Relationship between monocyte-to-lymphocyte ratio and coronary plaque vulnerability in patients with acute coronary syndrome: an optical coherence tomography study

Background The importance of monocyte-to-lymphocyte ratio (MLR) has been indicated in the initiation and progression of coronary artery disease (CAD). However, few previous researches demonstrated the relationship between MLR and plaque vulnerability. We aimed to investigate coronary plaque vulnerability in patients with acute coronary syndrome (ACS) by optical coherence tomography (OCT). Methods A total of 72 ACS patients who underwent coronary angiography and OCT test in Beijing Anzhen hospital were included in this retrospective study. The plaque vulnerability and plaque morphology were assessed by OCT. Results FCT: thickness of cap; ROC: receiver operating characteristic; LLC: low lymphocyte count; AMI: acute myocardial infarction; IVUS: intravascular

Conclusion Circulating MLR level has potential value in identifying the presence of vulnerable plaque in patients with ACS. MLR, as a non-invasive biomarker of in ammation, may be valuable in revealing plaque vulnerability. Key words Monocyte-to-lymphocyte ratio, Optical coherence tomography, Plaque vulnerability Background As the most severe form of coronary artery disease (CAD), acute coronary syndrome (ACS) is the leading cause of mortality in recent decades. Development of a thrombus on the background of an atherosclerotic plaque rupture, which subsequently lead to arterial occlusion, is the major mechanism responsible for the majority of ACS. In recent years, it has been widely realized that it is plaque composition rather than plaque size or stenosis severity signi cant for plaque rupture and subsequent thrombosis [1] . Vulnerable plaques were described as plaques with thin brous caps and large lipid core.
In ammation reaction plays a signi cant role in the occurrence and progress of atherosclerosis [2,3] . Previous researches have demonstrated that increased level of monocytes, neutrophils and decreased level lymphocytes have closely association with the cardiovascular adverse outcomes [4,5a] . Recently, Hanhua Ji, et al reported that monocyte-to-lymphocyte ratio (MLR) was an independent risk factor of the presence of CAD, and signi cantly associated with coronary lesion severity [6,7] . Furthermore, Chen H, et al reported that MLR was an independent predictor for major adverse cardiovascular event (MACE) in patients with non-ST segment elevated myocardial infarction (NSTEMI) [7] .It was also demonstrated that increased MLR level was associated with higher morality in patients with acute heart failure [8] . However, the relationship between the MLR level and plaque vulnerability has not been su ciently evaluated. On account of high resolution (10μm),optical coherence tomography (OCT) can accurately identify the features of coronary atherosclerotic plaques and observe microstructure of coronary arteries such as brous cap, lipid nucleus and intravascular thrombosis of plaques. Meanwhile, OCT has advantages in differentiating the thin cap bro-atheroma (TCFA) [9,10] . In this regard, our present study aimed to identify the relationship between MLR level and plaque vulnerability evaluated by OCT in patients with ACS.

Laboratory analysis
For all patients, venous blood samples were tested by the laboratory of Beijing Anzhen hospital. The MLR value was measured as the ratio of monocyte counts to lymphocyte counts.

Coronary angiography and analysis
Coronary angiography (CAG) is performed through radial or femoral artery pathways. All patients received a loading dose of aspirin (300 mg) and clopidogrel (300 mg) at least 6 hours before angiography. The images were analysis by two cardiac doctors.

OCT analysis
OCT imaging was obtained using the commercially available, frequency-domain C7-XR OCT Intravascular Imaging System (St Jude Medical, St Paul, MN) and Dragon y catheter (Light Lab Imaging, Inc). OCT images and measurements were analyzed using light Lab Imaging by two independent cardiac doctors. The indistinct OCT image and length of vessel images less 50mm was excluded.
OCT was used to identify the presence of brous plaques, lipid plaques, calci ed plaques, macrophage in ltration, microchannels, cholesterol crystals, plaque rupture and thrombus, as well as the imaging characteristics of lipid plaques [11][12][13][14][15] .Normal vascular structure as shown in gure 1A.Lipid plaque was de ned as a diffusely bordered, signal-poor region with an overlying signal-rich bands. Each plaque was at least 5mm apart from each other or at least 5mm away from the scaffold edge. When the lipid plaque was identi ed, the Lipid length was measured with the system's own measuring tool, and the Lipid core arc of the Lipid plaque was measured every 1mm. At the thinnest place of the lipid plaque brous cap, measured three times the thickness of the brous cap and calculated the average as the plaque thickness of brous cap (FCT).The length of the lipid plaque, maximum lipid core Angle (Max lipid arc), average lipid core Angle (Mean lipid arc) and lipid index (de ned as average lipid arc times lipid plaque length) were recorded respectively. Thin cap bro-atheroma (TCFA) (as shown in gure 1B) was de ned as a lipid plaque whose thickness of brous was less than 65 μm and whose lipid core was larger than two quadrants. Macrophages was de ned as dots or bands with high re ection and strong attenuation, and they often form radial shadows after dots of high signal (as shown in gure 1C). Fibrous plaque was de ned as a signal-rich region with scattering (as shown in gure 1D). Calci ed plaques were de ned as low-signal or non-uniform areas with sharp edges (as shown in gure 1E). The micro channel was de ned as a 50-300μm, low-signal and sharp edge hollow-like structure, which needs to be observed in at least 3 continuous sections (as shown in gure 1F). Cholesterol crystals was de ned as thin linear regions with high signal intensity and low attenuation. Plaque rupture was de ned as the disruption of plaque brous cap continuity with cavity formation. Intracoronary thrombus was de ned as a mass (diameter 250 μm) attached to the luminal surface or oating within the lumen.

Statistical analysis
All Statistical analyses were performed by SPSS 21.0. Measurement data in accordance with normal distribution was mean±standard deviation, the comparison between groups using t-Test. Non-normal distribution of the measurement data using the median (25 th to 75 th percentiles), the comparison between the group using Mann-Whitney U test. Counting data using example (%), the comparison between groups is X 2 test or Fisher exact test. Receiver operating characteristic (ROC) curve analysis was performed to examine the diagnostic value of MLR in predicting the OCT-detected TCFA. Liner regression analysis was used to assess the correlations between MLR and FCT. Multivariatelogistic regression analysis was carried out to identify the independent risk factors of the TCFA. Variables that had a clinically possible relation with TCFA or appeared to be signi cant in univariate model were introduced into multivariable analysis. Both were bilateral tests and p<0.05 were statistically signi cant.

Results
We nally enrolled 72 patients with 120 plaques in this study. ROC

Baseline clinical characteristics
The baseline demographic and clinical characteristics between the two groups were shown in Tables 1  and 2. The patients in high MLR group showed higher ages (P = 0.005) and lower BMI values (P = 0.035).
The proportion of previous dyslipidemia was lower in high MLR group (36.1% vs 61.1%, P = 0.034).
Compared with the low MLR group, those with high MLR had higher CRP (P = 0.023), monocyte count (P < 0.001) and lower triglyceride level (P = 0.049), lymphocyte count (P < 0.001). There was no statistical signi cance in other index between two groups.

Coronary angiography and OCT image ndings
No prominent difference was found in the number and characteristics of lesions between the high and low MLR group, as shown in Table 3.
76 vessels were screened and OCT images were recorded. OCT images showed that the ratio of TCFA  Table 5. Furthermore, a prominently negative liner relation was found between MLR and FCT (R = 0.225, P < 0.005) (Figure 2).

Risk factors of TCFA
According to the presence of OCT-detected TCFA, patients were classi ed into TCFA group (25 patients) and non-TCFA group (47 patients). Patients with TCFA had signi cantly higher MLR level compared with those without TCFA (P = 0.001) (Figure 3). Multivariatelogistic regression analysis showed that MLR was the independent risk factor of TCFA, after adjusting for confounding factors (OR: 3.316, 95% CI: 1.448-7.593, p = 0.005), as shown in Table 6.

Discussion
In recent years, many advances have been made in the identi cation and prediction of coronary plaque stability using non-invasive biomarkers. Our trial was the rst to use the OCT test to evaluate the relationship between MLR levels and coronary plaque vulnerability in ACS patients. In the present study of ACS patients with high MLR, coronary plaque presented more vulnerable features, characterizing as thinner FCT, greater maximum lipid angle and longer lipid plaque length. Meanwhile, the incidence of plaque rupture and OCT-detected TCFA were higher in high MLR group. And high MLR level was demonstrated to be an independent risk factor of TCFA.
MLR is the ratio of monocytes to lymphocytes and the combined indicator of the two in ammatory factors. As the major mechanism of atherosclerosis, in ammation response has a great impact on the occurrence and development of atherosclerosis lesions, and eventually result in plaque instability and severely adverse outcomes related to CAD [16,17] . One general feature of atherosclerotic lesions is the presence of in ammatory cells, such as macrophages, T cells. Monocytes and lymphocytes in the leukocyte subtype are crucial cells in the in ammatory process. Previous studies have shown that monocytes are closely related to many vital steps in cardiovascular pathology and atherosclerosis. Macrophages, derived from the circulating monocytes, in uence the initiation and progression of atherosclerotic plaque by participating foam cells formation and inducing the production of in ammatory cytokines, matrix metalloproteinases, and reactive oxidative species [18][19][20] . A novel therapeutic target associated with monocytes phenotype modulation has been used for cardiovascular diseases prevention and treatment [21] . Low lymphocyte count (LLC) has been found to be a critical factor in atherosclerotic progress where in ammation responses plays a vital role. A decrease in lymphocyte cells was related to worse clinical outcomes in patients with cardiovascular disease, especially ACS. Furthermore, it has been demonstrated that increased lymphocytes apoptosis has negative impact on ejection fraction, area of myocardial necrosis and microvascular function in patients with ACS, especially acute myocardial infarction (AMI) [22][23][24] . As a combination of two different biomarkers of in ammation, MLR can provide additive value to the evaluation of cardiovascular diseases. It could be concluded that high monocytes, low lymphocytes or both are associated with higher risk of CAD. Meanwhile, MLR is a widely available, inexpensive, and non-invasive biomarker.
Coronary unstable plaques are more prone to plaque rupture with subsequent thrombogenesis leading to episodes of acute coronary syndrome. Lipid-rich plaques with thin brous caps are the highest-risk and most vulnerable plaque phenotype. The TCFA, which characterizes as thin brous cap and a great lipid angle, tends to cause major adverse cardiovascular events [25] . OCT, as a high-resolution imaging modality, has great advantages in identifying TCFA and providing detailed information about the lipid plaque, such as FCT. Meanwhile, OCT enables in vivo visualization of plaque burden, plaque morphology and response to therapy. In the present study, the plaque in high MLR group showed more vulnerable characteristics which had more prone to plaque rupture, leading to the occurrence of adverse cardiac events. And a conspicuously negative relation was found between MLR and FCT. The measurement of the necrotic lipid core and the FCT play crucial roles in the dynamics of plaque destabilization. The pathophysiological mechanisms between elevated MLR and plaque vulnerability may be related to immune-in ammatory response [1][2][3] . In ammation, proteolysis, and decreased collagen ber content are likely to lead to plaque rupture.
The presence of vulnerable plaques is the main cause of ACS [26] . Previous study demonstrated that MLR level was independently with TCFA in the patients with stable angina. The vulnerable plaque was identi ed by intravascular ultrasound (IVUS), which cannot provide speci c morphology characteristics of plaque, such as the thickness of plaque components [27] . In this study, the advantage of OCT in identifying vulnerable plaques was utilized to provide a direct link between MLR and the vulnerability of coronary atherosclerotic plaques in ACS patients, such as FCT and lipid core angle. The present study found the incidence of TCFA and plaque rupture were higher in high MLR group. After adjusting for potential confounding factors, MLR was an independent predictable factor of TCFA. Based on ROC curve analysis, the cut-off value for MLR level was 0.21, with a sensitivity of 72.0% and speci city of 66.1% for predicting TCFA. Compared with monocytes count or lymphocytes count, we considered that MLR, as a combination of two in ammatory indicators, may has a greater advantage in identifying plaque vulnerability in ACS patients.

Study limitations
The current study has several potential limitations. Firstly, this study was a small sample and singlecenter retrospective study, this was a major limitation and source of bias for the ndings. More trials are needed to con rm this result. Secondly, we were concerned with atherosclerotic plaques in culprit vessels, but not non-target lesions. Thirdly, the study was a cross-sectional trial, but no follow-up data and prognostic implications were analyzed. Despite these limitations, this study rstly explored the value of MLR level in vulnerability to coronary plaque in ACS patients.

Conclusion
In conclusion, the coronary atherosclerotic plaques in high MLR group exhibited more vulnerable characteristics. Meanwhile, elevated MLR level was independently with OCT-detected TCFA in culprit vessels in patients with ACS. These results re ected that circulating MLR, as a noninvasive and widely available in ammation biomarker, may has a potential value for determining atherosclerotic plaque vulnerability in the setting of ACS.          Correlation analysis of monocyte-to-lymphocyte ratio(MLR) and brous cap thickness (FCT). The MLR was negatively associated with FCT (R = 0.255, P =0.005).