WHAT IS VIRAL TROPISM?
When HIV attaches to a CD4 cell it is going to infect, it uses molecules on the cell surface. These are called receptors or chemokine co-receptors. The first receptor HIV uses is the CD4 molecule. The virus then uses a “co-receptor.” This is either a CCR5 molecule or a CXCR4 molecule. The virus usually uses one co-receptor or the other. HIV that uses the CCR5 co-receptor is called “CCR5 tropic” or “R5 tropic.” However, viral tropism can be CCR5, CXCR4, or “dual/mixed” (or D/M) if some of the sample of virus uses each co-receptor. When people are first infected, their virus is usually CCR5 tropic. As HIV infection progresses, tropism usually changes to include more CXCR4 tropism. Researchers thought that the co-receptors would be a good target for anti-HIV drugs. There is now an approved antiviral medication active against CCR5-tropic HIV. This attachment inhibitor is maraviroc (see fact sheet 462.) Maraviroc only works against R5-tropic HIV. There are many other attachment inhibitors i
