c-Rel is a selective activator of a novel IL-4/CD40 responsive element in the human Ig γ4 germline promoter

Induction of isotype switching to a specific C_H gene correlates with the transcriptional activation of the same gene in germline (GL) configuration. Expression of correctly spliced GL transcripts is necessary to target a switch region for recombination. In human B cells, the IgE and IgG4 isotypes are both induced by IL-4 through sequential switching, but are functionally antagonistic because IgG4 appears to have IgE-blocking activity. In order to understand the molecular mechanisms that regulate IgG4 production, we undertook a systematic analysis of the γ4 GL promoter. A HindIII/NaeI region (-421/+474) highly conserved in the human γ locus mediated the synergistic activation of a reporter gene in response to IL-4 and CD40 cross-linking. STAT6 binding to the proximal γ4 GL promoter was essential for both IL-4-induced activation and CD40-dependent enhancement of transcription. Of note, a 45bp region (-76/-32) centered around the STAT6 binding motif drove robust synergistic activation of a heterologous fos promoter upon stimulation with IL-4 and CD40 cross-linking. This finding suggested that the (-76/-32) region may contain a novel IL-4/CD40 responsive element (RE). Electrophoretic mobility shift assays (EMSA) analysis using BL-2 nuclear extracts and in vitro translated NF-κB/Rel family proteins revealed the presence of a motif that overlaps the 5′ end of the STAT6 element and binds selectively c-Rel. A mutation that abrogated c-Rel, but not STAT6, binding strongly impaired the CD40-induced enhancement of IL-4-dependent γ4 GL transcription in reporter assays. These results indicate that c-Rel is selectively involved in the CD40-dependent activation of the IL-4/CD40 RE in the proximal γ4 GL promoter.