BCR-intrinsic regulation of memory B cell responses
Final Report Abstract
The B cell antigen receptor (BCR) is involved in almost every aspect of B cell biology and controls antigen‐independent aspects such as development and survival of the cells as well as their antigen‐ triggered activation and differentiation into plasma cells. Unlike other immunoreceptors, the BCR can change its molecular composition owing to the genetic rearrangement of immunoglobulin (Ig) genes, a process known as Ig class‐switch recombination (CSR). As a result antigen‐experienced B cells like germinal center B cells and memory B cells may express BCR isoforms that differ from those found on naïve B cells in the isotype of membrane‐bound Ig (mIg). Whereas CSR does not alter the antigen specificity of the mIg molecule, it can equip the BCR with additional signaling functions that are encoded in short peptide sequences in the intracellular domains of mIgG and mIgE that we termed immunoglobulin tail tyrosine (ITT) motifs. The central aims of this project were the characterization of memory‐type BCR‐specific signaling mechanisms (with a focus on ITT motifs) and their impact on the biology of memory B cells in vivo. To address the function of ITT signaling on the biology of IgG‐switched B cells, we analyzed a novel mouse model system, in which we had inactivated the ITT motif specifically in mIgG1 (mIgG1‐YF mice). This allowed us to analyze the impact of ITT signaling on IgG1 antibody responses and on the formation and duration of IgG1 memory. The results revealed that the ITT signaling motif is a B cell‐ intrinsic driver of IgG antibody production and furthermore promotes the longevity of IgG‐switched memory B cells in vivo. In a parallel approach, we investigated the molecular mechanisms of ITT signaling, focusing on mIgE‐BCRs. IgE antibodies are notorious for their capacity to cause allergic reactions that in extreme cases can become life‐threatening anaphylactic reactions. Probably because of these powerful and thus potentially dangerous properties, the produciton of IgE antibodies is tightly regulated in healthy individuals. The impact of the mIgE‐BCR on these regulatory processes is only poorly understood. We addressed the functional principle of mIgE‐ITT signaling using a range of biochemical and genetic model systems and were able to demonstrate a critical role of two related adaptor proteins, Grb2 and GRAP, for mIgE‐ITT signaling. While investigating the functional roles of the two adaptor proteins for ITT‐mediated signal amplification of mIgE‐BCRs, we came across a more general function of these molecules, which is their key contribution to couple the BCR of all mIg isotypes to the central Ras‐Mek‐Erk MAP kinase signaling pathway in human B cells. This result was somewhat unexpected since B cells from the mouse were previously shown to employ a different way to couple BCR engagement to activation of Ras and Erk. Finally, we identified a regulatory mechanism that is also specific for human (and other primate) B cells, which might restrict the production of (potentially anaphylactic) IgE antibodies. This mechanism involves the retention of fully assembled mIgE‐BCR complexes in the endoplasmic retention, as a consequence of which the amount of cell surface‐expressed mIgE is strongly diminished. Mechanistically, ER retention of human and primate mIgE‐BCRs is brought about by incorporation of an extended extracellular membrane‐proximal domain (EMPD) in mIgE by alternative splicing of primary εm Ig heavy chain‐encoding transcripts. This extended EMPD is exclusively found in humans and primates and adopts an intrinsically unstructured conformation, which presumably is not tolerated by the ER quality control system and is thus not allowed to get efficiently transported to the cell surface. Consequently, limited surface expression of EMPD‐containing mIgE‐BCRs is associated with a strongly reduced capacity to signal activation of the IgE‐switched B cells.
Publications
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2015. Reactivation of IgG‐switched memory B cells by BCR‐intrinsic signal amplification promotes IgG antibody production. Nature Communications 6, 8575
Lutz, J., Dittmann, K., Bösl, M.R., Winkler, T.H., Wienands, J. & Engels, N.
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2016. The Memory Function of the B Cell Antigen Receptor. Current Topics in Microbiology and Immunology 393:107‐121
Wienands, J. & Engels, N.
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2018. Grb2 and GRAP connect the B cell antigen receptor to Erk MAP kinase activation in human B cells. Scientific Reports 8, 4244
Vanshylla, K., Bartsch, C., Hitzing, C., Krümpelmann, L., Wienands, J. & Engels, N.
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2018. The extracellular membrane‐proximal domain of membrane‐bound IgE restricts B cell activation by limiting B cell antigen receptor surface expression. European journal of immunology 48:441‐453
Vanshylla, K., Opazo, F., Gronke, K., Wienands, J. & Engels, N.
