Interferons: cell signalling, immune modulation, antiviral responses and virus countermeasures

24 Pages·2000·0.58 MB·English

JournalofGeneralVirology(2000),81,2341–2364. PrintedinGreatBritain
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Interferons: cell signalling, immune modulation, antiviral
responses and virus countermeasures
S. Goodbourn,1 L. Didcock2 and R. E. Randall2
1DepartmentofBiochemistryandImmunology,StGeorge’sHospitalMedicalSchool,UniversityofLondon,LondonSW170RE,UK
2BiomolecularSciencesBuilding,NorthHaugh,UniversityofStAndrews,FifeKY169TS,UK
Introduction Type I IFN (IFN-a}b) and type II IFN (IFN-c) share no
obviousstructuralhomology.However,functionalsimilarities
Toestablishinfectionsinvivo,virusesmustreplicateinthe
exist due to a broad overlap in the types of genes that they
faceofpowerfulimmunedefencemechanismsincludingthose
induce(reviewedinStarketal.,1998;summarizedinFig.1).It
induced by interferons (IFNs). The effectiveness of the IFN
is clear that IFNs can induce transcription of a significant
response has led to many viruses developing specific
number of genes. In addition to the well-characterized gene
mechanismsthatantagonizetheproductionoractionsofIFNs.
products described below, large-scale screening using oligo-
Indeed, in order to replicate efficiently in vivo, it seems likely
nucleotide arrays has identified several novel human IFN-
thatallvirusesmust,atleasttoadegree,havesomemeansof induciblegenesthatareinducedbyeitherIFN-a}borIFN-cor
circumventing the IFN response either by limiting IFN
both (Der et al., 1998). The importance of IFN in mediating
production or by blocking IFN actions. However, virus
responsestovirusinfectionsisestablishedbythefactthatmice
countermeasures to the IFN response are rarely absolute and lackingIFN-a}b(Mulleretal.,1994;Fietteetal.,1995;Hwang
the IFN response, by limiting virus spread, buys time for the
et al., 1995; Rousseau et al., 1995; Steinhoff et al., 1995; van
generation of an acquired immune response to the invading
denBroek et al., 1995a,b; Garcia-Sastre et al., 1998; Mrkic et
virus.Nevertheless,thespeedandefficiencybywhichagiven
al., 1998; Yeow et al., 1998; Cousens et al., 1999; Grieder &
virus circumvents the IFN response may be critical deter-
Vogel, 1999; Grob et al., 1999; Johnson & Roehrig,
minants in its host range and pathogenicity. In part A of this
1999;Nunez,1999)orIFN-c(Huangetal.,1993;Mulleretal.,
article,wereviewhowvirusinfectionsleadtotheproduction
1994; Fiette et al., 1995; van den Broek et al., 1995a,b;
ofIFNs(section1),howIFNsinducethetranscriptionoftheir
Bovolentaetal.,1999;Cantinetal.,1999;Dormanetal.,1999;
targetgenes(section2)andhowthesetargetgenesexerttheir
Grobetal.,1999;Nunez,1999;Tayetal.,1999)receptorsare
antiviral effects (section 3). Part B of this article reviews the
unable to mount efficient responses to a large number of
strategiesusedbyvirusestoinhibitIFNproduction(section4),
viruses.Importantly,thereareoftendifferencesintherequire-
IFN signalling (section 5) and}or specific antiviral functions
ments for the two types of IFN in resolving specific virus
(section 6).
infections, and the systems are non-redundant in many cases.
The IFNs are a large family of multifunctional secreted
BothtypesofIFNstimulatean‘antiviralstate’intargetcells,
proteins involved in antiviral defence, cell growth regulation
wherebythereplicationofvirusisblockedorimpaireddueto
and immune activation. The IFNs may be classified into two
the synthesis of a number of enzymes that interfere with
distincttypes.TypeIIFNsareproducedindirectresponseto
cellular and virus processes. Both types of IFN can also slow
virus infection and consist of the products of the IFN-a
the growth of target cells or make them more susceptible to
multigene family, which are predominantly synthesized by
apoptosis,therebylimitingtheextentofvirusspread.Finally,
leukocytes, and the product of the IFN-b gene, which is
both types of IFN have profound immunomodulatory effects
synthesizedbymostcelltypesbutparticularlybyfibroblasts.
and stimulate the adaptive response. However, whilst both
Type II IFN consists of the product of the IFN-c gene and, IFN-a}bandIFN-cinfluencethepropertiesofimmuneeffector
rather than being induced directly by virus infection, is
cells,theyshowsignificantdifferences,anditistheseextended
synthesizedinresponsetotherecognitionofinfectedcellsby
cytokine functions that probably account for the different
activatedTlymphocytesandnaturalkiller(NK)cells(reviewed
spectrums of antiviral activities of the two types of IFN.
in Vilcek & Sen, 1996).
A. Production and actions of IFNs
Authorsforcorrespondence:SteveGoodbourn(fax›44208725 1. VirusinductionofIFNgenes
2992;e-mails.goodbourn!sghms.ac.uk)andRickRandall(fax›44 The induction of IFN-b expression by virus infection of
1334462595;e-mailrer!st-andrews.ac.uk) fibroblastoidcellshasbeenthesubjectofintensiveresearch.It
CDEB
0001-7157#2000SGM
SS..GGooooddbboouurrnn,,LL..DDiiddccoocckkaannddRR..EE..RRaannddaallll
Fig.1.Thebiologicalpropertiesofa/b
andcIFNs.IFNsa/bandcbindto
specificsurfacereceptorsonprimary
targetcellsandinducethetranscription
ofavarietyofgenesthatmountan
antiviralresponse.Itischaracteristicof
thesegeneproductsthattheyoften
dependuponviraldsRNAasaco-factor
inordertoensurethattheyareonly
activeunderconditionsofinfection.Thus,
PKRand2«–5«oligoadenylatesynthetase
(OAS)aresynthesizedasinactive
precursors(PKRiandOASi,respectively)
andareactivatedbydsRNA(PKRaand
OASa,respectively).Onceactivated,
thesegeneproductsshutdown
translation.IFNscanalsoinducethe
synthesisofgeneproductsthatarrestthe
cellcycle(e.g.p21,aninhibitorofG /S
1
phase-specificcyclin-dependentkinases),
thusblockingvirusreplication,orinduce
apro-apoptoticstate(e.g.procaspases).
Finally,IFNscaninducethesynthesisof
proteinsthatareinvolvedinthe
processingandpresentationofvirus
proteinstoCD8+cytotoxicTlymphocytes
(CTLs)(e.g.MHCclassIproteins,
componentsoftheproteasomeand
peptidetransportermolecules).Both
typesofIFNalsohaveprofound
immunomodulatoryeffectsthatdiffer
betweentypes,andthesearediscussed
inthetext.
is generally assumed that the inducer is intracellular double- (Kumar et al., 1994; Offermann et al., 1995), although the
strandedRNA(dsRNA),providedbytheviralgenomeitselfor biological role for this is unclear.
formedasaresultofreplicationorconvergenttranscriptionof NF-jBbindstotheIFN-bpromoteraspartofamultiprotein
viral genomes (reviewed in Jacobs & Langland, 1996). The transcription-promoting complex called the ‘enhanceasome’
induction of IFN-b occurs primarily at the level of trans- (reviewed in Thanos, 1996), which also contains HMG-I}Y,
criptionalinitiation(seeFig.2).Thekeyinductioneventisthe ATF-2 homodimers or ATF-2}c-Jun heterodimers (Du et al.,
redistribution from the cytoplasm to the nucleus of the 1993) and a factor that binds to positive regulatory domain I
transcriptionfactorNF-jB(Lenardoetal.,1989;Visvanathan& (PRDI).Althoughthelatterwouldappeartobeamemberof
Goodbourn, 1989). NF-jB plays a role in the transcriptional the interferon regulatory factor (IRF) family, its identity
inductionofmanyimmunomodulatorygenes,includingother remains the subject of debate, having been suggested to be
cytokines,MHCclassIandcelladhesionmolecules(reviewed IRF-1(Miyamotoetal.,1988;Fujitaetal.,1989a;Watanabeet
inBaldwin,1996).NF-jBisnormallyheldinaquiescentstate al., 1991; Reis et al., 1992; Matsuyama et al., 1993), ISGF3
in the cytoplasm by association with an inhibitor molecule (Yoneyamaetal.,1996),IRF-3(Satoetal.,1998b;Schaferetal.,
called IjB. Upon receipt of a wide range of stress signals 1998; Weaver et al., 1998; Yoneyama et al., 1998) or a
[for example lipopolysaccharide, tumour necrosis factor combination of IRF-3 and IRF-7 (Wathelet et al., 1998). Since
(TNF), interleukin (IL)-1 and viral dsRNA], IjB becomes many of the IRF proteins bind both PRD I and the closely
phosphorylatedbyaspecificmulticomponent IjBkinase and, related IFN-stimulated response element (ISRE; an element
in turn, the phosphorylated IjB becomes ubiquitinated by an that is found in genes that are transcriptionally responsive to
E3 ubiquitin ligase. The ubiquitinated IjB is itself a target for type I IFN–see section 2), there may be some functional
degradation by proteasomes and, once the inhibitory IjB is overlap in the properties of these proteins. One consequence
destroyed,theassociatedNF-jBisfreedfromrestraintandcan of this overlap may be to ensure that virus infections cannot
enterthenucleusandactivatetranscription(reviewedinIsrael, blockIFN-binductioncompletelybyinhibitinganysingleIRF
2000). Exposure to dsRNA activates NF-jB via the dsRNA- (see section 4).
dependentproteinkinaseR(PKR)(Maranetal.,1994;Yanget IFN-acanalsobeinducedbyvirusinfectioninfibroblastoid
al., 1995; see section 3), which activates the IKKb subunit of cells, and the promoters of several IFN-a genes have been
the multicomponent IjB kinase (Chu et al., 1999; Zamanian- studiedindetail(reviewedinPitha&Au,1995).UnlikeIFN-b,
Daryoushetal.,2000).PKRcanalsophosphorylateIjBdirectly the IFN-a promoters do not have an NF-jB site, but contain
CDEC
RReevviieeww::VViirruussiinntteerraaccttiioonnsswwiitthhtthheeIIFFNNssyysstteemm
Fig.2.TranscriptionalinductionoftheIFN-bgene.VirusreplicationgivesrisetodsRNA,whichisabletoactivatePKRand
perhapsadditionalcellularkinases.PKRinturnactivatestheIjBkinaseandindirectlyleadstotheactivationofthe
immunomodulatorytranscriptionfactorNF-jB.TogetherwithATF-2andamember(s)oftheIRFfamily,NF-jBassembleson
theIFN-bpromoterwiththehelpofseveralcopiesoftheaccessoryfactorHMG-I/Ytoformamultifactorialcomplexcalledthe
‘enhanceasome’.Componentsoftheenhanceasomemakecontactswithfactorsthatarepartofthebasaltranscriptional
machineryand,bystabilizinginteractionswiththismachineryandcausingalocal‘remodelling’ofthechromatin,recruitRNA
polymeraseIItothepromotertobringabouttranscriptionoftheIFN-bgene.Seetextfordetails.
elements that are related to the PRD I- and ATF-2-binding promoter is under the control of two distinct regulatory
sites,aswellasdistinctelements.IFN-agenesarenotableto elements (proximal and distal; Aune et al., 1997). In contrast,
beinducedinembryonicfibroblastsderivedfrommicelacking onlythedistalelementisactivatedinCD8+cells,leadingtoa
both copies of the IFN-b gene, implying that fibroblasts significantlyweakerresponsethanthatseeninCD4+cells.The
depend upon IFN receptor activation by IFN-b for IFN-a proximal element is activated by complexes containing c-Jun
production (Erlandsson et al., 1998). It is thought that IFN-b andATF-2,whilstthedistalelementisactivatedbyGATA-3
works by inducing the synthesis of IRF-7, which, following andATF-1(Penixetal.,1996;Zhangetal.,1998a).Thesignal
activation by virus infection, leads to stimulation of IFN-a transduction mechanisms involved in activating transcription
transcription (Au et al., 1998; Marie et al., 1998; Sato et al., oftheIFN-cgenearepoorlycharacterized,butinvolvethep38
1998a;Yeowetal.,2000).IFN-aisalsoinducedinleukocytes and JNK2 mitogen-activated protein kinase (MAP kinase)
by virus infection. The induction mechanism is poorly pathways(Rinconetal.,1998;Yangetal.,1998;Luetal.,1999).
characterized in the case of these cells, but is clearly distinct IFN-c production in response to antigen stimulation is
from induction in fibroblasts, since IFN-b production is not enhancedmarkedlybyIL-12orIL-18,cytokinesproducedby
required (Erlandsson et al., 1998). activatedantigen-presentingcells(reviewedinOkamuraetal.,
IFN-c is produced by Th1 CD4+ helper T cells and by 1998). Although neither IL-12 nor IL-18 alone can stimulate
nearly all CD8+ cells, as a result of transcriptional activation IFN-cproductionsignificantlyinunstimulatedTcells,together
induced by exposure to antigen-presenting cells (reviewed in thesecytokinescanstimulateIFN-cproductioninanantigen-
Young, 1996). In naive and memory CD4+ T cells, the IFN-c independent manner (Tominaga et al., 2000). The molecular
CDED
SS..GGooooddbboouurrnn,,LL..DDiiddccoocckkaannddRR..EE..RRaannddaallll
Fig.3.SignallingpathwaysactivatedbyIFN-a/bandIFN-c.ThebiologicalactivitiesofIFNsareinitiatedbybindingtotheir
cognatereceptors.Thisleadstotheactivationofreceptor-associatedtyrosinekinases,asdiscussedinthetext.Thesekinases
phosphorylatemembersoftheSTATfamilyoftranscriptionfactors,whichcanenterthenucleusand,eitherontheirownorin
combinationwithp48,bindtothepromotersoftargetgenesandbringaboutgene-specifictranscriptionalactivation.Seetext
fordetails.
basisofthisisunknown,butmayinvolveactivationofSTAT4 genes contain a unique element called the gamma activation
byIL-12and NF-jBby IL-18, andit mayalsoinvolvean up- sequence (GAS), which contains the consensus sequence
regulation of the IL-18 receptor by IL-12 (Yoshimoto et al., TTNCNNNAA.
1998). IFN-c is also produced by activated NK cells in an TheIFN-a}breceptoriscomposedoftwomajorsubunits,
antigen-independentmannerandthisisalsodependentonIL- IFNAR1 and IFNAR2 (reviewed in Mogensen et al., 1999).
12productionbyantigen-presentingcellsandisstimulatedby Priortostimulation,thecytoplasmicdomainsofIFNAR1and
IL-18 (Singh et al., 2000). IFNAR2 are respectively associated with the ‘Janus« tyrosine
kinasesTyk2(Colamonicietal.,1994)andJak1(Novicketal.,
1994). IFNAR2 is also associated with the ‘signal transducer
2. SignaltransductioninresponsetoIFNs and activator of transcription« (STAT) molecule STAT2 (Li et
ThebiologicalactivitiesofIFNsareinitiatedbythebinding al.,1997).OnIFN-a}bbinding,IFNAR1andIFNAR2associate,
ofIFN-a}bandIFN-ctotheircognatereceptorsonthesurface facilitating the transphosphorylation and activation of Tyk2
of cells, which results in the activation of distinct but related and Jak1 (Novick et al., 1994). Tyk2 then phosphorylates the
signalling pathways, known as the Jak}STAT pathways tyrosineatposition466(Tyr%’’)onIFNAR1(Colamonicietal.,
(reviewed in Stark et al., 1998; summarized in Fig. 3). The 1994), creating a new docking site for STAT2 through the
ultimate outcome of this signalling is the activation of latter’s SH2 domain (Yan et al., 1996). STAT2 is then
’*!
transcription of target genes that are normally expressed at phosphorylated by Tyk2 at Tyr and serves as a platform
lowlevelsorarequiescent.Theupstreamregulatorysequences (Leung et al., 1995; Qureshi et al., 1996) for the recruitment
of most IFN-a}b-inducible genes contain a variation of the of STAT1 (also through its SH2 domain), which is subse-
consensus sequence [GAAAN(N)GAAA] called the ISRE, quently phosphorylated on Tyr(!" (Shuai et al., 1993). The
whilst the upstream regulatory regions of IFN-c-inducible phosphorylated STAT1}STAT2 heterodimers thus formed
CDEE
RReevviieeww::VViirruussiinntteerraaccttiioonnsswwiitthhtthheeIIFFNNssyysstteemm
dissociatefromthereceptorandaretranslocatedtothenucleus phosphorylationistofacilitateinteractionofSTAT1withthe
through an unknown mechanism, where they associate with basal transcriptional machinery. Recent studies have revealed
the DNA-binding protein p48 (Veals et al., 1992) to form a importantconnectionsbetweenSTAT1andtheCREB-binding
heterotrimericcomplexcalledISGF3,whichbindstheISREof protein (CBP)}p300 transcription factors. The CBP}p300
IFN-a}b-responsivegenes.p48isamemberoftheIRFfamily familyoftranscriptionfactorspotentiatetheactivityofseveral
and is sometimes referred to as IRF-9; it should be stressed groups of transcription factors (reviewed in Janknecht &
that,liketheIFN-bpromoterelementPRDI,theISREsequence Hunter,1996).BoththeC-andN-terminaldomainsofSTAT1
canalsobeboundbyothermembersoftheIRFfamily,notably have been shown to bind CBP}p300 (Zhang et al., 1996).
IRF-1 and IRF-2, and this may have profound biological STAT1 also interacts with the chromatin-associated protein
consequences (see below). MCM5inaSer(#(-dependentmanner(Zhangetal.,1998b)and
IFN-c receptors are composed of at least two major with Nmi, a protein that acts to enhance the association
polypeptides, IFNGR1 and IFNGR2 (reviewed in Bach et al., between STAT1 and CBP}p300 (Zhu et al., 1999). Although
1997). In unstimulated cells, IFNGR1 and IFNGR2 do not STAT2 does not contain a MAP kinase consensus site and is
pre-associatestronglywithoneanother(Bachetal.,1996),but notknowntobeserine-phosphorylatedinresponsetoIFN,it
their intracellular domains specifically associate with the alsobindsCBP}p300andfacilitatesinteractionwiththebasal
Janus kinases Jak1 and Jak2, respectively (Kotenko et al., transcriptional machinery (Bhattacharya et al., 1996).
1995; Sakatsume et al., 1995; Bach et al., 1996; Kaplan et al., A second form of STAT1 (STAT1b) can be derived by
1996). Binding of the dimeric IFN-c to the receptor triggers differentialsplicing.STAT1bcontainsthetyrosineatposition
receptordimerization,whichbringsJak1andJak2moleculeson 701andisrecruitedtothereceptorcomplex,becomestyrosine-
adjacent receptor molecules into close proximity (Greenlund phosphorylated and binds DNA. However, STAT1b differs
etal.,1994,1995;Igarashietal.,1994;Bachetal.,1996);Jak2 fromthepredominantformofSTAT1(STAT1a)bylackingthe
(#(
is thus activated and in turn activates Jak1 by trans- C-terminal 38 amino acids that include Ser and, thus, it
phosphorylation(Briscoeetal.,1996).TheactivatedJaksthen cannotactivatetranscription(Schindleretal.,1992;Shuaietal.,
phosphorylate a tyrosine-containing sequence near the C 1993). The function of STAT1b is not clear. Although it can
%%! %%%
terminus of IFNGR1 (Tyr –Tyr ), thereby forming paired become incorporated into ISGF3 complexes that retain their
binding sites for STAT1 that interact through their SH2 transcriptional activation potential as a result of STAT2
domains(Greenlundetal.,1994,1995;Igarashietal.,1994)and function (Muller et al., 1993), the consequences of a potential
arephosphorylatedatTyr(!",neartheCterminus(Shuaietal., STAT1a}STAT1bheterodimerhavenotbeenestablished,but
1993, 1994; Greenlund et al., 1994; Heim et al., 1995). The these might well down-regulate transcription.
phosphorylated STAT1 proteins dissociate from the receptor Recently, several other proteins have been identified that
and form a homodimer, through SH2 domain–tyrosine may also be required for IFN signalling. For example, the
phosphate recognition, which translocates to the nucleus tyrosine phosphatase SHP-2, which pre-associates with
through a poorly characterized mechanism (Sekimoto et al., IFNAR1, is phosphorylated in response to IFN-a}b and, in
1996). Active STAT1 homodimers, also called gamma- transfectionexperiments,adominant-negativeformofSHP-2
activatedfactor(GAF),bindtospecificGASelementsofIFN- inhibits the IFN-a}b-induced expression of a reporter gene
c-induciblegenes(reviewedbyStarketal.,1998)andstimulate (David et al., 1996). IFN-a}b treatment also induces the
transcription.IFN-a}bcanalsoinducetheformationofSTAT1 phosphorylationandactivationofcytosolicphospholipaseA2
homodimers, albeit less efficiently than IFN-c (Haque & (CPLA2),aneventthatrequiresJak1andthep38MAPkinase
Williams, 1994), although the mechanism whereby STAT1 (Goh et al., 1999). The demonstration that CPLA2 inhibitors
homodimers are activated by IFN-a}b remains obscure. canblocktheexpressionofISRE-containinggenesinducedby
The transactivation function of STAT1 depends upon IFN-a}b implies that CPLA2 also plays a role in the trans-
phosphorylation of Ser(#( (Wen et al., 1995) by akinase with activation of ISRE-containing genes (Hannigan & Williams,
MAP-like specificity. The identity of this kinase remains 1991; Flati et al., 1996).
controversial,althoughitmaydifferbetweencelltypes.Thus, As discussed above, other members of the IRF family can
(#(
p38 kinase has been shown to be important for Ser bind ISRE sequences and our understanding of IFN-mediated
phosphorylation in response to IFN-a}b and IFN-c in mouse signal transduction is complicated by the fact that some of
fibroblasts (Goh et al., 1999) but not in response to IFN-c in these IRF proteins are inducible by IFNs. Thus, both IFN-a}b
macrophages (Kovarik et al., 1999). Furthermore, the pro- and IFN-c can induce IRF-1, which can then serve to sustain
tein tyrosine kinase Pyk2 has recently been shown to be a expression of genes that contain ISREs. Indeed, IRF-1-
(#(
critical mediator of the Jak-dependent activation of Ser dependent gene expression in response to IFNs has been
phosphorylation of STAT1 in IFN-c, but not IFN-a}b, observed in a number of cases (see for example Kimura
signalling (Takaoka et al., 1999). It has also recently been et al., 1994; Chatterjee-Kishore et al., 1998; Kano et al.,
shownthatPKRplaysaroleinSer(#(phosphorylation(Ramana 1999;Salkowskietal.,1999;Karlsenetal.,2000).Thiscangive
etal.,2000),butthisisunlikelytobedirect.TheroleofSer(#( rise to complex patterns of gene expression whereby, for
CDEF
SS..GGooooddbboouurrnn,,LL..DDiiddccoocckkaannddRR..EE..RRaannddaallll
example,IFN-ccaninducethesynthesisofgenesthatlackGAS containsalloftheconservedmotifsforproteinkinaseactivity
sitesviatheinductionofIRF-1(seeforexampleLechleitneret (Meursetal.,1990).PKRisnormallyinactive,butisactivated
al., 1998; Foss & Prydz, 1999; Piskurich et al., 1999). by binding to dsRNA or other polyanions (Meurs et al.,
In contrast to the mechanism of IFN signal transduction, 1990; Katze et al., 1991; George et al., 1996), whereupon it
little is known about the mechanism of signal attenuation. undergoes a conformational change that leads to the un-
SeveralIRFproteins,includingIRF-2(Haradaetal.,1989)and masking of the catalytic domain. The active form of PKR is
the IFN-consensus sequence-binding protein (ICSBP, also postulatedtobeadimer,withtwoPKRmoleculesbindingone
calledIRF-8;Nelsonetal.,1993),areknowntobindISREsand molecule of dsRNA; the juxtaposed PKR molecules trans-
negatively regulate expression, and may help to prevent phosphorylate each other on several serines and threonines.
expressionintheabsenceofIFNordown-regulatetheinduced PKRactivationisdecreasedwhenlargeamountsofdsRNAare
response. IFN-induced proteins play a major role in signal present,duetosaturationofdsRNA-bindingsitesandashiftin
attenuation, since protein synthesis inhibitors prolong the the equilibrium towards monomers. There are no sequence
transcription of IFN-induced genes (Friedman et al., 1984; requirements for the dsRNA, although some RNAs are more
Larner et al., 1986). One group of proteins with the po- potent activators than others. However, there are size
tential to fulfil this role is the SOCS}JAB}SSI family, which requirements, with at least 50 base pairs of duplex being
areinduciblebyIFN-candseveralothercytokinesandbindto necessary for activation (reviewed in Robertson & Mathews,
and inhibit activated Jaks, leading to signal down-regulation 1996).
(Endoetal.,1997;Nakaetal.,1997;Starretal.,1997;Starr& Activated PKR has a number of important cell-regulatory
Hilton, 1999). activities. Firstly, it phosphorylates the a subunit of the
Activation by STAT1 is usually transient, as a result of eukaryotic translation initiation factor eIF2 and prevents the
dephosphorylation by a tyrosine phosphatase (Igarashi et al., recyclingofinitiation factors(Meurset al.,1992;reviewed in
1993; Haque et al., 1995). However, it is not known whether Clemens & Elia, 1997). In the initial step of translation, the
thephosphataseactsonphosphorylatedSTATsinthenucleus initiator Met-tRNA is recruited to the 40S ribosomal subunit
or on phosphorylatedJaks or receptor subunitsat the plasma via an interaction with GTP-bound eIF2 (which consists of
membrane. The tyrosine phosphatase SHP-1 has been shown three subunits, a,b and c). This complex then interacts with
to be associated reversibly with IFNAR-1 after IFN-a stimu- mRNA,otherinitiationfactorsandthelargeribosomalsubunit
lation(Davidetal.,1995)andJak1andSTAT1phosphorylation toformapre-initiationcomplex,withsubsequenthydrolysisof
is increased significantly in macrophages isolated from mice the GTP molecule bound to eIF2 and release of GDP-bound
that lack SHP-1 activity compared with normal control eIF2. In order to participate in another round of translational
macrophages(Haque&Williams,1998),suggestingthatSHP- initiation,theGDPboundtoeIF2mustbeexchangedforGTP,
1 may play a role in signal attenuation. In addition to down- a reaction that is catalysed by the guanine exchange factor,
regulationbydephosphorylation,STAT1isturnedoverby a eIF2B.PhosphorylatedeIF2ainteractsstronglywitheIF2Band
mechanism involving proteasome-mediated degradation, but traps it such that it cannot mediate the recycling of eIF2
there is no evidence that this process is important in the (Ramaiahetal.,1994;reviewedbyClemens&Elia,1997).Since
regulation of STAT1 function (Kim & Maniatis, 1996). eIF2B is present in limiting amounts, translation is inhibited.
PKR also plays a role in mediating signal transduction in
response to dsRNA and other ligands (reviewed in Williams,
3. Theantiviralresponse
1999). For example, the transcription factor NF-jB, which is
The best-characterized IFN-inducible components of the
essentialformediatinginductionoftheIFN-bgene,isactivated
antiviral response are PKR and the 2«–5« oligoadenylate
by PKR in response to dsRNA (see section 1). PKR has also
synthetases, although it is clear that other factors may be
been proposed to influence the activity of the transcription
involved, especially molecules that regulate the cell cycle or
factorsSTAT1(Wongetal.,1997;Ramanaetal.,2000),IRF-1
cell death and thereby limit the extent of virus replication. In
(Kumar et al., 1997) and p53 (Cuddihy et al., 1999a,b),
manycases,IFN-inducibleenzymesareinactiveuntilexposed
although the details of the activation events remain to be
to virus infection, thus ensuring that uninfected cells do not
clarified.TheelevatedlevelsofPKRthatwouldbefoundina
sufferunduetrauma.Itisthoughtthatthevirusco-factorthat
cell exposed to IFN would cause an enhancement of these
activatestheseIFN-inducibleenzymesisdsRNA(reviewedin
signaltransductionevents,whichmayhelptoacceleratevirus
Jacobs & Langland, 1996).
clearance. For example, enhanced activation of NF-jB ac-
(i) dsRNA-dependentproteinkinaseR(PKR).TheIFN-inducible tivation would lead to increased cytokine, chemokine and
PKR is a serine}threonine kinase with multiple functions in MHC class I presentation.
control of transcription and translation (reviewed in Clemens PKR also aids in the clearance of virus infection by
& Elia, 1997). The PKR protein has two well-characterized mediatingapoptosis.IthasbeenshownthatdsRNA(andthus
domains, an N-terminal regulatory domain that contains the virus infection) can trigger apoptosis directly (Der et al.,
dsRNA-binding site and a C-terminal catalytic domain that 1997;King&Goodbourn,1998;Tanakaetal.,1998)andthere
CDEG
RReevviieeww::VViirruussiinntteerraaccttiioonnsswwiitthhtthheeIIFFNNssyysstteemm
is considerable evidence that this effect works through PKR (reviewedinSilverman&Cirino,1997)andantiviraleffectsof
(Takizawa et al., 1996; Der et al., 1997; reviewed in Jagus IFN-a are indeed impaired in RNase L−/− mice (Zhou et al.,
et al., 1999; Tan & Katze, 1999), although PKR-independent 1997).RNaseLmayalsoplayaroleinapoptosis,sinceRNase
mechanismsalsooperateforsomeviruses(Balachandranetal., L−/−miceshowdefectsinapoptosisinseveraltissues(Zhouet
2000). The downstream targets for PKR-mediated apoptosis al.,1997)whilstactivationofRNaseLinducesapoptosis(Diaz-
remain to be identified, but overexpression of PKR has been Guerra et al., 1997). Although the exact role of RNase L in
shown to induce apoptosis through a Bcl2- and caspase- apoptosis is not clear, it seems likely that the 2«–5« oligo-
dependentmechanism(Leeetal.,1997).Intriguingly,although adenylate synthetase}RNase L system may contribute to the
micewithatargettedknockoutofthedsRNA-bindingdomain antiviralactivityofIFNbyinducingapoptosisofinfectedcells
of PKR are sensitive to virus-induced apoptosis (Yang et al., (Zhou et al., 1997; Castelli et al., 1998a,b).
1995), mice with a targetted knockout of the PKR catalytic
domain are not (Abraham et al., 1999). (iii) Alternative antiviral pathways. The IFN-inducible Mx
PKRalsoplaysaroleinmediatingtheapoptoticeffectsof proteinsarehighlyconserved,largeGTPaseswithhomology
dsRNA in an indirect manner. In this case, effects on protein to dynamin and have been found in all vertebrate species
synthesisareimportant(Srivastavaetal.,1998;Giletal.,1999), examinedsofar,includingmammals,birdsandfish(reviewed
asareeffectsonthetranscriptionfactorNF-jB(Giletal.,1999). in Staeheli et al., 1993; Arnheiter et al., 1995). Mx proteins
Exposure of cells to dsRNA also enhances apoptosis by interfere with virus replication, probably by inhibiting the
inducing the synthesis of Fas (Takizawa et al., 1995; trafficking or activity of virus polymerases (Stranden et al.,
Balachandran et al., 1998; Fujimoto et al., 1998) and Fas 1993),therebyimpairingthegrowthofawiderangeofRNA
receptor(Fujimotoetal.,1998)inamannerthatdependsupon virusesatthelevelofvirustranscriptionandatotherstepsin
PKR(Balachandranetal.,1998).Finally,theapoptoticeffectsof theviruslife-cycle.ThemurinenuclearproteinMx1hasbeen
TNF on promonocytic U937 cells require p53 to ensure a shown to suppress the growth of members of the Ortho-
response to activated PKR (Yeung et al., 1996). myxoviridae(Staehelietal.,1986,1988;Halleretal.,1995).The
Although there is abundant evidence that PKR plays a human cytoplasmic protein MxA inhibits the growth of
major role in regulating virus infection, PKR is not sufficient members of several RNA families, including the Ortho-
to mediate the full antiviral response. Thus, mice with myxoviridae (Pavlovic et al., 1990, 1992; Frese et al., 1995,
homozygous disruptions of the PKR gene (Yang et al., 1997),Paramyxoviridae(Schneider-Schauliesetal.,1994;Zhao
1995; Abraham et al., 1999) still show resistance to virus etal.,1996),Rhabdoviridae(Pavlovicetal.,1990),Bunyaviridae
infection, although the wild-type but not the PKR-deficient (Freseetal.,1996;Kanervaetal.,1996)andTogaviridae(Landis
animalsareprotectedtosomeextentbyinjectionofdsRNAat etal.,1998).MutantformsofMxproteinslackingtheabilityto
virus doses that are normally lethal (Yang et al., 1995). bindorhydrolyseGTPfailtosuppressvirusreplication.Hefti
et al. (1999) have analysed the behaviour of transgenic mice
(ii) The 2«–5« oligoadenylate synthetase system. 2«–5« oligo- that constitutively express the human MxA gene in a mouse
adenylatesynthetasesareagroupofenzymesthatareinduced backgroundlackingtheIFN-a}breceptorandhaveshownthat
by IFNs in mammalian cells and catalyse the synthesis from the MxA protein protects mice against Thogoto virus, La
ATPof oligomers (threetofiveunits)of adenosine linkedby Crosse virus and Semliki Forest virus.
phosphodiesterbondsintheunusualconformationof2«to5« Recent studies involving the generation of mice that are
(2«5«A;Kerr&Brown,1978).The2«5«Amoleculesbindwith triply deficient in RNase L, PKR and Mx1 indicate that there
high affinity to endoribonuclease L (RNase L) and induce its areadditionalantiviraleffectsofIFNs(Zhouetal.,1999).Other
activation via dimerization. Activated RNase L catalyses the factors that clearly play a role in the IFN-induced antiviral
cleavage of single-stranded RNA including mRNA, thereby response are caspases (see below) and the dsRNA-dependent
leading to inhibition of protein synthesis (reviewed in adenosine deaminase (ADAR). The enzyme ADAR recog-
Silverman, 1997). It has recently been demonstrated that nizes dsRNA as a substrate and unwinds it as a result of
RNase L also cleaves 28S ribosomal RNA in a site-specific systematically replacing adenosines with inosine (Bass et al.,
manner, leading to ribosomal inactivation and thus trans- 1989;Polson&Bass,1994;O’Connelletal.,1995;Patterson
lationalinhibition(Iordanovetal.,2000).Since2«5«Aishighly et al., 1995). Since many viral RNAs go through a dsRNA-
labile,theactivationofRNaseLdependsuponlocallyactivated based replicative intermediate, this has the effect of being
2«–5«oligoadenylatesynthetasewithinthecell,thusensuring mutagenic, and there are several reports of genomic substi-
thatvirustranscriptsaredestroyedpreferentiallyovercellular tutionsconsistentwiththisactivity(Bassetal.,1989;Cattaneo,
mRNAs, since they are in the vicinity of the activator (viral 1994;Casey&Gerin,1995;Hajjar&Linial,1995;Horikami&
dsRNA; Nilsen & Baglioni, 1979). Moyer, 1995; Polson et al., 1996). It has also been suggested
The2«–5«oligoadenylatesynthetase}RNaseLsystemhas that an inosine-specific ribonucleasecouldact in concert with
beensuggestedtoplayaroleintheantiviraleffectsofIFN-a}b ADAR to destroy modified viral RNAs (Scadden & Smith,
againstvacciniavirus,reovirusandencephalomyocarditisvirus 1997).
CDEH
SS..GGooooddbboouurrnn,,LL..DDiiddccoocckkaannddRR..EE..RRaannddaallll
(iv) Antiproliferative activities of IFNs. IFNs can inhibit cell complex between E2F and p202 is unable to bind DNA and
growth and thereby inhibit the replication of some viruses. hence there is a loss of stimulation of transcription of genes
However,thesensitivityofcellstotheantiproliferativeeffects importantfortheG –Stransition.Sincethep202proteinalso
"
of IFNs is very cell-type dependent. For example, growth of contains a transcriptional repression domain (Johnstone et al.,
the Daudi B cell line is arrested completely by as little as 1 1998), any recruitment to DNA would also shut down gene
unit}ml IFN-a}b, whereas many cell types are largely un- expression.Finally, IFNshavebeenshownrecently todown-
responsiveatanydosetested.Becauseofthepotentialclinical regulate directly the transcription of c-myc, an essential gene
importance of the cytostatic properties of IFN, the negative productthatisrequiredtodrivecellcycleprogression(Ramana
regulation of growth has been studied intensively and a et al., 2000).
numberof aspectsof thisprocess havebeendescribed.There
(v) Control of apoptosis. IFNs, like other cytokines, can have
is evidence to support a role for PKR and RNase L in the
either pro- or anti-apoptotic activities depending on various
antiproliferative functions of IFNs. The amount of PKR can
factorsincludingthestateofcelldifferentiation.Forexample,
vary according to the state of growth of mammalian cells in
IFN-c induces apoptosis of murine pre-B cells but inhibits
culture and this appears to correlate with the level of eIF2a
apoptosis of chronic lymphocytic leukaemia cells (Buschle et
phosphorylation(reviewedinJaramilloetal.,1995),suggesting
al.,1993;Grawunderetal.,1993;Rojasetal.,1996).However,
that, even in the absence of viral dsRNA, PKR can exhibit
whenacellisinfectedwithavirus,amajorfunctionofIFNis
residualactivity, presumablydue tothe presenceof a cellular
to ensure that the cell is triggered to undergo apoptosis
activator. Additionally, overexpression of PKR is growth
(Tanakaetal.,1998).IFNappearstodothisbyinducingapro-
suppressiveand}ortoxicininsect,mammalianandyeastcells
apoptotic state in uninfected cells (reviewed in Schindler,
(Koromilasetal.,1992;Chongetal.,1992;Deveretal.,1993),
1998). As discussed above, IFN-induction of PKR and the
an effect which can also be shown to be due to eIF2a
2«5«A system plays a major role in the apoptosis response.
phosphorylation.Overexpressionofthe40kDaformof2«–5«
However,IFNhasalsobeendemonstratedtoinducecaspase1
oligoadenylate synthetase has been shown to reduce growth
(Chin et al., 1997), caspase 3 (Subramaniam et al., 1998) and
rates of transfected cells (Chebath et al., 1987; Rysiecki et al.,
caspase 8 (Balachandran et al., 2000) and thus to enhance the
1989; Coccia et al., 1990) and expression of a dominant-
sensitivity of cells to virus-induced apoptosis. IFN-c has also
negativemutantofRNaseLinmurineSVT2cellsinhibitedthe
been shown to influence the sensitivity to apoptosis by
antiproliferativeeffectofIFNonthesecells(Hasseletal.,1993).
inducing both Fas and Fas ligand (Xu et al., 1998).
IFNscanalsoexertnegativeregulationofthecellcycleat
a more direct level. IFNs have been shown to up-regulate (vi) Immunomodulatory functions of IFNs. Nearly all phases of
specificallythelevelsofthecyclin-dependentkinaseinhibitor innateandadaptiveimmuneresponsesareaffectedprofoundly
p21 (Chin et al., 1996; Subramaniam & Johnson, 1997; byIFNs.AllIFNfamilymemberssharetheabilitytoenhance
Subramaniam et al., 1998), which plays a crucial role in theexpressionofMHCclassIproteinsandtherebytopromote
theprogressionfromG intoSphase(reviewedinHarperetal., CD8+ T cell responses (reviewed in Boehm et al., 1997). In
"
1993;Garteletal.,1996).Whenp21levelsareelevated,cyclin- contrast, only IFN-c is capable of inducing the expression of
dependent kinase activity is turned off and consequently the MHCclassIIproteins,thuspromotingCD4+Tcellresponses.
phosphorylationoftheretinoblastomageneproduct(pRb)and IFNsplayanimportantroleinantigenprocessingbyregulating
therelatedpocketproteinsissuppressed(Sangfeltetal.,1999). theexpressionofmanyproteinsinvolvedinthegenerationof
SincehypophosphorylatedpRbandtherelatedpocketproteins antigenic peptides to be displayed in association with MHC
interact strongly with the E2F family of transcription factors, class I proteins. IFN-c modifies the activity of proteasomes
there is a consequent increase in the pRB-}pocket protein- (reviewed in York & Rock, 1996) such that they enhance the
bound E2F complexes (Iwase et al., 1997; Kirch et al., generation of peptides that bind class I MHC proteins. In
1997;Furukawaetal.,1999).Thesignificanceofthisisthatfree unstimulated cells, the proteasome contains the three en-
E2F is required for the transcription of many genes that are zymatic subunits x, y and z. However, following IFN-c
neededfortransitionfromG toSphaseandthustheelevation treatment of cells, the transcription of the x, y and z genes is
"
of pRB-}pocket protein-bound E2F complexes results in a decreased and the transcription of three additional genes
block to the cell cycle. encoding enzymatic proteasome subunits LMP2, LMP7 and
Another major IFN-inducible activity that can act as a MECL1 is increased. This results in the formation of
potentrepressorofthecellcycleisthep202geneproductand proteasomes with different substrate specificities, thereby
related members of its ‘200 family’ (Kingsmore et al., altering the types of peptide produced and subsequently
1989;Lemboetal.,1995;Gutterman&Choubey,1999).The presented to the immune system. IFN-c also increases the
p202 product can bind both hypophosphorylated pRb expression of TAP1 and TAP2, which are involved in the
(Choubey & Lengyel, 1995) and members of the E2F transfer of peptides (generated by the proteasome) from the
transcription family (Choubey et al., 1996; Choubey & cytoplasm into the endoplasmic reticulum to bind nascent
Gutterman, 1997) as well as complexes containing both. The MHC class I proteins (Trowsdale et al., 1990; Epperson et al.,
CDEI
RReevviieeww::VViirruussiinntteerraaccttiioonnsswwiitthhtthheeIIFFNNssyysstteemm
1992).Thus,IFNsenhanceimmunogenicitybyincreasingthe al., 1999), whilst IFN-a}b appears to be able to promote the
repertoireandquantityofpeptidesdisplayedtoCD8+Tcells. survival of activated T cells directly (Marrack et al., 1999).
IFN-calsoplaysanimportantroleinregulatingthebalance
betweenTh1andTh2cells.Firstly,itincreasesthesynthesisof
B. Virus countermeasures to the IFN
IL-12 in antigen-presenting cells (Dighe et al., 1995; Flesch et
response
al., 1995; Murphy et al., 1995). IL-12 is the primary effector
that drives developing CD4+ T cells to become Th1 cells 4. InhibitionofIFNproduction
(Hsiehetal.,1993;Trinchieri,1995).Secondly,IFN-cprevents Viruses vary considerably in their ability to induce IFN.
the development of Th2 cells by inhibitingthe production of This may simply reflect the amounts of dsRNA produced
IL-4, which is required for Th2 cell formation (Gajewski & during their replication cycles (in general, DNA viruses
Fitch,1988;Szaboetal.,1995).IFN-calsoplaysanimportant producelessdsRNA than RNA viruses and are therefore less
role in macrophage activation (Adams & Hamilton, 1984; potentinducersofIFN;reviewedinJacobs&Langland,1996)
Buchmeier & Schreiber, 1985; Dalton et al., 1993; Huang or it may reflect the fact that many viruses produce dsRNA-
etal.,1993).Onceactivated,macrophagesuseavarietyofIFN- bindingproteinsaspartoftheirlife-cycle.Thesequestrationof
c-induced mechanisms to kill microbial targets. The most dsRNAcouldinhibittheinductionofIFN-a}bandmightalso
important of these mechanisms involve the production of act to minimize the dsRNA-dependent activation of antiviral
reactiveoxygenandreactivenitrogenintermediates.Reactive gene products like PKR, 2«–5« oligoadenylate synthetase and
oxygenintermediatesaregeneratedasproductsoftheenzyme ADAR,aswellasdsRNA-dependentapoptosis.Forexample,
NADPHoxidase,theassemblyofwhichisinducedbyIFN-c. thereovirusmajoroutercapsidproteinr3isadsRNA-binding
Reactive nitrogen intermediates, especially nitric oxide (NO), protein(Lloyd&Shatkin,1992;Yue&Shatkin,1997;reviewed
aregeneratedinmurinemacrophagesasaresultoftheIFN-c- in Jacobs & Langland, 1998), as is the rA protein of avian
dependent transcription of the gene encoding the inducible reovirus (Martinez-Costas et al., 2000). Reovirus strains vary
form of nitric oxide synthase (iNOS), which catalyses NO significantly in their ability to induce IFN-a}b (reviewed in
formation (MacMicking et al., 1997). Samuel,1998); although this has notyet been shownto be a
In addition to affecting humoral immunity indirectly by functionofvariationinther3protein,itisinterestingtonote
regulating the development of specific T helper cell subsets, that strain differences in IFN sensitivity have been linked to
IFNs can have direct effects on B cells by regulating differencesindsRNAaffinityofther3protein(Bergeronetal.,
developmentandproliferation,immunoglobulin(Ig)secretion 1998).The multifunctional NS1 proteinof influenza virus (Lu
and Ig heavy-chain switching. Since different Ig isotypes et al., 1995), the E3L protein of vaccinia virus (Chang et al.,
promote distinct effector functions in the host, IFNs can 1992) and products of the NSP3 gene of porcine rotaviruses
facilitateinteractionsbetweenthehumoralandcellulareffector (Langlandetal.,1994)alsobinddsRNAandanumberofother
limbs of the immune response and increase the host defence virusesthathavebeenreportedtoblockIFNproductionatthe
against certain bacteria and viruses by selectively enhancing transcriptional level may also do so by sequestering dsRNA
the production of certain Ig isotypes while inhibiting the [e.g. the core antigen of hepatitis B virus (HBV); Twu &
production of others (Snapper & Paul, 1987; Snapper et al., Schloemer, 1989; Whitten et al., 1991]. The sequestration of
1988, 1992). dsRNAbyviralproteinsmighthaveawiderroleinprotecting
A major immunomodulatory function of IFN-a}b is to the virus from antiviral mechanisms; dsRNA-activated PKR
enhance the cytotoxicity of NK cells (reviewed in Reiter, can activate NF-jB and induce the synthesis of immuno-
1993; Biron et al., 1999) by up-regulating the levels of modulatory genes in addition to IFN-a}b.
perforins (Mori et al., 1998; Kaser et al., 1999). IFN-a}b also SincetheactivationofNF-jBbyinfectionisakeytrigger
acts to stimulate the proliferation of NK cells to a limited toinducingIFN-a}btranscriptionandotherimmuneresponses,
degree,apparentlyviatheinductionofIL-15frommonocytes} it would perhaps not be surprising to find that many viruses
macrophages (Ogasawara et al., 1998; Fawaz et al., 1999; encoded inhibitors of NF-jB activation or function. Indeed,
Gosselin et al., 1999; Sprent et al., 1999). NK cells also Africanswinefevervirus(ASFV)encodesahomologueofIjB
synthesize and secrete IFN-c in response to a combination of thatinhibitstheactivityofNF-jB(Powelletal.,1996;Revilla
IL-12andIL-15,whicharereleasedfrominfectedmonocytes} etal.,1998).However,itiswellestablishedthatNF-jB,aswell
macrophages (Doherty et al., 1996; Fehniger et al., 1999). as inducing proinflammatory cytokines, also induces anti-
However,IFN-a}bblockstheproductionofIL-12byinfected apoptoticgenes(Liuetal.,1996;Wuetal.,1996;Wangetal.,
monocytes (reviewed in Biron et al., 1999) and thus prevents 1996;VanAntwerpetal.,1996;reviewedinVanAntwerpet
NKcellsfromproducingIFN-c.Thebiologicalreasons,ifany, al.,1998;Foo&Nolan,1999)andanyvirusthatblocksNF-jB
behind this are unclear. Finally, IFN-a}bs also play a role in activationmayleaveitselfsusceptibletoenhancedinductionof
stimulating the adaptive responses; IFN-induced IL-15 can apoptosis. Interestingly, ASFV infections are indeed charac-
stimulate the division of memory T cells (Tough et al., 1996; terizedbyasignificantdegreeofapoptosis(Ouraetal.,1998).
Zhangetal.,1998c;reviewedinToughetal.,1999;Sprentet The increased risk of apoptosis associated with inhibition of
CDEJ
SS..GGooooddbboouurrnn,,LL..DDiiddccoocckkaannddRR..EE..RRaannddaallll
NF-jBmaybecircumventedbyviralgeneproductsthatactto
blockapoptosis;suchgeneproductsarewidespread(reviewed 5. InhibitionofIFNsignalling
in Cuff & Ruby, 1996; Gillet & Brun, 1996). Thereareclearadvantagestovirusesinhavingtheability
Another major strategy for blocking IFN-a}b production toblockIFNsignalling.Sincetherearecomponentsincommon
wouldbetotargettheactivitiesoftheIRFtranscriptionfactors betweensignallingpathways,itispossibleforavirustoblock
that bind to the PRD I region of the IFN-b promoter. IFN-a}borIFN-csignallingorboth.Usingsuchstrategies,not
Intriguingly,theE6proteinofhumanpapillomavirustype 16 onlywouldtheinductionofcellularantiviralenzymes,suchas
(HPV-16) binds IRF-3 and can inhibit its virus-induced PKR,2«–5«oligoadenylatesynthetaseandMx,beinhibitedbut
transcriptional activation function (Ronco et al., 1998). How- therewouldalsobenoup-regulationofclassIMHCmolecules
ever, induction of IFN-b is not blocked completely by the E6 withininfectedcells,makingthempoorertargetsforcytotoxic
protein, suggesting that other cellular factors can substitute Tcells.Furthermore,virus-infectedcellswouldberesistantto
functionallyforIRF-3,andindeed,asdiscussedabove,thereare the actions of IFNs regardless of whether the IFNs were
several lines of evidence consistent with this hypothesis. The produced by infected cells or by activated leukocytes.
potentialsubstitutesforIRF-3includeIRF-1(Miyamotoetal., BlockingtheIFNsignallingpathwayscouldoccuratseveral
1988; Fujita et al., 1989a; Watanabe et al., 1991; Reis et al., levelsandthereisaccumulatingevidencethatvirusescanblock
1992; Matsuyama et al., 1993) and ISGF3 (Yoneyama et al., at most, if not all, stages (Table 1). Several poxviruses have
1996), but these factors can themselves be targetted by virus beenshowntoencodesolubleIFN-receptorhomologuesthat
functions.Forexample,IRF-1istargettedbytheK9ORFgene bind and sequester IFNs, thereby preventing their biological
product of human herpesvirus-8 (HHV-8) (Zimring et al., activity. For example, functional IFN-c receptors are secreted
1998),whilsttheE7proteinofHPV-16interactswiththep48 by cells infected with rabbit myxoma virus, ectromelia virus,
subunit of ISGF3 and prevents binding to DNA (Barnard & cowpoxvirus,camelpoxvirusandvacciniavirus(Uptonetal.,
McMillan,1999).Perhapstheplethoraoffactorsthatcanbind 1992;Mossmanetal.,1995;Alcami&Smith,1995).Vaccinia
tothePRDIregionoftheIFN-bpromoterreflectsaneedofthe virusandmostotherorthopoxvirusesalsoencodesolubleIFN-
cell to be able to circumvent virus blockades. a}breceptorhomologues(Symonsetal.,1995;Colamoniciet
In addition to specific transcription factor blocks, viruses al., 1995). It appears that the vIFN-a}b receptor of vaccinia
may inhibit the production of IFN by generally down- virus can also bind to the surface of cells and inhibit IFN
regulating host mRNA production or protein synthesis, and activity. Intriguingly, highly attenuated strains of vaccinia
there is some evidence that these apparently non-specific virus do not secrete the IFN-a}b receptor, consistent with its
effectscanaffectviruspathogenicity.Forexample,mutationin importance in virus pathogenesis. Interestingly, in terms of
thegeneencodingthematrixMproteinofvesicularstomatitis virus host range, both the IFN-a}b and IFN-c receptor
virus (which in wild-type virus causes a general inhibition of homologuessecretedbypoxvirusesoftenhaveabroadspecies
host-cell transcription) leads to an attenuated virus with specificity, unlike their cellular counterparts.
efficient IFN-b-inducing properties (Ferran & Lucas-Lenard, Human cytomegalovirus (HCMV) has been shown to
1997).Similarly,thefoot-and-mouthdiseasevirusLproteinase disruptIFNsignallingbydecreasingthelevelsofJak1andp48
geneencodesaproteinthatshutsoffhost-cellproteinsynthesis byamechanisminvolvingtheproteasome(Milleretal.,1998,
andmutationofthisgeneissufficienttogenerateanattenuated 1999),whereastheTantigenofmurinepolyomavirus(MPyV)
strainthatinduceselevatedlevelsofIFN-a}b(Chinsangaramet binds to Jak1 thereby blocking the activation of the IFN-a}b
al., 1999). andIFN-csignallingpathways(Weihuaetal.,1998).TheSTAT
Virusesmayalsohavemoresubtleandindirectmethodsfor andp48proteinsthatformpartofIFN-inducibletranscription
reducingthelevelofIFNproduced.Forexample,Epstein–Barr complexesaretargetsforinhibitionbyseveralviruses.TheV
virus(EBV)producesahomologueofIL-10(Hsuetal.,1990). protein of the paramyxovirus simian virus 5 (SV5) targets
Normally,IL-10isproducedbytheTh2subsetofThelpercells STAT1 for proteasome-mediated degradation (Didcock et al.,
and one of its biological functions is to inhibit the ability of 1999b),therebypreventingtheformationofISGF3andGAF
monocytes and macrophages to activate Th1 cells by down- complexes; indeed, at least part of the host range of SV5
regulatingtheexpressionofclassIIMHCmolecules.Activated appears to be determined by the ability to mediate STAT1
Th1cellsproduceanumberofcytokines,includingIFN-c,that degradation(Didcocketal.,1999a).Surprisingly,whilstmumps
arecriticalfortheinductionofclassicalcell-mediatedimmune virusalsoprobablytargetsSTAT1fordegradation(Yokosawa
responses,includingcytotoxicTlymphocytes.Ithastherefore etal.,1998),humanparainfluenzavirus2(hPIV2)(avirusvery
beenproposedthatEBVproducesthehomologueofIL-10in closely related to SV5 and mumps) targets STAT2 (Young et
order to induce an inappropriate and less-effective immune al., 2000). As a consequence, whilst SV5 and mumps virus
response against the virus (Bejarano & Masucci, 1998). block both IFN-a}b and IFN-c signalling, hPIV2 blocks only
Similarly,humanherpesvirus-6mayup-regulateIL-10,thereby IFN-a}bsignalling.Sendaivirus(Didcocketal.,1999b;Yokoo
causingimmunodysregulationbycausingashiftfromaTh1to etal.,1999)andhPIV3alsoblockIFN-a}bandIFN-csignalling,
a Th2 cytokine profile (Arena et al., 1999). althoughtherewasnoevidencewiththesevirusesthateither
CDFA