NLRP3 inflammasomes are required for atherogenesis and activated by cholesterol crystals

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Abstract
A study in atherosclerosis-prone (apolipoprotein E-deficient) mice on a high cholesterol diet shows that small cholesterol crystals appear in the earliest stages of atherogenesis, and that these crystals can activate the NLRP3 inflammasome in phagocytes. This suggests that therapeutic strategies that reduce cholesterol crystal deposition or block the inflammasome pathway may have anti-atherosclerotic activity. During atherosclerosis, crystals of cholesterol accumulate in atherosclerotic plaques. But are they a consequence or a cause of the inflammation associated with the disease? Here it is shown that small cholesterol crystals appear early in the development of atherosclerosis, and that they act as an endogenous danger signal, causing inflammation by activating the NLRP3 inflammasome pathway. Cholesterol crystals thus seem to be an early cause, rather than a late consequence, of inflammation. The inflammatory nature of atherosclerosis is well established but the agent(s) that incite inflammation in the artery wall remain largely unknown. Germ-free animals are susceptible to atherosclerosis, suggesting that endogenous substances initiate the inflammation1. Mature atherosclerotic lesions contain macroscopic deposits of cholesterol crystals in the necrotic core, but their appearance late in atherogenesis had been thought to disqualify them as primary inflammatory stimuli. However, using a new microscopic technique, we revealed that minute cholesterol crystals are present in early diet-induced atherosclerotic lesions and that their appearance in mice coincides with the first appearance of inflammatory cells. Other crystalline substances can induce inflammation by stimulating the caspase-1-activating NLRP3 (NALP3 or cryopyrin) inflammasome2,3, which results in cleavage and secretion of interleukin (IL)-1 family cytokines. Here we show that cholesterol crystals activate the NLRP3 inflammasome in phagocytes in vitro in a process that involves phagolysosomal damage. Similarly, when injected intraperitoneally, cholesterol crystals induce acute inflammation, which is impaired in mice deficient in components of the NLRP3 inflammasome, cathepsin B, cathepsin L or IL-1 molecules. Moreover, when mice deficient in low-density lipoprotein receptor (LDLR) were bone-marrow transplanted with NLRP3-deficient, ASC (also known as PYCARD)-deficient or IL-1α/β-deficient bone marrow and fed on a high-cholesterol diet, they had markedly decreased early atherosclerosis and inflammasome-dependent IL-18 levels. Minimally modified LDL can lead to cholesterol crystallization concomitant with NLRP3 inflammasome priming and activation in macrophages. Although there is the possibility that oxidized LDL activates the NLRP3 inflammasome in vivo, our results demonstrate that crystalline cholesterol acts as an endogenous danger signal and its deposition in arteries or elsewhere is an early cause rather than a late consequence of inflammation. These findings provide new insights into the pathogenesis of atherosclerosis and indicate new potential molecular targets for the therapy of this disease.