B lymphocyte development and transcription regulation in vivo

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

The transcription factor Bright associates with Bruton's tyrosine kinase, the defective protein in immunodeficiency disease

Binding of the transcription factor Bright to Ig heavy chain loci after B cell activation is associated with increased heavy chain transcription. We now report that Bright coprecipitates with Bruton's tyrosine kinase (Btk), the defective enzyme in X-linked immunodeficiency disease (xid). Furthermore, we observed Btk in the nucleus of activated murine B cells, and mobility shift assays suggest that it is a component of the Bright DNA-binding complex. While BRIGHT protein was synthesized in activated spleen cells from xid mice, it did not bind DNA or associate stably with Btk. These data suggest that deficiencies in BRIGHT DNA-binding activity may contribute to the defects in Ig production seen in xid mice.

Transcriptional regulation of lymphocyte lineage commitment

The development of T cells and B cells from pluripotent hematopoietic precursors occurs through a stepwise narrowing of developmental potential that ends in lineage commitment. During this process, lineage-specific genes are activated asynchronously, and lineage-inappropriate genes, although initially expressed, are asynchronously turned off. These complex gene expression events are the outcome of the changes in expression of multiple transcription factors with partially overlapping roles in early lymphocyte and myeloid cell development. Key transcription factors promoting B-cell development and candidates for this role in T-cell development are discussed in terms of their possible modes of action in fate determination. We discuss how a robust, stable, cell-type-specific gene expression pattern may be established in part by the interplay between endogenous transcription factors and signals transduced by cytokine receptors, and in part by the network of effects of particular transcription factors on each other.

Two models of murine B lymphopoiesis: a correlation

During B cell genesis in mouse bone marrow (BM), precursor B cells pass through a series of developmental stages that have been defined by changes in expression of various marker molecules. The use of dissimilar phenotypic criteria in different laboratories, however, has led to the formulation of disparate models of B lymphopoiesis not fully reconciled with one another. We have directly compared two such models, one based on expression of intracellular mu heavy chain of IgM (c mu) and terminal deoxynucleotidyl transferase (TdT), the other monitoring cell surface leukosialin (CD43), heat-stable antigen (HSA; CD24) and the ectopeptidase BP-1. Each model uses cell surface B220 glycoprotein (CD45RA) to denote the B cell lineage. We have examined the cellular composition of four sorted BM fractions by immunofluorescent labeling of CD43, HSA and BP-1, using immunofluorescence microscopy of cytocentrifuged fractions to quantitate precursor B cell populations expressing either c mu or TdT. The results reveal a range of B cell differentiation stages within individual sorted BM fractions, providing a cross-reference between these two analytical methods and contributing to a unified model of B cell development in mouse BM.

Expression of Bright at Two Distinct Stages of B Lymphocyte Development

The B cell regulator of Ig heavy chain transcription (Bright) is a DNA-binding protein that was originally discovered in a mature Ag-specific B cell line after stimulation with IL-5 and Ag. It binds to the intronic heavy chain enhancer and 5′ of the V1 S107 family VH promoter. Several studies suggested that Bright may increase transcription of the heavy chain locus, and expression in cell lines was limited to those representing mature B cells. We have now analyzed normal hemopoietic tissues for the expression of Bright during B lymphocyte differentiation. We expected to find Bright expression in a subset of mature spleen cells, but also observed Bright in a subset of normal B lymphocytic progenitors in both adult bone marrow (BM) and in fetal liver as early as day 12 of gestation. Bright was also expressed in the small percentage of CD4low cells in the thymus that are newly arrived from the BM and are not yet committed to the T lymphocyte lineage, but was not observed at later stages of T cell differentiation in either the spleen or thymus. Bright mRNA was not detected in the immature B lymphocytes that initially populate the spleen after migration from the BM. In addition, new splice variants of Bright were observed in fetal tissues. Thus, Bright expression is highly regulated in normal murine lymphocytes and occurs both early and late during B cell differentiation. These findings may have important implications for the function of Bright in regulating Ig transcription.