The protein encoded by this gene belongs to the CD28 and CTLA-4 cell-surface receptor family. It forms homodimers and plays an important role in cell-cell signaling, immune responses, and regulation of cell proliferation. [provided by RefSeq, Jul 2008],disease:Defects in ICOS are the cause of ICOS deficiency (ICOSD) [MIM:607594]. ICOSD is a form of common variable immunodeficiency (CVID) characterized by recurrent bacterial infections of the respiratory and digestive tracts characteristic of humoral immunodeficiency. There is absence of other complicating features of CVID such as splenomegaly, autoimmune phenomena, or sarcoid-like granulomas and absence of clinical signs of overt T-cell immunodeficiency. A severe disturbance of the T-cell-dependent B-cell maturation occurs in secondary lymphoid tissue. B-cells exhibit a naive IgD+/IgM+ phenotype and the numbers of IgM memory and switched memory B-cells are substantially reduced.,function:Enhances all basic T-cell responses to a foreign antigen, namely proliferation, secretion of lymphokines, up-regulation of molecules that mediate cell-cell interaction, and effective help for antibody secretion by B-cells. Essential both for efficient interaction between T and B-cells and for normal antibody responses to T-cell dependent antigens. Does not up-regulate the production of interleukin-2, but superinduces the synthesis of interleukin-10. Prevents the apoptosis of pre-activated T-cells. Plays a critical role in CD40-mediated class switching of immunoglobin isotypes.,induction:By phorbol myristate acetate (PMA) and ionomycin. Up-regulated early on T-cells and continues to be expressed into the later phases of T-cell activation.,online information:ICOS mutation db,PTM:N-glycosylated.,similarity:Contains 1 Ig-like V-type (immunoglobulin-like) domain.,subunit:Homodimer, disulfide-linked.,tissue specificity:Activated T-cells. Highly expressed on tonsillar T-cells, which are closely associated with B-cells in the apical light zone of germinal centers, the site of terminal B-cell maturation. Expressed at lower levels in thymus, lung, lymph node and peripheral blood leukocytes. Expressed in the medulla of fetal and newborn thymus.,
