Background: Despite the manageable safety and encouraging efficacy of donor-derived CD7 chimeric antigen receptor (CAR) T cells in relapsed or refractory T-cell acute lymphoblastic leukemia (r/r T-ALL) (Pan et al. J Clin Oncol 2021;39:3340-3351), a considerable proportion of responding patients eventually relapsed with CD7 antigen loss. CAR T cells targeting another antigen, CD5, which is expressed on blasts of over 80% T-ALL cases, may be capable of treating these patients. Here we present early safety and efficacy results of a phase I trial of donor-derived CD5 CAR T cells in T-ALL. Methods: CD5 CAR T cells that resist fratricide by deletion of CD5 gene (Preclinical data in Dai et al. Mol Ther 2021;29:2707-2722) were manufactured. Patients with prior stem cell transplantation (SCT) (group A) received CAR T cells from prior SCT donors, while patients without SCT history (group B) received CAR T cells from new donors who also provided stem cells for transplantation post CAR T therapy. The trial using bayesian optimal interval phase I/II design to explore optimal biological dose (OBD) from the initial dose of 1×10⁶ (±20%) CAR T cells/kg in each group. If manufactured cells were not sufficient, patients could be treated at a low dose of 5×10⁵ (± 20%)/kg. The primary endpoint was safety with efficacy secondary. Results: Five patients who had CD7-negative relapsed after CD7 CAR therapy were enrolled and received prior SCT donor-derived CD5 CAR T cells between Oct 8^th, 2021 and Dec 14^th, 2021. Four patients were on initial dose level, while one patient was on low dose level. This early report has approved by the Data and Safety Monitoring Committee, and indicated the OBD in group A was 1×10⁶ (±20%) /kg. No DLT occurred. Adverse events within 30 days included grade 3-4 hematologic toxicity in all (100%) which already existed before enrollment, grade 1-2 cytokine release syndrome in 4 (80%), grade 1 graft-versus-host disease in 1 (20%), grade 2 rash maculo-papular in 3 (60%). One patient developed late grade 5 epstein-barr virus infection accompanied by hemophagocytic lymphohistiocytosis at 2.7 months post-infusion. All five patients achieved complete remission at day 30, and remained MRD-negative with a median follow-up time of 2.7 (range, 1.8-4.1) months. In three patients, low level of CD7 CAR T cells were still detectable for one month post-infusion, but CD5 CAR T cells dominated and persisted well until last visit. Patient CD5-positive healthy T cells were depleted, while CD5-negative T cells increased to a median count of 133 (range, 13-299)/μl at one month, although they were still much lower than normal level. Conclusions: We report the safety and efficacy of donor-derived CD5 CAR T cells in r/r T-ALL. And the depletion of CD5-positive healthy T cells was commonly observed. Longer follow-up assessment is needed to determine the durable remission and the functional immune system reconstitution. Clinical trial information: NCT05032599.