Phenotypic analysis of human fetal Treg. (A) CD25 high (top row) or negative (bottom row) T cells from fetal thymus were analyzed for cell surface expression of the indicated markers. Percentages of positive cells among CD4⁺CD3⁺ cells are indicated. Gray histograms represent the background staining with isotype-matched irrelevant mAbs. (B) Mean frequencies ± SD of positive cells for each indicated marker among CD3⁺CD4⁺CD25^hi Treg's and CD3⁺CD4⁺CD25^- cells from thymus, spleen, liver, and bone marrow cells. Each marker was tested in 3 to 11 fetuses (13 to 29 WGs), except for GITR in liver that was tested once in a 19.5-WG fetus. ^*P < .05 by Student t test.