Tissue tropism

Tissue tropism is a term most often used in virology to define the cells and tissues of a host which support growth of a particular virus. Bacteria and other parasites may also be referred to as having a tissue tropism.

Some viruses have a broad tissue tropism and can infect many types of cells and tissues. Other viruses may infect primarily a single tissue. For example Rabies virus affects primarily neuronal tissue, and Hepatitis primarily affects liver tissue.

Factors influencing viral tissue tropism include:

• The presence of cellular receptors permitting viral entry.
• Availability of transcription factors involved in viral replication.
• The molecular nature of the viral tropogen.

The cellular receptors are the proteins found on a cell or viral surface. These receptors are like keys allowing the viral cell fuse with a cell, or attach itself to a cell. The way that these proteins are acquired is through similar process to that of an infection cycle.

-Example- Infection Cycle (how Tissue Tropism works)

• Note # 1: The cycle discussed bellow is similar to the HIV cycle
• Note # 2: All cells in the human body have an array of Glycoprotein that are used to identify Cell A from Cell B, and that serve to identify Cell A to Cell A in case of White blood cells, they would state that a certain cell is native to the system.
• Note # 3: It is these Protein "keys" that the viruses use to 'target' and Identify their proper hosts.

Steps of Infections Cycle (how Tissue Tropism works)
- Virus with GPX enters body (where GP - glycoprotein and X is the numeric value given to the GP)
- Viral Cell 'targets' cell with a GPX receptors
- Viral Cell fuses with the cell and dumps it's contents into it
- Reverse Transcription occurs
- Viral DNA is incorporated with host DNA via Viral Enzyme
- Production of RNA and Viral Protein
- Viral particle is assembled
- Viral particle budds out of the cell taking a chunk of the cell membrane with it acquiring a new tissue with all the receptors it needs to continue Tissue Tropism

  Example: HIV has a gp120 which is precisely what the CD4 marker is on the surface of the macrophages and
     
           T cells, thus HIV can enter T cells and macrophages

References

Raven, Peter H.(2008). "Biology 8th Edition". New York, McGraw-Hill.