Recent Advances in Fluorescent Angioscopy for Molecular Imaging of Human Atherosclerotic Coronary Plaque

PURPOSE OF REVIEW

In vivo imaging of the native substances, including lipoproteins, that comprise human atherosclerotic plaques is currently beyond the scope of any available imaging techniques. Color and near-infrared fluorescent angioscopy (CFA and NIRFA, respectively) systems have been recently developed for molecular imaging of lipoproteins within the human coronary arterial wall ex vivo and/or in vivo. The author reviews recent findings on lipoprotein deposition in human coronary plaques obtained by these imaging techniques.

RECENT FINDINGS

Using specific biomarkers, native pro-atherogenic substances such as oxidized low-density lipoprotein (ox-LDL), LDL, triglycerides (TG), apolipoprotein B-100 (ApoB-100), and lysophosphatidylcholine (LPC), and the anti-atherogenic substance such as high-density lipoprotein (HDL) were visualized by CFA, and LDL and cholesterol by NIRFA, in coronary plaques obtained from autopsy subjects. The relationship between incidence and plaque morphology differed for each substance. The incidence of ox-LDL and LDL on color fluorescence microscopy correlated well with that observed using immunohistochemical techniques. During coronary catheterization in patients, ox-LDL, LDL, and HDL in coronary plaques were visualized by CFA or NIRFA.

CONCLUSIONS

Using CFA or NIRFA, the distribution of the major native pro-atherogenic and anti-atherogenic lipoproteins and their components within human coronary plaques can be evaluated ex vivo and/or in vivo. Fluorescent angioscopy could help our understanding of the molecular mechanisms of coronary atherosclerosis and in the evaluation of the effects of therapy targeting the substances comprising atherosclerotic coronary plaques.

Imaging of native high-density lipoprotein in human coronary plaques by color fluorescent angioscopy

BACKGROUND

High-density lipoprotein (HDL) plays a key role in reverse cholesterol transport, and halts the progression of atherosclerosis. The aim of the present study was to visualize native HDL in the human coronary arterial wall.

METHODS AND RESULTS

The fluorescence characteristics of HDL were investigated by color fluorescent microscopy (CFM) using excitation at 470 nm and emission at 515 nm with Fast green dye (FG) as the biomarker. HDL in 30 normal coronary segments, and in 25 white and 25 yellow plaques in excised human coronary arteries, was visualized by color fluorescent angioscopy (CFA) and CFM. Localization of HDL visualized by CFM was compared with that stained by immunostaining using an anti-HDL antibody. FG elicited a characteristic brown fluorescence of HDL. By CFA, the percent incidence of HDL in normal segments, white (early stage of plaque growth) and yellow (advanced stage of plaque growth) plaques was, respectively, 33%, 76% (P<0.05 vs. normal segments and yellow plaques) and 21%. Localization of HDL visualized by CFM did not differ from that stained by immunostaining.

CONCLUSIONS

In the human coronary arterial wall, HDL deposits infrequently in normal segments, but increasingly deposits with plaque formation, and decreases in the advanced stage of plaque growth.

Localization of native high-density lipoprotein and its relation to plaque morphology in human coronary artery

High-density lipoprotein (HDL) plays a key role in reverse cholesterol transport, and halts the progression of atherosclerosis. However, its localization in human vascular wall is not well understood. We discovered that by exciting at 470-nm and emitting at 515-nm light wavelengths, Fast green dye (FG) elicits brown fluorescence characteristic of HDL only. Therefore, the localization of native HDL in normal segments and plaques in excised human coronary artery was investigated by scanning their transected surface with color fluorescent microscopy (CFM) using FG as a biomarker, and the relationships between the localization of HDL and morphology of plaques and normal segments classified by conventional angioscopy and histology were examined. The % incidence of HDL in 13 normal segments (NS) with thin (≤ 200 µm) intima, 28 NS with thick (200 µm <) intima, 41 white plaques (early stage of plaque growth), 15 yellow plaques (Y) without necrotic core (NC), and 20 Y with NC (advanced stage of plaque growth), was 30, 71 (P < 0.05 versus NS with thin intima and Y with NC), 83 (P < 0.05 versus NS with thin intima and Y with NC), 60, and 35, respectively. HDL begins to deposit in human coronary arterial wall in the early stage of atherosclerosis and deposits increase with plaque growth, but HDL decreases in plaques at an advanced stage of growth.

Molecular imaging of apolipoprotein B-100 in human coronary plaques by color fluorescent angioscopy and microscopy

Apolipoprotein B-100 (ApoB-100) is an important risk factor for coronary artery disease. However, its localization in human coronary plaques is not well understood. The present study was performed to visualize ApoB-100 in human coronary artery wall. Deposition of native ApoB-100 in excised human coronary plaques and normal segments classified by conventional angioscopy was investigated by color fluorescent angioscopy (CFA) and microscopy (CFM) using Nile blue dye (NB) which elicits a golden fluorescence characteristic of ApoB-100 as a biomarker. By CFA, the % incidence of ApoB-100 was 20 in 40 normal segments, 38 in 42 white, and 11 in 35 yellow plaques (P < 0.05 versus white plaques). There was no significant difference in detection sensitivity between CFA and luminal surface scan by CFM. By CFM transected surface scan, ApoB-100 deposited in superficial, deep, and/or in both layers. Deposition in both layers was frequently observed in white plaques and yellow plaques without necrotic core (NC), less frequently in normal segments, and rarely in yellow plaques with NC. (1) Taking into consideration the well known process of plaque growth, the results suggest that ApoB-100 begins to deposit before plaque formation, increasingly deposits with plaque growth, and disappears after necrotic core formation. (2) CFA is feasible for imaging of ApoB-100 in human coronary artery wall.

Molecular imaging of low-density lipoprotein in human coronary plaques by color fluorescent angioscopy and microscopy

Objectives

Low-density lipoprotein (LDL) is an important risk factor for coronary artery disease. However, its localization in human coronary plaques is not well understood. The present study was performed to visualize LDL in human coronary artery wall.

Methods

(1) The fluorescence characteristic of LDL was investigated by color fluorescent microscopy (CFM) with excitation at 470-nm and emission at 515-nm using Nile blue dye (NB) as a biomarker. (2) Native LDL in 40 normal segments, 42 white plaques and 35 yellow plaques (20 with necrotic core) of human coronary arteries was investigated by color fluorescent angioscopy (CFA) and CFM.

Results

(1) NB elicited a brown, golden and red fluorescence characteristic of LDL, apolipoprotein B-100, and lysophosphatidylcholine/triglyceride, respectively. (2) The % incidence of LDL in normal segments, white, and yellow plaques was 25, 38 and 14 by CFA and 42, 42 and 14 by CFM scan of their luminal surface, respectively, indicating lower incidence (p<0.05) of LDL in yellow plaques than white plaques, and no significant differences in detection sensitivity between CFA and CFM. By CFM transected surface scan, LDL deposited more frequently and more diffusely in white plaques and yellow plaques without necrotic core (NC) than normal segments and yellow plaques with NC. LDL was localized to fibrous cap in yellow plaques with NC. Co-deposition of LDL with other lipid components was observed frequently in white plaques and yellow plaques without NC.

Conclusions

(1) Taken into consideration of the well-known process of coronary plaque growth, the results of the present study suggest that LDL begins to deposit before plaque formation; increasingly deposits with plaque growth, often co-depositing with other lipid components; and disappears after necrotic core formation. (2) CFA is feasible for visualization of LDL in human coronary artery wall.

Fluffy luminal surface of the non-stenotic culprit coronary artery in patients with acute coronary syndrome: an angioscopic study

BACKGROUND

Approximately 15% of acute coronary syndrome (ACS) cases have no significant coronary stenosis. Mechanisms underlying the attacks are, however, unknown.

METHODS AND RESULTS

The clinical study had 254 patients with ACS; 38 patients (31 females and 7 males; aged 51.0 ± 8.0 years) had no significant coronary stenosis on angiography. They underwent a dye-staining angioscopy of the suspected culprit coronary artery using Evans blue, which selectively stains fibrin and damaged endothelial cells. A fluffy coronary luminal surface was observed in the suspected culprit artery in all 38 patients. The fluffy luminal surface was stained blue with Evans blue. In animal experiments involving 5 beagles, 10% hydrogen peroxide solution was injected into the iliac arteries to damage endothelial cells, which was then followed by blood reperfusion, and then the artery was examined by intravascular microscopy and histology. In the beagles, the arterial segment, where the thrombus had been formed, exhibited a fluffy luminal surface after a washout of the thrombus, and the surface was stained blue. Histologically, the fluffy surfaces were composed of damaged endothelial cells attached by multiple fibrin threads and platelets.

CONCLUSIONS

It was considered that the coronary segment exhibiting a fluffy luminal surface was the culprit lesion and that the fluffy surface was caused by residual thrombi after dispersion of an occlusive thrombus, which had formed on the damaged endothelial cells.

Visualization of lipid components in human coronary plaques using color fluorescence angioscopy

BACKGROUND

If oxidized low-density lipoprotein (oxLDL), LDL, lysophosphatidylcholine (LPC) and apolipoprotein B (apoB) can be visualized simultaneously, their roles in the initiation, progression and destabilization of atherosclerotic plaques can be objectively evaluated.

METHODS AND RESULTS

(1) The fluorescence characteristic of each atherogenic substance was investigated by microscopy using a band-pass filter (470 nm) and a band-absorption filter (520 nm) with homidium bromide (Ho) and trypan blue (TB) as indicators. (2) 50 excised human coronary plaques were classified by their autofluorescence into green, greenish-yellow and yellow, and the localization of oxLDL, LDL, LPC and apoB were investigated by color fluorescence angioscopy (CFA). The plaque colors were white, yellow and glistening yellow by conventional angioscopy. (1) OxLDL and LDL exhibited golden fluorescence, whereas LPC and apoB exhibited red fluorescence. (2) By CFA, 16 of 19 greenish-yellow and 1 of 8 yellow plaques exhibited red and golden fluorescence in a mosaic pattern, indicating co-deposition of oxLDL/LDL and LPC/apoB; 3 greenish-yellow and 7 yellow plaques exhibited red fluorescence, indicating solitary deposition of apoB; 23 green plaques infrequently exhibited these fluorescence colors.

CONCLUSIONS

OxLDL/LDL and LPC/apoB were successfully visualized as co-deposited in greenish-yellow autofluorescence plaques, but only LPC/apoB in yellow autofluorescence plaques.