Treg cells comprise a larger proportion of the CD4⁺repertoire in patients with MC. (A) The percentage of CD4⁺CD25⁺CD127^lowFoxP3⁺ Treg cells within the CD4⁺ T-cell pool is shown in relation to chimerism status. Patients with MC demonstrate a higher frequency of Treg cells (8.7%, n = 29) compared with patients with FC (4.7%, n = 35; *P = .03). (B) The absolute number of Treg cells was similar between FC and MC cohorts at 2.6 × 10⁶/mL (n = 35) and 2.7 × 10⁶/mL (n = 29), respectively. (C) The number of Treg cells is inversely correlated to the degree of donor T-cell chimerism. Pearson’s coefficient r = −0.25, *P = .04 (n = 64). (D) The number of host and donor-derived Treg cells within patients with MC calculated using flowRNA probes specific to the KDM5D gene on the Y chromosome. No differences were seen between the 2 groups (n = 11). (E) Interleukin-10 and transforming growth factor-β production by Treg cells from patients with FC and MC. (F) The number of CD4⁺ T cells within peripheral blood in the first year after transplant in relation to chimerism status. No differences were seen between the 2 groups (FC, n = 35; MC, n = 29). (G) The number of CD8⁺ T cells within peripheral blood in the first year after transplant in relation to chimerism status. No differences were seen between the 2 groups (FC, n = 35; MC, n = 29). All graphs show the mean and standard error. Analyses were undertaken with 2-tailed Mann-Whitney U tests (A-B) and Wilcoxon matched pairs signed rank test (C).