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. 2014 Dec 24;9(6):2098-111.
doi: 10.1016/j.celrep.2014.11.024. Epub 2014 Dec 11.

A Pathway Switch Directs BAFF Signaling to Distinct NFκB Transcription Factors in Maturing and Proliferating B Cells

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Free PMC article

A Pathway Switch Directs BAFF Signaling to Distinct NFκB Transcription Factors in Maturing and Proliferating B Cells

Jonathan V Almaden et al. Cell Rep. .
Free PMC article

Abstract

BAFF, an activator of the noncanonical NFκB pathway, provides critical survival signals during B cell maturation and contributes to B cell proliferation. We found that the NFκB family member RelB is required ex vivo for B cell maturation, but cRel is required for proliferation. Combined molecular network modeling and experimentation revealed Nfkb2 p100 as a pathway switch; at moderate p100 synthesis rates in maturing B cells, BAFF fully utilizes p100 to generate the RelB:p52 dimer, whereas at high synthesis rates, p100 assembles into multimeric IκBsome complexes, which BAFF neutralizes in order to potentiate cRel activity and B cell expansion. Indeed, moderation of p100 expression or disruption of IκBsome assembly circumvented the BAFF requirement for full B cell expansion. Our studies emphasize the importance of p100 in determining distinct NFκB network states during B cell biology, which causes BAFF to have context-dependent functional consequences.

Figures

Figure 1
Figure 1. BAFF Enhances BCR-Controlled B Cell Expansion
(A) Schematic of the distinct canonical and noncanonical NFκB pathways identified in B cells. Antigenic stimulation leads to release of cRel-containing dimers into the nucleus, resulting in activation of cell division programs (Grumont et al., 1998, 1999). BAFF activates noncanonical RelB dimers and B cell survival. Thus, this model suggests that BAFF costimulation results in enhanced expansion by augmenting survival of B cells. (B) In vitro proliferation of CFSE-labeled wild type B cells and stimulated for 3 days with anti-IgM alone (black) or anti-IgM + BAFF ligand (red) and analyzed by FACS. Live cell numbers gated by exclusion of 7AADHi population. (C) The CFSE proliferation profiles of wild-type B cells stimulated with either anti-IgM or anti-IgM plus BAFF analyzed with FlowMax software tool to calculate the fraction of B cells responding to stimulation (pF), the average time to division of undivided (Tdiv0), and average time to death of undivided (Tdie0) cells. (D) Representative histograms of cell survival analyzed by 7AAD staining following 24 hr of stimulation by anti-IgM alone (black) or anti-IgM + BAFF ligand (red). (E) Time course averages of B cells viability stimulated with anti-IgM alone or anti-IgM + BAFF ligand. Live cells were identified by gating out 7AADHi population. FACS plots in (B) and (D) are representative of at least three experiments. Bar graphs in (C) summarize best-fit cellular parameters with the lognormal SD. Error bars in (E) SD; n = 3. *p < 0.05, **p < 0.01 by Student’s t test. See also Figure S1.
Figure 2
Figure 2. In the Context of BCR-Stimulated B Cells, BAFF-Induced Hyperexpression Shows a cRel Signature
(A) Venn diagram of gene sets identified by RNA-seq analysis (NCBI GEO accession number GSE54588) that are induced >2-fold over unstimulated wild-type B cells (t = 0 hr) following 30 hr of stimulation with anti-IgM alone (black), BAFF alone (blue), or anti-IgM and BAFF (orange). (B) Heatmap of “hyper” expressed genes, defined as induced >log2(0.5) by anti-IgM at 30 hr over unstimulated wild-type B cells (t = 0 hr) and possess an additional >log2(0.5) greater expression in the anti-IgM and BAFF costimulation condition at the same time point. Of these genes, we identified a few genes involved in survival and cell cycle. (C) Graphic representation of prosurvival and cell cycle genes in RNA-seq analysis in (B). The cRel:p50-binding motif is found overrepresented within the regulatory regions of hyperexpressed genes, regardless of the biological process they are categorized into. (D) Quantitative PCR of wild-type B cells stimulated with either anti-IgM alone or in costimulation conditions. Relative expression normalized to the basal level is shown. (E) Quantitative PCR of wild-type and cRel-deficient B cells costimulated with anti-IgM and BAFF for 24 hr. **p < 0.01 denotes significantly higher in wild-type condition than Crel−/− counterpart by Student’s t test. (F) Time course of NFκB DNA-binding activities in anti-IgM alone, BAFF alone, and anti-IgM plus BAFF costimulated B cells. Nuclear extracts from wild-type B cells activated by indicated stimuli were collected and subjected to RelA, cRel, and RelB EMSA (for cRel EMSA, RelA and RelB antibodies are incubated together with nuclear extracts in order to shift away these NFκB-containing dimers, leaving cRel species untouched; see Experimental Procedures). Signals quantified and graphed relative to their respective resting cells (left). EMSA is representative of five experiments. AU, arbitrary units.
Figure 3
Figure 3. Costimulation of BCR and BAFF-R Result in Activation of cRel and RelB Dimers, but Enhanced Expansion Phenotype Is Dependent Solely on cRel
(A) Representative FACS plot of B cells derived from cRel (orange) and RelB (blue)-deficient mice cultured with BAFF ligand for 3 days. Percentage of surviving B cells (7AAD) was assessed by FACS and graphed on left; wild-type (black), Relb−/− (blue) and Crel−/− (orange). (B and C) Total splenic wild-type, Relb−/−, and Crel−/− B cells are stained with CFSE and then stimulated with anti-IgM plus BAFF. Cell proliferation is examined at the indicated time points by FACS. (D) Time course for CFSE-labeled magnetically isolated follicular B cells (CD23+) from wild-type, Relbdb/db, and Crel−/− B cells stimulated with anti-IgM + BAFF ligand. Live cells numbers were identified by gating out 7AADHi population and graphed on left. (E) FlowMax analysis of CFSE proliferation profiles for CD23+ wild-type, Relbdb/db, and Crel−/− B cells in (D). Bar graphs of pF0, Tdiv0, and Tdie0 with the lognormal SD provided. FACS plots in (A)–(D) are representative of at least two independent experiments. Error bars represent SD; n = 3. *p < 0.05; **p < 0.01 by Student’s t test. See also Figure S2.
Figure 4
Figure 4. BAFF Releases IκBδ Inhibition of cRel in BCR-Stimulated B Cells
(A) Proposed mechanism of IκBδ inhibition of cRel in BCR-stimulated B cells. In resting B cells, BAFF-R stimulation activates the NIK/IKK1 kinase complex that results in p100 processing, which allows for RelB:p52 nuclear translocation (Figure 3A). In activated B cells, continuous BCR stimulation activates cRel dimers, resulting in the expression of p100, which functions as a substrate for oligomerization to form the inhibitor IκBδ, which binds cRel, forming a negative feedback loop. During BAFF costimulation, the p100 buildup is prevented, allowing for persistent “superactivated” cRel. (B) Cytoplasmic protein levels of p100 in stimulated wild-type B cells were measured by immunoblot. (C) Immunoblots of cRel coimmunoprecipitates prepared from wild-type B cells stimulated with anti-IgM, BAFF ligand, or anti-IgM and BAFF. Samples were normalized to cRel protein levels to allow for comparison of distinct conditions (left). Whole-cell lysates (input) are also presented. IP, immunoprecipitation. (D) Computational simulations are shown for the NEMO/IKK2-inducing stimulus anti-IgM (left column), for the NIK/IKK1-inducing stimulus BAFF (middle column), and a combination of both (right column), namely NEMO/IKK2 and NIK/IKK1 activity profiles (top, black), NFκB family members cRel and RelB nuclear fold activities (middle, red), and total cellular protein levels of IκBα, IκBε, and ανδ p100 (IκBδ substrate; bottom, blue). Gel images in (B) and (C) are representative of at least four experiments. See also Figures S3–S6 and Tables S1–S3.
Figure 5
Figure 5. IκBδ Limits the Proliferative Capacity of BCR-Stimulated B Cells
(A) Model simulation of fold p100 protein levels (log2) in wild-type (black) and Nfkb2 heterozygous (green) B cells stimulated with anti-IgM alone. (B) Immunoblot of p100 protein expression in wild-type and Nfkb2+/− B cells stimulated with anti-IgM alone. Blots were quantified and plotted (log2, right). (C) Computational prediction of p100:cRel complex following 24 hr of anti-IgM stimulation in wild-type (black) and Nfkb2+/− (green) B cells. (D) Representative immunoblot blot for coimmunoprecipitation of p100:cRel complex from wild-type and Nfkb2+/− B cells following 24 hr of stimulation with either anti-IgM alone or anti-IgM plus BAFF ligand. Samples normalized to cRel protein levels (bottom). Immunoblots were quantified and graphed (right). (E) Model simulation of nuclear cRel protein levels in wild-type and Nfkb2 heterozygous B cells stimulated with anti-IgM alone (log2). (F) cRel DNA-binding activities in wild-type and Nfkb2 heterozygous B cells induced by anti-IgM alone were monitored by EMSA. EMSA was quantified and graphed (log2, right). (G) Time course of cell proliferation for CD23+ wild-type (black) and Nfkb2+/− (green) B cells labeled with CFSE and 7AAD. (H) FlowMax analysis of CFSE proliferation assay in (G). (I) T-dependent Ag responses in Nfkb2+/− mice immunized with NP-KLH, and serum levels of anti-NP total Ig and specific Ig isotypes at 10 and 28 days postimmunization measured by ELISA (n = 5). Indicated p values are calculated by two-way ANOVA. Unless stated, all experiments in Figure 5 used total splenic B cells. Immunoblots and FACS plot for (B), (D), (F), and (G) are representative of at least three experiments. Error bars in (D) SD; n = 3. Bar graphs in (H) summarize best-fit cellular parameters with the lognormal SD; *p < 0.05; **p < 0.01. See also Figure S7.
Figure 6
Figure 6. Nfkb1/p105 Limits the Proliferative Capacity of B Cells by Stabilizing p100/IκBδ
(A) Proposed schematic of Nfkb1 and Nfkb2 protein fates in wild-type and Nfkb1−/− B cells. In normal wild-type B cells, p105 and p100 mainly function in the NFκB-signaling system as precursors of p50 and p52. p105 is processed into p50 and dimerizes predominantly with cRel and RelA and, to a lesser extent, RelB. p100 processing yields p52, which in turn dimerizes with RelB primarily. A second function of p100 is to form the inhibitory molecule IκBδ (left). In Nfkb1−/− B cells, p105/p50 absence leaves cRel and RelA molecules without their primary binding partner. This in turn shifts the available pool of p100 to be processed into p52 in order to overcome the loss of p50; as a result, less p100 is accessible to form IκBδ (right). (B) Immunoblots of p52 whole-cell expression in wild-type and Nfkb1−/− B cells stimulated with anti-IgM. WCE, whole-cell extract. (C) Immunoblots of cytoplasmic p100 expression in wild-type and Nfkb1−/− B cells stimulated with anti-IgM. CE, cytoplasmic extract. (D) Immunoblot of nuclear cRel for wild-type and Nfkb1−/− B cells stimulated with anti-IgM. Nuclear marker USF2 is used as a loading control. NE, nuclear extract. (E) Immunoblots of immunoprecipitates to monitor p100/IκBδ (and p52) associated with cRel during BCR stimulation time course. cRel was immunoprecipitated from B cell whole-cell extracts and prepared at indicated time points. Whole-cell lysates (input) are also shown; α-tubulin serves as loading control. (F) FACS analysis for in vitro proliferation of wild type (black) and Nfkb1−/− (purple) B cells labeled with CFSE and stimulated for 3 days with anti-IgM alone. Live-cell numbers gated by exclusion of 7AADHi population. (G) FlowMax analysis of CFSE proliferation profiles for wild-type (black) versus Nfkb1−/− (purple) B cells in (F). Immunoblots and FACS plots for (B)–(F) are representative of at least three experiments. Bar graphs in (G) summarize best-fit cellular parameters with the lognormal SD. See also Figure S7.

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