Leukocyte rolling and adhesion via ICAM-1 signals to endothelial permeability. Focus on "Leukocyte rolling and adhesion both contribute to regulation of microvascular permeability to albumin via ligation of ICAM-1".

THE VASCULAR ENDOTHELIUM plays a vital role in the inflammatory response by induction and surface expression of adhesion molecules and chemokines. At sites of acute inflammation, blood flow is increased and postcapillary venules exhibit increased permeability (leakiness) and support the influx of blood leukocytes, primarily neutrophils. The relationship between neutrophil influx and increased vessel permeability has been a topic of investigation for decades. Many investigations have reported a direct neutrophil-dependent increase in vessel permeability during inflammation using in vivo and in vitro models (reviewed in Ref. 4). One aspect that has received increased attention is whether endothelial cell adhesion molecules contribute to the regulation of vessel permeability, in addition to their well-characterized role in neutrophil rolling and adhesion. Intercellular adhesion molecule-1 (ICAM-1, also known as CD54) is an immunoglobulin superfamily member that is abundantly expressed on the endothelial cell surface and is enriched at endothelial cell borders in vivo and in vitro (2, 12, 14, 17). ICAM-1 surface expression is upregulated in endothelial cells by proinflammatory cytokines like TNF, IL-1 , IFN, or bacterial endotoxins, and ICAM-1 serves as a receptor for leukocyte 2-integrins (LFA-1 and Mac-1) (reviewed in Ref. 15). In an experiment of nature, patients with leukocyte adhesion deficiency-1 (LAD-1) have a severe primary immune deficiency in which blood neutrophils fail to localize to sites of inflammation or injury. This defect occurs because patients’ neutrophils either lack 2-integrins or contain mutations in these molecules. Studies using neutrophils from LAD-1 patients or 2-integrin-deficient mice clearly demonstrate that these cells have normal selectin-dependent rolling on activated endothelium but fail to stably arrest and transmigrate. Additional studies have demonstrated that endothelial-expressed ICAM-1 contributes to both kinetics of leukocyte rolling and arrest in vivo as determined by intravital microscopy of the cremaster muscle microcirculation in ICAM-1 / mice (16). In this issue of American Journal of Physiology-Cell Physiology, Sumagin and colleagues (18) use elegant in vivo studies to demonstrate that microvascular permeability in the cremaster muscle model is regulated by leukocyte engagement of ICAM-1. The authors monitored permeability (Ps) in cremaster microcirculation by efflux of luminal fluorescent-tagged albumin by fluorescence confocal intravital microscopy. Prior studies by Sarelius and colleagues (19) in the same model demonstrated that ICAM-1 engagement induced a localized increase in vessel permeability in unstimulated arterioles and in TNF-stimulated venules. In agreement with this finding, 4 h of TNFstimulation of arterioles and venules in ICAM-1 knockout (KO) animals did not result in increased vessel permeability as compared with wild-type (WT) mice under identical conditions. Similar results were seen with 2-integrin KO mice. In the current study, the authors provide further evidence linking ICAM-1 expression to regulation of endothelial permeability. Comparisons between unstimulated venules and TNF-treated venules reveal that neutrophil rolling regulates venule permeability in unstimulated venules while leukocyte adhesion regulates venule permeability in TNF-stimulated venules. The 2-integrin-blocking antibodies, which inhibit neutrophil arrest on endothelium, reduced the vessel permeability, and this was further reduced by treatment with a combination of 2-integrin and P-selectin blocking antibodies that inhibits both rolling and arrest. Interestingly, the reduction in venule permeability observed by blockade of P-selectin and 2-integrin function were suggested to be due to insufficient ICAM-1 engagement due to the absence of rolling leukocytes. To examine the role that neutrophils play, circulating neutrophils were depleted by antineutrophil GR-1 antibody treatment. Notably, depletion of neutrophils caused a marked decrease in permeability of unstimulated venules and permeability was returned to “baseline” following antibody ligation of ICAM-1 in neutrophil-depleted venules. Although suggestive, these data do not conclusively rule out a contribution of neutrophilsecreted products in regulation of vessel permeability. Further evidence for the importance of ICAM-1 engagement in regulation of venule permeability was provided by injecting secondary antibodies to cross-link ICAM-1 antibody in unstimulated venules in the presence of rolling leukocytes. Antibody cross-linking of ICAM-1 is a widely used approach to mimic ICAM-1 clustering that occurs beneath stably adherent and transmigrating neutrophils in vivo and in vitro (14, 17). Secondary mAb cross-linking of ICAM-1 induced increases in permeability in unstimulated venules to levels seen in TNFstimulated venules. This observation indicates that the degree of ICAM-1 cross-linking is commensurate with the level of vessel permeability. Thus, as more leukocytes interact with the venule wall and begin to adhere, the more venule permeability increases. Cross-linking of VCAM-1, an inducible adhesion molecule expressed in unstimulated cremaster venules and arterioles that minimally supports neutrophil interactions, did not alter permeability in unstimulated venules or arterioles (Fig. 5 in Ref. 18). These data establish an important role for neutrophil interactions with ICAM-1 in regulating vessel permeability and also provide evidence that the degree of Address for reprint requests and other correspondence: F. W. Luscinskas, Dept. of Pathology, Brigham and Women’s Hospital, 77 Avenue Louis Pasteur, NRB 752P, Boston, MA 02115 (e-mail: [email protected]). Am J Physiol Cell Physiol 301: C777–C779, 2011; doi:10.1152/ajpcell.00250.2011. Editorial Focus