The necrotic core is not the only compositional change affecting plaque size and stability. Advanced plaques are also marked by the presence of cholesterol crystals. Interestingly, some of the crystals are derived from erythrocytes, whose membranes are the richest in free cholesterol among all cells in the body. Intraplaque hemorrhage has emerged as a significant contributing factor to enlargement of the necrotic core ³⁸ . The source of hemorrhage is thought to arise from leaky new capillaries that infiltrate the plaque as futile neovascularization attempts in response to a hypoxic environment created by increased lesion burden and inflammatory macrophages ³⁹ . The capillaries within the plaque typically lack an intact basement membrane, are poorly stabilized by surrounding pericytes, and show less than tight endothelial junctions, all factors likely responsible for their inability to hold contents.

Macrophage engulfment of cholesterol crystals or de novo formation of intracellular cholesterol crystals will induce lysosomal destabilization and release of cathepsin B to the cytoplasm, which activates a multimolecular signaling complex known as the nucleotide-binding leucine-rich repeat-containing pyrin receptor 3 (NLRP3) inflammasome ⁴⁰ . Activation of the NLRP3 inflammasome results in caspase-1-mediated production of interleukin-1 beta (IL-1β) and ultimately IL-6, which amplifies the inflammatory cascade ⁴¹ . The significance of this discovery needs to be stressed, as it offers a mechanistic relationship between hypercholesterolemia and vascular inflammation ⁴² . The importance of cholesterol crystals within foam cells extends beyond its ability to augment inflammation. Crystalline cholesterol may also provoke plaque rupture by physical disruption of the fibrous cap ⁴³ .

Abela and Aziz ^{44, 45} and Kellner-Weibel et al. ^{44, 45} investigated the role of crystalline cholesterol in advanced atherosclerotic lesions. They observed that crystallization of cholesterol can result in sharp-edged cholesterol crystals with the potential to penetrate biological membranes. They hypothesized that these cholesterol crystals could induce plaque rupture by mechanical perforation of the outer layers of atherosclerotic plaques. To support this hypothesis, they used scanning electron microscopy to demonstrate cholesterol crystals perforating the arterial intima in patients who had died from acute coronary syndromes ⁴⁶ . The authors found no cases of cholesterol crystal perforation in subjects with severe atherosclerosis but without acute cardiac events. These pioneering studies were the first to suggest that cholesterol crystals can trigger plaque disruption and vascular injury. However, although these studies are compelling, it is not entirely clear whether cholesterol crystals are causally linked, or are merely bystanders, to plaque rupture.

The focus of this review has been on experimental models of atherosclerosis spanning numerous decades. However, several orthogonal lines of evidence have now clearly established the link between lipids and ASCVD. Starting with the visionary Framingham Heart Study in 1948, numerous large epidemiological studies performed around the globe provided highly reproducible results ^{47– 51} . The consistency of the epidemiology was truly stunning and suggested the association of LDL-C with ASCVD. Demonstration of the causal role of LDL with ASCVD emerged from genetics (familial hypercholesterolemia, genome-wide association studies, and Mendelian randomization studies). Individuals with genetically elevated LDL-C are at high risk for ASCVD, whereas individuals with genetically low LDL-C are at exquisitely low risk for ASCVD. The results of the large prospective, double-blind, randomized, placebo-controlled statin mega-trials further supported the notion that LDL is causal in ASCVD, although many investigators for years attributed the benefits of statins to their “pleiotropic” effects ^{52– 57} . The results of the IMPROVE-IT trial (Improved Reduction of Outcomes: Vytorin Efficacy International Trial) finally created a wedge between certainty and doubt ⁵⁸ . All things considered, there is now unequivocal evidence that cholesterol-rich apoB-containing lipoproteins are inextricably linked with ASCVD and are the principal drivers of this process. For two and half decades, statins enjoyed a privileged status; they were considered the most effective class of drugs to reduce LDL-C and ASCVD events to which no additional drug impacted outcomes. The IMPROVE-IT trial ushered in the new era where LDL lowering with non-statin agents has demonstrated the ability to add to the benefits of statin therapy ⁵⁸ . This has brought in renewed energy for the discovery of novel cholesterol-lowering strategies. Within the last decade, investigators have successfully linked genetic insights to molecular pathways, allowing the swift development of a new class of potent LDL-C-lowering drugs, the proprotein convertase subtilisin-kexin type 9 (PCSK9) inhibitors ^{59– 61} . These agents hold the potential to transform ASCVD risk reduction given their tremendous LDL-lowering power ⁶² . However, is it likely that the epidemic of cardiovascular disease will be stopped by an LDL-lowering agent usually started when the patient is near to, or has already had, the first ischemic event? We think not. The real revolution in the prevention and management of ASCVD will arrive with tools that prohibit plaque development (needed by large numbers of relatively young and healthy individuals) and tools that induce plaque regression (needed by patients with established disease). These tools are likely to affect parietal processes, such as endothelial function, inflammatory responses, macrophage survival and egress, and lipid efflux. At last check, nothing in the literature forecasts the arrival of these tools in practice anytime soon.