Cytopathogenicity of classical swine fever virus (CSFV) depends on the presence of defective particles containing a subgenomic (sg) RNA with a defined deletion. In a previous report we described the spontaneous generation of this sg RNA and therefore of cytopathogenic (cp) CSFV in porcine kidney cell cultures persistently infected with CSFV. Frequently, some cells survived the CPE and could be further propagated. They remained positive for viral antigen and continued to shed complete virus and in most cases also defective virus particles. SK-6 cells that had survived the CPE (CPE(surv)cells) were used to investigate these findings further. In contrast to persistently infected cells that had not experienced a CPE, CPE(surv) cells were protected from the CPE when superinfected with cp CSFV or with cp bovine viral diarrhoea virus. Similarly, cells which were rescued and further propagated after acute infection with cp CSFV also proved to be protected from the CSFV-induced CPE. When either virus obtained from CPE(surv) cells that had spontaneously lost the sg RNA or virus from which defective particles had been removed was used to establish persistently infected cells, these cells were also protected from the CPE after superinfection with cp CSFV. These findings suggest that the virus contained in CPE(surv) cells confers on the host cell the ability to resist the CSFV-induced CPE. However, when naive cells were infected with supernatants from CPE(surv)cells that contained defective virus particles, the CPE reappeared within three to five virus passages, indicating that the sg RNA retained its cytopathogenic potential.


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