In vivo hepatic adenoviral gene delivery occurs independently of the coxsackievirus-adenovirus receptor

The investigation of oncolytic viruses in the field of cancer therapy

Oncolytic viruses (OVs) have emerged as a potential strategy for tumor treatment due to their ability to selectively replicate in tumor cells, induce apoptosis, and stimulate immune responses. However, the therapeutic efficacy of single OVs is limited by the complexity and immunosuppressive nature of the tumor microenvironment (TME). To overcome these challenges, engineering OVs has become an important research direction. This review focuses on engineering methods and multi-modal combination therapies for OVs aimed at addressing delivery barriers, viral phagocytosis, and antiviral immunity in tumor therapy. The engineering approaches discussed include enhancing in vivo immune response, improving replication efficiency within the tumor cells, enhancing safety profiles, and improving targeting capabilities. In addition, this review describes the potential mechanisms of OVs combined with radiotherapy, chemotherapy, cell therapy and immune checkpoint inhibitors (ICIs), and summarizes the data of ongoing clinical trials. By continuously optimizing engineering strategies and combination therapy programs, we can achieve improved treatment outcomes and quality of life for cancer patients.

Oncolytic virotherapy: basic principles, recent advances and future directions

Oncolytic viruses (OVs) have attracted growing awareness in the twenty-first century, as they are generally considered to have direct oncolysis and cancer immune effects. With the progress in genetic engineering technology, OVs have been adopted as versatile platforms for developing novel antitumor strategies, used alone or in combination with other therapies. Recent studies have yielded eye-catching results that delineate the promising clinical outcomes that OVs would bring about in the future. In this review, we summarized the basic principles of OVs in terms of their classifications, as well as the recent advances in OV-modification strategies based on their characteristics, biofunctions, and cancer hallmarks. Candidate OVs are expected to be designed as "qualified soldiers" first by improving target fidelity and safety, and then equipped with "cold weapons" for a proper cytocidal effect, "hot weapons" capable of activating cancer immunotherapy, or "auxiliary weapons" by harnessing tactics such as anti-angiogenesis, reversed metabolic reprogramming and decomposing extracellular matrix around tumors. Combinations with other cancer therapeutic agents have also been elaborated to show encouraging antitumor effects. Robust results from clinical trials using OV as a treatment congruously suggested its significance in future application directions and challenges in developing OVs as novel weapons for tactical decisions in cancer treatment.

Engineering Cancer Selective Virotherapies: Are the Pieces of the Puzzle Falling into Place?

Advances in gene therapy, synthetic biology, cancer genomics, and patient-derived cancer models have expanded the repertoire of strategies for targeting human cancers using viral vectors. Novel capsids, synthetic promoters, and therapeutic payloads are being developed and assessed through approaches such as rational design, pooled library screening, and directed evolution. Ultimately, the goal is to generate precision-engineered viruses that target different facets of tumor cell biology, without compromising normal tissue and organ function. In this study, we briefly review the opportunities for engineering cancer selectivity into viral vectors at both the cell extrinsic and intrinsic level. Such stringently tumor-targeted vectors can subsequently act as platforms for the delivery of potent therapeutic transgenes, including the exciting prospect of immunotherapeutic payloads. These have the potential to eradicate nontransduced cells through stimulation of systemic anticancer immune responses, thereby side-stepping the inherent challenge of achieving gene delivery to the entire cancer cell population. We discuss the importance of using advanced primary human cellular models, such as patient-derived cultures and organoids, to enable rapid screening and triage of novel candidates using disease-relevant models. We believe this combination of improved delivery and selectivity, through novel capsids and promoters, coupled with more potent choices for the combinations of immunotherapy-based payloads seems capable of finally delivering innovative new gene therapies for oncology. Many pieces of the puzzle of how to build a virus capable of targeting human cancers appear to be falling into place.

Efficient Intravenous Tumor Targeting Using the αvβ6 Integrin-Selective Precision Virotherapy Ad5NULL-A20

We previously developed a refined, tumor-selective adenovirus, Ad5NULL-A20, harboring tropism ablating mutations in each major capsid protein, to ablate all native means of infection. We incorporated a 20-mer peptide (A20) in the fiber knob for selective infection via αvβ6 integrin, a marker of aggressive epithelial cancers. Methods: To ascertain the selectivity of Ad5NULL-A20 for αvβ6-positive tumor cell lines of pancreatic and breast cancer origin, we performed reporter gene and cell viability assays. Biodistribution of viral vectors in mice harboring xenografts with low, medium, and high αvβ6 levels was quantified by qPCR for viral genomes 48 h post intravenous administration. Results: Ad5NULL-A20 vector transduced cells in an αvβ6-selective manner, whilst cell killing mediated by oncolytic Ad5NULL-A20 was αvβ6-selective. Biodistribution analysis following intravenous administration into mice bearing breast cancer xenografts demonstrated that Ad5NULL-A20 resulted in significantly reduced liver accumulation coupled with increased tumor accumulation compared to Ad5 in all three models, with tumor-to-liver ratios improved as a function of αvβ6 expression. Conclusions: Ad5NULL-A20-based virotherapies efficiently target αvβ6-integrin-positive tumors following intravenous administration, validating the potential of Ad5NULL-A20 for systemic applications, enabling tumor-selective overexpression of virally encoded therapeutic transgenes.

The Fiber Knob Protein of Human Adenovirus Type 49 Mediates Highly Efficient and Promiscuous Infection of Cancer Cell Lines Using a Novel Cell Entry Mechanism

The human adenovirus (HAdV) phylogenetic tree is diverse, divided across seven species and comprising over 100 individual types. Species D HAdV are rarely isolated with low rates of preexisting immunity, making them appealing for therapeutic applications. Several species D vectors have been developed as vaccines against infectious diseases, where they induce robust immunity in preclinical models and early phase clinical trials. However, many aspects of the basic virology of species D HAdV, including their basic receptor usage and means of cell entry, remain understudied. Here, we investigated HAdV-D49, which previously has been studied for vaccine and vascular gene transfer applications. We generated a pseudotyped HAdV-C5 presenting the HAdV-D49 fiber knob protein (HAdV-C5/D49K). This pseudotyped vector was efficient at infecting cells devoid of all known HAdV receptors, indicating HAdV-D49 uses an unidentified cellular receptor. Conversely, a pseudotyped vector presenting the fiber knob protein of the closely related HAdV-D30 (HAdV-C5/D30K), differing in four amino acids from HAdV-D49, failed to demonstrate the same tropism. These four amino acid changes resulted in a change in isoelectric point of the knob protein, with HAdV-D49K possessing a basic apical region compared to a more acidic region in HAdV-D30K. Structurally and biologically we demonstrate that HAdV-D49 knob protein is unable to engage CD46, while potential interaction with coxsackievirus and adenovirus receptor (CAR) is extremely limited by extension of the DG loop. HAdV-C5/49K efficiently transduced cancer cell lines of pancreatic, breast, lung, esophageal, and ovarian origin, indicating it may have potential for oncolytic virotherapy applications, especially for difficult to transduce tumor types.IMPORTANCE Adenoviruses are powerful tools experimentally and clinically. To maximize efficacy, the development of serotypes with low preexisting levels of immunity in the population is desirable. Consequently, attention has focused on those derived from species D, which have proven robust vaccine platforms. This widespread usage is despite limited knowledge in their basic biology and cellular tropism. We investigated the tropism of HAdV-D49, demonstrating that it uses a novel cell entry mechanism that bypasses all known HAdV receptors. We demonstrate, biologically, that a pseudotyped HAdV-C5/D49K vector efficiently transduces a wide range of cell lines, including those presenting no known adenovirus receptor. Structural investigation suggests that this broad tropism is the result of a highly basic electrostatic surface potential, since a homologous pseudotyped vector with a more acidic surface potential, HAdV-C5/D30K, does not display a similar pantropism. Therefore, HAdV-C5/D49K may form a powerful vector for therapeutic applications capable of infecting difficult to transduce cells.

Hitting the Target but Missing the Point: Recent Progress towards Adenovirus-Based Precision Virotherapies

More people are surviving longer with cancer. Whilst this can be partially attributed to advances in early detection of cancers, there is little doubt that the improvement in survival statistics is also due to the expansion in the spectrum of treatments available for efficacious treatment. Transformative amongst those are immunotherapies, which have proven effective agents for treating immunogenic forms of cancer, although immunologically "cold" tumour types remain refractive. Oncolytic viruses, such as those based on adenovirus, have great potential as anti-cancer agents and have seen a resurgence of interest in recent years. Amongst their many advantages is their ability to induce immunogenic cell death (ICD) of infected tumour cells, thus providing the alluring potential to synergise with immunotherapies by turning immunologically "cold" tumours "hot". Additionally, enhanced immune mediated cell killing can be promoted through the local overexpression of immunological transgenes, encoded from within the engineered viral genome. To achieve this full potential requires the development of refined, tumour selective "precision virotherapies" that are extensively engineered to prevent off-target up take via native routes of infection and targeted to infect and replicate uniquely within malignantly transformed cells. Here, we review the latest advances towards this holy grail within the adenoviral field.

The regulatory role of PGC1α-related coactivator in response to drug-induced liver injury

PGC1α-Related Coactivator (PRC) is a transcriptional coactivator promoting cytokine expression in vitro in response to mitochondrial injury and oxidative stress, however, its physiological role has remained elusive. Herein we investigate aspects of the immune response function of PRC, first in an in vivo thioacetamide (TAA)-induced mouse model of drug-induced liver injury (DILI), and subsequently in vitro in human monocytes, HepG2, and dendritic (DC) cells. TAA treatment resulted in the dose-dependent induction of PRC mRNA and protein, both of which were shown to correlate with liver injury markers. Conversely, an adenovirus-mediated knockdown of PRC attenuated this response, thereby reducing hepatic cytokine mRNA expression and monocyte infiltration. Subsequent in vitro studies with conditioned media from HepG2 cells overexpressing PRC, activated human monocytes and monocyte-derived DC, demonstrated up to 20% elevated expression of CD86, CD40, and HLA-DR. Similarly, siRNA-mediated knockdown of PRC abolished this response in oligomycin stressed HepG2 cells. A putative mechanism was suggested by the co-immunoprecipitation of Signal Transducer and Activator of Transcription 1 (STAT1) with PRC, and induction of a STAT-dependent reporter. Furthermore, PRC co-activated an NF-κB-dependent reporter, indicating interaction with known major inflammatory factors. In summary, our study indicates PRC as a novel factor modulating inflammation in DILI.

Innate immunity to adenovirus: lessons from mice

Adenovirus is a highly evolutionary successful pathogen, as it is widely prevalent across the animal kingdom, infecting hosts ranging from lizards and frogs to dolphins, birds, and humans. Although natural adenovirus infections in humans rarely cause severe pathology, intravenous injection of high doses of adenovirus-based vectors triggers rapid activation of the innate immune system, leading to cytokine storm syndrome, disseminated intravascular coagulation, thrombocytopenia, and hepatotoxicity, which individually or in combination may cause morbidity and mortality. Much of the information on exactly how adenovirus activates the innate immune system has been gathered from mouse experimental systems. Intravenous administration of adenovirus to mice revealed mechanistic insights into cellular and molecular components of the innate immunity that detect adenovirus particles, activate pro-inflammatory signaling pathways and cytokine production, sequester adenovirus particles from the bloodstream, and eliminate adenovirus-infected cells. Collectively, this information greatly improved our understanding of mechanisms of activation of innate immunity to adenovirus and may pave the way for designing safer adenovirus-based vectors for therapy of genetic and acquired human diseases.

Overexpression of NRK1 ameliorates diet- and age-induced hepatic steatosis and insulin resistance

NAD⁺ is a co-enzyme in redox reactions and a substrate required for activity of various enzyme families, including sirtuins and poly(ADP-ribose) polymerases. Dietary supplementation of NAD⁺ precursors nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR) protects against metabolic disease, neurodegenerative disorders and age-related physiological decline in mammals. Here we sought to identify the roles of nicotinamide riboside kinase 1 (NRK1) plays in regulating hepatic NAD⁺ biosynthesis and lipid metabolism. Using adenovirus mediated gene transduction to overexpress or knockdown NRK1 in mouse liver, we have demonstrated that NRK1 is critical for maintaining hepatic NAD⁺ levels and triglyceride content. We have further shown that the hepatic expression of Nmrk1 mRNA is significantly decreased either in mice treated with high-fat diet or in aged mice. However, adenoviral delivery of NRK1 in these diet- and age-induced mice elevates hepatic NAD⁺ levels, reduces hepatic steatosis, and improves glucose tolerance and insulin sensitivity. Our results provide important insights in targeting NRK1 for treating hepatic steatosis.

Designer Oncolytic Adenovirus: Coming of Age

The licensing of talimogene laherparepvec (T-Vec) represented a landmark moment for oncolytic virotherapy, since it provided unequivocal evidence for the long-touted potential of genetically modified replicating viruses as anti-cancer agents. Whilst T-Vec is promising as a locally delivered virotherapy, especially in combination with immune-checkpoint inhibitors, the quest continues for a virus capable of specific tumour cell killing via systemic administration. One candidate is oncolytic adenovirus (Ad); it’s double stranded DNA genome is easily manipulated and a wide range of strategies and technologies have been employed to empower the vector with improved pharmacokinetics and tumour targeting ability. As well characterised clinical and experimental agents, we have detailed knowledge of adenoviruses’ mechanisms of pathogenicity, supported by detailed virological studies and in vivo interactions. In this review we highlight the strides made in the engineering of bespoke adenoviral vectors to specifically infect, replicate within, and destroy tumour cells. We discuss how mutations in genes regulating adenoviral replication after cell entry can be used to restrict replication to the tumour, and summarise how detailed knowledge of viral capsid interactions enable rational modification to eliminate native tropisms, and simultaneously promote active uptake by cancerous tissues. We argue that these designer-viruses, exploiting the viruses natural mechanisms and regulated at every level of replication, represent the ideal platforms for local overexpression of therapeutic transgenes such as immunomodulatory agents. Where T-Vec has paved the way, Ad-based vectors now follow. The era of designer oncolytic virotherapies looks decidedly as though it will soon become a reality.

Ad5NULL-A20: A Tropism-Modified, αvβ6 Integrin-Selective Oncolytic Adenovirus for Epithelial Ovarian Cancer Therapies

Purpose: Virotherapies are maturing in the clinical setting. Adenoviruses (Ad) are excellent vectors for the manipulability and tolerance of transgenes. Poor tumor selectivity, off-target sequestration, and immune inactivation hamper clinical efficacy. We sought to completely redesign Ad5 into a refined, tumor-selective virotherapy targeted to αvβ6 integrin, which is expressed in a range of aggressively transformed epithelial cancers but nondetectable in healthy tissues.Experimental Design: Ad5_NULL-A20 harbors mutations in each major capsid protein to preclude uptake via all native pathways. Tumor-tropism via αvβ6 targeting was achieved by genetic insertion of A20 peptide (NAVPNLRGDLQVLAQKVART) within the fiber knob protein. The vector's selectivity in vitro and in vivo was assessed.Results: The tropism-ablating triple mutation completely blocked all native cell entry pathways of Ad5_NULL-A20 via coxsackie and adenovirus receptor (CAR), αvβ3/5 integrins, and coagulation factor 10 (FX). Ad5_NULL-A20 efficiently and selectively transduced αvβ6⁺ cell lines and primary clinical ascites-derived EOC ex vivo, including in the presence of preexisting anti-Ad5 immunity. In vivo biodistribution of Ad5_NULL-A20 following systemic delivery in non-tumor-bearing mice was significantly reduced in all off-target organs, including a remarkable 10⁷-fold reduced genome accumulation in the liver compared with Ad5. Tumor uptake, transgene expression, and efficacy were confirmed in a peritoneal SKOV3 xenograft model of human EOC, where oncolytic Ad5_NULL-A20-treated animals demonstrated significantly improved survival compared with those treated with oncolytic Ad5.Conclusions: Oncolytic Ad5_NULL-A20 virotherapies represent an excellent vector for local and systemic targeting of αvβ6-overexpressing cancers and exciting platforms for tumor-selective overexpression of therapeutic anticancer modalities, including immune checkpoint inhibitors. Clin Cancer Res; 24(17); 4215-24. ©2018 AACR.

Pseudotyped αvβ6 integrin-targeted adenovirus vectors for ovarian cancer therapies

Encouraging results from recent clinical trials are revitalizing the field of oncolytic virotherapies. Human adenovirus type 5 (HAdV-C5/Ad5) is a common vector for its ease of manipulation, high production titers and capacity to transduce multiple cell types. However, effective clinical applications are hindered by poor tumor-selectivity and vector neutralization. We generated Ad5/kn48 by pseudotyping Ad5 with the fiber knob domain from the less seroprevalent HAdV-D48 (Ad48). The vector was shown to utilize coxsackie and adenovirus receptor (CAR) but not CD46 for cell entry. A 20-amino acid peptide NAVPNLRGDLQVLAQKVART (A20) was inserted into the Ad5. Luc HI loop (Ad5.HI.A20) and Ad5/kn48 DG loop (Ad5/kn48.DG.A20) to target a prognostic cancer cell marker, αvβ6 integrin. Relative to the Ad5.Luc parent vector, Ad5.HI.A20, Ad5.KO1.HI.A20 (KO1, ablated CAR-binding) and Ad5/kn48.DG.A20 showed ~ 160-, 270- and 180-fold increased transduction in BT-20 breast carcinoma cells (αvβ6high). Primary human epithelial ovarian cancer (EOC) cultures derived from clinical ascites provided a useful ex vivo model for intraperitoneal virotherapy. Ad5.HI.A20, Ad5.KO1.HI.A20 and Ad5/kn48.DG.A20 transduction was ~ 70-, 60- and 16-fold increased relative to Ad5.Luc in EOC cells (αvβ6high), respectively. A20 vectors transduced EOC cells at up to ~ 950-fold higher efficiency in the presence of neutralizing ovarian ascites, as compared to Ad5.Luc. Efficient transduction and enhanced cancer-selectivity via a non-native αvβ6-mediated route was demonstrated, even in the presence of pre-existing anti-Ad5 immunity. Consequently, αvβ6-targeted Ad vectors may represent a promising platform for local intraperitoneal treatment of ovarian cancer metastases.

Defining a Novel Role for the Coxsackievirus and Adenovirus Receptor in Human Adenovirus Serotype 5 Transduction In Vitro in the Presence of Mouse Serum

Human adenoviral serotype 5 (HAdV-5) vectors have predominantly hepatic tropism when delivered intravascularly, resulting in immune activation and toxicity. Coagulation factor X (FX) binding to HAdV-5 mediates liver transduction and provides protection from virion neutralization in mice. FX is dispensable for liver transduction in mice lacking IgM antibodies or complement, suggesting that alternative transduction pathways exist. To identify novel factor(s) mediating HAdV-5 FX-independent entry, we investigated HAdV-5 transduction in vitro in the presence of serum from immunocompetent C57BL/6 or immunocompromised mice lacking IgM antibodies (Rag 2^-/- and NOD-scid-gamma [NSG]). Sera from all three mouse strains enhanced HAdV-5 transduction of A549 cells. While inhibition of HAdV-5-FX interaction with FX-binding protein (X-bp) inhibited transduction in the presence of C57BL/6 serum, it had negligible effect on the enhanced transduction observed in the presence of Rag 2^-/- or NSG serum. Rag 2^-/- serum also enhanced transduction of the FX binding-deficient HAdV-5HVR5*HVR7*E451Q (AdT*). Interestingly, Rag 2^-/- serum enhanced HAdV-5 transduction in a FX-independent manner in CHO-CAR and SKOV3-CAR cells (CHO or SKOV3 cells transfected to stably express human coxsackievirus and adenovirus receptor [CAR]). Additionally, blockade of CAR with soluble HAdV-5 fiber knob inhibited mouse serum-enhanced transduction in A549 cells, suggesting a potential role for CAR. Transduction of HAdV-5 KO1 and HAdV-5/F35 (CAR binding deficient) in the presence of Rag 2^-/- serum was equivalent to that of HAdV-5, indicating that direct interaction between HAdV-5 and CAR is not required. These data suggest that FX may protect HAdV-5 from neutralization but has minimal contribution to HAdV-5 transduction in the presence of immunocompromised mouse serum. Alternatively, transduction occurs via an unidentified mouse serum protein capable of bridging HAdV-5 to CAR.IMPORTANCE The intravascular administration of HAdV-5 vectors can result in acute liver toxicity, transaminitis, thrombocytopenia, and injury to the vascular endothelium, illustrating challenges yet to overcome for HAdV-5-mediated systemic gene therapy. The finding that CAR and potentially an unidentified factor present in mouse serum might be important mediators of HAdV-5 transduction highlights that a better understanding of the complex biology defining the interplay between adenovirus immune recognition and cellular uptake mechanisms is still required. These findings are important to inform future optimization and development of HAdV-5-based adenoviral vectors for gene therapy.

Oncolytic viruses: From bench to bedside with a focus on safety

Oncolytic viruses are a relatively new class of anti-cancer immunotherapy agents. Several viruses have undergone evaluation in clinical trials in the last decades, and the first agent is about to be approved to be used as a novel cancer therapy modality. In the current review, an overview is presented on recent (pre)clinical developments in the field of oncolytic viruses that have previously been or currently are being evaluated in clinical trials. Special attention is given to possible safety issues like toxicity, environmental shedding, mutation and reversion to wildtype virus.

Incorporation of Peptides Targeting EGFR and FGFR1 into the Adenoviral Fiber Knob Domain and Their Evaluation as Targeted Cancer Therapies

Oncolytic virotherapies based on adenovirus 5 (Ad5) hold promise as adjunctive cancer therapies; however, their efficacy when delivered systemically is hampered by poor target cell specificity and preexisting anti-Ad5 immunity. Ovarian cancer represents a promising target for virotherapy, since the virus can be delivered locally into the peritoneal cavity. Both epidermal growth factor receptor (EGFR) and fibroblast growth factor receptor 1 (FGFR1) are overexpressed in the majority of human tumors, including ovarian cancer. To generate adenoviral vectors with improved tumor specificity, we generated a panel of Ad5 vectors with altered tropism for EGFR and FGFR, rather than the natural Ad5 receptor, hCAR. We have included mutations within AB loop of the viral fiber knob (KO1 mutation) to preclude interaction with hCAR, combined with insertions in the HI loop to incorporate peptides that bind either EGFR (peptide YHWYGYTPQNVI, GE11) or FGFR1 (peptides MQLPLAT, M*, and LSPPRYP, LS). Viruses were produced to high titers, and the integrity of the fiber protein was validated by Western blotting. The KO1 mutation efficiently ablated hCAR interactions, and significantly increased transduction was observed in hCAR^low/EGFR^high cell lines using Ad5.GE11, while transduction levels using Ad5.M* or Ad5.LS were not increased. In the presence of physiological concentrations of human blood clotting factor X (hFX), significantly increased levels of transduction via the hFX-mediated pathway were observed in cell lines, but not in primary tumor cells derived from epithelial ovarian cancer (EOC) ascites samples. Ad5-mediated transduction of EOC cells was completely abolished by the presence of 2.5% serum from patients, while, surprisingly, incorporation of the GE11 peptide resulted in significant evasion of neutralization in the same samples. We thus speculate that incorporation of the YHWYGYTPQNVI dodecapeptide within the fiber knob domain may provide a novel means of circumventing preexisting Ad5 immunity that warrants further investigation.

Oncolytic Adenovirus: Strategies and Insights for Vector Design and Immuno-Oncolytic Applications

Adenoviruses (Ad) are commonly used both experimentally and clinically, including oncolytic virotherapy applications. In the clinical area, efficacy is frequently hampered by the high rates of neutralizing immunity, estimated as high as 90% in some populations that promote vector clearance and limit bioavailability for tumor targeting following systemic delivery. Active tumor targeting is also hampered by the ubiquitous nature of the Ad5 receptor, hCAR, as well as the lack of highly tumor-selective targeting ligands and suitable targeting strategies. Furthermore, significant off-target interactions between the viral vector and cellular and proteinaceous components of the bloodstream have been documented that promote uptake into non-target cells and determine dose-limiting toxicities. Novel strategies are therefore needed to overcome the obstacles that prevent efficacious Ad deployment for wider clinical applications. The use of less seroprevalent Ad serotypes, non-human serotypes, capsid pseudotyping, chemical shielding and genetic masking by heterologous peptide incorporation are all potential strategies to achieve efficient vector escape from humoral immune recognition. Conversely, selective vector arming with immunostimulatory agents can be utilized to enhance their oncolytic potential by activation of cancer-specific immune responses against the malignant tissues. This review presents recent advantages and pitfalls occurring in the field of adenoviral oncolytic therapies.

Chapter two--Adenovirus strategies for tissue-specific targeting

Cancer gene therapy approaches have benefited greatly from the utilization of molecular-based therapeutics. Of these, adenovirus-based interventions hold much promise as a platform for targeted therapeutic delivery to tumors. However, a barrier to this progression is the lack of native adenovirus receptor expression on a variety of cancer types. As such, any adenovirus-based cancer therapy must take into consideration retargeting the vector to nonnative cellular surface receptors. Predicated upon the knowledge gained in native adenovirus biology, several strategies to transductionally retarget adenovirus have emerged. Herein, we describe the biological hurdles as well as strategies utilized in adenovirus transductional targeting, covering the progress of both adapter-based and genetic manipulation-based targeting. Additionally, we discuss recent translation of these targeting strategies into a clinical setting.

A mouse model for HIV-1 entry

Passive transfer of neutralizing antibodies against HIV-1 can prevent infection in macaques and seems to delay HIV-1 rebound in humans. Anti-HIV antibodies are therefore of great interest for vaccine design. However, the basis for their in vivo activity has been difficult to evaluate systematically because of a paucity of small animal models for HIV infection. Here we report a genetically humanized mouse model that incorporates a luciferase reporter for rapid quantitation of HIV entry. An antibody's ability to block viral entry in this in vivo model is a function of its bioavailability, direct neutralizing activity, and effector functions.

Sstr2A: a relevant target for the delivery of genes into human glioblastoma cells using fiber-modified adenoviral vectors

Glioblastomas are the most aggressive of the brain tumors occurring in adults and children. Currently available chemotherapy prolongs the median survival time of patients by only 4 months. The low efficiency of current treatments is partly owing to the blood-brain barrier, which restricts the penetration of most drugs into the central nervous system. Locoregional treatment strategies thus become mandatory. In this context, viral tools are of great interest for the selective delivery of genes into tumoral cells. Gliomas express high levels of type 2 somatostatin receptors (sstr2A), pinpointing them as suitable targets for the improvement of transduction efficiency in these tumors. We designed a new adenoviral vector based on the introduction of the full-length somatostatin (SRIF (somatotropin release-inhibiting factor)) sequence into the HI loop of the HAdV fiber protein. We demonstrate that (i) HAdV-5-SRIF uptake into cells is mediated by sstr2A, (ii) our vector drives high levels of gene expression in cells expressing endogenous sstr2A, with up to 65-fold enhancement and (iii) low doses of HAdV-5-SRIF are sufficient to infect high-grade human primary glioblastoma cells. Adenoviral vectors targeting SRIF receptors might thus represent a promising therapeutic approach to brain tumors.

Interaction between mouse adenovirus type 1 and cell surface heparan sulfate proteoglycans

Application of human adenovirus type 5 (Ad5) derived vectors for cancer gene therapy has been limited by the poor cell surface expression, on some tumor cell types, of the primary Ad5 receptor, the coxsackie-adenovirus-receptor (CAR), as well as the accumulation of Ad5 in the liver following interaction with blood coagulation factor X (FX) and subsequent tethering of the FX-Ad5 complex to heparan sulfate proteoglycan (HSPG) on liver cells. As an alternative vector, mouse adenovirus type 1 (MAV-1) is particularly attractive, since this non-human adenovirus displays pronounced endothelial cell tropism and does not use CAR as a cellular attachment receptor. We here demonstrate that MAV-1 uses cell surface heparan sulfate proteoglycans (HSPGs) as primary cellular attachment receptor. Direct binding of MAV-1 to heparan sulfate-coated plates proved to be markedly more efficient compared to that of Ad5. Experiments with modified heparins revealed that the interaction of MAV-1 to HSPGs depends on their N-sulfation and, to a lesser extent, 6-O-sulfation rate. Whereas the interaction between Ad5 and HSPGs was enhanced by FX, this was not the case for MAV-1. A slot blot assay demonstrated the ability of MAV-1 to directly interact with FX, although the amount of FX complexed to MAV-1 was much lower than observed for Ad5. Analysis of the binding of MAV-1 and Ad5 to the NCI-60 panel of different human tumor cell lines revealed the preference of MAV-1 for ovarian carcinoma cells. Together, the data presented here enlarge our insight into the HSPG receptor usage of MAV-1 and support the development of an MAV-1-derived gene vector for human cancer therapy.

Gene therapy: light is finally in the tunnel

After two decades of ups and downs, gene therapy has recently achieved a milestone in treating patients with Leber's congenital amaurosis (LCA). LCA is a group of inherited blinding diseases with retinal degeneration and severe vision loss in early infancy. Mutations in several genes, including RPE65, cause the disease. Using adeno-associated virus as a vector, three independent teams of investigators have recently shown that RPE65 can be delivered to retinal pigment epithelial cells of LCA patients by subretinal injections resulting in clinical benefits without side effects. However, considering the whole field of gene therapy, there are still major obstacles to clinical applications for other diseases. These obstacles include innate and immune barriers to vector delivery, toxicity of vectors and the lack of sustained therapeutic gene expression. Therefore, new strategies are needed to overcome these hurdles for achieving safe and effective gene therapy. In this article, we shall review the major advancements over the past two decades and, using lung gene therapy as an example, discuss the current obstacles and possible solutions to provide a roadmap for future gene therapy research.

Generation of a replication-deficient recombinant human adenovirus type 35 vector using bacteria-mediated homologous recombination

The use of adenovirus type 35 (Ad35) as a vector in vaccine and gene therapy studies is promising due to its broad cell tropism and low seroprevalence in humans. However, to date, a simple and effective system for producing recombinant Ad35 (rAd35) has not been well developed. This report describes a two-plasmid Ad35-Easy system to facilitate the production of recombinant Ad35 (rAd35). The system employed the pAd35-shuttle vector for foreign gene transfer and the pAd35-backbone vector to provide the Ad35 genomic backbone. A 293-Ad35E1B cell line was used to trans-complement rAd35 replication. rAd35 plasmids were obtained through homologous recombination following co-transformation of E. coli BJ5183 cells with recombinant pAd35-shuttle vectors harboring foreign genes. rAd35 viruses were obtained directly by transfecting 293-Ad35E1B cells with foreign gene-containing rAd35 plasmids and the pAd35-backbone vector. The production of E1 deficient rAd35 was evaluated by transfecting the 293-Ad35E1B cells with the rAd35 plasmid containing the enhanced green fluorescent protein (EGFP) gene. The virus grew effectively at a yield comparable to that of wild type Ad35 in HEp2 cells, indicating that the Ad35-Easy system is an efficient method for rapid production of rAd35 in sufficient quantities for vaccine development or gene therapy.

The AAV9 receptor and its modification to improve in vivo lung gene transfer in mice

Vectors based on adeno-associated virus (AAV) serotype 9 are candidates for in vivo gene delivery to many organs, but the receptor(s) mediating these tropisms have yet to be defined. We evaluated AAV9 uptake by glycans with terminal sialic acids (SAs), a common mode of cellular entry for viruses. We found, however, that AAV9 binding increased when terminal SA was enzymatically removed, suggesting that galactose, which is the most commonly observed penultimate monosaccharide to SA, may mediate AAV9 transduction. This was confirmed in mutant CHO Pro-5 cells deficient in the enzymes involved in glycoprotein biogenesis, as well as lectin interference studies. Binding of AAV9 to glycans with terminal galactose was demonstrated via glycan binding assays. Co-instillation of AAV9 vector with neuraminidase into mouse lung resulted in exposure of terminal galactose on the apical surface of conducting airway epithelial cells, as shown by lectin binding and increased transduction of these cells, demonstrating the possible utility of this vector in lung-directed gene transfer. Increasing the abundance of the receptor on target cells and improving vector efficacy may improve delivery of AAV vectors to their therapeutic targets.

Tropism-modification strategies for targeted gene delivery using adenoviral vectors

Achieving high efficiency, targeted gene delivery with adenoviral vectors is a long-standing goal in the field of clinical gene therapy. To achieve this, platform vectors must combine efficient retargeting strategies with detargeting modifications to ablate native receptor binding (i.e. CAR/integrins/heparan sulfate proteoglycans) and “bridging” interactions. “Bridging” interactions refer to coagulation factor binding, namely coagulation factor X (FX), which bridges hepatocyte transduction in vivo through engagement with surface expressed heparan sulfate proteoglycans (HSPGs). These interactions can contribute to the off-target sequestration of Ad5 in the liver and its characteristic dose-limiting hepatotoxicity, thereby significantly limiting the in vivo targeting efficiency and clinical potential of Ad5-based therapeutics. To date, various approaches to retargeting adenoviruses (Ad) have been described. These include genetic modification strategies to incorporate peptide ligands (within fiber knob domain, fiber shaft, penton base, pIX or hexon), pseudotyping of capsid proteins to include whole fiber substitutions or fiber knob chimeras, pseudotyping with non-human Ad species or with capsid proteins derived from other viral families, hexon hypervariable region (HVR) substitutions and adapter-based conjugation/crosslinking of scFv, growth factors or monoclonal antibodies directed against surface-expressed target antigens. In order to maximize retargeting, strategies which permit detargeting from undesirable interactions between the Ad capsid and components of the circulatory system (e.g. coagulation factors, erythrocytes, pre-existing neutralizing antibodies), can be employed simultaneously. Detargeting can be achieved by genetic ablation of native receptor-binding determinants, ablation of “bridging interactions” such as those which occur between the hexon of Ad5 and coagulation factor X (FX), or alternatively, through the use of polymer-coated “stealth” vectors which avoid these interactions. Simultaneous retargeting and detargeting can be achieved by combining multiple genetic and/or chemical modifications.

Adenoviruses with an αvβ integrin targeting moiety in the fiber shaft or the HI-loop increase tumor specificity without compromising antitumor efficacy in magnetic resonance imaging of colorectal cancer metastases

Background

Colorectal cancer is often a deadly disease and cannot be cured at metastatic stage. Oncolytic adenoviruses have been considered as a new therapeutic option for treatment of refractory disseminated cancers, including colorectal cancer. The safety data has been excellent but tumor transduction and antitumor efficacy especially in systemic administration needs to be improved.

Methods

Here, the utility of αvβ integrin targeting moiety Arg-Gly-Asp (RGD) in the Lys-Lys-Thr-Lys (KKTK) domain of the fiber shaft or in the HI-loop of adenovirus serotype 5 for increased tumor targeting and antitumor efficacy was evaluated. To this end, novel spleen-to-liver metastatic colorectal cancer mouse model was used and the antitumor efficacy was evaluated with magnetic resonance imaging (MRI).

Results

Both modifications (RGD in the HI-loop or in the fiber shaft) increased gene transfer efficacy in colorectal cancer cell lines and improved tumor-to-normal ratio in systemic administration of the vector.

Conclusions

Antitumor potency was not compromised with RGD modified viruses suggesting increased safety profile and tumor specificity.

Influence of chimeric human-bovine fibers on adenoviral uptake by liver cells and the antiviral immune response

Human adenoviruses (HAdV) are widely used for in vitro and in vivo gene transfer. Viral hepatotropism, inflammatory responses and neutralization by pre-existing antibodies (NAbs) are obstacles for clinical applications of HAdV vectors. Although the multifactorial events leading to innate HAdV toxicity are far from being elucidated, there is a consensus that the majority of intravenously injected-HAdV vectors is sequestered by Kuppfer cells, probably independently of coagulation factors. In this study, we show that the adenoviral-associated humoral and innate cytokine immune responses are significantly reduced when HAdV-5 vector carrying human bovine chimeric fibers (HAdV-5-F2/BAdV-4) is intravenously injected into mice. Fiber pseudotyping modified its interaction with blood coagulation factors, as FIX and FX no longer mediate the infection of liver cells by HAdV-5-F2/BAdV-4. As a consequence, at early time points post-infection, several cytokines and chemokines (IFN-gamma, IL-6, IP-10, MCP-1, RANTES and MP1beta) were found to be present at lower levels in the plasma of mice that had been intravenously injected with HAdV-5-F2/BAdV-4 compared with mice injected with the parental vector HAdV-5. Moreover, genetic modification of the fiber allowed HAdV-5-F2/BAdV-4 to partially escape neutralization by NAbs.

Adenovirus

Of the 53 different human adenovirus (HAdV) serotypes belonging to species A-G, a significant number are associated with acute respiratory, gastrointestinal and ocular infections. Replication-defective HAdV-5-based vectors also continue to play a significant role in gene transfer trials and in vaccine delivery efforts in the clinic. Although significant progress has been made from studies of AdV biology, we still have an incomplete understanding of AdV's structure as well as its multifactorial interactions with the host. Continuing efforts to improve knowledge in these areas, as discussed in this chapter, will be crucial for revealing the mechanisms of AdV pathogenesis and for allowing optimal use of AdV vectors for biomedical applications.

Mouse adenovirus type 1 and human adenovirus type 5 differ in endothelial cell tropism and liver targeting

BACKGROUND

For adenovirus vectors derived from human serotype 5 (Ad5), the efficiency and safety after intravascular delivery is hindered by their sequestration in nontarget tissues, predominantly the liver. The latter is largely dictated by adenovirus binding to blood coagulation zymogens. In addition, several target cells, such as endothelial and smooth muscle cells, are difficult to transduce by Ad5 due to the low expression of the primary coxsackie-adenovirus receptor (CAR). Therefore, alternative adenovirus serotypes are being explored.

METHODS

In the present study, we assessed the tropism of mouse adenovirus type 1 (MAV-1), a nonhuman adenovirus for which cellular attachment is CAR-independent.

RESULTS

The typical replication of MAV-1 in endothelial cells as observed in vivo was not reflected in elevated attachment to primary and continuous endothelial cells in cell culture. Remarkably, MAV-1 displayed a higher affinity for primary human smooth muscle cells than recombinant Ad5 (rAd5). Attachment of MAV-1 to human and mouse cells of hepatocyte origin was not altered by physiological concentrations of human coagulation factor XI (FXI) or the vitamin K-dependent FIX, FX and FVII. By contrast, attachment of Ad5-derived vectors was enhanced at least eight-fold by FX. Using surface plasmon resonance, MAV-1 was shown to directly associate with human FX and murine FX and FIX but, opposite to rAd5, this interaction did not lead to enhanced cellular attachment. In intravenously injected severe combined immunodeficiency mice, distribution of MAV-1 to the liver was markedly lower than that observed with rAd5.

CONCLUSIONS

Our data on the tropism of MAV-1 suggest that this virus may find utility in the field of gene therapy.

Targeting of adenovirus serotype 5 pseudotyped with short fiber from serotype 41 to c-erbB2-positive cells using bispecific single-chain diabody

The purpose of the current study was to alter the broad native tropism of human adenovirus for virus targeting to c-erbB2-positive cancer cells. First, we engineered a single-chain antibody (scFv) against the c-erbB2 oncoprotein into minor capsid protein IX (pIX) of adenovirus serotype 5 (Ad5) in a manner commensurate with virion integrity and binding to the soluble extracellular c-erbB2 domain. To ablate native viral tropism and facilitate binding of the pIX-incorporated scFv to cellular c-erbB2, we replaced the Ad5 fiber with the Ad41 short (41s) fiber devoid of all known cell-binding determinants. The resultant Ad5F41sIX6.5 vector demonstrated increased cell binding and gene transfer as compared to the Ad5F41s control; however, this augmentation of virus infectivity was not c-erbB2 specific. Incorporation of a six-histidine (His(6)) peptide into the C-terminus of the 41s fiber protein resulted in markedly increased Ad5F41s6H infectivity in 293AR cells, which express a membrane-anchored scFv against the C-terminal oligohistidine tag, as compared to the Ad5F41s vector and the parental 293 cells. These data suggested that a 41s-fiber-incorporated His(6) tag could serve for attachment of an adapter protein designed to guide Ad5F41s6H infection in a c-erbB2-specific manner. We therefore engineered a bispecific scFv diabody (scDb) combining affinities for both c-erbB2 and the His(6) tag and showed its ability to provide up to 25-fold increase of Ad5F41s6H infectivity in c-erbB2-positive cells. Thus, Ad5 fiber replacement by a His(6)-tagged 41s fiber coupled with virus targeting mediated by an scDb adapter represents a promising strategy to confer Ad5 vector tropism for c-erbB2-positive cancer cells.

Redundant and synergistic mechanisms control the sequestration of blood-born adenovirus in the liver

Human adenovirus (Ad) is a ubiquitous pathogen causing a wide range of diseases. Although the interactions of human Ad serotype 5 (Ad5) with susceptible cells in vitro are known in great detail, host factors controlling the tissue specificity of Ad5 infection in vivo remain poorly understood. Here, we analyzed the mechanisms of sequestration by the liver for blood-born human Ads and Ad5-based vectors. Our data suggest that several known mechanisms that lead to Ad5 sequestration by the liver become engaged in a redundant, sequential, and synergistic manner to ensure the rapid clearance of circulating virus particles from the blood. These mechanisms include (i) trapping of the virus by liver residential macrophages, Kupffer cells; (ii) Ad5 hepatocyte infection via blood factor-hexon interactions; and (iii) Ad5 penton RGD motif-mediated interactions with liver endothelial cells and hepatocytes, mediating virus retention in the space of Disse. More important, we show that when all of these mechanisms are simultaneously inactivated via mutations of Ad5 capsid proteins and pharmacological interventions, virus sequestration by the liver is markedly reduced. Therefore, our study is the first demonstration of the principal possibility of ablating the sequestration of blood-born Ad in the liver via specific inactivation of a defined set of mechanisms that control this process.

Comparative analysis of vector biodistribution, persistence and gene expression following intravenous delivery of bovine, porcine and human adenoviral vectors in a mouse model

Nonhuman adenoviruses including bovine adenovirus serotype 3 (BAd3) and porcine adenovirus serotype 3 (PAd3) can circumvent pre-existing immunity against human adenovirus serotype 5 (HAd5) and are being developed as alternative vectors for gene delivery. To assess the usefulness of these vectors for in vivo gene delivery, we compared biodistribution, persistence, state of vector genome, and transgene and vector gene expression by replication-defective BAd3 and PAd3 vectors with those of HAd5 vector in a FVB/n mouse model following intravenous inoculation. BAd3 vector efficiently transduced the heart, kidney and lung in addition to the liver and spleen and persisted for a longer duration compared to PAd3 or HAd5 vectors. Biodistribution of PAd3 vector was comparable to that of HAd5 vector but showed more rapid vector clearance. Only linear episomal forms of BAd3, PAd3, and HAd5 vector genomes were detected. All three vectors efficiently expressed the green fluorescent protein (GFP) transgene proportionate to the vector genome copy number in various tissues. Furthermore, leaky expression of vector genes, both the early (E4) and the late (hexon) was observed in all three vectors and gradually declined with time. These results suggest that BAd3 and PAd3 vectors could serve as an alternative or supplement to HAd5 for gene delivery applications.

Coagulation factors IX and X enhance binding and infection of adenovirus types 5 and 31 in human epithelial cells

Most adenoviruses bind directly to the coxsackie and adenovirus receptor (CAR) on target cells in vitro, but recent research has shown that adenoviruses can also use soluble components in body fluids for indirect binding to target cells. These mechanisms have been identified upon addressing the questions of how to de- and retarget adenovirus-based vectors for human gene and cancer therapy, but the newly identified mechanisms also suggest that the role of body fluids and their components may also be of importance for natural, primary infections. Here we demonstrate that plasma, saliva, and tear fluid promote binding and infection of adenovirus type 5 (Ad5) in respiratory and ocular epithelial cells, which corresponds to the natural tropism of most adenoviruses, and that plasma promotes infection by Ad31. By using a set of binding and infection experiments, we also found that Ad5 and Ad31 require coagulation factors IX (FIX) or X (FX) or just FIX, respectively, for efficient binding and infection. The concentrations of these factors that were required for maximum binding were 1/100th of the physiological concentrations. Preincubation of virions with heparin or pretreatment of cells with heparinase I indicated that the role of cell surface heparan sulfate during FIX- and FX-mediated adenovirus binding and infection is mechanistically serotype specific. We conclude that the use of coagulation factors by adenoviruses may be of importance not only for the liver tropism seen when administering adenovirus vectors to the circulation but also during primary infections by wild-type viruses of their natural target cell types.

Development and evaluation of a novel gene delivery vehicle composed of adenovirus serotype 35

The capacity of gene delivery vehicles is considered to be a critical factor determining the success of gene therapy. To date, various types of gene delivery vehicle have been developed. Among them, recombinant adeno-virus (Ad) vectors have potential that has favored their worldwide use in vitro and in vivo. Conventional Ad vectors are composed of subgroup C Ad serotype 5 (Ad5), although it has been clarified that the drawbacks of Ad5 vectors are a high seroprevalence of Ad5 in adults and low transduction efficiencies in cells lacking the primary receptor for Ad5, coxsackievirus and adenovirus receptor. To overcome these problems, we developed a novel Ad vector fully composed of Ad serotype 35 (Ad35). Ad35 vectors show a wide tropism for human cells because Ad35 binds to human CD46, which is ubiquitously expressed on almost all human cells, as a primary receptor. In addition, anti-Ad5 antibodies do not inhibit Ad35 vector-mediated transduction and the seroprevalence of Ad35 in adults is lower than that of Ad5. This paper reviews our studies on the development and evaluation of Ad35 vectors. Ad vectors derived from other Ad serotypes different from Ad5, including Ad35, are expected to be gene delivery vehicles alternative to conventional Ad5 vectors.

Effect of neutralizing sera on factor x-mediated adenovirus serotype 5 gene transfer

The deployment of adenovirus serotype 5 (Ad5)-based vectors is hampered by preexisting immunity. When such vectors are delivered intravenously, hepatocyte transduction is mediated by the hexon-coagulation factor X (FX) interaction. Here, we demonstrate that human sera efficiently block FX-mediated cellular binding and transduction of Ad5-based vectors in vitro. Neutralizing activity correlated well with the ability to inhibit Ad5-mediated liver transduction, suggesting that prescreening patient sera in this manner accurately predicts the efficacy of Ad5-based gene therapies. Neutralization in vitro can be partially bypassed by pseudotyping with Ad45 fiber protein, indicating that a proportion of neutralizing antibodies are directed against the Ad5 fiber.

Substitution of hexon hypervariable region 5 of adenovirus serotype 5 abrogates blood factor binding and limits gene transfer to liver

Liver tropism potentially leading to massive hepatocyte transduction and hepatotoxicity still represents a major drawback to adenovirus (Ad)-based gene therapy. We previously demonstrated that substitution of the hexon hypervariable region 5 (HVR5), the most abundant capsid protein, constituted a valuable platform for efficient Ad retargeting. The use of different mouse strains revealed that HVR5 substitution also led to dramatically less adenovirus liver transduction and associated toxicity, whereas HVR5-modified Ad were still able to transduce different cell lines efficiently, including primary hepatocytes. We showed that HVR5 modification did not significantly change Ad blood clearance or liver uptake at early times. However, we were able to link the lower liver transduction to enhanced HVR5-modified Ad liver clearance and impaired use of blood factors. Most importantly, HVR5-modified vectors continued to transduce tumors in vivo as efficiently as their wild-type counterparts. Taken together, our data provide a rationale for future design of retargeted vectors with a safer profile.

Polyinosinic acid enhances delivery of adenovirus vectors in vivo by preventing sequestration in liver macrophages

Adenovirus is among the preferred vectors for gene therapy because of its superior in vivo gene-transfer efficiency. However, upon systemic administration, adenovirus is preferentially sequestered by the liver, resulting in reduced adenovirus-mediated transgene expression in targeted tissues. In the liver, Kupffer cells are responsible for adenovirus degradation and contribute to the inflammatory response. As scavenger receptors present on Kupffer cells are responsible for the elimination of blood-borne pathogens, we investigated the possible implication of these receptors in the clearance of the adenovirus vector. Polyinosinic acid [poly(I)], a scavenger receptor A ligand, was analysed for its capability to inhibit adenovirus uptake specifically in macrophages. In in vitro studies, the addition of poly(I) before virus infection resulted in a specific inhibition of adenovirus-induced gene expression in a J774 macrophage cell line and in primary Kupffer cells. In in vivo experiments, pre-administration of poly(I) caused a 10-fold transient increase in the number of adenovirus particles circulating in the blood. As a consequence, transgene expression levels measured in different tissues were enhanced (by 5- to 15-fold) compared with those in animals that did not receive poly(I). Finally, necrosis of Kupffer cells, which normally occurs as a consequence of systemic adenovirus administration, was prevented by the use of poly(I). No toxicity, as measured by liver-enzyme levels, was observed after poly(I) treatment. From our data, we conclude that poly(I) can prevent adenovirus sequestration by liver macrophages. These results imply that, by inhibiting adenovirus uptake by Kupffer cells, it is possible to reduce the dose of the viral vector to diminish the liver-toxicity effect and to improve the level of transgene expression in target tissues. In systemic gene-therapy applications, this will have great impact on the development of targeted adenoviral vectors.

Adenovirus serotype 5 hexon is critical for virus infection of hepatocytes in vivo

Human species C adenovirus serotype 5 (Ad5) is the most common viral vector used in clinical studies worldwide. Ad5 vectors infect liver cells in vivo with high efficiency via a poorly defined mechanism, which involves virus binding to vitamin K-dependent blood coagulation factors. Here, we report that the major Ad5 capsid protein, hexon, binds human coagulation factor X (FX) with an affinity of 229 pM. This affinity is 40-fold stronger than the reported affinity of Ad5 fiber for the cellular receptor coxsackievirus and adenovirus receptor, CAR. Cryoelectron microscopy and single-particle image reconstruction revealed that the FX attachment site is localized to the central depression at the top of the hexon trimer. Hexon-mutated virus bearing a large insertion in hexon showed markedly reduced FX binding in vitro and failed to deliver a transgene to hepatocytes in vivo. This study describes the mechanism of FX binding to Ad5 and demonstrates the critical role of hexon for virus infection of hepatocytes in vivo.

Modification of pIX or hexon based on fiberless Ad vectors is not effective for targeted Ad vectors

Adenovirus (Ad) vector application in gene therapy is limited by its naïve tropism. We previously developed protein IX (pIX)-modified and hexon-modified Ad vectors in order to alter Ad vector tropism. However, these modified Ad vectors failed to infect cells with the foreign ligands displayed in the pIX or hexon. We hypothesized that steric hindrance by fiber proteins might have prevented the ligand-mediated transduction, as fibers are the outmost capsid proteins of Ad vectors. Therefore, we generated a series of fiberless Ad vectors and investigated their gene expression properties. Unexpectively, however, pIX- or hexon-modified fiberless Ad vector did not achieve any gene expression (the gene expression level by these vectors was similar to the background level). These results might be caused by the fact that the fiberless particles were weaker against physical burdens. To the best of our knowledge, this study is the first reported attempt to develop fiberless Ad vectors containing foreign ligands in the pIX or hexon region. The drawback of the lower stability of fiberless Ad vectors must be overcome to develop targeted Ad vectors based on such vectors. This study could provide basic information for the development of effective targeted Ad vectors.

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.

Novel strategies in tailoring human adenoviruses into therapeutic cancer gene therapy vectors

Gene therapy is a novel approach for the treatment of cancer that has so far not been realized. The scope of this review is to try to define the remaining barriers to the successful use of adenovirus vectors for gene and viral therapy of human tumors and to suggest solutions whereby these barriers can be bypassed. It is the conviction of the authors that too many studies have been performed in animal models that are not sufficiently comprehensive to allow conclusions to be drawn for application in humans. For example, in the case of the murine experimental model, in which most studies have been performed, mice are devoid of circulating antibodies to adenovirus type 5 and adenovirus cannot replicate in mouse cells. While the problems are real enough, as witnessed by the quite limited success in human trials, some of the solutions that will be suggested here are hypothetical and have not as yet been tried, even in animals. The review has no ambition to be exhaustive but is intended as a contribution in order to forward the field of gene therapy vectors for systemic clinical application.

Fiber shaft-chimeric adenovirus vectors lacking the KKTK motif efficiently infect liver cells in vivo

The molecular mechanisms governing the infectivity of adenovirus (Ad) toward specific cell and tissue types in vivo remain poorly understood. The direct Ad binding to hepatic heparan sulfate proteoglycans via the KKTK motif within the fiber shaft domain was suggested to be the major mechanism of Ad liver cell infection in vivo. Here, we describe the generation and in vitro and in vivo infectivity studies of Ad5-based vectors possessing long Ad31- or Ad41-derived fiber shaft domains, which lack the KKTK motif. We found that all the critical early steps of Ad infection, including attachment to the cellular receptor, internalization, and virus genome transfer into the nucleus, occurred with similar levels of efficiency for fiber shaft-chimeric vectors and unmodified Ad5. Upon intravenous delivery into mice, fiber shaft-chimeric vectors accumulated in liver tissue, transduced liver cells, and induced the production of proinflammatory cytokines (tumor necrosis factor alpha and interleukin-6) and the chemokine monocyte chemoattractant protein 1 at levels indistinguishable from those observed for Ad5. Thus, our data provide evidence that the Ad5 fiber shaft amino acid sequence does not play any substantial role in determining adenovirus infectivity toward hepatic cells in vivo. The data obtained contribute to improving our understanding of the molecular mechanisms determining Ad infectivity and biodistribution in vivo and may aid in designing novel Ad-based vectors for gene therapy applications.

Development of PEGylated adenovirus vector with targeting ligand

For effective gene therapy, a vector system that transduces the therapeutic gene into target cells efficiently and safely is essential. Adenovirus (Ad) vectors frequently are used for gene therapy research, especially cancer gene therapy, because of their high transduction efficiency. However, broad clinical utility of Ad vectors have not yet been achieved owing to problems related to several properties inherent to Ads. Systemic administration of Ad vectors leads to acute virus accumulation and undesirable transgene expression in the liver, with subsequent inefficient systemic cancer-targeted therapy and pronounced hepatotoxicity. Furthermore, most people have Ad-neutralizing antibodies, which hamper gene expression efficiency. Chemical conjugation of Ad surface with polyethylene glycol (PEG) (PEGylation) is one of the promising strategies to overcome these problems. Furthermore, PEGylation of Ad vectors with targeting ligands on the tip of PEG, which alter the transfection range of Ad vectors will improve the safety and efficiency of Ad gene-delivery vectors. In this review, we describe the molecular biology of Ads and outline this PEGylation approach including our data.

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.