AN INTRONIC SILENCER REGULATES B-LYMPHOCYTE CELL-SPECIFIC AND STAGE-SPECIFIC EXPRESSION OF THE HUMAN-COMPLEMENT RECEPTOR-TYPE 2 (CR-2, CD21) GENE

Human CR2 (CD21) is a B lymphocyte protein whose surface expression is restricted primarily to the mature cell stage during development, To study the transcriptional mechanisms that govern cell-and stage-restri cted CR2 expression, we first performed transient transfection analysi s using constructs extending from -5 kb to +75 bp (-5 kb/+75) in the C R2 promoter, The promoter was found to he broadly active, with no evid ence of cell-or stage-specific reporter gene expression, However, the addition of a 2,5-kb intronic gene segment (containing a DNase I hyper sensitive site) to the (-5-kh/+75) construct resulted in appropriate r eporter gene expression, defined as the silencing of the (-5-kb/+75) p romoter activity only in non-CR2-expressing cells, Interestingly, appr opriate reporter gene expression required stable transfection of the c onstructs in cell lines, suggesting nuclear matrix or chromatin intera ctions may be important for appropriate CR2 gene expression, Important ly, transgenic mice also required the intronic silencer to generate ly mphoid tissue-specific reporter gene expression, Some transgenic found er lines did net demonstrate reporter gene expression, however, indica ting that additional transcriptional regulatory elements are present i n other regions of the CR2 gene, In summary, these data support the hy pothesis that human CR2 expression is regulated primarily by an intron ic silencer with lineage-and B cell stage-specific activity.