Differential Uptake of Conventional and Polyethylene Glycol Modified-Alkylphosphocholine-Liposomes by J 774A.1 Murine Macrophages

Multilamellar liposomes from different types of alkylphosphocholines (APC), which had been shown to activate macrophages, were prepared in the presence or absence of polyethylene glycol and the uptake of these multilamellar vesicles (MLV) by J 774A. 1 macrophages was examined. Using l-hydroxypyrene-3,6,8-trisulfonic acid (HPTS), a pH sensitive, highly fluorescent dye, it is possible to microscopically differentiate between liposomes adhered to the outer cell membrane and liposomes endocytosed by macrophages (Finkelstein and Weissmann 1978; Straubinger et al. 1983). In order to quantify the intracellular liposome-associated HPTS, fluorescence of cell lysates was measured at +/-390. We found that phagocytosis of APC-MLV by macrophages is phospholipid type (DCP, TPC, HPC, EPC) and size dependent. Total liposome uptake increased at 37 degrees C for all types of liposomes, plateauing at 4-6 hours, whereas very little liposome attachment could be measured at 4 degrees C. MLV liposomes are incorporated into J 774A.1 macrophages more rapidly and effectively than SUV. DPPE- or DSPE-PEG was used to produce sterical stabilization of the liposomes. The addition of these lipids led to an almost complete inhibition of liposomal uptake by the macrophages over a time period of 24 hours, independent of the PEG-lipid type and -acyl chain length used for liposome preparation. The hypothesis that PEG-like lipids can be powerful tools to prevent APC-MLV from being taken up by the RES and rapidly eliminated from circulation is supported by the present data.