Long-Term T Cell Expansion Results in Increased Numbers of Central Memory T Cells with Sustained Functional Properties for Adoptive T Cell Therapy

Success of adoptive T cell therapy (ATT) is dependent on sufficient numbers of T cells and the characteristics of the final T cell product. In several studies, clinical grade CD19 CAR T cell products could not be generated from about 6-30% patients, particularly if they were isolated from older or heavily pretreated diffuse large B cell lymphoma (DLBCL) patients. In cyclophosphamide/fludarabine-lymphodepleted patients with persistent or progressive disease a sequential second dose of T cells has been shown to be effective resulting in tumor regression. Here we investigated to what extend T cell numbers could be increased via prolonged expansion with standard cytokines IL-7/IL-15 and how transcriptome and function of central memory T cells (Tcm) longitudinally change during culture. Method: Murine and human T cells were cultured with the cytokine combination IL-7/IL-15. Short-term expanded (ST, one week) and long-term expanded (LT) CD8+ (4 weeks) and CD4+ (3 weeks) T cells were compared for proliferation capacity (CFSE), extent of apoptosis (AnnexinV), up-regulation of T cell inhibitory receptors (TIRs) and cytokine expression pattern after in vitro re-stimulation upon anti-CD3/CD28 stimulation. Further, RNA sequencing of ST and LT expanded murine CD8+ and CD4+ Tcm followed by unsupervised hierarchical clustering, principal component analysis (PCA) and differential expression analysis was performed. In vivo mouse models were used to analyze engraftment, persistence and anti-tumor capacity applying our bioluminescent dual-luciferase reporter mouse (BLITC - bioluminescent imaging of T cells) allowing us to monitor migration, expansion (RLuc luciferase) and activation (NFAT-driven Click-beetle luciferase) of adoptively transferred T cells in vivo. Finally, we analyzed the expansion and in vitro properties of T cells from healthy donors and DLBCL patients. Results: There was a 50-fold increase of T cells in LT vs. ST culture, the Tcmproportion was extended and stem cell markers were comparable or even higher expressed in LT expanded T cells. Differential analysis revealed 2786 (CD8) and 912 (CD4) with statistically significant expression alterations with generally only moderate effect size when comparing LT and ST expanded T cells. Interestingly, the dynamically modified genes largely overlapped for CD8 and CD4 T cells suggesting culture-associated changes. Comparable RLuc signals and T cells counts in peripheral lymph nodes (LN) and spleen indicate similar eng