Stimulation of OX40 on CD4⁺GFP(Foxp3)⁺ Tregs abrogates their suppressor functions. (A) CD4⁺GFP(Foxp3)⁻ T effector cells sorted from wt and OX40KO foxp3gfpKI mice were stimulated with anti-CD3 plus wt APCs. T effector cell proliferation in the presence or absence of wt CD4⁺GFP(Foxp3)⁺ Tregs at different Tregs to T effector ratios was shown. Data shown are mean (CPM ± SD) of triplicate assays. (B) OX40 deficient CD4⁺GFP(Foxp3)⁻ T effector cells were stimulated with anti-CD3 plus wt APCs or OX40Ltg APCs in the presence or absence of wt CD4⁺GFP(Foxp3)⁺ Tregs. Cell proliferation was determined 3 days later by 3H-TdR uptake. Data shown are mean (CPM ± SD) of triplicate assays. (C) OX40 deficient CD4⁺GFP(Foxp3)⁻ T effector cells were stimulated with anti-CD3 plus OX40Ltg APCs. In these cultures, graded numbers of wt or OX40KO CD4⁺GFP(Foxp3)⁺ Tregs were added, and cell proliferation was determined 3 days later by 3H-TdR uptake. Data shown are mean (CPM ± SD) of triplicate assays. In all the suppression assays, representative data of 3 independent experiments are shown.