Relapse after treatment with CD19 CAR T cells. (A) Cumulative incidence of CD19⁺ (red line) and CD19^– (blue line) relapse after CD19 CAR T-cell therapy. (B-C) Patients who subsequently developed CD19⁺ relapse had a lower peak (4.34 vs 5.18 log₁₀ transgene copies per μg of DNA; P = .034) and AUC from day 28 to day 90 (AUC28-90; 5.47 vs 6.60 log₁₀ transgene copies per μg of DNA; P = .0042) of CAR T cells in the blood compared with those who developed CD19^– relapse. (B) Each thin line represents a single patient; the bold lines represent the averaged data using LOESS curve fitting approximation with the standard error shown in pink and light blue. (D) Swimmer plots demonstrating CAR T-cell counts in the blood from day 0 to day 600 in patients who either did not relapse (left) or who relapsed with CD19⁺ (center) or CD19^– (right) disease. The timing of B-cell recovery (Ο), relapse (+), allogeneic HCT (⋄), and death (Δ) are shown. Patients 17, 27, 29, and 34 died on days 608, 818, 1079, and 1174, respectively. All patients who developed CD19^– relapse (right panel) had detectable levels of CAR T cells in the blood at the last time point before relapse and ongoing B-cell aplasia. CAR T-cell counts in the blood were measured by qPCR to detect FlapEF1α and are reported as transgene copies per μg of DNA in the color-coded legend. Patients 24, 25, 26, 28, 30, 32, 33, 34, 35, and 36 received a second infusion of CAR T cells after relapse.