Activation of the NF-kappaB pathway by adeno-associated virus (AAV) vectors and its implications in immune response and gene therapy

Single tyrosine mutation in AAV8 and AAV9 capsids is insufficient to enhance gene delivery to skeletal muscle and heart

Site-directed mutations of tyrosine (Y) to phenylalanine (F) on the surface of adeno-associated viral (AAV) capsids have been reported as a simple method to greatly enhance gene transfer in vitro and in vivo. To determine whether the Y-to-F mutation could also enhance AAV8 and AAV9 gene transfer in skeletal muscle and heart to facilitate muscular dystrophy gene therapy, we investigated four capsid mutants of AAV8 (Y447F or Y733F) and AAV9 (Y446F or Y731F). The mutants and their wild-type control AAV8 and AAV9 capsids were used to package reporter genes (luciferase or β-galactosidase) resulting in similar vector yields. To evaluate gene delivery efficiencies, especially in muscle and heart, the vectors were compared side by side in a series of experiments in vivo in two different strains of mice, the outbred ICR and the inbred C57BL/6. Because AAV8 and AAV9 are among the most effective in systemic gene delivery, we first examined the mutant and wild-type vectors in neonatal mice by intraperitoneal injection, or in adult mice by intravenous injection. To our surprise, no statistically significant differences in transgene expression were observed between the mutant and wild-type vectors, regardless of the reporter genes, vector doses, and the ages and strains of mice used. In addition, quantitative analyses of vector DNA copy number in various tissues from mice treated with mutant and wild-type vectors also showed similar results. Finally, direct intramuscular injection of the above-described vectors with the luciferase gene into the hind limb muscles revealed the same levels of gene expression between mutant and wild-type vectors. Our results thus demonstrate that a single mutation of Y447F or Y733F on capsids of AAV8, and of Y446F or Y731F on AAV9, is insufficient to enhance gene delivery to the skeletal muscle and heart.

Advances in Overcoming Immune Responses following Hemophilia Gene Therapy

Both Clinical trials and pre-clinical experiments for hemophilia gene therapy showed that it is important to overcome potential immune responses against gene transfer vectors and/or transgene products to ensure the success of gene therapy. Recently various approaches have been investigated to prevent or modulate such responses. Gene transfer vectors have been specifically engineered and immunosuppressive regimens have been administered to avoid or manipulate the immune responses against the vectors. In order to prevent cytotoxic lymphocyte or antibody formation induced by transgene expression, novel approaches have been developed, including methods to manipulate antigen presentation, development of variant genes encoding less immunogenic proteins or gene transfer protocols to evade immune responses, as well as immunosuppressive strategies to target either T and/or B cell responses. Most of these successful protocols involve the induction of activated regulatory T cells to create a regulatory immune environment during tolerance induction. Recent development of these strategies to evade vector-specific immune responses and induce long-term immune tolerance specific to the transgene product will be discussed.

Self-complementary adeno-associated viral vectors for gene therapy of hemophilia B: progress and challenges

Therapies currently used for hemophilia involve injection of protein concentrates that are expensive, invasive and associated with side effects such as development of neutralizing antibodies (inhibitors) that diminish therapeutic efficacy. Gene transfer is an attractive alternative to circumvent these issues. However, until now, clinical trials using gene therapy to treat hemophilia have failed to demonstrate sustained efficacy, although a vector based on a self-complementary adeno-associated virus has recently shown promise. This article will briefly outline a novel gene-transfer approach using self-complementary adeno-associated viral vectors using hemophilia B as a target disorder. This approach is currently being evaluated in the clinic. We will provide an overview of the development of self-complementary adeno-associated virus vectors as well as preclinical and clinical data with this vector system.

Innate Immune Responses to AAV Vectors

Gene replacement therapy by in vivo delivery of adeno-associated virus (AAV) is attractive as a potential treatment for a variety of genetic disorders. However, while AAV has been used successfully in many models, other experiments in clinical trials and in animal models have been hampered by undesired responses from the immune system. Recent studies of AAV immunology have focused on the elimination of transgene-expressing cells by the adaptive immune system, yet the innate immune system also has a critical role, both in the initial response to the vector and in prompting a deleterious adaptive immune response. Responses to AAV vectors are primarily mediated by the TLR9–MyD88 pathway, which induces the production of pro-inflammatory cytokines by activating the NF-κB pathways and inducing type I IFN production; self-complementary AAV vectors enhance these inflammatory processes. Additionally, the alternative NF-κB pathway influences transgene expression in cells transduced by AAV. This review highlights these recent discoveries regarding innate immune responses to AAV and discusses strategies to ablate these potentially detrimental signaling pathways.

Antisense-mediated RNA targeting: versatile and expedient genetic manipulation in the brain

A limiting factor in brain research still is the difficulty to evaluate in vivo the role of the increasing number of proteins implicated in neuronal processes. We discuss here the potential of antisense-mediated RNA targeting approaches. We mainly focus on those that manipulate splicing (exon skipping and exon inclusion), but will also briefly discuss mRNA targeting. Classic knockdown of expression by mRNA targeting is only one possible application of antisense oligonucleotides (AON) in the control of gene function. Exon skipping and inclusion are based on the interference of AONs with splicing of pre-mRNAs. These are powerful, specific and particularly versatile techniques, which can be used to circumvent pathogenic mutations, shift splice variant expression, knock down proteins, or to create molecular models using in-frame deletions. Pre-mRNA targeting is currently used both as a research tool, e.g., in models for motor neuron disease, and in clinical trials for Duchenne muscular dystrophy and amyotrophic lateral sclerosis. AONs are particularly promising in relation to brain research, as the modified AONs are taken up extremely fast in neurons and glial cells with a long residence, and without the need for viral vectors or other delivery tools, once inside the blood brain barrier. In this review we cover (1). The principles of antisense-mediated techniques, chemistry, and efficacy. (2) The pros and cons of AON approaches in the brain compared to other techniques of interfering with gene function, such as transgenesis and short hairpin RNAs, in terms of specificity of the manipulation, spatial, and temporal control over gene expression, toxicity, and delivery issues. (3) The potential applications for Neuroscience. We conclude that there is good evidence from animal studies that the central nervous system can be successfully targeted, but the potential of the diverse AON-based approaches appears to be under-recognized.

miRNA cassettes in viral vectors: problems and solutions

The discovery of RNA interference (RNAi), an evolutionary conserved gene silencing mechanism that is triggered by double stranded RNA, has led to tremendous efforts to use this technology for basic research and new RNA therapeutics. RNAi can be induced via transfection of synthetic small interfering RNAs (siRNAs), which results in a transient knockdown of the targeted mRNA. For stable gene silencing, short hairpin RNA (shRNA) or microRNA (miRNA) constructs have been developed. In mammals and humans, the natural RNAi pathway is triggered via endogenously expressed miRNAs. The use of modified miRNA expression cassettes to elucidate fundamental biological questions or to develop therapeutic strategies has received much attention. Viral vectors are particularly useful for the delivery of miRNA genes to specific target cells. To date, many viral vectors have been developed, each with distinct characteristics that make one vector more suitable for a certain purpose than others. This review covers the recent progress in miRNA-based gene-silencing approaches that use viral vectors, with a focus on their unique properties, respective limitations and possible solutions. Furthermore, we discuss a related topic that involves the insertion of miRNA-target sequences in viral vector systems to restrict their cellular range of gene expression. This article is part of a Special Issue entitled: MicroRNAs in viral gene regulation.

The genome of self-complementary adeno-associated viral vectors increases Toll-like receptor 9-dependent innate immune responses in the liver

Although adeno-associated viral (AAV) vectors have been successfully used in hepatic gene transfer for treatment of hemophilia and other diseases in animals, adaptive immune responses blocked long-term transgene expression in patients on administration of single-stranded AAV serotype-2 vector. More efficient vectors have been developed using alternate capsids and self-complimentary (sc) genomes. This study investigated their effects on the innate immune profile on hepatic gene transfer to mice. A mild and transient up-regulation of myeloid differentiation primary response gene (88), TLR9, TNF-α, monocyte chemotactic protein-1, IFN-γ inducible protein-10, and IFN-α/β expression in the liver was found after single-stranded AAV vector administration, regardless of the capsid sequence. In contrast, scAAV vectors induced higher increases of these transcripts, upregulated additional proinflammatory genes, and increased circulating IL-6. Neutrophil, macrophage, and natural killer cell liver infiltrates were substantially higher on injection of scAAV. Some but not all of these responses were Kupffer cell dependent. Independent of the capsid or expression cassette, scAAV vectors induced dose-dependent innate responses by signaling through TLR9. Increased innate responses to scAAV correlated with stronger adaptive immune responses against capsid (but not against the transgene product). However, these could be blunted by transient inhibition of TLR9.