Background aims. Haplo-identical hematopoietic stem cell transplantation (HSCT) with add-back of donor lymphocytes
expressing the herpes simplex virus thymidine kinase suicide gene (TK cells) is one of the most widely applied promising new
gene therapy approaches. However, the immunological status of added-back TK cells after HSCT has yet to be well
characterized. Methods. We investigated TK cells through the use of flow cytometry, T-cell receptor (TCR) Vb repertoire
spectratyping and linear amplification-mediated polymerase chain reaction followed by insertion site analysis in a patient
enrolled in our clinical trial. Results. A comparison of onset with remission of acute graft-versus-host disease confirmed that
TK cells were predominantly eliminated and that proliferative CD8þ non-TK cells were also depleted in response to
ganciclovir administration. The TCR Vb-chain repertoire of both TK cells and noneTK cells markedly changed after
administration of ganciclovir, and, whereas the TCR repertoire of noneTK cells returned to a normal spectratype long after
transplantation, that of TK cells remained skewed. With the long-term prophylactic administration of acyclovir, TK cells
oligoclonally expanded and the frequency of spliced variants of TK cells increased. Known cancer-associated genes were not
evident near the oligoclonally expanded herpes simplex virus (HSV)-TK insertion sites. Conclusions. We demonstrate
obvious differences in immunological status between TK cells and non-TK cells. In addition, we speculate that long-term
prophylactic administration of acyclovir increases the risk of oligoclonal expansion of spliced forms of TK cells.