Paracellular diapedesis, a key step in leukocyte recruitment to the site of inflammation, occurs at endothelial junctions and is regulated by highly coordinated interactions between leukocytes and endothelium. We found that CD157, a glycosylphosphatidylinositol-anchored ectoenzyme belonging to the NADase/ADPribosyl cyclase family, plays a crucial role for neutrophil diapedesis, because its ligation with specific monoclonal antibodies (both on neutrophils or endothelial cells) results in altered neutrophil movement on the apical surface of endothelium and, ultimately, in loss of diapedesis. Realtime microscopy revealed that CD157 behaves as a sort of compass during the interaction between neutrophils and endothelial cells; indeed, following CD157 ligation, neutrophils appear disoriented, meandering toward junctions where they eventually stop without transmigrating. These findings are relevant in vivo because CD157-deficient neutrophils obtained from patients with paroxysmal nocturnal hemoglobinuria are characterized by a severely impaired diapedesis.
IntroductionLeukocyte extravasation, a crucial step in immune responses and inflammatory reactions, is governed by sequential molecular interactions between leukocytes circulating in the bloodstream and endothelial cells lining the vascular wall. 1,2 This cascade involves initial tethering and rolling steps, prevalently mediated by selectinbased adhesive events 3 and a subsequent firm adhesion mediated by integrins. 4 Then, leukocytes polarize, move to endothelial cell junctions, and migrate through them in sequential processes referred to as locomotion and transendothelial migration (or diapedesis), respectively. 5,6 In contrast to leukocyte rolling and firm adhesion to endothelium, the molecular details of leukocyte diapedesis have not been completely defined. A small number of molecules have been identified that control the different steps of neutrophil and monocyte transendothelial migration; these include CD31, CD99, VE-cadherin, and junctional adhesion molecules (JAM-1, -2, and -3). [7][8][9][10] However, data derived from in vitro and in vivo models suggest that other molecules are involved in the process of diapedesis. 11,12 On molecular identification of these, several, unexpectedly, turned out to be ectoenzymes. 13 We addressed our attention to CD157 surface enzyme, a glycosylphosphatidylinositol (GPI)-linked molecule encoded by a member of the NADase/ ADP-ribosyl cyclase (cADPR) gene family, which also includes CD38. [14][15][16] The findings that CD157 is expressed both by neutrophils and endothelial cells and is implicated in neutrophil chemotaxis 17 were highly indicative of potential involvement in transendothelial migration and, eventually, in neutrophil recruitment to the inflammatory site.This work demonstrates that CD157 is (1) constitutively expressed at vascular interendothelial junctions and its expression is not affected by cell activation, (2) is crucial to transendothelial migration of neutrophils, and (3) orchestrates neutrophil journey thr...