Binding of dipalmitoyl-phosphatidylcholine liposomes to lung epithelial type II cells and other surfaces

To better understand the interaction of phosphatidylcholine liposomes with lung type II cells, their association with rat type II pneumocytes in primary culture was investigated. With a filtration assay technique free and cellularly bound liposomes were separated and an apparent binding site with a Bmax of 171 nmol/mg protein and a Kd of 470 nmol/l was detected. Bound liposomes were displacable by an excess of unlabelled liposomes, were sensitive to EDTA and trypsin treatment and were not significantly competed with by dioleyl-phosphatidylcholine liposomes. However, a site with identical characteristics was demonstrated in experiments performed with blanc dishes in the absence of cells. This site was generated by liposomes bound to the blanc culture dish, which could not be removed by extensive washing, but which were released by EDTA and trypsin and were trapped during filtration. These problems were overcome by a centrifugation assay technique. Then non-specific binding was less than 1% and cellular binding was inhibited dose-dependently by liposomes made from various phospholipids. These studies may facilitate attempts to direct liposomes as carriers to specific cellular targets in the lungs.