CLPs generate significantly more thymic donor chimerism than MPPs in unirradiated IL-7R^−/− and F1 recipients. The BM populations indicated were sorted as in Figure 1C, and BM equivalents (3.8 × 10⁴ Flk2⁺CD27⁺, 9.5 × 10⁵ non-Flk2⁺CD27⁺, 2.9 × 10⁴ CLP, 1.9 × 10⁴ MPP per IL-7R^−/− recipient; 6 × 10⁴ CLP, 4.5 × 10⁴ MPP per F1 recipient) were injected into unirradiated IL-7R^−/− or F1 recipients. (A) Flk2⁺CD27⁺ cells, and within that population, CLPs represent the major source of prethymic progenitors, 14 days after injection into IL-7R^−/− mice. Arrows in the top panel indicate gating hierarchy. (B) Quantification of donor thymocyte chimerism in IL-7R^−/− mice from MPPs and CLPs, as shown in panel A. Each symbol represents the chimerism of an individual mouse, and bars represent the average. Data are pooled from 2 separate experiments of 3 recipients each. As determined by the Student t test, differences in chimerism were significant (P = .001). (C) Representative plots of all gated donor thymocytes 14 days after transfer of MPP or CLP from CD45.2, Thy1.2 mice into nonirradiated F1 recipients (CD45.1, Thy1.1x CD45.2, Thy1.2). Donor-gated thymocytes are shown (CD45.2⁺ CD45.1⁻ Thy1.2⁺ Thy1.1⁻). CLP-derived thymocytes are more numerous and have progressed to the DP developmental stage, whereas MPP-derived progeny are relatively rare and are less mature. These plots are representative of 6 mice in 2 separate experiments.