Revisiting Erk signaling following B cell antigen receptor activation by different stimulatory agents

Revisiting Erk signaling following B cell antigen receptor activation by different stimulatory agents

Bartsch, Caren

Doctoral thesis

Date of Examination: 2016-09-15

Date of issue: 2016-09-19

Advisor: Wienands, Jürgen Prof. Dr.

Referee: Wienands, Jürgen Prof. Dr.

Referee: Groß, Uwe Prof. Dr.

Persistent Address: http://hdl.handle.net/11858/00-1735-0000-002B-7C04-C

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Abstract

English

B cell antigen receptor (BCR) ligation leads to activation of a number of intracellular signaling events that decide the fate of the B cell and thus is a key event in humoral immunity. BCR activation is tightly controlled by different inhibiting and activating coreceptors and Fc receptors. Dysregulation or imbalance between inhibiting and activating receptors can result in autoimmunity. The recombinant soluble (s)FcγRIIB has been tested in clinical trials as a promising treatment option for autoimmune diseases. Besides competing with membrane-bound Fcγ receptors for pathogenic immune-complexes, sFcγRIIB is also suggested to interfere with BCR signaling by binding to membrane IgG on memory B cells. Here, I showed that multimeric variants of recombinant sFcγRIIB induce BCR signaling events in human IgG-B cells, most probably by direct interaction with the IgG-BCR. BCR signaling induced by multimeric sFcγRIIB indicated an alternative pathway for activation of extracellular signal regulated kinase (Erk). Based on these findings, I revisited the Erk pathway in human B cells. According to current knowledge based on chicken and mouse B cells, BCR-mediated Erk activation is mainly mediated via diacylglycerol (DAG) and Ras guanyl nucleotide-releasing proteins (RasGRPs) and thus requires activation of phosphoinositide phospholipase C (PLC) γ2. In contrast, my data from human B cells showed no correlation between PLCγ activity and Erk activation, revealing species specific differences between chicken, mice and human. Instead, I showed that Erk activation depends on the growth factor receptor bound protein 2 (Grb2). This led to the suggestion that Erk activation in human B cells relies on the Grb2-Son of sevenless (Sos) signaling axis as described for many other receptor systems. The involvement of Grb2 in BCR-induced Erk activation raised the question for a Grb2-membrane anchor, linking the activated BCR to Erk activation. Indirect recruitment of Grb2 to the BCR complex via the adapter protein SHC1 turned out to be not important for Erk activation. Thus, I investigated another potential Grb2 membrane anchor, namely the recently-discovered Fc receptor for IgM (FcµR). In experiments with chimeric IgG2a FcµR receptors, I showed that the cytoplasmic domain of the FcµR indeed harbors a Grb2 binding site, which confers costimulatory functions, at least to the chimeric protein. Rigorous elucidation of BCR signaling provides important knowledge to understand the molecular biology of lymphomagenesis and autoimmune disorders for the development of potential therapeutics.

Keywords: Erk; BCR signaling; Grb2; Sos; RasGRP; B lymphocyte; Fc receptors