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

Gene therapy for hemophilia - From basic science to first approvals of "one-and-done" therapies

Realistic paths to gene therapy for the X-linked bleeding disorder hemophilia started to materialize in the mid 1990s, resulting in disease correction in small and large animal models. Out of a diversity of approaches, in vivo adeno-associated viral (AAV) gene transfer to hepatocytes emerged as the most promising strategy, eventually forming the basis for multiple advanced clinical trials and regulatory approval of two products for the treatment of hemophilia B (coagulation factor IX deficiency) and one for hemophilia A (factor VIII deficiency). Ideally, gene therapy is effective with a single administration, thus providing therapeutic factor levels over a period of years, without the need for frequent injections. Overcoming multiple obstacles, some not predicted by preclinical studies, sustained partial to complete correction of coagulation for several years to an entire decade has now been documented in patients, with observation ongoing. A hyperactive form of FIX improved efficacy in hemophilia B, and superior engineered variants of FVIII are emerging. Nonetheless, challenges remain, including pre-existing immunity to AAV capsids, toxicities, inter-patient variability in response to treatment, and difficulty in obtaining durable therapeutic expression of FVIII. In alternative approaches, in vivo gene editing and ex vivo gene therapies targeting hemopoietic cells are in development.

Myeloid sarcoma with RBM15::MRTFA (MKL1) mimicking vascular neoplasm

Extramedullary involvement of acute myeloid leukemia (AML), aka myeloid sarcoma, is a rare phenomenon in acute megakaryoblastic leukemia with RBM15:: MRTFA(MKL1) fusion, which might mimic non-hematologic malignancies. A 7-month-old infant presented with leukocytosis, hepatosplenomegaly, multiple lymphadenopathies, and a solid mass in the right thigh. Initially, the patient was diagnosed with a malignant vascular tumor regarding the expression of vascular markers from the biopsy of the right thigh lesion that was performed after the inconclusive bone marrow biopsy. The second bone marrow biopsy, which was performed due to the partial response to sarcoma treatment, showed hypercellular bone marrow with CD34 and CD61-positive spindle cell infiltration and > 20% basophilic blasts with cytoplasmic blebs. RNA sequencing of soft tissue biopsy revealed the presence of RBM15::MRTFA(MKL1) fusion. Based on these findings, myeloid sarcoma/AML with RBM15::MRTFA(MKL1) fusion diagnosis was made. AML with RBM15::MRTFA(MKL1) fusion can initially present as extramedullary lesions and might cause misdiagnosis of non-hematologic malignancies.

Norepinephrine versus phenylephrine affects prethrombotic response in patients undergoing cesarean section under spinal anesthesia: a randomized, double-blind, controlled study

BACKGROUND

Venous thromboembolism is one of the most common and serious complications of cesarean section in parturients. Norepinephrine (NE) has been shown to activate coagulation. The aim of this study was to compare the effect of a fixed-rate prophylactic NE infusion and a fixed-rate prophylactic phenylephrine (PHE) infusion under spinal anesthesia for cesarean section on the prethrombotic response.

MATERIALS AND METHODS

Sixty-six women undergoing cesarean section under spinal anesthesia were randomly assigned to the NE group or PHE group, starting simultaneously with the administration of the subarachnoid solution, a 'study drug' solution containing either NE or PHE was pumped intravenously at a constant rate of 15 ml/h until the end of the operation. Plasma coagulation factor VIII activity (FVIII: C), Fibrinogen, and D-dimer levels were measured in blood samples obtained on admission to the operating theater and at the end of the procedure.

RESULTS

Compared with preoperative levels, there were no significant differences in postoperative fibrinogen and D-dimer levels in the NE group, except for a decrease in FVIII: C levels ( P =0.003). However, postoperative levels of FVIII: C ( P =0.009), fibrinogen ( P =0.035), and D-dimer ( P =0.025) were increased in the NE group compared with postoperative levels in the PHE group.

CONCLUSIONS

NE does not affect the maternal prethrombotic response and can be safely used in cesarean sections. Compared with PHE infusion, NE infusion increased the level of coagulation molecules, suggesting that NE maybe more beneficial for women with high intraoperative bleeding requiring hemostasis.

Platelet generation in vivo and in vitro

ABSTRACT

Platelets play crucial roles in hemostasis, thrombosis, and immunity, but our understanding of their complex biogenesis (thrombopoiesis) is currently incomplete. Deeper insight into the mechanisms of platelet biogenesis inside and outside the body is fundamental for managing hematological disorders and for the development of novel cell-based therapies. In this article, we address the current understanding of in vivo thrombopoiesis, including mechanisms of platelet generation from megakaryocytes (proplatelet formation, cytoplasmic fragmentation, and membrane budding) and their physiological location. Progress has been made in replicating these processes in vitro for potential therapeutic application, notably in platelet transfusion and bioengineering of platelets for novel targeted therapies. The current platelet-generating systems and their limitations, particularly yield, scalability, and functionality, are discussed. Finally, we highlight the current controversies and challenges in the field that need to be addressed to achieve a full understanding of these processes, in vivo and in vitro.

Packaging of supplemented urokinase into alpha granules of in vitro-grown megakaryocytes for targeted nascent clot lysis

ABSTRACT

Fibrinolytics delivered into the general circulation lack selectivity for nascent thrombi, reducing efficacy and increasing the risk of bleeding. Urokinase-type plasminogen activator (uPA) transgenically expressed within murine platelets provided targeted thromboprophylaxis without causing bleeding but is not clinically feasible. Recent advances in generating megakaryocytes prompted us to develop a potentially clinically relevant means to produce "antithrombotic" platelets from CD34+ hematopoietic stem cell-derived in vitro-grown megakaryocytes. CD34+ megakaryocytes internalize and store in alpha granules (α-granules) single-chain uPA (scuPA) and a plasmin-resistant thrombin-activatable variant (uPAT). Both uPAs colocalized with internalized factor V (FV), fibrinogen and plasminogen, low-density lipoprotein receptor-related protein 1 (LRP1), and interferon-induced transmembrane protein 3, but not with endogenous von Willebrand factor (VWF). Endocytosis of uPA by CD34+ megakaryocytes was mediated, in part, via LRP1 and αIIbβ3. scuPA-containing megakaryocytes degraded endocytosed intragranular FV but not endogenous VWF in the presence of internalized plasminogen, whereas uPAT-megakaryocytes did not significantly degrade either protein. We used a carotid artery injury model in nonobese diabetic-severe combined immunodeficiency IL2rγnull (NSG) mice homozygous for VWFR1326H (a mutation switching binding VWF specificity from mouse to human glycoprotein Ibα) to test whether platelets derived from scuPA- or uPAT-megakaryocytes would prevent thrombus formation. NSG/VWFR1326H mice exhibited a lower thrombotic burden after carotid artery injury compared with NSG mice unless infused with human platelets or megakaryocytes, whereas intravenous injection of uPA-megakaryocytes generated sufficient uPA-containing human platelets to lyse nascent thrombi. These studies describe the use of in vitro-generated megakaryocytes as a potential platform for delivering uPA or other ectopic proteins within platelet α-granules to sites of vascular injury.

Recent Advances in Gene Therapy for Hemophilia: Projecting the Perspectives

One of the well-known X-linked genetic disorders is hemophilia, which could be hemophilia A as a result of a mutation in the F8 (factor VIII) gene or hemophilia B as a result of a mutation in the F9 (factor IX) gene, leading to insufficient levels of the proteins essential for blood coagulation cascade. In patients with severe hemophilia, factor VIII or factor IX activities in the blood plasma are considerably low, estimated to be less than 1%. This is responsible for spontaneous or post-traumatic bleeding episodes, or both, leading to disease complications and death. Current treatment of hemophilia relies on the prevention of bleeding, which consists of expensive lifelong replacement infusion therapy of blood plasma clotting factors, their recombinant versions, or therapy with recombinant monoclonal antibodies. Recently emerged gene therapy approaches may be a potential game changer that could reshape the therapeutic outcomes of hemophilia A or B using a one-off vector in vivo delivery and aim to achieve long-term endogenous expression of factor VIII or IX. This review examines both traditional approaches to the treatment of hemophilia and modern methods, primarily focusing on gene therapy, to update knowledge in this area. Recent technological advances and gene therapeutics in the pipeline are critically reviewed and summarized. We consider gene therapy to be the most promising method as it may overcome the problems associated with more traditional treatments, such as the need for constant and expensive infusions and the presence of an immune response to the antibody drugs used to treat hemophilia.

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.

Packaging of supplemented urokinase into naked alpha-granules of in vitro -grown megakaryocytes for targeted therapeutic delivery

Our prior finding that uPA endogenously expressed and stored in the platelets of transgenic mice prevented thrombus formation without causing bleeding, prompted us to develop a potentially clinically relevant means of generating anti-thrombotic human platelets in vitro from CD34 + hematopoietic cell-derived megakaryocytes. CD34 + -megakaryocytes internalize and store in α-granules single-chain uPA (scuPA) and a uPA variant modified to be plasmin-resistant, but thrombin-activatable, (uPAT). Both uPAs co-localized with internalized factor V (FV), fibrinogen and plasminogen, low-density lipoprotein receptor-related protein 1 (LRP1), and interferon-induced transmembrane protein 3 (IFITM3), but not with endogenous von Willebrand factor (VWF). Endocytosis of uPA by CD34 + -\megakaryocytes was mediated in part via LRP1 and αIIbβ3. scuPA-containing megakaryocytes degraded endocytosed intragranular FV, but not endogenous VWF, in the presence of internalized plasminogen, whereas uPAT-megakaryocytes did not significantly degrade either protein. We used a carotid-artery injury model in NOD-scid IL2rγnull (NSG) mice homozygous for VWF R1326H (a mutation switching binding VWF specificity from mouse to human glycoprotein IbmlIX) to test whether platelets derived from scuPA-MKs or uPAT-Mks would prevent thrombus formation. NSG/VWF R1326H mice exhibited a lower thrombotic burden after carotid artery injury compared to NSG mice unless infused with human platelets or MKs, whereas intravenous injection of either uPA-containing megakaryocytes into NSG/VWF R1326H generated sufficient uPA-containing human platelets to lyse nascent thrombi. These studies suggest the potential to deliver uPA or potentially other ectopic proteins within platelet α-granules from in vitro- generated megakaryocytes.

Key points

Unlike platelets, in vitro-grown megakaryocytes can store exogenous uPA in its α-granules.uPA uptake involves LRP1 and αIIbβ3 receptors and is functionally available from activated platelets.

Altered cleavage of human factor VIII at the B-domain and acidic region 3 interface enhances expression after gene therapy in hemophilia A mice

BACKGROUND

Variants of human factor VIII (hFVIII) have been developed to further understand the structure and function of hFVIII and improve gene-based therapeutics. We have previously characterized several hFVIII variants of the furin cleavage site (1645-1648) with improved secretion. We have also identified a second cleavage site in the acidic region 3 (a3) (1657-1658) that becomes the primary hFVIII intracellular cleavage position in the absence of the furin site. We tested a hypothesis that modification of this site may confer additional functional advantages to hFVIII.

OBJECTIVES

The aim of this study was to conduct the biochemical and functional characterization of hFVIII variants of the furin cleavage site, the a3 cleavage site, or in combination, both in vitro and in vivo after AAV mediated gene therapy.

METHODS

Recombinant hFVIII variants of the furin cleavage site (hFVIII-Δ3), the a3 cleavage site (hFVIII-S1657P/D1658E [SP/DE]), or in combination (hFVIII-Δ3-SP/DE) were purified and characterized in vitro and in vivo.

RESULTS

Recombinant hFVIII-Δ3, hFVIII-SP/DE, and hFVIII-Δ3-SP/DE variants all had comparable specific activity to B-domain deleted (BDD) hFVIII. Hemophilia A mice tolerant to hFVIII did not develop immune responses to hFVIII after protein challenge with these variants or after adeno-associated virus (AAV) delivery. Following AAV delivery, hFVIII-Δ3-SP/DE resulted in expression levels that were 2- to 5-fold higher than those with hFVIII-BDD in hemophilia A mice.

CONCLUSION

The novel hFVIII-Δ3-SP/DE variant of the furin and a3 cleavage sites significantly improved secretion compared with hFVIII-BDD. This key feature of the Δ3-SP/DE variant provides a unique strategy that can be combined with other approaches to further improve factor VIII expression to achieve superior efficacy in AAV-based gene therapy for hemophilia A.

Immunothrombosis and the Role of Platelets in Venous Thromboembolic Diseases

Venous thromboembolism (VTE) is the third leading cardiovascular cause of death and is conventionally treated with anticoagulants that directly antagonize coagulation. However, recent data have demonstrated that also platelets play a crucial role in VTE pathophysiology. In the current review, we outline how platelets are involved during all stages of experimental venous thrombosis. Platelets mediate initiation of the disease by attaching to the vessel wall upon which they mediate leukocyte recruitment. This process is referred to as immunothrombosis, and within this novel concept inflammatory cells such as leukocytes and platelets directly drive the progression of VTE. In addition to their involvement in immunothrombosis, activated platelets can directly drive venous thrombosis by supporting coagulation and secreting procoagulant factors. Furthermore, fibrinolysis and vessel resolution are (partly) mediated by platelets. Finally, we summarize how conventional antiplatelet therapy can prevent experimental venous thrombosis and impacts (recurrent) VTE in humans.

Platelet Mechanobiology Inspired Microdevices: From Hematological Function Tests to Disease and Drug Screening

Platelet function tests are essential to profile platelet dysfunction and dysregulation in hemostasis and thrombosis. Clinically they provide critical guidance to the patient management and therapeutic evaluation. Recently, the biomechanical effects induced by hemodynamic and contractile forces on platelet functions attracted increasing attention. Unfortunately, the existing platelet function tests on the market do not sufficiently incorporate the topical platelet mechanobiology at play. Besides, they are often expensive and bulky systems that require large sample volumes and long processing time. To this end, numerous novel microfluidic technologies emerge to mimic vascular anatomies, incorporate hemodynamic parameters and recapitulate platelet mechanobiology. These miniaturized and cost-efficient microfluidic devices shed light on high-throughput, rapid and scalable platelet function testing, hematological disorder profiling and antiplatelet drug screening. Moreover, the existing antiplatelet drugs often have suboptimal efficacy while incurring several adverse bleeding side effects on certain individuals. Encouraged by a few microfluidic systems that are successfully commercialized and applied to clinical practices, the microfluidics that incorporate platelet mechanobiology hold great potential as handy, efficient, and inexpensive point-of-care tools for patient monitoring and therapeutic evaluation. Hereby, we first summarize the conventional and commercially available platelet function tests. Then we highlight the recent advances of platelet mechanobiology inspired microfluidic technologies. Last but not least, we discuss their future potential of microfluidics as point-of-care tools for platelet function test and antiplatelet drug screening.

Ectopic Expression of FVIII in HPCs and MSCs Derived from hiPSCs with Site-Specific Integration of ITGA2B Promoter-Driven BDDF8 Gene in Hemophilia A

Hemophilia A (HA) is caused by mutations in the coagulation factor VIII (FVIII) gene (F8). Gene therapy is a hopeful cure for HA; however, FVIII inhibitors formation hinders its clinical application. Given that platelets promote coagulation via locally releasing α-granule, FVIII ectopically expressed in platelets has been attempted, with promising results for HA treatment. The B-domain-deleted F8 (BDDF8), driven by a truncated ITGA2B promoter, was targeted at the ribosomal DNA (rDNA) locus of HA patient-specific induced pluripotent stem cells (HA-iPSCs). The F8-modified, human induced pluripotent stem cells (2bF8-iPSCs) were differentiated into induced hematopoietic progenitor cells (iHPCs), induced megakaryocytes (iMKs), and mesenchymal stem cells (iMSCs), and the FVIII expression was detected. The ITGA2B promoter-driven BDDF8 was site-specifically integrated into the rDNA locus of HA-iPSCs. The 2bF8-iPSCs were efficiently differentiated into 2bF8-iHPCs, 2bF8-iMKs, and 2bF8-iMSCs. FVIII was 10.31 ng/10⁶ cells in lysates of 2bF8-iHPCs, compared to 1.56 ng/10⁶ cells in HA-iHPCs, and FVIII was 3.64 ng/10⁶ cells in 2bF8-iMSCs lysates, while 1.31 ng/10⁶ cells in iMSCs with CMV-driven BDDF8. Our results demonstrated a high expression of FVIII in iHPCs and iMSCs derived from hiPSCs with site-specific integration of ITGA2B promoter-driven BDDF8, indicating potential clinical prospects of this platelet-targeted strategy for HA gene therapy.

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.

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.

Nongenotoxic antibody-drug conjugate conditioning enables safe and effective platelet gene therapy of hemophilia A mice

Gene therapy offers the potential to cure hemophilia A (HA). We have shown that hematopoietic stem cell (HSC)-based platelet-specific factor VIII (FVIII) (2bF8) gene therapy can produce therapeutic protein and induce antigen-specific immune tolerance in HA mice, even in the presence of inhibitory antibodies. For HSC-based gene therapy, traditional preconditioning using cytotoxic chemotherapy or total body irradiation (TBI) has been required. The potential toxicity associated with TBI or chemotherapy is a deterrent that may prevent patients with HA, a nonmalignant disease, from agreeing to such a protocol. Here, we describe targeted nongenotoxic preconditioning for 2bF8 gene therapy utilizing a hematopoietic cell-specific antibody-drug conjugate (ADC), which consists of saporin conjugated to CD45.2- and CD117-targeting antibodies. We found that a combination of CD45.2- and CD117-targeting ADC preconditioning was effective for engrafting 2bF8-transduced HSCs and was favorable for platelet lineage reconstitution. Two thirds of HA mice that received 2bF8 lentivirus-transduced HSCs under (CD45.2+CD117)-targeting ADC conditioning maintained sustained therapeutic levels of platelet FVIII expression. When CD8-targeting ADC was supplemented, chimerism and platelet FVIII expression were significantly increased, with long-term sustained platelet FVIII expression in all primary and secondary recipients. Importantly, immune tolerance was induced and hemostasis was restored in a tail-bleeding test, and joint bleeding also was effectively prevented in a needle-induced knee joint injury model in HA mice after 2bF8 gene therapy. In summary, we show for the first time efficient engraftment of gene-modified HSCs without genotoxic conditioning. The combined cocktail ADC-mediated hematopoietic cell-targeted nongenotoxic preconditioning that we developed is highly effective and favorable for platelet-specific gene therapy in HA mice.

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.

Mouse models of hemostasis

Hemostasis is the normal process that produces a blood clot at a site of vascular injury. Mice are widely used to study hemostasis and abnormalities of blood coagulation because their hemostatic system is similar in most respects to that of humans, and their genomes can be easily manipulated to create models of inherited human coagulation disorders. Two of the most widely used techniques for assessing hemostasis in mice are the tail bleeding time (TBT) and saphenous vein bleeding (SVB) models. Here we discuss the use of these methods in the evaluation of hemostasis, and the advantages and limits of using mice as surrogates for studying hemostasis in humans.

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.

Escape or Fight: Inhibitors in Hemophilia A

Replacement therapy with coagulation factor VIII (FVIII) represents the current clinical treatment for patients affected by hemophilia A (HA). This treatment while effective is, however, hampered by the formation of antibodies which inhibit the activity of infused FVIII in up to 30% of treated patients. Immune tolerance induction (ITI) protocols, which envisage frequent infusions of high doses of FVIII to confront this side effect, dramatically increase the already high costs associated to a patient's therapy and are not always effective in all treated patients. Therefore, there are clear unmet needs that must be addressed in order to improve the outcome of these treatments for HA patients. Taking advantage of preclinical mouse models of hemophilia, several strategies have been proposed in recent years to prevent inhibitor formation and eradicate the pre-existing immunity to FVIII inhibitor positive patients. Herein, we will review some of the most promising strategies developed to avoid and eradicate inhibitors, including the use of immunomodulatory drugs or molecules, oral or transplacental delivery as well as cell and gene therapy approaches. The goal is to improve and potentiate the current ITI protocols and eventually make them obsolete.

Bivalent property of intra-platelet VWF in liver regeneration and HCC recurrence: A prospective multicenter study

BACKGROUND AND AIMS

A striking difference has been observed in structure and functional properties between plasma and platelet von Willebrand factor (VWF). While the existing evidence has revealed a clinical relevance of plasma VWF-Ag in liver regeneration (LR) and different cancers, this study was designed to explore the properties of intra-platelet (IP) and serum VWF-Ag in patients with hepatocellular carcinoma (HCC) undergoing partial hepatectomy.

METHODS

A total of 40 patients undergoing partial hepatectomy were prospectively recruited from 3 institutions. VWF-Ag concentrations were evaluated mainly in serum and platelet extracts. Patients were followed-up for postoperative liver dysfunction and HCC recurrence.

RESULTS

We observed a post-resection increase in the concentration of VWF-Ag in serum and platelet. Patients with postoperative liver dysfunction had substantially reduced serum and IP VWF-Ag concentrations. After a 2-year follow-up, patients with higher post-resection serum and IP VWF-Ag concentrations were found to develop early HCC recurrence. Likewise, IP VWF-Ag was able to independently predict post-resection early HCC recurrence.

CONCLUSION

This multicenter, prospective, pilot study demonstrates a bivalent property of IP VWF in LR and oncological outcome; low preoperative VWF appeared to have a negative association on post-resection liver dysfunction, whereas, patients with higher post-resection VWF-Ag concentrations were found to have early HCC recurrence.

Transfusion of Platelets Loaded With Recombinant ADAMTS13 (A Disintegrin and Metalloprotease With Thrombospondin Type 1 Repeats-13) Is Efficacious for Inhibiting Arterial Thrombosis Associated With Thrombotic Thrombocytopenic Purpura

Objective- ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats-13) cleaves VWF (von Willebrand factor). This process is essential for hemostasis. Severe deficiency of plasma ADAMTS13 activity, most commonly resulting from autoantibodies against ADAMTS13, causes thrombotic thrombocytopenic purpura. Therapeutic plasma exchange is the standard of care to date, which removes autoantibodies and replenishes ADAMTS13. However, such a therapy is often ineffective to raise plasma ADAMTS13 activity, and in-hospital mortality rate remains as high as 20%. Approach and Results- To overcome the inhibition by autoantibodies, we developed a novel approach by delivering rADAMTS13 (recombinant ADAMTS13 ) using platelets as vehicles. We show that both human and murine platelets can uptake rADAMTS13 ex vivo. The endocytosed rADAMTS13 within platelets remains intact, active, and is stored in α-granules. Under arterial shear (100 dyne/cm²), the rADAMTS13 in platelets is released and effectively inhibits platelet adhesion and aggregation on a collagen-coated surface in a concentration-dependent manner. Transfusion of rADAMTS13-loaded platelets into Adamts13^-/- mice dramatically reduces the rate of thrombus formation in the mesenteric arterioles after FeCl₃ injury. An ex vivo transfusion of rADAMTS13-loaded platelets to a reconstituted whole blood containing plasma from a patient with immune-mediated thrombotic thrombocytopenic purpura and the cellular components (eg, erythrocytes and leukocytes) from a healthy individual, as well as a fresh whole blood obtained from a patient with congenital or immune-mediated thrombotic thrombocytopenic purpura also dramatically reduces the rate of thrombus formation under arterial flow. Conclusions- Our results demonstrate that transfusion of rADAMTS13-loaded platelets may be a novel and potentially effective therapeutic approach for arterial thrombosis, associated with congenital and immune-mediated thrombotic thrombocytopenic purpura.

Prothrombotic response to norepinephrine infusion, mimicking norepinephrine stress-reactivity effects, is partly mediated by α-adrenergic mechanisms

BACKGROUND

Stress-induced prothrombotic changes are mediated by the sympathetic nervous system and critically involved in mental triggering of acute coronary syndromes, but the underlying psychobiology is not fully understood. We tested the hypothesis that a norepinephrine (NE) infusion to mimic effects of stress-induced NE release on blood coagulation elicits prothrombotic changes and examined to what extent these would be mediated by an alpha-adrenergic mechanism.

METHODS AND RESULTS

In a single-blind placebo-controlled within-subjects design, 24 middle-aged, non-smoking, non-obese and normotensive men participated in three experimental trials with an interval between one and two weeks. Each trial applied two sequential infusions of 1 and 15 min duration with varying substances [i.e., saline as placebo, the non-specific α-blocker phentolamine (2.5 mg/min), and NE (5 μg/min)]: trial 1=saline + saline; trial 2=saline + NE, and trial 3=phentolamine + NE. Plasma levels of clotting factor VIII activity (FVIII:C), fibrinogen, and D-dimer were assessed from blood samples collected immediately before and 1 min and 20 min after infusion procedures. Compared to saline + saline, saline + NE induced increases over time in FVIII:C, fibrinogen, and D-dimer levels. With phentolamine + NE, fibrinogen levels remained increased compared to saline + saline, but changes in FVIII:C and D-dimer levels were no more different. Coagulation changes did not differ between saline + NE and phentolamine + NE.

CONCLUSIONS

NE infusion activates blood coagulation. The resulting prothrombotic state could be one psychobiological mechanism underlying mental triggering of acute coronary syndromes. Blockade of α-adrenergic receptors partly attenuated NE effects on coagulation and could be implied to have preventive potential in susceptible individuals.

The impact of GPIbα on platelet-targeted FVIII gene therapy in hemophilia A mice with pre-existing anti-FVIII immunity

Essentials Platelet-specific FVIII gene therapy is effective in hemophilia A mice even with inhibitors. The impact of platelet adherence via VWF/GPIbα binding on platelet gene therapy was investigated. GPIbα does not significantly affect platelet gene therapy of hemophilia A with inhibitors. Platelet gene therapy induces immune tolerance in hemophilia A mice with pre-existing immunity. SUMMARY: Background We have previously demonstrated that von Willebrand factor (VWF) is essential in platelet-specific FVIII (2bF8) gene therapy of hemophilia A (HA) with inhibitory antibodies (inhibitors). At the site of injury, platelet adherence is initiated by VWF binding to the platelet GPIb complex. Objective To investigate the impact of GPIbα on platelet gene therapy of HA with inhibitors. Methods Platelet-FVIII expression was introduced by 2bF8 lentivirus (2bF8LV) transduction of hematopoietic stem cells (HSCs) from GPIbαnull (Ibnull ) mice or rhF8-primed FVIIInull (F8null ) mice followed by transplantation into lethally irradiated rhF8-primed F8null recipients. Animals were analyzed by flow cytometry, FVIII assays and the tail bleeding test. Results After transplantation, 99% of platelets were derived from donors. The macrothrombocytopenia phenotype was maintained in F8null mice that received 2bF8LV-transduced Ibnull HSCs (2bF8-Ibnull /F8null ). The platelet-FVIII expression level in 2bF8-Ibnull /F8null recipients was similar to that obtained from F8null mice that received 2bF8LV-transduced F8null HSCs (2bF8-F8null /F8null ). The tail bleeding test showed that the remaining hemoglobin level in the 2bF8-Ibnull /F8null group was significantly higher than in the F8null control group, but there was no significant difference between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The half-life of inhibitor disappearance time was comparable between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The rhF8 re-challenge did not elicit a memory immune response once inhibitor titers dropped to undetectable levels after 2bF8 gene therapy. Conclusion GPIbα does not significantly impact platelet gene therapy of HA with inhibitors. 2bF8 gene therapy restores hemostasis and promotes immune tolerance in HA mice with pre-existing immunity.

Platelets loaded with liposome-encapsulated thrombin have increased coagulability

Essentials Platelet transfusions can have limited efficacy during hemorrhage associated with coagulopathy. Thrombin can be shielded by encapsulation into nanoliposomes and delivered to platelets ex vivo. Loading platelets with liposomal thrombin improved several aspects of platelet coagulability. Platelets loaded with liposomal thrombin can overcome some coagulopathic deficiencies in vitro.

SUMMARY

Background Platelets are integral to clot formation and are often transfused to stop or prevent bleeding. However, transfusions of platelets are not always effective, particularly in the most severe cases of hemorrhage. Nanoparticle systems have been developed to mimic platelets but inherently lack important aspects of platelet function, which limits their potential effectiveness. Objectives Increasing the natural coagulability of transfusable platelets could increase their efficacy during treatment of severe hemorrhage. Thrombin is a potent platelet agonist that currently cannot be used intravenously because of the risk of thrombosis. We hypothesized that delivery of thrombin to ex vivo platelets via liposomal encapsulation would enable transfusable platelets to become more coagulable in response to platelet agonists. Methods Thrombin was encapsulated into nanoliposomes and delivered to platelets ex vivo. Platelet coagulability was measured by monitoring platelet activation, clot contraction, clot time and clot stability in several in vitro assays. These parameters were also measured under conditions where coagulation is compromised, including during acidosis, antiplatelet drugs, hemophilia A and trauma-induced coagulopathy. Results Liposomal thrombin was endocytosed and used by platelets ex vivo but was not secreted upon activation. These modified platelets became more sensitive and responsive to agonists and improved clotting time even under conditions that normally cause platelet dysfunction or have impaired coagulation. Conclusions Several aspects of platelet function were enhanced by ex vivo delivery of liposomal thrombin.

Current status of blood 'pharming': megakaryoctye transfusions as a source of platelets

PURPOSE OF REVIEW

Donor-derived platelets have proven to be of hemostatic value in many clinical settings. There is a fear that the need for platelets may outgrow the donor pool in first-world countries. Moreover, there are other challenges with donor platelets that add to the impetus to find an alternative platelet source, especially after the megakaryocyte cytokine thrombopoietin was identified. Megakaryocytes have since been differentiated from numerous cell sources and the observed released platelet-like particles (PLPs) have led to calls to develop such products for clinical use. The development of megakaryocytes from embryonic stem cell also supported the concept of developing nondonor-based platelets.

RECENT FINDINGS

Several groups have claimed that nondonor-based platelets derived from in-vitro grown megakaryocytes may soon become available to supplement or replace donor-derived products, but their number and quality has been wanting. A possible alternative of directly infusing megakaryocytes that release platelets in the lungs - similar to that recently shown for endogenous megakaryocytes - has been proposed.

SUMMARY

This present review will describe the present state-of-the-art in generating and delivering nondonor-derived platelets. Progress has been slow, but advances in our ability to generate human megakaryocytes in culture, generate PLPs from these cells, and test the functionality of the resultant platelets in vitro and in vivo have identified important remaining challenges and raised alternative potential solutions.

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.

Gene Therapy for Coagulation Disorders

Molecular genetic details of the human coagulation system were among the first successes of the genetic revolution in the 1980s. This information led to new molecular diagnostic strategies for inherited disorders of hemostasis and the development of recombinant clotting factors for the treatment of the common inherited bleeding disorders. A longer term goal of this knowledge has been the establishment of gene transfer to provide continuing access to missing or defective hemostatic proteins. Because of the relative infrequency of inherited coagulation factor disorders and the availability of safe and effective alternative means of management, the application of gene therapy for these conditions has been slow to realize clinical application. Nevertheless, the tools for effective and safe gene transfer are now much improved, and we have started to see examples of clinical gene therapy successes. Leading the way has been the use of adeno-associated virus-based strategies for factor IX gene transfer in hemophilia B. Several small phase 1/2 clinical studies using this approach have shown prolonged expression of therapeutically beneficial levels of factor IX. Nevertheless, before the application of gene therapy for coagulation disorders becomes widespread, several obstacles need to be overcome. Immunologic responses to the vector and transgenic protein need to be mitigated, and production strategies for clinical grade vectors require enhancements. There is little doubt that with the development of more efficient and facile strategies for genome editing and the application of other nucleic acid-based approaches to influence the coagulation system, the future of genetic therapies for hemostasis is bright.

FLI1 level during megakaryopoiesis affects thrombopoiesis and platelet biology

Friend leukemia virus integration 1 (FLI1), a critical transcription factor (TF) during megakaryocyte differentiation, is among genes hemizygously deleted in Jacobsen syndrome, resulting in a macrothrombocytopenia termed Paris-Trousseau syndrome (PTSx). Recently, heterozygote human FLI1 mutations have been ascribed to cause thrombocytopenia. We studied induced-pluripotent stem cell (iPSC)-derived megakaryocytes (iMegs) to better understand these clinical disorders, beginning with iPSCs generated from a patient with PTSx and iPSCs from a control line with a targeted heterozygous FLI1 knockout (FLI1^+/-). PTSx and FLI1^+/- iMegs replicate many of the described megakaryocyte/platelet features, including a decrease in iMeg yield and fewer platelets released per iMeg. Platelets released in vivo from infusion of these iMegs had poor half-lives and functionality. We noted that the closely linked E26 transformation-specific proto-oncogene 1 (ETS1) is overexpressed in these FLI1-deficient iMegs, suggesting FLI1 negatively regulates ETS1 in megakaryopoiesis. Finally, we examined whether FLI1 overexpression would affect megakaryopoiesis and thrombopoiesis. We found increased yield of noninjured, in vitro iMeg yield and increased in vivo yield, half-life, and functionality of released platelets. These studies confirm FLI1 heterozygosity results in pleiotropic defects similar to those noted with other critical megakaryocyte-specific TFs; however, unlike those TFs, FLI1 overexpression improved yield and functionality.

Targeting expression to megakaryocytes and platelets by lineage-specific lentiviral vectors

Essentials Platelet phenotypes can be modified by lentiviral transduction of hematopoietic stem cells. Megakaryocyte-specific lentiviral vectors were tested in vitro and in vivo for restricted expression. The glycoprotein 6 vector expressed almost exclusively in megakaryocytes. The platelet factor 4 vector was the strongest but with activity in hematopoietic stem cells.

SUMMARY

Background Lentiviral transduction and transplantation of hematopoietic stem cells (HSCs) can be utilized to modify the phenotype of megakaryocytes and platelets. As the genetic modification in HSCs is transmitted onto all hematopoietic progenies, transgene expression from the vector should be restricted to megakaryocytes to avoid un-physiologic effects by ectopic transgene expression. This can be achieved by lentiviral vectors that control expression by lineage-specific promoters. Methods In this study, we introduced promoters of megakaryocyte/platelet-specific genes, namely human glycoprotein 6 (hGP6) and hGP9, into third generation lentiviral vectors and analyzed their functionality in vitro and in vivo in bone marrow transplantation assays. Their specificity and efficiency of expression was compared with lentiviral vectors utilizing the promoters of murine platelet factor 4 (mPf4) and hGP1BA, both with strong activity in megakaryocytes (MKs) used in earlier studies, and the ubiquitously expressing phosphoglycerate kinase (hPGK) and spleen focus forming virus (SFFV) enhancer/promoters. Results Expression from the mPf4 vector in MKs and platelets was the strongest similar to expression from the viral SFFV promoter, however, the mPf4 vector, also exhibited considerable off-target expression in hematopoietic stem and progenitor cells. In contrast, the newly generated hGP6 vector was highly specific to megakaryocytes and platelets. The specificity was also retained when reducing the promoter size to 350 bp, making it a valuable new tool for lentiviral expression in MKs/platelets. Conclusion MK-specific vectors express preferentially in the megakaryocyte lineage. These vectors can be applied to develop murine models to study megakaryocyte and platelet function, or for gene therapy targeting proteins to platelets.

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.

Novel factor VIII variants with a modified furin cleavage site improve the efficacy of gene therapy for hemophilia A

Essentials Factor (F) VIII is an inefficiently expressed protein. Furin deletion FVIII variants were purified and characterized using in vitro and in vivo assays. These minimally modified novel FVIII variants have enhanced function. These variants provide a strategy for increasing FVIII expression in hemophilia A gene therapy.

SUMMARY

Background The major challenge for developing gene-based therapies for hemophilia A is that human factor VIII (hFVIII) has intrinsic properties that result in inefficient biosynthesis. During intracellular processing, hFVIII is predominantly cleaved at a paired basic amino acid cleaving enzyme (PACE) or furin cleavage site to yield a heterodimer that is the major form of secreted protein. Previous studies with B-domain-deleted (BDD) canine FVIII and hFVIII-R1645H, both differing from hFVIII by a single amino acid at this site, suggested that these proteins are secreted mainly in a single polypeptide chain (SC) form and exhibit enhanced function. Objective We hypothesized that deletion(s) of the furin site modulates FVIII biology and may enhance its function. Methods A series of recombinant hFVIII-furin deletion variants were introduced into hFVIII-BDD [Δ1645, 1645-46(Δ2), 1645-47(Δ3), 1645-48(Δ4), or Δ1648] and characterized. Results In vitro, recombinant purified Δ3 and Δ4 were primarily SC and, interestingly, had 2-fold higher procoagulant activity compared with FVIII-BDD. In vivo, the variants also have improved hemostatic function. After adeno-associated viral (AAV) vector delivery, the expression of these variants is 2-4-fold higher than hFVIII-BDD. Protein challenges of each variant in mice tolerant to hFVIII-BDD showed no anti-FVIII immune response. Conclusions These data suggest that the furin deletion hFVIII variants are superior to hFVIII-BDD without increased immunogenicity. In the setting of gene-based therapeutics, these novel variants provide a unique strategy to increase FVIII expression, thus lowering the vector dose, a critical factor for hemophilia A gene therapy.

Hemophilia A gene therapy via intraosseous delivery of factor VIII-lentiviral vectors

Current treatment of hemophilia A (HemA) patients with repeated infusions of factor VIII (FVIII; abbreviated as F8 in constructs) is costly, inconvenient, and incompletely effective. In addition, approximately 25 % of treated patients develop anti-factor VIII immune responses. Gene therapy that can achieve long-term phenotypic correction without the complication of anti-factor VIII antibody formation is highly desired. Lentiviral vector (LV)-mediated gene transfer into hematopoietic stem cells (HSCs) results in stable integration of FVIII gene into the host genome, leading to persistent therapeutic effect. However, ex vivo HSC gene therapy requires pre-conditioning which is highly undesirable for hemophilia patients. The recently developed novel methodology of direct intraosseous (IO) delivery of LVs can efficiently transduce bone marrow cells, generating high levels of transgene expression in HSCs. IO delivery of E-F8-LV utilizing a ubiquitous EF1α promoter generated initially therapeutic levels of FVIII, however, robust anti-FVIII antibody responses ensued neutralized functional FVIII activity in the circulation. In contrast, a single IO delivery of G-FVIII-LV utilizing a megakaryocytic-specific GP1bα promoter achieved platelet-specific FVIII expression, leading to persistent, partial correction of HemA in treated animals. Most interestingly, comparable therapeutic benefit with G-F8-LV was obtained in HemA mice with pre-existing anti-FVIII inhibitors. Platelets is an ideal IO delivery vehicle since FVIII stored in α-granules of platelets is protected from high-titer anti-FVIII antibodies; and that even relatively small numbers of activated platelets that locally excrete FVIII may be sufficient to promote efficient clot formation during bleeding. Additionally, combination of pharmacological agents improved transduction of LVs and persistence of transduced cells and transgene expression. Overall, a single IO infusion of G-F8-LV can generate long-term stable expression of hFVIII in platelets and correct hemophilia phenotype for long term. This approach has high potential to permanently treat FVIII deficiency with and without pre-existing anti-FVIII antibodies.

Endothelial Cell-Derived von Willebrand Factor Is the Major Determinant That Mediates von Willebrand Factor-Dependent Acute Ischemic Stroke by Promoting Postischemic Thrombo-Inflammation

OBJECTIVE

von Willebrand factor (VWF), which is synthesized in endothelial cells and megakaryocytes, is known to worsen stroke outcome. In vitro studies suggest that platelet-derived VWF (Plt-VWF) is biochemically different from the endothelial cell-derived VWF (EC-VWF). However, little is known about relative contribution of different pools of VWF in stroke.

APPROACH AND RESULTS

Using bone marrow transplantation, we generated chimeric Plt-VWF mice, Plt-VWF mice that lack ADAMTS13 in platelets and plasma (Plt-VWF/Adamts13(-/-)), and EC-VWF mice to determine relative contribution of different pools of VWF in stroke. In brain ischemia/reperfusion injury model, we found that infarct size and postischemic intracerebral thrombo-inflammation (fibrin(ogen) deposition, neutrophil infiltration, interleukin-1β, and tumor necrosis factor-α levels) within lesions were comparable between EC-VWF and wild-type mice. Infarct size and postischemic thrombo-inflammation were comparable between Plt-VWF and Plt-VWF/Adamts13(-/-) mice, but decreased compared with EC-VWF and wild-type mice (P<0.05) and increased compared with Vwf(-/-) mice (P<0.05). Susceptibility to FeCl3 injury-induced carotid artery thrombosis was comparable between wild-type and EC-VWF mice, whereas Plt-VWF and Plt-VWF/Adamts13(-/-) mice exhibited defective thrombosis. Although most of the injured vessels did not occlude, slope over time showed that thrombus growth rate was increased in both Plt-VWF and Plt-VWF/Adamts13(-/-) mice compared with Vwf(-/-) mice (P<0.05), but decreased compared with wild-type or EC-VWF mice.

CONCLUSIONS

Plt-VWF, either in presence or absence of ADAMTS13, partially contributes to VWF-dependent injury and postischemic thrombo-inflammation after stroke. EC-VWF is the major determinant that mediates VWF-dependent ischemic stroke by promoting postischemic thrombo-inflammation.

An in vitro study of the effects of t-PA and tranexamic acid on whole blood coagulation and fibrinolysis

AIMS

Acute traumatic coagulopathy is characterised by fibrinolysis and low fibrinogen. It is unclear how much fibrinogenolysis contributes to reduce fibrinogen levels. The study aim was to: investigate in vitro the effects of tissue-plasminogen activator (t-PA) and tranexamic acid (TXA) on coagulation and fibrinolysis.

METHODS

Whole blood was spiked with varying t-PA concentrations. Clauss fibrinogen levels and thrombelastography (TEG, Haemonetics) were performed, including functional fibrinogen level (FLEV). TXA effects were assessed using four TXA concentrations. Recorded parameters from kaolin activated TEG included maximal amplitude (MA), clot strength (G), percentage lysis (LY). Plasmin-antiplasmin complex (PAP), endogenous thrombin potential (ETP), prothrombin fragment 1+2 (PF1+2), factor V and factor VIII levels were all measured.

RESULTS

t-PA induced fibrinolysis: it increased PAP and LY, but decreased MA and G. t-PA induced fibrinogenolysis, with a concentration-dependant decrease in fibrinogen from 2.7 (2.6-3.1) to 0.8 (0.8-0.9) g/L with 60 nM t-PA. FLEV and fibrinogen levels were well correlated. High t-PA doses increased PF1+2, decreased ETP of 19% and FVIII of 63% but not FV. TXA had no effect on plasmin generation as evidenced by no change in PAP. It corrected LY, MA and G and partly protected fibrinogen against fibrinogenolysis: 0.03 mg/mL TXA reduced the fibrinogen fall induced by t-PA 20 nM from 43% to 14%. TXA halved the FVIII fall and increased ETP.

CONCLUSIONS

t-PA induced plasminogen activation and fibrinogenolysis in a concentration-dependant manner. TXA did not affect plasmin activation but reduced fibrinogenolysis. These results suggest that TXA given early in bleeding patients may prevent fibrinogenolysis.

A chimeric platelet-targeted urokinase prodrug selectively blocks new thrombus formation

The use of fibrinolytic agents to prevent new thrombus formation is limited by an increased risk of bleeding due to lysis of hemostatic clots that prevent hemorrhage in damaged blood vessels. We sought to develop an agent that provides thromboprophylaxis without carrying a significant risk of causing systemic fibrinolysis or disrupting hemostatic clots. We previously showed that platelet (PLT) α granule-delivered urokinase plasminogen activator (uPA) is highly effective in preventing thrombosis, while being associated with little systemic fibrinolysis or bleeding. Here, we generated a chimeric prodrug composed of a single-chain version of the variable region of an anti-αIIbβ3 mAb fused to a thrombin-activatable, low-molecular-weight pro-uPA (PLT/uPA-T). PLT/uPA-T recognizes human αIIbβ3 on both quiescent and activated platelets and is enzymatically activated specifically by thrombin. We found that this prodrug binds tightly to human platelets even after gel filtration, has a prolonged half-life in mice transgenic for human αIIb compared with that of uPA-T, and prevents clot formation in a microfluidic system. Importantly, in two murine injury models, PLT/uPA-T did not lyse preexisting clots, even when administration was delayed by as little as 10 minutes, while it concurrently prevented the development of nascent thrombi. Thus, PLT/uPA-T represents the prototype of a platelet-targeted thromboprophylactic agent that selectively targets nascent over preexisting thrombi.

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.

Megakaryocyte- and megakaryocyte precursor-related gene therapies

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

Comparison of platelet-derived and plasma factor VIII efficacy using a novel native whole blood thrombin generation assay

BACKGROUND

We have recently developed a successful gene therapy approach for hemophilia A in which factor VIII (FVIII) expression is targeted to platelets by the αIIb promoter. Levels of platelet-expressed FVIII (2bF8) achieved by gene therapy may vary between individuals due to differences in ex vivo transduction and gene expression efficiency. Accurate assays to evaluate 2bF8 efficacy are desirable.

OBJECTIVE

To compare the hemostatic efficacy of 2bF8 with replacement therapy over a wide therapeutic dose range.

METHODS

Efficacy of 2bF8 was assessed using a new transgenic mouse model expressing high 2bF8 levels (LV18(tg) ). Blood from LV18(tg) mice or FVIII(null) mice infused with recombinant FVIII was mixed with FVIII(null) blood at different ratios ex vivo to achieve several concentrations of 2bF8 or plasma FVIII. Samples were evaluated with a novel native whole blood thrombin generation assay that uses recalcified whole blood without the addition of tissue factor to initiate coagulation.

RESULTS

FVIII dose dependency was observed in all five thrombin generation parameters. While the total amount of thrombin generated was similar, 2bF8 significantly accelerated thrombin generation compared with plasma FVIII. Remarkably, a 10-fold lower dose of 2bF8 than plasma FVIII (0.2% vs. 2%) significantly shortened the onset and peak of thrombin generation compared with FVIII(null) blood.

CONCLUSION

Using a new transgenic mouse model, we showed that the novel native whole blood thrombin generation assay established here can be used to monitor platelet targeted FVIII gene therapy. The higher therapeutic efficacy of 2bF8 compared with factor replacement therapy seemed to be due to acceleration of thrombin generation.

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.