Suitability of Epstein-Barr virus-based episomal vectors for expression of cytokine genes in human lymphoma cells

Plasmids carrying the Epstein-Barr Virus (EBV) latent gene EBNA1 and the EBV latent origin of replication (oriP) stay in transfected human cells as autonomously replicating extrachromosomal genetic units. They thus might represent a suitable fool for cytokine gene introduction into human tumor cells with the prospect of therapeutic anti-tumor vaccination. The aim of this study was to analyze whether such plasmids permit stable and efficient expression of cytokine genes in human non-Hodgkin lymphoma cells. We tested physical stability and expression levels of plasmids carrying EBNA1 and oriP for episomal maintenance, immunoglobulin light chain enhancer elements for augmentation of expression, and cytokine or marker genes after introduction into human NHL cell lines in vitro and in vivo after inoculation into nude mice. Data obtained with these EBV-based vectors were compared with another plasmid, not carrying EBNA1 and oriP. cDNAs coding for GM-CSF, IL6, TNF alpha, the chloramphenicolacetyl-transferase (CAT) and the beta-galactosidase (lacZ) gene were transfected into the EBV-positive Burkitt's lymphoma cell line BL60 and the EBV-negative B cell lymphoma cell line BJA-B. EBV-derived vectors permitted a high, host cell independent transfection efficiency and high and host cell independent levels of expression. After removal of the selection pressure (hygromycin B) cytokine expression could be detected for several weeks in vitro and in vivo but, however, declined continuously. These experiments suggest that episomal BC-derived vectors represent an effective tool for cytokine gene transfer in human lymphoma cells.