Influenza A Virus NS1 Protein Inhibits the NLRP3 Inflammasome

The inflammasome is a molecular platform that stimulates the activation of caspase-1 and the processing of pro-interleukin (IL)-1β and pro-IL-18 for secretion. The NOD-like receptor family, pyrin domain containing 3 (NLRP3) protein is activated by diverse molecules and pathogens, leading to the formation of the NLRP3 inflammasome. Recent studies showed that the NLRP3 inflammasome mediates innate immunity against influenza A virus (IAV) infection. In this study, we investigated the function of the IAV non-structural protein 1 (NS1) in the modulation of NLRP3 inflammasome. We found that NS1 proteins derived from both highly pathogenic and low pathogenic strains efficiently decreased secretion of IL-1β and IL-18 from THP-1 cells treated with LPS and ATP. NS1 overexpression significantly impaired the transcription of proinflammatory cytokines by inhibiting transactivation of the nuclear factor-κB (NF-κB), a major transcription activator. Furthermore, NS1 physically interacted with endogenous NLRP3 and activation of the NLRP3 inflammasome was abrogated in NS1-expressing THP-1 cells. These findings suggest that NS1 downregulates NLRP3 inflammasome activation by targeting NLRP3 as well as NF-κB, leading to a reduction in the levels of inflammatory cytokines as a viral immune evasion strategy.

炎性体（inflammasome）是一类可激活半胱天冬氨酸蛋白酶-1（caspase-1），并介导pro-白细胞介素-1β（pro-IL-1β）与pro-白细胞介素-18（pro-IL-18）加工及分泌的分子平台。核苷酸结合寡聚化结构域样受体蛋白3（NLRP3）可被多种分子与病原体激活，进而组装形成NLRP3炎性体。近期研究表明，NLRP3炎性体可介导针对甲型流感病毒（IAV）感染的固有免疫应答。本研究探讨了甲型流感病毒非结构蛋白1（NS1）对NLRP3炎性体的调控作用。研究发现，高致病性与低致病性毒株来源的NS1蛋白，均可有效抑制经脂多糖（LPS）与三磷酸腺苷（ATP）处理的THP-1细胞中IL-1β与IL-18的分泌。NS1过表达可通过抑制核心转录激活因子核因子κB（NF-κB）的反式激活，显著削弱促炎细胞因子的转录水平。进一步实验显示，NS1可与内源性NLRP3发生物理相互作用；在表达NS1的THP-1细胞中，NLRP3炎性体的激活被完全阻断。上述结果提示，NS1可通过靶向NLRP3与NF-κB下调NLRP3炎性体的激活，进而降低炎性细胞因子水平，以此作为病毒的免疫逃逸策略。