Murine B cells regulate serum IgE levels in a CD23-dependent manner.

The manifestations of allergic disorders are closely tied to the biologic effects of IgE activation with Ag. In immediate hypersensitivity reactions, IgE effector function requires prior binding to innate immune cells, primarily mast cells and basophils, with the blood acting as a reservoir for unbound IgE. As the severity of allergic disease is proportional to the size of this unbound IgE pool, we hypothesized that cellular mechanisms exist to limit the size and/or enhance the clearance of free IgE molecules. We examined this in mice by engineering a reporter IgE molecule that allowed us to track the fate of IgE molecules in vivo. The absence of FcεRI-expressing cells did not affect serum IgE levels, but B cells regulated serum IgE by controlling the size of the free IgE pool. B cells captured IgE by direct binding to the low-affinity IgE receptor, CD23. These data indicate a mechanism regulating serum IgE and additionally clarify the role of CD23 in this process.