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,§
*
Institute of Immunology, Departments of
Pathology and
Urology, University Hospital Benjamin Franklin, Free University Berlin, Berlin, Germany;
§
Berufsgenossenschaftliches Forschungsinstitut für Arbeitsmedizin, Bochum, Germany; and
¶
Department of Internal Medicine IV, Friedrich-Alexander-University, Erlangen, Germany
OX40 is a member of the TNF receptor family, expressed on activated
T cells. It is the only costimulatory T cell molecule known to be
specifically up-regulated in human T cell leukemia virus type-I
(HTLV-I)-producing cells. In a T cell line, OX40 surface expression was
shown to be induced by HTLV-I Tax alone. To understand molecular
mechanisms of OX40 gene regulation and modulation by HTLV-I Tax, we
have cloned the human OX40 gene and analyzed its 5'-flanking region. By
reporter gene analysis with progressive 5' deletions from nucleotides
-1259 to -64, we have defined a 157-bp DNA fragment as a minimal
promoter for constitutive expression. In addition, we show that in the
OX40+ cell line, Co, Tax is able to further increase OX40
surface expression. Up-regulation of OX40 promoter activity by Tax
requires two upstream NF-
B sites, which are not active in the
constitutive OX40 expression. Their deletion abrogates Tax
responsiveness in reporter gene analysis. The site-directed mutagenesis
of each NF-B site demonstrates that cooperative NF-B binding is a
prerequisite for Tax-directed activity as neither site alone is
sufficient for a full Tax responsiveness of the OX40 promoter. Upon Tax
expression, both sites bind p65 and c-Rel. These data provide new
insight into the direct regulation of OX40 by Tax and add to our
understanding of the possible role of the OX40/OX40 ligand system in
the proliferation of HTLV-I+ T cells.
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