Antibodies to cholesterol: biological implications of antibodies to lipids

Injection of silicone gel or silicone oil intraperitoneally into BALB/c mice induced the formation of antibodies that reacted by ELISA with highly purified crystalline cholesterol and, to a much lesser extent, antibodies that reacted with a phospholipid (dimyristoyl phosphatidylglycerol). Although IgM and IgG antibodies to cholesterol were detected, the titers of IgG antibodies were low when compared with IgM. The titers of IgM antibodies to cholesterol in certain sera exhibited activities that reached baseline values at dilutions as high as 1:5000, thus making them equivalent to titers that have been previously published for ascites fluid containing murine monoclonal antibodies to cholesterol. The antibodies to cholesterol induced by silicone compounds are indistinguishable in their binding to crystalline cholesterol from naturally-occurring antibodies to cholesterol in normal human serum. They are also indistinguishable from antibodies induced by a proposed vaccine to cholesterol that is currently in late preclinical development for prevention of hypercholesterolemia in humans. The anti-cholesterol vaccine, which consists of liposomes heavily laden with cholesterol as an antigen and lipid A as an adjuvant, induces antibodies that react with low density lipoproteins (LDL) and opsonize them for removal by liver macrophages. It appears that silicone gel or silicone oil causes recruitment and adsorption of cholesterol at the injection site, and also serves as an adjuvant that may have immunostimulant properties similar to lipid A for inducing antibodies to lipids. Antibodies to lipids such as cholesterol or phospholipids are not harmful to intact cell membranes because of steric hindrance from surrounding lipids and larger macromolecules that block binding of the antibodies.