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BACKGROUND. Chimeric antigen receptor (CAR) T cell immunotherapy has resulted in complete remission (CR) and durable response in highly refractory patients. However, logistical complexity and high costs of manufacturing autologous viral products limit CAR T cell availability. METHODS. We report the early results of a phase I/II trial in B cell acute lymphoblastic leukemia (B-ALL) patients relapsed after allogeneic hematopoietic stem cell transplantation (HSCT) using donor-derived CD19 CAR T cells generated with the Sleeping Beauty (SB) transposon and differentiated into cytokine-induced killer (CIK) cells. RESULTS. The cellular product was produced successfully for all patients from the donor peripheral blood (PB) and consisted mostly of [CD3.sup.+] lymphocytes with 43% CAR expression. Four pediatric and 9 adult patients were infused with a single dose of CAR T cells. Toxicities reported were 2 grade I and 1 grade II cytokine-release syndrome (CRS) cases at the highest dose in the absence of graft-versus-host disease (GVHD), neurotoxicity, or dose-limiting toxicities. Six out of 7 patients receiving the highest doses achieved CR and CR with incomplete blood count recovery (CRi) at day 28. Five out of 6 patients in CR were also minimal residual disease negative (MRD). Robust expansion was achieved in the majority of the patients. CAR T cells were measurable by transgene copy PCR up to 10 months. Integration site analysis showed a positive safety profile and highly polyclonal repertoire in vitro and at early time points after infusion. CONCLUSION. SB-engineered CAR T cells expand and persist in pediatric and adult B-ALL patients relapsed after HSCT. Antileukemic activity was achieved without severe toxicities. TRIAL REGISTRATION. ClinicalTrials.gov NCT03389035. FUNDING. This study was supported by grants from the Fondazione AIRC per la Ricerca sul Cancro (AIRC); Cancer Research UK (CRUK); the Fundacion Cientifica de la Asociacion Espanola Contra el Cancer (FC AECC); Ministero Della Salute; Fondazione Regionale per la Ricerca Biomedica (FRRB)., Introduction Substantial efforts over the past few years have led chimeric antigen receptor (CAR) T cell therapy to success in relapsed and refractory (r/r) B cell malignancies. The early concept [...]