HIV: structure, replication and treatment

In a nutshell

The Human Immunodeficiency Virus (HIV) is a virus that affects the immune system and eventually causes acquired immune deficiency syndrome (AIDS). It uses the host cell to replicate and invade other cells. Currently, there is no universal cure for HIV but it can be managed by taking antiviral drugs.

Structure

The structure of a HIV particle is shown below.

Biology; The immune system; KS5 Year 12; HIV: structure, replication and treatment — 3. A HIV particle.

	Component	Description
1.	RNA	Two single-strands of RNA make up the virus' genetic material.
2.	Attachment proteins	These are peg-like and help HIV attach to the host's helper T cells.
4.	Reverse transcriptase (enzyme)	An enzyme needed for viral replication.
5.	Capsid	A protein layer encloses the genetic material.
6.	Lipid envelope	An extra outer layer is made from the cell membrane of the previous host cell.

HIV replication

As with all viruses, HIV cannot replicate itself. Instead, it uses its genetic material to instruct the host cell to produce the components it needs to make new HIV. After infection, HIV enters the bloodstream and circulates around the body.

Biology; The immune system; KS5 Year 12; HIV: structure, replication and treatment

1.	The attachment protein on HIV attaches to the receptor molecule called CD4. CD4 is mainly found on T helper cells.
2.	The lipid envelope fuses with the cell-surface membrane of the T cell. The capsid uncoats and the RNA is released into the cell's cytoplasm (E).
3.	Reverse transcriptase converts the viral RNA template (A) into a molecule of complementary DNA (B).
4.	This is then made into double-stranded DNA so it can be inserted into human DNA.
5.	Host cell enzymes then create messenger RNA (mRNA). This mRNA contains the instructions for making new viral proteins and RNA to go into the new viral particle.
6.	The mRNA passes out of the nucleus (C) through the nuclear pore.
7.	The mRNA then uses the host's protein synthesis machinery to make the HIV particle (D).
8.	The HIV particles then break away from the helper T cell and take a piece of the cell-surface membrane to form their lipid envelope.
9.	The virus is then free to circulate in the blood and infect other T helper cells.

AIDS

During initial HIV infection, a person may experience flu-like symptoms as the virus is replicating rapidly in the body. After the rapid replication, the virus often goes into dormancy and this is known as the latency period. During this time the infected person will not experience any symptoms. The latency period can last for several years.

Healthy people have a normal helper T cell count of $800-1200$ T cells per $mm^3$ of blood. As HIV targets these T cells, this level can drop dramatically. When it reaches less than $200$ cells per $mm^3$ of blood, the person is said to have acquired immune deficiency syndrome (AIDS). T cells are crucial to immunity, so not having enough can make a person very susceptible to infections and cancers.

Many AIDS sufferers initially develop minor infections, such as those of mucous membranes and respiratory infections. As the immune system gets weaker, these infections become more serious like tuberculosis. During the late stages of AIDS, a person can get very serious infections such as toxoplasmosis of the brain. These infections ultimately lead to death.

Note: A person is not killed by HIV or AIDS. It is the resulting infections and diseases that cause ill health and death.

Antibiotics and viruses

Antibiotics are ineffective against viral infections. Typically, antibiotics target metabolic processes and structures such as cell walls and enzymes in bacterial cells.

Example

Penicillin is an antibiotic that works by targeting the enzyme required for the synthesis and assembly of the peptide cross-linkages in bacterial cell walls. This weakens the wall and makes them unable to withstand pressure. Water enters the cell by osmosis and this cause the bacterium to burst and die.

Viruses don't have many of their own enzymes and lack their own metabolic pathways and cell structures because they rely on the host cell. As a result, antibiotics are ineffective against viruses. Equally, as viruses are often within an organism's own cell, the antibiotics cannot reach them.

Treating HIV

Currently, there is no universal cure or vaccine for HIV. It can however be managed with antiviral drugs known as antiretroviral therapy. One type of antiretroviral drug called reverse transcriptase inhibitors targets reverse transcriptase, as this is not found normally in human cells. These drugs are very effective at managing HIV and many people on antiretroviral therapy have 'undetectable' status', meaning the virus cannot be detected in the blood and passed on.

Curiosity: Four people have been cured of HIV. Three of these received a stem cell transplant and one received experimental drugs.

Prevention

As will most communicable diseases, the best way to control infection is to prevent its spread. This can be done by having protected sex (e.g., using condoms) and not sharing needles. This is because HIV is spread via bodily fluids such as semen and blood.