Tisagenlecleucel (CTL019) is an anti-CD19 autologous Chimeric Antigen Receptor (CAR) T-cell therapy, which has shown dramatic early results in advanced ALLs. Early loss of B-cell aplasia (recovery of B-cells in marrow/ peripheral blood within 6 months after infusion), a marker of the loss or non-functionality of the CAR T-cells, is associated to a very high risk of relapse. A reinfusion of CTL019, even after Fludarabine-Cyclophosphamide reconditioning, frequently fails to induce further expansion as observed in UPENN studies and in the Robert Debré Hospital experience. Non-persistence of CAR T-cells may be due to immune- mediated rejection or environment-mediated suppression of their growth. Evidence for increased PD-1 expression in CAR T-cells between infusion and peak expansion has been demonstrated in clinical samples. Preclinical data and few clinical data support a role of PD- 1-PD-L1 blockade in improving the effectiveness of CAR T-cell therapy. The objectives of this phase I/II study is to determine the safety, efficacy and feasibility of Nivolumab (Opdivo®)- an anti-PD1 treatment- combined to tisagenlecleucel in a cohort of relapsed or refractory B-ALL patients, aged 1-25 years old, previously treated by tisagenlecleucel (Kymriah®), with a demonstrated early loss of B-cell aplasia (within 6 months), a surrogate marker of the loss of CAR T-cells or their non- functionality. More specifically, the main objectives are: • In cohort 1 that includes patients with a MRD negative disease status combined to an early loss (within 6 months) of B-cell aplasia : To determine the optimal starting time of Nivolumab (Opdivo®) in terms of safety and efficacy among 4 candidate time points (day 14, day 11, day 5, and day - 1). • In cohort 2 that includes relapsed patients with an early loss (within 6 months) of B-cell aplasia : To estimate the feasibility in terms of safety and efficacy of a very early start of nivolumab (day-1), prior to the reinfusion of tisagenlecleucel