Effects of High-Dietary Salt on Immune Cells and Microbiome: Results from a Randomized Clinical Trial

Background: High dietary salt intake ranks among the most prominent risk factors and has been tied to hypertension and mortality. The mechanisms responsible are incompletely understood and have been extended to inflammatory and microbiome-associated processes. Building on preliminary data, the present study investigates whether a moderate increase in salt intake in healthy subjects primes host physiology to a more transiently unstable state, potentially leading to changes in the microbiome-immune axis. Methods: We conducted a prospective, randomized, double-blinded trial to evaluate the effect of high dietary salt in healthy participants (NCT03024567), where the intervention group (n = 19) was given 6g of salt (NaCl) in addition to their daily normal salt intake, essentially doubling the recommended salt intake against a placebo group (n=19) for 14 days. Clinical parameters, stool and PBMC were collected at baseline and day 14. Shotgun metagenomic sequencing, metabolomics (stool and serum), and single-cell sequencing (CITEseq) of whole PBMCs and CD4+ sorted T cells was performed. Results: The results confirm previous findings showing an increase in the dissimilarity of the microbiome composition under a high salt diet. Differential abundance of the microbiome's functional space, as quantified by functional gut-specific modules, picked up a salt-specific module (M00494, NatK-NatR (sodium extrusion) two-component regulatory system) in the salt group. Furthermore, among all immune cell subsets, we detected 500 common differently expressed genes (DEGs) solely in the salt group. Most of which belonged to to mitochondrial respiration and energy metabolism. Interestingly, we found appr. 8000 DEG in naïve T helper cells and 4000 DEGs in naïve cytotoxic T cells. Commonly, we detected a gene set enrichment for mitochondrial respiration in naïve T cell populations (CD4, CD8), functionally validated by Real Time Cell Metabolic Analysis (Seahorse). Conclusion: Our study is among the first to utilize different omics techniques to identify high-salt-induced changes in the microbiome and immunome in healthy individuals that may be relevant to the development of pathological conditions in the long term.