Rapamycin rescues loss-of-function in blood-brain barrier-interacting regulatory T cells

In many autoimmune diseases, FOXP3+ regulatory T cells (Tregs) skew towards a pro-inflammatory and non-suppressive phenotype and are therefore unable to control the exaggerated autoimmune response. This may largely impact the success of autologous Treg therapy which is currently under investigation for treatment of autoimmune diseases, including multiple sclerosis (MS). Thus, there is a need to ensure in vivo stability of Tregs before successful Treg therapy can be applied. Using a murine genetic fate-mapping model, we demonstrate that inflammatory exFOXP3 T cells accumulate in the central nervous system (CNS) during experimental autoimmune encephalomyelitis (EAE). In a human in vitro BBB model, we discovered that interaction with inflamed blood-brain barrier (BBB)-endothelial cells induces loss of suppressive function in Tregs. Transcriptome analysis further revealed that Tregs which migrated across inflamed BBB-endothelial cells in vitro have a pro-inflammatory Th1/17 signature and upregulate the mTORC1 signaling pathway compared to non-migrated Tregs. These findings suggest that interaction with BBB-endothelial cells is sufficient to affect Treg function, and that transmigration triggers an additive pro-inflammatory phenotype switch, which was also seen in CNS-derived exFOXP3 T cells of EAE mice. In vitro treatment of migrated human Tregs with the clinically-approved mTORC1 inhibitor rapamycin completely restored the loss of suppressive function. Finally, flow cytometric analysis indicated an enrichment of inflammatory, less suppressive CD49d+ Tregs in the cerebrospinal fluid of MS patients, thereby underscoring the relevance of our findings for human disease. In sum, our findings provide firm evidence that the inflamed BBB affects human Treg stability, which can be restored using a mTORC1 inhibitor. These insights can help in significantly improving the efficacy of autologous Treg therapy of MS. ### Competing Interest Statement The authors have declared no competing interest.