Treatment with Rapamycin Can Restore Regulatory T Cell Function in IPEX Patients.

Abstract. Background Immune-dysregulation, Polyendocrinopathy, Enteropathy, X-linked (IPEX) syndrome is a lethal disease caused by mutations in a transcription factor critical for the function of thymus-derived (t) regulatory T cells (Tregs), i.e., Forkhead-box-P3 (FOXP3), resulting in impaired Treg function and autoimmunity. At present, hematopoietic stem cell transplantation is the therapy of choice for IPEX patients. If not available, multiple immunosuppressive regimens have been used with poor disease-free survival at long-term follow-up. Rapamycin has been shown to suppress peripheral T cells while sparing Tregs expressing wild-type FOXP3, thereby proving beneficial in the clinical setting of immune-dysregulation. However, the mechanisms of immunosuppression selective to IPEX patients’ Tregs are unclear. Objective To determine the cellular and molecular basis of the clinical benefit observed under rapamycin treatment in six IPEX patients with different FOXP3 mutations. Methods We tested phenotype and function of FOXP3-mutated Tregs from rapamycin-treated IPEX patients by flow cytometry and in vitro suppression assays, and the gene expression profile of rapamycin-conditioned Tregs by droplet-digital-PCR. Results Clinical and histological improvements in patients correlated with partially restored Treg function, independent of FOXP3 expression or Treg frequency. Expression of Tumor-Necrosis-Factor-Receptor-Superfamily-Member18 (TNFRSF18, GITR) and Epstein-Barr-virus-Induced3 (EBI3, an IL-35 subunit) in patients’ Tregs increased during treatment as compared to that of Tregs from untreated healthy subjects. Furthermore inhibition of GITR and Ebi3 partially reverted in vitro suppression by in vivo rapamycin-conditioned Tregs. Conclusion Rapamycin is able to impact on Treg suppressive function via a FOXP3-independent mechanism, thus sustaining the clinical improvement observed in IPEX patients under rapamycin treatment.