| The inflammatory response is the primary defense used by mammals against invading microorganisms and foreign pathogens, as well as a vital mechanism for the clearance of damaged cells. The success of this defense, in which leukocytes are recruited from the bloodstream to the extravascular space where they act aggressively against these targets, is largely dependent on adhesive interactions and signalling between leukocytes and the layer of endothelial cells (ECs) that line the inside of blood vessels. Using intravital microscopy to observe the behavior of leukocytes following firm adhesion in blood-perfused venules of cremaster muscle in anesthetized mice, this work makes several important contributions to the current knowledge of the inflammatory response. A crucial finding is that leukocytes exhibit a likelihood to adhere to the endothelium near the junctions between ECs that is largely based on EC size and shape, which are vessel-diameter dependent. Furthermore, following arrest from rolling, and firm adhesion, the majority of adherent leukocytes exhibit significant intralumenal crawling before either undergoing transendothelial diapedesis or detachment back to the free-stream. Intralumenal crawling of leukocytes is mediated, in part, by the cell adhesion molecule PECAM-1, as ligation of endothelial PECAM-1 affected both the percentage of the population of firmly adherent leukocytes participating in crawling, and decreased the velocity with which they crawled. The study shows that the venular vasculature exhibits regional heterogeneity in its capacity to support leukocyte transendothelial diapedesis. Further, the study demonstrates significant regional heterogeneity in leukocyte adhesion density that correlates with both ICAM-1 and PECAM-1 expression, suggesting that a level of control of the inflammatory response is through variable expression of endothelial cell adhesion molecules. Endothelial PECAM-1 is not constrained to the junctional region, but is also significantly expressed on the non junctional surface of ECs, thus has the capacity to mediate leukocyte crawling along the venular wall. Intralumenal crawling is hypothesized to be a mechanism facilitating leukocyte movement to regions supporting diapedesis. Overall, this work advances the understanding of key interactions between leukocytes and endothelial cells following firm adhesion that are critical for the successful completion of the inflammatory response, a primary mechanism of innate immune defense. |
