Rapid immune reconstitution after a reduced‐intensity conditioning regimen and a CD3‐depleted haploidentical stem cell graft for paediatric refractory haematological malignancies

Summary The main obstacles to successful haploidentical haematopoietic stem cell transplantation from a mismatched family member donor are delayed immune reconstitution, vulnerability to infections and severe graft‐versus‐host disease (GvHD). We designed a reduced‐intensity conditioning regimen that excluded total body irradiation and anti‐thymocyte globulin in order to expedite immune reconstitution after a CD3‐depleted haploidentical stem cell transplant. This protocol was used to treat 22 paediatric patients with refractory haematological malignancies. After transplantation, 91% of the patients achieved full donor chimaerism. They also showed rapid recovery of CD3+ T‐cells, T‐cell receptor (TCR) excision circle counts, TCRβ repertoire diversity and natural killer (NK)‐cells during the first 4 months post‐transplantation, compared with those results from a group of patients treated with a myeloablative conditioning regimen. The incidence and extent of viremia were limited and no lethal infection was seen. Only 9% of patients had grade 3 acute GvHD, while 27% patients had grade 1 and another 27% had grade 2 acute GvHD. This well‐tolerated regimen appears to accelerate immune recovery and shorten the duration of early post‐transplant immunodeficiency, thereby reducing susceptibility to viral infections. Rapid T‐cell reconstitution, retention of NK‐cells in the graft and induction of low grade GvHD may also enhance the potential anti‐cancer immune effect.