PD-1^−/− graft-induced GVHD in target organs, especially in the thymus. (A) 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). Recipient mice were euthanized on day 14 after HSCT, and the skin, liver, colon, and thymus were harvested. (B) Representative images of the skin, liver, and colon from recipient mice with hematoxylin and eosin (H&E) staining are shown (magnification, ×200 for skin and liver, ×400 for colon). (C) Pathological scores for the skin, liver, and colon on day 14 after HSCT are shown (PD-1^+/+ vs PD-1^−/−, P < .05). (D) Representative images of thymuses with H&E staining, lymphocyte gate of thymuses, and absolute number of double-positive (DP) cells in thymuses (PD-1^+/+ vs PD-1^−/−, P < .01). (E) Chimerism analysis of CD8 or CD4 single-positive conventional T cells (PD-1^+/+ vs PD-1^−/−, P < .001). Green, red, and blue indicate donor stem cell–, host-, and donor graft–derived cells, respectively. (D-E) Data are representative of 2 independent experiments. Data are shown as the mean ± standard error of the mean. *P < .05, **P < .01, ***P < .001.