Type III interferons suppress influenza A virus infection independently of STAT activation by triggering cell death

Abstract Type III interferons (IFN-λ1–λ4) are known to limit influenza infections in vivo and are non-redundant to type I interferons (IFN-α and IFN-β). Here, we demonstrated in vitro that type III interferons secreted by infected epithelial cells limit infection with influenza A virus (IAV). This effect occurs independently of STAT1 and STAT2 activation. Knockout of the IFN-λ receptor subunit IFNLR1 results in a slight reduction of STAT1/2 phosphorylation during infection with IAV, but leads to a substantially increased fraction of IAV-infected cells and higher levels of viral RNA and protein in the cell population. In contrast, knockout of the IFN-β receptor subunit IFNAR1 results in a pronounced reduction of STAT1/2 phosphorylation but a smaller increase in IAV proliferation. We propose that type III interferons limit the spread of IAV by promoting death of IAV-infected cells and demonstrate that the ratio of dying to infected cells is lower in IFNLR1 KO cells compared to wild-type cells. Overall, our results suggest an additional non-transcriptional role of type III interferons in the control of viral infection. Key points IFNAR1 deficiency leads to a significant reduction of STAT1/2 activation in cell populations infected with IAV and a small decrease in IAV proliferation. IFNLR1 deficiency leads to a small reduction of STAT1/2 activation in cell populations infected with IAV and a significant increase in IAV proliferation. IFN-λ controls the proliferation of IAV (but not RSV) independently of STAT1/2 signaling. IFN-λ signaling increases the ratio of the dead to the infected cells, likely promoting death in IAV-infected cells.