Immune responses to AAV vectors: overcoming barriers to successful gene therapy

Adeno-associated virus vectors and neurological gene therapy

Gene therapy has strong potential for treating a variety of genetic disorders, as demonstrated in recent clinical trials. There is unfortunately no scarcity of disease targets, and the grand challenge in this field has instead been the development of safe and efficient gene delivery platforms. To date, approximately two thirds of the 1800 gene therapy clinical trials completed worldwide have used viral vectors. Among these, adeno-associated virus (AAV) has emerged as particularly promising because of its impressive safety profile and efficiency in transducing a wide range of cell types. Gene delivery to the CNS involves both considerable promise and unique challenges, and better AAV vectors are thus needed to translate CNS gene therapy approaches to the clinic. This review discusses strategies for vector design, potential routes of administration, immune responses, and clinical applications of AAV in the CNS.

Optogenetic stimulation of the auditory pathway

Auditory prostheses can partially restore speech comprehension when hearing fails. Sound coding with current prostheses is based on electrical stimulation of auditory neurons and has limited frequency resolution due to broad current spread within the cochlea. In contrast, optical stimulation can be spatially confined, which may improve frequency resolution. Here, we used animal models to characterize optogenetic stimulation, which is the optical stimulation of neurons genetically engineered to express the light-gated ion channel channelrhodopsin-2 (ChR2). Optogenetic stimulation of spiral ganglion neurons (SGNs) activated the auditory pathway, as demonstrated by recordings of single neuron and neuronal population responses. Furthermore, optogenetic stimulation of SGNs restored auditory activity in deaf mice. Approximation of the spatial spread of cochlear excitation by recording local field potentials (LFPs) in the inferior colliculus in response to suprathreshold optical, acoustic, and electrical stimuli indicated that optogenetic stimulation achieves better frequency resolution than monopolar electrical stimulation. Virus-mediated expression of a ChR2 variant with greater light sensitivity in SGNs reduced the amount of light required for responses and allowed neuronal spiking following stimulation up to 60 Hz. Our study demonstrates a strategy for optogenetic stimulation of the auditory pathway in rodents and lays the groundwork for future applications of cochlear optogenetics in auditory research and prosthetics.

Cardiovascular gene therapy for myocardial infarction

INTRODUCTION

Cardiovascular gene therapy is the third most popular application for gene therapy, representing 8.4% of all gene therapy trials as reported in 2012 estimates. Gene therapy in cardiovascular disease is aiming to treat heart failure from ischemic and non-ischemic causes, peripheral artery disease, venous ulcer, pulmonary hypertension, atherosclerosis and monogenic diseases, such as Fabry disease.

AREAS COVERED

In this review, we will focus on elucidating current molecular targets for the treatment of ventricular dysfunction following myocardial infarction (MI). In particular, we will focus on the treatment of i) the clinical consequences of it, such as heart failure and residual myocardial ischemia and ii) etiological causes of MI (coronary vessels atherosclerosis, bypass venous graft disease, in-stent restenosis).

EXPERT OPINION

We summarise the scheme of the review and the molecular targets either already at the gene therapy clinical trial phase or in the pipeline. These targets will be discussed below. Following this, we will focus on what we believe are the 4 prerequisites of success of any gene target therapy: safety, expression, specificity and efficacy (SESE).

DNA cleavage enzymes for treatment of persistent viral infections: recent advances and the pathway forward

Treatment for most persistent viral infections consists of palliative drug options rather than curative approaches. This is often because long-lasting viral DNA in infected cells is not affected by current antivirals, providing a source for viral persistence and reactivation. Targeting latent viral DNA itself could therefore provide a basis for novel curative strategies. DNA cleavage enzymes can be used to induce targeted mutagenesis of specific genes, including those of exogenous viruses. Although initial in vitro and even in vivo studies have been carried out using DNA cleavage enzymes targeting various viruses, many questions still remain concerning the feasibility of these strategies as they transition into preclinical research. Here, we review the most recent findings on DNA cleavage enzymes for human viral infections, consider the most relevant animal models for several human viral infections, and address issues regarding safety and enzyme delivery. Results from well-designed in vivo studies will ideally provide answers to the most urgent remaining questions, and allow continued progress toward clinical application.

Passive immunization against HIV/AIDS by antibody gene transfer

Despite tremendous efforts over the course of many years, the quest for an effective HIV vaccine by the classical method of active immunization remains largely elusive. However, two recent studies in mice and macaques have now demonstrated a new strategy designated as Vectored ImmunoProphylaxis (VIP), which involves passive immunization by viral vector-mediated delivery of genes encoding broadly neutralizing antibodies (bnAbs) for in vivo expression. Robust protection against virus infection was observed in preclinical settings when animals were given VIP to express monoclonal neutralizing antibodies. This unorthodox approach raises new promise for combating the ongoing global HIV pandemic. In this article, we survey the status of antibody gene transfer, review the revolutionary progress on isolation of extremely bnAbs, detail VIP experiments against HIV and its related virus conduced in humanized mice and macaque monkeys, and discuss the pros and cons of VIP and its opportunities and challenges towards clinical applications to control HIV/AIDS endemics.

Use of genetically modified mesenchymal stem cells to treat neurodegenerative diseases

The transplantation of mesenchymal stem cells (MSCs) for treating neurodegenerative disorders has received growing attention recently because these cells are readily available, easily expanded in culture, and when transplanted, survive for relatively long periods of time. Given that such transplants have been shown to be safe in a variety of applications, in addition to recent findings that MSCs have useful immunomodulatory and chemotactic properties, the use of these cells as vehicles for delivering or producing beneficial proteins for therapeutic purposes has been the focus of several labs. In our lab, the use of genetic modified MSCs to release neurotrophic factors for the treatment of neurodegenerative diseases is of particular interest. Specifically, glial cell-derived neurotrophic factor (GDNF), nerve growth factor (NGF), and brain derived neurotrophic factor (BDNF) have been recognized as therapeutic trophic factors for Parkinson's, Alzheimer's and Huntington's diseases, respectively. The aim of this literature review is to provide insights into: (1) the inherent properties of MSCs as a platform for neurotrophic factor delivery; (2) the molecular tools available for genetic manipulation of MSCs; (3) the rationale for utilizing various neurotrophic factors for particular neurodegenerative diseases; and (4) the clinical challenges of utilizing genetically modified MSCs.

B-Cell Depletion is Protective Against Anti-AAV Capsid Immune Response: A Human Subject Case Study

Gene therapy strategies for congenital myopathies may require repeat administration of adeno-associated viral (AAV) vectors due to aspects of the clinical application, such as: (i) administration of doses below therapeutic efficacy in patients enrolled in early phase clinical trials; (ii) progressive reduction of the therapeutic gene expression over time as a result of increasing muscle mass in patients treated at a young age; and (iii) a possibly faster depletion of pathogenic myofibers in this patient population. Immune response triggered by the first vector administration, and to subsequent doses, represents a major obstacle for successful gene transfer in young patients. Anti-capsid and anti-transgene product related humoral and cell-mediated responses have been previously observed in all preclinical models and human subjects who received gene therapy or enzyme replacement therapy (ERT) for congenital myopathies. Immune responses may result in reduced efficacy of the gene transfer over time and/or may preclude for the possibility of re-administration of the same vector. In this study, we evaluated the immune response of a Pompe patient dosed with an AAV1-GAA vector after receiving Rituximab and Sirolimus to modulate reactions against ERT. A key finding of this single subject case report is the observation that B-cell ablation with rituximab prior to AAV vector exposure results in non-responsiveness to both capsid and transgene, therefore allowing the possibility of repeat administration in the future. This observation is significant for future gene therapy studies and establishes a clinically relevant approach to blocking immune responses to AAV vectors.

Safety profile of recombinant adeno-associated viral vectors: focus on alipogene tiparvovec (Glybera®)

There has been great interest over the past two decades in developing gene therapies (GTs) to treat a variety of diseases; however, translating research findings into clinical treatments have proved to be a challenge. A major milestone in the development of GT has been achieved with the approval of alipogene tiparvovec (Glybera(®)) in Europe for the treatment of familial lipoprotein lipase deficiency. At this important stage with the evolution of GT into the clinic, this review will examine the safety aspects GT with adeno-associated virus (AAV) vectors. The topics that will be covered include acute reactions, immunological reactions to the AAV capsid and expressed transgene, viral biodistribution and shedding, DNA integration and carcinogenicity. These safety aspects of GT will be discussed with a focus on alipogene tiparvovec, in addition to other AAV vector GT products currently in clinical development.

Beyond humanization and de-immunization: tolerization as a method for reducing the immunogenicity of biologics

Immune responses to some monoclonal antibodies (mAbs) and biologic proteins interfere with their efficacy due to the development of anti-drug antibodies (ADA). In the case of mAbs, most ADA target 'foreign' sequences present in the complementarity determining regions (CDRs). Humanization of the mAb sequence is one approach that has been used to render biologics less foreign to the human immune system. However, fully human mAbs can also drive immunogenicity. De-immunization (removing epitopes) has been used to reduce biologic protein immunogenicity. Here, we discuss a third approach to reducing the immunogenicity of biologics: introduction of Treg epitopes that stimulate Treg function and induce tolerance to the biologic protein. Supplementing humanization (replacing xeno-sequences with human) and de-immunization (reducing T effector epitopes) with tolerization (introducing Treg epitopes) where feasible, as a means of improving biologics 'quality by design', may lead to the development of ever more clinically effective, but less immunogenic, biologics.

Gene therapy on the move

The first gene therapy clinical trials were initiated more than two decades ago. In the early days, gene therapy shared the fate of many experimental medicine approaches and was impeded by the occurrence of severe side effects in a few treated patients. The understanding of the molecular and cellular mechanisms leading to treatment- and/or vector-associated setbacks has resulted in the development of highly sophisticated gene transfer tools with improved safety and therapeutic efficacy. Employing these advanced tools, a series of Phase I/II trials were started in the past few years with excellent clinical results and no side effects reported so far. Moreover, highly efficient gene targeting strategies and site-directed gene editing technologies have been developed and applied clinically. With more than 1900 clinical trials to date, gene therapy has moved from a vision to clinical reality. This review focuses on the application of gene therapy for the correction of inherited diseases, the limitations and drawbacks encountered in some of the early clinical trials and the revival of gene therapy as a powerful treatment option for the correction of monogenic disorders.

Vascular delivery of rAAVrh74.MCK.GALGT2 to the gastrocnemius muscle of the rhesus macaque stimulates the expression of dystrophin and laminin α2 surrogates

Overexpression of GALGT2 in skeletal muscle can stimulate the glycosylation of α dystroglycan and the upregulation of normally synaptic dystroglycan-binding proteins, some of which are dystrophin and laminin α2 surrogates known to be therapeutic for several forms of muscular dystrophy. This article describes the vascular delivery of GALGT2 gene therapy in a large animal model, the rhesus macaque. Recombinant adeno-associated virus, rhesus serotype 74 (rAAVrh74), was used to deliver GALGT2 via the femoral artery to the gastrocnemius muscle using an isolated focal limb perfusion method. GALGT2 expression averaged 44 ± 4% of myofibers after treatment in macaques with low preexisting anti-rAAVrh74 serum antibodies, and expression was reduced to 9 ± 4% of myofibers in macaques with high preexisting rAAVrh74 immunity (P < 0.001; n = 12 per group). This was the case regardless of the addition of immunosuppressants, including prednisolone, tacrolimus, and mycophenolate mofetil. GALGT2-treated macaque muscles showed increased glycosylation of α dystroglycan and increased expression of dystrophin and laminin α2 surrogate proteins, including utrophin, plectin1, agrin, and laminin α5. These experiments demonstrate successful transduction of rhesus macaque muscle with rAAVrh74.MCK.GALGT2 after vascular delivery and induction of molecular changes thought to be therapeutic in several forms of muscular dystrophy.

Developments in the treatment of hemophilia B: focus on emerging gene therapy

Hemophilia B is a genetic disorder that is characterized by a deficiency of clotting factor IX (FIX) and excessive bleeding. Advanced understanding of the pathophysiology of the disease has led to the development of improved treatment strategies that aim to minimize the acute and long-term complications of the disease. Patients with hemophilia B are ideal candidates for gene therapy, mostly because a small increase in protein production can lead to significantly decreased bleeding diathesis. Although human clotting FIX was cloned and sequenced over 30 years ago, progress toward achieving real success in human clinical trials has been slow, with long-term, therapeutically relevant gene expression only achieved in one trial published in 2011. The history of this extensive research effort has revealed the importance of the interactions between gene therapy vectors and multiple arms of the host immune system at multiple stages of the transduction process. Different viral vector systems each have unique properties that influence their ability to deliver genes to different tissues, and the data generated in several clinical trials testing different vectors for hemophilia have guided our understanding toward development of optimal configurations for treating hemophilia B. The recent clinical success implementing a novel adeno-associated virus vector demonstrated sufficient FIX expression in patients to convert a severe hemophilia phenotype to mild, an achievement which has the potential to profoundly alter the impact of this disease on human society. Continued research should lead to vector designs that result in higher FIX activity at lower vector doses and with reduced host immune responses to the vector and the transgene product.

T-cell dependent immunogenicity of protein therapeutics: Preclinical assessment and mitigation

Protein therapeutics hold a prominent and rapidly expanding place among medicinal products. Purified blood products, recombinant cytokines, growth factors, enzyme replacement factors, monoclonal antibodies, fusion proteins, and chimeric fusion proteins are all examples of therapeutic proteins that have been developed in the past few decades and approved for use in the treatment of human disease. Despite early belief that the fully human nature of these proteins would represent a significant advantage, adverse effects associated with immune responses to some biologic therapies have become a topic of some concern. As a result, drug developers are devising strategies to assess immune responses to protein therapeutics during both the preclinical and the clinical phases of development. While there are many factors that contribute to protein immunogenicity, T cell- (thymus-) dependent (Td) responses appear to play a critical role in the development of antibody responses to biologic therapeutics. A range of methodologies to predict and measure Td immune responses to protein drugs has been developed. This review will focus on the Td contribution to immunogenicity, summarizing current approaches for the prediction and measurement of T cell-dependent immune responses to protein biologics, discussing the advantages and limitations of these technologies, and suggesting a practical approach for assessing and mitigating Td immunogenicity.

Immunological Monitoring to Rationally Guide AAV Gene Therapy

Recent successes with adeno-associated virus (AAV)-based gene therapies fuel the hope for new treatments for hereditary diseases. Pre-existing as well as therapy-induced immune responses against both AAV and the encoded transgenes have been described and may impact on safety and efficacy of gene therapy approaches. Consequently, monitoring of vector- and transgene-specific immunity is mandated and may rationally guide clinical development. Next to the humoral immune response, the cellular response is central in our understanding of the host reaction in gene therapy. But in contrast to the monitoring of antibodies, which has matured over many decades, sensitive and robust monitoring of T cells is a relatively new development. To make cellular immune assessments fit for purpose, investigators need to know, control and report the critical assay variables that influence the results. In addition, the quality of immune assays needs to be continuously adjusted to allow for exploratory hypothesis generation in early stages and confirmatory hypothesis validation in later stages of clinical development. The concept of immune assay harmonization which includes use of field-wide benchmarks, harmonization guidelines, and external quality control can support the context-specific evolution of immune assays. Multi-center studies pose particular challenges to sample logistics and quality control of sample specimens. Cooperative groups need to define if immune assessments should be performed in one central facility, in peripheral labs or including a combination of both. Finally, engineered reference samples that contain a defined number of antigen-specific T cells may become broadly applicable tools to control assay performance over time or across institutions.