Background
During the development of coronary atherosclerosis, atherosclerotic plaque enlargement accompanied with narrowing of arterial luminal results in a series of chronic ischemic manifestations. More seriously, plaque instability and ensuing rupture with superimposed thrombi eventually lead to myocardial infarction and sudden death [
1]. Thin-cap fibroatheroma (TCFA) is the most common phenotype of vulnerable plaque and is the precursor of plaque rupture. Although a greater understanding of lesion dimensions favors the clinical detection of TCFA in coronary artery disease (CAD) patients, coronary angiography alone could not provide comprehensive information on plaque morphology beyond luminal narrowing. Current clinical studies have suggested that optical coherence tomography (OCT) is one of the most preferred invasive approaches that allow high-resolution (10 μm) tomographic intra-arterial imaging [
2,
3].
As professional antigen-presenting cells, monocytes and dendritic cells (DCs) possess the ability to recognize and present antigens to T cells [
4]. Human circulating monocytes could be categorized into three subsets according to different expression of membrane receptors CD14 and CD16: a numerically dominant CD14++CD16- subset (Mon1), an intermediate CD14++CD16+ subset (Mon2), and a non-classical CD14 + CD16++ subset (Mon3) [
5,
6]. Given the heterogeneity of circulating monocytes, the exact identity of atherosclerosis-related monocyte subsets deserves careful consideration. In this regard, some studies demonstrated that CD16 positive monocytes were associated with the incidence of myocardial infarction and in-stent restenosis [
7,
8]. Furthermore, Rogacev et al. [
9] observed that Mon2 subsets independently predicted the incidence of cardiovascular disease. Conversely, an earlier study by Shantsila et al. [
10] did not find a marked correlation between monocyte subsets and the incidence of CAD. To our best knowledge, the relationship between monocyte subsets and atherosclerotic progress has not been examined.
Depending on the origin, location and function, DCs comprise two heterogeneous subpopulations. For human, plasmacytoid DCs (pDCs) express the surface marker blood dendritic cell antigen (BDCA)-2, while myeloid DCs (mDCs) could be categorized into mDC1 subsets with BDCA-1 and mDC2 subsets with BDCA-3 [
11]. Recent reviews have helped solidify the knowledge of pDCs and mDCs in atherosclerosis [
12]. Indeed, several clinical trials attempting to address whether circulating DC subsets correlate with the incidence and severity of CAD reached contradictory results [
13,
14].
Toward this end, we sought to explore which subsets of DCs and monocytes are associated with the emergence, vulnerability and rupture of atherosclerotic plaques identified by coronary angiography and OCT.
Discussion
In the present study, we investigated the association of circulating inflammatory cells with morphological characteristics of atherosclerotic plaque observed by coronary angiography and OCT. The results herein demonstrated that elevated CD14++CD16+ Mon2 subsets were associated with plaque vulnerability and rupture determined by angiography and OCT.
There were a growing number of researches regarding the relationships among circulating DCs, inflammation and their effects on coronary plaque, whereas conflicting results were reported. While Yilmaz et al. [
13] and Kretzschmar et al. [
20] observed markedly decreases in circulating mDCs, pDCs and total DCs in stable CAD and STEMI patients, Wen et al. [
21] found a reduction in circulating mDCs, but not pDCs, in patients with unstable CAD. The disparate findings among published literatures may be explained as follow. Total DCs make up only approximate a 0.4 percentage of leukocytes in peripheral blood. It is thus conceivable that the absolute numbers of circulating DC subsets may fluctuate wildly because of changes in total leukocyte counts in different disease states and errors in operation. In our study, we observed that UAP and STEMI patients had a lower level of circulating mDC2s when compared with controls without coronary plaque. Human mDC2s share most features with murine CD8α + DCs, while mDC1s represent human counterpart of murine CD8α- DCs [
22]. In response to viral infections and inflammatory stimuli, mDC1s activates CD4+ T lymphocytes and possess the capacity to promote Treg differetiation [
23]. Conversely, mDC2s prime CD8+ T lymphocytes [
24]. The mechanisms of decreased circulating mDC2s in CAD are unclear; however, the possible theories have been postulated. In former studies, decreased circulating DCs in CAD were shown to be accompanied with a resident DC increase in the inflamed plaques and infarct myocardium, inferring that circulating DC precursor recruitment to inflamed plaques was a part of the underlying disease process [
20,
25]. Additionally, the pathological study showed a high density of resident DCs with frequent T lymphocyte contacts in unstable plaques, suggesting that tissue-resident DCs, at least in part, facilitated local T lymphocyte activation and aggravated atherosclerotic development [
26,
27]. Collectively, it is likely that decreased circulating mDC2s in CAD patients are caused by their recruitment into inflamed lesions and infarct myocardium. Subsequently, mature mDC2s in response to inflammatory stimuli within vulnerable plaques promote CD8+ T lymphocyte activation and aggravate atherosclerotic development [
27].
According to the severity of symptoms, some published studies grouped CAD patients into patients with stable angina pectoris (SAP) and with acute coronary syndrome (ACS). They reported that mDCs or pDCs were dramatically declined as CAD became more serious [
13]. However, these subjects were not classified according to the plaque morphology in their coronary arteries. Although TCFA determined by OCT are found more prevalent in patients with ACS than in those with SAP, there are approximate 20% patients with SAP that have TCFA observed by OCT. [
2] In agreement with the previous meta-analysis, the prevalence of TCFA is markedly higher in ACS group than that in SAP group [
28]. TCFA harboring in patients with SAP seems to be neglected by clinical presentations but gradually evolve into plaque rupture [
29]. Therefore, we applied OCT, which allows for the accurate assessment of plaque characteristics, to objectively identify the different stages of atherosclerosis. It should be noted that plaque rupture detected by OCT accounts for over 40% ACS, while the other ACS are caused by plaque erosion and calcified nodule [
30]. As compared with plaque rupture, plaque erosion and calcified nodule are more prevalent in fibrous plaque than those in lipid plaque and TCFA [
31]. Therefore, apart from vulnerable plaque characterized as TCFA, culprit plaque erosion and calcification pertain to fibrous plaque also contribute to the incidence of ACS. Additionally, TCFA alone is unable to correctly predict the adverse events in CAD patients. In contrast, TCFA combined with traditional risk factors is referred to be more feasible for evaluate the prognosis of CAD [
28]. In patients undergoing coronary angiography, mDC2s were decreased in UAP and STEMI patients when compared with healthy controls, whereas the percentage and absolute number of mDC2s remained similar between UAP and STEMI patients. Moreover, as reported in the OCT subgroup, mDC2s were not significantly altered between patients with and without TCFA. Collectively, these results implied that decreased circulating mDC2s were associated with the presence of coronary plaque but not altered during plaque destabilization and rupture. Though CD11c + DCs were found to reside in vulnerable plaques in mice and humans, it was uncertain whether these resident DCs were derived from circulating common DC precursors or monocytes [
25]. Moreover, the specific markers are needed to distinguish the different subsets of resident DCs in coronary plaques. [
12]
Current studies unveil that monocyte subsets have different propensities to migrate the vascular wall and differentiate into particular subsets of DCs and macrophages located in plaques. [
32,
33] This recognition prompted researchers to determine the association between distinct monocyte subsets and CAD. It is worth noting that some laboratories only referred to two monocyte subpopulations (CD14 + CD16- and CD14 + CD16+ monocytes). The consistent result of these studies was that the count of CD16 positive subsets was higher in patients with CAD and MI as compared with healthy controls.[
34,
35] Rogacev et al. [
9] found that the count of CD14++CD16+ Mon2 subsets was an independent predictor of adverse cardiovascular events. Tapp et al. [
7] reported that circulating Mon2 subsets were prominently elevated in STEMI patients compared with patients with stable CAD and healthy volunteers. Our work extends the results of earlier studies on CAD patients diagnosed by coronary angiography. We showed that elevated CD14++CD16+ Mon2 subsets and reduced CD14 + CD16++ Mon3 subsets were associated with the presence and progression of coronary plaques determined by coronary angiography and OCT. In addition, we found that the proportion of circulating Mon3 subsets were positively correlated with fibrous cap thickness and negatively correlated with arc of lipid core, whereas none of monocyte subsets were significantly correlated with plaque volume. Considering that plaque composition, rather than plaque volume, plays an important role in the plaque disruption and subsequent thrombosis that leads to acute cardiovascular events [
1], higher proportion and absolute number of Mon2 may provide important information to predict the presence of TCFA and the progression of atherosclerosis in coronary arteries.
Pathological studies have pushed forward the recognition of human monocyte heterogeneity and biological functions [
5,
32]. Mechanistically, the intermediate Mon2 subsets expressing high levels of Tie-2 and CXCR4 possess pro-angiogenic properties [
36,
37]. Recent studies uncovered that neovascularization of vasa vasorum within plaques facilitate macrophage infilitration and precipitate plaque vulnerability and rupture [
38,
39]. Another important mechanism is that circulating monocytes eventually differentiate into resident DCs/macrophages in atheroma and myocardium, and continue to function during the development of atherosclerosis and myocardial infarction [
40]. In this regard, our results revealed that Mon1 and Mon2 were highly related to the serological markers of plaque rupture and myocardial infarction. Murine counterparts of human circulating monocytes could be delineated as Ly-6C
hi and Ly-6C
lo monocytes. In murine experiments, Ly-6C
hi monocytes substantially increased in peripheral blood of Apoe−/− mice fed a high-fat diet and differentiated into pro-inflammatory M1-type macrophages, while Ly-6C
lo monocytes differentiated more readily into anti-inflammatory M2-type macrophages [
41]. Taken together, these phenomena suggest that both higher Mon2 counts may precipitate plaque destabilization and rupture.
Study limitations
There were some limitations associated with the present study. First, the study population was relatively small and the study design was cross-sectional in nature. Therefore, we were unable to distinguish whether or not the variation in circulating monocyte subsets is causative of atherosclerotic process. There is a need for prospective studies that investigate the predictive value of circulating DC and monocyte subsets for the development of coronary plaque. Additionally, currently, there was no standardization in the measurement of circulating DC and monocyte subsets.