Introduction
Cardiovascular diseases are the leading causes of worldwide morbidity and mortality. In consequence, understanding the precise contribution of the mechanisms involved in cardiovascular tissue injury and repair is of prominent importance. In this sense, increasing evidences reveal that innate immune response plays a critical and complex role throughout the acute inflammation and regenerative process triggered after cardiac or vascular injury. As such, leukocytosis and monocytosis have been associated with cardiovascular diseases in numerous epidemiological studies, prompting speculation on the functional importance of these cells (1). The goal of this review is to summarize the complex immunobiology of mononuclear phagocytic cells and their relevance to the pathogenesis of cardiovascular diseases, highlighting the effect of major immune modulators.

Origin
In the last decade, important advances in the knowledge of macrophage origin have triggered an essential conceptual progress in the mononuclear phagocyte system field. Parabiosed mice and genetic fate-mapping experiments have revealed that the majority of resident macrophages in healthy tissues are established from the yolk sac and fetal liver before birth (2–4). This cellular compartment locally self-maintains throughout life within the tissue and is independent from the hematopoietic input. On the other hand, during adulthood tissue-infiltrating macrophages can develop from circulating monocytes. The recruitment of monocytes is associated with pathological, but also with homeostatic response. Macrophages derived from monocytes display a short lifespan, although exceptions have also been reported. Embryonic- and adult-derived macrophages generally coexist in a given tissue, and their respective number correlates with the origin and records of their tissue of residence. Seminal experiments have demonstrated that embryonic- and monocyte-derived macrophages make different functional contributions in homeostatic conditions or following challenge. In this sense, Levine and coworkers have revealed that embryonic-derived macrophages are clue for cardiac recovery after injury (5). Their results suggest that targeting specific macrophage lineages could have important therapeutic implications in order to improve treatments for heart diseases.
Adult resident macrophage compartments seem to be independent from monocyte recruitment in a given tissue, as was demonstrated by parabiotic experiments. Monocyte compartments of the parabionts reach, with time, considerable chimerism in the joined circulation. However, tissue macrophages failed to equilibrate even after several months of parabiosis, suggesting the absence of an ongoing steady-state contribution of bone marrow-derived cells to adult tissue macrophage compartments (3). Furthermore, recruited monocytes are short-lived effector cells in tissues and assumed different roles that have yet to be better defined; emerging reports suggest, for example, that monocytes can promote angiogenesis and arteriogenesis (6). In addition, it was recently reported that inflammatory Ly6Chigh monocytes persist during the steady state without commitment toward macrophage or dendritic cell (DC) fates and might contribute to antigen transport toward lymph nodes (7). Remarkably, macrophage effector function also needs to be tailored to its tissue of residence, an adaptation that is driven by the local microenvironment and by the inflammatory history of a given tissue.

Phenotypes and Functions of Monocytes
Monocytes/macrophages are very plastic cells and can acquire distinct phenotypes and activation states under the influence of different microenvironments. Although several authors have shown that macrophages treated with different stimuli display altered phenotypes or functional capacities, many of these studies are limited considering that they compare only one particular activation state with non-polarized macrophages. Although macrophage activation was initially seen as a dichotomy between classically and alternatively activated states, it is now clear that the spectrum of macrophage activation states is much more diverse.
Monocytes develop in steady state in the bone marrow from hematopoietic precursors, and they enter the circulation via CCR2 receptor. In mice, circulating monocytes are phenotypically and functionally heterogeneous and can be defined according to Ly6C expression marker (Figure 1). In mice, during steady-state conditions, about 50–60% of circulating monocytes belongs to the Ly6Chigh CCR2high CX3CR1low CD62L+ subset. These inflammatory or classical monocytes have a relatively short-circulating lifespan and are preferentially recruited to injured/inflamed tissues where they maturate to macrophages. The remaining non classical (Ly6Clow CCR2low CX3CR1high CD62L−) subset patrols blood vessels and accumulates at low numbers in the steady state (8). The number of Ly6Chigh monocytes rises during inflammation, at expenses of enhanced monocytopoiesis in the bone marrow and spleen (1, 9–11). Regarding the origin, evidence shows that monocyte subpopulations do not arise from separate progenitors, but rather convert from the Ly6Chigh to Ly6Clow subset (12) (Figure 1).
Figure 1  Murine monocyte and macrophage subsets. Murine monocytes develop from a common myeloid progenitor and leave the bone marrow through the chemokine receptor CCR2. In bloodstream, circulating monocytes are phenotypically and functionally heterogeneous. Non-classical monocytes (Ly6Clow CCR2low CX3CR1high CD62L−) patrol the vasculature and accumulate at low numbers in the steady state. Inflammatory or classical (Ly6Chigh CCR2high CX3CR1low CD62L+) monocytes have a relatively short-circulating lifespan and preferentially accumulate in inflammatory sites where they give rise to inflammatory M1 macrophages (F4/80+ CD11b+ CD86+ CD206−). M1 macrophage subset has high microbicidal capacity due to their ability to produce inflammatory cytokines [TNF, IL-1β], reactive oxygen species (ROS) secretion and the expression of iNOS enzyme that metabolizes arginine to arginine-derived killer molecule NO. Non-classical monocytes can be recruited to tissue and differentiate to M2 macrophages (F4/80+ CD11b+ CD86− CD206+), which secrete anti-inflammatory cytokines (IL-10, IL-4) and contribute to tissue repair mechanisms.   In humans, circulating monocytes can be segregated into three major subsets based on the expression of CD14 and CD16 (13). Over the past decades, human circulating monocytes had been separated into two subpopulations based on CD16 expression, the CD14+ CD16− and CD14+ CD16+ monocyte subsets (hereby designated as CD16− and CD16+, respectively). A new nomenclature defines three monocyte populations, where the minor CD16+ subset is further separated into two subpopulations (Figure 2). The intermediate subset expresses relatively high levels of CD14 coupled with low CD16 expression (CD14++ CD16+), while the non-classical subset expresses low levels of CD14 with high expression of CD16 (CD14+ CD16++). The subset with high CD14 and no CD16 expression, termed classical subset (CD14++ CD16−), constitute approximately 85–90% of human monocytes in the peripheral blood. Several studies have demonstrated that circulating CD16+ monocytes are found in large numbers in patients with inflammation processes (14) and infectious diseases (15, 16).
Figure 2  Human circulating monocyte subsets. Human blood monocytes can be separated into three subsets according to the CD16 and CD14 expression: classical monocytes (CD14++ CD16−), which represent the majority of circulating monocytes produce the anti-inflammatory IL-10 upon stimulation; intermediate monocytes (CD14++ CD16+); and non-classical monocytes (CD14+ CD16++), which secrete inflammatory cytokines such as IL-1β, IL-12, TNF, and antimicrobial molecules [nitric oxide (NO) and reactive oxygen species (ROS)].   Analyses based on hierarchical clustering of gene expression profiles of human monocyte subsets have been a matter of debate. Cros et al. reported that the classical and intermediate subpopulations were most closely related among the subsets, while the most distant was the non-classical subset (17). However, other independent groups described that both CD16+ subpopulations are more closely related (18, 19). Indeed, the number of genes that were significantly different between both CD16+ subsets was the lowest among the three subpopulations. Until now, there is poor agreement on the effector functions of the three monocyte subsets. In this sense, it was reported that TNF is produced mainly by CD16+ monocytes (intermediate and non-classical subsets) (20, 21). However, recent results have evidenced that TNF is produced by the three subsets. While Cros et al. (17) described that non-classical monocytes were poor producers of several cytokines [TNF, interleukin (IL)-1β, CCL2, IL-10, IL-8, IL-6, and CCL3] in response to LPS, other reports showed that the stimulation of intermediate subset with LPS produced the most TNF, IL-1β, and IL-6. In agreement, others also showed that intermediate monocytes produced the most TNF and IL-1β upon treatment with LPS and that this subset is the highest producer of TNF when cocultured with pre-activated T cells (22). In addition, Wong et al. (18) showed that non-classical monocytes upon LPS stimulation produced the highest levels of TNF and IL-1β, while equivalent amounts of IL-6 and IL-8 were produced by all three subsets. Other reports also recognized non-classical monocyte population as the greatest producer of TNF (21). Hence, it seems that non-classical monocytes constitute the subset capable of producing inflammatory cytokines in response to TLR ligands. There are also controversies on which monocyte subset is the major producer of the anti-inflammatory cytokine IL-10. In this sense, although it was demonstrated that intermediate monocytes produced the most IL-10 in response to LPS and zymosan (23), some recent reports showed that classical monocytes produced the most IL-10 (17, 18, 24). Further studies are necessary to clarify this relevant question.

Phenotypes and Functions of Macrophages
Several studies on macrophage biology have focused on the particular functions that these cells acquire after tissue accumulation. Macrophages are highly plastic and can adapt to environmental stimuli, displaying either a classically activated (M1) or alternatively activated (M2) profile, which represent extremes of a spectrum of functional phenotypes (25, 26). M1 macrophages are activated by pro-inflammatory Th1 cytokines and LPS. Besides being antigen-presenting cells, the M1 subset shows an enhanced microbicidal capacity attributable to the production of reactive oxygen species (ROS) (such as hydrogen peroxide, superoxide, nitric oxide (NO), and peroxynitrite) and inflammatory cytokines (TNF, IL-1β, IL-12, and IL-23). On the other extreme, M2 macrophages are activated by Th2 cytokines (IL-4 and IL-13) or by anti-inflammatory mediators (IL-10), enhancing the arginase activity and mannose receptor (CD206) expression, to promote wound healing and reduce Th1 response. However, the M2 subset development can also be detrimental to host tissue, leading to fibrosis when their matrix-enhancing activity is not regulated (27). Although embryonic and monocyte-derived macrophages likely are on a continuum spectrum that lies between (and outside of) the M1 and M2 classifications, the terminology nevertheless has been helpful in order to elucidate macrophage heterogeneity. When Ly6Chigh monocytes are recruited to atherosclerotic lesions, they mature to F4/80+ macrophages. In a persistent inflammatory environment, these Ly6Chigh monocyte-derived macrophages contribute to oxidative stress and are inflammatory by producing IL-1β and TNF (9). In this respect, Ly6Chigh monocyte-derived cells are M1 macrophages. In the setting of inflammation resolution, M1 macrophages are replaced by M2 repairing macrophages. Although it has been proposed that M1 to M2 conversion occurs locally (28–30), M2 macrophages also could derive from non-classical and less inflammatory Ly6Clow monocytes (12). Otherwise, M2 macrophages may arise through direct differentiation of Ly6Chigh monocytes in a microenvironment that favors wound healing. This option should be further explored because it is enticing for potential therapeutic reasons.
Cardiac resident macrophages coexpress M1 and M2 markers suggesting no specific polarization (31). However, within myocardium macrophages respond to systemic Th2 environment induced by helminth parasites infection, adopting an M2 phenotype associated with enhanced fibrosis. In this model, the increased amount of cardiac macrophages relies on recruitment of Ly6ChighCCR2+ monocytes instead of IL-4-induced expansion. In this sense, Jenkins et al. have shown that IL-4 and IL-13 through IL-4Rα not only activate macrophages but also cause proliferative expansion of resident macrophages (32, 33). Moreover, IL-33 also induces macrophages proliferation, but in an IL-4Rα-signaling-independent manner (34), suggesting that the number and activation state of cardiac macrophages largely depend on mediators locally produced.