| dc.description.abstracteng | Robust memory antibody responses are a distinguishing feature of humoral immunity and are typically marked by elevated serum immunoglobulin (Ig) titers that rapidly and effectively neutralize the invading pathogen. However, relatively little is known about the molecular mechanisms that govern the activation of memory B cells. In particular, the biology of IgE-expressing memory B cells remains elusive due to the scarcity of this B cell population, despite the clinical importance of IgE as a major driver for immediate hypersensitivity reactions. By analyzing human membrane bound IgE (mIgE) in B cell lines, this work focused on the molecular composition and signaling features of mIgE-containing B cell antigen receptors (BCRs).
A previously described mIg-intrinsic motif called the immunoglobulin tail tyrosine (ITT), which is exclusively present in the cytoplasmic tails of mIgG and mIgE, is known to augment IgG1 responses in a mouse model system. The mIgG-ITT amplifies BCR signaling via a Grb2-Btk complex that enhances intracellular Ca2+ mobilization. However, I found that unlike the mIgG-ITT which exclusively signals via the adaptor protein Grb2, the mIgE-ITT also employs the Grb2 related adaptor protein (Grap), which together with Grb2, plays an important role in mIgE-BCR signaling. The absence of these adaptor proteins severely impairs the capability of mIgE-BCRs to mobilize Ca2+ and activate the mitogen activated protein (MAP) kinase extracellular signal regulated kinase (ERK) pathway, both of which are central pathways for B cell activation, proliferation and differentiation. Therefore, by enhancing BCR signaling via the ITT-Grb2/Grap module, mIgE-BCRs lower the activation threshold of mIgE-expressing B cells.
Furthermore, a new role for Grb2 and Grap in the canonical immunoreceptor tyrosine-based activation motif (ITAM) signaling pathway of ITT-less BCRs was revealed, since mIgM-BCRs also require these proteins for optimal Ca2+ signaling. Grb2 and Grap contribute to ITAM-induced Ca2+ mobilization through improved activation of the Ca2+ initiation complex proteins Syk, SLP65 and PLC2. Therefore, Grb2 and Grap are not only necessary for mIgE- but also mIgM-BCR signaling in human B cells.
Intriguingly, unlike the mIgM molecule, the human mIgE molecule can be expressed on the surface of B cells in the absence of the Ig/Ig heterodimer, a characteristic that depends on the cytoplasmic tail of mIgE. The mIgE-ITT motif is signaling competent in the absence of Ig/Ig and requires the catalytic activity of the tyrosine kinase Syk. These results highlight differences in signaling mechanisms employed by different BCR isotypes. While mIgM-expressing B cells are restricted by the Ig/Ig heterodimer for BCR expression and signaling, mIgE-expressing B cells can utilize the ITT motif independently of Ig/Ig for activation of signaling pathways. This non-canonical mIgE signaling via the ITT motif provides preliminary insight into novel signaling mechanisms that could be regulating the fate of the IgE memory and plasma B cell compartment. | de |