Antibody neutralization poses a barrier to intravitreal adeno-associated viral vector gene delivery to non-human primates

Permanent transduction of retinal ganglion cells by rAAV2-retro

Adeno-associated virus (AAV) is widely used as a vector for delivery of gene therapy. Long term therapeutic benefit depends on perpetual expression of the wild-type gene after transduction of host cells by AAV. To address this issue in a mass population of identified single cells, 4 rats received an injection of a 1:1 mixture of rAAV2-retro-hSyn-EGFP and rAAV2-retro-hSyn-mCherry into each superior colliculus. After the virus was transported retrogradely to both retinas, serial fundus imaging was performed at days 14, 45, 211, and 375 to visualize individual fluorescent ganglion cells. The location of each cell was plotted to compare labeling at each time point. In 12/16 comparisons, 97% or more of the cells identified in the initial baseline fundus image were still labeled at day 375. In 4 cases the percentage was lower, but in these cases the apparent reduction in the number of labeled cells at day 375 was attributable to the lower quality of follow-up fundus images, rather than true loss of transgene expression. These data indicate that retinal ganglion cells transduced by rAAV2-retro are transduced permanently.

The dose-response relationship of subretinal gene therapy with rAAV2tYF-CB-hRS1 in a mouse model of X-linked retinoschisis

Purpose

X-linked retinoschisis (XLRS), due to loss-of-function mutations in the retinoschisin (RS1) gene, is characterized by a modest to severe decrease in visual acuity. Clinical trials for XLRS utilizing intravitreal (IVT) gene therapy showed ocular inflammation. We conducted a subretinal dose-response preclinical study using rAAV2tYF-CB-hRS1 utilizing the Rs1 knockout (Rs1-KO) mouse to investigate short- and long-term retinal rescue after subretinal gene delivery.

Methods

Rs1-KO mice were subretinally injected with 2 μL of rAAV2tYF-CB-hRS1 vector with 8E9 viral genomes (vg)/eye, 8E8 vg/eye, 8E7 vg/eye, or sham injection, and compared to untreated eyes. Reconstitution of human RS1 protein was detected using western blotting. Analysis of retinal function by electroretinography (ERG) and structural analysis by optical coherence tomography (OCT) were performed at 1, 2, 3, 5, 7, and 12 months post injection (MPI). Immunohistochemistry (IHC) was performed to evaluate cone rescue on the cellular level. Functional vision was evaluated using a visually guided swim assay (VGSA).

Results

Western blotting analysis showed human RS1 protein expression in a dose-dependent manner. Quantification of western blotting showed that the RS1 protein expression in mice treated with the 8E8 vg dose was near the wild-type (WT) expression levels. ERG demonstrated dose-dependent effects: At 1 MPI the 8E8 vg dose treated eyes had higher light-adapted (LA) ERG amplitudes in 3.0 flash and 5 Hz flicker compared to untreated (p < 0.0001) and sham-treated eyes (p < 0.0001) which persisted until the 12 MPI endpoint, consistent with improved cone function. ERG b-wave amplitudes were higher in response to dark-adapted (DA) 0.01 dim flash and 3.0 standard combined response (SCR) compared to sham-treated (p < 0.01) and untreated eyes (p < 0.001) which persisted until 3 MPI, suggesting short-term improvement of the rod photoreceptors. All injections, including sham-treated, resulted in a cyst severity score of 1 (no cavities), with significant reductions compared to untreated eyes up to 3 MPI (p < 0.05). The high and low dose groups showed inconsistent ERG improvements, despite reduced cyst severity, emphasizing the dose-dependent nature of gene augmentation's efficacy and the tenuous connection between cyst reduction and ERG improvement. IHC data showed a significant cone rescue in eyes treated with the 8E8 vg dose compared to sham-treated and untreated eyes. VGSA showed better functional vision in 8E8 vg dose treated mice. Eyes treated with the highest dose showed occasional localized degeneration in the outer nuclear layer.

Conclusion

Our data suggest that a dose of 8E8 vg/eye subretinally improves retinal function and structure in the Rs1-KO mouse. It improves cone function, rod function, and reduces cyst severity. Sham treatment resolves schisis cysts, but 8E8 vg/eye is needed for optimal retinal electrical function rescue. These findings offer a promising path for clinical translation to human trials.

A novel capsid-XL32-derived adeno-associated virus serotype prompts retinal tropism and ameliorates choroidal neovascularization

Gene therapy has been adapted, from the laboratory to the clinic, to treat retinopathies. In contrast to subretinal route, intravitreal delivery of AAV vectors displays the advantage of bypassing surgical injuries, but the viral particles are more prone to be nullified by the host neutralizing factors. To minimize such suppression of therapeutic effect, especially in terms of AAV2 and its derivatives, we introduced three serine-to-glycine mutations, based on the phosphorylation sites identified by mass spectrum analysis, to the XL32 capsid to generate a novel serotype named AAVYC5. Via intravitreal administration, AAVYC5 was transduced more effectively into multiple retinal layers compared with AAV2 and XL32. AAVYC5 also enabled successful delivery of anti-angiogenic molecules to rescue laser-induced choroidal neovascularization and astrogliosis in mice and non-human primates. Furthermore, we detected fewer neutralizing antibodies and binding IgG in human sera against AAVYC5 than those specific for AAV2 and XL32. Our results thus implicate this capsid-optimized AAVYC5 as a promising vector suitable for a wide population, particularly those with undesirable AAV2 seroreactivity.

AAV2-antiVEGFscFv gene therapy for retinal neovascularization

Retinal neovascularization (NV) may lead to irreversible vision impairment, the main treatment for which is the inhibition of vascular endothelial growth factor (VEGF). Existing drugs show limited clinical benefits because of their high prices and short half-lives, which increase the financial burden and medical risks to patients. Gene therapy on the basis of adeno-associated viruses is a promising approach to overcome these limitations because of the nonintegrative nature, low immunogenicity, and potential long-term gene expression of adeno-associated viruses. In this study, we constructed a novel recombinant adeno-associated virus with the single-chain fragment variable (scFv) fragment of the anti-VEGF antibody, AAV2-antiVEGFscFv, consisting of the VH and VL structural domains of IgG. AAV2-antiVEGFscFv effectively inhibited NV, retinal leakage, and retinal detachment in oxygen-induced retinopathy (OIR) mice, Tet/opsin/VEGF double-transgenic mice, and VEGF-induced rabbit NV models. AAV2-antiVEGFscFv also significantly suppressed VEGF-induced inflammation. Furthermore, we showed that AAV2-antiVEGFscFv could be sustainably expressed for a prolonged period and exhibited low immunotoxicity in vivo. This study indicates that AAV2-antiVEGFscFv could be a potential approach for NV treatment and provides strong support for preclinical research.

PRPF31-retinitis pigmentosa: Challenges and opportunities for clinical translation

Mutations in pre-mRNA processing factor 31 cause autosomal dominant retinitis pigmentosa (PRPF31-RP), for which there is currently no efficient treatment, making this disease a prime target for the development of novel therapeutic strategies. PRPF31-RP exhibits incomplete penetrance due to haploinsufficiency, in which reduced levels of gene expression from the mutated allele result in disease. A variety of model systems have been used in the investigation of disease etiology and therapy development. In this review, we discuss recent advances in both in vivo and in vitro model systems, evaluating their advantages and limitations in the context of therapy development for PRPF31-RP. Additionally, we describe the latest approaches for treatment, including AAV-mediated gene augmentation, genome editing, and late-stage therapies such as optogenetics, cell transplantation, and retinal prostheses.

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.

Innate and adaptive AAV-mediated immune responses in a mouse model of Duchenne muscular dystrophy

High systemic doses of adeno-associated viruses (AAVs) have been associated with immune-related serious adverse events (SAEs). Although AAV was well tolerated in preclinical models, SAEs were observed in clinical trials, indicating the need for improved preclinical models to understand AAV-induced immune responses. Here, we show that mice dual-dosed with AAV9 at 4-week intervals better recapitulate aspects of human immunity to AAV. In the model, anti-AAV9 immunoglobulin G (IgGs) increased in a linear fashion between the first and second AAV administrations. Complement activation was only observed in the presence of high levels of both AAV and anti-AAV IgG. Myeloid-derived pro-inflammatory cytokines were significantly induced in the same pattern as complement activation, suggesting that myeloid cell activation to AAV may rely on the presence of both AAV and anti-AAV IgG complexes. Single-cell RNA sequencing of peripheral blood mononuclear cells confirmed that activated monocytes were a primary source of pro-inflammatory cytokines and chemokines, which were significantly increased after a second AAV9 exposure. The same activated monocyte clusters expressed both Fcγ and complement receptors, suggesting that anti-AAV-mediated activation of myeloid cells through Fcγ receptors and/or complement receptors is one mechanism by which anti-AAV antigen complexes may prime antigen-presenting cells and amplify downstream immunity.

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.

Lentiviral delivered aflibercept OXB-203 for treatment of neovascular AMD

Neovascular age-related macular degeneration (nAMD) is a leading cause of blindness in the aging population, with vascular endothelial growth factor (VEGF) playing a key role. Treatment with recombinant anti-VEGFs is the current standard of care; however, it is only effective for 1-2 months at a time and requires re-administration. Gene therapy could pave the way for stable, long-term expression of therapeutic anti-VEGF with a single dose, reducing the frequency of treatment and potentially improving clinical outcomes. As such, we have developed OXB-203, a lentiviral-based gene therapy encoding the anti-VEGF protein aflibercept. Aflibercept derived from OXB-203 exhibited comparable in vitro binding characteristics to VEGF as recombinant aflibercept. Furthermore, its biological potency was demonstrated by the equivalent inhibition of VEGF-induced human umbilical vein endothelial cell (HUVEC) proliferation and tubule formation as recombinant aflibercept. In a rat choroidal neovascularization (CNV) model of nAMD, a single subretinal administration of OXB-203 reduced laser-induced CNV lesion areas analogous to an intravitreal bolus of recombinant aflibercept. Finally, in a head-to-head comparative study, aflibercept derived from OXB-203 was shown to be expressed at significantly higher levels in ocular tissues than from an AAV8-aflibercept vector following a single subretinal delivery to rats. These findings support the therapeutic potential of OXB-203 for the management of nAMD.

X-Linked Retinoschisis

X-linked retinoschisis (XLRS) is an inherited vitreoretinal dystrophy causing visual impairment in males starting at a young age with an estimated prevalence of 1:5000 to 1:25,000. The condition was first observed in two affected brothers by Josef Haas in 1898 and is clinically diagnosed by characteristic intraretinal cysts arranged in a petaloid "spoke-wheel" pattern centered in the macula. When clinical electroretinogram (ERG) testing began in the 1960s, XLRS was noted to have a characteristic reduction of the dark-adapted b-wave amplitude despite normal or usually nearly normal a-wave amplitudes, which became known as the "electronegative ERG response" of XLRS disease. The causative gene, RS1, was identified on the X-chromosome in 1997 and led to understanding the molecular and cellular basis of the condition, discerning the structure and function of the retinoschisin protein, and generating XLRS murine models. Along with parallel development of gene delivery vectors suitable for targeting retinal diseases, successful gene augmentation therapy was demonstrated by rescuing the XLRS phenotype in mouse. Two human phase I/II therapeutic XLRS gene augmentation studies were initiated; and although these did not yield definitive improvement in visual function, they gave significant new knowledge and experience, which positions the field for further near-term clinical testing with enhanced, next-generation gene therapy for XLRS patients.

Goats with low levels of AAV antibody may serve as candidates for large animal gene therapy

AAV vector-mediated gene therapy has been proposed as a feasible strategy for several eye diseases. However, AAV antibodies in the serum prior to treatment hinder the transduction efficiency and thus the therapeutic effect. Therefore, it is necessary to evaluate AAV antibodies in the serum before gene therapy. As large animals, goats are more closely related to humans than rodents and more economically available than nonhuman primates. Here, we first evaluated the AAV2 antibody serum level in rhesus monkeys before AAV injection. Then, we optimized a cell-based neutralizing antibody assay for detecting AAV antibodies in the serum of Saanen goats and evaluated the consistency of the cell-based neutralizing antibody assay and ELISA for goat serum antibody evaluation. The cell-based neutralizing antibody assay showed that the percentage of macaques with low antibody levels was 42.86%; however, there were no macaques with low antibody levels when the serum was evaluated by ELISA. The proportion of goats with low antibody levels was 56.67% according to the neutralizing antibody assay and 33. 33% according to the ELISA, and McNemar's test showed that the results of the two assays were not significantly different (P = 0.754), but that their consistency is poor (Kappa = 0.286, P = 0.114). Moreover, longitudinal evaluation of serum antibodies before and after intravitreal injection of AAV2 in goats revealed that the level of AAV antibodies increased and transduction inhibition subsequently increased, as reported in humans, indicating that transduction inhibition should be taken into account at different stages of gene therapy. In summary, starting with an evaluation of monkey serum antibodies, we optimized a detection method of goat serum antibodies, providing an alternative large animal model for gene therapy, and our serum antibody measurement method may be applied to other large animals.

Bruch's Membrane: A Key Consideration with Complement-Based Therapies for Age-Related Macular Degeneration

The complement system is crucial for immune surveillance, providing the body's first line of defence against pathogens. However, an imbalance in its regulators can lead to inappropriate overactivation, resulting in diseases such as age-related macular degeneration (AMD), a leading cause of irreversible blindness globally affecting around 200 million people. Complement activation in AMD is believed to begin in the choriocapillaris, but it also plays a critical role in the subretinal and retinal pigment epithelium (RPE) spaces. Bruch's membrane (BrM) acts as a barrier between the retina/RPE and choroid, hindering complement protein diffusion. This impediment increases with age and AMD, leading to compartmentalisation of complement activation. In this review, we comprehensively examine the structure and function of BrM, including its age-related changes visible through in vivo imaging, and the consequences of complement dysfunction on AMD pathogenesis. We also explore the potential and limitations of various delivery routes (systemic, intravitreal, subretinal, and suprachoroidal) for safe and effective delivery of conventional and gene therapy-based complement inhibitors to treat AMD. Further research is needed to understand the diffusion of complement proteins across BrM and optimise therapeutic delivery to the retina.

Bioengineering strategies for restoring vision

Late-stage retinal degenerative disease involving photoreceptor loss can be treated by optogenetic therapy, cell transplantation and retinal prostheses. These approaches aim to restore light sensitivity to the retina as well as visual perception by integrating neuronal responses for transmission to the cortex. In age-related macular degeneration, some cell-based therapies also aim to restore photoreceptor-supporting tissue to prevent complete photoreceptor loss. In the earlier stages of degeneration, gene-replacement therapy could attenuate retinal-disease progression and reverse loss of function. And gene-editing strategies aim to correct the underlying genetic defects. In this Review, we highlight the most promising gene therapies, cell therapies and retinal prostheses for the treatment of retinal disease, discuss the benefits and drawbacks of each treatment strategy and the factors influencing whether functional tissue is reconstructed and repaired or replaced with an electronic device, and summarize upcoming technologies for enhancing the restoration of vision.

Developing New Vectors for Retinal Gene Therapy

Since their discovery over 55 years ago, adeno-associated virus (AAV) vectors have become powerful tools for experimental and therapeutic in vivo gene delivery, particularly in the retina. Increasing knowledge of AAV structure and biology has propelled forward the development of engineered AAV vectors with improved abilities for gene delivery. However, major obstacles to safe and efficient therapeutic gene delivery remain, including tropism, inefficient and untargeted gene delivery, and limited carrying capacity. Additional improvements to AAV vectors will be required to achieve therapeutic benefit while avoiding safety issues. In this review, we provide an overview of recent methods for engineering-enhanced AAV capsids, as well as remaining challenges that must be overcome to achieve optimized therapeutic gene delivery in the eye.

Every nano-step counts: a critical reflection on do's and don'ts in researching nanomedicines for retinal gene therapy

INTRODUCTION

Retinal disease affects millions of people worldwide, generating a massive social and economic burden. Current clinical trials for retinal diseases are dominated by gene augmentation therapies delivered with recombinant viruses as key players. As an alternative, nanoparticles hold great promise for the delivery of nucleic acid therapeutics as well. Nevertheless, despite numerous attempts, 'nano' is in practice not as successful as aspired and major breakthroughs in retinal gene therapy applying nanomaterials are yet to be seen.

AREAS COVERED

In this review, we summarize the advantages of nanomaterials and give an overview of nanoparticles designed for retinal nucleic acid delivery up to now. We furthermore critically reflect on the predominant issues that currently limit nano to progress to the clinic, where faulty study design and the absence of representative models play key roles.

EXPERT OPINION

Since the current approach of in vitro - in vivo experimentation is highly inefficient and creates misinformation, we advocate for a more prominent role for ex vivo testing early on in nanoparticle research. In addition, we elaborate on several concepts, including systematic studies and open science, which could aid in pushing the field of nanomedicine beyond the preclinical stage.

Intraocular neutralizing antibodies against aflibercept in patients with age related macular degeneration

PURPOSE

To detect immunoglobulins in aqueous humour of AMD patients after repeated administration of intravitreal aflibercept.

PATIENTS AND METHODS

Twenty-one patients (age: 77.85 ± 9.21 years) previously treated with intravitreal aflibercept due to wet type age-related macular degeneration (AMD group) and 18 age-matched control subjects (age: 69.75 ± 12.67 years) were included in this study. Patients in the AMD group received a mean of 5 intravitreal injections (min: 1 max: 17) prior to the cataract surgery. Samples of aqueous humour (50 μl) were obtained by anterior chamber paracentesis as the first step of routine cataract surgery. The IgG content of the samples was analysed by an in-house developed ELISA system.

RESULTS

A significant increase in nonspecific IgG levels in the AMD group was detected compared to the control group (13.37 ± 6.65 vs. 9.44 ± 6.55 μg/ml; p = 0.03). In 11 patients, intraocular anti-aflibercept immunoglobulins could be detected (0.05 ± 0.01 μg/ml) which was significantly higher than the limit of detection for anti-aflibercept (0.04 μg/ml; p = 0.001). No correlation was found between the number of injections or the type of CNV and the aqueous level of anti-aflibercept (r = 0.02; p = 0.95).

CONCLUSION

According to our results, penetration of non-specific systemic antibodies through the impaired blood-retinal barrier is higher in patients with neovascular AMD than in subjects with an intact structural barrier. Evaluation of neutralizing antibodies to anti-VEGF agents in the aqueous humour can lead us to understanding tachyphylaxis and changes in intraocular immune mechanisms due to AMD.

Intravitreal Delivery of rAAV2tYF-CB-hRS1 Vector for Gene Augmentation Therapy in Patients with X-Linked Retinoschisis: 1-Year Clinical Results

PURPOSE

To evaluate the safety and efficacy of rAAV2tYF-CB-hRS1, a recombinant adeno-associated virus vector expressing retinoschisin (RS1), in individuals with retinal disease caused by mutations in the RS1 gene.

DESIGN

Open-label, phase I/II dose-escalation clinical trial.

SUBJECTS

Twenty-two adults and 5 children with X-linked retinoschisis (XLRS), aged 10 to 79 years, were enrolled.

METHODS

The participants received an intravitreal (IVT) injection of rAAV2tYF-CB-hRS1, at 1 of 3 dose levels, in the poorer-seeing eye and were followed up for a minimum of 1 year after treatment.

MAIN OUTCOME MEASURES

The primary safety measures were local (ocular) or systemic (nonocular) adverse events (AEs) during the 12-month period after study agent administration. Efficacy was assessed based on measures of best-corrected visual acuity (BCVA), schisis cavity volume, static perimetry visual field testing, and electroretinography (ERG).

RESULTS

The IVT administration of rAAV2tYF-CB-hRS1 was generally safe at each of the dose levels. There were no AEs resulting in early termination, and no dose-limiting toxicities were reported. The most common ocular AEs observed were related to ocular inflammation (blurred vision, visual impairment, and the presence of vitreous cells, keratic precipitates, vitreous floaters, anterior chamber cells, and vitreous haze). Ocular inflammation was generally either mild or moderate in severity and responsive to standard immunosuppressive therapy, except in 3 participants (all in the highest-dose group) who developed chronic uveitis, which required prolonged therapy. Two patients experienced retinal detachments. There was no overall improvement in BCVA, visual fields, or ERG in the study eye compared with that in the fellow eye for any dose group. Variable changes in the cystic cavity volume over time were similar in the study and fellow eyes.

CONCLUSIONS

Gene augmentation therapy with rAAV2tYF-CB-hRS1 for XLRS was generally safe and well tolerated but failed to demonstrate a measurable treatment effect. The clinical trial is ongoing through 5 years of follow-up to assess its long-term safety.

Outer retinal transduction by AAV2-7m8 following intravitreal injection in a sheep model of CNGA3 achromatopsia

Sheep carrying a mutated CNGA3 gene exhibit diminished cone function and provide a naturally occurring large animal model of achromatopsia. Subretinal injection of a vector carrying the CNGA3 transgene resulted in long-term recovery of cone function and photopic vision in these sheep. Research is underway to develop efficacious vectors that would enable safer transgene delivery, while avoiding potential drawbacks of subretinal injections. The current study evaluated two modified vectors, adeno-associated virus 2-7m8 (AAV2-7m8) and AAV9-7m8. Intravitreal injection of AAV2-7m8 carrying enhanced green fluorescent protein under a cone-specific promoter resulted in moderate photoreceptor transduction in wild-type sheep, whereas peripheral subretinal delivery of AAV9-7m8 resulted in the radial spread of the vector beyond the point of deposition. Intravitreal injection of AAV2-7m8 carrying human CNGA3 in mutant sheep resulted in mild photoreceptor transduction, but did not lead to the clinical rescue of photopic vision, while day-blind sheep treated with a subretinal injection exhibited functional recovery of photopic vision. Transgene messenger RNA levels in retinas of intravitreally treated eyes amounted to 4-23% of the endogenous CNGA3 levels, indicating that expression levels >23% are needed to achieve clinical rescue. Overall, our results indicate intravitreal injections of AAV2.7m8 transduce ovine photoreceptors, but not with sufficient efficacy to achieve clinical rescue in CNGA3 mutant sheep.

Advancements in ocular gene therapy delivery: vectors and subretinal, intravitreal, and suprachoroidal techniques

INTRODUCTION

: Ocular gene therapy represents fertile ground for rapid innovation, with ever-expanding therapeutic strategies, molecular targets, and indications.

AREAS COVERED

: Potential indications for ocular gene therapy have classically focused on inherited retinal disease (IRD), but more recently include acquired retinal diseases, such as neovascular age-related macular degeneration, geographic atrophy and diabetic retinopathy. Ocular gene therapy strategies have proliferated recently, and include gene augmentation, gene inactivation, gene editing, RNA modulation, and gene-independent gene augmentation. Viral vector therapeutic constructs include adeno-associated virus and lentivirus and continue to evolve through directed evolution and rationale design. Ocular gene therapy administration techniques have expanded beyond pars plana vitrectomy with subretinal injection to intravitreal injection and suprachoroidal injection.

EXPERT OPINION

: The success of treatment for IRD, paired with the promise of clinical research in acquired retinal diseases and in administration techniques, has raised the possibility of in-office gene therapy for common retinal disorders within the next five to ten years.

Gene Therapy for Rhodopsin Mutations

Mutations in RHO, the gene for rhodopsin, account for a large fraction of autosomal-dominant retinitis pigmentosa (adRP). Patients fall into two clinical classes, those with early onset, pan retinal photoreceptor degeneration, and those who experience slowly progressive disease. The latter class of patients are candidates for photoreceptor-directed gene therapy, while former may be candidates for delivery of light-responsive proteins to interneurons or retinal ganglion cells. Gene therapy for RHO adRP may be targeted to the mutant gene at the DNA or RNA level, while other therapies preserve the viability of photoreceptors without addressing the underlying mutation. Correcting the RHO gene and replacing the mutant RNA show promise in animal models, while sustaining viable photoreceptors has the potential to delay the loss of central vision and may preserve photoreceptors for gene-directed treatments.

Tropism of the Novel AAVBR1 Capsid Following Subretinal Delivery

A serious limitation of current adeno-associated viral (AAV) capsids employed for subretinal delivery is achieving adequate lateral spread beyond the injection site, required for the efficient delivery of gene therapy to the outer retina and/or RPE. AAVBR1 is a unique AAV with exceptional tropism for CNS microvasculature following systemic delivery. Here, we used in vivo and ex vivo analysis to show that subretinal delivery of AAVBR1.GFP in mice achieves superior tropism to RPE and outer retina than either AAV2.GFP or AAV8.GFP, two of the most common capsids used for subretinal delivery. At a low (5 × 108 vg) subretinal dose, the AAVBR1.GFP signal was visible by 48 h and significantly surpassed peak fluorescence of other AAVs in retina and RPE. The co-injection of AAVBR1.GFP with the AAVBR1-specific heptapeptide, NRGTEWD, significantly blocked the AAVBR1.GFP signal, but had no effect on AAV2.GFP fluorescence, confirming that AAVBR1’s enhanced tropism for RPE and outer retina derives from this 7AA modification within the capsid-binding motif. Enhanced dispersal and consequent transduction suggest that AAVBR1 can be employed at a lower dosage than the standard AAV2 capsid to achieve equivalent expression for gene therapy, warranting further evaluation of its utility as a therapeutic vehicle for subretinal delivery.

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.

Fundus imaging of retinal ganglion cells transduced by retrograde transport of rAAV2-retro

Access of adeno-associated virus (AAV) to ganglion cells following intravitreal injection for gene therapy is impeded by the internal limiting membrane of the retina. As an alternative, one could transduce ganglion cells via retrograde transport after virus injection into a retinal target nucleus. It is unknown if recombinant AAV2-retro (rAAV2-retro), a variant of AAV2 developed specifically for retrograde transport, is capable of transducing retinal ganglion cells. To address this issue, equal volumes of rAAV2-retro-hSyn-EGFP and rAAV2-retro-hSyn-mCherry were mixed in a micropipette and injected into the rat superior colliculus. The time-course of viral transduction was tracked by performing serial in vivo fundus imaging. Cells that were labeled by the fluorophores within the first week remained consistent in distribution and relative signal strength on follow-up imaging. Most transduced cells were double-labeled, but some were labeled by only EGFP or mCherry. Fundus images were later aligned with retinal wholemounts. Ganglion cells in the wholemounts matched precisely the cells imaged by fundus photography. As seen in the fundus images, ganglion cells in wholemounts were sometimes labeled by only EGFP or mCherry. Overall, there was detectable label in 32-41% of ganglion cells. Analysis of the number of cells labeled by 0, 1, or 2 fluorophores, based on Poisson statistics, yielded an average of 0.66 virions transducing each ganglion cell. Although this represents a low number relative to the quantity of virus injected into the superior colliculus, the ganglion cells showed sustained and robust fluorescent labeling. In the primate, injection of rAAV2-retro into the lateral geniculate nucleus might provide a viable approach for the transduction of ganglion cells, bypassing the obstacles that have prevented effective gene delivery via intravitreal injection.

Challenges to Gene Editing Approaches in the Retina

Retinal gene therapy has recently been at the cutting edge of clinical development in the diverse field of genetic therapies. The retina is an attractive target for genetic therapies such as gene editing due to the distinctive anatomical and immunological features of the eye, known as immune privilege, so that inherited retinal diseases (IRDs) have been studied in several clinical studies. Thus, rapid strides are being made toward developing targeted treatments for IRDs. Gene editing in the retina faces a group of heterogenous challenges, including editing efficiencies, off-target effects, the anatomy of the target organ, immune responses, inactivation, and identifying optimal application methods. As clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated nuclease (Cas) based technologies are at the forefront of current gene editing advances, their specific editing efficiency challenges and potential off-target effects were assessed. The immune privilege of the eye reduces the likelihood of systemic immune responses following retinal gene therapy, but possible immune responses must not be discounted. Immune responses to gene editing in the retina may be humoral or cell mediated, with immunologically active cells, including microglia, implicated in facilitating possible immune responses to gene editing. Immunogenicity of gene therapeutics may also lead to the inactivation of edited cells, reducing potential therapeutic benefits. This review outlines the broad spectrum of potential challenges currently facing retinal gene editing, with the goal of facilitating further advances in the safety and efficacy of gene editing therapies.

Subretinal Therapy: Technological Solutions to Surgical and Immunological Challenges

Recent advances in ocular gene and cellular therapy rely on precisely controlled subretinal delivery. Due to its inherent limitations, manual delivery can lead to iatrogenic damage to the retina, the retinal pigment epithelium, favor reflux into the vitreous cavity. In addition, it suffers from lack of standardization, variability in delivery and the need to maintain proficiency. With or without surgical damage, an eye challenged with an exogenous viral vector or transplanted cells will illicit an immune response. Understanding how such a response manifests itself and to what extent immune privilege protects the eye from a reaction can help in anticipating short- and long-term consequences. Avoidance of spillover from areas of immune privilege to areas which either lack or have less protection should be part of any mitigation strategy. In that regard, robotic technology can provide reproducible, standardized delivery which is not dependent on speed of injection. The advantages of microprecision medical robotic technology for precise targeted deliveries are discussed.

Delivery of nVEGFi using AAV8 for the treatment of neovascular age-related macular degeneration

Inhibition of vascular endothelial growth factor (VEGF) is the standard therapy for neovascular age-related macular degeneration (nAMD). However, anti-VEGF agents used in the clinic require repeated injections, causing adverse effects. Gene therapy could provide sustained anti-VEGF levels after a single injection, thereby drastically decreasing the treatment burden and improving visual outcomes. In this study, we developed a novel VEGF Trap, nVEGFi, containing domains 1 and 2 of VEGFR1 and domain 3 of VEGFR2 fused to the Fc portion of human IgG. The nVEGFi had a higher expression level than aflibercept under the same expression cassettes of adeno-associated virus (AAV)8 in vitro and in vivo. nVEGFi was found to be noninferior to aflibercept in binding and blocking VEGF in vitro. AAV8-mediated expression of nVEGFi was maintained for at least 12 weeks by subretinal delivery in C57BL/6J mice. In a mouse laser-induced choroidal neovascularization (CNV) model, 4 × 10⁸ genome copies of AAV8-nVEGFi exhibited a significantly increased reduction in the CNV area compared with AAV8-aflibercept (78.1% vs. 63.9%, p < 0.05), while causing no structural or functional changes to the retina. In conclusion, this preclinical study showed that subretinal injection of AAV8-nVEGFi was long lasting, well tolerated, and effective for nAMD treatment, supporting future translation to the clinic.

Systemic and local immune responses to intraocular AAV vector administration in non-human primates

Positive clinical outcomes in adeno-associated virus (AAV)-mediated retinal gene therapy have often been attributed to the low immunogenicity of AAVs and immune privilege of the eye. However, several recent studies have shown potential for inflammatory responses. The current understanding of the factors contributing to inflammation, such as the pre-existence of serum antibodies against AAVs and their contribution to increases in antibody levels post-injection, is incomplete. The parameters that regulate the generation of new antibodies in response to the AAV capsid or transgene after intraocular injections are also insufficiently described. This study is a retrospective analysis of the pre-existing serum antibodies in correlation with changes in antibody levels after intraocular injections of AAV in non-human primates (NHPs) of the species Macaca fascicularis. In NHP serums, we analyzed the binding antibody (BAB) levels and a subset of these called neutralizing antibodies (NABs) that impede AAV transduction. We observed significantly higher pre-existing serum BABs against AAV8 compared with other serotypes and a dose-dependent increase in BABs and NABs in the serums collected post-injection, irrespective of the serotype or the mode of injection. Lastly, we were able to demonstrate a correlation between the serum BAB levels with clinical grading of inflammation and levels of transgene expression.

High concordance of ELISA and neutralization assays allows for the detection of antibodies to individual AAV serotypes

Prescreening of participants in clinical trials that use adeno-associated virus (AAV) vectors is required to identify naive participants, as preexisting neutralizing antibodies can limit the efficacy of AAV gene therapies. The presence of antibodies to individual AAV serotypes is typically detected by neutralization assay. To streamline the screening process, we compared an ELISA-based screening method with a neutralization assay for the detection of antibodies against AAV1, AAV8, and AAV9 in a collection of 50 rhesus macaque sera and 20 human sera. We observed a high level of concordance between the two assays (Pearson r > 0.8) for all three serotypes in both sample sets. We thus investigated pre- vs post-vector inoculation sera samples from rhesus macaques that received AAV1 or AAV8 vector inoculations for cross-reactive anti-AAV antibodies. All 12 macaques seroconverted to the vector they received, but many also reacted to the other serotypes. Our results validate an easy-to-use ELISA for reliable detection of antibodies to individual serotypes of AAV. Our results also demonstrate that an antibody response post-AAV inoculation may partially cross-react with other AAV serotypes. Overall, these results suggest that either assay can be used by academic labs for prescreening samples for preexisting anti-AAV antibodies.

Advances in Ophthalmic Optogenetics: Approaches and Applications

Recent advances in optogenetics hold promise for vision restoration in degenerative eye diseases. Optogenetics refers to techniques that use light to control the cellular activity of targeted cells. Although optogenetics is a relatively new technology, multiple therapeutic options are already being explored in pre-clinical and phase I/II clinical trials with the aim of developing novel, safe, and effective treatments for major blinding eye diseases, such as glaucoma and retinitis pigmentosa. Optogenetic approaches to visual restoration are primarily aimed at replacing lost or dysfunctional photoreceptors by inserting light-sensitive proteins into downstream retinal neurons that have no intrinsic light sensitivity. Such approaches are attractive because they are agnostic to the genetic causes of retinal degeneration, which raises hopes that all forms of retinal dystrophic and degenerative diseases could become treatable. Optogenetic strategies can also have a far-reaching impact on translational research by serving as important tools to study the pathogenesis of retinal degeneration and to identify clinically relevant therapeutic targets. For example, the CRY-CIBN optogenetic system has been recently applied to animal models of glaucoma, suggesting a potential role of OCRL in the regulation of intraocular pressure in trabecular meshwork. As optogenetic strategies are being intensely investigated, it appears crucial to consider the opportunities and challenges such therapies may offer. Here, we review the more recent promising optogenetic molecules, vectors, and applications of optogenetics for the treatment of retinal degeneration and glaucoma. We also summarize the preliminary results of ongoing clinical trials for visual restoration.

Gene-Based Therapeutics for Acquired Retinal Disease: Opportunities and Progress

Acquired retinal diseases such as age-related macular degeneration and diabetic retinopathy rank among the leading causes of blindness and visual loss worldwide. Effective treatments for these conditions are available, but often have a high treatment burden, and poor compliance can lead to disappointing real-world outcomes. Development of new treatment strategies that provide more durable treatment effects could help to address some of these unmet needs. Gene-based therapeutics, pioneered for the treatment of monogenic inherited retinal disease, are being actively investigated as new treatments for acquired retinal disease. There are significant advantages to the application of gene-based therapeutics in acquired retinal disease, including the presence of established therapeutic targets and common pathophysiologic pathways between diseases, the lack of genotype-specificity required, and the larger potential treatment population per therapy. Different gene-based therapeutic strategies have been attempted, including gene augmentation therapy to induce in vivo expression of therapeutic molecules, and gene editing to knock down genes encoding specific mediators in disease pathways. We highlight the opportunities and unmet clinical needs in acquired retinal disease, review the progress made thus far with current therapeutic strategies and surgical delivery techniques, and discuss limitations and future directions in the field.

Prevalence of pre-existing neutralizing antibodies against AAV serotypes 1, 2, 5, 6, 8, and 9 in sera of different pig strains

Pre-existing neutralizing antibodies (NAb) to adeno-associated virus (AAV) may diminish the efficacy of AAV-based therapies depending on the titer. To support gene therapy studies in pigs, the seroprevalence of NAb to AAV 1, 2, 5, 6, 8, and 9 serotypes were assessed in the sera of 3 different strains of pigs consisting of 60 Norsvin Topigs-20 strain, 22 Gottingen minipigs, and 40 Yucatan minipigs. Cell-based NAb assays were developed for various AAV serotypes. The sera were tested for NAb in a Lec-2 cell line for AAV9 vector and in a COS-7 cell line for the other AAV serotypes. In the 60 Topigs-20 strain aged 2 to 4 years old, 100% were positive for AAV2 NAb, 45 % positive for AAV6 NAb, and ~20% positive for each of AAV1, 5, 8, and 9 NAb. These data showed that approximately 80% of Norsvin Topigs-20 pigs evaluated were seronegative for pre-existing NAb to the AAV1, 5, 8, and 9 serotypes, respectively. In 22 Gottingen minipigs at 5-6 months of age, serum AAV-serotype specific NAb co-existed with that of various other AAV serotypes at 32 to 46 % between two serotypes. These results suggested that coexisting NAb resulted either from multiple AAV serotype co-infection or from one (or more) serotypes that can cross-react with other AAV serotypes in some minipigs. Among the 40 Yucatan minipigs, 20 of the minipigs were less than 3 months old and were all negative for NAb against AAV5, 8 and 9, and only one of these 20 pigs was positive to AAV1 and 6. We further determined the titers in those positive pigs and found most Gottingen minipigs had low titer at 1:20, whereas some of Topigs-20 pigs had titers between 1:80 to 1: 320, and some of Yucatan pigs had titers between 1: 160 to 1: 640. These results suggested that the majority of the pigs in the three strains would be amenable to gene therapy study using AAV1, AAV5, AAV8, and AAV9 and that prescreening on circulating AAV antibodies could be helpful before inclusion of pigs into studies.

CRISPR/Cas System and Factors Affecting Its Precision and Efficiency

The diverse applications of genetically modified cells and organisms require more precise and efficient genome-editing tool such as clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas). The CRISPR/Cas system was originally discovered in bacteria as a part of adaptive-immune system with multiple types. Its engineered versions involve multiple host DNA-repair pathways in order to perform genome editing in host cells. However, it is still challenging to get maximum genome-editing efficiency with fewer or no off-targets. Here, we focused on factors affecting the genome-editing efficiency and precision of CRISPR/Cas system along with its defense-mechanism, orthologues, and applications.

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.

Pre-retinal delivery of recombinant adeno-associated virus vector significantly improves retinal transduction efficiency

Intravitreal injection is the most widely used injection technique for ocular gene delivery. However, vector diffusion is attenuated by physical barriers and neutralizing antibodies in the vitreous. The 13-lined ground squirrel (13-LGS), as in humans, has a larger relative vitreous body volume than the more common rodent models such as rats and mice, which would further reduce transduction efficiency with the intravitreal injection route. We report here a “pre-retinal” injection approach that leads to detachment of the posterior hyaloid membrane and delivers vector into the space between vitreous and inner retina. Vectors carrying a ubiquitously expressing mCherry reporter were injected into the deep vitreous or pre-retinal space in adult wild-type 13-LGSs. Then, adeno-associated virus (AAV)-mediated mCherry expression was evaluated with non-invasive imaging, immunofluorescence, and flow cytometry. Compared to deep vitreous delivery, pre-retinal administration achieved pan-retinal gene expression with a lower vector dose volume and significantly increased the number of transduced cone photoreceptors. These results suggest that pre-retinal injection is a promising tool in the development of gene therapy strategies in animal models and is a potential approach for use in human research, particularly in younger individuals with an intact posterior hyaloid membrane and stable vitreous.

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.

An Optimized Treatment Protocol for Subretinal Injections Limits Intravitreal Vector Distribution

Purpose

Subretinal injections (SRis) are commonly used in retinal gene therapy procedures to deliver adeno-associated virus (AAV) to photoreceptors and retinal pigment epithelial cells. We present an optimized surgical protocol to minimize off-target application of AAV in the vitreous, which in turn reduces the risk of extensive biodistribution and inflammation, ultimately leading to enhanced safety of the therapy.

Design

Experimental animal research study.

Participants

Eight cynomolgus monkeys (Macaca fascicularis).

Methods

Subretinal injections with an AAV2/8 vector were performed. The animals were allocated to 2 different vector dose groups (1×10^ˆ11 and 5×10^ˆ11 viral genomes [vg]). Samples of intravitreal fluid were taken at the end of the SRi procedure and again after a 3-minute lavage (wash-out) with balanced salt solution (BSS).

Main Outcome Measures

Intravitreal vector genome copies were analyzed with quantitative polymerase chain reaction and compared between groups.

Results

Even uneventful SRi leads to dissemination of millions of AAV particles (0.1–0.7% of viral vector loading dose) into the vitreous cavity. Three minutes of lavage led to a substantial decrease (on average 96%) of intravitreal vector load.

Conclusions

Multiple studies have shown that the intravitreal space is not as immune privileged as the subretinal space. Intravitreal AAV particles disseminate into the bloodstream, lead to increased biodistribution into lymphatic tissue, and help to stage an immune response with implications for both safety and efficacy. Therefore, minimizing off-target vector application after reflux of vector from the subretinal space is of significant interest. We show that a simple lavage of intravitreal fluid efficiently decreases the intravitreal vector load. Such a step should be considered when performing subretinal gene therapy.

The future of retinal gene therapy: evolving from subretinal to intravitreal vector delivery

Inherited retinal degenerations are a leading and untreatbale cause of blindness, and as such they are targets for gene therapy. Numerous gene therapy treatments have progressed from laboratory research to clinical trails, and a pioneering gene therapy received the first ever FDA approval for treating patients. However, currently retinal gene therapy mostly involves subretinal injection of the therapeutic agent, which treats a limited area, entails retinal detachment and other potential complications, and requires general anesthesia with consequent risks, costs and prolonged recovery. Therefore there is great impetus to develop safer, less invasive and cheapter methods of gene delivery. A promising method is intravitreal injection, that does not cause retinal detachment, can lead to pan-retinal transduction and can be performed under local anesthesia in out-patient clinics. Intravitreally-injected vectors face several obstacles. First, the vector is diluted by the vitreous and has to overcome a long diffusion distance to the target cells. Second, the vector is exposed to the host's immune response, risking neutralization by pre-existing antibodies and triggering a stronger immune response to the injection. Third, the vector has to cross the inner limiting membrane which is both a physical and a biological barrier as it contains binding sites that could cause the vector's sequestration. Finally, in the target cell the vector is prone to proteasome degradation before delivering the transgene to the nucleus. Strategies to overcome these obstacles include modifications of the viral capsid, through rational design or directed evolution, which allow resistance to the immune system, enhancement of penetration through the inner limiting membrane or reduced degradation by intracellular proteasomes. Furthermore, physical and chemical manipulations of the inner limiting membrane and vitreous aim to improve vector penetration. Finally, compact non-viral vectors that can overcome the immunological, physical and anatomical and barriers have been developed. This paper reviews ongoing efforts to develop novel, safe and efficacious methods for intravitreal delivery of therapeutic genes for inherited retinal degenerations. To date, the most promising results are achieved in rodents with robust, pan-retinal transduction following intravitreal delivery. Trials in larger animal models demonstrate transduction mostly of inner retinal layers. Despite ongoing efforts, currently no intravitreally-injected vector has demonstrated outer retinal transduction efficacy comparable to that of subretinal delivery. Further work is warranted to test promising new viral and non-viral vectors on large animal models of inherited retinal degenerations. Positive results will pave the way to development of the next generation of treatments for inherited retinal degeneration.

Dendritic calcium signals in rhesus macaque motor cortex drive an optical brain-computer interface

Calcium imaging is a powerful tool for recording from large populations of neurons in vivo. Imaging in rhesus macaque motor cortex can enable the discovery of fundamental principles of motor cortical function and can inform the design of next generation brain-computer interfaces (BCIs). Surface two-photon imaging, however, cannot presently access somatic calcium signals of neurons from all layers of macaque motor cortex due to photon scattering. Here, we demonstrate an implant and imaging system capable of chronic, motion-stabilized two-photon imaging of neuronal calcium signals from macaques engaged in a motor task. By imaging apical dendrites, we achieved optical access to large populations of deep and superficial cortical neurons across dorsal premotor (PMd) and gyral primary motor (M1) cortices. Dendritic signals from individual neurons displayed tuning for different directions of arm movement. Combining several technical advances, we developed an optical BCI (oBCI) driven by these dendritic signalswhich successfully decoded movement direction online. By fusing two-photon functional imaging with CLARITY volumetric imaging, we verified that many imaged dendrites which contributed to oBCI decoding originated from layer 5 output neurons, including a putative Betz cell. This approach establishes new opportunities for studying motor control and designing BCIs via two photon imaging.

Amount of Green Fluorescent Protein in the Anterior Chamber after Intravitreal Injection of Triple-Mutated Self-Complementary AAV2 Vectors is Not Affected by Previous Vitrectomy Surgery

BACKGROUND

The adeno-associated virus (AAV) vector is a promising vector for ocular gene therapy. Surgical internal limiting membrane peeling before AAV vector administration is useful for efficient retinal transduction. However, no report has investigated localization of AAV vectors after administration into a post-vitrectomy eye. This study investigated the effects of vitrectomy surgery on intravitreal-injected AAV vector-mediated gene expression in the anterior segment and examined the presence of neutralizing antibodies (NAbs) in serum before and after AAV vector administration.

METHODS

Of six eyes from three female cynomolgus monkeys, four were vitrectomized (Group VIT) and two were non-vitrectomized (Group IV). All eyes were injected with 50 μL of triple-mutated self-complementary AAV2 vector (1.9 × 10¹³ v.g./mL) encoding green fluorescent protein (GFP). NAbs in the serum were examined before administration and at 2 and 6 weeks after administration. GFP expression was analyzed at 19 weeks after administration.

RESULTS

Immunohistological analysis showed no GFP expression in the trabecular meshwork in any eye. The GFP genome copy in two slices of the anterior segment was 2.417 (vector genome copies/diploid genome) in Group VIT and 4.316 (vector genome copies/diploid genome) in group IV. The NAb titer was 1:15.9 (geometric mean) before administration, 1:310.7 at 2 weeks after administration, and 1:669.4 at 6 weeks after administration.

CONCLUSION

Previous vitrectomy surgery did not affect gene expression in the anterior segment after intravitreal injection of AAV vectors.

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.

Novel AAV capsids for intravitreal gene therapy of photoreceptor disorders

Gene therapy using recombinant adeno-associated virus (rAAV) vectors to treat blinding retinal dystrophies has become clinical reality. Therapeutically impactful targeting of photoreceptors still relies on subretinal vector delivery, which detaches the retina and harbours substantial risks of collateral damage, often without achieving widespread photoreceptor transduction. Herein, we report the development of novel engineered rAAV vectors that enable efficient targeting of photoreceptors via less invasive intravitreal administration. A unique in vivo selection procedure was performed, where an AAV2-based peptide-display library was intravenously administered in mice, followed by isolation of vector DNA from target cells after only 24 h. This stringent selection yielded novel vectors, termed AAV2.GL and AAV2.NN, which mediate widespread and high-level retinal transduction after intravitreal injection in mice, dogs and non-human primates. Importantly, both vectors efficiently transduce photoreceptors in human retinal explant cultures. As proof-of-concept, intravitreal Cnga3 delivery using AAV2.GL lead to cone-specific expression of Cnga3 protein and rescued photopic cone responses in the Cnga3-/- mouse model of achromatopsia. These novel rAAV vectors expand the clinical applicability of gene therapy for blinding human retinal dystrophies.

Host Immune Responses after Suprachoroidal Delivery of AAV8 in Nonhuman Primate Eyes

The suprachoroid is a potential space located between the sclera and choroid of the eye, which provides a novel route for ocular drug or viral vector delivery. Suprachoroidal injection of adeno-associated virus (AAV)8 using transscleral microneedles enables widespread transgene expression in eyes of nonhuman primates, but may cause intraocular inflammation. We characterized the host humoral and cellular immune responses after suprachoroidal delivery of AAV8 expressing green fluorescent protein (GFP) in rhesus macaques, and found that it can induce mild chorioretinitis that resolves after systemic corticosteroid administration, with recovery of photoreceptor morphology, but persistent immune cell infiltration after 3 months, corresponding to a loss of GFP expression from retinal pigment epithelial cells, but persistent expression in scleral fibroblasts. Suprachoroidal AAV8 triggered B cell and T cell responses against GFP, but only mild antibody responses to the viral capsid compared to intravitreal injections of the same vector and dose. Systemic biodistribution studies showed lower AAV8 levels in liver and spleen after suprachoroidal injection compared with intravitreal delivery. Our findings suggest that suprachoroidal AAV8 primarily triggers host immune responses to GFP, likely due to sustained transgene expression in scleral fibroblasts outside the blood-retinal barrier, but elicits less humoral immune reactivity to the viral capsid than intravitreal delivery due to lower egress into systemic circulation. As GFP is not native to primates and not a clinically relevant transgene, suprachoroidal AAV delivery of human transgenes may have significant translational potential for retinal gene therapy.