A 10 patient case report on the impact of plasmapheresis upon neutralizing factors against adeno-associated virus (AAV) types 1, 2, 6, and 8

Hemophilia B and gene therapy: a new chapter with etranacogene dezaparvovec

ABSTRACT

The US Food and Drug Administration (FDA)'s authorization of etranacogene dezaparvovec (Hemgenix) is a significant milestone, constituting not only the first FDA approval of a gene therapy for hemophilia but also the first approval of a liver-targeted adeno-associated virus vector gene therapy. This review summarizes the nonclinical studies and clinical development that supported regulatory clearance. Similar to other gene therapies for single gene disorders, both the short-term safety and the phenotypic improvement were unequivocal, justifying the modest-sized safety and efficacy database, which included 57 participants across the phase 2b (3 participants) and phase 3 (54 participants) studies. The most common adverse reactions included liver enzyme elevation, headache, flu-like symptoms, infusion-related reactions, creatine kinase elevation, malaise, and fatigue; these were mostly transient. One participant had hepatocellular carcinoma on a study-mandated liver ultrasound conducted 1 year after vector infusion; molecular analysis of the resected tumor showed no evidence of vector-related insertional mutagenesis as the etiology. A remarkable 96% of participants in the phase 3 trial were able to stop factor IX (FIX) prophylaxis, with the study demonstrating noninferiority to FIX prophylaxis in terms of the primary end point, annualized bleeding rate. Key secondary end points such as the annualized infusion rate, which declined by 97%, and the plasma FIX activity level at 18 months after infusion, with least squares mean increase of 34.3 percentage points compared with baseline, were both clinically and statistically significant. The FDA's landmark approval of Hemgenix as a pioneering treatment for hemophilia stands on the shoulders of >20 years of gene therapy clinical research and heralds a promising future for genomic medicines.

AAV mediated gene therapy for haemophilia B: From the early attempts to modern trials

Early gene therapy clinical trials for the treatment of Haemophilia B have been instrumental to our global understanding of gene therapy and have significantly contributed to the rapid expansion of the field. The use of adeno-associated viruses (AAVs) as vectors for gene transfer has successfully led to therapeutic expression of coagulation factor IX (FIX) in severe haemophilia B patients. Expression of FIX has remained stable following a single administration of vector for up to 8 years at levels that are clinically relevant to reduce the incidence of spontaneous bleeds and have permitted a significant change in the disease management with reduction or elimination of the need for coagulation factor concentrates. These trials have also shed light on several concerns around AAV-mediated gene transfer such as the high prevalence of pre-existing immunity against the vector capsid as well as the elevation of liver transaminases that is associated with a loss of FIX transgene expression in some patients. However, this field is advancing very rapidly with the development of increasingly more efficient strategies to overcome some of these obstacles and importantly raise the possibility of a functional cure, which has been long sought after. This review overviews the evolution of gene therapy for haemophilia B over the last two decades.

Gene targeting in adult organs using in vivo cleavable donor plasmids for CRISPR-Cas9 and CRISPR-Cas12a

The CRISPR-Cas system for in vivo genome editing is a powerful tool for gene therapy against several diseases. We have previously developed the pCriMGET_9-12a system, an in vivo cleavable donor plasmid for precise targeted knock-in of exogenous DNA by both Cas9 and Cas12a. Here, we show that the pCriMGET_9-12a system can be applied for in vivo in-frame knock-in of exogenous DNA in adult mouse liver by hydrodynamic delivery of the targeting plasmids. The in vivo cleavable pCriMGET_9-12a donor plasmids significantly increased the knock-in efficiency of both CRISPR-Cas9 and CRISPR-Cas12a in the adult mouse liver compared to uncleavable donor plasmids. This strategy also achieved in-frame reporter gene knock-in without indel mutations. Therefore, in vivo gene targeting using the pCriMGET_9-12a system may contribute to the establishment of safer, more precise, versatile and efficient gene therapy methods in adult organs.

Using an In Vivo Mouse Model to Determine the Exclusion Criteria of Preexisting Anti-AAV9 Neutralizing Antibody Titer of Pompe Disease Patients in Clinical Trials

The efficacy of adeno-associated virus (AAV)-based gene therapy is dependent on effective viral transduction, which might be inhibited by preexisting immunity to AAV acquired from infection or maternal delivery. Anti-AAV neutralizing Abs (NAbs) titer is usually measured by in vitro assay and used for patient enroll; however, this assay could not evaluate NAbs' impacts on AAV pharmacology and potential harm in vivo. Here, we infused a mouse anti-AAV9 monoclonal antibody into Balb/C mice 2 h before receiving 1.2 × 1014 or 3 × 1013 vg/kg of rAAV9-coGAA by tail vein, a drug for our ongoing clinical trials for Pompe disease. The pharmacokinetics, pharmacodynamics, and cellular responses combined with in vitro NAb assay validated the different impacts of preexisting NAbs at different levels in vivo. Sustained GAA expression in the heart, liver, diaphragm, and quadriceps were observed. The presence of high-level NAb, a titer about 1:1000, accelerated vector clearance in blood and completely blocked transduction. The AAV-specific T cell responses tended to increase when the titer of NAb exceeded 1:200. A low-level NAbs, near 1:100, had no effect on transduction in the heart and liver as well as cellular responses, but decreased transduction in muscles slightly. Therefore, we propose to preclude patients with NAb titers > 1:100 from rAAV9-coGAA clinical trials.

Rational immunosilencing of a promiscuous T-cell epitope in the capsid of an adeno-associated virus

Owing to the immunogenicity of adeno-associated viruses (AAVs), gene therapies using AAVs face considerable obstacles. Here, by leveraging ex vivo T-cell assays, the prediction of epitope binding to major histocompatibility complex class-II alleles, sequence-conservation analysis in AAV phylogeny and site-directed mutagenesis, we show that the replacement of amino acid residues in a promiscuous and most immunodominant T-cell epitope in the AAV9 capsid with AAV5 sequences abrogates the immune responses of peripheral blood mononuclear cells to the chimaeric vector while preserving its functions, potency, cellular specificity, transduction efficacy and biodistribution. This rational approach to the immunosilencing of capsid epitopes promiscuously binding to T cells may be applied to other AAV vectors and epitope regions.

Understanding AAV vector immunogenicity: from particle to patient

Gene therapy holds promise for patients with inherited monogenic disorders, cancer, and rare genetic diseases. Naturally occurring adeno-associated virus (AAV) offers a well-suited vehicle for clinical gene transfer due to its lack of significant clinical pathogenicity and amenability to be engineered to deliver therapeutic transgenes in a variety of cell types for long-term sustained expression. AAV has been bioengineered to produce recombinant AAV (rAAV) vectors for many gene therapies that are approved or in late-stage development. However, ongoing challenges hamper wider use of rAAV vector-mediated therapies. These include immunity against rAAV vectors, limited transgene packaging capacity, sub-optimal tissue transduction, potential risks of insertional mutagenesis and vector shedding. This review focuses on aspects of immunity against rAAV, mediated by anti-AAV neutralizing antibodies (NAbs) arising after natural exposure to AAVs or after rAAV vector administration. We provide an in-depth analysis of factors determining AAV seroprevalence and examine clinical approaches to managing anti-AAV NAbs pre- and post-vector administration. Methodologies used to quantify anti-AAV NAb levels and strategies to overcome pre-existing AAV immunity are also discussed. The broad adoption of rAAV vector-mediated gene therapies will require wider clinical appreciation of their current limitations and further research to mitigate their impact.

Liver directed adeno-associated viral vectors to treat metabolic disease

The liver is the metabolic center of the body and an ideal target for gene therapy of inherited metabolic disorders (IMDs). Adeno-associated viral (AAV) vectors can deliver transgenes to the liver with high efficiency and specificity and a favorable safety profile. Recombinant AAV vectors contain only the transgene cassette, and their payload is converted to non-integrating circular double-stranded DNA episomes, which can provide stable expression from months to years. Insights from cellular studies and preclinical animal models have provided valuable information about AAV capsid serotypes with a high liver tropism. These vectors have been applied successfully in the clinic, particularly in trials for hemophilia, resulting in the first approved liver-directed gene therapy. Lessons from ongoing clinical trials have identified key factors affecting efficacy and safety that were not readily apparent in animal models. Circumventing pre-existing neutralizing antibodies to the AAV capsid, and mitigating adaptive immune responses to transduced cells are critical to achieving therapeutic benefit. Combining the high efficiency of AAV delivery with genome editing is a promising path to achieve more precise control of gene expression. The primary safety concern for liver gene therapy with AAV continues to be the small risk of tumorigenesis from rare vector integrations. Hepatotoxicity is a key consideration in the safety of neuromuscular gene therapies which are applied at substantially higher doses. The current knowledge base and toolkit for AAV is well developed, and poised to correct some of the most severe IMDs with liver-directed 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.

Infusion reactions to adeno-associated virus (AAV)-based gene therapy: Mechanisms, diagnostics, treatment and review of the literature

The use of adeno-associated virus (AAV) vectors in gene therapy has demonstrated great potential in treating genetic disorders. However, infusion-associated reactions (IARs) pose a significant challenge to the safety and efficacy of AAV-based gene therapy. This review provides a comprehensive summary of the current understanding of IARs to AAV therapy, including their underlying mechanisms, clinical presentation, and treatment options. Toll-like receptor activation and subsequent production of pro-inflammatory cytokines are associated with IARs, stimulating neutralizing antibodies (Nabs) and T-cell responses that interfere with gene therapy. Risk factors for IARs include high titers of pre-existing Nabs, previous exposure to AAV, and specific comorbidities. Clinical presentation ranges from mild flu-like symptoms to severe anaphylaxis and can occur during or after AAV administration. There are no established guidelines for pre- and postadministration tests for AAV therapies, and routine laboratory requests are not standardized. Treatment options include corticosteroids, plasmapheresis, and supportive medications such as antihistamines and acetaminophen, but there is no consensus on the route of administration, dosage, and duration. This review highlights the inadequacy of current treatment regimens for IARs and the need for further research to improve the safety and efficacy of AAV-based gene therapy.

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.

Analysis of Adeno-Associated Virus Serotype 8 (AAV8)-antibody complexes using epitope mapping by molecular imprinting leads to the identification of Fab peptides that potentially evade AAV8 neutralisation

Gene therapy is a promising approach for treating genetic disorders by delivering therapeutic genes to replace or correct malfunctioning genes. However, the introduced gene therapy vector can trigger an immune response, leading to reduced efficacy and potential harm to the patient. To improve the efficiency and safety of gene therapy, preventing the immune response to the vector is crucial. This can be achieved through the use of immunosuppressive drugs, vector engineering to evade the immune system, or delivery methods that bypass the immune system altogether. By reducing the immune response, gene therapy can deliver therapeutic genes more effectively and potentially cure genetic diseases. In this study, a novel molecular imprinting technique, combined with mass-spectrometry and bioinformatics, was used to identify four antigen-binding fragments (Fab) sequences of Adeno-Associated Virus (AAV) - neutralising antibodies capable of binding to AAV. The identified Fab peptides were shown to prevent AAV8's binding to antibodies, demonstrating their potential to improve gene therapy efficiency by preventing the immune response.

Barriers in Heart Failure Gene Therapy and Approaches to Overcome Them

With the growing prevalence and incidence of heart failure worldwide, investigation and development of new therapies to address disease burden are of great urgency. Gene therapy is one promising approach for the management of heart failure, but several barriers currently exclude safe and efficient gene delivery to the human heart. These barriers include the anatomical and biological difficulty of specifically targeting cardiomyocytes, the vascular endothelium, and immunogenicity against administered vectors and the transgene. We review approaches taken to overcome these barriers with a focus on vector modification, evasion of immune responses, and heart-targeted delivery techniques. While various modifications proposed to date show promise in managing some barriers, continued investigation into improvements to existing therapies is required to address transduction efficiency, duration of transgene expression, and immune response.

Immunogenicity of Recombinant Adeno-Associated Virus (AAV) Vectors for Gene Transfer

Recombinant adeno-associated viruses (AAVs) have emerged as promising gene delivery vehicles resulting in three US Food and Drug Administration (FDA) and one European Medicines Agency (EMA)-approved AAV-based gene therapies. Despite being a leading platform for therapeutic gene transfer in several clinical trials, host immune responses against the AAV vector and transgene have hampered their widespread application. Multiple factors, including vector design, dose, and route of administration, contribute to the overall immunogenicity of AAVs. The immune responses against the AAV capsid and transgene involve an initial innate sensing. The innate immune response subsequently triggers an adaptive immune response to elicit a robust and specific response against the AAV vector. AAV gene therapy clinical trials and preclinical studies provide important information about the immune-mediated toxicities associated with AAV, yet studies suggest preclinical models fail to precisely predict the outcome of gene delivery in humans. This review discusses the contribution of the innate and adaptive immune response against AAVs, highlighting the challenges and potential strategies to mitigate these responses, thereby enhancing the therapeutic potential of AAV gene therapy.

Rescue of infant progressive familial intrahepatic cholestasis type 3 mice by repeated dosing of AAV gene therapy

Background & Aims

Gene therapy using recombinant adeno-associated virus (rAAV) vector carrying multidrug resistance protein 3 (MDR3) coding sequence (AAV8-MDR3) represents a potential curative treatment for progressive familial intrahepatic cholestasis type 3 (PFIC3), which presents in early childhood. However, patients with the severest form of PFIC3 should receive treatment early after detection to prevent irreversible hepatic fibrosis leading ultimately to liver transplantation or death. This represents a challenge for rAAV-based gene therapy because therapeutic efficacy is expected to wane as rAAV genomes are lost owing to hepatocyte division, and the formation of AAV-specific neutralising antibodies precludes re-administration. Here, we tested a strategy of vector re-administration in infant PFIC3 mice with careful evaluation of its oncogenicity - a particular concern surrounding rAAV treatment.

Methods

AAV8-MDR3 was re-administered to infant Abcb4 ^-/- mice 2 weeks after a first dose co-administered with tolerogenic nanoparticles carrying rapamycin (ImmTOR) given at 2 weeks of age. Eight months later, long-term therapeutic efficacy and safety were assessed with special attention paid to the potential oncogenicity of rAAV treatment.

Results

Co-administration with ImmTOR mitigated the formation of rAAV-specific neutralising antibodies and enabled an efficacious second administration of AAV8-MDR3, resulting in stable correction of the disease phenotype, including a restoration of bile phospholipid content and healthy liver function, as well as the prevention of liver fibrosis, hepatosplenomegaly, and gallstones. Furthermore, efficacious repeat rAAV administration prevented the appearance of liver malignancies in an animal model highly prone to developing hepatocellular carcinoma.

Conclusions

These outcomes provide strong evidence for rAAV redosing through co-administration with ImmTOR, as it resulted in a long-term therapeutic effect in a paediatric liver metabolic disorder, including the prevention of oncogenesis.

Impact and implications

Redosing of gene therapy for inborn hepatobiliary disorders may be essential as effect wanes during hepatocyte division and renewal, particularly in paediatric patients, but the approach may carry long-term risks of liver cancer. Viral vectors carrying a therapeutic gene exerted a durable cure of progressive familial intrahepatic cholestasis type 3 in infant mice and reduced the risk of liver cancer only following a second administration.

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.

Mitigating Serious Adverse Events in Gene Therapy with AAV Vectors: Vector Dose and Immunosuppression

Gene transfer with high doses of adeno-associated viral (AAV) vectors has resulted in serious adverse events and even death of the recipients. Toxicity could most likely be circumvented by repeated injections of lower and less toxic doses of vectors. This has not been pursued as AAV vectors induce potent neutralizing antibodies, which prevent cell transduction upon reinjection of the same vector. This review discusses different types of immune responses against AAV vectors and how they offer targets for the elimination or inhibition of vector-specific neutralizing antibodies. Such antibodies can be circumvented by using different virus serotypes for sequential injections, they can be removed by plasmapheresis, or they can be destroyed by enzymatic degradation. Antibody producing cells can be eliminated by proteasome inhibitors. Drugs that inhibit T-cell responses, B-cell signaling, or presentation of the vector's antigens to B cells can prevent or reduce induction of AAV-specific antibodies. Combinations of different approaches and drugs are likely needed to suppress or eliminate neutralizing antibodies, which would then allow for repeated dosing. Alternatively, novel AAV vectors with higher transduction efficacy are being developed and may allow for a dose reduction, although it remains unknown if this will completely address the problem of high-dose adverse events.

AAV vectors applied to the treatment of CNS disorders: Clinical status and challenges

In recent years, adeno-associated virus (AAV) has become the most important vector for central nervous system (CNS) gene therapy. AAV has already shown promising results in the clinic, for several CNS diseases that cannot be treated with drugs, including neurodegenerative diseases, neuromuscular diseases, and lysosomal storage disorders. Currently, three of the four commercially available AAV-based drugs focus on neurological disorders, including Upstaza for aromatic l-amino acid decarboxylase deficiency, Luxturna for hereditary retinal dystrophy, and Zolgensma for spinal muscular atrophy. All these studies have provided paradigms for AAV-based therapeutic intervention platforms. AAV gene therapy, with its dual promise of targeting disease etiology and enabling 'long-term correction' of disease processes, has the advantages of immune privilege, high delivery efficiency, tissue specificity, and cell tropism in the CNS. Although AAV-based gene therapy has been shown to be effective in most CNS clinical trials, limitations have been observed in its clinical applications, which are often associated with side effects. In this review, we summarized the therapeutic progress, challenges, limitations, and solutions for AAV-based gene therapy in 14 types of CNS diseases. We focused on viral vector technologies, delivery routes, immunosuppression, and other relevant clinical factors. We also attempted to integrate several hurdles faced in clinical and preclinical studies with their solutions, to seek the best path forward for the application of AAV-based gene therapy in the context of CNS diseases. We hope that these thoughtful recommendations will contribute to the efficient translation of preclinical studies and wide application of clinical trials.

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.

Progress, and prospects in the therapeutic armamentarium of persons with congenital hemophilia. Defining the place for liver-directed gene therapy

In persons with congenital severe hemophilia A (HA) living in high-income countries, twice weekly intravenous infusions of extended half-life (EHL) factor VIII (FVIII) products, or weekly/biweekly/monthly subcutaneous injections of emicizumab are the gold standard home treatments to grant days without hurdles and limitations. Once weekly/twice monthly infusions of EHL Factor IX (FIX) products achieve the same target in severe hemophilia B (HB). Gene therapy, which is likely to be licensed for clinical use within 1-2 years, embodies a shift beyond these standards. At an individual patient level, a single functional gene transfer leads to a > 10-yr almost full correction of the hemostatic defect in HB and to a sustained (3-6-yrs) expression of FVIII sufficient to discontinue exogenous clotting factor administrations. At the doses employed, the limited liver toxicity of systemically infused recombinant adeno-associated virus (rAAV) vectors is documented by long-term (12-15 yrs) follow-ups, and pre-existing high-titer neutralizing antibodies to the AAV5 vector are no longer an exclusion criterion for effective transgene expression with this vector. A safe durable treatment that converts a challenging illness to a phenotypically curable disease, allows persons to feel virtually free from the fears and the obligations of hemophilia for years/decades. Along with patient organizations and health care professionals, communicating to government authorities and reimbursement agencies the liberating potential of this substantial innovation, and disseminating across the Centers updated information on benefits and risks of this strategy, will align expectations of different stakeholders and establish the notion of a potentially lifelong cure of hemophilia.

Multiplexing AAV Serotype-Specific Neutralizing Antibodies in Preclinical Animal Models and Humans

The accurate assessment of AAV-specific pre-existing humoral immunity due to natural viral infection is critical for the efficient use of clinical gene therapy. The method described in the present study applies equivalent infection conditions to each AAV serotype (AAV1, AAV2, AAV3, AAV5, AAV6, AAV7, AAV8, AAV9, AAV10, and AAVAnc80L65). In the current study, we validated the assay by assessing AAV-neutralizing antibody titers in a limited cohort of random human donors and well-established preclinical large animal models, including dogs and non-human primates (NHPs). We achieved a rapid and accurate evaluation of neutralizing titers for each individual subject that can be used for clinical enrollment based on specific AAV serotypes and individualized selection of the most suitable AAV serotype for each specific patient.

Preclinical studies in Krabbe disease: A model for the investigation of novel combination therapies for lysosomal storage diseases

Krabbe disease (KD) is a lysosomal storage disease (LSD) caused by mutations in the galc gene. There are over 50 monogenetic LSDs, which largely impede the normal development of children and often lead to premature death. At present, there are no cures for LSDs and the available treatments are generally insufficient, short acting, and not without co-morbidities or long-term side effects. The last 30 years have seen significant advances in our understanding of LSD pathology as well as treatment options. Two gene therapy-based clinical trials, NCT04693598 and NCT04771416, for KD were recently started based on those advances. This review will discuss how our knowledge of KD got to where it is today, focusing on preclinical investigations, and how what was discovered may prove beneficial for the treatment of other LSDs.

Long-term follow-up of liver-directed, adeno-associated vector-mediated gene therapy in the canine model of hemophilia A

Questions remain concerning the long-term efficacy, safety, and site(s) of transgene expression following adeno-associated vector (AAV) therapy. We report a long-term follow-up of 8 (male = 4, hemizygous, and female = 4, homozygous) dogs with severe hemophilia A treated with a single portal vein infusion of a B-domain-deleted (BDD)-canine FVIII (cFVIII) AAV vector (median dose = 1.25 × 1013 vg/kg, AAV2 = 4, AAV6 = 3, and AAV8 = 1). After a median follow-up of 10.8 years (8.2-12.0 years), persistent FVIII:C (median one-stage = 12.7%, chromogenic = 7.2%) was seen in all responding dogs (n = 6), with improvement in annualized bleed rates (pre = 3.9 vs post = 0.3 event per year; P = .003). Anti-AAV capsid neutralizing antibodies (nAbs) toward the dosed capsid were detected throughout the study, with limited cross-reactivity to other capsids. nAb titers for all capsid serotypes declined with time, although they remained at levels precluding redosing with the same capsid. AAV-BDD-cFVIII DNA was detected in the liver of all dogs (median = 0.15 vg per diploid genome), with lower levels in the spleen in 4 dogs (median = 0.005 vg per diploid genome). Consistent with the liver-specific promoter, BDD-cFVIII mRNA was only detected in the liver. Postmortem examination demonstrated no evidence of chronic liver disease or liver malignancy. Persistent FVIII expression and an improved bleeding phenotype was seen for more than a decade after vector delivery. This is the longest follow-up reported in a preclinical model supporting long-term efficacy and safety of AAV-mediated gene therapy.

AAV vectors: The Rubik's cube of human gene therapy

Defective genes account for ∼80% of the total of more than 7,000 diseases known to date. Gene therapy brings the promise of a one-time treatment option that will fix the errors in patient genetic coding. Recombinant viruses are highly efficient vehicles for in vivo gene delivery. Adeno-associated virus (AAV) vectors offer unique advantages, such as tissue tropism, specificity in transduction, eliciting of a relatively low immune responses, no incorporation into the host chromosome, and long-lasting delivered gene expression, making them the most popular viral gene delivery system in clinical trials, with three AAV-based gene therapy drugs already approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA). Despite the success of AAV vectors, their usage in particular scenarios is still limited due to remaining challenges, such as poor transduction efficiency in certain tissues, low organ specificity, pre-existing humoral immunity to AAV capsids, and vector dose-dependent toxicity in patients. In the present review, we address the different approaches to improve AAV vectors for gene therapy with a focus on AAV capsid selection and engineering, strategies to overcome anti-AAV immune response, and vector genome design, ending with a glimpse at vector production methods and the current state of recombinant AAV (rAAV) at the clinical level.

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.

rAAV immunogenicity, toxicity, and durability in 255 clinical trials: A meta-analysis

Recombinant Adeno-associated virus (rAAV) is one of the main delivery vectors for gene therapy. To assess immunogenicity, toxicity, and features of AAV gene therapy in clinical settings, a meta-analysis of 255 clinical trials was performed. A total of 7,289 patients are planned to be dosed. AAV2 was the most dominantly used serotype (29.8%, n=72), and 8.3% (n=20) of trials used engineered capsids. 38.7% (n=91) of trials employed neutralizing antibody assays for patient enrollment, while 15.3% (n=36) used ELISA-based total antibody assays. However, there was high variability in the eligibility criteria with cut-off tiers ranging from 1:1 to 1:1,600. To address potential immunogenicity, 46.3% (n=118) of trials applied immunosuppressants (prophylactic or reactive), while 32.7% (n=18) of CNS and 37.5% (n=24) of ocular-directed trials employed immunosuppressants, possibly due to the immune-privileged status of CNS and retina. There were a total of 11 patient deaths across 8 trials, and 18 out of 30 clinical holds were due to toxicity findings in clinical studies. 30.6% (n=78) of trials had treatment-emergent serious adverse events (TESAEs), with hepatotoxicity and thrombotic microangiopathy (systemic delivery) and neurotoxicity (CNS delivery) being the most prominent. Additionally, the durability of gene therapy may be impacted by two distinct decline mechanisms: 1) rapid decline presumably due to immune responses; or 2) gradual decline due to vector dilution. The durability varied significantly depending on disease indication, dose, serotypes, and patient individuals. Most CNS (90.0%) and muscle trials (73.3%) achieved durable transgene expression, while only 43.6% of ocular trials had sustained clinical outcomes. The rAAV production system can affect rAAV quality and thus immunogenicity and toxicity. Out of 186 trials that have disclosed production system information, 63.0% (n=126) of trials used the transient transfection of the HEK293/HEK293T system, while 18.0% (n=36) applied the baculovirus/Sf9 (rBac/Sf9) system. There were no significant differences in TESAEs and durability between AAV generated by rBac/Sf9 and HEK293/HEK293T systems. In summary, rAAV immunogenicity and toxicity poses significant challenges for clinical development of rAAV gene therapies, and it warrants collaborative efforts to standardize monitoring/measurement methods, design novel strategies to overcome immune responses, and openly share relevant information.

Immune Responses to Gene Editing by Viral and Non-Viral Delivery Vectors Used in Retinal Gene Therapy

Inherited retinal diseases (IRDs) are a leading cause of blindness in industrialized countries, and gene therapy is quickly becoming a viable option to treat this group of diseases. Gene replacement using a viral vector has been successfully applied and advanced to commercial use for a rare group of diseases. This, and the advances in gene editing, are paving the way for the emergence of a new generation of therapies that use CRISPR-Cas9 to edit mutated genes in situ. These CRISPR-based agents can be delivered to the retina as transgenes in a viral vector, unpackaged transgenes or as proteins or messenger RNA using non-viral vectors. Although the eye is considered to be an immune-privileged organ, studies in animals, as well as evidence from clinics, have concluded that ocular gene therapies elicit an immune response that can under certain circumstances result in inflammation. In this review, we evaluate studies that have reported on pre-existing immunity, and discuss both innate and adaptive immune responses with a specific focus on immune responses to gene editing, both with non-viral and viral delivery in the ocular space. Lastly, we discuss approaches to prevent and manage the immune responses to ensure safe and efficient gene editing in the retina.

Pre-existing humoral immunity and complement pathway contribute to immunogenicity of adeno-associated virus (AAV) vector in human blood

AAV gene transfer is a promising treatment for many patients with life-threatening genetic diseases. However, host immune response to the vector poses a significant challenge for the durability and safety of AAV-mediated gene therapy. Here, we characterize the innate immune response to AAV in human whole blood. We identified neutrophils, monocyte-related dendritic cells, and monocytes as the most prevalent cell subsets able to internalize AAV particles, while conventional dendritic cells were the most activated in terms of the CD86 co-stimulatory molecule upregulation. Although low titers (≤1:10) of AAV neutralizing antibodies (NAb) in blood did not have profound effects on the innate immune response to AAV, higher NAb titers (≥1:100) significantly increased pro-inflammatory cytokine/chemokine secretion, vector uptake by antigen presenting cells (APCs) and complement activation. Interestingly, both full and empty viral particles were equally potent in inducing complement activation and cytokine secretion. By using a compstatin-based C3 and C3b inhibitor, APL-9, we demonstrated that complement pathway inhibition lowered CD86 levels on APCs, AAV uptake, and cytokine/chemokine secretion in response to AAV. Together these results suggest that the pre-existing humoral immunity to AAV may contribute to trigger adverse immune responses observed in AAV-based gene therapy, and that blockade of complement pathway may warrant further investigation as a potential strategy for decreasing immunogenicity of AAV-based therapeutics.

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.

Immunogenicity assessment of AAV-based gene therapies: An IQ consortium industry white paper

Immunogenicity has imposed a challenge to efficacy and safety evaluation of adeno-associated virus (AAV) vector-based gene therapies. Mild to severe adverse events observed in clinical development have been implicated with host immune responses against AAV gene therapies, resulting in comprehensive evaluation of immunogenicity during nonclinical and clinical studies mandated by health authorities. Immunogenicity of AAV gene therapies is complex due to the number of risk factors associated with product components and pre-existing immunity in human subjects. Different clinical mitigation strategies have been employed to alleviate treatment-induced or -boosted immunogenicity in order to achieve desired efficacy, reduce toxicity, or treat more patients who are seropositive to AAV vectors. In this review, the immunogenicity risk assessment, manifestation of immunogenicity and its impact in nonclinical and clinical studies, and various clinical mitigation strategies are summarized. Last, we present bioanalytical strategies, methodologies, and assay validation applied to appropriately monitor immunogenicity in AAV gene therapy-treated subjects.

Prevalence of Anti-AAV9 Antibodies in Adult Patients with Spinal Muscular Atrophy

5q-associated spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder that leads to progressive muscle atrophy and weakness. The disease is caused by a homozygous deletion or mutation in the survival of motor neuron 1 gene (SMN1), resulting in insufficient levels of SMN protein. Onasemnogene abeparvovec-xioi (OA) is a non-replicating vector based on adeno-associated virus serotype 9 (AAV9) that contains the full-length human SMN1 gene. Recently, OA was approved for the treatment of SMA by the U.S. Food and Drug Administration and the European Medicines Agency. Since the presence of neutralizing antibodies caused by previous natural exposure to wild-type AAVs may impair the efficiency of AAV-mediated gene transfer, and thus reduce the therapeutic benefit of the gene therapy, an AAV9-binding antibody titer of >1:50 was defined as a surrogate exclusion criterion in pivotal OA clinical trials. However, these studies were exclusively conducted in infants and children. Since data on anti-AAV9 antibody titers in adults are generally sparse and not available for adult patients with SMA, we determined the prevalence of anti-AAV9 antibodies in sera of adult individuals with SMA to evaluate the feasibility of AAV9-mediated gene therapy in this cohort. In our study population of 69 adult patients with SMA type 2 and type 3 from four German academic sites, only three patients (4.3%) had an elevated anti-AAV9 antibody titer of >1:50. The prevalence of anti-AAV9 antibodies did not increase with age. The low and age-independent prevalence of anti-AAV9 antibodies in our cohort provides evidence that gene therapy with intravenous administered recombinant AAV9 vectors (rAAV9) might be feasible in adult patients with SMA, regardless of the patients' sex, SMA type, walking ability, or ventilatory status. This could also apply to the treatment of other inherited neurological diseases with rAAV9.

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.

AAV-vector based gene therapy for mitochondrial disease: progress and future perspectives

Mitochondrial diseases are a group of rare, heterogeneous diseases caused by gene mutations in both nuclear and mitochondrial genomes that result in defects in mitochondrial function. They are responsible for significant morbidity and mortality as they affect multiple organ systems and particularly those with high energy-utilizing tissues, such as the nervous system, skeletal muscle, and cardiac muscle. Virtually no effective treatments exist for these patients, despite the urgent need. As the majority of these conditions are monogenic and caused by mutations in nuclear genes, gene replacement is a highly attractive therapeutic strategy. Adeno-associated virus (AAV) is a well-characterized gene replacement vector, and its safety profile and ability to transduce quiescent cells nominates it as a potential gene therapy vehicle for several mitochondrial diseases. Indeed, AAV vector-based gene replacement is currently being explored in clinical trials for one mitochondrial disease (Leber hereditary optic neuropathy) and preclinical studies have been published investigating this strategy in other mitochondrial diseases. This review summarizes the preclinical findings of AAV vector-based gene replacement therapy for mitochondrial diseases including Leigh syndrome, Barth syndrome, ethylmalonic encephalopathy, and others.

Cryo-electron Microscopy of Adeno-associated Virus

Adeno-associated virus (AAV) has a single-stranded DNA genome encapsidated in a small icosahedrally symmetric protein shell with 60 subunits. AAV is the leading delivery vector in emerging gene therapy treatments for inherited disorders, so its structure and molecular interactions with human hosts are of intense interest. A wide array of electron microscopic approaches have been used to visualize the virus and its complexes, depending on the scientific question, technology available, and amenability of the sample. Approaches range from subvolume tomographic analyses of complexes with large and flexible host proteins to detailed analysis of atomic interactions within the virus and with small ligands at resolutions as high as 1.6 Å. Analyses have led to the reclassification of glycan receptors as attachment factors, to structures with a new-found receptor protein, to identification of the epitopes of antibodies, and a new understanding of possible neutralization mechanisms. AAV is now well-enough characterized that it has also become a model system for EM methods development. Heralding a new era, cryo-EM is now also being deployed as an analytic tool in the process development and production quality control of high value pharmaceutical biologics, namely AAV vectors.

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.

The effect of rapamycin and ibrutinib on antibody responses to adeno-associated virus vector-mediated gene transfer

Adeno-associated virus (AAV) vector-mediated gene transfer is lessening the impact of monogenetic disorders. Human AAV gene therapy recipients commonly mount immune responses to AAV or the encoded therapeutic protein, which requires transient immunosuppression. Most efforts to date have focused on blunting AAV capsid-specific T cell responses, which have been implicated in elimination of AAV transduced cells. Here we explore the use of immunosuppressants, rapamycin given alone or in combination with ibrutinib to inhibit AAV vector- or transgene product-specific antibody responses. Our results show that rapamycin or ibrutinib given alone reduce primary antibody responses against AAV capsid but the combination of rapamycin and ibrutinib is more effective, blunts recall responses, and reduces numbers of circulating antibody-secreting plasma cells. The drugs fail to lower B cell memory formation or to reduce the inhibitory effects of pre-existing AAV capsid-specific antibodies on transduction efficiency.

Delivery of CRISPR-Cas tools for in vivo genome editing therapy: Trends and challenges

The discovery of clustered regularly interspaced short palindromic repeats (CRISPR) genome editing technology opened the door to provide a versatile approach for treating multiple diseases. Promising results have been shown in numerous pre-clinical studies and clinical trials. However, a safe and effective method to deliver genome-editing components is still a key challenge for in vivo genome editing therapy. Adeno-associated virus (AAV) is one of the most commonly used vector systems to date, but immunogenicity against capsid, liver toxicity at high dose, and potential genotoxicity caused by off-target mutagenesis and genomic integration remain unsolved. Recently developed transient delivery systems, such as virus-like particle (VLP) and lipid nanoparticle (LNP), may solve some of the issues. This review summarizes existing in vivo delivery systems and possible solutions to overcome their limitations. Also, we highlight the ongoing clinical trials for in vivo genome editing therapy and recently developed genome editing tools for their potential applications.

Selective Anti-AAV Antibody Depletion by Hemapheresis and Immunoadsorption

One of the greatest barriers for the use of adeno-associated vectors (AAV) in gene therapy is the presence of pre-existing antibodies against AAV in the general population. Since many of the anti-AAV antibodies have the ability to neutralize the transduction target tissues, even patients with low antibody titers must be excluded from clinical trials or therapy. In recent years, various methods have been proposed to overcome this problem, unfortunately with limited success. In this chapter, we describe in detail a protocol for hemapheresis with an immunoadsorption matrix to remove specifically anti-AAV antibodies in an in vivo rat model. Furthermore, this chapter describes in detail the methods to determine the efficiency of hemapheresis and immunoadsorption.

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.

Capsid-Engineering for Central Nervous System-Directed Gene Therapy with Adeno-Associated Virus Vectors

Closing the gap in knowledge on the cause of neurodegenerative disorders is paving the way toward innovative treatment strategies, among which gene therapy has emerged as a top candidate. Both conventional gene therapy and genome editing approaches are being developed, and a great number of human clinical trials are ongoing. Already 2 years ago, the first gene therapy for a neurodegenerative disease, spinal muscular atrophy type 1 (SMA1), obtained market approval. To realize such innovative strategies, gene therapy delivery tools are key assets. Here, we focus on recombinant adeno-associated virus (AAV) vectors and report on strategies to improve first-generation vectors. Current efforts focus on the viral capsid to modify the host-vector interaction aiming at increasing the efficacy of target cell transduction, at simplifying vector administration, and at reducing the risk of vector dose-related side effects.

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.

Overcoming innate immune barriers that impede AAV gene therapy vectors

The field of gene therapy has made considerable progress over the past several years. Adeno-associated virus (AAV) vectors have emerged as promising and attractive tools for in vivo gene therapy. Despite the recent clinical successes achieved with recombinant AAVs (rAAVs) for therapeutics, host immune responses against the vector and transgene product have been observed in numerous preclinical and clinical studies. These outcomes have hampered the advancement of AAV gene therapies, preventing them from becoming fully viable and safe medicines. The human immune system is multidimensional and complex. Both the innate and adaptive arms of the immune system seem to play a concerted role in the response against rAAVs. While most efforts have been focused on the role of adaptive immunity and developing ways to overcome it, the innate immune system has also been found to have a critical function. Innate immunity not only mediates the initial response to the vector, but also primes the adaptive immune system to launch a more deleterious attack against the foreign vector. This Review highlights what is known about innate immune responses against rAAVs and discusses potential strategies to circumvent these pathways.

Emerging Immunogenicity and Genotoxicity Considerations of Adeno-Associated Virus Vector Gene Therapy for Hemophilia

Adeno-associated viral (AAV) vector gene therapy has shown promise as a possible cure for hemophilia. However, immune responses directed against AAV vectors remain a hurdle to the broader use of this gene transfer platform. Both innate and adaptive immune responses can affect the safety and efficacy of AAV vector-mediated gene transfer in humans. These immune responses may be triggered by the viral capsid, the vector's nucleic acid payload, or other vector contaminants or excipients, or by the transgene product encoded by the vector itself. Various preclinical and clinical strategies have been explored to overcome the issues of AAV vector immunogenicity and transgene-related immune responses. Although results of these strategies are encouraging, more efficient approaches are needed to deliver safe, predictable, and durable outcomes for people with hemophilia. In addition to durability, long-term follow-up of gene therapy trial participants will allow us to address potential safety concerns related to vector integration. Herein, we describe the challenges with current methodologies to deliver optimal outcomes for people with hemophilia who choose to undergo AAV vector gene therapy and the potential opportunities to improve on the results.

Challenges Posed by Immune Responses to AAV Vectors: Addressing Root Causes

Host immune responses that limit durable therapeutic gene expression and cause clinically significant inflammation remain a major barrier to broadly successful development of adeno-associated virus (AAV)-based human gene therapies. In this article, mechanisms of humoral and cellular immune responses to the viral vector are discussed. A perspective is provided that removal of pathogen-associated molecular patterns in AAV vector genomes to prevent the generation of innate immune danger signals following administration is a key strategy to overcome immunological barriers.

Vectored Immunotherapeutics for Infectious Diseases: Can rAAVs Be The Game Changers for Fighting Transmissible Pathogens?

Conventional vaccinations and immunotherapies have encountered major roadblocks in preventing infectious diseases like HIV, influenza, and malaria. These challenges are due to the high genomic variation and immunomodulatory mechanisms inherent to these diseases. Passive transfer of broadly neutralizing antibodies may offer partial protection, but these treatments require repeated dosing. Some recombinant viral vectors, such as those based on lentiviruses and adeno-associated viruses (AAVs), can confer long-term transgene expression in the host after a single dose. Particularly, recombinant (r)AAVs have emerged as favorable vectors, given their high in vivo transduction efficiency, proven clinical efficacy, and low immunogenicity profiles. Hence, rAAVs are being explored to deliver recombinant antibodies to confer immunity against infections or to diminish the severity of disease. When used as a vaccination vector for the delivery of antigens, rAAVs enable de novo synthesis of foreign proteins with the conformation and topology that resemble those of natural pathogens. However, technical hurdles like pre-existing immunity to the rAAV capsid and production of anti-drug antibodies can reduce the efficacy of rAAV-vectored immunotherapies. This review summarizes rAAV-based prophylactic and therapeutic strategies developed against infectious diseases that are currently being tested in pre-clinical and clinical studies. Technical challenges and potential solutions will also be discussed.