Rebecca Milburn Lo Hon Yun

Human immunodeficiency virus (HIV) is a member of the retrovirus family, it is responsible to the disease acquired immunodeficiency syndrome (AIDS). With this disease, the immune system of the patient begins to fail, leading to life-threatening opportunistic infections. HIV can be transferred mainly by body fluids: blood, semen, vaginal fluid, pre-ejaculate or breast milk. HIV is present in both free virus particles and virus with infected immune cells.

Morphology of HIV

This is a computer generated image, the big object is a human CD4+ white blood cell while the small spots on its surface an spiky blue objects in the foreground represent HIV particles. Outside of a human cell, HIV exists as roughly spherical particles (sometimes called virions). The surface of each particle is studded with lots of little spikes.

An HIV particle is around 100-150 billionths of a metre in diameter. That's about the same as:

  • 4 millionths of an inch
  • one twentieth of the length of an E. coli bacterium
  • one seventieth of the diameter of a human CD4+ white blood cell.
HIV particles are much smaller than bacteria, so cannot be seen through an ordinary microscope. They can been seen through an electron microscope. HIV particles surround themselves with a coat of fatty material known as the viral envelope (or membrane). Projecting from this are around 72 little spikes, which are formed from the proteins gp120 and gp41. Just below the viral envelope is a layer called the matrix, which is made from the protein p17.

HIV life cycle

1. Entry

HIV is only capable of replicating inside of human cells. The process typically begins when a virus particle bumps into a cell that carries on its surface a special protein called CD4. The spikes on the surface of the virus particle stick to the CD4 and allow the viral envelope to fuse with the cell membrane. The contents of the HIV particle are then released into the cell, leaving the envelope behind.

2. Reverse Transcription and Integration

When the HIV enters the cell, HIV enzyme reverse transcrptase converts the viral RNA into DNA. Once inside the cell, the HIV enzyme reverse transciptase converts the viral RNA into DNA, which is compatible with human genetic material. This DNA is transported to the cell's nucleus, where it is spliced into the human DNA by the HIV enzyme integrase. Once integrated, the HIV DNA is known as provirus.

This electron microscope photo shows newly formed HIV particles budding from a human cell

HIV provirus may lie dormant within a cell for a long time. But when the cell becomes activated, it treats HIV genes in much the same way as human genes. First it converts them into messenger RNA (using human enzymes). Then the messenger RNA is transported outside the nucleus, and is used as a blueprint for producing new HIV proteins and enzymes.

3. Assembly, Budding and Maturation

 The messenger RNAs produced by the cells contain all the genetic material for the HIV. Among the strands of messenger RNA produced by the cell are complete copies of HIV genetic material. The RNA come together with newly synthesized HIV proteins and enzymes to form new viral particles, which are then released from the cell. The enzyme protease plays a vital role at this stage of the HIV life cycle by cutting up long strands of protein into smaller pieces, which are used to construct mature viral cores. The newly matured HIV particles are ready to infect another cell and begin the replication process all over again. In this way the virus quickly spreads through the human body. And once a person is infected, they can pass HIV on to others in their bodily fluids.

The Disease and it’s effects

HIV mainly infects important cells in the human immune system like the helper cells, macrophages and dendritic cells. HIV infection leads to low levels of CD4+ T cells through three main mechanisms. Firstly, direct viral killing of infected cells; secondly, increased rates of apoptosis in infected cells; and thirdly, killing of infected CD4+ T cells by CD8 cytotoxic lymphocytes that recognize infected cells. As the CD4+T cell numbers decline below a critical level, cell-mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections.

People infected with HIV eventually develop AIDS. These people usually die from opportunistic infections associated with the progressive failure of the immune system. Without treatment, nine out of ten people progress with AIDS after about 10-15 years.  Many progress much sooner. Treatment with anti-retrovirals increases the life expectancy of people infected with HIV. Even after HIV has progressed to diagnosable AIDS, the average survival time with antiretroviral therapy (as of 2005) is estimated to be more than 5 years. Without antiretroviral therapy, death normally occurs within a year. It is hoped that current and future treatments may allow HIV-infected individuals to achieve a life expectancy approaching that of the general public.

*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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