Editing Viral Vector Engineering (section)

== 2. Tropism and Pseudotyping ==

'''Tropism''' refers to a virus's natural affinity for infecting a specific type of cell or tissue. This is determined by the specific proteins on the outside of the virus (the capsid or envelope) binding to specific receptors on the surface of the target cell.

For example, the natural HIV virus (a lentivirus) has a strict tropism for CD4+ T-cells because its envelope protein perfectly matches the CD4 receptor.

To make viral vectors more useful, engineers use a technique called '''pseudotyping'''. They keep the core machinery of one virus but swap its outer envelope/capsid proteins with those from a completely different virus. 
* ''Example:'' Lentiviral vectors are frequently pseudotyped with the VSV-G envelope protein (borrowed from the Vesicular Stomatitis Virus). VSV-G binds to a receptor found on almost all mammalian cells, transforming the lentivirus from a highly specific vector into a broad-spectrum delivery tool.