Plays a role in cell-cell interaction, cell migration, and angiogenesis. Through interaction with HHLA2, costimulates T-cells in the context of TCR-mediated activation. Enhances T-cell proliferation and cytokine production via an AKT-dependent signaling cascade. May form homophilic interactions that could regulate cell-cell interaction. Interacts with CACNB2, DST, MIA and NCKIPSD. Interacts with HHLA2. Widely expressed, mainly by epithelial and endothelial cells, including bronchial epithelial cells of lung, breast glandular and lobular epithelia cells, urothelium of the bladder, skin epidermis, epithelium of gastrointestinal, rectum, endometrial glands of the uterus, ureter, fallopian tube epithelium, colonic epithelium, small bowl epithelium, stomach epithelium, including both chief and parietal cells, trophoblastic epithelium of placenta, and pancreatic acinar cells (at protein level). Consistently expressed in veins and arteries (at protein level). Not detected in thyroid, cerebellum, cerebral cortex and thymus (at protein level). Expressed in lymphoid organs, with highest levels in thymus, spleen, peripheral blood lymphocytes and liver. In the thymus, expressed in CD4+ and CD8+ single- and double-positive cells, but not in immature CD4- and CD8- double-negative cells (at protein level). In peripheral blood mononuclear cells, highly expressed on CD56+ or CD16+ natural killer cells and CD3+ T-cells(at protein level). Not detected on B-cells(at protein level). Expressed in tonsils (at protein level). Repetitive stimulation of naive T-cells, including with IL2 and antibodies against CD3 and CD28 or repetitive antigenic exposure, leads to progressive and irreversible loss of expression. N-glycosylated.