PD-1^−/− donor graft–derived effector T cells exhibited high proliferative capacity. (A) Lethally irradiated BDF1 recipients received transplants of 10 × 10⁶ PD-1^+/+ B6 or PD-1^−/− B6 SPs (n = 5 per group). All animals were euthanized, and spleen cells were harvested on day 3. (B) Representative image of cell division of graft-derived CD8 T cells and the population of graft-derived T cells. (C) Percentage of proliferated graft-derived T cells (divided ≥2) (CD8 T cells, PD-1^+/+ vs PD-1^−/−, P < .05). (D) Ki-67 expression of graft-derived T cells (CD8 T, PD-1^+/+ vs PD-1^−/−, P < .05; CD4 Tcons, PD-1^+/+ vs PD-1^−/−, P < .05). Data are expressed as the mean ± standard error of the mean. *P < .05. CFSE, carboxyfluorescein diacetate succinimidyl ester; ns, not significant.