An assay for measuring functional activated thrombin-activatable fibrinolysis inhibitor in plasma

Assays to quantify fibrinolysis: strengths and limitations. Communication from the International Society on Thrombosis and Haemostasis Scientific and Standardization Committee on fibrinolysis

Fibrinolysis is a series of enzymatic reactions that degrade insoluble fibrin. Plasminogen activators convert the zymogen plasminogen to the active serine protease plasmin, which cleaves and solubilizes crosslinked fibrin clots into fibrin degradation products. The quantity and quality of fibrinolytic enzymes, their respective inhibitors, and clot structure determine overall fibrinolysis. The quantity of protein can be measured by antigen-based assays, and both quantity and quality can be assessed using functional assays. Furthermore, variations of commonly used assays have been reported, which are tailored to address the role(s) of specific fibrinolytic factors and cellular elements (eg, platelets, neutrophils, and red blood cells). Although the concentration and/or activity of a protein can be quantified, how these individual components contribute to the overall fibrinolysis outcome can be challenging to determine. This difficulty is due to temporal changes within and around the thrombi during the clot breakdown, particularly the fibrin matrix structure, and composition. Furthermore, terms such as "fibrinolytic activity/potential," "plasminogen activation," and "plasmin activity" are often used interchangeably despite having different definitions. The purpose of this review is to 1) summarize the assays measuring fibrinolysis activity and potential, 2) facilitate the interpretation of data generated by these assays, and 3) summarize the strengths and limitations of these assays.

Biomarkers of coagulation, endothelial function, and fibrinolysis in critically ill patients with COVID-19: A single-center prospective longitudinal study

BACKGROUND

Immunothrombosis and coagulopathy in the lung microvasculature may lead to lung injury and disease progression in coronavirus disease 2019 (COVID-19). We aim to identify biomarkers of coagulation, endothelial function, and fibrinolysis that are associated with disease severity and may have prognostic potential.

METHODS

We performed a single-center prospective study of 14 adult COVID-19(+) intensive care unit patients who were age- and sex-matched to 14 COVID-19(-) intensive care unit patients, and healthy controls. Daily blood draws, clinical data, and patient characteristics were collected. Baseline values for 10 biomarkers of interest were compared between the three groups, and visualized using Fisher's linear discriminant function. Linear repeated-measures mixed models were used to screen biomarkers for associations with mortality. Selected biomarkers were further explored and entered into an unsupervised longitudinal clustering machine learning algorithm to identify trends and targets that may be used for future predictive modelling efforts.

RESULTS

Elevated D-dimer was the strongest contributor in distinguishing COVID-19 status; however, D-dimer was not associated with survival. Variable selection identified clot lysis time, and antigen levels of soluble thrombomodulin (sTM), plasminogen activator inhibitor-1 (PAI-1), and plasminogen as biomarkers associated with death. Longitudinal multivariate k-means clustering on these biomarkers alone identified two clusters of COVID-19(+) patients: low (30%) and high (100%) mortality groups. Biomarker trajectories that characterized the high mortality cluster were higher clot lysis times (inhibited fibrinolysis), higher sTM and PAI-1 levels, and lower plasminogen levels.

CONCLUSIONS

Longitudinal trajectories of clot lysis time, sTM, PAI-1, and plasminogen may have predictive ability for mortality in COVID-19.

Carboxypeptidase U (CPU, TAFIa, CPB2) in Thromboembolic Disease: What Do We Know Three Decades after Its Discovery?

Procarboxypeptidase U (proCPU, TAFI, proCPB2) is a basic carboxypeptidase zymogen that is converted by thrombin(-thrombomodulin) or plasmin into the active carboxypeptidase U (CPU, TAFIa, CPB2), a potent attenuator of fibrinolysis. As CPU forms a molecular link between coagulation and fibrinolysis, the development of CPU inhibitors as profibrinolytic agents constitutes an attractive new concept to improve endogenous fibrinolysis or to increase the efficacy of thrombolytic therapy in thromboembolic diseases. Furthermore, extensive research has been conducted on the in vivo role of CPU in (the acute phase of) thromboembolic disease, as well as on the hypothesis that high proCPU levels and the Thr/Ile325 polymorphism may cause a thrombotic predisposition. In this paper, an overview is given of the methods available for measuring proCPU, CPU, and inactivated CPU (CPUi), together with a summary of the clinical data generated so far, ranging from the current knowledge on proCPU concentrations and polymorphisms as potential thromboembolic risk factors to the positioning of different CPU forms (proCPU, CPU, and CPUi) as diagnostic markers for thromboembolic disease, and the potential benefit of pharmacological inhibition of the CPU pathway.

Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) inhibition stimulates the fibrinolytic rate in different in vitro models

Essentials AZD9684 is a potent inhibitor of carboxypeptidase U (CPU, TAFIa, CPB2). The effect of AZD9684 on fibrinolysis was investigated in four in vitro systems. The CPU system also attenuates fibrinolysis in more advanced hemostatic systems. The size of the observed effect on fibrinolysis is dependent on the exact experimental conditions.

SUMMARY

Background Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) is a basic carboxypeptidase that attenuates fibrinolysis. This characteristic has raised interest in the scientific community and pharmaceutical industry for the development of inhibitors as profibrinolytic agents. Objectives Little is known about the contribution of CPU to clot resistance in more advanced hemostatic models, which include blood cells and shear stress. The aim of this study was to evaluate the effects of the CPU system in in vitro systems for fibrinolysis with different grades of complexity. Methods The contribution of the CPU system was evaluated in the following systems: (i) plasma clot lysis; (ii) rotational thromboelastometry (ROTEM) in whole blood; (iii) front lysis with confocal microscopy in platelet-free and platelet-rich plasma; and (iv) a microfluidic system with whole blood under arterial shear stress. Experiments were carried out in the presence or absence of AZD9684, a specific CPU inhibitor. Results During plasma clot lysis, addition of AZD9684 resulted in 33% faster lysis. In ROTEM, the lysis onset time was decreased by 38%. For both clot lysis and ROTEM, an AZD9684 dose-dependent response was observed. CPU inhibition in front lysis experiments resulted in 47% and 50% faster lysis for platelet-free plasma and platelet-rich plasma, respectively. Finally, a tendency for faster lysis was observed only in the microfluidic system when AZD9684 was added. Conclusions Overall, these experiments provide novel evidence that the CPU system can also modulate fibrinolysis in more advanced hemostatic systems. The extent of the effects appears to be dependent upon the exact experimental conditions.

Pro-inflammatory cytokines reduce human TAFI expression via tristetraprolin-mediated mRNA destabilisation and decreased binding of HuR

Thrombin activatable fibrinolysis inhibitor (TAFI) is the zymogen form of a basic carboxypeptidase (TAFIa) with both anti-fibrinolytic and anti-inflammatory properties. The role of TAFI in inflammatory disease is multifaceted and involves modulation both of specific inflammatory mediators as well as of the behaviour of inflammatory cells. Moreover, as suggested by in vitro studies, inflammatory mediators are capable of regulating the expression of CPB2, the gene encoding TAFI. In this study we addressed the hypothesis that decreased TAFI levels observed in inflammation are due to post-transcriptional mechanisms. Treatment of human HepG2 cells with pro-inflammatory cytokines TNFα, IL-6 in combination with IL-1β, or with bacterial lipopolysaccharide (LPS) decreased TAFI protein levels by approximately two-fold over 24 to 48 hours of treatment. Conversely, treatment of HepG2 cells with the anti-inflammatory cytokine IL-10 increased TAFI protein levels by two-fold at both time points. We found that the mechanistic basis for this modulation of TAFI levels involves binding of tristetraprolin (TTP) to the CPB2 3′-UTR, which mediates CPB2 mRNA destabilisation. In this report we also identified that HuR, another ARE-binding protein but one that stabilises transcripts, is capable of binding the CBP2 3’UTR. We found that pro-inflammatory mediators reduce the occupancy of HuR on the CPB2 3’-UTR and that the mutation of the TTP binding site in this context abolishes this effect, although TTP and HuR appear to contact discrete binding sites. Interestingly, all of the mediators tested appear to increase TAFI protein expression in THP-1 macrophages, likewise through effects on CPB2 mRNA stability.

The mRNA encoding TAFI is alternatively spliced in different cell types and produces intracellular forms of the protein lacking TAFIa activity

TAFI (thrombin-activatable fibrinolysis inhibitor) is a pro-carboxypeptidase, encoded by the CPB2 gene in humans that links the coagulation cascade to fibrinolysis and inflammation. The liver is the main source for plasma TAFI, and TAFI expression has been documented in platelets and monocyte-derived macrophages. A recent study reported an alternatively spliced CPB2 mRNA variant lacking exon 7 (Δ7) in HepG2 cells and liver. Another study identified a CPB2 mRNA variant lacking exon 7 and a 52 bp deletion in exon 11 (Δ7+11) in human hippocampus. We have examined alternative splicing of CPB2 mRNA in various cell types by RT-PCR and have assessed the functional properties of TAFI variants encoded by these transcripts by recombinant expression in mammalian cells. We identified the Δ7 exon skipping event in liver, Dami megakaryoblasts, THP-1-derived macrophages, peripheral blood mononuclear cells, platelets, testis, cerebellum, and SH-SY5Y neuroblastoma cells. The Δ11 alternative splicing event was notably absent in liver cells. We also detected a novel exon Δ7+8 skipping event in liver and megakaryocytes. Of note, we detected non-alternatively spliced CPB2 transcripts in brain tissues, suggesting the expression of full-length TAFI in brain. Experiments using cultured mammalian cells transfected with wild-type CPB2-, Δ7-, Δ7+11 -, and_Δ11 -cDNA revealed that alternatively spliced TAFI is stored inside the cells, cannot be activated by thrombin-thrombomodulin, and does not have TAFIa activity. The alternative splicing events clearly do not give rise to a secreted protein with basic carboxy-peptidase activity, but the intracellular forms may possess novel functions related to intracellular proteolysis.

Plasma levels of carboxypeptidase U (CPU, CPB2 or TAFIa) are elevated in patients with acute myocardial infarction

BACKGROUND

Two decades after its discovery, carboxypeptidase U (CPU, CPB2 or TAFIa) has become a compelling drug target in thrombosis research. However, given the difficulty of measuring CPU in the blood circulation and the demanding sample collecton requirements, previous clinical studies focused mainly on measuring its inactive precursor, proCPU (proCPB2 or TAFI).

OBJECTIVES

Using a sensitive and specific enzymatic assay, we investigated plasma CPU levels in patients presenting with acute myocardial infarction (AMI) and in controls.

METHODS

In this case-control study, peripheral arterial blood samples were collected from 45 patients with AMI (25 with ST segment elevation myocardial infarction [STEMI], 20 with non-ST segment elevation myocardial infarction [NSTEMI]) and 42 controls. Additionally, intracoronary blood samples were collected from 11 STEMI patients during thrombus aspiration. Subsequently, proCPU and CPU plasma concentrations in all samples were measured by means of an activity-based assay, using Bz-o-cyano-Phe-Arg as a selective substrate.

RESULTS

CPU activity levels were higher in patients with AMI (median LOD-LOQ, range 0-1277 mU L(-1) ) than in controls (median < LOD, range 0-128 mU L(-1) ). No correlation was found between CPU levels and AMI type (NSTEMI [median between LOD-LOQ, range 0-465 mU L(-1) ] vs. STEMI [median between LOD-LOQ, range 0-1277 mU L(-1) ]). Intracoronary samples (median 109 mU L(-1) , range 0-759 mU L(-1) ) contained higher CPU levels than did peripheral samples (median between LOD-LOQ, range 0-107 mU L(-1) ), indicating increased local CPU generation. With regard to proCPU, we found lower levels in AMI patients (median 910 U L(-1) , range 706-1224 U L(-1) ) than in controls (median 1010 U L(-1) , range 753-1396 U L(-1) ).

CONCLUSIONS

AMI patients have higher plasma CPU levels and lower proCPU levels than controls. This finding indicates in vivo generation of functional active CPU in patients with AMI.

Active PAI-1 as marker for venous thromboembolism: case-control study using a comprehensive panel of PAI-1 and TAFI assays

BACKGROUND

Both activated Thrombin Activatable Fibrinolysis Inhibitor (TAFI) and active Plasminogen Activator Inhibitor-1 (PAI-1) attenuate fibrinolysis and may therefore contribute to the pathophysiology of Venous ThromboEmbolism (VTE). Whether increased TAFI and/or PAI-1 concentrations are associated with VTE is unclear.

OBJECTIVE

To study an association of impaired fibrinolysis and VTE using a comprehensive panel of in-house developed assays measuring intact TAFI, activation peptide of TAFI (AP-TAFI), PAI-1 antigen, endogenous PAI-1:t-PA complex (PAI-1:t-PA) and active PAI-1 levels in 102 VTE patients and in 113 healthy controls (HC).

RESULTS

Active PAI-1 was significantly higher in VTE patients compared to HC (20.9 [9.6-37.8] ng/ml vs. 6.2 [3.5-9.7] ng/ml, respectively). Active PAI-1 was the best discriminator with an area under the ROC curve and 95% confidence interval (AUROC [95%CI]) of 0.84 [0.79-0.90] compared to 0.75 [0.68-0.72] for PAI-1:t-PA, 0.65 [0.58-0.73] for PAI-1 antigen, 0.62 [0.54-0.69] for AP-TAFI and 0.51 [0.44-0.59] for intact TAFI. Using ROC analysis, we defined an optimal cut-off of 12.8 ng/ml for active PAI-1, with corresponding sensitivity of 71 [61-79] % and specificity of 89 [82-94] %. A lack of association with the time between VTE event and sample collection or with the intake of anticoagulant treatment suggests that active PAI-1 levels are sustainable high in VTE patients.

CONCLUSIONS

This case-control study emphasizes the clinical importance of measuring active PAI-1 instead of PAI-1 antigen and identifies active PAI-1 as a potential marker of VTE. Prognostic studies will need to address the clinical significance of active PAI-1 as biomarker.

Modulation of inflammatory and hemostatic markers in obstructive sleep apnea patients treated with mandibular advancement splints: a parallel, controlled trial

STUDY OBJECTIVE

Obstructive sleep apnea (OSA) is associated with systemic inflammation and a hypercoagulable state. The current study aim was to investigate whether mandibular advancement splint (MAS) therapy affects inflammatory and hemostatic parameters in patients with mild-to-moderate OSA.

METHODS

Twenty-two patients with mild-to-moderate OSA and 16 control subjects were studied. OSA subjects were treated with a titratable MAS for 6 months. Baseline plasma C-reactive protein, interleukin-1β, interleukin-10, interleukin-6, P-selectin, fibrinogen, D-dimer, plasminogen activator inhibitor-1 (PAI-1), thrombin-antithrombin complex, activated thrombin-activatable fibrinolysis inhibitor (TAFIa), 6-keto-PGF1α, glucose, and fibrin clot lysis time (CLT) were measured in all subjects. After 3 months of MAS therapy, measurements were repeated for the 22 patients, and after 6 months all measurements were repeated for all study subjects.

RESULTS

MAS treatment reduced significantly AHI at 3 months (24 vs 13.1/h) and further improved it at 6 months (13.1 vs 7.05/h). Compared with controls, OSA subjects had a significant higher baseline mean levels of fibrinogen, TAFIa, 6-keto-PGF1α, and glucose. MAS treatment significantly improved levels of IL-1β, D-dimer, TAFIa, and CLT. Despite residual apneas, MAS treatment group presented similar measured homeostatic and inflammatory levels to controls except for glucose.

CONCLUSION

Treatment with MAS in mild-to-moderate OSA subjects improves the inflammatory profile and homeostatic markers.

CITATION

Niżankowska-Jędrzejczyk A; Almeida FR; Lowe AA; Kania A; Nastałek P; Mejza F; Foley JH; Niżankowska-Mogilnicka E; Undas A. Modulation of inflammatory and hemostatic markers in obstructive sleep apnea patients treated with mandibular advancement splints: a parallel, controlled trial.

Insights into thrombin activatable fibrinolysis inhibitor function and regulation

Fibrinolysis is initiated when the zymogen plasminogen is converted to plasmin via the action of plasminogen activators. Proteolytic cleavage of fibrin by plasmin generates C-terminal lysine residues capable of binding both plasminogen and the plasminogen activator, thereby stimulating plasminogen activator-mediated plasminogen activation and propagating fibrinolysis. This positive feedback mechanism is regulated by activated thrombin activatable fibrinolysis inhibitor (TAFIa), which cleaves C-terminal lysine residues from the fibrin surface, thereby decreasing its cofactor activity. TAFI can be activated by thrombin alone, but the rate of activation is accelerated when in complex with thrombomodulin. Plasmin is also known to activate TAFI. TAFIa has no known physiologic inhibitors and consequently, its primary regulatory mechanism involves its intrinsic thermal instability. The rate of TAFI activation and stability of the active form, TAFIa, function in maintaining its concentration above the threshold value required to down-regulate fibrinolysis. Although all methods to quantify TAFI or TAFIa have their limitations, epidemiologic studies have indicated that elevated TAFI levels are correlated with an increased risk of venous thrombosis. Major efforts have been made to develop TAFI inhibitors that can either directly interfere with TAFIa activity or impair its activation. However, the anti-inflammatory properties of TAFIa might complicate the development and application of a TAFIa inhibitor that aims to increase the efficiency of thrombolytic therapy.

Activated thrombin activatable fibrinolysis inhibitor (TAFIa) is associated with inflammatory markers in inflammatory bowel diseases TAFIa level in patients with IBD

BACKGROUND

Thrombin activatable fibrinolysis inhibitor (TAFI) has been reported to be involved in the pathogenesis and progression of inflammatory bowel disease (IBD). Activated TAFI (TAFIa) attenuates fibrinolysis by cleaving C-terminal lysine residues thus down-regulating plasminogen activation. To date, no reports on TAFIa in IBD have been published.

METHODS

Plasma levels of TAFIa were measured using a functional assay in 55 consecutive patients with ulcerative colitis (UC) and 50 with Crohn's disease (CD). Associations of TAFIa with disease activity, hemostatic variables and inflammatory markers were assessed.

RESULTS

Plasma TAFIa was higher in CD patients than in those with UC. The disease activity correlated positively with TAFIa levels in the UC group, but not in the CD group. In UC patients, there were positive correlations of TAFIa with white blood cells, C-reactive protein and fibrinogen and an inverse correlation with albumin. In the CD group, a positive correlation was shown for C-reactive protein, fibrinogen and platelet count, while a negative correlation was noted for albumin.

CONCLUSIONS

This study is the first to show that TAFIa is increased in CD patients compared with UC and its levels are associated with inflammatory markers in both forms of IBD. These findings fit in the hypothesis that TAFIa may be a marker of active IBD, and in particular of active UC.

Thrombin activatable fibrinolysis inhibitor activation and bleeding in haemophilia A

Individuals with haemophilia A exhibit bleeding tendencies that are not always predicted by their factor (F)VIII level. It has been suggested that bleeding in haemophilia is due not only to defective prothrombin activation but also aberrant fibrinolysis. Thrombin activatable fibrinolysis inhibitor (TAFI) activation was measured in tissue factor (TF)-initiated blood coagulation in blood samples of 28 haemophiliacs and five controls. Reactions were quenched over time with FPRck and citrate and assayed for TAFIa and thrombin-antithrombin (TAT). The TAFIa potential (TP), TAFI activation rate and the TAFIa level at 20 min (TAFIa(20 min)) was extracted from the TAFI activation progress curve. In general, the time course of TAFI activation follows thrombin generation regardless of FVIII activity and as expected the rate of TAFI activation and TP decreases as FVIII decreases. The magnitude of TP was similar among the control subjects and subjects with <11% FVIII. In severe subjects with <1% FVIII at the time of blood collection, the TAFIa(20 min) was inversely and significantly correlated with haemarthrosis (-0.77, P = 0.03) and total bleeds (-0.75, P = 0.03). In all cases, TAFIa(20 min) was more strongly correlated with bleeding than TAT levels at 20 min. Overall, this study shows that TAFI activation in whole blood can be quantified and related to the clinical bleeding phenotype. Measuring TAFIa along with thrombin generation can potentially be useful to evaluate the differential bleeding phenotype in haemophilia A.

Identification of human thrombin-activatable fibrinolysis inhibitor in vascular and inflammatory cells

TAFI (thrombin-activatable fibrinolysis inhibitor) is a carboxypeptidase zymogen originally identified in plasma. The TAFI pathway helps to regulate the balance between the coagulation and fibrinolytic cascades. Activated TAFI (TAFIa) can also inactivate certain pro-inflammatory mediators, suggesting that the TAFI pathway may also regulate communication between coagulation and inflammation. Expression in the liver is considered to be the source of plasma TAFI. TAFI has also been identified in platelets and CPB2 (the gene encoding TAFI) mRNA has been detected in megakaryocytic cell lines as well as in endothelial cells. We have undertaken a quantitative analysis of CPB2 mRNA and TAFI protein in extrahepatic cell types relevant to vascular disease. Using RT-PCR and quantitative RT-PCR, we detected CPB2 mRNA in the human megakaryoblastic cell lines MEG-01 and Dami, the human monocytoid cell line THP-1 as well as THP-1 cells differentiated into a macrophage-like phenotype, and in primary human umbilical vein and coronary artery endothelial cells. CPB2 mRNA abundance in MEG-01, Dami, and THP-1 cells was modulated by the state of differentiation of these cells. Using a recently developed TAFIa assay, we detected TAFI protein in the lysates of the human hepatocellular carcinoma cell line HepG2 as well as in MEG-01 and Dami cells and in the conditioned medium of HepG2 cells, differentiated Dami cells, and THP-1 macrophages. We have obtained clear evidence for extrahepatic expression of TAFI, which has clear implications for the physiological and pathophysiological functions of the TAFI pathway.

Kinetics of activated thrombin-activatable fibrinolysis inhibitor (TAFIa)-catalyzed cleavage of C-terminal lysine residues of fibrin degradation products and removal of plasminogen-binding sites

Partial digestion of fibrin by plasmin exposes C-terminal lysine residues, which comprise new binding sites for both plasminogen and tissue-type plasminogen activator (tPA). This binding increases the catalytic efficiency of plasminogen activation by 3000-fold compared with tPA alone. The activated thrombin-activatable fibrinolysis inhibitor (TAFIa) attenuates fibrinolysis by removing these residues, which causes a 97% reduction in tPA catalytic efficiency. The aim of this study was to determine the kinetics of TAFIa-catalyzed lysine cleavage from fibrin degradation products and the kinetics of loss of plasminogen-binding sites. We show that the k(cat) and K(m) of Glu(1)-plasminogen (Glu-Pg)-binding site removal are 2.34 s(-1) and 142.6 nm, respectively, implying a catalytic efficiency of 16.21 μm(-1) s(-1). The corresponding values of Lys(77)/Lys(78)-plasminogen (Lys-Pg)-binding site removal are 0.89 s(-1) and 96 nm implying a catalytic efficiency of 9.23 μm(-1) s(-1). These catalytic efficiencies of plasminogen-binding site removal by TAFIa are the highest of any TAFIa-catalyzed reaction with a biological substrate reported to date and suggest that plasmin-modified fibrin is a primary physiological substrate for TAFIa. We also show that the catalytic efficiency of cleavage of all C-terminal lysine residues, whether they are involved in plasminogen binding or not, is 1.10 μm(-1) s(-1). Interestingly, this value increases to 3.85 μm(-1) s(-1) in the presence of Glu-Pg. These changes are due to a decrease in K(m). This suggests that an interaction between TAFIa and plasminogen comprises a component of the reaction mechanism, the plausibility of which was established by showing that TAFIa binds both Glu-Pg and Lys-Pg.

Secretion and antifibrinolytic function of thrombin-activatable fibrinolysis inhibitor from human platelets

BACKGROUND

The thrombin-activatable fibrinolysis inhibitor (TAFI) is a zymogen first characterized in human plasma that is activated through proteolytic cleavage by thrombin, thrombin in complex with thrombomodulin, or plasmin. Active TAFI attenuates fibrinolysis by removing C-terminal lysine residues from partially degraded fibrin, thereby inhibiting a potent positive feedback loop in the fibrinolytic cascade. The existence of a separate pool of TAFI within platelets has been described.

OBJECTIVES AND METHODS

We aimed to confirm the presence of TAFI in the medium of washed, thrombin-stimulated platelets and to evaluate the characteristics of platelet TAFI by western blot analysis and with a quantitative assay for activated TAFI. We also assessed the ability of platelet TAFI to inhibit fibrinolysis in vitro, using a platelet-rich thrombus lysis assay.

RESULTS

Our data are consistent with the presence of TAFI in the α-granules of resting platelets. In contrast to previous reports, platelet TAFI is very similar in electrophoretic mobility to plasma-derived TAFI. We also show, for the first time, that platelet-derived TAFI is capable of attenuating platelet-rich thrombus lysis in vitro independently of plasma TAFI. Moreover, we demonstrate additive effects on thrombolysis of platelet-derived TAFI and TAFI present in plasma.

CONCLUSIONS

Taken together, these observations indicate that the secretion of platelet-derived TAFI can augment the concentrations of TAFI already present in plasma to enhance attenuation of the fibrinolytic cascade. This could be significant in regions of vascular damage or pathologic thrombosis, where activated platelets are known to accumulate.

Measurement of carboxypeptidase U (active thrombin-activatable fibrinolysis inhibitor) in plasma: Challenges overcome by a novel selective assay

This article introduces a novel assay for the measurement of carboxypeptidase U (CPU) in plasma using the selective CPU substrate Bz-o-cyano-Phe-Arg (N-benzoyl-ortho-cyano-phenylalanyl-arginine), thereby limiting the interference of plasma carboxypeptidase N (CPN) as well as the intrinsic activity of procarboxypeptidase U (proCPU). A limit of detection of 0.05 U/L (10 pM) was reached. In addition, the current assay has the advantage of being easy to perform and shows excellent linearity and variability, rendering it a useful tool in the screening of samples for the presence of CPU in several patient populations and encouraging in-depth exploration of the pathophysiological role of the proCPU/CPU system.

Soluble thrombomodulin partially corrects the premature lysis defect in FVIII-deficient plasma by stimulating the activation of thrombin activatable fibrinolysis inhibitor

BACKGROUND

Previous work by others has shown that premature clot lysis occurs in plasmas deficient in components of the intrinsic pathway, due to a failure to activate thrombin activatable fibrinolysis inhibitor (TAFI). This suggests the hypothesis that bleeding in hemophilia is due not only to defective coagulation but also enhanced fibrinolysis. These studies were carried out to quantify the extent of TAFI activation over time in normal plasma (NP) and factor VIII deficient plasma (FVIII-DP) and to determine whether soluble thrombomodulin (sTM) can correct the lysis defect in FVIII-DP.

METHODS

The time courses of TAFI activation in both NP and FVIII-DP were monitored after clotting with thrombin, PCPS and Ca(2+), +/- sTM. Clotting and lysis were measured turbidometrically and TAFIa using a functional assay.

RESULTS

Premature lysis that occurs in FVIII-DP is corrected by mixing deficient plasma with 10% NP. However, this does not fully correct the defect in TAFI activation. FVIII-DP must be mixed with up to 50% NP to attain the same TAFIa potential as NP. In FVIII-DP, sTM can correct the defect in TAFIa-dependent prolongation of lysis at low tPA concentrations and partially correct this defect at high tPA concentrations.

CONCLUSIONS

TAFI activation increases as the concentration of FVIII increases. FVIII at a level of 10% fully corrects the lysis defect in spite of the extent of TAFI activation being only one half that obtained with 100% FVIII. In addition, sTM increases TAFI activation sufficiently to correct the premature lysis defect in FVIII-DP.

Thrombin-activable fibrinolysis inhibitor zymogen does not play a significant role in the attenuation of fibrinolysis

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) plays a significant role in the prolongation of fibrinolysis. During fibrinolysis, plasminogen is activated to plasmin, which lyses a clot by cleaving fibrin after selected arginine and lysine residues. TAFIa attenuates fibrinolysis by removing the exposed C-terminal lysine residues. It was recently reported that TAFI zymogen possesses sufficient carboxypeptidase activity to attenuate fibrinolysis through a mechanism similar to TAFIa. Here, we show with a recently developed TAFIa assay that when thrombin is used to clot TAFI-deficient plasma supplemented with TAFI, there is some TAFI activation. The extent of activation was dependent upon the concentration of zymogen present in the plasma, and lysis times were prolonged by TAFIa in a concentration-dependent manner. Potato tuber carboxypeptidase inhibitor, an inhibitor of TAFIa but not TAFI, abolished the prolongation of lysis in TAFI-deficient plasma supplemented with TAFI zymogen. In addition, TAFIa but not TAFI catalyzed release of plasminogen bound to soluble fibrin degradation products. The data presented confirm that TAFI zymogen is effective in cleaving a small substrate but does not play a role in the attenuation of fibrinolysis because of its inability to cleave plasmin-modified fibrin degradation products.