Excess Salt Intake Activates IL-21-dominant Autoimmune Diabetogenesis via A Salt-regulated Ste20-related Proline/alanine-rich Kinase in CD4 T Cells

The fundamental mechanisms whereby a diet affects susceptibility to or modifies autoimmune diseases are poorly understood. Despite excess dietary salt intake acts as a risk factor for autoimmune diseases, little information exists on the impact of salt intake on type 1 diabetes. To elucidate the potential effect of high-salt intake on autoimmune diabetes, non-obese diabetic (NOD) mice were fed with a high-salt diet (HSD) or a normal-salt diet from 6 to 12 weeks of age and monitored for diabetes development. Our results revealed that HSD accelerated diabetes progression with more severe insulitis in NOD mice in a CD4⁺ T cell-autonomous manner when compared to NSD group. Moreover, expression of IL-21 and SPAK in splenic CD4⁺ T cells from HSD-fed mice was significantly upregulated. Accordingly, we generated T cell-specific SPAK knockout (CKO) NOD mice and demonstrated that SPAK deficiency in T cells significantly attenuated diabetes development in NOD mice by downregulating IL-21 expression in CD4⁺ T cells. Furthermore, HSD-triggered diabetes acceleration was abolished in HSD-fed SPAK CKO mice when compared to HSD-fed NOD mice, suggesting an essential role of SPAK in salt-exacerbated T cell pathogenicity. Finally, pharmacological inhibition of SPAK activity using a specific SPAK inhibitor (closantel) in NOD mice ameliorated diabetogenesis, further illuminating the potential of a SPAK-targeting immunotherapeutic approach for autoimmune diabetes. Here, we illustrate that a substantial association between salt sensitivity and the functional impact of SPAK on T cell pathogenicity serves as a central player linking high-salt intake influences to immune-pathophysiology of diabetogenesis in NOD mice.