Smooth Muscle Cells Healing Atherosclerotic Plaque Disruptions Are of Local, Not Blood, Origin in Apolipoprotein E Knockout Mice

Abstract

Background— Signs of preceding episodes of plaque rupture and smooth muscle cell (SMC)–mediated healing are common in atherosclerotic plaques, but the source of the healing SMCs is unknown. Recent studies suggest that activated platelets adhering to sites of injury recruit neointimal SMCs from circulating bone marrow–derived progenitor cells. Here, we analyzed the contribution of this mechanism to plaque healing after spontaneous and mechanical plaque disruption in apolipoprotein E knockout (apoE −/− ) mice. Methods and Results— To determine the origin of SMCs after spontaneous plaque disruption, irradiated 18-month-old apoE −/− mice were reconstituted with bone marrow cells from enhanced green fluorescent protein (eGFP) transgenic apoE −/− mice and examined when they died up to 9 months later. Plaque hemorrhage, indicating previous plaque disruption, was widely present, but no bone marrow–derived eGFP + SMCs were detected. To examine the origin of healing SMCs in a model that recapitulates more features of human plaque rupture and healing, we developed a mechanical technique that produced consistent plaque disruption, superimposed thrombosis, and SMC-mediated plaque healing in apoE −/− mice. Mechanical plaque disruption was produced in irradiated apoE −/− mice reconstituted with eGFP + apoE −/− bone marrow cells and in carotid bifurcations cross-grafted between apoE −/− and eGFP + apoE −/− mice. Apart from few non–graft-derived SMCs near the anastomosis site in 1 transplanted carotid bifurcation, no SMCs originating from outside the local arterial segment were detected in healed plaques. Conclusions— Healing SMCs after atherosclerotic plaque disruption are derived entirely from the local arterial wall and not circulating progenitor cells in apoE −/− mice.