Porcine cells persistently infected with classical swine fever virus protected from pestivirus-induced cytopathic effect

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 (CPEsurv cells) were used to investigate these findings further. In con-trast to persistently infected cells that had not experienced a CPE, CPEsurv cells were protected from the CPE when superinfected with cp CSFV or with cp bovine viral diarrhoea virus. Similarly, cells whic