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 (By similarity). Interacts with CTLA4 (in vitro). Additional isoforms seem to exist. Isoform 1 is widely expressed (brain, heart, kidney, liver, lung, pancreas, placenta, skeletal muscle, bone marrow, colon, ovary, prostate, testis, lymph nodes, leukocytes, spleen, thymus and tonsil), while isoform 2 is detected only in lymph nodes, leukocytes and spleen. Expressed on activated monocytes and dendritic cells. Constitutive expression is further enhanced by treatment with TNF in peripheral blood B-cells and monocytes, while it is decreased in dendritic cells. Belongs to the immunoglobulin superfamily. BTN/MOG family. Extended N-terminus. The sequence differs from that shown in position 300 onward for unknown reason.