SIRT1-Regulated Abnormal Acetylation of FOXP3 Induces Regulatory T-Cell Function Defect in Hashimoto's Thyroiditis

Background: Hashimoto's thyroiditis (HT) is an autoimmune thyroid disease characterized by low expression of transcription factor Forkhead Box P3 (FOXP3) and functional deficiency of a cluster of differentiation regulatory T cells (Tregs). This study aimed to investigate the mechanism of Treg dysfunction in HT. Methods: The number of CD4+CD25+FOXP3+ T cells was determined by flow cytometry. Expression of FOXP3 and Sirtuin type 1 (SIRT1) was evaluated by Western blot analysis. Acetylation of FOXP3 was analyzed by immunoprecipitation and Western blot analysis. The suppressive function of Treg was analyzed by the 5,6-carboxyfluorescein diacetate succinimidyl ester (CFSE) assay. Results: The percentage of CD4+CD25+FOXP3+ T cells, expression of FOXP3, and FOXP3 acetylation level in the HT group were significantly lower than in the control groups. Conversely, SIRT1 expression was significantly higher in the HT group than in the other two groups. After Ex-527 treatment, the CD4+CD25+FOXP3+ T cells percentage, FOXP3 expression, and FOXP3 acetylation level in the HT group were significantly increased. HT Tregs exhibited less suppressive activity, but Ex-527 treatment significantly increased their suppressive activity. Conclusions: The findings demonstrate that the reduced FOXP3 expression level and Treg function defect in HT patients are regulated by SIRT1-mediated abnormal FOXP3 acetylation. Ex-527 may upregulate the FOXP3 acetylation level and subsequently increase the number and suppressive function of Treg cells.