Cutaneous Na+ Storage Strengthens the Antimicrobial Barrier Function of the Skin and Boosts Macrophage-Driven Host Defense

• Na+ accumulates at site of bacterial skin infections in humans and in mice
• Salt boosts classical macrophage (MΦ) activation and wards off infection
• Salt increases NOS2 activity in MΦ via p38/MAPK and NFAT5 signaling
• High-salt diet promotes skin Na+ storage and ameliorates cutaneous leishmaniasis

SummaryImmune cells regulate a hypertonic microenvironment in the skin; however, the biological advantage of increased skin Na+ concentrations is unknown. We found that Na+ accumulated at the site of bacterial skin infections in humans and in mice. We used the protozoan parasite Leishmania major as a model of skin-prone macrophage infection to test the hypothesis that skin-Na+ storage facilitates antimicrobial host defense. Activation of macrophages in the presence of high NaCl concentrations modified epigenetic markers and enhanced p38 mitogen-activated protein kinase (p38/MAPK)-dependent nuclear factor of activated T cells 5 (NFAT5) activation. This high-salt response resulted in elevated type-2 nitric oxide synthase (Nos2)-dependent NO production and improved Leishmania major control. Finally, we found that increasing Na+ content in the skin by a high-salt diet boosted activation of macrophages in a Nfat5-dependent manner and promoted cutaneous antimicrobial defense. We suggest that the hypertonic microenvironment could serve as a barrier to infection.

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