PD-1 blockade enhances IL-2–induced proliferation of expanded human Tregs and promotes apoptosis. Purified CD45RA⁺ naïve Tregs and Tcons labeled with carboxyfluorescein succinimidyl ester (CFSE) were stimulated with IL-2, anti-CD3/28 antibody, or anti-PD-L1 antibody for 4 days. Representative flow cytometry histograms were used to quantify CFSE dilution and (A) identify PD-1⁺ cells and (B) identify annexin-V⁺ cells. (C) Percentage of annexin-V⁺ apoptotic cells within Tcon and Treg populations stimulated with IL-2, anti-CD3/28 antibody, or anti-PD-L1 antibody for 4 days. Data are obtained from 1 experiment.