P-selectin- and CD18-mediated recruitment of canine neutrophils under conditions of shear stress

Neutrophil mobilization at sites of inflammation or thrombosis involves the participation of several adhesion molecules expressed on neutrophils and vascular endothelial cells. Local vascular damage with disruption of the endothelium results in adhesion of platelets to the exposed subendothelium, and these platelets could also participate in neutrophil recruitment. This initial phase of mobilization could be followed by heterotypic aggregation to recruit more leukocytes in the area. The present study first examined the interactions of adherent canine platelets and flowing canine neutrophils using an in vitro system that simulates vascular flow conditions. Results showed that collagen-adherent platelets express the adhesion molecule P-selectin on their surface and can support neutrophil arrest (612 ± 43 neutrophils/mm2) at shear stresses of approximately 2.5 dynes/cm2. Both transient adhesion (manifested by a rolling-type behavior) and complete arrest were observed. These interactions could be totally inhibited by a monoclonal antibody directed against platelet P-selectin (24 ± 18 neutrophils/mm2) but not by a monoclonal antibody against neutrophil CD18 (625 ± 46 neutrophils/mm2). Additionally, under shear mixing conditions (700 RPM), canine blood leukocytes exhibited aggregation (>80% singlets recruited into aggregates after 5 minutes), and this process does not involve P-selectin but is dependent on the neutrophil integrin CD 18. However, stimulation of the blood with platelet-activating factor (5-20 ng/ml) induced a rapid aggregation with a significantly greater number of aggregates when compared with stirring alone (68.3% ± 3.2% versus 35.2% ± 6.3% at 1 minute, P < 0.05), and this aggregation was both P-selectin and CD 18 dependent. Overall, these two mechanisms of leukocyte recruitment (neutrophil arrest on adherent platelets and aggregation) could act sequentially and in a cooperative manner to bring into close contact platelets and neutrophils at sites of inflammation and thrombosis in pathologic conditions in the dog.