Ability of IL-2– or IL-4– and allo(PVG)–activated CD4⁺CD25⁺ Tregs to suppress in vitro proliferation of CD4⁺CD25⁻ T cells in MLCs. Responses to specific PVG and third-party Lewis stimulators were compared. The effect of specific suppression was evident at ratios of activated CD4⁺CD25⁺ Tregs to CD4⁺CD25⁻ T cells of 1:16 and 1:32, respectively. At ratios above this, there was non–Ag-specific suppression by both IL-2 and IL-4 allo-Ag–activated Tregs. (A,B) Proliferation of 1 × 10⁵ naive CD4⁺CD25⁻ T cells to PVG and Lewis APC cultured with naive (A) and IL-2/PVG–activated (B) CD4⁺CD25⁺ Tregs in different ratio; no Tregs (□), IL-2/PVG–activated CD4⁺CD25⁺ T regs, and CD4⁺CD25⁻ T cells at ratios of 1:16 (squlf), 1:4 (▨), and 1:1 (▩). (C,D) Proliferation of 1 × 10⁵ naive CD4⁺CD25⁻ T cells to PVG and Lewis APCs cultured with naive (C) and IL-4/PVG–activated (D) CD4⁺CD25⁺ Tregs in different ratios; no CD4⁺CD25⁺ Tregs (□), IL-4/PVG–activated CD4⁺CD25⁺ Tregs, and CD4⁺CD25⁻ T cells at ratios of 1:32 (■), 1:4 (▨), and 1:1 (▩). Significant differences at *P < .01; **P < .05.