Inflammasome activation has been recently recognized to play a central role in the development of drug-induced and obesity-associated liver disease. However, the sources and mechanisms of inflammasome mediated liver damage remain poorly understood. Our aim was to investigate the effect of NLRP3 inflammasome activation on the liver using novel mouse models. We generated global and myeloid cell specific conditional mutant Nlrp3 knock-in mice expressing the D301N Nlrp3 mutation (ortholog of D303N in human NLRP3) resulting in a constitutively activated NLRP3. To study the presence and significance of NLRP3 initiated pyroptotic cell death, we separated hepatocytes from non-parenchymal cells and developed a novel flow cytometry-based (FACS) strategy to detect and quantify pyroptosis in vivo based on detection of active caspase1 and propidium iodide (PI) positive cells. Liver inflammation was quantified histologically, by FACS and via gene expression analysis. Liver fibrosis was assessed by Sirius-Red-staining and qPCR for markers of hepatic stellate cell-(HSC)-activation. NLRP3 activation resulted in shortened survival, poor growth, and severe liver inflammation; characterized by neutrophilic infiltration and HSC-activation with collagen deposition in the liver. These changes were partially attenuated by treatment with anakinra, an interleukin-1 receptor antagonist. Notably, hepatocytes from global Nlrp3 mutant mice showed marked hepatocyte pyroptotic cell death with more than a fivefold increase in active caspase1-PI double positive cells. Myeloid cell restricted mutant NLRP3 activation resulted in a less severe liver phenotype in the absence of detectable pyroptotic hepatocyte cell death. Conclusions Our data demonstrates that global and to a lesser extent myeloid-specific NLRP3 inflammasome activation results in severe liver inflammation and fibrosis, while identifying hepatocyte pyroptotic cell death as a novel mechanism of NLRP3 mediated liver damage.

SUMMARY Cryopyrin (NALP3) mediates formation of the inflammasome, a protein complex responsible for cleavage of pro-IL-1β to its active form. Mutations in the cryopyrin gene, NLRP3, cause the autoinflammatory disease spectrum: cryopyrin-associated periodic syndromes (CAPS). The central role of IL-1β in CAPS is supported by the remarkable response to IL-1 targeted therapy. We developed two novel Nlrp3 mutant knock-in mouse strains to model CAPS to examine the role of other inflammatory mediators and adaptive immune responses in an innate immune driven disease. These mice had systemic inflammation and poor growth, similar to some human CAPS patients, and demonstrated early mortality, primarily mediated by myeloid cells. Mating these mutant mice to various knock-out backgrounds confirmed the mouse disease phenotype required an intact inflammasome, was only partially dependent on IL-1β, and was independent of T cells. This data suggests CAPS are true inflammasomopathies and provide insight for more common inflammatory disorders.

NLR inflammasomes, caspase 1 activation platforms critical for processing key pro-inflammatory cytokines, have been implicated in the development of nonalcoholic fatty liver disease (NAFLD). As the direct role of the NLRP3 inflammasome remains unclear, we tested effects of persistent NLRP3 activation as a contributor to NAFLD development and, in particular, as a modulator of progression from benign hepatic steatosis to steatohepatitis during diet-induced NAFLD. Gain of function tamoxifen-inducible Nlrp3 knock-in mice allowing for in vivo temporal control of NLRP3 activation and loss of function Nlrp3 knockout mice were placed on short-term choline-deficient amino acid-defined (CDAA) diet, to induce isolated hepatic steatosis or long-term CDAA exposure, to induce severe steatohepatitis and fibrosis, respectively. Expression of NLRP3 associated proteins was assessed in liver biopsies of a well-characterized group of patients with the full spectrum of NAFLD. Nlrp3−/− mice were protected from long-term feeding CDAA-induced hepatomegaly, liver injury, and infiltration of activated macrophages. More importantly, Nlrp3−/−mice showed marked protection from CDAA-induced liver fibrosis. After 4 weeks on CDAA diet, wild-type (WT) animals showed isolated hepatic steatosis while Nlrp3 knock-in mice showed severe liver inflammation, with increased infiltration of activated macrophages and early signs of liver fibrosis. In the liver samples of patients with NAFLD, inflammasome components were significantly increased in those patients with nonalcoholic steatohepatitis (NASH) when compared to those with non-NASH NAFLD with mRNA levels of pro-IL1 beta correlated to levels of COL1A1. Our study uncovers a crucial role for the NLRP3 inflammasome in the development of NAFLD. These findings may lead to novel therapeutic strategies aimed at halting the progression of hepatic steatosis to the more severe forms of this disease.

ORMDL3 (orosomucoid like 3) has been strongly linked with asthma in genetic association studies. As allergen challenge induces lung ORMDL3 expression in WT mice, we have generated human ORMDL3 Zona Pellucida 3 Cre (hORMDL3zp3-Cre) mice that overexpress human ORMDL3 universally to investigate the role of ORMDL3 in regulating airway inflammation and remodeling. These hORMDL3zp3-Cre mice have significantly increased levels of airway remodeling including increased airway smooth muscle, subepithelial fibrosis, and mucus. hORMDL3zp3-Cre mice had spontaneous increased AHR to methacholine compared to WT mice. This increased airway remodeling was associated with selective activation of the Unfolded Protein Response pathway transcription factor ATF6 (but not Ire1 or PERK). The ATF6 target gene SERCA2b, implicated in airway remodeling in asthma, was strongly induced in the lungs of hORMDL3zp3-Cre mice. In addition, increased levels of expression of genes associated with airway remodeling (TGF-β1, ADAM8) were detected in airway epithelium of these mice. Increased levels of airway remodeling preceded increased levels of airway inflammation in hORMDL3zp3-Cre mice. hORMDL3zp3-Cre mice had increased levels of IgE, with no change in levels of IgG, IgM, and IgA. These studies provide evidence that ORMDL3 plays an important role in vivo in airway remodeling potentially through ATF6 target genes such as SERCA2b, and/or through ATF6 independent genes (TGF-β1, ADAM8).