Lentivirus is a genus of viruses of the Retroviridae family, characterized by a long incubation
period. Lentiviruses can deliver a significant amount of viral RNA into the DNA of the host
cell and have the unique ability among retroviruses of being able to infect non-dividing cells,
so they are one of the most efficient methods of a gene delivery vector. HIV, SIV, FIV,
EIAV, and Visna are all examples of lentiviruses.
Classification Five serogroups of lentiviruses are recognized,
reflecting the vertebrate hosts with which they are associated. The primate lentiviruses
are distinguished by the use of CD4 protein as a receptor and the absence of dUTPase.
Some groups have cross-reactive gag antigens. Antibodies to gag antigens in lions and other
large felids indicate the existence of other viruses related to FIV and the ovine/caprine
lentiviruses. Description is on taxonomic level of genus.
Morphology The virions are enveloped, slightly pleomorphic,
spherical and measure 80–100 nm in diameter. Projections of envelope make the surface appear
rough, or tiny spikes may be dispersed evenly over the surface. The nucleocapsids are isometric.
The nucleoids are concentric and rod-shaped, or shaped like a truncated cone.
Genome organization and replication Features of the genome: infectious viruses
have three main genes coding for the viral proteins in the order: 5´-gag-pol-env-3´.
There are two regulatory genes, tat and rev. There are additional accessory genes depending
on the virus whose products are involved in regulation of synthesis and processing viral
RNA and other replicative functions. The Long terminal repeat is about 600 nt long, of which
the U3 region is 450, the R sequence 100 and the U5 region some 70 nt long.
Viral proteins involved in early stages of replication include Reverse Transcriptase
and Integrase. Reverse Transcriptase is the virally encoded RNA-dependent DNA polymerase.
The enzyme uses the viral RNA genome as a template for the synthesis of a complementary
DNA copy. Reverse transcriptase also has RNaseH activity for destruction of the RNA-template.
Integrase binds both the viral cDNA generated by reverse transcriptase and the host DNA.
Integrase processes the LTR before inserting the viral genome into the host DNA. Tat acts
as a trans-activator during transcription to enhance initiation and elongation. The
Rev responsive element acts post-transcriptionally, regulating mRNA splicing and transport to
the cytoplasm. Proteome
The lentiviral proteome consists of five major structural proteins and 3-4 non-structural
proteins. Structural proteins listed here by protein size:
Gp120 glycosylated surface envelope protein SU, encoded by the viral gene env. Largest
120000 Da. Gp41 glycosylated transmembrane envelope protein
TM, also encoded by the viral gene env. 2nd largest 41000 Da.
P24 non-glycosylated capsid protein CA, encoded by the viral gene gag. 3rd largest 24000 Da.
P17 non-glycosylated matrix protein MA, also encoded by gag. 4th largest 17000 Da.
Non-glycosylated capsid protein NC, also encoded by gag. 5th largest 7000-11000 Da.
The envelope proteins SU and TM are glycosylated in at least some lentiviruses, if not all
of them. Glycosylation seems to play a structural role in the concealment and variation of antigenic
sites necessary for the host to mount an immune system response.
Non-structural proteins listed here: Reverse transcriptase encoded by the pol gene.
Protein size 66000 Da. Integrase IN also encoded by the pol gene.
Protein size 32000 Da. Protease PR encoded by the pro gene. dUPTase
DU, the role of which is still unknown. Protein size 14000 Da.
Antigenic properties Serological Relationships: Antigen determinants
are type-specific and group-specific. Antigen determinants that possess type-specific reactivity
are found on the envelope. Antigen determinants that possess type-specific reactivity and
are involved in antibody mediated neutralization are found on the glycoproteins. Cross-reactivity
has been found among some species of the same serotype, but not between members of different
genera. Classification of members of this taxon is infrequently based on their antigenic
properties. Biological
Symptoms and host range: The virus' hosts are found in the orders Primates, Perissodactyla,
Carnivora, and Artiodactyla Transmission: Transmitted by means not involving
a vector. Geographic distribution: Worldwide.
Physicochemical and physical properties General
Buoyant density 1.16–1.18 g cm−3 in sucrose Virions sensitive to heat, detergents, and
formaldehyde Infectivity not affected by irradiation
Classed as having class C morphology Nucleic Acid
Virions contain 2% nucleic acid Genome consists of a dimer
Virions contain one molecule of linear positive-sense single stranded RNA.
Total genome length is of one monomer 9200 nt
Genome sequence has terminal repeated sequences; long terminal repeats (of about 600 nt
The 5' end of the genome has a cap Cap sequence of type 1 m7G5ppp5'GmpNp
3' end of each monomer has a poly tract; 3'-terminus has a tRNA-like structure
Encapsidated nucleic acid are solely genomic 2 copies packed per particle.
There are 11 proteins Virions contain 60% protein
Fivestructural virion proteins have been found so far
Lipids: Virions contain 35% lipid. Carbohydrates: Other compounds detected in
the particles 3% carbohydrates. Practical applications
Lentivirus is primarily a research tool used to introduce a gene product into in vitro
systems or animal models. Large-scale collaborative efforts are underway to use lentiviruses to
block the expression of a specific gene using RNA interference technology in high-throughput
formats. The expression of short-hairpin RNA reduces the expression of a specific gene,
thus allowing researchers to examine the necessity and effects of a given gene in a model system.
These studies can be a precursor to the development of novel drugs which aim to block a gene-product
to treat diseases. Another common application is to use a lentivirus
to introduce a new gene into human or animal cells. For example, a model of mouse hemophilia
is corrected by expressing wild-type platelet-factor VIII, the gene that is mutated in human hemophilia.
Lentiviral infection has advantages over other gene-therapy methods including high-efficiency
infection of dividing and non-dividing cells, long-term stable expression of a transgene,
and low immunogenicity. Lentiviruses have also been successfully used for transfection
of diabetic mice with the gene encoding PDGF, a therapy being considered for use in humans.
These treatments, like most current gene therapy experiments, show promise but are yet to be
established as safe and effective in controlled human studies. Gammaretroviral and lentiviral
vectors have so far been used in more than 300 clinical trials, addressing treatment
options for various diseases. Notes
References Ryan KJ, Ray CG, ed.. Sherris Medical Microbiology:
An Introduction to Infectious Diseases. New York: McGraw Hill. ISBN 0-8385-8529-9.
Desport, M, ed.. Lentiviruses and Macrophages: Molecular and Cellular Interactions. Caister
Academic Press. ISBN 978-1-904455-60-8. Further reading
ICTV taxonomy of Lentivirus "Lentiviruses In Ungulates. I. General Features,
History And Prevalence". Bulgarian Journal of Veterinary Medicine 9: 175–181. 2006.
Tim Ravenscroft. "Are Lentiviral Vectors on Cusp of Breakout?". Genetic Engineering & Biotechnology
News. pp. 54–55. Retrieved 2008-07-06. "(subtitle) Rapidly emerging technology has
potential to treat hemophilia, AIDS, and Cancer" External links
Viralzone: Lentivirus