Recent evidence from chemical analysis of tissue electrolyte and water composition has shown that body Na+ content in experimental animals is not constant, does not always readily equilibrate with water, and cannot be exclusively controlled by the renal blood purification process. Instead, large amounts of Na+ are stored in the skin and in skeletal muscle. Quantitative non-invasive detection of Na+ reservoirs with 23NaMRI suggests that this mysterious Na+ storage is not only an animal research curiosity, but also exists in humans. In clinical studies, tissue Na+ storage is closely associated with essential hypertension. In animal experiments, modulation of reservoir tissue Na+ content leads to predictable blood pressure changes. The available evidence thus suggests that the patho(?)-physiological process of Na+ storage might be even of relevance for human health and disease.

B lymphocytes, as a central part of adaptive immune responses, have the ability to fight against an almost unlimited numbers of pathogens. Impairment of B cell development, activation and differentiation to antibody secreting plasma cells can lead to malignancy, allergy, autoimmunity and immunodeficiency. However, the impact of environmental factors, such as hyperosmolality or osmotic stress caused by varying salt concentrations in different lymphoid organs, on these processes is not well-understood. Here, we report that B cells respond to osmotic stress in a biphasic manner. Initially, increased osmolality boosted B cell activation and differentiation as shown by an untimely downregulation of Pax5 as well as upregulation of CD138. However, in the second phase, we observed an increase in cell death and impaired plasmablast differentiation. Osmotic stress resulted in impaired class switch to IgG1, inhibition of phosphorylation of p38 mitogen-activated kinase and a delayed NFAT5 response. Overall, these findings demonstrate the importance of microenvironmental hyperosmolality and osmotic stress caused by NaCl for B cell activation and differentiation.