STAT3 activity is kept low

because of cross-inhibition by STAT1. Thus, the contribution of

STAT3 to IFN-g function is limited in most contexts and is best

revealed in cells deficient in STAT1 (Gil et al., 2001; Qing and

Stark, 2004). STAT1 suppresses IFNGR-mediated activation of

STAT3, at least in part by competing for the STAT docking site

within the IFNGR cytoplasmic domain. Because receptor dock-

ing is a prerequisite for activation by tyrosine phosphorylation,

the prediction of the competition for docking sites model is

Immunity 31, October 16, 2009 ª2009 Elsevier Inc. 545

Immunity

Review

that STAT1 suppresses STAT3 tyrosine phosphorylation down-

stream of IFNGR or other receptors. Several reports using cell

lines support this model (Costa-Pereira et al., 2002; Qing and

Stark, 2004), but suppression of STAT3 tyrosine phosphorylation

by STAT1 appears to be context dependent, and in primary

macrophages it is clear that IFN-g and STAT1 suppress STAT3

function without suppressing its tyrosine phosphorylation (Her-

rero et al., 2003). Conceivably, STAT1 could suppress STAT3

function by displacing STAT3 from binding at target gene

promoters; in the case of promoter binding by the STAT1b iso-

form that does not contain a transcription activation domain,

such binding would lead to inhibition of transcription. There is,

however, very limited evidence to support mechanisms that

involve competition for binding to target DNA elements or for

transcriptional coactivators.

An alternative explanation for how STAT1 can inhibit STAT3

function without suppressing STAT3 tyrosine phosphorylation

is sequestration of STAT3 away from active complexes into

STAT1:STAT3 heterodimers. This will result in diminished

amounts of STAT3:STAT3 homodimers, such as those activated

by IL-10, that are transcriptionally active and functional. It is

possible that STAT1:STAT3 heterodimers are less transcription-

ally active than STAT3 homodimers or bind to alternative

promoters. Sequestration of STAT3 into STAT1:STAT3 hetero-

dimers is increased in cells that have been primed and that

express increased amounts of STAT1; near complete sequestra-

tion of STAT3 into STAT1:STAT3 heterodimers in primed cells

correlates with diminished STAT3 function (Herrero et al.,

2003). Under these conditions of dimerization with excess

STAT1, STAT3 can be retained in the cytoplasm, with diminished

target gene expression secondary to decreased nuclear translo-

cation (Hu et al., 2005b). In addition to suppressing STAT3

homodimer formation, incorporation of STAT3 into STAT1:

STAT3 heterodimers can result in diminished formation of other

active STAT3-containing complexes, such as STAT3-Jun

complexes important for activation of specific target genes

(Ivanov et al., 2001). Interestingly, this sequestration model by

which STAT1 inhibits transcription factors extends to inhibition

of RUNX2 and NF-kB by STAT1 binding and subsequent trap-

ping of these transcription factors in the cytoplasm (Kim et al.,

2003; Kramer et al., 2006). Finally, it is possible that STAT1 can

bind to STAT3 target genes and directly suppress transcription

by recruiting transcriptional repressors. An interesting area for

future investigation will be to determine whether STAT1 can

indeed directly repress gene transcription, in contrast to the

indirect mechanisms that have been described previously and

reviewed here. It will also be important to determine mechanisms

by which IFN-g and STAT1 inhibit STAT3-mediated IL-6, IL-21,

and IL-23 function during Th17 cell differentiation.

Role in Autoimmune DiseasesAutoimmune diseases are characterized by the development of

autoimmunity against self-antigens, together with an effector

phase characterized by chronic inflammation and attendant

tissue damage. Many autoimmune diseases, such as rheuma-

toid arthritis (RA), multiple sclerosis (MS), inflammatory bowel

disease, psoriasis, and lupus nephritis, are characterized by

the presence of activated macrophages at sites of inflammation

and disease. These macrophages exhibit an ‘‘M1’’ classically

546 Immunity 31, October 16, 2009 ª2009 Elsevier Inc.

activated phenotype and are believed to be key players in path-

ogenesis via production of cytokines such as TNF, IL-1, and IL-6

(Mosser and Edwards, 2008). Thus, on the basis of its macro-

phage-activating properties, IFN-g has been considered an

attractive candidate pathogenic cytokine in autoimmune

diseases. Several mouse models of autoimmune diseases,

such as collagen-induced arthritis (CIA) and EAE, were originally

thought to be predominantly Th1 cell mediated, further support-

ing the notion that IFN-g is pathogenic. However, consistent with

the pleiotropic activating and suppressive functions of IFN-g

described above, it is now clear that IFN-g has both promoting

and suppressive effects in autoimmune diseases. Most strik-

ingly, IFN-g suppresses Th17 cell-mediated autoimmunity in

mice and can have both augmenting and suppressive effects

on autoimmunity and on the effector inflammatory phase of auto-

immune diseases (Kelchtermans et al., 2008), depending on the

specific disease and the timing, location, and intensity of IFN-g

action.

It is now clear on the basis of genetic evidence that EAE and

CIA are Th17 cell-mediated disease models. In EAE, genetic

ablation of IFN-g or the IFNGR results in increased morbidity

and mortality (Ferber et al., 1996; Willenborg et al., 1996). Exac-

erbated disease in the absence of IFN-g signaling is associated

with massive central nervous system infiltrates composed of

neutrophils and macrophages (Ferber et al., 1996; Willenborg

et al., 1996). In CIA, deficiency of IFNGR leads to accelerated

onset and increased incidence of disease (Manoury-Schwartz

et al., 1997; Vermeire et al., 1997). Joint lesions of IFNGR defi-

cient mice in CIA are characterized by increased infiltration of

neutrophils and macrophages, with increased tissue destruction

and bone erosion (Manoury-Schwartz et al., 1997; Vermeire

et al., 1997). In both EAE and CIA, the protective role of IFN-g

has been attributed to its suppression of Th17 cell responses,

and this notion is supported by evidence that IL-17 antibodies

attenuate arthritis in IFN-g-deficient animals in two different

models (Chu et al., 2007; Kelchtermans et al., 2009). However,

as discussed above, attenuation of disease by IFN-g is also

probably mediated by additional protective mechanisms such

as suppression of production of chemokines, cytokines, and

tissue-destructive enzymes (Guedez et al., 2001; Kelchtermans

et al., 2007; Willenborg et al., 1999), infiltration of inflammatory

cells, and differentiation of osteoclasts; modulation of Treg cell

function may also be important and IFN-g-induced Treg cell

subsets may specifically attenuate Th1cell-mediated pathology

while allowing Th17 cell-mediated pathology to progress. Finally,

strong activation of the IL-12-IFN-g axis leads to enhanced IL-10

production by Th1 cells, which constitutes another feedback

loop that can attenuate disease pathology (Saraiva et al., 2009).

Although IFN-g is clearly protective in EAE and CIA, it is overly

simplistic to conclude that IFN-g plays a protective role in multiple

sclerosis and rheumatoid arthritis on the basis of its role in these

two acute neutrophil-dominated models of chronic human

autoimmune diseases that exhibit a more complex and often

different pathology. Indeed, administration of IFN-g induces

exacerbations of MS in humans (Panitch et al., 1987), and IFN-

g is pathogenic in other models of RA, such as proteoglycan-

induced arthritis, and in CIA when complete Freund’s adjuvant

(CFA) is not used during disease induction (Finnegan et al.,

2002; Matthys et al., 1999). Even in CIA induced with standard

Immunity

Review

CFA-utilizing protocols, exogenous IFN-g can exacerbate

disease depending on whether it is provided locally or systemi-

cally and on timing of administration (Boissier et al., 1995).

More recent work (Kroenke et al., 2008; Luger et al., 2008; Stein-

man, 2008) supports the fact that both Th1 and Th17 cells can

contribute to pathogenesis of EAE and experimental allergic

uveitis (EAU). The predominant pathogenic Th cell type is deter-

mined by the methods used to induce disease, especially by

the use of adjuvants such as CFA that contain various TLR

ligands. Th17 cell-mediated disease was characterized by

neutrophil-rich infiltrates. In contrast, Th1 cell disease had

predominant macrophage infiltrates, which is more characteristic

of MS, RA, and many human autoimmune diseases. Thus, a more

balanced role for Th1 cells and IFN-g in autoimmune diseases is

emerging, with a mixed picture in which Th1 and Th17 cells can

coexist and contribute to pathology. This mixed picture is consis-

tent with lineage plasticity and coexpression of IFN-g and IL-17

by certain Th cells as discussed above and is supported by

data showing coexpression of IFN-g and IL-17 in various models

and diseases, including RA, systemic lupus erythematosus (SLE),

EAE, Crohn’s disease, and psoriasis. One recent study shows

that IFN-g actually contributes to induction of Th17 cell migration

and differentiation in the context of psoriasis, suggesting that

IFN-g may play a positive role in Th17 cell responses (Kryczek

et al., 2008). Overall, a large body of work highlights the complex

interplay between Th1 cells-IFN-g and Th17 cells in vivo and

suggests that IFN-g could differentially regulate Th17 cell

responses under different disease conditions.

A pathogenic role of Th1 cells and IFN-g in autoimmune

diseases raises the question of mechanisms by which IFN-g

contributes to pathogenesis. Given the above discussion, a

good candidate mechanism is IFN-g-mediated activation of

macrophages and other cell types at sites of inflammation, and

thus augmentation of the effector inflammatory component of

autoimmune diseases. In this scenario, the activating and

priming functions of IFN-g that lead to increased inflammatory

cytokine production and abrogate homeostatic mechanisms

contribute to disease pathology. Indeed, we and others have

provided evidence supporting IFN-g-mediated priming of

macrophages in human RA and mouse models of lupus nephritis

(Hu et al., 2002; Wang et al., 2008). In support of a role for IFN-g

in augmenting inflammation in autoimmune diseases, local

administration or tissue-specific transgene-mediated expres-

sion of IFN-g at inflammatory sites exacerbates disease in

arthritis and autoimmune diabetes models. Additional support

for a role for IFN-g in the effector phase of autoimmune disease

is provided by genetic evidence showing that deletion of the Ifng

gene ameliorates nephritis in the MRL/lpr model of SLE in which

nephritis is dependent on pathogenic macrophages (Baccala

et al., 2005). Importantly, autoimmunity did not appear to be

diminished in IFN-g-deficient animals, supporting the idea that

IFN-g can boost inflammation and tissue destruction in the

kidney independently of the autoimmune process. However,

there is also evidence that IFN-g can suppress the inflammatory

effector phase of autoimmunity. The clearest example may be

the increased severity of arthritis in IFN-g-deficient mice in the

K/BxN model that is induced by passive transfer of autoanti-

bodies and does not depend on acquired immunity (Wu et al.,

2007). Conversely, systemic administration of exogenous IFN-g

suppressed K/BxN arthritis. The mechanism by which IFN-g

suppresses K/BxN arthritis is inhibition of neutrophil infiltration

of joints, although it is possible that direct attenuation of tissue

destruction and osteoclastogenesis could also play a role.

The complex role of IFN-g in autoimmune diseases has impor-

tant therapeutic implications. A detailed understanding of key

pathogenic processes will be required to determine whether

blocking endogenous IFN-g or administering exogenous IFN-g

may be efficacious, and at which point in the disease process,

given that IFN-g may ameliorate or exacerbate disease under

different circumstances. It will be equally important to under-

stand the interplay between Th1 and Th17 cell responses in

specific autoimmune diseases. Blockade of solely IFN-g or

Th17 cell cytokines may result only in partial therapeutic efficacy

and a shift to a different pathology. In diseases in which Th1 and

Th17 cells work together, blocking both may be required for

effective therapy. Indeed, the striking beneficial effects of anti-

bodies against IL-12 p40 in diseases such as Crohn’s disease

and psoriasis may be explained by attenuation of both Th1 and

Th17 cell responses (Ghosh et al., 2006; Nestle et al., 2009). It

will be interesting to see the effects of IL-12 p40 blockade in

autoimmune diseases such as MS and RA.

ACKNOWLEDGMENTS

X.H. is supported by a Within Our Reach rheumatoid arthritis research grantfrom the American College of Rheumatology, and L.B.I. is supported by grantsfrom the National Institutes of Health.

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