Adenovirus (Ad) as Vaccine-vectors

Advances in recombinant DNA technology, genomics and immunology have greatly affected the vaccine development process. The genetic manipulation of microbes makes them suitable as vectors for gene delivery, providing a unique opportunity for vaccine design. With the in-depth study of various microbes, viral vectors are becoming an important immuno-prophylactic tool in modern medical arsenals. Among various vaccine vectors, adenovirus vectors are being extensively investigated and show great promise as vaccine vectors.

Introduction of Adenovirus Structure

The adenovirus (Ad) virion is a nonenveloped icosahedral particle about 70-90 nm in size with an outer protein shell surrounding an inner nucleoprotein core. Adenoviruses are double-stranded DNA viruses with a genome of 34~43 kb, a size that is amenable for easy manipulation. The facets of the viral capsid are composed primarily of trimers of hexon protein and many other minor components, including protein IIIa (pIIIa), pVI, pVIII, and pIX. The apex of the capsid is composed of a penta-base, which acts to anchor fibrin, which is part responsible for attaching the virion to the cell surface. The genome encodes five early gene transcription units (designated E1a, E1b, E2, E3, and E4) and a late transcription unit that can be subdivided into L1 through L5. The early transcription units play a key role in viral DNA replication and evasion of host immunosurveillance while the late transcription units primarily encode viral structural components.

The structure of adenovirus – Creative Biolabs

Fig.1 The structure of adenovirus.

Adenoviruses as Vaccine Vectors

To date, most efforts have been focused on vectors derived from an adenovirus of the human serotype 5 (AdHu5), which are ultimately used as human vaccines, while some non-human adenoviruses such as bovine, porcine and ovine adenoviruses have also been explored for veterinary use. The early Ad gene transfer vector deletes the E3 domain and its deletion does not cause replication defects in the virus. Most currently used vectors are deleted in E1 and E3 genes. The E1 deletion renders the virus replication-defective, while the E3 deletions may affect the interaction of the vector with the host immune system. Due to the lack of a transcription product of the E3 gene, this effect is expected to be negligible in the E1 deleted Ad vector. Nevertheless, E3 deletion can be used to increase the packaging capacity of the Ad vector. Although E3 is not required for viral replication, E1 is essential and must be trans-complementary in a suitable packaging cell line for viral propagation.

Multiple sites for insertion of foreign genes have been identified in the genome of various Ad. In general, the dual plasmid strategy for homologous recombination in bacteria to produce a full-length recombinant Ad genome followed by transfection of a suitable El complement cell line has become the primary method for Ad vector construction. After co-transfection of the shuttle plasmid and wild-type genomic DNA, the Ad vector can be rescued in a human E1 complement cell line.

Strategies for the Design of Adenoviral Vectors

Two strategies are currently used primarily in the design and development of recombinant adenoviral vectors. The first involves homologous recombination between the adenoviral genome and the shuttle plasmid expressing the transgene cassette in the mammalian system. The second method is to clone the transgene cassette directly into the adenoviral backbone without the need for homologous recombination.

Method 1: homologous recombination in mammalian systems


Development of adenoviral-vectored vaccines by homologous recombination. The most widely used strategy in the generation of E1-deleted human adenoviral vectors (homologous recombination in a viral packaging cell line) is described

Fig.2 Development of adenoviral-vectored vaccines by homologous recombination. The most widely used strategy in the generation of E1-deleted human adenoviral vectors (homologous recombination in a viral packaging cell line) is described. (Afkhami, et al. 2016)

Method 2: direct molecular cloning of the adenoviral genome


Strategies for developing adenoviral vector vaccines by direct cloning. An adenoviral vector is produced by direct in vitro molecular cloning of the entire adenovirus. This represents another strategy for generating adenoviral vectors that bypass the need for homologous recombination.

Fig.3 Strategies for developing adenoviral vector vaccines by direct cloning. An adenoviral vector is produced by direct in vitro molecular cloning of the entire adenovirus. This represents another strategy for generating adenoviral vectors that bypass the need for homologous recombination. (Afkhami, et al. 2016)

The Advantages of Adenovirus Vectors

Ad vectors offer several advantages as vaccine vectors and achieve the most important criteria for an ideal vaccine vector in terms of efficacy, safety, and stability. Specifically includes the following advantages:

  • Can infect a wide range of both actively dividing as well as post-mitotic quiescent mammalian cells.
  • Transgene expression is usually robust and can be further enhanced by the use of strong heterologous promoters.
  • The production of various Ad vectors is fairly simple, and the vector can be easily grown on a large scale in tissue culture.
  • The vector genome does not integrate into the host chromosome and mostly remains epichromosomal, thus making their use safe without the potential risk of insertional mutagenesis.
  • Ad vectors induce strong immunity when administered via parenteral (subcutaneous, intravenous, intramuscular or intraperitoneal) or mucosal (oral or intranasal) routes.

The adenoviral genome has good characteristics and replication-defective viruses can be constructed by deleting key regions of the viral genome, which increases their predictability and reduces unwanted side effects. Creative Biolabs has developed a safe and efficient vaccine vector by genetically modifying the adenoviral genome to lay the foundation for the treatment of complex human diseases.

Reference

  1. Afkhami S, et al. (2016). Methods and clinical development of adenovirus-vectored vaccines against mucosal pathogens. Molecular Therapy Methods & Clinical Development, 3(C):16030.

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