Improved retroviral packaging lines derived from spleen necrosis virus.

Using highly efficient gene expression vectors, we constructed new retroviral packaging lines derived from spleen necrosis virus. Core proteins are expressed from the murine leukemia virus promoter and enhancer followed by the tripartite leader sequence of an adenovirus. Using different plasmids for envelope expression, we found that the efficiency of vector transduction is dependent on the level of gag-pol expression. The level of envelope expression did not have a measurable impact on vector virus titers. The new helper cell lines do not contain any sequences homologous to vector genomes. They transduce standard retrovirus vectors with titers up to 10(6) colony forming units per milliliter of supernatant tissue culture medium. No replication-competent virus was observed.