Biochemical, Morphological, and Ultrastructural Studies on the Uptake of Liposomes by Murine Macrophages1

The interaction of multilamellar liposomes with mouse peri toneal macrophages cultured in vitro has been examined. The principal mechanism of liposome uptake by these cells is by phagocytic engulfment. Studies with radiolabeled liposomes demonstrated that they are incorporated into macrophages as intact structures and that treatment of macrophages with inhib itors of phagocytosis prevents liposome uptake. Incubation of macrophages with liposomes containing encapsulated fluorescein-labeled bovine serum albumin resulted in localization of fluorescence within discrete cytoplasmic vacuoles. Ultrastruc tural observations confirmed that liposomes were internalized and were enclosed within phagosomes. Electron microscopy also revealed that, by 24 hr following phagocytosis, adjacent phagosomes containing liposomes prepared from bovine brain phosphatidylserine, egg phosphatidylcholine, and lysolecithin (mol ratio, 4.95/4.95/0.1) fused within the cytoplasm. In con trast, phagosomes containing neutral liposomes consisting solely of egg phosphatidylcholine did not fuse and remained as discrete single structures. Negatively charged bovine brain phosphatidylserine/egg phosphatidylcholine/lysolecithin lipo somes were phagocytosed at a much faster rate (12 times faster) than were neutral egg phosphatidylcholine liposomes. not possess the specialized phagocytic abilities of macro phages, it is by no means certain that the pattern(s) of liposome uptake observed in nonphagocytic cells will apply to macro phages. In this study, we have examined the mechanism of uptake of MLV liposomes by murine PEC cultured in vitro. MLV liposomes were chosen for study because this type of liposome has been found to localize not only in the liver and spleen but also in the lung (5). Data presented here indicate that the predominant pathway for uptake of liposomes by macrophages is by phago cytosis and that there are major differences in the rate of phagocytic uptake of neutral and negatively charged lipo somes. The behavior of internalized liposomes also is influ enced by their lipid composition, and phagosomes containing negatively charged, but not neutral, MLV undergo extensive intracellular fusion. These differences in the uptake and intracellular behavior of neutral and charged liposomes may con tribute to differences in the kinetics of macrophage activation, as described in the accompanying paper (6), in which activation occurs significantly faster when MAP is encapsulated in nega tively charged MLV. MATERIALS AND METHODS