CHARACTERIZATION OF A CIS-ACTING REGULATORY ELEMENT WHICH SILENCES EXPRESSION OF THE CLASS II-A BETA-GENE IN EPITHELIUM

`Class II major histocompatibility complex (MHC) genes encode for alph a/beta chain pairs that are constitutively expressed principally on ma ture B cells and dendritic cells in mice. These gene products are easi ly induced on macrophages with cytokines, and may also aberrantly appe ar on the surface of epithelium during immune injury. The appearance o f class II determinants in parenchymal tissue potentially renders thes e somatic cells capable of antigen presentation to circulating CD4(+) T lymphocytes, and their absence may be protective for normal tissues expressing self-antigens. The low surface class II expression observed on parenchymal cells generally correlates with low levels of mRNA, su ggesting that transcription rate is a major element in class II regula tion. To understand the transcriptional mechanism maintaining low basa l surface expression of class II in somatic cells, we transiently tran sfected mini-gene reporter constructs to study the regulation of the m urine A beta promoter in a cultured renal epithelial cell line. We des cribe here a negative cis-acting regulatory region located between -55 2 and -489 bp upstream of the A beta cap site that silences the transc riptional activity of the AP promoter in epithelial cells in an orient ation-dependent manner, and is also able to silence a heterologous pro moter. This region is not active in class II-expressing B cells (BAL-1 7) in culture, but is functional in two other murine class II-negative cell lines, fibroblasts and thymoma T cells. Using competition electr ophoretic mobility shift assays, we have localized the core protein bi nding site within this region to an 8-10-bp response element, designat ed A beta NRE, at -543 to -534 bp. A nuclear extract from BAL-17 cells does not bind to this element. Mutation of this site abrogates the tr anscriptional silencing activity of the region. We conclude that the t ranscription of class II-A beta in parenchymal cells, and some lymphoc ytes, can be actively repressed by an upstream silencing element.