Characterization of 15-dpc FL cells for T-cell progenitor activity: predominance of TCP in the B220^lo

c-kit⁺CD19⁻ fraction. FL cells from 15-dpc C57BL/6 embryos were depleted of erythroid precursors and analyzed for expression of CD19, c-kit, and B220. After flow cytometric sorting of the different subpopulations defined by these 3 markers, wild-type FL cells were tested for TCP activity in an FTOC performed under limiting dilution conditions. (A) Flow cytometric plots are shown for wild-type and nu/nu C57BL/6 FL cells. (B) Reanalysis of sorted cells from populations a and e. Sorted cells were consistently more than 98% pure. (C) The total number of TCPs in 15-dpc FL was previously determined14 and reconfirmed here. From the frequency of TCPs and the fraction that they represent in total FL cells, we calculated the total number of TCPs in each population. The bars show the percentage of the total TCP activity in each fraction. (D) Analysis of surface phenotype of sorted cells from population e after staining with indicated antibodies (solid lines). Data are representative of at least 3 independent experiments.