Restriction ofPorcine Parvovirus Replication inNonpermissive Cells

Swinetesticle (ST)cells andMadin-Darby canine kidney(MDCK)cells differ intheir ability tosupport replication ofporcine parvovirus (PPV). Viral replication eventsinSTcells, apermissive cell type,andMDCK cells, a nonpermissive cell type, were compared inan attempt toelucidate putative mechanisms ofrestrictive virus replication. Radiolabeled PPVboundtothecell surface ofbothcell typesequally wellandthebinding was showntobePPVspecific, indicating thattherestriction was notatthecell surface level. Incontrast, profound differences inintracellular events inPPVreplication were observed between these twocell types. Synthesis of viral DNA was limited inMDCK cells inthatthepercentage ofcells withreplicative-form DNA asdetermined bystrand-specific probeinsitu hybridization was approximately 100-fold lower inMDCK cells thaninSTcells atthesame multiplicity ofinfection. Northern (RNA)blot analysis, using oligonucleotide probes derived from bothstructural andnonstructural protein-coding regions ofthePPV genome, revealed fourPPV mRNA transcripts frominfected STcells. Comparatively, RNA species fromthestructural protein coding region were actively transcribed inMDCK cells, butsynthesis ofRNA species fromthenonstructural protein coding region was negligible. Immunoprecipitation ofviral polypeptides revealed thethree characteristic structural polypeptides, VP1,VP2,andVP3,along withthenonstructural polypeptide, NS-1, fromSTcells. Incontrast, neither viral structural ornonstructural polypeptides nor progeny virions were produced fromMDCK cells. Thedata suggest thatmechanisms controlling permissivenessofcells toPPVinfection areassociated withthelevel of viral DNA replication, RNAtranscription, andviral antigen expression butnotabsorption tothecell surface. Porcine parvovirus (PPV), characterized asamemberof theautonomous parvoviruses, causes fetal death andmummification inswinefetuses (25,29).Thevirion contains a 5-kblinear, minus-polarity single-stranded DNA genome (33) encapsidated within asimple icosahedral protein coat composed ofthree structural polypeptides (84, 64,and60 kDa)(32). Inaddition tothethree structural polypeptides, at least onenonstructural polypeptide (86kDa)isproduced within infected cells (35). Similar toother parvoviruses, PPV undergoes replication through duplex replicative-form (RF) intermediates whichformthetemplate forsingle-stranded DNA progeny andRNA transcripts. Onlytheminusstrand ispacked into virion particles (33, 49). Inaddition tovirion particles containing DNA,designated full particles (density of1.39g/ml), noninfectious emptyparticles without viral DNA (density of1.30g/ml) aresynthesized andreleased fromPPV-infected cells (11, 32). Parvoviruses causearangeofclinical manifestation in various hostspecies, suchasinuteroinfection andfetal death (pigs [PPV] andhumans[B19]), enteritis (dogs [canine parvovirus], minks[mink enteritis virus], andcats[feline panleukopenia virus]), andaplastic crisis (humans [B19]) (3, 4,39,47,48). Comparative studies haverevealed aprofound variation inpathogenicity amongPPVisolates (10, 22,31, 34). Moreover, other clinical manifestation besides reproductive failure, suchasdermatitis (26) andenteric diseases (16,18,57),werereported tobeassociated withPPV. Dissection ofthemechanisms controlling thepermissiveness ofdifferent cells andtissues tosupport replication ofthis particular virus istherefore fundamental toanoverall understanding ofvirus-induced disease.