Generation of an adenoviral vaccine vector based on simian adenovirus 21

In vivo editing of the pan-endothelium by immunity evading simian adenoviral vector

Biological applications deriving from the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 site-specific nuclease system continue to impact and accelerate gene therapy strategies. Safe and effective in vivo co-delivery of the CRISPR/Cas9 system to target somatic cells is essential in the clinical therapeutic context. Both non-viral and viral vector systems have been applied for this delivery matter. Despite elegant proof-of-principle studies, available vector technologies still face challenges that restrict the application of CRISPR/Cas9-facilitated gene therapy. Of note, the mandated co-delivery of the gene-editing components must be accomplished in the potential presence of pre-formed anti-vector immunity. Additionally, methods must be sought to limit the potential of off-target editing. To this end, we have exploited the molecular promiscuities of adenovirus (Ad) to address the key requirements of CRISPR/Cas9-facilitated gene therapy. In this regard, we have endeavored capsid engineering of a simian (chimpanzee) adenovirus isolate 36 (SAd36) to achieve targeted modifications of vector tropism. The SAd36 vector with the myeloid cell-binding peptide (MBP) incorporated in the capsid has allowed selective in vivo modifications of the vascular endothelium. Importantly, vascular endothelium can serve as an effective non-hepatic cellular source of deficient serum factors relevant to several inherited genetic disorders. In addition to allowing for re-directed tropism, capsid engineering of nonhuman primate Ads provide the means to circumvent pre-formed vector immunity. Herein we have generated a SAd36. MBP vector that can serve as a single intravenously administered agent allowing effective and selective in vivo editing for endothelial target cells of the mouse spleen, brain and kidney. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding author upon reasonable request.

A novel intradermal tattoo-based injection device enhances the immunogenicity of plasmid DNA vaccines

In recent years, tattooing technology has shown promising results toward evaluating vaccines in both animal models and humans. However, this technology has some limitations due to variability of experimental evaluations or operator procedures. The current study evaluated a device (intradermal oscillating needle array injection device: IONAID) capable of microinjecting a controlled dose of any aqueous vaccine into the intradermal space. IONAID-mediated administration of a DNA-based vaccine encoding the glycoprotein (GP) from the Ebola virus resulted in superior T- and B-cell responses with IONAID when compared to single intramuscular (IM) or intradermal (ID) injection in mice. Moreover, humoral immune responses, induced after IONAID vaccination, were significantly higher to those obtained with traditional passive DNA tattooing in guinea pigs and rabbits. This device was well tolerated and safe during HIV vaccine delivery in non-human primates (NHPs), while inducing robust immune responses. In summary, this study shows that the IONAID device improves vaccine performance, which could be beneficial to the animal and human health, and importantly, provide a dose-sparing approach (e.g., monkeypox vaccine).

The use of adenoviral vectors in gene therapy and vaccine approaches

Adenovirus was first identified in the 1950s and since then this pathogenic group of viruses has been explored and transformed into a genetic transfer vehicle. Modification or deletion of few genes are necessary to transform it into a conditionally or non-replicative vector, creating a versatile tool capable of transducing different tissues and inducing high levels of transgene expression. In the early years of vector development, the application in monogenic diseases faced several hurdles, including short-term gene expression and even a fatality. On the other hand, an adenoviral delivery strategy for treatment of cancer was the first approved gene therapy product. There is an increasing interest in expressing transgenes with therapeutic potential targeting the cancer hallmarks, inhibiting metastasis, inducing cancer cell death or modulating the immune system to attack the tumor cells. Replicative adenovirus as vaccines may be even older and date to a few years of its discovery, application of non-replicative adenovirus for vaccination against different microorganisms has been investigated, but only recently, it demonstrated its full potential being one of the leading vaccination tools for COVID-19. This is not a new vector nor a new technology, but the result of decades of careful and intense work in this field.

Engineered Sleeping Beauty Transposon as Efficient System to Optimize Chimp Adenoviral Production

Sleeping Beauty (SB) is the first DNA transposon employed for efficient transposition in vertebrate cells, opening new applications for genetic engineering and gene therapies. A transposon-based gene delivery system holds the favourable features of non-viral vectors and an attractive safety profile. Here, we employed SB to engineer HEK293 cells for optimizing the production of a chimpanzee Adenovector (chAd) belonging to the Human Mastadenovirus C species. To date, chAd vectors are employed in several clinical settings for infectious diseases, last but not least COVID-19. A robust, efficient and quick viral vector production could advance the clinical application of chAd vectors. To this aim, we firstly swapped the hAd5 E1 with chAd-C E1 gene by using the CRISPR/Cas9 system. We demonstrated that in the absence of human Ad5 E1, chimp Ad-C E1 gene did not support HEK293 survival. To improve chAd-C vector production, we engineered HEK293 cells to stably express the chAd-C precursor terminal protein (ch.pTP), which plays a crucial role in chimpanzee Adenoviral DNA replication. The results indicate that exogenous ch.pTP expression significantly ameliorate the packaging and amplification of recombinant chAd-C vectors thus, the engineered HEK293ch.pTP cells could represent a superior packaging cell line for the production of these vectors.

Ebolavirus: Comparison of Survivor Immunology and Animal Models in the Search for a Correlate of Protection

Ebola viruses are enveloped, single-stranded RNA viruses belonging to the Filoviridae family and can cause Ebola virus disease (EVD), a serious haemorrhagic illness with up to 90% mortality. The disease was first detected in Zaire (currently the Democratic Republic of Congo) in 1976. Since its discovery, Ebola virus has caused sporadic outbreaks in Africa and was responsible for the largest 2013–2016 EVD epidemic in West Africa, which resulted in more than 28,600 cases and over 11,300 deaths. This epidemic strengthened international scientific efforts to contain the virus and develop therapeutics and vaccines. Immunology studies in animal models and survivors, as well as clinical trials have been crucial to understand Ebola virus pathogenesis and host immune responses, which has supported vaccine development. This review discusses the major findings that have emerged from animal models, studies in survivors and vaccine clinical trials and explains how these investigations have helped in the search for a correlate of protection.

Genomics-based re-examination of the taxonomy and phylogeny of human and simian Mastadenoviruses: an evolving whole genomes approach, revealing putative zoonosis, anthroponosis, and amphizoonosis

With the advent of high-resolution and cost-effective genomics and bioinformatics tools and methods contributing to a large database of both human (HAdV) and simian (SAdV) adenoviruses, a genomics-based re-evaluation of their taxonomy is warranted. Interest in these particular adenoviruses is growing in part due to the applications of both in gene transfer protocols, including gene therapy and vaccines, as well in oncolytic protocols. In particular, the re-evaluation of SAdVs as appropriate vectors in humans is important as zoonosis precludes the assumption that human immune system may be naïve to these vectors. Additionally, as important pathogens, adenoviruses are a model organism system for understanding viral pathogen emergence through zoonosis and anthroponosis, particularly among the primate species, along with recombination, host adaptation, and selection, as evidenced by one long-standing human respiratory pathogen HAdV-4 and a recent re-evaluation of another, HAdV-76. The latter reflects the insights on amphizoonosis, defined as infections in both directions among host species including "other than human", that are possible with the growing database of nonhuman adenovirus genomes. HAdV-76 is a recombinant that has been isolated from human, chimpanzee, and bonobo hosts. On-going and potential impacts of adenoviruses on public health and translational medicine drive this evaluation of 174 whole genome sequences from HAdVs and SAdVs archived in GenBank. The conclusion is that rather than separate HAdV and SAdV phylogenetic lineages, a single, intertwined tree is observed with all HAdVs and SAdVs forming mixed clades. Therefore, a single designation of "primate adenovirus" (PrAdV) superseding either HAdV and SAdV is proposed, or alternatively, keeping HAdV for human adenovirus but expanding the SAdV nomenclature officially to include host species identification as in ChAdV for chimpanzee adenovirus, GoAdV for gorilla adenovirus, BoAdV for bonobo adenovirus, and ad libitum.

The Ebola virus glycoprotein and its immune responses across multiple vaccine platforms

Introduction: For over 40 years, ebolaviruses have been responsible for sporadic outbreaks of severe and often fatal hemorrhagic fever in humans and nonhuman primates across western and central Africa. In December 2013, an unprecedented Ebola virus (EBOV) epidemic began in West Africa and resulted in the largest outbreak to date. The past and current epidemics in West Africa and the Democratic Republic of the Congo has focused attention on the potential vaccine platforms developed over the past 20 years.Areas covered: This review summarizes the extraordinary progress using a variety of vaccination platforms including DNA, subunit, and several viral vector approaches, replicating and non-replicating, incorporating the primary antigen of EBOV, the glycoprotein. These vaccine constructs have shown varying degrees of protective efficacy in the 'gold-standard' nonhuman primate model for EBOV infections and were immunogenic in human clinical trials.Expert commentary: A number of these vaccine platforms have moved into phase III clinical trials over the past years and with the recent approval of the first EBOV vaccine in the European Union and the USA there is a strong potential to prevent future outbreaks/epidemics of EBOV infections on the scale of the West African epidemic.

Ebola: Lessons on Vaccine Development

The West African Ebola virus (EBOV) epidemic has fast-tracked countermeasures for this rare, emerging zoonotic pathogen. Until 2013-2014, most EBOV vaccine candidates were stalled between the preclinical and clinical milestones on the path to licensure, because of funding problems, lack of interest from pharmaceutical companies, and competing priorities in public health. The unprecedented and devastating epidemic propelled vaccine candidates toward clinical trials that were initiated near the end of the active response to the outbreak. Those trials did not have a major impact on the epidemic but provided invaluable data on vaccine safety, immunogenicity, and, to a limited degree, even efficacy in humans. There are plenty of lessons to learn from these trials, some of which are addressed in this review. Better preparation is essential to executing an effective response to EBOV in the future; yet, the first indications of waning interest are already noticeable.

Rapid Cloning of Novel Rhesus Adenoviral Vaccine Vectors

Human and chimpanzee adenovirus vectors are being developed to circumvent preexisting antibodies against common adenovirus vectors such as Ad5. However, baseline immunity to these vectors still exists in human populations. Traditional cloning of new adenovirus vaccine vectors is a long and cumbersome process that takes 2 months or more and that requires rare unique restriction enzyme sites. Here we describe a novel, restriction enzyme-independent method for rapid cloning of new adenovirus vaccine vectors that reduces the total cloning procedure to 1 week. We developed 14 novel adenovirus vectors from rhesus monkeys that can be grown to high titers and that are immunogenic in mice. All vectors grouped with the unusual adenovirus species G and show extremely low seroprevalence in humans. Rapid cloning of novel adenovirus vectors is a promising approach for the development of new vector platforms. Rhesus adenovirus vectors may prove useful for clinical development.IMPORTANCE To overcome baseline immunity to human and chimpanzee adenovirus vectors, we developed 14 novel adenovirus vectors from rhesus monkeys. These vectors are immunogenic in mice and show extremely low seroprevalence in humans. Rhesus adenovirus vectors may prove useful for clinical development.

Ebola and Marburg virus vaccines

The filoviruses, Ebola virus (EBOV), and Marburg virus (MARV), are among the most pathogenic viruses known to man and the causative agents of viral hemorrhagic fever outbreaks in Africa with case fatality rates of up to 90%. Nearly 30,000 infections were observed in the latest EBOV epidemic in West Africa; previous outbreaks were much smaller, typically only affecting less than a few hundred people. Compared to other diseases such as AIDS or Malaria with millions of cases annually, filovirus hemorrhagic fever (FHF) is one of the neglected infectious diseases. There are no licensed vaccines or therapeutics available to treat EBOV and MARV infections; therefore, these pathogens can only be handled in maximum containment laboratories and are classified as select agents. Under these limitations, a very few laboratories worldwide conducted basic research and countermeasure development for EBOV and MARV since their respective discoveries in 1967 (MARV) and 1976 (EBOV). In this review, we discuss several vaccine platforms against EBOV and MARV, which have been assessed for their protective efficacy in animal models of FHF. The focus is on the most promising approaches, which were accelerated in clinical development (phase I-III trials) during the EBOV epidemic in West Africa.

A prime-boost immunization regimen based on a simian adenovirus 36 vectored multi-stage malaria vaccine induces protective immunity in mice

Malaria remains a considerable burden on public health. In 2015, the WHO estimates there were 212 million malaria cases causing nearly 429,000 deaths globally. A highly effective malaria vaccine is needed to reduce the burden of this disease. We have developed an experimental vaccine candidate (PyCMP) based on pre-erythrocytic (CSP) and erythrocytic (MSP1) stage antigens derived from the rodent malaria parasite P. yoelii. Our protein-based vaccine construct induces protective antibodies and CD4+ T cell responses. Based on evidence that viral vectors increase CD8+ T cell-mediated immunity, we also have tested heterologous prime-boost immunization regimens that included human adenovirus serotype 5 vector (Ad5), obtaining protective CD8+ T cell responses. While Ad5 is commonly used for vaccine studies, the high prevalence of pre-existing immunity to Ad5 severely compromises its utility. Here, we report the use of the novel simian adenovirus 36 (SAd36) as a candidate for a vectored malaria vaccine since this virus is not known to infect humans, and it is not neutralized by anti-Ad5 antibodies. Our study shows that the recombinant SAd36PyCMP can enhance specific CD8+ T cell response and elicit similar antibody titers when compared to an immunization regimen including the recombinant Ad5PyCMP. The robust immune responses induced by SAd36PyCMP are translated into a lower parasite load following P. yoelii infectious challenge when compared to mice immunized with Ad5PyCMP.

Ebola virus vaccines - reality or fiction?

For 40 years ebolaviruses have been responsible for sporadic outbreaks of severe and often fatal hemorrhagic fever in humans and nonhuman primates. In December 2013 an unprecedented Zaire ebolavirus epidemic began in West Africa. Although "patient zero" has finally been reached after 2 years, the virus is again causing disease in the region. Currently there are no licensed vaccines or therapeutic countermeasures against ebolaviruses; however, the epidemic in West Africa has focused attention on the potential vaccine platforms developed over the past 15 years. There has been remarkable progress using a variety of platforms including DNA, subunit, and several viral vector approaches, replicating and non-replicating, which have shown varying degrees of protective efficacy in the "gold-standard" nonhuman primate models for Ebolavirus infections. A number of these vaccine platforms have moved into clinical trials over the past year with the hope of finding an efficacious vaccine to prevent future outbreaks/epidemics of Ebola hemorrhagic fever on the scale of the West African epidemic.

Xenogenic Adenoviral Vectors

Many nonhuman adenoviruses (AdVs) of simian, bovine, porcine, canine, ovine, murine, and fowl origin are being developed as gene delivery systems for recombinant vaccines and gene therapy applications. In addition to circumventing preexisting human AdV (HAdV) immunity, nonhuman AdV vectors utilize coxsackievirus-adenovirus receptor or other receptors for vector internalization, thereby expanding the range of cell types that can be targeted. Nonhuman AdV vectors also provide excellent platforms for veterinary vaccines. A specific nonhuman AdV vector when used in its species of origin could provide an excellent animal model for evaluating the vector efficacy and pathogenesis. These vectors are useful in prime–boost approaches with other AdV vectors or with other gene delivery systems including DNA immunization and viral or bacterial vectors. When multiple vector inoculations are required, nonhuman AdV vectors could supplement HAdV or other viral vectors.

Assessment of chimpanzee adenovirus serotype 63 neutralizing antibodies prior to evaluation of a candidate malaria vaccine regimen based on viral vectors

Prior to a chimpanzee adenovirus-based (ChAd63) malarial vaccine trial, sera were collected to assess ChAd63-specific neutralizing antibody titers in Banfora (Burkina Faso). The low neutralizing antibody titers reported in both adults and children (median titers, 139.1 and 35.0, respectively) are encouraging for the potential use of ChAd63 as a malarial vaccine vector.

Ebola virus vaccines: an overview of current approaches

Ebola hemorrhagic fever is one of the most fatal viral diseases worldwide affecting humans and nonhuman primates. Although infections only occur frequently in Central Africa, the virus has the potential to spread globally and is classified as a category A pathogen that could be misused as a bioterrorism agent. As of today there is no vaccine or treatment licensed to counteract Ebola virus infections. DNA, subunit and several viral vector approaches, replicating and non-replicating, have been tested as potential vaccine platforms and their protective efficacy has been evaluated in nonhuman primate models for Ebola virus infections, which closely resemble disease progression in humans. Though these vaccine platforms seem to confer protection through different mechanisms, several of them are efficacious against lethal disease in nonhuman primates attesting that vaccination against Ebola virus infections is feasible.

Simian adenovirus type 35 has a recombinant genome comprising human and simian adenovirus sequences, which predicts its potential emergence as a human respiratory pathogen

Emergent human and simian adenoviruses (HAdVs) may arise from genome recombination. Computational analysis of SAdV type 35 reveals a genome comprising a chassis with elements mostly from two simian adenoviruses, SAdV-B21 and -B27, and regions of high sequence similarity shared with HAdV-B21 and HAdV-B16. Although recombination direction cannot be determined, the presence of these regions suggests prior infections of humans by an ancestor of SAdV-B35, and/or vice versa. Absence of this virus in humans may reflect non-optimal conditions for zoonosis or incomplete typing, e.g., limited epitope-based. The presence of both a critical viral replication element found in HAdV genomes and genes that are highly similar to ones in HAdVs suggest the potential to establish in a human host. This allows a prediction that this virus may be a nascent human respiratory pathogen. The recombination potential of human and simian adenovirus genomes should be considered in the use of SAdVs as vectors for gene delivery in humans.

Computational analysis of four human adenovirus type 4 genomes reveals molecular evolution through two interspecies recombination events

Computational analysis of human adenovirus type 4 (HAdV-E4), a pathogen that is the only HAdV member of species E, provides insights into its zoonotic origin and molecular adaptation. Its genome encodes a domain of the major capsid protein, hexon, from HAdV-B16 recombined into the genome chassis of a simian adenovirus. Genomes of two recent field strains provide a clue to its adaptation to the new host: recombination of a NF-I binding site motif, which is required for efficient viral replication, from another HAdV genome. This motif is absent in the chimpanzee adenoviruses and the HAdV-E4 prototype, but is conserved amongst other HAdVs. This is the first report of an interspecies recombination event for HAdVs, and the first documentation of a lateral partial gene transfer from a chimpanzee AdV. The potential for such recombination events are important when considering chimpanzee adenoviruses as candidate gene delivery vectors for human patients.

Current ebola vaccines

INTRODUCTION

Ebolaviruses cause severe viral hemorrhagic fever in humans and non-human primates (NHPs), with case fatality rates of up to 90%. Currently, neither a specific treatment nor a vaccine licensed for use in humans is available. However, a number of vaccine candidates have been developed in the last decade that are highly protective in NHPs, the gold standard animal model for ebola hemorrhagic fever.

AREAS COVERED

This review analyzes a number of scenarios for the use of ebolavirus vaccines, discusses the requirements for ebolavirus vaccines in these scenarios and describes current ebolavirus vaccines. Among these vaccines are recombinant adenoviruses, recombinant vesicular stomatitis viruses (VSVs), recombinant human parainfluenza viruses and virus-like particles. Interestingly, one of these vaccine platforms, based on recombinant VSVs, has also demonstrated post-exposure protection in NHPs.

EXPERT OPINION

The most pressing remaining challenge is now to move these vaccine candidates forward into human trials and toward licensure. In order to achieve this, it will be necessary to establish the mechanisms and correlates of protection for these vaccines, and to continue to demonstrate their safety, particularly in potentially immunocompromised populations. However, already now there is sufficient evidence that, from a scientific perspective, a vaccine protective against ebolaviruses is possible.

Construction of gene transfer vectors based on simian adenovirus 7

The complete nucleotide sequence of an isolate of simian adenovirus 7 (SAdV-7) was determined. The genome organization of this isolate was found to be similar to that of other primate adenoviruses with two principal notable points: severe truncation of the E1A and E1B 19K proteins and an E3 region encoding only the 12.5K homologue. The viral gene products of SAdV-7 are most closely related to simian adenovirus 1 (SAdV-1), and like SAdV-1, are related to the human adenovirus species HAdV-F, such as the enteric adenoviruses HAdV-40 and HAdV-41 and the recently defined HAdV-G (HAdV-52). Two kinds of gene transfer vectors were made: a replication-competent SAdV-7-based vector with no genomic deletion, and a standard replication-incompetent vector deleted for E1. Importantly, the E1-deleted vector could be propagated to high titre by trans-complementation in human HEK 293 cells.

Progress in filovirus vaccine development: evaluating the potential for clinical use

Marburg and Ebola viruses cause severe hemorrhagic fever in humans and nonhuman primates. Currently, there are no effective treatments and no licensed vaccines; although a number of vaccine platforms have proven successful in animal models. The ideal filovirus vaccine candidate should be able to provide rapid protection following a single immunization, have the potential to work postexposure and be cross-reactive or multivalent against all Marburg virus strains and all relevant Ebola virus species and strains. Currently, there are multiple platforms that have provided prophylactic protection in nonhuman primates, including DNA, recombinant adenovirus serotype 5, recombinant human parainfluenza virus 3 and virus-like particles. In addition, a single platform, recombinant vesicular stomatitis virus, has demonstrated both prophylactic and postexposure protection in nonhuman primates. These results demonstrate that achieving a vaccine that is protective against filoviruses is possible; the challenge now is to prove its safety and efficacy in order to obtain a vaccine that is ready for human use.

Creation of a panel of vectors based on ape adenovirus isolates

BACKGROUND

We recently reported the isolation and sequencing of 30 novel adenoviruses from chimpanzees, bonobos and gorillas. These adenoviruses are promising candidates for the purpose of expanding the repertoire of adenoviral serotypes that can be used to create vectors for circumventing pre-existing neutralizing antibodies in human populations. We thus aimed to create vectors from 20 of the newly isolated adenoviruses.

METHODS

Plasmid molecular clones were created that harbored the complete E1-deleted genomes from 20 of the newly isolated ape adenoviruses belonging to species B, C and E. The plasmids were transfected into human embryonic kidney (HEK) 293 cells to rescue vectors. We also tested normal human sera to determine the extent of pre-existing cross-neutralizing anti-adenovirus neutralizing antibodies.

RESULTS

Twelve vectors could be rescued and expanded following transfection into HEK 293 cells with yields (from fifty 150-mm culture dishes) that ranged from 3 × 10(11) to 7 × 10(13) viral particles. Sera from 50 normal human donors were tested for the presence of neutralizing activity against 21 of the newly isolated ape adenoviruses. Cross-neutralizing activity was generally low, although outliers with high neutralizing activity were frequently detected. Species B ape adenoviruses generally showed the least cross-neutralization with antibodies present in the human sera that were tested.

CONCLUSIONS

E1-deleted adenovirus vectors can be created from a wide variety of ape adenoviruses that can be rescued and propagated in HEK 293 cells. The prevalence of pre-existing antibodies that can neutralize these adenoviruses in human populations is low.

A porcine adenovirus with low human seroprevalence is a promising alternative vaccine vector to human adenovirus 5 in an H5N1 virus disease model

Human adenovirus 5 (AdHu5) vectors are robust vaccine platforms however the presence of naturally-acquired neutralizing antibodies may reduce vector efficacy and potential for re-administration. This study evaluates immune responses and protection following vaccination with a replication-incompetent porcine adenovirus 3 (PAV3) vector as an alternative vaccine to AdHu5 using an avian influenza H5N1 disease model. Vaccine efficacy was evaluated in BALB/c mice following vaccination with different doses of the PAV3 vector expressing an optimized A/Hanoi/30408/2005 H5N1 hemagglutinin antigen (PAV3-HA) and compared with an AdHu5-HA control. PAV3-HA rapidly generated antibody responses, with significant neutralizing antibody titers on day 21, and stronger cellular immune responses detected on day 8, compared to AdHu5-HA. The PAV3-HA vaccine, administered 8 days before challenge, demonstrated improved survival and lower virus load. Evaluation of long-term vaccine efficacy at 12 months post-vaccination showed better protection with the PAV3-HA than with the AdHu5-HA vaccine. Importantly, as opposed to AdHu5, PAV3 vector was not significantly neutralized by human antibodies pooled from over 10,000 individuals. Overall, PAV3-based vector is capable of mediating swift, strong immune responses and offer a promising alternative to AdHu5.

An efficient method of directly cloning chimpanzee adenovirus as a vaccine vector

Adenoviral vectors have shown great promise as vaccine carriers and in gene transfer to correct underlying genetic diseases. Traditionally, construction of adenoviral vectors is complex and time consuming. In this paper, we provide an improved method for efficient generation of novel adenoviral vectors by using direct cloning. We introduce a feasible and detailed protocol for the development of chimpanzee adenoviruses (Ads) as molecular clones, as well as for the generation of recombinant virus from the molecular clones. Recombinant viruses are genetically stable and induce potent immune responses in animals. Generation of new Ad molecular clones or new recombinant Ad can be achieved in 2 months or 2 weeks, respectively.

Adenoviral producer cells

Adenovirus (Ad) vectors, in particular those of the serotype 5, are highly attractive for a wide range of gene therapy, vaccine and virotherapy applications (as discussed in further detail in this issue). Wild type Ad5 virus can replicate in numerous tissue types but to use Ad vectors for therapeutic purposes the viral genome requires modification. In particular, if the viral genome is modified in such a way that the viral life cycle is interfered with, a specific producer cell line is required to provide trans-complementation to overcome the modification and allow viral production. This can occur in two ways; use of a producer cell line that contains specific adenoviral sequences incorporated into the cell genome to trans-complement, or use of a producer cell line that naturally complements for the modified Ad vector genome. This review concentrates on producer cell lines that complement non-replicating adenoviral vectors, starting with the historical HEK293 cell line developed in 1977 for first generation Ad vectors. In addition the problem of replication-competent adenovirus (RCA) contamination in viral preparations from HEK293 cells is addressed leading to the development of alternate cell lines. Furthermore novel cell lines for more complex Ad vectors and alternate serotype Ad vectors are discussed.

PEGylated Adenoviruses: From Mice to Monkeys

Covalent modification with polyethylene glycol (PEG), a non-toxic polymer used in food, cosmetic and pharmaceutical preparations for over 60 years, can profoundly influence the pharmacokinetic, pharmacologic and toxciologic profile of protein and peptide-based therapeutics. This review summarizes the history of PEGylation and PEG chemistry and highlights the value of this technology in the context of the design and development of recombinant viruses for gene transfer, vaccination and diagnostic purposes. Specific emphasis is placed on the application of this technology to the adenovirus, the most potent viral vector with the most highly characterized toxicity profile to date, in several animal models.

Applying genomic and bioinformatic resources to human adenovirus genomes for use in vaccine development and for applications in vector development for gene delivery

Technological advances and increasingly cost-effect methodologies in DNA sequencing and computational analysis are providing genome and proteome data for human adenovirus research. Applying these tools, data and derived knowledge to the development of vaccines against these pathogens will provide effective prophylactics. The same data and approaches can be applied to vector development for gene delivery in gene therapy and vaccine delivery protocols. Examination of several field strain genomes and their analyses provide examples of data that are available using these approaches. An example of the development of HAdV-B3 both as a vaccine and also as a vector is presented.

Molecular detection of novel adenoviruses in fecal specimens of captive monkeys with diarrhea in China

Adenovirus (AdV) has been recently detected among monkeys with diarrhea in a major research primate colony in China. To better assess disease burden and epidemiology of adenoviruses in the colony, we examined the prevalence of this virus in fecal specimens by PCR using broadly reactive hexon gene-specific primers. Of the 29 strains that were characterized by sequence and phylogenetic analysis, we identified a broad spectrum of simian AdV (SAdV) types, including species SAdV-A (n=14) and HAdV-G (n=9). Six additional strains represented two genetic clusters distantly related to other known SAdVs. A better understanding of the epidemiology of SAdVs and their potential role in gastroenteritis is critical to the implementation of advanced prevention strategies against AdV infection in captive primates.

Overcoming innate host resistance to vaccination: employing a genetically distinct strain of murine cytomegalovirus avoids vector-mediated resistance to virally vectored immunocontraception

The laboratory strain of murine cytomegalovirus (MCMV), K181, has been successfully engineered as a vaccine expressing murine zona pellucida 3 (mZP3) for viral vectored immunocontraception (VVIC) in mice. However, certain laboratory strains of mice are resistant to infection with K181 and therefore demonstrate resistance to VVIC. Cmv1 is the best characterised innate resistance mechanism to MCMV and was first described in C57BL/6 mice. Resistance in C57BL/6 mice is due to early and strong activation of natural killer (NK) cells by an MCMV gene product, m157, that binds directly to the NK cell activating receptor Ly49H. In this study a wild strain of MCMV, G4, which expresses a variant m157 incapable of activating Ly49H, was engineered to express murine zona pellucida 3 (mZP3) and assessed for its ability to sterilise female C57BL/6 mice. When infected with K181-mZP3 female C57BL/6 mice remained fully fertile. In contrast, female C57BL/6 mice were sterilised by a single intraperitoneal inoculation of G4-mZP3. Infertility was induced by G4-mZP3 in three strains of mice that express Ly49H, on two different histocompatibility-2 (H-2) backgrounds. Finally, enhanced immunocontraception was observed in mice expressing H-2(k) mediated resistance to MCMV when infected with G4-mZP3 compared to K181-mZP3. These data indicate that when using viral vaccine vectors, variant vector strains may be used to circumvent powerful innate immune responses against the vector and promote effective vaccination. This study highlights the importance of vaccine vector genetics in vaccination strategies.

Prevalence of serum neutralizing antibodies against chimpanzee adenovirus 63 and human adenovirus 5 in Kenyan children, in the context of vaccine vector efficacy

Vaccination against Plasmodium falciparum malaria could reduce the worldwide burden of this disease, and decrease its high mortality in children. Replication-defective recombinant adenovirus vectors carrying P. falciparum epitopes may be useful as part of a vaccine that raises cellular immunity to the pre-erythrocytic stage of malaria infection. However, existing immunity to the adenovirus vector results in antibody-mediated neutralization of the vaccine vector, and reduced vaccine immunogenicity. Our aim was to examine a population of children who are at risk from P. falciparum malaria for neutralizing immunity to replication-deficient recombinant chimpanzee adenovirus 63 vector (AdC63), compared to human adenovirus 5 vector (AdHu5). We measured 50% and 90% vector neutralization titers in 200 individual sera, taken from a cohort of children from Kenya, using a secreted alkaline phosphatase neutralization assay. We found that 23% of the children (aged 1-6 years) had high-titer neutralizing antibodies to AdHu5, and 4% had high-titer neutralizing antibodies to AdC63. Immunity to both vectors was age-dependent. Low-level neutralization of AdC63 was significantly less frequent than AdHu5 neutralization at the 90% neutralization level. We conclude that AdC63 may be a useful vector as part of a prime-boost malaria vaccine in children.

First step in characterization of cis-acting sequences involved in fowl adenovirus 1 (CELO) packaging and its effect on the development of a helper-dependent vector strategy

Adenovirus-based vectors are widely developed for potential utilization as vectors in vaccine and gene therapy strategies. We focused on developing a helper-dependent adenoviral (HD-Ad) vector for the potential use of CELO, a member of the Aviadenovirus genus, in avian species vaccination. Our aim was to localize sequences which could play an essential role in CELO genome encapsidation and, when deleted, was unable to produce viruses to develop a helper CELO virus. A panel of 6 mutants with deletions between nt 80 and 350 of the CELO genome was constructed and characterized for its ability to produce viable virus. To develop a helper-dependent adenoviral vector derived from CELO, a helper virus was developed by inserting loxP sequences around the region containing the identified putative packaging sequences. A LMH (Leghorn Male Hepatocarcinoma) cell line expressing Cre recombinase was developed to allow the excision of this region. We demonstrated that the region from nt 200 to 250 was important and the region from nt 250 to 300 at the left end of the CELO genome was essential for virus encapsidation. We also showed that the loxP-flanked region was efficiently removed in a Cre expressing cell line to produce a candidate helper virus.

Parasite vaccines: The new generation

Parasites cause some of the most devastating and prevalent diseases in humans and animals. Moreover, parasitic infections increase mortality rates of other serious non-parasitic infections caused by pathogens such as HIV-1. The impact of parasitic diseases in both industrialised and developing countries is further exacerbated by the resistance of some parasites to anti-parasitic drugs and the absence of efficacious parasite vaccines. Despite years of research, much remains to be done to develop effective vaccines against parasites. This review focuses on the more recent vaccine strategies such as DNA and viral vector-based vaccines that are currently being used to develop vaccines against parasites. Obstacles yet to be overcome and possible advantages and disadvantages of these vaccine modalities are also discussed.

Partial protection against H5N1 influenza in mice with a single dose of a chimpanzee adenovirus vector expressing nucleoprotein

The development of adenoviral vectors based on non-human serotypes such as the chimpanzee adenovirus simian adenovirus 24 (AdC7) may allow for their utilization in populations harboring neutralizing antibodies to common human serotypes. Because adenoviral vectors can be used to generate potent T cell responses, they may be useful as vaccines against pandemic influenza such as may be caused by the H5N1 strains that are currently endemic in avian populations. The influenza nucleoprotein (NP) is known to provide MHC Class I restricted epitopes that are effective in evoking a cytolytic response. Because there is only low sequence variation in NP sequences between different influenza strains, a T cell vaccine may provide heterosubtypic protection against a spectrum of influenza A strains. An AdC7 vector expressing the influenza A/Puerto Rico/8/34 NP was tested for its efficacy in protecting BALB/c mice against two H5N1 strains and compared to a conventional human adenovirus serotype 5 vaccine. The AdC7 NP vaccine elicited a strong anti-NP T cell response. When tested in a mouse challenge model, there was improved survival following challenge with two strains of H5N1 that have caused human outbreaks, Vietnam/1203/04 and Hong Kong/483/97, although the improved survival reached statistical significance only with the strain from Vietnam.

Phylogenetic analysis and structural predictions of human adenovirus penton proteins as a basis for tissue-specific adenovirus vector design

The penton base is a major capsid protein of human adenoviruses (HAdV) which forms the vertices of the capsid and interacts with hexon and fiber protein. Two hypervariable loops of the penton are exposed on the capsid surface. Sequences of these and 300 adjacent amino acid residues of all 51 HAdV and closely related simian adenoviruses were studied. Adjacent sequences and predicted overall secondary structure were conserved. Phylogenetic analysis revealed clustering corresponding to the HAdV species and recombination events in the origin of HAdV prototypes. All HAdV except serotypes 40 and 41 of species F exhibited an integrin binding RGD motif in the second loop. The lengths of the loops (HVR1 and RGD loops) varied significantly between HAdV species with the longest RGD loop observed in species C and the longest HVR1 in species B. Long loops may permit the insertion of motifs that modify tissue tropism. Genetic analysis of HAdV prime strain p17'H30, a neutralization variant of HAdV-D17, indicated the significance of nonhexon neutralization epitopes for HAdV immune escape. Fourteen highly conserved motifs of the penton base were analyzed by site-directed mutagenesis of HAdV-D8 and tested for sustained induction of early cytopathic effects. Thus, three new motifs essential for penton base function were identified additionally to the RGD site, which interacts with a secondary cellular receptor responsible for internalization. Therefore, our penton primary structure data and secondary structure modeling in combination with the recently published fiber knob sequences may permit the rational design of tissue-specific adenoviral vectors.

Effect of preexisting immunity to adenovirus human serotype 5 antigens on the immune responses of nonhuman primates to vaccine regimens based on human- or chimpanzee-derived adenovirus vectors

In this study we compared a prime-boost regimen with two serologically distinct replication-defective adenovirus (Ad) vectors derived from chimpanzee serotypes C68 and C1 expressing Gag, Pol, gp140, and Nef of human immunodeficiency virus type 1 with a regimen in which replication-defective Ad vectors of the human serotype 5 (AdHu5) were given twice. Experiments were conducted in rhesus macaques that had or had not been preexposed to antigens of AdHu5. There was no significant difference in T-cell responses tested from peripheral blood of the different groups, although responses were overall highest in nonpreexposed animals immunized with the chimpanzee Ad vectors. Preexisting immunity to AdHu5 completely inhibited induction of transgene product-specific antibodies by the AdHu5 vectors without affecting antibody responses to the chimpanzee vectors. Upon euthanasia, T-cell responses were tested from a number of tissues. Preexisting immunity to AdHu5, commonly found in humans, changed the homing pattern of vaccine-induced T cells. In AdHu5-preexposed animals vaccinated with the chimpanzee Ad vectors, frequencies of transgene-specific T cells were higher in spleens than in blood, and in most preexposed animals vaccinated either with AdHu5 vectors or chimpanzee adenovirus vectors, frequencies of such T cells were exceptionally high in livers. The latter results indicate that analysis of T-cell responses solely from blood mononuclear cells of vaccine recipients may not suffice to compare the potencies of different vaccine regimens.

A CD46-binding chimpanzee adenovirus vector as a vaccine carrier

A replication-defective chimeric vector based on the chimpanzee adenovirus serotype C1 was developed and tested as a vaccine carrier in mice. The AdC1 virus is closely related to human adenoviruses of subgroup B2 and uses CD46 for cell attachment. To overcome poor growth of E1-deleted AdC1 vectors on cell lines that provide the E1 of adenovirus of the human serotype 5 (AdHu5) virus in trans, the inverted terminal repeats and some of the early genes of AdC1 were replaced with those from AdC5, a chimpanzee origin adenovirus of subfamily E. The chimeric AdC1/C5 vector efficiently transduces CD46-expressing mouse dendritic cells (DCs) in vitro and initiates their maturation. Transduction of DCs in vivo is inefficient in CD46 transgenic mice. The AdC1/C5 vector induces transgene product-specific B- and CD8(+) T-cell responses in mice. Responses are slightly higher in wild-type mice than in CD46 transgenic mice. Transgene product-specific T-cell responses elicited by the AdC1/C5 vector can be increased by priming or boosting with a heterologous adenovirus vector. Pre-existing immunity to adenovirus of the common human serotype 5 does not affect induction of cell-mediated immune responses by the AdC1/C5 vector. This vector provides an additional tool in a repertoire of adenovirus-based vaccine vectors.