New insights into hematopoietic cell transplantation in ALL: Who should be transplanted, when, and how

Advancements in multi-agent chemotherapy through clinical trials conducted by multi-institutional collaborative groups worldwide, along with risk stratification using molecular genetic features and treatment responses, including minimal residual disease, have significantly improved outcomes for children and adolescents with acute lymphoblastic leukemia (ALL) over the past half-century. However, for a certain number of high-risk patients, including those with relapsed or refractory disease, for whom existing chemotherapy alone is insufficient for cure, allogeneic hematopoietic cell transplantation (HCT) has provided a potential opportunity for leukemia cure. For these patients, the appropriate selection of donor and stem cell source, conditioning regimen, timing of transplantation, and comprehensive supportive care, including effective graft-versus-host disease prophylaxis, are prerequisites for successful HCT. While HCT from a human leukocyte antigen (HLA)-matched sibling has traditionally been the preferred option, less than 25 % of patients currently have such a donor in developed countries. Consequently, alternative donor HCT options, such as those from matched unrelated donors identified through high-resolution HLA typing, unrelated cord blood donors, and more recently, haploidentical donors using post-transplant cyclophosphamide or TCRαβ+/CD19+ cell-depleted grafts, are providing broader access to HCT for patients lacking matched sibling donors. Nonetheless, HCT carries the risk of various acute and late toxicities. In particular, the use of myeloablative conditioning with total body irradiation, a standard in pediatric ALL, is associated with significant long-term sequelae. As our understanding of the pathophysiology of the disease improves and novel molecular targeted therapies and immunotherapies are developed, the indication for HCT in pediatric ALL is becoming more selective, leading to a gradual decrease in the number of transplants performed. However, further optimization and evolution of allogeneic HCT are needed to both maximize its anti-leukemia effects and minimize transplant-related complications, as there remain cases that undoubtedly require HCT for the cure of leukemia.