Proliferation and suppressive activity of human fetal CD4⁺CD25⁺ T cells. Results from 19.5-WG fetus are given as mean counts per minute (cpm) ± SD results of triplicate cultures and are representative of 4 independent experiments performed on 4 fetuses aged 16, 19.5, 29, and 29 WGs. (A) CD4⁺CD8^-CD25^- (50 000) and increasing numbers of CD4⁺CD8^-CD25⁺ thymocytes were cocultured at the indicated ratios of CD4⁺CD8^-CD25⁺/CD4⁺CD8^-CD25^- cells and stimulated by irradiated allogeneic fetal splenocytes (20.5 WGs). (B) Proliferation of 40 000 purified CD4⁺, CD4⁺CD25^-, or CD4⁺CD25⁺ splenocytes stimulated by irradiated allogeneic splenocytes. All the differences between CD4⁺, CD4⁺CD25⁺, and CD4⁺CD25^- splenocyte proliferations are statistically significant (P < .05 by Student t test).