Semliki Forest virus (SFV) established a persistent, non-cytopathic infection in cultured cells with no effect on cumulative cell number as compared with control cultures. All cells were initially infected by SFV as judged by infective centre and immunofluorescence assay and released approx. 50 to 70 p.f.u./cell in the initial 24 h after infection. At 12 h after infection there was a ‘shut-down’ of virus-specific RNA synthesis followed by a sharp decline in the percentage of cells infected, stabilizing at about 2% by 48 h. Significant amounts of virus were released from about 2% of cells in persistently infected cultures. At 8 to 10 days a small-plaque, attenuated variant of SFV appeared in infected cultures; it replaced ‘wild-type’ virus completely by day 16.

The establishment of persistence could not be related to (i) the release of interferon-like activity from infected cells, (ii) the presence of a directly antiviral agent in infected cells or medium, or (iii) changes in the total levels or subcellular distribution of acid phosphatase.

Subculturing persistently infected cells stimulated cell division and promoted a (reduced) burst of virus production; simultaneous superinfection with SFV gave no increase in virus yields.


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