Open sesame! CXCR4 blockade recruits neutrophils into the plaque.

The importance of inflammation in the initiation and development of atherosclerosis has been extensively studied over the last decades.1,2 In particular, much effort has been devoted to elucidating the role of chemokines and their receptors, which play a central part in the atherogenic immune response. Indeed, interfering with the chemokine– chemokine receptor system emerged as both an exciting tool to dissect the complex inflammatory response and a promising therapeutic target.3 Although participation of other inflammatory cells such as macrophages and T cells is well documented, less attention has been paid to the involvement of neutrophils. Neutrophil infiltrates are seldom observed within human atherosclerotic plaques in comparison with other inflammatory cells. However, neutrophils have been detected at the sites of plaque erosion or rupture on atherectomy specimens from the patients with unstable angina and on autopsy samples from the patients with acute myocardial infarction.4 Moreover, the increase in proinflammatory molecules occurring with coronary artery diseases correlates with the peripheral neutrophil count,5 which, hence, may be useful as a predictor of complex coronary stenosis or myocardial infarction. A histological analysis of human carotid endarterectomy specimens suggested that intraplaque hemorrhage could convey neutrophils into the atherosclerotic lesion, spreading into the necrotic core, thus participating in its protease enrichment.6 Although these clinical studies have suggested the potential contribution of neutrophils to plaque progression and vulnerability, few animal studies have been performed to determine the role of neutrophils in the pathogenesis of atherosclerosis. The molecular mechanism by which neutrophils accumulate at the advanced plaques remains largely unknown. In the current issue of Circulation Research, Zernecke et al provide compelling evidence that neutrophils participate in atherosclerotic lesion development.7 Neutrophils are a critical component of the innate immune response against a wide array of infectious agents.8 Furthermore, neutrophils may be a major contributor to tissue damage in inflammatory diseases, such as rheumatoid arthritis and adult respiratory distress syndrome.8 Neutrophils are normally produced exclusively is the bone marrow, and their release into the systemic circulation is tightly regulated to maintain homeostatic levels in blood. Although the mechanisms regulating neutrophil homeostasis and trafficking are poorly understood, recent evidence suggests that the chemokines receptor CXCR4 and its ligand CXCL12 (stromal-derived factor 1, SDF-1) play a critical role in the retention of granulocytic precursors within the bone marrow microenvironment.8–10 Zernecke et al hypothesized that blockade of CXCR4/ CXCL12 signaling could enable neutrophil recruitment to the atherosclerotic lesions. To investigate the role of the CXCR4/ CXCL12 axis in the pathogenesis of atherosclerosis, the authors performed (1) long-term administration of the CXCR4 antagonist AMD3465 in apolipoprotein E–null (ApoE / ) mice, (2) depletion of CXCR4 expression in the bone marrow of ApoE / and LDL receptor–null (LDLR / ) mice, (3) systemic infusion of neutrophils coupled to fluorescently labeled latex beads into ApoE / mice, and (4) systemic administration of an anti–polymorphonuclear leukocyte antibody into ApoE / mice. Treatment of ApoE / mice with constant delivery of the CXCR4 antagonist AMD3465 led to an increase in the relative number of neutrophils in the bone marrow, accompanied by an increased production of lysozyme, especially in mature neutrophils. AMD3465 also increased neutrophils in the peripheral blood with an appearance of more immature band population. The authors report that AMD3465 treatment did not modify CXCR4 expression, phagocytosis, reactive oxygen species production, calcium flux regulation, or adhesion properties of circulating neutrophils. However, CXCR4 blockade not only increased neutrophil migration from the bone marrow but also decreased the returning of senescent neutrophils back to the bone marrow after they circulated.7,10 Thus, the global rate of both fresh and senescent circulating neutrophils was increased. Subsequently, AMD3465 increased the number of neutrophils in aortic root plaques and adventitia. The relative content of neutrophils among all inflammatory plaque cells increased, whereas that of macrophage or T cells was unchanged. The infiltrated neutrophils were functionally active, as attested by their production of myeloperoxidase, gelatinase-associated lipocalin (NGAL), matrix metalloproteinase-9, and elastase. Exaggerated infiltration of neutrophils was associated with upregulation of interferon, the CXCR2 ligand CXCL1, and tissue factor. Furthermore, AMD3465 increased the proportion of apoptotic neutrophils, mainly located on the luminal side of the atherosclerotic plaque, as well as that of macrophages. As a whole, these phenomena led to a significant increase of atherosclerotic lesion and necrotic core area size. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Departments of Cardiovascular Medicine (J.S., M.S.) and Advanced Clinical Science and Therapeutics (M.S.), University of Tokyo, Graduate School of Medicine, Japan. Correspondence to Dr Masataka Sata, Department of Cardiovascular Medicine, University of Tokyo Graduate School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. E-mail [email protected] (Circ Res. 2008;102:154-156.) © 2008 American Heart Association, Inc.