Absolute requirement of FLT3L but not TSLP in perinatal B-1 lymphopoiesis. (A) FACS profiles and mean (SD) numbers of total lin⁻ AA4.1⁺B220⁺CD19⁺ cells and lin⁻ AA4.1⁺CD19⁺B220^−/low B-1 progenitors in the BM of 1-day-old WT littermate and Tpte2^−/− (n = 8 and n = 6, 2-3 litters) mice. (B) Mean (SD) numbers of total lin⁻AA4.1⁺B220⁺CD19⁺ cells and B-1 progenitors in 1-day-old BM of littermate Il7^−/− (n = 6) and Tpte2^−/−Il7^−/− (n = 5). (C) FACS profiles and (D) mean (SD) numbers of total lin⁻AA4.1⁺B220⁺CD19⁺ cells and lin⁻ AA4.1⁺CD19⁺B220^−/low B-1 progenitors in the BM of 1-day-old WT (n = 16), Flt3l^−/− (n = 12), Il7^−/− (n = 10), and Flt3l^−/−Il7^−/− (n = 11) mice (each from at least 2 litters). Numbers in FACS profiles show the percentages of cells within the indicated gates of total BM cells (mean values from all analyzed mice). For panel C, a range of 20 764 to 137 438, 36 500 to 87 584, 22 367 to 74 650, and 16 809 to 143 764 cell events were acquired for the WT, Flt3l^−/−, Il7^−/−, and Flt3l^−/−Il7^−/− samples, respectively. # The majority of Flt3l^−/−Il7^−/− mice had no detectable B-1 progenitors by FACS, and none of the mice had more than 100 cells per 2 tibias (***P < .001).