Anticancer suicide gene therapy using herpes simplex virus-thymidine kinase (HSV-tk) and ganciclovir (GCV) features the unique advantage of being able to elicit brisk host immune response against tumors and the host response reportedly can be potentiated with the co-expression of other appropriate immune- or apoptosis-related genes. We introduced a novel antiapoptotic gene, bfl-1, to test its applicability in the HSV-tk/GCV system. CT-26 murine colon cancer cells transfected with HSV-tk, alone or in combination with bcl-xL or bfl-1, were either grown in vitro or injected into syngeneic mice, followed by GCV administration. The co-expression of bfl-1 was associated with the upregulation of CD95 and CD40 ligand (CD40L) in vitro and with pronounced intratumoral T-lymphocyte infiltration in vivo. These results add to the previous findings that antiapoptotic genes can be used as an adjunctive component in the HSV-tk/GCV system to enhance host immune response against tumors. (C) 2003 Elsevier Science (USA). All rights reserved.