CD278 is a 47-57 kD protein, also known as inducible costimulatory molecule (ICOS), or H4. This protein is homologous to the CD28/CTLA-4 proteins. It forms homodimers and is expressed on activated T cells and a subset of thymocytes. It is able to costimulate T cells proliferation. In addition, ICOS has been shown to be involved in humoral immune responses (B cell germinal center formation). The ICOS ligand is B7-H2. ICOS stimulation has been shown to potentiate TCR-mediated IL-4 and IL-10 production and has been proposed to play a role in Th2 cell development. It has also been shown to be involved in airway tolerance and the downregulation of pulmonary inflammation.