Medical Microbiology

430 8 VirusesasHumanPathogen

ofacellulardoublelipidlayerinwhichareintegratedthehepatitisBsurface

(HBs)antigen,a 25 kDapolypeptide,anditsprecursorstagesPreS1(40kDa)

andPreS2(33kDa).(Fig.8. 10 ).Thisenvelopeenclosestheactualcapsid,

whichconsistsofthehepatitisBcore(HBc)antigenwith 21 kDaandcontains

thegenometogetherwiththeDNApolymerase(areversetranscriptase,

p.385).Thecomplete,infectiousvirion,alsoknownasaDaneparticleafter

itsdiscoverer,hasadiameterof 42 nm,theinnerstructure 27 nm.Thevirus

replicatesinlivercells.TheDaneparticlesandtheHBsantigen,butnotthe

HBcantigen,arereleasedintothebloodstream,wherebytheHBsantigenis

presentintwodifferentforms,afilamentousparticleapproximately 22!

100 nmandasphericalformwithadiameterofabout 22 nm.Afurtherviral

proteinistheHBeantigen,whichrepresentsaposttranslational,truncated

formoftheHBcantigenandisnolongercapableofspontaneouscapsidfor-

mation.Itisalsoreleasedfromthehepaticcellsintotheblood.

HepatitisBMutants

Usingmolecularbiologicalmethodsrefinedinrecentyears,moreandmoreHBV

mutantshavebeenfoundwithoneormoreaminoacidexchangesincertainpro-

teins.HBsorPreSmutantsareso-called“escape”mutantsthatcancauseanew

infectionorrecidivationdespiteimmuneprotectionbyantibodiestoHBs.Similarly,

pre-HBVcorHBVcmutantscanleadtoareactivationofHBVreplicationandthus

toachronichepatitis,sincetheyblockformationoftheHBeantigenandthusthe

pointofattackforthecellularimmunedefenses.TheseHBcmutantsarefrequently

observedunderinterferontherapy.

HepatitisBVirus

PreS1 PreS2 HBs antigen

Fig.8. 10 Thecapsid,whichconsistsof

HbcandHbeantigens,enclosestheen-

tireDNAantisensestrand,theincom-

pletesensestrand,andthereversetran-

scriptase(notshownhere).Theenve-

lopecontainsthethreeformsofthe

Hbsantigen:PreS1=completeprotein,

PreS2=shortenedformofPreS1,HBs

antigen=HBsurfaceantigeninthe

propersense,shortenedformofPreS2.

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Kayser, Medical Microbiology © 2005 Thieme