Flow-enhanced cell adhesion is an unexplained phenomenon that might result from a transport-dependent increase in on-rates or a force-dependent decrease in off-rates of adhesive bonds. L-selectin requires a threshold shear to support leukocyte rolling on P-selectin glycoprotein ligand-1 (PSGL-1) and other vascular ligands. Low forces decrease L-selectin–PSGL-1 off-rates (catch bonds), whereas higher forces increase off-rates (slip bonds). We determined that a force-dependent decrease in off-rates dictated flow-enhanced rolling of L-selectin–bearing microspheres or neutrophils on PSGL-1. Catch bonds enabled increasing force to convert short-lived tethers into longer-lived tethers, which decreased rolling velocities and increased the regularity of rolling steps as shear rose from the threshold to an optimal value. As shear increased above the optimum, transitions to slip bonds shortened tether lifetimes, which increased rolling velocities and decreased rolling regularity. Thus, force-dependent alterations of bond lifetimes govern L-selectin–dependent cell adhesion below and above the shear optimum. These findings establish the first biological function for catch bonds as a mechanism for flow-enhanced cell adhesion.

A record anisotropy barrier (319 cm(-1) ) for all d-f complexes was observed for a unique Fe(II) -Dy(III) -Fe(II) single-molecule magnet (SMM), which possesses two asymmetric and distorted Fe(II) ions and one quasi-D5h Dy(III) ion. The frozen magnetization of the Dy(III) ion leads to the decreased Fe(II) relaxation rates evident in the Mössbauer spectrum. Ab initio calculations suggest that tunneling is interrupted effectively thanks to the exchange doublets.

The immunological synapse is a stable intercellular structure that specializes in substance and signal transfer from one immune cell to another. Its formation is regulated in part by the diffusion of adhesion and signaling molecules into, and their binding of countermolecules in the contact area. The stability of immunological synapses allows receptor-ligand interactions to approximate chemical equilibrium despite other dynamic aspects. We have developed a mathematical model that describes the coupled reaction-diffusion process in an established immunological synapse. In this study, we extend a previously described contact area fluorescence recovery after photobleaching (FRAP) experiment to test the validity of the model. The receptor binding activity and lateral mobility of fluorescently labeled, lipid-anchored ligands in the bilayer resulted in their accumulation, as revealed by a much higher fluorescence intensity inside the contact area than outside. After complete photobleaching of the synapse, fluorescence recovery requires ligands to dissociate and rebind, and to diffuse in and out of the contact area. Such a FRAP time course consequently provides information on reaction and diffusion, which can be extracted by fitting the model solution to the data. Surprisingly, reverse rates in the two-dimensional contact area were at least 100-fold slower than in three-dimensional solution. As previously reported in immunological synapses, a significant nonrecoverable fraction of fluorescence was observed with one of two systems studied, suggesting some ligands either dissociated or diffused much more slowly compared with other ligands in the same synapse. The combined theory and experiment thus provides a new method for in situ measurements of kinetic rates, diffusion coefficients, and nonrecoverable fractions of interacting molecules in immunological synapses and other stable cell-bilayer junctions.