Cre levels limit packaging signal excision efficiency in the Cre/loxP helper-dependent adenoviral vector system

Application of a split-Cre system for high-capacity adenoviral vector amplification

BACKGROUND AND AIMS

High-capacity adenoviral vectors (HC-AdV) show extended DNA payload and stability of gene expression in vivo due to the absence of viral coding sequences. However, production requires methods to trans-complement viral proteins, usually through Helper Viruses (HV). The Cre/loxP system is frequently employed to remove the packaging signal in HV genomes, in order to avoid their encapsidation. However, chronic exposure to the Cre recombinase in packaging cells is detrimental. We have applied the dimerizable Cre system to overcome this limitation.

METHODS AND RESULTS

Cre was split in two fragments devoid of recombinase function (N-terminal 244 and C-terminal 99 amino-acids). In one version of the system, interaction with both moieties was favored by rapamycin-dependent heterodimerization domains (DiCre). Other version contained only Cre sequences (oCre). We generated packaging cells and HVs expressing the complementary fragments and studied their performance for HC-AdV production. We found that both conformations avoided interference with the growth of packaging cells, and the oCre system was particularly suitable for HC-AdV amplification.

CONCLUSIONS

The split-Cre system improves the performance of packaging cells and can reduce the time and cost of HC-AdV amplification up to 30% and 15%, respectively. This may contribute to the standardization of HC-AdV production.

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.

Adenovirus-Mediated Gene Delivery: Potential Applications for Gene and Cell-Based Therapies in the New Era of Personalized Medicine

With rapid advances in understanding molecular pathogenesis of human diseases in the era of genome sciences and systems biology, it is anticipated that increasing numbers of therapeutic genes or targets will become available for targeted therapies. Despite numerous setbacks, efficacious gene and/or cell-based therapies still hold the great promise to revolutionize the clinical management of human diseases. It is wildly recognized that poor gene delivery is the limiting factor for most in vivo gene therapies. There has been a long-lasting interest in using viral vectors, especially adenoviral vectors, to deliver therapeutic genes for the past two decades. Among all currently available viral vectors, adenovirus is the most efficient gene delivery system in a broad range of cell and tissue types. The applications of adenoviral vectors in gene delivery have greatly increased in number and efficiency since their initial development. In fact, among over 2,000 gene therapy clinical trials approved worldwide since 1989, a significant portion of the trials have utilized adenoviral vectors. This review aims to provide a comprehensive overview on the characteristics of adenoviral vectors, including adenoviral biology, approaches to engineering adenoviral vectors, and their applications in clinical and pre-clinical studies with an emphasis in the areas of cancer treatment, vaccination and regenerative medicine. Current challenges and future directions regarding the use of adenoviral vectors are also discussed. It is expected that the continued improvements in adenoviral vectors should provide great opportunities for cell and gene therapies to live up to its enormous potential in personalized medicine.

Helper-Dependent Adenoviral Vectors and Their Use for Neuroscience Applications

Neuroscience research has been revolutionized by the use of recombinant viral vector technology from the basic, preclinical and clinical levels. Currently, multiple recombinant viral vector types are employed with each having its strengths and weaknesses depending on the proposed application. Helper-dependent adenoviral vectors (HdAd) are emerging as ideal viral vectors that solve a major need in the neuroscience field: (1) expression of transgenes that are too large to be packaged by other viral vectors and (2) rapid onset of transgene expression in the absence of cytotoxicity. Here, we describe the methods for large-scale production of HdAd viral vectors for in vivo use with neurospecific transgene expression.

Canine helper-dependent vectors production: implications of Cre activity and co-infection on adenovirus propagation

The importance of Cre recombinase to minimize helper vector (HV) contamination during helper-dependent adenovirus vectors (HDVs) production is well documented. However, Cre recombinase, by inducing DNA double-strand breaks (DSBs), can cause a reduced proliferation and genotoxic effects in cultured cells. In this work, Cre-expressing cell stability, co-infection and their relation to adenovirus amplification/HV contamination were evaluated to develop a production protocol for HD canine adenovirus type 2 (CAV-2) vectors. Long-term Cre expression reduced the capacity of MDCK-E1-Cre cells to produce CAV-2 by 7-fold, although cell growth was maintained. High HDV/HV MOI ratio (5:0.1) led to low HV contamination without compromising HDV yields. Indeed, such MOI ratio was sufficient to reduce HV levels, as these were similar either in MDCK-E1 or MDCK-E1-Cre cells. This raises the possibility of producing HDVs without Cre-expressing cells, which would circumvent the negative effects that this recombinase holds to the production system. Here, we show how Cre and MOI ratio impact adenovirus vectors yields and infectivity, providing key-information to design an improved manufacturing of HDV. Potential mechanisms to explain how Cre is specifically impacting cell productivity without critically compromising its growth are presented.

A rapid Q-PCR titration protocol for adenovirus and helper-dependent adenovirus vectors that produces biologically relevant results

Adenoviruses are employed in the study of cellular processes and as expression vectors used in gene therapy. The success and reproducibility of these studies is dependent in part on having accurate and meaningful titers of replication competent and helper-dependent adenovirus stocks, which is problematic due to the use of varied and divergent titration protocols. Physical titration methods, which quantify the total number of viral particles, are used by many, but are poor at estimating activity. Biological titration methods, such as plaque assays, are more biologically relevant, but are time consuming and not applicable to helper-dependent gene therapy vectors. To address this, a protocol was developed called "infectious genome titration" in which viral DNA is isolated from the nuclei of cells ~3 h post-infection, and then quantified by Q-PCR. This approach ensures that only biologically active virions are counted as part of the titer determination. This approach is rapid, robust, sensitive, reproducible, and applicable to all forms of adenovirus. Unlike other Q-PCR-based methods, titers determined by this protocol are well correlated with biological activity.

Impact of E1 and Cre on adenovirus vector amplification: developing MDCK CAV-2-E1 and E1-Cre transcomplementing cell lines

Adenovirus vectors have been extensively studied through the manipulation of viral genome. However, little attention is being paid to their producer cell-lines; cells are selected according to virus yields, neglecting the expression profile of transcomplementing gene products underlying cell performance. This work evaluates the impact of E1 (E1A and E1B) and Cre recombinase levels in the production of E1-deleted and helper-dependent canine adenovirus type 2 (CAV-2) vectors using MDCK cells. E1A and E1B gene expression and Cre activity were evaluated in different cell clones and compared with the corresponding cell productivity and susceptibility to oxidative stress injury. CAV-2 production was proportional to E1A expression (the highest levels of E1A corresponding to productivities of 3000–5000 I.P./cell), while E1B prolonged host cell viability after infection, conferring protection against apoptosis. Cre recombinase counteracted E1B anti-apoptotic properties, however viral production was maintained under high levels of Cre. Yet, Cre recombinase side effects can be reduced using cell lines with lower Cre-activities, without compromising the excision efficiency of helper vector packaging signal. These results highlight the influence of transcomplementing gene products on CAV-2 producer cell line performance, and the ability to express high levels of E1A and E1B as an important feature for cell line establishment and high adenovirus titers.

Vector and helper genome rearrangements occur during production of helper-dependent adenoviral vectors

Helper-dependent adenoviral vectors (HD Ad) hold extreme promise for gene therapy of human diseases. All viral genes are deleted in HD Ad vectors, and therefore, the presence of a helper virus is required for their production. Current methods to minimize helper contamination in large-scale preparations rely on the use of the Cre/loxP system. The inclusion of loxP sites flanking the packaging signal results in its excision in the presence of Cre recombinase, preventing helper genome encapsidation. It is well established that the level of Cre recombinase activity is important in determining the degree of helper contamination. However, there is little information on other mechanisms that could also play an important role. We have generated several HD Ad vectors containing a rapalog-inducible system to regulate transgene expression, or LacZ under the control of the elongation factor 1 α promoter. Large-scale production of these vectors resulted in abundant helper contamination. Viral DNA analysis revealed the presence of rearrangements between vector and helper genomes. The rearrangements involved a helper DNA molecule with a fragment of the left arm of the HD Ad vector, including its ITR, packaging signal, and some stuffer sequence. Overall, our data suggest that helper DNA molecules that accumulate after Cre recombinase activity are prone to rearrangements, resulting in helper genomes that have incorporated a packaging signal from the vector. Helper particles with rearranged genomes have a growth advantage. This study identifies a novel mechanism leading to helper contamination during helper-dependent adenoviral vector production.

Efficient production of adenovirus vector lacking genes of virus-associated RNAs that disturb cellular RNAi machinery

First-generation adenovirus vectors (FG AdVs) are widely used in basic studies and gene therapy. However, virus-associated (VA) RNAs that act as small-interference RNAs are indeed transcribed from the vector genome. These VA RNAs can trigger the innate immune response. Moreover, VA RNAs are processed to functional viral miRNAs and disturb the expressions of numerous cellular genes. Therefore, VA-deleted AdVs lacking VA RNA genes would be advantageous for basic studies, both in vitro and in vivo. Here, we describe an efficient method of producing VA-deleted AdVs. First, a VA RNA-substituted “pre-vector” lacking the original VA RNA genes but alternatively possessing an intact VA RNA region flanked by a pair of FRTs was constructed. VA-deleted AdVs were efficiently obtained by infecting 293hde12 cells, which highly express FLP, with the pre-vector. The resulting transduction titers of VA-deleted AdVs were sufficient for practical use. Therefore, VA-deleted AdVs may be substitute for current FG AdV.

Influence of loxP insertion upstream of the cis-acting packaging domain on adenovirus packaging efficiency

First-generation AdV enables efficient gene transduction, although its immunogenicity is an important problem in vivo. Helper-dependent AdV (HD-AdV) is one possible solution to this problem. The construction of HD-AdV requires a helper virus, in which the viral packaging domain is flanked by two inserted loxP to hamper its packaging in Cre-expressing 293 cells. Here, we constructed 19L viruses containing loxP at 191 nt from the left end of the genome upstream of the packaging domain, 15L viruses bearing loxP at 143 nt, and a control ΔL virus lacking loxP at these positions. The 19L position is used worldwide, and the 15L position has been reported to result in a lower titer than that of 19L. When the titers were compared for six pairs of 19L and 15L AdV, the 19L AdV produced titers similar to, or sometimes lower than, the 15L and ΔL AdV, unlike the results of previous reports. We next chose one pair of 15L and 19L AdV that produced titers similar to that of ΔL and a competitor AdV lacking loxP for use in a competition assay. When a small amount of the competitor AdV was co-infected, both the 15L and the 19L AdV, but not ΔL, gradually became minority components during subsequent viral passages. Therefore, the loxP insertions at 143 nt and 191 nt decreased the viral packaging efficiency.

Altering the Ad5 packaging domain affects the maturation of the Ad particles

We have previously described a new family of mutant adenoviruses carrying different combinations of attB/attP sequences from bacteriophage PhiC31 flanking the Ad5 packaging domain. These novel helper viruses have a significantly delayed viral life cycle and a severe packaging impairment, regardless of the presence of PhiC31 recombinase. Their infectious viral titers are significantly lower (100–1000 fold) than those of control adenovirus at 36 hours post-infection, but allow for efficient packaging of helper-dependent adenovirus. In the present work, we have analyzed which steps of the adenovirus life cycle are altered in attB-helper adenoviruses and investigated whether these viruses can provide the necessary viral proteins in trans. The entry of attB-adenoviral genomes into the cell nucleus early at early timepoints post-infection was not impaired and viral protein expression levels were found to be similar to those of control adenovirus. However, electron microscopy and capsid protein composition analyses revealed that attB-adenoviruses remain at an intermediate state of maturation 36 hours post-infection in comparison to control adenovirus which were fully mature and infective at this time point. Therefore, an additional 20–24 hours were found to be required for the appearance of mature attB-adenovirus. Interestingly, attB-adenovirus assembly and infectivity was restored by inserting a second packaging signal close to the right-end ITR, thus discarding the possibility that the attB-adenovirus genome was retained in a nuclear compartment deleterious for virus assembly. The present study may have substantive implications for helper-dependent adenovirus technology since helper attB-adenovirus allows for preferential packaging of helper-dependent adenovirus genomes.

Self-inactivating helper virus for the production of high-capacity adenoviral vectors

Standard methods for producing high-capacity adenoviral vectors (HC-Ads) are based on co-infection with a helper adenovirus (HV). To avoid HV encapsidation, its packaging signal (Ψ) is flanked by recognition sequences for recombinases expressed in the producing cells. However, accumulation of HV and low yield of HC-Ad are frequently observed, due in part to insufficient recombinase expression. We describe here a novel HV (AdTetCre) in which Ψ is flanked by loxP sites that can be excised by a chimeric MerCreMer recombinase encoded in the same viral genome. Efficient modulation of cleavage was obtained by simultaneous control of MerCreMer expression using a tet-on inducible system, and translocation to the nucleus by 4-hydroxytamoxifen (TAM). Encapsidation of AdTetCre was strongly inhibited by TAM plus doxycicline. Using AdTetCre and 293Cre4 cells for the production of HC-Ads, we found that cellular and virus-encoded recombinases cooperate to minimize HV contamination. The method was highly reproducible and allowed the routine production of different HC-Ads in a medium-scale laboratory setting in adherent cells, with titers >10¹⁰ infectious units and <0.1% HV contamination. The residual HVs lacked Ψ and were highly attenuated. We conclude that self-inactivating HVs based on virally encoded recombinases are promising tools for the production of HC-Ads.

Production of adenovirus vectors and their use as a delivery system for influenza vaccines

IMPORTANCE OF THE FIELD

With the emergence of highly pathogenic avian influenza H5N1 viruses that have crossed species barriers and are responsible for lethal infections in humans in many countries, there is an urgent need for the development of effective vaccines which can be produced in large quantities at a short notice and confer broad protection against these H5N1 variants. In order to meet the potential global vaccine demand in a pandemic scenario, new vaccine-production strategies must be explored in addition to the currently used egg-based technology for seasonal influenza.

AREAS COVERED IN THIS REVIEW

Adenovirus (Ad) based influenza vaccines represent an attractive alternative/supplement to the currently licensed egg-based influenza vaccines. Ad-based vaccines are relatively inexpensive to manufacture, and their production process does not require either chicken eggs or labor-intensive and time-consuming processes necessitating enhanced biosafety facilities. Most importantly, in a pandemic situation, this vaccine strategy could offer a stockpiling option to reduce the response time before a strain-matched vaccine could be developed.

WHAT THE READER WILL GAIN

This review discusses Ad-vector technology and the current progress in the development of Ad-based influenza vaccines.

TAKE HOME MESSAGE

Ad vector-based influenza vaccines for pandemic preparedness are under development to meet global vaccine demand.

Helper-Dependent Adenoviral Vectors

Helper-dependent adenoviral vectors are devoid of all viral coding sequences, possess a large cloning capacity, and can efficiently transduce a wide variety of cell types from various species independent of the cell cycle to mediate long-term transgene expression without chronic toxicity. These non-integrating vectors hold tremendous potential for a variety of gene transfer and gene therapy applications. Here, we review the production technologies, applications, obstacles to clinical translation and their potential resolutions, and the future challenges and unanswered questions regarding this promising gene transfer technology.

Adenoviral producer cells

Adenovirus (Ad) vectors, in particular those of the serotype 5, are highly attractive for a wide range of gene therapy, vaccine and virotherapy applications (as discussed in further detail in this issue). Wild type Ad5 virus can replicate in numerous tissue types but to use Ad vectors for therapeutic purposes the viral genome requires modification. In particular, if the viral genome is modified in such a way that the viral life cycle is interfered with, a specific producer cell line is required to provide trans-complementation to overcome the modification and allow viral production. This can occur in two ways; use of a producer cell line that contains specific adenoviral sequences incorporated into the cell genome to trans-complement, or use of a producer cell line that naturally complements for the modified Ad vector genome. This review concentrates on producer cell lines that complement non-replicating adenoviral vectors, starting with the historical HEK293 cell line developed in 1977 for first generation Ad vectors. In addition the problem of replication-competent adenovirus (RCA) contamination in viral preparations from HEK293 cells is addressed leading to the development of alternate cell lines. Furthermore novel cell lines for more complex Ad vectors and alternate serotype Ad vectors are discussed.

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.

Methods for the production of helper-dependent adenoviral vectors

Helper-dependent adenoviral vectors (HDAd) are deleted of all viral coding sequences and have demonstrated tremendous potential for gene therapy by providing long-term, high-level transgene expression in the absence of chronic toxicity. Thus, HDAd are superior to early generation Ad for gene therapy of genetic diseases where long-term transgene expression is required. This chapter describes in detail the rescue, amplification, and large-scale production of HDAd.

Differential amplification of adenovirus vectors by flanking the packaging signal with attB/attP-PhiC31 sequences: implications for helper-dependent adenovirus production

Current strategies to amplify helper-dependent adenovirus, based on excision of the packaging signal, do not routinely reduce helper adenovirus contamination below 1%. Here, we have tested if reducing the efficiency of the packaging process of the helper adenovirus could impair its packaging without affecting helper-dependent adenovirus production. Interestingly, insertion of attB/attP-PhiC31 sequences flanking the packaging signal significantly lengthens adenovirus cycle up to 60 h without reducing virus viability or production yield. This delay occurs in the absence of PhiC31 recombinase indicating that other mechanisms different from excision of packaging signal must be involved. In addition, at 36 h post-coinfection helper-dependent adenovirus are efficiently produced, while production levels of helper attB/attP-modified adenovirus are 100-1000 times lower than controls. Therefore, these results suggest that attB/attP-mediated packaging impairment of the adenovirus genome is an attractive strategy to significantly reduce helper adenovirus contamination in helper-dependent adenovirus preparations, which in turn would facilitate scaling-up processes for clinical grade preparations.

A convenient plasmid system for construction of helper-dependent adenoviral vectors and its application for analysis of the breast-cancer-specific mammaglobin promoter

BACKGROUND

Helper-dependent (HD) adenovirus (Ad) vectors, deleted of all viral coding sequences, have a higher cloning capacity, improved performance of tissue-specific promoters, and reduced toxicity in animals relative to first-generation Ad vectors, making these vectors promising tools for gene transfer in vitro and in vivo. However, the large size of HDAd precursor plasmids renders them relatively difficult to manipulate due to the paucity of unique restriction enzyme sites suitable for transgene insertion and to the size constraints imposed by the viral packaging machinery.

METHODS

We have constructed a series of HDAd precursor plasmids that allows cassette insertion at a unique site in the vector backbone. We have tested whether these vector backbones will support the tissue-specificity of inserted expression cassettes in a study of the activity of the potentially breast-cancer-specific mammaglobin promoter and enhancer.

RESULTS

We report here the generation of a series of HDAd precursor plasmids, both with and without an additional reporter expression cassette, that were designed to accommodate a wide range in size of inserted DNA. The system was validated for transcriptional targeting studies by demonstrating the tissue-specificity and activity of the mammaglobin promoter rescued using this precursor system. In addition, we have extended our previous studies on the mammaglobin promoter by demonstrating that two copies of the mammaglobin enhancer fused to the minimal promoter surpassed the activity of the single enhancer/promoter by at least 10-fold in breast cancer cells while maintaining only minimal expression in normal cells both in vitro and in a mouse tumor model.

CONCLUSIONS

This versatile plasmid system simplifies the construction of HDAd vectors and was valuable in demonstrating the targeting potential of the mammaglobin promoter for breast cancer gene therapy.

Quantification of high-capacity helper-dependent adenoviral vector genomes in vitro and in vivo, using quantitative TaqMan real-time polymerase chain reaction

First-generation adenoviral (Ad) and high-capacity adenoviral (HC-Ad) vectors are efficient delivery vehicles for transferring therapeutic transgenes in vivo into tissues/organs. The initial successes reported with adenoviral vectors in preclinical trials have been limited by immune-related adverse side effects. This has been, in part, attributed to the use of poorly characterized preparations of adenoviral vectors and also to the untoward immune adverse side effects elicited when high doses of these vectors were used. HC-Ads have several advantages over Ads, including the lack of viral coding sequences, which after infection and uncoating, makes them invisible to the host's immune system. Another advantage is their large cloning capacity (up to approximately 35 kb). However, accurate characterization of HC-Ad vectors, and of contaminating replication-competent adenovirus (RCA) or helper virus, is necessary before these preparations can be used safely in clinical trials. Consequently, the development of accurate, simple, and reproducible methods to standardize and validate adenoviral preparations for the presence of contaminant genomes is required. By using a molecular method that allows accurate, reproducible, and simultaneous determination of HC-Ad, contaminating helper virus, and RCA genome copy numbers based on real-time quantitative PCR, we demonstrate accurate detection of these three genomic entities, within CsCl-purified vector stocks, total DNA isolated from cells transduced in vitro, and from brain tissue infected in vivo. This approach will allow accurate assessment of the levels and biodistribution of HC-Ad and improve the safety and efficacy of clinical trials.

Production and formulation of adenovirus vectors

Adenovirus vectors have attracted considerable interest over the past decade, with ongoing clinical development programs for applications ranging from replacement therapy for protein deficiencies to cancer therapeutics to prophylactic vaccines. Consequently, considerable product, process, analytical, and formulation development has been undertaken to support these programs. For example, "gutless" vectors have been developed in order to improve gene transfer capacity and durability of expression; new cell lines have been developed to minimize recombination events; production conditions have been optimized to improve volumetric productivities; analytical techniques and scaleable purification processes have advanced towards the goal of purified adenovirus becoming a "well-characterized biological"; and liquid formulations have been developed which maintain virus infectivity at 2-8 degrees C for over 18 months. These and other advances in the production of adenovirus vectors are discussed in detail in this review. In addition, the needs for the next decade are highlighted.

Physical and infectious titers of helper-dependent adenoviral vectors: a method of direct comparison to the adenovirus reference material

Accurate measurements of the physical and infectious titers of adenoviral vectors are crucial for evaluating preclinical studies and for the safety and efficacy of clinical studies. Unfortunately, there are no standardized methods of measurement, consequently variable and unreliable values are the result. Furthermore, infectious titers of helper-dependent adenoviral vectors (HDAd) are difficult to measure because traditional cytopathic effect assays cannot be employed, thus hindering their potential clinical application. In response to this problem, a fully characterized Adenovirus Reference Material (ARM) has been developed to be used as a reference standard for clinical grade adenoviral vectors. However, no specific protocols for this purpose have been provided. To fulfill this important need, we have developed a simple assay involving co-infection of 293 cells with the adenoviral vector and the ARM to permit direct comparisons of their physical and infectious titers. We demonstrate, using a HDAd, that this co-infection assay is reliable, sensitive, and reproducible. Importantly, this assay is inherently unaffected by variables that plague other methods of determining vector titers. This assay is applicable to all human serotype 5 adenoviral vectors and will permit reliable comparisons within and between studies as well as meet an important prerequisite for clinical studies.

Helper-Dependent Adenoviral Vectors for Gene Therapy

Helper-dependent adenoviral vectors possess a number of characteristics that make them attractive gene therapy vectors. These vectors are completely devoid of viral coding sequences and are able to mediate high-efficiency transduction in vivo to direct sustain high-level transgene expression with negligible chronic toxicity. This review focuses on advances in helper-dependent adenoviral vector technology, selected examples of in vivo studies of particular interest, and the issue of vector-mediated acute toxicity.

Helper-dependent adenoviral vectors in experimental gene therapy

In the majority of potential applications gene therapy will require an effective transfer of a transgene in vivo resulting in high-level and long-term transgene expression, all in the absence of significant toxicity or inflammatory responses. The most efficient vehicles for delivery of foreign genes to the target tissues are modified adenoviruses. Adenoviral vectors of the first generation, despite the high infection efficacy, have an essential drawback: they induce strong immune response, which leads to short term expression of the transgene, and limits their usefulness in clinical trials. In contrast, helper-dependent adenoviral vectors (HdAd) lacking all viral coding sequences display only minimal immunogenicity and negligible side-effects, allowing for long-term transgene expression. Thus, HdAd vehicles have become the carrier of choice for adenoviral vector-mediated experimental gene therapy, effectively used in animal models for delivery of transgenes into the liver, skeletal muscle, myocardium or brain. Strong and long-lasting expression of therapeutic genes has allowed for successful treatment of dyslipidemias, muscular dystrophy, obesity, hemophilia, and diabetes. Additionally, the large cloning capacity of HdAd, up to 37 kb, facilitates the use of physiologically regulated, endogenous promoters, instead of artificial viral promoter sequences. This enables also generation of the single vectors expressing multiple genes, which can be potentially useful for treatment of polygenic diseases. In this review we characterize the basic features of HdAd vectors and describe some of their experimental and potential clinical applications.

Helper-dependent adenovirus vectors elicit intact innate but attenuated adaptive host immune responses in vivo

Helper-dependent adenovirus (HD-Ad) vectors with all adenoviral genes deleted mediate very long-term expression of therapeutic transgenes in a variety of animal models of disease. These vectors are associated with reduced toxicity and improved safety relative to traditional early region 1 deletion first-generation Ad (FG-Ad) vectors. Many studies have clearly demonstrated that FG-Ad vectors induce innate and adaptive immune responses in vivo; however, a comprehensive analysis of host immune responses to HD-Ad vectors has not yet been performed. In DBA/2 mice, intravenous injection of HD-Ad vectors encoding LacZ (HD-AdLacZ) or a murine secreted alkaline phosphatase (HD-AdSEAP) induced an early expression of inflammatory cytokine and chemokine genes in the liver, including interferon-inducible protein 10, macrophage inflammatory protein 2, and tumor necrosis factor alpha, and were expressed in a pattern similar to that induced by FG-Ad vectors encoding AdSEAP. Like AdSEAP, and consistent with the pattern of cellular gene expression, HD-AdLacZ and HD-AdSEAP induced the recruitment of CD11b-positive leukocytes to the transduced liver within hours of administration. AdSEAP also induced a second phase of liver inflammation, consisting of inflammatory gene expression and CD3-positive lymphocytic infiltrates 7 days posttransduction. In contrast, beyond 24 h no infiltrates or expression of inflammatory genes was detected in the livers of mice receiving HD-AdSEAP. Despite the lack of liver inflammation at 7 days, Ad-specific cytotoxic T lymphocytes could be detected in mice receiving HD-AdSEAP. This lack of liver inflammation was not due to reduced transduction since levels of transgene expression and the amounts of vector DNA in the liver were equivalent in mice receiving HD-AdSEAP and AdSEAP. These results demonstrate that HD-Ad vectors induce intact innate but attenuated adaptive immune responses in vivo.

Development of a size-restricted pIX-deleted helper virus for amplification of helper-dependent adenovirus vectors

Helper-dependent adenovirus vectors (hdAd), which are deleted of all viral protein-coding sequences, can mediate long-term expression of a therapeutic transgene and lead to life-long, phenotypic correction in animal models of genetic disease. Here, we describe a new system for the generation of hdAd, which utilizes the DNA size restrictions imposed on an Ad virion deleted of protein IX (pIX): such virions are reported to package up to only approximately 35 kb of viral DNA. A pIX(-) helper virus (approximately 37.3 kb) was easily grown on complementing 293pIX cells. Upon infection of noncomplementing cells, this virus was not capable of forming infectious virions, but provided replicative and packaging functions for propagation of a 30-kb hdAd. The pIX(-) helper virus was effective in amplifying an hdAd and, in combination with Cre-mediated excision in the viral-packaging signal, resulted in a 1000-fold reduction in helper virus contamination in hdAd stocks compared to Cre/lox alone, as determined by plaque assay. However, through slot blot analysis of DNA isolated from virions, we determined that the ratio of hdAd to helper DNA was 500:1, similar to the ratio observed when using Cre/lox alone. Surprisingly, a large amount of the 37.3-kb helper DNA was being packaged into the pIX-deleted virions, but these virions were incapable of establishing productive infections in plaque assays, for reasons which are still unclear. Nevertheless, the pIX(-) hdAd generated in this system infected cells and expressed a transgene at levels similar to those obtained with a pIX(+) hdAd. These data suggest that, although further studies are necessary to characterize the nature of the defective helper virions formed in this system, deletion of pIX from the helper virus genome does provide an effective method to prevent recovery of functional helper virus during hdAd amplification.

Improved system for helper-dependent adenoviral vector production

Helper-dependent adenoviral vectors (HDAds) are devoid of all viral coding sequences and have demonstrated tremendous potential for gene therapy by providing increased cloning capacity (up to 37 kb) and long-term, high-level transgene expression in vivo with negligible toxicity. Currently, the most widely used method of producing HDAds is the Cre/loxP system developed by Graham and co-workers. However, two major obstacles currently hinder progress of this promising technology: (1) the difficulty of large-scale vector production and (2) helper virus (HV) contamination. We have developed an improved producer cell line, HV, and protocols that have successfully addressed these problems. With this system, >1 x 10(13) viral particles (vp) can be easily produced from 3 liters of cells within 2 weeks of vector rescue, with specific yields of >10,000 vp/cell and with exceedingly low HV contamination of 0.4-0.1% without relying on density-based vector purification and 0.02-0.01% following CsCl purification. This new system represents a major improvement over the original method in terms of simplicity, speed, vector yield, and purity, and it will significantly improve our ability to assess this promising gene therapy technology, especially in large animal models and, ultimately, for clinical applications.

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.

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.

Targeting calcium cycling proteins in heart failure through gene transfer

Our understanding of cardiac excitation-contraction coupling has improved significantly over the last 10 years. Furthermore, defects in the various steps of excitation-contraction coupling that characterize cardiac dysfunction have been identified in human and experimental models of heart failure. The various abnormalities in ionic channels, transporters, kinases and various signalling pathways collectively contribute to the 'failing phenotype.' However, deciphering the causative changes continues to be a challenge. An important tool in dissecting the importance of the various changes in heart failure has been the use of cardiac gene transfer. To achieve effective cardiac gene transfer a number of obstacles remain, including appropriate vectors for gene delivery, appropriate delivery systems, and a better understanding of the biology of the disease. In this review, we will examine our current understanding of these various factors. Gene transfer provides not only a potential therapeutic modality but also an approach to identifying and validating molecular targets.