Targeting of liposomes to cells expressing CD4 using glycosylphosphatidylinositol-anchored gp120. Influence of liposome composition on intracellular trafficking.

To test the concept that glycosylphosphatidylinositol (GPI)-anchored proteins might be useful as targeting molecules for liposomes, we engineered a GPI-anchored form of gp120 from human immunodeficiency virus type 1 (termed gp120DAF) using the GPI signal of decay-accelerating factor (DAF). We show that (i) purified gp120DAF spontaneously inserts into liposome membranes via the GPI anchor; (ii) liposomes bearing gp120DAF bind specifically to cells expressing CD4, the cellular receptor for gp120; and (iii) the receptor-bound liposomes are internalized and recycle in Chinese hamster ovary cells. To test whether the lipid composition of the liposome affects any of these processes, we compared small unilamellar liposomes containing only phosphatidylcholine and cholesterol in a 7:1 molar ratio with artificial viral envelopes that mimic the lipid composition of human immunodeficiency virus type 1. We show that when tagged with gp120DAF, both liposome preparations bind specifically to cells expressing CD4, and both are endocytosed. However, artificial viral envelope liposomes are transported to late endosomes or lysosomes in the cell interior, whereas phosphatidylcholine:cholesterol liposomes are confined to a population of vesicles that remain close to the plasma membrane. Since the binding and internalization of both liposome preparations are mediated by the same receptor, we conclude that the lipid composition of the liposome profoundly influences the subsequent intracellular trafficking of the liposome-receptor complex.