Medical Microbiology

394 7 GeneralVirology

LatentInfection

Inthisinfectiontype,thevirus(oritsgenome)isintegratedinacell,butnoviral

progeniesareproduced.Thecellisaccordinglynotdamagedandthemacro-

organismdoesnotmanifestdisease.Thisformofinfectionisfound,forinstance,

withtheadenovirusgroupandinparticulartheherpesviruses,whichcanre-

mainlatentforlongperiodsinthehumanbody.Latencyprotectstheseviruses

fromimmunesystemactivityandthusispartoftheirsurvivalstrategy.How-

ever,avarietyofinitiatingevents(seeChapter8,p. 41 9)caninitiatealyticcycle

leadingtomanifestdiseaseanddisseminationofthevirus.Repeatedactivation

ofalatentvirusistermedrecidivation(e.g.,herpeslabialis).

TumorTransformation

Infectionsbyanumberofvirusesdonotresultineventualhostcelldeath,but

rathercausetumortransformationofthecell.Thismeansthecellisalteredin

manyways,e.g.,initsgrowthproperties,morphology,andmetabolism.Fol-

lowinganinfectionwithDNAtumorviruses,thetypeofhostcellinfected

determineswhetherthecellreactionwillbeatumortransformation,viral

replicationorlyticcycle.Thetransformationthattakesplaceafterinfection

withanRNAtumorviruseitherinvolvesnoviralreplication(nonpermissive

infection)orthecellproducesnewvirusesbutremainsvital(permissivein-

fection).

CarcinogenicRetroviruses(“Oncoviruses”)

Genomestructureandreplicationoftheoncoviruses.Thegenomesofall

oncovirusespossessgag(group-specificantigen),pol(enzymaticactivities:

polymerasecomplexwithreversetranscriptase,integrase,andprotease),

andenv(envelopeglycoproteins)genes.Thesecodingregionsareflanked

bytwocontrolsequencesimportantforregulatoryfunctionscalledLTR(=

longterminalrepeats),Fig. 7. 7 .Thesesequenceshaveapromoter/enhancer

functionandareresponsibleforbothreversetranscriptionandinsertionof

theviralgenomeintothecellDNA.Certainoncovirusespossessaso-called

“oncgene”insteadofthepolregion(oncgene=oncogene,referstoacellular

genesegmentacquiredbyrecombination,seebelow).Thesevirusesalsoof-

tenhaveincompletegagand/orenvregions.Suchvirusesaredefectiveand

requireahelpervirustoreplicate(complementation,seep. 39 1).Anexcep-

tiontothisprincipleistheRoussarcomavirus,whichpossessesbothanonc

geneandacompletesetofviralgenesandcanthereforereplicateitself.

7

Kayser, Medical Microbiology © 2005 Thieme