(A) Representative spleens of Rag2^−/−γ_c^−/− mice 3 weeks after receiving an i.v. infusion of 1 × 10⁶ MEC1 or MEC1-ROR1 cells. The spleen of an age-matched, nonengrafted Rag2^−/−γ_c^−/− mouse (Nor) is shown for comparison. (B) MEC1 or MEC1-ROR1 cells were collected from the marrow or spleens of mice engrafted 3 weeks earlier with MEC1 or MEC1-ROR1 cells. The fluorescence of cells stained with 4A5–Alexa Fluor 647 (ordinate) and anti-CD19-PE (abscissa) are shown in the contour plots. The percentages at the top right of each contour plot indicate the proportions of cells with fluorescence above the threshold indicated by the dotted line. (C) Representative spleens of Rag2^−/−γ_c^−/− mice 3 weeks after receiving an i.v. infusion of 1 × 10⁶ MEC1-ROR1 cells and treatment with either Ctrl-IgG or UC-961. The spleen of an age-match nonengrafted Rag2^−/−γ_c^−/− mouse (Nor) is shown for comparison. (D) MEC1-ROR1 cells were collected from the marrow or spleens of mice engrafted 3 weeks earlier with MEC1-ROR1 and then treated with either Ctrl-IgG or UC-961, as indicated on the right. Cells were stained with 4A5–Alexa Fluor 647 and anti–CD19-PE to identify the MEC1-ROR1 cells, as in panel B. Percentages in the top right of each contour plot indicate the proportions of cells with fluorescence above the threshold indicated by the dotted line. (E) Bars indicate the average numbers of CD19⁺ human leukemia cells harvested from the marrow (left) or spleen (right) of mice engrafted 3 weeks earlier with MEC1 cells (white bars) or MEC1-ROR1 cells (black bars). Some groups of animals were treated with Ctrl-IgG or UC-961, as indicated at the bottom of each histogram. Data are shown as mean ± SD (n = 5). **P < 0.01; ***P < 0.001, as determined by 2-tailed Student’s t test.