Effects of combined IL-2 therapy and PD-1 blockade on Treg expansion in vivo. Wild-type C57BL/6 mice received vehicle (plus isotype antibody), IL-2 (plus isotype antibody), anti-PD-1 (aPD-1) antibody (plus vehicle control), or IL-2 plus anti-PD-1 antibody. Anti-PD-1 antibody (250 μg) was administrated intraperitoneally twice per week for a total of 4 injections beginning on the first day of IL-2 treatment. IL-2–treated groups received IL-2 at 5000 IU once per day for 14 days. Peripheral blood cells were collected and analyzed at days 0, 4, 8, 11, and 15. (A) Increase of the percentage of Ki-67⁺ proliferating Tregs from the baseline level of each group during therapy. (B) Increase of the percentage of Tregs during therapy from the baseline level of each group with 4 mice per group per experiment. Data are representative of 2 independent experiments and expressed as means +/– SEM. *P < .05.