The N-Terminal Domain of Thrombomodulin Sequesters HMGB1: A Novel Anti-Inflammatory Mechanism.

Thrombomodulin (TM) is an endothelial anticoagulant cofactor that promotes thrombin-mediated formation of activated protein C (APC), the latter an enzyme with potent anti-coagulant and anti-inflammatory properties. We have found that the N-terminal, lectin-like domain (D1) of thrombomodulin has unique anti-inflammatory properties. Thrombomodulin, via D1, binds high mobility group-B1 DNA binding protein (HMGB1), a factor closely associated with necrotic cell damage following its release from the nucleus, thereby preventing leukocyte activation in vitro, and ultraviolet radiation-induced cutaneous inflammation and lipopolysaccharide-induced lethality in vivo. Our data also demonstrate anti-inflammatory properties of a peptide spanning the D1 domain of TM and suggest its therapeutic potential. These findings highlight a novel mechanism through which an endothelial cofactor, TM, suppresses inflammation; i.e., sequestration of mediators thereby preventing their interaction with cell surface receptors on effector cells in the vasculature.

Results: TM binds HMGB1 and prevents expression of pro-inflammatory activity. Our co-culture studies of leukocytes and HUVEC, and results in the cutaneous irritation model suggested that early release of a mediator, such as HMGB1, might contribute importantly to cellular activation in inflammation at later time points. In this context, TM might have the ability to decrease HMGB1-mediated inflammatory events. Binding studies using surface plasmon resonance (SPR), performed to directly assess the interaction of TM and immobilized HMGB1, demonstrated dose-dependent binding in the nanomolar range (K_d ~232 nM). Furthermore, addition of rhs-TM decreased, in a dose-dependent manner, the binding of HMGB1 to RAGE through the its N-terminal domain, but not anti-coagulant domain.

TM and the N-terminal-derived TM peptide have anti-inflammatory effects in settings where HMGB1 is a likely key mediator. In HMGB1-mediated skin inflammation model, systemic administration of rhs-TM, its lectin-like domain and sRAGE resulted in a significant blunting of the inflammatory response. In contrast, the effect of anti-coagulant domain, although showing a trend toward decreased ear swelling, did not achieve statistical significance (anticoagulant domain has anti-inflammatory effects in vivo that probably reflect its ability to support thrombin-mediated activation of protein C; the latter does not occur in vitro after inactivation of the protein C zymogen by heat treatment). In view of recent data suggesting a link between HMGB1 released from injured tissue and endotoxin-induced lethality in mice, we also tested whether rhs-TM and its lectin-like domain might also have protective effects in this model. We employed a dose of intraperitoneal (IP) LPS (10 mg/kg) resulting in 100% lethality by 96 hrs. Systemic (IP) treatment of animals with anti-HMGB1 IgY had a protective effect with respect to lethality at 4 days, whereas the same regimen of nonimmune IgY was without effect. Similarly, IP administration of rhs-TM and its N-teminal lectin domain, but not anti-coagulant domain had complete protective effects compared with anti-HMGB1 IgY.

Conclusion: Our findings have elucidated an unexpected anti-inflammatory property of TM residing in the D1 domain, namely binding of HMGB1.