RelB/p50 dimers are differentially regulated by tumor necrosis factor-alpha and lymphotoxin-beta receptor activation: critical roles for p100.

peer reviewedTumor necrosis factor-alpha (TNF-alpha) and lymphotoxin-beta receptor (LTbetaR) signaling both play important roles in inflammatory and immune responses through activation of NF-kappaB. Using various deficient mouse embryonic fibroblast cells, we have compared the signaling pathways leading to NF-kappaB induction in response to TNF-alpha and LTbetaR activation. We demonstrate that LTbetaR ligation induces not only RelA/p50 dimers but also RelB/p50 dimers, whereas TNF-alpha induces only RelA/p50 dimers. LTbetaR-induced binding of RelB/p50 requires processing of p100 that is mediated by IKKalpha but is independent of IKKbeta, NEMO/IKKgamma, and RelA. Moreover, we show that RelB, p50, and p100 can associate in the same complex and that TNF-alpha but not LTbeta signaling increases the association of p100 with RelB/p50 dimers in the nucleus, leading to the specific inhibition of RelB DNA binding. These results suggest that the alternative NF-kappaB pathway based on p100 processing may account not only for the activation of RelB/p52 dimers but also for that of RelB/p50 dimers and that p100 regulates the binding activity of RelB/p50 dimers via at least two distinct mechanisms depending on the signaling pathway involved

The B-cell antigen receptor integrates adaptive and innate immune signals

B cells respond to antigens by engagement of their B-cell antigen receptor (BCR) and of coreceptors through which signals from helper T cells or pathogen-associated molecular patterns are delivered. We show that the proliferative response of B cells to the latter stimuli is controlled by BCR-dependent activation of phosphoinositidyl 3-kinase (PI-3K) signaling. Glycogen synthase kinase 3 beta and Foxo1 are two PI-3K-regulated targets that play important roles, but to different extents, depending on the specific mitogen. These results suggest a model for integrating signals from the innate and the adaptive immune systems in the control of the B-cell immune response

A role for the nuclear receptor NR2F6 in peritoneal B cell homeostasis

B cells are key mediators of humoral immunity. Mature B cells fall into various sub-classes that can be separated by their ontogeny, expression of cell surface markers, anatomical location, and function. B1 subsets play important roles in natural immunity and constitute the majority of B cells in newborns. In the adult, B1 cells predominate in the pleural and peritoneal cavities, while the mature B2 follicular subset makes up the major fraction of B cells in lymphoid tissue, although important subsets of antibody-secreting B1 cells are also present at these sites. B1 cells are the main producers of natural IgM but can also contribute to elimination of some pathogens, while B2 cells primarily mediate response to foreign antigens. The differential molecular underpinning of the B1 and B2 subsets remains incompletely understood. Here we demonstrate that germline-deficiency of the orphan nuclear receptor NR2F6 causes a partial loss of B1b and B2 B cells in the peritoneum while leaving peritoneal B1a cells unaltered. A competitive bone marrow chimera in Nr2f6(+/+) host mice produced similar numbers of Nr2f6(+/+) and Nr2f6(-/-) peritoneal B1b and B2 cells. The proliferation of Nr2f6(-/-) peritoneal B cells was not altered, while the migration marker CXCR5 was reduced on all subsets but Beta7-integrin was reduced only on peritoneal B1b and B2 cells. Similarly, B1b and B2 but not B1a cells, exhibited significantly reduced survival

Canonical NF-κB signaling is uniquely required for the long-term persistence of functional mature B cells

Although canonical NF-κB signaling is crucial to generate a normal mature B-cell compartment, its role in the persistence of resting mature B cells is controversial. To resolve this conflict, we ablated NF-κB essential modulator (NEMO) and IκB kinase 2 (IKK2), two essential mediators of the canonical pathway, either early on in B-cell development or specifically in mature B cells. Early ablation severely inhibited the generation of all mature B-cell subsets, but follicular B-cell numbers could be largely rescued by ectopic expression of B-cell lymphoma 2 (Bcl2), despite a persisting block at the transitional stage. Marginal zone (MZ) B and B1 cells were not rescued, indicating a possible role of canonical NF-κB signals beyond the control of cell survival in these subsets. When canonical NF-κB signaling was ablated specifically in mature B cells, the differentiation and/or persistence of MZ B cells was still abrogated, but follicular B-cell numbers were only mildly affected. However, the mutant cells exhibited increased turnover as well as functional deficiencies upon activation, suggesting that canonical NF-κB signals contribute to their long-term persistence and functional fitness

NIK overexpression amplifies, whereas ablation of its TRAF3-binding domain replaces BAFF:BAFF-R-mediated survival signals in B cells

BAFF-R-dependent activation of the alternative NF-κB pathway plays an essential role in mature B cell survival. Mutations leading to overexpression of NIK and deletion of the TRAF3 gene are implicated in human multiple myeloma. We show that overexpression of NIK in mouse B lymphocytes amplifies alternative NF-κB activation and peripheral B cell numbers in a BAFF-R-dependent manner, whereas uncoupling NIK from TRAF3-mediated control causes maximal p100 processing and dramatic hyperplasia of BAFF-R-independent B cells. NIK controls alternative NF-κB signaling by increasing the protein levels of its negative regulator TRAF3 in a dose-dependent fashion. This mechanism keeps NIK protein levels below detection even when they cause B cell hyperplasia, so that contributions of NIK to B cell pathologies can easily be overlooked

Development of immunoglobulin k-chain-positive B cells, but not editing of immunoglobulin j-chain, depends on NF-jB signals A R T I C L E S

By genetically ablating IjB kinase (IKK)-mediated activation of the transcription factor NF-jB in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin k-chain-positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin j-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-jB signaling. During the first phase, in which NF-jB signaling is dispensable, predominantly j-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly k-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding j-chain. This second phase of development is dependent on NF-jB signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2. It is well established that the NF-kB family of transcription factors is critical to B cell physiology 1,2 . Activation of NF-kB by the alternative pathway, which is mediated by NF-kB-inducing kinase and the inhibitor of NF-kB kinase 1 (IKK1; A001170) downstream of interactions between B cell-activation factor of the tumor necrosis factor family (BAFF) and BAFF-receptor, is essential for mature B cell survival 3 . In addition, mature B cells depend on continuous signaling through the canonical NF-kB pathway, in which activation of the IKK complex, which consists of IKK1, IKK2 (A001172) and NF-kB essential modulator (NEMO; A001628), is central 1 . In contrast, the function of NF-kB signaling in B cell development remains unclear 1 and is indeed highly controversial. Initial experiments addressed that issue in mice lacking one or two individual NF-kB transcription factors. Whereas the generation of mature B cells is generally impaired in most of these mutant mice, the effects are often mild in B cell progenitors and it has remained unresolved whether these defects are B cell autonomous 2 . Notably, genetic ablation of the BAFF-receptor or IKK1 seems not to affect B cell development in the bone marrow, at least in terms of proportions of cells at the various developmental stages 1,3 ; the same is true for ablation of the canonical pathway by knockout of IKK2 or NEMO specifically in B cell