Retroviral-Mediated Transfer of the Green Fluorescent Protein Gene Into Murine Hematopoietic Cells Facilitates Scoring and Selection of Transduced Progenitors In Vitro and Identification of Genetically Modified Cells In Vivo

We have investigated the utility of the green fluorescent protein (GFP) to serve as a marker to assess retroviral gene transfer into hematopoietic cells and as a tool to identify and enrich for cells expressing high levels of the vector-encoded transcript. GFP, by virtue of a naturally occurring chromophore encoded in its primary sequence, displays autonomous fluorescence, thus eliminating the need for antibody or cytochemical staining to detect its expression. A bicistronic murine stem cell virus (MSCV)-based retroviral vector was constructed containing the GFP cDNA and a mutant, human dihydrofolate reductase gene. High-titer, ecotropic retroviral producer cells free of replication competent virus were generated and used to transduce murine bone marrow cells by cocultivation. Within 24 hours after completion of the transduction procedure, a high proportion (40% to 70%) of the marrow cells were intensely fluorescent compared to mock-transduced cells or cells transduced with a control retrovirus. Erythroid and myeloid hematopoietic colonies derived from GFP-transduced marrow were easily scored for retroviral gene transfer by direct in situ fluorescence microscopy. Clonogenic progenitors expressing increased levels of antifolate drug resistance could be enriched from the GFP-transduced marrow population by fluorescence activated cell sorting of cells expressing high levels of GFP. In vivo, splenic hematopoietic colonies and peripheral blood cells from animals transplanted with GFP-transduced marrow displayed intense fluorescence. These results show that GFP is an excellent marker for scoring and tracking gene-modified hematopoietic cells and for allowing rapid selection and enrichment of transduced cells expressing high levels of the transgene.