Neutrophils are the all-terrain vehicle of the innate immune system because of their ability to gain entry into tissues and organs, and thus, play an essential role in host defense. Exactly how this marvel of nature works is still incompletely understood. In the last two to three years, new players and processes have been identified in the endothelial - leukocyte adhesion cascade. Novel signaling pathways have been discovered in both the endothelium and the neutrophil that regulate various steps in the recruitment process. This review focuses on these emerging pathways and the mechanisms that regulate neutrophil recruitment across endothelium.

Leukocytes rolling on selectins extrude thin membrane tethers that might stabilize rolling velocities despite marked alterations in wall shear stress. To test this hypothesis, we used differential interference contrast videomicroscopy to visualize formation and breakage of membrane tethers as neutrophils rolled on P-selectin under flow. Neutrophils rapidly increased tether number as wall shear stress rose and decreased tether number as wall shear stress declined. Membrane tethers invariably accompanied slower, more uniform rolling steps that translated into lower mean rolling velocities and variances in velocity. Unexpectedly, neutrophils, but not fixed cells or microspheres bearing selectin ligands, rolled progressively more slowly and uniformly over time. Scanning electron microscopy revealed that neutrophils developed larger, more complex tether structures as they rolled for longer periods. These data provide evidence that neutrophils stabilize selectinmediated rolling by rapidly adjusting tether number in response to changes in wall shear stress. Gradual remodeling of tether architecture may further reduce rolling velocities, facilitating integrindependent deceleration and arrest on inflamed vascular surfaces.P-selectin glycoprotein ligand 1 ͉ adhesion ͉ inflammation ͉ hydrodynamic flow

To simulate the effects of shear stress in regions of the vasculature prone to developing atherosclerosis, we subjected human umbilical vein endothelial cells to reversing shear stress to mimic the hemodynamic conditions at the wall of the carotid sinus, a site of complex, reversing blood flow and commonly observed atherosclerosis. We compared the effects of reversing shear stress (time-average: 1 dyn/cm(2), maximum: +11 dyn/cm(2), minimum: -11 dyn/cm(2), 1 Hz), arterial steady shear stress (15 dyn/cm(2)), and low steady shear stress (1 dyn/cm(2)) on gene expression, cell proliferation, and monocyte adhesiveness. Microarray analysis revealed that most differentially expressed genes were similarly regulated by all three shear stress regimens compared with static culture. Comparisons of the three shear stress regimens to each other identified 138 genes regulated by low average shear stress and 22 genes regulated by fluid reversal. Low average shear stress induced increased cell proliferation compared with high shear stress. Only reversing shear stress exposure induced monocyte adhesion. The adhesion of monocytes was partially inhibited by the incubation of endothelial cells with ICAM-1 blocking antibody. Increased heparan sulfate proteoglycan expression was observed on the surface of cells exposed to reversing shear stress. Heparinase III treatment significantly reduced monocyte adhesion. Our results suggest that low steady shear stress is the major impetus for differential gene expression and cell proliferation, whereas reversing flow regulates monocyte adhesion.