FLT3 ligand is redundant for maintenance of MZB and B1 cell compartments with age and post-transplantation. Representative FACS plots of IgM⁺CD23^loCD43⁺ B1 (A) and IgM^hiCD21^hiCD23^loIgD^lo MZB (B) cells from spleens of 12- to 15-week-old WT and FL^−/− mice. Numbers shown represent frequencies of gated cell populations of total spleen cells. Mean (SEM) numbers of total IgM⁺ B cells (C), B1 cells (D), and MZ B cells (E) in the spleens of 12- to 15-weekold (n = 5), 25-week-old (n = 5), and 16- to 18-month-old (n = 5-11) WT and FL^−/− mice. Mean (SEM) numbers (from 10 fetuses per genotype) of total B220⁺IgM⁺ B cells (F) and B220⁺IgM⁺CD5⁺NK1.1⁻ B1a cells (G) in the fetal liver of day 18.5 WT and FL^−/− mice. Lethally irradiated adult WT and FL^−/− mice (both CD45.2) were transplanted with WT and FL^−/− day 14.5 fetal liver cells (both CD45.1), respectively, and 17 weeks after transplantation their spleens were analyzed for donor-derived total B220⁺IgM⁺ (H) and B1 (IgM⁺CD23^loCD43⁺) cells (I). Mean (SEM) values from 9 recipients per genotype. **P < .01, ***P < .001, ns indicates not significant compared with WT mice.