Differential effects of liposome-incorporation on liver macrophage activating potencies of rough lipopolysaccharide, lipid A, and muramyl dipeptide. Differences in susceptibility to lysosomal enzymes
Daemen, T.; Veninga, A.; Dijkstra, J.; Scherphof, G.
Journal of Immunology 142(7): 2469-2474
1989
ISSN/ISBN: 0022-1767 PMID: 2926140 Document Number: 326520
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.