T Cell Suicide Gene Therapy Prompts Thymic Renewal in Adults After Haploidentical Hematopoietic Stem Cell Transplantation in the Absence of Post-Transplant Immunesuppression

The genetic modification of T cells with a suicide gene grants a mechanism of control of Graft-versus-Host Disease (GvHD), allowing safe infusion of donor lymphocytes after partially HLA-incompatible Hematopoietic Stem Cell Transplantation (HSCT). In the TK007 phase I-II clinical trial, which enrolled a total of 54 adults with hematologic malignancies, 22 of the 28 treated patients experienced a rapid and sustained immune recovery after T cell-depleted HSCT and serial infusions of purified donor T cells expressing the Herpes Simplex Virus Tymidine Kinase suicide gene (TK cells; Ciceri and Bonini et al., Lancet Oncology, 2009). In these patients, after a first wave of circulating TK cells, the majority of T cells supporting long-term immune reconstitution did not carry the suicide gene and displayed high numbers of naïve lymphocytes, leading us to hypothesize a thymus-dependent development of T cells, occurring only upon TK cell engraftment.

Thymic function was investigated in a total of 31 patients enrolled in the TK007 trial (median age 55 years), which were compared to a cohort of adult patients receiving non T cell-depleted haploidentical transplantation (n=31), and to healthy pediatric and adult subjects. T cell subsets and the proportion of CD31⁺ recent thymic emigrants amongst CD4 naïve T cells were measured by immunophenotypic analysis. Single joint T cell Receptor Excision Circles (sjTREC) were quantified by qPCR. The volume of the biologically active thymus was assessed by chest CT scans. Serum concentration of cytokines was assessed by a multiplex luminex-based assay. Pathogen-specific immunity was quantified by interferon-γ ELISpot.

After the infusion of TK cells we documented a significant increase in peripheral blood sjTRECs as compared to the pre-HSCT determination (p = 0.02), suggesting an improved thymic output. Importantly, in line with that, only in TK007 patients almost the totality of CD4 naïve T cells circulating after transplantation were CD31⁺, thus bona fide recent thymic emigrants (89.54±9.55% at immune reconstitution, 81.84±15.9% at 6 months after HSCT, and 79.55±16.66% at 12 months after HSCT). Accordingly, a substantial expansion of the active thymic tissue was observed at chest tomography scans as compared to the pre-HSCT counterparts (p < 0.0001). A peculiar observation, possibly linked to the renewal of thymic activity and unique to the TK007 patients who achieved immune reconstitution, was the documentation of a peak in the serum level of interleukin-7, reproducibly occurring after each infusion of suicide gene-modified cells and anticipating the appearance of the newly generated T cells. Ultimately, the development of a wide repertoire of T cells in the patient thymus from donor precursors ensured a long-term protective immunity against pathogens, as exemplified by the preservation of a physiological and protective response against viruses both ex vivo and in vivo, even after the elimination of the infused TK cells in case of GvHD.

Our data from TK007 patients show that the infusion of genetically modified donor T cells after transplantation can drive the recovery of thymic activity in adults, leading to long-term immune reconstitution. On the lead of the encouraging biological and clinical results of the phase I-II clinical trial, demonstrating a dramatic decrease in late infectious mortality, a multicenter, phase III clinical trial (TK008 study) to assess the efficacy of TK cells in the context of haploidentical HSCT for leukemia started in 2010 at the San Raffaele Institute, and is currently expanding to multiple centers throughout Europe and US. Main endpoints of this randomized phase III trial are disease free survival and overall survival. The first TK008 patients randomized to receive suicide gene-modified cells showed recovery of thimyc activity and concomitantly achieved a rapid and robust T cell immune reconstitution.

Bonini:MolMed SpA: Consultancy.