Pharmacological modulation of humoral immunity in a nonhuman primate model of AAV gene transfer for hemophilia B

Adeno-associated virus as a delivery vector for gene therapy of human diseases

Adeno-associated virus (AAV) has emerged as a pivotal delivery tool in clinical gene therapy owing to its minimal pathogenicity and ability to establish long-term gene expression in different tissues. Recombinant AAV (rAAV) has been engineered for enhanced specificity and developed as a tool for treating various diseases. However, as rAAV is being more widely used as a therapy, the increased demand has created challenges for the existing manufacturing methods. Seven rAAV-based gene therapy products have received regulatory approval, but there continue to be concerns about safely using high-dose viral therapies in humans, including immune responses and adverse effects such as genotoxicity, hepatotoxicity, thrombotic microangiopathy, and neurotoxicity. In this review, we explore AAV biology with an emphasis on current vector engineering strategies and manufacturing technologies. We discuss how rAAVs are being employed in ongoing clinical trials for ocular, neurological, metabolic, hematological, neuromuscular, and cardiovascular diseases as well as cancers. We outline immune responses triggered by rAAV, address associated side effects, and discuss strategies to mitigate these reactions. We hope that discussing recent advancements and current challenges in the field will be a helpful guide for researchers and clinicians navigating the ever-evolving landscape of rAAV-based gene therapy.

Circumventing B Cell Responses to Allow for Redosing of Adeno-Associated Virus Vectors

Adeno-associated virus (AAV)-mediated gene therapy has made significant progress in the last few decades. Nevertheless, challenges imposed by the immune system remain. The very high doses of AAV vectors used for some disorders have resulted in serious adverse events (SAEs) or even deaths, demonstrating that AAV vector doses that can safely be injected into patients are limited and for some indications below the therapeutic dose. Currently used immunosuppressive drugs have not prevented the SAEs, indicating that it may be prudent to treat patients with repeated transfer of moderate doses rather than a single injection of high doses of AAV vectors. The former approach has been avoided as AAV vectors elicit neutralizing antibodies that prevent successful reapplication of serologically crossreactive vectors. Immunosuppressive regimens that block B cell responses to AAV vectors or treatments that remove AAV neutralizing antibodies thus need to be developed to allow for a shift from toxic single-dose injections of AAV vectors to repeated treatments with more moderate and safe doses. Preventing or blocking antibody responses would also allow for redosing of patients with declining transgene product expression, or for effective AAV-mediated gene transfer into patients with the pre-existing neutralizing antibodies.

Current limitations of gene therapy for rare pediatric diseases: Lessons learned from clinical experience with AAV vectors

Gene therapy using adeno-associated viral (AAV) vectors is a promising therapeutic strategy for multiple inherited diseases. Following intravenous injection, AAV vectors carrying a copy of the missing gene or the genome-editing machinery reach their target cells and deliver the genetic material. Several clinical trials are currently ongoing and significant success has already been achieved with at least six AAV gene therapy products with market approval in Europe and the United States. Nonetheless, clinical trials and preclinical studies have uncovered several limitations of AAV gene transfer, which need to be addressed in order to improve the safety and enable the treatment of the largest patient population. Limitations include the occurrence of immune-mediated toxicities, the potential loss of correction in the long run, and the development of neutralizing antibodies against AAV vectors preventing re-administration. In this review, we summarize these limitations and discuss the potential technological developments to overcome them. © 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.

Readministration of high-dose adeno-associated virus gene therapy vectors enabled by ImmTOR nanoparticles combined with B cell-targeted agents

Tolerogenic ImmTOR nanoparticles encapsulating rapamycin have been demonstrated to mitigate immunogenicity of adeno-associated virus (AAV) gene therapy vectors, enhance levels of transgene expression, and enable redosing of AAV at moderate vector doses of 2 to 5E12 vg/kg. However, recent clinical trials have often pushed AAV vector doses 10-fold to 50-fold higher, with serious adverse events observed at the upper range. Here, we assessed combination therapy of ImmTOR with B cell-targeting drugs for the ability to increase the efficiency of redosing at high vector doses. The combination of ImmTOR with a monoclonal antibody against B cell activation factor (aBAFF) exhibited strong synergy leading to more than a 5-fold to 10-fold reduction of splenic mature B cells and plasmablasts while increasing the fraction of pre-/pro-B cells. In addition, this combination dramatically reduced anti-AAV IgM and IgG antibodies, thus enabling four successive AAV administrations at doses up to 5E12 vg/kg and at least two AAV doses at 5E13 vg/kg, with the transgene expression level in the latter case being equal to that observed in control animals receiving a single vector dose of 1E14 vg/kg. Similar synergistic effects were seen with a combination of ImmTOR and a Bruton's tyrosine kinase inhibitor, ibrutinib. These results suggest that ImmTOR could be combined with B cell-targeting agents to enable repeated vector administrations as a potential strategy to avoid toxicities associated with vector doses above 1E14 vg/kg.

Efficient removal of antibodies to adeno-associated viruses by immunoadsorption

BACKGROUND

Gene therapies based on adeno-associated viruses (AAV) are a therapeutic option to successfully treat monogenetic diseases. However, the influence of pre-existing immunity to AAV can compromise the application of AAV gene therapy, most notably by the presence of neutralizing antibodies (NAb) to AAV.

METHODS

In the following study, we investigated to what extent the treatment by immunoadsorption (IA) would reduce the levels of human anti-AAV antibodies to AAV2 and AAV5. To that end, we screened blood sera from 40 patients receiving IA treatment because of underlying autoimmune disease or transplant rejection, with detectable AAV-antibodies in 23 patients (22 by NAb detection, and 1 additionally by anti-AAV5 ELISA analysis).

RESULTS

Our results show that IA efficiently depleted anti-AAV2 NAb with a mean reduction of 3.92 ± 1.09 log2 titer steps (93.4%) after three to five single IA treatments, 45% of seropositive subjects had an anti-AAV2 titer below the threshold titer of 1:5 after the IA treatment series. Anti-AAV5 NAb were reduced to below the threshold titer of 1:5 in all but one of five seropositive subjects. Analysis of total anti-AAV5 antibodies by ELISA demonstrated an anti-AAV5 antibody reduction over the IA treatment series of 2.67 ± 1.16 log2 titer steps (84.3%).

CONCLUSION

In summary, IA may represent a safe strategy to precondition patients with pre-existing anti-AAV antibodies to make this population eligible for an effective AAV-based gene therapy.

MiR133b-mediated inhibition of EGFR-PTK pathway promotes rAAV2 transduction by facilitating intracellular trafficking and augmenting second-strand synthesis

Recombinant adeno-associated virus (rAAV) is an extremely attractive vector in the in vivo delivery of gene therapy as it is safe and its genome is simple. However, challenges including low permissiveness to specific cells and restricted tissue specificity have hindered its clinical application. Based on the previous studies, epidermal growth factor receptor-protein tyrosine kinase (EGFR-PTK) negatively regulated rAAV transduction, and EGFR-positive cells were hardly permissive to rAAV transduction. We constructed a novel rAAV-miRNA133b vector, which co-expressed miRNA133b and transgene, and investigated its in vivo and in vitro transduction efficiency. Confocal microscopy, live-cell imaging, pharmacological reagents and labelled virion tracking were used to analyse the effect of miRNA133b on rAAV2 transduction and the underlying mechanisms. The results demonstrated that miRNA133b could promote rAAV2 transduction and the effects were limited to EGFR-positive cells. The increased transduction was found to be a direct result of decreased rAAV particles degradation in the cytoplasm and enhanced second-strand synthesis. ss-rAAV2-miRNA133b vector specifically increased rAAV2 transduction in EGFR-positive cells or tissues, while ss-rAAV2-Fluc-miRNA133b exerted an antitumor effect. rAAV-miRNA133b vector might emerge as a promising platform for delivering various transgene to treat EGFR-positive cell-related diseases, such as non-small-cell lung cancer.

Recent Advancements in AAV-Vectored Immunoprophylaxis in the Nonhuman Primate Model

Monoclonal antibodies (mAbs) are important treatment modalities for preventing and treating infectious diseases, especially for those lacking prophylactic vaccines or effective therapies. Recent advances in mAb gene cloning from naturally infected or immunized individuals has led to the development of highly potent human mAbs against a wide range of human and animal pathogens. While effective, the serum half-lives of mAbs are quite variable, with single administrations usually resulting in short-term protection, requiring repeated doses to maintain therapeutic concentrations for extended periods of time. Moreover, due to their limited time in circulation, mAb therapies are rarely given prophylactically; instead, they are generally administered therapeutically after the onset of symptoms, thus preventing mortality, but not morbidity. Adeno-associated virus (AAV) vectors have an established record of high-efficiency in vivo gene transfer in a variety of animal models and humans. When delivered to post-mitotic tissues such as skeletal muscle, brain, and heart, or to organs in which cells turn over slowly, such as the liver and lungs, AAV vector genomes assume the form of episomal concatemers that direct transgene expression, often for the lifetime of the cell. Based on these attributes, many research groups have explored AAV-vectored delivery of highly potent mAb genes as a strategy to enable long-term expression of therapeutic mAbs directly in vivo following intramuscular or intranasal administration. However, clinical trials in humans and studies in nonhuman primates (NHPs) indicate that while AAVs are a powerful and promising platform for vectored immunoprophylaxis (VIP), further optimization is needed to decrease anti-drug antibody (ADA) and anti-capsid antibody responses, ultimately leading to increased serum transgene expression levels and improved therapeutic efficacy. The following review will summarize the current landscape of AAV VIP in NHP models, with an emphasis on vector and transgene design as well as general delivery system optimization. In addition, major obstacles to AAV VIP, along with implications for clinical translation, will be discussed.

IgG-cleavage protein allows therapeutic AAV gene delivery in passively immunized MPS IIIA mice

The widespread pre-existing αAAV-Abs in humans pose a critical challenge in translation of AAV gene therapy. The IgG degrading enzyme of Streptococci (IdeS) is demonstrated to specifically cleave IgG of humans and other species (not mouse). This study developed a modified new modified IdeS protein product (IdeSop). When incubated in vitro, IdeSop was shown to completely cleave human and rabbit IgGs within 6 h. To test IdeSop in a disease setting, we established a rabbitized αAAV9-Ab+ mouse by an IV infusion of purified acute αAAV9-Ab+ rabbit IgG into MPS IIIA mice, resulting in serum αAAV9-IgG at 1:6,400 and αAAV9-nAbs at 1:800. IdeSop-Ab-cleavage was shown to be dose-dependent. An IV IdeSop infusion at the effective doses resulted in rapid IgG depletion and clearance of pre-existing αAAV9-IgG and αAAV9-nAbs in rabbitized αAAV9-Abs+ MPS IIIA mice. Importantly, an IV injection of a high dose AAV9-hSGSHop vector (5 × 1013vg/kg) at 24 h post IdeSop treatment led to transduction as effective in αAAV9-Abs+ MPS IIIA mice, as in αAAV9-Abs-negative controls. We believe that transient IdeSop administration may offer a great tool to address the pre-existing-αAAV-Abs for the translation of rAAV gene therapy to treat diseases in humans, making effective rAAV gene therapy available to all patients in need.

Immune responses in the mammalian inner ear and their implications for AAV-mediated inner ear gene therapy

Adeno-associated virus (AAV)-mediated inner ear gene therapy is a promising treatment option for hearing loss and dizziness. Several studies have shown that AAV-mediated inner ear gene therapy can be applied to various mouse models of hereditary hearing loss to improve their auditory function. Despite the increase in AAV-based animal and clinical studies aiming to rescue auditory and vestibular functions, little is currently known about the host immune responses to AAV in the mammalian inner ear. It has been reported that the host immune response plays an important role in the safety and efficacy of viral-mediated gene therapy. Therefore, in order for AAV-mediated gene therapy to be successfully and safely translated into patients with hearing loss and dizziness, a better understanding of the host immune responses to AAV in the inner ear is critical. In this review, we summarize the current knowledge on host immune responses to AAV-mediated gene therapy in the mammalian inner ear and other organ systems. We also outline the areas of research that are critical for ensuring the safety and efficacy of AAV-mediated inner ear gene therapy in future clinical and translational studies.

Binding and neutralizing anti-AAV antibodies: Detection and implications for rAAV-mediated gene therapy

Assessment of anti-adeno-associated virus (AAV) antibodies in patients prior to systemic gene therapy administration is an important consideration regarding efficacy and safety of the therapy. Approximately 30%-60% of individuals have pre-existing anti-AAV antibodies. Seroprevalence is impacted by multiple factors, including geography, age, capsid serotype, and assay type. Anti-AAV antibody assays typically measure (1) transduction inhibition by detecting the neutralizing capacity of antibodies and non-antibody neutralizing factors, or (2) total anti-capsid binding antibodies, regardless of neutralizing activity. Presently, there is a paucity of head-to-head data and standardized approaches associating assay results with clinical outcomes. In addition, establishing clinically relevant screening titer cutoffs is complex. Thus, meaningful comparisons across assays are nearly impossible. Although complex, establishing screening assays in routine clinical practice to identify patients with antibody levels that may impact favorable treatment outcomes is achievable for both transduction inhibition and total antibody assays. Formal regulatory approval of such assays as companion diagnostic tests will confirm their suitability for specific recombinant AAV gene therapies. This review covers current approaches to measure anti-AAV antibodies in patient plasma or serum, their potential impact on therapeutic safety and efficacy, and investigative strategies to mitigate the effects of pre-existing anti-AAV antibodies in patients.

Adeno-Associated Virus Gene Therapy for Hemophilia

In vivo gene therapy is rapidly emerging as a new therapeutic paradigm for monogenic disorders. For almost three decades, hemophilia A (HA) and hemophilia B (HB) have served as model disorders for the development of gene therapy. This effort is soon to bear fruit with completed pivotal adeno-associated viral (AAV) vector gene addition trials reporting encouraging results and regulatory approval widely anticipated in the near future for the current generation of HA and HB AAV vectors. Here we review the clinical development of AAV gene therapy for HA and HB and examine outstanding questions that have recently emerged from AAV clinical trials for hemophilia and other monogenic disorders.

Evading and overcoming AAV neutralization in gene therapy

Adeno-associated virus (AAV)-derived viral vectors are a promising platform for the delivery of curative, life-changing therapies to a huge number of patients with monogenic disorders. There are currently over 250 clinical trials ongoing worldwide. However, for these therapies to benefit as many patients as possible, techniques must be developed to treat those with pre-existing immunity and to potentially allow re-administration of a dose in the future, should efficacy wane over time. This review discusses the current state and prospects of technologies to evade and overcome these immune responses and allow successful treatment of the greatest number of patients possible.

Immune profiling of adeno-associated virus response identifies B cell-specific targets that enable vector re-administration in mice

Adeno-associated virus (AAV) vector-based gene therapies can be applied to a wide range of diseases. AAV expression can last for months to years, but vector re-administration may be necessary to achieve life-long treatment. Unfortunately, immune responses against these vectors are potentiated after the first administration, preventing the clinical use of repeated administration of AAVs. Reducing the immune response against AAVs while minimizing broad immunosuppression would improve gene delivery efficiency and long-term safety. In this study, we quantified the contributions of multiple immune system components of the anti-AAV response in mice. We identified B-cell-mediated immunity as a critical component preventing vector re-administration. Additionally, we found that IgG depletion alone was insufficient to enable re-administration, suggesting IgM antibodies play an important role in the immune response against AAV. Further, we found that AAV-mediated transduction is improved in µMT mice that lack functional IgM heavy chains and cannot form mature B-cells relative to wild-type mice. Combined, our results suggest that B-cells, including non-class switched B-cells, are a potential target for therapeutics enabling AAV re-administration. Our results also suggest that the µMT mice are a potentially useful experimental model for gene delivery studies since they allow repeated dosing for more efficient gene delivery from AAVs.

A versatile toolkit for overcoming AAV immunity

Recombinant adeno-associated virus (AAV) is a promising delivery vehicle for in vivo gene therapy and has been widely used in >200 clinical trials globally. There are already several approved gene therapy products, e.g., Luxturna and Zolgensma, highlighting the remarkable potential of AAV delivery. In the past, AAV has been seen as a relatively non-immunogenic vector associated with low risk of toxicity. However, an increasing number of recent studies indicate that immune responses against AAV and transgene products could be the bottleneck of AAV gene therapy. In clinical studies, pre-existing antibodies against AAV capsids exclude many patients from receiving the treatment as there is high prevalence of antibodies among humans. Moreover, immune response could lead to loss of efficacy over time and severe toxicity, manifested as liver enzyme elevations, kidney injury, and thrombocytopenia, resulting in deaths of non-human primates and patients. Therefore, extensive efforts have been attempted to address these issues, including capsid engineering, plasmapheresis, IgG proteases, CpG depletion, empty capsid decoy, exosome encapsulation, capsid variant switch, induction of regulatory T cells, and immunosuppressants. This review will discuss these methods in detail and highlight important milestones along the way.

MSD-based assays facilitate a rapid and quantitative serostatus profiling for the presence of anti-AAV antibodies

Adeno-associated virus (AAV) vector applications are often limited by capsid-directed humoral immune responses, mainly through neutralizing antibodies (NAbs), which are present throughout the human population due to natural AAV infections. Currently, antibody levels are often quantified via ELISA-based protocols or by cellular NAb assays and less frequently by in vivo NAb assays in mice. These methods need optimization for each serotype and are often not applicable to AAV variants with poor in vitro transduction. To tackle these limitations, we have established Meso Scale Discovery (MSD)-based assays for the quantification of binding antibodies (BAbs) and NAbs against the three most commonly used AAV serotypes, AAV2, AAV8, and AAV9. Both assays detect anti-AAV-IgG_1–3 with high sensitivity and consistency as shown in a screen of sera from 40 healthy human donors. Subsequently, BAb and NAb titers were determined for identification of seronegative animals in a non-human primate (NHP) cohort. Moreover, the MSD-based BAb assay protocol was extended to a panel of 14 different AAV serotypes. In summary, our platform allows a rapid and quantitative assessment of the immunological properties of any natural or engineered AAV variant irrespective of transduction efficiency and enables high-throughput screens.

Overcoming the Challenges Imposed by Humoral Immunity to AAV Vectors to Achieve Safe and Efficient Gene Transfer in Seropositive Patients

One of the major goals of in vivo gene transfer is to achieve long-term expression of therapeutic transgenes in terminally differentiated cells. The extensive clinical experience and the recent approval of Luxturna^® (Spark Therapeutics, now Roche) and Zolgensma^® (AveXis, now Novartis) place vectors derived from adeno-associated viruses (AAV) among the best options for gene transfer in multiple tissues. Despite these successes, limitations remain to the application of this therapeutic modality in a wider population. AAV was originally identified as a promising virus to derive gene therapy vectors because, despite infecting humans, it was not associated with any evident disease. Thee large proportion of AAV infections in the human population is now revealing as a limitation because after exposure to wild-type AAV, anti-AAV antibodies develops and may neutralize the vectors derived from the virus. Injection of AAV in humans is generally well-tolerated although the immune system can activate after the recognition of AAV vectors capsid and genome. The formation of high-titer neutralizing antibodies to AAV after the first injection precludes vector re-administration. Thus, both pre-existing and post-treatment humoral responses to AAV vectors greatly limit a wider application of this gene transfer modality. Different methods were suggested to overcome this limitation. The extensive preclinical data available and the large clinical experience in the control of AAV vectors immunogenicity are key to clinical translation and to demonstrate the safety and efficacy of these methods and ultimately bring a curative treatment to patients.

Single-Dose Intrathecal Dorsal Root Ganglia Toxicity of Onasemnogene Abeparvovec in Cynomolgus Monkeys

Intravenous onasemnogene abeparvovec is approved for the treatment of spinal muscular atrophy in children < 2 years. For later-onset patients, intrathecal onasemnogene abeparvovec may be advantageous over intravenous administration. Recently, microscopic dorsal root ganglion (DRG) changes were observed in nonhuman primates (NHPs) following intrathecal onasemnogene abeparvovec administration. To characterize these DRG findings, two NHP studies evaluating intrathecal onasemnogene abeparvovec administration were conducted: a 12-month study with a 6-week interim cohort and a 13-week study with a 2-week interim cohort. The latter investigated the potential impact of prednisolone or rituximab plus everolimus on DRG toxicity. An additional 6-month, single-dose, intravenous NHP study conducted in parallel evaluated onasemnogene abeparvovec safety (including DRG toxicity) with or without prednisolone coadministration. Intrathecal onasemnogene abeparvovec administration was well tolerated and not associated with clinical observations. Microscopic onasemnogene abeparvovec-related changes were observed in the DRG and trigeminal ganglion (TG) and included mononuclear cell inflammation and/or neuronal degeneration, which was colocalized with high vector transcript expression at 6 weeks postdose. Incidence and severity of DRG changes were generally decreased after 52 weeks compared with 6 weeks postdose. Other onasemnogene abeparvovec-related microscopic findings of axonal degeneration, mononuclear cell infiltrates and/or gliosis in the spinal cord, dorsal spinal nerve root/spinal nerves, and/or peripheral nerves were absent or found at decreased incidences and/or severities after 52 weeks. DRG and/or TG microscopic findings following intravenous onasemnogene abeparvovec dosing included minimal to slight neuronal degeneration and mononuclear cell inflammation at 6 weeks and 6 months postdose. Nervous system microscopic findings following intrathecal onasemnogene abeparvovec (≥1.2 × 10¹³ vg/animal) trended toward resolution after 52 weeks, supporting nonprogression of changes, including in the DRG. Onasemnogene abeparvovec-related DRG findings were not associated with electrophysiology changes and were not ameliorated by prednisolone or rituximab plus everolimus coadministration. The pathogenesis is possibly a consequence of increased vector genome transduction and/or transgene expression.

Preventing packaging of translatable P5-associated DNA contaminants in recombinant AAV vector preps

Recombinant adeno-associated virus (rAAV) vectors are increasingly being used for clinical gene transfer and have shown great potential for the treatment of several monogenic disorders. However, contaminant DNA from producer plasmids can be packaged into rAAV alongside the intended expression cassette-containing vector genome. The consequences of this are unknown. Our analysis of rAAV preps revealed abundant contaminant sequences upstream of the AAV replication (Rep) protein driving promoter, P5, on the Rep-Cap producer plasmid. Characterization of P5-associated contaminants after infection showed transfer, persistence, and transcriptional activity in AAV-transduced murine hepatocytes, in addition to in vitro evidence suggestive of integration. These contaminants can also be efficiently translated and immunogenic, revealing previously unrecognized side effects of rAAV-mediated gene transfer. P5-associated contaminant packaging and activity were independent of an inverted terminal repeat (ITR)-flanked vector genome. To prevent incorporation of these potentially harmful sequences, we constructed a modified P5-promoter (P5-HS), inserting a DNA spacer between an Rep binding site and an Rep nicking site in P5. This prevented upstream DNA contamination regardless of transgene or AAV serotype, while maintaining vector yield. Thus, we have constructed an rAAV production plasmid that improves vector purity and can be implemented across clinical rAAV applications. These findings represent new vector safety and production considerations for rAAV gene therapy.

New approaches to moderate CRISPR-Cas9 activity: Addressing issues of cellular uptake and endosomal escape

CRISPR-Cas9 is rapidly entering molecular biology and biomedicine as a promising gene-editing tool. A unique feature of CRISPR-Cas9 is a single-guide RNA directing a Cas9 nuclease toward its genomic target. Herein, we highlight new approaches for improving cellular uptake and endosomal escape of CRISPR-Cas9. As opposed to other recently published works, this review is focused on non-viral carriers as a means to facilitate the cellular uptake of CRISPR-Cas9 through endocytosis. The majority of non-viral carriers, such as gold nanoparticles, polymer nanoparticles, lipid nanoparticles, and nanoscale zeolitic imidazole frameworks, is developed with a focus toward optimizing the endosomal escape of CRISPR-Cas9 by taking advantage of the acidic environment in the late endosomes. Among the most broadly used methods for in vitro and ex vivo ribonucleotide protein transfection are electroporation and microinjection. Thus, other delivery formats are warranted for in vivo delivery of CRISPR-Cas9. Herein, we specifically revise the use of peptide and nanoparticle-based systems as platforms for CRISPR-Cas9 delivery in vivo. Finally, we highlight future perspectives of the CRISPR-Cas9 gene-editing tool and the prospects of using non-viral vectors to improve its bioavailability and therapeutic potential.

Adeno-Associated Viruses (AAV) and Host Immunity - A Race Between the Hare and the Hedgehog

Adeno-associated viruses (AAV) have emerged as the lead vector in clinical trials and form the basis for several approved gene therapies for human diseases, mainly owing to their ability to sustain robust and long-term in vivo transgene expression, their amenability to genetic engineering of cargo and capsid, as well as their moderate toxicity and immunogenicity. Still, recent reports of fatalities in a clinical trial for a neuromuscular disease, although linked to an exceptionally high vector dose, have raised new caution about the safety of recombinant AAVs. Moreover, concerns linger about the presence of pre-existing anti-AAV antibodies in the human population, which precludes a significant percentage of patients from receiving, and benefitting from, AAV gene therapies. These concerns are exacerbated by observations of cellular immune responses and other adverse events, including detrimental off-target transgene expression in dorsal root ganglia. Here, we provide an update on our knowledge of the immunological and molecular race between AAV (the “hedgehog”) and its human host (the “hare”), together with a compendium of state-of-the-art technologies which provide an advantage to AAV and which, thus, promise safer and more broadly applicable AAV gene therapies in the future.

ImmTOR nanoparticles enhance AAV transgene expression after initial and repeat dosing in a mouse model of methylmalonic acidemia

A major barrier to adeno-associated virus (AAV) gene therapy is the inability to re-dose patients due to formation of vector-induced neutralizing antibodies (Nabs). Tolerogenic nanoparticles encapsulating rapamycin (ImmTOR) provide long-term and specific suppression of adaptive immune responses, allowing for vector re-dosing. Moreover, co-administration of hepatotropic AAV vectors and ImmTOR leads to an increase of transgene expression even after the first dose. ImmTOR and AAV Anc80 encoding the methylmalonyl-coenzyme A (CoA) mutase (MMUT) combination was tested in a mouse model of methylmalonic acidemia, a disease caused by mutations in the MMUT gene. Repeated co-administration of Anc80 and ImmTOR was well tolerated and led to nearly complete inhibition of immunoglobulin (Ig)G antibodies to the Anc80 capsid. A more profound decrease of plasma levels of the key toxic metabolite, plasma methylmalonic acid (pMMA), and disease biomarker, fibroblast growth factor 21 (FGF21), was observed after treatment with the ImmTOR and Anc80-MMUT combination. In addition, there were higher numbers of viral genomes per cell (vg/cell) and increased transgene expression when ImmTOR was co-administered with Anc80-MMUT. These effects were dose-dependent, with the higher doses of ImmTOR providing higher vg/cell and mRNA levels, and an improved biomarker response. Combining of ImmTOR and AAV can not only block the IgG response against capsid, but it also appears to potentiate transduction and enhance therapeutic transgene expression in the mouse model.

Synthetic Biology: Emerging Concepts to Design and Advance Adeno-Associated Viral Vectors for Gene Therapy

Three recent approvals and over 100 ongoing clinical trials make adeno-associated virus (AAV)-based vectors the leading gene delivery vehicles in gene therapy. Pharmaceutical companies are investing in this small and nonpathogenic gene shuttle to increase the therapeutic portfolios within the coming years. This prospect of marking a new era in gene therapy has fostered both investigations of the fundamental AAV biology as well as engineering studies to enhance delivery vehicles. Driven by the high clinical potential, a new generation of synthetic-biologically engineered AAV vectors is on the rise. Concepts from synthetic biology enable the control and fine-tuning of vector function at different stages of cellular transduction and gene expression. It is anticipated that the emerging field of synthetic-biologically engineered AAV vectors can shape future gene therapeutic approaches and thus the design of tomorrow's gene delivery vectors. This review describes and discusses the recent trends in capsid and vector genome engineering, with particular emphasis on synthetic-biological approaches.

Humoral immune responses to AAV gene therapy in the ocular compartment

Viral vectors can be utilised to deliver therapeutic genes to diseased cells. Adeno-associated virus (AAV) is a commonly used viral vector that is favoured for its ability to infect a wide range of tissues whilst displaying limited toxicity and immunogenicity. Most humans harbour anti-AAV neutralising antibodies (NAbs) due to subclinical infections by wild-type virus during infancy and these pre-existing NAbs can limit the efficiency of gene transfer depending on the target cell type, route of administration and choice of serotype. Vector administration can also result in de novo NAb synthesis that could limit the opportunity for repeated gene transfer to diseased sites. A number of strategies have been described in preclinical models that could circumvent NAb responses in humans, however, the successful translation of these innovations into the clinical arena has been limited. Here, we provide a comprehensive review of the humoral immune response to AAV gene therapy in the ocular compartment. We cover basic AAV biology and clinical application, the role of pre-existing and induced NAbs, and possible approaches to overcoming antibody responses. We conclude with a framework for a comprehensive strategy for circumventing humoral immune responses to AAV in the future.

Immunomodulation in Administration of rAAV: Preclinical and Clinical Adjuvant Pharmacotherapies

Recombinant adeno-associated virus (rAAV) has attracted a significant research focus for delivering genetic therapies to target cells. This non-enveloped virus has been trialed in many clinical-stage therapeutic strategies but important obstacle in clinical translation is the activation of both innate and adaptive immune response to the protein capsid, vector genome and transgene product. In addition, the normal population has pre-existing neutralizing antibodies against wild-type AAV, and cross-reactivity is observed between different rAAV serotypes. While extent of response can be influenced by dosing, administration route and target organ(s), these pose concerns over reduction or complete loss of efficacy, options for re-administration, and other unwanted immunological sequalae such as local tissue damage. To reduce said immunological risks, patients are excluded if they harbor anti-AAV antibodies or have received gene therapy previously. Studies have incorporated immunomodulating or suppressive regimens to block cellular and humoral immune responses such as systemic corticosteroids pre- and post-administration of Luxturna^® and Zolgensma^®, the two rAAV products with licensed regulatory approval in Europe and the United States. In this review, we will introduce the current pharmacological strategies to immunosuppress or immunomodulate the host immune response to rAAV gene therapy.

Current Clinical Applications of In Vivo Gene Therapy with AAVs

Hereditary diseases are caused by mutations in genes, and more than 7,000 rare diseases affect over 30 million Americans. For more than 30 years, hundreds of researchers have maintained that genetic modifications would provide effective treatments for many inherited human diseases, offering durable and possibly curative clinical benefit with a single treatment. This review is limited to gene therapy using adeno-associated virus (AAV) because the gene delivered by this vector does not integrate into the patient genome and has a low immunogenicity. There are now five treatments approved for commercialization and currently available, i.e., Luxturna, Zolgensma, the two chimeric antigen receptor T cell (CAR-T) therapies (Yescarta and Kymriah), and Strimvelis (the gammaretrovirus approved for adenosine deaminase-severe combined immunodeficiency [ADA-SCID] in Europe). Dozens of other treatments are under clinical trials. The review article presents a broad overview of the field of therapy by in vivo gene transfer. We review gene therapy for neuromuscular disorders (spinal muscular atrophy [SMA]; Duchenne muscular dystrophy [DMD]; X-linked myotubular myopathy [XLMTM]; and diseases of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, Canavan disease, aromatic l-amino acid decarboxylase [AADC] deficiency, and giant axonal neuropathy), ocular disorders (Leber congenital amaurosis, age-related macular degeneration [AMD], choroideremia, achromatopsia, retinitis pigmentosa, and X-linked retinoschisis), the bleeding disorder hemophilia, and lysosomal storage disorders.

Early Phase Clinical Immunogenicity of Valoctocogene Roxaparvovec, an AAV5-Mediated Gene Therapy for Hemophilia A

Evaluation of immune responses to adeno-associated virus (AAV)-mediated gene therapies prior to and following dose administration plays a key role in determining therapeutic safety and efficacy. This report describes up to 3 years of immunogenicity data following administration of valoctocogene roxaparvovec (BMN 270), an AAV5-mediated gene therapy encoding human B domain-deleted FVIII (hFVIII-SQ) in a phase 1/2 clinical study of adult males with severe hemophilia A. Patients with pre-existing humoral immunity to AAV5 or with a history of FVIII inhibitors were excluded from the trial. Blood plasma and peripheral blood mononuclear cell (PBMC) samples were collected at regular intervals following dose administration for assessment of humoral and cellular immune responses to both the AAV5 vector and transgene-expressed hFVIII-SQ. The predominant immune response elicited by BMN 270 administration was largely limited to the development of antibodies against the AAV5 capsid that were cross-reactive with other common AAV serotypes. No FVIII inhibitor responses were observed within 3 years following dose administration. In a context of prophylactic or on-demand corticosteroid immunosuppression given after vector infusion, AAV5 and hFVIII-SQ peptide-specific cellular immune responses were intermittently detected by an interferon (IFN)-γ and tumor necrosis factor (TNF)-α FluoroSpot assay, but they were not clearly associated with detrimental safety events or changes in efficacy measures.

Timing of Intensive Immunosuppression Impacts Risk of Transgene Antibodies after AAV Gene Therapy in Nonhuman Primates

Adeno-associated virus (AAV) vector gene therapy is a promising treatment for a variety of genetic diseases, including hemophilia. Systemic administration of AAV vectors is associated with a cytotoxic immune response triggered against AAV capsid proteins, which if untreated can result in loss of transgene expression. Immunosuppression (IS) with corticosteroids has limited transgene loss in some AAV gene therapy clinical trials, but was insufficient to prevent loss in other studies. We used a nonhuman primate model to evaluate intensive T cell-directed IS combined with AAV-mediated transfer of the human factor IX (FIX) gene. Early administration of rabbit anti-thymocyte globulin (ATG) concomitant with AAV administration resulted in the development of anti-FIX antibodies, whereas delayed ATG by 5 weeks administration did not. The anti-FIX immune response was associated with increases in inflammatory cytokines, as well as a skewed Th17/regulatory T cell (Treg) ratio. We conclude that the timing of T cell-directed IS is critical in determining transgene-product immunogenicity or tolerance. These data have implications for systemically administered AAV gene therapy being evaluated for hemophilia A and B, as well as other genetic diseases.

Long-Term Follow-Up of the First in Human Intravascular Delivery of AAV for Gene Transfer: AAV2-hFIX16 for Severe Hemophilia B

Adeno-associated virus (AAV) vectors are a leading platform for gene-based therapies for both monogenic and complex acquired disorders. The success of AAV gene transfer highlights the need to answer outstanding clinical questions of safety, durability, and the nature of the human immune response to AAV vectors. Here, we present longitudinal follow-up data of subjects who participated in the first trial of a systemically delivered AAV vector. Adult males (n = 7) with severe hemophilia B received an AAV2 vector at doses ranging from 8 × 10¹⁰ to 2 × 10¹² vg/kg to target hepatocyte-specific expression of coagulation factor IX; a subset (n = 4) was followed for 12-15 years post-vector administration. No major safety concerns were observed. There was no evidence of sustained hepatic toxicity or development of hepatocellular carcinoma as assessed by liver transaminase values, serum α-fetoprotein, and liver ultrasound. Subjects demonstrated persistent, increased AAV neutralizing antibodies (NAbs) to the infused AAV serotype 2 (AAV2) as well as all other AAV serotypes tested (AAV5 and AAV8) for the duration of follow-up. These data represent the longest available longitudinal follow-up data of subjects who received intravascular AAV and support the preliminary safety of intravascular AAV administration at the doses tested in adults. Data demonstrate, for the first time, the persistence of high-titer, multi-serotype cross-reactive AAV NAbs for up to 15 years post- AAV vector administration. Our observations are broadly applicable to the development of AAV-mediated gene therapy.

IgG-cleaving endopeptidase enables in vivo gene therapy in the presence of anti-AAV neutralizing antibodies

Neutralizing antibodies to adeno-associated virus (AAV) vectors are highly prevalent in humans^1,2, and block liver transduction^3-5 and vector readministration⁶; thus, they represent a major limitation to in vivo gene therapy. Strategies aimed at overcoming anti-AAV antibodies are being studied⁷, which often involve immunosuppression and are not efficient in removing pre-existing antibodies. Imlifidase (IdeS) is an endopeptidase able to degrade circulating IgG that is currently being tested in transplant patients⁸. Here, we studied if IdeS could eliminate anti-AAV antibodies in the context of gene therapy. We showed efficient cleavage of pooled human IgG (intravenous Ig) in vitro upon endopeptidase treatment. In mice passively immunized with intravenous Ig, IdeS administration decreased anti-AAV antibodies and enabled efficient liver gene transfer. The approach was scaled up to nonhuman primates, a natural host for wild-type AAV. IdeS treatment before AAV vector infusion was safe and resulted in enhanced liver transduction, even in the setting of vector readministration. Finally, IdeS reduced anti-AAV antibody levels from human plasma samples in vitro, including plasma from prospective gene therapy trial participants. These results provide a potential solution to overcome pre-existing antibodies to AAV-based gene therapy.

Successful Transduction with AAV Vectors after Selective Depletion of Anti-AAV Antibodies by Immunoadsorption

Gene therapy with adeno-associated virus (AAV)-based vectors shows great promise for the gene therapeutic treatment of a broad array of diseases. In fact, the treatment of genetic diseases with AAV vectors is currently the only in vivo gene therapy approach that is approved by the US Food and Drug Administration (FDA). Unfortunately, pre-existing antibodies against AAV severely limit the patient population that can potentially benefit from AAV gene therapy, especially if the vector is delivered by intravenous injection. Here, we demonstrate that we can selectively deplete anti-AAV antibodies by hemapheresis combined with AAV9 particles coupled to Sepharose beads. In rats that underwent hemapheresis and immunoadsorption, luciferase expression was dramatically increased in the hearts and fully restored in the livers of these rats. Importantly, our method can be readily adapted for the use in clinical AAV gene therapy.

Immune Response Mechanisms against AAV Vectors in Animal Models

Early preclinical studies in rodents and other species did not reveal that vector or transgene immunity would present a significant hurdle for sustained gene expression. While there was early evidence of mild immune responses to adeno-associated virus (AAV) in preclinical studies, it was generally believed that these responses were too weak and transient to negatively impact sustained transduction. However, translation of the cumulative success in treating hemophilia B in rodents and dogs with an AAV2-F9 vector to human studies was not as successful. Despite significant progress in recent clinical trials for hemophilia, new immunotoxicities to AAV and transgene are emerging in humans that require better animal models to assess and overcome these responses. The animal models designed to address these immune complications have provided critical information to assess how vector dose, vector capsid processing, vector genome, difference in serotypes, and variations in vector delivery route can impact immunity and to develop approaches for overcoming pre-existing immunity. Additionally, a comprehensive dissection of innate, adaptive, and regulatory responses to AAV vectors in preclinical studies has provided a framework that can be utilized for development of immunomodulatory therapies to overcome or bypass immune responses and for developing strategic approaches toward engineering stealth AAV vectors that can circumvent immunity.

Immune-orthogonal orthologues of AAV capsids and of Cas9 circumvent the immune response to the administration of gene therapy

Protein-based therapeutics can activate the adaptive immune system and lead to the production of neutralizing antibodies and to cytotoxic-T-cell-mediated clearance of the treated cells. Here, we show that the sequential use of immune-orthogonal orthologues of the CRISPR-associated protein 9 (Cas9) and of adeno-associated viruses (AAVs) eludes adaptive immune responses and enables effective gene editing from repeated dosing. We compared total sequence similarities and predicted binding strengths to class-I and class-II major-histocompatibility-complex proteins for 284 DNA-targeting and 84 RNA-targeting CRISPR effectors, and for 167 AAV VP1-capsid-protein orthologues. We predict the absence of cross-reactive immune responses for 79% of the DNA-targeting Cas orthologs, which we validate for three Cas9 orthologs in mice, yet anticipate broad immune cross-reactivity among the AAV serotypes. We also show that efficacious in vivo gene editing is uncompromised when using multiple dosing with orthologues of AAVs and Cas9 in mice previously immunized against the AAV vector and the Cas9 payload. Multiple dosing with protein orthologues may allow for sequential regimens of protein therapeutics that circumvent pre-existing immunity or induced immunity.

Threshold for Pre-existing Antibody Levels Limiting Transduction Efficiency of Systemic rAAV9 Gene Delivery: Relevance for Translation

Widespread anti-AAV antibodies (Abs) in humans pose a critical challenge for the translation of AAV gene therapies, limiting patient eligibility. In this study, non-human primates (NHPs) with pre-existing αAAV Abs were used to investigate the impact of αAAV9 Ab levels on the transduction efficiency of rAAV9 via systemic delivery. No significant differences were observed in vector genome (vg) biodistribution in animals with ≤1:400 total serum αAAV9-IgG compared to αAAV9-Ab-negative animals, following an intravenous (i.v.) rAAV9-hNAGLU^op (codon-optimized human α-N-acetylglucosaminidase coding sequence cDNA) injection. Serum αAAV9-IgG at >1:400 resulted in a >200-fold decrease in vg in the liver, but had no significant effect on vg levels in brain and most of the peripheral tissues. Although tissue NAGLU activities declined significantly, they remained above endogenous levels. Notably, there were higher vg copies but lower NAGLU activity in the spleen in NHPs with >1:400 αAAV9 Abs than in those with ≤1:400 Abs. We demonstrate here the presence of a threshold of pre-existing αAAV9 Abs for diminishing the transduction of i.v.-delivered AAV vectors, supporting the expansion of patient eligibility for systemic rAAV treatments. Our data also indicate that high pre-existing αAAV9 Abs may promote phagocytosis and that phagocytized vectors are not processed for transgene expression, suggesting that effectively suppressing innate immunity may have positive impacts on transduction efficiency in individuals with high Ab titers.

Low Seroprevalence of Neutralizing Antibodies Targeting Two Clade F AAV in Humans

To assess the therapeutic utility of AAVHSC15 and AAVHSC17, two recently described Clade F adeno-associated viruses (AAVs), the seroprevalence of neutralizing antibodies (NAbs) to these AAVs was assessed in a representative human population and compared to that of AAV9. NAb levels were measured in 100 unique human sera of different races (34, Black, 33 Caucasian, and 33 Hispanic) and sex (49% female, 51% male) collected within the United States. Fifty-six sera were tested in Huh7 cells and 44 sera were tested in 2V6.11 cells with vectors packaged with either a CMV-promoter upstream of LacZ or a CBA-promoter upstream of Firefly Luciferase, respectively. For AAVHSC15, AAVHSC17, and AAV9, 24/100 (24%), 21/100 (21%), and 17/100 (17%), respectively, of all sera tested were seropositive for NAbs using 50% inhibition of cellular transduction at a 1/16 dilution of serum as cutoff for seropositivity. Only 6% of positive sera had titers of 1/150 to 1/340, indicating that the majority of positive sera were of low titer. Significant cross-reactivity of NAbs across all three AAV serotypes was observed. These data show that approximately 80% of humans evaluated were seronegative for pre-existing NAbs to the AAV serotypes tested, suggesting that the vast majority of human subjects would be amenable to therapeutic intervention with Clade F AAVs.

Adeno-associated viral gene therapy corrects a mouse model of argininosuccinic aciduria

Argininosuccinic aciduria (ASA) is the second most common genetic disorder affecting the urea cycle. The disease is caused by deleterious mutations in the gene encoding argininosuccinate lyase (ASL); total loss of ASL activity results in severe neonatal onset of the disease, which is characterized by hyperammonemia within a few days of birth that can rapidly progress to coma and death. The long-term complications of ASA, such as hypertension and neurocognitive deficits, appear to be resistant to the current treatment options of dietary restriction, arginine supplementation, and nitrogen scavenging drugs. Treatment-resistant disease is currently being managed by orthotopic liver transplant, which shows variable improvement and requires lifetime immunosuppression. Here, we developed a gene therapy strategy for ASA aimed at alleviating the symptoms associated with urea cycle disruption by providing stable expression of ASL protein in the liver. We designed a codon-optimized human ASL gene packaged within adeno-associated virus serotype 8 (AAV8) as a vector for targeted delivery to the liver. To evaluate the therapeutic efficacy of this approach, we utilized a murine hypomorphic model of ASA. Neonatal administration of AAV8 via the temporal facial vein extended survival in ASA hypomorphic mice, although not to wild-type levels. Intravenous injection into adolescent hypomorphic mice led to increased survival and body weight and correction of metabolites associated with the disease. Our results demonstrate that AAV8 gene therapy is a viable approach for the treatment of ASA.

Exposure to wild-type AAV drives distinct capsid immunity profiles in humans

Recombinant adeno-associated virus (AAV) vectors have been broadly adopted as a gene delivery tool in clinical trials, owing to their high efficiency of transduction of several host tissues and their low immunogenicity. However, a considerable proportion of the population is naturally exposed to the WT virus from which AAV vectors are derived, which leads to the acquisition of immunological memory that can directly determine the outcome of gene transfer. Here, we show that prior exposure to AAV drives distinct capsid immunity profiles in healthy subjects. In peripheral blood mononuclear cells (PBMCs) isolated from AAV-seropositive donors, recombinant AAV triggered TNF-α secretion in memory CD8+ T cells, B cell differentiation into antibody-secreting cells, and anti-capsid antibody production. Conversely, PBMCs isolated from AAV-seronegative individuals appeared to carry a population of NK cells reactive to AAV. Further, we demonstrated that the AAV capsid activates IL-1β and IL-6 cytokine secretion in monocyte-related dendritic cells (moDCs). IL-1β and IL-6 blockade inhibited the anti-capsid humoral response in vitro and in vivo. These results provide insights into immune responses to AAV in humans, define a possible role for moDCs and NK cells in capsid immunity, and open new avenues for the modulation of vector immunogenicity.

Antigen-selective modulation of AAV immunogenicity with tolerogenic rapamycin nanoparticles enables successful vector re-administration

Gene therapy mediated by recombinant adeno-associated virus (AAV) vectors is a promising treatment for systemic monogenic diseases. However, vector immunogenicity represents a major limitation to gene transfer with AAV vectors, particularly for vector re-administration. Here, we demonstrate that synthetic vaccine particles encapsulating rapamycin (SVP[Rapa]), co-administered with AAV vectors, prevents the induction of anti-capsid humoral and cell-mediated responses. This allows successful vector re-administration in mice and nonhuman primates. SVP[Rapa] dosed with AAV vectors reduces B and T cell activation in an antigen-selective manner, inhibits CD8⁺ T cell infiltration in the liver, and efficiently blocks memory T cell responses. SVP[Rapa] immunomodulatory effects can be transferred from treated to naive mice by adoptive transfer of splenocytes, and is inhibited by depletion of CD25⁺ T cells, suggesting a role for regulatory T cells. Co-administration of SVP[Rapa] with AAV vector represents a powerful strategy to modulate vector immunogenicity and enable effective vector re-administration.

Safety of Intradiaphragmatic Delivery of Adeno-Associated Virus-Mediated Alpha-Glucosidase (rAAV1-CMV-hGAA) Gene Therapy in Children Affected by Pompe Disease

A first-in-human trial of diaphragmatic gene therapy (AAV1-CMV-GAA) to treat respiratory and neural dysfunction in early-onset Pompe disease was conducted. The primary objective of this study was to assess the safety of rAAV1-CMV-hGAA vector delivered to the diaphragm muscle of Pompe disease subjects with ventilatory insufficiency. Safety was assessed by measurement of change in serum chemistries and hematology, urinalysis, and immune response to GAA and AAV, as well as change in level of health. The data demonstrate that the AAV treatment was safe and there were no adverse events related to the study agent. Adverse events related to the study procedure were observed in subjects with lower baseline neuromuscular function. All adverse events were resolved before the end of the study, except for one severe adverse event determined not to be related to either the study agent or the study procedure. In addition, an anti-capsid and anti-transgene antibody response was observed in all subjects who received rAAV1-CMV-hGAA, except for subjects who received concomitant immunomodulation to manage reaction to enzyme replacement therapy, as per their standard of care. This observation is significant for future gene therapy studies and serves to establish a clinically relevant approach to blocking immune responses to both the AAV capsid protein and transgene product.

Gene Therapy With Regulatory T Cells: A Beneficial Alliance

Gene therapy aims to replace a defective or a deficient protein at therapeutic or curative levels. Improved vector designs have enhanced safety, efficacy, and delivery, with potential for lasting treatment. However, innate and adaptive immune responses to the viral vector and transgene product remain obstacles to the establishment of therapeutic efficacy. It is widely accepted that endogenous regulatory T cells (Tregs) are critical for tolerance induction to the transgene product and in some cases the viral vector. There are two basic strategies to harness the suppressive ability of Tregs: in vivo induction of adaptive Tregs specific to the introduced gene product and concurrent administration of autologous, ex vivo expanded Tregs. The latter may be polyclonal or engineered to direct specificity to the therapeutic antigen. Recent clinical trials have advanced adoptive immunotherapy with Tregs for the treatment of autoimmune disease and in patients receiving cell transplants. Here, we highlight the potential benefit of combining gene therapy with Treg adoptive transfer to achieve a sustained transgene expression. Furthermore, techniques to engineer antigen-specific Treg cell populations, either through reprogramming conventional CD4⁺ T cells or transferring T cell receptors with known specificity into polyclonal Tregs, are promising in preclinical studies. Thus, based upon these observations and the successful use of chimeric (IgG-based) antigen receptors (CARs) in antigen-specific effector T cells, different types of CAR-Tregs could be added to the repertoire of inhibitory modalities to suppress immune responses to therapeutic cargos of gene therapy vectors. The diverse approaches to harness the ability of Tregs to suppress unwanted immune responses to gene therapy and their perspectives are reviewed in this article.

Adeno-Associated Virus Gene Therapy: Translational Progress and Future Prospects in the Treatment of Heart Failure

Despite advances in treatment over the past decade, heart failure remains a significant public health burden and a leading cause of death in the developed world. Gene therapy provides a promising approach for preventing and reversing cardiac abnormalities, however, clinical application has shown limited success to date. A substantial effort is being invested into the development of recombinant adeno-associated viruses (AAVs) for cardiac gene therapy as AAV gene therapy offers a high safety profile and provides sustained and efficient transgene expression following a once-off administration. Due to the physiological, anatomical and genetic similarities between large animals and humans, preclinical studies using large animal models for AAV gene therapy are crucial stepping stones between the laboratory and the clinic. Many molecular targets selected to treat heart failure using AAV gene therapy have been chosen because of their potential to regulate and restore cardiac contractility. Other genes targeted with AAV are involved with regulating angiogenesis, beta-adrenergic sensitivity, inflammation, physiological signalling and metabolism. While significant progress continues to be made in the field of AAV cardiac gene therapy, challenges remain in overcoming host neutralising antibodies, improving AAV vector cardiac-transduction efficiency and selectivity, and optimising the dose, route and method of delivery.

Emerging Issues in AAV-Mediated In Vivo Gene Therapy

In recent years, the number of clinical trials in which adeno-associated virus (AAV) vectors have been used for in vivo gene transfer has steadily increased. The excellent safety profile, together with the high efficiency of transduction of a broad range of target tissues, has established AAV vectors as the platform of choice for in vivo gene therapy. Successful application of the AAV technology has also been achieved in the clinic for a variety of conditions, including coagulation disorders, inherited blindness, and neurodegenerative diseases, among others. Clinical translation of novel and effective "therapeutic products" is, however, a long process that involves several cycles of iterations from bench to bedside that are required to address issues encountered during drug development. For the AAV vector gene transfer technology, several hurdles have emerged in both preclinical studies and clinical trials; addressing these issues will allow in the future to expand the scope of AAV gene transfer as a therapeutic modality for a variety of human diseases. In this review, we will give an overview on the biology of AAV vector, discuss the design of AAV-based gene therapy strategies for in vivo applications, and present key achievements and emerging issues in the field. We will use the liver as a model target tissue for gene transfer based on the large amount of data available from preclinical and clinical studies.

Enhanced liver gene transfer and evasion of preexisting humoral immunity with exosome-enveloped AAV vectors

Results from clinical trials of liver gene transfer for hemophilia demonstrate the potential of the adeno-associated virus (AAV) vector platform. However, to achieve therapeutic transgene expression, in some cases high vector doses are required, which are associated with a higher risk of triggering anti-capsid cytotoxic T-cell responses. Additionally, anti-AAV preexisting immunity can prevent liver transduction even at low neutralizing antibody (NAb) titers. Here, we describe the use of exosome-associated AAV (exo-AAV) vectors as a robust liver gene delivery system that allows the therapeutic vector dose to be decreased while protecting from preexisting humoral immunity to the capsid. The in vivo efficiency of liver targeting of standard AAV8 or AAV5 and exo-AAV8 or exo-AAV5 vectors expressing human coagulation factor IX (hF.IX) was evaluated. A significant enhancement of transduction efficiency was observed, and in hemophilia B mice treated with 4 × 10¹⁰ vector genomes per kilogram of exo-AAV8 vectors, a staggering ∼1 log increase in hF.IX transgene expression was observed, leading to superior correction of clotting time. Enhanced liver expression was also associated with an increase in the frequency of regulatory T cells in lymph nodes. The efficiency of exo- and standard AAV8 vectors in evading preexisting NAbs to the capsid was then evaluated in a passive immunization mouse model and in human sera. Exo-AAV8 gene delivery allowed for efficient transduction even in the presence of moderate NAb titers, thus potentially extending the proportion of subjects eligible for liver gene transfer. Exo-AAV vectors therefore represent a platform to improve the safety and efficacy of liver-directed gene transfer.

Overcoming the Host Immune Response to Adeno-Associated Virus Gene Delivery Vectors: The Race Between Clearance, Tolerance, Neutralization, and Escape

Immune responses in gene therapy with adeno-associated virus (AAV) vectors have been the object of almost two decades of study. Although preclinical models helped to define and predict certain aspects of interactions between the vector and the host immune system, most of our current knowledge has come from clinical trials. These studies have allowed development of effective interventions for modulating immunotoxicities associated with vector administration, resulting in therapeutic advances. However, the road to full understanding and effective modulation of immune responses in gene therapy is still long; the determinants of the balance between tolerance and immunogenicity in AAV vector-mediated gene transfer are not fully understood, and effective solutions for overcoming preexisting neutralizing antibodies are still lacking. However, despite these challenges, the goal of reliably delivering effective gene-based treatments is now in sight.

Optimized AAV rh.10 Vectors That Partially Evade Neutralizing Antibodies during Hepatic Gene Transfer

Of the 12 common serotypes used for gene delivery applications, Adeno-associated virus (AAV)rh.10 serotype has shown sustained hepatic transduction and has the lowest seropositivity in humans. We have evaluated if further modifications to AAVrh.10 at its phosphodegron like regions or predicted immunogenic epitopes could improve its hepatic gene transfer and immune evasion potential. Mutant AAVrh.10 vectors were generated by site directed mutagenesis of the predicted targets. These mutant vectors were first tested for their transduction efficiency in HeLa and HEK293T cells. The optimal vector was further evaluated for their cellular uptake, entry, and intracellular trafficking by quantitative PCR and time-lapse confocal microscopy. To evaluate their potential during hepatic gene therapy, C57BL/6 mice were administered with wild-type or optimal mutant AAVrh.10 and the luciferase transgene expression was documented by serial bioluminescence imaging at 14, 30, 45, and 72 days post-gene transfer. Their hepatic transduction was further verified by a quantitative PCR analysis of AAV copy number in the liver tissue. The optimal AAVrh.10 vector was further evaluated for their immune escape potential, in animals pre-immunized with human intravenous immunoglobulin. Our results demonstrate that a modified AAVrh.10 S671A vector had enhanced cellular entry (3.6 fold), migrate rapidly to the perinuclear region (1 vs. >2 h for wild type vectors) in vitro, which further translates to modest increase in hepatic gene transfer efficiency in vivo. More importantly, the mutant AAVrh.10 vector was able to partially evade neutralizing antibodies (~27-64 fold) in pre-immunized animals. The development of an AAV vector system that can escape the circulating neutralizing antibodies in the host will substantially widen the scope of gene therapy applications in humans.

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.