Rat alveolar macrophages are susceptible to activation by free and liposome-encapsulated lymphokines.

Alveolar macrophages (AM) obtained from F344 rats were rendered tumoricidal by incubation in vitro with cellfree culture supernatant fluids rich in macrophage-activating factor (MAF) activity harvested from mitogen-stimulated F344 rat lymphocytes. AM activated by this procedure destroyed syngeneic, allogeneic, and xenogeneic tumor cells but were not cytotoxic for nonneoplastic cells. MAF was encapsulated in multilamellar lipid vesicles (liposomes) and its ability to render AM tumoricidal was compared with that of free (unencapsulated) MAF. Liposome-encapsulated MAF rendered AM cytotoxic at concentrations up to 16,000 times lower than free MAF. These data demonstrate that AM can respond in vitro to lymphokines and that MAF encapsulated within liposomes is far more efficient in rendering AM tumoridical than free MAF.