Type I IFNs directly inhibit T reg cell proliferation and activity during LCMV infection. (a, Left) Ratio of Ifnar1^−/−/WT CD4⁺Foxp3⁺ T reg cells in spleens of mixed bone marrow chimeric mice left uninfected or 7 dpi with LCMV Armstrong. (a, Right) Absolute number of WT (black) and Ifnar1^−/− CD4⁺Foxp3⁺ T reg cells in spleens of mixed bone marrow chimeric mice left uninfected or 7 dpi. (b, Left) Summary of Ki-67 expression by WT (black) and Ifnar1^−/− (gray) CD4⁺Foxp3⁺ T reg cells from spleens of mixed bone marrow chimeric mice left uninfected or 7 dpi. (b, Right) Representative flow cytometry analysis of Ki-67 expression by gated CD45.1⁺ WT (black) and CD45.2⁺ Ifnar1^−/− (gray) CD4⁺Foxp3⁺ T reg cells in spleens of mixed bone marrow chimeric mice 7 dpi. (c) Summary of Ki-67 expression by WT (black) and Ifnar1^−/− (gray) CD8⁺ and CD4⁺Foxp3⁻ T cells from spleens of mixed bone marrow chimeric mice left uninfected or 7 dpi. (d) Representative flow cytometry analysis and summaries of the indicated markers expressed by WT (black) and Ifnar1^−/− (gray) CD4⁺Foxp3⁺ T reg cells in spleens of mixed bone marrow chimeric mice left uninfected or 7 dpi with LCMV-Armstrong. (e, Left) Ratio of Ifnar1^−/−/WT CD4⁺Foxp3⁺ T reg cells in small intestinal LP (SI-LP) of mixed bone marrow chimeric mice left uninfected or 7 dpi with LCMV Armstrong. (e, Right) Absolute number of WT (black) and Ifnar1^−/− CD4⁺Foxp3⁺ T reg cells in SI-LP of mixed bone marrow chimeric mice left uninfected or 7 dpi. (f, Left) Summary of Ki-67 expression by WT (black) and Ifnar1^−/− (gray) CD4⁺Foxp3⁺ T reg cells from SI-LP of mixed bone marrow chimeric mice left uninfected or 7 dpi. (f, Right) Representative flow cytometry analysis of Ki-67 expression by gated CD45.1⁺ WT (black) and CD45.2⁺ Ifnar1^−/− (gray) CD4⁺Foxp3⁺ T reg cells in SI-LP of mixed bone marrow chimeric mice 7 dpi. Statistical significance was determined using two-tailed paired t test when comparing WT and Ifnar1^−/− cells within the same chimeric mouse. Two-tailed unpaired Student’s t test was used when comparing cells from different mice. n = 4 per group. Data are representative of 6 (a–d) or 2 (e and f) independent experiments with 3–4 mice per group. *, P < 0.05; **, P < 0.005; ***, P < 0.0001. All data are presented as the mean values ± SEM.