Lentivirus

Classification

Five serogroups of lentiviruses are recognized, reflecting the vertebrate hosts with which they are associated (primates, sheep and goats, horses, domestic cats, and cattle). 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 (e.g., the ovine, caprine, and feline lentiviruses). Antibodies to gag antigens in lions and other large felids indicate the existence of other as yet unidentified viruses related to feline lentivirus and the ovine/caprine lentiviruses.

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 (about 8 nm) may be dispersed evenly over the surface. The nucleocapsids (cores) 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 (e.g., for HIV-1: vif, vpr, vpu, nef) whose products are involved in regulation of synthesis and processing viral RNA and other replicative functions. The Long terminal repeat (LTR) 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 (3 in the primate lentiviruses).

Structural proteins listed by size:

1. Gp120 surface envelope protein SU, encoded by the viral gene env. 120000 Da (Daltons).
2. Gp41 transmembrane envelope protein TM, also encoded by the viral gene env. 41000 Da.
3. P24 capsid protein CA, encoded by the viral gene gag. 24000 Da.
4. P17 matrix protein MA, also encoded by gag. 17000 Da.
5. P9 capsid protein NC, also encoded by gag. 7000-11000 Da.

Enzymes:

1. Reverse transcriptase RT encoded by the pol gene. Protein size 66000 Da.
2. Integrase IN also encoded by the pol gene. Protein size 32000 Da.
3. Protease PR encoded by the pro gene.
4. dUTPase DU encoded by the pro gene (part of pol gene in some viruses), the role of which is still unknown. Protein size 14000 Da.

Gene regulatory proteins:

1. Tat
2. Rev

Accessory proteins:

1. Nef
2. Vpr
3. Vif
4. Vpu/Vpx
5. p6

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.

Epidemiology

Physicochemical and physical properties

Classed as having class C morphology

Use as gene delivery vectors

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 (shRNA) 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 (platelet-derived growth factor), 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.