Human immunodeficiency viruses (HIV)

Biology : Viruses

Classification: Group VI virus 

Commonly known as HIV.

This refers to a pair of closely related viruses known as HIV1 and HIV2.

 

Human pathogenesis

• HIV infection
• Acquired immunodeficiency syndrome

 

T lymphocyte physiology

The cells of the human body have evolved to display their infected status to immune cells. For intracellular pathogens, the immune system has a specialised answer: T lymphocytes (particularly CD4+ and CD8+ cells).

• CD4+ (helper) T cells spread information between immune cells. The information is specific to the antigen (molecular signature) of the pathogen. 
• CD8+ (cytotoxic) cells recognise dysfunctional host cells and kill them.

Active T lymphocytes can form an immunological synapse with naive T lymphocytes for information transmission.

 

HIV pathophysiology

HIV is a relatively successful pathogen. It achieved success by mastering the tool of targeted immunosuppression.

Most viruses replicate inside non-immune cells (e.g. epithelial cells), but HIV dedicated its replication toolkit to target immune cells. Many types of cell (e.g. macrophages) can become infected, but the primary target is the CD4+ T lymphocyte. In HIV infection, the immunological synapse provides a bridge between non-infected T cells and infected T cells.

As the viral titre (population in the blood) rises, the CD4+ T lymphocyte count falls. As the lymphocyte count falls, the antiviral defences of the body weaken. The body continuously generates new lymphocytes, replenishing the supply of HIV incubators. The host is asymptomatic for a relatively long time, but their ability to clear HIV decreases. This provides sufficient time for existing hosts to unknowingly transmit HIV to new hosts. Eventually the host becomes symptomatic.

 

Rapid evolution

HIV has a fast replication cycle and up to 10¹⁰ new virions are generated within a host every day. Combined with a high mutation rate, the result is very high genetic variability.

Antiviral therapy with a single agent would provide a selection pressure to the large population and resistance against this agent would rapidly evolve. The new strains could be transmitted to a new host, rendering the agent useless on a global scale.

 

Taxonomy

• Family: Retrovirus
• Genus: Lentivirus
• Species:
• HIV-1: High virulence, high infectivity and global prevalence
• HIV-2: Low virulence, low infectivity and West African prevalence

 

Origin

HIV is closely related to SIV (simian immunodeficiency virus). It likely spread to humans (zoonosis) from the ingestion of infected meat. Alternatively both viruses may be descendants of a virus which affected the common ancestors of all primate families.