Induction of Epstein-Barr virus kinases to sensitize tumor cells to nucleoside analogues

The presence of Epstein-Barr virus (EBV) in the tumor cells of some EBV-ass ociated malignancies may facilitate selective killing of these tumor cells. We show that treatment of an EBV+ Burkitt's lymphoma cell line with 5-azac ytidine led to a dose-dependent induction of EBV lytic antigen expression, including expression of the viral thymidine kinase (TK) and phosphotransfer ase (PT). Azacytidine treatment for 24 h modestly sensitized the cell line to all nucleosides tested. To better characterize EBV TK with regard to var ious nucleoside analogues, we expressed EBV TK in stable cell clones. Two E BV TK expressing clones were moderately sensitive to high doses of acyclovi r and penciclovir (PCV) (62.5 to 500 muM) and to lower doses of ganciclovir (GCV) and bromovinyldeoxyuridine (BVdU) (10 to 100 muM) compared to a cont rol clone and were shown to phosphorylate GCV. Similar experiments in a tra nsient overexpression system showed more killing of cells transfected with the EBV TK expression vector than of cells transfected with the control mut ant vector (50 muM GCV for 4 days). A putative PT was also studied in the t ransient transfection system and appeared similar to the TK in phosphorylat ing GCV and conferring sensitivity to GCV, but not in BVdU or PCV-mediated cell killing. Induction of EBV kinases in combination with agents such as G CV merits further evaluation as an alternative strategy to gene therapy for selective killing of EBV-infected cells.