Flt3L enhances thymopoiesis following BMT. (A) Normal thymus-bearing C57BL/6 underwent BMT as described in “Materials and methods” and the legend to Figure 3; 5 μg flt3L or buffer only was administered intraperitoneally daily from day 1 to 28. On day 29, CD45RB expression on CD4⁺/CD45.1⁺ BM-derived splenocytes was evaluated. No significant differences in CD45RB expression on CD4⁺/CD45.1 BM-derived T cells between flt3L-treated groups (black line) and sham-treated groups (gray line) were seen. Dotted line is the isotype control. Shown are representative animals from each group. Consistent results were seen in 3 separate experiments. (B) Total; CD4/CD8 double-positive (DP); CD4 single-positive (SP); CD8 SP; and triple-negative (TN) thymocytes at day 14 following BMT as described in panelA. Open bars represent buffer-treated and gray bars represent flt3L-treated animals. Gating on CD45.1 cells derived from BM; n = 13 per group. The graph shows median values with standard error. (C) BMT was performed as described for panel A except the BM inocula was composed of 5 × 10⁶ TCD B6CD45.1⁺ BM cells and 5 × 10⁶ TCD BM cells derived from HY TCR Tg⁺ animals. On day 29, the number of TCR Tg⁺ splenocytes was enumerated. P = .012, n = 7 in the sham-treated group and n = 4 in the flt3L-treated group. Data are representative of 2 separate experiments. Median values with standard error are shown. (D) BMT was performed as described for panel C.Absolute number of TRECs from sorted CD4⁺ and CD8⁺ T cells was enumerated on day 29 as described in “Materials and methods.”