A BIFUNCTIONAL CONTROL ELEMENT IN THE HUMAN IGE GERMLINE PROMOTER INVOLVED IN REPRESSION AND IL-4 ACTIVATION

One of the first steps during Ig heavy chain isotype switching to IgE is the IL-4 induced synthesis of IgE germline transcripts. To further characterize the molecular mechanism of the IL-4 action, the regulator y DNA elements involved in the control of expression of these transcri pts were analyzed. Transient transfection of a B cell tumor line revea led the presence of a 15 bp IL-4 responsive cis-acting element (IL-4RE ) highly homologous to an IL-4 response element in the human CD23b pro moter. An IL-4 induced DNA binding protein specifically interacted wit h this sequence, Point mutations within that sequence not only abolish ed IL-4 inducibility of reporter constructs but also prevented binding of the nuclear factor to the mutated sequence. A stretch of 16 nucleo tides just upstream of the IL-4RE contributed to IL-4 inducibility and formed nucleoprotein complexes with constitutive factors. All reporte r constructs containing the functional IL-4RE were transcriptionally v ery weak but could be readily activated upon IL-4 induction. Transfect ion of constructs containing the mutated IL-4RE or plasmids lacking th at sequence displayed a high constitutive promoter activity and were I L-4 unresponsive. These data suggest that in the absence of the cytoki ne the activity of the IgE germline promoter is actively repressed thr ough the action of the IL-4RE. The same sequence appears to be critica lly involved in the IL-4 induced activation of the promoter via the bi nding of a cytokine induced transcription factor.