Environmentally relevant doses of fluoride exposure mediate kidney injury by affecting Tregs/Teffs homeostasis

Although global implementation of water improvement and fluoride reduction measures has led to an overall decline in environmental fluoride exposure, the risk of damage to non-skeletal tissues, including the kidneys, may persist even at concentrations below the WHO drinking water safety guideline of 1.5 mg/L. Mounting evidence indicates that excessive activation of the immune system and inflammatory infiltration are important contributors to kidney injury, among which the balance between Tregs and Teffs is critical for maintaining renal function homeostasis. However, it remains unclear whether Tregs/Teffs imbalance is involved in fluoride-induced kidney injury under real-world environmentally relevant exposure levels. This study aimed to explore the association among fluoride exposure, Tregs/Teffs imbalance, and renal injury, and to evaluate the clinical value of the Tregs/Teffs ratio as a predictive biomarker for fluoride-induced renal damage. We conducted a cross-sectional survey in drinking water type of fluorosis in China, and measured urinary fluoride, renal function markers (NAG and MALB), the peripheral blood Tregs/Teffs ratio, and the expression levels of P65 and C-rel in Tregs. Simultaneously, a drinking water fluorosis model was established in Wistar rats with five fluoride concentration gradients (0, 10, 25, 50, and 100 mg/L), and the rats were exposed for 6 and 12 weeks. The Tregs/Teffs ratios in the spleen and kidney, as well as the expression levels of P65, p-P65, PKC-θ, p-PKC-θ, IL-1β, IL-6, and IL-2 in renal Tregs, were measured. Based on the population data, a nomogram model for predicting renal dysfunction was constructed and internally validated. The results of the field survey showed that an elevated peripheral blood Tregs/Teffs ratio was associated with an increased risk of NAG and MALB abnormalities and significantly mediated fluoride-induced renal dysfunction. In the fluoride-exposed rat model, kidney injury was closely associated with dynamic changes in the renal Tregs/Teffs ratio, and the levels of P65, p-P65, PKC-θ, p-PKC-θ, IL-1β, IL-2, and IL-6 in renal Tregs were altered. The risk prediction model for renal dysfunction, constructed by combining urinary fluoride and the Tregs/Teffs ratio, demonstrated good discrimination in both the training set and the internal validation set. Calibration curves, decision curve analysis, and clinical impact curves further confirmed that the model had satisfactory calibration and clinical net benefit. These results indicate that fluoride-induced kidney injury is dynamically regulated by Tregs/Teffs imbalance, that the Tregs/Teffs ratio combined with urinary fluoride holds promise as a biomarker for predicting fluoride-induced renal dysfunction, and that changes in P65 may represent a potential regulatory mechanism in this process.