Role of FLT3 ligand in regeneration of B-cell progenitors after BM transplantation. Lethally irradiated adult WT mice were transplanted with 2 × 10⁵ unfractionated BM cells from 9- to 10-week-old WT mice, whereas FL^−/− recipient mice were transplanted with 2 × 10⁵ unfractionated BM cells from FL^−/− mice. In all experiments, donor- and recipient-derived BM cells could be separated based on expression of different CD45 isoforms. The frequency and absolute numbers of donor-derived B-cell progenitors in the BM of transplanted mice were determined 16 weeks after transplantation. (A) FACS profiles from representative mice showing donor-derived (gated on CD45.1 vs CD45.2) pre-pro-B (B220⁺CD43⁺AA4.1⁺CD19⁻), pro-B (B220⁺CD43⁺AA4.1⁺CD19⁺), and pre-B (B220⁺CD43⁻IgM⁻) cells. Numbers indicate percentage of total donor-derived cells within the indicated gates or quadrants. (B) Bar graphs show mean (SEM) numbers of donor-derived pre-pro-B, pro-B, and pre-B cells (per 2 tibiae and 2 femora) from 7 recipient mice of each genotype from 1 of 2 representative experiments. **P < .01, ***P < .001, compared with WT mice.