Thymic reconstitution occurs in the absence of canonical early T-lineage progenitors at early time points after BMT. (A) Reconstitution of donor-derived CD4⁺CD8⁺ DP thymocytes after BMT. BM cells (10⁵ Ly5^SJL) were transplanted into lethally irradiated (900 rads) B6 recipients. The absolute number of Ly5^SJL-derived CD4⁺CD8⁺ DP thymocytes was assessed between 2 and 10 weeks after BMT. ▵ represent data from individual mice (3-8 mice/time point). The thin line is drawn between the mean value for each time point. (B) CD4/CD8 profile of donor-derived thymocytes at 3 weeks after BMT. The left contour plot is gated on donor-derived cells. The right contour plot shows the profile of a control B6 thymus. Numbers indicate the percentage of cells in each quadrant. (C) Emergence of Lin^–CD25^–c-Kit⁺ ETPs and Lin^–c-Kit^–CD25⁺ DN3 thymocytes after BMT. ▵ and ▴ represent individual data from the same mice as in panel A (▵, ETP; ▴, DN3). (D) Flow cytometric analysis of donor-derived Lin^– thymocytes at 3 weeks after BMT, using 10⁵ BM cells, in comparison with control B6 thymus. (E) Donor-derived Lin^– thymocytes at 1, 2, or 3 weeks after BMT, using a high dose of BM cells (15 × 10⁶). Numbers indicate the percentage of cells in the boxed regions.