Retroviral producer cells are generated by the introduction of a viral genome into "helper" cell lines containing all the necessary components for viral packaging and the release of infectious particles. The selection of high-titer vector producer cells is most efficient if the vector genome encodes a selectable marker, while it is extremely tedious to select high-titer producer clones if the transgene cannot be detected and selected directly. Here we describe the development of a screening system that uses reversible integration of lox-P-flanked eGFP as a qualitative and quantitative marker gene in two different vector systems, greatly facilitating the selection of viral producer cells. After selection and titration of high-titer viral producer cells based on eGFP expression, the eGFP gene could be removed from the provirus by transient introduction of Cre-recombinase into the producer cells, thus allowing the production of therapeutic relevant vectors expressing solely the gene of interest. However, after removal of the marker gene a slight but consistent increase in viral titers compared to the respective control vectors was found, independent of the transgene or backbone used. The single lox-P site retained in the vector backbone does not affect gene expression level or fidelity of RNA processing.
