Impact of real-time shedding on binding kinetics of membrane-remaining L-selectin to PSGL-1.

L-selectin shedding induced by various cytokines is crucial in activating neutrophils (PMNs) in inflammatory cascade. While the real-time shedding in vivo lasts about 10 min after PMN activation, the impact of time-dependent shedding on binding kinetics of membrane-remaining L-selectins to its ligands is poorly understood at transient or steady state. Here we developed an in vitro L-selectin shedding dynamics approach, together with competitive assays of cell adhesion, and proposed a theoretical model for quantifying the impact of real-time shedding on the binding kinetics of membrane-remaining L-selectins to PSGL-1. Our data indicated that the extent of L-selectin shedding on PMA activation is higher but the terminating time is longer for Jurkat cells than those for human PMNs. Meanwhile, fMLF or IL-8 stimulation yields the longer terminating time than that on PMA stimulation but results in the similar shedding extent for PMNs. L-selectin shedding reduces L-selectin-PSGL-1-mediated cell adhesion in three ways, decreasing membrane-anchored L-selectins, increasing soluble L-selectins competitively binding to ligands, and presenting conformational alteration of membrane-remaining L-selectin themselves. Compared with those on intact cells, the binding affinities of membrane-remaining L-selectin-PSGL-1 pair were all enhanced at initial and lowered at late shedding phase for both PMN and Jurkat cells even with varied transition time point. The rolling velocities of both PMNs and Jurkat cells were increased following mechanically- or biochemically-induced shedding of L-selectin under shear flow. These findings help to further understand the function of time-dependent L-selectin shedding during inflammation cascade.