Testing Phase of Trial:

        Inclusion Criteria:

          -  Pre-B-ALL in complete remission (CR), <5% blasts on most recent bone marrow aspirate
             determined by morphologic assessment, with an intention to proceed to allogeneic HSCT.
             Eligible participants can be in 1st CR or greater. Presence of detectable MRD by flow
             cytometry or other techniques in patients that are in morphologic remission prior to
             transplant is permitted.

          -  Detectable MRD measured by flow cytometry or other molecular techniques is acceptable
             for enrollment in patients with <5% blasts.

          -  Patients with either Philadelphia chromosome positive or negative B-ALL are eligible

          -  Documented expression of CD19 on the lymphoblast population as measured by
             flow-cytometry if patient has received prior CD19-directed therapy.

          -  Eligibility for HSCT along with conditioning regimen and donor selection will be
             determined according to the treating centre's policy.

          -  Patients must be age ≥1 years and have a baseline performance status of ECOG ≤ 2
             (adult) or Lansky ≥ 50% (pediatric).

          -  Patient with chronic hepatitis B (Hep B surface antigen or Hep B Core antibody
             reactive) are eligible if they are receiving treatment to prevent reactivation (e.g.
             lamivudine, tenofovir) and have undetectable serum Hepatitis B DNA

          -  Patients (or legally acceptable designate) must provide written consent.

          -  Female patients of childbearing potential should be willing to use 2 methods of birth
             control or be surgically sterile, or abstain from heterosexual activity for the course
             of the study from the time of informed consent signature date until 3 months after
             completion of study treatment. Patients of childbearing potential are those who have
             not been surgically sterilized or have not been free from menses for > 1 year.

        Exclusion Criteria:

          -  Inability to comply with study procedures.

          -  Active central nervous system (CNS) involvement or other extramedullary disease at the
             time of enrollment.

          -  Uncontrolled infection until resolved.

          -  Burkitt lymphoma/leukemia or mixed phenotype leukemia.

          -  Chronic infection with Hepatitis C. Previously treated Hepatitis C with undetectable
             Hepatitis C RNA for six months or longer is acceptable.

          -  HIV 1/2 Infection.

        Treatment Phase of Trial:

        Inclusion Criteria:

          -  Detectable MRD ≥ 10^-4 leukemic cells/TNC on a bone marrow aspirate done on day +56,
             +100, +180 or day +270.

          -  Morphologic remission on bone marrow from same date (on day +56, +100, +180 or day
             +270)

          -  Patients must be age ≥1 years and have a baseline performance status of ECOG ≤ 2
             (adult) or Lansky ≥ 50% (pediatric) documented within 7 days of enrollment.

          -  Patients with either Philadelphia chromosome positive or negative B-ALL are eligible

          -  Documented expression of CD19 on the lymphoblast population as measured by
             flow-cytometry if patient has received prior CD19-directed therapy.

          -  Patient with chronic hepatitis B (Hep B surface antigen or Hep B Core antibody
             reactive) are eligible if they are receiving treatment to prevent reactivation (e.g.
             lamivudine, tenofovir) and have undetectable serum Hepatitis B DNA

          -  Adequate organ, liver and renal function including: Total bilirubin ≤ 1.5 x upper
             limit of normal (ULN), eGFR >30 mL/min/1.73 m, Alkaline phosphatase ≤ 2.5 x ULN, Serum
             lipase ≤ 1.5 x ULN

          -  Patients (or legally acceptable designate) must provide written consent.

        Exclusion Criteria:

          -  Active acute GVHD (grade II-IV) or active moderate-severe chronic GVHD (NIH Grade) at
             the time of MRD detection are ineligible treatment phase until GVHD resolves or
             quiescent as determined by the treating physician.

          -  Uncontrolled infection until resolved.

          -  Chronic infection with Hepatitis C. Previously treated Hepatitis C with undetectable
             Hepatitis C RNA for six months or longer is acceptable.

          -  HIV 1/2 Infection.

          -  Extramedullary or CNS disease or the time of MRD detection.