Membrane Bound IgE: The Key Receptor to Restrict High IgE Levels

The membrane form of immunoglobulins (mIg) is expressed on the surface of B lymphocytes from a very early developmental stage in the bone marrow (pre B cell) until the cell finally differentiates into a plasma cell. mIgs associate with other transmembrane proteins to form the B cell antigen receptor complex (BCR). Numerous studies have confirmed that signaling through the BCR not only steers the B cell through development, but also ensures its survival in the periphery as a fully matured cell. During development in the bone marrow, mIg is restricted to the IgM isotype, but as soon as the B cell leaves the bone marrow to populate peripheral lymphoid organs like spleen, lymph nodes, or intestinal mucosal tissue, it starts to express IgD and in later stages of maturation IgG, IgE and IgA. Engagement of the B cell receptor triggers signals that control affinity maturation, memory induction, differentiation and other physiological processes in B cells. We showed that truncation of the cytoplasmic tail of mIgE in vivo results in lower serum IgE levels, decreased numbers of IgE-secreting plasma cells, and abrogation of specific secondary responses correlating with a defect in the selection of high-affinity antibodies during the germinal centre reaction. We conclude that the BCR is necessary at all times during antibody responses not only for the maturation process, but also for the expansion of antigen specific B cells. As a result of these findings it is obvious that the IgE antigen receptor could be a useful target molecule for therapeutic inter-