Monocytes, like all leukocytes, undergo a series of sequential steps during extravasation from blood into tissues: tethering, rolling, adhesion and diapedesis. We have discovered an essential step, which we call locomotion, in which the monocyte moves from a site of firm adhesion to the nearest junction to begin diapedesis. Blocking CD11a-CD18 and CD11b-CD18 on human monocytes or adhesion molecules ICAM-1 and ICAM-2 on endothelial cells prevented the monocytes from reaching junctions. The blocked monocytes spun in circles as if they were unable to direct their movement despite being able to adhere and polarize normally. This step fills a gap in the paradigm of extravasation as a multistep process.
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