THE TRANSCRIPTION FACTOR-B CELL-SPECIFIC ACTIVATOR PROTEIN (BSAP) ENHANCES BOTH IL-4-MEDIATED AND CD40-MEDIATED ACTIVATION OF THE HUMAN EPSILON GERMLINE PROMOTER

Induction of isotype switching to a particular C-H gene correlates wit h the transcriptional activation of the same gene in germline configur ation, Induction of correctly spliced germline transcripts is necessar y to target a switch region for recombination and switching, Different cytokines activate transcription at different germline promoters. Bec ause binding sites for the B cell-specific transcription factor BSAP a re located upstream of several switch regions in the Ig locus, BSAP mi ght play a role in isotype switching by regulating germline transcript ion. We investigated whether BSAP plays a role in the transcriptional regulation of the epsilon germline promoter in human B cells, We ident ified human EBV-negative B cell lines that express epsilon germline tr anscripts upon stimulation with IL-4. Electrophoretic mobility shift a ssay analysis showed that the human E germline promoter binds BSAP. BS AP activity was expressed constitutively and was not affected by stimu lation 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-reg ulation of human epsilon germline transcription, Furthermore, epsilon germline promoter activity was abrogated in REH cells that express a B SAP polypeptide truncated in the trans-activation domain, Among the tr anscription 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 ind uction of IgE switching.