Monocytes circulate in the blood and migrate to inflammatory tissues, but their functions can be either detrimental or beneficial, depending on their phenotypes. In humans, classical monocytes are inflammatory cluster of differentiation (CD)14++CD16CCR2++ cells originated from the bone marrow or spleen reservoirs and comprise ≥92% of monocytes. Intermediate monocytes (CD14++CD16+CCR2+) are involved in the production of anti-inflammatory cytokines [such as interleukin (IL)-10], reactive oxygen species (ROS), and proinflammatory mediators [such as tumor necrosis factor-α (TNF-α) and IL-1β). Nonclassical monocytes (CD14+CD16++CCR2) are patrolling cells involved in tissue repair and debris removal from the vasculature. Many studies in both humans and animals have shown the importance of monocyte chemoattractant protein-1 (MCP-1) and its receptor [chemokine receptor of MCP-1 (CCR2)] in pathologies, such as atherosclerosis and myocardial infarction (MI). This review presents the importance of these monocyte subsets in cardiovascular diseases (CVDs), and sheds light on new strategies for the blocking of the MCP-1/CCR2 axis as a therapeutic goal for treating vascular disorders.

Monocyte heterogeneity

Monocytes are large circulating cells that originate from a common myeloid precursor in the bone marrow and give rise to macrophages and dendritic cells, which circulate in the blood for approximately 72 h and then migrate into tissues [1]. In addition, the spleen is also a monocyte reservoir that can contribute to tissue injury responses, but the studies focusing on the role of the spleen in this scenario are still scarce [2,3]. Some studies have shown that monocytes are highly plastic and might differentiate according to microenvironment changes [4,5].

In humans, the heterogeneity of monocytes was first described by Passlick et al. [6], with the cell surface expression of cluster of differentiation (CD)14 [lipopolysaccharide (LPS) receptor] and CD16 (FcγIII receptor). These cells play a pivotal role in inflammation and exhibit both detrimental and beneficial activities in different conditions, such as atherosclerosis and myocardial infarction (MI). Furthermore, the disease stage and risk factors might differentially affect the monocyte subtypes in humans [7]. Recently, other staining protocols proved that even the CD16+ population is heterogeneous in its functions, resulting in the classification of monocytes as ‘classical’, ‘nonclassical’, and ‘intermediate’ monocytes (Table 1) [1,4].

Table 1
Monocyte subsets in human and mouse blood [1,34]
Monocytes Human Mouse Functions 
Classical CD14++CD16CCR2++ Ly6C++CD43+CCR2++ Inflammatory cells that respond to many stimuli originated from damaged/infected tissue and produce inflammatory cytokines [3,10]. 
Intermediate CD14++CD16+CCR2+ Ly6C++CD43++CCR2++ Highly phagocytic cells that produce high levels of ROS and inflammatory mediators [25,26]. 
Nonclassical CD14+CD16++CCR2 Ly6C+CD43++CCR2 Reparative/patrolling cells that remove debris from vasculature and produce high levels of anti-inflammatory factors [14]. 
Monocytes Human Mouse Functions 
Classical CD14++CD16CCR2++ Ly6C++CD43+CCR2++ Inflammatory cells that respond to many stimuli originated from damaged/infected tissue and produce inflammatory cytokines [3,10]. 
Intermediate CD14++CD16+CCR2+ Ly6C++CD43++CCR2++ Highly phagocytic cells that produce high levels of ROS and inflammatory mediators [25,26]. 
Nonclassical CD14+CD16++CCR2 Ly6C+CD43++CCR2 Reparative/patrolling cells that remove debris from vasculature and produce high levels of anti-inflammatory factors [14]. 

Classical monocytes [CD14++CD16 in humans and lymphocyte antigen 6 complex (Ly6C)++CD43+ in mice] [2] are considered inflammatory cells, which express high levels of CCR2 (chemokine receptor of monocyte chemoattractant protein-1, MCP-1); these cells comprise ≥92% of monocytes in humans, whereas the frequency in mice is approximately 50% [8,9]. These cells are recruited from the bone marrow and splenic reservoirs to sites of active inflammation and respond to different stimuli generated by damaged and/or infected tissue by producing inflammatory cytokines, such as interleukin (IL)-1, IL-12, and tumor necrosis factor-α (TNF-α) [3,10].

In contrast, ‘nonclassical’ monocytes (CD14+CD16++ in humans and Ly6C+CD43++ in mice) [11] do not express CCR2 and can remove debris from the vasculature (reparative/patrolling monocytes) [12,13]. This subset has an extended intravascular and tissue presence [13] and produces low levels of CD14 and proinflammatory cytokines, as well as high levels of anti-inflammatory factors. However, the literature is limited concerning the capacity of these cells to differentiate into effector macrophage populations. Some studies suggest that nonclassical monocytes can repair the vasculature or remove lipids from the blood in atherosclerotic disease [14].

Some of the nonclassical monocytes are derived from classical monocytes [1517], but they might also arise independently [18]. However, some authors argue that there is no experimental evidence to support the differentiation between classical and nonclassical monocytes beyond the distinct morphology, in vivo functions, and signal transduction pathways [19].

Both classical and nonclassical monocytes are related to atherosclerosis; however, a higher percentage of classical monocytes enter the plaque. On the other hand, some studies speculate that nonclassical monocytes, even in small numbers, might be cholesterol transporters in hypercholesterolemia, in both humans and mice [9,20,21].

‘Intermediate’ monocytes (CD14++CD16+ in humans and Ly6C++CD43++ in mice) [11] are the least characterized monocyte subset, although recent studies have highlighted its importance in two global health diseases: human immunodeficiency virus-1 (HIV-1) infection [22] and cardiovascular diseases (CVDs) [23]. These cells express CCR2 and were identified during cytokine-induced in vitro differentiation from the intermediate phenotype (CD14++CD16+) to the nonclassical phenotype (CD14+CD16++) [1,24]. Intermediate monocytes seem to be highly phagocytic, an important capacity in some conditions such as in the removal of apoptotic cells after MI [25]. Furthermore, these cells produce high levels of reactive oxygen species (ROS) [26] and inflammatory mediators (such as TNF-α and IL-1β). Skrzeczynska-Moncznik et al. [24] reported increased production of the anti-inflammatory cytokine IL-10 in human intermediate monocytes upon in vitro LPS stimulation.

Studies in both humans and animals suggested that under inflammatory conditions, the emergence of the three monocyte subsets is a gradual process (Figure 1) in which classical monocytes leave the bone marrow and spleen and might give rise to intermediate monocytes in a few days followed by nonclassical monocytes [4], suggesting that intermediate monocytes might be a transient stage of differentiation. These data were reinforced by a study in humans that shows a shorter telomere length in nonclassical monocytes, which are considered the most mature cells [26]; however, evidence for this process of differentiation is still lacking.

Monocyte subsets, differentiation, and recruitment to the atherosclerotic plaque

Figure 1
Monocyte subsets, differentiation, and recruitment to the atherosclerotic plaque

Some studies suggest that classical monocytes originate both intermediate and nonclassical monocytes in the bloodstream, under inflammatory conditions. Bone marrow and spleen represent sources of these cells. Classical and intermediate subsets express the chemokine receptor CCR2, related to atherosclerosis development. On the other hand, the nonclassical subset is noninflammatory and does not express CCR2 [24,30].

*Circulating CD14+ monocytes can give rise to different cell types, for example endothelial-like cells, suggesting that these monocytes can be multipotent precursors of distinct cell populations [2729].

Figure 1
Monocyte subsets, differentiation, and recruitment to the atherosclerotic plaque

Some studies suggest that classical monocytes originate both intermediate and nonclassical monocytes in the bloodstream, under inflammatory conditions. Bone marrow and spleen represent sources of these cells. Classical and intermediate subsets express the chemokine receptor CCR2, related to atherosclerosis development. On the other hand, the nonclassical subset is noninflammatory and does not express CCR2 [24,30].

*Circulating CD14+ monocytes can give rise to different cell types, for example endothelial-like cells, suggesting that these monocytes can be multipotent precursors of distinct cell populations [2729].

Most articles used CD14, CCR2, and CD16 to classify the monocyte subtypes [1,4]. On the other hand, other protocols consider intermediate and nonclassical monocytes as a single subtype [31]. In other studies [8,32], the authors used a granulocyte marker (Gr1) and the following definition: Gr1highCCR2high CX3C chemokine receptor 1 (CX3CR1)low for classical monocytes and Gr1lowCCR2lowCX3CR1high for nonclassical monocytes.

The heterogeneity of the definitions for monocytes subsets creates difficulty in comparing studies and increases confusion in the nomenclature of the monocyte subtypes; the different antibodies used for identifying these cells and the variability among species are other challenging issues. Furthermore, there are many discrepancies in the monocyte classification in the literature, mainly related to intermediate monocytes.

Some laboratories use additional antibodies, such as CD115 and CD62L [8,33]. In addition, as noted by Ziegler-Heitbrock et al. [1], possible contaminants of the population of interest must be excluded, most of which concern the very low frequency of intermediate and nonclassical monocytes (approximately 15% of all monocytes). Indeed, some studies use terms such as ‘inflammatory’ or ‘proinflammatory’ monocytes, which do not represent the same populations in humans and mice [1].

Monocytes, MCP-1/CCR2, and CVD

MCP-1, also known as CCL2 (chemokine ligand 2), is a chemoattractant that is involved in the migration of endothelial cells and monocyte recruitment to inflammatory sites [34,35]. Its receptor, CCR2, is present on the surface of monocytes/macrophages, endothelial cells, leukocytes, and smooth muscle cells [36,37]; it is also expressed in basophils, dendritic and natural killer cells, and activated T lymphocytes [38].

The role of CCR2 in monocyte recruitment is well described in the literature. Some studies have shown that monocytes do not migrate in response to MCP-1 in CCR2−/− mice; these cells are less capable of adhering to the endothelium, suggesting the importance of CCR2 in the strong adhesion of monocytes to the endothelium [39].

Han et al. [39] treated cultured human monocytic leukemia cell line (THP-1) cells with low-density lipoprotein (LDL) and observed increased CCR2 expression. These results were confirmed in vivo. In the same article, the authors showed a 2-fold increase in CCR2 expression in monocytes isolated from individuals with hypercholesterolemia compared with normal patients and observed a significant correlation between CCR2 expression and plasma concentration of LDL. The authors concluded that proatherogenic lipoproteins increase the chemotactic motility and recruitment of monocytes to the vessel wall and might accelerate the progression of atherosclerosis.

A recently published study by Yang et al. [40] evaluated the effect of intensive treatment with atorvastatin on monocyte recruitment in patients with unstable angina. The results showed lower levels of CCR2 on the cell surface of CD14+ monocytes after percutaneous coronary intervention (PCI) in the intensive treatment group compared with the control group, as well as a reduction in MCP-1 monocyte chemotaxis. These findings suggest that intensive atorvastatin treatment exerts anti-inflammatory activity, since it down-regulated CCR2 expression and reduced CD14+ monocyte-induced inflammation. A mouse study conducted by Swirski et al. [41] found similar results, as the authors showed that the anti-inflammatory effects of statins beyond lowering LDL levels could reduce the number of classical monocytes.

Atherosclerosis

Atherosclerosis is an inflammatory disease in which circulating oxidized lipids activate endothelial cells, resulting in monocyte recruitment and differentiation into macrophages that are rich in lipids (foam cells), followed by lesion formation in the arterial wall [42].

The literature shows, in mice, that classical monocytes are precursors of ‘inflammatory’ M1 macrophages. In addition, studies suggest that nonclassical monocytes differentiate into resident or ‘anti-inflammatory’ M2 macrophages that can block the inflammatory process in atherosclerotic plaques [14,43,44]. Furthermore, a mouse study suggested that monocyte recruitment is important only during the early stages of atherosclerosis and that the local macrophage proliferation is responsible for atherosclerotic progression [45]. These findings suggest that the origin of M1 and M2 macrophages in atherosclerotic plaques is still unclear and that additional studies are needed to elucidate the processes involved in monocyte recruitment (as well as the consequent differentiation into M1 and M2 macrophages) in early stage lesions and in the chronic stages of this disease [46].

In 1998, Gu et al. [47] evaluated the relationship between MCP-1 and atherosclerosis in low-density lipoprotein receptor (LDL-R) and MCP-1 knockout mice fed a cholesterol-enriched diet. They found fewer macrophages entering the aortic wall, suggesting that there was less monocyte recruitment and that the atherosclerotic plaque was reduced. In the same year, Boring et al. [48] found high levels of macrophages in the subendothelial space in CCR2+/+/apolipoprotein E (apoE)+/+ mice fed a high-fat diet for 5 weeks. However, fewer macrophages were found in the aortas of CCR2−/−/apoE+/+ animals. These results suggest the importance of CCR2 in the recruitment of monocytes/macrophages to the arterial wall. However, these studies evaluated macrophages and not the three monocyte subsets, which is a limitation since the specialized immunological functions of these cells are different and they have the capacity to rapidly adapt to modified microenvironments [25].

Many observations in both mice and humans have associated increased CCR2/MCP-1 expression with the progression of atherosclerosis, because increased circulating MCP-1 levels are related to increased risk of CVD. Schober et al. [49] evaluated the effect of CCR2 deficiency on neointima formation and monocyte recruitment in hyperlipidemic mice subjected to wire-induced carotid artery injury. At 28 days after injury, they found a 47% reduction in the neointimal area in the CCR2−/−/apoE−/− animals compared with CCR2+/+/apoE−/− mice. Furthermore, the plaque had a more stable phenotype in CCR2 deficient animals, reinforcing the importance of CCR2 in the development of atherosclerosis.

Swirski et al. [41] evaluated classical and nonclassical monocytes in C57BL/6 wild-type (apoE+/+) and apoE−/− mice fed either regular chow or a Western diet (high in cholesterol and fat) for 25 weeks. They found a 14-fold increase in the classical (inflammatory) monocytes in the hyperlipidemic animals without differences in the number of nonclassical monocytes (repair cells). These findings were reinforced by quantification of the monocyte subtypes in the bone marrow, peripheral blood, and spleen in mice fed a Western diet, in which monocytosis promoted increased survival and proliferation as well as impaired conversion from classical to nonclassical monocytes. The authors also showed that classical monocytes adhered to the activated endothelium, accumulated in the atherosclerotic plaques, and gave rise to lesional macrophages, confirming the inflammatory role of this monocyte subset.

Nagareddy et al. [50] evaluated monocyte tracking in mice with diabetes, which is the main risk factor related to atherosclerosis. The authors showed that the recruitment of Ly6-Chi monocytes into lesions causes the increase in macrophage numbers in hyperglycemia and results in worse lesion progression, highlighting the importance of persistent monocytosis during the development of atherosclerotic plaques in individuals with diabetes. A similar study was performed by the same group in 2014 [51], with a focus on obesity, which is another important risk factor in atherosclerosis. The authors found that chronically inflamed visceral adipose tissue in obese mice is related to increased monocyte recruitment from the bone marrow, confirming the relationship between obesity and monocytosis.

Berg et al. [52] evaluated human monocyte subsets in the context of atherosclerosis by measuring the intima media thickness of the carotid artery. There was no correlation between classical monocytes (CD14++CD16CCR2+) and the burden of atherosclerosis before a first event (MI or stroke). However, the authors found higher levels of classical monocytes in the group with previous CV events, suggesting that these cells might predict CV events.

Some data have demonstrated that both classical and nonclassical monocyte subtypes can enter atherosclerotic plaques in mice [53]. However, there are few data describing the role of nonclassical monocytes in the development of atherosclerosis, especially concerning human studies, although the literature suggests a role of these cells in the repair of the vasculature in atherosclerosis and in the removal of lipids from the circulation (i.e. patrolling function) [44]. In addition, McArdle et al. [54] proved the existence of nonclassical monocytes in apoE−/− mice in the vessel lumen via intravital imaging.

A recent study developed by Wildgruber et al. [7] evaluated the three monocyte subsets in humans with peripheral artery disease (PAD). There were no changes in the number of classical monocytes (CD14++CD16CCR2+) related to the different stages of PAD, although there was a decrease in nonclassical monocytes (CD14+CD16++CCR2) with advancing PAD, suggesting an impaired healing process. On the other hand, the number of intermediate monocytes (CD14++CD16+CCR2+) increased with the progression of PAD, indicating the proinflammatory function of this monocyte subset. The authors argued that intermediate monocytes might be useful biomarkers for disease progression, particularly in PAD.

Myocardial infarction

Studies evaluating the relationship between monocyte levels and MI in both humans and mice indicate that the environment can favor the recruitment of different monocytes and that some subsets might have stronger associations with MI than others. Based on this affirmation, some studies have shown a sequential mobilization of different monocyte subtypes after MI related to the release of subset-specific chemokines [5,55,56]. Varol et al. [17] showed in mice that in the absence of inflammation, classical monocytes return to the bone marrow and might give rise to other subsets of monocytes.

The increase in macrophage levels after MI depends on the recruitment of monocytes originating from hematopoietic progenitor cells in the bone marrow and spleen, as well as from local macrophage proliferation. Mouse studies have shown that the spleen produces classical monocytes 8 weeks after a large MI compared with stable conditions, suggesting that splenic reservoirs might be useful as a rich peripheral source of circulating monocytes [2].

Nahrendorf et al. [5] showed that classical monocytes in mice predominate up to the 3rd day after MI and exhibit phagocytic, proteolytic, and inflammatory activities. On the other hand, nonclassical monocytes have a peak on the 7th day after MI and are involved with myocardial healing, via myofibroblasts accumulation, angiogenesis, collagen deposition, and expression of the chemokine CX3CR1. Based on these findings in mice, the authors suggested that patients with atherosclerosis and monocytosis might have an impaired capacity for efficient myocardial healing. In this situation, a higher number of classical monocytes than nonclassical monocytes in the infarcted area would be expected.

Some evidence in the literature has shown the central role of monocytes in restenosis after balloon angioplasty and stent implantation in individuals following ST-elevation myocardial infarction (STEMI). Tsujioka et al. [56] evaluated the monocyte subset counts in patients after acute myocardial infarction (AMI) soon after hospital admission for up to 12 days and found high levels of classical and nonclassical monocytes after AMI in patients successfully treated with PCI. The peak levels of classical but not nonclassical monocytes were negatively correlated to the extent of myocardial salvage as observed 7 days after AMI, indicating the importance of classical subsets during the early stages of recovery after MI. On the other hand, Liu et al. [57] quantified the monocyte subsets in patients 12 days after STEMI (to avoid possible interferences of myocardial necrosis). The authors found a positive correlation between the circulating nonclassical monocytes and in-stent restenosis, suggesting that this subtype has clinical significance as a marker of late in-stent restenosis in patients with STEMI. However, neither study described a mechanistic explanation for the results obtained both in the early and late stages after MI, and large-scale clinical trials that either confirm or refute these findings are necessary.

Tapp et al. [55] evaluated the three monocyte subsets from the acute phase to the recovery phase following STEMI. There were higher levels of classical and mainly intermediate monocytes (CD14++CD16+CCR2+) on day 1 following STEMI, without differences in nonclassical monocytes, which supports the hypothesis that in humans, this subset has other functions compared with classical and intermediate monocyte subsets. This study also showed that CCR2 expression in classical and mainly intermediate monocytes was significantly increased in STEMI compared with control groups. In addition, there was a change in the intermediate monocyte phenotype following STEMI, including an increase in CD14 and CCR2 expression levels and a reduction in CD16 expression. There was a significant reduction in the intermediate monocyte count by day 7 after MI, suggesting that this subset is most functionally active in the first few days after an MI event.

MCP-1/CCR2 blockade as a therapeutic strategy

Studies showing the impact of CCR2 in CVD have suggested that this receptor can be a promising therapeutic target after MI and in atherosclerotic disease. The healing phase is crucial for better outcomes post-MI, since impaired healing can cause an increase in the infarct area and more left ventricular remodeling. In this sense, the literature has suggested that targeting CCR2 with small interfering RNAs (siRNAs) may be useful in the treatment of CVD and other disorders. In apoE−/− mice, reduced levels of classical monocytes have been observed in the infarct area 4 days after coronary ligation. In the same study, the researchers applied CCR2 siRNA in vivo and found a reduction in the number of classical monocytes (which express CCR2) at the sites of inflammation, without differences in the nonclassical monocyte subset; this subset is related to the receptor CX3CR1 and does not express CCR2. The authors concluded that CCR2 siRNA could be useful as a therapeutic tool to reduce left ventricular remodeling and improve post-MI heart failure in the animals. However, the authors highlighted that targeting innate immune cells such as monocytes should be done with caution, because these cells have not only disease-promoting but also protective functions [58].

Increased levels of CCR2 were found in heart tissue biopsies from patients with myocarditis. This disease is the leading cause of sudden death among young adults. In this study, the effects of CCR2 siRNA in mice with autoimmune myocarditis were tested and the data showed a reduction in classical monocytes, suggesting that blocking CCR2 expression could be useful in the treatment of myocarditis. According to the authors, there is high variability among the clinical manifestations of this disease and the results concerning immunosuppressives are disappointing, which reinforces the blockade of CCR2 activity as a potentially useful approach in the treatment of myocarditis. On the other hand, the authors argued that treatment by silencing CCR2 (siCCR2) probably is unsuitable for patients with active viral myocarditis, because monocytes are important in the defense against infection [59].

Monoclonal antibodies have also been used as a possible atherosclerosis treatment since they might block CCR2 function. Gilbert et al. [60] evaluated the effects of the monoclonal antibody MLN1202, which binds to CCR2 and inhibits MCP-1 binding, in patients with at least two CV risk factors and high-sensitivity C-reactive protein (hsCRP) levels >3 mg/l (a biomarker of CVD). There was decrease in hsCRP levels, elevated serum MCP-1 levels, and a reversible reduction in circulating monocytes, although this study did not discriminate the three monocyte subsets. Indeed, only one CCR2 ligand (MCP-1) has been evaluated and the effects of other ligands are still unknown.

Okamoto et al. [61] examined the effects of the CCR2 antagonist, TLK-19705, on atherosclerosis and diabetic nephropathy in mouse models. This antagonist inhibits chemotaxis induced by MCP-1 both in vitro and in vivo. The authors found that treatment with this inhibitor reduced the area of the atherosclerotic lesions in apoE−/− mice. Similar findings were obtained with the antagonist propagermanium (PG) in THP-1 cells, [62]; PG also prevented tissue damage by inhibiting monocyte recruitment in mice [63] and likely blocking MCP-1/CCR2 activity in a manner similar to TLK-19705. These results suggest a useful role for both antagonists as therapeutic tools in inflammatory conditions.

Pharmacological inhibition of MCP-1 might also represent an approach to prevent detrimental vascular remodeling and to minimize damage after MI. Liehn et al. [64] used a competitive mutant MCP-1 protein (PA508) in vitro as a possible antagonist for modulating monocyte infiltration in atherosclerotic lesions. They found reduced levels of monocyte chemotaxis and migration. Furthermore, the authors showed that treating mice under myocardial ischemia/reperfusion with PA508 maintained heart function and led to a reduction in the size of the area affected by MI. These data indicate favorable effects of PA508 related to pathological conditions and that this competitor might be a new drug for the treatment of vascular diseases.

Usui et al. [65] used an MCP-1 inhibitor to determine the role of MCP-1 in restenotic changes after balloon injury in rats. They found both reduced inflammation and cell proliferation, which indicated the importance of MCP-1 in neointimal formation after balloon injury. The authors highlighted the need for studies evaluating the safety of long-term MCP-1 inhibition, since it may affect systemic immunoprotection ability in humans.

Frangogiannis et al. [66] evaluated MCP-1 antibody neutralization in the pathogenesis of fibrotic cardiomyopathy in mice; this condition is common upon repetitive ischemia/reperfusion injury. Animals treated with the antibody had reduced collagen deposition after 7 days of reperfusion, compared with vehicle-treated mice. This inhibition also promoted amelioration in left ventricular dysfunction after brief repetitive ischemia/reperfusion, which was related to a reduction in macrophage infiltration. The authors highlighted that these findings may be helpful in treating patients with cardiomyopathy; however, there are risks of blocking MCP-1 activity, which include collateral vessel formation due to the recruitment of macrophages that release angiogenic factors.

In recent studies, disappointing results related to inhibition of the MCP-1/CCR2 axis were reported [67]. The use of INCB-3344, a CCR2 small molecule antagonist in apoE−/− mice, did not reduce either the lesion size or the advancement of the lesion; the authors argued that the antagonists might not promote a total effect, similar to the blockade with antibodies in apoE−/− mice. In another study [68], the endogenous murine CCR2 gene was replaced with human CCR2 (huCCR2ki) and the authors analyzed the effect of the CCR2 antagonist GSK1344386B on atherosclerotic development, accelerated by a Western diet and infusion of angiotensin II. There was no reduction in the aortic root lesion area, although the authors observed a decreased macrophage area by 30% in these animals. According to the authors, these inhibitors might be related to the blockade of multiple other chemokine signaling pathways, unrelated to inflammatory processes. Both studies suggest that data concerning the antagonistic efficacy of these small molecules in CVD are inconclusive and additional research is necessary.

Some limitations related to therapeutic advantages of MCP-1/CCR2 antibodies and antagonists to treat CVD have been described in the literature. One point of consideration is the contribution of off-target effects. For example, siCCR2 may not be completely specific to inflammatory monocytes, as other cells (such as natural killer cells) depend on CCR2 for their recruitment. Furthermore, some studies describe other off-target effects, such as toxicity [58] and the potential for compensatory activity by other receptors in response to blocking CCR2 function [67]. Indeed, some small molecule antagonists, such as TAK-779 are not specific for CCR2 and might inhibit other chemokines, such as CXCR3 and CCR5.

According to Tapp et al. [55], the CCR2 sequence homology between the human and murine homologs is limited (approximately 85%) and there is divergence concerning the CCR2 ligands. The blockade of multiple chemokines is related to their affinity for multiple receptors, which remains a challenge in the development of chemokine antagonists.

Conclusions

The findings presented in the present study suggest that there is heterogeneity among monocyte subtypes, with discrepant results when the studies are compared. In addition, in conditions such as atherosclerosis and MI, the studies suggest an inflammatory phenotype related to classical monocytes and a reparative (e.g. patrolling) role related to nonclassical monocytes; intermediate monocytes have been poorly evaluated, with some protocols reporting increased production of the anti-inflammatory cytokine IL-10 by these cells, whereas others described the production of ROS and proinflammatory mediators such as TNF-α and IL-1β.

There is still inconclusive information related to how these monocyte subsets are generated, but there are currently three theories: (1) whether classical monocytes directly give rise to the other subtypes; (2) whether intermediate monocytes might be differentiated from nonclassical subtypes depending on environmental changes and vice versa; and (3) whether intermediate monocytes originate from macrophages inside the plaque and which type of macrophage (M1, M2, or both).

Although there is a standardization of the different monocyte subsets (mainly concerning classical and nonclassical monocytes) via CD14 and CD16 expression, comparing the studies is still difficult because of the different protocols used (i.e. additional antibodies beyond CD14 and CD16, fresh or frozen samples, variable gate strategies).

Many studies highlight the role of the MCP-1/CCR2 axis in promoting atherosclerosis and contributing to tissue damage after MI. In this sense, blocking or silencing MCP-1/CCR2 might be useful in the treatment of vascular diseases. However, this therapeutic strategy should be done with caution, since there are some limitations and consequences, including the following: (1) off-target effects; (2) siRNA delivery to the appropriate tissue; (3) other cells beyond monocytes/macrophages that express CCR2; (4) local targeting of a specific process such as monocyte recruitment blockade, while avoiding systemic changes; (5) the safety and efficacy of extended blocking/silencing of MCP-1/CCR2; and (6) the specific consequences of blocking/silencing MCP-1/CCR2 regarding the levels of the classical and intermediate monocyte subsets. Explanations concerning these issues are still unknown.

Therefore, additional experiments and clinical studies in larger cohorts might be helpful to better elucidate the controversies related to monocyte subsets, beyond the safety and benefits of MCP-1/CCR2 blocking/silencing as a therapeutic target for the treatment of CVD.

Competing interests

The authors declare that there are no competing interests associated with the manuscript.

Abbreviations

     
  • AMI

    acute myocardial infarction

  •  
  • ApoE

    apolipoprotein E

  •  
  • CCL2

    chemokine ligand 2

  •  
  • CCR2

    chemokine receptor of monocyte chemoattractant protein-1

  •  
  • CD

    cluster of differentiation

  •  
  • CVD

    cardiovascular disease

  •  
  • CX3CR1

    CX3C chemokine receptor 1

  •  
  • Gr1

    granulocyte marker

  •  
  • GSK1344386B

    CCR2 antagonist

  •  
  • HIV-1

    human immunodeficiency virus-1

  •  
  • hsCRP

    high-sensitivity C-reactive protein

  •  
  • IL

    interleukin

  •  
  • INCB-3344

    CCR2 antagonist

  •  
  • LDL

    low-density lipoprotein

  •  
  • LDL-R

    low-density lipoprotein receptor

  •  
  • LPS

    lipopolysaccharide

  •  
  • Ly6C

    lymphocyte antigen 6 complex

  •  
  • MCP-1

    monocyte chemoattractant protein-1

  •  
  • MI

    myocardial infarction

  •  
  • MLN1202

    monoclonal antibody

  •  
  • PA508

    MCP-1 competitor

  •  
  • PAD

    peripheral artery disease

  •  
  • PCI

    percutaneous coronary intervention

  •  
  • PG

    propagermanium

  •  
  • ROS

    reactive oxygen species

  •  
  • siCCR2

    silencing CCR2

  •  
  • siRNA

    small interfering RNA

  •  
  • STEMI

    ST-elevation myocardial infarction

  •  
  • TAK-779

    small molecule antagonist

  •  
  • THP-1

    human monocytic leukemia cell line

  •  
  • TLK-19705

    CCR2 antagonist

  •  
  • TNF-α

    tumor necrosis factor-α

References

References
1
Ziegler-Heitbrock
,
L.
,
Ancuta
,
P.
,
Crowe
,
S.
,
Dalod
,
M.
,
Grau
,
V.
,
Hart
,
D.N.
et al.  (
2010
)
Nomenclature of monocytes and dendritic cells in blood
.
Blood
116
,
e74
e80
[PubMed]
2
Sager
,
H.B.
,
Hulsmans
,
M.
,
Lavine
,
K.J.
,
Moreira
,
M.B.
,
Heidt
,
T.
,
Courties
,
G.
et al.  (
2016
)
Proliferation and recruitment contribute to myocardail macrophage expansion in chronic heart failure
.
Circ. Res.
119
,
853
864
[PubMed]
3
Swirski
,
F.K.
(
2011
)
The spatial and developmental relationships in the macrophage family
.
Arterioscler. Thromb. Vasc. Biol.
31
,
1517
1522
[PubMed]
4
Yang
,
J.
,
Zhang
,
L.
,
Yu
,
C.
,
Yang
,
X.F.
and
Wang
,
H.
(
2014
)
Monocyte and macrophage differentiation: circulation inflammatory monocyte as biomarker for inflammatory diseases
.
Biomark. Res.
2
,
1
[PubMed]
5
Nahrendorf
,
M.
,
Swirski
,
F.K.
,
Aikawa
,
E.
,
Stangenberg
,
L.
,
Wurdinger
,
T.
,
Figueiredo
,
J.L.
et al.  (
2007
)
The healing myocardium sequentially mobilizes two monocyte subsets with divergent and complementary functions
.
J. Exp. Med.
204
,
3037
3047
[PubMed]
6
Passlick
,
B.
,
Flieger
,
D.
and
Ziegler-Heitbrock
,
H.W.
(
1989
)
Identification and characterization of a novel monocyte subpopulation in human peripheral blood
.
Blood
74
,
2527
2534
[PubMed]
7
Wildgruber
,
M.
,
Aschenbrenner
,
T.
,
Wendorff
,
H.
,
Czubba
,
M.
,
Glinzer
,
A.
,
Haller
,
B.
et al.  (
2016
)
The “intermediate” CD14++CD16+ monocyte subset increases in severe peripheral artery disease in humans
.
Sci. Rep.
6
,
39483
[PubMed]
8
Geissmann
,
F.
,
Jung
,
S.
and
Littman
,
D.
(
2003
)
Blood monocytes consist of two principal subsets with distinct migratory properties
.
Immunity
19
,
71
82
[PubMed]
9
Gautier
,
E.L.
,
Jakubzick
,
C.
and
Randolph
,
G.J.
(
2009
)
Regulation of the migration and survival of monocyte subsets by chemokine receptors and its relevance to atherosclerosis
.
Arterioscler. Thromb. Vasc. Biol.
29
,
1412
1418
[PubMed]
10
Meisner
,
J.K.
,
Song
,
J.
and
Price
,
R.J.
(
2012
)
Arteriolar and venular remodeling are differentially regulated by bone-marrow-derived cell-specific CX3CR1 and CCR2 expression
.
PloS One
7
,
e46312
[PubMed]
11
Ziegler-Heitbrock
,
H.W.L.
(
1996
)
Heterogeneity of human blood monocytes: the CD14+CD16+ subpopulation
.
Immunol. Today
17
,
424
428
[PubMed]
12
Carlin
,
L.M.
,
Stamatiades
,
E.G.
,
Auffray
,
C.
,
Hanna
,
R.N.
,
Glover
,
L.
,
Vizcay-Barrena
,
G.
et al.  (
2013
)
Nr4a1-dependent ly6c(low) monocytes monitor endotelial cells and orchestrate their disposal
.
Cell
153
,
362
375
[PubMed]
13
Auffray
,
C.
,
Fogg
,
D.
,
Garfa
,
M.
,
Elain
,
G.
,
Join-Lambert
,
O.
,
Kayal
,
S.
et al.  (
2007
)
Monitoring of blood vessels and tissues by a population of monocytes with patrolling behavior
.
Science
317
,
666
670
[PubMed]
14
Thomas
,
G.
,
Tacke
,
R.
,
Hedrick
,
C.C.
and
Hanna
,
R.N.
(
2015
)
Nonclassical patrolling monocyte function in the vasculature
.
Arterioscler. Thromb. Vasc. Biol.
35
,
1306
1316
[PubMed]
15
Sunderkotter
,
C.
,
Nikolic
,
T.
,
Dillon
,
M.J.
,
Van Rooijen
,
N.
,
Stehling
,
M.
,
Drevets
,
D.A.
et al.  (
2004
)
Subpopulations of mouse blood monocytes differ in maturation stage and inflammatory response
.
J. Immunol.
172
,
4410
4417
[PubMed]
16
Tacke
,
F.
,
Ginhoux
,
F.
,
Jakubzick
,
C.
,
van Rooijen
,
N.
,
Merad
,
M.
and
Randolph
,
G.J.
(
2006
)
Immature monocytes acquire antigens from other cells in the bone marrow and present them to T cells after maturing in the periphery
.
J. Exp. Med.
203
,
583
597
[PubMed]
17
Varol
,
C.
,
Landsman
,
L.
,
Fogg
,
D.K.
,
Greenshtein
,
L.
,
Gildor
,
B.
,
Margalit
,
R.
et al.  (
2007
)
Monocytes give rise to mucosal, but not splenic, conventional dendritic cells
.
J. Exp. Med.
204
,
171
180
[PubMed]
18
Geissmann
,
F.
,
Auffray
,
C.
,
Palframan
,
R.
,
Wirrig
,
C.
,
Ciocca
,
A.
,
Campisi
,
L.
et al.  (
2008
)
Blood monocytes: distinct subsets, how they relate to dendritic cells, and their possible roles in the regulation of T-cell responses
.
Immunol. Cell Biol.
86
,
398
408
[PubMed]
19
Cros
,
J.
,
Cagnard
,
N.
,
Woollard
,
K.
,
Patey
,
N.
,
Zhang
,
S.Y.
,
Senechal
,
B.
et al.  (
2010
)
Human cd14dim monocytes patrol and sense nucleic acids and viruses via tlr7 and tlr8 receptors
.
Immunity
33
,
375
386
[PubMed]
20
Chou
,
M.Y.
,
Fogelstrand
,
L.
,
Hartvigsen
,
K.
,
Hansen
,
L.F.
,
Woelkers
,
D.
,
Shaw
,
P.X.
et al.  (
2009
)
Oxidation-specific epitopes are dominant targets of innate natural antibodies in mice and humans
.
J. Clin. Invest.
119
,
1335
1349
[PubMed]
21
Mosig
,
S.
,
Rennert
,
K.
,
Krause
,
S.
,
Kzhyshkowska
,
J.
,
Neunübel
,
K.
,
Heller
,
R.
et al.  (
2009
)
Different functions of monocyte subsets in familial hypercholesterolemia: potential function of CD14+CD16+ monocytes in detoxification of oxidized LDL
.
FASEB J.
23
,
866
874
[PubMed]
22
Jaworowski
,
A.
,
Kamwendo
,
D.D.
,
Ellery
,
P.
,
Sonza
,
S.
,
Mwapasa
,
V.
,
Tadesse
,
E.
et al.  (
2007
)
CD16+ monocyte subset preferentially harbors HIV-1 and is expanded in pregnant Malawian women with Plasmodium falciparum malaria and HIV-1 infection
.
J. Infect. Dis.
196
,
38
42
[PubMed]
23
Wrigley
,
B.J.
,
Shantsila
,
E.
,
Tapp
,
L.D.
and
Lip
,
G.Y.
(
2013
)
CD14++CD16+ monocytes in patients with acute ischaemic heart failure
.
Eur. J. Clin. Invest.
43
,
121
130
[PubMed]
24
Skrzeczynska-Moncznik
,
J.
,
Bzowska
,
M.
,
Loseke
,
S.
,
Grage-Griebenow
,
E.
,
Zembala
,
M.
and
Pryjma
,
J.
(
2008
)
Peripheral blood CD14high CD16+ monocytes are main producers of IL-10
.
Scand. J. Immunol.
67
,
152
159
[PubMed]
25
Zawada
,
A.M.
,
Rogacev
,
K.S.
,
Rotter
,
B.
,
Winter
,
P.
,
Marell
,
R.R.
,
Fliser
,
D.
et al.  (
2011
)
SuperSAGE evidence for CD14++CD16+ monocytes as a third monocyte subset
.
Blood
118
,
e50
e61
[PubMed]
26
Merino
,
A.
,
Buendia
,
P.
,
Martin-Malo
,
A.
,
Aljama
,
P.
,
Ramirez
,
R.
and
Carracedo
,
J.
(
2011
)
Senescent CD14+CD16+ monocytes exhibit proinflammatory and proatherosclerotic activity
.
J. Immunol.
186
,
1809
1815
[PubMed]
27
Das
,
A.
,
Sinha
,
M.
,
Datta
,
S.
,
Izumi
,
K.
,
Yasuoka
,
H.
,
Ogawa
,
Y.
et al.  (
2015
)
Monocyte and macrophage plasticity in tissue repair and regeneration
.
Am. J. Pathol.
185
,
2596e2606
28
Kuwana
,
M.
,
Okasaki
,
Y.
,
Kodama
,
H.
,
Izumi
,
K.
,
Yasuoka
,
H.
,
Ogawa
,
Y.
et al.  (
2003
)
Human circulating CD14+ monocytes as a source of progenitors that exhibit mesenchymal cell differentiation
.
J. Leukoc. Biol.
74
,
833
845
[PubMed]
29
Seta
,
N.
and
Kuwana
,
M.
(
2007
)
Human circulating monocytes as multipotential progenitors
.
Keio J. Med.
56
,
41
47
[PubMed]
30
Oh
,
J.
,
Riek
,
A.E.
,
Weng
,
S.
,
Petty
,
M.
,
Kim
,
D.
,
Colonna
,
M.
et al.  (
2012
)
Endoplasmic reticulum stress controls M2 macrophage differentiation and foam cell formation
.
J. Biol. Chem.
287
,
11629
11641
[PubMed]
31
Shantsila
,
E.
,
Wrigley
,
B.
,
Tapp
,
L.
,
Apostolakis
,
S.
,
Montoro-Garcia
,
S.
,
Drayson
,
M.T.
et al.  (
2011
)
Immunophenotypic characterization of human monocyte subsets: possible implications for cardiovascular disease pathophysiology
.
J. Thromb. Haemost.
9
,
1056
1066
[PubMed]
32
Engel
,
D.R.
,
Maurer
,
J.
,
Tittel
,
A.
,
Weisheit
,
C.
,
Cavlar
,
T.
,
Schumak
,
B.
et al.  (
2008
)
CCR2 mediates homeostatic and inflammatory release of Gr1high monocytes from the bone marrow, but is dispensable for bladder infiltration in bacterial urinary tract infection
.
J. Immunol.
181
,
5579
5586
[PubMed]
33
Serbina
,
N.V.
,
Salazar-Mather
,
T.P.
,
Biron
,
C.A.
,
Kuziel
,
W.A.
and
Pamer
,
E.G.
(
2003
)
TNF/iNOS-producing dendritic cells mediate innate immune defense against bacterial infection
.
Immunity
19
,
59
70
[PubMed]
34
Weber
,
K.S.
,
Nelson
,
P.J.
,
Grone
,
H.J.
and
Weber
,
C.
(
1999
)
Expression of CCR2 by endothelial cells: implications for MCP-1 mediated wound injury repair and in vivo inflammatory activation of endothelium
.
Arterioscler. Thromb. Vasc. Biol.
19
,
2085
2093
[PubMed]
35
Hoyer
,
F.F.
,
Giesen
,
M.K.
,
Nunes França
,
C.
,
Lütjohann
,
D.
,
Nickenig
,
G.
and
Werner
,
N.
(
2012
)
Monocytic microparticles promote atherogenesis by modulating inflammatory cells in mice
.
J. Cell. Mol. Med.
16
,
2777
2788
[PubMed]
36
Murphy
,
P.M.
(
1996
)
Chemokine receptors: structure, function and role in microbial pathogenesis
.
Cytokine Growth Factor Ver
7
,
47
64
37
Schecter
,
A.D.
,
Calderon
,
T.M.
,
Berman
,
A.B.
,
McManus
,
C.M.
,
Fallon
,
J.T.
,
Rossikhina
,
M.
et al.  (
2000
)
Human vascular smooth muscle cells possess functional CCR5
.
J. Biol. Chem.
275
,
5466
5471
[PubMed]
38
Feria
,
M.
and
Díaz-González
,
F.
(
2006
)
The CCR2 receptor as a therapeutic target
.
Expert Opin. Ther. Pat.
16
,
49
57
39
Han
,
K.H.
,
Han
,
K.O.
,
Green
,
S.R.
and
Quehenberger
,
O.
(
1999
)
Expression of the monocyte chemoattractant protein-1 receptor CCR2 is increased in hypercholesterolemia. Differential effects of plasma lipoproteins on monocytes function
.
J. Lipid Res.
40
,
1053
1063
[PubMed]
40
Yang
,
J.
,
Liu
,
C.
,
Zhang
,
L.
,
Liu
,
Y.
,
Guo
,
A.
,
Shi
,
H.
et al.  (
2015
)
Intensive atorvastatin therapy attenuates the inflammatory responses in monocytes of patients with unstable angina undergoing percutaneous coronary intervention via peroxisome proliferator-activated receptor γ activation
.
Inflammation
38
,
1415
1423
[PubMed]
41
Swirski
,
F.K.
,
Libby
,
P.
,
Aikawa
,
E.
,
Alcaide
,
P.
,
Luscinskas
,
F.W.
,
Weissleder
,
R.
et al.  (
2007
)
Ly-6Chi monocytes dominate hypercholesterolemia-associated monocytosis and give rise to macrophages in atheroma
.
J. Clin. Invest.
117
,
195
205
[PubMed]
42
Libby
,
P.
(
2002
)
Inflammation in atherosclerosis
.
Nature
420
,
868
874
[PubMed]
43
Ley
,
K.
,
Miller
,
Y.I.
and
Hedrick
,
C.C.
(
2011
)
Monocyte and macrophage dynamics during atherogenesis
.
Arterioscler. Thromb. Vasc. Biol.
31
,
1506
1516
[PubMed]
44
Woollard
,
K.J.
and
Geissmann
,
F
(
2010
)
Monocytes in atherosclerosis: subsets and functions
.
Nat. Rev. Cardiol.
7
,
77
86
[PubMed]
45
Robbins
,
C.S.
,
Hilgendorf
,
I.
,
Weber
,
G.F.
,
Theurl
,
I.
,
Iwamoto
,
Y.
,
Figueiredo
,
J.L.
et al.  (
2013
)
Local proliferation dominates lesional macrophage accumulation in atherosclerosis
.
Nat. Med.
19
,
1166
1172
[PubMed]
46
Moore
,
K.
,
Sheedy
,
F.
and
Fisher
,
E.
(
2013
)
Macrophages in atherosclerosis: a dynamic balance
.
Nat. Rev. Immunol.
13
,
709
721
[PubMed]
47
Gu
,
L.
,
Okada
,
Y.
,
Clinton
,
S.
,
Gerard
,
C.
,
Sukhova
,
G.K.
,
Libby
,
P.
et al.  (
1998
)
Absence of monocyte chemoattractant protein-1 reduces atherosclerosis in low density lipoprotein receptor-deficient mice
.
Mol. Cell
2
,
275
281
[PubMed]
48
Boring
,
L.
,
Gosling
,
J.
,
Cleary
,
M.
and
Charo
,
I.F.
(
1998
)
Decreased lesion formation in CCR2(−/−) mice reveals a role for chemokines in the initiation of atherosclerosis
.
Nature
394
,
894
897
[PubMed]
49
Schober
,
A.
,
Zernecke
,
A.
,
Liehn
,
E.A.
,
von Hundelshausen
,
P.
,
Knarren
,
S.
,
Kuziel
,
W.A.
et al.  (
2004
)
Crucial role of the CCL2/CCR2 axis in neointimal hyperplasia after arterial injury in hyperlipidemic mice involves early monocyte recruitment and CCL2 presentation on platelets
.
Circ. Res.
95
,
1125
1133
[PubMed]
50
Nagareddy
,
P.R.
,
Murphy
,
A.J.
,
Stirzaker
,
R.A.
,
Hu
,
Y.
,
Yu
,
S.
,
Miller
,
R.G.
et al.  (
2013
)
Hyperglycemia promotes myelopoiesis and impairs the resolution of atherosclerosis
.
Cell Metab.
17
,
695
708
[PubMed]
51
Nagareddy
,
P.R.
,
Kraakman
,
M.
,
Masters
,
S.L.
,
Stirzaker
,
R.A.
,
Gorman
,
D.J.
,
Grant
,
R.W.
et al.  (
2014
)
Adipose tissue macrophages promote myelopoiesis and monocytosis in obesity
.
Cell Metab.
19
,
821
835
[PubMed]
52
Berg
,
K.E.
,
Ljungcrantz
,
I.
,
Andersson
,
L.
,
Bryngelsson
,
C.
,
Hedblad
,
B.
,
Fredrikson
,
G.N.
et al.  (
2012
)
Elevated CD14++CD16 monocytes predict cardiovascular events
.
Circ. Cardiovasc. Genet.
5
,
122
131
[PubMed]
53
Tacke
,
F.
,
Alvarez
,
D.
,
Kaplan
,
T.
,
Jakubzick
,
C.
,
Spanbroek
,
R.
,
Llodra
,
J.
et al.  (
2007
)
Monocyte subsets differentially employ ccr2, ccr5, and cx3cr1 to accumulate within atherosclerotic plaques
.
J. Clin. Invest.
117
,
185
194
[PubMed]
54
McArdle
,
S.
,
Chodaczek
,
G.
,
Ray
,
N.
and
Ley
,
K.
(
2015
)
Intravital live cell triggered imaging system reveals monocyte patrolling and macrophage migration in atherosclerotic arteries
.
J. Biomed. Opt.
20
,
26005
[PubMed]
55
Tapp
,
L.D.
,
Shantsila
,
E.
,
Wrigley
,
B.J.
,
Pamukcu
,
B.
and
Lip
,
G.Y.
(
2012
)
The CD14++CD16+ monocyte subset and monocyte-platelet interactions in patients with ST-elevation myocardial Michaudinfarction
.
J. Thromb. Haemost.
10
,
1231
1241
[PubMed]
56
Tsujioka
,
H.
,
Imanishi
,
T.
,
Ikejima
,
H.
,
Kuroi
,
A.
,
Takarada
,
S.
,
Tanimoto
,
T.
et al.  (
2009
)
Impact of heterogeneity of human peripheral blood monocyte subsets on myocardial salvage in patients with primary acute myocardial infarction
.
J. Am. Coll. Cardiol.
54
,
130
138
[PubMed]
57
Liu
,
Y.
,
Imanishi
,
T.
,
Ikejima
,
H.
,
Tsujioka
,
H.
,
Ozaki
,
Y.
,
Kuroi
,
A.
et al.  (
2010
)
Association between circulating monocyte subsets and in-stent restenosis after coronary stent implantation in patients with ST-elevation myocardial infarction
.
Circ. J.
74
,
2585
2591
[PubMed]
58
Majmudar
,
M.D.
,
Keliher
,
E.J.
,
Heidt
,
T.
,
Leuschner
,
F.
,
Truelove
,
J.
,
Sena
,
B.F.
et al.  (
2013
)
Monocyte-directed RNAi targeting CCR2 improves infarct healing in atherosclerosis-prone mice
.
Circulation
127
,
2038
2046
[PubMed]
59
Leuschner
,
F.
,
Courties
,
G.
,
Dutta
,
P.
,
Mortensen
,
L.J.
,
Gorbatov
,
R.
,
Sena
,
B.
et al.  (
2015
)
Silencing of CCR2 in myocarditis
.
Eur. Heart J.
36
,
1478
1488
[PubMed]
60
Gilbert
,
J.
,
Lekstrom-Himes
,
J.
,
Donaldson
,
D.
,
Lee
,
Y.
,
Hu
,
M.
,
Xu
,
J.
et al.  (
2011
)
MLN1202 Study Group. Effect of CC chemokine receptor 2 CCR2 blockade on serum C-reactive protein in individuals at atherosclerotic risk and with a single nucleotide polymorphism of the monocyte chemoattractant protein-1 promoter region
.
Am. J. Cardiol.
107
,
906
911
[PubMed]
61
Okamoto
,
M.
,
Fuchigami
,
M.
,
Suzuki
,
T.
and
Watanabe
,
N.
(
2012
)
A novel C-C chemokine receptor 2 antagonist prevents progression of albuminuria and atherosclerosis in mouse models
.
Biol. Pharm. Bull.
35
,
2069
2074
[PubMed]
62
Yokochi
,
S.
,
Hashimoto
,
H.
,
Ishiwata
,
Y.
,
Shimokawa
,
H.
,
Haino
,
M.
,
Terashima
,
Y.
et al.  (
2001
)
An anti-inflammatory drug, propagermanium, may target GPI-anchored proteins associated with na MCP-1 receptor, CCR2
.
J. Interferon Cytokine Res.
21
,
389
398
[PubMed]
63
Yamashita
,
T.
,
Kawashima
,
S.
,
Ozaki
,
M.
,
Namiki
,
M.
,
Inoue
,
N.
,
Hirata
,
K.
et al.  (
2002
)
Propagermanium reduces atherosclerosis in apolipoprotein E knockout mice via inhibition of macrophage infiltration
.
Arterioscler. Thromb. Vasc. Biol.
22
,
969
974
[PubMed]
64
Liehn
,
E.A.
,
Piccinini
,
A.M.
,
Koenen
,
R.R.
,
Soehnlein
,
O.
,
Adage
,
T.
,
Fatu
,
R.
et al.  (
2010
)
A new monocyte chemotactic protein-1/chemokine CC motif ligand-2 competitor limiting neointima formation and myocardial ischemia/reperfusion injury in mice
.
J. Am. Col. Cardiol.
56
,
1847
1857
65
Usui
,
M.
,
Egashira
,
K.
,
Ohtani
,
K.
,
Kataoka
,
C.
,
Ishibashi
,
M.
,
Hiasa
,
K.
et al.  (
2002
)
Anti-monocyte chemoattractant protein-1 gene therapy inhibits restenotic changes (neointimal hyperplasia) after balloon injury in rats and monkeys
.
FASEB J.
16
,
1838
1840
[PubMed]
66
Frangogiannis
,
N.G.
,
Dewald
,
O.
,
Xia
,
Y.
,
Ren
,
G.
,
Haudek
,
S.
,
Leucker
,
T.
et al.  (
2007
)
Critical role of monocyte chemoattractant protein-1/CC chemokine ligand 2 in the pathogenesis of ischemic cardiomyopathy
.
Circulation
115
,
584
592
[PubMed]
67
Aiello
,
R.J.
,
Perry
,
B.D.
,
Bourassa
,
P.A.
,
Robertson
,
A.
,
Weng
,
W.
,
Knight
,
D.R.
et al.  (
2010
)
CCR2 receptor blockade alters blood monocyte subpopulations but does not affect atherosclerotic lesions in apoE−/− mice
.
Atherosclerosis
208
,
370
375
[PubMed]
68
Olzinski
,
A.R.
,
Turner
,
G.H.
,
Bernard
,
R.E.
,
Karr
,
H.
,
Cornejo
,
C.A.
,
Aravindhan
,
K.
et al.  (
2010
)
Pharmacological inhibition of C-C chemokine receptor 2 decreases macrophage infiltration in the aortic root of the human C-C chemokine receptor 2/apolipoproteinE-/- mouse: magnetic resonance imaging assessment
.
Arterioscler. Thromb. Vasc. Biol.
30
,
253
259
[PubMed]