Neutrophil CD44 rafts and rolls.

Functional E-selectin ligands identified on murine neutrophils include E-selectin ligand-1 (ESL-1), PSGL-1, and CD44. So far, PSGL-1 is the only ligand reported to support neutrophil slow rolling over inflamed venules.1,2 In this issue of Blood, Yago et al3 identify CD44 and new components to an existing intracellular kinase signaling cascade in neutrophils that activates LFA-1 integrins to enable E-selectin–mediated slow rolling. These studies report that E-selectin engagement of either CD44 or PSGL-1 in bone marrow leukocytes ( 90% neutrophils) triggered a sequential tyrosine kinase cascade consisting of (1) Src family kinases Hck, Lyn and Fgr, (2) Syk, (3) Btk, and (4) p38, and led to LFA-1 engagement of ICAM-1 (see figure) for slow rolling in vivo and in vitro. The importance of these pathways to neutrophil recruitment was corroborated by neutrophil “competitive” adoptive transfer studies where cells treated with inhibitors and/or cells from knockout animals exhibited defective slow rolling on TNF–treated venules in the cremaster muscle or diminished recruitment in a model of thioglycollate-induced peritonitis in wild-type mice. Furthermore, they corroborated many of their findings with human neutrophils, which also exhibit slower rolling on E-selectin plus ICAM-1 than on E-selectin alone via SFK, Syk, and p38 and lipid raft– mediated LFA-1 activation under flow conditions in vitro. E-selectin, P-selectin, and L-selectin constitute the selectin gene family. E-selectin is exclusively expressed in the vascular endothelium. Early studies in E-selectin knockout (E-sel / ) mice revealed that peripheral blood neutrophils rolled at higher velocities in TNF–treated venules of the cremaster muscle4 and that neutrophils exhibited a significant decrease in stable adhesion to inflamed dermal and mesenteric microvessels.5 But E-selectin null animals did not exhibit a leukocyte recruitment defect in a bacterial or thioglycollate peritonitis or delayed-type hypersensitivity models of inflammation, which is due to compensation by endothelialexpressed Pselectin.6,7 E-selectin– dependent slow rolling led to longer transit times of leukocytes rolling in inflamed venules and longer transit time was postulated to facilitate recruitment of neutrophils to sites of inflammation.6 Meanwhile, work in other labs with different models discovered leukocyte PSGL-1 engagement by Eor P-selectin–transmitted signals through its cytoplasmic tail to prime integrin activation and initiate gene transcription.8,9 These data and other accumulating evidence led to the idea that leukocyte integrins interact with endothelial cell ICAM-1 to support slower leukocyte rolling on inflamed venules and facilitate arrest in the presence of appropriate chemokines presented by the apical surface of the endothelium. However, the molecular mechanism underlying E-selectin– dependent “slow” rolling velocity remained undefined until 2007 when studies by Zarbock, Ley, and colleagues shed light on the problem.1,2 Their elegant studies used a combination of gene knockout animals and an ex vivo whole blood perfusion chamber to reveal E-selectin engagement of neutrophil PSGL-1–triggered LFA-1 binding of ICAM-1 through sequential activation of Syk, DAP12/FcR , and p38 kinase in combination with chemokine receptor signaling pathways to promote leukocyte slow rolling. In this issue, Yago and colleagues sought to extend these observations by exploring the role of CD44 in signal transduction and activation of LFA-1 integrins in leukocyte to enable slow rolling in venules in vivo and in vitro on immobilized E-selectin and ICAM-1 adhesion Model for E-selectin–mediated slow rolling. The circled numbers represent new signaling components identified in this paper. Neutrophils rolling on E-selectin engage both CD44 and PSGL-1 to initiate signaling through a common pathway that requires lipid rafts, the cytoplasmic domain of PSGL-1, all three SFKs, the ITAM adaptors DAP12 and FcR , the Tec kinase Btk, and p38. This signaling cascade activates integrin LFA-1 to a conformation that enables slow rolling but not arrest on ICAM-1. PSGL-1 and CD44 may not be located in the same raft domains as depicted in the figure. See the complete figure in the article by Yago et al on page 485.