Liver-Directed but Not Muscle-Directed AAV-Antibody Gene Transfer Limits Humoral Immune Responses in Rhesus Monkeys

Reprogramming human B cells with custom heavy-chain antibodies

The immunoglobulin locus of B cells can be reprogrammed by genome editing to produce custom or non-natural antibodies that are not induced by immunization. However, current strategies for antibody reprogramming require complex expression cassettes and do not allow for customization of the constant region of the antibody. Here we show that human B cells can be edited at the immunoglobulin heavy-chain locus to express heavy-chain-only antibodies that support alterations to both the fragment crystallizable domain and the antigen-binding domain, which can be based on both antibody and non-antibody components. Using the envelope protein (Env) from the human immunodeficiency virus as a model antigen, we show that B cells edited to express heavy-chain antibodies to Env support the regulated expression of B cell receptors and antibodies through alternative splicing and that the cells respond to the Env antigen in a tonsil organoid model of immunization. This strategy allows for the reprogramming of human B cells to retain the potential for in vivo amplification while producing molecules with flexibility of composition beyond that of standard antibodies.

In vivo mRNA expression of a multi-mechanistic mAb combination protects against Staphylococcus aureus infection

Single monoclonal antibodies (mAbs) can be expressed in vivo through gene delivery of their mRNA formulated with lipid nanoparticles (LNPs). However, delivery of a mAb combination could be challenging due to the risk of heavy and light variable chain mispairing. We evaluated the pharmacokinetics of a three mAb combination against Staphylococcus aureus first in single chain variable fragment scFv-Fc and then in immunoglobulin G 1 (IgG1) format in mice. Intravenous delivery of each mRNA/LNP or the trio (1 mg/kg each) induced functional antibody expression after 24 h (10-100 μg/mL) with 64%-78% cognate-chain paired IgG expression after 3 days, and an absence of non-cognate chain pairing for scFv-Fc. We did not observe reduced neutralizing activity for each mAb compared with the level of expression of chain-paired mAbs. Delivery of the trio mRNA protected mice in an S. aureus-induced dermonecrosis model. Intravenous administration of the three mRNA in non-human primates achieved peak serum IgG levels ranging between 2.9 and 13.7 μg/mL with a half-life of 11.8-15.4 days. These results suggest nucleic acid delivery of mAb combinations holds promise and may be a viable option to streamline the development of therapeutic antibodies.

Influence of AAV vector tropism on long-term expression and Fc-γ receptor binding of an antibody targeting SARS-CoV-2

Long-acting passive immunization strategies are needed to protect immunosuppressed vulnerable groups from infectious diseases. To further explore this concept for COVID-19, we constructed Adeno-associated viral (AAV) vectors encoding the human variable regions of the SARS-CoV-2 neutralizing antibody, TRES6, fused to murine constant regions. An optimized vector construct was packaged in hepatotropic (AAV8) or myotropic (AAVMYO) AAV capsids and injected intravenously into syngeneic TRIANNI-mice. The highest TRES6 serum concentrations (511 µg/ml) were detected 24 weeks after injection of the myotropic vector particles and mean TRES6 serum concentrations remained above 100 µg/ml for at least one year. Anti-drug antibodies or TRES6-specific T cells were not detectable. After injection of the AAV8 particles, vector mRNA was detected in the liver, while the AAVMYO particles led to high vector mRNA levels in the heart and skeletal muscle. The analysis of the Fc-glycosylation pattern of the TRES6 serum antibodies revealed critical differences between the capsids that coincided with different binding activities to murine Fc-γ-receptors. Concomitantly, the vector-based immune prophylaxis led to protection against SARS-CoV-2 infection in K18-hACE2 mice. High and long-lasting expression levels, absence of anti-drug antibodies and favourable Fc-γ-receptor binding activities warrant further exploration of myotropic AAV vector-based delivery of antibodies and other biologicals.

Recent Advances Using Genetic Therapies Against Infectious Diseases and for Vaccination

The development of prophylatic or therapeutic medicines for infectious diseases is one of the priorities for health organizations worldwide. Innovative solutions are required to achieve effective, safe, and accessible treatments for most if not all infectious diseases, particularly those that are chronic in nature or that emerge unexpectedly over time. Genetic technologies offer versatile possibilities to design therapies against pathogens. Recent developments such as mRNA vaccines, CRISPR gene editing, and immunotherapies provide unprecedented hope to achieve significant results in the field of infectious diseases. This review will focus on advances in this domain, showcasing the cross-fertilization with other fields (e.g., oncology), and addressing some of the logistical and economic concerns important to consider when making these advances accessible to diverse populations around the world.

Expanding the Reach of Monoclonal Antibodies: A Review of Synthetic Nucleic Acid Delivery in Immunotherapy

Harnessing the immune system to combat disease has revolutionized medical treatment. Monoclonal antibodies (mAbs), in particular, have emerged as important immunotherapeutic agents with clinical relevance in treating a wide range of diseases, including allergies, autoimmune diseases, neurodegenerative disorders, cancer, and infectious diseases. These mAbs are developed from naturally occurring antibodies and target specific epitopes of single molecules, minimizing off-target effects. Antibodies can also be designed to target particular pathogens or modulate immune function by activating or suppressing certain pathways. Despite their benefit for patients, the production and administration of monoclonal antibody therapeutics are laborious, costly, and time-consuming. Administration often requires inpatient stays and repeated dosing to maintain therapeutic levels, limiting their use in underserved populations and developing countries. Researchers are developing alternate methods to deliver monoclonal antibodies, including synthetic nucleic acid-based delivery, to overcome these limitations. These methods allow for in vivo production of monoclonal antibodies, which would significantly reduce costs and simplify administration logistics. This review explores new methods for monoclonal antibody delivery, including synthetic nucleic acids, and their potential to increase the accessibility and utility of life-saving treatments for several diseases.

Reprogramming human B cells with custom heavy chain antibodies

We describe a genome editing strategy to reprogram the immunoglobulin heavy chain (IgH) locus of human B cells to express custom molecules that respond to immunization. These heavy chain antibodies (HCAbs) comprise a custom antigen-recognition domain linked to an Fc domain derived from the IgH locus and can be differentially spliced to express either B cell receptor (BCR) or secreted antibody isoforms. The HCAb editing platform is highly flexible, supporting antigen-binding domains based on both antibody and non-antibody components, and also allowing alterations in the Fc domain. Using HIV Env protein as a model antigen, we show that B cells edited to express anti-Env HCAbs support the regulated expression of both BCRs and antibodies, and respond to Env antigen in a tonsil organoid model of immunization. In this way, human B cells can be reprogrammed to produce customized therapeutic molecules with the potential for in vivo amplification.

Seroprevalence of Adeno-Associated Virus Neutralizing Antibodies in Males with Duchenne Muscular Dystrophy

Adeno-associated virus (AAV)-based gene therapies are emerging strategies in Duchenne muscular dystrophy (DMD) treatment. Exposure to wild-type AAV can lead to development of neutralizing antibodies (NAbs) and blocking of AAV transduction, thereby limiting the delivery of AAV vector-based gene therapy. Therefore, it is imperative to check for the presence of AAV NAbs in a patient who is a candidate for gene therapy. We prospectively enrolled 101 genetically confirmed males with DMD (median age 11 years, 48% ambulatory and 59% on steroids) and performed AAV neutralization assays against AAV2, AAV8, AAV9, and AAVrh74 serotypes. The serotype analysis showed that AAV9 (36%) and AAVrh74 (32%) seroprevalence was lower compared with AAV2 (56%) and AAV8 (47%). Interestingly, age was not correlated with NAb titer for any of the capsids. NAb responses were observed at a higher frequency in African American participants and at a lower frequency in Caucasian participants for all four serotypes. Further analysis showed no significant differences in NAb titers regardless of serotype and whether participants were taking steroids or not. Finally, we observed higher AAV8, AAV9, and AAVrh74 seroprevalence and significantly higher AAV2 and AAV8 NAb titers in participants who were ambulatory compared with nonambulatory participants. Overall, these data identify AAV9 and AAVrh74 as the two serotypes with lower pre-existing NAb titers in this study's cohort of 101 males with DMD, possibly showing their utility in future gene therapy applications in treatment of this cohort of patients with DMD.

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

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

Adeno-associated virus-vectored delivery of HIV biologics: the promise of a "single-shot" functional cure for HIV infection

The ability of immunoglobulin-based HIV biologics (Ig-HIV), including broadly neutralizing antibodies, to suppress viral replication in pre-clinical and clinical studies illustrates how these molecules can serve as alternatives or adjuncts to antiretroviral therapy for treating HIV infection. However, the current paradigm for delivering Ig-HIVs requires repeated passive infusions, which faces both logistical and economic challenges to broad-scale implementation. One promising way to overcome these obstacles and achieve sustained expression of Ig-HIVs in vivo involves the transfer of Ig-HIV genes to host cells utilizing adeno-associated virus (AAV) vectors. Because AAV vectors are non-pathogenic and their genomes persist in the cell nucleus as episomes, transgene expression can last for as long as the AAV-transduced cell lives. Given the long lifespan of myocytes, skeletal muscle is a preferred tissue for AAV-based immunotherapies aimed at achieving persistent delivery of Ig-HIVs. Consistent with this idea, recent studies suggest that lifelong immunity against HIV can be achieved from a one-time intramuscular dose of AAV/Ig-HIV vectors. However, realizing the promise of this approach faces significant hurdles, including the potential of AAV-delivered Ig-HIVs to induce anti-drug antibodies and the high AAV seroprevalence in the human population. Here we describe how these host immune responses can hinder AAV/Ig-HIV therapies and review current strategies for overcoming these barriers. Given the potential of AAV/Ig-HIV therapy to maintain ART-free virologic suppression and prevent HIV reinfection in people living with HIV, optimizing this strategy should become a greater priority in HIV/AIDS research.

Strategies to improve safety profile of AAV vectors

Adeno-associated virus (AAV) vectors are currently used in four approved gene therapies for Leber congenital amaurosis (Luxturna), spinal muscular atrophy (Zolgensma), aromatic L-amino acid decarboxylase deficiency (Upstaza) and Haemophilia A (Roctavian), with several more therapies being investigated in clinical trials. AAV gene therapy has long been considered extremely safe both in the context of immunotoxicity and genotoxicity, but recent tragic deaths in the clinical trials for X-linked myotubular myopathy and Duchenne's muscular dystrophy, together with increasing reports of potential hepatic oncogenicity in animal models have prompted re-evaluation of how much trust we can place on the safety of AAV gene therapy, especially at high doses. In this review we cover genome and capsid engineering strategies that can be used to improve safety of the next generation AAV vectors both in the context of immunogenicity and genotoxicity and discuss the gaps that need filling in our current knowledge about AAV vectors.

Single-dose AAV vector gene immunotherapy to treat food allergy

Immunotherapies for patients with food allergy have shown some success in limiting allergic responses. However, these approaches require lengthy protocols with repeated allergen dosing and patients can relapse following discontinuation of treatment. The purpose of this study was to test if a single dose of an adeno-associated virus (AAV) vector can safely prevent and treat egg allergy in a mouse model. AAV vectors expressing ovalbumin (OVA) under an ubiquitous or liver-specific promoter were injected prior to or after epicutaneous sensitization with OVA. Mice treated with either AAV8-OVA vector were completely protected from allergy sensitization. These animals had a significant reduction in anaphylaxis mediated by a reduction in OVA-specific IgE titers. In mice with established OVA allergy, allergic responses were mitigated only in mice treated with an AAV8-OVA vector expressing OVA from an ubiquitous promoter. In conclusion, an AAV vector with a liver-specific promoter was more effective for allergy prevention, but higher OVA levels were necessary for reducing symptoms in preexisting allergy. Overall, our AAV gene immunotherapy resulted in an expansion of OVA-specific FoxP3⁺ CD4⁺ T cells, an increase in the regulatory cytokine IL-10, and a reduction in the IgE promoting cytokine IL-13.

Safety and tolerability of AAV8 delivery of a broadly neutralizing antibody in adults living with HIV: a phase 1, dose-escalation trial

Adeno-associated viral vector-mediated transfer of DNA coding for broadly neutralizing anti-HIV antibodies (bnAbs) offers an alternative to attempting to induce protection by vaccination or by repeated infusions of bnAbs. In this study, we administered a recombinant bicistronic adeno-associated virus (AAV8) vector coding for both the light and heavy chains of the potent broadly neutralizing HIV-1 antibody VRC07 (AAV8-VRC07) to eight adults living with HIV. All participants remained on effective anti-retroviral therapy (viral load (VL) <50 copies per milliliter) throughout this phase 1, dose-escalation clinical trial ( NCT03374202 ). AAV8-VRC07 was given at doses of 5 × 1010, 5 × 1011 and 2.5 × 1012 vector genomes per kilogram by intramuscular (IM) injection. Primary endpoints of this study were to assess the safety and tolerability of AAV8-VRC07; to determine the pharmacokinetics and immunogenicity of in vivo VRC07 production; and to describe the immune response directed against AAV8-VRC07 vector and its products. Secondary endpoints were to assess the clinical effects of AAV8-VRC07 on CD4 T cell count and VL and to assess the persistence of VRC07 produced in participants. In this cohort, IM injection of AAV8-VRC07 was safe and well tolerated. No clinically significant change in CD4 T cell count or VL occurred during the 1-3 years of follow-up reported here. In participants who received AAV8-VRC07, concentrations of VRC07 were increased 6 weeks (P = 0.008) and 52 weeks (P = 0.016) after IM injection of the product. All eight individuals produced measurable amounts of serum VRC07, with maximal VRC07 concentrations >1 µg ml-1 in three individuals. In four individuals, VRC07 serum concentrations remained stable near maximal concentration for up to 3 years of follow-up. In exploratory analyses, neutralizing activity of in vivo produced VRC07 was similar to that of in vitro produced VRC07. Three of eight participants showed a non-idiotypic anti-drug antibody (ADA) response directed against the Fab portion of VRC07. This ADA response appeared to decrease the production of serum VRC07 in two of these three participants. These data represent a proof of concept that adeno-associated viral vectors can durably produce biologically active, difficult-to-induce bnAbs in vivo, which could add valuable new tools to the fight against infectious diseases.

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.

Durability of transgene expression after rAAV gene therapy

Recombinant adeno-associated virus (rAAV) gene therapy has the potential to transform the lives of patients with certain genetic disorders by increasing or restoring function to affected tissues. Following the initial establishment of transgene expression, it is unknown how long the therapeutic effect will last, although animal and emerging human data show that expression can be maintained for more than 10 years. The durability of therapeutic response is key to long-term treatment success, especially since immune responses to rAAV vectors may prevent re-dosing with the same therapy. This review explores the non-immunological and immunological processes that may limit or improve durability and the strategies that can be used to increase the duration of the therapeutic effect.

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.

Genome edited B cells: a new frontier in immune cell therapies

Cell therapies based on reprogrammed adaptive immune cells have great potential as "living drugs." As first demonstrated clinically for engineered chimeric antigen receptor (CAR) T cells, the ability of such cells to undergo clonal expansion in response to an antigen promotes both self-renewal and self-regulation in vivo. B cells also have the potential to be developed as immune cell therapies, but engineering their specificity and functionality is more challenging than for T cells. In part, this is due to the complexity of the immunoglobulin (Ig) locus, as well as the requirement for regulated expression of both cell surface B cell receptor and secreted antibody isoforms, in order to fully recapitulate the features of natural antibody production. Recent advances in genome editing are now allowing reprogramming of B cells by site-specific engineering of the Ig locus with preformed antibodies. In this review, we discuss the potential of engineered B cells as a cell therapy, the challenges involved in editing the Ig locus and the advances that are making this possible, and envision future directions for this emerging field of immune cell engineering.

Advances in cell and gene therapy for HIV disease: it is good to be specific

PURPOSE OF REVIEW

Tremendous advances in cell and gene therapy may soon realize the goal of treating and possibly curing HIV disease. These advances rely on new technologies for cell engineering and new strategies for product manufacturing that are targeting the most important immune deficits in HIV and promising to reconstitute protective, antiviral immunity and achieve natural suppression of HIV disease.

RECENT FINDINGS

We summarize important advances in vectored passive immunity, e.g., directing in vivo expression of protective antibodies or antiviral proteins, B cell engineering to overcome the inadequate humoral immune response to HIV, and T cell engineering that is breaking new ground using viral vector modification of HIV specific T cells. These innovative approaches build on a substantial history of gene and cell therapy research in HIV disease.

SUMMARY

Cell and gene therapy for HIV disease has been an area of tremendous innovation during the nearly two decades since early reports showed evidence for modulating disease. Recent efforts are building on the early experiences, closing gaps in previous approaches, and moving closer to effective treatment. Products approaching or already in clinical trials hold great promise for achieving durable suppression of HIV that will revolutionize therapy and offering hope to infected individuals that disease may be controlled without lifelong dependence on antiretroviral medications.

VIDEO ABSTRACT

http://links.lww.com/COHA/A15.

Promise and Progress of an HIV-1 Cure by Adeno-Associated Virus Vector Delivery of Anti-HIV-1 Biologics

Despite the success of antiretroviral therapy (ART) at suppressing HIV-1 infection, a cure that eradicates all HIV-1-infected cells has been elusive. The latent viral reservoir remains intact in tissue compartments that are not readily targeted by the host immune response that could accelerate the rate of reservoir decline during ART. However, over the past decade, numerous broadly neutralizing antibodies (bNAbs) have been discovered and characterized. These bNAbs have also given rise to engineered antibody-like inhibitors that are just as or more potent than bNAbs themselves. The question remains whether bNAbs and HIV-1 inhibitors will be the effective “kill” to a shock-and-kill approach to eliminate the viral reservoir. Additional research over the past few years has sought to develop recombinant adeno-associated virus (rAAV) vectors to circumvent the need for continual administration of bNAbs and maintain persistent expression in a host. This review discusses the advancements made in using rAAV vectors for the delivery of HIV-1 bNAbs and inhibitors and the future of this technology in HIV-1 cure research. Numerous groups have demonstrated with great efficacy that rAAV vectors can successfully express protective concentrations of bNAbs and HIV-1 inhibitors. Yet, therapeutic concentrations, especially in non-human primate (NHP) models, are not routinely achieved. As new studies have been reported, more challenges have been identified for utilizing rAAV vectors, specifically how the host immune response limits the attainable concentrations of bNAbs and inhibitors. The next few years should provide improvements to rAAV vector delivery that will ultimately show whether they can be used for expressing bNAbs and HIV-1 inhibitors to eliminate the HIV-1 viral reservoir.

Towards Physiologically and Tightly Regulated Vectored Antibody Therapies

Cancers represent highly significant health issues and the options for their treatment are often not efficient to cure the disease. Immunotherapy strategies have been developed to modulate the patient's immune system in order to eradicate cancerous cells. For instance, passive immunization consists in the administration at high doses of exogenously produced monoclonal antibodies directed either against tumor antigen or against immune checkpoint inhibitors. Its main advantage is that it provides immediate immunity, though during a relatively short period, which consequently requires frequent injections. To circumvent this limitation, several approaches, reviewed here, have emerged to induce in vivo antibody secretion at physiological doses. Gene delivery vectors, such as adenoviral vectors or adeno-associated vectors, have been designed to induce antibody secretion in vivo after in situ cell modification, and have driven significant improvements in several cancer models. However, anti-idiotypic antibodies and escape mutants have been detected, probably because of both the continuous expression of antibodies and their expression by unspecialized cell types. To overcome these hurdles, adoptive transfer of genetically modified B cells that secrete antibodies either constitutively or in a regulated manner have been developed by ex vivo transgene insertion with viral vectors. Recently, with the emergence of gene editing technologies, the endogenous B cell receptor loci of B cells have been modified with the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated endonuclease (Cas-9) system to change their specificity in order to target a given antigen. The expression of the modified BCR gene hence follows the endogenous regulation mechanisms, which may prevent or at least reduce side effects. Although these approaches seem promising for cancer treatments, major questions, such as the persistence and the re-activation potential of these engineered cells, remain to be addressed in clinically relevant animal models before translation to humans.

Long-Term Delivery of an Anti-SIV Monoclonal Antibody With AAV

Long-term delivery of anti-HIV monoclonal antibodies using adeno-associated virus (AAV) holds promise for the prevention and treatment of HIV infection. We previously reported that after receiving a single administration of AAV vector coding for anti-SIV antibody 5L7, monkey 84-05 achieved high levels of AAV-delivered 5L7 IgG1 in vivo which conferred sterile protection against six successive, escalating dose, intravenous challenges with highly infectious, highly pathogenic SIVmac239, including a final challenge with 10 animal infectious doses (1). Here we report that monkey 84-05 has successfully maintained 240-350 μg/ml of anti-SIV antibody 5L7 for over 6 years. Approximately 2% of the circulating IgG in this monkey is this one monoclonal antibody. This monkey generated little or no anti-drug antibodies (ADA) to the AAV-delivered antibody for the duration of the study. Due to the nature of the high-dose challenge used and in order to rule out a potential low-level infection not detected by regular viral loads, we have used ultrasensitive techniques to detect cell-associated viral DNA and RNA in PBMCs from this animal. In addition, we have tested serum from 84-05 by ELISA against overlapping peptides spanning the whole envelope sequence for SIVmac239 (PepScan) and against recombinant p27 and gp41 proteins. No reactivity has been detected in the ELISAs indicating the absence of naturally arising anti-SIV antibodies; moreover, the ultrasensitive cell-associated viral tests yielded no positive reaction. We conclude that macaque 84-05 was effectively protected and remained uninfected. Our data show that durable, continuous antibody expression can be achieved after one single administration of AAV and support the potential for lifelong protection against HIV from a single vector administration.

AAV Vector Immunogenicity in Humans: A Long Journey to Successful Gene Transfer

Gene therapy with adeno-associated virus (AAV) vectors has demonstrated safety and long-term efficacy in a number of trials across target organs, including eye, liver, skeletal muscle, and the central nervous system. Since the initial evidence that AAV vectors can elicit capsid T cell responses in humans, which can affect the duration of transgene expression, much progress has been made in understanding and modulating AAV vector immunogenicity. It is now well established that exposure to wild-type AAV results in priming of the immune system against the virus, with development of both humoral and T cell immunity. Aside from the neutralizing effect of antibodies, the impact of pre-existing immunity to AAV on gene transfer is still poorly understood. Herein, we review data emerging from clinical trials across a broad range of gene therapy applications. Common features of immune responses to AAV can be found, suggesting, for example, that vector immunogenicity is dose-dependent, and that innate immunity plays an important role in the outcome of gene transfer. A range of host-specific factors are also likely to be important, and a comprehensive understanding of the mechanisms driving AAV vector immunogenicity in humans will be key to unlocking the full potential of in vivo gene therapy.