Young BMM macrophages produce higher levels of CXCL13. (A) Western blot of protein lysates of macrophages (MΦ) from young vs old mice stained with an antibody to CXCL13 (13 kDa). The macrophages had been cultured for 3 days. The blot is representative of 3 independent western blots. Concentration of CXCL13 pg/mL in flushed bone (P = .007; Student t test) (B) or CM from macrophages (P < .0001; Student t test) (C) from healthy young (red) vs aged (blue) mice (n = 3). Macrophages had been cultured for 7 days. (D-F) Concentration of CXCL13 pg/mL in the plasma of peripheral blood (P = .03; Student t test, n = 3-4) (D), flushed BM (P = .03; Student t test, n = 4-5) (E), or CM of macrophages (P = .05; Student t test, n = 3) (F) from young (red) vs old (blue) nonirradiated BALB/c recipient mice transplanted with BCR-ABL1–transduced BM on day 18 after transplantation. The macrophages had been cultured for 3 days before the harvest of the CM. The P values are as indicated (Student t test, n = 3). (G) Percentage of GFP⁺ (BCR-ABL1⁺) BP-1⁺ CXCR5⁺ cells of total leukocytes in the peripheral blood (PB), BM, and spleen of young (red) vs old (blue) mice with fully established B-ALL (day 20 after transplantation) (P = .03 in PB and spleen; Student t test, n = 3). (H) MFI of CXCR5 on BCR-ABL1⁺ BA/F3 cells and the human B-ALL cell lines NALM6, SMS-SB, Reh, RCH-ACV, and SUP-B15 (n = 5). (I) Percentage of CXCR5⁺ cells of human CD45⁺ CD19⁺ cells from the peripheral blood of healthy individuals vs patients with B-ALL (n = 5-9).