Peripheral apoptosis delays T-cell reconstitution after allogeneic TCD-BMT. Lethally irradiated (1100 cGy) 8- to 10-week-old LP mice were transplanted with 5 × 10⁶ TCD B6 BM and harvested at various intervals after transplantation. LP mice that did not receive transplants were also analyzed. Values represent mean (± SEM). n = 5-10 mice per group, results from 2 or more independent experiments. Donor B6 cells were gated as Ly9.1-negative because of the allelic difference between the B6 and LP strains. (A) Splenic donor T cells were gated as Ly9.1⁻ cells, and apoptosis was determined by annexin-V staining on days 14, 21, 28, and 56 after transplantation and in control animals that did not receive transplants. (B) Cleaved caspase 8 and 9 activity in donor Ly9.1⁻ T cells and control animals that did not receive transplants was determined on day 14 after transplantation. (C) Donor-derived Ly9.1⁻ naive (CD44^lo) and effector/memory (CD44^hi) T cells were analyzed by annexin-V staining on days 14 and 56 after transplantation. Control animals that did not receive transplants were also analyzed. (D) Cell surface Fas expression on donor-derived Ly9.1⁻ splenic CD4⁺ and CD8⁺ T cells was determined at days 21 and 56 after transplantation. Control animals that did not receive transplants were also analyzed. (E) Intracellular levels of Bcl-2 in donor Ly9.1⁻ CD4⁺ and CD8⁺ T cells were determined on days 14, 21, 28, and 56 after transplantation. Control animals that did not receive transplants were also analyzed. (F) Intracellular levels of Bcl-X_L in donor Ly9.1⁻ CD4⁺ and CD8⁺ T cells were determined at days 14 and 56 after transplantation. Control animals that did not receive transplants were also analyzed.