Haptoglobin inhibits the pro-inflammatory activity of HMGB1 (P4177)

Abstract HMGB1 is a cytokine mediator in the pathogenesis of inflammatory diseases including sepsis (Yang et al, BBA, 2009). Recently we demonstrated that haptoglobin (Hp), a naturally occurring serum protein, binds to HMGB1 and suppresses HMGB1-stimulated TNF and IL-8 release in cultured macrophages. Hp knockout mice subjected to cecal ligation and puncture (CLP)-induced sepsis had significantly higher serum HMGB1 levels and higher mortality rates compared to wild type animals (75% survival in wild type vs. 34% in Hp knockout mice; P<0.05). Wild type mice subjected to CLP and received injections of Hp were twice as likely to survive (63% survival in Hp-treated vs. 33% in vehicle control; P<0.05), suggesting the therapeutic potential of exogenous Hp in sepsis. Hp is composed of alpha and beta subunits in a polymeric form (Yueh, J. Chromat B Life Sci. 2007). Structure-functional analysis revealed that Hp beta subunit alone is sufficient to recapitulate effects of Hp to neutralize HMGB1 in vitro in macrophages. The survival advantage of Hp in CLP-induced sepsis was also fully reproduced by Hp beta. Surface plasmon resonance analysis showed that Hp beta binds HMGB1 with high affinity (Kd = 29 nM). Thus, our data reveal unexpected roles of Hp as an endogenous antagonist of HMGB1, preventing the harmful HMGB1-induced inflammation in sepsis. The essential domain of Hp maps to the beta subunit, making it a target in the design of therapeutics.