Adenovirus type 5 with modified hexons induces robust transgene-specific immune responses in mice with pre-existing immunity against adenovirus type 5

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.

SARS-CoV-2 (Covid-19) vaccines structure, mechanisms and effectiveness: A review

The world has been suffering from COVID-19 disease for more than a year, and it still has a high mortality rate. In addition to the need to minimize transmission of the virus through non-pharmacological measures such as the use of masks and social distance, many efforts are being made to develop a variety of vaccines to prevent the disease worldwide. So far, several vaccines have reached the final stages of safety and efficacy in various phases of clinical trials, and some, such as Moderna/NIAID and BioNTech/Pfizer, have reported very high safety and protection. The important point is that comparing different vaccines is not easy because there is no set standard for measuring neutralization. In this study, we have reviewed the common platforms of COVID-19 vaccines and tried to present the latest reports on the effectiveness of these vaccines.

Adenoviral vector vaccine platforms in the SARS-CoV-2 pandemic

Adenoviral vectors have been explored as vaccine agents for a range of infectious diseases, and their ability to induce a potent and balanced immune response made them logical candidates to apply to the COVID-19 pandemic. The unique molecular characteristics of these vectors enabled the rapid development of vaccines with advanced designs capable of overcoming the biological challenges faced by early adenoviral vector systems. These successes and the urgency of the COVID-19 situation have resulted in a flurry of candidate adenoviral vector vaccines for COVID-19 from both academia and industry. These vaccines represent some of the lead candidates currently supported by Operation Warp Speed and other government agencies for rapid translational development. This review details adenoviral vector COVID-19 vaccines currently in human clinical trials and provides an overview of the new technologies employed in their design. As these vaccines have formed a cornerstone of the COVID-19 global vaccination campaign, this review provides a full consideration of the impact and development of this emerging platform.

New vaccine production platforms used in developing SARS-CoV-2 vaccine candidates

The threat of the current coronavirus disease pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is accelerating the development of potential vaccines. Candidate vaccines have been generated using existing technologies that have been applied for developing vaccines against other infectious diseases. Two new types of platforms, mRNA- and viral vector-based vaccines, have been gaining attention owing to the rapid advancement in their methodologies. In clinical trials, setting appropriate immunological endpoints plays a key role in evaluating the efficacy and safety of candidate vaccines. Updated information about immunological features from individuals who have or have not been exposed to SARS-CoV-2 continues to guide effective vaccine development strategies. This review highlights key strategies for generating candidate SARS-CoV-2 vaccines and considerations for vaccine development and clinical trials.

Molecular retargeting of antibodies converts immune defense against oncolytic viruses into cancer immunotherapy

Virus-neutralizing antibodies are a severe obstacle in oncolytic virotherapy. Here, we present a strategy to convert this unfavorable immune response into an anticancer immunotherapy via molecular retargeting. Application of a bifunctional adapter harboring a tumor-specific ligand and the adenovirus hexon domain DE1 for engaging antiadenoviral antibodies, attenuates tumor growth and prolongs survival in adenovirus-immunized mice. The therapeutic benefit achieved by tumor retargeting of antiviral antibodies is largely due to NK cell-mediated triggering of tumor-directed CD8 T-cells. We further demonstrate that antibody-retargeting (Ab-retargeting) is a feasible method to sensitize tumors to PD-1 immune checkpoint blockade. In therapeutic settings, Ab-retargeting greatly improves the outcome of intratumor application of an oncolytic adenovirus and facilitates long-term survival in treated animals when combined with PD-1 checkpoint inhibition. Tumor-directed retargeting of preexisting or virotherapy-induced antiviral antibodies therefore represents a promising strategy to fully exploit the immunotherapeutic potential of oncolytic virotherapy and checkpoint inhibition.

Microbubble-mediated delivery of human adenoviruses does not elicit innate and adaptive immunity response in an immunocompetent mouse model of prostate cancer

Background

Gene transfer to malignant sites using human adenoviruses (hAds) has been limited because of their immunogenic nature and host specificity. Murine cells often lack some of the receptors needed for hAds attachment, thus murine cells are generally non-permissive for human adenoviral infection and replication, which limits translational studies.

Methods

We have developed a gene transfer method that uses a combination of lipid-encapsulated perfluorocarbon microbubbles and ultrasound to protect and deliver hAds to a target tissue, bypassing the requirement of specific receptors.

Results

In an in vitro model, we showed that murine TRAMP-C2 and human DU145 prostate cancer cells display a comparable expression pattern of receptors involved in hAds adhesion and internalization. We also demonstrated that murine and human cells showed a dose-dependent increase in the percentage of cells transduced by hAd-GFP (green fluorescent protein) after 24 h and that GFP transgene was efficiently expressed at 48 and 72 h post-transduction. To assess if our image-guided delivery system could effectively protect the hAds from the immune system in vivo, we injected healthy immunocompetent mice (C57BL/6) or mice bearing a syngeneic prostate tumor (TRAMP-C2) with hAd-GFP/MB complexes. Notably, we did not observe activation of innate (TNF-α and IL-6 cytokines), or adaptive immune response (neutralizing antibodies, INF-γ+ CD8⁺ T cells).

Conclusions

This study brings us a step closer to demonstrating the feasibility of murine cancer models to investigate the clinical translation of image guided site-specific adenoviral gene therapy mediated by ultrasound-targeted microbubble destruction.

Electronic supplementary material

The online version of this article (10.1186/s12967-019-1771-0) contains supplementary material, which is available to authorized users.

Capsid Engineering of Adenovirus Vectors: Overcoming Early Vector-Host Interactions for Therapy

Adenovirus-based vectors comprise the most frequently used vector type in clinical studies to date. Both intense lab research and insights from the clinical trials reveal the importance of a comprehensive understanding of vector-host interactions. Especially for systemic intravenous adenovirus vector delivery, it is paramount to develop safe and efficacious vectors. Very early vector-host interactions that take place in blood long before the first cell is being transduced are phenomena triggered by the surface, shape, and size of the adenovirus vector particles. Not surprisingly, a multitude of different technologies ranging from genetics to chemistry has been developed to alter the adenovirus vector surface. In this review, we discuss the most important technologies and evaluate them for their suitability to overcome hurdles imposed by early vector-host interactions.

Progress in Adenoviral Capsid-Display Vaccines

Adenoviral vectored vaccines against infectious diseases are currently in clinical trials due to their capacity to induce potent antigen-specific B- and T-cell immune responses. Heterologous prime-boost vaccination with adenoviral vector and, for example, adjuvanted protein-based vaccines can further enhance antigen-specific immune responses. Although leading to potent immune responses, these heterologous prime-boost regimens may be complex and impact manufacturing costs limiting efficient implementation. Typically, adenoviral vectors are engineered to genetically encode a transgene in the E1 region and utilize the host cell machinery to express the encoded antigen and thereby induce immune responses. Similarly, adenoviral vectors can be engineered to display foreign immunogenic peptides on the capsid-surface by insertion of antigens in capsid proteins hexon, fiber and protein IX. The ability to use adenoviral vectors as antigen-display particles, with or without using the genetic vaccine function, greatly increases the versatility of the adenoviral vector for vaccine development. This review describes the application of adenoviral capsid antigen-display vaccine vectors by focusing on their distinct advantages and possible limitations in vaccine development.

Hazard Characterization of Modified Vaccinia Virus Ankara Vector: What Are the Knowledge Gaps?

Modified vaccinia virus Ankara (MVA) is the vector of choice for human and veterinary applications due to its strong safety profile and immunogenicity in vivo. The use of MVA and MVA-vectored vaccines against human and animal diseases must comply with regulatory requirements as they pertain to environmental risk assessment, particularly the characterization of potential adverse effects to humans, animals and the environment. MVA and recombinant MVA are widely believed to pose low or negligible risk to ecosystem health. However, key aspects of MVA biology require further research in order to provide data needed to evaluate the potential risks that may occur due to the use of MVA and MVA-vectored vaccines. The purpose of this paper is to identify knowledge gaps in the biology of MVA and recombinant MVA that are of relevance to its hazard characterization and discuss ongoing and future experiments aimed at providing data necessary to fill in the knowledge gaps. In addition, we presented arguments for the inclusion of uncertainty analysis and experimental investigation of verifiable worst-case scenarios in the environmental risk assessment of MVA and recombinant MVA. These will contribute to improved risk assessment of MVA and recombinant MVA vaccines.

Human Adenovirus Serotype 3 Vector Packaged by a Rare Serotype 14 Hexon

Recombinant adenovirus serotype 3 (rAd3), which infects cells through the receptor desmoglein 2 (DSG2), has been investigated as a vector for gene therapy or vaccination. However, pre-existing anti-vector immunity may limit the practical application of rAd3. In this study, we investigated the seroprevalence and neutralizing antibody (NAb) titers to Ad3 and alternate serotypes in normal healthy adults in southern China. Sera samples had a high seroprevalence (80.00%) against Ad3 and Ad7 (85.83%), compared with Ad14 (22.50%). Furthermore, 19.17% and 25.83% of samples had high-titer neutralizing antibodies to Ad3 and Ad7, respectively, compared with 3.33% against Ad14. We constructed a chimeric adenovirus, rAd3H14, designed to evade anti-vector immunity by replacing the enhanced green fluorescent protein (EGFP)-expressing hexon of the rAd3EGFP vector with a hexon from Ad14. The chimeric vector rAd3H14 was not neutralized in vitro efficiently by Ad3 NAbs using sera from mice and normal healthy human volunteers. Furthermore, in contrast to the unmodified vector rAd3EGFP, rAd3H14 induced robust antibody responses against EGFP in mice with high levels of pre-existing anti-Ad3 immunity. In conclusion, the chimeric vector rAd3H14 may be a useful alternative vector in adult populations with a high prevalence of Ad3 NAbs.

A Plasmodium Promiscuous T Cell Epitope Delivered within the Ad5 Hexon Protein Enhances the Protective Efficacy of a Protein Based Malaria Vaccine

A malaria vaccine is a public health priority. In order to produce an effective vaccine, a multistage approach targeting both the blood and the liver stage infection is desirable. The vaccine candidates also need to induce balanced immune responses including antibodies, CD4+ and CD8+ T cells. Protein-based subunit vaccines like RTS,S are able to induce strong antibody response but poor cellular reactivity. Adenoviral vectors have been effective inducing protective CD8+ T cell responses in several models including malaria; nonetheless this vaccine platform exhibits a limited induction of humoral immune responses. Two approaches have been used to improve the humoral immunogenicity of recombinant adenovirus vectors, the use of heterologous prime-boost regimens with recombinant proteins or the genetic modification of the hypervariable regions (HVR) of the capsid protein hexon to express B cell epitopes of interest. In this study, we describe the development of capsid modified Ad5 vectors that express a promiscuous Plasmodium yoelii T helper epitope denominated PyT53 within the hexon HVR2 region. Several regimens were tested in mice to determine the relevance of the hexon modification in enhancing protective immune responses induced by the previously described protein-based multi-stage experimental vaccine PyCMP. A heterologous prime-boost immunization regime that combines a hexon modified vector with transgenic expression of PyCMP followed by protein immunizations resulted in the induction of robust antibody and cellular immune responses in comparison to a similar regimen that includes a vector with unmodified hexon. These differences in immunogenicity translated into a better protective efficacy against both the hepatic and red blood cell stages of P. yoelii. To our knowledge, this is the first time that a hexon modification is used to deliver a promiscuous T cell epitope. Our data support the use of such modification to enhance the immunogenicity and protective efficacy of adenoviral based malaria vaccines.

Immunogenicity and efficacy in mice of an adenovirus-based bicistronic rotavirus vaccine expressing NSP4 and VP7

NSP4 and VP7 are important functional proteins of rotavirus. Proper combination of viral gene expression is favorable to improving the protection effect of subunit vaccine. In the present study, We evaluated the immunogenicity and efficacy of the bicistronic recombinant adenovirus (rAd-NSP4-VP7) and two single-gene expressing adenoviruses (rAd-NSP4, rAd-VP7). The three adenovirus vaccines were used to immunize mice by intramuscular or intranasal administration. The data showed significant increases in serum antibodies, T lymphocyte subpopulations proliferation, and cytokine secretions of splenocyte in all immunized groups. However, the serum IgA and neutralizing antibody levels of the rAd-NSP4-VP7 or rAd-VP7 groups were significantly higher than those of the rAd-NSP4, while the splenocyte numbers of IFN-γ secretion in the rAd-NSP4-VP7 or rAd-NSP4 groups was greater than that of the rAd-VP7. Furthermore, the efficacy evaluation in a suckling mice model indicated that only rAd-NSP4-VP7 conferred significant protection against rotavirus shedding challenge. These results suggest that the co-expression of NSP4 and VP7 in an adenovirus vector induce both humoral and cell-mediated immune responses efficiently, and provide potential efficacy for protection against rotavirus disease. It is possible to represent an efficacious subunits vaccine strategy for control of rotavirus infection and transmission.

The influence of delivery vectors on HIV vaccine efficacy

Development of an effective HIV/AIDS vaccine remains a big challenge, largely due to the enormous HIV diversity which propels immune escape. Thus novel vaccine strategies are targeting multiple variants of conserved antibody and T cell epitopic regions which would incur a huge fitness cost to the virus in the event of mutational escape. Besides immunogen design, the delivery modality is critical for vaccine potency and efficacy, and should be carefully selected in order to not only maximize transgene expression, but to also enhance the immuno-stimulatory potential to activate innate and adaptive immune systems. To date, five HIV vaccine candidates have been evaluated for efficacy and protection from acquisition was only achieved in a small proportion of vaccinees in the RV144 study which used a canarypox vector for delivery. Conversely, in the STEP study (HVTN 502) where human adenovirus serotype 5 (Ad5) was used, strong immune responses were induced but vaccination was more associated with increased risk of HIV acquisition than protection in vaccinees with pre-existing Ad5 immunity. The possibility that pre-existing immunity to a highly promising delivery vector may alter the natural course of HIV to increase acquisition risk is quite worrisome and a huge setback for HIV vaccine development. Thus, HIV vaccine development efforts are now geared toward delivery platforms which attain superior immunogenicity while concurrently limiting potential catastrophic effects likely to arise from pre-existing immunity or vector-related immuno-modulation. However, it still remains unclear whether it is poor immunogenicity of HIV antigens or substandard immunological potency of the safer delivery vectors that has limited the success of HIV vaccines. This article discusses some of the promising delivery vectors to be harnessed for improved HIV vaccine efficacy.

Developments in Viral Vector-Based Vaccines

Viral vectors are promising tools for gene therapy and vaccines. Viral vector-based vaccines can enhance immunogenicity without an adjuvant and induce a robust cytotoxic T lymphocyte (CTL) response to eliminate virus-infected cells. During the last several decades, many types of viruses have been developed as vaccine vectors. Each has unique features and parental virus-related risks. In addition, genetically altered vectors have been developed to improve efficacy and safety, reduce administration dose, and enable large-scale manufacturing. To date, both successful and unsuccessful results have been reported in clinical trials. These trials provide important information on factors such as toxicity, administration dose tolerated, and optimized vaccination strategy. This review highlights major viral vectors that are the best candidates for clinical use.

Hexon hypervariable region-modified adenovirus type 5 (Ad5) vectors display reduced hepatotoxicity but induce T lymphocyte phenotypes similar to Ad5 vectors

Hexon modification of adenovirus type 5 (Ad5) vectors with the hypervariable regions (HVRs) of Ad48 has been shown to allow Ad5HVR48 vectors to circumvent the majority of the preexisting Ad5-neutralizing antibodies. However, it remains unclear whether modifying hexon HVRs impacts innate or adaptive immune responses elicited by this vector. In this study, we investigated the influence of the HVR substitution of Ad5 on innate and adaptive immune responses following vaccination. Ad5HVR48 displayed an intermediate level of innate immune cytokines and chemokines relative to those of Ad5 and Ad48, consistent with its chimeric nature. Hepatotoxicity was observed after Ad5 immunization but not after Ad5HVR48 or Ad48 immunization. However, the CD8(+) T-cell responses elicited by Ad5HVR48 vectors displayed a partially exhausted phenotype, as evidenced by the sustained expression of programmed death 1 (PD-1), decreased effector-to-central memory conversion, and reduced memory recall responses, similar to those elicited by Ad5 vectors and in contrast to those induced by Ad48 vectors. Taken together, these results indicate that although Ad5HVR48 largely bypasses preexisting Ad5 neutralizing antibodies and shows reduced hepatotoxicity compared to that of Ad5, it induces adaptive immune phenotypes that are functionally exhausted similar to those elicited by Ad5.

A recombinant adenovirus-based vector elicits a specific humoral immune response against the V3 loop of HIV-1 gp120 in mice through the "Antigen Capsid-Incorporation" strategy

Background

Due to potential advantages, human adenoviral vectors have been evaluated pre-clinically as recombinant vaccine vectors against several cancers and infectious diseases, including human immunodeficiency virus (HIV) infection. The V3 loop of HIV-1 glycoprotein 120 (gp120) contains important neutralizing epitopes and plays key roles in HIV entry and infectivity.

Methods

In order to investigate the humoral immune response development against portions of the V3 loop, we sought to generate four versions of adenovirus (Ad)-based V3 vectors by incorporating four different antigen inserts into the hypervariable region 1 (HVR1) of human adenovirus type 5 (hAd5) hexon. The strategy whereby antigens are incorporated within the adenovirus capsid is known as the “Antigen Capsid-Incorporation” strategy.

Results

Of the four recombinant vectors, Ad-HVR1-lgs-His₆-V3 and Ad-HVR1-long-V3 had the capability to present heterologous antigens on capsid surface, while maintaining low viral particle to infectious particle (VP/IP) ratios. The VP/IP ratios indicated both high viability and stability of these two vectors, as well as the possibility that V3 epitopes on these two vectors could be presented to immune system. Furthermore, both Ad-HVR1-lgs-His₆-V3 and Ad-HVR1-long-V3 could, to some extent escape the neutralization by anti-adenovirus polyclonal antibody (PAb), but rather not the immunity by anti-gp120 (902) monoclonal antibody (MAb). The neutralization assay together with the whole virus enzyme-linked immunosorbent assay (ELISA) suggested that these two vectors could present V3 epitopes similar to the natural V3 presence in native HIV virions. However, subsequent mice immunizations clearly showed that only Ad-HVR1-lgs-His₆-V3 elicited strong humoral immune response against V3. Isotype ELISAs identified IgG2a and IgG2b as the dominant IgG isotypes, while IgG1 comprised the minority.

Conclusions

Our findings demonstrated that human adenovirus (hAd) vectors which present HIV antigen via the “Antigen Capsid-Incorporation” strategy could successfully elicit antigen-specific humoral immune responses, which could potentially open an avenue for the development of Ad-based HIV V3 vaccines.

Adenovirus-based vaccines for fighting infectious diseases and cancer: progress in the field

The field of adenovirology is undergoing rapid change in response to increasing appreciation of the potential advantages of adenoviruses as the basis for new vaccines and as vectors for gene and cancer therapy. Substantial knowledge and understanding of adenoviruses at a molecular level has made their manipulation for use as vaccines and therapeutics relatively straightforward in comparison with other viral vectors. In this review we summarize the structure and life cycle of the adenovirus and focus on the use of adenovirus-based vectors in vaccines against infectious diseases and cancers. Strategies to overcome the problem of preexisting antiadenovirus immunity, which can hamper the immunogenicity of adenovirus-based vaccines, are discussed. When armed with tumor-associated antigens, replication-deficient and oncolytic adenoviruses can efficiently activate an antitumor immune response. We present concepts on how to use adenoviruses as therapeutic cancer vaccines and consider some of the strategies used to further improve antitumor immune responses. Studies that explore the prospect of adenoviruses as vaccines against infectious diseases and cancer are underway, and here we give an overview of the latest developments.

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.

Adenovirus-Based Vectors for the Development of Prophylactic and Therapeutic Vaccines

Emerging and reemerging infectious diseases as well as cancer pose great global health impacts on the society. Vaccines have emerged as effective treatments to prevent or reduce the burdens of already developed diseases. This is achieved by means of activating various components of the immune system to generate systemic inflammatory reactions targeting infectious agents or diseased cells for control/elimination. DNA virus-based genetic vaccines gained significant attention in the past decades owing to the development of DNA manipulation technologies, which allowed engineering of recombinant viral vectors encoding sequences for foreign antigens or their immunogenic epitopes as well as various immunomodulatory molecules. Despite tremendous progress in the past 50 years, many hurdles still remain for achieving the full clinical potential of viral-vectored vaccines. This chapter will present the evolution of vaccines from “live” or “attenuated” first-generation agents to recombinant DNA and viral-vectored vaccines. Particular emphasis will be given to human adenovirus (Ad) for the development of prophylactic and therapeutic vaccines. Ad biological properties related to vaccine development will be highlighted along with their advantages and potential hurdles to be overcome. In particular, we will discuss (1) genetic modifications in the Ad capsid protein to reduce the intrinsic viral immunogenicity, (2) antigen capsid incorporation for effective presentation of foreign antigens to the immune system, (3) modification of the hexon and fiber capsid proteins for Ad liver de-targeting and selective retargeting to cancer cells, (4) Ad-based vaccines carrying “arming” transgenes with immunostimulatory functions as immune adjuvants, and (5) oncolytic Ad vectors as a new therapeutic approach against cancer. Finally, the combination of adenoviral vectors with other non-adenoviral vector systems, the prime/boost strategy of immunization, clinical trials involving Ad-based vaccines, and the perspectives for the field development will be discussed.

Identification of a suppressor mutation that improves the yields of hexon-modified adenovirus vectors

We have generated hexon-modified adenovirus serotype 5 (Ad5) vectors that are not neutralized by Ad5-specific neutralizing antibodies in mice. These vectors are attractive for the advancement of vaccine products because of their potential for inducing robust antigen-specific immune responses in people with prior exposure to Ad5. However, hexon-modified Ad5 vectors displayed an approximate 10-fold growth defect in complementing cells, making potential vaccine costs unacceptably high. Replacing hypervariable regions (HVRs) 1, 2, 4, and 5 with the equivalent HVRs from Ad43 was sufficient to avoid Ad5 preexisting immunity and retain full vaccine potential. However, the resulting vector displayed the same growth defect as the hexon-modified vector carrying all 9 HVRs from Ad43. The growth defect is likely due to a defect in capsid assembly, since DNA replication and late protein accumulation were normal in these vectors. We determined that the hexon-modified vectors have a 32°C cold-sensitive phenotype and selected revertants that restored vector productivity. Genome sequencing identified a single base change resulting in a threonine-to-methionine amino acid substitution at the position equivalent to residue 342 of the wild-type protein. This mutation has a suppressor phenotype (SP), since cloning it into our Ad5 vector containing all nine hypervariable regions from Ad43, Ad5.H(43m-43), increased yields over the version without the SP mutation. This growth improvement was also shown for an Ad5-based hexon-modified vector that carried the hexon hypervariable regions of Ad48, indicating that the SP mutation may have broad applicability for improving the productivity of different hexon-modified vectors.

Prospects for oral replicating adenovirus-vectored vaccines

Orally delivered replicating adenovirus (Ad) vaccines have been used for decades to prevent adenovirus serotype 4 and 7 respiratory illness in military recruits, demonstrating exemplary safety and high efficacy. That experience suggests that oral administration of live recombinant Ads (rAds) holds promise for immunization against other infectious diseases, including those that have been refractory to traditional vaccination methods. Live rAds can express intact antigens from free-standing transgenes during replication in infected cells. Alternatively, antigenic epitopes can be displayed on the rAd capsid itself, allowing presentation of the epitope to the immune system both prior to and during replication of the virus. Such capsid-display rAds offer a novel vaccine approach that could be used either independently of or in combination with transgene expression strategies to provide a new tool in the search for protection from infectious disease.

Using multivalent adenoviral vectors for HIV vaccination

Adenoviral vectors have been used for a variety of vaccine applications including cancer and infectious diseases. Traditionally, Ad-based vaccines are designed to express antigens through transgene expression of a given antigen. For effective vaccine development it is often necessary to express or present multiple antigens to the immune system to elicit an optimal vaccine as observed preclinically with mosaic/polyvalent HIV vaccines or malaria vaccines. Due to the wide flexibility of Ad vectors they are an ideal platform for expressing large amounts of antigen and/or polyvalent mosaic antigens. Ad vectors that display antigens on their capsid surface can elicit a robust humoral immune response, the “antigen capsid-incorporation” strategy. The adenoviral hexon protein has been utilized to display peptides in the majority of vaccine strategies involving capsid incorporation. Based on our abilities to manipulate hexon HVR2 and HVR5, we sought to manipulate HVR1 in the context of HIV antigen display for the first time ever. More importantly, peptide incorporation within HVR1 was utilized in combination with other HVRs, thus creating multivalent vectors. To date this is the first report where dual antigens are displayed within one Ad hexon particle. These vectors utilize HVR1 as an incorporation site for a seven amino acid region of the HIV glycoprotein 41, in combination with six Histidine incorporation within HVR2 or HVR5. Our study illustrates that these multivalent antigen vectors are viable and can present HIV antigen as well as His₆ within one Ad virion particle. Furthermore, mouse immunizations with these vectors demonstrate that these vectors can elicit a HIV and His₆ epitope-specific humoral immune response.

Hexon-modified recombinant E1-deleted adenovirus vectors as dual specificity vaccine carriers for influenza virus

To determine if an ordered and repetitive display of an epitope promoted induction of superior antibody responses, we compared B-cell responses to an influenza A virus epitope that was either encoded as a transgene by an adenovirus (Ad) vector or expressed on the vector's surface. To this end, we constructed a panel of influenza A virus vaccines based on chimpanzee-derived replication-defective adenovirus (AdC) vectors of serotype SAd-V25 also called AdC68. AdC68 vectors were modified to express a linear B-cell epitope of the ectodomain of matrix 2 (M2e) within variable regions 1 (VR1) or 4 (VR4) of the adenovirus hexon. Additional vectors with wild-type or M2e-modified hexon encoded M2e fused to the influenza A virus nucleoprotein (NP) as a transgene product. Hexon-modified vectors were tested for immunogenicity and efficacy in mice in comparison to vectors with native hexon expressing the M2e-NP fusion protein. Upon priming, vectors expressing M2e within VR1 of hexon induced M2e-specific antibody responses of higher magnitude and avidity than those carrying M2e within VR4 or vectors expressing the M2e as part of a transgene product. CD8(+) T-cell responses to the transgenic NP were comparable between vectors. M2e-specific antibody responses could be boosted by a second dose of the VR1 hexon-modified vector but not by repeated immunization with the VR4 hexon-modified vector.

Vaccines expressing the innate immune modulator EAT-2 elicit potent effector memory T lymphocyte responses despite pre-existing vaccine immunity

The mixed results from recent vaccine clinical trials targeting HIV-1 justify the need to enhance the potency of HIV-1 vaccine platforms in general. Use of first-generation recombinant adenovirus serotype 5 (rAd5) platforms failed to protect vaccinees from HIV-1 infection. One hypothesis is that the rAd5-based vaccine failed due to the presence of pre-existing Ad5 immunity in many vaccines. We recently confirmed that EAT-2-expressing rAd5 vectors uniquely activate the innate immune system and improve cellular immune responses against rAd5-expressed Ags, inclusive of HIV/Gag. In this study, we report that use of the rAd5-EAT-2 vaccine can also induce potent cellular immune responses to HIV-1 Ags despite the presence of Ad5-specific immunity. Compared to controls expressing a mutant SH2 domain form of EAT-2, Ad5 immune mice vaccinated with an rAd5-wild-type EAT-2 HIV/Gag-specific vaccine formulation significantly facilitated the induction of several arms of the innate immune system. These responses positively correlated with an improved ability of the vaccine to induce stronger effector memory T cell-biased, cellular immune responses to a coexpressed Ag despite pre-existing anti-Ad5 immunity. Moreover, inclusion of EAT-2 in the vaccine mixture improves the generation of polyfunctional cytolytic CD8(+) T cell responses as characterized by enhanced production of IFN-γ, TNF-α, cytotoxic degranulation, and increased in vivo cytolytic activity. These data suggest a new approach whereby inclusion of EAT-2 expression in stringent human vaccination applications can provide a more effective vaccine against HIV-1 specifically in Ad5 immune subjects.

Comparison of two high-throughput assays for quantification of adenovirus type 5 neutralizing antibodies in a population of donors in China

Background

The presence of various levels of Adenovirus serotype 5 neutralizing antibodies (Ad5NAb) is thought to contribute to the inconsistent clinical results obtained from vaccination and gene therapy studies. Currently, two platforms based on high-throughput technology are available for Ad5NAb quantification, chemiluminescence- and fluorescence-based assays. The aim of this study was to compare the results of two assays in the seroepidemiology of Ad5NAb in a local population of donors.

Methodology/Principal Findings

The fluorescence-based neutralizing antibody detection test (FRNT) using recombinant Ad5-EGFP virus and the chemiluminescence-based neutralizing antibody test (CLNT) using Ad5-Fluc were developed and standardized for detecting the presence of Ad5NAb in serum samples from the population of donors in Beijing and Anhui provinces, China. First, the overall percentage of people positive for Ad5NAb performed by CLNT was higher than that obtained by FRNT (85.4 vs 69.9%, p<0.001). There was an 84.5% concordance between the two assays for the 206 samples tested (144 positive in both assays and 30 negative in both assays). All 32 discordant sera were CLNT-positive/FRNT-negative and were confirmed positive by western blot. Secondly, for all 144 sera positive by both assays, the two assays showed high correlation (r = 0.94, p<0.001) and close agreement (mean difference: 0.395 log₁₀, 95% CI: −0.054 log₁₀ to 0.845 log₁₀). Finally, it was found by both assays that there was no significant difference observed for titer or prevalence by gender (p = 0.503 vs 0.818, for two assays); however, age range (p = 0.049 vs 0.010) and geographic origin (p = 0.007 vs 0.011) were correlated with Ad5NAb prevalence in northern regions of China.

Conclusion

The CLNT assay was relatively more simple and had higher sensitivity than the FRNT assay for determining Ad5NAb titers. It is strongly suggested that the CLNT assay be used for future epidemiological studies of Ad5NAb in other localities.

Subcutaneous immunization with recombinant adenovirus expressing influenza A nucleoprotein protects mice against lethal viral challenge

Current influenza vaccines mainly induce strain-specific neutralizing antibodies and need to be updated each year, resulting in significant burdens on vaccine manufacturers and regulatory agencies. Genetic immunization strategies based on the highly conserved nucleoprotein (NP) of influenza have attracted great attention as NP could induce heterosubtypic immunity. It is unclear, however, whether different forms of vectors and/or vaccination regimens could have contributed to the previously reported discrepancies in the magnitude of protection of NP-based genetic vaccinations. Here, we evaluated a plasmid DNA vector (pNP) and a recombinant adenovirus vector (rAd-NP) containing the NP gene through various combinations of immunization regimens in mice. We found that pNP afforded only partial protection even after 4 injections, with full protection against lethal challenge achieved only with the fourth boost using rAd-NP. Alternatively, only two doses of rAd-NP delivered subcutaneously were needed to induce an enhanced immune response and completely protect the animals, a finding which, to our knowledge, has not been reported before.

Immunogenic comparison of chimeric adenovirus 5/35 vector carrying optimized human immunodeficiency virus clade C genes and various promoters

Adenovirus vector-based vaccine is a promising approach to protect HIV infection. However, a recent phase IIb clinical trial using the vector did not show its protective efficacy against HIV infection. To improve the vaccine, we explored the transgene protein expression and its immunogenicity using optimized codon usage, promoters and adaptors. We compared protein expression and immunogenicity of adenovirus vector vaccines carrying native or codon usage-optimized HIV-1 clade C gag and env genes expression cassettes driven by different promoters (CMV, CMVi, and CA promoters) and adapters (IRES and F2A). The adenovirus vector vaccine containing optimized gag gene produced higher Gag protein expression and induced higher immune responses than the vector containing native gag gene in mice. Furthermore, CA promoter generated higher transgene expression and elicited higher immune responses than other two popularly used promoters (CMV and CMVi). The second gene expression using F2A adaptor resulted in higher protein expression and immunity than that of using IRES and direct fusion protein. Taken together, the adenovirus vector containing the expression cassette with CA promoter, optimized HIV-1 clade C gene and an F2A adaptor produced the best protein expression and elicited the highest transgene-specific immune responses. This finding would be promising for vaccine design and gene therapy.

A rapid generation of adenovirus vector with a genetic modification in hexon protein

The generation of hexon-modified adenovirus vector has proven difficult. In this paper, we developed a novel method for rapid generation of hexon-modified adenoviral vector via one step ligation in vitro followed by quick white/blue color screening. The new system has the following features. First, eGFP expression driven by the CMV promoter in E1 region functions as a reporter to evaluate the tropism of hexon-modified adenovirus in vitro. Second, it has two unique restriction enzyme sites with sticky ends located in the hexon HVR5 region. Third, a lacZ expression cassette under the control of plac promoter is placed between the two restriction enzyme sites, which allows recombinants to be selected using blue/white screening. To prove the principle of the method, genetically modified adenoviruses were successfully produced by insertion of NGR, RGD or Tat PTD peptide into hexon HVR5. Furthermore, the transduction efficiency of the Tat PTD modified virus was shown to be a significant enhancement in A172 and CHO-K1 cells. In conclusion, the novel system makes the production of truly retargeted vectors more promising, which would be of substantial benefit for cancer gene therapy.

Adenovirus serotype 5-specific neutralizing antibodies target multiple hexon hypervariable regions

The immunogenicity of adenovirus serotype 5 (Ad5) vectors has been shown to be suppressed by neutralizing antibodies (NAbs) directed primarily against the hexon hypervariable regions (HVRs). We previously reported that replacing all seven HVRs with those from the rare serotype virus Ad48 resulted in a chimeric Ad5HVR48(1-7) vector that largely evaded preexisting Ad5 immunity in mice and rhesus monkeys. In this study, we evaluated the extent to which Ad5-specific NAbs are directed against various HVRs. We constructed partial HVR-chimeric Ad5 vectors with only a subset of HVRs exchanged, and we utilized these vectors in both NAb assays and murine immunogenicity studies with and without baseline Ad5 immunity. Our results demonstrate that Ad5-specific NAbs target multiple HVRs, suggesting that replacing all HVRs is required to optimize evasion of anti-Ad5 immunity. These data have important implications for the development of novel vectors for both vaccines and gene therapy.

The challenge for gene therapy: innate immune response to adenoviruses

Adenoviruses are the most commonly used vectors for gene therapy. Despite the promising safety profile demonstrated in clinical trials, the efficacy of using adenoviruses for gene therapy is poor. A major hurdle to adenoviral-mediated gene therapy is the innate immune system. Cell-mediated recognition of viruses via capsid components or nucleic acids has received significant attention, principally thought to be regulated by the toll-like receptors (TLRs). Antiviral innate immune responses are initiated by the infected cell, which activates the interferon (IFN) response to block viral replication, while simultaneously releasing chemokines to attract neutrophils, mononuclear- and natural killer-cells. While the IFN and cellular recruitment pathways are activated and regulated independently of each other, both are required to overcome immune escape mechanisms by adenoviruses. Recent work has shown that the generation of adenoviral vectors lacking specific transcriptionally-active regions decreases immune system activation and increases the chance for immune escape. In this review, we elucidate how adenoviral vector modifications alter the IFN and innate inflammatory pathway response and propose future targets with clinically-translational relevance.

Potent antitumor immunity generated by a CD40-targeted adenoviral vaccine

In situ delivery of tumor-associated antigen (TAA) genes into dendritic cells (DC) has great potential as a generally applicable tumor vaccination approach. Although adenoviruses (Ad) are an attractive vaccine vehicle in this regard, Ad-mediated transduction of DCs is hampered by the lack of expression of the Ad receptor CAR on the DC surface. DC activation also requires interaction of CD40 with its ligand CD40L to generate protective T-cell-mediated tumor immunity. Therefore, to create a strategy to target Ads to DCs in vivo, we constructed a bispecific adaptor molecule with the CAR ectodomain linked to the CD40L extracellular domain via a trimerization motif (CFm40L). By targeting Ad to CD40 with the use of CFm40L, we enhanced both transduction and maturation of cultured bone marrow-derived DCs. Moreover, we improved transduction efficiency of DCs in lymph node and splenic cell suspensions in vitro and in skin and vaccination site-draining lymph nodes in vivo. Furthermore, CD40 targeting improved the induction of specific CD8(+) T cells along with therapeutic efficacy in a mouse model of melanoma. Taken together, our findings support the use of CD40-targeted Ad vectors encoding full-length TAA for in vivo targeting of DCs and high-efficacy induction of antitumor immunity.

Vaccination with Human Induced Pluripotent Stem Cells Creates an Antigen-Specific Immune Response Against HIV-1 gp160

Induced pluripotent stem cells (iPSCs) are artificially derived from somatic cells that have been transduced with defined reprogramming factors. A previous report has indicated the possibility of using iPSCs as an immune stimulator to generate antigen-specific immunity. In our current study, we have investigated whether human iPSCs (hiPSCs) have the ability to enhance specific immune response against a human immunodeficiency virus type 1 (HIV-1) antigen in a xenogenic mouse model. Our results show that BALB/c mice immunized with hiPSCs transduced with an adenoviral vector encoding HIV-1 gp160 exhibited prominent antigen-specific cellular immune responses. We further found that pre-treatment of hiPSCs with ionizing radiation promotes the secretion of pro-inflammatory cytokines such as interleukin-1 alpha (IL-1α), IL-12, and IL-18. These cytokines might promote the activation of antigen-presenting cells and the effective induction of cellular immunity. Our present findings thus demonstrate that a hiPSCs-based vaccine has the potential to generate cellular immunity against viral antigens such as HIV-1 gp160 in a xenogenic condition.

Current progress in the development of a prophylactic vaccine for HIV-1

Since its discovery and characterization in the early 1980s as a virus that attacks the immune system, there has been some success for the treatment of human immunodeficiency virus-1 (HIV-1) infection. However, due to the overwhelming public health impact of this virus, a vaccine is needed urgently. Despite the tireless efforts of scientist and clinicians, there is still no safe and effective vaccine that provides sterilizing immunity. A vaccine that provides sterilizing immunity against HIV infection remains elusive in part due to the following reasons: 1) degree of diversity of the virus, 2) ability of the virus to evade the hosts' immunity, and 3) lack of appropriate animal models in which to test vaccine candidates. There have been several attempts to stimulate the immune system to provide protection against HIV-infection. Here, we will discuss attempts that have been made to induce sterilizing immunity, including traditional vaccination attempts, induction of broadly neutralizing antibody production, DNA vaccines, and use of viral vectors. Some of these attempts show promise pending continued research efforts.

Anti-adenovirus humoral responses influence on the efficacy of vaccines based on epitope display on adenovirus capsid

The efficacy of recombinant adenoviruses (Ads) vaccine vectors is diminished by the high prevalence of anti-Ad antibodies (Abs) that hampers gene transfer. Epitope display on Ad capsid constitutes an alternative approach to bypass anti-Ad Ab capacity from blocking antigen expression. To understand the role of the epitope insertion site, an ovalbumin-derived epitope was genetically inserted into either Ad hexon or fiber proteins. Hexon-modified Ads triggered higher anti-ovalbumin Ab responses after one injection but surprisingly fiber-modified Ads were by far more potent after two or several administrations. Our data unravel a role for anti-Ad humoral immunity in controlling anti-epitope humoral responses.

Capsid-incorporation of antigens into adenovirus capsid proteins for a vaccine approach

Some viral vectors are potent inducers of cellular and humoral responses; therefore, viral vectors can be used to vaccinate against cancer or infectious diseases. This report will focus on adenovirus (Ad)-based vectors. Traditional viral-vector vaccination embodies the concept that the vector uses the host-cell machinery to express antigens that are encoded as transgenes within the viral vector. Several preclinical successes have used this approach in animal model systems. However, in some instances, these conventional Ad-based vaccines have yielded suboptimal clinical results. These suboptimal results are ascribed, in part, to preexisting Ad serotype 5 (Ad5) immunity. To address this issue, the "antigen capsid-incorporation" strategy has been developed to circumvent the drawbacks associated with conventional transgene expression of antigens by Ad vectors. This strategy embodies the incorporation of antigenic peptides within the capsid structure of viral vectors. Incorporating immunogenic peptides into the Ad capsid offers potential advantages. Importantly, vaccination by means of the antigen capsid-incorporated approach results in a strong humoral response, similar to the response generated by native Ad capsid proteins. This strategy also allows for the boosting of antigenic specific responses. This strategy may be the way forward for improved vaccine schemes, especially for those infections requiring a strong humoral antigenic response.

Replacing adenoviral vector HVR1 with a malaria B cell epitope improves immunogenicity and circumvents preexisting immunity to adenovirus in mice

Although adenovirus (Ad) has been regarded as an excellent vaccine vector, there are 2 major drawbacks to using this platform: (a) Ad-based vaccines induce a relatively weak humoral response against encoded transgenes, and (b) preexisting immunity to Ad is highly prevalent among the general population. To overcome these obstacles, we constructed an Ad-based malaria vaccine by inserting a B cell epitope derived from a Plasmodium yoelii circumsporozoite (CS) protein (referred to as the PyCS-B epitope) into the capsid proteins of WT/CS-GFP, a recombinant Ad expressing P. yoelii CS protein and GFP as its transgene. Multiple vaccinations with the capsid-modified Ad induced a substantially increased level of protection against subsequent malaria challenge in mice when compared with that of unmodified WT/CS-GFP. Increased protection correlated with augmented antibody responses against the PyCS-B epitope expressed in the capsid. Furthermore, replacement of hypervariable region 1 (HVR1) of the Ad capsid proteins with the PyCS-B epitope circumvented neutralization of the modified Ad by preexisting Ad-specific antibody, both in vivo and in vitro. Importantly, the immunogenicity of the Ad-containing PyCS-B epitope in the HVR1 and a P. yoelii CS transgene was maintained. Overall, this study demonstrates that the HVR1-modifed Ad vastly improves upon Ad as a promising malaria vaccine platform candidate.

HIV antigen incorporation within adenovirus hexon hypervariable 2 for a novel HIV vaccine approach

Adenoviral (Ad) vectors have been used for a variety of vaccine applications including cancer and infectious diseases. Traditionally, Ad-based vaccines are designed to express antigens through transgene expression of a given antigen. However, in some cases these conventional Ad-based vaccines have had sub-optimal clinical results. These sub-optimal results are attributed in part to pre-existing Ad serotype 5 (Ad5) immunity. In order to circumvent the need for antigen expression via transgene incorporation, the “antigen capsid-incorporation” strategy has been developed and used for Ad-based vaccine development in the context of a few diseases. This strategy embodies the incorporation of antigenic peptides within the capsid structure of viral vectors. The major capsid protein hexon has been utilized for these capsid incorporation strategies due to hexon's natural role in the generation of anti-Ad immune response and its numerical representation within the Ad virion. Using this strategy, we have developed the means to incorporate heterologous peptide epitopes specifically within the major surface-exposed domains of the Ad capsid protein hexon. Our study herein focuses on generation of multivalent vaccine vectors presenting HIV antigens within the Ad capsid protein hexon, as well as expressing an HIV antigen as a transgene. These novel vectors utilize HVR2 as an incorporation site for a twenty-four amino acid region of the HIV membrane proximal ectodomain region (MPER), derived from HIV glycoprotein gp41 (gp41). Our study herein illustrates that our multivalent anti-HIV vectors elicit a cellular anti-HIV response. Furthermore, vaccinations with these vectors, which present HIV antigens at HVR2, elicit a HIV epitope-specific humoral immune response.

Overcoming pre-existing adenovirus immunity by genetic engineering of adenovirus-based vectors

Adenovirus (Ad)-based vectors offer several benefits showing their potential for use in a variety of vaccine applications. Recombinant Ad-based vaccines possess potent immunogenic potential, capable of generating humoral and cellular immune responses to a variety of pathogen-specific antigens expressed by the vectors. Ad5 vectors can be readily produced, allowing for usage in thousands of clinical trial subjects. This is now coupled with a history of safe clinical use in the vaccine setting. However, traditional Ad5-based vaccines may not be generating optimal antigen-specific immune responses, and generate diminished antigen-specific immune responses when pre-existing Ad5 immunity is present. These limitations have driven initiation of several approaches to improve the efficacy of Ad-based vaccines, and/or allow modified vaccines to overcome pre-existing Ad immunity. These include: generation of chemically modified Ad5 capsids; generation of chimeric Ads; complete replacement of Ad5-based vaccine platforms with alternative (human and non-human origin) Ad serotypes, and Ad5 genome modification approaches that attempt to retain the native Ad5 capsid, while simultaneously improving the efficacy of the platform as well as minimizing the effect of pre-existing Ad immunity. Here we discuss recent advances in- and limitations of each of these approaches, relative to their abilities to overcome pre-existing Ad immunity.