(A) WT mice (n = 4–5) were treated by the subcutaneous route with 3 µg IFN-α, 3 µg IFN-λ or saline solution 18 hr before intranasal challenge with 103 TCID50 of SeV in a 40 µl volume. Groups of mice were sacrificed on days 2, 4 or 6 post infection (dp.i.), and viral titers in nasal swabs (left panel), upper airways (middle panel) and lungs (right panel) were determined by the TCID50 method. Symbols represent means ±SD. Red or blue asterisks indicate statistically significant differences between mock and IFN-α- or IFN-λ-treated groups, respectively; purple asterisks indicate differences between IFN-α- or IFN-λ-treated groups. Statistical analysis: One-way ANOVA with Tukey’s multiple comparisons; p-values: ***p<0.001, **p<0.01, *p<0.05. (B) Ifnar1−/− (blue triangles, n = 5–6) or Ifnlr1−/− mice (red squares, n = 5) were treated by the intranasal route with 2 µg IFN-λ or 2 µg IFN-α, respectively, before intranasal challenge with 4 × 105 PFU of Udorn in a 40 µl volume. Mice treated with saline (Ø) served as controls. Mice were sacrificed at 24 hr (left panel) or 72 hr (right panel) post infection (p.i.), and viral titers in the upper airways and lungs were determined by plaque assay. Symbols represent individual mice, and bars represent means ± SEM. Statistical analysis: One-way ANOVA with Tukey’s multiple comparisons; p-values: ***p<0.001, **p<0.01, *p<0.05. (C) IFN-mediated induction of Mx1 was determined by stimulating differentiated primary airway epithelial cells derived from mouse tracheae for the indicated time points with 1 ng/ml of either IFN-α or IFN-λ. Mx1 induction was assessed by RT-qPCR; values are represented as gene expression levels relative to unstimulated controls (mock). Symbols represent single wells; line indicates mean. Representative data of two independent experiments is shown. Statistical analysis: Two-way ANOVA; asterisks indicate significant differences between IFN-α- and IFN-λ-treated cells. P-values: ***p<0.001, **p<0.01, *p<0.05.