CAR-T-19 Injection in the Treatment of CD19-positive Relapsed/Refractory B-ALL

This is a multiple-center, single-arm, open-label study. After meeting the eligibility criteria and enrolling on the trial, patients will undergo leukapheresis for collection of autologous lymphocytes, patients will then proceed to lymphodepleting chemotherapy with cyclophosphamide 300mg/m\^2 and fludarabine 30mg/m\^2 for 3 consecutive days followed by the infusion of CD19 CAR T-cells at a target dose of 2.5 x10\^6 cells/kg（range 0.8-2.5×10\^6 cells/kg）.