Production of Antibodies Against Phosphocholine, Phosphatidylcholine, Sphingomyelin, and Lipid A by Injection of Liposomes Containing Lipid A

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Abstract
Liposomes were investigated as a combined carrier and adjuvant to promote the immunogenicity of lipid A derived from endotoxin. Lipid A-containing liposomes, or lipid A alone, were injected once into rabbits either subcutaneously or i.v. Antiserum activity was measured by complement-dependent damage to appropriate indicator liposomes containing trapped glucose. Lipid A in liposomes produced a greater immune response against lipid A than that obtained by injecting identical amounts of lipid A alone, and the response to the liposome-lipid A combination was equivalent to that obtained by a frequently used alternative method, in which the antigen consisted of acid-treated bacterial cells coated with lipid A. Antiserum, or purified IgG antibodies, from rabbits immunized with liposomes containing lipid A also reacted with liposomes lacking lipid A. The antibodies were inhibited by phosphocholine, and the activity was adsorbed by phosphatidylcholine, or ceramide phosphocholine (sphingomyelin). The anti-phosphocholine antibodies did not cross-react with lipid A, but their binding to liposomes was inhibited by the presence of anti-lipid A antibodies on the surface of the liposomes. An “anti-liposome” response was not observed when either lipid A alone, acid-treated bacterial cells coated with lipid A, or liposomes lacking lipid A were injected. We conclude that one injection of liposomes containing lipid A produces an immune response against lipid A that is equivalent to that produced by other methods of immunization. Injection of liposomes containing lipid A also produces an immune response against phosphocholine, phosphatidylcholine, and sphingomyelin.