High Na+ Environments Impair Phagocyte Oxidase-Dependent Antibacterial Activity of Neutrophils

Abstract

Infection and inflammation can augment local Na+ abundance. These increases in local Na+ levels boost proinflammatory and antimicrobial macrophage activity and can favor polarization of T cells towards a proinflammatory Th17 phenotype. Although neutrophils play an important role in fighting intruding invaders, the impact of increased Na+ on the antimicrobial activity of neutrophils remains elusive. Here we show that, in neutrophils, increases in Na+ (high salt, HS) impair the ability of human and murine neutrophils to eliminate Escherichia coli and Staphylococcus aureus. High salt caused reduced spontaneous movement, degranulation and impaired production of reactive oxygen species (ROS) while leaving neutrophil viability unchanged. High salt enhanced the activity of the p38 mitogen-activated protein kinase (p38/MAPK) and increased the interleukin (IL)-8 release in a p38/MAPK-dependent manner. Whereas inhibition of p38/MAPK did not result in improved neutrophil defense, pharmacological blockade of the phagocyte oxidase (PHOX) or its genetic ablation mimicked the impaired antimicrobial activity detected under high salt conditions. Stimulation of neutrophils with phorbol-12-myristate-13-acetate (PMA) overcame high salt-induced impairment in ROS production and restored antimicrobial activity of neutrophils. Hence, we conclude that high salt-impaired PHOX activity results in diminished antimicrobial activity. Our findings suggest that increases in local Na+ represent an ionic checkpoint that prevents excessive ROS production of neutrophils, which decreases their antimicrobial potential and could potentially curtail ROS-mediated tissue damage.

High Salt Induces a Delayed Activation of Human Neutrophils

“…The reasons underlying the discrepancy between our results and those reported by Hatanata and coworkers are not clear, since similar experimental conditions were used in both studies. On the other hand, and consistent with our observations, Krampert and coworkers (29) recently reported that high salt induced IL-8 production and boosted IL-8 production of neutrophils infected with E.coli. However, contrasting with our results, Krampert and coworkers observed that high salt boosted IL-8 production after a short incubation period, i.e., 1.5 h, time at which we observed a profound inhibition in the production of IL-8 triggered by LPS.…”

“…However, contradictory observations have also been published. In fact, different studies have reported that high salt enhances (26) or suppresses neutrophil antibacterial activity (27)(28)(29). Similar contrasting findings have been reported regarding the ability of high salt to modulate the production of NETs (29,30) as well as the release of IL-8 by neutrophils (29,31).…”

“…Most of previous published reports have shown that high salt suppresses neutrophil function. In fact, high salt has shown to inhibit the up-regulation of CD11b/CD18 expression induced by LPS (23), the chemotactic response (25), the bactericidal activity (27)(28)(29), the exocytosis of the content of the different neutrophil granules as well as the release of ROS and neutrophil extracellular traps induced by different stimuli (24,28,29,38). By contrast, it has been reported that high salt induces the release of neutrophil extracellular traps (30) and enhances host response to bacterial challenge in experimental models, by increasing receptorindependent production of ROS by neutrophils (26).…”

The modulatory effect of high salt on immune cells and related diseases

“…Consistently, high NaCl can impaire the antimicrobial capacities of both human and murine neutrophils. In human neutrophils, the reduced bactericidal activity was due to the decreased reactive oxygen species (ROS) production 81 . Furthermore, an HSD in mice can increase the urea accumulation in the renal medulla and elevate the glucocorticoid level in the blood, resulting in the suppressed neutrophil antibacterial functionality.…”

“…In human neutrophils, the reduced bactericidal activity was due to the decreased reactive oxygen species (ROS) production. 81 Furthermore, an HSD in mice can increase the urea accumulation in the renal medulla and elevate the glucocorticoid level in the blood, resulting in the suppressed neutrophil antibacterial functionality. Healthy humans who accepted an HSD also showed hyperglucocorticoidism and inhibited neutrophil function.…”