Rapid construction of adenoviral vectors by lambda phage genetics

Getting genetic access to natural adenovirus genomes to explore vector diversity

Recombinant vectors based on the human adenovirus type 5 (HAdV5) have been developed and extensively used in preclinical and clinical studies for over 30 years. However, certain restrictions of HAdV5-based vectors have limited their clinical applications because they are rather inefficient in specifically transducing cells of therapeutic interest that lack the coxsackievirus and adenovirus receptor (CAR). Moreover, enhanced vector-associated toxicity and widespread preexisting immunity have been shown to significantly hamper the effectiveness of HAdV-5-mediated gene transfer. However, evolution of adenoviruses in the natural host is driving the generation of novel types with altered virulence, enhanced transmission, and altered tissue tropism. As a consequence, an increasing number of alternative adenovirus types were identified, which may represent a valuable resource for the development of novel vector types. Thus, researchers are focusing on the other naturally occurring adenovirus types, which are structurally similar but functionally different from HAdV5. To this end, several strategies have been devised for getting genetic access to adenovirus genomes, resulting in a new panel of adenoviral vectors. Importantly, these vectors were shown to have a host range different from HAdV5 and to escape the anti-HAdV5 immune response, thus underlining the great potential of this approach. In summary, this review provides a state-of-the-art overview of one essential step in adenoviral vector development.

Fowl Adenovirus-Based Vaccine Platform

Nonpathogenic fowl adenoviruses (FAdVs) are amenable for engineering multivalent vaccine platforms due to large stretches of nonessential DNA sequences in their genomes. We describe the generation of FAdV-9-based vaccine platforms by targeted homologous recombination in an infectious clone (pPacFAdV-9 or wild type FAdmid) containing the entire viral genome in a cosmid vector. The viral DNA is subsequently released from the cosmid by restriction enzyme digestion followed by transfection in a chicken hepatoma cell line (CH-SAH). Virus is harvested, propagated, and verified for foreign gene expression.

A Dual-Modality Herpes Simplex Virus 2 Vaccine for Preventing Genital Herpes by Using Glycoprotein C and D Subunit Antigens To Induce Potent Antibody Responses and Adenovirus Vectors Containing Capsid and Tegument Proteins as T Cell Immunogens

We evaluated a genital herpes prophylactic vaccine containing herpes simplex virus 2 (HSV-2) glycoproteins C (gC2) and D (gD2) to stimulate humoral immunity and UL19 (capsid protein VP5) and UL47 (tegument protein VP13/14) as T cell immunogens. The HSV-2 gC2 and gD2 proteins were expressed in baculovirus, while the UL19 and UL47 genes were expressed from replication-defective adenovirus vectors. Adenovirus vectors containing UL19 and UL47 stimulated human and murine CD4(+) and CD8(+) T cell responses. Guinea pigs were either (i) mock immunized; (ii) immunized with gC2/gD2, with CpG and alum as adjuvants; (iii) immunized with the UL19/UL47 adenovirus vectors; or (iv) immunized with the combination of gC2/gD2-CpG/alum and the UL19/UL47 adenovirus vectors. Immunization with gC2/gD2 produced potent neutralizing antibodies, while UL19 and UL47 also stimulated antibody responses. After intravaginal HSV-2 challenge, the mock and UL19/UL47 adenovirus groups developed severe acute disease, while 2/8 animals in the gC2/gD2-only group and none in the combined group developed acute disease. No animals in the gC2/gD2 or combined group developed recurrent disease; however, 5/8 animals in each group had subclinical shedding of HSV-2 DNA, on 15/168 days for the gC2/gD2 group and 13/168 days for the combined group. Lumbosacral dorsal root ganglia were positive for HSV-2 DNA and latency-associated transcripts for 5/8 animals in the gC2/gD2 group and 2/8 animals in the combined group. None of the differences comparing the gC2/gD2-only group and the combined group were statistically significant. Therefore, adding the T cell immunogens UL19 and UL47 to the gC2/gD2 vaccine did not significantly reduce genital disease and vaginal HSV-2 DNA shedding compared with the excellent protection provided by gC2/gD2 in the guinea pig model.

IMPORTANCE

HSV-2 infection is a common cause of genital ulcer disease and a significant public health concern. Genital herpes increases the risk of transmission and acquisition of HIV-1 infection 3- to 4-fold. A herpes vaccine that prevents genital lesions and asymptomatic genital shedding will have a substantial impact on two epidemics, i.e., both the HSV-2 and HIV-1 epidemics. We previously reported that a vaccine containing HSV-2 glycoprotein C (gC2) and glycoprotein D (gD2) reduced genital lesions and asymptomatic HSV-2 genital shedding in guinea pigs, yet the protection was not complete. We evaluated whether adding the T cell immunogens UL19 (capsid protein VP5) and UL47 (tegument protein VP13/14) would enhance the protection provided by the gC2/gD2 vaccine, which produces potent antibody responses. Here we report the efficacy of a combination vaccine containing gC2/gD2 and UL19/UL47 for prevention of genital disease, vaginal shedding of HSV-2 DNA, and latent infection of dorsal root ganglia in guinea pigs.

Adenoviruses isolated from wild gorillas are closely related to human species C viruses

We have isolated and cultured three distinct adenoviruses from wild gorillas. Phylogenetic analysis grouped the viruses with human adenovirus species C based on DNA polymerase, hexon, and E4ORF6 genes. The three wild gorilla adenoviruses clustered with the other species C captive gorilla adenoviruses, forming a branch separate from human and chimpanzee/bonobo adenoviruses. Animal sera to the three newly isolated viruses did not cross-neutralize, demonstrating serological distinctiveness. The human adenovirus 5 fiber knob blocked infection, suggesting use of the Coxsackie and Adenovirus Receptor. These viruses may provide viral vectors with properties distinct from chimpanzee adenovirus and human adenovirus vectors.

Characterization of human adenovirus 35 and derivation of complex vectors

Background

Replication-deficient recombinant adenoviral vectors based on human serotype 35 (Ad35) are desirable due to the relatively low prevalence of neutralizing antibodies in the human population. The structure of the viral genome and life cycle of Ad35 differs from the better characterized Ad5 and these differences require differences in the strategies for the generation of vectors for gene delivery.

Results

Sequences essential for E1 and E4 function were identified and removed and the effects of the deletions on viral gene transcription were determined. In addition, the non-essential E3 region was deleted from rAd35 vectors and a sequence was found that did not have an effect on viability but reduced viral fitness. The packaging capacity of rAd35 was dependent on pIX and vectors were generated with stable genome sizes of up to 104% of the wild type genome size. These data were used to make an E1-, E3-, E4-deleted rAd35 vector. This rAd35 vector with multiple gene deletions has the advantages of multiple blocks to viral replication (i.e., E1 and E4 deletions) and a transgene packaging capacity of 7.6 Kb, comparable to rAd5 vectors.

Conclusions

The results reported here allow the generation of larger capacity rAd35 vectors and will guide the derivation of adenovirus vectors from other serotypes.

Potent immune responses and in vitro pro-inflammatory cytokine suppression by a novel adenovirus vaccine vector based on rare human serotype 28

Adenovirus vaccine vectors derived from rare human serotypes have been shown to be less potent than serotype 5 (Ad5) at inducing immune responses to encoded antigens. To identify highly immunogenic adenovirus vectors, we assessed pro-inflammatory cytokine expression, binding to the CD46 receptor, and immunogenicity. Species D adenoviruses uniquely suppressed pro-inflammatory cytokines and induced high levels of type I interferon. Thus, it was unexpected that a vector derived from a representative serotype, Ad28, induced significantly higher transgene-specific T cell responses than an Ad35 vector. Prime-boost regimens with Ad28, Ad35, Ad14, or Ad5 significantly boosted T cell and antibody responses. The seroprevalence of Ad28 was confirmed to be <10% in the United States. Together, this shows that a rare human serotype-based vector can elicit strong immune responses, which was not predicted by in vitro results.

Gene transfer into neural cells in vitro using adenoviral vectors

Adenoviral vectors are excellent vehicles to transfer genes into the nervous system due to their ability to transduce dividing and nondividing cells, their ability to be grown to very high titers, and their relatively large insert capacity. Also, adenoviral vectors can sustain very long-term transgene expression in the CNS of rodents and in neurons and glial cells in culture. Successful gene transfer into the nervous system is dependent on the development, production, and quality control of vector preparations, which need to be of the highest quality. This unit provides protocols to clone, rescue, amplify, and purify first-generation adenoviral vectors. Detailed quality control assays are provided to ensure that vector preparations are devoid of contamination from replication-competent adenovirus and lipopolysaccharides. Also included are methodologies related to adenoviral-mediated gene transfer into neurons and glial cells in culture, and the analysis of transgene expression using immunocytochemistry, enzymatic assays, and fluorescence-activated cell sorting (FACS) analysis.

Rescue and production of vaccine and therapeutic adenovirus vectors expressing inhibitory transgenes

Expression of certain transgenes from an adenovirus vector can be deleterious to its own replication. This can result in the inhibition of virus rescue, reduced viral yields, or, in the worst case, make it impossible to construct a vector expressing the inhibiting transgene product. A gene regulation system based on the tet operon was used to allow the rescue and efficient growth of adenovectors that express transgenes to high levels. A key advantage to this system is that repression of transgene expression is mediated by the packaging cell line, thus, expression of regulatory products from the adenovector are not required. This provides a simple, broadly applicable system wherein transgene repression is constitutive during vector rescue and growth and there is no effect on adenovector-mediated expression of gene products in transduced cells. Several high-level expression vectors based on first- and second-generation adenovectors were rescued and produced to high titer that otherwise could not be grown. Yields of adenovectors expressing inhibitory transgene products were increased, and the overgrowth of cultures by adenovectors with nonfunctional expression cassettes was prevented. The gene regulation system is a significant advancement for the development of adenovirus vectors for vaccine and other gene transfer applications.

Local gene delivery of tumor necrosis factor alpha can impact primary tumor growth and metastases through a host-mediated response

TNFerade is a replication incompetent adenovector designed to express human TNFalpha under control of the Egr-1 radiation and chemotherapy enhanced promoter, and is currently in Phase II/III clinical testing. Data from Phase I clinical testing of TNFerade in a limited set of melanoma patients suggested the potential to impact distal metastases following intratumoral injections of TNFerade. These clinical observations and the multiple potential mechanisms of TNFerade led us to hypothesize local treatment with TNFerade + radiation may impact metastatic disease. We explored this hypothesis in preclinical models using the spontaneously metastatic, syngeneic B16F10 murine melanoma model. Established subcutaneous B16F10 tumors were treated with intratumoral injections of TNFerade and localized 2 Gy fractionated radiation therapy, modeling the clinical treatment regimen. Following 10-14 days of treatment, mice were evaluated for metastases development in the iliac and axillary lymph nodes. Comparisons of metastatic burden to control groups indicated TNFerade +/- radiation suppressed the formation of metastases in the lymph nodes. Additional experiments in TNF receptor knockout mice, where the only possible effects are on tumor cells containing the TNFalpha receptor, indicate TNFerade's local and distal activities are critically dependent on a host-mediated response. These data provide direct preclinical evidence local therapy of a solid tumor with TNFerade can also reduce metastatic disease, in addition to effects on the treated lesion. Furthermore, our finding of a host dependant response(s) for TNFerade at both the treated tumor and on lymph node metastases suggest the potential for broad activity independent of tumor histology.

Novel adenovirus vaccine vectors based on the enteric-tropic serotype 41

Replication-defective adenovirus vectors, primarily developed from serotype 5 (Ad5) viruses, have been widely used for gene transfer and vaccination approaches. Vectors based on other serotypes of adenovirus could be used in conjunction with, or in place of, Ad5 vectors. In this study, Ad41, an enteric adenovirus usually described as 'non-cultivable' or 'fastidious,' has been successfully cloned, rescued and propagated on 293-ORF6 cells. The complementation capabilities of this cell line allow generation of Ad41 vectors at titers comparable to those obtained for Ad5 vectors. Mice immunized with an Ad41 vector containing an HIV envelope (Env) gene mounted anti-Env cellular and humoral immune responses. Ad41-Env vectors appear to be particularly attractive when used in heterologous prime-boost regimens, where they induce significantly higher cellular immune responses to HIV-Env than Ad5-based regimens. Ad41-based constructs are attractive vaccine vectors alone or in combination with Ad5 adenovectors, since each vector type can provide circumvention of pre-existing immunity to the other.

Transposon-assisted cloning and traceless mutagenesis of adenoviruses: Development of a novel vector based on species D

Until recently, adenovirus (Ad)-mediated gene therapy was almost exclusively based on human Ad type 5 (Ad5). Preexisting immunity and the limited, coxsackievirus and adenovirus receptor-dependent tropism of Ad5 stimulated attempts to exploit the natural diversity in tropism of the other 50 known human Ad serotypes. Aiming in particular at immunotherapy and vaccination, we have screened representative serotypes from different Ad species for their ability to infect dendritic cells. Ad19a, an Ad from species D, was selected for development as a new vector for vaccination and cancer gene therapy. To clone and manipulate its genome, we have developed a novel methodology, coined "exposon mutagenesis," that allows the rapid and precise introduction of virtually any genetic alteration (deletions, point mutations, or insertions) into recombinant Ad bacterial artificial chromosomes. The versatility of the system was exemplified by deleting the E3 region of Ad19a, by specifically knocking out expression of a species-specific E3 gene, E3/49K, and by reinserting E3/49K into an E3 null Ad19a mutant. The technology requires only limited sequence information and is applicable to other Ad species. Therefore, it should be extremely valuable for the analysis of gene functions from any Ad species. In addition, a basic, replication-defective E1- and E3-deleted Ad19a vector expressing GFP (Ad19aGFP) was generated. This new vector based on species D Ads exhibits a very promising tropism for lymphoid and muscle cells and shows great potential as an alternative vector for transduction of cell types that are resistant to or only poorly transduced by conventional Ad5-based vectors.

Adenovirus IVa2 protein plays an important role in transcription from the major late promoter in vivo

Adenovirus IVa2 protein is essential and multifunctional, with roles in encapsidation and transcriptional activation of the Major Late Promoter (MLP), but the importance of the transcriptional function to viability has not been assessed. To address this question, viral genomes with multiple nonbinding mutations in the MLP downstream elements DE1 and DE2, alone or in combination with nonbinding mutations in the UPE (USF0), were constructed. The results show that DE1/2 and the UPE are functionally redundant, suggesting an important role of IVa2 protein in the activation of the MLP in vivo. Previously, a virus (vIVa2) expressing a 40-kDa IVa2 isoform was created. Neither the DE1/2 mutations nor the USF0 mutations could be recovered in this genetic background. These results suggest that this 40-kDa isoform can play a role in transcription.

A 40 kDa isoform of the type 5 adenovirus IVa2 protein is sufficient for virus viability

The multifunctional IVa2 protein is essential for adenovirus replication [J. Virol. 77 (2003) 3586], but the relative importance of the transcriptional and encapsidation functions is unknown. As part of a study of IVa2 function, we created a set of mutations in the IVa2 gene in the correct location in the viral genome. Unexpectedly, an opal stop codon at position 6 was recovered in virus twice. Isolate #2 showed defective viral replication, but produced late proteins at almost wild-type levels. Analysis of IVa2 mRNA showed an additional species, larger and more abundant than the equivalent wild-type species. It was a hybrid of the 5' UTR of L3 23 kDa attached to the IVa2 second exon, so that M75 is the 5' proximal methionine. This mRNA arises from a corresponding hybrid DNA, present in the virus stock. A protein of approximately 40 kDa, consistent with translation from the hybrid mRNA, was detected. It is able to bind to the packaging sequence and to the MLP downstream elements (DE1/2). Isolate #8 was more defective in replication than #2. No hybrid mRNA or DNA was detected, but it also produces a 40 kDa isoform, which is present in wild-type-infected cells. Mutational analysis of M75 and M101 revealed that the 40 kDa isoform is produced by initiation at Met75. This might be the origin of the previously unidentified 40 kDa factor present in the heterodimer DEF-A, which binds to DE1 and DE2a.

Adenovirus vector library: an approach to the discovery of gene and protein function

A method was developed to generate a complex cDNA expression library within an adenovirus type 5 (Ad5)-based vector backbone, termed AdLibrary. Construction of the AdLibrary entailed the conversion of an Ad5 genome-containing cosmid to infectious virus particles. The Ad5 genome was modified by replacing the E1A and E1B genes with a Rous sarcoma virus-driven expression cassette. Conversion was accomplished by liberating the viral genome by restriction enzyme digestion and transfection in HEK 293 cells, which support the growth of E1A/E1B-deficient virus. A test AdLibrary demonstrated the possibility of converting and identifying a marker gene present at a frequency of 1/105 in the cosmid library. To demonstrate the utility of this technology, an AdLibrary was used to isolate a viral gene by its biological function. Virus growth was selected for with an AdLibrary on A549 cells, which do not complement for E1A/E1B function. The AdLibrary was generated with cDNAs derived from HeLa cells productively infected with Ad5. A cDNA corresponding to Ad5 E1A 13S was selected and isolated from the AdLibrary using this strategy. Since multiple genes are assayed simultaneously, this technology should expedite the discovery of genes affecting defined biological activities. This AdLibrary approach provides an opportunity to exploit the efficient gene delivery capabilities of adenovirus vectors for the rapid discovery of gene and protein function.

Development of an Ad7 cosmid system and generation of an Ad7deltaE1deltaE3HIV(MN) env/rev recombinant virus

A strategy to circumvent immune responses to adenovirus (Ad) resulting from natural infection or repeated vector administrations involves sequential use of vectors from different Ad serotypes. To further develop an Ad-HIV recombinant AIDS vaccine approach, a replication-defective recombinant Ad from a non-subgroup C virus was required. Using a cosmid system, we generated an Ad7deltaE1deltaE3HIV(MN) env/rev recombinant virus and compared expression of the inserted HIV genes with a similarly constructed replication-competent Ad7deltaE3HIV(MN)env/rev recombinant. Ad7deltaE1deltaE3HIV(MN)env/rev expressed both HIV env and rev gene products. The envelope protein was correctly processed and functional, mediating syncytia formation of Ad7deltaE1deltaE3HIV(MN) env/rev-infected cells and CD4(+) T lymphocytes. Ad7deltaE1deltaE3HIV(MN)env/rev could be amplified on 293-ORF6 cells, containing the E4 ORF6 gene, shown earlier to support production of an Ad7 vector lacking the E1a gene. The utility of this cell line is now extended to the production of replication-defective Ad7 recombinants lacking E1a, E1b, and protein IX genes. Sequential immunizations with Ad-HIV recombinants based in different Ad serotypes have been shown to effectively elicit both humoral and cellular HIV-specific immune responses. The recombinant Ad7deltaE1deltaE3HIV(MN)env/rev will be useful in such AIDS vaccine strategies. Further, these studies have created new cosmid vectors that can be applied to generation of single- or double-deleted Ad7 recombinants with foreign genes inserted into the E1 and/or E3 regions.