Nuclear TRAF3 is a negative regulator of CREB in B cells.

The adaptor protein TNF receptor-associated factor 3 (TRAF3) regulates signaling through B-lymphocyte receptors, including CD40, BAFF receptor, and Toll-like receptors, and also plays a critical role inhibiting B-cell homoeostatic survival. Consistent with these findings, loss-of-function human TRAF3 mutations are common in B-cell cancers, particularly multiple myeloma and B-cell lymphoma. B cells of B-cell-specific TRAF3(-/-) mice (B-Traf3(-/-)) display remarkably enhanced survival compared with littermate control (WT) B cells. The mechanism for this abnormal homeostatic survival is poorly understood, a key knowledge gap in selecting optimal treatments for human B-cell cancers with TRAF3 deficiency. We show here for the first time to our knowledge that TRAF3 is a resident nuclear protein that associates with the transcriptional regulator cAMP response element binding protein (CREB) in both mouse and human B cells. The TRAF-C domain of TRAF3 was necessary and sufficient to localize TRAF3 to the nucleus via a functional nuclear localization signal. CREB protein was elevated in TRAF3(-/-) B cells, without change in mRNA, but with a decrease in CREB ubiquitination. CREB-mediated transcriptional activity was increased in TRAF3-deficient B cells. Consistent with these findings, Mcl-1, an antiapoptotic target of CREB-mediated transcription, was increased in the absence of TRAF3 and enhanced Mcl-1 was suppressed with CREB inhibition. TRAF3-deficient B cells were also preferentially sensitive to survival inhibition with pharmacologic CREB inhibitor. Our results identify a new mechanism by which nuclear TRAF3 regulates B-cell survival via inhibition of CREB stability, information highly relevant to the role of TRAF3 in B-cell malignancies.

TRAF3, ubiquitination, and B-lymphocyte regulation.

The signaling adapter protein tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) is both modified by and contributes to several types of ubiquitination events. TRAF3 plays a variety of context-dependent regulatory roles in all types of immune cells. In B lymphocytes, TRAF3 contributes to regulation of signaling by members of both the TNFR superfamily and innate immune receptors. TRAF3 also plays a unique cell type-specific and critical role in the restraint of B-cell homeostatic survival, a role with important implications for both B-cell differentiation and the pathogenesis of B-cell malignancies. This review focuses upon the relationship between ubiquitin and TRAF3, and how this contributes to multiple functions of TRAF3 in the regulation of signal transduction, transcriptional activation, and effector functions of B lymphocytes.

Author Correction: TRAF3 regulates the oncogenic proteins Pim2 and c-Myc to restrain survival in normal and malignant B cells.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

TRAF3 regulates the oncogenic proteins Pim2 and c-Myc to restrain survival in normal and malignant B cells.

TRAF3 is a versatile intracellular adapter protein with multiple context-specific roles. Uniquely in B cells, TRAF3 deficiency enhances survival and increases the risk of transformation, as loss of TRAF3 is observed in several types of B cell cancers. Here, we report a new mechanism for TRAF3 in the restraint of B cell survival. We found that TRAF3 deficiency was associated with induction of the pro-survival kinase Pim2 in mouse primary B cells and human malignant B cell lines. The increase in Pim2 was independent of NF-κB2 activation but was ameliorated with inhibition of STAT3 expression or function. TRAF3 deficiency also led to a Pim2-dependent increase in c-Myc protein levels and was associated with reduced c-Myc ubiquitination. TRAF3-deficient primary B cells were less sensitive to cell death induced by the Pim inhibitors SGI-1776 and TP-3654. Interestingly, human malignant B cell lines with low expression of TRAF3 were more sensitive to Pim inhibition-induced cell death. Combination treatment of TRAF3-deficient B cells and B cell tumor lines with c-Myc inhibitors enhanced their sensitivity to Pim inhibition, suggesting a possible therapeutic strategy. TRAF3 thus suppresses a Pim2-mediated B cell survival axis, which can be a potential target for treatment of B cell malignancies.

Self-reactive B cells in the GALT are actively curtailed to prevent gut inflammation.

Immune homeostasis in the gut associated lymphoid tissues (GALT) is critical to prevent the development of inadvertent pathologies. B cells as the producers of antibodies and cytokines plays an important role in maintaining the GALT homeostasis. However, the mechanism by which B cells specifically direct their responses towards non-self-antigens and become ignorant to self-antigens in the GALT is not known. Therefore, we developed a novel mouse model by expressing Duck Egg Lysozyme (DEL) in gut epithelial cells in presence of HEL reactive B cells. Notably, we observed a transient activation and rapid deletion of self-reactive B cells in Peyers Patches and Mesenteric lymph nodes upon self-antigen exposure. The survival of self-reactive B cells upon exposure to their self-antigen was partially rescued by blocking receptor editing but could be completely rescued by stronger survival signal like ectopic expression of BCL2. Importantly, rescuing the self-reactive B cells promoted production of auto-antibodies and gut inflammation. Mechanistically, we identify a specific activation of TGFß signaling in self-reactive B cells in the gut and a critical role of this pathway in maintaining peripheral tolerance. Collectively, our studies describe functional consequences and fate of self-reactive B cells in GALT and provide novel mechanistic insights governing self-tolerance of B cells in the gut.

Co-expression Networks Identify DHX15 RNA Helicase as a B Cell Regulatory Factor.

Genome-wide co-expression analysis is often used for annotating novel gene functions from high-dimensional data. Here, we developed an R package with a Shiny visualization app that creates immuno-networks from RNAseq data using a combination of Weighted Gene Co-expression Network Analysis (WGCNA), xCell immune cell signatures, and Bayesian Network Learning. Using a large publicly available RNAseq dataset we generated a Gene Module-Immune Cell (GMIC) network that predicted causal relationships between DEAH-box RNA helicase (DHX)15 and genes associated with humoral immunity, suggesting that DHX15 may regulate B cell fate. Deletion of DHX15 in mouse B cells led to impaired lymphocyte development, reduced peripheral B cell numbers, and dysregulated expression of genes linked to antibody-mediated immune responses similar to the genes predicted by the GMIC network. Moreover, antigen immunization of mice demonstrated that optimal primary IgG1 responses required DHX15. Intrinsic expression of DHX15 was necessary for proliferation and survival of activated of B cells. Altogether, these results support the use of co-expression networks to elucidate fundamental biological processes.

TRAF3 deficiency promotes metabolic reprogramming in B cells.

The adaptor protein TNF receptor-associated factor 3 (TRAF3) is a critical regulator of B lymphocyte survival. B cell-specific TRAF3 deficiency results in enhanced viability and is associated with development of lymphoma and multiple myeloma. We show that TRAF3 deficiency led to induction of two proteins important for glucose metabolism, Glut1 and Hexokinase 2 (HXK2). This was associated with increased glucose uptake. In the absence of TRAF3, anaerobic glycolysis and oxidative phosphorylation were increased in B cells without changes in mitochondrial mass or reactive oxygen species. Chemical inhibition of glucose metabolism or glucose deprivation substantially attenuated the enhanced survival of TRAF3-deficient B cells, with a decrease in the pro-survival protein Mcl-1. Changes in Glut1 and Mcl-1 levels, glucose uptake and B cell number in the absence of TRAF3 were all dependent upon NF-κB inducing kinase (NIK). These results indicate that TRAF3 deficiency suffices to metabolically reprogram B cells, a finding that improves our understanding of the role of TRAF3 as a tumor suppressor, and suggests potential therapeutic strategies.

The adaptor protein TRAF3 inhibits interleukin-6 receptor signaling in B cells to limit plasma cell development.

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is an adaptor protein that inhibits signaling by CD40 and by the receptor for B cell-activating factor (BAFF) and negatively regulates homeostatic B cell survival. Loss-of-function mutations in TRAF3 are associated with human B cell malignancies, in particular multiple myeloma. The cytokine interleukin-6 (IL-6) supports the differentiation and survival of normal and neoplastic plasma cells. We found that mice with a deficiency in TRAF3 specifically in B cells (B-Traf3(-/-) mice) had about twice as many plasma cells as did their littermate controls. TRAF3-deficient B cells had enhanced responsiveness to IL-6, and genetic loss of IL-6 in B-Traf3(-/-) mice restored their plasma cell numbers to normal. TRAF3 inhibited IL-6 receptor (IL-6R)-mediated signaling by facilitating the association of PTPN22 (a nonreceptor protein tyrosine phosphatase) with the kinase Janus-activated kinase 1 (Jak1), which in turn blocked phosphorylation of the transcription factor STAT3 (signal transducer and activator of transcription 3). Consistent with these results, the number of plasma cells in the PTPN22-deficient mice was increased compared to that in the wild-type mice. Our findings identify TRAF3 and PTPN22 as inhibitors of IL-6R signaling in B cells and reveal a previously uncharacterized role for TRAF3 in the regulation of plasma cell differentiation.

A Complex Relationship between TRAF3 and Non-Canonical NF-κB2 Activation in B Lymphocytes.

The adaptor protein TRAF3 restrains B cell activating factor receptor (BAFFR) and CD40-mediated activation of the NF-κB2 pathway in B cells. Mice lacking TRAF3 specifically in B cells revealed the critical role of TRAF3 in restraining homeostatic B cell survival. Furthermore, loss-of-function mutations of the traf3 gene have been associated with human B cell malignancies, especially multiple myeloma (MM). It has been proposed that receptor-induced TRAF3 degradation leads to stabilization of the NF-κB inducing kinase (NIK), and subsequent NF-κB2 activation. However, it is unclear how receptor-mediated TRAF3 degradation or loss-of-function contributes to B cell-specific NF-κB2 activation. In the current study, we employed two complementary models to address this question. One utilized a mutant traf3 gene found in a human MM-derived cell line called LP1. The LP1 mutant TRAF3 protein lacks the TRAF-N and TRAF-C domains. Consistent with the paradigm described, expression of LP1 TRAF3 in B cells promoted higher basal levels of NF-κB2 activation compared to Wt TRAF3. However, LP1 did not associate with TRAF2, CD40, or BAFFR, and no LP1 degradation was observed following receptor engagement. Interestingly, LP1 showed enhanced NIK association. Thus, TRAF3 degradation becomes dispensable to activate NF-κB2 when it is unable to associate with TRAF2. In a second model, we examined several mutant forms of BAFFR that are unable to induce NF-κB2 activation in B cells. Signaling to B cells by each of these BAFFR mutants, however, induced levels of TRAF3 degradation similar to those induced by Wt BAFFR. Thus, in B cells, receptor-mediated TRAF3 degradation is not sufficient to promote NF-κB2 activation. We thus conclude that there is not a simple linear relationship in B lymphocytes between relative levels of cellular TRAF3, induced TRAF3 degradation, NIK activation, and NF-κB2 activation.