Patients with relapsed or refractory CD 19+ leukemia who have achieved remission after CD19 CAR-T cell treatment sometimes relapse because the CD 19 CAR-T cells decrease in number over time. Study PLAT-03 will test whether administering T cell antigen presenting cells (T-APCs) at intervals following treatment with CAR-T cells improves CD 19 CAR-T cell persistence and reduces the incidence of leukemia relapse.