Purpose of review: Leukocyte extravasation is a multistep process that is regulated at various levels. This review will highlight recent findings that define new regulatory mechanisms and novel activities in the process of leukocyte docking to the endothelium and diapedesis of leukocytes through the endothelial barrier of the vessel wall.
Recent findings: Within the past 2-3 years, novel regulatory mechanisms have been identified that control or balance leukocyte extravasation at different steps of the extravasation process. First evidence was established for differences in the roles of intracellular factors that bind to integrins and support their activation. A cytokine was found that counteracts the activation of leukocyte integrins. Not only leukocyte integrins but also their ligands on endothelial cells were shown to arrange in clusters while supporting leukocyte-endothelial interactions. Recent progress was made in determining in vivo the route of leukocyte diapedesis through the endothelium of the blood vessel wall. Finally, novel mechanisms were found that control the opening of the endothelial barrier during diapedesis and others that determine directionality of diapedesis.
Summary: Recent progress in our understanding of leukocyte extravasation has unraveled novel steps and mechanisms that control this process in vivo. These findings provide new insights into the mechanisms that balance the entry of leukocytes into tissue.