**Author details**

Hiroshi Iwata1 and Masanori Aikawa1,2\*

\*Address all correspondence to: maikawa@rics.bwh.harvard.edu

1 The Center for Interdisciplinary Cardiovascular Sciences, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA

2 The Center for Excellence in Vascular Biology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA

### **References**

**Figure 5. Possible transdifferentiation processes between SMC and macrophage lineages in the atherosclerotic pla‐ ques.** The evidence suggests possible transdifferentiation between cells in the SMC and monocyte/macrophage lineag‐ es within the plaque. Various studies have demonstrated this concept in two directions: from SMC to macrophage foam cells (Ref. 20-25) and from monocytes to smooth muscle–like cells (Ref. 13, 14, 18, 19, 111,) and addressed its po‐

The evidence has used cutting-edge technologies, particularly in mouse models, to propose the substantial heterogeneity of SMCs and monocytes/macrophages. Due to technical diffi‐ culties in identifying SMC and monocyte/macrophage lineages in lesions, addressing the origin and the functionality of each cell type remains challenging. Lineage tracing of lesional cells in humans particularly requires highly sophisticated technologies. Gomez et al. recently reported a rigorous method with detection of histone modification at specific gene loci of SM-MHC gene [133]. Such specific cell lineage tracing methods will serve as powerful tools to provide insights into the crosstalk between SMCs and macrophages in human atherosclerosis. In addition, the use of multidisciplinary strategies, involving in vitro models, animal experi‐ ments, human samples, and more systemic approaches such as network analysis may help to unfold complex mechanisms for human atherogenesis. In addition, such strategies may identify new classes of therapeutic targets for atherosclerosis and its devastating complications

tential contribution to the pathogenesis of vascular disease.

246 Muscle Cell and Tissue

**13. Conclusions and future perspective**


[23] Allahverdian S, Chehroudi AC, McManus BM, Abraham T and Francis GA. Contri‐ bution of intimal smooth muscle cells to cholesterol accumulation and macrophagelike cells in human atherosclerosis. *Circulation*. 2014;129:1551-9.

[11] Asahara T, Murohara T, Sullivan A, Silver M, van der Zee R, Li T, Witzenbichler B, Schatteman G and Isner JM. Isolation of putative progenitor endothelial cells for an‐

[12] Merkulova-Rainon T, Broqueres-You D, Kubis N, Silvestre JS and Levy BI. Towards the therapeutic use of vascular smooth muscle progenitor cells. *Cardiovasc Res*.

[13] Sata M, Saiura A, Kunisato A, Tojo A, Okada S, Tokuhisa T, Hirai H, Makuuchi M, Hirata Y and Nagai R. Hematopoietic stem cells differentiate into vascular cells that

[14] Iwata H, Manabe I, Fujiu K, Yamamoto T, Takeda N, Eguchi K, Furuya A, Kuro-o M, Sata M and Nagai R. Bone marrow-derived cells contribute to vascular inflammation but do not differentiate into smooth muscle cell lineages. *Circulation*.

[15] Iwata H, Manabe I and Nagai R. Lineage of bone marrow-derived cells in atheroscle‐

[16] Iwata H and Nagai R. Novel immune signals and atherosclerosis. *Curr Atheroscler*

[17] Iwata H and Sata M. Origin of cells that contribute to neointima growth. *Circulation*.

[18] Sugiyama S, Kugiyama K, Nakamura S, Kataoka K, Aikawa M, Shimizu K, Koide S, Mitchell RN, Ogawa H and Libby P. Characterization of smooth muscle-like cells in

[19] Shimizu K, Sugiyama S, Aikawa M, Fukumoto Y, Rabkin E, Libby P and Mitchell RN. Host bone-marrow cells are a source of donor intimal smooth- muscle-like cells

[20] Rong JX, Shapiro M, Trogan E and Fisher EA. Transdifferentiation of mouse aortic smooth muscle cells to a macrophage-like state after cholesterol loading. *Proc Natl*

[21] Bentzon JF, Weile C, Sondergaard CS, Hindkjaer J, Kassem M and Falk E. Smooth muscle cells in atherosclerosis originate from the local vessel wall and not circulating progenitor cells in ApoE knockout mice. *Arterioscler Thromb Vasc Biol*.

[22] Feil S, Fehrenbacher B, Lukowski R, Essmann F, Schulze-Osthoff K, Schaller M and Feil R. Transdifferentiation of vascular smooth muscle cells to macrophage-like cells

circulating human peripheral blood. *Atherosclerosis*. 2006;187:351-62.

in murine aortic transplant arteriopathy. *Nat Med*. 2001;7:738-41.

participate in the pathogenesis of atherosclerosis. *Nat Med*. 2002;8:403-9.

giogenesis. *Science*. 1997;275:964-7.

2012;95:205-14.

248 Muscle Cell and Tissue

2010;122:2048-57.

*Rep*. 2012;14:484-90.

2008;117:3060-1.

2006;26:2696-702.

rosis. *Circ Res*. 2013;112:1634-47.

*Acad Sci U S A*. 2003;100:13531-6.

during atherogenesis. *Circ Res*. 2014;115:662-7.


[47] Ridker PM, Howard CP, Walter V, Everett B, Libby P, Hensen J and Thuren T. Effects of interleukin-1beta inhibition with canakinumab on hemoglobin A1c, lipids, C-reac‐ tive protein, interleukin-6, and fibrinogen: a phase IIb randomized, placebo-control‐ led trial. *Circulation*. 2012;126:2739-48.

[35] Kuro-o M, Nagai R, Tsuchimochi H, Katoh H, Yazaki Y, Ohkubo A and Takaku F. Developmentally regulated expression of vascular smooth muscle myosin heavy

[36] Nagai R, Kuro-o M, Babij P and Periasamy M. Identification of two types of smooth muscle myosin heavy chain isoforms by cDNA cloning and immunoblot analysis. *J*

[37] Nagai R, Larson DM and Periasamy M. Characterization of a mammalian smooth muscle myosin heavy chain cDNA clone and its expression in various smooth mus‐

[38] Aikawa M, Rabkin E, Okada Y, Voglic SJ, Clinton SK, Brinckerhoff CE, Sukhova GK and Libby P. Lipid lowering by diet reduces matrix metalloproteinase activity and increases collagen content of rabbit atheroma: a potential mechanism of lesion stabili‐

[39] Allahverdian S, Pannu PS and Francis GA. Contribution of monocyte-derived macro‐ phages and smooth muscle cells to arterial foam cell formation. *Cardiovasc Res*.

[40] Lipskaia L, Pourci ML, Delomenie C, Combettes L, Goudouneche D, Paul JL, Capiod T and Lompre AM. Phosphatidylinositol 3-kinase and calcium-activated transcrip‐ tion pathways are required for VLDL-induced smooth muscle cell proliferation. *Circ*

[41] Ewart MA, Kennedy S, Macmillan D, Raja AL, Watt IM and Currie S. Altered vascu‐ lar smooth muscle function in the ApoE knockout mouse during the progression of

[42] Van Vre EA, Ait-Oufella H, Tedgui A and Mallat Z. Apoptotic cell death and effero‐

[43] Lacolley P, Regnault V, Nicoletti A, Li Z and Michel JB. The vascular smooth muscle cell in arterial pathology: a cell that can take on multiple roles. *Cardiovasc Res*.

[44] Clarke MC, Talib S, Figg NL and Bennett MR. Vascular smooth muscle cell apoptosis induces interleukin-1-directed inflammation: effects of hyperlipidemia-mediated in‐

[45] Yu H, Clarke MC, Figg N, Littlewood TD and Bennett MR. Smooth muscle cell apop‐ tosis promotes vessel remodeling and repair via activation of cell migration, prolifer‐

[46] Alexander MR, Moehle CW, Johnson JL, Yang Z, Lee JK, Jackson CL and Owens GK. Genetic inactivation of IL-1 signaling enhances atherosclerotic plaque instability and reduces outward vessel remodeling in advanced atherosclerosis in mice. *J Clin Invest*.

ation, and collagen synthesis. *Arterioscler Thromb Vasc Biol*. 2011;31:2402-9.

cytosis in atherosclerosis. *Arterioscler Thromb Vasc Biol*. 2012;32:887-93.

chain isoforms. *J Biol Chem*. 1989;264:18272-5.

cle types. *Proc Natl Acad Sci U S A*. 1988;85:1047-51.

atherosclerosis. *Atherosclerosis*. 2014;234:154-61.

hibition of phagocytosis. *Circ Res*. 2010;106:363-72.

*Biol Chem*. 1989;264:9734-7.

250 Muscle Cell and Tissue

zation. *Circulation*. 1998;97:2433-44.

2012;95:165-72.

*Res*. 2003;92:1115-22.

2012;95:194-204.

2012;122:70-9.


[73] Zawada AM, Rogacev KS, Rotter B, Winter P, Marell RR, Fliser D and Heine GH. Su‐ perSAGE evidence for CD14++CD16+ monocytes as a third monocyte subset. *Blood*. 2011;118:e50-61.

[59] Veillard NR, Steffens S, Burger F, Pelli G and Mach F. Differential expression pat‐ terns of proinflammatory and antiinflammatory mediators during atherogenesis in

[60] Mestas J and Ley K. Monocyte-endothelial cell interactions in the development of

[61] Steinberg D. Atherogenesis in perspective: hypercholesterolemia and inflammation

[62] Deguchi JO, Aikawa M, Tung CH, Aikawa E, Kim DE, Ntziachristos V, Weissleder R and Libby P. Inflammation in atherosclerosis: visualizing matrix metalloproteinase

[63] Hansson GK. Inflammation, atherosclerosis, and coronary artery disease. *N Engl J*

[64] Hansson GK and Hermansson A. The immune system in atherosclerosis. *Nat Immu‐*

[65] Stoger JL, Goossens P and de Winther MP. Macrophage heterogeneity: relevance and functional implications in atherosclerosis. *Curr Vasc Pharmacol*. 2010;8:233-48.

[66] Geissmann F, Jung S and Littman DR. Blood monocytes consist of two principal sub‐

[67] Geissmann F, Gordon S, Hume DA, Mowat AM and Randolph GJ. Unravelling mon‐

[68] Weber C, Zernecke A and Libby P. The multifaceted contributions of leukocyte sub‐ sets to atherosclerosis: lessons from mouse models. *Nat Rev Immunol*. 2008;8:802-15.

[69] Gautier EL, Jakubzick C and Randolph GJ. Regulation of the migration and survival of monocyte subsets by chemokine receptors and its relevance to atherosclerosis. *Ar‐*

[70] Weber C, Belge KU, von Hundelshausen P, Draude G, Steppich B, Mack M, Franken‐ berger M, Weber KS and Ziegler-Heitbrock HW. Differential chemokine receptor ex‐ pression and function in human monocyte subpopulations. *J Leukocyte Biol*.

[71] Auffray C, Fogg D, Garfa M, Elain G, Join-Lambert O, Kayal S, Sarnacki S, Cumano A, Lauvau G and Geissmann F. Monitoring of blood vessels and tissues by a popula‐

[72] Gordon S and Taylor PR. Monocyte and macrophage heterogeneity. *Nat Rev Immu‐*

tion of monocytes with patrolling behavior. *Science*. 2007;317:666-70.

sets with distinct migratory properties. *Immunity*. 2003;19:71-82.

onuclear phagocyte heterogeneity. *Nat Rev Immunol*. 2010;10:453-60.

mice. *Arterioscler Thromb Vasc Biol*. 2004;24:2339-44.

atherosclerosis. *Trends Cardiovasc Med*. 2008;18:228-32.

action in macrophages in vivo. *Circulation*. 2006;114:55-62.

as partners in crime. *Nat Med*. 2002;8:1211-7.

*terioscler Thromb Vasc Biol*. 2009;29:1412-8.

*Med*. 2005;352:1685-95.

*nol*. 2011;12:204-12.

252 Muscle Cell and Tissue

2000;67:699-704.

*nol*. 2005;5:953-64.


Anderson DG, Libby P, Swirski FK, Weissleder R and Nahrendorf M. In vivo silenc‐ ing of the transcription factor IRF5 reprograms the macrophage phenotype and im‐ proves infarct healing. *J Am Coll Cardiol*. 2014;63:1556-66.


mulation and organization in mouse atherosclerotic plaques. *Circulation*. 2005;112:2708-15.


Anderson DG, Libby P, Swirski FK, Weissleder R and Nahrendorf M. In vivo silenc‐ ing of the transcription factor IRF5 reprograms the macrophage phenotype and im‐

[86] Nishimura S, Manabe I, Nagasaki M, Eto K, Yamashita H, Ohsugi M, Otsu M, Hara K, Ueki K, Sugiura S, Yoshimura K, Kadowaki T and Nagai R. CD8+ effector T cells contribute to macrophage recruitment and adipose tissue inflammation in obesity.

[87] Murray PJ, Allen JE, Biswas SK, Fisher EA, Gilroy DW, Goerdt S, Gordon S, Hamil‐ ton JA, Ivashkiv LB, Lawrence T, Locati M, Mantovani A, Martinez FO, Mege JL, Mosser DM, Natoli G, Saeij JP, Schultze JL, Shirey KA, Sica A, Suttles J, Udalova I, van Ginderachter JA, Vogel SN and Wynn TA. Macrophage activation and polariza‐

[88] Robbins CS, Chudnovskiy A, Rauch PJ, Figueiredo JL, Iwamoto Y, Gorbatov R, Etz‐ rodt M, Weber GF, Ueno T, van Rooijen N, Mulligan-Kehoe MJ, Libby P, Nahrendorf M, Pittet MJ, Weissleder R and Swirski FK. Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions. *Circulation*.

[89] Nahrendorf M, Swirski FK, Aikawa E, Stangenberg L, Wurdinger T, Figueiredo JL, Libby P, Weissleder R and Pittet MJ. The healing myocardium sequentially mobilizes two monocyte subsets with divergent and complementary functions. *J Exper Med*.

[90] Liu J, Thewke DP, Su YR, Linton MF, Fazio S and Sinensky MS. Reduced macro‐ phage apoptosis is associated with accelerated atherosclerosis in low-density lipo‐

[91] Tabas I. Consequences and therapeutic implications of macrophage apoptosis in atherosclerosis: the importance of lesion stage and phagocytic efficiency. *Arterioscler*

[92] Bhatia VK, Yun S, Leung V, Grimsditch DC, Benson GM, Botto MB, Boyle JJ and Haskard DO. Complement C1q reduces early atherosclerosis in low-density lipopro‐

[93] Tabas I. Macrophage death and defective inflammation resolution in atherosclerosis.

[94] Fukumoto Y, Deguchi JO, Libby P, Rabkin-Aikawa E, Sakata Y, Chin MT, Hill CC, Lawler PR, Varo N, Schoen FJ, Krane SM and Aikawa M. Genetically determined re‐ sistance to collagenase action augments interstitial collagen accumulation in athero‐

[95] Deguchi JO, Aikawa E, Libby P, Vachon JR, Inada M, Krane SM, Whittaker P and Ai‐ kawa M. Matrix metalloproteinase-13/collagenase-3 deletion promotes collagen accu‐

protein receptor-null mice. *Arterioscler Thromb Vasc Biol*. 2005;25:174-9.

tein receptor-deficient mice. *Am J Pathol*. 2007;170:416-26.

tion: nomenclature and experimental guidelines. *Immunity*. 2014;41:14-20.

proves infarct healing. *J Am Coll Cardiol*. 2014;63:1556-66.

*Nat Med*. 2009;15:914-20.

254 Muscle Cell and Tissue

2012;125:364-74.

2007;204:3037-47.

*Thromb Vasc Biol*. 2005;25:2255-64.

*Nat Rev Immunol*. 2010;10:36-46.

sclerotic plaques. *Circulation*. 2004;110:1953-9.


growth cells circulating in blood and vasculogenic smooth muscle-like cells in vivo. *Atherosclerosis*. 2008;198:29-38.


[123] Hong KM, Burdick MD, Phillips RJ, Heber D and Strieter RM. Characterization of human fibrocytes as circulating adipocyte progenitors and the formation of human adipose tissue in SCID mice. *FASEB J*. 2005;19:2029-31.

growth cells circulating in blood and vasculogenic smooth muscle-like cells in vivo.

[110] Richardson MR and Yoder MC. Endothelial progenitor cells: quo vadis? *J Mol Cell*

[111] Han CI, Campbell GR and Campbell JH. Circulating bone marrow cells can contrib‐

[112] Saiura A, Sata M, Hirata Y, Nagai R and Makuuchi M. Circulating smooth muscle

[113] Yu H, Stoneman V, Clarke M, Figg N, Xin HB, Kotlikoff M, Littlewood T and Bennett M. Bone marrow-derived smooth muscle-like cells are infrequent in advanced pri‐ mary atherosclerotic plaques but promote atherosclerosis. *Arterioscler Thromb Vasc Bi‐*

[114] Alexander MR and Owens GK. Epigenetic control of smooth muscle cell differentia‐ tion and phenotypic switching in vascular development and disease. *Annu Rev Physi‐*

[115] Ebihara Y, Masuya M, Larue AC, Fleming PA, Visconti RP, Minamiguchi H, Drake CJ and Ogawa M. Hematopoietic origins of fibroblasts: II. In vitro studies of fibro‐

[116] Mori L, Bellini A, Stacey MA, Schmidt M and Mattoli S. Fibrocytes contribute to the myofibroblast population in wounded skin and originate from the bone marrow. *Exp*

[117] Bellini A and Mattoli S. The role of the fibrocyte, a bone marrow-derived mesenchy‐ mal progenitor, in reactive and reparative fibroses. *Lab Invest*. 2007;87:858-70.

[118] Reilkoff RA, Bucala R and Herzog EL. Fibrocytes: emerging effector cells in chronic

[119] Curnow SJ, Fairclough M, Schmutz C, Kissane S, Denniston AK, Nash K, Buckley CD, Lord JM and Salmon M. Distinct types of fibrocyte can differentiate from mono‐

[120] Niedermeier M, Reich B, Rodriguez Gomez M, Denzel A, Schmidbauer K, Gobel N, Talke Y, Schweda F and Mack M. CD4+ T cells control the differentiation of Gr1+

[121] Pilling D, Fan T, Huang D, Kaul B and Gomer RH. Identification of markers that dis‐ tinguish monocyte-derived fibrocytes from monocytes, macrophages, and fibro‐

[122] He Q, Wan C and Li G. Concise review: multipotent mesenchymal stromal cells in

nuclear cells in the presence and absence of serum. *PLoS One*. 2010;5:e9730.

monocytes into fibrocytes. *Proc Natl Acad Sci U S A*. 2009;106:17892-7.

progenitor cells contribute to atherosclerosis. *Nat Med*. 2001;7:382-3.

ute to neointimal formation. *J Vasc Res*. 2001;38:113-9.

blasts, CFU-F, and fibrocytes. *Exp Hematol*. 2006;34:219-29.

inflammation. *Nat Rev Immunol*. 2011;11:427-35.

*Atherosclerosis*. 2008;198:29-38.

*Cardiol*. 2011;50:266-72.

256 Muscle Cell and Tissue

*ol*. 2011;31:1291-9.

*ol*. 2012;74:13-40.

*Cell Res*. 2005;304:81-90.

blasts. *PLoS One*. 2009;4:e7475.

blood. *Stem Cells*. 2007;25:69-77.


**Section 4**

**Skin Pigmentation**
