Defining a transcriptional fingerprint of murine splenic B-cell development

The E26 Transformation-Specific Family Transcription Factor Spi-C Is Dynamically Regulated by External Signals in B Cells

Spi-C is an E26 transformation-specific transcription factor closely related to PU.1 and Spi-B. Spi-C has lineage-instructive functions important in B cell development, Ab-generating responses, and red pulp macrophage generation. This research examined the regulation of Spi-C expression in mouse B cells. To determine the mechanism of Spic regulation, we identified the Spic promoter and upstream regulatory elements. The Spic promoter had unidirectional activity that was reduced by mutation of an NF-κB binding site. Reverse transcription-quantitative PCR analysis revealed that Spic expression was reduced in B cells following treatment with cytokines BAFF + IL-4 + IL-5, anti-IgM Ab, or LPS. Cytochalasin treatment partially prevented downregulation of Spic. Unstimulated B cells upregulated Spic on culture. Spic was repressed by an upstream regulatory region interacting with the heme-binding regulator Bach2. Taken together, these data indicate that Spi-C is dynamically regulated by external signals in B cells and provide insight into the mechanism of regulation.

Lineage-instructive functions of the E26-transformation-specific-family transcription factor Spi-C in immune cell development and disease

Cell fate decisions during hematopoiesis are the consequence of a complex mixture of inputs from cell-intrinsic and cell-extrinsic factors. In rare cases, expression of a single transcription factor, or a few key factors, may be sufficient to dictate lineage differentiation in a precursor cell. The E26-transformation-specific-family transcription factor Spi-C has emerged as an example of a lineage-instructive factor involved in the generation of mature, specialized subsets of both myeloid and lymphoid cells. Spi-C can instruct differentiation of splenic precursors into red pulp macrophages responsible for phagocytosing senescent red blood cells. In the B cell compartment, Spi-C acts as a key regulator of cell fate decisions at the pro-B to pre-B cell stage and for plasma cell differentiation. Spi-C regulates key genes including Nfkb1, Bach2, Syk, and Blnk to regulate cell cycle entry and B cell differentiation. Here, we review the biology of the lineage-instructive transcription factor Spi-C and its contribution to mechanisms of disease in macrophages and B cells. This article is categorized under: Cancer > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology Infectious Diseases > Genetics/Genomics/Epigenetics.

Functional characterization and genomic studies of a novel murine submandibular gland epithelial cell line

A better understanding of the normal and diseased biology of salivary glands (SG) has been hampered, in part, due to difficulties in cultivating and maintaining salivary epithelial cells. Towards this end, we have generated a mouse salivary gland epithelial cell (mSGc) culture system that is well-suited for the molecular characterization of SG cells and their differentiation program. We demonstrate that mSGc can be maintained for multiple passages without a loss of proliferation potential, readily form 3D-spheroids and importantly express a panel of well-established salivary gland epithelial cell markers. Moreover, mSGc 3D-spheroids also exhibit functional maturation as evident by robust agonist-induced intracellular calcium signaling. Finally, transcriptomic characterization of mSGc by RNA-seq and hierarchical clustering analysis with adult organ RNA-seq datasets reveal that mSGc retain most of the molecular attributes of adult mouse salivary gland. This well-characterized mouse salivary gland cell line will fill a critical void in the field by offering a valuable resource to examine various mechanistic aspects of mouse salivary gland biology.

The Role of TLR4 on B Cell Activation and Anti-2GPI Antibody Production in the Antiphospholipid Syndrome

High titer of anti-β₂-glycoprotein I antibodies (anti-β₂GPI Ab) plays a pathogenic role in antiphospholipid syndrome (APS). Numerous studies have focused on the pathological mechanism in APS; however, little attention is paid to the immune mechanism of production of anti-β₂GPI antibodies in APS. Our previous study demonstrated that Toll-like receptor 4 (TLR4) plays a vital role in the maturation of bone marrow-derived dendritic cells (BMDCs) from the mice immunized with human β₂-glycoprotein I (β₂GPI). TLR4 is required for the activation of B cells and the production of autoantibody in mice treated with β₂GPI. However, TLR4 provides a third signal for B cell activation and then promotes B cells better receiving signals from both B cell antigen receptor (BCR) and CD40, thus promoting B cell activation, surface molecules expression, anti-β₂GPI Ab production, and cytokines secretion and making B cell functioning like an antigen presenting cell (APC). At the same time, TLR4 also promotes B cells producing antibodies by upregulating the expression of B-cell activating factor (BAFF). In this paper, we aim to review the functions of TLR4 in B cell immune response and antibody production in autoimmune disease APS and try to find a new way for the prevention and treatment of APS.

Distinct Transcriptomic Features are Associated with Transitional and Mature B-Cell Populations in the Mouse Spleen

Splenic transitional B-cells (T1 and T2) are selected to avoid self-reactivity and to safeguard against autoimmunity, then differentiate into mature follicular (FO-I and FO-II) and marginal zone (MZ) subsets. Transcriptomic analysis by RNA-seq of the five B-cell subsets revealed T1 cell signature genes included RAG suggesting a potential for receptor revision. T1 to T2 B-cell differentiation was marked by a switch from Myb to Myc, increased expression of the PI3K adapter DAP10 and MHC class II. FO-II may be an intermediate in FO-I differentiation and may also become MZ B-cells as suggested by principle component analysis. MZ B-cells possessed the most distinct transcriptome including down-regulation of CD45 phosphatase-associated protein (CD45-AP/PTPRC-AP), as well as upregulation of IL-9R and innate molecules TLR3, TLR7, and bactericidal Perforin-2 (MPEG1). Among the endosomal TLRs, stimulation via TLR3 further enhanced Perforin-2 expression exclusively in MZ B-cells. Using gene-deleted and overexpressing transgenic mice we show that IL-9/IL-9R interaction resulted in rapid activation of STAT1, 3, and 5, primarily in MZ B-cells. Importantly, CD45-AP mutant mice had reduced transitional and increased mature MZ and FO B-cells, suggesting that it prevents premature entry of transitional B-cells to the mature B-cell pool or their survival and proliferation. Together, these findings suggest, developmental plasticity among splenic B-cell subsets, potential for receptor revision in peripheral tolerance whereas enhanced metabolism coincides with T2 to mature B-cell differentiation. Further, unique core transcriptional signatures in MZ B-cells may control their innate features.

Zfp318 regulates IgD expression by abrogating transcription termination within the Ighm/Ighd locus

The protein Zfp318 is expressed during the transition of naive B cells from an immature to mature state. To evaluate its role in mature B cell functions, a conditional gene deficiency in Zfp318 was created and deleted in bone marrow lineages via Vav-Cre. B cell development was minimally altered in the absence of the protein, although transitional 2 (T2) B cell populations were depressed in the absence of Zfp318. Intriguingly, the analysis of IgM and IgD expression by maturing and mature naive B cells demonstrated an elevated level of IgM gene products and a virtual loss of IgD products. Transcriptome analysis of Zfp318-deficient B cells revealed that only two gene products showed altered expression in the absence of Zfp318 (Ighd and Sva), demonstrating a remarkable specificity of Zfp318 action. In the absence of Zfp318, Ighm/Ighd transcripts, which would normally encode IgM and IgD from heterogeneous nuclear RNA transcripts via alternative splicing, lack intron and exon sequences from the IgD (Ighd)-encoding region. This finding indicates that Zfp318, in a novel manner, functions by repressing recognition of the transcriptional termination site at the 3' end of the terminal IgM-encoding exon, allowing for synthesis of the complete Ighm/Ighd heterogeneous nuclear RNA.

Zinc-finger protein ZFP318 is essential for expression of IgD, the alternatively spliced Igh product made by mature B lymphocytes

IgD and IgM are produced by alternative splicing of long primary RNA transcripts from the Ig heavy chain (Igh) locus and serve as the receptors for antigen on naïve mature B lymphocytes. IgM is made selectively in immature B cells, whereas IgD is coexpressed with IgM when the cells mature into follicular or marginal zone B cells, but the transacting factors responsible for this regulated change in splicing have remained elusive. Here, we use a genetic screen in mice to identify ZFP318, a nuclear protein with two U1-type zinc fingers found in RNA-binding proteins and no known role in the immune system, as a critical factor for IgD expression. A point mutation in an evolutionarily conserved lysine-rich domain encoded by the alternatively spliced Zfp318 exon 10 abolished IgD expression on marginal zone B cells, decreased IgD on follicular B cells, and increased IgM, but only slightly decreased the percentage of B cells and did not decrease expression of other maturation markers CD21, CD23, or CD62L. A targeted Zfp318 null allele extinguished IgD expression on mature B cells and increased IgM. Zfp318 mRNA is developmentally regulated in parallel with IgD, with little in pro-B cells, moderate amounts in immature B cells, and high levels selectively in mature follicular B cells. These findings identify ZFP318 as a crucial factor regulating the expression of the two major antibody isotypes on the surface of most mature B cells.

Bone marrow-induced Mef2c deficiency delays B-cell development and alters the expression of key B-cell regulatory proteins

The Mef2 family transcriptional regulator Mef2c (myocyte enhancer factor 2c) is highly expressed in maturing bone marrow and peripheral mature B-cells. To evaluate the role of this transcription factor in B-cell development, we generated a B-cell-specific conditional deletion of Mef2c using the Mb-1-Cre transgene that is expressed during the early stages of immunoglobulin rearrangement. Young mice possessing this defect demonstrated a significant impairment in B-cell numbers in bone marrow and spleen. This phenotype was evident in all B-cell subsets; however, as the animals mature, the deficit in the peripheral mature B-cell compartments was overcome. The absence of Mef2c in mature B-cells led to unique CD23+ and CD23- subsets that were evident in Mef2c knockout primary samples as well as Mef2c-deficient cultured, differentiated B-cells. Genome-wide expression analysis of immature and mature B-cells lacking Mef2c indicated altered expression for a number of key regulatory proteins for B-cell function including Ciita, CD23, Cr1/Cr2 and Tnfsf4. Chromatin immunoprecipitation analysis confirmed Mef2c binding to the promoters of these genes indicating a direct link between the presence (or absence) of Mef2c and altered transcriptional control in mature B-cells.

The in vitro derivation of phenotypically mature and diverse B cells from immature spleen and bone marrow precursors

The capacity of immature B cells of the spleen and bone marrow to differentiate in vitro into cells representing mature end stage cells was investigated using B-cell activating factor belonging to the TNF family (BAFF) and Notch pathway activators. Immature splenic and bone marrow B cells were found, in the presence of both of these activators, to mature into cells with follicular mature (FM) and marginal zone (MZ) cell phenotypes. Such cells were functionally responsive to B-cell-specific activation. The derivation in vitro of cells with an MZ phenotype was more robust from CD23(-) populations than CD23(+) immature/transitional B cells, suggesting a direct immature/T1 B cell to MZ cell differentiation pathway. Transcript analysis of the in vitro-derived B-cell populations demonstrated expression profiles similar to maturing B cells in vivo. FACS-purified populations of B220(+)CD19(+)CD21(-)CD23(-) cells from bone marrow of 2-wk-old mice gave rise to populations of CD21(+)CD23(-) cells with MZ cell phenotypes as well as CD21(+)CD23(+) cells with FM cell phenotypes in percentages similar to those found in vivo. These data suggest that the commitment to an MZ and FM B cell phenotype is set prior to immature B-cell release from the marrow.

CD21 signaling via C3 regulates Purkinje cell protein 4 expression

Complement receptor proteins CR2 (CD21) and CR1 (CD35) have been identified as components of the murine B cell co-receptor complex. Gene expression profiles between naïve WT, C3-/-, and CD21/35-/- B cells demonstrate enhanced expression of a Ca(2+)-modulating gene, Pcp4, in WT mice compared to the complement-deficient animals. Increased expression of Pcp4 is also coincident with B cell maturation into end stage phenotypes. Prolonged activation of B cells via cross-linking of the BCR (but not CR1/CR2 alone) leads to increased expression of Pcp4 and suppressed Ca(2+) release. In total these data demonstrate that the expression of Pcp4 in naïve resting mature B cells is dependent upon tonic stimulation from the CR1/CR2 proteins via a C3 ligand, and that antigen specific B cell activation can also elevate Pcp4 expression that is coincident with suppression of calcium-dependent responses.