Calcium Release-Activated Calcium Modulator ORAI1-Sensitive Serine Dehydratase Regulates Fatty Acid-Induced CD4+ Th17/Treg Imbalance in Dairy Cows

High concentrations of free fatty acids (FFAs) caused by negative energy balance render the cow more prone to inflammatory diseases in part due to an imbalance in the types of immune cells and their specific functions. We previously demonstrated that ORAI calcium release-activated calcium modulator 1 (ORAI1) was associated with increased CD4⁺ Th17 content, but the precise mechanisms remain unclear. The purpose of this study was to evaluate the efficacy of FFAs on CD4⁺ T cell inflammatory response. High FFAs in dairy cows caused the transcript level of the pro-inflammatory factor IL-17A, plasma concentration of IL-17A, and amount of intracellular IL-17A to increase while the transcript levels and intracellular amount of the anti-inflammatory factor FOXP3 were downregulated. These changes indicated Th17/Treg imbalance and inflammation in dairy cows with high FFA. Moreover, ORAI1 and SDS abundance was elevated in dairy cows with high FFAs by transcriptomics, QPCR, and Western blot. Knockdown of SDS (siSDS) did not alter ORAI1 expression in CD4⁺ T cells from high-FFA cows, while it decreased the expression of inflammatory factors. Transfection of CD4⁺ T cells using siRNA knockdown for ORAI1 (siORAI1) revealed that SDS and inflammatory factor abundance decreased. Serine can be catabolized to pyruvate by the action of serine dehydratase (SDS). Data from this study suggested that high FFAs caused by negative energy balance after calving regulates the Th17/Treg balance via SDS, but SDS does not regulate ORAI1 abundance. The above data suggested a pro-inflammatory mechanism in CD4⁺ T cells regulated by the ORAI1-sensitive SDS pathway in early postpartum cows experiencing high-FFA conditions. Thus, targeting this pathway may represent a new therapeutic and interventional approach for preventing immune-related disorders around parturition.