Leukocytes can enhance platelet-mediated aggregation and thromboxane release via interaction of P-selectin glycoprotein ligand 1 with P-selectin.
BACKGROUND
Platelet--leukocyte conjugates have been observed in patients with unstable coronary syndromes and after cardiopulmonary bypass. In vitro, the binding of platelet P-selectin to leukocyte P-selectin glycoprotein ligand-1 (PSGL1) mediates conjugate formation; however, the hemostatic implications of these cell--cell interactions are unknown. The aims of this study were to determine the ability of leukocytes to modulate platelet agonist--induced aggregation and secretion in the blood milieu, and to investigate the role of P-selectin and PSGL-1 in mediating these responses.


METHODS
Blood was drawn from healthy volunteers for in vitro analysis of platelet agonist--induced aggregation, secretion (adenosine triphosphate, beta-thromboglobulin, and thromboxane), and platelet-leukocyte conjugate formation. Experiments were performed on live cells in whole blood or plasma to simulate physiologic conditions. Whole-blood impedance and optical aggregometry, flow cytometry, and enzyme-linked immunosorbent assays were performed in the presence and absence of blocking antibodies to P-selectin and PSGL1. The platelet-specific agonists, thrombin receptor activating peptide and adenosine diphosphate, were used to elicit platelet activation responses.


RESULTS
Inhibition of platelet--leukocyte adherence by P- selectin and PSGL1 antibodies decreased agonist--induced aggregation in whole blood. The presence of leukocytes in platelet-rich plasma increased aggregation, and this increase was attenuated by P-selectin blocking antibodies. Data from flow cytometry confirmed that platelet-leukocyte conjugate formation contributed to aggregation responses. Blocking antibodies reduced platelet agonist--induced thromboxane release but had no impact on adenosine triphosphate and beta-thomboglobulin secretion.


CONCLUSIONS
Leukocytes can enhance platelet agonist--induced aggregation and thromboxane release in whole blood and platelet-rich plasma under shear conditions in vitro. Interaction of platelet P-selectin with leukocyte PSGL1 contributes substantially to these effects.
Faraday
N
N
Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. nfaraday@jhmi.edu
Scharpf
R B
RB
Dodd-o
J M
JM
Martinez
E A
EA
Rosenfeld
B A
BA
Dorman
T
T
D000328:Adult; D000906:Antibodies / Q000502:physiology; D001792:Blood Platelets / Q000378:metabolism*; D005260:Female; D006801:Humans; D007962:Leukocytes / Q000502:physiology*; D008297:Male; D008562:Membrane Glycoproteins / Q000276:immunology / Q000502:physiology*; D019007:P-Selectin / Q000276:immunology / Q000502:physiology*; D010974:Platelet Aggregation / Q000502:physiology*; D013931:Thromboxanes / Q000378:metabolism*
D000906:Antibodies; D008562:Membrane Glycoproteins; D019007:P-Selectin; C084882:P-selectin ligand protein; D013931:Thromboxanes