Model thrombi formed under flow reveal the role of factor XIII-mediated cross-linking in resistance to fibrinolysis

Factor XIII is a key molecule at the intersection of coagulation and fibrinolysis as well as inflammation and infection control

Factor XIII (FXIII) is a transglutaminase consisting of two catalytic A subunits (FXIII-A) and two non-catalytic B subunits (FXIII-B) in plasma. FXIII-B protects FXIII-A from its clearance. FXIII-A is also present as a homodimer inside megakaryocytes/platelets and monocytes/macrophages. Although possible functions of intracellular FXIII-A have been proposed, these remain to be established. Intra- and extra-cellular FXIIIs support platelet adhesion and spreading as well as clot retraction, suggesting that FXIII is important for the stabilization of platelet-fibrin clots. Intra- and extra-cellular FXIIIs also support immobilization and killing of bacteria as well as phagocytosis by macrophages. Thus, FXIII may function in innate immunity. Congenital FXIII deficiency due to defective F13-A genes manifests as a life-long bleeding tendency, abnormal wound healing, and recurrent miscarriage. Although congenital FXIII-B deficiency used to be thought rare, reports of such cases have increased recently. As the bleeding tendency is often mild, patients with FXIII-B deficiency may be overlooked by physicians. Patients with acquired FXIII deficiency, in particular those with autoimmune hemorrhaphilia due to anti-FXIII antibodies, are on the increase, at least in Japan. It is important to diagnose such cases as early as possible, and to treat them with immunosuppression in combination with FXIII replacement therapy as their bleeding symptoms can be life-threatening.

Measurement of fibrin concentration by fast field-cycling NMR

The relaxation of (1)H nuclei due to their interaction with quadrupolar (14)N nuclei in gel structures is measured using fast field-cycling NMR. This phenomenon called quadrupolar dips has been reported in different (1)H-(14)N bond-rich species. In this study, we have studied quadrupolar dips in fibrin, an insoluble protein that is the core matrix of thrombi. Fibrin was formed by the addition of thrombin to fibrinogen in 0.2% agarose gel. T(1)-dispersion curves were measured using fast field-cycling NMR relaxometry, over the field range of 1.5-3.5 MHz (proton Larmor frequency), and were analyzed using a curve-fitting algorithm. A linear increase of signal amplitude with increasing fibrin concentration was observed. This agrees with the current theory that predicts a linear relationship of signal amplitude with the concentration of contributing (14)N spins in the sample. Interestingly, fibrin formation gave rise to the signal, regardless of crosslinking induced by the transglutaminase factor XIIIa. To investigate the effect of proteins that might be trapped in the thrombi in vivo, the plasma protein albumin was added to the fibrin gel, and an increase in the quadrupolar signal amplitude was observed. This study can potentially be useful for thrombi classification by fast field-cycling MRI techniques.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Ultrasound-enhanced rt-PA thrombolysis in an ex vivo porcine carotid artery model

Ultrasound is known to enhance recombinant tissue plasminogen activator (rt-PA) thrombolysis. In this study, occlusive porcine whole blood clots were placed in flowing plasma within living porcine carotid arteries. Ultrasonically induced stable cavitation was investigated as an adjuvant to rt-PA thrombolysis. Aged, retracted clots were exposed to plasma alone, plasma containing rt-PA (7.1 ± 3.8 μg/mL) or plasma with rt-PA and Definity® ultrasound contrast agent (0.79 ± 0.47 μL/mL) with and without 120-kHz continuous wave ultrasound at a peak-to-peak pressure amplitude of 0.44 MPa. An insonation scheme was formulated to promote and maximize stable cavitation activity by incorporating ultrasound quiescent periods that allowed for the inflow of Definity®-rich plasma. Cavitation was measured with a passive acoustic detector throughout thrombolytic treatment. Thrombolytic efficacy was measured by comparing clot mass before and after treatment. Average mass loss for clots exposed to rt-PA and Definity® without ultrasound (n = 7) was 34%, and with ultrasound (n = 6) was 83%, which constituted a significant difference (p < 0.0001). Without Definity® there was no thrombolytic enhancement by ultrasound exposure alone at this pressure amplitude (n = 5, p < 0.0001). In the low-oxygen environment of the ischemic artery, significant loss of endothelium occurred but no correlation was observed between arterial tissue damage and treatment type. Acoustic stable cavitation nucleated by an infusion of Definity® enhances rt-PA thrombolysis without apparent treatment-related damage in this ex vivo porcine carotid artery model.

The antifibrinolytic function of factor XIII is exclusively expressed through α₂-antiplasmin cross-linking

Factor XIII (FXIII) generates fibrin-fibrin and fibrin-inhibitor cross-links. Our flow model, which is sensitive to cross-linking, was used to assess the effects of FXIII and the fibrinolytic inhibitor, α₂-antiplasmin (α₂AP) on fibrinolysis. Plasma model thrombi formed from FXIII or α₂AP depleted plasma lysed at strikingly similar rates, 9-fold faster than pooled normal plasma (PNP). In contrast, no change was observed on depletion of PAI-1 or thrombin activatable fibrinolysis inhibitor (TAFI). Inhibition of FXIII did not further enhance lysis of α₂AP depleted thrombi. Addition of PNP to FXIII or α₂AP depleted plasmas normalized lysis. Lysis rate was strongly inversely correlated with total cross-linked α₂AP in plasma thrombi. Reconstitution of FXIII into depleted plasma stabilized plasma thrombi and normalized γ-dimers and α-polymers formation. However, the presence of a neutralizing antibody to α₂AP abolished this stabilization. Our data show that the antifibrinolytic function of FXIII is independent of fibrin-fibrin cross-linking and is expressed exclusively through α₂AP.

Factor XIII combined with recombinant factor VIIa: a new means of treating severe hemophilia A

BACKGROUND

Abnormal thrombin generation is considered the key defect in hemophilia. Conventional treatment seeks to correct this using coagulation factor replacement or bypassing agents, for example recombinant factor VIIa (rFVIIa). Previous studies demonstrate abnormal FXIII activation in patients with hemophilia. FXIII activation is essential for formation of structurally normal, stable clots.

OBJECTIVES

The present study challenges the hypothesis that in hemophilia the use of plasma-derived FXIII (pdFXIII) in combination with rFVIIa will produce a greater improvement in clot stability than promotion of thrombin generation alone.

METHODS

Fourteen individuals with severe hemophila A were enrolled. Whole blood was spiked ex vivo with buffer, rFVIIa (2 μg mL(-1)) or rFVIIa (2 μg mL(-1)) plus pdFXIII (10 μg mL(-1)). Whole blood thromboelastometry assessed clot stability, after activation with tissue factor (TF) (0.15 pm) plus tissue-type plasminogen activator (tPa) (2 nm). The primary outcome measure of clot stability was area under the elasticity curve (AUEC).

RESULTS

The combination of pdFXIII and rFVIIa significantly improved clot stability as measured by AUEC (P < 0.05) compared with rFVIIa alone.

CONCLUSION

The use of pdFXIII resulted in superior clot stability compared with solely enhancing thrombin generation and we suggest that increasing thrombin generation alone fails to fully correct dysregulation of clot-stabilizing mechanisms associated with bleeding disorders. Hemorrhage control in hemophilia may be improved using clot stabilizing drugs. FXIII shows potential as a novel agent.