Highlights d PLZF is required for the generation and differentiation of iNKT cells d A hypomorphic allele of PLZF alters iNKT cell development and subset composition d Quantitative difference in PLZF protein abundance determines iNKT lineage fate

The oral mucosa is a critical barrier tissue that protects the oral cavity against invading pathogens and foreign antigens. Interestingly, inflammation in the oral cavity is rarely observed, indicating that overt immune activation in this site is actively suppressed. Whether Foxp3+ Treg cells are involved in controlling immunity of the oral mucosa, however, is not fully understood. Here, we show that the oral mucosa is highly enriched for Foxp3+ Treg cells, and that oral mucosa Treg cells are phenotypically distinct from those of LN or spleen, as they expressed copious amounts of the tissue-retention molecule CD103 and unusually high-levels of CTLA-4. Acute depletion of Foxp3+ Treg cells had catastrophic effects, resulting in dramatic infiltration of activated effector T cells that were associated with autoimmunity and tissue destruction of the oral mucosa. Moreover, adoptive transfer of naïve CD4 T cells revealed that the oral mucosa is highly ineffective in inducing Foxp3 Treg cells in situ, so that it depends on recruitment and migration of exogenous Treg cells to populate this mucosal site. Collectively, these results demonstrate a previously unappreciated role and a distinct developmental pathway for Foxp3+ Treg cells in the oral mucosa, which were essential to control local tissue immunity.