The transcription factor E2A controls the initiation of B lymphopoiesis, which is arrested at the pre-pro-B cell stage in E2A-deficient mice. Here, we demonstrate by conditional mutagenesis that E2A is essential for the development of pro-B, pre-B, and immature B cells in the bone marrow. E2A is, however, dispensable for the generation of mature B cells and plasma cells in peripheral lymphoid organs. In contrast, germinal center B cell development is impaired in the absence of E2A despite normal AID expression and class-switch recombination. Molecular analysis revealed that E2A is required not only for initiating but also for maintaining the expression of Ebf1, Pax5, and the B cell gene program in pro-B cells. Notably, precocious Pax5 transcription from the Ikzf1 locus promotes pro-B cell development in E2A-deficient mice, demonstrating that ectopic Pax5 expression is sufficient to activate the B lymphoid transcription program in vivo in the absence of E2A.

Pax5 controls the identity and development of B cells by repressing lineage-inappropriate genes and activating B-cell-specific genes. Here, we used genome-wide approaches to identify Pax5 target genes in pro-B and mature B cells. In these cell types, Pax5 bound to 40% of the cisregulatory elements defined by mapping DNase I hypersensitive (DHS) sites, transcription start sites and histone modifications. Although Pax5 bound to 8000 target genes, it regulated only 4% of them in pro-B and mature B cells by inducing enhancers at activated genes and eliminating DHS sites at repressed genes. Pax5-regulated genes in pro-B cells account for 23% of all expression changes occurring between common lymphoid progenitors and committed pro-B cells, which identifies Pax5 as an important regulator of this developmental transition. Regulated Pax5 target genes minimally overlap in pro-B and mature B cells, which reflects massive expression changes between these cell types. Hence, Pax5 controls B-cell identity and function by regulating distinct target genes in early and late B lymphopoiesis. The EMBO Journal (2012) (Nutt and Kee, 2007). The helix-loop-helix protein E2A and the early B-cell factor EBF1 specify the B-cell lineage by activating the expression of B-lymphoid genes in pre-pro-B cells Treiber et al, 2010). Pax5 subsequently controls B-cell commitment at the transition to the pro-B cell stage by restricting the developmental potential of lymphoid progenitors to the B-cell lineage, as shown by the fact that Pax5-deficient pro-B cells are still able to differentiate into most hematopoietic cell types in vitro and in vivo (Nutt et al, 1999;Medvedovic et al, 2011). At the molecular level, Pax5 fulfills a dual role by repressing B-lineage-inappropriate genes to suppress alternative lineage options and by simultaneously activating B-cell-specific genes to promote B-cell development (Nutt et al, 1999;Medvedovic et al, 2011). Gene expression analyses of wild-type and Pax5-deficient pro-B cells identified 110 Pax5-repressed and 170 Pax5-activated genes, which code for key regulatory and structural proteins involved in transcriptional control, receptor signalling, adhesion, migration and immune function (Delogu et al, 2006;Schebesta et al, 2007;Pridans et al, 2008). Pax5 regulates these gene expression changes by inducing active chromatin at activated target genes and eliminating active chromatin at repressed genes in pro-B cells . Notably, Pax5 induces these chromatin and transcription changes by recruiting chromatin-remodelling, histone-modifying and basal transcription factor complexes to its target genes, which identifies Pax5 as an epigenetic regulator of B-cell commitment .Pax5 is expressed throughout B-cell development from pro-B cells in the bone marrow to mature B cells in peripheral lymphoid organs (Fuxa and Busslinger, 2007), where it plays an important role in the generation and function of distinct mature B-cell types (Horcher et al, 2001;Medvedovic et al, 2011). Pax5 is essential for maintaining the B-cell gene exp...

Background Combined immunodeficiencies (CIDs) denote inborn errors of T-cell immunity with T cells present but quantitatively or functionally deficient. Impaired humoral immunity, either due to a primary B cell defect or secondary to the T-cell defect, is also frequent. Consequently, patients with CID display severe infections and/or autoimmunity. The specific molecular, cellular, and clinical features of many types of CID remain unknown. Methods We performed genetic and cellular immunological studies in five unrelated children who shared a history of early-onset invasive bacterial and viral infections, with lymphopenia and defective T-, B-, and NK-cell responses. Two patients died early in childhood, whereas the other three underwent allogeneic hematopoietic stem cell transplantation with normalization of T cell function and clinical improvement. Results We identified bi-allelic mutations in the Dedicator Of Cytokinesis 2 (DOCK2) gene in these five patients. RAC1 activation was impaired in T cells. Chemokine-induced migration and actin polymerization were defective in T, B, and NK cells. NK-cell degranulation was also affected. The production of interferon (IFN)-α and -λ by peripheral blood mononuclear cells (PBMCs) was diminished following virus infection. Moreover, in DOCK2-deficient fibroblasts, virus replication was increased and there was enhanced virus-induced cell death, which could be normalized by treatment with IFN-α2β or upon expression of wild-type DOCK2. Conclusions Autosomal recessive DOCK2 deficiency is a Mendelian disorder with pleiotropic defects of hematopoietic and non-hematopoietic immunity. Children with clinical features of CID, especially in the presence of early-onset, invasive infections may have this condition.

Coreceptor expression is tightly regulated during thymocyte development. Deletion of specific Cd8 enhancers leads to variegated expression of CD8alphabeta heterodimers in double-positive thymocytes. Here we show CD8 variegation is correlated with an epigenetic 'off' state, linking Cd8 enhancer function with chromatin remodeling of the adjacent genes Cd8a and Cd8b1 (Cd8). The zinc finger protein MAZR bound the Cd8 enhancer and interacted with the nuclear receptor corepressor N-CoR complex in double-negative thymocytes. MAZR was downregulated in double-positive and CD8 single-positive thymocytes. 'Enforced' expression of MAZR led to impaired Cd8 activation and variegated CD8 expression. Our results demonstrate epigenetic control of the Cd8 loci and identify MAZR as an important regulator of Cd8 expression.

Gain- and loss-of-function analyses reveal that the transcription factor EBF1 is required for normal differentiation and function of marginal zone, B-1, follicular, and germinal center B cells in mice.

CD4/CD8 lineage specification of thymocytes is linked with coreceptor expression. Previously, the transcription factor MAZR was identified as an important regulator of Cd8 gene expression. Here we show that variegated CD8 expression by loss of Cd8 enhancers is reverted in MAZRdeficient mice, confirming that MAZR negatively regulates the Cd8 loci during the transition to the double-positive (DP) stage. Moreover, loss of MAZR led to a partial redirection of MHC class I-restricted thymocytes into CD4 + helper-like T cells correlating with derepression of ThPOK, a central transcription factor for helper-lineage development. MAZR bound the ThPOK silencer, indicating direct regulation of the ThPOK locus by MAZR. Thus, MAZR is part of the transcription factor network regulating CD8 lineage differentiation of DP thymocytes.

Chromatin modifications, such as reversible histone acetylation, play a key role in the regulation of T cell development and function. However, the role of individual histone deacetylases (HDACs) in T cells is less well understood. In this article, we show by conditional gene targeting that T cell-specific loss of HDAC1 led to an increased inflammatory response in an in vivo allergic airway inflammation model. Mice with HDAC1-deficient T cells displayed an increase in all critical parameters in this Th2-type asthma model, such as eosinophil recruitment into the lung, mucus hypersecretion, parenchymal lung inflammation, and enhanced airway resistance. This correlated with enhanced Th2 cytokine production in HDAC1-deficient T cells isolated from diseased mice. In vitro-polarized HDAC1-deficient Th2 cells showed a similar enhancement of IL-4 expression, which was evident already at day 3 of Th2 differentiation cultures and restricted to T cell subsets that underwent several rounds of cell divisions. HDAC1 was recruited to the Il4 gene locus in ex vivo isolated nonstimulated CD4+ T cells, indicating a direct control of the Il4 gene locus. Our data provide genetic evidence that HDAC1 is an essential HDAC that controls the magnitude of an inflammatory response by modulating cytokine expression in effector T cells.

Busslinger et al. showed that the transcription factors E2A and E2-2 control the expression of genes required for the development of GC B cells and plasma cells.