Reduced T potential of aged BM in short-term assays. (A) Ly5^B6 donor BM from 2 month-old and 18- to 24-month-old mice was depleted of CD4⁺, CD8⁺, and B220⁺ cells, and 20 × 10⁶ of the remaining cells were injected intravenously into nonirradiated mice. Recipients were analyzed at 3 weeks for chimerism in ETP (Lin^lo c-Kit^hi CD25⁻), DN2 (Lin^lo c-Kit^hi CD25⁺), DN3 (Lin^lo c-Kit⁻ CD25⁺), and DP (CD4⁺ CD8⁺) populations in the thymus, granulocytes in the blood (Mac-1⁺ Gr-1⁺), and Pro B (B220⁺ CD43⁺ CD19⁺ AA4.1⁺) cells in the BM by flow cytometry. (B) Summary of experiments shown in panel A. Data represent 7 mice per group across 3 independent experiments. Error bars represent 1 SEM (*P ≤ .01). Absolute numbers of donor-derived thymocytes were also significantly different (young: 1.1 × 10⁶ ± 0.17 × 10⁶; aged: 0.28 × 10⁶ ± 0.072 × 10⁶; P < .001). (C) BM cells (5 × 10⁵ to 1 × 10⁶) were injected into the thymi of nonirradiated mice. Thymi were harvested and analyzed for donor-derived DP thymocytes 21 days later. Data are combined from 3 independent experiments each, with 18 to 20 total mice per group (*P ≤ .001). Error bars represent 1 SEM.