Immunoliposomes that target endothelium in vitro are dependent on lipid raft formation

Lipid rafts are plasma membrane microdomains rich in cholesterol, sphingolipids, and cell surface receptors. Recent studies demonstrated the upregulation and localization of two receptors, intercellular cell adhesion molecule-1 (ICAM, CD54) and endothelial leukocyte adhesion molecule-1 (E-selectin, CD64E), within lipid raft microdomains of inflamed or injured endothelial cells (ECs). We hypothesized that the localization of ICAM and E-selectin within lipid rafts may be essential for drug delivery vehicles labeled with antibodies against ICAM (aICAM) and E-selectin (aE-selectin). To eliminate localization of cell surface receptors, ECs were treated with a cholesterol depleting drug, methyl-β-cyclodextrin. We also tested if antibody mobility and the ratio of aICAM to aE-selectin on immunoliposomes influenced binding to lipid-raft-depleted cells. Liposomes were prepared from either 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC, C(18:1), T(m) = -20 °C) or 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC, C(16:0), T(m) = 42 °C) which are in the liquid crystalline and gel phase at 37 °C, respectively. Mobility and the aICAM:aE-selectin ratio influenced cellular binding only when lipid rafts form. In the absence of lipid rafts, cellular binding of both DOPC and DPPC immunoliposomes was reduced to the nonspecific binding level. These results, which were obtained under static conditions, suggest that the presence of lipid rafts in ECs is critical for targeted drug delivery.