Enhancement of Liposomal Model Membrane Immunogenicity by Incorporation of Lipid A

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Abstract
Incorporation of lipid A into sphingomyelin-cholesterol-dicetylphosphate liposomes stimulates the humoral response of AKR mice (and other inbred strains) to model membranes that are also sensitized with the synthetic amphipathic antigen, dinitrophenyl-ε-aminocaproylphosphatidylethanolamine (DNP-Cap-PE). This effect is observed with liposomes containing varying amounts of DNP-Cap-PE and the magnitude depends on the quantity of lipid A that is incorporated. No significant increase in the frequency of direct anti-DNP plaque-forming cells (PFC) is apparent in the spleens of mice immunized with a mixture of DNP-Cap-PE sensitized liposomes (lacking lipid A) and liposomes prepared with lipid A (lacking DNP-Cap-PE). Thus, enhancement of liposomal immunogenicity by lipid A is contingent on its presence in the same bilayers that contain DNP-Cap-PE. These results further indicate that covalent attachment of lipid A to an antigenic determinant is not required to obtain an increased hapten-specific response. Three lines of evidence support the conclusion that stimulation can be attributed to the known mitogenic effect of lipid A on bone marrow-derived (B) lymphocytes. 1) Incorporation of alkali-treated lipid A, which is nonmitogenic, does not enhance the immunogenicity of DNP-Cap-PE sensitized liposomes in AKR mice. 2) Conversely, lipid A incorporation had no effect on anti-DNP PFC production in C3H/HeJ mice, a strain possessing B cells that are not responsive to this mitogen. 3) In contrast, lipid A did increase the response to DNP-Cap-PE sensitized liposomes in either nude or thymectomized mice that are deficient in thymus-derived (T) lymphocytes. The available data suggest that modulation of liposomal immunogenicity by lipid A may be an effective method for enhancing antibody formation against lipid antigens.