Effect of OX40 costimulation on the induction of new CD4⁺Foxp3⁺ Tregs from T effector cells. (A) CD4⁺GFP(Foxp3)⁻ T effector cells were sorted from wt foxp3gfpKI mice and OX40KO foxp3gfpKI mice, The T effector cells were stimulated with anti-CD3 plus APCs in the presence or absence of TGF-β for 2 to 5 days. Induction of new GFP(Foxp3)⁺ T cells in the CD4⁺ fraction was determined by FACS. The dot plot shown is one of 3 individual experiments 4 days after the culture. Numbers in quadrants are the relative percentage of cells in each region given by the flow cytometer. (B) Induction of new GFP(Foxp3)⁺ Tregs calculated from 3 individual experiments. The conversion shown is the mean ± SD of 3 independent experiments at different time points. (C) Suppression of T effector cell proliferation by CD4⁺GFP(Foxp3)⁺ Tregs converted from either the wt or the OX40KO T effector cells. Data shown are representative of 3 independent experiments. Error bars represent SD of triplicate assays.