LFA-1 and Mac-1 Define Characteristically Different Intralumenal Crawling and Emigration Patterns for Monocytes and Neutrophils In Situ

Abstract

To exit blood vessels, most (∼80%) of the lumenally adhered monocytes and neutrophils crawl toward locations that support transmigration. Using intravital confocal microscopy of anesthetized mouse cremaster muscle, we separately examined the crawling and emigration patterns of monocytes and neutrophils in blood-perfused unstimulated or TNF-α–activated venules. Most of the interacting cells in microvessels are neutrophils; however, in unstimulated venules, a greater percentage of the total monocyte population is adherent compared with neutrophils (58.2 ± 6.1% versus 13.6 ± 0.9%, adhered/total interacting), and they crawl for significantly longer distances (147.3 ± 13.4 versus 61.8 ± 5.4 μm). Intriguingly, after TNF-α activation, monocytes crawled for significantly shorter distances (67.4 ± 9.6 μm), resembling neutrophil crawling. Using function-blocking Abs, we show that these different crawling patterns were due to CD11a/CD18 (LFA-1)- versus CD11b/CD18 (Mac-1)-mediated crawling. Blockade of either Mac-1 or LFA-1 revealed that both LFA-1 and Mac-1 contribute to monocyte crawling; however, the LFA-1–dependent crawling in unstimulated venules becomes Mac-1 dependent upon inflammation, likely due to increased expression of Mac-1. Mac-1 alone was responsible for neutrophil crawling in both unstimulated and TNF-α–activated venules. Consistent with the role of Mac-1 in crawling, Mac-1 block (compared with LFA-1) was also significantly more efficient in blocking TNF-α–induced extravasation of both monocytes and neutrophils in cremaster tissue and the peritoneal cavity. Thus, mechanisms underlying leukocyte crawling are important in regulating the inflammatory responses by regulating the numbers of leukocytes that transmigrate.

Patrolling monocytes promote intravascular neutrophil activation and glomerular injury in the acutely inflamed glomerulus

“…In contrast, it was necessary to inhibit both β 2 and α 4 integrins to reduce monocyte retention. Under inflammatory conditions in vessels of the renal interstitium and cremaster muscle, monocytes have been shown to use both LFA-1 and Mac-1 for crawling and, in some circumstances, VLA-4 for adhesion (6,22). In the glomerulus, LFA-1 and Mac-1 showed distinct roles, in that LFA-1 was more important for initial recruitment of monocytes whereas Mac-1 contributed to their retention.…”

“…Previous studies have examined the molecular basis of intraluminal crawling of patrolling monocytes in organs such as the skin, mesentery, and muscle (5,6,22). Constitutive monocyte migration in these locations has been found to be dependent on LFA-1 and CX 3 CR1.…”

“…Evidence indicates that nonclassical monocytes perform a specialized form of immune homeostasis, serving as initiators of acute inflammatory responses and contributing to tissue remodeling (5,6,13,(17)(18)(19). A unique aspect of the function of these cells is their capacity to undergo extensive intravascular crawling or "patrolling," a constitutive phenomenon that has now been observed in a wide range of organs (5,6,(20)(21)(22). A growing body of evidence indicates that this form of immune surveillance allows Significance Nonclassical monocytes patrol the microvascular lumen in numerous organs in behavior thought to represent a form of immune surveillance.…”

Intravascular Leukocyte Chemotaxis: The Rules of Attraction

“…The crawling neutrophils were searching the endothelium for optimal sites for transmigration, since inhibition of crawling significantly delayed neutrophil transmigration (Phillipson M. et al, 2006;Sumagin R. et al, 2010).…”

“…Intravascular crawling of neutrophils is dependent on the leukocyte  2 -integrin Mac-1 and its ligand ICAM-1 on endothelial cells (Phillipson M. et al, 2006;Sumagin R. et al, 2010). Compared to LFA-1, Mac-1 binds a wider spectrum of ligands, including complement fragment iC3b, fibrinogen, fibronectin, laminin, collagen, myeloperoxidase, elastase, JAM-B and -C to name just a few (Simon S. I., Green C. E., 2005; Kelly M. et al, 2007;Luo B. H. et al, 2007) suggesting that this integrin might have other roles apart from intravascular crawling.…”