New insights into plaque stabilisation by lipid lowering

Thrombosis on the substrate of a disrupted plaque causes most acute coronary events. The physical integrity of the plaque thus governs the most important clinical manifestations of atherosclerosis. Of particular importance is the extracellular matrix of the fibrous capsule overlying the thrombogenic core of the atheroma. Stable atheroma generally have thick fibrous caps, and smaller lipid cores than lesions that have ruptured. Accumulating evidence supports a key role for inflammation as another critical determinant of the stability of human atherosclerotic plaques. Plaques that rupture usually have more abundant leucocytic infiltrates than those considered stable. Inflammatory mediators such as cytokines can influence several biological processes that regulate the stability of the plaque's fibrous cap, and thus its resistance to rupture. For example, interferon-gamma produced by activated T lymphocytes within atheroma inhibits the production of interstitial forms of collagen by human vascular smooth muscle cells. Inflammatory cytokines such as interleukin-1, tumour necrosis factor (TNF) and CD-40 ligand (a cell surface homologue of TNFalpha) can also elicit the expression by macrophages and smooth muscle cells of proteolytic enzymes that can weaken the extracellular matrix. We have hypothesised that lipid lowering reduces stimuli for the inflammatory response within the complex atherosclerotic lesion. Recent studies in rabbits with experimentally produced atherosclerosis have indeed shown that lipid lowering can (i) reduce macrophage numbers, (ii) decrease expression of the collagenolytic enzyme MMP-1, and (iii) reinforce the plaque's fibrous skeleton by increasing the content of interstitial collagen. By reducing local inflammation, lipid lowering can thus stabilise the plaque's fibrous cap, rendering the atheroma less prone to rupture and to precipitate thrombotic complications. These observations provide a mechanistic basis for understanding the marked reduction in acute coronary events and cerebrovascular accidents observed in patients treated with agents that reduce plasma lipids.