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The phosphatidylinositol 3-kinase (PI3K) signaling cascade downstream of the B cell receptor (BCR) signalosome is essential for B cell maturation. Proper signaling strength is maintained through the PI3K negative regulator phosphatase and tensin homolog (PTEN). Although a role for microRNA (miRNA)-dependent control of the PTEN-PI3K axis has been described, the contribution of individual miRNAs to the regulation of this crucial signaling modality in mature B lymphocytes remains to be elucidated. Our analyses reveal that ablation of miR-29 specifically in B lymphocytes results in an increase in PTEN expression and dampening of the PI3K pathway in mature B cells. This dysregulation has a profound impact on the survival of B lymphocytes and results in increased class switch recombination and decreased plasma cell differentiation. Furthermore, we demonstrate that ablation of one copy of Pten is sufficient to ameliorate the phenotypes associated with miR-29 loss. Our data suggest a critical role for the miR-29-PTEN-PI3K regulatory axis in mature B lymphocytes., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

B-1a cells are long-lived, self-renewing innate-like B cells that predominantly inhabit the peritoneal and pleural cavities. In contrast to conventional B-2 cells, B-1a cells have a receptor repertoire that is biased towards bacterial and self-antigens, promoting a rapid response to infection and clearing of apoptotic cells. Although B-1a cells are known to primarily originate from fetal tissues, the mechanisms by which they arise has been a topic of debate for many years. Here we show that in the fetal liver versus bone marrow environment, reduced IL-7R/STAT5 levels promote immunoglobulin kappa gene recombination at the early pro-B cell stage. As a result, differentiating B cells can directly generate a mature B cell receptor (BCR) and bypass the requirement for a pre-BCR and pairing with surrogate light chain. This 'alternate pathway' of development enables the production of B cells with self-reactive, skewed specificity receptors that are peculiar to the B-1a compartment. Together our findings connect seemingly opposing lineage and selection models of B-1a cell development and explain how these cells acquire their unique properties.

The immunoglobulin heavy chain (Igh) locus features a dynamic chromatin landscape to promote class switch recombination (CSR), yet the mechanisms that regulate this landscape remain poorly understood. CHD4, a component of the chromatin remodeling NuRD complex, directly binds H3K9me3, an epigenetic mark present at the Igh locus during CSR. We find that CHD4 is essential for early B cell development but is dispensable for the homeostatic maintenance of mature, naive B cells. However, loss of CHD4 in mature B cells impairs CSR because of suboptimal targeting of AID to the Igh locus. Additionally, we find that CHD4 represses p53 expression to promote B cell proliferation. This work reveals distinct roles for CHD4 in B cell development and CSR and links the H3K9me3 epigenetic mark with AID recruitment to the Igh locus., (Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved.)

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1 -/- mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1 -/- large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

Phenotypic differences among substrains of laboratory mice due to spontaneous mutations or pre-existing genetic variation confound the interpretation of targeted mutagenesis experiments and contribute to challenges with reproducibility across institutions. Notably, C57BL/6 Hsd mice and gene-targeted mice that have been backcrossed to this substrain have been reported to harbor a duplication in exons 28 and 29 of Dock2 In this study, we demonstrate the presence of this Dock2 variant in the widely used Nod2 -/- mice bred at our institution displayed multiple B cell defects including deficiencies in recirculating B cells, marginal zone B cells, and B1a cells in vivo, as well as defects in class switch recombination in vitro. However, we found that these effects are due to the Nod2 -/- mice bred at our institution displayed multiple B cell defects including deficiencies in recirculating B cells, marginal zone B cells, and B1a cells in vivo, as well as defects in class switch recombination in vitro. However, we found that these effects are due to the Dock2 variant and are independent of Nod2 variant or B cell defects. Finally, we show that Nod2 mice display the same B cell defects as mice harboring the -/- variant, confirming that the variant is a loss-of-function mutation and is sufficient to explain the alterations to the B cell compartment observed in Dock2 mice. Our findings highlight the effects of confounding mutations from widely used inbred strains on gene-targeted mice and reveal new functions of DOCK2 in B cells.Dock2 -/- mice display the same B cell defects as mice harboring the Dock2 variant, confirming that the variant is a loss-of-function mutation and is sufficient to explain the alterations to the B cell compartment observed in Nod2 -/- mice. Our findings highlight the effects of confounding mutations from widely used inbred strains on gene-targeted mice and reveal new functions of DOCK2 in B cells., (Copyright © 2018 by The American Association of Immunologists, Inc.)

B lymphocytes are essential for an efficient immune response against a variety of pathogens. A large fraction of hematologic malignancies is of B cell origin, suggesting that the development and activation of B cells need to be tightly regulated. In recent years, increasing evidence has emerged demonstrating that microRNAs (miRNAs) - a class of non-coding RNAs that control gene expression - are involved in the regulation of B cell development and function. We provide here an overview of the current knowledge on the role of miRNAs and their relevant targets in B cell development, B cell activation, and B cell malignant transformation., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

B cell development is a tightly regulated process dependent on sequential rearrangements of immunoglobulin loci that encode the antigen receptor. To elucidate the role of microRNAs (miRNAs) in the orchestration of B cell development, we ablated all miRNAs at the earliest stage of B cell development by conditionally targeting the enzymes critical for RNAi in early B cell precursors. Absence of any one of these enzymes led to a block at the pro- to pre-B cell transition due to increased apoptosis and a failure of pre-B cells to proliferate. Expression of a Bcl2 transgene allowed for partial rescue of B cell development, however, the majority of the rescued B cells had low surface immunoglobulin expression with evidence of ongoing light chain editing. Our analysis revealed that miRNAs are critical for the regulation of the PTEN-AKT-FOXO1 pathway that in turn controls Rag expression during B cell development., (Copyright © 2016 The Author(s). Published by Elsevier Inc. All rights reserved.)

There is ample evidence that somatic cell differentiation during development is accompanied by extensive DNA demethylation of specific sites that vary between cell types. Although the mechanism of this process has not yet been elucidated, it is likely to involve the conversion of 5mC to 5hmC by Tet enzymes. We show that a Tet2/Tet3 conditional knockout at early stages of B-cell development largely prevents lineage-specific programmed demethylation events. This lack of demethylation affects the expression of nearby B-cell lineage genes by impairing enhancer activity, thus causing defects in B-cell differentiation and function. Thus, tissue-specific DNA demethylation appears to be necessary for proper somatic cell development in vivo.

Unlabelled: A salient feature of pancreatic ductal adenocarcinoma (PDAC) is an abundant fibroinflammatory response characterized by the recruitment of immune and mesenchymal cells and the consequent establishment of a protumorigenic microenvironment. Here, we report the prominent presence of B cells in human pancreatic intraepithelial neoplasia and PDAC lesions as well as in oncogenic Kras-driven pancreatic neoplasms in the mouse. The growth of orthotopic pancreatic neoplasms harboring oncogenic Kras was significantly compromised in B-cell-deficient mice (μMT), and this growth deficiency could be rescued by the reconstitution of a CD1d(hi)CD5(+) B-cell subset. The protumorigenic effect of B cells was mediated by their expression of IL35 through a mechanism involving IL35-mediated stimulation of tumor cell proliferation. Our results identify a previously unrecognized role for IL35-producing CD1d(hi)CD5(+) B cells in the pathogenesis of pancreatic cancer and underscore the potential significance of a B-cell/IL35 axis as a therapeutic target., Significance: This study identifies a B-cell subpopulation that accumulates in the pancreatic parenchyma during early neoplasia and is required to support tumor cell growth. Our findings provide a rationale for exploring B-cell-based targeting approaches for the treatment of pancreatic cancer., (©2015 American Association for Cancer Research.)

Antibody repertoires are known to be shaped by selection for antigen binding. Unexpectedly, we now show that selection also acts on a non-antigen-binding antibody region: the heavy-chain variable (VH)-encoded "elbow" between variable and constant domains. By sequencing 2.8 million recombined heavy-chain genes from immature and mature B-cell subsets in mice, we demonstrate a striking gradient in VH gene use as pre-B cells mature into follicular and then into marginal zone B cells. Cells whose antibodies use VH genes that encode a more flexible elbow are more likely to mature. This effect is distinct from, and exceeds in magnitude, previously described maturation-associated changes in heavy-chain complementarity determining region 3, a key antigen-binding region, which arise from junctional diversity rather than differential VH gene use. Thus, deep sequencing reveals a previously unidentified mode of B-cell selection.

To explore the role of Dicer-dependent control mechanisms in B lymphocyte development, we ablated this enzyme in early B cell progenitors. This resulted in a developmental block at the pro- to pre-B cell transition. Gene-expression profiling revealed a miR-17 approximately 92 signature in the 3'UTRs of genes upregulated in Dicer-deficient pro-B cells; a top miR-17 approximately 92 target, the proapoptotic molecule Bim, was highly upregulated. Accordingly, B cell development could be partially rescued by ablation of Bim or transgenic expression of the prosurvival protein Bcl-2. This allowed us to assess the impact of Dicer deficiency on the V(D)J recombination program in developing B cells. We found intact Ig gene rearrangements in immunoglobulin heavy (IgH) and kappa chain loci, but increased sterile transcription and usage of D(H) elements of the DSP family in IgH, and increased N sequence addition in Igkappa due to deregulated transcription of the terminal deoxynucleotidyl transferase gene.

Mature B cells replace the mu constant region of the H chain with a downstream isotype in a process of class switch recombination (CSR). Studies suggest that CSR induction is limited to activated mature B cells in the periphery. Recently, we have shown that CSR spontaneously occur in B lymphopoiesis. However, the mechanism and regulation of it have not been defined. In this study, we show that spontaneous CSR occurs at all stages of B cell development and generates aberrant joining of the switch junctions as revealed by: 1) increased load of somatic mutations around the CSR break points, 2) reduced sequence overlaps at the junctions, and 3) excessive switch region deletion. In addition, we found that incidence of spontaneous CSR is increased in cells carrying VDJ rearrangements. Our results reveal major differences between spontaneous CSR in developing B cells and CSR induced in mature B cells upon activation. These differences can be explained by deregulated expression or function of activation-induced cytidine deaminase early in B cell development.

Germinal centers (GCs) represent the main sites for the generation of high-affinity, class-switched antibodies during T cell-dependent antibody responses. To study gene function specifically in GC B cells, we generated Cgamma1-cre mice in which the expression of Cre recombinase is induced by transcription of the Ig gamma1 constant region gene segment (Cgamma1). In these mice, Cre-mediated recombination at the fas, Igbeta, IgH, and Rosa26 loci occurred in GC B cells as early as 4 days after immunization with T cell-dependent antigens and involved >85% of GC B cells at the peak of the GC reaction. Less than 2% of IgM(+) B cells showed Cre-mediated recombination. These cells carried few Ig somatic mutations, expressed germ-line Cgamma1- and activation-induced cytidine deaminase-specific transcripts and likely include GC B cell founders and/or plasma cell precursors. Cre-mediated recombination involved most IgG1, but also a fraction of IgG3-, IgG2a-, IgG2b-, and IgA-expressing GC and post-GC B cells. This result indicates that a GC B cell can transcribe more than one downstream C(H) gene before undergoing class switch recombination. The efficient induction of Cre expression in GC B cells makes the Cgamma1-cre allele a powerful tool for the genetic analysis of these cells, as well as, in combination with a suitable marker for Cre-mediated recombination, the tracking of class-switched memory B and plasma cells in vivo. To expedite the genetic analysis of GC B cells, we have established Cgamma1-cre F(1) embryonic stem cells, allowing further rounds of gene targeting and the cloning of compound mutants by tetraploid embryo complementation.

V(D)J recombination at the immunoglobulin heavy chain (IgH) locus follows the 12/23 rule to ensure the correct assembly of the variable region gene segments. Here, we report characterization of an in vivo model that allowed us to study recombination violating the 12/23 rule, namely a mouse strain lacking canonical D elements in its IgH locus. We demonstrate that VH to JH joining can support the generation of all B cell subsets. However, the process is inefficient in that B cells and antibodies derived from the DH-less allele are not detectable if the latter is combined with a wild-type IgH allele. There is no preferential usage of any particular VH gene family or JH element in VHJH junctions, indicating that 23/23-guided recombination is possible, but is a low frequency event at the IgH locus in vivo.