EXPERIMENTAL AND MOLECULAR PATHOLOGY 33, 302-315 (1980) Ultrastructural Studies on the Localization of IgG in the Aortic Endothelium and Subendothelial lntima of Atherosclerotic and Nonatherosclerotic Rabbits’ GORAN K. HANSSON,~G~RAN BONDJERS, ANDERS BYLOCK, AND LENA HJALMARSSON Arterial Biology Group, Departments of Histology and Internal Medicine I, University of Gdteborg, Gateborg, Sweden Received March 24, 1980, and in revised form July 11, 1980 Immunoelectron microscopy with peroxidase-conjugated Fab fragments of anti-IgG was used for studying the localization of IgG in the aortic endothelium and subendothelial intima of atherosclerotic and nonatherosclerotic rabbits. Small amounts of IgG were found in the cell coat, in caveolae and vesicles, and also in intercellular clefts of endotheiial cells from normocholesterolemic rabbits. Injured endothelial cells exhibited prominent accumulations of IgG in the cytoplasmic matrix, possibly due to leakage through plasma membrane defects. In atherosclerotic lesions from hypercholesterolemic rabbits, there was a striking increase in the amount of IgG-reactive material in the cell coat and vesicles of intact endothelial cells. Also in these animals, injured endothelial cells were characterized by a cytoplasmic IgG accumulation. There were prominent IgG depositions in the subendothelial zone of the lesions. IgG was adhering to collagen fibers, and also coating the surfaces of subendothelial foam cells. The pathophysiological significance of an interaction between such intimal IgG and phagocytes is discussed. INTRODUCTION Experimental evidence suggests that endothelial injury is an initial step in the formation of atherosclerotic lesions (Bondjers ef al., 1977; Ross ef al., 1977). Endothelial injury may be induced in several ways, e.g., by hemodynamic strain (Fry, 1968), hypercholesterolemia (Ross and Harker, 1976; Goode et al., 1977), hypertension (Iwanaga et al., 1969; Schwartz and Benditt, 1977), or immune com- plex disease (Minick and Murphy 1973; Sharma and Geer, 1977). Ultrastructural observations on injured endothelial cells in unmanipulated ani- mals suggest a characteristic sequence of events, ranging from minor signs of injury to overt cell death (Gerrity et al., 1977). Injured endothelial cells may also be identified due to their inability to exclude anionic dyes such as trypan blue, Evans blue, and nigrosin (Bjorkerud and Bondjers, 1972). With a double-staining technique using fluorescent antibodies and a dye exclusion test, we found that injured endothelial cells contain immunoglobulin IgG (Hansson et al., 1979). In the present study, we have extended the investigations on injured endothelial cells by using peroxidase-labeled Fab fragments of specific antibodies against rabbit IgG in combination with electron microscopy. We demonstrate the ultrastructural localization of IgG in aortic endothelial cells and subendothelium of nor- mocholesterolemic and hypercholesterolemic rabbits. ’ This investigation was supported by grants from the Swedish Medical Research Council (Project 4531). the Swedish National Association against Heart and Chest Diseases, Goteborgs Lilkaresallskap, and the University of Goteborg. *To whom correspondence should be addressed at: Department of Histology, University of Gbteborg, Box 33031, S-400 33 Goteborg, Sweden. 302 00144800/80/060302-14$2.00/O Copyright @ 1980 by Academic Press, Inc. AU rights of reproduction in any form reserved.