Effects of Lipid A on the Immunologic Properties of Liposomal Model Membranes

Abstract

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 (1) a marked enhancement of the hapten-specific response; (2) abolition of the requirement of critical threshold epitope density for liposomal immunogenicity; and (3) 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.