Intraluminal crawling of neutrophils to emigration sites: a molecularly distinct process from adhesion in the recruitment cascade

The prevailing view is that the beta(2)-integrins Mac-1 (alpha(M)beta(2), CD11b/CD18) and LFA-1 (alpha(L)beta(2), CD11a/CD18) serve similar biological functions, namely adhesion, in the leukocyte recruitment cascade. Using real-time and time-lapse intravital video-microscopy and confocal microscopy within inflamed microvessels, we systematically evaluated the function of Mac-1 and LFA-1 in the recruitment paradigm. The chemokine macrophage inflammatory protein-2 induced equivalent amounts of adhesion in wild-type and Mac-1-/-mice but very little adhesion in LFA-1(-/-) mice. Time-lapse video-microscopy within the postcapillary venules revealed that immediately upon adhesion, there is significant intraluminal crawling of all neutrophils to distant emigration sites in wild-type mice. In dramatic contrast, very few Mac-1(-/-) neutrophils crawled with a 10-fold decrease in displacement and a 95% reduction in velocity. Therefore, Mac-1(-/-) neutrophils initiated transmigration closer to the initial site of adhesion, which in turn led to delayed transmigration due to movement through nonoptimal emigration sites. Interestingly, the few LFA-1(-/-) cells that did adhere crawled similarly to wild-type neutrophils. Intercellular adhesion molecule-1 but not intercellular adhesion molecule-2 mediated the Mac-1- dependent crawling. These in vivo results clearly delineate two fundamentally different molecular mechanisms for LFA-1 and Mac-1 in vivo, i.e., LFA-1-dependent adhesion followed by Mac-1-dependent crawling, and both steps ultimately contribute to efficient emigration out of the vasculature.