Shear stress applied on T cells approaching endothelial junctions trigger membrane protrusions within junctions and into neighboring endothelial cells. T cells locomotive on HUVECs in the presence of apical CXCL12 (10 ng/mL) and basal CCL5 (100 ng/mL) were subjected to continuous application of shear or to shear-free conditions (for 2 minutes), as described in “Materials and methods” (see also Figure 2). After 2 minutes, HUVEC monolayers and adherent T cells were immediately fixed and processed for transmission electron microscopy. (A, i-ii) Cross-sections of 2 representative T cells reaching endothelial junctions under continuous shear flow. (B) Representative T cell under shear-free conditions, showing contact over the junction without pseudopodial extensions. Arrows denote the endothelial junctions in each panel. Scale is indicated by the 1-μM bar in each panel. The magnification of panels A, B, and C is 11 000×, 16 000×, and 11 000×, respectively.