Helper-dependent adenoviral vectors mediate therapeutic factor VIII expression for several months with minimal accompanying toxicity in a canine model of severe hemophilia A

Non-viral and viral delivery systems for hemophilia A therapy: recent development and prospects

Recent advancements have focused on enhancing factor VIII half-life and refining its delivery methods, despite the well-established knowledge that factor VIII deficiency is the main clotting protein lacking in hemophilia. Consequently, both viral and non-viral delivery systems play a crucial role in enhancing the quality of life for hemophilia patients. The utilization of viral vectors and the manipulation of non-viral vectors through targeted delivery are significant advancements in the field of cellular and molecular therapies for hemophilia. These developments contribute to the progression of treatment strategies and hold great promise for improving the overall well-being of individuals with hemophilia. This review study comprehensively explores the application of viral and non-viral vectors in cellular (specifically T cell) and molecular therapy approaches, such as RNA, monoclonal antibody (mAb), and CRISPR therapeutics, with the aim of addressing the challenges in hemophilia treatment. By examining these innovative strategies, the study aims to shed light on potential solutions to enhance the efficacy and outcomes of hemophilia therapy.

Gene therapy approaches for equine osteoarthritis

With an intrinsically low ability for self-repair, articular cartilage injuries often progress to cartilage loss and joint degeneration resulting in osteoarthritis (OA). Osteoarthritis and the associated articular cartilage changes can be debilitating, resulting in lameness and functional disability both in human and equine patients. While articular cartilage damage plays a central role in the pathogenesis of OA, the contribution of other joint tissues to the pathogenesis of OA has increasingly been recognized thus prompting a whole organ approach for therapeutic strategies. Gene therapy methods have generated significant interest in OA therapy in recent years. These utilize viral or non-viral vectors to deliver therapeutic molecules directly into the joint space with the goal of reprogramming the cells' machinery to secrete high levels of the target protein at the site of injection. Several viral vector-based approaches have demonstrated successful gene transfer with persistent therapeutic levels of transgene expression in the equine joint. As an experimental model, horses represent the pathology of human OA more accurately compared to other animal models. The anatomical and biomechanical similarities between equine and human joints also allow for the use of similar imaging and diagnostic methods as used in humans. In addition, horses experience naturally occurring OA and undergo similar therapies as human patients and, therefore, are a clinically relevant patient population. Thus, further studies utilizing this equine model would not only help advance the field of human OA therapy but also benefit the clinical equine patients with naturally occurring joint disease. In this review, we discuss the advancements in gene therapeutic approaches for the treatment of OA with the horse as a relevant patient population as well as an effective and commonly utilized species as a translational model.

Bioengineering hemophilia A-specific microvascular grafts for delivery of full-length factor VIII into the bloodstream

Hemophilia A (HA) is a bleeding disorder caused by mutations in the F8 gene encoding coagulation factor VIII (FVIII). Current treatments are based on regular infusions of FVIII concentrates throughout a patient's life. Alternatively, viral gene therapies that directly deliver F8 in vivo have shown preliminary successes. However, hurdles remain, including lack of infection specificity and the inability to deliver the full-length version of F8 due to restricted viral cargo sizes. Here, we developed an alternative nonviral ex vivo gene-therapy approach that enables the overexpression of full-length F8 in patients' endothelial cells (ECs). We first generated HA patient-specific induced pluripotent stem cells (HA-iPSCs) from urine epithelial cells and genetically modified them using a piggyBac DNA transposon system to insert multiple copies of full-length F8. We subsequently differentiated the modified HA-iPSCs into competent ECs with high efficiency, and demonstrated that the cells (termed HA-FLF8-iECs) were capable of producing high levels of FVIII. Importantly, following subcutaneous implantation into immunodeficient hemophilic (SCID-f8ko) mice, we demonstrated that HA-FLF8-iECs were able to self-assemble into vascular networks, and that the newly formed microvessels had the capacity to deliver functional FVIII directly into the bloodstream of the mice, effectively correcting the clotting deficiency. Moreover, our implant maintains cellular confinement, which reduces potential safety concerns and allows effective monitoring and reversibility. We envision that this proof-of-concept study could become the basis for a novel autologous ex vivo gene-therapy approach to treat HA.

High-Capacity Adenoviral Vectors: Expanding the Scope of Gene Therapy

The adaptation of adenoviruses as gene delivery tools has resulted in the development of high-capacity adenoviral vectors (HC-AdVs), also known, helper-dependent or "gutless". Compared with earlier generations (E1/E3-deleted vectors), HC-AdVs retain relevant features such as genetic stability, remarkable efficacy of in vivo transduction, and production at high titers. More importantly, the lack of viral coding sequences in the genomes of HC-AdVs extends the cloning capacity up to 37 Kb, and allows long-term episomal persistence of transgenes in non-dividing cells. These properties open a wide repertoire of therapeutic opportunities in the fields of gene supplementation and gene correction, which have been explored at the preclinical level over the past two decades. During this time, production methods have been optimized to obtain the yield, purity, and reliability required for clinical implementation. Better understanding of inflammatory responses and the implementation of methods to control them have increased the safety of these vectors. We will review the most significant achievements that are turning an interesting research tool into a sound vector platform, which could contribute to overcome current limitations in the gene therapy field.

Translational Potential of Immune Tolerance Induction by AAV Liver-Directed Factor VIII Gene Therapy for Hemophilia A

Hemophilia A (HA) is an X-linked bleeding disorder due to deficiencies in coagulation factor VIII (FVIII). The major complication of current protein-based therapies is the development of neutralizing anti-FVIII antibodies, termed inhibitors, that block the hemostatic effect of therapeutic FVIII. Inhibitors develop in about 20-30% of people with severe HA, but the risk is dependent on the interaction between environmental and genetic factors, including the underlying F8 gene mutation. Recently, multiple clinical trials evaluating adeno-associated viral (AAV) vector liver-directed gene therapy for HA have reported promising results of therapeutically relevant to curative FVIII levels. The inclusion criteria for most trials prevented enrollment of subjects with a history of inhibitors. However, preclinical data from small and large animal models of HA with inhibitors suggests that liver-directed gene therapy can in fact eradicate pre-existing anti-FVIII antibodies, induce immune tolerance, and provide long-term therapeutic FVIII expression to prevent bleeding. Herein, we review the accumulating evidence that continuous uninterrupted expression of FVIII and other transgenes after liver-directed AAV gene therapy can bias the immune system toward immune tolerance induction, discuss the current understanding of the immunological mechanisms of this process, and outline questions that will need to be addressed to translate this strategy to clinical trials.

Innate immunity to adenovirus: lessons from mice

Adenovirus is a highly evolutionary successful pathogen, as it is widely prevalent across the animal kingdom, infecting hosts ranging from lizards and frogs to dolphins, birds, and humans. Although natural adenovirus infections in humans rarely cause severe pathology, intravenous injection of high doses of adenovirus-based vectors triggers rapid activation of the innate immune system, leading to cytokine storm syndrome, disseminated intravascular coagulation, thrombocytopenia, and hepatotoxicity, which individually or in combination may cause morbidity and mortality. Much of the information on exactly how adenovirus activates the innate immune system has been gathered from mouse experimental systems. Intravenous administration of adenovirus to mice revealed mechanistic insights into cellular and molecular components of the innate immunity that detect adenovirus particles, activate pro-inflammatory signaling pathways and cytokine production, sequester adenovirus particles from the bloodstream, and eliminate adenovirus-infected cells. Collectively, this information greatly improved our understanding of mechanisms of activation of innate immunity to adenovirus and may pave the way for designing safer adenovirus-based vectors for therapy of genetic and acquired human diseases.

Protein-Engineered Coagulation Factors for Hemophilia Gene Therapy

Hemophilia A (HA) and hemophilia B (HB) are X-linked bleeding disorders due to inheritable deficiencies in either coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Recently, gene therapy clinical trials with adeno-associated virus (AAV) vectors and protein-engineered transgenes, B-domain deleted (BDD) FVIII and FIX-Padua, have reported near-phenotypic cures in subjects with HA and HB, respectively. Here, we review the biology and the clinical development of FVIII-BDD and FIX-Padua as transgenes. We also examine alternative bioengineering strategies for FVIII and FIX, as well as the immunological challenges of these approaches. Other engineered proteins and their potential use in gene therapy for hemophilia with inhibitors are also discussed. Continued advancement of gene therapy for HA and HB using protein-engineered transgenes has the potential to alleviate the substantial medical and psychosocial burdens of the disease.

Improving miRNA Delivery by Optimizing miRNA Expression Cassettes in Diverse Virus Vectors

The RNA interference pathway is an evolutionary conserved post-transcriptional gene regulation mechanism that is exclusively triggered by double-stranded RNA inducers. RNAi-based methods and technologies have facilitated the discovery of many basic science findings and spurred the development of novel RNA therapeutics. Transient induction of RNAi via transfection of synthetic small interfering RNAs can trigger the selective knockdown of a target mRNA. For durable silencing of gene expression, either artificial short hairpin RNA or microRNA encoding transgene constructs were developed. These miRNAs are based on the molecules that induce the natural RNAi pathway in mammals and humans: the endogenously expressed miRNAs. Significant efforts focused on the construction and delivery of miRNA cassettes in order to solve basic biology questions or to design new therapy strategies. Several viral vectors have been developed, which are particularly useful for the delivery of miRNA expression cassettes to specific target cells. Each vector system has its own unique set of distinct properties. Thus, depending on the specific application, a particular vector may be most suitable. This field was previously reviewed for different viral vector systems, and now the recent progress in the field of miRNA-based gene-silencing approaches using lentiviral vectors is reported. The focus is on the unique properties and respective limitations of the available vector systems for miRNA delivery.

Gene therapy for monogenic liver diseases: clinical successes, current challenges and future prospects

Over the last decade, pioneering liver-directed gene therapy trials for haemophilia B have achieved sustained clinical improvement after a single systemic injection of adeno-associated virus (AAV) derived vectors encoding the human factor IX cDNA. These trials demonstrate the potential of AAV technology to provide long-lasting clinical benefit in the treatment of monogenic liver disorders. Indeed, with more than ten ongoing or planned clinical trials for haemophilia A and B and dozens of trials planned for other inherited genetic/metabolic liver diseases, clinical translation is expanding rapidly. Gene therapy is likely to become an option for routine care of a subset of severe inherited genetic/metabolic liver diseases in the relatively near term. In this review, we aim to summarise the milestones in the development of gene therapy, present the different vector tools and their clinical applications for liver-directed gene therapy. AAV-derived vectors are emerging as the leading candidates for clinical translation of gene delivery to the liver. Therefore, we focus on clinical applications of AAV vectors in providing the most recent update on clinical outcomes of completed and ongoing gene therapy trials and comment on the current challenges that the field is facing for large-scale clinical translation. There is clearly an urgent need for more efficient therapies in many severe monogenic liver disorders, which will require careful risk-benefit analysis for each indication, especially in paediatrics.

Gene therapy with helper-dependent adenoviral vectors: lessons from studies in large animal models

Helper-dependent adenoviral vectors (HDAd) are deleted of all viral genes and they can efficiently transduce a wide variety of dividing and non-dividing cells to mediate high transgene expression levels. Unlike early generation adenoviral vectors, the absence of viral genes in HDAd results in long-term transgene expression without chronic toxicity and permits a large cloning capacity of 36 kb. Moreover, HDAd genomes exist extra-chromosomally thus minimizing the risks of germline transmission and insertional mutagenesis. For these reasons, HDAd offers tremendous potential for in vivo gene therapy. This chapter reviews preclinical studies using HDAd in large animal models to assess safety and efficacy in a wide variety of gene therapy applications.

A Novel Platform for Immune Tolerance Induction in Hemophilia A Mice

Hemophilia A (HA) is an X-linked bleeding disease caused by factor VIII (FVIII) deficiency. We previously demonstrated that FVIII is produced specifically in liver sinusoid endothelial cells (LSECs) and to some degree in myeloid cells, and thus, in the present work, we seek to restrict the expression of FVIII transgene to these cells using cell-specific promoters. With this approach, we aim to limit immune response in a mouse model by lentiviral vector (LV)-mediated gene therapy encoding FVIII. To increase the target specificity of FVIII expression, we included miRNA target sequences (miRTs) (i.e., miRT-142.3p, miRT-126, and miRT-122) to silence expression in hematopoietic cells, endothelial cells, and hepatocytes, respectively. Notably, we report, for the first time, therapeutic levels of FVIII transgene expression at its natural site of production, which occurred without the formation of neutralizing antibodies (inhibitors). Moreover, inhibitors were eradicated in FVIII pre-immune mice through a regulatory T cell-dependent mechanism. In conclusion, targeting FVIII expression to LSECs and myeloid cells by using LVs with cell-specific promoter minimized off-target expression and immune responses. Therefore, at least for some transgenes, expression at the physiologic site of synthesis can enhance efficacy and safety, resulting in long-term correction of genetic diseases such as HA.

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.

Canine models of inherited bleeding disorders in the development of coagulation assays, novel protein replacement and gene therapies

Animal models of inherited bleeding disorders are important for understanding disease pathophysiology and are required for preclinical assessment of safety prior to testing of novel therapeutics in human and veterinary medicine. Experiments in these animals represent important translational research aimed at developing safer and better treatments, such as plasma-derived and recombinant protein replacement therapies, gene therapies and immune tolerance protocols for antidrug inhibitory antibodies. Ideally, testing is done in animals with the analogous human disease to provide essential safety information, estimates of the correct starting dose and dose response (pharmacokinetics) and measures of efficacy (pharmacodynamics) that guide the design of human trials. For nearly seven decades, canine models of hemophilia, von Willebrand disease and other inherited bleeding disorders have not only informed our understanding of the natural history and pathophysiology of these disorders but also guided the development of novel therapeutics for use in humans and dogs. This has been especially important for the development of gene therapy, in which unique toxicities such as insertional mutagenesis, germ line gene transfer and viral toxicities must be assessed. There are several issues regarding comparative medicine in these species that have a bearing on these studies, including immune reactions to xenoproteins, varied metabolism or clearance of wild-type and modified proteins, and unique tissue tropism of viral vectors. This review focuses on the results of studies that have been performed in dogs with inherited bleeding disorders that closely mirror the human condition to develop safe and effective protein and gene-based therapies that benefit both species.

Delivery of full-length factor VIII using a piggyBac transposon vector to correct a mouse model of hemophilia A

Viral vectors have been used for hemophilia A gene therapy. However, due to its large size, full-length Factor VIII (FVIII) cDNA has not been successfully delivered using conventional viral vectors. Moreover, viral vectors may pose safety risks, e.g., adverse immunological reactions or virus-mediated cytotoxicity. Here, we took advantages of the non-viral vector gene delivery system based on piggyBac DNA transposon to transfer the full-length FVIII cDNA, for the purpose of treating hemophilia A. We tested the efficiency of this new vector system in human 293T cells and iPS cells, and confirmed the expression of the full-length FVIII in culture media using activity-sensitive coagulation assays. Hydrodynamic injection of the piggyBac vectors into hemophilia A mice temporally treated with an immunosuppressant resulted in stable production of circulating FVIII for over 300 days without development of anti-FVIII antibodies. Furthermore, tail-clip assay revealed significant improvement of blood coagulation time in the treated mice.piggyBac transposon vectors can facilitate the long-term expression of therapeutic transgenes in vitro and in vivo. This novel gene transfer strategy should provide safe and efficient delivery of FVIII.

Omental implantation of BOECs in hemophilia dogs results in circulating FVIII antigen and a complex immune response

Ex vivo gene therapy strategies avoid systemic delivery of viruses thereby mitigating the risk of vector-associated immunogenicity. Previously, we delivered autologous factor VIII (FVIII)-expressing blood outgrowth endothelial cells (BOECs) to hemophilia A mice and showed that these cells remained sequestered within the implanted matrix and provided therapeutic levels of FVIII. Prior to translating this strategy into the canine (c) model of hemophilia A, we increased cFVIII transgene expression by at least 100-fold with the use of the elongation factor 1 alpha (EF1α) promoter and a strong endothelial enhancer element. BOECs isolated from hemophilia A dogs transduced with this lentiviral vector express levels of cFVIII ranging between 1.0 and 1.5 U/mL per 10(6) cells over 24 hours. Autologous BOECs have been implanted into the omentum of 2 normal and 3 hemophilia A dogs. These implanted cells formed new vessels in the omentum. All 3 hemophilia A dogs treated with FVIII-expressing autologous BOECs developed anti-FVIII immunoglobulin G2 antibodies, but in only 2 of the dogs were these antibodies inhibitory. FVIII antigen levels >40% in the absence of FVIII coagulant function were detected in the circulation for up to a year after a single gene therapy treatment, indicating prolonged cellular viability and synthesis of FVIII.

Animal models of hemophilia and related bleeding disorders

Animal models of hemophilia and related diseases are important for the development of novel treatments and to understand the pathophysiology of bleeding disorders in humans. Testing in animals with the equivalent human disorder provides informed estimates of doses and measures of efficacy, which aids in design of human trials. Many models of hemophilia A, hemophilia B, and von Willebrand disease (VWD) have been developed from animals with spontaneous mutations (hemophilia A dogs, rats, sheep; hemophilia B dogs; and VWD pigs and dogs), or by targeted gene disruption in mice to create hemophilia A, B, or VWD models. Animal models have been used to generate new insights into the pathophysiology of each bleeding disorder and also to perform preclinical assessments of standard protein replacement therapies, as well as novel gene transfer technology. The differences both between species and in underlying causative mutations must be considered in choosing the best animal for a specific scientific study.

A novel cell-sheet technology that achieves durable factor VIII delivery in a mouse model of hemophilia A

Gene- or cell-based therapies aimed at creating delivery systems for coagulation factor VIII (FVIII) protein have emerged as promising options for hemophilia A treatment. However, several issues remain to be addressed regarding the efficacies and adverse events of these new classes of therapies. To improve an existing cell-based therapy involving the subcutaneous transplantation of FVIII-transduced blood outgrowth endothelial cells (BOECs), we employed a novel cell-sheet technology that allows individual dispersed cells to form a thin and contiguous monolayer without traditional bioabsorbable scaffold matrices. Compared to the traditional methodology, our cell-sheet approach resulted in longer-term and 3–5-fold higher expression of FVIII (up to 11% of normal) in recipient hemophilia A mice that lacked a FVIII humoral immune response due to transient immunosuppression with cyclophosphamide. Histological studies revealed that the transplanted BOEC sheets were structured as flat clusters, supporting the long-term expression of therapeutic FVIII in plasma from an ectopic subcutaneous space. Our novel tissue-engineering approach using genetically modified BOEC sheets could aid in development of cell-based therapy that will allow safe and effective in vivo delivery of functional FVIII protein in patients with hemophilia A.

Gene therapy for hemophilia

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

A Novel Adenoviral Hybrid-vector System Carrying a Plasmid Replicon for Safe and Efficient Cell and Gene Therapeutic Applications

In dividing cells, the two aims a gene therapeutic approach should accomplish are efficient nuclear delivery and retention of therapeutic DNA. For stable transgene expression, therapeutic DNA can either be maintained by somatic integration or episomal persistence of which the latter approach would diminish the risk of insertional mutagenesis. As most monosystems fail to fulfill both tasks with equal efficiency, hybrid-vector systems represent promising alternatives. Our hybrid-vector system synergizes high-capacity adenoviral vectors (HCAdV) for efficient delivery and the scaffold/matrix attachment region (S/MAR)–based pEPito plasmid replicon for episomal persistence. After proving that this plasmid replicon can be excised from adenovirus in vitro, colony forming assays were performed. We found an increased number of colonies of up to sevenfold in cells that received the functional plasmid replicon proving that the hybrid-vector system is functional. Transgene expression could be maintained for 6 weeks and the extrachromosomal plasmid replicon was rescued. To show efficacy in vivo, the adenoviral hybrid-vector system was injected into C57Bl/6 mice. We found that the plasmid replicon can be released from adenoviral DNA in murine liver resulting in long-term transgene expression. In conclusion, we demonstrate the efficacy of our novel HCAdV-pEPito hybrid-vector system in vitro and in vivo.

Recent progress in gene therapy for hemophilia

Hemophilia A and B are X-linked monogenic disorders caused by deficiencies in coagulation factor VIII (FVIII) and factor IX (FIX), respectively. Current treatment for hemophilia involves intravenous infusion of clotting factor concentrates. However, this does not constitute a cure, and the development of gene-based therapies for hemophilia to achieve prolonged high level expression of clotting factors to correct the bleeding diathesis are warranted. Different types of viral and nonviral gene delivery systems and a wide range of different target cells, including hepatocytes, skeletal muscle cells, hematopoietic stem cells (HSCs), and endothelial cells, have been explored for hemophilia gene therapy. Adeno-associated virus (AAV)-based and lentiviral vectors are among the most promising vectors for hemophilia gene therapy. Stable correction of the bleeding phenotypes in hemophilia A and B was achieved in murine and canine models, and these promising preclinical studies prompted clinical trials in patients suffering from severe hemophilia. These studies recently resulted in the first demonstration that long-term expression of therapeutic FIX levels could be achieved in patients undergoing gene therapy. Despite this progress, there are still a number of hurdles that need to be overcome. In particular, the FIX levels obtained were insufficient to prevent bleeding induced by trauma or injury. Moreover, the gene-modified cells in these patients can become potential targets for immune destruction by effector T cells, specific for the AAV vector antigens. Consequently, more efficacious approaches are needed to achieve full hemostatic correction and to ultimately establish a cure for hemophilia A and B.

Porcine model of hemophilia A

Hemophilia A is a common X chromosome-linked genetic bleeding disorder caused by abnormalities in the coagulation factor VIII gene (F8). Hemophilia A patients suffer from a bleeding diathesis, such as life-threatening bleeding in the brain and harmful bleeding in joints and muscles. Because it could potentially be cured by gene therapy, subhuman animal models have been sought. Current mouse hemophilia A models generated by gene targeting of the F8 have difficulties to extrapolate human disease due to differences in the coagulation and immune systems between mice and humans. Here, we generated a porcine model of hemophilia A by nuclear transfer cloning from F8-targeted fibroblasts. The hemophilia A pigs showed a severe bleeding tendency upon birth, similar to human severe hemophiliacs, but in contrast to hemophilia A mice which rarely bleed under standard breed conditions. Infusion of human factor VIII was effective in stopping bleeding and reducing the bleeding frequency of a hemophilia A piglet but was blocked by the inhibitor against human factor VIII. These data suggest that the hemophilia A pig is a severe hemophilia A animal model for studying not only hemophilia A gene therapy but also the next generation recombinant coagulation factors, such as recombinant factor VIII variants with a slower clearance rate.

A TLR and non-TLR mediated innate response to lentiviruses restricts hepatocyte entry and can be ameliorated by pharmacological blockade

Lentiviral vector (LV)-mediated gene transfer is a promising method of gene therapy. We previously reported that systemic injection of HIV-based LV triggers a transient inflammatory response. Here, we carried out studies to better characterize this response, and to develop a strategy to overcome the adverse effects of interferon (IFN) on LV-mediated gene transfer. We profiled gene expression in the liver after LV administration using deep-sequencing (RNA-seq), and identified several innate response pathways. We examined the response to LV in MyD88-TRIF knockout mice, which are incapable of toll-like receptor (TLR) signaling. Unexpectedly, the IFN response to LV was not reduced in the liver indicating that a non-TLR pathway can recognize LV in this organ. Indeed, blocking reverse transcription with azidothymidine (AZT) reduced the IFN response only in the liver, suggesting that proviral DNA can be a trigger. To block the inflammatory response, we pretreated mice with a short course of dexamethasone (Dex). At 4 hours post-treatment, all the IFN-induced genes were normalized. By blocking the inflammatory response, hepatocyte transduction was dramatically increased, which in turn doubled the level of human factor IX (FIX) produced by a hepatocyte-specific LV. Our studies uncover new insights into LV-induced immune responses in the liver, and provide a means to increase the safety and efficiency of LV-mediated gene transfer.

Endothelial progenitor cell-based therapy for hemophilia A

As shown by the results of both pre-clinical and clinical studies reported in past decades, the goal of establishing an effective and successful gene therapy for hemophilia A remains feasible and realistic. However, at this time, no single approach has been shown to be clearly superior, and a number of recurring challenges remain to be overcome. Given the persistent problems presented by the host immune response to systemic in vivo gene delivery, and the additional obstacles of inadequate transgene delivery and expression, we propose a re-evaluation of an ex vivo gene transfer approach that utilizes a genetically modified stem cell population. In this strategy, autologous blood outgrowth endothelial progenitor cells are obtained from hemophilic animals, into which a normal copy of the factor VIII gene is introduced via an engineered virus. Cell numbers are expanded in culture prior to their re-implantation under the skin of the hemophilic animals in an artificially developed supporting environment. Follow-up assessment of the treatment involves the general evaluation of clotting activity, the specific measurement of factor VIII levels in the blood, and clinical observation.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

miRNA cassettes in viral vectors: problems and solutions

The discovery of RNA interference (RNAi), an evolutionary conserved gene silencing mechanism that is triggered by double stranded RNA, has led to tremendous efforts to use this technology for basic research and new RNA therapeutics. RNAi can be induced via transfection of synthetic small interfering RNAs (siRNAs), which results in a transient knockdown of the targeted mRNA. For stable gene silencing, short hairpin RNA (shRNA) or microRNA (miRNA) constructs have been developed. In mammals and humans, the natural RNAi pathway is triggered via endogenously expressed miRNAs. The use of modified miRNA expression cassettes to elucidate fundamental biological questions or to develop therapeutic strategies has received much attention. Viral vectors are particularly useful for the delivery of miRNA genes to specific target cells. To date, many viral vectors have been developed, each with distinct characteristics that make one vector more suitable for a certain purpose than others. This review covers the recent progress in miRNA-based gene-silencing approaches that use viral vectors, with a focus on their unique properties, respective limitations and possible solutions. Furthermore, we discuss a related topic that involves the insertion of miRNA-target sequences in viral vector systems to restrict their cellular range of gene expression. This article is part of a Special Issue entitled: MicroRNAs in viral gene regulation.

Helper-dependent adenoviral vectors for liver-directed gene therapy

Helper-dependent adenoviral (HDAd) vectors devoid of all viral-coding sequences are promising non-integrating vectors for liver-directed gene therapy because they have a large cloning capacity, can efficiently transduce a wide variety of cell types from various species independent of the cell cycle and can result in long-term transgene expression without chronic toxicity. The main obstacle preventing clinical applications of HDAd for liver-directed gene therapy is the host innate inflammatory response against the vector capsid proteins that occurs shortly after intravascular vector administration resulting in acute toxicity, the severity of which is dependent on vector dose. Intense efforts have been focused on elucidating the factors involved in this acute response and various strategies have been investigated to improve the therapeutic index of HDAd vectors. These strategies have yielded encouraging results with the potential for clinical translation.

HIV-1-derived lentiviral vectors directly activate plasmacytoid dendritic cells, which in turn induce the maturation of myeloid dendritic cells

Lentiviral vectors (LV) can induce type I interferon (IFN I) production from murine plasmacytoid dendritic cells (pDC), but not myeloid (my)DC. Here, we investigated whether this mechanism is conserved in human DC. MyDC and pDC were isolated from peripheral blood and transduced with increasing vector concentrations. Compared with in vitro differentiated monocyte-derived DC, the transduction efficiency of peripheral blood DC was low (ranging from <1% to 45%), with pDC showing the lowest susceptibility to LV transduction. Phenotype and function of myDC were not directly modified by LV transduction; by contrast, pDC produced significant levels of IFN-α and tumor necrosis factor-α. pDC activation was dependent on functional vector particles and was mediated by Toll-like receptor 7/9 triggering. Coculture of myDC with pDC in the presence of LV resulted in myDC activation, with CD86 up-regulation and interleukin-6 secretion. These findings demonstrate that the induction of transgene-specific immunity is triggered by an innate immune response with pDC activation and consequent myDC maturation, a response that closely resembles the one induced by functional viruses. This information is important to design strategies aimed at using LV in humans for gene therapy, where adverse immune responses must be avoided, or for cancer immunotherapy, where inducing immunity is the goal.

Production of adenovirus vectors and their use as a delivery system for influenza vaccines

IMPORTANCE OF THE FIELD

With the emergence of highly pathogenic avian influenza H5N1 viruses that have crossed species barriers and are responsible for lethal infections in humans in many countries, there is an urgent need for the development of effective vaccines which can be produced in large quantities at a short notice and confer broad protection against these H5N1 variants. In order to meet the potential global vaccine demand in a pandemic scenario, new vaccine-production strategies must be explored in addition to the currently used egg-based technology for seasonal influenza.

AREAS COVERED IN THIS REVIEW

Adenovirus (Ad) based influenza vaccines represent an attractive alternative/supplement to the currently licensed egg-based influenza vaccines. Ad-based vaccines are relatively inexpensive to manufacture, and their production process does not require either chicken eggs or labor-intensive and time-consuming processes necessitating enhanced biosafety facilities. Most importantly, in a pandemic situation, this vaccine strategy could offer a stockpiling option to reduce the response time before a strain-matched vaccine could be developed.

WHAT THE READER WILL GAIN

This review discusses Ad-vector technology and the current progress in the development of Ad-based influenza vaccines.

TAKE HOME MESSAGE

Ad vector-based influenza vaccines for pandemic preparedness are under development to meet global vaccine demand.

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.

Repeated oral administration of chitosan/DNA nanoparticles delivers functional FVIII with the absence of antibodies in hemophilia A mice

BACKGROUND

Current treatment of hemophilia A is expensive and involves regular infusions of factor (F)VIII concentrates. The supply of functional FVIII is further compromised by the generation of neutralizing antibodies. Thus, the development of an alternative safe, cost effective, non-invasive treatment that circumvents immune response induction is desirable.

OBJECTIVES

To evaluate the feasibility of oral administration of chitosan nanoparticles containing FVIII DNA to provide sustainable FVIII activity in hemophilia A mice.

METHODS

Nanoparticles were characterized for morphology, DNA protection and transfection efficiency. Oral administration of nanoparticles containing canine FVIII in C57Bl/6 FVIII(-/-) hemophilia A mice was evaluated for biodistribution, plasma FVIII activity and phenotypic correction. Sustainable FVIII expression was elucidated after repeated nanoparticle administration. Immune responses to repeated oral nanoparticle administration were also investigated.

RESULTS

Chitosan nanoparticles had a particle size range of 200-400 nm and protected DNA from endonuclease and pH degradation. In addition, nanoparticles transfected HEK 293 cells resulted in expression of eGFP, luciferase and FVIII. Hemophilia A mice that ingested chitosan nanoparticles demonstrated transient canine FVIII expression reaching > 100 mU 1 day after treatment, together with partial phenotypic correction. The delivered FVIII plasmid DNA was detected in the intestine and, to a lesser extent, in the liver. Importantly, repeated weekly administrations restored FVIII activity. Furthermore, inhibitors and non-neutralizing FVIII antibodies were not detectable.

CONCLUSIONS

Repeat oral administration of FVIII DNA formulated in chitosan nanoparticles resulted in sustained FVIII activity in hemophilic mice, and thus may provide a non-invasive alternative treatment for hemophilia A.

Gene therapy with helper-dependent adenoviral vectors: current advances and future perspectives

Recombinant Adenoviral vectors represent one of the best gene transfer platforms due to their ability to efficiently transduce a wide range of quiescent and proliferating cell types from various tissues and species. The activation of an adaptive immune response against the transduced cells is one of the major drawbacks of first generation Adenovirus vectors and has been overcome by the latest generation of recombinant Adenovirus, the Helper-Dependent Adenoviral (HDAd) vectors. HDAds have innovative features including the complete absence of viral coding sequences and the ability to mediate high level transgene expression with negligible chronic toxicity. This review summarizes the many aspects of HDAd biology and structure with a major focus on in vivo gene therapy application and with an emphasis on the unsolved issues that these vectors still presents toward clinical application.

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

Hyperactive sleeping beauty transposase enables persistent phenotypic correction in mice and a canine model for hemophilia B

Sleeping Beauty (SB) transposase enables somatic integration of exogenous DNA in mammalian cells, but potency as a gene transfer vector especially in large mammals has been lacking. Herein, we show that hyperactive transposase system delivered by high-capacity adenoviral vectors (HC-AdVs) can result in somatic integration of a canine factor IX (cFIX) expression-cassette in canine liver, facilitating stabilized transgene expression and persistent haemostatic correction of canine hemophilia B with negligible toxicity. We observed stabilized cFIX expression levels during rapid cell cycling in mice and phenotypic correction of the bleeding diathesis in hemophilia B dogs for up to 960 days. In contrast, systemic administration of an inactive transposase system resulted in rapid loss of transgene expression and transient phenotypic correction. Notably, in dogs a higher viral dose of the active SB transposase system resulted into transient phenotypic correction accompanied by transient increase of liver enzymes. Molecular analysis of liver samples revealed SB-mediated integration and provide evidence that transgene expression was derived mainly from integrated vector forms. Demonstrating that a viral vector system can deliver clinically relevant levels of a therapeutic protein in a large animal model of human disease paves a new path toward the possible cure of genetic diseases.

Large-scale production of high-quality helper-dependent adenoviral vectors using adherent cells in cell factories

The most efficient and widely used system for generating helper-dependent adenoviral vectors (HDAds) is the Cre/loxP system developed by Graham and co-workers (Parks, R.J., Chen, L., Anton, M., Sankar, U., Rudnicki, M.A., and Graham, F.L. [ 1996 ]. Proc. Natl. Acad. Sci. U. S. A. 93, 13565-13570). Alternative systems have been developed for HDAd production, but all are limited by the technical complexity of a three-component vector production system for reproducibly generating large quantities of adenovirus with high infectivity and low helper virus (HV) contamination. Recently, these problems were addressed by Ng and co-workers (Palmer, D., and Ng, P. [ 2003 ]. Mol Ther. 8, 846-852), who developed an improved system that combines the use of a suspension-adapted producer cell line expressing high levels of Cre recombinase, a HV resistant to mutation, and a refined purification protocol. With this system, >1 x 10(13) highly infectious vector particles are easily produced without vector genome rearrangements and having very low HV contamination levels. However, the Ng system incorporates a spinner flask culture system that involves considerable time, effort, and tissue culture medium to produce HDAds. We have an alternative system to obtain comparable quantities with equivalent quality to the spinner flask approach but requiring reduced labor and lower volumes of medium. This method utilizes a 10-chamber cell factory with adherent cells to produce high infectivity of HDAds with minimal HV contamination while improving yield and reducing technical complexity, effort, and medium requirements. This system is easily translatable to the production of clinical-grade HDAds for human trials.

Recombinant canine B-domain-deleted FVIII exhibits high specific activity and is safe in the canine hemophilia A model

Production of recombinant B-domain-deleted canine factor VIII (cFVIII-BDD) unexpectedly revealed superior protein yields with 3-fold increased specific activity relative to human FVIII-BDD (hFVIII-BDD). We also determined that activated cFVIII-BDD is more stable than activated hFVIII-BDD. Furthermore, cFVIII-BDD is efficient at inducing hemostasis in human plasma containing FVIII inhibitors. Infusion of cFVIII-BDD in hemophilia A dogs resulted in correction of the disease phenotype with a pharmacokinetic profile similar to clinical experience with hFVIII-BDD. Notably, immune tolerance challenges with cFVIII-BDD in young and adult hemophilia A dogs did not induce the formation of neutralizing or nonneutralizing antibodies to cFVIII. These data establish the framework to quantitatively investigate the efficacy and safety in preclinical studies of novel therapies for hemophilia A.

A rapid protocol for construction and production of high-capacity adenoviral vectors

High-capacity adenoviral vectors (HC-AdVs) lacking all viral coding sequences were shown to result in long-term transgene expression and phenotypic correction in small and large animal models. It has been established that HC-AdVs show significantly reduced toxicity profiles compared with early-generation adenoviral vectors. Furthermore, with capsid-modified HC-AdV becoming available, we are just starting to understand the full potential of this vector system. However, for many researchers, the wide-scale use of HC-AdV is hampered by labor-intensive and complex production procedures. Herein, we provide a feasible and detailed protocol for efficient generation of HC-AdV. We introduce an efficient cloning strategy for the generation of recombinant HC-AdV vector genomes. Infection and amplification of the HC-AdV are performed in a spinner culture system. For purification, we routinely apply cesium chloride gradients. Finally, we describe various methods for establishing vector titers. Generation of high-titer HC-AdV can be achieved in 3 weeks.

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.

Thromboelastography reflects global hemostatic variation among severe haemophilia A dogs at rest and following acute exercise

The heterogeneity among severe haemophilia A patients reflects on variable tendencies for bleeding and also variable responses to FVIII therapy. This variability cannot be detected or predicted by routine coagulation tests. Thromboelastography (TEG) has recently been evaluated for assessing hemostatic patterns in haemophiliacs and proved valuable in monitoring therapy and/or prophylaxis, however, usually only in limited small case series. Exercise is an important component of overall haemophilia care, however, in severe haemophiliacs there is an increased risk of bleeding. The availability of a validated hemostatic test to evaluate the influence of exercise would be advantageous. This study has used TEG analysis to evaluate the global hemostatic status of a group of severe haemophilia A dogs at rest and after a standardized period of exercise. The study demonstrated significant inter and intra-individual variations based on TEG patterns at rest and following acute exercise as well as significant improvement of global hemostasis after exercise in the majority of tested dogs. The study supports the utilization of TEG in assessment of the hemostatic pattern in severe haemophilia A and provides a potential for utilizing TEG evaluation in managing exercise regimens for haemophilia care.

Phenotypic correction of ornithine transcarbamylase deficiency using low dose helper-dependent adenoviral vectors

BACKGROUND

Helper-dependent adenoviral vectors (HDAd) can mediate long-term phenotypic correction in the ornithine transacarbamylase (OTC)-deficient mice model with negligible chronic toxicity. However, the high doses required for metabolic correction will result in systemic inflammatory response syndrome in humans. This acute toxicity represents the major obstacle for clinical applications of HDAd vectors for the treatment of OTC deficiency. Strategies for reducing the dose necessary for disease correction are highly desirable because HDAd acute toxicity is clearly dose-dependent.

METHODS

We analysed a potent expression cassette and the hydrodynamic injection for the ability to reduce the HDAd dose necessary for phenotypic correction in OTC-deficient spf-ash mice.

RESULTS

We have developed a vector containing a potent expression cassette expressing the OTC transgene, which allowed phenotypic correction at lower doses. Our results suggest that vector expressing greater OTC levels allows correction of orotic acid overproduction at lower doses that make clinical translation more relevant. We were able to further reduce the minimal effective dose by delivering the vector through the hydrodynamic injection technique.

CONCLUSIONS

Vectors containing the expression cassette used in the present study, combined with other strategies for improving HDAd therapeutic index, will likely permit application of these vectors for the treatment of OTC deficiency as well as other urea cycle disorders.

Gene therapy for haemophilia

The ultimate goal of gene therapy is the replacement of a defective gene sequence with a corrected version to eliminate disease for the lifetime of the patient. This challenging task is not yet accomplished, however significant progress is evident. An initial spate of clinical trials attempting the treatment of haemophilia with gene transfer primarily resulted in the demonstration of good safety profiles, but without efficacy. Subsequent reengineering of vector plasmids and delivery systems resulted in markedly improved outcomes in animal models of the disease. The most recent clinical trial for the treatment of haemophilia B with gene transfer showed transient achievement of efficacy in the highest dose cohort tested, but also exposed a previously hidden barrier to the future success of these treatments. The progress and problems of gene therapies for haemorrhagic disorders will be discussed. This review will concentrate on approaches in or near clinical application.

Antigen-specific tolerance of human alpha1-antitrypsin induced by helper-dependent adenovirus

As efficient and less toxic virus-derived gene therapy vectors are developed, a pressing problem is to avoid immune response to the therapeutic gene product. Secreted therapeutic proteins potentially represent a special problem, as they are readily available to professional antigen-presenting cells throughout the body. Some studies suggest that immunity to serum proteins can be avoided in some mouse strains by using tissue-specific promoters. Here we show that expression of human alpha1-antitrypsin (AAT) was nonimmunogenic in the immune-responsive strain C3H/HeJ, when expressed from helper-dependent (HD) vectors using ubiquitous as well as tissue-specific promoters. Coadministration of less immunogenic HD vectors with an immunogenic first-generation vector failed to immunize, suggesting immune suppression rather than immune stealth. Indeed, mice primed with HD vectors were tolerant to immune challenge with hAAT emulsified in complete Freund's adjuvant. Such animals developed high-titer antibodies to coemulsified human serum albumin, showing that tolerance was antigen specific. AAT-specific T cell responses were depressed in tolerized animals, suggesting that tolerance affects both T and B cells. These results are consistent with models of high-dose tolerance of B cells and certain other suppressive mechanisms, and suggest that a high level of expression from HD vectors can be sufficient to induce specific immune tolerance to serum proteins.

Gene therapy for treatment of inherited haematological disorders

Gene therapy, a molecular medicine based on vector-mediated transfer of therapeutic genes, holds promise for a cure of monogenetic inherited diseases. In recent years, tremendous progress has been reported in the treatment of haematological disorders: clinical trials in severe combined immune deficiencies have been successful by using retroviral vectors to express target genes in haematopoietic stem cells, which after transplantation efficiently reconstituted the immune system concomitant with substantial improvement in the clinical status of patients. Conversely, unexpected adverse events were also encountered. In other work, progress towards clinical studies on ex vivo gene transfer for Fanconi anaemia and haemoglobinopathies has been made. Each approach features a unique treatment strategy and also faces various impediments to success. In the case of the X-linked bleeding disorder haemophilia, several Phase I/II clinical trials were conducted, including in vivo administration of viral vectors to skeletal muscle and liver. Adeno-associated viral gene transfer of coagulation Factor IX has been documented in human subjects, reaching therapeutic levels after infusion into a hepatic blood vessel. However, sustained expression of therapeutic levels (as shown in large animal models of haemophilia) has not yet been achieved in humans. In general, long-term follow-up will be important for assessment of the safety of all existing gene therapy strategies.

Ex Vivo Gene Therapy for Hemophilia A That Enhances Safe Delivery and Sustained In Vivo Factor VIII Expression from Lentivirally Engineered Endothelial Progenitors

Novel therapeutic strategies for hemophilia must be at least as effective as current treatments and demonstrate long-term safety. To date, several small clinical trials of hemophilia gene transfer have failed to show the promise of preclinical evaluations. Therefore, we wanted to develop and evaluate the feasibility of a novel ex vivo gene transfer strategy whereby cells derived from progenitor cells are engineered to express factor VIII (FVIII) and then implanted subcutaneously to act as a depot for FVIII expression. Circulating blood outgrowth endothelial cells (BOECs) were isolated from canine and murine blood and transduced with a lentiviral vector encoding the canine FVIII transgene. To enhance safety, these cells were implanted subcutaneously in a Matrigel scaffold, and the efficacy of this strategy was compared with i.v. delivery of engineered BOECs in nonhemophilic nonobese diabetic/severe combined immunodeficiency mice. Therapeutic levels of FVIII persisted for 15 weeks, and these levels of stable expression were extended to 20 weeks when the cytomegalovirus promoter was replaced with the thrombomodulin regulatory element. Subsequent studies in immunocompetent hemophilic mice, pretreated with tolerizing doses of FVIII or with transient immunosuppression, showed therapeutic FVIII expression for 27 weeks before the eventual return to baseline levels. This loss of transgene expression appears to be due to the disappearance of the implanted cells. The animals treated with either of the two tolerizing regimens did not develop anti-FVIII antibodies. Biodistribution analysis demonstrated that BOECs were retained inside the subcutaneous implants. These results indicate, for the first time, that genetically modified endothelial progenitor cells implanted in a subcutaneous scaffold can provide sustained therapeutic levels of FVIII and are a promising and safe treatment modality for hemophilia A. Disclosure of potential conflicts of interest is found at the end of this article.

Gene therapy for liver enzyme deficiencies: what have we learned from models for Crigler-Najjar and tyrosinemia?

The liver is the site of numerous metabolic inherited diseases. It has unique features that make it compliant to various gene therapy approaches. Many vector types and gene delivery strategies have been evaluated during the past 20 years in a number of animal models of metabolic liver diseases. However, the complete cure of inherited liver deficiencies by gene therapy in relevant animal models were only reported recently. These successes were achieved thanks to major advances in vector technology. In this review, we will focus on Crigler-Najjar disease and hereditary tyrosinemia, two paradigmatic examples of the two categories of enzymatic liver deficiencies: type I, in which the genetic defect does not affect liver histology; and type II, in which liver lesions are present.

Correction of murine hemophilia A and immunological differences of factor VIII variants delivered by helper-dependent adenoviral vectors

Bioengineering of the factor VIII (FVIII) molecule has led to the production of variants that overcome poor secretion and/or rapid inactivation. We tested six modified FVIII variants for in vivo efficacy by expressing them from helper-dependent adenoviral (HD-Ad) vectors. We constructed a wild-type (WT) variant, a B-domain-deleted (BDD) variant, a point mutant for improved secretion (F309S), a variant with a partial B-domain deletion for improved secretion (N6), a combination of the point mutant and partial BDD variant (F309N6), and an inactivation-resistant (IR8) FVIII variant. All the constructs expressed functional protein after injection of high-dose HD-Ad. Activity ranged from 20 to 50% with WT, to approximately 100% with the N6 and F309N6 variants. Interestingly, mice treated with N6 showed long-term FVIII activity and phenotypic correction for up to 74 weeks, with low anti-FVIII antibody titer. Importantly, the N6 variant was therapeutically efficacious even after a 50% reduction of viral dose, thereby indicating that transgene modification itself can improve the dose efficacy of HD-Ad. This finding is significant, because dose efficacy is a key factor in clinical application. In summary, bioengineering of the FVIII molecule may be an effective approach to improving the safety, immunogenicity, and efficacy of HD-Ad gene therapy in hemophilia A (HA).

Immune responses against tetracycline-dependent transactivators affect long-term expression of mouse erythropoietin delivered by a helper-dependent adenoviral vector

BACKGROUND

Helper-dependent adenoviral (HD-Ad) vectors give rise to sustained gene expression after delivery in a variety of organisms. In particular, we previously documented persistent expression of erythropoietin (EPO) in mice after a single intramuscular (i.m.) injection of a HD-Ad vector harboring the mouse EPO cDNA.

METHODS

We use the same vector harboring the tetracycline (tet)-dependent transactivator (rtTA2S-M2) and silencer (tTS) and mouse EPO cDNA to analyze the capacity of the dual tet-dependent transactivator system to control long-term EPO gene expression and to study the effect of an eventual immune response against these artificial proteins after i.m. delivery in immuno-competent mice.

RESULTS

In the present study we demonstrate that i.m. injection of this vector in immuno-competent mice generates a cellular immune response to the rtTA2S-M2 transcription factor. This response curtails the duration of mouse EPO expression in mice, presumably by destroying the cells that co-express transcription factors and the therapeutic gene. Nonetheless, regulation of mouse EPO secretion was maintained during the entire experimental period, both when the vector dosage was reduced and when the tet-dependent transcription factors were put under the control of a muscle-specific promoter.

CONCLUSIONS

Delivery of the tet transactivators using as vehicle a HD-Ad vector induced an immune response directed against the transactivators themselves, causing short-term therapeutic transgene expression. Regulated, long-term therapeutic transgene expression was, however, obtained by reducing the vector dose or expressing the transactivators under the control of a muscle-specific promoter.

Genetic engineering for haemophilia A

At first sight, haemophilia A would appear to be an ideal candidate for treatment by gene therapy. There is a single gene defect; cells in different parts of the body, but especially the liver, produce Factor VIII, and only 5% of normal levels of Factor VIII are necessary to prevent the serious symptoms of bleeding. This review attempts to outline the status of gene therapy at present and efforts that have been made to overcome the difficulties and remaining problems that require solving. Undoubtedly, success will be achieved, but it is likely that considerably more work will be necessary before experimental models can be introduced into the clinic with any likelihood of success. The most successful results in animals that may have clinical application were from introducing the Factor VIII gene to newborn animals before antibodies are produced, presumably inducing a state of tolerance.