Influenza A virus infection activates caspase-8 to enhance innate antiviral immunity by cleaving CYLD and blocking TAK1 and RIG-I deubiquitination

Caspase-8, an aspartate-specific cysteine protease that primarily functions as an initiator Caspase to induce Apoptosis, can downregulate innate immunity in part by cleaving RIPK1 and IRF3. However, patients with Caspase-8 mutations or deficiency develop immunodeficiency and are prone to viral infections. The molecular mechanism underlying this controversy remains unknown. Whether Caspase-8 enhances or suppresses Antiviral responses against influenza A virus (IAV) Infection remains to be determined. Here, we report that Caspase-8 is readily activated in A549 and NL20 cells infected with the H5N1, H5N6, and H1N1 subtypes of IAV. Surprisingly, Caspase-8 deficiency and two Caspase-8 inhibitors, Z-VAD and Z-IETD, do not enhance but rather downregulate Antiviral innate immunity, as evidenced by decreased TBK1, IRF3, IκBα, and p65 phosphorylation, decreased IL-6, IFN-β, MX1, and ISG15 gene expression; and decreased IFN-β production but increased virus replication. Mechanistically, Caspase-8 cleaves and inactivates CYLD, a tumor suppressor that functions as a Deubiquitinase. Caspase-8 inhibition suppresses CYLD cleavage, RIG-I and TAK1 ubiquitination, and innate immune signaling. In contrast, CYLD deficiency enhances IAV-induced RIG-I and TAK1 ubiquitination and innate Antiviral immunity. Neither Caspase-3 deficiency nor treatment with its inhibitor Z-DEVD affects CYLD cleavage or Antiviral innate immunity. Our study provides evidence that Caspase-8 activation in two human airway epithelial cell lines does not silence but rather enhances innate immunity by inactivating CYLD.