Defining In Vivo Transcription Factor Complexes of the Murine CD21 and CD23 Genes

Transcriptional Control of Mature B Cell Fates

The mature naïve B cell repertoire consists of three well-defined populations: B1, B2 (follicular B, FOB), and marginal zone B (MZB) cells. FOB cells are the dominant mature B cell population in the secondary lymphoid organs and blood of both humans and mice. The driving forces behind mature B lineage selection have been linked to B cell receptor (BCR) signaling strength and environmental cues, but how these fate-determination factors are transcriptionally regulated remains poorly understood. We summarize emerging data on the role of transcription factors (TFs) - particularly the ETS and IRF families - in regulating MZB and FOB lineage selection. Indeed, genomic analyses have identified four major groups of target genes that are crucial for FOB differentiation, revealing previously unrecognized pathways that ultimately determine biological responses specific to this lineage.

Recent studies on cellular and molecular mechanisms in Alzheimer’s disease: focus on epigenetic factors and histone deacetylase

Alzheimer’s disease (AD) is one of the most common neurodegenerative disorders mainly affecting elderly people. It is characterized by progressive loss of memory and cognitive function. More than 95% of AD cases are related to sporadic or late-onset AD (LOAD). The etiology of LOAD is still unclear. It has been reported that environmental factors and epigenetic alterations play a significant role in AD pathogenesis. Furthermore, recently, genome-wide association studies (GWAS) identified 10 novel risk genes:ABCA7,APOE,BIN1,CD2AP,CD33,CLU,CR1,MS4A6A,MS4A4E, andPICALM, which play an important role for LOAD. In this review, the therapeutic approaches of AD by epigenetic modifications have been discussed. Nowadays, HDAC inhibitors have clinically proven its activity for epigenetic modifications. Furthermore, we try to establish the relationship between HDAC inhibitors and above mentioned LOAD risk genes. Finally, we are hoping that this review may open new area of research for AD treatment.

Altered Marginal Zone B Cell Selection in the Absence of IκBNS

Marginal zone (MZ) B cells reside in the splenic MZ and play important roles in T cell-independent humoral immune responses against blood-borne pathogens. IκBNS-deficient bumble mice exhibit a severe reduction in the MZ B compartment but regain an MZ B population with age and, thus, represent a valuable model to examine the biology of MZ B cells. In this article, we characterized the MZ B cell defect in further detail and investigated the nature of the B cells that appear in the MZ of aged bumble mice. Flow cytometry analysis of the splenic transitional B cell subsets demonstrated that MZ B cell development was blocked at the transitional-1 to transitional-2-MZ precursor stage in the absence of functional IκBNS. Immunohistochemical analysis of spleen sections from wild-type and bumble mice revealed no alteration in the cellular MZ microenvironment, and analysis of bone marrow chimeras indicated that the MZ B cell development defect in bumble mice was B cell intrinsic. Further, we demonstrate that the B cells that repopulate the MZ in aged bumble mice were distinct from age-matched wild-type MZ B cells. Specifically, the expression of surface markers characteristic for MZ B cells was altered and the L chain Igλ⁺ repertoire was reduced in bumble mice. Finally, plasma cell differentiation of sorted LPS-stimulated MZ B cells was impaired, and aged bumble mice were unable to respond to NP-Ficoll immunization. These results demonstrate that IκBNS is required for an intact MZ B cell compartment in C57BL/6 mice.

JNK1 negatively controls antifungal innate immunity by suppressing CD23 expression

Opportunistic fungal infections are a leading cause of death among immune-compromised patients, and there is a pressing need to develop new antifungal therapeutic agents because of toxicity and resistance to the antifungal drugs currently in use. Although C-type lectin receptor- and Toll-like receptor-induced signaling pathways are key activators of host antifungal immunity, little is known about the mechanisms that negatively regulate host immune responses to a fungal infection. Here we found that JNK1 activation suppresses antifungal immunity in mice. We showed that JNK1-deficient mice had a significantly higher survival rate than wild-type control mice in response to Candida albicans infection, and the expression of JNK1 in hematopoietic innate immune cells was critical for this effect. JNK1 deficiency leads to significantly higher induction of CD23, a novel C-type lectin receptor, through NFATc1-mediated regulation of the CD23 gene promoter. Blocking either CD23 upregulation or CD23-dependent nitric oxide production eliminated the enhanced antifungal response found in JNK1-deficient mice. Notably, JNK inhibitors exerted potent antifungal therapeutic effects in both mouse and human cells infected with C. albicans, indicating that JNK1 may be a therapeutic target for treating fungal infection.

Differential requirements for the canonical NF-κB transcription factors c-REL and RELA during the generation and activation of mature B cells

Signaling through the canonical NF-κB pathway is critical for the generation and maintenance of mature B-cells and for antigen-dependent B-cell activation. c-REL (rel) and RELA (rela) are the downstream transcriptional activators of the canonical NF-κB pathway. Studies of B-cells derived from constitutional rel knockout mice and chimeric mice repopulated with rela^−/− fetal liver cells provided evidence that the subunits can have distinct roles during B-cell development. However, the B-cell-intrinsic functions of c-REL and RELA during B-cell generation and antigen-dependent B-cell activation have not been determined in vivo. To clarify this issue, we crossed mice with conditional rel and rela alleles individually or in combination to mice that express Cre-recombinase in B-cells. We here report that, whereas single deletion of rel or rela did not impair mature B-cell generation and maintenance, their simultaneous deletion led to a dramatic reduction of follicular and marginal zone B-cells. Upon T-cell-dependent immunization, B-cell-specific deletion of the c-REL subunit alone abrogated the formation of germinal centers (GC), whereas rela deletion did not affect GC formation. T-independent responses were strongly impaired in mice with B-cell-specific deletion of rel, and only modestly in mice with RELA-deficient B-cells. Our findings identify differential requirements for the canonical NF-κB subunits c-REL and RELA at distinct stages of mature B-cell development. The subunits are jointly required for the generation of mature B-cells. During antigen-dependent B-cell activation, c-REL is the critical subunit required for the initiation of the GC-reaction and for optimal T-independent antibody responses, with RELA being largely dispensable at this stage.

Canonical NF-κB signaling is uniquely required for the long-term persistence of functional mature B cells

Although canonical NF-κB signaling is crucial to generate a normal mature B-cell compartment, its role in the persistence of resting mature B cells is controversial. To resolve this conflict, we ablated NF-κB essential modulator (NEMO) and IκB kinase 2 (IKK2), two essential mediators of the canonical pathway, either early on in B-cell development or specifically in mature B cells. Early ablation severely inhibited the generation of all mature B-cell subsets, but follicular B-cell numbers could be largely rescued by ectopic expression of B-cell lymphoma 2 (Bcl2), despite a persisting block at the transitional stage. Marginal zone (MZ) B and B1 cells were not rescued, indicating a possible role of canonical NF-κB signals beyond the control of cell survival in these subsets. When canonical NF-κB signaling was ablated specifically in mature B cells, the differentiation and/or persistence of MZ B cells was still abrogated, but follicular B-cell numbers were only mildly affected. However, the mutant cells exhibited increased turnover as well as functional deficiencies upon activation, suggesting that canonical NF-κB signals contribute to their long-term persistence and functional fitness.

B-cell survival and development controlled by the coordination of NF-κB family members RelB and cRel

Targeted deletion of BAFF causes severe deficiency of splenic B cells. BAFF-R is commonly thought to signal to nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB)-inducing kinase dependent noncanonical NF-κB RelB. However, RelB-deficient mice have normal B-cell numbers. Recent studies showed that BAFF also signals to the canonical NF-κB pathway, and we found that both RelB and cRel are persistently activated, suggesting BAFF signaling coordinates both pathways to ensure robust B-cell development. Indeed, we report now that combined loss of these 2 NF-κB family members leads to impaired BAFF-mediated survival and development in vitro. Although single deletion of RelB and cRel was dispensable for normal B-cell development, double knockout mice displayed an early B-cell developmental blockade and decreased mature B cells. Despite disorganized splenic architecture in Relb(-/-)cRel(-/-) mice, generation of mixed-mouse chimeras established the developmental phenotype to be B-cell intrinsic. Together, our results indicate that BAFF signals coordinate both RelB and cRel activities to ensure survival during peripheral B-cell maturation.

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.

Generation of a novel Cr2 gene allele by homologous recombination that abrogates production of Cr2 but is sufficient for expression of Cr1

The enhancing effects of the complement system for humoral immunity have primarily focused upon the recognition of complement-bound foreign antigens by a co-receptor complex of the antigen-specific B cell receptor (BCR) and complement receptor 2 (Cr2). In vivo experiments using Cr2 gene deficient mice (which lack the expression of both the Cr1 and Cr2 proteins) do demonstrate depressed humoral responses to immunization but cannot be used to define specific contributions of the singular Cr1 or Cr2 proteins on B cell functions. To study the effect of a Cr2 deficiency in a Cr1 sufficient environment we created a mouse line in which the alternative splice site required for the expression of the Cr2 isoform was removed. This mouse line, Cr2KO, still expressed Cr1 on B cells but was deficient for the full length Cr2 protein. Surprisingly a new alternative splice within the Cr2 gene created a truncated product that encoded a novel protein termed iCr2 that was expressed on the surface of the cells. The Cr2KO mouse thus provides a new model system for the analysis of Cr1 and Cr2 functions in the immune response of the mouse.

Optimal germinal center B cell activation and T-dependent antibody responses require expression of the mouse complement receptor Cr1

Follicular dendritic cells (FDCs) and complement receptor (Cr)1 and complement receptor (Cr)2 are important for the generation of humoral immunity. Cr1/2 expression on B cells and FDCs was shown to provide a secondary signal for B cell activation, to facilitate transport of Ag in immune follicles, and to enhance retention of immune complexes by FDCs. We show in this study that murine B cells predominantly express the Cr2 product from the Cr2 gene, whereas FDCs almost exclusively express the Cr1 isoform generated from the Cr2 gene. To define the specific role of Cr1, we created an animal that maintains normal cell-restricted expression of Cr2 but does not express Cr1. Cr1-deficient (Cr1KO) mice develop normal B1 and B2 immature and mature B cell subsets and have normal levels of naive serum Abs but altered levels of natural Abs. Immunization of the Cr1KO animal demonstrates deficient Ab responses to T-dependent, but not T-independent, Ags. Germinal centers from the immunized Cr1KO animal possess a deficiency in activated B cells, similar to that seen for animals lacking both Cr1 and Cr2 or C3. Finally, animals lacking only Cr1 respond similarly to wild-type animals to infections with Streptococcus pneumoniae, a pathogen to which animals lacking C3 or both Cr1 and Cr2 are particularly sensitive. Altogether, these data suggest that the production of Cr1, primarily by FDCs, is critical in the generation of appropriately activated B cells of the germinal center and the generation of mature Ab responses.

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.

Critical role of TLR2 and TLR4 in autoantibody production and glomerulonephritis in lpr mutation-induced mouse lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by pathogenic autoantibodies directed against nuclear Ags and immune complex deposits in damaged organs. Environmental factors have been thought to play a role in the onset of the disease. The recognition of these factors is mediated by TLRs, in particular TLR2 and TLR4 which bind pathogen-associated molecular patterns of Gram(+) and Gram(-) bacteria, respectively. We attempted to determine the role of these TLRs in SLE by creating TLR2- or TLR4-deficient C57BL/6(lpr/lpr) mice. These mice developed a less severe disease and fewer immunological alterations. Indeed, in C57BL/6(lpr/lpr)-TLR2 or -TLR4-deficient mice, glomerular IgG deposits and mesangial cell proliferation were dramatically decreased and antinuclear, anti-dsDNA, and anti-cardiolipin autoantibody titers were significantly reduced. However, the response against nucleosome remained unaffected, indicating a role of TLR2 and TLR4 in the production of Abs directed against only certain categories of SLE-related autoantigens. Analysis of B cell phenotype showed a significant reduction of marginal zone B cells, particularly in C57BL/6(lpr/lpr)-TLR4-deficient mice, suggesting an important role of TLR4 in the sustained activation of these cells likely involved in autoantibody production. Interestingly, the lack of TLR4 also affected the production of cytokines involved in the development of lupus disease.

Deletion of putative intronic control sequences does not alter cell or stage specific expression of Cr2

The expression of the mouse Cr2 gene has been shown to be restricted to mature B cells, follicular dendritic cells and, in some reports, to a minor population of activated T cells. In this report, we demonstrate that the expression of antigen(s) recognized by the anti-CR2 antibody on the surface of T cells is co-incident with T cell apoptotic death. Two distinct regions of the Cr2 gene have been implicated as critical for specific expression, the promoter region at the transcription start site and a control region within the first intron of the gene, approximately 1500 bp from the transcription start site. We have created a mouse that is lacking this intronic control sequence which, in the wild type (WT) mouse, contains multiple known binding sites for RBP-jkappa, Oct, NFAT and YY1 proteins. The analysis of this mouse named Cr2iDelta (Cr2 intron deletion) demonstrated normal tissue specific expression of the Cr2 gene including a lack of expression in mouse T cells. B cell expression of the Cr2 gene products, CR1 and CR2, is normal compared to WT, and the FDC of these mice continue to express Cr2 gene products. Therefore the intronic control region of the Cr2 gene, defined in transfection-based reporter gene assays as instrumental in controlling the cell specific expression profile of Cr2, does not influence the expression of the Cr2 gene in vivo nor alter the relative production of the CR1 and CR2 proteins via alternative slicing of Cr2 gene products.

Cell- and stage-specific chromatin structure across the Complement receptor 2 (CR2/CD21) promoter coincide with CBF1 and C/EBP-beta binding in B cells

Stringent developmental transcription requires multiple transcription factor (TF) binding sites, cell-specific expression of signaling molecules, TFs and co-regulators and appropriate chromatin structure. During B-lymphopoiesis, human Complement receptor 2 (CR2/CD21) is detected on immature and mature B cells but not on B cell precursors and plasma cells. We examined cell- and stage-specific human CR2 gene regulation using cell lines modeling B-lymphopoiesis. Chromatin accessibility assays revealed a region between -409 and -262 with enhanced accessibility in mature B cells and pre-B cells, compared to either non-lymphoid or plasma cell-types, however, accessibility near the transcription start site (TSS) was elevated only in CR2-expressing B cells. A correlation between histone acetylation and CR2 expression was observed, while histone H3K4 dimethylation was enriched near the TSS in both CR2-expressing B cells and non-expressing pre-B cells. Candidate sites within the CR2 promoter were identified which could regulate chromatin, including a matrix attachment region associated with CDP, SATB1/BRIGHT and CEBP-beta sites as well as two CBF1 sites. ChIP assays verified that both CBF1 and C/EBP-beta bind the CR2 promoter in B cells raising the possibility that these factors facilitate or respond to alterations in chromatin structure to control the timing and/or level of CR2 transcription.

Development of immunoglobulin lambda-chain-positive B cells, but not editing of immunoglobulin kappa-chain, depends on NF-kappaB signals

By genetically ablating IκB kinase (IKK)-mediated NF-κB activation in the B cell lineage, and by analyzing a mouse mutant in which Igλ⁺ B cells are generated in the absence of rearrangements in Igk, we define two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly Igκ⁺ B cells are generated and undergo efficient receptor editing. In the second phase, predominantly Igλ⁺ B cells are generated, whose development is ontogenetically timed to occur after Igk rearrangements. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the pro-survival factor Bcl2.

c-Abl-deficient mice exhibit reduced numbers of peritoneal B-1 cells and defects in BCR-induced B cell activation

A role for c-Abl in B cell development and signaling has been suggested by previous work showing that c-Abl-deficient mice have defects in bone marrow B cell development and that c-Abl-deficient B cells are hypoproliferative in response to antigen receptor stimulation. Here we show that in addition to defects in early B cell development, c-Abl-deficient mice have defects in peripheral B cell development, including reduced percentages of peritoneal B-1 cells as well as transitional and marginal zone B cells in the spleen. It has been shown that c-Abl kinase activity increases upon B cell receptor (BCR) stimulation and that one of the targets of tyrosine phosphorylation by c-Abl is CD19. However, the consequences of c-Abl activity on B cell activation and CD19 signaling remain unknown. Here, we show that c-Abl-deficient splenic B cells exhibit reduced calcium flux in response to CD19 cross-linking, consistent with a role for c-Abl in CD19-dependent signaling. Additionally, we show that c-Abl-deficient B cells are defective in their ability to be activated in response to antigen receptor engagement, suggesting a functional role for c-Abl in BCR-dependent activation signaling pathways.

Signalling of the BCR is regulated by a lipid rafts-localised transcription factor, Bright

Regulation of BCR signalling strength is crucial for B-cell development and function. Bright is a B-cell-restricted factor that complexes with Bruton's tyrosine kinase (Btk) and its substrate, transcription initiation factor-I (TFII-I), to activate immunoglobulin heavy chain gene transcription in the nucleus. Here we show that a palmitoylated pool of Bright is diverted to lipid rafts of resting B cells where it associates with signalosome components. After BCR ligation, Bright transiently interacts with sumoylation enzymes, blocks calcium flux and phosphorylation of Btk and TFII-I and is then discharged from lipid rafts as a Sumo-I-modified form. The resulting lipid raft concentration of Bright contributes to the signalling threshold of B cells, as their sensitivity to BCR stimulation decreases as the levels of Bright increase. Bright regulates signalling independent of its role in IgH transcription, as shown by specific dominant-negative titration of rafts-specific forms. This study identifies a BCR tuning mechanism in lipid rafts that is regulated by differential post-translational modification of a transcription factor with implications for B-cell tolerance and autoimmunity.

Defining a transcriptional fingerprint of murine splenic B-cell development

B-cell development occurs in a stepwise fashion that can be followed by the expression of B cell-specific surface markers. In this study, we wished to identify proteins that could contribute to the changes in expression of such markers. By using RNA from freshly isolated B220+ cells, we hoped to reduce the effect of artifacts that occur during the isolation and amplification steps necessary to use flow cytometry analysis-sorted subsets in microarray experiments. Analyses comparing expression patterns from B220+ 2-week bone marrow (pro-B, pre-B, immature B cells), 2-week spleen (predominantly transitional cells) and 8-week spleen (mainly mature B cells) yielded hundreds of genes. We also examined the B cell-activating factor (BAFF)-dependent effects on immature splenic B cells by comparing expression patterns in the spleen between 2-week A/J vs 2-week A/WySnJ mice, which lack functional BAFF receptor signaling. Genes that showed the expression differences between spleen and bone marrow samples were then analyzed through quantitative PCR on B-cell subsets isolated using two different sorting protocols. A comparison of the results from our study with the results from other analyses showed not only some overlap of preferentially expressed genes but also an expansion of other genes potentially involved in B-cell development.

Intrafollicular location of marginal zone/CD1d(hi) B cells is associated with autoimmune pathology in a mouse model of lupus

Marginal zone (MZ) B cells contain a large number of autoreactive clones and the expansion of this compartment has been associated with autoimmunity. MZ B cells also efficiently transport blood-borne antigen to the follicles where they activate T cells and differentiate into plasma cells. Using the B6.NZM2410.Sle1.Sle2.Sle3 (B6.TC) model of lupus, we show that the IgM+ CD1d(hi)/MZ B-cell compartment is expanded, and a large number of them reside inside the follicles. Contrary to the peripheral B-cell subset distribution and their activation status, the intrafollicular location of B6.TC IgM+ CD1d(hi)/MZ B cells depends on both bone marrow- and stromal-derived factors. Among the factors responsible for this intrafollicular location, we have identified an increased response to CXCL13 by B6.TC MZ B cells and a decreased expression of VCAM-1 on stromal cells in the B6.TC MZ. However, the reduced number of MZ macrophages observed in B6.TC MZs was independent of the IgM+ CD1d(hi)/B-cell location. B7-2 but not B7-1 deficiency restored IgM+ CD1d(hi)/MZ B-cell follicular exclusion in B6.TC mice, and it correlated with tolerance to dsDNA and a significant reduction of autoimmune pathology. These results suggest that follicular exclusion of IgM+ CD1d(hi)/MZ B cells is an important B-cell tolerance mechanism, and that B7-2 signaling is involved in breaching this tolerance checkpoint.

The regulation of the B-cell gene expression programme by Pax5

The activity of the transcription factor paired box gene 5 (Pax5) is essential for many aspects of B lymphopoiesis including the initial commitment to the lineage, immunoglobulin rearrangement, pre-B cell receptor signalling and maintaining cell identity in mature B cells. Deregulated or reduced Pax5 activity has also been implicated in B-cell malignancies both in human disease and mouse models. Candidate gene approaches and biochemical analysis have revealed that Pax5 regulates B lymphopoiesis by concurrently activating B cell-specific gene expression as well as repressing the expression of genes, many of which are associated with non-B cell lineages. These studies have been recently complemented with more exhaustive microarray studies, which have identified and validated a large panel of Pax5 target genes. These target genes reveal a gene regulatory network, with Pax5 at its centre that controls the B-cell gene expression programme.