Luminal Foam Cell Accumulation Is Associated With Smooth Muscle Cell Death in the Intimal Thickening of Human Saphenous Vein Grafts

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Abstract
Background Occlusion of saphenous vein grafts is a major problem after coronary artery bypass graft surgery. Diffuse intimal thickening develops in all implanted aortocoronary saphenous vein grafts within 6 months to 1 year. In some regions of the thickened intima, foam cells accumulate along the luminal margin. This particular morphology resembles the morphology of unstable atherosclerotic plaques as they occur in coronary arteries. In the present study, we focused on the possible topographic relation between luminal foam cell accumulation and cell death of smooth muscle cells (SMCs) within the adjacent thickened intima. Methods and Results Segments of occluded and suboccluded implanted human aortocoronary saphenous vein grafts were obtained during reintervention coronary artery bypass graft surgery in 30 patients. In the regions of the vein grafts with luminal foam cell accumulation, the percentage of SMC α-actin immunoreactive area of the superficial intimal thickening was 6±1.4%, which was different from the 17.6±2.3% of the deep intimal thickening. A strong negative correlation between the number of foam cell nuclei and the percentage of SMC α-actin immunoreactive area in the adjacent superficial intimal thickening was present ( r =−.77, P <.001). Within the superficial intimal thickening, cytoplasmic and DNA fragmentation could be detected, which points to apoptotic cell death. A fraction of the cytoplasmic fragments fitted the ultrastructural characteristics of matrix vesicles and showed pronounced calcium and phosphorus accumulation as demonstrated with the use of x-ray microanalysis. Conclusions The close spatial relation among foam cell accumulation, pronounced intimal SMC loss, and cell death suggests the presence of a foam cell–derived factor that can induce cell death in the SMC population of the intimal thickening. The depletion of the intimal SMC population could promote plaque rupture and thrombotic complications in the grafts.