Summary
The contact system is a plasma protease cascade initiated by factor XII (FXII) that activates the proinflammatory kallikrein‐kinin system and the procoagulant intrinsic coagulation pathway. ...Anionic surfaces induce FXII zymogen activation to form proteolytically active FXIIa. Bacterial surfaces also have the ability to activate contact system proteins, indicating an important role for host defense using the cooperation of the inflammatory and coagulation pathways. Recent research has shown that inorganic polyphosphate found in platelets activates FXII in vivo and can induce coagulation in pathological thrombus formation. Experimental studies have shown that interference with FXII provides thromboprotection without a therapy‐associated increase in bleeding, renewing interest in the FXIIa‐driven intrinsic pathway of coagulation as a therapeutic target. This review summarizes how the contact system acts as the cross‐road of inflammation, coagulation, and innate immunity.
Essentials
How the Alzheimer's disease (AD) peptide β‐amyloid (Aβ) disrupts neuronal function in the disease is unclear.
Factor (F) XII initiates blood clotting via FXI, and thrombosis has been ...implicated in AD.
Aβ triggers FXII‐dependent FXI and thrombin activation, evidence of which is seen in AD plasma.
Aβ‐triggered clotting could contribute to neuronal dysfunction in AD and be a novel therapeutic target.
Summary
Background
β‐Amyloid (Aβ) is a key pathologic element in Alzheimer's disease (AD), but the mechanisms by which it disrupts neuronal function in vivo are not completely understood. AD is characterized by a prothrombotic state, which could contribute to neuronal dysfunction by affecting cerebral blood flow and inducing inflammation. The plasma protein factor XII triggers clot formation via the intrinsic coagulation cascade, and has been implicated in thrombosis.
Objectives
To investigate the potential for Aβ to contribute to a prothrombotic state.
Methods and results
We show that Aβ activates FXII, resulting in FXI activation and thrombin generation in human plasma, thereby establishing Aβ as a possible driver of prothrombotic states. We provide evidence for this process in AD by demonstrating decreased levels of FXI and its inhibitor C1 esterase inhibitor in AD patient plasma, suggesting chronic activation, inhibition and clearance of FXI in AD. Activation of the intrinsic coagulation pathway in AD is further supported by elevated fibrin levels in AD patient plasma.
Conclusions
The ability of Aβ to promote coagulation via the FXII‐driven contact system identifies new mechanisms by which it could contribute to neuronal dysfunction and suggests potential new therapeutic targets in AD.
The classic intrinsic pathway of coagulation is triggered by contact activation of the plasma protease factor (F)XII, followed by sequential proteolytic activation of FX1 and FIX. While a key ...mechanism for initiating coagulation in some clinically useful in vitro assays, the absence of abnormal bleeding associated with congenital FXII deficiency indicates that the intrinsic pathway is not important for normal blood coagulation in vivo. However, recent work with mice lacking FXII or FXI suggest that these proteases make important contributions to formation of pathologic intravascular thrombi. In models of arterial injury, FXII or FXI null mice are protected from formation of platelet rich occlusive thrombi to a degree similar to that seen in FIX deficient mice (a model for the severe bleeding disorder hemophilia B) or to wild type mice treated with high dose heparin. FXII or FXI deficiency does not appear to prevent the initiation of thrombus formation in these models, but instead causes significant thrombus instability that prevents occlusion of the vessel. These findings raise the possibility that a pathway similar or identical to the intrinsic pathway may operate in vivo under some circumstances. Furthermore, the disproportionate importance of FXII and FXI to occlusive thrombus formation compared to normal hemostasis makes these proteases attractive candidates for therapeutic inhibitors to treat or prevent thromboembolic disorders.
See also Shapiro S, Laffan M. Making contact with microparticles. This issue, pp 1352–4.
Summary. Background: The procoagulant properties of microparticles (MPs) are due to the of the presence of ...phosphatidylserine (PS) and tissue factor (TF) on their surface. The latter has been demonstrated especially on MPs derived from monocytes.
Objectives: To investigate the relative contribution of TF and factor (F)XII in initiating coagulation on MPs derived from monocytes, platelets and erythrocytes.
Methods: Microparticles were isolated from calcium ionophore‐stimulated platelets, erythrocytes and monocytic THP‐1 cells. MPs were quantified, characterized for cell‐specific antigens and analyzed for TF, PS exposure and their thrombin‐generating potential.
Results: The MP number was not proportional to PS exposure and the majority of the MPs exposed PS. TF activity was undetectable on platelet‐ and erythrocyte‐derived MPs (< 1 fm nm−1 PS), whereas monocyte‐derived MPs exposed TF (32 fm nm−1 PS). Platelet‐, erythrocyte‐ and monocyte‐derived MPs, but not purified phospholipids, initiated thrombin generation in normal plasma in the absence of an external trigger (lag time < 11 min). Deficiency or inhibition of FVII had no effect on thrombin generation induced by platelet‐ and erythrocyte‐derived MPs, but interfered with monocyte MP‐triggered coagulation. Platelet‐ and erythrocyte‐derived MPs completely failed to induce thrombin generation in FXII‐deficient plasma. In contrast, monocyte‐derived MPs induced similar thrombin generation in normal vs. FXII‐deficient plasma.
Conclusion: MPs from platelets and erythrocytes not only propagate coagulation by exposing PS but also initiate thrombin generation independently of TF in a FXII‐dependent manner. In contrast, monocyte‐derived MPs trigger coagulation predominantly via TF.
Plasma prekallikrein is the liver-derived precursor of the trypsin-like serine protease plasma kallikrein (PK) and circulates in plasma bound to high molecular weight kininogen. The zymogen is ...converted to PK by activated factor XII. PK drives multiple proteolytic reaction cascades in the cardiovascular system such as the intrinsic pathway of coagulation, the kallikrein-kinin system, the fibrinolytic system, the renin-angiotensin system and the alternative complement pathway. Here, we review the biochemistry and cell biology of PK and focus on recent in vivo studies that have established important functions of the protease in procoagulant and proinflammatory disease states. Targeting PK offers novel strategies not previously appreciated to interfere with thrombosis and vascular inflammation in a broad variety of diseases.
Summary
Background
Acute thrombotic microangiopathies (TMAs) are characterized by excessive microvascular thrombosis and are associated with markers of neutrophil extracellular traps (NETs) in ...plasma. NETs are composed of DNA fibers and promote thrombus formation through the activation of platelets and clotting factors.
Objective
The efficient removal of NETs may be required to prevent excessive thrombosis such as in TMAs. To test this hypothesis, we investigated whether TMAs are associated with a defect in the degradation of NETs.
Methods and Results
We show that NETs generated in vitro were efficiently degraded by plasma from healthy donors. However, NETs remained stable after exposure to plasma from TMA patients. The inability to degrade NETs was linked to a reduced DNase activity in TMA plasma. Plasma DNase1 was required for efficient NET degradation and TMA plasma showed decreased levels of this enzyme. Supplementation of TMA plasma with recombinant human DNase1 restored NET‐degradation activity.
Conclusions
Our data indicate that DNase1‐mediated degradation of NETs is impaired in patients with TMAs. The role of plasma DNases in thrombosis is, as of yet, poorly understood. Reduced plasma DNase1 activity may cause the persistence of pro‐thrombotic NETs and thus promote microvascular thrombosis in TMA patients.
Plasma protein factor XII (FXII) activates the procoagulant and proinflammatory contact system that drives both the kallikrein–kinin system and the intrinsic pathway of coagulation. When zymogen FXII ...comes into contact with negatively charged surfaces, it auto‐activates to the serine proteaseactivated FXII (FXIIa). Recently, various in vivo activators of FXII have been identified including heparin, misfolded protein aggregates, polyphosphate and nucleic acids. Murine models have established a central role of FXII in arterial and venous thrombosis. Despite its central function in thrombosis, deficiency in FXII does not impair haemostasis in animals and humans. In a preclinical cardiopulmonary bypass system in large animals, the FXIIa‐blocking antibody 3F7 prevented thrombosis; however, in contrast to traditional anticoagulants, bleeding was not increased. In addition to its function in thrombosis, FXIIa initiates formation of the inflammatory mediator bradykinin. This mediator increases vascular leak, causes vasodilation, and induces chemotaxis with implications for septic, anaphylactic and allergic disease states. Therefore, targeting FXIIa appears to be a promising strategy for thromboprotection without associated bleeding risks but with anti‐inflammatory properties.
•Cytomegalovirus induces endothelial cell interleukin-11 secretion.•Viral replication drives interleukin-11 upregulation at the transcriptional level.•First report of any biological agent that ...induces endothelial cell IL-11 production.
Endothelial cells (EC) are critical sites of human cytomegalovirus (hCMV) infection in vivo. Infection can induce the production of various EC cytokines, such as interleukin (IL-)6, which can have autocrine and/or paracrine effector functions. Here, we report that hCMV induces the production of EC IL-11, a relatively understudied member of the IL-6-type cytokine family. We detail temporal EC IL-11 translation and protein secretion dynamics in response to hCMV infection, and reveal distinct differences compared to EC IL-6. Viral replication had markedly opposing effects on the regulation of these closely related cytokines, representing a major driving force behind IL-11 production, whilst concurrently suppressing IL-6 expression. This is the first report of any biological agent that stimulates EC IL-11 production.
Abstract Objective Proliferation of smooth muscle cells (SMCs) can stabilize atherosclerotic lesions but the molecular mechanisms that regulate this process in humans are largely unknown. We have ...previously shown that heparan sulfate proteoglycans (HSPGs), such as perlecan, regulate SMC growth in animal models by modulating heparin-binding mitogens. Since perlecan is expressed at low levels in human atherosclerosis, we speculated that the effect of heparan sulfate (HS) in human disease was rather influenced by HS degradation and investigated the expression of heparanase (HPSE) in human carotid endarterectomies. Methods and results Gene expression analysis from 127 endarterectomies in the BiKE database revealed increased expression of HPSE in carotid plaques compared with normal arteries, and a further elevation in symptomatic lesions. Increased HPSE protein expression in symptomatic plaque tissue was verified by tissue microarrays. HPSE mRNA levels correlated positively with expression of inflammatory markers IL-18, RANTES and IL-1β, and also T-cell co-stimulatory molecules, such as B7.2, CD28, LFA-1 and 4-1BB. Previously reported single nucleotide polymorphisms within HPSE were associated with differential mRNA expression in plaques. Immunohistochemistry revealed that inflammatory cells were major producers of HPSE in plaque tissue. HPSE immunoreactivity was also observed in SMCs adjacent to the necrotic core and was co-localized to deposits of fibrin. Conclusions This study demonstrates increased expression of HPSE in human atherosclerosis associated with inflammation, coagulation and plaque instability. Since HS can regulate SMC proliferation and influence plaque stability, the findings suggest that HPSE degradation of HS take part in the regulation of SMC function in human atherosclerosis.
Inflammation triggered by the deposition of LDL (low-density lipoprotein) in the arterial wall leads to the development of atherosclerosis. Regulatory T (Treg) cells inhibit vascular inflammation ...through the induction of immune tolerance toward LDL-related antigens. However, tolerogenic mechanisms that promote the generation of LDL-specific Treg cells in vivo remain unclear.
We identified LDL-specific T cells by activation-induced marker expression and analyzed expression profiles and suppressive functions of TCR (T-cell antigen receptor)-transgenic T cells upon repetitive transfer into antigen-transgenic mice via flow cytometry.
We investigated the naturally occurring Treg-cell response against human LDL in standard chow diet-fed mice that are transgenic for human ApoB100 (apolipoprotein B100). We found that IL (interleukin)-10 expression in LDL-specific T cells from spleen increases with age, albeit LDL-specific populations do not enlarge in older mice. To investigate the generation of IL-10-producing LDL-specific T cells, we transferred naive CD4+ T cells recognizing human ApoB100 from TCR-transgenic mice into human ApoB100-transgenic mice. Adoptive transfer of human ApoB100-specific T cells induced immune tolerance in recipient mice and effectively inhibited activation of subsequently transferred naive T cells of the same specificity in vivo. Moreover, repetitive transfers increased the population of Treg type 1 cells that suppress ApoB100-specific responses via IL-10. In a translational approach, LDL-specific Treg type 1 cells from blood of healthy donors suppressed the activation of monocytic THP-1 cells in an IL-10-dependent manner.
We show that repetitive transfer of naive ApoB100-specific T cells and recurrent LDL-specific T-cell stimulation induces Treg type 1 cell-mediated immune tolerance against LDL in vivo. Our results provide insight into the generation of autoantigen-specific anti-inflammatory T cells under tolerogenic conditions.
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