Evolution of virus and defective-interfering RNAs in BHK cells persistently infected with Sindbis virus.

We analyzed a BHK cell line persistently infected with Sindbis virus for 16 months and a virus (Sin-16) cloned from these cells. Sin-16 virus was resistant to the defective interfering particles present in the original infection. We found that (i) cells infected with Sin-16 were impaired in the processing of a viral precursor glycoprotein, (ii) high-multiplicity passaging of Sin-16 gave rise to a variant that was able to generate and be inhibited by defective-interfering particles to which the original Sin-16 virus was resistant, and (iii) the persistently infected culture contained a heterogeneous mixture of defective Sindbis virus RNAs which were not packaged into extracellular particles. To determine whether these intracellular RNAs could interfere with the replication of Sin-16, we analyzed cells that were cloned from the persistently infected culture. One clone (A3) synthesized a single defective viral RNA which was lost with continued passaging in culture. Infection of A3 cells with Sin-16 showed that the presence of the defective RNA greatly enhanced cell survival and led to enrichment of this RNA. In contrast, cured cells were highly susceptible to killing by Sin-16, and survivors did not synthesize this RNA. Thus, A3 cells were not genetically altered in their response to Sin-16, but were protected from the cytopathic effects of infection by an RNA with the characteristics of a defective-interfering RNA.