The Life Cycle of HIV
HIV and AIDS
HIV (human immunodeficiency virus) is a virus that in time can lead to AIDS (acquired immunodeficiency syndrome). HIV attacks and weakens the immune system, specifically the immune cells that are called CD4 cells or T-cells. Unlike many other viral infections, our bodies are unable to eliminate HIV completely.
Eventually, when the virus has destroyed so many CD4 cells that the immune system is largely non-functional, the damage leads to AIDS, the most advanced stage of HIV. A person with AIDS is susceptible to any number of infections due to one’s damaged immune system.
Transmission of HIV
The Centers for Disease Control and Prevention (CDC) reports that in the United States, HIV is largely transmitted from one person to another through unprotected sex and through shared drug needles. While rare, the virus can also be transmitted though a blood transfusion.
Infection occurs in parts of the body that are lined with a mucus membrane. Here, the virus attaches to immune cells, which are then transported to the lymph nodes. From the lymph nodes, the virus can begin to infect CD4 cells, the cells that coordinate the body’s immune response.
The HIV Life Cycle Begins with Attachment
Scientists have yet to determine why the human immune system cannot get rid of HIV. To better understand the virus and develop treatments, researchers have worked toward determining the virus’s life cycle in the body.
The first stage is attachment, or binding. This is when the virus finds and attaches to the outside of a CD4 cell. HIV finds and binds to CD4 cells via a receptor. The virus functions like a key that fits into a specific lock, or receptor, on the outside of the immune cell.
The Virus Enters the Host Cell
Once HIV has found and attached to a receptor on a CD4 cell, it has to enter the cell in order to infect it and replicate itself. This stage is called fusion. The outer wall of the virus, called the envelope, fuses with the membrane of the CD4 cell. This fusion allows the virus to get inside of the immune cell with its genetic material.
Reverse Transcription: Genetic Conversion
Within the cells of the human body, our genetic material is in the form of a molecule called deoxyribonucleic acid, or DNA. HIV’s genetic material is the form of ribonucleic acid (RNA) instead of DNA.
To take over a human immune cell, the virus has to convert its RNA to DNA. This process occurs using an enzyme called reverse transcriptase. The virus releases this enzyme once it has entered the CD4 cell so it can begin the conversion process, called reverse transcription.
Integration: Taking Over the DNA
Once reverse transcription is complete and the virus has its genetic material in the form of DNA, it can enter the CD4 cell’s nucleus. This is where the cell keeps its DNA. Inside the nucleus, the virus joins its DNA to the cell’s DNA. In other words, it integrates its genetic material into that of the host cell. The HIV uses an enzyme called integrase to do this.
Transcription and Translation: Building HIV Proteins
Once the virus has integrated its DNA into that of the host cell, it can hijack its machinery and begin to manufacture copies of its own genetic material and its own proteins. The virus uses the enzymes and other structures inside the CD4 cell to replicate itself.
Assembly and Budding: The Infection Progresses
With all the parts needed to make more viruses, thanks to the machinery of the CD4 cell, copies of the virus begin to assemble inside the cell. The viruses are simple, and need only their copied RNA and some proteins to assemble into new viral packages.
Once they are fully put together, the new viruses leave the CD4 cell through a process called budding. The viruses push against the membrane of the CD4 cell to get out and take part of the membrane with them to create their own envelopes. These new viruses can now infect other CD4 cells and repeat the life cycle all over again.
Treatments Target Stages of the HIV Life Cycle
Through understanding the life cycle of HIV, researchers have developed treatments that target the virus. While there is no cure, the drugs have been successful in keeping patients with HIV healthy for many years.
Some drugs target the receptors on CD4 cells, preventing HIV from binding to them. Others inhibit reverse transcriptase to prevent the virus from converting its RNA into DNA. And some medications prevent the enzyme integrase from allowing HIV to get into the CD4 cell nucleus.
● HIV Life Cycle. (2009, November 18). U.S. Department of Health and Human Services. Retrieved December 2, 2013 from http://www.aids.gov/hiv-aids-basics/just-diagnosed-with-hiv-aids/hiv-in-your-body/hiv-lifecycle/
● HIV Overview. (2013, October 16). National Institutes of Health. Retrieved December 2, 2013, from http://aidsinfo.nih.gov/education-materials/fact-sheets/19/73/the-hiv-life-cycle
● HIV Transmission. (2013, June 3). Centers for Disease Control and Prevention. Retrieved December 2, 2013, from http://www.cdc.gov/hiv/basics/transmission.html
● What is HIV/AIDS?. (2012, June 6). U.S. Department of Health and Human Services. Retrieved December 2, 2013, from http://www.cdc.gov/hiv/prevention/research/prep/