Chronic vacancy of intrathymic progenitor niches after BMT. (A) Thymi from unirradiated, CCR7/CCR9 DKO, or 2 weeks post-BMT mice were analyzed for the frequency of Kit^high thymic progenitors. Shown are representative plots gated on Lin⁻ thymocytes. (B) Sorted CD45.2 donor BM Lin⁻Kit⁺Flt3^high progenitors were injected intrathymically into either unirradiated CD45.1 hosts (left column) or CD45.1 hosts that had been lethally irradiated and transplanted with 2.5 × 10⁵ CD45.1 BM cells 2 weeks earlier (right column). Recipient thymi were analyzed for donor contributions 2 weeks later. Shown are representative plots of thymic DN3 cells (top row) or DP cells (bottom row). (C) The mean numbers ± SEM of donor DN3 cells (left) and DP cells (right) in the recipient thymi described in panel B are shown (top row). The same experiment was also performed with recipients that had undergone BMT 4 weeks (middle row) or 10 weeks (bottom row) before intrathymic injection. *P < .05, compared with the corresponding unirradiated control. **P < .01 compared with the corresponding unirradiated control. N = 3 to 10 per group.