Long-Term Immunological Profile of Patients Treated with Haploidentical HSCT and TK-Cells to Study the Requirements of Memory T Cell Persistence

BACKGROUND: Suicide gene therapy applied to haploidentical hematopoietic stem cell transplantation (haplo-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.

RESULTS: We studied 9 adult patients who underwent haplo-HSCT and infusion of purified suicide-gene modified donor T cells (median dose: 1.9x10⁷ cells/kg, range:0.9x10⁶-39.5x10⁶) for high-risk hematologic malignancies between 1995 and 2010 (TK patients). At a median follow-up of 7,4 years (range 3.2-12.3), all patients are in complete remission. Two out of 9 patients (22%) experienced GvHD in the early phase post immune reconstitution; in all cases, ganciclovir (GCV) 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.

A complete recovery of NK cells, B lymphocytes and αβ or γδ T cells was observed. The CD8+ and CD4+ T cell compartment of TK patients were characterized by level of naïve and memory cell comparable to age and sex matched healthy controls. The quantification of CD4+ CD31+ CD62L+ CD45RA+ CD95- recent thymic emigrants and measure of single joint T-cell receptor excision circles demonstrated that the normalization of the T cell compartment was supported by a completely recovered thymic output.

TK-cells were detected in all patients (100%), at low levels (median=4cells/uL). Ex vivo selection of pure TK-cells after polyclonal stimulation and LNGFR-purification confirmed the presence of functional transduced cells, thus directly demonstrating the ability of memory T cells to persist for years. Of notice TK-cells could be retrieved also in patients successfully treated with GCV for GvHD, thus confirming the selective action of GCV only on proliferating TK-cells. Accordingly, GCV sensitivity was preserved in long-term persisting TK-cells, independently from their differentiation phenotype. TK-cells circulating in patients displayed a memory phenotype comprising effector memory (T_EM), central memory (T_CM) and stem memory (T_SCM) T cells and exhibited a low level of Ki-67 positivity, thus suggesting the maintenance of a pool of gene modified memory cells through homeostatic proliferation.

The number of TK-cells circulating 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 months after infusion, suggesting that antigen recognition is dominant in driving in vivo expansion and persistence of memory T cells. We evaluated whether the phenotype of infused TK-cells was able to affect the long-term fate of gene-modified memory T cells. We observed that the number of infused T_SCM cells positively correlated with early TK-cell expansion and with their long-term persistence, suggesting that T_SCMmight play a privileged role in the generation of a long-lasting immunological memory.

CONCLUSION: These data show that a complete and physiological donor-derived immune system is restored in adult surviving long-term after suicide gene therapy. After infusion, gene modified cells persist for up to 12 years in treated patients. This setting can be exploited to investigate the requirements at the basis of the generation of a long-lasting immunological memory in vivo. 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.