A clinical trial evaluated the safety and efficacy of CD19 CAR-T cells manufactured from defined CD4+ and CD8+ T cell subsets in adults with B cell acute lymphoblastic leukemia (B-ALL). The study enrolled 32 patients who received CD19 CAR-T cells in a defined CD4+CD8+ composition after lymphodepletion chemotherapy. The defined composition product was remarkably potent, with 27 of 29 patients (93%) achieving bone marrow (BM) remission. High CAR-T cell doses and tumor burden increased the risk of severe cytokine release syndrome (CRS) and neurotoxicity. Serum biomarkers were identified to test early intervention strategies in high-risk patients. Risk-stratified CAR-T cell dosing based on BM disease burden reduced toxicity. CD8+ T cell-mediated immune responses against the CAR transgene limited CAR-T cell persistence and increased relapse risk. Adding fludarabine to the lymphodepletion regimen improved CAR-T cell persistence and disease-free survival. The study demonstrated that a defined composition CAR-T cell product enabled identification of factors correlating with CAR-T cell expansion, persistence, and toxicity. This approach facilitated the design of lymphodepletion and CAR-T cell dosing strategies that mitigated toxicity and improved disease-free survival. The trial registered on ClinicalTrials.gov (NCT01865617) and was funded by various grants. The results showed that CD19 CAR-T cells of defined composition achieved a high rate of complete remission (CR) and disease-free survival, particularly in patients with high BM tumor burden. The study also highlighted the importance of selecting defined T cell subsets for CAR-T cell manufacturing to achieve reproducible potency and to identify correlations between cell dose and efficacy or toxicity. The findings suggest that CD19 CAR-T cells of defined composition can be successfully manufactured and administered to patients following allogeneic hematopoietic cell transplantation (HCT). The study underscores the potential of CD19 CAR-T cells in treating refractory B-ALL, including those with extramedullary disease or relapse after HCT. The results also emphasize the need for further research to understand the mechanisms of relapse and to optimize CAR-T cell dosing and lymphodepletion regimens to improve therapeutic outcomes.A clinical trial evaluated the safety and efficacy of CD19 CAR-T cells manufactured from defined CD4+ and CD8+ T cell subsets in adults with B cell acute lymphoblastic leukemia (B-ALL). The study enrolled 32 patients who received CD19 CAR-T cells in a defined CD4+CD8+ composition after lymphodepletion chemotherapy. The defined composition product was remarkably potent, with 27 of 29 patients (93%) achieving bone marrow (BM) remission. High CAR-T cell doses and tumor burden increased the risk of severe cytokine release syndrome (CRS) and neurotoxicity. Serum biomarkers were identified to test early intervention strategies in high-risk patients. Risk-stratified CAR-T cell dosing based on BM disease burden reduced toxicity. CD8+ T cell-mediated immune responses against the CAR transgene limited CAR-T cell persistence and increased relapse risk. Adding fludarabine to the lymphodepletion regimen improved CAR-T cell persistence and disease-free survival. The study demonstrated that a defined composition CAR-T cell product enabled identification of factors correlating with CAR-T cell expansion, persistence, and toxicity. This approach facilitated the design of lymphodepletion and CAR-T cell dosing strategies that mitigated toxicity and improved disease-free survival. The trial registered on ClinicalTrials.gov (NCT01865617) and was funded by various grants. The results showed that CD19 CAR-T cells of defined composition achieved a high rate of complete remission (CR) and disease-free survival, particularly in patients with high BM tumor burden. The study also highlighted the importance of selecting defined T cell subsets for CAR-T cell manufacturing to achieve reproducible potency and to identify correlations between cell dose and efficacy or toxicity. The findings suggest that CD19 CAR-T cells of defined composition can be successfully manufactured and administered to patients following allogeneic hematopoietic cell transplantation (HCT). The study underscores the potential of CD19 CAR-T cells in treating refractory B-ALL, including those with extramedullary disease or relapse after HCT. The results also emphasize the need for further research to understand the mechanisms of relapse and to optimize CAR-T cell dosing and lymphodepletion regimens to improve therapeutic outcomes.