Retroviral suicide gene vectors have successfully been used in clinical studies to improve the safety of adoptive immunotherapy with allogeneic T lymphocytes in the treatmentAdoptive immunotherapy based on donor leukocyte infusions (DLI) with or after allogeneic hematopoietic transplantation is a promising concept in the treatment of malignant 1,2 and infectious diseases. 3,4 In addition, donor T lymphocytes significantly contribute to preventing graft rejection and to enhancing post-transplant immune competence. However, infusion of allogeneic T lymphocytes is frequently associated with an immune reaction against host tissue which may lead to a severe, potentially lethal graft-versus-host disease (GVHD).
5Consequently, much effort has been made to separate the 'graft-versus-malignancy' effects, eg 'graft-versus-leukemia' (GVL), from GVH reactions. 5,6 One concept relies on a gene therapy approach for the control of GVHD. 7,8 This strategy is based on retroviral vector-mediated introduction of a so-called 'suicide gene' Herpes simplex virus thymidine kinase (HSVtk) into T cells allowing their in vivo elimination in the event of GVHD. The available clinical experience supports the conclusion that infusion of suicide gene-modified T cells is feasible. [8][9][10] Although the approach has been successful in several patients, impaired T cell function seems to be a major shortcoming. 10,11 Other problems such as low retroviral gene transfer efficiencies into primary T lymphocytes and gen- Correspondence: C Baum, Experimental Cell Therapy, OE5120, Department of Hematology and Oncology, Hannover Medical School, 30623