Novel human gene transfer vectors: evaluation of wild-type and recombinant animal adenoviruses in human-derived cells

Understanding Post Entry Sorting of Adenovirus Capsids; A Chance to Change Vaccine Vector Properties

Adenovirus vector-based genetic vaccines have emerged as a powerful strategy against the SARS-CoV-2 health crisis. This success is not unexpected because adenoviruses combine many desirable features of a genetic vaccine. They are highly immunogenic and have a low and well characterized pathogenic profile paired with technological approachability. Ongoing efforts to improve adenovirus-vaccine vectors include the use of rare serotypes and non-human adenoviruses. In this review, we focus on the viral capsid and how the choice of genotypes influences the uptake and subsequent subcellular sorting. We describe how understanding capsid properties, such as stability during the entry process, can change the fate of the entering particles and how this translates into differences in immunity outcomes. We discuss in detail how mutating the membrane lytic capsid protein VI affects species C viruses' post-entry sorting and briefly discuss if such approaches could have a wider implication in vaccine and/or vector development.

COVID-19 Vaccine Frontrunners and Their Nanotechnology Design

Humanity is experiencing a catastrophic pandemic. SARS-CoV-2 has spread globally to cause significant morbidity and mortality, and there still remain unknowns about the biology and pathology of the virus. Even with testing, tracing, and social distancing, many countries are struggling to contain SARS-CoV-2. COVID-19 will only be suppressible when herd immunity develops, either because of an effective vaccine or if the population has been infected and is resistant to reinfection. There is virtually no chance of a return to pre-COVID-19 societal behavior until there is an effective vaccine. Concerted efforts by physicians, academic laboratories, and companies around the world have improved detection and treatment and made promising early steps, developing many vaccine candidates at a pace that has been unmatched for prior diseases. As of August 11, 2020, 28 of these companies have advanced into clinical trials with Moderna, CanSino, the University of Oxford, BioNTech, Sinovac, Sinopharm, Anhui Zhifei Longcom, Inovio, Novavax, Vaxine, Zydus Cadila, Institute of Medical Biology, and the Gamaleya Research Institute having moved beyond their initial safety and immunogenicity studies. This review analyzes these frontrunners in the vaccine development space and delves into their posted results while highlighting the role of the nanotechnologies applied by all the vaccine developers.

Bovine adenovirus-3 as a vaccine delivery vehicle

The use of vaccines is an effective and relatively inexpensive means of controlling infectious diseases, which cause heavy economic losses to the livestock industry through animal loss, decreased productivity, treatment expenses and decreased carcass quality. However, some vaccines produced by conventional means are imperfect in many respects including virulence, safety and efficacy. Moreover, there are no vaccines for some animal diseases. Although genetic engineering has provided new ways of producing effective vaccines, the cost of production for veterinary use is a critical criterion for selecting the method of production and delivery of vaccines. The cost effective production and intrinsic ability to enter cells has made adenovirus vectors a highly efficient tool for delivery of vaccine antigens. Moreover, adenoviruses induce both humoral and cellular immune responses to expressed vaccine antigens. Since nonhuman adenoviruses are species specific, the development of animal specific adenoviruses as vaccine delivery vectors is being evaluated. This review summarizes the work related to the development of bovine adenovirus-3 as a vaccine delivery vehicle in animals, particularly cattle.

Circumventing antivector immunity: potential use of nonhuman adenoviral vectors

Adenoviruses are efficient gene delivery vectors based on their ability to transduce a wide variety of cell types and drive high-level transient transgene expression. While there have been advances in modifying human adenoviral (HAdV) vectors to increase their safety profile, there are still pitfalls that need to be further addressed. Preexisting humoral and cellular immunity against common HAdV serotypes limits the efficacy of gene transfer and duration of transgene expression. As an alternative, nonhuman AdV (NHAdV) vectors can circumvent neutralizing antibodies against HAdVs in immunized mice and monkeys and in human sera, suggesting that NHAdV vectors could circumvent preexisting humoral immunity against HAdVs in a clinical setting. Consequently, there has been an increased interest in developing NHAdV vectors for gene delivery in humans. In this review, we outline the recent advances and limitations of HAdV vectors for gene therapy and describe examples of NHAdV vectors focusing on their immunogenicity, tropism, and potential as effective gene therapy vehicles.

Bovine adenovirus 3 core protein precursor pVII localizes to mitochondria, and modulates ATP synthesis, mitochondrial Ca2+ and mitochondrial membrane potential

Viruses modulate the functions of mitochondria by translocating viral proteins to the mitochondria. Subcellular fractionation and sensitivity to proteinase K/Triton X-100 treatment of mitochondrial fractions of bovine adenovirus (BAdV)-3-infected/transfected cells suggested that core protein pVII localizes to the mitochondria and contains a functional mitochondrial localization signal. Moreover, mitochondrial localization of BAdV-3 pVII appears to help in the retention of mitochondrial Ca(2+), inducing a significant increase in the levels of ATP and maintaining the mitochondrial membrane potential (MMP) in transfected cells. In contrast, mitochondrial localization of BAdV-3 pVII has no significant effect on the levels of cytoplasmic Ca(2+) and reactive oxygen species production in the transfected cells. Consistent with these results, expression of pVII in transfected cells treated with staurosporine decreased significantly the activation of caspase-3. Our results suggested that BAdV-3 pVII localizes to mitochondria, and interferes with apoptosis by inhibiting loss of the MMP and by increasing mitochondrial Ca(2+) and ATP production.

Phoenix rising: gene therapy makes a comeback

Despite the first application of gene therapy in 1990, gene therapy has until recently failed to meet the huge expectations set forth by researchers, clinicians, and patients, thus dampening enthusiasm for an imminent cure for many life-threatening genetic diseases. Nonetheless, in recent years we have witnessed a strong comeback for gene therapy, with clinical successes in young and adult subjects suffering from inherited forms of blindness or from X-linked severe combined immunodeficiency disease. In this review, various gene therapy vectors progressing into clinical development and pivotal advances in gene therapy trials will be discussed.

Mapping of nuclear import signal and importin α3 binding regions of 52K protein of bovine adenovirus-3

The L1 region of bovine adenovirus (BAdV)-3 encodes a non-structural protein designated 52K. Anti-52K serum detected a protein of 40kDa, which localized to the nucleus but not to the nucleolus in BAdV-3-infected or transfected cells. Analysis of mutant 52K proteins suggested that three basic residues ((105)RKR(107)) of the identified domain (amino acids (102)GMPRKRVLT(110)) are essential for nuclear localization of 52K. The nuclear import of a GST-52K fusion protein utilizes the classical importin α/β-dependent nuclear transport pathway. The 52K protein is preferentially bound to the cellular nuclear import receptor importin α3. Although deletion of amino acid 102-110 is sufficient to abrogate the nuclear localization of 52K, amino acid 90-133 are required for interaction with importin-α3 and localizing a cytoplasmic protein to the nucleus. These results suggest that 52K contains a bipartite NLS, which preferentially utilize an importin α3 nuclear import receptor-mediated pathway to transport 52K to the nucleus.

Efficient replication and generation of recombinant bovine adenovirus-3 in nonbovine cotton rat lung cells expressing I-SceI endonuclease

BACKGROUND

The rigorous evaluation of recombinant bovine adenovirus (BAdV)-3 as a gene delivery vector requires quick and efficient method of isolating recombinants. This requires both a suitable cell line and an efficient method of rescuing recombinant BAdV-3. To facilitate rapid isolation of recombinant BAdV-3, we have developed an efficient system for generating recombinants using newly identified nonbovine cell line permissive for replication of BAdV-3.

METHODS

Nonbovine cotton rat lung (CRL) cells in comparison to Madin-Darby bovine kidney cells and VIDO R2 cells were analyzed for the production of progeny virus and DNA transfection efficiency. In addition, lentiviral expression system was used to generate stable nonbovine CRL cell line expressing endonuclease I-SceI as examined by western blotting. Transfection of this cell line with circular or linear plasmid containing full-length BAdV-3 genome was used to generate recombinant BAdV-3.

RESULTS

We demonstrate that nonbovine CRL cells are permissive for replication of BAdV-3 and can be efficiently transfected with plasmid DNA. Second, we constructed CRL cell line (VIDO DT1) expressing an intron-encoding endonuclease I-SceI. Finally, we demonstrate that transfection of VIDO DT1 cells with a circular plasmid containing recombinant BAdV-3 genome flanked by I-SceI recognition sites can efficiently rescue recombinant virus.

CONCLUSIONS

The use of circular molecular clones together with I-SceI endonuclease expressing, BAdV-3 permissive CRL cell line not only increased the viral genome transfection efficiency, but also reduced the viral rescue time and amount of DNA required for rescuing recombinant BAdV-3s.

Oncolytic gene therapy for canine cancers: teaching old dog viruses new tricks

The use of viruses to treat cancer has been studied for decades. With the advancement of molecular biology, viruses have been modified and genetically engineered to optimize their ability to target cancer cells. Canine viruses, such as distemper virus and adenovirus, are being exploited for the treatment of canine cancer as the dog has proven to be a good comparative model for human cancer research and proof of concept investigations. In this review, we introduce the concept of oncolytic viruses and describe some of the preliminary attempts to use oncolytic viruses for the treatment of canine cancer.

Bovine adenovirus-3 E1A coding region contain cis-acting DNA packaging motifs

To elucidate further the regulation of E1 gene transcription and viral DNA packaging, we constructed and analyzed mutant BAdV-3s in which the deletion of sequences between left ITR and E1A ATG codon was combined with the functional blocking of E1A gene expression by introducing deletion mutations into E1A open reading frame (ORF). The results suggest that E1A coding region contains cis-acting packaging motifs for efficient encapsidation of BAdV-3 DNA into preformed empty capsids. In addition, E1A is not required for the transcription of E1B.

E1A promoter of bovine adenovirus type 3

Conserved motifs of eukaryotic gene promoters, such as TATA box and CAAT box sequences, of E1A of human adenoviruses (e.g human adenovirus 5) lie between the left inverted terminal repeat (ITR) and the ATG of E1A. However, analysis of the left end of the bovine adenovirus 3 (BAdV-3) genome revealed that the conserved sequences of the E1A promoter are present only in the ITR. As such, the promoter activity of ITR was tested in the context of a BAdV-3 vector or a plasmid-based system. Different regions of the left end of the BAdV-3 genome initiated transcription of the red fluorescent protein gene in a plasmid-based system. Moreover, BAdV-3 mutants in which the open reading frame of E1A was placed immediately downstream of the ITR produced E1A transcript and could be propagated in non-E1A-complementing Madin-Darby bovine kidney cells. These results suggest that the left ITR contains the sole BAdV-3 E1A promoter.

293T cells expressing simian virus 40 T antigen are semi-permissive to bovine adenovirus type 3 infection

Human cells do not normally support productive bovine adenovirus type 3 (BAdV-3) infection. Here, the outcome of BAdV-3 infection of both 293 cells and 293 cells modified to constitutively express the simian virus 40 (SV-40) T antigen (293T cells) was studied. Whereas BAdV-3 could efficiently infect 293 cells, there was a block in virus DNA replication, late-gene expression and virus production. In contrast, replication and efficient virus production could be detected in 293T cells infected with BAdV-3 or transfected with a replication-competent genomic BAdV-3 clone (pFBAV304). Early-phase gene expression was detected readily in both BAdV-3-infected 293 and 293T cells. However, the progression to efficient viral DNA synthesis and late-phase protein synthesis occurred only in 293T cells. Electron microscopy and virus growth kinetics demonstrated the formation of progeny virus in 293T cells. The SV-40 T antigens act to overcome a barrier in BAdV-3 DNA replication in 293 cells.

An adenovirus serotype 5 vector with fibers derived from ovine atadenovirus demonstrates CAR-independent tropism and unique biodistribution in mice

Many clinically important tissues are refractory to adenovirus (Ad) infection due to negligible levels of the primary Ad5 receptor the coxsackie and adenovirus receptor CAR. Thus, development of novel CAR-independent Ad vectors should lead to therapeutic gain. Ovine atadenovirus type 7, the prototype member of genus Atadenovirus, efficiently transduces CAR-deficient human cells in vitro, and systemic administration of OAdV is not associated with liver sequestration in mice. The penton base of OAdV7 does not contain an RGD motif, implicating the long-shafted fiber molecule as a major structural dictate of OAdV tropism. We hypothesized that replacement of the Ad5 fiber with the OAdV7 fiber would result in an Ad5 vector with CAR-independent tropism in vitro and liver "detargeting" in vivo. An Ad5 vector displaying the OAdV7 fiber was constructed (Ad5Luc1-OvF) and displayed CAR-independent, enhanced transduction of CAR-deficient human cells. When administered systemically to C57BL/6 mice, Ad5Luc1-OvF reporter gene expression was reduced by 80% in the liver compared to Ad5 and exhibited 50-fold higher gene expression in the kidney than the control vector. To our knowledge, this is the first report of a fiber-pseudotyped Ad vector that simultaneously displays decreased liver uptake and a distinct organ tropism in vivo. This vector may have future utility in murine models of renal disease.

Fluorescently tagged canine adenovirus via modification with protein IX-enhanced green fluorescent protein

Canine adenovirus type 2 (CAV2) has become an attractive vector for gene therapy because of its non-pathogenicity and the lack of pre-existing neutralizing antibodies against this virus in the human population. Additionally, this vector has been proposed as a conditionally replicative adenovirus agent under the control of an osteocalcin promoter for evaluation in a syngeneic, immunocompetent canine model with spontaneous osteosarcoma. In this study, a CAV2 vector labelled with the fluorescent capsid fusion protein IX-enhanced green fluorescent protein (pIX-EGFP) was developed. Expression of the fluorescent fusion-protein label in infected cells with proper nuclear localization, and incorporation into virions, could be detected. The labelled virions could be visualized by fluorescence microscopy; this was applicable to the tracking of CAV2 infection, as well as localizing the distribution of the vector in tissues. Expression of pIX-EGFP could be exploited to detect the replication and spread of CAV2. These results indicate that pIX can serve as a platform for incorporation of heterologous proteins in the context of a canine adenovirus xenotype. It is believed that capsid-labelled CAV2 has utility for vector-development studies and for monitoring CAV2-based oncolytic adenovirus replication.

Development of nonhuman adenoviruses as vaccine vectors

Human adenoviral (HAd) vectors have demonstrated great potential as vaccine vectors. Preclinical and clinical studies have demonstrated the feasibility of vector design, robust antigen expression and protective immunity using this system. However, clinical use of adenoviral vectors for vaccine purposes is anticipated to be limited by vector immunity that is either preexisting or develops rapidly following the first inoculation with adenoviral vectors. Vector immunity inactivates the vector particles and rapidly removes the transduced cells, thereby limiting the duration of transgene expression. Due to strong vector immunity, subsequent use of the same vector is usually less efficient. In order to circumvent this limitation, nonhuman adenoviral vectors have been proposed as alternative vectors. In addition to eluding HAd immunity, these vectors possess most of the attractive features of HAd vectors. Several replication-competent or replication-defective nonhuman adenoviral vectors have been developed and investigated for their potential as vaccine-delivery vectors. Here, we review recent advances in the design and characterization of various nonhuman adenoviral vectors, and discuss their potential applications for human and animal vaccination.

Bovine adenovirus type 3 internalization is independent of primary receptors of human adenovirus type 5 and porcine adenovirus type 3

Usefulness of adenoviral vectors derived from human adenovirus (HAd) type 5 (HAd5) is mainly limited by wide prevalence of preexisting anti-HAd5 immunity as well as non-specific tissue tropism of these vectors. As an alternative, non-human adenoviral vectors including bovine adenovirus type 3 (BAd3) are currently being investigated. Non-prevalence of BAd3 in humans and its ability to evade preexisting HAd immunity are some of the features that make BAd3 a promising vector for human gene delivery. BAd3 appears to have a tissue tropism distinct from that of HAd5 and also the repertoire of cells efficiently transduced by BAd3 is different. We performed antibody-mediated receptor blocking experiments to show that BAd3 internalization was independent of coxsackievirus-adenovirus receptor, the primary determinant of HAd5 tropism, or integrin alpha(v)beta3, a secondary molecule involved in HAd5 entry. Using homologous and heterologous knob-mediated competition assays with recombinant knobs of HAd5, porcine adenovirus type 3 (PAd3), or BAd3, we observed that BAd3 internalization was independent of the primary receptors of HAd5 and PAd3. These results provide support for further exploration of BAd3 vectors for designing targeted vectors for human gene therapy.

Deletion of open reading frames 9, 10 and 11 from the avian adenovirus CELO genome: effect on biodistribution and humoral responses

In this study, the in vivo effect of the 3·6 kbp deletion of the three open reading frames (ORF) 9, 10 and 11 found at the right end of the CELO genome was examined. Groups of chickens were inoculated oronasally with 105–107 p.f.u. per animal of wild-type virus and two recombinant CELO strains (rCELO) expressing luciferase and secreted alkaline phosphatase (SEAP). The tissue biodistribution, assessed by PCR, was similar for both wild-type and recombinant viruses. The infectious viral particle titre was determined by a p.f.u. counting method and the antibody responses to the CELO vector and the SEAP antigen were evaluated by ELISA. Infectious particle titres in tissues from chickens inoculated with the wild-type CELO virus increased up to 6 days post-inoculation, and declined until 11 days while titres in organs from chickens inoculated with the rCELO strain were low and only detectable at 4 days post-inoculation. Moreover, although anti-CELO antibody levels were three times lower in sera from chickens inoculated with rCELO, antibodies directed to the heterologous SEAP antigen were detected. Based on these results, no differences in tropism were observed, but the level of production of viral particles and the humoral responses appeared to decrease. Viruses replicate less efficiently with a deletion performed at the right end of the CELO genome. Nevertheless, the presence of antibodies directed to heterologous antigens makes the CELO virus an advantageous candidate for avian vaccination.

Comparative transduction efficiencies of human and nonhuman adenoviral vectors in human, murine, bovine, and porcine cells in culture

Clinical usefulness of human Ad serotype 5 (HAd5) based vectors is limited primarily because of preexisting Ad immunity and lack of targeting to specific cell types. Alternative vectors based on less prevalent HAd serotypes as well as nonhuman adenoviruses such as porcine Ad serotype 3 (PAd3) and bovine Ad serotype 3 (BAd3) are being developed to overcome these shortcomings. Using virus neutralization assay, we examined whether preexisting Ad immunity in humans would cross-neutralize PAd3 or BAd3. To further evaluate the potential of PAd3 and BAd3 vectors as gene delivery vehicles, we compared their transduction efficiencies in a panel of human, murine, bovine, and porcine cell lines to those obtained with a HAd5 vector. Transduction by the HAd5 vector in the majority of human cell lines correlated with the expression levels of coxsackievirus-adenovirus receptor (CAR), the primary HAd5 receptor; while transduction by PAd3 and BAd3 vectors was CAR-independent. The results suggest that PAd3 and BAd3 vectors are promising gene delivery vehicles for human gene therapy as well as for recombinant vaccines for human and animal use.

An adenovirus vector with a chimeric fiber derived from canine adenovirus type 2 displays novel tropism

Many clinically relevant tissues are refractory to Ad5 transduction because of negligible levels of the primary Ad5 receptor, the coxsackie and adenovirus receptor (CAR). Thus, development of Ad vectors that display CAR-independent tropism could lead directly to therapeutic gain. The Toronto strain of canine adenovirus type 2 (CAV2) exhibits native tropism that is augmented by, but not fully dependent upon, CAR for cellular transduction. We hypothesized that an Ad5 vector containing the nonhuman CAV2 knob would provide expanded tropism and constructed Ad5Luc1-CK, an E1-deleted Ad5 vector encoding the fiber knob domain from CAV2. Ad5Luc1-CK gene delivery to CAR-deficient cells was augmented up to 30-fold versus the Ad5 control vector, and correlated with increased cell surface binding. Further, we confirmed the importance of cellular integrins to Ad5Luc1-CK transduction. Herein, we present the rationale, design, purification, and characterization of a novel tropism modified, infectivity-enhanced Ad vector.

Role of Bovine Adenovirus-3 33K protein in viral replication

The L6 region of bovine adenovirus type (BAdV)-3 encodes a nonstructural protein named 33K. To identify and characterize the 33K protein, rabbit polyclonal antiserum was raised against a 33K-GST fusion protein expressed in bacteria. Anti-33K serum immunoprecipitated a protein of 42 kDa in in vitro translated and transcribed mRNA of 33K. However, three proteins of 42, 38, and 33 kDa were detected in BAdV-3 infected cells. To determine the role of this protein in virus replication, a recombinant BAV-33S1 containing insertional inactivation of 33K (a stop codon created at the seventh amino acid of 33K ORF) was constructed. Although BAV-33S1 could be isolated, the mutant showed a severe defect in the production of progeny virus. Inactivation of the 33K gene showed no effect on early and late viral gene expression in cells infected with BAV-33S1. However, formation of mature virions was significantly reduced in cells infected with BAV-33S1. Surprisingly, insertional inactivation of 33K at amino acid 97 (pFBAV-33.KS2) proved lethal for virus production. Although expression of early or late genes was not affected, no capsid formation could be observed in mutant DNA-transfected cells. These results suggest that 33K is required for capsid assembly and efficient DNA capsid interaction.

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A canine conditionally replicating adenovirus for evaluating oncolytic virotherapy in a syngeneic animal model

Oncolytic adenoviruses, which selectively replicate in and subsequently kill cancer cells, have emerged as a promising approach for treatment of tumors resistant to other modalities. Although preclinical results have been exciting, single-agent clinical efficacy has been less impressive heretofore. The immunogenicity of adenoviruses, and consequent premature abrogation of replication, may have been a partial reason. Improving the oncolytic potency of agents has been hampered by the inability to study host-vector interactions in immune-competent systems, since human serotype adenoviruses do not productively replicate in animal tissues. Therefore, approaches such as immunomodulation, which could result in sustained replication and subsequently increased oncolysis, have not been studied. Utilizing the osteocalcin promoter for restricting the replication of a canine adenovirus to dog osteosarcoma cells, we generated and tested the first nonhuman oncolytic adenovirus. This virus effectively killed canine osteosarcoma cells in vitro and yielded a therapeutic benefit in vivo. Canine osteosarcoma is the most frequent malignant disease in large dogs, with over 8000 cases in the United States annually, and there is no curative treatment. Therefore, immunomodulation for increased oncolytic potency could be studied with clinical trials in this population. This could eventually translate into human trials.

Biology of ovine adenovirus infection of nonpermissive cells

Nonhuman adenoviruses, including those of the genus Atadenovirus, have the potential to serve as vectors for vaccine and gene therapy applications in humans, since they are resistant to preexisting immunity induced by human adenoviruses in the majority of the population. In this study, we elucidate the outcome of infection by ovine adenovirus type 7 isolate 287 (OAdV) of several nonovine cell types. We show here that OAdV infects a wide range of nonovine cells but is unable to complete its replication cycle in any of them. In nonovine, nonfibroblast cells, viral replication is blocked at an early stage before the onset of, or early in, DNA replication. Some fibroblasts, on the other hand, allow viral DNA replication but block virus production at a later stage during or after the translation of late viral proteins. Late viral proteins are expressed in cells where viral DNA replication takes place, albeit at a reduced level. Significantly, late proteins are not properly processed, and their cellular distribution differs from that observed in infected ovine cells. Thus, our results clearly show that OAdV infection of all nonovine cells tested is abortive even if significant viral DNA replication occurs. These findings have significant positive implications with respect to the safety of the vector system and its future use in humans.

Development and characterization of bovine x human hybrid cell lines that efficiently support the replication of both wild-type bovine and human adenoviruses and those with E1 deleted

The 293 cell line that was generated by transforming human embryonic kidney cells with human adenovirus type 5 (HAV5) early region 1 (E1) sequences is an excellent host for generating and growing HAV5 recombinants with E1 deleted, but it does not support the replication of bovine adenovirus type 3 (BAV3). Madin-Darby bovine kidney (MDBK), an established bovine cell line, is an excellent host for growing and plaquing BAV3. For the purpose of combining the unique characteristics of these two cell lines (293 and MDBK), we generated a number of bovine x human hybrid (BHH) cell lines. Comparison of three BHH hybrid clones-BHH3, BHH8, and BHH2C-with 293-Puro (puromycin-resistant 293 cells) and MDBK-Neo (G418-resistant MDBK cells) cell lines for total cellular DNA content, species-specific surface markers, isoenzyme analysis, and karyotyping indicate that they are hybrid in nature. BHH clones constitutively expressed the E1 proteins (E1A, E1B-21kDa, and E1B-55kDa) of HAV5 and efficiently supported the replication of both wild-type and replication-incompetent bovine or human adenoviruses. Transient gene expression experiments with a plasmid encoding the bacterial beta-galactosidase gene demonstrated that BHH cell hybrids seem to have better transfection efficiencies than either of the parental cell lines. These cell lines will be useful for isolating and growing replication-competent human or bovine adenovirus recombinants with E1 deleted and for the study of cellular or viral factors important for viral replication. The development of somatic cell hybrids appears to be a simple way of combining some of the desirable characteristics present separately in two parental cell lines.

Diversity and complexity of CD8+ T cell responses against a single epitope of adenovirus E1B

This study describes the characteristics of the immune responses against adenovirus in C57BL/6 mice. CTL responses could be induced against E1Bp of adenovirus type 5, when whole viruses were immunized. A panel of E1Bp-specific CTL clones showed a wide range of T cell avidity. Recognition of the E1Bp peptide and a panel of variant peptides containing a single alanine substitution by CTL clones revealed that the fine specificity of the CTL response was quite diverse, rather than being limited to a certain clonal preference. Moreover, the variant peptides with a substitution at the TCR contact residue had antagonistic properties to some of the CTL clones, while being agonistic to others, reflecting the extensive diversity of the T cells. These results imply that the functional diversity of T cells to even a single epitope should be considered in manipulating immunity to viruses and in developing adoptive immunotherapy for immunocompromised individuals.

Induction of protective immunity to bovine herpesvirus type 1 in cattle by intranasal administration of replication-defective human adenovirus type 5 expressing glycoprotein gC or gD

Replication-defective human adenoviruses type 5 (HAd5) expressing the bovine herpesvirus type 1 (BHV-1) glycoprotein gC or gD under the control of the human cytomegalovirus immediate-early promoter/enhancer (AdCMVgC or AdCMVgD) or the 5' regulatory region of the human desmin gene (AdDESMgC or AdDESMgD) were generated. A preliminary experiment performed on rabbits showed that the intranasal administration of AdCMV elicited higher levels of BHV-1 neutralizing antibodies than the intramuscular administration of AdDESM. The obtained results allowed to select the replication-defective AdCMVgC and AdCMVgD for further assessment of their potential as a recombinant vaccine in cattle. Calves were injected intranasally twice 3 weeks apart with either AdCMVgC or AdCMVgD or a combination of these two recombinants or a commercially available live vaccine for comparison. The highest BHV-1 neutralizing antibody titres were obtained with AdCMVgD followed by the live vaccine and to a lower extent with the combination of the two recombinants (AdCMVgC+AdCMVgD). Calves were protected against intranasal BHV-1 challenge performed 3 weeks after the second immunization. In view of the obtained results, recombinant HAd5 may be developed as an intranasal vaccine vector in cattle administrated either alone or sequentially with non-human adenovirus-based vectors.

Characterization of a cDNA encoding the bovine coxsackie and adenovirus receptor

Non-human adenoviruses such as bovine adenovirus type 3 (BAV-3) that do not replicate in human cells but can infect human cells in culture could provide an attractive alternative to human adenoviral vectors for gene therapy. In addition, a large-animal model for genetic diseases can be very useful for the assessment of the efficacy of adenovector-mediated gene delivery in man. Recombinant human subgroup C adenovectors use the coxsackie and adenovirus receptor (CAR) to enter their target cells. Through RT-PCR and sequencing we determined the complete coding sequence of bovine CAR which serves as the primary adenoviral attachment site on bovine cells. A multiple sequence alignment, involving all the previously identified CAR species (man, mouse, rat, pig, and dog) showed that bovine CAR was most related to porcine CAR (92% nucleotide similarity) and demonstrated a highly conserved adenovirus binding Ig1 domain.

Bovine adenovirus type 3 E1B(small) protein is essential for growth in bovine fibroblast cells

In order to study the function of bovine adenovirus type 3 (BAV-3) E1A and E1B(small) proteins, we constructed two mutants: (a) BAV102A carries an in-frame deletion in the coding region for the E1A protein (nt 831-1080); (b) BAV102B carries an insertion of triple stop codons in the E1B region (nt 1654, 178 bp downstream of the E1B(small) start codon), which stops the translation of the E1B(small) gene. BAV102A virus could grow to the wild-type BAV-3 titer in transformed cell line VIDO R2 (HAV-5 E1 transformed) cells, but no progeny virus could be found in fetal bovine retina cells (FBRC). RT-PCR and Western blot analysis showed that neither mRNA transcripts nor protein expression of early genes [E1B(small) and DNA binding protein (DBP)] could be detected in BAV102A infected FBRC. The BAV102B grew 1.5 log less than wild-type BAV-3 in FBRC; however, no BAV102B progeny virus could be observed in bovine fibroblast (BFB) cells. No appreciable difference was observed in DBP transcript synthesis between wild-type BAV-3- or BAV102B-infected FBRC. However, compared to wild-type BAV-3, BAV102B viral DNA synthesis and fiber gene expression were found to be slightly reduced in FBRC. In contrast, compared to wild-type BAV-3, DBP transcripts and viral DNA synthesis were drastically reduced in BAV102B-infected BFB cells. In addition, no fiber gene expression could be detected in BAV102B-infected BFB cells. These results suggest that BAV-3 E1A is essential for virus replication and is required for activating the transcription of other BAV-3 early genes. However, the requirement for E1B(small) protein for BAV-3 replication appears to be cell type-dependent.

Construction of avian adenovirus CELO recombinants in cosmids

The avian adenovirus CELO is a promising vector for gene transfer applications. In order to study this potentiality, we developed an improved method for construction of adenovirus vectors in cosmids that was used to engineer the CELO genome. For all the recombinant viruses constructed by this method, the ability to produce infectious particles and the stability of the genome were evaluated in a chicken hepatocarcinoma cell line (LMH cell line). Our aim was to develop a replication-competent vector for vaccination of chickens, so we first generated knockout point mutations into 16 of the 22 unassigned CELO open reading frames (ORFs) to determine if they were essential for virus replication. As the 16 independent mutant viruses replicated in our cellular system, we constructed CELO genomes with various deletions in the regions of these nonessential ORFs. An expression cassette coding for the enhanced green fluorescent protein (eGFP) was inserted in place of these deletions to easily follow expression of the transgene and propagation of the vector in cell monolayers. Height-distinct GFP-expressing CELO vectors were produced that were all replication competent in our system. We then retained the vector backbone with the largest deletion (i.e., 3.6 kb) for the construction of vectors carrying cDNA encoding infectious bursal disease virus proteins. These CELO vectors could be useful for vaccination in the chicken species.

Circumvention of vector-specific neutralizing antibody response by alternating use of human and non-human adenoviruses: implications in gene therapy

To determine whether non-human adenovirus-specific antibodies are cross-neutralizing, rabbit and mouse anti-human adenovirus type 5 (HAd5), anti-bovine adenovirus type 3 (BAd3), and anti-porcine adenovirus type 3 (PAd3) sera were used in cross-virus neutralization assays. Adenovirus neutralizing antibodies were found to be virus-specific, suggesting that virus neutralizing epitope differs significantly in HAd5, BAd3, and PAd3. To further investigate whether immunity to an HAd5-derived vector could be circumvented by the use of non-human adenoviruses in vivo, mice were first immunized either intranasally or intraperitoneally with HAd5, BAd3, PAd3, or BAd3 + PAd3, and after development of adenovirus-specific antibodies, animals were inoculated with the HAd5 recombinant (AdCA36lacZ) containing the bacterial beta-galactosidase gene under the control of murine cytomegalovirus immediate-early promoter. Virus-inoculated animals developed virus-specific IgG and IgA antibodies. LacZ expression in animals initially primed with HAd5 was significantly reduced (P < 0.05), suggesting that the immune response against the vector could prevent the transgene expression following subsequent inoculation of the same vector, whereas LacZ expression in mice initially primed with BAd3, PAd3, or BAd3 + PAd3 was significantly higher (P > 0.05) than that obtained in HAd5-primed animals. Our results suggest that HAd5-, BAd3-, or PAd3-based vectors may be used sequentially for human gene therapy or vaccine production as a means to avoid immunity to the vector.