Subendothelial proteins and platelet adhesion. von Willebrand factor and fibronectin, not thrombospondin, are involved in platelet adhesion to extracellular matrix of human vascular endothelial cells.

Endothelial cell matrix contained von Willebrand factor (VWF), fibronectin, and thrombospondin. The role of these proteins in the adhesion of platelets was investigated by preincubation of the matrix with specific antibodies and subsequent perfusion with human blood. When perfusions were performed with platelets in a human albumin solution (HAS) platelet adhesion was similar to that with normal plasma, indicating that proteins in the matrix can fully support adhesion. Preincubation of the matrix with a monoclonal antibody to VWF and perfusion with HAS showed a nearly complete inhibition of platelet adhesion at 1300 s-1, indicating a role for matrix-bound VWF at high shear rates and no requirement for VWF in plasma. Preincubation of the matrix with antihuman fibronectin F(ab')2 showed a slight inhibition of adhesion. The same result was obtained with perfusions with fibronectin-free plasma, and an untreated matrix. Preincubation with antifibronectin F(ab')2 and perfusion with fibronectin-free plasma showed a significant inhibition of platelet adhesion at all shear rates. These results indicate that fibronectin is required for adhesion at all shear rates. Preincubation of the matrix with different antibodies against human platelet thrombospondin showed no inhibition of platelet adhesion at all wall shear rates. Thrombospondin in the matrix is evidently not required for platelet adhesion.