Inducible Costimulator(ICOS) is a member of the growing CD28 family of immune costimulatory receptors. Other family members are CD28, CTLA4 and PD1. ICOS shares approximately 39% amino acid similarity with CD 28 and CTLA4. Mouse and human ICOS share approximately 72% amino acid identity. ICOS is expressed on most CD45RO+ cells. ICOS expression is up-regulated within approximately 24-48 hours of activation on Th primed cells. B7-H2, a member of the B7 family of costimulatory ligands, has been identified as the ICOS ligand. The B7-H2/ ICOS interaction appears to play roles in T cell dependent B cell activation and Th differentiation. In addition, ICOS is more potent in the induction of IL-10 production, acytokine important for suppressive function of T regulatory cells.