Evolution of foam cells in subcutaneous rabbit carrageenan granulomas: II. Tissue and macrophage lipid composition

This study describes the lipid composition of differentiating macrophage-derived foam cells in the inflammatory carrageenan granuloma. In this model, macrophages exposed in vivo to diet-induced hypercholesterolemia progressively accumulate electron-translucent lipid inclusions; and at 14 and 28 days, many assume the morphologic features of arterial plaque foam cells. Subcutaneous carrageenan granulomas were induced in 24 pellet-fed (NC) and 24 cholesterol-fed (HC) rabbits, and tissue was harvested at 4, 14, and 28 days. Total (TC) and free cholesterol (FC), cholesteryl esters (CEs), CE fatty acids, triglycerides (TGs), and phospholipids (PLs) were measured on lipid extracts from tissue. TC, FC, and CEs were also measured on isolated, cultured granuloma macrophages. Tissue TCs and FCs were significantly elevated in HC relative to NC rabbits at both 14 and 28 days (P < 0.005 and P < 0.01, respectively). CE accumulation in HC granuloma tissue was 80-fold greater at 14 days and 178-fold greater at 28 days (P < 0.005), compared with NC granulomas. Oleic acid (18:1), the principal CE fatty acid in both NC and HC granulomas, accounted for significantly more (P < 0.05) of the total CE fatty acids in HC (48%) relative to NC granulomas (37%). No net accumulation of TG was observed with time in NC or HC animals. Although diet did not influence tissue PL content, significant increases (P < 0.05) were observed at 14 days in NC rabbits and at 14 and 28 days in HC rabbits relative to 4-day levels. CE accumulation was significantly greater in cultured macrophages isolated from HC granulomas at 14 days (P < 0.001) and 28 days (P < 0.01). These findings have demonstrated the significant accumulation of CEs in both HC granuloma tissue and in cultured HC macrophage/foam cells in vivo. The carrageenan granuloma model has, we believe, considerable potential for defining mechanisms responsible for CE accumulation in the differentiating macrophage-derived foam cell.