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* Servei d’Immunologia, Institut d’Investigacions Biomèdiques August Pi i Sunyer, Hospital Clínic, Barcelona, Spain;
Department of Medicine, Division of Rheumatology and Immunology, Medical University of South Carolina, Charleston, SC 29425;
Molecular Medicine Unit, University of Leeds, Saint James University Hospital, Leeds, United Kingdom;
Biología Molecular, Hospital de la Princesa, Madrid, Spain; and
¶ Unitat d’Immunologia, Departament de Biologia Cellular i Anatomia Patològica, Facultat de Medicina, Universitat de Barcelona, Barcelona, Spain
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.
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