WHAT ARE LENTIVIRUSES? - BIOLOGY - 2025

The lentivirus, from the Latin meaning slow Lenti, are viruses that require a long time from months to years, from initial infection to the onset of the disease. These viruses belong to the genus Lentivirus and to the retroviruses (Retroviridae family), which have an RNA genome that is transcribed into DNA by reverse transcriptase (TR).

In nature, lentiviruses are present in primates, ungulates, and felines. For example, in primates there are two phylogenetically related lineages: simian immunodeficiency viruses (SIV) and human immunodeficiency viruses (HIV). Both are the causative agents of acquired immunodeficiency syndrome (AIDS).

Source: PhD Dre at English Wikipedia

Lentivectors, obtained from lentiviruses, have been widely used for basic research in biology, functional genomics, and gene therapy.

Stages in the life cycle of retroviruses

The life cycle of all retroviruses begins with the body's binding to a specific receptor on the cell surface, followed by internalization of the virus through endocytosis.

The cycle continues with the shedding of the virus coat and the formation of a viral nucleus protein complex (VNC), which consists of the viral genome associated with viral and cellular proteins. The composition of the complex changes over time and is related to the conversion, by TR, of the invader genome into a DNA double helix.

The integration of the virus genome into that of the cell will depend on the ability of the viral genome to penetrate the host nucleus. VNC reorganization plays an important role in importation into the nucleus, although important cellular proteins, such as transportin-SR2 / TNPO3, importin-alpha3, and importin7 also play a role.

Viral proteins, such as integrase, and host cell transcription factors, such as LEDCF, are key in the integration of the viral genome.

This uses the host cell machinery to transcribe and translate viral proteins and to assemble virions, releasing them into extracellular space.

From lentivirus to lentivector

The genome of retroviruses has three open reading frames (MLA) for the different viral elements. For example, capsidia and matrix (gag gene), enzymes (pol gene), and envelope (env gene).

The construction of a viral vector consists of the elimination of some genes of the wild virus, such as those related to virulence. In this way, a viral vector can infect eukaryotic cells, retro-transcribe, integrate into the genome of the host eukaryotic cell, and express the transgene (inserted therapeutic gene) without causing disease.

One method of lentivector construction is transient transfection. It is based on the use of viral minigenomes (called constructs) that carry only the genes of interest. Transient transfection consists of the independent delivery of constructs.

Some retrovectors have only main elements for the assembly of viral particles, called non-functional retrovectors. They are used to transfect packaging cells.

Vectors with a transgene expression cassette are capable of infecting, transforming cells (transduction), and expressing the transgene.

The use of separate constructs is intended to avoid recombination events that could restore the wild type phenotype.

Lentivector technology

Lentivector technology has wide use in basic biology and translational studies for stable transgene overexpression, site-directed gene editing, persistent gene silencing, stem cell modification, transgenic animal generation, and induction of pluripotent cells.

Lentivectors are easy to handle and produce systems. They are irreversibly and securely integrated into the host genome. They infect cells that are or are not dividing.

They show tropism towards certain tissues, facilitating therapy. They do not express viral proteins, which is why they have low immunogenicity. They can send complex genetic elements.

In basic research, HIV-based lentivectors have been used as RNA interference delivery systems (RNAi) to eliminate the function of a specific gene, thus allowing the study of interaction with other different ones.

Lentivectors obtained from HIV

In the early 1990s, the first lentivectors were built from HVI-1, which is closely related to chimpanzee SIV. HVI-1 is responsible for AIDS worldwide.

The first generation of lentivectors has a significant part of the HIV genome. It includes gal and pol genes, and several additional viral proteins. This generation was created using two constructs. One of them, which expresses Env, supplies the packaging functions. Another expresses all MLAs, with the exception of Env.

The transfer vector consists of an expression cassette marked by two types of long repeats (LTRs) and genes necessary for packaging and reverse transcription.

The second generation of packaging vectors lack most accessory genes and retain Tat and Rev. These genes were removed in the third generation and provided by a fourth construct.

Third generation transfer vectors are composed of two packaging constructs. One encodes gal and pol. Another encodes rev. A third construct encodes the envelope, which is derived from VSV-G. The one encoding the gene of interest contains inactivated LTR lentiviral sequences to prevent recombination.

In the latter case, the transcriptional regulatory elements increase the performance of the transfer genes.

Lentivectors obtained from other viruses

The HIV-2 virus is closely related to the gray magabey SIV (SIV _SM), and is responsible for AIDS in West Africa. First and second generation vectors have been obtained from this virus.

In a similar way to HVI-1, vectors have been constructed from SIV _SM, EIAV (infectious anemia virus in horses), FIV (feline immunodeficiency virus) and BIV (bovine immunodeficiency virus (BIV) three generations EIAV-based vectors have been developed for clinical use.

First and third generation vectors have been constructed from the caprine arthritis-encephalitis virus (CAEV). While first generation vectors have been constructed from the SIV of the African green monkey.

References

1. Da Silva, FH, Dalberto, TP, Beyer Nardi, N. 2006. Beyond retrovirus infection: HIV meets gene therapy, Genetics and Molecular Biology, 29, 367–379.
2. Durand, S., Cimarelli, A. 2011. The Inside Out of Lentiviral Vector. Viruses, 3: 132-159.
3. Mátrai, J., Chuah, MKL, Van den Driessche, T. 2010. Recent advances in lentiviral vector development and applications. Molecular Therapy, 18: 477–490.
4. Milone, MC, O'Doherty, U. 2018. Clinical use of lentiviral vectors. Leukemia, 32, 1529–1541.
5. Sakuma, T., Barry, MA, Ikeda, Y. 2012. Lentiviral vectors: basic to translational. Biochemical Journal, 443, 603-618.