NEUTROPHIL-NEUTROPHIL INTERACTIONS UNDER HYDRODYNAMIC SHEAR-STRESS INVOLVE L-SELECTIN AND PSGL-1 - A MECHANISM THAT AMPLIFIES INITIAL LEUKOCYTE ACCUMULATION ON P-SELECTIN IN-VITRO

Leukocytes attach to and roll on inflamed endothelium and on leukocyte monolayers that form on the endothelial cells. Leukocyte-leukocyte in teractions occurring under hydrodynamic shear stress are mediated by b inding of L-selectin to unknown sialomucin-like glycoproteins. We show that purified neutrophil PSGL-1, a sialomucin glycoprotein that serve s as a ligand for both P- and E-selectin, can also support the attachm ent and rolling of free flowing neutrophils in vitro. Neutrophil rolli ng on PSGL-1 was abolished by the anti-L-selectin mAb DREG200 and by t he anti-PSGL-1 mAb PL1, indicating that L-selectin can interact direct ly with PSGL-1. Neutrophil rolling on neutrophil monolayers was also b locked by PL1 (60+/-9% SEM inhibition); however, DREG200 blocked more efficiently (93+/-7% SEM inhibition), suggesting that other L-selectin ligands may exist on the neutrophil surface. These studies demonstrat e that PSGL-1 on the neutrophil surface is a major functional ligand f or L-selectin. The avidity of this L-selectin-dependent adhesion event was sufficient to allow individual neutrophils rolling on P-selectin to capture free flowing neutrophils, which progressed to form linear s trings and discrete foci of rolling neutrophils. Neutrophil accumulati on on P-selectin accelerated with time as a result of neutrophil-assis ted capture of free flowing neutrophils. When neutrophil-neutrophil in teractions were blocked by DREG200, neutrophils accumulated on P-selec tin in a random pattern and at a uniform rate. Thus, leukocyte-assiste d capture of flowing leukocytes may play an important role in amplifyi ng the rate of initial leukocyte recruitment at sites of inflammation.