ICOS and neoplasm: Inducible T-cell co-stimulator (ICOS; CD278) and its unique ligand (ICOSL) form a complex and two-faced immune machinery, which contributes to both anti-tumor responses/immune stimulation and potentiation of immunosuppression.1, , –4 ICOSL is constitutively expressed by antigen-presenting cells including B cells, macrophages, and dendritic cells, along with many somatic cells, while ICOS is only expressed on a small fraction of resting T cells at low levels after activation.1 ICOS on T cells interacts with ICOSL on antigen-presenting cells.