Implication of the tetraspanin CD9 in the immune system and cancer

Tetraspanins are considered as “adapter” or “facilitating” proteins and, through their participation in complexes, they modulate key cellular functions like cell fusion, adhesion, migration, differentiation and signal transduction. The organization of the tetraspanin web, based on different association levels determined by their resistance to detergent solubilization, is described. In particular, tetraspanins participating in the regulation of the Immune System through interactions with the B- and T-cell receptors, the class I and class II MHC antigens, and co-receptors such as CD2, CD4, CD5, CD8, or CD19 are analyzed. At last, the role of CD9 in myeloid and lymphoid cell function, its relevance to HIV infection, and the importance of tetraspanin association with integrins to cancer progression are described in detail.