Expression of human factor VIII under control of the platelet-specific alphaIIb promoter in megakaryocytic cell line as well as storage together with VWF

Platelet-targeted gene therapy induces immune tolerance in hemophilia and beyond

Blood platelets have unique storage and delivery capabilities. Platelets play fundamental roles in hemostasis, inflammatory reactions, and immune responses. Beyond their functions, platelets have been used as a target for gene therapy. Platelet-targeted gene therapy aims to deliver a sustained expression of neo-protein in vivo by genetically modifying the target cells, resulting in a cure for the disease. Even though there has been substantial progress in the field of gene therapy, the potential development of immune responses to transgene products or vectors remains a significant concern. Of note, multiple preclinical studies using platelet-specific lentiviral gene delivery to hematopoietic stem cells in hemophilia have demonstrated promising results with therapeutic levels of neo-protein that rescue the hemorrhagic bleeding phenotype and induce antigen-specific immune tolerance. Further studies using ovalbumin as a surrogate protein for platelet gene therapy have shown robust antigen-specific immune tolerance induced via peripheral clonal deletions of antigen-specific CD4- and CD8-T effector cells and induction of antigen-specific regulatory T (Treg) cells. This review discusses platelet-targeted gene therapy, focusing on immune tolerance induction.

DMAG, a novel countermeasure for the treatment of thrombocytopenia

Background

Thrombocytopenia is one of the most common hematological disease that can be life-threatening caused by bleeding complications. However, the treatment options for thrombocytopenia remain limited.

Methods

In this study, giemsa staining, phalloidin staining, immunofluorescence and flow cytometry were used to identify the effects of 3,3ʹ-di-O-methylellagic acid 4ʹ-glucoside (DMAG), a natural ellagic acid derived from Sanguisorba officinalis L. (SOL) on megakaryocyte differentiation in HEL cells. Then, thrombocytopenia mice model was constructed by X-ray irradiation to evaluate the therapeutic action of DMAG on thrombocytopenia. Furthermore, the effects of DMAG on platelet function were evaluated by tail bleeding time, platelet aggregation and platelet adhesion assays. Next, network pharmacology approaches were carried out to identify the targets of DMAG. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to elucidate the underling mechanism of DMAG against thrombocytopenia. Finally, molecular docking simulation, molecular dynamics simulation and western blot analysis were used to explore the relationship between DAMG with its targets.

Results

DMAG significantly promoted megakaryocyte differentiation of HEL cells. DMAG administration accelerated platelet recovery and megakaryopoiesis, shortened tail bleeding time, strengthened platelet aggregation and adhesion in thrombocytopenia mice. Network pharmacology revealed that ITGA2B, ITGB3, VWF, PLEK, TLR2, BCL2, BCL2L1 and TNF were the core targets of DMAG. GO and KEGG pathway enrichment analyses suggested that the core targets of DMAG were enriched in PI3K–Akt signaling pathway, hematopoietic cell lineage, ECM-receptor interaction and platelet activation. Molecular docking simulation and molecular dynamics simulation further indicated that ITGA2B, ITGB3, PLEK and TLR2 displayed strong binding ability with DMAG. Finally, western blot analysis evidenced that DMAG up-regulated the expression of ITGA2B, ITGB3, VWF, p-Akt and PLEK.

Conclusion

DMAG plays a critical role in promoting megakaryocytes differentiation and platelets production and might be a promising medicine for the treatment of thrombocytopenia.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-021-00404-1.

Generation of HLA Universal Megakaryocytes and Platelets by Genetic Engineering

Patelet transfusion refractoriness remains a relevant hurdle in the treatment of severe alloimmunized thrombocytopenic patients. Antibodies specific for the human leukocyte antigens (HLA) class I are considered the major immunological cause for PLT transfusion refractoriness. Due to the insufficient availability of HLA-matched PLTs, the development of new technologies is highly desirable to provide an adequate management of thrombocytopenia in immunized patients. Blood pharming is a promising strategy not only to generate an alternative to donor blood products, but it may offer the possibility to optimize the therapeutic effect of the produced blood cells by genetic modification. Recently, enormous technical advances in the field of in vitro production of megakaryocytes (MKs) and PLTs have been achieved by combining progresses made at different levels including identification of suitable cell sources, cell pharming technologies, bioreactors and application of genetic engineering tools. In particular, use of RNA interference, TALEN and CRISPR/Cas9 nucleases or nickases has allowed for the generation of HLA universal PLTs with the potential to survive under refractoriness conditions. Genetically engineered HLA-silenced MKs and PLTs were shown to be functional and to have the capability to survive cell- and antibody-mediated cytotoxicity using in vitro and in vivo models. This review is focused on the methods to generate in vitro genetically engineered MKs and PLTs with the capacity to evade allogeneic immune responses.

Characteristics of the Thrombus Formation in Transgenic Mice with Platelet-Targeted Factor VIII Expression

Platelet-targeted FVIII gene therapy can efficiently recover bleeding phenotype for hemophilia A (HA), yet characteristics of thrombus formation with this ectopic expression of factor VIII (FVIII) in platelets remain unclear. Here, we generated 2bF8^trans mice restrictively expressing human B-domain-deleted FVIII (hBDD FVIII) in platelets on a hemophilic (FVIII^null) mice background. The results showed no statistical difference in clot strength and stability between wild-type (WT) and 2bF8^trans mice, but with a prolonged reaction time (R-time), by thromboelastography. Fluid dynamics analysis showed that at the shear rates of 500 to 1,500 s^-1, where physiological hemostasis often develops, the thrombi formed in 2bF8^trans mice were more stable than those in FVIII^null mice, while at high pathological shear rates (2,500 s^-1), mimicking atherosclerosis, thrombus size and fibrin deposition in 2bF8^trans mice were less than those in WT mice. Thrombus morphology analysis showed that there was a locally concentrated deposition of fibrin in thrombus at the injured site and fibrin co-localized with activated platelets in 2bF8^trans mice. Moreover, a higher ratio of fibrin to platelets was found in thrombus from 2bF8^trans mice following laser-induced injury in cremaster arterioles, which might be the underlying mechanism of thrombus stability in 2bF8^trans mice at physiological arterial circumstance. These observations suggest that specific morphological features of the thrombi might contribute to the efficacy and safety of platelet-targeted FVIII gene therapy for HA.

Activated factor X targeted stored in platelets as an effective gene therapy strategy for both hemophilia A and B

BACKGROUND

Treatment of hemophiliacs with inhibitors remains challenging, and new treatments are in urgent need. Coagulation factor X plays a critical role in the downstream of blood coagulation cascade, which could serve as a bypassing agent for hemophilia therapy. Base on platelet-targeted gene therapy for hemophilia by our and other groups, we hypothesized that activated factor X (FXa) targeted stored in platelets might be effective in treating hemophilia A (HA) and B (HB) with or without inhibitors.

METHODS

To achieve the storage of FXa in platelets, we constructed a FXa precursor and used the integrin αIIb promoter to control the targeted expression of FXa precursor in platelets. The expression cassette (2bFXa) was carried by lentivirus and introduced into mouse hematopoietic stem and progenitor cells (HSPCs), which were then transplanted into HA and HB mice. FXa expression and storage in platelets was examined in vitro and in vivo. We evaluated the therapeutic efficacy of platelet-stored FXa by tail bleeding assays and the thrombelastography. In addition, thrombotic risk was assessed in the recipient mice and the lipopolysaccharide induced inflammation mice.

RESULTS

By transplanting 2bFXa lentivirus-transduced HSPCs into HA and HB mice, FXa was observed stably stored in platelet α-granules, the stored FXa is releasable and functional upon platelet activation. The platelet-stored FXa can significantly ameliorate bleeding phenotype in HA and HB mice as well as the mice with inhibitors. Meanwhile, no FXa leakage in plasma and no signs of increased risk of hypercoagulability were found in transplantation recipients and lipopolysaccharide induced septicemia recipients.

CONCLUSIONS

Our proof-of-principle data indicated that target expression of the FXa precursor to platelets can generate a storage pool of FXa in platelet α-granules, the platelet-stored FXa is effective in treating HA and HB with inhibitors, suggesting that this could be a novel choice for hemophilia patients with inhibitors.

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.

Physiological Roles of the von Willebrand Factor-Factor VIII Interaction

Von Willebrand factor (VWF) and coagulation factor VIII (FVIII) circulate as a complex in plasma and have a major role in the hemostatic system. VWF has a dual role in hemostasis. It promotes platelet adhesion by anchoring the platelets to the subendothelial matrix of damaged vessels and it protects FVIII from proteolytic degradation. Moreover, VWF is an acute phase protein that has multiple roles in vascular inflammation and is massively secreted from Weibel-Palade bodies upon endothelial cell activation. Activated FVIII on the other hand, together with coagulation factor IX forms the tenase complex, an essential feature of the propagation phase of coagulation on the surface of activated platelets. VWF deficiency, either quantitative or qualitative, results in von Willebrand disease (VWD), the most common bleeding disorder. The deficiency of FVIII is responsible for Hemophilia A, an X-linked bleeding disorder. Here, we provide an overview on the role of the VWF-FVIII interaction in vascular physiology.

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.

Production and characterization of polyclonal antibodies to human recombinant domain B-free antihemophilic factor VIII

Moroctocog alpha is a human B-domain deleted recombinant factor VIII (BDDrFVIII), which represents a new generation of pure antihemophilic products. We describe here an optimized procedure for polyclonal anti-FVIII-antibody production with the use of BDDrFVIII as an immunogen. The main immunochemical characteristics of the produced antibodies and their potential biomedical applications are also reported. Rabbits were immunized with BDDrFVIII as an emulsion with Freund's adjuvant or with antigen immobilized in polyacrylamide gel (PAAG). Antibody titers in immune sera were assayed by enzyme-linked immunosorbent assay (ELISA). IgG purification was performed by afine chromatography on protein A-sepharose. Immune sera and IgG were tested by immunoblotting with the use of human plasma of healthy donors and people with hemophilia A, platelet lysates, and commercial plasma-derived concentrates as sources of FVIII-related antigens. FVIII-producing human umbilical vein cells were processed for immunocytochemical staining with the use of purified anti-FVIII-antibodies. Immunization of rabbits with PAAG-trapped antigen induced more potent immune response compared to the standard immunization procedure with Freund's adjuvant. The lowest working amount of immune IgG, measured by ELISA, was ~50 ng. Immunoblotting demonstrated that anti-BDDrFVIII antibodies effectively recognize the whole FVIII molecule (320 kDa), as well as different truncated polypeptides thereof, and are suitable for immunocytochemical analysis of FVIII-producing cells. An optimized procedure for the production of polyclonal antibodies against FVIII with the use of PAAG-immobilized BDDrFVIII (moroctocog alpha) was proposed and successfully validated. The produced antibodies are suitable for detecting and measuring FVIII-related antigens and may have various biomedical applications.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Platelet-targeted gene therapy with human factor VIII establishes haemostasis in dogs with haemophilia A

It is essential to improve therapies for controlling excessive bleeding in patients with haemorrhagic disorders. As activated blood platelets mediate the primary response to vascular injury, we hypothesize that storage of coagulation Factor VIII within platelets may provide a locally inducible treatment to maintain haemostasis for haemophilia A. Here we show that haematopoietic stem cell gene therapy can prevent the occurrence of severe bleeding episodes in dogs with haemophilia A for at least 2.5 years after transplantation. We employ a clinically relevant strategy based on a lentiviral vector encoding the ITGA2B gene promoter, which drives platelet-specific expression of human FVIII permitting storage and release of FVIII from activated platelets. One animal receives a hybrid molecule of FVIII fused to the von Willebrand Factor propeptide-D2 domain that traffics FVIII more effectively into α-granules. The absence of inhibitory antibodies to platelet-derived FVIII indicates that this approach may have benefit in patients who reject FVIII replacement therapies. Thus, platelet FVIII may provide effective long-term control of bleeding in patients with haemophilia A.

Lentivirus-mediated platelet gene therapy of murine hemophilia A with pre-existing anti-factor VIII immunity

BACKGROUND

The development of inhibitory antibodies, referred to as inhibitors, against exogenous factor VIII in a significant subset of patients with hemophilia A remains a persistent challenge to the efficacy of protein replacement therapy. Our previous studies using the transgenic approach provided proof-of-principle that platelet-specific expression could be successful in treating hemophilia A in the presence of inhibitory antibodies.

OBJECTIVE

To investigate a clinically translatable approach for platelet gene therapy of hemophilia A with pre-existing inhibitors.

METHODS

Platelet FVIII expression in preimmunized FVIII(null) mice was introduced by transplantation of lentivirus-transduced bone marrow or enriched hematopoietic stem cells. FVIII expression was determined with a chromogenic assay. The transgene copy number per cell was quantitated with real-time PCR. Inhibitor titer was measured with the Bethesda assay. Phenotypic correction was assessed by the tail clipping assay and an electrolytically induced venous injury model. Integration sites were analyzed with linear amplification-mediated PCR.

RESULTS

Therapeutic levels of platelet FVIII expression were sustained in the long term without evoking an anti-FVIII memory response in the transduced preimmunized recipients. The tail clip survival test and the electrolytic injury model confirmed that hemostasis was improved in the treated animals. Sequential bone marrow transplants showed sustained platelet FVIII expression resulting in phenotypic correction in preimmunized secondary and tertiary recipients.

CONCLUSIONS

Lentivirus-mediated platelet-specific gene transfer improves hemostasis in mice with hemophilia A with pre-existing inhibitors, indicating that this approach may be a promising strategy for gene therapy of hemophilia A even in the high-risk setting of pre-existing inhibitory antibodies.

SK-HEP cells and lentiviral vector for production of human recombinant factor VIII

Hemophilia A is caused by a deficiency in coagulation factor VIII. Recombinant factor VIII can be used as an alternative although it is unavailable for most patients. Here, we describe the production of a human recombinant B-domain-deleted FVIII (rBDDFVIII) by the human cell line SK-HEP-1, modified by a lentiviral vector rBDDFVIII was produced by recombinant SK-HEP cells (rSK-HEP) at 1.5-2.1 IU/10(6) in 24 h. The recombinant factor had increased in vitro stability when compared to commercial pdFVIII. The functionality of rBDDFVIII was shown by its biological activity and by tail-clip challenge in hemophilia A mice. The rSK-HEP cells grew in a scalable system and produced active rBDDFVIII, indicating that this platform production can be optimized to meet the commercial production scale needs.

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.

Correction of murine Bernard-Soulier syndrome by lentivirus-mediated gene therapy

Bernard-Soulier syndrome (BSS) is an inherited bleeding disorder caused by a defect in the platelet glycoprotein (GP) Ib-IX-V complex. The main treatment for BSS is platelet transfusion but it is often limited to severe bleeding episodes or surgical interventions due to the risk of alloimmunization. We have previously reported successful expression of human GPIbα (hGPIbα) in human megakaryocytes using a lentiviral vector (LV) encoding human GP1BA under control of the platelet-specific integrin αIIb promoter (2bIbα). In this study, we examined the efficacy of this strategy for the gene therapy of BSS using GPIbα(null) as a murine model of BSS. GPIbα(null) hematopoietic stem cells (HSC) transduced with 2bIbα LV were transplanted into lethally irradiated GPIbα(null) littermates. Therapeutic levels of hGPIbα expression were achieved that corrected the tail bleeding time and improved the macrothrombocytopenia. Sequential bone marrow (BM) transplants showed sustained expression of hGPIbα with similar phenotypic correction. Antibody response to hGPIbα was documented in 1 of 17 total recipient mice but was tolerated without any further treatment. These results demonstrate that lentivirus-mediated gene transfer can provide sustained phenotypic correction of murine BSS, indicating that this approach may be a promising strategy for gene therapy of BSS patients.

Thrombin generation assay using factor IXa as a trigger to quantify accurately factor VIII levels in haemophilia A

BACKGROUND

The available methods for measuring factor VIII (FVIII) activity suffer reportedly from lack of sensitivity and precision in the < 1 IU dL(-1) range. This precludes correlation of clinical phenotype with FVIII levels.

OBJECTIVES

To study a possible association between clinical phenotype in patients with FVIII levels < 1 IU dL(-1).

METHODS/RESULTS

The FIXa-driven FVIII assay (FVIII-CAT) has a detection limit of 0.05 IU dL(-1). For the range of 0-2 IU dL(-1) FVIII, the intra-assay coefficient of variation (CV) is around 2% and the inter-assay CV is about 8%. We tested 30 hemophiliacs with FVIII:C between < 1 and 6 IU dL(-1) as measured in the one-stage clotting assay using the FVIII-CAT assay. For genetic defects related to moderate hemophilia, the FVIII-CAT test finds FVIII levels that are in good agreement with those determined with the one-stage assay. Of the 21 hemophilic patients with FVIII < 1 IU dL(-1), four patients exhibited a mild bleeding phenotype. When we applied TF-initiated thrombin generation, patients with a mild clinical phenotype showed significantly higher endogenous thrombin potentials.

CONCLUSION

The novel developed FVIII assay measures accurately FVIII levels below 1 IU dL(-1). Its application demonstrated that the clinical heterogeneity in individuals with < 1 IU dL(-1) FVIII is not associated with their FVIII level.

Gene therapy strategies for hemophilia: benefits versus risks

Hemophilia is an inherited bleeding disorder caused by a deficiency of functional clotting factors VIII or IX in the blood plasma. The drawbacks of the classical protein substitution therapy fueled interest in alternative treatments by gene therapy. Hemophilia has been recognized as an ideal target disease for gene therapy because a relatively modest increase in clotting factor levels can result in a significant therapeutic benefit. Consequently, introducing a functional FVIII or FIX gene copy into the appropriate target cells could ultimately provide a cure for hemophilic patients. Several cell types have been explored for hemophilia gene therapy, including hepatocytes, muscle, endothelial and hematopoietic cells. Both nonviral and viral vectors have been considered for the development of hemophilia gene therapy, including transposons, γ-retroviral, lentiviral, adenoviral and adeno-associated viral vectors. Several of these strategies have resulted in stable correction of the bleeding diathesis in hemophilia A and B murine as well as canine models, paving the way towards clinical trials. Although clotting factor expression has been detected in hemophilic patients treated by gene therapy, the challenge now lies in obtaining prolonged therapeutic FVIII or FIX levels in these patients. This review highlights the benefits and potential risks of the different gene therapy strategies for hemophilia that have been developed.

Unique strategies for therapeutic gene transfer in haemophilia A and haemophilia BWFH State-of-the-Art Session on Therapeutic Gene Transfer Buenos Aires, Argentina

SUMMARY

Gene therapy of haemophilia has been initiated through a number of approaches including expression in muscle, liver and omental implanted fibroblasts, or i.v. injection of an expression construct under the control of a ubiquitous promoter. In all these approaches, the goal was to have factor VIII (FVIII) or factor IX (FIX) synthesized so that it restored the levels of the missing protein in blood. The three talks in this session are totally, or at least in part, directed at strategies that may be clinically effective even in the absence of correction of the missing plasma clotting factor, although the haematopoietic stem cell or blood outgrowth endothelial cell therapy could achieve plasma correction as well. Two of the approaches achieve localized coagulation factor expression without necessarily correcting the systemic defect--one is with synthesis of FVIII or FIX within the joint space and the other is with the local release of FVIII (or FIX) by platelets at the site of vascular injury. All of the three approaches have demonstrated efficacy in small animal models and are now the subject of larger animal studies. None has yet to progress to human trials.

Targeting FVIII expression to endothelial cells regenerates a releasable pool of FVIII and restores hemostasis in a mouse model of hemophilia A

The natural cell type(s) that synthesize and release factor VIII (FVIII) into the circulation are still not known with certainty. In vitro studies indicate that artificial expression of FVIII in endothelial cells produces an intracellular pool of FVIII that can be mobilized together with its carrier protein, von Willebrand factor (VWF), by agonists. Here, we show that expression of human B-domain deleted FVIII (hFVIII) in the vascular endothelium of otherwise FVIII-deficient mice results in costorage of FVIII and VWF in endothelial Weibel-Palade bodies and restores normal levels and activity of FVIII in plasma. Stored FVIII was mobilized into the circulation by subcutaneous administration of epinephrine. Human FVIII activity in plasma was strictly dependent on the presence of VWF. Endothelial-specific expression of hFVIII rescued the bleeding diathesis of hemophilic mice lacking endogenous FVIII. This hemostatic function of endothelial cell-derived hFVIII was suppressed in the presence of anti-FVIII inhibitory antibodies. These results suggest that targeting FVIII expression to endothelial cells may establish a releasable pool of FVIII and normalize plasma FVIII level and activity in hemophilia A, but does not prevent the inhibitory effect of anti-FVIII antibodies on the hemostatic function of transgene-derived hFVIII as is seen with platelet-derived FVIII expression.

Factor IX ectopically expressed in platelets can be stored in alpha-granules and corrects the phenotype of hemophilia B mice

We developed 2bF9 transgenic mice in a hemophilia B mouse model with the expression of human factor IX (FIX) under control of the platelet-specific integrin alphaIIb promoter, to determine whether ectopically expressing FIX in megakaryocytes can enable the storage of FIX in platelet alpha-granules and corrects the murine hemophilia B phenotype. FIX was detected in the platelets and plasma of 2bF9 transgenic mice by both antigen and activity assays. Approximately 90% of total FIX in blood was stored in platelets, most of which is releasable on activation of platelets. Immunostaining demonstrated that FIX was expressed in platelets and megakaryocytes and stored in alpha-granules. All 2bF9 transgenic mice survived tail clipping, suggesting that platelet-derived FIX normalizes hemostasis in the hemophilia B mouse model. This protection can be transferred by bone marrow transplantation or platelet transfusion. However, unlike our experience with platelet FVIII, the efficacy of platelet-derived FIX was limited in the presence of anti-FIX inhibitory antibodies. These results demonstrate that releasable FIX can be expressed and stored in platelet alpha-granules and that platelet-derived FIX can correct the bleeding phenotype in hemophilia B mice. Our studies suggest that targeting FIX expression to platelets could be a new gene therapy strategy for hemophilia B.

Alternative strategies for gene therapy of hemophilia

Hemophilia A and B are monogenic disorders that were felt to be ideal targets for initiation of gene therapy. Although the first hemophilia gene therapy trial has been over 10 years ago, few trials are currently actively recruiting. Although preclinical studies in animals were promising, levels achieved in humans did not achieve long-term expression at adequate levels to achieve cures. Transplantation as a source of cellular replacement therapy for both hemophilia A and B have been successful following liver transplantation in which the recipient produces normal levels of either factor VIII (FVIII) or factor IX (FIX). Most of these transplants have been conducted for the treatment of liver failure rather than for "curing" hemophilia. There are a variety of new strategies for delivering the missing clotting factor through ectopic expression of the deficient protein. One approach uses hematopoietic stem cells using either a nonspecific promoter or using a lineage-specific promoter. An alternative strategy includes enhanced expression in endothelial cells or blood-outgrowth endothelial cells. An additional approach includes the expression of FVIII or FIX intraarticularly to mitigate the intraarticular bleeding that causes much of the disability for hemophilia patients. Because activated factor VII (FVIIa) can be used to treat patients with inhibitory antibodies to replacement clotting factors, preclinical gene therapy has been performed using platelet- or liver-targeted FVIIa expression. All of these newer approaches are just beginning to be explored in large animal models. Whereas improved recombinant replacement products continue to be the hallmark of hemophilia therapy, the frequency of replacement therapy is beginning to be addressed through longer-acting replacement products. A safe cure of hemophilia is still the desired goal, but many barriers must still be overcome.

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.

Methods for genetic modification of megakaryocytes and platelets

During recent decades there have been major advances in the fields of thrombosis and haemostasis, in part through development of powerful molecular and genetic technologies. Nevertheless, genetic modification of megakaryocytes and generation of mutant platelets in vitro remains a highly specialized area of research. Developments are hampered by the low frequency of megakaryocytes and their progenitors, a poor efficiency of transfection and a lack of understanding with regard to the mechanism by which megakaryocytes release platelets. Current methods used in the generation of genetically modified megakaryocytes and platelets include mutant mouse models, cell line studies and use of viruses to transform primary megakaryocytes or haematopoietic precursor cells. This review summarizes the advantages, limitations and technical challenges of such methods, with a particular focus on recent successes and advances in this rapidly progressing field including the potential for use in gene therapy for treatment of patients with platelet disorders.

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.

Intracellular cotrafficking of factor VIII and von Willebrand factor type 2N variants to storage organelles

Weibel-Palade bodies (WPBs) are the endothelial storage organelles that are formed upon von Willebrand factor (VWF) expression. Apart from VWF, WPBs contain a variety of hemostatic and inflammatory proteins. Some of these are thought to be targeted to WPBs by directly interacting with VWF in the secretory pathway. Previous studies have demonstrated that coexpression of factor VIII (FVIII) with VWF results in costorage of both proteins. However, whether cotrafficking is driven by intracellular FVIII-VWF assembly has remained unclear. We now have addressed this issue using recombinant VWF type 2N variants that are known to display reduced FVIII binding in the circulation. Binding studies using purified fluorescent FVIII and VWF type 2N variants revealed FVIII binding defects varying from moderate (Arg854Gln, Cys1060Arg) to severe (Arg763Gly, Thr791Met, Arg816Trp). Upon expression in HEK293 cells, all VWF variants induced formation of WPB-like organelles that were able to recruit P-selectin, as well as FVIII. WPBs containing FVIII did not display their typical elongated shape, suggesting that FVIII affects the organization of VWF tubules therein. The finding that VWF type 2N variants are still capable of cotargeting FVIII to storage granules implies that trafficking of WPB cargo proteins does not necessarily require high-affinity assembly with VWF.

Lentivirus-mediated platelet-derived factor VIII gene therapy in murine haemophilia A

BACKGROUND

Previous studies from our laboratory have demonstrated that lineage-targeted synthesis of factor VIII (FVIII) under the direction of the platelet-specific integrin alphaIIb gene promoter (2bF8) can correct the murine haemophilia A phenotype even in the presence of high titer inhibitory antibodies in a transgenic mouse model.

OBJECTIVE

In this study, we assessed the efficacy of using a genetic therapy approach to correct haemophilia A in FVIII-deficient (FVIII(null)) mice by transplantation of bone marrow (BM) transduced with a lentivirus (LV)-based gene transfer cassette encoding 2bF8.

RESULTS

Functional FVIII activity (FVIII:C) was detected in platelet lysates from treated mice and the levels were similar to 2bF8 heterozygous transgenic mice. Mice transplanted with 2bF8 LV-transduced BM survived tail clipping and we did not detected inhibitory or non-inhibitory FVIII antibodies over the period of this study (11 months). Furthermore, BM transferred from the primary transplant recipients into FVIII(null) secondary recipients demonstrated sustained platelet-FVIII expression leading to correction of the haemophilia A phenotype showing that gene transfer occurred within long-term repopulating haematopoietic stem cells.

CONCLUSIONS

These results demonstrate that ectopic expression of FVIII in platelets by lentivirus-mediated bone marrow transduction/transplantation may be a promising strategy for gene therapy of haemophilia A in humans.

Factor VIII ectopically targeted to platelets is therapeutic in hemophilia A with high-titer inhibitory antibodies

Inhibitory immune response to exogenously infused factor VIII (FVIII) is a major complication in the treatment of hemophilia A. Generation of such inhibitors has the potential to disrupt gene therapy for hemophilia A. We explore what we believe to be a novel approach to overcome this shortcoming. Human B-domain-deleted FVIII (hBDDFVIII) was expressed under the control of the platelet-specific alphaIIb promoter in platelets of hemophilic (FVIIInull) mice to create 2bF8trans mice. The FVIII transgene product was stored in platelets and released at the site of platelet activation. In spite of the lack of FVIII in the plasma of 2bF8trans mice, the bleeding phenotype of FVIIInull mice was corrected. More importantly, the bleeding phenotype was corrected in the presence of high inhibitory antibody titers introduced into the mice by infusion or by spleen cell transfer from recombinant hBDDFVIII-immunized mice. Our results demonstrate that this approach to the targeted expression of FVIII in platelets has the potential to correct hemophilia A, even in the presence of inhibitory immune responses to infused FVIII.

Regulated release of VWF and FVIII and the biologic implications

von Willebrand factor (VWF) performs a critical function in platelet binding at the site of vascular injury and also serves as the carrier protein for coagulation factor FVIII (FVIII), protecting it from proteolytic degradation in plasma. Both proteins undergo rapid, regulated release in response to DDAVP administration in patients with mild hemophilia A or von Wille-brand disease. Here, we attempt to summarize our current understanding of the establishment of the regulated storage pool of VWF and FVIII. The data presented indicate that regulated secretion of both proteins occurs only if there is endogenous synthesis of FVIII together with VWF.

Biosynthesis of FVIII in megakaryocytic cells: improved production and biochemical characterization

Haemophilia A is an attractive target for gene therapy. We designed a haemophilia A gene therapy strategy involving the genetic modification of haematopoietic stem cells to achieve tissue-specific expression of a factor VIII (FVIII) transgene in the megakaryocytic lineage. Platelets would then serve as vehicles to store the expressed FVIII and deliver the coagulation factor at the site of vascular injury. A local correction of the haemostasis defect could, therefore, be expected following platelet activation and secretion. In this study, we demonstrated that a model of haematopoietic cell lines (Dami cells) could produce a correctly processed FVIII. FVIII transgenes were placed under the control of the human platelet glycoprotein IIb (GPIIb) promoter and used for stable transfection of the Dami megakaryocytic cell line. The highest FVIII production was obtained when the FVIII transgene contained a factor IX intron 1 gene sequence inserted in the FVIII intron 1 and 13 sites. Reverse transcription polymerase chain reaction demonstrated that the splicing of these introns was complete. Recombinant FVIII (rFVIII) produced in Dami cells was a biologically active molecule (specific activity: 5664 IU/mg) that was correctly glycosylated and sulphated. This recombinant FVIII protein exhibited biochemical characteristics after deglycosylation or thrombin activation that were comparable to a commercially available B-domainless rFVIII. These results demonstrate the advantages of a modified FVIII transgene and represent the first biochemical characterization of megakaryocyte-produced FVIII.

Targeting platelet GPIbalpha transgene expression to human megakaryocytes and forming a complete complex with endogenous GPIbbeta and GPIX

Bernard-Soulier Syndrome (BSS) is a severe congenital platelet disorder that results from a deficiency of the platelet membrane glycoprotein (GP) Ib/IX complex that is composed of four subunits (GPIbalpha, GPIbbeta, GPIX, and GPV). Mutations in either GPIbalpha, GPIbbeta, or GPIX can result in BSS with many of the known mutations occurring in GPIbalpha. In this study, we have developed a gene therapy strategy to express hemagglutinin (HA)-tagged GPIbalpha in megakaryocytes and potentially correct a hereditary deficiency. To direct GPIbalpha expression in megakaryocytic lineage cells, we designed a GPIbalpha cassette where human GPIbalpha cDNA was placed under control of the megakaryocytic/platelet-specific alphaIIb promoter and inserted into a lentiviral vector. Human CD34+ peripheral blood cells (PBC) and Dami cells were transduced with alphaIIb-HA-GPIbalpha-WPT virus. Flow cytometry analysis demonstrated that 50.1% of the megakaryocytes derived from CD34+ stem cells and 97.3% of Dami cells were transduced and expressed transgene GPIbalpha protein. Immunoprecipitation with Western blot analysis demonstrated that transgene protein associated with endogenous GPIbbeta and GPIX proteins. To address further the lineage-specific expression of the alphaIIb-HA-GPIbalpha construct, three cell lines, Dami, AtT-20 and HepG2, were transfected with GPIbalpha expression plasmids and analyzed by confocal microscopy. The results demonstrated that among these three cell lines, the tissue-specific alphaIIb promoter was active only in Dami cells. Thus, GPIbalpha can be efficiently and specifically expressed in the megakaryocytic compartment of hematopoietic cells and the transgene product associates with endogenous GPIbbeta and GPIX forming a complete complex. This strategy could potentially be utilized for gene therapy of BSS.

How to roll an endothelial cigar: the biogenesis of Weibel-Palade bodies

Weibel-Palade bodies (WPB) are the regulated secretory organelles of endothelial cells. These cigar-shaped membrane-bound structures function in both hemostasis and inflammation but their biogenesis is poorly understood. Here, we review what is currently known about their formation. The content of WPBs is dominated by the hemostatic factor von Willebrand factor (VWF), whose complex biogenesis ends in the formation of high molecular weight multimers. VWF is also organized into proteinaceous tubules which underlie the striated interior of WPBs as seen in the EM. VWF expression is necessary for formation of WPBs, and its heterologous expression can even lead to the specific recruitment of WPB membrane proteins, including the leukocyte receptor P-selectin, the tetraspanin CD63, and Rab27a. Unusually, the VWF propeptide is implicated in the biogenesis of WPBs, being essential for formation of the storage compartment. The elongation of the cigars and the formation of the tubules are determined by non-covalent interactions between pro- and mature VWF proteins. Surprisingly, high molecular weight multimers seem neither necessary nor sufficient to trigger formation of a storage compartment, and do not seem to have any role in WPB biogenesis. Von Willebrand's disease, usually caused by mutations within VWF, has provided many of the insights into the way in which VWF drives the formation of these organelles.

Induction of megakaryocytes to synthesize and store a releasable pool of human factor VIII

von Willebrand factor (VWF) is a complex plasma glycoprotein that modulates platelet adhesion at the site of a vascular injury, and it also serves as a carrier protein for factor (F)VIII. As megakaryocytes are the only hematopoietic lineage to naturally synthesize and store VWF within alpha-granules, this study was performed to determine if expression of a FVIII transgene in megakaryocytes could lead to trafficking and storage of FVIII with VWF in platelet alpha-granules. Isolex selected CD34+ cells from human G-CSF mobilized peripheral blood cells (PBC) and murine bone marrow were transduced with a retrovirus encoding the B-domain deleted form of human FVIII (BDD-FVIII). Cells were then induced with cytokines to form a population of multiple lineages including megakaryocytes. Chromogenic analysis of culture supernatant from FVIII-transduced human cells demonstrated synthesis of functional FVIII. Treatment of cells with agonists of platelet activation (ADP, epinephrine, and thrombin receptor-activating peptide) resulted in the release of VWF antigen and active FVIII into the supernatant from transduced cells. Immunofluorescence analysis of cultured human and murine megakaryocytes revealed a punctate pattern of staining for FVIII that was consistent with staining for VWF. Electron microscopy of transduced megakaryocytes using immunogold-conjugated antibodies colocalized FVIII and VWF within the alpha-granules. FVIII retained its association with VWF in human platelets isolated from the peripheral blood of NOD/SCID mice at 2-6 weeks post-transplant of transduced human PBC. These results suggest feasibility for the development of a locally inducible secretory pool of FVIII in platelets of patients with hemophilia A.

Factor VIII ectopically expressed in platelets: efficacy in hemophilia A treatment

Activated platelets release their granule content in a concentrated fashion at sites of injury. We examined whether ectopically expressed factor VIII in developing megakaryocytes would be stored in alpha-granules and whether its release from circulating platelets would effectively ameliorate bleeding in a factor VIIInull mice model. Using the proximal glycoprotein 1b alpha promoter to drive expression of a human factor VIII cDNA construct, transgenic lines were established. One line had detectable human factor VIII that colocalizes with von Willebrand factor in platelets. These animals had platelet factor VIII levels equivalent to 3% to 9% plasma levels, although there was no concurrent plasma human factor VIII detectable. When crossed onto a factor VIIInull background, whole blood clotting time was partially corrected, equivalent to a 3% correction level. In a cuticular bleeding time study, these animals also had only a partial correction, but in an FeCl3 carotid artery, thrombosis assay correction was equivalent to a 50% to 100% level. These studies show that factor VIII can be expressed and stored in platelet alpha-granules. Our studies also suggest that platelet-released factor VIII is at least as potent as an equivalent plasma level and perhaps even more potent in an arterial thrombosis model.

Gene therapy for platelet disorders: studies with Glanzmann's thrombasthenia

Current research aimed at correcting platelet defects are designed to further our knowledge in the use of hematopoietic stem cells for gene therapies of hemorrhagic disorders. Information gained from these studies may be directly applicable to treatment of disorders affecting platelets (e.g. Glanzmann's thrombasthenia, Bernard Soulier syndrome, gray platelet syndrome, and von Willebrand disease) as well as other disorders affecting distinct hematopoietic cell lineages. This work specifically addresses three questions: (i) can bone marrow stem cells be given sufficient genetic information to induce abnormal megakaryocytes to synthesize transgene products that help newly formed platelets to participate in normal hemostasis? (ii) can the newly synthesized receptor be maintained as a platelet-specific protein at therapeutic levels for a reasonable period of time? and (iii) will newly expressed proteins be tolerated by the immune system or become a target for B- and T-cell mediated immunity resulting in the premature destruction and clearing of the genetically altered megakaryocytes and platelets? Answers to these questions should indicate the feasibility of targeting platelets with genetic therapies that will in turn enable better management of patients with inherited bleeding disorders. The long-range benefit of this research will be an improved understanding of the regulation of protein expression during normal megakaryocytopoiesis, and the accumulation of additional scientific knowledge about normal platelet function and the way in which platelets and other cells recognize and interact with each other.