Erling Falk

Coronary Plaque Disruption

Coronary atherosclerosis is by far the most frequent cause of ischemic heart disease, and plaque disruption with superimposed thrombosis is the main cause of the acute coronary syndromes of unstable angina, myocardial infarction, and sudden death.1 2 3 4 5 Therefore, for event-free survival, the vital question is not why atherosclerosis develops but rather why, after years of indolent growth, it suddenly becomes complicated by life-threatening thrombosis. The composition and vulnerability of plaque rather than its volume or the consequent severity of stenosis produced have emerged as being the most important determinants for the development of the thrombus-mediated acute coronary syndromes; lipid-rich and soft plaques are more dangerous than collagen-rich and hard plaques because they are more unstable and rupture-prone and highly thrombogenic after disruption.6 This review will explore potential mechanisms responsible for the sudden conversion of a stable atherosclerotic plaque to an unstable and life-threatening atherothrombotic lesion—an event known as plaque fissuring, rupture, or disruption.7 8 nnAtherosclerosis is the result of a complex interaction between blood elements, disturbed flow, and vessel wall abnormality, involving several pathological processes: inflammation, with increased endothelial permeability, endothelial activation, and monocyte recruitment9 10 11 12 13 14 ; growth, with smooth muscle cell (SMC) proliferation, migration, and matrix synthesis15 16 ; degeneration, with lipid accumulation17 18 ; necrosis, possibly related to the cytotoxic effect of oxidized lipid19 ; calcification/ossification, which may represent an active rather than a dystrophic process20 21 ; and thrombosis, with platelet recruitment and fibrin formation.1 22 23 Thrombotic factors may play a role early during atherogenesis, but a flow-limiting thrombus does not develop until mature plaques are present, which is why thrombosis often is classified as a complication rather than a genuine component of atherosclerosis. nn### Mature Plaques: Atherosis and Sclerosis nnAs the name atherosclerosis implies, mature …

Macrophage infiltration in acute coronary syndromes. Implications for plaque rupture.

BackgroundRupture of atherosclerotic plaques is probably the most important mechanism underlying the sudden onset of acute coronary syndromes. Macrophages may release lytic enzymes that degrade the fibrous cap and therefore produce rupture of the atherosclerotic plaque. This study was designed to quantify macrophage content in coronary plaque tissue from patients with stable and unstable coronary syndromes. Methods and ResultsHematoxylin and eosin and immuno-staining with anti-human macrophage monoclonal antibody (PG-M1) were performed. Computerized planimetry was used to analyze 26 atherectomy specimens comprising 524 pieces of tissue from 8 patients with chronic stable angina, 8 patients with unstable angina, and 10 patients with non-Q-wave myo-cardial infarction. Total plaque area was 417±87 mm2× 10−2 in patients with stable angina, 601±157 mm2×10−2 in patients with unstable angina, and 499±87 mm2× 10−2in patients with non-Q-wave myocardial infarction (P=NS). The macrophage-rich area was larger in plaques from patients with unstable angina (61±18 mm2x 102) and non-Q-wave myocardial infarction (87±32 mm2× 10−2) than in plaques from patients with stable angina (14±5 mm2×10−2) (P=.024). The percentage of the total plaque area occupied by macrophages was also larger in patients with unstable angina (13.3 ±5.6%) and non-Q-wave myocardial infarction (14.6±4.6%) than in patients with stable angina (3.14±1%) (P=.018). Macrophagerich sclerotic tissue was largest in patients with non-Q-wave myocardial infarction (67±30 mm2× 1010−2) and unstable angina (55±19 mm2× 10−2) than in patients with stable angina (11.5±4.1 mm2× 10−2) (P=.046). Macrophage-rich atheromatous gruel was also larg-est in patients with non-Q-wave myocardial infarction (15±4 mm2×10−2) than in patients with unstable angina (3.3±1.7 mm2×10−2) or stable angina (2.4±1.2 mm2× 10−2) (P=.026). ConclusionsMacrophage-rich areas are more frequently found in patients with unstable angina and non-Q-wave myocardial infarction. This suggests that macrophages are a marker of unstable atherosclerotic plaques and may play a significant role in the pathophysiology of acute coronary syndromes.

Characterization of the relative thrombogenicity of atherosclerotic plaque components: Implications for consequences of plaque rupture☆

OBJECTIVESnThe purpose of this study was to determine whether different components of human atherosclerotic plaques exposed to flowing blood resulted in different degrees of thrombus formation.nnnBACKGROUNDnIt is likely that the nature of the substrate exposed after spontaneous or angioplasty-induced plaque rupture is one factor determining whether an unstable plaque proceeds rapidly to an occlusive thrombus or persists as a nonocclusive mural thrombus. Although observational data show that plaque rupture is a potent stimulus for thrombosis, and exposed collagen is suggested to have a predominant role in thrombosis, the relative thrombogenicity of different components of human atherosclerotic plaques is not well established.nnnMETHODSnWe investigated thrombus formation on foam cell-rich matrix (obtained from fatty streaks), collagen-rich matrix (from sclerotic plaques), collagen-poor matrix without cholesterol crystals (from fibrolipid plaques), atheromatous core with abundant cholesterol crystals (from atheromatous plaques) and segments of normal intima derived from human aortas at necropsy. Specimens were mounted in a tubular chamber placed within an ex vivo extracorporeal perfusion system and exposed to heparinized porcine blood (mean [+/- SEM] activated partial thromboplastin time ratio 1.5 +/- 0.04) for 5 min under high shear rate conditions (1,690 s-1). Thrombus was quantitated by measurement of indium-labeled platelets and morphometric analysis. Under similar conditions, substrates were perfused with heparinized human blood (2 IU/ml) in an in vitro system, and thrombus formation was similarly evaluated.nnnRESULTSnThrombus formation on atheromatous core was up to sixfold greater than that on other substrates, including collagen-rich matrix (p = 0.0001) in both heterologous and homologous systems. Although the atheromatous core had a more irregular exposed surface and thrombus formation tended to increase with increasing roughness, the atheromatous core remained the most thrombogenic substrate when the substrates were normalized by the degree of irregularity as defined by the roughness index (p = 0.002).nnnCONCLUSIONSnThe atheromatous core is the most thrombogenic component of human atherosclerotic plaques. Therefore, plaques with a large atheromatous core content are at high risk of leading to acute coronary syndromes after spontaneous or mechanically induced rupture because of the increased thrombogenicity of their content.

Mechanisms of Plaque Formation and Rupture

Atherosclerosis causes clinical disease through luminal narrowing or by precipitating thrombi that obstruct blood flow to the heart (coronary heart disease), brain (ischemic stroke), or lower extremities (peripheral vascular disease). The most common of these manifestations is coronary heart disease, including stable angina pectoris and the acute coronary syndromes. Atherosclerosis is a lipoprotein-driven disease that leads to plaque formation at specific sites of the arterial tree through intimal inflammation, necrosis, fibrosis, and calcification. After decades of indolent progression, such plaques may suddenly cause life-threatening coronary thrombosis presenting as an acute coronary syndrome. Most often, the culprit morphology is plaque rupture with exposure of highly thrombogenic, red cell-rich necrotic core material. The permissive structural requirement for this to occur is an extremely thin fibrous cap, and thus, ruptures occur mainly among lesions defined as thin-cap fibroatheromas. Also common are thrombi forming on lesions without rupture (plaque erosion), most often on pathological intimal thickening or fibroatheromas. However, the mechanisms involved in plaque erosion remain largely unknown, although coronary spasm is suspected. The calcified nodule has been suggested as a rare cause of coronary thrombosis in highly calcified and tortious arteries in older individuals. To characterize the severity and prognosis of plaques, several terms are used. Plaque burden denotes the extent of disease, whereas plaque activity is an ambiguous term, which may refer to one of several processes that characterize progression. Plaque vulnerability describes the short-term risk of precipitating symptomatic thrombosis. In this review, we discuss mechanisms of atherosclerotic plaque initiation and progression; how plaques suddenly precipitate life-threatening thrombi; and the concepts of plaque burden, activity, and vulnerability.

Remodeling Rather Than Neointimal Formation Explains Luminal Narrowing After Deep Vessel Wall Injury Insights From a Porcine Coronary (Re)stenosis Model

BACKGROUNDnOversized balloon dilatation of normal porcine coronary arteries usually heals without stenosis formation.nnnMETHODS AND RESULTSnWith the purpose of developing a stenotic model and examining the mechanisms of luminal narrowing after angioplasty, we produced a circumferential deep vessel wall injury by inflating and withdrawing an oversized chain-encircled angioplasty balloon in the left anterior descending coronary artery (LAD) of 20 pigs. Three pigs died and did not complete the study. In 8 pigs (group 1), serial coronary arteriography was performed. The lumen diameter (mean+/-SD) before dilation was 3.4 +/- .4 mm; after dilation, 4.2 +/- 0.6 mm; and at follow-ups 2 and 4 weeks later, 1.6 +/- 0.4 mm (P<.0001). In 9 pigs (group 2) examined postmortem 3 weeks after dilatation, histology revealed that the injury was deep (out to adventitia) in all arteries and completely circumferential (360 degrees) in all but two arteries. Adventitia was markedly thickened as a result of neoadventitial formation. Injury correlated strongly with neointimal formation (middle LAD, r=.71, P=.00001, but neither injury nor neointima correlated with lumen size (r=.14, P=.46 and r=.34, P=.07, respectively); ie, neointimal formation did not explain late luminal narrowing. Lumen size, however, did correlate strongly with vessel size (r=.74, P=000005). The late loss in lumen diameter observed angiographically in group 1 substantially exceeded that caused by neointimal formation seen by histology in group 2.nnnCONCLUSIONSnThe chain-encircled angioplasty balloon produced a circumferential deep vessel wall injury that healed by luminal narrowing. In this porcine model, arterial remodeling was more important than neointimal formation in late luminal narrowing.

Smooth Muscle Cells in Atherosclerosis Originate From the Local Vessel Wall and Not Circulating Progenitor Cells in ApoE Knockout Mice

Objective—Recent studies of bone marrow (BM)-transplanted apoE knockout (apoE−/−) mice have concluded that a substantial fraction of smooth muscle cells (SMCs) in atherosclerosis arise from circulating progenitor cells of hematopoietic origin. This pathway, however, remains controversial. In the present study, we reexamined the origin of plaque SMCs in apoE−/− mice by a series of BM transplantations and in a novel model of atherosclerosis induced in surgically transferred arterial segments. Methods and Results—We analyzed plaques in lethally irradiated apoE−/− mice reconstituted with sex-mismatched BM cells from eGFP+apoE−/− mice, which ubiquitously express enhanced green fluorescent protein (eGFP), but did not find a single SMC of donor BM origin among ≈10 000 SMC profiles analyzed. We then transplanted arterial segments between eGFP+apoE−/− and apoE−/− mice (isotransplantation except for the eGFP transgene) and induced atherosclerosis focally within the graft by a recently invented collar technique. No eGFP+ SMCs were found in plaques that developed in apoE−/− artery segments grafted into eGFP+apoE−/− mice. Concordantly, 96% of SMCs were eGFP+ in plaques induced in eGFP+apoE−/− artery segments grafted into apoE−/− mice. Conclusions—These experiments show that SMCs in atherosclerotic plaques are exclusively derived from the local vessel wall in apoE−/− mice.

Unreliable Assessment of Necrotic Core by Virtual Histology Intravascular Ultrasound in Porcine Coronary Artery Disease

Background—Intravascular ultrasound–derived virtual histology (VH IVUS) is used increasingly in clinical research to assess composition and vulnerability of coronary atherosclerotic lesions. However, the ability of VH IVUS to quantify individual plaque components, in particular the size of the destabilizing necrotic core, has never been validated. We tested for correlation between VH IVUS necrotic core size and necrotic core size by histology in porcine coronary arteries with human-like coronary disease. Methods and Results—In adult atherosclerosis-prone minipigs, 18 advanced coronary lesions were assessed by VH IVUS in vivo followed by postmortem microscopic examination (histology). We found no correlation between the size of the necrotic core determined by VH IVUS and histology. VH IVUS displayed necrotic cores in lesions lacking cores by histology. Conclusions—We found no correlation between necrotic core size determined by VH IVUS and real histology, questioning the ability of VH IVUS to detect rupture-prone plaques, so-called thin-cap fibroatheromas.

Unreliable Assessment of Necrotic Core by VH TM IVUS in Porcine Coronary Artery Disease

Background—Intravascular ultrasound–derived virtual histology (VH IVUS) is used increasingly in clinical research to assess composition and vulnerability of coronary atherosclerotic lesions. However, the ability of VH IVUS to quantify individual plaque components, in particular the size of the destabilizing necrotic core, has never been validated. We tested for correlation between VH IVUS necrotic core size and necrotic core size by histology in porcine coronary arteries with human-like coronary disease. Methods and Results—In adult atherosclerosis-prone minipigs, 18 advanced coronary lesions were assessed by VH IVUS in vivo followed by postmortem microscopic examination (histology). We found no correlation between the size of the necrotic core determined by VH IVUS and histology. VH IVUS displayed necrotic cores in lesions lacking cores by histology. Conclusions—We found no correlation between necrotic core size determined by VH IVUS and real histology, questioning the ability of VH IVUS to detect rupture-prone plaques, so-called thin-cap fibroatheromas.

Association of Multiple Cellular Stress Pathways With Accelerated Atherosclerosis in Hyperhomocysteinemic Apolipoprotein E-Deficient Mice

Background—A causal relation between hyperhomocysteinemia (HHcy) and accelerated atherosclerosis has been established in apolipoprotein E–deficient (apoE−/−) mice. Although several cellular stress mechanisms have been proposed to explain the atherogenic effects of HHcy, including oxidative stress, endoplasmic reticulum (ER) stress, and inflammation, their association with atherogenesis has not been completely elucidated. Methods and Results—ApoE−/− mice were fed a control or a high-methionine (HM) diet for 4 (early lesion group) or 18 (advanced lesion group) weeks to induce HHcy. Total plasma homocysteine levels and atherosclerotic lesion size were significantly increased in early and advanced lesion groups fed the HM diet compared with control groups. Markers of ER stress (GRP78/94, phospho-PERK), oxidative stress (HSP70), and inflammation (phospho-IκB-&agr;) were assessed by immunohistochemical staining of these atherosclerotic lesions. GRP78/94, HSP70, and phospho-IκB-&agr; immunostaining were significantly increased in the advanced lesion group fed the HM diet compared with the control group. HSP47, an ER-resident molecular chaperone involved in collagen folding and secretion, was also increased in advanced lesions of mice fed the HM diet. GRP78/94 and HSP47 were predominantly localized to the smooth muscle cell–rich fibrous cap, whereas HSP70 and phospho-IκB-&agr; were observed in the lipid-rich necrotic core. Increased HSP70 and phospho-IκB-&agr; immunostaining in advanced lesions of mice fed the HM diet are consistent with enhanced carotid artery dihydroethidium staining. Interestingly, GRP78/94 and phospho-PERK were markedly increased in macrophage foam cells from early lesions of mice fed the control or the HM diet. Conclusions—Multiple cellular stress pathways, including ER stress, are associated with atherosclerotic lesion development in apoE−/− mice.

Chronic Renal Failure Accelerates Atherogenesis in Apolipoprotein E–Deficient Mice

Cardiovascular mortality is 10 to 20 times increased in patients with chronic renal failure (CRF). Risk factors for atherosclerosis are abundant in patients with CRF. However, the pathogenesis of cardiovascular disease in CRF remains to be elucidated. The effect of CRF on the development of atherosclerosis in apolipoprotein E-deficient male mice was examined. Seven-week-old mice underwent 5/6 nephrectomy (CRF, n = 28), unilateral nephrectomy (UNX, n = 24), or no surgery (n = 23). Twenty-two weeks later, CRF mice showed increased aortic plaque area fraction (0.266 +/- 0.033 versus 0.045 +/- 0.006; P < 0.001), aortic cholesterol content (535 +/- 62 versus 100 +/- 9 nmol/cm(2) intimal surface area; P < 0.001), and aortic root plaque area (205,296 +/- 22,098 versus 143,662 +/- 13,302 micro m(2); P < 0.05) as compared with no-surgery mice; UNX mice showed intermediate values. The plaques from uremic mice contained CD11b-positive macrophages and showed strong staining for nitrotyrosine. Systolic BP and plasma homocysteine concentrations were similar in uremic and nonuremic mice. Plasma urea and cholesterol concentrations were elevated 2.6-fold (P < 0.001) and 1.5-fold (P < 0.001) in CRF compared with no-surgery mice. Both variables correlated with aortic plaque area fraction (r(2) = 0.5, P < 0.001 and r(2) = 0.3, P < 0.001, respectively) and with each other (r(2) = 0.5, P < 0.001). On multiple linear regression analysis, only plasma urea was a significant predictor of aortic plaque area fraction. In conclusion, the present findings suggest that uremia markedly accelerates atherogenesis in apolipoprotein E-deficient mice. This effect could not be fully explained by changes in BP, plasma homocysteine levels, or total plasma cholesterol concentrations. Thus, the CRF apolipoprotein E-deficient mouse is a new model for studying the pathogenesis of accelerated atherosclerosis in uremia.