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.