The intracellular half-lives of nonreplicating nucleocapsids of DI particles of wild type and mutant strains of vesicular stomatitis virus.

We have used defective interfering (DI) particles purified free of all infectious virus to determine the intracellular biological stability of nonreplicating nucleocapsids of vesicular stomatitis virus (VSV). Following infection of BHK-21 cells or tc-7 cells with a low multiplicity of pure DI particles, we superinfected them at varying times afterward with a high multiplicity of infectious helper virus to allow replication of those DI particle nucleocapsids retaining biological activity. Careful quantitation of DI particles in the yields from each time point showed that the biological half-life of intracellular VSV Indiana wild type DI particle nucleocapsids was 6 hr in BHK-21 cells and 5.3 hr in tc-7 cells (not significantly different). However, a DI particle from a temperature-sensitive mutant (tsG31) of VSV exhibited a biological nucleocapsid half-life of 12.5 hr and a DI particle isolated following 5 years of persistent infection had a half-life of 18 hr. These findings have significance for the stability of DI particle activity in vivo during acute infections where virus and DI particles are not always present together in the same cells due to cycling interactions. The increased half-life of DI nucleocapsids after years of persistent infection contrasts with the decreased stability and debilitation generally observed in infectious virus from persistent infection. Finally, transcapsidation studies showed that the intracellular half-life differences between DI particles are due mainly to their RNA genomes rather than to the N protein which encapsidates them.