Accumulation of ceroid-like pigments in macrophages cultured with phosphatidylcholine liposomes in vitro.

When mouse peritoneal macrophages as well as P388D1 cells, an established macrophage-like cell line, were cultured with liposomes composed of rat liver phosphatidylcholine and phosphatidylserine, storage of fluorescent products, ceroid-like pigments, within those cells was observed with light and fluorescence microscopy, and fluorescence spectrophotometry. The amounts of thiobarbituric acid-reactive substances and fluorescent products in macrophages were increased gradually to reach a maximal level to between 6 and 8 days of culture. The involvement of peroxidation of liposomal lipids in the formation of the pigments was further suggested by the 6 days that incorporation of alpha-tocopherol into liposomes decreased the storage of the pigments. No appreciable formation of the pigments was observed in macrophages cultured with liposomes containing dipalmitoylphosphatidylcholine instead of rat liver phosphatidylcholine. The fluorescent products formed in cultured cells were found in lipid-soluble and -insoluble fractions. Lipid-insoluble fluorescent products had an excitation maximum at 360 nm and a fluorescence maximum at 430 nm in SDS-aqueous solution (pH 7.4) and the intensity of the fluorescence was quenched at base pH, but it was not changed in acidic media. These findings indicate that the macrophages can store Schiff base fluorescent substances formed by the reaction between peroxidation products of exogenous lipids and amino compounds in the cells, under some pathological conditions.