Objective-Although matrix metalloproteinase-9 (MMP-9) has been implicated in atherosclerotic plaque instability, the exact role it plays in the plaque development and progression remains largely unknown. We generated apolipoprotein E (apoE)-deficient (apoE Ϫ/Ϫ ) MMP-9 -deficient (MMP-9 Ϫ/Ϫ ) mice to determine the mechanisms and the main cell source of MMP-9 responsible for the plaque composition during accelerated atherosclerotic plaque formation. Methods and Results-Three weeks after temporary carotid artery ligation revealed that while on a Western-type diet, apoE Ϫ/Ϫ MMP-9 Ϫ/Ϫ mice had a significant reduction in intimal plaque length and volume compared with apoEMMP-9 ϩ/ϩ mice. The reduction in plaque volume correlated with a significantly lower number of intraplaque cells of resident cells and bone marrow-derived cells. To determine the cellular origin of MMP-9 in plaque development, bone marrow transplantation after total-body irradiation was performed with apoE Ϫ/Ϫ MMP-9 ϩ/ϩ and apoE Ϫ/Ϫ MMP-9mice, which showed that only MMP-9 derived from resident arterial cells is required for plaque development. Key Words: atherosclerosis Ⅲ MMP-9 Ⅲ bone marrow Ⅲ mouse Ⅲ compartmentalization S everal matrix metalloproteinases (MMPs), namely MMP-1, MMP-2, MMP-3, MMP-7, MMP-9, MMP-11, and MMP-14, are present in human atherosclerotic lesions, leading to the speculation that overexpression of these enzymes is linked to atherogenesis. [1][2][3][4][5][6] However, experimental models of atherosclerosis demonstrated that mice deficient in apolipoprotein E (apoE) and MMP-3 had larger atherosclerotic plaque, whereas apoE-deficient (apoE Ϫ/Ϫ ) mice overexpressing human MMP-1 had a reduction in plaque size. 1,7 Moreover, inactivation of tissue inhibitor of metalloproteinase 1, which can inhibit MMP-1, MMP-3, MMP-9, and MMP-11 activity and is overexpressed in human atherosclerotic lesions, reduced the plaque size in apoE Ϫ/Ϫ mice. 1 Thus, it is likely that some MMPs are involved in atherogenic processes, whereas others function to inhibit plaque formation.
Conclusions-MMP-9 is derived from resident arterial cells and isMMP-9 (gelatinase B) is expressed in late atherosclerotic lesions in humans and has been suggested to mediate plaque instability, a leading cause of acute coronary syndrome and stroke. 2,3 Studies in humans have revealed that polymorphisms in the MMP-9 promoter, which enhance expression, correlate with the development and progression of coronary atherosclerosis. 8,9 Studies with apoE Ϫ/Ϫ MMP-9 Ϫ/Ϫ mice have demonstrated that MMP-9 is critical to intimal plaque size. 10,11 To determine the mechanisms and the main cell source of MMP-9 responsible for the plaque composition during accelerated atherosclerotic plaque formation, we also cross-bred apoE Ϫ/Ϫ mice with MMP-9 Ϫ/Ϫ mice. We demonstrate that although MMP-9 expression is derived mostly from bone marrow cells, MMP-9 derived from resident arterial cells dictates the overall plaque composition. Therefore, MMP-9 activity associated with the resident cells (compartmental...