Cholesterol accumulation in myeloid cells activates the NLRP3 inflammasome, which enhances neutrophil accumulation and neutrophil extracellular trap formation in atherosclerotic plaques. Patients with Tangier disease, who have increased myeloid cholesterol content, showed markers of inflammasome activation, suggesting human relevance.

Summary Autoimmune diseases such as systemic lupus erythematosus (SLE) are associated with increased cardiovascular disease and reduced plasma high-density lipoproteins (HDL) levels. HDL mediates cholesterol efflux from immune cells via the ATP Binding Cassette Transporters A1 and G1 (ABCA1/G1). The significance of impaired cholesterol efflux pathways in autoimmunity is unknown. We observed that Abca1/g1 deficient mice develop enlarged lymph nodes (LNs) and glomerulonephritis suggestive of SLE. This lupus-like phenotype was recapitulated in mice with knockouts of Abca1/g1 in dendritic cells (DCs) but not in macrophages or T-cells. DC-Abca1/g1 deficiency increased LN and splenic CD11b+ DCs, which displayed cholesterol accumulation and inflammasome activation, increased cell surface levels of the GM-CSF receptor, and enhanced inflammatory cytokine secretion. Consequently, DC-Abca1/g1 deficiency enhanced T-cell activation, Th1 and Th17-cell polarization. NLRP3 inflammasome deficiency diminished the enlarged LNs and enhanced Th1-cell polarization. These findings identify an essential role of DC cholesterol efflux pathways in maintaining immune tolerance.

Objective: HDL (high-density lipoprotein) infusion reduces atherosclerosis in animal models and is being evaluated as a treatment in humans. Studies have shown either anti- or proinflammatory effects of HDL in macrophages, and there is no consensus on the underlying mechanisms. Here, we interrogate the effects of HDL on inflammatory gene expression in macrophages. Approach and Results: We cultured bone marrow–derived macrophages, treated them with reconstituted HDL or HDL isolated from APOA1 Tg ;Ldlr −/− mice, and challenged them with lipopolysaccharide. Transcriptional profiling showed that HDL exerts a broad anti-inflammatory effect on lipopolysaccharide-induced genes and proinflammatory effect in a subset of genes enriched for chemokines. Cholesterol removal by POPC (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine) liposomes or β-methylcyclodextrin mimicked both pro- and anti-inflammatory effects of HDL, whereas cholesterol loading by POPC/cholesterol-liposomes or acetylated LDL (low-density lipoprotein) before HDL attenuated these effects, indicating that these responses are mediated by cholesterol efflux. While early anti-inflammatory effects reflect reduced TLR (Toll-like receptor) 4 levels, late anti-inflammatory effects are due to reduced IFN (interferon) receptor signaling. Proinflammatory effects occur late and represent a modified endoplasmic reticulum stress response, mediated by IRE1a (inositol-requiring enzyme 1a)/ASK1 (apoptosis signal-regulating kinase 1)/p38 MAPK (p38 mitogen-activated protein kinase) signaling, that occurs under conditions of extreme cholesterol depletion. To investigate the effects of HDL on inflammatory gene expression in myeloid cells in atherosclerotic lesions, we injected reconstituted HDL into Apoe −/− or Ldlr −/− mice fed a Western-type diet. Reconstituted HDL infusions produced anti-inflammatory effects in lesion macrophages without any evidence of proinflammatory effects. Conclusions: Reconstituted HDL infusions in hypercholesterolemic atherosclerotic mice produced anti-inflammatory effects in lesion macrophages suggesting a beneficial therapeutic effect of HDL in vivo.

Nonalcoholic fatty liver disease is a metabolic disorder commonly associated with obesity. A subset of nonalcoholic fatty liver disease patients further develops nonalcoholic steatohepatitis that is characterized by chronic liver injury, inflammation, and fibrosis. Recent work has implicated the autophagy pathway in the mobilization and oxidation of triglycerides from lipid droplets. However, whether impaired autophagy in hepatocytes drives excess fat accumulation in the liver remains controversial. In addition, the role of autophagy in protecting the liver from gut endotoxin-induced injury has not been elucidated. Here we generated mice with liver-specific autophagy deficiency by the conditional deletion of focal adhesion kinase family kinase-interacting protein of 200 kDa (also called Rb1cc1), a core subunit of the mammalian autophagy related 1 complex. To our surprise, mice lacking FIP200 in hepatocytes were protected from starvation- and high-fat diet-induced fat accumulation in the liver and had decreased expression of genes involved in lipid metabolism. Activation of the de novo lipogenic program by liver X receptor was impaired in FIP200-deficient livers. Furthermore, liver autophagy was stimulated by exposure to low doses of lipopolysaccharides and its deficiency-sensitized mice to endotoxin-induced liver injury. Together these studies demonstrate that hepatocyte-specific autophagy deficiency per se does not exacerbate hepatic steatosis. Instead, autophagy may play a protective role in the liver after exposure to gut-derived endotoxins and its blockade may accelerate nonalcoholic steatohepatitis progression.

Objective Plasma high-density-lipoproteins (HDL) have several putative anti-atherogenic effects, including preservation of endothelial functions. This is thought to be mediated, in part, by the ability of HDL to promote cholesterol efflux from endothelial cells (ECs). The ATP binding cassette transporters A1 and G1 (ABCA1 and ABCG1) interact with HDL to promote cholesterol efflux from ECs. To determine the impact of endothelial cholesterol efflux pathways on atherogenesis, we prepared mice with endothelial-specific knockout of Abca1 and/or Abcg1. Approach and Results Generation of mice with EC-ABCA1 and ABCG1 deficiency required crossbreeding Abca1fl/flAbcg1fl/flLdlr−/− mice with the Tie2Cre strain, followed by irradiation and transplantation of Abca1fl/flAbcg1fl/fl bone marrow to abrogate the effects of macrophage ABCA1 and ABCG1 deficiency induced by Tie2Cre. After 20–22 weeks of Western type diet (WTD), both single EC-Abca1 and Abcg1 deficiency increased atherosclerosis in the aortic root and whole aorta. Combined EC-Abca1/g1 deficiency caused a significant further increase in lesion area at both sites. EC-Abca1/g1 deficiency dramatically enhanced macrophage lipid accumulation in the branches of the aorta that are exposed to disturbed blood flow, decreased aortic eNOS activity, and increased monocyte infiltration into the atherosclerotic plaque. Abca1/g1 deficiency enhanced LPS-induced inflammatory gene expression in mouse aortic ECs, which was recapitulated by ABCG1 deficiency in human aortic ECs. Conclusions These studies provide direct evidence that endothelial cholesterol efflux pathways mediated by ABCA1 and ABCG1 are non-redundant and athero-protective, reflecting preservation of eNOS activity and suppression of endothelial inflammation, especially in regions of disturbed arterial blood flow.