Inhibition of STAT5 targets Lmo2^Tg pre-LSCs. (A-C) Treatment schematic and absolute numbers of DN3a T-cell progenitors (A), levels of activated STAT5 (pSTAT5) in DN3a cells (B) and representative flow cytometry analysis of T-cell populations (C) in the thymus of 6-week old Lmo2^Tg mice following treatment with vehicle or pimozide. Mean ± SEM, Student t test; ∗∗P < .01 and ∗∗∗P < .001 compared with vehicle. (D) Pre-LSC frequency within the DN3a thymocyte population of Lmo2^Tg mice treated with vehicle or pimozide assessed by limiting dilution assays. Mice were scored positive when T-cell lineage reconstitution was more than 1%, as previously described.⁵⁶ Pre-LSC frequencies (95% confidence intervals) were calculated from 2 biological replicates. (E) Treatment schematic and absolute numbers of DN3a cells in the thymus of 6-week-old Lmo2^Tg mice following administration of 2 rounds of vehicle and pimozide, alone or combined with VXL chemotherapy. Mean ± SEM, 2-way ANOVA with Tukey correction test; ∗∗∗P < .001 compared with vehicle; ^###P < .001, as compared with VXL. (F-G) Fold expansion of DN3a thymocytes (F) and immunophenotype of donor-derived (Cd45.2⁺) thymocytes (G) from Lmo2^Tg mice treated with either vehicle, pimozide, VXL, or combination therapy, analyzed 8 weeks posttransplant into sublethally irradiated Cd45.1⁺ recipients. Immature CD4⁻CD8⁻ (DN), CD4⁺CD8⁺ DP, and mature CD4⁺CD8⁻ and CD4⁻CD8⁺ SP populations are indicated. Mean ± SEM, 2-way ANOVA with Bonferroni correction test; ∗P < .05, ∗∗P < .01, and ∗∗∗P < .001 compared with vehicle; ^##P < .001 compared with VXL.