CD123 is an excellent target in primary AML in vivo. (A) Primary AML xenograft model. NSGS mice were sublethally irradiated on day −1 and injected with 5 to 10 × 10⁶ primary AML blasts via the tail vein on day 0. Engraftment was confirmed by flow cytometric measurement of live mouse CD45^neg human CD45^dim CD123⁺ cells in the peripheral blood, usually occurring around day 14. Mice were then injected with CART123 cells or control T cells (CART19 or UTD T cells) and bled weekly to quantify AML burden. (B) Analysis of peripheral blood from mice 8 to 15 days after receiving T cells demonstrated marked reduction of circulating UPN 024 blasts. Note that residual CD45^bright T cells are poorly detectable in some CART123 mice at this time point, correlating with the rapid rise and fall of peripheral CART123 cells in response to clearance of AML; P < .0001 (Student t test). (C) Composite survival plot of mice from 3 independent experiments. (D) In vivo upregulation of CD123 on AML blasts. Upon injection into NSGS mice treated with control UTD T cells, the UPN034 sample upregulated CD123 expression in comparison with blood measurements obtained prior to (day 13 [D13], left) and after (day 27 [D27], right) T-cell injection. The CD45^bright CD123⁻ population represents adoptively transferred human T cells. MFI of CD123 is shown at top right. All mice receiving control T cells subsequently died of disease, and all CART123-treated mice were long-term survivors (these are a subset of the mice in Figure 4C). Results are representative of 3 independent experiments.