Inducible co-stimulator (ICOS), also called ALIM and H4, is a homodimeric protein and member of the CD28 family of immune co-stimulatory receptors. Other family members are CD28, CTLA-4 and PD-1. ICOS 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. Unlike the constitutively expressed CD28, ICOS has to be de novo induced on the T cell surface, does not up-regulate the production of IL-2, but superinduces the synthesis of IL-10, a B cell-differentiation factor (1). In vivo, ICOS is highly expressed on tonsillar T cells, which are closely associated with B cells in the apical light zone of germinal centres, the site of terminal B cell maturation. ICOS regulates CD28-dependent and CD28-independent CD4 (+) subset (Th1 and Th2) responses (2). It is a major regulator of the adaptive immune system and therefore, has a protective role in inflammatory autoimmune diseases (3).

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