No abstract

Inhalation of silica crystals causes inflammation in the alveolar space. Prolonged silica exposure can lead to the development of silicosis, an irreversible, fibrotic pulmonary disease. The mechanisms by which silica and other crystals activate immune cells are not well understood. Here, we demonstrate that silica and aluminum salt crystals activate the NALP3 inflammasome. NALP3 activation requires crystal phagocytosis and crystal uptake leads to lysosomal damage and rupture. Sterile lysosomal damage is also sufficient to induce NALP3 activation and inhibition of phagosomal acidification or cathepsin B impairs NALP3 activation. These results indicate that the NALP3 inflammasome can sense lysosomal damage induced by various means as an endogenous danger signal.

Autophagy, a cellular process for organelle and protein turnover, regulates innate immune responses. We demonstrate that depletion of autophagic proteins microtubule associated protein-1 light chain 3B (LC3B) and Beclin 1 enhances caspase-1 activation and secretion of interleukin-1β and interleukin-18. Autophagic protein depletion promoted accumulation of dysfunctional mitochondria and cytosolic translocation of mitochondrial DNA (mtDNA) in response to lipopolysaccharide (LPS) and ATP in macrophages. Release of mtDNA into the cytosol depended on the NALP3 inflammasome and mitochondrial ROS. Cytosolic mtDNA contributed to IL-1β and IL-18 secretion in response to LPS and ATP. LC3B-deficient mice produced more caspase-1-dependent cytokines in two sepsis models and were susceptible to LPS-induced mortality. Our study suggests that autophagic proteins regulate NALP3-dependent inflammation by preserving mitochondrial integrity.

The IL-1 family cytokines are regulated on transcriptional and posttranscriptional levels. Pattern recognition and cytokine receptors control pro-IL-1β transcription whereas inflammasomes regulate the proteolytic processing of pro-IL-1β. The NLRP3 inflammasome, however, assembles in response to extracellular ATP, pore-forming toxins, or crystals only in the presence of proinflammatory stimuli. How the activation of gene transcription by signaling receptors enables NLRP3 activation remains elusive and controversial. In this study, we show that cell priming through multiple signaling receptors induces NLRP3 expression, which we identified to be a critical checkpoint for NLRP3 activation. Signals provided by NF-κB activators are necessary but not sufficient for NLRP3 activation, and a second stimulus such as ATP or crystal-induced damage is required for NLRP3 activation.

The transcription factors interferon regulatory factor 3 (IRF3) and NF-kappaB are required for the expression of many genes involved in the innate immune response. Viral infection, or the binding of double-stranded RNA to Toll-like receptor 3, results in the coordinate activation of IRF3 and NF-kappaB. Activation of IRF3 requires signal-dependent phosphorylation, but little is known about the signaling pathway or kinases involved. Here we report that the noncanonical IkappaB kinase homologs, IkappaB kinase-epsilon (IKKepsilon) and TANK-binding kinase-1 (TBK1), which were previously implicated in NF-kappaB activation, are also essential components of the IRF3 signaling pathway. Thus, IKKepsilon and TBK1 have a pivotal role in coordinating the activation of IRF3 and NF-kappaB in the innate immune response.