Extracellular Histone Released from Leukemic Cells Increases Their Adhesion to Endothelium and Protects them from Spontaneous and Chemotherapy-Induced Leukemic Cell Death

Elevation of circulating neutrophil extracellular traps in endometrial cancer: Poor prognostic value of cell-free double-stranded DNA

OBJECTIVE

Neutrophils produce neutrophil extracellular traps (NETs) by releasing nuclear contents into the extracellular environment. NETs are associated with systemic inflammation and cancer development and progression. We aimed to investigate whether NET markers are associated with the prognosis of endometrial cancer.

METHODS

Circulating levels of three NET markers (histone-DNA complex, cell-free double-stranded DNA (dsDNA), and neutrophil elastase) were measured in 98 patients with endometrial cancer who underwent surgery as primary treatment between January 2015 and June 2018 and 45 healthy women. Area under the receiver operating characteristic curve (AUC) analyses were conducted to investigate the diagnostic and prognostic utility of the markers for endometrial cancer.

RESULTS

Patients with endometrial cancer showed significantly higher levels of the three NET markers than those in healthy controls. In discriminating endometrial cancer patients from healthy controls, the three NET markers showed AUC values in the following order: cell-free dsDNA (0.832; 95 % CI, 0.760-0.889), histone-DNA complex (0.740; 95 % CI, 0.660-0.809), and neutrophil elastase (0.689; 95 % CI, 0.607-0.764), comparable to those of CA-125 (0.741; 95 % CI, 0.659-0.813). Multivariate analysis adjusting for FIGO stage, histology, and lymphovascular space invasion, and lymph node involvement revealed that cell-free dsDNA level (cutoff: 95.2 ng/mL) was an independent prognostic marker for poor progression-free (adjusted HR, 2.75; 95 % CI, 1.096.92; P = 0.032) and overall survival (adjusted HR, 11.51; 95 % CI, 2.0664.22; P = 0.005) for patients with endometrial cancer.

CONCLUSION

High levels of circulating NET markers were observed in patients with endometrial cancer. Cell-free dsDNA levels may play a role as prognostic markers for endometrial cancer.

A neutrophil extracellular trap-related risk score predicts prognosis and characterizes the tumor microenvironment in multiple myeloma

Multiple myeloma (MM) is a distinguished hematologic malignancy, with existing studies elucidating its interaction with neutrophil extracellular traps (NETs), which may potentially facilitate tumor growth. However, systematic investigations into the role of NETs in MM remain limited. Utilizing the single-cell dataset GSE223060, we discerned active NET cell subgroups, namely neutrophils, monocytes, and macrophages. A transcriptional trajectory was subsequently constructed to comprehend the progression of MM. Following this, an analysis of cellular communication in MM was conducted with a particular emphasis on neutrophils, revealing an augmentation in interactions albeit with diminished strength, alongside abnormal communication links between neutrophils and NK cells within MM samples. Through the intersection of differentially expressed genes (DEGs) between NET active/inactive cells and MM versus healthy samples, a total of 316 genes were identified. This led to the development of a 13-gene risk model for prognostic prediction based on overall survival, utilizing transcriptomics dataset GSE136337. The high-risk group manifested altered immune infiltration and heightened sensitivity to chemotherapy. A constructed nomogram for predicting survival probabilities demonstrated encouraging AUCs for 1, 3, and 5-year survival predictions. Collectively, our findings unveil a novel NET-related prognostic signature for MM, thereby providing a potential avenue for therapeutic exploration.

GSK484, an inhibitor of peptidyl arginine deiminase 4, increases the radiosensitivity of colorectal cancer and inhibits neutrophil extracellular traps

INTRODUCTION

Colorectal cancer (CRC) is the third most common malignancy and a major cause of cancer-related deaths. Peptidyl arginine deiminase 4 (PAD4 or PADI4) is expressed in neutrophils that, when activated, can drive the formation of neutrophil extracellular traps (NETs). PAD4 has been found to be upregulated in CRC patients and to predict a poor prognosis. This study is aimed at exploring the role of PAD4 inhibitor (GSK484) in NET formation and radioresistance in CRC.

METHODS

Reverse transcriptase quantitative polymerase chain reaction and western blotting were used to measure PAD4 expression in CRC tissues and cells. GSK484, an inhibitor of PAD4, was investigated in the following functional assays in vitro: western blotting, clonogenic survival, colony formation, TUNEL, flow cytometry and transwell assays. Nude mouse xenograft models were applied to evaluate the effect of GSK484 on tumor growth in CRC in vivo. The formation of NETs influenced by GSK484 was also investigated.

RESULTS

We showed upregulation of PAD4 mRNA and protein in CRC tissues and cells. High expression of PAD4 was related to a poor prognosis in CRC patients. GSK484 treatment promoted the radiosensitivity of CRC cells and induced cell death by promoting DNA double-strand breaks. Rescue experiments further verified that GSK484 inhibited the effects of PAD4 overexpression in irradiated CRC cells. Moreover, GSK484 injection strengthened the radiosensitivity of CRC and inhibited NET formation in vivo.

CONCLUSIONS

PAD4 inhibitor GSK484 promotes the radiosensitivity of CRC and inhibits NET formation in vivo and in vitro.

Circulating Histones to Detect and Monitor the Progression of Cancer

Liquid biopsies have emerged as a minimally invasive cancer detection and monitoring method, which could identify cancer-related alterations in nucleosome or histone levels and modifications in blood, saliva, and urine. Histones, the core component of the nucleosome, are essential for chromatin compaction and gene expression modulation. Increasing evidence suggests that circulating histones and histone complexes, originating from cell death or immune cell activation, could act as promising biomarkers for cancer detection and management. In this review, we provide an overview of circulating histones as a powerful liquid biopsy approach and methods for their detection. We highlight current knowledge on circulating histones in hematologic malignancies and solid cancer, with a focus on their role in cancer dissemination, monitoring, and tumorigenesis. Last, we describe recently developed strategies to identify cancer tissue-of-origin in blood plasma based on nucleosome positioning, inferred from nucleosomal DNA fragmentation footprint, which is independent of the genetic landscape.

Diagnostic and prognostic role of circulating neutrophil extracellular trap markers and prekallikrein in patients with high-grade serous ovarian cancer

Objective

Tumor-promoting inflammation is among the hallmarks of cancer. Prekallikrein is among the acute-phase reactants in the inflammatory response; moreover, neutrophils release nuclear contents into the extracellular space to create neutrophil extracellular traps (NET). We aimed to investigate the diagnostic and prognostic utilities of circulating plasma NET markers and prekallikrein for high-grade serous ovarian cancer (HGSOC).

Methods

Circulating levels of three NET markers (histone-DNA complex, cell-free DNA, and neutrophil elastase) and prekallikrein were measured in 75 patients with HGSOC and 23 healthy controls. We used an area under the receiver operating characteristic curve (AUC) analysis to investigate their diagnostic and prognostic utilities for HGSOC.

Results

Compared with healthy controls, patients with HGSOC showed significantly higher levels of the three NET markers and prekallikrein. Patients with advanced-stage HGSOC showed significantly higher levels of the cell-free DNA (87.4 vs. 79.5 ng/ml; P = 0.013), compared with those with early-stage HGSOC. Further, the levels of histone-DNA complex, neutrophil elastase, and prekallikrein did not significantly differ according to the cancer stage. All markers showed significant diagnostic utility. Notably, a logistic regression-based model that comprised all four markers showed the strongest diagnostic power (AUC, 0.966; 95% confidence interval [CI], 0.933-1.000). Specifically, neutrophil elastase was identified as an independent poor prognostic factor for overall survival (adjusted hazard ratio [aHR], 10.17; 95% CI, 1.09-94.97; P = 0.042) and progression-free survival (aHR, 14.47; 95% CI, 1.52-137.35; P = 0.020) in patients with HGSOC.

Conclusions

The levels of the three NET markers and prekallikrein might be novel diagnostic and prognostic markers for HGSOC.

Extracellular histones trigger oxidative stress-dependent induction of the NF-kB/CAM pathway via TLR4 in endothelial cells

Extracellular histones have been reported to aggravate different pathophysiological processes by increasing vascular permeability, coagulopathy, and inflammation. In the present study, we elucidate how extracellular histones (10–100 µg/mL) concentration dependently increase cytosolic reactive oxygen species (ROS) production using human umbilical vein endothelial cells (HUVECs). Furthermore, we identify cyclooxygenase (COX) and NADPH oxidase (NOX) activity as sources of ROS production in extracellular histone-treated HUVEC. This COX/NOX-mediated ROS production is also involved in enhanced NF-kB activity and cell adhesion molecules (VCAM1 and ICAM1) expression in histone-treated HUVEC. Finally, by using different toll-like receptor (TLR) antagonists, we demonstrate the role of TLR4 in CAMs overexpression triggered by extracellular histones in endothelial cells. In conclusion, our data suggest that through TLR4 signaling, extracellular histones increase endothelial cell activation, a mechanism involving increased COX- and NOX-mediated ROS. These findings increase our understanding on how extracellular histones enhance systemic inflammatory responses in diseases in which histone release occurs as part of the pathological processes.

Histones: The critical players in innate immunity

The highly conserved histones in different species seem to represent a very ancient and universal innate host defense system against microorganisms in the biological world. Histones are the essential part of nuclear matter and act as a control switch for DNA transcription. However, histones are also found in the cytoplasm, cell membranes, and extracellular fluid, where they function as host defenses and promote inflammatory responses. In some cases, extracellular histones can act as damage-associated molecular patterns (DAMPs) and bind to pattern recognition receptors (PRRs), thereby triggering innate immune responses and causing initial organ damage. Histones and their fragments serve as antimicrobial peptides (AMPs) to directly eliminate bacteria, viruses, fungi, and parasites in vitro and in vivo. Histones are also involved in phagocytes-related innate immune response as components of neutrophil extracellular traps (NETs), neutrophil activators, and plasminogen receptors. In addition, as a considerable part of epigenetic regulation, histone modifications play a vital role in regulating the innate immune response and expression of corresponding defense genes. Here, we review the regulatory role of histones in innate immune response, which provides a new strategy for the development of antibiotics and the use of histones as therapeutic targets for inflammatory diseases, sepsis, autoimmune diseases, and COVID-19.

Neutrophil extracellular traps: New players in cancer research

NETs are chromatin-derived webs extruded from neutrophils as a result of either infection or sterile stimulation using chemicals, cytokines, or microbes. In addition to the classical role that NETs play in innate immunity against infection and injuries, NETs have been implicated extensively in cancer progression, metastatic dissemination, and therapy resistance. The purpose of this review is to describe recent investigations into NETs and the roles they play in tumor biology and to explore their potential as therapeutic targets in cancer treatment.

Characterization of the B-Cell Epitopes of Echinococcus granulosus Histones H4 and H2A Recognized by Sera From Patients With Liver Cysts

Cystic echinococcosis (CE) is a zoonotic disease worldwide distributed, caused by the cestode Echinococcus granulosus sensu lato (E. granulosus), with an incidence rate of 50/100,000 person/year and a high prevalence in humans of 5-10%. Serology has variable sensitivity and specificity and low predictive values. Antigens used are from the hydatid fluid and recombinant antigens have not demonstrated superiority over hydatid fluid. A cell line called EGPE was obtained from E. granulosus sensu lato G1 strain from bovine liver. Serum from CE patients recognizes protein extracts from EGPE cells with higher sensitivity than protein extracts from hydatid fluid. In the present study, EGPE cell protein extracts and supernatants from cell colonies were eluted from a protein G affinity column performed with sera from 11 CE patients. LC-MS/MS proteomic analysis of the eluted proteins identified four E. granulosus histones: one histone H4 in the cell extract and supernatant, one histone H2A only in the cell extract, and two histones H2A only in the supernatant. This differential distribution of histones could reflect different parasite viability stages regarding their role in gene transcription and silencing and could interact with host cells. Bioinformatics tools characterized the linear and conformational epitopes involved in antibody recognition. The three-dimensional structure of each histone was obtained by molecular modeling and validated by molecular dynamics simulation and PCR confirmed the presence of the epitopes in the parasite genome. The three histones H2A were very different and had a less conserved sequence than the histone H4. Comparison of the histones of E. granulosus with those of other organisms showed exclusive regions for E. granulosus. Since histones play a role in the host-parasite relationship they could be good candidates to improve the predictive value of serology in CE.

Identification of Biomarkers in Patients with Thrombotic Thrombocytopenic Purpura Presenting with Large and Small Ischemic Stroke

Background

Thrombotic thrombocytopenic purpura (TTP) is a rare blood disorder resulting in organ damage including ischemic strokes. We sought to characterize the neuroimaging patterns of stroke in a large cohort of patients with immune-mediated TTP (iTTP) and determined their associations with clinical and laboratory parameters and outcomes.

Methods

We analyzed the Alabama TTP Registry who had laboratory confirmation of acute iTTP. We reviewed the neuroimaging patterns of those with ischemic stroke on MRI, clinical information, and laboratory results. Small ischemic strokes were ≤20 mm in their maximum diameter in the axial plane. Large ischemic strokes were >20 mm. Student t test, Mann-Whitney U test, and χ² test were all used for data analysis.

Results

Of 108 iTTP patients, 21 had ischemic stroke on neuroimaging. The median platelet count in these patients was 12 × 10⁹/L (interquartile range, IQR, 8.8–21 × 10⁹/L), plasma ADAMTS13 activity 1.8 U/dL (IQR 0–4.5 U/dL), and the mean plasma level of anti-ADAMTS13 IgG was 6,595.8 U/mL (SD 3,448.9 U/mL). Comparison between patients with large ischemic strokes (n = 10) and small ischemic strokes (n = 11) revealed that patients with small stroke were older (p = 0.043) and had higher plasma levels of citrullinated histone 3 (p = 0.006) and histone/DNA complex (p = 0.014) than those with large strokes. There were no significant differences between 2 stroke groups in mortality or exacerbation.

Conclusions

iTTP patients can present with large ischemic strokes and are usually younger. Further research should be performed in assessing different etiologies of iTTP-associated stroke based on neutrophil extracellular trap formation biomarkers (e.g., histone markers) seen in small ischemic stroke.

A narrative review of central nervous system involvement in acute leukemias

Acute leukemias (both myeloid and lymphoblastic) are a group of diseases for which each year more successful therapies are implemented. However, in a subset of cases the overall survival (OS) is still exceptionally low due to the infiltration of leukemic cells in the central nervous system (CNS) and the subsequent formation of brain tumors. The CNS involvement is more common in acute lymphocytic leukemia (ALL), than in adult acute myeloid leukemia (AML), although the rates for the second case might be underestimated. The main reasons for CNS invasion are related to the expression of specific adhesion molecules (VLA-4, ICAM-1, VCAM, L-selectin, PECAM-1, CD18, LFA-1, CD58, CD44, CXCL12) by a subpopulation of leukemic cells, called “sticky cells” which have the ability to interact and adhere to endothelial cells. Moreover, the microenvironment becomes hypoxic and together with secretion of VEGF-A by ALL or AML cells the permeability of vasculature in the bone marrow increases, coupled with the disruption of blood brain barrier. There is a single subpopulation of leukemia cells, called leukemia stem cells (LSCs) that is able to resist in the new microenvironment due to its high adaptability. The LCSs enter into the arachnoid, migrate, and intensively proliferate in cerebrospinal fluid (CSF) and consequently infiltrate perivascular spaces and brain parenchyma. Moreover, the CNS is an immune privileged site that also protects leukemic cells from chemotherapy. CD56/NCAM is the most important surface molecule often overexpressed by leukemic stem cells that offers them the ability to infiltrate in the CNS. Although asymptomatic or with unspecific symptoms, CNS leukemia should be assessed in both AML/ALL patients, through a combination of flow cytometry and cytological analysis of CSF. Intrathecal therapy (ITT) is a preventive measure for CNS involvement in AML and ALL, still much research is needed in finding the appropriate target that would dramatically lower CNS involvement in acute leukemia.

Endogenous PAD4 in Breast Cancer Cells Mediates Cancer Extracellular Chromatin Network Formation and Promotes Lung Metastasis

Peptidyl arginine deiminase 4 (PAD4/PADI4) is a posttranslational modification enzyme that converts protein arginine or mono-methylarginine to citrulline. The PAD4-mediated hypercitrullination reaction in neutrophils causes the release of nuclear chromatin to form a chromatin network termed neutrophil extracellular traps (NET). NETs were first described as antimicrobial fibers that bind and kill bacteria. However, it is not known whether PAD4 can mediate the release of chromatin DNA into the extracellular space of cancer cells. Here, we report that murine breast cancer 4T1 cells expressing high levels of PADI4 can release cancer extracellular chromatin networks (CECN) in vitro and in vivo. Deletion of Padi4 using CRISPR/Cas9 abolished CECN formation in 4T1 cells. Padi4 deletion from 4T1 cells also reduced the rate of tumor growth in an allograft model, and decreased lung metastasis by 4T1 breast cancers. DNase I treatment, which degrades extracellular DNA including CECNs, also reduced breast to lung metastasis of Padi4 wild-type 4T1 cells in allograft experiments in the Padi4-knockout mice. We further demonstrated that DNase I treatment in this mouse model did not alter circulating tumor cells but decreased metastasis through steps after intravasation. Taken together, our genetic studies show that PAD4 plays a cell autonomous role in cancer metastasis, thus revealing a novel strategy for preventing cancer metastasis by inhibiting cancer cell endogenous PAD4. IMPLICATIONS: This study shows that PADI4 can mediate the formation of CECNs in 4T1 cells, and that endogenous PADI4 plays an essential role in breast cancer lung metastasis. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/molcanres/18/5/735/F1.large.jpg.

Cancer cell-induced neutrophil extracellular traps promote both hypercoagulability and cancer progression

Introduction

Neutrophils can generate extracellular net-like structures by releasing their DNA–histone complexes and antimicrobial peptides, which is called neutrophil extracellular traps (NETs). Various stimuli can induce NET formation. In particular, neutrophils and NET formation are abundant in tumor tissue. This study investigated how cancer cells induce NET formation and whether this NET formation promotes plasma thrombin generation and cancer progression.

Methods

Induction of NET formation by a pancreatic cancer cell line (AsPC-1) was assessed by measuring the histone–DNA complex level. The endogenous thrombin potential (ETP) was measured by thrombin generation assay. In vitro migration, invasion, and tubule formation assays were performed. The circulating levels of NET markers and hypercoagulability markers were assessed in 62 patients with pancreatobiliary malignancy and 30 healthy controls.

Results

AsPC-1 significantly induced NET formation in a dose-dependent manner. Conditioned medium (CM) from AsPC-1 also induced NETs. Interestingly, NET-formation was abolished by heat-inactivated CM, but not by lipid-extracted CM, suggesting an important role of protein components. A reactive oxygen species inhibitor did not inhibit cancer cell–induced NET formation, but prostaglandin E1 (PGE1, cyclic adenosine monophosphate inducer) and antithrombin did. NETs significantly increased ETP of normal plasma. Of note, NETs promoted cancer cell migration and invasion as well as angiogenesis, which were inhibited by histone-binding agents (heparin, polysialic acid), a DNA-degrading enzyme, and Toll-like receptor neutralizing antibodies. In patients with pancreatobiliary malignancy, elevated NET markers correlated well with hypercoagulability makers.

Conclusion

Our findings indicate that cancer cell–induced NET formation enhances both hypercoagulability and cancer progression and suggest that inhibitors of NET formation such as PGE1 and antithrombin can be potential therapeutics to reduce both hypercoagulability and cancer progression.

Elevated extracellular trap formation and contact system activation in acute leukemia

Leukemic cells release their nuclear contents into the extracellular space upon activation. The released nuclear contents, called extracellular traps, can activate the contact system of coagulation. This study accessed the extent of contact system activation, the levels of extracellular traps, and coagulation activation in hematologic malignancies including acute leukemia. In 154 patients with hematologic malignancies (acute leukemia, n = 29; myelodysplastic syndrome, n = 20; myeloproliferative neoplasms, n = 69; plasma cell myeloma, n = 36) and 48 normal controls, the levels of coagulation factors (fibrinogen and factor VII, VIII, IX, and XII), D-dimer, thrombin generation, extracellular trap markers (histone-DNA complex, cell-free dsDNA, leukocyte elastase), and contact system markers (activated factor XII [XIIa], high-molecular-weight kininogen, prekallikrein, bradykinin) were measured. Patients with acute leukemia showed the highest levels of peak thrombin, extracellular trap markers, and factor XIIa. Factor XIIa level was significantly associated with the presence of acute leukemia. The histone-DNA complex and cell-free dsDNA were revealed as significant associated factors with the factor XIIa level. Three markers of extracellular traps and two markers of thrombin generation significantly contributed to the hemostatic abnormalities in hematologic malignancies. Contact system was activated in acute leukemia and its activation was significantly associated with the extent of extracellular trap formation. This finding suggests that extracellular traps might be a major source of contact system activation and therapeutic strategies targeting extracellular trap formation or contact system activation may be beneficial in acute leukemia.

Damage-associated molecular patterns in intensive care unit patients with acute liver injuries: A prospective cohort study

Acute liver injury (ALI) is frequently detected in an intensive care unit (ICU) and reportedly affects prognosis. Experimental animal studies suggested that increased extracellular histone and high morbidity group box-1 (HMGB1) levels might contribute to ALI development. Whether these damage-associated molecular patterns (DAMPs) play a crucial role in ALI remains unclear in the human clinical setting.We consecutively enrolled the patients admitted to our ICU. The patients with ALI were included in the analysis together with those without ALI by using frequency matching. Extracellular histone, HMGB1, soluble thrombomodulin (sTM), and interleukin-6 (IL-6) levels were measured in plasma collected at ICU admission. ALI was defined as an acute elevation in serum aminotransferase levels to >200 IU/L.A total of 805 patients were enrolled. Twenty ALI and forty non-ALI patients were analyzed. Plasma histone levels were significantly higher in the ALI group than in the non-ALI group, whereas HMGB1 levels were significantly lower in the ALI group. Furthermore, sTM was significantly increased in the ALI patients, whereas IL-6 levels were comparable between the groups. Multivariate logistic regression analysis demonstrated that histones were independently associated with ALI. There was no significant impact of ALI on in-hospital mortality.Extracellular histones showed an independent association with ALI. Histone elevation might be one of the possible pathogenic mechanisms in the development of ALI of ICU patients.

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.

Molecular actions of heparin and their implications in preventing pre-eclampsia

Pre-eclampsia, a hypertensive disorder of pregnancy, continues to be a significant cause of global maternal morbidity. Low-dose aspirin remains the only standard-of-care prophylactic therapy for preventing pre-eclampsia, but is limited in efficacy. Heparin and its derivatives may further enhance the efficacy of aspirin therapy to prevent pre-eclampsia, but the mechanisms mediating this augmentative effect are not known. Although heparin is an anticoagulant agent, it also possesses many anticoagulant-independent properties that may be relevant in the prevention of pre-eclampsia, including effects on placental, vascular and inflammatory function. This review summarizes the non-anticoagulant properties of heparin, and extrapolates how these actions may influence the trajectory of pre-eclampsia pathogenesis as a means of pathway-specific therapy.

Proteomic Analysis of Extracellular HMGB1 Identifies Binding Partners and Exposes Its Potential Role in Airway Epithelial Cell Homeostasis

The release of damage-associated molecular patterns (DAMPs) by airway epithelial cells is believed to play a crucial role in the initiation and development of chronic airway conditions such as asthma and chronic obstructive pulmonary disease (COPD). Intriguingly, the classic DAMP high-mobility group box-1 (HMGB1) is detected in the culture supernatant of airway epithelial cells under basal conditions, indicating a role for HMGB1 in the regulation of epithelial cellular and immune homeostasis. To gain contextual insight into the potential role of HMGB1 in airway epithelial cell homeostasis, we used the orthogonal and complementary methods of high-resolution clear native electrophoresis, immunoprecipitation, and pull-downs coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) to profile HMGB1 and its binding partners in the culture supernatant of unstimulated airway epithelial cells. We found that HMGB1 presents exclusively as a protein complex under basal conditions. Moreover, protein network analysis performed on 185 binding proteins revealed 14 that directly associate with HMGB1: amyloid precursor protein, F-actin-capping protein subunit alpha-1 (CAPZA1), glyceraldehyde-3 phosphate dehydrogenase (GAPDH), ubiquitin, several members of the heat shock protein family (HSPA8, HSP90B1, HSP90AA1), XRCC5 and XRCC6, high mobility group A1 (HMGA1), histone 3 (H3F3B), the FACT (facilitates chromatin transcription) complex constituents SUPT1H and SSRP1, and heterogeneous ribonucleoprotein K (HNRNPK). These studies provide a new understanding of the extracellular functions of HMGB1 in cellular and immune homeostasis at the airway mucosal surface and could have implications for therapeutic targeting.