The
inflammasome represents a molecular platform for innate immune regulation and controls proinflammatory
cytokine production. The NLRP3
inflammasome is comprised of NLRP3, ASC, and
procaspase-1. When the NLRP3
inflammasome is activated, it causes ASC speck formation and caspase-1 activation, resulting in the maturation of interleukin-1β (IL-1β). The NLRP3
inflammasome is regulated at multiple levels, with one level being posttranslational modification. Interestingly, ubiquitination of ASC has been reported to be indispensable for the activation of the NLRP3
inflammasome. Influenza A virus (IAV)
infection induces NLRP3
inflammasome-dependent IL-1β secretion, which contributes to the host
antiviral defense. However, IAVs have evolved multiple antagonizing mechanisms, one of which is executed by viral NS1
protein to suppress the NLRP3
inflammasome. In this study, we compared IL-1β production in porcine alveolar macrophages in response to IAV
infection and found that the 2009 pandemic H1N1 induced less IL-1β than
swine influenza viruses (SIVs). Further study revealed that the NS1 C terminus of pandemic H1N1 but not that of SIV was able to significantly inhibit NLRP3
inflammasome-mediated IL-1β production. This inhibitory function was attributed to impaired ASC speck formation and suppression of ASC ubiquitination. Moreover, we identified two target
lysine residues, K110 and K140, which are essential for both porcine ASC ubiquitination and NLRP3
inflammasome-mediated IL-1β production. These results revealed a novel mechanism by which the NS1
protein of the 2009 pandemic H1N1 suppresses NLRP3
inflammasome activation.IMPORTANCE Influenza A virus (IAV)
infection activates the NLRP3
inflammasome, resulting in the production of IL-1β, which contributes to the host innate immune response. ASC, an adaptor
protein of NLRP3, forms specks that are critical for
inflammasome activation. Here, we report that the NS1 C terminus of the 2009 pandemic H1N1 has functions to suppress porcine IL-1β production by inhibiting ASC speck formation and ASC ubiquitination. Furthermore, the ubiquitination sites on porcine ASC were identified. The information gained here may contribute to an in-depth understanding of porcine
inflammasome activation and regulation in response to different IAVs, helping to further enhance our knowledge of innate immune responses to influenza virus
infection in pigs.