1 Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado Health Sciences Center, Denver, Colorado 2 Central Institute, Shino-Test Corporation, Sagamihara, Kanagawa, Japan 3 Department of Laboratory and Molecular Medicine, Faculty of Medicine, Kagoshima University, Kagoshima, Japan 4 Department of Medicine, Keio University, School of Medicine, Tokyo, Japan 5 Department of Internal Medicine, Chung Ang University College of Medicine, Seoul, Korea 6 Service d'Anesthesie-Réanimation et Unité Propre de Recherche de l'Enseignment Superieur-Equipe d'Accueil, Hospital de Bicêtre, Le Kremlin Bicetre, France 7 Department of Anesthesiology, Chonnam University Medical School, Gwangju, Korea Submitted 21 September 2004 ; accepted in final form 3 January 2005 High mobility group box 1 (HMGB1) is a novel late mediator of inflammatory responses that contributes to endotoxin-induced acute lung injury and sepsis-associated lethality. Although acute lung injury is a frequent complication of severe blood loss, the contribution of HMGB1 to organ system dysfunction in this setting has not been investigated. In this study, HMGB1 was detected in pulmonary endothelial cells and macrophages under baseline conditions. After hemorrhage, in addition to positively staining endothelial cells and macrophages, neutrophils expressing HMGB1 were present in the lungs. HMGB1 expression in the lung was found to be increased within 4 h of hemorrhage and then remained elevated for more than 72 h after blood loss. Neutrophils appeared to contribute to the increase in posthemorrhage pulmonary HMGB1 expression since no change in lung HMGB1 levels was found after hemorrhage in mice made neutropenic with cyclophosphamide. Plasma concentrations of HMGB1 also increased after hemorrhage. Blockade of HMGB1 by administration of anti-HMGB1 antibodies prevented hemorrhage-induced increases in nuclear translocation of NF- B in the lungs and pulmonary levels of proinflammatory cytokines, including keratinocyte-derived chemokine, IL-6, and IL-1 . Similarly, both the accumulation of neutrophils in the lung as well as enhanced lung permeability were reduced when anti-HMGB1 antibodies were injected after hemorrhage. These results demonstrate that hemorrhage results in increased HMGB1 expression in the lungs, primarily through neutrophil sources, and that HMGB1 participates in hemorrhage-induced acute lung injury. high mobility group box 1; nuclear factor- B; neutrophils Address for reprint requests and other correspondence: E. Abraham, Univ. of Colorado Health Sciences Center, 4200 E. Ninth Ave., Box C272, Denver, CO 80262 (E-mail: Edward.Abraham{at}uchsc.edu )