Letter by Hibbert et al Regarding Article, “Smooth Muscle Cells Healing Atherosclerotic Plaque Disruptions Are of Local, Not Blood, Origin in Apolipoprotein E Knockout Mice”
To the Editor:
We congratulate Bentzon and colleagues on their interesting study of the origin of smooth muscle cells (SMCs) in atherosclerotic lesions in the context of plaque hemorrhage and mechanical disruption.1 Although their study supports the notion that SMCs originate from the vessel wall, there are significant limitations that should be addressed.
In their initial experiment, apolipoprotein E knockout mice received bone marrow transplants from green fluorescent protein-positive donors and then were followed until death (mean interval, 6 months). Two major concerns arise from this experiment. First, the timeframe from transplantation to death is problematic, as marrow repopulation by endogenous cells is known to begin as early as 10 weeks after transplantation.2 Hence, one plausible explanation for the lack of green fluorescent protein-positive bone marrow–derived SMCs in vascular lesions may be that the marrow is chimeric because of reconstitution by endogenous marrow cells. Also, the recipient mice were 18 months of age at time of transplantation. Mature atherosclerotic lesions with previous hemorrhage undoubtedly developed before the bone marrow transplantation, and the investigators cannot be sure that they are studying hemorrhagic lesions that developed before or after bone marrow transplantation.
Second, the authors1 address the origin of SMCs after mechanical disruption of mature plaques in endogenous and transplanted carotid artery bifurcations. Although these studies may also be limited by the age of the apolipoprotein E knockout mice at the time of transplantation, there are other methodological issues. Elegant studies performed decades ago clearly show that the vasa vasorum of carotid arteries arises from terminal branches of adjacent arteries (eg, ophthalmic).3 Hence, the harvesting of carotid artery bifurcations ultimately results in disruption of the vasa vasorum and leads to vessel wall hypoxia—a phenomenon previously shown to induce atherosclerotic lesions4 and neointimal growth of endogenous cells—thereby abrogating healing by blood-borne (circulatory) cells.
To date, the most intricate and sophisticated studies to question the origin of SMCs in vascular lesions have been performed by Hillebrands and colleagues5 in cardiac and aortic transplants. In addition to microscopy and its inherent limitations, the authors provide compelling data from microdissected SMCs that demonstrate the presence of Y chromosomes in male recipients with female grafts, which is persuasive evidence of a circulatory origin for intimal SMCs. Despite technical differences, the divergence of results reported may reflect discrepancies inherent in the animal models studied, findings that do not reflect the human condition. Notably, studies of human lesions have not shown significant proliferation,6 which raises questions about the assertion that local sources alone account for the marked accumulation of SMCs after mechanical injury. Finally, should one assume that SMCs in atherosclerotic lesions are derived from mural SMCs, lesions that develop over decades would necessarily contravene the Hayflick limit by requiring many fold replications.
Thus, with these comments, we disagree with the conclusions of the authors1 and suggest that the evidence of exclusive local origin of SMCs in healing atherosclerotic lesions is far from conclusive. Indeed, these studies simply highlight the need to embrace more definitive techniques to advance our understanding of the pathogenesis of neoinitimal formation.
Bentzon JF, Sondergaard CS, Kassem M, Falk E. Smooth muscle cells healing atherosclerotic plaque disruptions are of local, not blood, origin in apolipoprotein E knockout mice. Circulation. 2007; 116: 2053–2061.
Svilvassy SJ, Humphries RK, Lansdrop PM, Eaves AC, Eaves CJ. Quantitative assay for totipotent reconstituting hematopoietic stem cells by a competitive repopulation strategy. Proc Natl Acad Sci U S A. 1990; 87: 8736–8740.
Clarke JA. An x-ray microscopic study of the normal root of neck arteries in man. Thorax. 1965; 20: 270–274.
O’Brien ER, Alpers CE, Stewart DK, Ferguson M, Tran N, Gordon D, Benditt EP, Hinohara T, Simpson JB, Schwartz SM. Proliferation in primary and restenotic coronary atherectomy tissue: implications for anti-proliferative therapy. Circ Res. 1993; 73: 223–231.