Efficient rescue of gutted adenovirus genomes allows rapid production of concentrated stocks without negative selection

High-Capacity Adenoviral Vectors: Expanding the Scope of Gene Therapy

The adaptation of adenoviruses as gene delivery tools has resulted in the development of high-capacity adenoviral vectors (HC-AdVs), also known, helper-dependent or "gutless". Compared with earlier generations (E1/E3-deleted vectors), HC-AdVs retain relevant features such as genetic stability, remarkable efficacy of in vivo transduction, and production at high titers. More importantly, the lack of viral coding sequences in the genomes of HC-AdVs extends the cloning capacity up to 37 Kb, and allows long-term episomal persistence of transgenes in non-dividing cells. These properties open a wide repertoire of therapeutic opportunities in the fields of gene supplementation and gene correction, which have been explored at the preclinical level over the past two decades. During this time, production methods have been optimized to obtain the yield, purity, and reliability required for clinical implementation. Better understanding of inflammatory responses and the implementation of methods to control them have increased the safety of these vectors. We will review the most significant achievements that are turning an interesting research tool into a sound vector platform, which could contribute to overcome current limitations in the gene therapy field.

Manufacturing of adenovirus vectors: production and purification of helper dependent adenovirus

Adenoviral vector (AdV) of the third generation also known as helper-dependent adenoviral vector (HDV) is an attractive delivery system for gene therapy applications. However, obtaining high quality-grade HDV in sufficient amount remains a challenge that hampers the extensive use of this vector in preclinical and clinical studies. Here we review recent progress in the large-scale manufacturing of HDV. The production of HDV is now amenable to large-scale volume with reduced process duration under optimized rescue and co-infection conditions. Also, efficient downstream processing of HDV with acceptable recovery of HDV and minimal contamination by the helper virus is described.

An efficient process for the purification of helper-dependent adenoviral vector and removal of helper virus by iodixanol ultracentrifugation

The preparation of large amount of purified helper-dependent adenoviral vector material is hampered by the lack of development of downstream processes with proven records on separation and recovery efficiencies. In order to facilitate the use of clinical-grade helper-dependent virus material for large-scale in vivo studies, a three-step purification scheme consisting of (1) an anion-exchange chromatography for initial capturing of virus, (2) a shallow iodixanol density gradient ultracentrifugation for the removal of helper virus from helper-dependent virus, and (3) a size-exclusion chromatography for the removal of iodixanol and residual protein contaminants as a polishing step was developed. The use of a fast iodixanol density ultracentrifugation step was highly effective in separating infectious helper-dependent virus from contaminating helper virus. The overall downstream processing scheme gave 80% infectious particle yield. The contamination ratio of helper virus in the helper-dependent virus preparation are reduced from 2.57 to 0.03% corresponding to a reduction of helper virus by factors of 85 by two iodixanol purification steps. It was also demonstrated that size-exclusion chromatography is an excellent step for the removal of iodixanol and polishing of the final helper-dependent virus preparation.

From the first to the third generation adenoviral vector: what parameters are governing the production yield?

Human adenoviral viral vector serotype 5 (AdV) is presently the primary viral vector used in gene therapy trials. Advancements in AdV process development directly contribute to the clinical application and commercialization of the AdV gene delivery technology. Notably, the development of AdV production in suspension culture has driven the increase in AdV volumetric and specific productivity, therefore providing large quantities of AdV required for clinical studies. This review focuses on detailing the viral, cell and cell culture parameters governing the productivity of the three generations of AdV vectors.

Gene therapy as a therapeutic approach for the treatment of rheumatoid arthritis: innovative vectors and therapeutic genes

In recent years, significant progress has been made in the treatment of rheumatoid arthritis (RA). In addition to conventional therapy, novel biologicals targeting tumour necrosis factor-alpha have successfully entered the clinic. However, the majority of the patients still has some actively inflamed joints and some patients suffer from side-effects associated with the high systemic dosages needed to achieve therapeutic levels in the joints. In addition, due to of the short half-life of these proteins there is a need for continuous, multiple injections of the recombinant protein. An alternative approach might be the use of gene transfer to deliver therapeutic genes locally at the site of inflammation. Several viral and non-viral vectors are being used in animal models of RA. The first gene therapy trials for RA have already entered the clinic. New vectors inducing long-term and regulated gene expression in specific tissue are under development, resulting in more efficient gene transfer, for example by using distinct serotypes of viral vectors such as adeno-associated virus. This review gives an overview of some promising vectors used in RA research. Furthermore, several therapeutic genes are discussed that could be used for gene therapy in RA patients.

Genome size and structure determine efficiency of postinternalization steps and gene transfer of capsid-modified adenovirus vectors in a cell-type-specific manner

Adenovirus serotype 5 (Ad5) vectors containing Ad B-group fibers have become increasingly popular as gene transfer vectors because they efficiently transduce human cell types that are relatively refractory to Ad5 infection. So far, most B-group fiber-containing vectors have been first-generation vectors, deleted of E1 and/or E3 genes. Transduction with these vectors, however, results in viral gene expression and is associated with cytotoxicity and immune responses against transduced cells. To circumvent these problems, we developed fiber-chimeric Ad vectors devoid of all viral genes that were produced either by the homologous recombination of first-generation vectors or by using the Cre/lox-based helper virus system. In this study we compared early steps of infection between first-generation (35-kb genome) and Ad vectors devoid of all viral genes with genome sizes of 28 kb and 12.6 kb. All vectors possessed an Ad35-derived fiber knob domain, which uses CD46 as a primary attachment receptor. Using immortalized human hematopoietic cell lines and primary human CD34-positive hematopoietic cells, we found that the Ad genome size did not affect the efficiency of virus attachment to and internalization into cells. Furthermore, independently of the genome length and structure, all vectors migrated to the nucleus through late endosomal and lysosomal cellular compartments. However, the vector containing the short 12.6-kb genome was unable to efficiently escape from endosomes and deliver its DNA into the nucleus. Moreover, compared to other vectors, these Ad particles were less stable and had an abnormal capsid protein composition, including a lack of capsid-stabilizing protein IX. Our data indicate that the size and structure of the packaged viral genomes can affect the integrity of Ad particles, which in turn results in lower infectivity of Ad vectors.

Long-term in vivo transduction of neurons throughout the rat CNS using novel helper-dependent CAV-2 vectors

Numerous genetic and environmental causes, variable pathophysiologies, and the blood-brain barrier create a formidable challenge for the study and treatment of neurodegenerative diseases affecting the central nervous system. Although there are many intracellular strategies to address neurodegeneration, for example, which transgene to use, one fundamental criterion for the long-term survival of neurons may be their genetic modification. Here, we describe the generation and in vivo efficacy of helper-dependent canine adenovirus (CAV-2) vectors that preferentially transduced neurons and efficiently trafficked via axonal retrograde transport. We used a flexible strategy and the synergy between Cre/loxP and nonlethal packaging-defective helper vectors to generate high titer helper-dependent vector stocks. One year after striatal injections in the rat brain, we found stable, high-level expression in striatal neurons, ~50% of the dopaminergic neurons of the substantia nigra, and the cholinergic neurons in the basal nuclei of Meynert. Due to the intrinsic properties of helper-dependent CAV-2 vectors (27-kb cloning capacity; low preexisting, innate, and induced immunogenicity; retrograde transport; and long-term transgene expression), they will aid fundamental and applied studies in neurobiology. Moreover, helper-dependent CAV-2 vectors may be clinically relevant for the treatment of many neurodegenerative diseases.

A gene-deleted adenoviral vector results in phenotypic correction of canine hemophilia B without liver toxicity or thrombocytopenia

Many approaches for treating hemophilia via gene transfer have been attempted in large animal models but all have potential drawbacks. Recombinant adenoviral vectors offer high-efficiency transfer of an episomal vector but have been plagued by the cytotoxicity/immunogenicity of early-generation vectors that contain viral genes. In our current study, we have used a nonintegrating helper-dependent (HD) adenoviral vector for liver-directed gene transfer to achieve hemostatic correction in a dog with hemophilia B. We measured plasma canine factor IX (cFIX) concentrations at a therapeutic range for up to 2.5 months and normalization of the whole blood clotting time (WBCT) for about a month. This was followed by a decrease and stabilized partial correction for 4.5 months. Hepatic gene transfer of a slightly lower dose of the HD vector resulted in WBCTs that were close to normal for 2 weeks, suggesting a dose threshold effect in dogs. In sharp contrast to other studies using first- or second-generation adenoviral vectors, we observed no vector-related elevation of liver enzymes, no fall in platelet counts, and normal liver histology. Taken together, this study demonstrates that injection of an adenoviral HD vector results in complete but transient phenotypic correction of FIX deficiency in canine models with no detectable toxicity.

Episomal persistence of recombinant adenoviral vector genomes during the cell cycle in vivo

Previously we showed that recombinant adenoviral helper-dependent (HD) vectors result in long-term transgene expression levels in vivo which slowly declined by 95% over a period of 1 year. In this study, we further establish that this was not predominantly immune mediated. To determine if cell turnover was responsible for the loss of transgene expression, we induced rapid hepatocyte cell cycling in mouse liver, by performing a surgical two-thirds partial hepatectomy. We observed a 55 and 65% reduction in transgene expression levels and a 50 and 71% loss of vector genomes for the HD vector and the first-generation adenoviral vector. In sharp contrast, in nonviral, episomal plasmid DNA-injected mice, transgene expression levels and DNA copy numbers decreased by 95 and 99%, respectively. These findings suggest that cell division alone was not the primary reason for the slow decrease in transgene expression levels and that recombinant adenoviral vectors have a more robust mechanism for maintaining persistence during cell cycling. Several potential mechanisms are proposed.

Optimization of the generation and propagation of gutless adenoviral vectors

Adenoviral vectors devoid of all viral coding regions are referred to by many names, including gutless vectors. Gutless vectors display reduced toxicity and immunogenicity, increased duration of transgene expression, and increased coding capacity compared to early generation vectors, which contain the majority of the viral backbone genes. However, the production of gutless vectors at a scale and purity suitable for clinical use has limited the utility of this technology. In this work we describe the optimization of the production of gutless vectors. We constructed an improved helper virus and generated an alternative gutless vector producer cell line, PERC6-Cre. We demonstrated increased gutless vector yields, minimal helper virus contamination, and no replication-competent adenovirus contamination using the optimized system. Furthermore, the PERC6-Cre cells were adapted to serum-free suspension culture and high-titer gutless vector preparations were produced using bioreactor technology, suggesting the feasibility of gutless vector scale-up for clinical use. Finally, we observed that helper virus lacking a packaging signal could be packaged at a low frequency, revealing an inherent limitation to the differential packaging strategy for gutless vector propagation.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.