Suicide gene therapy applied to allogeneic hematopoietic stem cell transplantation (allo-HSCT) is one of the widest clinical applications of gene therapy. By the infusion of donor lymphocytes transduced to express the Herpes Simplex Virus Thymidine Kinase (TK) suicide gene, patients achieve a rapid immune reconstitution and substantial protection against tumor recurrence. TK-cells are promptly eliminated in case of graft versus host disease (GvHD), with complete resolution of the adverse reaction. In previous studies, we showed that TK-cell infusions are necessary and sufficient to promote the generation of a fast, polyclonal and full competent T cell repertoire. In the present work we characterize the immunological profile of a cohort of long-term survivors after suicide gene therapy and we studied the long-term fate of TK-cells to shed light on memory T cell dynamics after transplantation.


We studied 14 adult patients who underwent allo-HSCT (haploidentical HSCT: n=11; HLA-identical HSCT n=3) and infusion of purified suicide-gene modified donor T cells (median dose: 1.9x107 cells/kg, range:0.9x106-2.8x108) for high-risk hematologic malignancies between 1995 and 2010. At a median follow-up of 8,7 years (range 3-17), all patients are in complete remission. Five out of 14 patients experienced GvHD in the early phase post immune reconstitution; in all cases, ganciclovir administration proved effective in abrogating the adverse reaction. No symptoms or complications related to GvHD were observed during the long-term follow up, and none of the patient is receiving immunosuppressive drugs.

We observed a complete recovery of NK cells, comprising of mature (CD56+CD16+) and immature (CD56+CD16-) NK cells. Interestingly the proportion of B cells circulating long-term in patients was significantly higher than that observed in age-related healthy controls (p<0.0001). Full recovery of CD3, including CD4 and CD8 cell counts was observed in this long-term analysis. The youngest patients (age range: 22-34 years) showed naïve and memory frequencies similar to age-matched controls. Conversely, in oldest patients (age range: 44-66 years) the frequency of naïve T cells was inferior to age-matched healthy subjects (p=0.0038), and was compensated by a larger proportion of central memory and effector memory cells. Nevertheless, we observed a high percentage of recent thymic emigrants, suggesting a full recovery of thymic output not only in young but also in old patients. Stem memory CD4 and CD8 T cell counts were similar to that of healthy controls, independently from age. CMV-specific T cells, quantified by dextramer staining, were detected in CMV+ patients.

TK-cells were detected in the majority of analyzed patients (90%), at low levels (median=0,43%±6,9%). Ex vivo selection of pure TK-cells after polyclonal stimulation and NGFR-purification confirmed the presence of functional transduced cells, thus directly demonstrating the ability of memory T cells to persist for years. The proportion of TK-cells detectable at the longest follow-up did not correlate with the number of infused cells, nor patients or donors’ age, but instead with the peak of TK-cells observed within the first 3 months after infusion, suggesting that antigen recognition is dominant in driving in vivo expansion and persistence of memory T cells. Of notice TK-cells could be retrieved also in patients successfully treated with ganciclovir for GvHD, thus confirming the selective action of ganciclovir only on proliferating TK-cells. Accordingly, ganciclovir sensitivity was preserved in long-term persisting TK-cells, independently from their differentiation phenotype.

While infused TK-cells displayed a predominant effector memory phenotype, gene modified T cells persisting long-term were enriched for central memory (CD45RA-CD62L+) and stem memory (CD45RA+CD62L+CD95+) phenotypes, suggesting the higher ability of these T cell subsets to persist and shape the immunological profile long-term in treated patients.


These data show that a complete donor-derived immune system is restored in adult surviving long-term after suicide gene therapy. After infusion, gene modified cells persist for up to 14 years in treated patients. Further studies on TK-cell TCR repertoire and vector integrations are currently being performed to elucidate the in vivo dynamics of infused memory T cells.


Valtolina:MolMed S.p.A: Employment. Traversari:MolMed S.p.A: Employment. Bordignon:MolMed S.p.A: Employment. Bonini:MolMed S.p.A: Consultancy.

Author notes


Asterisk with author names denotes non-ASH members.

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