Salt: the microbiome, immune function and hypertension

Credit: Carl Conway/Macmillan Publishers Limited

Previous studies have demonstrated a role for pro-inflammatory T cells — particularly T helper 17 (T_H17) cells — in the development of hypertension. The generation of pathogenic T_H17 cells is regulated by specific gut microbiota and can be stimulated by a high-salt environment, but the effects of salt on the gut microbiome are unknown. New research now shows that a high-salt diet (HSD) affects the abundance of intestinal bacteria in mice, with depletion of Lactobacillus murinus. Treatment of mice with L. murinus reduced T_H17 cell numbers and prevented salt-sensitive hypertension, while a pilot study in humans showed that a high-salt challenge reduced intestinal survival of Lactobacillus spp., increased T_H17 cell numbers and increased blood pressure. “Our results extend the existing knowledge on the effects of HSD to another compartment of the body — the microbiome,” say researchers Dominik Müller and Nicola Wilck. “In addition to the known direct effect of salt and the ionic microenvironment on T_H17 cells, microbiome-mediated mechanisms could represent a novel route by which salt acts upon the body, particularly the immune system.”

Müller explains that the effect of salt on the immune system and hypertension has been a long-standing focus of his research. “Increasing evidence suggests that the microbiome is shaped by diet, with possible consequences for the host organism and the immune system — it is interesting that the microbiome has been overlooked with regard to diets rich in salt,” adds Wilck. To assess the effect of a HSD on the composition of the microbiome, the researchers performed 16S ribosomal DNA sequencing of faecal pellets from mice fed a normal-salt diet (NSD) or a HSD, which identified L. murinus as the species most strongly altered in response to a HSD. In vitro studies confirmed the ability of NaCl to inhibit the growth of L. murinus and several human-associated Lactobacillus spp.

Microbiome-mediated mechanisms could represent a novel route by which salt acts upon the body

To assess the effects of L. murinus depletion on physiological responses, the researchers administered L. murinus to two mouse models of disease — experimental autoimmune encephalomyelitis (EAE) and salt-sensitive hypertension. In mice with EAE, a disease that has been linked to alterations in the gut microbiome, administration of a HSD increased numbers of intestinal T_H17 cells and exacerbated the disease course. Administration of L. murinus by oral gavage normalized T_H17 cell numbers and attenuated disease severity. To assess the mechanisms for this effect, the researchers investigated the presence of indole metabolites. Lactobacillus spp. can metabolize tryptophan to indole metabolites, and in accordance with the depletion of L. murinus, the researchers noted lower levels of indole metabolites in faeces of mice fed a HSD — an effect that was reversed by L. murinus supplementation. In vitro, the indole metabolite ILA significantly reduced T_H17 polarization, suggesting that salt alters T_H17 cells by affecting the abundance of L. murinus tryptophan metabolites. In mice with salt-sensitive hypertension, administration of L. murinus reduced blood pressure and T_H17 cell frequencies compared with levels in mice fed a HSD.

Finally, the researchers assessed the applicability of their findings in mice to humans by administering slow-release NaCl tablets (equating to 6 g NaCl per day on top of their normal eating habits). This salt challenge increased blood pressure from baseline and induced a significant increase in the number of peripheral blood lymphocyte T_H17 cells. Use of full shotgun metagenomic sequencing of faecal samples demonstrated a loss of Lactobacillus spp. in response to the salt challenge, which is in line with the findings in mice and highlights the microbiome as a salt-sensitive compartment. Müller says that they are planning further studies to assess the mechanisms that underly the effect of Lactobacillus spp. on T_H17 cells. “We are also aiming to perform controlled clinical studies in humans with hypertension and multiple sclerosis to test the effect of Lactobacillus-containing probiotics on these diseases,” he adds.