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Activation of tumoricidal properties in mouse macrophages by lymphokines encapsulated in liposomes
Abstract:
Cell-free culture supernatants rich in macrophage-activating factor (MAF) activity obtained from mitogen-stimulated 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 X C3H F1 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 p-nitrophenyl-2-O-alpha-L-fucopyranosyl-beta-D-galactopyranoside, 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 alpha-L-fucosidase or occupation of the MAF receptor on macrophages by fucose-binding plant lectins (Ulex europaeus 1 and Lotus tetragonolobus agglutinins). Also, populations of nontumoricidal inflammatory tissue macrophages, which were inherently unresponsive to free MAF, would 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.
Insights
Liposome-encapsulated macrophage-activating factor (MAF) effectively activates macrophages to kill tumor cells at significantly lower concentrations. This method overcomes macrophage refractoriness and suggests MAF acts intracellularly for tumoricidal activity.
Area of Science:
- Immunology
- Cell Biology
- Drug Delivery Systems
Background:
- Macrophage-activating factor (MAF) induces macrophage cytotoxicity against tumor cells.
- Liposomes are potential carriers for delivering bioactive molecules to cells.
- Understanding MAF delivery mechanisms is crucial for cancer immunotherapy.
Purpose of the Study:
- To compare the efficacy of liposome-encapsulated MAF versus free MAF in activating macrophages for tumoricidal activity.
- To investigate the protective effect of liposomes on MAF activity.
- To explore MAF's mechanism of action in rendering macrophages tumoricidal.
Main Methods:
- Encapsulation of MAF from rat lymphocytes into liposomes of varying sizes and lipid compositions.
- In vitro assessment of macrophage cytotoxicity against tumor cells using liposome-encapsulated MAF and free MAF.
- Dose-response studies to determine effective MAF concentrations.
- Evaluation of MAF activity in the presence of MAF inhibitors and on refractory macrophages.
Main Results:
- Liposome-encapsulated MAF induced significant in vitro cytotoxicity in normal mouse macrophages against tumor cells.
- Encapsulated MAF was effective at concentrations at least 20,000 times lower than free MAF.
- Liposomes protected MAF from inhibition and enabled activation of MAF-refractory macrophages.
- Inflammatory macrophages unresponsive to free MAF became tumoricidal upon treatment with liposome-encapsulated MAF.
Conclusions:
- Liposome encapsulation enhances MAF efficacy and overcomes macrophage resistance to activation.
- The findings suggest that MAF exerts its tumoricidal effect by acting on intracellular targets within macrophages.