Defined Central Memory and Stem Memory T Cell Phenotype of CD4 and CD8 CAR T Cells for the Treatment of CD19+ Acute Lymphoblastic Leukemia in an Automated Closed System

Relapsed and refractory B-precursor acute lymphoblastic leukemia (B-ALL) remains a major therapeutic problem. Chimeric antigen receptor (CAR) modified T cells targeting CD19 are promising treatment options for these patients with the potential to induce hematological remission in adult and pediatric patients with refractory B-ALL. Despite the promising data, some patients do not respond to T-cell treatment. Until now it is not possible to fully understand and predict critical factors for response or non-response, but proliferation and persistence of CAR T cells in vivo is an essential precondition for treatment efficacy. Central memory T cells (Tcm) and stem cell-like memory T cells (Tscm) are known to be the best candidates for a sustained in vivo expansion after T-cell therapy with small cell doses. Therefore, we set up a protocol for the generation of anti-CD19 CAR T cells in a closed system that is compliant to current GMP regulations. Starting samples were mononuclear cells from pediatric ALL patients at diagnosis and under chemotherapy using up to 100cc peripheral blood. After separation for CD4+/CD8+ cells, T cells were activated with anti-CD3/CD28 beads. The lentiviral vector encoded the anti-CD19 single-chain variable fragment, 4-1BB (CD137) co-stimulation and T cell receptor (TCR) zeta chain. The whole process including separation of T cells, activation, transduction and cultivation was performed in a closed and fully automated system. Despite a broad variety in cellular composition including high blast counts, low cell numbers and a rather exhausted phenotype in the starting fraction, a robust T-cell composition was achieved at day five after activation with a mean of 63% CD4+ and 37% CD8+ T cells and a transduction rate of up to 38 %. The vast majority of CAR T cells were of a Tcm (47%) and Tscm (44%) phenotype leading to a strong proliferative potential of more than 100-fold expansion. In addition, a reduced sensitivity to inhibitory signals was documented (programmed cell death protein 1 (PD-1) expression ≤10%). CAR T cells showed effective cytotoxic functionality when co-cultured with CD19+ target cells with only little background of the un-transduced control. At an effector to target ratio of 5:1 up to 80 % of the CD19+ target cells were killed. In addition, a significant release of Interferon gamma (IFN-ɣ), Tumor necrosis factor (TNF-α) and Interleukin-2 (IL-2) was detected upon recognition of the target cell lines, confirming a strong and target-specific Th1 response. In conclusion, generation of CAR T cells from small pediatric blood samples was feasible in a closed GMP-compatible fully automated system. Despite variety of cell numbers, cellular composition and T cell phenotype in the starting sample, a uniform T cell product of Tcm and Tscm could be produced with a balanced CD4 / CD8 ratio leading to high expansion potential and functionality of the T cell graft.