Thrombin contributes to protective immunity in pneumonia-derived sepsis via fibrin polymerization and platelet-neutrophil interactions

Topical adjunctive treatment with flagellin augments pulmonary neutrophil responses and reduces bacterial dissemination in multidrug-resistant K. pneumoniae infection

Objective

Antimicrobial resistance is an emerging problem and multi-drug resistant (MDR) Klebsiella pneumoniae (K. pneumoniae) represents an enormous risk of failing therapy in hospital-acquired pneumonia. The current study aimed to determine the immunomodulatory effect of topical flagellin in addition to antibiotic treatment during respiratory infection evoked by hypervirulent antibiotic-susceptible and antibiotic-resistant K. pneumoniae in mice.

Methods

C57BL6 mice were inoculated intranasally with hypervirulent K. pneumoniae (K2:O1) which was either antibiotic-susceptible or multi-drug resistant. Six hours after infection, mice were treated with antibiotics intraperitoneally and flagellin or vehicle intranasally. Mice were sacrificed 24 hours after infection. Samples were analyzed for bacterial loads and for inflammatory and coagulation markers.

Results

Flagellin therapy induced neutrophil influx in the lung during antibiotic-treated pneumonia evoked by either antibiotic-susceptible or -resistant K. pneumoniae. The pulmonary neutrophil response was matched by elevated levels of neutrophil-attracting chemokines, neutrophil degranulation products, and local coagulation activation. The combined therapy of effective antibiotics and flagellin did not impact K. pneumoniae outgrowth in the lung, but decreased bacterial counts in distant organs. Neutrophil depletion abrogated the flagellin-mediated effect on bacterial dissemination and local coagulation responses.

Conclusion

Topical flagellin administration as an adjunctive to antibiotic treatment augments neutrophil responses during pneumonia evoked by MDR-K. pneumoniae, thereby reducing bacterial dissemination to distant organs.

Lung endothelium, tau, and amyloids in health and disease

Lung endothelia in the arteries, capillaries, and veins are heterogeneous in structure and function. Lung capillaries in particular represent a unique vascular niche, with a thin yet highly restrictive alveolar-capillary barrier that optimizes gas exchange. Capillary endothelium surveys the blood while simultaneously interpreting cues initiated within the alveolus and communicated via immediately adjacent type I and type II epithelial cells, fibroblasts, and pericytes. This cell-cell communication is necessary to coordinate the immune response to lower respiratory tract infection. Recent discoveries identify an important role for the microtubule-associated protein tau that is expressed in lung capillary endothelia in the host-pathogen interaction. This endothelial tau stabilizes microtubules necessary for barrier integrity, yet infection drives production of cytotoxic tau variants that are released into the airways and circulation, where they contribute to end-organ dysfunction. Similarly, beta-amyloid is produced during infection. Beta-amyloid has antimicrobial activity, but during infection it can acquire cytotoxic activity that is deleterious to the host. The production and function of these cytotoxic tau and amyloid variants are the subject of this review. Lung-derived cytotoxic tau and amyloid variants are a recently discovered mechanism of end-organ dysfunction, including neurocognitive dysfunction, during and in the aftermath of infection.

Platelet ITGA2B inhibits caspase-8 and Rip3/Mlkl-dependent platelet death though PTPN6 during sepsis

Platelets play an important role in the pathogenesis of sepsis and platelet transfusion is a therapeutic option for sepsis patients, although the exact mechanisms have not been elucidated so far. ITGA2B encodes the αIIb protein in platelets, and its upregulation in sepsis is associated with increased mortality rate. Here, we generated a Itga2b (Q887X) knockin mouse, which significantly reduced ITGA2B expression of platelet and megakaryocyte. The decrease of ITGA2B level aggravated the death of septic mice. We analyzed the transcriptomic profiles of the platelets using RNA sequencing. Our findings suggest that ITGA2B upregulates PTPN6 in megakaryocytes via the transcription factors Nfkb1 and Rel. Furthermore, PTPN6 inhibits platelet apoptosis and necroptosis during sepsis by targeting the Ripk1/Ripk3/Mlkl and caspase-8 pathways. This prevents Kupffer cells from rapidly clearing activated platelets, and eventually maintains vascular integrity during sepsis. Our findings indicate a new function of ITGA2B in the regulation of platelet death during sepsis.

Fusobacterium nucleatum Promotes Megakaryocyte Maturation in Patients with Gastric Cancer via Inducing the Production of Extracellular Vesicles Containing 14-3-3ε

Fusobacterium nucleatum colonization contributes to the occurrence of portal vein thrombosis in patients with gastric cancer (GC). However, the underlying mechanism by which F. nucleatum promotes thrombosis remains unclear. In this study, we recruited a total of 91 patients with GC and examined the presence of F. nucleatum in tumor and adjacent non-tumor tissues by fluorescence in situ hybridization and quantitative PCR. Neutrophil extracellular traps (NETs) were detected by immunohistochemistry. Extracellular vesicles (EVs) were extracted from the peripheral blood and proteins in the EVs were identified by mass spectrometry (MS). HL-60 cells differentiated into neutrophils were used to package engineered EVs to imitate the EVs released from NETs. Hematopoietic progenitor cells (HPCs) and K562 cells were used for megakaryocyte (MK) in vitro differentiation and maturation to examine the function of EVs. We observed that F. nucleatum-positive patients had increased NET and platelet counts. EVs from F. nucleatum-positive patients could promote the differentiation and maturation of MKs and had upregulated 14-3-3 proteins, especially 14-3-3ε. 14-3-3ε upregulation promoted MK differentiation and maturation in vitro. HPCs and K562 cells could receive 14-3-3ε from the EVs, which interacted with GP1BA and 14-3-3ζ to trigger PI3K-Akt signaling. In conclusion, we identified for the first time that F. nucleatum infection promotes NET formation, which releases EVs containing 14-3-3ε. These EVs could deliver 14-3-3ε to HPCs and promote their differentiation into MKs via activation of PI3K-Akt signaling.

Coagulopathy and thromboembolic events a pathogenic mechanism of COVID-19 associated with mortality: An updated review

During 2019, the SARS-CoV-2 emerged from China, and during months, COVID-19 spread in many countries around the world. The expanding data about pathogenesis of this virus could elucidate the exact mechanism by which COVID-19 caused death in humans. One of the pathogenic mechanisms of this disease is coagulation. Coagulation disorders that affect both venous and arterial systems occur in patients with COVID-19. The possible mechanism involved in the coagulation could be excessive inflammation induced by SARS-CoV-2. However, it is not yet clear well how SARS-CoV-2 promotes coagulopathy. However, some factors, such as pulmonary endothelial cell damage and some anticoagulant system disorders, are assumed to have an important role. In this study, we assessed conducted studies about COVID-19-induced coagulopathy to obtain clearer vision of the wide range of manifestations and possible pathogenesis mechanisms.

Development of Host-Cleavable Antibody-Bactericide Conjugates against Extracellular Pathogens

Novel antimicrobial agents with potent bactericidal activity are needed to treat infections caused by multidrug-resistant (MDR) extracellular pathogens, such as Pseudomonas aeruginosa. Antimicrobial peptides (AMPs) and peptidomimetics are promising alternatives to traditional antibiotics, but their therapeutic use is limited due to the lack of specificity and resulting off-target effects. The incorporation of an antibody into the drug design would alleviate these challenges by localizing the AMP to the target bacterial cells. Antibody-drug conjugates (ADCs) have already achieved clinical success as anticancer therapeutics, due to the ability of the antibody to deliver the payload directly to the cancer cells. This strategy involves the selective delivery of highly cytotoxic drugs to the target cells, which enables a broad therapeutic window. This platform can be translated to the treatment of infections, whereby an antibody is used to deliver an antimicrobial agent to the bacterial antigen. Herein, we propose the development of an antibody-bactericide conjugate (ABC) in which the antibacterial oligothioetheramide (oligoTEA), BDT-4G, is coupled to an anti-P. aeruginosa antibody via a cleavable linker. The drug BDT-4G was chosen based on its efficacy against a range of P. aeruginosa isolates and its ability to evade mechanisms conferring resistance to the last-resort agent polymyxin B. We demonstrate that the ABC binds to the bacterial cell surface, and following cleavage of the peptide linker, the oligoTEA payload is released and exhibits antipseudomonal activity.

Network pharmacology- and molecular simulation-based exploration of therapeutic targets and mechanisms of heparin for the treatment of sepsis/COVID-19

Critically infected patients with COVID-19 (coronavirus disease 2019) are prone to develop sepsis-related coagulopathy as a result of a robust immune response. The mechanism underlying the relationship between sepsis and COVID-19 is largely unknown. LMWH (low molecular weight heparin) exhibits both anti-inflammatory and anti-coagulating properties that result in a better prognosis of severely ill patients with COVID-19 co-associated with sepsis-induced coagulopathy or with a higher D-dimer value. Heparin-associated molecular targets and their mechanism of action in sepsis/COVID-19 are not well understood. In this work, we characterize the pharmacological targets, biological functions and therapeutic actions of heparin in sepsis/COVID-19 from the perspective of network pharmacology. A total of 38 potential targets for heparin action against sepsis/COVID-19 and 8 core pharmacological targets were identified, including IL6, KNG1, CXCL8, ALB, VEGFA, F2, IL10 and TNF. Moreover, enrichment analysis showed that heparin could help in treating sepsis/COVID-19 through immunomodulation, inhibition of the inflammatory response, regulation of angiogenesis and antiviral activity. The pharmacological effects of heparin against these targets were further confirmed by molecular docking and simulation analysis, suggesting that heparin exerts effective binding capacity by targeting the essential residues in sepsis/COVID-19. Prospective clinical practice evaluations may consider the use of these key prognostic indicators for the treatment of sepsis/COVID-19.Communicated by Ramaswamy H. Sarma.

Unique crosstalk between platelet and leukocyte counts during treatment for acute coronary syndrome: A retrospective observational study

In the pathophysiology of acute coronary syndrome (ACS), platelet (PLT) and neutrophil (Neu) crosstalk may be important for activating coagulation and inflammation. It has been speculated that PLTs and Neu may affect each other's cell counts; however, few studies have investigated this hypothesis. In this study, we measured changes in blood cell counts in 245 patients with ACS during treatment and investigated the mutual effects of each blood cell type. Path diagrams were drawn using structural equation modeling, and temporal changes in the count of each blood cell type and the relevance of these changes were analyzed. Throughout the treatment period, the numbers of all blood cell types (red blood cells [RBCs], leukocytes, and PLTs) were associated with each other before and after treatment. A detailed examination of the different cell types revealed that the PLT count at admission had a significant positive effect on the leukocyte (especially Neu) count after treatment. Conversely, the leukocyte (especially Neu) count at admission had a significant positive effect on the PLT count after treatment. During ACS, PLTs and leukocytes, especially Neu, stimulate each other to increase their numbers. The formation of a PLT-leukocyte complex may increase coagulation activity and inflammation, which can lead to a further increase in the counts of both blood cell types.

Fibrinolytic Proteins and Factor XIII as Predictors of Thrombotic and Hemorrhagic Complications in Hospitalized COVID-19 Patients

Introduction

In a prospective cohort of hospitalized COVID-19 patients, an extensive characterization of hemostatic alterations by both global and specific assays was performed to clarify mechanisms underlying the coagulopathy and identify predictive factors for thrombotic and hemorrhagic events during hospitalization.

Materials and Methods

Intensive care unit (ICU; n = 46) and non-ICU (n = 55) patients were enrolled, and the occurrence of thrombotic and hemorrhagic events was prospectively monitored. At study inclusion, thromboelastometry together with the measurement of specific coagulation proteins and hypercoagulation markers was performed.

Results

Patients (median age 67 years) showed significantly shorter clot formation time together with greater maximum clot firmness by thromboelastometry, increased levels of F1 + 2 and D-dimer, as biomarkers of hypercoagulability, and of procoagulant factors V, VIII, IX, XI, and fibrinogen, while FXIII was significantly reduced. The concentration of fibrinolytic proteins, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor type 1 (PAI-1) were elevated in the overall cohort of patients. Many of these hemostatic alterations were significantly greater in ICU compared to non-ICU subjects and, furthermore, they were associated with inflammatory biomarker elevation [i.e., interleukin 6 (IL-6), C-reactive protein (CRP), neutrophil to lymphocyte ratio (NLR), and procalcitonin]. After enrollment, 7 thrombosis and 14 major bleedings occurred. Analysis of clinical and biological data identified increased t-PA, PAI-1, and NLR values as independent predictive factors for thrombosis, while lower FXIII levels were associated with bleeding.

Conclusion

This study demonstrates alterations in all different hemostatic compartments analyzed, particularly in severe COVID-19 conditions, that strongly correlated with the inflammatory status. A potential role of fibrinolytic proteins together with NLR and of FXIII as predictors of thrombotic and hemorrhagic complications, respectively, is highlighted.

Outpatient and inpatient anticoagulation therapy and the risk for hospital admission and death among COVID-19 patients

BACKGROUND

Coronavirus disease 2019 (COVID-19) is associated with a hypercoagulable state. Limited data exist informing the relationship between anticoagulation therapy and risk for COVID-19 related hospitalization and mortality.

METHODS

We evaluated all patients over the age of 18 diagnosed with COVID-19 in a prospective cohort study from March 4th to August 27th, 2020 among 12 hospitals and 60 clinics of M Health Fairview system (USA). We investigated the relationship between (1) 90-day anticoagulation therapy among outpatients before COVID-19 diagnosis and the risk for hospitalization and mortality and (2) Inpatient anticoagulation therapy and mortality risk.

FINDINGS

Of 6195 patients, 598 were immediately hospitalized and 5597 were treated as outpatients. The overall case-fatality rate was 2•8% (n = 175 deaths). Among the patients who were hospitalized, the inpatient mortality was 13%. Among the 5597 COVID-19 patients initially treated as outpatients, 160 (2.9%) were on anticoagulation and 331 were eventually hospitalized (5.9%). In a multivariable analysis, outpatient anticoagulation use was associated with a 43% reduction in risk for hospital admission, HR (95% CI = 0.57, 0.38-0.86), p = 0.007, but was not associated with mortality, HR (95% CI=0.88, 0.50 - 1.52), p = 0.64. Inpatients who were not on anticoagulation (before or after hospitalization) had an increased risk for mortality, HR (95% CI = 2.26, 1.17-4.37), p = 0.015.

INTERPRETATION

Outpatients with COVID-19 who were on outpatient anticoagulation at the time of diagnosis experienced a 43% reduced risk of hospitalization. Failure to initiate anticoagulation upon hospitalization or maintaining outpatient anticoagulation in hospitalized COVID-19 patients was associated with increased mortality risk.

FUNDING

No funding was obtained for this study.

Protease-Activated Receptor-1 Antagonist Protects Against Lung Ischemia/Reperfusion Injury

Protease-activated receptor (PAR)-1 is a thrombin-activated receptor that plays an essential role in ischemia/reperfusion (IR)-induced acute inflammation. PAR-1 antagonists have been shown to alleviate injuries in various IR models. However, the effect of PAR-1 antagonists on IR-induced acute lung injury (ALI) has not yet been elucidated. This study aimed to investigate whether PAR-1 inhibition could attenuate lung IR injury. Lung IR was induced in an isolated perfused rat lung model. Male rats were treated with the specific PAR-1 antagonist SCH530348 (vorapaxar) or vehicle, followed by ischemia for 40 min and reperfusion for 60 min. To examine the role of PAR-1 and the mechanism of SCH530348 in lung IR injury, western blotting and immunohistochemical analysis of lung tissue were performed. In vitro, mouse lung epithelial cells (MLE-12) were treated with SCH530348 or vehicle and subjected to hypoxia-reoxygenation (HR). We found that SCH530348 decreased lung edema and neutrophil infiltration, attenuated thrombin production, reduced inflammatory factors, including cytokine-induced neutrophil chemoattractant-1, interleukin-6 and tumor necrosis factor-α, mitigated lung cell apoptosis, and downregulated the phosphoinositide 3-kinase (PI3K), nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways in IR-injured lungs. In addition, SCH530348 prevented HR-induced NF-κB activation and inflammatory chemokine production in MLE12 cells. Our results demonstrate that SCH530348 exerts protective effects by blocking PAR-1 expression and modulating the downstream PI3K, NF-κB and MAPK pathways. These findings indicate that the PAR-1 antagonist protects against IR-induced ALI and is a potential therapeutic candidate for lung protection following IR injury.

Liver-Dependent Lung Remodeling during Systemic Inflammation Shapes Responses to Secondary Infection

Systemic duress, such as that elicited by sepsis, burns, or trauma, predisposes patients to secondary pneumonia, demanding better understanding of host pathways influencing this deleterious connection. These pre-existing circumstances are capable of triggering the hepatic acute-phase response (APR), which we previously demonstrated is essential for limiting susceptibility to secondary lung infections. To identify potential mechanisms underlying protection afforded by the lung-liver axis, our studies aimed to evaluate liver-dependent lung reprogramming when a systemic inflammatory challenge precedes pneumonia. Wild-type mice and APR-deficient littermate mice with hepatocyte-specific deletion of STAT3 (hepSTAT3-/-), a transcription factor necessary for full APR initiation, were challenged i.p. with LPS to induce endotoxemia. After 18 h, pneumonia was induced by intratracheal Escherichia coli instillation. Endotoxemia elicited significant transcriptional alterations in the lungs of wild-type and hepSTAT3-/- mice, with nearly 2000 differentially expressed genes between genotypes. The gene signatures revealed exaggerated immune activity in the lungs of hepSTAT3-/- mice, which were compromised in their capacity to launch additional cytokine responses to secondary infection. Proteomics revealed substantial liver-dependent modifications in the airspaces of pneumonic mice, implicating a network of dispatched liver-derived mediators influencing lung homeostasis. These results indicate that after systemic inflammation, liver acute-phase changes dramatically remodel the lungs, resulting in a modified landscape for any stimuli encountered thereafter. Based on the established vulnerability of hepSTAT3-/- mice to secondary lung infections, we believe that intact liver function is critical for maintaining the immunological responsiveness of the lungs.

Hypoxia-Inducible Factor-1α in Macrophages, but Not in Neutrophils, Is Important for Host Defense during Klebsiella pneumoniae-Induced Pneumosepsis

Hypoxia-inducible factor- (HIF-) 1α has been implicated in the ability of cells to adapt to alterations in oxygen levels. Bacterial stimuli can induce HIF1α in immune cells, including those of myeloid origin. We here determined the role of myeloid cell HIF1α in the host response during pneumonia and sepsis caused by the common human pathogen Klebsiella pneumoniae. To this end, we generated mice deficient for HIF1α in myeloid cells (LysM-cre × Hif1α^fl/fl) or neutrophils (Mrp8-cre × Hif1α^fl/fl) and infected these with Klebsiella pneumoniae via the airways. Myeloid, but not neutrophil, HIF1α-deficient mice had increased bacterial loads in the lungs and distant organs after infection as compared to control mice, pointing at a role for HIF1α in macrophages. Myeloid HIF1α-deficient mice did not show increased bacterial growth after intravenous infection, suggesting that their phenotype during pneumonia was mediated by lung macrophages. Alveolar and lung interstitial macrophages from LysM-cre × Hif1α^fl/fl mice produced lower amounts of the immune enhancing cytokine tumor necrosis factor upon stimulation with Klebsiella, while their capacity to phagocytose or to produce reactive oxygen species was unaltered. Alveolar macrophages did not upregulate glycolysis in response to lipopolysaccharide, irrespective of HIF1α presence. These data suggest a role for HIF1α expressed in lung macrophages in protective innate immunity during pneumonia caused by a common bacterial pathogen.

The Long Pentraxin PTX3 Controls Klebsiella Pneumoniae Severe Infection

Klebsiella pneumoniae is a common pathogen in human sepsis. The emergence of multidrug-resistant K. pneumoniae strains represents a major clinical challenge in nosocomial and community acquired infections. The long pentraxin PTX3, a key component of humoral innate immunity, is involved in resistance to selected pathogens by promoting opsonophagocytosis. We investigated the relevance of PTX3 in innate immunity against K. pneumoniae infections using Ptx3^-/- mice and mouse models of severe K. pneumoniae infections. Local and systemic PTX3 expression was induced following K. pneumoniae pulmonary infection, in association with the up-regulation of TNF-α and IL-1β. PTX3 deficiency in mice was associated with higher bacterial burden and mortality, release of pro-inflammatory cytokines as well as IL-10 in the lung and systemically. The analysis of the mechanisms responsible of PTX3-dependent control of K. pneumoniae infection revealed that PTX3 did not interact with K. pneumoniae, or promote opsonophagocytosis. The comparison of susceptibility of wild-type, Ptx3^-/-, C3^-/- and Ptx3^-/-/C3^-/- mice to the infection showed that PTX3 acted in a complement-independent manner. Lung histopathological analysis showed more severe lesions in Ptx3^-/- mice with fibrinosuppurative, necrotizing and haemorrhagic bronchopneumonia, associated with increased fibrin deposition in the lung and circulating fibrinogen consumption. These findings indicate that PTX3 contributes to the control of K. pneumoniae infection by modulating inflammatory responses and tissue damage. Thus, this study emphasizes the relevance of the role of PTX3 as regulator of inflammation and orchestrator of tissue repair in innate responses to infections.

Neutrophil extracellular traps promote gastric cancer metastasis by inducing epithelial‑mesenchymal transition

The risks of tumor recurrence following the successful resection of the primary tumor have been known for decades; however, the precise mechanisms underlying treatment failures remain unknown. The formation of neutrophil extracellular traps (NETs) has increasingly been demonstrated to be associated with thrombi formation in cancer patients, as well as with the development and metastasis of cancer. The present study demonstrated that the level of peripheral blood NETs in patients with gastric cancer (GC) was associated with tumor progression, and patients with stage III/IV disease exhibited significant differences compared with the healthy controls and patients with stage I/II disease, which may be associated with an increased risk of metastasis. In addition, plasma from patients with stage III/IV GC was more prone to stimulate neutrophils to form NETs; thus, it was hypothesized that the formation of NETs may be affected by the tumor microenvironment. A higher deposition of NETs in GC tissues compared with normal resection margins was also identified. In vitro, following treatment with phorbol myristate acetate, which promotes the formation of NETs, or with DNAse-1/GSK-484, which inhibits the formation of NETs, it was found that the tumor migratory ability was altered; however, no significant changes were observed in cell proliferation and cell cycle progression. Epithelial-mesenchymal transition (EMT) is a key event associated with dissemination and metastasis in GC pathogenesis. Finally, the present study demonstrated that NETs promote a more aggressive mesenchymal phenotype and promote the progression of GC in vitro and in vivo. On the whole, to the best of our knowledge, the present study reports a previously unknown role of NETs in the regulation of GC, which is associated with EMT and migration. Therefore, targeting NETs may prove to be therapeutically beneficial.

Complement component factor B has thrombin-like activity

Serine proteases are fundamental components of biology, including innate immunity, which is systematically orchestrated in an orderly, balanced fashion in the healthy host. Such serine proteases are found in two well-recognized pathways of an innate immune network, coagulation and complement. Both pathways, if uncontrolled due to a variety of causes, are pathogenic in numerous diseases, including coagulation disorders and infectious diseases. Previous studies have reported sequence homologies, functional similarities and interplay between these two pathways with some implications in health and disease. The current study newly reveals that complement component factor B (Bf), the second component of the alternative complement pathway, has thrombin-like activity, which is supported by a characteristic homology of the trypsin-like domain of Bf to that of thrombin. Moreover, we newly report that the trypsin-like domain of Bf is closely related to Limulus clotting factor C, the LPS sensitive clotting factor of the innate immune system. We will also discuss potential implications of our findings in diseases.

Pathogenesis, Diagnosis, and Treatment of Hemostatic Disorders in COVID-19 Patients

The novel coronavirus infection named COVID-19 was first detected in Wuhan, China, in December 2019, and it has been responsible for significant morbidity and mortality in scores of countries. At the time this article was being written, the number of infected and deceased patients continued to grow worldwide. Most patients with severe forms of the disease suffer from pneumonia and pulmonary insufficiency; in many cases, the disease is generalized and causes multiple organ failures and a dysfunction of physiological systems. One of the most serious and prognostically ominous complications from COVID-19 is coagulopathy, in particular, decompensated hypercoagulability with the risk of developing disseminated intravascular coagulation. In most cases, local and diffuse macro- and microthromboses are present, a condition which causes multiple-organ failure and thromboembolic complications. The causes and pathogenic mechanisms of coagulopathy in COVID-19 remain largely unclear, but they are associated with systemic inflammation, including the so-called cytokine storm. Despite the relatively short period of the ongoing pandemic, laboratory signs of serious hemostatic disorders have been identified and measures for specific prevention and correction of thrombosis have been developed. This review discusses the causes of COVID-19 coagulopathies and the associated complications, as well as possible approaches to their early diagnosis, prevention, and treatment.

Thrombin-Fibrin(ogen) Interactions, Host Defense and Risk of Thrombosis

Fibrinogen is a well-known risk factor for arterial and venous thrombosis. Its function is not restricted to clot formation, however, as it partakes in a complex interplay between thrombin, soluble plasma fibrinogen, and deposited fibrin matrices. Fibrinogen, like thrombin, participates predominantly in hemostasis to maintain vascular integrity, but executes some important pleiotropic effects: firstly, as observed in thrombin generation experiments, fibrin removes thrombin from free solution by adsorption. The adsorbed thrombin is protected from antithrombins, notably α2-macroglobulin, and remains physiologically active as it can activate factors V, VIII, and platelets. Secondly, immobilized fibrinogen or fibrin matrices activate monocytes/macrophages and neutrophils via Mac-1 interactions. Immobilized fibrin(ogen) thereby elicits a pro-inflammatory response with a reciprocal stimulating effect of the immune system on coagulation. In contrast, soluble fibrinogen prohibits recruitment of these immune cells. Thus, while fibrin matrices elicit a procoagulant response, both directly by protecting thrombin and indirectly through the immune system, high soluble fibrinogen levels might protect patients due to its immune diminutive function. The in vivo influence of the 'protective' plasma fibrinogen versus the 'pro-thrombotic' fibrin matrices on thrombosis should be explored in future research.

Regulation, Activation and Function of Caspase-11 during Health and Disease

Caspase-11 is a pro-inflammatory enzyme that is stringently regulated during its expression and activation. As caspase-11 is not constitutively expressed in cells, it requires a priming step for its upregulation, which occurs following the stimulation of pathogen and cytokine receptors. Once expressed, caspase-11 activation is triggered by its interaction with lipopolysaccharide (LPS) from Gram-negative bacteria. Being an initiator caspase, activated caspase-11 functions primarily through its cleavage of key substrates. Gasdermin D (GSDMD) is the primary substrate of caspase-11, and the GSDMD cleavage fragment generated is responsible for the inflammatory form of cell death, pyroptosis, via its formation of pores in the plasma membrane. Thus, caspase-11 functions as an intracellular sensor for LPS and an immune effector. This review provides an overview of caspase-11—describing its structure and the transcriptional mechanisms that govern its expression, in addition to its activation, which is reported to be regulated by factors such as guanylate-binding proteins (GBPs), high mobility group box 1 (HMGB1) protein, and oxidized phospholipids. We also discuss the functional outcomes of caspase-11 activation, which include the non-canonical inflammasome, modulation of actin dynamics, and the initiation of blood coagulation, highlighting the importance of inflammatory caspase-11 during infection and disease.

Platelets as key players in inflammation and infection

PURPOSE OF REVIEW

This review highlights recent insights into the role of platelets in acute inflammation and infection.

RECENT FINDINGS

Platelets exhibit intravascular crawling behavior and can collect and bundle bacteria. In addition, platelets are key in promoting intravascular thrombus formation in infection, a process termed 'immunothrombosis', which contributes to pathogen containment, but also potentially damages the host. Platelets are at the nexus of leukocyte recruitment and activation, yet they are at the same time crucial in preventing inflammation-associated hemorrhage and tissue damage. This multitasking requires specific receptors and pathways, depending on stimulus, organ and effector function.

SUMMARY

New findings highlight the complex interplay of innate immunity, coagulation and platelets in inflammation and infection, and unravel novel molecular pathways and effector functions. These offer new potential therapeutic approaches, but require further extensive research to distinguish treatable proinflammatory from host-protective pathways.

Does fibrinogen serve the host or the microbe in Staphylococcus infection?

PURPOSE OF REVIEW

Fibrin(ogen) is a multifunctional clotting protein that not only has critical roles in hemostasis but is also important in inflammatory processes that control bacterial infection. As a provisional extracellular matrix protein, fibrin(ogen) functions as a physical barrier, a scaffold for immune cell migration, or as a spatially-defined cue to drive inflammatory cell activation. These mechanisms contribute to overall host antimicrobial defense against infection. However, numerous bacterial species have evolved mechanisms to manipulate host fibrin(ogen) to promote microbial virulence and survival. Staphylococcal species, in particular, express numerous virulence factors capable of engaging fibrin(ogen), promoting fibrin formation, and driving the dissolution of fibrin matrices.

RECENT FINDINGS

Recent studies have highlighted both new insights into the molecular mechanisms involved in fibrin(ogen)-mediated host defense and pathogen-driven virulence. Of particular interest is the role of fibrin(ogen) in forming host protective biofilms versus pathogen protective barriers and biofilms as well as the role of fibrin(ogen) in mediating direct host antimicrobial responses.

SUMMARY

Current data suggest that the role of fibrin(ogen) in staphylococcal infection is highly context-dependent and that better defining the precise cellular and molecular pathways activated will provide unique opportunities of therapeutic intervention to better treat Staphylococcal disease.

Caspase-11 contributes to pulmonary host defense against Klebsiella pneumoniae and local activation of coagulation

Klebsiella (K.) pneumoniae is a common cause of gram-negative pneumonia and sepsis. Caspase-11 is an intracellular receptor for lipopolysaccharide and regulates pyroptosis, a specific form of inflammatory cell death, which aids in host defense against intracellular gram-negative bacteria. Recently, caspase-11 has also been implicated in blood coagulation. Previously, we found that local fibrin formation contributes to protective immunity against Klebsiella infection of the lung. The aim of the present study was to determine the role of caspase-11 in host defense during K. pneumoniae-evoked pneumonia and sepsis. Therefore, we infected wild-type and caspase-11-deficient (Casp11^-/-) mice with a low-dose K. pneumoniae via the airways to induce a gradually evolving pneumosepsis. Casp11^-/- mice displayed increased bacterial numbers in the lung 12 h and 48 h after inoculation. Analysis of pulmonary IL-1α, IL-1β, and TNF levels showed reduced IL-1α levels in bronchoalveolar lavage fluid and increased TNF levels in the lung of Casp11^-/- mice at 48 h after inoculation. Lung γH2AX staining (marker for cell death), lung pathology and neutrophil influx in the lung, as well as bacterial dissemination and organ damage, however, were not altered in Casp11^-/- mice after Klebsiella infection. Strikingly, analysis of cross-linked fibrin and D-dimer (markers for coagulation) revealed significantly less fibrin formation in the lungs of Casp11^-/- mice at either time point after Klebsiella infection. These data reveal that caspase-11 contributes to protective immunity against K. pneumoniae possibly by activation of blood coagulation in the lung.

Inhibition of triggering receptor expressed on myeloid cells-1 impairs thrombin generation

BACKGROUND

New evidence has shown the link between inflammation and thrombosis. Triggering receptor expressed on myeloid cells-1 (TREM-1) is an immunoreceptor expressed mostly on neutrophils and monocytes/macrophages. TREM-1 acts as an amplifier of the inflammatory response, and its pharmacological inhibition displays protective effects in various models of inflammatory disorders, in particular by dampening coagulation abnormalities and thrombocytopenia observed during acute inflammation.

OBJECTIVES

We aimed to decipher the role of TREM-1 in fostering thrombin generation.

METHODS

We measured thrombin generation (TG) by the use of calibrated automated thrombography with whole blood, and isolated primary human neutrophils and monocytes upon stimulation with lipopolysaccharide (LPS). Tissue factor (TF) expression was measured by flow cytometry and its activity by ELISA. Phosphatidylserine (PtdSer) exposure was determined by flow cytometry. A dodecapeptide (LR12) was used as a specific inhibitor of TREM-1.

RESULTS

LPS increased TG, TF expression, and activity, as well as the exposure of PtdSer on the surface of monocytes. LR12 dampened TF activity through the decrease of PtdSer exposure, leading to a reduction of thrombin generation.

CONCLUSIONS

TREM-1 inhibition decreases thrombin generation and could be an interesting target for the development of new inhibitors of leukocyte-associated thrombotic activity.

Prekallikrein inhibits innate immune signaling in the lung and impairs host defense during pneumosepsis in mice

Prekallikrein (PKK, also known as Fletcher factor and encoded by the gene KLKB1 in humans) is a component of the contact system. Activation of the contact system has been implicated in lethality in fulminant sepsis models. Pneumonia is the most frequent cause of sepsis. We sought to determine the role of PKK in host defense during pneumosepsis. To this end, mice were infected with the common human pathogen Klebsiella pneumoniae via the airways, causing an initially localized infection of the lungs with subsequent bacterial dissemination and sepsis. Mice were treated with a selective PKK‐directed antisense oligonucleotide (ASO) or a scrambled control ASO for 3 weeks prior to infection. Host response readouts were determined at 12 or 36 h post‐infection, including genome‐wide messenger RNA profiling of lungs, or mice were followed for survival. PKK ASO treatment inhibited constitutive hepatic Klkb1 mRNA expression by >80% and almost completely abolished plasma PKK activity. Klkb1 mRNA could not be detected in lungs. Pneumonia was associated with a progressive decline in PKK expression in mice treated with control ASO. PKK ASO administration was associated with a delayed mortality, reduced bacterial burdens, and diminished distant organ injury. While PKK depletion did not influence lung pathology or neutrophil recruitment, it was associated with an upregulation of multiple innate immune signaling pathways in the lungs already prior to infection. Activation of the contact system could not be detected, either during infection in vivo or at the surface of Klebsiella in vitro. These data suggest that circulating PKK confines pro‐inflammatory signaling in the lung by a mechanism that does not involve contact system activation, which in the case of respiratory tract infection may impede early protective innate immunity. © 2019 Authors. Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Role of tissue factor in the procoagulant and antibacterial effects of human adipose-derived mesenchymal stem cells during pneumosepsis in mice

Background

Adult mesenchymal stem cells (MSCs) improve the host response during experimental sepsis in animals. MSCs from various sources express a procoagulant activity that has been linked to the expression of tissue factor. This study sought to determine the role of tissue factor associated with adipose-derived MSCs (ASCs) in their procoagulant and antibacterial effects during pneumonia-derived sepsis.

Methods

Mice were infused intravenously with ASCs or vehicle after infection with the common human pathogen Klebsiella pneumoniae via the airways.

Results

Infusion of freshly cultured or cryopreserved ASCs induced the expression of many genes associated with tissue factor signaling and coagulation activation in the lungs. Freshly cultured and cryopreserved ASCs, as well as ASC lysates, exerted procoagulant activity in vitro as determined by a fibrin generation assay, which was almost completely inhibited by an anti-tissue factor antibody. Infusion of cryopreserved ASCs was associated with a rise in plasma thrombin-antithrombin complexes (indicative of coagulation activation) and formation of multiple thrombi in the lungs 4 h post-infusion. Preincubation of ASCs with anti-tissue factor antibody prior to infusion prevented the rise in plasma thrombin-antithrombin complex concentrations but did not influence thrombus formation in the lungs. ASCs reduced bacterial loads in the lungs and liver at 48 h after infection, which was not influenced by preincubation with anti-tissue factor antibody. At this late time point, microthrombi in the lungs were not detected anymore.

Conclusion

These data indicate that ASC-associated tissue factor is responsible for systemic activation of coagulation after infusion of ASCs but not for the formation of microthrombi in the lungs or antibacterial effects.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1391-x) contains supplementary material, which is available to authorized users.

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.

Inhibition of the extrinsic or intrinsic coagulation pathway during pneumonia-derived sepsis

Pneumonia is the most frequent cause of sepsis, and Klebsiella pneumoniae is a common pathogen in pneumonia and sepsis. Infection is associated with activation of the coagulation system. Coagulation can be activated by the extrinsic and intrinsic routes, mediated by factor VII (FVII) and factor XII (FXII), respectively. To determine the role of FVII and FXII in the host response during pneumonia-derived sepsis, mice were treated with specific antisense oligonucleotide (ASO) directed at FVII or FXII for 3 wk before infection with K. pneumoniae via the airways. FVII ASO treatment strongly inhibited hepatic FVII mRNA expression, reduced plasma FVII to ~25% of control, and selectively prolonged the prothrombin time. FXII ASO treatment strongly suppressed hepatic FXII mRNA expression, reduced plasma FXII to ~20% of control, and selectively prolonged the activated partial thromboplastin time. Lungs also expressed FVII mRNA, which was not altered by FVII ASO administration. Very low FXII mRNA levels were detected in lungs, which were not modified by FXII ASO treatment. FVII ASO attenuated systemic activation of coagulation but did not influence fibrin deposition in lung tissue. FVII ASO enhanced bacterial loads in lungs and mitigated sepsis-induced distant organ injury. FXII inhibition did not affect any of the host response parameters measured. These results suggest that partial inhibition of FVII, but not of FXII, modifies the host response to gram-negative pneumonia-derived sepsis.

Nbeal2 Deficiency Increases Organ Damage but Does Not Affect Host Defense During Gram-Negative Pneumonia-Derived Sepsis

Objective- Nbeal2-/- mice, a model of human gray platelet syndrome, have reduced neutrophil granularity and impaired host defense against systemic Staphylococcus aureus infection. We here aimed to study the role of Nbeal2 deficiency in both leukocytes and platelets during gram-negative pneumonia and sepsis. Approach and Results- We studied the role of Nbeal2 in platelets and leukocytes during murine pneumonia and sepsis by Klebsiella pneumoniae. Apart from platelet α-granule deficiency and reduced neutrophil granularity, also monocyte granularity was reduced in Nbeal2-/- mice, whereas plasma levels of MPO (myeloperoxidase), elastase, NGAL (neutrophil gelatinase-associated lipocalin), and MMP-9 (matrix metalloproteinase 9), and leukocyte CD11b expression were increased. Nbeal2-/- leukocytes showed unaltered in vitro antibacterial response and phagocytosis capacity against Klebsiella, and unchanged reactive nitrogen species and cytokine production. Also during Klebsiella pneumonia and sepsis, Nbeal2-/- mice had similar bacterial growth in lung and distant body sites, with enhanced leukocyte migration to the bronchoalveolar space. Despite similar infection-induced inflammation, organ damage was increased in Nbeal2-/- mice, which was also seen during endotoxemia. Platelet-specific Nbeal2 deficiency did not influence leukocyte functions, indicating that Nbeal2 directly modifies leukocytes. Transfusion of Nbeal2-/- but not of Nbeal2+/+ platelets into thrombocytopenic mice was associated with bleeding in the lung but similar host defense, pointing at a role for platelet α-granules in maintaining vascular integrity but not host defense during Klebsiella pneumosepsis. Conclusions- These data show that Nbeal2 deficiency-resulting in gray platelet syndrome-affects platelets, neutrophils, and monocytes, with intact host defense but increased organ damage during gram-negative pneumosepsis.

Role of Peptidylarginine Deiminase 4 in Neutrophil Extracellular Trap Formation and Host Defense during Klebsiella pneumoniae-Induced Pneumonia-Derived Sepsis

Peptidylarginine deiminase 4 (PAD4) catalyzes citrullination of histones, an important step for neutrophil extracellular trap (NET) formation. We aimed to determine the role of PAD4 during pneumonia. Markers of NET formation were measured in lavage fluid from airways of critically ill patients. NET formation and host defense were studied during pneumonia-derived sepsis caused by Klebsiella pneumoniae in PAD4^+/+ and PAD4^-/- mice. Patients with pneumosepsis, compared with those with nonpulmonary disease, showed increased citrullinated histone 3 (CitH3) levels in their airways and a trend toward elevated levels of NET markers cell-free DNA and nucleosomes. During murine pneumosepsis, CitH3 levels were increased in the lungs of PAD4^+/+ but not of PAD4^-/- mice. Combined light and electron microscopy showed NET-like structures surrounding Klebsiella in areas of CitH3 staining in the lung; however, these were also seen in PAD4^-/- mice with absent CitH3 lung staining. Moreover, cell-free DNA and nucleosome levels were mostly similar in both groups. Moreover, Klebsiella and LPS could still induce NETosis in PAD4^-/- neutrophils. Both groups showed largely similar bacterial growth, lung inflammation, and organ injury. In conclusion, these data argue against a major role for PAD4 in NET formation, host defense, or organ injury during pneumonia-derived sepsis.

The coagulation system in host defense

The blood coagulation system and immune system of higher organisms are thought to have a common ancestral origin. During infections, the blood coagulation system is activated and components of the hemostatic system are directly involved in the immune response and immune system modulations. The current view is that the activation of coagulation is beneficial for infections with bacteria and viruses. It limits pathogen dissemination and supports pathogen killing and tissue repair. On the other hand, over-activation can lead to thrombosis with subsequent depletion of hemostatic factors and secondary bleeding. This review will summarize the current knowledge on blood coagulation and pathogen infection with focus on most recent studies of the role of the different parts of the blood coagulation system in selected bacterial and viral infections.

Measuring and interpreting platelet-leukocyte aggregates

Platelets, besides their specialised role in haemostasis and atherothrombosis, actively modulate innate and adaptive immune responses with crucial roles in immune surveillance, inflammation and host defence during infection. An important prerequisite for platelet-mediated changes of immune functions involves direct engagement with different types of leukocytes. Indeed, increased platelet-leukocyte aggregates (PLAs) within the circulation and/or locally at the site of inflammation represent markers of many thrombo-inflammatory diseases, such as cardiovascular diseases, acute lung injury, renal and cerebral inflammation. Therefore, measurement of PLAs could provide an attractive and easily accessible prognostic and/or diagnostic tool for many diseases. To measure PLAs in different (patho-)physiological settings in human and animal models flow cytometric and microscopic approaches have been applied. These techniques represent complementary tools to study different aspects relating to the involvement of leukocyte subtypes and molecules, as well as location of PLAs within tissues, dynamics of their interactions and/or dynamic changes in leukocyte and platelet behaviour. This review summarises various approaches to measure and interpret PLAs and discusses potential experimental factors influencing platelet binding to leukocytes. Furthermore, we summarise insights gained from studies regarding the underlying mechanism of platelet-leukocyte interactions and discuss implications of these interactions in health and disease.

Immunohaemostasis: a new view on haemostasis during sepsis

Host infection by a micro-organism triggers systemic inflammation, innate immunity and complement pathways, but also haemostasis activation. The role of thrombin and fibrin generation in host defence is now recognised, and thrombin has become a partner for survival, while it was seen only as one of the "principal suspects" of multiple organ failure and death during septic shock. This review is first focused on pathophysiology. The role of contact activation system, polyphosphates and neutrophil extracellular traps has emerged, offering new potential therapeutic targets. Interestingly, newly recognised host defence peptides (HDPs), derived from thrombin and other "coagulation" factors, are potent inhibitors of bacterial growth. Inhibition of thrombin generation could promote bacterial growth, while HDPs could become novel therapeutic agents against pathogens when resistance to conventional therapies grows. In a second part, we focused on sepsis-induced coagulopathy diagnostic challenge and stratification from "adaptive" haemostasis to "noxious" disseminated intravascular coagulation (DIC) either thrombotic or haemorrhagic. Besides usual coagulation tests, we discussed cellular haemostasis assessment including neutrophil, platelet and endothelial cell activation. Then, we examined therapeutic opportunities to prevent or to reduce "excess" thrombin generation, while preserving "adaptive" haemostasis. The fail of international randomised trials involving anticoagulants during septic shock may modify the hypothesis considering the end of haemostasis as a target to improve survival. On the one hand, patients at low risk of mortality may not be treated to preserve "immunothrombosis" as a defence when, on the other hand, patients at high risk with patent excess thrombin and fibrin generation could benefit from available (antithrombin, soluble thrombomodulin) or ongoing (FXI and FXII inhibitors) therapies. We propose to better assess coagulation response during infection by an improved knowledge of pathophysiology and systematic testing including determination of DIC scores. This is one of the clues to allocate the right treatment for the right patient at the right moment.

Platelet glycoprotein VI aids in local immunity during pneumonia-derived sepsis caused by gram-negative bacteria

Platelet collagen receptor glycoprotein VI (GPVI) and podoplanin receptor C-type lectin-like receptor 2 (CLEC2) are receptors implicated in platelet activation that both signal via an immunoreceptor tyrosine-based activation motif. Platelets are necessary for host defense and prevention of hemorrhage during sepsis, but the role of platelet GPVI and CLEC2 herein is unknown. To investigate this, we infected mice depleted of platelet GPVI or CLEC2 by antibody treatment or GPVI^-/- mice with the common human sepsis pathogen Klebsiella pneumoniae via the airways to induce pneumonia-derived sepsis. The GPVI ligand collagen and the CLEC2 ligand podoplanin were constitutively present in the lung, whereas the GPVI ligands fibrin and histone were induced during pneumonia. During late-stage infection, both mice depleted of GPVI and GPVI^-/- mice showed increased bacterial growth in lungs, and GPVI^-/- mice also showed increased bacterial growth in distant body sites. Despite higher bacterial loads, GPVI-depleted mice showed reduced platelet numbers, platelet activation, and platelet-leukocyte complex formation in the bronchoalveolar space. Consistently, in human whole blood, GPVI stimulation of platelets increased platelet-leukocyte complex formation and leukocyte activation, which was accompanied by enhanced phagocytosis of Klebsiella GPVI-depleted mice showed increased lung hemorrhage during infection, but not to the extent observed in platelet-depleted mice, and lung bleeding was not significantly different between GPVI^-/- and wild-type mice. CLEC2 depletion did not affect any of the responses during pneumonia. These results suggest that platelet GPVI, but not CLEC2, contributes to local host defense during pneumonia-derived sepsis by enhancing leukocyte function.