PTCy restored the balanced reconstitution of T-cell subsets and prevented tissue damage. Sublethally irradiated BDF1 recipient mice received transplants of 5 × 10⁶ PD-1^+/+ B6 or PD-1^−/− B6 SPs and 5 × 10⁶ Ly 5.1 B6 TCD-BM cells (n = 5 per group). All recipient mice were administered 50 mg/kg of PTCy intraperitoneally on day 3 after HSCT. (A) Representative images of the skin, liver, and colon from recipient mice with H&E staining are shown (magnification, ×200 for skin and liver, ×400 for colon). (B) Pathology scores of the skin, liver, and colon on day 14 after HSCT are shown. (C) Representative images of thymuses with H&E stains, lymphocyte gate of thymuses, and absolute number of DP cells in thymuses. (D) Chimerism analysis of CD8 or CD4 SP cells. Green, red, and blue indicate donor stem cell–, host-, and donor graft–derived cells, respectively. (E) Numbers of donor spleen–derived CD4 Tcons and donor spleen–derived Tregs after HSCT (CD4 Tcons, PD-1^+/+ vs PD-1^−/−, P < .05). (F) Annexin V expression of spleen-derived T cells on day 14 after HSCT. Data are expressed as the mean ± standard error of the mean. *P < .05.