FLT3 ligand is dispensable for adult HSC reconstitution and expansion after BM transplantation. (A) Experimental design for HSC reconstitution and expansion experiments: 2 × 10⁵ BM cells from 9- to 11-week-old WT and Fl^−/− mice (CD45.1) were transplanted into lethally irradiated (900-975 cGy) adult WT and Fl^−/− primary (1°) recipients (CD45.2), respectively. At 16 weeks after transplantation, 5 × 10⁶ BM cells from 1° recipients were transplanted into secondary (2°) lethally irradiated WT (CD45.2) recipients along with 5 × 10⁵ WT competitor BM cells (CD45.2). (B) Test cell–derived total (Tot), B, T, and myeloid (M) reconstitution in PB at 16 weeks after transplantation in 1° recipients, presented as total number of each cell type per milliliter blood. Data represent mean (SEM) values from 30 to 35 recipient mice of each genotype, from 5 independent experiments. (C) Mean (SEM) total number of test cell–derived myeloid (Mac-1⁺) cells in the BM (2 tibiae and 2 femora) of WT and Fl^−/− 1° recipients at 16 weeks after transplantation (13 recipient mice per genotype from 2 independent experiments). (D) Mean (SEM) total number of test cell–derived LSKCD150⁺ cells in the BM (2 tibiae and 2 femora) of WT and Fl^−/− 1° recipients at 16 weeks after transplantation (7-8 recipient mice per genotype). (E) Test cell–derived PB total, B-, T-, and M-cell reconstitution 12 to 15 weeks after transplantation of 2° recipients. Data are expressed as mean (SEM) percentage of test cell reconstitution (20-22 recipient mice per genotype, from 3 independent experiments). (F) Mean (SEM) total number of test cell–derived myeloid (Mac-1⁺) cells in the BM (2 tibiae and 2 femora) of WT and Fl^−/− 2° recipients at 14 weeks after transplantation (7-8 recipient mice per genotype). (G) Mean (SEM) total number of test cell–derived LSKCD150⁺ cells in the BM (2 tibiae and 2 femora) of WT and Fl^−/− 2° recipients at 14 weeks after transplantation (7-8 recipient mice per genotype). ns indicates not significant; **P < .01, ***P < .001.