病毒学双语版课件Cha课件

AcademicPress,2000.,Pathogenesis,LearningObjectives:Oncompletingthissession,youshouldbeableto:Explaintheconceptofpathogenesisinthecontextofvirusinfections.Discussthemolecularbasisforvirus-inducedimmunodeficiency(includingAIDS)&celltransformationbyviruses.Understandthewaysinwhichvirusinfectionmayresultincellularinjury.,AcademicPress,2000.,Pathogenicity,Pathogenicity,thecapacityofoneorganismtocausediseaseinanother,isacomplex&variablephenomenon.Atthesimplestlevelthereisthequestion,whatisdisease?Anall-embracingdefinitionwouldbethatitisadeparturefromthenormalphysiologicalparametersofanorganism.Thiscouldrangefromatransient&veryminorconditionsuchasaslightlyelevatedtemperatureorrathersubjectivefeelingsoflethargytochronicpathologicconditionswhicheventuallyresultindeath.Anyoftheseconditionsmayresultfromalargenumberofinternalorexternalsources.However,thereisrarelyonesinglefactorwhichcausesadisease.Mostdiseasestatesaremulti-factorialatoneleveloranother.,AcademicPress,2000.,VirusDiseases,Twocomponentsareinvolved:thedirecteffectsofvirusreplicationtheeffectsofbodilyresponsestotheinfectionThecourseofanyvirusinfectionisdeterminedbyadelicate&dynamicbalancebetweenthehost&thevirus,asistheextent&severityofviruspathogenesis.Insomevirusinfections,mostofthepathologicsymptomsobservedareattributablenottovirusreplication,buttotheside-effectsoftheimmuneresponse.Inflammation,fever,headaches&skinrashesarenotusuallycausedbyvirusesthemselves,butbythecellsoftheimmunesystemduetothereleaseofpotentchemicalssuchasinterferons&interleukins.Inthemostextremecases,noneofthepathologiceffectsofcertaindiseasesarecauseddirectlybythevirus,exceptthatitspresencestimulatestheactivationoftheimmunesystem.,AcademicPress,2000.,VirusPathogenesis,Viruspathogenesisisanabnormal&fairlyraresituation-themajorityofvirusinfectionsaresilent&donotresultinoutwardsignsofdisease.Itissometimessaidthatviruseswoulddisappeariftheykilledtheirhosts-thisisnotnecessarilytrue.Itispossibletoimagineviruseswithahit-and-runstrategy,movingquicklyfromonedyinghosttothenext&relyingoncontinuingcirculationfortheirsurvival.Nevertheless,thereisacleartendencyforvirusesnottoinjuretheirhostsifpossible.Ideally,aviruswouldnotevenprovokeanimmuneresponsefromitshost,oratleastbeabletohidetoavoidtheeffects.Threemajoraspectsofviruspathogenesismustbeconsidered:directcelldamageresultingfromvirusreplication,damageresultingfromimmuneactivationorsuppression,&celltransformationcausedbyviruses.,AcademicPress,2000.,MechanismsofCellularInjury,Virusinfectionresultsinanumberofchangeswhicharedetectablebyvisualorbiochemicalexaminationofinfectedcells.Thesechangesresultfromtheproductionofvirusproteins&nucleicacids,butalsofromalterationstothebiosyntheticcapabilitiesofcells.Intracellularparasitismbyvirusessequesterscellularapparatussuchasribosomes&rawmaterialswhichwouldnormallybedevotedtosynthesizingmoleculesrequiredbythecell.Eukaryoticcellsmustcarryoutconstantmacromolecularsynthesis,whethertheyaregrowing&dividingorinastateofquiescence.,AcademicPress,2000.,MechanismsofCellularInjury,Agrowingcellclearlyneedstomanufacturemoreproteins,morenucleicacids&moreofallofitsmyriadcomponentstoincreaseitssizebeforedividing.Thefunctionofallcellsisregulatedbycontrolledexpressionoftheirgeneticinformation&thesubsequentdegradationofthemoleculesproduced.Suchcontrolreliesonadelicate&dynamicbalancebetweensynthesis&decaywhichdeterminestheintracellularlevelsofalltheimportantmoleculesinthecell.Thisisparticularlytrueofthecontrolofthecellcycle,whichdeterminesthebehaviourofdividingcells.,AcademicPress,2000.,MechanismsofCellularInjury,Anumberofcommonphenotypicchangescanberecognizedinvirus-infectedcells.Thesechangesareoftenreferredtoasthecytopathiceffects(c.p.e.)ofavirus&include:1)Alteredshape2)Detachmentfromthesubstrate3)Lysis4)Membranefusion5)Membranepermeability6)Inclusionbodies7)Apoptosis,AcademicPress,2000.,Shutoff,Anumberofviruseswhichcausecelllysisexhibitaphenomenonknownasshutoffearlyininfection.Shutoffisthesudden&dramaticcessationofmosthostcellmacromolecularsynthesis.Inpoliovirus-infectedcells,thisistheresultofproductionofthevirus2Aprotein.Thismoleculeisaproteasewhichcleavesthep220componentofeIF-4F,acomplexofproteinsrequiredforcap-dependenttranslationofmessengerRNAsbyribosomes.SincepoliovirusRNAdoesnothavea5methylatedcapbutismodifiedbytheadditionoftheVPgprotein,virusRNAcontinuestobetranslated.Inpoliovirus-infectedcells,thedissociationofmRNAs&polyribosomesfromthecytoskeletoncanbeobserved&thisisthereasonfortheinabilityofthecelltotranslateitsownmessages.Afewhoursaftertranslationceases,lysisofthecelloccurs.,AcademicPress,2000.,CellDamagebyAdenoviruses,Inthecaseofadenoviruses,thepentonprotein(partoftheviruscapsid)hasatoxiceffectoncells.Althoughitspreciseactiononcellsisnotknown,additionofpurifiedpentonproteintoculturedcellsresultsintheirrapiddeath.Toxinproductionbypathogenicbacteriaisacommonphenomenon,butthisistheonlywell-establishedcaseofavirus-encodedmoleculewithatoxin-likeaction.TheadenovirusE3-11.6KproteinissynthesizedinsmallamountsfromtheE3promoteratearlystagesofinfection&inlargeamountsfromthemajorlatepromoteratlatestagesofinfection.E3-11.6Kisrequiredforthelysisofadenovirus-infectedcells&thereleaseofvirusparticlesfromthenucleus.,AcademicPress,2000.,MembraneFusion,Membranefusionistheresultofvirus-encodedproteinsrequiredforinfectionofcells,typicallytheglycoproteinsofenvelopedviruses.OneofthebestknownexamplesofsuchaproteincomesfromSendaivirus(aparamyxovirus),whichhasbeenusedtoinducecellfusionduringtheproductionofmonoclonalantibodies.Atleastnineoftheelevenknownherpessimplexvirus(HSV/HHV-1)glycoproteinshavebeencharacterisedregardingtheirroleinvirusreplication.Anumberoftheseproteinsareinvolvedinfusionofthevirusenvelopewiththecellmembrane&alsoincellpenetration.ProductionoffusedsyncytiaisacommonfeatureofHSVinfection.,AcademicPress,2000.,CellFusionbyHIV,InfectionofCD4+cellswithsomebutnotallisolatesofHIVcausescell-cellfusion&theproductionofsyncytiaorgiantcells.Theproteinresponsibleforthisisthetransmembraneenvelopeglycoproteinofthevirus(gp41)&thedomainneartheaminoterminusresponsibleforthisfusogenicactivityhasbeenidentifiedbymoleculargeneticanalysis.BecauseHIVinfectsCD4+cells&itisthereductioninthenumberofthesecrucialcellsoftheimmunesystemwhichisthemostobviousdefectinAIDS,itwasinitiallybelievedthatdirectkillingofthesecellsbytheviruswasthebasisforthepathogenesisofAIDS.AlthoughdirectcellkillingbyHIVundoubtedlyoccursinvivo,itisnowbelievedthatthepathogenesisofAIDSisconsiderablymorecomplex.Manyanimalretrovirusesalsocausecellkilling&inmostcases,itappearsthattheenvelopeproteinofthevirusisrequired,althoughtheremaybemorethanonemechanisminvolved.,AcademicPress,2000.,CellFusionbyHIV,AcademicPress,2000.,Viruses&Immunodeficiency,Atleasttwogroupsofviruses,herpesviruses&retroviruses,directlyinfectthecellsoftheimmunesystem.Thishasimportantconsequencesfortheoutcomeoftheinfection&fortheimmunesystemofthehost.Herpessimplexvirus(HSV)establishesasystemicinfection,spreadingviathebloodstreaminassociationwithplatelets,butdoesnotshowparticulartropismforcellsoftheimmunesystem.However,Herpessaimirii&Mareksdiseasevirusareherpesviruseswhichcauselymphoproliferativediseases(butnotclonaltumours)inmonkeys&chickens,respectively.Themostrecentlydiscoveredhumanherpesviruses,humanherpesvirus-6(HHV-6),HHV-7&HHV-8allinfectlymphocytes.,AcademicPress,2000.,InfectionofImmunecellsbyHerpesviruses,Epstein-BarrvirusinfectionofB-cellsleadstoimmortalization&proliferation,resultinginglandularfeverormononucleosis,adebilitatingbutbenigncondition.EBVwasfirstidentifiedinalymphoblastoidcelllinederivedfromBurkittslymphomaand,inrareinstances,EBVinfectionmayleadtotheformationofamalignanttumour.WhilesomeherpesvirusessuchasHSVarenotablycytopathic,mostofthelymphotropicherpesvirusesdonotcauseasignificantdegreeofcellularinjury.Infectionofthedelicatecellsoftheimmunesystemmayperturbtheirnormalfunction.Becausetheimmunesystemisinternallyregulatedbycomplexnetworksofinterlinkingsignals,relativelysmallchangesincellularfunctioncanresultinitscollapse.Alterationofthenormalpatternofproductionofcytokinescouldhaveprofoundeffectsonimmunefunction.Thetrans-regulatoryproteinsinvolvedinthecontrolofherpesvirusgeneexpressionmayalsoaffectthetranscriptionofcellulargenes-theeffectsofherpesvirusesonimmunecellsaremorecomplexthanjustcellkilling.,AcademicPress,2000.,RetrovirusPathogenesis,Retrovirusescauseavarietyofpathogenicconditionsincludingparalysis,arthritis,anaemia&malignantcellulartransformation.Asignificantnumberofretrovirusesinfectthecellsoftheimmunesystem.Althoughtheseinfectionsmayleadtoadiversearrayofdiseases&haematopoeticabnormalitiessuchasanaemia&lymphoproliferation,themostcommonlyrecognizedconsequenceofretrovirusinfectionistheformationoflymphoidtumours.However,somedegreeofimmunodeficiency,rangingfromverymildtoquitesevere,isacommonconsequenceoftheinterferencewiththeimmunesystemresultingfromthepresenceofalymphoidormyeloidtumour.,AcademicPress,2000.,AcquiredImmunodeficiencySyndrome(AIDS),Themostprominentaspectofvirus-inducedimmunodeficiencyisacquiredimmunodeficiencysyndrome(AIDS),aconsequenceofinfectionwithHumanimmunodeficiencyvirus(HIV),amemberofthegenusLentivirusoftheRetroviridae.Thereareanumberofsimilarlentiviruseswhichcauseimmunodeficiencydiseasesinanimals.Unlikeinfectionbyothertypesofretrovirus,HIVinfectiondoesnotdirectlyresultintheformationoftumours.SometumourssuchasB-celllymphomasaresometimesseeninAIDSpatientsbuttheseareconsequenceofthelackofimmunesurveillancewhichisresponsibleforthedestructionoftumoursinhealthyindividuals.TheclinicalcourseofAIDSislong&veryvariable.AgreatnumberofdifferentabnormalitiesoftheimmunesystemareseeninAIDS.Asaresultofthebiologyoflentivirusinfections,thepathogenesisofAIDSishighlycomplex.,AcademicPress,2000.,AIDSPathogenesis,ItisstillnotcompletelyclearhowmuchofthepathologyofAIDSiscauseddirectlybythevirus&howmuchiscausedbytheimmunesystem.TherearenumerousmodelsthathavebeensuggestedtoexplainhowHIVcausesimmunodeficiency.Thesemechanismsarenotmutuallyexclusive&itisprobablethattheunderlyinglossofCD4+cellsinAIDSismultifactorial.,AcademicPress,2000.,DirectCellKilling:,Thiswastheearliestmechanismsuggested,basedonthebehaviouroflaboratoryisolatesofHIV.CellfusionresultinginsyncytiumformationisoneofthemajormechanismsofcellkillingbyHIVinvitro.However,differentisolatesofHIVvaryconsiderablyintheextenttowhichtheypromotethefusionofinfectedcells.SubsequentexperimentssuggestedtheremaynotbesufficientviruspresentinAIDSpatientstoaccountforallthedamageseen,althoughkillingofCD4+cellsmaycontributetotheoverallpathogenesisofAIDSinsomecircumstances.Recently,ithasbecomeclearthatuptohalfoftheCD4+cellsinthebodymaybeinfectedwithHIV,sotheideaofdirectcellkillinghasbeenre-examined,butinlightofinductionofapoptosisratherthanbycellfusion.,AcademicPress,2000.,IndirectKillingofHIV-InfectedCells:,Indirecteffectsofinfection,e.g.disturbancesincellbiochemistry&cytokineproduction,mayalsoaffecttheregulationoftheimmunesystem.However,theexpressionofvirusantigensonthesurfaceofinfectedcellsleadstoindirectkillingbytheimmunesystem-effectivelyatypeofautoimmunity.Theextentofthisactivityisdependentonthevirusload&replicationkineticsininfectedindividuals.,AcademicPress,2000.,AntigenicDiversity,Thistheoryproposesthatthecontinualgenerationofnewantigenicvariantseventuallyswamps&overcomestheimmunesystem,leadingtoitscollapse.ThereisnodoubtthatnewantigenicvariantsofHIVconstantlyariseduringthelongcourseofAIDSbecauseofthelowfidelityofreversetranscription.Itisenvisagedthattheremightbearatcheteffect,witheachnewvariantcontributingtotheslightbutirreversibledeclineinimmunefunction.Becauseofthewayvirusinfectionsarehandledbytheimmunesystem,itisprobablethatvariationofT-cellepitopesontargetproteinsrecognizedbyCTLaremoreimportantthanB-cellepitopeswhichgeneratetheantibodyresponsetoaforeignantigen.Recently,ithasbeenshownthatthereisasimplerelationshipbetweenvirusload&survivaltime,&thatapatientcanwithstandonly1,300viralyearsofHIV(i.e.copiesofthevirusgenome/mlxsurvivaltimeinyears).,AcademicPress,2000.,AntigenicDiversityofHIV,AcademicPress,2000.,T-CellAnergy:,Anergyisanimmunologicallyunresponsivestateinwhichlymphocytesarepresentbutnotfunctionallyactive.Thisisusuallyduetoincompleteactivationsignals&maybeanimportantregulatorymechanismintheimmunesystem,e.g.toleranceofselfantigens.InAIDS,anergycouldbeinducedduetoHIVinfection,erferencewithcytokineexpression.Thereisexperimentalinvitroevidencethatgp120-CD4interactionsresultinanergyduetointerferencewithsignaltransduction.ManyAIDSpatientsareanergic,i.e.failtomountadelayed-typehypersensitivity(DTH)responsetoskin-testantigens.ImpairedDTHresponsesaredirectlyrelatedtodecreasingCD4+T-lymphocytecounts.However,thereisnostrongevidencethatthisphenomenonisdirectlyrelatedtoanyaspectofHIVinfectioninvivoratherthantogeneraldepletionofimmunefunctions.,AcademicPress,2000.,Apoptosis:,LikeT-cellanergy,apoptosiscouldpotentiallybeinducedinlargenumbersofuninfectedcellsbyfactorsreleasedfromamuchsmallernumberofHIV-infectedcells.InadditiontoclonaldeletionasanormalpartoftheevolutionoftheT-cellrepertoire,apoptosismaybeinducedfollowingT-cellactivationasanegativeregulatorymechanismtocontrolthestrength&durationoftheimmuneresponse.HIVinfectionofT-cellsinducesanactivatedphenotype,e.g.surfaceexpressionofCD45&HLA-DRmarkers,whichsuggeststhatthesecellsmaybeinevitablydoomedduetoactivationoftheapoptosispathway.BecauseHIVestablishesapersistentinfection,itisbynomeansclearthatapoptosishasanentirelynegativeeffect-inductionofcelldeathmaywelllimitvirusproduction&slowdownthecourseofinfection.SeveralHIVproteinshavebeenidentifiedasbothinducers&repressorsofapoptosisundervariouscircumstances.However,theproportionofCD4+TcellsinthelaterstagesofapoptosisisabouttwofoldhigherinHIV-1infectedindividualsthaninuninfectedpeople.,AcademicPress,2000.,Superantigens:,Superantigensaremoleculeswhichshort-circuittheimmunesystem,resultinginmassiveactivationofT-cellsratherthantheusual,carefullycontrolledresponsetoforeignantigens.Itisbelievedthattheydothisbybindingtoboththevariableregionoftheb-chainoftheT-cellreceptor(Vb)&toMHCclassIImolecules,cross-linkingtheminanon-specificway.ThisresultsinpolyclonalT-cellactivationratherthantheusualsituationwhereonlythefewclonesofT-cellsresponsivetoaparticularantigenpresentedbytheMHCclassIImoleculeareactivated.Theover-responseoftheimmunesystemproducedresultsinautoimmunityaswholefamiliesofT-cellswhichbindsuperantigensareactivated,&immunosuppressionastheactivatedcellsarekilledbyotheractivatedT-cellsorundergoapoptosis.NosuperantigenhasbeenconclusivelyidentifiedinHIV,despiteintensiveinvestigation,thusthepracticalrelevanceofsuperantigensinAIDSisindoubt.,AcademicPress,2000.,Superantigens:,AcademicPress,2000.,TH1/TH2Imbalance:,ImmunologicaltheorysuggeststhattherearetwotypesofCD4+T-helper(TH)cell:TH1cellswhichpromotethecellmediatedresponse&TH2cellswhichpromotethehumoralresponse.ThistheorysuggeststhatearlyinHIVinfection,TH1-responsiveT-cellspredominate&areeffectiveincontrolling(butnoteliminating)thevirus.Atsomepoint,a(relative)lossoftheTH1responseoccurs&TH2HIV-responsivecellspredominate.ThehypothesisisthereforethattheTH2-dominatedhumoralresponseisnoteffectiveatmaintainingHIVreplicationatalowlevel&thevirusloadbuildsup,resultinginAIDS.Althoughthisislargelyatheoreticalproposalwhichhasnotbeenproved,thisthinkingisshapingourunderstandingoftheimmuneresponsetomanydifferentpathogens,notjustHIV.However,noexperimentalstudyhasdemonstratedanactualswitchfromtheTH1toTH2patternofcytokineexpression&secretionthatisassociatedwithdiseaseprogression,sothereisnoevidencefortheinvolvementofthesemechanismsinAIDS.,AcademicPress,2000.,TH1/TH2Imbalance:,AcademicPress,2000.,VirusLoad&ReplicationKinetics:,Experimentsinvolvingaccuratequantitationoftheamountofvirusininfectedindividualshaverevealedthatmuchhighervirusloadsarepresentthanwasoriginallymeasuredbylesssensitivetechniques.Usingquantitativepolymerasechainreaction(PCR)methodstoaccuratelymeasuretheamountofviruspresent&determinehowtheselevelschangewhenpatientsaretreatedwithdrugswhichinhibitHIVreplication,ithasbeenshownthat:Continuous&highlyproductivereplicationofHIVoccursinallinfectedindividuals,althoughtheratesofvirusproductionvarybyupto70-foldindifferentindividuals.Theaveragehalf-lifeofanHIVparticleinvivois2.1days.Upto109-1010HIVparticlesareproducedeachday.Anaverageof2x109newCD4+cellsareproducedeachday.,AcademicPress,2000.,AIDSPathogenesis,Contrarytowhatwasinitiallybelieved,thereisaverydynamicsituationinHIV-infectedpeopleinvolvingcontinuousinfection,destruction&replacementofCD4+cells.BillionsofnewCD4+cellsareproduced,infected&killedeachday.AbetterunderstandingofthepathogenesisofAIDSand,inparticular,theroleoftheimmunesystemintheearlystagesofthediseaseisvitaltopermitthedevelopmentofmoreappropriatetherapiesforAIDS.Highlyactiveantiretroviraltherapy(HAART)hasadramaticeffectonsuppressingvirusproduction&temporarilyrestoresCD4cellpopulations.Unfortunately,becauseofthelonghalflifeoflatently-infectedcells,theestimatedtimetocompletelyeradicateHIVfromthebodyisroughly12-60years-somethingwhichisnotpresentlyachievableduetothefailureoftherapyasaresultofdrugtoxicity&virusresistance.,AcademicPress,2000.,Virus-RelatedDiseases,Thereareanumberofhumandiseasesyndromesforwhichvirusinfectionsarebelievedtobeanecessaryprerequisite.Insomeinstances,thelinkbetweenaparticularvirus&apathologicalconditioniswellestablished,butitisclearthatthepathogenesisofthediseaseiscomplex&alsoinvolvestheimmunesystemofthehost.Inothercases,thepathogenicinvolvementofaparticularvirusislesscertain,&inafewinstances,ratherspeculative.,AcademicPress,2000.,SubacuteSclerosingPanencephalitis(SSPE),Althoughtheincidenceofmeaslesvirusinfectionhasbeenreducedsharplybyvaccination,measlesstillcauseshundredsofthousandsofdeathseachyear.Thenormalcourseofmeaslesvirusinfectionisanacutefebrileillnessduringwhichthevirusspreadsthroughoutthebody,infectingmanytissues.Thevastmajorityofpeoplespontaneouslyrecoverfromthediseasewithoutanylastingharm.Inrarecases(about1in2000),measlesmayprogresstoasevereencephalitis.Thisisstillanacutecond