Regulatory T Cells Across Oral Diseases: A Narrative Review on a Functional Perspective

Regulatory T cells (Tregs) maintain peripheral immune tolerance and are involved in various oral diseases with markedly different characteristics. In the oral field, Tregs have largely been discussed through reports on their distribution and numbers, and interpretations of whether their role is protective or pathogenic. Current identification markers do not directly assess the suppressive function, and reports on this function remain limited. This review focuses on the suppressive function of Tregs in the organization of oral diseases. Cells may express FOXP3 without exerting stable suppressive function, and the suppressive capacity of the same FOXP3⁺ cells may vary with TSDR status and the inflammatory environment. In Sjögren's disease, oral lichen planus, periodontitis, and peri-implantitis, inflammation does not resolve even when Treg numbers are increased or maintained. In contrast, Treg suppressive function is enhanced in oral squamous cell carcinoma. Medication-related osteonecrosis of the jaw differs in that a reduction in Tregs induced by the causative drugs is itself what governs the condition. Attending to the functional state allows these conditions to be organized coherently and supports a therapeutic conception of Treg control that differs in direction according to the disease: weakening the function where it is enhanced, and restoring it where it is impaired.