Caspase-8 是一种天冬氨酸特异性半胱氨酸蛋白酶，主要作为诱导细胞凋亡的起始 caspase，可通过裂解 RIPK1 和 IRF3 部分下调先天免疫。然而，具有 caspase-8 突变或缺陷的患者会出现免疫缺陷，并且容易受到病毒感染。这一争议背后的分子机制仍不清楚。 caspase-8 是否增强或抑制针对甲型流感病毒 (IAV) 感染的抗病毒反应仍有待确定。在这里，我们报道 caspase-8 在感染 H5N1、H5N6 和 H1N1 IAV 亚型的 A549 和 NL20 细胞中很容易被激活。令人惊讶的是，caspase-8 缺陷和两种 caspase-8 抑制剂 Z-VAD 和 Z-IETD 不会增强而是下调抗病毒先天免疫，TBK1、IRF3、IκBα 和 p65 磷酸化降低、IL-6 降低、 IFN-β、MX1 和 ISG15 基因表达；并减少了 IFN-β 的产生，但增加了病毒复制。从机制上讲，caspase-8 会裂解并灭活 CYLD（一种具有去泛素酶功能的肿瘤抑制因子）。 Caspase-8 抑制可抑制 CYLD 裂解、RIG-I 和 TAK1 泛素化以及先天免疫信号传导。相反，CYLD 缺陷增强 IAV 诱导的 RIG-I 和 TAK1 泛素化和先天抗病毒免疫。 caspase-3 缺乏或其抑制剂 Z-DEVD 治疗均不会影响 CYLD 裂解或抗病毒先天免疫。我们的研究提供的证据表明，两种人气道上皮细胞系中的 caspase-8 激活不会沉默，而是通过灭活 CYLD 来增强先天免疫。© 2024。作者。

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.© 2024. The Author(s).