Foam cell formation containing lipid droplets enriched with free cholesterol by hyperlipidemic serum

A monoclonal antibody, ASH1a/256C (256C), which binds to atherosclerotic le sions in Watanabe heritable hyperlipidemic rabbit (WHHL) aorta in vivo, rec ognizes complex structures of phosphatidylcholine mixed with neutral lipids . In the present study, a cell culture system is described in which foam ce lls express 256C-positive lipid droplets. J774.1 macrophages were incubated in the presence of a small volume of WHHL serum for 24 h to produce foam c ells, which were then incubated without the WHHL serum for 3 days. Oil red O-positive lipid droplets appeared on day 1, and were present in the cells during the whole incubation period. The lipid droplets in the cells were po sitively immunostained with antibody 256C on day 4, although they were nega tive on day 1. Expression of the antigenic lipid droplets was also induced by the addition of acetylated LDL or sera from patients with hyperlipidemia . When foam cells were induced by the addition of WHHL serum, cellular cont ent of cholesteryl ester was greatly increased but then decreased to near b asal levels by day 4. Concomitantly, cellular free cholesterol increased du ring the culture period, indicating that the cholesteryl ester changes to f ree cholesterol by day 4. The lipid droplets in the foam cells on day 4 wer e positively stained with filipin, a fluorescent probe for free cholesterol , as well as with 256C antibody, indicating that free cholesterol is enrich ed in antigenic lipid droplets. These observations suggest that hydrolysis and rearrangement of cellular cholesterol take place in foam cells to form complex structures of phosphatidylcholine and free cholesterol in lipid dro plets.