Blimp1 controls plasma cell physiology and function
ABSTRACT: Antibody-secreting plasma cells are the terminal stage of the B-cell lineage. Plasma cell differentiation requires a major resetting of gene expression to silence the B cell transcriptional program, whilst establishing secretory function and long-term survival. The transcription factors Blimp1 and Irf4 are essential for the initial differentiation of activated B cells to antibody-secreting cells, however their function in mature plasma cells remains elusive. We have found that while Irf4 was essential for plasma cell survival, Blimp1 was dispensable. Blimp1-deficient cells retained the unique plasma cell transcriptional signature, but lost the ability to secrete antibody or to maintain the characteristic size and ultrastructure of plasma cells. Blimp1 was required for full expression of many components of the unfolded protein response (UPR), including Xbp1 and Atf6, as well as for the appropriate processing of Igh mRNA. The overlap of Blimp1 and Xbp1 function was restricted to the UPR genes, with Blimp1 uniquely regulating activity of the mTOR pathway, plasma cell size and morphology. These studies establish Blimp1 as a major regulator of the UPR pathway that is also required for the unique metabolic requirements of plasma cells enabling the secretion of protective antibody. RNA-seq was performed on wild type, Blimp1-/- and Xbp1-/- mouse plasma cells. Between two to four biological replicates were generated and sequenced for each sample.
Project description:The activated B-cell (ABC) to plasmablast transition encompasses the cusp of antibody-secreting cell (ASC) differentiation. We explore this transition with integrated analysis in human cells, focusing on changes that follow removal from CD40-mediated signals. Within hours of input signal loss, cell growth programs shift toward enhanced proliferation, accompanied by ER-stress response, and up-regulation of ASC features. Clustering of genomic occupancy for IRF4, BLIMP1, XBP1, and CTCF with histone marks identifies a dichotomy: XBP1 and IRF4 link to induced but not repressed gene modules in plasmablasts, whereas BLIMP1 links to modules of ABC genes that are repressed, but not to activated genes. Between ABC and plasmablast states, IRF4 shifts away from AP1/IRF composite elements while maintaining occupancy at IRF and ETS/IRF elements. This parallels the loss of BATF expression, which is identified as a potential BLIMP1 target. In plasmablasts, IRF4 acquires an association with CTCF, a feature maintained in plasma cell myeloma lines. Thus, shifting occupancy links IRF4 to both ABC and ASC gene expression, whereas BLIMP1 occupancy links to repression of the activation state.
Project description:During plasma cell differentiation there is activation of UPR gene expression that has been termed the physiological UPR of plasma cell differentiation. This is canonically thought to be downstream of Blimp1-mediated increases in immunoglobulin gene production and activation of the Xbp1 transcription factor by the RNA splicing activity of the ER-stress sensor IRE1α. Having observed UPR-affiliated gene expression prior to upregulation of Blimp1 as reported in the companion linked series, we endeavored to determine the necessity of Xbp1 activity in the early remodeling of the ER in nascent plasma cells. To this end, we mated mice harboring a floxed exon 2 of the Xbp1 gene (Xbp1flox) to mice expressing a tamoxifen-inducible cre recombinase under the control of the human CD20 promoter (hCD20-TamCre). These mice and their hCD20-TamCre-Xbp1wt litermates were fed oral tamoxifen in their diet for two weeks and follicular B cells were prepared for in-vitro plasma cell differentiation studies. We report here that prior to plasma cell differentiation as measured by CD138 expression, which coincides temporally with Blimp1 expression, there is no defect in UPR-affiliated gene expression in cells lacking Xbp1. Overall design: Samples are follicular B cells from Xbp1-floxed (n=5) and Xbp1-wildtype (n=3) mice expressing tamoxifen inducible Cre under the CD20 promoter after tamoxinen treatment, stimulated in culture to produce plasma cells. cDNA was prepared from RNA with 3' polyA capture method. Transcript abundance was measured by Kallisto psuedo-alignment and normalized (voom) and differential expression was assessed using limma.
Project description:The ZFP36/Tis11 family of zinc-finger proteins regulate cellular processes by binding to adenine uridine rich elements in the 3' untranslated regions of various mRNAs and promoting their degradation. We show here that ZFP36L1 expression is largely extinguished during the transition from B cells to plasma cells, in a reciprocal pattern to that of ZFP36 and the plasma cell transcription factor, BLIMP1. Enforced expression of ZFP36L1 in the mouse BCL1 cell line blocked cytokine-induced differentiation while shRNA-mediated knock-down enhanced differentiation. Reconstruction of regulatory networks from microarray gene expression data using the ARACNe algorithm identified candidate mRNA targets for ZFP36L1 including BLIMP1. Genes that displayed down-regulation in plasma cells were significantly over-represented (P?=?<0.0001) in a set of previously validated ZFP36 targets suggesting that ZFP36L1 and ZFP36 target distinct sets of mRNAs during plasmacytoid differentiation. ShRNA-mediated knock-down of ZFP36L1 in BCL1 cells led to an increase in levels of BLIMP1 mRNA and protein, but not for mRNAs of other transcription factors that regulate plasmacytoid differentiation (xbp1, irf4, bcl6). Finally, ZFP36L1 significantly reduced the activity of a BLIMP1 3' untranslated region-driven luciferase reporter. Taken together, these findings suggest that ZFP36L1 negatively regulates plasmacytoid differentiation, at least in part, by targeting the expression of BLIMP1.
Project description:CCAAT/enhancer-binding protein beta (C/EBPbeta), also known as nuclear factor-interleukin-6 (NF-IL6), is a transcription factor that plays an important role in the regulation of growth and differentiation of myeloid and lymphoid cells. Mice deficient in C/EBPbeta show impaired generation of B lymphocytes. We show that C/EBPbeta regulates transcription factors critical for proliferation and survival in multiple myeloma. Multiple myeloma cell lines and primary multiple myeloma cells strongly expressed C/EBPbeta, whereas normal B cells and plasma cells had little or no detectable levels of C/EBPbeta. Silencing of C/EBPbeta led to down-regulation of transcription factors such as IRF4, XBP1, and BLIMP1 accompanied by a strong inhibition of proliferation. Further, silencing of C/EBPbeta led to a complete down-regulation of antiapoptotic B-cell lymphoma 2 (BCL2) expression. In chromatin immunoprecipitation assays, C/EBPbeta directly bound to the promoter region of IRF4, BLIMP1, and BCL2. Our data indicate that C/EBPbeta is involved in the regulatory network of transcription factors that are critical for plasma cell differentiation and survival. Targeting C/EBPbeta may provide a novel therapeutic strategy in the treatment of multiple myeloma.
Project description:How activated B cells build biosynthetic pathways and organelle structures necessary for subsequent robust antibody secretion is still unclear. The dominant model holds that nascent plasma cells adapt to increased antibody synthesis by activating the unfolded protein response (UPR) under the control of the transcription factor Xbp1. Here, by analyzing gene expression in activated B cells with or without plasma cell-inductive signals, we find that follicular B cells up-regulate a wide array of UPR-affiliated genes before initiating antibody secretion; furthermore, initial transcription of these loci requires the mTORC1 kinase adaptor, Raptor, but not Xbp1. Transcriptomic analyses of resting marginal zone B cells, which generate plasma cells with exceptionally rapid kinetics, reinforce these results by revealing the basal expression of UPR-affiliated mRNA networks without detectable Xbp1 activity. We thus conclude that B cells utilize mTORC1 to prepare for subsequent plasma cell function, before the onset of antibody synthesis.
Project description:Plasma cell differentiation requires silencing of B cell transcription, while it establishes antibody-secretory function and long-term survival. The transcription factors Blimp-1 and IRF4 are essential for the generation of plasma cells; however, their function in mature plasma cells has remained elusive. We found that while IRF4 was essential for the survival of plasma cells, Blimp-1 was dispensable for this. Blimp-1-deficient plasma cells retained their transcriptional identity but lost the ability to secrete antibody. Blimp-1 regulated many components of the unfolded protein response (UPR), including XBP-1 and ATF6. The overlap in the functions of Blimp-1 and XBP-1 was restricted to that response, with Blimp-1 uniquely regulating activity of the kinase mTOR and the size of plasma cells. Thus, Blimp-1 was required for the unique physiological ability of plasma cells that enables the secretion of protective antibody.
Project description:Early UPR-affiliated gene expression occurs before Blimp1 upregulation and independent of canonical ER-stress activated Xbp1. In linked companion studies we detailed how Xbp1-independent activation of early plasma cell specific UPR-affiliated gene expression occurs prior to Blimp1 upregulation. Having observed that these genes overlapped with canonical mTORC1 signature genes and that PC-poised marginal zone B cells have higher base line mTORC1 signaling we designed this experiment to assess the dependence of early ER-remodeling on mTORC1 signaling in plasma cell differentiation. To this end we mated mice harboring a floxed allele for the mTORC1 adapter Raptor (B6.Rptor-flox) to mice expressing a tamoxifen-inducible cre recombinase under the control of the human CD20 promoter (B6.hCD20-TamCre). These mice and their hCD20-TamCre-Rptor-wt litermates were fed oral tamoxifen in their diet for two weeks and follicular B cells were prepared for in-vitro plasma cell differentiation studies. We report here that as early as 24 hours of stimulation in culture we see a marked defect in the ability of Rptor null B cells to upregulate UPR-affiliated genes associated with plasma cell differentiation. Overall design: Samples are follicular B cells from Rptor-floxed (n=3) and Rptor-wildtype (n=3) mice expressing tamoxifen inducible Cre under the CD20 promoter after tamoxinen treatment, stimulated in culture to produce plasma cells. cDNA was prepared from RNA with 3' polyA capture method. Transcript abundance was measured by Kallisto psuedo-alignment and normalized (voom) and differential expression was assessed using limma.
Project description:The unfolded protein response (UPR) is a conserved stress-signaling pathway activated after accumulation of unfolded proteins within the endoplasmic reticulum (ER). Active UPR signaling leads to unconventional, enzymatic splicing of XBP1 mRNA enabling expression of the transcription factor XBP1s to control ER homeostasis. While IRE1 has been identified as the endoribonuclease required for cleavage of this mRNA, the corresponding ligase in mammalian cells has remained elusive. Here, we report that RTCB, the catalytic subunit of the tRNA ligase complex, and its co-factor archease mediate XBP1 mRNA splicing both in vitro and in vivo. Depletion of RTCB in plasma cells of Rtcb(fl/fl) Cd23-Cre mice prevents XBP1s expression, which normally is strongly induced during plasma cell development. RTCB-depleted plasma cells show reduced and disorganized ER structures as well as severe defects in antibody secretion. Targeting RTCB and/or archease thus represents a promising strategy for the treatment of a growing number of diseases associated with elevated expression of XBP1s.
Project description:Humoral immunity requires the generation of high-affinity antibodies, which involves the generation of germinal centres (GC) promoting class switch and affinity maturation of antigen-specific B cells, and the differentiation of long-lived plasma cells. This multi-layered process is tightly controlled by the activity of a transcriptional network including Bcl6, essential for the development of GC, and Blimp1, required for the differentiation of plasma cells. Here, we reveal an additional layer of complexity by demonstrating that dynamic changes in E-protein activity mediated by Id3 govern both GC and plasma cell differentiation. We show that down-regulation of Id3 expression in B cells in essential for releasing E2A and E2-2, the combined activity of which is required for both GC B cell and plasma cell differentiation. We demonstrate that this pathway controls the expression of multiple key factors required for antigen-induced B cell differentiation, including Blimp1, Xbp1, Mef2b and CXCR4 and is therefore critical for establishing the transcriptional network that controls GC B cell and plasma cell differentiation. Transcriptional profiling of wild type, Id3 knockout and E2A/E22 double knockout B cells using RNA sequencing
Project description:Little is known about the earliest transcriptomic events in plasma cell differentiation. Indeed, activation of UPR-affiliated gene expression is seen in in-vitro plasma cell differentiation prior to Blimp1 upregulation and in the absence of Xbp1 as we show in linked companion series. In this study, Blimp1 reporter mice were immunized with the T-independent antigen NP-LPS four days prior to spleen harvest and naïve B cells, NP-binding activated B cells and NP-binding plasma cells were analyzed by bulk RNA-seq. Here we report the activation of ER remodeling gene expression in antigen-specific activated B cells in-vivo prior to upregulation of Blimp1 transcription. Overall design: Samples are three celltypes from three biological replicates (mice) 4 days post NP-LPS immunization. cDNA was prepared from RNA with 3' polyA capture method. Transcript abundance was measured by Kallisto psuedo-alignment and normalized (voom) and differential expression was assessed using limma.