Background
Venous thrombosis is a thrombotic disorder that is a major cause of worldwide mortality and morbidity. In venous thrombosis, thrombi generally form within the venous valve pockets in the absence of overt vessel damage. How thrombus formation is initiated in venous thrombosis remains unclear.

Aims
To investigate the influence of alterations to venous flow patterns upon endothelial cell phenotype and subsequent interactions between endothelial-derived von Willebrand factor (VWF) and platelets, as well as the platelet-dependent capture and activation of neutrophils.

Methods
Endothelial cells were analysed using a variety of different fluidic devices to explore the influence of different flow patterns upon cell function. Real-time analysis of platelet binding to VWF under different flow patterns was explored in these devices, and leukocyte binding to primed platelets quantified.

Results
At venous shear stress, platelets bound to endothelial-released VWF via GPIbα. Under unidirectional shear stress, this led to formation of characteristic VWF-platelet strings. However, when multidirectional/aberrant shear stress was applied (either in linear channels or valve pocket regions of venous valve fluidic channels) tangling of VWF-platelet cables occurred. These VWF-platelet tangles resisted proteolysis by ADAMTS13. Subsequently, neutrophils interacted with these VWF-bound platelets, leading to intracellular Ca2+ release within neutrophils, a precursor for neutrophil activation, and neutrophil extracellular trap (NET) formation. Blockade of both interactions between αIIbβ3-SLC44A2 and P selectin-PSGL-1 reduced neutrophil interactions on platelet-VWF strings. Furthermore, blockade of SLC44A2, but not P-selectin, reduced NET formation.

Conclusions
Under venous shear conditions, platelets bind VWF strings and facilitate neutrophil capture. This involves αIIbβ3-SLC44A2 and P selectin-PSGL-1 and provides a model for how venous thrombosis may be initiated in the absence of vessel damage. The importance of neutrophil capture (and NET release) on platelets and/or endothelial cells suggests novel potential therapeutic targets for the protection against venous thrombosis.