Cooperation between lateral ligand mobility and accessibility for receptor recognition in selectin-induced cell rolling.

Selectin-induced leukocyte rolling along the endothelial surface is an essential step in the immune response. Several in vitro studies showed that this cell rolling is a highly regulated adhesion phenomenon, controlled by the kinetics and forces of selectin-ligand interactions. In the flow chamber study presented here, we focused on the requirements on the ligand structure in this context. A series of neoglycolipids bearing the binding epitope Sialyl Lewis X was synthesized and used as artificial ligands. These lipids differed in their spacer structures between headgroup and membrane anchor, resulting in a gradual variation in accessibility and mobility of the binding epitope when immobilized in model membranes. Consequently, analysis of cell rolling along such membranes allowed correlation of ligand structures and functionality. All model membranes containing such ligands were further characterized by film balance measurements, epifluorescence, and atomic force microscopy. Generally, the glycolipids exhibited a high tendency for lateral aggregation, but the resulting clusters were of different morphology. This was also reflected by strong differences in the rolling experiments. Our results confirm that, in addition to a sufficient headgroup accessibility, the cell rolling process is governed by two further interdependent factors: (i) the headgroup flexibility caused by the intramolecular uncoupling between the headgroup and the hydrophobic moiety due to introduction of a spacer, and (ii) the stiffness of the molecules resulting from their supramolecular arrangement in clustered assemblies. Since both factors are influenced simultaneously by the spacer modification, we present for the first time a clear correlation between structural aspects of selectin ligands and their ability to mediate cell rolling. This might help to develop a better understanding for the function of the natural selectin ligands.