The prevailing view is that the β 2 -integrins Mac-1 (α M β 2 , CD11b/CD18) and LFA-1 (α L β 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.

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are both dependent on the interaction of the leukocyte integrin, Mac-1, and a heretofore unknown platelet counterreceptor. Here, we identify the platelet counterreceptor as glycoprotein (GP) Ibα, a component of the GP Ib-IX-V complex, the platelet von Willebrand factor (vWf) receptor. THP-1 monocytic cells and transfected cells that express Mac-1 adhered to GP Ibα–coated wells. Inhibition studies with monoclonal antibodies or receptor ligands showed that the interaction involves the Mac-1 I domain (homologous to the vWf A1 domain), and the GP Ibα leucine-rich repeat and COOH-terminal flanking regions. The specificity of the interaction was confirmed by the finding that neutrophils from wild-type mice, but not from Mac-1–deficient mice, bound to purified GP Ibα and to adherent platelets, the latter adhesion being inhibited by pretreatment of the platelets with mocarhagin, a protease that specifically cleaves GP Ibα. Finally, immobilized GP Ibα supported the rolling and firm adhesion of THP-1 cells under conditions of flow. These observations provide a molecular target for disrupting leukocyte–platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.