The Pathophysiology of Atherosclerosis
Atherosclerosis is a chronic inflammatory disease of the arterial wall that leads to coronary artery disease, stroke, and peripheral arterial disease. For decades, it was viewed simply as a lipid-storage disease where cholesterol accumulated on arterial walls. Modern cardiovascular science has demonstrated that atherosclerosis is an active, lipid-driven inflammatory process that involves endothelial dysfunction, lipoprotein retention, macrophage activation, and smooth muscle cell proliferation.
Endothelial Dysfunction: The Initial Insult
The arterial wall is lined by a single layer of endothelial cells that maintains vascular tone, inhibits platelet aggregation, and controls leukocyte entry. Atherosclerosis begins when this layer is damaged. Causes of endothelial injury include mechanical shear stress from hypertension, chemical toxins from smoking (such as acrolein), advanced glycation end-products in diabetes, and elevated circulating levels of ApoB-containing lipoproteins. Damaged endothelial cells decrease their production of protective nitric oxide and increase their expression of adhesion molecules, such as vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1).
Lipoprotein Retention and Chemical Modification
Once the endothelial barrier is compromised, circulating lipoproteins (primarily LDL, but also VLDL, IDL, and remnants) enter the subendothelial space, the intima. Here, these lipoproteins bind to chondroitin sulfate proteoglycans in the extracellular matrix. This trapping prevents them from returning to circulation. Retained LDL is exposed to reactive oxygen species, myeloperoxidase, and lipoxygenases, which modify the particle’s proteins and lipids to form oxidized LDL (oxLDL). Oxidized LDL is highly inflammatory and immunogenic, signaling the body to initiate an immune response.
Monocyte Recruitment and Foam Cell Formation
Oxidized LDL stimulates endothelial cells to release monocyte chemoattractant protein-1 (MCP-1) and colony-stimulating factors. These molecules recruit circulating monocytes to the site of injury. The monocytes migrate through the endothelium into the intima, where they differentiate into macrophages. To clear the foreign lipids, macrophages express scavenger receptors, such as scavenger receptor-A1 (SR-A1) and CD36. Unlike LDL receptors, scavenger receptors are not downregulated by high intracellular cholesterol levels. Macrophages continuously engulf oxLDL, eventually becoming packed with cholesteryl esters and taking on a foamy appearance under the microscope, which is why they are called foam cells.
Plaque Progression and Necrotic Core Formation
As foam cells accumulate, they form a “fatty streak,” the earliest visible lesion of atherosclerosis. Over time, these foam cells undergo apoptosis. If the clearance of dead cells (efferocytosis) is impaired, the dying cells release their lipid contents, forming a lipid-rich necrotic core. Macrophages and T-cells in the plaque secrete inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta). These cytokines stimulate smooth muscle cells in the arterial media to migrate into the intima, proliferate, and secrete collagen and elastin. This forms a “fibrous cap” that walls off the necrotic core from the bloodstream.
💡 💡 Plaque Stability vs. Vulnerability
Clinical events are rarely caused by gradual arterial narrowing. Instead, they are typically triggered by the rupture of a “vulnerable plaque.” Vulnerable plaques are characterized by a thin fibrous cap, a large lipid-rich necrotic core, and high concentrations of inflammatory macrophages. Stable plaques have a thick fibrous cap and a smaller necrotic core, making them less likely to rupture.
Plaque Rupture and Thrombosis
Inflammatory cells in the plaque release matrix metalloproteinases (MMPs), enzymes that degrade collagen in the fibrous cap. When the cap is sufficiently weakened, mechanical stress from blood flow can cause it to rupture. This exposes the highly thrombogenic necrotic core to circulating blood. The exposure of tissue factor and collagen triggers platelet activation and the coagulation cascade, forming a thrombus. If the thrombus blocks the coronary artery, it causes a myocardial infarction; if it blocks a cerebral artery, it causes an ischemic stroke.
Therapeutic Interventions and Plaque Regression
Large-scale clinical trials have shown that lowering LDL-C can stabilize plaques and reduce cardiovascular events. The GLAGOV trial, which evaluated the PCSK9 inhibitor evolocumab, showed that intensive LDL-C lowering can lead to regression of coronary atherosclerosis. Anti-inflammatory therapies, as demonstrated in the CANTOS trial using the IL-1beta inhibitor canakinumab, have also shown that targeting the inflammatory pathway reduces cardiovascular events without altering lipid levels. Clinicians use markers like Apolipoprotein B to track and manage atherogenic particle concentrations.
💡 Frequently Asked Questions (FAQ)
📚 References & Sources
- Libby P. (2021). The changing landscape of atherosclerosis. Nature, 592(7855), 524-533.
- Nicholls SJ, et al. (2016). Effect of Evolocumab on Progression of Coronary Disease: The GLAGOV Randomized Clinical Trial. JAMA, 316(22), 2373-2384.
- Ridker PM, et al. (2017). Antiinflammatory Therapy with Canakinumab for Atherosclerotic Disease. New England Journal of Medicine, 377(12), 1119-1131.