Viruses are tiny microbes that can infect cells. Once in a cell, they use cellular components to replicate.
They can be classified according to several factors, including:
- the type of genetic material they use (DNA or RNA)
- the method they use to replicate within the cell
- their shape or structural features
Retroviruses are a type of virus in the viral family called Retroviridae. They use RNA as their genetic material and are named for a special enzyme that’s a vital part of their life cycle — reverse transcriptase.
There are many technical differences between viruses and retroviruses. But generally, the main difference between the two is how they replicate within a host cell.
Here’s a look at the steps of the life cycle of human immunodeficiency virus (HIV) to help illustrate how retroviruses replicate:
- Attachment. The virus binds to a receptor on the surface of the host cell. In the case of HIV, this receptor is found on the surface of immune cells called CD4 T cells.
- Entry. The envelope surrounding the HIV particle fuses with the membrane of the host cell, allowing the virus to enter the cell.
- Reverse transcription. HIV uses its reverse transcriptase enzyme to turn its RNA genetic material into DNA. This makes it compatible with the host cell’s genetic material, which is vital for the next step of the life cycle.
- Genome integration. The newly synthesized viral DNA travels to the cell’s control center, the nucleus. Here, a special viral enzyme called integrase is used to insert the viral DNA into the host cell’s DNA.
- Replication. Once its DNA has been inserted to the host cell’s genome, the virus uses the host cell’s machinery to produce new viral components, such as viral RNA and viral proteins.
- Assembly. The newly made viral components combine close to the cell surface and begin to form new HIV particles.
- Release. The new HIV particles push out from the surface of the host cell, forming a mature HIV particle with the help of another viral enzyme called protease. Once outside the host cell, these new HIV particles can go on to infect other CD4 T cells.
The key steps that differentiate retroviruses from viruses are reverse transcription and genome integration.
There are three retroviruses that can affect humans:
HIV is transmitted through bodily fluids and needle sharing. In addition, mothers can transmit the virus to children through childbirth or breastfeeding.
Because HIV attacks and destroys CD4 T cells, which are very important for helping the body fight infections, the immune system gets progressively weaker and weaker.
If an HIV infection isn’t managed through medication, a person can develop acquired immunodeficiency syndrome (AIDS). AIDS is the last stage of the HIV infection and can lead to the development of opportunistic infections and tumors, which can be life-threatening.
Human T-cell lymphotropic virus (HTLV) types 1 and 2
HTLV1 and 2 are closely related retroviruses.
HTLV1 is found mostly in Japan, the Caribbean, and parts of Africa. It’s transmitted through sexual contact, blood transfusions, and needle sharing. Mothers can also transmit the virus to their child through breastfeeding.
HTLV1 is associated with the development of acute T cell leukemias. It’s also associated with a neurological disorder affecting the spinal cord called HTLV1-associated myelopathy/tropical spastic paraparesis.
Less is known about HTLV2, which is mostly found in North, Central, and South America. It’s transmitted in the same ways as HLTV1 and is likely linked to neurodegenerative disease and the development of certain blood cancers.
Currently, there’s no cure for retroviral infections. But a variety of treatments can help to keep them managed.
Specific antiviral medications, called antiretroviral therapy (ART), are available for the management of HIV.
ART can help to reduce viral load in a person with HIV. Viral load refers to the amount of HIV that’s detectable in a person’s blood.
People undergoing ART take a combination of medications. Each of these medications targets the virus in different ways. This is important because the virus easily mutates, which can make it resistant to certain medications.
ART works to target a retrovirus by interfering with their replication process.
Since there’s currently no cure for HIV, people undergoing ART will need to do so throughout their life. Although ART cannot eliminate HIV completely, it can reduce viral load to undetectable levels.
HTLV1 and HTLV2 treatment
A combination of the drugs interferon and zidovudine may also be used. Both of these drugs help to prevent retroviruses from attacking new cells and replication.
Retroviruses are a type of virus that use a special enzyme called reverse transcriptase to translate its genetic information into DNA. That DNA can then integrate into the host cell’s DNA.
Once integrated, the virus can use the host cell’s components to make additional viral particles.