Formation and Deposition of Leukocyte Microparticles during Rolling

During inflammation and thrombosis, interactions of selectins with cell-surface adhesion molecules allow the tethering and rolling of leukocytes on blood vessel walls. Leukocytes express L-selectin, whereas activated endothelial cells express P- and E-selectin. Leukocyte rolling allows regional sampling of chemokines and other mediators of leukocyte targeting, which leads to integrin-dependent arrest and emigration of leukocytes into the underlying tissues. Rolling requires the rapid formation and rapid dissociation of selectin-ligand bonds that are subjected to tensile forces applied by wall shear stress. We have previously demonstrated the rapid formation of membrane tethers as neutrophils roll on P-selectin. Normally these membrane tethers retract back to the cell body, however we have also observed that membrane tethers can break and deposit membrane microparticle (< 1 &mum) or microfragment (> 1 &mum) on the underlying surface. Breakage of membrane tethers was observed when neutrophils rolled over a wide range of P-selectin site densities (170-1096 sites/&mum2) and shear rates (100-800s-1). We demonstrate that the frequency of membrane tether formation (40-100%) and microparticle deposition (0-40%) both increase as the wall shear stress increases, while there was a small effect of ligand density. We also demonstrate that deposited membrane microfragments may serve as adhesion points for other cells.