Background: The mononegavirus superfamily (Mononegavirales) comprises three families, Rhabdoviridae, Paramyxoviridae and Filoviridae. These viruses possess a single stranded negative sense RNA as the genome. Recent success in the recovery of infectious virus from a transfected cDNA of mononegaviruses including Sendai virus, a prototypic paramyxovirus, is opening the possibility of their genetic engineering. However, infectious viruses have been recovered only by initiating the infectious cycle with cDNA directing the synthesis of antigenomic positive sense (+)RNA. Starting with genomic negative sense (-)RNA has been unsuccessful. Furthermore, the recovery efficiency has often been extremely low.
Results: We describe here an analogous system that allows recovery of Sendai virus at a high rate, from cells in which the transfected cDNA and plasmids to support the synthesis of viral nucleocapsid protein and RNA polymerases are coexpressed by vaccinia virus-driven bacteriophage T7 polymerase. Our system was able to recover the virus from cDNA directing not only (+)RNA but also (-)RNA. Moreover, using this system, we succeeded in recovery of the virus by transfection of in vitro synthesized (+)RNA or (-)RNA. This improved virus recovery appeared to be accomplished by supplying the supporting plasmids at an optimal ratio and by minimizing the cytopathic effect of the vaccinia virus by specific inhibitors. In addition, it was probably critical that our cDNAs were constructed to generate viral authentic RNAs without adding T7 promoter-specific nucleotides to the 5' ends. An immediate application of the system was demonstrated by the creation of a candidate vaccine strain with a predetermined attenuating mutation in the cleavage-activation site of the viral fusion glycoprotein.
Conclusion: We have established methods which greatly improve the recovery of Sendai virus from cDNA. There is essentially no absolute obstacle to recovery of the virus from the (-)RNA template. Even the complete full length RNA chain in the naked form appears to be properly encapsidated to become a functional template.