High Salt (NaCl) Reduces The Activation And Function Of Alternatively Activated (M2) Macrophages Via Epigenetic Modifications

High intake of dietary salt (sodium chloride; NaCl) is attributed to the development of hypertension and cardiovascular disease, and has been proposed to be a cause for the rapid increase in autoimmune diseases in western civilisations. We have recently shown that NaCl has a pro-inflammatory effect and boosts the activation of Th17 cells in vitro, with mice fed a high salt diet having an accelerated and more severe experimental autoimmune encephalomyelitis. In this study, we examine how the activation of alternatively activated (M2) macrophages is affected by NaCl. In stark contrast to our study with Th17 cells, we find that high salt dose-dependently decreased M2 activation in IL-4+IL-13 stimulated BM-derived mouse macrophages. Genes important for M2 activation, including Mrc1, Arg1, Ym1, Fizz1 and PD-L2, all had a blunted expression in the presence of NaCl; an effect which was not observed in tonicity controls (mannitol or urea), implying a specific action of NaCl. Transcription factors important for modulating M2 function (Irf4 and Klf4) were similarly affected. In contrast to our previous findings in Th17 cells, the effect of salt on M2 activation was not mediated via Sgk-1. To explore the mechanism for the effect of NaCl on M2 activation we performed gene expression analysis simultaneously with genome wide epigenetic modification analysis (ChIP-seq for H3K4me3 and H4ac). The results of this revealed that NaCl modulated epigenetic marks at genes important for M2 activation, and additionally identified new genes which were affected. Finally, we asked if these salt-mediated changes in M2 gene profiles, translated into effects on their function. Using an in vitro assay, we found that NaCl-treated M2 macrophages have a reduced ability to suppress the activation of effector T cells. Our study reveals a novel effect of NaCl on M2 activation and function, and gives support to the notion that the modulation of immune cell function by high dietary salt is relevant to hypertension and autoimmune diseases.