3006 Background: Genetically modified T cells derived from distinct T cell subsets differ in the capacity to persist after adoptive transfer. We are conducting the first phase I/II clinical trial in which patients (pts) with CD19+ B cell malignancies receive T cells comprised of a defined composition of CD8+ TCM and CD4+ T cells engineered to express a CD19 CAR. Methods: CD8+ TCM and CD4+ T cells were separately enriched from each patient, transduced with a CD19 CAR lentivirus and expanded in vitro. The cell product for infusion was formulated in a 1:1 ratio of CD8+:CD4+ CAR+ T cells and infused at one of three dose levels (2x105 – 2x107 CAR-T cells/kg) after lymphodepleting chemotherapy. Results: Thirty-seven pts with ALL (n = 20), NHL (n = 14) or CLL (n = 3) have been treated and 33/37 received a product that conformed to a prescribed CD8+:CD4+ composition. There was no serious acute infusional toxicity. Severe cytokine release syndrome (sCRS) consisting of fever, hypotension, coagulopathy and neurotoxicity associated with elevated serum IFN-γ and IL-6 was only observed in ALL pts with high tumor burden. One ALL patient treated at the highest cell dose died of complications associated with sCRS. No NHL or CLL pts had sCRS. Eighteen of 20 ALL pts were evaluated for response, with 15 (83%) achieving complete marrow remission by high resolution flow cytometry. Clinical responses in NHL included complete (n = 1) or partial (n = 6) remission in 7/13 pts. Two of 3 CLL pts achieved marrow remission by flow cytometry. The peak level and duration of persistence of both CD4+ and CD8+ CAR-T cells were associated with clinical response. We are investigating the impact of distinct lymphodepletion regimens on CAR-T cell proliferation and persistencein vivo. A T cell immune response to the murine CD19-specific scFv component of the CAR transgene was detected in a subset of pts with limited CAR-T cell persistence. Conclusions: Adoptive immunotherapy with CD19 CAR-T cells of defined subset composition is feasible and safe in a majority of heavily pretreated pts with refractory B cell malignancies and has potent anti-tumor activity at low cell doses. CAR-T cell doses for phase II studies in ALL and NHL cohorts have been determined. Clinical trial information: NCT01865617.