Semliki Forest Virus-Mediated Production of Retroviral Vector RNA in Retroviral Packaging Cells

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Abstract
Retroviral vectors are efficient tools for gene transfer studies. Their major advantage is that they can permanently integrate the transgene into the target cell's genome. However, because of the compulsory nuclear expression phase of their life cycle, it can be difficult for retroviruses to carry complex expression cassettes. In a attempt to mimic the structural features of most eukaryotic genes and obtain a potentially self-amplifying system for retrovirus production, we tested the feasibility of Semliki Forest virus (SFV) expression to mediate cytoplasmic synthesis of retrovirus vector RNA. An equivalent of a retrovirus virion RNA (retrovirus vector cassette, RVC) was cloned under the SFV 26S promoter, and full-length chimeric SFV-RVC RNA was produced in vitro. This RNA was introduced into retrovirus packaging cells, either via electroporation or transduction in SFV virions, and supernatants were analyzed for the presence of biologically active retroviruses. We demonstrate that this strategy can be used for cytoplasmic retrovirus production. The resulting viral particles are fully functional; they can transduce target cells, undergo reverse transcription, and integrate into genomic DNA. We also demonstrate that the SFV virion-based RVC delivery into packaging cells can yield high transient titers, in this case more than 105 G418R cfu/ml. This study shows that a simple, one-plasmid, heterologous viral RNA production system can be used to create functional retroviral RNA outside the cell nucleus. Wahlfors et al. report that retroviral vector RNA can be efficiently synthesized in the cytoplasm of retroviral vector packaging cell lines using an alphavirus-based expression system. This method liberates vector design from the constraints imposed by synthesis of RNA in the cell nucleus.