HIV

The human immunodeficiency virus, commonly called HIV, is a retrovirus that primarily infects vital components of the human immune system such as CD4+ T cells, macrophages and dendritic cells. It also directly and indirectly destroys CD4+ T cells. As CD4+ T cells are required for the proper functioning of the immune system, when enough CD4+ cells have been destroyed by HIV, the immune system barely works, leading to AIDS. HIV also directly attacks certain human organs, such as the kidneys, the heart and the brain leading to acute renal failure, cardiomyopathy, dementia and encephalopathy. Many of the problems faced by people infected with HIV results from the failure of the immune system to protect them from certain opportunistic infections and cancers.

Genetic variability of HIV

One of the major characteristics of HIV is its high genetic variability as a result of its fast replication cycle and the high error rate and recombinogenic properties of reverse transcriptase. This means that different genomic combinations may be generated within an individual who is infected by genetically different HIV strains. Recombination results when a cell is simultaneously infected by two different strains of HIV and one RNA transcript from two different viral strains are encapsidated into the same virion particle. This virion then infects a new cell where it undergoes replication. During this phase, the reverse transcriptase, by jumping back and forth between the two different RNA templates, will generate a newly synthesized retroviral DNA sequence that is a recombinant between the two parental genomes. This recombination is most obvious when it occurs between subtypes.

Related Topics:
Replication cycle - Genomic

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Three groups of HIV-1 have been identified on the basis of differences in env: M, N and O {{ref|Thomson}}(Figure 7). Group M is the most prevalent and is subdivided into eight subtypes, based on the whole genome, that are each geographically distinct {{ref|Carr}}. The most prevalent are subtypes B (found predominantly in North America and Europe), A and D (found predominantly in Africa), and C (found predominantly in Africa and Asia) (Figure 8); these subtypes form branches in the phylogenetic tree representing the lineage of the M group of HIV-1 (Figure 7). Coinfection with distinct subtypes gives rise to circulating recombinant forms (CRFs).

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The first CRFs to be isolated were the AG recombinant from west and central Africa, the AGI recombinant from Cyprus and Greece, the AB recombinant from Russia, and the AE virus from Southeast Asia. However, the parent subtype E of CRF01_AE has not yet been identified and its status as a recombinant is in dispute.

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In 2000, the last year in which an analysis of global subtype prevalence was made, 47.2% of infections worldwide were of subtype C, 26.7% were of subtype A/CRF02_AG, 12.3% were of subtype B, 5.3% were of subtype D, 3.2% were of CRF_AE, and the remaining 5.3% were composed of other subtypes and CRFs {{ref|Osmanov}} (Figure 8). Almost 95% of all HIV research currently taking place is focused on subtype B, while a few laboratories focus on other subtypes.

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