Type I IFNs directly inhibit T reg cell proliferation and activity in vitro. (a, Left) T reg cell proliferation assay showing CPDye dilution of gated WT (left) or Ifnar1^−/− (right) CD4⁺Foxp3⁺ T reg cells cultured with WT (top) or Ifnar1^−/− APCs (bottom) in the absence (gray) or presence (black) of 50 U/ml IFN-β after 72 h of culture with soluble CD3 and IL-2. (a, Right) Division index (top) and absolute number (bottom) of WT (black) or Ifnar1^−/− (gray) T reg cells after 72 h of culture in the indicated conditions. (b, Left) In vitro suppression assay showing CPDye dilution of Ifnar1^−/− CD4⁺Foxp3⁻ T_conv cells in the presence of WT or Ifnar1^−/− T reg cells at the indicated T reg/T_conv cell ratios in the absence (gray) or presence (black) of 50 U/ml IFN-β after 72 h of culture with soluble CD3 and irradiated Ifnar1^−/− APCs. (b, Right) Absolute number of and percent suppression by WT (black) or Ifnar1^−/− (gray) T reg cells at decreasing T reg/T_conv cell ratios after 72 h culture in the absence (−) or presence (+) of 50 U/ml IFN-β. Data are representative of 3 independent experiments.