Pseudotypes of avian sarcoma viruses with the envelope properties of vesicular stomatitis virus.

Upon superinfection of cells producing Rous sarcoma virus (RSV) with temperature-sensitive mutants of vesicular stomatitis virus (VSV), two kinds of pseudotype viruses are produced: VSV genomes within particles bearing the envelope antigens of RSV, denoted VSV(RSV), and RSV genomes within particles bearing the envelope antigens of VSV, denoted RSV(VSV). The VSV(RSV) pseudotypes are recognized as the fraction of plaque-forming units resistant to neutralization by antiserum to VSV or, in the case of thermolabile envelope mutants of VSV, resistant to heat inactivation; they possess the host range restrictions of RSV and are neutralized by antisera specific to the RSV subgroup. The RSV(VSV) pseudotypes are recognized as the fraction of focus-forming units which transforms chick cells resistant to infection with the strain of RSV used. Both kinds of pseudotypes are produced concomitantly with VSV synthesis. VSV(RSV) particles comprise up to 12% of the VSV progeny titer and RSV(VSV) up to 1% of the RSV titer, but pseudotype fractions varied according to the VSV mutant used for superinfection. The proportions of pseudotypes in harvests of mixed infections are not reduced by filtration through 0.2-μm pore size filters to eliminate large aggregates of virus particles, and pseudotypes are not formed by mixing pure-grown RSV and VSV particles in vitro. VSV acts as a helper virus for BH-RSV(-), which is defective in envelope antigen, but not for αBH-RSV(-), which is also defective in RNA-directed DNA polymerase activity. The titer of BH-RSV(VSV) is enhanced by the presence of the avian leukosis helper virus, RAV-1, and more than 90% of this mixed pseudotype stock is neutralized by antiserum to either VSV or RAV-1, indicating that the RSV particles bear a mosaic of both VSV and RAV-1 envelope antigens. RSV(VSV) pseudotypes transform cells of four out of five mammalian species tested. Like RSV of subgroup D and B77, the focus-forming titer of RSV(VSV) assayed on mammalian cells is 1000-fold lower than on chick cells.