UCB-derived T cells genetically modified to express CD19R and TK genes are selectively eliminated by ganciclovir in vitro and in vivo. (A) UCB-derived T cells expressing TK gene can be eradicated by 5 μM GCV in vitro. The control Hy⁺TK^– UCB-derived T-cell line was genetically modified with the plasmid HyMP1-pEK.²¹  Initially, 4 × 10⁴ T cells plated/well in triplicate, and after 14 days the average viable cell count is presented as percentage of mean viable cells ± SD. (B) UCB-derived CD8⁺ CD19R⁺ffLucHyTK⁺ T cells can be ablated by GCV in vivo. T cells were stimulated in vivo with CD19⁺ tumor and exogenous rhIL-2 to promote T-cell persistence. Data are shown for 5 mice in each group (broken lines indicate mice that received PBS; solid lines, mice that received GCV). The background bioluminescence (mice that were injected with luciferin but which received no T cells) over the 12-day experiment for both the groups of mice receiving GCV or PBS was approximately 3 × 10⁶ ± 10⁶ a p/s/cm²/sr (mean ± SD) as represented by the shaded box. Genetically modified T-cell in vitro ffLuc-enzyme activity was 1.6 ± 0.1 CPM/cell (mean ± SD), compared with 0.008 ± 0.004 CPM/cell (mean ± SD) background ffLuc-enzyme activity in ffLuc^neg T cells.