Oral lichen planus is a chronic inflammatory disorder of the oral mucosa that represents T cell-mediated autoimmune diseases. The regulation and roles of carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1), a novel immune molecule, in the immunopathogenesis of T cell-mediated autoimmune diseases remain unclear. In the current paper, CEACAM1 was found to be overexpressed in peripheral T cells and epithelial cells in oral lichen planus patients. A fraction of infiltrating inflammatory mononuclear cells in the lamina propria of the oral lichen planus mucosa also expressed CEACAM1. Importantly, for the first time, CEACAM1 expression in T cells and in normal human oral keratinocytes was demonstrated to be regulated differently by osteopontin in vitro. Furthermore, the apoptosis of oral keratinocytes and activated T cells can be markedly suppressed by CEACAM1-specific monoclonal antibodies. In conclusion, OPN-regulated CEACAM1 expression may play a critical role in the immunopathogenesis of oral lichen planus.