Abelson-transformed PI3Kδ^−/− cells induce leukemia in mice with an increased latency. (A) Kaplan-Meier plot of RAG2^−/− mice after transplantation of 10⁶ transformed cells (3 independently derived cell lines per genotype were injected into n = 8 for PI3Kδ^+/− and n = 6 for PI3Kδ^−/−). Mice that had been injected with transformed PI3Kδ^−/− cells developed leukemia significantly later as determined by a log-rank test (median survival, 13 vs 20 days; P = .005). (B) H&E stains of blood smears (top), spleens (middle), and livers (bottom) from RAG2^−/− mice after injection of v-abl–transformed cells (magnification, ×100, Zeiss AxioImager 21 [Jena, Germany], 10× objective, NA 0.25, air; camera: Pixelink Color, 1600 × 1200; software: PixelINK Capture 3.0). (C) Spleen, bone marrow, and blood were analyzed for infiltration with CD19⁺CD43⁺ leukemic cells by FACS. Tissue infiltration was slightly lower in mice that had received PI3Kδ^−/− cells without reaching statistical significance (data represent means ± SEM). (D) [⁵¹Cr]-release assay using IL-2–expanded wt NK cells as effectors and Abelson-transformed leukemic cells as targets. PI3Kδ^−/− leukemic cells were significantly better eradicated by wt NK cells than PI3Kδ^+/− leukemic target cells at any effector-target cell ratio tested (for ratios 20:1, 10:1, 5:1, and 2:1, P = .02, P = .02, P = .003, and P = .006 as determined in an unpaired 2-tailed t test). (E) Quantification of pan–Rae-1, Mult-1, and MHC I expression by FACS of in vitro–derived Abelson-transformed cell lines. PI3Kδ^−/− leukemic cells showed a significantly higher surface expression of pan-Rae1 when compared with PI3Kδ^+/− leukemic cells (mean fluorescence intensity [MFI] of 1082 ± 199 vs 639 ± 259; P = .035 in an unpaired 2-tailed t test; n = 4 for each genotype). n.s., indicates P > .05; *P < .05; and **P < .01.