L-plastin supports macrophage NLRP3 inflammasome activation and pulmonary fibrosis

Abstract

Abstract NLRP3 inflammasome induces IL-1β production and pyroptotic cell death in macrophages to fight against invading pathogens. However, excessive inflammasome activation promotes tissue fibrosis and inflammatory diseases such as acute lung injury, liver injury, and acute respiratory distress syndrome (ARDS). NLRP3 inflammasome activation has been extensively studied, but exact regulatory molecular mechanisms remain unclear. We have found that the actin-bundling protein L-plastin (LPL) is required for efficient activation of the NLRP3 inflammasome pathway. In ex-vivo stimulation, lung resident alveolar macrophages (AMs) lacking LPL produced less IL-1β and IL-18 upon NLRP3 stimulation. LPL−/− AMs from ASC-citrine reporter mice exhibited reduced ASC speck formation after NLRP3 activation. Similarly, bone-marrow derived macrophages (BMDMs) displayed reduced ASC oligomerization. Downstream of ASC oligomerization, caspase-1 activity and downstream IL-1β and pyroptotic gasdermin-D processing were also reduced in LPL-deficient macrophages. Further, LPL−/− mice displayed reduced peritonitis after monosodium urate (MSU) treatment, an experimental NLRP3-induced inflammatory disease model. Since AMs are crucial for NLRP3-mediated lung fibrosis, we next examined NLRP3-dependent bleomycin-induced lung injury. We found that LPL−/− mice responded mildly to bleomycin and exhibited significantly reduced fibrosis development. Thus, we identify a previously unknown significant function of LPL in regulating NLRP3 inflammasome signaling in macrophages. This finding presents LPL as a potential target for diagnosis and treatment for NLRP3-associated inflammatory disorders such as ARDS, COPD and lung fibrosis.