Introduction Current available treatments are limited once patients with B-ALL relapse following allogeneic hematopoietic stem cell transplantation (allo-HSCT). While chimeric antigen receptor (CAR) T-cell therapy offers a chance of remission, long-term outcomes for these patients remain poor. The benefit of bridging into a second transplant after CAR T-cell therapy remains inconclusive and available data are limited. Here, we report the long-term outcomes of 23 B-ALL patients who chose to undergo a second allo-HSCT after achieving complete remission (CR) from CAR T-cell therapy. Methods From April 2017 to April 2020, 23 R/R B-ALL patients (median age of 20 years, ranging from 3 to 58 years) who relapsed after first allo-HSCT received CAR T-cell therapy. The data were aggregated from seven different clinical trials (www.clinicaltrials.gov NCT03173417, NCT02546739 NCT03825718, NCT03825731, NCT03952923, NCT04100187 and www.chictr.org.cn ChiCTR1800016541). Patients' first transplant sources were HLA-identical sibling (n=5), matched-unrelated donor (MUD) (n=1), and haploidentical donors (haplo) (n=17). Eight of the 23 patients had disease relapse within 6 months following the first transplant. The median time from first transplant to CAR T-cell infusion was 261days (range: 117~2181 days). Before CAR T-cell infusion, patients' median bone marrow (BM) blasts by morphology were about 72.5% (1.5%-94.5%) including 12 patients with BM blasts >70% (5 with BM blasts >90%). Three of the 23 patients (13%) had received at least one prior donor lymphocyte infusion. No patients had active graft-versus-host disease (GVHD) prior to CAR T-cell therapy. Second generation CAR T-cells were generated by using purified T-cells from transplant donors (n=15) or patients (n=8). Twenty-two patients received T-cells modified with CD19-targeting CAR T-cells containing either a 4-1BB (n=18) or a CD28 co-stimulatory domain (n=4), and one patient received CD19-CD22 dual specificity CAR T-cells. All patients received a conditioning regimen of IV fludarabine (30mg/m2/d) and cyclophosphamide (250mg/m2/d) for 3 days followed by a single CAR T-cell infusion with a median dose of 3×105 cells/kg (1×105-6×105 cells/kg) in 21 patients. Two patients received a second CAR T-cell infusion in 2-3 months (1/3×105 cells/kg dose). Post CAR-T therapy, all patients bridged into a consolidation second transplantation with conventional myeloablative pre-transplantation conditioning regimens including 15 patients who received total body irradiation-based and 7 patients that received a busulfan-based conditioning regimen. Cyclosporin A, short-term methotrexate, and mycophenolate mofetil were used for GVHD prophylaxis. Results Patients' characteristics are shown in Table 1. On Day 30 post CAR-T-cell infusion, 23/23 (100%) patients achieved minimal residual disease (MRD)-negative CR. A total of 16/23 (69.6%) patients developed cytokine release syndrome (CRS) of which 14/23 (60.9%) had Grade I-II and 2/23 (8.7%) had Grade III CRS. Two patients had Grade III neurotoxicity. All patients with MRD-negative status subsequently bridged into a second transplant (2 from MUD and 21 from haplo donors) with a median interval time of 67 days (39- 329 days) from CAR T-cell therapy to a second transplant. At a median follow-up time of 258 days (84-978 days), no patients relapsed, which was encouraging. Five of 23 patients (21.7%) died from transplant-related mortality (TRM) at a median time of 295 days (103-372 days) (1 from GVHD and 4 from infection). The 1-year overall survival (OS) was 68.0% and 2-year OS was 54.4% (Fig.1). While there was a trend towards a more efficacious OS for patients whose CAR T-cells were derived from donors rather than from patients themselves but the number are too small to reach statistical significance (1-year OS 83.9% vs. 64.3%, 2-year OS 83.9% vs. 42.9%, P=0.739. Fig.2). After the 2nd transplant, four patients developed GVHD. Conclusions Our study demonstrates that even for R/R B-ALL patients who have relapsed following a first allo-HSCT , an MRD-negative CR status can still be achieved through CAR T-cell cell therapy without increasing CRS or neurotoxicity, making consolidation second allo-HSCT feasible for these patients. CAR T-cell therapy combined with a consolidation second HSCT are effective for these heavily pre-treated patients with an encouraging prospect for long-term survival. Disclosures No relevant conflicts of interest to declare.