Alleviating Effects of Citrus Polymethoxyflavones on Autoimmune Thyroiditis via IL-17 Signaling Pathway

Autoimmune thyroid diseases (AITDs), characterized by chronic thyroid inflammation and autoantibody production, currently lack targeted therapies, highlighting the need for natural immunomodulators. This study systematically investigated the immunomodulatory effects of citrus flavonoids, particularly polymethoxyflavones (PMFs), on AITDs. Flavonoid extracts from six citrus varieties were screened on Nthy-ori3-1 cells. Extracts from Ougan, Eureka lemon, and Newhall sweet orange significantly reduced thyroid peroxidase antibody (TPO-Ab) levels by 25%–34% (p < 0.001), demonstrating potential anti-AITDs activity. High-performance liquid chromatography identified and quantified thirteen flavonoids, among which eight representative compounds were further evaluated. Tangeretin exhibited the strongest activity, lowering TPO-Ab levels by 64% (p < 0.0001) and downregulating interleukin 1 beta (IL1B), interleukin 6 (IL6), and tumor necrosis factor-α (TNF) by 1.9-fold–2.5-fold in an AITDs cell model. Mechanistically, PMFs, specifically tangeretin and nobiletin, alleviated AITDs by modulating the IL-17 signaling pathway, regulating key regulators such as TNF receptor-associated factor 3 (TRAF3), TRAF3-interacting protein 2 (Act1), nuclear factor kappa B subunit 1 (NFKB1), interleukin 1 beta (IL-1β), interleukin 6 (IL6), and tumor necrosis factor-α (TNF-α). In a NaI-induced murine AITDs model, oral tangeretin administration (20 and 100 mg/kg/day body weight) for 8 weeks significantly reduced serum TPO-Ab levels (p < 0.05), thyroid follicular destruction, and lymphocyte infiltration. Notably, this study is the first to systematically evaluate the immunoregulatory effects of citrus PMFs in AITDs, demonstrating their potential as natural, food-derived therapeutic agents. These findings provide valuable insights into PMFs as novel immunomodulators, paving the way for food-based interventions in autoimmune diseases.