CD19CAR T cell products of defined composition can be reproducibly generated from heavily treated leukemic pediatric patients
O. Finney1, H. Brakke1, S. Rhea1, R. Hicks1, B. Futrell1, D. Doolittle1, D. Li3, R. Gardner2, M. Jensen1
1Seattle Children's Research Institute, Seattle, WA; 2Seattle Children's Hospital, Seattle, WA 3Juno Therapeutics, Seattle, WA
Chimeric antigen receptor (CAR) T cell therapy has shown remarkable results in the treatment of leukemia, yet the prerequisites of a successful therapy are largely unknown. Using specimens from the Pediatric Leukemia Adoptive Therapy study (NCT02028455), we studied the impact of patient-intrinsic and T-cell intrinsic factors on CD19CAR T cell function in vivo.
Apheresed PBMC were immunomagnetically purified into CD4 and CD8 T cell subsets, activated with antiCD3x28 CTS beads, lentivirally transduced, then propagated in homeostatic cytokines. Following lymphodepleting chemotherapy patients were dosed with a 1:1 ratio of CD4:CD8 CD19CAR+ T cells. Here we report on the phenotypic and functional attributes of the starting CD4 and CD8 T cell repertoire and infusion product as analyzed by multiparametric flow cytometry.
T cells isolated from heavily treated patients exhibited a high degree of variability with respect to differentiation/lineage and activation/exhaustion markers. The manufactured products however displayed enhanced homogeneity of markers and were enriched for CCR7+ and CD27+ cells. In vivo, CAR T cell expansion was observed in 42/43 patients, and 40/43 patients had a documented MRD-negative CR within 21 days. The percentage of final product cells expressing CD45RA correlated with peak expansion, but this was not related to therapeutic outcome or graft longevity, as measured by B cell aplasia (BCA). However, the percentage of cytokine-producing final cellular CD8+ product cells positively correlated with BCA duration. Patient-intrinsic factors, such as CD19 antigen burden at time of lymphodepletion, had the largest positive impact on BCA. Strategies to extend CAR T cell persistence are focused on delivering vaccinations of CD19-expressing T cells to re-induce proliferation of CAR T cells in vivo.