The T_reg compartment is expanded in patients with leukemia prior to SCT compared to healthy donors. PBMCs were stained with monoclonal antibodies to CD3, CD4, and FOXP3, and the absolute and relative frequencies of CD4⁺ and CD4⁺FOXP3⁺ T cells were studied in patients with leukemia before SCT (□) and their respective donors (○) (A-B). PBMCs were obtained from donors prior to G-CSF administration for stem cell mobilization. In addition, we examined FOXP3 gene expression in purified CD4⁺ T cells from patients and donors (C). Patients with leukemia had fewer CD4⁺ T cells compared to healthy donors (n = 16) (A); however, the T_reg compartment of patients with leukemia was relatively expanded as demonstrated by increased percentage of CD4⁺FOXP3⁺ T cells (n = 12) (B) and increased numbers of FOXP3 mRNA copies (n = 5) (C). (D) Patients were further stratified into high-risk (n = 11) (▪) and low-risk (n = 5) (□) disease as defined in Table 1. There was no significant association between FOXP3 mRNA expression and disease status. Bars represent median values. NS indicates not significant.