Oxidized Low-Density Lipoprotein Immune Complex Priming of the Nlrp3 Inflammasome Involves TLR and FcγR Cooperation and Is Dependent on CARD9

Abstract

Oxidized LDL (oxLDL) is known to activate inflammatory responses in a variety of cells, especially macrophages and dendritic cells (DCs). Interestingly, much of the oxLDL in circulation is complexed to antibodies and these resulting immune complexes (ICs) are a prominent feature of chronic inflammatory disease such as atherosclerosis, Type-2 diabetes, systemic lupus erythematosus and rheumatoid arthritis. Levels of oxLDL-ICs often correlate with disease severity, and past studies have demonstrated that oxLDL-ICs elicit potent inflammatory responses in macrophages. Here we show that bone marrow-derived dendritic cells (BMDCs) incubated with oxLDL-ICs for 24 hrs secrete significantly more IL-1β compared to BMDCs treated with free oxLDL, while there was no difference in levels of TNFα or IL-6. Treatment of BMDCs with oxLDL-ICs increased expression of inflammasome-related genes Il1a, Il1b, and Nlrp3; and pre-treatment with a caspase 1 inhibitor decreased IL-1β secretion in response to oxLDL-ICs. This inflammasome priming was due to oxLDL-IC signaling via multiple receptors as inhibition of CD36, TLR4 and FcγR significantly decreased IL-1β secretion in response to oxLDL-ICs. Signaling through these receptors converged on the adaptor protein CARD9, a component of the CARD9-Bcl10-MALT1 signalosome complex involved in NF-κB translocation. Finally, oxLDL-IC-mediated IL-1β production resulted in increased Th17 polarization and cytokine secretion. Collectively, these data demonstrate that oxLDL-ICs induce inflammasome activation through a separate and more robust mechanism than oxLDL alone, and that these ICs may be immunomodulatory in chronic disease and not just biomarkers of severity.

Western Diet Triggers NLRP3-Dependent Innate Immune Reprogramming

“…Intriguingly, this analysis revealed that oxLDL-induced trained immunity was influenced by several independent genetic polymorphisms in the putative regulatory region of PYCARD, the gene encoding the principal inflammasome adaptor protein ASC, and also in the gene for the endogenous IL-1 inhibitory molecule IL-1RAP. Together with a large array of studies linking NLRP3 to the inflammatory response in metabolic diseases (Duewell et al, 2010; Rhoads et al, 2017; Sheedy et al, 2013), this provided evidence for a potential key role of the NLRP3 inflammasome pathway in the induction of trained immunity under conditions of WD feeding. Strongly supporting this hypothesis, we observed an almost complete protection from WD-induced changes in systemic inflammation, induction of hematopoiesis and functional and transcriptomic reprogramming of GMPs isolated from Ldlr −/− /NLRP3 −/− mice.…”

<p>Ox-LDL Causes Endothelial Cell Injury Through ASK1/NLRP3-Mediated Inflammasome Activation via Endoplasmic Reticulum Stress</p>

“…26 In recent years, numerous studies have confirmed that NLRP3 inflammasomes can be activated by two abundant components of atherosclerotic plaque (ox-LDLs and crystalline cholesterol). 26,[47][48][49] Hoseini et al reported that high numbers of inflammasomes were present in atherosclerotic plaque and ox-LDL-induced cells in vitro. 26 However, the mechanism by which inflammasomes function in atherosclerosis is complex, and involves several signaling pathways, including the JNK-1 and ASK1 pathways.…”

FcγRIIb on CD11c+ cells modulates serum cholesterol and triglyceride levels and differentially affects atherosclerosis in male and female Ldlr mice

“…Despite this, oxLDL-ICs have only recently been implicated in disease pathogenesis. OxLDL-ICs were found to be more potent primers of the NLRP3 inflammasome and elicited greater IL-1β production than free oxLDL in bone marrow-derived dendritic cells (DCs) [4]. However, the mechanism behind oxLDL-ICs enhanced inflammatory effects is unknown.…”

“…FcγRs are found on antigen presenting cells (APCs) such as macrophages, DCs, and B-cells and have high affinity for the Fc region of IgG antibodies. In vitro studies demonstrate that oxLDL-ICs can signal through Fc receptors in both human macrophage cells lines and murine bone marrow-derived DCs, likely in concert with toll-like receptor 4 and the scavenger receptor CD36 [4,5]. FcγRs can be either activating or inhibitory, containing an immunoreceptor tyrosine-based activation motif or an immunoreceptor tyrosine inhibitory motif, respectively [6].…”

“…Additionally, polymorphisms in Fcgr2b are associated with susceptibility to systemic lupus erythematosus (SLE) and rheumatoid arthritis in humans [9][10][11]. Although the inhibitory FcγRIIb is expressed at lower levels compared to the activating FcγRs in bone marrow-derived DCs, several studies support a role for FcγRIIb in regulating DC function [4]. In a bovine type II collagen-induced model, CD11c-conditional FcγRIIb knockout (KO) mice were more susceptible to developing arthritis [12].…”