Effects of lipid A on the immunologic properties of liposomal model membranes.

Incorporation of lipid A influences the immunogenic properties of liposomal model membranes that carry N-(hapten)-substituted derivatives of phosphatidylethanolamine as antigenic determinants. These effects of lipid A include a marked enhancement of the hapten-specific response; abolition of the requirement of critical threshold epitope density for liposomal immunogenicity; and conversion of liposomes from a thymus-independent type 2 to a thymus-independent type 1 immunogen. Recent experiments demonstrate that an optimal in vitro response to liposomes--although originally classified as thymus-independent on the basis of conventional criteria--requires two T cell factors, interleukin-2 and interleukin-X, which are also involved in the response to a classic thymus-dependent immunogen. Incorporation of lipid A has no qualitative effect on the dependence of liposomal immunogenicity on these lymphokines.