Roles for thrombin and fibrin(ogen) in cytokine/chemokine production and macrophage adhesion in vivo

Aberrant coagulation causes a hyper-inflammatory response in severe influenza pneumonia

Influenza A virus (IAV) infects the respiratory tract in humans and causes significant morbidity and mortality worldwide each year. Aggressive inflammation, known as a cytokine storm, is thought to cause most of the damage in the lungs during IAV infection. Dysfunctional coagulation is a common complication in pathogenic influenza, manifested by lung endothelial activation, vascular leak, disseminated intravascular coagulation and pulmonary microembolism. Importantly, emerging evidence shows that an uncontrolled coagulation system, including both the cellular (endothelial cells and platelets) and protein (coagulation factors, anticoagulants and fibrinolysis proteases) components, contributes to the pathogenesis of influenza by augmenting viral replication and immune pathogenesis. In this review, we focus on the underlying mechanisms of the dysfunctional coagulatory response in the pathogenesis of IAV.

Regulation of alveolar procoagulant activity and permeability in direct acute lung injury by lung epithelial tissue factor

Tissue factor (TF) initiates the extrinsic coagulation cascade in response to tissue injury, leading to local fibrin deposition. Low levels of TF in mice are associated with increased severity of acute lung injury (ALI) after intratracheal LPS administration. However, the cellular sources of the TF required for protection from LPS-induced ALI remain unknown. In the current study, transgenic mice with cell-specific deletions of TF in the lung epithelium or myeloid cells were treated with intratracheal LPS to determine the cellular sources of TF important in direct ALI. Cell-specific deletion of TF in the lung epithelium reduced total lung TF expression to 39% of wild-type (WT) levels at baseline and to 29% of WT levels after intratracheal LPS. In contrast, there was no reduction of TF with myeloid cell TF deletion. Mice lacking myeloid cell TF did not differ from WT mice in coagulation, inflammation, permeability, or hemorrhage. However, mice lacking lung epithelial TF had increased tissue injury, impaired activation of coagulation in the airspace, disrupted alveolar permeability, and increased alveolar hemorrhage after intratracheal LPS. Deletion of epithelial TF did not affect alveolar permeability in an indirect model of ALI caused by systemic LPS infusion. These studies demonstrate that the lung epithelium is the primary source of TF in the lung, contributing 60-70% of total lung TF, and that lung epithelial, but not myeloid, TF may be protective in direct ALI.

Retained Blood Syndrome after Cardiac Surgery: A New Look at an Old Problem

Retained blood occurs when drainage systems fail to adequately evacuate blood during recovery from cardiothoracic surgery. As a result, a spectrum of mechanical and inflammatory complications can ensue in the acute, subacute, and chronic setting. The objectives of this review were to define the clinical syndrome associated with retained blood over the spectrum of recovery and to review existing literature regarding how this may lead to complications and contributes to poor outcomes. To better understand and prevent this constellation of clinical complications, a literature review was conducted, which led us to create a new label that better defines the clinical entity we have titled retained blood syndrome. Analysis of published reports revealed that 13.8% to 22.7% of cardiac surgical patients develop one or more components of retained blood syndrome. This can present in the acute, subacute, or chronic setting, with different pathophysiologic mechanisms active at different times. The development of retained blood syndrome has been linked to other clinical outcomes, including the development of postoperative atrial fibrillation and infection and the need for hospital readmission. Grouping multiple objectively measurable and potentially preventable postoperative complications that share a common etiology of retained blood over the continuum of recovery demonstrates a high prevalence of retained blood syndrome. This suggests the need to develop, implement, and test clinical strategies to enhance surgical drainage and reduce postoperative complications in patients undergoing cardiothoracic surgery.

Pravastatin inhibits fibrinogen- and FDP-induced inflammatory response via reducing the production of IL-6, TNF-α and iNOS in vascular smooth muscle cells

Atherosclerosis is a chronic inflammatory response of the arterial wall to pro‑atherosclerotic factors. As an inflammatory marker, fibrinogen directly participates in the pathogenesis of atherosclerosis. Our previous study demonstrated that fibrinogen and fibrin degradation products (FDP) produce a pro‑inflammatory effect on vascular smooth muscle cells (VSMCs) through inducing the production of interleukin‑6 (IL‑6), tumor necrosis factor‑α (TNF‑α) and inducible nitric oxide synthase (iNOS). In the present study, the effects of pravastatin on fibrinogen‑ and FDP‑induced expression of IL‑6, TNF‑α and iNOS were observed in VSMCs. The results showed that pravastatin dose‑dependently inhibited fibrinogen‑ and FDP‑stimulated expression of IL‑6, TNF‑α and iNOS in VSMCs at the mRNA and protein level. The maximal inhibition of protein expression of IL‑6, TNF‑α and iNOS was 46.9, 42.7 and 49.2% in fibrinogen‑stimulated VSMCs, and 50.2, 49.8 and 53.6% in FDP‑stimulated VSMCs, respectively. This suggests that pravastatin has the ability to relieve vascular inflammation via inhibiting the generation of IL‑6, TNF‑α and iNOS. The results of the present study may aid in further explaining the beneficial effects of pravastatin on atherosclerosis and related cardiovascular diseases. In addition, they suggest that application of pravastatin may be beneficial for prevention of atherosclerosis formation in hyperfibrinogenemia.

Increased Levels of NF-kB-Dependent Markers in Cancer-Associated Deep Venous Thrombosis

Several studies highlight the role of inflammatory markers in thrombosis as well as in cancer. However, their combined role in cancer-associated deep vein thrombosis (DVT) and the molecular mechanisms, involved in its pathophysiology, needs further investigations. In the present study, C-reactive protein, interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1β), matrix metalloproteases-9 (MMP-9), vascular endothelial growth factor (VEGF), tissue factor (TF), fibrinogen and soluble P-selectin, were analyzed in plasma and in monocyte samples from 385 cancer patients, of whom 64 were concomitantly affected by DVT (+). All these markers were higher in cancer patients DVT+ than in those DVT-. Accordingly, significantly higher NF-kB activity was observed in cancer patients DVT+ than DVT-. Significant correlation between data obtained in plasma and monocyte samples was observed. NF-kB inhibition was associated with decreased levels of all molecules in both cancer DVT+ and DVT-. To further demonstrate the involvement of NF-kB activation by the above mentioned molecules, we treated monocyte derived from healthy donors with a pool of sera from cancer patients with and without DVT. These set of experiments further suggest the significant role played by some molecules, regulated by NF-kB, and detected in cancer patients with DVT. Our data support the notion that NF-kB may be considered as a therapeutic target for cancer patients, especially those complicated by DVT. Treatment with NF-kB inhibitors may represent a possible strategy to prevent or reduce the risk of DVT in cancer patients.

Innate Immunity and Biomaterials at the Nexus: Friends or Foes

Biomaterial implants are an established part of medical practice, encompassing a broad range of devices that widely differ in function and structural composition. However, one common property amongst biomaterials is the induction of the foreign body response: an acute sterile inflammatory reaction which overlaps with tissue vascularisation and remodelling and ultimately fibrotic encapsulation of the biomaterial to prevent further interaction with host tissue. Severity and clinical manifestation of the biomaterial-induced foreign body response are different for each biomaterial, with cases of incompatibility often associated with loss of function. However, unravelling the mechanisms that progress to the formation of the fibrotic capsule highlights the tightly intertwined nature of immunological responses to a seemingly noncanonical “antigen.” In this review, we detail the pathways associated with the foreign body response and describe possible mechanisms of immune involvement that can be targeted. We also discuss methods of modulating the immune response by altering the physiochemical surface properties of the biomaterial prior to implantation. Developments in these areas are reliant on reproducible and effective animal models and may allow a “combined” immunomodulatory approach of adapting surface properties of biomaterials, as well as treating key immune pathways to ultimately reduce the negative consequences of biomaterial implantation.

Autoinflammatory diseases: a possible cause of thrombosis?

Autoinflammatory diseases are a group of disorders due to acquired or hereditary disfunction of innate immune system and characterized by systemic or localized manifestations. The prototype is Familial Mediterranean Fever, a monogenic hereditary disorder, whose causing gene (MeFV gene) was identified in 1997 and opened the way to a new fascinanting chapter of rheumatology. A growing body of monogenic and poligenic autoinflammatory disorders has been described since then.

Arterial and venous thrombosis is a common medical problem, with significant morbidity and mortality. Strong evidences from basic research and clinical epidemiological studies support the theory that inflammation and thrombosis can be associated.

Because of their recurrent/chronic inflammatory nature, autoinflammatory diseases are a putative cause of thrombotic manifestations. In the present work, we reviewed the available evidences about monogenic autoinflammatory disorders, complicated by thrombotic manifestations.

Chronic Rhinosinusitis and the Coagulation System

Chronic rhinosinusitis (CRS) is one of the most common chronic diseases in adults and severely affects quality of life in patients. Although various etiologic and pathogenic mechanisms of CRS have been proposed, the causes of CRS remain uncertain. Abnormalities in the coagulation cascade may play an etiologic role in many diseases, such as asthma and other inflammatory conditions. While studies on the relationship between asthma and dysregulated coagulation have been reported, the role of the coagulation system in the pathogenesis of CRS has only been considered following recent reports. Excessive fibrin deposition is seen in nasal polyp (NP) tissue from patients with chronic rhinosinusitis with nasal polyp (CRSwNP) and is associated with activation of thrombin, reduction of tissue plasminogen activator (t-PA) and upregulation of coagulation factor XIII-A (FXIII-A), all events that can contribute to fibrin deposition and crosslinking. These findings were reproduced in a murine model of NP that was recently established. Elucidation of the mechanisms of fibrin deposition may enhance our understanding of tissue remodeling in the pathophysiology of NP and provide new targets for the treatment of CRSwNP.

Laboratory markers indicating gastrointestinal involvement of henoch-schönlein purpura in children

PURPOSE

To determine clinically useful biochemical markers reflecting disease activity and/or gastrointestinal (GI) tract involvement in Henoch-Schönlein purpura (HSP).

METHODS

A total of 185 children with HSP and 130 controls were included. Laboratory data indicating inflammation, standard coagulation, and activated coagulation were analyzed for the HSP patients, including measurements of the hemoglobin level, white blood cell (WBC) count, absolute neutrophil count (ANC), platelet count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) level, prothrombin time, activated partial thromboplastin time, and fibrinogen, D-dimer, and fibrin degradation product (FDP) levels. The clinical scores of the skin, joints, abdomen, and kidneys were assessed during the acute and convalescence phases of HSP.

RESULTS

The WBC count, ANC, ESR, and CRP, fibrinogen, D-dimer, and FDP levels were significantly higher in the acute phase compared with the convalescent phase of HSP (p<0.05). The total clinical scores were more strongly correlated with the D-dimer (r=0.371, p<0.001) and FDP (r=0.369, p<0.001) levels than with inflammatory markers, such as the WBC count (r=0.241, p=0.001), ANC (r=0.261, p<0.001), and CRP (r=0.260, p<0.001) levels. The patients with GI symptoms had significantly higher ANC (median [interquartile range], 7,138.0 [4,446.4-9,470.0] vs. 5,534.1 [3,263.0-8,153.5], p<0.05) and CRP (0.49 [0.15-1.38] vs. 0.23 [0.01-0.67], p<0.05), D-dimer (2.63 [1.20-4.09] vs. 1.75 [0.62-3.39]), and FDP (7.10 [0.01-13.65] vs. 0.10 [0.01-7.90], p<0.05) levels than those without GI symptoms.

CONCLUSION

D-dimer and FDPs are more strongly associated with disease activity and more consistently reflect GI involvement than inflammatory markers during the acute phase of HSP.

Minimizing immunosuppression in islet xenotransplantation

Pancreatic islet transplantation is a promising treatment option for Type 1 diabetes, but organ supply shortage limits its wide adoption. Pig islets are the most promising alternative source and many important measures such as donor animal selection, pig islet production release criteria, preclinical data and zoonosis surveillance prior to human clinical trials have been put forward as a consensus through the efforts of the International Xenotransplantation Association. To bring pig islet transplantation to clinical reality, the development of clinically applicable immunosuppression regimens and methods to minimize immunosuppression to reduce side effects should be established. This review encompasses immune rejection mechanisms in islet xenotransplantation, immunosuppression regimens that have enabled long-term graft survival in pig-to-nonhuman primate experiments and strategies for minimizing immunosuppression in islet xenotransplantation. By thoroughly examining the drugs that are currently available and in development and their individual targets within the immune response, the best strategy for enabling clinical trials of pig islets for Type 1 diabetes will be proposed.

The characteristics of thrombin in osteoarthritic pathogenesis and treatment

Osteoarthritis (OA) is a mechanical abnormality associated with degradation of joints. It is characterized by chronic, progressive degeneration of articular cartilage, abnormalities of bone, and synovial change. The most common symptom of OA is local inflammation resulting from exogenous stress or endogenous abnormal cytokines. Additionally, OA is associated with local and/or systemic activation of coagulation and anticoagulation pathways. Thrombin plays an important role in the stimulation of fibrin deposition and the proinflammatory processes in OA. Thrombin mediates hemostatic and inflammatory responses and guides the immune response to tissue damage. Thrombin activates intracellular signaling pathways by interacting with transmembrane domain G protein coupled receptors (GPCRs), known as protease-activated receptors (PARs). In pathogenic mechanisms, PARs have been implicated in the development of acute and chronic inflammatory responses in OA. Therefore, discovery of thrombin signaling pathways would help us to understand the mechanism of OA pathogenesis and lead us to develop therapeutic drugs in the future.

Fibrinogen and factor XIII at the intersection of coagulation, fibrinolysis and inflammation

Fibrinogen and factor XIII are two essential proteins that are involved directly in fibrin gel formation as the final step of a sequence of reactions triggered by a procoagulant stimulus. Haemostasis is the most obvious function of the resulting fibrin clot. Different variables affect the conversion of fibrinogen to fibrin as well as the mode of fibrin polymerisation and fibrin crosslinking, hereby, critically influencing the architecture of the resulting fibrin network and consequently determining its mechanical strength and resistance against fibrinolysis. Due to fibrinogen's structure with a multitude of domains and binding motifs the fibrin gel allows for complex interactions with other coagulation factors, with profibrinolytic as well as antifibrinolyic proteins, with complement factors and with various cellular receptors. These interactions enable the fibrin network to control its own further state (i. e. expansion or degradation), to influence innate immunity, and to function as a scaffold for cell migration processes. During the whole process of fibrin gel formation biologically active peptides and protein fragments are released that additionally influence cellular processes via chemotaxis or by modulating cell-cell interactions. Thus, it is not surprising that fibrinogen and factor XIII in addition to their haemostatic function influence innate immunity as well as cell-mediated reactions like wound healing, response to tissue injury or inflammatory processes. The present review summarises current knowledge of fibrinogen's and factor XIII's function in coagulation and fibrinolysis giving special emphasis on their relation to inflammation control.

Fibrin, a scaffold material for islet transplantation and pancreatic endocrine tissue engineering

Fibrin is derived from fibrinogen during injury to produce a blood clot and thus promote wound repair. Composed of different domains, including Arg-Gly-Asp amino acid motifs, fibrin is used extensively as a hydrogel and sealant in the clinic. By binding to cell surface receptors like integrins and acting as a supportive 3D scaffold, fibrin has been useful in promoting cell differentiation, proliferation, function, and survival. In particular, fibrin has been able to maintain islet cell architecture, promote beta cell insulin secretion, and islet angiogenesis, as well as inducing a protective effect against cell death. During islet transplantation, fibrin improved neovascularization and islet function. These improvements resulted in reduced number of transplanted islets necessary to reverse diabetes. Therefore, fibrin, as a biocompatible and biodegradable scaffold, should be considered during subcutaneous islet transplantation and beta cell expansion in vitro to ensure maintenance of islet cell function, proliferation, and survival to develop effective cell-based therapies for the treatment of diabetes.

Deletion of Braun lipoprotein and plasminogen-activating protease-encoding genes attenuates Yersinia pestis in mouse models of bubonic and pneumonic plague

Currently, there is no FDA-approved vaccine against Yersinia pestis, the causative agent of bubonic and pneumonic plague. Since both humoral immunity and cell-mediated immunity are essential in providing the host with protection against plague, we developed a live-attenuated vaccine strain by deleting the Braun lipoprotein (lpp) and plasminogen-activating protease (pla) genes from Y. pestis CO92. The Δlpp Δpla double isogenic mutant was highly attenuated in evoking both bubonic and pneumonic plague in a mouse model. Further, animals immunized with the mutant by either the intranasal or the subcutaneous route were significantly protected from developing subsequent pneumonic plague. In mice, the mutant poorly disseminated to peripheral organs and the production of proinflammatory cytokines concurrently decreased. Histopathologically, reduced damage to the lungs and livers of mice infected with the Δlpp Δpla double mutant compared to the level of damage in wild-type (WT) CO92-challenged animals was observed. The Δlpp Δpla mutant-immunized mice elicited a humoral immune response to the WT bacterium, as well as to CO92-specific antigens. Moreover, T cells from mutant-immunized animals exhibited significantly higher proliferative responses, when stimulated ex vivo with heat-killed WT CO92 antigens, than mice immunized with the same sublethal dose of WT CO92. Likewise, T cells from the mutant-immunized mice produced more gamma interferon (IFN-γ) and interleukin-4. These animals had an increasing number of tumor necrosis factor alpha (TNF-α)-producing CD4(+) and CD8(+) T cells than WT CO92-infected mice. These data emphasize the role of TNF-α and IFN-γ in protecting mice against pneumonic plague. Overall, our studies provide evidence that deletion of the lpp and pla genes acts synergistically in protecting animals against pneumonic plague, and we have demonstrated an immunological basis for this protection.

New insights into the role of Plg-RKT in macrophage recruitment

Plasminogen (PLG) is the zymogen of plasmin, the major enzyme that degrades fibrin clots. In addition to its binding and activation on fibrin clots, PLG also specifically interacts with cell surfaces where it is more efficiently activated by PLG activators, compared with the reaction in solution. This results in association of the broad-spectrum proteolytic activity of plasmin with cell surfaces that functions to promote cell migration. Here, we review emerging data establishing a role for PLG, plasminogen receptors and the newly discovered plasminogen receptor, Plg-RKT, in macrophage recruitment in the inflammatory response, and we address mechanisms by which the interplay between PLG and its receptors regulates inflammation.

Optical imaging of fibrin deposition to elucidate participation of mast cells in foreign body responses

Mast cell activation has been shown to be an initiator and a key determinant of foreign body reactions. However, there is no non-invasive method that can quantify the degree of implant-associated mast cell activation. Taking advantage of the fact that fibrin deposition is a hallmark of mast cell activation around biomaterial implants, a near infrared probe was fabricated to have high affinity to fibrin. Subsequent in vitro testing confirmed that this probe has high affinity to fibrin. Using a subcutaneous particle implantation model, we found significant accumulation of fibrin-affinity probes at the implant sites as early as 15 min following particle implantation. The accumulation of fibrin-affinity probes at the implantation sites could also be substantially reduced if anti-coagulant - heparin was administered at the implant sites. Further studies have shown that subcutaneous administration of mast cell activator - compound 48/80 - prompted the accumulation of fibrin-affinity probes. However, implant-associated fibrin-affinity probe accumulation was substantially reduced in mice with mast cell deficiency. The results show that our fibrin-affinity probes may serve as a powerful tool to monitor and measure the extent of biomaterial-mediated fibrin deposition and mast cell activation in vivo.

Superior in vivo compatibility of hydrophilic polymer coated prosthetic vascular grafts

PURPOSE

Protein adsorption, cell adhesion and graft patency was compared in hydrophilic versus hydrophobic polymer-coated prosthetic vascular grafts. We hypothesize that in vivo compatibility of hydrophilic polymer-coated prosthetic vascular grafts is superior to in vivo compatibility of hydrophobic grafts.

METHODS

A pairwise side-to-side common carotid artery interposition graft was placed eight female landrace goats (mean weight 55 kg). Protein adsorption was assessed using Western Blot in two hydrophilic and two hydrophobic grafts harvested after three days. Graft patency was monitored for 28 days in six goats with continuous wave Doppler ultrasonography. Adherence of endothelial cells, leukocytes and platelets was determined with ELISA and compared between the two graft types after 28 days.

RESULTS

After three days, more ApoA-I, albumin and VEGF and less fibrin adsorbed to hydrophilic grafts. After 28 days, compared to hydrophobic grafts, higher numbers of endothelial cells were present on hydrophilic grafts (P=0.016), and less thrombocytes and leukocytes (P=0.012 and 0.024, respectively). Two out of eight hydrophobic grafts lost patency, while none of the hydrophilic grafts failed (P=0.157).

CONCLUSIONS

Hydrophilic polymer-coated vascular grafts have superior in vivo compatibility when compared to hydrophobic grafts as characterized by reduced platelet and leukocyte adherence as well as higher endothelialization.

Isoflurane on brain inflammation

Brain inflammation may play an important role in the pathophysiology of early brain injury after subarachnoid hemorrhage (SAH). Our aim was to demonstrate brain inflammation development and to determine whether isoflurane, a clinically available volatile anesthetic agent, prevents brain inflammation after SAH. This study used 162 8-week-old male CD-1 mice. We induced SAH with endovascular perforation in mice and randomly assigned animals to sham-operated (n=21), SAH+vehicle-air (n=35) and SAH+2% isoflurane (n=31). In addition to the evaluation of brain injury (neurological scores, brain edema and Evans blue dye extravasation), brain inflammation was evaluated by means of expression changes in markers of inflammatory cells (ionized calcium binding adaptor molecule-1, myeloperoxidase), cytokines (tumor necrosis factor [TNF]-α, interleukin-1β), adhesion molecules (intercellular adhesion molecule [ICAM]-1, P-selectin), inducers of inflammation (cyclooxygenase-2, phosphorylated c-Jun N-terminal kinase [p-JNK]) and endothelial cell activation (von Willebrand factor) at 24h post-SAH. Sphingosine kinase inhibitor (N, N-dimethylsphingosine [DMS]) and sphingosine-1-phosphate receptor-1/3 antagonist (VPC23019) were used to block isoflurane's effects (n=22, each). SAH caused early brain injury, which was associated with inflammation so that all evaluated markers of inflammation were increased. Isoflurane significantly inhibited both brain injury (P<0.001, respectively) and inflammation (myeloperoxidase, P=0.022; interleukin-1β, P=0.002; TNF-α, P=0.015; P-selectin, P=0.010; ICAM-1, P=0.016; p-JNK, P<0.001; cyclooxygenase-2, P=0.003, respectively). This beneficial effect of isoflurane was abolished with DMS and VPC23019. Isoflurane may suppress post-SAH brain inflammation possibly via the sphingosine-related pathway.

Potential role of oral anticoagulants in the treatment of patients with coronary artery disease: focus on dabigatran

The pharmacologic management of patients with high-risk coronary artery disease consists of aspirin and a P2Y12 receptor inhibitor. Chronic oral anticoagulation with warfarin is the major treatment strategy to attenuate thromboembolism or stroke in patients with deep vein thrombosis, pulmonary embolism, heart failure and atrial fibrillation. A substantial percentage of the latter group of patients have coronary artery disease and may require stenting with long-term dual antiplatelet therapy in addition to therapy with warfarin to reduce arterial ischemic events in addition to stroke. These new oral anticoagulants have been developed for long-term therapy to overcome the limitations of warfarin. Dabigatran is a direct thrombin inhibitor and its role in patients with acute coronary syndrome is being explored.

The effect of platelet lysate fibrinogen on the functionality of MSCs in immunotherapy

Human platelet lysate (PL) represents an attractive alternative to fetal bovine serum (FBS) for the ex vivo expansion of human mesenchymal stromal cells (MSCs). However, there is controversy whether MSCs propagated in unfractionated PL retain their immunosuppressive properties. Since fibrinogen can be a major component of PL, we hypothesized that the fibrinogen content in PL negatively affects the suppressor function of MSCs. Pools of outdated plateletpheresis products underwent a double freeze-thaw centrifugation and filtration to produce unfractionated platelet lysates (uPL), followed by a temperature controlled clotting procedure to produce a fibrinogen depleted platelet lysate (fdPL). Fibrinogen depletion affected neither the mitogenic properties of PL or growth factor content, however fdPL was less prone to develop precipitate over time. Functionally, fibrinogen interacted directly with MSCs, dose dependently increased IL-6, IL-8 and MCP-1 protein production, and compromised the ability of MSCs to up-regulate indoleamine dioxygenase (IDO), as well as, mitigate T-cell proliferation. Similarly uPL expanded MSCs showed a reduced capability of inducing IDO and suppressing T-cell proliferation compared to FBS expanded MSCs. Replacing uPL with fdPL largely restored the immune modulating effects of MSCs. Together these data suggest that fibrinogen negatively affects the immunomodulatory functions of MSCs and fdPL can serve as non-xenogenic mitogenic supplement for expansion of clinical grade MSCs for immune modulation.

Modulation of neutrophil NETosis: interplay between infectious agents and underlying host physiology

The ability of neutrophils and other leucocyte members of the innate immune system to expel their DNA into the extracellular environment in a controlled manner in order to trap and kill pathogenic microorganisms lead to a paradigm shift in our understanding of host microbe interactions. Surprisingly, the neutrophil extracellular trap (NET) cast by neutrophils is very wide and extends to the entrapment of viruses as well as multicellular eukaryotic parasites. Not unexpectedly, it has emerged that pathogenic microorganisms can employ a wide array of strategies to avoid ensnarement, including expression of DNAse enzymes that destroy the lattice backbone of NETs. Alternatively, they may use molecular mimicry to avoid detection or trigger events leading to the expression of immune modulatory cytokines such as IL-10, which dampen the NETotic response of neutrophils. In addition, the host microenvironment may contribute to the innate immune response by the production of lectin-like molecules that bind to bacteria and promote their entrapment on NETs. An example of this is the production of surfactant protein D by the lung epithelium. In addition, pregnancy provides a different challenge, as the mother needs to mount an effective response against pathogens, without harming her unborn child. An examination of these decoy and host response mechanisms may open the path for new therapies to treat pathologies mediated by overt NETosis.

Association of microparticles and preeclampsia

Preeclampsia (PE) is a syndrome characterized by poor placentation and endothelial dysfunction. The diagnosis for this syndrome is based in hypertension and proteinuria presented after the 20th week of pregnancy. Despite intensive research, PE is still one of the leading causes of maternal mortality, although reliable screening tests or effective treatments of this disease have yet to be proposed. Microparticles (MPs) are small vesicles released after cell activation or apoptosis, which contain membrane proteins that are characteristic of the original parent cell. MPs have been proven to play key role in thrombosis, inflammation, and angiogenesis, as well as to mediate cell-cell communication by transferring mRNAs and microRNA from the cell of origin to target cells. Placenta-derived syncytiotrophoblast MPs are one of the most increased MPs during PE and may play an important role in the pathogenesis of this syndrome. Therefore, a better overall understanding of the role of MPs in PE may be useful for new clinical diagnoses and therapeutic approaches.

Increased expression of factor XIII-A in patients with chronic rhinosinusitis with nasal polyps

BACKGROUND

Profound edema or formation of a pseudocyst containing plasma proteins is a prominent characteristic of nasal polyps (NP). However, the mechanisms underlying NP retention of plasma proteins in the submucosa remain unclear. Recently, we reported that impairment of fibrinolysis causes excessive fibrin deposition in NP and this might be involved in the retention of plasma proteins. Although the coagulation cascade plays a critical role in fibrin clot formation at extravascular sites, the expression and role of coagulation factors in NP remain unclear.

OBJECTIVE

The objective of this study was to investigate the expression of coagulation factors in patients with chronic rhinosinusitis (CRS).

METHODS

Sinonasal tissues were collected from patients with CRS and control subjects. We assayed mRNA for factor XIII-A (FXIII-A) by using real-time PCR and measured FXIII-A protein by means of ELISA, immunohistochemistry, and immunofluorescence.

RESULTS

FXIII-A mRNA levels were significantly increased in NP tissue from patients with CRS with NP (P < .001) compared with uncinate tissue from patients with CRS or control subjects. Similarly, FXIII-A protein levels were increased in NP. Immunofluorescence analysis revealed that FXIII-A expression in inflammatory cells and FXIII-A(+) cell numbers were significantly increased in NP. Most FXIII-A staining was observed within CD68(+)/CD163(+) M2 macrophages in NP. Levels of FXIII-A correlated with markers of M2 macrophages, suggesting that M2 macrophages are major FXIIIA-producing cells in NP.

CONCLUSION

Overproduction of FXIII-A by M2 macrophages might contribute to the excessive fibrin deposition in the submucosa of NP, which might contribute to the tissue remodeling and pathogenesis of CRS with NP.

Fibrin facilitates both innate and T cell-mediated defense against Yersinia pestis

The Gram-negative bacterium Yersinia pestis causes plague, a rapidly progressing and often fatal disease. The formation of fibrin at sites of Y. pestis infection supports innate host defense against plague, perhaps by providing a nondiffusible spatial cue that promotes the accumulation of inflammatory cells expressing fibrin-binding integrins. This report demonstrates that fibrin is an essential component of T cell-mediated defense against plague but can be dispensable for Ab-mediated defense. Genetic or pharmacologic depletion of fibrin abrogated innate and T cell-mediated defense in mice challenged intranasally with Y. pestis. The fibrin-deficient mice displayed reduced survival, increased bacterial burden, and exacerbated hemorrhagic pathology. They also showed fewer neutrophils within infected lung tissue and reduced neutrophil viability at sites of liver infection. Depletion of neutrophils from wild-type mice weakened T cell-mediated defense against plague. The data suggest that T cells combat plague in conjunction with neutrophils, which require help from fibrin to withstand Y. pestis encounters and effectively clear bacteria.

A small molecule inhibitor to plasminogen activator inhibitor 1 inhibits macrophage migration

OBJECTIVE

Macrophage (Mϕ) migration rests on the adhesion/detachment between Mϕ surface components and extracellular matrixes, and the contribution of numerous inflammatory disorders. Plasminogen activator inhibitor (PAI)-1, a serine protease inhibitor, influences Mϕ motility through an action distinct from its classical modulation of the plasmin-based fibrinolytic process. We rely here on a small molecule PAI-1 inhibitor (TM5275) to investigate the role of PAI-1 in Mϕ migration in the pathogenesis of renal injury.

APPROACH AND RESULTS

Mϕ migration was inhibited both in vitro and in vivo by TM5275. It was also reduced in T-cell-deficient nude mice, but not in PAI-1-deficient mice. Mϕ migration hinged on the interaction of PAI-1 with low-density lipoprotein receptor-related protein, an interaction prevented by TM5275, but not with vitronectin, urokinase-type plasminogen activator, or tissue-type plasminogen activator. Fed to rats with anti-Thy-1-induced nephritis, TM5275 significantly decreased Mϕ accumulation and ameliorated the progression of renal injury.

CONCLUSIONS

These findings suggest that a small molecule PAI-1 inhibitor represents a novel class of anti-inflammatory agents targeting Mϕ migration by the inhibition of the interaction of PAI-1 with low-density lipoprotein receptor-related protein.

Genetic elimination of the binding motif on fibrinogen for the S. aureus virulence factor ClfA improves host survival in septicemia

Fibrinogen can support host antimicrobial containment/clearance mechanisms, yet selected pathogens appear to benefit from host procoagulants to drive bacterial virulence. Here, we explored the hypothesis that host fibrin(ogen), on balance, supports Staphylococcus aureus infection in the context of septicemia. Survival studies following intravenous infection in control and fibrinogen-deficient mice established the overall utility of host fibrin(ogen) to S. aureus virulence. Complementary studies in mice expressing mutant forms of fibrinogen-retaining clotting function, but lacking either the bacterial ClfA (Fibγ(Δ5)) binding motif or the host leukocyte integrin receptor αMβ2 (Fibγ(390-396A)) binding motif, revealed the preeminent importance of the bacterial ClfA-fibrin(ogen) interaction in determining host survival. Studies of mice lacking platelets or the platelet integrin receptor subunit αIIb established that the survival benefits observed in Fibγ(Δ5) mice were largely independent of platelet αIIbβ3-mediated engagement of fibrinogen. Fibγ(Δ5) mice exhibited reduced bacterial burdens in the hearts and kidneys, a blunted host proinflammatory cytokine response, diminished microscopic tissue damage, and significantly diminished plasma markers of cardiac and other organ damage. These findings indicate that host fibrin(ogen) and bacterial ClfA are dual determinants of virulence and that therapeutic interventions at the level of fibrinogen could be advantageous in S. aureus septicemia.

Citrullination enhances the pro-inflammatory response to fibrin in rheumatoid arthritis synovial fibroblasts

OBJECTIVE

Fibrin deposits are characteristic of the synovial tissues in rheumatoid arthritis (RA). Once citrullinated, fibrin becomes an autoantigen and is thought to contribute in this way to perpetuate the disease. Our study aimed to analyse the responses of RA synovial fibroblasts (RASF) to native and citrullinated fibrin.

METHODS

The transcriptome induced by fibrin in RASF was approached with whole-genome-based gene expression arrays. The upregulation of selected pro-inflammatory genes by fibrin was confirmed in additional primary cell cultures using quantitative PCR and ELISA. Citrullination reactions were carried out with recombinant human peptidylarginine deiminases (PAD) 2 and 4.

RESULTS

In the whole-genome array native fibrin was found to modulate the gene expression profile of RASF, particularly upregulating mRNA levels of several pro-inflammatory cytokines. The induction of interleukin (IL)-6 and IL-8 by fibrin was confirmed in additional samples at both the mRNA and the protein level. Blocking and knockdown experiments showed the participation of toll-like receptor (TLR)4 in the induction of both cytokines. As compared with the native macromolecule, PAD2-citrullinated fibrin induced significantly higher expression of the pro-inflammatory cytokines in these cells.

CONCLUSIONS

Our results suggest that fibrin mediates inflammatory responses in RASF via a TLR4 pathway. In this way, fibrin and particularly its citrullinated form may contribute to sustain the cytokine burst in RA.

Thrombosis as an intravascular effector of innate immunity

Thrombosis is the most frequent cause of mortality worldwide and is closely linked to haemostasis, which is the biological mechanism that stops bleeding after the injury of blood vessels. Indeed, both processes share the core pathways of blood coagulation and platelet activation. Here, we summarize recent work suggesting that thrombosis under certain circumstances has a major physiological role in immune defence, and we introduce the term immunothrombosis to describe this process. Immunothrombosis designates an innate immune response induced by the formation of thrombi inside blood vessels, in particular in microvessels. Immunothrombosis is supported by immune cells and by specific thrombosis-related molecules and generates an intravascular scaffold that facilitates the recognition, containment and destruction of pathogens, thereby protecting host integrity without inducing major collateral damage to the host. However, if uncontrolled, immunothrombosis is a major biological process fostering the pathologies associated with thrombosis.

Excessive fibrin deposition in nasal polyps caused by fibrinolytic impairment through reduction of tissue plasminogen activator expression

RATIONALE

Nasal polyps (NPs) are characterized by intense edema or formation of pseudocysts filled with plasma proteins, mainly albumin. However, the mechanisms underlying NP retention of plasma proteins in their submucosa remain unclear.

OBJECTIVES

We hypothesized that formation of a fibrin mesh retains plasma proteins in NPs. We assessed the fibrin deposition and expression of the components of the fibrinolytic system in patients with chronic rhinosinusitis (CRS).

METHODS

We assessed fibrin deposition in nasal tissue from patients with CRS and control subjects by means of immunofluorescence. Fibrinolytic components, d-dimer, and plasminogen activators were measured using ELISA, real-time PCR, and immunohistochemistry. We also performed gene expression and protein quantification analysis in cultured airway epithelial cells.

MEASUREMENTS AND MAIN RESULTS

Immunofluorescence data showed profound fibrin deposition in NP compared with uncinate tissue (UT) from patients with CRS and control subjects. Levels of the cross-linked fibrin cleavage product protein, d-dimer, were significantly decreased in NP compared with UT from patients with CRS and control subjects, suggesting reduced fibrinolysis (P < 0.05). Expression levels of tissue plasminogen activator (t-PA) mRNA and protein were significantly decreased in NP compared with UT from patients with CRS and control subjects (P < 0.01). Immunohistochemistry demonstrated clear reduction of t-PA in NP, primarily in the epithelium and glands. Th2 cytokine-stimulated cultured airway epithelial cells showed down-regulation of t-PA, suggesting a potential Th2 mechanism in NP.

CONCLUSIONS

A Th2-mediated reduction of t-PA might lead to excessive fibrin deposition in the submucosa of NP, which might contribute to the tissue remodeling and pathogenesis of CRS with nasal polyps.

Reactive carbonyl compounds (RCCs) cause aggregation and dysfunction of fibrinogen

Fibrinogen is a key protein involved in coagulation and its deposition on blood vessel walls plays an important role in the pathology of atherosclerosis. Although the causes of fibrinogen (fibrin) deposition have been studied in depth, little is known about the relationship between fibrinogen deposition and reactive carbonyl compounds (RCCs), compounds which are produced and released into the blood and react with plasma protein especially under conditions of oxidative stress and inflammation. Here, we investigated the effect of glycolaldehyde on the activity and deposition of fibrinogen compared with the common RCCs acrolein, methylglyoxal, glyoxal and malondialdehyde. At the same concentration (1 mmol/L), glycolaldehyde and acrolein had a stronger suppressive effect on fibrinogen activation than the other three RCCs. Fibrinogen aggregated when it was respectively incubated with glycolaldehyde and the other RCCs, as demonstrated by SDS-PAGE, electron microscopy and intrinsic fluorescence intensity measurements. Staining with Congo Red showed that glycolaldehyde- and acrolein-fibrinogen distinctly formed amyloid-like aggregations. Furthermore, the five RCCs, particularly glycolaldehyde and acrolein, delayed human plasma coagulation. Only glycolaldehyde showed a markedly suppressive effect on fibrinogenesis, none did the other four RCCs when their physiological blood concentrations were employyed, respectively. Taken together, it is glycolaldehyde that suppresses fibrinogenesis and induces protein aggregation most effectively, suggesting a putative pathological process for fibrinogen (fibrin) deposition in the blood.

Preliminary study on the effects of ageing cold oxygen plasma treated PET/PP with respect to protein adsorption

Surfaces of polyethylene terephthalate (PET) and polypropylene (PP) have been modified by oxygen plasma. The surface hydrophilicity and changes in topography during up to 90 days storage in water and in dry air in a desiccator were analysed by dynamic contact angle test and atomic force microscopy (AFM). Clear ageing effects on the plasma treated surface were observed as increases in contact angle and changes in roughness as functions of increasing storage time. However, the effect of oxygen plasma treatment to increase the hydrophilicity of surface was still evident on the treated surfaces even after 90 days storage either in dry air or in water. In protein adsorption experiments, human serum albumin (HSA) and fibrinogen (Fg) were adsorbed on untreated and oxygen plasma treated PET and PP surfaces. The quantified ATR-FTIR results showed that both HSA and Fg adsorption on PET and PP surfaces decreased after oxygen plasma treatment, with the effect most evident for HSA. Although for both proteins adsorption increased with ageing, the amount of adsorbed proteins was still lower than untreated surface at 30 days. This suggests the shelf life of oxygen plasma treated samples could be as long as 30 days.

Thrombin induces an inflammatory phenotype in a human brain endothelial cell line

In this study, we utilized the human brain endothelial cell line, hCMEC/D3, to determine the effects of the coagulation factor, thrombin, on the human blood-brain barrier (BBB). We show that thrombin increased the mRNA and cell surface levels of ICAM-1 and VCAM-1 in hCMEC/D3 cells. Thrombin similarly upregulated several chemokines implicated in human neurological conditions. Additionally, the paracellular permeability of the human BBB in vitro was also increased following thrombin treatment. Overall, this study demonstrates that thrombin can effectively induce an inflamed phenotype in an in vitro human BBB.

Inflammation and haemostasis

Inflammation and haemostasis are interrelated pathophysiologic processes that considerably affect each other. In this bidirectional relationship, inflammation leads to activation of the haemostatic system that in turn also considerably influences inflammatory activity. Such, the haemostatic system acts in concert with the inflammatory cascade creating an inflammation-haemostasis cycle in which each activated process promotes the other and the two systems function in a positive feedback loop. The extensive crosstalk between immune and haemostatic systems occurs at level of all components of the haemostatic system including vascular endothelial cells, platelets, plasma coagulation cascade, physiologic anticoagulants and fibrinolytic activity. During inflammatory response, inflammatory mediators, in particular proinflammatory cytokines, play a central role in the effects on haemostatic system by triggering its disturbance in a number of mechanisms including endothelial cell dysfunction, increased platelet reactivity, activation of the plasma coagulation cascade, impaired function of physiologic anticoagulants and suppressed fibrinolytic activity. The two examples of pathophysiologic processes in which the tight interdependent relationship between inflammation and haemostasis considerably contribute to the pathogenesis and/or progression of disease are systemic inflammatory response to infection or sepsis and acute arterial thrombosis as a consequence of ruptured atherosclerotic plaque. Close links between inflammation and haemostasis help explain the prothrombotic tendency in these two clinical conditions in which inflammation shifts the haemostatic activity towards procoagulant state by the ability of proinflammatory mediators to activate coagulation system and to inhibit anticoagulant and fibrinolytic activities. This review summarizes the current knowledge of the complex interactions in the bidirectional relationship between inflammation and haemostasis.

Metalloproteinase-mediated Shedding of Integrin β2 promotes macrophage efflux from inflammatory sites

Macrophage exiting from inflammatory sites is critical to limit the local innate immune response. With tissue insult, resident tissue macrophages rapidly efflux to lymph nodes where they modulate the adaptive immune response, and inflammatory macrophages attracted to the site of injury then exit during the resolution phase. However, the mechanisms that regulate macrophage efflux are poorly understood. This study has investigated soluble forms of integrin β2 whose levels are elevated in experimental peritonitis at times when macrophages are exiting the peritoneum, suggesting that its proteolytic shedding may be involved in macrophage efflux. Both constitutive and inducible metalloproteinase-dependent shedding of integrin β2 from mouse macrophages are demonstrated. Soluble integrin β2 is primarily released as a heterodimeric complex with αM that retains its ability to bind its ligands intracellular adhesion molecule-1, fibrin, and collagen and thus may serve as a soluble antagonist. In a model of accelerated exiting, administration of a metalloproteinase inhibitor prevents macrophage efflux by 50% and impedes loss of macrophage integrin β2 from the cell surface. Exiting of peritoneal macrophages in mice lacking integrin β2 is accelerated, and antibody disruption of integrin β2-substrate interactions can reverse 50% of the metalloprotease inhibitor blockade of macrophage exiting. Thus, our study demonstrates the ability of metalloproteinase-mediated shedding of integrin β2 to promote macrophage efflux from inflammatory sites, and the release of soluble integrin heterodimers may also limit local inflammation.

Fibrinogen as a key regulator of inflammation in disease

The interaction of coagulation factors with the perivascular environment affects the development of disease in ways that extend beyond their traditional roles in the acute hemostatic cascade. Key molecular players of the coagulation cascade like tissue factor, thrombin, and fibrinogen are epidemiologically and mechanistically linked with diseases with an inflammatory component. Moreover, the identification of novel molecular mechanisms linking coagulation and inflammation has highlighted factors of the coagulation cascade as new targets for therapeutic intervention in a wide range of inflammatory human diseases. In particular, a proinflammatory role for fibrinogen has been reported in vascular wall disease, stroke, spinal cord injury, brain trauma, multiple sclerosis, Alzheimer's disease, rheumatoid arthritis, bacterial infection, colitis, lung and kidney fibrosis, Duchenne muscular dystrophy, and several types of cancer. Genetic and pharmacologic studies have unraveled pivotal roles for fibrinogen in determining the extent of local or systemic inflammation. As cellular and molecular mechanisms for fibrinogen functions in tissues are identified, the role of fibrinogen is evolving from a marker of vascular rapture to a multi-faceted signaling molecule with a wide spectrum of functions that can tip the balance between hemostasis and thrombosis, coagulation and fibrosis, protection from infection and extensive inflammation, and eventually life and death. This review will discuss some of the main molecular links between coagulation and inflammation and will focus on the role of fibrinogen in inflammatory disease highlighting its unique structural properties, cellular targets, and signal transduction pathways that make it a potent proinflammatory mediator and a potential therapeutic target.

Thrombin as a multi-functional enzyme. Focus on in vitro and in vivo effects

Thrombin is the central protease in the coagulation cascade and one of the most extensively studied of all enzymes. In addition to its recognised role in the coagulation cascade and haemostasis, thrombin is known to have multiple pleiotropic effects, which mostly have been shown only in in vitro studies: it plays a role in inflammation and cellular proliferation and displays a mitogen activity on smooth muscle cells and endothelial cells, predominantly by activation of angiogenesis. In vivo , thrombin effects were examined in animal models of intravenous or intraarterial thrombin infusion. An extensive literature search regarding in vivo data showed that i) thrombin administered as a bolus causes microembolism, ii) thrombin infused slowly at steady-state conditions (up to 1.6 U/kg/min) leads to bleeds but not to intravascular clotting, iii) large quantity of thrombin infused at low rates (0.05 U/kg/min) does not have any measurable effect, and iv) thrombin increases vascular permeability leading to tissue damage. Although several decades of research on thrombin functions have provided a framework for understanding the biology of thrombin, animal and human studies with use of newer laboratory techniques are still needed to confirm the pleiotropic thrombin functions shown in in vitro studies.

Tissue factor, blood coagulation, and beyond: an overview

Emerging evidence shows a broad spectrum of biological functions of tissue factor (TF). TF classical role in initiating the extrinsic blood coagulation and its direct thrombotic action in close relation to cardiovascular risks have long been established. TF overexpression/hypercoagulability often observed in many clinical conditions certainly expands its role in proinflammation, diabetes, obesity, cardiovascular diseases, angiogenesis, tumor metastasis, wound repairs, embryonic development, cell adhesion/migration, innate immunity, infection, pregnancy loss, and many others. This paper broadly covers seminal observations to discuss TF pathogenic roles in relation to diverse disease development or manifestation. Biochemically, extracellular TF signaling interfaced through protease-activated receptors (PARs) elicits cellular activation and inflammatory responses. TF diverse biological roles are associated with either coagulation-dependent or noncoagulation-mediated actions. Apparently, TF hypercoagulability refuels a coagulation-inflammation-thrombosis circuit in “autocrine” or “paracrine” fashions, which triggers a wide spectrum of pathophysiology. Accordingly, TF suppression, anticoagulation, PAR blockade, or general anti-inflammation offers an array of therapeutical benefits for easing diverse pathological conditions.

Plasma carboxypeptidase B downregulates inflammatory responses in autoimmune arthritis

The immune and coagulation systems are both implicated in the pathogenesis of rheumatoid arthritis (RA). Plasma carboxypeptidase B (CPB), which is activated by the thrombin/thrombomodulin complex, plays a procoagulant role during fibrin clot formation. However, an antiinflammatory role for CPB is suggested by the recent observation that CPB can cleave proinflammatory mediators, such as C5a, bradykinin, and osteopontin. Here, we show that CPB plays a central role in downregulating C5a-mediated inflammatory responses in autoimmune arthritis. CPB deficiency exacerbated inflammatory arthritis in a mouse model of RA, and cleavage of C5a by CPB suppressed the ability of C5a to recruit immune cells in vivo. In human patients with RA, genotyping of nonsynonymous SNPs in the CPB-encoding gene revealed that the allele encoding a CPB variant with longer half-life was associated with a lower risk of developing radiographically severe RA. Functionally, this CPB variant was more effective at abrogating the proinflammatory properties of C5a. Additionally, expression of both CPB and C5a in synovial fluid was higher in patients with RA than in those with osteoarthritis. These findings suggest that CPB plays a critical role in dampening local, C5a-mediated inflammation and represents a molecular link between inflammation and coagulation in autoimmune arthritis.

Thrombin and vascular inflammation

Vascular endothelium is a key regulator of homeostasis. In physiological conditions it mediates vascular dilatation, prevents platelet adhesion, and inhibits thrombin generation. However, endothelial dysfunction caused by physical injury of the vascular wall, for example during balloon angioplasty, acute or chronic inflammation, such as in atherothrombosis, creates a proinflammatory environment which supports leukocyte transmigration toward inflammatory sites. At the same time, the dysfunction promotes thrombin generation, fibrin deposition, and coagulation. The serine protease thrombin plays a pivotal role in the coagulation cascade. However, thrombin is not only the key effector of coagulation cascade; it also plays a significant role in inflammatory diseases. It shows an array of effects on endothelial cells, vascular smooth muscle cells, monocytes, and platelets, all of which participate in the vascular pathophysiology such as atherothrombosis. Therefore, thrombin can be considered as an important modulatory molecule of vascular homeostasis. This review summarizes the existing evidence on the role of thrombin in vascular inflammation.

Protective roles for fibrin, tissue factor, plasminogen activator inhibitor-1, and thrombin activatable fibrinolysis inhibitor, but not factor XI, during defense against the gram-negative bacterium Yersinia enterocolitica

Septic infections dysregulate hemostatic pathways, prompting coagulopathy. Nevertheless, anticoagulant therapies typically fail to protect humans from septic pathology. The data reported in this work may help to explain this discrepancy by demonstrating critical protective roles for coagulation leading to fibrin deposition during host defense against the Gram-negative bacterium Yersinia enterocolitica. After i.p. inoculation with Y. enterocolitica, fibrinogen-deficient mice display impaired cytokine and chemokine production in the peritoneal cavity and suppressed neutrophil recruitment. Moreover, both gene-targeted fibrinogen-deficient mice and wild-type mice treated with the anticoagulant coumadin display increased hepatic bacterial burden and mortality following either i.p. or i.v. inoculation with Y. enterocolitica. Mice with low tissue factor activity succumb to yersiniosis with a phenotype similar to fibrin(ogen)-deficient mice, whereas factor XI-deficient mice show wild-type levels of resistance. Mice deficient in plasminogen activator inhibitor-1 or thrombin-activatable fibrinolysis inhibitor display modest phenotypes, but mice deficient in both plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor succumb to yersiniosis with a phenotype resembling fibrin(ogen)-deficient mice. These findings demonstrate critical protective roles for the tissue factor-dependent extrinsic coagulation pathway during host defense against bacteria and caution that therapeutics targeting major thrombin-generating or antifibrinolytic pathways may disrupt fibrin-mediated host defense during Gram-negative sepsis.