Enhanced transgene expression in primitive hematopoietic progenitor cells and embryonic stem cells efficiently transduced by optimized retroviral hybrid vectors.

Oncoretroviral vectors have been successfully used in gene therapy trials, yet low transduction rates and loss of transgene expression are still major obstacles for their application. To overcome these problems we modified the widely used Moloney murine leukemia virus-derived retroviral vector pMX by replacing the 3'LTR with the spleen focus-forming virus LTR and inserting the woodchuck hepatitis B virus post-translational regulatory element. To compare requirements crucial for efficient transgene expression, we generated the hybrid retroviral vectors pMOWS and pOWS that harbor the complete murine embryonic stem cell virus (MESV)-leader sequence or a shortened MESV-leader not comprising primer binding site (PBS) and splice donor (SD). Applying these retroviral vectors significantly augmented transgene expression in hematopoietic cell lines and progenitor cells. For transduction of murine embryonic stem (ES) cells the retroviral vector pMOWS that harbors the MESV-PBS and -SD was superior resulting in 65% green fluorescent protein (GFP) expressing ES cells. Surprisingly, in murine and human primitive hematopoietic progenitor cells (HPC), the highest efficiency of up to 66% GFP expressing cells was achieved with pOWS, a retroviral vector that retains the negative regulatory element coinciding with the MoMuLV-PBS. In summary our hybrid retroviral vectors facilitate significantly improved transgene expression in multipotent cells and thus possess great potential for reconstituting genes in primary cells of disease models, as well as for gene therapy.