Increased plasma IL-15 level and CAR-expressing NK cells by repetitive administration of CAR NK cells. (A) Experimental scheme for quantification of IL-15 production and CAR NK cell persistence in vivo. (B) Bioluminescent imaging of tumor-bearing mice injected with the indicated CAR NK cells; n = 3. 70-IL-15(2) indicates mice treated with 2 dosages of CD70-CAR-IL-15 NK cells. (C) The quantification of tumor luminescence measured at the indicated days after tumor implantation and NK cell injection. ∗P < .05, ∗∗P < .01, and ∗∗∗P < .001; n = 3. P values were determined by 2-way ANOVA. (D) Serial measurement of IL-15 levels in mouse serum on the indicated days; n = 3. (E) The percentage of CAR-expressing NK cells in the peripheral blood of mice on day 15 after injection, analyzed by flow cytometry. ∗∗P < .01, ∗∗∗P < .001, and ∗∗∗∗P < .0001. P values were determined by 2-way ANOVA; n = 3. (F) Representative flow cytometry plots showing the percentage of tumor (BFP⁺) and NK (CD56⁺) cells in the bone marrow at day 25 after the first CAR NK cell injection (top). Representative flow cytometry plots showing the percentage of bone marrow CAR⁺ cells gated in CD56⁺ NK cells at day 25 after the first CAR NK-cell injection (bottom).