title: In vitro activation of rat liver macrophages to tumoricidal activity by free or liposome-encapsulated muramyl dipeptide.
abstract: We investigated the in vitro activation of rat liver macrophages to a tumoricidal state with free and liposome-encapsulated immunomodulators. The cytolytic activity of liver macrophages was determined by a radioactivity release assay using murine B16 melanoma cells, labeled with [methyl-3H]thymidine. Exposure of the liver macrophages to concentrations of 50 micrograms of free, nonencapsulated, muramyl dipeptide (MDP) per ml resulted in maximal levels of tumor cell lysis of approximately 20%. Encapsulation of the MDP within liposomes (multilamellar vesicles, 0.3 to 0.5 micron in diameter, consisting of egg phosphatidylcholine, cholesterol, and dicetylphosphate, 4:5:1) not only caused a 500-fold reduction in the amount of MDP required to obtain the same levels of cytolysis but also increased the maximally obtainable level of cytolysis more than 2-fold. A synergistic effect of lipopolysaccharide and free or encapsulated MDP on cytolytic activity was observed when the macrophages were exposed to a combination of the two agents simultaneously. Besides causing tumor cell lysis, activated macrophages were also able to suppress tumor cell proliferation by 80 to 90% as determined by a [methyl-3H]thymidine incorporation assay. With a fixed amount of MDP, encapsulated in different amounts of liposomal lipid, the extent of macrophage activation was found to increase with a larger amount of encapsulating lipid. This increase in macrophage activation may be the result of a sustained intracellular release of encapsulated MDP from the liposomes. Liposome structure and composition will thus be important parameters in the in vivo application of liposomes as carriers of immunoactive substances.
mesh_terms: D000119:Acetylmuramyl-Alanyl-Isoglutamine / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D002453:Cell Cycle / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D003602:Cytotoxicity, Immunologic; D005260:Female; D007111:Immunity, Cellular / Q000187:drug effects; D008070:Lipopolysaccharides / Q000494:pharmacology; D008081:Liposomes / Q000008:administration & dosage; D008099:Liver / Q000166:cytology / Q000276:immunology*; D008262:Macrophage Activation / Q000187:drug effects; D008264:Macrophages / Q000276:immunology*; D009374:Neoplasms, Experimental / Q000276:immunology*; D051381:Rats
chemical_list: D008070:Lipopolysaccharides; D008081:Liposomes; D000119:Acetylmuramyl-Alanyl-Isoglutamine