The distribution of solid liposomes of different size on the surface of epithelial cells

Interaction of large multilamellar and small sonicated liposomes with epithelial sheets in culture has been studied. Liposomes were comprised of dipalmitoyl- or distearoylphosphatidylcholine and were solid at 37 degrees. Sodium fluorescein was entrapped in liposomes and their localization on the cell surface was studied by fluorescence microscopy. The distribution of liposomes depends on their size. Large liposomes adhere uniformly all over the surface of the sheet, which proved to be nonadhesive either for other cells or for inert particles. Small liposomes adhere mostly along the margins of the sheet. When the cells are partly detached from each other by mild EDTA (ethylene diamine tetraacetic acid) treatment, the newly accessible cell margins become capable of binding small liposomes. The binding of both types of liposomes is trypsin-sensitive; however large liposomes, bound along the edges of the cellular sheet, are removed slower than those adhering to the inner parts of the sheet. Competition experiments show that large and small liposomes bind to the same sites on the surface of the sheet. Liposome-acceptor proteins are suggested to exist on the surface of epithelial cells. We suppose that these proteins are concentrated along the cell margins and are involved in cell-cell contacts.