Adenovirus Vector System and Vaccine Development

Adenovirus Vector

Adenovirus (AdV) is an enveloped virus with icosahedral particles, 70-90 nanometers in diameter, containing a linear double-stranded DNA genome of about 36 kb. AdV can be isolated from a variety of vertebrate hosts including humans, mice and non-human primates. Humans infected with AdV present with flu-like symptoms, sore throat, diarrhea and vomiting.

The traditional AdV vector is composed of species C adenovirus serotype 5 (Ad5). The E1A gene is located next to the left inverted terminal repeat (ITR), which is the first gene to be transcribed after infection. It is essential for viral replication, and can activate the early viral gene transcription. Therefore, deletion of E1A renders the AdV vector incapable of replication. The recombinant AdV vector usually delete the E3 gene from the virus genome to improve the ability of transgene insertion, and transgene expression cassettes are inserted into E1 or E3 deleted regions. The recombinant AdV vector can stably express Ad5 E1 protein in HEK293 cells.

Figure 1. Schematic diagram of an Ad vector vaccine
(Source: Sakurai F, et al. 2022)

As research progressed, the AdV genome was progressively modified to improve safety and therapeutic efficacy. The immune response was reduced by progressively removing non-essential DNA regions from the replication region (first generation) to the gene (third generation). As a result, the insertion capacity of the AdV gene was improved from approximately 7kb to 36kb.

Adenovirus Vector Vaccine

Based on the interaction between AdV and the immune system, adenovirus vectors have been used to develop vaccines against viral pathogens. This class of vaccines deletes the E1 gene of adenovirus resulting in defective viral replication, which impairs the ability of AdV to multiply in host cells without affecting the infectivity of the virus, and then inserts a transgenic expression cassette into the D1 locus. The commonly used genetic engineering methods for inserting antigens are BAC-based recombinant engineering, plasmid-based homologous recombination in E. coli, and in vitro Gateway recombination. Human Ad5 vector was the earliest AdV vector used in preclinical and clinical vaccine trials. Due to the existence of neutralizing antibodies against human Ad5, researchers began to use non-human AdVs, such as chimpanzee adenovirus vectors (ChAds), porcine adenovirus vectors, and ovine adenovirus vectors. Among them, ChAds are most commonly used in clinical trials, are easily cultured in human cell lines, and have a low seropositivity rate in humans. ChAds have been used to develop vaccines against a wide range of infectious diseases, including AIDS, Ebola, hepatitis C, Zika, Middle East Respiratory Syndrome Coronavirus and Severe Acute Respiratory Syndrome Coronavirus.

Ebola/COVID-19 vaccine

The Ebola vaccine is the first adenovirus vaccine approved for human prevention purposes. An AdV serotype 26 (Ad26) vector expressing Ebola virus glycoproteins has been approved by the European Union in a program that uses the Ad26 vector as the primary vaccine and varicella virus encoding glycoproteins from Ebola, Sudan, Marburg, and Tai Forest viruses nucleoprotein as a booster vaccine. In addition, ChAd3 vector vaccine and Ad5 vector vaccine expressing Ebola virus glycoproteins have been shown to be effective in inducing anti-Ebola virus immunity.

Most of the COVID-19 vaccine candidates currently being evaluated in large-scale clinical trials utilize recombinant adenovirus vector platform technologies, such as HAd5, HAd26, and ChAdOx-1 vectors. Most of these vaccine candidates are designed to express the S protein or RBD of SARS-CoV-2. The natural tropism of adenoviruses for respiratory mucosa is an advantage in the development of adenovirus vector-based COVID-19 vaccines, but the efficacy of HAd5-based vaccine may be reduced because there may be pre-immunization against human adenovirus. To overcome this deficiency, the researchers used adenovirus vectors from other species. The results of research on ChAdOx-1 show that it has safety and immunogenicity.

Influenza vaccine

The widely used traditional influenza virus vaccines (live attenuated virus vaccine, inactivated virus vaccine, split virus vaccine) have relatively low immune efficiency, narrow coverage, and only target specific strains. Adenovirus vectors can stimulate humoral immunity and induce broad cross-reactive T cell immunity against conserved antigens, and many studies have used adenovirus vector platforms to develop universal influenza vaccines. For example, antibodies are induced by targeting the HA stalk region or extracellular region of relatively conserved M2 ion channels, or by targeting NP or M1 to induce a wide range of cross-reactive T cells. Influenza vaccines based on HAd5 expressing HA have demonstrated safety and immunogenicity through human clinical trials. The ideal influenza vaccine should induce both broad cross-reactive T cell immunity and neutralizing antibodies, and this protective immunity can be sustained for a long time, and in this regard, the development of influenza vaccines based on adenovirus vector platforms is a strong choice.

HIV-1 vaccine

The high immunogenicity of adenovirus vectors enhances the efficacy of HIV-1 vaccines, and studies have demonstrated that adenovirus vector-based HIV-1 vaccines show very good preventive effects against infection in non-human primates. When a large-scale STEP trial of an HAd5-based vaccine was conducted in humans, instead of the expected efficacy of the vaccine, there was a trend toward increased HIV-1 infection in HAd5-seropositive volunteers. Although the true cause of the results has not been explained, the researchers suggest that vaccination-induced CD4 T-cell activation may have contributed to the increase in HIV-1 infection.

Adenovirus ELISA Kits

Adenovirus Antigens

Adenovirus Antibodies