The transcription factor Foxo1 controls germinal center B cell proliferation in response to T cell help

Regulation of Decay Accelerating Factor Primes Human Germinal Center B Cells for Phagocytosis

Germinal centers (GC) are sites for extensive B cell proliferation and homeostasis is maintained by programmed cell death. The complement regulatory protein Decay Accelerating Factor (DAF) blocks complement deposition on host cells and therefore also phagocytosis of cells. Here, we show that B cells downregulate DAF upon BCR engagement and that T cell-dependent stimuli preferentially led to activation of DAF^lo B cells. Consistent with this, a majority of light and dark zone GC B cells were DAF^lo and susceptible to complement-dependent phagocytosis, as compared with DAF^hi GC B cells. We could also show that the DAF^hi GC B cell subset had increased expression of the plasma cell marker Blimp-1. DAF expression was also modulated during B cell hematopoiesis in the human bone marrow. Collectively, our results reveal a novel role of DAF to pre-prime activated human B cells for phagocytosis prior to apoptosis.

GSK3 Restrains Germinal Center B Cells to Form Plasma Cells

B cells in the germinal center (GC) are programmed to form plasma cells (PCs) or memory B cells according to signals received by receptors that are translated to carry out appropriate activities of transcription factors. However, the precise mechanism underlying this process to complete the GC reaction is unclear. In this study, we show that both genetic ablation and pharmacological inhibition of glycogen synthase kinase 3 (GSK3) in GC B cells of mice facilitate the cell fate decision toward PC formation, accompanied by acquisition of dark zone B cell properties. Mechanistically, under stimulation with CD40L and IL-21, GSK3 inactivation synergistically induced the transcription factors Foxo1 and c-Myc, leading to increased levels of key transcription factors required for PC differentiation, including IRF4. This GSK3-mediated alteration of transcriptional factors in turn facilitated the dark zone transition and consequent PC fate commitment. Our study thus reveals the upstream master regulator responsible for interpreting external cues in GC B cells to form PCs mediated by key transcription factors.

Cyclin D3 Governs Clonal Expansion of Dark Zone Germinal Center B Cells

Germinal center (GC) B cells surge in their proliferative capacity, which poses a direct risk for B cell malignancies. G1- to S-phase transition is dependent on the expression and stability of D-type cyclins. We show that cyclin D3 expression specifically regulates dark zone (DZ) GC B cell proliferation. B cell receptor (BCR) stimulation of GC B cells downregulates cyclin D3 but induces c-Myc, which subsequently requires cyclin D3 to exert GC expansion. Control of DZ proliferation requires degradation of cyclin D3, which is dependent on phosphorylation of residue Thr283 and can be bypassed by cyclin D3^T283A hyperstabilization as observed in B cell lymphoma. Thereby, selected GC B cells in the light zone potentially require disengagement from BCR signaling to accumulate cyclin D3 and undergo clonal expansion in the DZ.

Mutations of cyclin D3 occur in B cell lymphomas, which derive from highly proliferating germinal center (GC) B cells. Ramezani-Rad et al. show that cyclin D3 in GC B cells is controlled by B cell receptor signaling and is required for proliferation of dark zone GC B cells.

A rheostat sets B-cell receptor repertoire selection to distinguish self from non-self

In bone marrow VDJ-recombination continuously generates original repertoires of immature B cells expressing IgM-B cell receptor (BcR), in which each cell recognizes the wide variety of self and non-self antigens with an individually different spectrum of avidities. High avidity self-reactive B cells try to edit their BcRs by secondary or multiple V_L-rearrangements to J_L-rearrangements. If they do not manage to change their self reactivity, they are deleted by apoptosis. Low avidity self-reactive B cells are anergized, while B cells with no avidity to self are ignored. A rheostat crosslinking antigen-binding BcRs, self antigen complexed with pentameric IgM and Fcμ-receptor monitors high, low or no binding. PI3K and PTEN are the effectors of this self antigen-sensing device. In mature B cells this rheostat continues to function in the activation of resting B cells by foreign antigens which crosslink BcR, antigen and pentameric IgM with Fcμ-receptors.

NR4A nuclear receptors restrain B cell responses to antigen when second signals are absent or limiting

Antigen stimulation (signal 1) triggers B cell proliferation, and primes B cells to recruit, engage, and respond to T cell help (signal 2). Failure to receive signal 2 within a defined time window results in B cell apoptosis, yet the mechanisms that enforce dependence upon co-stimulation are incompletely understood. Nr4a1-3 encode a small family of orphan nuclear receptors that are rapidly induced by B cell antigen receptor (BCR) stimulation. Here we showed that Nr4a1 and Nr4a3 play partially redundant roles to restrain B cell responses to antigen in the absence of co-stimulation, and do so in part by repressing expression of BATF and consequently MYC. The NR4A family also restrains B cell access to T cell help by repressing expression of the T cell chemokines CCL3 and CCL4, as well as CD86 and ICAM1. Such NR4A-mediated regulation plays a role specifically under conditions of competition for limiting T cell help.

Effect of Qihuang Decoction Combined with Enteral Nutrition on Postoperative Gastric Cancer of Nutrition and Immune Function

Objective

Early nutritional support in patients with gastric cancer can improve their nutritional status, but the impact on immune function has not been confirmed. This study aimed to analyze the effects of Qihuang decoction combined with enteral nutrition on nutrition and the immune function of postoperative gastric cancer.

Methods

120 patients with postoperative gastric cancer in the study group and 117 in the control group were selected as the study subjects from our hospital at random. Indications of nutrition and immune and the rates of complications were compared the day before surgery and 1, 3, 7, and 14 days after surgery.

Results

Indications of nutrition except hemoglobin (HB) in the study group were significantly higher than those before operation and the albumin (ALB) and prealbumin (TP) were significantly increased 7 and 14 days after surgery (P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 and P < 0.001 versus P < 0.001 and P < 0.001) and the protein (PA) 3, 7, and 14 days after surgery (P=0.011, P=0.002, and P=0.022) in the study group compared to those in the control group. Cellular and humoral immunity indications in the study group are significantly higher than those before operation compared to those in the control group, and the CD3⁺, CD4⁺, and CD4⁺/CD8⁺ were significantly increased 7 and 14 days after surgery (P=0.027 and P < 0.001 versus P=0.008 and P < 0.001 versus P=0.010 and P < 0.001) and IgA, IgG, and IgM 3, 7, and 14 days after surgery in the study group (P < 0.001, P < 0.001, and P < 0.001 versus P < 0.001, P < 0.002, and P < 0.001 versus P < 0.001, P < 0.001, and P < 0.001). The complications such as abdominal, lung, wound, and urinary infection were also significantly decreased (P^χ2=0.017; P < 0.001 and P < 0.001 versus P < 0.001 and P < 0.001) and the protein (PA) 3, 7, and 14 days after surgery (P=0.011, P=0.002, and P=0.022) in the study group compared to those in the control group. Cellular and humoral immunity indications in the study group are significantly higher than those before operation compared to those in the control group, and the CD3⁺, CD4⁺, and CD4⁺/CD8⁺ were significantly increased 7 and 14 days after surgery (P=0.027 and P < 0.001 versus P=0.008 and P < 0.001 versus P=0.010 and P < 0.001) and IgA, IgG, and IgM 3, 7, and 14 days after surgery in the study group (P < 0.001, P < 0.001, and P < 0.001 versus P < 0.001, P < 0.002, and P < 0.001 versus P < 0.001, P < 0.001, and P < 0.001). The complications such as abdominal, lung, wound, and urinary infection were also significantly decreased (P^χ2=0.017; P < 0.001 and P < 0.001 versus P < 0.001 and P < 0.001) and the protein (PA) 3, 7, and 14 days after surgery (P=0.011, P=0.002, and P=0.022) in the study group compared to those in the control group. Cellular and humoral immunity indications in the study group are significantly higher than those before operation compared to those in the control group, and the CD3⁺, CD4⁺, and CD4⁺/CD8⁺ were significantly increased 7 and 14 days after surgery (P=0.027 and P < 0.001 versus P=0.008 and P < 0.001 versus P=0.010 and P < 0.001) and IgA, IgG, and IgM 3, 7, and 14 days after surgery in the study group (P < 0.001, P < 0.001, and P < 0.001 versus P < 0.001, P < 0.002, and P < 0.001 versus P < 0.001, P < 0.001, and P < 0.001). The complications such as abdominal, lung, wound, and urinary infection were also significantly decreased (P^χ2=0.017; P < 0.001 and P < 0.001 versus P < 0.001 and P < 0.001) and the protein (PA) 3, 7, and 14 days after surgery (P=0.011, P=0.002, and P=0.022) in the study group compared to those in the control group. Cellular and humoral immunity indications in the study group are significantly higher than those before operation compared to those in the control group, and the CD3⁺, CD4⁺, and CD4⁺/CD8⁺ were significantly increased 7 and 14 days after surgery (P=0.027 and P < 0.001 versus P=0.008 and P < 0.001 versus P=0.010 and P < 0.001) and IgA, IgG, and IgM 3, 7, and 14 days after surgery in the study group (P < 0.001, P < 0.001, and P < 0.001 versus P < 0.001, P < 0.002, and P < 0.001 versus P < 0.001, P < 0.001, and P < 0.001). The complications such as abdominal, lung, wound, and urinary infection were also significantly decreased (P^χ2=0.017; P^χ2=0.036; P^χ2=0.041; P^χ2=0.004).

Conclusions

Qihuang decoction combined with enteral nutrition can promote the absorption of enteral nutrition with improving the immune and reducing complications of infection.

T and B-cell signaling in activated PI3K delta syndrome: From immunodeficiency to autoimmunity

Phosphatidylinositol 3 kinases (PI3K) are a family of lipid kinases that are activated by a variety of cell-surface receptors, and regulate a wide range of downstream readouts affecting cellular metabolism, growth, survival, differentiation, adhesion, and migration. The importance of these lipid kinases in lymphocyte signaling has recently been highlighted by genetic analyses, including the recognition that both activating and inactivating mutations of the catalytic subunit of PI3Kδ, p110δ, lead to human primary immunodeficiencies. In this article, we discuss how studies on the human genetic disorder "Activated PI3K-delta syndrome" and mouse models of this disease (Pik3cd^E1020K/+ mice) have provided fundamental insight into pathways regulated by PI3Kδ in T and B cells and their contribution to lymphocyte function and disease, including responses to commensal bacteria and the development of autoimmunity and tumors. We highlight critical roles of PI3Kδ in T follicular helper cells and the orchestration of the germinal center reaction, as well as in CD8⁺ T-cell function. We further  present data demonstrating the ability of the AKT-resistant FOXO1^AAA mutant to rescue IgG1 class switching defects in Pik3cd^E1020K/+ B cells, as well as data supporting a role for PI3Kδ in promoting multiple T-helper effector cell lineages.

Linking signaling and selection in the germinal center

Germinal centers (GC) are sites of rapid B-cell proliferation in response to certain types of immunization. They arise in about 1 week and can persist for several months. In GCs, B cells differentiate in a unique way and begin to undergo somatic mutation of the Ig V regions at a high rate. GC B cells (GCBC) thus undergo clonal diversification that can affect the affinity of the newly mutant B-cell receptor (BCR) for its driving antigen. Through processes that are still poorly understood, GCBC with higher affinity are selectively expanded while those with mutations that inactivate the BCR are lost. In addition, at various times during the extended GC reaction, some GCBC undergo differentiation into either long-lived memory B cells (MBC) or plasma cells. The cellular and molecular signals that govern these fate decisions are not well-understood, but are an active area of research in multiple laboratories. In this review, we cover both the history of this field and focus on recent work that has helped to elucidate the signals and molecules, such as key transcription factors, that coordinate both positive selection as well as differentiation of GCBC.

Histone deacetylase 3 controls a transcriptional network required for B cell maturation

Histone deacetylase 3 (Hdac3) is a target of the FDA approved HDAC inhibitors, which are used for the treatment of lymphoid malignancies. Here, we used Cd19-Cre to conditionally delete Hdac3 to define its role in germinal center B cells, which represent the cell of origin for many B cell malignancies. Cd19-Cre-Hdac3-/- mice showed impaired germinal center formation along with a defect in plasmablast production. Analysis of Hdac3-/- germinal centers revealed a reduction in dark zone centroblasts and accumulation of light zone centrocytes. RNA-seq revealed a significant correlation between genes up-regulated upon Hdac3 loss and those up-regulated in Foxo1-deleted germinal center B cells, even though Foxo1 typically activates transcription. Therefore, to determine whether gene expression changes observed in Hdac3-/- germinal centers were a result of direct effects of Hdac3 deacetylase activity, we used an HDAC3 selective inhibitor and examined nascent transcription in germinal center-derived cell lines. Transcriptional changes upon HDAC3 inhibition were enriched for light zone gene signatures as observed in germinal centers. Further comparison of PRO-seq data with ChIP-seq/exo data for BCL6, SMRT, FOXO1 and H3K27ac identified direct targets of HDAC3 function including CD86, CD83 and CXCR5 that are likely responsible for driving the light zone phenotype observed in vivo.

TGFβ signaling in germinal center B cells promotes the transition from light zone to dark zone

GCs are polarized into LZ and DZ to allow for the spatial separation of selection and proliferation. In this study, we describe the previously unknown function of TGFβ in promoting the transition of GCBs from LZ to DZ.

B cells in germinal centers (GCs) cycle between light zone (LZ) and dark zone (DZ). The cues in the GC microenvironment that regulate the transition from LZ to DZ have not been well characterized. In Peyer’s patches (PPs), transforming growth factor-β (TGFβ) promotes IgA induction in activated B cells that can then differentiate into GC B cells. We show here that TGFβ signaling occurs in B cells in GCs and is distinct from signaling that occurs in activated B cells in PPs. Whereas in activated B cells TGFβ signaling is required for IgA induction, in the GC it was instead required for the transition from LZ to DZ. In the absence of TGFβ signaling, there was an accumulation of LZ GC B cells and reduced antibody affinity maturation likely due to reduced activation of Foxo1. This work identifies TGFβ as a microenvironmental cue that is critical for GC homeostasis and function.

Excessive CD11c+Tbet+ B cells promote aberrant TFH differentiation and affinity-based germinal center selection in lupus

Excessive self-reactive and inadequate affinity-matured antigen-specific antibody responses have been reported to coexist in lupus, with elusive cellular and molecular mechanisms. Here, we report that the antigen-specific germinal center (GC) response-a process critical for antibody affinity maturation-is compromised in murine lupus models. Importantly, this defect can be triggered by excessive autoimmunity-relevant CD11c⁺Tbet⁺ age-associated B cells (ABCs). In B cell-intrinsic Ship-deficient (ShipΔB) lupus mice, excessive CD11c⁺Tbet⁺ ABCs induce deregulated follicular T-helper (T_FH) cell differentiation through their potent antigen-presenting function and consequently compromise affinity-based GC selection. Excessive CD11c⁺Tbet⁺ ABCs and deregulated T_FH cell are also present in other lupus models and patients. Further, over-activated Toll-like receptor signaling in Ship-deficient B cells is critical for CD11c⁺Tbet⁺ ABC differentiation, and blocking CD11c⁺Tbet⁺ ABC differentiation in ShipΔB mice by ablating MyD88 normalizes T_FH cell differentiation and rescues antigen-specific GC responses, as well as prevents autoantibody production. Our study suggests that excessive CD11c⁺Tbet⁺ ABCs not only contribute significantly to autoantibody production but also compromise antigen-specific GC B-cell responses and antibody-affinity maturation, providing a cellular link between the coexisting autoantibodies and inadequate affinity-matured antigen-specific antibodies in lupus models and a potential target for treating lupus.

Forkhead box transcription factors as context-dependent regulators of lymphocyte homeostasis

Lymphocytes have evolved to react rapidly and robustly to changes in their local environment by using transient adaptations and by regulating their terminal differentiation programmes. Forkhead box transcription factors (FTFs) can direct leukocyte-specific responses, and their functional diversification promotes a high degree of context-dependent specification. Many, often antagonistic, FTFs have overlapping expression patterns and can thereby compete for binding to the same chromosomal target sequences. Multiple molecular mechanisms also connect extracellular signals to the expression and functionality of specific FTFs and, in this way, fine-tune their activity. Through these diverse mechanisms, FTFs can function as context-dependent rheostats responding to diverse environmental stimuli. Focusing on the various mechanisms by which their functional activity is modulated, as well as on their mechanisms of action, we discuss how specific FTFs control lymphocyte function, allowing for the establishment and maintenance of immune homeostasis.

Protein Amounts of the MYC Transcription Factor Determine Germinal Center B Cell Division Capacity

High-affinity B cell selection in the germinal center (GC) is governed by signals delivered by follicular helper T (Tfh) cells to B cells. Selected B cells undergo clonal expansion and affinity maturation in the GC dark zone in direct proportion to the amount of antigen they capture and present to Tfh cells in the light zone. Here, we examined the mechanisms whereby Tfh cells program the number of GC B cell divisions. Gene expression analysis revealed that Tfh cells induce Myc expression in light-zone B cells in direct proportion to antigen capture. Conditional Myc haplo-insufficiency or overexpression combined with cell division tracking showed that MYC expression produces a metabolic reservoir in selected light-zone B cells that is proportional to the number of cell divisions in the dark zone. Thus, MYC constitutes the GC B cell division timer that when deregulated leads to emergence of B cell lymphoma.

The quantity of CD40 signaling determines the differentiation of B cells into functionally distinct memory cell subsets

In mice, memory B (B_mem) cells can be divided into two subpopulations: CD80^hi B_mem cells, which preferentially differentiate into plasma cells; and CD80^lo B_mem cells, which become germinal center (GC) B cells during a recall response. We demonstrate that these distinct responses can be B-cell-intrinsic and essentially independent of B-cell receptor (BCR) isotypes. Furthermore, we find that the development of CD80^hi B_mem cells in the primary immune response requires follicular helper T cells, a relatively strong CD40 signal and a high-affinity BCR on B cells, whereas the development of CD80^lo B_mem cells does not. Quantitative differences in CD40 stimulation were enough to recapitulate the distinct B cell fate decisions in an in vitro culture system. The quantity of CD40 signaling appears to be translated into NF-κB activation, followed by BATF upregulation that promotes B_mem cell differentiation from GC B cells.

Pten controls B-cell responsiveness and germinal center reaction by regulating the expression of IgD BCR

In contrast to other B‐cell antigen receptor (BCR) classes, the function of IgD BCR on mature B cells remains largely elusive as mature B cells co‐express IgM, which is sufficient for development, survival, and activation of B cells. Here, we show that IgD expression is regulated by the forkhead box transcription factor FoxO1, thereby shifting the responsiveness of mature B cells towards recognition of multivalent antigen. FoxO1 is repressed by phosphoinositide 3‐kinase (PI3K) signaling and requires the lipid phosphatase Pten for its activation. Consequently, Pten‐deficient B cells expressing knock‐ins for BCR heavy and light chain genes are unable to upregulate IgD. Furthermore, in the presence of autoantigen, Pten‐deficient B cells cannot eliminate the autoreactive BCR specificity by secondary light chain gene recombination. Instead, Pten‐deficient B cells downregulate BCR expression and become unresponsive to further BCR‐mediated stimulation. Notably, we observed a delayed germinal center (GC) reaction by IgD‐deficient B cells after immunization with trinitrophenyl‐ovalbumin (TNP‐Ova), a commonly used antigen for T‐cell‐dependent antibody responses. Together, our data suggest that the activation of IgD expression by Pten/FoxO1 results in mature B cells that are selectively responsive to multivalent antigen and are capable of initiating rapid GC reactions and T‐cell‐dependent antibody responses.

Zinc finger-IRF composite elements bound by Ikaros/IRF4 complexes function as gene repression in plasma cell

The transcription factor (TF) interferon regulatory factor-4 (IRF4) promotes both germinal center (GC) reactions and plasma cell (PC) differentiation by binding to alternative DNA motifs including AP-1-IRF composite elements, Ets-IRF composite elements (EICEs), and interferon sequence response elements (ISREs). Although all of these motifs mediate transcriptional activation by IRF4, it is still unknown how some of the IRF4 target genes are downregulated upon PC differentiation. Here, we revealed a molecular mechanism of IRF4-mediated gene downregulation during PC differentiation. By combining IRF4 chromatin immunoprecipitation sequence and gene expression analysis, we identified zinc finger-IRF composite elements (ZICEs) in IRF4 binding regions aligned with genes whose expression was downregulated in PCs. The zinc finger TFs Ikaros and Aiolos were identified as IRF4 binding partners in PCs, and Ikaros but not Aiolos was essential for IRF4 binding to the ZICE sequence and for PC differentiation. The Ebf1 gene, which positively controls B-cell activation and GC reactions, was identified as one of the Ikaros/IRF4 target genes. Importantly, while the ZICE embeds the ISRE motif, IRF4 bound the ZICE motif as heterodimers with Ikaros for repression of target genes, which include Ebf1 In contrast, if the zinc finger motif is juxtaposed to the EICE motif, the Ikaros/PU.1/IRF4 complex functioned to activate target gene expression. Our findings revealed a novel mode of IRF4 activity upon PC differentiation where upon forming an Ikaros/IRF4 DNA-bound complex, a subset of genes is repressed.

Sin1/mTORC2 regulate B cell growth and metabolism by activating mTORC1 and Myc

Proper control of B cell growth and metabolism is crucial for B-cell-mediated immunity, but the underlying molecular mechanisms remain incompletely understood. In this study, Sin1, a key component of mTOR complex 2 (mTORC2), specifically regulates B cell growth and metabolism. Genetic ablation of Sin1 in B cells reduces the cell size at either the transitional stage or upon antigen stimulation and severely impairs metabolism. Sin1 deficiency also severely impairs B-cell proliferation, antibody responses, and anti-viral immunity. At the molecular level, Sin1 controls the expression and stability of the c-Myc protein and maintains the activity of mTORC1 through the Akt-dependent inactivation of GSK3 and TSC1/2, respectively. Therefore, our study reveals a novel and specific role for Sin1 in coordinating the activation of mTORC2 and mTORC1 to control B cell growth and metabolism.

Molecular regulation of peripheral B cells and their progeny in immunity

In this review, Boothby et al. summarize some salient advances toward elucidation of the molecular programming of the fate choices and function of B cells in the periphery. They also note unanswered questions that pertain to differences among subsets of B lymphocytes and plasma cells.

Mature B lymphocytes are crucial components of adaptive immunity, a system essential for the evolutionary fitness of mammals. Adaptive lymphocyte function requires an initially naïve cell to proliferate extensively and its progeny to have the capacity to assume a variety of fates. These include either terminal differentiation (the long-lived plasma cell) or metastable transcriptional reprogramming (germinal center and memory B cells). In this review, we focus principally on the regulation of differentiation and functional diversification of the “B2” subset. An overview is combined with an account of more recent advances, including initial work on mechanisms that eliminate DNA methylation and potential links between intracellular metabolites and chromatin editing.

Expression of the Plasma Cell Transcriptional Regulator Blimp-1 by Dark Zone Germinal Center B Cells During Periods of Proliferation

Long-lived plasma cells (PCs) develop in germinal centers (GCs) by the differentiation of affinity matured B cells. Antibody affinity maturation involves iterative rounds of somatic hypermutation in dark zones (DZs) and selection in light zones (LZs), however the details of where, when and how PC commitment occurs are not well-understood. Fate bifurcation at the time of selection is one possibility, with the very highest affinity GC B cells differentiating as an alternative to DZ re-entry. However, how this model fits with a need to also retain these clones in the response is not clear. Here, we show that subsets of bona fide DZ cells express the plasma cell master regulator Blimp-1 at low levels during periods of proliferation. Ex vivo culture experiments demonstrate that these cells are not yet committed to plasma cell differentiation but that they may be sensitized to go down that route. Contrary to models in which T cells directly select GC B cells to begin expressing Blimp-1, we found that expression of this transcriptional regulator occurred even when follicular helper T cells were ablated. We speculate that Blimp-1 may be induced during proliferation in the DZ, and that as such single selected cells might give rise to both GC and post-GC progeny.

Nuclear FOXO1 promotes lymphomagenesis in germinal center B cells

Forkhead box class O1 (FOXO1) acts as a tumor suppressor in solid tumors. The oncogenic phosphoinositide-3-kinase (PI3K) pathway suppresses FOXO1 transcriptional activity by enforcing its nuclear exclusion upon AKT-mediated phosphorylation. We show here abundant nuclear expression of FOXO1 in Burkitt lymphoma (BL), a germinal center (GC) B-cell–derived lymphoma whose pathogenesis is linked to PI3K activation. Recurrent FOXO1 mutations, which prevent AKT targeting and lock the transcription factor in the nucleus, are used by BL to circumvent mutual exclusivity between PI3K and FOXO1 activation. Using genome editing in human and mouse lymphomas in which MYC and PI3K cooperate synergistically in tumor development, we demonstrate proproliferative and antiapoptotic activity of FOXO1 in BL and identify its nuclear localization as an oncogenic event in GC B-cell–derived lymphomagenesis.

NaF Reduces KLK4 Gene Expression by Decreasing Foxo1 in LS8 Cells

Decreased expression and increased phosphorylation of Forkhead box o1 (Foxo1) in ameloblasts were observed both in vivo and in vitro when treated by fluoride. The present study aims to investigate the possible relationship between Foxo1 and enamel matrix proteinases, matrix metalloproteinase 20 (MMP20), and kallikrein 4 (KLK4), in NaF-treated ameloblasts. Ameloblast-like cells (LS8 cells) were exposed to NaF at selected concentration (0/2 mM) for 24 h. Gene overexpression and silencing experiments were used to up- and down-regulate Foxo1 expression. The expression levels of Foxo1, MMP20, and KLK4 were detected by quantitative real-time PCR and western blot. Dual luciferase reporter assay was performed to evaluate the regulation of Foxo1 on the transcriptional activity of KLK4 promoter. The results showed that KLK4 expression was decreased in LS8 cells treated by NaF, while MMP20 expression was not changed. Foxo1 activation led to significantly up-regulation of KLK4 in LS8 cells under NaF condition. Knockout of Foxo1 markedly decreased klk4 expression in mRNA level, and intensified inhibition occurred in LS8 cells when combined with NaF treatment. However, the variation trend of MMP20 was not clear. Dual luciferase reporter assay showed that Foxo1 activation enhanced the transcriptional activity of KLK4 promoter. These findings suggest that the decrease of Foxo1 expression induced by high fluoride was a cause for low KLK4 expression.

Cbl Ubiquitin Ligases Control B Cell Exit from the Germinal-Center Reaction

Selective expansion of high-affinity antigen-specific B cells in germinal centers (GCs) is a key event in antibody affinity maturation. GC B cells with improved affinity can either continue affinity-driven selection or exit the GC to differentiate into plasma cells (PCs) or memory B cells. Here we found that deleting E3 ubiquitin ligases Cbl and Cbl-b (Cbls) in GC B cells resulted in the early exit of high-affinity antigen-specific B cells from the GC reaction and thus impaired clonal expansion. Cbls were highly expressed in GC light zone (LZ) B cells, where they promoted the ubiquitination and degradation of Irf4, a transcription factor facilitating PC fate choice. Strong CD40 and BCR stimulation triggered the Cbl degradation, resulting in increased Irf4 expression and exit from GC affinity selection. Thus, a regulatory cascade that is centered on the Cbl ubiquitin ligases ensures affinity-driven clonal expansion by connecting BCR affinity signals with differentiation programs.

New concepts in follicular lymphoma biology: From BCL2 to epigenetic regulators and non-coding RNAs

The molecular pathogenesis of follicular lymphoma (FL) was partially revealed 3 decades ago, with the discovery of the translocation that brings BCL2 under the influence of immunoglobulin heavy chain enhancers in a vast majority of cases. Despite the importance of this seminal observation, it has become increasingly clear that additional genetic alterations need to occur to trigger neoplastic transformation and disease progression. The evolution of FL involves developmental arrest and disruption of the normal function of one or more of epigenetic regulators including KMT2D/MLL2, EZH2, CBP/CREBBP, p300/EP300, and HIST1H1 in >95% of cases. B-cells "arrested" in germinal centers acquire dozens of additional genetic aberrations that influence key pathways controlling their physiological development including B Cell Receptor (BCR) signaling, PI3K/AKT, TLR, mTOR, NF-κB, JAK/STAT, MAPK, CD40/CD40L, chemokine, and interleukin signaling. Additionally, most cases of FL do not result from linear accumulation of genomic aberrations, but rather evolve from a common progenitor cell population by diverse evolution, creating multiple FL subclones in one patient. Moreover, one of the subclones might acquire a combination of aberrations involving genes controlling cell survival and proliferation including MDM2, CDKN2A/B, BCL6, MYC, TP53, β2M, FOXO1, MYD88, STAT3, or miR-17-92, and this can lead to the transformation of an initially indolent FL to an aggressive lymphoma (2%-3% risk per year). The complexity of the disease is also underscored by the importance of its interactions with the microenvironment that can substantially influence disease development and prognosis. Interpreting individual aberrations in relation to their impact on normal processes, their frequency, position in the disease evolution, and the consequences of their (co)occurrence, are the basis for understanding FL pathogenesis. This is necessary for the identification of patients with risk of early progression or transformation, for the development of novel targeted therapies, and for personalized treatment approaches. In this review, we summarize recent knowledge of molecular pathways and microenvironmental components involved in FL biology, and discuss them in the context of physiological B-cell development, FL evolution, and targeted therapies.

The Transcriptional Regulation of Germinal Center Formation

Germinal centers (GCs) are essential structures of the humoral immune response, which form in the periphery in response to T cell dependent antigens. During the GC reaction, B cells undergo critical differentiation steps, which ultimately lead to the generation of antibodies with altered effector function and higher affinity for the selected antigen. Remarkably, many of the B cell tumors have their origin in the GCs; thus, understanding how the formation of these structures is regulated or deregulated is of high medical importance. This review gives an overview of the transcription factors that have been linked to the generation of GCs, and of their roles in the process.

Interleukin-10 from CD4+ follicular regulatory T cells promotes the germinal center response

CD4⁺ follicular regulatory T (T_fr) cells suppress B cell responses through modulation of follicular helper T (T_fh) cells and germinal center (GC) development. We found that T_fr cells can also promote the GC response through provision of interleukin-10 (IL-10) after acute infection with lymphocytic choriomeningitis virus (LCMV). Sensing of IL-10 by B cells was necessary for optimal development of the GC response. GC B cells formed in the absence of T_reg cell-derived IL-10 displayed an altered dark zone state and decreased expression of the transcription factor Forkhead box protein 1 (FOXO1). IL-10 promoted nuclear translocation of FOXO1 in activated B cells. These data indicate that T_fr cells play a multifaceted role in the fine-tuning of the GC response and identify IL-10 as an important mediator by which T_fr cells support the GC reaction.

Cell Cycle-Associated CXCR4 Expression in Germinal Center B Cells and Its Implications on Affinity Maturation

Adaptation of antibody-mediated immunity occurs in germinal centers (GC). It is where affinity maturation, class switching, memory and plasma cell differentiation synergize to generate specific high-affinity antibodies that aid both to clear and protect against reinfection of invading pathogens. Within GCs, light and dark zone are two compartments instrumental in regulating this process, by segregating T cell-dependent selection and differentiation from generation of GC B cells bearing hypermutated antigen receptors. Spatial segregation of GC B cells into the two zones relies on the chemokine receptor CXCR4, with textbooks attributing high and low expression to a dark and light zone phenotype. Interestingly, this bipolarity is not reflected in the CXCR4 expression profile of GC B cells, which is highly variable and unimodal, indicating a continuum of intermediate CXCR4 levels rather than a binary dark or light zone phenotype. Here, analysis of published BrdU pulse-chase data reveals that throughout cell cycle, average CXCR4 expression in GC B cells steadily increases close to twofold, scaling with cell surface area. CXCR4 expression in recently divided GC B cells in G0/G1 or early S phase shows intermediate levels compared to cells in G2M phase, consistent with their smaller size. The lowest number of CXCR4 receptors are displayed by relatively aged GC B cells in G0/G1 or early S phase. The latter, upon progressing through S phase, however, ramp up relative CXCR4 expression twice as much as recently divided cells. Twelve hours after the BrdU pulse, labeled GC B cells, while initially in S phase, are desynchronized in terms of cell cycle and match the CXCR4 profile of unlabeled cells. A model is discussed in which CXCR4 expression in GC B cell increases with cell cycle and cell surface area, with highest levels in G2 and M phase, coinciding with GC B cell receptor signaling in G2 and immediately preceding activation-induced cytidine deaminase (AID) activity in early G1. In the model, GC B cells compete for CXCL12 expression on the basis of their CXCR4 expression, gaining a relative advantage as they progress in cell cycle, but loosing the advantage at the moment they divide.

Insulin suppresses the production of fibroblast growth factor 23 (FGF23)

Fibroblast growth factor 23 (FGF23) is produced by bone cells and regulates renal phosphate and vitamin D metabolism, as well as causing left ventricular hypertrophy. FGF23 deficiency results in rapid aging, whereas high plasma FGF23 levels are found in several disorders, including kidney or cardiovascular diseases. Regulators of FGF23 production include parathyroid hormone (PTH), calcitriol, dietary phosphate, and inflammation. We report that insulin and insulin-like growth factor 1 (IGF1) are negative regulators of FGF23 production. In UMR106 osteoblast-like cells, insulin and IGF1 down-regulated FGF23 production by inhibiting the transcription factor forkhead box protein O1 (FOXO1) through phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB)/Akt signaling. Insulin deficiency caused a surge in the serum FGF23 concentration in mice, which was reversed by administration of insulin. In women, a highly significant negative correlation between FGF23 plasma concentration and increase in plasma insulin level following an oral glucose load was found. Our results provide strong evidence that insulin/IGF1-dependent PI3K/PKB/Akt/FOXO1 signaling is a powerful suppressor of FGF23 production in vitro as well as in mice and in humans.

Tailoring Immune Responses toward Autoimmunity: Transcriptional Regulators That Drive the Creation and Collusion of Autoreactive Lymphocytes

T-dependent humoral immune responses to infection involve a collaboration between B and CD4 T cell activation, migration, and co-stimulation, thereby culminating in the formation of germinal centers (GCs) and eventual differentiation into memory cells and long-lived plasma cells (PCs). CD4 T cell-derived signals drive the formation of a tailored B cell response. Downstream of these signals are transcriptional regulators that are the critical enactors of immune cell programs. In particular, a core group of transcription factors regulate both B and T cell differentiation, identity, and function. The timing and expression levels of these transcription factors are tightly controlled, with dysregulated expression correlated to immune cell dysfunction in autoimmunity and lymphomagenesis. Recent studies have significantly advanced our understanding of both extrinsic and intrinsic regulators of autoreactive B cells and antibody-secreting PCs in systemic lupus erythematosus, rheumatoid arthritis, and other autoimmune conditions. Yet, there are still gaps in our understanding of the causative role these regulators play, as well as the link between lymphoid responses and peripheral damage. This review will focus on the genesis of immunopathogenic CD4 helper and GC B cells. In particular, we will detail the transcriptional regulation of cytokine and chemokine receptor signaling during the pathogenesis of GC-derived autoimmune conditions in both murine models and human patients.

The RNA-binding protein PTBP1 is necessary for B cell selection in germinal centers

Antibody affinity maturation occurs in germinal centres (GC) where B cells cycle between the light zone (LZ) and the dark zone. In the LZ GC B cells bearing immunoglobulins with the highest affinity for antigen receive positive selection signals from T helper cells that promotes their rapid proliferation. Here we show that the RNA binding protein PTBP1 is necessary for the progression of GC B cells through late S-phase of the cell cycle and for affinity maturation. PTBP1 is required for the proper expression of the c-MYC-dependent gene program induced in GC B cells receiving T cell help and directly regulates the alternative splicing and abundance of transcripts increased during positive selection to promote proliferation.

Regulation of B cell fate, survival, and function by mitochondria and autophagy

B cells are responsible for protective antibody production after differentiation into antibody-secreting cells during humoral immune responses. From early B cell development in the bone marrow, to their maturation in the periphery, activation in the germinal center, and differentiation into plasma cells or memory B cells, B cells display ever-changing functions and properties. Autophagy and mitochondria play important roles in B cell development, activation, and differentiation to accommodate the phenotypic and environmental changes encountered over the lifetime of the cell. Among their many functions, mitochondria and autophagy generate energy, mediate cell survival, and produce/eliminate reactive oxygen species that can serve as signal molecules to regulate differentiation. As B cells mature and differentiate into plasma or memory cells, both autophagic and mitochondrial functions undergo significant changes. In this review, we aim to provide an overview of the role of the autophagosome and mitochondria in regulating B cell fate, survival, and function. Moreover, we will discuss the interplay between these two highly metabolic organelles during B cell development, maturation, and differentiation.