Periodontitis is a chronic immuno-inflammatory disease in which the disruption of the balance between host and microbiota interactions is key to the onset and progression of the disease. The immune homeostasis associated with periodontal health requires a regulated immuno-inflammatory response, during which the presence of regulatory T cells (Tregs) is essential to ensure a controlled response that minimizes collateral tissue damage. Since Tregs modulate both innate and adaptive immunity, pathological conditions that may resolve by the acquisition of immuno-tolerance, such as periodontitis, may benefit by the use of Treg immunotherapy. In recent years, many strategies have been proposed to take advantage of the immuno-suppressive capabilities of Tregs as immunotherapy, including theex vivoandin vivomanipulation of the Treg compartment. Ongoing research in both basic and translational studies let us gain a better understanding of the diversity of Treg subsets, their phenotypic plasticity, and suppressive functions, which can be used as a substrate for new immunotherapies. Certainly, as our knowledge of Treg biology increases, we will be capable to develop new therapies designed to enhance the stability and function of Tregs during periodontitis.

Among the mechanisms of suppression that T regulatory (Treg) cells exert to control the immune responses, the secretion of small extracellular vesicles (sEV) has been recently proposed as a novel contact‐independent immunomodulatory mechanism. Previous studies have demonstrated that Treg cells produce sEV, including exosomes, able to modulate the effector function of CD4+ T cells, and antigen presenting cells (APCs) such as dendritic cells (DCs) through the transfer of microRNA, cytokines, the production of adenosine, among others. Previously, we have demonstrated that Neuropilin‐1 (Nrp1) is required for Tregs‐mediated immunosuppression mainly by impacting on the phenotype and function of effector CD4+ T cells. Here, we show that Foxp3+ Treg cells secrete sEV, which bear Nrp1 in their membrane. These sEV modulate effector CD4+ T cell phenotype and proliferation in vitro in a Nrp1‐dependent manner. Proteomic analysis indicated that sEV obtained from wild type (wt) and Nrp1KO Treg cells differed in proteins related to immune tolerance, finding less representation of CD73 and Granzyme B in sEV obtained from Nrp1KO Treg cells. Likewise, we show that Nrp1 is required in Treg cell‐derived sEV for inducing skin transplantation tolerance, since a reduction in graft survival and an increase on M1/M2 ratio were found in animals treated with Nrp1KO Treg cell‐derived sEV. Altogether, this study describes for the first time that Treg cells secrete sEV containing Nrp1 and that this protein, among others, is necessary to promote transplantation tolerance in vivo via sEV local administration.

Regulatory T cell (Treg)‐derived extracellular vesicles (EVs) represent a contact‐independent mechanism by which Tregs suppress dysregulated immune responses. These EVs carry diverse immunomodulatory molecules, including CD73, an ectoenzyme that hydrolyses AMP into adenosine. Adenosine subsequently acts as a potent immunosuppressive mediator that inhibits effector CD4⁺ T cell activation and controls pathological inflammation. Periodontitis is a highly prevalent inflammatory disease characterised by the accumulation of IL‐17A‐expressing CD4⁺ T cells in response to dysbiotic oral bacterial biofilms, ultimately leading to RANKL‐mediated alveolar bone resorption and tooth loss. We tested the hypothesis that CD73⁺ Treg‐derived EVs, isolated from Tregs induced with polarising cytokines in the presence of retinoic acid, could limit inflammation and prevent alveolar bone loss in periodontitis. Our findings demonstrate that Tregs induced with polarising cytokines in the presence of retinoic acid express high levels of CD73 and secrete adenosine‐producing suppressive CD73 ⁺ EVs. Furthermore, local administration of these CD73⁺ Treg‐derived EVs in a murine periodontitis model reduced activated CD4⁺ T cell infiltration, decreased IL‐17A and RANKL expression, and attenuated osteoclast‐mediated alveolar bone loss. In conclusion, retinoic acid‐induced Treg‐derived EVs suppress CD4⁺ T cell‐driven inflammation and ameliorate periodontitis, at least in part through CD73/adenosine‐dependent immunomodulatory mechanisms.