**4. The role of SMC-derived foam cells**

Pathological changes in the atherosclerotic intima include increased modification of lipoproteins (e.g., oxidized LDL) and SMC uptake of modified lipoproteins. Foam cell formation by SMCs, in addition to macrophage-derived foam cells, may represent a pivotal Dynamic Interplay Between Smooth Muscle Cells and Macrophages in Vascular Disease http://dx.doi.org/10.5772/61089 237

**3. Existence of immature SMCs in vascular disease**

236 Muscle Cell and Tissue

the repopulation of immature SMC by mature SMC.

is where those intimal SMCs originate from.

**4. The role of SMC-derived foam cells**

SMCs exhibit the diversity of phenotypes in the vascular lesions. A panel of antibodies for SMC differentiation markers enables to identify a wide spectrum of their phenotypes in experimental and human vascular diseases. In the arterial lesions after acute mechanical injury and in chronic atherosclerotic plaques of experimental animals, intimal SMC often express lower levels of SM-MHC isoforms SM1 and SM2, whereas the medial SMC usually stain positively for SM-MHC and α-actin. While SM1 expression appears at the late stage of vascular development, SM2 expression increases later, particularly after birth [2, 35-37]. SM2 thus serves as a sensitive marker of mature SMC. Aikawa et al. demonstrated that intimal SMC of the rabbit aorta 4 months after mechanical injury and the initiation of a high-cholesterol diet showed an immature phenotype as gauged by decreased SM2 immunoreactivity (Figure 1, top panels) [38]. Interestingly, dietary cholesterol lowering for 8 or 16 months induced the increased expression of SM2 in intimal SMC to the levels similar to those of medial SMC (Figure 1, bottom panels). Such SMC plasticity may depend on microenvironmental cues. Alterna‐ tively, different microenvironment may affect the balance of SMC subpopulations. Thus, the recovery of SM2 expression by lipid lowering may result from either "redifferentiation" of the same group of intimal SMC, the expansion of a subset of SMC with a mature phenotype, or

In human coronary arteries, SMC begin to lose their SM2 expression even in the physiological intima of young adults with no obvious signs of atherosclerotic changes such as macrophage accumulation [28]. In more advanced plaques, immature SMCs often exist in areas where macrophages accumulate. In Figure 2, SM2 was undetectable in SMC identified by SMα-actin and SM1. Co-existence of intraplaque microvessels and macrophages may indicate active recruitment of circulating monocytes into this area. A pro-inflammatory microenvironment produced by activated macrophages via the release of IL-1β, PDGF-BB, and other factors and production of oxidative stress may have promoted phenotypic modulation of SMC in this region. As we discuss later, some of these immature SMC may have originated from a subset of monocytes. Alternatively, some SMCs may have transdifferentiated into macrophage-like cells. Understanding of such crosstalk and potential interchangeability between SMCs and macrophages may provide important insight into the identification of new therapeutic targets. Of note, "redifferentiation" or "repopulation" of intimal SMC after mechanical injury also occurs in human coronary arteries. The differentiation state of SMC reduces a first few months after percutaneous coronary intervention while it increases over time [27]. The key question

Pathological changes in the atherosclerotic intima include increased modification of lipoproteins (e.g., oxidized LDL) and SMC uptake of modified lipoproteins. Foam cell formation by SMCs, in addition to macrophage-derived foam cells, may represent a pivotal

**Figure 1. Accumulation of immature SMC in the arterial intima.** Top panels: SM α-actin is expressed in a wide range of differentiated states of SMC, while mature SMCs produce SM2. In the atherosclerotic rabbit aorta 4 months after mechanical injury and the initiation of a high-cholesterol diet, medial SMC stained positively for SM α-actin and SM2. SM2 was undetectable in intima SMC identified by SM α-actin antibody, indicating their immature phenotype. Bottom panels: Dietary cholesterol lowering for 16 months increased SM2 immunoreactivity in the intima. The data indicate highly plastic nature of SMCs ("re-differentiation"). Alternatively, a repopulation of intimal cells by a more mature SMC subset may have resulted in increased SM2-positive cells, although their origin is unknown.

step in the transition of physiological intimal thickenings into nascent atherosclerotic lesions. [39]. Atherogenic lipoproteins also promote SMC growth by modulating calcium (Ca2+) signaling [40, 41]. SMC foam cell formation may, in part, result from the increased uptake and impaired clearance of lipids [42]. SMCs within intimal thickenings express increased levels of receptors regulating endocytosis of modified lipoproteins, including scavenger receptor A (SR-A), CD36, lectin-type oxidized LDL receptor 1, and low-density lipopro‐ tein receptor-related protein 1 (LRP1) [43]. In parallel, the expression of ATP-binding cassette transporter A1 (ABCA1) and apolipoprotein A1, key molecules for reverse cholesterol transport, decreases in intimal SMCs [39].

**Figure 2. Decreased SMC differentiation in macrophage-rich regions of human coronary arteries.** Immunohisto‐ chemistry of SMC differentiation markers in the left ascending coronary artery (LAD) of a 78-year-old male had devel‐ oped the thickened intima. SM α-actin covers a wide spectrum of SMC differentiation. SMC-specific myosin heavy chains isoforms are definitive markers of differentiated SMC. In particular, SM2 identifies mature SMC. In this athero‐ sclerotic human artery, medial SMC stained positively for all three markers, suggesting an apparently "normal" phe‐ notype. Intima SMC exhibited the diversity of SMC phenotype. Some SM α-actin expressing cells were not immunoreactive for SM1 and SM2 antibodies. Especially, many SMC did not express the detectable level SM2 in the area where many macrophages (CD68) accumulated. They may have undergone phenotypic modulation ("de-differen‐ tiation") due to a pro-inflammatory microenvironment induced by neighboring macrophages. As an alternative possi‐ bility, some of these SM2-negative cells probably co-expressed SM α-actin, SM1, and CD68, an intermediate phenotype between SMC and macrophage lineages. Of note, intraplaque microvessels surrounded by macrophages may have re‐ cruited a subset of activated monocytes as precursor cells for smooth muscle–like cells of the bone marrow origin.

In lipid-laden intimal SMCs, cholesterol accumulation induces cell death. SMC death and subsequent necrosis promote a series of pro-inflammatory events: the release of pro-inflam‐ matory cytokines including monocyte chemo-attractant protein-1 (MCP-1) and IL-1β from the dying and surrounding SMCs [44]; migration and proliferation of adjacent SMCs [45], MCP-1 mediated monocyte infiltration. MCP-1 and IL-1β modulate SMC phenotype, growth, and MMP production [46]. Such a cascade of events accelerates a positive feedback loop of vascular inflammation. To assess the anti-inflammatory and anti-atherosclerotic effects of a monoclonal anti-human IL-1β antibody, a randomized, placebo-controlled trial entitled CANTOS is currently ongoing in high-risk cardiovascular patients [47].
