A new defective retroviral vector system based on the Bryan strain of Rous sarcoma virus

We have constructed a helper cell line and vector system based on the Bryan high titer (BH) strain of Rous sarcoma virus (RSV). BH-RSV is a defective virus which lacks an env gene; however, if env is supplied in trans, it replicates to a very high titer. Like BH-RSV, the vector contains gag and pol genes and lacks an env gene. The helper cell line supplies env in trans and permits the production of infectious virions. To construct the helper cell line the subgroup A env gene from the Schmidt-Ruppin-A (SRA) RSV was stably transfected into Qt6 cells, a chemically transformed quail fibroblast line. To minimize homology between the vector and helper cell line, transcription of the env gene is driven by a MuLV LTR, and 3' processing is controlled by the simian virus 40 (SV40) polyadenylation signal. This combination of vector and helper cells can be used to produce high-titer viral stocks in which recombinant replication-competent virus have not been detected even when the stocks were used to inoculate chickens. This system should be useful for developing transgenic chickens, studying cell lineage, and introducing genes into cultured cells.