Inclusion Criteria Pre-emptive arm

        Children (18 years or younger) with CD19+ precursor B cell ALL fulfilling one of the
        following criteria who are undergoing an allogeneic stem cell transplant from an
        EBV-seropositive donor:

        In first remission, if at least one of the following criteria are met:

          -  t(9;22) and MRD positive (BCR-ABL/ABL ratio > 0.01%) after HR3 block of EsPhALL or
             pre-HSCT or

          -  Infant ALL age < 6 months at diagnosis with MLL gene rearrangement and either
             presenting wcc >300 x 10^9/L or poor steroid early response (i.e circulating blast
             count >1x10^9/L following 7 day steroid pre-phase of Interfant 06) or

          -  Resistant disease (> 30% blasts at end of induction treatment day 28-33) in subsequent
             morphological CR or

          -  High level bone marrow MRD (> 1 in 1000) at week 12 ALL-BFM 2000/AIEOP BFM ALL
             2009/EORTC 58951 protocols, week 12-15 of FRALLE A or at week 14 of UKALL2011

        Relapsed patients if at least one of the following criteria are met:

          -  Very early (< 18 months from diagnosis) bone marrow or extramedullary relapse in
             second CR or

          -  Early (within 6 months of finishing therapy) isolated bone marrow relapse with bone
             marrow MRD > 1 in 100 at day 35 of reinduction in second CR or

          -  Early (within 6 months of finishing therapy) bone marrow or combined relapse with high
             level bone marrow MRD (> 1 in 1000) at the end of consolidation therapy (week 12-13
             UKALL R3/INTREALL and COOPRALL protocols, prior to protocol M in BFM relapse protocol
             (ALL-REZ BFM 2002) and after Protocol II-IDA in AIEOP LLA Rec 2003)or

          -  Any relapse of infant or Philadelphia-positive ALL in morphological complete remission

          -  Any patient being transplanted in 3rd or greater CR

        These patients have a high (> 50%) risk of relapse and will be monitored for evidence of
        MRD in bone marrow aspirates (monthly for months 1-6, 6 weekly months 7.5-12 post HSCT) for
        the first year post-transplant. Patients who become MRD +ve in the marrow at a level
        minimum 5 x 10-4 (or BCR-ABL/ABL ratio 0.05% in Ph+ve ALL patients with no IgH MRD marker)
        but are in morphological remission (<5% blasts in BM) will be eligible to be treated
        pre-emptively with CD19ζ transduced CTL

        Prophylaxis arm

        Additionally, any patient (≤ 18 years) with ALL relapsing in the bone marrow (isolated or
        combined) after myeloablative allogeneic HSCT who achieves morphological remission after
        re-induction and who is a candidate for second HSCT at one of the participating centres is
        eligible to receive CD19ζ transduced CTL prophylactically

          -  Stem cell donors must be EBV sero-positive and HLA-matched (8/8 HLA A,B,C and DR at
             medium resolution typing) or a single antigenic/allelic (7/8) mismatch with the
             recipient

          -  A life expectancy of at least 12 weeks

          -  Karnofsky score of >60% if >10 years old or Lansky performance score of >60 if ≤ 10
             years old

          -  Patients must have transduced donor-derived EBV-specific CTLs with 15% or higher
             expression of CD19ζ determined by flow-cytometry which meet the specified release
             criteria

          -  Informed written consent indicating that patients are aware this is a research study
             and have been told of its possible benefits and toxic side effects

        Exclusion Criteria

          -  Patients with CD19 negative precursor B cell ALL

          -  EBV seronegative or > single antigenic/allelic HLA-mismatched donor

          -  Active acute GVHD overall Grade 2 or higher or significant chronic GVHD requiring
             systemic steroids at the time of scheduled infusion of transduced CTL will be excluded
             until the patient is GVHD-free and off steroids

          -  Pre-existing severe lung disease (FEV1 or FVC < 50% predicted) pre-HSCT or an oxygen
             requirement of >28% O2 supplementation or active pulmonary infiltrates on chest X-ray
             at the time scheduled for transduced CTL infusion

          -  Serum bilirubin >3 times the upper limit of normal or an AST or ALT > 5 times the
             upper limit of normal

          -  Serum creatinine >3 times upper limit of normal

          -  Active severe intercurrent infection at the time of transduced CTL infusion (if
             present consult with Chief investigator).

          -  Patients in whom transduced donor-derived EBV-specific CTLs don't meet release
             criteria

          -  Serologically positive for Hepatitis B, C or HIV pre-HSCT

          -  Females of childbearing age with a positive pregnancy test

          -  Known allergy to DMSO