Brief Introduction of Recombinant Lentiviruses

Recombinant lentiviruses (rLVs) can infect both replicating and quiescent cells and have become an important class of gene delivery vectors in research field as well as in gene therapies. rLVs offer the advantage of a large packaging capacity. They can encompass gene encoding sequences, non-coding RNA (shRNA, sgRNA, lncRNA) sequences to overexpress / knock down / knock out target genes in cell cultures or animal models.

To ensure the safety of the users, all rLVs are engineered to be replication incompetent. The components responsible for virus replication has been removed and rLVs are produced by co-transfection of 3 ~ 4 plasmids (transfer, packaging and envelope) containing the components necessary for virus production into the packaging cell line.

rLVs can be further divided into different subgroups based on the viral backbone, envelope glycoprotein and its integration ability.

The most widely used rLVs are human immunodeficiency virus (HIV)-based lentiviruses. Feline Immunodeficiency Virus (FIV), closely related to HIV, has also been engineered into a vector for gene transfer.

The host range of lentiviruses is determined by the envelope glycoprotein which can interact with receptors displaying on host cell surface. The VSV-G (Vesicular Stomatitis Virus Glycoprotein) envelope protein is commonly used in lentiviral particle production because it confers broad tropism over a range of species and cell types. VSV-G also increases viral particle stability which allows virus supernatant to be concentrated through high-speed centrifugation to obtain viruses with high titer. However, VSV-G expression in cells can induce cytotoxicity. Therefore, non-toxic viral envelopes have been investigated. The most prominent alternative that has been identified is feline endogenous retrovirus (RD114) glycoprotein. Compared with VSV-G, this envelope protein has lower transduction efficiency but also possesses a wide host cell tropism and provides increased particle stability. The tropism of rLVs can also be expanded or altered by pseudotyped with other glycoproteins for specific targeting, e.g., using rabies virus-derived glycoprotein for target the central nervous system.

The first developed rLVs can integrate into the host cell chromosome to achieve long-term stable transgene expression from integrated gene cassettes. However, as transgene integration is randomly occurring, it carries a risk of disrupting (silencing or activating) native gene expression through insertional mutagenesis and may change host cell properties or functions. The development of non-integrating rLVs provides us with an opportunity to reduce the risk of random-insertion-induced mutation. The integration is mediated by the lentiviral integrase protein, non-integrating rLVs are obtained by loss-of-function modifications of integrase. Thus, non-integrating rLVs also known as integrase-deficient lentivirus (IDLV). IDLV don't integrate into host cell genome and is beneficial in cases in which transient gene expression is desired.

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