Characterization of classical swine fever virus associated with defective interfering particles containing a cytopathogenic subgenomic RNA isolated from wild boar

Classical swine fever virus (CSFV) strain WB82, isolated from a wild boar i n 1982, induced a distinct cytopathic effect (CPE) in primary swine testicl e cell culture and in most of the porcine cell lines. This strain of CSFV w as found to be composed of two biotypes, cytopathogenic (cp) CSFV, as a min or population, and noncytopathogenic (noncp) CSFV, as a major population. T he noncp CSFV (designated strain WB82/E+) was obtained by biological clonin g, and it showed the exaltation of Newcastle disease virus phenomenon. In N orthern blot analysis and RT-PCR assay, CSFV RNA with a subgenomic (sg) len gth was detected in addition to full-length viral RNA only in the cells in which a CPE had been revealed, These RNAs represent the genomes of typical defective interfering (DI) particles because of the strict dependence on a complementing helper virus and interference with replication of the helper virus. The sg RNA, which exhibits the genomes of the DI particles, lacked t he nucleotides of the viral genomic region from N-pro to NS2 (4764 bases). When extracted sg RNA was transfected to the cells infected with the WB82/E + strain, a distinct CPE was observed. Interestingly, the CPE was observed in cells infected with other heterologous noncp CSFV ALD and GPE(-) strains by sg RNA transfection. The results suggested that these noncp CSFVs act a s helper viruses for the replication of sg RNA (DI particles). It was also shown that the cytopathogenicity of strain WB82 is caused by apoptosis.