CLPs generate thymocytes with faster kinetics than MPPs. (A) MPPs and CLPs were sorted from the BM of CD45.1 mice as indicated in Figure 1C. BM equivalents (1.4 × 10⁴ CLPs and 4 × 10³ MPPs per mouse) were transplanted into sublethally irradiated CD45.2 mice along with syngeneic lineage-depleted BM (∼ 1.8 × 10⁵ cells). Representative plots of recipient thymuses at the indicated time points are shown. Arrows in the top panels indicate gating hierarchy. CLP chimerism is evident at day 7, whereas MPP chimerism becomes apparent at day 14. Plots are representative of one of 3 mice in 1 of 3 experiments. For each thymus, 2.5 × 10⁶ events were collected and displayed. In plots with rare events, pixel size has been increased to facilitate visualization. (B) Quantification of donor chimerism pooled from 3 time course experiments as in panel A. Each symbol represents the chimerism of 1 mouse (● indicates MPP recipient: ▲, CLP recipient); bars represent averages of each group. CLPs drive a wave of thymopoiesis that starts by day 7 and peaks at day 21, whereas MPP-derived thymopoiesis starts later and lasts longer. Positive donor gates were drawn using control mice, which were injected in parallel with only syngeneic BM, and the percentage of cells that fell into the donor Ly5⁺,Thy1.1⁺ gate of these control animals was zero. As determined by the Student t test, differences in chimerism between MPP and CLP were significant at days 7, 14, and 28 (P = .048, .022, and 10⁻⁴, respectively). (C) Faster development of CLP-derived thymocytes. The graph shows the percentage of donor thymocytes in each recipient that have reached the CD4⁺CD8⁺ (DP) stage of development. By day 14, an average of 68% of the CLP-derived thymocytes had reached the DP stage, compared with only 19% of the MPP-derived cells. The decrease in the percentage of CLP-derived DP thymocytes at day 28, relative to MPP, reflects their further maturation to the CD4⁺ and CD8⁺ SP stages, as seen in panel A. As determined by the Student t test, differences in DP chimerism were significant at days 14 and 28 (P = .006 and .018, respectively).