Post-BMT splenic T-lineage progenitors are most efficiently derived from Flt3⁺ LSK BM cells. Progenitor populations were sorted from Ly5^SJL BM and transplanted into lethally irradiated B6 recipient mice together with 10⁵ B6 BM cells to ensure survival. The following populations were assessed: Flt3^– LSK cells, Flt3⁺ LSK cells, and CLPs (Lin^–IL-7Rα⁺AA4.1⁺Sca-1^loc-Kit^lo).^47,48  The number of cells injected was as follows: Flt3⁺ LSK, 500; Flt3^– LSK, 250; CLPs, 750. These numbers were chosen to be close to or higher than the respective number of each progenitor subset in 10⁵ BM cells. (A) Donor versus host Ly5 staining of spleen cell suspensions at day 16 after BMT. The high percentage of Ly5^– cells is due to the presence of erythroid progenitors. (B) Myeloid (Gr-1/CD11b⁺) and B-lineage (CD19⁺) progeny of the sorted B6.Ly5^SJL progenitor subsets. (C) Generation of T-lineage progenitors as defined in Figure 2. (D) Relative potency Flt3^– LSK, Flt3⁺ LSK cells, and CLPs in the generation of SpT cells. Each triangle represents an individual recipient (n = 6). The mean is shown as a horizontal bar. The difference between each progenitor subset was statistically significant (P < .01, Student t test).