HLA-Haploidentical TCR αβ/CD19-Depleted Hematopoietic Stem Cell Transplantation in Children with Fanconi Anemia

Introduction. Allogeneic HSCT currently represents the only consolidated curative approach for Fanconi anemia (FA) patients, with best results observed in the HLA-identical sibling setting. For patients lacking an HLA-matched related or unrelated donor, haploidentical HSCT virtually assures the opportunity for nearly all patients to benefit from HSCT, offering the advantage of immediate accessibility to the transplant procedure. In order to overcome the limitation of delayed immune recovery, historically associated with this type of allograft, in the last few years we developed a novel method of ex vivo graft manipulation, consisting of the negative depletion of T-cell receptor (TCR) αβ+ T-lymphocytes and CD19 B-cells from peripheral blood stem cells (PBSC) grafts (ClinicalTrial.gov identifier: NCT01810120) (Bertaina el al., Blood 2014). Here we report our analysis in a subgroup of FA patients given TCRαβ/CD19-depleted haploidentical HSCT at our Institution.

Patients and methods. Ten consecutive FA patients (6 girls and 4 boys) underwent a TCRαβ/CD19-depleted HSCT from an HLA-haploidentical relative between September 2011 and July 2015. Median age at diagnosis was 6.6 (range 2.7-22.0) years and median age at time of transplantation was 8.1 (range 4.4-22.2) years. The conditioning regimen included Cyclophosphamide 300 mg/m²/day and Fludarabine 30 mg/m²/day for 4 consecutive days (days -6 to -3), with 200 cGy single-dose TBI. Pretransplantation Fresenius^® ATG was administered at a dose of 4 mg/kg/day for 3 consecutive days (days -5 to -3) in order to prevent both graft failure and graft-versus-host disease (GVHD). All patients received Rituximab 200 mg/m² to reduce the risk of Epstein-Barr virus-related post-transplant lymphoproliferative disorders. Selective removal of TCRαβ+ and B-cells was performed on G-CSF-mobilized donor PBSC through labeling with biotinylated anti-TCRαβ antibodies and anti-CD19 antibodies, followed by incubation with anti-biotin antibodies conjugated to paramagnetic beads (CliniMACS; Miltenyi Biotec, Bergisch Gladbach, Germany). No immunosuppressive therapy was administered as post-transplantation prophylaxis against GVHD.

Results. The TCRαβ/CD19-depletedgrafts contained a median of 20.40 x10⁶/kg (range 15.80-33.40) CD34⁺ cells, 5.60 x10⁶/kg (range 1.78-69.60) CD3⁺ lymphocytes, 0.021 x10⁶/kg (range 0.002-0.043) TCRαβ+ lymphocytes, 5.60 x10⁶/kg (range 1.78-69.60) TCRγδ+ lymphocytes, 0.036 x10⁶/kg (range 0.013-0.079) CD20⁺ lymphocytes, and 45.30 x10⁶/kg (range 16.2-177.0) NK cells. Engraftment with sustained full donor chimerism was achieved in 9 out of 10 patients, the cumulative incidence of graft rejection being 10% (95% CI, 0-26.8). The patient who rejected his first allograft achieved a complete engraftment after a second HSCT from one-antigen mismatched unrelated donor. No secondary graft failures were observed. The median time for neutrophil and platelet engraftment was 12 days (range, 9-15) and 9 days (range, 8-12), respectively. No patient experienced acute or chronic GVHD in the follow-up period. No transplant-related deaths occurred in our cohort. With a median follow-up of 28 months (range 13.2-39.1), the Kaplan-Meier estimates of OS and DFS were both 100%, while the EFS probability was 90% (95% CI, 47.3-98.5).

Discussion. These data suggest that haploidentical HSCT after removal of TCRαβ+ and CD19+ lymphocytes is able to guarantee engraftment with excellent OS and DFS in patients affected by FA. Moreover, given the very low incidence of both acute and chronic GVHD, which has been shown to contribute to the increased risk of developing late post-transplantation malignancies in FA patients, this approach can be considered a very attractive option for FA patients in need of an allograft and lacking an HLA-identical family donor.