Two NFAT Transcription Factor Binding Sites Participate in the Regulation of CD95 (Fas) Ligand Expression in Activated Human T Cells

Abstract

Antigen receptor engagement on T lymphocytes activates transcription factors important for stimulating cytokine gene expression. This is critical for clonal expansion of antigen-specific T cells and propagation of immune responses. Additionally, under some conditions antigen receptor stimulation initiates apoptosis of T lymphocytes through the induced expression of CD95 ligand and its receptor. Here we demonstrate that the transcription factor, NFAT, which is critical for the inducible expression of many cytokine genes, also plays a critical role in the regulation of T cell receptor-mediated CD95 ligand expression. Two sites within the CD95 ligand promoter, identified through DNase I footprinting, bind NFAT proteins from nuclear extracts of activated T cells. Although both sites appear important for optimal expression of CD95 ligand in activated T cells, mutational analysis suggests that the distal NFAT site plays a more significant role. Furthermore, these sites do not appear to be required for constitutive CD95 ligand expression in Sertoli cells.Adaptive immune responses require the activation of T lymphocytes through antigen-specific T cell receptor (TCR) 1 stimulation. Signaling events initiated by TCR ligation lead to the activation of transcription factors that regulate expression of cytokine genes, such as IL-2. This is important for the clonal expansion of antigen-specific T cells and propagation of immune responses (1, 2). However, once the antigenic stimulus has been cleared, the expanded population of cells must be eliminated to prevent accumulation of excessive lymphocytes (2, 3). Recently, it has been proposed that one mechanism by which this occurs is through the induced expression of CD95 ligand (4 -7). Once expressed, CD95 ligand engages its receptor, CD95, also expressed on the population of activated lymphocytes (8). In the absence of costimulatory signals that can delay apoptosis (9 -12), CD95 ligation rapidly initiates the programmed cell death machinery thus efficiently eliminating excessive activated lymphocytes (8). Additionally, autoreactive T cells that are inappropriately activated in the periphery are believed to undergo apoptosis through a similar process (13).The significance of CD95 ligand expression in the process of activation-induced cell death has been highlighted by recent studies that demonstrate that blocking CD95/CD95 ligand interactions prevents apoptosis of TCR-stimulated lymphocytes (5, 7). Interestingly, in addition to its inducible expression on activated lymphocytes, CD95 ligand is constitutively expressed on epithelial cells within the eye and Sertoli cells within the testes (14 -16). Constitutive CD95 ligand expression participates in maintenance of the "immune privileged" status of these tissues by inducing apoptosis in infiltrating, CD95-bearing, activated lymphocytes (17). Despite improved understanding of the important physiological roles for CD95 ligand in immune privileged sites and in controlling T cell homeostasis, little is yet known about the regulation of ...

Increased proliferation of CD8+ T cells in SAP‐deficient mice is associated with impaired activation‐induced cell death

“…This may be due to complex regulation of FasL transcription. The major positive regulators controlling FasL transcription are NFAT, AP-1, Egr2/3, c-Myc and NF-jB, among which only the activation of NF-jB p50 has been shown to be defective in SAP -/-CD4 + T cells, whereas NFAT and AP-1 activation are normal [52][53][54][55]. Thus, it is unlikely that FasL induction in SAP -/-CD8 + T cells will be significantly lower than that in WT cells.…”

The Regulation of CD95 (Fas) Ligand Expression in Primary T Cells: Induction of Promoter Activation in CD95LP-Luc Transgenic Mice

“…This segment of the promoter extends in a 5Ј direction from the translational start site and encompasses all previously identified response elements (15)(16)(17)(18)(19)(20)(21)(22)(23). In the present study we have used this strain, designated CD95LP-Luc, to examine the regulation of CD95L promoter induction of luciferase activity (CD95L/luciferase) in a variety of primary murine T cell populations.…”

“…Using the Jurkat T cell line as a model system, we and others have demonstrated a role for NF-AT in mediating transcriptional activation of the CD95L promoter (15)(16)(17)(18). Other transcription factors, such as NF-B, Sp1, and Egr-2 and -3, also contribute to inducible CD95L promoter activity (19 -22).…”

“…Thus, most studies have measured either CD95L mRNA levels, generally by RT-PCR, or protein expression, indirectly through bioassays. Studies of the CD95L promoter have relied almost exclusively on transient or stable transfections of promoter reporter constructs into T cell lines (15)(16)(17)(18)(19)(20)(21)(22)(23)25).…”

Hepatitis B Virus X Protein Induced Expression of the Nur77 Gene

“…direct binding site for Nur77 (NBRE or NurRE) was not found in the promoter of FasL (34) indicating that the Nur77 may activate FasL by an indirect mechanism. Instead, the nuclear factors of activated T-cells (NFATs) directly binds an enhancer sequence on FasL promoter that plays a key role on the expression of FasL (36,37). Recently Lara-Pezzi et al (35) showed that HBx induced the dephosphorylation of the nuclear factor of activated T cells (NFAT), which caused translocation and transcriptional activation of the protein through a CsA-dependent pathway.…”