Use of a novel cross-linking method to modify adenovirus tropism

Targeted Adenoviral Vectors I

Human adenovirus (Ad) has been used extensively to develop gene transfer vectors for vaccine and gene therapy applications. A major factor limiting the efficacy of the current generation of Ad vectors is their inability to accomplish specific gene delivery to the cells of interest. Transductional targeting strategies seek to redirect virus binding to the appropriate cellular receptor to increase infection efficiency in selected cell types to achieve therapeutic intervention. These efforts mainly focused on incorporating targeting ligands by means of chemical conjugation or genetic modification of Ad capsid proteins and using bispecific adapter molecules to mediate virus recognition of target cells. This review summarizes current progress in Ad tropism modification maneuvers that embody genetic capsid modification and adapter-based approaches that have encouraging implications for further development of advanced vectors suitable for clinical translation.

Retargeting of viruses to generate oncolytic agents

Oncolytic virus therapy is based on the ability of viruses to effectively infect and kill tumor cells without destroying the normal tissues. While some viruses seem to have a natural preference for tumor cells, most viruses require the modification of their tropism to specifically enter and replicate in such cells. This review aims to describe the transductional targeting strategies currently employed to specifically redirect viruses towards surface receptors on tumor cells. Three major strategies can be distinguished; they involve (i) the incorporation of new targeting specificity into a viral surface protein, (ii) the incorporation of a scaffold into a viral surface protein to allow the attachment of targeting moieties, and (iii) the use of bispecific adapters to mediate targeting of a virus to a specified moiety on a tumor cell. Of each strategy key features, advantages and limitations are discussed and examples are given. Because of their potential to cause sustained, multiround infection—a desirable characteristic for eradicating tumors—particular attention is given to viruses engineered to become self-targeted by the genomic expression of a bispecific adapter protein.

Molecular disruption of NBS1 with targeted gene delivery enhances chemosensitisation in head and neck cancer

BACKGROUND

a fibroblast growth factor 2 (FGF2)-targeted adenoviral system can alter viral tropism and allow for improved transduction and reduced systemic toxicity. This study is to investigate if the FGF2-targeted adenoviral mutant Nijmegen breakage syndrome 1 (FGF2-Ad-NBS1) gene transfer can enhance cisplatin chemosensitisation not only by targeting DNA repair, but also through the induction of antiangiogenesis, whereas at the same time reducing toxicities in treating head and neck squamous cell carcinoma (HNSCC).

METHODS

the human HNSCC cell line was treated in vitro and in a nude mouse xenograft model. We conducted verification of binding ability of mutant NBS1 and downregulation of MRN complex, evaluation of transduction efficiency and combined antitumour activities. The antiangiogenesis mechanism was also investigated. Finally, we estimated the distribution of adenoviral vector in the liver.

RESULTS

the mutant NBS1 protein retains the binding ability and effectively suppresses the expression level of the MRN in infected cells. Transduction efficiency in vitro and cisplatin chemosensitisation were upregulated. The FGF2-Ad-NBS1 also showed detargeting the viral vectors away from the liver. The downregulation of NF-κB expression was supposed to correlate with increased antiangiogenesis.

CONCLUSIONS

FGF2-targeted adenoviral system enhances the cisplatin chemosensitisation of mutant NBS1 and may avoid viral-associated liver toxicities.

Retargeting of adenovirus vectors through genetic fusion of a single-chain or single-domain antibody to capsid protein IX

Adenovirus (Ad) vectors are the most commonly used system for gene therapy applications, due in part to their ability to infect a wide array of cell types and tissues. However, many therapies would benefit from the ability to target the Ad vector only to specific cells, such as tumor cells for cancer gene therapy. In this study, we investigated the utility of capsid protein IX (pIX) as a platform for the presentation of single-chain variable-fragment antibodies (scFv) and single-domain antibodies (sdAb) for virus retargeting. We show that scFv can be displayed on the capsid through genetic fusion to native pIX but that these molecules fail to retarget the virus, due to improper folding of the scFv. Redirecting expression of the fusion protein to the endoplasmic reticulum (ER) results in correct folding of the scFv and allows it to recognize its epitope; however, ER-targeted pIX-scFv was incorporated into the Ad capsid at a very low level which was not sufficient to retarget virus infection. In contrast, a pIX-sdAb construct was efficiently incorporated into the Ad capsid and enhanced virus infection of cells expressing the targeted receptor. Taken together, our data indicate that pIX is an effective platform for presentation of large targeting polypeptides on the surface of the virus capsid, but the nature of the ligand can significantly affect its association with virions.

Co-delivery of doxorubicin and plasmid by a novel FGFR-mediated cationic liposome

In our previous study, we developed a novel cationic liposome, which was modified with truncated human basic fibroblast growth factor (tbFGF) peptide. This tbFGF-mediated cationic liposome could deliver chemotherapeutic agents or gene specifically to FGFRs on tumors and obtained higher transfection efficiency than plain cationic liposomes. In order to investigate whether this novel cationic liposome could achieve a synergistic/combined anti-tumor effect as a co-delivery system, we simultaneously delivered doxorubicin (DOX) and the plasmid encoding the phosphorylation-defective mouse survivin threonine 34-->alanine mutant (Msurvivin T34A plasmid) to the same cells through this cationic liposome. As a result, an enhanced antiproliferative activity in vitro has been achieved by delivering DOX and DNA simultaneously to the Lewis lung carcinoma cells (LLC) using this liposome. The concentration of DOX in the co-delivery system which caused 50% killing was nearly 3-fold lower than that of the free DOX. Furthermore, the co-delivery system suppressed tumor growth more efficiently than either DOX or the Msurvivin T34A plasmid alone in the Lewis lung carcinoma-bearing C57BL/6 mice. After 18 days of treatment with the co-delivery system, the average tumor volume in mice was decreased by 80%, which was higher than liposomal DOX (70%, P<0.05) and Msurvivin T34A plasmid (41%, P<0.01). The co-delivery system also caused 15 days delay of tumor growth, which was longer than the other treatment groups. In conclusion, this novel cationic liposome is an efficient vector to simultaneously deliver drugs and DNA to the same cells in vitro and in vivo.

Lac-regulated system for generating adenovirus 5 vaccine vectors expressing cytolytic human immunodeficiency virus 1 genes

Adenovirus (Ad) vectors have been developed as human immunodeficiency-1 (HIV-1) vaccine vectors because they consistently induce immune responses in preclinical animal models and human trials. Strong promoters and codon-optimization are often used to enhance vaccine-induced HIV-1 gene expression and immunogenicity. However, if the transgene is inherently cytotoxic in the cell line used to produce the vector, and is expressed at high levels, it is difficult to rescue a stable Ad HIV-1 vaccine vector. Therefore we hypothesized that generation of Ad vaccine vectors expressing cytotoxic genes, such as HIV-1 env, would be more efficient if expression of the transgene was down-regulated during Ad rescue. To test this hypothesis, a Lac repressor-operator system was applied to regulate expression of reporter luciferase and HIV-1 env transgenes during Ad rescue. The results demonstrate that during Ad rescue, constitutive expression of the Lac repressor in 293 cells reduced transgene expression levels to approximately 5% of that observed in the absence of regulation. Furthermore, Lac-regulation translated into more efficient Ad rescue compared to traditional 293 cells. Importantly, Ad vectors rescued with this system showed high levels of transgene expression when transduced into cells that lack the Lac repressor protein. The Lac-regulated system also facilitated the rescue of modified Ad vectors that have non-native receptor tropism. These tropism-modified Ad vectors infect a broader range of cell types than the unmodified Ad, which could increase their effectiveness as a vaccine vector. Overall, the Lac-regulated system described here (i) is backwards compatible with Ad vector methods that employ bacterial-mediated homologous recombination, (ii) is adaptable for the engineering of tropism-modified Ad vectors, and (iii) does not require co-expression of regulatory genes from the vector or the addition of exogenous chemicals to induce or repress transgene expression. This system therefore could facilitate the development of Ad-based vaccine candidates that otherwise would not be feasible to generate.

Retargeting polymer-coated adenovirus to the FGF receptor allows productive infection and mediates efficacy in a peritoneal model of human ovarian cancer

BACKGROUND

Transductional targeting of adenovirus following systemic or regional delivery remains one of the most difficult challenges for cancer gene medicine. The numerical excess and anatomical advantage of normal (non-cancer) cells in vivo demand far greater detargeting than is necessary for studies using single cell populations in vitro, and this must be coupled with efficient retargeting to cancer cells.

METHODS

Adenovirus (Ad5) particles were coated with reactive poly[N-(2-hydroxypropyl)methacrylamide] copolymers, to achieve detargeting, and retargeting ligands were attached to the coating. Receptor-mediated infection was characterised in vitro and anticancer efficacy was studied in vivo.

RESULTS

Polymer coating prevented the virus binding any cellular receptors and mediated complete detargeting in vitro and in vivo. These fully detargeted vectors were efficiently retargeted with the model ligand FGF2 to infect FGFR-positive cells. Specific transduction activity was the same as parental virus, and intracellular routing appeared unaffected. Levels of transduction were up to 100-fold greater than parental virus on CAR negative cells. This level of specificity permitted good efficacy in intraperitoneal cancer virotherapy, simultaneously decreasing peritoneal adhesions seen with parental virus. Following intravenous delivery FGF2 mediated unexpected binding to erythrocytes, improving circulation kinetics, but preventing the targeted virus from leaving the blood stream.

CONCLUSIONS

Polymer cloaking enables complete adenovirus detargeting, providing a versatile platform for receptor-specific retargeting. This approach can efficiently retarget cancer virotherapy in vivo. Ligands should be selected carefully, as non-specific interactions with non-target cells (e.g. blood cells) can deplete the pool of therapeutic virus available for targeting disseminated disease.

PEGylated adenovirus for targeted gene therapy

Bifunctional polyethylene glycol (PEG) molecules provide a novel approach to retargeting viral vectors without the need to genetically modify the vector. Modification of the surface of adenovirus with heterofunctional PEG allows further modification of the capsid with ligands. In addition, heterofunctional PEG modification ablates the normal tropism of the virus and reduces transduction of non-target tissues in vivo. Moreover, the addition of PEG chains to the surface of the virus shields antigen-binding sites, significantly reducing the susceptibility of the virus to antibody neutralization. Finally, T cell subsets from mice exposed to the PEGylated vector demonstrate a marked decrease in Th1 and Th2 responses, suggesting that PEG modification may help reduce the immune response to the vector.

Specific and efficient targeting of adenovirus vectors to macrophages: application of a fusion protein between an adenovirus-binding fragment and avidin, linked to a biotinylated oligonucleotide

BACKGROUND

The application of serotype 5 adenoviruses (Ad5) in macrophages is hampered by the absence of the endogenous coxsackie adenovirus receptor (CAR).

METHODS

To overcome this limitation, we first generated a linker protein consisting of the virus-binding domain of CAR and the C-terminus of avidin. Second, to target macrophages, this linker protein was equipped with the biotinylated (bio) oligonucleotide dA6G10, which was previously shown to display a high affinity for the scavenger receptor A (SR-A).

RESULTS

As compared to nontargeted virus, the linker protein equipped with bio-dA6G10 showed a 500-fold increased reporter gene expression in mouse macrophage RAW264.7 cells. A linker protein equipped with a bio-dA16 control oligonucleotide was inactive. Moreover, the bio-dA6G10-equipped linker showed a 390-fold increased luciferase expression in the macrophage cell line J774 and 276- and 150-fold increased reporter gene expression in primary peritoneal and bone marrow (BM)-derived macrophages, respectively. Using BM-derived macrophages from SR-A knockout mice, it was shown that the dA6G10-mediated uptake is predominantly SR-A-mediated.

CONCLUSIONS

Thus, we have developed a novel tool to link biotinylated ligands to a virus-binding fragment of CAR and have exploited this linker protein to extend the applicability of Ad5 to infect transformed and primary macrophages.

Delivery of viral vectors to tumor cells: extracellular transport, systemic distribution, and strategies for improvement

It is a challenge to deliver therapeutic genes to tumor cells using viral vectors because (i) the size of these vectors are close to or larger than the space between fibers in extracellular matrix and (ii) viral proteins are potentially toxic in normal tissues. In general, gene delivery is hindered by various physiological barriers to virus transport from the site of injection to the nucleus of tumor cells and is limited by normal tissue tolerance of toxicity determined by local concentrations of transgene products and viral proteins. To illustrate the obstacles encountered in the delivery and yet limit the scope of discussion, this review focuses only on extracellular transport in solid tumors and distribution of viral vectors in normal organs after they are injected intravenously or intratumorally. This review also discusses current strategies for improving intratumoral transport and specificity of viral vectors.

Viral gene therapy strategies: from basic science to clinical application

A major impediment to the successful application of gene therapy for the treatment of a range of diseases is not a paucity of therapeutic genes, but the lack of an efficient non-toxic gene delivery system. Having evolved to deliver their genes to target cells, viruses are currently the most effective means of gene delivery and can be manipulated to express therapeutic genes or to replicate specifically in certain cells. Gene therapy is being developed for a range of diseases including inherited monogenic disorders and cardiovascular disease, but it is in the treatment of cancer that this approach has been most evident, resulting in the recent licensing of a gene therapy for the routine treatment of head and neck cancer in China. A variety of virus vectors have been employed to deliver genes to cells to provide either transient (eg adenovirus, vaccinia virus) or permanent (eg retrovirus, adeno-associated virus) transgene expression and each approach has its own advantages and disadvantages. Paramount is the safety of these virus vectors and a greater understanding of the virus-host interaction is key to optimizing the use of these vectors for routine clinical use. Recent developments in the modification of the virus coat allow more targeted approaches and herald the advent of systemic delivery of therapeutic viruses. In the context of cancer, the ability of attenuated viruses to replicate specifically in tumour cells has already yielded some impressive results in clinical trials and bodes well for the future of this approach, particularly when combined with more traditional anti-cancer therapies.

Oncolytic viral therapies

Molecular research has vastly advanced our understanding of the mechanism of cancer growth and spread. Targeted approaches utilizing molecular science have yielded provocative results in the treatment of cancer. Oncolytic viruses genetically programmed to replicate within cancer cells and directly induce toxic effect via cell lysis or apoptosis are currently being explored in the clinic. Safety has been confirmed and despite variable efficacy results several dramatic responses have been observed with some oncolytic viruses. This review summarizes results of clinical trials with oncolytic viruses in cancer.

Gene Transfer Mediated by Native versus Fibroblast Growth Factor–Retargeted Adenoviral Vectors into Lung Cancer Cells

We compared native Adenoviral (Ad) vectors to a basic Fibroblast Growth Factor-retargeted Adenovirus (FGF2-Ad) for gene delivery into a diverse panel of lung cancer cells in vitro and xenografts in vivo. Cells were first evaluated for vector-specific receptor expression. Marked variations of surface coxsackie-adenovirus receptor (CAR), but relatively similar levels of alpha v integrin and FGF receptor expression were evident. Transduction efficiency by Ad directly correlated (R = 0.77, 95% CI 0.28-0.94, P = 0.0085) with CAR, but not with alpha v integrin expression. Transduction efficiency by FGF2-Ad did not correlate with the measured FGF receptor expression. Blocking studies indicated that gene transfer by FGF2-Ad occurred by a CAR-independent pathway, and could be inhibited by free FGF in a dose-dependent manner. Ad-antiserum inhibited FGF2-Ad gene transfer, suggesting that the Ad-component was needed for post-entry DNA-delivery. Soluble heparin sulfate proteoglycans (HSPG) or alpha v integrin blockers marginally decreased FGF2-Ad transduction. Both Ad and FGF2-Ad equally transduced CAR-positive non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) cells. By contrast, FGF2-Ad had a distinct transduction advantage in CAR-deficient NSCLC cells. This improvement in transduction of CAR-deficient cells by FGF2-Ad persisted in vivo. These data justify the need for an improved FGF2-Ad vector for clinical use in CAR-deficient lung cancer.

Targeting of vaccinia virus using biotin-avidin viral coating and biotinylated antibodies

INTRODUCTION

To test a general method for altering the tropism of viral vectors, we conjugated targeting antibody to the surface of recombinant vaccinia virus with a biotin-avidin-biotin linker and assessed the resulting infectivity in target cells and controls.

MATERIALS AND METHODS

We biotinylated a vaccinia viral vector and used avidin to crosslink the biotinylated viral surface to a biotinylated antibody specific for a molecule on the surface of a target cell. In an in vitro model system, we coated a recombinant vaccinia construct containing the E. coli beta-galactosidase gene with antibody to the murine class I MHC molecule Db. Target cells were B78H1 murine melanoma cells transduced with either the Db gene or, as a control, the Kb gene. Infectivity was assessed by staining target cells with x-gal to demonstrate expression of virally delivered beta-galactosidase. This technique was also assessed in a second system with vaccinia/beta-gal targeted to the murine B7.2 molecule. The infectivity of the resulting construct was assessed for murine SA1 fibrosarcoma cells transfected with the B7.2 gene and for wild-type, B7.2-negative SA1. Experiments were repeated in each system with similar results.

RESULTS

This strategy demonstrated antibody-mediated viral targeting in both the B78H1 and the SA1 models. Importantly, addition of the targeting coat diminished the infectivity of the modified vaccinia for control cells but preserved infectivity for targeted cells. In the B78H1 system, Db-targeted vaccinia consistently had 2- to 3-fold greater infectivity for B78H1Db than B78H1Kb. Increasing the number of avidin molecules used per virion in the synthesis of the viral coat led to greater selectivity but decreased overall infectivity. In the SA1 system, B7.2-targeted vaccinia demonstrated completely ablated infectivity for control SA1 cells, but maintained infectivity for target SA1/B7.2 cells.

CONCLUSIONS

Recombinant viral vectors such as vaccinia may be coated with biotin/avidin and linked to biotinylated antibodies to preferentially target specific cell types in vitro. Such an approach may be useful in targeting recombinant lytic viruses to tumors for destruction and in immune up-regulation in vivo. Similarly, this approach may enhance nonlytic viruses for gene therapy applications.

Kinetic analysis and modeling of firefly luciferase as a quantitative reporter gene in live mammalian cells

Firefly luciferase has proven to be a highly sensitive and quantitative reporter gene for studying gene delivery and regulation, and its recent use in live cells and organisms promises to further expand its utility. However, the intracellular behavior and properties of the enzyme are not well characterized. Specifically, information on the intracellular kinetics and stability of luciferase activity is necessary for real-time luminescence counts from live cells to be quantitatively meaningful. Here, we report a dynamic analysis of luciferase activity in the context of living mammalian cells. We have determined the relative light units measured in living cells to be proportional to that found in cell lysate. We have also calculated the K(m) of luciferase in living cells to be approximately 1 mM, a value much higher than the 10 microM found for pure enzyme in vitro. In addition, a 2-hour half-life of luciferase activity in live cells was measured in real time. Finally, we have modeled luciferase activity in live cells for the purposes of understanding and translating the luciferase signal into a more effective metric of gene expression and cell behavior.

Use of adenovirus protein IX (pIX) to display large polypeptides on the virion--generation of fluorescent virus through the incorporation of pIX-GFP

The adenovirus (Ad) protein IX (pIX) is a minor component of the Ad capsid and associates with the hexons that make up the facets of the icosahedron. In this study, we investigated whether a large protein tag could be fused to pIX without compromising the Ad vector itself. As proof-of-principle, we generated a pIX-green fluorescent protein (GFP) fusion protein. We show that a virus encoding the pIX-GFP can be generated and that pIX-GFP fusion protein was incorporated into the Ad capsid as efficiently as native pIX. In tissue culture, translocation of Ad/pIX-GFP from the outside of the cell to the nucleus could be followed using fluorescence microscopy, and the timing of migration to the nucleus was similar to that previously reported for Ad. We also could track the virus after injection into the tibialis anterior muscle of mice. Shortly after injection, the majority of the Ad/pIX-GFP accumulated in pockets adjacent to the muscle fibers, with some migration of the virus between fibers. Our ability to attach GFP to the Ad virion, through fusion to pIX, provides a valuable tool for virus tracking in vitro and in vivo. Moreover, our data indicate that pIX can be used as a platform to anchor proteins to the Ad capsid, such as large ligands for cell-type-specific targeting of the vector.

Gene transduction and cell entry pathway of fiber-modified adenovirus type 5 vectors carrying novel endocytic peptide ligands selected on human tracheal glandular cells

Monolayers of cystic fibrosis transmembrane conductance regulator (CFTR)-deficient human tracheal glandular cells (CF-KM4) were subjected to phage biopanning, and cell-internalized phages were isolated and sequenced, in order to identify CF-KM4-specific peptide ligands that would confer upon adenovirus type 5 (Ad5) vector a novel cell target specificity and/or higher efficiency of gene delivery into airway cells of patients with cystic fibrosis (CF). Three different ligands, corresponding to prototypes of the most represented families of phagotopes recovered from intracellular phages, were designed and individually inserted into Ad5-green fluorescent protein (GFP) (AdGFP) vectors at the extremities of short fiber shafts (seven repeats [R7]) terminated by scissile knobs. Only one vector, carrying the decapeptide GHPRQMSHVY (abbreviated as QM10), showed an enhanced gene transduction of CF-KM4 cells compared to control nonliganded vector with fibers of the same length (AdGFP-R7-knob). The enhancement in gene transfer efficiency was not specific to CF-KM4 cells but was observed in other mammalian cell lines tested. The QM10-liganded vector was referred to as AdGFP-QM10-knob in its knobbed version and as AdGFP-QM10 in its proteolytically deknobbed version. AdGFP-QM10 was found to transduce cells with a higher efficiency than its knob-bearing version, AdGFP-QM10-knob. Consistent with this, competition experiments indicated that the presence of knob domains was not an absolute requirement for cell attachment of the QM10-liganded vector and that the knobless AdGFP-QM10 used alternative cell-binding domains on its capsid, including penton base capsomer, via a site(s) different from its RGD motifs. The QM10-mediated effect on gene transduction seemed to take place at the step of endocytosis in both quantitative and qualitative manners. Virions of AdGFP-QM10 were endocytosed in higher numbers than virions of the control vector and were directed to a compartment different from the early endosomes targeted by members of species C Ad. AdGFP-QM10 was found to accumulate in late endosomal and low-pH compartments, suggesting that QM10 acted as an endocytic ligand of the lysosomal pathway. These results validated the concept of detargeting and retargeting Ad vectors via our deknobbing system and redirecting Ad vectors to an alternative endocytic pathway via a peptide ligand inserted in the fiber shaft domain.

Helper-Dependent Adenoviral Vectors Containing Modified Fiber for Improved Transduction of Developing and Mature Muscle Cells

Adenoviruses (Ads) have shown great utility as vectors for the delivery of genes to mammalian cells, partly because of their ability to infect a wide range of different cell types independent of the replicative state of the cell. However, Ads do not transduce mature muscle efficiently because of low levels of the natural viral primary receptor, the coxsackie virus and adenovirus receptor, on the surface of adult muscle cells. In this study, we have addressed whether incorporation of polylysine [p(K)] or arginine-glycine-aspartic acid (RGD) placed in the H-I loop of the adenoviral fiber protein can improve helper-dependent Ad vector (hdAd) transduction of mature muscle cells. We show that incorporation of the p(K) motif into the fiber of early region 1 (E1)-deleted Ad results in enhanced transduction of undifferentiated and differentiated C2C12 cells relative to a virus, containing a wild-type fiber (12- and 21-fold enhancement, respectively). Incorporation of the RGD motif resulted in only a 60-70% increase in transduction efficiency in these cells. The two fiber modifications were then incorporated into helper viruses for use in the Cre-lox system for generating hdAd, and the resulting retargeted Ad vectors, which encoded the beta-galactosidase reporter gene (beta-Gal), demonstrated enhanced transduction of C2C12 cells in culture. Although hdAdpK also showed enhanced infection of mature mouse muscle in vivo, hdAdRGD did not. All hdAd vectors elicited only minor anti-Ad immune responses, compared with an E1-deleted control vector, but each vector elicited strong anti-beta-Gal immunoreactivity. Our results demonstrate that hdAd with modified cell tropism can be generated efficiently and, in the case of polylysine-modified hdAd, can lead to improved transduction of adult muscle cells in vivo.

Synthesis of adenoviral targeting molecules by intein-mediated protein ligation

Adenoviral vectors infect cells through the binding of capsid proteins to cell-surface receptors. The ubiquitous expression of adenoviral receptors in human tissues represents an obstacle toward the development of systemically deliverable vectors for cancer therapy, since effective therapy may require delivery to specific sites. For these reasons, major efforts are directed toward the elimination of the native tropism combined with identification of ligands that bind to tumor-specific cell-surface proteins. Highthroughput technologies have identified potential targeting ligands, which need to be evaluated for their ability to retarget adenovirus to alternative receptors. Here, we present a strategy that permits the routine analysis of adenoviral targeting ligands. We use intein-mediated protein ligation as a means to produce functional biological molecules, that is, adenoviral targeting molecules that function as adapters between cellular receptors and the adenovirus fiber protein. We demonstrate the versatility of the present system by conjugating targeting ligands that differ in size and nature including an apolipoprotein E synthetic peptide, the basic fibroblast growth factor and folic acid. The resulting adenoviral targeting molecules mediate adenoviral gene delivery in cells that express the corresponding receptor.

Adaptable modification of adenoviral tropism using a bifunctional ligand protein

In order to target recombinant adenovirus (AdV), we have developed a new strategy using a fusion ligand protein comprising coxsackievirus/adenovirus receptor (CAR), and the antibody Fc-binding domain from protein A in vitro testing with this ligand shows that it blocks viral gene transduction and, when coupled with anti-ICAM-1 IgG, redirects AdV to endothelial cells that are induced to express ICAM-1. Because the protein A Fc-binding domain will bind to any immunoglobulin, the current strategy can be adapted to target a wide variety of tissues or cells as long as an antibody species that recognizes a membrane marker on target tissue or cell is present. This concept may be further expanded to other viruses that employ peptide receptors. These membrane receptors can be fused to the Fc-binding domain to create a variety of bifunctional ligands for targeting recombinant viruses in gene therapy.

Mechanism of restriction of normal and cystic fibrosis transmembrane conductance regulator-deficient human tracheal gland cells to adenovirus infection and ad-mediated gene transfer

CF-KM4 (cystic fibrosis transmebrane conductance regulator-deficient) and MM-39 (healthy) cells, two serous cell lines from submucosal tracheal glands, were found to be poorly susceptible to adenovirus (Ad)5 infection and Ad5-mediated gene transduction. The major limiting steps apparently resided in the primary events of Ad5 interaction, i.e., cell attachment and entry. Both CF-KM4 and MM-39 cells failed to express the Coxsackie-Ad receptor (CAR), and experimental data suggested that alpha[2-->6]-linked sialic acid residues of sialoglycoproteins (SAGP) in CF-KM4 cells, and heparan sulfate glycosaminoglycans (HS-GAG) in MM-39, were used as receptors by Ad5 virions. Ad5 attached to SAGP and HS-GAG receptors via its fiber knob domain, but entered the cells via a penton base- and Arg-Gly-Asp (RGD)-integrin-independent pathway. The block to Ad5-mediated gene transfer in MM-39 and KM4 cells could be overcome by conferring to the vector a novel cell-binding specificity. Thus, Ad5 vectors carrying a stretch of 7-lysine residues genetically inserted at the C-terminus of the fiber knob were found to transduce MM-39 cells with a 10- to 20-fold higher efficiency than the original vectors, but the transduction of CF-KM4 was not significantly improved. Retargeting Ad5 to integrin receptors via RGD peptide ligands, inserted at the extremity of the fiber shaft, resulted in a transducing efficiency of 20- and 50-fold higher in MM-39 and KM4 cells, respectively, compared with Ad5 vectors carrying fibers terminated by their natural knob domain.

Strategies for muscle-specific targeting of adenoviral gene transfer vectors

Currently, adenoviral transfer of therapeutic genes such as dystrophin is hampered by low transduction efficiency of adult skeletal muscle. This is largely due to the lack of appropriate virus attachment receptors on the myofiber surface. Recent studies in transgenic mice revealed that upregulation of Coxsackie- and adenovirus receptor improves gene transfer efficiency by approximately ten-fold. Conversely, the vector load that needed to be administered to achieve sufficient gene transfer could be lowered significantly. Reduced viral vector loads may help to control virally mediated toxicity and immunogenicity. To date, there are no drugs or methods known to increase Coxsackie- and adenovirus receptor expression in skeletal muscle that would be easily applicable in humans. However, alternative strategies such as vector retargeting are currently being investigated that may allow for an increase in binding of adenoviral vectors to skeletal muscle. Recent experiments have shown that directed mutagenesis of the adenoviral fiber knob allows for a significant reduction in Coxsackie- and adenovirus receptor binding and for introduction of a new binding domain. Therefore, vector retargeting towards efficient and specific infection of skeletal muscle may be achieved by directed genetic alteration of adenoviral capsid proteins.

Adenoviruses in oncology: a viable option?

The feasibility of using adenoviruses for gene therapy has been under close scrutiny recently, as it has become clear that significant toxicity can result from the strong immune response created by intravenous administration of large doses of first generation adenovirus vectors. This suggests that other vectors could be more useful for treatment of metabolic and hereditary disease, where widespread transduction is often necessary for effective gene replacement, and the viability of target cells is important. However, promising recent results in human cancer trials have confirmed that adenoviruses can be very useful in oncology. For cancer treatment, the unparalleled transduction efficacy of adenovirus in dividing and dormant cells is a major benefit. As the goal in cancer gene therapy is to kill infected tumour cells, long-term transgene expression is not necessary. In addition, the immune response generated against infected cells could be useful for eradicating uninfected tumour. Importantly, more than 670 cancer patients have been treated with adenovirus intratumorally, intra-arterially, intraperitoneally and intravenously with very manageable adverse effects and no unexpected severe or lethal toxicity. Currently, the most promising approaches are based on replication-competent agents that allow efficient tumour penetration because of their capacity for tissue-specific replication. In addition to transcriptional control, it is becoming clear that targeting is necessary for efficient tumour transduction and less infection of normal tissues. Exciting results are anticipated when the first selectively replicating targeted adenoviruses go to clinical trials. In conclusion, intense gene therapy and virological research have suggested that while other vectors could be more useful for treatment of hereditary disease, adenoviruses are highly promising and safe agents for oncology, as suggested in a number of early phase clinical trials.

Genetic retargeting of adenovirus vectors: functionality of targeting ligands and their influence on virus viability

BACKGROUND

We studied the ability of adenovirus type 5 (Ad5) to encapsidate new cellular ligands carried by their fibers to yield functional retargeted vectors for gene therapy. Recombinant Ad5 fibers containing shaft repeats 1 to 7 and an extrinsic trimerization motif, and terminated by its native knob or amino acid motifs containing RGD, have been rescued into infectious virions.

METHODS

Polypeptide ligands of cell surface molecules, including single-chain antibodies or epidermal growth factor, were cloned into recombinant fibers. Phenotypic analysis of fiber constructs and rescuing into the Ad5 genome were performed. Recombinant viruses were characterized with reference to fiber content, growth rate and infectivity.

RESULTS

A major limiting factor for recovering viable recombinant Ad5 carrying fiber-fused polypeptide ligands was apparently the ability of the ligand to fold correctly within the cellular cytoplasm. This constraint has previously not been systematically evaluated in the literature. Phenotypic analysis of the fiber-ligand fusions showed that their degree of cytoplasmic solubility correlated with their ability to yield viable Ad5 vectors. Our results suggested that the fiber manipulations diminish virus growth rate, probably through different, opposing effects: (i) the reduced shaft length increases fiber solubility in the absence of the knob but (ii) diminishes virus entry, and (iii) the absence of the knob alters the overall protein composition of the virion and decreases its fiber copy number.

CONCLUSIONS

Based on our findings, cytoplasmic solubility and cytoplasmic ligand reactivity of fiber-ligand fusion proteins are the best prediction criterion for viability and recovery of genetically retargeted Ad vectors.

Targeting of suicide gene delivery in pancreatic cancer cells via FGF receptors

Pancreatic ductal adenocarcinomas (PDACs) overexpress various cell-surface tyrosine kinase receptors, including the type I high-affinity fibroblast growth factor receptor (FGFR-1). The purpose of this study was to determine whether FGFR-targeted gene therapy is feasible in this disorder. Accordingly, the effects of a conjugate consisting of fibroblast growth factor (FGF)-2 linked to a Fab' fragment against the adenovirus knob region were evaluated in human pancreatic cancer cell lines treated with an adenoviral vector containing the herpes simplex virus thymidine kinase (AdTK) gene. An adenoviral vector containing the firefly luciferase reporter gene (AdLuc) served to assess infection efficiency, and was initially tested in L6 rat myoblasts. In parental L6 cells that express exceedingly low levels of high-affinity FGFRs, transduction with AdLuc was enhanced 7- to 10-fold with the FGF2-Fab' conjugate, whereas in L6 cells transfected to express FGFR-1, it was enhanced 39- to 52-fold. The pancreatic cancer cell lines expressed variable levels of the four high-affinity FGF receptors, and exhibited 2- to 34-fold increases in gene transduction in the presence of the FGF2-Fab' conjugate. In the absence of FGF2-Fab' there was no correlation between surface binding of FGF2 and AdLuc transduction efficiency, whereas in the presence of FGF2-Fab', enhanced AdLuc transduction efficiency correlated with greater surface binding of FGF2. In the absence of AdTK, all the cell lines were insensitive to ganciclovir, whereas after AdTK transduction, only ASPC-1 and PANC-1 cells were resistant to ganciclovir even in the presence of FGF2-Fab'. Ganciclovir-mediated inhibition was dependent on the conjugate in CAPAN-1 and COLO-357 cells, but was independent of the conjugate in T3M4 and MIA-PaCa-2 cells. Real-time quantitative PCR of laser-captured cancer cells revealed high levels of various FGFR mRNA species in six of seven PDAC tumor samples. These findings indicate that transduction efficiency with FGF2-Fab' in pancreatic cancer cells is independent of native adenoviral transduction efficiency and is greatest in cells that exhibit concomitant expression of various high-affinity FGFRs. In view of the overexpression of high-affinity FGFRs in the cancer cells in PDAC, our findings also suggest that the combined use of AdTK, ganciclovir, and FGF2-Fab' may ultimately be a promising therapeutic approach in a subgroup of patients with PDAC.

Emerging new therapies for chemotherapy-resistant cancer using adenoviral vectors

The treatment of cancer by genetic manipulation of either the tumor itself or the patient as a whole offers new avenues for the treatment of otherwise refractory cancers. Gene therapy seeks to correct underlying genetic defects in malignant tissue or to augment the host defense response or to promote selectivity of other therapies. Many innovative and exciting genetic targets have been recently identified. However, the field as a whole is still constrained by limitations of gene delivery. The most common vector for gene delivery is modified adenovirus. In this review, we survey a sampling of current therapeutic approaches that depend upon adenoviral delivery vehicles and outline the advantages and disadvantages of this vector system.

Head and neck cancer: gene therapy approaches. Part 1: adenoviral vectors

Treatment options for recurrent or refractory head and neck cancer are limited. The goal of gene therapy is to introduce new genetic material into cancer cells without affecting toxicity to surrounding malignant cells. The most common vehicles for delivery of genes are adenoviruses. Adenoviruses gain access to malignant and normal cell cytoplasm via viral ligand binding to a unique cell surface receptor (the coxsackie adenovirus receptor [CAR]). However, this receptor is not cancer specific. Genetic modification of adenoviral DNA can create cancer specific targeting. Adenoviruses can be modified to express cancer specific ligands thereby focusing binding to malignant tissue. Furthermore, adenoviral delivered genes can be put under cancer specific promoter control to further limit gene expression in malignant tissue. Increased antitumour activity from such modifications has been demonstrated preclinically and several clinical trials have been completed demonstrating safety and clinical activity of non-replicating and conditional replicating adenoviral vector thereby opening the door for gene delivery and cancer specific targeting.

Genetic retargeting of adenovirus: novel strategy employing "deknobbing" of the fiber

For efficient and versatile use of adenovirus (Ad) as an in vivo gene therapy vector, modulation of the viral tropism is highly desirable. In this study, a novel method to genetically alter the Ad fiber tropism is described. The knob and the last 15 shaft repeats of the fiber gene were deleted and replaced with an external trimerization motif and a new cell-binding ligand, in this case the integrin-binding motif RGD. The corresponding recombinant fiber retained the basic biological functions of the natural fiber, i.e., trimerization, nuclear import, penton formation, and ligand binding. The recombinant fiber bound to integrins but failed to react with antiknob antibody. For virus production, the recombinant fiber gene was rescued into the Ad genome at the exact position of the wild-type (WT) fiber to make use of the native regulation of fiber expression. The recombinant virus Ad5/FibR7-RGD yielded plaques on 293 cells, but the spread through the monolayer was two to three times delayed compared to WT, and the ratio of infectious to physical particles was 20 times lower. Studies on virus tropism showed that Ad5/FibR7-RGD was able to infect cells which did not express the coxsackie-adenovirus receptor (CAR), but did express integrins. Ad5/FibR7-RGD virus infectivity was unchanged in the presence of antiknob antibody, which neutralized the WT virus. Ad5/FibR7-RGD virus showed an expanded tropism, which is useful when gene transfer to cells not expressing CAR is needed. The described method should also make possible the construction of Ad genetically retargeted via ligands other than RGD.

Retargeting of adenoviral vectors to neurons using the Hc fragment of tetanus toxin

The Hc fragment of tetanus toxin (Hc) retains the specific nerve cell binding and transport properties of the holotoxin, but lacks any toxicity. We are investigating the potential for utilising its neurotropism for targeted gene delivery to the central nervous system. Previously we reported the use of Hc-polylysine conjugates for selective gene transfer into neuronal cells in vitro. However, as attempts to apply these constructs in vivo were not successful, we have extended these studies to modification of the tropism of adenoviral vectors. Either Hc-polylysine conjugates or the Fab fragment of a neutralising anti-knob antibody covalently bound to Hc were attached to the virus. Infection of neuronal and non-neuronal cell lines with retargeted virus showed highly increased neuronal cell selectivity, but no significant enhancement of gene delivery into these cells. High concentrations of free Hc blocked the infectivity of the retargeted vector efficiently. Intramuscular injection of retargeted virus into mouse tongues resulted in selective gene transfer to the neurons of the hypoglossal nucleus, where no pathological changes were observed. As differentiated neurons do not undergo cell division, appropriate vectors carrying a thymidine kinase gene, which allows selective elimination of dividing cells, may be exploitable for the treatment of tumours of the central nervous system. The demonstrated suitability of the Hc fragment of tetanus toxin as targeting moiety for viral vectors also indicates a potential for gene therapy of inherited neurodegenerative diseases such as spinal muscular atrophy.

Ectodomain of coxsackievirus and adenovirus receptor genetically fused to epidermal growth factor mediates adenovirus targeting to epidermal growth factor receptor-positive cells

Human adenovirus (Ad) is extensively used for a variety of gene therapy applications. However, the utility of Ad vectors is limited due to the low efficiency of Ad-mediated gene transfer to target cells expressing marginal levels of the Ad fiber receptor. Therefore, the present generation of Ad vectors could potentially be improved by modification of Ad tropism to target the virus to specific organs and tissues. The fact that coxsackievirus and adenovirus receptor (CAR) does not play any role in virus internalization, but functions merely as the virus attachment site, suggests that the extracellular part of CAR might be utilized to block the receptor recognition site on the Ad fiber knob domain. We proposed to design bispecific fusion proteins formed by a recombinant soluble form of truncated CAR (sCAR) and a targeting ligand. In this study, we derived sCAR genetically fused with human epidermal growth factor (EGF) and investigated its ability to target Ad infection to the EGF receptor (EGFR) overexpressed on cancer cell lines. We have demonstrated that sCAR-EGF protein is capable of binding to Ad virions and directing them to EGFR, thereby achieving targeted delivery of reporter gene. These results show that sCAR-EGF protein possesses the ability to effectively retarget Ad via a non-CAR pathway, with enhancement of gene transfer efficiency.

Ligand-mediated retargeting of recombinant adenovirus for gene transfer in vivo

The development of efficient and safe methods for in vivo gene transfer is central to the success of gene therapy. Recombinant adenoviral vectors, although highly efficient, are limited by the host immune response, potential safety hazards due to obligatory cotransfer of viral proteins, and their broad tissue tropism. Here, we demonstrate in an animal model that host range and tissue tropism of a recombinant adenovirus from a distant species can be modified by complexing adenovirus with a cell-specific ligand. Thus, a replication-deficient lacZ recombinant human adenovirus, which naturally does not infect avian cells, allowed highly efficient and specific gene transfer to the liver of ducks in vivo when complexed with N-acetylglucosamine, a ligand for the chicken hepatic lectin. This combination of ligand-mediated receptor targeting with adenoviral uptake and intracellular processing of a given gene represents a novel approach to gene therapy of inherited and acquired liver diseases.

Targeted transfer of polyethylenimine-avidin-DNA bioconjugates to hematopoietic cells using biotinylated monoclonal antibodies

Here we examine whether attachment of biotinylated antibodies to proteins on the cell surface increases the transfection efficiency of polyethylenimine-avidin-DNA bioconjugate gene transfer. Preliminary experiments were performed to compare avidin endocytosis into cells incubated with biotinylated antibodies. Antibody biotinylation resulted in the endocytosis of avidin-FITC into nearly 100% of cells compared with no detectable binding or entry into unbiotinylated cells. Gene transfer was accomplished with avidin conjugated to polyethylenimine (PEI) at a molar ratio of 4:1 (PA4). Plasmid DNA encoding the green fluorescent protein (GFP) gene was condensed on the PA4, and transfection efficiencies were measured by flow cytometry as the percentage of cells that fluoresced at levels greater than two standard deviations above the negative control. Gene transfer efficiencies were compared among K562, HEL, and Jurkat leukemia cell lines. Control transfections with DNA alone or untargeted PEI-DNA resulted in </=2% GFP positive cells. Targeting PEI-avidin-DNA to antibody biotinylated cells increased transfection efficiency several fold over untargeted PEI. For each cell type, the increase in transfection efficiency was not significantly different among four biotinylated antibodies tested (antiCD55, antiCD59, antiCD71, and antiCD98). These data suggest biotinylated antibodies may be useful for targeting polyethylenimine-avidin mediated gene transfer.

Latest developments in gene transfer technology: achievements, perspectives, and controversies over therapeutic applications

Over the last decade, more than 300 phase I and phase II gene-based clinical trials have been conducted worldwide for the treatment of cancer and monogenic disorders. Lately, these trials have been extended to the treatment of AIDS and, to a lesser extent, cardiovascular diseases. There are 27 currently active gene therapy protocols for the treatment of HIV-1 infection in the USA. Preclinical studies are currently in progress to evaluate the possibility of increasing the number of gene therapy clinical trials for cardiopathies, and of beginning new gene therapy programs for neurologic illnesses, autoimmuno diseases, allergies, regeneration of tissues, and to implement procedures of allogeneic tissues or cell transplantation. In addition, gene transfer technology has allowed for the development of innovative vaccine design, known as genetic immunization. This technique has already been applied in the AIDS vaccine programs in the USA. These programs aim to confer protective immunity against HIV-1 transmission to individuals who are at risk of infection. Research programs have also been considered to develop therapeutic vaccines for patients with AIDS and generate either preventive or therapeutic vaccines against malaria, tuberculosis, hepatitis A, B and C viruses, influenza virus, La Crosse virus, and Ebola virus. The potential therapeutic applications of gene transfer technology are enormous. However, the effectiveness of gene therapy programs is still questioned. Furthermore, there is growing concern over the matter of safety of gene delivery and controversy has arisen over the proposal to begin in utero gene therapy clinical trials for the treatment of inherited genetic disorders. From this standpoint, despite the latest significant achievements reported in vector design, it is not possible to predict to what extent gene therapeutic interventions will be effective in patients, and in what time frame.

Construction of a pseudoreceptor that mediates transduction by adenoviruses expressing a ligand in fiber or penton base

Modification of adenovirus to achieve tissue specific targeting for the delivery of therapeutic genes requires both the ablation of its native tropism and the introduction of specific, novel interactions. Inactivation of the native receptor interactions, however, would cripple the virus for growth in production cells. We have developed an alternative receptor, or pseudoreceptor, for the virus which might allow propagation of viruses with modified fiber proteins that no longer bind to the native adenovirus receptor (coxsackievirus/adenovirus receptor [CAR]). We have constructed a membrane-anchored single-chain antibody [m-scFv(HA)] which recognizes a linear peptide epitope (hemagglutinin [HA]). Incorporation of HA within the HI loop of the fiber protein enabled the modified virus to transduce pseudoreceptor expressing cells under conditions where fiber-CAR interaction was blocked or absent. The pseudoreceptor mediated virus transduction with an efficiency similar to that of CAR. In addition, the HA epitope mediated virus transduction through interaction with the m-scFv(HA) when it was introduced into penton base. These findings indicate that cells expressing the pseudoreceptor should support production of HA-tagged adenoviruses independent of retaining the fiber-CAR interaction. Moreover, they demonstrate that high-affinity targeting ligands may function following insertion into either penton base or fiber.

Tumor-specific gene transfer via an adenoviral vector targeted to the pan-carcinoma antigen EpCAM

The utility of adenoviral vectors for cancer therapy is limited due to their lack of specificity for tumor cells. In order to target adenovirus to tumor, the natural tropism of the adenovirus should be ablated and replaced by a tumor-specific binding domain. To this end, a neutralizing anti-fiber antibody conjugated to an anti-EpCAM antibody was created that targets the adenovirus to the EpCAM antigen present on tumor cells. The EpCAM antigen was chosen as the target because this antigen is highly expressed on a variety of adenocarcinomas of different origin such as breast, ovary, colon and lung, whereas EpCAM expression is limited in normal tissues. In these studies, the EpCAM-targeted adenovirus was shown to infect specifically cancer cell lines of different origin expressing EpCAM such as ovary, colon and head and neck. Gene transfer was blocked by excess anti-EpCAM antibody and dramatically reduced in EpCAM negative cell lines, thus showing the specificity of the EpCAM-targeted adenovirus. Importantly, infection with targeted adenovirus was independent of CAR, which is the natural receptor for adenovirus binding, since blocking of CAR with recombinant fiber knob did not affect infection with targeted adenovirus. Apart from the cancer cell lines, the efficacy of targeted viral infection was studied in freshly isolated primary human colon cancer cells. As colon cancer predominantly metastasizes to liver, and adenovirus has a high tropism for hepatocytes, we also sought to determine if the EpCAM-targeted adenovirus showed reduced infectivity of human liver cells. The bispecific antibody could successfully mediate gene transfer to primary human colon cancer cells, whereas it almost completely abolished infection of liver cells. This work thus demonstrates that EpCAM-targeted adenoviral vectors can be specifically directed to a wide variety of adenocarcinomas. This approach may prove to be useful for selective gene therapy of cancer.

Fiber swap between adenovirus subgroups B and C alters intracellular trafficking of adenovirus gene transfer vectors

Following receptor binding and internalization, intracellular trafficking of adenovirus (Ad) among subgroups B and C is different, with significant amounts of Ad serotype 7 (Ad7) (subgroup B) virions found in cytoplasm during the initial hours of infection while Ad5 (subgroup C) virions rapidly translocate to the nucleus. To evaluate the role of the fiber in these differences, we examined intracellular trafficking of Ad5, Ad7, and Ad5f7 (a chimeric vector composed of the Ad5 capsid with the fiber replaced by the Ad7 fiber) by conjugating Ad capsids directly with Cy3 fluorescent dye, permitting the trafficking of the capsids to be examined by fluorescence microscopy. The human lung carcinoma cell line A549 was infected with Cy3-conjugated viruses for 10 min followed by a 1-h incubation. Ad5 virions rapidly translocated to the nucleus (within 1 h of infection), while Ad7 virions were widely distributed in the cytoplasm at the same time point. Interestingly, chimeric Ad5f7 virions behaved similarly to Ad7 but not Ad5. In this regard, the percentages of nuclear localization of Ad5, Ad7, and Ad5f7 at 1 h following infection were 72% +/- 4%, 32% +/- 6%, and 38% +/- 2%, respectively. Consistent with these observations, fluorescence in situ hybridization demonstrated that most of the Ad5 DNA was detected at the nucleus after 1 h, but at the same time point, DNA of Ad7 and Ad5f7 was distributed in both the nucleus and cytoplasm. Quantification of the kinetics of Ad genomic DNA delivery to the nucleus using a fluorogenic probe-based PCR assay (TaqMan PCR) demonstrated that the percentages of nuclear association of Ad5 DNA and Ad5f7 DNA at 1 h postinfection were 80% +/- 13% and 43% +/- 1%, respectively. Although it has been generally accepted that Ad fiber protein mediates attachment of virions to cells and that fibers dissociate during endocytic uptake, these data suggest that in addition to mediating binding to the cell surface, fiber likely modulates intracellular trafficking as well.

Human PBMC-derived dendritic cells transduced with an adenovirus vectorinduce cytotoxic T-lymphocyte responses against a vector-encoded antigen in vitro

Dendritic cells (DC) are among the most potent antigen-presenting cells known and play an important role in the initiation of antigen-specific T-lymphocyte responses. Several recent studies have demonstrated that DC expressing vector-encoded tumor-associated antigens can induce protective and therapeutic immunity in murine cancer models. In the current study we set out to examine in vitro the utility of adenovirus vectors in the transduction of human DC for the induction of antigen-specific T-lymphocyte responses against a defined vector-encoded antigen. DC were derived from the adherent fraction of PBMC by culture in defined medium containing GM-CSF and IL-4. A replication-defective E1/E3-deleted type 5 adenovirus vector encoding bacterial beta-galactosidase (beta-gal) under the transcriptional control of a CMV promoter was used to transduce DC at multiplicities of infection (MOI) up to 1000. While high MOI were required to achieve efficient transduction there was no significant effect on DC morphology, immunophenotype or potency in allogeneic lymphocyte proliferation assays. Furthermore, transduced DC-induced antigen-specific CTL activity against adenoviral proteins and more significantly, the vector-encoded antigen beta-gal. These data clearly demonstrate the potential of adenovirus vectors in anticancer DC vaccine strategies and provide an important link between existing animal data and human clinical application.

A system for the propagation of adenoviral vectors with genetically modified receptor specificities

The development of genetically modified adenovirus (Ad) vectors with specificity for a single cell type will require both the introduction of novel tropism determinants and the ablation of endogenous tropism. Consequently, it will not be possible to exploit the native cellular entry pathway in the propagation of these targeted Ad vectors. Based on the concept that Ad enters cells by a two-step process in which a primary receptor serves as a high affinity binding site for the Ad fiber knob, with subsequent internalization mediated by alpha v integrins, we designed two artificial primary receptors. The extracellular domain of one of these synthetic receptors was derived from a single-chain antibody (sFv) with specificity for Ad5 knob, while the second receptor consisted of an icosapeptide identified by biopanning a phage display library against Ad5 knob. Expression of either of these artificial virus-binding receptors in fiber receptor-negative cells possessing alpha v integrins conferred susceptibility to Ad infection. We then created a novel mechanism for cell binding by genetically modifying both the vector and the target cell. In this approach, six histidine (His) residues were incorporated at the C-terminal of the Ad fiber protein. The resultant Ad vector was able to infect nonpermissive cells displaying the cognate artificial receptor, containing an anti-His sFv. This strategy, comprising a genetically engineered Ad virion and a modified cell line, should be useful in the propagation of targeted Ad vectors that lack the ability to bind the native fiber receptor.

Inducible genetic suppression of neuronal excitability

Graded, reversible suppression of neuronal excitability represents a logical goal of therapy for epilepsy and intractable pain. To achieve such suppression, we have developed the means to transfer "electrical silencing" genes into neurons with sensitive control of transgene expression. An ecdysone-inducible promoter drives the expression of inwardly rectifying potassium channels in polycistronic adenoviral vectors. Infection of superior cervical ganglion neurons did not affect normal electrical activity but suppressed excitability after the induction of gene expression. These experiments demonstrate the feasibility of controlled ion channel expression after somatic gene transfer into neurons and serve as the prototype for a novel generalizable approach to modulate excitability.

Recombinant herpes simplex virus type 1 engineered for targeted binding to erythropoietin receptor-bearing cells

The utility of recombinant herpes simplex virus type 1 (HSV-1) vectors may be expanded by manipulation of the virus envelope to achieve cell-specific gene delivery. To this end, an HSV-1 mutant virus deleted for glycoprotein C (gC) and the heparan sulfate binding domain of gB (KgBpK-gC-) was engineered to encode different chimeric proteins composed of N-terminally truncated forms of gC and the full-length erythropoietin hormone (EPO). Biochemical analyses demonstrated that one gC-EPO chimeric molecule (gCEPO2) was posttranslationally processed, incorporated into recombinant HSV-1 virus (KgBpK-gCEPO2), and neutralized with antibodies directed against gC or EPO in a complement-dependent manner. Moreover, KgBpK-gCEPO2 recombinant virus was specifically retained on a soluble EPO receptor column, was neutralized by soluble EPO receptor, and stimulated proliferation of FD-EPO cells, an EPO growth-dependent cell line. FD-EPO cells were nevertheless refractory to productive infection by both wild-type HSV-1 and recombinant KgBpK-gCEPO2 virus. Transmission electron microscopy of FD-EPO cells infected with KgBpK-gCEPO2 showed virus endocytosis leading to aborted infection. Despite the lack of productive infection, these data provide the first evidence of targeted HSV-1 binding to a non-HSV-1 cell surface receptor.