Regulation of human epsilon germline transcription: role of B-cell-specific activator protein

Myb proteins repress human Ig epsilon germline transcription by inhibiting STAT6-dependent promoter activation

Cytokine-dependent induction of correctly spliced germline (GL) transcripts is required to target the appropriate switch region for class switch recombination. GL transcription is linked to the cell cycle and the number of cell divisions through mechanisms that have not been defined. The human proximal epsilon GL promoter contains an IL-4 responsive element (IL-4RE) that binds STAT6 and is sufficient to confer IL-4 inducibility to a heterologous promoter in transient transfection studies. We show herein that the IL-4RE contains a novel Myb binding motif that overlaps the 3' end of the STAT6 palindrome. EMSA analysis showed binding to the IL-4RE of endogenous Myb proteins expressed in BL-2 B cells and Jurkat T cells. However, double occupancy of a probe spanning both STAT6 and Myb binding motifs could not be detected. Thus, binding of either factor may prevent protein/DNA interactions at the other site, raising the possibility that Myb binding may interfere with STAT6-dependent activation of the IL-4RE. Indeed, cotransfection of A-Myb or c-Myb expression vectors in HEK293 and BL-2 cells suppressed STAT6-dependent transcription from a reporter construct containing four copies of the IL-4RE cloned upstream of a minimal thymidine kinase promoter. Most importantly, overexpression of A-Myb was sufficient to suppress IL-4-induced endogenous epsilon GL transcription in BL-2 cells. Our results indicate that Myb proteins, which are known to act as cell cycle sensors, may play an important mechanistic role in the in vivo regulation of epsilon GL transcription in human B cells.

BSAP can repress enhancer activity by targeting PU.1 function

PU.1 and BSAP are transcription factors crucial for proper B-cell development. Absence of PU.1 results in loss of B, T, and myeloid cells, while absence of BSAP results in an early block in B-cell differentiation. Both of these proteins bind to the immunoglobulin kappa chain 3' enhancer, which is developmentally regulated during B-cell differentiation. We find here that BSAP can repress 3' enhancer activity. This repression can occur in plasmacytoma lines or in a non-B-cell line in which the enhancer is activated by addition of the appropriate enhancer binding transcription factors. We show that the transcription factor PU.1 is a target of the BSAP-mediated repression. Although PU.1 and BSAP can physically interact through their respective DNA binding domains, this interaction does not affect DNA binding. When PU.1 function is assayed in isolation on a multimerized PU.1 binding site, BSAP targets a portion of the PU.1 transactivation domain (residues 7 to 30) for repression. The BSAP inhibitory domain (residues 358 to 385) is needed for this repression. Interestingly, the coactivator protein p300 can eliminate this BSAP-mediated repression. We also show that PU.1 can inhibit BSAP transactivation and that this repression requires PU.1 amino acids 7 to 30. Transfection of p300 resulted in only a partial reversal of PU.1-mediated repression of BSAP. When PU.1 function is assayed in the context of the immunoglobulin kappa chain 3' enhancer and associated binding proteins, BSAP represses PU.1 function by a distinct mechanism. This repression does not require the PU.1 transactivation or PEST domains and cannot be reversed by p300 expression. The possible roles of BSAP and PU.1 antagonistic activities in hematopoietic development are discussed.

Dendritic Cells Interact Directly with Naive B Lymphocytes to Transfer Antigen and Initiate Class Switching in a Primary T-Dependent Response

Dendritic cells (DC) are thought to initiate Ab synthesis by activation of T cells, which then provide cytokine and cell-bound “help” to B cells. Here, we provide evidence that DC can capture and retain unprocessed Ag in vitro and in vivo, and can transfer this Ag to naive B cells to initiate a specific Ab response. The response is skewed with 4- to 13-fold higher titers of IgG than IgM, and the predominant subclasses of Ab produced in naive animals are those associated with Th2-type responses. Ag retention and the skew in class switching is a physiologic phenomenon because DC loaded with Ag in vivo and isolated 24 h later initiated a class-switched, Ag-specific Ab response in naive animals. In vitro studies confirmed that DC provide naive B cells with signals that are essential for the synthesis of class-switched Ab. Taken together, these observations show that DC have an important role in the initiation of Ab synthesis by direct interaction with B cells.

Lymphotoxin-alpha is an important autocrine factor for CD40 + interleukin-4-mediated B-cell activation in normal and atopic donors

Stimulation of human B cells with anti-CD40 + interleukin-4 (IL-4) results not only in proliferation and immunoglobulin E (IgE)-production, but also increased production of the cytokine lymphotoxin-alpha (LT-alpha) (formerly also known as tumour necrosis factor-beta (TNF-beta)). Here, we studied the role of LT-alpha (TNF-beta) in B cells following stimulation with anti-CD40 + IL-4 from normal versus atopic donors. Anti-CD40 + IL-4 stimulation of peripheral blood mononuclear cells (PBMC) from atopic donors resulted in enhanced production of soluble LT-alpha (TNF-beta) and increased membrane LT-alpha (TNF-beta) expression on the B cells compared with normal donors. Functional evaluation of LT-alpha (TNF-beta) in CD40 + IL-4-stimulated B cells shows that recombinant LT-alpha (TNF-beta) induces proliferation of B cells and enhances CD40 + IL-4-mediated B-cell proliferation and IgE synthesis in both normal and atopic donors in a dose-dependent manner. These findings were supported by semiquantitative analysis of epsilon-germline transcripts using reverse transcription-polymerase chain reaction (RT-PCR) showing increased epsilon-germline transcription in the presence of LT-alpha. Furthermore, addition of anti-LT-alpha (anti-TNF-beta) to CD40 + IL-4-stimulated B cells partially inhibited proliferation and IgE synthesis in a dose-dependent manner indicating a role of endogenous LT-alpha (TNF-beta) production by B cells during continued CD40 + IL-4 stimulation. These data suggest that LT-alpha (TNF-beta) plays a potentially significant role during B-cell proliferation and IgE synthesis. Moreover, LT-alpha (TNF-beta) production seems to be differentially regulated in B cells from normal and atopic donors.