Retroviral vectors have been considered the most promising vehicles for the transfer of therapeutic genes into cells of the hematopoietic system. In clinical studies, however, cases of leukemoid clonogenic expansion of the transduced cells in vivo caused by semirandom integration of the foreign DNA in the genome have changed the focus to other types of vectors. In addition to their superior safety profile, a higher packaging capacity might be an advantage of non-viral vectors in certain applications. Prolonged transgene expression of non-viral vectors can be achieved by inclusion of Epstein-Barr virus (EBV)-derived elements mediating episomal replication and retention. Furthermore, a variety of cis acting elements have been explored in an attempt to enhance gene transfer efficiency. Our study confirmed that prolonged transgene expression can be achieved in B-lymphoid cells with EBV-derived vectors containing the EBV latent gene EBNA-1 and the EBV latent origin of replication oriP. In addition, we demonstrated that the inclusion of enhancer elements of the immunoglobulin κ light chain (Ei and E3') associated with its matrix attachment region (MAR) resulted in a 10-fold increase in transgene expression in B-lymphoid cells. It can be concluded that these elements are generally useful modules for improving the efficiencies of non-viral vectors in the B-lymphoid lineage.

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