Donor T Cells Genetically Modified with a Novel Suicide Gene (inducible Caspase 9, iC9) Expand and Persist over Time after Post-Allograft Infusion in Patients Given αβ T-Cell and B-Cell Depleted HLA-Haploidentical Allogeneic Stem Cell Transplantation (αβ haplo-HSCT) Contributing to Accelerate Immune Recovery

Background: αβ haplo-HSCT is an effective option for children with non-malignant disorders (Bertaina et al., Blood 2014) or acute leukemia (Locatelli et al. Blood 2017) and in need of an urgent allograft. In patients given this type of HSCT, recovery of adaptive immunity is, however, suboptimal. With the aim of accelerating reconstitution of adaptive immunity, we evaluated an innovative approach based on post-transplant infusion of titrated numbers of donor αβ T cells, transduced with the suicide gene iC9 (BPX-501 cells). Since the transduced gene contains sequences for the CD19 marker, BPX-501 cells are CD3+/CD19+ and, thus, can be easily tracked in peripheral blood. We report data on immune recovery of children transplanted at Bambino Gesù Children's Hospital, Rome, and enrolled in a multicenter prospective phase I-II trial on BPX-501 cells (sponsored by Bellicum Pharmaceuticals®, ClinicalTrials.gov identifier: NCT02065869).

Patients and study design: Included in this analysis were 108 children. Median age at transplant was 5.6 years (range, 0.3-18) and the male/female ratio was 61/47. Patients were subdivided according to the original disorder in 5 groups: group A (53 children with acute leukemia), group B (22 with primary immune-deficiency, PIDs), group C (14 with erythroid disorders), group D (7 with Fanconi anemia) and group E (12 with other diseases). No patient was given any post-transplant graft-versus-host disease (GvHD) prophylaxis. Nine children were enrolled in the phase I portion of the trial consisting of 3 cohorts, receiving escalating doses of BPX-501 cells of 2.5x10⁵, 5x10⁵, and 1x10⁶ cells/kg, respectively. The remaining 99 patients included in the phase II portion received 1x10⁶ BPX-501 cells/kg. Per protocol, BPX-501 cells were to be infused within the first month after αβ haplo-HSCT. We evaluated the absolute number of circulating CD3+, CD3+/CD4+, CD3+/CD8+, CD19+, and NK cells at 1, 2, 3, 6, 9, 12 and 24 months after αβ haplo-HSCT. We also analyzed the number of BPX-501 cells, αβ T cells and γδ T cells at the same time points. The impact of patient gender, age (0-2, 2-6, 6-12 and >12 years), original disease, use of total body irradiation (TBI) during conditioning regimen, occurrence of cytomegalovirus (CMV) reactivation, of acute GvHD and of leukemia relapse (only in group A patients), on recovery of different lymphocyte subsets was investigated, as well. Data were expressed as mean value+SEM (standard error of mean). Six patients given Rimiducid, the agent activating iC9, were censored at time of drug infusion.

Results: After infusion, BPX-501 cells expanded progressively reaching a peak at 9 months after the allograft, when their mean value was 144+36/μL. In the 16 patients with 2-year follow-up, the mean number of BPX-501 cells was still 62+23/μL. Details on expansion/persistence of BPX-501 cells, as well as on recovery of CD3+, CD4+ and CD8+ T cells, are shown in Figure 1A and 1B, respectively. A mean number of CD3+ cells greater than 1.000/μL was reached at 6 months after transplantation. Figure 1C depicts the progressive recovery of B cells. NK and γδ T cells promptly recovered after the allograft; the number of αβ T cells overcomes that of γδ T cells at 2 months after transplant. CMV reactivation promoted a statistically better expansion of BPX-501 cells (see also Figure 1D). This effect was paralleled by an improved recovery of both CD3+ and αβ T cells in patients who experienced CMV reactivation. A statistically higher number of both CD3+ and αβ T cells was observed at 6 and 12 months after αβ haplo-HSCT in patients who did not receive TBI. Neither acute GvHD nor leukemia recurrence occurrence statistically affected the number of BPX-501 cells. Children with an age comprised between 0-2 years or with PIDs had a better recovery of both CD3+ and αβ T cells at 6, 9 and 12 months after the allograft.

Conclusions: Our results indicate that BPX-501 cells infused after αβ haplo-HSCT expand in vivo and persist over time, contributing to fasten adaptive immunity recovery. Their peak of expansion is reached at 9 months after the allograft, but they are consistently detected also at 2 years after infusion. CMV reactivation is the main driver of BPX-501 cell expansion, this finding suggesting that BPX-501 cells cooperate to the viral clearance. Overall, the whole pattern of immune recovery of these 108 children is improved as compared with that of patients given αβ haplo-HSCT without BPX-501 cell infusion.

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