OxLDL and substrate stiffness promote neutrophil transmigration by enhanced endothelial cell contractility and ICAM-1

Elevated levels of oxLDL in the bloodstream and increased vasculature stiffness are both associated with cardiovascular disease in patients. However, it is not known how oxLDL and subendothelial matrix stiffness together regulate an immune response. Here, we used an in vitro model of the vascular endothelium to explore the combined effects of oxLDL and subendothelial matrix stiffening on neutrophil transmigration. We prepared fibronectin-coated polyacrylamide gels of varying stiffness and plated human umbilical vein endothelial cells (ECs) onto the gels. We observed that oxLDL treatment of the endothelium promoted neutrophil transmigration (from <1% to 26% on soft 0.87 kPa substrates), with stiffer substrates further promoting transmigration (54% on 5 kPa and 41% on 280 kPa). OxLDL exposure enhanced intercellular adhesion molecule-1 (ICAM-1) expression on the endothelium, which was likely responsible for the oxLDL-induced transmigration. Importantly, inhibition of MLCK-mediated EC contraction reduced transmigration to ∼9% on all substrates and eliminated the effects of subendothelial matrix stiffness. In addition, large holes, thousands of square microns in size, formed in monolayers on stiff substrates following transmigration, indicating that oxLDL treatment and subsequent neutrophil transmigration caused serious damage to the endothelium. Our results reveal that an interplay between ICAM-1 and MLCK-dependent contractile forces mediates neutrophil transmigration through oxLDL-treated endothelium. Thus, microvasculature stiffness, which likely varies depending on tissue location and health, is an important regulator of the transmigration step of the immune response in the presence of oxLDL.