Critical role of FLT3 ligand in B-cell regeneration after BM transplantation. Lethally irradiated adult WT mice were transplanted with 2 × 10⁵ unfractionated BM cells from 9- to 11-week-old WT mice (WT into WT), and FL^−/− recipient mice were transplanted with 2 × 10⁵ unfractionated FL^−/− BM cells (FL^−/− into FL^−/−). In all experiments, donor- and recipient-derived blood cells could be separated based on expression of different CD45 isoforms (“Materials and methods”). Donor-derived B- and T-cell reconstitution of PB at (A) 3, (B) 5, and (C) 16 weeks after transplantation. FACS plots show donor-derived B-cell (B220⁺ or B220⁺IgM⁺) and T-cell (CD4/CD8⁺) reconstitution from representative mice transplanted with WT and FL^−/− cells. Numbers indicate percentage of total donor-derived cells within the indicated quadrants. Bar graphs show mean (standard error of mean [SEM]) values from 19-27 recipient mice of each genotype, from 3-4 independent experiments. Donor-derived B cells (B220⁺CD19⁺) in the spleen (D) and BM (per 2 tibiae and 2 femora) (E) 16 weeks after transplantation. Mean (SEM) values from 9-21 recipients and 2-3 independent experiments. **P < .01, ***P < .001, compared with WT mice.