Sindbis virus DNA-based expression vectors: utility for in vitro and in vivo gene transfer

Several DNA-based Sindbis virus vectors were constructed to investigate the feasibility and potential applications for initiating the virus life cycle in cells transfected directly with plasmid DNA. These vectors, when transfected into mammalian cells, have been used to produce virus, to express heterologous genes, and to produce infectious vector particles. This approach involved the conversion of a self-replicating vector RNA (replicon) into a layered DNA-based expression system. The first layer includes a eukaryotic RNA polymerase II expression cassette that initiates nuclear transcription of an RNA which corresponds to the Sindbis virus vector replicon. Following transport of this RNA from the nucleus to the cytoplasm, the second layer, autocatalytic amplification of the vector, proceeds according to the Sindbis virus replication cycle and results in expression of the heterologous gene. The Sindbis virus DNA vectors expressed reporter genes in transfected cells at levels that were comparable to those of in vitro-transcribed RNA replicons and were approximately 10-fold higher than the levels produced by conventional RNA polymerase II-dependent plasmids in which the promoter and reporter gene were linked directly. Reporter gene expression was also observed in rodent muscle following injection with Sindbis virus DNA vectors. In a second application, packaged vector particles were produced in cells cotransfected with complementing replicon and defective helper DNAs. The Sindbis virus-derived DNA vectors described here increase the utility of alphavirus-based vector systems in general and also provide a vector with broad potential applications for genetic immunization.