Cellular FXIII in Human Macrophage-Derived Foam Cells

Macrophages express the A subunit of coagulation factor XIII (FXIII-A), a transglutaminase which cross-links proteins through Nε-(γ-L-glutamyl)-L-lysyl iso-peptide bonds. Macrophages are major cellular constituents of the atherosclerotic plaque; they may stabilize the plaque by cross-linking structural proteins and they may become transformed into foam cells by accumulating oxidized LDL (oxLDL). The combination of oxLDL staining by Oil Red O and immunofluorescent staining for FXIII-A demonstrated that FXIII-A is retained during the transformation of cultured human macrophages into foam cells. ELISA and Western blotting techniques revealed that the transformation of macrophages into foam cells elevated the intracellular FXIII-A content. This phenomenon seems specific for macrophage-derived foam cells; the transformation of vascular smooth muscle cells into foam cells fails to induce a similar effect. FXIII-A containing macrophages are abundant in the atherosclerotic plaque and FXIII-A is also present in the extracellular compartment. The protein cross-linking activity of FXIII-A in the plaque was demonstrated using an antibody labeling the iso-peptide bonds. Cells showing combined staining for FXIII-A and oxLDL in tissue sections demonstrated that FXIII-A-containing macrophages within the atherosclerotic plaque are also transformed into foam cells. Such cells may contribute to the formation of lipid core and the plaque structurization.

Activation mechanism dependent surface exposure of cellular factor XIII on activated platelets and platelet microparticles

BACKGROUND

Platelets contain a high amount of potentially active A subunit dimer of coagulation factor XIII (cellular FXIII; cFXIII). It is of cytoplasmic localization, not secreted, but becomes translocated to the surface of platelets activated by convulxin and thrombin (CVX+Thr).

OBJECTIVE

To explore the difference in cFXIII translocation between receptor mediated and non-receptor mediated platelet activation and if translocation can also be detected on platelet-derived microparticles. Our aim was also to shed some light on the mechanism of cFXIII translocation.

METHODS

Gel-filtered platelets were activated by CVX+Thr or Ca2+ -ionophore (calcimycin). The translocation of cFXIII and phosphatidylserine (PS) to the surface of activated platelets and platelet-derived microparticles was investigated by flow cytometry, immunofluorescence, and immune electron microscopy. Fluo-4-AM fluorescence was used for the measurement of intracellular Ca2+ concentration.

RESULTS

Receptor mediated activation by CVX+Thr exposed cFXIII to the surface of more than 60% of platelets. Electron microscopy revealed microparticles with preserved membrane structure and microparticles devoid of labeling for membrane glycoprotein CD41a. cFXIII was observed on both types of microparticles but was more abundant in the absence of CD41a. Rhosin, a RhoA inhibitor, significantly decreased cFXIII translocation. Non-receptor mediated activation of platelets by calcimycin elevated intracellular Ca2+ concentration, induced the translocation of PS to the surface of platelets and microparticles, but failed to expose cFXIII.

CONCLUSIONS

The elevation of intracellular Ca2+ concentration is sufficient for the translocation of PS from the internal layer of the membrane, while the translocation of cFXIII from the platelet cytoplasm requires additional receptor mediated mechanism(s).

An international collaborative study to assign value for Total Factor XIII-B Subunit Antigen to the WHO 1st International Standard for Factor XIII Plasma, (02/206): Communication from the ISTH SSC Subcommittee on Factor XIII and Fibrinogen

BACKGROUND

Factor XIII (FXIII)-B subunit measurements are required for the diagnosis and characterization of the type of FXIII deficiency. Furthermore, therapy for FXIII-A deficiency with recombinant FXIII (rFXIII-A) relies on available FXIII-B.

OBJECTIVE

To carry out a collaborative study to calibrate and assign value to the current WHO 1st International Standard (IS) FXIII Plasma for Total FXIII-B subunit, relative to locally collected normal plasma pools.

METHODS

Laboratories were instructed to use a validated method (specific ELISA antibodies provided) for assessment of Total FXIII-B subunit antigen potency. All laboratories used this method with one laboratory using an additional in-house method. Nine data sets were received from seven laboratories (37 assays in total), which provided a total of 35 valid estimates for this new assignment. Total FXIII-B subunit estimates were calculated relative to locally collected normal plasma pools, using an arbitrary value of 1.00 unit of Total FXIII-B subunit per ml, for each pool.

RESULTS

Combination of results produced an overall mean of 0.98 units/mL with an inter-laboratory variability (geometric coefficients of variation - GCV%) of 18.3% [95% confidence interval: 0.86-1.11]. Real-time and bench stability studies indicated good stability and preservation of the FXIII-B subunit analyte in the WHO 1st IS FXIII Plasma (02/206).

CONCLUSION

Following agreement by study participants, ISTH/SSC Experts, WHO-ISTH Liaison Group and the SSC Board, the WHO/ECBS established the current WHO 1st IS Factor XIII plasma (NIBSC code 02/206) by additionally assigning it with a Total FXIII-B subunit antigen value of 0.98 IU/ampoule, in October 2019.

Effect of α2-plasmin inhibitor heterogeneity on the risk of venous thromboembolism

INTRODUCTION

Alpha2-plasmin inhibitor (α2-PI) has a heterogeneous composition in the plasma. Both N- and C-terminal cleavages occur that modify the function of the molecule. C-terminal cleavage converts the plasminogen-binding form (PB-α2-PI) to a non-plasminogen-binding form (NPB-α2-PI). N-terminal cleavage by soluble fibroblast activation protein (sFAP) results in a form shortened by 12 amino acids, which is more quickly cross-linked to fibrin. The p.Arg6Trp polymorphism of α2-PI affects N-terminal cleavage. In this work, we aimed to investigate the association between α2-PI heterogeneity and the risk of venous thromboembolism.

MATERIALS AND METHODS

Two hundred and eighteen patients with venous thromboembolism (VTE) and the same number of age and sex-matched healthy controls were enrolled. Total-α2-PI, PB-α2-PI and NPB-α2-PI antigen levels, α2-PI activity, sFAP antigen levels and p.Arg6Trp polymorphism were investigated.

RESULTS

Total-α2-PI and NPB-α2-PI levels were significantly elevated in VTE patients, while PB-α2-PI levels did not change. Elevated NPB-α2-PI levels independently associated with VTE risk (adjusted OR: 9.868; CI: 4.095-23.783). Soluble FAP levels were significantly elevated in the VTE group, however, elevated sFAP levels did not show a significant association with VTE risk. The α2-PI p.Arg6Trp polymorphism did not influence VTE risk, however, in the case of elevated sFAP levels the carriage of Trp6 allele associated with lower VTE risk.

CONCLUSION

Our results showed that the elevation of total-α2-PI levels in VTE is caused by the elevation of NPB-α2-PI levels. Elevated sFAP level or p.Arg6Trp polymorphism alone did not influence VTE risk. However, an interaction can be detected between the polymorphism and high sFAP levels.

Autoimmune factor XIII deficiency with unusual laboratory and clinical phenotype

Hemorrhagic diathesis due to anti-factor XIII (FXIII) autoantibody is a rare but severe disorder. Challenges of the diagnosis and treatment is demonstrated by the case of a 67-year-old female without previous bleeding history, who suffered a huge muscular hematoma. Without blank subtraction 18% plasma FXIII activity was measured; however, after correction for blank the activity was below the limit of detection and the lack of fibrin cross-linking in the patient's plasma confirmed the latter result. FXIII-A2 antigen was not detectable by enzyme-linked immunosorbent assay (ELISA); however, it was well detected by western blotting. The autoantibody showed high affinity toward FXIII-A2 . Its considerable inhibitory activity was demonstrated by high titer in Bethesda units and the low immunoglobulin G concentration required for inhibition. The main biochemical effect was the inhibition of Ca2+ -induced activation. Eradication therapy was only partially successful. Four months after the last hemorrhagic event the patient suffered deep vein thrombosis complicated by pulmonary embolism.

N-glycosylation of blood coagulation factor XIII subunit B and its functional consequence

BACKGROUND

The protective/inhibitory B subunits of coagulation factor XIII (FXIII-B) is a ~80 kDa glycoprotein containing two N-glycosylation sites. Neither the structure nor the functional role of the glycans on FXIII-B has been explored.

OBJECTIVE

To reveal the glycan structures linked to FXIII-B, to design a method for deglycosylating the native protein, to find out if deglycosylation influences the dimeric structure of FXIII-B and its clearance from the circulation.

METHODS

Asparagine-linked carbohydrates were released from human FXIIII-B by PNGase F digestion. The released N-linked oligosaccharides were fluorophore labeled and analyzed by capillary electrophoresis. Structural identification utilized glycan database search and exoglycosidase digestion based sequencing. The structure of deglycosylated FXIII-B was investigated by gel filtration. The clearance of deglycosylated and native FXIII-B from plasma was compared in FXIII-B knock out mice.

RESULTS

PNGase F completely removed N-glycans from the denatured protein. Deglycosylation of the native protein was achieved by repeated digestion at elevated PNGase F concentration. The total N-glycan profile of FXIII-B featured nine individual structures; three were fucosylated and each structure contained at least one sialic acid. Deglycosylation did not change the native dimeric structure of FXIII-B, but accelerated its clearance from the circulation of FXIII-B knock out mice.

CONCLUSION

Characterization of the glycan moieties attached to FXIII-B is reported for the first time. Complete deglycosylation of the native protein was achieved by a deglycosylation workflow. The associated glycan structure is not required for FXIII-B dimer formation, but it very likely prolongs the half-life of FXIII-B in the plasma.

The factor XIII-A Val34Leu polymorphism decreases whole blood clot mass at high fibrinogen concentrations

BACKGROUND

Factor XIII (FXIII) promotes fibrin crosslinking and red blood cell (RBC) retention in clots. The FXIII-A polymorphism, Val34Leu, is associated with protection against venous thrombosis. This effect is hypothesized to result from fibrinogen concentration-dependent changes in fibrin structure. Effects of the FXIII-A Val34Leu polymorphism in whole blood clots have not been investigated.

AIM

Characterize effects of FXIII-A Val34Leu polymorphism and fibrinogen on whole blood clots.

METHODS

We isolated platelet-poor plasmas from human donors (FXIIIVal/Val , FXIIIVal/Leu , FXIIILeu/Leu ), reconstituted plasmas with platelets and RBCs, and triggered clotting. We assessed contributions of gender, age, clotting times, thrombin generation, FXIII activity, FXIII-A Val34Leu polymorphism, and fibrinogen to clot mass. We also reconstituted FXIII-depleted plasma with platelets, RBCs, and purified FXIIIVal/Val or FXIIILeu/Leu , varied fibrinogen, and characterized effects on clot mass.

RESULTS

Clot mass was associated with age, fibrinogen, prothrombin time, and thrombin generation. Clots reconstituted with plasmas from individuals with FXIII-AVal/Val and FXIII-AVal/Leu did not differ in mass from clots with FXIII-ALeu/Leu . However, clots containing a 34Val allele demonstrated a fibrinogen concentration-dependent increase in mass, whereas clots with homozygous 34Leu did not. In plasmas with high fibrinogen, mass was higher for clots with 34Val alleles compared with clots with homozygous 34Leu. In clots reconstituted with purified FXIII, increasing fibrinogen enhanced clot mass in the presence of 34Val, but decreased mass in the presence of 34Leu.

CONCLUSIONS

FXIII 34Leu mitigates the effect of elevated fibrinogen on whole blood clot mass. The Val34Leu polymorphism may protect against venous thrombosis by reducing clot mass.

A new ELISA method for the measurement of total α2-plasmin inhibitor level in human body fluids

The ever-increasing research efforts to develop new antithrombotic therapies have led to the reassessment of the role of alpha-2-plasmin inhibitor (α₂-PI) in pathological conditions. In particular, experimental stroke studies have suggested correlation between increased free α2-PI level and mortality. However there are only a small number of well-characterized and specific assays available for the measurements of free α₂-PI. In plasma α₂-PI undergoes both N- and/or C-terminal cleavages resulting four isoforms with modified susceptibility to FXIII catalyzed cross-linking to fibrin and/or loss of plasmin(ogen) binding. Present paper describes a new sandwich ELISA method for the determination of free total α₂-PI in plasma and other body fluids. A newly generated biotinylated monoclonal antibody recognizes and captures all the four N- and/or C-terminally modified isoforms of α₂-PI while HRPO-labeled polyclonal anti-α₂-PI antibody detects the captured antigen. Performing the 2-step assay in streptavidin-coated microplate can be completed within three hours. The assay is well reproducible, total (within laboratory) imprecision in the normal, pathological and very low ranges were 7.4%, 9.1% and < 19%, respectively. When examining the plasma samples of 197 healthy volunteers, 100 acute ischemic stroke patients and 102 patients with venous thrombosis, strong correlation was observed between total α2-PI antigen levels and α2-PI activity for each group. Using the assay a reference interval of 45-86 mg/L was established for total α₂-PI mass concentration in the plasma. α₂-PI levels were also measured in cerebrospinal fluid samples of 47 individuals the median value and range was 132 (36-379) μg/L. In conclusion, our ELISA enables accurate and fast measurement of total free α₂-PI in human body fluids.

Promotion of the Crosslinking of Fibrin and α2-antiplasmin by Platelets

Factor XIII (FXIII) is of high importance in the regulation of fibrinolysis. It crosslinks α2-antiplasmin (α2AP) and fibrin and by this way protects fibrin from the prompt elimination by plasmin. Although FXIII of platelets has been implicated in this protective mechanism, the role of platelets and platelet FXIII in the crosslinking process is far from being elucidated. As demonstrated by SDS PAGE and by immunoblotting for α2AP, intact normal platelets resuspended in FXIII-free plasma or FXIII-free fibrinogen solution catalyzed the crosslinking of fibrin chains and also the crosslinking of α2AP to fibrin α-chains. With FXIII-deficient platelets no crosslinking reaction could be observed indicating that the crosslinking with normal platelets was, indeed, due to platelet FXIII and not to another, putative platelet transglutaminase. However, the crosslinking of α2AP to fibrin induced by the FXIII of intact platelets resuspended in FXIII-free plasma was considerably less extensive than the crosslinking carried out by the FXIII of normal plasma in the presence of FXIII-free platelets. Furthermore, the replacement of FXIII-free platelets by normal platelets in normal FXIII-containing plasma resulted in little, if any, difference in the crosslinking process. When crosslinking was induced by highly purified plasma FXIII the presence of intact FXIII-free platelets significantly accelerated the formation of α-chain polymers as well as the incorporation of α2AP-fibrin α-chain hetero-dimer into these polymers. The results indicate that, in physiological conditions, platelet FXIII plays only a minor role in the crosslinking of α2AP and fibrin; however, platelets, independently of their FXIII content, promote the crosslinking reaction by providing a catalytic surface on which the formation of highly crosslinked fibrin polymers is accelerated.

Transformation of Cellular Factor XIII into an Active Zymogen Transglutaminase in Thrombin-Stimulated Platelets

The cellular form of blood coagulation factor XIII (FXIII) is present in platelets, monocytes and macrophages. During long-term stimulation of platelets by thrombin cellular FXIII becomes activated and crosslinks proteins, however, the mechanism of its activation has not been elucidated. It was shown that, contrary to plasma FXIII, the intracellular activation of platelet FXIII does not involve proteolysis. FXIII remained intact in thrombin-activated platelets, i.e., the activation peptide was not removed from the molecule. Part of the zymogen FXIII molecules, however, assumed an active configuration as was demonstrated both by the measurement of transglutaminase activity and by active-site-SH titration. These findings clearly indicate that during platelet activation, when intracellular Ca2+ concentration is raised, a slow non-proteolytic transformation of FXIII zymogen into an active transglutaminase occurs.

Platelet Factor XIII Becomes Active without the Release of Activation Peptide during Platelet Activation

The potentially active A subunit of factor XIII of blood coagulation has also been detected in platelets and monocytes/macrophages though the exact function of this cellular protransglutaminase has not yet been elucidated. In physiological conditions the first step in the activation of plasma factor XIII is the removal of an activation peptide from the N-terminal end of subunit A by thrombin. The A subunit then, in the presence of Ca2+, dissociates from the inhibitory B subunit and assumes an active conformation. Cellular factor XIII, which lacks B subunit, can be proteolytically activated in vitro by thrombin and the intracellular Ca2+ sensitive protease, calpain, in the same way as plasma factor XIII subunit A, and calpain has been suggested as the intracellular protease involved in the activation of cellular factor XIII in platelets. In the present experiments it was shown by SDS PAGE that during long-term stimulation of platelets with thrombin nondisulfide-crosslinked high M r protein polymers not penetrating the concentrating gel were formed. The lack of these polymers in thrombin-stimulated factor XIII deficient platelets clearly indicated that their formation in normal platelets was due to factor XIII that became active during platelet activation. However, no release of the activation peptide could be detected by Western blotting during this process. Similarly, no proteolytic cleavage of factor XIII was detectable when platelets were stimulated by Ca2+ ionophore through this stimulus activated calpain as it was clearly demonstrated by the breakdown of major intracellular calpain substrates. The results indicate: 1) during thrombin induced platelet activation factor XIII becomes active and crosslinks platelet protein, 2) platelet factor XIII is not an intracellular substrate of calpain, 3) cellular factor XIII could be activated without the proteolytic removal of activation peptide. It is presumed that the nonproteolytic pathway for the activation of cellular factor XIII, we reported most recently, might have physiological implications under such conditions.

Factors of the Extrinsic Pathway of Blood Coagulation in Tumour Associated Macrophages

Despite the growing evidence implicating intratumoural fibrin formation in the progression of malignant tumours, the origin of coagulation factors that participate in extravascular clotting has not been elucidated. Using immunohistochemical methods we attempted to detect and localize clotting factors of extrinsic pathway in different human malignant tumours. Coagulation factors II, V, VII and X (FII, FV, FVII and FX) were detected in a huge number of cells showing spindle-shaped or stellate morphology. By double immunohistochemical labellings it was demonstrated that cells containing these clotting factors express monocyte/macrophage differentiation marker antigens recognized by Leu M3, Ki M7 and DAKO-macrophage monoclonal antibodies, i.e. they represent monocyte-derived, phagocytic tumour associated macrophages (TAMs). These findings suggest that TAMs can be viewed as clot cells, which in addition to the initiation of extravascular clotting by expressing procoagulant activity can also provide all the components necessary for the extrinsic thrombin formation.

Fibrinolysis Resistant Fibrin Deposits in Lymph Nodes with Hodgkin's Disease

Extravasal fibrin deposition is frequently observed within and around tumorous tissues and has been implicated in various aspects of tumor growth. However, no adequate information has been available on the mechanism how intratumoral interstitial fibrin deposits escape a prompt elimination by the fibrinolytic system. In this study we provide immunomorphological evidence showing that fibrin deposits in lymph nodes with Hodgkin's disease are stabilized and made resistant to fibrinolysis by factor XIII (FXIII) of blood coagulation. By double immuno- fluorescent labelling systems fibrin deposits were simultaneously stained for α2-antiplasmin (α2-AP), the main physiological inhibitor of fibrinolysis and in a number of nodular areas they were also labelled for plasmin (ogen). The detection of α2-antiplasmin-plasmin complex-neoantigen (α2-AP-P-Neo) revealed that α2-AP reacted with plasmin, i.e., α2-AP covalently linked to fibrin indeed inhibited intratumoral fibrinolysis. In addition to fibrin deposits FXIII was also found in cellular elements characterized earlier as tumor associated macrophages. These cells were attached to fibrin strands suggesting that they are involved in the intratumoral fibrin formation and might be a source of fibrin stabilizing factor in the tumor stroma.

Monocytes of Patients Congenitally Deficient in Plasma Factor XIII Lack Factor XIII Subunit A Antigen and Transglutaminase Activity

Monocytes isolated from patients with severe deficiency in plasma Factor XIII of blood coagulation (FXIII) were tested for FXIII antigen and transglutaminase activity. By immunoperoxi- dase method the patients’ monocytes, in contrast to normal controls, showed no reaction with a monospecific antibody against FXIII subunit ª.This result was confirmed by immuno- blotting technique, as well. In addition, tissue macrophages tested in one of the patients were also exempt of FXIII subunit ª antigen. The transglutaminase activity in FXIII deficient monocytes was below the limit of the detection of the dansylcadaverine incorporation assay. The results suggest that FXIII subunitªof monocytes/macrophages and its plasma and platelet counterparts are closely related or identical proteins and demonstrate that the transglutaminase activity in monocytes is of FXIII origin and tissue transglutaminase is present, if at all, only in insignificant amount.

Auto- and alloantibodies against factor XIII: laboratory diagnosis and clinical consequences

Acquired FXIII deficiencies caused by autoantibodies against FXIII subunits represent rare but very severe bleeding diatheses. Alloantibodies in FXIII-deficient patients also cause life-threatening bleeding complications, but they develop extremely rarely. In this review we provide an overview of the diagnosis and classification of anti-FXIII antibodies and analyze 48 patients with autoimmune FXIII deficiency and four additional FXIII-deficient patients who developed anti-FXIII alloantibody. The patients were collected from peer-reviewed publications from which relevant data could be extracted. With the exception of two cases the antibodies were directed against FXIII-A. The difficulties in the diagnosis of FXIII deficiency in the presence of anti-FXIII antibodies are discussed and a scheme for the functional classification of the anti-FXIII antibodies is recommended. The three main categories are neutralizing and non-neutralizing antibodies and antibodies with combined effect. The methods being used for detecting and quantifying the inhibitory effect on FXIII activation and on the transglutaminase activity of activated FXIII are summarized and techniques for the classification of neutralizing anti-FXIII antibodies are outlined. The importance of clearance studies in these cases is emphasized. Binding assays, useful for the identification of non-neutralizing and combined type antibodies, were collected from the literature and their informative power is demonstrated by examples. The most frequently occurring bleeding symptoms in patients with anti-FXIII antibodies were soft tissue bleeding; intracranial bleedings also occurred, but less frequently than in inherited FXIII deficiency. Treatment of such patients is extremely challenging; the main aim should be eradication of the antibody.

Effect of factor XIII levels and polymorphisms on the risk of myocardial infarction in young patients

Factor XIII (FXIII) stabilizes and protects the fibrin network. Its role in myocardial infarction (MI) is still to be clarified. To evaluate the association of FXIII levels with MI in young patients and to investigate how the FXIII-A p.Val34Leu, FXIII-B p.His95Arg, and IVS11, c.1952 + 144 C>G (Intron K) polymorphisms influence FXIII levels and MI risk. Patients with ST elevation MI below 40 years of age (MI, n = 119), age-matched clinical controls (CC, n = 101) without MI and coronary artery disease, and healthy controls (HC, n = 120) were investigated for FXIII activity, FXIII-A₂B₂, FXIII-B concentrations and for the polymorphisms. FXIII activity and FXIII-A₂B₂ antigen were significantly elevated in MI. FXIII activity and antigen were significantly elevated in Arg95, while decreased in Intron K "G" carriers. Smoking had an independent increasing effect on FXIII activity and FXIII-A₂B₂ antigen. Intron K C>G polymorphism significantly decreased the risk of MI in patients with elevated fibrinogen. Among the investigated factors Intron K C>G polymorphism and smoking have the most powerful effect on FXIII levels and on the risk of MI in the young. The effect of smoking on coronary thrombus formation may partially be attributed to its FXIII increasing effect.

A novel homozygous mutation (1619delC) in GPIIb gene associated with Glanzmann thrombasthenia, the decay of GPIIb-mRNA and the synthesis of a truncated GPIIb unable to form complex with GPIIIa

The absence of agonist-induced platelet aggregation and the lack of fibrinogen receptor (GPIIb/IIIa) on the platelet surface demonstrated that the severe hemorrhagic complications of a child of Romany descent were caused by Glanzmann thrombasthenia. DNA sequencing revealed a novel homozygous deletion of a cytosine (1619delC) in the GPIIb gene causing a frameshift and predicting a novel stop codon at position 533 following 24 altered amino acids. Both parents possessed the same deletion in heterozygous form. The amount of GPIIb mRNA in the patient’s platelets was 0.06% of the amount measured in control platelets. Neither GPIIb nor its truncated form could be detected in the platelets of the patient by Western blotting, while a small amount of GPIIIa was demonstrated. Quantitative flow cytometric analysis showed an elevated number of vitronectin receptors, a component of which is GPIIIa, on the patient’s platelets. The surface expression of vitronectin receptor on thrombasthenic, but not on normal platelets was further increased by activation with thrombin receptor agonist peptide. BHK cells transfected with wild type GPIIIa and mutated GPIIb failed to express any mature GPIIb or pro-GPIIb. Immunoprecipitation with a polyclonal antibody recognizing both GPIIb and GPIIIa recovered a 60 kDa truncated form of GPIIb. This band was absent when immunoprecipitation was carried out with an antibody recognizing GPIIIa, suggesting that the truncated protein, lacking calf-1, calf-2 domains and major part of the thigh domain, is unable to form complex with GPIIIa.

A Simple, Quick One-step ELISA Assay for the Determination of Complex Plasma Factor XIII (A2B2)

A new highly sensitive sandwich ELISA assay was developed for the determination of plasma factor XIII (FXIII). Plasma FXIII is a tetrameric complex of two types of subunits (A2B2). A biotinylated monoclonal capture-antibody against the B-subunit and a peroxidase-labelled monoclonal tag-antibody against the A-subunit were added to the plasma dilution and the amount of the complex attached to streptavidincoated microplate was quantitated by measuring peroxidase activity. Only the tetrameric plasma FXIII reacted in the assay, non-complexed A or B subunits showed no reaction. The assay is linear up-to 40 µg/L of FXIII in the assay mixture. It is a quick one-step assay which can be performed within two hours. At normal and low FXIII concentration within batch reproducibility was 2.0% and 3.3%, day to day variation was 5.5% and 8.7%, respectively. Its high sensitivity allows reliable measurement at FXIII concentrations below 1% of normal average. Plasma samples can be stored for the assay at –20° C for at least one month. Plasma levels of healthy individuals were normally distributed and no gender difference was observed. A reference interval of 14-28 mg/L (67-133%) was established.

Effect of Val34Leu Polymorphism on the Activation of the Coagulation Factor XIII-A

Coagulation factor XIII (FXIII) is a protransglutaminase involved in the last step of the coagulation cascade by stabilising the fibrin clot. Recently, a common variation (FXIII Val34Leu) has been associated with a decreased risk of myocardial infarction and deep venous thrombosis. Val34Leu is critically located near the thrombin activation site of FXIII-A. In this study we investigated its effects on the activation of FXIII. Both recombinant and platelet-derived FXIII Val34Leu variants were shown to be more susceptible to thrombin cleavage than the wild type FXIII. The rate of enzymatic activation of FXIII Val34Leu was found increased, however, the specific activity of fully activated wild type FXIII and the Val34Leu mutant did not differ. During the course of thrombin-induced activation of FXIII fibrin γ-chain dimerisation and α-chain polymerisation developed more rapidly with the Val34Leu mutant. The increased rate of fibrin stabilisation brought about by the Val34Leu FXIII seems to be paradoxically associated with a protective effect against pathological thrombosis.

Abbreviations: AP, activation peptide of factor XIII; FXIII, blood coagulation factor XIII; FXIII-A, factor XIII subunit A; FXIII-A’, proteolytically activated subunit A; FXIII-B, factor XIII subunit B; SDS PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis

Deficiency Causing Mutations and Common Polymorphisms in the Factor XIII-A Gene

Blood coagulation factor XIII (FXIII), for its unique features, used to be considered a stepchild among clotting factors. In contrast to other proenzyme clotting factors, it is the precursor of a transglutaminase and not of a proteolytic enzyme. By cross-linking peptide-bound glutamine and lysine side chains transglutaminases make, rather than break, peptide bonds. Activated FXIII (FXIIIa) cross-links fibrin α-chains and γ-chains and covalently attaches α2-plasmin inhibitor to fibrin α-chains to strengthen fibrin mechanically and to protect it from fibrinolysis. In addition to being a clotting factor, FXIII is also an intracellular proenzyme present in platelets and monocytes/macrophages. The plasma factor is of heterotetrameric structure consisting of two potentially active A (FXIII-A) and two inhibitory/protective B (FXIII-B) subunits (A2B2), while its cellular counterpart is a dimer of FXIII-A. Plasma FXIII is activated by the concerted action of thrombin and Ca2+. Thrombin cleaves FXIII-A at Arg37-Gly38, then in the presence of Ca2+ FXIII-B dissociates and FXIII-A assumes an active configuration (reviewed in ref. 1).

Factor XIII Val34Leu variant protects against coronary artery disease

Several studies suggested that Val34Leu variant of factor XIII (FXIII) might have a protective effect against coronary artery disease (CAD), but studies not supporting these findings have also been published. The authors performed a meta-analysis of 16 studies on 5,346 cases and 7,053 controls that investigated the association between Val34Leu polymorphism and CAD defined as history of myocardial infarction or significant stenosis on a coronary artery assessed by coronary angiography. Because of the heterogeneity of the study-specific results, the pooled effect estimates were calculated by a random-effects empirical Bayes model. The combined odds ratios for CAD were 0.82 (95% confidence interval [95% Cl] 0.73, 0.94) for the heterozygotes of the FXIIIVal34Leu variant, 0.89 (95% CI 0.69, 1.13) for the homozygotes, and 0.81 (95% CI 0.70, 0.92) for the heterozygotes and homozygotes combined. The results were essentially the same when only myocardial infarction was considered as outcome. The beneficial effect of the polymorphism might be smaller than the effect estimates obtained in this metaanalysis, because the analysis raised the possibility of publication bias. Data published in the literature suggest that gene-gene and gene-environmental interactions might significantly influence the protective effect of FXIII-AVal34Leu polymorphism.

Leukemic lymphoblasts, a novel expression site of coagulation factor XIII subunit A

Blood coagulation factor XIII (FXIII) is a protransglutaminase circulating asa tetramer formed by two types of subunits (A2B2). The intracellular dimeric form of FXIII (A2) is present in platelets, megakaryocytes, monocytes and macrophages and has been detected in mono-and megakaryocytic leukemias. The aim of our study was to investigate FXIII-A expression in newly diagnosed B cell acute lymphoblastic leukemia (ALL) samples. We examined 47 de novo ALL cases of B cell origin by triple color labeling with flow cytometry. FXIII-A was detected by a FITC conjugated monoclonal antibody combined with CD34 and CD45 staining. In selected cases FXIII-A was investigated on slides prepared from blasts and visualized with a fluorescent microscope. In addition, blasts were studied by Western blot analysis and FXIII-A was measured by a highly sensitive ELISA method. By flow cytometry 19 samples of the 47 cases were found to be FXIII-A positive. Antigen concentration was 3.11± 1.19 fg/blast, while normal lymphoid precursors and mature lymphocytes from B-CLL did not contain FXIII-A. In the lysate of lymphoblasts that were positive by flow cytometry, a single band (82 kDa) corresponding to FXIII-A was detected on Western blots. Confocal laser scanning microscopic examination revealed the presence of FXIII-A in the cytoplasm of these lymphoblasts. This novel expression site of FXIII-A in leukemic lymphoblasts can be utilized as a diagnostic tool and may also gain functional significance in B-lineage ALL.

Down-regulation of activated factor XIII by polymorphonuclear granulocyte proteases within fibrin clot

Activated clotting factors are down-regulated by two major mechanisms which involve protease inhibitors or proteolytic degradation. To date, no down-regulating mechanism for activated factor XIII (FXIIIa) has been demonstrated. As the hemostatic plug contains polymorphonuclear granulocytes (PMNs) rich in proteolytic enzymes, we tested if these proteases are released in fibrin clots, and become involved in the down-regulation of FXIIIa.The supernatant of stimulated granulocytes proteolytically degraded and inactivated FXIIIa. In the fibrin clot formed from fibrinogen solution elastase, cathepsin G and matrix metalloprotease-9 (MMP-9) were released from granulocytes without any external stimulus. PMN proteases released in fibrin clot exerted a fibrinolytic effect and almost completely de-graded both FXIII subunits.The elastase inhibitor, ONO 5046, partially inhibited the proteolytic degradation of FXIII in PMNsupplemented fibrin clots. Cathepsin G and MMP-9 inhibitors provided less protection; in these cases intermediate split products accumulated.The proteolytic degradation of FXIII by PMNs was also significant when the clot was made from whole plasma. The main plasma protease inhibitor, α1-antitrypsin, provided only partial protection. In the fibrin clot which contained α1-antitrypsin FXIIIa was degraded by PMN proteases significantly faster than cross-linked fibrin.The results suggest that the degradation of FXIII subunits by the concerted action of PMN proteases released within the clot represents a novel mechanism for the down-regulation of FXIIIa.

Cleavage of factor XIII by human neutrophil elastase results in a novel active truncated form of factor XIII A subunit

The first step in the activation of plasma factor XIII (FXIII) is the cleavage of R37-G38 bond in FXIIIA subunit (FXIII-A) by thrombin, which makes the subsequent formation of an active transglutaminase possible. No active truncated form of FXIII-A, other than G38-FXIII-A, has been identified. In contrast to thrombin, which has a preference toward arginine residues, human neutrophil elastase (HNE) cleaves peptide bonds at small side-chain aliphatic amino acids, preferably at valine. As there are several valine residues close to the thrombin cleavage-site, we tested if an active truncated FXIII-A was formed during fragmentation of FXIII by HNE. It was demonstrated by Western blotting and transglutaminase assay that HNE induced a limited cleavage of FXIII-A resulting in the activation of both plasma and cellular FXIII; the maximal transglutaminase activities were 52.5% and 67.4% of thrombin-activated FXIII, respectively. After the relatively rapid activation a much slower inactivation occurred. HNE-activated FXIII cross-linked fibrin γ- and α-chains in the clot formed by batroxobin moojeni. MALDI-TOF analysis of the cleaved fragments and N-terminal Edman degradation of the truncated protein identified V39-N40 as the primary cleavagesite and N40-FXIII-A as the active form. No primary cleavage occurred at V34, V35, V47, V50 residues. FXIII-A V34L polymorphism, which increases the rate of FXIII-A cleavage by thrombin, was without effect on FXIII activation by HNE. Molecular modeling located the primary HNE cleavage-site in the middle of the flexible and accessible Q32-L45 loop and showed that other neighboring valine residues were in less favorable position.

Binding of plasma factor XIII to thrombin-receptor activated human platelets

Platelet-bound coagulation factor XIII (FXIII) is targeted and concentrated at the site where platelet-rich thrombi are formed. Previous studies were in disagreement about the nature of FXIII binding to platelets. In this study, thrombin-receptor activating peptide (TRAP)-stimulated human whole blood and washed platelets were analysed by flow cytometry for the binding of FXIII using a monoclonal antibody against the A subunit of FXIII (FXIII-A). Here, we demonstrate that FXIII-A positivity significantly increased on activated platelets in whole blood compared to unstimulated sample, but not in washed platelets. GPIIb/IIIa receptor plays an essential role in FXIII binding, as fibrinogen receptor antagonist eptifibatide and fibrinogen binding inhibitor RGDS tetrapeptide significantly prevented the binding of FXIII. Furthermore, stimulated platelets from a patient with severe type I Glanzmann thrombasthenia showed insignificant FXIII-A positivity versus healthy controls. In addition, basal negligible amount of FXIII on washed platelets was only slightly increased when highly purified plasma FXIII (FXIII-A2B2) was added upon platelet activation by TRAP. Similarly, no remarkable FXIII-A positivity was observed when we used FXIII-A2B2 with γA/γA fibrinogen. However, γA/γ' fibrinogen significantly augmented FXIII binding on TRAP-stimulated platelets in the presence of non-activated FXIII-A2B2. We conclude that FXIII-A2B2 of plasma origin binds to thrombin-receptor activated platelets via GPIIb/IIIa receptor-bound fibrinogen with γ’-chain and is not capable of direct platelet binding.

Clinical and laboratory characteristics of antithrombin deficiencies: A large cohort study from a single diagnostic center

INTRODUCTION

Inherited antithrombin (AT) deficiency is a heterogeneous disease. Due to low prevalence, only a few studies are available concerning genotype-phenotype associations. The aim was to describe the clinical, laboratory and genetic characteristics of AT deficiency in a large cohort including children and to add further laboratory data on the different sensitivity of functional AT assays.

PATIENTS AND METHODS

Non-related AT deficient patients (n=156) and their family members (total n=246) were recruited. Clinical and laboratory data were collected, the mutation spectrum of SERPINC1 was described. Three different AT functional assays were explored.

RESULTS

Thirty-one SERPINC1 mutations including 11 novel ones and high mutation detection rate (98%) were detected. Heparin binding site deficiency (type IIHBS) was the most frequent (75.6%) including AT Budapest3 (ATBp3), AT Padua I and AT Basel (86%, 9% and 4% of type IIHBS, respectively). Clinical and laboratory phenotypes of IIHBS were heterogeneous and dependent on the specific mutation. Arterial thrombosis and pregnancy complications were the most frequent in AT Basel and AT Padua I, respectively. Median age at the time of thrombosis was the lowest in ATBp3 homozygotes. The functional assay with high heparin concentration and pH7.4 as assay conditions had low (44%) sensitivity for ATBp3 and it was insensitive for AT Basel and Padua I.

CONCLUSION

Type IIHBS deficiencies behave differently in clinical and laboratory phenotypes from each other and from other AT deficiencies. Heparin concentration and pH seem to be the key factors influencing the sensitivity of AT functional assays to IIHBS.

Minimal factor XIII activity level to prevent major spontaneous bleeds

Essentials A strong association between bleeding severity and FXIII activity level (FXIII:C) was shown. The range 5-30 IU dL-1 of FXIII:C was associated with a high variability of bleeding severity. The PROspective study confirmed the association between FXIII:C activity and bleeding severity.

A FXIII

C of 15 IU dL-1 is a proposed target to start prophylaxis for prevention of major bleeding.

SUMMARY

Background Congenital factor XIII (FXIII) deficiency is a rare bleeding disorder associated with significant bleeding manifestations. The European Network of Rare Bleeding Disorders (EN-RBD) study, performed from 2007 to 2010, showed a strong association between bleeding severity and FXIII activity in plasma of patients with FXIII deficiency. Among these patients, variable levels of FXIII activity, from undetectable to 30%, were associated with a wide range of bleeding severity. Objectives and patients The present cross-sectional study, in the frame of the PRO-RBDD project, a prospective cohort study, analyzed data of 64 patients with FXIII deficiency and different types of clinical and laboratory severity. Results The results of this analysis confirmed that FXIII coagulant activity in plasma is well associated with clinical severity of patients. In addition, 15 IU dL-1 of FXIII activity was identified to be the level under which the probability of spontaneous major bleeding sharply increases (from 50% for levels of 15 IU dL-1 to more than 90% for levels of 5 IU dL-1 or lower). Conclusion The PRO-RBDD study suggests a FXIII coagulant activity level of 15 IU dL-1 as a target to start prophylaxis in order to prevent major bleedings, such as central nervous system or gastrointestinal tract hemorrhages.

Factor XIII levels and factor XIII B subunit polymorphisms in patients with venous thromboembolism

BACKGROUND

The association of plasma factor XIII (FXIII) level with venous thromboembolism (VTE) is still controversial and the effect of sex and FXIII B subunit (FXIII-B) polymorphisms in this respect have not been explored.

OBJECTIVES

1/ To determine FXIII activity and antigen levels in patients with a history of VTE and how they are influenced by sex and FXIII-B polymorphisms. 2/ To explore the association of FXIII levels and FXIII-B polymorphisms with the risk of VTE.

METHODS

218 VTE patients and equal number of age and sex matched controls were enrolled in the study. FXIII activity was measured by ammonia release assay; FXIII-A₂B₂ and FXIII-B levels were determined by ELISAs. FXIII-B polymorphisms were identified by RT-PCR using melting point analysis.

RESULTS

Adjusted FXIII activity and FXIII-A₂B₂ antigen levels were significantly higher in females with a history of VTE than in the respective controls. FXIII-B levels were significantly lower in male VTE patients than in controls. FXIII-A₂B₂ antigen levels in the upper tertile increased the risk of VTE in females (adjusted OR: 2.52; CI: 1.18-5.38). Elevated FXIII-B antigen level had a protective effect only in males (adjusted OR: 0.19; CI: 0.08-0.46). FXIII-B Intron K c.1952+144 C>G polymorphism significantly lowered FXIII activity, FXIII-A₂B₂ and FXIII-B antigen levels in both groups. FXIII-B polymorphisms did not influence the risk of VTE.

CONCLUSIONS

In VTE patients the changes of FXIII level and their effect on the risk of VTE show considerable sex-specific differences. Intron K polymorphism results in decreased FXIII levels, but does not influence the risk of VTE.

Antithrombin Debrecen (p.Leu205Pro) - Clinical and molecular characterization of a novel mutation associated with severe thrombotic tendency

INTRODUCTION

Hereditary antithrombin (AT) deficiency is a rare thrombophilic disorder with heterogeneous genetic background and various clinical presentations. In this study we identified a novel AT mutation. Genotype-phenotype correlations, molecular characteristics and thrombotic manifestations of the mutation were investigated.

MATERIALS AND METHODS

Thirty-one members of a single family were included. Clinical data was collected regarding thrombotic history. The mutation was identified by direct sequencing of the SERPINC1 gene. HEK293 cells were transfected with wild type and mutant SERPINC1 plasmids. Western blotting, ELISA and functional amidolytic assay were used to detect wild type and mutant AT. After double immunostaining, confocal laser scanning microscopy was used to localize mutant AT in the cells. Molecular modeling was carried out to study the structural-functional consequences of the mutation.

RESULTS

Unprovoked venous thrombotic events at early age, fatal first episodes and recurrences were observed in the affected individuals. The median AT activity was 59%. Genetic analysis revealed heterozygous form of the novel mutation p.Leu205Pro (AT Debrecen). The mutant AT was expressed and synthesized in HEK293 cells but only a small amount was secreted. The majority was trapped intracellularly in the trans‑Golgi and 26S proteasome. The mutation is suspected to cause considerable structural distortion of the protein. The low specific activity of the mutant AT suggested functional abnormality.

CONCLUSIONS

AT Debrecen was associated with very severe thrombotic tendency. The mutation led to misfolded AT, impaired secretion and altered function. Detailed clinical and molecular characterization of a pathogenic mutation might provide valuable information for individualized management.

Severe bleeding diatheses in an elderly patient with combined type autoantibody against factor XIII A subunit; novel approach to the diagnosis and classification of anti-factor XIII antibodies

INTRODUCTION

Acquired factor XIII (FXIII) deficiency due to autoantibody is a rare, severe bleeding diathesis. Its laboratory diagnosis and classification represents a difficult task.

AIM

Introduction of novel approaches into the diagnosis and characterization of anti-FXIII autoantibody and demonstration of their use in the diagnosis of a patient with autoimmune FXIII deficiency.

METHODS

Factor XIII activity, FXIII antigen levels and the titre of anti-FXIII-A antibody were monitored throughout the course of the disease. FXIII activity was measured by ammonia release assay; FXIII-A₂ B₂ complex, total and free FXIII-B concentrations were determined by ELISAs. The binding constant for the interaction of the autoantibody with recombinant FXIII-A₂ (rFXIII-A₂ ) and FXIII-A₂ B₂ was determined by surface plasmon resonance (SPR). The inhibitory capacity of IgG was expressed as the concentration exerting 50% inhibition of FXIII activation/activity (IC50). The truncation of FXIII-A by thrombin was monitored by western blotting. The inhibition of Ca²⁺ -induced FXIII activation and active FXIII (FXIIIa) were assessed by FXIII activity assay.

RESULTS

The antibody bound to rFXIII-A₂ and FXIII-A₂ B₂ with high affinity and accelerated the decay of supplemented FXIII concentrate. An IC50 value of 170.1 μg IgG·mL^-1 indicated effective FXIII neutralization. The main neutralizing effect of the autoantibody was the inhibition of FXIIIa. After 2 months, due to combined therapeutic modalities, the autoantibody disappeared and FXIII activity significantly elevated.

CONCLUSION

The anti-FXIII-A autoantibody exerted a combined effect including inhibition of FXIIIa and acceleration of FXIII decay in the plasma. IC50 and binding constant determinations added important information to the characterization of the autoantibody.

Assessment of Factor XIII

Blood coagulation factor XIII (FXIII) is essential for maintaining hemostasis. The absence of FXIII results in severe bleeding diathesis, which without prophylaxis frequently leads to fatal bleeding. As the usual hemostasis screening tests remain normal, the diagnosis of FXIII deficiency needs specific tests. Here, we describe FXIII activity determination by the ammonia release assay, which is the first-line test in the diagnostic algorithm for FXIII deficiency. The method for another activity test, the undeservedly rarely used fibrin cross-linking assay, is also presented. Further tests used for the classification of FXIII deficiencies, measurement of FXIII activity in platelets, ELISAs for the measurement of complex plasma FXIII (FXIII-A₂B₂) antigen and for FXIII-A₂ in plasma and platelets are also included. Detailed description of the methods for the detection and measurement of neutralizing auto- and alloantibodies is also provided.

Regulation of plasma factor XIII levels in healthy individuals; a major impact by subunit B intron K c.1952+144 C>G polymorphism

BACKGROUND

The regulation of plasma factor XIII (FXIII) levels in healthy individuals has been only partially explored. The identification of major non-genetic and genetic regulatory factors might provide important information on the contribution of FXIII to the risk of cardio/cerebrovascular diseases.

OBJECTIVES

To determine the effect of age, smoking, BMI, fibrinogen concentration on plasma FXIII activity, complex FXIII antigen (FXIII-A₂B₂) and total FXIII-B subunit (tFXIII-B) level, to correlate FXIII-B level with the other two FXIII parameters and to assess the variation of FXIII levels in carriers of major FXIII subunit polymorphisms.

METHODS

268 healthy individuals were enrolled in the study. FXIII activity was measured by the ammonia release assay; FXIII-A₂B₂ and tFXIII-B were determined by ELISAs. FXIII-A p.Val34Leu, FXIII-B p.His95Arg and FXIII-B intron K c.1952+144 C>G polymorphisms were identified by RT-PCR using melting point analysis with fluorescence resonance energy transfer detection.

RESULTS

All investigated FXIII parameters showed significant positive correlation with age and fibrinogen level; gender and BMI influenced only tFXIII-B. A highly significant positive correlation was demonstrated between tFXIII-B and the other FXIII parameters. FXIII-A p.Val34Leu polymorphism had only slight, if any effect on FXIII levels. The FXIII-B Arg95 allele moderately increased all three FXIII parameters, but the effect became statistically significant only after adjustment. The FXIII-B intron K G allele drastically decreased FXIII levels, and it seemed to be in synergism with the FXIII-A Leu34 allele.

CONCLUSIONS

Plasma FXIII levels are subjected to multifactorial regulation, in which age, fibrinogen level and FXIII-B intron K polymorphism are major determinants.

Neutralizing autoantibody against factor XIII A subunit resulted in severe bleeding diathesis with a fatal outcome - characterization of the antibody

Essentials Autoantibody against factor XIII (FXIII) is a rare but severe acquired hemorrhagic diathesis. In an elderly patient, anti-FXIII-A antibody led to severe bleedings with fatal outcome. The neutralizing autoantibody bound to FXIII with high affinity (Ka≈10(9) m(-1) ). The dominant effect of the autoantibody was the inhibition of activated FXIII.

SUMMARY

Autoantibodies may develop against the catalytic A subunit of factor XIII (FXIII-A) or the carrier B subunit (FXIII-B). Autoimmune FXIII-A deficiency was diagnosed in an elderly (75 years) patient with severe bleeding symptoms. The patient had 3% FXIII activity, and unmeasurable FXIII-A2 B2 and FXIII-A antigens in the plasma, whereas, in the platelet lysate, activity and FXIII-A antigen values were normal. As revealed by western blotting, FXIII antigen was present in the plasma, but the autoantibody interfered with the immunoassays. A mixing study indicated the presence of inhibitor with a titer of 63.2 Bethesda units (BU). The patient's IgG bound to FXIII-A2 B2 and to FXIII-A2 with equally high affinity (Ka in the range of 10(9) m(-1) ). It exerted a multiple inhibitory effect on FXIII activation/activity (IC50: 50 μg mL(-1) ). Immunosupressive therapy gradually decreased the autoantibody titer to 8.0 BU, but FXIII activity remained very low, and, owing to recurrent bleeding, the patient died.

Diagnosis and Management of Congenital and Acquired FXIII Deficiencies

Inherited deficiency of FXIII A subunit (FXIII-A) is a rare (1:2,000,000) but very severe bleeding diathesis. The incidence is much higher in communities where the practice of consanguineous marriage is combined with founder effect mutation. Because of the high risk of intracranial bleeding, life-long prophylaxis, preferably using FXIII concentrate, is mandatory. In FXIII-B subunit deficiency the bleeding diathesis is mild to moderate. FXIII deficiency is frequently associated with impaired wound healing. Women suffering from FXIII deficiency cannot carry pregnancies to term; in severe cases spontaneous abortion occurs in the first trimester. Plasma-derived heat-inactivated FXIII concentrate and recombinant FXIII-A are available for prophylaxis; a 4 weekly dose of 35 to 40 U/kg is recommended and a trough level of greater than 5% FXIII activity should be aimed for. During pregnancy, 2 weekly prophylaxis with a target trough level of greater than 10% is recommended, and during labor FXIII activity should exceed 30%. During surgical procedures, the target should be higher than 50% FXIII activity. Alloantibodies make FXIII deficiency difficult to manage, but fortunately they are extremely rare. Acquired FXIII deficiency may involve both subunits. Autoantibodies against FXIII subunits also manifest in severe bleeding complication with a relatively high mortality rate. The first-line test in the diagnosis of FXIII deficiency should be a quantitative functional assay based on the measurement of ammonia release or amine incorporation. The sensitivity of the traditional clot solubility assay is not sufficiently robust to enable proper screening. Antigen assays are needed for the classification of FXIII deficiencies. In the case of anti-FXIII antibodies, the diagnostic armory should be supplemented by a mixing test/Bethesda-type inhibitor assay and by assays that detect/measure the binding of antibodies to FXIII and to its subunits.

Founder effect is responsible for the p.Leu131Phe heparin-binding-site antithrombin mutation common in Hungary: phenotype analysis in a large cohort

BACKGROUND

Antithrombin (AT) is a key regulator of the coagulation. In type II deficiency, the heparin-binding-site defect (type II HBS) is considered to be relatively low thrombosis risk.

OBJECTIVES

Our aims were to search for SERPINC1 mutation(s) and to describe the clinical and laboratory phenotype of a large number of AT Budapest3 (ATBp3, p.Leu131Phe) carriers and confirm the presence of a founder effect.

PATIENTS/METHODS

AT-deficient patients were recruited and carriers of ATBp3, n = 102 (63 families) were selected. To investigate the founder effect, eight intragenic single nucleotide polymorphisms, a 5'-length dimorphism, and five microsatellite markers were detected. Clinical and laboratory data of the patients were collected and analyzed.

RESULTS

In AT deficiency, 16 different causative mutations were found, and the great majority of patients were of type II HBS subtype. Most of them (n = 102/118, 86.5%) carried the ATBp3 mutation. The ATBp3 mutant allele was associated with one single haplotype, while different haplotypes were detected in the case of normal allele. The anti-factor Xa-based AT activity assay that we used could detect all ATBp3 patients with high sensitivity in our cohort. ATBp3 homozygosity (n = 26) was associated with thrombosis at a young age and conferred a high thrombotic risk. Half of the heterozygotes (n = 41/76, 53.9%) also had venous and/or arterial thrombosis, and pregnancy complications were also recorded.

CONCLUSION

In Hungary, the founder mutation, ATBp3, is the most common AT deficiency. Our study is the first in which the clinical characterization of ATBp3 mutation was executed in a large population.

The lack of aspirin resistance in patients with coronary artery disease

Background

Aspirin resistance established by different laboratory methods is still a debated problem. Using COX1 specific methods no aspirin resistance was detected among healthy volunteers. Here we tested the effect of chronic aspirin treatment on platelets from patients with stable coronary artery disease. The expression of COX2 mRNA in platelets and its influences on the effect of aspirin was also investigated.

Methods

One hundred and forty four patients were enrolled in the study. The direct measurement of COX1 acetylation was carried out by monoclonal antibodies specific to acetylated and non-acetylated COX1 (acCOX1 and nacCOX1) using Western blotting technique. Arachidonic acid (AA) induced TXB₂ production by platelets was measured by competitive immunoassay. AA induced platelet aggregation, ATP secretion and VerifyNow Aspirin Assay were also performed. COX2 and COX1 mRNA expression in platelets were measured in 56 patients by RT-qPCR.

Results

In 138 patients only acCOX1 was detected, in the remaining six patients nacCOX1 disappeared after a compliance period. AA induced TXB₂ production by platelets was very low in all patients including the 6 patients after compliance. AA induced platelet aggregation, secretion and with a few exceptions the VerifyNow Assay also demonstrated the effect of aspirin. Smoking, diabetes mellitus and inflammatory conditions did not influence the results. The very low amount of COX2 mRNA detected in 39 % of the investigated platelets did not influence the effect of aspirin.

Conclusions

No aspirin resistance was detected among patients with stable coronary artery disease. COX2 expression in platelets did not influence the effect of aspirin.

Evaluation of flow cytometric HIT assays in relation to an IgG-Specific immunoassay and clinical outcome

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a severe side effect of heparin treatment caused by platelet activating IgG antibodies generated against the platelet factor 4 (PF4)-heparin complex. Thrombocytopenia and thrombosis are the leading clinical symptoms of HIT.

METHODS

The clinical pretest probability of HIT was evaluated by the 4T score system. Laboratory testing of HIT was performed by immunological detection of antibodies against PF4-heparin complex (EIA) and two functional assays. Heparin-dependent activation of donor platelets by patient plasma was detected by flow cytometry. Increased binding of Annexin-V to platelets and elevated number of platelet-derived microparticles (PMP) were the indicators of platelet activation.

RESULTS

EIA for IgG isotype HIT antibodies was performed in 405 suspected HIT patients. Based on negative EIA results, HIT was excluded in 365 (90%) of cases. In 40 patients with positive EIA test result functional tests were performed. Platelet activating antibodies were detected in 17 cases by Annexin V binding. PMP count analysis provided nearly identical results. The probability of a positive flow cytometric assay result was higher in patients with elevated antibody titer. 71% of patients with positive EIA and functional assay had thrombosis.

CONCLUSIONS

EIA is an important first line laboratory test in the diagnosis of HIT; however, HIT must be confirmed by a functional test. Annexin V binding and PMP assays using flow cytometry are functional HIT tests convenient in a clinical diagnostic laboratory. The positive results of functional assays may predict the onset of thrombosis. © 2016 International Clinical Cytometry Society.

Progressive chromogenic anti-factor Xa assay and its use in the classification of antithrombin deficiencies

BACKGROUND

Antithrombin (AT) is a slow-acting progressive inhibitor of activated clotting factors, particularly thrombin and activated factor X (FXa). However, the presence of heparin or heparan sulfate accelerates its effect by several magnitudes. AT deficiency, a severe thrombophilia, is classified as type I (quantitative) and type II (qualitative) deficiency. In the latter case mutations may influence the reactive site, the heparin binding-site (HBS) and exert pleiotropic effect. Heterozygous type II-HBS deficiency is a less severe thrombophilia than other heterozygous subtypes. However, as opposed to other subtypes, it also exists in homozygous form which represents a very high risk of venous thromboembolism.

METHODS

A modified anti-FXa chromogenic AT assay was developed which determines both the progressive (p) and the heparin cofactor (hc) activities, in parallel. The method was evaluated and reference intervals were established. The usefulness of the assay in detecting type II-HBS AT deficiency was tested on 78 AT deficient patients including 51 type II-HBS heterozygotes and 18 homozygotes.

RESULTS

Both p-anti-FXa and hc-anti-FXa assays showed excellent reproducibility and were not influenced by high concentrations of triglyceride, bilirubin and hemoglobin. Reference intervals for p-anti-FXa and hc-anti-FXa AT activities were 84%-117% and 81%-117%, respectively. Type II-HBS deficient patients demonstrated low (heterozygotes) or very low (homozygotes) hc-anti-FXa activity with normal or slightly decreased p-anti-FXa activity. The p/hc ratio clearly distinguished wild type controls, type II-HBS heterozygotes and homozygotes.

CONCLUSIONS

Concomitant determination of p-anti-FXa and hc-anti-FXa activities provides a reliable, clinically important diagnosis of type II-HBS AT deficiency and distinguishes between homozygotes and heterozygotes.