Rotation favors the formation of multiple L-selectin bonds.

Leukocytes have a better chance of sticking to neighboring cells with the right ligands if they rotate on their way by, based on results from Dwir et al. (page 649).Flowing leukocytes are captured via L-selectin interactions with carbohydrate ligands on vessel walls and subsets of other leukocytes. L-selectin is an unusual adhesive molecule—shear stress improves its adherance to ligands, whereas most adhesive bonds are destabilized by shear forces. Previous experiments have shown that L-selectin tethers are not formed below a critical shear threshold. But using new high temporal resolution videomicroscopy, Dwir et al. show that very short-lived bonds, too transient to detect previously, are indeed formed below this threshold.

Above the shear threshold, leukocyte tethers were stabilized more than 10-fold. Stabilization was not induced when higher viscosity was used to raise the shear force without raising the flow rate. Shear may stabilize adhesive tethers because initially bound cells can rotate past the substrate fast enough to form additional L-selectin bonds before the first bond is broken. So, although individual bonds may be destabilized by higher shear force, the benefits obtained by multiple bonds are much larger. ▪