Rapid Cloning of Novel Rhesus Adenoviral Vaccine Vectors

Isolation of a recombinant simian adenovirus encoding the human adenovirus G52 hexon suggests a simian origin for human adenovirus G52

Human adenoviruses (HAdVs) are causative agents of morbidity and mortality throughout the world. These double-stranded DNA viruses are phylogenetically classified into seven different species (A-G). HAdV-G52, originally isolated in 2008 from a patient presenting with gastroenteritis, is the sole human-derived member of species G. Phylogenetic analysis previously suggested that HAdV-G52 may have a simian origin, indicating a potential zoonotic spillover into humans. However, evidence of HAdV-G52 in either human or simian populations has not been reported since. Here, we describe the isolation and in vitro characterization of rhesus (rh)AdV-69, a novel simian AdV with clear evidence of recombination with HAdV-G52, from the stool of a rhesus macaque. Specifically, the rhAdV-69 hexon capsid protein is 100% identical to that of HAdV-G52, whereas the remainder of the genome is most similar to rhAdV-55, sharing 95.36% nucleic acid identity. A second recombination event with an unknown adenovirus (AdV) is evident at the short fiber gene. From the same sample, we also isolated a second, highly related recombinant AdV (rhAdV-68) that harbors a distinct hexon gene but nearly identical backbone compared to rhAdV-69. In vitro, rhAdV-68 and rhAdV-69 demonstrate comparable growth kinetics and tropisms in human cell lines, nonhuman cell lines, and human enteroids. Furthermore, we show that coinfection of highly related AdVs is not unique to this sample since we also isolated coinfecting rhAdVs from two additional rhesus macaque stool samples. Our data collectively contribute to elucidating the origins of HAdV-G52 and provide insights into the frequency of coinfections and subsequent recombination in AdV evolution.IMPORTANCEUnderstanding the host origins of adenoviruses (AdVs) is critical for public health as transmission of viruses from animals to humans can lead to emergent viruses. Recombination between animal and human AdVs can also produce emergent viruses. HAdV-G52 is the only human-derived member of the HAdV G species. It has been suggested that HAdV-G52 has a simian origin. Here, we isolated from a rhesus macaque, a novel rhAdV, rhAdV-69, that encodes a hexon protein that is 100% identical to that of HAdV-G52. This observation suggests that HAdV-G52 may indeed have a simian origin. We also isolated a highly related rhAdV, differing only in the hexon gene, from the same rhesus macaque stool sample as rhAdV-69, illustrating the potential for co-infection of closely related AdVs and recombination at the hexon gene. Furthermore, our study highlights the critical role of whole-genome sequencing in understanding AdV evolution and monitoring the emergence of pathogenic AdVs.

The Biodistribution of Replication-Defective Simian Adenovirus 1 Vector in a Mouse Model

The administration route affects the biodistribution of a gene transfer vector and the expression of a transgene. A simian adenovirus 1 vector carrying firefly luciferase and GFP reporter genes (SAdV1-GFluc) were constructed, and its biodistribution was investigated in a mouse model by bioluminescence imaging and virus DNA tracking with real-time PCR. Luciferase activity and virus DNA were mainly found in the liver and spleen after the intravenous administration of SAdV1-GFluc. The results of flow cytometry illustrated that macrophages in the liver and spleen as well as hepatocytes were the target cells. Repeated inoculation was noneffective because of the stimulated serum neutralizing antibodies (NAbs) against SAdV-1. A transient, local expression of low-level luciferase was detected after intragastric administration, and the administration could be repeated without compromising the expression of the reporter gene. Intranasal administration led to a moderate, constant expression of a transgene in the whole respiratory tract and could be repeated one more time without a significant increase in the NAb titer. An immunohistochemistry assay showed that respiratory epithelial cells and macrophages in the lungs were transduced. High luciferase activity was restricted at the injection site and sustained for a week after intramuscular administration. A compromised transgene expression was observed after a repeated injection. When these mice were intramuscularly injected for a third time with the human adenovirus 5 (HAdV-5) vector carrying a luciferase gene, the luciferase activity recovered and reached the initial level, suggesting that the sequential use of SAdV-1 and HAdV-5 vectors was practicable. In short, the intranasal inoculation or intramuscular injection may be the preferred administration routes for the novel SAdV-1 vector in vaccine development.

A Novel Simian Adenovirus Associating with Human Adeno-virus Species G Isolated from Long-Tailed Macaque Feces

Metagenomics has demonstrated its capability in outbreak investigations and pathogen surveillance and discovery. With high-throughput and effective bioinformatics, many disease-causing agents, as well as novel viruses of humans and animals, have been identified using metagenomic analysis. In this study, a VIDISCA metagenomics workflow was used to identify potential unknown viruses in 33 fecal samples from asymptomatic long-tailed macaques (Macaca fascicularis) in Ratchaburi Province, Thailand. Putatively novel astroviruses, enteroviruses, and adenoviruses were detected and confirmed by PCR analysis of long-tailed macaque fecal samples collected from areas in four provinces, Ratchaburi, Kanchanaburi, Lopburi, and Prachuap Khiri Khan, where humans and monkeys live in proximity (total n = 187). Astroviruses, enteroviruses, and adenoviruses were present in 3.2%, 7.5%, and 4.8% of macaque fecal samples, respectively. One adenovirus, named AdV-RBR-6-3, was successfully isolated in human cell culture. Whole-genome analysis suggested that it is a new member of the species Human adenovirus G, closely related to Rhesus adenovirus 53, with evidence of genetic recombination and variation in the hexon, fiber, and CR1 genes. Sero-surveillance showed neutralizing antibodies against AdV-RBR-6-3 in 2.9% and 11.2% of monkeys and humans, respectively, suggesting cross-species infection of monkeys and humans. Overall, we reported the use of metagenomics to screen for possible new viruses, as well as the isolation and molecular and serological characterization of the new adenovirus with cross-species transmission potential. The findings emphasize that zoonotic surveillance is important and should be continued, especially in areas where humans and animals interact, to predict and prevent the threat of emerging zoonotic pathogens.

Characterization of monkey adenoviruses with three fiber genes

Although the occurrence of three fiber genes in monkey adenoviruses had already been described, the relatedness of the "extra" fibers have not yet been discussed. Here we report the genome analysis of two simian adenovirus (SAdV) serotypes from Old World monkeys and the phylogenetic analysis of the multiple fiber genes found in these and related AdVs. One of the newly sequenced serotypes (SAdV-2), isolated from a rhesus macaque (Macaca mulatta), was classified into species Human mastadenovirus G (HAdV-G), while the other serotype (SAdV-17), originating from a grivet (Chlorocebus aethiops), classified to Simian mastadenovirus F (SAdV-F). We identified unique features in the gene content of these SAdVs compared to those typical for other members of the genus Mastadenovirus. Namely, in the E1B region of SAdV-2, the 19K gene was replaced by an ITR repetition and a copy of the E4 ORF1 gene. Among the 37 genes in both SAdVs, three genes of different lengths, predicted to code for the cellular attachment proteins (the fibers), were found. These proteins exhibit high diversity. Yet, phylogenetic calculations of their conserved parts could reveal the probable evolutionary steps leading to the multiple-fibered contemporary HAdV and SAdV species. Seemingly, there existed (a) common ancestor(s) with two fiber genes for the lineages of the AdVs in species SAdV-B, -E, -F and HAdV-F, alongside a double-fibered ancestor for today's SAdV-C and HAdV-G, which later diverged into descendants forming today's species. Additionally, some HAdV-G members picked up a third fiber gene either to the left-hand or to the in-between position from the existing two. A SAdV-F progenitor also obtained a third copy to the middle, as observed in SAdV-17. The existence of three fiber genes in these contemporary AdVs brings novel possibilities for the design of optimised AdV-based vectors with potential multiple target binding abilities.

Development of Oncolytic Vectors Based on Human Adenovirus Type 6 for Cancer Treatment

Human Adenovirus type 6 (HAdV-C6) is a promising candidate for the development of oncolytic vectors as it has low seroprevalence and the intrinsic ability to evade tissue macrophages. However, its further development as a therapeutic agent is hampered by the lack of convenient cloning methods. We have developed a novel technology when a shuttle plasmid carrying the distal genome parts with modified E1A and E3 regions is recombined in vitro with the truncated HAdV-C6 genome. Using this approach, we have constructed a novel Ad6-hT-GM vector controlled by the hTERT promoter and expressing granulocyte-macrophage colony-stimulating factor (GM-CSF) instead of 6.7K and gp19K E3 proteins. We have demonstrated that control by the hTERT promoter may result in delayed viral replication, which nevertheless does not significantly change the cytotoxic ability of recombinant viruses. The insertion of the transgene by displacing the E3-6.7K/gp19K region does not drastically change the expression patterns of E3 genes; however, mild changes in expression from major late promoter were observed. Finally, we have demonstrated that the treatment of human breast cancer xenografts in murine models with Ad6-hT-GM significantly decreased the tumor volume and improved survival time compared to mock-treated mice.

CELO Fiber1 Knob Is a Promising Candidate to Modify the Tropism of Adenoviral Vectors

Fowl adenovirus 4 (FAdV-4) has the potential to be constructed as a gene transfer vector for human gene therapy or vaccine development to avoid the pre-existing immunity to human adenoviruses. To enhance the transduction of FAdV-4 to human cells, CELO fiber1 knob (CF1K) was chosen to replace the fiber2 knob in FAdV-4 to generate recombinant virus F2CF1K-CG. The original FAdV4-CG virus transduced 4% human 293 or 1% HEp-2 cells at the multiplicity of infection of 1000 viral particles per cell. In contrast, F2CF1K-CG could transduce 98% 293 or 60% HEp-2 cells under the same conditions. Prokaryotically expressed CF1K protein blocked 50% transduction of F2CF1K-CG to 293 cells at a concentration of 1.3 µg/mL while it only slightly inhibited the infection of human adenovirus 5 (HAdV-5), suggesting CF1K could bind to human cells in a manner different from HAdV-5 fiber. The incorporation of CF1K had no negative effect on the growth of FAdV-4 in the packaging cells. In addition, CF1K-pseudotyped HAdV-41 could transduce HEp-2 and A549 cells more efficiently. These data indicated that CF1K had the priority to be considered when there is a need to modify adenovirus tropism.

No Genus-Specific Gene Is Essential for the Replication of Fowl Adenovirus 4 in Chicken LMH Cells

Essential genus-specific genes have not been discovered for fowl adenovirus (FAdV), which hampers the development of FAdV-based vectors and attenuated FAdV vaccines. Reverse genetics approaches were employed to construct FAdV-4 mutants carrying deletions or frameshift mutations covering the whole left and right ends of the viral genome. The results of virus rescue and plaque forming experiments illustrated that all the 22 designated ORFs (open reading frames) were dispensable for the replication of FAdV-4 in chicken hepatoma Leghorn male hepatoma (LMH) cells and primary embryo hepatocytes. RNA-seq data demonstrated that ORF28 and ORF29 were not protein-encoding genes, and suggested a promoter (RP1) and an intron in these regions, respectively. The promoter activity of RP1 was further confirmed by reporter gene expression experiments. GAM-1-deleted FAdV-4 formed small plaques, while deletion of GAM-1 together with ORF22 resulted in even smaller ones in LMH cells. Simultaneous deletion of ORF28, ORF29, and GAM-1 led to growth defect of FAdV-4. These facts implied that genus-specific genes contributed to and synergistically affected viral replication, although no single one was essential. Notably, replication of FAdV-4 mutants could be different in vitro and in vivo. XGAM1-CX19A, a GAM-1-deleted FAdV-4 that replicated efficiently in LMH cells, did not kill chicken embryos because virus propagation took place at a very low level in vivo. This work laid a solid foundation for FAdV-4 vector construction as well as vaccine development, and would benefit viral gene function study. IMPORTANCE Identification of viral essential genes is important for adenoviral vector construction. Deletion of nonessential genes enlarges cloning capacity, deletion of essential genes makes a replication-defective vector, and expression of essential genes in trans generates a virus packaging cell line. However, the genus-specific essential genes in FAdV have not been identified. We constructed adenoviral plasmid carrying deletions covering all 22 genus-specific ORFs of FAdV-4, and found that all virus mutants could be rescued and amplified in chicken LMH cells except those that had defects in key promoter activity. These genus-specific genes affected virus growth, but no single one was indispensable. Dysfunction of several genus-specific genes at the same time could make FAdV-4 vectors replication-defective. In addition, the growth of FAdV-4 mutants could be different in LMH cells and in chicken embryos, suggesting the possibility of constructing attenuated FAdV-4 vaccines.

Restriction-Assembly: A Solution to Construct Novel Adenovirus Vector

Gene therapy and vaccine development need more novel adenovirus vectors. Here, we attempt to provide strategies to construct adenovirus vectors based on restriction-assembly for researchers with little experience in this field. Restriction-assembly is a combined method of restriction digestion and Gibson assembly, by which the major part of the obtained plasmid comes from digested DNA fragments instead of PCR products. We demonstrated the capability of restriction-assembly in manipulating the genome of simian adenovirus 1 (SAdV-1) in this study. A PCR product of the plasmid backbone was combined with SAdV-1 genomic DNA to construct an infectious clone, plasmid pKSAV1, by Gibson assembly. Restriction-assembly was performed repeatedly in the steps of intermediate plasmid isolation, modification, and restoration. The generated adenoviral plasmid was linearized by restriction enzyme digestion and transfected into packaging 293 cells to rescue E3-deleted replication-competent SAdV1XE3-CGA virus. Interestingly, SAdV1XE3-CGA could propagate in human chronic myelogenous leukemia K562 cells. The E1 region was similarly modified to generate E1/E3-deleted replication-defective virus SAdV1-EG. SAdV1-EG had a moderate gene transfer ability to adherent mammalian cells, and it could efficiently transduce suspension cells when compared with the human adenovirus 5 control vector. Restriction-assembly is easy to use and can be performed without special experimental materials and instruments. It is highly effective with verifiable outcomes at each step. More importantly, restriction-assembly makes the established vector system modifiable, upgradable and under sustainable development, and it can serve as the instructive method or strategy for the synthetic biology of adenoviruses.

COVID-19 Vaccines: Adenoviral Vectors

The worldwide pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to the unprecedented pace of development of multiple vaccines. This review evaluates how adenovirus (Ad) vector platforms have been leveraged in response to this pandemic. Ad vectors have been used in the past for vaccines against other viruses, most notably HIV and Ebola, but they never have been produced, distributed, or administered to humans at such a large scale. Several different serotypes of Ads encoding SARS-CoV-2 Spike have been tested and found to be efficacious against COVID-19. As vaccine rollouts continue and the number of people receiving these vaccines increases, we will continue to learn about this vaccine platform for COVID-19 prevention and control.

Protective Efficacy of Rhesus Adenovirus COVID-19 Vaccines against Mouse-Adapted SARS-CoV-2

The global COVID-19 pandemic has sparked intense interest in the rapid development of vaccines as well as animal models to evaluate vaccine candidates and to define immune correlates of protection. We recently reported a mouse-adapted SARS-CoV-2 virus strain (MA10) with the potential to infect wild-type laboratory mice, driving high levels of viral replication in respiratory tract tissues as well as severe clinical and respiratory symptoms, aspects of COVID-19 disease in humans that are important to capture in model systems. We evaluated the immunogenicity and protective efficacy of novel rhesus adenovirus serotype 52 (RhAd52) vaccines against MA10 challenge in mice. Baseline seroprevalence is lower for rhesus adenovirus vectors than for human or chimpanzee adenovirus vectors, making these vectors attractive candidates for vaccine development. We observed that RhAd52 vaccines elicited robust binding and neutralizing antibody titers, which inversely correlated with viral replication after challenge. These data support the development of RhAd52 vaccines and the use of the MA10 challenge virus to screen novel vaccine candidates and to study the immunologic mechanisms that underscore protection from SARS-CoV-2 challenge in wild-type mice. IMPORTANCE We have developed a series of SARS-CoV-2 vaccines using rhesus adenovirus serotype 52 (RhAd52) vectors, which exhibit a lower seroprevalence than human and chimpanzee vectors, supporting their development as novel vaccine vectors or as an alternative adenovirus (Ad) vector for boosting. We sought to test these vaccines using a recently reported mouse-adapted SARS-CoV-2 (MA10) virus to (i) evaluate the protective efficacy of RhAd52 vaccines and (ii) further characterize this mouse-adapted challenge model and probe immune correlates of protection. We demonstrate that RhAd52 vaccines elicit robust SARS-CoV-2-specific antibody responses and protect against clinical disease and viral replication in the lungs. Further, binding and neutralizing antibody titers correlated with protective efficacy. These data validate the MA10 mouse model as a useful tool to screen and study novel vaccine candidates, as well as the development of RhAd52 vaccines for COVID-19.

An update on preclinical pregnancy models of Zika virus infection for drug and vaccine discovery

INTRODUCTION

Congenital Zika syndrome is caused by Zika virus (ZIKV) infection during pregnancy and can culminate in structural and neurological defects in the fetus, including a spectrum of symptoms such as brain calcifications, hydrocephalus, holoprosencephaly, lissencephaly, ventriculomegaly, and microcephaly. Using animal models to study ZIKV infection during pregnancy represents a critical tool for understanding ZIKV pathophysiology, drug testing, vaccine development, and prevention of vertical transmission.

AREAS COVERED

In this review, the authors cover state-of-the-art preclinical pregnancy models of ZIKV infection for drug discovery and vaccine development to prevent vertical transmission.

EXPERT OPINION

The discovery of drugs against ZIKV infection represents an urgent necessity, and until now, no effective drug that can prevent the effects of vertical transmission has been tested in humans. Even after six years of the ZIKV outbreak in Brazil, no drugs or vaccines have been approved for use in humans. In part, this failure could be related to the lack of translatability from available preclinical models to humans.

Protective efficacy of rhesus adenovirus COVID-19 vaccines against mouse-adapted SARS-CoV-2

The global COVID-19 pandemic has sparked intense interest in the rapid development of vaccines as well as animal models to evaluate vaccine candidates and to define immune correlates of protection. We recently reported a mouse-adapted SARS-CoV-2 virus strain (MA10) with the potential to infect wild-type laboratory mice, driving high levels of viral replication in respiratory tract tissues as well as severe clinical and respiratory symptoms, aspects of COVID-19 disease in humans that are important to capture in model systems. We evaluated the immunogenicity and protective efficacy of novel rhesus adenovirus serotype 52 (RhAd52) vaccines against MA10 challenge in mice. Baseline seroprevalence is lower for rhesus adenovirus vectors than for human or chimpanzee adenovirus vectors, making these vectors attractive candidates for vaccine development. We observed that RhAd52 vaccines elicited robust binding and neutralizing antibody titers, which inversely correlated with viral replication after challenge. These data support the development of RhAd52 vaccines and the use of the MA10 challenge virus to screen novel vaccine candidates and to study the immunologic mechanisms that underscore protection from SARS-CoV-2 challenge in wild-type mice.

IMPORTANCE

We have developed a series of SARS-CoV-2 vaccines using rhesus adenovirus serotype 52 (RhAd52) vectors, which exhibits a lower seroprevalence than human and chimpanzee vectors, supporting their development as novel vaccine vectors or as an alternative Ad vector for boosting. We sought to test these vaccines using a recently reported mouse-adapted SARS-CoV-2 (MA10) virus to i) evaluate the protective efficacy of RhAd52 vaccines and ii) further characterize this mouse-adapted challenge model and probe immune correlates of protection. We demonstrate RhAd52 vaccines elicit robust SARS-CoV-2-specific antibody responses and protect against clinical disease and viral replication in the lungs. Further, binding and neutralizing antibody titers correlated with protective efficacy. These data validate the MA10 mouse model as a useful tool to screen and study novel vaccine candidates, as well as the development of RhAd52 vaccines for COVID-19.

Fiber modifications enable fowl adenovirus 4 vectors to transduce human cells

Background

Pre‐existing immunities hamper the application of human adenovirus (HAdV) vectors in gene therapy or vaccine development. Fowl adenovirus (FAdV)‐based vector might represent an alternative.

Methods

An intermediate plasmid containing FAdV‐4 fiber genes, pMD‐FAV4Fs, was separated from FAdV‐4 adenoviral plasmid pKFAV4GFP. An overlap extension polymerase chain reaction (PCR) was employed for fiber modification in pMD‐FAV4Fs, and the modified fibers were restored to generate new adenoviral plasmids through restriction‐assembly. FAdV‐4 vectors were rescued and amplified in chicken LMH cells. Fluorescence microscopy and flow cytometry were used to evaluate the gene transfer efficiency. The amount of viruses binding to cells was determined by a real‐time PCR. A plaque‐forming assay and one‐step growth curve were used to evaluate virus growth.

Results

Four sites in the CD‐, DE‐, HI‐ and IJ‐loop of fiber1 knob could tolerate the insertion of exogenous peptide. The insertion of RGD4C peptide in the fiber1 knob significantly promoted FAdV‐4 transduction to human adherent cells such as 293, A549 and HEp‐2, and the insertion to the IJ‐loop demonstrated the best performance. The replacement of the fiber2 knob of FAdV‐4 with that of HAdV‐35 improved the gene transfer to human suspension cells such as Jurkat, K562 and U937. Fiber‐modified FAdV‐4 vectors could transduce approximately 80% human cells at an acceptable multiplicity of infection. Enhanced gene transfer mainly resulted from increased virus binding. Fiber modifications did not significantly influence the growth of recombinant FAdV‐4 in packaging cells.

Conclusions

As a proof of principle, it was feasible to enhance gene transduction of FAdV‐4 vectors to human cells by modifying the fibers.

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.

Non-Human Primate-Derived Adenoviruses for Future Use as Oncolytic Agents?

Non-human primate (NHP)-derived adenoviruses have formed a valuable alternative for the use of human adenoviruses in vaccine development and gene therapy strategies by virtue of the low seroprevalence of neutralizing immunity in the human population. The more recent use of several human adenoviruses as oncolytic agents has exhibited excellent safety profiles and firm evidence of clinical efficacy. This proffers the question whether NHP-derived adenoviruses could also be employed for viral oncolysis in human patients. While vaccine vectors are conventionally made as replication-defective vectors, in oncolytic applications replication-competent viruses are used. The data on NHP-derived adenoviral vectors obtained from vaccination studies can only partially support the suitability of NHP-derived adenoviruses for use in oncolytic virus therapy. In addition, the use of NHP-derived adenoviruses in humans might be received warily given the recent zoonotic infections with influenza viruses and coronaviruses. In this review, we discuss the similarities and differences between human- and NHP-derived adenoviruses in view of their use as oncolytic agents. These include their genome organization, receptor use, replication and cell lysis, modulation of the host’s immune responses, as well as their pathogenicity in humans. Together, the data should facilitate a rational and data-supported decision on the suitability of NHP-derived adenoviruses for prospective use in oncolytic virus therapy.

Expanding the Spectrum of Adenoviral Vectors for Cancer Therapy

Adenoviral vectors (AdVs) have attracted much attention in the fields of vaccine development and treatment for diseases such as genetic disorders and cancer. In this review, we discuss the utility of AdVs in cancer therapies. In recent years, AdVs were modified as oncolytic AdVs (OAs) that possess the characteristics of cancer cell-specific replication and killing. Different carriers such as diverse cells and extracellular vesicles are being explored for delivering OAs into cancer sites after systemic administration. In addition, there are also various strategies to improve cancer-specific replication of OAs, mainly through modifying the early region 1 (E1) of the virus genome. It has been documented that oncolytic viruses (OVs) function through stimulating the immune system, resulting in the inhibition of cancer progression and, in combination with classical immune modulators, the anti-cancer effect of OAs can be even further enforced. To enhance the cancer treatment efficacy, OAs are also combined with other standard treatments, including surgery, chemotherapy and radiotherapy. Adenovirus type 5 (Ad5) has mainly been explored to develop vectors for cancer treatment with different modulations. Only a limited number of the more than 100 identified AdV types were converted into OAs and, therefore, the construction of an adenovirus library for the screening of potential novel OA candidates is essential. Here, we provide a state-of-the-art overview of currently performed and completed clinic trials with OAs and an adenovirus library, providing novel possibilities for developing innovative adenoviral vectors for cancer treatment.

Site-directed modification of adenoviral vector with combined DNA assembly and restriction-ligation cloning

Commonly used and well accepted approaches are lacking for site-directed modification of adenoviral vectors. Here, we attempt to introduce an easy-to-implement strategy for such purpose with an example of establishing a replication competent adenoviral vector system from pKAd5 plasmid, an infectious clone of human adenovirus 5 (HAdV-5). PCR products of GFP expression cassette and plasmid backbone were fused with the EcoRI/NdeI-digested fragment of pKAd5 to generate a modified intermediate plasmid pMDXE3GA by DNA assembly. NdeI-digested fragment of pMDXE3GA was brought back to pKAd5 to form the adenoviral plasmid pKAd5XE3GA by restriction-ligation cloning. Recombinant adenovirus HAdV5-XE3GA was rescued, amplified and purified. The expression of GFP and the propagation of virus in adherent HEp-2 and suspension K562 cells were investigated. Expression of target gene was significantly enhanced in both cell lines infected with HAdV5-XE3GA due to virus replication. However, propagation of virus could not sustain in culture of K562 cells. Shuttle plasmid pSh5RC-GFP was constructed to facilitate exchange of transgene. In summary, the strategy of combined DNA assembly and restriction-ligation cloning is functional, cost-effective and suitable for genetic modification of adenovirus.

Dose Effects of Recombinant Adenovirus Immunization in Rodents

Recombinant adenovirus type 5 (rAd) has been used as a vaccine platform against many infectious diseases and has been shown to be an effective vaccine vector. The dose of the vaccine varies significantly from study to study, making it very difficult to compare immune responses and vaccine efficacy. This study determined the immune correlates induced by serial dilutions of rAd vaccines delivered intramuscularly (IM) and intranasally (IN) to mice and rats. When immunized IM, mice had substantially higher antibody responses at the higher vaccine doses, whereas, the IN immunized mice showed a lower response to the higher rAd vaccine doses. Rats did not show dose-dependent antibody responses to increasing vaccine doses. The IM immunized mice and rats also showed significant dose-dependent T cell responses to the rAd vaccine. However, the T cell immunity plateaued in both mice and rats at 10⁹ and 10¹⁰ vp/animal, respectively. Additionally, the highest dose of vaccine in mice and rats did not improve the T cell responses. A final vaccine analysis using a lethal influenza virus challenge showed that despite the differences in the immune responses observed in the mice, the mice had very similar patterns of protection. This indicates that rAd vaccines induced dose-dependent immune responses, especially in IM immunized animals, and that immune correlates are not as predictive of protection as initially thought.

Zika virus vaccines

The recent epidemic of Zika virus (ZIKV) in the Americas has revealed the devastating consequences of ZIKV infection, particularly in pregnant women. Congenital Zika syndrome, characterized by malformations and microcephaly in neonates as well as developmental challenges in children, highlights the need for the development of a safe and effective vaccine. Multiple vaccine candidates have been developed and have shown promising results in both animal models and phase I clinical trials. However, important challenges remain for the clinical development of these vaccines. In this Progress article, we discuss recent preclinical studies and lessons learned from first-in-human clinical trials with ZIKV vaccines.

Immunogenicity and Cross-Reactivity of Rhesus Adenoviral Vectors

Adenovirus (Ad) vectors are being investigated as vaccine candidates, but baseline antivector immunity exists in human populations to both human Ad (HuAd) and chimpanzee Ad (ChAd) vectors. In this study, we investigated the immunogenicity and cross-reactivity of a panel of recently described rhesus adenoviral (RhAd) vectors. RhAd vectors elicited T cells with low exhaustion markers and robust anamnestic potential. Moreover, RhAd vector immunogenicity was unaffected by high levels of preexisting anti-HuAd immunity. Both HuAd/RhAd and RhAd/RhAd prime-boost vaccine regimens were highly immunogenic, despite a degree of cross-reactive neutralizing antibodies (NAbs) between phylogenetically related RhAd vectors. We observed extensive vector-specific cross-reactive CD4 T cell responses and more limited CD8 T cell responses between RhAd and HuAd vectors, but the impact of vector-specific cellular responses was far less than that of vector-specific NAbs. These data suggest the potential utility of RhAd vectors and define novel heterologous prime-boost strategies for vaccine development.

IMPORTANCE To date, most adenoviral vectors developed for vaccination have been HuAds from species B, C, D, and E, and human populations display moderate to high levels of preexisting immunity. There is a clinical need for new adenoviral vectors that are not hindered by preexisting immunity. Moreover, the development of RhAd vector vaccines expands our ability to vaccinate against multiple pathogens in a population that may have received other HuAd or ChAd vectors. We evaluated the immunogenicity and cross-reactivity of RhAd vectors, which belong to the poorly described adenovirus species G. These vectors induced robust cellular and humoral immune responses and were not hampered by preexisting anti-HuAd vector immunity. Such properties make RhAd vectors attractive as potential vaccine vectors.