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

The inflammasome recognizes cytosolic microbial and host DNA and triggers an innate immune response

The innate immune system recognizes nucleic acids during infection and tissue damage. Whereas viral RNA is detected by endosomal toll-like receptors (TLR3, TLR7, TLR8) and cytoplasmic RIG-I and MDA5, endosomal TLR9 and cytoplasmic DAI bind DNA, resulting in the activation of nuclear factor-kappaB and interferon regulatory factor transcription factors. However, viruses also trigger pro-inflammatory responses, which remain poorly defined. Here we show that internalized adenoviral DNA induces maturation of pro-interleukin-1beta in macrophages, which is dependent on NALP3 and ASC, components of the innate cytosolic molecular complex termed the inflammasome. Correspondingly, NALP3- and ASC-deficient mice display reduced innate inflammatory responses to adenovirus particles. Inflammasome activation also occurs as a result of transfected cytosolic bacterial, viral and mammalian (host) DNA, but in this case sensing is dependent on ASC but not NALP3. The DNA-sensing pro-inflammatory pathway functions independently of TLRs and interferon regulatory factors. Thus, in addition to viral and bacterial components or danger signals in general, inflammasomes sense potentially dangerous cytoplasmic DNA, strengthening their central role in innate immunity.

Antigen-specific tolerance of human alpha1-antitrypsin induced by helper-dependent adenovirus

As efficient and less toxic virus-derived gene therapy vectors are developed, a pressing problem is to avoid immune response to the therapeutic gene product. Secreted therapeutic proteins potentially represent a special problem, as they are readily available to professional antigen-presenting cells throughout the body. Some studies suggest that immunity to serum proteins can be avoided in some mouse strains by using tissue-specific promoters. Here we show that expression of human alpha1-antitrypsin (AAT) was nonimmunogenic in the immune-responsive strain C3H/HeJ, when expressed from helper-dependent (HD) vectors using ubiquitous as well as tissue-specific promoters. Coadministration of less immunogenic HD vectors with an immunogenic first-generation vector failed to immunize, suggesting immune suppression rather than immune stealth. Indeed, mice primed with HD vectors were tolerant to immune challenge with hAAT emulsified in complete Freund's adjuvant. Such animals developed high-titer antibodies to coemulsified human serum albumin, showing that tolerance was antigen specific. AAT-specific T cell responses were depressed in tolerized animals, suggesting that tolerance affects both T and B cells. These results are consistent with models of high-dose tolerance of B cells and certain other suppressive mechanisms, and suggest that a high level of expression from HD vectors can be sufficient to induce specific immune tolerance to serum proteins.

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.

Flt3L and TK gene therapy eradicate multifocal glioma in a syngeneic glioblastoma model

The disseminated characteristics of human glioblastoma multiforme (GBM) make it a particularly difficult tumor to treat with long-term efficacy. Most preclinical models of GBM involve treatment of a single tumor mass. For therapeutic outcomes to translate from the preclinical to the clinical setting, induction of an antitumor response capable of eliminating multifocal disease is essential. We tested the hypothesis that expression of Flt3L (human soluble FMS-like tyrosine kinase 3 ligand) and TK (herpes simplex virus type 1-thymidine kinase) within brain gliomas would mediate regression of the primary, treated tumor mass and a secondary, untreated tumor growing at a distant site from the primary tumor and the site of therapeutic vector injection. In both the single-GBM and multifocal-GBM models used, all saline-treated control animals succumbed to tumors by day 22. Around 70% of the animals bearing a single GBM mass treated with an adenovirus expressing Flt3L (AdFlt3L) and an adenovirus expressing TK (AdTK + GCV) survived long term. Approximately 50% of animals bearing a large primary GBM that were implanted with a second GBM in the contralateral hemisphere at the same time the primary tumors were being treated with AdFlt3L and AdTK also survived long term. A second multifocal GBM model, in which bilateral GBMs were implanted simultaneously and only the right tumor mass was treated with AdFlt3L and AdTK, also demonstrated long-term survival. While no significant difference in survival was found between unifocal and multifocal GBM-bearing animals treated with AdFlt3L and AdTK, both treatments were statistically different from the saline-treated control group (p < 0.05). Our results demonstrate that combination therapy with AdFlt3L and AdTK can eradicate multifocal brain tumor disease in a syngeneic, intracranial GBM model.

Helper-dependent adenovirus for the gene therapy of proliferative retinopathies: stable gene transfer, regulated gene expression and therapeutic efficacy

BACKGROUND

Ocular neovascular disorders, such as diabetic retinopathy and age-related macular degeneration, are the principal causes of blindness in developed countries. Current treatments are of limited efficacy, whereas a therapy based on intraocular gene transfer of angiostatic factors represents a promising alternative. For the first time we have explored the potential of helper-dependent adenovirus (HD-Ad), the last generation of Ad vectors, in the therapy of retinal neovascularization.

METHODS

We first analyzed efficiency and stability of intraretinal gene transfer following intravitreous injection in mice. A HD-Ad vector expressing green fluorescent protein (GFP) under the control of the cytomegalovirus (CMV) promoter (HD-Ad/GFP) was compared with a first-generation (E1/E3-deleted) Ad vector carrying an identical GFP expression cassette (FG-Ad/GFP). We also constructed HD-Ad vectors expressing a soluble form of the VEGF receptor (sFlt-1) in a constitutive (HD-Ad/sFlt-1) or doxycycline (dox)-inducible (HD-Ad/S-M2/sFlt-1) manner and tested their therapeutic efficacy upon intravitreous delivery in a rat model of oxygen-induced retinopathy (OIR).

RESULTS

HD-Ad/GFP promoted long-lasting (up to 1 year) transgene expression in retinal Müller cells, in marked contrast with the short-term expression observed with FG-Ad/GFP. Intravitreous injection of HD-Ad vectors expressing sFlt-1 resulted in detectable levels of sFlt-1 and inhibited retinal neovascularization by more than 60% in a rat model of OIR. Notably, the therapeutic efficacy of the inducible vector HD-Ad/S-M2/sFlt-1 was strictly dox-dependent.

CONCLUSIONS

HD-Ad vectors enable stable gene transfer and regulated expression of angiostatic factors following intravitreous injection and thus are attractive vehicles for the gene therapy of neovascular diseases of the retina.

One-year expression from high-capacity adenoviral vectors in the brains of animals with pre-existing anti-adenoviral immunity: clinical implications

The main challenge of gene therapy is to provide long-term, efficient transgene expression. Long-term transgene expression from first generation adenoviral vectors (Advs) delivered to the central nervous system (CNS) is elicited in animals not previously exposed to adenovirus (Ad). However, upon systemic immunization against Ad, transgene expression from a first generation Adv is abolished. High-capacity Advs (HC-Advs) provide sustained very long-term transgene expression in the brain, even in animals pre-immunized against Ad. In this study, we tested the hypothesis that a HC-Adv in the brain would allow for long-term transgene expression, for up to 1 year, in the brain of mice immunized against Ad prior to delivery of the vector to the striatum. In naïve animals, the expression of beta-galactosidase from Adv or HC-Adv was sustained for 1 year. In animals immunized prior to vector delivery, expression from a first generation Adv was abolished. These results point to a very long-term HC-Adv-mediated transgene expression in the brain, even in animals that had been immunized systemically against Ad before the delivery of HC-Adv into the brain. This study therefore indicates the utility of HC-Adv as a powerful gene therapy vector for chronic neurological disorders, even in patients who had been pre-exposed to Ad prior to gene therapy.

Early STAT1 activation after systemic delivery of HSV amplicon vectors suppresses transcription of the vector-encoded transgene

The herpes simplex virus (HSV) amplicon vector is a powerful gene delivery vehicle that can accommodate up to 150 kilobase of exogenous DNA. However, amplicon-mediated transgene expression is often transient outside the nervous system. In order to define the role of host immune responses in silencing amplicon-encoded transgenes, we evaluated the kinetics of cytokine-/chemokine-expression after tail-vein injection of a luciferase-encoding amplicon into mice. Type I interferons (IFNs) were induced earliest, within an hour after injection, and several other cytokines/chemokines were subsequently upregulated in the livers of wild-type (WT) mice. When the same experiment was performed in signal transducers and activators of transcription 1 (STAT1)-knockout (KO) mice, the levels of type I IFN expression were significantly lower and chemokine induction was almost non-existent. Importantly, STAT1-KO mice exhibited significantly higher and more sustained luciferase activity than did the WT mice, which is attributable to increased transcriptional activity rather than increased copy numbers of luciferase-encoding vector DNA. Further studies using primary cultured fibroblasts derived from WT and STAT1-KO mice revealed the significance of STAT1 signaling in transcriptional silencing of the amplicon-encoded transgene in vitro. Our results indicate that type I IFNs induced by systemic delivery of HSV amplicon vectors initiate a cascade of immune responses and suppress transgene expression at the transcriptional level by activation of STAT1.

Current advances and future challenges in Adenoviral vector biology and targeting

Gene delivery vectors based on Adenoviral (Ad) vectors have enormous potential for the treatment of both hereditary and acquired disease. Detailed structural analysis of the Ad virion, combined with functional studies has broadened our knowledge of the structure/function relationships between Ad vectors and host cells/tissues and substantial achievement has been made towards a thorough understanding of the biology of Ad vectors. The widespread use of Ad vectors for clinical gene therapy is compromised by their inherent immunogenicity. The generation of safer and more effective Ad vectors, targeted to the site of disease, has therefore become a great ambition in the field of Ad vector development. This review provides a synopsis of the structure/function relationships between Ad vectors and host systems and summarizes the many innovative approaches towards achieving Ad vector targeting.

Adenovirus vector delivery stimulates natural killer cell recognition

We report that delivery of first-generation replication-deficient adenovirus (RDAd) vectors into primary human fibroblasts is associated with the induction of natural killer (NK) cell-mediated cytolysis in vitro. RDAd vector delivery induced cytolysis by a range of NK cell populations including the NK cell clone NKL, primary polyclonal NK lines and a proportion of NK clones (36 %) in autologous HLA-matched assays. Adenovirus-induced cytolysis was inhibited by antibody blocking of the NK-activating receptor NKG2D, implicating this receptor in this function. NKG2D is ubiquitously expressed on NK cells and CD8⁺ T cells. Significantly, γ-irradiation of the vector eliminated the effect, suggesting that breakthrough expression from the vector induces at least some of the pro-inflammatory responses of unknown aetiology following the application of RDAd vectors during in vivo gene delivery.

Pseudo-hydrodynamic delivery of helper-dependent adenoviral vectors into non-human primates for liver-directed gene therapy

Helper-dependent adenoviral vectors (HDAds) are attractive for liver-directed gene therapy because they can mediate long-term, high-level transgene expression without chronic toxicity. However, systemic delivery requires high vector doses for efficient hepatic transduction, resulting in dose-dependent acute toxicity. Clearly, strategies to improve hepatic transduction with low vector doses are needed. In this regard, we have previously shown that hydrodynamic injection of helper-dependent adenoviral vectors into mice results in increased hepatic transduction, reduced systemic vector dissemination, and reduced pro-inflammatory cytokines compared with conventional injection and thus has the potential to improve dramatically the therapeutic index of helper-dependent adenoviral vectors. Unfortunately, the rapid, large-volume injection used in this method cannot be applied to larger animals. Therefore, we have developed a novel balloon occlusion catheter-based method to mimic hydrodynamic injection of helper-dependent adenoviral vectors into non-human primates that does not require rapid, large-volume injection. Using a low, clinically relevant vector dose, this minimally invasive method results in high-efficiency hepatic transduction with minimal toxicity and stable long-term transgene expression for at least 413 days.

Toll-like Receptor 9 Triggers an Innate Immune Response to Helper-dependent Adenoviral Vectors

A major obstacle to the clinical application of systemic adenoviral gene replacement therapy is the host innate immune response. Although recent studies have attempted to characterize the cellular basis for this response to systemically administered helper-dependent adenoviral vector (HD-Ad), the underlying molecular components of the innate immune repertoire required to recognize the viral vector have yet to be identified. Here, we show that primary macrophages can sense HD-Ad vectors via the Toll-like Receptor 9 (TLR9) and respond by increasing pro-inflammatory cytokine secretion. Moreover, TLR9 sensing is involved in the rapid innate immune response to HD-Ad in vivo. TLR9 deficiency attenuates the innate immune response to HD-Ad, whereas TLR9 blockade reduces the acute inflammatory response after intravenous injection of the vector. Moreover, HD-Ad upregulates TLR9 gene expression independent of TLR9 function, suggesting that additional innate signaling pathways work cooperatively with TLR9. The identification of the components of the innate immune response to adenovirus will facilitate the development of combinatorial therapy directed at increasing the maximal tolerated dose of systemically delivered adenoviral vectors.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Innate immune response to adenoviral vectors is mediated by both Toll-like receptor-dependent and -independent pathways

Recombinant adenoviral vectors have been widely used for gene therapy applications and as vaccine vehicles for treating infectious diseases such as human immunodeficiency virus disease. The innate immune response to adenoviruses represents the most significant hurdle in clinical application of adenoviral vectors for gene therapy, but it is an attractive feature for vaccine development. How adenovirus activates innate immunity remains largely unknown. Here we showed that adenovirus elicited innate immune response through the induction of high levels of type I interferons (IFNs) by both plasmacytoid dendritic cells (pDCs) and non-pDCs such as conventional DCs and macrophages. The innate immune recognition of adenovirus by pDCs was mediated by Toll-like receptor 9 (TLR9) and was dependent on MyD88, whereas that by non-pDCs was TLR independent through cytosolic sensing of adenoviral DNA. Furthermore, type I IFNs were pivotal in innate and adaptive immune responses to adenovirus in vivo, and type I IFN blockade diminished immune responses, resulting in more stable transgene expression and reduction of inflammation. These findings indicate that adenovirus activates innate immunity by its DNA through TLR-dependent and -independent pathways in a cell type-specific fashion, and they highlight a critical role for type I IFNs in innate and adaptive immune responses to adenoviral vectors. Our results that suggest strategies to interfere with type I IFN pathway may improve the outcome of adenovirus-mediated gene therapy, whereas approaches to activate the type I IFN pathway may enhance vaccine potency.

Sustained phenotypic correction in a mouse model of hypoalphalipoproteinemia with a helper-dependent adenovirus vector

We examined the efficacy and host response to the adenovirus (Ad)-mediated delivery of human apolipoprotein A-I (APOA1) gene to the liver of APOA1(-/-) mice. Administration of a first-generation vector (FGAd-AI) resulted in a transient appearance of APOA1 in plasma and induced an anti-APOA1 antibody titer, whereas treatment with a helper-dependent vector (HDAd-AI) resulted in sustained APOA1 expression without inducing an antibody titer. With these results, we studied the effects of FGAd vectors on APOAI expression by HDAd-AI vector. Co-treatment with an FGAd vector inhibited HDAd-AI- mediated APOA1 expression independent of transgene cassettes, but only FGAd-AI induced a humoral response. Furthermore, APOA1 mRNA levels in mice co-treated with FGAd vectors were much lower than those expected from the vector copy number, suggesting that DNA of FGAd vectors interferes with the HDAd-AI vector's APOA1 promoter. A single treatment with an HDAd-AI vector produced a supraphysiological plasma APOA1 level that gradually declined to about half the normal human level over the course of 2 years, associated with a plasma cholesterol level that is persistently higher than that in controls. This investigation provides the proof of principle that liver-directed HDAd gene delivery is effective for the long-term phenotypic correction of monogenic hypoalphalipoproteinemia.

Interaction of adenovirus type 5 fiber with the coxsackievirus and adenovirus receptor activates inflammatory response in human respiratory cells

The innate immune response to adenovirus (Ad)-derived gene transfer vectors has been shown to initiate immediately after interaction of Ad with respiratory epithelial cells, through the induction of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and JNK mitogen-activated protein kinase (MAPK), nuclear factor kappaB (NF-kappaB), and different proinflammatory genes. Ad serotypes 2 or 5 (Ad2/5) enter respiratory epithelia after initial binding of fiber with the coxsackievirus-adenovirus receptor (CAR) or, alternatively, with cell surface heparan sulfate glycosaminoglycans. Ad2/5 internalization is triggered by binding of penton base to cellular RGD-binding integrins. Here we investigated the role of the Ad5 surface domain proteins constituting the vector capsid, namely, the fiber, the penton base, and the hexon, on the transmembrane signals leading to the transcription of the different proinflammatory genes in the human respiratory A549 cell line. Interaction of Ad fiber with CAR activates both ERK1/2 and JNK MAPK and the nuclear translocation of NF-kappaB, whereas no activation was observed after exposing A549 cells to penton base and hexon proteins. Moreover, interaction of Ad fiber with CAR, but not heparan sulfate proteoglycans, promotes transcription of the chemokines interleukin-8, GRO-alpha, GRO-gamma, RANTES, and interferon-inducible protein 10. These results identify the binding of Ad5 fiber with the cellular CAR as a key proinflammatory activation event in epithelial respiratory cells that is independent of the transcription of Ad5 genes.

Intrahepatic injection of adenovirus reduces inflammation and increases gene transfer and therapeutic effect in mice

Recombinant adenoviruses (Ad) are among the most extensively used vectors for liver gene transfer. One of the major limitations for the clinical application of these vectors is the inflammatory immune response associated with systemic administration of high dose of virus. We evaluated the effect of Ad administration route on the inflammatory immune response and liver transgene expression. We compared direct intrahepatic injection (IH) with the systemic administration via tail vein (IV). IH injection of Ad resulted in a lower inflammatory response and a higher transgene expression. When a relatively low dose of virus was used, IV administration resulted in no detectable protein expression but production of proinflammatory cytokines. In contrast, IH administration induced high levels of transgene expression and no inflammation, although we detected a transient hypertransaminemia, which fully resolved within days. Furthermore, IH injection resulted in a faster protein expression being more intense at the site of injection, whereas IV administration caused slower but diffuse liver expression. IH injection also reduced the spreading of the virus to other organs. Independently of the route, depletion of Kupffer cells significantly enhanced the transduction efficiency of Ad. This effect was stronger when using IV injection, indicating that IH injection partially overcomes Kupffer cell phagocytic activity. Moreover, the antitumor efficacy of high-capacity-Ad encoding murine interleukin-12 (IL-12) was significantly greater when the vector was administered by IH injection than when given IV. In conclusion, IH injection of adenovirus represents a safe and efficient administration route for clinical applications of gene therapy targeting the liver.

Improved hepatic transduction, reduced systemic vector dissemination, and long-term transgene expression by delivering helper-dependent adenoviral vectors into the surgically isolated liver of nonhuman primates

Helper-dependent adenoviral vectors (HDAds) are attractive vectors for liver-directed gene therapy because they can mediate sustained, high-level transgene expression without chronic toxicity. However, high vector doses are required to achieve efficient hepatic transduction by systemic delivery because of a nonlinear dose response. Unfortunately, such high doses result in systemic vector dissemination and dose-dependent acute toxicity with potentially severe and lethal consequences. We hypothesize that the threshold to efficient hepatic transduction may be circumvented by delivering the vector into the surgically isolated liver via the portal vein. Total hepatic isolation was achieved by occluding hepatic inflow from the portal vein and hepatic artery and by occluding hepatic venous outflow at the inferior vena cava. We demonstrate in nonhuman primates that this approach resulted in significantly higher efficiency hepatic transduction with reduced systemic vector dissemination compared with systemic intravascular delivery. This method of delivery was associated with transient acute toxicity, the severity of which was variable. Importantly, stable, high levels of transgene expression were obtained for at least 665 days for one baboon and for at least 560 days for two baboons with no evidence of long-term toxicity.

Adenovirus: from foe to friend

Human adenoviruses (HAdVs) can cause mild respiratory, gastrointestinal, urogenital and ocular disease. Knowledge about HAdVs has been expanding for more than five decades putting them amongst the most-studied viruses. This continued interest stems, to a great extent, from the fact that these double-stranded DNA viruses have proven to be a versatile tool to probe the basic phenomena of eukaryotic cells. HAdV research has led to the discovery of, for instance, RNA splicing and greatly contributed to our knowledge of processes as fundamental as replication, transcription and translation. Moreover, the transformation of rodent cells by HAdVs has provided a system to unravel the molecular pathways that control cell proliferation. As a result, the genetic organisation of these agents is known in great detail allowing the straightforward manipulation of their genomes. In addition, the virus itself became renowned for its ability to produce large amounts of progeny and to efficiently infect mammalian cells regardless of their cell cycle status. These features contributed to the broad use of recombinant HAdVs as gene carriers particularly in in vivo settings where the vast majority of target cells are post-mitotic. The most advanced type of HAdV vectors can accommodate up to 37 kb of foreign DNA and are devoid of viral genes. With the aid of these high-capacity HAdV vectors large physiologically responsive transcriptional elements and/or genes can be efficiently introduced into target cells while minimising adaptive immune responses against the transduced cells. This article provides information on HAdV especially on the aspects pertinent to the design, production and performance of its recombinant forms. The development and characteristics of the main HAdV-based vector types are also briefly reviewed.

Modulation of TNFalpha, a determinant of acute toxicity associated with systemic delivery of first-generation and helper-dependent adenoviral vectors

Understanding the determinants of the host innate immune response to systemic administration of adenoviral (Ad) vectors is critical for clinical gene therapy. Acute toxicity occurs within minutes to hours after vector administration and is characterized by activation of innate immune responses. Our data indicate that in mice, indicators of vector toxicity include elevations of cytokine levels, liver transaminase levels and thrombocytopenia. To discern potential targets for blunting this host response, we evaluated genetic factors in the host response to systemically administered first-generation Ad vectors (FGV) and helper-dependent Ad vectors (HDV) containing beta-galactosidase expression cassettes. A preliminary screen for modulation of vector-induced thrombocytopenia revealed no role for interferon-gamma, mast cells or perforin. However, vector-induced thrombocytopenia and interleukin 6 (IL-6) expression are less evident in tumor necrosis factor alpha (TNFalpha)-deficient mice. Moreover, we also demonstrated that TNFalpha blockade via antibody or huTNFR:Fc pretreatment attenuates both thrombocytopenia (>40% increase in platelet count) and IL-6 expression (>80% reduction) without affecting interleukin 12 , liver enzymes, hematological indices or vector transduction in a murine model. Our data indicate that the use of HDV, in combination with clinically approved TNFalpha immunomodulation, may represent an approach for improving the therapeutic index of Ad gene therapy for human clinical trials.

Insertional mutagenesis at positions 520 and 584 of adeno-associated virus type 2 (AAV2) capsid gene and generation of AAV2 vectors with eliminated heparin- binding ability and introduced novel tropism

Recombinant adeno-associated virus (AAV) vectors are promising in the context of gene therapy because of their ability to mediate efficient gene transfer and stable gene expression. AAV2 uses heparin sulfate as its primary receptor, which is widely expressed on the various tissues and organs. This limits the application of AAV2 in targeting specific tissues. To make an AAV2 vector with modified tropism, we constructed various AAV2 capsid mutants by inserting RGD-4C peptide at position 520 and/or at position 584. Eight mutants were generated, identified, and characterized. Heparin-binding ability was completely abrogated in five mutants, and partially reduced in three mutants. Solid-phase ELISA and gene transduction assays confirmed that the novel tropism is determined by the introduced RGD epitope, which binds to cellular integrin receptor. Our observations suggest that simultaneous modification at both sites, tentatively involved in heparin binding, results in altered tropism and improved transduction efficiency in vitro.

From pathogen to medicine: HIV-1-derived lentiviral vectors as vehicles for dendritic cell based cancer immunotherapy

Over the years, the unique capacity of dendritic cells (DC) for efficient activation of naive T cells has led to their extensive use in cancer immunotherapy protocols. In order to be able to fulfil their role as antigen-presenting cells, the antigen of interest needs to be efficiently introduced and subsequently correctly processed and presented by the DC. For this purpose, a variety of both viral and non-viral antigen-delivery systems have been evaluated. Amongst those, HIV-1-derived lentiviral vectors have been used successfully to transduce DC. This review considers the use of HIV-1-derived lentiviral vectors to transduce human and murine DC for cancer immunotherapy. Lentivirally transduced DC have been shown to present antigenic peptides, prime transgene-specific T cells in vitro and elicit a protective cytotoxic T-lymphocyte (CTL) response in animal models. Different parameters determining the efficacy of transduction are considered. The influence of lentiviral transduction on the DC phenotype and function is described and the induction of immune responses by lentivirally transduced DC in vitro and in vivo is discussed in detail. In addition, direct in vivo administration of lentiviral vectors aiming at the induction of antigen-specific immunity is reviewed. This strategy might overcome the need for ex vivo generation and antigen loading of DC. Finally, future perspectives towards the use of lentiviral vectors in cancer immunotherapy are presented.

Lentiviral Vectors for Use in the Central Nervous System

Lentiviral vectors have been used extensively as gene transfer tools for the central nervous system throughout the past decade since they transduce most cell types in the brain, resulting in high-level and long-term transgene expression. This review discusses some of the recent progress in this field, including preclinical gene therapy experiments in disease models, development of regulated vectors, and the application of siRNA's using lentiviral vectors. We also describe some of the features that make lentiviral vectors a likely candidate for human gene therapy in the brain.

Canine adenovirus vectors for lung-directed gene transfer: efficacy, immune response, and duration of transgene expression using helper-dependent vectors

A major hurdle to the successful clinical use of some viral vectors relates to the innate, adaptive, and memory immune responses that limit the efficiency and duration of transgene expression. Some of these drawbacks may be circumvented by using vectors derived from nonhuman viruses such as canine adenovirus type 2 (CAV-2). Here, we evaluated the potential of CAV-2 vectors for gene transfer to the respiratory tract. We found that CAV-2 transduction was efficient in vivo in the mouse respiratory tract, and ex vivo in well-differentiated human pulmonary epithelia. Notably, the in vivo and ex vivo efficiency was poorly inhibited by sera from mice immunized with a human adenovirus type 5 (HAd5, a ubiquitous human pathogen) vector or by human sera containing HAd5 neutralizing antibodies. Following intranasal instillation in mice, CAV-2 vectors also led to a lower level of inflammatory cytokine secretion and cellular infiltration compared to HAd5 vectors. Moreover, CAV-2 transduction efficiency was increased in vitro in human pulmonary cells and in vivo in the mouse respiratory tract by FK228, a histone deacetylase inhibitor. Finally, by using a helper-dependent CAV-2 vector, we increased the in vivo duration of transgene expression to at least 3 months in immunocompetent mice without immunosuppression. Our data suggest that CAV-2 vectors may be efficient and safe tools for long-term clinical gene transfer to the respiratory tract.

Gutless adenovirus: last-generation adenovirus for gene therapy

Last-generation adenovirus vectors, also called helper-dependent or gutless adenovirus, are very attractive for gene therapy because the associated in vivo immune response is highly reduced compared to first- and second-generation adenovirus vectors, while maintaining high transduction efficiency and tropism. Nowadays, gutless adenovirus is administered in different organs, such as the liver, muscle or the central nervous system achieving high-level and long-term transgene expression in rodents and primates. However, as devoid of all viral coding regions, gutless vectors require viral proteins supplied in trans by a helper virus. To remove contamination by a helper virus from the final preparation, different systems based on the excision of the helper-packaging signal have been generated. Among them, Cre-loxP system is mostly used, although contamination levels still are 0.1-1% too high to be used in clinical trials. Recently developed strategies to avoid/reduce helper contamination were reviewed.

The efficacy and safety of gene transfer into the porcine liver in vivo by HVJ (Sendai virus) liposome

BACKGROUND

Gene transfer systems using viral vectors are efficient; however, most viral vectors also tend to evoke immunologic reactions, thereby clinically causing serial side effects. HVJ-liposome vector is a hybrid vector consisting of liposome and an inactivated Sendai virus (Hemmagglutinating Virus of Japan [HVJ]), which has been reported to be less immunogenic and can also be repeatedly administered. We examined the usefulness of this vector for hepatic gene therapy in a pig model.

METHODS

Genes encoding beta-galactosidase and luciferase were used as reporter genes. The pigs were injected with the reporter gene loaded-HVJ-liposome into the portal vein under total vascular exclusion of the liver. The transfection efficiencies were then assessed by beta-galactosidase staining, a luciferase assay, and RT-PCR for LacZ mRNA. Biochemical and histologic analyses were performed to evaluate tissue toxicity after gene transfer.

RESULTS

The luciferase gene expression in the liver reached its highest level at 7 days after transfection. It continued to be detected up to 28 days after transfection, while all pigs remained healthy throughout the observation period. The transfection efficiency was 15% in the hepatocytes according to beta-galactosidase staining. Extrahepatic transgene expression was slightly observed in the lung and kidney, but not in the spleen or ovary.

CONCLUSIONS

These data suggest for the first time that the use of the HVJ-liposome vector is a safe and feasible modality for liver-directed gene transfer in pigs, and it might therefore be suitable for clinical gene therapy trials.

Readministration of helper-dependent adenovirus to mouse lung

Adenovirus vectors (Ad) are widely used in gene therapy studies, including those aimed at treating cystic fibrosis lung disease. Various approaches have been investigated to blunt the host immune response to Ad, including development of helper-dependent (HD) Ad. The host cytotoxic T-cell response to HD-Ad is generally lower than to earlier-generation Ad. However, antibodies are formed which could inhibit the efficacy of HD-Ad readministration. In this first study of HD-Ad readministration to the lung, we found that a second administration of HD-Ad to mice was possible with minimal loss of transgene expression. In contrast, when first-generation (FG) Ad was administered initially, followed by HD-Ad or FG-Ad, transgene expression was reduced. Significantly lower concentrations of antibodies against Ad were found in lung lavage fluid and serum from mice that received two doses of HD-Ad (when the initial HD-Ad lacked a transgene), compared to mice that received FG-Ad followed by HD-Ad. These data suggest that readministration of HD-Ad for lung gene therapy may be feasible.

Frequency, proliferation, and activation of human memory T cells induced by a nonhuman adenovirus

Multiple human adenovirus (HAd) infections during childhood generate a memory T-cell (T(M)) response, which is the primary defense against HAd-induced morbidity. This cellular memory creates a conundrum for the potential clinical use of HAd-derived vectors: vector-mediated gene transfer is efficient in immunologically naïve mammals but will be compromised by memory immunity when using vectors derived from ubiquitous human pathogens. The potential lack of cellular and humoral memory is one reason we developed vectors from canine adenovirus serotype 2 (CAV-2). Here, we assayed human peripheral blood mononuclear cells for a T(M) response that could be stimulated by CAV-2 virion and individual capsid proteins. We found that less than half of the donors harbored a proliferating T(M) response directed against the CAV-2 virion (versus >85% against HAd5) in spite of a conserved antigenic Adenoviridae epitope in the CAV-2 hexon. When CAV-2 induced proliferation, it was 2.3- to >10-fold lower than HAd5 depending on the assay. The primary proliferating cells appeared to be memory (CD45RO+) CD4+ lymphocytes, differentiated into Th1 gamma interferon-producing cells, with a frequency that was up to 66-fold lower than that obtained for HAd5. We also compared CAV-2 to prototype HAd from five of the six human species and found that CAV-2-induced cellular proliferation was similar to that found with rare HAd serotypes. Individual CAV-2 capsid proteins also induced less proliferation than their HAd5 homologues. Our data suggest that CAV-2 vectors may be safer (i.e., less immunogenic) for gene transfer but are not without a theoretical risk in a subset of potential patients.

Akt/protein kinase B activation by adenovirus vectors contributes to NFkappaB-dependent CXCL10 expression

In gene therapy, the innate immune system is a significant barrier to the effective application of adenovirus (Ad) vectors. In kidney epithelium-derived (REC) cells, serotype 5 Ad vectors induce the expression of the chemokine CXCL10 (IP-10), a response that is dependent on NFkappaB. Compared to the parental vector AdLuc, transduction with the RGD-deleted vector AdL.PB resulted in reduced CXCL10 activation despite increasing titers, implying that RGD-alpha(V) integrin interactions contribute to adenovirus induction of inflammatory genes. Akt, a downstream effector of integrin signaling, was activated within 10 min of transduction with Ad vectors in a dose-dependent manner. Akt activation was not present following transduction with AdL.PB, confirming the importance of capsid-alpha(V) integrin interactions in Ad vector Akt activation. Inhibition of the phosphoinositide-3-OH kinase/Akt pathway by Wortmannin or Ly294002 compounds decreased Ad vector induction of CXCL10 mRNA. Similarly, adenovirus-mediated overexpression of the dominant negative AktAAA decreased CXCL10 mRNA expression compared to the reporter vector AdLacZ alone. The effect of Akt on CXCL10 mRNA expression occurred via NFkappaB-dependent transcriptional activation, since AktAAA overexpression and Ly294002 both inhibited CXCL10 and NFkappaB promoter activation in luciferase reporter experiments. These results show that Akt plays a role in the Ad vector activation of NFkappaB and CXCL10 expression. Understanding the mechanism underlying the regulation of host immunomodulatory genes by adenovirus vectors will lead to strategies that will improve the efficacy and safety of these agents for clinical use.

Sustained phenotypic correction of canine hemophilia B after systemic administration of helper-dependent adenoviral vector

We have evaluated the potential of liver-directed, helper-dependent adenoviral (HDAd) vector-mediated gene therapy in the hemophilia B dog. Two dogs were injected intravenously with HDAd (3 x 10(12) VP/kg) bearing a liver-restricted canine coagulation factor IX (FIX) expression cassette. After injection, the whole blood clotting time for both dogs declined from >60 min to </=20 min for at least 604 and 446 days, respectively. Peak FIX activities of 34.1 and 129.2% were detected at 12x14 days and then slowly declined to 2 to 5% by 120 days and stabilized at these therapeutic levels for at least 418 and 257 days. For one dog, a peak FIX level of 500 ng/ml was achieved and stabilized at >170 ng/ml for at least 256 days. For the other dog, a peak FIX level of 1258 ng/ml was achieved and stabilized at >400 ng/ml for at least 213 days. Inhibitor formation was not evident in either animal. Importantly, whereas untreated hemophilia B dogs suffer five or six spontaneous bleeds per year, the treated dogs suffered no such bleeds postinjection. Significantly, this study is the first to demonstrate long-term phenotypic correction of a genetic disorder in a large animal with HDAd. Although no evidence of chronic toxicity was observed in either animal, systemic vector administration at 3 x 10(12) VP/kg was accompanied by acute, albeit transient and variable laboratory abnormalities (alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, creatine phosphokinase, and platelet counts). The results of this study highlight both the potential benefit and the risk associated with systemic intravascular delivery of high-dose HDAd for liver-directed gene therapy.

Use of a murine secreted alkaline phosphatase as a non-immunogenic reporter gene in mice

BACKGROUND

The development of any vector system as a gene delivery system requires its optimization in vitro and in vivo. Preliminary studies frequently involve the use of a reporter gene, which allows for the rapid and simple assay of vector function through monitoring expression levels of the reporter gene. However, evaluation of vector efficacy can be compromised by immune responses directed against immunogenic reporter proteins.

METHODS

We have cloned a murine secreted alkaline phosphatase (mSEAP), and explored its use as a reporter gene in the context of an early region 1 (E1)-deleted adenovirus (Ad) vector. Studies involved characterization of gene expression in vitro and in vivo, and immunological responses after gene delivery to mice.

RESULTS

In tissue culture, we show that mSEAP is easily measured quantitatively using a sensitive, commercially available chemiluminescent assay, or visualized directly using histological staining. The level of transgene expression from AdmSEAP was similar to that observed for an Ad vector encoding the human placental secreted alkaline phosphatase (hSEAP). After intravenous administration in mice, AdmSEAP continued to express at high levels for the duration of the experiment (1 month), whereas expression from AdhSEAP declined to background levels over the course of the experiment. Although cytotoxic T-lymphocytes were not detected against either the murine or human SEAP proteins in mice, antibodies were readily detected against the human protein. No antibodies were detected to mSEAP.

CONCLUSIONS

Taken together, these data illustrate that mSEAP is a sensitive, non-immunogenic reporter gene for preclinical mouse studies.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

Primary adenovirus-specific cytotoxic T lymphocyte response occurs after viral clearance and liver enzyme elevation

The virus-specific cytotoxic T lymphocyte (CTL) response is a major obstacle to effective delivery of adenovirus gene therapy. However, its relative role in viral clearance, transgene elimination and hepatotoxicity remains unclear. In this paper, we present an analysis of viral clearance and liver toxicity in relation to the induction of the virus-specific CD8 T-cell response revealed by an MHC class I tetramer. A surprisingly high number of tetramer+ CD8 T cells were found in the liver and lung and reached peak values at days 8 and 10, respectively, post-infection. Nearly 100% of these tetramer+ CD8 T cells expressed high levels of granzyme B and IFNgamma. Remarkably, liver viral load and liver enzyme elevation peaked early, at days 2 and 4, respectively, post-infection, before the specific CTL response was detectable. After generation of CTLs, there was only minimal liver damage or further decrease in virus titer. These results indicated that the primary peak response of tetramer+ CTLs does not correlate with the elimination of adenovirus or liver cytotoxic response.

Gene therapy: twenty-first century medicine

Broadly defined, the concept of gene therapy involves the transfer of genetic material into a cell, tissue, or whole organ, with the goal of curing a disease or at least improving the clinical status of a patient. A key factor in the success of gene therapy is the development of delivery systems that are capable of efficient gene transfer in a variety of tissues, without causing any associated pathogenic effects. Vectors based upon many different viral systems, including retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses, currently offer the best choice for efficient gene delivery. Their performance and pathogenicity has been evaluated in animal models, and encouraging results form the basis for clinical trials to treat genetic disorders and acquired diseases. Despite some initial success in these trials, vector development remains a seminal concern for improved gene therapy technologies.

Rapid Kupffer cell death after intravenous injection of adenovirus vectors

When adenovirus vectors are injected intravenously, they are quickly taken up by Kupffer cells in the liver. We report that this causes rapid necrosis of Kupffer cells in mice at doses of 10(11) particles/kg or higher. By 10 min after intravenous vector injection, Kupffer cells were permeable to propidium iodide and trypan blue. This coincided with a sharp rise in serum lactate dehydrogenase. Ultrastructural examination showed degeneration of Kupffer cells, including complete disappearance of chromatin by 1 h. After an initial intravenous injection of vector, dead Kupffer cells were unable to take up a second dose of vector, and hepatic transgene expression from the second dose was augmented. Death of Kupffer cells did not affect serum levels of IL-6 or IL-12. There was no immediate change in the number of Kupffer cells in the liver, but a significant decline was found by 4 h after injection of vector. Interestingly, substantial numbers of vector-containing Kupffer cells were found in pulmonary capillaries, indicating that they had been swept out of the liver. Together these results show that an intravenous injection of adenovirus vector causes synchronous and surprisingly rapid Kupffer cell death.

Evaluation of biodistribution and safety of adenovirus vectors containing group B fibers after intravenous injection into baboons

Vectors containing group B adenovirus (Ad) fibers are able to efficiently transduce gene therapy targets that are refractory to infection with standard Ad serotype 5 (Ad5) vectors, including malignant tumor cells, hematopoietic stem cells, and dendritic cells. Preliminary studies in mice indicate that, after intravenous injection, B-group fiber-containing Ads do not efficiently transduce most organs and cause less acute toxicity than Ad5 vectors. However, biodistribution and safety studies in mice are of limited value because the mouse analog of the B-group Ad receptor, CD46, is expressed only in the testis, whereas in humans, CD46 is expressed on all nucleated cells. Unlike mice, baboons have CD46 expression patterns and levels that closely mimic those in humans. We conducted a biodistribution and toxicity study of group B Ad fiber-containing vectors in baboons. Animals received phosphate-buffered saline, Ad5-bGal (a first-generation Ad5 vector), or B-group fiber-containing Ads (Ad5/35-bGal and Ad5/11-bGal) at a dose of 2 x 10(12) VP/kg, and vector biodistribution and safety was analyzed over 3 days. The amount of Ad5/35-bGal and Ad5/11-bGal vector genomes was in most tissues one to three orders of magnitude below that of Ad5. Significant Ad5/35- and Ad5/11-mediated transgene (beta-galactosidase) expression was seen only in the marginal zone of splenic follicles. Compared with the animal that received Ad5-bGal, all animals injected with B-group fiber-containing Ad vectors had lower elevations in serum proinflammatory cytokine levels. Gross and histopathology were normal in animals that received B-group Ad fiber-containing Ads, in contrast to the Ad5-infused animal, which showed widespread endothelial damage and inflammation. In a further study, a chimeric Ad5/35 vector carrying proapoptotic TRAIL and Ad E1A genes under tumor-specific regulation was well tolerated in a 30-day toxicity study. No major clinical, serologic, or pathologic abnormalities were noticed in this animal.

PEGylated helper-dependent adenoviral vectors: highly efficient vectors with an enhanced safety profile

Transgene expression from helper-dependent adenoviral (HD-Ad) vectors is effective and long lasting, but not permanent. Their use is also limited by the host response against capsid proteins that precludes successful gene expression upon readministration. In this report, we test the hypothesis that PEGylation of HD-Ad reduces its toxicity and promotes transgene expression upon readministration. PEGylation did not compromise transduction efficiency in vitro and in vivo and reduced peak serum IL-6 levels two-fold. IL-12 and TNF-alpha levels were reduced three- and seven-fold, respectively. Thrombocytopenia was not detected in mice treated with the PEGylated vector. Serum transaminases were not significantly elevated in mice treated with either vector. Mice immunized with 1 x 10(11) particles of unmodified HD-Ad expressing human alpha-1 antitrypsin (hA1AT) were rechallenged 28 days later with 8 x 10(10) particles of unmodified or PEG-conjugated vector expressing beta-galactosidase. Trace levels of beta-galactosidase (52.23+/-19.2 pg/mg protein) were detected in liver homogenates of mice that received two doses of unmodified HD-Ad. Mice rechallenged with PEGylated HD-Ad produced significant levels of beta-galactosidase (5.1+/-0.4 x 10(5) pg/mg protein, P=0.0001). This suggests that PEGylation of HD-Ad vectors may be appropriate for their safe and efficient use in the clinic.

A complementation method for functional analysis of mammalian genes

Our progress in understanding mammalian gene function has lagged behind that of gene identification. New methods for mammalian gene functional analysis are needed to accelerate the process. In yeast, the powerful genetic shuffle system allows deletion of any chromosomal gene by homologous recombination and episomal expression of a mutant allele in the same cell. Here, we report a method for mammalian cells, which employs a helper-dependent adenoviral (HD-Ad) vector to synthesize small hairpin (sh) RNAs to knock-down the expression of an endogenous gene by targeting untranslated regions (UTRs). The vector simultaneously expresses an exogenous version of the same gene (wild-type or mutant allele) lacking the UTRs for functional analysis. We demonstrated the utility of the method by using PRPF3, which encodes the human RNA splicing factor Hprp3p. Recently, missense mutations in PRPF3 were found to cause autosomal-dominant Retinitis Pigmentosa, a form of genetic eye diseases affecting the retina. We knocked-down endogenous PRPF3 in multiple cell lines and rescued the phenotype (cell death) with exogenous PRPF3 cDNA, thereby creating a genetic complementation method. Because Ad vectors can efficiently transduce a wide variety of cell types, and many tissues in vivo, this method could have a wide application for gene function studies.

Molecular, cellular, and pharmacological therapies for Duchenne/Becker muscular dystrophies

Although the molecular defect causing Duchenne/Becker muscular dystrophy (DMD/BMD) was identified nearly 20 years ago, the development of effective therapeutic strategies has nonetheless remained a daunting challenge. Over the years, a variety of different approaches have been explored in an effort to compensate for the lack of the DMD gene product called dystrophin. This review not only presents some of the most promising molecular, cellular, and pharmacological strategies but also highlights some issues that need to be addressed before considering their implementation. Specifically, we describe current strategies being developed to exogenously deliver healthy copies of the dystrophin gene to dystrophic muscles. We present the findings of several studies that have focused on repairing the mutant dystrophin gene using various approaches. We include a discussion of cell-based therapies that capitalize on the use of myoblast or stem cell transfer. Finally, we summarize the results of several studies that may eventually lead to the development of appropriate drug-based therapies. In this context, we review our current knowledge of the mechanisms regulating expression of utrophin, the autosomal homologue of dystrophin. Given the complexity associated with the dystrophic phenotype, it appears likely that a combinatorial approach involving different therapeutic strategies will be necessary for the appropriate management and eventual treatment of this devastating neuromuscular disease.

Evaluation of polyethylene glycol modification of first-generation and helper-dependent adenoviral vectors to reduce innate immune responses

Adenoviruses are robust gene delivery vectors in vivo, but are limited by their propensity to provoke strong innate and adaptive responses. Previous work has demonstrated that polyethylene glycol (PEG) modification of adenovirus can protect the vectors from preexisting and adaptive immune responses by reducing protein-protein interactions. To test whether PEGylation can reduce innate immune responses to adenovirus by reducing their interactions with immune cells, first-generation (FG-Ad) and helper-dependent (HD-Ad) Ad5 vectors were PEGylated with SPA-PEG and tested in vitro and in vivo. We demonstrate that increasing PEGylation ablated in vitro transduction, but surprisingly had no negative effect on the level or distribution of in vivo gene delivery. This poor in vitro transduction could be rescued in part by physically forcing the PEGylated vectors onto cells, suggesting that physiological forces in vivo may enable transduction via heparin sulfate proteoglycan and integrin interactions. While transduction remained the same as for unmodified vectors, the PEGylated vectors reduced innate IL-6 responses by 70 and 50% in vivo for FG-Ad and HD-Ad. These reduced innate responses paralleled similar reductions in vector uptake by macrophages in vitro and Kupffer cells in vivo. These data suggest that PEGylation of Ad vectors can reduce innate immune responses without reducing transduction in vivo. These data also suggest that nonspecific vector uptake by macrophages and Kupffer cells may be critically involved in the initial activation of innate immune responses.

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.

Induction and Inhibition of Innate Inflammatory Responses by Adenovirus Early Region Proteins

First-generation adenovirus (Ad) gene therapy vectors deleted for the E1A, E1B, and E3 regions and carrying foreign genes under the control of strong foreign promoters induce high-level innate inflammatory responses within the first 24 hrs after transduction. Both uptake of the capsid and expression of gene products encoded by the vector contribute to the innate inflammatory response. Natural infections by Ad are frequently asymptomatic, suggesting that Ad has potent methods of inhibiting inflammation. The inability of Ad vectors to counter inflammatory responses suggests that the products of the Ad genes deleted in vector construction play critical roles in inhibiting these responses. Genetic analysis of the roles of Ad early region gene functions in vivo demonstrated that a virus made replication-incompetent by deletion of the preterminal protein gene and deleted for the transcriptional activation function of E1A effectively inhibits the innate inflammatory processes induced by Ad vectors. The mechanism(s) by which the Ad early region proteins inhibit inflammation is complex, as certain early region proteins can promote as well as inhibit inflammation, depending on the genetic context of the virus. Understanding of the roles of the Ad gene products in the induction and inhibition of innate inflammatory functions offers potential for the development of non-inflammatory vectors as well as for understanding of the mechanisms by which inflammation is regulated.

Inhibition and regression of atherosclerotic lesions

Atherosclerosis, once believed to be a result of a slow, irreversible process resulting from lipid accumulation in arterial walls, is now recognized as a dynamic process with reversibility. Liver-directed gene therapy for dyslipidemia aims to treat patients who are not responsive to currently available primary and secondary prevention. Moreover, gene therapy strategies have also proved valuable in studying the dynamics of atherosclerotic lesion formation, progression, and remodeling in experimental animals. Recent results on the long-term effect of gene therapy suggest that hepatic expression of therapeutic genes suppresses inflammation and has profound effects on the nature of the atherogenic process.

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.

The Innate Immune Response to Adenovirus Vectors

Gene therapy is a clinical strategy that may potentially treat an array of genetic and nongenetic diseases, as well as a novel method for drug delivery and vaccination. To these ends, adenovirus vectors are a promising means to deliver specific genes of interest into the patient. A major limitation of the use of adenovirus vectors is the host immune response. Adenovirus vectors induce the innate arm of the immune system that results in inflammation of transduced tissues and efficient clearance of administered vectors. Unlike adaptive immunity, the innate response is mediated by the adenovirus particle and does not require viral transcription. In vivo, the innate immune response involves the induction of cytokines and activation of effector leukocytes that comprise the host response to these agents. A number of interactions with leukocytes and with epithelial and endothelial cells are essential in triggering the host response to adenovirus vectors. Signal transduction via MAP kinases and NF-kappaB-mediated gene transcription are triggered during early virus-cell interactions and are key events in the innate recognition of adenovirus vector transduction. This review aims to describe data examining cellular and molecular mechanisms involved in the adenovirus-mediated innate immune response.