Human herpesvirus 6 (Roseola)

Human herpesvirus 6, or HHV-6, is a double stranded DNA virus which belongs to the Herpesviridae family, subfamily Betaherpesvirinae, genus Roseolovirus. HHV-6 primary infection is the cause of the common childhood disease called roseola infantum, also known as exanthema subitum or sixth disease.

Human herpesvirus 6 is a double stranded linear DNA virus surrounded by an icosahedral capsid, which is a spherical protein shell made up of 20 equilateral triangular faces.

The capsid is covered by a protein layer called the tegument, and finally enclosed in an envelope, which is a lipid membrane that contains viral glycoproteins and is acquired from the nuclear membrane of host cells.

Now, HHV-6 infection begins when a person comes in contact with respiratory secretions from another person who contains the virus.

Once in the body, the virus attaches to dendritic cells, which are antigen-presenting cells that can be found in the skin, nose, lungs, stomach and intestines.

These cells are able to process antigens like viral proteins and present them on the cell surface to the T cells.

So, when the virus attaches to dendritic cells, it activates them, and they migrate to the lymph nodes where they interact with CD4+ T lymphocytes.

When dendritic cells and T cells interact, the virus gets ingested by T cells and they get infected.

Inside T cells, the virus can replicate most efficiently, so it undergoes the lytic cycle, where its DNA gets transcribed and translated by cellular enzymes, to form viral proteins, which are packaged into new viruses that can leave the cell destroyed and subsequently infect neighboring cells.

The virus can also replicate into a wide variety of other cells such as monocytes, macrophages, natural killer cells, astrocytes, megakaryocytes and glial cells - but not as efficiently as inside T cells.

Finally, HHV-6 can go into a latent state in monocytes, where the virus just hangs out inside the cell instead of killing it.

This may sound harmless, but in people with immunosuppression, especially after a bone marrow or organ transplant, the virus can reactivate and cause serious infections such as encephalitis.