IDENTIFICATION AND CHARACTERIZATION OF A HUMAN CD4 SILENCER

Using transgenic mice, we have identified a human CD4 silencer contain ed within a 484-bp Fragment in the first intron of the CD-I gene. Furt her experiments have mapped a lineage-specific silencing activity to a region of 190 bp. This region contains two protein-binding sites dete cted by deoxyribonuclease I footprinting analyses. Tested in transient transfection assays. these two DNA elements showed significant silenc ing activity restricted to the CD8 phenotype. In CD4 cells. either no clear effect (FP I) or strong enhancing activity (FP II) was observed by transient transfection assays. Despite the lineage-specific activit y of these two elements. electrophoretic mobility shift assays (EMSA) showed similar levels of protein binding to the silencer element FP I in CD4 and CD8 T cells. Base substitutions in the FP I fragment abolis hed the silencing activity ill transfected CD8 cells as well as the pr otein binding in ERISA. suggesting an important role of this protein-D NA interaction in CD4 gene regulation.