Transcriptional Regulation of Human CD5: Important Role of Ets Transcription Factors in CD5 Expression in T Cells

Abstract

CD5 is a surface receptor constitutively expressed on thymocytes and mature T and B-1a cells. CD5 expression is tightly regulated during T and B cell development and activation processes. In this study we shown that the constitutive expression of CD5 on human T cells correlates with the presence of a DNase I-hypersensitive (DH) site at the 5′-flanking region of CD5. Human CD5 is a TATA-less gene for which 5′-RACE analysis shows multiple transcriptional start sites, the most frequent of which locates within an initiator sequence. Luciferase reporter assays indicate that a 282-bp region upstream of the initiation ATG displays full promoter activity in human T cells. Two conserved Ets-binding sites (at positions −239 and −185) were identified as functionally relevant to CD5 expression by site-directed mutagenesis, EMSAs, and cotransfection experiments. A possible contribution of Sp1 (−115 and −95), c-Myb (−177), and AP-1-like (−151) motifs was also detected. Further DH site analyses revealed an inducible DH site 10 kb upstream of the human CD5 gene in both T and B CD5+ cells. Interestingly, a 140-bp sequence showing high homology with a murine inducible enhancer is found within that site. The data presented indicate that the 5′-flanking region of human CD5 is transcriptionally active in T cells, and that Ets transcription factors in conjunction with other regulatory elements are responsible for constitutive and tissue-specific CD5 expression.