The purpose of this study is to test if the CD19 CAR T cell product tisagenlecleucel (also referred to as CTL019, or Kymriah™) is safe and has beneficial effects in children and young adults who have newly diagnosed high-risk B-cell Acute Lymphoblastic Leukemia (HR B-ALL) that remains MRD-positive after 2 cycles of chemotherapy. The study is enrolling children and young adults between the ages of 1-25 who have received induction and consolidation chemotherapy for HR B-ALL but were still found to have remaining disease (MRD). If you agree to join in this study, you will get one dose of the tisagenlecleucel cells administered by intravenous infusion and be followed for leukemia response. This study also involves blood draws and assessments. Reimbursement for travel may be provided. If you have questions or would like to learn more about the study, please contact the Cancer Immunotherapy Program at (215) 425-7193. The Principal Investigator is Dr. Shannon Maude and the Immunotherapy Nurse Navigator is Brooke Leibfreid, RN.