Rapid recovery of T_Reg cells is due to conversion of donor CD4⁺CD25⁻ T cells into CD4⁺CD25⁺Foxp3⁺ T_Reg cells in recipients after transplantation. Syngeneic transplantations were set up as described. On day 0, a cohort of recipients (Thy1.2⁺) received FACS-purified CD4⁺CD25⁻ T cells from Thy1.1⁺ tumor–bearing mice and Thy1.2⁺ stem cells (Thy1.1⁺CD4⁺CD25⁻ cells + Thy1.2⁺ stem cells). Others received only stem cells from Thy1.1⁺ donor mice (Thy1.1⁺ stem cells). Seven, 14, and 21 days later, splenocytes were stained with anti-Thy1.1, anti-CD4, anti-CD25, and anti-Foxp3 antibodies and subjected to FACS analysis. (A) The mean absolute number (± SD; n = 4) of CD4⁺CD25⁺Foxp3⁺ T cells per spleen in each group is indicated. (B) The percentages of CD4⁺Thy1.1⁺ and CD4⁺Thy1.1⁻ T cells among total lymphocytes at day 14 are indicated. (C) The percentages of CD25⁺Foxp3⁺ T cells among CD4⁺Thy1.1⁺ or CD4⁺Thy1.1⁻ T cells at day 14 are indicated. Representative results of 2 independent experiments are shown.