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.

Life cycle of HIV

Viral entry to the cell

The interaction between the gp120, coreceptor and CD4 provokes conformational changes in gp120 that exposes a previously buried portion of the transmembrane glycoprotein, gp41, and allows access of the V3 loop of gp120 to the coreceptor. gp41 causes the fusion of the viral envelope and the host-cell envelope, allowing the capsid to enter the target cell. The exact mechanism by which gp41 causes the fusion is still largely unknown {{ref|Chan}}{{ref|Wyatt}}.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Once HIV has bound to the CD4+ T-cell a viral protein known as gp41 penetrates the cell membrane and the HIV RNA and various enzymes including but not limited to reverse transcriptase, integrase and protease are injected into the cell.

Related Topics:
RNA - Enzymes

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Viral replication and transcription

Once the viral capsid has entered the cell, an enzyme called reverse transcriptase liberates the single-stranded (+)RNA from the attached viral proteins and copies it into a negatively sensed viral complementary DNA of 9 kb pairs (cDNA) (Figure 5). This process of reverse transcription is extremely error prone and it is during this step that mutations (such as drug resistance) are likely to arise. The reverse transcriptase then makes a complementary DNA strand to form a double-stranded viral DNA intermediate (vDNA). This new vDNA is then transported into the nucleus. The integration of the proviral DNA into the host genome is carried out by another viral enzyme called integrase. This is called the latent stage of HIV infection {{ref|Zheng}}.

Related Topics:
Enzyme - Reverse transcriptase - RNA - DNA - Nucleus - Genome - Integrase

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

To actively produce virus, certain transcription factors need to be present in the cell. The most important is called NF-kB (NF Kappa B) and is present once the T cells becomes activated. This means that those cells most likely to be killed by HIV are in fact those currently fighting infection.

Related Topics:
Transcription factors - NF-kB

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The production of the virus is regulated, like that of many viruses. Initially the integrated provirus is copied to mRNA which is then spliced into smaller chunks. These small chunks produce the regulatory proteins Tat (which encourages new virus production) and Rev. As Rev accumulates it gradually starts to inhibit mRNA splicing {{ref|Pollard}}. At this stage the structural proteins Gag and Env are produced from the full-length mRNA. Additionally the full-length RNA is actually the virus genome, so it binds to the Gag protein and is packaged into new virus particles.

Related Topics:
Provirus - MRNA - Spliced - Tat - Rev - Splicing

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Interestingly, HIV-1 and HIV-2 appear to package their RNA differently; HIV-1 will bind to any appropriate RNA whereas HIV-2 will preferentially bind to the mRNA which was used to create the Gag protein itself. This may mean that HIV-1 is better able to mutate (HIV-1 causes AIDS faster than HIV-2 and is the majority species of the virus).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Viral assembly and release

The final step of the viral cycle is the assembly of new HIV-1 virions, begins at the plasma membrane of the host cell. The Env polyprotein (gp160) goes through the endoplasmic reticulum and is transported to the Golgi complex where it is cleaved by protease and processed into the two HIV envelope glycoproteins gp41 and gp120. These are transported to the plasma membrane of the host cell where gp41 anchors the gp120 to the membrane of the infected cell. The Gag (p55) and Gag-Pol (p160) polyproteins also associate with the inner surface of the plasma membrane along with the HIV genomic RNA as the forming virion begins to bud from the host cell. Maturation either occurs in the forming bud or in the immature virion after it buds from the host cell. During maturation, HIV proteases (proteinases) cleave the polyproteins into individual functional HIV proteins and enzymes. The various structural components then assemble to produce a mature HIV virion {{ref|Gelderblom}}. This step can be inhibited by drugs. The virus is then able to infect another cell.

Related Topics:
Endoplasmic reticulum - Golgi - Protease - Plasma membrane

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

There are two forms of the virus:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Immature form

When the virus leaves the cell it is not infectious and the inner part of the virus particle contains a spherical core (stains dark on electron micrographs). There are spikes on the outer membrane that are the Env proteins (gp120 and gp41) (Figure 6). Sometimes a virus can be seen during the process of budding, when it looks like a dark arc sitting under the cell membrane. The Env proteins link together in groups of three (trimers).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Mature form

Once the virus protease has cleaved the gag proteins, the core rearranges into a truncated cone (like a traffic cone sliced at an angle across the top). Some reports also show a small filament linking the core to the membrane. The envelope spikes are often much rarer on mature particles, because they are easily dislodged. It is the mature conical core that makes HIV easily identifiable.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~