The starting day of NKTR-255 affects in vivo efficacy of human CD19 CAR T cells. NSG mice were injected IV with 5 × 10⁵ Raji cells. Seven days later, tumor-bearing mice received 0.8 × 10⁶ CD19 CAR T cells (1:1, CD8⁺:CD4⁺) IV. Cohorts of mice (n = 5 per group) received either CAR T cells only, or CAR T cells and NKTR-255 at 0.3 mg/kg IV starting on day 7, 14, or 21 after CAR T-cell infusion followed by weekly doses. One control group received NKTR-255 without CAR T cells. (A) Schematic of the experimental setup. (B) Bioluminescence imaging of Raji tumor burden at indicated time points. (C) Average tumor radiance grouped by the NKTR-255 start day. (D) Kaplan-Meier survival curve. Log-rank (Mantel-Cox tests) were used to compare differences between groups. (E) AUC of CD8⁺ (left) or CD4⁺ (right) CAR T-cell counts obtained by flow cytometry from days 0 to 28. Mann-Whitney tests were used to compare the differences between the groups. ns, not significant.