Medical Microbiology

384 7 GeneralVirology

Adsorption.Virusparticlescanonlyinfectcellspossessingsurface“recep-

tors”specifictotheparticularvirusspecies.Whenavirusencounterssuch

acell,itadsorbstoiteitherwiththecapsidor,inenvelopedviruses,bymeans

ofenvelopeproteins.Itisthereforethereceptorsonacellthatdetermine

whetheritcanbeinfectedbyacertainvirus.

Receptors

Someaspectsofthenatureofthereceptorsareknown.Thesearemoleculesthat

playimportantrolesinthelifeofthecellorintercellularcommunication,e.g.,mol-

eculesoftheimmunoglobulinsuperfamily(CD4:receptorforHIV;ICAM-1:receptor

forrhinoviruses),thecomplement(C3)receptorthatisalsothereceptorforthe

Epstein-Barrvirus,orglycoproteinsthecellularfunctionsofwhicharenotyet

known.

Practicalconsequencesarisefromthisgrowingknowledgeaboutthereceptors:on

theonehand,itaidsinthedevelopmentofantiviraltherapeuticsdesignedtoinhibit

theadsorptionofthevirusestotheirtargetcells.Ontheotherhand,thegenetic

informationthatcodesforcertainreceptorscanbeimplantedintocellsorexperi-

mentalanimals,renderingthemsusceptibletovirusestowhichtheywouldnor-

mallyberesistant.Anexampleofthisapplicationistheuseinexperimentalstudies

oftransgenicmicerenderedsusceptibletopoliovirusesinsteadofprimates(e.g.,on

vaccinetesting).

Penetrationanduncoating.Virusesadsorbedtothecellsurfacereceptors

thenpenetrateintothecellbymeansofpinocytosis(aprocessalsoknown

asviropexis).Inenvelopedviruses,theenvelopemayalsofusewiththecell

membrane,releasingthevirusintothecytoplasm.Adsorptionofsuchanen-

velopedvirustotwocellsatthesametimemayresultincellfusion.Thenext

step,knownasuncoating,involvesthereleaseofthenucleicacidfromthe

capsidandisapparently(exceptinthesmallpoxvirus)activatedbycellular

enzymes,possiblywithacontributionfromcellmembranesaswell.Theex-

actmechanism,whichwouldhavetoincludepreservationofthenucleicacid

intoto,isnotknownforallviruses.

Replicationofthenucleicacid.Differentprocessesareobservedcorrespond-

ingtothetypesandconfigurationsoftheviralgenome(Fig. 7. 5 ).

— DNAviruses:thereplicationofviralDNAtakesplaceinthecellnucleus

(exception:poxviruses).Someviruses(e.g.,herpesviruses)possessrepli-

casesoftheirown.ThesmallerDNAviruses(e.g.,polyomaviruses),which

donotcarryinformationfortheirownDNApolymerase,codeforpoly-

peptidesthatmodifythecellularpolymerasesinsuchawaythatmainly

viralDNAsequencesarereplicated.

7

Kayser, Medical Microbiology © 2005 Thieme