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. During pregnancy, may function to skew the cytokine of maternal T-cells toward immunoprotective Th2 phenotype. Interacts with CTLA4 (in vitro). Additional isoforms seem to exist. Isoform 1 highest expression in lymphoid tissues, such as spleen (mostly in the marginal zone), lymph nodes (particularly in the cortex and in both primary and secondary follicles), thymus (predominantly in the medulla) and Peyer patches (mostly in the follicles), lower levels in many non-lymphoid tissues, such as brain, heart, kidney, liver, lung, skeletal muscle and testis. Present on freshly isolated splenic B-cells, T-cells, dendritic cells and macrophages. The expression of isoform 2 is restricted to heart, spleen and kidney. Detected early in hemopoiesis: in the yolk sac at 11.5 and 12.5 dpc and, to a lesser extent, in the liver at 14.5 dpc. Belongs to the immunoglobulin superfamily. BTN/MOG family.