Protective effect of the long pentraxin PTX3 against histone-mediated endothelial cell cytotoxicity in sepsis

Emerging therapeutic strategies targeting extracellular histones for critical and inflammatory diseases: an updated narrative review

Extracellular histones are crucial damage-associated molecular patterns involved in the development and progression of multiple critical and inflammatory diseases, such as sepsis, pancreatitis, trauma, acute liver failure, acute respiratory distress syndrome, vasculitis and arthritis. During the past decade, the physiopathologic mechanisms of histone-mediated hyperinflammation, endothelial dysfunction, coagulation activation, neuroimmune injury and organ dysfunction in diseases have been systematically elucidated. Emerging preclinical evidence further shows that anti-histone strategies with either their neutralizers (heparin, heparinoids, nature plasma proteins, small anion molecules and nanomedicines, etc.) or extracorporeal blood purification techniques can significantly alleviate histone-induced deleterious effects, and thus improve the outcomes of histone-related critical and inflammatory animal models. However, a systemic evaluation of the efficacy and safety of these histone-targeting therapeutic strategies is currently lacking. In this review, we first update our latest understanding of the underlying molecular mechanisms of histone-induced hyperinflammation, endothelial dysfunction, coagulopathy, and organ dysfunction. Then, we summarize the latest advances in histone-targeting therapy strategies with heparin, anti-histone antibodies, histone-binding proteins or molecules, and histone-affinity hemoadsorption in pre-clinical studies. Finally, challenges and future perspectives for improving the clinical translation of histone-targeting therapeutic strategies are also discussed to promote better management of patients with histone-related diseases.

Humoral pathways of innate immune regulation in granuloma formation

The humoral arm of mammalian innate immunity regulates several molecular mechanisms involved in resistance to pathogens, inflammation, and tissue repair. Recent studies highlight the crucial role played by humoral mediators in granulomatous inflammation. However the molecular mechanisms linking the function of these soluble molecules to the initiation and maintenance of granulomas remain elusive. We propose that humoral innate immunity coordinates fundamental physiological processes in macrophages which, in turn, initiate activation and transformation events that enable granuloma formation. We discuss the involvement of humoral mediators in processes such as immune activation, phagocytosis, metabolism, and tissue remodeling, and how these can dictate macrophage functionality during granuloma formation. These advances present opportunities for discovering novel disease factors and developing targeted, more effective treatments for granulomatous diseases.

Pentraxin 3: A promising therapeutic target for cardiovascular diseases

Cardiovascular disease (CVD) is the primary global cause of death, and inflammation is a crucial factor in the development of CVDs. The acute phase inflammatory protein pentraxin 3 (PTX3) is a biomarker reflecting the immune response. Recent research indicates that PTX3 plays a vital role in CVDs and has been investigated as a possible biomarker for CVD in clinical trials. PTX3 is implicated in the progression of CVDs through mechanisms such as exacerbating vascular endothelial dysfunction, affecting angiogenesis, and regulating inflammation and oxidative stress. This review summarized the structure and function of PTX3, focusing on its multifaceted effects on CVDs, such as atherosclerosis, myocardial infarction, and hypertension. This may help in explaining the varying PTX3 functions and usage, as well as in utilizing target organs to manage diseases. Moreover, elucidating the opposite role of PTX3 in the cardiovascular system will demonstrate the therapeutic and predictive potential in human diseases.

The pentraxin family in autoimmune disease

The pentraxins represent a family of multifunctional proteins composed of long and short pentamers. The latter includes serum amyloid P component (SAP) and C-reactive protein (CRP) whereas the former includes neuronal PTX1 and PTX2 (NPTX1 and NPTX2, respectively), PTX3 and PTX4. These serve as a bridge between adaptive immunity and innate immunity and a link between inflammation and immunity. Similarities and differences between long and short pentamers are examined and their roles in autoimmune disease are discussed. Increased CRP and PTX3 could indicate the activity of rheumatoid arthritis, systemic lupus erythematosus or other autoimmune diseases. Mechanistically, CRP and PTX3 may predict target organ injury, regulate bone metabolic immunity and maintain homeostasis as well as participate in vascular endothelial remodeling. Interestingly, PTX3 is pleiotropic, being involved in inflammation and tissue repair. Given the therapeutic potential of PTX3 and CRP, targeting these factors to exert a beneficial effect is the focus of research efforts. Unfortunately, studies on NPTX1, NPTX2, PTX4 and SAP are scarce and more research is clearly needed to elaborate their potential roles in autoimmune disease.

The Polyanionic Drug Suramin Neutralizes Histones and Prevents Endotheliopathy

Drugs are needed to protect against the neutrophil-derived histones responsible for endothelial injury in acute inflammatory conditions such as trauma and sepsis. Heparin and other polyanions can neutralize histones but challenges with dosing or side effects such as bleeding limit clinical application. In this study, we demonstrate that suramin, a widely available polyanionic drug, completely neutralizes the toxic effects of individual histones, but not citrullinated histones from neutrophil extracellular traps. The sulfate groups on suramin form stable electrostatic interactions with hydrogen bonds in the histone octamer with a dissociation constant of 250 nM. In cultured endothelial cells (Ea.Hy926), histone-induced thrombin generation was significantly decreased by suramin. In isolated murine blood vessels, suramin abolished aberrant endothelial cell calcium signals and rescued impaired endothelial-dependent vasodilation caused by histones. Suramin significantly decreased pulmonary endothelial cell ICAM-1 expression and neutrophil recruitment caused by infusion of sublethal doses of histones in vivo. Suramin also prevented histone-induced lung endothelial cell cytotoxicity in vitro and lung edema, intra-alveolar hemorrhage, and mortality in mice receiving a lethal dose of histones. Protection of vascular endothelial function from histone-induced damage is a novel mechanism of action for suramin with therapeutic implications for conditions characterized by elevated histone levels.

Clusterin Neutralizes the Inflammatory and Cytotoxic Properties of Extracellular Histones in Sepsis

Rationale: Extracellular histones, released into the surrounding environment during extensive cell death, promote inflammation and cell death, and these deleterious roles have been well documented in sepsis. Clusterin (CLU) is a ubiquitous extracellular protein that chaperones misfolded proteins and promotes their removal. Objectives: We investigated whether CLU could protect against the deleterious properties of histones. Methods: We assessed CLU and histone expression in patients with sepsis and evaluated the protective role of CLU against histones in in vitro assays and in vivo models of experimental sepsis. Measurements and Main Results: We show that CLU binds to circulating histones and reduces their inflammatory, thrombotic, and cytotoxic properties. We observed that plasma CLU levels decreased in patients with sepsis and that the decrease was greater and more durable in nonsurvivors than in survivors. Accordingly, CLU deficiency was associated with increased mortality in mouse models of sepsis and endotoxemia. Finally, CLU supplementation improved mouse survival in a sepsis model. Conclusions: This study identifies CLU as a central endogenous histone-neutralizing molecule and suggests that, in pathologies with extensive cell death, CLU supplementation may improve disease tolerance and host survival.

Cell-free histones and the cell-based model of coagulation

The cell-based model of coagulation remains the basis of our current understanding of clinical hemostasis and thrombosis. Its advancement on the coagulation cascade model has enabled new prohemostatic and anticoagulant treatments to be developed. In the past decade, there has been increasing evidence of the procoagulant properties of extracellular, cell-free histones (CFHs). Although high levels of circulating CFHs released following extensive cell death in acute critical illnesses, such as sepsis and trauma, have been associated with adverse coagulation outcomes, including disseminated intravascular coagulation, new information has also emerged on how its local effects contribute to physiological clot formation. CFHs initiate coagulation by tissue factor exposure, either by destruction of the endovascular barrier or induction of endoluminal tissue factor expression on endothelia and monocytes. CFHs can also bind prothrombin directly, generating thrombin via the alternative prothrombinase pathway. In amplifying and augmenting the procoagulant signal, CFHs activate and aggregate platelets, increase procoagulant material bioavailability through platelet degranulation and Weibel-Palade body exocytosis, activate intrinsic coagulation via platelet polyphosphate release, and induce phosphatidylserine exposure. CFHs also inhibit protein C activation and downregulate thrombomodulin expression to reduce anti-inflammatory and anticoagulant effects. In consolidating clot formation, CFHs augment the fibrin polymer to confer fibrinolytic resistance and integrate neutrophil extracellular traps into the clot structure. Such new information holds the promise of new therapeutic developments, including improved targeting of immunothrombotic pathologies in acute critical illnesses.

Natural polyphenols convert proteins into histone-binding ligands

Antioxidants are sensitive to oxidation and are immediately converted into their oxidized forms that can react with proteins. We have recently found that proteins incubated with oxidized vitamin C (dehydroascorbate) gain a new function as a histone-binding ligand. This finding led us to predict that antioxidants, through conversion to their oxidized forms, may generally have similar functions. In the present study, we identified several natural polyphenols as a source of histone ligands and characterized the mechanism for the interaction of protein-bound polyphenols with histone. Through screening of 25 plant-derived polyphenols by assessing their ability to convert bovine serum albumin into histone ligands, we identified seven polyphenols, including (-)-epigallocatechin-3-O-gallate (EGCG). Additionally, we found that the histone tail domain, which is a highly charged and conformationally flexible region, is involved in the interaction with the polyphenol-modified proteins. Further mechanistic studies showed the involvement of a complex heterogeneous group of the polyphenol-derived compounds bound to proteins as histone-binding elements. We also determined that the interaction of polyphenol-modified proteins with histones formed aggregates and exerted a protective effect against histone-mediated cytotoxicity toward endothelial cells. These findings demonstrated that histones are one of the major targets of polyphenol-modified proteins and provide important insights into the chemoprotective functions of dietary polyphenols.

STAT3 Activates the Pentraxin 3 Gene in Chronic Lymphocytic Leukemia Cells

Pentraxin-related protein 3 (PTX3), commonly produced by myeloid and endothelial cells, is a humoral pattern recognition protein of the innate immune system. Because PTX3 plasma levels of patients with chronic lymphocytic leukemia (CLL) are high and most circulating cells in patients with CLL are CLL cells, we reasoned that CLL cells produce PTX3. Western immunoblotting revealed that low-density cells from seven of seven patients with CLL produce high levels of PTX3, flow cytometry analysis revealed that the PTX3-producing cells are B lymphocytes coexpressing CD19 and CD5, and confocal microscopy showed that PTX3 is present in the cytoplasm of CLL cells. Because STAT3 is constitutively activated in CLL cells, and because we identified putative STAT3 binding sites within the PTX3 gene promoter, we postulated that phosphorylated STAT3 triggers transcriptional activation of PTX3. Immunoprecipitation analysis of CLL cells' chromatin fragments showed that STAT3 Abs precipitated PTX3 DNA. STAT3 knockdown induced a marked reduction in PTX3 expression, indicating a STAT3-induced transcriptional activation of the PTX3 gene in CLL cells. Using an EMSA, we established and used a dual-reporter luciferase assay to confirm that STAT3 binds the PTX3 gene promoter. Downregulation of PTX3 enhanced apoptosis of CLL cells, suggesting that inhibition of PTX3 might benefit patients with CLL.

PTX3 Protects Intestinal Mucosal Barrier Damage in Sepsis Through Toll-Like Receptor Signaling Pathway

This study aims to confirm the protective effect of Pentraxin 3 (PTX3) on intestinal mucosal barrier damage in sepsis in animal and cell models and explore its mechanism. Analysis of the GSE147775 gene set revealed that the level of PTX3 was upregulated in the lipopolysaccharide (LPS)-induced rat sepsis model. The mice sepsis model was established by cecal ligation perforation (CLP), and the cell inflammation model was induced by LPS. Cell apoptosis and the expression of apoptosis-related protein were detected by flow cytometry and Western blotting. The PTX3 level was significantly upregulated in the mice sepsis model. Intestinal mucosal barrier damage was aggravated and inflammatory factor expression was upregulated after PTX3 downregulation in sepsis mice. After upregulation of PTX3, intestinal mucosal barrier damage was alleviated and inflammatory factor expression was decreased in sepsis mice. Further data mining suggested that the anti-inflammatory effect of PTX3 might be realized through inhibition of the toll-like receptor (TLR) signaling pathway. Moreover, compared with the LPS group, downregulation of PTX3 increased cell apoptosis and the levels of BCL2-associated X (Bax), myeloperoxidase (MPO), tumor necrosis factor-alfa (TNF-α), interleukin 1 beta (IL-1β), and interferon-gamma (IFN-γ), and decreased the levels of B-cell lymphoma-2 (Bcl-2), zona occludens (ZO)-1, and occludin. On the contrary, overexpression of PTX3 reduced cell apoptosis and the levels of Bax, MPO, TNF-α, IL-1β, and IFN-γ. Moreover, downregulation of PTX3 reversed the inhibitive effects on cell apoptosis and inflammation and promotive effects on the levels of Zo-1 and occludin induced by CLI-095 (a TLR signaling pathway inhibitor). In the CLP-induced mice sepsis model and LPS-induced cell inflammation model, PTX3 inhibits inflammatory response and reduces intestinal mucosal barrier damage through the TLR signaling pathway.

The Protective Role of the Long Pentraxin PTX3 in Spontaneously Hypertensive Rats with Heart Failure

Pentraxin 3 (PTX3) is synthesized locally and released into the circulation, reflecting local inflammation in the cardiovascular system. Therefore, we conducted a study to explore the effect of PTX3 in spontaneously hypertensive heart failure (SHHF) rats. Sprague Dawley (SD) and SHHF rats were treated with recombinant PTX3 protein, and the blood pressure (BP) and echocardiographic parameters were collected. Radioimmunoassay, enzyme immunoassay and enzyme-linked immunosorbent assay (ELISA) were applied to detect plasma levels of atrial/B-type natriuretic peptide (ANP/BNP) and PTX3. The pathological changes in the myocardial tissues were observed by hematoxylin and eosin (HE) and Masson stainings. The mRNA and protein expressions were detected by quantitative real-time reverse-transcription polymerase chain reaction (qPCR) and western blotting. Cardiomyocyte apoptosis was evaluated by TUNEL staining and DNA fragmentation test. Increased plasma concentrations of PTX3 were found in SHHF rats compared with SD rats, which was further enhanced by recombinant PTX3 protein. After injection with recombinant PTX3 protein, the heart function was improved in SHHF rats with the decreased systolic and diastolic BP, and the reduced plasma levels of ANP and BNP. Moreover, PTX3 improved the myocardial damage and interstitial fibrosis in SHHF rats with reduced cardiomyocyte apoptosis and decreased mRNA expressions of pro-inflammatory factors in myocardial tissues. PTX3 could decrease the BP and plasma levels of ANP and BNP in SHHF rats, as well as improve the inflammation, cardiomyocyte apoptosis, and pathological changes of myocardial tissues, suggesting it may be a useful intervention in the treatment of SHHF.

Exosomes from adipose tissue-derived mesenchymal stem cells ameliorate histone-induced acute lung injury by activating the PI3K/Akt pathway in endothelial cells

BACKGROUND

Mesenchymal stem cells (MSCs), including adipose-derived mesenchymal stem cells (ADSCs), have been shown to attenuate organ damage in acute respiratory distress syndrome (ARDS) and sepsis; however, the underlying mechanisms are not fully understood. In this study, we aimed to explore the potential roles and molecular mechanisms of action of ADSCs in histone-induced endothelial damage.

METHODS

Male C57BL/6 N mice were intravenously injected with ADSCs, followed by histones or a vehicle. The mice in each group were assessed for survival, pulmonary vascular permeability, and histological changes. A co-culture model with primary human umbilical vein endothelial cells (HUVECs) exposed to histones was used to clarify the paracrine effect of ADSCs. Overexpression and inhibition of miR-126 ADSCs were also examined as causative factors for endothelial protection.

RESULTS

The administration of ADSCs markedly improved survival, inhibited histone-mediated lung hemorrhage and edema, and attenuated vascular hyper-permeability in mice. ADSCs were engrafted in the injured lung and attenuated histone-induced endothelial cell apoptosis. ADSCs showed endothelial protection (via a paracrine effect) and Akt phosphorylation in the histone-exposed HUVECs. Notably, increased Akt phosphorylation by ADSCs was mostly mediated by exosomes in histone-induced cytotoxicity and lung damage. Moreover, the expression of miR-126 was increased in exosomes from histone-exposed ADSCs. Remarkably, the inhibition of miR-126 in ADSCs failed to increase Akt phosphorylation in histone-exposed HUVECs.

CONCLUSION

ADSC-derived exosomes may exert protective effects on endothelial cells via activation of the PI3K/Akt pathway.

Extracellular histones in lung dysfunction: a new biomarker and therapeutic target?

Extracellular histones released from injured or dying cells following trauma and other severe insults can act as potent damage-associated molecular patterns. In fact, elevated levels of histones are present in human circulation in hyperinflammatory states such as acute respiratory distress syndrome and sepsis. The molecular mechanisms owing to histone-induced pathologies are at the very beginning of elucidating. However, neutralization of histones with antibodies, histone-binding or histone-degrading proteins, and heparan sulfates have shown promising therapeutic effects in pre-clinical acute respiratory distress syndrome and sepsis models. Various cell types undergoing necrosis and apoptosis or activated neutrophils forming neutrophil extracellular traps have been implicated in excessive release of histones which further augments tissue injury and may culminate in multiple organ failure. At the molecular level, an uncontrolled inflammatory cascade has been considered as the major event; however, histone-activated coagulation and thrombosis represent additional pathologic events reflecting coagulopathy. Furthermore, epigenetic regulation and chemical modifications of circulating histones appear to be critically important in their biological functions as evidenced by increased cytotoxicity associated with citrullinated histone. Herein, we will briefly review the current knowledge on the role of histones in acute respiratory distress syndrome and sepsis, and discuss the future potential of anti-histone therapy for treatment of these life-threatening disorders.

Expression and Purification of Full-Length and Domain-Fragment Recombinant Pentraxin 3 (PTX3) Proteins from Mammalian and Bacterial Cells

Although cell-based protein expression systems enable us a certain amount of protein suitable for subsequent biological experiments to be obtained, aggregates of the protein of interest are sometimes encountered during the purification procedure. Pentraxin 3 (PTX3), a member of the pentraxin family that is classified as a carbohydrate-binding protein based on its structure, comprises one of the humoral arms of the pattern recognition receptors that play an important role in the innate immune response. PTX3 comprises two domains; an N-terminal domain and a C-terminal domain. The C-terminal domain containing pentraxin signature has similar biological functions as other pentraxins such as C-reactive protein (CRP) and serum amyloid-P component (SAP). On the other side, the N-terminal domain is specific to PTX3. A supply of the PTX3 protein in full length or partial fragments is thus essential for the elucidation of its biological functions. Here we describe the expression and purification of recombinant PTX3. An arginine-containing buffer is essential for the elution of bacterially expressed PTX3 N-terminal domain to minimize aggregation. This method allows high-yield purification of full-length or domain-fragment recombinant PTX3 proteins for biological study.

The Long Pentraxin PTX3 as a Humoral Innate Immunity Functional Player and Biomarker of Infections and Sepsis

The first line of defense in innate immunity is provided by cellular and humoral mediators. Pentraxins are a superfamily of phylogenetically conserved humoral mediators of innate immunity. PTX3, the first long pentraxin identified, is a soluble pattern recognition molecule rapidly produced by several cell types in response to primary pro-inflammatory signals and microbial recognition. PTX3 acts as an important mediator of innate immunity against pathogens of fungal, bacterial and viral origin, and as a regulator of inflammation, by modulating complement activation and cell extravasation, and facilitating pathogen recognition by myeloid cells. In sepsis, PTX3 plasma levels are associated with severity of the condition, patient survival, and response to therapy. In combination with other established biomarkers, PTX3 could improve stratification of sepsis patients and thus, complement the system of classification and monitoring of this disease.

Calcineurin inhibitors reduce NFAT-dependent expression of antifungal pentraxin-3 by human monocytes

Calcineurin (CN) inhibitors are effective clinical immunosuppressants but leave patients vulnerable to potentially fatal fungal infections. This study tested the hypothesis that CN inhibition interferes with antifungal immune defenses mediated by monocytes. We showed that NFAT is expressed by human monocytes, and is activated by exposure to fungal ligands. We confirmed that NFAT translocation potently activated target gene transcription using a human monocytic reporter cell line. Inhibition of CN‐NFAT by cyclosporine A significantly reduced monocyte production of TNF‐α, IL‐10, and MCP‐1 proteins in response to pattern recognition receptor ligands as well as to Aspergillus fumigatus conidia. Moreover, we revealed that human monocytes express the antifungal protein pentraxin‐3 under control of NFAT. In conclusion, clinical CN inhibitors have the potential to interfere with the novel NFAT‐dependent pentraxin‐3 pathway as well as antifungal cytokine production in human monocytes, thereby impeding monocyte‐mediated defenses against fungal infection in immune‐suppressed patients.

Source of Circulating Pentraxin 3 in Septic Shock Patients

Sepsis, which is the leading cause of death in intensive care units (ICU), has been acknowledged as a global health priority by the WHO in 2017. Identification of biomarkers allowing early stratification and recognition of patients at higher risk of death is crucial. One promising biomarker candidate is pentraxin-3 (PTX3); initially elevated and persistently increased plasma concentration in septic patients has been associated with increased mortality. PTX3 is an acute phase protein mainly stored in neutrophil granules. These cells are responsible for rapid and prompt release of PTX3 in inflammatory context, but the cellular origin responsible for successive days' elevation in sepsis remains unknown. Upon inflammatory stimulation, PTX3 can also be produced by other cell types, including endothelial and immune cells. As in septic patients immune alterations have been described, we therefore sought to investigate whether such cells participated in the elevation of PTX3 over the first days after septic shock onset. To address this point, PTX3 was measured in plasma from septic shock patients at day 3 after ICU admission as well as in healthy volunteers (HV), and the capacity of whole blood cells to secrete PTX3 after inflammatory stimulation was evaluated ex vivo. A significantly mean higher (100-fold) concentration of plasma PTX3 was found in patients compared to HV, which was likely due to the inflammation-induced initial release of the pre-existing PTX3 reservoir contained in neutrophils. Strikingly, when whole blood was stimulated ex vivo with LPS no significant difference between patients and HV in PTX3 release was found. This was in contrast with TNFα which decreased production was illustrative of the endotoxin tolerance phenomenon occurring in septic patients. Then, the release of PTX3 protein from a HV neutrophil-free PBMC endotoxin tolerance model was investigated. At the transcriptional level, PTX3 seems to be a weakly tolerizable gene similar to TNFα. Conversely, increased protein levels observed in anergy condition reflects a non-tolerizable phenotype, more likely to an anti-inflammatory marker. Hence, altered immune cells still have the ability to produce PTX3 in response to an inflammatory trigger, and therefore circulating white blood cell subset could be responsible of the sustained PTX3 plasma levels over the first days of sepsis setting.

Endothelial Dysfunction Is Associated with Mortality and Severity of Coagulopathy in Patients with Sepsis and Disseminated Intravascular Coagulation

The role of the endothelium in sepsis-associated disseminated intravascular coagulation (DIC) is multifaceted and may contribute substantially to disease severity and outcome. The purpose of this study was to quantify measures of endothelial function, including markers of activation (endocan, Angiopoietin-2 [Ang-2], and von Willebrand Factor), endogenous anticoagulants (tissue factor pathway inhibitor and protein C), and damage-associated factors (High Mobility Group Box 1 [HMGB-1]) in the plasma of patients with sepsis and DIC, and to determine the relationship of these factors with severity of illness and outcome. Plasma samples were collected from 103 adult patients with sepsis within 48 hours of intensive care unit admission. Biomarker levels were measured using commercially available, standardized methods. Disseminated intravascular coagulation was diagnosed according to the International Society of Thrombosis and Hemostasis scoring algorithm. Twenty-eight-day mortality was used as the primary end point. In this study, endothelial damage and dysfunction were associated with the severity of coagulopathy and mortality in DIC patients. Loss of the endogenous anticoagulant protein C and elevation in the vascular regulator Ang-2 were associated with the development of overt DIC. In addition to Ang-2 and protein C, endocan, a biomarker of endothelial activation, and HMGB-1, a mediator of endothelial damage and activation, were significantly associated with mortality. This underscores the contribution of the endothelium to the pathogenesis of sepsis-associated DIC.

Biology, role and therapeutic potential of circulating histones in acute inflammatory disorders

Histones are positively charged nuclear proteins that facilitate packaging of DNA into nucleosomes common to all eukaryotic cells. Upon cell injury or cell signalling processes, histones are released passively through cell necrosis or actively from immune cells as part of extracellular traps. Extracellular histones function as microbicidal proteins and are pro‐thrombotic, limiting spread of infection or isolating areas of injury to allow for immune cell infiltration, clearance of infection and initiation of tissue regeneration and repair. Histone toxicity, however, is not specific to microbes and contributes to tissue and end‐organ injury, which in cases of systemic inflammation may lead to organ failure and death. This review details the processes of histones release in acute inflammation, the mechanisms of histone‐related tissue toxicity and current and future strategies for therapy targeting histones in acute inflammatory diseases.

The long pentraxin PTX3: A prototypical sensor of tissue injury and a regulator of homeostasis

Tissue damage frequently occurs. The immune system senses it and enforces homeostatic responses that lead to regeneration and repair. The synthesis of acute phase molecules is emerging as a crucial event in this program. The prototypic long pentraxin PTX3 orchestrates the recruitment of leukocytes, stabilizes the provisional matrix in order to facilitate leukocyte and stem progenitor cells trafficking, promotes swift and safe clearance of dying cells and of autoantigens, limiting autoimmunity and protecting the vasculature. These non-redundant actions of PTX3 are necessary for the resolution of inflammation. Recent studies have highlighted the mechanisms by which PTX3 adapts the functions of innate immune cells, orchestrates tissue repair and contributes to select the appropriate acquired immune response in various tissues. Conversely, PTX3 continues to be produced in diseases where the inflammatory response does not resolve. It is therefore a valuable biomarker for more precise and personalized stratification of patients, often independently predicting clinical evolution and outcome. There is strong promise for novel therapies based on understanding the mechanisms with which PTX3 plays its homeostatic role, especially in regulating leukocyte migration and the resolution of inflammatory processes.

DNA and factor VII-activating protease protect against the cytotoxicity of histones

Circulating histones have been implicated as major mediators of inflammatory disease because of their strong cytotoxic effects. Histones form the protein core of nucleosomes; however, it is unclear whether histones and nucleosomes are equally cytotoxic. Several plasma proteins that neutralize histones are present in plasma. Importantly, factor VII-activating protease (FSAP) is activated upon contact with histones and subsequently proteolyzes histones. We aimed to determine the effect of FSAP on the cytotoxicity of both histones and nucleosomes. Indeed, FSAP protected against histone-induced cytotoxicity of cultured cells in vitro. Upon incubation of serum with histones, endogenous FSAP was activated and degraded histones, which also prevented cytotoxicity. Notably, histones as part of nucleosome complexes were not cytotoxic, whereas DNA digestion restored cytotoxicity. Histones in nucleosomes were inefficiently cleaved by FSAP, which resulted in limited cleavage of histone H3 and removal of the N-terminal tail. The specific isolation of either circulating nucleosomes or free histones from sera of Escherichia coli challenged baboons or patients with meningococcal sepsis revealed that histone H3 was present in the form of nucleosomes, whereas free histone H3 was not detected. All samples showed signs of FSAP activation. Markedly, we observed that all histone H3 in nucleosomes from the patients with sepsis, and most histone H3 from the baboons, was N-terminally truncated, giving rise to a similarly sized protein fragment as through cleavage by FSAP. Taken together, our results suggest that DNA and FSAP jointly limit histone cytotoxicity and that free histone H3 does not circulate in appreciable concentrations in sepsis.

Acute ethanol intoxication suppresses pentraxin 3 expression in a mouse sepsis model involving cecal ligation and puncture

Acute ethanol intoxication impairs immunological reactions and increases the risk of sepsis; however, the underlying mechanism remains unclear. Pentraxin (PTX) 3 is a humoral pattern recognition receptor whose levels rapidly increase in response to inflammation. PTX3 production is triggered by tumor necrosis factor (TNF)-α and is mediated by c-Jun N-terminal kinase (JNK). As PTX3 exerts protective effects against sepsis as well as acute lung injury, we investigated whether acute ethanol exposure exacerbates sepsis by altering PTX3 expression. Sepsis was induced in C57/BL6 mice by cecal ligation and puncture (CLP) after ethanol/saline administration. Survival rates were significantly lower in ethanol-treated than in saline-treated mice. Increased vascular permeability and attenuation of PTX3 expression were observed in the lungs of ethanol-treated mice 4 h after CLP. Concomitant with a delayed increase of plasma TNF-α in ethanol-treated mice, plasma PTX3 was also suppressed in the early phase of sepsis. Although TNF-α level in ethanol-treated mice exceeded that in saline-treated mice 16 h after CLP, PTX3 levels were still suppressed in the former group. JNK phosphorylation in lung tissue was suppressed in both groups 4 and 16 h after CLP. Furthermore, JNK phosphorylation in ethanol-treated human umbilical vein endothelial cells was suppressed even in the presence of exogenous TNF-α, resulting in inhibition of PTX3 mRNA and protein expression. Our results suggest that ethanol suppresses de novo PTX3 synthesis via two mechanisms - i.e., suppression of TNF-α production and inhibition of JNK phosphorylation. PTX3 suppression may therefore contribute to exacerbation of sepsis in acute ethanol intoxication.

Pentraxins in the activation and regulation of innate immunity

Humoral fluid phase pattern recognition molecules (PRMs) are a key component of the activation and regulation of innate immunity. Humoral PRMs are diverse. We focused on the long pentraxin PTX3 as a paradigmatic example of fluid phase PRMs. PTX3 acts as a functional ancestor of antibodies and plays a non-redundant role in resistance against selected microbes in mouse and man and in the regulation of inflammation. This molecule interacts with complement components, thus modulating complement activation. In particular, PTX3 regulates complement-driven macrophage-mediated tumor progression, acting as an extrinsic oncosuppressor in preclinical models and selected human tumors. Evidence collected over the years suggests that PTX3 is a biomarker and potential therapeutic agent in humans, and pave the way to translation of this molecule into the clinic.

Competition between histone and transcription factor binding regulates the onset of transcription in zebrafish embryos

Upon fertilization, the genome of animal embryos remains transcriptionally inactive until the maternal-to-zygotic transition. At this time, the embryo takes control of its development and transcription begins. How the onset of zygotic transcription is regulated remains unclear. Here, we show that a dynamic competition for DNA binding between nucleosome-forming histones and transcription factors regulates zebrafish genome activation. Taking a quantitative approach, we found that the concentration of non-DNA-bound core histones sets the time for the onset of transcription. The reduction in nuclear histone concentration that coincides with genome activation does not affect nucleosome density on DNA, but allows transcription factors to compete successfully for DNA binding. In agreement with this, transcription factor binding is sensitive to histone levels and the concentration of transcription factors also affects the time of transcription. Our results demonstrate that the relative levels of histones and transcription factors regulate the onset of transcription in the embryo.

DOI:http://dx.doi.org/10.7554/eLife.23326.001

The DNA in a fertilized egg contains all the information required to form an animal’s body. In order for the animal to develop properly, particular genes encoded in the DNA are only active at specific times. The DNA is wrapped around proteins called histones, which allows the DNA to be tightly packed inside the cell. However, histones can block other proteins called transcription factors from binding to the DNA to activate the genes. Young embryos initially develop with all of their genes switched off, relying on the nutrients and other molecules provided by their mother. After some time, the embryo starts to switch on its own genes to take control of its own development, but it was not clear how this happens.

Joseph et al. investigated how genes are activated in zebrafish embryos, which are often used as models to study how animals develop. The experiments show that competition between histones and transcription factors for binding to DNA controls when genes are switched on. In young fish embryos, there are so many histones present that transcription factors have no opportunity to bind to DNA. Over time, however, the numbers of histones decrease, allowing transcription factors to bind to DNA and switch on genes.

Histones and transcription factors regulate the activity of genes throughout the life of the animal. Therefore, competition between these two types of protein may also control gene activity in other situations. A better understanding of how gene activity is controlled could allow researchers to more easily grow different types of cell in the laboratory or to reprogram specific cells in the body. As such, these new findings may aid the development of therapies to regenerate organs or tissues that have been damaged by injury or disease.

DOI:http://dx.doi.org/10.7554/eLife.23326.002

Depletion of myeloid cells exacerbates hepatitis and induces an aberrant increase in histone H3 in mouse serum

Tissue-resident macrophages and bone marrow (BM)-derived monocytes play a crucial role in the maintenance of tissue homeostasis; however, their contribution to recovery from acute tissue injury is not fully understood. To address this issue, we generated an acute murine liver injury model using hepatocyte-specific Cflar-deficient (Cflar^Hep-low ) mice. Cellular FLICE-inhibitory protein expression was down-regulated in Cflar-deficient hepatocytes, which thereby increased susceptibility of hepatocytes to death receptor-induced apoptosis. Cflar^Hep-low mice developed acute hepatitis and recovered with clearance of apoptotic hepatocytes at 24 hours after injection of low doses of tumor necrosis factor α (TNFα), which could not induce hepatitis in wild-type (WT) mice. Depletion of Kupffer cells (KCs) by clodronate liposomes did not impair clearance of dying hepatocytes or exacerbate hepatitis in Cflar^Hep-low mice. To elucidate the roles of BM-derived monocytes and neutrophils in clearance of apoptotic hepatocytes, we examined the effect of depletion of these cells on TNFα-induced hepatitis in Cflar^Hep-low mice. We reconstituted Cflar^Hep-low mice with BM cells from transgenic mice in which human diphtheria toxin receptor (DTR) was expressed under control of the lysozyme M (LysM) promoter. TNFα-induced infiltration of myeloid cells, including monocytes and neutrophils, was completely ablated in LysM-DTR BM-reconstituted Cflar^Hep-low mice pretreated with diphtheria toxin, whereas KCs remained present in the livers. Under these experimental conditions, LysM-DTR BM-reconstituted Cflar^Hep-low mice rapidly developed severe hepatitis and succumbed within several hours of TNFα injection. We found that serum interleukin-6 (IL-6), TNFα, and histone H3 were aberrantly increased in LysM-DTR BM-reconstituted, but not in WT BM-reconstituted, Cflar^Hep-low mice following TNFα injection.

CONCLUSION

These findings indicate an unexpected role of myeloid cells in decreasing serum IL-6, TNFα, and histone H3 levels via the suppression of TNFα-induced hepatocyte apoptosis. (Hepatology 2017;65:237-252).

The Protective Effect against Extracellular Histones Afforded by Long-Pentraxin PTX3 as a Regulator of NETs

Pentraxin 3 (PTX3) is a soluble pattern recognition molecule that plays critical roles in innate immunity. Its fundamental functions include recognition of microbes, activation of complement cascades, and opsonization. The findings that PTX3 is one of the component proteins in neutrophil extracellular traps (NETs) and binds with other NET proteins imply the importance of PTX3 in the NET-mediated trapping and killing of bacteria. As NETs play certain critically important host-protective roles, aberrant NET production results in tissue damage. Extracellular histones, the main source of which is considered to be NETs, are mediators of septic death due to their cytotoxicity toward endothelial cells. PTX3 protects against extracellular histones-mediated cytotoxicity through coaggregation. In addition to the anti-bacterial roles performed in coordination with other NET proteins, PTX3 appears to mitigate the detrimental effect of over-activated NETs. A better understanding of the role of the PTX3 complexes in NETs would be expected to lead to new strategies for maintaining a healthy balance between the helpful bactericidal and undesirable detrimental activities of NETs.

The Immune System in Tissue Environments Regaining Homeostasis after Injury: Is "Inflammation" Always Inflammation?

Inflammation is a response to infections or tissue injuries. Inflammation was once defined by clinical signs, later by the presence of leukocytes, and nowadays by expression of "proinflammatory" cytokines and chemokines. But leukocytes and cytokines often have rather anti-inflammatory, proregenerative, and homeostatic effects. Is there a need to redefine "inflammation"? In this review, we discuss the functions of "inflammatory" mediators/regulators of the innate immune system that determine tissue environments to fulfill the need of the tissue while regaining homeostasis after injury.

Pentraxin-3 Attenuates Renal Damage in Diabetic Nephropathy by Promoting M2 Macrophage Differentiation

As one of the most important long-term complications of diabetes, diabetic nephropathy (DN) is the major cause of end-stage renal disease and high mortality in diabetic patients. The long pentraxin 3 (Ptx3) is a member of a superfamily of conserved proteins characterized by a cyclic multimeric structure and a conserved C-terminal domain. Several clinical investigations have demonstrated that elevated plasma Ptx3 levels are associated with cardiovascular and chronic kidney diseases (CKD). However, the therapeutic effect of Ptx3 on DN has never been investigated. In our current study, we showed a crucial role for Ptx3 in attenuating renal damage in DN. In our mouse hyperglycemia-induced nephropathy model, Ptx3 treatment showed significantly increased expression of nephrin, acetylated nephrin, and Wilm's tumor-1 protein (WT-1) when compared with control. The number of CD4(+) T cells, CD8(+) T cells, Ly6G(+) neutrophils, and CD11b(+) macrophages were all significantly lower in the Ptx3-treated group than that in the control group in DN. The IL-4 and IL-13 levels in the Ptx3-treated group were markedly higher than that in the control group in DN. Correspondingly, the Ptx3-treated group showed increased numbers of Arg1- or CD206-expressing macrophages compared with the control group. Furthermore, inhibition of Ptx3-treated macrophages abrogated the alleviated renal damage induced by Ptx3 treatment. In conclusion, Ptx3 attenuates renal damage in DN by promoting M2 macrophage differentiation.

Pentraxin 3: an immune modulator of infection and useful marker for disease severity assessment in sepsis

The acute phase protein pentraxin 3 (PTX3) is a pattern recognition receptor involved in regulation of the host immune response. This relatively newly discovered member of the pentraxin superfamily elicits both immunostimulatory and immunoregulatory functions preventing autoimmune pathology and orchestrated clearance of pathogens through opsonization of damage- and pathogen-associated molecular patterns (DAMP/PAMP). Thus, PTX3 has been described as a possible evolutionary precursor to immunoglobulins. While shown to provide protection against specific bacterial and fungal pathogens, persistent elevation of PTX3 levels following initial onset of infection appear to predict poor patient outcome and may contribute to disease sequelae such as tissue damage and coagulopathy. Measurement of PTX3 following onset of sepsis may improve patient risk assessment and thus be useful in guiding subsequent therapeutic interventions including steroidal anti-inflammatory and altered antibiotic therapies. In this review, we summarize the role of PTX3 in inflammatory syndromes and its utility as a marker of sepsis disease severity.

Therapeutic Targets in Sepsis: Past, Present, and Future

Antibiotics and fluids have been standard treatment for sepsis since World War II. Many molecular mediators of septic shock have since been identified. In models of sepsis, blocking these mediators improved organ injury and decreased mortality. Clinical trials, however, have failed. The absence of new therapies has been vexing to clinicians, clinical researchers, basic scientists, and the pharmaceutical industry. This article examines the evolution of sepsis therapy and theorizes about why so many well-reasoned therapies have not worked in human trials. We review new molecular targets for sepsis and examine trial designs that might lead to successful treatments for sepsis.

Circulating Extracellular Histones Are Clinically Relevant Mediators of Multiple Organ Injury

Extracellular histones are a damage-associated molecular pattern (DAMP) involved in the pathogenesis of various diseases. The mechanisms of histone-mediated injury in certain organs have been extensively studied, but an understanding of the pathophysiological role of histone-mediated injury in multiple organ injury remains elusive. To elucidate this role, we systemically subjected C57BL/6 mice to various doses of histones and performed a chronological evaluation of the morphological and functional changes in the lungs, liver, and kidneys. Notably, histone administration ultimately led to death after a dose-dependent aggravation of multiple organ injury. In chronological studies, pulmonary and hepatic injuries occurred within 15 minutes, whereas renal injuries presented at a later phase, suggesting that susceptibility to extracellular histones varies among organs. Histones bound to pulmonary and hepatic endothelial cells immediately after administration, leading to endothelial damage, which could be ameliorated by pretreatment with heparin. Furthermore, release of another DAMP, high-mobility group protein box 1, followed the histone-induced tissue damage, and an antibody against the molecule ameliorated hepatic and renal failure in a late phase. These findings indicate that extracellular histones induce multiple organ injury in two progressive stages-direct injury to endothelial cells and the subsequent release of other DAMPs-and that combination therapies against extracellular histones and high-mobility group protein box 1 may be a promising strategy for treating multiple organ injury.

Sepsis and ARDS: The Dark Side of Histones

Despite advances in management over the last several decades, sepsis and acute respiratory distress syndrome (ARDS) still remain major clinical challenges and the leading causes of death for patients in intensive care units (ICUs) due to insufficient understanding of the pathophysiological mechanisms of these diseases. However, recent studies have shown that histones, also known as chromatin-basic structure proteins, could be released into the extracellular space during severe stress and physical challenges to the body (e.g., sepsis and ARDS). Due to their cytotoxic and proinflammatory effects, extracellular histones can lead to excessive and overwhelming cell damage and death, thus contributing to the pathogenesis of both sepsis and ARDS. In addition, antihistone-based treatments (e.g., neutralizing antibodies, activated protein C, and heparin) have shown protective effects and have significantly improved the outcomes of mice suffering from sepsis and ARDS. Here, we review researches related to the pathological role of histone in context of sepsis and ARDS and evaluate the potential value of histones as biomarkers and therapeutic targets of these diseases.

The long pentraxin PTX3 promotes fibrocyte differentiation

Monocyte-derived, fibroblast-like cells called fibrocytes are associated with fibrotic lesions. The plasma protein serum amyloid P component (SAP; also known as pentraxin-2, PTX2) inhibits fibrocyte differentiation in vitro, and injections of SAP inhibit fibrosis in vivo. SAP is a member of the pentraxin family of proteins that includes C-reactive protein (CRP; PTX1) and pentraxin-3 (PTX3). All three pentraxins are associated with fibrosis, but only SAP and CRP have been studied for their effects on fibrocyte differentiation. We find that compared to SAP and CRP, PTX3 promotes human and murine fibrocyte differentiation. The effect of PTX3 is dependent on FcγRI. In competition studies, the fibrocyte-inhibitory activity of SAP is dominant over PTX3. Binding competition studies indicate that SAP and PTX3 bind human FcγRI at different sites. In murine models of lung fibrosis, PTX3 is present in fibrotic areas, and the PTX3 distribution is associated with collagen deposition. In lung tissue from pulmonary fibrosis patients, PTX3 has a widespread distribution, both in unaffected tissue and in fibrotic lesions, whereas SAP is restricted to areas adjacent to vessels, and absent from fibrotic areas. These data suggest that the relative levels of SAP and PTX3 present at sites of fibrosis may have a significant effect on the ability of monocytes to differentiate into fibrocytes.

Inter-α inhibitor protein and its associated glycosaminoglycans protect against histone-induced injury

Extracellular histones are mediators of tissue injury and organ dysfunction; therefore they constitute potential therapeutic targets in sepsis, inflammation, and thrombosis. Histone cytotoxicity in vitro decreases in the presence of plasma. Here, we demonstrate that plasma inter-α inhibitor protein (IAIP) neutralizes the cytotoxic effects of histones and decreases histone-induced platelet aggregation. These effects are mediated through the negatively charged glycosaminoglycans (GAGs) chondroitin sulfate and high-molecular-weight hyaluronan (HMW-HA) associated with IAIP. Cell surface anionic glycosaminoglycans heparan sulfate and HA protect the cells against histone-mediated damage in vitro. Surface plasmon resonance showed that both IAIP and HMW-HA directly bind to recombinant histone H4. In vivo neutralization of histones with IAIP and HMW-HA prevented histone-induced thrombocytopenia, bleeding, and lung microvascular thrombosis, decreased neutrophil activation, and averted histone-induced production of inflammatory cytokines and chemokines. IAIP and HMW-HA colocalized with histones in necrotic tissues and areas that displayed neutrophil extracellular traps. Increasing amounts of IAIP-histone complexes detected in the plasma of septic baboons correlated with increase in histones and/or nucleosomes and consumption of plasma IAIP. Our data suggest that IAIP, chondroitin sulfate, and HMW-HA are potential therapeutic agents to protect against histone-induced cytotoxicity, coagulopathy, systemic inflammation, and organ damage during inflammatory conditions such as sepsis and trauma.