Bright (B-cell regulator of immunoglobulin heavy chain transcription) binding to immunoglobulin heavy chain loci after B-cell activation is associated with increased heavy chain transcription. Our earlier reports demonstrated that Bright coimmunoprecipitates with Bruton's tyrosine kinase (Btk) and that these proteins associate in a DNA-binding complex in primary B cells. B cells from immunodeficient mice with a mutation in Btk failed to produce stable Bright DNA-binding complexes. In order to determine if Btk is important for Bright function, a transcription activation assay was established and analyzed using real-time PCR technology. Cells lacking both Bright and Btk were transfected with Bright and/or Btk along with an immunoglobulin heavy chain reporter construct. Immunoglobulin gene transcription was enhanced when Bright and Btk were coexpressed. In contrast, neither Bright nor Btk alone led to activation of heavy chain transcription. Furthermore, Bright function required both Btk kinase activity and sequences within the pleckstrin homology domain of Btk. Bright was not appreciably phosphorylated by Btk; however, a third tyrosine-phosphorylated protein coprecipitated with Bright. Thus, the ability of Bright to enhance immunoglobulin transcription critically requires functional Btk.Bright (B-cell regulator of immunoglobulin [Ig] heavy chain transcription) is a B-cell-restricted transcription factor that binds specific A-T-rich sequences. The protein consists of an acidic amino-terminal domain, a DNA-binding A-T-rich interaction domain, a putative transactivation domain, and a protein interaction domain (18). The carboxyl-terminal domain of Bright currently has no assigned function. Bright was originally identified in an antigen-specific B-cell line, BCg3R-1d, as a mobility-shifted complex induced after stimulation with interleukin-5 (IL-5) and antigen (54). Binding sites for Bright were originally identified 5Ј of the basal promoter of the V1 S107 gene but also exist within the matrix association regions on either side of the intronic enhancer (53, 55). Bright binding to the 5Ј-flanking sequences of the V1 S107 variable heavy chain (V H ) promoter correlated with two-to sixfold increases in heavy chain mRNA levels in response to 55). Deletion of Bright binding sites flanking the V1 promoter resulted in lack of antigen-and IL-5-stimulated heavy chain transcription (55). Bright expression is tightly regulated in normal murine lymphocytes, occurring in pre-B cells and late stages of B-cell differentiation (58). However, Bright is not present in detectable amounts in immature B cells, suggesting that it may not play a role in maintenance of Ig expression (58). On the other hand, Bright activity is induced in B cells activated in response to lipopolysaccharide (LPS), CD40 ligand stimulation, and anti-CD38 (55, 59). These data suggest that Bright enhances Ig heavy chain transcription above basal levels following B-cell activation.Our earlier results revealed that Bruton's tyrosine kinase (Btk) associates with Brig...

The B cell-restricted transcription factor, B cell regulator of IgH transcription (Bright), up-regulates Ig H chain transcription 3- to 7-fold in activated B cells in vitro. Bright function is dependent upon both active Bruton’s tyrosine kinase and its substrate, the transcription factor, TFII-I. In mouse and human B lymphocytes, Bright transcription is down-regulated in mature B cells, and its expression is tightly regulated during B cell differentiation. To determine how Bright expression affects B cell development, transgenic mice were generated that express Bright constitutively in all B lineage cells. These mice exhibited increases in total B220+ B lymphocyte lineage cells in the bone marrow, but the relative percentages of the individual subpopulations were not altered. Splenic immature transitional B cells were significantly expanded both in total cell numbers and as increased percentages of cells relative to other B cell subpopulations. Serum Ig levels, particularly IgG isotypes, were increased slightly in the Bright-transgenic mice compared with littermate controls. However, immunization studies suggest that responses to all foreign Ags were not increased globally. Moreover, 4-wk-old Bright-transgenic mice produced anti-nuclear Abs. Older animals developed Ab deposits in the kidney glomeruli, but did not succumb to further autoimmune sequelae. These data indicate that enhanced Bright expression results in failure to maintain B cell tolerance and suggest a previously unappreciated role for Bright regulation in immature B cells. Bright is the first B cell-restricted transcription factor demonstrated to induce autoimmunity. Therefore, the Bright transgenics provide a novel model system for future analyses of B cell autoreactivity.

Bright/ARID3a/Dril1, a member of the ARID family of transcription factors, is expressed in a highly regulated fashion in B lymphocytes, where it enhances immunoglobulin transcription three-to sixfold. Recent publications from our lab indicated that functional, but not kinase-inactive, Bruton's tyrosine kinase (Btk) is critical for Bright activity in an in vitro model system, yet Bright itself is not appreciably tyrosine phosphorylated. These data suggested that a third protein, and Btk substrate, must contribute to Bright-enhanced immunoglobulin transcription. The ubiquitously expressed transcription factor TFII-I was identified as a substrate for Btk several years ago. In this work, we show that TFII-I directly interacts with human Bright through amino acids in Bright's protein interaction domain and that specific tyrosine residues of TFII-I are essential for Bright-induced activity of an immunoglobulin reporter gene. Moreover, inhibition of TFII-I function in a B-cell line resulted in decreased heavy-chain transcript levels. These data suggest that Bright functions as a three-component protein complex in the immunoglobulin locus and tie together previous data indicating important roles for Btk and TFII-I in B lymphocytes.The transcription factor Bright (B-cell regulator of immunoglobulin H [IgH] transcription)/ARID3a/Dril1 is a B-cellspecific protein first discovered in a mature mouse B-cell line, BCg3R1-d, as a mobility-shifted protein complex that caused three-to sixfold increases in heavy-chain mRNA levels in response to stimulation with a T-dependent antigen and interleukin-5 (42, 43). Bright binds to AϩT-rich regions of the intronic heavy-chain enhancer previously identified as matrix association regions and to regions 5Ј of some variable heavychain (V H ) promoters, including the V1 S107 family gene (15,43). The cDNA for Bright was isolated in 1995, and the protein was shown to interact with DNA as a multimeric complex that included multiple copies of Bright (15). The Bright protein structure consists of an acidic N-terminal domain of unknown function, a DNA-binding AϩT-rich interaction domain (ARID), a putative transactivation domain, a protein interaction domain containing a helix-turn-helix region, and a small carboxyl-terminal domain with no assigned function.Earlier studies indicated that Bruton's tyrosine kinase (Btk), the defective enzyme in X-linked immunodeficiency disease in both mice and humans, is a component of the Bright DNAbinding complex (28, 44). X-linked immunodeficiency disease in mice, or X-linked agammaglobulinemia (XLA) in humans, results in blocks in B-lymphocyte development that ultimately lead to a deficient production of serum antibodies (9,33,40). Patients with XLA are unable to fight normal bacterial infections without frequent intravenous Ig treatments. Although Btk was identified as the genetic defect in XLA many years ago, the mechanism by which Btk deficiencies lead to early blocks at the pro-B-to pre-B-lymphocyte stages in humans remains unclear.Recently, an in vitro mode...

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