Adenoviruses (Ads) are non-enveloped viruses with a diameter between 90 to 100 nm. The Ad particle consists of an icosahedral protein shell surrounding a protein core that contains a double-stranded DNA (dsDNA) genome. The icosahedral capsid is composed of 252 capsomeres (240 hexons and 12 pentons), and proteins VI, VIII, IX, and IIIa are bridging proteins that stabilize hexon proteins within the capsid. The Ad particle core contains four proteins, of which V, VII, and mu proteins are attached to DNA, and the terminal protein (TP) is covalently attached to the ends of the dsDNA linear genome. The Ad genome contains two inverted terminal repeats (ITRs) at both ends and carries four early transcription units E1 to E4 and five late transcription units L1 to L5.
Ads are classified into seven species A-G. Human Ads are classified into more than 100 subtypes, including serotypes 1-52 and genotypes 53-103. Ads among the different species enter the host cells via different receptors on the cell surface, including the coxsackie/adenovirus receptor (CAR), differentiation cluster 46 (CD46), desmoglein-2 (DSG2), sialic acid (SA), and heparin sulfate proteoglycan (HSPG). Attachment of Ad to the host cell surface is mediated by the interaction between the Ad fiber protein and the receptor. The early stage of Ad infection starts with clathrin-mediated endocytosis, followed by viral DNA translocation to the nucleus, expression of early and late genes, and assembly and release of new viral particles.
Ad vectors are ideal for gene delivery due to their non-integrated nature, large transgene capacity, and the option to produce replication-competent and replication-deficient vectors. Molecular manipulation of the Ad genome is easier than that of other viral vectors. Some Ad vectors are low or non-toxic to humans and possess high transduction efficiency for both replicating and non-replicating cells. Large quantities of Ad vectors can be grown and purified at very high titers at a reasonable cost. In addition, Ad vectors possess the minimal risk of insertional mutagenesis due to their inability to integrate into the host genome. There are different generations of Ad vectors. For the first generation of Ad vectors, the E1 region is completely deleted, so that inserts of up to 5 kb can be placed into the E1 region. To increase the transgene capacity up to 8 kb, the E3 gene region can additionally be deleted. In addition to deletions in the E1 and E3 regions, early gene regions E2 and/or E4 in the second-generation Ad vectors are deleted to increase packaging capacity. The third generation of adenovirus vectors, called gutless or gutted Ad, are deleted of all viral coding sequences, resulting in exogenous DNA packaging up to 36kb.
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