Salt augments macrophage-driven host defense in the skin

Immune cells regulate a hypertonic microenvironment in the skin; however, possible functions of increased skin Na+ concentrations are unknown. We found that Na+ accumulated at the site of bacterial skin infections in humans and in mice. We used the protozoan parasite Leishmania (L.) 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 L. major elimination. Finally, we found that increasing Na+ content in the skin by a high-salt diet boosted activation of macrophages in an Nfat5-dependent manner and promoted cutaneous antimicrobial defense. We suggest that the hypertonic microenvironment could serve as a barrier to infection.