Preparation and characterization of dipalmitoylphosphatidylcholine liposomes containing interleukin-2

Human recombinant interleukin-2 (IL-2) has been associated or mixed with small unilamellar vesicles prepared from dipalmitoylphosphatidylcholine (DPPC). Whatever the mode of IL-2 introduction, a considerable proportion of the added protein was associated with the liposomes, as determined by gel filtration and ultrafiltration/centrifugation, suggesting that IL-2 can interact with the lipid bilayer as well as being entrapped within the aqueous phase. Moreover, IL-2 prevented the aggregation/fusion of the vesicles at 4 degrees C. Liposome-associated protein was partially protected from digestion by pepsin, especially at the C-terminal, since no fluorescence emission from the tryptophan in this region was detected in the resulting peptides after separation by HPLC. Such systems could constitute a sustained release form of IL-2 for immunotherapy.