Thrombomodulin Binds Lethal Cytokine HMGB1 and Neutralizes Its Pro-inflammatory Activity: Novel Cytoprotective Action of Thrombomodulin

Abstract number: OR170

Maruyama¹ I, Abeyama¹ K, Ito¹ T, Biswas¹ KK, Yamada² S, Hashiguchi¹ T

¹¹Department of Laboratory and Vascular Medicine, Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima 890–8520, Japan ¹¹Department of Laboratory and Vascular Medicine, Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima 890–8520, Japan ²²Central Institute, Shino-Test Corporation, Kanagawa 229–0011, Japan

Thrombomodulin (TM) is an endothelial membrane cofactor which converts thrombin, a procoagulant protease to an anticoagulant. TM is composed by 5 domains such as N-terminus lectin-like domain (D1), 6 consecutive EGF-like domain (D2), O-glycosylation site rich domain (D3), transmembrane domain (D4) and cytoplasmic domain (D5). Thrombin binds to the 3rd, 4th, 5th-EGF-like domains of TM and thus loses its procoagulant and platelet-activating activities. Moreover, the thrombin-TM complex efficiently activates protein C which acts as an anticoagulant and anti-inflammatory protease. On the other hand, HMGB1, a DNA-binding protein, is released passively from the necrotic cells at the site of injury and inflammation, where thrombin is also generated. The released HMGB1 acts as a pro-inflammatory cytokine through multi-ligand receptor RAGE. Thus, circulating HMGB1 is recently considered as a lethal factor in sepsis, organ failures and so on. However, recent reports also demonstrate that the localized HMGB1 has endogenous adjuvant and stem-cell inducing activity in the injury sites. Therefore, there may be a system to localize the released HMGB1 in the injury sites and thus prevented from entering into the circulation. Here we report that TM can bind free HMGB1 through its D1 lectin like domain, thereby preventing leukocyte activation in vitro, and ultraviolet radiation-induced cutaneous inflammation and lipopolysaccharide-induced lethality in vivo. We also demonstrate that recombinant TM or D1 domain of TM possesses the anti-inflammatory properties suggesting the therapeutic potentiality of TM. These findings highlight a novel mechanism by which an endothelial cofactor, TM, suppresses inflammation by sequestrating inflammatory mediators and thereby preventing their interaction with cell surface receptors on effector cells in the vasculature.