Activation of Tumoricidal Properties in Mouse Macrophages by Lymphokines Encapsulated in Liposomes

Cell-free culture supernatants rich in macrophage-acti-vating factor (MAF) activity obtained from mitogen-stimu-lated F344 rat lymphocytes have been encapsulated within liposomes of differing size and lipid composition and their ability to render normal mouse macrophages cytotoxic for tumor cells in vitro has been compared with that of unencapsulated (free) MAF added to the extracellular medium. Normal macrophages from C57BL/6, C3H/Hen, and C57BL/ 6 × C3H F, mice treated with liposome-encapsulated MAF exhibited significant in vitro cytotoxicity against syngeneic and allogeneic tumor cells but did not kill nontumorigenic normal cells. Dose-response measurements revealed that liposome-encapsulated MAF was able to render macrophages tumoricidal at concentrations of at least 20,000 times lower than free MAF. Liposomes containing MAF were able to activate macrophages in the presence of ρ-nitrophenyl-2-O-a-L-fucopyranosyl-0-D-galactopyrano-side, a potent inhibitor of free MAF, indicating that encapsulated MAF was protected within liposomes and that liposome-mediated activation was not caused by small amounts of MAF released into the culture medium from “leaky” liposomes. Liposome-encapsulated MAF was also able to activate macrophages which were refractory to activation by free MAF following either removal of presumably surface receptors for MAF by pronase and/or α-L-fucosidase or occupation of the MAF receptor on macrophages by fucose-binding plant lectins (Ulex europaeus I and Lotus tetragonolobus agglutinins). Also, populations of nontumoricidal inflammatory tissue macrophages, which were inherently unresponsive to free MAF, could be rendered tumoricidal in vitro by incubation with liposome-encapsulated MAF. Collectively, the data suggest that MAF can render macrophages tumoricidal by acting on intracellular sites.