Platelet-delivered factor VIII provides limited resistance to anti-factor VIII inhibitors

Intrauterine lethality in Tfpi gene disrupted mice is differentially suppressed during mid- and late-gestation by platelet TFPIα overexpression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein required for murine embryonic development. Intrauterine lethality of Tfpi-/- mice occurs at mid- and late gestation, the latter of which is associated with severe cerebrovascular defects. Megakaryocytes produce only the TFPIα isoform, which is stored within platelets and released upon activation.

OBJECTIVES

To examine biological activities of platelet TFPIα (pTFPIα) by characterizing effects of pTFPIα overexpression in Tfpi-/- mice.

METHODS

Transgenic mice overexpressing pTFPIα were generated and crossed onto the Tfpi-/- background. Genetic and histological analyses of embryos were performed to investigate the function of pTFPIα during embryogenesis.

RESULTS

The transgene (Tg) increased pTFPIα 4- to 5-fold without altering plasma TFPI in adult Tfpi+/+ and Tfpi+/- mice but did not rescue Tfpi-/- mice to wean. Analyses of the impact of pTFPIα overexpression on Tfpi-/- survival, however, were complicated by linkage between the Tg integration site and the endogenous Tfpi locus on chromosome 2. Strain-specific genetic interactions also modulated Tfpi-/- embryonic survival. After accounting for these underlying genetic factors, pTFPIα overexpression completely suppressed mid-gestational lethality of Tfpi-/- embryos but had no effect on development of cerebrovascular defects during late gestation resulting in their lack of survival to wean.

CONCLUSIONS

pTFPIα overexpression rescued Tfpi-/- embryos from mid-gestational but not late gestational lethality. The prevalence of underlying genetic factors complicating analyses within our study illustrates the importance of meticulously characterizing transgenic mouse models to avoid spurious interpretation of results.

The severe spontaneous bleeding phenotype in a novel hemophilia A rat model is rescued by platelet FVIII expression

Previous studies have shown that platelet-specific factor VIII (FVIII) expression (2bF8) restores hemostasis and induces immune tolerance in hemophilia A (HA) mice even with preexisting inhibitors. Here we investigated for the first time whether platelet FVIII expression can prevent severe spontaneous bleeding in rat HA, a model mimicking the frequent spontaneous bleeding in patients with severe HA. A novel FVIII-/- rat model in a Dahl inbred background (Dahl-FVIII-/-) with nearly the entire rat FVIII gene inverted was created by using a CRISPR/Cas9 strategy. There was no detectable FVIII in plasma. Spontaneous bleeding in the soft tissue, muscles, or joints occurred in 100% of FVIII-/- rats. Sixty-one percent developed anti-FVIII inhibitors after ≥2 doses of recombinant human FVIII infusion. However, when 2bF8 transgene was crossed into the FVIII-/- background, none of the resulting 2bF8tg+FVIII-/- rats (with platelet FVIII levels of 28.26 ± 7.69 mU/108 platelets and undetectable plasma FVIII) ever had spontaneous bleeding. When 2bF8tg bone marrow (BM) was transplanted into FVIII-/- rats, only 1 of 7 recipients had a bruise at the early stage of BM reconstitution, but no other spontaneous bleeding was observed during the study period. To confirm that the bleeding diathesis in FVIII-/- rats was ameliorated after platelet FVIII expression, rotational thromboelastometry and whole-blood thrombin generation assay were performed. All parameters in 2bF8tg BM transplantation recipients were significantly improved compared with FVIII-/- control rats. Of note, neither detectable levels of plasma FVIII nor anti-FVIII inhibitors were detected in 2bF8tg BM transplantation recipients. Thus, platelet-specific FVIII expression can efficiently prevent severe spontaneous bleeding in FVIII-/- rats with no anti-FVIII antibody development.

RNAi targeting heparin cofactor II promotes hemostasis in hemophilia A

Hemophilia A is a hemorrhagic disease due to congenital deficiencies of coagulation factor VIII (FVIII). Studies show that hemophilia patients with anticoagulant deficiency present less severe hemorrhagic phenotypes. We aimed to find a new therapeutic option for hemophilia patients by RNA interference (RNAi) targeting heparin cofactor II (HCII), a critical anticoagulant protein inactivating the thrombin. The optimal small interfering RNA (siRNA) was conjugated to an asialoglycoprotein receptor ligand (N-acetylgalactosamine [GalNAc]-HCII), promoting targeted delivery to the liver. After administration, GalNAc-HCII demonstrated effective, dose-dependent, and persistent HCII inhibition. After 7 days, in normal mice, GalNAc-HCII reduced HCII levels to 25.04% ± 2.56%, 11.65% ± 2.41%, and 6.50% ± 1.73% with 2, 5, and 10 mg/kg GalNAc-HCII, respectively. The hemostatic ability of hemophilia mice in the GalNAc-HCII-treated group significantly improved, with low thrombus formation time in the carotid artery thrombosis models and short bleeding time in the tail-clipping assays. After repeated administration, the prolonged activated partial thromboplastin time (APTT) was reduced. A 30 mg/kg dose did not cause pathological thrombosis. Our study confirmed that GalNAc-HCII therapy is effective for treating hemophilia mice and can be considered a new option for treating hemophilia patients.

Enhancing therapeutic efficacy of in vivo platelet-targeted gene therapy in hemophilia A mice

Our previous studies demonstrated that intraosseous (IO) infusion of lentiviral vectors (LVs) carrying a modified B domain-deleted factor VIII (FVIII) transgene driven by a megakaryocyte-specific promoter (GP1Bα promoter; G-F8/N6-LV) successfully transduced hematopoietic stem cells (HSCs) to produce FVIII stored in the platelet α-granules. Platelet FVIII corrected the bleeding phenotype with limited efficacy in hemophilia A (HemA) mice with and without preexisting anti-FVIII inhibitors. The present study sought to further enhance the therapeutic efficacy of this treatment protocol by increasing both the efficiency of LV transduction and the functional activity of platelet FVIII. A combined drug regimen of dexamethasone and anti-CD8α monoclonal antibody enhanced the percentage of transduced bone marrow and HSCs over time. In G-F8/N6-LV-treated HemA mice, significant improvement in phenotypic correction was observed on day 84. To improve platelet FVIII functionality, genes encoding FVIII variant F8X10K12 with increased expression or F8N6K12RH with increased functional activity compared with F8/N6 were incorporated into LVs. Treatment with G-F8X10K12-LV in HemA mice produced a higher level of platelet FVIII but induced anti-FVIII inhibitors. After treatment with combined drugs and IO infusion of G-F8/N6K12RH-LV, HemA mice showed significant phenotypic correction without anti-FVIII inhibitor formation. These results indicate that new human FVIII variant F8/N6K12RH combined with immune suppression could significantly enhance the therapeutic efficacy of in vivo platelet-targeted gene therapy for murine HemA via IO delivery. This protocol provides a safe and effective treatment for hemophilia that may be translatable to and particularly beneficial for patients with preexisting inhibitory antibodies to FVIII.

Platelet-Targeted FVIII Gene Therapy Restores Hemostasis and Induces Immune Tolerance for Hemophilia A

Platelets are small anucleated blood components primarily described as playing a fundamental role in hemostasis and thrombosis. Over the last decades, increasing evidence has demonstrated the role of platelets in modulating inflammatory reactions and immune responses. Platelets harbor several specialized organelles: granules, endosomes, lysosomes, and mitochondria that can synthesize proteins with pre-stored mRNAs when needed. While the functions of platelets in the immune response are well-recognized, little is known about the potential role of platelets in immune tolerance. Recent studies demonstrate that platelet-specific FVIII gene therapy can restore hemostasis and induce immune tolerance in hemophilia A mice, even mice with preexisting anti-FVIII immunity. Here, we review the potential mechanisms by which platelet-targeted FVIII gene therapy restores hemostasis in the presence of anti-FVIII inhibitory antibodies and induces immune tolerance in hemophilia A.

Infused factor VIII-expressing platelets or megakaryocytes as a novel therapeutic strategy for hemophilia A

B-domainless factor VIII (FVIII) ectopically expressed in megakaryocytes (MKs) is stored in α granules of platelets (pFVIII) and is capable of restoring hemostasis in FVIII^null mice, even in the presence of circulating inhibitors. However, our prior studies have shown that this ectopically expressed pFVIII can injure developing MKs. Moreover, the known risks of prolonged thrombocytopenia after bone marrow transplantation are significant challenges to the use of this strategy to treat individuals with severe hemophilia A and particularly those with intractable clinically relevant inhibitors. Because of these limitations, we now propose the alternative therapeutic pFVIII strategy of infusing pFVIII-expressing MKs or platelets derived from induced pluripotent stem cells (iPSCs). pFVIII-expressing iPSC-derived MKs, termed iMKs, release platelets that can contribute to improved hemostasis in problematic inhibitor patients with hemophilia A. As proof of principle, we demonstrate that hemostasis can be achieved in vitro and in vivo with pFVIII-expressing platelets and show prolonged efficacy. Notably, pFVIII-expressing platelets are also effective in the presence of inhibitors, and their effect was enhanced with recombinant FVIIa. Human pFVIII-expressing iMKs improved hemostasis in vitro, and derived platelets from infused human pFVIII-expressing iMKs improved hemostasis in FVIII^null mice. These studies indicate the potential therapeutic use of recurrent pFVIII-expressing MK or platelet infusions with prolonged hemostatic coverage that may be additive with bypassing agents in hemophilia A patients with neutralizing inhibitors.

Platelet-inspired therapeutics: current status, limitations, clinical implications, and future potential

Recent research has been successful in demonstrating the importance of the addition of platelets to the field of cell-mediated therapeutics, by making use of different platelet forms to design modalities able to positively impact a wide range of diseases. A key obstacle hindering the success of conventional therapeutic interventions is their inability to produce targeted treatment, resulting in a number of systemic side effects and a longer duration for the onset of action to occur. An additional challenge facing current popular therapeutic interventions is biocompatibility of the system, resulting in the decline of patient compliance to treatment. In an attempt to address these challenges, the past few decades have been witness to the discovery and innovation of precision therapy, in order to achieve targeted treatment for an array of conditions, thereby superseding alternative mechanisms of treatment. Platelet-mediated therapeutics, as well as employing platelets as drug delivery vehicles, are key components in advancing precision therapy within research and in clinical settings. This novel approach is designed with the objective that the platelets retain their original structure and functions within the body, thereby mitigating biocompatibility challenges. In this article, we review the current significant impact that the addition of platelet-inspired systems has made on the field of therapeutics; explore certain limitations of each system, together with ideas on how to overcome them; and discuss the clinical implications and future potential of platelet-inspired therapeutics. Graphical abstract.

Factor XIII deficiency does not prevent FeCl3-induced carotid artery thrombus formation in mice

BACKGROUND

The compositions of venous (red blood cell-rich) and arterial (platelet-rich) thrombi are mediated by distinct pathophysiologic processes; however, fibrin is a major structural component of both. The transglutaminase factor XIII (FXIII) stabilizes fibrin against mechanical and biochemical disruption and promotes red blood cell retention in contracted venous thrombi. Previous studies have shown factor XIII (FXIII) inhibition decreases whole blood clot mass and therefore, may be a therapeutic target for reducing venous thrombosis. The role of FXIII in arterial thrombogenesis is less studied, and the particular contribution of platelet FXIII remains unresolved.

OBJECTIVE

To determine whether FXIII reduction prevents experimental arterial thrombogenesis.

METHODS

Using wild-type mice and mice with genetically imposed deficiency in FXIII, we measured thrombus formation and stability following ferric chloride-induced arterial thrombosis. We also determined the impact of FXIII on the mass of contracted platelet-rich plasma clots.

RESULTS

Following vessel injury, F13a+/+ , F13a+/- , and F13a-/- mice developed occlusive arterial thrombi. FXIII deficiency did not significantly reduce the incidence or prolong the time to occlusion. FXIII deficiency also did not alter the timing of reflow events or decrease platelet-rich clot mass.

CONCLUSIONS

FXIII does not significantly alter the underlying pathophysiology of experimental arterial thrombus formation.

Platelet-Targeted Gene Therapy for Hemophilia

Gene therapy is an attractive approach for disease treatment. Since platelets are abundant cells circulating in blood with the distinctive abilities of storage and delivery and fundamental roles in hemostasis and immunity, they could be a unique target for gene therapy of diseases. Recent studies have demonstrated that ectopic expression of factor VIII (FVIII) in platelets under control of the platelet-specific promoter results in FVIII storage together with its carrier protein von Willebrand factor (VWF) in α-granules and the phenotypic correction of hemophilia A. Importantly, the storage and sequestration of FVIII in platelets appears to effectively restore hemostasis even in the presence of functional-blocking inhibitory antibodies. This review summarizes studies on platelet-specific gene therapy of hemophilia A as well as hemophilia B.

Factor XIII in plasma, but not in platelets, mediates red blood cell retention in clots and venous thrombus size in mice

The transglutaminase factor XIII (FXIII) stabilizes clots against mechanical and biochemical disruption and is essential for hemostasis. In vitro and in vivo models of venous thrombosis demonstrate that FXIII mediates clot size by promoting red blood cell (RBC) retention. However, the key source of FXIII and whether FXIII activity can be reduced to suppress thrombosis without imposing deleterious hemostatic consequences are 2 critical unresolved questions. FXIII is present in multiple compartments, including plasma (FXIII_plasma) as a heterotetramer of A₂ and B₂ subunits and platelets (FXIII_plt) as an A₂ homodimer. We determined the role of the FXIII compartment and level in clot contraction, composition, and size in vitro and using in vivo models of hemostasis and venous thrombosis. Reducing overall FXIII levels decreased whole blood clot weight but did not alter thrombin generation or contraction of platelet-rich plasma clots. In reconstituted platelet-rich plasma and whole blood clot contraction assays, FXIII_plasma, but not FXIII_plt, produced high-molecular-weight fibrin crosslinks, promoted RBC retention, and increased clot weights. Genetically imposed reduction of FXIII delayed FXIII activation and fibrin crosslinking, suggesting FXIII levels mediate the kinetics of FXIII activation and activity and that the timing of these processes is a critical determinant of RBC retention during clot formation and contraction. A 50% reduction in FXIII_plasma produced significantly smaller venous thrombi but did not increase bleeding in tail transection or saphenous vein puncture models in vivo. Collectively, these findings suggest that partial FXIII reduction may be a therapeutic strategy for reducing venous thrombosis.

Induced Pluripotent Stem Cell-Derived Megakaryocytes and Platelets for Disease Modeling and Future Clinical Applications

Platelets, derived from megakaryocytes, are anucleate cytoplasmic discs that circulate in the blood stream and play major roles in hemostasis, inflammation, and vascular biology. Platelet transfusions are used in a variety of medical settings to prevent life-threatening thrombocytopenia because of cancer therapy, other causes of acquired or inherited thrombocytopenia, and trauma. Currently, platelets used for transfusion purposes are donor derived. However, there is a drive to generate nondonor sources of platelets to help supplement donor-derived platelets. Efforts have been made by many laboratories to generate in vitro platelets and optimize their production and quality. In vitro-derived platelets have the potential to be a safer, more uniform product, and genetic manipulation could allow for better treatment of patients who become refractory to donor-derived units. This review focuses on potential clinical applications of in vitro-derived megakaryocytes and platelets, current methods to generate and expand megakaryocytes from pluripotent stem cell sources, and the use of these cells for disease modeling.

Targeting factor VIII expression to platelets for hemophilia A gene therapy does not induce an apparent thrombotic risk in mice

Essentials Platelet-Factor (F) VIII gene therapy is a promising treatment in hemophilia A. This study aims to evaluate if platelet-FVIII expression would increase the risk for thrombosis. Targeting FVIII expression to platelets does not induce or elevate thrombosis risk. Platelets expressing FVIII are neither hyper-activated nor hyper-responsive.

SUMMARY

Background Targeting factor (F) VIII expression to platelets is a promising gene therapy approach for hemophilia A, and is successful even in the presence of inhibitors. It is well known that platelets play important roles not only in hemostasis, but also in thrombosis and inflammation. Objective To evaluate whether platelet-FVIII expression might increase thrombotic risk and thereby compromise the safety of this approach. Methods In this study, platelet-FVIII-expressing transgenic mice were examined either in steady-state conditions or under prothrombotic conditions induced by inflammation or the FV Leiden mutation. Native whole blood thrombin generation assay, rotational thromboelastometry analysis and ferric chloride-induced vessel injury were used to evaluate the hemostatic properties. Various parameters associated with thrombosis risk, including D-dimer, thrombin-antithrombin complexes, fibrinogen, tissue fibrin deposition, platelet activation status and activatability, and platelet-leukocyte aggregates, were assessed. Results We generated a new line of transgenic mice that expressed 30-fold higher levels of platelet-expressed FVIII than are therapeutically required to restore hemostasis in hemophilic mice. Under both steady-state conditions and prothrombotic conditions induced by lipopolysaccharide-mediated inflammation or the FV Leiden mutation, supratherapeutic levels of platelet-expressed FVIII did not appear to be thrombogenic. Furthermore, FVIII-expressing platelets were neither hyperactivated nor hyperactivatable upon agonist activation. Conclusion We conclude that, in mice, more than 30-fold higher levels of platelet-expressed FVIII than are required for therapeutic efficacy in hemophilia A are not associated with a thrombotic predilection.

Gene therapy for immune tolerance induction in hemophilia with inhibitors

The development of inhibitors, i.e. neutralizing alloantibodies against factor (F) VIII or FIX, is the most significant complication of protein replacement therapy for patients with hemophilia, and is associated with both increased mortality and substantial physical, psychosocial and financial morbidity. Current management, including bypassing agents to treat and prevent bleeding, and immune tolerance induction for inhibitor eradication, is suboptimal for many patients. Fortunately, there are several emerging gene therapy approaches aimed at addressing these unmet clinical needs of patients with hemophilia and inhibitors. Herein, we review the mounting evidence from preclinical hemophilia models that the continuous uninterrupted expression of FVIII or FIX delivered as gene therapy can bias the immune system towards tolerance induction, and even promote the eradication of pre-existing inhibitors. We also discuss several gene transfer approaches that directly target immune cells in order to promote immune tolerance. These preclinical findings also shed light on the immunologic mechanisms that underlie tolerance induction.

The immunogenicity of platelet-derived FVIII in hemophilia A mice with or without preexisting anti-FVIII immunity

Evidence shows that factor VIII (FVIII) ectopically expressed in platelets (2bF8) is therapeutic in FVIII(null) mice even with anti-FVIII inhibitory antibodies (inhibitors). If current efforts to generate platelets in vitro succeed, genetically manipulated platelets containing FVIII may be used therapeutically in hemophilia A patients with inhibitors. One important concern is the immunogenicity of platelet-derived FVIII. To address this concern, we infused 2bF8 transgenic (2bF8(Tg)) platelets into naïve FVIII(null) mice weekly for 8 weeks. No anti-FVIII antibodies were detected in the infused animals during the study course. We then explored whether platelet-derived FVIII is immunogenic in FVIII(null) mice with inhibitors. The 2bF8(Tg) platelets were transfused into rhF8-primed FVIII(null) mice, resulting in no augmentation of anti-FVIII antibodies. To investigate whether preconditioning affects the immune response, animals were sublethally irradiated and subsequently transfused with 2bF8(Tg) platelets. No anti-FVIII antibodies were detected in the recipients after platelet infusions. Following further challenge with rhF8, the inhibitor titer in this group was significantly lower than in naïve FVIII(null) mice utilizing the same immunization protocol. Thus, our data demonstrate that infusion of platelets containing FVIII triggers neither primary nor memory anti-FVIII immune response in FVIII(null) mice and that sublethal irradiation plus 2bF8(Tg) platelet infusion suppresses anti-FVIII immune response in FVIII(null) mice.

Platelet-delivered therapeutics

We have proposed that modified platelets could potentially be used to correct intrinsic platelet defects as well as for targeted delivery of therapeutic molecules to sights of vascular injury. Ectopic expression of proteins within α-granules prior to platelet activation has been achieved for several proteins, including urokinase, factor (F) VIII, and partially for FIX. Potential uses of platelet-directed therapeutics will be discussed, focusing on targeted delivery of urokinase as a thromboprophylactic agent and FVIII for the treatment of hemophilia A patients with intractable inhibitors. This presentation will discuss new strategies that may be useful in the care of patients with vascular injury as well as remaining challenges and limitations of these approaches.

New approaches to gene and cell therapy for hemophilia

Hemophilia is considered suitable for gene therapy because it is caused by a single gene abnormality, and therapeutic coagulation factor levels may vary across a broad range. Recent success of hemophilia B gene therapy with an adeno-associated virus (AAV) vector in a clinical trial showed the real prospect that, through gene therapy, a cure for hemophilia may become a reality. However, AAV-mediated gene therapy is not applicable to patients with hemophilia A at present, and neutralizing antibodies against AAV reduce the efficacy of AAV-mediated strategies. Because patients that benefit from AAV treatment (hemophilia B without neutralizing antibodies) are estimated to represent only 15% of total patients with hemophilia, the development of basic technologies for hemophilia A and those that result in higher therapeutic effects are critical. In this review, we present an outline of gene therapy methods for hemophilia, including the transition of technical developments thus far and our novel techniques.

Platelet-delivered ADAMTS13 inhibits arterial thrombosis and prevents thrombotic thrombocytopenic purpura in murine models

ADAMTS13 metalloprotease cleaves von Willebrand factor (VWF), thereby inhibiting platelet aggregation and arterial thrombosis. An inability to cleave ultralarge VWF resulting from hereditary or acquired deficiency of plasma ADAMTS13 activity leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). Plasma exchange is the most effective initial therapy for TTP to date. Here, we report characterization of transgenic mice expressing recombinant human ADAMTS13 (rADAMTS13) in platelets and its efficacy in inhibiting arterial thrombosis and preventing hereditary and acquired antibody-mediated TTP in murine models. Western blotting and fluorescent resonance energy transfer assay detect full-length rADAMTS13 protein and its proteolytic activity, respectively, in transgenic (Adamts13(-/-)Plt(A13)), but not in wild-type and Adamts13(-/-), platelets. The expressed rADAMTS13 is released on stimulation with thrombin and collagen, but less with 2MesADP. Platelet-delivered rADAMTS13 is able to inhibit arterial thrombosis after vascular injury and prevent the onset and progression of Shigatoxin-2 or recombinant murine VWF-induced TTP syndrome in mice despite a lack of plasma ADAMTS13 activity resulting from the ADAMTS13 gene deletion or the antibody-mediated inhibition of plasma ADAMTS13 activity. These findings provide a proof of concept that platelet-delivered ADAMTS13 may be explored as a novel treatment of arterial thrombotic disorders, including hereditary and acquired TTP, in the presence of anti-ADAMTS13 autoantibodies.

Gene therapy for hemophilia

Hemophilia is an X-linked inherited bleeding disorder consisting of two classifications, hemophilia A and hemophilia B, depending on the underlying mutation. Although the disease is currently treatable with intravenous delivery of replacement recombinant clotting factor, this approach represents a significant cost both monetarily and in terms of quality of life. Gene therapy is an attractive alternative approach to the treatment of hemophilia that would ideally provide life-long correction of clotting activity with a single injection. In this review, we will discuss the multitude of approaches that have been explored for the treatment of both hemophilia A and B, including both in vivo and ex vivo approaches with viral and nonviral delivery vectors.

Apoptotic effects of platelet factor VIII on megakaryopoiesis: implications for a modified human FVIII for platelet-based gene therapy

BACKGROUND

Ectopically expressed B-domainless factor VIII in megakaryocytes is stored in α-granules, is effective in a number of murine hemostatic models, and is protected from circulating inhibitors. However, this platelet (p) FVIII has different temporal-spatial availability from plasma FVIII, with limited efficacy in other murine hemostatic models.

OBJECTIVES AND METHODS

We sought to improve pFVIII hemostatic efficacy by expressing canine (c) FVIII, which has higher stability and activity than human (h) FVIII in FVIII(null) mice.

RESULTS AND CONCLUSIONS

We found that pcFVIII was more effective than phFVIII at restoring hemostasis, but peak pcFVIII antigen levels were lower and were associated with greater megakaryocyte apoptosis than phFVIII. These new insights suggest that pFVIII gene therapy strategies should focus on enhancing activity rather than levels. We previously showed that modification of the PACE/furin cleavage site in hFVIII resulted in secretion of hFVIII primarily as a single-chain molecule with increased biological activity. In megakaryocytes, this variant was expressed at the same level as phFVIII with a lentiviral bone marrow transplant approach to reconstitute FVIII(null) mice, but was more effective, resulting in near-normal hemostasis in the cremaster laser injury model. These studies may have implications for pFVIII gene therapy in hemophilia A.

Advancements in gene transfer-based therapy for hemophilia A

Gene therapy has promised clinical benefit to those suffering with hemophilia A, but this benefit has not yet been realized. However, during the past two decades, basic and applied gene therapy research has progressed and the goal of gene therapy for hemophilia A is once again in our sights. The hemophilia A patient population suffers from a disease that requires invasive, lifelong management, is exorbitantly expensive to treat, has geographically limited treatment access and can become untreatable due to immune reactions to the treatment product. Subsequent to the cloning of the factor VIII gene and cDNA in the early 1980s, academic and commercial research laboratories began to pursue gene transfer-based therapies to supplement or supplant the available protein replacement therapy. However, to date, clinical trials for gene therapy of hemophilia A have been unsuccessful. Three trials have been conducted with each having tested a different gene-transfer strategy and each demonstrating that there is a considerable barrier to achieving sustained expression of therapeutic amounts of factor VIII. Recent progress has been made in gene-transfer technology and, relevant to hemophilia A, towards increasing the biosynthetic efficiency of factor VIII. These advances are now being combined to develop novel strategies to treat and possibly cure hemophilia A.

Gene therapy for hemophilia

Hemophilia A and B are X-linked monogenic disorders resulting from deficiencies of factor VIII and FIX, respectively. Purified clotting factor concentrates are currently intravenously administered to treat hemophilia, but this treatment is non-curative. Therefore, gene-based therapies for hemophilia have been developed to achieve sustained high levels of clotting factor expression to correct the clinical phenotype. Over the past two decades, different types of viral and non-viral gene delivery systems have been explored for hemophilia gene therapy research with a variety of target cells, particularly hepatocytes, hematopoietic stem cells, skeletal muscle cells, and endothelial cells. Lentiviral and adeno-associated virus (AAV)-based vectors are among the most promising vectors for hemophilia gene therapy. In preclinical hemophilia A and B animal models, the bleeding phenotype was corrected with these vectors. Some of these promising preclinical results prompted clinical translation to patients suffering from a severe hemophilic phenotype. These patients receiving gene therapy with AAV vectors showed long-term expression of therapeutic FIX levels, which is a major step forwards in this field. Nevertheless, the levels were insufficient to prevent trauma or injury-induced bleeding episodes. Another challenge that remains is the possible immune destruction of gene-modified cells by effector T cells, which are directed against the AAV vector antigens. It is therefore important to continuously improve the current gene therapy approaches to ultimately establish a real cure for hemophilia.

Structure-function and regulation of ADAMTS-13 protease

ADAMTS-13, a plasma reprolysin-like metalloprotease, cleaves von Willebrand factor (VWF). Severe deficiency of plasma ADAMTS-13 activity results in thrombotic thrombocytopenic purpura (TTP), while mild to moderate deficiencies of plasma ADAMTS-13 activity are emerging risk factors for developing myocardial and cerebral infarction, pre-eclampsia, and malignant malaria. Moreover, Adamts13(-/-) mice develop more severe inflammatory responses, leading to increased ischemia/perfusion injury and formation of atherosclerosis. Structure-function studies demonstrate that the N-terminal portion of ADAMTS-13 (MDTCS) is necessary and sufficient for proteolytic cleavage of VWF under various conditions and attenuation of arterial/venous thrombosis after oxidative injury. The more distal portion of ADAMTS-13 (TSP1 2-8 repeats and CUB domains) may function as a disulfide bond reductase to prevent an elongation of ultra-large VWF strings on activated endothelial cells and inhibit platelet adhesion/aggregation on collagen surface under flow. Remarkably, the proteolytic cleavage of VWF by ADAMTS-13 is accelerated by FVIII and platelets under fluid shear stress. A disruption of the interactions between FVIII (or platelet glycoprotein 1bα) and VWF dramatically impairs ADAMTS-13-dependent proteolysis of VWF in vitro and in vivo. These results suggest that FVIII and platelets may be physiological cofactors regulating VWF proteolysis. Finally, the structure-function and autoantibody mapping studies allow us to identify an ADAMTS-13 variant with increased specific activity but reduced inhibition by autoantibodies in patients with acquired TTP. Together, these findings provide novel insight into the mechanism of VWF proteolysis and tools for the therapy of acquired TTP and perhaps other arterial thrombotic disorders.

Progress toward inducing immunologic tolerance to factor VIII

A major problem in treating hemophilia A patients with therapeutic factor VIII (FVIII) is that 20% to 30% of these patients produce neutralizing anti-FVIII antibodies. These antibodies block (inhibit) the procoagulant function of FVIII and thus are termed "inhibitors." The currently accepted clinical method to attempt to eliminate inhibitors is immune tolerance induction (ITI) via a protocol requiring intensive FVIII treatment until inhibitor titers drop. Although often successful, ITI is extremely costly and is less likely to succeed in patients with high-titer inhibitors. During the past decade, significant progress has been made in clarifying mechanisms of allo- and autoimmune responses to FVIII and in suppression of these responses. Animal model studies are suggesting novel, less costly methods to induce tolerance to FVIII. Complementary studies of anti-FVIII T-cell responses using blood samples from human donors are identifying immunodominant T-cell epitopes in FVIII and possible targets for tolerogenic efforts. Mechanistic experiments using human T-cell clones and lines are providing a clinically relevant counterpoint to the animal model studies. This review highlights recent progress toward the related goals of lowering the incidence of anti-FVIII immune responses and promoting durable, functional immune tolerance to FVIII in patients with an existing inhibitor.

Factor VIII inhibitors: von Willebrand factor makes a difference in vitro and in vivo

BACKGROUND

The important association between von Willebrand factor (VWF) and factor VIII (FVIII) has been investigated for decades, but the effect of VWF on the reactivity of FVIII inhibitory antibodies, referred to as inhibitors, is still controversial.

OBJECTIVE

To investigate the interaction among VWF, FVIII and FVIII inhibitory antibodies.

METHODS

Three sources of inhibitors were used for in vitro studies, including the plasma from immunized VWF(null) FVIII(null) mice, purified plasma IgG from human inhibitor patients, or human monoclonal antibody from inhibitor patients' B-cell clones. Inhibitors were incubated with recombinant human FVIII (rhFVIII) either with or without VWF. The remaining FVIII activity was determined by chromogenic assay and inhibitor titers were determined. For in vivo studies, inhibitors and rhFVIII were infused into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test.

RESULTS

VWF has a dose-dependent protective effect on FVIII, limiting inhibitor inactivation of FVIII in both mouse and human samples. A preformed complex of VWF with FVIII provides more effective protection from inhibitors than competitive binding of antibodies and VWF to FVIII. The protective effect of VWF against FVIII inactivation by inhibitors was further confirmed in vivo by infusing inhibitors and FVIII into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test.

CONCLUSION

Our results demonstrate that VWF exerts a protective effect, reducing inhibitor inactivation of FVIII, both in vitro and in vivo.

Platelet and endothelial expression of clotting factors for the treatment of hemophilia

Hemostasis is achieved by the coordinate interaction of plasma, platelets, and vascular endothelium. Coagulation factors circulate in plasma with synthesis in liver and in endothelium. Interaction between Factor VIII (FVIII) and von Willebrand factor (VWF) in plasma is critically important, but there remains some question about whether this relationship is first established within the endothelial cell or in plasma. When FVIII is expressed with VWF in a cell that stores VWF, FVIII will also be stored and released. The manuscript will summarize some studies in which gene therapy exploits this relationship between VWF and FVIII to achieve hemostasis even in the presence of circulating inhibitory antibodies to FVIII. VWF is critical to this efficacy in the presence of inhibitors. Since FIX expression in platelets is effective for hemophilia B, efficacy in the presence of inhibitory antibodies to FIX was not achieved and emphasized the importance of VWF to the efficacy of platelet FVIII expression. These approaches have been studied in murine models but will need further study before this approach can be attempted clinically.

Ectopic platelet-delivered factor (F) VIII for the treatment of Hemophilia A: Plasma and platelet FVIII, is it all the same?

Hemophilia A is the most common inherited bleeding diathesis and is due to a deficiency of functional coagulation factor (F) VIII. Most patients have a severe deficiency and require a program of prophylactic plus acute infusions of recombinant FVIII to prevent significant joint and other target organ damage. One of the greatest challenges remaining in the care of these patients is that one fifth to third of the patients develop inhibitors to the infused proteins. While a significant portion of such inhibitors can be either overcome or the inhibitors eliminated, some patients with persistent and significant titers of inhibitors need to rely on second tier therapies that are not as effective at preventing significant bleeding morbidity or mortality. A number of groups have been developing therapeutic strategies for FVIII gene therapy for this disorder. Virtually all of these therapies have in common a rise in the plasma level of FVIII, and interpretation of their efficacy is straightforward related to levels achieved. However, several groups have also shown that FVIII can be ectopically expressed in developing megakaryocytes, where although plasma FVIII levels remain undetectable, this FVIII can be released and be effective at sites of platelet activation. Moreover, it is clear that this platelet (p) FVIII is protected to a degree from inhibitors, making pFVIII a particularly attractive strategy for gene therapy for hemophilia A. Yet at the same time, we have shown that pFVIII has a different availability and distribution in a growing thrombus than plasma FVIII. The clinical implications and challenges of these findings as murine and canine hemophilia A preclinical studies go forward with pFVIII are discussed.

Gene therapy for haemophilia: prospects and challenges to prevent or reverse inhibitor formation

Monogenic hereditary diseases, such as haemophilia A and B, are ideal targets for gene therapeutic approaches. While these diseases can be treated with protein therapeutics, such as factor VIII (FVIII) or IX (FIX), the notion that permanent transfer of the genes encoding these factors can cure haemophilia is very attractive. An underlying problem with a gene therapy approach, however, is the patient's immune response to the therapeutic protein (as well as to the transmission vector), leading to the formation of inhibitory antibodies. Even more daunting is reversing an existing immune response in patients with pre-existing inhibitors. In this review, we will describe the laboratory and clinical progress, and the challenges met thus far, in achieving the goal of gene therapy efficacy, with a focus on the goal of tolerance induction.

Gene therapy for haemophilia: a long and winding road

BACKGROUND

Cure, or improvement of disease phenotype, has been a long-term goal in the treatment of haemophilia. An obvious strategy for achieving this goal is the use of gene therapy.

OBJECTIVES

This paper summarises prior and current clinical trials of gene therapy for haemophilia A and B, and briefly describes additional strategies in pre-clinical development.

RESULTS AND CONCLUSIONS

Approximately 50 human subjects with severe haemophilia A or B have been enrolled in seven different trials of gene therapy. These have used plasmids, retroviral, adenoviral, and AAV vectors, directed to autologous fibroblasts, skeletal muscle, liver, and other target cell types accessed by intravenous injection of vector. Four separate trials have used AAV vectors, three of these targeting liver. Data from animal models suggest that several different gene replacement strategies may eventually yield long-term expression of factor at therapeutic levels, and that in situ correction of gene defects in hepatocytes may eventually be a therapeutic option.

Platelets as delivery systems for disease treatments

Platelets are small, anucleate, discoid shaped blood cells that play a fundamental role in hemostasis. Platelets contain a large number of biologically active molecules within cytoplasmic granules that are critical to normal platelet function. Because platelets circulate in blood through out the body, release biological molecules and mediators on demand and participate in hemostasis as well as many other pathophysiologic processes, targeting expression of proteins of interest to platelets and utilizing platelets as delivery systems for disease treatment would be a logical approach. This paper reviews the genetic therapy for inherited bleeding disorders utilizing platelets as delivery system, with a particular focus on platelet-derived FVIII for hemophilia A treatment.

Delivery of nucleic acid therapeutics by genetically engineered hematopoietic stem cells

Several populations of adult human stem cells have been identified, but only a few of these are in routine clinical use. The hematopoietic stem cell (HSC) is arguably the most well characterized and the most routinely transplanted adult stem cell. Although details regarding several aspects of this cell's phenotype are not well understood, transplant of HSCs has advanced to become the standard of care for the treatment of a range of monogenic diseases and several types of cancer. It has also proven to be an excellent target for genetic manipulation, and clinical trials have already demonstrated the usefulness of targeting this cell as a means of delivering nucleic acid therapeutics for the treatment of several previously incurable diseases. It is anticipated that additional clinical trials will soon follow, such as genetically engineering HSCs with vectors to treat monogenic diseases such as hemophilia A. In addition to the direct targeting of HSCs, induced pluripotent stem (iPS) cells have the potential to replace virtually any engineered stem cell therapeutic, including HSCs. We now know that for the broad use of genetically modified HSCs for the treatment of non-lethal diseases, e.g. hemophilia A, we must be able to regulate the introduction of nucleic acid sequences into these target cells. We can begin to refine transduction protocols to provide safer approaches to genetically manipulate HSCs and strategies are being developed to improve the overall safety of gene transfer. This review focuses on recent advances in the systemic delivery of nucleic acid therapeutics using genetically modified stem cells, specifically focusing on i) the use of retroviral vectors to genetically modify HSCs, ii) the expression of fVIII from hematopoietic stem cells for the treatment of hemophilia A, and iii) the use of genetically engineered hematopoietic cells generated from iPS cells as treatment for disorders of hematopoiesis.

In vivo efficacy of platelet-delivered, high specific activity factor VIII variants

Ectopically expressed, human B-domainless (hB) factor 8 (F8) in platelets improves hemostasis in hemophilia A mice in several injury models. However, in both a cuticular bleeding model and a cremaster laser arteriole/venule injury model, there were limitations to platelet-derived (p) hBF8 efficacy, including increased clot embolization. We now address whether variants of F8 with enhanced activity, inactivation resistant F8 (IR8) and canine (c) BF8, would improve clotting efficacy. In both transgenic and lentiviral murine model approaches, pIR8 expressed at comparable levels to phBF8, but pcBF8 expressed at only approximately 30%. Both variants were more effective than hBF8 in cuticular bleeding and FeCl(3) carotid artery models. However, in the cremaster injury model, only pcBF8 was more effective, markedly decreasing clot embolization. Because inhibitors of F8 are stored in platelet granules and IR8 is not protected by binding to von Willebrand factor, we also tested whether pIR8 was effective in the face of inhibitors and found that pIR8 is protected from the inhibitors. In summary, pF8 variants with high specific activity are more effective in controlling bleeding, but this improved efficacy was inconsistent between bleeding models, perhaps reflecting the underlying mechanism(s) for the increased specific activity of the studied F8 variants.

Functional aspects of factor VIII expression after transplantation of genetically-modified hematopoietic stem cells for hemophilia A

BACKGROUND

Major complications with respect to the development of gene therapy treatments for hemophilia A include low factor VIII (fVIII) expression and humoral immune responses resulting in inhibitory anti-fVIII antibodies. We previously achieved sustained curative fVIII activity levels in hemophilia A mice after nonmyeloablative transplantation of genetically-modified hematopoietic stem cells (HSCs) encoding a B-domain deleted porcine fVIII (BDDpfVIII) transgene with no evidence of an immune response.

METHODS

Mouse HSCs were transduced using MSCV-based recombinant virus encoding BDDpfVIII and transplanted into hemophilia A mice. Transplanted mice were followed for donor cell engraftment, fVIII expression and activity, and generation of anti-fVIII immune response.

RESULTS

We now show that: (i) the protein expressed by hematopoietic cells has a specific activity similar to that of purified protein; (ii) BDDpfVIII expressed from hematopoietic cells effectively induces thrombus formation, which is shown using a new method of in vivo analysis of fVIII function; (iii) naïve and pre-immunized mice receiving HSC gene therapy are nonresponsive to challenges with recombinant human fVIII; (iv) nonresponsiveness is not broken after stringent challenges with BDDpfVIII; and (v) T cells from these mice are unresponsive to BDDpfVIII presentation. Furthermore, stem cells isolated from donors with high titer anti-human fVIII antibodies show no defects in donor cell engraftment or the ability to express BDDpfVIII.

CONCLUSIONS

These results demonstrate that HSC gene therapy can be an effective alternative treatment for individuals with hemophilia A and may benefit patients by inducing immunological nonresponsiveness to fVIII replacement products.

A ferric chloride induced arterial injury model used as haemostatic effect model

A number of experimental bleeding models have been applied to animal models of haemophilia in order to evaluate the acute haemostatic effect of procoagulant compounds. In contrast, in vivo thrombosis models (including the FeCl(3) induced injury model) have mainly been used to study antithrombotic pharmacological intervention. However, as there are limitations to existing bleeding models and as new recombinant FVIII, FIX, and FVIIa variants with increased and prolonged activity are generated there is an increasing need for new and optimized in vivo animal models for testing the efficacy of these haemostatic drug candidates. This led us to look at existing thrombosis models in a new perspective. We have studied the effect of a FeCl(3) induced arterial injury in both F8-KO and F9-KO mice using optimized conditions where exposure to FeCl(3) induces occlusion within 4.2 +/- 0.2 min in wild type mice with a normal coagulation system. In contrast, no occlusion was observed in haemophilic mice providing a therapeutic window in the model making it suitable for pharmacological testing of therapeutic intervention. We demonstrate that replacement therapy with a clinical relevant dose of rFVIII (Advate 20-80 U kg(-1)) and rFIX [(0.75 mg kg(-1) BeneFIX) approximately 50 IU kg(-1)] restored coagulation and normalized the time to occlusion following FeCl(3) induced injury in F8-KO mice and restored coagulation and nearly normalized the time to occlusion in F9-KO mice. In conclusion, we have demonstrated that under optimized conditions the FeCl(3) induced arterial injury model provides a therapeutic window that makes it an useful effect model for evaluation of the haemostatic potential of procoagulant drugs.

Supplying clotting factors from hematopoietic stem cell-derived erythroid and megakaryocytic lineage cells

Systemically distributed proteins such as clotting factors have been traditionally expressed from muscle or liver to achieve therapeutic, long-term expression. As long-lived cell capable of generating an abundant progeny, hematopoietic stem cells (HSCs) also merit consideration for this purpose. To be clinically relevant, this approach would require that hematopoietic cells be capable of expressing high levels of functional, secreted proteins, that the risk of insertional oncogenesis be minimized, and that sufficient stem cell engraftment be achieved following minimally invasive conditioning. Recent reports demonstrate the feasibility of expressing either factor IX (FIX) or factor VIII (FVIII) in erythroblasts and platelets using lineage-restricted vectors, resulting in effective treatments in mouse models of hemophilia. The erythroid system is especially powerful in providing high protein output, yielding FIX levels approaching 1 micro g/ml per vector copy in the plasma of long-term hematopoietic chimeras, a secretion level that vastly outperforms any other current mammalian constitutive or long-terminal repeat (LTR)-driven vector system. These early but promising studies raise the prospect of further developing these strategies for clinical investigation.

Protein replacement therapy and gene transfer in canine models of hemophilia A, hemophilia B, von willebrand disease, and factor VII deficiency

Dogs with hemophilia A, hemophilia B, von Willebrand disease (VWD), and factor VII deficiency faithfully recapitulate the severe bleeding phenotype that occurs in humans with these disorders. The first rational approach to diagnosing these bleeding disorders became possible with the development of reliable assays in the 1940s through research that used these dogs. For the next 60 years, treatment consisted of replacement of the associated missing or dysfunctional protein, first with plasma-derived products and subsequently with recombinant products. Research has consistently shown that replacement products that are safe and efficacious in these dogs prove to be safe and efficacious in humans. But these highly effective products require repeated administration and are limited in supply and expensive; in addition, plasma-derived products have transmitted bloodborne pathogens. Recombinant proteins have all but eliminated inadvertent transmission of bloodborne pathogens, but the other limitations persist. Thus, gene therapy is an attractive alternative strategy in these monogenic disorders and has been actively pursued since the early 1990s. To date, several modalities of gene transfer in canine hemophilia have proven to be safe, produced easily detectable levels of transgene products in plasma that have persisted for years in association with reduced bleeding, and correctly predicted the vector dose required in a human hemophilia B liver-based trial. Very recently, however, researchers have identified an immune response to adeno-associated viral gene transfer vector capsid proteins in a human liver-based trial that was not present in preclinical testing in rodents, dogs, or nonhuman primates. This article provides a review of the strengths and limitations of canine hemophilia, VWD, and factor VII deficiency models and of their historical and current role in the development of improved therapy for humans with these inherited bleeding disorders.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

Erythroid-specific human factor IX delivery from in vivo selected hematopoietic stem cells following nonmyeloablative conditioning in hemophilia B mice

We have developed a lentiviral vector system for human factor IX (hFIX) gene transfer in hematopoietic stem cells (HSCs) that provides erythroid cell-derived systemic protein delivery following nonmyeloablative conditioning and in vivo methylguanine methyltransferase (MGMT) drug selection. After bone marrow transplantation under moderate Busulfan conditioning, the initial hFIX expression in the chimeras was minimally detectable. However, the hFIX levels rose sharply following in vivo MGMT-drug selection and eventually reached a level that is considered curative in hemophilia B therapy (>500 ng/ml). The rise of hFIX levels was proportional to the increase in vector copy (VC) number in peripheral blood cells. High levels of hFIX expression were maintained in serially engrafted mice chimeras for 18 months. Importantly, high-level hFIX expression by erythroid cells did not result in anemia or adversely affect red blood cell counts. The prospect of combining reduced intensity conditioning, a presumably lowered risk of insertional mutagenesis due to low VC number requirement and erythroid-restricted transgene expression, as well as long-term protein expression at high level, strongly supports the potential applicability of adult stem cell-based gene therapy in nonlethal blood or metabolic disorders, as demonstrated here for hemophilia.

Analysis of the spatial and temporal characteristics of platelet-delivered factor VIII-based clots

Normally factor (F) VIII is not expressed in megakaryocytes, but when human FVIII was transgenically expressed in murine megakaryocytes, it was stored in platelet alpha-granules and released at sites of injury. This platelet FVIII (pFVIII) is effective in correcting hemostasis, even in the presence of circulating inhibitors, so it offers a potential gene therapy strategy for hemophilia A. To understand clot development by pFVIII, we have examined clot response to laser injury in both cremaster arterioles and venules in FVIII(null) mice either infused with FVIII or transgenic for pFVIII. In both sets of vessels, pFVIII is at least as effective as infused FVIII. However, there are temporal and spatial differences in fibrin and platelet accumulation within clots depending on how FVIII is delivered. These differences may be related to the temporal and spatial distribution of the alpha-granular-released FVIII within the developing clot, and may explain the increased frequency and size of embolic events seen with pFVIII. These observations may not only have implications for the use of pFVIII in gene therapy for hemophilia A, but may also have physiologic consequences, explaining why many procoagulant factors are delivered both in the plasma and in platelet alpha-granules.