CD278, also called inducible costimulator (ICOS) or CRP1 (CD28-related protein-1), is a member of the growing CD28 family of immune costimulatory receptors. Other family members are CD28, CTLA-4 and PD-1. ICOS is expressed on most CD45RO⁺ cells. ICOS expression is upregulated 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.