Association between PD-1 expression and leukemic relapse. Absolute CD4⁺ T-cell counts (A) and absolute CD8⁺ T-cell counts (B) were measured by FACS in PBMC samples obtained at 6 months after UCBT. Frequencies of PD-1⁺ CD8⁺ T cells were measured in PBMC samples obtained at 2 months (C) and 6 months (D) after UCBT. Statistical significance was tested with the Mann-Whitney U test. Box-and-whisker plots depict the median, interquartile range, and range. n represents the number of subjects. NR indicates nonrelapsers; and R, relapsers.