Adenovirus Biology Research Articles

Current Views on Oncolytic Adenoviruses for Cancer Therapy

Abstract: Oncolytic viruses replicate and spread in tumors at the same time, resulting in increased cytotoxicity and the reversal of tumor immune suppression. Among other viruses, recombinant adenoviruses that replicated in tumor cells were clinically tested via intratumoral or systemic administration. Although oncolytic virus replication kills tumor cells on its own, it may also activate the immune system, which can aid in tumor control. Viruses can be modified to improve their selectivity and effectiveness. Adenovirus genomes can be easily designed to incorporate various tumor-targeting pathways and therapeutic transgenes to improve antitumor properties. Poor tumor targeting, intratumoral expansion, and virocentric immune responses are all linked to low efficacy. As a result, more effective oncolytic adenoviruses that can be used alone or in combination with chemotherapy or immunotherapy are needed. Oncolytic Adenovirus (OAds) has long been considered a potential biotherapeutic agent against various cancers due to its ability to replicate cancer cells while remaining dormant in healthy cells selectively. Several preclinical studies using genetic engineering technology have increased antitumor OAds in various cancers in recent years. Systemic OAds administration is hampered by poor targeting tropism to healthy tissues, low-level ad receptors on tumor cells, and pre-existing neutralizing antibodies. Various discoveries have been made to overcome these barriers, including stem cells, nanoparticles, polymer shielding, extracellular vesicles, hydrogels, and microparticles (MPs). These carriers may improve Oncolytic viruses’ therapeutic efficacy by improving transfection, circulatory survival, cellular interactions, specific targeting, and immune response. The structure and biology of adenoviruses, the different types of OAds, and the efficacy of different carriers in the systemic administration of OAds were all examined in this review.

A brief review on the molecular biology of human adenoviruses

Human adenoviruses infection causes diseases worldwide in all age groups and genders, which is associated with a wide range of diseases affecting the gastrointestinal tract, respiratory tract, urinary tract, and the eye, but they are often isolated from the pharynx and stool of asymptomatic children. However, in developing countries, diarrhea is a major cause of morbidity and mortality; and after rotaviruses, human adenoviruses are considered to be the second most important cause of viral infantile diarrhea. Also, human adenoviruses cause fatal acute respiratory distress syndrome in healthy adults and are especially fatal in infants and immune-compromised individuals. This review summarizes both classical and contemporary discoveries in the study of human adenoviruses at the molecular level, with particular emphasis on viral receptors, capsid proteins, nucleic acid, and genome properties as well as the molecular interactions governing the virion assembly. In this article, we provide insightful information concerning the molecular aspects of human adenoviruses. This would develop an understanding of the virus and serve as a powerful tool in identifying new approaches for the prevention and treatment of adenoviral infection.

A Single Amino Acid Switch in the Adenoviral DNA Binding Protein Abrogates Replication Center Formation and Productive Viral Infection.

ABSTRACTAdenoviruses are very efficient high-capacity vaccine vectors and are common gene delivery systems. Despite their extensive use in preclinical models and clinical trials over the past decades, adenoviral vectors still require optimization. To achieve that, more thorough characterizations of adenoviral genes and gene products, as well as pathogen-host interactions, are indispensable. The adenoviral DNA binding protein (DBP) is a key regulatory protein involved in various cellular and viral processes. Here, we show that single amino acid exchange mutations in human adenovirus C5 (HAdV-C5) DBP strongly influence adenoviral replication by altering interaction with the cellular ubiquitination machinery. Specifically, phenotypic analyses of DBP mutants demonstrate that single amino acid substitutions can regulate interactions with the cellular USP7 deubiquitinase, impede viral DNA synthesis, and completely abolish viral late protein expression and progeny production. Importantly, cells infected with the DBP mutant UBM5 consistently lack DBP-positive replication centers (RCs), which are usually formed during the transition from the early to the late phase of infection. Our findings demonstrate that DBP regulates a key step at the onset of the late phase of infection and that this activity is unambiguously linked to the formation and integrity of viral RCs. These data provide the experimental basis for future work that targets DBP and its interference with the formation of viral RCs during productive infection. Consequently, this work will have immediate impact on DNA virus and adenovirus research in general and, potentially, also on safety optimization of existing and development of novel adenoviral vectors and anti-adenoviral compounds.

Restriction-Assembly: A Solution to Construct Novel Adenovirus Vector.

Gene therapy and vaccine development need more novel adenovirus vectors. Here, we attempt to provide strategies to construct adenovirus vectors based on restriction-assembly for researchers with little experience in this field. Restriction-assembly is a combined method of restriction digestion and Gibson assembly, by which the major part of the obtained plasmid comes from digested DNA fragments instead of PCR products. We demonstrated the capability of restriction-assembly in manipulating the genome of simian adenovirus 1 (SAdV-1) in this study. A PCR product of the plasmid backbone was combined with SAdV-1 genomic DNA to construct an infectious clone, plasmid pKSAV1, by Gibson assembly. Restriction-assembly was performed repeatedly in the steps of intermediate plasmid isolation, modification, and restoration. The generated adenoviral plasmid was linearized by restriction enzyme digestion and transfected into packaging 293 cells to rescue E3-deleted replication-competent SAdV1XE3-CGA virus. Interestingly, SAdV1XE3-CGA could propagate in human chronic myelogenous leukemia K562 cells. The E1 region was similarly modified to generate E1/E3-deleted replication-defective virus SAdV1-EG. SAdV1-EG had a moderate gene transfer ability to adherent mammalian cells, and it could efficiently transduce suspension cells when compared with the human adenovirus 5 control vector. Restriction-assembly is easy to use and can be performed without special experimental materials and instruments. It is highly effective with verifiable outcomes at each step. More importantly, restriction-assembly makes the established vector system modifiable, upgradable and under sustainable development, and it can serve as the instructive method or strategy for the synthetic biology of adenoviruses.

Adenovirus Biology, Recombinant Adenovirus, and Adenovirus Usage in Gene Therapy.

Gene therapy is currently in the public spotlight. Several gene therapy products, including oncolytic virus (OV), which predominantly replicates in and kills cancer cells, and COVID-19 vaccines have recently been commercialized. Recombinant adenoviruses, including replication-defective adenoviral vector and conditionally replicating adenovirus (CRA; oncolytic adenovirus), have been extensively studied and used in clinical trials for cancer and vaccines. Here, we review the biology of wild-type adenoviruses, the methodological principle for constructing recombinant adenoviruses, therapeutic applications of recombinant adenoviruses, and new technologies in pluripotent stem cell (PSC)-based regenerative medicine. Moreover, this article describes the technology platform for efficient construction of diverse “CRAs that can specifically target tumors with multiple factors” (m-CRAs). This technology allows for modification of four parts in the adenoviral E1 region and the subsequent insertion of a therapeutic gene and promoter to enhance cancer-specific viral replication (i.e., safety) as well as therapeutic effects. The screening study using the m-CRA technology successfully identified survivin-responsive m-CRA (Surv.m-CRA) as among the best m-CRAs, and clinical trials of Surv.m-CRA are underway for patients with cancer. This article also describes new recombinant adenovirus-based technologies for solving issues in PSC-based regenerative medicine.

Conserved E1B-55K SUMOylation in Different Human Adenovirus Species Is a Potent Regulator of Intracellular Localization.

Over the past decades, studies on the biology of human adenoviruses (HAdVs) mainly focused on the HAdV prototype species C type 5 (HAdV-C5) and revealed fundamental molecular insights into mechanisms of viral replication and viral cell transformation. Recently, other HAdV species are gaining more and more attention in the field. Reports on large E1B proteins (E1B-55K) from different HAdV species showed that these multifactorial proteins possess strikingly different features along with highly conserved functions. In this work, we identified potential SUMO-conjugation motifs (SCMs) in E1B-55K proteins from HAdV species A to F. Mutational inactivation of these SCMs demonstrated that HAdV E1B-55K proteins are SUMOylated at a single lysine residue that is highly conserved among HAdV species B to E. Moreover, we provide evidence that E1B-55K SUMOylation is a potent regulator of intracellular localization and p53-mediated transcription in most HAdV species. We also identified a lysine residue at position 101 (K101), which is unique to HAdV-C5 E1B-55K and specifically regulates its SUMOylation and nucleo-cytoplasmic shuttling. Our findings reveal important new aspects on HAdV E1B-55K proteins and suggest that different E1B-55K species possess conserved SCMs while their SUMOylation has divergent cellular effects during infection. IMPORTANCE E1B-55K is a multifunctional adenoviral protein and its functions are highly regulated by SUMOylation. Although functional consequences of SUMOylated HAdV-C5 E1B-55K are well studied, we lack information on the effects of SUMOylation on homologous E1B-55K proteins from other HAdV species. Here, we show that SUMOylation is a conserved posttranslational modification in most of the E1B-55K proteins, similar to what we know about HAdV-C5 E1B-55K. Moreover, we identify subcellular localization and regulation of p53-dependent transcription as highly conserved SUMOylation-regulated E1B-55K functions. Thus, our results highlight how HAdV proteins might have evolved in different HAdV species with conserved domains involved in virus replication and differing alternative functions and interactions with the host cell machinery. Future research will link these differences and similarities to the diverse pathogenicity and organ tropism of the different HAdV species.

Adenovirus Core Proteins: Structure and Function.

Adenoviruses have served as a model for investigating viral-cell interactions and discovering different cellular processes, such as RNA splicing and DNA replication. In addition, the development and evaluation of adenoviruses as the viral vectors for vaccination and gene therapy has led to detailed investigations about adenovirus biology, including the structure and function of the adenovirus encoded proteins. While the determination of the structure and function of the viral capsid proteins in adenovirus biology has been the subject of numerous reports, the last few years have seen increased interest in elucidating the structure and function of the adenovirus core proteins. Here, we provide a review of research about the structure and function of the adenovirus core proteins in adenovirus biology.

Adenoviral vector-based platforms for developing effective vaccines to combat respiratory viral infections.

Since the development of the first vaccine against smallpox over two centuries ago, vaccination strategies have been at the forefront of significantly impacting the incidences of infectious diseases globally. However, the increase in the human population, deforestation and climate change, and the rise in worldwide travel have favored the emergence of new viruses with the potential to cause pandemics. The ongoing severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) pandemic is a cruel reminder of the impact of novel pathogens and the suboptimal capabilities of conventional vaccines. Therefore, there is an urgent need to develop new vaccine strategies that allow the production of billions of doses in a short duration and are broadly protective against emerging and re‐emerging infectious diseases. Extensive knowledge of the molecular biology and immunology of adenoviruses (Ad) has favored Ad vectors as platforms for vaccine design. The Ad‐based vaccine platform represents an attractive strategy as it induces robust humoral and cell‐mediated immune responses and can meet the global demand in a pandemic situation. This review describes the status of Ad vector‐based vaccines in preclinical and clinical studies for current and emerging respiratory viruses, particularly coronaviruses, influenza viruses and respiratory syncytial viruses.

Human species D adenovirus hexon capsid protein mediates cell entry through a direct interaction with CD46

Human adenovirus species D (HAdV-D) types are currently being explored as vaccine vectors for coronavirus disease 2019 (COVID-19) and other severe infectious diseases. The efficacy of such vector-based vaccines depends on functional interactions with receptors on host cells. Adenoviruses of different species are assumed to enter host cells mainly by interactions between the knob domain of the protruding fiber capsid protein and cellular receptors. Using a cell-based receptor-screening assay, we identified CD46 as a receptor for HAdV-D56. The function of CD46 was validated in infection experiments using cells lacking and overexpressing CD46, and by competition infection experiments using soluble CD46. Remarkably, unlike HAdV-B types that engage CD46 through interactions with the knob domain of the fiber protein, HAdV-D types infect host cells through a direct interaction between CD46 and the hexon protein. Soluble hexon proteins (but not fiber knob) inhibited HAdV-D56 infection, and surface plasmon analyses demonstrated that CD46 binds to HAdV-D hexon (but not fiber knob) proteins. Cryoelectron microscopy analysis of the HAdV-D56 virion-CD46 complex confirmed the interaction and showed that CD46 binds to the central cavity of hexon trimers. Finally, soluble CD46 inhibited infection by 16 out of 17 investigated HAdV-D types, suggesting that CD46 is an important receptor for a large group of adenoviruses. In conclusion, this study identifies a noncanonical entry mechanism used by human adenoviruses, which adds to the knowledge of adenovirus biology and can also be useful for development of adenovirus-based vaccine vectors.

Delivery systems for enhancing oncolytic adenoviruses efficacy.

Oncolytic adenovirus (OAds) has long been considered a promising biotherapeutic agent against various types of cancer owing to selectively replicate in and lyse cancer cells, while remaining dormant in healthy cells. In the last years, multiple (pre)clinical studies using genetic engineering technologies enhanced OAds anti-tumor effects in a broad range of cancers. However, poor targeting delivery, tropism toward healthy tissues, low-level expression of Ad receptors on tumor cells, and pre-existing neutralizing antibodies are major hurdles for systemic administration of OAds. Different vehicles have been developed for addressing these obstacles, such as stem cells, nanoparticles (NPs) and shielding polymers, extracellular vesicles (EVs), hydrogels, and microparticles (MPs). These carriers can enhance the therapeutic efficacy of OVs through enhancing transfection, circulatory longevity, cellular interactions, specific targeting, and immune responses against cancer. In this paper, we reviewed adenovirus structure and biology, different types of OAds, and the efficacy of different carriers in systemic administration of OAds.

Imaging the adenovirus infection cycle.

Incoming adenoviruses seize control of cytosolic transport mechanisms to relocate their genome from the cell periphery to specialized sites in the nucleoplasm. The nucleus is the site for viral gene expression, genome replication, and the production of progeny for the next round of infection. By taking control of the cell, adenoviruses also suppress cell-autonomous immunity responses. To succeed in their production cycle, adenoviruses rely on well-coordinated steps, facilitated by interactions between viral proteins and cellular factors. Interactions between virus and host can impose remarkable morphological changes in the infected cell. Imaging adenoviruses has tremendously influenced how we delineate individual steps in the viral life cycle, because it allowed the development of specific optical markers to label these morphological changes in space and time. As technology advances, innovative imaging techniques and novel tools for specimen labeling keep uncovering previously unseen facets of adenovirus biology emphasizing why imaging adenoviruses is as attractive today as it was in the past. This review will summarize past achievements and present developments in adenovirus imaging centered on fluorescence microscopy approaches.

Adenovirus Infection of Human Enteroids Reveals Interferon Sensitivity and Preferential Infection of Goblet Cells.

Human adenoviruses (HAdV) are significant human pathogens. Although only a subset of HAdV serotypes commonly cause gastroenteritis in humans, most HAdV species replicate in the gastrointestinal tract. Knowledge of the complex interaction between HAdVs and the human intestinal epithelium has been limited by the lack of a suitable cell culture system containing relevant cell types. Recently, this need has been met by the stable and prolonged cultivation of primary intestinal epithelial cells as enteroids. Human enteroids have been used to reveal novel and interesting aspects of rotavirus, norovirus, and enterovirus replication, prompting us to explore their suitability for HAdV culture. We found that both prototype strains and clinical isolates of enteric and nonenteric HAdVs productively replicate in human enteroids. HAdV-5p, a respiratory pathogen, and HAdV-41p, an enteric pathogen, are both sensitive to type I and III interferons in human enteroid monolayers but not A549 cells. Interestingly, HAdV-5p, but not HAdV-41p, preferentially infected goblet cells. And, HAdV-5p but not HAdV-41p was potently neutralized by the enteric human alpha-defensin HD5. These studies highlight new facets of HAdV biology that are uniquely revealed by primary intestinal epithelial cell culture.IMPORTANCE Enteric adenoviruses are a significant cause of childhood gastroenteritis worldwide, yet our understanding of their unique biology is limited. Here we report robust replication of both prototype and clinical isolates of enteric and respiratory human adenoviruses in enteroids, a primary intestinal cell culture system. Recent studies have shown that other fastidious enteric viruses replicate in human enteroids. Therefore, human enteroids may provide a unified platform for culturing enteric viruses, potentially enabling isolation of a greater diversity of viruses from patients. Moreover, both the ability of interferon to restrict respiratory and enteric adenoviruses and a surprising preference of a respiratory serotype for goblet cells demonstrate the power of this culture system to uncover aspects of adenovirus biology that were previously unattainable with standard cell lines.

Impact of Adenovirus infection in host cell metabolism evaluated by 1H-NMR spectroscopy

Adenovirus-based vectors are powerful vehicles for gene transfer applications in vaccination and gene therapy. Although highly exploited in the clinical setting, key aspects of the adenovirus biology are still not well understood, in particular the subversion of host cell metabolism during viral infection and replication. The aim of this work was to gain insights on the metabolism of two human cell lines (HEK293 and an amniocyte-derived cell line, 1G3) after infection with an adenovirus serotype 5 vector (AdV5). In order to profile metabolic alterations, we used 1H-NMR spectroscopy, which allowed the quantification of 35 metabolites in cell culture supernatants with low sample preparation and in a relatively short time. Significant differences between both cell lines in non-infected cultures were identified, namely in glutamine and acetate metabolism, as well as by-product secretion. The main response to AdV5 infection was an increase in glucose consumption and lactate production rates. Moreover, cultures performed with or without glutamine supplementation confirmed the exhaustion of this amino acid as one of the main causes of lower AdV5 production at high cell densities (10- and 1.5-fold less specific yields in HEK293 and 1G3 cells, respectively), and highlighted different degrees of glutamine dependency of adenovirus replication in each cell line. The observed metabolic alterations associated with AdV5 infection and specificity of the host cell line can be useful for targeted bioprocess optimization.

Intracellular Signaling and Desmoglein 2 Shedding Triggered by Human Adenoviruses Ad3, Ad14, and Ad14P1.

We recently discovered that desmoglein 2 (DSG2) is a receptor for human adenovirus species B serotypes Ad3, Ad7, Ad11, and Ad14. Ad3 is considered to be a widely distributed human pathogen. Ad3 binding to DSG2 triggers the transient opening of epithelial junctions. Here, we further delineate the mechanism that leads to DSG2-mediated epithelial junction opening in cells exposed to Ad3 and recombinant Ad3 fiber proteins. We identified an Ad3 fiber knob-dependent pathway that involves the phosphorylation of mitogen-activated protein (MAP) kinases triggering the activation of the matrix-metalloproteinase ADAM17. ADAM17, in turn, cleaves the extracellular domain of DSG2 that links epithelial cells together. The shed DSG2 domain can be detected in cell culture supernatant and also in serum of mice with established human xenograft tumors. We then extended our studies to Ad14 and Ad14P1. Ad14 is an important research and clinical object because of the recent appearance of a new, more pathogenic strain (Ad14P1). In a human epithelial cancer xenograft model, Ad14P1 showed more efficient viral spread and oncolysis than Ad14. Here, we tested the hypothesis that a mutation in the Ad14P1 fiber knob could account for the differences between the two strains. While our X-ray crystallography studies suggested an altered three-dimensional (3D) structure of the Ad14P1 fiber knob in the F-G loop region, this did not significantly change the fiber knob affinity to DSG2 or the intracellular signaling and DSG2 shedding in epithelial cancer cells. A number of widely distributed adenoviruses use the epithelial junction protein DSG2 as a receptor for infection and lateral spread. Interaction with DSG2 allows the virus not only to enter cells but also to open epithelial junctions which form a physical barrier to virus spread. Our study elucidates the mechanism beyond virus-triggered junction opening with a focus on adenovirus serotype 3. Ad3 binds to DSG2 with its fiber knob domain and triggers intracellular signaling that culminates in the cleavage of the extracellular domain of DSG2, thereby disrupting DSG2 homodimers between epithelial cells. We confirmed this pathway with a second DSG2-interacting serotype, Ad14, and its recently emerged strain Ad14P1. These new insights in basic adenovirus biology can be employed to develop novel drugs to treat adenovirus infection as well as be used as tools for gene delivery into epithelial tissues or epithelial tumors.

Human adenovirus 52 uses sialic acid-containing glycoproteins and the coxsackie and adenovirus receptor for binding to target cells.

Most adenoviruses attach to host cells by means of the protruding fiber protein that binds to host cells via the coxsackievirus and adenovirus receptor (CAR) protein. Human adenovirus type 52 (HAdV-52) is one of only three gastroenteritis-causing HAdVs that are equipped with two different fiber proteins, one long and one short. Here we show, by means of virion-cell binding and infection experiments, that HAdV-52 can also attach to host cells via CAR, but most of the binding depends on sialylated glycoproteins. Glycan microarray, flow cytometry, surface plasmon resonance and ELISA analyses reveal that the terminal knob domain of the long fiber (52LFK) binds to CAR, and the knob domain of the short fiber (52SFK) binds to sialylated glycoproteins. X-ray crystallographic analysis of 52SFK in complex with 2-O-methylated sialic acid combined with functional studies of knob mutants revealed a new sialic acid binding site compared to other, known adenovirus:glycan interactions. Our findings shed light on adenovirus biology and may help to improve targeting of adenovirus-based vectors for gene therapy.

Mutation of the fiber shaft heparan sulphate binding site of a 5/3 chimeric adenovirus reduces liver tropism.

Natural tropism to the liver is a major obstacle in systemic delivery of adenoviruses in cancer gene therapy. Adenovirus binding to soluble coagulation factors and to cellular heparan sulphate proteoglycans via the fiber shaft KKTK domain are suggested to cause liver tropism. Serotype 5 adenovirus constructs with mutated KKTK regions exhibit liver detargeting, but they also transduce tumors less efficiently, possibly due to altered fiber conformation. We constructed Ad5/3lucS*, a 5/3 chimeric adenovirus with a mutated KKTK region. The fiber knob swap was hypothesized to facilitate tumor transduction. This construct was studied with or without additional coagulation factor ablation. Ad5/3lucS* exhibited significantly reduced transduction of human hepatic cells in vitro and mouse livers in vivo. Combination of coagulation factor ablation by warfarinization to Ad5/3lucS* seemed to further enhance liver detargeting. Cancer cell transduction by Ad5/3lucS* was retained in vitro. In vivo, viral particle accumulation in M4A4-LM3 xenograft tumors was comparable to controls, but Ad5/3lucS* transgene expression was nearly abolished. Coagulation factor ablation did not affect tumor transduction. These studies set the stage for further investigations into the effects of the KKTK mutation and coagulation factor ablation in the context of 5/3 serotype chimerism. Of note, the putative disconnect between tumor transduction and transgene expression could prove useful in further understanding of adenovirus biology.

Aggresome Formation by the Adenoviral Protein E1B55K Is Not Conserved among Adenovirus Species and Is Not Required for Efficient Degradation of Nuclear Substrates

Much of the work on the basic molecular biology of human adenoviruses has been carried out on a very limited number of the more than 60 serotypes, primarily the highly related species C viruses adenovirus type 5 (Ad5) and Ad2 and, to some extent, Ad12 of species A. Until recently, it has been widely assumed that insights obtained with these model viruses were representative of all human adenoviruses. Recent studies on the E3 ubiquitin ligase formed by the viral E1B55K and E4orf6 proteins with a cellular Cullin-based complex indicated that although all species form such a functional complex, significant variations exist in terms of complex composition and the substrates that are degraded. In the present report we conducted a comprehensive analysis of the localization of E1B55K products from representatives of six of the seven adenovirus species in the presence and the absence of the corresponding E4orf6 protein. We found that although in some species E1B55K localized in aggresomes, such was not always the case, suggesting that these structures are not necessary for the efficient degradation of substrates. In addition, differences were evident in the localization of E1B55K, although all forms readily associated with PML. Finally, Ad5 E1B55K was seen to localize in close proximity to Rab11, a marker for the endosomal recycling compartment, and both focused at the microtubule organizing center. These findings suggest that E1B55K from some species may employ the transport system utilized by the membrane recycling pathway to assemble aggresomes and the possibility that this structure might then affect recycling of cell surface components.

Gene therapy for cardiovascular disease: Perspectives and potential

Cardiovascular disease is the most frequent cause of mortality in the western world, accounting for over 800,000 premature deaths per year in the EU alone. Cardiovascular disease is the second most common application for gene therapy clinical trials, which most frequently employ adenovirus serotype 5 (Ad5)-based vectors as delivery vehicles. Although interactions of Ad5 vectors with circulating proteins and cells can limit their efficacy after systemic administration, local gene delivery strategies show great potential in the cardiovascular setting, notably in the context of vascular delivery. Here we review the pathogenesis of bypass graft failure and in-stent restenosis, identifying potential therapeutic targets and discussing recent advances in the field of adenovirus biology and retargeting that, in concert, will potentially translate in coming years to more effective gene therapies for cardiovascular applications.

Manipulation of Adenovirus Interactions with Host Factors for Gene Therapy Applications

Nanomedicine based on the use of adenovirus vectors for therapeutic gene delivery shows broad potential. Specific targeting for many gene therapy applications, such as metastatic cancers or cardiovascular diseases requires intravascular delivery of the vector. However, a major barrier to successful adenovirus vector targeting follows systemic delivery, as upon contact with the bloodstream the virus interacts with a variety of host proteins, in particular coagulation factor X, which mediates profound liver gene transfer. This inherent hepatic tropism combined with macrophage scavenging minimizes the efficacy of the virus at the desired sites and induces toxic side effects. Understanding the complex, multifaceted interactions of adenovirus with host factors is of vital importance to the design of safer vectors with improved efficacy and pharmacokinetic profiles. Increased knowledge of adenovirus biology provides the opportunity to develop innovative strategies to detarget the virus from the liver following intravascular delivery and redirect the vector to disease areas.

Advances and future challenges in adenoviral vector pharmacology and targeting.

Adenovirus is a robust vector for therapeutic applications, but its use is limited by our understanding of its complex in vivo pharmacology. In this review we describe the necessity of identifying its natural, widespread, and multifaceted interactions with the host since this information will be crucial for efficiently redirecting virus into target cells. In the rational design of vectors, the notion of overcoming a sequence of viral “sinks” must be combined with re-targeting to target populations with capsid as well as shielding the vectors from pre-existing or toxic immune responses. It must also be noted that most known adenoviral pharmacology is deduced from the most commonly used serotypes, Ad5 and Ad2. However, these serotypes may not represent all adenoviruses, and may not even represent the most useful vectors for all purposes. Chimeras between Ad serotypes may become useful in engineering vectors that can selectively evade substantial viral traps, such as Kupffer cells, while retaining the robust qualities of Ad5. Similarly, vectorizing other Ad serotypes may become useful in avoiding immunity against Ad5 altogether. Taken together, this research on basic adenovirus biology will be necessary in developing vectors that interact more strategically with the host for the most optimal therapeutic effect.