CPEsurv cells are protected from the CPE when
superinfected with cytopathogenic CSFV
The data obtained so far indicated that in CPEsurv cells the
ratio between sg and genomic RNA was lower compared with
cells acutely infected with cp CSFV (Fig. 1). This suggests that
the absence of a CPE in CPEsurv cells is a consequence of
down-regulation of sg RNA synthesis. To investigate this
hypothesis further we examined whether a CPE could be
induced in such cells by superinfection with cp CSFV.
The four CPEsurv cultures A.1}a, A.1}b, B.2}a, and B.2}b
(see Fig. 1, lanes 7±10), which had been passaged up to 95
times after the CPE had occurred, were superinfected with
serial dilutions of cp vA187-1. As shown in Fig. 3(rows 1±4),
these cells were completely protected from the CPE. This was
also the case for cultures derived from SK-6 cells which were
rescued after acute infection with cp CSFV (data not shown). In
contrast, a persistently infected SK-6 culture that had not
experienced a CPE (row 5) and a mock-infected culture (row 6)
showed a CPE at virus dilutions up to 10−#. The apparent
partial destruction of the monolayer observed after staining of
cells infected with undiluted virus in rows 1±4 is not considered
to be a virus-speci®c CPE, as concluded from microscopic
examination of the cells and from the observation that cell