Inflammasome Activation and Neutrophil Extracellular Traps in Atherosclerosis

Abstract

The deposition of cholesterol containing cholesterol crystals and the infiltration of immune cells are features of atherosclerosis. Although the role of cholesterol crystals in the progression of atherosclerosis have long remained unclear, recent studies have clarified the involvement of cholesterol crystals in inflammatory responses. Cholesterol crystals activate the NLRP3 inflammasome, a molecular complex involved in the innate immune system. Activation of NLRP3 inflammasomes in macrophages cause pyroptosis, which is accompanied by the release of inflammatory cytokines such as IL-1β and IL-1α. Furthermore, NLRP3 inflammasome activation drives neutrophil infiltration into atherosclerotic plaques. Cholesterol crystals trigger NETosis against infiltrated neutrophils, a form of cell death characterized by the formation of neutrophil extracellular traps (NETs), which, in turn, prime macrophages to enhance inflammasome-mediated inflammatory responses. Colchicine, an anti-inflammatory drug effective in cardiovascular disease, is expected to inhibit cholesterol crystal-induced NLRP3 inflammasome activation and neutrophil infiltration. In this review, we illustrate the reinforcing cycle of inflammation that is amplified by inflammasome activation and NETosis.

Keywords: Colchicine; Interleukin; NETosis; Neutrophils; Pyroptosis.

Fig.1. Mechanisms of canonical NLRP3 inflammasome activation

The canonical NLRP3 inflammasome complex is composed of NLRP3, ASC, and caspase-1. Upstream HAMPs, such as K⁺ efflux, mitochondrial ROS production, and lysosomal destabilization, promote the assembly of the NLRP3 inflammasome complex. Active caspase-1 processes the inflammatory cytokines IL-1β and IL-18, and the pore-forming protein GSDMD. GSDMD-pore mediate the release of IL-1β, IL-18, and IL-1α. The Ca^2＋ influx induced by GSDMD pores activates calpain to promote IL-1α processing. The GSDMD pore formation induces NINJ1 oligomerization, which causes plasma membrane rupture. The NLRP3 inflammasome also activates caspase-8 and promotes the GSDME processing of downstream apoptotic caspases.

Fig.2. Non-canonical NLRP3 inflammasome activation

The non-canonical NLRP3 inflammasome is initiated by inflammatory caspases including human caspase-4 and murine caspase-11. Stimulation with LPS activates caspase-4/11 and promotes the processing of GSDMD. Ion mobilization by GSDMD-pore mediates various cellular processes; Ca^2＋ influx causes the processing of IL-1α, ESCRT-mediated membrane repair, and microvesicle formation. K^＋ efflux promotes assembly of the NLRP3 inflammasome and processing of IL-1β and IL-18. Pannexin 1 has also been suggested to be a mediator of the K^＋ efflux induced by caspase-4/11 activation.

Fig.3. Inflammatory response triggered by cholesterol crystals in atherosclerotic plaque

Crystals and particles such as cholesterol crystals and CPPs activate the NLRP3 inflammasome. Canonical and non-canonical NLRP3 inflammasome activation induces neutrophil infiltration into atherosclerotic plaques. Cholesterol crystals induce NETosis in infiltrated neutrophils. The released NETs upregulate the expression of proIL-1β to augment inflammasome-dependent inflammatory responses. NETs can be a source of dsDNA that activates the AIM2 inflammasome. Colchicine, which inhibits neutrophil infiltration and crystal-induced NLRP3 inflammasome activation, is a potential therapeutic option to prevent inflammatory responses triggered by cholesterol crystals in atherosclerotic plaques.