Taxonomy and Epidemiology
The causative agent belongs to the family Retroviridae and the genus Lentivirus (lymphotropic viruses). The infection has a tendency toward rapid epidemic spread. Currently, two main variants of the virus are distinguished, which differ in their structural and antigenic characteristics:
- HIV-1: Discovered in 1983 by the French virologist Luc Montagnier. It is the primary causative agent of the infection worldwide.
- HIV-2: Discovered in 1985 in West Africa.
Virion Ultrastructure
HIV is a complex virus. Its structure is best examined from the center to the outer envelopes:
- Nucleocapsid (Core): Has a truncated cone shape and is built from approximately 2,000 copies of the p24 protein. Inside the capsid are packaged two molecules of +ssRNA, nucleoproteins (p7, p6, and p9 proteins), and an essential enzymatic complex comprising reverse transcriptase, integrase, and protease.
- Matrix Layer: Located directly beneath the capsid. Formed by the matrix protein p17.
- Supercapsid (Envelope): Represents a bilayer lipid membrane that the virus acquires from the host cell membrane during budding. The membrane is penetrated by exactly 72 glycoprotein spikes (trimers). Each such spike consists of a transmembrane glycoprotein gp41 embedded in the envelope and a surface glycoprotein gp120 projecting outward.
Genome Organization
The HIV genome is represented by two RNA strands. All genetic information is distributed among 3 structural and 7 regulatory genes (tat, rev, nef, vif, vpr, vpu, vpx), which control reproduction processes and pathogenesis.
Key structural genes:
- gag (group antigen): encodes the synthesis of internal structures (matrix, capsid, and nucleocapsid proteins).
- pol: responsible for the formation of replication enzymes (reverse transcriptase/revertase, integrase, protease, RNase).
- env: encodes external envelope proteins—glycoproteins gp120 and gp41.
Antigenic Structure and Variability
In response to the infection, the human body produces antibodies against all products of the env, gag, and pol genes. The group-specific antigen p24 elicits the most significant immune response.
The main antigenic determinant of the virus is the surface glycoprotein gp120, which contains a domain for binding to the CD4 receptor and coreceptors on the surface of target cells.
Important feature: The region of the env gene encoding gp120 is hypervariable. Due to extremely high mutability during each viral replication cycle, new antigenic variants of gp120 are formed. This allows the pathogen to effectively evade the immune system.
HIV Variants and Classification
The primary causative agent (HIV-1) is classified based on the env gene. It is divided into three groups: M, N, and O. Groups N and O are localized primarily in West Africa, whereas group M dominates the rest of the world and includes 10 subtypes (from A to K). Subtype A prevails in Eastern Europe and parts of Asia.
HIV-1 and HIV-2 have a number of significant genetic and antigenic differences:
| Structure | HIV-1 | HIV-2 |
|---|---|---|
| Regulatory gene | vpu | vpx |
| Surface glycoprotein | gp120 | gp140 / gp105 |
| Transmembrane glycoprotein | gp41 | gp36 |
| Capsid protein | p24 | p26 |
Cultivation and Resistance
HIV is incapable of growing on standard nutrient media; its propagation requires special cell cultures.
The virus is relatively sensitive to environmental factors. It is destroyed by sunlight, UV radiation, 70% alcohol, and all standard disinfectants. Heating to 56 °C causes a sharp decline in infectivity, and a temperature of 70 °C inactivates the virus within 10 minutes.
Nevertheless, the virus can persist for a long time in biological fluids:
- In a frozen state (in semen or serum)—survives for several years.
- In dried plasma at room temperature (23–27 °C)—retains infectious activity for up to 7 days.