Gene therapy is an approach to treat human diseases by stable transfer of exogenous genes. Stem cell-based gene therapy for congenital blood disorders is accompanied by the risk for leukemia development conferred by insertional oncogenesis. The safety of retroviral gene transfer may be increased by including a suicide gene in the therapeutic vector to eliminate adverse events. Mice were challenged with leukemia cells transformed by proviral integration of a murine oncogene that simultaneously expressed the HSV-TK suicide gene or hyperactive mutants thereof (SR39 and sc39). After treatment with the drug substrate Ganciclovir, leukemia developed in mice given clonal cells expressing HSV-TK, but not SR39 or sc39. In vitro Ganciclovir resistance was observed in heterogeneously transduced leukemia pools, and single nucleotide changes or partial loss of the suicide gene were identified as mechanisms of drug escape. However, Ganciclovir treatment resulted in 80-100% survival of mice challenged with partially resistant leukemia pools expressing modified HSV-TK variants with improved biological activity.

Retroviral gene transfer can be exploited for immunotherapy for cancer by transducing autologous tumor cells with immunostimulatory molecules. Interleukin-12 is a potent adjuvant for cell-based tumor vaccines, and to avoid toxic side effects of recombinant IL-12 administration, murine acute lymphoblastic leukemia (ALL) cells were stably transduced with IL-12 expressing vectors. Local IL-12 secretion of cellular vaccines efficiently rejected primary leukemia challenges in mice and was slightly superior to systemic IL-12 administration in immunologic memory formation, but anti-tumor effects were highly sentitive to IL-12 dosage. We provide evidence that a combination of transgenic IL-12 expression in conjunction with a "triple" immunomodulatory transgene expression panel (CD80, GM-CSF and CD40L) may present the most potent cellular vaccination strategy to induce primary tumor rejection and long-lasting memory formation. Our "triple" transduced leukemia vaccine was also shown to be highly efficient in leukemia rejection one and three months after syngeneic bone marrow transplantation. Survival rates were similar or improved compared to untransplanted mice although there were slight deficiencies in numerical immune reconstitution early after transplantation. Vaccination in a post-BMT setting might therefore be beneficial to fight minimal residual disease and prevent leukemia relapse after chemotherapy.