Arch Virol (2002) 147: 43–58
Kinetics of porcine circovirus type 2 replication
A. K. Cheung
and S. R. Bolin
Virus and Prion Diseases of Livestock Research Unit, National Animal Disease
Center, USDA, Agricultural Research Service, Ames, Iowa, U.S.A.
Animal Health Diagnostic Laboratory, College of Veterinary Medicine,
Michigan State University, Lansing, Michigan, U.S.A.
Accepted August 13, 2001
Summary. The kinetics of porcine circovirus type 2 (PCV2) replication in PK15
cells was examined. During productive infection, viral antigens, RNA transcripts
and progeny viruses all increased in a time dependent manner.Viral antigens were
observed in a few cells at 18 hour postinfection (h p.i.) and cell-free progeny
viruses began to appear at about 30 h p.i. Viral transcripts were detected by 18 h
p.i. and the capsid protein RNA of 950 nucleotides (nt) was the most abundant
RNA species. Two other RNAs of sizes 750 and 450 nt, derived from the predicted
replication associated protein (Rep) gene region, were also detected. These two
RNAs share 3
common nucleotide sequences and they are transcribed in the same
orientation as the proposed unspliced Rep RNA or the recently described Rep’
RNA. The 35 kD capsid protein was observed at 30 h p.i. by Western blot anal-
ysis and itappeared to be the mostimmunodominantprotein in swine exposed
to PCV2. The capsid proteins of PCV type 1 and PCV2 each contain a nuclear
localization signal sequence capable of targeting a reporter protein to the nucleoli
of transfected cells when the capsid proteins were expressed as 3
tides. Although previous reports indicated that PCV2 capsid proteins localized
predominantly in the nuclei of infected cells, we observed an abundant amount
of PCV2 capsid proteins in the cytoplasm of many cells of the infected cultures.
The cells that exhibited cytoplasmic capsid proteins also contained virus nucleic
acids, indicating that these proteins were synthesized by the infected cells and
not through uptake from the culture medium. Elucidation of the changes that af-
fect the localization pattern of PCV2 capsid proteins, nuclear versus cytoplasmic,
requires further investigation.