Natalya M. Ananyeva

Inhibitors in hemophilia A: mechanisms of inhibition, management and perspectives.

Factor VIII (FVIII) replacement therapy remains the mainstay in hemophilia A care. The major complication of replacement therapy is formation of antibodies, which inhibit FVIII activity, thus dramatically reducing treatment efficiency. The present review summarizes the accumulated knowledge on epitopes of FVIII inhibitors and mechanisms of their inhibitory effects. FVIII inhibitors most frequently target the A2, C2 and A3 domains of FVIII and interfere with important interactions of FVIII at various stages of its functional pathway; a class of FVIII inhibitors inactivates FVIII by proteolysis. We discuss therapeutic approaches currently used for treatment of hemophilia A patients with inhibitors and analyze the factors that influence the outcome. The choice between options should depend on the level of inhibitors and consideration of efficacy, safety, and availability of particular regimens. Advances of basic science open avenues for alternative targeted, specific and long-lasting treatments, such as the use of peptide decoys for blocking FVIII inhibitors, bypassing them with human/porcine FVIII hybrids, neutralizing FVIII-reactive CD4+ T cells with anti-clonotypic antibodies, or inducing immune tolerance to FVIII with the use of universal CD4+ epitopes or by genetic approaches.

Initiation and propagation of coagulation from tissue factor-bearing cell monolayers to plasma: initiator cells do not regulate spatial growth rate*

Summary.  Exposure of tissue factor (TF)‐bearing cells to blood is the initial event in coagulation and intravascular thrombus formation. However, the mechanisms which determine thrombus growth remain poorly understood. To explore whether the procoagulant activity of vessel wall‐bound cells regulates thrombus expansion, we studied in vitro spatial clot growth initiated by cultured human cells of different types in contact pathway‐inhibited, non‐flowing human plasma. Human aortic endothelial cells, smooth muscle cells, macrophages and lung fibroblasts differed in their ability to support thrombin generation in microplate assay with peaks of generated thrombin of 60 ± 53 nmol L−1, 135 ± 57 nmol L−1, 218 ± 55 nmol L−1 and 407 ± 59 nmol L−1 (mean ± SD), respectively. Real‐time videomicroscopy revealed the initiation and spatial growth phases of clot formation. Different procoagulant activity of cell monolayers was manifested as up to 4‐fold difference in the lag times of clot formation. In contrast, the clot growth rate, which characterized propagation of clotting from the cell surface to plasma, was largely independent of cell type (≤ 30% difference). Experiments with factor VII (FVII)‐, FVIII‐, FX‐ or FXI‐deficient plasmas and annexin V revealed that (i) cell surface‐associated extrinsic Xase was critical for initiation of clotting; (ii) intrinsic Xase regulated only the growth phase; and (iii) the contribution of plasma phospholipid surfaces in the growth phase was predominant. We conclude that the role of TF‐bearing initiator cells is limited to the initial stage of clot formation. The functioning of intrinsic Xase in plasma provides the primary mechanism of sustained and far‐ranging propagation of coagulation leading to the physical expansion of a fibrin clot.

Oxidized LDL Mediates the Release of Fibroblast Growth Factor-1

Fibroblast growth factor-1 (FGF-1) and lipoproteins play an important role in atherogenesis. In the present study, we explored a possible mechanism by which abnormal lipid metabolism could be linked to the proliferative aspects of the disease. We tested oxidized LDL (oxLDL) as a possible pathophysiological mediator of the release of FGF-1, using FGF-1-transfected mouse NIH 3T3 cells and FGF-1-transfected rabbit smooth muscle cells, and compared it with the release caused by elevated temperature. Immunoblot analysis showed that oxLDL induced the release of FGF-1 in a concentration-dependent manner from 10 to 100 micrograms/mL. The effect correlated with the extent of oxidative modification of LDL and was maximal within 4 hours of exposure of cells to oxLDL. In contrast to the temperature stress-induced FGF-1 secretion pathway, FGF-1 released in response to oxLDL (1) appeared in the conditioned medium as a monomer, (2) appeared independently of the presence of either actinomycin D or cycloheximide, and (3) was neither enhanced nor inhibited by brefeldin A. We did not detect cell loss, significant morphological changes, changes in growth characteristics, or other indications of lethal toxicity in oxLDL-treated cells. Although the level of lactate dehydrogenase activity was elevated after oxLDL exposure, the calculations showed that > 90% of the FGF-1 was released by viable cells. We propose that oxLDL-induced FGF-1 release is mediated by sublethal and apparently transient changes in cell membrane permeability. In the environment of an atherosclerotic lesion, oxLDL-induced FGF-1 release may be among the mediators of endothelial and smooth muscle cell proliferation.

The future of recombinant coagulation factors.

Summary.  Hemophilias A and B are X chromosome‐linked bleeding disorders, which are mainly treated by repeated infusions of factor (F)VIII or FIX, respectively. In the present review, we specify the limitations in expression of recombinant (r)FVIII and summarize the bioengineering strategies that are currently being explored for constructing novel rFVIII molecules characterized by high efficiency expression and improved functional properties. We present the strategy to prolong FVIII lifetime by disrupting FVIII interaction with its clearance receptors and demonstrate how construction of human‐porcine FVIII hybrid molecules can reduce their reactivity towards inhibitory antibodies. While the progress in improving rFIX is impeded by low recovery rates, the authors are optimistic that the efforts of basic science may ultimately lead to higher efficiency of replacement therapy of both hemophilias A and B.

Haemophilia A: effects of inhibitory antibodies on factor VIII functional interactions and approaches to prevent their action

Factor VIII (FVIII) is an essential component of the intrinsic pathway of blood coagulation. Normal functioning of FVIII requires its interactions with other components of the coagulation cascade. In the circulation, it exists as a complex with von Willebrand factor (vWF). Upon activation by thrombin or activated factor X (FXa), activated FVIII (FVIIIa) functions as a cofactor for the serine protease factor IXa. Their complex assembled on the phospholipid surface activates FX to FXa, which consequently participates in formation of thrombin, the key protease of the coagulation cascade. Genetic deficiency in FVIII results in a coagulation disorder haemophilia A, which is treated by infusions of FVIII products. Approximately 25–30% of patients develop antibodies inhibiting FVIII activity (FVIII inhibitors). The major epitopes of inhibitors are located within the A2, C2 and A3 domains of the FVIII molecule. The inhibitory effects of antibodies are manifested at various stages of the FVIII functional pathway, including FVIII binding to vWF, activation of FVIII by thrombin, and FVIIIa incorporation into the Xase complex. We summarize the current knowledge of the FVIII sites involved in interaction with its physiological ligands and different classes of inhibitory antibodies and describe their inhibitory mechanisms. We outline the strategies aimed to overcome the effects of inhibitory antibodies such as development of human/porcine FVIII molecules, resistant to inhibitors. We also discuss approaches to modulate the antibody response, as well as efforts to develop a long‐term immunotolerance to FVIII protein.

Factor VIII – novel insights into form and function

Factor VIII (FVIII) is a key cofactor in blood coagulation, deficiency of which results in the serious bleeding disorder haemophilia A. The traditional treatment for haemophilia is by replacement therapy using plasma-derived or recombinant FVIII products. Recent developments in FVIII biochemistry have improved our understanding of how FVIII interacts with other macromolecules in the performance of its normal procoagulant function and how naturally occurring mutations result in the spectrum of clinical phenotypes found in haemophilia A. This review describes elements of recent studies on the structure of the FVIII molecule (as determined by X-ray crystallography, electron crystallography and modelling studies), its interaction with plasma von Willebrand factor and how this relates to the activation state of FVIII, interactions with procoagulant phospholipid surfaces and factor IXa in the formation of the tenase complex and with the low-density lipoprotein receptor-related protein (LRP), a hepatic cell-surface receptor now known to be at least partly responsible for FVIII clearance from the circulation. Our improving understanding of the life cycle of the FVIII molecule carries with it the potential for better treatment for haemophilia, either by augmentation of its functional characteristics or by suppression of the natural clearance mechanisms.

Two subpopulations of thrombin‐activated platelets differ in their binding of the components of the intrinsic factor X‐activating complex

Summary.  Binding of fluorescein‐labeled coagulation factors IXa, VIII, X, and allophycocyanin‐labeled annexin V to thrombin‐activated platelets was studied using flow cytometry. Upon activation, two platelet subpopulations were detected, which differed by 1–2 orders of magnitude in the binding of the coagulation factors and by 2–3 orders of magnitude in the binding of annexin V. The percentage of the high‐binding platelets increased dose dependently of thrombin concentration. At 100 nm of thrombin, platelets with elevated binding capability constituted ∼4% of total platelets and were responsible for the binding of ∼50% of the total bound factor. Binding of factors to the high‐binding subpopulation was calcium‐dependent and specific as evidenced by experiments in the presence of excess unlabeled factor. The percentage of the high‐binding platelets was not affected by echistatin, a potent aggregation inhibitor, confirming that the high‐binding platelets were not platelet aggregates. Despite the difference in the coagulation factors binding, the subpopulations were indistinguishable by the expression of general platelet marker CD42b and activation markers PAC1 (an epitope of glycoprotein IIb/IIIa) and CD62P (P‐selectin). Dual‐labeling binding studies involving coagulation factors (IXa, VIII, or X) and annexin V demonstrated that the high‐binding platelet subpopulation was identical for all coagulation factors and for annexin V. The high‐binding subpopulation had lower mean forward and side scatters compared with the low‐binding subpopulation (∼80% and ∼60%, respectively). In its turn, the high‐binding subpopulation was not homogeneous and included two subpopulations with different scatter values. We conclude that activation by thrombin induces the formation of two distinct subpopulations of platelets different in their binding of the components of the intrinsic fX‐activating complex, which may have certain physiological or pathological significance.

Development of Improved Factor VIII Molecules and New Gene Transfer Approaches for Hemophilia A

Hemophilia A, the most common inherited bleeding disorder, is caused by deficiency or functional defects in coagulation factor VIII (fVIII). Conventional treatment for this disease involves intravenous infusions of plasma-derived or recombinant fVIII products. Although replacement therapy effectively stops the bleeding episodes, it has a risk of transmission of viral blood-borne diseases and development of neutralizing antibodies that inactivate the administered fVIII protein. Hemophilia A is an attractive candidate for application of gene therapy approaches because the therapeutic window is wide and even modest elevation of fVIII levels will correct the hemophilic phenotype. Ongoing preclinical investigations utilize animal models of hemophilia A, including genetically fVIII-deficient mice and naturally fVIII-deficient dogs, to optimize vectors, transgenes and target cell populations for Phase I clinical trials. In this review, we outline the progress in understanding the mechanisms of fVIII turnover, which provides a basis for development of improved fVIII molecules with prolonged half-life in the circulation. We discuss the possibility of incorporating these improved fVIII molecules as transgenes into self-inactivating lentiviral vectors carrying chromatin insulator sequences, representing a new generation of gene delivery vehicle, to target hematopoietic stem cells and endothelial cells. The use of hematopoietic stem cells as the target cell population may prevent inhibitor formation to transduced fVIII by induction of immune tolerance. Alternatively, endothelial cells may support optimal synthesis of fVIII and myeloablative conditioning of patients with radiation or chemotherapy may not be required for efficient engraftment of the engineered cells. Collectively, these proposed advances represent promising prophylactic strategies toward long-term correction of the coagulation defect in this progressively debilitating, life-threatening disease.

Mathematical Models of Blood Coagulation and Platelet Adhesion: Clinical Applications

At present, computer-assisted molecular modeling and virtual screening have become effective and widely-used tools for drug design. However, a prerequisite for design and synthesis of a therapeutic agent is determination of a correct target in the metabolic system, which should be either inhibited or stimulated. Solution of this extremely complicated problem can also be assisted by computational methods. This review discusses the use of mathematical models of blood coagulation and platelet-mediated primary hemostasis and thrombosis as cost-effective and time-saving tools in research, clinical practice, and development of new therapeutic agents and biomaterials. We focus on four aspects of their application: 1) efficient diagnostics, i.e. theoretical interpretation of diagnostic data, including sensitivity of various clotting assays to the changes in the coagulation system; 2) elucidation of mechanisms of coagulation disorders (e.g. hemophilias and thrombophilias); 3) exploration of mechanisms of action of therapeutic agents (e.g. recombinant activated factor VII) and planning rational therapeutic strategy; 4) development of biomaterials with non-thrombogenic properties in the design of artificial organs and implantable devices. Accumulation of experimental knowledge about the blood coagulation system and about platelets, combined with impressive increase of computational power, promises rapid development of this field.

Comparison of the properties of phospholipid surfaces formed on HPA and L1 biosensor chips for the binding of the coagulation factor VIII.

Binding of a coagulation factor VIII to phosphatidylserine-containing membranes is critical for exerting its cofactor activity. The use of surface plasmon resonance allows studying factor VIII interaction with immobilized phospholipids. In the present study we compared factor VIII-binding properties of phospholipid surfaces immobilized on L1 and HPA Biacore chips in the form of a flexible bilayer and rigid monolayer, respectively. We demonstrated that immobilized phospholipid surfaces with physiological contents of PS and PE formed on L1 but not on HPA chip closely mimic intact phospholipid vesicles in their factor VIII and thrombin-activated factor VIII (factor VIIIa) binding properties.