Ligand for the T-cell-specific cell surface receptor ICOS. Acts as a costimulatory signal for T-cell proliferation and cytokine secretion; induces also B-cell proliferation and differentiation into plasma cells. Could play an important role in mediating local tissue responses to inflammatory conditions, as well as in modulating the secondary immune response by co- stimulating memory T-cell function. Belongs to the immunoglobulin superfamily. BTN/MOG family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.