The role of TRAF3 in B cell survival and activation
Creators: name: Gokhale, Samantha
Publisher: name: No Publisher Supplied
Year: 2022

description: Tumor necrosis factor receptor 3 (TRAF3) is a cytoplasmic adaptor protein critical to the homeostasis and function of B lymphocytes. TRAF3 is an integral signaling protein for B-cell activating factor receptor (BAFF-R), required to maintain B cell survival, and CD40, a receptor necessary for triggering T-dependent B cell activation and subsequent antibody immune response. The loss of TRAF3 enables constitutive activation of NF-κB2 downstream of these receptors and consequently promotes aberrant survival signaling. However, further mechanistic understanding of how TRAF3 regulates B cell survival and activation remains elusive. To address this, we exploited multiple “omics” approaches, including transcriptomics, metabolomics, lipidomics, and proteomics, to identify key molecular players that are regulated by TRAF3 in B lymphocytes. Our transcriptomic data revealed that B cell TRAF3 deficiency resulted in overexpression of the enzyme choline kinase α (Chkα) which subsequently increased the production of phosphocholine (P-Cho) and phospholipids, specifically phosphatidylcholine (PC)and phosphatidylethanolamine (PE). The elevated Chkα-P-cho-PC/PE signaling pathway was demonstrated to be essential for the extended survival phenotype of TRAF3-/- B cells. Interestingly, our proteomic study identified mitochondrial fission factor (MFF) as a novel TRAF3-binding partner in B cells. Our detailed mechanistic investigation elucidated that increased levels of mitochondrial TRAF3 led to decreased MFF phosphorylation and ubiquitination, altered mitochondrial morphology, decreased mitochondrial respiration, increased mitochondrial ROS production and membrane permeabilization, resulting in apoptosis. Furthermore, our transcriptomic data also identified IL-9R as a downstream target strikingly up-regulated in TRAF3-/- B cells, which prompted us to further pursue how TRAF3 regulates B cell activation in response to IL-9 stimulation, in the absence or presence of B cell antigen receptor (BCR) crosslinking and IL-4. We found that IL-9 induced STAT3 phosphorylation and IgM production in TRAF3-deficient, but not in wild type, B lymphocytes. Notably, IL-9 together with BCR crosslinking plus IL-4 significantly potentiated Ig isotype switching to IgG1 through inducing γ1 germline transcription, γ1switch circle generation and AID expression in TRAF3-deficient B cells. We further demonstrated that blocking the JAK-STAT3 signaling pathways abrogated the CSR to IgG1induced by IL-9 in combination with BCR crosslinking plus IL-4. Our findings thus revealed that TRAF3 suppresses B cell activation and class switch recombination (CSR) by inhibiting the IL-9R -JAK-STAT3 signaling axis and that in the absence of TRAF3, B cell activation and CSR no longer require cognate T cell help. In conclusion, my dissertation research illustrates previously unknown regulatory mechanisms of TRAF3 in B cell survival and activation, highlighting its importance in B cell physiology. Our findings bear significant implications for the understanding and treatment of a variety of human diseases involving aberrant B cell survival and activation, including B cell malignancies and autoimmune disorders. Indeed, our studies elucidated multiple TRAF3-regulated signaling pathways and downstream signaling components that present could serve as new therapeutic potential targets for the treatment of B cell malignancies and autoimmune diseases.