Replication and Binding Strategy
|All viruses of the Circoviridae family are thought to have a similar replication strategy. A circular, double stranded replicative form of DNA intermediate is produced using host cell DNA polymerases during the S phase of cell division. The replicative form serves as template for generation of viral ssDNA using the rolling circle replication mechanism (Todd et al., 2005). Viral DNA intermediates are generated in the nuclei and require host cell enzymes for completion of the replication cycle (Tischer et al., 1987). Virions are assembled in both nuclei and cytoplasm and released from infected cells in the absence of viral cytopathic effects.
Misinzo et al. (2005) studied the binding characteristics of PCV2 in the porcine monocytic line 3D4/31 and found that PCV2 enters 3D4/31 cells predominantly via clathrin-mediated endocytosis and requires an acidic environment for infection.
Meerts et al. (2005a) found that PCV2 capsid protein was expressed between 6 to 12 hours post inoculation and nuclear relocation occurred around 12 to 24 hours post inoculation in PK-15 cells. In porcine alveolar macrophages or fetal cardiomyocytes, nuclear localized antigens appeared approximately at 48 hours post inoculation and in fewer cells compared to PK-15 cells.
When seven different PCV2 strains were compared in PK-15 cells, it was found that abortion-associated strains had different replication kinetics compared with PMWS or PDNS-associated PCV2 isolates. A higher number of infected cells were observed at 24 hours post inoculation and the percentage of infected cells with nuclear localized antigens was lower compared to that of the other strains. The same authors also found differences in susceptibility to PCV2 infection between porcine alveolar macrophages from different pigs (Meerts et al., 2005a).
It has been demonstrated that treatment of PK-15 cell cultures with IFN-γ causes a 20 times higher production of PCV2 progeny (Meerts et al., 2005b). The enhancing effect of IFN-γ on PCV2 infection was found to be due to increased internalization of PCV2 virion-like particles. Expression of PCV2 proteins in infected cells were not altered by IFN-γ treatment (Meerts et al., 2005b).
During productive infection of PK-15 cells, nine RNAs, capsid RNA, five Rep-associated RNAs (Rep, Rep’, Rep3a, Rep3b, and Rep3c), and three NS-associated RNAs (NS515, NS672, and NS0) are synthesized by PCV2 (Cheung, 2003c), and it was demonstrated that Rep and Rep’ are essential for PCV2 replication (Cheung, 2003b). PCV1 and PCV2 were found to differ from each other in expression levels of NS- and Rep3c-associated RNAs (Cheung, 2003a).
Cheung AK: The essential and nonessential transcription units for viral protein synthesis and DNA replication of porcine circovirus type 2. Virology 313:452-459, 2003b
Cheung AK: Transcriptional analysis of porcine circovirus type 2. Virology 305:168-180, 2003c
Meerts P, Misinzo G, McNeilly F, Nauwynck HJ: Replication kinetics of different porcine circovirus 2 strains in PK-15 cells, fetal cardiomyocytes and macrophages. Arch Virol. 3:427-441, 2005a
Meerts P, Misinzo G, Nauwynck HJ: Enhancement of porcine circovirus 2 replication in porcine cell lines by IFN-gamma before and after treatment and by IFN-alpha after treatment. J Interferon Cytokine Res. 25:684-693, 2005b
Misinzo G, Meerts P, Bublot M, Mast J, Weingartl HM, Nauwynck HJ: Binding and entry characteristics of porcine circovirus 2 in cells of the porcine monocytic line 3D4/31. J Gen Virol. 86:2057-2068, 2005
Tischer I, Peters D, Rasch R, Pociuli S: Replication of porcine circovirus: induction by glucosamine and cell cycle dependence. Arch Virol. 96:39-57, 1987
Todd D, Bendinelli M, Biagini P, Hino S, Mankertz A, Mishiro S, Niel C, Okamoto H, Raidal S, Ritchie BW, Teo GC:Circoviridae.In: C. M. Fauquet, M. A. Mayo, J. Maniloff, U. Desselberger, L.A. Ball (Eds.): Virus taxonomy: Eighth report of the international Committee on Taxonomy of Viruses, Elsevier Academic Press, San Diego:327-334, 2005