Structure and binding properties of monoclonal antibodies to core histones from autoimmune mice

The Impact of Extracellular Histones and Absence of Toll-like Receptors on Cardiac Functional and Electrical Disturbances in Mouse Hearts

In polymicrobial sepsis, the extracellular histones, mainly released from activated neutrophils, significantly contribute to cardiac dysfunction (septic cardiomyopathy), as demonstrated in our previous studies using Echo-Doppler measurements. This study aims to elucidate the roles of extracellular histones and their interactions with Toll-like receptors (TLRs) in cardiac dysfunction. Through ex vivo assessments of ECG, left ventricle (LV) function parameters, and in vivo Echo-Doppler studies in mice perfused with extracellular histones, we aim to provide comprehensive insights into the mechanisms underlying sepsis-induced cardiac dysfunction. Langendorff-perfused hearts from both wild-type and TLR2, TLR3, or TLR4 knockout (KO) mice were examined. Paced mouse hearts were perfused with histones to assess contractility and relaxation. Echo-Doppler studies evaluated cardiac dysfunction after intravenous histone injection. Histone perfusion caused defects in contractility and relaxation, with TLR2 and TLR3 KO mice being partially protected. Specifically, TLR2 KO mice exhibited the greatest reduction in Echo-Doppler abnormalities, while TLR4 KO exacerbated cardiac dysfunction. Among individual histones, H1 induced the most pronounced abnormalities in cardiac function, apoptosis of cardiomyocytes, and LDH release. Our data highlight significant interactions between histones and TLRs, providing insights into histones especially H1 as potential therapeutic targets for septic cardiomyopathy. Further studies are needed to explore specific histone-TLR interactions and their mechanisms.

Extracellular histones exacerbate heat stroke-induced liver injury by triggering hepatocyte pyroptosis and liver injury via the TLR9-NLRP3 pathway

BACKGROUND

Severe heat stroke is often complicated by multiple organ failure, including liver injury. Recent evidence indicates that the underlying mechanism constitutes sterile inflammation triggered by cell damage, in which hepatocyte NOD-like receptor family pyrin domain-containing 3 inflammasome activation and pyroptosis play key roles. As extracellular histones act as damage-associated molecular patterns and mediate tissue toxicity and inflammation, we aimed to investigate whether extracellular histones contribute to inducing hepatocyte pyroptosis following heat stroke, promoting the development of liver inflammation and injury, and elucidate the potential underlying mechanisms.

METHODS

Exogenous histones were administered to AML-12 murine hepatocytes or male aged 8-12 week mice following hyperthermic treatment (at 39 °C in a chamber with 60 % relative humidity). Prior to heat exposure, endogenous histones were neutralized using neutralizing antibodies, inflammasomes were inhibited by RNA silencing, and Toll-like receptor 9 was modulated using a pharmacological agonist or antagonist. Inflammasome assembly, caspase-1 activation, histological changes, and liver enzyme levels were measured. Statistical comparison of more than two groups was performed using one-way ANOVA with Tukey's post-hoc testing. The correlations were analyzed using Pearson's correlation test. All experiments were repeated thrice. A p-value < 0.05 was considered significant.

RESULTS

Heat stroke induced histone release into the extracellular space at levels correlating with liver injury. Moreover, extracellular histones augmented heat stroke-induced liver injury both in vitro and in vivo in a dose- and time-dependent manner, whereas neutralizing histones conferred protection following heat stroke. Histones mediated NOD-like receptor family pyrin domain-containing 3 inflammasome activation through the Toll-like receptor 9 signaling pathway, which resulted in hepatocyte pyroptosis and liver inflammation.

CONCLUSIONS

Our findings show that histones are critical mediators of hepatocyte pyroptosis that aggravate liver injury in a heat stroke setting. Therefore, we suggest extracellular histones as potential therapeutic targets to limit heat stroke-induced cell death and liver injury.

Externalized histones fuel pulmonary fibrosis via a platelet-macrophage circuit of TGFβ1 and IL-27

Externalized histones erupt from the nucleus as extracellular traps, are associated with several acute and chronic lung disorders, but their implications in the molecular pathogenesis of interstitial lung disease are incompletely defined. To investigate the role and molecular mechanisms of externalized histones within the immunologic networks of pulmonary fibrosis, we studied externalized histones in human and animal bronchoalveolar lavage (BAL) samples of lung fibrosis. Neutralizing anti-histone antibodies were administered in bleomycin-induced fibrosis of C57BL/6 J mice, and subsequent studies used conditional/constitutive knockout mouse strains for TGFβ and IL-27 signaling along with isolated platelets and cultured macrophages. We found that externalized histones (citH3) were significantly (P < 0.01) increased in cell-free BAL fluids of patients with idiopathic pulmonary fibrosis (IPF; n = 29) as compared to healthy controls (n = 10). The pulmonary sources of externalized histones were Ly6G+CD11b+ neutrophils and nonhematopoietic cells after bleomycin in mice. Neutralizing monoclonal anti-histone H2A/H4 antibodies reduced the pulmonary collagen accumulation and hydroxyproline concentration. Histones activated platelets to release TGFβ1, which signaled through the TGFbRI/TGFbRII receptor complex on LysM+ cells to antagonize macrophage-derived IL-27 production. TGFβ1 evoked multiple downstream mechanisms in macrophages, including p38 MAPK, tristetraprolin, IL-10, and binding of SMAD3 to the IL-27 promotor regions. IL-27RA-deficient mice displayed more severe collagen depositions suggesting that intact IL-27 signaling limits fibrosis. In conclusion, externalized histones inactivate a safety switch of antifibrotic, macrophage-derived IL-27 by boosting platelet-derived TGFβ1. Externalized histones are accessible to neutralizing antibodies for improving the severity of experimental pulmonary fibrosis.

Extracellular Histone-Induced Protein Kinase C Alpha Activation and Troponin Phosphorylation Is a Potential Mechanism of Cardiac Contractility Depression in Sepsis

Reduction in cardiac contractility is common in severe sepsis. However, the pathological mechanism is still not fully understood. Recently it has been found that circulating histones released after extensive immune cell death play important roles in multiple organ injury and disfunction, particularly in cardiomyocyte injury and contractility reduction. How extracellular histones cause cardiac contractility depression is still not fully clear. In this work, using cultured cardiomyocytes and a histone infusion mouse model, we demonstrate that clinically relevant histone concentrations cause significant increases in intracellular calcium concentrations with subsequent activation and enriched localization of calcium-dependent protein kinase C (PKC) α and βII into the myofilament fraction of cardiomyocytes in vitro and in vivo. Furthermore, histones induced dose-dependent phosphorylation of cardiac troponin I (cTnI) at the PKC-regulated phosphorylation residues (S43 and T144) in cultured cardiomyocytes, which was also confirmed in murine cardiomyocytes following intravenous histone injection. Specific inhibitors against PKCα and PKCβII revealed that histone-induced cTnI phosphorylation was mainly mediated by PKCα activation, but not PKCβII. Blocking PKCα also significantly abrogated histone-induced deterioration in peak shortening, duration and the velocity of shortening, and re-lengthening of cardiomyocyte contractility. These in vitro and in vivo findings collectively indicate a potential mechanism of histone-induced cardiomyocyte dysfunction driven by PKCα activation with subsequent enhanced phosphorylation of cTnI. These findings also indicate a potential mechanism of clinical cardiac dysfunction in sepsis and other critical illnesses with high levels of circulating histones, which holds the potential translational benefit to these patients by targeting circulating histones and downstream pathways.

Damage associated molecular patterns and neutrophil extracellular traps in acute pancreatitis

Previous researches have emphasized a trypsin-centered theory of acute pancreatitis (AP) for more than a century. With additional studies into the pathogenesis of AP, new mechanisms have been explored. Among them, the role of immune response bears great importance. Pro-inflammatory substances, especially damage-associated molecular patterns (DAMPs), play an essential role in activating, signaling, and steering inflammation. Meanwhile, activated neutrophils attach great importance to the immune defense by forming neutrophil extracellular traps (NETs), which cause ductal obstruction, premature trypsinogen activation, and modulate inflammation. In this review, we discuss the latest advances in understanding the pathological role of DAMPs and NETs in AP and shed light on the flexible crosstalk between these vital inflammatory mediators. We, then highlight the potentially promising treatment for AP targeting DAMPs and NETs, with a focus on novel insights into the mechanism, diagnosis, and management of AP.

Histone H4 induces heparan sulfate degradation by activating heparanase in chlorine gas-induced acute respiratory distress syndrome

Background

Heparan sulfate (HS) degradation mediates pulmonary endothelial hyper-permeability and acute pulmonary edema during acute respiratory distress syndrome (ARDS). The aim of this study was to examine whether histone H4 induced HS degradation by activating heparanase (HPSE) in chlorine gas (Cl₂)-induced ARDS.

Methods

Acute lung injury was induced by Cl₂ exposure or histone H4 injection in C57BL/6 mice. Histone H4 in bronchoalveolar lavage fluid (BALF) and plasma was measured by ELISA. HS degradation was measured by immunostaining, ELISA, and flow cytometry. HPSE mRNA and protein were measured by real-time qPCR and western blot analysis, respectively, at preset timepoints. The HPSE inhibitor OGT2115 and specific siRNAs were used to study the role of HPSE during HS degradation caused by Cl₂ exposure or histone H4 challenge. Blocking antibodies against TLR1, TLR2, TLR4, or TLR6 were used in vitro to investigate which signaling pathway was involved. The transcriptional regulation of HPSE was studied vis-à-vis NF-κB, which was assessed by nuclear translocation of NF-κB p65 and phosphorylation of I-κBα protein.

Results

Histone H4 in BALF and plasma increased evidently after Cl₂ inhalation. Cl₂ exposure or histone H4 challenge caused obvious acute lung injury in mice, and the pulmonary glycocalyx was degraded evidently as observed from endothelial HS staining and measurement of plasma HS fragments. Pretreatment with OGT2115, an HPSE inhibitor, relieved the acute lung injury and HS degradation caused by Cl₂ exposure or histone H4 challenge. Targeted knockdown of HPSE by RNA interference (RNAi) significantly inhibited histone H4 induced HS degradation in HPMECs, as measured by immunofluorescence and flow cytometry. By inducing phosphorylation of I-κB α and nuclear translocation of NF-κB p65, histone H4 directly promoted mRNA transcription and protein expression of HPSE in a dose-dependent manner. Additionally, a blocking antibody against TLR4 markedly inhibited both activation of NF-κB and expression of HPSE induced by histone H4.

Conclusions

Histone H4 is a major pro-inflammatory mediator in Cl₂-induced ARDS in mice, and induces HS degradation by activating HPSE via TLRs- and NF-κB-signaling pathways.

Extracellular Histones Promote Pulmonary Fibrosis in Patients With Coal Workers' Pneumoconiosis

OBJECTIVE

This investigation assessed the profibrotic role that extracellular histones play in the pathogenesis of coal workers' pneumoconiosis (CWP).

METHODS

The correlation of extracellular histones with small opacity profusion (SOP) and transforming growth factor-β (TGF-β) was analyzed. The stimulating effect of extracellular histones on pulmonary fibroblast was assessed in vitro.

RESULTS

The levels of extracellular histones in plasma were positively correlated with SOP and TGF-β in the coal miners investigated. Plasma collected from patients with CWP caused apparent lung fibroblast proliferation, while anti-H4 antibody antagonized the stimulating effect of the patient plasma by blocking histone H4. In vitro experiments showed that extracellular histones directly stimulated fibroblast proliferation.

CONCLUSION

Consistent with our hypothesis, the concentrations of extracellular histones were indices of the severity of pulmonary fibrosis in simple CWP, and extracellular histones-targeted intervention could inhibit the proliferation of lung fibroblast.

Histone H4 aggravates inflammatory injury through TLR4 in chlorine gas-induced acute respiratory distress syndrome

Background

Chlorine gas (Cl₂) exposure remains a public health concern in household, occupational, and transportation accidents around the world. The death rate associated with acute respiratory distress syndrome (ARDS) caused by high concentrations of Cl₂ is very high, mainly because the pathogenesis of ARDS remains unclear. Histone H4 has been identified as an important endogenous pro-inflammatory molecule. The present study aimed to examine the pathogenic role of histone H4 in Cl₂-induced ARDS.

Methods

ARDS was induced by Cl₂ exposure in male C57BL/6 mice. Circulating histone H4, blood gas, pulmonary edema, endothelial activation, and neutrophil infiltration were measured during acute lung injury (ALI). Histone H4 or anti-H4 antibody was administered through the tail vein 1 h prior to Cl₂ exposure to study the pathogenic role of histone H4. Toll-like receptor 2 knock-out (Tlr2-KO) and Tlr4-KO mice were used in conjunction with blocking antibody against TLR1, TLR2, TLR4, or TLR6 to explore the mechanism involved in histone H4-mediated injury.

Results

Cl₂ exposure induced a concentration-dependent ALI. The levels of circulating histone H4 were positively correlated with Cl₂ concentrations. Pretreatment with intravenous histone H4 further aggravated lethality rate, blood gas, endothelial activation, and neutrophil infiltration, while anti-H4 antibody showed protective effects. Tlr4 deficiency improved lethality rate, blood gas, and pulmonary edema, and prevented endothelial and neutrophil activation caused by Cl₂ exposure. More importantly, Tlr4 gene deletion greatly diminished the effect of histone H4 or anti-H4 antibody observed in wild-type (WT) mice. The impact of Tlr2 on inflammatory injury was not significant. The role of TLRs was also validated by endothelial activation mediated by histone H4 in vitro.

Conclusions

Circulating histone H4 played a pro-inflammatory role in ARDS caused by Cl₂. TLR4 was closely involved in histone H4-mediated inflammatory injury. Therefore, intervention targeting histone H4 is potentially protective.

Quantitative proteomic analysis in serum of Takifugu rubripes infected with Cryptocaryon irritans

Cryptocaryon irritans can cause cryptocaryonosis (white spot disease) in marine fish but the pathogenesis of the disease is unclear. In this work, we used high-throughput proteomics to identify differentially expressed proteins in the serum of Takifugu rubripes challenged with C. irritans. By using quantitative proteomic assays combined with Tandem Mass Tag-labeled quantitative proteomic analysis, we identified a total of 2088 differentially abundant proteins (1706 proteins were quantified, p < 0.05, fold-change threshold ≥ 2), including 21 up-regulated and 44 down-regulated. Combined with STRING-based functional analysis, we ultimately obtained eight proteins including glucokinase-like, integrin beta-1-like isoform X2, H4, H2A.V, histone H1-like, histone H2AX-like, histone H2B 1/2-like and myosin-9 isoform X1, which could be considered as potential biomarkers for T. rubripes immune responses. Eight proteins that were selected to validate significant differentially expressed genes at the proteomic level were consistent with qPCR at the transcriptomic level. The proteins identified in our work may serve as candidates for elucidating the molecular mechanism of cryptocaryonosis in T. rubripes. Our collective findings could provide new insights into searching for disease-specific targets and biomarkers, which may be effective indicators of C. irritans infection in T. rubripes.

Complement as a Major Inducer of Harmful Events in Infectious Sepsis

There is abundant evidence that infectious sepsis both in humans and mice with polymicrobial sepsis results in robust activation of complement. Major complement activation products involved in sepsis include C5a anaphylatoxin and its receptors (C5aR1 and C5aR2) and, perhaps, the terminal complement activation product, C5b-9. These products (and others) also cause dysfunction of the innate immune system, with exaggerated early proinflammatory responses, followed by decline of the innate immune system, leading to immunosuppression and multiorgan dysfunction. Generation of C5a during sepsis also leads to activation of neutrophils and macrophages and ultimate appearance of extracellular histones, which have powerful proinflammatory and prothrombotic activities. The distal complement activation product, C5b-9, triggers intracellular Ca fluxes in epithelial and endothelial cells. Histones activate the NLRP3 inflammasome, products of which can damage cells. C5a also activates MAPKs and Akt signaling pathways in cardiomyocytes, causing buildup of [Ca]i, defective action potentials and substantial cell dysfunction, resulting in cardiac and other organ dysfunction. Cardiac dysfunction can be quantitated by ECHO-Doppler parameters. In vivo interventions that block these complement-dependent products responsible for organ dysfunction in sepsis reduce the intensity of sepsis. The obvious targets in sepsis are C5a and its receptors, histones, and perhaps the MAPK pathways. Blockade of C5 has been considered in sepsis, but the FDA-approved antibody (eculizumab) is known to compromise defenses against neisseria and pneumonococcal bacteria, and requires immunization before the mAb to C5 can be used clinically. Small molecular blocking agents for C5aRs are currently in development and may be therapeutically effective for treatment of sepsis.

Heparin improves alveolarization and vascular development in hyperoxia-induced bronchopulmonary dysplasia by inhibiting neutrophil extracellular traps

The objective of this study was to assess the role of NETs in BPD of hyperoxia-induced rat model and the effect of heparin on alveolarization and vascular development in BPD. The neonatal rats exposed to 90% oxygen continuously for 7 days to mimic BPD, meanwhile, the rats were injected by different doses of histones to evaluate the impact on lung injury. The newborn rats exposed to hyperoxia were injected by different doses of heparin (250 U/kg, 500 U/kg) or anti-H4 antibody to evaluate the effect of heparin. Histones and hyperoxia impaired alveolarization with the increase of mean linear intercept (MLI) and the decrease of radial alveolar count (RAC), decreased lung angiogenesis with the decrease expression of VEGF, and increased the expression of NETs, histones and pro-inflammatory factor. However, low dose heparin (250U/kg) administration enhanced survival, improved alveolarization and vascular development in hyperoxia-induced BPD, as well as reduced expression of NETs, histones and pro-inflammatory factor. We concluded that heparin improves alveolarization and vascularization in BPD by inhibiting NETs.

Evidence for the important role of inflammation in xenotransplantation

There is increasing evidence of a sustained state of systemic inflammation after pig-to-nonhuman primate (NHP) xenotransplantation (that has been termed systemic inflammation in xenograft recipients [SIXR]). Increases in inflammatory markers, e.g., C-reactive protein, histones, serum amyloid A, D-dimer, cytokines, chemokines, and a decrease in free triiodothyronine, have been demonstrated in the recipient NHPs. The complex interactions between inflammation, coagulation, and the immune response are well-recognized, but the role of inflammation in xenograft recipients is not fully understood. The evidence suggests that inflammation can promote the activation of coagulation and the adaptive immune response, but the exact mechanisms remain uncertain. If prolonged xenograft survival is to be achieved, anti-inflammatory strategies (e.g., the administration of anti-inflammatory agents, and/or the generation of genetically-engineered organ-source pigs that are protected from the effect of inflammation) may be necessary to prevent, control, or negate the effect of the systemic inflammation that develops in xenograft recipients. This may allow for a reduction in the intensity of exogenous immunosuppressive therapy. If immunological tolerance to a xenograft is to be obtained, then control of inflammation may be essential.

An Investigation of Extracellular Histones in Pig-To-Baboon Organ Xenotransplantation

BACKGROUND

Serum (extracellular) histone levels are increased in inflammatory states and in the presence of coagulation dysfunction, for example, trauma, chemical/ischemic injury, infection. There is increasing evidence of a systemic inflammatory response associated with the presence of a pig xenograft in a nonhuman primate. We evaluated extracellular histone levels in baboons with various pig xenografts.

METHODS

We measured serum histones in baboons with pig heterotopic heart (n = 8), life-supporting kidney (n = 5), orthotopic liver (n = 4), and artery patch (n = 9) grafts by enzyme-linked immunosorbent assay. C-reactive protein (CRP), free triiodothyronine (fT3), serum amyloid A (SAA), and platelet counts were also measured, all of which may provide an indication of an inflammatory state. We investigated the effect of histones on platelet aggregation and on cytotoxicity of pig cells in vitro.

RESULTS

Serum histones increased when baboons developed consumptive coagulopathy (eg, thrombocytopenia) or infection. CRP levels tended to be higher and fT3 levels lower when consumptive coagulopathy developed. Measurement of SAA correlated fairly well with CRP and indicated the state of inflammation. Treatment of the recipient with tocilizumab reduced the level of serum histones, CRP, and SAA, and increased the level of fT3 and platelet counts. In vitro, histone-induced platelet aggregation and endothelial cell apoptosis were both significantly reduced by the NF-κB pathway inhibitor, parthenolide.

CONCLUSIONS

These noninvasive assays may be useful for monitoring the health status of nonhuman primate recipients of pig organ grafts and may help in management after xenotransplantation. Tocilizumab and NF-κB inhibitors might prove valuable in reducing the inflammatory response to a pig xenograft.

B Cell Tolerance to Deiminated Histones in BALB/c, C57BL/6, and Autoimmune-Prone Mouse Strains

Deimination, a posttranslational modification of arginine to citrulline carried out by peptidylarginine deiminases, may compromise tolerance of self-antigens. Patients with connective tissue autoimmunity, particularly rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), or Felty’s syndrome, present with autoantibodies to deiminated histones (dH), which thus form a category of antibodies to citrullinated protein antigens (ACPA). In general, ACPA are a sensitive diagnostic for RA and may form in response to the release of nuclear chromatin (DNA plus dH) from granulocytes, usually referred to as neutrophil extracellular traps. The aim of this study was to examine spontaneously autoimmune mice for autoantibodies and T cell responses to dH. We compared IgG binding to deiminated and non-deiminated histones (nH) by ELISA and Western blotting in spontaneously autoimmune strains of (NZB × NZW) F₁ and NZM2410 together with their derivative congenic strains, C57BL/6.Sle1 and C57BL/6.Sle1.Sle3, which display profound autoreactivity against nuclear self-antigens. The splenocyte proliferation against the two antigens was determined in the spontaneously autoimmune (NZB × NZW) F₁ strain from which other autoimmune strains used in the study were derived. Immunizations with dH and nH were attempted in BALB/c mice to assess their splenocyte response. Splenocytes from BALB/c mice and from autoimmune mice at the time of conversion to autoimmunity proliferated strongly in response to dH, yet serum IgG from autoimmune (NZB × NZW) F₁, NZM2410, and C57BL/6.Sle1.Sle3 mice displayed a remarkable bias against binding to dH. At the time of seroconversion, the antibodies already exhibited preference for nH, and only nH were recovered from circulating immune complexes. Analysis of histone deimination showed constitutive deimination in thymic extracts from C57BL/6 and C57BL/6.Sle1.Sle2.Sle3 triply congenic mice and in spleens of autoimmune triply congenic mice. Our study demonstrates that tolerance mechanisms against dH are intact in BALB/c and C57BL/6 mice and continue to be effective in mice with overt autoimmunity to nH. We conclude that, in contrast to human RA and SLE patients, where we frequently observe autoantibodies against dH, autoimmune mice maintain strong tolerance mechanisms to prevent the development of autoantibodies to dH.

Pulmonary endothelial activation caused by extracellular histones contributes to neutrophil activation in acute respiratory distress syndrome

BACKGROUND

During the acute respiratory distress syndrome (ARDS), neutrophils play a central role in the pathogenesis, and their activation requires interaction with the endothelium. Extracellular histones have been recognized as pivotal inflammatory mediators. This study was to investigate the role of pulmonary endothelial activation during the extracellular histone-induced inflammatory response in ARDS.

METHODS

ARDS was induced in male C57BL/6 mice by intravenous injection with lipopolysaccharide (LPS) or exogenous histones. Concurrent with LPS administration, anti-histone H4 antibody (anti-H4) or non-specific IgG was administered to study the role of extracellular histones. The circulating von Willebrand factor (vWF) and soluble thrombomodulin (sTM) were measured with ELISA kits at the preset time points. Myeloperoxidase (MPO) activity in lung tissue was measured with a MPO detection kit. The translocation of P-selectin and neutrophil infiltration were measured by immunohistochemical detection. For in vitro studies, histone H4 in the supernatant of mouse lung vascular endothelial cells (MLVECs) was measured by Western blot. The binding of extracellular histones with endothelial membrane was examined by confocal laser microscopy. Endothelial P-selectin translocation was measured by cell surface ELISA. Adhesion of neutrophils to MLVECs was assessed with a color video digital camera.

RESULTS

The results showed that during LPS-induced ARDS extracellular histones caused endothelial and neutrophil activation, as seen by P-selectin translocation, release of vWF, an increase of circulating sTM, lung neutrophil infiltration and increased MPO activity. Extracellular histones directly bound and activated MLVECs in a dose-dependent manner. On the contrary, the direct stimulatory effect of exogenous histones on neutrophils was very limited, as measured by neutrophil adhesion and MPO activity. With the contribution of activated endothelium, extracellular histones could effectively activating neutrophils. Both inhibiting the endothelial activation with an anti-toll like receptor (TLR) antibody and inhibiting the interaction of the endothelium with neutrophil using an anti-P-selectin antibody decreased the degree of neutrophil activation.

CONCLUSIONS

Extracellular histones are pro-inflammatory mediators in LPS-induced ARDS in mice. In addition to direct action to neutrophils, extracellular histones promote neutrophil adhesion and subsequent activation by first activating the pulmonary endothelium via TLR signaling. Thus, endothelial activation is important for extracellular histone-induced inflammatory injury.

Organ distribution of histones after intravenous infusion of FITC histones or after sepsis

Histones appear in plasma during infectious or non-infectious sepsis and are associated with multiorgan injury. In the current studies, intravenous infusion of histones resulted in their localization in major organs. In vitro exposure of mouse macrophages to histones caused a buildup of histones on cell membranes followed by localization into cytosol and into the nucleus. After polymicrobial sepsis (cecal ligation and puncture), histones appeared in plasma as well as in a multiorgan pattern, peaking at 8 h followed by decline. In lungs, histones and neutrophils appeared together, with evidence for formation of neutrophil extracellular traps (NETs), which represent an innate immune response to trap and kill bacteria and other infectious agents. In liver, there was intense NET formation, featuring linear patterns containing histones and strands of DNA. When neutrophils were activated in vitro with C5a or phorbol myristate acetate, NET formation ensued. While formation of NETs represents entrapment and killing of infectious agents, the simultaneous release from neutrophils of histones often results in tissue/organ damage.

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.

Pro-Coagulant Endothelial Dysfunction Results from EHEC Shiga Toxins and Host Damage-Associated Molecular Patterns

Hemolytic uremic syndrome (HUS) from enterohemorrhagic Escherichia coli infection is a leading cause of kidney failure in otherwise healthy U.S. children. The bacterial Shiga toxins (Stx) induce the characteristic coagulopathy of HUS, but the damage to toxin-receptor expressing cells and organ injury due to ischemia likely also releases inflammatory damage-associated molecular patterns (DAMPs), which may exacerbate injury along with the toxins. To examine this, human aortic and renal glomerular cell anti-coagulant and barrier functions were studied after in vitro challenge with Stx1, Stx2, and DAMPs. There was significant loss of surface anti-coagulant protein C pathway molecules, increased expression of pro-thrombotic PAR1 and reduced protein C activation capability by 15-27%. Histones nearly completely prevented the activated protein C protection of endothelial cells from thrombin-induced permeability. In mice, lethal Stx2 challenge elevated plasma HMGB1 (day 2, 321 ± 118%; p < 0.01) and extracellular histones (day 3, 158 ± 62%; p < 0.01). Mice colonized with Stx2-expressing Citrobacter rodentium developed increased HMGB1 (day 5, 155 ± 55%; p < 0.01) and histones (day 3, 378 ± 188%; p < 0.01). Anti-histone antibody reduced both DAMPs to baseline, but was not sufficient to improve survival outcome or kidney function. Together, these data suggest a potential role Stx to produce DAMPs, and DAMPs to produce endothelial injury and a pro-thrombotic environment.

EPCR-dependent PAR2 activation by the blood coagulation initiation complex regulates LPS-triggered interferon responses in mice

Infection and inflammation are invariably associated with activation of the blood coagulation mechanism, secondary to the inflammation-induced expression of the coagulation initiator tissue factor (TF) on innate immune cells. By investigating the role of cell-surface receptors for coagulation factors in mouse endotoxemia, we found that the protein C receptor (ProcR; EPCR) was required for the normal in vivo and in vitro induction of lipopolysaccharide (LPS)-regulated gene expression. In cultured bone marrow-derived myeloid cells and in monocytic RAW264.7 cells, the LPS-induced expression of functionally active TF, assembly of the ternary TF-VIIa-Xa initiation complex of blood coagulation, and the EPCR-dependent activation of protease-activated receptor 2 (PAR2) by the ternary TF-VIIa-Xa complex were required for the normal LPS induction of messenger RNAs encoding the TLR3/4 signaling adaptor protein Pellino-1 and the transcription factor interferon regulatory factor 8. In response to in vivo challenge with LPS, mice lacking EPCR or PAR2 failed to fully initiate an interferon-regulated gene expression program that included the Irf8 target genes Lif, Iigp1, Gbp2, Gbp3, and Gbp6. The inflammation-induced expression of TF and crosstalk with EPCR, PAR2, and TLR4 therefore appear necessary for the normal evolution of interferon-regulated host responses.

Role of extracellular histones in the cardiomyopathy of sepsis

The purpose of this study was to define the relationship in polymicrobial sepsis (in adult male C57BL/6 mice) between heart dysfunction and the appearance in plasma of extracellular histones. Procedures included induction of sepsis by cecal ligation and puncture and measurement of heart function using echocardiogram/Doppler parameters. We assessed the ability of histones to cause disequilibrium in the redox status and intracellular [Ca(2+)]i levels in cardiomyocytes (CMs) (from mice and rats). We also studied the ability of histones to disturb both functional and electrical responses of hearts perfused with histones. Main findings revealed that extracellular histones appearing in septic plasma required C5a receptors, polymorphonuclear leukocytes (PMNs), and the Nacht-, LRR-, and PYD-domains-containing protein 3 (NLRP3) inflammasome. In vitro exposure of CMs to histones caused loss of homeostasis of the redox system and in [Ca(2+)]i, as well as defects in mitochondrial function. Perfusion of hearts with histones caused electrical and functional dysfunction. Finally, in vivo neutralization of histones in septic mice markedly reduced the parameters of heart dysfunction. Histones caused dysfunction in hearts during polymicrobial sepsis. These events could be attenuated by histone neutralization, suggesting that histones may be targets in the setting of sepsis to reduce cardiac dysfunction.

Neutrophil Extracellular Trap-Related Extracellular Histones Cause Vascular Necrosis in Severe GN

Severe GN involves local neutrophil extracellular trap (NET) formation. We hypothesized a local cytotoxic effect of NET-related histone release in necrotizing GN. In vitro, histones from calf thymus or histones released by neutrophils undergoing NETosis killed glomerular endothelial cells, podocytes, and parietal epithelial cells in a dose-dependent manner. Histone-neutralizing agents such as antihistone IgG, activated protein C, or heparin prevented this effect. Histone toxicity on glomeruli ex vivo was Toll-like receptor 2/4 dependent, and lack of TLR2/4 attenuated histone-induced renal thrombotic microangiopathy and glomerular necrosis in mice. Anti-glomerular basement membrane GN involved NET formation and vascular necrosis, whereas blocking NET formation by peptidylarginine inhibition or preemptive anti-histone IgG injection significantly reduced all aspects of GN (i.e., vascular necrosis, podocyte loss, albuminuria, cytokine induction, recruitment or activation of glomerular leukocytes, and glomerular crescent formation). To evaluate histones as a therapeutic target, mice with established GN were treated with three different histone-neutralizing agents. Anti-histone IgG, recombinant activated protein C, and heparin were equally effective in abrogating severe GN, whereas combination therapy had no additive effects. Together, these results indicate that NET-related histone release during GN elicits cytotoxic and immunostimulatory effects. Furthermore, neutralizing extracellular histones is still therapeutic when initiated in established GN.

Cutting edge: critical role for C5aRs in the development of septic lymphopenia in mice

In the early stages of sepsis, lymphocytes undergo apoptosis, resulting in lymphopenia and immunosuppression. The trigger for septic lymphopenia is unknown. Using the polymicrobial model of murine sepsis, we investigated the role of C5a receptors in septic lymphopenia. In wild-type mice, cecal ligation and puncture resulted in splenocyte apoptosis and significant lymphopenia after 3 d, which was not observed in C5aR1(-/-) or C5aR2(-/-) mice. Our data show that mouse neutrophils exposed to recombinant mouse C5a cause release of histones in a dose-dependent and time-dependent manner. Histone levels in spleen were significantly elevated following cecal ligation and puncture but were reduced by the absence of C5aR1. Histones induced significant lymphocyte apoptosis in vitro. Ab-mediated neutralization of histones prevented the development of lymphopenia in sepsis. Together, these results describe a new pathway of septic lymphopenia involving complement and extracellular histones. Targeting of this pathway may have therapeutic benefit for patients with sepsis or other serious illness.

Lung inflammation and damage induced by extracellular histones

Despite decades of research, acute respiratory distress syndrome (ARDS) remains an important clinical challenge due to an incomplete understanding of the pathophysiological mechanisms. No FDA-approved drug therapy currently exists for treatment of humans with ARDS. There is accumulating evidence in rodents and humans suggesting that extracellular histones are strong drivers of inflammation and tissue damage. We recently described an important role for extracellular histones during acute lung injury (ALI) in mice (Bosmann et al., FASEB J. 27:5010-5021 (2013)). Extracellular histones were detected in bronchoalveolar lavage fluids (BALF) from patients with ARDS but not in BALF from non-ARDS patients in intensive care units. Extracellular histones were also detected in BALF from mice during experimental ALI. The presence of extracellular histones was dependent on the two C5a receptors (C5aR and C5L2) and availability of neutrophils. Extracellular histones were highly pro-inflammatory, and caused severe damage to respiratory function. Intratracheal instillation of histones resulted in pro-inflammatory mediator production, epithelial cell damage, disturbances in alveolar-capillary gas exchange, lung consolidation, activation of the coagulation cascade, and in some cases, death. Antibody-mediated neutralization of extracellular histones attenuated C5a-induced ALI. Together, these data suggested a prominent role for extracellular histones in the pathophysiology of ALI. The predominant source of histones in ALI may be neutrophils that have been activated by C5a to form neutrophil extracellular traps (NETs). Therapeutic targeting of extracellular histones may provide a novel approach to combat ARDS in humans.

Dexmedetomidine: a review of applications for cardiac surgery during perioperative period

Cardiac surgery is associated with a high incidence of cardiovascular and other complications during the perioperative period that translate into increased mortality and prolonged hospital stays. Safe comprehensive perioperative management is required to eliminate these adverse events. Dexmedetomidine is a selective α2-adrenoreceptor agonist that has been described as an ideal medication in the perioperative period of cardiac surgery. The major clinical effects of dexmedetomidine in this perioperative period can be summarized as attenuating the hemodynamic response, cardioprotective effects, antiarrhythmic effects, sedation in the ICU setting, treatment of delirium, and procedural sedation. Although there are some side effects of dexmedetomidine, it is emerging as an effective therapeutic agent in the management of a wide range of clinical conditions with an efficacious, safe profile. The present review serves as an overview update in the diverse applications of dexmedetomidine for cardiac surgery during the perioperative period.

N-acetyl-heparin attenuates acute lung injury caused by acid aspiration mainly by antagonizing histones in mice

Acute lung injury (ALI) is the leading cause of death in intensive care units. Extracellular histones have recently been recognized to be pivotal inflammatory mediators. Heparin and its derivatives can bind histones through electrostatic interaction. The purpose of this study was to investigate 1) the role of extracellular histones in the pathogenesis of ALI caused by acid aspiration and 2) whether N-acetyl-heparin (NAH) provides more protection than heparin against histones at the high dose. ALI was induced in mice via intratracheal instillation of hydrochloric acid (HCl). Lethality rate, blood gas, myeloperoxidase (MPO) activity, lung edema and pathological changes were used to evaluate the degree of ALI. Heparin/NAH was administered intraperitoneally, twice a day, for 3 days or until death. Acid aspiration caused an obvious increase in extracellular histones. A significant correlation existed between the concentration of HCl aspirated and the circulating histones. Heparin/NAH (10 mg/kg) improved the lethality rate, blood gas, MPO activity, lung edema and pathological score. At a dose of 20 mg/kg, NAH still provided protection, however heparin tended to aggravate the injury due to hemorrhagic complications. The specific interaction between heparin and histones was verified by the binding assay. In summary, high levels of extracellular histones can be pathogenic in ALI caused by acid aspiration. By neutralizing extracellular histones, heparin/NAH can offer similar protection at the moderate doses. At the high dose, NAH provides better protection than heparin.

Critical role for the NLRP3 inflammasome during acute lung injury

The inflammasome is a key factor in innate immunity and senses soluble pathogen and danger-associated molecular patterns as well as biological crystals (urate, cholesterol, etc.), resulting in expression of IL-1β and IL-18. Using a standard model of acute lung injury (ALI) in mice featuring airway instillation of LPS, ALI was dependent on availability of NLRP3 as well as caspase-1, which are known features of the NLRP3 inflammasome. The appearance of IL-1β, a product of NLRP3 inflammasome activation, was detected in bronchoalveolar lavage fluids (BALF) in a macrophage- and neutrophil-dependent manner. Neutrophil-derived extracellular histones appeared in the BALF during ALI and directly activated the NLRP3 inflammasome. Ab-mediated neutralization of histones significantly reduced IL-1β levels in BALF during ALI. Inflammasome activation by extracellular histones in LPS-primed macrophages required NLRP3 and caspase-1 as well as extrusion of K(+), increased intracellular Ca(2+) concentration, and generation of reactive oxygen species. NLRP3 and caspase-1 were also required for full extracellular histone presence during ALI, suggesting a positive feedback mechanism. Extracellular histone and IL-1β levels in BALF were also elevated in C5a-induced and IgG immune complex ALI models, suggesting a common inflammatory mechanism. These data indicate an interaction between extracellular histones and the NLRP3 inflammasome, resulting in ALI. Such findings suggest novel targets for treatment of ALI, for which there is currently no known efficacious drug.

Extracellular histones in tissue injury and inflammation

Neutrophil NETosis is an important element of host defense as it catapults chromatin out of the cell to trap bacteria, which then are killed, e.g., by the chromatin's histone component. Also, during sterile inflammation TNF-alpha and other mediators trigger NETosis, which elicits cytotoxic effects on host cells. The same mechanism should apply to other forms of regulated necrosis including pyroptosis, necroptosis, ferroptosis, and cyclophilin D-mediated regulated necrosis. Beyond these toxic effects, extracellular histones also trigger thrombus formation and innate immunity by activating Toll-like receptors and the NLRP3 inflammasome. Thereby, extracellular histones contribute to the microvascular complications of sepsis, major trauma, small vessel vasculitis as well as acute liver, kidney, brain, and lung injury. Finally, histones prevent the degradation of extracellular DNA, which promotes autoimmunization, anti-nuclear antibody formation, and autoimmunity in susceptible individuals. Here, we review the current evidence on the pathogenic role of extracellular histones in disease and discuss how to target extracellular histones to improve disease outcomes.

Circulating histones exacerbate inflammation in mice with acute liver failure

Circulating histones are a newly recognized mediator implicated in various inflammatory diseases. It is likely that the release of histones, from dying hepatocytes or inflammatory leukocytes, into the circulation initiates and amplifies inflammation during the course of acute liver failure (ALF). In this study, we investigated a putative pathogenic role of circulating histones in a murine model of ALF induced by D-galactosamine (GalN) plus lipopolysaccharide (LPS). Hepatic function and histological indexes, myeloperoxidase (MPO) activity, hepatocyte apoptosis and the levels of circulating histone were measured in GalN/LPS-treated mice. GalN/LPS caused severe liver damage and a notable increase in plasma concentration of circulating histones. To further assess the role of circulating histones in our model, we administered exogenous histones and anti-histone H4 antibody. Notably, exogenous histones aggravated GalN/LPS-induced hepatotoxicity, whereas anti-histone antibody significantly protected mice. Circulating histones may serve as both a functional marker of ALF activity and as an inflammatory mediator contributing to the progression of ALF. Blockade of circulating histones shows potent protective effects, suggesting a potential therapeutic strategy for ALF.

Histones trigger sterile inflammation by activating the NLRP3 inflammasome

Sterile cell death mediated inflammation is linked to several pathological disorders and involves danger recognition of intracellular molecules released by necrotic cells that activate different groups of innate pattern recognition receptors. Toll-like receptors directly interact with their extrinsic or intrinsic agonists and induce multiple proinflammatory mediators. In contrast, the NLRP3 inflammasome is rather thought to represent a downstream element integrating various indirect stimuli into proteolytic cleavage of interleukin (IL)-1β and IL-18. Here, we report that histones released from necrotic cells induce IL-1β secretion in an NLRP3-ASC-caspase-1-dependent manner. Genetic deletion of NLRP3 in mice significantly attenuated histone-induced IL-1β production and neutrophil recruitment. Furthermore, necrotic cells induced neutrophil recruitment, which was significantly reduced by histone-neutralizing antibodies or depleting extracellular histones via enzymatic degradation. These results identify cytosolic uptake of necrotic cell-derived histones as a triggering mechanism of sterile inflammation, which involves NLRP3 inflammasome activation and IL-1β secretion via oxidative stress.

Extracellular histones are essential effectors of C5aR- and C5L2-mediated tissue damage and inflammation in acute lung injury

We investigated how complement activation promotes tissue injury and organ dysfunction during acute inflammation. Three models of acute lung injury (ALI) induced by LPS, IgG immune complexes, or C5a were used in C57BL/6 mice, all models requiring availability of both C5a receptors (C5aR and C5L2) for full development of ALI. Ligation of C5aR and C5L2 with C5a triggered the appearance of histones (H3 and H4) in bronchoalveolar lavage fluid (BALF). BALF from humans with ALI contained H4 histone. Histones were absent in control BALF from healthy volunteers. In mice with ALI, in vivo neutralization of H4 with IgG antibody reduced the intensity of ALI. Neutrophil depletion in mice with ALI markedly reduced H4 presence in BALF and was highly protective. The direct lung damaging effects of extracellular histones were demonstrated by airway administration of histones into mice and rats (Sprague-Dawley), which resulted in ALI that was C5a receptor-independent, and associated with intense inflammation, PMN accumulation, damage/destruction of alveolar epithelial cells, together with release into lung of cytokines/chemokines. High-resolution magnetic resonance imaging demonstrated lung damage, edema and consolidation in histone-injured lungs. These studies confirm the destructive C5a-dependent effects in lung linked to appearance of extracellular histones.

Circulating histones are mediators of trauma-associated lung injury

RATIONALE

Acute lung injury is a common complication after severe trauma, which predisposes patients to multiple organ failure. This syndrome largely accounts for the late mortality that arises and despite many theories, the pathological mechanism is not fully understood. Discovery of histone-induced toxicity in mice presents a new dimension for elucidating the underlying pathophysiology.

OBJECTIVES

To investigate the pathological roles of circulating histones in trauma-induced lung injury.

METHODS

Circulating histone levels in patients with severe trauma were determined and correlated with respiratory failure and Sequential Organ Failure Assessment (SOFA) scores. Their cause-effect relationship was studied using cells and mouse models.

MEASUREMENTS AND MAIN RESULTS

In a cohort of 52 patients with severe nonthoracic blunt trauma, circulating histones surged immediately after trauma to levels that were toxic to cultured endothelial cells. The high levels were significantly associated with the incidence of acute lung injury and SOFA scores, as well as markers of endothelial damage and coagulation activation. In in vitro systems, histones damaged endothelial cells, stimulated cytokine release, and induced neutrophil extracellular trap formation and myeloperoxidase release. Cellular toxicity resulted from their direct membrane interaction and resultant calcium influx. In mouse models, cytokines and markers for endothelial damage and coagulation activation significantly increased immediately after trauma or histone infusion. Pathological examinations showed that lungs were the predominantly affected organ with edema, hemorrhage, microvascular thrombosis, and neutrophil congestion. An anti-histone antibody could reduce these changes and protect mice from histone-induced lethality.

CONCLUSIONS

This study elucidates a new mechanism for acute lung injury after severe trauma and proposes that circulating histones are viable therapeutic targets for improving survival outcomes in patients.

Platelets induce neutrophil extracellular traps in transfusion-related acute lung injury

There is emerging evidence that platelets are major contributors to inflammatory processes through intimate associations with innate immune cells. Here, we report that activated platelets induce the formation of neutrophil extracellular traps (NETs) in transfusion-related acute lung injury (TRALI), which is the leading cause of death after transfusion therapy. NETs are composed of decondensed chromatin decorated with granular proteins that function to trap extracellular pathogens; their formation requires the activation of neutrophils and release of their DNA in a process that may or may not result in neutrophil death. In a mouse model of TRALI that is neutrophil and platelet dependent, NETs appeared in the lung microvasculature and NET components increased in the plasma. We detected NETs in the lungs and plasma of human TRALI and in the plasma of patients with acute lung injury. In the experimental TRALI model, targeting platelet activation with either aspirin or a glycoprotein IIb/IIIa inhibitor decreased NET formation and lung injury. We then directly targeted NET components with a histone blocking antibody and DNase1, both of which protected mice from TRALI. These data suggest that NETs contribute to lung endothelial injury and that targeting NET formation may be a promising new direction for the treatment of acute lung injury.

Histones from dying renal cells aggravate kidney injury via TLR2 and TLR4

In AKI, dying renal cells release intracellular molecules that stimulate immune cells to secrete proinflammatory cytokines, which trigger leukocyte recruitment and renal inflammation. Whether the release of histones, specifically, from dying cells contributes to the inflammation of AKI is unknown. In this study, we found that dying tubular epithelial cells released histones into the extracellular space, which directly interacted with Toll-like receptor (TLR)-2 (TLR2) and TLR4 to induce MyD88, NF-κB, and mitogen activated protein kinase signaling. Extracellular histones also had directly toxic effects on renal endothelial cells and tubular epithelial cells in vitro. In addition, direct injection of histones into the renal arteries of mice demonstrated that histones induce leukocyte recruitment, microvascular vascular leakage, renal inflammation, and structural features of AKI in a TLR2/TLR4-dependent manner. Antihistone IgG, which neutralizes the immunostimulatory effects of histones, suppressed intrarenal inflammation, neutrophil infiltration, and tubular cell necrosis and improved excretory renal function. In summary, the release of histones from dying cells aggravates AKI via both its direct toxicity to renal cells and its proinflammatory effects. Because the induction of proinflammatory cytokines in dendritic cells requires TLR2 and TLR4, these results support the concept that renal damage triggers an innate immune response, which contributes to the pathogenesis of AKI.

Extracellular chromatin is an important mediator of ischemic stroke in mice

OBJECTIVE

Recently, a growing number of studies have revealed a prothrombotic and cytotoxic role for extracellular chromatin. Cerebral ischemia/reperfusion injury is characterized by a significant amount of cell death and neutrophil activation, both of which may result in the release of chromatin. The goal of this study was to assess the effect of extracellular chromatin in ischemic stroke using a mouse model of transient middle cerebral artery occlusion.

METHODS AND RESULTS

Similar to reports in stroke patients, we observed increased levels of circulating nucleosomes and DNA after ischemic stroke in mice. In addition, we observed that general hypoxia also augmented extracellular chromatin. We hypothesized that targeting extracellular chromatin components would be protective in ischemic stroke. Indeed, treatment with recombinant human DNase 1 significantly improved stroke outcome. Neutralization of histones using an antihistone antibody was also protective as evidenced by smaller infarct volumes, whereas increasing levels of extracellular histones via histone infusion exacerbated stroke outcome by increasing infarct size and worsening functional outcome.

CONCLUSIONS

Our results indicate that extracellular chromatin is generated and is detrimental during cerebral ischemia/reperfusion in mice. Targeting DNA and histones may be a new therapeutic strategy to limit injury resulting from ischemic stroke.

Plasmodium falciparum histones induce endothelial proinflammatory response and barrier dysfunction

Plasmodium falciparum is a protozoan parasite of human erythrocytes that causes the most severe form of malaria. Severe P. falciparum infection is associated with endothelial activation and permeability, which are important determinants of the outcome of the infection. How endothelial cells become activated is not fully understood, particularly with regard to the effects of parasite subcomponents. We demonstrated that P. falciparum histones extracted from merozoites (HeH) directly stimulated the production of IL-8 and other inflammatory mediators by primary human dermal microvascular endothelial cells through a signaling pathway that involves Src family kinases and p38 MAPK. The stimulatory effect of HeH and recombinant P. falciparum H3 (PfH3) was abrogated by histone-specific antibodies. The release of nuclear contents on rupture of infected erythrocytes was captured by live cell imaging and confirmed by detecting nucleosomes in the supernatants of parasite cultures. HeH and recombinant parasite histones also induced endothelial permeability through a charge-dependent mechanism that resulted in disruption of junctional protein expression and cell death. Recombinant human activated protein C cleaved HeH and PfH3 and abrogated their proinflammatory effects. Circulating nucleosomes of both human and parasite origin were detected in the plasma of patients with falciparum malaria and correlated positively with disease severity. These results support a pathogenic role for both host- and pathogen-derived histones in P. falciparum-caused malaria.

Epigenetic histone code and autoimmunity

The multiple inter-dependent post-translational modifications of histones represent fine regulators of chromatin dynamics. These covalent modifications, including phosphorylation, acetylation, ubiquitination, deimination, and methylation, affect therefore the numerous processes involving chromatin, such as replication, repair, transcription, genome stability, and cell death. Specific enzymes introducing modified residues in histones are precisely regulated, and a single amino acid residue can be subjected to a single or several, independent modifications. Disruption of histone post-translational modifications perturbs the pattern of gene expression, which may result in disease manifestations. It has become evident in recent years that apoptosis-modified histones exert a central role in the induction of autoimmunity, for example in systemic lupus erythematosus and rheumatoid arthritis. Certain histone post-translational modifications are linked to cell death (apoptotic and non-apoptotic cell death) and might be involved in lupus in the activation of normally tolerant lymphocyte subpopulations. In this review, we discuss how these modifications can affect the antigenicity and immunogenicity of histones with potential consequences in the pathogenesis of autoimmune diseases.

Identification of new pathogenic players in lupus: autoantibody-secreting cells are present in nephritic kidneys of (NZBxNZW)F1 mice

An important hallmark of systemic lupus erythematosus is the production of autoantibodies specific for nuclear Ags, among which nucleosomes and their constituents, DNA and histones. It is widely admitted that some of these autoantibodies contribute largely in lupus pathogenesis because of their nephritogenic potential. However, the underlying mechanisms are still debated. In this study, we analyzed the autoimmune response against histone H2B during the course of the disease in lupus-prone (NZBxNZW)F1 mice, both in lymphoid organs and kidneys, and we assessed its potential involvement in lupus pathogenicity. We found that the N-terminal region of histone H2B represents a preferential target for circulating autoantibodies, which kinetics of appearance positively correlates with disease development. Furthermore, immunization of preautoimmune (NZBxNZW)F1 mice with H2B peptide 1-25 accelerates the disease. Kidney eluates from diseased (NZBxNZW)F1 mice do contain IgG Abs reacting with this peptide, and this H2B sequence was found to be accessible to specific Ab probes in Ag-containing deposits detected in nephritic kidneys. Finally, compared with control normal mice and to young preautoimmune (NZBxNZW)F1 animals, the frequency of cells secreting autoantibodies reacting with peptide 1-25 was significantly raised in the spleen and bone marrow and most importantly on a pathophysiological point of view, locally, in nephritic kidneys of diseased (NZBxNZW)F1 mice. Altogether our results demonstrate the existence in (NZBxNZW)F1 mice of both a systemic and local B cell response targeting the N-terminal region of histone H2B, and highlight the potential implication of this nuclear domain in lupus pathology.

Extracellular histones are major mediators of death in sepsis

Hyperinflammatory responses can lead to a variety of diseases, including sepsis. We now report that extracellular histones released in response to inflammatory challenge contribute to endothelial dysfunction, organ failure and death during sepsis. They can be targeted pharmacologically by antibody to histone or by activated protein C (APC). Antibody to histone reduced the mortality of mice in lipopolysaccharide (LPS), tumor necrosis factor (TNF) or cecal ligation and puncture models of sepsis. Extracellular histones are cytotoxic toward endothelium in vitro and are lethal in mice. In vivo, histone administration resulted in neutrophil margination, vacuolated endothelium, intra-alveolar hemorrhage and macro- and microvascular thrombosis. We detected histone in the circulation of baboons challenged with Escherichia coli, and the increase in histone levels was accompanied by the onset of renal dysfunction. APC cleaves histones and reduces their cytotoxicity. Co-infusion of APC with E. coli in baboons or histones in mice prevented lethality. Blockade of protein C activation exacerbated sublethal LPS challenge into lethality, which was reversed by treatment with antibody to histone. We conclude that extracellular histones are potential molecular targets for therapeutics for sepsis and other inflammatory diseases.

Anti-nuclear antibody reactivity in lupus may be partly hard-wired into the primary B-cell repertoire

When monoclonal ANAs and non-ANAs generated from a genetically simplified mouse model of lupus, B6.Sle1, were recently compared, the ANAs exhibited three sequence motifs in their immunoglobulin heavy chains, including increased cationicity in CDR3 ("motif A"), reduced anionicity in CDR2 ("motif B") and increased aspartate at H50 ("motif C"). The present study was designed to elucidate the extent to which these ANA-associated sequence motifs might be hard-wired into the primary B-cell repertoire in lupus. The immunoglobulin heavy chain sequence of total splenic B-cells, follicular B-cells and marginal zone B-cells from B6.Sle1 congenic mice and C57BL/6 controls were amplified by single-cell PCR and compared. Analysis of the primary immunoglobulin heavy chain repertoire indicated that the first two sequence motifs "A" and "B" were already encoded in the naïve repertoire of B6.Sle1(z) mice, whereas the third motif "C" was introduced in part by somatic mutation. Site-directed mutagenesis confirmed that non-anionic CDR2 and cationic CDR3 residues in the immunoglobulin heavy chain facilitated nuclear antigen binding in concert, whereas aspartate at H50 strongly vetoed DNA-binding, while preserving nucleosome reactivity. Hence, anti-nuclear antibodies appear to arise as a consequence of two distinct processes-genetically programmed selection of specific CDR charge motifs into the primary immunoglobulin repertoire, with secondary contribution from somatic mutation. Polymorphisms in the lupus susceptibility gene Ly108 that impair central B-cell tolerance may be mechanistically responsible for these early repertoire differences in lupus.

Molecular hallmarks of anti-chromatin antibodies associated with the lupus susceptibility locus, Sle1

Anti-nuclear antibodies constitute the hallmark of lupus. The NZM2410-derived Sle1 lupus susceptibility interval on murine chromosome 1 breaches tolerance, leading to the emergence of anti-nuclear autoantibodies targeting nucleosomes. However, little is known about the molecular structure of the anti-nucleosome autoantibodies from this genetically simplified mouse model of lupus. In this study, the immunoglobulin heavy chain and light chain sequences of 50 anti-nuclear monoclonal antibodies derived from five B6.Sle1(z) mice were compared to non-nuclear antibody controls. Compared to two different sets of non-nuclear antibodies, anti-nucleosome antibodies derived from B6.Sle1(z) congenic mice exhibited a high degree of clonal expansion and three distinct sequence motifs in their heavy chains - cationic CDR3 stretches, non-anionic CDR2 regions, and an increased frequency of aspartate residues at H50, which together increased the likelihood of an antibody being chromatin-reactive by approximately 4-fold.

Histone H4 is a major component of the antimicrobial action of human sebocytes

Antimicrobial peptides, such as cathelicidin and beta defensins, directly kill microbes and have been detected in human sebaceous glands and cell lines. Despite the presence of several such peptides, the apparent abundance of these is insufficient for direct killing of most skin pathogens. In this study, we sought to determine which molecules provide the majority of antimicrobial peptide activity in human sebocytes. Acid-soluble protein extracts of SEB-1 sebocytes were separated by reverse-phase high-performance liquid chromatography and were assayed for their capacity to inhibit the growth of Staphylococcus aureus. Antimicrobial activity was isolated in a single major fraction and identified to be histone H4 by mass spectrometry and western blot analysis. The importance of histone H4 in the antimicrobial activity of sebocytes was confirmed by a specific neutralizing antibody and by direct demonstration that recombinant histone H4 had antimicrobial activity against S. aureus and Propionibacterium acnes. In addition, histone H4 enhanced the antimicrobial action of free fatty acids in human sebum. Taken together, these results indicate that the release of histone H4 by holocrine secretion from the sebaceous gland may play an important role in innate immunity.

An active mechanism flanks and modulates the export of the small ribosomal subunits

The modalities of export of the ribosomal subunits from the nucleolus to the nuclear pores have been only partially clarified since it is not yet clear whether the movements depend purely on diffusion or also from an active process. Recently, we suggested the existence of an active transport mechanism of a subset (10-12%) of the small ribosomal subunits (SSU) (Cisterna et al. in 2006, Faseb J). Here, we give further evidence that an active, motor protein-mediated process exists for the SSU transport from the nucleolus to the nuclear pore. We demonstrate that the blockade of ATP synthesis and antibody-mediated inhibition of nuclear myosin or actin induce structural and functional modifications of the nucleolus, suggestive of transcriptional activity decrease. Moreover, both treatments induce a significant retention of RNA inside the nucleus and an accumulation of ribosomal subunits in the granular component. We suggest that the existence of this secondary, active mechanism of SSU transport might be utilized by the cell when a more rapid and directional export is needed.

Nucleosomes are exposed at the cell surface in apoptosis

Apoptotic cells are considered the source of DNA, histones, and nucleoprotein complexes that drive the production of autoantibodies in systemic lupus erythematosus. However, the role of apoptotic cells in the activation of the immune system is not clear. To explore interactions that may initiate or sustain the production of anti-nuclear autoantibodies, we characterized the binding of a large panel of monoclonal autoantibodies to apoptotic cells. Autoantibodies to DNA, individual core histones, histone-DNA complexes, or the native nucleosome core particle revealed a consistent and specific binding pattern in confocal microscopy. Immunoreactive epitopes were detected in the cytoplasm and accumulated along the surface of the fragmenting nucleus in a caspase-dependent manner. Ag-Ab complexes on nuclear fragments that had emerged from the plasma membrane were accessible to anti-isotype-reactive microparticles. Moreover, autoantibodies specific for the nucleosome core or its molecular components selectively precipitated a complex of core histones and DNA from the cytosol at 4 h after induction of apoptosis. These observations identify distinct steps in the release of nucleosomes from the nucleus and their exposure at the cell surface. Furthermore, the results indicate a direct role for nucleosomes in the execution of apoptosis, clearance of apoptotic cells, and regulation of anti-nuclear autoantibody production.

Molecular signatures of antinuclear antibodies-contributions of heavy chain CDR residues

A database of the Ig heavy chains of 143 anti-ssDNA, 103 anti-dsDNA and 23 anti-nucleosome antinuclear antibodies (ANAs) was constructed, with no clonal overlap, gleaning from published literature. In comparison to the Kabat database of antibodies (N>3600), ANAs (total=269) demonstrated several significant changes, particularly in the incidence of charged or polar residues, in their CDR regions. In particular, anti-dsDNA ANAs differed significantly from anti-ssDNA ANAs in having (a) more 'D' residues at H31 and more 'Y' residues at H33, in CDR1, (b) significantly different distributions of charged or polar residues at H53, H55 and H56 of CDR2, and (c) more 'R' residues at H95-H100 of CDR3. Whereas, the differences in CDR1 and CDR3 are likely to characterize anti-dsDNA ANAs encoded by all VH families, the sequence differences in CDR2 are likely to be VH family specific. Finally, among anti-dsDNA ANAs, there was an enrichment of VH1/J558 germline genes (notably, VH 45.21.1), which bear germline-encoded amino acid residues in their CDR regions that may potentially facilitate nuclear antigen binding. This ANA heavy chain database thus constitutes a useful resource for analyzing the molecular requirements for nuclear antigen reactivity.

Endotoxin-neutralizing antimicrobial proteins of the human placenta

Microbial colonization and infection of placental tissues often lead to adverse pregnancy outcomes such as preterm birth, a leading cause of neonatal morbidity and mortality. The fetal membranes of the placenta, a physical and active barrier to microbial invasion, encapsulate the fetus and secure its intrauterine environment. To examine the innate defense system of the human placenta, antimicrobial peptides were isolated from the fetal membranes of human placenta and characterized biochemically. Two salt-resistant antimicrobial host proteins were purified to homogeneity using heparin-affinity and reversed-phase HPLC. Characterization of these proteins revealed that they are identical to histones H2A and H2B. Histones H2A and H2B showed dose-dependent inhibition of the endotoxin activity of LPS and inhibited this activity by binding to and therefore blocking both the core and lipid A moieties of LPS. Consistent with a role for histones in the establishment of placental innate defense, histones H2A and H2B were highly expressed in the cytoplasm of syncytiotrophoblasts and amnion cells, where the histone proteins were localized mainly to the epithelial surface. Furthermore, culturing of amnion-derived WISH cells led to the constitutive release of histone H2B, and histones H2A and H2B contribute to bactericidal activity of amniotic fluid. Our studies suggest that histones H2A and H2B may endow the epithelium of the placenta with an antimicrobial and endotoxin-neutralizing barrier against microorganisms that invade this immune-privileged site.

Cathepsin D produces antimicrobial peptide parasin I from histone H2A in the skin mucosa of fish

Parasin I is a potent 19-residue antimicrobial peptide isolated from the skin mucus of wounded catfish (Parasilurus asotus). Here we describe the mechanism of parasin I production from histone H2A in catfish skin mucosa on epidermal injury. Cathepsin D is found to exist in the mucus as an inactive proenzyme (procathepsin D), and a metalloprotease, induced on injury, cleaves procathepsin D to generate active cathepsin D. This activated form of cathepsin D then cleaves the Ser19-Arg20 bond of histone H2A to produce parasin I. Immunohistochemical analysis reveals that unacetylated histone H2A, a precursor of parasin I, and procathepsin D are present in the cytoplasm of epithelial mucous cells and that parasin I is produced on the mucosal surface on epidermal injury. Western blot analysis shows that parasin I is also present in the skin mucus of other fish species. Furthermore, parasin I shows good antimicrobial activity against fish-specific bacterial pathogens. Taken together, these results indicate that cathepsin D and a metalloprotease participate in the production of parasin I from histone H2A and that parasin I contributes to the innate host defense of the fish against invading microorganisms.

Immunogenetic analysis of variable regions encoding AB1 and gamma-type AB2 antibodies from the NeuGc-containing ganglioside family

The variable regions from P3, a murine monoclonal antibody (MAb) against NeuGc-containing gangliosides, and two anti-idiotype MAbs directed to P3 MAb were cloned and sequenced. Comparisons with previously reported sequences showed that P3 is a germline antibody encoded by genes from the V(H)Q52 and V(kappa)19 families. Analysis of nucleotides at the heavy chain CDR3 (H-CDR3) showed the presence of an extensive 3' N region that contains almost 50% of the nucleotides of this CDR. In addition, amino acid sequence analysis of the H-CDRs of this MAb revealed the presence of three arginines, two of which are present in the H-CDR3, that could be involved in the interaction of P3 MAb with its electronegative epitope on gangliosides. Anti-idiotype 1E10, which seems to define a "regulatory" idiotope on P3 MAb (it induces Id+ Ab3), represents a germline Ab2 that belongs to the V(H)J558 and V(kappa)10 gene families. By contrary, the anti-idiotype 3B11 is an extensively mutated antibody that belongs to the V(H)3660 and V(kappa)4/5 gene families, defining a "private" idiotope on P3 MAb. Even when different V genes contribute to the variable regions of 1E10 and 3B11 MAbs, they share an acidic motif E/D-D-Y/D-Y-D in H-CDR3, suggesting that both Ab2s recognize paratope positive residues on the Ab1. Therefore, complementary electrostatic interactions involving H-CDR3 from both Ab1 and Ab2, might provide a clue to understand the molecular basis for the generation of gamma-type anti-idiotype antibodies to V regions recognizing glycolylated ganglioside antigens.