The Impact of a High-Salt Diet on Macrophage Development and Function in the Gut and the Liver of Mice

The prevalence of unhealthy diets poses a significant global challenge to public health and development. Among dietary risks, excessive salt intake remains a major concern. High-salt diets (HSD) have been linked to numerous health issues, including elevated blood pressure, cardiovascular diseases, and gastric cancer. Importantly, macrophages, the key immune-regulating phagocytes, have emerged as central mediators of HSD-associated adverse effects. The gut-liver axis, being the primary interface for nutritional and microbial content, is particularly vulnerable to HSD-induced stress, promoting tissue damage and persistent inflammatory responses. However, the role of macrophages in mediating these adaptations remains elusive. In this study, we investigated how short- and long-term HSD affects gut and liver macrophages in mice. Long-term HSD induced sex-dependent structural and functional changes across the gut-liver axis, including ileal pathology, gut microbial imbalance, and hepatic lipid accumulation. Functional assays showed increased intestinal permeability, indicating epithelial barrier damage, and impaired recovery in a colitis model, potentially mediated by altered macrophage phagocytic Fcγ receptors (FcγRs). Single-nucleus RNA sequencing of the ileum revealed transcriptional changes in volume regulation pathways, notably the Wnk1-dependent chloride-sensing pathway. Further 3D imaging showed increased macrophage cell volume in both tissues following long-term HSD, potentially affecting their function. High-dimensional flow cytometry enabled detailed characterization of gut and liver macrophage subsets, highlighting an increase in MHCII⁺Cx3cr1⁺CD13^⁻ macrophages in the ileum of male mice and a reduction in Kupffer cells, the liver-resident macrophages, in females. A novel fate-mapping model to track fetal- and bone marrow-derived macrophages uncovered an enhanced monocyte-driven replacement of fetal-derived macrophages in both tissues with niche-specific dynamics. Functional and transcriptomic analyses further revealed dysregulated macrophage phagocytic activity in both organs, associated with FcγRIIb expression. In conclusion, these findings demonstrate that long-term HSD systemically alters macrophage function and development in the gut and liver, contributing to tissue damage and inflammatory-like responses linked to HSD. This underscores the important role of macrophages and the mechanisms they mediate in HSD-associated diseases.