EVIDENCE FOR A CD40 RESPONSE ELEMENT, DISTINCT FROM THE IL-4 RESPONSE ELEMENT, IN THE GERMLINE EPSILON-PROMOTER

Engagement of CD40 by its ligand induces transcription of unrearranged Ig heavy chain genes, an initial step in switch recombination. The following studies were undertaken to understand the molecular basis of this response, co-culture of Sf9 cells expressing membrane-bound CD40 ligand (CD40L) encoded by recombinant baculovirus with EBV-transformed B cell lines induced germline transcription of the epsilon gene in the absence of cytokines. To identify a putative CD40 response element, a reporter construct consisting of the 777 bp of the 5' flank of the human I epsilon region linked to the chloramphenicol acetyl transferase (CAT) gene was stably transfected into a cell lines. Stimulation with either CD4OL-expressing Sf9 cells or IL-4 induced CAT activity. Deletional analysis of this promoter region confirmed that an IL-4 response element was present at -122 to -28 bp from the major transcription initiation site. A second element was identified within a 63 bp segment 3' to the IL-4-responsive element that was responsive to CD40 ligation. These results indicate that the germline epsilon promoter contains a CD40 response element that is distinct from that accounting for IL-4 responsiveness. Activity of this response element may explain the capacity of ligation of CD40 to induce germline epsilon transcripts in the absence of cytokines.