Mannitol upregulates monocyte HLA-DR, monocyte and neutrophil CD11b, and inhibits neutrophil apoptosis.

D-Mannitol is a substance widely used for both clinical applications as a strong diuretic and in basic research as a purportedly inert osmotic control substance. However, recent experiments have shown that mannitol is able to decrease neutrophil apoptosis in vitro by more than 25%. The aim of the current study was to assess mannitol's effects on two immune cell activation markers; CD11b on neutrophils and monocytes, and HLA-DR on monocytes. Exposure of diluted whole blood (1:10 in RPMI 1640) to increasing concentrations of mannitol for 24 h was associated with a significant increase in both monocyte and neutrophil CD11b expression in non-lipopolysaccharide (LPS)-stimulated samples. Monocyte HLA-DR, but not neutrophil HLA-DR, was significantly higher in the presence of 16.5 mM/l mannitol, whereas isoosmolar NaCl had no effect. Levels of IL-6 and TNF-alpha were not affected by mannitol in our model. All reagents were tested negative for endotoxin contamination. Together, these data indicate that mannitol may directly interact with neutrophils and monocytes. Further systematic evaluation is indicated to define the precise immunomodulating actions of mannitol, and to assess these effects in patients receiving such therapy.