Shope fibroma virus-induced facilitation of vesicular stomatitis virus adsorption and replication in nonpermissive cells

Abstract

The ability of Shope fibroma virus (SFV) to facilitate the replication of vesicular stomatitis virus (VSV) in a strain of domestic rabbit kidney (DRK 3) cells was investigated. Without SFV present, cultures of DRK 3 cells were nonpermissive for VSV when challenged at a low multiplicity of 20 infectious particles/cell; no infectious virus was produced, no viral antigens were synthesized, no transcription of the viral genome was observed, and no cytopathic effects (CPE) were induced. When the multiplicity of VSV was increased to 1000 infectious particles/cell, the cultures were abortively infected. At this high multiplicity, while still no infectious VSV was produced and viral antigens remained undetectable by immunofluorescence, there appeared to be a transient primary transcription of the viral genome and the virus demonstrated a uv-sensitive ability to induce CPE. Upon infection with SFV, the DRK 3 cells became as permissive as Vero cells for complete replication of VSV, even at low-input multiplicities of infection. Conversion to this permissive state required early synthesis of DNA-dependent RNA, which was dependent on de novo synthesis of protein. The need for synthesis of this RNA was discontinuous; by 6 hr after SFV infection its synthesis was no longer necessary for VSV to replicate. By 24 hr after infection with SFV, 89% of the cells in DRK 3 cultures were superinfected when challenged with VSV at a multiplicity of 1000, but only 2% were superinfected when the input multiplicity of VSV was 20. This difference appeared to reflect a time-dependent lag in the ability of SFV-infected cells to adsorb VSV efficiently. Kinetic experiments revealed that little if any VSV was adsorbed by DRK 3 cells unless they were first infected with SFV for several days. By 3 days after SFV infection, adsorption of VSV was significantly enhanced and a majority of the cells were infected when challenged with only 20 infectious VSV particles/cell. These results indicate that DRK 3 cells are nonpermissive for VSV alone because extracellular adsorption of the virus is extremely inefficient and intracellular viral replication is abortive. Facilitation of VSV by SFV in these cells therefore appears to involve (i) modification of the plasma membrane to enhance VSV attachment and, (ii) elimination of intracellular restrictions to VSV replication.