Transduction and selection of human T cells with novel CD34/thymidine kinase chimeric suicide genes for the treatment of graft-versus-host disease

Clinical trials evaluating the herpes simplex virus thymidine kinase (HSV-tk)/ganciclovir (GCV) suicide gene therapy system for the control of graft-versus-host disease (GVHD) have been limited by low transduction efficiencies and inefficient selection procedures. In this study, we designed and evaluated a novel chimeric suicide gene consisting of the extracellular and transmembrane domains of human CD34 and full-length HSV-tk (ΔCD34-tk). High-efficiency transfer of ΔCD34-tk to primary human T cells was accomplished after a single exposure to VSV-G-pseudotyped, Moloney murine leukemia virus-based retrovirus 48 h after activation of human PBMCs with anti-CD3 and anti-CD28 antibodies immobilized on magnetic beads. Using an optimized 5-day transduction and selection procedure, transduction efficiencies averaged 71%, with isolation purities greater than 95% and yields exceeding 90%. The immunoselected T cells were selectively eliminated by GCV (IC₅₀ ∼3 nM), maintained a normal subset composition, exhibited a polyclonal TCR Vβ family repertoire, and contained 5 or 6 vector copies per transduced cell when optimally transduced. No increase in GCV sensitivity was observed upon incorporation of highly active mutant HSV-tk enzymes into the ΔCD34-tk suicide gene. T cells modified with the ΔCD34-tk gene using the optimized protocol should improve the overall efficacy of the HSV-tk/GCV suicide gene therapy method of GVHD control.