Cholesterol Crystals Induce Complement-Dependent Inflammasome Activation and Cytokine Release

Abstract

Inflammation is associated with development of atherosclerosis, and cholesterol crystals (CC) have long been recognized as a hallmark of atherosclerotic lesions. CC appear early in the atheroma development and trigger inflammation by NLRP3 inflammasome activation. In this study we hypothesized whether CC employ the complement system to activate the inflammasome-caspase-1 leading to release of mature IL-1β, and if complement activation regulates CC-induced cytokine production. We here describe that CC activated both the classical and alternative complement pathways and C1q was found to be crucial for the activation. CC employed C5a in the release of a number of cytokines in whole blood, including IL-1β and TNF. CC induced minimal amounts of cytokines in C5-deficient whole blood, until reconstituted with C5. Furthermore, C5a and TNF in combination acted as a potent primer for CC-induced IL-1β release by increasing IL-1β transcripts. CC-induced complement activation resulted in up-regulation of Complement receptor 3 (CD11b/CD18) leading to phagocytosis of CC. Also, CC mounted a complement-dependent production of reactive oxygen species and active caspase-1. We conclude that CC employs the complement system to induce cytokines and activate the inflammasome-caspase-1 by regulating several cellular responses in human monocytes. In light of this, complement inhibition might be an interesting therapeutic approach for treatment of atherosclerosis.

Molecular Regulation of Cell Fate in Cerebral Ischemia: Role of the Inflammasome and Connected Pathways

“…21 Moreover, these different factors influence each other and signaling pathways have been identified between the TLRs, the NLRs, and the complement system. [39][40][41][42] Clinical studies have identified a time window of 4.5 hours for systemic thrombolysis and early recanalization and reperfusion in ischemic stroke as mandatory. 43 Reperfusion after cerebral ischemia restores the necessary blood flow after insufficient supply with energy metabolites and oxygen, and it also contributes to an inflammatory response with invading immune cells, activated endothelial cells, and thereby leads to a further progress of ischemic injury by inflammatory mechanisms.…”

“…Thus, cholesterol crystals use the complement system to induce cytokines and activate the inflammasome. 41 The activation of the inflammasome is also triggered by dysfunction of intracellular organelles, such as the mitochondria 19 or the lysosome. Accordingly, lysosomal membrane rupture after exposure to cell-toxic aggregates such as monosodium urate, or amyloid-beta in Alzheimer's disease activate the inflammasome.…”

Cholesterol Crystals Activate the Lectin Complement Pathway via Ficolin-2 and Mannose-Binding Lectin: Implications for the Progression of Atherosclerosis

“…Cholesterol crystallizes in a bilayer structure with an end-for-end arrangement of approximately parallel molecules (2). CC are known to activate the complement system (3)(4)(5) and induce complement-dependent inflammasome activation and cytokine release in phagocytes in vitro (1,6,7).…”

Role of C5a-C5aR axis in the development of atherosclerosis

“…In addition, C5a can directly activate inflammatory cells, causing the release of cytokines, expression of related receptors, secretion of lysosomal enzymes from macrophages, proliferation of T lymphocytes, and degranulation of mast cells [28]. Recently, C5a was found involved in cholesterol crystal-induced production of cytokines (including IL-1β and tumor necrosis factor), complement receptor 3, reactive oxygen species and active caspase 1 [29].…”