The transcription factor B cell-specific activator protein (BSAP) enhances both IL-4- and CD40-mediated activation of the human epsilon germline promoter.
Induction of isotype switching to a particular C(H) gene correlates with the transcriptional activation of the same gene in germline configuration. Induction of correctly spliced germline transcripts is necessary to target a switch region for recombination and switching. Different cytokines activate transcription at different germline promoters. Because binding sites for the B cell-specific transcription factor BSAP are located upstream of several switch regions in the Ig locus, BSAP might play a role in isotype switching by regulating germline transcription. We investigated whether BSAP plays a role in the transcriptional regulation of the epsilon germline promoter in human B cells. We identified human EBV-negative B cell lines that express epsilon germline transcripts upon stimulation with IL-4. Electrophoretic mobility shift assay analysis showed that the human epsilon germline promoter binds BSAP. BSAP activity was expressed constitutively and was not affected by stimulation with IL-4 and/or anti-CD40 mAb. Reporter assays with constructs containing a luciferase gene driven by the epsilon germline promoter, with or without mutations in the BSAP binding site, showed that BSAP plays a role in both IL-4-dependent induction and CD40-mediated up-regulation of human epsilon germline transcription. Furthermore, epsilon germline promoter activity was abrogated in REH cells that express a BSAP polypeptide truncated in the trans-activation domain. Among the transcription factors that regulate epsilon germline expression, BSAP is unique, in that it is B cell-specific and is at the merging point of two signaling pathways that are distinct but both critical for the induction of IgE switching.
Thienes
C P
CP
Molecular Immunoregulation Unit, DIBIT, San Raffaele Scientific Institute, Milan, Italy.
De Monte
L
L
Monticelli
S
S
Busslinger
M
M
Gould
H J
HJ
Vercelli
D
D
D001483:Base Sequence; D019013:CD40 Antigens / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000502:physiology*; D005803:Genes, Immunoglobulin / Q000502:physiology*; D006801:Humans; D007134:Immunoglobulin Switch Region / Q000502:physiology; D015847:Interleukin-4 / Q000502:physiology*; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000502:physiology*; D051757:PAX5 Transcription Factor; D011401:Promoter Regions, Genetic / Q000502:physiology*; D014157:Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic; D015533:Transcriptional Activation
D019013:CD40 Antigens; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D051757:PAX5 Transcription Factor; C497281:PAX5 protein, human; D014157:Transcription Factors; D015847:Interleukin-4