Abstract CD23 is a trans-membrane protein belonging to the C-type lectin family, widely expressed in the immune system. Originally identified as the low-affinity receptor for IgE, CD23 was later shown to also bind CD21, MHCII and various integrins, and to have pleiotropic roles, including modulation of cell proliferation, differentiation, Ig and cytokine secretion. Expression of membrane CD23 (mCD23) is regulated at least in part by its shedding from the cell surface via activity of metalloproteinases, the most important of which is ADAM10. Interestingly, secreted CD23 (sCD23) isoforms can exert immunomodulatory functions as soluble factors. B cells upregulate mCD23 and serum sCD23 levels have been found to be elevated in inflammatory and autoimmune conditions, including rheumatoid arthritis, and in B cell chronic lymphocytic leukemia. However, the mechanisms responsible for CD23 modulation on B cells are still unclear. Here we show that mCD23 is upregulated on mouse and human B cells found in inflamed lymphoid tissues. Furthermore, in vitro experiments demonstrate that B cell mCD23 upregulation is strictly dependent on soluble signals provided by stromal cells from inflamed, but not non-inflamed lymph nodes, and can be reversed by ADAM10-specific inhibitors. We propose therefore that the balance between B cell mCD23 and sCD23 expression in inflammatory conditions is modulated by microenvironmental signals provided by stromal cells within lymphoid tissues.