Phloretin Modulates STING Ubiquitination Degradation to Restrain Dendritic Cell Overactivation and Alleviate...

Background/Objectives: Sjögren's Syndrome (SS) is an autoimmune disorder with impaired exocrine gland function. Its pathogenesis remains elusive, but aberrant innate/adaptive immunity and dysregulated interferon signaling are core features. Current SS research lacks exploration of innate immune mechanisms and therapeutic targets. Phloretin, a natural dihydrochalcone derivative, inhibits excessive innate immune activation, yet its role and mechanism in SS remain unreported. Methods: This study aimed to explore the therapeutic efficacy of phloretin against Sjögren's syndrome (SS) and elucidate the regulatory mechanism underlying its effects on dendritic cells (DCs) and the Stimulator of Interferon Genes (STING)-driven innate immune cascade. Clinical specimens were utilized to analyze DC infiltration and STING expression in the labial glands of SS patients. In vivo experiments were conducted on NOD/Ltj SS mice with phloretin treatment, using hydroxychloroquine as a positive control. For in vitro studies, DCs were stimulated with poly I:C to verify the functional effects of phloretin and its regulatory role in the STING/TANK-binding kinase 1 (TBK1)/Interferon Regulatory Factor 3 (IRF3) signaling. Salivary secretion, glandular inflammatory responses, and systemic immune overactivation were evaluated in SS mice. In poly I:C-stimulated DCs, cell activation, migration, and co-stimulatory molecule expression were measured. The regulatory effects of phloretin on the STING signaling cascade and STING ubiquitin-mediated degradation were examined by Western blotting. Phloretin, initially identified as a candidate compound via virtual screening, was subjected to thermal shift assay (CETSA), drug affinity responsive target stability (DARTS) and molecular dynamics simulation (MD) for direct binding verification with STING. The STING overexpression rescue experiment further corroborates that phloretin modulates innate immune responses in a STING-dependent manner. Results: Clinical results revealed obvious infiltration of CD11c⁺STING⁺ DCs and CD11c⁺p-TBK1⁺ DCs in labial gland lesions of SS patients. In NOD/Ltj SS mice, phloretin treatment recovered salivary secretion, relieved glandular inflammatory injury, and restrained excessive activation of innate and adaptive immune responses, showing comparable therapeutic effects compared with hydroxychloroquine. In vitro experiments confirmed that phloretin could alleviate poly I:C-induced abnormal activation and migration of DCs, as well as reduce the expression of co-stimulatory molecules. Mechanistically, phloretin was found to interfere with the STING signaling cascade, alter the ubiquitination level of STING protein, and further inhibit the abnormal activation of DCs, which may contribute to its protective effects against Sjögren's syndrome. Notably, results from MD, CETSA and DARTS collectively validate the interaction between phloretin and STING protein, providing solid molecular-level evidence for its regulatory effect on STING signaling transduction. Conclusions: DCs and STING-driven innate immunity critically contribute to SS progression. Phloretin effectively ameliorates SS-related immune disorders by restraining DC overactivation and STING signaling, supporting it as a promising candidate agent for SS treatment.