Inducible T-cell costimulator is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the ICOS gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

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CD278 or ICOS (Inducible T-cell COStimulator) is a CD28-superfamily costimulatory molecule that is expressed on activated T cells. It is thought to be important for T_h2 cells in particular.

ICOS knockout phenotype

Compared to wild-type naïve T cells, ICOS-/- T cells activated with plate-bound anti-CD3 have reduced proliferation and IL-2 secretion(1). The defect in proliferation can be rescued by addition of IL-2 to the culture, suggesting the proliferative defect is due either to ICOS-mediated IL-2 secretion or the activation of similar signaling pathways between ICOS and IL-2. In terms of Th1 and Th2 cytokine secretion, ICOS-/- CD4+ T cell activated in vitro have reduced IL-4 secretion, but similar IFN-g secretion. Similarly, CD4+ T cells purified from ICOS-/- mice immunized with the protein keyhole limpet hemocyanin (KLH) in alum or Complete Freunds Adjuvant have attenuated IL-4 secretion, but similar IFN-g and IL-5 secretion when recalled with KLH. These data are similar to an airway hypersensitivity model showing similar IL-5 secretion, but reduced IL-4 secretion in response to sensitization with Ova protein, indicating a defect in Th2 cytokine secretion, but not a defect in Th1 differentiation as both IL-4 and IL-5 are Th2-associated cytokines. In agreement with reduced Th2 responses, ICOS-/- mice have reduced germinal center formation and IgG1 and IgE antibody titers in response to immunization.