The efficiency of induction of B-ALL in young vs old BMM differs. (A) White blood cell (WBC) count × 10³/μL in the peripheral blood of young (red) or old (blue) nonirradiated BALB/c recipient mice on day 13 of transplantation of 2 × 10⁶ BCR-ABL1–transduced BM in the B-ALL model (P = .01; Student t test, n = 7-9). (B) Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ cells in peripheral blood of young (red) or old (blue) nonirradiated BALB/c recipient mice on day 13 of transplantation of B-ALL–initiating cells, as in panel A (P = .03; Student t test, n = 7). (C) Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ cells in peripheral blood of young (red) or old (blue) nonirradiated BALB/c recipient mice on days 13 and 19 of transplantation, as in panel A (P = .008; Student t test, n = 3). A stable or decreasing tumor load in peripheral blood due to migration of leukemia cells to lymphatic tissue has been described¹⁷ and is not unusual (unpublished data, D.S.K.). (D) Spleen weights of young (red) vs old (blue) nonirradiated BALB/c recipient mice on day 18 of transplantation of BCR-ABL1–transduced BM in the B-ALL model. The spleen weight is presented as organ weight (g) normalized by mouse body weight (g) (P = .03; Student t test, n = 7). Kaplan-Meier–style survival curves for young (red) or old (blue) BALB/c (E) and C57BL/6J (F) nonirradiated recipients of 2 × 10⁶ non–5-FU–pretreated BCR-ABL1–transduced BM cells in the retroviral transduction/transplantation model of B-ALL. The differences in survival were statistically significant (P = .02; log-rank test, n = 18-19; and P = .04; log-rank test, n = 7-8, respectively). Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ cells in peripheral blood on day 13 after transplantation (P = .03; Student t test, n = 7) (G) and Kaplan-Meier–style survival curve (P = .01; log-rank test, n = 7) (H) of young (red) or old (blue) nonirradiated BALB/c recipient mice transplanted with 1 × 10⁶ (young) or 2 × 10⁶ (old) B-ALL–initiating cells. The young mice had an average weight of 15 g and the old mice of 30 g. (I-J) Percentage of human CD45⁺ leukocytes in the peripheral blood of young vs old nonobese diabetic severe combined immunodeficiency interleukin-2 receptor γ knockout mice transplanted with 2 × 10⁶ human B-ALL cells on day 34 after transplantation (P = .007; Student t test, n = 9-10) (I) and over time (days 19 and 34 after transplantation) (J). Individual patients in panel I are indicated in distinct colors. (K) Kaplan-Meier–style survival curves for young (red) or old (blue) nonobese diabetic severe combined immunodeficiency interleukin-2 receptor γ knockout mice transplanted with 2 × 10⁶ human B-ALL cells (P = .04; log-rank test, n = 9-10). (L) Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ cells in the peripheral blood (PB), BM, spleen, and liver of young (red) or old (blue) nonirradiated BALB/c recipient mice transplanted with BCR-ABL1–transduced BM on day 18 after transplantation (P values as indicated; Student t test, n = 6-8). (M) Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ cells of total BM cells that homed to the BM (P = .008; Student t test) and spleen (not significant [n.s.]; Student t test) of young (red) vs old (blue) nonirradiated BALB/c recipient mice 18 hours after transplantation (n = 3) in the B-ALL model; 5 × 10⁶ BCR-ABL1–transduced BM cells had been transplanted.