Clinical Outcomes after Allogeneic Hematopoietic Stem Cell Transplantation in Patients with Transfusion-Dependent β-Thalassemia Treated at the Bambino Gesù Children's Hospital, Rome, Italy

β-thalassemia is one of the most common monogenic blood disorders worldwide, and is highly prevalent in Mediterranean countries. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has been the only curative treatment for transfusion-dependent β-thalassemia (TDT; the most severe disease form) for many years, but it is limited by donor availability and has a significant risk of morbidity and mortality.

We conducted a chart review of patients with β-thalassemia who underwent allo-HSCT (N=80) at the Bambino Gesù Children's Hospital, Rome, Italy, between March 2011 and August 2018. Median (range) age at allo-HSCT was 5.5 (0.3-20.0) years [<12 years: n=70 (87.5%), ≥12-18 years: n=7 (8.8%), ≥18 years: n=3 (3.8%)] and all patients but one had TDT [n=79 (98.8%)]. Prior to allo-HSCT, patients received a median (range) of 17.4 (4-52) transfusions per year (n=64) and had a median (range) serum ferritin concentration of 1217 (135-9123) ng/mL and a median (interquartile range) hemoglobin level of 10.3 (9.3-11.4) g/dL. All patients had received regular iron chelation therapy prior to transplantation.

In total, 18 (22.5%), 28 (35.0%), and 34 (42.5%) patients received allo-HSCT from human leukocyte antigen (HLA)-identical sibling donors, HLA-haploidentical donors, and unrelated donors (fully matched donor: n=28, donor with a single HLA disparity: n=6), respectively. Of these donors, 42 (52.5%) were carriers for thalassemia-associated mutations. In total, 53 (66.3%) donors and 35 (43.8%) recipients were cytomegalovirus-positive. Bone marrow was the stem cell source in 51 cases (63.8%), while 28 patients received an alphabeta T-cell depleted peripheral blood haploidentical HSCT (35.0%); the remaining child (1.3%) received both bone marrow and cord blood from the same related donor.

All patients continued to receive transfusions immediately after allo-HSCT; however, only 7 (8.8%) received a transfusion in the 3 to 12-month post-transplantation period (2 due to underlying disease; 5 due to other reasons including GI bleeding). Median (range) time to reach transfusion-free status was 3.8 (1.1-47.8) weeks. Median (interquartile range) hemoglobin levels at 6 and 12 months after allo-HSCT were 10.9 (10.2-11.9) and 11.9 (10.6-13.0) g/dL, respectively.

The cumulative incidences of primary and secondary graft failure were 10.0% and 12.5% at 24 months (HLA-identical donor: 0% and 11.1%, haploidentical donor: 17.9% and 3.6%, unrelated donor: 8.8% and 20.6%). Eleven out of 14 patients experiencing graft failure were successfully rescued with a second allograft, while 2 patients were not retransplanted due to parental decision and 1 patient died after the engraftment of the second allograft. Eight patients developed grade II-IV acute graft-versus-host disease (GVHD) and one patient developed moderate chronic GVHD. Cumulative incidence rates of grades II-IV and III-IV acute GVHD were 12.7% and 8.0% at 24 months (HLA-identical donor: 0% and 0%, haploidentical donor: 7.3% and 0%, unrelated donor: 23.8% and 18.8%). Three patients (3.8%) died of transplant-related causes (1 case each of hemophagocytic lymphohistiocytosis, sepsis, and multi-organ failure [the patient receiving the second allograft]) with a median (range) time from transplantation to death of 8.7 (3.7-11.0) months. Of these patients, all had been transplanted from an unrelated donor and 2 had reached sustained full-donor chimerism. The probability of overall and event-free (event defined as either death or primary/secondary graft failure) survival was 96.2% and 81.2% at 24 months (HLA-identical sibling donor: 100% and 88.9%, haploidentical donor: 100% and 78.6%, unrelated donor: 91.2% and 79.4%). The probability of thalassemia-free survival (event defined as either death or primary/secondary graft failure not rescued by a second allograft) was 93.7% at 24 months (HLA-identical sibling donor: 100%, haploidentical donor: 92.9%, unrelated donor: 91.2%).

In this large single-center cohort of children with predominantly TDT, allo-HSCT led to beneficial outcomes for most patients, resulting in the discontinuation of transfusions with 93.7% of patients being thalassemia-free. Nevertheless, HSCT is still associated with GVHD, graft failure, and mortality, and only 22.5% of patients had an HLA-identical sibling donor, illustrating a key limitation of allo-HSCT. Emerging research is addressing such barriers to treatment.