Frequencies of PD-1⁺ CD8⁺ and PD-1⁺ CD4⁺ T cells after UCBT. (A) PD-1 expression was analyzed by FACS on CD8⁺ T cells ex vivo. Gates were set on CD8^hi to exclude CD8⁺ natural killer cells. (B) PD-1 expression was analyzed by FACS on CD4⁺ T cells ex vivo. (C) Correlation between the frequency of PD-1⁺ CD8⁺ T cells and absolute CD8⁺ T-cell counts throughout the follow-up period. (D) Correlation between the frequency of PD-1⁺ CD8⁺ T cells and absolute CD4⁺ T-cell counts throughout the follow-up period. Box-and-whisker plots depict the median, interquartile range, and range of frequencies in graft inoculums (G), UCBT recipients, healthy children (c), and healthy adults (AB). Statistical significance was tested with Spearman rank correlation test or 1-way ANOVA and Bonferroni multiple comparison test. *P < .05; **P < .01; ***P < .001. n represents the number of subjects.