Herpesvirus saimiri-based gene delivery vectors maintain heterologous expression throughout mouse embryonic stem cell differentiation in vitro

In order to achieve a high efficiency of gene delivery into rare cell types like stem cells the use of viral vectors is presently without alternative. An ideal stem cell gene therapy vector would be able to infect primitive p rogenitor cells and sustain or activate gene expression in differentiated p rogeny. However, many viral vectors are inactivated when introduced in deve loping systems where cell differentiation occurs. To this end, we have deve loped a mouse in vitro model for testing herpesvirus saimiri (HVS)-based ge ne therapy vectors. We demonstrate here for the first time that HVS is able to infect totipotent mouse embryonic stem (ES) cells with high efficiency. We have transduced ES cells with a recombinant virus carrying the enhanced green fluorescent protein (EGFP) gene and the neomycin resistance gene (Ne oR) driven by a CMV promoter and the SV40 promoter, respectively. ES cells maintain the viral episomal genome and can be terminally differentiated int o mature haematopoietic cells. Moreover, heterologous gene expression is ma intained throughout in vitro differentiation. Besides its obvious use in ge ne therapy, this unique expression system has wide ranging applications in studies aimed at understanding gene function and expression in cell differe ntiation and development.