CD9 is a member of the cell membrane associated tetraspanin family and has been shown to have a wide array of functions, including promotion of MHC clustering, antigen presentation, T cell activation, cell adhesion, motility, growth and differentiation, signal transduction, tumor formation and egg/sperm fusion. CD9 is ubiquitously expressed in mammalian tissues and its roles are cell type dependent. CD9 is a typical interferon stimulated gene and further associated with MHC II and the immune system and inflammation in general, as has been shown in mammals and to a lesser extent in fish. In mammals, some viruses, such as influenza, coronavirus and hepatitis C, exploit CD9 for exit of new virus particles from host cells. In contrast, increased expression of CD9 can limit HIV-I virus budding.
- Interferon signaling
- Antiviral immune response