Background and aims:
As part of the innate immune response, NLRP3 inflammasomes are involved in the process of sterile inflammation, IL-1β release and are key mediators of inflammation-related vascular diseases, such as atherosclerosis. Recent data showed the existence of extracellular inflammasomes released from monocytes during pyroptotic cell death. Their biological function in the vascular system is still not known. Here, we tested the hypothesis that extracellular NLRP3 inflammasome particles are internalized by primary human vascular cells, such as coronary artery endothelial cells (HCAEC) and smooth muscle cells (HCASMC) and induce atherogenic effects.

Methods and Result:

Fluorescently labelled NLRP3 inflammasome particles were isolated from a mutant NLRP3- YFP cell line and used to treat vascular cells for 4 and 24 h. Immunhistologic and expressional analyses showed that extracellular NLRP3 particles are internalized by vascular cells, where they induce caspase-1 (1.9- fold) and IL-1β (1.5- fold) activation. Transcriptomic analysis revealed increased expression level of pro-inflammatory adhesion molecules (ICAM1, CADM1), NLRP3 and genes involved in cytoskleleton organization. The inflammasome- induced gene expression was not dependent on caspase-1 activation. Instead, the effects were abrogated by blocking NFκB activation. Genes, upregulated by extracellular NLRP3 were validated in human carotid artery atheromatous plaques. Extracellular NLRP3 inflammasome particles promoted HCASMC migration (2.6- fold) and extracellular matrix production (collagen type 1: 1.3- fold; fibronectin: 2.1- fold) and exerted a chemoattractive effect on HCASMC (1.6- fold) and monocytes (2.3- fold).

Conclusions:
Extracellular NLRP3 inflammasomes are internalized into vascular cells where they induce pro-inflammatory and pro-atherogenic effects representing a novel mechanism of cell- cell communication and perpetuation of inflammation in atherosclerosis. Therefore, extracellular NLRP3 inflammasomes may be useful to improve the diagnosis of inflammatory diseases and the development of novel anti-inflammatory therapeutic strategies.