Lentiviruses are single stranded RNA (ssRNA) viruses that belong to the retrovirus family. Retroviruses contain reverse transcriptase that converts the retroviral RNA genome into double-stranded DNA. The DNA then migrates to the nucleus, where it integrates with the host genome. Lentiviruses contain two copies of positive stranded RNA encompassed by capsid and envelope with a diameter between 80 to 120 nm. All retroviruses encode three equivalent genes, including gag, pol, and env, which are flanked by long terminal repeats (LTRs). The three primary genes, gag, pol, and env, are respectively responsible for encoding the viral core, reverse transcriptase, integrase and protease, and viral envelope protein, which are necessary for viral replication, integration, and expression. Unlike simple retroviruses, lentiviruses possess vif, vpr, vpu, and nef as accessory genes and tat and rev as regulatory genes, which encode proteins that contribute to infection, binding, and release.
Lentiviruses hold significant inherent advantages as gene delivery vectors because of their ability to transduce non-dividing cells, permanently transform target cell genomes, and allow stable, long-term transgene expression. Unlike most retroviruses that infect only dividing cells, lentiviruses are capable of infecting both dividing and non‐dividing cells. Especially, lentiviral vectors show an enhanced proneness to transduce terminally differentiated tissues from neuronal origin and the normal function of infected cells is unaffected both in vitro and in vivo. Long-term expression of the transgene can be achieved using lentiviral vectors because the lentivirus is integrated into the host genome. In addition, lentiviral vectors can accommodate relatively large sequences, and most of the transferred genes can be cloned into lentiviruses, which expands the scope of application of lentiviral vectors. A major advantage of lentiviruses is that they are not inherently immunogenic and rarely cause immune response from the host. In addition, lentiviral vectors are compatible with a variety of transcription promoters, including promoters of housekeeping genes and promoters of specific cells or tissues.
The first generation of lentiviral vectors consists of three plasmids, one carrying the gene of interest, one containing all essential genes except env, and one containing env. The second-generation lentiviral vectors maintain the three-plasmid system and the safety is improved by removing accessory genes that are not essential for vector production. In the second-generation vectors, regulatory genes are still maintained. The third-generation lentiviral vectors further improve safety by splitting the viral genome into separate plasmids. In this system, the gag and pol genes are encoded on a different plasmid from that of the rev or env genes, resulting in a vector consisting of three separate plasmids containing the viral sequences required for packaging.
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