An alternative procedure for the preparation of immunogenic liposomal model membranes

This investigation describes a new procedure for the preparation of immunogenic liposomes which circumvents the need to synthesize the N-(hapten)-substituted derivatives of phosphatidylethanolamine that were previously employed for this purpose. The method is based on the generation of liposomes containing the N-hydroxysuccinimide (NHS) esters of either palmitic acid, cholesteryl-hemisuccinate, or N-succinyl-phosphatidylethanolamine. Reaction of these preformed liposomes with a hapten that possesses a substitutable amino group (e.g., DNP-lysine) results in covalent attachment of the hapten to the lipid bilayers. As a consequence of this binding, the liposomes can elicit formation of hapten-specific plaque-forming cells in mice. The reliability of this procedure is indicated by the fact that these liposomes share the essential immunological properties of liposomes sensitized by incorporation of N-substituted phosphatidylethanolamine derivatives (e.g., DNP-Cap-PE). Thus, the magnitude of the response was found to be dependent on: (a) the presence of lipid A in the liposomes; (b) the phospholipid composition of the liposomes; (c) the distance separating the DNP determinant from the liposomal surface. Additional applications of liposomes, which contain the NHS esters, are indicated.