The role of BH3-only proteins in B cell survival

B cell activating factor of the tumor necrosis factor (TNF) family (BAFF, also known as BlyS) is a critical regulator of mature B cell survival. Lack of BAFF signaling causes B cell death and lymphopenia while excessive signaling fosters autoimmune pathology and B cell lymphomagenesis. BAFF can signal via three distinct TNF receptors (R), transmembrane activator and cyclophilin ligand interactor (TACI), B cell maturation antigen (BCMA) and BAFF-receptor (BAFF-R, also known as BR3), the latter being most critical for B cell maturation and survival. BAFF-R ligation leads to the activation of a number of key signaling molecules including ERK/MAPK, PKCß, AKT and NF-kB that regulate metabolic fitness and survival of developing and mature B cells, the latter reportedly by regulating the expression of anti-apoptotic Bcl2 family members. Consistently, overexpression of Bcl2 can restore survival of some but not all B cell subsets in the absence of BAFF. The pro-survival function of Bcl2 and its homologues is antagonized by pro-apoptotic members within the family, so-called "BH3-only proteins" such as Bim, a key-regulator of lymphocyte homeostasis. Bim induction triggered by ligation of the B cell receptor (BCR) in in vitro model systems of negative selection can be suppressed by BAFF. Consistently, bim-/- and BAFF-transgenic mice both develop sings of autoimmunity. Besides fostering autoimmunity, BAFF-signaling is also exploited by B cell tumors facilitating cell survival and growth by so far undefined molecular mechanisms. Again, repression of Bim appears to be a putative mechanism, since its loss accelerates Myc-driven B lymphomagenesis in mice and associates with disease progression and drug-resistance in men. However, B cell survival does not depend on Bim alone nor does B cell lymphomagenesis or drug-resistance in humans associate exclusively with loss of this particular BH3-only protein. A number of reports suggest that other members of this family, such as Puma, Bad or Bmf can contribute in a stimulus and cell type specific manner. Here, I propose to study the role of BH3-only proteins in BAFF-regulated B cell maturation and survival in more detail. For biochemical analysis and cell signaling studies we will take advantage of the IgM-sensitive and BAFF- responsive murine B lymphoma line WEHI231, primary splenic mouse and human B cells, as well as cells from patients suffering from B cell chronic lymphocytic leukemia (B-CLL). As a genetic tool, we will exploit mice devoid of mature B cells due to transgenic expression of a TACI-Ig fusion protein or loss of the BAFF-R gene, crossed onto different BH3-only protein-deficient backgrounds, in an attempt to restore B cell maturation and function. Finally, we intend to test the efficacy of BAFF-depletion alone and in combination with a BH3-mimetic, ABT-737, in an experimental model of autoimmunity and cancer. These studies will yield new insights as to how BAFF controls B cell survival, maturation and function, as well as related pathologies and may provide a rational for the use of "BH3-mimetics", small molecule drugs that mimic BH3-only protein function, together with BAFF-neutralizing agents in the treatment of autoimmune disorders and/or malignancies that associate with high levels of this cytokine.

B cells are a central component of our immune system producing antibodies. B cells arise in the bone marrow in response to different signals and are tolerized towards our own body that is a relevant means to preclude autoimmunity. After contact with foreign structures from bacteria or viruses, B cells are activated in order to secrete antibodies, or to turn into memory B cells with the ability to be reactivated in a highly effective manner when encountering the same pathogens, even after many years. Errors in establishing B cell tolerance, or errors during B cell activation, however, can have fatal consequences, leading to the development of autoimmunity or cancer. Maturation and activation of B cells are orchestrated in response to a large number of different signaling molecules produced in our body, such as, for example, BAFF and APRIL. These molecules, in turn, control the level of other proteins that are critical regulators of cell death or survival, and hence, in an indirect manner, affect the fate of developing as well as activated B cells. Aims of our study was to define the impact of the B cell survival factor BAFF on B cell development and function in the context of its ability to influence expression of Bcl2 family proteins. Furthermore, we were aiming to get first insights into whether these proteins may be suitable targets for the treatment of B cell-driven pathologies that correlate with high levels of Bcl2 pro-survival factors. Overall, we anticipated that this should deepen our understanding of B cell development and function and reveal first hints as to whether joint targeting of Bcl2 family proteins and BAFF-signaling would be an effective means to treat B cell-driven pathologies. Taken together, our studies yielded the following new insights: 1) A certain subgroup of the Bcl2 family, referred to as BH3-only proteins, controls B cell numbers in our body. 2) The BH3-only proteins Bim and Bmf are essential for establishing B cell tolerance and act as a barrier against autoimmunity, but also against malignant disease. 3) BAFF counteracts the function of Bim and Bmf to promote B cell survival during development and activation. 4) Neutralizing BAFF can delay the onset of B cell diseases that are characterized by increased Bcl2 expression levels. 5) BAFF signalling can affect the efficacy of anti-cancer drugs commonly used for the treatment of chronic lymphocytic leukemia (CLL) and components of this signalling pathway may be used as prognostic markers to predict therapy success in CLL.