title: Differential effects of liposome-incorporation on liver macrophage activating potencies of rough lipopolysaccharide, lipid A, and muramyl dipeptide. Differences in susceptibility to lysosomal enzymes.
abstract: We investigated the in vitro activation of rat liver macrophages to a tumor-cytotoxic state with muramyl dipeptide (MDP), rough LPS (Re-LPS) and lipid A in both a free and liposome-encapsulated form. The tumor cytotoxic state of the liver macrophages was determined with a [methyl-3H]thymidine release assay using C26 colon adenocarcinoma cells as target cells. As was shown previously, the encapsulation of MDP within multi-lamellar phospholipid vesicles greatly enhanced the activating potency of the drug; by contrast, encapsulation of Re-LPS or lipid A significantly reduced the activation of macrophages as compared to the free form of these agents. At a dose of 1 ng of free Re-LPS per ml a significant induction of tumor cell lysis was observed whereas a maximal level was obtained at a concentration of approximately 10 ng/ml. By encapsulation of Re-LPS in liposomes the activating potency diminished 20- to 100-fold. The minimal concentration required to induce detectable macrophage activation with free lipid A was 10 ng/ml, while liposome-encapsulated lipid A did not induce any detectable tumor cell lysis up to a concentration of 200 ng/ml. After a 1-h pre-incubation with a lysosomal fraction from rat liver at pH 4.8, the macrophage-activating potency of Re-LPS and lipid A was diminished by up to 95% whereas MDP remained fully active under these conditions. We conclude that, due to endocytic uptake of liposome-incorporated Re-LPS and lipid A and subsequent intralysosomal degradation, these immunomodulators are inactivated with respect to their potency to activate liver macrophages to tumor cytotoxicity.
authors.affiliation: Laboratory of Physiological Chemistry, University of Groningen, The Netherlands.
authors.forename: T
authors.initials: T
authors.lastname: Daemen
authors.forename: A
authors.initials: A
authors.lastname: Veninga
authors.forename: J
authors.initials: J
authors.lastname: Dijkstra
authors.forename: G
authors.initials: G
authors.lastname: Scherphof
mesh_terms: D000119:Acetylmuramyl-Alanyl-Isoglutamine / Q000494:pharmacology; D000276:Adjuvants, Immunologic / Q000494:pharmacology*; D000818:Animals; D003602:Cytotoxicity, Immunologic / Q000187:drug effects; D005260:Female; D008050:Lipid A / Q000494:pharmacology; D008070:Lipopolysaccharides / Q000494:pharmacology; D008081:Liposomes / Q000276:immunology*; D008099:Liver / Q000276:immunology*; D008247:Lysosomes / Q000201:enzymology*; D008262:Macrophage Activation / Q000187:drug effects*; D008264:Macrophages / Q000201:enzymology / Q000276:immunology; D051379:Mice; D008807:Mice, Inbred BALB C; D051381:Rats; D011919:Rats, Inbred Strains; D014407:Tumor Cells, Cultured / Q000187:drug effects
chemical_list: D000276:Adjuvants, Immunologic; D008050:Lipid A; D008070:Lipopolysaccharides; D008081:Liposomes; D000119:Acetylmuramyl-Alanyl-Isoglutamine