We used one of the CPEsurv SK-6 cultures to examine the
correlation between protection from the CPE and down regulation
of sg RNA synthesis (Fig. 4).
Uninfected SK-6 cells
(a), SK-6 cells persistently infected with vA187-1 that had not
experienced a CPE (b) and the CPEsurv cell line SK-6 vA187-
1}a (see Fig. 1, lane 1) that had survived the CSFV-induced
CPE (c) were infected with cp vA187-1 at an m.o.i. of 2 (lanes
7±9) and RNA was analysed by Northern blotting at 20 h p.i.
As a control, the same cells were either mock-infected (lanes
1±3) or infected with ncp vA187-1 at an m.o.i. of 2 (lanes 4±6).
Analysis of the cultures revealed large amounts of CSFVspeci
®c RNA when uninfected cells were infected with ncp
vA187-1 (lane 4), whereas there was no change in the amount
of viral RNA or the ratio of genomic to sg RNA in the two
persistently infected cultures before (lanes 2 and 3) or after
superinfection with ncp vA187-1 (lanes 5 and 6). However,
when the same three cultures were superinfected with cp
vA187-1 a striking difference was observed between the two
persistently infected cultures. Similarly to uninfected cells (lane
7), persistently infected cells which had not experienced a CPE
produced large amounts of sg RNA (lane 8). This demonstrated
that the latter culture was still infectable with CSFV and that in
these cells the replication of sg RNA was not down-regulated,
resulting in a rapid appearance of the CPE. In contrast, CPEsurv
cells produced only small amounts of sg RNA (lane 9),
consistent with the absence of a CPE after superinfection of
such cells with cp vA187-1 (Fig. 3