Gene transfer by retrovirus vectors occurs only in cells that are actively replicating at the time of infection.

Intravesical liposome-mediated interleukin-2 gene therapy in orthotopic murine bladder cancer model

“…26 In addition, the generation of retroviral vectors is complex and associated with safety issues for human trials that include the production of replication-competent recombinant virus. 27 Because of these limitations of the retroviral vector system as a transfection vehicle, we have successfully utilized liposome-mediated plasmid DNA transfer technique to primary cultures of human bladder carcinoma ex vivo as well as established bladder cancer cell lines, suggesting the clinical feasibility of this strategy. 11 In our current study, using an optimized 30 min transfection protocol, we demonstrated high levels of murine IL-2 production after the in vitro transfection of VR1110 plasmid DNA into MBT-2 cells.…”

Gene therapy in the CNS

“…82 These are enveloped RNA viruses which can transfer genes to a wide spectrum of dividing cell types. 83 Vector production utilizes packaging cells which express the retroviral gag-pol-env genes and can continually release vectors. The vectors bear up to 8.5 kb of transgenes flanked by retroviral long terminal repeat (LTR) regions, a virion packaging signal (psi), and a primer binding site for reverse transcription.…”

Differences among nonhuman primates in susceptibility to bone marrow progenitor transduction with retrovirus vectors

“…Although there is a steady increase in the total number of cells in these cultures (data not shown), it is possible that the rate of cell division, especially at the level of progenitors, declines over time at this temperature. As cell division is required for provirus integration, 46 such a decline would have a strong effect on transduction rates. Because HSC from such disparate species as mice, 33 and dogs, 45 appear to continue cycling during culture at 33°C, it is possible that the rate of transduction in this more primitive population may continue to increase with the time in culture.…”