Negatively regulates T-cell-mediated immune response by inhibiting T-cell activation, proliferation, cytokine production and development of cytotoxicity. When expressed on the cell surface of tumor macrophages, plays an important role, together with regulatory T-cells (Treg), in the suppression of tumor-associated antigen-specific T-cell immunity. Involved in promoting epithelial cell transformation (By similarity). Expressed at the cell surface. A soluble form has also been detected (By similarity). N-glycosylated. Belongs to the immunoglobulin superfamily. BTN/MOG family.