Comparison of the E3 and L3 regions for arming oncolytic adenoviruses to achieve a high level of tumor-specific transgene expression

Advances of Recombinant Adenoviral Vectors in Preclinical and Clinical Applications

Adenoviruses (Ad) have the potential to induce severe infections in vulnerable patient groups. Therefore, understanding Ad biology and antiviral processes is important to comprehend the signaling cascades during an infection and to initiate appropriate diagnostic and therapeutic interventions. In addition, Ad vector-based vaccines have revealed significant potential in generating robust immune protection and recombinant Ad vectors facilitate efficient gene transfer to treat genetic diseases and are used as oncolytic viruses to treat cancer. Continuous improvements in gene delivery capacity, coupled with advancements in production methods, have enabled widespread application in cancer therapy, vaccine development, and gene therapy on a large scale. This review provides a comprehensive overview of the virus biology, and several aspects of recombinant Ad vectors, as well as the development of Ad vector, are discussed. Moreover, we focus on those Ads that were used in preclinical and clinical applications including regenerative medicine, vaccine development, genome engineering, treatment of genetic diseases, and virotherapy in tumor treatment.

Engineered Human Adenoviruses of Species B and C Report Early, Intermediate Early, and Late Viral Gene Expression

Adenoviruses (AdVs) are being developed for oncolytic or vaccination therapy against existing and emerging conditions. Well-characterized replication-competent human and human primate AdVs expressing multiple payloads are desirable, but their replication in rodent models is limited. To score the timing of adenoviral gene expression in cell cultures, we developed fully replication-competent transcriptional reporter viruses for HAdV-C5, -B3, and -B35. The picornavirus-derived 2A sequence, which induces cotranslational peptide splitting and reinitiation (skipping), was linked to GFP and the fused sequence was inserted C-terminal of the early gene E1A, the intermediate early gene protein IX and the late fiber gene. The 2A peptide induced ribosomal skipping during translation of the messenger RNA (mRNA) and gave rise to GFP from the corresponding viral promoters, as shown by immunoblotting and flow cytometry analyses of human and rodent cells. In human cells, both species B and C AdV exhibited highest reporter expression for fiber, followed by protein IX and lowest for E1A. Inoculation with either HAdV-C5 or -B3/35 viruses encoding protein IX- or fiber-GFP gave rise to higher GFP levels in hamster than mouse cells. Remarkably, despite rather low 2A ribosomal skipping efficiency of ∼50% for E1A-2A-GFP, protein IX-2A-GFP, and fiber-2A-GFP, unprocessed protein IX-2A-GFP and fiber-2A-GFP fusion proteins were efficiently incorporated into HAdV-B3 virions, respectively. These data indicate that the B3 C-termini of protein IX and fiber can be considered for retargeting engineered oncolytic or vaccination vectors, or for antigen display. The variable expression levels of transgenes from different subviral promoters may be used to improve oncolytic AdV vectors expressing therapeutic genes.

Generation and Characterization of a Replication-Competent Human Adenovirus Type 55 Encoding EGFP

Human adenovirus 55 (HAdV-55) has recently caused outbreaks of acute respiratory disease (ARD), posing a significant public threat to civilians and military trainees. Efforts to develop antiviral inhibitors and quantify neutralizing antibodies require an experimental system to rapidly monitor viral infections, which can be achieved through the use of a plasmid that can produce an infectious virus. Here, we used a bacteria-mediated recombination approach to construct a full-length infectious cDNA clone, pAd55-FL, containing the whole genome of HadV-55. Then, the green fluorescent protein expression cassette was assembled into pAd55-FL to replace the E3 region to obtain a recombinant plasmid of pAd55-dE3-EGFP. The rescued recombinant virus rAdv55-dE3-EGFP is genetically stable and replicates similarly to the wild-type virus in cell culture. The virus rAdv55-dE3-EGFP can be used to quantify neutralizing antibody activity in sera samples, producing results in concordance with the cytopathic effect (CPE)-based microneutralization assay. Using an rAdv55-dE3-EGFP infection of A549 cells, we showed that the assay could be used for antiviral screening. Our findings suggest that the rAdv55-dE3-EGFP-based high-throughput assay provides a reliable tool for rapid neutralization testing and antiviral screening for HAdV-55.

Concepts in Oncolytic Adenovirus Therapy

Oncolytic adenovirus therapy is gaining importance as a novel treatment option for the management of various cancers. Different concepts of modification within the adenovirus vector have been identified that define the mode of action against and the interaction with the tumour. Adenoviral vectors allow for genetic manipulations that restrict tumour specificity and also the expression of specific transgenes in order to support the anti-tumour effect. Additionally, replication of the virus and reinfection of neighbouring tumour cells amplify the therapeutic effect. Another important aspect in oncolytic adenovirus therapy is the virus induced cell death which is a process that activates the immune system against the tumour. This review describes which elements in adenovirus vectors have been identified for modification not only to utilize oncolytic adenovirus vectors into conditionally replicating adenoviruses (CRAds) that allow replication specifically in tumour cells but also to confer specific characteristics to these viruses. These advances in development resulted in clinical trials that are summarized based on the conceptual design.

Beyond cancer cells: Targeting the tumor microenvironment with gene therapy and armed oncolytic virus

Cancer gene therapies are usually designed either to express wild-type copies of tumor suppressor genes or to exploit tumor-associated phenotypic changes to endow selective cytotoxicity. However, these approaches become less relevant to cancers that contain many independent mutations, and the situation is made more complex by our increased understanding of clonal evolution of tumors, meaning that different metastases and even regions of the same tumor mass have distinct mutational and phenotypic profiles. In contrast, the relatively genetically stable tumor microenvironment (TME) therefore provides an appealing therapeutic target, particularly since it plays an essential role in promoting cancer growth, immune tolerance, and acquired resistance to many therapies. Recently, a variety of different TME-targeted gene therapy and armed oncolytic strategies have been explored, with particular success observed in strategies targeting the cancer stroma, reducing tumor vasculature, and repolarizing the immunosuppressive microenvironment. Herein, we review the progress of these TME-targeting approaches and try to highlight those showing the greatest promise.

Arming Oncolytic Adenoviruses: Effect of Insertion Site and Splice Acceptor on Transgene Expression and Viral Fitness

Oncolytic adenoviruses (OAds) present limited efficacy in clinics. The insertion of therapeutic transgenes into OAds genomes, known as "arming OAds", has been the main strategy to improve their therapeutic potential. Different approaches were published in the decade of the 2000s, but with few comparisons. Most armed OAds have complete or partial E3 deletions, leading to a shorter half-life in vivo. We generated E3+ OAds using two insertion sites, After-fiber and After-E4, and two different splice acceptors linked to the major late promoter, either the Ad5 protein IIIa acceptor (IIIaSA) or the Ad40 long fiber acceptor (40SA). The highest transgene levels were obtained with the After-fiber location and 40SA. However, the set of codons of the transgene affected viral fitness, highlighting the relevance of transgene codon usage when arming OAds using the major late promoter.

Effect of Transgene Location, Transcriptional Control Elements and Transgene Features in Armed Oncolytic Adenoviruses

Clinical results with oncolytic adenoviruses (OAds) used as antitumor monotherapies show limited efficacy. To increase OAd potency, transgenes have been inserted into their genome, a strategy known as "arming OAds". Here, we review different parameters that affect the outcome of armed OAds. Recombinant adenovirus used in gene therapy and vaccination have been the basis for the design of armed OAds. Hence, early region 1 (E1) and early region 3 (E3) have been the most commonly used transgene insertion sites, along with partially or complete E3 deletions. Besides transgene location and orientation, transcriptional control elements, transgene function, either virocentric or immunocentric, and even the codons encoding it, greatly impact on transgene levels and virus fitness.

Chapter eight--Oncolytic adenoviruses for cancer immunotherapy: data from mice, hamsters, and humans

Adenovirus is one of the most commonly used vectors for gene therapy and two products have already been approved for treatment of cancer in China (Gendicine(R) and Oncorine(R)). An intriguing aspect of oncolytic adenoviruses is that by their very nature they potently stimulate multiple arms of the immune system. Thus, combined tumor killing via oncolysis and inherent immunostimulatory properties in fact make these viruses in situ tumor vaccines. When further engineered to express cytokines, chemokines, tumor-associated antigens, or other immunomodulatory elements, they have been shown in various preclinical models to induce antigen-specific effector and memory responses, resulting both in full therapeutic cures and even induction of life-long tumor immunity. Here, we review the state of the art of oncolytic adenovirus, in the context of their capability to stimulate innate and adaptive arms of the immune system and finally how we can modify these viruses to direct the immune response toward cancer.

Oncolytic Viruses for Cancer Therapy: Overcoming the Obstacles

Targeted therapy of cancer using oncolytic viruses has generated much interest over the past few years in the light of the limited efficacy and side effects of standard cancer therapeutics for advanced disease. In 2006, the world witnessed the first government-approved oncolytic virus for the treatment of head and neck cancer. It has been known for many years that viruses have the ability to replicate in and lyse cancer cells. Although encouraging results have been demonstrated in vitro and in animal models, most oncolytic viruses have failed to impress in the clinical setting. The explanation is multifactorial, determined by the complex interactions between the tumor and its microenvironment, the virus, and the host immune response. This review focuses on discussion of the obstacles that oncolytic virotherapy faces and recent advances made to overcome them, with particular reference to adenoviruses.

Armed replicating adenoviruses for cancer virotherapy

Conditionally replicating adenoviruses (CRAds) have many advantages as agents for cancer virotherapy and have been safely used in human clinical trials. However, replicating adenoviruses have been limited in their ability to eliminate tumors by oncolysis. Thus, the efficacy of these agents must be improved. To this end, CRAds have been engineered to express therapeutic transgenes that exert antitumor effects independent of direct viral oncolysis. These transgenes can be expressed under native gene control elements, in which case placement within the genome determines the expression profile, or they can be controlled by exogenous promoters. The therapeutic transgenes used to arm replicating adenoviruses can be broadly classified into three groups. There are those that mediate killing of the infected cell, those that modulate the tumor microenvironment and those with immunomodulatory functions. Overall, the studies to date in animal models have shown that arming a CRAd with a rationally chosen therapeutic transgene can improve its antitumor efficacy over that of an unarmed CRAd. However, a number of obstacles must be overcome before the full potential of armed CRAds can be realized in the human clinical context. Hence, strategies are being developed to permit intravenous delivery to disseminated cancer cells, overcome the immune response and enable in vivo monitoring of the biodistribution and activity of armed CRAds.

Novel immunocompetent murine tumor model for evaluation of conditionally replication-competent (oncolytic) murine adenoviral vectors

Oncolytic adenoviral vectors that express immunostimulatory transgenes are currently being evaluated in clinic. Preclinical testing of these vectors has thus far been limited to immunodeficient xenograft tumor models since human adenoviruses do not replicate effectively in murine tumor cells. The effect of the immunostimulatory transgene on overall virus potency can therefore not be readily assessed in these models. Here, a model is described that allows the effective testing of mouse armed oncolytic adenovirus (MAV) vectors in immunocompetent syngeneic tumor models. These studies demonstrate that the MAV vectors have a high level of cytotoxicity in a wide range of murine tumor cells. The murine oncolytic viruses were successfully armed with murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) by a novel method which resulted in vectors with a high level of tumor-specific transgene expression. The mGM-CSF-armed MAV vectors showed an improved level of antitumor potency and induced a systemic antitumor immune response that was greater than that induced by unarmed parental vectors in immunocompetent syngeneic tumor models. Thus, the oncolytic MAV-1 system described here provides a murine homolog model for the testing of murine armed oncolytic adenovirus vectors in immunocompetent animals. The model allows evaluation of the impact of virus replication and the host immune response on overall virus potency and enables the generation of translational data that will be important for guiding the clinical development of these viruses.