NKTR-255 increases the proliferation, survival, and expansion of CAR T cells in the presence of antigen. Human CD19 CAR T cells were generated from healthy donors (n = 2-4) and assayed on days 14 to 16 after start of manufacturing. CD8⁺ and CD4⁺ CAR T cells were independently cocultured with CD19-expressing K562 cells at the indicated E:T ratios and concentrations of NKTR-255 for 4 days. CD8⁺ (A) and CD4⁺ (B) CAR T cells were labeled with CFSE before coculture, and the percentage of divided cells was determined by flow cytometry using CFSE dilution. (C) BCL-2 expression (MFI) in CD8⁺ (top) and CD4⁺ (bottom) CAR T cells was measured by intracellular flow cytometry. (D) Fold expansion of CD8⁺ (top) and CD4⁺ (bottom) CAR T cells was determined by the fold change in absolute cell counts from days 0 to day 4 by flow cytometry using counting beads. (B-E) Figures show mean ± SEM.