**10. Circulating fibrocytes**

inflammatory responses of macrophages may destabilize atherosclerotic plaques. The pro‐ duction of MMPs by macrophages may degrade collagen in the fibrous cap and make plaques susceptible for plaque rupture and thrombosis. Fukumoto et al. and Deguchi et al. used genetically altered mouse strains to provide the first in vivo direct evidence for the role of collagenases of the MMP family for the loss of fibrillar collagen within the intima [94, 95].

As mentioned, emerging data have proposed the heterogeneity of macrophages. A subpopu‐ lation of T lymphocytes (Th1 cells) secretes such as IFNγ, IL-2, IL-12, and TNFα and promotes the activation of macrophages toward a pro-inflammatory phenotype (M1). Th2 cytokines (e.g., L-4 and IL-13) induce an alternative form of activation toward a non/anti-inflammatory (M2) phenotype. The balance of such macrophage polarization (M1/M2 balance) may affect plaque outcome [71]. The high M1/M2 ratio in atherosclerotic plaques may induce lesion formation and plaque vulnerability [80, 96]. The evidence has identified switching of macro‐ phage phenotypes from M1 to M2 during the regression of atherosclerosis or in response to anti-inflammatory therapies [84, 97]. Proinflammatory M1 macrophages also induce SMC

Alternatively activated M2 macrophages may generally exert anti-atherogenic effects. M2 cells suppress Th1 inflammatory responses. TGF-β released by M2 macrophages may inhibit the recruitment of inflammatory cells and the development of atherosclerosis [99]. M2 macro‐ phages also release IL-10, which inhibits the production of inflammatory cytokines from T lymphocytes and other macrophages. M2 macrophages suppress inflammatory milieu by clearing apoptotic cells and tissue debris [100, 101]. During the repair process after tissue injury, M2 cell may promote fibrosis [102], [103]. This action may potentially be beneficial in

While the M1/M2 paradigm has clear relationships between stimulators and downstream effects and has thus served as a useful mechanistic model, the evidence suggests that macro‐ phages are more diverse. In particular, M2 may further contain various forms of macrophage activation, e.g, M2a to M2d [104]. Mox macrophages develop in response to atherogenic phospholipids and have lower phagocytotic and chemotactic capacity than do conventional M1 and M2 cells [104, 105]. Mhem cells, induced by intraplaque hemorrhage, often associate with atherothrombotic complications [105, 106]. CXC chemokine ligand 4 induces M4, a recently proposed subtype of atherogenic macrophages [107]. The heterogeneity of macro‐ phages thus seems to be more complex than previously proposed. Furthermore, in vivo functional significance of each macrophage subpopulation remains incompletely understood. Recently, new terms more specific to each stimulator were proposed, e.g. M(IFNγ), M(LPS),

Studies have identified circulating SM progenitor cells (SMPCs) and EPCs that can acquire SMC-like or EC-like phenotypes in mouse and human: [11, 13, 108]. These cell types share similar surface markers and functions with myeloid cells [109, 110] and SMCs and ECs,

proliferation [98].

242 Muscle Cell and Tissue

M(IL-10), and M(IL-4) [87].

plaque instability via thickening the fibrous cap [104].

**9. Smooth Muscle Progenitor Cells (SMPCs)**

Fibrocytes, BM-derived mesenchymal progenitors [115, 116], coexpress markers of hemato‐ poietic stem cells, the monocyte-lineage, and fibroblasts. Fibrocytes may participate in various diseases, including inflammatory bowel diseases, allergy, and pulmonary and liver fibrosis. They produce extracellular matrix components as well as matrix-degrading enzymes and further differentiate into myofibroblast-like cells [117, 118]. Human fibrocytes also express genes, including Tall-like receptor 4, IL-1β, CCL2, CCL3, CCL7, CCL22, and C5aR, suggesting that they mediate inflammatory responses [119]. They may originate from CD14+ BM–derived monocytes in humans [119] and from the Gr1+CD115+CD11b+ monocyte population in mice [120], suggesting that circulating fibrocytes are a transitional cell population between mono‐

**Figure 4. Bone marrow CD11b+Ly-6C+ cells migrate to the arterial wall and express SM α-actin.** CD11b+Ly-6C+ cells within the bone marrow of SM α-actin-EGFP mice were sorted and adoptively transferred into wild-type mice with femoral arteries that had been subjected to wire-induced arterial injury. Four weeks after the injury, EGFP+ cells corre‐ sponding to CD11b+Ly-6C+-derived SM α-actin+ cells (arrows) were found in the walls of the injured vessel. Scale bar=50 µm. i Indicates intima; m, media; and a, adventitia (Modified from Ref 14 by Iwata et al.).

cytes and fibroblasts. Considerable overlaps exist in the gene expression profiles among human monocytes, macrophages, fibrocytes, and fibroblasts [121]. Human fibrocytes also may differentiate into cells with characteristics of adipocytes, chondrocytes, and osteoblasts [122, 123]. In human peripheral blood, 0.1% to 0.5% of nucleated cells are circulating fibrocytes that express type 1 and 2 collagens, vimentin, and SMα-actin [124]. Because no single marker can unequivocally identify fibrocytes, the combined use of collagen and other surface markers, including CD34, CD45, and CD68, is a common approach. More recent studies have used a combination of CD45RO, 25F9, and S100A8/A9 or CD49.

The fibrous cap of human atherosclerotic lesions contains fibrocytes expressing procollagen I and CD34 [125]. Subendothelial SMα-actin–positive myofibroblasts expressing the monocyte marker CD68 have been found in lipid-rich areas of the atherosclerotic intima in human aorta [126]. The overexpression of TGF-β1 resulted in the increased accumulation of fibrocytes in atherosclerotic plaques of Apoe-/- mice [127]. The pro-inflammatory monocyte subset CD14<sup>+</sup> + CD16− CCR2high may be precursors of fibrocytes. The expression profile of marker genes indicates considerable overlaps between fibrocytes, SMPC, smooth muscle–like cells, and monocytes/macrophages, suggesting the importance to clarify the relationship in lineages and functions between these cell types to unfold intertwined mechanisms for atherosclerosis and provide insight into the development of new classes of therapeutics.
