Neutrophil transendothelial migration is independent of tight junctions and occurs preferentially at tricellular corners

Abstract

Since macromolecular permeability between endothelial cells is regulated by tight junctions (zonula occludens), we wished to determine whether they also regulate neutrophil transendothelial migration. HUVEC monolayers, a commonly used model for studying leukocyte transmigration, were characterized using electric cell substrate impedance sensing and transmission electron microscopy. We show that culture medium containing endothelial cell growth supplement (50 microg/ml) was sufficient and necessary for the development of endothelial tight junctions. The frequency with which tight junctions were observed by transmission electron microscopy was further increased (twofold) by culturing HUVEC monolayers in a 1:1 mixture of endothelial medium and astrocyte-conditioned medium. These astrocyte-conditioned HUVEC monolayers showed a >1.5-fold increase in transcellular electrical resistance. The extent of neutrophil migration across IL-1-treated (10 U/ml for 4 h) HUVEC monolayers was the same whether tight junctions were present or absent, and the molecular requirements for neutrophil transmigration (CD18 and intercellular adhesion molecule-1) were unaffected by culturing in astrocyte-conditioned medium. Immunostaining for proteins associated with the intercellular junctional domain (occludin, ZO-1, cadherin, beta-catenin, gamma-catenin, and platelet-endothelial cell adhesion molecule-1) was localized to the endothelial borders, regardless of the culture conditions. Discontinuities were observed in the border staining for occludin, ZO-1, cadherin, and beta-catenin at the tricellular corner where the borders of three endothelial cells intersected. Significantly, 75% of neutrophil migration across IL-1-treated HUVEC monolayers occurred at tricellular corners. It appears that neutrophils preferentially migrate around endothelial tight junctions by crossing at tricellular corners rather than passing through the tight junctions that lie between two endothelial cells.