acLDL binding and endocytosis by macrophages and macrophage foam cells in situ

The direct binding and internalization of acLDL by monocyte-derived macrophages associated with atherosclerotic lesions in the White Carneau pigeon were observed. Using an organ culture labeling protocol, pigeon thoracic aortae containing atherosclerotic lesions were incubated with colloidal gold-acLDL conjugates. When analyzed by electron microscopy acLDL-gold was bound and internalized by monocyte-derived macrophages residing on the lumenal surface of the endothelium. Macrophage foam cells located within the intima and projecting from the lesion into the arterial lumen also bound and internalized acLDL. In contrast, acLDL was not observed in spherical (nonactivated) monocytes adherent to the lesion surface. Colabeling experiments with acLDL in combination with beta VLDL resulted in the preferential (> 90%) binding and internalization of acLDL by macrophages and macrophage foam cells. The results of this study indicated that macrophages located on the endothelial surface were able to bind and internalize lipoprotein, although this binding and internalization was enhanced in foam cells spanning the endothelium to the subendothelial intima. Acetyl-LDL binding and internalization was effectively inhibited through competition studies using an excess of unconjugated acLDL. Through the direct observation of lipoprotein binding by macrophages and foam cells, we have demonstrated the expression of a functional receptor for modified lipoprotein on monocyte-derived macrophages during different stages of association with foam cell lesions.