![]() ![]() Clinical PMWS was also reproduced in conventional pigs coinfected with PCV2 and either porcine parvovirus (PPV) or porcine reproductive and respiratory syndrome virus (PRRSV) ( 57 T. Recently, clinical PMWS was reproduced in cesarean-derived-colostrum-deprived pigs and in specific-pathogen-free (SPF) pigs inoculated with PCV2 alone ( 28, 37). Initial attempts to reproduce clinical PMWS in conventional pigs by PCV2 inoculation were unsuccessful ( 12, 22, 33). The ORF2 gene of both PCV1 and PCV2 encodes the major immunogenic capsid protein ( 15, 48, 49). The pathogenic PCV2 shares a very similar genomic organization with the nonpathogenic PCV1. Sequence analyses revealed that PMWS-associated PCV2 shares about 75% nucleotide sequence identity with the nonpathogenic PCV1. The complete genomic sequence of PMWS-associated PCV2 has been determined ( 23, 26, 41). The primary causative agent of PMWS is a pathogenic strain of PCV designated PCV2 ( 2, 5, 9, 11, 19, 21, 22, 23, 43, 46). The PK-15-derived PCV did not produce clinical disease in experimentally inoculated pigs, and thus the virus was considered to be nonpathogenic ( 3, 63) and was designated PCV1. Although antibodies to PCV have been found in many animal species including humans, mice, cattle, and pigs ( 1, 17, 18, 30, 43, 50, 64, 65), little is known regarding the pathogenesis of PCV in these animal species ( 55, 65). Recently, three novel human circoviruses have been discovered, including transfusion-transmitted virus (TTV), SEN virus, and TTV-like minivirus ( 14, 44, 45, 51, 61, 67, 70). There are also three plant circoviruses, banana bunchy top virus, coconut foliar decay virus, and subterranean clover stunt virus ( 13, 42). PCV belongs to the Circoviridae family along with other animal circoviruses such as Psittacine beak and feather disease virus, Chicken anemia virus ( 13), and Columbid circovirus ( 40). The PCV genome contains at least two potentially functional open reading frames (ORFs): ORF1 (930 bp) encodes the Rep protein involved in viral replication and ORF2 (690 bp) encodes the immunogenic capsid protein ( 15, 23, 39, 49). PCV is a small icosahedral nonenveloped virus with a single-stranded circular DNA genome of about 1.76 kb. Porcine circovirus (PCV) was first discovered as a noncytopathic contaminant of the porcine kidney cell culture PK-15 ( 62, 66). Future studies are warranted to evaluate the usefulness of the chimeric PCV1-2 infectious DNA clone as a genetically engineered live-attenuated vaccine against PCV2 infection and PMWS. These data indicated that the chimeric PCV1-2 virus with the immunogenic ORF2 capsid gene of pathogenic PCV2 cloned into the nonpathogenic PCV1 genomic backbone induces a specific antibody response to the pathogenic PCV2 capsid antigen but is attenuated in pigs. Gross and microscopic lesions in various tissues of animals inoculated with the PCV2 infectious DNA clone were significantly more severe than those found in pigs inoculated with PCV1, chimeric PCV1-2, and reciprocal chimeric PCV2-1 infectious DNA clones. Group 2 and 5 pigs all seroconverted to PCV1 antibody. As expected, seroconversion to antibodies to the PCV2 capsid antigen was detected in group 3 and group 4 pigs. Group 3 pigs were each similarly injected with 200 μg of the PCV2 infectious DNA clone, group 4 pigs were each injected with 200 μg of the chimeric PCV1-2 infectious DNA clone, and group 5 pigs were each injected with 200 μg of the reciprocal chimeric PCV2-1 infectious DNA clone. Group 2 pigs were each injected in the superficial inguinal lymph nodes with 200 μg of the PCV1 infectious DNA clone. Group 1 pigs received phosphate-buffered saline as the negative control. To evaluate the immunogenicity and pathogenicity of the chimeric infectious DNA clones, 40 specific-pathogen-free (SPF) pigs were randomly assigned into five groups of eight pigs each. The PCV1, PCV2, and chimeric PCV1-2 and PCV2-1 DNA clones were all shown to be infectious in PK-15 cells, and their growth characteristics in vitro were determined and compared. ![]() A reciprocal chimeric PCV2-1 DNA clone was also constructed by replacing the PCV2 capsid gene with that of PCV1 in the backbone of the PCV2 genome. The chimeric PCV1-2 clone contains the PCV2 capsid gene cloned in the backbone of the nonpathogenic PCV1 genome. We report here the construction and characterization of two chimeric infectious DNA clones of PCV1 and PCV2. Porcine circovirus type 2 (PCV2) is the primary causative agent of postweaning multisystemic wasting syndrome (PMWS), whereas the ubiquitous porcine circovirus type 1 (PCV1) is nonpathogenic for pigs. ![]()
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