Antitumor capacity of NK cells expressing mbIL15. (A) Results of 24-hour cytotoxicity assays with mbIL15- and mock-transduced NK cells from 9 donors against the Nalm-6, U937, K562, Daudi, SKBR3, and ES8 cell lines at 1:4 and 1:1 E:T ratio (15 experiments at each ratio; P < .001 for both). Results obtained with individual cell lines in 4-hour and 24-hour cytotoxicity assays are shown in supplemental Figure 2. (B) NK cells expressing mbIL15 have an increased release of lytic granules in the presence of target cells. Percentage of CD107a⁺ NK cells after 4-hour cytotoxicity assays at 1:1 E:T. Results with NK cells from 3 donors against 2 cell lines are shown (P = .002). (C) NK cells expressing mbIL15 exert antitumor activity in vivo. NOD/scid-IL2RGnull mice were i.p. injected with 1 × 10⁴ U937 cells labeled with luciferase. In 3 mice, no treatment was given (No NK), while 4 mice i.p. received mock-transduced NK cells (1 × 10⁷) on days 3 and 7, and 4 other mice received mbIL15-transduced NK cells at the same dose and schedule. Results of in vivo imaging of tumor growth are shown (ventral images). (D) Overall survival comparisons of mice in the different treatment groups. Mice were killed when bioluminescence reached 1 × 10¹¹ photons/second. P values for log-rank test of the 3 curves and for comparisons between each of 2 curves are shown.