CD278

Inducible T-cell costimulator is an is_associated_with::immune checkpoint protein that in humans is encoded by the ICOS is_associated_with::gene.

CD278 or ICOS (Inducible T-cell COStimulator) is a CD28-superfamily costimulatory molecule that is expressed on activated is_associated_with::T cells. It is thought to be important for Th2 cells in particular.

Function
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.

ICOS knockout phenotype
Compared to wild-type naïve T cells, ICOS-/- is_associated_with::T cell s activated with plate-bound anti-CD3 have reduced proliferation and IL-2 secretion. 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 is_associated_with::Th1 and is_associated_with::Th2 cytokine secretion, ICOS-/- is_associated_with::CD4+ T cell activated is_associated_with::in vitro have reduced is_associated_with::IL-4 secretion, but similar is_associated_with::IFN-g secretion. Similarly, CD4+ T cells purified from ICOS-/- mice immunized with the protein is_associated_with::keyhole limpet hemocyanin (KLH) in is_associated_with::alum or Complete is_associated_with::Freund's Adjuvant have attenuated IL-4 secretion, but similar IFN-g and is_associated_with::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 is_associated_with::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 is_associated_with::IgG1 and is_associated_with::IgE antibody titers in response to immunization.

Combination therapy
Ipilimumab patients expressed increased ICOS+ T cells in tumor tissues and blood. The increase served as a pharmacodynamic biomarker of anti-CTLA-4 treatment. In wild-type C57BL/6 mice, anti-CTLA-4 treatment resulted in tumor rejection in 80 to 90% of subjects, but in gene-targeted mice that were deficient for either ICOS or its ligand, the efficacy was less than 50%. An agonistic stimulus for the ICOS pathway during anti-CTLA-4 therapy resulted in an increase in efficacy that was about four to five times as large as that of control treatments. As of 2015 antibodies for ICOS were not available for clinical testing.