A chimpanzee-origin adenovirus vector expressing the rabies virus glycoprotein as an oral vaccine against inhalation infection with rabies virus

Recombinant chimpanzee adenovirus expressing spike protein protects chickens against infectious bronchitis virus

Infectious bronchitis (IB) is an acute and highly contagious disease caused by avian infectious bronchitis virus (IBV), resulting in significant economic losses in the global poultry industry. In this study, we utilized a replication-incompetent adenovirus vector derived from chimpanzees for the first time to express the S gene of IBV. The adenovirus was successfully rescued and demonstrated convenient production, good growth performance, and stability on HEK293 A cells. Morphologically, the recombinant adenovirus (named PAD-S) appeared normal under transmission electron microscopy, and efficient expression of the exogenous gene was confirmed through immunofluorescence analysis and immunoblotting. Administration of PAD-S via ocular and nasal routes induced a strong immune response in the chicken population, as evidenced by specific antibody and cytokine measurements. PAD-S was unable to replicate within chickens and showed low pre-existing immunity, demonstrating high safety and environmental friendliness. The robust immune response triggered by PAD-S immunization effectively suppressed viral replication in various tissues, alleviating clinical symptoms and tissue damage, thus providing complete protection against viral challenges in the chicken population. In conclusion, this study successfully developed an IBV candidate vaccine strain that possesses biosafety, high protective efficacy, and ease of production.

Construction and application of adenoviral vectors

Adenoviral vectors have been widely used as vaccine candidates or potential vaccine candidates against infectious diseases due to the convenience of genome manipulation, their ability to accommodate large exogenous gene fragments, easy access of obtaining high-titer of virus, and high efficiency of transduction. At the same time, adenoviral vectors have also been used extensively in clinical research for cancer gene therapy and treatment of diseases caused by a single gene defect. However, application of adenovirus also faces a series of challenges such as poor targeting, strong immune response against the vector itself, and they cannot be used repeatedly. It is believed that these problems will be solved gradually with further research and technological development in related fields. Here, we review the construction methods of adenoviral vectors, including "gutless" adenovirus and discuss application of adenoviral vectors as prophylactic vaccines for infectious pathogens and their application prospects as therapeutic vaccines for cancer and other kinds of chronic infectious disease such as human papillomavirus, hepatitis B virus, and hepatitis C virus.

A regimen compression strategy for commercial vaccines leveraging an injectable hydrogel depot technology for sustained vaccine exposure

Equitable global access to vaccines requires we overcome challenges associated with complex immunization schedules and their associated economic burdens that hinder delivery in under resourced environments. The rabies vaccine, for example, requires multiple immunizations for effective protection and each dose is cost prohibitive, and therefore inaccessibility disproportionately impacts low- and middle-income countries. In this work we developed an injectable hydrogel depot technology for sustained delivery of commercial inactivated rabies virus vaccines. In a mouse model, we showed that a single immunization of a hydrogel-based rabies vaccine elicited comparable antibody titers to a standard prime-boost bolus regimen of a commercial rabies vaccine, despite these hydrogel vaccines comprising only half of the total dose delivered in the bolus control. Moreover, these hydrogel-based vaccines elicited similar antigen-specific T-cell responses and neutralizing antibody responses compared to the bolus vaccine. Notably, we demonstrated that while addition of a potent clinical TLR4 agonist adjuvant to the gels slightly improved binding antibody responses, inclusion of this adjuvant to the inactivated virion vaccine was detrimental to neutralizing responses. Taken together, these results suggest that these hydrogels can enable an effective regimen compression and dosesparing strategy for improving global access to vaccines.

Development and characterization of a vector system based on the simian adenovirus type 25

Technological versatility and the humoral and cellular immune response induction capacity have conditioned wide spread of adenoviral vectors as vaccine and gene therapy drugs. However, vaccination with Sputnik V made a significant portion of the population immune to the types 5 and 26 (Ad5 and Ad26) recombinant human adenovirus vectors, which are some of the most frequently used bases for candidate vaccines. Today, vaccine designers tend to select alternative adenovirus serotypes as platforms to develop vaccines against new pathogens on. A good example is simian adenovirus type 25 (SAd25), which belongs to subgroup E. It is genetically distant from Ad5 and exhibits extremely low seroprevalence in human beings, which makes it an appealing alternative vaccine vector. The purpose of this work was to design and study a new vaccine platform based on simian adenovirus type 25. We relied on the advanced methods of molecular biology and virology to construct and make recombinant adenoviruses; the phylogenetic analysis in the context of this study was enabled with bioinformatic methods. The resulting recombinant adenoviral vector can effectively replicate itself in the HEK293 cell line (human embryonic kidney cells). This work substantiates the expediency of further investigation into the SAd25 vector as a platform for development of the prevention vaccines against various infectious diseases.

Preclinical evaluation of safety and immunogenicity of a primary series intranasal COVID-19 vaccine candidate (BBV154) and humoral immunogenicity evaluation of a heterologous prime-boost strategy with COVAXIN (BBV152)

Most if not all vaccine candidates developed to combat COVID-19 due to SARS-CoV-2 infection are administered parenterally. As SARS-CoV-2 is transmitted through infectious respiratory fluids, vaccine-induced mucosal immunity could provide an important contribution to control this pandemic. ChAd-SARS-CoV-2-S (BBV154), a replication-defective chimpanzee adenovirus (ChAd)-vectored intranasal (IN) COVID-19 vaccine candidate, encodes a prefusion-stabilized version of the SARS-CoV-2 spike protein containing two proline substitutions in the S2 subunit. We performed preclinical evaluations of BBV154 in mice, rats, hamsters and rabbits. Repeated dose toxicity studies presented excellent safety profiles in terms of pathology and biochemical analysis. IN administration of BBV154 elicited robust mucosal and systemic humoral immune responses coupled with Th1 cell-mediated immune responses. BBV154 IN vaccination also elicited potent variant (omicron) cross neutralization antibodies. Assessment of anti-vector (ChAd36) neutralizing antibodies following repeated doses of BBV154 IN administration showed insignificant titers of ChAd36 neutralizing antibodies. However, the immune sera derived from the same animals displayed significantly higher levels of SARS-CoV-2 virus neutralization (p<0.003). We also evaluated the safety and immunogenicity of heterologous prime-boost vaccination with intramuscular (IM) COVAXIN-prime followed by BBV154 IN administration. COVAXIN priming followed by BBV154 IN-booster showed an acceptable reactogenicity profile comparable to the homologous COVAXIN/COVAXIN or BBV154/BBV154 vaccination model. Heterologous vaccination of COVAXIN-prime and BBV154 booster also elicited superior (p<0.005) and cross variant (omicron) protective immune responses (p<0.013) compared with the homologous COVAXIN/COVAXIN schedule. BBV154 has successfully completed both homologous and heterologous combination schedules of human phase 3 clinical trials and received the restricted emergency use approval (in those aged above 18 years) from the Drugs Controller General of India (DCGI).

The use of viral vectors in vaccine development

Vaccines represent the single most cost-efficient and equitable way to combat and eradicate infectious diseases. While traditional licensed vaccines consist of either inactivated/attenuated versions of the entire pathogen or subunits of it, most novel experimental vaccines against emerging infectious diseases employ nucleic acids to produce the antigen of interest directly in vivo. These include DNA plasmid vaccines, mRNA vaccines, and recombinant viral vectors. The advantages of using nucleic acid vaccines include their ability to induce durable immune responses, high vaccine stability, and ease of large-scale manufacturing. In this review, we present an overview of pre-clinical and clinical data on recombinant viral vector vaccines and discuss the advantages and limitations of the different viral vector platforms.

A Novel Rabies Vaccine Based on a Recombinant Bovine Herpes Virus Type 1 Expressing Rabies Virus Glycoprotein

Rabies is a highly prevalent zoonotic disease and a public health threat worldwide. Currently licensed rabies vaccines are effective but less is known which would protect cattle. This study describes the construction of a novel recombinant bovine herpes virus type I (BHV-1) expressing rabies virus glycoprotein (RABV G) instead of its gE glycoprotein (gE) by CRISPR-Cas9 and homologous recombination technology (BHV-1-ΔgE-G). Insertion of the RABV G gene is stable after 20 rounds of in vitro passaging and the recombinant virus replicates to high titers in MDBK cells. The RABV G expresses in the recombinant virus-infected cells and on the virion surface of BHV-1-ΔgE-G. One single immunization with BHV-1-ΔgE-G-activated dendritic cells (DCs) and B cells furthermore induced a protective immune response in mice against severe lethal challenge infection. A protective level of RABV-specific virus-neutralizing antibody (VNA) was detected in intramuscular immunized mice and cattle without any clinical symptoms. This research demonstrated that the BHV-1 vector-based RABV vaccine is a potential candidate for cattle.

Novel approaches for vaccine development

Vaccines are critical tools for maintaining global health. Traditional vaccine technologies have been used across a wide range of bacterial and viral pathogens, yet there are a number of examples where they have not been successful, such as for persistent infections, rapidly evolving pathogens with high sequence variability, complex viral antigens, and emerging pathogens. Novel technologies such as nucleic acid and viral vector vaccines offer the potential to revolutionize vaccine development as they are well-suited to address existing technology limitations. In this review, we discuss the current state of RNA vaccines, recombinant adenovirus vector-based vaccines, and advances from biomaterials and engineering that address these important public health challenges.

A chimpanzee adenoviral vector-based rabies vaccine protects beagle dogs from lethal rabies virus challenge

Rabies continues to poses serious threats to the public health in many countries. The development of novel inexpensive, safe and effective vaccines has become a high priority for rabies control worldwide. We previously generated a novel recombinant rabies vaccine by cloning rabies virus glycoprotein into a chimpanzee adenoviral vector, termed ChAd68-Gp. The present study evaluated the immune responses and protection afforded by this vaccine in beagle dogs. The results demonstrated that intramuscular immunization with both low-dose and high-dose of ChAd68-Gp induced strong immune responses and provided complete protection in beagles even at low-dose. However, when administered orally, high-dose vaccination was protective while low-dose vaccination was ineffective. Further investigation indicated that the low-pH value of gastric juice in the stomach of beagles might decompose the adenovirus. Therefore, suitable formulation for adenovirus-based oral vaccine should be considered and developed. The chimpanzee adenovirus-vectored rabies vaccine ChAd68-Gp warrants extensive test for clinical application.

Development of novel vaccine vectors: Chimpanzee adenoviral vectors

Adenoviral vector has been employed as one of the most efficient means against infectious diseases and cancer. It can be genetically modified and armed with foreign antigens to elicit specific antibody responses and T cell responses in hosts as well as engineered to induce apoptosis in cancer cells. The chimpanzee adenovirus-based vector is one kind of novel vaccine carriers whose unique features and non-reactivity to pre-existing human adenovirus neutralizing antibodies makes it an outstanding candidate for vaccine research and development. Here, we review the different strategies for constructing chimpanzee adenoviral vectors and their applications in recent clinical trials and also discuss the oncolytic virotherapy and immunotherapy based on chimpanzee adenoviral vectors.

Seroprevalence of Neutralizing Antibodies against Human Adenovirus Type-5 and Chimpanzee Adenovirus Type-68 in Cancer Patients

Since the preclinical results about chimpanzee adenovirus serotype-68 (AdC68)-based vaccine showed an encouraging results, it reminded us that AdC68 may be a suitable cancer vaccine vector. Previous study indicated that the seroprevalence of neutralizing antibodies (NAbs) against adenovirus was different between cancer patients and healthy volunteers. Knowledge regarding the prevalence rates of AdC68 NAbs for cancer patients is lacking. Therefore, assessing the preexistence of NAbs against AdC68 in cancer patients could provide useful insights for developing future AdC68-based cancer vaccines. In this study, 440 patients with different pathological types of tumors and 204 healthy adult volunteers were enrolled to evaluate the NAbs against AdC68 and human adenovirus serotype-5 (AdHu5). The seroprevalence of NAbs against AdC68 was much lower than that against AdHu5 in cancer subjects (43.64 vs. 67.05%, P < 0.01). The seroprevalence rates of NAbs to AdC68 in the cancer subjects were statistically higher than those detected in the healthy adult volunteers (43.64 vs. 23.53%, P = 0.000). The seroprevalence rates of AdC68 NAbs were much lower in lung, laryngeal, esophageal, and cervical cancer patients compared with oropharyngeal, colon, and rectal cancer patients. Furthermore, the seroprevalence rates of AdC68 NAbs were much lower in lung adenocarcinoma patients than in lung squamous cell carcinoma patients (35.00 vs. 70.00%, P < 0.05). No significant difference in the AdC68 NAbs among patients with different clinical stages of cancer was detected. The percentage of NAbs against AdC68 was significantly lower than that against AdHu5 (P < 0.05) in stage-I, -II, and -III cancer patients. No significant difference between the percentage of NAbs against AdC68 and AdHu5 in the subjects with stage-IV cancer was detected. The study also demonstrated the distribution of AdHu5 and AdC68 NAb titers for the positive samples. It showed that very low NAb titers against AdC68 with respect to AdHu5 in both healthy subjects and cancer subjects, especially in lung, laryngeal, esophageal, gastric, and cervical carcinomas. Also, the titer of NAbs against AdC68 was significantly lower than that against AdHu5 in the same clinical stage and age group (P < 0.05). Taken together, the present study showed that NAbs against AdC68 is much lower than AdHu5, especially in lung adenocarcinoma, laryngeal cancer, esophageal cancer, and cervical cancer patients. These results provided strong support for candidating AdC68 as a suitable vector of cancer vaccines.

Chimpanzee adenoviral vectors as vaccines - challenges to move the technology into the fast lane

INTRODUCTION

In recent years, replication-defective chimpanzee-derived adenoviruses have been extensively evaluated as genetic vaccines. These vectors share desirable properties with human adenoviruses like the broad tissue tropism and the ease of large-scale manufacturing. Additionally, chimpanzee adenoviruses have the advantage to overcome the negative impact of pre-existing anti-human adenovirus immunity. Areas covered: Here the authors review current pre-clinical research and clinical trials that utilize chimpanzee-derived adenoviral vectors as vaccines. A wealth of studies are ongoing to evaluate different vector backbones and administration routes with the aim of improving immune responses. The challenges associated with the identification of an optimal chimpanzee vector and immunization strategies for different immunological outcomes will be discussed. Expert commentary: The demonstration that chimpanzee adenoviruses can be safely used in humans has paved the way to the use of a whole new array of vectors of different serotypes. However, so far no predictive signature of vector immunity in humans has been identified. The high magnitude of T cell responses elicited by chimpanzee adenoviruses has allowed dissecting the qualitative aspects that may be important for protective immunity. Ultimately, only the results from the most clinically advanced products will help establish the efficacy of the vaccine vector platform in the field of disease prevention.

Current state and challenges in developing oral vaccines

While vaccination remains the most cost effective strategy for disease prevention, communicable diseases persist as the second leading cause of death worldwide. There is a need to design safe, novel vaccine delivery methods to protect against unaddressed and emerging diseases. Development of vaccines administered orally is preferable to traditional injection-based formulations for numerous reasons including improved safety and compliance, and easier manufacturing and administration. Additionally, the oral route enables stimulation of humoral and cellular immune responses at both systemic and mucosal sites to establish broader and long-lasting protection. However, oral delivery is challenging, requiring formulations to overcome the harsh gastrointestinal (GI) environment and avoid tolerance induction to achieve effective protection. Here we address the rationale for oral vaccines, including key biological and physicochemical considerations for next-generation oral vaccine design.

Both haemagglutinin-specific antibody and T cell responses induced by a chimpanzee adenoviral vaccine confer protection against influenza H7N9 viral challenge

Since 2013, the outbreak or sporadic infection of a new reassortant H7N9 influenza virus in China has resulted in hundreds of deaths and thousands of illnesses. An H7N9 vaccine is urgently needed, as a licensed human vaccine against H7N9 influenza is currently not available. Here, we developed a recombinant adenovirus-based vaccine, AdC68-H7HA, by cloning the H7N9 haemagglutinin (HA) gene into the chimpanzee adenoviral vector AdC68. The efficacy of AdC68-H7HA was evaluated in mice as well as guinea pigs. For comparison, an H7N9 DNA vaccine based on HA was also generated and tested in mice and guinea pigs. The results demonstrated that both AdC68-H7HA and the DNA vaccine prime-adenovirus boost regimen induced potent immune responses in animals and completely protected mice from lethal H7N9 influenza viral challenge. A post-immunization serum transfer experiment showed that antibody responses could completely protect against lethal challenge, while a T cell depletion experiment indicated that HA-specific CD8⁺ T cells responses also contributed to protection. Therefore, both HA-specific humoral immunity and cellular immunity play important roles in the protection. These data suggest that the chimpanzee adenovirus expressing HA is a promising vaccine candidate for H7N9 virus or other influenza viral subtypes.

Simian adenoviruses as vaccine vectors

Replication incompetent human adenovirus serotype 5 (HAdV-C5) has been extensively used as a delivery vehicle for gene therapy proteins and infectious disease antigens. These vectors infect replicating and nonreplicating cells, have a broad tissue tropism, elicit high immune responses and are easily purified to high titers. However, the utility of HAdV-C5 vectors as potential vaccines is limited due to pre-existing immunity within the human population that significantly reduces the immunogenicity of HAdV-C5 vaccines. In recent years, adenovirus vaccine development has focused on simian-derived adenoviral vectors, which have the desirable vector characteristics of HAdV-C5 but with negligible seroprevalence in the human population. Here, we discuss recent advances in simian adenovirus vaccine vector development and evaluate current research specifically focusing on clinical trial data.

Current Status and Development of Vaccines and Other Biologics for Human Rabies Prevention

Rabies is a neglected viral zoonosis with the highest case fatality of any infectious disease. Pasteur's historical accomplishments during the late 19(th) century began the process of human vaccine development, continuing to evolve into the 21(st) century. Over the past 35 years, great improvements occurred in the production of potent tissue culture vaccines and the gradual removal from the market of unsafe nerve tissue products. Timely and appropriate administration of modern biologics virtually assures survivorship, even after severe exposures. Nevertheless, in the developing world, if not provided for free nationally, the cost of a single course of human prophylaxis exceeds the average monthly wage of the common worker. Beyond traditional approaches, recombinant, sub-unit and other novel methods are underway to improve the availability of safe, effective and more affordable rabies biologics.

Xenogenic Adenoviral Vectors

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

Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what's important?

Pre-existing immunity against human adenovirus (HAd) serotype 5 derived vector in the human population is widespread, thus hampering its clinical use. Various components of the immune system, including neutralizing antibodies (nAbs), Ad specific T cells and type I IFN activated NK cells, contribute to dampening the efficacy of Ad vectors in individuals with pre-existing Ad immunity. In order to circumvent pre-existing immunity to adenovirus, numerous strategies, such as developing alternative Ad serotypes, varying immunization routes and utilizing prime-boost regimens, are under pre-clinical or clinical phases of development. However, these strategies mainly focus on one arm of pre-existing immunity. Selection of alternative serotypes has been largely driven by the absence in the human population of nAbs against them with little attention paid to cross-reactive Ad specific T cells. Conversely, varying the route of immunization appears to mainly rely on avoiding Ad specific tissue-resident T cells. Finally, prime-boost regimens do not actually circumvent pre-existing immunity but instead generate immune responses of sufficient magnitude to confer protection despite pre-existing immunity. Combining the above strategies and thus taking into account all components regulating pre-existing Ad immunity will help further improve the development of Ad vectors for animal and human use.

Clinical development of Ebola vaccines

The ongoing outbreak of Ebola virus disease in West Africa highlighted the lack of a licensed drug or vaccine to combat the disease and has renewed the urgency to develop a pipeline of Ebola vaccines. A number of different vaccine platforms are being developed by assessing preclinical efficacy in animal models and expediting clinical development. Over 15 different vaccines are in preclinical development and 8 vaccines are now in different stages of clinical evaluation. These vaccines include DNA vaccines, virus-like particles and viral vectors such as live replicating vesicular stomatitis virus (rVSV), human and chimpanzee adenovirus, and vaccinia virus. Recently, in preliminary results reported from the first phase III trial of an Ebola vaccine, the rVSV-vectored vaccine showed promising efficacy. This review charts this rapidly advancing area of research focusing on vaccines in clinical development and discusses the future opportunities and challenges faced in the licensure and deployment of Ebola vaccines.

Adenoviral vectors as novel vaccines for influenza

OBJECTIVES

Influenza is a viral respiratory disease causing seasonal epidemics, with significant annual illness and mortality. Emerging viruses can pose a major pandemic threat if they acquire the capacity for sustained human-to-human transmission. Vaccination reduces influenza-associated mortality and is critical in minimising the burden on the healthcare system. However, current vaccines are not always effective in at-risk populations and fail to induce long-lasting protective immunity against a range of viruses.

KEY FINDINGS

The development of 'universal' influenza vaccines, which induce heterosubtypic immunity capable of reducing disease severity, limiting viral shedding or protecting against influenza subtypes with pandemic potential, has gained interest in the research community. To date, approaches have focused on inducing immune responses to conserved epitopes within the stem of haemagglutinin, targeting the ectodomain of influenza M2e or by stimulating cellular immunity to conserved internal antigens, nucleoprotein or matrix protein 1.

SUMMARY

Adenoviral vectors are potent inducers of T-cell and antibody responses and have demonstrated safety in clinical applications, making them an excellent choice of vector for delivery of vaccine antigens. In order to circumvent pre-existing immunity in humans, serotypes from non-human primates have recently been investigated. We will discuss the pre-clinical development of these novel vectors and their advancement to clinical trials.

Development of chimpanzee adenoviruses as vaccine vectors: challenges and successes emerging from clinical trials

Replication-defective chimpanzee adenovirus vectors are emerging as a promising new class of genetic vaccine carriers. Chimpanzee adenovirus vectors have now reached the clinical stage and appear to be endowed with all the properties needed for human vaccine development, including high quality and magnitude of the immune response induced against the encoded antigens, good safety and ease of manufacturing on a large-scale basis. Here the authors review the recent findings of this novel class of adenovirus vectors and compare their properties to other clinical stage vaccine vectors derived from poxvirus, alphavirus and human adenovirus.

Generation and immunity testing of a recombinant adenovirus expressing NcSRS2-NcGRA7 fusion protein of bovine Neospora caninum

Neospora caninum is the etiologic agent of bovine neosporosis, which affects the reproductive performance of cattle worldwide. The transmembrane protein, NcSRS2, and dense-granule protein, NcGRA7, were identified as protective antigens based on their ability to induce significant protective immune responses in murine neosporosis models. In the current study, NcSRS2 and NcGRA7 genes were spliced by overlap-extension PCR in a recombinant adenovirus termed Ad5-NcSRS2-NcGRA 7, expressing the NcSRS2-NcGRA7 gene, and the efficacy was evaluated in mice. The results showed that the titer of the recombinant adenovirus was 10(9)TCID50/ml. Three weeks post-boost immunization (w.p.b.i.), the IgG antibody titer in sera was as high as 1:4,096. IFN-γ and IL-4 levels were significantly different from the control group (P<0.01). This research established a solid foundation for the development of a recombinant adenovirus vaccine against bovine N. caninum.

Airway delivery of an adenovirus-based Ebola virus vaccine bypasses existing immunity to homologous adenovirus in nonhuman primates

Anti-adenovirus serotype 5 antibodies are capable of neutralizing adenovirus serotype 5-based vaccines. In mice and guinea pigs, intranasal delivery of adenovirus serotype 5-based vaccine bypasses induced adenovirus serotype 5 preexisting immunity, resulting in protection against species-adapted Ebola virus challenge. In this study, nonhuman primates were vaccinated with adenovirus serotype 5-based vaccine either intramuscularly or via the airway route (intranasally/intratracheally) in the presence or absence of adenovirus serotype 5 preexisting immunity. Immune responses were evaluated to determine the effect of both the vaccine delivery route and preexisting immunity before and after a lethal Ebola virus (Zaïre strain Kikwit 95) challenge. Intramuscular vaccination fully protected nonhuman primates in the absence of preexisting immunity, whereas the presence of preexisting immunity abrogated vaccine efficacy and resulted in complete mortality. In contrast, the presence of preexisting immunity to adenovirus serotype 5 did not alter the survival rate of nonhuman primates receiving the adenovirus serotype 5-based Ebola virus vaccine in the airway. This study shows that airway vaccination with adenovirus serotype 5-based Ebola virus vaccine can efficiently bypass preexisting immunity to adenovirus serotype 5 and induce protective immune responses, albeit at lower efficacy than that using an intramuscular vaccine delivery route.

A novel rabies vaccine based on a recombinant parainfluenza virus 5 expressing rabies virus glycoprotein

Untreated rabies virus (RABV) infection leads to death. Vaccine and postexposure treatment have been effective in preventing RABV infection. However, due to cost, rabies vaccination and treatment have not been widely used in developing countries. There are 55,000 human death caused by rabies annually. An efficacious and cost-effective rabies vaccine is needed. Parainfluenza virus 5 (PIV5) is thought to contribute to kennel cough, and kennel cough vaccines containing live PIV5 have been used in dogs for many years. In this work, a PIV5-vectored rabies vaccine was tested in mice. A recombinant PIV5 encoding RABV glycoprotein (G) (rPIV5-RV-G) was administered to mice via intranasal (i.n.), intramuscular (i.m.), and oral inoculation. The vaccinated mice were challenged with a 50% lethal challenge dose (LD(50)) of RABV challenge virus standard 24 (CVS-24) intracerebrally. A single dose of 10(6) PFU of rPIV5-RV-G was sufficient for 100% protection when administered via the i.n. route. The mice vaccinated with a single dose of 10(8) PFU of rPIV5-RV-G via the i.m. route showed very robust protection (90% to 100%). Intriguingly, the mice vaccinated orally with a single dose of 10(8) PFU of rPIV5-RV-G showed a 50% survival rate, which is comparable to the 60% survival rate among mice inoculated with an attenuated rabies vaccine strain, recombinant LBNSE. This is first report of an orally effective rabies vaccine candidate in animals based on PIV5 as a vector. These results indicate that rPIV5-RV-G is an excellent candidate for a new generation of recombinant rabies vaccine for humans and animals and PIV5 is a potential vector for oral vaccines.

Impact of systemic or mucosal immunity to adenovirus on Ad-based Ebola virus vaccine efficacy in guinea pigs

BACKGROUND

Approximately 35% of the North American population and an estimated 90% of the sub-Saharan African population have antibodies against adenovirus serotype 5 (AdHu5) that are capable of neutralizing AdHu5-based vaccines. In mice, intranasal delivery of AdHu5 expressing the Zaire ebolavirus glycoprotein human adenovirus serotype 5 (Ad) containing the genes for the Zaire ebolavirus glycoprotein (ZGP) under the expressional control of a cytomegalovirus immediate early promoter (CMV)) can bypass systemic preexisting immunity, resulting in protection against mouse-adapted Zaire ebolavirus (Mayinga 1976).

METHODS

Guinea pigs administered an adenovirus-based Ebola virus vaccine either intramuscularly or intranasally in the presence of systemically or mucosally induced adenovirus immunity were challenged with a lethal dose of guinea pig-adapted Zaire ebolavirus (Mayinga 1976) (GA-ZEBOV). The humoral immune response was assayed to determine the effect of vaccine delivery route and preexisting immunity.

RESULTS

Intramuscular or intranasal vaccination fully protected guinea pigs against a lethal GA-ZEBOV challenge. However, intramuscular vaccination in animals with systemically induced preexisting immunity resulted in low survival following challenge. Interestingly, intranasal vaccination protected guinea pigs with systemic preexisting immunity to AdHu5. Mucosal adenoviral immunity induced by intranasal administration of AdHu5 decreased protection following intranasal vaccination with the first-generation but not with the second-generation vaccine.

CONCLUSIONS

Intranasal vaccination is an effective vaccine delivery route in the presence of systemic and, to a lower extent, mucosal preexisting immunity to the vaccine vector in guinea pigs.

Emerging cancer vaccines: the promise of genetic vectors

Therapeutic vaccination against cancer is an important approach which, when combined with other therapies, can improve long-term control of cancer. In fact, the induction of adaptive immune responses against Tumor Associated Antigens (TAAs) as well as innate immunity are important factors for tumor stabilization/eradication. A variety of immunization technologies have been explored in last decades and are currently under active evaluation, such as cell-based, protein, peptide and heat-shock protein-based cancer vaccines. Genetic vaccines are emerging as promising methodologies to elicit immune responses against a wide variety of antigens, including TAAs. Amongst these, Adenovirus (Ad)-based vectors show excellent immunogenicity profile and have achieved immunological proof of concept in humans. In vivo electroporation of plasmid DNA (DNA-EP) is also a desirable vaccine technology for cancer vaccines, as it is repeatable several times, a parameter required for the long-term maintenance of anti-tumor immunity. Recent findings show that combinations of different modalities of immunization (heterologous prime/boost) are able to induce superior immune reactions as compared to single-modality vaccines. In this review, we will discuss the challenges and requirements of emerging cancer vaccines, particularly focusing on the genetic cancer vaccines currently under active development and the promise shown by Ad and DNA-EP heterologous prime-boost.

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

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

Simian recombinant adenovirus delivered by the mucosal route modulates gammadelta T cells from murine genital tract

Although it has been demonstrated that mucosal immunization using vectors such as simian adenovirus (AdC) stimulates robust adaptive immune responses, there remains a paucity of information on the modulation of innate immune responses by such vectors. Using an established murine model of intravaginal immunization (Ivag), we have investigated whether mucosal gammadelta T cells participate in immune responses induced by AdC vectors. gammadelta T cell numbers were found to be increased in the vaginal tract. Moreover, gammadelta T cells isolated from the genital tract showed an activated phenotype and enhanced expression of cytokine gene. Altogether, our results demonstrate that AdC modulates gammadelta T cell responses and suggest that this cell population may influence immune responses following vaginal immunization.

Engineered adenovirus serotypes for overcoming anti-vector immunity

Adenovirus (Ad)-based gene transfer has been successfully utilised in gene therapy and vaccine applications. To date, an increasing number of human clinical trials utilise recombinant Ad-based vectors as a gene transfer platform. In particular, progress has been made recently in utilising Ad-based vectors as a vaccine platform in HIV, cancer immunotherapy approaches and in vaccination for other infections. Despite these successes, the scientific and bio-industrial communities have recently recognised that innate and pre-existing immunity against Ad vectors can constitute a serious obstacle to the development and application of this technology. It is essential to overcome vector-mediated immune responses, such as production of inflammatory cytokines and pre-existing immunity to Ad, because the induction of these responses not only shortens the period of gene expression but also leads to serious side effects. This review focuses on the biology of Ad infection and the approaches that are being adopted to overcome immunity against the Ad-based vectors.

Novel vaccines to human rabies

Rabies, the most fatal of all infectious diseases, remains a major public health problem in developing countries, claiming the lives of an estimated 55,000 people each year. Most fatal rabies cases, with more than half of them in children, result from dog bites and occur among low-income families in Southeast Asia and Africa. Safe and efficacious vaccines are available to prevent rabies. However, they have to be given repeatedly, three times for pre-exposure vaccination and four to five times for post-exposure prophylaxis (PEP). In cases of severe exposure, a regimen of vaccine combined with a rabies immunoglobulin (RIG) preparation is required. The high incidence of fatal rabies is linked to a lack of knowledge on the appropriate treatment of bite wounds, lack of access to costly PEP, and failure to follow up with repeat immunizations. New, more immunogenic but less costly rabies virus vaccines are needed to reduce the toll of rabies on human lives. A preventative vaccine used for the immunization of children, especially those in high incidence countries, would be expected to lower fatality rates. Such a vaccine would have to be inexpensive, safe, and provide sustained protection, preferably after a single dose. Novel regimens are also needed for PEP to reduce the need for the already scarce and costly RIG and to reduce the number of vaccine doses to one or two. In this review, the pipeline of new rabies vaccines that are in pre-clinical testing is provided and an opinion on those that might be best suited as potential replacements for the currently used vaccines is offered.

New insights on adenovirus as vaccine vectors

Adenovirus (Ad) vectors were initially developed for treatment of genetic diseases. Their usefulness for permanent gene replacement was limited by their high immunogenicity, which resulted in rapid elimination of transduced cells through induction of T and B cells to antigens of Ad and the transgene product. The very trait that excluded their use for sustained treatment of genetic diseases made them highly attractive as vaccine carriers. Recently though results showed that Ad vectors based on common human serotypes, such as serotype 5, may not be ideal as vaccine carriers. A recently conducted phase 2b trial, termed STEP trial, with an AdHu5-based vaccine expressing antigens of human immunodeficiency virus 1 (HIV-1) not only showed lack of efficacy in spite of the vaccine's immunogenicity, but also suggested an increased trend for HIV acquisition in individuals that had circulating AdHu5 neutralizing antibodies prior to vaccination. Alternative serotypes from humans or nonhuman primates (NHPs), to which most humans lack pre-existing immunity, have been vectored and may circumvent the problems encountered with the use of AdHu5 vectors in humans. In summary, although Ad vectors have seen their share of setbacks in recent years, they remain viable tools for prevention or treatment of a multitude of diseases.

Strategies to overcome host immunity to adenovirus vectors in vaccine development

The first clinical evaluations of adenovirus (Ad)-based vectors for gene therapy were initiated in the mid-1990s and led to great anticipation for future utility. However, excitement surrounding gene therapy, particularly Ad-based therapy, was diminished upon the death of Jesse Gelsinger, and recent discouraging results from the HIV vaccine STEP trial have brought efficacy and safety issues to the forefront again. Even so, Ad vectors are still considered among the safest and most effective vaccine vectors. Innate and pre-existing immunity to Ad mediate much of the acute toxicities and reduced therapeutic efficacies observed following vaccination with this vector. Thus, innovative strategies must continue to be developed to reduce Ad-specific antigenicity and immune recognition. This review provides an overview and critique of the most promising strategies, including results from preclinical trials in mice and nonhuman primates, which aim to revive the future of Ad-based vaccines.

A preclinical animal model to assess the effect of pre-existing immunity on AAV-mediated gene transfer

Hepatic adeno-associated virus (AAV)-serotype 2-mediated gene transfer results in sustained transgene expression in experimental animals but not in human subjects. We hypothesized that loss of transgene expression in humans might be caused by immune memory mechanisms that become reactivated upon AAV vector transfer. Here, we tested the effect of immunological memory to AAV capsid on AAV-mediated gene transfer in a mouse model. Upon hepatic transfer of an AAV2 vector expressing human factor IX (hF.IX), mice immunized with adenovirus (Ad) vectors expressing AAV8 capsid before AAV2 transfer developed less circulating hF.IX and showed a gradual loss of hF.IX gene copies in liver cells as compared to control animals. This was not observed in mice immunized with an Ad vectors expressing AAV2 capsid before transfer of rAAV8-hF.IX vectors. The lower hF.IX expression was primarily linked to AAV-binding antibodies that lacked AAV-neutralizing activity in vitro rather than to AAV capsid-specific CD8(+) T cells.

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

A novel chimpanzee serotype-based adenoviral vector as delivery tool for cancer vaccines

The use of adenovirus (Ad) as vaccine vectors is hindered by pre-existing immunity to human Ads in most of the human population. In order to overcome this limitation, uncommon alternative Ad serotypes need to be utilized. In this study, an E1-E3 deleted recombinant Ad based on the chimpanzee serotype 3 (ChAd3) was engineered to express human carcinoembryonic antigen (CEA) protein or rat neu extracellular/transmembrane domains (ECD.TM). ChAd3 vectors were tested in CEA transgenic (CEA.Tg) and BALB/NeuT mice, which show immunologic tolerance to these antigens. ChAd3 is capable of inducing an immune response comparable to that of hAd5 serotype-based vectors, thus breaking tolerance to tumor associated antigens (TAAs) and achieving anti-tumor effects. Of importance is that ChAd3 can overcome hAd5 pre-existing immunity and work in conjunction with DNA electroporation (DNA-EP) and other Ad vaccines based on common human serotypes.

Nasal delivery of an adenovirus-based vaccine bypasses pre-existing immunity to the vaccine carrier and improves the immune response in mice

Pre-existing immunity to human adenovirus serotype 5 (Ad5) is common in the general population. Bypassing pre-existing immunity could maximize Ad5 vaccine efficacy. Vaccination by the intramuscular (I.M.), nasal (I.N.) or oral (P.O.) route with Ad5 expressing Ebola Zaire glycoprotein (Ad5-ZGP) fully protected naïve mice against lethal challenge with Ebola. In the presence of pre-existing immunity, only mice vaccinated I.N. survived. The frequency of IFN-γ+ CD8+ T cells was reduced by 80% and by 15% in animals vaccinated by the I.M. and P.O. routes respectively. Neutralizing antibodies could not be detected in serum from either treatment group. Pre-existing immunity did not compromise the frequency of IFN-γ+ CD8+ T cells (3.9±1% naïve vs. 3.6±1% pre-existing immunity, PEI) nor anti-Ebola neutralizing antibody (NAB, 40±10 reciprocal dilution, both groups). The number of INF-γ+ CD8+ cells detected in bronchioalveolar lavage fluid (BAL) after I.N. immunization was not compromised by pre-existing immunity to Ad5 (146±14, naïve vs. 120±16 SFC/million MNCs, PEI). However, pre-existing immunity reduced NAB levels in BAL by ∼25% in this group. To improve the immune response after oral vaccination, the Ad5-based vaccine was PEGylated. Mice given the modified vaccine did not survive challenge and had reduced levels of IFN-γ+ CD8+ T cells 10 days after administration (0.3±0.3% PEG vs. 1.7±0.5% unmodified). PEGylation did increase NAB levels 2-fold. These results provide some insight about the degree of T and B cell mediated immunity necessary for protection against Ebola virus and suggest that modification of the virus capsid can influence the type of immune response elicited by an Ad5-based vaccine.

Adenoviral vectors persist in vivo and maintain activated CD8+ T cells: implications for their use as vaccines

CD8(+) T cell-numbers rapidly expand and then contract after exposure to their cognate antigen. Here we show that the sustained frequencies of transgene product-specific CD8(+) T cells elicited by replication-defective adenovirus vectors are linked to persistence of low levels of transcriptionally active adenovirus vector genomes at the site of inoculation, in liver, and lymphatic tissues. Continuously produced small amounts of antigen maintain fully active effector CD8(+) T cells, while also allowing for their differentiation into central memory cells. The long-term persistence of adenoviral vectors may be highly advantageous for their use as vaccines against pathogens for which T-cell-mediated protection requires both fully activated T cells for immediate control of virus-infected cells and central memory CD8(+) T cells that, because of their higher proliferative capacity, may be suited best to eliminate cells infected by pathogens that escaped the initial wave of effector T cells.

Intramuscular rather than oral administration of replication-defective adenoviral vaccine vector induces specific CD8+ T cell responses in the gut

Gut-associated lymphoid tissue (GALT) is the primary replication site for HIV-1, resulting in a pronounced CD4(+) T cell loss in this tissue during primary infection. A mucosal vaccine that generates HIV-specific CD8(+) T cells in the gut could prevent the establishment of founder populations and broadcasting of virus. Here, we immunized mice orally and systemically with a chimpanzee derived adenoviral vector expressing HIV gag (AdC68gag) and measured frequencies of gag-specific interferon-gamma (IFN-gamma) producing CD8(+) T cells in the GALT. A single oral administration was inefficient at eliciting responses in the mesenteric lymph nodes and Peyer's Patches, while a single intramuscular administration elicited strong systemic and detectable mucosal responses. The gag-specific CD8(+) T cell responses were present in both acute and memory phases following intramuscular administration.