BackgroundExtracellular histones were recently identified as an inflammatory mediator involved in the pathogenesis of various organ injuries. This study aimed to examine extracellular histone levels and their clinical implications in acute respiratory distress syndrome (ARDS) patients and to explore histone-mediated effects through ex-vivo investigations.MethodsExtracellular histones, cytokine profiles and clinical data from 96 ARDS patients and 30 healthy volunteers were obtained. Human bronchial epithelial cells (BEAS-2B), human pulmonary artery endothelial cells (HPAEC), and human monocytic U937 cells were exposed to bronchoalveolar lavage fluid (BALF) collected from ARDS patients, and cellular damage and cytokine production were assessed. Furthermore, the effect of histone-targeted interventions by heparin or anti-histone antibody was evaluated.ResultsPlasma and BALF extracellular histone levels were much higher in ARDS patients than in healthy controls. There was a significant association between extracellular histones and ARDS severity and mortality. In addition, extracellular histones correlated with an evident systemic inflammation detected in ARDS patients. Ex-vivo analysis further showed that ARDS patient’s BALF remarkably induced epithelial and endothelial cell damage and stimulated cytokine production in the supernatant of U937 cells. The adverse effects on these cells could be abrogated by heparin or anti-histone antibody.ConclusionsExtracellular histones in ARDS patients are excessively increased and may contribute to disease aggravation by inducing cellular damage and promoting systemic inflammation. Targeting extracellular histones may provide a promising approach for treating ARDS.Electronic supplementary materialThe online version of this article (10.1186/s12931-017-0651-5) contains supplementary material, which is available to authorized users.

Acute liver failure (ALF) is a life-threatening systemic disorder. Here we investigated the impact of circulating histones, recently identified inflammatory mediators, on systemic inflammation and liver injury in murine models and patients with ALF. We analyzed histone levels in blood samples from 62 patients with ALF, 60 patients with chronic liver disease, and 30 healthy volunteers. We incubated patients' sera with human L02 hepatocytes and monocytic U937 cells to assess cellular damage and cytokine production. d-galactosamine plus lipopolysaccharide (GalN/LPS), concanavalin A (ConA), and acetaminophen (APAP) were given to C57BL/6N mice to induce liver injury, respectively, and the pathogenic role of circulating histones was studied. Besides, the protective effect of nonanticoagulant heparin, which can bind histones, was evaluated with in vivo and ex vivo investigations. We observed that circulating histones were significantly increased in patients with ALF, and correlated with disease severity and mortality. Significant systemic inflammation was also pronounced in ALF patients, which were associated with histone levels. ALF patients' sera induced significant L02 cell death and stimulated U937 cells to produce cytokines, which were abrogated by nonanticoagulant heparin. Furthermore, circulating histones were all released remarkably in GalN/LPS, ConA, and APAP-treated mice, and associated with high levels of inflammatory cytokines. Heparin reduced systemic inflammation and liver damage in mice, suggesting that it could interfere with histone-associated liver injury. Collectively, these findings demonstrate that circulating histones are critical mediators of systemic inflammation and cellular damage in ALF, which may be potentially translatable for clinical use.

Hemorrhagic shock (HS) promotes the development of systemic inflammatory response syndrome (SIRS) and organ injury by activating and priming the innate immune system for an exaggerated inflammatory response through, as of yet, unclear mechanisms. IL-1β also plays an important role in the development of post-HS SIRS and active IL-1β production is tightly controlled by the inflammasome. Pyrin, a protein of 781 amino-acids with pyrin domain (PYD) at the N-terminal, negatively regulates inflammasome activation through interaction with nucleotide-binding oligomerization domain-like receptor protein (NLRP). Expression of pyrin can be induced by LPS and cytokines, and IL-10 is a known potent inducer of pyrin expression in macrophages. In the present study, we tested the hypothesis that HS downregulates IL-10, and therefore decreases pyrin expression to promote inflammasome activation and subsequent IL-1β processing and secretion in the lungs. Our results show that LPS, while activating Nlrp3 inflammasome in the lungs, also induced pyrin expression, which in turn suppressed inflammasome activation. More importantly, LPS-mediated upregulation of IL-10 enhanced pyrin expression, which serves, particularly in later phases, as a potent negative feedback mechanism regulating inflammasome activation. However, HS-mediated suppression of IL-10 expression in alveolar macrophages (AM) attenuated the upregulation of pyrin in AM and lung endothelial cells, and thereby significantly enhanced inflammasome activation and IL-1β secretion in the lungs. This study demonstrates a novel mechanism by which HS suppresses negative feedback regulation of Nlrp3 inflammasome to enhance IL-1β secretion in response to subsequent LPS challenge, and so primes for inflammation.