Neutrophil Adhesion to Fibrinogen and Fibrin Under Flow Conditions Is Diminished by Activation and L-Selectin Shedding

Abstract

The adhesion of neutrophils (polymorphonuclear leukocytes [PMNs]) to immobilized fibrinogen/fibrin is mediated by b2-integrins. However, the influence of physiologic flow con- ditions on neutrophil adhesion to these surfaces is poorly defined. In this report, the effect of flow and neutrophil acti- vation on adhesion to immobilized fibrinogen and fibrin was examined. For the evaluation of (the distribution of) neutro- phil adhesion, real-time video-assisted microscopy and custom- made software were used. Under flow conditions, ad- herent neutrophils appeared to support the subsequent margination of other neutrophils, thereby enhancing the ad- herence of these cells to fibrin. Consequently, neutrophils adhered in clusters, especially at higher shear stresses (eg, cluster index 1.4 at shear 80 mPa). Preactivation of PMNs with fMLP (10Ï7 mol/L) or 4b-phorbol, 12-myristate, 13-ace- tate (PMA; 100 ng/mL) resulted in approximately 50% inhibi- tion of adhesion to fibrin and a more random distribution (cluster index Ú0.5). L-selectin antibodies or neuraminidase treatment of PMNs also inhibited adhesion and clustering, indicating a role for L-selectin. Under static conditions, no clustering appeared and PMN activation with fMLP or PMA caused threefold and sevenfold increased adhesion, respec- tively. Under these conditions, anti-L-selectin antibodies or neuraminidase did not affect adhesion. These results indi- cate that, under flow conditions, adherent neutrophils support adhesion of flowing neutrophils by L-selectin-mediated cell-cell interactions. Preactivated neutrophils, with lowered L-selectin expression, are less susceptible for this interac- tion. By this mechanism, adhered leukocytes can modulate the recruitment of leukocytes to the vessel wall at sites of inflammation.