Expansion of Tregs by autologous, IDO-expressing, mature moDCs involves cell-to-cell contact, CD80/86 ligation, and IL-2. CD4⁺CD25⁺ T cells were assessed for their content of Foxp3⁺CD127^neg T cells at baseline (left panels), after 6 days' stimulation by autologous DCs (Auto MLR, middle panels), and after 6 days' stimulation by autologous moDCs (A) with transwell separation of moDCs from responder T cells (transwell separation), (B) after opsonization of moDCs with anti-CD80 and anti-CD86 a priori (α-CD80/86 opsonized), and (C) in the presence of neutralizing anti–IL-2 (α-IL-2 antibody; right panel for each condition). Isotype-matched, nonreactive antibodies were added to the control auto-MLRs in panels B and C. (D) Pooled data are shown for each experimental variable in rows A through C (mean ± SEM; number of independent experiments and P value indicated on each graph).