The regulation of CD5 expression in murine T cells

We further characterize the CD5 promoter and identify minimal and regulatory regions on the CD5 promoter. Using a luciferase reporter system, we show that a 43 bp region on the CD5 promoter regulates CD5 expression in resting mouse thymoma EL4 T cells and that an Ets binding site within the 43 bp region mediates the CD5 expression. In addition, we show that Ets-1, a member of the Ets family of transcription factors, recognizes the Ets binding site in the electrophoretic mobility shift assay (EMSA). This Ets binding site is directly responsible for the increase in reporter activity when co-transfected with increasing amounts of Ets-1 expression plasmid.We also identify two additional evolutionarily-conserved regions in the CD5 promoter (CD5X and CD5Y) and demonstrate the respective roles of the each region in the regulation of CD5 transcription.Our studies define a minimal and regulatory promoter for CD5 and show that the CD5 expression level in T cells is at least partially dependent on the level of Ets-1 protein. Based on the findings in this report, we propose a model of CD5 transcriptional regulation in T cells.The murine CD5 protein (Ly-1), a 67 kD membrane-associated glycoprotein found on all T cells and at low levels on B-1a cells, a CD5-expressing subset of B lymphocytes [1,2,3,4]. CD5 levels on the developmental and functional subsets range are ordered in a characteristic fashion: Thymic CD4+ T > Splenic CD4+ T > Thymic CD8+ T > Splenic CD8+ T > Thymic CD4+CD8+ T >> B-1a cells [5, 6]. The CD5 surface and mRNA expression levels in these subsets are highly correlated, suggesting that CD5 expression is primarily regulated at the transcriptional level.Structurally, CD5 belongs to the scavenger receptor family of proteins based on the homology of its three extracellular scavenger receptor cysteine-rich (SRCR) domains to other family members [7]. Several potential CD5 ligands have been reported so far: the pan-B cell marker, CD72 [8]; an as yet unidentified lympho