Question:

We previously demonstrated that CD40 ligand-CD40 interaction induces a strong von Willebrand factor (vWF) multimer release from functionally intact endothelial cells (ECs) which might be important in early atherosclerosis. We now hypothesise that this release of ultra-large vWF (ULVWF) multimers mediates platelet adhesion and reinforces the recruitment and transmigration of circulating monocytes at sites of accelerated blood flow.

Methods:

Microfluidic devices were used to apply laminar shear stress to an intact EC monolayer. Human umbilical vein ECs were seeded into ibidi µ-slides, perfused at 2.5 or 10 dyn/cm² and stimulated with histamine or soluble CD40L (sCD40L). Freshly isolated fluorescent labelled monocytes and/or platelets were added and superfused over the ECs. Cell adhesion was monitored using fluorescence microscopy or reflection interference contrast microscopy (RICM).

Results:

Upon exposure to sCD40L, deposition of ULVWF multimers comparable to histamine stimulation was monitored on the EC surface with platelets adhering to these multimers in a string-like fashion. The number of vWF-platelet strings formed over time as well as the number of attached platelets was significantly increased at 10 as compared to 2.5 dyn/cm². Moreover, rolling of monocytes on platelet-decorated vWF multimers followed by firm adhesion was observed.

Conclusions:

The stronger platelet adhesion to ULVWF multimers at arterial shear stress levels may enable monocyte recruitment and diapedesis in arteries and arterioles which normally is impossible due to the high shear rate. This may be relevant for early artherosclerotic lesions by augmenting monocyte transmigration into the subendothelial space and differentiation into pro-inflammatory macrophages.