Sterile inflammatory insults are known to activate innate immunity and propagate organ damage through the recognition of extracellular Damage Associated Molecular Pattern (DAMP) molecules. Although DAMPs, such as endogenous DNA and nuclear High Mobility Group Box 1, have been shown to be critical in sterile inflammation, the role of nuclear histone proteins has not yet been investigated. We report that endogenous histones function as DAMPs following ischemic injury through the pattern recognition receptor Toll-Like Receptor 9 (TLR9) to initiate inflammation. Using an in vivo model of hepatic ischemia/reperfusion (I/R) injury, we show that levels of circulating histones are significantly higher after I/R, and that histone neutralization significantly protects against injury. Injection of exogenous histones exacerbates I/R injury through cytotoxic effects mediated by TLR9 and MyD88. In addition, histone administration increases TLR9 activation, while neither TLR9 nor MyD88 mutant mice respond to exogenous histones. Furthermore, we demonstrate in vitro that extracellular histones enhance DNA-mediated TLR9 activation in immune cells through a direct interaction. Conclusions these novel findings reveal that histones represent a new class of DAMP molecules and they serve as a crucial link between initial damage and activation of innate immunity during sterile inflammation.

The mobilization and extracellular release of nuclear high mobility group box-1 (HMGB1) by ischemic cells activates inflammatory pathways following liver ischemia/reperfusion (I/R) injury. In immune cells such as macrophages, post-translational modification by acetylation appears to be critical for active HMGB1 release. Hyperacetylation shifts its equilibrium from a predominant nuclear location toward cytosolic accumulation and subsequent release. However, mechanisms governing its release by parenchymal cells such as hepatocytes are unknown. In this study, we found that serum HMGB1 released following liver I/R in vivo is acetylated, and that hepatocytes exposed to oxidative stress in vitro also released acetylated HMGB1. Histone deacetylases (HDACs) are a family of enzymes that remove acetyl groups and control the acetylation status of histones and various intracellular proteins. Levels of acetylated HMGB1 increased with a concomitant decrease in total nuclear HDAC activity, suggesting that suppression in HDAC activity contributes to the increase in acetylated HMGB1 release after oxidative stress in hepatocytes. We identified the isoforms HDAC1 and HDAC4 as critical in regulating acetylated HMGB1 release. Activation of HDAC1 was decreased in the nucleus of hepatocytes undergoing oxidative stress. In addition, HDAC1 knockdown with siRNA promoted HMGB1 translocation and release. Furthermore, we demonstrate that HDAC4 is shuttled from the nucleus to cytoplasm in response to oxidative stress, resulting in decreased HDAC activity in the nucleus. Together, these findings suggest that decreased nuclear HDAC1 and HDAC4 activities in hepatocytes following liver I/R is a mechanism that promotes the hyperacetylation and subsequent release of HMGB1. High Mobility Group Box Protein 1 (HMGB1)3 is a ubiquitously expressed nuclear molecule that functions as a structural protein of chromatin (1). In addition to its nuclear role, HMGB1 also functions as an inflammatory cytokine when released from necrotic cells or actively secreted from stressed cells. Its proinflammatory properties were first highlighted in experiments showing that HMGB1 is actively secreted by activated macrophages, serving as a late mediator of lethality in sepsis (2). Whereas HMGB1 is involved in the late systemic inflammatory response to sepsis, our laboratory demonstrated that HMGB1 is a central and necessary mediator of organ damage following acute, sterile organ injury (3, 4). HMGB1 is rapidly mobilized and released by hepatocytes in the setting of hepatic ischemia and reperfusion injury. Extracellular HMGB1 functions as a damage-associated molecular pattern (DAMP) molecule and activates proinflammatory signaling pathways by activating pattern recognition receptors including Toll-like receptor 4 (TLR4) and the receptor for advanced glycation end-products (RAGE) (5, 6). Mounting evidence suggests HMGB1 may also function to facilitate the recognition of other immune co-activators such as LPS, DNA, and IL-1 through avid binding to these molecules (7-9).Thoro...

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Neutrophil extracellular traps (NETs) facilitate the extracellular killing of pathogens. However, excessive NETs formation and poor degradation are associated with exacerbated immune responses and tissue injury. In this study, we investigated the role of NETs in lipopolysaccharide (LPS)-mediated acute lung injury (ALI) and assessed the use of DNase I, for the treatment of ALI. Additionally, we focused on the controversial issue of whether LPS directly induces NETs release in vitro. NETs formation was detected in murine ALI tissue in vivo and was associated with increased NETs markers, citrullinated-histone H3 tissue levels and NET-DNA levels in BALF. Treatment with DNase I significantly degraded NETs and reduced citrullinated-histone H3 levels, which protected against ALI and ameliorated pulmonary oedema and total protein in BALF. In addition, DNase I significantly reduced IL-6 and TNF-α levels in plasma and BALF. In vitro, LPS-activated platelets rather than LPS alone efficiently induced NETs release. In conclusion, NETs formed during LPS-induced ALI, caused organ damage and initiated the inflammatory response. NETs degradation by DNase I promoted NET-protein clearance and protected against ALI in mice; thus, DNase I may be a new potential adjuvant for ALI therapy. Specifically, LPS induced NETs formation in an indirect manner via platelets activation.