Assessment of CD25 expression on donor graft CD4⁺ and CD8⁺ T cells. Three-color flow cytometry was used to assess CD25 expression on CD4⁺ and CD8⁺ T cells in donor PBSC grafts. (A) CD25 expression is illustrated using dot plots and histograms on a donor PBSC graft with minimal expression of CD25. The left panels illustrate the expression of CD25 on CD4⁺ T cells (top) and CD8⁺ T cells (bottom). The right panels depict histograms used to quantitate CD25 expression: The gray histograms represent staining with a CD25-specific antibody; open histograms, isotope control staining on a paired sample. CD25⁺ T cells constituted 0.72% of CD4⁺ T cells (top) and 0.78% of CD8⁺ T cells (bottom). (B) CD25 expression is similarly illustrated for a donor with relatively high CD25 expression on graft CD4⁺ and CD8⁺ T cells (26.5%, top panel, upper right quadrant; and 9.5%, bottom panel, upper right quadrant).