Stable expression of the vaccinia virus K1L gene in rabbit cells complements the host range defect of a vaccinia virus mutant.

Modified vaccinia virus Ankara (MVA), having acquired genomic deletions during passage in chicken embryo fibroblasts, is highly attenuated and unable to productively infect most mammalian cell lines. Multiplication in rabbit kidney-derived RK13 cells, but not other nonpermissive cells, can be restored by insertion of the vaccinia virus K1L gene into the MVA genome. During nonproductive infection of RK13 cells by MVA, transcription of representative viral early genes was revealed by Northern (RNA) blotting, whereas synthesis of an intermediate mRNA and replication of viral DNA could not be detected. Despite the persistence of viral early mRNA for at least several hours, synthesis of virus-induced polypeptides occurred only during the first hour and was followed by abrupt inhibition of all protein synthesis. Transfection of RK13 cells with a eukaryotic expression plasmid that contained the K1L gene allowed MVA infection to proceed to late stages of viral protein synthesis. Moreover, RK13 cell lines that stably expressed the K1L gene were permissive for MVA as well as a K1E deletion mutant of the WR strain of vaccinia virus. This is the first description of the complementation of a poxvirus mutant by cells that stably express a viral gene.