AAVrh.10 immunogenicity in mice and humans. Relevance of antibody cross-reactivity in human gene therapy

Recombinant Adeno-Associated Virus Vectors for Gene Therapy of the Central Nervous System: Delivery Routes and Clinical Aspects

The Central Nervous System (CNS) is vulnerable to a range of diseases, including neurodegenerative and oncological conditions, which present significant treatment challenges. The blood-brain barrier (BBB) restricts molecule penetration, complicating the achievement of therapeutic concentrations in the CNS following systemic administration. Gene therapy using recombinant adeno-associated virus (rAAV) vectors emerges as a promising strategy for treating CNS diseases, demonstrated by the registration of six gene therapy products in the past six years and 87 ongoing clinical trials. This review explores the implementation of rAAV vectors in CNS disease treatment, emphasizing AAV biology and vector engineering. Various administration methods-such as intravenous, intrathecal, and intraparenchymal routes-and experimental approaches like intranasal and intramuscular administration are evaluated, discussing their advantages and limitations in different CNS contexts. Additionally, the review underscores the importance of optimizing therapeutic efficacy through the pharmacokinetics (PK) and pharmacodynamics (PD) of rAAV vectors. A comprehensive analysis of clinical trials reveals successes and challenges, including barriers to commercialization. This review provides insights into therapeutic strategies using rAAV vectors in neurological diseases and identifies areas requiring further research, particularly in optimizing rAAV PK/PD.

Prevalence Study of Cellular Capsid-Specific Immune Responses to AAV2, 4, 5, 8, 9, and rh10 in Healthy Donors

Recombinant adeno-associated virus (rAAV) vectors appear, more than ever, to be efficient viral vectors for in vivo gene transfer as illustrated by the approvals of 7 drugs across Europe and the United States. Nevertheless, preexisting immunity to AAV capsid in humans remains one of the major limits for a successful clinical translation. Whereas a preexisting humoral response to AAV capsid is well documented, the prevalence of preexisting capsid-specific T cell responses still needs to be studied and characterized. In this study, we investigated the prevalence of AAV-specific circulating T cells toward AAV2, 4, 5, 8, 9, and rh10 in a large cohort of healthy donors using the standard IFNγ ELISpot assay. We observed the highest prevalence of preexisting cellular immunity to AAV9 serotype followed by AAV8, AAV4, AAV2, AAVrh10, and AAV5 independently of the donors' serological status. An in-depth analysis of T cell responses toward the 2 most prevalent serotypes 8 and 9 shows that IFNγ secretion is mainly mediated by CD8 T cells for both serotypes. A polyfunctional analysis reveals different cytokine profiles between AAV8 and AAV9. Surprisingly, no IL-2 secretion was mediated by anti-AAV9 immune cells suggesting that these cells may rather be exhausted or terminally differentiated than cytotoxic T cells. Altogether, these results suggest that preexisting immunity to AAV may vary depending on the serotype and support the necessity of using multiparametric monitoring methods to better characterize anticapsid cellular immunity and foresee its impact in rAAV-mediated clinical trials.

Liver-directed gene therapy for inherited metabolic diseases

Gene therapy clinical trials are rapidly expanding for inherited metabolic liver diseases whilst two gene therapy products have now been approved for liver based monogenic disorders. Liver-directed gene therapy has recently become an option for treatment of haemophilias and is likely to become one of the favoured therapeutic strategies for inherited metabolic liver diseases in the near future. In this review, we present the different gene therapy vectors and strategies for liver-targeting, including gene editing. We highlight the current development of viral and nonviral gene therapy for a number of inherited metabolic liver diseases including urea cycle defects, organic acidaemias, Crigler-Najjar disease, Wilson disease, glycogen storage disease Type Ia, phenylketonuria and maple syrup urine disease. We describe the main limitations and open questions for further gene therapy development: immunogenicity, inflammatory response, genotoxicity, gene therapy administration in a fibrotic liver. The follow-up of a constantly growing number of gene therapy treated patients allows better understanding of its benefits and limitations and provides strategies to design safer and more efficacious treatments. Undoubtedly, liver-targeting gene therapy offers a promising avenue for innovative therapies with an unprecedented potential to address the unmet needs of patients suffering from inherited metabolic diseases.

AAV Vectors Pseudotyped with Capsids from Porcine and Bovine Species Mediate In Vitro and In Vivo Gene Delivery

Adeno-associated virus (AAV) vectors are among the most widely used delivery vehicles for in vivo gene therapy as they mediate robust and sustained transgene expression with limited toxicity. However, a significant impediment to the broad clinical success of AAV-based therapies is the widespread presence of pre-existing humoral immunity to AAVs in the human population. This immunity arises from the circulation of non-pathogenic endemic human AAV serotypes. One possible solution is to use non-human AAV capsids to pseudotype transgene-containing AAV vector genomes of interest. Due to the low probability of human exposure to animal AAVs, pre-existing immunity to animal-derived AAV capsids should be low. Here, we characterize two novel AAV capsid sequences: one derived from porcine colon tissue and the other from a caprine adenovirus stock. Both AAV capsids proved to be effective transducers of HeLa and HEK293T cells in vitro. In vivo, both capsids were able to transduce the murine nose, lung, and liver after either intranasal or intraperitoneal administration. In addition, we demonstrate that the porcine AAV capsid likely arose from multiple recombination events involving human- and animal-derived AAV sequences. We hypothesize that recurrent recombination events with similar and distantly related AAV sequences represent an effective mechanism for enhancing the fitness of wildtype AAV populations.

Validation of Anti-Adeno Associated Virus Serotype rh10 (AAVrh.10) Total and Neutralizing Antibody Immunogenicity Assays

Immunogenicity assessment of Adeno-Associated Virus (AAV) vectors is a critical part of gene therapy drug development. Whether the assays are used for inclusion/exclusion criteria or to monitor the safety and efficacy of the gene therapy, they are critical bioanalytical assessments. While total anti-AAV assays are perceived as easier to develop and implement than neutralizing anti-AAV assays, the gene therapy field is still nascent, and it is not yet clear which of the assays should be implemented at what stage of drug development. Recently AAVrh.10 has gained interest for use in gene therapies targeting cardiac, neurological, and other diseases due to its enhanced transduction efficiency. There is limited information on anti-AAVrh.10 antibodies and their clinical impact; thus, the information presented herein documents the validation of both a total antibody assay (TAb) and a neutralizing antibody (NAb) assay for anti-AAVrh.10 antibodies. In this manuscript, the validation was performed in accordance with the 2019 FDA immunogenicity guidance with additional evaluations to comply with CLIA where applicable. The AAVrh.10 TAb and NAb assays were compared in terms of sensitivity, drug tolerance, and precision, along with a concordance analysis using the same individual serum samples. This comparison gave insight into the utility of each format as a screening assay for inclusion into clinical studies.

In-depth comparison of Anc80L65 and AAV9 retinal targeting and characterization of cross-reactivity to multiple AAV serotypes in humans

Anc80L65 is a synthetic, ancestral adeno-associated virus that has high tropism toward retinal photoreceptors after subretinal injection in mice and non-human primates. We characterized, for the first time, the post-intravitreal cell-specific transduction profile of Anc80L65 compared with AAV9. Here we use Anc80L65 and AAV9 to intravitreally deliver a copy of the gene encoding GFP into WT C57Bl/6J mice. GFP expression was driven by one of two clinically relevant promoters, chicken β actin (CB) or truncated MECP2 (P546). After qualitative assessment of relative GFP expression, we found Anc80L65 and AAV9 to have similar transduction profiles. Through the development of a novel method for quantifying GFP-positive retinal cells, we found Anc80L65 to have higher tropism in Müller glia and AAV9 to have higher tropism in horizontal cells. In addition, we found P546 to promote GFP expression at a more moderate level compared with the high levels seen under the CB promoter. Finally, for the first time, we characterized Anc80L65 cross-reactivity in human sera; 83% of patients with AAV2 pre-existing antibodies were found to be seropositive for Anc80L65. This study demonstrates the expanded therapeutic applications of Anc80L65 to treat retinal disease and provides the first insights to Anc80L65 pre-existing immunity in humans.

Intracerebral lentiviral ABCD1 gene therapy in an early disease onset ALD mouse model

X-linked adrenoleukodystrophy (ALD) is a genetic disorder of the ABCD1 gene. We aimed to treat ALD via direct intracerebral injection of lentiviral ABCD1 (LV.ABCD1). Lentiviral vectors (LVs) were injected into the brain of wild type mice to access toxicities and biodistribution. Confocal microscopy illustrated supraphysiological ABCD1 expression surrounding the injection sites, and LVs were also detected in the opposite site of the unilaterally injected brain. In multi-site bilateral injections (4, 6, 8, and 9 sites), LV.ABCD1 transduced most brain regions including the cerebellum. Investigation of neuronal loss, astrogliosis and microglia activation did not detect abnormality. For efficacy evaluation, a novel ALD knockout (KO) mouse model was established by deleting exons 3 to 9 of the ABCD1 gene based on CRISPR/Cas9 gene editing. The KO mice showed behavioral deficit in open-field test (OFT) and reduced locomotor activities in rotarod test at 6 and 7 months of age, respectively. We treated 3-month-old KO mice with bilateral LV.ABCD1 injections into the external capsule and thalamus. ABCD1 expression was detected 15 days later, and the impaired motor ability was gradually alleviated. Our studies established an early onset ALD model and illustrated neurological improvement after LV.ABCD1 intracerebral injection without immunopathological toxicity.

Challenges in the pharmacological treatment of patients under suspicion of chronic traumatic encephalopathy: A review

Chronic traumatic encephalopathy (CTE) is caused by progressive neurodegeneration associated with repetitive head impacts. This disease is more common in professionals who practice contact sports, resulting in a concussion and subconcussive trauma. CTE is characterized by the accumulation of hyperphosphorylated tau protein in neurons, astrocytes, and frontotemporal lobe degeneration. Symptoms are usually nonspecific and overlap with other neurodegenerative diseases, such as Alzheimer's disease and frontotemporal dementia, making it difficult to provide drug treatment for patients with this comorbidity. Therefore, the objective of this article is to present an updated review of the pharmacological treatment of chronic traumatic encephalopathy and its challenges.

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.

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.

AAV capsid engineering identified two novel variants with improved in vivo tropism for cardiomyocytes

AAV vectors are promising delivery tools for human gene therapy. However, broad tissue tropism and pre-existing immunity against natural serotypes limit their clinical use. We identified two AAV capsid variants, AAV2-THGTPAD and AAV2-NLPGSGD, by in vivo AAV2 peptide display library screening in a murine model of pressure overload-induced cardiac hypertrophy. Both variants showed significantly improved efficacy in in vivo cardiomyocyte transduction compared with the parental serotype AAV2 as indicated by a higher number of AAV vector episomes in the nucleus and significant improved transduction efficiency. Both variants also outcompeted the reference serotype AAV9 regarding cardiomyocyte tropism, reaching comparable cardiac transduction efficiencies accompanied with liver de-targeting and decreased transduction efficiency of non-cardiac cells. Capsid modification influenced immunogenicity as sera of mice treated with AAV2-THGTPAD and AAV2-NLPGSGD demonstrated a poor neutralization capacity for the parental serotype and the novel variants. In a therapeutic setting, using the long non-coding RNA H19 in low vector dose conditions, novel AAV variants mediated superior anti-hypertrophic effects and revealed a further improved target-to-noise ratio, i.e., cardiomyocyte tropism. In conclusion, AAV2-THGTPAD and AAV2-NLPGSGD are promising novel tools for cardiac-directed gene therapy outperforming AAV9 regarding the specificity and therapeutic efficiency of in vivo cardiomyocyte transduction.

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.

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.

Changing Fate: Reprogramming Cells via Engineered Nanoscale Delivery Materials

The incorporation of nanotechnology in regenerative medicine is at the nexus of fundamental innovations and early-stage breakthroughs, enabling exciting biomedical advances. One of the most exciting recent developments is the use of nanoscale constructs to influence the fate of cells, which are the basic building blocks of healthy function. Appropriate cell types can be effectively manipulated by direct cell reprogramming; a robust technique to manipulate cellular function and fate, underpinning burgeoning advances in drug delivery systems, regenerative medicine, and disease remodeling. Individual transcription factors, or combinations thereof, can be introduced into cells using both viral and nonviral delivery systems. Existing approaches have inherent limitations. Viral-based tools include issues of viral integration into the genome of the cells, the propensity for uncontrollable silencing, reduced copy potential and cell specificity, and neutralization via the immune response. Current nonviral cell reprogramming tools generally suffer from inferior expression efficiency. Nanomaterials are increasingly being explored to address these challenges and improve the efficacy of both viral and nonviral delivery because of their unique properties such as small size and high surface area. This review presents the state-of-the-art research in cell reprogramming, focused on recent breakthroughs in the deployment of nanomaterials as cell reprogramming delivery tools.

Rational design of AAVrh10-vectored ACE2 functional domain to broadly block the cell entry of SARS-CoV-2 variants

The frequently emerging SARS-CoV-2 variants have weakened the effectiveness of existing COVID-19 vaccines and neutralizing antibody therapy. Nevertheless, the infections of SARS-CoV-2 variants still depend on angiotensin-converting enzyme 2 (ACE2) receptor-mediated cell entry, and thus the soluble human ACE2 (shACE2) is a potential decoy for broadly blocking SARS-CoV-2 variants. In this study, we firstly generated the recombinant AAVrh10-vectored shACE2 constructs, a kind of adeno-associated virus (AAV) serotype with pulmonary tissue tropism, and then validated its inhibition capacity against SARS-CoV-2 infection. To further optimize the minimized ACE2 functional domain candidates, a comprehensive analysis was performed to clarify the interactions between the ACE2 orthologs from various species and the receptor binding domain (RBD) of SARS-CoV-2 spike (S) protein. Based on the key interface amino acids, we designed a series of truncated ACE2 orthologs, and then assessed their potential affinity to bind to SARS-CoV-2 variants RBD in silico. Of note, we found that the 24-83aa fragment of dog ACE2 (dACE2_24-83) had a higher affinity to the RBD of SARS-CoV-2 variants than that of human ACE2. Importantly, AAVrh10-vectored shACE2 or dACE2_24-83 constructs exhibited a broadly blockage breadth against SARS-CoV-2 prototype and variants in vitro and ex vivo. Collectively, these data highlighted a promising therapeutic strategy against SARS-CoV-2 variants.

Testing preexisting antibodies prior to AAV gene transfer therapy: rationale, lessons and future considerations

Given the increasing number of gene transfer therapy studies either completed or underway, there is growing attention to the importance of preexisting adaptive immunity to the viral vectors used. The recombinant viral vectors developed for gene transfer therapy share structural features with naturally occurring wild-type virus. Antibodies generated against viral vectors obtained through a previous exposure to wild-type virus can potentially compromise transgene expression by blocking transduction, thereby limiting the therapeutic efficacy of the gene transfer therapy; they may also pose potential safety concerns. Therefore, systemic gene transfer delivery requires testing patients for preexisting antibodies. Two different assays have been used: (1) binding assays that focus on total antibodies (both neutralizing and non-neutralizing) and (2) neutralizing assays that detect neutralizing antibodies. In this review we focus on adeno-associated virus-based gene therapies, describing the immune response that occurs to naturally occurring adeno-associated viruses, the implications for patients with this exposure, the assays used to detect preexisting immune responses, and strategies to circumvent preexisting adaptive immunity to expand the patient base that could benefit from such therapies.

Age-stratified adeno-associated virus serotype 3 neutralizing and total antibody prevalence in hemophilia A patients from India

Gene therapy using adeno-associated virus (AAV) vector offers a new treatment option for individuals with monogenetic disorders. The major bottleneck is the presence of pre-existing anti-AAV antibodies, which impacts its use. Even very low titers of neutralizing antibodies (NAb) to capsids from natural AAV infections have been reported to inhibit the transduction of intravenously administered AAV in animal models and are associated with limited efficacy in human trials. Assessing the level of pre-existing NAb is important for determining the primary eligibility of patients for AAV vector-based gene therapy clinical trials. Techniques used to screen AAV-antibodies include AAV capsid ELISA and transduction inhibition assay (TIA) for detecting total capsid-binding (TAb) and Nab, respectively. In this study, we screened 521 individuals with hemophilia A from India for TAb and NAb using ELISA and TIA, respectively. The prevalence of TAb and NAb in hemophilia A patients from India were 96 % and 77.5%, respectively. There was a significant increase in anti-AAV3 NAb prevalence with age in the hemophilia A patient group from India. There was a trend in anti-AAV3 TAb positivity between the pediatric age group (94.4%) and the adult age group (97.4%). This article is protected by copyright. All rights reserved.

Gene Therapy Advances: A Meta-Analysis of AAV Usage in Clinical Settings

Adeno-associated viruses (AAVs) are the safest and most effective gene delivery vehicles to drive long-term transgene expression in gene therapy. While animal studies have shown promising results, the translatability of AAVs into clinical settings has been partly limited due to their restricted gene packaging capacities, off-target transduction, and immunogenicity. In this study, we analysed over two decades of AAV applications, in 136 clinical trials. This meta-analysis aims to provide an up-to-date overview of the use and successes of AAVs in clinical trials, while evaluating the approaches used to address the above challenges. First, this study reveals that the speed of novel AAV development has varied between therapeutic areas, with particular room for improvement in Central Nervous System disorders, where development has been slow. Second, the lack of dose-dependent toxicity and efficacy data indicates that optimal dosing regimes remain elusive. Third, more clinical data on the effectiveness of various immune-modulation strategies and gene editing approaches are required to direct future research and to accelerate the translation of AAV-mediated gene therapy into human applications.

Delivering gene therapy for mucopolysaccharide diseases

Mucopolysaccharide diseases are a group of paediatric inherited lysosomal storage diseases that are caused by enzyme deficiencies, leading to a build-up of glycosaminoglycans (GAGs) throughout the body. Patients have severely shortened lifespans with a wide range of symptoms including inflammation, bone and joint, cardiac, respiratory and neurological disease. Current treatment approaches for MPS disorders revolve around two main strategies. Enzyme replacement therapy (ERT) is efficacious in treating somatic symptoms but its effect is limited for neurological functions. Haematopoietic stem cell transplant (HSCT) has the potential to cross the BBB through monocyte trafficking, however delivered enzyme doses limit its use almost exclusively to MPSI Hurler. Gene therapy is an emerging therapeutic strategy for the treatment of MPS disease. In this review, we will discuss the various vectors that are being utilised for gene therapy in MPS as well as some of the most recent gene-editing approaches undergoing pre-clinical and clinical development.

Efficacy of AAV serotypes to target Schwann cells after intrathecal and intravenous delivery

To optimize gene delivery to myelinating Schwann cells we compared clinically relevant AAV serotypes and injection routes. AAV9 and AAVrh10 vectors expressing either EGFP or the neuropathy-associated gene GJB1/Connexin32 (Cx32) under a myelin specific promoter were injected intrathecally or intravenously in wild type and Gjb1-null mice, respectively. Vector biodistribution in lumbar roots and sciatic nerves was higher in AAVrh10 injected mice while EGFP and Cx32 expression rates and levels were similar between the two serotypes. A gradient of biodistribution away from the injection site was seen with both intrathecal and intravenous delivery, while similar expression rates were achieved despite higher vector amounts injected intravenously. Quantified immune cells in relevant tissues were similar to non-injected littermates. Overall, AAV9 and AAVrh10 efficiently transduce Schwann cells throughout the peripheral nervous system with both clinically relevant routes of administration, although AAV9 and intrathecal injection may offer a more efficient approach for treating demyelinating neuropathies.

Structural Study of Aavrh.10 Receptor and Antibody Interactions

Recombinant adeno-associated virus (rAAV) vectors are one of the leading tools for the delivery of therapeutic genes in human gene therapy applications. For a successful transfer of their payload, the AAV vectors have to circumvent potential preexisting neutralizing host antibodies and bind to the receptors of the target cells. Both of these aspects have not been structurally analyzed for AAVrh.10. Here, cryo-electron microscopy and three-dimensional image reconstruction were used to map the binding site of sulfated N-acetyllactosamine (LacNAc; previously shown to bind AAVrh.10) and a series of four monoclonal antibodies (MAbs). LacNAc was found to bind to a pocket located on the side of the 3-fold capsid protrusion that is mostly conserved to AAV9 and equivalent to its galactose-binding site. As a result, AAVrh.10 was also shown to be able to bind to cell surface glycans with terminal galactose. For the antigenic characterization, it was observed that several anti-AAV8 MAbs cross-react with AAVrh.10. The binding sites of these antibodies were mapped to the 3-fold capsid protrusions. Based on these observations, the AAVrh.10 capsid surface was engineered to create variant capsids that escape these antibodies while maintaining infectivity. IMPORTANCE Gene therapy vectors based on adeno-associated virus rhesus isolate 10 (AAVrh.10) have been used in several clinical trials to treat monogenetic diseases. However, compared to other AAV serotypes little is known about receptor binding and antigenicity of the AAVrh.10 capsid. Particularly, preexisting neutralizing antibodies against capsids are an important challenge that can hamper treatment efficiency. This study addresses both topics and identifies critical regions of the AAVrh.10 capsid for receptor and antibody binding. The insights gained were utilized to generate AAVrh.10 variants capable of evading known neutralizing antibodies. The findings of this study could further aid the utilization of AAVrh.10 vectors in clinical trials and help the approval of the subsequent biologics.

AAV ablates neurogenesis in the adult murine hippocampus

Recombinant adeno-associated virus (rAAV) has been widely used as a viral vector across mammalian biology and has been shown to be safe and effective in human gene therapy. We demonstrate that neural progenitor cells (NPCs) and immature dentate granule cells (DGCs) within the adult murine hippocampus are particularly sensitive to rAAV-induced cell death. Cell loss is dose dependent and nearly complete at experimentally relevant viral titers. rAAV-induced cell death is rapid and persistent, with loss of BrdU-labeled cells within 18 hr post-injection and no evidence of recovery of adult neurogenesis at 3 months post-injection. The remaining mature DGCs appear hyperactive 4 weeks post-injection based on immediate early gene expression, consistent with previous studies investigating the effects of attenuating adult neurogenesis. In vitro application of AAV or electroporation of AAV2 inverted terminal repeats (ITRs) is sufficient to induce cell death. Efficient transduction of the dentategyrus (DG)- without ablating adult neurogenesis- can be achieved by injection of rAAV2-retro serotyped virus into CA3. rAAV2-retro results in efficient retrograde labeling of mature DGCs and permits in vivo two-photon calcium imaging of dentate activity while leaving adult neurogenesis intact. These findings expand on recent reports implicating rAAV-linked toxicity in stem cells and other cell types and suggest that future work using rAAV as an experimental tool in the DG and as a gene therapy for diseases of the central nervous system should be carefully evaluated.

Prevalence of Adeno-Associated Virus 3 Capsid Binding and Neutralizing Antibodies in Healthy and Hemophilia B Individuals from India

Adeno-associated virus (AAV) vector-based gene therapy offers a new treatment option for individuals with hemophilia. Pre-existing anti-AAV antibodies significantly impact the use of AAV vectors. Even relatively low titers of AAV neutralizing antibodies (NAb) from natural AAV infections against the capsid have been shown to inhibit the transduction of intravenously administered AAV in animal models and were associated with limited efficacy in human trials. This is important for determining the primary eligibility of patients for AAV vector-based gene therapy clinical trials. Current techniques to screen AAV antibodies include AAV capsid enzyme-linked immunosorbent assay (ELISA) for total antibodies and a transduction inhibition assay (TIA) for NAb. This study developed and screened total capsid binding anti-AAV3 antibodies by using ELISA and determined NAb levels by TIA using mCherry flow cytometry in healthy individuals with hemophilia B in India. One hundred and forty-three apparently healthy controls and 92 individuals with hemophilia B were screened. The prevalence of total and NAb in healthy controls was 79.7% and 65%, respectively; the prevalence of total and NAb in patients with hemophilia B for AAV3 was 92.4% and 91.3%, respectively.

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.

Adeno-associated virus serotype 9 antibodies in patients screened for treatment with onasemnogene abeparvovec

Spinal muscular atrophy is a progressive, recessively inherited monogenic neurologic disease, the genetic root cause of which is the absence of a functional survival motor neuron 1 gene. Onasemnogene abeparvovec (formerly AVXS-101) is an adeno-associated virus serotype 9 vector-based gene therapy that delivers a fully functional copy of the human survival motor neuron gene. We report anti-adeno-associated virus serotype 9 antibody titers for patients with spinal muscular atrophy when they were screened for eligibility in the onasemnogene abeparvovec clinical trials (intravenous and intrathecal administration) and managed access programs (intravenous). Through December 31, 2019, 196 patients and 155 biologic mothers were screened for anti-adeno-associated virus serotype 9 binding antibodies with an enzyme-linked immunosorbent assay. Of these, 15 patients (7.7%) and 23 biologic mothers (14.8%) had titers >1:50 on their initial screening tests. Eleven patients (5.6%) had elevated titers on their final screening tests. The low percentage of patients with exclusionary antibody titers indicates that most infants with spinal muscular atrophy type 1 should be able to receive onasemnogene abeparvovec. Retesting may identify patients whose antibody titers later decrease to below the threshold for treatment, and retesting should be considered for patients with anti-adeno-associated virus serotype 9 antibody titers >1:50.

Megalencephalic Leukoencephalopathy: Insights Into Pathophysiology and Perspectives for Therapy

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. It is mainly caused by recessive mutations in MLC1 and HEPACAM (also called GLIALCAM) genes. These disease variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. Besides, dominant mutations in HEPACAM were also identified in a subtype of MLC patients (MLC2B) with a remitting phenotype. MLC1 and GlialCAM proteins form a complex mainly expressed in brain astrocytes at the gliovascular interface and in Bergmann glia at the cerebellum. Both proteins regulate several ion channels and transporters involved in the control of ion and water fluxes in glial cells, either directly influencing their location and function, or indirectly regulating associated signal transduction pathways. However, the MLC1/GLIALCAM complex function and the related pathological mechanisms leading to MLC are still unknown. It has been hypothesized that, in MLC, the role of glial cells in brain ion homeostasis is altered in both physiological and inflammatory conditions. There is no therapy for MLC patients, only supportive treatment. As MLC2B patients show an MLC reversible phenotype, we speculated that the phenotype of MLC1 and MLC2A patients could also be mitigated by the re-introduction of the correct gene even at later stages. To prove this hypothesis, we injected in the cerebellar subarachnoid space of Mlc1 knockout mice an adeno-associated virus (AAV) coding for human MLC1 under the control of the glial-fibrillary acidic protein promoter. MLC1 expression in the cerebellum extremely reduced myelin vacuolation at all ages in a dose-dependent manner. This study could be considered as the first preclinical approach for MLC. We also suggest other potential therapeutic strategies in this review.

Cerebellar Astrocyte Transduction as Gene Therapy for Megalencephalic Leukoencephalopathy

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. There is no therapy for MLC patients, only supportive treatment. We show here a preclinical gene therapy approach for MLC using the Mlc1 knock-out mouse. An adeno-associated virus coding for human MLC1 under the control of the glial fibrillary acidic protein promoter was injected in the cerebellar subarachnoid space of Mlc1 knock-out and wild-type animals at 2 months of age, before the onset of the disease, as a preventive approach. We also tested a therapeutic strategy by injecting the animals at 5 months, once the histopathological abnormalities are starting, or at 15 months, when they have progressed to a more severe pathology. MLC1 expression in the cerebellum restored the adhesion molecule GlialCAM and the chloride channel ClC-2 localization in Bergmann glia, which both are mislocalized in Mlc1 knock-out model. More importantly, myelin vacuolation was extremely reduced in treated mice at all ages and correlated with the amount of expressed MLC1 in Bergmann glia, indicating not only the preventive potential of this strategy but also its therapeutic capacity. In summary, here we provide the first therapeutic approach for patients affected with MLC. This work may have also implications to treat other diseases affecting motor function such as ataxias.

Isolating Human Monoclonal Antibodies Against Adeno-Associated Virus From Donors With Pre-existing Immunity

With the advent of single B-cell cloning technology, we can isolate antibodies against virtually any antigen to study the interaction of a given pathogen with the immune system and develop novel therapeutic strategies. Antibodies directed against the capsid of adeno-associated viruses (AAV) are a significant obstacle to effectively leveraging AAV as a gene-delivery vector in seropositive individuals. In order to design next-generation vectors that can evade neutralization by these antibodies, studies have mapped the epitopes of mouse monoclonal antibodies generated by immunization with AAV. Although these studies provide critical information regarding capsid immunogenicity, they cannot address (1) differences in the antibody repertoire generated in humans following AAV natural infection; or (2) how reactions can vary when generated in response to vector administration. Here, we isolated and evaluated a panel of novel, fully human anti-AAV antibodies by cloning single memory B cells from a seropositive normal donor. We have validated the utility of this approach to study AAV immunology. Our goal is to leverage this knowledge to design novel AAV variants that can effectively transduce target tissues in individuals with AAV-neutralizing antibodies.

T-Cell Dependent Immunogenicity of Protein Therapeutics Pre-clinical Assessment and Mitigation-Updated Consensus and Review 2020

Immune responses to protein and peptide drugs can alter or reduce their efficacy and may be associated with adverse effects. While anti-drug antibodies (ADA) are a standard clinical measure of protein therapeutic immunogenicity, T cell epitopes in the primary sequences of these drugs are the key drivers or modulators of ADA response, depending on the type of T cell response that is stimulated (e.g., T helper or Regulatory T cells, respectively). In a previous publication on T cell-dependent immunogenicity of biotherapeutics, we addressed mitigation efforts such as identifying and reducing the presence of T cell epitopes or T cell response to protein therapeutics prior to further development of the protein therapeutic for clinical use. Over the past 5 years, greater insight into the role of regulatory T cell epitopes and the conservation of T cell epitopes with self (beyond germline) has improved the preclinical assessment of immunogenic potential. In addition, impurities contained in therapeutic drug formulations such as host cell proteins have also attracted attention and become the focus of novel risk assessment methods. Target effects have come into focus, given the emergence of protein and peptide drugs that target immune receptors in immuno-oncology applications. Lastly, new modalities are entering the clinic, leading to the need to revise certain aspects of the preclinical immunogenicity assessment pathway. In addition to drugs that have multiple antibody-derived domains or non-antibody scaffolds, therapeutic drugs may now be introduced via viral vectors, cell-based constructs, or nucleic acid based therapeutics that may, in addition to delivering drug, also prime the immune system, driving immune response to the delivery vehicle as well as the encoded therapeutic, adding to the complexity of assessing immunogenicity risk. While it is challenging to keep pace with emerging methods for the preclinical assessment of protein therapeutics and new biologic therapeutic modalities, this collective compendium provides a guide to current best practices and new concepts in the field.

Adeno-associated virus-binding antibodies detected in cats living in the Northeastern United States lack neutralizing activity

Cats are a critical pre-clinical model for studying adeno-associated virus (AAV) vector-mediated gene therapies. A recent study has described the high prevalence of anti-AAV neutralizing antibodies among domestic cats in Switzerland. However, our knowledge of pre-existing humoral immunity against various AAV serotypes in cats is still limited. Here, we show that, although antibodies binding known AAV serotypes (AAV1 to AAV11) are prevalent in cats living in the Northeastern United States, these antibodies do not necessarily neutralize AAV infectivity. We analyzed sera from 35 client-owned, 20 feral, and 30 specific pathogen-free (SPF) cats for pre-existing AAV-binding antibodies against the 11 serotypes. Antibody prevalence was 7 to 90% with an overall median of 50%. The AAV-binding antibodies showed broad reactivities with other serotypes. Of 44 selected antibodies binding AAV2, AAV6 or AAV9, none exhibited appreciable neutralizing activities. Instead, AAV6 or AAV9-binding antibodies showed a transduction-enhancing effect. AAV6-binding antibodies were highly prevalent in SPF cats (83%), but this was primarily due to cross-reactivity with preventive vaccine-induced anti-feline panleukopenia virus antibodies. These results indicate that prevalent pre-existing immunity in cats is not necessarily inhibitory to AAV and highlight a substantial difference in the nature of AAV-binding antibodies in cats living in geographically different regions.

Comparative Analysis of the Capsid Structures of AAVrh.10, AAVrh.39, and AAV8

Adeno-associated viruses (AAVs) from clade E are often used as vectors in gene delivery applications. This clade includes rhesus isolate 10 (AAVrh.10) and 39 (AAVrh.39) which, unlike representative AAV8, are capable of crossing the blood-brain barrier (BBB), thereby enabling the delivery of therapeutic genes to the central nervous system. Here, the capsid structures of AAV8, AAVrh.10 and AAVrh.39 have been determined by cryo-electron microscopy and three-dimensional image reconstruction to 3.08-, 2.75-, and 3.39-Å resolution, respectively, to enable a direct structural comparison. AAVrh.10 and AAVrh.39 are 98% identical in amino acid sequence but only ∼93.5% identical to AAV8. However, the capsid structures of all three viruses are similar, with only minor differences observed in the previously described surface variable regions, suggesting that specific residues S269 and N472, absent in AAV8, may confer the ability to cross the BBB in AAVrh.10 and AAVrh.39. Head-to-head comparison of empty and genome-containing particles showed DNA ordered in the previously described nucleotide-binding pocket, supporting the suggested role of this pocket in DNA packaging for the Dependoparvovirus The structural characterization of these viruses provides a platform for future vector engineering efforts toward improved gene delivery success with respect to specific tissue targeting, transduction efficiency, antigenicity, or receptor retargeting.IMPORTANCE Recombinant adeno-associated virus vectors (rAAVs), based on AAV8 and AAVrh.10, have been utilized in multiple clinical trials to treat different monogenetic diseases. The closely related AAVrh.39 has also shown promise in vivo As recently attained for other AAV biologics, e.g., Luxturna and Zolgensma, based on AAV2 and AAV9, respectively, the vectors in this study will likely gain U.S. Food and Drug Administration approval for commercialization in the near future. This study characterized the capsid structures of these clinical vectors at atomic resolution using cryo-electron microscopy and image reconstruction for comparative analysis. The analysis suggested two key residues, S269 and N472, as determinants of BBB crossing for AAVrh.10 and AAVrh.39, a feature utilized for central nervous system delivery of therapeutic genes. The structure information thus provides a platform for engineering to improve receptor retargeting or tissue specificity. These are important challenges in the field that need attention. Capsid structure information also provides knowledge potentially applicable for regulatory product approval.

Anti-Phospho-Tau Gene Therapy for Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disorder caused by repetitive trauma to the central nervous system (CNS) suffered by soldiers, contact sport athletes, and civilians following accident-related trauma. CTE is a CNS tauopathy, with trauma-induced inflammation leading to accumulation of hyperphosphorylated forms of the microtubule-binding protein Tau (pTau), resulting in neurofibrillary tangles and progressive loss of neurons. At present, there are no therapies to treat CTE. We hypothesized that direct CNS administration of an adeno-associated virus (AAV) vector coding for an anti-pTau antibody would generate sufficient levels of anti-pTau in the CNS to suppress pTau accumulation thus interrupting the pathogenic process. Using a serotype AAVrh.10 gene transfer vector coding for a monoclonal antibody directed against pTau, we demonstrate the feasibility of this strategy in a murine CTE model in which pTau accumulation was elicited by repeated traumatic brain injury (TBI) using a closed cortical impact procedure over 5 days. Direct delivery of AAVrh.10 expression vectors coding for either of the two different anti-pTau antibodies to the hippocampus of these TBI mice significantly reduced pTau levels across the CNS. Using doses that can be safely scaled to humans, the data demonstrate that CNS administration of AAVrh.10anti-pTau is effective, providing a new strategy to interrupt the CTE consequences of TBI.

Advances in the Treatment of Neuronal Ceroid Lipofuscinosis

Neuronal ceroid lipofuscinoses (NCL) represent a class of neurodegenerative disorders involving defective lysosomal processing enzymes or receptors, leading to lysosomal storage disorders, typically characterized by observation of cognitive and visual impairments, epileptic seizures, ataxia, and deterioration of motor skills. Recent success of a biologic (Brineura®) for the treatment of neurologic manifestations of the central nervous system (CNS) has led to renewed interest in therapeutics for NCL, with the goal of ablating or reversing the impact of these devastating disorders. Despite complex challenges associated with CNS therapy, many treatment modalities have been evaluated, including enzyme replacement therapy, gene therapy, stem cell therapy, and small molecule pharmacotherapy. Because the clinical endpoints for the evaluation of candidate therapies are complex and often reliant on subjective clinical scales, the development of quantitative biomarkers for NCLs has become an apparent necessity for the validation of potential treatments. We will discuss the latest findings in the search for relevant biomarkers for assessing disease progression. For this review, we will focus primarily on recent pre-clinical and clinical developments for treatments to halt or cure these NCL diseases. Continued development of current therapies and discovery of newer modalities will be essential for successful therapeutics for NCL.

AREAS COVERED

The reader will be introduced to the NCL subtypes, natural histories, experimental animal models, and biomarkers for NCL progression; challenges and different therapeutic approaches, and the latest pre-clinical and clinical research for therapeutic development for the various NCLs. This review corresponds to the literatures covering the years from 1968 to mid-2019, but primarily addresses pre-clinical and clinical developments for the treatment of NCL disease in the last decade and as a follow-up to our 2013 review of the same topic in this journal.

EXPERT OPINION

Much progress has been made in the treatment of neurologic diseases, such as the NCLs, including better animal models and improved therapeutics with better survival outcomes. Encouraging results are being reported at symposiums and in the literature, with multiple therapeutics reaching the clinical trial stage for the NCLs. The potential for a cure could be at hand after many years of trial and error in the preclinical studies. The clinical development of enzyme replacement therapy (Brineura® for CLN2), immunosuppression (CellCept® for CLN3), and gene therapy vectors (for CLN1, CLN2, CLN3, and CLN6) are providing encouragement to families that have a child afflicted with NCL. We believe that successful therapies in the future may involve the combination of two or more therapeutic modalities to provide therapeutic benefit especially as the patients grow older.

AAVrh-10 transduces outer retinal cells in rodents and rabbits following intravitreal administration

Recombinant adeno-associated virus (rAAV) has been widely used for gene delivery in animal models and successfully applied in clinical trials for treating inherited retinal disease. Although subretinal delivery of AAVs can effectively transduce photoreceptors and/or retinal pigmental epithelium (RPE), cells most affected by inherited retinal diseases, the procedure is invasive and complicated, and only delivers the gene to a limited retinal area. AAVs can also be delivered intravitreally to the retina, a much less invasive nonsurgical procedure. However, intravitreal administration of non-modified AAV serotypes tends to transduce only ganglion cells and inner nuclear layer cells. To date, most non-modified AAV serotypes that have been identified are incapable of efficiently transducing photoreceptors and/or RPE when delivered intravitreally. In this study, we investigate the retinal tropism of AAVrh10 vector administered by intravitreal injection to mouse, rat, and rabbit eyes. Our results demonstrate that AAVrh10 is capable of transducing not only inner retinal cells, but also outer retinal cells in all three species, though the transduction efficiency in rabbit was low. In addition, AAVrh10 preferentially transduced outer retinal cells in mouse models of retinal disease. Therefore, AAVrh10 vector could be a useful candidate to intravitreally deliver genes to photoreceptor and RPE cells.

Metabotropic glutamate type 5 receptor requires contactin-associated protein 1 to control memory formation

The hippocampus is a key brain region for memory formation. Metabotropic glutamate type 5 receptors (mGlu5R) are strongly expressed in CA1 pyramidal neurons and fine-tune synaptic plasticity. Accordingly, mGlu5R pharmacological manipulation may represent an attractive therapeutic strategy to manage hippocampal-related neurological disorders. Here, by means of a membrane yeast two-hybrid screening, we identified contactin-associated protein 1 (Caspr1), a type I transmembrane protein member of the neurexin family, as a new mGlu5R partner. We report that mGlu5R and Caspr1 co-distribute and co-assemble both in heterologous expression systems and in rat brain. Furthermore, downregulation of Caspr1 in rat hippocampal primary cultures decreased mGlu5R-mediated signaling. Finally, silencing Caspr1 expression in the hippocampus impaired the impact of mGlu5R on spatial memory. Our results indicate that Caspr1 plays a pivotal role controlling mGlu5R function in hippocampus-dependent memory formation. Hence, this new protein-protein interaction may represent novel target for neurological disorders affecting hippocampal glutamatergic neurotransmission.

Multifunctional CRISPR-Cas9 with engineered immunosilenced human T cell epitopes

The CRISPR-Cas9 system has raised hopes for developing personalized gene therapies for complex diseases. Its application for genetic and epigenetic therapies in humans raises concerns over immunogenicity of the bacterially derived Cas9 protein. Here we detect antibodies to Streptococcus pyogenes Cas9 (SpCas9) in at least 5% of 143 healthy individuals. We also report pre-existing human CD8+T cell immunity in the majority of healthy individuals screened. We identify two immunodominant SpCas9 T cell epitopes for HLA-A*02:01 using an enhanced prediction algorithm that incorporates T cell receptor contact residue hydrophobicity and HLA binding and evaluated them by T cell assays using healthy donor PBMCs. In a proof-of-principle study, we demonstrate that Cas9 protein can be modified to eliminate immunodominant epitopes through targeted mutation while preserving its function and specificity. Our study highlights the problem of pre-existing immunity against CRISPR-associated nucleases and offers a potential solution to mitigate the T cell immune response.

Possible immunogenicity of the Cas9 protein raises concerns about therapeutic applications. Here the authors identify pre-existing CD8+T-cell immunity in healthy individuals and in response modify Cas9 to remove the immunodominant epitopes.

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.

Development of a Chemiluminescent ELISA Method for the Detection of Total Anti-Adeno Associated Virus Serotype 9 (AAV9) Antibodies

Recombinant adeno associated viruses (rAAV) have become an important tool for the delivery of gene therapeutics due to long-standing safety and success in clinical trials. Since humans often become exposed to AAVs and develop anti-AAV antibodies (Abs), a potential impediment to the success of gene therapeutics is neutralization of the viral particle before it has had a chance to bind and enter target cells to release the transgene. Identification of subjects with preexisting Abs having neutralizing potential, and exclusion of such subjects from clinical studies is expected to enhance drug efficacy. In vitro cell-based reporter assays are most often employed to determine the level of neutralizing antibodies in a given population. Such assays measure the ability of the Abs to prevent viral binding and entry into cells by engaging epitopes on the viral capsid involved in host cell receptor binding. In general, cell-based assays are low throughput and labor intensive and may suffer from high variability and low sensitivity issues. In contrast, enzyme-linked immunosorbent assays (ELISAs) are simpler, less variable, and have higher throughput. Demonstrating a correlation between neutralizing Abs assessed by a cell-based assay and total binding Abs measured in an ELISA will enable the use and substitution of the latter for screening and exclusion of subjects. In this work, we describe the development of a highly sensitive, specific, robust, and reproducible chemiluminescent ELISA method for the detection of total anti-AAV9 Abs. Using this method, we analyzed the prevalence of preexisting anti-AAV9 Abs in 100 serum samples from heart disease patients. Analysis of neutralizing Abs in the same samples using an in vitro cell-based assay showed a strong correlation between total anti-AAV9 Abs and neutralizing Abs, indicating the feasibility of using the total Ab ELISA in the future for patient screening and exclusion.

Unraveling the Complex Story of Immune Responses to AAV Vectors Trial After Trial

Over the past decade, vectors derived from adeno-associated virus (AAV) have established themselves as a powerful tool for in vivo gene transfer, allowing long-lasting and safe transgene expression in a variety of human tissues. Nevertheless, clinical trials demonstrated how B and T cell immune responses directed against the AAV capsid, likely arising after natural infection with wild-type AAV, might potentially impact gene transfer safety and efficacy in patients. Seroprevalence studies have evidenced that most individuals carry anti-AAV neutralizing antibodies that can inhibit recombinant AAV transduction of target cells following in vivo administration of vector particles. Likewise, liver- and muscle-directed clinical trials have shown that capsid-reactive memory CD8⁺ T cells could be reactivated and expanded upon presentation of capsid-derived antigens on transduced cells, potentially leading to loss of transgene expression and immune-mediated toxicities. In celebration of the 25th anniversary of the European Society of Gene and Cell Therapy, this review article summarizes progress made during the past decade in understanding and modulating AAV vector immunogenicity. While the knowledge generated has contributed to yield impressive clinical results, several important questions remain unanswered, making the study of immune responses to AAV a priority for the field of in vivo transfer.

AAVrh.10-Mediated APOE2 Central Nervous System Gene Therapy for APOE4-Associated Alzheimer's Disease

Alzheimer's disease (AD) is a progressive degenerative neurological disorder affecting nearly one in nine elderly people in the United States. Population studies have shown that an inheritance of the apolipoprotein E (APOE) variant APOE4 allele increases the risk of developing AD, whereas APOE2 homozygotes are protected from late-onset AD. It was hypothesized that expression of the "protective" APOE2 variant by genetic modification of the central nervous system (CNS) of APOE4 homozygotes could reverse or prevent progressive neurologic damage. To assess the CNS distribution and safety of APOE2 gene therapy for AD in a large-animal model, intraparenchymal, intracisternal, and intraventricular routes of delivery to the CNS of nonhuman primates of AAVrh.10hAPOE2-HA, an AAVrh.10 serotype coding for an HA-tagged human APOE2 cDNA sequence, were evaluated. To evaluate the route of delivery that achieves the widest extent of APOE2 expression in the CNS, the expression of APOE2 in the CNS was evaluated 2 months following vector administration for APOE2 DNA, mRNA, and protein. Finally, using conventional toxicology assays, the safety of the best route of delivery was assessed. The data demonstrated that while all three routes are capable of mediating ApoE2 expression in AD relevant regions, intracisternal delivery of AAVrh.10hAPOE2-HA safely mediated wide distribution of ApoE2 with the least invasive surgical intervention, thus providing the optimal strategy to deliver vector-mediated human APOE2 to the CNS.

AAV Natural Infection Induces Broad Cross-Neutralizing Antibody Responses to Multiple AAV Serotypes in Chimpanzees

Cross-sectional studies of primates have revealed that natural neutralizing antibody (NAb) responses to adeno-associated viruses (AAV) span multiple serotypes. This differs from the phenotype of the NAb response to an AAV vector delivered to seronegative nonhuman primates that is typically restricted to the administered AAV serotype. To better understand the mechanism by which natural AAV infections result in broad NAb responses, we conducted a longitudinal study spanning 10 years in which we evaluated serum-circulating AAV NAb levels in captive-housed chimpanzees. In a cohort of 25 chimpanzees we identified 3 distinct groups of animals: those that never seroconverted to AAV (naïve), those that were persistently seropositive (chronic), and those that seroconverted during the 10-year period (acute). For the chronic group we found a broad seroresponse characterized by NAbs reacting to multiple AAV serotypes. A similar cross-neutralization pattern of NAbs was observed in the acute group. These data support our hypothesis that a single natural infection with AAV induces a broadly cross-reactive NAb response to multiple AAV serotypes.

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.

Site-Specific PEGylated Adeno-Associated Viruses with Increased Serum Stability and Reduced Immunogenicity

Adeno-associated virus (AAV) is one of the most extensively studied and utilized viral vectors in clinical gene transfer research. However, the serum instability and immunogenicity of AAV vectors significantly limit their application. Here, we endeavored to overcome these limitations by developing a straightforward approach for site-specific PEGylation of AAV via genetic code expansion. This technique includes incorporation of the azide moiety into the AAV capsid protein followed by orthogonal and stoichiometric conjugation of a variety of polyethylene glycols (PEGs) through click chemistry. Using this approach, only the chosen site(s) was consistently PEGylated under mild conditions, preventing nonselective conjugation. Upon a series of in vitro examinations, AAVs conjugated with 20-kD PEG at sites Q325+1, S452+1, and R585+1 showed a 1.7- to 2.4-fold stability improvement in pooled human serum and a nearly twofold reduction in antibody recognition. Subsequent animal research on Sprague Dawley rats displayed a promising 20% reduction in antibody inducement and a higher virus titer in the blood. Together, our data demonstrate successful protection of an AAV vector from antibody neutralization and blood clearance, thereby increasing the efficiency of therapeutic gene delivery.

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

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

Adeno-associated viral gene therapy for mucopolysaccharidoses exhibiting neurodegeneration

The mucopolysaccharidoses (MPS) are a subgroup of lysosomal storage disorders that are caused by mutations in the genes involved in glycosaminoglycan breakdown. Multiple organs and tissues are affected, including the central nervous system. At present, hematopoietic stem cell transplantation and enzyme replacement therapies are approved for some of the (non-neurological) MPS. Treatments that effectively ameliorate the neurological aspects of the disease are being assessed in clinical trials. This review will focus on the recent outcomes and planned viral vector-mediated gene therapy clinical trials, and the pre-clinical data that supported these studies, for MPS-I (Hurler/Scheie syndrome), MPS-II (Hunter syndrome), and MPS-IIIA and -IIIB (Sanfilippo syndrome).

Identification of liver-specific enhancer-promoter activity in the 3' untranslated region of the wild-type AAV2 genome

Vectors based on adeno-associated virus type 2 (AAV2) are powerful tools for gene transfer and genome editing applications. The level of interest in this system has recently surged in response to reports of therapeutic efficacy in human clinical trials, most notably for those in patients with hemophilia B (ref. 3). Understandably, a recent report drawing an association between AAV2 integration events and human hepatocellular carcinoma (HCC) has generated controversy about the causal or incidental nature of this association and the implications for AAV vector safety. Here we describe and functionally characterize a previously unknown liver-specific enhancer-promoter element in the wild-type AAV2 genome that is found between the stop codon of the cap gene, which encodes proteins that form the capsid, and the right-hand inverted terminal repeat. This 124-nt sequence is within the 163-nt common insertion region of the AAV genome, which has been implicated in the dysregulation of known HCC driver genes and thus offers added insight into the possible link between AAV integration events and the multifactorial pathogenesis of HCC.

Gene therapy with modified U1 small nuclear RNA

More than 15% of all disease-causing mutations result in mRNA splicing defects. U1 snRNA binds to the 5' splice site (5'ss) through base pairing. Mutation-adapted U1 snRNA (with compensatory U1 snRNA changes) and exon-specific U1 snRNA (complementary to intronic sequences) have been shown to suppress 5'ss mutations in cellular and animal models. Areas covered: The history, mechanism of action, and efficacy of U1 snRNA-mediated gene therapy are covered. The clinical utility of this technology and its limitations will be discussed. Expert commentary: Recently, gene therapies with mutation-adapted U1 snRNAs have been conducted on animal models, including aromatic l-amino acid decarboxylase deficiency and spinal muscular atrophy. However, although U1-mediated therapy has the advantage of maintaining the regulated expression of defective genes, its accuracy and efficacy needs to be improved before clinical application of this technique is possible.

Induction of T-Cell Infiltration and Programmed Death Ligand 2 Expression by Adeno-Associated Virus in Rhesus Macaque Skeletal Muscle and Modulation by Prednisone

Use of adeno-associated virus (AAV) to transduce genes into skeletal muscles can be associated with T-cell responses to viral capsid and/or to transgenic protein. Intramuscular mononuclear cell infiltrates primarily consisting of CD8+ T cells and also containing FOXP3+ regulatory T cells were present in rhesus macaque skeletal muscle treated with rAAVrh74.MCK.GALGT2 by vascular delivery. Administration of oral prednisone prior to AAV gene delivery and throughout the study reduced such infiltrates by 60% at 24 weeks post AAV delivery compared with AAV-treated animals not receiving prednisone, regardless of the presence of pre-existing AAV serum antibodies at the time of treatment. The majority of CD8+ T cells in AAV-treated muscles expressed activated caspase 3 and programmed cell death protein 1 (PD1), suggesting ongoing programmed cell death. AAV-transduced skeletal muscles also had elevated expression of programmed death ligand 2 (PDL2) on skeletal myofibers, and this increase in expression extended to muscles where transgene was not overexpressed. These data demonstrate that prednisone can reduce the extent of intramuscular T-cell infiltrates in AAV-treated muscles, which may aid in achieving long-term transgene expression, as may the induction of PDL2 expression on skeletal myofibers to promote PD1-mediated programmed T-cell death.